Colorants and auxiliaries
ORGANIC CHEMISTRY AND APPLICATION PROPERTIES
Second Edition
Volume 2 – Auxiliaries
Edited by John Shore
Formerly of BTTG/Shirley and ICI Dyes (now DyStar), Manchester, UK
2002
Society of Dyers and Colourists
iii
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Copyright © 2002 Society of Dyers and Colourists. All rights reserved. No part of this publication
may be reproduced, stored in a retrieval system or transmitted in any form or by any means without
the prior permission of the copyright owners.
Published by the Society of Dyers and Colourists, PO Box 244, Perkin House, 82 Grattan Road,
Bradford, West Yorkshire BD1 2JB, England, on behalf of the Dyers’ Company Publications
Trust.
This book was produced under the auspices of the Dyers’ Company Publications Trust. The Trust
was instituted by the Worshipful Company of Dyers of the City of London in 1971 to encourage
the publication of textbooks and other aids to learning in the science and technology of colour and
coloration and related fields. The Society of Dyers and Colourists acts as trustee to the fund.
Typeset by the Society of Dyers and Colourists and printed by Hobbs The Printers, Hampshire, UK.
ISBN 0 901956 78 3
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Contributors
John Shore
Formerly of BTTG/Shirley and ICI Dyes (now DyStar), Manchester, UK
Terence M Baldwinson
Formerly Dye & Information Service Manager, Yorkshire Chemicals plc, Leeds, UK
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Contents
Preface
ix
CHAPTER 8
Functions and properties of dyeing and printing auxiliaries
8.1
8.2
The need for auxiliaries 471
The general types and characteristics of auxiliaries 474
References 476
CHAPTER 9
The chemistry and properties of surfactants
9.1
9.2
9.3
9.4
9.5
9.6
9.7
9.8
Introduction 477
Hydrophiles 477
Hydrophobes 477
Anionic surfactants 479
Cationic surfactants 485
Nonionic surfactants 486
Amphoteric surfactants 489
The general properties of surfactants 490
References 496
CHAPTER 10
Classification of dyeing and printing auxiliaries by function
10.1
10.2
10.3
10.4
10.5
10.6
10.7
10.8
Electrolytes and pH control 497
Sequestering agents 505
Macromolecular complexing agents 519
Enzymes 539
Preparation of substrates 553
Dispersing and solubilising agents 636
Levelling and retarding agents 642
Thickening agents, migration inhibitors and hydrotropic
agents used in printing and continuous dyeing 645
Treatments to alter dyeing properties or enhance fastness 664
Agents for fibre lubrication, softening, antistatic effects,
soil release, soil repellency and bactericidal activity 705
Foaming and defoaming agents 744
References 750
10.9
10.10
10.11
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471
477
497
CHAPTER 11
Fluorescent brightening agents
760
11.1
11.2
11.3
11.4
11.5
11.6
11.7
11.8
11.9
11.10
11.11
11.12
11.13
Introduction 760
Mode of action of a fluorescent brightener 761
Evaluation of FBAs: measurement of whiteness 765
General factors influencing FBA performance 768
Chemistry and applications of FBAs 770
Brighteners for cellulosic substrates 770
Brighteners for cellulose acetate and triacetate fibres 781
Brighteners for nylon 784
Brighteners for wool 788
Brighteners for polyester fibres 790
Brighteners for acrylic fibres 799
Brighteners in detergent formulations 803
Analysis of FBAs 809
References 811
CHAPTER 12
Auxiliaries associated with main dye classes
12.1
12.2
12.3
12.4
12.5
12.6
12.7
12.8
12.9
Introduction 813
Acid dyes 813
Azoic components 820
Basic dyes 824
Direct dyes 832
Disperse dyes 837
Reactive dyes 856
Sulphur dyes 882
Vat dyes 893
References 913
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813
Preface to Volume 2
This Second Edition of a textbook first published in 1990 forms part of a series on colour
and coloration technology initiated by the Textbooks Committee of the Society of Dyers and
Colourists under the aegis of the Dyers’ Company Publications Trust Management
Committee, which administers the trust fund generously provided by the Worshipful
Company of Dyers.
The initial objective of this series of books has been to establish a coherent body of
explanatory information on the principles and application technology of relevance for
students preparing to take the Associateship examinations of the Society. This particular
book has been directed specifically to the subject areas covered by Section A of Paper B: the
organic chemistry and application of dyes and pigments and of the auxiliaries used with
them in textile coloration processes. However, many qualified chemists and colourists
interested in the properties of colorants and their auxiliaries have found the First Edition
useful as a work of reference. For several reasons it has been convenient to divide the
material into two separate volumes: 1. Colorants, 2. Auxiliaries. Although fluorescent
brighteners share some features in common with colorants, they have been treated as
auxiliary products in this book.
This second volume of the book collects together a remarkable quantity and variety of
factual information linking the application properties of auxiliary products in textile
coloration and related processes to as much as is known of the chemical structure of these
agents. The environmental impact of auxiliary products has become of major importance
and developments during the 1990s have necessitated substantial modification and
expansion of the text of this volume. The opportunity has also been taken to highlight novel
chemical types of auxiliaries that are under evaluation to overcome or avoid many of the
drawbacks shown by traditional products. Thus the two volumes of this Second Edition are
now approximately equal in size, whereas in the 1990 edition Volume 2 was only about half
as big as its sibling.
Virtually all of this development and improvement of Volume 2, especially in the much
expanded Chapters 10 and 12, is thanks to the thorough and painstaking work of Terry
Baldwinson, who has carefully sifted through an extensive yet scattered range of primary
sources. Our grateful thanks are due to John Holmes and Catherine Whitehouse for their
patient copy editing and to the publications staff of the Society, especially Carol Davies, who
have prepared all the material in this new edition for publication.
JOHN SHORE
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Chapters in Volume 1
Chapter 1
Classification and general properties of colorants
Chapter 2
Organic and inorganic pigments; solvent dyes
Chapter 3
Dye structure and application properties
Chapter 4
Chemistry of azo colorants
Chapter 5
Chemistry and properties of metal-complex and mordant dyes
Chapter 6
Chemistry of anthraquinonoid, polycyclic and miscellaneous colorants
Chapter 7
Chemistry of reactive dyes
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471
CHAPTER 8
Functions and properties of dyeing and printing
auxiliaries
Terence M Baldwinson
8.1 THE NEED FOR AUXILIARIES
There is hardly a dyeing or printing process of commercial importance that can be
adequately operated by the use of dyes and water alone. Practically every colorant–substrate
system requires the use of additional products, known as auxiliaries, to ensure its reliable
functioning and control. This was the case even centuries ago, when the use of natural vat
and mordant dyes depended entirely on the proper, albeit rule of thumb, use of additives.
These controlled pH, reduction, oxidation and mordanting to enable the dyes to be applied
to the natural fibres of those days. Many of the auxiliaries, like the dyes and fibres, were of
natural origin. Dung and urine [1] were among the agents used, and soap was clearly the
first surfactant to be employed. Indeed, from the standpoint of today, one can only wonder
at the degree of purely empirical expertise so successfully developed and applied by the
ancient dyers and printers.
Our current level of understanding is clearly a phenomenal advance on the ancient arts,
yet our need for auxiliaries remains. For example, even before dyeing or printing the
substrate must be cleaned and wetted. Products are needed to convert non-substantive vat
and sulphur dyes to substantive forms, to help stabilise the conditions that bring about the
substantivity, and then to reconvert the dyes to their insoluble forms in the substrate
(sections 1.6.1 and 1.6.2). Mordant dyes still require the appropriate chelating agents, as
well as other agents to create and maintain the optimum chelating conditions (section 5.8).
Printers still need thickening agents to facilitate the localised application of dyes. Inevitably,
however, the present-day plethora of dyes, fibres and coloration processes has created
additional reasons for the use of auxiliaries, whilst the concurrent evolution of the chemical
industry has satisfied these needs as they arose. Moreover, a vastly more comprehensive
understanding of the physico–chemical processes involved has enabled auxiliaries to be
precisely engineered for specific purposes.
Hand in hand with this theoretical knowledge, practical evaluation has become
increasingly sophisticated. Nevertheless, it is often difficult to differentiate between
auxiliaries promoted purely for commercial reasons and those that serve a definite technical
need. Dye manufacturers are acutely aware of the positive part played by auxiliaries in
helping to sell dyes, and dyers today are under constant pressure to use more of them. Some
additives offer cost savings by improving reproducibility and minimising reprocessing;
nevertheless it is all too tempting to incorporate too many products without critically
evaluating their efficacy, thus inevitably and unnecessarily increasing processing costs.
Consequently it is more important than ever that the dyer or printer understands the
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FUNCTIONS AND PROPERTIES OF DYEING AND PRINTING AUXILIARIES
functions of auxiliary products and is equipped to evaluate their use realistically and to
monitor it continually.
As has been implied already, functional demands for auxiliaries continue to grow, with
each dye–fibre system and dyeing or printing process having particular needs. The primary
functions of auxiliaries are:
(a) to prepare or improve the substrate in readiness for coloration by
– scouring, bleaching and desizing
– wetting
– enhancing the whiteness by a fluorescent brightening effect
(b) to modify the sorption characteristics of colorants by
– acceleration
– retardation
– creating a blocking or resist effect
– providing sites for sorption
– unifying otherwise divergent rates of sorption
– improving or resisting the migration of dyes
(c) to stabilise the application medium by
– improving dye solubility
– stabilising a dispersion or solution
– thickening a print paste or pad liquor
– inhibiting or promoting foaming
– forming an emulsion
– scavenging or minimising the effects of impurities
– preventing or promoting oxidation or reduction
(d) to protect or modify the substrate by
– creating or resisting dyeability
– lubricating the substrate
– protecting against the effects of temperature and other processing conditions
(e) to improve the fastness of dyeings, as in
– the aftertreatment of direct or reactive dyes
– the aftertreatment of acid dyes on nylon
– the chroming of mordant dyes on wool or nylon
– giving protection against atmospheric influences, as in UV absorbers or inhibitors of
gas-fume fading
– back-scouring or reduction clearing
(f) to enhance the properties of laundering formulations (fluorescent brightening agents).
Some auxiliaries fulfil more than one of the above functions. For example, an auxiliary to
improve dye solubility may also accelerate (or retard) a coloration process, or an emulsifying
agent may also act as a thickening agent; pH-control agents may both stabilise a system and
also affect the rate of dye sorption.
Thus the range of auxiliaries available is very large indeed, covering a multiplicity of uses
for all stages of textile processing. However, a factor which has assumed great importance
regarding the use of auxiliaries in recent years is that of their effects on the environment. In
view of the extensive portfolio of products and processes, it is not surprising that good
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THE NEED FOR AUXILIARIES
473
environmental management is complex. Undesirable effects from the use of auxiliaries may
become evident during handling, through effluent discharge to surface waters, through
discharge to the atmosphere (e.g. via stenter gases), through consumer contact with the
finished product (e.g. skin sensitivity) or during the eventual disposal of solid wastes (e.g.
incineration or landfill). All these factors need careful consideration in the selection of
auxiliaries at all stages of processing. Compliance with good environmental practice may be
voluntary (preferably) or enforced by legislation, some countries having introduced quite
extensive and stringent requirements [2,3].
Many factors need to be considered: acute toxicity to mammals, toxicity to aquatic
organisms (fish, daphniae, algae) and waste water bacteria, biodegradability (aerobic or
anaerobic), abiotic degradability (hydrolysis, photolysis, oxidation), ground mobility,
bioaccumulation, carcinogenicity, mutagenicity and teratogenicity.
The textile wet processing industry produces particularly heavy discharges of effluent;
hence the responsibility placed on it for environmentally good behaviour is indeed an
onerous one, both technically and financially [4]. The preparation processes of desizing,
scouring and bleaching, together with their associated wash-off processes, inevitably produce
a heavy biological oxygen demand in the effluent [5]. Hence there has been, and continues
to be, much research effort to improve the environmental performance of these areas.
There are two general approaches to good environmental practice. The first, termed ‘endof-pipe’ solutions, requires all unacceptable matter to be removed from the effluent, or at
least to be reduced to acceptable levels. This is relatively difficult and expensive, requiring
the appropriate treatment facilities. The second attempts to minimise the need for end-ofpipe treatments by reducing the hazardous nature of the effluent in the first place. This can
be achieved, for example, by recycling and reusing useful constituents, especially reducing
water volumes through the use of low liquor ratios, and reducing the toxicity of the effluent
by selecting ‘green’ chemicals and processing methods. In some cases, the volume of effluent
can be reduced by combining some processes, e.g. desizing, scouring and bleaching. The
possibilities inherent in the second approach have stimulated much research work amongst
manufacturers and suppliers of auxiliaries, in terms of finding ‘greener’ products and more
acceptable processes for their use. These efforts will continue for the foreseeable future.
It is important to remember that auxiliaries nowadays are most frequently supplied as
more or less complex mixtures. It is essential from an environmental point of view to
consider the influence of the subsidiary components in the branded product as well as of the
main substances. For example, even a small amount of solvent added to improve the stability
of an auxiliary may pose problems of flammability or toxic vapours, necessitating careful
storage and labelling. Constant monitoring of products and processes is necessary, since
environmental requirements are continually changing as more information, matched by
increasing awareness of hazards, becomes available. It has been suggested that the main
concerns to date have been in response to controlling listed substances rather than in
tackling fundamental environmental problems [5].
Wragg [6] has provided the following basic environmental check-list for textile wet
processing:
(1) Is the product free from species and/or by-products that are on the various control lists?
Lists of controlled chemicals are being continually extended, together with controls on
their use. Usage of heavy metals, solvents and AOX-containing products will gradually
decrease.
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FUNCTIONS AND PROPERTIES OF DYEING AND PRINTING AUXILIARIES
(2) Is the manufacturing process suitable for safety and environmental controls? Processing
cycles are being placed under greater control in terms of machine operation. Auxiliary
usage will be more specific and accurate to avoid over-consumption and to minimise
waste.
(3) Has the product or its components been assessed at any time for acute toxicity,
carcinogenicity or irritant properties?
(4) Are the materials easily dealt with in waste treatment systems?
(5) Has the product or its components been assessed at any time for toxicity to aquatic
species?
The overall result of environmental awareness has been to increase the interplay of
devolution and evolution. Devolution has seen increasing restrictions, sometimes amounting
to a complete ban, on the use of certain substances (e.g. alkylphenolethoxylates, which were
once widely used) and a corresponding evolution of new products which, at least for the
present, are environmentally acceptable. This interplay between devolution and evolution is
likely to continue indefinitely. Environmental factors as they affect specific types of auxiliary
will be dealt with under the relevant sections of this volume.
8.2 THE GENERAL TYPES AND CHARACTERISTICS OF AUXILIARIES
An auxiliary has been defined [7] as ‘a chemical or formulated chemical product which
enables a processing operation in preparation, dyeing, printing or finishing to be carried out
more effectively, or which is essential if a given effect is to be obtained’. It is much harder to
devise a classification system for auxiliaries than it is for dyes. This is undoubtedly one of the
main reasons why there has been no incentive to produce an auxiliaries index comparable
with the Colour Index. It is difficult enough to put together a comprehensive yet manageable
list of general application types; it becomes even more difficult to classify them chemically,
especially as many of them are more or less complex mixtures, are of imprecisely known
structure or are the subject of a good deal of trade confidentiality. Although the Society of
Dyers and Colourists has shown reluctance to be involved in this area, a very useful biennial
trade publication [8] has made considerable progress in the ordered listing of currently
available commercial products. This first appeared in 1967. The seventeenth edition (2000)
lists currently available products by trade name, application and suppliers. Unfortunately
this publication does not include a listing by chemical type. Although the first section does
give whatever chemical detail the manufacturers are prepared to divulge, these tend to be
bland, broadly based descriptions such as ‘nonionic aqueous emulsion of modified wax’ or
‘quaternary ammonium compound, cationic’. In spite of these shortcomings it remains an
indispensable guide to the vast range of products on the market today.
The broadest classification of auxiliaries is achieved simply by dividing them into nonsurfactants and surfactants, as detailed below.
Non-surfactants include simple electrolytes, acids and bases, both inorganic and organic.
Examples include sodium chloride, sodium acetate, sulphuric acid, acetic acid and sodium
carbonate, together with complex salts (such as sodium dichromate, copper(II) sulphate,
sodium ethylenediaminetetra-acetate, sodium hexametaphosphate), oxidising agents
(hydrogen peroxide, sodium chlorite) and reducing agents (sodium dithionite, sodium
sulphide). Anionic polyelectrolytes such as sodium alginate or carboxymethylcellulose, used
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THE GENERAL TYPES AND CHARACTERISTICS OF AUXILIARIES
475
mainly as thickening agents and migration inhibitors, also fall within the class of nonsurfactants; so too do sorption accelerants such as o-phenylphenol, butanol and
methylnaphthalene, although they normally require an emulsifier to stabilise them in
aqueous media. Fluorescent brightening agents (FBAs) form another large class of nonsurfactant auxiliaries (see Chapter 11).
Surfactants are, in general, substantially organic in nature and structurally more complex
than most non-surfactants. It is difficult to define surfactants in a manner sufficiently
precise to satisfy everyone. However, for the purposes of this book an adequate definition of
a surfactant is given by the Society’s Terms and Definitions Committee [7]: ‘an agent,
soluble or dispersible in a liquid, which reduces the surface tension of the liquid’. In
coloration processes this reduction in surface tension usually takes place at a liquid/liquid or
liquid/solid interface, although liquid/gas interfaces are also occasionally important. In
general, a dramatic lowering of surface tension can be brought about by a relatively small
amount of surfactant; as little as 0.2 g/l of a soap such as sodium oleate will more than halve
the surface tension of water. This physical effect in solution is attributed to the molecular
orientation potential of a relatively small hydrophilic moiety (a hydrophile) having strong
polar forces, juxtaposed with a relatively large (usually linear) hydrophobic moiety (a
hydrophobe) having relatively weak electrostatic forces (Figure 8.1). In aqueous solution or
dispersion the polar hydrophile tends to be oriented into the body of the aqueous phase,
whilst the hydrophobe, by nature subjected to forces of repulsion by the aqueous phase, is
oriented towards (or at) the interfacial boundary, which may be that between the solution
and air or between the solution and a fibrous (or other) substrate.
The surfactants used as textile auxiliaries can be divided into four major groups,
depending on the type and distribution of the polar forces, an arrangement broadly
resembling the ionic classification of dyes. The general scheme is shown in Table 8.1.
Strongly
polar
hydrophile
Weakly polar hydrophobe
Figure 8.1 Schematic diagram of surfactant
Table 8.1 General classification of surfactants
Degree of ionic charge on the
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Hydrophile
(associated ion)
Class of surfactant
Hydrophobe
Anionic
Cationic
Nonionic
Amphoteric
Weakly negative
Strongly positive
Weakly positive
Strongly negative
Uncharged
Uncharged
These possess balanced negative and positive charges,
one or other of which dominates in solution depending
on the pH
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FUNCTIONS AND PROPERTIES OF DYEING AND PRINTING AUXILIARIES
REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
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H T Pratt, Text. Chem. Colorist, 19 (1987) 23.
S Helman, Melliand Textilber., 72 (1991) 567.
W Baumann, U Engler,. W Keller and W Schefer, Textilverediung, 27 (1992) 392.
M Lomas, J.S.D.C., 109 (1993) 10.
J Park and J Shore, J.S.D.C., 100 (1984) 383.
P Wragg, J.S.D.C., 110 (1994) 137
Colour terms and definitions (Bradford: SDC, 1988).
Index to textile auxiliaries, 17th Edn (Bradford: World Textile Publications, 2000).
476
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477
CHAPTER 9
The chemistry and properties of surfactants
Terence M Baldwinson
9.1 INTRODUCTION
A surfactant was defined in Chapter 8 as: ‘an agent, soluble or dispersible in a liquid, which
reduces the surface tension of the liquid’ [1]. It is helpful to visualise surfactant molecules as
being composed of opposing solubility tendencies. Thus, those effective in aqueous media
typically contain an oil-soluble hydrocarbon-based chain (the hydrophobe) and a smaller
water-solubilising moiety which may or may not confer ionic character (the hydrophile).
The limitations of space do not permit a comprehensive detailed treatment of the chemistry
of surfactants. The emphasis is therefore on a broad-brush discussion of the principal types
of surfactant encountered in textile preparation and coloration processes. Comprehensive
accounts of the chemistry and properties of surfactants are available [2–13]. A useful and
lucid account of the chemistry and technology of surfactant manufacturing processes is
given by Davidsohn and Milwidsky [14].
9.2 HYDROPHILES
The basic purpose of the hydrophile is to confer solubility (aqueous solubility is always to be
understood unless otherwise stated). The simple moieties most often employed are as
follows:
(a) in anionic surfactants: sodium, potassium or ammonium cations, associated with
negatively charged groups on the hydrophobe such as carboxylate, sulphonate, sulphate
or phosphate
(b) in cationic surfactants: chloride, bromide or methosulphate ions, juxtaposed with, for
example, positively charged quaternary nitrogen atoms
(c) in nonionic surfactants: ethylene oxide or propylene oxide moieties.
More complex hydrophilic moieties are sometimes encountered, however, such as mono-,
di- and tri-ethanolamine and the corresponding isopropanolamines in anionic surfactants.
Morpholine, once employed, is now obsolete owing to its toxicity.
9.3 HYDROPHOBES
There is a much wider choice of hydrophobes. Most are based on substantially linear longchain alkanes, either saturated or unsaturated. These were originally obtained from naturally
occurring fats and oils such as castor, fish, olive, sperm, coconut and tallow oils, but these
sources were later superseded by petroleum products which at that time were cheaper. More
recently, not only has the price of crude oil escalated, but there has also been a growing
477
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THE CHEMISTRY AND PROPERTIES OF SURFACTANTS
awareness of the finite and diminishing nature of this resource. In 1995, some 75% fossilsourced raw materials were used in the production of synthetic anionic surfactants (90% if
lignosulphonates are excluded) [8], but it is foreseen that more biological materials will be
used in the future. It is evident that for some time there has been a systematic and large
expansion of vegetable oil production, especially in South East Asia [8]. Typical vegetable
oils include: tallow, coconut, palm kernel, palm, soybean, linseed, cotton, rape and
sunflower. Most cationic surfactants are still obtained from petrochemical olefins, alcohols,
paraffins and aromatics, although some are derived from fatty acids [9].
The most common hydrophobes used as the basis for surfactants are those containing
eight to eighteen carbon atoms, such as those listed as carboxylates in Table 9.1. Some
hydrophobes are aromatic (benzene or naphthalene) moieties, often containing lower alkyl
substituents; dodecylbenzene (9.1) is a common example. Alkyl-substituted toluenes,
xylenes and phenols, and mono- and di-alkylated naphthalenes (9.2 and 9.3), are also used.
Table 9.1 Examples of hydrophobes
No. of
carbon
atoms
Chemical
name
Trivial name and formula
8
Octanoate
Caprylate
CH3(CH2)6COO
10
Decanoate
Caprate
CH3(CH2)8COO
12
Dodecanoate
Laurate
CH3(CH2)10COO
12
9-Dodecenoate
Lauroleate
CH3CH2CH=CH(CH2)7COO
14
Tetradecanoate
Myristate
CH3(CH2)12COO
14
9-Tetradecenoate
Myristoleate
CH3(CH2)3CH=CH(CH2)7COO
15
Pentadecanoate
Isocetate
CH3(CH2)13COO
16
Hexadecanoate
Palmitate
CH3(CH2)14COO
16
9-Hexadecenoate
Palmitoleate
CH3(CH2)5CH=CH(CH2)7COO
17
Heptadecanoate
Margarate
CH3(CH2)15COO
18
Octadecanoate
Stearate
CH3(CH2)16COO
18
9-Octadecenoate
Oleate
CH3(CH2)7CH=CH(CH2)7COO
18
9,12-Octadecadienoate
Linoleate
CH3(CH2)4(CH=CHCH2)2(CH2)6COO
18
9,12,15-Octadecatrienoate
Linolenate
CH3CH2(CH=CHCH2)3(CH2)6COO
18
12-Hydroxy-9-octadecenoate
Ricinoleate
CH3(CH2)5CH(OH)CH2CH=CH(CH2)7COO
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ANIONIC SURFACTANTS
C12H25
C8H17
479
C4H9
H9C4
9.2
9.1
9.3
The hydrophobes are usually, though not always, used in the form of acids, alcohols,
esters or amines. Commercial products rarely contain a single pure hydrophobe, however;
most are mixtures containing a range of hydrophobes, since the raw materials from which
they are made are generally themselves mixtures of homologues. For example, a batch of
coconut oil, a rich source of the lauric hydrophobe, may have the approximate composition
shown in Table 9.2, although the proportions of the individual components may vary by
1–3% between batches. As is the general rule in naturally occurring fats and waxes, only
even-numbered carbon compounds are present; odd-numbered ones have to be made by
synthesis. Clearly, a surfactant produced from such a mixture will contain a very large, and
variable, number of homologues and isomers. Hence two products with the same nominal
constitution, but from different manufacturers, often differ in details of composition and
properties. This is one fundamental reason why a chemical classification of auxiliaries,
analogous to that for dyes in the Colour Index, would be extremely difficult to devise.
Table 9.2 Approximate hydrophobe composition of
coconut oil
Trivial
name
No. of
carbon atoms
Amount
(%)
Caproate
Caprylate
Caprate
Laurate
Myristate
Palmitate
Stearate
Oleate
Linoleate
6
8
10
12
14
16
18
18
18
0.5
7.0
6.5
49.5
17.0
8.5
2.5
6.5
2.0
Any hydrophobe can yield each of the main (i.e. anionic, cationic, nonionic or
amphoteric) types of surfactant in much the same way as the same chromogenic system can
be used in anionic, basic or disperse dyes. This will be demonstrated in the following
sections, dealing with each class of surfactant, using the cetyl-containing (C 16H33)
hydrophobe.
9.4 ANIONIC SURFACTANTS
Until recently this class accounted for by far the largest number of surfactants used in
preparation and coloration processes. This dominance is now challenged by the much
increased use of nonionic types. The essential feature of the class is a long-chain
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THE CHEMISTRY AND PROPERTIES OF SURFACTANTS
hydrophobe linked through an anionic grouping – usually carboxylate, sulphate (sulphuric
ester) or sulphonate, but occasionally phosphate, carboxymethyl or other group – to a
relatively small cation, generally sodium, although ammonium, potassium and other cations
are also used.
Carboxylates (9.4, where R is the long-chain hydrophobe and X the cation) represent the
oldest type of surfactants, since they could be obtained from naturally occurring fats and oils
long before the advent of the petrochemical industry; sodium heptadecanoate (9.5), for
example, incorporates the cetyl group as hydrophobe. Sodium stearate, sodium palmitate
and sodium oleate are the simplest carboxylates generally used as surfactants. Alkylaryl
compounds (9.6) are also known.
_
_
R
COO
X
+
_
C16H33COO
H25C12
+
Na
Na +
9.5
9.4
COO
9.6
Many carboxylates are used in the form of soaps, obtained by alkaline saponification of
triglyceride fats and waxes of general formula 9.7. The three carboxylic ester groups
(RCOO) may carry the same or different hydrophobes, generally containing eight to 22
carbon atoms, the most common being laurate, palmitate and stearate among the saturated
types, and oleate and linoleate among the unsaturated ones. At ambient temperatures the
unsaturated fats tend to be liquids and the saturated ones solids.
R
CH2
R1
COO
OOC
R2
OOC
R3
CH
CH2
HC
CH
CH
CH2
9.7
_
COO
_
COO
Na+
+
Na
9.8
Particularly important as wetting agents are the disodium alkenylsuccinates (9.8), in
which the saturated R group may contain from three to fourteen carbon atoms. The
surfactant properties of these carboxylates, as with other types of surfactant, are dependent
on the number of carbon atoms in the hydrophobe. Significant surfactant properties begin to
appear in the C8 compounds, although the C8–C12 carboxylates are wetting agents rather
than detergents. Better detergency and emulsifying properties become evident with C12–C18
alkyl groups. Solubility decreases with increasing length of the alkyl group; the solubility of
soaps, for example, reaches its useful limit with the C22 compounds. The major disadvantage
of the carboxylates is that they tend to be precipitated by acids and hard water, since the free
acids and the calcium and magnesium salts of the carboxylates are insoluble. This
disadvantage provided the main technical reason for finding alternative products that
showed tolerance to a wider range of processing conditions.
Modified carboxylates, in which the carboxylate moiety forms part of a carboxymethoxy
group, are also available. These are made by reaction of selected nonionic surfactants with
chloroacetic acid. The result is a useful hybrid range, lacking the sensitivity of simple
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ANIONIC SURFACTANTS
481
carboxylates to calcium and magnesium whilst retaining excellent detergency; these
compounds are more stable to electrolytes than are the conventional nonionics and more
suitable for use at high temperatures as they are not susceptible to cloud point problems
(section 9.8.2).
Sulphates or sulphuric esters of the long-chain fatty acids were the first alternative to the
carboxylates. They are essentially the half esters of sulphuric acid (9.9); the ester
incorporating the cetyl hydrophobe (9.10) belongs to the important class of fatty alcohol
sulphates. Such sulphates, using C8–C18 hydrophobes, are common.
R
_
+
OSO3 Na
_
C16H33OSO3 Na +
9.10
9.9
Just as there are mono-, di- and tri-carboxylate surfactants, the sulphates can also be
prepared from products bearing mono-, di- and tri-hydrophobes. Indeed, the first sulphates
to be used were analogous to soaps in that they were the sulphation products of triglycerides.
Although their chemistry can be represented in simple terms, it is worth re-stating that most
commercial products are highly complex mixtures. For example, a sulphated triglyceride may
contain the following:
– the sulphated glyceride proper
– sulphated free fatty acid
– unsulphated glyceride
– unsulphated fatty acid
– inorganic salts
– traces of glycerol.
The range of hydrophobes present may also be unexpectedly broad, since the raw materials
often consist of mixtures of symmetrical and/or mixed glycerides. As little as 60% of an oil
may be sulphatable; sulphation is never carried to theoretical completion and is often far
below 100%. With these provisos in mind, the chemistry of the sulphated oils can be
considered.
Many oils are used as starting materials: olive, castor, tallow, neatsfoot, cotton seed, rape
seed and corn oils are examples. Sulphated olive oil was the first sulphated oil to be
produced and was used as a mordant in dyeing as long ago as 1834. Sulphation usually
occurs at the double bonds of any unsaturated fatty acids in the glyceride (Scheme 9.1). On
the other hand, in the preparation of the best-known of these products, Turkey Red Oil or
sulphated (often wrongly termed ‘sulphonated’) castor oil, sulphation of the main
component, the glyceride of ricinoleic acid (12-hydroxy-9-octadecenoic acid), takes place
preferentially at the hydroxy group rather than at the double bond (Scheme 9.2). Such
products possess useful wetting, emulsifying and dye-levelling properties.
CH
CH
+ H2SO4
CH2
CH
_
OSO3
Scheme 9.1
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+
H
482
THE CHEMISTRY AND PROPERTIES OF SURFACTANTS
CH
CH2
CH
CH
CH
CH2 CH
OSO3 H+
+ H2SO4
CH
_
OH
Scheme 9.2
At the present time, however, the long-chain alcohol sulphates already mentioned, such
as structure 9.10, and particularly the sulphated ethers are of greater importance. The
stability of the sulphates to mildly acidic conditions and to hard water is much better than
that of the carboxylates and is sufficient for most purposes. Under more stringent acidic
conditions, however, hydrolysis may take place.
Another type of sulphated product, an ester sulphate, can be prepared by esterifying a
fatty acid such as ricinoleic or oleic acid with a short-chain (C3–C5) alcohol and then
sulphating. Such products are particularly useful foaming, wetting and emulsifying agents; an
example is sulphated butyl ricinoleate (9.11).
CH3(CH2)5CHCH2CH
OSO H+
CH(CH 2)7COO(CH 2)3CH3
3
9.11
More recent developments amongst anionic surfactants are the sulphated polyethers or
alcohol poly(oxyethylene) sulphates (9.12, 9.13), prepared by ethoxylating the fatty alcohol
to give a polyether containing a terminal hydroxy group that is then sulphated. Aromatic
hydrophobes may also be used to produce, for example, alkylphenol poly(oxyethylene)
sulphates. In a general sense, a poly(oxyethylene) sulphate can be viewed as a partly anionic
and partly nonionic surfactant, although the degree of ethloxylation of these products is
generally much lower than that of the purely nonionic surfactants. Hence they are
sometimes referred to as ‘lightly ethoxylated alcohol sulphates’; again, their actual
composition may be a good deal more complex than indicated by their nominal structural
formulae. There has been increasing use of these derivatives in domestic detergents.
R
_
OSO3
(OCH2CH2)x
Na+
C16H33
(OCH2CH2)x
_
OSO3
Na+
9.13
9.12
Sulphonated anionic surfactants have the general structure 9.14, which should be
compared with that of the sulphates (9.9). As well as the simple alkyl derivatives such as
structure 9.15, aromatic and particularly alkylated aromatic (alkylaryl) types are technically
and commercially important. Indeed, sodium dodecylbenzenesulphonate (9.16) has long been
of great importance in domestic washing powders. Although it is no longer the only surfactant
used in domestic washing powders, its economy, efficacy and environmental properties are
such that it is likely to remain the dominant anionic surfactant in heavy-duty concentrated
powder detergents for some time [15]. Environmental studies have shown that 90% of such
linear alkylbenzenesulphonates are removed by conventional sewage treatment, the remainder
being virtually completely biodegradable in topsoil with half-life values varying from 3 to 25
days [16,17].
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ANIONIC SURFACTANTS
_
RSO3 Na+
_
+
C16H33SO3 Na
9.14
9.15
483
_
SO3 Na +
H25C12
9.16
Nowadays these compounds are usually blended with other surfactants, including
nonionic types (section 9.6). In 1990 a typical low- or non-phosphate domestic detergent
contained 7% linear alkylbenzenesulphonate and 6% nonionic fatty alcohol ethoxylate [16].
There is increasing use of the long-chain fatty alcohol poly(oxyethylene) sulphates
previously described (e.g. 9.12) as a partial or complete replacement for linear
alkylbenzenesulphonates [15] since they are made from renewable feedstocks such as tallow
and palm oil [16].
Naphthalene and other aromatic hydrophobes are also used to produce sulphonates, such
as structure 9.17. Of greater importance, however, are the more complex condensation
products that form the basis of many excellent dispersing, resist and aftertreating (syntan)
agents. Typical examples are the condensation products of naphthalenesulphonates with
formaldehyde, and the lignosulphonates derived from pulping processes; these are described
in more detail in section 10.6.1.
C3H7
_
+
SO3 Na
9.17
Sulphosuccinates are of particular interest not only for their technical properties but also
because structurally they combine the two hydrophile functions described earlier – the
sulphonate and carboxylate moieties – in a single molecule (9.18). The sulphosuccinate
diesters, however, are probably of greater commercial importance in textile processing than
are the monoesters. The most important example is sodium dioctylsulphosuccinate (9.19),
but the dinonyl, dimethylamyl and di-isobutyl analogues are also used commercially. As
usual, a wide choice of hydrophobes is available and includes alcohols, lightly ethoxylated
alcohols, alkanolamides and combinations of these.
COOC8H17
COOR
H2C
CH
_
_
+
SO3 Na
COO
9.18
Na+
H2C
_
SO3
CH
+
Na
COOC8H17
9.19
Phosphate esters (9.20) represent a different class of hydrophile-characterised anionic
surfactants; mono- or di-esters can be formed depending on whether one or two alkyl groups
are present. Most phosphate esters are based on alcohols and especially their ethoxylates,
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484
THE CHEMISTRY AND PROPERTIES OF SURFACTANTS
including aliphatic and alkylaryl types. Whereas the sulphates tend to be based on lightly
ethoxylated alcohols, the phosphate esters are also made from more highly ethoxylated
products. Commercial products are complex mixtures (9.21) of monoester, diester, free
phosphoric acid and free nonionic surfactant [18].
O
O
R
O
P
R
OH
O
P
R
O
OH
OH
9.20
O
O
R(OCH2CH2)xO
P
R(OCH2CH2)xO
OH
P
O(CH2CH2O)xR
OH
OH
Monoester surfactant
Diester surfactant
9.21
O
HO
P
OH
R(OCH2CH2)xOH
OH
Free nonionic
surfactant
Free phosphoric
acid
Phosphate esters are particularly useful for their alkali stability and wettability. It has been
shown [18] that a certain amount of ethoxylation of the hydrophobe is required to obtain
alkali resistance, and that as the relative molecular mass (Mr) of the hydrophobe increases,
the proportion of ethylene oxide required also increases. The greater the degree of
ethoxylation, the greater the degree of alkali resistance, few materials showing good alkali
resistance with less than six moles of ethylene oxide per mole of hydrophobe. The same
study [18] showed that wetting power decreased with increasing Mr of the hydrophobe.
Incorporating 2–3 moles of ethylene oxide per mole of hydrophobe seems to give optimum
wetting. It appears that as one attempts to increase the rate of wetting, alkali tolerance
decreases.
Other anionic surfactant types include the alkylisethionates (9.22), N-acylsarcosides
(9.23), N-acyltaurides (9.24) and perfluorinated carboxylates, sulphonates (e.g. 9.25),
sulphates and phosphates [13].
O
_
R
O
CH2CH2SO3
9.22
chpt9(2).pmd
484
R
+
Na
C
_
N
CH2COO
Na
+
CH3
9.23
15/11/02, 15:43
CATIONIC SURFACTANTS
O
R1
C
N
_
CH2CH2SO3
F3C
+
Na
CF2
F3C
R2
C
CF3
C
C
CF2
F3C
485
O
_
SO3 Na+
CF3
9.24
9.25
9.5 CATIONIC SURFACTANTS
By far the most important types of cationic surfactant used in textile processing are the
quaternary ammonium salts (9.26), in which R is usually a long-chain hydrophobe and R1,
R2, R 3 are lower alkyl groups. The most common anions in these and other cationic
surfactants are chloride and bromide: thus cetyltrimethylammonium chloride (9.27) is
typical of this class of cationic surfactants. In fact, however, all four alkyl groups on the
nitrogen atom can be varied to alter the balance of properties of the products. In the
alkyldimethylmethallylammonium chlorides (9.28), an unsaturated aliphatic group is used.
Aromatic components are also used, as in the important alkyldimethylbenzylammonium
chlorides (9.29), and both the aromatic nucleus and the alkyl groups in such products may
contain substituents (9.30 and 9.31). As important as the quaternary ammonium surfactants
are the pyridinium salts (9.32; R is a long-chain alkyl group), such as cetylpyridinium
chloride (9.33).
+
R
N
CH3
CH3
_
R2
+
+
R1
X
H33C16
N
R3
_
CH3
R
Cl
N
CH3
CH3
9.26
_
CH2
CH2
+
R
N
+
Cl
_
CH3
Cl
CH2
CH3
9.28
9.27
CH3
C
R
CH3
N
_
Cl
CH2
CH3
9.29
9.30
+
CH3
R
N
CH2CH2OH
CH3
N C16H33
+ _
Cl
N R
+ _
X
_
Cl
9.32
9.33
9.31
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486
THE CHEMISTRY AND PROPERTIES OF SURFACTANTS
Imidazoles can be quaternised to yield cationic surfactants (such as structures 9.34 and
9.35). Long-chain alkyl primary, secondary and tertiary amines can also be used as cationic
surfactants, but their use in textile processing is limited as a result of their insolubility in
other than acidic aqueous media. The range of products available as cationic surfactants is
truly enormous, including, for example, such complex products as alkylated mono- and diguanidines and polyamines (9.36) containing more than one basic nitrogen atom.
CH2CH3
CH3
H2C
N
H2C
N
_
+C
R1
+C
Cl
_
R
Cl
N
N
CH2CH2
NHCO
R2
CH2
9.34
9.35
R
(NHCH2CH2)xNH2
9.36
Many of these cationic products, including the quaternary amines and imidazoles, can be
ethoxylated (9.37, 9.38), forming cationic analogues of the ethoxysulphates and
ethoxyphosphates in the anionic series. They are essentially cationic/nonionic hybrid
surfactants, variously described in manufacturers’ promotional literature as ‘modified
cationic’, ‘weakly cationic’ or even ‘modified nonionic’. Their value lies in the fact that the
cationic nature can be controlled by varying not only the alkyl substituents but also the
degree of ethoxylation. In addition the ethoxylate moiety confers useful emulsifying
properties. Fluoro-containing cationic surfactants (9.39) can also be obtained [13].
+
CH3
CH3
R
N
_
(CH2CH2O)xH
H2C
Cl
(CH2CH2O)xH
H2C
R
_
Cl
N
(CH2CH2O)xH
9.37
CH3
C7F15
N
+C
CONH
(CH2)3
9.39
9.38
N CH3
+
CH3
_
I
9.6 NONIONIC SURFACTANTS
Nearly all nonionic surfactants contain the same type of hydrophobes as do anionic and
cationic surfactants, with solubilisation and surfactant properties arising from the addition of
ethylene oxide to give a product having the general formula 9.40. Usually, depending on the
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NONIONIC SURFACTANTS
487
hydrophobe, aqueous solubility and detergent properties begin to be evident when x = 6, but
in theory the degree of ethoxylation can be continued almost indefinitely. Optimal
surfactant properties are generally found when x = 10–15, although higher homologues (for
example, x = 50) are known. The ‘lightly ethoxylated’ sulphates mentioned earlier usually
contain only 2–4 oxyethylene units per molecule. Thus a typical nonionic surfactant can be
represented by structure 9.41.
R
(OCH2CH2)xOH
H33C16
(OCH2CH2)12OH
9.41
9.40
Although there are other types of nonionic surfactant, the great majority are adducts of
ethylene oxide with hydrophobes derived from three sources:
– fatty alcohols and alkylphenols
– fatty acids
– fatty amines and amides.
For many years the most common of these have been adducts with p-nonyl- and
p-octylphenol, and to a lesser extent 2,4-dinonylphenol, p-dodecylphenol and
1-alkylnaphthols. Since the hydrophobes used may be variable products conforming to an
average nominal structure, and since the quoted degree of ethoxylation can also only be
regarded as an average value, products having the same name (such as, for example,
p-nonylphenol dodecaoxyethylene) may in fact differ in detailed composition and properties
when obtained from different manufacturers. These provisos should be borne in mind when
considering the examples below, even though there is a trend in some cases towards the
manufacture of narrower fractions.
An example of an alcohol-based nonionic (9.41) has already been given. An alkylphenol
adduct (9.42) is essentially similar; both alcohols and phenols give rise to the relatively
strong and stable ether link, a valuable property of this type of product. Analogues based on
alkylthiols (9.43) may also be used.
(OCH2CH2)xOH
H19C9
R
(OCH2CH2)xOH
H17C8
9.42
S
(CH2CH2O)xH
(OCH2CH2)xOH
9.44
9.43
Polyfunctional alcohols of varying complexity, such as polyethylene glycols (9.44) and
polypropylene glycols of varying chain length, also provide useful nonionic agents. A
polypropylene glycol molecule has a hydroxy group at each end to which ethylene oxide can
be added, forming random segments of poly(oxyethylene) and poly(oxypropylene). This
results in block copolymers, which can be engineered by control of starting materials and
processing conditions to give products specifically suited to a wide variety of purposes by
virtue of wide variations in segment length and degree of polymerisation.
Whereas the alcohol and phenol derivatives are characterised by ether linkages, adducts
of ethylene oxide with fatty acids give rise to both monoesters (9.45) and diesters. These are
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THE CHEMISTRY AND PROPERTIES OF SURFACTANTS
H33C16CO(OCH2CH2)xOH
H23C11NH(CH2CH2O)xH
9.45
9.46
(CH2CH2O)xH
(CH2CH2O)xH
H23C11
N
H23C11
H23C11
C
(CH2CH2O)xH
NH(CH2CH2O)xH
C
N
O
(CH2CH2O)xH
O
9.47
9.49
9.48
less stable than the ethers in strongly acidic or alkaline media, however, hydrolysing to the
original fatty acid and polyethylene glycol.
Adducts of ethylene oxide with fatty amines can yield mono- (9.46) or di-substituted
(9.47) products, as can the adducts with fatty amides (9.48, 9.49). In practice the products
formed are far from being as simple or as symmetrical as represented by these formulae since,
amongst other things, the ethylene oxide addition takes place randomly.
A recent introduction in the area of nonionic surfactants is the alkylglycoside series.
These are long-chain acetals of saccharides (9.50). Commercial products currently have an
average alkyl chain length of 10–12 carbon atoms. These are manufactured from nonpetroleum sources, being synthesised from glucose and fatty alcohols. Such acetals are
regarded as eco-friendly, being said to be completely biodegradable [19] and having low skin
irritancy. These surfactants possess wetting, foaming and detergency properties similar to
those of the corresponding alcohol ethoxylates but with higher solubility in water and in
solutions of electrolytes. They are soluble and stable in sodium hydroxide solutions and show
no inverse solubility characteristics.
CH2OH
CH
HO
CH
O
CH
CH
CH
OH
OH
O
CnH2n+1
9.50
The nonionic types so far discussed form the great majority used in textile processing. Of
course, a great many more can be synthesised, as the possible range of permutations and
combinations is truly enormous. Given appropriate conditions, ethylene oxide will react with
almost any proton-donating compound, but the choice in practice is restricted by economic
factors. Not all nonionic surfactants are ethoxylates, however. Analogous propylene oxide
adducts are known; rather more different products include sucrose and sorbitan esters,
alkanolamides and fatty amine oxides. The fatty acid esters of compounds such as sucrose
and sorbitol exhibit surfactant properties. Some, such as the sorbitan fatty esters, are
insoluble in water but being oil-soluble they can be used as emulsifiers in oil-based systems,
or they can be ethoxylated to render them water-soluble. Mention has already been made of
fatty amide poly(oxyethylene) adducts formed by condensation of a fatty acid with an
alkanolamine which is then ethoxylated; some complex alkanolamides have in themselves
(i.e. without ethoxylation) some surfactant properties, however. They are made by the
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AMPHOTERIC SURFACTANTS
489
reaction of a fatty acid (such as lauric acid or a coconut fatty acid) with a secondary
alkanolamine (such as diethanolamine) to yield an amide, which then reacts further with
diethanolamine to give the water-soluble alkanolamide surfactant. Typical fatty amine oxides
(9.51 and 9.52) are derived, for example, from the peroxide oxidation of tertiary amines
containing at least one fatty-chain group.
O
CH3
H25C12
N
O
O
N
C12H25
CH3
9.51
9.52
9.7 AMPHOTERIC SURFACTANTS
As mentioned in Table 8.1, amphoteric surfactants contain both an anionic and a cationic
group. In acidic media they tend to behave as cationic agents and in alkaline media as
anionic agents. Somewhere between these extremes lies what is known as the isoelectric
point (not necessarily, or even commonly, at pH 7), at which the anionic and cationic
properties are counterbalanced. At this point the molecule is said to be zwitterionic and its
surfactant properties and solubility tend to be at their lowest. These products have acquired
a degree of importance as auxiliaries in certain ways [20–25], particularly as levelling agents
in the application of reactive dyes to wool.
The simplest type is represented by the higher alkylaminoacids, such as compound 9.53;
disubstituted amines can also be synthesised (9.54).
Ethoxylated products can also feature as amphoteric surfactants; an example is compound
9.55, an alkylamine poly(oxyethylene) sulphate. Of particular interest in textile processing are
the trisubstituted alkylamino acids known as betaines; N-alkylbetaines (9.56; R = C8–C16
alkyl) and acylaminoalkylbetaines (9.57; R = C10–C16 alkyl) are typical [30].
Sulphate and sulphonate analogues of the carboxylates, such as the sulphobetaine 9.58,
can also be used as amphoteric agents.
_
+
H33C16NH2CH2COO
+
H33C16HN
9.53
CH2COOH
(CH2CH2O)xH
_
CH2COO
H33C16
9.54
+
HN
_
(CH2CH2O)xCH2CH2OSO3
R
CH3
_
+
N CH2COO
9.55
CH3
9.56
R
CONHCH 2CH2CH2
CH3
_
+
N CH2COO
CH3
9.57
R
CH3
_
+
N CH2CH2CH2SO3
CH3
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9.58
490
THE CHEMISTRY AND PROPERTIES OF SURFACTANTS
9.8 THE GENERAL PROPERTIES OF SURFACTANTS
9.8.1 Effects on the environment
The widespread use of these products focused attention on their environmental properties
long ago, owing to the persistent foam-creating tendency of many surfactants when
discharged. However, the surfactants industry has a very good track record of responding to
environmental problems, stretching back as long ago as the 1960s; that is, quite some time
before the present environmental bandwagon began to roll. Karsa has provided a pointed
reminder of this: ‘The most significant development in the West in the 1960s was the
growing environmental awareness and concern for biodegradable components to overcome
problems at sewage treatment plants and foam in watercourses. The result was an underpublicised and often forgotten fact that industry on both sides of the Atlantic voluntarily
changed from branched-chain alkylbenzenesulphonates’. Since then, ‘detergents have been
based on biodegradable components, contrary to the impression given with some of the
information supplied with today’s ‘green’ detergents, which would have one believe
biodegradability is something new and exclusive to these products. This was among the first
major environmental moves by any industry and was ten years ahead of any UK or EEC
legislation’ [16].
Major works dealing with environmental aspects of surfactants are available [26–30].
The excellent biodegradability of the linear alkylarylsulphonates has already been
mentioned (section 9.4). The alcohol sulphates have low toxicity and alcohol
poly(oxyethylene) sulphates are even less toxic. Alkane sulphonates have high COD, BOD
and an MBAS degradation rate of 90%. Polyether carboxylates have excellent
environmental properties and are non-toxic to the extent that they are used in cosmetics
and household detergents. Sodium-α-olefin sulphonates show rapid biodegradation due to
their linear structures. The α-sulphomonocarboxylic esters show good to excellent
environmental properties and are also used in cosmetics and household detergents.
Sulphosuccinates generally show 90% biodegradation after seven days and have a long
history of safe use, being ranked as relatively non-toxic. Phosphorus-containing anionics are
very mild to the skin and are used in cosmetics, shampoos and lotions.
The toxicology of perfluorinated surfactants varies greatly; most are harmless, whilst some
are amongst the most toxic non-proteins known, the structural differences between the two
often being relatively slight. Hence caution is needed in their use, even though they are so
strongly surface-active that they can be used in much smaller quantities than other
surfactants.
Cationic alkylammonium surfactants have shown 94% biodegradability [27].
Amongst the nonionics, the use of linear primary alcohol ethoxylates has grown rapidly
since the 1970s, due in very large measure to their high degree of biodegradability under
most test procedures, both rapid primary and ultimate degradation [28]. Biodegradation of
such products is retarded by branching of the alkyl chain, this being cumulative. It is also
retarded in secondary alcohol structures, by the addition of about 3 equivalents of propylene
oxide to the ethoxylate moiety and by an ethoxylate chain of more than 20 units. However,
as Talmage points out [28], products containing these features are not present in the simple
alcohol ethoxylates most commonly used in detergent formulations.
In contrast to the above trends, during the 1980s and 1990s there has been considerable
environmental concern over the alleged effects of the nonionic nonylphenol ethoxylates.
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THE GENERAL PROPERTIES OF SURFACTANTS
491
The concern seemed to be centred around possible bioaccumulation and the properties of
nonylphenol itself, potentially one of the major metabolites (products of biodegradation)
released during the environmental breakdown of nonylphenol ethoxylates. The incomplete
biodegradation was attributed to branching in the nonyl group and the presence of the
aromatic phenyl ring. In the 1980s, particularly in Europe, there were calls for restrictions
and bans on the use of nonylphenol ethoxylates. This concern, not surprisingly, led to much
careful and detailed research from which has evolved a clarified and much less alarmist
picture.
This research has been excellently reported by Naylor [31], who pointed out the
limitations of laboratory methods (the extent of biodegradation of nonylphenol ethoxylates
has been variously reported from 0% to 100%!) and the critical importance of determining
biodegradability in conventional waste water plants using improved and streamlined
analytical methods. This work, on American rivers and treatment plants, showed that
nonylphenol ethoxylates exhibited high treatability under conditions of extremely high
loadings on waste water treatment plants. It was confirmed that nonylphenol is indeed the
metabolite of highest toxicity but it is not a significant metabolite except under anaerobic
conditions. It was shown that nonylphenol ethoxylates are extensively biodegraded (92.5–
99.8% removal rates) in secondary treatment. Re-aeration studies have shown that
nonylphenol and nonylphenol ethoxylates contained in sewage sludge degrade when the
sludge is applied to soil.
Thus there is a strong basis for the conclusion that nonylphenol ethoxylates are highly
biodegradable, do not accumulate in water, sediment or aquatic organisms and do not pose a
credible threat to the environment. Hence, in 1995, Naylor [31] was able to say that, in
America, nonylphenol ethoxylates were by far the most important alkylphenol ethoxylates,
accounting for 80% of the total volume and commonly found in formulations for fibre sizing,
spinning, weaving, scouring and dyeing, as well as for water-based paints, inks, adhesives and
many institutional and household cleaning products.
Finally, in considering the environmental properties of surface-active auxiliaries
generally, it should be borne in mind that they are more or less complex mixtures and hence
the presence of other components, such as solvents, electrolytes or sequestrants, needs to be
considered in addition to the surfactants present.
9.8.2 Application properties
Anionic and cationic products generally tend to interact with each other, usually
diminishing the surface-active properties of both and often resulting in precipitation of the
complex formed. Amphoteric compounds can also be incompatible with anionics in acid
solution but are generally compatible with cationics and nonionics. Interaction between
anionic and cationic agents can sometimes be prevented by addition of a nonionic. In some
cases, if an ethoxylated sulphate or phosphate is used as the anionic component a cationic
compound produces no obvious precipitation, since the oxyethylene chain acts as dispersant
for any complex that may be formed.
The main disadvantages of the carboxylates are their tendency to react with calcium and
magnesium ions in hard water to give insoluble precipitates and their insolubility in acidic
media, although they generally have good wetting and detergent properties. The
acylsarcosides are less affected by calcium and magnesium ions, however, whilst the
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THE CHEMISTRY AND PROPERTIES OF SURFACTANTS
carboxymethyl surfactants are unaffected. The sulphates were specifically developed to
overcome the drawbacks of the carboxylates and, like the phosphates, are stable towards
calcium and magnesium ions. As well as being outstanding detergents, the sulphonates are
also unaffected by strongly acidic or alkaline conditions, and the higher-alkyl members have
useful lubricating properties. On the other hand, the sulphates can be hydrolysed by acid
and sulphated monoglycerides can also be hydrolysed by alkali. Their wetting properties tend
to be inferior to those of the sulphonates but they are particularly valuable as emulsifying
agents, especially in combination with nonionics. The sulphosuccinates have a high
propensity to foaming and their solubility is not generally good, but the monoesters have
good detergency properties and the diesters are particularly rapid wetting agents.
As a group, the phosphates have good stability to acid and alkali for most purposes, have
low foaming and good detergency properties and are biodegradable. They tend to be better
wetting agents than the sulphates and their solubility in organic solvents makes them useful
in, for example, dry cleaning. The perfluoroalkyl anionic surfactants are very expensive, but
are powerful surfactants at very low concentrations and are stable in chemically hostile
environments; they also exhibit surface activity in organic solvents.
Cationic agents generally are less useful than anionics as detergents but they have useful
properties as softeners, germicides and emulsifiers.
Nonionic agents are generally compatible with both anionic and cationic types. They are
also stable to calcium and magnesium ions. With the exception of the fatty acid esters, which
are readily hydrolysed by acid and alkali, they are stable and effective over a wide range of pH
values. A particular characteristic of nonionic surfactants is their inverse solubility: as the
temperature rises the solubility decreases, until a point is reached at which the surfactant
attains its limiting solubility and therefore begins to precipitate out, causing cloudiness of the
solution. The temperature at which this occurs, known as the cloud point, depends on the
number of oxyethylene units in the nonionic molecule in relation to the length of the
hydrophobe. Thus, for any given hydrophobe, the cloud point increases with the increasing
degree of ethoxylation; for example, dodecanol heptaoxyethylene C12H25(OCH2CH2)7OH has
a cloud point of 59 °C, while that of the undecaoxyethylene homologue is 100 °C. Conversely,
for a fixed number of oxyethylene units, the cloud point decreases with increasing size of the
hydrophobe. The cloud points of nonionic agents are also generally lowered by the presence of
electrolytes, the effect varying with the electrolyte and its concentration. It is important to
bear this in mind when choosing nonionic agents for use in electrolyte-containing processes.
This inverse solubility arises from the solubilisation of the nonionic molecules by hydrogen
bonding of water with the ether oxygen atoms (9.59). As the temperature rises, the energy
within these bonds becomes insufficient to maintain their cohesion and dehydration takes
place, with a consequent decrease in solubility. A knowledge of the cloud point of a surfactant
is useful, not only because of solubility effects but also because the surface activity tends to be
optimal just below the cloud point.
O
H
O
H
O
H
H
CH2
CH2
CH2
O
O
CH2
n
H
9.59
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H
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THE GENERAL PROPERTIES OF SURFACTANTS
493
The tendency of nonionics to produce foam varies. Some, such as the block copolymers,
are even used as defoamers. Their wetting, detergency and emulsifying properties also vary
widely, depending to a large extent on the balance between the hydrophobic and hydrophilic
(oxyethylene) portions.
The amphoteric agents exhibit excellent compatibility with inorganic electrolytes and
with acids and alkalis. Such is their stability in strongly acidic solution that they are even
used in cleaning compositions based on hydrofluoric acid [14].
9.8.3 The theory of surface activity
The physico-chemical theory of surface activity is a vast field and no more than broad
principles can be touched on here; major reference sources exist for those who require more
detail of the relationship between chemical structure and the various surfactant properties
such as wetting, detergency and emulsification-solubilisation [32–36].
Surface activity is generally related to the balance between the hydrophobic and
hydrophilic portions of the molecule. For example, among the anionic surfactants C8–C12
alkyl hydrophobes tend to be predominantly wetting agents, whilst the C12–C18 homologues
exhibit better detergency and emulsifying properties. The alkylsuccinates and
sulphosuccinates are particularly powerful wetting agents. Clearly, as the hydrophobic
character of the surfactant is increased, aqueous solubility decreases and oil solubility
increases. Thus the balance between the hydrophobic and hydrophilic moieties of a
surfactant is a critical factor in determining its major characteristics. This is referred to as
the hydrophile–lipophile balance, or HLB (the term ‘lipophile’, of course, being analogous to
‘hydrophobe’). Whilst the HLB value is of general use in expressing the characteristics of a
surfactant, it is of particular value in describing the formation of emulsions. For some
general purposes the HLB can be used qualitatively (referring, for instance, to low, medium
or high HLB), but for more precise work it is preferable to use a quantifying scale. Such a
scale, put forward in the 1940s [37], covers a range of values from zero (the lipophilic or
hydrophobic extreme) to a hydrophilic extreme of 20 or higher, with a value of 10
approximately representing the point at which the hydrophilic and hydrophobic portions are
in balance. This scale is especially useful in describing the properties of the nonionic
ethoxylates. For example, a low HLB value (4–6) signifies a predominance of hydrophobic
groups, indicating that the surfactant is lipophilic and should be suited for preparing waterin-oil emulsions. A value in the 7–9 range indicates good wetting properties. As the value
shifts towards increased hydrophilicity other properties predominate, values of 8–18 being
typical for surfactants that will give oil-in-water emulsions, and values of 13–15 for
surfactants that show useful detergency. The HLB values required for solubilising properties
are generally in the range 10–18.
The HLB of a relatively pure poly(oxyethylene) adduct can be calculated from theoretical
data [37]. For these agents the HLB is an indication of percentage by mass of the
hydrophilic portion, divided by five to give a conveniently small number. For example, if the
hydrophilic portion of a purely hypothetical nonionic agent accounted for 100% of the
molecule (such a product cannot, of course, exist), its HLB is 20. Similarly, a more plausible
product in which 85% of the molecule is accounted for by the hydrophilic portion has an
HLB of 85/5 = 17. The ICI Americas Inc. method of calculating the theoretical HLB of a
sorbitan monolaurate nonionic having 20 oxyethylene units per molecule is given in
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THE CHEMISTRY AND PROPERTIES OF SURFACTANTS
Equation 9.1 (total relative molecular mass = 1226, of which 1044 is contributed by the
hydrophilic portion) [37].
HLB =
1044
1
™ 100 ™ = 17.0
1226
5
(9.1)
As explained earlier, however, the actual constitution of a surfactant rarely conforms to its
nominal structure. Consequently the theoretical method of calculation is of limited utility,
practical methods being more reliable. The HLB value may be determined directly by
analysis or by comparison with a range of surfactants of known HLB values. An analytical
method for the sorbitan monolaurate described above uses Equation 9.2 [37].
SÛ
Ë
Ë 45.5 Û
HLB = 20 Ì 1 - Ü = 20 Ì1 Ü = 16.7
Í
Í
AÝ
276 Ý
(9.2)
where S is the saponification number of the ester and A is the acid number of the recovered
acid. The saponification value of a product is the mass in milligrams of potassium hydroxide
required to saponify one gram of the product; it can be found by saponification of the
product with an excess of potassium hydroxide, followed by back-titration of the remaining
alkali with hydrochloric acid. The acid value of an acid is the number of milligrams of
potassium hydroxide required to neutralise a standard quantity, and can again be found by
titration. The comparative methods should always be used for the nonionic surfactants that
are not based on ethylene oxide, and also for ionic surfactants since the hydrophilic
influence of the ionic group exceeds that indicated by the mass percentage basis (this can
lead to apparent HLB values higher than 20).
Once the HLB values of a range of surfactants are known it is an easy matter to calculate
the HLB value of a mixture as follows:
Individual HLB
45% of surfactant A 16.7
35% of surfactant B 4.0
20% of surfactant C 9.6
Fractional HLB
0.45 × 16.7 = 7.52
0.35 × 4.0 = 1.40
0.20 × 9.6 = 1.92
Total HLB = 10.84
When preparing an emulsion, emulsification tends to be most efficient when the HLB of the
agent matches that of the oil phase. Often a mixture of surfactants makes a more efficient
emulsifying agent than a single product having the same HLB value as the mixture;
similarly, if the oil phase to be emulsified is itself a mixture, its components will each
contribute to the effective HLB value. It is this effective HLB that is the main criterion in
designing a suitable emulsifying system. The effective HLB value can be found by carrying
out preliminary emulsification tests with agents of known HLB values. A useful procedure
[37] uses two such emulsifying agents of widely differing HLB values mixed in various
proportions so as to give a range of intermediate HLB values. The HLB value of the mixture
that gives the best emulsion of the oil phase under test then corresponds to the effective
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THE GENERAL PROPERTIES OF SURFACTANTS
495
HLB value of the oil phase. Further tests can then be carried out with different chemical
types of agents around this effective HLB value in order to find the optimum emulsifying
system.
9.8.4 Micelle formation
Surface tension/N m–1
All surfactants in solution tend to form more or less ordered agglomerates of molecules,
known as micelles. Pure water has a surface tension of about 72 × 10–3 N/m. As surfactant
is added gradually to it, the surface tension falls quite rapidly (Figure 9.1) until, at a certain
concentration of surfactant, it begins to level off more or less sharply. At the point at which
this levelling out takes place, the critical micelle concentration (CMC in Figure 9.1), the
surfactant molecules begin to orient themselves in clusters within the body of the solution,
these clusters being more or less lamellar or spherical (Figure 9.2).
Concentration of surfactant/g l–1
CMC
Figure 9.1 Surface tension of water against surfactant concentration
Lamellar
Spherical
Figure 9.2 Micelle formation
In water the surfactant molecules orient themselves with their hydrophobes at the centre
of the cluster. The CMC is typically quite low, perhaps 0.5–0.2 g/l. At concentrations lower
than this the molecules orient themselves only at the interfaces of the solution, and it is this
effect which brings about the lowering of surface tension. Once the CMC is reached the
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THE CHEMISTRY AND PROPERTIES OF SURFACTANTS
interfaces become saturated and as the concentration increases micellar clusters of
molecules begin to form in the bulk of the solution; there is little further reduction in
surface tension beyond the CMC, nor are there changes in the other surfactant properties
such as wetting and foaming. In general, the CMC decreases with increasing size of the
hydrophobe, and the CMCs of nonionic agents tend to be lower than those of ionic types,
since with the nonionics micelles can form more easily in the absence of polar charges. This
ability to form micelles is vital to the efficacy of surfactants as emulsifying, dispersing and
solubilising agents.
REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
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13.
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22.
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25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
chpt9(2).pmd
Colour terms and definitions (Bradford: SDC, 1988).
Kirk-Othmer encyclopedia of chemical technology, 3rd Edn., Vol. 22 (New York: Wiley, 1983).
M R Porter and M Porter, Handbook of surfactants, 2nd Edn. (Glasgow: Blackie Academic and Professional,
1994).
D R Karsa, J M Goody and P J Donnelly, Surfactants applications directory (Glasgow: Blackie Academic and
Professional, 1991).
M R Porter and M Porter, Recent developments in the technology of surfactants (Glasgow: Blackie Academic and
Professional, 1991).
K Y Lai, Liquid detergents (New York: Marcel Dekker, 1996).
M J Rosen, Surfactants and interfacial phenomena, 2nd Edn. (New York: Wiley, 1989).
H W Stache, Anionic surfactants: organic chemistry (New York: Marcel Dekker, 1995).
J M Richmond, Cationic surfactants: organic chemistry (New York: Marcel Dekker, 1990).
V M Nace, Nonionic surfactants: polyoxyalkylene block copolymers (New York: Marcel Dekker, 1996).
E G Lomax, Amphoteric surfactants, 2nd Edn. (New York: Marcel Dekker, 1996).
I Piirma, Polymeric surfactants (New York: Marcel Dekker, 1992).
E Kissa, Fluorinated surfactants: synthesis, properties, applications (New York: Marcel Dekker, 1993).
A S Davidsohn and B Milwidsky, Synthetic detergents, 7th Edn. (Harlow: Longman, 1987).
G Bevan, Rev. Prog. Coloration, 27 (1997) 1.
D R Karsa, Rev. Prog. Coloration, 20 (1990) 70.
Summary of Aachen conference, Alkylbenzenesulphonates in the environment, J. Amer. Oil Chem. Soc., 66
(1989) 748; full proceedings in Tenside Surf. Det., (Apr/May 1989).
A J O’Lenick and J K Parkinson, Text. Chem. Colorist, 27 (Nov 1995) 17.
R H Mehta and A R Mehta, Colourage, 43/45 (1996) 49.
A Riva and J Cegarra, J.S.D.C., 103 (1987) 32.
H Egli, Textilveredlung, 8 (1973) 495.
W Mosimann, Text. Chem. Colorist, 1 (1969) 182.
J Cegarra, Proc. IFATCC, Barcelona (1975).
J Cegarra, A Riva and L Aizpurua, J.S.D.C., 94 (1978) 394.
J Cegarra and A Riva, Melliand Textilber., 64 (1983) 221.
D R Karsa and M R Porter, Biodegradability of surfactants (Glasgow: Blackie Academic and Professional, 1995).
M J Schwuger, Detergents in the environment (New York: Marcel Dekker, 1996).
S S Talmage, Environmental and human safety of major surfactants (London: Lewis Publications, 1994).
C Gloxhuber and K Kunstler, Anionic surfactants: biochemistry, toxicology, dermatology, 2nd Edn. (New York:
Marcel Dekker, 1995).
P Schöberl, K J Bock and L Huber, Tenside Surf. Det., 25 (1988) 86.
C G Naylor, Text. Chem. Colorist, 27 (Apr l995) 29.
K Tsujii, Surface activity: principles, phenomena and applications (San Diego: Academic Press, 1998).
J C Berg, Wettability (New York: Marcel Dekker, 1993).
A K Chattopadhyay and K L Mittal, Surfactants in solution (New York: Marcel Dekker, 1996).
S D Christian and J F Scamehorn, Solubilisation in surfactant aggregates (New York: Marcel Dekker, 1995).
A W Neumann and J K Spelt, Applied surface thermodynamics (New York: Marcel Dekker, 1996).
The HLB system – a time-saving guide to emulsifier selection, Publication 103–3 10M (Wilmington: ICI Americas,
1984).
496
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497
CHAPTER 10
Classification of dyeing and printing auxiliaries
by function
Terence M Baldwinson
10.1 ELECTROLYTES AND pH CONTROL
The simplest auxiliaries of all are the neutral electrolytes such as sodium chloride and
sodium sulphate. These are used in large quantities for dyeing cellulosic materials with
direct or reactive dyes and wool with anionic dyes. The major effect of electrolytes on dyes
of this type is to increase the degree of aggregation of the dye anions in solution by the
common-ion effect, the degree of aggregation varying markedly with dye structure (section
3.1.2). The electrolyte suppresses ionisation of the dye in solution, thereby effectively
reducing its solubility in the dyebath and modifying the equilibrium in favour of movement
of dye anions from the solution into the fibre. The objective, of course, is to use the
optimum amount of salt to give the required rate and degree of exhaustion of the dyebath;
too little electrolyte is ineffective whilst too much may aggregate the dye to an extent that
may inhibit its diffusion into the fibre, thus giving a tendency to surface coloration only, or
even bringing about precipitation. The aggregating effect of electrolytes varies, sodium
chloride having a stronger effect than sodium sulphate, but it is generally decreased by
raising the temperature.
This effect, which we may term the ‘salting-on’ effect, is the result of interactions
between electrolyte and dye. However, there may also be interactions between electrolyte
and fibre, giving rise to a positive levelling action as electrolyte anions compete with dye
anions for the cationic sites in the fibre. Ionic surfactants (Table 8.1) can of course be
regarded as electrolytes, although by hydrophobic interactions they tend to form micelles in
concentrated solution and hence may be referred to as colloidal electrolytes. In some
respects their levelling action is analogous to that of simple inorganic electrolytes – that is,
ionic hydrophobes compete with dye ions of similar charge for sites of opposite charge in the
fibre.
Electrolytes are used to promote the exhaustion of direct or reactive dyes on cellulosic
fibres; they may also be similarly used with vat or sulphur dyes in their leuco forms. In the
case of anionic dyes on wool or nylon, however, their role is different as they are used to
facilitate levelling rather than exhaustion. In these cases, addition of electrolyte decreases
dye uptake due to the competitive absorption of inorganic anions by the fibre and a decrease
in ionic attraction between dye and fibre. In most discussions of the effect of electrolyte on
dye sorption, attention is given only to the ionic aspects of interaction. In most cases, this
does not create a problem and so most adsorption isotherms of water-soluble dyes are
interpreted on the basis of Langmuir or Donnan ionic interactions only. There are, however,
some observed cases of apparently anomalous behaviour of dyes with respect to electrolytes
that cannot be explained by ionic interactions alone.
497
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
The fact is, ionic interaction between dyes, fibres and electrolytes is only part of the story.
As Yang [1] has pointed out, hydrophobic interactions also need to be taken into
consideration. Whilst this has been accepted for many years in relation to dye–fibre
interactions, the extension of the concept to interactions involving neutral electrolytes is
novel.
Yang quotes as one of several examples the fact that sodium chloride has a stronger effect
than sodium sulphate on decreasing the uptake of CI Acid Red 1 by nylon, which cannot be
explained on the basis of ionic interaction alone. It can, however, be explained in terms of
the effect of the electrolytes on hydrophobic interaction, the same explanation also being
applied to other examples. A lyotropic series is used to explain the effectiveness of
hydrophobic interactions, which always coexist with ionic interactions. A semi-quantitative
representation of the lyotropic series is shown in Figure 10.1. In such a series, neutral ions
have little influence on hydrophobic interactions. Kosmotropes increase hydrophobic
interaction and therefore tend to increase dye adsorption, whilst chaotropes decrease both
hydrophobic interaction and dye adsorption. Thus, in the example quoted above of CI Acid
Red 1 on nylon, sodium chloride has a stronger effect on decreasing dye adsorption than the
more kosmotropic sodium sulphate. On this basis, Yang has introduced a modified Donnan
model that quantitatively predicts the various effects of electrolytes on either decreasing or
increasing dye adsorption.
anions
SO42– > CH3COO– > Cl– > Br– NO–3 SCN–
H2PO–4
cations
Li+ > Na+ > K+ > Rb+ Cs+
kosmotropes
(water
structure-makers)
N
E
U
T
R
A
L
chaotropes
(water
structure-breakers)
Figure 10.1 Lyotropic series: effectiveness of hydrophobic interactions [1]
R
CH3
+
N (CH2)n
_
CH3 X
CH3
+
N R
CH3 X
_
n = 3–12
R = n-propyl, n-butyl or benzyl
X = halide, e.g. bromide
10.1
Rather more complex compounds that are currently being researched are the bolaform
electrolytes [2–4]. Bolaform electrolytes are organic compounds possessing two cationic or
two anionic groups linked by a flexible hydrocarbon chain; the terminal groups may be
aliphatic or aromatic (e.g. as in 10.1). Their interaction with sulphonated monoazo dyes in
the presence of poly(vinylpyrrolidone) as substrate has been studied in detail. However, it
remains to be seen what commercial developments take place with these interesting
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ELECTROLYTES AND
pH CONTROL
499
compounds. It seems likely that they would be used in complex formation rather than in the
more traditional roles associated with electrolytes in textile processing.
In whatever role electrolytes are used, their effects on the environment need to be
considered, particularly when discharged to effluent. High salt loading is undesirable in
waste water and sodium sulphate in particular causes corrosion of concrete pipes. It thus
makes sense to choose electrolytes carefully and to use the minimum amounts consistent
with obtaining the desired effects. Automatic dosing is helpful in this respect. The use of
shorter liquor ratios has been promoted on the grounds of economy (less water to heat and
less liquor to treat subsequently). However, it should not be overlooked that when dyeing in
a short liquor more rinsing baths are required to give the same residual concentration as at a
longer liquor ratio. Weible [5] has demonstrated this effect for the washing-off of reactive
dyes from fabric having a retention capacity of 4 l/kg using 60 g/l of electrolyte. At a liquor
ratio of 20:1, some 12 g/l and 2.4 g/l of electrolyte are found in the first and second rinses
respectively. These rise to 30 g/l and 15 g/l when a liquor ratio of 8:1 is used. Equation 10.1
was used by Weible to calculate these concentrations.
Cs =
Cs =
c =
FV =
R =
cR
(g/l)
FV
(10.1)
concentration in rinsing bath (g/l)
concentration in treatment bath (g/l)
liquor ratio (l/kg)
retention capacity of goods (l/kg)
More detailed information on attempts to reduce the impact of electrolytes on the
environment is given under the individual dye classes discussed in Chapter 12.
The great majority of coloration processes demand some control over the treatment pH,
which varies from strongly alkaline in the case of vat, sulphur or reactive dyes, to strongly
acidic for levelling acid dyes. The concept of pH is a familiar one; its theoretical derivation
can be found in all standard physical chemistry textbooks and has been particularly well
explained in relation to coloration processes [6,7] both in theory and in practice. We are
concerned here essentially with the chemistry of the products used to control pH and their
mode of action. It has been stated [7] that: ‘Unfortunately, pH control appears simple and
easy to carry out. Add acid and the pH decreases; add base (alkali) and the pH increases.
However, pH is the most difficult control feature in any industry’.
The control of pH in textile coloration processes is ensured by three fundamentally
different techniques:
(a) the maintenance of a relatively high degree of acidity or alkalinity
(b) the control of pH within fairly narrow tolerances mainly in the near-neutral region
(c) the gradual shifting of the pH as a dyeing proceeds.
Approach (a) is normally the easiest to control, and is used in the application of levelling
acid and 1:1 metal-complex dyes to wool or nylon, and of the reactive, sulphur or vat dyes to
cellulosic fibres. The agents traditionally used are the stronger acids and alkalis such as
sulphuric, hydrochloric and formic acids, sodium carbonate and sodium hydroxide. In
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
certain operations, particularly fixation in steam (as in printing), steam-volatile acids are
replaced with non-volatile products such as citric acid. Use of approach (a) can lead to the
misconception already mentioned, that pH is easy to control, particularly as the dye–
substrate systems involved are not normally sensitive to minor pH shifts. Nevertheless,
strong acids and alkalis can react to produce quite drastic changes in pH; this can occur, for
example, when alkali is carried over from wool scouring into initially acidic dyebaths.
Wool and nylon absorb acid from dyebaths, thereby inducing a change in the dyebath pH,
but wool will absorb significantly more acid than nylon [8] – a factor to be borne in mind
when comparing results on these two fibres, especially in those systems using ‘half-milling’
acid dyes for which the controlling agent is generally the weaker acetic acid; such systems
represent a compromise between approaches (a) and (b) and are moderately sensitive to
change in pH. In this area, the organic formic and acetic acids are of interest. Formic, of
course, is a stronger acid than acetic. Hence, acetic acid has been traditionally the preferred
choice for the adjustment of slightly acidic media, down to about pH 4, whereas formic was
the choice below this level. It has been demonstrated, however, that for general purposes
formic acid is preferred to acetic acid, particularly on economical and environmental
grounds [9]. Formic acid has an extremely low BOD, being biodegraded to carbon dioxide
and water. In any case, being a much stronger acid, smaller amounts are needed, thus giving
less load for disposal. For example, in order to obtain pH values of 4.5, 4.0 and 3.3, the
amount of formic acid 85% needed is, respectively, about 62%, 50% and 12% of that of
acetic acid 80%. Other advantages of formic acid are that it has a more powerful
neutralising effect than acetic acid and it is less corrosive than mineral acids.
Approach (b) needs greater awareness of the factors that not only determine pH but also
help to stabilise it against interference. Most of the dye–fibre systems requiring approach (b)
are operated in the near-neutral region (pH 4–9) and are much more sensitive to minor
changes in pH. In addition, the pH of the water supply may vary, or drift during heating.
Even the pH of pure water changes on heating, from 7.47 at 0 °C to 7.00 at 24 °C and 6.13
at 100 °C, but that of the process water used in dyehouses and printworks can change much
more drastically, most commonly showing an increase. Changes in pH on heating may
counteract the intended response of process liquors, especially in the central pH range
associated with approach (b); even more critical can be the effect of any acids or alkalis
carried over from previous processes.
The dye–fibre systems of obvious interest for approach (b) are milling acid and 1:2 metalcomplex dyes on wool or nylon, basic dyes on acrylic fibres and disperse dyes on various
fibres. With wool and nylon there is often some overlap with approach (c) (section 12.2).
Where control is not too critical, simple electrolytes of weak bases with strong acids
(such as ammonium sulphate) or strong bases with weak acids (such as sodium acetate) are
often used to produce slightly acidic or slightly alkaline media respectively. Ammonium
acetate is also commonly used, producing a less acidic effect than ammonium sulphate.
Occasionally acetic acid and sodium carbonate are used, necessitating careful control and
monitoring. These simple expedients are not suitable for systems requiring more sensitive
control, however, and use of single electrolytes such as ammonium sulphate or sodium
acetate more properly belong to control systems based on approach (c). More precise control
is achieved by the use of buffering systems. By the use of electrolyte pairs, these systems set
the initial pH and exert a protective action that tends to resist changes arising from
contaminants entering by way of the substrate or the water supply.
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501
Buffering systems are generally based on combinations of:
– a weak acid together with the salt of this acid formed from a strong base, or
– a weak base together with the salt of this base formed from a strong acid.
The most commonly used example of the first type is acetic acid/sodium acetate, which
functions well over the pH range 3.8–5.8. Acetic acid with ammonium acetate is also used
although it is less effective, especially in those boiling dyebaths from which ammonia can
escape into the atmosphere, thus allowing the pH to fall. Such acetate buffers have the
advantage of low cost. Somewhat more expensive are the phosphate buffers, of which the
most commonly used is a mixture of sodium dihydrogen orthophosphate (NaH2PO4) with
disodium hydrogen orthophosphate (Na2HPO4). Here, as with most polybasic acid systems,
the distinction between the acid and its salt seems blurred at first sight. In fact, sodium
dihydrogen phosphate is the ‘acting acid’ and disodium hydrogen phosphate is its salt. The
tribasic orthophosphoric acid and its three salts can be used to produce a series of buffers,
each active within a particular pH range:
– orthophosphoric acid and the monosodium salt, main buffering region pH 2.5–3.5
– the mono- and di-sodium salts, main buffering region pH 6–8
– the di- and tri-sodium salts, main buffering region pH 10.5–11.
This can be seen from the titration curve for phosphoric acid [6] shown in Figure 10.2. In
practice the mono- and di-sodium salt system is used most extensively, since this covers the
pH range over which precise control is most often needed. These phosphate buffers are
more resistant than the acetate systems to temperature-induced changes.
12
Na3PO4
10
Na2HPO4
pH
8
Main
buffer region
6
NaH2PO4
pH 6.2–8.2
4
2
H3PO4
Alkali added
Figure 10.2 Orthophosphate buffer system
The most common buffering system containing a weak base together with its salt formed
with a strong acid is ammonia with ammonium sulphate. Some useful buffers are obtained
from combinations of unrelated acids or bases with salts. The following combinations find
occasional use in textile coloration processes, but the acetates and orthophosphates are most
frequently used:
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
– pyrophosphoric acid (H4P2O7) and its salts (pH 3–9)
– orthoboric acid (H3BO3), sodium tetraborate (borax Na2B4O7) and sodium hydroxide
(pH 8.1–10.1)
– citric acid and sodium hydroxide (pH 2.1–6.4)
– sodium carbonate and sodium bicarbonate (pH 9.3–11.3).
The pyrophosphate buffer is of particular technical interest as it can be used over the
relatively wide range of pH 3–9. Unlike the orthophosphate titration curve, that for the
tetrabasic pyrophosphate system is almost straight [6]. This linearity (Figure 10.3) means
that effective buffering action is available across the whole pH range simply by using various
pairs of ionised components and varying their proportions; even so, however, it does not
seem to be widely used.
9
Na4P2O7
pH
7
5
3
Strong acid added
Figure 10.3 Pyrophosphate buffer system
The mechanism of buffering can be described by reference to the acetic acid/sodium
acetate system. In aqueous solution sodium acetate can be considered to be practically
completely ionised (Scheme 10.1), the equilibrium being wholly to the right-hand side.
Since acetic acid is a weak acid it is only slightly ionised, and the equilibrium represented by
Scheme 10.2 lies mainly to the left-hand side. This low degree of ionisation is even further
suppressed in the presence of sodium acetate as a result of the common ion (in this case
acetate) effect operating through the law of mass action. The undissociated acetic acid is, in
effect, a ‘bank’ of hydrogen and acetate ions that can be brought into play as a neutralising
mechanism when either acidic or alkaline chemicals enter the system (either by deliberate
addition or adventitiously). If a small amount of an acidic solute is added to the mixture, the
added hydrogen ions combine with acetate ions to form undissociated acetic acid, which has
only a minimal effect on the pH of the system. If an alkaline solute is added to the buffer,
the added hydroxide ions react with the bank of hydrogen ions to form undissociated water
and so again the ionic balance and hence the pH remain essentially the same. The
mechanisms of other buffering systems are similar: buffering action is increased by adding
more of the components, keeping their proportions constant.
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ELECTROLYTES AND
CH3COONa
Na + CH3COO
CH3COOH
H + CH 3COO
pH CONTROL
503
Scheme 10.1
Scheme 10.2
Approach (c) for pH control involves a deliberate shift of pH during the processing cycle,
in a consistent direction rather than randomly. Systems of this type are particularly useful for
non-migrating anionic dyes on wool or nylon and have long been known in this connection.
More recently, similar systems have been adopted for reactive dyes on cellulosic fibres. The
simplest and most widely used of these systems consist of the salts of strong acids with weak
bases or of strong bases with weak acids, examples being ammonium sulphate and sodium
acetate respectively. Ammonium sulphate, for instance, dissociates in aqueous media to
yield the dominant strong-acid species of sulphuric acid, so lowering the pH (Scheme 10.3)
at a rate that increases with temperature, especially when the ammonia formed can be
released from an open dyebath. Ammonium acetate functions in the same way but does not
yield as great a pH shift. Similarly, a solution of sodium acetate tends to produce the
dominant strong-alkali species of sodium hydroxide (Scheme 10.4), thus increasing the pH.
(NH4)2SO4 + 2H2O
H2SO4 + 2NH4OH
NH4OH
NH3 + H2O
2H + SO42–
Scheme 10.3
CH3COONa + HOH
NaOH + CH3COOH
Na + OH
Scheme 10.4
Acetic acid (b.p. 118 °C) is not boiled off from open dyebaths as readily as ammonia but
is rapidly flashed off in steam or dry heat processes, thus developing the maximum degree of
alkalinity under these conditions. The sodium salts of less volatile acids, such as sodium
citrate, can be used to develop a lower degree of alkalinity.
If the process demands a gradual shift from about pH 9 to a slightly acidic pH, ammonium
sulphate together with ammonia can be used. This gives a safer, more uniform development
of acidity than can be achieved by making additions of acid to an alkaline bath, although the
degree of acidity developed will clearly depend on the ease with which ammonia can escape
from the system. In enclosed or partially enclosed machines this system does not function so
efficiently [10–12].
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
Another method of obtaining a pH shift in the direction of acidity is to use an organic
ester that hydrolyses to the alcohol and acid under the conditions of processing. Ethyl
lactate (Scheme 10.5) and diethyl tartrate (10.2) have been recommended for applying
milling and chrome dyes to wool [13]. 2-Hydroxyethyl chloroacetate (10.3) and
γ-butyrolactone (10.4), which hydrolyses during processing to give 4-hydroxybutyric acid
(Scheme 10.6), have also been recommended. Such hydrolysable esters may be used alone,
beginning at a near-neutral pH, but more likely in conjunction with an alkali to give a
higher starting pH. Thus γ-butyrolactone and sodium tetraborate (borax), giving a pH shift
from about 8 to 5.6, have been recommended for the dyeing of wool [14], as has
2-hydroxyethyl chloroacetate with sodium hydroxide for the dyeing of nylon [15]. Such
hydrolysable esters are sometimes sold under proprietary trade names. The disadvantages of
hydrolysable esters have been their higher cost, a limited pH range and, where the dyebath
is to be reused, the need for increasing quantities of ester to overcome the buffering effect
caused by the accumulation of salts [7,16]. Interest in these systems has declined due to
environmental pressures on the one hand and the increased availability and sophistication of
automatic dosing and monitoring systems on the other.
H3C
O
CH
H2O
H3C
C
C
HO
OCH2CH3
HO
O
CH
OH + HOCH2CH3
Scheme 10.5
O
HO
C
OCH2CH3
HC
HC
HO
O
OCH2CH3
C
C
ClH2C
O
10.2
OCH2CH2OH
10.3
O
O
C
O
CH2
CH2
CH2
H2O
C
HO
CH2
HO
CH2
CH2
10.4
Scheme 10.6
The advantages of automatic metering and monitoring devices were well described by
Mosimann [17] and have recently been re-emphasised [9]. Such devices are clearly of great
value in environmental terms since they are crucial in ensuring that the minimum quantity
of agent is used, thus reducing the effluent load to a minimum. The use of strong acids and
bases for control of pH-shift systems is obviously fraught with difficulties where the
operation is carried out manually. If a sophisticated automatic monitoring and dosing system
is used, however, the use of such compounds has certain very worthwhile advantages:
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SEQUESTERING AGENTS
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(a) The adjusting chemicals are the cheapest available;
(b) The entire range of pH values can be controlled by using just two chemicals;
(c) Since a buffering system is not built up in the bath, the pH can be shifted in any
direction to any degree, easily and with the minimum addition of chemicals;
(d) As a result of (c), any variation in the intrinsic pH of the substrate or water supply can
be easily neutralised;
(e) Exhausted dyebaths can often be reused as there is no build-up of buffering agent; and
(f) There are no environmental problems.
It should be noted that automatic dosing and monitoring of dyes and chemicals can be used
generally and is not restricted to pH control.
10.2 SEQUESTERING AGENTS
The tendency of soaps and other carboxylates to form insoluble complexes with calcium and
magnesium ions in hard water is mentioned in sections 9.4 and 9.8.2. Apart from decreasing
the efficiency of the anionic surfactant, deposition of such insoluble complexes on the
textile substrate can cause problems in subsequent processing and particularly in coloration.
Even trace amounts of certain transition-metal or alkaline-earth elements may cause
processing difficulties. The formation of ‘iron spots’, particularly in bleaching, is well known:
multivalent transition-metal cations catalyse the decomposition of hydrogen peroxide
(although divalent calcium and magnesium ions have a stabilising effect) and localised
staining or tendering of the fibre may occur. In coloration trace-metal ions can react with
certain dyes, giving rise to precipitation, discoloration, unlevel dyeing and reduced fastness.
The processing water is the most obvious source of such extraneous metal ions, but other
potential sources should not be overlooked. For example, trace metals may be dissolved from
the surfaces of machinery and fittings. The substrate may already contain such metals, as
may also any chemicals or dyes used. Hence these problems cannot always be avoided simply
by ensuring the supply of suitable water – indeed, the overzealous treatment of water can
actually lead to the presence of troublesome aluminium ions that were not originally
present! Such problems can be solved using chemicals that react preferentially with the
metal ions, effectively preventing them from interfering with the mainstream reaction or
process. Such chemicals are aptly known as sequestering agents. Other terms frequently
used in the literature include the derivative ‘sequestrants’ and ‘complexing agents’, although
complexing does cover a wider field than just metal–ion chelation with which we are
concerned here.
Sequestering agents work by a mechanism of complex formation, often in the form of
chelation. A chelating agent contains substituents suitably located to form one or more
chelate rings by electron donation to the metal ion (section 5.2), the resulting complex
remaining soluble and innocuous under the conditions of processing. The most useful
donating atoms are nitrogen, as found in amines or substituted amines, and oxygen in the
form of carboxyl, phosphate or ionised hydroxy groups. As in the formation of dye–metal
chelates (such as chrome mordant and metal-complex dyes), at least two electron-donating
atoms in the sequestering agent structure must be arranged so that a stable ring can be
formed with the metal ion, the highest stability resulting from five- and six-membered rings.
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
A great many chemicals exhibit sequestering capability but not all are of commercial
value in textile processing. Earlier literature [18,19] mentioned three main types:
– aminopolycarboxylates
– phosphates, mainly inorganic
– hydroxycarboxylates.
However, environmental awareness, in addition to commercial and technical exploitation,
has resulted in considerable activity in this area, leading to a greatly expanded range of
products in recent years, as well as some conflicting statements with regard to their
environmental properties. The scheme of classification adopted here is as follows:
– aminopolycarboxylates and their analogues, e.g. hydroxyaminocarboxylates
– phosphates and phosphonates
– hydroxycarboxylates
– polyacrylic acids and derivatives.
10.2.1 Aminopolycarboxylates and their analogues
These are powerful chelating agents, having good environmental properties [20]. Important
members include:
– ethylenediaminetetra-acetic acid
EDTA
(10.5)
– diethylenetriaminepenta-acetic acid
DTPA
(10.6)
– nitrilotriacetic acid
NTA
(10.7)
These products are sold as free acids or sodium salts. Analogues of these aminopolycarboxylic acids include the hydroxyaminocarboxylic acids. These structures are derived
O
Na +
O
_
C
O
CH2
N
_
Na+ O
C
C
H2C
CH2CH2
N
H2C
CH2
O
_
+
O Na
_
O Na+
C
O
10.5
EDTA
O
O
Na+
_
+
Na
C
O
O
_
C
CH2
O
CH2
C
H2C
_
O Na+
N
N
Na +
CH2CH2
N
_
+
O Na
_
O
C
O
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506
H2C
CH2
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SEQUESTERING AGENTS
507
O
Na+
Na+
_
C
O
O
CH2
N
_
O
C
CH2
C
CH2
_
+
O Na
10.7
O
NTA
by replacing one or more carboxymethyl groups of the aminopolycarboxylate by a
hydroxyethyl group. Examples include:
N-(hydroxyethyl)ethylenediaminetriacetic acid
HEDTA (10.8)
in which one of the carboxymethyl groups of EDTA has been replaced by a hydroxyethyl
group
N,N-bis(hydroxyethyl)glycine
DEG
(10.9)
in which two of the carboxymethyl groups of NTA have been replaced by hydroxyethyl
groups.
O
O
_
C
+
Na
O
CH2
C
H2C
N
CH2CH2
CH2
HO
_
O Na+
HO
CH2
CH2
N
H2C
CH2
C
10.8
N
_
O Na+
HO
CH2
CH2
O
HEDTA
O
CH2
C
_ +
O Na
10.9
DEG
These compounds are not persistent in the environment, NTA degrading slightly more
quickly than EDTA, DTPA or HEDTA [20].
These aminopolycarboxylates act as sequestering agents by forming complexes in which
each metal ion is chelated into one or more five-membered rings. It is often assumed that
one molecule of sequestering agent interacts with one metal ion and for many practical
purposes this is a valid assumption. The nature of the complexes actually formed, however,
may depend on other factors such as the pH of the medium. It is difficult to represent such
structures in detail, particularly as water of solvation is usually involved. It is convenient to
adopt a simplified representation, omitting the water of solvation, as for the EDTA–calcium
complex shown in structure 10.10, in which the arrows represent coordination bonds and
the calcium ion is held by three five-membered rings. At pH values below 11 the structure
tends to be more like that shown in 10.11, which also resembles the complex formed with
NTA (10.12).
O
C
O
Na+
_ C
O
H2C
H2C
_
O
_
O
Ca2+
N
N
CH2CH2
O
C
CH2
CH2
O
C
_
O Na+
10.10
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
O
O
C
_
O
CH2
2+
Ca
_
N
C
CH2CH2
_
2+
Ca
N
C
O
C
O
O
N
Ca
O
C
CH2
2+
_
H2C
CH2
O
O
_
O
H2C
_
O
CH2
C
CH2
O
_
O
Na+
C
O
10.11
10.12
A more elaborate representation of an EDTA-metal complex (10.13), which gives some
indication of the three-dimensional aspects of the structure, shows a complex of five fivemembered rings [18]. A similar representation of a DTPA-metal complex shows a system of
eight five-membered rings.
O
C
CH2
O
O
C
CH2
O
N
CH2
M
CH2
O
N
C
O
CH2
CH2
O
C
10.13
O
10.2.2 Phosphates and phosphonates
Various polyphosphates are effective sequestering agents under appropriate conditions. The
best known of these is sodium hexametaphosphate (10.14), the cyclic hexamer of sodium
orthophosphate. Further examples are the cyclic trimer sodium trimetaphosphate (10.15), as
well as the dimeric pyrophosphate (10.16), the trimeric tripolyphosphate (10.17) and other
linear polyphosphates (10.18). All of these polyanions function by withdrawing the
troublesome metal cation into an innocuous and water-soluble complex anion by a process
of ion exchange as shown in Scheme 10.7 for sodium hexametaphosphate. Hence these
compounds are sometimes referred to as ion-exchange agents.
The disadvantage of the polyphosphates is that at the temperatures (100 °C or higher)
used in many textile processes they can be hydrolysed into simpler phosphates that cannot
retain the metal atom in the sequestered form. For example, dicalcium disodium
hexametaphosphate hydrolyses on prolonged boiling to yield the insoluble calcium
orthophosphate. This is one of the main reasons why polyphosphate sequestrants are used
much less extensively than the more versatile and stable aminopolycarboxylates.
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SEQUESTERING AGENTS
_
+
O Na
O
+
Na
Na+
P
O
_
P
O
O
O
O
_
P
O
O
O
O
O
P
O
_
O
O
P
P
Ca2+
_
O
_
O Na +
P
_
+
O Na
O
O _
P O Na
O
+ 2 Ca2+
P
O _
O
O
O
O
O
_
+
O Na
O
O
P
O
509
O
P
Ca2+
_
O
O
P _
O Na+
+
+ 4 Na
10.14
Sodium hexametaphosphate
Scheme 10.7
_
O Na+
O
+
Na
_
O
O
P
P
O
Na+
O
O
P
O
O
_
O
Na+
_
P
O
O
Na+
O
O
_
P
O
_
_
O Na+
Na+
10.16
Sodium pyrophosphate
10.15
Sodium trimetaphosphate
Na+
_
O
O
O
_
O P
_
Na+ O
O
P O P O
_
_
O Na+ O Na+
+
Na
O
_
O P
_
Na+ O
+
Na
O
O
P
O
O
_
+
O Na
P
_
O Na+
_
O
Na+
n
10.17
10.18
Sodium tripolyphosphate
Sodium polyphosphate
A structural compromise between these two types of compound can also give products
with sequestering properties, although they are phosphonates rather than phosphates, since
they contain C–P rather than C–O–P linkages. Examples of these aminopolyphosphonates
are:
– ethylenediaminetetramethylphosphonic acid
EDTMP
(10.19)
– diethylenetriaminepentamethylphosphonic acid
DETMP
(10.20)
– nitrilotrimethylphosphonic acid
ATMP
(10.21)
– hydroxyethylethylenediaminetrimethylphosphonic acid
HEDTMP (10.22)
– hexamethylenediaminetetramethylphosphonic acid
HMDTMP (10.23)
Two sequestrants of the phosphonate class unrelated to aminopolycarboxylic acids are:
1-hydroxyethane-l,1-diphosphonic acid
HEDP
(10.24)
2-phosphonobutane-1,2,4-tricarboxylic acid
PBTC
(10.25)
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
O
HO
O
P
CH2
HO
HO
HO
CH2CH2
N
P
CH2
H2C
P
N
OH
OH
H2C
P
OH
O
10.19
O
OH
EDTMP
HO
P
P
O
P
CH2
CH2
HO
H2C
P
N
OH
OH
H2C
P
N
HO
HO
HO
O
HO
O
CH2CH2
N
CH2CH2
CH2
10.20
O
OH
OH
O
O
DETMP
HO
P
CH2
HO
HO
O
HO
O
P
CH2
HO
HO
CH2
N
CH2CH2
CH2
H2C
P
N
OH
OH
H2C
P
HO
N
P
OH
O
CH2
OH
CH2
O
10.21
ATMP
OH
O
10.22
HEDTMP
O
HO
O
P
CH2
HO
HO
HO
P
N
CH2CH2CH2CH2CH2CH2
H2C
P
N
OH
OH
H2C
CH2
10.23
O
P
OH
OH
O
HMDTMP
OH
O
HO
O
CH3
O
P
C
P
HO
chpt10(2).pmd
OH
OH
O
C
OH
CH2CH2
HO
O
C
10.24
10.25
HEDP
PBTC
510
C
O
C
P
CH2 OH
OH
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P
OH
OH
SEQUESTERING AGENTS
511
The consumption of these phosphonates in textile processing is small in relation to that of
the aminopolycarboxylates; they are mainly used in detergent formulations [21,22] as
sodium, potassium, ammonium or alkanolamine salts.
Environmentally, phosphates generally have been a sensitive issue, not least because they
can cause eutrophication of watercourses, and the situation is still not resolved completely.
No aerobic or anaerobic bacterium has been found to date that will biodegrade
aminopolyphosphonates under the treatment conditions used today, yet these products are
not biologically persistent. They are partially eliminated photolytically, partially absorbed in
sediment or eliminated by precipitation [23,24]. They show 50–80% elimination in the
Zahn–Wellens test. They show low aquatic toxicity and are non-toxic to humans, animals
and plants. Detailed ecological properties are listed by Schöberl and Huber [25]. Held [26]
has investigated the ecological behaviour of sequestering agents based on phosphonic acids
in detail, concluding that although they contain phosphorus they do show ecological
advantages compared with other types and thus their use is justified.
10.2.3 Hydroxycarboxylates
The hydroxycarboxylic acids provide a range of sequestering agents of which the best known
are citric (10.26), tartaric (10.27) and gluconic (10.28) acids. The toxic oxalic acid (10.29)
is now rarely used. However, these acids are much less important as sequestering agents for
textile processes than either the aminopolycarboxylates or the polyphosphates.
Hydroxycarboxylates are easily biodegraded but do have a high COD. It has been pointed
out [20] that glucoheptanoic acid (2,3,4,5,6,7-hexahydroxyheptanoic acid; 10.30) is also
used in the USA, on the grounds that this compound is less prone to browning at high
temperatures although it possesses no other advantages, having less binding power as well as
being more expensive than other hydroxycarboxylic acids.
O
C
HO
H
OH H
C
C
O
C
O
C
OH
H C H
O
H
C
10.26
Citric acid
HO
H
OH
C
C
OH H
O
C
10.27
Tartaric acid
OH
O
HO
H
H
OH H
OH
C
C
C
C
H
OH H
C
O
C
C
HO
10.28
Gluconic acid
OH
OH H
O
C
OH
10.29
Oxalic acid
H
HO
H
OH H
OH H
C
C
C
C
H
OH H
C
OH H
C
OH
O
C
OH
10.30
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
10.2.4 Polyacrylic acids and their derivatives
Recent research has led to some more complex sequestering agents, particularly the
polymeric carboxylic acids referred to as polycarboxylates. These are, in effect,
polyelectrolytes and as such have close similarities to the products described later as
dispersing and solubilising agents (section 10.6), thickening agents or migration inhibitors
(section 10.8). Common monomers used in the production of these compounds, either as
homopolymers or as copolymers with each other, include acrylamide (10.31) and various
unsaturated acids (10.32–10.34). The common and essential feature of these monomers is
the carbon–carbon double bond.
H
H2C
H
C
H2C
CONH2
H
CH3
C
H2C
C
COOH
H
C
HOOC
COOH
10.31
10.32
10.33
Acrylamide
Acrylic acid
Methacrylic acid
C
COOH
10.34
Maleic acid
Polymers which have been suggested for use as sequestering agents [23,27] include:
– poly(butadiene-1,2-dicarboxylic acid)
EMA
(10.35)
which is an ethylene–maleic acid copolymer
– poly(α-hydroxyacrylic acid)
PHAS
(10.36)
– poly(3-hydroxymethylhexatriene-1,3,5-tricarboxylic acid)
(10.37)
which is a copolymer of acrylic acid with hydroxymethacrylic acid
– poly(3-formylhexatriene-1,3,5-tricarboxylic acid)
(10.38)
which is a copolymer of acrylic acid with 2-formylacrylic acid.
OH
CH2
CH2
CH
HOOC
CH2
CH
C
COOH
COOH
n
n
10.35
10.36
EMA
PHAS
CH2OH
CH2
CH
CH2
COOH
C
CH2
COOH
CH
COOH
n
10.37
CHO
CH2
CH
CH2
COOH
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C
CH2
COOH
CH
COOH
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SEQUESTERING AGENTS
513
These agents are generally described as ion-exchange reagents rather than complex-forming
chemicals. They tend to operate by sequestering the metal by an ion-exchange mechanism
and as a result of their polyelectrolytic character they keep the complex dispersed.
Oligomers with a molecular mass of 1200–8000 (i.e. relatively low) are said to give optimum
sequestering power. Polymers of high molecular mass (i.e. Mr = 106–10 7) are useful as
flocculating agents or migration inhibitors.
Although these acrylic oligomers and polymers show little decomposition in effluent
treatment, they pose no significant threat to the environment since they can be removed
easily by adsorption on activated sludge or by precipitation as an insoluble calcium complex
[23]. Exhaustive tests have not revealed any adverse environmental influence. Their aquatic
toxicity is negligible and toxicity to warm-blooded mammals is slight. Mutagenic,
carcinogenic or teratogenic effects have not been found so far.
A further development [27] is the formation of so-called sugar–acrylate copolymers in
which acrylic acid is copolymerised with glucose or other saccharides. Unlike other
sequestering agents these polymers are said to be readily biodegradable, this being the main
reason for their development.
An unusual type of sequestering agent is triethanolamine (10.39). This compound is
cheap and exclusively useful for complexing iron(III) in strongly alkaline solutions, e.g. up to
18% sodium hydroxide. It does in fact remain active as a complexing agent even in more
strongly alkaline solutions although solubility can be a problem.
CH2CH2OH
HOCH2CH2
N
CH2CH2OH
10.39
Triethanolamine
10.2.5 The action of sequestering agents
Sequestering agents are often used rather indiscriminately, in amounts far in excess of the
stoichiometric quantities required by the particular set of conditions. Instructions often
simply state ‘add 0.5–1.0 g/l of a suitable sequestering agent such as EDTA’. Whilst this is
convenient for most purposes, it is worth bearing in mind that the action of sequestering
agents is governed by physico-chemical factors that, among other things, determine a
hierarchy of efficacy. When the type and concentration of trace-metal ions to be
sequestered is known, a more discriminatory approach can be adopted regarding the choice
of agent. In some cases, including the treatment of water, this more precise specification of
type and quantity can be important.
Little more need be said here about the simple ion-exchange reactions such as that
between sodium hexametaphosphate and calcium ions (Scheme 10.7). It is useful, however,
to consider in more detail those reactions involving chelation (Scheme 10.8). This is a
reversible reaction, the equilibrium being dependent on the process pH and the
concentrations of the reacting species (Equation 10.2). While chelated complexes are less
stable at higher temperatures, this effect can be ignored in practice. The factors involved
have been discussed in some considerable detail by Engbers and Dierkes [20,23].
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
Sequestering agent (SA) + Metal ion (MI)
Chelated complex (CC)
Scheme 10.8
Ks =
[CC]
[SA][MI]
(10.2)
The stability of the complex is generally expressed in terms of its stability constant, which is
the logarithm of the equilibrium constant (Ks in Equation 10.2) of the reaction in Scheme
10.8. A high stability constant indicates a powerful sequestering effect. For example,
amongst the aminopolycarboxylates the stability constant for a given metal ion generally
increases in the order: NTA < HEDTA < EDTA < DTPA. Metals can also be listed in
order of increasing stability constant: Mg2+ < Ca2+ < Mn2+ < Al3+ < Zn2+ < Co3+ <
Pb2+ < Cu2+ < Ni3+ < Fe3+. The stability constants at 25 °C for six metals with four
different aminopolycarboxylates are shown in Figure 10.4.
30
NTA
HEDTA
EDTA
DTPA
log Ks
20
10
Mg(II)
Ca(II)
Fe(II)
Zn(II)
Cu(II)
Fe(III)
Figure 10.4 Stability constants of aminopolycarboxylate chelates [20]
Thus for the series of sequestering agents and metal ions mentioned, the magnesium–
NTA complex has the lowest stability and iron(III)–DTPA the highest. This scale of values
effectively constitutes a displacement series. This means, in general, that in any system
containing more than one metal it is the metal forming the most stable complex (that is, the
complex having the highest stability constant) that chelates preferentially. When these ions
have been completely chelated, any remaining sequestering agent then begins to sequester
the metal that forms the complex having the next highest stability constant. Similarly if
iron(III) enters a system in which, for example, calcium is already chelated, the iron will
displace the calcium since the iron complex has the higher stability constant. The calcium
will only remain chelated if sufficient sequestering agent is present to sequester both iron
and calcium.
Adding protons or hydroxide ions to the system will influence the position of the
chelation equilibrium. The stability constant of a complex is thus influenced by the pH of
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SEQUESTERING AGENTS
515
the system and pH is an important consideration in the choice of sequestering agents. Figure
10.5 provides a very good illustration of the high sensitivity of the conditional stability
constant (Kc) to the pH of the system, using the same four aminopolycarboxylates in the
presence of iron(II) and iron(III). The pH-related stability constant is known as the
conditional stability constant, log K c. The effects of pH on the conditional stability
constants of the same four aminopolycarboxylates with other metal ions are shown in
Figures 10.6 to 10.9.
20
log Kc
15
10
5
1
3
5
7
9
11
13
pH
DTPA-Fe(III)
DTPA-Fe(II)
ETDA-Fe(III)
EDTA-Fe(II)
HEDTA-Fe(III)
HEDTA-Fe(II)
NTA-Fe(III)
NTA-Fe(II)
Figure 10.5 Effect of pH on the conditional stability constants at 25 °C of Fe(III) and Fe(II) chelates of
aminopolycarboxylic acids [20]
20
DTPA-Fe(III)
DTPA-Cu(II)
DTPA-Fe(II)
log Kc
15
10
DTPA-Zn(II)
5
DTPA-Ca(II)
DTPA-Mg(II)
1
3
5
7
9
11
13
pH
Figure 10.6 Effect of pH on the conditional stability constants at 25 °C of metal chelates of DTPA [20]
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
20
EDTA-Fe(III)
EDTA-Fe(II)
EDTA-Ca(II)
EDTA-Cu(II)
EDTA-Zn(II)
EDTA-Mg(II)
log Kc
15
10
5
1
3
5
7
9
11
13
pH
Figure 10.7 Effect of pH on the conditional stability constants at 25 °C of metal chelates of EDTA [20]
20
HEDTA-Fe(III)
HEDTA-Fe(II)
HEDTA-Ca(II)
HEDTA-Cu(II)
HEDTA-Zn(II)
HEDTA-Mg(II)
log Kc
15
10
5
1
3
5
7
9
11
13
pH
Figure 10.8 Effect of pH on the conditional stability constants at 25 °C of metal chelates of HEDTA
[20]
20
NTA-Fe(III)
NTA-Cu(II)
NTA-Fe(II)
15
NTA-Zn(II)
log Kc
NTA-Ca(II)
NTA-Mg(II)
10
5
1
3
5
7
9
11
13
pH
Figure 10.9 Effect of pH on the conditional stability constants at 25 °C of metal chelates of NTA [20]
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It should not be overlooked that stability constants can be affected significantly by the
presence of electrolytes. For example, the stability constant of the Ca–EDTA complex
changes from 12 in distilled water to 10.5 in 0.1N KCl and to 8.5 in 1.0N KCl. The
inorganic polyphosphates tend to be most efficient under slightly acidic conditions whilst
the aminopolycarboxylates generally work best under neutral or alkaline conditions,
although they still show some usefulness, and are used, at pH values of around 4.5.
Generalisations like these can be misleading, however, since efficiency varies from one metal
ion to another at different pH values for each sequestering agent. The phosphates are good
sequestrants for magnesium and calcium but are considerably less effective for trivalent
cations, which can be successfully sequestered with NTA, EDTA and DTPA up to about pH
9. At higher pH values iron(III) tends to be precipitated from these complexes. It was
mainly for this reason that the hydroxyaminocarboxylates were developed, this basically
being their main use. For example, HEDTA will sequester iron(III) ions at pH 9 and DEG
works well at pH 12. Although effective with most metal ions, DEG will not sequester
calcium or magnesium, and HEDTA is also not as efficacious with these hard water ions as
are the aminopolycarboxylates. At pH values above 12 iron(III) can be sequestered with
triethanolamine (10.39), either alone or together with EDTA.
Most divalent and trivalent ions, with the exception of the alkaline-earth metals, are
effectively chelated by the hydroxycarboxylates citric and tartaric acid, and citric acid will
also sequester iron in the presence of ammonia. Another hydroxycarboxylate, gluconic acid,
is especially useful in caustic soda solution and as a general-purpose sequestering agent.
Clearly, the efficiency of sequestering action must be optimised for a specific set of
conditions. Thought needs to be given especially to the pH of the system and to whether
broad-spectrum or specific sequestering is required. The extent of knowledge of the tracemetal ions present will determine whether a precise addition or an arbitrary excess of agent
is appropriate. Finally, in some circumstances problems can arise from the use of certain
sequestering agents that can remove the coordinated metal from a dye chromogen with
subsequent changes in shade or fastness properties. Metal-complex acid dyes and mordanted
dyes are obviously vulnerable, but also many direct and reactive dyes contain coordinated
copper atoms.
10.2.6 The uses of sequestering agents
Notwithstanding the comments made above in relation to the need to adopt a more or less
sophisticated approach to the selection and use of sequestering agents to target known
contamination by trace metals, an attempt will now be made to provide general
recommendations [28]. This approach will also indicate the wide range of uses for these
agents in textile wet processing.
Sized warp yarns sometimes contain metal salts or complexes (e.g. copper or zinc
compounds) as fungicides or bactericides and these can interfere with enzyme action in
subsequent desizing. Phosphonates such as ATMP, DETMP, EDTMP and PBTC are suitable
for use here, sequestering the heavy metals at pH 6.8–7.0, the effectiveness of each agent
being dependent on the type of enzyme and the metal ions present. The use of the
phosphonates HEDP or PBTC helps to reduce cellulose damage to a minimum in oxidative
desizing with persulphate.
In the kier boiling of cotton, the action of sodium hydroxide can be intensified by the use
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
of sequestering agents. The phosphonates EDTMP, HEDP and PBTC have been
recommended. In practice, however, the tendency is to use synergistic mixtures, such as:
– phosphonate and gluconate
– phosphonate and triethanolamine
– aminopolycarboxylate and gluconate or triethanolamine.
Polycarboxylates may also be added to increase dispersing power and so reduce the
possibility of incrustations. Fine-tuning will again depend on the process details, the
machine type and the degree of scouring necessary.
Phosphonates are useful additions in acidification treatments after alkaline processing to
assist in the removal of metal compounds that have limited solubility in alkali.
Under the strongly alkaline conditions of mercerising, addition of either gluconate or
triethanolamine with a little HEDP is useful. The presence of a polycarboxylate helps to
prevent precipitation on machine components.
Certain transition-metal salts catalyse redox reactions, leading to uneven treatment and
perhaps damage to the substrate. Sequestering agents are therefore employed to complex
these metal ions and so to inhibit their catalytic activity. In reductive bleaching with
dithionite, PBTC acts as a stabiliser at pH 5.5–6.5 and EDTA also gives good results. At
higher pH values, aminopolyphosphonates (e.g. EDTMP) and aminopolycarboxylates are
useful. Only triethanolamine is effective at pH 13. For oxidative bleaching with hypochlorite
or chlorite, the amine oxides of ATMP and PBTC may be used. Aminopolycarboxylates are
less suitable, whilst the amine oxide of NTA is unsuitable.
In the stabilisation of peroxide with silicate, the use of a polycarboxylate, perhaps in
combination with a polyphosphonate such as DETMP (or PBTC in a lesser amount) helps
amongst other things in preventing fibre damage and incrustations on fabric or machine.
Conversely, aminopolyphosphonates such as EDTMP or DETMP may themselves be
suitable as stabilisers in the absence of silicate. Combinations of sequestering agents may
also be used to obtain a synergistic effect, the following mixtures having been suggested for
use with silicates:
– DETMP, HEDP and either gluconate or triethanolamine
– DETMP, DTPA and gluconate.
Polycarboxylates may also be added to help prevent incrustations. It should be borne in
mind, however, that magnesium is an essential component in most cases of stabilisation in
peroxide systems, so any mixture of sequestrants should have minimum binding effect on
this metal ion.
In the pretreatment and dyeing of synthetic fibres, the aminopolyphosphonates can assist
in the removal of oligomers.
Some dyes contain a coordinated transition metal as an essential part of their
chromogenic structure and this must be left undisturbed by any sequestrant used to complex
extraneous metal ions in the system. Hence a balance of properties is needed, phosphates
and hydroxycarboxylates being useful. It is claimed that polycarboxylates can be molecularly
engineered to give the required balance of properties.
Wool is exceptionally prone to absorb metal ions, particularly in the weathered tips of the
fibres, leading to shade differences on subsequent dyeing, especially if chelatable dyes are
used. Hence sequestering agents can be essential additions to scouring, rinsing and dyeing
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MACROMOLECULAR COMPLEXING AGENTS
519
baths in order to remove this absorbed metal. However, it is difficult to say which
sequestrant or mixture of sequestrants gives the best results. Careful laboratory tests need to
be carried out beforehand, taking into account the chemical structure of the dyes to be used,
the type of metal ions involved, the pH value of the dyebath and the type and concentration
of electrolyte present.
In many dyeing and printing operations, livelier, more intense colours can be obtained
with better reproducibility and perhaps better fastness to rubbing by careful choice of
sequestering agent for use in the coloration process. The addition of triethanolamine with
vat dyes can be beneficial in helping to prevent unnecessary loss of reducing agent or overoxidation. Triethanolamine together with EDTMP or HEDP can be used where the
concentration of alkali is less than 10 g/l NaOH (or 22 ml/l caustic soda liquor 38°Bé).
Bronzing of sulphur dyeings can often be prevented using triethanolamine with
polyphosphate, polyphosphonate or polycarboxylate. Azoic combinations can be very
sensitive to metal contamination. EDTA, or perhaps combinations of polyphosphate,
polyphosphonate or polycarboxylate, can help in the solubilisation of naphtholates and in
the stabilisation of their colloidal solutions, whilst EDTA, for example, can assist in
protecting solutions of diazonium salts from metal-induced catalytic decomposition.
Polyphosphates, or in lower amounts the very effective polyphosphonates, are helpful in
applications of fluorescent brightening agents. Sequestering agents can be useful additions in
the afterwashing of dyeings and prints, for example polyphosphates, polyphosphonates or
polycarboxylates such as, amongst others, a polyacrylate of molecular mass 3000–4000 or an
acrylic–maleic acid copolymer. In the soaping of vat or azoic dyeings, recrystallisation is
accelerated and rubbing fastness improved by the use of sequestering agents, examples being
mixtures of polyphosphonates and polycarboxylates, such as HEDP and an acrylic–maleic
acid copolymer.
The sugar–acrylate polymers [27] are recommended for applications similar to those
mentioned above for polycarboxylate polymers and copolymers.
10.3 MACROMOLECULAR COMPLEXING AGENTS
Macromolecular complexing agents have featured a good deal in recent research. Although
they do not yet appear to have attained any significant commercial use, they possess
interesting properties, not least their environmental advantages, that offer potential for
future exploitation.
The term macromolecule includes all large molecules, including textile fibres and
polymers. The polyelectrolytes used as dispersing agents (section 10.6) or as thickening
agents and migration inhibitors (section 10.8) are examples of linear macromolecules. Cyclic
macromolecules are also known. An important feature of such macromolecules often
responsible for their functioning as auxiliaries is their ability to form complexes, particularly
with dyes or fibrous polymer segments. In the case of linear macromolecules, the complexes
are generally formed by multipoint attachment with the smaller entity situated alongside the
macromolecule. Cyclic macromolecules, on the other hand, may exhibit the interesting
property of complexing another compound within its centre, the macromolecule completely
surrounding the complexed entity. Thus such agents have some functional similarity with
sequestering and chelating agents. However, whereas sequestering and chelating agents are
generally used to complex simple metal ions, the macrocyclic complexing agents are usually
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
engineered to complex with bigger molecules such as dyes, polymer segments or surfactants
and do not act as simple metal-sequestering agents. It is these properties that have
stimulated much research into the possible uses of macrocyclic complexing agents as
auxiliaries in coloration processes or as agents for helping to clean up textile wastes. It is
particularly interesting that certain macrocyclic agents can be obtained from natural
replenishable sources. Four types of macrocyclic complexing agents are considered here:
– cyclodextrins
– cucurbituril
– crown ethers
– liposomes such as phosphatidylcholine derivatives.
The discussion, however, is relatively brief in view of the fact that little commercial
development seems to have taken place so far.
10.3.1 Cyclodextrins
Cyclodextrins are obtained by enzymatic depolymerisation and extraction from starch. They
comprise rings of D-glucose units and α-, β-, or γ-cyclodextrin can be obtained [29],
depending on whether 6, 7 or 8 glucose units are present in the ring (10.40). The dimensions
of the outer and inner surfaces increase as the number of glucose units increases (Figure
10.10). The important characteristic feature of these cylindrical structures that influences their
properties lies in the different nature of their surfaces, the outer being essentially hydrophilic
whilst the inner is essentially hydrophobic [29–31]. Thus the outer surface confers aqueous
solubility whilst the inner surface is capable of attracting suitably configured hydrophobic
moieties into the cavity, thus forming a complex, sometimes referred to as an inclusion
complex. Obviously, the size of the hydrophobic moiety to be complexed must be such that it
can fit into the cavity of the cyclodextrin used (i.e. α, β or γ).
In addition, the versatility of cyclodextrin macromolecules as a group is enhanced by the
fact that derivatives of cyclodextrins can be prepared [32]. This is achieved, for example, by
OH
O
O
HO
OH
O
O
HO
OH
O
OH
HO
OH
OH
HO
HO
OH
HO
O
10.40a
O
α-Cyclodextrin
(6 units)
HO
OH
O
HO
O
O
OH
O
O
HO
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MACROMOLECULAR COMPLEXING AGENTS
521
OH
O
O
O
HO
O
HO
OH
OH
OH
HO
OH
O
HO
O
O
O
HO
OH
HO
OH
OH
HO
HO
OH
O
10.40b
HO OH
O
O
β-Cyclodextrin
(7 units)
O
O
O
OH
HO
HO
O
HO
OH
O
O
O
O
HO
OH
OH
O
O
HO
HO
HO
OH
HO
OH
HO
OH
OH
O
10.40c
O
OH
O
HO
HO
HO
γ-Cyclodextrin
(8 units)
OH
O
OH
O
O
HO
O
O
OH
O
HO
-cyclodextrin
-cyclodextrin
-cyclodextrin
1370
1530
1690
500
650
850
780
780
Figure 10.10 Dimensions of cyclodextrins [29]
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
substituting to various degrees the hydrogen atoms of the hydroxy groups (Table 10.1).
Introduction of a hydroxypropyltrimethylammonium chloride grouping gives a cationic
derivative, whilst the monochlorotriazinyl substituent gives rise to a derivative that is
analogous to a reactive dye. This cyclodextrin derivative, therefore, is capable of reacting
with fibrous macromolecules that contain nucleophilic groups.
Table 10.1 Typical derivatives of β-cyclodextrin [32]
R Group
β-cyclodextrin
H
CH2
CH
CH3
2-hydroxypropyl
CH2OH
2,3-dihydroxypropyl (glyceryl)
CH2CH2CH2CH3
2-hydroxyhexyl
OH
CH2
CH
OH
CH2
CH
OH
CH2
CH
CH2
O CH2CH2CH2CH3
n-butylglyceryl
CH2
O CH2CHCH2CH2CH2CH3
2-ethylhexylglyceryl
OH
CH2
CH
OH
CH2
CH
CH2CH3
CH2
O
phenylglyceryl
CH2
O
o-tolylglyceryl
OH
CH2
CH
OH
H3C
CH2
COONa
CH2
CH
sodium carboxymethyl
CH3
OH
CH2
N CH3
CH3
2-hydroxypropyltrimethylammonium
Cl
Cl
N
monochlorotriazinyl
N
N
ONa
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MACROMOLECULAR COMPLEXING AGENTS
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OR
O
O
RO
OR
O
O
RO OR
OR
RO
OR
RO
O
O
RO
OR
RO
OR
RO
RO
O
RO
O
OR
O
OR
OR
O
O
RO
O
O
O
RO
Once complex formation has taken place, the physico-chemical nature of both the
cyclodextrin and the complexed substance is changed. For example, complexing diminishes
the vapour pressure of volatile compounds and increases the stability of compounds that are
sensitive to light or air [32]. The aqueous solubility of some sparingly soluble compounds
can be increased by complexing, this being attributable to the influence of the hydrophilic
exterior of the cyclodextrin. Complexing with cyclodextrin can also increase the hydrolytic
stability of some compounds, including dyes [29]. The functionality of cyclodextrin
complexes, like that of metal-sequestrant complexes, is governed to a large extent by the
stability constant of the complex and this is markedly influenced by stereochemical factors.
Cyclodextrins have ecologically advantageous properties. Not only are they produced
from natural and replenishable sources, they are biodegradable, non-toxic and possess no
allergenic potential [32,33]. They are commercially available in bulk quantities at an
optional degree of purity and have been used for many years in pharmaceuticals.
The general uses of cyclodextrins (i.e. non-textile as well as textile) have been reviewed
[30]. Research into possible textile applications has been ongoing since the 1950s and has
covered many aspects, from preparation of substrates through coloration processes to
finishing, as well as effluent treatment. A brief review of more recent textile-related research
is given here to demonstrate the wide range of potential applications of these interesting
products.
Substrate preparation
It has been claimed that complexes of β-cyclodextrin with anionic surfactants, notably
higher fatty alcohol ethoxylates, improve scouring efficiency on cotton and wool in
laboratory-scale processing [34]. Residual surfactants carried over from preparation can
have undesirable effects in subsequent processing. When cyclodextrins complex with
surfactants, their surface activity is reduced. Hence cyclodextrins are potentially useful for
the removal of residual amounts of surfactants from substrates [35]. The use of α- and
β-cyclodextrins has been studied in this context with one cationic, one anionic and four
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
nonionic agents. α-Cyclodextrin tended to form weaker complexes than β-cyclodextrin.
Methylated β-cyclodextrin was especially useful. The hollow interior of a β-cyclodextrin
molecule is capable of the optimum accommodation of a benzene ring and is thus
particularly good for complexing ethoxylated phenols.
Acid dyes
The formation of complexes of the fluorescent tracer dye ammonium 1-phenylaminonaphthalene-8-sulphonate (10.41) with cyclodextrins has been investigated with
favourable results, especially in environmental studies [33]. The ability of this dye to
complex with cyclodextrins has been exploited mainly as an analytical tool in the study of
cyclodextrin applications, since its fluorescence is easily measured. The interaction of α-, βand γ-cyclodextrins with azo acid dyes containing alkyl chains of different lengths has been
reported [36,37]. The formation and isolation of solid complexes between β-cyclodextrin
and CI Acid Red 42, CI Acid Blue 40 or Erionyl Bordeaux 5BLF (Ciba) have been reported
[29].
NH
SO3NH4
10.41
Basic dyes
The structure and formation constants of α-, β- and γ-cyclodextrin complexes with
azoniabetaine dyes have been studied, the formation constants decreasing in the order:
β-CD > γ-CD > α-CD [31]. Complexes of methylene blue (CI Basic Blue 9) with
cyclodextrins have been examined, β-cyclodextrin giving the highest stability constant. This
was indicative of almost ideal fitting of the dye molecule into the cyclodextrin cavity [38].
Complexing with cyclodextrins increased the fluorescence intensity of the dye, this effect
also being highest with β-cyclodextrin.
Direct dyes
The application to cotton of CI Direct Orange 46, Red 81 and Blue 71 at 90 °C in the
presence of a cyclodextrin, sodium chloride and borax (to give pH 9) gave results that varied
with the dye and auxiliary combination present [39]. The formation and isolation of solid
complexes between β-cyclodextrin and CI Direct Orange 40, Orange 46, Blue 86 or Green
26 and between γ-cyclodextrin and Green 26 have been reported [29]. The complexes of
β-cyclodextrin with Orange 46 or Green 26 were evaluated for the dyeing of cotton with
them in the presence of electrolyte at 90 °C. Greater amounts of the complexes were needed
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MACROMOLECULAR COMPLEXING AGENTS
525
to obtain depths matching those obtained conventionally using the uncomplexed dyes. It is
not clear whether complexing with the cyclodextrin interferes with the capability of the dye
to interact with the fibre in the usual way. The stability of these complexes does not appear
to be high.
Disperse dyes
Detailed dyeing studies have been carried out using complexes formed between disperse dyes
(Table 10.2) and cyclodextrins. One study [40,41] evaluated the complexing and subsequent
dyeing properties of five related disperse dyes (10.42) on polyester. The results indicated that
dyes which have no ortho substituents in the diazo component formed 1:1-complexes with
α-, β- and γ-cyclodextrins. Dyes having electron-withdrawing groups in both of these ortho
positions formed 1:1-complexes with β-cyclodextrin and 2:2-complexes with γ-cyclodextrin.
Dyes 3 to 5 have a substituted diazo grouping that is larger than the cavity of α-cyclodextrin
and so they are unable to form complexes with this macrocyclic molecule. It was suggested
that such complexing could be used as a retarding mechanism in dyeing with disperse dyes,
although these studies were restricted to a maximum temperature of 90 °C.
Table 10.2 Dyes used in complexing with cyclodextrins [40,41]
Dye
X
Y
R
MSCS
1
2
3
4
5
H
H
Cl
Cl
NO2
H
H
Cl
Cl
Br
CH2CH3
CH2CH2OH
CH3
CH2CH2OH
CH2CH2OH
4.35
4.35
7.29
7.29
8.21
MSCS maximum size of cross-section of the substituted ring in the diazo
component
X
O2N
R
N
N
N
CH2CH2OH
Y
10.42
Buschmann et al. [29,42] have studied the formation and isolation of solid complexes
between disperse dyes and cyclodextrins. Disperse dyes free from their usual diluents, such
as dispersing agents, were used. Complexes were formed as listed in Table 10.3. CI Disperse
Blue 79 did not form complexes with either β- or γ-cyclodextrin. The complexes with CI
Disperse Orange 11, Orange 29, Red 82, Violet 1, Blue 165, Resolin Red FRL or Resolin
Yellow 5GL were studied in the dyeing of polyester at 130 °C. An important aspect of these
dyeings was that they were carried out using only the dye–cyclodextrin complex in water,
without the addition of dispersing or levelling agents. Good level dyeings of high exhaustion
were obtained, even though complexing with cyclodextrins increases the aqueous solubility
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
of disperse dyes. Thus particularly important advantages are claimed for this process over
the traditional method of dyeing with disperse dyes: the fact that no dispersing or levelling
agents are needed coupled with higher exhaustion leads to much less pollution of the
effluent.
Table 10.3 Formation of complexes between cyclodextrins and
disperse dyes [42]
With β-cyclodextrin:
With γ-cyclodextrin:
CI Disperse Yellow 3
CI Disperse Yellow 42
CI Disperse Orange 11
CI Disperse Orange 29
CI Disperse Violet 1
CI Disperse Violet 31
CI Disperse Blue 56
CI Disperse Blue 165
Resolin Red FRL (DyStar)
Resolin Yellow 5GL (DyStar)
CI Disperse Orange 11
CI Disperse Orange 29
CI Disperse Red 82
CI Disperse Violet 1
CI Disperse Blue 56
CI Disperse Blue 165
Resolin Red FRL (DyStar)
Resolin Yellow 5GL (DyStar)
Reactive dyes
The formation and isolation of solid complexes between cyclodextrins and reactive dyes
have been reported, but no dyeing results were presented [29]. Complexes were formed
between β-cyclodextrin and CI Reactive Orange 16, Violet 5, Blue 38 or Blue 114 and
between γ-cyclodextrin and CI Reactive Blue 38 or Blue 114.
The use of β-cyclodextrin and its fibre-reactive heptasubstituted monochlorotriazine
derivative (10.43) has been studied in an effort to minimise the amount of the
environmentally undesirable hygroscopic agent urea that is necessary when printing with
reactive dyes [43]. Although this detailed research covered many variables as regards print
paste additives, it involved only one reactive dye. It was found that the 300 g/kg urea
normally used could be reduced to 75 g/kg when 40 g/kg of the reactive cyclodextrin
derivative was used or to zero when 80 g/kg of the cyclodextrin was used, comparable results
being obtained in all cases. Such cyclodextrins may function in several possible ways. The
exterior hydrophilic surface of the cyclodextrin may enhance dye solubility in the same way
as traditional hygroscopic agents and the hydrophobic cavity may assist this by complexing
with the dye. The reactive cyclodextrin derivative may first react with the fibre and then
attract further dye through its ability to absorb dye into its hydrophobic cavity.
Finishing
It has been demonstrated [32,44] that the various β-cyclodextrin derivatives shown in Table
10.1 can be applied to the surface of appropriate fibres by dyeing methods traditionally used
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MACROMOLECULAR COMPLEXING AGENTS
527
OR
O
O
O
RO
O
HO
OH
OH
HO
OH
OR
O
HO
O
O
O
OH
RO
HO
OH
HO
HO
OH
HO OH
O
OR
O
Cl
N
O
R=
N
N
O
O
O
ONa
RO
RO
10.43
for those fibres. When the agent has become attached to the fibre surface, the cavity of the
cyclodextrin derivative is still available for the accommodation of appropriate hydrophobic
guest compounds. Such guest chemicals may be, for example, perfumed, bactericidal,
waterproofing or stain-resist agents. Thus the concept offers a versatile potential for the
development of special finishing effects.
The mechanisms by which certain β-cyclodextrin derivatives can become successfully
attached to respective fibres are analogous to those operating between dyes and fibres. The
monochlorotriazine derivative can be applied to cellulosic fibres either:
– by padding and fixing for 5 minutes in saturated steam at 100 °C or 3 minutes contact
heat at 150 °C
– or by printing followed by drying for 2 minutes at 100 °C and steaming in saturated steam
for 8 minutes at 100 °C.
The protective action of the alginate thickening agent in the print paste is believed to play a
significant part in the success of this printing method. Application of the reactive
β-cyclodextrin by an exhaust method, as used for hot-dyeing monochlorotriazine reactive
dyes, was unsuccessful. This is probably because the cyclodextrin derivative, although a
monochlorotriazine, does not possess the structural features, such as a planar molecule and a
conjugated system of double bonds, that play an important role in the substantivity of
reactive dyes for cellulosic fibres. Nor was padding followed by a cold (15 h at 25 °C) or hot
(4 h at 80 °C) batching treatment successful. Several derivatives with hydrophilic (2,3dihydroxypropyl or 2-hydroxypropyl) or lipophilic (n-butylglyceryl, 2-ethylhexylglyceryl or
o-tolylglyceryl) substituents showed evidence of fixation to polyester by exhaust application
at 130 °C. The anionic sodium carboxymethyl derivative could be fixed to nylon and the
cationic 2-hydroxypropyltrimethylammonium derivative became fixed to an acrylic fibre by
exhaust application at atmospheric pressure and 95–98 °C.
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
Effluent treatment
Since cyclodextrins form complexes with various other substances, including many dyes and
surfactants, it is clear that they could be useful in effluent treatment. They are potentially
suitable for the reduction or removal of polluting substances either by immobilisation or by
solubilisation and extraction and thus can accelerate detoxification [30].
Summary
Cyclodextrins, discovered as long ago as 1930, have been the subject of much research since
the 1950s, covering the whole field from textile preparation to effluent treatment. Their
capability to form complexes with surfactants, dyes and segments of fibrous polymers is now
well-established, yet little or no commercial exploitation appears to have grown out of their
research and environmental potential. Most of the work has been done with β-cyclodextrin
and its derivatives, followed by γ-cyclodextrin. The cavity of α–cyclodextrin appears to be
too small for it to be widely used in textile applications. Thus there are limits to complexforming capability depending on the molecular size and nature of the cyclodextrin in
relation to the molecular size, nature and configuration of potential guest compounds.
Much of this research has been carried out with single guest compounds. Evidently, the
behaviour of these systems is highly specific, leading to a need for considerable fine-tuning
in extending the concept to more heterogeneous commercial conditions. For example, there
have been few investigations of cyclodextrins with typical trichromatic mixtures of dyes, in
which the compatible behaviour of all the components may be difficult to resolve. This is
implicit in the work of Yun et al. [45], who studied the compatibility of β-cyclodextrin with
27 water-soluble dyes. They demonstrated marked specificity in cyclodextrin–dye
interactions and found that this was influenced by electrolytes and surfactants, as well as by
pH. Complexing with β-cyclodextrin protected some dyes from the effects of salts and acids;
this could be desirable or problematic, depending on requirements. It was also found that
β-cyclodextrin improved the levelling of certain acid dyes [45].
Further problems may arise from the possible effects of residual cyclodextrins on
subsequent processes. For example, as described above, a cyclodextrin can be used to
remove residual surfactants from a fabric after scouring, yet it is possible that any residual
cyclodextrin could itself interfere with subsequent coloration or finishing processes to the
same extent as the surfactants that have been eliminated.
10.3.2 Cucurbituril
Cucurbituril, like cyclodextrin, is a macrocyclic complexing agent [46]. An advantage of this
compound is its potentially low cost, being made from glyoxal, urea and formaldehyde. It is a
cyclic hexamer (10.44) containing six acetyleneurein residues linked by methylene groups
between the nitrogen atoms and is configured like a wristband (10.45), composed of six
8-membered rings alternating with pairs of 5-membered rings. Also like cyclodextrin, this
cylindrical macromolecule has a hydrophobic cavity into which a dye molecule, or a part
thereof, can be accommodated, the chemical structure of the dye having a distinct influence
on the stability of the complex [46]. Cucurbituril can also complex with substances other
than dyes, including alkaline-earth and alkali metal ions [47].
In the case of a typical dyebath composition containing dye, electrolyte and surfactants,
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MACROMOLECULAR COMPLEXING AGENTS
529
O
O
O
N
N
C
N
H
H
H
C
N
cavity
N
C
O
N
C
H
N
O
O
N
N
N
N
N
N
N
N
N
N
N
N
N
C
N
N
O
C
H
O
10.44
H
6
O
O
O
10.45
Cucurbituril
and possibly alkaline-earth cations from hard water, there are many competing concurrent
interactions involving cucurbituril. For example, the following possibilities coexist [46]:
– the dye forms a complex with the cucurbituril and simultaneously is associated in micellar
complexes with the surfactant
– the surfactant forms a complex with cucurbituril
– the electrolyte ions interact with cucurbituril.
Each factor may affect the others, depending on relative concentrations and pH.
Cucurbituril requires quite strongly acidic conditions for solubilisation, hence its use in
textile processing is likely to be very limited. It has mostly been investigated in connection
with the removal of colour from textile effluents [46,48–53].
10.3.3 Crown ethers
Crown ethers and related structures are macrocyclic organic compounds generally composed
of repeating ethylene (CH2CH2) units separated by hetero atoms such as oxygen, nitrogen,
sulphur or phosphorus [54]. Other alkylene sub-units such as methylene (CH2) or propylene
(CH2CH2CH2) may also be included but are less common. In contrast to the cyclodextrins
and cucurbituril, these macrocyclic complexing agents possess an electron-rich and highly
polar cavity and a hydrophobic exterior. Usually they are readily soluble in organic solvents.
They have been known since the 1930s. Two typical structures are 10.46 and 10.47. Oxygen
is the hetero atom most commonly incorporated into the ring, but nitrogen (azacrowns),
sulphur (thiacrowns) or phosphorus (phosphacrowns) are also known. Organic moieties
sterically equivalent to the ethylene unit (such as 1,2-benzo) can be incorporated, as can
most carbohydrates with vicinal dihydroxy groupings. Crown ethers are usually named as
x-crown-y, from the number (x) of atoms composing the macrocyclic ring and the number
(y) of hetero atoms contained within it. If one of the oxygen atoms in structure 10.46 is
substituted by nitrogen, it becomes monoaza-12-crown-4.
Not all crown ethers have been tested for ecological or toxicological properties. Some are
irritants and some are known to be toxic, although those tested do not show high toxicity.
Nevertheless, amongst those that have not been tested, some may be hazardous to health.
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
H2C
H2C
H2C
H2C
CH2
O
O
O
O
H2C
CH2
O
H2C
CH2
CH2
O
O
O
O
CH2
CH2
H2C
O
H2C
10.46
12-crown-4
CH2
CH2
10.47
Dibenzo-18-crown-6
The functional characteristic of these compounds that is of interest from the viewpoint of
textile processing is their capability to accommodate alkaline-earth and alkali metal cations,
as well as a variety of other species, within their cavities. Stability constants (Equation 10.3)
are again used, both as a measure of ligand strength and as a hierarchical indicator of
complexing capability.
Ks =
kc
kd
(10.3)
kc = rate constant for complex formation
kd = rate constant for dissociation of the complex
Ks = stability constant
The stability constant is dependent, amongst other things, on the solvating medium. For
example, for a simple crown ether kc is usually very large and kd also large, but in nonpolar
solvents kd is much smaller than kc, so that KS increases with decreasing polarity of the
solvating medium.
It is generally accepted that for complexing to occur the cavity of the crown ether must
contain convergent binding sites (as, for example, the inwardly directed oxygen atoms in
10.46 and 10.47), whilst the entity to be complexed must have divergent binding sites. An
example is shown in 10.48, the formation of which is facilitated by the hydrogen atoms
diverging from the central nitrogen of the methylammonium cation. It is the three N–H–O
hydrogen bonds that stabilise the complex.
The potential of crown ethers for use as auxiliaries in textile coloration processes does not
appear to have been evaluated recently, although their potential to complex with alkaline-
H2C
H2C
O
H
H2C
H2C
O
CH2
H
CH2
+
N CH3 O
_
H Cl
CH2
O
CH2
10.48
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MACROMOLECULAR COMPLEXING AGENTS
531
CH2
O
CH2
H2C
O
CH2
H2C
D
N
H2C
CH2
O
H2C
CH2
O
CH2
10.49
D = Styryl dye chromogen
earth and alkali metal ions has been demonstrated with styryl dyes containing an aza-15crown-5 macroheterocyclic moiety (10.49) [55].
10.3.4 Liposomes
Liposomes, also known as lipid vesicles, are aqueous compartments enclosed by lipid bilayer
membranes [56,57]. Figure 10.11 shows how lipid bilayers are arranged in the liposome and
the lipid structures in large unilamellar vesicles and multilamellar vesicles. Lipids consist of
two components:
– an elongated hydrophobic moiety
– a hydrophilic end group.
polar
non polar
ca
y
vit
polar
Liposome
MLV
LUV
Figure 10.11 Liposome structures, including multilamellar vesicles (MLV) and large unilamellar
vesicles (LUV) [57]
When these lipids are dispersed in water, they spontaneously form bilayer membranes (also
called lamellae) which are composed of two monolayer sheets of lipid molecules with their
hydrophobic surfaces facing one another and their hydrophilic surfaces contacting the
aqueous medium. In the case of phospholipids such as phosphatidylcholine (10.50), the
structure consists of:
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
– hydrophobic component: two hydrocarbon chains (R1 and R2)
– hydrophilic component: glyceryl ester, phosphate and choline groups.
O
C
O
R1
CH2
O
CH
O
R2
C
O
CH2
O
P
O
_
CH2
O
CH2
CH3
+
N CH3
CH3
10.50
Phosphatidylcholine
These structures are effective encapsulating systems for either hydrophilic or hydrophobic
compounds. They can be obtained not only in uni- or multilamellar forms but also in
different particle sizes with varying degrees of aggregation. They are particularly useful in
biological and pharmacological applications. Recent research in various areas of textile wet
processing has revealed further potential in these sectors. However, the methods of
preparing liposomes so far reported are not readily adaptable to commercial processing
conditions, as can be seen from the following typical procedures used by de la Maza et al.
[57–60]. Clearly, substantial development work is needed before these techniques become
compatible with bulk-scale textile wet processing.
Large unilamellar vesicle liposomes
Reverse-phase evaporation in a nitrogen atmosphere was used to prepare lipids. A lipid film
previously formed was redissolved in diethyl ether and an aqueous phase containing the
dyebath components added to the phospholipid solution. The resulting two-phase system
was sonicated at 70 W and 5 °C for 3 minutes to obtain an emulsion. The solvent was
removed at 20 °C by rotary evaporation under vacuum, the material forming a viscous gel
and then an aqueous solution. The vesicle suspension was extruded through a polycarbonate
membrane to obtain a uniform size distribution (400 nm).
Multilamellar vesicle liposomes
A lipid film was formed from a chloroform solution of egg phosphatidylcholine by rotary
evaporation in a nitrogen atmosphere and under vacuum. An aqueous phase containing the
dyebath components was then added to the lipid film. The solution was swirled to transfer
the lipid from the flask and to disperse lipid aggregates; glass beads being added to facilitate
dispersion. The resulting milky suspension was centrifuged for 5 minutes and then extruded
through a polycarbonate membrane to obtain a uniform size distribution (400 or 800 nm).
Liposomes made from pure phosphatidylcholine or containing lipids that are found in the
cell membrane complex of wool (e.g. cholesterol) have been used to encapsulate aqueous
chlorine solutions in chlorination processes [61,62]. The results showed improvements in
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MACROMOLECULAR COMPLEXING AGENTS
533
the uniformity and homogeneity of oxidative treatment, minimising degradation of the wool
and facilitating subsequent treatments.
The application of acid dyes to wool using liposomes has also been researched. The dyes
used were the milling acid dye CI Acid Blue 90 [57] and the neutral-dyeing 1:2 metalcomplex dye CI Acid Yellow 129 [60]. Dyeing conditions were 90 °C and pH 5.5, using
various ratios of phosphatidylcholine:dye. In the work with CI Acid Blue 90, both uni- and
multilamellar vesicles were used. Dye exhaustion decreased with increasing concentration of
phospholipid (Figure 10.12) but the amount of bonded dye increased with increasing lipid
concentration (Table 10.4). The percentage of dye bonded to wool (Cb) was expressed by
Equation 10.4:
Cb =
100(Ca - C e )
Ca
(10.4)
where Ca = mg/g dye absorbed by wool
and Ce = mg/g dye extracted from wool by ethanol and ammonia.
100
100
A
B
80
Dye exhaustion/%
Dye exhaustion/%
80
60
40
20
60
40
20
20
60
100
20
60
Time/min
Concentration (mmol/l)
100
Time/min
0.0
0.5
1.0
2.0
4.0
Figure 10.12 Exhaustion of CI Acid Blue 90 by untreated wool in dyeing with LUV (A) and MLV (B)
liposomes [57]
Table 10.4 Amounts of dye bonded to wool using
LUV and MLV liposomes at different lipid concentrations with CI Acid Blue 90 [57]
Bonded dye (%)
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(mmol/1)
LUV
MLV
4.0
2.0
1.0
0.5
0
84
78
77
68
62
77
74
73
68
62
533
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
The work with CI Acid Yellow 129 used only unilamellar vesicles. The liposomes again
suppressed exhaustion but increased dye–fibre bonding, leading to better fastness properties.
It is claimed that liposomes can be used to control the rate of exhaustion.
The application of CI Disperse Violet 1 to wool with phosphatidylcholine [58] and
phosphatidylcholine/cholesterol [59] liposomes has been investigated. Figures 10.13 and
10.14 show that exhaustion decreases with increasing concentration of liposome, an effect
which may be used to control exhaustion rate. It is claimed that liposomes enhance the dye
dispersion efficiency, being superior to conventional dispersing agents. Dye–fibre bonding
forces and levelling of the dye are also said to be improved.
Exploration of the use of liposomes in wool processing stems from the similarity that
exists between the bilayer structure of the cell membrane complex of wool and that of the
liposomes. Merino wool contains about 1% by weight of lipids, these forming the
hydrophobic barrier of the cell membrane complex. Cholesterol is one of the main lipid
100
Dye exhaustion/%
80
60
40
Phosphatidylcholine lipid
concn (mmol/l)
20
20
60
40
0.5
1.0
1.5
2.5
80
100
120
Time/min
Figure 10.13 Exhaustion rates of CI Disperse Violet 1 on untreated wool in dyeing using liposomes at
different lipid concentrations and constant dye concentration [58]
100
Dye exhaustion/%
80
60
40
Phosphatidylcholine/cholesterol
lipid concn (mmol/l)
20
1.25
2.5
20
40
60
3.0
80
100
120
Time/min
Figure 10.14 Exhaustion rates of CI Disperse Violet 1 on untreated wool during dyeing in the
presence of MLV liposomes at different lipid concentrations and constant dye concentration [59]
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MACROMOLECULAR COMPLEXING AGENTS
535
components in wool; hence its use in combination with phosphatidylcholine (Figure 10.14).
One of the ideas behind this research, which remains valid despite limited commercial
prospects as yet, is to focus attention away from electrostatic forces of attraction towards
hydrophobic interactions, which are now accorded greater importance. In dyeing, for
example, the idea is that the hydrophobic liposome will encapsulate dye molecules by means
of hydrophobic interaction, the liposome–dye complex then being absorbed into the
hydrophobic centre of the cell membrane complex via further hydrophobic interaction. This
is why claims are made for increased hydrophobic bonding of the dyes to the fibre.
Along similar lines, synthetic cationic (10.51) and anionic (10.52) double-chain
surfactant vesicles have been investigated for the dyeing of polyester with a monoazo
disperse dye [63]. The results were moderately encouraging from technological, economical
and environmental viewpoints, although problems and inconsistencies were observed. It is
often difficult to explain results with disperse dyes on the basis of structural chemistry alone,
since they can be influenced by variations in dispersion characteristics and instability during
dyeing.
CH3(CH2)10CH2
CH3
CH3(CH2)10CH2
N+
CH3
10.51
_
CH3(CH2)14CH2
O
CH3(CH2)14CH2
O
X
_
O Na +
P
O
10.52
X = Br or Cl
10.3.5 Chitin, chitosan and their derivatives
Chitin is the second most important natural polysaccharide produced by biosynthesis,
exceeded only by cellulose, to which it is closely related in structure. It was first isolated by
Braconnot in 1811 and thus its ‘original and spectacular’ properties have been recognised for
a long time [64]. Chitin is found in crabs, lobsters and other crustaceans, spiders and other
arthropodic insects, and the cell walls of fungi. Like cellulose (10.53), chitin (10.54) is a 1,4β-D-glucopyranose. Both have a linear sequence of pyranose rings linked by 1,4-glycosidic
bonds, a non-reducing end group and a reducing end group in the cyclic hemiacetal form.
The characteristic difference between them is that chitin has an acetylamino group in the
2-position, compared with the 2-hydroxy group in cellulose. Chitin exists in three
polymorphic forms, depending on the directions of adjacent polymer chains, the alternating
α-chitin structure being the most common [64].
CH2OH
CH2OH
CH2OH
O
OH
O
O
O
O
OH
OH
HO
OH
OH
OH
n–2
10.53
Cellulose
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536
CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
CH2OH
CH2OH
CH2OH
O
O
O
O
OH
O
OH
OH
OH
HO
NHCOCH3
NHCOCH3
NHCOCH3
n–2
10.54
Chitin
Chitosan (10.55) is a derivative of chitin made by alkaline hydrolysis resulting in
deacetylation to give a primary amino group in the 2-position. Chitin is less hydrophilic than
cellulose, whilst chitosan is more basic than either of the others. Structures 10.53–10.55 are
representative only. Variations occur, depending on the source of the chitin and its
treatment during and after harvesting. Chain length, average molecular mass and molecular
mass distribution vary. There is also the question of impurities and the degree of
deacetylation of chitosan, which is usually 75–95% [65]. Knittel and Schollmeyer [65] have
outlined the nature, properties and uses of chitin and chitosan, including indications of their
uses in textile processes [65]. Roberts has provided an excellent textbook [64]. There are
also the proceedings of two symposia, although these do not deal with textile processing
applications [66,67].
CH2OH
CH2OH
CH2OH
O
OH
O
O
O
O
OH
OH
OH
HO
NH2
NH2
NH2
n–2
10.55
Chitosan
Numerous substituted derivatives of chitin and chitosan are known [67]; some important
examples are shown in Scheme 10.9. The possibility of forming either anionic (5,7,8,11) or
cationic (9,12) derivatives should be noted. The O-carboxymethyl (5) and N-carboxymethyl
(11) polymers are of particular interest as they have stronger complex-forming capabilities
with metal ions than either unsubstituted chitosan or EDTA [65]. In practice, derivatives
formed by substitution via the 2-amino group of chitosan are more common than those
substituted via the 6-hydroxy position of the glucopyranose grouping [65].
Chitosan features far more than chitin in research into applications. This is largely due to
their difference in solubility characteristics, chitosan being more amenable to practical
manipulation. Chitin is in fact rather more intractable than cellulose, since it is insoluble in
those solvents, such as cuprammonium hydroxide, that are commonly used to dissolve
cellulose. Chitin is soluble in hot concentrated solutions of certain inorganic salts capable of
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MACROMOLECULAR COMPLEXING AGENTS
CH2OCOR
CH2ONa
O
O
OCOR
4
3
CH2OCH2COONa
CH2OCH2CH2OH
6
NHCOCH3
2
NHCOCH3
7
O
OH
NH2
11
O
O
O
OPO3H2
NHCOCH3
8
NHCH2COONa
CH2OH
CH2OPO3H2
O
OSO3Na
O
O
O
O
CH2OH
OH
CH2OSO3Na
O
NHCOR
10
O
O
1
O
OH
+NH3
_
OCOR
9
CH2OH
OH
NHCOCH3
OH
NHCOCH3
O
O
5
O
OH
CH2OH
O
O
O
O
ONa
NHCOCH3
OH
CH2OH
CH2OH
O
NHCOCH3
O
OH
R
12
+
N CH3
CH3
_
X
R = alkyl
Typical derivatives of chitin (1) and chitosan (2) [67]: alkali-chitin (3), O-acylchitin (4),
O-carboxymethylchitin (5), O-hydroxyethylchitin (6), chitin O-sulphate (7), chitin O-phosphate (8),
chitosan salt (9), N-acylchitosan (10), N-carboxymethylchitosan (11), trialkylammonium salt (12)
Scheme 10.9
a high degree of hydration, the order of effectiveness being: lithium thiocyanate > calcium
thiocyanate > calcium iodide > calcium bromide > calcium chloride. Chitin also dissolves,
with some degradation, in concentrated hydrochloric acid, sulphuric acid (some
O-sulphation taking place) or phosphoric acid, but not in nitric acid. Certain organic
carboxylic acids, such as formic, dichloroacetic or trichloroacetic, will also dissolve chitin.
Chitosan, on the other hand, interacts with inorganic acids to yield cationic
polyelectrolytes, their solubility depending on the nature of the anion. Thus it is soluble in
dilute hydrochloric, hydrobromic, hydroiodic, nitric or perchloric acid, but may be
precipitated from hydrochloric or hydrobromic solutions as the acid strength is increased.
Chitosan forms water-soluble salts with most carboxylic acids. Hence it is chitosan, rather
than chitin, that has come to the fore in a remarkably wide range of end-uses, including
such diverse fields as medicine, personal care, contact lenses, biotechnology, food,
agriculture, effluent treatment, analysis, textile finishes and coatings. Although usage in
textiles is relatively small as yet, its availability, environmental compatibility and remarkable
versatility offer considerable potential.
Both chitin and chitosan are manufactured commercially on a large scale. Chitosan is
available in powder, gel, solution, film, membrane, fibre and bead forms. Interest in all forms
and levels of purity is high and continuing to expand [67]. Chitosan is produced from amply
replenishable biological sources and is readily biodegradable, non-toxic and non-allergenic.
It has bactericidal and fungicidal properties and actively promotes wound-healing.
The ability of chitosan to form complexes is of particular interest. Being slightly basic, it
will readily form complexes with anionic compounds. Initially it forms into micelles with
small amounts of anionic surfactants, leading to precipitation of a complex as the
concentration of the anionic surfactant increases. Chitosan will complex with anionic
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
polyelectrolytes leading to the formation of polycationic/polyanionic complexes of high
molecular mass. This capability to complex with anionic substances is further enhanced if
cationic derivatives of chitosan are used. The degree and strength of complexing depends
on:
– the nature and ionic strengths of the cationic and anionic species
– the spacing of the charged ionic groups, as influenced by the relative molecular masses
and spatial configurations of the components.
This property is clearly of interest for the removal of anionic substances from effluent
streams, for example. However, since such complexing often results in an increase in
viscosity as complexing proceeds, such systems can be used to produce gels or viscous
liquids. Hence there is the possibility of using these complexes as print–paste or pad–liquor
additives to control migration. Weakly basic chitosan or its more strongly basic derivatives
will complex with anionic fibres and can therefore be used as finishes or pretreatments to
modify selected properties of the fibres. They are already used, for example, in hair sprays or
for complexing with and isolating proteins.
It is not surprising, therefore, that chitosan and its basic derivatives will complex with
anionic dyes. Giles et al. [68,69] researched the use of chitosan for the removal of dyes from
effluent as long ago as 1958. The binding capacity of chitosan for anionic dyes is pHdependent, but it has been reported [65] that in effluent treatment as much as 10 g dye per
kg chitosan can be complexed at pH values above about 6.5. Similarly, chitosan has been
used for the aftertreatment of direct dyeings on cotton to improve their fastness.
The complexing of chitosan and its basic derivatives with anionic substances is paralleled
by compatibility with cationic and nonionic compounds. Similarly, the anionic derivatives of
chitosan show complex formation with cationic agents and are compatible with anionic and
nonionic compounds. The capability of these chitosan derivatives to complex with certain
metal ions, notably those of the transition series, is also important, having possibilities for
the removal of metal salts from effluent. The hierarchy in terms of binding capacity is:
Cr(III) < Cr(II) < Pb(II) < Mn(II) < Cd(II) < Ni(II) < Fe(II) < Co(II).
Chitosan will readily react with formaldehyde via its primary amino groups [65]. The
capability of chitosan and its basic derivatives to complex with anionic fibres has already
been mentioned. In this context, the bactericidal and fungicidal properties of these chitosan
compounds are useful. The fact that fibre-reactive chitosan derivatives can be prepared
further increases these possibilities. Chitosan compounds containing long-chain alkyl groups
exhibit fabric-softening properties and can be incorporated into finishing formulations for
this purpose. The fact that charged chitosan derivatives can interact with appropriate fibre
types gives scope for their use as levelling agents and to modify dye absorption in either a
positive or negative sense, depending on circumstances and dyeing requirements. For
example, they are claimed to reduce dye uptake variations between mature and dead cotton.
The use of chitosan derivatives in print pastes, to reduce the content of the environmentally unfavourable hygroscopic agent urea necessary when applying reactive dyes, has been
evaluated [43]. It was found that in recipes normally requiring 300 g/kg urea this could be
reduced to 75 g/kg by adding either 20 g/kg chitosan or 4 g/kg N-hexylchitosan. Although this
did not give a significant increase in dye yield, the replacement of most of the urea by a
biodegradable chitosan polymer offered significant promise.
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10.3.6 Summary
In conclusion, it is noteworthy that cyclodextrins, liposomes and chitin derivatives are all
readily available from renewable biochemical sources and offer advantages of
biodegradability and safety in use. However, it needs to be borne in mind that this fact alone
does not necessarily mean that they are entirely environmentally innocuous in the long run.
Demands on resources for the husbanding and processing of bioforms that may be necessary
in order to sustain demand for commercially viable qualities and quantities can exert
deleterious effects, not least because they may give by-products that present problems of
utilisation or disposal [70].
10.4 ENZYMES
10.4.1 Structure and properties of enzymes
Enzymes are proteins, i.e. sequences of amino acids linked by peptide bonds. The sequence
of amino acids within the polypeptide chain is characteristic of each enzyme. This leads to a
specific three-dimensional conformation for each enzyme in which the molecular chains are
folded in such a way that certain key amino acids are situated in specific strategic locations.
This folded arrangement, together with the positioning of key amino acids, gives rise to the
remarkable catalytic activity associated with enzymes.
Their molecular masses range from about 10 000 to more than 1 000 000. Each enzyme
can catalyse an indefinite amount of chemical change without itself being consumed or
degraded by the reaction, although most enzymes lose their activity gradually under the
conditions of use due to an inherent instability. Enzymes are produced by all living cells and
are of two types:
– exoenzymes: these are expelled by the manufacturing cell into the surrounding
environment, where they can break down organic compounds such as proteins, starches
and fats into more soluble components of lower molecular mass; and
– endoenzymes: these remain within the living cell and are transformed or broken down by
the action of coenzymes to produce relatively large amounts of energy and the cell
components needed for cell processes.
Clearly, it is the exoenzymes that are of interest in textile processing, an area which has seen
considerable development in recent years. Originally used only in the preparatory processes
of scouring and desizing, they are now also used to modify textile surfaces in finishing as well
as in effluent treatment.
Although enzymes are present in living systems they can exhibit powerful activity under
certain conditions. Their behaviour within the cells of a correctly functioning (i.e. healthy)
living entity is controlled by a sophisticated biochemical system designed to maintain
optimum activity according to the needs of the living cells. In an external application such
as textile processing, such biochemical control is not in place. Hence care is needed in the
use of enzymes. Repeated inhalation of enzyme dust is associated with a comparatively high
risk of respiratory allergy in susceptible persons. Undue exposure can cause irritation of
moist skin, eyes and mucous membranes. The manufacture and use of enzymes is usually
government-controlled.
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More than 3000 different enzymes have been extracted from animals, plants and microorganisms. Traditionally, they have been used in impure form since purification is expensive
and pure enzymes may be difficult to store and use. There is usually an optimum
temperature and pH for maximum activity of an enzyme. Outside these optimum conditions,
activity may simply be held in check or the enzyme may become ‘denatured’, i.e. altered in
such a way that activity is lost permanently, although some forms of denaturing are
reversible. Many enzymes are also sensitive to transition-metal ions, the effect being specific
to particular metal ions and enzymes. In some cases, certain metal ions are essential for the
stability and/or activity of an enzyme. In other cases, metal ions may inhibit the activity of
an enzyme. Similarly, certain organic compounds can act as enzyme inhibitors or activators.
An enzyme consists of a polypeptide chain with a particular spatial configuration specific
to that sequence of amino acids. The molecule twists and turns, forming structural features
that are catalytically active, these being known as active sites. There may be more than one
active site per enzyme molecule. Sometimes an auxiliary catalyst, known as a coenzyme, is
also needed. Apparently, only the relevant active site of the enzyme comes into contact with
the substrate and is directly involved in the catalysed reaction. The active site consists of
only a few amino acid residues. These are not necessarily adjacent to one another in the
peptide chain but may be brought into proximity by the characteristic folding of the enzyme
structure. The active site may also include the coenzyme. The remainder of the enzyme
molecule fulfils the essential function of holding the components of the active site in their
appropriate relative positions and orientation.
Thus the alkaline protease obtained from Bacillus licheniformis with a molecular mass of
about 27 000 consists of 274 amino acid residues and has serine and histidine as active sites.
Pancreatic trypsin with a molecular mass of about 24 000 contains 230 amino acid residues
and also has serine and histidine as active sites. Papain (molecular mass about 23 000 and
211 amino acid residues) has cysteine and histidine as active sites.
The molecular folding of the backbone chain of the enzyme, as well as the distribution
and content of amino acids [71] plays a decisive role in determining the characteristic
specificity of an enzyme with regard to its reactions. This folding is markedly affected by
temperature, for example. As the temperature rises, the chain gradually unfolds until a point
is reached at which the enzyme becomes ‘denatured’ and the catalytic activity is lost.
Most enzymes are highly specific, catalysing only one specific reaction. They may act
upon only one isomer of a particular compound and are then described as stereospecific.
Others are less specific, being able to catalyse several (usually related) reactions. Part of the
active site is involved in binding to the substrate and another part is responsible for making
or breaking chemical bonds. It appears that, for some enzymes, binding of the substrate
produces a change in conformation which brings the key functional group of the enzyme
into the required position for reaction to take place. Thus there must exist between the
enzyme and the substrate a close stereochemical fit or complementarity, analogous to the
situation between a lock and its key. Regarding stereospecific behaviour, certain enzymes
exhibit a remarkable ability to discriminate between asymmetric right-hand and left-hand
molecular configurations.
Enzymes may be named trivially or more formally. Trivial naming tends to predominate in
industry and two trivial systems exist:
(1) a suffix (-in or -ain) is added to a root indicating the source of the enzyme, e.g. papain
from papaya or pancreatin from pancreatic cells
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(2) the suffix -ase is added to a root indicative of the substrate or reaction involved, e.g.
lactase acts on lactose, cellulase hydrolyses cellulose and glucose oxidase oxidises
glucose.
In formal nomenclature, a decimal numbering system is used. Only a brief description can be
given here; more complete accounts can be found elsewhere [72,73]. The system requires
four numbers. The first number gives the class of enzyme according to the following scheme:
(1) Oxidoreductases
(2) Transferases
(3) Hydrolases
(4) Lyases
(5) Isomerases
(6) Ligases.
The second and third numbers give the subclass according to the type of reaction which is
characteristic of the enzyme, e.g.
1. Oxidoreductases
1.1 acting on the CH–OH group of donors
1.1.3 with O2 as acceptor
The fourth number is the serial number of the enzyme in its subclass.
With regard to the specificity of enzymes, there are four main types:
(1) Enzymes that catalyse the reaction of only one substrate are known as absolute
enzymes.
(2) Stereospecific enzymes catalyse reactions with one type of optical isomer but may also
react with a series of related compounds of the same configuration. Many proteolytic
enzymes hydrolyse only peptide bonds linking laevorotatory (L-) amino acids.
(3) Enzymes that react with a specific type of ester linkage are known as general
hydrolysing enzymes. Thus lipases hydrolyse a wide range of organic esters. Generally,
phosphatases will break down phosphate esters into phosphoric acid and an alcohol.
(4) This group is characterised by enzyme attack at a certain specific point in a molecule.
Examples are:
(a) Some proteolytic enzymes act at a location where the adjacent amino acid (e.g.
phenylalanine) contains a benzene ring.
(b) Some hydrolytic enzymes attack the interior bonds of a molecule. Thus α-amylase
attacks the mid-chain region of the starch molecule and of glucosidic fragments
formed from starch.
(c) Other hydrolytic enzymes attack the end groups of saccharidic macromolecules.
Thus β-amylase attacks the end groups in starch molecules, splitting off two
glucose units in the form of a maltose residue. Amyloglucosidase attacks the nonreducing ends of starch or its hydrolysis products to split off single glucose units.
As mentioned above, certain metal ions may be necessary for activity or stability. Thus
calcium is needed for bacterial α-amylase. Magnesium or cobalt is needed with glucose
isomerase. Calcium stabilises the starch-liquifying bacterial α-amylases but inactivates the
glucose isomerase that may be used subsequently. Many enzymes contain an additional non-
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
protein component, referred to as a coenzyme or prosthetic group. This may be an organic
molecule, a vitamin derivative or a metal ion. In most cases the coenzyme participates
directly in the catalytic reaction.
The four main groups of enzyme activity mentioned above are covered by the six classes
of enzymes already listed.
Oxidoreductases
Such enzymes catalyse reactions involving electron transfer. Oxidases use molecular oxygen
as an electron acceptor (Scheme 10.10). Dehydrogenases remove hydrogen atoms from the
substrate and transfer them to an acceptor other than oxygen.
CH2OH
CH2OH
CH O
HO
CH
glucose
CH OH
+ O2
oxidase
CH O
CH
HO
C
O
CH CH
CH CH
OH OH
OH OH
10.56
10.57
D-Gluconolactone
D-Glucose
Scheme 10.10
Transferases
These catalysts bring about the transfer of a particular chemical group from one substance to
another. Examples of groups that can be transferred include alkyl, formyl, carboxyl,
aldehyde, keto, acyl, glucosyl, nitrogen-, phosphorus- or sulphur-containing groups. Thus a
transaminase transfers an amino group and a transmethylase transfers a methyl group.
Scheme 10.11 shows the transfer of an amino group using a transaminase.
R
H
C
R
R′
NH2
+
C
O
COOH
COOH
α-Amino acid
C
transaminase
R′
O
+
H
COOH
C
COOH
α-Keto acid
Scheme 10.11
Hydrolases
Enzymes in this group are capable of hydrolysing a substrate. Examples include:
Substrates
starch
proteins
nucleic acids
fats
esters (e.g. phosphates)
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Types of enzyme
amylases
proteinases and peptidases
nucleases
lipases
esterases (e.g. phosphatases)
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ENZYMES
543
Scheme 10.12 shows the action of cholinesterase in the hydrolysis of acetylcholine (10.58)
to choline (10.59).
CH3
H3C
N
CH3
CH3
H3C
N
CH2
H2O
CH2
cholinesterase
O
C
CH3
CH2
CH2
+ CH3COOH
OH
O
10.59
Choline
CH3
10.58
Acetylcholine
Scheme 10.12
Lyases
These enzymes catalyse the non-hydrolytic cleavage of bonds in a substrate to remove
specific functional groups. Examples include decarboxylases, which remove carboxylic acid
groups as carbon dioxide, dehydrases, which remove water, and aldolases. The
decarboxylation of pyruvic acid (10.60) to form acetaldehyde (10.61) takes place in the
presence of pyruvic decarboxylase (Scheme 10.13), which requires the presence of thiamine
pyrophosphate and magnesium ions for activity.
CH3
C
pyruvic
O
COOH
10.60
Pyruvic acid
decarboxylase
and coenzyme
CH3
C
O
+ CO2
H
10.61
Acetaldehyde
Scheme 10.13
Isomerases
These catalysts facilitate the interconversion of isomeric compounds and include racemases,
optimerases, cis-trans isomerases, intramolecular oxidoreductases and intramolecular
transferases. Scheme 10.14 shows the conversion of an aldehyde to a ketone by triose
phosphate isomerase.
Ligases or synthetases
Such enzymes catalyse the condensation of specific compounds, accompanied by the
breakdown of a pyrophosphate bond in adenosine triphosphate (10.64). Adenosine is the
condensation product of a pentose (D-ribofuranose) and a purine (adenine). Scheme 10.15
shows the action of glutamine synthetase on a mixture of L-glutamic acid (10.65) and
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
CHO
H
CH2OH
triose
OH
C
CH2
O
O
P
CH2
phosphate
isomerase
OH
O
C
O
O
OH
P
OH
OH
10.62
10.63
D-Glyceraldehyde phosphate
Dihydroxyacetone phosphate
Scheme 10.14
NH2
N
O
HO
P
N
O
N
CH2
HO
N
O
HC
HO
CH
O
HC
P
O
CH
O
O
P
HO
OH
OH
10.64
Adenosine triphosphate
COOH
CONH2
CH2
+ NH3 + ATP
CH2
H
C
CH2
glutamine
NH2
H
COOH
10.64
+ ADP + H3PO4
CH2
synthetase
C
NH2
COOH
10.66
L-Glutamine
10.65
L-Glutamic acid
Scheme 10.15
ammonia. Adenosine triphosphate (ATP) is converted into the diphosphate (ADP) and
phosphoric acid is formed.
More comprehensive accounts of enzyme chemistry, behaviour and technology are
available [71,73–75]. In addition, general biochemistry textbooks contain more or less
detailed accounts of these topics.
10.4.2 Enzyme applications in textile processing
Enzymes have traditionally been closely associated with the desizing of cellulosic fabrics. In
recent years, however, the sphere of possible, if not actual, uses has widened considerably.
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545
Further developments can reasonably be expected, particularly as much of this vigorous
research is motivated by environmental concerns. An overview of this activity is given
below.
Mercerisation, scouring and alkali boiling of cellulosic fibres
The traditional mercerising of cotton presents quite hostile conditions for enzymes. Hence it
is not surprising that little use of enzymes has been reported, either in traditional
mercerising or as an alternative means of obtaining similar effects. Cegarra [76] has
concluded that, because of the strongly alkaline nature of mercerising solutions and the
resultant transformation of the cellulose structure, it seems rather unlikely that enzymes will
provide alternatives to alkali in the near future. Even so, it is pertinent to study the effects
that mercerisation may have on any subsequent enzyme treatment. It has been shown [77]
that enzymatic hydrolysis is accelerated on cellulose that has been mercerised without
tension compared with stretch mercerisation.
Possible uses of enzymes in scouring or alkali boiling offer somewhat more scope, although
conditions can still be rather hostile as regards alkalinity and temperature. Nevertheless,
there are some emergent signs. Various enzymes, such as cellulases, pectinases, lipases and
proteases, have been compared [77], leading to the tentative conclusion that cellulases give
the best results for the removal of impurities, together with slightly inferior whiteness, a
similar loss in strength and less contaminated effluents, compared with the traditional
alkaline scour. Use of either a pectinase or a pectinase/cellulase mixture for the removal of
pectin from cotton has also been studied [78]. Effective removal using such enzymes was
found at 40 °C and pH 4.5, giving a higher degree of whiteness than alkaline washing at the
boil for the same degree of cellulose degradation. Cellulase can facilitate the alkaline
scouring of viscose [79], enabling the concentration of alkali (36–60 g/l) traditionally used
to be reduced by 5–10 g/l, and giving, moreover, a more uniform and consistent swelling
process than when alkali is used alone. Another study [80] also demonstrated possibilities
for using enzyme formulations in cotton scouring.
Desizing of cellulosic fabrics
The enzymatic desizing of cellulosic fabrics is a long-established standard process.
Amylolytic enzymes are used to convert any type of starch size into water-soluble products
without affecting the cellulosic fibres. Using enzymes in their natural or modified state,
products are available to allow desizing at 20–70 °C, 70–90 °C or 85–115 °C [81]. Cegarra
[76] has intimated that, given the availability of such products, further studies are likely to
be concentrated on formulations allowing simultaneous desizing and scouring in an alkaline
medium, replacing the present two-stage process.
Nevertheless, research continues to explore improved desizing processes. Advantages
have been claimed for lipases [82] and traditional amylase desizing can be improved with the
help of a thermostable lipase, giving both technical and environmental advantages [83].
Cotton sized with poly (vinyl alcohol) (PVA) is generally desized in water at about 80 °C.
However, a mixture of two different PVA-degrading enzymes gives equivalent desizing at
only 30–55 °C and pH 8.0. Extending the enzymatic treatment time to 4–6 hours (compared
with one hour) resulted in minimum residual PVA [84]. Environmental benefits were also
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
found, since the PVA content in the liquid waste after desizing for four hours was negligible.
Advantages for oxidoreductases over amylolytic enzymes have been observed, since they
break down lignin impurities and are effective over a wider range of temperature and of pH
[85].
Bleaching of cellulosic fibres
The possibility of catalysing the action of hydrogen peroxide by enzymes is an interesting
one, but the need to avoid fibre damage is critical and so such catalysis by a peroxidase is
not currently practical. However, the careful use of glucose oxidase in conjunction with an
enzymatic desizing process is reported [86] to permit the novel and eco-friendly use of
starch-containing effluent liquors for subsequent bleaching. This allows the use of hydrogen
peroxide as the oxidising agent, together with gluconic acid (10.28) which has outstanding
sequestering properties and good biodegradability. Ecological and economic advantages are
claimed, including minimising the effluent pollution load, reducing chemical consumption
and processing under mild conditions.
After bleaching it is important to ensure that the fibre does not contain residual hydrogen
peroxide since this can interfere with subsequent processes, particularly coloration. Certain
enzymes, particularly catalases, used to eliminate peroxide are bio-friendly and time-saving
[87], thus having significant advantages over traditional methods [88]. Such a technique
can be used in either batchwise or continuous washing-off after bleaching to give rapid and
complete decomposition of any residual peroxide [89].
Dyeing of cellulosic fibres
Enzyme processing before, during or after dyeing is an active area of study. Enzyme
pretreatment may have beneficial or adverse effects on subsequent coloration. The action of
enzymes during coloration may improve the coloration process or provide a combined
process, such as desizing/coloration or coloration/biofinishing. With the emergence of
biofinishing techniques, it is important to know how such enzyme treatments are affected by
any prior coloration process. Most of the published work deals with enzyme pretreatments or
aftertreatments.
In one study [90], enzyme pretreatment increased colour yield without affecting fastness
properties. However, pretreatment of cellulosic fibres with cellulase lowered the subsequent
fixation of homobifunctional triazine reactive dyes but did not impair the fixation of other
types of reactive dyes [91]. Another study suggested that the enhanced brightness of
reactive dyeings was greater with triazine dyes than with vinylsulphone types when cotton
was pretreated or aftertreated with cellulase [92].
The enzyme biofinishing of cotton after dyeing was found to be inhibited by direct or
reactive dyes but not by vat dyes [93]. In another investigation of reactive dyes in this
context, biofinishing was variously influenced by the type of dye–fibre bond, the type of
chromogen, the presence of metal ion, the number of reactive groups per molecule and even
by the dye application method [91]. Yet another study [94] showed that cellulase
pretreatment boosts dye exhaustion and cellulase aftertreatment increases the apparent
depth of the dyeing. Interactions between cellulase enzyme pre- or post-treatments and
reactive or direct dyes have been studied by Buschle-Diller et al. [95], with the objectives of
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547
elucidating the mechanism of enzymatic degradation and specifying optimum conditions for
a combined dyeing/biofinishing process.
Denim washing
The practice of stonewashing of dyed cotton denim fabrics to give a ‘distressed’ or washeddown appearance was traditionally carried out with pumice stones. This was labourintensive, time-consuming, caused abrasion of the fabric surface and created debris. Several
authors have described how the stonewashed effect can be produced more advantageously
using cellulase enzymes to partially or wholly replace the stones [76,96–101]. The enzymes
provide a controllable means of surface attack of the fibres, thus bringing about the desired
uneven appearance. Advantages claimed include savings in time and labour, much less
fabric abrasion and no debris. The enzymes used are neutral or acidic cellulase preparations,
which may contain endo-, exo- or beta-gluconases. The finisher can exploit the differing
characteristics of acidic and neutral cellulases by employing washing procedures that take
advantage of each type of formulation [98].
Biofinishing of cellulosic fabrics
Biofinishing, or ‘biopolishing’ as it is more popularly known, is similar to denim washing in
its use of cellulase enzymes, although the effects intended are quite different. The process is
designed to eliminate, by dissolution, the cellulosic fibrils projecting from the surface of the
fabric. This treatment results in [76]:
– a cleaner, smoother surface
– a softer, cooler feel
– improved resistance to pilling
– brighter, sometimes deeper colours.
The precise effects obtained are dependent on the fabric quality, the type of cellulase
enzyme and the application conditions, but no mechanical forces are involved in removal of
the fibrils. The process has attracted considerable attention and is now one of the main
methods of defibrillating lyocell fabrics [94,101–114]. Simultaneous treatment with cellulase
and protease enzymes has been applied to the biofinishing of wool/cotton blends [115].
Acidic cellulases at pH 4.5–5.5 and 45–55 °C or neutral cellulases at pH 6–8 and 50–60 °C
are effective in biofinishing [106,107]. Heavier fabrics and lower enzyme concentrations need
longer treatment times but 30–60 minutes is a typical duration. The treatment is terminated
by inactivating the enzyme, either by raising the pH to 10 or by increasing the temperature to
75 °C for 10–15 minutes. The process is usually monitored by assessing the weight loss of the
fabric; a weight loss of 3–5% usually represents an adequately finished effect without excessive
loss of fabric strength [107]. Dissolution of the cellulose involves depolymerisation as
illustrated in Scheme 10.16 [107].
Wool processing
It is more difficult to control the enzymatic processing of wool. Hence there is a greater
danger of fibre damage compared with cellulosic fibres. Since cellulose is a highly crystalline
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
CH2OH
CH2OH
O
O
OH
O
HO
OH
OH
OH
OH
cellulase
H2O
enzyme
CH2OH
CH2OH
O
O
OH
OH
+
HO
HO
OH
OH
OH
OH
Scheme 10.16
material possessing only limited amorphous regions, it is relatively easy to restrict the action
of enzymes to the surface of the fibre and to the amorphous material, thus leaving the
strength of the fibre unchanged [116] In the case of wool, however, proteases and lipases
catalyse the degradation of different components of the fibre. Proteases, having diffused into
the interior of the fibre, hydrolyse parts of the endocuticle and proteins in the cell
membrane complex. This is difficult to control and can lead to serious damage of the fibre.
SEM micrographs have shown the complete damage of wool fibres and released cortical cells
characteristic of uncontrolled attack by protease enzymes [116].
Three types of enzyme may be selected for the treatment of protein fibres [76,99]:
(1) Proteases, which can be classified as either peptidases or proteinases. These cleave
polypeptide chains eventually into their component amino acids. Peptidases can be
further classified as endopeptidases (which act on the main-chain amido groups along
the polypeptide molecule) or as exopeptidases (which act only at terminal amino acid
residues).
(2) Lipases, which mainly hydrolyse fatty esters, especially triglyceride esters of fatty acids.
(3) Lipoprotein lipases, which act on the lipoproteic bonds of lipoproteins (combinations of
proteins with fatty ester molecules), thus breaching the hydrophobic barriers formed by
these compounds.
The most widely used of these types are the proteases, but the others may be useful in some
circumstances. A characteristic feature is that individual enzymes are highly specific in their
action, so that although one protease may yield the required effect, another may fail to do so.
Bleaching of wool
A serine protease that is stable to hydrogen peroxide and is active in an alkaline medium has
been found and marketed [76,117,118]. In fact this enzyme becomes more active with
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increasing concentration of peroxide. This enzyme increases the whiteness of wool directly
by decolorising the natural yellowish hue of the fibres. Hence, depending on the degree of
whiteness required, this enzyme can be used either alone or in combination with hydrogen
peroxide to effect the bleaching of wool. Serine protease can also be applied with bleaching
agents that operate by a reductive mechanism.
Carbonising of wool
The traditional method of carbonising with sulphuric acid is environmentally undesirable
and can easily lead to fibre damage. Hence it is not surprising that research has been
directed towards alternatives in which enzymes are used to remove the cellulosic impurities
from wool. Cellulases and lignases are mainly used but others have been proposed [116]:
(1) Removal of plant impurities by hydrolases, lyases or oxidoreductases.
(2) Cellulolytic and pectinolytic enzymes used to reduce the amount of sulphuric acid
required.
(3) Incubation of wool with cellulases facilitated subsequent removal of burr with no
chemical or physical damage to the wool.
(4) Application of a mixture of cellulases, pectinases and lignases, again without damage to
the wool.
Dyeing of wool
The effects of enzyme treatments on the subsequent dyeability of wool have been evaluated.
One investigation included both chlorinated and unchlorinated wool [119]. Wool was
treated with a protease at 50 °C and pH 7.5, followed by dyeing with CI Reactive Reds 28
and 116. The enzyme-treated wool showed more rapid dyeing and higher absorption with no
effect on fastness. These effects were greater on the chlorinated wool than on the
unchlorinated control. Alternatively, the enzyme-treated wool could be dyed at a lower
temperature. The effect of pretreatment with a neutral protease on dyeing with acid dyes
has also been examined [90,120], increased colour yields again being observed. It is
essential, of course, to determine whether the economies of increased yield or lower dyeing
temperature exceed the additional cost of enzyme treatment, and whether the durability of
the wool is adversely affected.
Shrink-resist finishing of wool
This is an area of considerable research activity, comparable with the enzymatic
stonewashing and biopolishing of cotton. However, there has been less success in translating
this research into commercial processes. Evidently, the technical use of enzymes for wool
fabrics will not become widespread for another five to ten years [121]. Since certain enzymes
can remove cuticular scales from wool fibres, it is not surprising that they are of interest for
shrink-resist finishing, either alone or in combination with traditional chlorination or resinapplication processes. Interest in this area is acute, because of the environmental
disadvantages of chlorination procedures. These yield absorbable organohalogen (AOX) byproducts, which accumulate in the effluent and ultimately may give rise to toxicity problems
in the food chain if taken up by aquatic organisms [116]. Hence there is considerable
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commercial potential for an enzymatic descaling process that could wholly or even partially
replace chlorination. The critical factor is to achieve the optimal degree of descaling
reproducibility, with minimal effect on fibre strength.
Ideally, the anti-felting effect should be achieved using ‘soft chemistry’ without
application of a synthetic resin and the entire process should be environmentally innocuous,
producing no harmful substances [116,122]. This ideal has yet to be attained. It was
originally thought that the large protease molecule would not be able to penetrate the fibre
cuticle. If so, attack would be limited, as with chlorination, to the cuticular scales with only
minor deterioration in mechanical properties attributable to damage in the interior [123]
This proved to be too simplistic a viewpoint, however, as some proteases even attack the
highly swellable cell membrane complex preferentially, possibly penetrating this region by
channelling beneath the cuticular scales [123–125]. Moreover, microscopic examination has
indicated that enzymatic action on wool is not uniform, some fibres remaining practically
intact whilst others are damaged considerably [126]. Most anti-felting investigations have
been carried out with proteases but other types have also been examined, e.g. a protein
disulphide isomerase which rearranges the disulphide bonds of cystine residues [127] and
transglutaminase which introduces new crosslinks into the keratin structure [128].
Protease activity towards wool can be increased by addition of sodium sulphite or bisulphite,
either with the enzyme treatment or as a pretreatment [122]. Pretreatment with oxidising
agents may also increase the effect of certain enzymes; hydrogen peroxide, dry chlorine,
peracetic or performic acid, wet chlorination, potassium permanganate and peroxymonosulphuric acid (H2SO5) have been used in this way [122]. Sulphite reduction increases
proteolytic activity by cleavage of cystine disulphide bonds in the cuticle to form thiosulphonic
acid groups, a reaction known as sulphitolysis [122,129,130]. When preceded by oxidative
treatment, the action of sulphite yields electron-withdrawing sulphonic acid groups in the
sulphur-rich cuticular layers, selectively activating the nucleophilic degradative reaction
catalysed by the protease and thus preferentially directing the enzyme action to the cuticle
[122,129,131]. Not all proteases are activated by sulphite, however [126].
Although the present situation and the way ahead appear uncertain, it is clear that
enzyme treatment alone does not fulfil the technical requirements for shrink-resist finishing.
Even with enzyme treatment, some degree of chlorination (with the attendant AOX
problems) and/or application of a resin will still be required. Two-stage or even three-stage
processes have been proposed [116]:
(1) (i) Treatment with permanganate; (ii) proteolytic enzyme treatment; This gave
complete descaling.
(2) (i) Treatment with papain (protease), monoethanolamine hydrosulphite and urea; (ii)
treatment with dichloroisocyanuric acid; (iii) a second enzymatic treatment.
(3) (i) A combined protease treatment; (ii) wet chlorination or oxidative treatment (using
sodium hypochlorite and potassium permanganate); (iii) application of a polymer.
(4) (i) Enzyme treatment; (ii) treatment in saturated steam.
(5) (i) Enzyme treatment; (ii) high-frequency radiation.
(6) The Schoeller Superwash 2000 process [132]: (i) so-called ‘black box’ pretreatment;
(ii) enzyme treatment; (iii) application of a low-AOX polyamide resin.
Most enzyme treatments of wool are carried out at about 50 °C for 30–60 minutes. The
amount of enzyme required depends on the specific enzyme type and its commercial
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strength. Optimal pH also depends on the enzyme type. In a study of sixteen commercial
proteases for which the optimal pH varied from 3 to 10.5 [122], it was found that only
papain (optimal pH 6.5–7) and alkaline proteases conferred shrink-resistance on sulphitetreated wool and these tended to cause too much fibre damage. It is thus clearly apparent
that in this area of enzyme activity there is still scope for further development to meet the
desired targets.
Biofinishing of wool
Enzymes can be used to modify the surface of wool fibres in order to improve lustre, softness,
smoothness or ‘warmth’ of the fabric. Since such processes involve attack on the cuticular
scales of the fibre, there is clearly a resemblance to shrink-resist treatments and similar
methods are used [116]:
(1) (i) Treatment with potassium permanganate, ammonium sulphate, acetic acid and
bisulphite; (ii) treatment with a proteolytic enzyme.
(2) Descaling by application of a heat-resistant neutral protease to confer a cashmere-like
feel.
(3) Combined use of dichloroisocyanurate and a proteolytic enzyme.
(4) Complete removal of degraded or damaged portions of the wool (not merely the
cuticle) using: (i) protease treatment; (ii) formic acid rinse and application of a
softener.
(5) (i) Treatment with dichloroisocyanurate; (ii) neutralising and incubating with papain;
(iii) steaming at 100 °C.
Only empirical tests have so far been carried out, however [76]. In a detailed but small-scale
study, various options were examined for the sequence: (i) oxidative treatment; (ii) protease
treatment; (iii) application of softener, including exhaust or pad application [133].
The following products were examined:
Oxidising agents
(a)
(b)
(c)
(d)
dichloroisocyanuric acid,
potassium peroxymonosulphate,
magnesium monoperoxyphthalate hexahydrate,
sodium hypochlorite.
Enzymes
Papain and four protease formulations that varied from neutral to alkaline as regards optimal
pH for activity.
Softeners
(a) a weakly cationic softener,
(b) a cationic silicone micro-emulsion,
(c) a cationic emulsion modified with a silicone elastomer.
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The results varied widely:
– descaling: from none to full
– fibre damage: from none to severe
– strength loss: from -6% (i.e. a slight increase in strength) to +30%
– Whiteness Index (original value -2): from -1.9 to +25.8.
Irrespective of the descaling effect, development of a ‘soft lustre’ depends on application of a
softener. These experiments were positive in demonstrating the possibility of descaling the
fibres and in perceptibly improving lustre under mild conditions. In particular, it was shown
that papain is effective at the remarkably low concentration of 50 mg/l, showing a high
degree of specificity after chlorination.
Degumming and desizing of silk
The use of enzymes in silk degumming or desizing is well-established [76,99,134–136]. In a
study of eight enzymes under optimised conditions [137], weight losses of 24 ± 3% were
observed in most cases but trypsin and pepsin gave extremely poor results. Increasing the
treatment time at the optimal concentration of enzyme gave no further significant weight
loss. There was no significant strength loss in the case of degummed silk and lustre was
improved. In earlier work, degumming with papain was as effective as alkaline degumming
and superior to other methods [138]. Nevertheless, all the parameters must be carefully
controlled to prevent attack on the silk itself [76].
Dyeing of silk
Enzymes have been used to facilitate the dyeing of silk with reactive dyes [139].
Application of enzymes in laundering
As surfactants are often used in textile processing, it is important to note that anionic or
cationic surfactants can inhibit the action of enzymes, as has been reported in the case of
cellulases used for the treatment of cotton [140]. Dyes can also inhibit enzyme activity: for
example, CI Direct Red 28 has been shown to have a much greater inhibitory effect than CI
Acid Orange 7 [141].
Many domestic and laundry washing formulations contain at least one enzyme. Alkaline
proteases, with serine active sites and optimal activity at pH 9–10.5, are mostly used [73].
Much attention is given to the degree of temperature toleration and to compatibility with
other components of the commercial product. In some countries (e.g. the USA) it is
sufficient to have temperature tolerance up to 50–55 °C, whereas elsewhere (e.g. in Europe)
toleration to 100 °C may be required for laundry detergent formulations. Papain, for
example, has broad activity and is thermally stable but is unsuitable, as is trypsin, on account
of incompatibility with perborate, many boosters and all bleaches. A protease derived from
Bacillus licheniformis is much more suitable, this being compatible with surfactants, chelating
agents such as phosphates, EDTA and NTA, as well as with fluorescent brightening agents
and perfumes.
Rather more offbeat investigations have centred on micro-organisms belonging to the group
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Archaea or Archaebacteria, which live in sulphurous waters around undersea volcanic vents.
An extraordinarily stable enzyme which functions even at 135 °C and survives at pH 3.2–12.7
has been identified [142]. This enzyme has been termed STABLE (stalk-associated
archaebacterial endoprotease). It is suggested that such exceptional stability may be
attributable to unusually large Mr and tight folding of the protein chain. Suggested uses
include washing powders and detergents, as well as industrial catalysts. It is even proposed that
such remarkable properties may have contributed to the early evolution of life on earth [142].
10.5 PREPARATION OF SUBSTRATES
By one of those perversities of terminology often encountered in textile wet processing, the
term ‘auxiliaries’ generally includes all chemicals used in preparation and finishing processes,
even though in these cases such chemicals often provide a primary rather than a secondary
(auxiliary) function as in coloration processes. Thus the chemicals used in preparation are
discussed in this section. It cannot be overstressed that the success of any coloration process
relies on the state of the substrate presented for coloration; moreover, thorough preparation
can often do much to reduce the need for auxiliaries in subsequent processes. Methods of
preparation for cellulosic and wool substrates are also discussed elsewhere [11,143]. In this
chapter the emphasis is on the chemistry of the products used rather than on the technology
of processing.
10.5.1 Scouring
The purpose of scouring is to reduce to an acceptable level the amounts of fats, waxes, oils
and dirt present. Apart from the aesthetic benefits of a clean substrate, the major technical
reason for scouring is to improve the extent and uniformity of absorbency for subsequent
processes, especially coloration. Usually the objective is the complete removal of all
extraneous matter but on occasion only partial removal is the aim, since a certain residue of
oils, for example, will aid such processes as spinning, weaving or knitting. Scouring is
particularly important with natural fibres, which obviously contain much more extraneous
matter than do synthetic fibres.
In scouring, surfactants function as primary, rather than auxiliary, agents. The basic
requirements are for good wetting power and detergency, the latter property generally
including the ability to remove, emulsify and suspend the extraneous matter in the liquor.
Not all effective detergents possess good wetting properties; hence a combination of surfaceactive agents to provide both wetting and detergency may be preferable. Detergency can be
significantly improved by the use of additional compounds usually referred to as ‘builders’,
the chief of which is undoubtedly alkali in the form of sodium carbonate or hydroxide.
Alkaline phosphates such as sodium orthophosphate, sodium pyrophosphate or sodium
tripolyphosphate may also be used in those countries where they do not contravene the local
environmental regulations. Alkalis function mainly through saponification of the waxes, fats
and oils on the substrate, thus. rendering them water-soluble and more amenable to removal
and suspension by detergents.
If the processing water or the substrate contains cations such as those of calcium,
magnesium or iron, a sequestering agent should be added to the scouring liquor. Apart from
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their ability to sequester metal ions, many of these agents also possess useful detergentenhancing powers. The aminopolycarboxylates are generally preferred, both for their
sequestering ability and for their stability in warm to hot alkaline liquors. Polyphosphates are
occasionally used at lower temperatures but are less efficient in alkaline media. When
incorporated in the scouring medium certain organic solvents, such as pine oil,
trichloroethylene, perchloroethylene, triethanolamine or glycols, can greatly aid the removal
of greasy matter, particularly mineral oil which may be a component of any lubricating oils
present on the substrate. However, the use of these additives must be in accordance with
local environmental regulations.
Soaps are occasionally still used for scouring, although anionic or nonionic synthetic
detergents are almost always preferred. Among the anionics, fatty alkyl sulphates,
sulphonates and phosphates are commonly used. Ethoxylated fatty alcohols are typical
nonionic scouring agents. Ethoxylated nonylphenol was once the most common nonionic
product used. This came under quite severe environmental scrutiny but, as explained in
section 9.8.1, there is now a good deal of evidence to suggest that nonylphenol derivatives
are not so environmentally damaging as at first thought.
Proprietary scouring agents range from single-component surfactants to complex,
specially formulated mixtures that contain some or all of the above mentioned types of
component matched to give a balanced or compatible product. As mentioned in section
9.8.3, better emulsifying properties are generally obtained with a carefully selected blend of
surfactants rather than with a single product. It has been demonstrated [144] that in the
case of ethoxylated nonylphenol and ethoxylated fatty alcohol surfactants, the broader the
molar mass distribution of the combined surfactants the better is the scouring efficiency, and
that it is inappropriate to aim at homogeneity of the nonionic formulation in scouring. In
selecting a suitable product, thought should be given to the ease with which it can be rinsed
out of the substrate and to any effects that residual quantities may have on subsequent
processing. Fabrics destined for printing, in particular, need the highest degree of uniform
absorbency and cleanliness [145], free from residual surfactants that may cause bleeding or
haloing of the printed design into the surrounding area.
Scouring is of crucial importance in wool processing: important, because the raw fibre
contains 20–60% of extraneous matter in the form of grease, suint, dirt, sand and vegetable
matter; and critical, because the fibre is so easily damaged by hot alkaline treatments. There
have been considerable changes in wool scouring practice, evolving mainly from
corresponding changes in the types of lubricants used on the fibre, although environmental
factors have also played a part. It has been pointed out that the pollution load from a wool
scouring mill can be similar in magnitude to the average discharge from a small town [11].
Hence there are heavy environmental pressures on wool scourers. This impact has provided
the impetus to develop systems which use as little water and energy as possible and reduce
effluent contamination through the recovery of some of the components.
Examples of such comprehensive systems are the WRONZ and Siroscour (CSIRO)
techniques, which use minimal volumes of water whilst producing wool of optimal quality.
The essential feature of the WRONZ treatment [146] is the passage of the greasy liquor
from the first stage through a heavy solids separating tank and a centrifugal separator, from
which the partially degreased liquor is returned to the first stage. The Siroscour system
depends on concentration destabilisation to increase centrifugal recovery of grease and dirt
and to reduce water usage even further [146]. In concentration destabilisation wool grease,
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dirt and suint salts are allowed to build up to very high levels in the scour bowls so that the
resulting unstable emulsion can be cracked easily by heating to 95 °C, allowing the grease
and dirt to be centrifugally separated. Separation occurs in three stages [147]:
(1) superficial dirt (easy to remove)
(2) grease
(3) persistent dirt (difficult to remove).
Advantages claimed for this process [148] include: improved whiteness, decrease in ash
content, improved grease recovery and quality, optimisation of water consumption, efficient
dirt removal from effluent and a reduction in treatment costs. An excellent review of wool
scouring (and of wool processing generally) up to 1984 is available [146].
Soap and alkali were traditionally used and very greasy wools were scoured with alkali
alone, forming a soap in situ by saponification of the wool grease. Such methods are now
rarely used, having been supplanted by nonionic surfactants under neutral or alkaline
conditions. Octa- and nona-ethoxylated nonylphenols are the preferred nonionics,
providing environmental considerations permit their use, since they are unsurpassed for
detergency. Alternatively, ethoxylated straight-chain fatty alcohols are preferred on the
grounds of superior biodegradability. The advantages of nonionic detergents over soaps
include greater efficiency under neutral conditions, stability in hard water, lower cost and
more efficient removal of grease (although this is one area where over-degreasing can be a
disadvantage). Syndets are desorbed more easily in difficult rinsing situations (in yarn
cheeses, for example), although they are not as efficient as soap for the suspension of dirt. In
place of the alkaline sodium carbonate, the tendency is to use neutral sodium sulphate as a
detergent builder.
Apart from the impurities present in raw wools, typical formulations [146] for lubricating
wool fibres are:
(1) water-miscible polyglycol lubricants; these are mainly used on carpet yarns and can be
readily removed using a neutral nonionic surfactant;
(2) mineral wool oil (mineral oil with a nonionic lubricant); normally extracted using a
nonionic surfactant, although in some cases a little alkali may also be useful;
(3) a combination of mineral oil, olein fatty acids and triglycerides; nonionic surfactant
with sodium carbonate can be used, but for more complete removal (as where
subsequent shrink-resist processes are carried out) soap and sodium carbonate must be
used;
(4) natural vegetable and/or animal oils; these are still used on woven worsteds scoured
traditionally with soap and sodium carbonate, although even here there is a gradual
trend towards more neutral systems.
Nowadays, proprietary mixtures of lubricating agents are formulated with ease of removal in
scouring very much in mind. Consequently, scouring processes are generally mild, using a
nonionic surfactant at about 50–60 °C [11].
Aqueous scouring is expensive in terms of water use and effluent treatment and it can
cause entanglement of delicate wool fibres. Solvent scouring offers an effective alternative
but it is essential that the solvent does not enter the environment. Earlier solvent-based
processes included the use of perchloroethylene in which 8–18% water had been emulsified
with a surfactant. Current processes are based on hexane (de Smet process), 1,1,1-
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trichloroethane (Toa/Asohi process) [11,149] or a commercial product called Triwool (ICI)
in the Wooltech process [150–152].
The Wooltech process is claimed to be environmentally friendly and to produce wool of
superior quality. The Triwool solvent is non-flammable, does not deplete ozone from the
upper atmosphere and is not a known carcinogen. The process specifically excludes water,
being designed primarily to avoid fibre entanglement. Current use is for the scouring of wool
tops, where it gives about 2–3% higher yields than aqueous scouring. The recovery of raw
wool grease is claimed to be 99%. The process offers environmental advantages over
conventional aqueous scouring and gives softer wool fibres of enhanced tensile strength and
elasticity, which is beneficial for spinning and weaving.
In the laundering of garments and household textiles, as opposed to the scouring of inprocess material, agents are often added to the liquor to prevent redeposition of soil
extracted from the fabric during washing, typical products being the hydroxyethyl,
hydroxybutylmethyl and carboxymethyl ethers of cellulose. It has been shown, using carbon
black soiling on shrink-resist treated wool, that the lowest redeposition was obtained with
hydroxybutylmethylcellulose and the highest with the carboxymethyl ether [153].
The other major natural fibre, cotton, contains a significant proportion of extraneous
matter such as seeds, fats, waxes, colouring matter and dirt, as well as substances such as
sizes and lubricants applied during processing. Unlike wool, however, it has outstanding
stability in alkali and withstands strongly alkaline treatments ranging from severe caustic
kier boiling to milder treatments with soap and soda [143,154]. It is difficult to detach the
effect of scouring from the complete sequence of desizing, scouring, mercerising and
bleaching, since they all contribute to improved absorbency and cleanliness. Traditional
caustic treatment in kiers is carried out at the boil or in some cases at up to 120 °C, using
1–2% o.w.f. alkali. This treatment bursts the seed motes and saponifies fats and waxes,
converting the fatty esters into sodium salts and glycerol. This in situ formation of soaps
naturally aids cleaning. Nevertheless, synthetic detergents are often added to aid
penetration through wetting and to increase detergency.
The surfactants selected must be highly stable in the strongly alkaline conditions, as well
as in hard water. Anionic surfactants of the fatty alkyl and alkylaryl sulphate types have been
preferred, although the use of phosphate esters is increasing. A synergistic mixture is
beneficial, one component (C10–C13) to aid wetting, the other (C14–C16 ) as a detergent.
The sulphosuccinates, often a first choice for wetting ability, cannot be used here as they are
hydrolysed under such strongly alkaline conditions. A sequestering agent is usually added in
order to remove metal ions that would create problems in subsequent bleaching. Addition of
a mild reducing agent guards against alkaline oxidative tendering of the fibre through
oxycellulose formation and also promotes a degree of bleaching. The reducing agent will also
reduce any iron(III) contamination to iron(II) ions, which are easier to remove by the
sequestering agent. A commercial kier boiling additive may contain some or all of these
components. Suppression of foam may also be a requirement. Semi-continuous and
continuous scouring systems are more common nowadays [154]. The auxiliary needs in
these pad–steam processes are generally the same as those for batch scouring, except that
the selection and balancing of components is much more critical in order to secure optimal
treatment during the short dwell times.
Contrary to the usual practice of scouring cotton and its blends under alkaline conditions,
McCaffrey and Santokhi [155] have cogently argued the case for scouring knitgoods under
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acidic conditions. This starts from the assertion that the degree of impurity removal
resulting from conventional alkaline scouring is probably not really necessary for cotton
knitgoods. In the case of knitgoods, as opposed to woven fabrics, a milder scour is desirable
to retain some of the natural fats and waxes, resulting in a fabric with a softer handle and
improved sewability [143]. It has been demonstrated [155] that acidic scouring (pH 4.5 with
acetic acid) in the presence of a self-emulsifiable knitting lubricant can give results as good
as traditional processes on knitgoods, except that impurity removal is inferior but
acceptable. In addition there are significant advantages, particularly from economical and
environmental aspects. Acidic scouring uses approximately one-third less water than an
alkaline scour. Alkaline processes often require a separate acidic rinse to neutralise the
alkali, whereas residual acid is easily rinsed out. Acidic scouring also offers an opportunity to
reduce chemical consumption and hence costs. These savings minimise effluent treatment
and reduce the effects of effluent on the environment. Acidic scouring gave a reduced
processing time and lower weight losses, both factors that contribute to improved
productivity.
Compared with wool and cotton, the scouring procedures for synthetic fibres are
relatively simple since these fibres contain fewer impurities. Most of these have at least some
degree of water solubility; the most important are sizes and lubricants. The major sizes used
are poly(vinyl alcohol), carboxymethylcellulose and poly(acrylic acid), all of which are
completely or partially water-soluble. Sometimes aliphatic polyesters are used.
Secondary acetate and triacetate fibres generally respond to a light scour with soap or
synthetic detergent, usually at 60–75 °C, this being sufficient to remove soil, oil, sighting
colour and any antistatic agent, although temperatures can range from 30 to 90 °C [156].
Anionic synthetic detergents, such as the poly(oxyethylene) sulphates, are preferred before
dyeing with disperse dyes since low cloud-point nonionic scouring agents, if carried over into
the dyeing process, can interfere with the stability of the dye dispersion at higher
temperatures. Addition of a sequestering agent is helpful in hard water. Care should be
taken if alkali is added, especially on secondary acetate, since these ester fibres can be
hydrolysed to cellulose under hot alkaline conditions. Nevertheless, in the S-finishing
process this alkali sensitivity is exploited to effect a carefully controlled surface
saponification of the fibres to improve drape and antistatic properties [157]. Sodium
hydroxide is applied together with an anionic surfactant to aid wetting and uniformity of
treatment. S-finishing is more usually carried out on triacetate fibres, reducing the total
acetyl content from about 62 to 59%.
When scouring synthetic fibres that are to be dyed with disperse dyes, nonionic scouring
agents are best avoided unless they are formulated to have a high cloud point and are known
not to adversely affect the dispersion properties of the dyes. Conversely, when scouring acrylic
fibres, anionic surfactants should be avoided [156] because they are liable to interfere with the
subsequent application of basic dyes. These fibres are usually scoured with an ethoxylated
alcohol, either alone or with a mild alkali such as sodium carbonate or a phosphate.
Polyamide and polyester fibres are generally scoured using an alkyl poly(oxyethylene)
sulphate and sodium carbonate. Some polyester qualities are subjected to a causticisation
treatment with sodium hydroxide in the presence of a cationic surfactant to give a lighter
fabric with a silkier handle [154,156]. This treatment involves etching (localised
saponification) of the polyester surface and is broadly analogous to the S-finish used on
triacetate fibres. The process has attracted considerable interest in recent years but its
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popularity is likely to decline as the availability of polyester microfibres increases. If the
weight loss during caustic reduction of polyester is restricted to 15–17% there is little effect
on the breaking strength or the viscosity of the fibre [158], since hydrolysis is limited to the
surface of the fibre. If the weight loss is greater than 20%, however, deeper layers of the fibre
are attacked and the fibre strength and viscosity are reduced. The crystalline regions of the
fibre show considerable resistance to hydrolysis. These factors, however, may vary
quantitatively depending on the structure and morphology of the particular polyester.
The cationic surfactant normally used with the alkali acts as a catalytic accelerant [159];
quaternary ammonium compounds are most often used. The kinetics of the process have
been studied [159], showing that careful control of all parameters is essential. The
treatment results in an increase in surface polarity of the fibre. The substantivity for dyes
and the rates of wetting-out and dyeing are increased. The increase in wettability arises from
the formation of free carboxyl and hydroxy groups [160]. Although sodium hydroxide is the
preferred alkali, other alkalis have been investigated [161], the order of activity being: KOH
> NaOH >> Na2CO3. Saponification also increased at lower liquor ratios.
Cationic accelerants vary in their efficacy [161]. Other types of accelerant have also been
evaluated. In one study [162], comparisons were made between tetra-ethylammonium
bromide, benzyltriethylammonium chloride, poly(diallyldimethylammonium chloride) and
the diethyldimethylammonium derivative of a benzenesulphonate polyglycol ester. It was
found that the cationic polymers had a greater effect than the simple quaternary ammonium
compounds of lower molecular mass. This effect was attributed to the capability of the
polymers to enter into hydrophobic interaction with the fibre surface. Ethylenediamine has
also been found to accelerate the alkaline hydrolysis of polyester [163].
Alkaline hydrolysis in a solvent (dimethylformamide, dimethylsulphoxide or dimethylacetamide) containing sodium hydroxide has been investigated [164]. Fabric geometry [165]
and the degree of heat setting of the polyester also influence the results. As the temperature
of heat setting was increased, the accelerating effect of dodecylbenzyldimethylammonium
chloride decreased [166]. Basic-dyeable polyester is particularly sensitive to alkaline
hydrolysis [167]. In some cases, saponification has been used to produce special effects such
as a leather-like finish [168].
10.5.2 Desizing
Desizing is an essential part of the purification process for woven fabrics. Sizes perform an
adhesive and lubricating function. After drying, the size forms a protective film on the
surface of the warp yarns, bonding the protruding fibrils to produce a smoother yarn with
improved tensile strength and abrasion resistance. The objective of sizing is to improve
weaving efficiency by reducing the number of yarn breakages, reducing frictional wear of
loom parts and allowing increased running speeds.
Individual size polymers may be used alone or in combination with one another and their
performance may be further improved by the addition of other components such as waxes
and lubricants. However, whilst sizing offers many benefits in the subsequent weaving of the
yarns, it is anathema as far as wet processing is concerned. A typical sized yarn may contain
as much as 34% of impurities, distributed as shown in Figure 10.15. These impurities can
interfere with wetting-out and with bleaching. They may also affect coloration processes.
Depending on the type of size and the dyes used, dye uptake may be increased or resisted;
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hence uneven distribution of size may lead to unlevel coloration. There may also be a
deleterious effect on fastness properties, since coloured size is likely to be attached to the
fibre only superficially. Consequently, it is invariably essential to remove sizes and lubricants
thoroughly before further wet processing.
Approx 4%
fats
oils
waxes
abraded metal
Up to 20%
size
Up to 10%
hemicellulose
pectins
proteins
seed husks
fruit capsules
colour
salts
Figure 10.15 Cotton warp yarn and its impurities [169]
This removal of size residues inevitably raises environmental questions. Unfortunately,
the various size polymers and their associated additives respond to different methods of
removal. It is therefore highly desirable to know which sizes and other components have
been used in a given case so that appropriate methods of removal can be formulated. This is
not always easy, particularly in commission dyehouses or printworks where sizing has been
carried out elsewhere. Analytical procedures are available but these require appropriate
facilities and expertise. Once desizing has been carried out there arises the question of how
to dispose of the effluent.
The range of size polymers available has expanded to the point of being extremely
complex [169], both in terms of the main types of size and the numerous combinational
possibilities that they represent. The present overview of salient aspects covers the
following: chemistry of size polymers, the use and properties of sizes, desizing methods,
analysis of size polymers and environmental aspects. A summary of the equipment used for
sizing is available [170].
Chemistry of size polymers
Recent reviews dealing with the chemistry of size polymers are available [169,171].
Sometimes a distinction is made [172] between primary sizes, secondary sizes and binders.
This distinction appears to be an arbitrary one depending mainly on the proportions present
in a mixture, although other factors may also be pertinent. There is a great deal of similarity
in essential chemistry between typical size polymers, thickening agents used in printing and
migration inhibitors used in continuous dyeing. An overall summary [169] of the main types
of size polymers available is given in Table 10.5. Similar but less comprehensive lists are
given elsewhere [171,172].
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
Table 10.5 Chemical types of main size polymers [169]
Vegetable products
Native
Starches
Resins
Vegetable gums
Modified
Mosses and algae
Pectin
Starch derivatives
Cellulose derivatives
Potato, maize, wheat, rice,
sago, tapioca
Arabic, Locust bean, Senegal,
Tragacanth
Moss starch, sodium alginate
Water-soluble starches
British gum
Starch ethers
Hydroxyethyl starch
Hydroxypropyl starch
Starch esters
Carboxymethyl starch
Phosphate starch
Starch carbamate
Cellulose esters
Carboxymethyl cellulose
Methyl cellulose
Animal products
Glue
Gelatin
Casein
Synthetic products
Polyvinyl compounds
Partially saponified
poly(vinyl acetate)
Fully saponified
poly(vinyl acetate)
Copolymers with
crotonic acid
Copolymers with
vinyl acetate
Acrylic acid copolymers with methacrylic acid
with acrylic acid esters
with acrylonitrile
Maleic acid copolymers with styrene
with ethyl vinyl ether
with butadiene
Poly(ethylene glycol)
copolymers
with isophthalic acid
It is important to recognise that the molecular characteristics (average molecular mass
and distribution, possible degree of substitution) of such polymers can be varied quite
widely, with attendant changes in properties. Thus polyacrylic acids and their salts may be
described as either sizes or binders. The distinction is related to the proportions used: to be
effective as a binder [171] such a polymer must constitute at least 10% of the dry weight of a
size formulation. A product marketed specifically as a binder may have molecular
characteristics that provide properties different from those of a similar product marketed as a
size component. For example, an acrylic size may be engineered specifically for adhesion and
film-forming properties whereas an acrylic binder may be designed to enhance film elasticity
when added in smaller quantities (about 10%) to a size formulation. It is therefore important
to bear in mind that different polymers based on similar chemistry can be engineered to
provide suitability for specific uses.
The chemistry of starches, galactomannans, modified starches, modified celluloses and
alginates is discussed in section 10.8.1 on natural thickeners. The main starches used are
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potato starch in Europe, maize starch in America, and rice, maize, tapioca or sago starches
in the Far East [169].
Animal glue is a complex colloidal mixture of proteins. The related gelatins are also
complex heterogeneous mixtures of proteins. They are strongly hydrophilic and rich in the
amino acids glycine, proline, lysine, hydroxyproline and hydroxylysine. Casein is a
phosphoprotein obtained from the milk of mammals.
Poly(vinyl alcohol) has the structure 10.67. Poly(vinyl acetate) is the fully esterified
derivative of poly(vinyl alcohol), in which the –OH groups are replaced by –OCOCH3
groups. As indicated in Table l0.5, commercial polyvinyl sizes are effectively copolymers of
poly(vinyl acetate) and poly(vinyl alcohol) that vary in the degree of saponification of the
ester groups. These products may comprise 100% of either polymer, or combinations of the
two monomers in any proportions. Crotonic acid (2-butenoic acid), widely used in the
preparation of resins, may also be a component. This compound exhibits cis–trans isomerism
(Scheme 10.17). The solid trans form is produced readily by catalysed rearrangement of the
liquid cis isomer.
CH2
CH
CH2
CH
OH
CH2
CH
OH
CH2
CH
OH
OH
10.67
H3C
H
C
H3C
C
C
COOH
H
COOH
H
Crotonic acid
mp 72°C
bp 180°C
C
H
Isocrotonic acid
mp 14°C
bp 169°C
Scheme 10.17
Polymers based on acrylic acid have gained considerable importance in recent years.
Their essential chemistry is discussed in section 10.8.2 on synthetic thickeners. Copolymers
of acrylic acid with acrylonitrile and methyl acrylate (10.68) contain a random distribution
of cyano, ester and acidic sidechain groups [169].
CH2
CH
CH2
C
O
CH
CH2
CH
C
CN
OCH3
CH
CH2
O
CH2
C
OH
O
CH
CH2
C
OCH3 O
CH
CN
OH
10.68
The polyester sizes used have a much lower average molecular mass than polyester fibres.
These structures (10.69) contain sulphonic acid groups and may be water-soluble or waterdispersible types. The degree of sulphonation is low [171]. If these resins are subjected to a
high pH, the sulphonate groups can be hydrolysed, giving an insoluble resin that is very
difficult to remove from the fibres.
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
O
C
R1
C
O
X1
O
O
O
R2
X2
n
C
O
R3
C
O
10.69
(1)
(2)
(3)
(4)
(5)
R1
X1
R2
X2
R3
=
=
=
=
=
Aromatic ring
—SO3–, —OCH2CH2CH2SO3– or —CO2CH2CH2SO3– substituent on the aromatic ring R1
CH2CH2 (n = 2–10), CH2CH2CH2 (n = 1–2) or CH2—cycloalkyl—CH2
—CH2OCH2CH2CH2SO3– substituent on aliphatic group R2, in which case X1 = H
Cycloaliphatic or aliphatic hydrocarbon of 2–6 carbon atoms
Rather more specialised sizes are used in certain applications. For example, a reactive
poly(dimethylsiloxane) (section 10.10.2) is recommended for the sizing of some industrial
textile fabrics [173].
The application and properties of sizes
As already mentioned, the essential aim of sizing is to increase productivity in weaving. This
is achieved through a reduction in yarn breakages that permits increased running speeds.
Indeed, the high speeds of modern weaving processes could not have been realised without
corresponding improvements in sizing technology. The most important requirements of a
size formulation can be summarised as follows [174]:
– high adhesion and good film-forming properties on the yarn, together with good elasticity
of the applied film
– low tendency to foam in the application liquor
– freedom from skin formation in the application liquor
– good storage stability
– good compatibility of wash-off liquors containing different size components
– appropriate compatibility with alkalis and bleaching agents if desizing is not carried out
separately from scouring and bleaching.
To these may be added economy and ease of removal from the substrate, as well as a
favourable response to effluent treatment.
The adhesive strength of a size film is an important consideration as it has a bearing on
stability during weaving [175]. Adhesive strength depends on such factors as type of size
polymer, additives present (e.g. wetting agent), sizing liquor temperature, yarn
characteristics, viscosity index of the size formulation and degree of saponification of
poly(vinyl acetate) sizes. Size dissolution rate is also an important factor; this needs to be
known if desizing is to be carried out effectively and efficiently [176].
Many factors will determine the choice of size type and formulation. A general scheme of
size use relative to different fibre types is given in Table 10.6. It is useful to classify sizes
according to those physico-chemical properties that largely determine the method of
desizing, as shown in Table 10.7. Evidently, some degree of synergistic chemistry is involved
in determining the specific suitability of size polymers for certain fibres. For example, there is
an obvious similarity of molecular structure between starch and cellulosic fibres, offering
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substantial scope for hydrogen bonding between hydroxy groups. The similarity in molecular
structure between polyester sizes and synthetic fibres facilitates hydrophobic bonding
between hydrocarbon segments of the polymer chains.
Table 10.6 Sizing agents for different substrates [169]
Size:
Natural
Synthetic
Substrate
Starch
CMC
Gums
Staple fibre yarns
Cellulosic
Polyester/cellulosic
Nylon/cellulosic
Wool
Polyester/wool
Polyester, nylon
+
o
o
o
o
o
+
o
o
o
o
o
+
o
o
Glue
AACo
AECo
PVA
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
Filament yarns
Viscose
Acetate
Triacetate
Nylon
Polyester
+
+
+
PE
PVCo
+
+
+
+
+ Alone
o Only in combination with synthetic size
CMC
AACo
AECo
PVCo
Carboxymethylcellulose
Acrylic acid copolymers
Acrylic ester copolymers
Polyvinyl copolymers
Gums
PVA
PE
Galactomannans
Poly(vinyl acetate)
Polyesters
Table 10.7 Size properties and methods of removal [169] (slightly modified)
Physico-chemical
characteristics
Type of size
Method of removal
Chemically degradable
starches
modified starches
enzymatic or oxidative
Water-soluble
acrylic acid copolymers
poly(vinyl acetate/alcohol)
carboxymethylcellulose
certain modified starches
rehydration and dissolution
Water-resistant
polyesters
certain acrylic acid copolymers
neutralisation and dispersion
Economic factors play a major part in the selection of sizes. For this reason, starch sizes
and their mixtures continue to be the most widely used, particularly on cellulosic substrates.
Nevertheless, more costly size polymers may be economically justifiable if this can be offset
by higher productivity in weaving. High productivity generally demands high elasticity and
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
strong adhesion, provided mostly, if not exclusively, by synthetic sizes. Water-jet weaving
machines require water-resistant sizes. Some sizes are adversely affected by high
temperatures (as in heat setting) or by treatment at inappropriate pH values and these
effects can make their removal more difficult.
The choice and combinations of different size components must take account of many
factors if optimum results are to be obtained. Much has been published regarding the
optimisation of size formulations in relation to desizing processes [177–183]. Cotton warp
yarns sized with starch are normally woven at high humidity (80% and above) to keep yarn
breakages low, as the starch film is brittle at low humidity. It has been shown [183],
however, that improved weavability at moderate relative humidity (e.g. 65%) can be
obtained using: (a) starch/acrylamide or hydroxyethyl starch at not less than 15% add-on; or
(b) poly(vinyl alcohol), which gave excellent results even at a low add-on of 5–6%.
Addition of acrylamide to starch improved the performance of cotton yarn more than
acrylamide alone, but addition of poly(vinyl alcohol) to starch lowered the performance of
the yarn compared with poly(vinyl alcohol) alone. Overall, taking into account economic
considerations, stringent pollution requirements and the needs of desizing, the singlecomponent hydroxyethyl starch showed optimum acceptability for weaving performance at
moderate relative humidity.
The incorporation of waxes or lubricants (section 10.10.1) is an important consideration
that should not be overlooked. These components exert a significant influence on
weavability and on the conditions and efficiency of desizing. The lubricant may be added as
a component of the size formulation, or applied separately by kiss-roll after sizing. Lubricants
may themselves be used as sizing agents, particularly on synthetic warp yarns. Indeed, it has
been argued [184] that such treatments give results comparable with traditional sizes as
regards fibre strength and weaving performance. They offer advantages of savings in heat
energy during drying and in manpower and space, as no special sizing unit is required.
Desizing methods
As indicated earlier in Table 10.7, it is helpful to select desizing methods according to
whether size polymers are chemically degradable, water-soluble or water-resistant.
Desizing by chemical decomposition is applicable to starch-based sizes. Since starch and its
hydrophilic derivatives are soluble in water, it might be assumed that a simple alkaline rinse
with surfactant would be sufficient to effect removal from the fibre. As is also the case with
some other size polymers, however, once the starch solution has dried to a film on the fibre
surface it is much more difficult to effect rehydration and dissolution. Thus controlled
chemical degradation is required to disintegrate and solubilise the size film without damaging
the cellulosic fibre. Enzymatic, oxidative and hydrolytic degradation methods can be used.
The traditional approach is enzymatic desizing, in which an α-amylase or diastase enzyme
is used to attack the 1:4-glycosidic links in the starch, breaking down the macromolecules
into small soluble saccharides such as maltose and glucose. Scheme 10.18 is a simplified
representation based on hydrolysis of the amylose component of starch; similar reactions
take place with the amylopectin component (section 10.8.1). In addition to the enzyme a
surfactant is required to ensure rapid and thorough wetting, including penetration of the size
film, and emulsification or solubilisation of lubricants. Nonionic surfactants are less likely to
deactivate the enzyme than anionic agents. The enzyme liquor is generally applied by
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impregnation immediately after singeing. With modern equipment running at speeds up to
170 m/min, the role of the surfactant is a critical one [169]. A carefully formulated mixture
of surfactants may give the best balance of properties for rapid and thorough wetting,
together with the removal of lubricant waxes or oils. The concentration of enzyme needed
depends on the type and amount of size present and particularly on the potency of the
commercial brand used.
CH2OH
CH2OH
CH2OH
OH
O
O
HO
OH
O
O
O
OH
OH
OH
OH
OH
n
Amylose component of starch
enzymatic
degradation
CH2OH
CH2OH
CH2OH
O
O
OH
O
HO
OH
OH
OH
O
OH
HO
OH
OH
Maltose
OH
Glucose
Scheme 10.18
Temperature and pH are critical parameters that are also brand-related. Enzymes are
available covering the range 20–120 °C [143,169] and from acidic to alkaline pH. Those
requiring mildly alkaline conditions are usually preferred, since the degraded fragments from
the branched amylopectin component of starch usually require such conditions for
solubilisation, rice and tapioca starches having rather higher amylopectin contents [143].
The presence of salt or calcium ions can also influence the behaviour of the enzyme.
Hickman [143] has summarised typical conditions (Table 10.8). The effect of electrolytes
(NaCl, Na2SO4, CaCl2 and MgCl2) on the aqueous solubility of saccharidic size polymers,
including potato starch, starch esters and galactomannans, has been studied in detail [185].
It is important to know whether a fungicide, such as a halogenated phenol, has been added
to protect the size formulation against mildew, since such fungicides are toxic to enzymes.
There are some circumstances in which enzymatic desizing is inefficient:
– if insufficient space is available for the batching of enzyme-impregnated fabric
– if branched-chain starches such as tapioca are present they can be difficult to degrade,
depending on the degree of ageing
– if fungicides have been used to protect the starch from mildew attack
– if oxidative desizing can be adopted and combined with scouring or bleaching, thus
minimising energy requirements.
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
Table 10.8 Optimum conditions for enzymatic desizing [143]
Effect of
Enzyme
Optimum pH
Optimum temperature
range (°C)
NaCl
Malt diastase
Pancreatic amylase
Bacterial amylase
Bacterial amylase
4.5–5.5
6.5–7.5
6.5–7.5
7.0–8.0
55–65
40–55
65–75
100–120
o
+
+
+
Ca ions Time
+
+
+
+
12–24 h
12–24 h
1–4 h
1–2 min
+ Improved desizing
o No effect
The primary alternative to enzymatic treatment is oxidative desizing. This is extremely
popular worldwide, especially in the Far East [169], and its popularity is increasing [143],
mainly for the economic reasons outlined above. The oxidants most often used are hydrogen
peroxide and sodium or potassium persulphate. There may be a considerable risk of fibre
damage by persulphate, however, as this oxidant tends to degrade both starch and cellulose.
The mechanism of degradation includes the hydrolysis of amylose (see Scheme 10.18) and
amylopectin, as well as the formation of aldehyde (10.70), carboxylic acid (10.71), keto
(10.72) and diketo (10.73) groups. There may also be ring cleavage [169] to give diacids
such as tartaric (10.74) and oxalic (10.75).
H
O
HO
C
O
HO
C
O
O
OH
HO
OH
OH
10.70
O
OH
HO
OH
HO
O
C
OH
HO
OH
O
10.71
O
HO
C
OH
10.72
O
C
O
OH
HO
OH
HO
O
O
HO
C
OH
O
10.73
10.74
OH
C
C
O
O
10.75
This oxidative reaction is generally carried out simultaneously with a caustic scour at
100–130 °C, offering the economic advantages of a combined scouring and desizing process.
If peroxide is selected as the oxidant, this also exerts a bleaching action. In addition to
oxidant and sodium hydroxide, the auxiliaries required include a magnesium salt, sodium
silicate or an organic stabiliser for the oxidant, a sequestering agent (e.g. DTPA;
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diethylenetriaminepenta-acetic acid) and a wetting agent. The role of the magnesium salt
and oxidant stabiliser is discussed elsewhere (section 10.5.3). Recommended conditions for
batchwise oxidative desizing at high temperature are given in Table 10.9.
Table 10.9 Oxidative desizing at high temperature [143]
Concentrations (% owf)
Magnesium sulphate heptahydrate
Sodium hydroxide (100%)
Stabiliser
DTPA (40% solution)
Hydrogen peroxide (35%)
or sodium persulphate
Wetting agent
Temperature (°C)
Time (min)
0.005
2–6
0–1
0.2
5–15
0.2–0.5
0.2–0.5
100
<15
0.005
2–4
0–1
0.2
5–15
0.2–0.5
0.2–0.5
100
15–60
0.005
9–12
0–1
0.2
5–15
0.2–0.5
0.2–0.5
120–130
1–2
In this table, the lower amounts of sodium hydroxide provide desizing only, whereas the
higher concentrations provide oxidative scouring. The concentrations of the various
components are critical to ensure efficiency of action without fibre damage. Hickman [143]
gives the following guidelines:
(1) Rapid desizing treatments require more critical control of alkali and oxidant
concentrations.
(2) Increased alkalinity for a given oxidant concentration tends to increase chemical
damage.
(3) Increased oxidant concentration above the minimum required for desizing increases
chemical damage.
(4) Persulphates promote desizing, rather than bleaching, and require more critical control
of concentration than does hydrogen peroxide.
(5) Mixing of persulphates and hydrogen peroxide is not recommended in pad–steam
desizing.
(6) To desize oxidatively by a batchwise process, the oxidant must be added when the
alkaline fabric reaches top temperature.
It is possible to degrade and solubilise starch size residues using cold peroxide and alkali but
batching times are long (typically 16–20 hours). This simultaneous desizing and bleaching
process requires 30 g/l sodium hydroxide (100%) and 50 ml/l hydrogen peroxide (35%),
together with wetting agent, detergent, stabiliser and sequestering agent. The desizing effect
can be improved by addition of persulphate without risk of significant fibre damage at this
temperature [169].
It has been demonstrated [186] that the inclusion of polyacrylamide in either enzymatic or
oxidative desizing formulations results in increased pick-up of the liquor by the sized warp
yarns. Desizing by hydrolytic degradation of starch during the traditional kier-boiling treatment
using 3°Bé sodium hydroxide liquor at 110 °C is now rarely encountered as it is a slow and
expensive process [169].
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
In Table 10.7 are listed the following size polymer types in the water-soluble category:
acrylic acid copolymers, poly(vinyl acetate/ alcohol), carboxymethylcellulose and certain
modified starches. It is necessary to clarify what is meant by water-soluble in this context.
All these polymers form colloidal solutions and the dissolving process is not instantaneous.
When films formed by such polymers come into contact with water, there is an initial period
during which they absorb water. The rate at which water diffuses into the polymer film
varies widely, depending on the polymer structure and the heat treatments that the film has
received. This phase is known as rehydration and is characterised by the formation of a gel.
Only when the polymer has absorbed sufficient water does it actually begin to disintegrate
and dissolve to give a viscous colloidal solution. Higher temperatures and the presence of
surfactants generally increase the rates of rehydration and dissolution. Figure 10.16
illustrates relative dissolution rates in cold water of various water-soluble size polymers
prepared as 60 × 15 mm films of 200 µm thickness [169]. The disintegration level represents
the completion of rehydration. Thus in any desizing process relying solely on aqueous
dissolution, it is always important to allow adequate time for rehydration.
Film thickness 200 m
Dimensions 60 × 15 mm
Dissolution
Disintegration
A
B
C
D
E
4
8
12
16
20
Treatment time/min
A
B
C
D
E
Size CB/CA
Starch ether
Partially saponified poly(vinyl acetate)
Carboxymethylcellulose, salt free
Fully saponified poly(vinyl acetate)
Figure 10.16 Dissolution rates of films of water-soluble sizing agents in water at 20 °C [169]
Not all modified starches are suitable for removal by aqueous dissolution alone. Such
modifications of natural starches are carried out to reduce solution viscosity, to improve
adhesion and ostensibly to enhance aqueous solubility. Commercial brands vary [169],
however, from readily soluble types to those of limited solubility. Indeed, some may be as
difficult to dissolve as potato starch if they have been overdried. It is thus very important to
be sure of the properties of any modified starch present. If there are any doubts about
aqueous dissolution, desizing should be carried out by enzymatic or oxidative treatment.
Even if the size polymer is sufficiently soluble, it is important to ensure that the washing-off
range is adequate. Whilst the above comments relate to modified starches, other size
polymers such as poly(vinyl acetate/alcohol) and acrylic acid copolymers vary from brand to
brand with regard to ease of dissolution.
Carboxymethylcellulose, an excellent film-former, is a highly effective size on cellulosic
substrates but has poor adhesion to synthetic fibres. It is easily desorbed, hot water generally
being sufficient, although surfactant and alkali are usually added to increase the efficiency of
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removal. A further advantage of carboxymethylcellulose is that heat setting does not affect
the ease of subsequent removal, nor is it sensitive to alkaline or acidic pH. Indeed, it has
been stated [171] that if anyone has a problem removing a carboxymethylcellulose size, it is
unlikely that they could remove much of anything from a piece of fabric! Nevertheless,
Angstmann and Bassing [169] caution that this size does require much water and an initial
swelling time.
The poor adhesion of carboxymethylcellulose to synthetic fibres means that where such
fibres are present, it can only be effective in combination with a synthetic size polymer
(Table 10.6). This needs to be taken into account when considering suitable desizing
procedures. If this cellulose derivative is to be used in conjunction with an electrolytesensitive acrylic acid copolymer, it is advisable to choose a salt-free carboxymethylcellulose.
Poly(vinyl acetate/alcohol) sizes are also described as water-soluble and are widely used,
either alone or in combination with most of the other types, across the whole range of fibres
and blends [169,171]. However, this category covers a wide range of commercial products,
differing greatly in quality and ease of removal. Indeed, some are quite difficult to remove,
thus necessitating careful selection [187]. Detailed studies of factors affecting the removal of
water-soluble sizes, particularly poly(vinyl alcohol) types, have been published [188–190].
Apart from molecular mass, the behaviour of polymers of this type depends strongly on
alcohol group content, usually expressed as the percentage degree of saponification of the
poly(vinyl acetate) from which it is derived. Poly(vinyl alcohol) forms a strong film that
shows good adhesion to cellulosic fibres but poor adhesion to polyester. These films can
usually be removed quite readily with hot water and detergent. The presence of a mild alkali
may be acceptable [171] but is perhaps best avoided [169]. Poly(vinyl alcohol) is sensitive to
alkali addition and is precipitated by strong alkali, making the residues very difficult to
remove. The wash-off parameters should be optimised for efficiency of size removal, since
high temperatures and long liquor ratios are environmentally and economically
unfavourable. The amount of size polymer applied and the presence of any additives need to
be carefully considered. Care should be taken in the heat setting of fabrics containing
poly(vinyl alcohol) size. Too high a temperature for too long a time induces crystallisation,
making the size residues much more difficult to remove.
Poly(vinyl alcohol) can be modified with crotonic acid (Scheme 10.17) to give size
copolymers that have higher solubility and lower sensitivity to alkali [169]. Although these
sizes are generally regarded as water-soluble, they are more readily removed by alkaline
oxidative desizing methods using either persulphate or peroxide [169], the polymer being
degraded into smaller segments as indicated in Scheme 10.19.
CH2
CH2
CH
CH
OH
OH
CH2
CH2
Na2S2O8
or H2O2
CH2
C
C
O
O
HO
CH2
C
O
Scheme 10.19
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C
OH
+
O
CH2
C
O
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
Many acrylic acid copolymers are water-soluble but unlike poly(vinyl alcohol) they are
not degraded by alkali. In fact they need alkali for effective desizing as they are more soluble
at alkaline pH than in neutral solutions. They are sensitive to acidic media, which should
not be used. Solubilisation occurs by the formation of sodium carboxylate groups from the
anionic polyacid. The polyelectrolyte formed in this way is readily soluble and shows a rapid
rate of dissolution. However, the presence of electrolytes such as magnesium or calcium salts
from hard water can inhibit removal [191].
The desizing of water-soluble size polymers can be summarised as follows. Batchwise or
continuous methods can be used; in both cases an adequate supply of hot water is needed
during the washing-off. Hot water and detergent are needed to remove poly(vinyl alcohol)
or carboxymethylcellulose. The addition of alkali may be beneficial with
carboxymethylcellulose. Alkali is essential with modified starches and acrylic acid
copolymers. Poly(vinyl alcohol) can be degraded effectively by alkaline oxidation.
Water-resistant sizes include polyesters and certain acrylic acid copolymers. These are
mainly used for the sizing of synthetic filament warp yarns, some formulations being
particularly useful in water-jet weaving. The acrylic acid size copolymers described as watersoluble are generally partially neutralised during manufacture to give partial formation of the
sodium salt; complete conversion to the sodium salt is carried out using alkali in desizing.
Water-dispersible types, on the other hand, are normally partially neutralised with ammonia
during manufacture and these are more suitable for water-jet weaving. When the sized yarn
is dried the ammonia is evolved, leaving the less soluble acrylic acid form. This lower
solubility is a significant advantage in water-jet weaving. A yarn sized with a sodium acrylate
copolymer will have a typical pH of 6–7 when dry, compared with pH 4–5 for an ammonium
acrylate type [171]. This difference in pH needs to be accounted for when desizing, so that
more alkali is needed for the desizing of ammonium acrylate copolymer sizes. Typical
desizing conditions include a detergent with up to 5 g/l sodium hydroxide or sodium
carbonate at 70–90 °C [171]. The pH should be carefully monitored in order to maintain
alkalinity throughout. A sequestering agent may be needed because transition-metal ions
can insolubilise acrylic acid copolymers, one atom of trace metal reacting with two or three
carboxylic acid groups in the size polymer.
Polyester sizes differ from acrylic acid sizes in the nature of the solubilising groups present,
namely sulphonic acid rather than carboxylic acid groups, solubility behaviour being
dependent on the degree of sulphonation with respect to the average molecular mass of the
polymer (10.76). In general, however, the degree of sulphonation is low (10.69) and the
polymer is rather sensitive to alkali. Too strongly alkaline conditions result in hydrolytic
cleavage of the ester groups (Scheme 10.20) to give some fragments without solubilising
groups and these are difficult to remove in desizing [169]. Consequently, although alkali and
detergent are needed for desizing, the amount of alkali is less than that needed for acrylic
acid sizes and the alkalinity should not be allowed to rise above pH 9. Typical conditions are
0.5–1.0 g/l sodium carbonate with detergent at 90–95 °C [169]. A sequestering agent may
also prove useful. Batchwise treatment, or more usually a pad-wash range, can be used.
Polyester resins can be highly beneficial as additives to other size polymers, although a great
deal of care and expertise is required in formulation [192]. Viscosity, for example, is an
important factor in the warp sizing process. The viscosity of some sizes, such as poly(vinyl
alcohol), is significantly affected by temperature fluctuations. The addition of a polyester resin
tends to minimise such changes in viscosity. Surface tension is another important parameter
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PREPARATION OF SUBSTRATES
SO3Na (=X1)
O
C
R1
O
C
O
X2
R2
X2
O
571
O
C
R3
O
C
O
O
C
SO3Na
O
R1
O
R2
C
O
O
(= X1)
10.76
hydrolytic cleavage
SO3Na
HO
C
O
R1
C
O
X2
HO
OH
HO
R2
OH
C
O
R3
C
OH
HO
O
C
O
HO
X2
R1
R2
C
OH
O
OH
SO3Na
In structure 10.76 (see also 10.69) either X1 or X2 contains a sulpho group, but not both.
R3 is an unsulphonated aliphatic group
Scheme 10.20
that is subject to variability and polyester resins can help to stabilise it within the optimum
range. Such resins have beneficial effects on rheological behaviour, film formation and film
characteristics, giving significant improvements in yarn abrasion resistance, good surface
lubrication and a softening of the surface fibres. Certain acrylic acid or vinyl copolymer resins
behave in a similar way to polyester resins in enhancing the performance of conventional size
formulations. The degree of expertise required in formulating such complex mixtures of size
polymers for optimal performance has led Mayfield [192] to emphasise the advantages of using
proprietary products formulated by specialists. These products may contain up to thirty
ingredients, including trace additives that can effectively influence the many components
present. These formulations can be very accurately combined and evaluated for chemical and
physico-chemical compatibility by specialist suppliers.
Analysis and monitoring of size polymers and desizing processes
The many technical factors involved in desizing and the need for economy and
environmental accountability emphasise the importance of monitoring and analysis.
However, only a brief outline can be given here. A review of analytical procedures and
simple laboratory methods for size determination is available [193]. Methods are given for
size determination directly on the fibre surface, for the extraction of components of low
molecular mass and for their subsequent estimation in solution.
Size polymers on polyester can be determined by staining tests with CI Basic Red 22, CI
Reactive Red 12, iodine/potassium iodide solution, or a mixed indicator. The extraction of
size components and their determination in solution using a variety of reagents to give a
characteristic coloration or a coloured precipitate has been described. Methods using
fluorescence spectroscopy with a fluorescent cationic dye (e.g. Pinacryptol Yellow or CI
Basic Orange 14) were also described.
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
An extremely useful technique for measuring the amount of size applied is non-contact
on-line determination of water absorption [194]. The moisture content of sized warps can be
derived from microwave absorption by the water present.
The identification of anionic poly(acrylic acid) sizes can be carried out by staining with a
fluorescent cationic dye (CI Basic Orange 14) followed by spectroscopic measurement of
excitation wavelength and fluorescence emission [195,196]. Such methods can also be used
(with CI Basic Orange 14 or CI Basic Red 1) to detect and estimate carboxymethylcellulose,
poly(vinyl alcohol) and starch derivatives [197].
A method is available, utilising on-line near-infrared reflectance spectroscopy, for
controlling the uniform application of poly(vinyl alcohol) size [198].
The desizing of cotton can be monitored using a dyeing test supported by assessment of
wettability [199,200]. The dyeing test requires a 10 g/l solution of CI Direct Red 83. This
test can usefully be supplemented by the TEGEWA drop test to determine wettability. A
simple method of estimating the efficiency of desizing with poly(vinyl alcohol) and starchbased sizes depends on the determination of TOC (total organic carbon) and COD
(chemical oxygen demand) [201].
Environmental aspects
Since size add-on is customarily in the region of 10–20%, desizing clearly produces quite a
sizeable pollution load. Although many size polymers are biodegradable, they exhibit high
biological oxygen demand (BOD) and chemical oxygen demand (COD). These high values
are compounded by the major contribution of size residues. High liquor ratios or copious
quantities of wash-off water facilitate the desizing process but these large volumes of waste
liquor pose additional difficulties quite apart from the initial cost, both financially and
environmentally. In Table 10.10 the water consumption, BOD and pollution load
contributed by desizing are compared with those from other wet processes on cotton [202].
Typical BOD and COD values determined for the main classes of size polymers are given in
Table 10.11 [203]. The COD of a typical cotton desizing effluent is composed of 42% from
sizing agents and fibre fragments, 40% from impurities in the cotton and 8% from
surfactants [204].
Table 10.10 Water and effluent data arising from the wet processing of
cotton [202]
Water consumption
(% of total)
Process
Desizing
Scouring
Bleaching
Mercerising
Dyeing
Printing
Washing-off
Finishing
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5
1
46
2
8
7
30
1
BOD
(% of total)
22
54
5
5
6
1
7
Pollution load
(% of total)
>50
10–25
3
<4
10–20
10–20
5
15
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PREPARATION OF SUBSTRATES
573
Table 10.11 Specific COD and BOD values found for important sizing agents [203]
Sizing agent
Specific COD
value (mg O2/g)
Specific BOD
value (mg O2/g)
Starch
Carboxymethylcellulose
Poly(vinyl alcohol)
Polyacrylate
Galactomannan
Polyester dispersion
Protein size
900–1000a
800–1000a
ca. 1700a
1350–1650a
1000–1150a
1600–1700a
1200a
500–600
50–90
30–80b
<50
400
<50
700–800
a Taking account of moisture content of commercial product
b With non-adapted inoculum
Clearly, one option to reduce the add-on is to use high-efficiency size formulations.
However, there is a limit to what can be achieved by this approach. Even if the add-on is
reduced to only 5%, the pollution load is still substantial. The two main options to facilitate
disposal are: (a) recovery of size polymers; and (b) biological effluent treatment. Recovery of
size polymers, particularly from water-soluble synthetic sizes, is based on extraction washing
using the minimum quantity of water. Recovery rates in the region of 50% have been quoted
for poly(vinyl alcohol) and carboxymethylcellulose size formulations. It is necessary to apply
one of three concentration techniques: precipitation, condensation or ultrafiltration [205].
Precipitation is only really possible with poly(vinyl alcohol) and is seldom applied to
textile effluents, normally only to eliminate size residues from effluent liquors. The
condensation technique exploits heating to drive off water. This is energy-intensive and is
therefore in decline. Ultrafiltration is long-established commercially and is the preferred
concentrating technique. The principle of ultrafiltration, capable of separating particles in
the range 0.05–0.15 µm, has been compared with other systems of filtration, such as reverse
osmosis, nanofiltration and microfiltration [206]. A sequence of treatments can be used,
such as ultrafiltration to recover most of the size followed by biological treatment of the
residue. In some cases ultrafiltration renders an effluent acceptable for discharge to the
municipal water treatment system, but this depends on local regulations.
One major advantage of ultrafiltration is that it can facilitate the reuse of recovered
materials [207]. In the case of sizing, however, this clearly depends crucially on the
composition of the recovered material. If a blend of sizing agents is present, it does not
necessarily follow that the recovered material will contain these components in the original
(required) proportions for reuse. Nor is reuse a possibility where desizing has been carried out
by a degradative process, such as enzymatic or oxidative desizing. Thus the scope for reuse is
favoured where single-component formulations of non-degraded size polymer can be
recovered. Simple formulations, however, may not meet all the weaving requirements. Hence
arguments for and against the relative merits of simple and complex formulations are common.
The basis of ultrafiltration is that a liquor is passed through a membrane many times until
the required concentration of the permeate is attained. Fouling of the membrane can be a
problem and regular cleaning and disinfection of the membrane is recommended.
Ultrafiltration of poly(vinyl alcohol) and starch sizes offers economic advantages over
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
discharge to the sewer [208,209]. A starch carbamate size effluent had its entry COD of
19 159 mg O2/l reduced by ultrafiltration to 672 mg O2/l, a recovery yield of almost 97%.
Starch ethers, such as hydroxypropyl and carboxymethyl derivatives, have given recovery
values of 92–97% [209]. Size polymers suitable for recovery by ultrafiltration must exhibit
the following characteristics: water solubility, thermal and mechanical stability,
bioresistance, good washing-out properties and low to moderate viscosity [210]. Poly(vinyl
alcohol), carboxymethylcellulose and certain acrylic sizes meet these demands. The ultimate
benefits of ultrafiltration from economic and environmental aspects are that not only can
size be recovered for reuse but also the water that passes through the membrane can be
recycled into the washing-off range [211].
Biological treatment of a desizing effluent usually involves a two-stage anaerobic
treatment using cultivated methane bacteria, possibly followed by a final aerobic treatment
[209]. Typically, a dwell time of 12–17 days will give about 80% degradation of a starch size.
Such treatment usually gives an effluent acceptable for discharge to the municipal sewage
system. Several factors other than the nature of the size polymer can affect the efficiency of
biological treatment, notably the degree of adaptation of the biological inoculum and the
temperature at which digestion takes place. The method of evaluation can also influence the
results obtained. Figures 10.17 and 10.18 illustrate the effects of adaptation of the inoculum
on the bioelimination of a modified poly(vinyl alcohol) size [212], as assessed by the
Sapromat [213] and Zahn–Wellens [214] test protocols respectively. The effect of digestion
temperature on the same polymer using optimally adapted inoculum is shown in Figure
10.19. Thus seasonal temperature fluctuations need to be considered. It is possible to
formulate mixtures of size polymers so that the effect of digestion temperature is minimised.
Data showing the effects of temperature on the rates of biodegradation of hydroxypropyl
starch and of a formulation containing poly(vinyl alcohol) and hydroxypropyl starch [215]
reveal that the polymer mixture is much less sensitive to the temperature of digestion. The
rate of influx and even of rainfall can also exert some influence on the rate of elimination.
100
Biodegradation/%
80
adapted
60
40
20
non–adapted
4
12
20
28
Time/days
Figure 10.17 Biodegradation rate curves for poly(vinyl alcohol) with adapted and non-adapted
inoculum according to the Sapromat test [212]
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PREPARATION OF SUBSTRATES
575
100
optimally adapted
DOC–elimination/%
80
60
40
20
non–adapted
2
4
6
Time/days
Figure 10.18 DOC-elimination rate curves for poly(vinyl alcohol) with optimally adapted and nonadapted inoculum according to the Zahn-Wellens test [212]
100
DOC–elimination/%
80
23 oC
15 oC
10 oC
8 oC
60
40
20
6
12
18
Time/days
Figure 10.19 DOC-elimination rate curves for poly(vinyl alcohol) with optimally adapted inoculum at
various temperatures [212]
The following scale has been proposed to assist discussions of biodegradability [216,217]:
Degree of biodegradation
after 14 days
80–100%
60–80%
0–60%
Description
easily biodegradable
moderately biodegradable
difficultly biodegradable
Many starches and modified starches are easily biodegradable, although some are less so. In
an evaluation of the biodegradability of ten starch-based sizes over seven days [218], four
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
were degraded to 95–100%, two to 80–95%, three to 60–80% and only one to less than 60%.
Whilst carboxymethyl starches are easily biodegradable [218], carboxymethylcelluloses are
generally difficult to degrade [211,217]. The biodegradation of carboxymethylcellulose can
be as low as 8–13% under aerobic conditions, although up to 54% can be eliminated under
anaerobic conditions [211]. Galactomannan sizes are biodegradable to 80% within seven
days [216,217].
It was mentioned earlier that poly(vinyl alcohol) sizes show wide variability in aqueous
solubility. They are also extremely variable with regard to biodegradation characteristics,
from 15% to 95%. It is not surprising, therefore, that they have been the subject of much
research [187,203,211,212,219] and not a little controversy. Reference 203 lists twelve
further references concerned with the microbial degradation of poly(vinyl alcohol).
Misunderstandings have undoubtedly arisen through a failure to appreciate the need for an
adequate digestion temperature and effective acclimatisation of the bacterium. There can be
a drastic fall in efficiency of degradation at temperatures lower than 10–12 °C, since the
desorption rate of the degrading micro-organisms at such temperatures exceeds the growth
rate [203,219] and bioactivity is lower in any case. The fluctuations are influenced by
variations in the composition of commercial size formulations, including the presence of
organic chemicals such as methanol and acetic acid remaining from the polymerisation
process, which may increase or decrease the biodegradability. In general, poly(vinyl alcohol)
sizes can be degraded or eliminated by an optimally adapted inoculum in an appropriate
sewage sludge phase, giving decomposition rates of 57–65% within seven days but with a
potential for up to 100% elimination [211,212]. Trials have been carried out at a low
temperature (8 °C) to demonstrate that even in winter poly(vinyl alcohol) can be eliminated
by more than 90%, admittedly at a slower rate [211,212].
Acrylic size polymers generally show poor biodegradability (typically 3–10%), although
recent research [211] has achieved up to 37% decomposition in special anaerobic cultures.
However, these anionic polyelectrolytes can be readily eliminated from effluents by
adsorption on sludge [220]. Acrylic size polymers may be precipitated in the third stage now
included in many water treatment plants for the removal of phosphates, using a
conventional inorganic precipitant such as iron(III) chloride. Some size formulations may
require the addition of a specific precipitant of this kind, whilst others may be removed by
bioelimination in the sludge. Polyester sizes may be disposed of by similar bioelimination
methods [220]. Deliberate precipitation creates a need for subsequent disposal, but with
acrylic size polymers this solid waste has high calorific value and is suitable for disposal by
incineration. An advantage of acrylic size polymers arising from their ease of removal by
precipitation is that they can be readily recycled, so that they need not enter the sewage
system. Ultrafiltration is the preferred method of recycling [220,221].
10.5.3 Bleaching
The three primary oxidants associated with textile bleaching are hydrogen peroxide (H2O2),
sodium hypochlorite (NaOC1) and sodium chlorite (NaClO2). There are other oxidising
and reducing agents occasionally used (or proposed for use) in bleaching; these will be dealt
with later. For the present, general factors affecting the use of the above three oxidising
agents will be discussed. Many technical factors govern the selection of one bleaching agent
over another:
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PREPARATION OF SUBSTRATES
–
–
–
–
–
–
–
577
generic type of fibre (cellulosic, wool, synthetic)
physical form of fibre (yarn, woven or knitted fabric)
product and process costs
stability, and therefore reliability
versatility of process (batchwise or continuous)
degree of whiteness obtained
extent of any fibre damage.
Environmental factors have now become important in bleaching and are having far-reaching
effects on the choice of bleaching agent. A particularly important parameter is the
absorbable organic halogen value, commonly referred to as AOX. This is generally expressed
as the mass of organohalogen compounds absorbed per unit mass of activated charcoal,
there being three main steps in the analytical procedure [222]:
(1) Absorption of all organic halogen compounds by activated charcoal, taking care to
avoid misleading results by uptake of inorganic salts such as sodium chloride
(2) Combustion of the activated charcoal and collection of the halogens thus released
(3) Quantitative determination of the halogens.
This aspect came to the fore with the well-known and much publicised discovery of the
potent toxin and carcinogen 2,3,7,8-tetrachlorodibenzo-p-dioxin (10.77), commonly but
erroneously referred to simply as ‘dioxin’, in effluent from chlorine-based bleaching processes
for wood pulp in papermaking. Although legislative requirements vary considerably
worldwide the trend has been to ban or severely restrict the discharge of AOX-containing
liquors. Typically, an AOX value of 0.5 mg/l must not be exceeded in waste water released
from a textile wet processing works. This compares with a typical limit for drinking water of
not more than 0.01 mg/l. In some legislative criteria there are limits on discharge from each
processing line in a works, as well as on overall discharge. Typical values for three oxidising
agents in cotton bleaching processes where no attempt has been made to minimise the AOX
value are listed in Table 10.12.
Cl
O
Cl
Cl
O
Cl
10.77
Table 10.12 AOX values for cotton bleached with various
oxidising agents [223]
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Oxidising agent
AOX (mg/l)
Sodium hypochlorite
Sodium chlorite
Hydrogen peroxide
27.00
2.40
0.92
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
The seriousness of the AOX problem is highlighted by the fact that there is currently no
economically acceptable solution for the specific removal of halogenated organic compounds
from effluents [224]. There are important technical reasons why, over the years, hydrogen
peroxide has progressively gained ascendancy over sodium hypochlorite as a bleaching
agent. In view of the AOX values in Table 10.12 it is clear that there will be additional
pressure, this time on ecological grounds, to continue the phasing out of hypochlorite
bleaching in favour of peroxide. This is in spite of the fact that there are some steps (to be
mentioned later) that can be taken to reduce the AOX values of effluent from hypochlorite
bleaching. The intention here is to concentrate discussion on the chemistry of the products
used. Hydrogen peroxide, because of its pre-eminence in bleaching technology, will be dealt
with first, followed by sodium hypochlorite, sodium chlorite and other bleaching agents. The
initial discussion is concerned mainly with cellulosic fibres, with some reference to synthetic
fibres; wool and silk are then dealt with separately. The processing details of bleaching and
the machinery used are dealt with elsewhere [11,143,225,226]. Excellent historical accounts
are also available [227,228].
Peroxide bleaching
The advantages favouring the pre-eminence of hydrogen peroxide (over 90% of cotton
goods are bleached with peroxide) include [143,225]:
(1) Stability and consistency of supply
Most of the hydrogen peroxide solution supplied for textile bleaching is acidic (pH 4.5–
5.0) because it shows maximum stability under these conditions. Additives are present
to increase its stability further at this pH.
(2) Environmentally friendly
As indicated above, peroxide does not contribute to AOX values and potentially
decomposes completely into water and oxygen during effluent treatment.
(3) Versatility of application
Peroxide can be used over a wide range of application conditions in batchwise and
continuous methods, the latter being predominant. The success of continuous peroxide
bleaching is attributable to the relatively rapid rate at which bleaching takes place,
although long-dwell processes are also established. Peroxide baths do not cause
significant corrosion of machinery. Peroxide gives a good white and can be used even
without prior scouring of the material. It is particularly suitable for combining with
other processes, e.g. scouring, desizing, application of fluorescent brighteners, as well as
the bleaching of many blends.
The technical disadvantages of hydrogen peroxide are relatively minor compared with the
process costs:
(1) Fibre damage
This may occur by free-radical formation, especially in the presence of transition-metal
ions such as those of iron or copper. Similar mechanisms can result in the
decomposition of peroxide but there are means of controlling or avoiding this problem.
(2) Quality of whiteness
Higher levels of whiteness and more attractive tones (i.e. neutral or bluish rather than
yellowish or reddish whites) are attainable with hypochlorite, although whites of the
highest quality are produced using the two bleaching agents sequentially.
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(3) Cost-effectiveness
If the costs for treating AOX-containing effluent are omitted, peroxide bleaching is
more costly than hypochlorite bleaching. Data has been presented [222] showing that
the chemical costs of a continuous bleaching process with alkaline hydrogen peroxide
are on average about six times higher than those from sodium hypochlorite. In
batchwise processing, peroxide can be twice as expensive as hypochlorite [222].
95
100
90
80
85
60
80
40
75
20
9.5
10.0
10.5
11.0
Residual H2O2/%
CIE whiteness
As mentioned above, hydrogen peroxide is available commercially as a stabilised liquid of
pH 4.5–5.5. Solid peroxides have been proposed, with claims of bleached fabrics with greater
strength and more stable whites [229], but it is difficult to foresee such solid peroxides
gaining a commercial foothold at the expense of liquid hydrogen peroxide. Hydrogen
peroxide exerts little bleaching action at pH 4.5–5.5 and requires activation to produce a
bleaching effect. The principal means of activation is alkali addition. Given the required
degree of alkalinity, temperature provides a further means of controlling the bleaching
action, such as an overnight dwell at ambient temperature or from 3 to 20 minutes at 100
°C. Figure 10.20 shows the degree of whiteness attained at various pH [226], together with
the corresponding curve for the amount of peroxide remaining in the treatment bath. This
classic activation curve for hydrogen peroxide shows that bleaching is generally best carried
out at pH 10.5–11.0. The amount of alkali required to give this pH will vary with the type of
process, sodium hydroxide being the alkali most commonly added, although sodium
carbonate or phosphate may be used.
11.5
Initial pH
Figure 10.20 Optimisation of pH in hydrogen peroxide bleaching [226]
Under alkaline conditions an additive is required for stabilisation of the peroxide, which
is necessary to avoid undesirably rapid decomposition with loss of bleaching efficiency and/
or damage to the fibre. Traditionally, the most common stabilising agents have been the
colloidal sodium silicates. The formulae of sodium silicates are best represented in terms of
the ratio of sodium oxide to silica, which is 1 in sodium metasilicate (Na2O:SiO2) and 2 in
the orthosilicate (2Na2O:SiO2). These silicates, however, are crystalline forms in which this
ratio is 1 or greater. In the colloidal forms originally preferred for peroxide bleaching the
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
ratio is less than 1. For example, in the so-called ‘alkaline glass’ form the ratio is 1:2, whilst
in the so-called ‘water glass’ form it varies from 1:1.6 to 1:1.38. The colloidal silicates are
efficient and economical stabilisers but care is needed to ensure efficient washing-off in
order to avoid silicaceous deposits on the fabric and equipment. A simple and rapid test
procedure has been developed to determine the relative suitability of stabilisers, with a view
to keeping the running surfaces of a continuous bleaching range free from deposits [230].
Although such colloidal forms have been preferred, the crystalline meta- and orthosilicates
can also be used and may provide easier washing-off.
The required degree of alkalinity is generally obtained by the addition of sodium
hydroxide, sodium carbonate or a phosphate, the amount of alkali varying with the type and
quantity of silicate used. Since the bleaching action yields acid, sufficient alkali is required
for neutralisation as well as absorption by the cellulose. The mechanism by which these
stabilisers act is complex, although the elements of buffering action and sequestering of
transition-metal ions, such as those of iron(III) and copper(II), undoubtedly contribute.
Magnesium ions also play an essential part in the mechanism and must be added (as the
sulphate, for example) if sufficient are not already present in the system. It is important to
recognise that whilst transition-metal ions catalyse the decomposition of peroxide, the
alkaline earth elements stabilise it. In the absence of calcium and magnesium even silicates
can act as bleach activators [19].
The problems associated with silicaceous deposits have led to the adoption of more costly
organic stabilising agents that also aid in plant cleaning and reduce the incidence of
reprocessing. These organic stabilisers are often commercially blended products which may
or may not contain magnesium salts [143], the three main types being aminopolycarboxylate
sequestering agents, protein degradation products and selected surfactants. The preferred
sequestering agents, in terms of both sequestering ability and stability to oxidation, are
DTPA (10.6), either as its sodium or its magnesium salt, and its hydroxy derivatives [18].
Relatively simple methods of evaluating the efficiency of stabilisers have been used, but
more reliable results are obtained with statistical experimental methods involving a realistic
simulation of the bleaching process [231,232].
The mechanism of peroxide bleaching
The details of this mechanism have been the subject of much debate over the years.
Dannacher and Schlenker [233] have reviewed the various hypotheses and applied
experimental criteria to establish the most likely mechanism. The fact that the bleaching
effect varies widely with reaction conditions suggests that the actual bleaching agent is not
hydrogen peroxide itself but another species liberated from the peroxide under the influence
of pH and temperature. This much is generally agreed; there is, however, much debate
regarding the liberated species. Since molecular oxygen has no bleaching effect, nascent or
atomic oxygen has been proposed as the active species on the basis that it is readily liberated
from the perhydroxide anion (HOO–), according to Scheme 10.21. Dannacher and
Schlenker reject this, since calculations show that the formation of oxygen atoms is
energetically highly unfavourable and is not expected to occur under bleaching conditions.
Singlet oxygen has also been proposed and this can indeed be formed, particularly in
mixtures of hydrogen peroxide and sodium hypochlorite. However, carefully designed
experiments showed that under these conditions sodium hypochlorite has neither a direct
chpt10(2).pmd
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PREPARATION OF SUBSTRATES
581
_
_
HO
H2O2
_
HOO
HO
+ [O]
H+
Scheme 10.21
_
CH2CH2COO
C
CH
O
O CH
CH
10.78
bleaching effect (because it is consumed in the reaction with peroxide) nor an indirect
bleaching effect via singlet oxygen. Dannacher and Schlenker also examined the effect of
singlet oxygen derived from the thermal decomposition of an endoperoxide (10.78). They
concluded that singlet oxygen has no bleaching effect when homogeneously dispersed in a
liquor and that alkaline hydrogen peroxide solution contains no singlet oxygen.
The active species most often cited in recent times has been the perhydroxide anion
mentioned previously (Scheme 10.21), even though it shows a decrease in redox potential
with increasing pH. Hydrogen peroxide exists in aqueous solution in a dissociated
equilibrium with the perhydroxide anion (HOO–) and the peroxo dianion (–O-O–), as in
Scheme 10.22. In view of the predominance of this hypothesis, Dannacher and Schlenker
subjected it to a thorough investigation, studying the bleaching effect of hydrogen peroxide
at 60 °C over the range of pH 2 to 13 and obtaining the results shown in Figure 10.21. If the
_
_
HO
H2O2
_
HO
_
O
HOO
_
O
H+
H+
Scheme 10.22
100
Experimental values
H2O2
HOO–
Stain removal/%
80
60
40
20
b
2
4
6
8
a
10
12
14
pH
Figure 10.21 Relative stain removal after 60 min bleaching at 60 °C [233] 6.5 × 10–3 mol/l hydrogen
peroxide in buffer solution. Relative share (%) of the protolytic forms of hydrogen peroxide in the total
amount at 60 °C (–O–O– negligible in this range)
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
perhydroxide anion were the effective agent one would expect the bleaching effect to
increase with concentration and thus with increase in pH beyond the equilibrium point of
pH 11. Figure 10.21, however, shows that at the point where the concentration of
perhydroxide anions begins to exceed that of undissociated hydrogen peroxide, the
bleaching effect decreases considerably, indicating that the perhydroxide anion is clearly not
the effective bleaching agent [233].
As shown in Scheme 10.23, the perhydroxide anion can give rise to both perhydroxyl and
hydroxyl free radicals. These free radicals have been proposed as the active bleaching
agents. Dannacher and Schlenker carried out bleaching tests in the presence of scavengers
for the hydroxyl free radical, 4-nitroso-N,N-dimethylaniline (10.79) and potassium
hexacyanoferrate(II) (10.80) separately at a concentration of 10 –3 mol/l [233]. The
bleaching effect was just as powerful in the presence of the scavengers as in their absence. It
was concluded that the hydroxyl free radical has no bearing on the bleaching effect of
hydrogen peroxide.
_
H
O
O
+
Perhydroxide
anion
H
H2O2
O
O
+
Perhydroxyl
radical
Hydrogen
peroxide
HO
+
Hydroxide
anion
HO
Hydroxyl
radical
Scheme 10.23
K+
CH3
O
N
N
K
CH3
+
NC
CN
NC Fe CN
NC
10.79
CN
K+
K+
10.80
Consideration of pKa values suggests that the perhydroxyl radical is dissociated almost
quantitatively in the optimum pH range for peroxide bleaching, giving rise to the conjugate
base .O–O– known as the superoxide radical ion (Scheme 10.24). When Dannacher and
Schlenker carried out tests using hydroquinone (10.81) as a scavenger for the superoxide
radical ion, it was found that the bleaching effect decreased with increasing concentration of
scavenger (Figure 10.22). These results suggested that the effective species in hydrogen
peroxide bleaching is the superoxide radical. The agreement between experimental and
calculated results for superoxide concentration at different pH values was taken as further
support for this mechanism [233].
_
HO
H
O
O
H
Perhydroxyl
radical
+
_
O
O
Superoxide
radical ion
Scheme 10.24
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PREPARATION OF SUBSTRATES
HO
583
OH
10.81
Hydroquinone
100
With 6.5 × 10–3 mol/l hydrogen peroxide
Without hydrogen peroxide
Stain removal/%
80
60
40
20
0
0.5
1
2
3
4
5
Hydroquinone concentration/mmol l–1
Figure 10.22 Effect of increasing hydroquinone concentration on the relative stain removal after 60
min at pH 10.5 [233]
However, these conclusions have been vigorously challenged by Spiro and Griffith [234].
The bleaching tests of Dannacher and Schlenker were carried out on cotton stained with
tea. Spiro and Griffith pointed out that tea stains are complex mixtures of coloured
polyphenolic compounds with unknown protonation constants. They claimed that kinetic
studies of peroxide bleaching are more easily interpreted using individual coloured
compounds with known properties such as phenolphthalein (10.82), alizarin (10.83) or
crocetin (10.84), a yellow polymethine colorant isolated from saffron. Spectrophotometric
studies by Spiro and Griffith of the bleaching of these compounds with hydrogen peroxide
demonstrated that none of these reaction rates was affected by addition of a scavenger for
hydroxyl and perhydroxyl radicals, which were therefore ruled out as effective species. They
do concur with Dannacher and Schlenker, however, that singlet oxygen can also be ruled
out on the basis of scavenging experiments. Offering a radically different interpretation,
Spiro and Griffith claimed that all the kinetic data can be explained quantitatively on the
assumption that the perhydroxide anion (at higher pH values) and hydrogen peroxide itself
(at lower pH values) are the only active oxidising species. Further studies of the bleaching of
malvidine chloride (10.85) at pH 1.5–4.0 showed that the only significant oxidant was the
hydrogen peroxide molecule, which was thus not as inert as Dannacher and Schlenker
apparently believed. Spiro and Griffith concluded that in the peroxide bleaching of a wide
range of colorants there is no need to invoke any oxidising species other than the
perhydroxide anion and molecular hydrogen peroxide.
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
HO
OH
O
OH
OH
O
C
O
O
10.83
Alizarin
10.82
Phenolphthalein
CH3
HO
C
C
CH3
CH
CH
CH
C
CH3
CH
CH
CH
CH
C
CH3
CH
CH
CH
O
C
OH
C
O
10.84
Crocetin
OH
OH
OCH3
+
_ O
HO
Cl
OH
10.85
OCH3
Thus the detailed mechanism of peroxide bleaching is not yet finally resolved. It should
be borne in mind, however, that the work of Dannacher and Schlenker was carried out at
60 °C on tea-stained cotton, whereas Spiro and Griffith studied the decolorisation of
individual colorants at 21–25 °C in the absence of a textile substrate.
Fibre tendering and process control
In an optimally controlled process free from transition-metal ions hydrogen peroxide
bleaching is remarkably safe, there being no reported detrimental effects of bleaching at
around 100 °C or for more than several hours [143]. Under such conditions, most of the
peroxide appears to be consumed in the oxidation of chain end units of the cellulose
macromolecule. The other major effect on the substrate is oxidation of secondary hydroxy to
keto groups, accompanied by the formation of very few aldehyde or carboxyl groups [235].
Owing to the relative inactivity of keto groups in cellulose, the bleached effect is stable
and is not susceptible to yellowing. This action of hydrogen peroxide is not entirely random
amongst all cellulosic hydroxy groups but is directed mainly towards the C3 hydroxy group
on randomly located glucose units in the amorphous regions of cellulose, as many as 8–17
keto groups being formed per macromolecule. The formation of keto groups is thought to
occur as a result of transient generation of the free radical form of the C3 hydroxy group, as
in Scheme 10.25. Free radicals can be formed from the C3 hydroxy groups following reaction
chpt10(2).pmd
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PREPARATION OF SUBSTRATES
585
between hydrogen peroxide and perhydroxide anions (Scheme 10.23) although other factors
may be involved, such as a localised concentration of radicals in the amorphous regions of
the fibre. A supply of free radicals can be generated by heat treatment, without requiring the
presence of transition-metal ions as a catalyst. The number of oxygen atoms consumed per
chain scission exceeds 100, compared with 26 for hypochlorite bleaching. The yield of
oxidised hydroxy groups is less than 10%, whereas it is about 40% for hypochlorite
treatment. Under these circumstances, the decomposition of hydrogen peroxide and its
oxidative modification of cellulose can be adequately regulated by a well-formulated
stabiliser, so that an unacceptable degree of fibre damage is avoided.
R
R
R
C
R
H
+
HO
R
C
OH
OH
R
R
C
OH
+
HO
R
C
+ H2O
+ H2O
O
Scheme 10.25
In the presence of certain transition-metal ions, however, notably Fe(II) or Fe(III), the
decomposition of hydrogen peroxide and its oxidative attack on cellulose are accelerated.
This results in uncontrolled damage, even to the extent in some cases of forming holes in a
fabric. Although sequestering agents and other stabilisers, such as silicates, can decrease
such severe degradation, in practice they cannot be relied upon to give consistent total
protection in the presence of catalysing metals. In such cases, a pretreatment specifically to
remove such metals before bleaching is recommended [143].
Various metals can be present in raw cotton, the amounts varying with source and
treatment (Table 10.13). There has been a trend in recent years towards increased metal
contamination of textiles, giving rise to a great deal of research in this area. Indeed, Reicher
[236] cites 29 other references in the period 1987–91 alone. Rotor spinning has become
more prevalent in recent years and this is known to give an increased incidence of localised
critical concentrations of metal contamination. Despite the overall complexity of metallic
contamination in cotton fibres, iron is the main culprit with regard to catalytic damage in
peroxide bleaching [236,237] and hence is the only one considered here.
Metal-ion catalysis of hydrogen peroxide decomposition can generate perhydroxyl and
hydroxyl free radicals as in Scheme 10.26 [235]. The catalytic effects of Fe2+ and Fe3+ ions
are found to be similar [235]. It is not necessary for the active catalyst to be dissolved [237],
as rust particles can be a prime cause of local damage. The degradative free-radical reaction
competes with the bleaching reaction, as illustrated in Scheme 10.27 [237]. Two adverse
consequences arise from the presence of free radicals:
– fibre damage
– rapid decomposition of hydrogen peroxide, leading to a corresponding loss of bleaching
action.
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
Table 10.13 Metal ion content of six different cotton samples obtained by
atomic absorption studies [236]
Sample:
Metal (mg/kg)
1
Al
Ba
Ca
Cu
Fe
K
Mg
Mn
Na
Ni
P
Pb
S
Sr
Ti
Zn
2
23
1.5
580
3
34
4500
490
3.2
220
nd
240
nd
340
5.4
0.3
4.5
3
21
32
1.5
1.6
600
840
4
2
100
110
4600
4500
510
520
3.5
4.0
230
230
1.2
nd
270
260
nd
nd
360
360
5.6
5.6
0.25
0.7
7.3
5.3
4
5
47
1.2
830
5
790
5300
530
6.7
330
2.1
330
7.6
430
5.9
1.4
35
6
25
2.0
580
3
36
4700
520
3.3
330
nd
270
nd
380
5.7
0.8
4.9
22
1.5
630
2
94
4700
550
3.7
320
1.0
270
nd
380
6.0
0.7
13
nd Not detected in practice (Ni < 1 mg/kg; Pb < 3 mg/kg)
H
O
O
H
O
_
O
H
+
+
_
Fe2+
Fe3+
HO
H
O
+ HO
O
+
+
Fe3+
Fe2+
Scheme 10.26
Alkaline stabiliser
H
O
O
H
_
H O O
Perhydroxide
anion
Bleaching
Fe ion catalyst
Hydrogen
peroxide
Fe ion catalyst
H O
Hydroxyl
radical
Scheme 10.27
Fibre damage
This is an important but very difficult area for research, particularly regarding the
formulation and evaluation of stabiliser systems. Non-reproducibility of results is a serious
problem and in commercial practice a single batch can unexpectedly give rise to damage
despite using the same protective measures that had been successful with apparently similar
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PREPARATION OF SUBSTRATES
587
batches. In research, it is difficult to obtain consistent metal-containing samples direct from
nature, whilst artificially contaminated specimens do not always give results directly
comparable with substrates contaminated naturally.
In both research and practice, critical localised concentrations of metal contamination
can be difficult to detect. Potassium hexacyanoferrate(II) (10.80) gives an intense deep blue
coloration with iron(III), permitting extremely sensitive detection of tiny iron spots even by
visual inspection. It is recommended as a quality control measure on batches of cotton
destined for bleaching [237]. However, in view of the random distribution of metal traces,
even the most sensitive test cannot guarantee freedom from contamination throughout a
batch of goods to be bleached.
Certain researchers have preferred soluble salts such as iron(III) nitrate [236] to
represent deliberate contamination, whilst others have used insoluble forms. However, even
iron(III) oxide in the form of rust is found to vary in catalytic activity depending on physical
form. Although uniform distribution of the contamination, at least below a relatively low
concentration, has been claimed to be less troublesome than localised concentrations, there
is not even agreement on this. A further complication is that different studies have been
carried out in either the absence or the presence of a cellulosic substrate. With these
provisos in mind, the catalytic behaviour of trace metals and the effects of some preventive
agents will be outlined.
The parameter most commonly monitored in this research is the extent of hydrogen
peroxide decomposition. Measurement of tensile strength and/or the degree of
polymerisation can be useful indicators of fibre damage. The effect of iron(III) ion
concentration in accelerating the rate of peroxide decomposition is shown in Figure 10.23,
using a system comprising hydrogen peroxide, sodium silicate and magnesium sulphate at 95
°C and pH 12. The effects of pH and Fe(III) concentration on decomposition are indicated
in Figure 10.24. Only slight changes in these variables can greatly influence the degree of
decomposition.
Hydrogen peroxide decomposition/%
100
80
60
5 mg/l
1 mg/l
0
40
20
5
10
15
20
25
30
Treatment time/min
Figure 10.23 Effect of Fe(III) ion concentration on rate of hydrogen peroxide decomposition in
absence of substrate [237]. Initial concentration 2.9 g/l H2O 2, Sodium silicate 5 g/l, Magnesium
sulphate 0.2 g/l, 95 °C, pH 12
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
Hydrogen peroxide decomposition/%
100
80
20 mg/l
10 mg/l
5 mg/l
1 mg/l
0
60
40
20
9
10
11
12
13
pH
Figure 10.24 Effects of pH and Fe(III) ion concentration on hydrogen peroxide decomposition in
absence of substrate [237]. Initial concentration 2.9 g/l H2O 2, Sodium silicate 5 g/l, Magnesium
sulphate 0.2 g/l, 95 °C, 30 min
The effect on the decomposition rate of replacing sodium silicate and magnesium
sulphate by a phosphonate stabiliser is shown in Figure 10.25. In this case, at a
concentration of 2 g/l of the specific phosphonate used proved more effective in retarding
Fe(III)-catalysed peroxide decomposition than the combination of 5 g/l sodium silicate and
0.2 g/l magnesium sulphate represented by Figure 10.23. However, doubling the
concentration of silicate and magnesium sulphate also brought about a considerable
improvement in stability (compare Figures 10.23 and 10.26).
Hydrogen peroxide decomposition/%
100
10 mg/l
5 mg/l
0
80
60
40
20
5
10
15
20
25
30
Treatment time/min
Figure 10.25 Effect of phosphate stabiliser on Fe(III)-catalysed decomposition of hydrogen peroxide
in absence of substrate [237]. Initial concentration 2.9 g/l H2O2, Phosphonate stabiliser 2 g/l, 95 °C,
pH 12
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PREPARATION OF SUBSTRATES
589
Hydrogen peroxide decomposition/%
100
5 mg/l
1 mg/l
0
80
60
40
20
5
10
15
20
25
30
Treatment time/min
Figure 10.26 Effect of magnesium silicate stabiliser on Fe(III)-catalysed decomposition of hydrogen
peroxide in absence of substrate [237]. Initial concentration 2.9 g/l H 2O 2, Sodium silicate 10 g/l,
Magnesium sulphate 0.4 g/l, 95 °C, pH 12
The results shown in Figures 10.23 to 10.26 were obtained in the absence of a textile
substrate. Those in Figure 10.27 were determined in the presence of cotton and involved
measurement of the degree of polymerisation of the cellulose [236]. These curves show the
effects of increasing the concentration (up to 40 g/l) of an unspecified commercial
sequestering agent on both peroxide decomposition and the degree of polymerisation of the
cellulose. The contaminant present on the cotton was iron(III) nitrate applied artificially. The
initial concentration (3 g/l) of sequestering agent present was that recommended by the
manufacturer. Under these conditions the loss of peroxide was 80%, accompanied by an
unacceptable lowering of the degree of polymerisation to less than 1000. It was necessary to
increase the concentration of complexing agent by several times the recommended amount in
order to obtain acceptable protection. Such an increase may be impossible to justify, however.
2500
A Peroxide decomposition
B Degree of polymerisation
80
A
2000
B
60
1500
40
1000
B
20
500
A
5
10
15
20
25
30
35
Average degree of polymerisation
Hydrogen peroxide decomposition/%
100
40
Sequestering agent/g/l
Figure 10.27 Effect of sequestering agent concentration on the decomposition of hydrogen peroxide
and degree of polymerisation of cotton cellulose [236]
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
It is evident that careful optimisation of all the parameters involved is essential for
efficient bleaching with minimal fibre damage. The need for close control of pH,
minimisation of liquor ratio and coordination of the component concentrations with
treatment time and temperature is critical [226]. Titration of samples of the liquor taken at
regular intervals has been the traditional method of analysis. An obvious disadvantage is the
unavoidable delay after sampling before corrective action can be taken. Hence the trend has
been towards continuous monitoring. A method of continuous on-line monitoring and
control of a continuous bleaching range, using measurement of pH, conductivity and
temperature, has been described [238].
The use of biosensors and chemosensors to monitor peroxide in continuous ranges has
been examined [239]. Biosensors incorporate electrochemical/enzymatic features with
oxygen detectors. Chemosensors depend on anodic or electrolytic oxidation and are more
robust under the conditions prevailing in the textile industry. This technique is limited to
peroxide concentrations from 0.01 to 100 mg/l, so dilution of the test sample from an
operating level of 10–50 g/l is necessary. Chemosensors offer a rapid means of determining
the concentration of peroxide. Being small, they can be fitted quite close to the point of
measurement with a direct link to the bleach liquor dispenser. In continuous flow
measurement, where liquor is transported continuously through the measuring instrument,
continual monitoring of the prevailing concentration is provided. By means of suitable
electronic control equipment connected to the sensor system, automatic logging of
measured values is possible and, if necessary, appropriate adjustment of the liquor can take
place immediately [239].
An alternative electrochemical method is based on the detection of hydrogen peroxide at
an electrode supplied with a reference voltage [240]. Many electrode materials, however,
give proportional measurements only up to 0.02 mol/l but a special electrode material of the
glassy carbon type provides linearity between current signal and peroxide concentration up
to 50 g/l. The measuring cell has five components: the measuring, reference and back
electrodes determine the current signal, whilst a pH and temperature probe provides
compensation for the pH- and temperature-dependence of the current signal. In spite of the
advantages offered by these systems most monitoring still depends on manual titration, with
manual intervention at the dispensing stage where necessary [240].
Typical peroxide bleaching bath formulations
The composition of bleaching formulations varies widely with the machinery available and
the condition of the substrate. Typical formulations and conditions have been given [143]
for batchwise processing (Table 10.14) and for continuous processing (Table 10.15). The
need to conserve energy in recent years has led to growing interest in cold pad–batch
peroxide bleaching, in which padded fabric is batched without uneven drainage or surface
drying for 15–24 hours at ambient temperature. Table 10.16 gives typical pad liquor
formulations [143,225,241]. A good wetting agent is required to ensure rapid and thorough
wetting of grey fabrics at 25–35 °C, together with an efficient detergent to assist in removal
of fats and waxes. A persulphate (up to 5 g/l) may also be added to assist desizing [143]. The
addition of a peroxide activator based on 1,2,4-triazole has been suggested as a means of
accelerating bleaching at about 30 °C [242].
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PREPARATION OF SUBSTRATES
591
Table 10.14 Recommended conditions for batchwise peroxide bleaching [143]
Kier
(% owf)
Additives
Magnesium sulphate heptahydrate
Wetting agent
Sodium silicate (79Tw)
Organic stabiliser
Caustic soda (100%)
Hydrogen peroxide (35% solution)
Liquor ratio
Temperature (°C)
Time (min)
Jig
(% owf)
2–3
Winch or
jet (g/l)
0.1
0.5–2
7
1–2
5–15
5–15
3–5
1–1.5
0.25–0.8
2–5
0.6–1.4
3–5
4:1
95
60–120
Package
machine (g/l)
3:1
95
60–120
15–20:1
95
60–120
0.1
0.5–2
2–7
0.5–2
5–15
5–15
8–10:1
90
60–120
Table 10.15 Recommended conditions for continuous peroxide bleaching [143]
J-box conveyor
(woven goods
in rope form)
Additives (g/l)
Magnesium sulphate
heptahydrate
Wetting agent
Sodium silicate (79Tw)
or organic stabiliser
Caustic soda (100%)
Hydrogen peroxide
(35% solution)
Liquor ratio
Temperature (oC)
Time (min)
0.1
2–5
Roller-bed
steamer
Pressure
steamer
0.1
2–5
Jemco
machine
0.1
2–5
5–10
2–5
10–20
5–15
5–10
2–5
15–30
45–60
1:1
95–98
60–120
1:1
95–98
10–30
0.1
0.5
J-box
(knitgoods)
0.1
5
1.5
4
10
5
30–45
2
45
1:1
120–140
1–2
10:1
95–100
40–60
1:1
95–98
60–90
The above recommendations are for caustic scoured or peroxide desized fabrics. For fabrics not so treated, the
amounts of caustic and peroxide should be increased by 50% [143]
Table 10.16 Recommended conditions for cold pad-batch peroxide
bleaching
References
chpt10(2).pmd
Additives (g/l)
225
Magnesium sulphate heptahydrate
DTPA (10.6) (40%)
Wetting agent
Organic stabiliser
Sodium silicate (79Tw)
Sodium hydroxide (100%)
Hydrogen peroxide (35%)
0.1
0.1
2–5
0–15
10–15
10–20
40–60
591
143,241
6–10
8–12
8–15
40–50
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
In some circumstances, hydrogen peroxide can be combined with an organic activator.
One such process suggests the use of hydrogen peroxide and urea for the bleaching of linen
[243]. Recommended additions are 7 g/l hydrogen peroxide, 8 g/l urea and 1 g/l nonionic
wetting agent for 2.5 hours at pH 6, 95 °C and 50:1 liquor ratio. Under these conditions
urea accelerates the decomposition of peroxide, which is normally slow at pH 6. In the
presence of urea, peroxide decomposition is believed to proceed according to Scheme 10.28
[243]. In this scheme, hydrogen peroxide and urea interact to form an unstable complex
which then decomposes to yield hydroxyl radicals. These attack more of the peroxide
molecules to yield perhydroxyl radicals, hydroxyl radicals and molecular oxygen. It is also
likely that urea interacts with perhydroxide anions from hydrogen peroxide according to
Scheme 10.29. This anionic complex of urea can be a further source of hydroxyl radicals.
The influence of temperature on the whiteness index of linen subjected to this process is
shown in Figure 10.28. The process may be applied to scoured or unscoured linen [243].
The success of peroxy bleach activators such as tetra-acetylethylenediamine (10.86) in
detergent formulations for low-temperature laundering has encouraged trials of this
approach in peroxide bleaching [244,245]. TAED is colourless, odourless, non-toxic, nonsensitising, non-mutagenic and stable on storage. During biological treatment it is degraded
to carbon dioxide, water, ammonia and nitrate; ethylenediamine is not detected on
biodegradation [245]. TAED seems to be an ideal peroxide activator in the neutral to
weakly alkaline range in textile bleaching, being entirely benign environmentally. Such an
H2N
H2N
C
+
O
H
O
O
C
H2N
H2N
C
C
H
O
O
O
O
H2N
OH
H2N
H2N
OH
H
O
H
O
+ 2
OH
H2N
HO
+ H2O2
H2O
+
O
+ H2O2
HO
+ H2O
H
O
O
+ O2
Scheme 10.28
H
O
O
H
H
H2N
_
C
O
+
H
O
+
+
H
_
O
_
O
H2N
C
O
H2N
H2N
O
O
O
H
Scheme 10.29
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H
PREPARATION OF SUBSTRATES
593
60
Scoured fabric
Unscoured fabric
Whiteness index
50
40
30
20
10
40
60
80
100
120
Temperature/oC
Figure 10.28 Effect of temperature on the whiteness index of linen bleached using urea-activated
hydrogen peroxide [243]. Treated with 7 g/l hydrogen peroxide and 8 g/l urea for 150 min at pH 6 and
50:1 liquor ratio
approach permits bleaching at lower temperatures and near-neutral pH values. Advantages
of this process over hot alkaline peroxide bleaching include:
– at neutral or weakly alkaline pH fibre damage is only slight and catalytic fibre damage is
almost unknown
– low temperatures (60 °C or lower) permit energy savings
– handle is improved, since cotton waxes remain on the fibre
– the process can be used for coloured goods, since colour bleeding is minimal at low
temperatures
– the process is suitable for regenerated cellulosic fibres, giving less swelling of cellulose
– the process is less sensitive to hard water.
The disadvantages include:
– TAED powder is difficult to dissolve in water
– disintegration of cotton husks is not as good as in alkaline bleaching.
The formulations listed in Table 10.17 have been suggested [244].
The mechanism of activation is believed to be as follows. In an alkaline medium,
hydrogen peroxide yields the perhydroxide anion (Scheme 10.22), which reacts with TAED
(10.86) to form diacetylethylenediamine (10.87) and the peracetate anion (10.88) as in
Scheme 10.30 [244]. At pH 8–9, the peracetate anion is in equilibrium with free peracetic
acid, as in Scheme 10.31 [244]. The peracetic acid reacts with the peracetate anion to form
nascent oxygen which is the active bleaching agent, as in Scheme 10.32 [244]. Further
possible activators suggested by Kleber [244] include:
PAG
penta-acetylglucose
BOBS
sodium p-benzoyloxybenzenesulphonate
NOBS
sodium n-nonanoyloxybenzenesulphonate
TAGU
tetra-acetylglycoluril
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
DADHT
PAP
diacetyldioxohexahydrotriazine
phthaloylaminoperoxycaproic acid.
Certain activated peroxide systems are specifically designed for bleaching wool and these
will be mentioned later.
The final step after peroxide bleaching is to ensure that the goods do not contain residual
peroxide. Reducing agents have been used traditionally for this purpose. However, the possibility of using environmentally friendly catalase enzymes should not be overlooked [87–89].
Table 10.17 Recommended conditions for TAED-activated peroxide
bleaching [244]
Pad-steam
Hydrogen peroxide (35%)
Sodium carbonate
TAED
Stabiliser
Wetting agent
Defoamer
5.16 ml/l
6.36 g/l
3.42 g/l
3 g/l
2 g/l
1–2 drops
Pad to 100% liquor pick-up and steam for 30 min
Cold pad–batch
Hydrogen peroxide (35%)
Sodium bicarbonate (10% w/v)
TAED
Stabiliser
Wetting agent
Defoamer
8.6 ml/l
84 ml/l
5.7 g/l
10 g/l
4 g/l
1–2 drops
Pad to 100% liquor pick-up and batch for 2 hours at ambient temperature
O
O
CH3C
CCH3
N
CH2CH2
N
CH3C
+
CCH3
O
2H
O
_
O
Perhydroxide
anion
O
10.86
TAED
O
O
2CH3C
_
O
Scheme 10.30
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10.88
Peracetate
anion
594
+
O
CCH3
CH3C
NH
CH2CH2
NH
10.87
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PREPARATION OF SUBSTRATES
O
O
CH3C
_
O
+ H2O
_
CH3C
+ HO
O
O
595
O
H
Scheme 10.31
O
O
CH3C
+
O
O
H
CH3C
O
O
O
_
O
+
CH3C
O
CH3C
H
_
O
+ 2 [O]
Scheme 10.32
Hypochlorite bleaching
As indicated in Table 10.12, bleaching with sodium hypochlorite is the most
environmentally damaging of all bleaching processes with regard to AOX values.
Consequently, despite the economical and technical benefits of this bleaching process, the
use of hypochlorite will continue to decline and may even be banned in some countries.
The advantages of this bleaching agent include:
(1) economical attractiveness
(2) lower risk of catalytic fibre damage, although some chemical damage can occur
depending on temperature and pH
(3) powerful bleaching action.
The disadvantages, in addition to the environmentally sensitive aspects already mentioned,
include:
(1) hypochlorite bleaching does not compete effectively with the rapid peroxide bleaching
process
(2) despite the intense whiteness that can be produced, hypochlorite bleached goods are
prone to subsequent yellowing on storage
(3) the substrate must be scoured before hypochlorite bleaching
(4) many dyes and fluorescent brighteners are destroyed or degraded by hypochlorite
bleaching
(5) stock solutions of sodium hypochlorite are unstable and must always be chemically
analysed before use.
Sodium hypochlorite is commercially available as an alkaline solution, normally containing
the equivalent of 12–14% available chlorine. However, this is so unstable that analytical
testing of its strength is always necessary before use. Calcium hypochlorite (bleaching
powder), stabilised by adding lime, has been used in the past but this product is no longer
used in textile bleaching.
The mechanism of hypochlorite bleaching appears to be considerably less controversial
than peroxide bleaching. The pH-related active species in sodium hypochlorite are shown in
Figure 10.29 and Scheme 10.33. The pH range 9–11 is the most suitable for hypochlorite
bleaching. The active bleaching species is the hypochlorite anion ClO–. In fact bleaching
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can be accelerated by lowering the pH but the liberation of hypochlorous acid HClO
dramatically increases damage to the fibre. This reaches a maximum at pH 6–8 and such
processes have never been exploited commercially. Higher temperatures also increase the
rate of bleaching and especially the extent of fibre damage, so that hypochlorite bleaching is
usually carried out at ambient temperature for several hours. The effect on cellulose is
mainly one of depolymerisation as a result of chain scission. Degradation includes the nonspecific formation of a minor proportion of aldehyde, keto and carboxyl groups, the lastnamed predominating at the usual pH of bleaching (Figure 10.30). The formation of
aldehyde groups, however, results in a tendency for the bleached fibre to yellow on storage.
In the traditional process, the optimum pH of 10–11 is carefully controlled by addition of
alkali, usually sodium carbonate. Typical bleach liquor formulations are given in Table 10.18.
100
Cl2
[ClO]–
HClO
Composition/%
80
60
40
20
HClO
HClO
2
4
6
8
9
10 11 12
pH
Functional groups/mequiv per 100 g per 10 equiv. O2
Figure 10.29 Effect of pH on the composition of sodium hypochlorite solutions [246]
1.0
COOH
0.8
0.6
0.4
CHO
0.2
CO
6
7
8
9
10
pH
Figure 10.30 Functional groups formed by oxidising cotton cellulose with sodium hypochlorite over
the pH range 5–10 [235]
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PREPARATION OF SUBSTRATES
pH > 10
_
H + + ClO
pH 5–6
HClO
HClO
+
H
+
_
+
_
+ ClO
+
Na
NaClO
597
pH < 2
+ H2O
Cl2
Cl
Scheme 10.33
Table 10.18 Recommended conditions for hypochlorite bleaching [143]
Cistern
Available Cl2 (g/l)
2–4
Sodium carbonate (g/l) 2–4
Treatment time (h)
3–4
Jig
Winch
Package
machine
J-box
conveyor
2–4
2–4
1–2
1–2
1–2
1–2
1–2
1–2
1–2
2–5
2–4
1–2
Despite its technical usefulness, hypochlorite bleaching faces severe environmental
pressures because it yields AOX values well in excess of permitted levels. The AOX value
observed increases with the active chlorine content of the bleach liquor (Figure 10.31) and
with the time of treatment (Figure 10.32).
The state of the substrate, including the source of the cotton and its degree of purity, has
a major influence on the AOX value. It has been shown [247] that under similar bleaching
conditions pure cotton cellulose gives an AOX value of 8–9 ppm, scoured or desized cottons
give intermediate values of 13–60 ppm and raw cotton 70 ppm or higher. Although pure
cellulose already gives a measurable AOX value, much higher values arise from impurities in
cotton. The concentration of hypochlorite applied and the treatment time can scarcely be
80
60
40
20
Whiteness
AOX content/ppm
1
2
3
4
5
6
7
8
Active chlorine/g/l
Figure 10.31 Influence of active chlorine concentration on AOX content and whiteness [247]
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
Rate of increase/%
80
60
40
AOX content
Whiteness
Chlorine consumption
20
50
150
100
200
Bleaching time/min
Figure 10.32 Rates of increase of whiteness, AOX content and chlorine consumption with time of
bleaching [247]
varied widely in practice, since they have been adjusted already to minimum values
determined by the degree of whiteness required and the economy of the process [222]. This
implies that attempts to reduce the AOX value must concentrate on removing impurities as
much as possible during the pretreatments (scouring, desizing and washing-off) before
hypochlorite bleaching [222,247,248].
The purification of cotton can be improved considerably by pretreatment with hydrogen
peroxide and this gives even lower AOX values after hypochlorite bleaching [222]. This
double bleaching, however, considerably increases process and chemical costs and demands
additional processing capacity. Würster and Conzelmann [222] have estimated that such
measures can lead to a reduction in pollution levels by about 70% at justifiable cost, offering
the possibility of bleaching cotton with hypochlorite without exceeding permissible AOX
loading of the effluent. Nevertheless, it seems inevitable that hypochlorite bleaching will
continue to decline on environmental grounds.
After bleaching with hypochlorite it is always necessary to remove or inactivate any
residual chlorine present. The importance of this is highlighted by the fact that the
concentration of undissociated hypochlorous acid reaches a maximum at pH 6, giving the
greatest risk of damage to the fibre. Hence it is essential to avoid lowering the pH to neutral
during washing off before an antichlor treatment has been given. The traditional antichlors
have been sodium bisulphite, or less often the sulphite or dithionite, but the current trend is
to use hydrogen peroxide (Scheme 10.34) on environmental grounds [143].
_
_
ClO
_
+ HSO3
Cl
_
ClO
+ H2O2
Cl
_
+ HSO4
_
+
H2O
+ O2
Scheme 10.34
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Chlorite bleaching
Sodium chlorite is commercially available as an alkaline powder (80%) or as an alkaline
liquid (26%) of specific gravity 1.25. As indicated in Table 10.12, chlorite gives considerably
lower AOX values than hypochlorite, although not as low as peroxide. Bleaching with
chlorite differs significantly from peroxide or hypochlorite bleaching, as it is generally carried
out under acidic conditions. The general advantages of chlorite bleaching include:
(1) no need for special cleaning treatments before bleaching, although such preparation
does give rise to lower AOX values
(2) the risk of chemical damage is low
(3) minimal scouring action of the acidic bleach results in lower weight losses
(4) softer handle and good sewability due to low degree of removal of fats and waxes
(5) this oxidising bleach is the least sensitive to accelerated damage by metallic
contamination
(6) washing off is easier than with alternative bleaching agents
(7) useful for synthetic fibres and particularly important for acrylic fibres.
The disadvantages include:
(1) toxic and unpleasant chlorine dioxide vapour can be liberated
(2) acidic chlorite solutions are highly corrosive and thus demand highly specialised and
expensive equipment
(3) no rapid chlorite bleaching process is available
(4) residual fats and waxes can be advantageous (see above), but they can also be
disadvantageous by giving lower absorbency
(5) many dyes and fluorescent brighteners are destroyed or degraded by chlorite bleaching.
As with hypochlorite, the mechanism of chlorite bleaching does not appear to be
controversial. The pH-related active species in sodium chlorite solution are shown in Figure
10.33. Initially, sodium chlorite hydrolyses in solution, as indicated in Scheme 10.35.
Chlorous acid, however, dissociates in water to a limited extent only. Acidic or acidreleasing agents, sometimes referred to as activators, are needed to lower the pH and raise
the concentration of chlorous acid to a level suitable for bleaching. However, as
demonstrated in Figure 10.33, the reactions in solution are complex. Evidently the chlorite
anions formed undergo the various reactions shown in Scheme 10.36, producing chlorine
dioxide vapour (ClO2), chlorate anions (ClO3–), chloride anions (Cl–) and oxygen. Figure
10.34 illustrates the amounts of chlorite anions, chlorine dioxide, chlorate anions (which
have no bleaching action) and oxygen produced at various pH values from a sodium chlorite
solution (1 g/l) after 1 hour at 95 °C in a stream of nitrogen [235]. This demonstrates very
clearly that decomposition according to Scheme 10.36 is very slow at pH 5 and almost
ceases above pH 6. Below pH 5 the decomposition products are mainly chlorine dioxide and
chlorate anions. Oxygen accounts for less than 5% of the products formed by
decomposition.
The rate of chlorite bleaching increases as pH decreases, but only between pH 2 and 9 is
the rate proportional to the concentration of chlorous acid present in solution. At low pH
values, evolution of the noxious and corrosive gas chlorine dioxide increases. In practice it is
necessary to keep the pH above 3 in order to minimise the formation of chlorine dioxide. It
is necessary to monitor the pH during chlorite bleaching because acid is liberated by the
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
100
[ClO2]–
80
Composition/%
HClO2
60
40
ClO2
20
HClO3
HClO2
2
4
6
8
10
pH
Figure 10.33 Effect of pH on the composition of sodium chlorite solutions [249]
12
[ClO2]–
Concentration/mmol l–1
10
8
6
ClO2
4
[ClO3]–
2
O2
2
3
4
6
5
pH
Figure 10.34 Decomposition of sodium chlorite solution as a function of pH [235]
NaClO2
+ H2O
NaOH
+ HClO2
Chlorous
acid
Sodium
chlorite
Scheme 10.35
_
5 ClO2
_
3 ClO2
_
ClO2
_
+ 2 H+
4 ClO2
_
2 ClO3
+ Cl
_
+ 2 HO
_
+ Cl
_
Cl
+ 2 [O]
Scheme 10.36
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PREPARATION OF SUBSTRATES
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reactions, creating a need for careful adjustment or compensation. Bleaching is generally
carried out at about pH 4. In some processes, particularly continuous or short liquor
methods, this pH shift is allowed for by setting the initial pH somewhat higher (pH 4.5–6.5).
Sodium nitrate is sometimes added to inhibit corrosion of stainless steel vessels to some
extent. Sodium dihydrogen phosphate is a useful pH buffer able to act as an activator for
sodium chlorite to improve the bleaching performance. Table 10.19 gives some typical
processing conditions.
Table 10.19 Recommended conditions for chlorite bleaching [143]
Additives (g/l)
Cistern
Sodium chlorite (80%)
Sodium dihydrogen phosphate
Sodium nitrate
pH
Temperature (°C)
Time (h)
Jig
2–4
0.5–2
2–6
0.5–2
1–2
3.8–4.2
80–85
3–4
3.8–4.2
80–85
1–2
Winch
or jet
Package
machine
1–3
1
1–3
2–4
0.5–1
1–2
3.8–4.2
80
1–2
3.8–4.2
80
1–2
J-box
conveyor
Cold
bleach
20
20–25
6–6.5
80–85
1–4
6–6.5
20
16–18
Aftertreatment with detergent (2–5 g/l) and sodium carbonate (2–5 g/l) often enhances
whiteness and may improve fabric absorbency, particularly if the goods have not been
scoured before bleaching. Antichlor treatment is unnecessary for white goods but may be
required before coloration. A convenient antichlor treatment involves combining the
detergent aftertreatment with sodium perborate, percarbonate or thiosulphate [143].
Traditional reductive antichlors such as sodium bisulphite are not recommended, since their
residues can be just as troublesome as chlorite residues.
Under optimum bleaching conditions, chlorite does not degrade cellulose but degradation
can occur from excessive oxidant concentration, prolonged treatment time or non-optimum
pH conditions, the main effect being depolymerisation. Formation of some aldehyde groups
is suspected, since the bleached goods can be susceptible to yellowing on storage.
The popularity of chlorite bleaching has always been restricted by the toxic and highly
corrosive nature of chlorine dioxide, which even attacks stainless steel. Hence equipment
costs for chlorite bleaching are high. Environmental aspects, in particular AOX values and
the toxicity of chlorine dioxide, will increasingly mitigate against the process in future. As
already noted (Table 10.12), chlorite bleaching reaches a significant AOX level, although
this is only about one-tenth of that produced by hypochlorite bleaching of the same sample.
It should also be borne in mind that chlorite bleaching is recommended for some synthetic
fibres as well as cellulosics. Kleber [224] considers that consent levels for AOX can easily be
met in practice and that chlorite bleaching will continue.
It is important to examine the influence of impurities or additives, such as sizes and
lubricants, since these are often prime sources of higher AOX values rather than the
substrate itself. It is equally important to assess the contribution of auxiliaries in the
bleaching bath to the total AOX value. Kleber [224] has reported several studies of systems
requiring compliance with an AOX consent of 3 ppm. Table 10.20 shows that: (a) synthetic
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
Table 10.20 Effect of surfactant addition on AOX values after
chlorite bleaching of synthetic fibres [224]
AOX value (ppm) after chlorite bleaching
Fibre
Without surfactant
With surfactant
Polyester
Polyamide
Acrylic
0.6
0.6
1.2
2.8
2.6
2.8
fibres are negligible contributors to AOX values; and (b) the inappropriate choice of an
auxiliary (in this case a surfactant) can have an adverse effect. Similarly, the fibre
manufacturer’s yarn finish may have a positive or negative influence on the AOX value.
Thus careful attention should be given to all yarn additives and to the auxiliaries used in
bleaching.
The possible benefits of prescouring to remove such contaminants should also be
considered. Alkaline pretreatments, including boiling off of cotton, have a profound effect
on AOX values after chlorite bleaching (Tables 10.21 and 10.22). It can be beneficial, from
the viewpoint of both AOX and whiteness, to follow a chlorite bleach with a peroxide
treatment. Linen yarns after an alkaline scour and chlorite bleach gave a whiteness value of
63.9 with an unacceptably high AOX value of 8.0 ppm. These results were improved to 78.5
and 1.2 ppm respectively after peroxide treatment [224].
Peracetic acid bleaching
In traditional peroxide bleaching, hydrogen peroxide is activated by alkali. Acids, both
inorganic and organic, can also be used to activate peroxide by the formation of a peracid.
Peracids can be effective oxidative bleaching agents and, at least potentially, offer an
alternative to the environmentally sensitive chlorine bleaches. Although known for quite
Table 10.21 Effect of washing on AOX and whiteness values before and
after chlorite bleaching [224]
Whiteness (460 nm)
Residual
fat (%)
Substrate
Cotton: untreated
washed
Viscose: untreated
washed
Nylon: untreated
washed
Acrylic: untreated
washed
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2.2
0.1
0.9
0.7
AOX
(ppm)
Before bleach
After bleach
3.8
2.4
6.2
2.8
2.6
1.2
2.8
1.5
54.6
61.5
68.7
72.0
82.3
81.6
70.9
72.5
81.3
87.7
81.1
84.1
80.6
84.9
75.5
78.5
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PREPARATION OF SUBSTRATES
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Table 10.22 Effect of desize and boil-off of cotton on AOX and whiteness values before
and after chlorite bleaching [224]
Whiteness (460 nm)
Pretreatment
AOX
(ppm)
Before bleach
After bleach
Chlorite
consumption
Bleach only
Desize* and bleach
Desize*, boil-off & bleach
4.4
7.2
2.8
58.0
61.2
62.7
87.9
89.1
89.9
51.4
57.3
55.0
* Starch sized and enzyme desized
some time, they have not achieved much use in practice, mainly because of their high cost
and difficulties in preparing and handling them. Recent environmental concerns, however,
have revived interest in their potential. Comparison with chlorine bleaches is one aspect
often highlighted, but ultimately they will have to compete both technically and
economically with peroxide bleaching. Peracetic acid is undoubtedly the peracid of greatest
interest for textile bleaching, although persulphate and others have also been evaluated.
Peracetic acid is produced in equilibrium with acetic acid by the reaction of acetic
anhydride with hydrogen peroxide, as in Scheme 10.37. Alternatively, peracetic acid can be
produced by acid-catalysed oxidation of acetic acid with hydrogen peroxide, as in Scheme
10.38. Characteristic of a peracid is the perhydroxide anion (H–O–O–), or in some instances
the peroxo dianion (–O–O –). Originally, the bleacher had a choice of using either
commercial peracetic acid (as a 38% solution) or preparing the peracetic acid in situ using
acetic anhydride as in Scheme 10.37. The 38% peracetic acid solution has a pungent odour,
attacks human skin and is highly inflammable; it requires special transport and storage
facilities. Preparation of the peracid in situ is both hazardous and unpleasant, as it requires
the handling and storage of acetic anhydride. One part of peroxide is reacted with six parts
of anhydride for 4 hours at ambient temperature to give 80% yield [244,250,251], during
which the explosive diacetyl peroxide may also be formed as a by-product, as in Scheme
10.39. Thus, although peracetic acid is environmentally friendly, hydrolysing to acetic acid
and oxygen, its manufacture and use do pose quite severe hazards. Hence it is hardly
surprising that it has been used only rarely.
More recently, manufacturers have introduced safer versions containing only 12–15%
active material [225]. Typical formulations of these so-called equilibrium mixtures are given
in Table 10.23, together with their flash points [251–253]. Although these lower-strength
O
CH3C
O
O
O
+ H2O2
CH3C
+
O
CH3C
O
CH3C
H
O
O
Acetic
anhydride
Peracetic
acid
Acetic
acid
Scheme 10.37
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
O
H
CH3C
+
O
CH3C
+ H2O2
O
+ H2O
O
H
O
H
Scheme 10.38
O
O
CH3C
CH3C
O
O
CH3C
O
O
CH3C
+
O
O
CH3C
+
O
H
O
H
CCH3
O
O
Acetic
anhydride
Peracetic
acid
Acetic
acid
Diacetyl
peroxide
Scheme 10.39
Table 10.23 Typical formulations of lower-strength
peracetic acid equilibrium mixtures
Peracetic acid
Hydrogen peroxide
Acetic acid
35–38%
7%
39%
15%
23%
16%
5%
27%
6%
Flash point (°C)
62
96
97
The remainder of these formulations consists of water, catalyst
(e.g. sulphuric acid) and stabiliser
O
O
CH3C
CH3C
O
Scheme 10.40
O
Peracetic
acid
+
O
O
H
H
Acetyl
radical
Perhydroxyl
radical
mixtures are less hazardous than the 38–40% strength, they still pose problems in use. Oral
ingestion, inhalation or contact with skin or mucous membranes leads to strong and
sustained cauterising or burning and eczema that is difficult to heal [254]. They are quite
stable when correctly stored but should not be stored in enclosed vessels or pipework, nor in
contact with catalysing metals: ventilated containers of stainless steel or aluminium of at
least 99.5% purity are suitable [254]. These hazard constraints are likely to continue to
restrict the use of peracetic acid in comparison with relatively hazard-free peroxide.
Bleaching is thought to take place through the perhydroxyl free radical (Scheme 10.40)
[223]. Various attempts have been made over the last decade to elucidate the mechanism
and demonstrate the potential of peracetic acid in bleaching. Temperature and pH are
critical parameters with regard to the rate and degree of bleaching on the one hand and the
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extent of fibre damage on the other. It is difficult, however, to be dogmatic about these
effects since the published results (and conclusions!) show variability because of the
different conditions and substrates selected by the researchers.
Examination of the influence of pH is complicated by the fact that the bleaching reaction
lowers the pH due to the liberation of acetic acid [254]. Hence the results are affected by
whether this effect is counteracted by the use of a buffer. It has been pointed out that this
pH shift increases with the content of bleachable material present [254]. In theory, optimal
bleaching should take place at pH 8.2, which corresponds to the pK value of peracetic acid
[253]. The results in Table 10.24 have been obtained on knitted cotton bleached for 30
minutes at 60 °C with 2.5 g/l peracetic acid, the original degree of polymerisation of the
cotton cellulose being about 2600. An investigation on linen [254] revealed no bleaching
under acidic conditions. Whiteness reached a maximum at pH 7–8 but there was no
increase beyond pH 8 as the peracetic acid decomposed to give acetic acid solution.
Likewise, pH 8 has been recommended for the bleaching of cotton/acrylic blends with 2–10
g/l peracetic acid at 75 °C in the presence of a phosphate buffer [252]. Conversely, pH 3–6
has been indicated for the bleaching of nylon carpets with 1–5 g/l peracetic acid for 15–45
minutes at 60–70 °C [252]. Somewhat conflicting results [255] have been obtained on a
starch-sized cotton fabric using 1% peracetic acid for 30 minutes at 75 °C and a long liquor
ratio (Figure 10.35).
Table 10.24 Effect of the pH of peracetic acid bleaching on the brightness and degree of polymerisation of
cotton cellulose [251,253]
pH
Brightness
value
Degree of
polymerisation
3
5
7
9
37
43
62
65
2650
2720
2630
2430
80
Whiteness
75
70
6
pH
10
8
Figure 10.35 Effect of pH on the whiteness of starch-sized cotton fabric in a one-stage pretreatment
with peracetic acid at a long liquor ratio [255]
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As indicated in Table 10.25, markedly different effects can be obtained at alkaline pH
values depending on whether sodium hydroxide or sodium carbonate is used to obtain the
required pH [256]. These results were obtained on desized and scoured cotton bleached
with 1% peracetic acid for 1 hour at 50 °C and 50:1 liquor ratio. It is evident that fibre
damage was much higher in the region of the pK value of peracetic acid when sodium
carbonate was used compared with sodium hydroxide. Sodium carbonate resulted in more
rapid decomposition of peracetic acid above pH 6, although this was not accompanied by
increased formation of the species active in bleaching, both alkalis giving virtually the same
degree of whiteness for a given pH. The optimum conditions in most cases are found at pH
6–7.
Table 10.25 Effect of alkali used with
peracetic acid at various pH values on the
degree of polymerisation of cotton cellulose
[256]
Degree of polymerisation
pH
Alkali:
4
5
6
7
8
9
Na2CO3
NaOH
2750
2800
2850
2680
1850
1350
2780
2550
2550
2720
2760
2800
Table 10.26 Effect of temperature of peracetic acid
bleaching on the brightness and degree of
polymerisation of cotton cellulose [251,253]
Temperature
(°C)
Brightness
value
Degree of
polymerisation
20
40
60
80
38
44
60
66
2650
2600
2550
2500
The effects of temperature are shown in Table 10.26 for the bleaching of knitted cotton
for 30 minutes at pH 6–7 with 2.5 g/l peracetic acid, the original degree of polymerisation
being about 2600 [251,253]. Thus both whiteness and fibre damage increase progressively
with bleaching temperature. In practice it is desirable to avoid excessive treatment
temperatures, the preferred range being 50–80 °C [251,253]. At higher temperatures a
pungent vapour is evolved.
Under optimum conditions of pH and temperature, a treatment time of 20–60 minutes is
generally adequate. The degree of whiteness increases with increasing concentration of
peracetic acid. The optimum concentration is dependent on the type of process and
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substrate, but relatively high concentrations of peracetic acid have little effect on the degree
of polymerisation. For example, increasing the oxidant concentration from 1.75 to 8.75 g/l
improved whiteness by about 25%, whilst the degree of polymerisation was lowered by only
about 9% [253]. This is in marked contrast to bleaching with sodium hypochlorite. Tests
with 100 mg/l iron showed that this transition metal has no significant influence on
whiteness or fibre damage [253] under otherwise optimum conditions. The addition of a
surfactant can improve wettability and penetration. Indeed, this may exert more influence
on whiteness than raising the treatment temperature [256].
In the context of peracid bleaching it is worthwhile recalling the reaction outlined in
Scheme 10.30, in which peracetic acid is produced in situ by the action of the activator
tetra-acetylethylenediamine (10.86) on hydrogen peroxide [244].
Peracetic acid bleaching is not widely practiced, so it is not possible to give typical
formulations and conditions. However, various process recommendations have been given
[251–256]. It has been demonstrated that peracetic acid bleaching combined with ultrasonic
treatment gave higher whiteness values and less fibre damage than conventional bleaching
with peroxide [257]. Low temperature bleaching with peracetic acid at 30 °C, catalysed by
incorporation of 2,2′-bipyridyl (10.89) in an alkyl sulphate surfactant, has been proposed
[258].
N
N
10.89
2,2′-Bipyridyl
Combined bleaching processes
Combinations of more than one bleaching process can be beneficial. The sequence of a
hypochlorite bleach followed by a peroxide bleach is common: the second stage can
considerably reduce the AOX value imparted by the preceding hypochlorite treatment. It
has also been suggested that the sodium hypochlorite in this two-stage sequence can
effectively be replaced by peracetic acid, lowering the AOX values even further. A review of
combined bleaching processes for weftknit cotton fabrics is available [225].
Monitoring bleaching processes
Accurate monitoring of bleaching processes is essential for efficiency, economy and
protection of the environment. The importance of checking raw materials, including the
substrate, should be obvious. There is also a need to monitor the actual process liquors and
to evaluate the results obtained on the substrate. A useful account of such procedures is
available [143].
Bleaching from the viewpoint of fibre type
Most cotton is bleached with peroxide, by far the greater proportion by continuous methods.
Synthetic fibres seldom require bleaching, but where it is necessary either peroxide or
chlorite bleaches are recommended. Most regenerated cellulosic fabrics are only bleached
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
for fluorescent whites and pastel shades. Any of the three oxidative bleaches may be used
[143]. Peroxide bleaching requires 5–15 g/l hydrogen peroxide (35%) at 80–100 °C,
depending on the type of machinery selected, together with sequestrant, caustic soda and
stabiliser for the peroxide. Chlorite bleaching on the winch requires 0.5–1.5 g/l sodium
chlorite (80%) at pH 4, treatment conditions being 45–90 minutes at 90–95 °C. Peracetic
acid (3 g/l) treatment for 60 minutes at 65 °C and pH 7 in the presence of sodium
hexametaphosphate is also suitable.
Wool bleaching
Wool poses special problems, although only approximately 1.5% of the world production of
wool is bleached directly. However, an unknown amount of so-called ‘top-up’ bleaching also
takes place, usually by adding hydrogen peroxide to the last bowl of the scouring process.
Reviews of wool bleaching, which includes oxidation, reduction and sequential oxidation–
reduction processes, are available [11,259,260]. A particular problem with wool lies not so
much in its initial natural colour as in its tendency to show yellowing under the influence of
light, dry heat or wet alkaline treatments. It is particularly important to realise that
bleaching often exacerbates this problem. The complex chemical mechanisms involved in
the yellowing of wool have been reviewed in detail [259] and will not be discussed further
here.
Oxidative bleaching of wool is invariably carried out with hydrogen peroxide. The active
species involved is likely to be the same as on cellulosic substrates but specific reactions with
wool amino acid residues must be considered. The primary reaction is oxidation of cystine
disulphide bonds leading to the formation of cysteic acid residues (Scheme 10.41). The
rupture of disulphide crosslinks, with attendant increase in urea-bisulphite and alkali
solubility values, adversely affects fibre properties. As the severity of bleaching conditions
increases, the urea-bisulphite solubility remains little changed but the relationships between
alkali solubility and cysteic acid (Figure 10.36) and between cystine and cysteic acid (Figure
10.37) are almost linear [259,261,262]. Tyrosine, tryptophan and methionine residues are
oxidised by hydrogen peroxide [259]. In order to retain commercially acceptable fibre
properties, the alkali solubility of bleached wool should not exceed 30% [259,263]. The
degree of attack on the fibre depends on the nature of the wool itself. Chlorinated wool is
more sensitive than untreated wool but Hercosett-treated wool is less sensitive (see Figure
10.38) [259,264].
As with cellulosic fibres, the bleaching of wool with peroxide requires careful
optimisation of several parameters: peroxide concentration, pH, temperature, treatment
time, choice and concentration of stabiliser and possibly choice and concentration of
activator. A typical formulation for batchwise bleaching of wool is given in Table 10.27.
Sodium pyrophosphate (10.16) or tripolyphosphate (10.17) are generally the stabilisers of
choice but silicates can also be used. Bleaching for two hours at 50–60 °C and pH 8.5–9 is
common, although in some cases careful optimisation can reduce the time to one hour [11].
Alkaline conditions are appropriate for cellulosic bleaching but may cause quite severe
damage with wool and thus careful control is necessary. Alternatively, acidic conditions (pH
5 at 80 °C) can be chosen, using a suitable peroxide activator to produce a peracid.
Only 15–25% of the hydrogen peroxide present is consumed in a typical bleaching
process. In order to minimise wastage, some bleachers reuse liquors several times,
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replenishing the bath with fresh peroxide after determining the deficit by titration. The
number of batches that can be bleached in the same replenished liquor is limited by
discoloration of the bath by impurities desorbed from the wool [259]. There is a further
economic benefit in that the peroxide replenishment decreases with each successive batch
(Figure 10.39), possibly because of the build-up in the bath of soluble proteins, which have a
stabilising effect.
H2O2
CH2
S
S
CH2
CH2
_
SO3H
HO3S
+
CH2
HO
Cysteic acid
Cystine
Scheme 10.41
Alkali solubility/%
80
60
40
20
1
2
4
3
Cysteic acid/%
Figure 10.36 Relationship between alkali solubility and cysteic acid content of peroxide-bleached
wool [259,261]
12
Cystine/%
11
10
9
8
1
2
3
4
Cysteic acid/%
Figure 10.37 Relationship between cystine and cysteic acid content of peroxide-bleached wool
[259,261]
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
40 oC
Alkali stability/%
Alkali stability/%
40
35
30
25
20
35
30
25
20
4
2
6
10
8
4
2
6
8
10
Time/h
Time/h
40 oC
Hercosett wool
Chlorinated wool
Untreated wool
50 oC
34
Whiteness index
34
Whiteness index
Chlorinated wool
Untreated wool
Hercosett wool
50 oC
40
33
32
31
30
33
32
31
30
4
2
6
10
8
4
2
Time/h
6
8
10
Time/h
Figure 10.38 Effect of treatment time on the alkali solubility and Jaquemart whiteness index of wool
bleached with 2-vol. hydrogen peroxide at 40 °C and 50 °C [259,264]
Table 10.27 Recommended conditions for the peroxide
bleaching of wool [259]
Hydrogen peroxide (35%)
Phosphate stabiliser
Wetting agent
10-30 ml/l
2-4 g/l
1 g/l
Treatment for 1–16 hours at 40–60 °C
Hydrogen peroxide 27%/ml
1400
1000
600
200
1
2
3
4
5
Batch number
Figure 10.39 Bleach bath regeneration in the peroxide bleaching of wool [259]
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A typical liquor formulation for pad application in a continuous or semi-continuous
system is given in Table 10.28. Batching for 24–48 hours is followed by backwashing,
although the use of radio-frequency heating to give a batch temperature of 50–60 °C can
reduce the treatment time to 2–4 hours.
Table 10.28 Recommended conditions for the pad
bleaching of wool [259]
Additives
g/kg
Hydrogen peroxide (35%)
Thickener
Isopropanol
Wetting agent
Formic acid
Antifoam
Fluorescent brightener (optional)
40
8
16
10
2
3
5
Although various processes may have been subjected to optimisation in recent years as a
result of economic pressures, a survey in the 1980s revealed disparities between different
sectors of the industry as summarised in Table 10.29 for batchwise wool bleaching methods.
Table 10.29 Worldwide practice of batchwise wool bleaching
[259]
Processing time:
<3 hours
>3 hours
Totals
Loose stock
Slubbing
Yarn
Piecegoods
17.8
5.1
11.6
4.7
34.3
10.5
10.2
5.8
52.1
15.6
21.8
10.5
100.0
As with the peroxide bleaching of cellulosics, the presence of transition metals in the
bleaching of wool with peroxide has a catalytic effect that can damage the fibre. This
catalytic effect has been exploited in a process for the bleaching of heavily pigmented wools.
The fibre is first mordanted with a ferrous salt in the presence of hypophosphorous acid
(H3PO2), then rinsed to remove the iron from the wool keratin but not from the melanin
pigment granules. Hydrogen peroxide is then added to destroy the pigment granules by a
free-radical mechanism [11,259,265].
Reductive bleaching of wool is mostly carried out with stabilised sodium dithionite.
Derivatives such as sodium formaldehyde-sulphoxylate (CI Reducing Agent 2) or zinc
formaldehyde-sulphoxylate (CI Reducing Agent 6) as well as thiourea dioxide (CI Reducing
Agent 11) are also used. The chemistry of these reducing agents is discussed in Chapter 12.
Apparently little is understood about the reactions between these reducing agents and the
pigments responsible for the natural colour of wool. Sulphitolysis of the wool is known to
take place, the reducing agent reacting with the disulphide bonds of cystine to give a Bunte
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
salt (Scheme 10.42) [266]. Rupture of the disulphide bonds results in an increase in alkali
solubility of the fibre, to which the formation of sulphonic groups also contributes.
C
CH
O
C
CH2
S
S
CH2
NH
_
HSO3
O
C
CH
CH
NH
NH
O
Cystine
C
_
S
CH2
SO3
+
HS
CH2
O
CH
NH
Bunte salt
Cysteine
Scheme 10.42
Typical conditions for bleaching with stabilised dithionite are given in Table 10.30
[11,259]. A warm rinse is then given, with an addition of 1 ml/l hydrogen peroxide (35%) in
the final rinse to eliminate sulphurous odours. The sulphoxylates are used at pH 3.0 for up
to 30 minutes at 90 °C, but these tend to produce a harsh handle as well as an unpleasant
residual odour. Thiourea dioxide (1–3 g/l) can be used at 80 °C and pH 7 for one hour [11].
A sequestering agent is added to prevent metal-catalysed decomposition of the thiourea
dioxide. The active bleaching species is sulphinic acid (Scheme 10.43) [11]. Thiourea
dioxide has less effect on the physical properties of the fibre than other reductive bleaching
agents [267]. Fluorescent brighteners can be applied together with a reductive bleach but
they may increase the subsequent tendency to photo-yellowing.
Table 10.30
Recommended conditions for the
reductive bleaching of wool [11,259]
H2N
Stabilised sodium dithionite
Wetting agent
2–5 g/l
1%
pH
Temperature
Time
5.5–6.0
45–65 °C
up to 1 hour
O
C
H2N
S
H2N
O
H2O
C
O
HN
Thiourea
dioxide
H2N
S
C
OH
Formamidinesulphinic acid
O
+
HO
S
OH
H2N
Urea
Sulphinic
acid
Scheme 10.43
Reductive bleaches are generally less costly than oxidative bleaches but tend to give a
greenish white compared with the reddish white tones from peroxide. Combining the two in
an oxidation/reduction sequence gives a more neutral white, this being known as ‘a full
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PREPARATION OF SUBSTRATES
bleach’. The sequence is generally carried out in separate baths. However, an interesting
attempt has been made to integrate the two processes [268–271]. This approach is of
significant interest chemically. A combination of hydrogen peroxide and thiourea is used but
the sequence of reactions is complex. Initially the wool is given a conventional oxidative
bleach with 8 g/l hydrogen peroxide (30%) and 2 g/l sodium pyrophosphate (10.16),
beginning at pH 9.0 and ambient temperature, then raising at 1 °C/minute to 60 °C and
maintaining at this temperature for one hour. Titration shows that only about 10% of the
peroxide is consumed and the redox potential is around +200 to +400 (i.e. oxidative). The
objective in the second stage is to make use of the residual peroxide. The pH is lowered to
8.2–8.7 and then to 5.0–5.5 with acetic acid, whilst still at 60 °C. Thiourea (1.68 g/l) is then
added and ten minutes is allowed for thiourea to react with peroxide to give the reducing
agent thiourea dioxide, as in Scheme 10.44. Adequate time at this pH for the reaction to
take place is essential. Ammonia is then added to give pH 6.8–7.2. The redox potential
becomes -600 to -700 (i.e. reductive) and reductive bleaching proceeds at 60 °C for 25
minutes. The thiourea dioxide formed in Scheme 10.44 is hydrolysed to urea and the active
reducing species, the sulphinate anion, during this phase, as in Scheme 10.45. The
sulphinate anions in turn react with wool and with the coloured pigments present on the
wool, being thereby oxidised to sulphate anions (Scheme 10.46). Any residual reducing
activity can be quenched by adding a small amount of peroxide.
H2N
H2N
C
H2N
pH 5
C
S
SH
+ 2 H2O2
HN
H2N
O
C
2 H2O +
S
HN
OH
Hydrogen
peroxide
Thiourea
H2N
O
C
H2N
O
Thiourea
dioxide
Scheme 10.44
H2N
O
C
H2N
H2O
S
O
_
O
H2N
_
C
S
O
O
+
Urea
_
O
613
_
O
Sulphinate
anion
_
O
O
S
+ 2 [O]
O
Sulphinate
anion
Scheme 10.46
S
H2N
Thiourea
dioxide
Scheme 10.45
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Sulphate
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
The control of conditions, particularly of pH and the molar quantities of reactants, is
critical; otherwise, variant reaction routes are possible. Unfortunately, thiourea is a
suspected carcinogen and this appears to be one reason why this two-stage redox process
has not been readily adopted, even though thiourea is easy.to handle and the bath can be
monitored colorimetrically to ensure complete conversion. In an attempt to overcome this
problem, it has been found [272] that a cyclic analogue of thiourea, namely thiocyanuric
acid or s-triazine-2,4,6-trithiol (10.90), can replace thiourea in this application.
Thiocyanuric acid is commercially available, being used as a heavy metal precipitant in
waste water treatment and as a vulcanising agent. It is best used in the bleaching process as
a 15% solution of the trisodium salt. The process is claimed to produce exceptional
whiteness with minimum fibre damage [272].
HS
N
N
SH
N
HS
10.90
Thiocyanuric acid
Semi-continuous and continuous reductive processes are best carried out with activated
hydroxymethanesulphinates, since dithionites (even when stabilised) are unsuitable due to
being oxidised too quickly by air [273].
The environmental aspects of wool bleaching have recently been reviewed [273]. There
is a tendency to replace phosphates by phosphate-free activators. Reductive bleaching
agents are not environmentally friendly as they are oxygen-depleting, but pollution by
dithionite is kept to acceptable levels if the residual concentration is less than 0.3 g/l.
Thiourea dioxide offers the advantage of contributing only half the sulphur load from
dithionite, but it also introduces detrimental nitrogen. The carcinogenic potential of
thiourea has already been mentioned.
Bleaching in particular is an area where many formulated commercial products are
offered. These have the advantage of providing the bleacher with a single (optimised)
product to simplify the preparation of bleach liquors, compared with having to store and
prepare several separate components. Little information is available on the detailed
constitution of such mixture products, although hints are sometimes given by the suppliers.
Some bleachers, however, may prefer the greater freedom to adjust liquors according to
requirements by the judicious use of separate components. Special considerations are
needed where bleaching is carried out in association with other processes or as a means of
‘brightening’ goods that have been already dyed.
10.5.4 Mercerising
Mercerising is an alkaline treatment often given to cotton yarns or fabrics, the objective
being to increase fibre lustre, strength and dyeability. These effects are brought about by
alkaline swelling of the fibres with or without tension. Accounts of practical aspects of
mercerising treatments are available [143,235,274]. Processing options include: cold or hot,
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wet or dry, chain or chainless and batchwise or continuous. Consideration must be given to
the position of mercerising in the overall sequence of preparation, since it may be carried
out on grey, partially prepared or fully prepared goods. The maximum degree of swelling
attainable decreases the later the stage at which mercerising is carried out. Accordingly,
mercerising of grey-state goods gives the maximum potential for swelling but fibre
penetration and uniformity of treatment are most difficult to achieve. Grey mercerising also
has the severe disadvantage of giving maximum fouling of the caustic soda liquor.
Mercerising is usually positioned either between desizing and boiling-off or between
boiling-off and bleaching [274,275]. Care should be taken to avoid carry-over of iron
impurities into the bleaching stage. It is also important to ensure thorough and uniform
rinsing after mercerising, as any localised residual alkali will lead to uneven bleaching [275].
Whilst these are generally the two most favourable positions for mercerising, their
disadvantage lies in interrupting the flow of the continuous preparation sequence, since
mercerising is the slowest pretreatment stage. This is where hot mercerising (60 °C) can
become an advantage, because the quicker rate of swelling allows this step to be more easily
incorporated within the processing range as a whole [275].
Mercerising after bleaching gives the least fouling of the liquors but increases the
possibility of fibre yellowing. Moreover, fibre swelling and absorptivity are less evident,
sometimes leading to problems in subsequent processing, particularly in continuous
processes where rapid uptake and maximum absorption are required [275].
In chain mercerising, the weft threads of the fabric are kept under tension on a clip
stenter. In a chainless merceriser the fabric dimensions are controlled by a series of rollers.
The fabric is usually woven slightly wider to allow for some weft shrinkage. Yarn is
mercerised in hanks between two movable rollers which create the required tension whilst
knitted constructions may be mercerised in either slit or tubular forms. Chain and chainless
mercerising have been compared [276].
It is important to be aware of the machine configuration as this influences the target
conditions, including the concentration of caustic soda applied and its subsequent washingoff. The three stages that are important from the viewpoint of auxiliaries are: mercerising
zone, stabilising zone and washing zone. In the mercerising zone the goods are impregnated
with caustic soda liquor and are subjected to the relevant means of controlling tension. In
the stabilising zone the concentration of alkali is reduced to a level at which the fabric again
becomes dimensionally stable, losing the plasticity imparted by the concentrated alkali in the
mercerising stage. The counterflow washing principle is applied to reduce the alkali
concentration. Only when dimensional stability has been restored are the goods ready for
the washing-off section in which residual alkali is removed and the fabric neutralised.
Traditionally, mercerising has been carried out cold (10–20 °C). This imparts the
maximum degree of swelling but this is attained at the slowest rate. Hot mercerising has
been introduced more recently and this is carried out at 60–70 °C. The characteristics of the
two processes are compared in Table 10.31 [274,277]. Table 10.32 presents a comparison of
these processes from the viewpoint of results obtained in relation to the stage at which
mercerising is carried out, as well as analogous trends for modal fibres [275,277]. The
characteristic feature of hot mercerisation is that the essential chemical and physical
changes do not take place at the higher initial temperature but in the subsequently cooled
fabric as it passes through a traditional tensioning process [143]. Hot mercerising, however,
has not yet achieved significant commercial success [143]. In cold mercerising, the
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
treatment time needs to be longer and is usually 40–45 seconds, compared with 25–35
seconds when mercerising at 60 °C [276]. The optimum effect is achieved when the fabric
takes on a glassy, transparent appearance just before the stabilising section. A combination
of hot and cold impregnation is possible [276].
Table 10.31 Comparison of hot mercerising with conventional cold mercerising of cotton [274,277]
Conventional mercerisation (10–20 °C)
Hot mercerisation (70 °C)
strong fibre swelling
slower swelling
slower relaxation
incomplete relaxation
higher residual shrinkage
surface swelling
unevenness
tighter fibre packing
firmer handle
slow NaOH diffusion
less lustre
less strongly swollen fibres
in surface zone of yarn
background less lustrous
less fibre swelling
rapid swelling
rapid relaxation
good relaxation
lower residual shrinkage
complete swelling
evenness
looser fibre packing
softer handle
rapid NaOH diffusion
optimised lustre
moderately swollen fibres
throughout yarn cross-section
background equivalent lustre
Table 10.32 Comparison of cold and hot mercerising processes for bleached and
unbleached cotton and for modal fibres [275,277]
Property
Substrate
Cold
mercerised
Hot
mercerised
Degree of polymerisation
grey cotton
bleached cotton
modal fibre
grey cotton
bleached cotton
modal fibre
grey cotton
bleached cotton
modal fibre
grey cotton
bleached cotton
modal fibre
grey cotton
bleached cotton
modal fibre
grey cotton
bleached cotton
modal fibre
grey cotton
bleached cotton
modal fibre
=
=
…
=
=
<
≤
>
>
<
<
>/<
<
<
<
≥
…
>
<
…
<
=
=
…
=
=
>
≥
<
<
>
>
>/<
>
>
>
≤
…
<
>
…
>
Breaking strength
Dyeability
Lustre
Dimensional stability
Handle (flexibility)
Crease recovery angle
= Similar to
< Less than
> Greater than the other method
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There is a so-called dry mercerisation process [275] in which the fabric is padded with
caustic soda liquor at 20–25 °C and then dried in a stenter at about 130 °C. An immersion
time in the pad trough of 7–10 seconds is sufficient but the goods need a total saturation
time in the alkaline liquor of 30–40 seconds, i.e. from the nip to entry into the drying zone.
The type of cotton and its condition determine to a large extent the degree and
uniformity of mercerisation. Fibres with a relatively rounded cross-section, exceptional
fineness and consistency tend to give the highest degree of mercerisation [274]. These
properties also play a part in determining the mercerising conditions.
Ideally, the maximum possible degree of mercerisation would be obtained if the goods
were repeatedly mercerised (for example, twice at around 70 °C and then for a third time at
10–15 °C) but such a procedure is economically impractical [274]. In practice it is essential
to aim for an optimum rather than the maximum degree of mercerisation, a compromise
between what is desirable or ultimately possible and what is economically feasible on
available machinery.
In the context of the above basic requirements of the process, the chemicals used are
sodium hydroxide as the primary agent and a surfactant-based auxiliary to aid rapid and
even penetration as an important secondary requirement.
The viscosity of solutions of sodium hydroxide increases with concentration and
decreases with temperature as shown in Figure 10.40. It is the higher viscosity of cold
concentrated solutions which makes fibre penetration so difficult in cold mercerising.
Nevertheless, despite the slower rate of penetration at low temperatures, the ultimate degree
of swelling is greater (Figure 10.41). Thus in this investigation, maximum swelling at 25 °C
was obtained with 4–6 mol/l sodium hydroxide solution. These curves illustrate that the
change in swelling behaviour with temperature becomes more critical at low concentrations
of alkali. Traditionally, solutions of sodium hydroxide in the 6.25–6.5 mol/l range have
generally been used in cold mercerising. In fact cellulose swells even in pure water but to a
lesser extent than in alkali, the swelling in water being reversible whereas alkali-induced
swelling is irreversible. Aqueous swelling affects only the readily accessible amorphous
regions of cotton, whilst alkali also greatly affects the crystalline regions.
8
A
A
B
C
Dynamic viscosity/10–3 Pa s
B
6
531 g/l NaOH/42 oBé
297 g/l NaOH/30 oBé
167 g/l NaOH/20 oBé
4
C
2
10
20
30
40
50
60
Temperature/oC
Figure 10.40 Change in viscosity of NaOH solutions with temperature [274]
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
2.0
Degree of swelling
1.8
1.6
0 oC
1.4
25 oC
1.2
100 oC
2
4
6
8
10
Sodium hydroxide concentration/mol l–1
Figure 10.41 Effect of temperature on degree of swelling of cotton fibres by sodium hydroxide (molar
sodium hydroxide solution contains 40 g/l NaOH) [278]
The mechanism is thought to be one of ionic hydration [143,279,280], arising from
replacement of some of the water of hydration by cellulosic hydroxy groups as shown in
Scheme 10.47. When a hydrated ion pair is absorbed by the cellulose, three molecules of
water are released and are replaced by three hydroxy groups. These are unlikely to be
located in the same anhydroglucose unit. The liberated water molecules now occupy a
greater volume than when associated with the ion pair, thus causing swelling of the fibre
[143]. It is important to realise that swelling involves only partial molecular disruption of
cellulose; complete disruption would produce dissolution or at least dispersion. Hydrogen
bonds in the crystal structure are ruptured but the van der Waals forces remain intact, thus
enabling the cellulosic matrix to behave as mobile sheets held in close contact by the van
der Waals forces. This parallel arrangement of the cellulose chains is enhanced by the
tension applied during mercerisation and the surface of individual fibres becomes smoother,
giving an increase in lustre [276].
_
+
Na OH .nH2O
Scheme 10.47
+
[cellulose](OH)3
_
+
[cellulose](OH)3.Na OH .(n–3)H2O + 3 H2O
The more highly crystalline the fibre, the more it will resist the mercerising treatment.
Initial swelling always takes place in the amorphous regions, followed by penetration of the
crystalline material by alkali. As indicated earlier in Figure 10.41, optimum conditions for
swelling are obtained with about 4–4.5 mol/l NaOH at 25 °C, this being the most favourable
concentration for penetration of the crystalline regions and thus for enhancing dye affinity. A
further optimum is reached at a concentration of 6.5–7.5 mol/l NaOH, since it is at this
concentration and 0 °C that the fibre cross-section changes from kidney-shaped to circular,
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giving enhanced lustre. This is why 6.25–6.5 mol/l NaOH has been the standard
concentration in traditional cold mercerising. Greif considers that a concentration of at least
6.75 mol/l and preferably 8.75 mol/l NaOH is necessary for the highest degree of mercerisation
[274]. In the so-called continuous addition mercerising process, a liquor concentration of
about 13 mol/l sodium hydroxide is used at 70 °C, applied to squeeze-wet goods [274]. This
facilitates quicker penetration and gives a higher degree of mercerisation. The active
concentration, however, falls to the usual level of about 6.75–8.75 mol/l after a dwell time of
about 15–25 seconds, since the fabric already contains about 60% moisture. Nevertheless, it is
claimed that a better effect is obtained by this technique of ‘dilution from above’.
Strictly speaking, regenerated cellulosic fibres cannot be ‘mercerised’ although they can
be given a ‘causticisation’ treatment. There is a critical concentration of caustic soda that
causes dissolution of regenerated cellulose: about 65 g/l. The much higher concentrations
used in conventional mercerising do not cause dissolution of cotton cellulose, of course.
Regenerated cellulosic fibres can be given a causticisation process using 15–50 g/l sodium
hydroxide. There is some advantage in using higher temperatures as swelling is thereby
reduced. Viscose staple fabrics for dresswear are often causticised to enhance dye receptivity,
increase the brilliance of colour and obtain a softer handle [279].
Addition of a wetting agent to the mercerising liquor gives better penetration and more
even treatment. However, the choice of wetting agent depends on the fabric to be
mercerised and the position of mercerising in the preparation sequence as a whole. The
need for this additive is greatest with grey yarn or piece goods. Goods that have been given
an alkaline boil-off or bleach already have much better wettability, so the need for a wetting
agent is not so great. Indeed, such goods may still be saturated from a previous process and if
fed directly into the mercerising liquor, there is no need for a wetting agent [280].
The properties of a wetting agent for mercerising can be summarised as follows [235,280]:
– good solubility and stability in 10M (400 g/l) sodium hydroxide solution; the stability
should be maintained under the conditions of alkali recovery by centrifugal separation or
vacuum evaporation
– high wetting ability and high efficiency at low concentrations in the strongly alkaline
solutions used; this is particularly critical in continuous processing and on grey goods
– low affinity for the fibre; together with high efficiency at low concentrations this aids
subsequent rinsing and recovery
– low foaming.
Suitable products include [235,280]:
– alkylarylsulphonates
– sulphated aliphatic alcohols, the most efficient being those of low molecular mass (i.e.
4–8 carbon atoms), such as sodium 2-ethylhexylsulphate (10.91); branched chains are
more efficient than linear ones [235]
– some short-chain alkylphosphonate esters, e.g. sodium methyloctylphosphonate.
CH3CH2CH2CH2
CHCH2OSO3Na
CH3CH2
10.91
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
A product such as 10.91 may require blending with about 10% each of butanol and
unsulphated 2-ethylhexanol to give an effective formulation in terms of solubility, stability
and wetting power. It is useful for a wetting agent to exert some degree of detergency across
the whole range of caustic soda concentrations, including the lower concentrations for
efficiency during washing-off [281].
Yarn shrinkage provides a good measure of the efficiency of a wetting agent under
mercerising conditions [280]. Figure 10.42 illustrates results for an effective product, yarn
shrinkage being complete in about 30 seconds.
20
B
15
Shrinkage/%
C
D
B
C
10
D
A
B
C
D
5
Water only
200 g/l NaOH
280 g/l NaOH
300 g/l NaOH
A
10
20
30
120
Time/s
Figure 10.42 Mercerising shrinkage rate using 5 g/l of a commercial wetting agent [280]
In the so-called stabilising zone, the concentration of the alkali is reduced, by a
counterflow washing system for example, until the fabric regains dimensional stability. In the
succeeding washing-off zone, most of the residual alkali is removed and the fabric is
neutralised, typically with acetic acid [274,276]. The stabilising zone generally gives a spent
liquor containing 40–80 g/l sodium hydroxide [274,282,283]. Desorption is more rapid the
better the preparation of the cotton and the remarkable claim has been made that it is
possible to remove more than 75% of the alkali in only 2 seconds [284]! Even after thorough
rinsing, however, there is always a small amount of residual bound alkali in the fibre [285].
The spent liquors may contain lint and residual size that can be removed by filtration.
Weakly alkaline liquors represent a cost problem, however. Although limited amounts of less
dilute liquor may be recycled and used in boiling-off or scouring, the major proportion becomes
a rather troublesome component of the effluent load. Neutralisation simply increases the salt
content of the effluent. Recovery of the alkali by vacuum evaporation is the usual procedure
[282,283].
Anhydrous liquid ammonia can also be used to enhance the absorption properties of
cotton [143].
10.5.5 Wool processing
Milling, a process peculiar to wool, is carried out to develop its felting propensity. Traditional
wool goods such as felt hats and blankets are milled under slightly acidic conditions, sulphuric
acid being the main agent. Acid milling is particularly useful for dyed goods, which may not
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PREPARATION OF SUBSTRATES
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have adequate fastness to neutral or alkaline treatments. Alkaline milling conditions are still
used for woven piece goods traditionally known as milling cloths, maximum milling taking
place using soap at pH 10. The higher-melting soaps, such as those based on tallow and palm
oils, have been preferred to give the required gelatinous solution and lubricating properties.
Greasy woollens are often milled in sodium carbonate solution, which saponifies the natural
grease to a soap. Even with such woven pieces, however, the trend is towards milling in almost
neutral conditions, for which milling aids based on nonionic and anionic surfactants are useful.
Some wool yarns are milled simply by tumble drying of the wet yarns, whilst knitted garments
are milled in rotary-type machines using a nonionic surfactant with sodium bicarbonate or
polyphosphate. Solvent-based systems in which a small amount of water is emulsified in the
solvent by an appropriate surfactant have also been used [146], this often forming part of a
sequence in which scouring, milling and shrink-resist finishing are all carried out in the same
machine.
Another process peculiar to wool is carbonising. This exploits differences between wool
keratin and cellulose in their response to strong acid, wool being substantially more stable
whereas cellulose is degraded. Hence strongly acidic conditions are required to remove
cellulosic impurities from wool. Dilute sulphuric acid (4–8%) is most commonly used [286].
Other mineral acids (e.g. hydrochloric acid) or inorganic compounds that are strongly acidic
in solution (e.g. aluminium chloride) may also be used. However, whilst aluminium chloride
has been used to carbonise wool blends containing other fibre types such as polyester or
cellulose acetate that are sensitive to sulphuric acid, it is not used commercially to any great
extent [286]. Arylsulphonic acids and thionyl chloride (SOCl2) are also suitable. Thionyl
chloride is hydrolysed to hydrochloric acid and sulphur dioxide, so it has been used in
carbonising either as a vapour or as a solution in perchloroethylene [286].
All carbonising processes involve the following steps [286]:
(1) Immersion (or spraying) to impregnate the wool with the appropriate acid solution
(2) Drying to concentrate the acid
(3) Baking to dehydrate and carbonise the cellulosic impurities
(4) Crushing and dedusting to remove the charred cellulosic debris
(5) Neutralisation of the residual acid.
As already mentioned, sulphuric acid is by far the most common carbonising agent. In
traditional processes, it is applied at 4–5% concentration with a dwell time of 3–5 minutes.
So-called rapid processes apply 7–8% sulphuric acid with very short dwell times, typically 5
seconds. When used alone, there is a danger that localised droplets of highly concentrated
sulphuric acid can be formed, with consequent damage to the wool. The critical conditions
for this to occur are met when the acid concentration reaches 40–45% [286–288].
Auxiliaries are generally used to prevent this localised damage and to ensure efficient
wetting and penetration. Many products have been suggested [286], surfactants being the
most important. These must be stable to the hot acidic treatment, of course. Anionic surfactants such as alkylbenzenesulphonates have been used, as well as nonionic types such as
nonylphenol polyoxyethylenes of 6 to 9 ethylene oxide units per molecule. Typically, the concentration of surfactant present is only 0.025–0.04%, depending on the composition of the
surfactant [286]. Mixtures containing a polyethoxylated nonionic and an alkylarylsulphonate
anionic, however, can significantly increase the risk of wool damage compared with the use of
either component separately [286,289].
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
Since wool is attacked most rapidly by sulphuric acid of intermediate concentration, it is
important that drying is carried out either at a relatively low temperature so that reaction of
the acid with wool is slow, or very quickly so that the time of exposure of the wool to the
critical acid concentrations is brief [146]. Ideally, all the sulphuric acid in the wool is
absorbed chemically as bound acid that causes little hydrolytic damage. It is the free acid
that can concentrate locally and cause serious degradation. The acid picked up by the
vegetable impurities, on the other hand, is free acid that has the desirable effect of beginning
the process of attacking the cellulose [286].
Large amounts of residual acid may cause damage to the wool, so that careful
neutralisation after baking is an essential and important stage of the process. Carbonised
fabrics allowed to accumulate without neutralisation at moderate humidity may suffer
considerable damage, so it is essential that neutralisation should take place as soon as
possible after carbonising. Neutralisation with ammonia or a mixture of ammonia and
ammonium acetate is achieved more rapidly than with sodium carbonate or sodium acetate;
the ammonia is best used cold [146,286,290].
The pH of carbonising effluent can be adjusted, of course, to meet discharge
requirements. However, this can lead to undesirably high levels of sulphate. The slime in
sewage pipes produced under anaerobic conditions in turn produces hydrogen sulphide from
the sulphates present. This hydrogen sulphide gas is oxidised on the sewer walls, promoting
the growth of sulphuric acid-producing anaerobic bacteria with consequent damage to
concrete pipes [286]. It is possible to minimise sulphate levels by coagulation with a
combination of aluminium salts and lime at pH 10 [286,291].
Mechanism of shrink-resist finishing
Shrink-resist processes for wool came into prominence with the growth in importance of
machine-washable wool. These processes have a decisive bearing on the selection of dyes as
regards fastness demands. Numerous approaches have been suggested, normally involving
oxidative modification of the epithelial scales, the application of a polymer to the fibre
surface, or a combination of both. In the case of polymer deposition, two approaches are
possible. The polymer may be applied either as a surface film masking the epithelial scales or
to link together neighbouring fibres in a process sometimes known as ‘spot-welding’. These
differences are clearly illustrated in Figure 10.43. Special equipment is needed for interfacial
condensation polymerisation, however, and a further restriction is that it can only be applied
Scale masking by polymer deposition
Scale linking by interfacial condensation polymerisation
Figure 10.43 Mechanisms of shrink-proofing by polymer application [292]
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to fabric, whereas polymer deposition can be used at any stage of wool processing. For these
reasons, polymer deposition remains by far the most popular method commercially. The
degradative modification or removal of the wool scales is sometimes referred to as
‘subtractive’ shrink-resist finishing, whilst polymer deposition techniques are described as
‘additive’ [293].
Subtractive shrink-resist treatments
The chemical basis of subtractive treatments is the oxidative breakdown of disulphide bonds
in the cystine-rich epithelial scales of wool to form cysteic acid residues (Scheme 10.41).
There is also some hydrolysis of amide groups in the main peptide chain. Acidified sodium
hypochlorite was originally the preferred oxidising agent. However, difficulties in achieving
consistent and uniform treatment without excessive fibre damage led to its replacement by
sodium dichloroisocyanurate (10.92), also applied under acidic conditions. The use of this
‘chlorine generator’ provides a more gradual and controlled release of chlorine than can be
achieved with hypochlorite. Additives include sodium dioctylsulphosuccinate as wetting
agent, as well as the acidifying medium such as acetic acid with a buffer such as sodium
formate/formic acid.
O
Cl
N
Cl
N
_
O Na+
N
O
10.92
Liposomes (section 10.3.4) have been suggested as auxiliary agents in wool chlorination
since they give improvements in the consistency and homogeneity of the oxidative
treatment, minimising degradation of the wool and facilitating subsequent treatments
[61,62].
Although chlorination with sodium dichloroisocyanurate is still by far the most
commonly used method of shrink-resist finishing, there is considerable concern over the
environmental influence of its AOX contribution. For this reason, its usefulness could
decline in future and there has been considerable investigation of alternatives to this
attractively cost-effective treatment.
One possibility is to use an atmosphere of nitrogen containing 3% fluorine gas, for which
a commercial-scale plant is available [294]. This dry fluorination process provides an
effective subtractive treatment capable of replacing chlorination. Since it requires a halogen
gas, it might be thought to be just as likely as chlorination to infringe AOX regulations.
However, organofluorides cannot be detected by the current AOX test method. In any case,
owing to the strength and stability of the carbon-fluorine bond, it is unlikely that
carcinogenic species are formed. Furthermore, as this is essentially a surface modification,
the formation of fluoride ions is limited. It is considered that current effluent legislation
covering fluoride ions should not restrict commercial adoption of this process [294]. The
design of the machine eliminates the risk of loss of gaseous fluorine.
Corona discharge, bombardment of the wool fibre surface with electrons of sufficient
energy to break covalent bonds, has also been applied to the improvement of shrink
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
resistance [294–296]. Collisions between electrons and molecules of oxygen and nitrogen in
the atmosphere results in the formation of ozone and oxides of nitrogen. Subsequent
reaction between free-valence species on the substrate surface and the corona atmosphere
leads to the formation of a polar surface. This can facilitate adhesion of polymer if the
subtractive stage is followed by an additive phase [294]. The plasma glow discharge
treatment of wool in non-polarised gases like air, oxygen or nitrogen, usually combined with
a polymer treatment, also shows promise as a zero-AOX treatment for the shrinkproofing of
wool [297–299]. The plasma treatment does not damage the fibres, yet considerably reduces
the felting potential of the wool, particularly by enhancing the effectiveness of polymer
treatment.
Enzymes have been proposed as a means of subtractive shrink-resist treatment. Their use
has been discussed already in section 10.4.2. There are difficulties, however, in the
commercially successful application of enzymes to wool at present.
Alternatives to sodium dichloroisocyanurate as oxidising agent include potassium
permanganate and peracids such as peracetic acid, peroximonosulphuric acid (Caro’s acid)
and peroximonophthalic acid, of which by far the most important in terms of current
interest is peroximonosulphuric acid (10.93), usually known as permonosulphuric acid The
pure acid can be obtained in crystalline form, but its salts are unstable. In a detailed study of
the effect of oxidising agents on wool, their relative reactivities generally corresponded with
the degree of shrink resistance but this was not related to redox potential [300], the order of
reactivity being: aqueous chlorine > dichloroisocyanurate = permonosulphate >
permanganate and salt > peracetic acid > permanganate > persulphate = hydrogen
peroxide. Permanganate does not produce adequate effects unless it is applied from
saturated salt solution [301], a situation hardly likely to make it a preferred commercial
choice.
O
O
H
O
O
S
O
H
10.93
Permonosulphuric acid looks quite promising and appears to have the greatest potential
of all other oxidants as an alternative to chlorination. The high reactivity of
permonosulphuric acid with wool makes it particularly suitable for continuous treatments,
where only a short time is available for reaction. The reactivity can be controlled by pH
adjustment (Figure 10.44), the most suitable range being pH 3–5. Lowering the pH
increases the rate of oxidation but also increases the likelihood of uneven treatment.
Compared with chlorination this product shows the following advantages, although it is less
effective in minimising felting of wool fibres [301]:
– No AOX problems
– No yellowing of the wool
– Superior uniformity of treatment
– Almost odourless
– Less likely to attack dyes by oxidative fading.
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5
pH 8
pH 6
pH 4
pH 2
Oxidant/%
4
3
2
1
5
10
20
30
40
50
60
Treatment time/min
Figure 10.44 Influence of pH value of treatment bath containing a commercial permonosulphate
formulation on the rate of reaction with dyed wool at 25 °C and 20:1 liquor ratio [301]
Degree of yellowing (DIN 6167)
30
29
A
A Dichloroisocyanurate
B Permonosulphate
28
27
26
B
B
25
A
1
2
3
4
5
6
Oxidant concentration/%
Figure 10.45 Variation of degree of yellowing of wool with concentration of commercial
dichloroisocyanurate and permonosulphate formulations [301]
The difference with regard to yellowing of the wool is quite marked (Figure 10.45).
Permonosulphuric acid is suitable for use at any processing stage and on all the usual dyeing
equipment. The goods are first treated in an acidic liquor at ambient temperature until
virtually all the active oxygen has been consumed. A reductive treatment with sodium
sulphite is then given in the same liquor at a slightly alkaline pH, followed by rinsing. There
is some conflict regarding the parameters involved but careful investigation [301] has led to
the following recommendations:
(1) After wetting out, the wool is treated in the same bath with 4–6% of a commercial
permonosulphuric acid formulation for 30–60 minutes at 25 °C and pH 4–5
(2) To the same bath are then added 0.5% sodium carbonate and 5% sodium sulphite (pH
8). The temperature is raised to 35–50 °C and maintained for 20 minutes.
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
Chlorination treatments, of course, are invariably followed by a reductive aftertreatment. In
the case of permonosulphuric acid, this is even more important as the sulphite treatment
significantly enhances the shrink resistance (Figure 10.46).
Shrinkage/%
40
30
20
10
0
1
2
3
4
5
6
7
8
9
10
Sodium sulphite/%
Figure 10.46 Influence of sulphite concentration on the felting shrinkage (IWS TM31) of dyed wool
treated with 5% of a commercial permonosulphate formulation [301]; untreated wool shrinkage 60%
Permonosulphuric acid treatment confers only a modest shrink-resist effect which usually
needs to be improved by a subsequent additive treatment. It has been suggested [300] that
the most likely mechanism for inhibiting felting by permonosulphuric acid treatment is the
removal of degraded protein from below the exocuticle, producing a modified surface with a
reduced differential friction. The direct formation from cystine residues of low
concentrations of Bunte salts has been confirmed, as indicated in Scheme 10.42.
The use of peroximonophthalic acid (10.94) has been reported as a shrink-resist agent
[302,303]. When comparing dichloroisocyanuric acid, permonosulphuric acid and
permonophthalic acid, it was observed [303] that dichloroisocyanuric acid reacts so rapidly
that it is difficult to control the evenness of chlorination. This treatment tends to cause
stiffening and yellowing of the wool. Consequently, it is used at lower concentrations than
would be needed for full shrink-resist properties, the balance being achieved by subsequent
application of a polymer. Permonosulphuric acid does not cause these problems but can only
be used successfully on woven cloth, unless a polymer is applied subsequently.
Permonophthalic acid does not exhibit the disadvantages of dichloroisocyanuric acid.
Furthermore, it can be applied to wool in all its forms to give an adequate degree of shrink
resistance without the need for subsequent polymer treatment.
O
C
O
H
C
O
O
H
O
10.94
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Since the handle of the wool is not impaired, permonophthalic acid can be applied at a
level that gives full protection from shrinkage, this being the reason why the cost of polymer
treatment can be avoided. Currently however, there is a major disadvantage: although
available for experimental work as its magnesium salt, this is expensive and not yet available
in commercial quantities. However, a process has been proposed [303] whereby it can be
readily and inexpensively prepared in about 80% yield at 25–30 °C by stirring together
phthalic anhydride and hydrogen peroxide in water for about an hour, maintaining the pH at
about 5. This process is analogous to that for the preparation of peracetic acid (Scheme
10.37), although phthalic anhydride is less hazardous and easier to handle than acetic
anhydride. Nevertheless, the stability of the resulting permonophthalic acid is poor, its
oxidising power decreasing through decomposition by about 5% per hour. As with
permonosulphuric acid, a subsequent reduction stage using sodium sulphite is needed to
eliminate residual oxidant.
Additive shrink-resist treatments
As mentioned previously, additive treatments involve the application of a polymer to the
fibre. This is usually prepared before application and contains reactive groups. However, it is
also possible to form the polymer in situ within the fibres. The traditional approach is to
apply the polymer after a subtractive oxidation treatment but environmental concern over
AOX problems is increasing demand for additive treatments that can stand alone. There is
no denying that the oxidative step can facilitate subsequent treatment with a polymer, since
the scission of cystine disulphide bonds to yield cysteic acid residues provides useful reactive
sites for crosslinking or anchoring the polymer.
The desirable properties of shrink-resist polymers are [301]:
(1) The treatment must impart maximum shrink resistance.
(2) High substantivity for wool. This is clearly linked with the foregoing requirement.
These two factors are particularly important when application takes place after an
AOX-free oxidative stage, since such treatments generally impart lower initial shrink
resistance than chlorine-based subtractive treatments. Indeed, these two requirements
may need to be fulfilled so effectively that the oxidative stage before polymer treatment
can be omitted.
(3) Uniform application properties on wool in all forms.
(4) The treatment should not lead to harshness or stiffness of the fabric, thus obviating the
need for a softener. Indeed, if the polymer itself provides a degree of softness, this is an
added bonus.
(5) Suitable for application by batchwise or continuous methods.
(6) Should have no adverse effects on frictional characteristics of wool fibres or yarns.
(7) Should have no deleterious effects on the colour or fastness of dyes.
(8) Should not contribute to AOX values.
No shrink-resist polymer developed so far meets all the above requirements [301]. There is
clearly some similarity with easy-care finishing of cotton. Although effective crosslinking
agents are readily available for application to cotton, the morphological complexity of the
wool fibre is such that an equally effective polymer has yet to be identified for wool
treatment [304].
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
Still the most widely used shrink-resist polymer treatment is that associated with the
Hercosett process. This treatment always follows on from an oxidative treatment with
dichloroisocyanuric acid. Despite its popularity, it is doubly suspect environmentally because
both the oxidative first stage and the polymer itself contribute to AOX values. The polymer
has aqueous solubility and is a cationic polyamide-epichlorohydrin resin, the
epichlorohydrin contributing to AOX values. Since the polymer is cationic it has
substantivity for the anionic sites in wool, including those produced by the oxidative
treatment. The polymer contains azetidinyl cationic groups, which are reactive with a
variety of nucleophiles leading to insolubilisation of the polymer [11]. Covalent bonding is
also possible through cysteine thiol groups.
Despite the popularity of the chlorination-Hercosett route, it is clear that AOX problems
will often enforce the adoption of alternatives, many of which have already been developed.
Non-AOX polymers include polyethers, polyurethanes, polysiloxanes, polyquaternary
compounds and multifunctional epoxides.
Polyethers of various types are of particular importance and include the following types:
– polyethers solubilised by reactive Bunte salts
– polyethers solubilised by carbamoyl sulphonate groups
– thiol-terminated polyethers
– aziridine-terminated polyethers.
The first two types are of long-standing commercial availability. They are applied using
magnesium chloride as catalyst and are crosslinked by addition of ammonia. An important
factor is that magnesium chloride induces a cloud point at about 50 °C, leading to the
physical form essential for functioning of the polymer [11].
Polyurethanes have also been used for many years. They can be applied from a solvent
such as perchloroethylene, but such solvents are increasingly under environmental scrutiny.
An aqueous polyurethane formulation is normally applied by padding, followed by baking at
150 °C using sodium carbonate as catalyst.
Polysiloxanes as shrink-resist finishes have been developed from their traditional uses as
softeners and water repellents; as such their chemistry is discussed in section 10.10.3. This
was a natural trend as many shrink-resist finishes tend to impart a harsh handle to wool.
Polyquaternary compounds are useful in that as well as conferring shrink resistance they
may also improve acid dye fastness to wet treatments.
A useful and detailed comparison between specific examples of a polyether, a cationic
polysiloxane and a polyquaternary compound is available [301]. This review includes details
of practical application via various processing routes available for loose stock, tops, yarn,
knitted garments and woven or knitted piece goods. As mentioned earlier no single polymer
fulfils all requirements and combinations of different types are sometimes used. Some
indication of this is given in Table 10.33.
Whilst many methods have been proposed for preventing shrinkage of wool fibres, no
satisfactory method operates without some damage to the hydrophobic nature of the fibre
surface [305,306]. Nevertheless, it has been claimed that treatment of wool with a
multifunctional epoxide, glycerol poly(glycidyl ether), in a saturated solution of sodium
chloride gives excellent shrink resistance without impairing the wool surface [305,306].
Apparently this polymer is able to crosslink the cuticular cells and decrease the prominence
of their edges. There must be doubts, however, about the commercial feasibility of using the
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Table 10.33 Comparison of typical shrink-resist polymers of the non-AOX types for application to wool
[301]
Polymer type
Advantages
Restrictions
Polyether with
reactive groups
Outstanding shrink-resist effect
on pre-oxidised wool
Durable soft handle
Suitable for piece goods,
garments and loose stock.
Cost advantage over
polysiloxane type
Restricted range of applications
Not applicable before dyeing
Special dissolving requirements
Polysiloxane with
amino groups
Semi-microemulsion
Most effective shrink-resist
silicone
Durable handle after finishing.
No crosslinking agent or
catalyst required
All processing stages suitable
Long liquor application
after pre-oxidation
Padding possible
without pre-oxidation
Costly but high-quality finish
Polyquaternary compound
Shrink-resist effect after
pre-oxidation
Exceptional improvement in
fastness
All processing stages suitable
Improves shrink-resist effect of
polyether and polysiloxane types
No improvement in handle
Shrink-resist effect weaker
than with other types
Odour possible on
chlorinated wool
saturated brine solution necessary to bring about effective treatment. Such a medium would
be difficult to handle, costly and environmentally undesirable, even though it is AOX-free.
The dyeing of wool with bifunctional reactive dyes can enhance its resistance to felting.
This observation has been exploited in a shrink-resist process using a specifically designed
‘colourless reactive dye’ as the effective agent [307,308]. The product developed is the
trifunctional reactive compound, dipotassium 2-chloro-4,6-bis(4′-sulphatoethylsulphonylanilino)-s-triazine, also known as XLC (10.95). This reactant has substantivity for wool and
imparts shrink resistance through crosslinking, being an example of a non-polymeric
additive treatment. The crosslinking activity is centred mainly in the low-sulphur
microfibrillar proteins through their high content of lysine and histidine residues. A
particularly interesting approach is to apply this compound together with reactive dyes, in
which case it reacts and crosslinks to an even greater extent than in the usually shorter
shrink-resist process [308].
Cl
N
KO3SO
CH2CH2SO2
NH
N
N
HN
SO2CH2CH2
10.95
XLC
chpt10(2).pmd
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630
CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
A variation of this technique utilises reactive surfactants based on Bunte salt acetate
esters of dodecanol and dodecane-1,12-diol [309]. Results using mono-, bi- and
trifunctional versions have been reported (10.96–10.98 respectively). These are watersoluble and highly surface-active compounds. They can be applied to wool by padding in
combination with 20 g/l sodium sulphite and a locust bean gum thickener to give 100%
pick-up, followed by batching at 20 °C for 24 hours. Rinsing completes the process. The
results in terms of shrink-proofing are shown in Figure 10.47. It is evident that reactant
efficiency increased with functionality: 15%, 10% and 3% of the mono-, bi- and
trifunctional agents respectively were required to obtain zero shrinkage, from which it is
predicted that only 1–2% of a tetrafunctional agent would be needed. The monofunctional
Area felting shrinkage/%
50
Monofunctional
Bifunctional
Trifunctional
40
30
20
10
0
1
3
5
8
10
12
15
Applied agent concentration/% owf
Figure 10.47 Effect on shrink resistance of functionality of agent applied by the pad–batch cold
process [309]
C12H25
O
C
CH2
NaO3SS
SSO3Na
CH2
C
O
CH2
O
O
10.96
Monofunctional
O
C
O
O
O
C
C
O
C12H24
CH
C12H24
C
CH2
SSO3Na
C
CH2
SSO3Na
O
OH
O
O
O
CH2
C
O
C12H24
O
10.98
Trifunctional
chpt10(2).pmd
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C
O
10.97
O
CH2
O
Bifunctional
CH2
NaO3SS
C12H24
15/11/02, 15:44
SSO3Na
PREPARATION OF SUBSTRATES
([wool]
S
S)n
[wool]
(NaO3SS)n
S
S
[wool]
[wool]
SNa
+
+ Na2SO3
[wool]
SSO3Na
SSO3Na
S
S
631
Wool protein Bunte salts
Dodecanol derivative
[wool]
(SSO3Na)n
Mixed disulphides
Scheme 10.48
agent in fact gave better results than expected, showing clear evidence in scanning electron
microscopy of agent–fibre spot-welding. This is believed to occur via the formation of mixed
disulphides from the dodecanol derivative and wool protein Bunte salts from the reaction of
disulphide-rich proteins in the wool with sulphite (Scheme 10.48) [309].
Mention has already been made of the effectiveness of corona or plasma treatment in
increasing the influence of subsequent or concurrent polymer treatment. As examples of
polymers used in this way, mention can be made of reactive cationic polysiloxane [294] and
polymerisation on the fibre of tetrafluoroethylene or hexafluoropropylene [299]. Water
repellency was also improved by the fluorinated polymers. Tetrafluoroethylene gave superior
shrink resistance; this polymer covered the scale edges of the wool, whereas this did not
occur with poly(hexafluoropropylene).
Shrink resistance can be achieved by graft polymerisation of vinyl monomers onto wool
[292]. Suitable monomers include methyl, ethyl and butyl methacrylate, particularly the
methyl ester. Polymerisation is initiated by a redox system, potassium bromate and cobalt(II)
acetate, with a co-solvent such as 2-butoxyethoxyethanol to improve both efficiency and
selectivity. Grafting of 100% monomer is possible in three hours or less at 50 °C. Ultimately
up to 950% can be achieved; such high levels, however, are impractical. There are some
rather severe obstacles to the commercial development of this process: methyl methacrylate
is flammable and is a respiratory irritant, whilst use of the co-solvent is also undesirable. It is
suggested [292], based on a limiting volume model, that the mechanism is characterised by
three stages: initial grafting and void-filling within the fibre, disruption of crystalline regions
and polymer growth on or through the fibre surface with formation of interfacial bonds (i.e.
spot-welding) at high levels of grafting.
Whilst elimination (by oxidation) or masking (by polymer deposition on the cuticular
scales) are the accepted mechanisms by which shrink resistance is achieved, there is
evidence that other factors need to be considered, particularly as it is possible to obtain a
shrink-resist effect without degradation or masking of the scales. A review is available [310]
of the mechanism of chlorine-based shrink-resist processes.
Whilst chlorine-based processes are well understood from a mechanistic viewpoint, there
are differences between these and the permonosulphuric acid processes. Understanding of
the mechanism of permonosulphuric acid treatment has improved in recent years but there
are still aspects requiring elucidation [300]. An important difference between these two
types of oxidative treatment is that chlorine-based processes lead to scale modification or
chpt10(2).pmd
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
destruction with zero differential friction as the target. Permonosulphuric acid processes,
even when followed by a sulphite treatment, leave the differential friction essentially the
same as untreated fibres, apart from some longitudinal striations at the base of the scales and
some detachment and rounding of scale edges [300].
Chlorine water acting on wool produces Allwörden bubbles or sacs by raising of the
epicuticle [11]. This occurs through the formation of osmotically active oxidation products.
Cleavage of peptide bonds and especially oxidation of the disulphide bonds to produce
sulphonic acid residues result in the formation of soluble peptides responsible for the
increase in osmotic pressure within the semi-permeable membrane. Permonosulphuric acid
treatment, with or without a sulphite aftertreatment, also forms Allwörden sacs, as do
hydrogen peroxide, potassium permanganate and peracetic acid [300]. On immersion in
water, wool treated with permonosulphuric acid shows major decreases in magnitude of
friction and differential friction compared with untreated fibres, especially following a
sulphite aftertreatment. This may be attributed to degraded protein at the fibre surface
acting as a lubricant or to changes in swelling properties of the wool surface.
The sulphite aftertreatment is particularly important with permonosulphuric acid
treatment. Evidence for the underlying mechanism is available from analysis of sulphur
oxidation products formed in the various processes (Table 10.34). It is evident from these
results that the concentration of [RSSO 3]– anionic groups necessary to change the
hydration of the fibre surface is achieved by the reaction of bisulphite with cystine monoxide
residues to give the required cysteine-S-sulphonate groups [311].
Table 10.34 Relative amounts of sulphur oxidation products formed during shrink-resist
processing of wool [11,311]
Treatment
Oxidation product
Frequency (cm–1)
Quantity
Chlorine-Hercosett
Permonosulphate
Permonosulphate +
bisulphite
Cysteic acid (RSO–3)
1042
***
**
Chlorine-Hercosett
Permonosulphate
Permonosulphate +
bisulphite
Cystine monoxide (RSOSR)
Chlorine-Hercosett
Permonosulphate
Permonosulphate +
bisulphite
Cysteine-S-sulphonate (RSSO–3)
*
1076
*
***
*
1024
**
*
***
In the case of polymer deposition, it has been pointed out [293] that the masking effect
at the scale edges may be less important than mutual adhesion of fibres in the yarn, since the
thickness of the polymer film (0.1 µm) is much less than the average height of scale edges
(1 µm). This effect is more analogous to spot-welding.
The particular requirements of shrink-resist processes in relation to wool fabric printing
have been described [312]. In addition to dimensional stability, there is a need for ease of
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PREPARATION OF SUBSTRATES
633
diffusion and absorption of dyes in printing. Chlorination is particularly effective because the
rapid reaction limits the oxidative attack to the fibre surface. Dye affinity and the
hydrophilicity of the fibre are increased by the anionic sulphonic acid groups formed and the
surface barrier to diffusion is lowered. However, this is an AOX-loading process.
Permonosulphuric acid, although AOX-free, is an inadequate preparation for printing even
with a polymer aftertreatment. Hydrogen peroxide, activated by the recyclable catalyst
sodium tungstate, offers a suitable process [312] that is AOX-free. Oxidation remains
concentrated at the fibre surface because of the high reactivity of peroxide, giving results
analogous to those from chlorination. A subsequent polymer treatment may also be given.
The process involves padding to 75% pick-up in a solution equivalent to 8% by weight of
sodium tungstate and 3% by weight of hydrogen peroxide. The padded fabric is rinsed after a
reaction time of two minutes and the tungstate can be recovered from the rinsing water.
10.5.6 Combined processes
The economics, at least on paper, of combining two or more processes to gain major savings
in time and energy have long been sufficiently attractive to motivate research in this
direction. For example, one-stage desize–scour, scour–bleach, desize–scour–bleach, scour–
dye and even (paradoxically enough) bleach–dye operations have been, and are, operated.
The major disadvantage of such combined processes stems from difficulties of operation,
especially compounded where there is a high probability of incompatibility, as in enzyme
desizing–bleaching or in bleaching–dyeing. In addition, any process that is combined with
scouring and/or desizing may be subject to interference from the products desorbed or
decomposed by those processes. Under these circumstances the choice of chemical additions
becomes critical. For example, in combined desizing–bleaching the enzyme must be stable to
oxidation and both processes must be operable under the same conditions of time,
temperature and pH. Similarly, other products added, such as surfactants for wetting and
detergency or metal ion sequestering agents, must fulfil their primary task and not interfere
with other functions, while themselves being unaffected by the conditions.
Scour–dye operations in particular need careful planning backed by detailed knowledge,
especially in regard to the effects of desorbed impurities on the stability and exhaustion of
the dyes. In the combined scouring and dyeing of wool or nylon with acid dyes the detergent
should ideally function as a levelling agent. The crucial factor in scour–dye processes
involving disperse dyes is stability of the dye dispersion in the presence of the detergent and
desorbed impurities. This becomes increasingly critical at higher temperatures (say 130 °C)
and with difficult application conditions, such as tightly woven fabrics on beams. Hence
such operations are more frequently used in jet dyeing, for example. Incompatible
surfactants and greasy soil can have disastrous effects on dye dispersions, leading to
agglomeration of dye particles and deposition of coloured oily stains on the substrate, with
extensive breakdown of the dispersion in severe cases. Surfactants can have a highly
selective effect on the rate and extent of exhaustion of disperse dyes and must therefore be
selected accordingly.
In spite of these difficulties successful combined processes are indeed operated, although
they require vigilant monitoring. The promised economies must not be squandered by lost
processing time and costs of damage and reprocessing. It would be surprising if the
manufacturers of dyes and chemicals were more enthusiastic than textile processors about
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
such combinations, in view of their understandable caution about guaranteeing the
behaviour of individual products in circumstances for which they are not intended. Now
that environmental and economic factors limit research into new products, the fusing of
normally sequential processes into a single one will remain a worthwhile technical and
economic goal.
Possibilities for combining the three main preparatory processes for cotton (desizing,
scouring and bleaching) remain economically attractive. One process for achieving this
involves treatment of loomstate cotton with an aqueous solution containing 3 g/l sodium
chlorite, 1 g/l hydrogen peroxide, 2 g/l nonionic wetting agent and 10 g/l disodium hydrogen
phosphate for 90 minutes at 95 °C, pH 10 and 20:1 liquor ratio [313]. In another process
the fabric is impregnated with 20 g/l sodium chlorite, 0.05 g/l potassium permanganate and
2 g/l nonionic wetting agent, followed by treatment for 30–60 minutes at 90 °C and pH 10
[314]. The concentrations of the oxidants, the pH and time of treatment were critical,
however. In a modification of this process, impregnation with 20 or 30 g/l sodium chlorite,
respectively 3 or 1 g/l potassium chromate and a nonionic wetting agent is followed by
treatment for 90 minutes at 90 °C and pH 6 [315]. All these processes are subject to the
usual criteria regarding machinery that is resistant to corrosion by sodium chlorite, whilst
the discharge of effluent containing chromate has environmental implications in many
countries.
An AOX-free alternative [316] is impregnation with 4 g/l hydrogen peroxide, 8 g/l urea
and 2 g/l nonionic wetting agent, then treatment for 60 minutes at 95 °C, pH 8 and 20:1
liquor ratio [316]. This results in a bleached fabric with excellent wettability and without
serious fibre degradation. The urea interacts with hydrogen peroxide to form an unstable
complex, which then decomposes to form hydroxyl and perhydroxyl radicals, according to
Scheme 10.28 [316]. Urea exhibits undesirable environmental characteristics in some
respects, however.
In all the above processes, the optimised quantities of the chemicals indicated will be
specific to the substrate quality evaluated. They would require further re-optimisation for
each substrate to take account of the type and concentration of size, the presence of other
impurities and the degree of natural yellowness. In particular, the amount of oxidant will
need to be adjusted to give the optimum balance between oxidative desizing and the degree
of bleaching required.
A commercially established system for the combined desizing, scouring and bleaching of
cellulosic fibres and blends is the Raco-Yet (Kieinewefers) system [317,318]. The fabric is
impregnated successively with 23–26 ml/kg sodium silicate (38°Bé), 32–40 g/kg sodium
hydroxide (100%), 20–26 ml/kg Cottoclarin AS and 40–65 ml/kg hydrogen peroxide (50%)
before steaming. Cottoclarin AS is designed specifically for this process, having excellent
wetting properties, freedom from foaming, high dispersing, emulsifying, complexing and
detergency powers. The process is applicable to a variety of size polymers and their mixtures,
including starch, hydroxypropyl starch, poly(acrylic acid), poly(vinyl alcohol),
carboxymethylcellulose and polyesters. Desizing, boiling-off and bleaching can be achieved
in 1–3 minutes. Savings in processing costs and exceptional flexibility of operation are
claimed, since recipe changes can be made in less than one minute. It is essential to carry
out a thorough analysis of each substrate and to adjust the recipe and processing speed
accordingly. Table 10.35 gives an indication of the flexibility of the process together with
typical results [318].
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PREPARATION OF SUBSTRATES
Table 10.35 Examples of application of the Raco-Yet system to cellulosic fabrics and blends [318]
Polyester/
cotton
65:35
170 g/m2
Cotton/
polyester
88:12
290 g/m2
Polyester/
viscose/
linen
70:20:10
185 g/m 2
Starch
None
Fabric
(130%
pick-up)*
100%
Cotton
320 g/m2
100%
Cotton
shirting
190 g/m2
Size type
Starch
None
PVA
26
26
23
26
26
40
40
32
40
40
26
26
20
26
26
65
65
40
65
65
Sodium silicate
(38 Be) ml/kg
Sodium
hydroxide
(100%) g/kg
Cottoclarin
AS ml/kg
Hydrogen
peroxide
(50%) ml/kg
Treatment
time (min)
Running
speed
(m/min)
Evaluation
2
1.5
60
Grey
Absorptivity
(mm)
After
15 seconds
0
After
30 seconds
0
After
60 seconds
0
Whiteness:
%REM
56.0
Berger
16.1
Degree of
polymerisation 2760
1.5
80
Treated
Grey
3
80
Treated
Grey
1.5
40
Treated Grey
80
Treated Grey
Treated
15
0
18
0
15
0
32
0
18
20
0
24
0
20
0
41
0
25
25
0
31
0
25
0
52
0
33
90.2
104.5
112.5
152.6
50.1
43.3
80.3
64.0
2280
86.1
75.7
61.8
20.1
81.2
70.8
3080 2670
* concentrations are per kg of fabric
It is possible to use an enzyme with hydrogen peroxide in a combined desize–bleach but
great care is needed in selection of the enzyme and optimisation of the concentrations
[319].
Mercerising (high concentrations of alkali; cold) is particularly difficult to combine with
desizing, scouring and peroxide bleaching (lower concentrations of alkali; hot).
Nevertheless, a combined one-stage desize, scour, bleach and slack mercerise process has
been attempted [320]. This involves impregnation of the fabric in a 10–30% sodium
hydroxide liquor containing 20 g/l hydrogen peroxide and 50 g/l trichloroethylene for
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
3 minutes, the results being dependent on impregnation temperature (20–100°c). The fabric
is squeezed and (without predrying) is heated to 120 °C, the results being influenced by the
duration of the heating step. It is claimed that, with a wet immersion treatment for
3 minutes at 40 °C and subsequent treatment for 30 seconds at 120 °C, the physical
properties of the treated cotton are similar to those from a conventional two-stage approach.
The use of trichloroethylene, of course, is highly sensitive both environmentally and as a
health hazard (toxic to the liver).
An electrochemical system combining scouring, mercerising and bleaching has been
proposed. It is a non-polluting method based on an electrochemical cell, the cathode of
which produces the base to mercerise and bleach, whilst the anode produces an acid to
neutralise the base remaining after mercerisation [321].
A mercerising-type effect and bleaching of viscose fabrics can be achieved simultaneously
using liquid ammonia, the bleaching agent being a peroxidated urea derivative that causes
less damage than conventional oxidants, together with improved dye absorbency [322].
Mercerising has been combined with vat dyeing in a continuous process [323]. Cotton
fabric is padded with an aqueous suspension of a vat dye in a sodium chloride solution
containing caustic soda for mercerising. After drying, the dyeing is developed by padding in
an alkaline solution of reducing agent, steaming and soaping.
Further combined processes involving dyeing include:
(1) Dyeing cotton yarn with selected direct dyes and simultaneous bleaching with peroxide.
It is claimed that the peroxide also increases the colour yield [324]
(2) Combined dyeing and easy-care finishing of cotton using bis-nicotinotriazine reactive
dyes and DMDHEU in a pad–dry-HT steam process [325]
(3) Combined dyeing and finishing of polyester/cotton using a liquid ammonia medium
[326]
(4) Dyeing polyester/cotton with reactive and disperse dyes and imparting a crease-resist
finish [327].
10.6 DISPERSING AND SOLUBILISING AGENTS
10.6.1 Dispersing agents
Dispersing agents are substances that promote the more or less uniform and stable
suspension of relatively small particles in a given matrix. We are concerned here with the
most common type of dispersion encountered in textile coloration, the solid-in-water
systems typified especially by disperse dye technology, as well as the insoluble forms of vat
and sulphur dyes. Pigments are also extremely important examples of solid-in-liquid
dispersions but form a specialised case fully dealt with in Chapter 2. Also excluded from this
section are other systems that depend mainly on a large increase in viscosity for their
suspending action; these are more appropriately dealt with in section 10.8. An in-depth
account is available [328], covering in particular the essentials of colloid science as
applicable to dispersions, the preparation of dispersions (solid-in-liquid and liquid-liquid) as
well as foams (section 10.11). An extensive account of the uses of dispersions is also
available [329]; this includes pigments and the incorporation of colorants in polymer melts
but is otherwise concerned with non-textile applications.
Any formulation of solid particles in a liquid medium is more or less unstable as a result of
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DISPERSING AND SOLUBILISING AGENTS
637
(a) gravitational settling effects and (b) attractive forces between particles tending to lead to
particles adhering, thus increasing the susceptibility of the system to gravitational effects.
Two aspects need to be considered: the initial preparation of the dispersion, and its
subsequent stabilisation during storage and use; only rarely will one agent satisfy the needs
of both. Individual dyes vary widely in their requirements and any given dye may require
different treatments depending, for example, on its microcrystalline form and the application
processes for which it is intended. Therefore specific types of dispersing agents, or mixtures
of them, are frequently needed to obtain the optimum dispersing action. There are two main
groups of such agents:
(1) Surfactants, mainly of the anionic and nonionic types
(2) Water-soluble polyelectrolytes, most usually of the anionic type.
The chemistry of surfactants has been described already. They usually play a subsidiary role
in dispersions involved in textile coloration. The polyelectrolytes may be conveniently
divided into two categories:
(1) Acrylic acid copolymers, sulphonated polyvinyl compounds, alginates and
carboxymethylcellulose. Some of these may require addition of other chemicals (e.g.
alkali) in order to ensure aqueous dissolution. These polymers are less important as
dispersing agents for disperse, vat and sulphur dyes than in areas such as pigment
applications and as thickeners in textile printing or migration inhibitors in continuous
dyeing (section 10.8).
(2) The condensation products of formaldehyde with arylsulphonates or lignosulphonates,
these being the major types of polyelectrolyte of interest in the manufacture and use of
disperse dyes [330,331].
As with surface-active agents, the detailed chemistry of these products is a good deal more
complicated than is indicated by the nominal structures frequently quoted. Most
commercial products are mixtures of which the nominal structure represents a basic type
only. Indeed, the detailed chemistry of the more complex products is still only partially
understood. These provisos should be borne in mind when considering the structures given
below.
The sulphonated aromatic condensation products form a large and varied group, since
formaldehyde will condense with many aromatic compounds [330], including sulphonated
arylamines, phenols and aliphatic ketones; the range of commercially important products is
relatively limited, however. One of the oldest is the condensation product of naphthalene-2sulphonic acid and formaldehyde (10.99), in which the degree of condensation is thought to
HOCH2
CH2
SO3Na
CH2
SO3Na
CH2
SO3Na
CH2OH
SO3Na
10.99
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638
CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
OH
OH
OH
CH2
HOCH2
OH
CH2OH
CH2
CH2
CH2SO3Na
n
CH2SO3Na
10.100
OH
OH
CH2OCH2
HOCH2
OH
CH2OH
CH2OCH2
CH2SO3Na
CH2SO3Na
n
CH2SO3Na
10.101
OH
HO
CH2SO3Na
CH2
HO
CH2SO3Na
CH2
NaO3S
10.102
OH
HO
CH2SO3Na
CH2
HO
CH2
NaO3S
10.103
SO3Na
correspond to between two and ten naphthalene units, although the quantitative
distribution of the condensates varies widely. Also of importance are the similarly structured
condensation products of (a) phenols with formaldehyde and sodium sulphite (structures
10.100 and 10.101, depending on molar ratios) and (b) p-cresol and 2-naphthol-6-sulphonic
acid with formaldehyde and sodium bisulphite (10.102 and 10.103). The types represented
by structures 10.100–10.103 are also widely used as syntans (section 10.9.4).
The lignosulphonates comprise a variable group of products derived from wood pulping.
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DISPERSING AND SOLUBILISING AGENTS
639
Their highly complex structures are only partially understood, although enough is known to
enable representative structures to be proposed showing the major functional groups. There
are two distinct processes whereby lignins are extracted. The first is an acidic digestion
process in which the wood is pulped with sulphite or bisulphite. The second is an alkaline
process, the so-called kraft process, in which the wood is treated [331] with sodium sulphide
in autoclaves at pH 13 and 160–175 °C. The product is precipitated by careful addition of
acid, filtered off and washed free from inorganic ions, then sulphonated to increase its
aqueous solubility. Structure 10.104 has been proposed as being representative of kraft lignin
prior to sulphonation [331]. An alternative partial representation of a lignosulphonate
structure [330] is that of structure 10.105, which shows sulphonation as having taken place
mainly at the CH=CH link. The molecular configuration is such as to give spherical
particles. The final nature of the product varies enormously depending, amongst other
things, on:
(a) purity, especially the content of electrolytes such as sodium sulphate
(b) the number of hydroxy groups present
(c) the degree of sulphonation
(d) the relative molecular mass (2000–1 000 000) and its distribution.
This wide variability need not be a disadvantage provided it can be controlled to give
reasonable consistency from batch to batch, since this enables products to be designed to
give the optimum efficiency for the particular application concerned. The kraft lignins
provide greater scope for modification, particularly of the degree of sulphonation and
molecular size, and are also much more amenable to production in low-electrolyte forms
[331]. Further information on the modification and behaviour of lignosulphonate dispersing
agents is given in section 12.6.1.
The basic similarity between these major types of dispersing agents and the surface-active
agents discussed earlier lies in their amphiphilic nature (the possession of a combination of
OH
OCH3
HOCH2
CH2
CH
OH
HS
CH3O
CH2OH
CH
OCH3
CH
H2C
OH
HOCH2CH2
CH
O
HC
CH
OCH3
OCH3
OCH3
HOOC
CH2
OH
HC
CH
HC
639
O
O
CH
OCH3
CH2
OCH3
chpt10(2).pmd
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C
HO
O
CH2
CH2
OH
CH2
HO
O
H2C
CH
HC
10.104
15/11/02, 15:44
O
640
CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
CH3O
SO3Na
HOCH2
O
HOCH2
O
CH
OH
NaO3S
CH
OCH3
HOCH2
CH
O
OH
CH
SO3Na
CH
O
CH
[lignin]
CH
H2C
CH
[lignin]
HO
SO3Na
CH3O
OCH3
CH
CH
HOCH2
OH
10.105
hydrophobic and hydrophilic moieties). The polyelectrolytes, however, are of much larger
molecular size than conventional surface-active agents.
When converting a conglomerate mass of relatively coarse particles into an aqueous
dispersion (as in the manufacture of disperse dyes) there are two broad phases to be
considered. In the first phase the dual aim is that of mechanically grinding the particles
down to the required size and of obtaining as narrow a range of particle size as possible
during the actual preparation of the dispersion. Maintaining these particles in a stabilised
suspension constitutes the second phase.
Particle size alone is not the main criterion; its distribution is equally important. This is
because all dispersions are metastable. As well as tending to settle as a result of gravitational
forces, there is also a thermodynamic tendency towards a reduction in the free energy of the
system. This is manifest as a continuing increase in particle size leading ultimately to a
severe deterioration in the dispersion quality (as already mentioned). Smaller particles tend
to be attracted towards larger particles, with which they then form even larger particles. The
opportunity for particle growth is therefore much less when all the particles are of similar
size than when the range of sizes is large.
The actual comminution of the coarse particles is usually carried out mechanically – for
example, by grinding an aqueous slurry of the colorant in a rotatory mill containing
relatively large, hard and inert grinding media such as pebbles. The process is facilitated by
efficient wetting of the particles and lowering of interfacial tension. It is therefore preferable
to add dispersing agents that have some surface activity and good wetting properties.
Microfissures are created as the particles are broken down. The surface-active properties of
the dispersing agent enable it to penetrate these microfissures, hindering agglomeration and
facilitating further comminution. The amphiphilic agent becomes adsorbed and oriented on
the surfaces of the particles, providing a protective sheath of like repellent charges, the
forces of which eventually exceed the forces of attraction between the particles and thus
stabilise the dispersion. This protective sheath becomes of critical importance during the
second phase by stabilising the suspension of particles, both in storage and in subsequent
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641
use. Hence different dispersing agents may well be required to provide optimum dispersing
and stabilising power in the two phases.
The dual amphiphilic nature of the polyelectrolyte condensates and lignosulphonates
described above, with their hydrophobic groups juxtaposed with many polarisable ionic
groups, renders them highly efficient dispersing agents. It is necessary, however, to make a
careful choice from the various grades of dispersing agents available with respect to their
molecular size and charge distribution. Optimum dispersing ability depends on matching the
steric, hydrophobic and ionic properties of the dispersing agents relative to the
characteristics of the particles to be dispersed. The forces that may be operative in
adsorption of surfactants onto disperse dye particles have been listed [330] as ion exchange,
ion pairing, hydrogen bonding, van der Waals dispersion forces, polarisation of π–electrons
on aromatic systems and hydrophobic interaction. It follows that anionic and nonionic
surfactants, judiciously selected, may be used in conjunction with the polyelectrolytes to aid
the dispersing mechanism [330].
10.6.2 Solubilisation
It is necessary to differentiate between simple solutions and the process of solubilisation in
colloidal solutions. A non-colloidal solution is a homogeneous single-phase system of a solute
dissolved in a solvent, examples being an aqueous solution of sodium chloride or a solution
of methylnaphthalene in acetone. The term solubilisation refers to the homogeneous mixing
of an otherwise insoluble agent, the solubilisate, into a liquid medium by addition of a
solubilising agent, invariably a surfactant. This agent acts as an amphiphilic bridge between
solubilisate and medium. For example, methylnaphthalene will not dissolve to any
significant extent in water, but its solubilisation in water to give an apparently clear colloidal
solution can be brought about by the use of a surface-active agent such as nonylphenol
poly(oxyethylene) sulphate. Hence the distinction between solution and solubilisation in
non-colloidal and colloidal situations generally.
Solubilisation can be viewed as one end of a reversible colloidal continuum that begins
with wetting and proceeds through dispersion or emulsification to solubilisation, each of
these stages being characterised by the size and nature of the particles. In this sense
solubilisation is an extension of emulsification (or dispersion) in which the proportion of
surfactant has been increased to the level where the discrete droplets (or particles) that
characterised the emulsion (or dispersion) have become completely absorbed into the
surfactant–medium phase. In some cases the surfactants used to produce an emulsion (or
dispersion) may need modification (a change of hydrophile–lipophile balance) before
complete solubilisation can be brought about. Similarly, if a solubilised system is diluted by
addition of the liquid medium, a point will usually be reached at which the solubilised
system changes to an emulsion (or dispersion). It is indeed possible to have all the stages of
wetting, emulsification (or dispersion) and solubilisation present at the same time to
different degrees.
It is beyond the scope of this section to discuss the complex physico-chemical parameters
of solubilisation in detail. Useful relevant works of reference are available [332–335]. It
follows, however, that since solubilisation is essentially an extension of emulsification (or
dispersion), the factors discussed in section 9.8.3 in regard to emulsification are also
pertinent to solubilisation. Theory in this area is a useful guide but much still depends on
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
empiricism. As in emulsification, each system to be solubilised will present its own specific
requirements with regard to the type and amount of surfactant(s) required. In some cases
solubilisation is aided by adding a small amount of a water-miscible solvent such as an
alcohol or glycol, although the environmental and the health and safety aspects of such
additions nowadays requires careful consideration. Conversely, the presence of electrolytes
can have a deleterious effect. Temperature too can be important; a system that is an
emulsion (or dispersion) at one temperature may become solubilised at a different (usually
higher) temperature.
There are two main approaches to solubilisation in textile wet processing. One is the
deliberate preparation of a solubilised product for use as an auxiliary agent, as in the
proprietary carriers formulated for dyeing polyester with disperse dyes. The other is as a
concomitant of the process itself, as in the solubilisation of fats and oils during scouring
processes and in the disperse dyeing process. In both situations more than one stage of the
colloidal continuum of wetting-dispersion/emulsification-solubilisation may be present at
any one time.
10.7 LEVELLING AND RETARDING AGENTS
Level dyeing problems can be divided into two broad categories [336]:
(1) Gross unlevelness throughout the material: this type of unlevelness is primarily related
to the dyeing equipment or process; the substrate is often uniform in properties, both
chemically and physically
(2) Localised unlevelness: this is primarily related to physical and/or chemical nonuniformity of the substrate; typical examples are barriness in nylon or polyester dyeing
and skitteriness in wool dyeing.
There are also two fundamental mechanisms that can contribute to a level dyeing:
(1) Control of rate of exhaustion of dye so that it is taken up evenly
(2) Migration of dye after initially unlevel sorption on the fibre.
Either or both of these mechanisms may operate to a greater or lesser extent in a given dye–
fibre system, although a general trend towards better fastness properties has dictated the use
of dyes that show low, if any, propensity to migration, thus placing the emphasis for level
dyeing on the control of exhaustion rate. Physical factors such as temperature and frequency
of liquor/substrate contact (governed by rate of liquor circulation in a jet, beam or package
machine) can be used to exert some degree of control over these mechanisms. Slower rates
of heating usually favour more even uptake of dye and higher temperatures tend to increase
migration or diffusion. In some cases level dyeing can be influenced by dyebath pH and/or
the presence of electrolytes. This section, however, is more concerned with the control of
levelness by means of chemical auxiliaries, generally known as levelling or retarding agents.
Since levelling agents are invariably surfactants, they may be anionic, cationic, nonionic
or amphoteric in nature. Sometimes combinations of these are used. The chemical structure
of commercial products is seldom revealed, however; hence only general principles can be
covered here. The main mechanisms by which levelling agents operate [337–341] are as
follows:
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643
(a) nonionic agents usually form water-soluble complexes with the dye, some degree of
solubilisation being involved
(b) ionic agents are primarily dye- or fibre-substantive; in the former case they tend to
form complexes with the dye and there is competition between the levelling agent and
the fibre for the dye, while in the latter case the competition is between levelling agent
and the dye for the fibre.
In complex formation the principle, as far as levelling action is concerned, is usually the
same irrespective of whether nonionic or ionic agents are used, although the mode of
complexing is different. The attractive forces between agent and dye create a
counterbalancing mechanism against dye–fibre attractive forces, restraining the uptake of
dye by the fibre. As the temperature of the dyebath increases the complex gradually breaks
down, progressively releasing the dye for more gradual sorption by the fibre. Clearly, for an
effective levelling agent that functions by this mechanism the stability of the agent–dye
complex, governed by forces of attraction between agent and dye, is crucial. If these forces
are so weak that a relatively unstable complex is formed, restraining or levelling action may
be inadequate. On the other hand, strong forces of attraction may result in a complex that is
too stable to break down as the temperature rises, so that the dye is effectively entrapped by
the agent in the solution phase and is not available for sorption by the fibre. The objective
therefore is to formulate the levelling agent so that it forms a dye complex of optimum,
rather than maximum, stability relative to the conditions of application. This is done by
adjusting the hydrophilic–lipophilic balance of the surfactant. The problem lies in the fact
that the dye–agent interaction is so specific that different members of a range of dyes may
each require a different balance. Hence commercial levelling agents may contain more than
one surfactant.
A difficulty that arises with ionic levelling agents is that they may form an insoluble
precipitate with ionic dyes of opposite charge; this can be obviated in various ways. In the
first instance attention should be paid to the concentration of the surfactant; where initial
addition of surfactant to the dyebath causes precipitation of the agent–dye complex, further
additions of surfactant often lead to its solubilisation. Alternatively, a further surfactant may
be added to solubilise the complex; a nonionic agent will not itself react with either the dye
or the original ionic surfactant to form a further insoluble complex, but its addition may
further complicate the relationship between the hydrophobic–hydrophilic balance of the
ionic agent and the dyes to be complexed. Due regard also needs to be paid to the cloud
point of the nonionic agent under the conditions of use. This does not preclude the use of a
relatively hydrophobic nonionic agent, since its cloud point may be effectively raised in the
presence of the ionic agent (subject to possible interference from any electrolytes or solvents
present in the dyeing system). Similarly, if there is any danger from the cloud point of a
nonionic surfactant used as the primary levelling agent (as with disperse dyes, for example),
a suitable anionic surfactant may be added to effectively raise the cloud point, again paying
due attention to any effect the anionic agent may have on the complexing–liberating
performance of the nonionic agent.
The third method of obviating precipitation of an ionic agent–ionic dye complex is to
choose what effectively amounts to a ‘modified’ ionic agent. Ethoxylated anionic and
ethoxylated cationic agents are particularly useful in this respect. The ethoxylation tends to
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
reduce the ionic character of the agent, thus giving rise to weaker but more controllable
forces of attraction for dye ions, and the oxyethylene chain can further function as a
dispersing–solubilising moiety for the agent–dye complex. In a sense this is basically similar
to using a mixture of ionic and nonionic agents as described above except that a single agent
is used, thus facilitating the aim of obtaining the optimum complexing–liberating balance.
Dye–agent complexes of lower net charge are formed when the ionic agent is added to
the ionic dye solution. As the concentration of agent is increased a point is reached at
which all the dye is complexed and its ionic charge has been neutralised. Beyond this point,
as more agent is added, the agent–dye complex takes on the charge of the complexing agent
(i.e. the opposite to that of the dye itself). This brings about a change in the partition
coefficient of the complex between water and organic solvents [336], modifying the
electrical and solution properties of the dye and so altering its affinity for the fibre.
Fibre-substantive levelling agents are usually of the same ionic type as the dye, that is
anionic agents are used with anionic dyes and cationic agents with cationic dyes, the aim being
to create a system in which levelling agent and dye both compete for the sorption sites in the
fibre. Just as the complexing type of levelling agent has to be carefully chosen so as to obtain
the optimum complexing–liberating properties, so must the competing type of levelling agent
be chosen such that its ionic power gives the optimum level of competition relative to the dye–
fibre system concerned. If the ionic power is too weak, it will not function as an effective
levelling agent; if it is too strong, it may exert blocking effects, preventing sorption of the dye.
Ideally the balance should be such that the smaller surfactant ions are adsorbed by the fibre
more quickly than are the larger dye ions, but the agent–fibre interaction needs to be weak
enough to permit subsequent displacement of the surfactant ions by the dye ions.
As the forces of dye–fibre interaction vary from one dye to another, the ionic power of the
levelling agent must be suitably adjusted through its hydrophilic–hydrophobic balance to give
the optimum properties. This can be done either by careful choice of a single surfactant or by
the use of mixtures, which has gained prominence in recent times. For example, the strongly
anionic character of a long-chain alkyl sulphate or sulphonate can be modified (toned down)
by mixing it with a more weakly anionic poly(oxyethylene) sulphate or with a nonionic agent.
Some levelling agents operate both by complexing and by competition. For example, in
the application of acid dyes a weakly cationic agent may be used to complex with the dye
and an anionic agent may also be used as a competing agent. This combination is more
versatile because unlevelness may arise from different mechanisms. Unlevelness arising from
process or equipment variables can often be controlled by dye–agent competition, whereas
localised dye uptake variations generally respond better to dye–agent complex formation.
Evidently, in this combined system the balance of properties is highly critical. In particular
the oppositely charged surfactants must not mutually precipitate; hence the more weakly
ionic ethoxylates are of particular interest, since the oxyethylene assists solubilisation of any
complex so formed. A purely nonionic agent may also be used to prevent coprecipitation of
the ionic types. Amphoteric agents, in a sense, fall within this combined system.
Theoretical considerations are clearly useful in formulating suitable levelling agents.
Nevertheless, a good deal of empiricism is always involved in formulating well-balanced agents
for specific dye–fibre systems. Table 10.36 shows the general types of levelling agents now
being offered and their uses; more detail is given in Chapter 12 relative to each class of dye.
Many, but not all, levelling agents promote migration of dye in addition to retarding
dyeing, such agents will obviously be a further aid to level dyeing. In some cases, however,
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645
Table 10.36 Levelling agent types and their uses
Recommended for use with
Type of levelling agent
Substrate
Dye classes
Nonionic
Cotton
Wool, nylon
Polyester
Direct, vat, azoic
Milling acid, metal-complex
Disperse
Nonionic/anionic
Polyester
Wool, nylon
Disperse
Milling acid, metal-complex
Nonionic/cationic
Wool
Acid, metal-complex, reactive, chrome
Anionic
Wool, nylon
Cotton
Polyester
Acid
Direct
Disperse
Weakly anionic
Polyester
Disperse
Anionic/cationic
Wool, nylon
Acid, metal-complex
Cationic
Acrylic
Wool, nylon
Basic
Acid, metal-complex, reactive
Weakly cationic
Wool, nylon
Acid, metal-complex, chrome
Cationic/polymeric
Cotton
Vat, sulphur
Amphoteric
Wool
Acid, metal-complex, reactive, chrome
higher concentrations of levelling agent are needed to obtain significant migration and this
may interfere unduly with dye sorption. Levelling agents are also widely used as stripping
agents, either alone for non-destructive desorption or together with reducing agents such as
sodium dithionite for destructive stripping. When used for this purpose, their hydrophilic–
hydrophobic balance is not as critical as when they are used simply as levelling agents. Thus
higher concentrations are often used in order to maximise rather than optimise desorption
of the dye.
It should not be overlooked that electrolytes can play an important part in levelling and
retardation. In recent times the use of bolaform electrolytes (section 10.1), cyclodextrins
(section 10.3.1) and liposomes (section 10.3.4) as complexing agents has been proposed.
10.8 THICKENING AGENTS, MIGRATION INHIBITORS AND
HYDROTROPIC AGENTS USED IN PRINTING AND CONTINUOUS DYEING
Most, if not all, textile printing and continuous dyeing processes entail the use of auxiliaries
that considerably increase the viscosity of the application medium compared with
conventional batchwise dyeing processes, the aim being to facilitate and stabilise the local
application of colour prior to its actual fixation to the fibre. Such auxiliaries are generally
known as thickening agents in printing and as migration inhibitors in padding operations.
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
They are characterised by undergoing marked macromolecular swelling in solution due to
solvation (hydration in aqueous systems). While the principal role of thickening agents is to
increase the viscosity of print pastes or pad liquors, certain other properties are also of
importance, such as stability and rheology of the print paste, adhesion and brittleness of the
dried thickener film, the effect on colour yield and penetration, ease of preparation and
removal, and not least cost. A succinct account of these factors and of their
interrelationships is available elsewhere [342]. The following discussion is restricted to
rheology of print pastes, since an understanding of the basic principles of fluid flow is
essential in appreciating the fundamental mechanism taking place during printing. The
number of recent publications dealing with rheology, particularly in relation to specific types
of thickening agents, is evidence of its importance, both as a concept and in current
research [343–352].
The essential fact concerning thickening agents is that they are viscoelastic, exhibiting
properties associated with both fluids and solids and showing what is known as pseudoplastic
(non-Newtonian) flow behaviour [352]. This is best understood by comparison with simple
liquids such as water or alcohol, which show Newtonian flow behaviour. The apparent
viscosity of Newtonian liquids does not change when a shear stress is applied (curve A in
Figure 10.48). All thickening agents, however, are highly viscous in a static state but
apparently show reduced viscosity when a shearing force is applied. This is indeed their
modus operandi: they must flow under shear to allow transfer through the screen, then
resume high viscosity when the shear is removed so that colorants remain where they have
been deposited.
Rate of shear/s–1 × 103
10
A
8
B
C
D
6
4
A
B, C
D
2
Newtonian
Shear thinning
Thixotropic
0
5
10
15
Shear stress/kPa
Figure 10.48 Typical flow curves demonstrating behaviour of viscous liquids [342]
Most thickening agents are of the shear thinning type represented by curve B, the
apparent viscosity progressively decreasing as the shear rate is increased. It is important that
this change is reversible, viscosity returning to its original level as soon as the shear is
removed. In some cases, shear thinning may not begin until a certain critical shear has been
applied (curve C). Thixotropic fluids (curve D) show time-dependent effects in that
apparent viscosity depends on both the rate and duration of shear, the return to original
viscosity being delayed. The opposite of shear thinning is shear thickening, often referred to
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647
as dilatant behaviour; such behaviour is clearly not suitable for textile printing. An
alternative method of representing flow behaviour is shown in Figure 10.49.
105
Apparent viscosity/mPas
Dilatant
104
Newtonian
103
102
Shear thinning
10
Shear stress/Pa
Figure 10.49 Relationships between apparent viscosity and shear stress [350]
Print pastes may be thickened by any of the following methods [342]:
(a) a relatively low concentration of a long-chain thickening agent
(b) a relatively high concentration of a shorter-chain thickener or one having a highly
branched structure
(c) an emulsion
(d) a finely dispersed solid such as bentonite (derived from clay).
The first two methods, particularly the first, are most frequently used today; combinations of
these methods are also possible.
Thickening agents can be of natural or synthetic origin. Various natural gums and
starches have been used traditionally in many printing styles. The materials from which they
are extracted are valuable sources of foodstuffs, so availability and cost can depend on
fluctuating demand from the food industry. The properties required of an ideal thickener
can be summarised as follows [352]:
(1) Compatibility with colorants and other auxiliaries
(2) Adequate solubility and good swelling properties in cold water
(3) Good washing-off properties
(4) High degree of purity and conformity to standard
(5) Non-dusting
(6) Biodegradable
(7) Non-toxic
(8) Manufactured from replenishable raw materials.
10.8.1 Natural thickeners
Natural thickeners are derived from plants by extraction from part of the plant itself or from a
plant secretion; their biosynthesis is now a possibility. These products are generally
polysaccharides and are thus closely related to cellulose. They consist of homo- or
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
heteropolymers of simple hexoses, most commonly glucose, mannose or galactose [353].
Linear and branched segments are normally present, the degree of branching being important
in relation to the technical properties of the product. Polysaccharides bear some structural
similarities to anionic polyelectrolyte dispersing agents (section 10.6.1) and sizing agents
(10.5.2). In particular, the nature of the side groups (mainly, though not always hydroxy or
carboxyl groups) has a decisive effect on viscosity and other technical properties. Some, such
as native starch, are used as extracted from their sources; others, such as starch ethers, are
derived by introducing substituents or undergoing controlled hydrolysis to lower their viscosity.
As with other polymeric auxiliaries already discussed, their detailed structure is still not
completely understood and the formulae given are only indicative of their structures.
Although native starch is less important nowadays as a thickening agent for textile
printing, some starch derivatives still make a significant contribution. Starch has two
components, both of which are made up of linked α-glucoside units (10.106). In amylose,
which accounts for some 20–30% of the polymer and has a relative molecular mass in the
range 2–6 × 105 the α-glucoside units are linked in a linear 1,4 arrangement (10.107).
Cellulose (10.108), by contrast, consists of β-glucoside chains. In amylopectin (Mr 4.5 × 104
to 4 × 105) the linear α-1,4-linked main chain is randomly branched at the 6-position every
15–30 glucose units to give an α-1,6-anchored side chain (10.109).
CH2OH
6
O
OH
OH
1
3
OH
HO
O
5
4
2
10.106
α-Glucoside unit
CH2OH
CH2OH
CH2OH
OH
OH
OH
O
O
HO
OH
O
O
O
OH
OH
OH
n
10.107
Amylose
CH2OH
CH2OH
CH2OH
O
O
O
O
O
HO
OH
OH
OH
OH
OH
n
10.108
Cellulose
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OH
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THICKENING AGENTS, MIGRATION INHIBITORS AND HYDROTROPIC AGENTS
CH2OH
O
O
O
OH
CH2OH
OH
CH2OH
CH2
O
O
O
OH
O
O
O
O
OH
OH
OH
OH
OH
10.109
Amylopectin
The amylose component is substantially crystalline, forming helical structures that uncoil
in an aqueous solution. It can also aggregate to give a gel or precipitate, an undesirable
phenomenon known as retrogradation. Amylose is completely hydrolysed by the β-amylase
enzyme. Amylopectin is substantially amorphous, having a globular structure that can
expand considerably in aqueous solution. Its branched chains give rise to a much more
stable solution, substantially free from retrogradation, and it is much more resistant to the
action of β-amylase. Starches containing little or no amylose are known as ‘waxy starches’.
The properties of starch can be improved from a printing viewpoint by conversion to
British Gum (10.110). This is done by a dry roasting treatment at 135–190 °C, accelerated
by trace quantities of acid, to give random hydrolysis of the 1,4-links to decrease the chain
length but with the formation of 1,6-links (branching). The effect is to increase the
solubility and stability although reducing characteristics, which can affect certain susceptible
dyes, are enhanced by formation of more aldehyde end groups. Control of the hydrolysis and
branching reactions yields a varied range of products.
CHO
CH2OH
O
O
CH2OH
O
O
O
OH
OH
OH
CH2
OH
OH
CHO
O
O
O
O
O
OH
O
O
HO
OH
OH
OH
10.110
British gum
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
Besides roasting, other methods of modifying starch are available. Etherification and
esterification, to give starch ethers and starch esters, are both practised although the ethers,
being resistant to hydrolysis in acidic or alkaline media, are much the more important as
thickening agents for textile printing. The starch may be first partially decomposed before
etherification and the degree of etherification itself may be varied. The most important
products are the carboxymethyl (10.111), hydroxyethyl (10.112) and methyl (10.113)
starches (the structures illustrated use the glucose unit as a model, showing the primary
hydroxy group substituted). The degree of alkylation is said to be low or high depending on
whether it is less or greater than 0.3 substituents per glucose (or other) unit; the products
are termed modified starches if the degree of substitution is low and starch derivatives if it is
high. Crossbonded starches can be obtained by treating, for example, a starch ether of low
degree of substitution with bifunctional agents such as ethylene oxide, propylene oxide,
epichlorohydrin or phosphates. The corresponding derivatives of cellulose can also be made
and used as thickening agents if the chain length is appropriate. The steric hindrance effect
of the substituents gives thickening agents of improved all-round properties and certain
derivatives have ousted their parent products in terms of commercial importance.
CH2OCH2COOH
CH2OCH2CH2OH
O
O
OH
HO
OH
CH2OCH3
OH
OH
HO
OH
10.111
O
OH
OH
HO
OH
10.112
OH
10.113
Galactomannans are another source of natural thickeners. Structurally related to starches
they are polysaccharides composed of main-chain mannose and side-chain galactose units as
in 10.114. Typical values are: locust bean gum (m = 3, n = 375) and guar gum (m = 1, n =
440). The distribution of galactose units varies with the source, as shown schematically in
10.115 [354]. Amongst other things, this distribution has an influence on ease of
dispersibility. For example, warm water is required to effect complete dispersion of locust
bean gum (the 1,4 form) but guar gum (the 1,2 form) disperses readily in cold water because
of decreased molecular association arising from the greater frequency of side-chain
substitution. As with the starches, modified gums can be obtained. In particular,
etherification improves the cold water dispersibility of locust bean gum. In Table 10.37
various derivatives of galactomannans are listed together with their main applications [354].
Locust bean gum forms an interesting and unusual crosslinked complex by association of
cis-dihydroxy groups in the mannose chains with borate ions, diagrammatically represented
in structure 10.116. This complex forms a gel, which has been made use of in printing with
vat dyes in a two-stage fixation process. The crosslinks are relatively weak, being in a state
of dynamic equilibrium, and are ruptured in the presence of hydrotropes such as glycerol.
The alginates derived from seaweed are of great importance as thickening agents. These
are based on alginic acid (10.117; n = 60–600) of which the major commercial salt is
sodium alginate, although calcium (particularly in mixture with sodium), magnesium and
ammonium alginates, as well as amine salts, are also available. Their exceptionally low
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CH2OH
O
HO
O
OH
CH2OH
CH2
OH
O
O
O
O
O
OH
OH
OH
OH
m
n
10.114
D-Galactomannoglycan
Polymannose
M
M
M
M
M
M
M
M
M
M
M
Galactomannan-1,5 (m = 4) Cassia gum from Cassia tora/obtusifolia seeds
M
M
M
M
M
G
M
M
M
M
M
G
M
G
Galactomannan-1,4 (m = 3) Carob gum from Ceratonia siliqua seeds
M
M
M
M
G
M
M
M
M
G
M
M
M
G
Galactomannan-1,3 (m = 2) Tara gum from Cesalpinia spinosa seeds
M
M
M
G
M
M
M
G
M
M
M
G
M
M
G
Galactomannan-1,2 (m = 1) Guar gum from Cyamopsis tetragonoloba seeds
M
M
G
M
M
G
M
M
G
M
M
G
M
M
G
M
G
Galactomannan-1,1 (m = 0)
M
M
M
M
M
M
M
M
M
M
M
G
G
G
G
G
G
G
G
G
G
G
10.115
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
Table 10.37 Composition and applications of galactomannan derivatives [354]
Galactomannan
Chemical variant
Main applications
-1,2
Unmodified
Carpet printing/dyeing:
acid, metal-complex dyes
Depolymerised
Carpet printing/dyeing:
acid, metal-complex dyes
Cotton, viscose: vat, direct, azoic dyes
Polyester: disperse dyes
Nylon: acid, metal-complex dyes
Acrylic fibres: basic dyes
Hydroxyethylated
Carpet printing/dyeing:
acid, metal-complex dyes
Cotton: African prints with azoics
Polyester: disperse dyes
Nylon: acid, metal-complex dyes
Acrylic fibres: basic dyes
Hydroxypropylated
Carpet printing/dyeing:
acid, metal-complex dyes
-1.4
-1,5
Additionally depolymerised
Sizing
Carboxymethylated
Carpet printing/dyeing:
acid, metal-complex dyes
Cotton: vat, limited reactive dyes
Hydroxyethylated
Carpet printing/dyeing:
acid, metal-complex dyes
Cotton: African prints with azoics
Polyester: disperse dyes
Nylon: acid, metal-complex dyes
Acrylic fibres: basic dyes
Wool, silk: acid, metal-complex dyes
Carboxymethylated
Cotton, viscose: vat dyes
Wool, silk: acid dyes
Hydroxypropylated
Additionally depolymerised
Sizing
Carboxymethylated
Carpet printing/dyeing:
acid, metal-complex dyes
Additionally depolymerised
Cotton, viscose: vat dyes
Wool: acid dyes
O
H
O
O
O
HOCH2
O
H
B
_
O
O
O
CH2OH
O
H
O
H
O
O
10.116
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653
reactivity with reactive dyes is a special advantage. This is a result of replacement of the
primary hydroxy groups by carboxyl groups. As well as being non-reactive towards reactive
dyes, the ionised carboxylate anions repel dye anions in alkaline or neutral media.
Carboxylated polymers form gels with multivalent metal ions. This behaviour, like the locust
bean gum borate complex mentioned earlier, has been exploited in the two-stage flash
ageing process for vat dyes in printing. Alginate esters (such as the hydroxypropyl ester)
have also been used.
COOH
COOH
COOH
O
O
O
OH
O
O
HO
OH
OH
OH
OH
OH
OH
n
10.117
Alginic acid
Other natural polysaccharides used as thickening agents include gum arabic, gum
tragacanth and xanthan gum, but these are of diminishing significance nowadays.
Research and development in the area of natural thickeners continues to be active.
Detailed studies of the rheology of starch derivatives, alginates, cellulose ethers and
vegetable gums [343,344] have shown that thickeners with low solids content form loose
networks with low convolution density whilst those with a high solids content show higher
convolution densities. This results in differences in tackiness, shear sensitivity and
viscoelastic properties, emphasising the major influence of flow properties and viscosity on
the quality of prints obtained. Starch carbamate esters and carboxymethyl carob gums of
varying degrees of substitution have been evaluated in the vat printing of cotton [355,356].
As the degree of substitution of the carboxymethyl carob gum was increased, the colour
strength of the prints increased. Such thickening agents, either alone or in combination with
alginates, showed shear thinning behaviour that became thixotropic after storage. Alkalimodified starches have also been assessed in vat printing [357,358]. The alkali-treated
starches showed higher aqueous solubility and so were more easily removed during washing
off, giving a fabric with a softer handle. Improved colour yields could also be obtained.
It has been shown that carboxymethylcellulose thickeners can effectively replace
emulsion systems in the application of pigments [359,360]. Research in Russia has been
directed towards finding a carboxymethylcellulose thickener for reactive printing that would
be efficient and economical, the latter resulting from the fact that low concentrations give
excellent printing properties that cannot be achieved with other known thickeners [361].
The rheology of carboxymethylcellulose thickeners, including storage for up to seven days,
has been studied in conjunction with their use in reactive printing [351], indicating suitable
conditions for achieving optimum penetration, depth of colour, sharpness of outline and
evenness of the prints.
A review of uses of galactomannan thickening agents, of which some 23 000 tons were
used worldwide in 1986, is available [354]. The use of borate ions to crosslink locust bean or
guar mannose chains has been mentioned already. It has been shown that addition of borax
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to dry guar gum delays the development of viscosity, making it easier to prepare large stocks
for continuous dispensing equipment [362]. The oxidative treatment of carob gum with
sodium hypochlorite resulted in improved rheological properties and increased aqueous
solubility [363]. No mention was made, however, of possible environmental problems arising
from the use of hypochlorite.
When applying reactive dyes, it is essential to use a thickener that does not react with the
dyes, alginates being by far the most used. The rheological behaviour of alginate thickeners
has been studied [346], showing that the gradient rule is not obeyed across the whole range
of shear rates. Alginate thickeners may deteriorate on storage, notably as a result of
biological attack. The significance of this for printing performance [364] and the addition
and effects of bactericidal preservatives (sodium o-chlorophenate or chlorinated m-cresol)
have been investigated [364]. Certain preservatives have a marked effect on the hues
obtained, this being greater with dry heat than with saturated steam fixation, but
formaldehyde does not give these effects [364].
The almost exclusive use of alginates with reactive dyes is threatened by uncertainty over
sourcing of raw material and by drastic fluctuations in price and quality [365]. Hence there
have been sustained efforts to find alternatives. In one study of various possibilities [366] a
synthetic acrylic thickener was chosen, but another investigation [367] showed that
although good results can be obtained with synthetic thickeners they cannot fully replace
alginates because of poor fastness to rubbing. It has been claimed [368] that
carboxymethylated derivatives of cellulose, guar gum or starch are quite suitable for reactive
printing and actually give better performance. Presumably the ionised carboxymethyl groups,
like the carboxyl groups in sodium alginate, inhibit reaction with the reactive dyes. Indeed,
it has been confirmed that carboxymethylated guar gum derivatives do not react with
reactive dyes, and that both carboxymethylated and unsubstituted guar thickeners give
satisfactory results with vinylsulphone reactive dyes, especially for pale grounds and fine
outlines [365].
10.8.2 Synthetic thickeners
Polymers based on acrylic acid have been known since the 1930s but it was not until the late
1970s that the use of thickening agents based on them came into prominence in textile
printing [369]. Typical repeat units are shown in 10.118 and 10.119. Commercial linear
products represented by 10.118 can have n = 50–750 but crosslinked grades of higher
relative molecular mass are also available. The products represented by 10.119 cover a range
of n values from 3200 to 30 000. Only the longer-chain grades are of significant interest for
textile printing in the form of their sodium or ammonium salts. However, the scope for
O
OH
O
C
CH2
CH
CH
CH2
CH2
CH
C
OH
O
10.118
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OH
C
CH2
CH
C
n
O
OH
n
10.119
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THICKENING AGENTS, MIGRATION INHIBITORS AND HYDROTROPIC AGENTS
O
O
OH
C
CH2
OH
C
CH
CH2
CH
CH2
CH
100
100
CH
CH
CH2
CH2
CH
CH2
CH
C
CH2
CH
100
O
OH
100
CH2
10.120
C
O
CH2
C
OH
O
CH
655
OH
100
forming copolymers and crosslinked variants is virtually limitless. A schematic
representation of a crosslinked copolymer, using acrylic acid and divinylbenzene at a molar
ratio of 100:1, is shown in 10.120 [370].
It is important to make a clear distinction between acrylic binders used in pigment
printing and acrylic thickeners. Binders are generally copolymers and usually contain an
integral crosslinking agent [371]. The thickeners also find their greatest use in pigment
printing. Their biggest drawback is their sensitivity to electrolytes, although this is less of a
problem in pigment printing than in printing with dyes. The sensitivity of poly(acrylic acid)
to electrolytes can be reduced by copolymerising with acrylamide [371], although only
relatively small proportions can be incorporated before a deterioration in thickening
efficiency occurs. Two important and interrelated parameters for acrylic thickeners are
relative molecular mass and degree of crosslinking. Simply increasing the molecular mass of
linear poly(acrylic acid) yields thickeners that give stringy pastes unsuitable for use in
printing. Hence a degree of crosslinking is necessary to minimise stringiness by decreasing
the water solubility and promoting dispersibility. Figure 10.50 illustrates the effect of
crosslinking for three acrylic acid polymers of the same molecular mass [371]. The balance
of molecular mass and degree of crosslinking influences other properties, such as degree of
penetration, levelness of ground colours and sharpness of the print.
These products are usually supplied to the printer as partially neutralised polyacids.
Further neutralisation is carried out by the printer when making up the print pastes. This
neutralisation is often a critical process. For certain applications, as with resin-bonded
pigments, neutralisation is carried out with ammonia. This has the advantage that during
subsequent baking the ammonia is driven off to liberate the free polyacid, which then
catalyses activation of the resin binder. In other cases neutralisation is carried out with nonvolatile alkalis such as sodium hydroxide. It is particularly important to use the latter in
reactive printing, since ammonia would be evaporated off during fixation leading to a
lowering of pH and consequently poor fixation. Moreover, reactive dyes can be deactivated
by reaction with ammonia to form their non-reactive amino derivatives.
The commercial success of acrylic thickeners in pigment printing is attributable to the
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
A
B
Viscosity
C
A
B
C
Highly crosslinked
Moderately crosslinked
Slightly crosslinked
Thickener concentration
Figure 10.50 Effect of crosslinking on thickener efficiency [371]
fact that they can be designed to give properties very similar to those of emulsion
thickenings. These were previously the only systems used in pigment printing and they are
dealt with in section 10.8.3. It is important to realise that an acrylic thickener intended for
use with pigment systems may be unsuitable for use with dyes. This is because commercial
thickeners, available as solutions, emulsions, liquid dispersions or powders, often contain
additional chemicals to improve their stability and performance in particular systems. For
pigment systems, for example, the thickener may also contain additives (surfactants or
polyelectrolyte dispersing assistants), such as the ammonium or sodium salt of linear
poly(acrylic acid) (10.121), which not only modify the behaviour of the acrylic thickener but
also assist dispersion of the pigment [371]. Surfactant additions are undesirable with
reactive dyes because they promote colour bleeding, whilst the ammonia is undesirable
because of deactivation of reactive groups, the lowering of pH that occurs by its evolution
during the fixation process and the subsequent difficulty in washing-off of the residual
thickener, now bereft of its solubilising ammonium ions.
CH2
CH
C
O
_
O M+
n
M = Na or NH4
10.121
A major drawback of synthetic thickeners when used with dyes is their sensitivity to
electrolytes. Most soluble dyes behave as highly ionised electrolytes and disperse dyes
contain anionic polyelectrolyte dispersing agents unless they have been formulated with
nonionic systems specifically for use with acrylic thickeners. Consequently there is a loss of
viscosity; this can be quite pronounced although it depends on circumstances, particularly
on the dye concentration. As already mentioned, this can be alleviated to some extent by
copolymerisation with acrylamide during manufacture. Otherwise it is necessary to try to
eliminate all electrolytes from the system or to increase the concentration of thickener. Such
measures have their limitations in practice, however. Alternative synthetic thickening
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657
agents include poly(vinyl alcohol) and copolymers of maleic anhydride with alkenes
(10.122).
CH2
CH2
CH
CH
C
O
C
O
O
n
10.122
A detailed comparative study of the rheological properties of four acrylic thickeners
varying in relative molecular mass from 1.25 × 10 6 to 4 × 10 6 and of two crosslinked
ethylene-maleic anhydride copolymers has been published [345]. In respect of some
properties, comparisons were also made with a starch ether and an alginate. Amongst other
factors, the influence of molecular mass was demonstrated (Figure 10.51), showing that the
higher the molecular mass of the acrylic polymer, the less the amount of thickener required
to achieve a given viscosity. Nevertheless, earlier comments in relation to the stringiness of
linear acrylic polymers should be borne in mind, i.e. factors other than viscosity need to be
considered. Viscosity develops as water is absorbed, causing swelling and rearrangement of
the polymer chains, a process that is assisted by, indeed is critically dependent on,
neutralisation. Figure 10.52 gives a schematic representation of this swelling and also
illustrates the similarity in behaviour with oil-in-water emulsions [371]. It is important that
the degree of crosslinking is the optimum required to maintain the polymer in this swollen
state and prevent it from dissolving, which would result in loss of desirable properties.
50
Acrylic
thickeners
Mr 4 × 106
Mr 3 × 106
Mr 1.75 × 106
Mr 1.25 × 106
Viscosity/Pa s
40
30
Crosslinked ethylene–
maleic anhydride
copolymers
20
No 1
No 2
10
0.2
0.6
1.0
1.4
Thickness concentration/%
Figure 10.51 Comparison of rheological behaviour of acrylic and copolymeric thickeners [345]
Base
Dispersed
polyacid/pH 4
Neutralised,
swollen
polyacid/pH 9
Kerosene
emulsion
in water
Figure 10.52 Schematic diagram illustrating swelling of polyacid thickener and comparison with oil-inwater emulsion [371]
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A novel concept [371] was evaluated to seek a replacement for alginates in printing with
reactive dyes. This approach utilised copolymers of poly(acrylic acid) onto which starch has
been grafted by free radical polymerisation, the free radical initiator being a potassium
persulphate redox system. Both native starch and starch pre-oxidised with sodium
hypochlorite were included in the study. It was found that an acrylic–starch graft copolymer
effectively replaced 25% sodium alginate, whilst a copolymer of poly(acrylic acid) and
oxidised starch replaced 50% sodium alginate. A detailed rheological study was presented
but the evaluation lacked economic analysis.
10.8.3 Emulsion thickeners
When immiscible liquids are emulsified the viscosity increases and this can be exploited to
prepare thickenings for textile printing. The emulsions used contain a hydrocarbon solvent
(usually white spirit), surfactant(s) and water; the oil phase must account for at least 70% of
the total volume [342]. The first pigment printing systems introduced in the late 1930s were
water-in-oil emulsions (that is, at least 70% of the product was water) in which typical
surfactants were ethoxylated alcohols, acids or amides with a low degree of ethoxylation,
perhaps 5–8 oxyethylene units per molecule, morpholine/oleic acid or lauric, palmitic and
stearic acid esters of sorbitol. Later, manufacturers developed oil-in-water emulsions for
which appropriate surfactants are higher ethoxylated alcohols, acids or amides, or a wide
variety of alkylaryl types. For any type of emulsion, the HLB of the emulsifying agent(s) is
clearly of great importance.
The size of the droplets in an emulsion is inversely related to its viscosity, typical
diameters ranging from 100 to 7000 nm. Theoretically no more than 75% of oil can be
incorporated in an aqueous emulsion, assuming uniformly spherical droplets, but distortion
due to packing allows significantly higher proportions of oil phase to be added. Presumably
the oil droplets are stabilised by a surrounding layer of like charges, the type and strength of
the charge depending on the surfactant(s) used. Consequently the stability of the emulsion
tends to be impaired by any additions that reduce the charge on the droplets.
Emulsion thickeners can be mixed with low concentrations of either natural or synthetic
thickeners, especially when applying fibre-substantive dyes rather than pigments; these
additions act as film formers, taking the place of the binder used with pigments to increase
retention of the dye by the substrate prior to fixation.
10.8.4 Continuous dyeing
The foregoing discussion has concentrated on the use of thickening agents in textile
printing. Similar types of product are used to thicken pad liquors in continuous dyeing
processes, although then they are normally described as migration inhibitors rather than
thickening agents. All polysaccharides mentioned previously, especially the alginates, locust
bean, guar and xanthan gums, modified starches and celluloses, can be used in continuous
dyeing, but by far the most widely used are the alginates. Concentrations tend to be
significantly lower than in printing since the dyeing process requires a lower viscosity,
permitting rapid and complete penetration into the fabric during padding. In addition to the
alginates, polyacrylates (10.8.2), polyacrylamides and polyethoxylates are also used [373].
Polyethoxylates are mixed polyglycol ethers of fatty alcohols with ethylene oxide (or
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659
propylene oxide), the nonionic block copolymers described in section 9.6 that operate by
virtue of a low cloud point (about 25 °C). The functions of these products are two-fold: (a)
they should favour the uniform application of dye by padding; and (b) they should
effectively inhibit any tendency of the dye to migrate during the subsequent intermediate
drying process. In the absence of an inhibitor, dye liquor tends to migrate towards hotter
regions of the fabric during drying, causing either patchiness or an undesirable two-sided
effect. Whilst viscosity plays an important part in facilitating the uniform absorption of dye
liquor, it is less effective than coagulation for inhibiting migration. The polyacrylates are
particularly effective [373].
In continuous dyeing agents to assist rapid wetting and even penetration of the fabric are
invariably added to pad liquors along with the migration inhibitor. Many types of surfactant
can be used, including phosphate esters (which are particularly effective [373]),
sulphonates, sulphates, sulphosuccinates and sulphated ethoxylates. Care should be taken to
ensure that the type and concentration of product chosen do not depress the degree of
fixation of the dyes. Wetting agents are rarely used in printing since they would tend to
promote bleeding or haloing of printed areas.
Methods available for assessing migration inhibitors have been reviewed [374,375].
Factors influencing dye migration during the drying phase include fibre type, liquor pick-up,
drying temperature, running speed, type of dyes and the various pad liquor additives present.
Those additives intended to increase substantivity and agglomeration of the dyes tend to
inhibit migration. Interaction between dyes is demonstrated by the finding [376] that, using
an alginate migration inhibitor on 50:50 polyester/cotton, the migration of an individual dye
can be inhibited by the presence of other dyes in the mixture that have larger particle sizes
or a greater tendency to flocculate. However, increasing the concentration of migration
inhibitor progressively neutralises this tendency. When the concentration is high enough,
specificity of particulate migration is eliminated as the migration of dyes in the mixture
approaches zero.
Polyacrylamides (10.123) of chain length (n) 7000 to 14 000, which is higher than
normally suitable for migration inhibitors, are useful pad liquor additives [373] in that they
increase liquor pick-up and sometimes colour yield, notably with pigments. When used with
azoic dyes on cellulosic fabrics, they can eliminate the need for the intermediate drying
process. In a study [377] using polyacrylonitrile saponified with alkali to form amide groups
capable of being crosslinked with formaldehyde, it was found that migration during drying
decreases as the degree of crosslinking is increased, this being attributed to the increased
structural density of the polymer films.
CH2
CH
C
O
NH2
n
10.123
The continuous dyeing of polyester/cotton blends inevitably results in staining of the
cotton by disperse dyes, this effect being greatly influenced by the chemical nature and
concentration of the migration inhibitor, the dyebath pH and the chemical nature and
concentration of the dyes, whereas the presence of wetting agent or neutral electrolyte does
not have much influence [378]. Normally in the dyeing of polyester/cotton blends a higher
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
dyeing temperature minimises staining of cotton, since the higher temperature facilitates
migration of disperse dye from the cotton to the polyester. Migration inhibitors can negate
this effect, however, so it is important to establish the optimum agent concentration
necessary to prevent migration but avoiding excessive concentrations that could lead to
increased staining of the cotton.
It has been shown that xanthan gum is an effective migration inhibitor for the application
of water-soluble chemicals, leading to uniform distribution and more reproducible fixation
[379]. Although this work was specifically concerned with the application of a soluble flame
retardant to polyester, suitability for the application of reactive dyes or resin finishes is also
claimed.
10.8.5 Hydrotropic agents
In many printing and some continuous dyeing processes the colour yield of dyes can often be
improved, sometimes markedly, by the use of an auxiliary that tends to increase the aqueous
solubility of the dye, particularly when using highly concentrated pastes or liquors that tend
to lose moisture under adverse conditions. There is clearly an analogy here with the
mechanism of solubilisation discussed in section 10.6.2, since the hydrotrope acts as an
amphiphilic bridge between the dye solubilisate and the aqueous medium. Surfactants can
therefore function as hydrotropes and are sometimes used as such in continuous dyeing
processes. Hydrotropes with much less powerfully surface-active properties are more suitable
for use in printing, where surfactants are normally avoided because their concomitant
powerful wetting properties would promote bleeding and haloing of the print.
By far the most important of these compounds are urea (10.124) and thiourea (10.125).
However, dye–fibre systems are so varied that many hydrotropes are of interest under
specific conditions. Typical examples that have been mentioned [380] include:
triethanolamine (10.126), N,N-diethylethanolamine (10.127), sodium N-benzy1sulphanilate
(10.128), sodium N,N-dibenzy1sulphanilate (10.129), ethanol, phenol (10.130), benzyl
alcohol (10.131), resorcinol (10.132), cyclohexanol (10.133), ethylene glycol, glycolic acid
(10.134), 2-ethoxyethanol (10.135), diethylene glycol (10.136), 2-ethoxyethoxyethanol
(10.137), 2-butoxyethoxyethanol (10.138), thiodiethylene glycol (10.139) and glycerol
(10.140), the last-named in particular being useful with practically all classes of dyes.
H2N
HOCH2CH2
H2N
C
H2N
O
C
N
S
H2N
CH3CH2
CH2CH2OH
N
HOCH2CH2
CH2CH2OH
CH3CH2
10.124
10.125
Thiourea
Urea
10.126
Triethanolamine
10.127
CH2
N
H2C
HN
SO3Na
10.128
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CH2
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661
HO
CH2OH
OH
OH
OH
10.131
10.130
10.133
Phenol
Cyclohexanol
10.132
Resorcinol
HOCH 2COOH
HOCH2CH2OCH2CH2OH
CH3CH2OCH2CH2OH
10.135
10.134
CH3CH2OCH2CH2OCH2CH2OH
10.136
CH3CH2CH2CH2OCH2CH2OCH2CH2OH
10.137
10.138
CH2OH
HOHC
CH2OH
HOCH2CH2SCH2CH2OH
10.140
10.139
Glycerol
Most hydrotropic agents, though not surfactants in the usual sense of the word, do
significantly lower the surface tension of water and this is an important prerequisite for their
solubilising action. Hydrogen bonds, together with weaker dipolar and van der Waals forces,
contribute to this interaction, the active centres in the hydrotropic molecules being protondonating groups (such as hydroxy, amino and amido groups) and proton-accepting atoms
(such as the nitrogen atom of a tertiary amine). For this reason hydrotropes can also be
added to the diluent system in the manufacture of certain dyes, particularly liquid brands, in
which they not only increase the apparent solubility of the dye but also help to prevent the
formation of surface skin. Another compound that has entered this area is 1-cyanoguanidine
(10.141) popularly known as dicyandiamide. The main mechanisms whereby hydrotropes
are believed to function have been mentioned above. However, there has been a good deal
of discussion regarding possible mechanisms, mainly in connection with the interaction of
urea with reactive dyes. Useful reviews of this topic are available [381,382].
H2N
C
NH
CN
HN
10.141
Dicyandiamide
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The growth of environmental awareness has certainly impinged on this area, restricting
the use of hydrotropes that are volatile during steaming or dry heat treatments, as well as
those suspected of endangering health. These problems are particularly acute with urea,
widely used for a long time and considered an essential hydrotropic assistant in printing and
continuous dyeing with reactive dyes. This is considered in more detail in section 12.7.1.
10.8.6 Environmental aspects
Reference to Table 10.10 in section 10.5.2 indicates printing to be responsible for some 10–
20% of the total pollution load from the textile wet processing of cotton. Other substrates,
as well as continuous dyeing, also make significant contributions. Useful reviews of the
environmental aspects of textile printing generally [383,384] and of pigment printing in
particular [385,386] are available. There are two aspects to be considered:
(1) airborne pollution from volatile components during steaming and dry heat treatments
(2) aqueous effluent pollution from washing-off and washdown procedures.
In both cases there are two basic methods of control:
(1) elimination or minimisation of offending products, which may involve total or partial
substitution by more benign products, or a redesigned process that does not require that
type of auxiliary
(2) remedial treatment of exhaust gases or effluent.
In any case, it is always sound practice to optimise the concentration of an auxiliary so that
no more than necessary is applied. This aim of using minimum quantities is assisted by the
trend towards more efficient thickeners. For example, in the 1960s it was common for
thickeners to be made up to a 20–30% concentration, whereas today 10% is more common
and it is predicted that this will fall to an average of about 5% within the next decade [383].
Airborne pollution can arise from volatile fractions during drying, as well as dry heat
curing or heat setting treatments. This aspect has been associated mostly, but not
exclusively, with the use of emulsion thickeners in pigment printing, considerable volumes
of organic compounds being released during drying and curing. Consequently, emulsion
printing is now seldom carried out in the major developed countries [383]. Those acrylic
thickeners that contain organic solvents or hydrocarbons are also sources of airborne
pollution, but thickeners free from such additives are now available [386]. One method of
dealing with the volatile emissions is incineration, but this is very costly. Scrubbing is highly
efficient with water-soluble volatiles but not with hydrocarbons: hence it is best to avoid
using polluting materials. A detailed account of the ecological factors in pigment printing is
available [385], indicating the obligation to minimise or eliminate emissions of residual
monomers, formaldehyde and unwanted solvents. Furthermore, alkylphenol ethoxylates
formerly used as emulsifiers can be replaced by fatty alcohol ethoxylates or, to some extent,
by anionic surfactants. Pigment printing also offers ecological benefits over other systems in
that it saves time and energy, and washing-off is unnecessary [385].
In general (excluding pigment printing), however, the thickeners or migration inhibitors
are present in waste waters, both from washing-off the dyeings or prints and from washingdown of equipment. The usual three methods of treatment (chemical degradation,
bioelimination and recycling) have to be considered for dealing with them. Natural
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THICKENING AGENTS, MIGRATION INHIBITORS AND HYDROTROPIC AGENTS
663
thickeners are not toxic in themselves but, in parallel with the analogous sizing agents
(Table 10.11 in section 10.5.2), they show high chemical and biological oxygen demand
[354,383,387]. Galactomannans [216,217,354] in particular are thought to have a good
future in respect of environmental impact. Since the naturally occurring polymers present
the least difficulties in environmental terms, it is thought that their derivatives will become
less important [383].
The treatment and disposal of these wastes is not only expensive but entails the loss of
potentially useful materials. Thus, as with sizing agents, recycling is an attractive
consideration [388,389]. The basic principle is precipitation of the thickener in a suitable
aqueous solvent system followed by isolation, often by ultrafiltration. Dyes can be separated
from the thickener either continuously or discontinuously [388]. In a detailed study [389],
the efficiency of precipitation of high-, moderate- and low-viscosity alginate thickeners in
four solvent systems was compared with that of carboxymethylated galactomannans or
celluloses and four synthetic thickeners, giving the results in Table 10.38. Thus all types of
alginate thickener can be isolated quantitatively from the wash water and recycled without
difficulty. These recycled thickeners are claimed [388,389] to give rheological properties and
printing results equivalent to those of the original products. Pure acrylic thickeners can also
be precipitated almost quantitatively [389].
Table 10.38 Precipitation (%) of printing thickeners using solvents [389]
Thickener
Ethanol
Low-viscosity alginate
Moderate-viscosity alginate
High-viscosity alginate
100
100
100
100
100
100
100
100
100
100
100
100
Carboxymethylgalactomannan
Low-viscosity
High-viscosity
>90
>90
>90
>90
>90
>90
70–90
70–90
Carboxymethylcellulose
Low-viscosity
High-viscosity
>90
>90
>90
>90
>90
>90
55–90
55–90
70–90
50
0
70–80
50
0
70–95
50–70
0
<20
<20
0
>90
>90
>90
>90
Synthetic thickener 1
Synthetic thickener 2
Synthetic thickener 3
Poly(acrylic acid)
Acetone
Aqueous
methanol
Methanol
Natural thickeners are prone to biological attack and degradation, leading to a loss of
thickening efficiency. It is common practice to add a small amount of a bactericide to
protect against attack during storage [383,387,390]. About thirty classes of compounds have
been listed as bactericides [387], but phenol derivatives (including chlorinated phenol,
m-cresol or o-phenylphenol) and formaldehyde have proved particularly suitable, added
either by the manufacturer of the thickening agent or by the printer during formulation of
the stock thickener. As bactericides, such compounds are by definition toxic and hence
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
environmentally undesirable if not actually prohibited in some countries. However, it has
been pointed out [383] that they are used only at very low concentrations (usually below
0.1%) that are just about sufficient for bacterial efficiency. Together with the thickener they
are then washed out with copious amounts of water, entering the effluent at such high
dilution that they no longer have bactericidal action and can even be biologically eliminated
during effluent treatment.
Mention has already been made of the environmental undesirability of many hydrotropes
(section 10.8.5) and this particularly applies to urea, the most widely used. This is discussed
in more detail in section 12.7.1 in connection with reactive dyes. Suffice to note that the
efficiency of urea as a hydrotrope is proving difficult to equal in every respect with
substitutes that are environmentally acceptable [382].
10.9 TREATMENTS TO ALTER DYEING PROPERTIES OR ENHANCE
FASTNESS
Treatments to improve the intrinsic fastness of dyeings have a long and prolific history. Most
of these are aftertreatments, although some are concurrent with the dyeing process as in the
application of UV absorbers to enhance light fastness. The treatments described in this
section are essentially concerned with improving fastness properties or fibre stability.
Treatments to modify the handle or other fibre properties are excluded, these being dealt
with in section 10.10. Furthermore, such treatments are not an essential integral part of the
dyeing process, thus excluding the afterchroming of chrome dyes and the oxidation of
sulphur and vat dyeings. Many processes have been developed for improving fastness but
relatively few are still in commercial use today. Their efficacy has to outweigh the extra
processing cost and time. The emphasis will be on processes of current commercial or
research interest. An excellent comprehensive review covering the period 1880–1980 is
already available [391]. Of growing interest, at least in the research sector, are fibre
pretreatments aimed primarily at modifying dyeing properties, although these may have
secondary benefits in terms of improved fastness. There has always been a good deal of
interest in pre- and aftertreatments amongst textile chemists and colourists, largely on
account of the innate interest of the chemistry and the challenges involved. Commercial
processors have been less enthusiastic, however, since these variations involve additional
costs and scheduling difficulties.
10.9.1 Pretreatments
One of the earliest fibre pretreatments for improving the dyeability of cotton is of course
mercerisation (section 10.5.4). However, more recent research interest in this area has been
generated by environmental concerns about reactive dyeing, aiming to enhance
substantivity for the modified fibre so that higher absorption and fixation are obtained. This
results in less dye (hydrolysed or still active) in the effluent. A further objective is to
minimise the usage of electrolyte in the application process. This area has been thoroughly
reviewed [392,393].
At the risk of over-simplification, most of the pretreatments covered by Lewis and
McIlroy will be summarised in Tables 10.39 to 10.42. Their review [393] and the original
works cited in it should be consulted for details, of course. The great majority of processes
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665
involve increasing the nitrogen content (i.e. the basicity) of the cellulose, there being an
analogy here with the better dye-sorption properties of wool that naturally contains such
built-in nitrogen. Table 10.39 presents structural modifications of cellulose not entailing the
application of a polymer. Most of these reactions involve amination in one form or another.
In fact interest in the amination of cellulose began in the 1920s, long predating current
environmental concerns regarding reactive dyes. Hence it is not surprising that some of the
earliest processes are environmentally suspect. Nevertheless, more acceptable processes
have evolved, making cellulose more substantive to anionic dyes and enabling much less
electrolyte and non-alkaline fixation conditions to be adopted for reactive dyeing.
It has been pointed out that aminoethylcellulose produced by a two-stage dry process
using 2-aminoethylsulphuric acid can even react covalently with hydrolysed vinylsulphone
dyes, thus enhancing fixation and making washing-off easier [394]. Another cationic
quaternary compound that has been used [395] is 3-chloro-2-hydroxypropyltrimethylammonium chloride (10.142). This compound and four analogous reactive cationic agents
(10.143) have been proposed [396], prepared by reaction of the appropriate amine with
epichlorohydrin.
The cationisation of cotton cellulose using these agents takes place in two stages
(Schemes 10.49 and 10.50), although from a practical viewpoint these occur concurrently in
a single process by exhaustion for 20 minutes at 80 °C in the presence of sodium hydroxide
as the base catalyst. In the first stage an epoxide is formed in the presence of alkali. In the
second stage this epoxide reacts with a hydroxy group in the cellulose. The cationised
cotton could be dyed with anionic dyes; only CI Acid Red 127 was used in this research.
The degree of substitution of the cellulose and the amount of dye absorbed both decreased
as the length of the hydrocarbon chains attached to the nitrogen atom was increased.
Regardless of hydrocarbon chain length, the light fastness was slightly higher on cotton than
on nylon or wool dyed with the same dye. Fastness to washing decreased with increasing
length of the hydrocarbon chains. The cationised cotton showed enhanced antibacterial
properties, the potency increasing with increasing hydrocarbon chain length [396].
CH3
CH3
+
N CH2
CH3
CH2Cl
_
Cl
CH
OH
10.142
R2
R1
+
N CH2
_
R3 Cl
CH2Cl
CH
Trimethylamine:
R1 = R2 = R3 = CH3
Triethylamine:
R1 = R2 = R3 = CH3CH2
Tripropylamine:
R1 = R2 = R3 = CH3CH2CH2
Tripentylamine:
R1 = R2 = R3 = CH3(CH2)4
OH
10.143
Dimethyltetradecylamine: R1 = R2 = CH3
R3 = CH3(CH2)13
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
R
R
+
N CH2
_
R Cl
CH2Cl
_
HO
CH
R
OH
R
+
N CH2
_
R Cl
CH2Cl
CH
_
O
R
CH2
+
R
Scheme 10.49
R
R
N
+
CH
N
CH2
R
Cl
_
_
+ Cl
O
CH2
CH2
R
Cl
_
CH
+
HO
[cellulose]
O
R
Scheme 10.50
R
+
N CH2
_
R Cl
CH2
O
[cellulose]
CH
OH
Table 10.39 Pretreatments to modify cotton cellulose by substitution reactions [393]
Pretreatment
reaction
Amination
Method
Comments
Tosylation, followed by treatment
with amines (Scheme 10.51)
All amination treatments give
improved dyeability
p-Nitrobenzoyl chloride, followed
by reduction of nitro (Scheme 10.52)
Acetylaminobenzenesulphonyl chloride
in nitrobenzene or chloroform,
followed by hydrolysis of the amide
(Scheme 10.53)
2-Chloroethylamine and alkali-treated
cellulose (Scheme 10.54)
Sodium 2-aminoethylsulphate in
aqueous alkali (Scheme 10.55)
More efficient in producing
2-aminoethylated cellulose.
Relatively inexpensive and
doesn’t require organic solvent
Sodium borohydride added to a
sodium 2-aminoethylsulphate
pad liquor
Gives white aminated cotton.
Borohydride reduces yellow
Schiff bases formed by reaction
of aldehyde groups with amino
groups
Polymerisation of ethyleneimine on
fibre by various methods
Improved dyeability
Diethylaminoethylation 2-Chloroethyldiethylamine
hydrochloride (Scheme 10.56)
Dyeable in absence of salt
with direct, reactive or
acid dyes
Continued on next page
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667
Table 10.39 Continued
Pretreatment
reaction
Method
Comments
Esterification
Inorganic or organic acids
Improved dyeability
Esterification and
amination
3-Chloropropionyl chloride,
followed by amine (Scheme 10.57)
Primary, secondary, tertiary
and quaternary derivatives
can be produced. Dyeable
with reactive dyes, neutral
to slightly acidic without salt
Amination
Epoxides in alkali, including ethylene
oxide, propylene oxide, glycidol
(2,3-epoxypropan-l-ol). Scheme 10.58
shows glycidyltrimethylammonium
chloride
Glycidyltrimethylammonium
chloride marketed to enhance
dyeability with direct and
reactive dyes
Introduction of
quaternary N groups
1,1-Dimethyl-3-hydroxyazetidinium
chloride in presence of strong
alkali by pad–bake (Scheme 10.59)
Dyeable with reactive dyes at
pH 7 without salt, giving
extremely high fixation
Acylation
Nicotinoyl-thioglycolate and alkali by
pad–bake (Scheme 10.60)
Dyeable with monochlorotriazine
reactive dyes at pH 3
without salt
Amination
N-Methylolacrylamide in presence
of Lewis acid catalyst. Further
modifications possible by
addition to double bond (Scheme 10.61)
Improved dyeability with
dichlorotriazine dyes at pH 5
without salt, giving 99%
fixation
Amines with durable press resins
Some improvements in
dyeability, especially with direct
dyes, but light fastness can be
a problem
[cellulose]
OH
[cellulose]
O
+ Cl
SO2
SO2
CH3
CH3
+ HCl
2 NH3
[cellulose]
NH2
+
+ NH4
O3S
CH3
Scheme 10.51
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
[cellulose]
OH
Cl
+
C
NO2
O
[cellulose]
O
C
NO2
+ HCl
O
reduction
[cellulose]
O
C
NH2
O
Scheme 10.52
[cellulose]
OH
[cellulose]
O
+
Cl
SO2
SO2
NHCOCH3
NHCOCH3
+ HCl
H2O
O
[cellulose]
O
SO2
NH2
C
+
HO
Scheme 10.53
[cellulose]
CH3
OH
+ ClCH2CH2NH2
[cellulose]
O
CH2CH2NH2
Scheme 10.54
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+ HCl
TREATMENTS TO ALTER DYEING PROPERTIES OR ENHANCE FASTNESS
[cellulose]
669
OH + H2NCH2CH2OSO3Na
130°C
NaOH 15 min
[cellulose]
O
CH2CH2NH2 + Na2SO4 + H2O
Scheme 10.55
CH3CH2
+ NH
_
+
CH2CH2Cl
HO
[cellulose]
CH3CH2 Cl
NaOH
CH3CH2
N
CH2CH2
O
[cellulose] + NaCl + H2O
CH3CH2
Scheme 10.56
[cellulose]
OH
+
Cl
C
CH2CH2Cl
O
NaOH
[cellulose]
O
C
CH2CH2Cl
+ NaCl + H2O
O
[cellulose]
O
C
O
+
CH2CH2Cl + NHR3
_
Cl
100°C
[cellulose]
+
NR3 + HO
_
Cl
C
CH2CH2Cl
O
Scheme 10.57
CH3
CH3
+
N CH2
CH3
Cl
CH2
+ HO
CH
_
[cellulose]
O
CH3
CH3
+
N CH2
_
CH3 Cl
CH2
CH
OH
Scheme 10.58
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O
[cellulose]
670
CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
H3C
H3C + CH2
N
CH2
H3C
_
Cl
CH2
N
H
+ Cl
H3C
CH
H2C
O
CH
OH
Scheme 10.59
N
+
C
S
HO
_
[cellulose]
N
HO
+ HS
CH2COONa
C
O
O
CH2COONa
[cellulose]
O
N
_
HO
+ H2O
C
S
+ HS
N
CH2COONa
C
O
CH2COONa
OH
O
Scheme 10.60
O
[cellulose]
OH
+
HO
CH2
NH
C
CH
CH2
ZnCl2, 150 °C
O
[cellulose]
O
Reaction with
CH2
NH
C
CH
CH2
+ H2O
Product
O
Ammonia
[cellulose]
O
CH2
NH
C
CH2CH2
NH2
CH2CH2
NHCH3
CH2CH2
N(CH3)2
CH2CH2
_
+
N(CH3)3X
CH2CH2
NHCH2CH2OH
O
Methylamine
[cellulose]
O
CH2
NH
C
O
Dimethylamine
[cellulose]
O
CH2
NH
C
O
Trimethylamine.HX
[cellulose]
O
CH2
NH
C
O
Ethanolamine
[cellulose]
O
CH2
NH
C
Scheme 10.61
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TREATMENTS TO ALTER DYEING PROPERTIES OR ENHANCE FASTNESS
671
A somewhat different substitution reaction involved treating cotton with an aqueous
solution containing 10 ml/l carbon disulphide, 1% sodium hydroxide and 0.5% wetting agent
[397]. This thiocarbonate pretreatment was followed by dyeing with a direct dye in the
presence of ammonium persulphate as free radical initiator, giving higher wet fastness than
on alkali-treated or untreated cotton. This was explained in terms of involvement of the
thiocarbonate groups in the formation of covalent bonds between cellulose and the direct
dye by a free radical mechanism.
Table 10.40 represents those pretreatments requiring application of a polymer, the
fundamental objective again being to increase basicity by incorporation of amino groups. A
point of interest is use of the cationic polymer Hercosett 125 (Hercules), formed by
condensation of adipic acid and diethylenetriamine to give a polyamide subsequently
partially crosslinked with epichlorohydrin, a resin more commonly associated with shrinkresist treatments of wool. Once again, improvements in dyeability are usually claimed but
retaining good light fastness appears to be a common problem. Fibre-reactive quaternary
ammonium compounds with chlorohydrin functionality have also been evaluated as epoxide
pretreatments for wool/cotton blends in an attempt to facilitate union dyeing using anionic
dyes for wool [398]. Since alkaline conditions were required, the integrity of the wool was
preserved by treatment at ambient temperature for 3 hours at pH 11.
Cotton grafted with 2-vinylpyridine followed by quaternisation using an excess of an alkyl
bromide or epichlorohydrin showed markedly increased exhaustion with direct dyes [399].
Grafting alone gave a substantial effect, with further slight improvements being conferred by
the quaternisation. Improved fastness to washing was also claimed.
Table 10.40 Pretreatments to modify cellulose using amino-containing polymers [393]
Pretreatment polymer
Method
Comments
Chitosan
Cationic polymer applied by
exhaustion from acidic solution
Improved coverage of immature
fibres, increased exhaustion but
decreased fastness unless further
treated with fibre-reactive
quaternary compound
Sandene (Clariant)
Cationic polymer applied by
exhaustion under alkaline
conditions
Enhanced dyeability with anionic
and reactive dyes, the latter
applied under neutral or slightly
acidic conditions. Reduced light
fastness and marked dulling
with some dyes
Hercosett 125 (Hercules)
reactive cationic polymer
formed by condensation
of adipic acid and
diethylenetriamine, then
partially crosslinked with
epichlorohydrin
Pad-dry-cure for 3 min at 100 °C.
Scheme 10.62 represents the
reactive and nucleophilic sites
that may exist on the surface of
the treated fibre
Dyeable neutral without salt; good
results with some high-reactivity
dyes (dichlorotriazine and
difluoropyrimidine) but not with
some other types
(monochlorotriazine and
dichloroquinoxaline). Washing
fastness very good but light
fastness lower
Continued on next page
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
Table 10.40 Continued
Pretreatment polymer
Method
Comments
Thiourea derivative
of Hercosett
(isothiouronium salt)
Scheme 10.63
Build-up good for dichlorotriazine
dyes. Dyes of other types gave
good results up to about 2%
applied depths
Ethylenediamine derivative
of Hercosett
Scheme 10.64
About 95% fixation of low- or
high-reactivity dyes under slightly
acidic conditions without salt, but
light fastness still inferior
Polyepichlorohydrin and
dimethylamine
Polymerisation of epichlorohydrin
Good yields with direct dyes using
in carbon tetrachloride with
only 2 g/l salt. Excellent build-up
boron trifluoride/ether catalyst,
with most reactive dyes; only 10%
then reaction with dimethylamine. of normal salt usage needed for
Applied to cotton by exhaust
low-reactivity dyes and none for
method or pad–dry.
highly reactive types. Washing
Scheme 10.65
fastness very good but light
fastness impaired.
+ CH2
N
CH2
_
Cl
NH
N
CH2
CH
CH2
+
H
_
HO
OH
OH
Azetidinium cation
+
NH2
+
2H
_
2 HO
N
CH
+
NH
Secondary amino
CH2
CH
CH2
+
NH
OH
Tertiary amino, secondary hydroxy
[cellulose]
OH
[cellulose]
O
Cellulose hydroxy
CH2
CH
CH2
N
Covalent link to cellulose
OH
Scheme 10.62
+ CH2
N
_ CH2
NH2
CH
OH
+
S
C
N
NH2
Cl
Thiourea
Azetidinium cation
CH2
CH
CH2
OH
Isothiouronium salt
Scheme 10.63
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NH2
+
S C
_
NH2
Cl
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TREATMENTS TO ALTER DYEING PROPERTIES OR ENHANCE FASTNESS
+ CH2
N
_ CH2
CH
OH
+
NH
_
Cl
CH2
CH
+
H2N
CH2CH2
NH2
NH
CH2CH2
NH2
673
Cl
+ CH2
N
_ CH2
CH
CH2
OH
OH
+
H2NCH2CH2NH2
+
CH2
CH
CH2
HO
Cl
N+
_
Cl
+
NH
CH2NHCH2CH2NH CH2
CH2
_
CH
CH
HO
OH
Cl
Cl
Scheme 10.64
O
H2C
CH
CH2Cl
BF3 , EtOEt
CH2
+
NH
_
CH2
CH
CCl4
O
CH2Cl
n
Epichlorohydrin
CH3
HN
CH3
CH2
CH
CH2
O
+
NH
CH3
_
CH3 Cl
n
Scheme 10.65
Certain pretreatments depend on introducing sulphur rather than nitrogen, these being
summarised in Table 10.41. So far these appear to have been much less successful than
nitrogenous treatments.
A more radical approach to pretreatment reverses the conventional reactive dyeing
concept by preparing a reactive cotton cellulose capable of reacting covalently with suitable
dyes containing, for example, aliphatic amino groups. In an initial attempt, cotton was
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
Table 10.41 Pretreatments to modify cellulose using sulphur-containing agents [393]
Pretreatment reaction
Comments
Cotton treated with bis(2-isocyanatoethyl)
disuphide in dimethylformamide at 80 °C,
followed by reduction with tri-n-butylphosphine
in methanol containing 10% water. This
gives the 2-mercaptoethylcarbamyl ester,
which is treated with methyl iodide to form
sulphonium salts. (Scheme 10.66).
The greater the sulphonium content, the greater
the uptake of direct dyes. Not an environmentally
acceptable pretreatment, however.
Alkali-treated cellulose immersed in an
arylsulphonium salt solution.
Dyeings with direct, reactive, sulphur and disperse
dyes at pH 5 showed improved colour strength
but detailed results were not reported.
Linen esterified with thioglycolic acid to
incorporate thiol groups, these being more
strongly nucleophilic than hydroxy groups.
Increased colour yields obtained with reactive dyes,
but not enough to give adequate solidity on
linen/wool blends.
[cellulose]
O
C
CH3I
NHCH2CH2SH
[cellulose]
O
C
NHCH2CH2
S
CH3
O
O
CH3I
[cellulose]
O
Scheme 10.66
C
NHCH2CH2
O
+
S CH3
_
CH3 I
O
[cellulose]
O
CH2
NH
C
CH
CH2
+
H2NCH2CH2NH
N
R
NH
N
N
D
HN
_
(SO3 )n
O
[cellulose]
O
CH2
NH
C
CH2CH2NHCH2CH2NH
N
Scheme 10.67
R
NH
N
N
HN
D
_
(SO3 )n
treated with N-methylolacrylamide (Scheme 10.61). This is capable of Lewis acid catalysed
etherification of hydroxy groups in cellulose and alkali-promoted Michael addition to
nucleophiles [393]. The modified cotton was reacted with aminoalkylated triazine dyes
(Scheme 10.67) at pH 10.5 in the presence of 80 g/l salt. In contrast to conventional
halotriazine reactive dyes, these are stable to hydrolysis under these conditions. However,
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675
build-up proceeded only up to a limit, beyond which the absorbed dye appeared to act as a
resist agent, preventing further uptake. In any case, this process still required the addition of
a high concentration of salt.
This approach was subsequently improved considerably by treating cotton with
2,4-dichloro-6-(2′-pyridinoethylamino)-s-triazine (Scheme 10.68). One of the chloro
substituents reacts under mild conditions with cellulose and the reactivity of the other
chloro is reduced [393], giving a fibre containing monochlorotriazine residues that can react
covalently with aminoalkylated dyes. Pad–batch pretreatment gave better results than
exhaustion, enabling application of the dye at pH 9 in absence of salt and giving similar
fastness to conventional dyeing. A highly significant advantage is that full use is made of the
dye applied because no dye hydrolysis can occur, resulting in shorter washing-off times and a
reduction in coloured effluent. However, the process requires special dyes and at present no
commercial range of these exists. Furthermore, there could be hydrolysis problems with the
pretreating agent or with the pretreated fibre. Any unlevelness in the pretreatment process
would clearly be disastrous in commercial terms.
[cellulose]
OH
Cl
+
N
N
+
N
_
NHCH2CH2
Cl
N
Cl
[cellulose]
O
N
+
N
_
NHCH2CH2
N
N
Cl
Cl
Scheme 10.68
A rather more perverse activity of researchers concerns their attempts to make cotton
dyeable with disperse dyes so that, for example, polyester/cotton blends could be dyed or
printed with a single dye class. This inevitably means invoking symbiotic chemistry; if cotton
is to become dyeable with hydrophobic disperse dyes, it must itself be made more
hydrophobic. This carries with it the risk that the very physical properties that make cotton
so desirable, and for which it is incorporated into blends with hydrophobic fibres, are
negated. Thus the moisture regain of a blend of polyester with hydrophobic cotton may be
such that it has no advantage over a fabric made entirely from polyester microfibre. It is
perhaps significant that moisture regain is hardly mentioned by researchers devising
treatments for making cotton more hydrophobic. Little is also said regarding light fastness,
although good fastness to washing is sometimes claimed [400].
Processes devised to make cotton hydrophobic are summarised in Table 10.42. These
processes are undoubtedly successful in conferring substantivity for disperse dyes but
attaining compatibility within a range of dyes across the entire colour gamut and on fibre
blends of various blend ratios could be a problem. In addition, ester bonds can be saponified
chpt10(2).pmd
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
during washing, typically losing 30% of the effect after one wash and 60–70% after five
washes. Hence commercial exploitation is subject to severe limitations.
Table 10.42 Pretreatments to make cotton more dyeable with disperse dyes [393,400]
Pretreatment reaction
Comments
Benzoylation with benzoyl chloride.
Proposed in the 1920s to confer easy-care
properties. In 1976 process patents introduced
for making cotton acceptable to disperse dyes
in transfer printing.
Water-soluble acylating esters sodium
benzoylthioglycolate and benzoylsalicylate
(Scheme 10.69).
Applied by padding with alkali and baking at 200 °C.
More acceptable for health and safety reasons
than the lachrymatory agent benzoyl chloride.
Aliphatic acid chlorides.
Increasing the length of the aliphatic chain increased
uptake of disperse dyes; maximum uptake needed
at least eleven C atoms per molecule.
Aromatic substituents in cellulose confer
greater disperse dye substantivity than
aliphatic substituents.
Benzoylation required a degree of substitution of
15–25% for optimum substantivity; 30–40% was
needed to give good fastness to washing.
Graft copolymerisation of styrene on partially
carboxymethylated cotton using
gamma radiation.
Higher graft yields of polystyrene gave higher
colour yields with disperse dyes.
O
C
SCH2COONa
+ HO
[cellulose]
_
HO
Sodium benzoylthioglycolate
O
C
O
[cellulose] + HSCH2COONa
O
[cellulose] + HO
O
C
O
+ HO
[cellulose]
_
HO
NaOOC
Sodium benzoylsalicylate
O
C
NaOOC
Scheme 10.69
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TREATMENTS TO ALTER DYEING PROPERTIES OR ENHANCE FASTNESS
Since wool dyeing is characterised by much higher levels of exhaustion than are typical
for the dyeing of cellulosic fibres, as well as much lower concentrations of electrolytes,
interest in pretreatments for wool is less concerned with the environmental aspects of
dyeing processes. The motivation to modify wool is usually to increase or decrease dye
absorption so that, for example, differential dyeing effects can be obtained [401]. As well as
overdyeing fabrics containing some predyed wool, possibilities in this area include the use of
prechromed and unchromed wool, pretreatment with a cationic agent to give increased dye
uptake or pretreatment with a syntan (section 10.9.4) to give a dye-resist effect. Attempts
around 1970 to commercialise the prechromed/unchromed system faltered as a result of
poor reproducibility. In any case, nowadays there would be environmental questions to be
addressed regarding chromium in the effluent. Pretreatment with a cationic agent or syntan
has economic benefits over the spinning of predyed wools in the production of heathereffect yarns.
Wool has been pretreated with glucose-derived crown ethers of the type shown in 10.144
and 10.145 [402]. Crosslinking occurs by interaction of amino groups in wool keratin with
the glucosidic hydroxy groups in the crown ether. Increased dyeability with reactive dyes was
achieved (Figure 10.53), a primary objective of the research being the low-temperature
dyeing of wool.
H2C
R
O
O
H2C
CH
O
CH2
O
CH
HC
CH2
HO
O
HC
HC
CH
H2C
CH
O
HO
OH
H2C
CH
CH2
O
H2C
CH2
O
HC
O
OH
HC
R = H or CH3
O
R
CH2
10.144
Bis-glucopyranoside-18-crown-6 ethers
O
O
O
MeO
OMe
O
O
O
O
O
O
O
O
O
O
O
O
O
O
OMe
O
O
10.145
Bis-glucopyranosido-24-crown-8 ethers
chpt10(2).pmd
677
O
O
O
O
O
O
O
O
O
MeO
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678
CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
100
Dye absorption/%
A
80
B
60
40
A Wool treated with glucosido
crown ether
B Untreated wool
20
30
60
90
110
130
Dyeing time/min
Figure 10.53 Rate of absorption of Lanasol Red G (Ciba; CI Reactive Red 83) by wool [402]
Wool dyeing at low temperatures has been enhanced by utilising the Maillard, Strecker
and modified Strecker (P-Mannich) reactions as pretreatments [403,404]. In a sense, the
interaction of wool with the glucose crown ethers already mentioned made use of the
Maillard reaction. This is the reaction between the oxo group of an aldose molecule and the
freely accessible amino groups of an amino acid or protein molecule, leading to the ultimate
formation of the tautomeric forms of an N-substituted 1-amino-1-deoxy-2-ketose via the
series of reactions shown in Scheme 10.70 [403]. This reaction is associated with the
browning observed when cooking saccharide-containing foods. Hence it is necessary to
control the conditions of application to wool so that yellowing or browning does not occur;
30 minutes at 90 °C gave satisfactory results [404]. The Strecker reaction is similar except
that a glucose-cyanohydrin is used (Scheme 10.71). The modified Strecker or P-Mannich
reaction typically uses dimethyl phosphite in place of the cyanohydrin [404], thus being
more acceptable on health and safety grounds for use in a dyehouse (Schemes 10.72 and
10.73).
R
NH2
+
H
NH
O
R
N
NH
R
C
CHOH
CH
CHOH
CHOH
CHOH
CHOH
CHOH
CHOH
CHOH
CHOH
CHOH
CHOH
CHOH
CHOH
CH2OH
CH2OH
CH2OH
Aldose
Aminated
aldose
Schiff
base
R
NH
C
CHOH
CHOH
CH2
CHOH
CHOH
OH
CHOH
CHOH
CH2OH
CH2OH
Enol
Keto
O
CH
CH
CH
OH
Aldose
amine
O
Amadori products
R = protein
Scheme 10.70
chpt10(2).pmd
R
COH
CH
HO
NH
CH2
HO
CH
R
CH
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TREATMENTS TO ALTER DYEING PROPERTIES OR ENHANCE FASTNESS
[wool]
[wool]
NaCN
+
H
C
CN
O
+
CHOH
C
H2N
CN
CH
(CH2)4
CH
NH
CHOH
CHOH
O
CHOH
CHOH
CHOH
CHOH
CHOH
CH2OH
CH2OH
Glucose
Cyanohydrin
NH
(CH2)4
CHOH
[wool]
CHOH
C
O
CH
NH
[wool]
CHOH
CHOH
lysine residue
CHOH
40°C
CH2OH
Scheme 10.71
COOH
CH3O
H2N(CH2)4CH
P
OH
OH
O
CH3
P
CH3O
OCH3
CH3
CH3O
CH3
C
O
+
CH3
CH3O
NH2
+
COOH
NH(CH2)4CH
O
NH2
CH3
P
CH3O
OCH3
CH3
L-Lysine
O
N-PhosphonomethylL-lysine
P
CH3O
H
Dimethyl phosphite
Scheme 10.72
[wool]
CH3O
CH3O
P
H
O
CH3O
C
CHOH
OH
P
C
O
+
H2N
(CH2)4
CH
CHOH
CHOH
+
O
CHOH
CH3O
CHOH
CH3O
O
P
H
P
CH
NH
C
O
NH(CH2)4
CHOH
[wool]
CHOH
CHOH
lysine residue
CHOH
CHOH
80°C
CHOH
CH2OH
CH2OH
[wool]
CH3O
CH3O
CHOH
CHOH
D-Glucose
CH3O
O
CH
NH
[wool]
CH2OH
Dimethyl
phosphite
Scheme 10.73
When the Maillard reaction was evaluated using 10 g/l glucose for 30 minutes at 90 °C
and 20:1 liquor ratio the fibre diameter increased by 3.5%; xylose gave an increase almost
twice as much but showed some yellowing. In this process accessibility of the fibre for dye
molecules is increased, since the glucose molecules penetrate between the peptide chains.
The reaction also introduces primary alcoholic groups, making the wool more dyeable with
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
the reactive dyes normally used on cellulosic fibres, as well as with bromoacrylamide reactive
dyes. Satisfactory dyeing was achieved at pH 5 and up to 80 °C on glucose-treated wool.
Penetration was rapid, enabling the dyeing time or temperature to be decreased. The buildup and fixation of reactive dyes were increased. The Strecker reaction utilising glucose–
cyanohydrin required only 20 minutes at 40 °C, whereas the modified Strecker reaction
using glucose–dimethyl phosphite needed 30 minutes at 80 °C. All treatments gave
improved dyeability, the order of efficacy being:
Modified Strecker > Strecker > Maillard
although differences were relatively slight. Dichlorotriazine dyes showed increased yield
after the modified Strecker reaction. There was no decrease in light fastness. Treatment with
dimethyl phosphite alone also improved dyeability but not to the same extent as glucose–
dimethyl phosphite. Penetration of the peptide chains by glucose–dimethyl phosphite is
shown in Scheme 10.74. The process is thought to occur in the following stages [404]:
(1) Penetration of dimethyl phosphite into the fibre, accompanied by decomposition of salt
linkages and elimination of structural water in a multi-step hydration process. New salt
linkages are formed between cationic groups in wool keratin and anionic dimethyl
phosphite.
(2) Penetration of glucose molecules into the loosened structure, causing further increase
in accessibility of the reactive sites.
(3) Formation of covalent bonds between dimethyl phosphite, glucose and amino groups in
wool keratin, stabilising the loosened fibre structure.
Imaginative use has been made of triazine and sulphatoethylsulphone reactive dye chemistry
in the application of pretreatments to nylon [405]. The concept resembles that used to
make cotton cellulose reactive before dyeing with aminoalkylated dyes, as discussed earlier
(Schemes 10.67 and 10.68). In this case, nylon becomes the reactive partner by
pretreatment with a reactive multifunctional crosslinking agent:
(a) 1,3,5-tris(acryloyl)hexahydro-s-triazine (10.146) or
(b) the trifunctional reactive compound XLC (10.95), already discussed as a shrink-resist
reactant for wool (section 10.5.5).
The pretreated nylon then undergoes covalent fixation of dyes containing aminoalkyl
groups. Interestingly, nylon treated with XLC showed markedly lower substantivity and
reactivity with conventional dyes. If the pretreated nylon was reacted with ammonia,
however, creating amino functionality at the reactive sites, normal reaction with a
conventional reactive dye was restored [405].
Many of the syntan aftertreatments described in section 10.9.4 can be applied as
pretreatments instead, mainly to confer dye-resist effects. This aspect is dealt with in section
10.9.4 rather than here.
10.9.2 Ultraviolet absorbers
The use of polyester and nylon upholstery fabrics in automobiles results in their prolonged
exposure to sunlight at high temperatures and humidities. This has created a demand for
dyeings of very high light fastness and for these dyed fibres to have exceptional resistance to
photodegradation. Although the selection of dyes of suitably high fastness under these
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Scheme 10.74
O
O
Lysine
Lysine
O
Serine
C
HN
C
HN
C
NH
CH
C
NH
CH
C
NH
CH
O
O
O
(CH2)4
_
O
(CH2)4
C
O
+
H3N
CH2
O
O
CH2
O
+
NH3
CH2
+
NH3
(CH2)4
H H
NH
NH
HN
C
CH
C
HN
C
CH
C
HN
C
CH
Serine
O
Aspartic acid
O
Lysine
O
O
O
O
C
HN
C
HN
C
NH
CH
C
NH
CH
C
NH
CH
O
O
(CH2)4
(CH2)4
CH2
O
O
O
CH2OH
CHOH
CHOH
CHOH
CHOH
CHOH
P
CH3O
CH3O
+
NH3
P
CHOH
CHOH
CHOH
CHOH
CH3O
CH3O
+
NH3
H
CHOH
CH2OH
_
O
O
+
H3N
H
C
O
CH2
O
(CH2)4
O
CH2
NH
NH
C
HN
C
CH
C
HN
CH
C
HN
C
CH
O
O
O
TREATMENTS TO ALTER DYEING PROPERTIES OR ENHANCE FASTNESS
681
682
CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
O
CH2
CH
C
N
H2C
N
N
CH2
CH
CH2
CH
CH2
C
CH2
O
C
O
10.146
extreme conditions is of primary importance, additional protection can be achieved using
so-called ultraviolet absorbers, these being colourless aromatic compounds with a high
propensity to absorb the troublesome UV radiation. Such products are usually applied during
dyeing and confer protection to both dyes and fibres. Although the main interest in UV
absorbers centres on polyester and nylon, they are also recommended for use on other fibres,
e.g. polypropylene, silk, wool (particularly bleached wools that have a tendency to
photoyellowing) and cotton. More recently, due to the increased incidence of skin
carcinomas induced by excessive exposure to sunlight, UV absorbers are being used together
with modifications of fabric construction to give increased resistance to the passage of
harmful rays through the fabric to the skin [406–408].
The principle involved is the same as that used in skin protection creams (that go under
the commonly used misnomer of sun protection screens), as well as finding extensive use in
plastics and surface coatings to enhance their durability to sunlight exposure. Two useful
reviews are available [409,410]. Table 10.43 gives a comparison of the components of solar
radiation. This illustrates very clearly the apparent paradox that those components present
in least quantity nevertheless pack the greatest punch in terms of photon energy. Thus out
of a total photon energy value of 1328 kJ/mol UV radiation of wavelengths 280–400 nm
accounts for 1065 kJ/mol (80%), despite the fact that it represents only 6% of the total
radiation intensity. This is the radiation that is primarily responsible for the degradation of
fibres and colorants. This is also the region, or at least part of it, in which UV absorbers have
to function. A useful definition [410] states that a UV absorber is a molecule that may be
incorporated within a host polymer in order to absorb ultraviolet radiation efficiently and
convert the energy into relatively harmless thermal energy, without itself undergoing any
Table 10.43 Intensity of global radiation (sum of direct and scattered radiation)
at the earth’s surface and its classification (summer, vertical incidence) [409]
Radiation intensity
Regions of
solar radiation
UV-B
UV-A
Visible
Infrared
chpt10(2).pmd
682
Wavelength (nm)
(W/m2)
(%)
Mean photon
energy (kJ/mol)
280–320
320–360
360–400
400–800
800–3000
5
27
36
580
472
0.5
2.4
3.2
51.8
42.1
400
350
315
200
63
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TREATMENTS TO ALTER DYEING PROPERTIES OR ENHANCE FASTNESS
683
irreversible chemical change or inducing any chemical change in the host macromolecules.
Thus UV absorbers do not simply scavenge UV radiation preferentially and become
themselves expended in the process, in the way that gas-fume fading inhibitors operate
(section 10.9.3). UV absorbers convert electronic excitation energy into thermal energy by a
rapid but reversible intramolecular proton transfer mechanism [410].
The fundamental nuclei of most of these compounds are represented by o-hydroxybenzophenone (10.147), o-hydroxybenzotriazole (10.148) or o-hydroxybenzotriazine
(10.149). All of these structures exhibit keto–enol tautomerism, the keto form being
favoured on irradiation. The characteristics of these skeletal structures can be modified by
introducing substituents to make them more suitable for specific purposes. For example
water-insoluble UV absorbers are preferred for application with disperse dyes for polyester,
whereas water-soluble sulphonated derivatives are more suitable for use with anionic dyes
on nylon. Thermal stability will be required if such agents are to be incorporated into molten
polymers or intended for pad–thermosol application to polyester. Typical variants, together
with other types of protective agents, will be mentioned later when dealing with specific
fibres. Even certain dyes can act as UV protective agents, particularly in heavy depths, but
other dyes may catalyse fading or fibre degradation [411]. Derivatives of certain dyes with
built-in UV absorber moieties have been produced specifically with the aim of obtaining dyes
of exceptionally high light fastness. For example, CI Disperse Yellow 33 (10.150; R = H) can
be converted to a benzophenone derivative (10.151) and a benzotriazole analogue (10.152)
of the important polyester dye CI Disperse Yellow 42 (10.150; R = Ph) is also readily
available [410]. In some instances there is also a need for an antioxidant; these are discussed
below in connection with nylon additives.
H
HO
O
hν
C
O
O
OH
C
OH
10.147
Enol
Keto
HO
H
hν
N
O
N
N
N
N
N
10.148
Enol
Keto
hν
N
O
N
N
N
R3
N
R2
683
R3
N
10.149
Enol
chpt10(2).pmd
H
R1
HO
R1
R2
Keto
15/11/02, 15:44
684
CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
HO
O2N
O2N
O
C
HN
HN
SO2
HN
OCH3
SO2
HN
R
10.151
10.150
O2S
HO
NH
O2N
N
N
NH
N
Cl
CH3
10.152
Polyamides are characteristically prone to photochemical and thermal degradation,
although much depends on the structure and purity of the polymer and the types and
concentrations of additives present. Although the titanium dioxide delustrant is an effective
UV absorber, it is also a sensitiser of the photodegradation of nylon. The decomposition is
initiated mainly by free radicals from sensitising impurities. Although most polyamides are
resistant to photochemical attack below 40–50 °C, they can degrade fairly rapidly at higher
temperatures, particularly with higher concentrations of titanium dioxide present and when
dyed in pale or medium depths. Certain metal-complex dyes, however, can act as
photostabilisers, especially in full depths. UV absorbers afford protection to nylon only at
black-panel temperatures below 40–50 °C. Since degradation is largely the result of
oxidative free-radical attack, protective effects are shown by antioxidant addition, either
alone or in conjunction with a UV absorber. Both agents may be incorporated during fibre
manufacture or can be applied during dyeing or finishing. Some commercial formulations
contain both types of agent.
Water-soluble UV absorbers are preferred for nylon, particularly sulphonated benzotriazoles
of the general type represented by 10.148, with halo, sulpho or sulphonated arylalkyl
substituents in the benzotriazole nucleus and alkyl, alkoxy or sulpho groups in the phenolic
nucleus. A specific example is provided by structure 10.153. Sulphonated derivatives of the
dihydroxybenzophenone structure 10.147 are also suitable. One study of the influence of UV
absorbers on light fastness involved a selection of nine acid dyes typical of those used in the
dyeing of nylon carpets. The efficacy of four water-soluble agents applied by exhaust dyeing
(10.154–10.156 and an undisclosed structure of the anionic o-hydroxyphenylbenzotriazole
type) and three water-insoluble types applied from solution in tetrachloroethylene
(10.157–10.159) was evaluated [412]. Some of the UV absorbers significantly improved light
fastness, but others gave significantly lower ratings. Overall, the behaviour of the UV absorbers
was dye- and hue-specific [412]. Conversely, in another investigation it was claimed that
stabilisers applied to nylon during dyeing can markedly improve fibre stability and light
fastness, independently of hue and depth of shade [413].
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TREATMENTS TO ALTER DYEING PROPERTIES OR ENHANCE FASTNESS
HO
CH3
HO
685
O
CH2CH3
CH
HO
N
C
OCH3
N
SO3Na
N
SO3Na
CH3O
10.154
SO3Na
10.153
HO
O
C
OCH3
SO3Na
HO
O
HO
C
N
10.155
O(CH2)7CH3
N
N
CH3
10.156
10.157
HO
O
O
C
N
CH
C
N
CH3CH2O
OCH3
CH3
10.158
10.159
The additional protection given to nylon by antioxidants has already been mentioned.
Since the need is to protect against oxidation by free radicals, antioxidants are essentially of
two types: peroxide decomposers and radical scavengers. Reviews of these products are
available [409,410,413]; these should be consulted for details of the mechanisms involved.
Peroxide decomposer types include compounds of manganese(II) or copper(I) and
copper(II) complexes, such as azomethine bridge derivatives of the type represented by
10.160, of which numerous water-soluble or water-insoluble variants are possible [409].
These products have a catalytic action and are therefore used in very small amounts.
Y
N
N
HC
CH
Cu
R
O
X
chpt10(2).pmd
R
O
10.160
685
X
R = H, halo, hydroxy, alkyl, alkoxy
X = H, sulpho, carboxyl, sulphonamide
Y = alkylene, cycloalkylene linkages
15/11/02, 15:44
686
CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
Conversely, radical scavengers have to be used in larger amounts because they lack the
regeneration capability of catalytic types. The principal product types [409] are sterically
hindered phenols (10.161) and sterically hindered amines (10.162). The latter are important
because they act not only as scavengers but also as peroxide decomposers [410]. Further
compounds include the semicarbazides 10.163 and 10.164, applicable by continuous and
exhaust methods respectively. The product 10.165 is suitable only for continuous
application. Most radical scavengers are suitable only for thermal stabilisation [410],
photostabilising scavengers being restricted to transition-metal chelates, especially nickel(II)
dithiocarbamates (10.166), and hindered amines such as 2,2,6,6-tetramethylpiperidine
(10.167). Water-soluble triazine derivatives of hindered amines (10.168) have also been
used [410].
H3C
CH3
C
HN
CH3
H3C
HO
R
A
CH3
R = H, oxy, hydroxy, acyl, alkyl, alkoxy
A = O or NH
X = H, sulpho, carboxyl
Y = alkylene, arylene, substituted arylene
(identical or different)
R = H, halo, alkyl, alkoxy
X = H, sulpho, carboxyl, sulphonamide
A = triazine or (CH2)nCONH linkages
H3C
CH3
NH
HN
X
10.162
10.161
H3C
HN
CH3
X
C
Y
N
N
H3C
H3C
N
N
A
H3C
X
N
H3C
R
Y
(CH2)6
NH
HN
N
N
CH3
C
C
SO3Na
H3C
NH
H3C
HN
C
NH
HN
O
CH3
N
C
CH3
O
O
O
10.163
10.164
OH
CH3CH2CH2CH2
N
CH2
S
N
H2C
Ni
C
S
CH3CH2CH2CH2
10.165
CH2CH2CH2CH3
S
C
S
N
CH2CH2CH2CH3
10.166
Cl
H3C
H3C
CH3
N
CH3
H3C
H
CH3
N
NH
N
N
N
HN
H
H3C
10.167
chpt10(2).pmd
686
CH3
10.168
15/11/02, 15:44
SO3Na
687
TREATMENTS TO ALTER DYEING PROPERTIES OR ENHANCE FASTNESS
Water-soluble UV absorbers are preferred on bleached wool, mainly to retard the
photoyellowing process. The sulphonated o-hydroxybenzotriazole type (10.169) is
particularly effective [414]. When applying such products by the exhaust process it is best to
avoid the use of sulphates as these lower the exhaustion, as does a pH higher than 5.5
[415,416]. These products are claimed to be fast to hand washing, shampooing and dry
cleaning [416]. Nevertheless, polymeric UV absorbers have been developed with the
intention of improving on the limited fastness of conventional types [417]. These have
typical UV absorber moieties attached to an acrylate, methacrylate or methacrylamide
polymer, examples of relevant monomers being 10.170–10.172.
HO
N
R
N
X
HO
N
X
O
O
10.169
C
C
OCH2O
C
CH2
H3C
R = alkyl, alkoxy, sulpho
X = H, halo, sulpho, sulphonated arylalkyl
10.170
H3C
H2C
HO
CH3
N
C
CH2
C
N
O
N
N
C
H3C
CH3
(CH2)4CH3
10.171
HO
H3C
CH3
C
O
O
CH3
N
CH2CH2
C
N
CH2CH2CH2
N
O
C
CH
CH
O
CH
CH2
CH2CH2
10.172
Polyester fibres generally show high resistance to photodegradation providing exposure in
the 280–320 nm range is avoided, so the main reason for using UV absorbers on this fibre is
to enhance light fastness, particularly to the standards required for automobile furnishings.
Indeed, marked improvements generally only become evident with prolonged exposure at
high ambient temperatures [409]. Water-insoluble benzophenones and benzotriazoles, such
as 10.173 and 10.174 respectively, are widely used. These products, however, have only
moderate fastness to sublimation. Where a high setting temperature or pad–thermosol
application is involved, o-hydroxyphenylbenzotriazoles of greater molecular mass offer
higher fastness to sublimation [409]. Water-insoluble mono- (10.149) or bis(o-hydroxy-
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
phenyl)triazines are also of interest. Derivatives of o-hydroxyphenylpyrimidine and
benzoxazin-4-one have also been patented [409]. In a recent series of papers [418,419], the
effectiveness of the following UV absorbers on polyester was demonstrated: 10.147, 10.156,
10.175 and a benzotriazole of undisclosed structure.
HO
OH
O
HO
H3C
C
CH3
N
C
CH3
N
CH3O
N
Cl
OCH3
HO
H3C
CH3
C
HO
CH3
N
O
N
Cl
CH3
10.174
10.173
C
N
C
H3C
10.175
CH2
CH3
CH3
O
10.176
Benzyl salicylate (10.176) at an applied concentration of 2% o.w.f. is reported [420] to
have given good protection from fading to two anthraquinone acid dyes on silk.
Water-soluble UV absorbers can be used on cotton to improve the photochemical
stability of the fibre or to protect the skin from UV radiation but this does not generally
improve the light fastness of dyeings. From a chemical viewpoint, bifunctional reactive UV
absorbers (10.177) for cotton are of particular interest [421]. The vat dye 10.178 containing
two imidazole rings is patented [422] for use by dyeing or printing on cellulosic fibres to
improve solar protection both to the wearer and to the fibre. The use of 10.177 in
combination with 10.178 is also patented [421]. Normal untreated cotton with a sun
protection factor of 5 can be improved to factor 28 after applying 10.178 and to factor 187
when this dye is used in combination with the reactive UV absorber 10.177.
Cl
OCH2CH3
O
N
NH C
C HN
NaO3S
NH
N
N
HN
SO2CH2CH2OSO3Na
O
10.177
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O
689
O
Cl
O
NH
HN
N
O
N
Cl
10.178
10.9.3 Gas-fume fading inhibitors
Not all disperse dyes on cellulose acetate fibres are resistant to oxides of nitrogen that may
be present in the atmosphere. Susceptible dyes, usually those containing primary amino or
secondary amino groups, can undergo quite profound changes in hue depending on the
reactivity or basicity of the susceptible group(s). The primary general mechanism of fading is
believed to be the formation of N-nitrosamines [423]. The problem is best avoided by using
dyes that do not fade, but this may not always be possible as these tend to be more costly
and less easy to apply. Some protection can be obtained by treatment of susceptible dyeings,
either during or after dyeing, with colourless agents that react preferentially with oxides of
nitrogen. Such agents are known as gas-fume inhibitors. Since they act as scavengers of the
acidic oxides of nitrogen, they need to be more basic in character than the dyes they protect.
Many such basic compounds, generally applied from aqueous solution or dispersion, have
been proposed [424].
The most popular and efficient are substantive to the fibre; typical examples are N,N′diphenylacetamidine (10.179), which tends to yellow on exposure to oxides of nitrogen, and
particularly the diphenylated diamines such as N,N′-diphenylethylenediamine (10.180),
which does not yellow. Non-substantive inhibitors applied by padding and drying, such as
triethanolamine (10.126) and melamine (10.181), have also been used despite the fact that
they are removed on washing. The demand for and commercial availability of gas-fume
inhibitors have declined.
NH
NH2
C
CH3
N
NH
N
HN
H2N
CH2CH2
10.180
10.179
N
N
10.181
NH2
Atmospheric ozone has also been reported as causing fading of certain dyes in some
countries [425,426]; diallyl phthalate (10.182) used as a carrier in the dyeing of cellulose
triacetate fibres, is said to be an effective ozone inhibitor [427]. Nylon, especially when dyed
with certain amino-substituted anthraquinone blue acid dyes, can also be susceptible to
ozone fading [428,429]. Selection of ozone-resistant dyes is obviously the best counteractive
measure, although hindered phenols (10.161) and hindered amines (10.162) are said to
provide some protection.
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
O
C
OCH2CH
CH2
C
OCH2CH
CH2
O
10.182
10.9.4 Aftertreatments and resist treatments for acid dyes
By far the most important aftertreatment for acid dyeings is the so-called syntan process
widely used on nylon, which has superseded the classic full back-tan process. This involved
treatment of the dyed nylon first with ‘tannic acid’ (a highly complex gallotannin or
polygalloylated glucose, such as structure 10.183), which hydrolyses to give digallic acid
(10.184) or gallic acid (10.185) [391,430], followed by further treatment with potassium
antimonyl tartrate (tartar emetic; 10.186). It has been replaced on grounds of its high cost,
instability to hot alkali, undesirable effects on fabric handle and light fastness, changes in
colour (usually dulling) during treatment, and diffusion and degradation of the antimonyl
tannate complex during subsequent steaming or dry heat setting, as well as for health and
environmental reasons. Its presumed mechanism of action is complex [391,430].
OH
OH
HO
O
C
OH
HO
OH
O
OH
OH
C
O
O
C
OH
CH2
O
CH
O
O
O
CH OH
C
HO
O
CH
O
O
C
CH
OH
O
O
CH
O
O
C
C
HO
HO
O
HO
HO
C
O
OH
OH
OH
OH
HO
OH
HO
10.183
HO
O
C
OH
O
OH
HO
C
O
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TREATMENTS TO ALTER DYEING PROPERTIES OR ENHANCE FASTNESS
O
OH
C
O
C
OH
HO
10.185
OH
_
O [SbO] +
CH
OH
CH
OH
C
O
691
10.186
_
O K+
The synthetic tanning agents (syntans) that have superseded the back-tan have the
advantage of being applied in a single process but they tend to be polyphenolic compounds
like the naturally derived gallotannins. In fact, compounds of this class have been described
in section 10.6.1 as condensation products of formaldehyde with sulphonated phenols,
naphthols or naphthylamines. Structures 10.100–10.103 are typical of these compounds
although, as mentioned earlier, they form a large and varied group. The precise structural
details of typical commercial syntans for nylon are hidden within a plethora of patent
literature, much of which pertains to their use as tanning agents for leather [391]. In
addition to the products mentioned in section 10.6.1 analogous sulphur-containing
products, such as those derived from thiophenols, and heavy-metal phenolates were formerly
used.
A brief simplified mechanism whereby syntans are thought to operate can be described, at
the risk of incompleteness. Their rapid sorption by the fibre under the (usually acidic)
conditions of application is largely the result of electrostatic attraction between negatively
charged sulphonate groups and protonated amino groups in the fibre. Hydrogen bonding
between uncharged polar groups, and hydrophobic bonding between the nonpolar moieties
in the syntan and the fibre, create conditions for the formation of complexes. Maximum
improvement in wet fastness results when the complexes are formed at the fibre surface,
since any treatment leading to diffusion of the syntan complex into the fibre tends to yield
lower fastness. Hence a ‘barrier effect’ seems to be involved. Possibly multilayers can be
formed via a Brunauer, Emmett and Teller (BET) mechanism. Some syntans will give a
modest improvement in the wet fastness of disperse dyes on nylon but even with acid dyes
the response to the syntan treatment in terms of improved wet fastness varies markedly from
dye to dye.
Empiricism has guided the screening of syntans for nylon. Both the affinity of the syntan
for the fibre and its diffusion rate are important, and the required values are obtained by
balancing the degree of sulphonation and range of molecular sizes present. Moreover, many
of these products are used not only as aftertreating agents for the improvement of wet
fastness, but also as blocking agents to inhibit absorption of dye either partially or completely
– for example, in the dyeing of nylon/wool blends to restrain the usually preferential uptake
of dye by the nylon, or to produce resist effects, particularly in printing. The balance of
properties required in the syntan may therefore vary somewhat and is influenced by the
characteristics of the dye–fibre system for which it is intended.
Studies on nylon, using commercial syntans of undisclosed structure [431–433] showed
that the absorption of syntan increased with decreasing pH, providing further evidence for
electrostatic interaction. Evidence was also found for the BET mechanism, indicating that
other forces are also operating, such as hydrophobic interaction. Uptake of syntan increased
with applied concentration and increasing temperature, this being attributable to the higher
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
kinetic energy of the syntan molecules and the greater extent of fibre swelling. Syntan
uptake was greater on nylon microfibres (greater surface area) than on conventional nylon
staple fibres, but the fastness to washing was inferior on microfibres. Application of a
cationic agent subsequent to the syntan treatment gave an additional significant
improvement in fastness, particularly where multiple washes were involved [433].
Unsulphonated 1:2 metal-complex dyes showed less desorption of dye than disulphonated
types after 1–3 washes but greater loss after 5–10 washes. The cost and inconvenience of
these two aftertreatments, however, would not be justifiable under commercial conditions.
Similar commercial syntans have been evaluated in dye–resist pretreatments on wool
[434]. Evidence for both kinds of agent–fibre interaction was again found. The resist effect
imparted to the wool was specific to the type of anionic dye applied, giving high resistance to
four hydrophilic dyes and low resistance towards two hydrophobic dyes. Syntan desorption
occurred during dyeing, indicating that these agents are relatively weakly attached to the
fibre. The desorbed syntan had a restraining effect on the uptake of all six dyes examined.
Section 10.9.1 includes an account of nylon pretreatments based on chemistry normally
associated with reactive dyes for cellulosic fibres. Similar chemistry has been used to develop
alternatives to syntans, both as aftertreatments to improve fastness and as pretreatments to
give dye-resist effects. The trifunctional reactive compound XLC (10.95), already discussed
as a shrink-resist reactant for wool (section 10.5.5), is hydrolysed under alkaline conditions
to the active vinylsulphone form (10.187). This trifunctional crosslinking agent has been
evaluated to improve the fastness of nylon dyed under mildly acidic conditions with specially
prepared anionic aminoalkylated triazine dyes [435]. Such dyes do not bond covalently with
nylon, but aftertreatment with this trifunctional reactive agent results in a significant degree
of covalent bonding with a consequent improvement in wet fastness. A cationic
aminoethyisulphonyl dye (10.188) was prepared and applied to nylon at the optimum pH 10,
followed by treatment with the parent sulphatoethylsulphone crosslinker (10.95). In this
way, the dye became fixed covalently via the agent to terminal amino groups in nylon,
resulting in improved fastness to washing [435].
Traditional syntan treatments are rather ineffective for improving the wet fastness of
anionic dyes on wool, mainly because of weakness of the interaction between syntan and
fibre. It is therefore not surprising that covalent reactive systems have been explored to find
more effective aftertreatments and dye-resist treatments for this fibre. In an initial study
Cl
N
H2C
CHSO2
NH
N
N
HN
SO2CH
10.187
CH2CH3
N
H2NCH2CH2SO2
N
N
+
N
_
CH2CH2
X
10.188
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TREATMENTS TO ALTER DYEING PROPERTIES OR ENHANCE FASTNESS
693
aimed at producing a system that would overcome the level dyeing problems associated with
reactive dyes on wool [436], a specially prepared aminoalkylated dye (not fibre-reactive) was
applied together with hexamethylenetetramine, from which formaldehyde was released
(Scheme 10.75) to give covalent bonding between the amino groupings in the dye and in
the fibre (Scheme 10.76). This process was successful up to a point but gave lower fixation
when dyeing in full depths. Certain chromogens also showed marked changes of hue.
N
H2C
CH2
CH2
H2C
N
+ 6 H2O
H2C
N
6 HCHO + 4 NH3
N
CH2
Scheme 10.75
[dye]
CH2CH2NH2
[dye]
Scheme 10.76
+ HCHO + H2N(CH2)4[wool]
CH2CH2NHCH2NH(CH2)4[wool]
This approach was subsequently improved [437] by dispensing with the formaldehyde
precursor and adding a trifunctional crosslinking agent to the dyebath after dye absorption
was complete. The crosslinking agent was 1,3,5-tris(acryloyl)hexahydro-s-triazine (10.146).
Scheme 10.77 represents an idealised reaction for the formation of dye–agent–wool linkages.
Since reaction takes place in a random manner, however, the derivatives 10.189–10.192
could equally well be produced. Products 10.191 and 10.192 need to be minimised, because
respectively they are linked only to the dye or to the fibre [437].
O
CH2
CH
C
N
2 [dye]CH2CH2NH2 +
H2C
N
CH
CH
CH2
+ H2N(CH2)4[wool]
C
CH2
O
N
CH2
CH2CH2NH(CH2)4[wool]
N
CH2
N
CH2
CH2
C
O
O
[dye]CH2CH2NHCH2CH2
C
H2C
[dye]CH2CH2NHCH2CH2
Scheme 10.77
chpt10(2).pmd
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C
O
C
O
693
15/11/02, 15:44
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
O
[dye]NHCH2CH2
O
C
[dye]NHCH2CH2
N
N
N
C
N
[dye]NHCH2CH2
C
CH2CH2X[wool]
O
C
[wool]XCH2CH2
C
N
O
O
X = NH or S
C
X = NH or S
O
CH2CH2X[wool]
N
10.189
10.190
O
O
[dye]NHCH2CH2
[wool]XCH2CH2
C
N
N
CH2CH2NH[dye]
N
N
[dye]NHCH2CH2
C
C
N
O
[wool]XCH2CH2
C
C
O
O
CH2CH2X[wool]
N
10.191
C
O
X = NH or S
10.192
Subsequently, two more cost-effective bifunctional crosslinking agents, both commercially available, were used to give the same quality of dyeings [438], these being N,N′methylene-bis-acrylamide (10.193) and a quaternised precursor (10.194). Idealised reactions
with functional groups in the dye and in wool are given in Schemes 10.78 and 10.79.
Although these bifunctional reagents are cheaper than the trifunctional product, dyeing has
to be carried out at pH 8 as their reactivities are lower than that of tris(acryloyl)hexahydros-triazine. However, this lower reactivity allows the bis-acrylamide to be added at the
beginning of the dyeing process. The quaternised derivative is a more effective crosslinking
agent, possibly because of its greater substantivity for wool under slightly alkaline dyeing
conditions, but it cannot be added at the beginning of dyeing because of its cationic
character.
There has been active interest in exploring the potential of reactive triazines as dye-resist
agents for wool. A model structure for reactive dye-resist agents (10.195) has been proposed
[439], where Z is a suitable high-reactivity group, NH is a bridging group, Ar is an aryl group
and [SO3] – is a solubilising and anionic dye-repelling group. Four dichlorotriazine
compounds (10.196–10.199) based on this model structure were found to be effective as
+ HX
[wool]
[dye]CH2CH2NHCH2CH2CONHCH2NHCOCH2CH2X
[wool]
[dye]CH2CH2NH2 + CH2
CHCONHCH2NHCO CH
CH2
10.193
X = NH or S
Scheme 10.78
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TREATMENTS TO ALTER DYEING PROPERTIES OR ENHANCE FASTNESS
_
+
A R3N CH2CH2CONHCH2NHCOCH2CH2
695
_
+
NR3 A
10.194
CH2
CHCONHCH2NHCOCH
CH2
_
+
+ 2 NHR3 A
10.193
+ HX
[dye]CH2CH2NH2
[wool]
[dye]CH2CH2NHCH2CH2CONHCH2NHCOCH2CH2X
_
X = NH or S
A = anion
[wool]
Scheme 10.79
dye-resist agents, although full white resist effects were difficult to achieve [439]. Pursuing
this line of research further, several more complex naphthylamine-based dichlorotriazine
dye-resist agents (10.200–10.206) were evaluated [440]. These agents each contained at
least one dichlorotriazine reactive moiety as well as a sulphonated naphthylamine system
that enabled the anionicity of the product to be varied. Using the trisulphonated dye CI
Acid Red 27 (10.207), the resist efficiency improved significantly with increasing degree of
sulphonation of the agent and with increasing relative molecular mass, enabling lower
concentrations of the more effective agents to be used. The upper limit of resist achieved
was 98.6%.
Cl
_
Z
NH
Ar
N
[SO3 ]n
N
NH
SO3Na
N
10.195
Cl
10.196
Cl
Cl
N
N
Cl
N
NH
NH
N
N
N
N
Cl
10.197
Cl
SO3Na
N
N
Cl
O
NH
C
N
HO
HN
Cl
10.199
chpt10(2).pmd
10.198
N
NH
SO3Na
695
COONa
Cl
N
N
NH
N
Cl
SO3Na
Cl
OH
N
10.200
15/11/02, 15:44
SO3Na
696
CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
Cl
N
N
O
NH
C
N
HN
Cl
SO3Na
10.201
NaO3S
Cl
N
O
N
NH
SO3Na
C
N
HN
Cl
SO3Na
10.202
NaO3S
Cl
N
N
O
NH
C
O
HN
N
C
Cl
HN
SO3Na
10.203
Cl
N
N
O
NH
C
N
O
HN
C
Cl
HN
SO3Na
10.204
NaO3S
Cl
Cl
SO3Na
N
N
N
NH
NH
N
N
N
Cl
NaO3S
HN
SO3Na
10.205
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TREATMENTS TO ALTER DYEING PROPERTIES OR ENHANCE FASTNESS
Cl
Cl
SO3Na
N
N
697
N
NH
NH
N
N
N
Cl
NaO3S
HN
SO3Na
10.206
NaO3S
O
NaO3S
SO3Na
HN
N
10.207
SO3Na
CI Acid Red 27
A rather different approach [441] to obtaining resist effects on wool involved evaluation
of the relatively simple compound sulphamic acid (10.208). From a comparison of curing
temperatures over the range 100–150 °C, it was found that only by curing at 140 °C or
higher was the sulphamic acid bound sufficiently to resist desorption during dyeing. There
are several possible routes of decomposition of sulphamic acid [441], leading to the
formation of various sulphonic acids and sulphonamides, as well as the possibility of scission
to form sulphur trioxide and ammonia (Scheme 10.80). The reactions of sulphamic acid
with wool are mainly through hydroxy groups and to a much lesser extent amino groups in
the fibre (Scheme 10.81). Thus the dye-resist effect is provided by the presence on the
modified fibre of anionic sulphate and sulphamate groups [442]. This approach was also
examined on silk [443]. Compared with wool, silk has a much smaller proportion of
nucleophilic amino groups but a substantial content of hydroxy groups with which
sulphamic acid reacts preferentially. Excellent dye-resist effects were obtained with acid, 1:2
metal-complex and reactive dyes, better than when a commercial dichlorotriazine reactant
(similar to 10.196) was used.
10.9.5 Aftertreatments for direct and reactive dyes on cellulosic fibres
The fact that the aftertreatment of direct dyes has a long history is not surprising since wet
fastness within this class is not particularly good. Their prime advantages are ease of
application and economy compared with dyes of higher fastness (reactive, sulphur or vat) –
hence the continued search for highly effective aftertreatments that improve wet fastness
O
H2N
S
O
chpt10(2).pmd
697
O
+
H3N S
OH
10.208
_
O
O
15/11/02, 15:44
698
CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
O
HO
S
O
NH2
+ H2N
S
O
O
O
O
H2N
S
+
OH
H2N
S
O
O
OH
HO
S
OH
H2N
S
OH
+ NH3
O
O
S
NH
O
S
OH
+ H2O
O
SO2
OH
HN
NH
O2S
SO2
+ 3H2O
NH
O
O
+
H3N S
NH
O
O
H2N
S
O
O
3
O
_
O
NH3 + SO 3
O
Scheme 10.80
[wool]
OH +
[wool]
NH2
HO3S
+
HO3S
NH2
[wool]
[wool]
NH2
_
SO3
O
NH
+
NH4
_ +
SO3 NH4
Scheme 10.81
without excessive additional cost. Some of the aftertreatments used for this purpose have
also been applied to reactive dyes, as will be described later.
One of the earliest aftertreatment processes employed diazotisation of free amino groups
in the adsorbed dye, followed by coupling (development) with a suitable component such as
a phenol, naphthol or amine; 2-naphthol, m-phenylenediamine, resorcinol and 1-phenyl-3methylpyrazol-5-one (10.209) were particularly popular. Obviously, this was only possible
with dyes containing a diazotisable amino group, an example being CI Direct Yellow 59, the
classic primuline (a mixture of structures 10.210a and b), which was converted on the fibre
from a greenish yellow to a bluish red by diazotisation and development with 2-naphthol.
The reverse approach, application of a dye containing a phenolic group and aftertreatment
with a solution of a diazotised amine, such as p-nitrobenzenediazonium chloride, was also
used.
N
O
N
CH3
10.209
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TREATMENTS TO ALTER DYEING PROPERTIES OR ENHANCE FASTNESS
699
SO3Na
H3C
S
a
S
C
C
N
NH2
N
SO3Na
H3C
S
S
C
b
S
C
N
C
N
NH2
N
10.210
Formaldehyde aftertreatment was employed to link together pairs of amino-substituted
dye molecules by a methylene bridge (10.211). The required reactivity of the sites in the dye
towards formaldehyde was ensured by pairs of o- and p-directing electron-donating groups
provided by resorcinol, m-phenylenediamine or 3-aminophenol.
Ar
Ar
N
N
N
H2N
CH2
NH2
N
NH2
H2N
10.211
A third approach utilised copper salts, especially copper(II) sulphate, in conjunction with
dyes containing chelatable groupings such as salicylic acid or o,o′-dihydroxyazo moieties.
Indeed, special ranges of ‘copperable’ direct dyes, for which the treatment with copper(II)
sulphate was really part of the dyeing process rather than an optional aftertreatment, were
introduced. In the past the main use of this chelation treatment was to enhance light
fastness, but it is little used for this purpose nowadays.
Since direct dyes have anionic structures, many cationic surfactants such as quaternary
ammonium compounds were used as aftertreatments to form surfactant–dye complexes of
reduced aqueous solubility and therefore higher wet fastness. The improved fastness related
only to non-detergent agencies such as perspiration and water, however. In soap-based
washing processes the stronger interaction between the anionic soap and the cationic agent
tended to cleave the dye–cation complex, thus effectively negating the aftertreatment even
after a single mild wash. The aftertreatment often brought about changes in hue and
reduced light fastness, although the latter could sometimes be countered by a combined or
subsequent treatment with a metal salt such as copper(II) sulphate, as described in the
preceding paragraph.
All the aftertreatment processes so far described have declined considerably in
commercial significance and are now rarely carried out. Nevertheless, their common
principle of creating on the fibre a dye–agent complex of larger size, reduced solubility and a
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
correspondingly lower rate of desorption has survived in today’s use of resin-type fixing
agents. The principle of cationic aftertreatment, in particular, has seen noteworthy
development.
The earliest polymeric cationic aftertreatments stemmed from the development of creaseresist finishes for cellulosic fibres. One such, promoted specifically for its colour fastness
improvements when applied as an aftertreatment to direct dyeings, was a condensation
product of formaldehyde with dicyandiamide (Scheme 10.82). Many similar compounds
followed, such as condensation products of formaldehyde with melamine (10.212),
poly(ethylene imine) with cyanuric chloride (10.213) and alkyl chlorides with poly(ethylene
imine) (10.214; R = alkyl).
NH
n H2N
C
N
C
NH
+ n HCHO + n HCl
CH2
NH
Cl
NH2
C
N
C
+ n H2O
N
n
Scheme 10.82
(CH3OCH2)2N
H(NHCH 2CH2)nNH
N
N
N(CH2OCH3)2
N
N
N
N
(CH3OCH2)2N
NH(CH2CH2NH)nH
H(NHCH 2CH2)nNH
10.212
10.213
_
+
RNH(CH 2CH2NH)nCH2CH2NH3 Cl
10.214
These condensates were an improvement on the simpler cationic surfactants mentioned
earlier. Their interaction still relies on electrostatic bonding between agent cation and dye
anion; hence the major weakness remains fastness to washing with anionic detergents. This
limitation of conventional cationic agents has been overcome by the development of bi-, triand tetra-functional agents which carry reactive groups capable of forming covalent bonds
by reaction with other suitable groups in the dye and/or the hydroxy groups in the cellulosic
fibre. Studies on the functionality and reactivity of various multifunctional crosslinking
agents [442] led to the selection of 1,3,5-tris(acyl)hexahydro-s-triazines (10.215) as being
particularly effective, their efficiency arising from the lability of the chloro substituents and
the strongly polarising effect of the carbonyl groups.
The tris(acryloyl) derivative (R is CH=CH2) was subsequently developed as a fixing
agent for use with the Basazol(BASF) dyes in the printing of cellulosic fibres, in conjunction
with urea as hydrotrope [444]. It has also been shown [445] that aftertreatment with 1,3,5tris(acryloyl)hexahydro-s-triazine or the tris (β-chloropropionyl) derivative improves the wet
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TREATMENTS TO ALTER DYEING PROPERTIES OR ENHANCE FASTNESS
701
O
C
R
N
R
N
C
N
O
C
R
10.215
O
R=
CH
CH2
CH(Cl)
C(Cl)
CH2Cl
or
CH2
CH2CH2Cl
fastness of direct dyes, providing the dyes contain suitable nucleophilic groups. The same
compounds have useful properties with reactive dyes (other than the Basazols) in giving
increased colour yield through fixation of hydrolysed dye, an aspect discussed in more detail
below.
The basic mechanism of action of multifunctional fixing agents has been well described
[446] (Figures 10.54 and 10.55). However, the multifunctional products are no better than
monofunctional types when used with conventional direct dyes that do not contain suitable
nucleophilic groups through which to link the fixing agent. Direct dyes suitable for forming
additional bonds have been termed reactant-fixable dyes and are mostly of the coppercomplex type. Thus brightness of shade is limited.
A range of bifunctional, trifunctional and tetrafunctional fixing agents was developed
[446,447] for use with a selected range of copper-complex (Indosol) dyes. The bifunctional
type, which reacts only with the dye, was applied in a fresh bath at about 60 °C and gave
fastness to washing at 50 °C through the formation of an extensive dye–agent complex
within the fibre. The trifunctional type additionally forms covalent bonds with cellulose and
is applied at 40 °C for about 15 minutes, followed by addition of alkali to bring about
reaction; this confers a higher degree of fastness to washing at 60 °C even with deep shades.
+
Monofunctional type
Bifunctional type
Trifunctional type
Tetrafunctional type
+
+
+
Figure 10.54 Fixing agents showing various degress of functionality [446]
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
cellulose
+
–
+
–
O3S–D
Monofunctional type
Bifunctional type
O3S–D
HO
O
Trifunctional type
+
–
O3S–D
HO
O
Tetrafunctional type
+
–
O3S–D
HO
O
Figure 10.55 Modes of reaction of the various fixing agents [446]
Tetrafunctional reactant resins confer the highest fastness, even to washing at the boil.
Typical polymers are made by the reaction of an amine such as diethylenetriamine with
cyanamide, dicyandiamide (especially), guanidine or biguanide. A cationic polymer of this
type is applied with an N-methylol reactant such as dimethyloldihydroxyethyleneurea and
an acid-liberating catalyst such as magnesium chloride to give a commercial product sold as
a cationic reactant resin. This formulation is applied to the dyeing by padding, at which
stage a dye–agent complex is believed to be formed. The fabric is then treated at 175–180
°C, resulting in covalent reaction between the cationic agent and the N-methylol groups as
well as crosslinking of cellulose chains by the N-methylol reactant, conferring not only
excellent wet fastness but also improved crease resistance and good dimensional stability.
The multifunctional agents offer an alternative aftertreatment for reactive dyeings [446],
if problems arise from the presence on the fibre of unfixed hydrolysed dye, normally removed
by ‘soaping’. This process is invariably lengthy, expensive and not always fully effective.
Multifunctional cationic reactants will react with unfixed (still reactive) dye and with
hydrolysed (hydroxy-containing) dye. Tri- and tetra-functional reactants will further fix
them by covalent bonding to the fibre. Improved wet fastness can also ensue when
tetrafunctional products are used, including considerably improved resistance to hydrolysis
of the dye–fibre bond and better fastness to treatments involving chlorine or perborates.
Deleterious effects on hue and light fastness still have to be carefully considered in the
selection of dyes for aftertreatment, however.
Most of the polymeric cationic products available [448] are based on the types described
in Table 10.44. The ideal aftertreating agent must fulfil many requirements [448]. There is a
high demand for:
– improved fastness to washing and other wet treatments
– low price
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–
–
–
–
–
–
–
703
applicability by exhaustion
applicability with various dye classes and on various fibres and blends
stability to electrolytes
capability to minimise migration of hydrolysed reactive dyes
stripping and overdyeing performance
ease of cleaning of stained machinery
biological elimination.
In addition, the ideal agent must not:
– cause significant changes of shade of dyeings
– increase chlorine retention or free formaldehyde content
– impair the handle, hydrophilicity, light fastness, gas-fume fastness or sewability of the
fabric
– be toxic to humans or irritate the skin.
Table 10.44 Classes of polymeric cationic agents for aftertreatment of dyed cellulosic fibres [448]
Amide–formaldehyde condensates
No defined structure
CH2 NH C NH C NH2 CH2
NH
NH X
Polybiguanides
NH
CH2
Poly(ethylene imine)
CH2
n
NH2 CH2 CH2
n
X
CH3
Quaternised poly(ethylene imine)
NH
CH2
CH2
N CH2 CH2
CH3
n
CH2
H2C
HC
Quaternised polyheterocycles
X
CH
CH2
H2C
N
CH3
H3C
X
n
CH2
Quaternised polyacrylamides
O
CH
C
CH3
NH CH2 CH2 N CH3
CH3
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
R
R
NH2 + HClO
NHCl
+ H2O
T < 40°C
R
R
NH
NCl + H2O
+ HClO
T < 40°C
R
R
Scheme 10.83
heat
R
NHCl
R
+ H2O
R
heat
+ HClO
R
NH
NCl + H2O
R
NH2
+ HClO
R
Scheme 10.84
Amines can give rise to chloramines during hypochlorite bleaching (Scheme 10.83). In
addition to increasing AOX values, this can result in cellulose oxidative degradation by
hypochlorite on subsequent hydrolysis of the chloramines (Scheme 10.84) [448].
Cationic fixing agents remain associated with the dyes on the fibre and are only partially
removed by subsequent washing. Skin irritation is therefore an important consideration and
in this respect they appear to have negligible influence. In common with cationic
surfactants, however, they are readily absorbed by protein material and show moderately
high toxicity to aquatic organisms, although interaction with excess anionic surfactant can
considerably reduce this problem. In vitro they can show poor biodegradability, BOD values
being typically not more than 10–20% of COD values, which are usually in the region of 40–
400 mg/g oxygen. However, they are highly absorbed by activated sludge, resulting in about
95–98% bioelimination accompanied by about 80% actual biodegradation.
Studies of the effectiveness of cationic polymers for aftertreatment of dyeings continue to
appear, although not all researchers disclose the detailed structures of the polymers
evaluated. One comparison involved three types of commercial fixing agents for improving
the fastness of six direct dyes on cotton, the agents being described as a dicyandiamide–
formaldehyde condensate, a cationic nitrogenous polymer and a long-chain polyamine-type
product. The long-chain polyamine imparted higher wet fastness than the dicyandiamide
type, but fastness characteristics were found to be influenced by the number and position of
solubilising and hydrogen bonding groups in the dye structures [449].
A polyacrylamide with a molecular mass of 1.87 × 105 was prepared by polymerising a 5%
w/v aqueous solution of acrylamide monomer in the presence of 0.15% w/w benzyl alcohol
and 0.025% w/w potassium persulphate for 45 minutes at 80 °C. This product was effective
in preventing the bleeding of direct dyes and hydrolysed reactive dyes from dyed cotton,
which was simply dipped in a 1% solution of the polyacrylamide and dried in air [450].
Ecological factors, as well as the improvement of wet fastness, were taken into
consideration in a study of viscose dyed with direct dyes (CI Direct Yellow 126, Red 83:1
and Blue 90) and aftertreated with three cationic agents described as monofunctional,
bifunctional and trifunctional [451]. For a system to be biodegradable, the BOD5/COD ratio
should be at least 0.5. For the systems examined, this ratio ranged from 0.00 to 0.38. Thus,
all of them proved difficult to treat, biodegradation taking at least 20 days. It was difficult to
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705
decide which of the three agents was least hazardous in environmental terms. The
bifunctional agent gave the best overall performance, since it was not more environmentally
hazardous than the others and gave the smallest colour change, together with good fastness
to washing at 60 °C.
There has been interest recently in comparing formaldehyde-free, low-formaldehyde and
conventional N-methylol fixing agents with direct [452] and reactive [453] dyes.
Advantages are claimed for formaldehyde-free products in giving less dye bleeding, less
change in shade and excellent fastness to light and wet treatments [453].
The effect of conventional cationic aftertreating agents in improving the fastness of direct
dyes can be further enhanced, albeit to only a small extent, by subsequent treatment with a
syntan [454]. This may result from the formation of a larger electrostatically linked complex
between the anionic syntan and the cationic fixing agent at the fibre surface, having low
aqueous solubility and slow diffusion behaviour. Traditionally, cationic fixing agents have been
applied to direct dyeings at neutral or slightly acidic pH. There is now some evidence [455]
that they can be somewhat more effective if applied under alkaline conditions. The reasons for
this remain speculative and more than one mechanism may be operating.
10.9.6 Aftertreatments for sulphur dyes on cellulosic fibres
The fastness of sulphur dyes to the increasingly severe conditions of washing currently
demanded, especially in the presence of peroxide-containing detergents, can sometimes be
improved by an alkylation treatment. The best-known products for this purpose are adducts
of epichlorohydrin with either ethylenediamine (especially) or ammonium salts. Typically
the procedure involves the application of 2–3% o.w.f. of a proprietary alkylating agent with
1–2% o.w.f. sodium carbonate (pH 10 and 30–40 °C), raising the temperature to 90–95 °C
and allowing the reaction to reach completion at this temperature, which it does after 10
minutes. This treatment usually replaces the traditional oxidation treatment, except for dyes
having a distinctly yellow leuco form.
There has been interest recently in extending the use of cationic fixing agents normally
used with direct dyes to sulphur dyes. Five cationic polymers markedly improved the fastness
to washing of solubilised sulphur, leuco sulphur and insoluble sulphur dyes when the cationic
treatment was applied by a simple exhaust method typical of their application to direct dyeings
[456]. The polymers were similarly effective when applied to leuco sulphur dyeings by pad–dry
and pad–flash cure methods [457]. The mechanisms operating were thought to resemble those
with direct dyes, i.e. the formation of electrostatically linked dye–agent complexes and/or the
formation of a peripheral layer of polycations restricting dye mobility. The use of a reactive cationic agent has also been examined [458], this being of a type used successfully as a pretreatment to enhance dyeability. Washing fastness of reoxidised, solubilised sulphur dyes when
aftertreated by an exhaust method was markedly improved. It was shown that the cationic
aftertreatment could replace the usual reoxidation procedure, giving enhanced fastness.
10.10 AGENTS FOR FIBRE LUBRICATION, SOFTENING, ANTISTATIC
EFFECTS, SOIL RELEASE, SOIL REPELLENCY AND BACTERICIDAL
ACTIVITY
Most of the title products are finishing agents rather than dyeing or printing auxiliaries.
Although in principle they could be applied before, during or after coloration (with the
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
exception of lubricants applied during textile manufacture), economic reasons firmly put the
preference on conjunct application with colorants wherever possible. Whilst not dyeing
auxiliaries as such, their inclusion often has a bearing on choice and behaviour of dyes as
well as on possible problems encountered during coloration (compatibility or otherwise of
the components) and afterwards (fastness properties). Many of these products have useful
activity in more than one function. Thus an agent (and especially a composite commercial
product) promoted chiefly as a softener may also have antistatic, yarn lubricating and soil
release properties, and it is useful to bear this diversity of function in mind.
10.10.1 Fibre lubricants
The main consideration here is lubricants applied during wet processing although some
consideration will also be given to those included during textile manufacture. The basic
requirement of a lubricant is that it should form a thin uniform protective coating around
the fibres to lower their surface friction and flexural rigidity, thus assisting a process such as
weaving or minimising the formation of durable creases during rope dyeing, particularly at
high temperatures. In general practice a lubricant may, and usually does, contain more than
one component, a typical composition comprising base lubricant(s), antistatic agent and
surfactant(s). HPLC provides a useful means for the analytical separation and identification
of the components of a lubricant formulation [459].
From the point of view of subsequent wet processing, it is usually preferable that fabrics
be free from fatty substances such as oils and waxes. From the weaver’s viewpoint, however,
it is usually contended that such fatty lubricants are essential to minimise wear of machine
parts and to prevent yarn abrasion and breakages, more especially where high-speed weaving
processes are concerned. Weaving lubricants of this kind are usually applied with the sizing
agent (section 10.5.2) and they must operate in conjunction, one of the main functions of
the lubricant being to increase the film-forming properties of the size polymer(s). However,
such fatty lubricants are difficult to remove and may complicate subsequent desizing,
bleaching and coloration processes. This practice of using fatty lubricants has been
questioned in recent times, based on evidence that such hydrophobic agents can actually
impair the performance of the size as well as creating difficulties in desizing [460–463].
This is the main reason why surfactants are often used with fatty lubricants, to improve
compatibility with size polymers and to assist emulsification and removal. However, arguments
have been advanced [463] for the use of carefully selected surfactants as lubricating agents.
The fatty or hydrophobic moiety of a surfactant produces a low coefficient of friction between
surfaces to which it is applied and so acts as a lubricant, whereas the hydrophilic moiety makes
for built-in ease of subsequent removal. In addition, the use of inorganic salts such as sodium
silicate and sodium carbonate, as routinely used in detergent formulations, enhances the
performance of the surfactant as a lubricant, a typical formulation comprising 0.6% surfactant,
0.6–1.0% sodium silicate and 0.5% sodium carbonate. It is claimed that such a composition
performs just as well as a fatty lubricant in sizing and yet it is much easier to remove during
subsequent scouring or desizing. Anionic and nonionic surfactants were compared but
unfortunately no details of structure were disclosed. Clearly a good deal of development work
would be required to evaluate a range of surfactants varying in hydrophobicity, in order to
obtain optimum performance with a specific size composition and substrate. This is essential in
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707
any case, irrespective of whether the surfactant is to replace a traditional lubricant or is in
support of one.
Compatibility with other components becomes a critical factor when lubricants are used
in the dyebath. The properties of the ideal dyebath lubricant have been summarised as
follows [464]:
(1) excellent fibre-to-fibre and fibre-to-metal lubrication;
(2) economical;
(3) no effect on other physical properties such as handle, water-repellency and absorbency;
(4) a foam suppressant or deaerating agent;
(5) no effect on reproducibility of dyeing;
(6) no effect on fastness properties;
(7) non-yellowing;
(8) easily washed out; and
(9) biodegradable.
When formulating a dyebath lubricant, particular attention must be paid to the type of
substrate (hydrophilic or hydrophobic) and to substantivity of the lubricant for that substrate.
Another important consideration is ionicity of the lubricant in relation to the components of
the dyebath, since this has such an important bearing on compatibility (clearly, anionic
lubricants should be avoided when basic dyes are used). Solubility and/or dispersibility in
relation to dyebath composition and the conditions of dyeing (temperature, pH, liquor ratio)
are also important, since the overall hydrophobic/hydrophilic balance has a major influence on
compatibility. The behaviour of the lubricant during drying can be just as critical as during
dyeing. Many lubricants promote undesirable thermomigration of disperse dyes on polyester
during high-temperature drying and heat setting, leading to lower wet fastness.
Natural products such as animal fats and vegetable oils still constitute an important share
of the lubricants market although synthetic types are gaining acceptance [464]. Natural fats
and oils include saponified fatty acids, fatty esters, fatty alcohols and fatty amides. Various
anionic groups are suitable, including carboxylate, phosphate, phosphonate, sulphate or
sulphonate, the last-named being the most widely preferred. Esterification of the fatty acids
is particularly useful. For example, ethoxylation with ethylene oxide enables products of
subtly graded character to be produced, depending on the degree of hydrophobicity of the
fatty acid and the degree of ethoxylation. Sulphonates offer greater stability at higher pH
and ionic strength but they can generate troublesome foaming.
Synthetic-based lubricants include polyacrylates, acrylamide/acrylic acid copolymers,
emulsified paraffin oils and waxes, modified silicones. The acrylates generally combine
excellent solubility in the dyebath with fabric lubrication and such polymers can be designed
to cover a wide range of solubility, rinsibility and lubricating performance. Ionicity can be
varied, whilst acrylic esters offer scope for incorporating other functionalities along the
polymer chain. Acrylic esters also allow the degree of anionicity to be varied. Acrylamide
groups generally result in increased stability, especially in acidic media where the amido
groups are partially protonated and thus mildly cationic, whilst in neutral media they behave
substantially as nonionic moieties.
Paraffin hydrocarbons of molecular mass 300 to 700 (i.e. 20–50 carbon atoms) require
emulsification with surfactants. They show good resistance to oxidation and yellowing.
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
However, the stability of these systems is critically dependent on effective emulsification.
Viscosity is usually low, a concomitant of this being that the emulsifier constitutes a
substantial proportion of the system. Ideally, the emulsifier should contribute to the
lubricating power of the product. Related to the paraffins are the even more hydrophobic
polyethylenes, primarily used as sewing lubricants or handle modifiers. These polymers may
be difficult to wash out, although their ionic character can be varied according to whether
anionic, cationic or nonionic emulsifying systems are chosen. Paraffin waxes and
polyethylenes have an advantage over paraffin oils in that they are terminated with a mildly
ionic functionality that assists their emulsification and, coupled with their high molecular
mass, increases substantivity. This tendency, however, can make them difficult to wash out
and leads to compatibility problems in dyeing. They are perhaps best regarded as softeners.
Alkoxylated polysiloxanes are a relatively new class of dyebath lubricants. They have
practically no substantivity for the substrate, yet combine adequate lubrication with water
solubility and easy rinsability. If the silicones contain primary hydroxy groups, these can be
modified by esterification, phosphation, phosphonation, sulphation, sulphonation or
carboxylation. These anionic substituents confer substantivity for various substrates without
losing rinsability. Anionic organic sulphates and sulphonates probably offer the best overall
properties for dyebath lubricants, whilst other types can be more suitable for selected
applications [464].
Certain problems associated with lubricants
The manufacture of textile fabrics for automotive trims normally requires the application of
a spin finish, a considerable percentage of which is still present in the finished fabric [465].
Residual lubricants can give rise to ‘fogging’ on the interior of car windows, caused by
condensation on the glass of volatile components present: other similar products, such as
antistats and softeners, may also be implicated. Fogging is measured by reflectance and is
expressed as the ratio of the reflectance values of a glass plate before and after exposure to
fogging (DIN 75021), a high value representing a low degree of fogging. Low-fogging
compounds typically give values of at least 90%; some measured values [465] are shown in
Table 10.45. Chemical type is not a direct indicator of propensity to fogging, since volatility
varies with hydrophobic/hydrophilic balance as determined by chain length and structural
type on the one hand and the degree of polar character, including ionicity, on the other.
Yellowing can occur with certain lubricants during drying or ageing. This phenomenon
has been investigated on wool and acrylic fibres and blends of them lubricated with a
Table 10.45 Fogging values of lubricants [465]
chpt10(2).pmd
Lubricant chemical type
Fogging value (%)
Alkylbenzene
Poly(ethylene glycol)
Fatty acid polyglycol ester No. 1
Fatty acid polyglycol ester No. 2
Poly(ethylene/propylene glycols)
Phosphate monoester/diester
36
86
55
95
>95
>90
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AGENTS FOR FIBRE LUBRICATION, SOFTENING, ANTISTATIC EFFECTS, SOIL RELEASE
709
cationic fatty acid condensation product, a cationic nitrogenous condensation product, a
quaternary stearylalkylamino ester, a nonionic oxyethylated carbonate/phosphate and a
polyglycol carbonate ester formulated with a nonionic surfactant [466]. Yellowing did not
occur when the lubricant manufacturer’s instructions regarding concentration and drying
temperature were followed. In some circumstances, however, such as radio-frequency drying,
cationic compounds can induce the formation of sparks that may result in yellowing,
indicating a need to reduce the power of the dryer and/or to use a lower concentration of
lubricant.
As already mentioned, some lubricants can be difficult to remove by washing and
surfactants are often added to overcome this problem [463]. Lubricants can impair fastness
properties, particularly those of disperse dyes. They may influence the uptake of dyes either
positively or negatively, although seldom seriously except where it results in unlevelness. For
example, knitting oils can increase the yield of relatively oleophilic reactive dyes on cotton
and yet with highly hydrophilic types they may cause dye-resist effects [467].
When considering the environmental aspects of lubricants, it is necessary to bear in mind
that commercial formulations are often more or less complex mixtures and that even small
amounts of toxicologically questionable components can lead to restrictions on use,
depending on local regulations. Thermally stable, readily biodegradable esterified oil
emulsions can offer a compromise solution, being environmentally preferable to unmodified
oils, fats and waxes, although problems can still arise in high-temperature processes such as
texturising or heat setting, as a result of cracking reactions leading to the formation of
harmful residues or condensates [468]. In order to overcome this problem of cracking,
attention must be paid to the susceptible rupture points in the molecule. This has been done
in the development [468,469] of a range of ecologically friendly lubricants by using
carbonate ester bridging groups to combine fatty alcohols or their ethoxylates with
poly(ethylene glycol) (10.216). Such products are readily water-soluble, self-emulsifiable,
thermally stable compounds with excellent cracking behaviour. Their lubricity is such that
they can be used in amounts 50–80% lower than normal. They show excellent
biodegradability and skin compatibility properties. Emission values are 10–20 times less than
with comparable esters or mineral oils. Environmentally friendly lubricants containing
alkylglucoside with emulsifier (1:2) or amine oxide with emulsifier (1:2) have also been
claimed [470].
O
R
O
(CH2CH2O)x
C
O
(OCH2CH2)y
O
C
(OCH2CH2)y
O
R
10.216
10.10.2 Antistatic agents
Antistatic agents are useful to prevent garments accumulating charges of static electricity,
particularly those made entirely from hydrophobic fibres. Whilst this is desirable for
domestic wear, it takes on added importance in many other sectors, when working in
explosion hazard areas, with micro-electronic components, in electronic data processing
environments, near textile dust filtering systems, or in certain areas of military and space
travel technology. Antistatic treatments may be impermanent (easily removed on washing)
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
or have a degree of durability. They operate through their hydrotropic properties, creating a
moist microclimate that discourages the build-up of static, or by having some degree of
conductivity, thus dissipating any localised static charge into a wider area of lower potential.
Impermanent antistats
Surfactants may exhibit a degree of hydrotropy and thus function as antistatic agents. They
are often used for this and for their emulsifying properties in conjunction with fibre
lubricants, or may be used alone in a dual capacity as lubricant and antistat. Suitably active
surfactants can be found amongst all four ionic types, some typical examples being [471]:
– quaternary ammonium derivatives of fatty acids
– polyethoxylated quaternary ammonium derivatives
– quaternised alkylenediamines
– alkyl sulphates, chlorides or phosphates
– mixtures of mono- and di-esters of phosphoric acid
– long chain amine oxides
– polyethoxylated and polypropoxylated nonionics.
Nonionic and cationic types are generally preferred, usually on grounds of fibre compatibility,
higher hygroscopicity and higher oil solubility. The quaternary ammonium derivatives of fatty
acids in particular impart static protection at a low level of application (i.e. 0.25%), whilst the
polyoxyethylated versions give higher solubility in water. The nonionic and cationic types
dominate particularly in spin finishes [471]. However, the cationic types may create
subsequent problems if anionic surfactants are used at the washing stage.
Impermanent antistatic agents that are not surface-active include triethanolamine,
glycerol in combination with potassium acetate, as well as inorganic salts such as lithium
chloride [471].
The major requirements for impermanent antistats are [47l]:
(1) effective at low levels of humidity;
(2) low volatility;
(3) non-corrosive;
(4) low toxicity; and
(5) no yellowing.
In addition, if used in spin finishes they must be thermally stable, oil-soluble and exhibit low
migration on the fibre.
Durable antistats
There are two types of durable antistat. The first (so-called external antistatic agent) is
applied at some stage after fibre manufacture whilst the second type is incorporated during
manufacture. The former type exhibits varying degrees of durability and is not usually as
durable as the latter.
External antistatic treatments generally involve the formation of a crosslinked polymer
containing ionic and/or hygroscopic groups, cationic polyelectrolytes containing poly(ethylene
oxide) units being widely used. Many crosslinking agents are suitable to insolubilise hydrophilic
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711
components within a fibre, including poly(ethylene glycol) dihalides, epoxy compounds,
cyanuric chloride, derivatives of piperazine, hexahydro-s-triazine, polyaziridines, polyacrylamide and polyamines [471].
Casein modified by graft copolymerisation with esters of acrylic acid and methacrylic acid
has been traditionally used for the coating of leather. Casein modified in this way by grafting
with ethyl acrylate [472] showed good film-forming properties on textile materials,
particularly as an antistatic agent in the backing of carpets and as an additive in pigment
printing. This polymer satisfied ecological requirements and its antistatic properties were
substantially enhanced by incorporating electroconductive materials such as carbon black
and powdered metals.
In another evaluation [473], durable antistatic properties were obtained using watersoluble quaternary ammonium polyamines applied to polyester by padding and curing at 160
°C. These products were prepared according to Scheme 10.85. Epichlorohydrin was
condensed with poly(ethylene glycol) 600 (known for its hygroscopicity and antistatic effect)
using boron trifluoride as catalyst to give the poly(ethylene glycol) diglycidyl ether. This was
then reacted with the highly electroconductive tetra-ethylenepentamine to give a watersoluble polyamine. Finally, in order to prevent gelation and to improve the stability in
aqueous solution, this polyamine was cationised as the phosphate salt.
An obvious means of increasing conductivity is to incorporate metals into the fabric.
Thus fabric can be sprayed with a liquid resembling metallic paint, containing micron-sized
metallic particles such as copper incorporated into a binder such as a polyester, epoxy or
acrylic resin. During curing the metallic particles come into contact with one another, thus
H2C
CH
CH2Cl
+ HO(CH2CH2O)nCH2CH2OH
+ ClCH2
CH
O
CH2
O
BF3
H2C
CH
CH2
O(CH2CH2O)nCH2CH2O
CH2
O
CH
CH2
O
H2N(CH2CH2NH)3CH2CH2NH2
H2C
CHCH2O(CH2CH2O)n+1CH2CHCH2NH(CH2CH2NH)4CH2CHCH2(OCH2CH2)n+1OCH2CH CH2
O
OH
O
OH
OH
O
P
OH
OH
HOCH2
+
+
CHCH2O(CH2CH2O)n+1CH2CHCH2NH2(CH2CH2NH2)4CH2CHCH2(OCH2CH2)n+1OCH2CH CH2OH
OH
OH
_
O
O
OH
Scheme 10.85
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711
OH
_
O
_
O
O
P
_
OH
_
O
O
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
providing excellent conductivity throughout the material [474]. The presence of copper,
however, may present environmental problems when waste material of this kind requires
disposal.
An unusual method of imparting antistatic properties to acrylic fibres involves
saponification of some of the nitrile groups to give amide and carboxyl groups, using sodium
hydroxide under exhaust (80 °C), pad–batch (varying within 15–120 minutes at 160–60 °C)
or pad–steam conditions. The pad–steam method in particular provides convenient surface
modification of acrylic fibres, giving antistatic properties together with improved soil-release
and improved dyeing properties with basic dyes [475]. This method may be substrate-specific
since acrylic fibres vary widely in their sensitivity to alkali, some being quite easily discoloured.
Methods of incorporating durable antistats during fibre manufacture include:
– incorporation of a hydrophilic component, such as a surfactant
– formation of a lattice of highly conductive components within the fibre.
This approach is outside the scope of the present chapter but details may be found elsewhere
[471,476,477].
10.10.3 Softeners
It is difficult to define unequivocally the quality of fabric handle or softness/firmness
differences, since this involves many factors. It is often linked with lubrication, especially as
similar products are often used for softening and lubrication. Whilst experienced assessors
can be quite remarkable in the extent to which they can grade and assess softeners simply by
means of a highly developed tactile sense, more objective methods are clearly desirable for
scientific investigations. Since many factors combine in producing an overall sense of
softness, it is not surprising that objective determination of softness involves more than one
parameter of measurement. The details of assessment are outside the scope of this chapter,
but descriptions and discussions are available elsewhere [478–481]. Suffice it to say here
that the Kawabata system has acquired considerable importance in quantifying various
aspects of fabric handle.
The oils and waxes described as lubricants in section 10.10.1, as well as talc, can be used
as softeners but have now been superseded by more effective products. These may be nonreactive or reactive and may be cationic, anionic, nonionic or amphoteric. Although many
compounds have been patented, by far the most important are cationic quaternary
ammonium compounds and various silicones. Until quite recently the field was led by the
cationic types but there is now evidence that aminofunctional polysiloxanes have become
the most important product group [482].
Before dealing with the detailed chemistry of softening agents, it is useful to consider
some general factors. The desirable properties of an ideal softener can be summarised as
follows [482]:
– easy to handle (suitable for pumping, stable on dilution)
– low foaming, stable to shearing, free from deposits on rollers
– compatible with other chemicals
– good exhaustion properties
– applicable by spraying
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–
–
–
–
–
713
stable to high temperatures, non-volatile especially in steam
no effect on shade or fastness
no yellowing
easily biodegradable
non-toxic, dermatologically harmless, non-corrosive
no restrictions for transport or storage.
No single product satisfies all the above requirements. Commercial softeners are usually
more or less complex mixtures, available as aqueous emulsions with a solids content of 15–
25%. As well as the softener component(s) other additives are also present, including the
emulsifying and/or dispersing agents necessary. It cannot be over-emphasised that emulsion
stability is just as important as softener effectiveness, since it is crucial to the performance of
the product. Nonionic emulsifying agents are especially useful, showing all-round
compatibility with other substances even if they are different in ionicity. Consideration
needs to be given to cloud point phenomena and stability to electrolytes when using
nonionic surfactants. In such cases the emulsifying system itself may be a mixture.
Softener emulsions are available in different commercial grades. Semimicro-emulsions
have an average particle size of <0.1 µm, allowing for penetration to the fibre core and
giving excellent distribution of the softener. Micro-emulsions have an average particle size of
<0.01 µm, giving the substrate an excellent inner softness and a distinctive surface
smoothness without looking greasy [482]. Micro-emulsions have good stability and a
decreased tendency for deposition on rollers. Critically optimised products are particularly
necessary when using micro-emulsions on high-speed equipment where high shearing forces
are developed [482]. Irrespective of the basic chemical type, the fabric handle can be
modified by varying the hydrophilic–hydrophobic balance of the softener. Thus increasing
hydrophobicity by incorporating a longer alkyl chain imparts an increasingly greasy handle to
the fabric.
The ionicity of the softener influences its substantivity for different fibres. Whilst this can
be significant in padding applications, it is of primary importance for application by exhaust
methods. Cationic softeners are highly substantive to acrylic fibres, whilst anionic products
are substantive to wool and nylon. Nonionic softeners, if water-soluble, have very low
substantivity for any substrate and are easily lost by washing. Insoluble types may be
substantive to polyester or cellulose acetate, as well as wool and nylon. The degree of
softening with the nonionic products is only moderate but their nonionic nature makes
them convenient for application simultaneously with any class of dyes or fluorescent
brightening agents. The low-substantivity agents are preferable for repeat application in
domestic washing.
There has been a trend towards so-called multifunctional softeners in which account is
taken of various other factors such as hydrophilicity, sewability, antistatic behaviour,
lubrication and shearing stability [482]. For example, in jet dyeing a product will require, in
addition to softening power:
– extremely high emulsion stability to cope with high pump and nozzle shearing forces at
high temperatures
– a suitable defoaming system
– good substantivity to ensure adequate exhaustion.
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
Conversely, if good sewability is required, suitable products need to have good lubrication
behaviour.
Quaternary ammonium compounds and other cationic softeners
Organic cationic softeners, as opposed to silicone derivatives, are usually quaternary
alkylammonium compounds, the most important over many years being dimethyldistearylammonium methosulphate (10.217), on grounds of economy and availability. There are
obviously many possible variants of such structures but typically the long-chain substituents
are within the range C16 to C22 and may be fully or partially saturated [483]. These softeners
possess aqueous solubility, substantivity for various fibres and, to some extent, antistatic and
water-repellency properties. These properties can be modified by varying the substituents on
the quaternary nitrogen atom. The softening effect is especially good. The balance of
properties can be controlled more precisely using analogous ethoxylated or propoxylated
amines (10.218), in which the degree of ethoxylation can also be varied. These are more
expensive but provide high-quality industrial softeners. Further compounds available include
quaternised imidazolines (10.219) and diamides or diurethanes containing a protonated
amino group (10.220; R = alkyl or alkoxy).
CH3
CH3
+
N (CH2)16CH3
(CH2)16CH3
CH3SO4
O
(CH2CH2O)nH
C
+
CH3 N CH2CH2NH
_
CH2CH2NH
CH3SO4
C R
O
10.218
10.217
R
R1
_
CH3SO4
+
HN
O
N
CH2CH2NH
C
O
10.219
R2
H +H
R C
N
HN CH2CH2 CH2CH2
_
X
10.220
O
C
R
NH
The specific properties of such compounds obviously depend on the precise nature of the
substituents but in general the degree of softening decreases as follows: dialkyldimethylammonium compounds > imidazolinium salts > aminoalkyl diamides or diurethanes [480].
In the case of the quaternised compounds the preferred anions are methosulphate or
ethosulphate, since these have a less corrosive effect on steel vessels than the chloride salts.
These compounds generally show maximum cation activity at around pH 3.5 but are usually
applied at higher pH values. In some cases the cationic softening agent is mixed with a
nonionic surfactant to serve as a lubricant/antistat, this being particularly common in fabric
softeners for domestic use. It is also worth noting that most quaternary ammonium
compounds have a degree of antibacterial activity.
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From a detailed study of three quaternary ammonium softeners based on hydrogenated
tallow oil (10.221–10.223), adsorption of these softeners by cotton and acrylic fibres is
shown in Figure 10.56. Adsorption of the three softeners by cotton did not vary greatly,
apparently following a Donnan equilibrium mechanism. On the acrylic substrate a saturation
point was reached at about 2 µmol of each softener per gram of fabric. A twenty-member
panel using tactile sensation found no significant difference between the products for
softness on either fibre. Measurement of bending rigidity indicated the following results for
the same quantity of applied softener:
on cotton fabric: imidazolinium quat > ester quat or alkyl quat
on acrylic fabric: ester quat or imidazolinium quat > alkyl quat.
Softener adsorption on cotton fabric
[NaCl] = 1.25 g/l, 30 oC, pH 4.5–5.0
Softener adsorption on acrylic fabric
[NaCl] = 1.25 g/l, 30 oC, pH 4.5–5.0
0.20
0.6
A Q
I Q
E Q
0.4
0.2
0.2
0.4
0.6
0.8
Softener concentration/%owf
Adsorbed softener/%owf
Adsorbed softener/%owf
0.8
E Q
I Q
0.15
A Q
0.10
0.05
0.2
0.4
0.6
Figure 10.56 Adsorption of cationic softeners by cotton and acrylic fabrics at 30 °C [483]
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Softener concentration/%owf
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
50
Rewettability/%
40
30
A Q
E Q
20
10
I Q
0.1
0.2
0.3
0.4
0.5
0.6
Softener concentration/%owf
Figure 10.57 Effect of cationic softeners on the rewettability of cotton fabric [483]; 20 °C, 60% relative
humidity
50
Semi–discharge time/s
40
30
20
10
E–Q A–Q
0.1
I–Q
E–Q A–Q
I–Q
E–Q A–Q
0.2
I–Q
0.4
Softener concentration/%owf
Figure 10.58 Effect of cationic softeners on the antistatic behaviour of acrylic fabric [483]; 20 °C, 60%
relative humidity
A problem associated with quaternary ammonium softeners is that they increase the
hydrophobic nature of the fibre and thus interfere with rewetting, this effect being
cumulative with each successive application of the softener. The rewettability of a cotton
fabric treated with the above three softeners is shown in Figure 10.57, wettability being
affected somewhat less by the alkyl or ester quat than by the imidazolinium quat. Antistatic
effects on the acrylic fabric are illustrated in Figure 10.58, showing wide variations between
the softeners at low concentrations [483]. Surprisingly, none of these softeners induced
significant thermomigration of three disperse dyes on polyester [483].
Micro-emulsion formulations of cationic softeners are available and are claimed to have
superior fibre penetration properties [484]. Figure 10.59 shows the sorption of a micro-
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emulsion formulation of dimethyldistearylammonium (10.217) chloride. Similar results were
obtained with micro-emulsion formulations of N,N-dialkyl-N-2-hydroxyethyl-Nmethylammonium methosulphate (10.224) and N,N-bis(stearamidoethyl)-2-methylimidazolinium methosulphate (10.225), although the highest rate constant values were
observed with the imidazoline derivative and the lowest with the dialkylhydroxyethylmethylammonium methosulphate (Table 10.46). Cotton washed 50 times in hard water
absorbed the softeners rather more strongly than unwashed cotton. It is thought that this
may be due to adsorption by the cotton of inorganic salts from the hard water and of acidic
residues from the detergent used.
Yellowing can occur with quaternary cationic softeners and this limits their use on white
fabrics. This problem can be overcome to some extent, provided drying or fixation
temperatures are not too high, using so-called pseudo-cationic softeners [482]. These
products are analogous to the so-called weakly cationic surfactants described in section 9.5.
60 oC
40 oC
32.5 oC
25 oC
Sorption/g per kg cotton
3.0
2.5
2.0
1.5
1.0
0.5
0.25
1
2
3
4
Time/min
Figure 10.59 Sorption kinetics of micro-emulsion cationic softener formulation at different
temperatures on cotton fabric [484]. Initial concentration 3 g/kg fibre, liquor ratio 15:1
CH3
_
CH2CH2OH
+
N (CH2)nCH3
(CH2)nCH3
CH3SO4
10.224
O
CH3(CH2)16
C
NHCH2CH2
CH3SO4–
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CH3
+
N
O
C
N
(CH2)16CH3
CH2CH2NH
10.225
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
Table 10.46 Rate constants for sorption of cationic softeners at various temperatures on
unwashed and washed cotton [484]
Rate constants
Softener
Dimethyldistearylammonium
(10.217) chloride
Dialkylhydroxyethylmethylammonium
methosulphate (10.224)
Bis(stearamidoethyl)methylimidazolinium
methosulphate (10.225)
Temperature
(°C)
Unwashed
cotton
Washed
50 times
25
32.5
40
60
0.42
0.55
0.64
0.83
0.72
0.79
1.14
1.70
25
32.5
40
60
0.46
0.50
0.55
0.61
0.57
0.63
0.71
0.81
25
32.5
40
60
0.73
0.80
0.91
1.03
0.95
1.10
1.16
1.47
Silicone softeners
Poly(dimethyl siloxane) (10.226) represents the simplest of the silicone softeners, which are
usually applied from emulsion. This polymer, however, is unreactive and is not substantive to
fibres. Therefore it is not fast to washing and is little used nowadays. More durable softeners
can be derived by modification of this fundamental siloxane. The so-called ‘conventionally
reactive’ silicones were the first to be introduced, typical examples being those containing
activated silanic hydrogen (10.227) or silanol (10.228) groups. Such polymers react with
crosslinking agents (10.229 being a typical example) during a curing process with an
organometallic catalyst, typically a zinc or zirconium alkanoate. A crosslinked polymer
network of high molecular mass is formed on the fibre, this being the mechanism necessary
to achieve a soft finish of high durability.
CH3
CH3
Si
CH3
CH3
Si
O
CH3
O
CH3
Si
n
CH3
CH3
CH3
CH3
Si
H
O
CH3
10.226
Si
CH3
O
Si
CH3
10.228
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O
CH3
Si
n
10.227
CH3
CH3
HO
Si
CH3
CH3
Si
O
n
CH3
OCH3
OH
CH3
Si
OCH3
OCH3
10.229
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The next development arose in the late 1970s with the introduction of aminofunctional
silicone softeners. These contain aminoalkyl groups attached to the poly(dimethyl siloxane)
backbone, leading to improved orientation and substantivity for the fibre [482]. This
favourable orientation, illustrated in Figure 10.60, leads to an extremely soft handle, often
described as ‘supersoft’. These polymers are highly cost-effective, very small amounts being
required to obtain the desired properties, thus giving both economic and environmental
benefits. Indeed, such are the advantages of these products that they have assumed a
dominant role in the marketplace. Mainly based on a single type of amino group, more than
90% of all commercially available aminosilicone softeners are in fact aminoethyliminopropyl
silicones (10.230) [485].
The softening effect of silicones results from their lubrication behaviour that affects both
the surface and the interior of the fibre. The behaviour of polysiloxanes of the 10.230 type
can be varied by adjusting the average values of x and y and the range of chain lengths
present. Further variations are possible by varying the R groups. In view of the technical and
Negative zeta potential of cellulosic fibre
Figure 10.60 Attachment and orientation of an aminofunctional polysiloxane on a cellulosic fibre
[482]
CH3
CH3
R
Si
O
CH3
Si
CH3
R
Si
O
x
CH3
O
(CH2)3
Si
R
CH3
NH
10.230
CH2
CH2
R = OH, CH3, OCH3
chpt10(2).pmd
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NH2
y
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
commercial importance of the aminofunctional polysiloxanes, it is worth exploring some
structure/property relationships [485–489] even though there is as yet no conclusive
correlation available for the most important aspect, that of softness as distinguished by
tactile sensation.
The presence of the aminofunctional group appears to be necessary to obtain a ‘supersoft’
handle. In a study [488] of nineteen variants of the aminoethyliminopropyl polysiloxane
structure (10.230) differing in amino group content (0 to 1.5 milli-eq./g), viscosity (300 to
21 000 mPas) and emulsion particle size (50 to 350 nm), good agreement was found between
test parameters and amino group content, for which there was a strong substrate dependence.
The cotton fabric softness showed a marked dependence on amino group content, giving an
optimum value at about 0.3–0.4 milli-eq./g polysiloxane. Polyester/cotton blends (65:35 to
35:65) gave an optimum at about 0.2 milli-eq./g with a strong dependence on amino content,
whereas a 100% polyester fabric showed a much weaker dependence together with a relatively
high optimum at 0.5–0.6 milli-eq./g. Emulsion particle size and viscosity played subordinate
roles, although it is known that very low viscosities (indicating short polymer chains) and very
small particle sizes can be detrimental to softness effects under certain conditions.
Epoxyfunctional siloxanes are also useful as softeners. These may be derived from
polysiloxane (10.231) or from aminopolysiloxanes (10.232). Further possibilities are represented by the polyalkoxylated epoxyfunctional silicones (10.233) and polyalkoxylated
aminofunctional silicones (10.234). However, it has been pointed out [485] that the
reaction of epichlorohydrin with aminopolysiloxanes is not very specific, since primary and
secondary amine groups are usually randomly epoxidised resulting in viscous products that
CH3
CH3
Si
CH3
O
CH3
Si
CH3
Si
O
CH3
Si
O
CH2
x
CH3
CH3
y
Si
CH3
CH3
CH3
O
CH3
CH3
Si
CH3
CH3
Si
O
x
10.231
CH2
Si
CH3
O
CH3
CH3
CH
CH2
O
CH
CH3
R
Si
CH2
10.232
O
CH3
CH2
Si
O
CH3
x
CH3
Si
O
CH3
y
CH2
CH3
Si
O
(CH2)n
z
CH3
CH3
(OCH2CH2)a(OCH2CH2CH2)bOCH3
10.233
CH2CH2NHCH2CH2NH2
CH3
CH3
Si
CH3
CH3
O
Si
CH2
Si
O
CH3
x
CH3
CH3
CH3
Si
O
y
Si
O
(CH2)n
CH3
CH3
z
(OCH2CH2)a(OCH2CH2CH2)bOCH3
10.234
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Si
O
NH
CH
O
CH3
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are difficult to emulsify. Thus they do not offer a significant improvement over the parent
aminopolysiloxanes.
Promising products, offering a variety of structural possibilities, are obtained by acylation
of aminopolysiloxanes [485]. Suitable acylating agents include anhydrides, lactones and carbonates (Scheme 10.86), of which acetic anhydride is the most economical. The optimum
combination of effects is obtained by 30–70% acylation; more than 70% substitution can
reduce the softening effect to the level associated with conventional poly(dimethyl
siloxane). No significant differences have been observed with respect to handle, whiteness
or water absorbency, depending on whether acylation is achieved using acetic anhydride,
butyrolactone or ethylene carbonate as the acylating agent. A slight decline in softness of
handle is observed with the acylated products compared with that from normal aminopolysiloxanes but this is compensated for by better whiteness, water absorbency and soil
release properties. A major drawback of the standard aminoethyliminopropyl polysiloxanes is
their tendency to show yellowing, resulting from the formation of chromogenic species by
oxidative thermal decomposition of the aminofunctional group. Acylation largely overcomes
this problem and also gives improvements in water absorbency and soil-release performance.
Si
(CH2)3NHCH2CH2NH
C
O
C
O
Si
CH2
CH2CH2CH2OH
O
O
CH2
CH3C
CH2
O
CH3C
(CH2)3NHCH2CH2NH2
O
Si
(CH2)3NHCH2CH2N H
C
O
O
C
O
Si
CH3
O
CH2
CH2
(CH2)3NHCH2CH2NH
C
OCH2CH2OH
O
Scheme 10.86
Further fine tuning of the properties of aminopolysiloxanes can be achieved [485] by
substitution of the siloxane units with primary, secondary or tertiary amino functions
(10.235). Whiteness, water absorbency and soil release properties are improved with
increasing degree of amino substitution from primary through to tertiary amines (Figure
10.61). The improvement in whiteness compared with the aminoethylimino reference
product is particularly noteworthy, this being attributed to retardation of the oxidative
thermal degradation because of the protective effect of alkylamino substitution. However,
only the secondary amines show softness values as good as those of the aminoethylimino
reference. The cyclohexylamino function, in particular, gives rise to a most useful
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
Handle
Reference
NHRNH2
Whiteness
Primary
NH2
Water
Soil
absorbency
release
Secondary
Tertiary
NHR
NR2
Figure 10.61 Effect of aminofunctional polysiloxanes on the physical properties of treated cotton
fabric [485]
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CH3
CH3
CH3
Si
Si
O
CH3
CH3
Si
O
CH3
x
723
CH3
Si
O
(CH2)3
O
CH3
CH3
y
CH
H2C
H3C
C
C
N
H3C
10.236
CH2
CH3
H
Barrier to rotation
(kcal/mol)
Bond dimensions
C
C
ca. 110o
O
C
ca. 110o
C
2.74
C
1.06
1.
Å
H3C—CH2—CH3
41
1.
54
Å
Typical compound
CH3
H3C—O—CH3
3Å
1.6
H3Si—O—SiH3
Si
O
ca. 144o
Si
0.32
Figure 10.62 Molecular dimensions of alkane, ether and siloxane chain segments [489]
combination of effects: it gives optimum whiteness, as well as improved water absorbency
and soil release (both properties superior to those of acylamino derivatives). In view of these
results, it is not surprising to learn that hindered secondary amino substituents (10.236) also
give rise to non-yellowing softeners [490].
An interesting attempt has been made to formulate a theoretical mechanism for the
softening and fibre-substantivity characteristics of aminopolysiloxanes [489]. Their
outstanding softening effects are the result of exceptionally high mobility of the polymer
segments, this lubrication behaviour affecting both the surface and the interior of the fibre.
Polyether chains are more flexible than polyalkyl chains, having a bond length of 1.41 Å and
a bond angle of about 110° (Figure 10.62), thus providing a lower barrier to rotation.
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
Polysiloxane chains have a longer bond length and a larger bond angle, giving rise to an
even lower barrier to rotation.
These softening agents have a binary nature; on the one hand there is the relatively
hydrophobic polysiloxane backbone of the polymer, whilst on the other there is the
aminoethyliminopropyl group capable of varying degrees of protonation. Increased
protonation gives enhanced hydrophilicity and an increasing number of positive electrostatic
charges. The hydrophobic backbone confers substantivity for hydrophobic fibres, whereas
the protonated amino groups provide for electrostatic attraction to negatively charged fibres.
These relationships are illustrated diagrammatically in Figures 10.63 to 10.68. In the case of
poly(dimethyl siloxane) on cotton (Figure 10.63), the fibre–water hydrogen bonding forces
are favoured and the weak polymer–fibre forces are inadequate to provide uniform
deposition of a polymer film over the fibre surface. Thus the lubricating action to decrease
fibre–fibre friction is inadequate and the handle of the finished fabric is firmer than
untreated cotton.
Cotton fibre surface
Dimethyl siloxane group
Figure 10.63 Poly(dimethyl siloxane) attached to cotton by weak polymer-fibre interactions [489]
Attachment to the cotton surface of an aminofunctional silicone containing relatively few
partly protonated amino substituents is illustrated in Figure 10.64. The strong interaction
between these groups and hydroxy groups in the cellulose does bring about some orientation
of the silicone polymer segments close to the fibre surface but coverage of the latter is
incomplete. The lubricating action of the polymer film, though more effective than in Figure
10.63, is somewhat limited and the fabric handle is not ‘supersoft’. Figure 10.65 shows the
much more effective distribution over the fibre surface of an aminofunctional silicone
containing the optimum proportion of partly protonated amino substituents. Coverage is
uniform and complete and the length of the silicone polymer segments between the
anchoring amino substituents is sufficient to permit their high flexibility to contribute to
optimum lubrication and a ‘supersoft’ handle. In the case of an aminofunctional silicone
containing an excessive proportion of partly protonated amino groups (Figure 10.66),
coverage is complete but the thin silicone film composed of short and inflexible polymer
segments between the anchoring points is inadequate to provide a ‘supersoft’ handle.
Figure 10.67 indicates the probable distribution of a silicone containing the optimum
content of aminoethyliminopropyl groups when applied to a polyester fibre surface. In this
case the attachment is through hydrophobic polymer–fibre interaction and the mobility of
the silicone chain segments is increased by electrostatic repulsion between neighbouring
cationic groups. Dependence of softness of the treated polyester fabric on the proportion of
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Cotton fibre surface
Dimethyl siloxane group
Aminoethyliminopropyl group (partly protonated)
Figure 10.64 Aminoethyliminopropyl silicone attached to cotton by too few cationic amino groups
[489]
Cotton fibre surface
Dimethyl siloxane group
Aminoethyliminopropyl group (partly protonated)
Figure 10.65 Aminoethyliminopropyl silicone attached to cotton by the optimum proportion of
cationic amino groups [489]
Cotton fibre surface
Dimethyl siloxane group
Aminoethyliminopropyl group (partly protonated)
Figure 10.66 Aminoethyliminopropyl silicone attached to cotton by too many cationic amino groups
[489]
Polyester fibre surface
Dimethyl siloxane group
Aminoethyliminopropyl group (partly protonated)
Figure 10.67 Probable orientation of an aminofunctional silicone on the surface of polyester [489]
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
Polyester fibre surface
Cotton fibre surface
Dimethyl siloxane group
Aminoethyliminopropyl group (partly protonated)
Figure 10.68 Probable orientation of an aminofunctional silicone between the surfaces of component
fibres in a polyester/cotton blend [489]
partly protonated amino groups present is less sensitive than in the case of cotton and the
optimum level of softness is reached at a higher proportion on polyester than on cotton.
These concepts are extended in Figure 10.68 to the behaviour of an aminofunctional
silicone applied to a polyester/cotton blend fabric. Hydrogen bonding and coulombic forces
of interaction between the partly protonated aminoethyliminopropyl groups of the silicone
and hydroxy groups at the cellulose surface are reinforced by hydrophobic interaction
between the dimethyl siloxane units and the polyester fibre surface. The mobility of the
silicone polymer segments is favoured by electrostatic repulsion between neighbouring
cationic groups and the optimum degree of softness is achieved at a relatively low proportion
of these groups, somewhat less than the corresponding optimum on cotton.
Although fabrics made from microfibres generally have a softer handle and better drape
than those from conventional fibres, these properties can be further improved to a
significant extent by the application of silicone softeners, the best results being obtained
with aminofunctional polysiloxanes [491].
Miscellaneous softening treatments
Many other products can be used as softeners but are less important commercially because
of greater cost and/or inferior properties. Examples are anionic surfactants such as longchain (C16–C22) alkyl sulphates, sulphonates, sulphosuccinates and soaps. These have rather
low substantivity and are easily washed out. Nonionic types of limited substantivity and
durability, usually applied by padding, include polyethoxylated derivatives of long-chain
alcohols, acids, glycerides, oils and waxes. They are useful where ionic surfactants would
pose compatibility problems and they exhibit useful antistatic properties, but they are more
frequently used as lubricants in combination with other softeners, particularly the cationics.
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Some amphoteric softeners such as amino acids (10.237) and sulphobetaines (10.238) are
more effective and durable than the nonionic types but less durable than the cationics;
moreover, they tend to be expensive. Other amphoteric types include the zwitterionic forms
of quaternised imidazolines (10.239); long-chain amine oxides (10.240) also exhibit
softening properties.
R
_
O
+
NH2
CH2CH2
R
R
O
+
N (CH2)n
S
R
O
C
O
10.237
10.238
R
+
HN
_
O
N
_
O
R
C
CH2CH2OCH2CH2
R
+
N
_
O
R
O
10.240
10.239
There are reactive softeners, some of which are N-methylol derivatives of long-chain
fatty amides (10.241) while others are triazinyl compounds (10.242). The N-methylol
compounds require baking with a latent acid catalyst to effect reaction, whereas dichlorotriazines require mildly alkaline fixation conditions. The N-methylol compounds are
sometimes useful for combination with crease-resist, durable-press, soil-release and waterrepellent finishes. In this context, the feasibility of using silane monomers such as methyltriethoxysilane (10.243), vinyltriethoxysilane (10.244), vinyltriacetylsilane (10.245) and
epoxypropyltrimethoxysilane (10.246) in crosslinking reactions to give crease-resist
properties and softness simultaneously has been investigated [492].
Cl
N
O
C
HOCH2
NH
HN
OCH2CH3
H3C
H2C
CH
OCH2CH3
O
10.244
OCH2CH3
OCH2CH3
10.243
10.242
OCH2CH3
Si
Si
Cl
O
CH
R
N
C
10.241
H2C
H3C
N
N
R
OCH2CH3
R
10.245
O
C
CH3
Si
C
C
O
CH3
CH2
OCH3
H3CO
Si
CH2CH2CH2 CH
OCH3
O
10.246
Softening treatments of a rather different nature include biofinishing enzyme treatments to
modify the fabric surface. This has been dealt with already in section 10.4.2. Even more
esoteric is the use of so-called telluric treatments using minerals (microliths) of precisely
defined lithological and metamorphic properties. A detailed account of these complex
materials is available [493]. In essence, an enzyme is micro-encapsulated within the mineral
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
microlith. Under the action of strong mechanical forces this crystal structure is broken open,
progressively releasing the enzyme. The process thus combines mechanical surface erosion of
the textile with biochemical modification.
Environmental aspects of softening treatments
This account is concerned with environmental aspects of the application of softeners rather
than their manufacture, although a discussion of environmental factors involved in the
production of quaternary ammonium softeners is available [494]. Environmental aspects of
cationic quaternary ammonium salts, nonionic surfactants and amphoteric compounds have
been dealt with already in section 9.8.1. With regard to cationic quaternary softeners (Figure
10.56), it has been reported that the ester 10.221 can be considered to be biodegradable, in
which respect it is superior to the tetra-alkylammonium salt 10.222 and the imidazolinium
salt 10.223 [483]. A paradox has also been pointed out [494]: as the water solubility of
quaternary compounds increases so does the rate of biodegradation and the fish toxicity, so
that the requirement for the maximum rate of biodegradation can only be met by developing
products that are more toxic to fish. Fish suffer through the interference of pollutants with
gill breathing. In some mammals, however, there is a possibility of developing a
subcutaneous toxicity that can cause neuromuscular problems and ultimately possible death.
In this sense, quaternised silicone polymers can become highly toxic if certain neuronal
distances are met, in the sense of a lock and key fit between them.
The environmental compatibility of silicone softeners is generally favourable [495,496].
The discussion here concerns only the silicone component of the formulation and not the
supporting emulsifying system. For the most part this is nonionic, preferably based on linear
ethoxylated fatty alcohols, although alkylphenol ethoxylates are still used in some countries
[496]. The salient points regarding the environmental influence of silicones can be
summarised as follows:
(1) Silicones are a minor part of discharges to waste waters.
(2) Although highly resistant to biodegradation by micro-organisms, poly(dimethyl
siloxane) derivatives are very effectively degraded via natural chemical processes [495]
such as catalysed hydrolysis and oxidation during soil contact to produce siloxanols and
silanols of lower molecular mass (Scheme 10.87). These are then susceptible to both
biological and abiotic decomposition, ultimately oxidising to natural silica.
CH3
CH3
Si
CH3
CH3
O
Si
CH3
Si
O
x
CH3
CH3
CH3
CH3
CH3
soil
water
HO
Si
CH3
O
H
+
CH3
y
Si
OH
CH3
y = mainly 1
Scheme 10.87
(3) Silicones are ecologically inert, having no effect on aerobic or anaerobic bacteria. Thus
they do not inhibit the biological processes taking place during waste water treatment.
(4) Non-volatile silicones do not bioconcentrate in aquatic media. Their large molecular
size prevents them from passing through the membranes of fish or other aquatic
creatures. They readily become attached to particulate matter and are effectively
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removed by the natural cleansing process of sedimentation. Elimination rates from
sewage sludge are very high.
(5) Silicones give insignificant BOD values. Tests on aquatic plant and animal life revealed
no measurable adverse effects even under highly exaggerated conditions. No significant
change in growth rates of algae, plankton or other marine organisms has been found.
(6) Silicones have not been found to pose a threat to insects or birds.
(7) Volatile silicones are broken down by oxidative chemical processes on entering the
atmosphere. The partially oxidised degradation products are less volatile and these are
scrubbed out of the atmosphere by rain or deposited on the ground to be further diluted
and degraded, the final products being natural silica, carbon dioxide and water.
(8) Volatile methylsiloxanes degrade quickly, the atmospheric lifetime being 10 to 30 days,
and have no potential to interfere with the ozone layer.
(9) There is no risk of forming compounds that contribute to AOX values.
(10) Formaldehyde can be produced only if the degradation temperature exceeds 200 °C and
even then the amounts produced are significantly less than those from carbon–carbon
polymers containing methyl groups.
10.10.4 Soil-release, soil-repellent and water-repellent agents
Agents to protect against soiling were developed following the increasing use of hydrophobic
fibres, particularly nylon and polyester, since experience demonstrated the tenacity of oily
stains and oil-bound dirt for these fibres. Durable press cotton fabrics also tended to soil
more easily than untreated fabrics. The subject of soiling and soil removal is more complex
than might at first appear [476,497–500] and involves such aspects as soil resistance, soil
adsorption, detergency, soil removal and soil re-deposition. We are concerned essentially
with soils attracted to, and bound mainly at the fibre surface, as opposed to particulate dirt
trapped within the interstices between fibres in the yarn. The primary objective is to modify
the fibre surface (a) to increase the resistance of the fibre to soiling in the first place (soil
repellency) and (b) to ensure that any soil that is deposited is more weakly bound and is
hence more easily removed in washing (soil release). Most soils, as expected from a
predominantly hydrophobic interaction, are held mainly by nonpolar bonding, although
electrostatic forces may come into play with, for example, coloured anionic stains from food
and beverages.
The essence of any soil-resistant treatment is to render the surface of the fibres more
hydrophilic. It also helps if the coating of the fibre is such as to reduce surface irregularity
and surface energy. Whilst the two aspects of soil repellency and soil release are interrelated,
the actual balance of these properties varies from finish to finish according to requirements.
In carpet treatments for example, which are normally given a shampoo rather than washed,
the emphasis must be on repellency, whereas soil release becomes of much greater
importance in textiles that are frequently washed.
Early soil-release agents, applied particularly to resin-finished cellulosic goods, were
water-soluble polymers, many being related to thickeners (section 10.8) such as starch,
hydroxypropyl starch, sodium carboxymethylcellulose, methylcellulose, hydroxyethylcellulose, alginates, poly(vinyl alcohol) and poly(vinylpyrrolidone). These functioned
essentially as temporary barriers and ‘preferential reservoirs’ for soil, which was thus easily
removed along with the finish in subsequent washing, when they then helped to minimise
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
re-deposition. Obviously these finishes were only temporarily effective. More durable
finishes have been developed and these are generally classified in three groups according to
whether they feature (a) carboxyl groups, (b) oxyethylene and/or hydroxy groups and (c)
fluorocarbon moieties. The fluorocarbon finishes in particular have also been developed as
water-repellent treatments.
The carboxylated polymers [476,499] include acrylic, methacrylic or maleic acid polymers
(all obviously anionic in character) applied mainly from aqueous emulsion and particularly
in combination with crease-resist or durable press resins. This type of chemistry has already
been discussed in section 10.8.2. A particularly common example is the copolymer of acrylic
acid with ethyl acrylate (10.247). In general the best balance of properties is obtained with
75–85% ethyl acrylate (y) and 25–15% acrylic acid (x), with an average chain length of
about 1300 (x + y) units; 65–85% ethyl acrylate with 35–15% methacrylic acid is also
suitable. When the content of the acidic comonomer increases above about 30% the
durability to washing tends to decrease, whilst longer chains tend to give a stiffer handle
[499].
CH2
CH
CH2
CH
C
O
C
OH x
O
y
OCH2CH3
10.247
Soil-release products containing oxyethylene or hydroxy groups may be anionic or
nonionic. Many less durable water-soluble polymers have been mentioned already, such as
the hydroxy-containing finishes poly(vinyl alcohol), starch, and derivatives of starch or
cellulose. When applied together with N-methylol reactants, as in easy-care finishing, they
give more durable soil-release properties. Typical of the oxyethylene-containing compounds
are poly(ethylene glycol) and poly(ethylene oxide) adducts of carboxylic acids, amines,
phenols and alcohols, which may be combined with hydroxy-reactive functional agents as
used in easy-care finishes, such as N-methylol reactants or isocyanates.
Essentially nonionic soil-release agents comprise polyesters, polyamides, polyurethanes,
polyepoxides and polyacetals. These have been used mainly on polyester and polyester/
cellulosic fabrics, either crosslinked to effect insolubilisation (if necessary) or by surface
adsorption at relatively low temperature. Polyester soil-release finishes have been most
important, particularly for polyester fibres and their blends with cellulosic fibres. These
finishes, however, have much lower relative molecular mass (1000 to 100 000) than
polyester fibres and hence contain a greater proportion of hydrophilic hydroxy groups. They
have been particularly useful for application in laundering processes. These essentially
nonionic polymers may be given anionic character by copolymerising with, for example, the
carboxylated polymers mentioned earlier; these hybrid types are generally applied with
durable press finishes.
Polyfluorinated chemicals now dominate in the fields of oil-repellent and water-repellent
finishes. The earlier so-called conventional polyfluorinated products were of the type
represented by poly(N-methylperfluoro-octanesulphonamidoethyl acrylate)(10.248) [499].
Such products presented a shield of closely packed fluoroalkyl groups at the fibre–air
interface, thus giving low-energy surfaces with excellent oleophobicity. These showed
excellent resistance to oil-based stains but were less satisfactory as soil-release agents during
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washing. The soil-release properties were subsequently considerably improved by
copolymerising these conventional fluorochemicals with hydrophilic moieties [476,498,499]
to give so-called hybrid block copolymers represented schematically by A–B–A–B–A–,
where A represents the perfluoroalkyl-containing segment and B the hydrophilic segment. A
typical example [498,499] is represented by structure 10.249, which is composed of
alternating perfluorinated units of the type shown in structure 10.248 with hydrophilic
oxyethylene moieties derived from the thiol-terminated copolymer of tetra-ethylene glycol
dimethacrylate and hydrogen sulphide (10.250).
CH2
CH
CH
CH2
C
O
CH
CH2
C
R
O
CH
CH2
C
R
O
CH3
R=
C
R
O
R
OCH2CH2
N
SO2(CF2)7CF3
n
10.248
O
H
CH
CH2
CH2
3
C
O
S
CH
C
O
O
(CH2CH2O)4
C
CH3
R
CH
CH2
S
CH2
CH
H
C
CH3
10
O
3
R
10.249
O
HS
CH2
CH
C
O
O
(CH2CH2O)4
C
CH
CH2
CH3
CH3
S
H
10
10.250
Once again these are only average schematic structures, in this case representing a block
copolymer of alternating segments. The hydrophilic segments in themselves show no
significant oil repellency and are not very effective as soil-release agents, yet when
incorporated into such hybrid structures they considerably improve the soil-release
properties without inhibiting the inherent soil-repellency of the perfluorinated segments.
This is said [499] to result from the capability of these hybrid polymers to orient a specific
moiety at the surface, depending on the polarity of the fibre–environment interface. Thus in
air the fibre–air interface is dominated by the closely packed perfluoroalkyl chains,
promoting good oil repellency, whilst in aqueous wash liquors it is the hydrophilic segments
that orient at the fibre–liquid interface, thus enhancing soil release. In either case the nonactive moiety is said to be collapsed below the surface. In this way, the lowest interfacial
energy, with respect to the particular environment, is attained in both cases.
The example used here incorporated a perfluorinated polyacrylate and a poly(oxyethylene) hydrophilic moiety. Other fluorochemical and hydrophilic moieties can be used
providing they display similar alternating surface orientation characteristics with respect to
air and water. The essential character of the hydrophilic unit is that it should have polar
groups capable of strong interaction with water, preferably by hydrogen bonding; examples
are hydroxy, carboxyl and ether oxygen. Usually C5–C18 perfluoroalkyl groups are used, but
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
individual products may contain a mixture of homologues. Thus there is tremendous scope
for designing a great variety of these complex copolymers.
It is now opportune to consider the structure–property relationships of fluorochemical
finishes in more detail [501,502]. Water repellency depends mainly on reducing the critical
surface energy of the fabric surface. This parameter must be less than that of the wetting
Table 10.47 Critical surface energies for low energy
surfaces [502]
Chemical groups
on the surfaces
Surface energy
(mN/m) at 20 °C
–CF3
–CF2–
–CH3
–CH2–
6
18
21
31
Table 10.48 Surface tension of a range of liquids and surface energies of a
range of textile fibres [502]
Liquid
Surface tension
(mN/m) at 20 °C
Water
Peanut oil
Olive oil
Petrol
n-Octane
n-Heptane
Fluorocarbons
72
40
32
26
22
20
10–15
Textile fibre
Surface energy
(mN/m) at 20 °C
Nylon
Wool
Cotton
Polyester
46
45
44
43
liquid in order to create a physico-chemical barrier against penetration of the fabric by the
liquid. Table 10.47 gives critical surface energies for various chemical groups at the surface
of a fibre from which it can be seen that the CF3 group is by far the most effective for
lowering this surface energy. Table 10.48 lists surface tension values of some liquids and
surface energy values of various fibres.
It is evident from these two tables that a high density of CF3 groups at the fibre surface
will lower the critical surface energy sufficiently to create a barrier to penetration of all the
liquids listed, particularly against water. It is also evident that perfluoroalkyl groups are
essential to guarantee resistance to oily liquids. Thus the presence of CF3 terminal groups is
crucial. Equally important, however, is the overall structure of the molecule [502]; the
perfluorinated segments should be long enough to maximise the CF3 group density on the
surface. It is this aspect that polyacrylic and polyurethane supporting structures have been
able to satisfy, the longest chains producing the lowest surface energies (Figure 10.69). The
results shown in Table 10.49 show how an increase in the perfluorinated segment length
gradually enhances the resistance to oils and, to a lesser extent, to water. It is these
considerations that have led to the preferred use of perfluoro-octyl or longer perfluorinated
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segments. Silicone emulsions, by comparison, typically give a surface tension of 25 mN/m
and hence act only as water repellents.
Surface energy/mN m–1
25
20
15
10
5
1
2
4
6
8
10
12
(x+1) in acrylic polymer
Figure 10.69 Change in critical surface energy with length of the perfluoroalkyl groups in acrylic
polymers [502]
Table 10.49 Oil and water repellency of cotton fabrics treated with
perfluorinated acrylic polymers [502]
Acrylic polymer
CH2
O
1% Polymer applied to printed cotton
CH
C
n
O CH2
R
Perfluorinated
group R
Oil repellency
(AATCC 118)
Spray test
(ISO 4920)
–CF3
–CF2CF3
–(CF2)2CF3
–(CF2)4CF3
–(CF2)6CF3
–(CF2)8CF3
0
3–4
6–7
7–8
7–8
8
50
70
70
70
70
80
A rather more novel yet logical development in fluorochemicals has been the emergence of
fluoro-silicone hybrid polymers [503,504]. A series of products (10.251–10.253) has been
extensively evaluated [503] for various properties, including liquid and solid surface energies,
micelle formation, wetting, contact angle, film-release and antifoam behaviour. A similar type
of product is tridecafluoro-octyltriethoxysilane (10.254). This has been applied to polyester
and to cotton fabrics, fixation being achieved by drying at ambient temperature [504].
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
CH3
Si
O
R
O
R
n
Si
O
O
CH2CH2CF3
R=
R
CH2CH2(CF2)3CF3
10.252
10.251
CH3
R1
Si
or
CH3
Si
O
CH3
2
CH2CH2(OCH2CH2)x
R2
OR3
10.253
R1
R2
R3
CH2CH2CF3
CH3
7
H
12
H
H
x
CH2CH2CF3
CH3
CH2CH2CF3
CH2CH2CF3
7
12
CH2CH2CF3
CH2CH2CF3
CH2CH2(CF2)3CF3
CH3
7
CH3
CH2CH2(CF2)3CF3
CH3
12
CH3
H
OCH2CH3
F3C(CF2)5CH2CH2
Si
OCH2CH3
OCH2CH3
10.254
For application of these fluorochemical finishes to textile fabrics, an extremely important
factor is their formulation into suitable aqueous emulsions or dispersions. The quality of the
formulation has a critical influence on stability during storage and application, as well as the
efficacy of treatment and durability [501,502]. In particular, the choice of surfactant(s) for
emulsifying or dispersing must ensure good stability with freedom from deposition on rollers,
yet must not impair the water and oil repellency of the finished fabric. No individual product
fulfils all requirements; hence specifically formulated products are available for certain fibre
types.
Application is mainly by padding followed by curing at 150–180 °C, although minimum
add-on techniques such as slop padding, spraying and foam application have been successful.
They can also be applied by discontinuous methods, such as exhaust or dip-spin [501].
These fluoropolymers are also used as the basis of so-called stain-blocking treatments,
applied especially to nylon floorcovering and upholstery [505–508]. In general, the
fluorochemical is used in conjunction with an anionic syntan resist agent of the type
described in section 10.9.4. The latter functions by blocking the cationic protonated amine
sorption sites in nylon. Thus the fluoropolymer repels oil-based soils and facilitates their
removal during cleaning, whilst the syntan inhibits electrostatic interaction between the
cationic sites and many coloured anionic substances in food, drinks and human/animal
excreta. The two product types may be applied later in the dyehouse, in which case the
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NaO3S
735
CH2OCH3
O
(CH2)zCH3
CH2
O
CH3(CH2)x
O
S
O
(CH2)yCH3
O
10.255
x, y, z = 1–5
effect is less durable. For maximum efficacy the two component types must be carefully
chosen after much empirical screening. Fluorochemicals having perfluoroalkyl groups
containing 10–12 carbon atoms and yielding an overall concentration level of 200–800 ppm
of fluorine on the weight of fibre appear to be optimal [508]. The syntans are typically
formaldehyde–phenolic condensation products; one product has the structure 10.255, which
is interesting in that all the hydroxy groups have been converted into alkylaryl ethers [508].
Polymers of methacrylic acid or maleic acid, either alone or as a blend or copolymer with
the sulphonated aryl–formaldehyde condensation products, have also been evaluated as
stain-blocking chemicals [508,509]. An interesting development is the use of a polystyrene–
maleic acid copolymer, this being unusual because of the absence of sulphonic acid groups
[508,510]. Although the maleic and methacrylic acid polymers do not have the durability of
the conventional syntans, they have the advantage that they are non-yellowing.
Although stain-blocking treatments were originally developed for nylon, there has been a
good deal of emphasis over the last decade on extending their use to wool carpets [511–
515]. Whilst syntans similar to those used on nylon are also suitable for wool, larger
amounts are required to block the greater number of dye sites in wool [512].
Environmental aspects of fluorochemical finishing agents
In certain circumstances, organofluorine compounds can lead to the generation of AOX
values, although a satisfactory method of measuring specific AOF values has yet to be
developed [516]. Typical results of the environmental analysis of twelve fluorochemicals are
shown in Table 10.50.
10.10.5 Bactericidal and insecticidal agents
There are three areas to consider:
(1) The use of insecticidal agents on wool to prevent attack by moth and beetle larvae.
(2) The use of bactericides to prevent biodegradation of chemicals such as thickening
agents.
(3) The use of bactericides to inhibit bacterial activity on textiles.
Insecticidal agents for wool
Lists of the principal types of insect that lay their eggs in wool are available [11,517].
Damage to the fibre is caused by the larvae that emerge from these eggs. Hence any
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
Table 10.50 Environmental analysis of fluorochemical agents at a product concentration of
1 g/l [516]
Product
no.
Solids
content (%)
1
2
3
4
5
6
7
8
9
10
11
12
21.2
40.9
20.3
18.7
5.3
17.0
18.5
18.0
32.3
10.1
23.4
31.2
AOX value
(mg/l)
0.18
5.6
0.18
<0.05
1.3
<0.05
0.10
0.06
19.0
0.08
<0.05
0.06
COD value
(mg/l)
BOD5 value
(mg/l)
Bacterial
toxicity
(% resistance)
275
620
250
345
305
300
510
365
805
220
425
355
140
15
60
30
160
55
45
90
335
5
55
85
26
18
21
66
6
23
20
29
27
42
50
54
insecticide used must be effective against these larvae. Since these products have no
insecticidal effect on insects that do not consume the wool, it seems likely that such
products act only through the digestive tract of the insect larvae. In addition to health and
safety considerations, fastness properties also need to be taken into account. Fastness
requirements on carpets are not so stringent as on machine-washable wool. Environmental,
as well as health and safety factors, have resulted in an almost total ban on the use of
dieldrin (10.256), the first compound to be used for this purpose. It proved to be toxic to
man, animals, fish and birds, and was highly persistent in the environment.
Cl
C
Cl
CH
O
C
CH
CH
Cl
CH2
C
CH
CH
Cl
C
HN
NH
Cl
Cl
C
CH
Cl
O
Cl
O
Cl
C
Cl
10.256
Dieldrin
NaO3S
10.257
Sulcofenuron
Environmental factors coupled with the relatively small size of the market are acting
restrictively against several other products that were formerly used. In an excellent review of
this subject [517], the compounds that have found use are divided into two categories. The
first category comprises those compounds that were developed specifically as wool
mothproofing agents, most of these being anionic multichlorinated aryl compounds; some of
these, for the reasons cited above, are no longer available. One of the best known is
sulcofenuron (10.257). The sulpho group confers water solubility and exhaustion behaviour
similar to those of an acid dye. This product is relatively expensive but has very good
chpt10(2).pmd
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737
fastness to washing and light. Sulcofenuron, up to a concentration of 4 g/kg wool, is one of
only three types of insect-resist agent permitted for the GuT scheme of ecolabelling [518].
Chlorphenylid (10.258) has also provided insect-resist treatments but AOX generation
during manufacture has led to the withdrawal of several such products since 1989. The
chloromethylsulphonamido group requires at least pH 10 for aqueous solubility as the
sodium salt. On acidification during application a dispersion of the free sulphonamide is
formed and this is absorbed mainly through weak nonpolar interactions and hydrogen bonds.
Combined chlorphenylid/sulcofenuron and chlorphenylid/flucofenuron (10.259) products
were formerly used but have been withdrawn since 1989. Chloromethylsulphonamidotrichlorobenzene (10.260) has also been withdrawn since that date. Thus of the products
developed specifically as insect-resist agents for wool only the sulcofenuron type remains in
any significant use.
Cl
O
Cl
C
Cl
O
NH
Cl
Cl
F3C
Cl
O2S
HN
NH
Cl
CF3
10.259
Flucofenuron
10.258
CH2Cl
Cl
Chlorphenylid
Cl
NHSO2CH2Cl
Cl
10.260
Compounds in the second group were originally developed as pesticides for agricultural use.
These products have proved efficaceous and amenable to formulation as wool insect-resist
agents. Most of them are pyrethroids, including permethrin (10.261), cyfluthrin (10.262) and
cyhalothrin (10.263). A further effective compound is the hexahydropyrimidine derivative
10.264. Permethrin and cyfluthrin (together up to 210 mg/kg wool) are the other two types of
insect-resist agent permitted for the GuT ecolabelling scheme [518].
H3C
CH3
C
O
HC
CH2
10.261
O
Cl
CH
CH
C
C
Cl
O
Permethrin
F
H3C
O
CH3
C
HC
CH
O
Cl
CH
CH
C
Cl
O
NC
10.262
Cyfluthrin
chpt10(2).pmd
737
15/11/02, 15:44
C
738
CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
H3C
CH3
C
O
HC
CH
O
CF3
CH
CH
C
C
Cl
O
NC
10.263
CH3
O
Cyhalothrin
Cl
NH
N
C
Cl
O
O
N
O
CH3
10.264
These synthetic pyrethroids mimic natural counterparts, of which the most important is
pyrethrin 1 (10.265). Unfortunately, the natural products lack the photochemical and
hydrolytic stability necessary for use as wool insect-resist agents. The synthetic products
have the required stability, yet retain the low mammalian toxicity and low environmental
retention of the natural products. Permethrin, however, is toxic to aquatic life and is
therefore subject to increasingly severe discharge limits. There is some evidence that
permethrin is less effective against larvae of a certain beetle. This can be compensated for by
using a combination of permethrin with the hexahydropyrimidine derivative 10.264. Some
possible alternative pyrethroids have been mentioned [517] as development products
(10.266–10.269).
O
H3C
CH3
HC
CH
Cl
H2C
CH
CH
CH
O
CH2
CH
C
CH3
C
Cl
O
10.265
Pyrethrin I
H3C
O
CH3
HC
CH
NC
CH3
CH
NC
C
O
O
C
Cl
O
Fenvalerate
738
CH
Br
10.267
chpt10(2).pmd
CH
C
Deltamethrin
CH
CH
O
10.266
H3C
O
Br
C
15/11/02, 15:44
739
AGENTS FOR FIBRE LUBRICATION, SOFTENING, ANTISTATIC EFFECTS, SOIL RELEASE
H3C
O
CH3
C
HC
CH
NC
O
C
CH3
C
CH3
O
10.268
Fenpropathrin
H3C
O
CH3
C
HC
CH
O
CH3
C
CH
C
CH3
C
NC
O
10.269
Cyphenothrin
OCH2CH3
O
C
H2C
O
CH2CH2S
CHSO2CH2CH2
O
P
X
SCH2CH2CH3
10.270
X = O or S
Other chemicals evaluated but not yet adopted commercially include organophosphorus
compounds, triphenyltin compounds, quaternary ammonium salts, imidazoles, benzimidazoles, carbamates and the precocene anti-juvenile hormones [517]. Although none of
the above has found use as an insect-resist agent, several have been used as antimicrobial
agents for textiles.
Vinylsulphone fibre-reactive, insect-resist agents have been described, in which the
insecticidal moiety is an organophosphorus grouping (10.270; X = O or S). The
vinylsulphone group, by virtue of its nucleophilic addition reactions with wool keratin,
confers excellent fastness. An interesting feature of these products is that they do not act as
insecticides on wool until they become activated by hydrolysis of the ester bond during
digestive processes within the insect.
A development reported recently [519] involves reduction of the cystine disulphide
bonds in wool with either thioglycolic acid or tetrakis(hydroxymethyl)phosphonium chloride
to form thiol groups, followed by crosslinking with bifunctional reactive dyes. This gave
improved insect resistance but had adverse effects on physical properties such as strength,
shrinkage and stiffness, thus limiting the potential of the process for commercial use.
Despite these interesting developments, it has been pointed out [11,517] that because of
the relatively small market and the costs of registration and ecotoxicological testing, it is
currently unlikely that novel agents designed specifically for wool could be marketed
economically. Any further advances are likely to be spin-offs from agricultural pesticide
developments.
Application of these agents is best carried out from the dyebath to achieve the highest
fastness ratings. This may not always be possible, however, and alternative stages (during
scouring or application of spinning lubricants) are available [11,517]. Particular care is
chpt10(2).pmd
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740
CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
necessary in the choice and application level of insect-resist agents when applying them to
fibre blends, since their partition behaviour between the component fibres varies.
Pyrethroids, for example, tend to partition in favour of nylon in wool/nylon blends [520]. It
is not surprising, given their aquatic toxicity, that these agents are under continual
environmental scrutiny [521]. In order to comply with minimum discharge requirements, it
is obviously helpful to be able to apply the minimum levels needed for adequate functionality
and this has led to the development of appropriate machinery and methods [522–524].
Bactericides for addition to fibres and other polymers
Bactericides can be added to microbially nutritious polymers, such as certain size polymers,
dispersing agents and thickening agents, in order to protect them against biodegradation.
For the same reason, they may be incorporated into man-made fibres for geotextiles and
awnings. They may be applied to medical fabrics, hosiery, underwear and sports clothing for
reasons of hygiene, in order to prevent infection, promote healing or prevent the
development of odours. A comprehensive index is available [525]. Although this index
covers all uses of antimicrobials, there is a section devoted to agents for textiles, in which
the following are listed:
– Ammonium zirconium carbonate
– 1-Capryl-2-hydroxyethylimidazoline (10.271)
– Cis-N-[(trichloromethyl)thio]-4-cyclohexene-1,2-dicarboximide (10.272; Captan)
– 2,2′-Dihydroxy-5,5′-1-dichlorodiphenylmethane (10.273; Dichlorophene)
– Diiodomethyl-p-tolylsulphone (10.274)
– Dimethylaminopropylricinoleamidobenzyl chloride (10.275)
– Lauryl/stearyltrimethylammonium bromide/chloride (10.276)
– Myristylamine (10.277)
– Sodium 2-mercaptobenzothiazole (10.278)
– 2,4,4′-Trichloro-2′-hydroxydiphenyl ether (10.279; Triclosan)
– Zinc 2-pyridinethiol-1-oxide (10.280; Zinc pyrithone).
O
O
C
(CH2)8CH3
OH
C
N
N
C
CH2CH2OH
N
S
C
Cl
Cl
10.271
I
SO2
CH2
Cl
O
H3C
HO
10.272
10.273
Captan
Dichlorophene
CH
I
10.274
Cl
CH2
NH
C
(CH2)7CH
O
chpt10(2).pmd
740
CHCH2CH(CH2)5CH3
CH3
OCH2CH2CH2N
10.275
Cl
Cl
CH3
15/11/02, 15:44
AGENTS FOR FIBRE LUBRICATION, SOFTENING, ANTISTATIC EFFECTS, SOIL RELEASE
R
CH3
+
N CH3
_
CH3 X
CH3(CH2)13NH2
N
C
10.277
R = lauryl / stearyl
X = bromide / chloride
741
_
+
S Na
S
Myristylamine
10.278
10.276
OH
N
Cl
O
O
Cl
Cl
Zn2+
S_
_
S
10.279
Triclosan
N
10.280
O
Zinc pyrithone
The environmental implications of adding biocides to polymers must always be borne in
mind since, by definition, all effective biocides are more or less toxic. Thus addition of a
biocide may render a normally biodegradable thickener less so. Hence biocides should be
used at as low a concentration as possible. Natural size polymers, thickening agents and
dispersing agents are particularly attractive targets for microbial attack, but degradation by
this route is more rapid wet than dry. Thus printing pastes and liquid disperse dyes are
sensitive targets. In the case of printing pastes, biological degradation on storage can lead to
a significant loss of viscosity and rheological malfunction. Sensitive thickening agents, for
example, can be protected by incorporating a biocide, usually at less than 0.1%, which is just
about sufficient for effectiveness [383].
It has been suggested [383] that manufacturers of thickening agents will cease to
incorporate preservatives in their products. The printer will then be responsible for selecting
and using, in just the required amount, a preservative that is still just tolerable under the
conditions of use. Formaldehyde has been widely used for this purpose but is now
ecologically undesirable. Phenolic compounds such as sodium pentachlorophenate, ophenylphenate or chloro-m-cresol have also been used. Such nucleophilic compounds can
adversely affect the yield and hue of certain reactive dyes [390]. If absolutely minimal
quantities are used, just sufficient for the required bacterial efficiency, it is possible for these
residues to be washed out with much water to give extremely dilute waste waters. These
traces no longer exert significant bacterial action and may even be biodegradable under
these conditions.
Interest in the application of biocides to textiles has increased in recent years. They may in
some cases be applied during manufacture of the fibre (in melt spinning) or as a finishing
treatment. Although numerous papers on this subject have been published, many are
unfortunately of little chemical interest as they disclose little other than commercial names.
Poly(ethylene glycol) (HO[CH2CH2O]nH) crosslinked with dimethyloldihydroxyethyleneurea
has been reported to give fabrics with antibacterial properties suitable for nonwoven protective
surgical apparel [526].
Triclosan (10.279) kills a wide range of bacteria that cause food poisoning, dysentery,
cholera, pneumonia, tetanus, meningitis, tuberculosis and sore throats. It also prevents the
development of bacterially related odours and kills the yeasts responsible for candida ulcers
chpt10(2).pmd
741
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742
CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
and athlete’s foot. This compound can be incorporated during fibre manufacture to give
durable antibacterial properties [527–529]. Despite the widespread use of Triclosan in
toothpastes and acne creams, it is reported that it can cause allergic dermatitis in susceptible
individuals, especially when used in products for the feet [525].
Poly(hexamethylenebiguanide hydrochloride) (10.281) has been used in the sanitisation
of swimming pools. For textiles, it has been formulated into a finish capable of providing a
range of antibacterial fabrics from medical products to odour-free socks [530,531]. This
agent is mainly of interest for cotton; the polymeric cationic structure exhibits high
substantivity for the negatively charged surface of the fibre. Application is by padding,
optimally at pH 7–8, and the performance can be improved by subsequent wet-on-wet
padding with an anionic fixing agent. No elevated curing temperature is required and
approximately 1% of the antimicrobial agent on the weight of cotton is optimal for
bactericidal performance. The product has a long history of use as a bactericide, exhibits low
toxicity and is environmentally acceptable, being bioeliminated by adsorption.
CH2CH2CH2CH2CH2CH2
NH
C
NH
NH
C
NH
_
average n = 12
+NH2 Cl
n
10.281
Novel bis-quaternary compounds have been reported for improving the microbial
resistance of wool [532]. These products are described as a new class of bis-quaternary
ammonium surfactants known as gemini quaternary ammonium compounds or bis-quats.
They consist of two saturated hydrocarbon chains and a complex polar group consisting of
two quaternary ammonium salts linked through an alkane spacer chain containing amide
and optional disulphide bonds. Bacterial efficacy on wool was confirmed for two products:
N,N′-bis(N-dodecyl-N,N-dimethylglycine)-1,4-diaminobutane dihydrochloride (10.282)
N,N′-bis(N-dodecyl-N,N-dimethylglycine)cystamine dihydrochloride (10.283). These
agents were applied at 0.0025–0.5% on the weight of wool by exhaustion from an aqueous
solution at 40 °C.
H3C(CH2)11
H3C
N
+
NHCH2
_
H3C
Cl
(CH2)11CH3
CH2CH2CH2CH2
N
C
C
O
10.282
O
H3C(CH2)11
H3C
+
NHCH2
_
H3C
Cl
chpt10(2).pmd
742
N
+ CH3
CH2NH
_ CH
3
Cl
(CH2)11CH3
CH2CH2
SS
CH2CH2
N
C
C
O
10.283
O
+ CH3
CH2NH
_ CH
3
Cl
15/11/02, 15:44
AGENTS FOR FIBRE LUBRICATION, SOFTENING, ANTISTATIC EFFECTS, SOIL RELEASE
743
The bacterial resistance of silk fibroin can be improved by treatment at low pH values in
aqueous solutions of metal ions and at high pH values in solutions of metal–amine
complexes, using untreated silk or silk pretreated with an aqueous solution of tannic acid
(10.183). Suitable metals include Cu, Zn, Ni, Fe and Ag [533]. It is to be expected that
treatment with such metal compounds will be subject to restrictions in environmentally
sensitive areas.
Antimicrobial agents may adversely affect the light fastness of nylon dyeings or cause
yellowing of the fibre. The six antimicrobial treatments listed in Table 10.51 have been
Table 10.51 Comparison of antimicrobial finishes on nylon [534]
Finish
Chemical class(es)
Structure(s)
1
2
3
4
5
6
Silicone quaternary ammonium salt
Silicone quaternary ammonium salt
Organo-tin
Phenolics (mixture)
Phenolic and organo-tin
Organo-tin and quaternary ammonium salt
10.284
10.285
10.286
10.273/10.287
10.279/10.288
10.288/10.289
(CH2)17CH3
H3C
(CH2)9CH3
OCH3
+
N CH2CH2CH2
_
CH3 Cl
Si
H3C
_
Cl (CH2)9CH3
OCH3
OCH3
10.284
Si
OCH3
OCH3
10.285
H3C (CH2)3
H3C(CH2)3
OCH3
+
N CH2CH2CH2
_
O
+
Sn H
CH
C
C
O
H3C (CH2)3
CH
(CH2)3CH3
_
O
+
H Sn (CH2)3CH3
O
(CH2)3CH3
10.286
OH
H3C (CH2)3
Cl
O
Cl
H3C(CH2)3
Sn
O
H3C (CH2)3
Cl
(CH2)3CH3
Sn
(CH2)3CH3
10.288
10.287
CH2
R
+
N CH3
_
CH3
Cl
R = n-alkyl
10.289
chpt10(2).pmd
743
(CH2)3CH3
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744
CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
evaluated in relation to these two factors, each being applied at the manufacturer’s
recommended concentration [534]. The effect on light fastness varied with each dye and
only a few dyes were studied; some dyes were more sensitive to specific antimicrobial
finishes. Overall, Finish 3 had the smallest effect on light fastness, followed by Finish 5.
Finishes 6 and 4 gave the greatest reduction in light fastness, Finish 4 also causing
appreciable discoloration of the undyed fibres.
10.11 FOAMING AND DEFOAMING AGENTS
10.11.1 Foaming agents
The idea of using a matrix of foam to transfer chemicals and colorants to textiles had its
origin in the growing need to conserve thermal energy and water in the aftermath of the socalled oil crisis and recession of the early 1970s. In a sense, it was an antidote to the feverish
work carried out in the late 1960s on solvent dyeing. Foam dyeing began with an elegant
short-liquor (about 1.5:1) dyeing process developed in 1972 mainly for garment dyeing in
rotary machines. The application of foams to textiles has been widely investigated
subsequently, although the degree of commercial acceptance has proved limited.
Nevertheless, potential exists for the foam-based application of lubricants, stiffening agents,
waxes, size polymers, mercerising liquors, durable press resins, water/oil repellents, softening
agents, shrink-resist resins, dyes and print pastes. Although continuous dyeing and printing,
mainly of carpeting, has attained significant commercial use, most foam processing is
confined to the application of finishes where concentration tolerances and evenness of
application are much less critical. A major account of the properties of foams and their
general industrial applications includes a chapter devoted to textile applications [535].
Apart from reproducibility and uniformity of application, one of the main problems
associated with foam application is dissolving or dispersing relatively large quantities of the
principal and auxiliary agents in a very small volume of water, followed by the difficulty of
maintaining compatibility of the components and the density and stability of the ‘loaded’
foam under such conditions. For example, resin finishing can typically require the
dissolution of about 600 g of resin, softener and catalyst in only 400 g of water. Nevertheless,
foam processing does offer advantages, notably in the conservation of water and energy and
the reduction of effluent problems.
The foam matrix used in textile wet processing is a stabilised air-in-water system. A foam
cannot be made with pure water, however; a foaming agent, usually a surfactant, is needed
to give a reasonably stable honeycomb matrix of air cells, each enclosed by a thin
viscoelastic film of liquid. Clearly, a reduction in surface tension is one important factor in
the creation of foam; others include the elasticity and viscosity of the film walls between the
bubbles, and the size and uniformity of the bubbles themselves. Drainage, by gravity, of the
liquid from the film walls leads to instability of the foam and tends to a maximum with larger
spherical bubbles. In a system consisting of variously sized bubbles, the smaller ones tend to
coalesce into the larger, which are thus further increased in size and become less stable
because of the increased propensity for liquid drainage. Thus, for maximum stability, the
bubbles should be as small and uniform as possible, leading to minimum diffusion of air from
bubble to bubble, and maximum entropic and electrical double layer repulsion. The ideal
state is not attainable in practice, however, and all foams are unstable to some degree. Nor is
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FOAMING AND DEFOAMING AGENTS
745
perfect foam stability particularly required in textile application, since at some stage during
the process collapse of the foam is generally desirable to ensure maximum deposition of
chemicals and/or colorants.
The foaming propensity of surfactants generally reaches a maximum at the critical micelle
concentration, beyond which there appears to be little further contribution to foam density.
Foam stabilisers are also added in some cases. The two important steps in the foam
treatment of textile materials are generating the foam and applying it to the substrate:
(1) Generation is generally by high-speed rotors, with metered air and liquid flows and
monitoring to control the density of the foam.
(2) Application of controlled amounts of foam to the substrate is by knife-on-roller, knifeon-blanket, floating knife, horizontal pad or furnishing roller with doctor blade, or by
squeegee across a printing screen.
Subsequent collapsing of the foam is generally by collapse onto the fabric (controlled to
some extent by the chemicals used), by vacuum suction of the foam into the fabric, or by
means of a pad nip.
The most important auxiliary used is, of course, the foaming agent. In theory any
surfactant that will form a stable foam can be used. In practice the choice is usually between
anionics (generally cheaper), nonionics, or a mixture of both. Consideration must be given
to overall compatibility as well as to foaming characteristics: for example, anionic agents
should generally be avoided when applying cationic products. Long-chain alcohol
sulphonates and ethoxylates, as well as sulphates and sulphosuccinates, have been used; a
typical selection is given below, the first three being anionic and the others nonionic:
– sodium lauryl sulphate
– ammonium lauryl sulphate
– sodium dioctylsulphosuccinate
– lauryl alcohol poly(oxyethylene)
– decanol poly(oxyethylene)
– tridecanol poly(oxyethylene).
Particularly effective is a mixture of anionic and nonionic agents, such as a mildly anionic
sulphated alcohol ethoxylate with a nonionic alcohol ethoxylate. Ideally, foaming agents
should:
– generate consistent foam easily
– show optimum and uniform wetting
– cover a wide range of wettability so as to be adaptable for different situations
– show little or no effect on colour fastness
– be compatible with the other components
– be biodegradable.
The main function of the foam stabilising agent is to reinforce the intercellular film wall by
contributing rheological characteristics of viscoelasticity. The increased viscosity may also
assist handling. The aim, as so often with auxiliaries, is to achieve an optimum balance. If the
bubbles are too thin and wet too quickly they will collapse prematurely, whilst too stable a film
could hinder uniform application. Examples of products used as foam stabilisers include
thickening agents such as the polysaccharides, hydroxyethylcellulose, methylcellulose,
chpt10(2).pmd
745
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
carboxymethylcellulose, poly(vinyl alcohol) and poly(acrylic acid), as well as other compounds
such as sodium tripolyphosphate, sodium hexametaphosphate (detergent ‘builders’) and
decanol. Ideally, foam stabilisers should increase the stability of the foam to the optimum
controllable level whilst also allowing for subsequent controllable collapse. They should be
compatible with the other components and effective at various concentrations, give
pseudoplastic solutions, and should not affect the drape and handle of the fabric. Electrolytes
can have positive or negative effects on foam stability [535]; for example, a low concentration
of phosphate ions increases the stability of a sodium laurate foam, whereas sodium chloride
decreases the stability.
10.11.2 Defoaming agents
All chemical systems, to varying degree, tend to reduce free energy and foams are no
exception. Hence foams are thermodynamically unstable, yet their stability varies from
almost instantaneous collapse to prolonged persistence. Although foam can be useful, there
are still many circumstances where its presence and persistence is enough of a nuisance to
create a need for foam-destruction products, known as defoaming agents or antifoams. An
authoritative reference work on the theory and applications of defoaming is available and
this includes a chapter on applications in textile dyeing [536]. Reference 535 contains a
chapter on the science and technology of silicone antifoams.
Just as foams are stabilised by decreasing the rate of liquid drainage from the film walls,
they can be destabilised by increasing this drainage, resulting in thinning and eventual
rupturing of the film. Defoaming agents generally effect this by two mechanisms, the basic
objective being to displace foam-stabilising substances from the liquid-air interface [537]:
(1) Spreading alone is sufficient with light foams of high blow ratio (those having a high
air-liquid ratio); in this case surfactants of relatively low surface tension (i.e. powerful
surfactants) will spread over the large surface area of the intercellular film and displace
the surfactants that are tending to stabilise the foam.
(2) Denser foams of low blow ratio additionally require penetration of the thicker aqueous
film by the defoaming agent. Such a defoamer consists of an emulsified hydrophobic
substance, which when added to the foaming system disperses fine droplets of insoluble
hydrophobic material within the liquid lamellar walls, thus entering the liquid-air
interface, aided to some extent by the solubilising action of the foaming agent(s). This
creates a weak link as a result of high interfacial tension, the foam then tending to
rupture at the interface between defoamer and foamer. In practice, the system is a finely
balanced one requiring careful formulation of the composite defoaming agent.
The main requirement of an effective defoamer [537] is that the agent should be insoluble
in the foaming system and should have a high rate of spreading. Spreading will be favoured if
the defoamer has a lower surface tension than that of the foaming system. The interfacial
tension between defoamer and foaming system must be high, but not so high as to inhibit
spreading. A low degree of attraction between defoamer and foaming system (i.e. a high
interfacial tension) is achieved by nonpolar defoamer systems that do not contribute
positively to the surface viscosity of the lamellar walls.
For maximum efficiency the defoamer should be added to the system as soon as foaming
chpt10(2).pmd
746
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FOAMING AND DEFOAMING AGENTS
747
becomes troublesome, since its active life-span is inevitably limited by the system’s
thermodynamic instability. Its principal action is on the liquid-air interface; therefore there is
little point in adding it before sufficient liquid-air interfaces are formed. Secondly, the
insoluble active ingredient of the defoamer, given the nature of the system in which it is
working, must eventually become more or less solubilised, or at least emulsified, into the bulk
liquid system, thus losing its activity and in some circumstances actually promoting foaming.
Defoamers are generally anionic or nonionic systems and fall into two groups. The first
group consists of water-soluble surfactants with polar and nonpolar moieties. These
compounds are effective only over a narrow range of conditions, functioning simply as
spreading agents, and are seldom used alone. Such surfactants are readily absorbed into the
bulk of the foaming system where, not surprisingly, they contribute to the foaming problem.
This system is more frequently used, however, as the vehicular or ‘carrier’ basis of the second
group of defoamers, which are much more widely used. These more active defoamers are
emulsions of water-insoluble silicones or organic-based compounds of low volatility and high
spreading power.
The general requirements for an ideal defoamer can be summarised [536]:
– eliminates existing foam as well as preventing further foam from forming
– easy to disperse in the dyebath
– does not react or interact with dyes or auxiliaries present in the bath
– chemically stable under dyeing conditions
– no deposition on fabric or machinery causing spotting or staining of the fabric
– no colour
– no odour
– stable to storage
– safe both to humans and to the environment.
The active organic-based defoamers include:
(1) Fatty acids, their glycerides and other esters, including fats, waxes and oils such as
mineral and vegetable oils; fatty alkylamines and acylamides. Alkaline earth metal or
aluminium salts of fatty acids tend to leave deposits on machinery [536].
(2) Higher alkanols, including the isomeric octyl alcohols (2-octanol and 2-ethylhexanol),
cyclohexanol, lauryl and cetyl alcohols. Aliphatic alcohols have relatively poor foam
control and have an odour that can be nauseous [536].
(3) Polyglycols, especially poly(propylene-1,2- or -1,3-glycol). Poly(oxyethylene) and
poly(oxypropylene) block copolymers have relatively poor foam control [536].
(4) Insoluble alkyl esters of phosphoric acid, especially tributyl phosphate. These phosphate
esters have relatively poor foam control [536].
Certain limitations of organic defoamers can be minimised by judicious formulation of
mixtures. The following system is said to overcome some of the drawbacks associated with
aluminium salts of fatty acids [536]:
– 87% Paraffin oil
– 6% 2-Ethyl-n-hexanol
– 4% Aluminium distearate
– 3% Phosphoric acid esterified with polyethoxylated p-nonylphenol.
chpt10(2).pmd
747
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748
CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
In the case of an aliphatic alcohol such as iso-octanol, a similar level of defoaming activity
together with a much less offensive odour can be achieved by using a propoxylated ester of a
branched aliphatic acid, such as propylene-1,2-glycol mononeodecanoate (10.290).
CH3
H3CCH2CH2CH2CH2CH2
C
O
C
CH3
CH3
O
CH2
CH
OH
10.290
The active ingredients in silicone-based defoamers have traditionally been a poly(alkyl
siloxane), especially poly(dimethyl siloxane) (10.226), and silica (SiO2); the latter may be
chemically bonded to the polysiloxane to render its surface hydrophobic. Some ‘spotting’
problems have been experienced with these defoamers owing to incompatibility of the
antifoam emulsion with certain dye dispersions, especially at high rates of shear in the hightemperature dyeing of polyester with disperse dyes. This created a poor reputation for the
early silicone antifoams in jet dyeing. The problems arose by destabilisation of the emulsion,
resulting in cracking out of hydrophobic components which caused staining or spotting of
the fabric, together with a drastic loss of defoaming action. The poly(dimethyl siloxane)
products exert little foam suppression power alone. Their foam inhibition properties only
become fully developed when combined with finely divided, hydrophobic silica particles
[538].
Further improvements have been made to the emulsion system and to the silicone
components themselves. Improved derivatives of poly(dimethyl siloxane) include [537] block
copolymers with poly(oxyethylene) and poly(oxypropylene) segments represented schematically by structure 10.291. The solubility and other characteristics of these alkoxylated silicones
(silicone polyglycols) can be adjusted by varying the proportions of dimethyl siloxane and
oxyalkylene units. A specific advantage claimed for these compounds in high-temperature
dyeing is their inverse solubility, analogous to the cloud point effect of nonionic surfactants. As
the dyebath temperature approaches its maximum the solubility of the defoamer decreases,
thus helping to maintain its effectiveness, whilst the increase in solubility on cooling after
completion of the dyeing is claimed to overcome potential problems of subsequent spotting.
The four most commonly used alkoxylated silicones are represented by structures
10.292–10.295 [536], where R is typically a methyl group, a is typically 1 and G is typically
represented by formulae such as –(CH2)3(OCH2CH2)x(OCH2CH2CH2)yOR or
–(OCH2CH2)x(OCH2CH2CH2)yOR, where R is an end-capping group such as methyl.
A more recent development is the use of hybrid fluorochemical silicones, such as
nonafluorohexyl-substituted siloxanes of the type represented by structure 10.296 [503].
CH3
CH3
Si
CH3
CH3
O
Si
CH3
CH3
CH3
Si
O
Si
O
y
x
CH3
CH3
R1(CH2CH2O)a(CH2CH2CH2O)b
R2
10.291
chpt10(2).pmd
748
15/11/02, 15:45
FOAMING AND DEFOAMING AGENTS
CH3
CH3
Si
CH3
CH3
CH3
CH3
Si
O
Si
O
G
m
Si
O
CH3
n
CH3
G
CH3
CH3
Si
Si
CH3
Si
O
G
10.292
m
CH3
Si
O
CH3
n
G
CH3
10.293
CH3
Ra Si
CH3
CH3
O
749
O
CH3
CH3
Si
O
CH3
Si
G
n
O
m
Si
CH3
CH3
4–a
10.294
CH3
Ra
Si
O
CH3
Si
O
CH3
Si
G
n
CH3
O
m
Si
G
CH3
CH3
Si
4–a
O
CH2CH2CF2CF2CF2CF3
n
10.295
10.296
These copolymers have been mentioned already in section 10.10.4 as versatile and highly
effective stain-resist, oil- and water-repellent finishing agents.
Antifoams are generally supplied for textile use as carefully formulated, relatively dilute
aqueous emulsions; this ensures that the hydrophobic phase is uniformly distributed within
the foaming system and also helps to safeguard against overdosing, with the attendant
danger of spotting. A typical emulsion generally contains emulsifying agent(s) and
thickening agent(s) in addition to water and the insoluble hydrophobic defoaming agent.
For emulsification, the most common system [537] is a combination of a low-HLB surfactant
with one of high HLB value, such as glyceryl monostearate or sorbitan monostearate with
poly(ethylene glycol) monostearate. Obviously, the surfactants selected need to be lowfoaming types and should provide an optimum level of emulsification, since overemulsification will tend to negate the activity of the defoamer by inhibiting its interaction
with the foaming system. The size of the defoamer droplets, largely determined by the
emulsification system and the degree of comminution during manufacture, is critical in
relation to the efficacy of the product; too small a size gives inadequate activity and if it is
too large the stability of the emulsion is adversely affected. The optimum droplet size
generally appears to be in the range 2 to 50 µm [537].
The function of the thickening agent is to increase the viscosity and so contribute to the
stability of the product, the aim again being to attain an optimum level of viscosity.
Thickening agents that do not gel at the high temperatures used in textile processing are
essential; hydroxyethylcellulose, alginates and synthetic poly(acrylic acid) derivatives may be
used. A small amount of a bactericide such as methyl p-hydroxybenzoate is often added to
safeguard against biological degradation during storage, particularly in the case of natural
thickening agents.
chpt10(2).pmd
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750
CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
Typical formulations of commercial composite antifoams have been detailed [536,537].
There are many products on the market but evaluation of their relative efficacy depends on
the foaming problems to be overcome. Not only does the chemical type of the active
defoamer have to be considered, but its state within the emulsion and the intrinsic
properties of the emulsion are also of crucial importance. Methods of evaluating defoamers
have been described [539,540].
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CLASSIFICATION OF DYEING AND PRINTING AUXILIARIES BY FUNCTION
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chpt10(2).pmd
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K J Baumert and P C Crews, AATCC Internat. Conf. & Exhib. (Oct 1994) 140.
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759
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FLUORESCENT BRIGHTENING AGENTS
CHAPTER 11
Fluorescent brightening agents
John Shore
11.1 INTRODUCTION
Partly from an association of whiteness with cleanliness and partly for aesthetic reasons to
achieve a more pleasing contrast with coloured goods, from the earliest times people have
searched for methods of producing ever whiter textiles. The oxidative bleaching of vegetable
fibres on exposure to air and sunlight, as well as the reductive bleaching of animal fibres by
the action of fumes from burning sulphur, have been known for thousands of years. Natural
blue minerals such as lapis lazuli (section 2.10.2), ground up and applied at very low
concentrations, have been used for centuries to mask any residual yellowish colour
remaining after bleaching, the overall greyish effect being perceived as ‘white’. Today
whiteness is achieved by a combination of chemical bleaching and the use of so-called
fluorescent brightening agents (FBAs).
Fluorescent hydroxy-substituted derivatives of coumarin (benzo-α-pyrone) occur
naturally, including aesculetin (6,7-dihydroxy) in horse chestnut, daphnetin (7,8-dihydroxy)
in daphne and umbelliferone (7-hydroxy) in spurge laurel. Paul Krais in 1929, in the first
recorded application of an FBA to textiles, carried out tests on half-bleached linen using an
aqueous extract of aesculetin-6-glucoside (11.1) from horse chestnut bark [1]. The addition
of blue-violet light to the total light reflected from the fabric produced a ‘whiter than white’
effect. Unfortunately, aesculetin showed poor fastness to light and washing. Nevertheless,
Krais’s work stimulated research in this area and in 1934 Paine and Radley patented the use
of 4,4′-bis(benzoylamino)stilbene-2,2′-disulphonic acid (11.2) in banknotes [2]. The first
commercial bis(triazinylamino)stilbene derivative (11.3) was introduced by I G Farben in
1940 as a brightener for cotton.
O
C
NH
SO3H
CH2OH
CH O
HO
CH
HC
CH
CH CH
OH OH
CH
O
HO3S
HO
O
11.1
Aesculetin-6-glucoside
O
HN
11.2
O
760
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C
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MODE OF ACTION OF A FLUORESCENT BRIGHTENER
761
NH
N
N
NH
N
SO3Na
HO
HC
CH
OH
NaO3S
N
HN
N
N
11.3
HN
CI Fluorescent Brightener 113
After the Second World War, development of synthetic FBAs was extremely rapid.
Several hundred commercial products, representing a wide variety of chemical types, have
since been marketed, with FBAs probably corresponding to approximately 10% of world
demand for dyestuffs. Several excellent books and reviews of the chemistry, application and
properties of FBAs have appeared [3–13].
FBAs are used to brighten not only textile materials but also paper, leather and plastics.
They are important constituents of household detergent formulations. More specialised
areas of application include lasers, liquid crystals and biological stains. By far the most
important uses for FBAs, however, are in applications to textiles and paper. Much of what
follows will be concerned with these two categories.
Descriptive terms such as ‘fluorescent whitening agents’, ‘optical brighteners’ and ‘optical
bleaches’ have all been used for the products described in this chapter as FBAs. Many of
these terms have validity and the term ‘fluorescent brightening agents’ is preferred here only
because it has been adopted in the indexes of Chemical Abstracts.
An FBA is a strongly fluorescent substance that absorbs ultraviolet radiation, emits light
in the blue-violet region of the visible spectrum and is substantive to the substrate for which
it is intended. The product should be applicable without undesirable side-effects, such as
staining or subsequent photosensitisation of degradation, on any other substrate that may be
present. The treated material should retain its properties during its working life and under
the conditions in which it will be found. For commercial success the product will also need
to be priced attractively and supplied to the customer in a form that is convenient and
practical to use. The marketed product and the active brightener that it contains must not
exhibit toxicity problems nor create an environmental hazard.
11.2 MODE OF ACTION OF A FLUORESCENT BRIGHTENER
All dyes absorb light. Fluorescent dyes re-emit the absorbed energy as light of longer
wavelengths. An FBA is a fluorescent chemical that absorbs in the ultraviolet region of the
spectrum and emits blue-violet light. A typical FBA shows maximum absorption at a
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FLUORESCENT BRIGHTENING AGENTS
wavelength between 340 and 380 nm and emits visible light at a maximum emission
between 425 and 450 nm. When describing a substrate treated with an FBA, the terms
‘remitted’, (reflected plus emitted) and ‘remission’ (reflectance plus emission), which take
into account the fluorescence component of the total light emitted from the brightened
substrate, are preferred to ‘reflected’ and ‘reflectance’.
When present on a substrate an effective FBA increases the apparent reflectance of the
article in the blue-violet region of the spectrum. The treated material remits more light in
the visible region than does an untreated white sample and thus appears ‘whiter than white’.
These effects are indicated in Figure 11.1, which illustrates the importance of thorough
preparation of the substrate to be brightened. Curve C represents the remission of an
unbleached ‘dirty’ fabric. On treatment with an FBA this material is brightened but the
treated sample (curve D) may be less bright than a clean but unbrightened fabric (curve A)
and much less bright than the same fabric after brightening (curve B). The details of fabric
preparation and bleaching processes are discussed fully elsewhere [14]. In some cases
brightening and bleaching can be carried out simultaneously and these possibilities will be
discussed later in this chapter.
B
Clean brightened article
Clean unbrightened article
Remission
A
‘Dirty’ unbrightened article
‘Dirty’ brightened article
C
D
Fluorescence
300
400
500
600
700
Wavelength/nm
Figure 11.1 Remission of brightened and unbrightened fabric
An efficient FBA must absorb strongly in the ultraviolet region and must also re-emit a
major proportion of the absorbed energy as visible light, that is, it must have a high
fluorescence efficiency. Although fluorescence can occur from the α-bonds of many organic
compounds, strong fluorescence is associated with π-bonded electrons. All FBAs therefore
contain a considerable number of conjugated double bonds.
Processes occurring during absorption and fluorescence are shown in Figure 11.2, where
chpt11(2).pmd
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MODE OF ACTION OF A FLUORESCENT BRIGHTENER
S0, S1, S2, … represent so-called singlet states in which all the electrons have paired spins,
and T1, T2, … represent triplet states in which two electrons have unpaired spins. The
energy levels of both ground (S0) and activated states (S1, S2, …) are subdivided into
vibrational and rotational energy levels. The vibrational energy levels are shown in Figure
11.2. Differences in rotational levels are very small and can be ignored for the present
discussion.
Internal
conversion
S2
T2
Thermal
deactivation
Intersystem
S1
crossing
S0
Intersystem crossing
Phosphorescence
Internal conversion
Absorption
Fluorescence
Energy
T1
S0
Figure 11.2 Absorption and fluorescence processes
When an FBA absorbs a photon of light an electron is raised from the ground state (S0)
of the molecule to one of its activated singlet states (S1, S2, …). Transitions from a singlet to
a triplet state are quantum-mechanically ‘forbidden’. Absorption occurs when the molecule
is in its ground state, the vibrational level of the activated state reached by absorption being
decided by the size of the quantum of energy (E) involved (E = hc/λ, where h is Planck’s
constant, c the velocity of light in a vacuum and λ the wavelength). Vibrational energy
levels are extremely close to each other and vibrational energy is lost very rapidly (within
about 10–12 s) before fluorescence occurs, when the molecule returns from the lowest
vibrational level of the activated state (S1) to one of the vibrational levels of the ground
state (S0) whilst simultaneously emitting a photon of light. Fluorescence lifetime is typically
about 10–9 s. Energy can also be lost from the activated singlet state by non-radiative
processes (internal conversion) or by ‘forbidden’ intersystem crossing to give the triplet
state. The triplet state in turn can lose energy, returning to the ground state by
phosphorescence or by a further radiationless intersystem crossing. Phosphorescence always
occurs at a longer wavelength than fluorescence because the energy difference between T1
and S 0 is less than that between S1 and S0. FBAs do not exhibit significant
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FLUORESCENT BRIGHTENING AGENTS
phosphorescence. For a more detailed discussion of the principles of fluorescence the reader
is referred to books by Lumb [15] and Lakowicz [16].
Typical absorption and fluorescence spectra are shown in Figure 11.3. Since energy is lost
in the activated state (S1) before fluorescence, the emission maximum always occurs at a
lower wavenumber than the absorption maximum. The difference, which is termed the
Stokes shift, can be calculated approximately from the absorption spectrum using the
Pestemer rule [17,18]. This rule states that the Stokes shift is 2.5 times the half-bandwidth
at the absorption maximum.
Relative absorption
or emission
Stokes
shift
Absorption
Fluorescence
emission
Halfbandwidth
Wavenumber
Figure 11.3 Typical absorption and emission curves for an FBA (polar solvent)
For the following reasons the Stokes shift of an FBA should not be too large:
(1) The closer the maximum wavelength of light absorption of an FBA approaches the
visible region, the greater is the energy content of sunlight at the earth’s surface that is
available for fluorescence excitation and the greater the potential fluorescence. An
FBA with a Stokes shift of 60 nm or less would have a maximum absorption at 370 nm
or longer and it would still show maximum fluorescence in the blue region of the
spectrum.
(2) Although absorption and fluorescence spectra are not always (or even normally)
symmetrical, a smaller Stokes shift reduces the chance of significant fluorescence in the
green or yellowish green regions of the spectrum. Green or yellowish green fluorescence
reduces whiteness.
A possible method for predicting absorption bandwidths of chromogenic molecules or FBAs
using PPP–MO theory (section 1.5) has been devised. It is based on the empirical linear
relationship stated by the Pestemer rule. Thus theoretical Stokes shifts are computed by the
PPP–MO method and related to bandwidths. The requisite MO parameters for various typical
absorption bands have been developed for use in these calculations. Reasonable correlation
between calculated and experimental half-bandwidth data was found, suggesting that this
approach has practical potential in predicting colour tone and brightness intensity [19].
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EVALUATION OF FBAs: MEASUREMENT OF WHITENESS
765
The quantum efficiency of fluorescence of a molecule is decided by the relative rates of
fluorescence, internal conversion and intersystem crossing to the triplet state. Up to the
present time it has proved impossible to predict these relative rates. Thus, whilst it is now
possible to calculate theoretically the wavelengths of maximum absorption and of maximum
fluorescence of an organic molecule, it remains impossible to predict which molecular
structures will be strong fluorescers. Design of new FBAs still relies on semi-empirical
knowledge plus the instinct of the research chemist.
11.3 EVALUATION OF FBAs: MEASUREMENT OF WHITENESS
Almost all man-made fibres destined for sale as white goods are producer-brightened by the
manufacturer and white textiles are almost always laundered using detergents containing
cellulose-substantive FBAs [20]. To evaluate an FBA it is necessary both to apply the
product to the desired substrate and to measure the whiteness of the treated material.
Measurement of the fluorescence intensity of the FBA-treated substrate provides useful
additional, although different, information.
Visual judgement of whiteness is highly subjective. Many factors, such as the observer’s
age, sex and colour perception, and even the hue of ‘white’ materials normally encountered
in the observer’s country, help to decide on individual preference for either a red-violet or a
blue-green shade of white.
Instrumental measurement of whiteness has been the subject of much research. The
parameters needed for unambiguous characterisation in the assessment of whiteness and tint
of fluorescent substrates have been reviewed [21]. The importance of seeking good
correlation between different instruments is stressed [20]. Various trials have demonstrated
that it is possible to adjust modern instruments used to measure the optical characteristics of
FBA-treated samples of paper so that the results agree with a standard deviation of the order
of one CIE whiteness unit [22].
Many whiteness (W) formulae have been proposed. All are based on CIE colour space
and the X,Y,Z tristimulus values. Three of these equations are those of Berger [23]
(Equation 11.1), Stensby [24] (Equation 11.2) and the CIE 1982 formula (Equation 11.3).
W = 3B + G - 3R
(11.1)
W = L + 3a - 3b
(11.2)
W = Y + 800( x n - x ) + 1700(yn - y)
(11.3)
The R,G,B values of the Berger formula measured by tristimulus colorimeters are linearly
related to the X,Y,Z tristimulus values of the CIE system. The Stensby formula incorporates
the L,a,b tristimulus values of the Hunter system. In the CIE 1982 formula, xn and yn are the
chromaticity coordinates of the D65 (2° or 10° observer) light source.
Information on the hue of whiteness is provided by dividing CIE colour space, in the
neighbourhood of the D65 achromatic point, into a series of parallel strips corresponding to
variations in hue of whiteness. The principle is illustrated in Figure 11.4. According to this
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FLUORESCENT BRIGHTENING AGENTS
system, a neutral white has a nuance (NU) value of zero. Greener shades have values
between 0 and +5, whereas violet shades have values between 0 and –5. The NU value can
be calculated from chromaticity data. Typical values giving a result corresponding to the
judgement of a standard observer are shown in Equation 11.4.
NU = -1132 x + 725y + 115.45
(11.4)
The brightened specimen under test must be carefully prepared and meticulous attention to
the instrumentation and state of equipment is necessary if reliable data are to be obtained.
The subject is complex and has been well reviewed by Griesser and co-workers [25,26].
Greener
NU values
positive
y
D65
achromatic
point
0.33
NU values
negative
More
violet
0.28
0.28
0.31
x
Figure 11.4 Colour space and hue of whiteness
To an average observer a sample of white material brightened with a low concentration of
an FBA will exhibit a bluish tone corresponding to a dominant wavelength in the vicinity of
467 nm. As the amount of FBA present in the substrate is increased this hue changes. At
first the sample may become slightly more violet but as the maximum possible whiteness is
approached there is a distinct change in hue towards green until the sample becomes
overloaded with FBA, whiteness falls and the material is perceived as coloured rather than
white. Typical effects are illustrated in Figure 11.5.
In industry a direct comparison for strength between two FBAs is frequently required.
Where both brighteners contain the same active component, or where they give a closely
similar shade of white, such a comparison presents little difficulty. Where the two products
yield quite different shades of white, however, comparisons between them are usually
meaningless. A typical situation is illustrated in Figure 11.6.
At concentrations below that yielding the maximum whiteness achievable there is an
approximately linear relationship between whiteness and the logarithm of the concentration
chpt11(2).pmd
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EVALUATION OF FBAs: MEASUREMENT OF WHITENESS
767
NU value
Brightened with green-hued FBA
+ve
0
Brightened with
violet-hued FBA
–ve
Whiteness
Figure 11.5 Variation of hue with whiteness
Violet-hued FBA 1
Whiteness
Violet-hued FBA 2
Green-hued FBA
Log (concn of FBA on weight of substrate)
Figure 11.6 Dependence of whiteness on concentration
of FBA present on the substrate. If two FBAs are similar in hue the straight line portions of
the graph are almost parallel and a single-strength relationship can readily be calculated. If
two FBAs differ in hue, however, their relative effectiveness changes with the degree of
whiteness, products that give greener hues generally being more effective at low
concentrations.
Methods for the instrumental measurement of whiteness are well established but visual
comparison remains important, even in well-equipped laboratories. Some degree of
quantification is achieved by the method of paired comparisons, in which a panel of
observers is presented with pairs of FBA-treated samples and asked to decide, without
undue delay, which is the brighter. The total of positive scores can be used as a measure of
whiteness and the results presented graphically as shown in Figure 11.7. Although time-
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FLUORESCENT BRIGHTENING AGENTS
consuming to carry out, a paired comparison series produces results that can be regarded
with considerable confidence and that usually correlate well with a comparison based on the
CIE whiteness formula.
No. of positive scores
Brightened with FBA 1
Brightened with FBA 2
Log (concn of FBA on weight of substrate)
Figure 11.7 Presentation of results of a paired comparison
11.4 GENERAL FACTORS INFLUENCING FBA PERFORMANCE
Apart from needing to be cost-effective, a good FBA must be capable of producing a high
level of whiteness. As the amount of FBA on a substrate is increased, whiteness increases
until a maximum value is reached (Figure 11.6). Further application of FBA results in lower
whiteness. On polyester the fall in whiteness with increasing concentration of FBA is
apparently not accompanied by a fall in total fluorescence [27]. On cotton both whiteness
and total fluorescence fall, the decrease in whiteness occurring before that in fluorescence
[28]. In general the main cause of the fall in whiteness with increasing concentration of
FBA is an increase in aggregation of the FBA on the substrate, resulting in a shift in
fluorescence hue. The effect is shown in Figure 11.8. Not surprisingly, FBAs that give a
greenish hue at concentrations below the maximum whiteness tend to produce a lower
maximum white than those exhibiting a violet hue. Other factors such as substantivity are of
considerable importance, of course. A computer-based expert system has been devised as an
aid to selection of the most suitable FBAs for specific applications to various fibres using the
processing equipment available in any given finishing works [29].
The presence of salts and additives can have an important influence on the performance
of an FBA. Traces of transition-metal ions such as iron and copper have an adverse effect on
fluorescence [30], but this can be controlled using conventional polyphosphate or EDTAtype sequestering agents [31]. Other salts, even sodium sulphate or sodium chloride, have
been claimed to enhance the fluorescence of FBAs in solution [32]. Apart from the normal
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GENERAL FACTORS INFLUENCING FBA PERFORMANCE
769
Relative fluorescence
Low concentration FBA
Medium concentration FBA
High concentration FBA
Wavelength
Figure 11.8 Effect of FBA concentration on fluorescence hue
effect of such electrolytes in minimising electrostatic repulsion between anionic FBAs and
the negatively charged surface of cellulose, their influence on the whiteness of a brightened
substrate is doubtful.
Surfactants, not surprisingly, exert a highly significant influence on the fluorescence of
FBAs in solution. This effect is associated with the critical micelle concentration of the
surfactant and may be regarded as a special type of solvent effect. Anionic surfactants have
almost no influence on the performance of anionic FBAs on cotton, but nonionic
surfactants may exert either positive or negative effects on the whiteness of the treated
substrate [33]. Cationic surfactants would be expected to have a negative influence, but this
is not always so [34]. No general rule can be formulated and each case has to be considered
separately.
The influence of additives needs to be kept under constant consideration when
formulating commercial brands of FBAs. Liquid formulations are of considerable commercial
importance and it is often necessary to use solvents such as diethylene glycol, poly(ethylene
glycol) and alkoxylated alkylphenols in order to achieve stable solutions. Surfactants can
help to stabilise FBA-resin bath combinations but in other circumstances they can adversely
affect the performance of the formulation, not only in its capacity to brighten the substrate
but also in the desired end-use of the brightened fabric. For example, nylon brightened by
the pad–thermosol method using a liquid formulation containing a large amount of an
alkoxylated polymer cannot be used as a substrate for colour printing unless previously
washed. Without a wash-off, diffuse prints are obtained.
Violet or bluish violet dyes can be used in combination with FBAs for shading purposes.
These shading dyes are used sparingly at no more than 2% of the weight of FBA applied.
They are of particular importance when the material to be brightened is slightly yellow. The
shading dye converts the pale yellow hue of the substrate to a perceived grey, enhancing the
effectiveness of the FBA. These effects can be considerable. Many types of violet or bluish
violet dye may be used. Typical examples include crystal violet (CI Basic Violet 3) used in
the brightening of paper stock or cotton, CI Disperse Violet 28 and CI Acid Violet 43
applied with cotton brighteners. Disperse dyes are also selected for shading with disperse
FBAs on polyester and with basic FBAs on acrylic fibres.
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FLUORESCENT BRIGHTENING AGENTS
On slightly yellow cotton the addition of a shading dye is particularly convenient, since
the substrate becomes bleached during the wash-wear cycle. Detergent formulations contain
FBAs that are substantive to cotton and loss of shading dye during washing is unlikely to
have a noticeable effect on the perceived whiteness of the article. Shading dyes are of
limited use with thoroughly prepared and well-bleached cotton. The importance of the
inherent degree of whiteness of cotton in determining its response to finishing processes has
been emphasised. The effects of dry heat on peroxide-bleached cotton fabrics treated with
brighteners that varied in substantivity were investigated. Remission spectroscopy was used
to analyse the origins of thermal yellowing [35].
FBAs applied in combination may show a synergistic effect. Synergism is, at present, only
of commercial interest with polyester brighteners. This phenomenon is discussed further in
section 11.10. Conversely, the presence of a trace impurity in an FBA formulation may
greatly reduce its effectiveness. In industrial laboratories much time and effort is expended
in developing processes to minimise the content of such impurities or even to eliminate
them completely.
11.5 CHEMISTRY AND APPLICATIONS OF FBAs
FBAs are available for application to all types of substrate. Thus there are anionic FBAs for
application to cellulosic materials in the presence of added salts, anionic types for
application to nylon or wool in the presence of acid, cationic types for acrylic fibres, disperse
types for polyester, and so on. Brighteners such as CI Fluorescent Brightener 104 (11.4),
capable in principle of being fixed to wool or cellulosic fibres by reaction with nucleophilic
groups in the substrate have been reported but have never achieved commercial importance.
Such FBAs in their reactive forms show diminished fluorescence because the presence of
labile halogen atoms leads to quenching. Hydrolysis or reaction with the fibre is
accompanied by development of fluorescence but an aftertreatment step is essential to
ensure that all the active chloro substituents present have been removed from the FBA
absorbed by the substrate [36]. The chemical structures of FBAs are many and varied
although, of course, they all contain some sort of extended π-electron system. In the
following discussion all the main chemical types are mentioned, but mainly in terms of
application rather than chemistry of preparation.
11.6 BRIGHTENERS FOR CELLULOSIC SUBSTRATES
The earliest FBAs were developed for application to paper. Even today larger tonnages of
brightener are marketed for application to cellulosics than to any other substrate.
11.6.1 FBAs for cotton
Brighteners are applied to cotton by methods similar to direct dyes. By far the most common
are triazinyl derivatives of diaminostilbenedisulphonic acid (DAS) of general formula 11.5,
where M is an alkali metal, ammonium or alkylammonium cation. Examples of groups R1
and R2 are shown in Table 11.1. Most suppliers of FBAs market such compounds, often
called DAST brighteners. Products in this class have sometimes been marketed because the
supplier needed to offer something different for commercial reasons, or to avoid infringing a
competitor’s patent, rather than for any real technological necessity.
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771
R2
O
N
N
N
SO3M
R1
NH
N
NH
N
N
N
HC
SO3Na
Cl
HC
CH
MO3S
CH
R1
N
NaO3S
HN
N
HN
Cl
N
N
11.5
N
R2
N
11.4
N
CI Fluorescent Brightener 104
O
Table 11.1 Important FBAs of type 11.5 used to brighten cotton
Substituents R1 and R2
SO3Na
Substantivity
Application
Lower
Padding
Higher
Exhaust
SO3Na
NH
NH
SO3Na
NH
N
O
NaO3S
SO3Na
N(CH2CH2OH)2
NH
SO3Na
NHCH2CH2OH
NH
NH
SO3Na
N
O
NH
N(CH2CH2OH)2
NH
N
CH3
CH2CH2OH
NH
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FLUORESCENT BRIGHTENING AGENTS
OH
ON
O2N
HN
2H
11.6
2H
SO3Na
SO3Na
SO3Na
CH3
CH3
CH3
NaOCl or
air/catalyst
self-condensation
H3C
O2N
NaO3S
O
SO3Na
HC
N
N
11.8
CH
SO3Na
Reduction
NaO3S
H2N
CH3
NO2
SO3Na
HC
CH
11.7
NaO3S
DAS
NH2
Scheme 11.1
DAS (11.7) is synthesised from 4-nitrotoluene-2-sulphonic acid (11.6) by the route
outlined in Scheme 11.1. An important factor in the preparation of DAST brighteners in
the purity necessary for good performance is the purity of the DAS used as starting material.
At one time DAS made in this way contained significant amounts of yellow azoxy
compounds similar to 11.8, which formed the main components of the obsolescent dye Sun
Yellow (CI Direct Yellow 11) made by the partial reduction and self-condensation of
intermediate 11.6. Today the major manufacturers supply DAS essentially free from these
undesirable impurities [37].
It is almost impossible to give a comprehensive list of all the FBAs of type 11.5 that have
appeared on the market since the first (11.3) of them was patented in 1940, but several
important commercial products are shown in Table 11.1. The less water-soluble products
have been widely used in the past as brighteners for detergent formulations and are generally
used to brighten cotton by exhaustion. The more soluble, less substantive types are usually
applied by padding in continuous bleaching, as a white ground for printing or in resin
finishing of woven goods for white garments or household textiles. Application methods
have been well described by Williamson [9].
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BRIGHTENERS FOR CELLULOSIC SUBSTRATES
773
Where the groups R1 and R2 as defined in Table 11.1 are both derived from amines,
variation of the amine has little influence on the hue of the white obtained. In general these
products give slightly violet shades of white and the main technical justification for the
existence of so many different structures in this class is the need for different levels of
substantivity. The compound in Table 11.1 where R2 is methoxy gives a distinctly violet
tone. All these products have light fastness values in the range 3–4 on cotton. In principle,
all the more substantive types may be applied in conjunction with a hydrogen peroxide
bleach. Their stability towards chlorine bleaches such as sodium hypochlorite varies, but
they are all essentially unstable towards these reagents [4].
Nine DAST-type FBAs, including six of those listed in Table 11.1, were exhausted onto
cotton fabric at two applied concentrations (0.05% and 0.5% on weight of fibre). Values of
exhaustion, fluorescence intensity and whiteness index were determined, as presented for
0.05% o.w.f. in Table 11.2. Results at the lower concentration facilitated more precise
comparisons between FBAs because of self-extinguishing of fluorescence and greater
variability in exhaustion at the higher level [38]. Structures containing disulphonated
anilino groups linked to either diethylamino or morpholino via the triazine rings were
characterised by high solubility, poor exhaustion and relatively disappointing increases in
whiteness and fluorescence intensity. Conversely, the combination of unsulphonated anilino
and methoxy substituents yielded high fluorescence and whiteness values in spite of only
moderate exhaustion, especially at the higher concentration. Average performance was
characteristic of those products containing monosulphonated anilino and hydroxyalkylamino groups.
The treated fabrics were exposed for various times in an Atlas Weatherometer and typical
values of loss in fluorescence intensity and whiteness are given in Table 11.3. Relatively
higher photostability was shown by the unsulphonated anilinotriazines, especially if
associated with alkoxy substituents. The least stable combinations were disulphonated
anilino groups with either diethylamino or morpholino i.e. those exhibiting poor exhaustion
and low whiteness index in Table 11.2. As in the latter table, the presence of
monosulphonated anilino and hydroxyalkylamino substituents on the triazine rings resulted
in average levels of performance.
A detailed study of the photostability of the DAST-type agent CI Fluorescent Brightener
85 on cellophane film was carried out recently. The initial fading reaction is a photosensitised trans–cis isomerisation of the stilbene grouping. The subsequent oxidative attack
on the molecule is concentrated on the vulnerable ethene linkage at the centre of this
moiety [39].
Substituted triazinyl derivatives of DAS are usually chosen for pad–dry–bake application to
cotton in conjunction with an easy-care or durable-press finish. In these mildly acidic
conditions (pH about 4) the FBA must show appreciable resistance towards the catalyst
(usually magnesium chloride) necessary to cure the resin. The less substantive products in the
upper half of Table 11.1 are important in this respect, as are compounds of type 11.9 where R
= OCH3 or CH3NCH2CH2OH. It is likely that the hydroxyethylamino groups present in
many of these compounds participate in condensation reactions with N-methylol groups in the
cellulose-reactant resin. The performance of an FBA applied in conjunction with a resin finish
can be modified and improved by careful formulation of the pad liquor but this lies beyond the
scope of the present chapter. Alternatively, FBA and resin can be applied in two separate
steps; most DAST-type brighteners would be suitable if applied in this way.
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FLUORESCENT BRIGHTENING AGENTS
Table 11.2 Exhaustion, fluorescence and whiteness shown by type 11.5 FBAs on cotton [38]
Increase in
fluorescence
intensity at
0.05% o.w.f.
Increase in
whiteness
index (ASTM)
at 0.05% o.w.f.
80
0.98
43
100
0.95
36
100
0.88
33
60
0.88
32
80
0.83
31
N(CH2CH2OH)2
80
0.78
28
N
O
40
0.82
26
N(CH2CH2OH)2
80
0.76
25
N(CH2CH3)2
40
0.69
19
Exhaustion (%)
at 0.05% o.w.f.
Substituents R1 and R2
NH
OCH3
NH
N(CH2CH2OH)2
NH
N
CH3
CH2CH2OH
SO3Na
NH
NHCH2CH2OH
NH
OCH2HC
CH3
OH
SO3Na
NH
SO3Na
NH
NaO3S
NH
SO3Na
SO3Na
NH
NaO3S
11.6.2 FBAs for paper
The paper industry is the second most important user of FBAs after the detergent industry,
most of the products applied to paper being of the DAST type.
Paper may be brightened during preparation, the FBA being added to pre-bleached pulp
before the paper sheet is laid down, or during a subsequent sizing operation. Approximately
one-third of the total FBAs used are applied to pulp and two-thirds at the sizing stage. An
FBA selected for addition to the pulp must show high substantivity at low temperature,
otherwise there would be excessive loss of brightener with the waste water from the process.
Resistance towards acidic conditions as low as pH 3 can also be important. Fillers used in
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BRIGHTENERS FOR CELLULOSIC SUBSTRATES
775
Table 11.3 Loss in fluorescence and whiteness of type 11.5 FBA-treated cotton on exposure to xenon
light [38]
Effect of 40 AATCC fading units
% Loss in
fluorescence
at 0.05% o.w.f.
% Loss in
whiteness (ASTM)
at 0.05% o.w.f.
N(CH2CH3)2
70
92
N
O
58
84
N(CH2CH2OH)2
49
75
NHCH2CH2OH
49
73
NH
N(CH2CH2OH)2
44
73
NH
N
46
69
45
67
44
66
44
65
Substituents R1 and R2
SO3Na
NH
NaO3S
SO3Na
NH
NaO3S
NH
SO3Na
SO3Na
NH
SO3Na
CH3
CH2CH2OH
NH
N(CH2CH2OH)2
CH3
OCH2HC
NH
OH
NH
OCH3
papermaking, such as alum, chalk or china clay, may cause loss of fluorescence and the type
and quantity of FBA added may have to be adjusted accordingly.
For use from the size press it is necessary for the FBA to be compatible with the chosen
size, such as starch, casein or urea-formaldehyde resin. Since sizes tend to be yellowish and
to absorb ultraviolet radiation, brighteners are generally less effective in sized paper.
The choice of FBAs and their methods of application to paper is highly complex, being
almost as much an art as a science [40]. Examples of important FBAs for use with paper are
listed in Table 11.4. This list is far from exhaustive, however, and there are other important
products of type 11.5 used as FBAs for paper.
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FLUORESCENT BRIGHTENING AGENTS
N(CH2CH2OH)2
N
N
NH
N
SO3Na
R
HC
CH
R
NaO3S
N
HN
N
N
11.9
N(CH2CH2OH)2
Table 11.4 Important FBAs of type 11.5 used to brighten paper
Substituents R1 and R2
Application
NH
N(CH2CH2OH)2
Pulp
N(CH2CH2OH)2
Pulp and size press
SO3Na
NH
CH2CH2OH
NH
N
SO3Na
CH2CH2CN
Pulp and size press
SO3Na
NH
N(CH2CH3)2
Size press
N(CH2CH2OH)2
Size press
NaO3S
NH2
11.6.3 Preparation of DAST-type FBAs
A major reason for the importance of DAST brighteners is their essentially straightforward
manufacture from readily available and inexpensive intermediates. Products with widely
different substituents and hence showing quite different application properties are easily
prepared in a three-step, one-pot synthesis starting from diaminostilbenedisulphonic acid
(11.7). The process is illustrated in Scheme 11.2.
By suitable choice of reaction conditions the chloro substituents of cyanuric chloride
(11.10) can be replaced in a stepwise fashion. In the first step DAS reacts with cyanuric
chloride at a temperature in the 0–20 °C range, ideally at pH 5–6. In the second step an
amine or alcohol (R1H) reacts within the range 20–50 °C under neutral or slightly alkaline
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BRIGHTENERS FOR CELLULOSIC SUBSTRATES
777
Cl
N
H2N
N
NH
N
SO3Na
HC CH
+ 2 Cl
SO3Na
Cl
Cl
N
step 1
HC CH
N
N
NaO3S
NaO3S
Cl
Cl
N
11.10
11.7
NH2
HN
2 R1H
N
N
step 2
Cl
Cl
R2
N
N
N
NH
N
N
NH
N
SO3Na
R1
SO3Na
R1
2 R2H
HC CH
step 3
NaO3S
R1
HC CH
NaO3S
R1
N
N
N
HN
N
HN
N
N
R2
Cl
Scheme 11.2
conditions. The third step with another amine or alcohol (R2H) is completed within the
range 50–100 °C under alkaline conditions (pH 8–9). The exact conditions selected for the
second and third steps depend on the nature of the attacking nucleophiles (R1H and R2H),
as well as the substituents already present in the chlorotriazine intermediates.
Scheme 11.2 illustrates the conventional sequence for the manufacture of DAST
brighteners. However, it is not always necessary and may not be desirable for DAS to be the
nucleophile selected for the first step. In principle the three nucleophiles can be reacted in
any order, but it is preferable for the most nucleophilic amine to react last in order to avoid
forcing conditions during removal of the last remaining chloro substituents. Alkylamines
react more readily than alcohols, thus ensuring that alkanolamines yield their
hydroxyalkylaminotriazine derivatives.
The mechanism of these bimolecular nucleophilic substitution reactions is shown in
Scheme 11.3 for the reaction between a primary amine and the intermediate
dichlorotriazine. A corresponding scheme can be drawn for reaction of a secondary amine,
an alcohol or any other nucleophile in any of the replacement steps. It follows from this
mechanism that the rate of reaction depends on:
(1) the nucleophilicity of the attacking species, the more nucleophilic reagents reacting
more quickly or under milder conditions
(2) the electronegativity of the substituent R1.
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FLUORESCENT BRIGHTENING AGENTS
H
N
R1
_ HN+ R
2
N
R1
Cl
slow
N
+
N
H2N
R2
N
N
Cl
Cl
Cl
N
R1
N
HN
N
R2
+ HCl
Cl
Scheme 11.3
The more electronegative the substituent, the more rapid the reaction or the milder the
conditions required. Thus the reaction rate decreases in the order: PhO > MeO > EtO >
PhNH > NH2 > MeNH > EtNH > Me2N > OH [4].
It also follows that protonation of the triazine ring makes it more susceptible to attack by
nucleophilic reagents unless the reagent itself is also protonated. If the triazine ring remains
unprotonated when a nucleophilic base, such as an alkylamine, is present as its acid salt the
reaction is slower, of course. Cyanuric chloride itself is a very weak base that becomes
protonated only under strongly acidic conditions. Thus step 1 in Scheme 11.2 can be carried
out in aqueous solution even at pH 2 without risk of undesirable hydrolysis of cyanuric
chloride, water being an extremely weak nucleophile.
The manufacture of several important brighteners containing alkoxytriazine groups, such
as the DAS derivative of highest substantivity in Table 11.1, does not follow the
conventional sequence. The first step involves reaction of cyanuric chloride with excess
methanol and excess acid acceptor, usually sodium bicarbonate. Under acidic conditions this
reaction takes a quite different course and can become dangerously violent (Scheme 11.4).
H3CO
N
NaHCO3
Cl
N
N
N
N
Cl
Cl
N
Cl
+ excess CH3OH
HO
N
OH
Cl
acid
conditions
11.10
N
+ 3 CH3Cl(g)
Methyl chloride gas
OH
Cyanuric chloride
Cyanuric acid
Scheme 11.4
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BRIGHTENERS FOR CELLULOSIC SUBSTRATES
779
In the preparation of other DAST brighteners it may be advantageous to avoid reacting
DAS with cyanuric chloride in the first step. It is difficult to suppress the reactivity of the
second chloro substituent completely and undesirable by-products of the general type 11.11
can be eliminated if DAS is made to react with a dichlorotriazine intermediate in the second
step. Very careful control of the reaction conditions, especially in steps 1 and 2, is also
necessary in order to avoid formation of partially hydrolysed by-products such as structures
11.12 and 11.13.
Unsymmetrical products of type 11.14 derived from DAS have been described but these
are much more complicated and expensive to prepare than those with symmetrical
structures. They have never become commercially important.
R2
R2
N
N
N
NH
N
HN
N
N
SO3Na
R1
HC
R1
NaO3S
CH
HC
CH
SO3Na
NaO3S
N
11.11
HN
NH
N
N
HO
R2
N
NH
N
N
SO3Na
R
HC
CH
R1
NaO3S
R2
R
HO
N
N
N
NH
CH
NaO3S
R3
N
N
HN
11.14
N
R4
chpt11(2).pmd
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N
R2
OH
SO3Na
HC
N
N
N
R1
N
HN
11.12
N
15/11/02, 15:46
11.13
780
FLUORESCENT BRIGHTENING AGENTS
11.6.4 Speciality FBAs for cotton
Two commercially important brighteners for cotton are not of the DAST type.
The distyryldiphenyl 11.15 is mainly of value for brightening cotton during laundering but
it can also be used to brighten cotton by exhaust application. It has high aqueous solubility
and has been recommended for use in combination with resin finishes, although its stability
at the padding stage is suspect. It has found further uses in the brightening of polyester/
cotton and nylon/cotton blends. Both components of a nylon/cotton blend are brightened
and on polyester/cotton the cellulosic component is brightened without any undesirable
staining of the polyester. Its light fastness on cotton is 4, which is slightly superior to that of
DAST brighteners. Compound 11.15 is also resistant to hypochlorite bleach but on cotton it
has limited fastness to washing in soft water. The effect of humidity on the photostability of
CI Fluorescent Brightener 359, a distyryidiphenyl structure similar to 11.15, has been
studied on cellophane film recently. A kinetic analysis of fading rates under these conditions
indicated participation in a bimolecular oxidative mechanism [41].
The manufacture of brightener 11.15 is shown in Scheme 11.5. The first
chloromethylation stage can only be accomplished safely in a plant specially designed to
CH2Cl
ZnCl2
HCHO + HCl
HOCH2Cl
O
CH2Cl
11.16
2 HOCH2Cl
ZnCl2
ClCH2
CH2Cl
Biphenyl
+ 2 H2O
2 P(OCH2CH3)3
Triethyl phosphite
OCH2CH3
O
P
OCH2CH3
CH2
CH2
P
+ 2 CH3CH2Cl
O
OCH2CH3
OCH2CH3
SO3Na
CHO
Base
SO3Na
HC
CH
HC
CH
Scheme 11.5
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780
NaO3S
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BRIGHTENERS FOR CELLULOSE ACETATE AND TRIACETATE FIBRES
ensure that the highly carcinogenic by-product bis(chloromethyl) ether (11.16) formed from
formaldehyde and hydrochloric acid does not escape. Otherwise the preparation proceeds
without undue difficulty.
A specific advantage of sulphonated distyrylarene brighteners such as 11.15 and similar
structures is that the ethene double bonds are readily oxidised during effluent treatment,
leading to the formation of soluble acids of low relative molecular mass (Scheme 11.6). Thus
it can be inferred that these brighteners and their degradation products are relatively
innocuous and are unlikely to accumulate in the environment.
SO3Na
O2 + H2O
SO3Na
COOH
2
free-radical
conditions
HC
+
CH
HC
OH
O
CH
C
HO
11.15
C
O
NaO3S
Scheme 11.6
The introduction of heterocyclic rings as terminal groups in the 4,4′-positions of the
stilbene nucleus intensifies the fluorescence of the conjugated system and shifts the
fluorescence maximum to a longer wavelength. The vic-triazole 11.17 is a premium product
and cotton brightened with it has a light fastness of 5. This structure is stable to bleaching
with hypochlorite or chlorite. It has adequate fastness to washing and a liquid formulation
has been marketed for use in combination with a resin finish. It is also of some importance
as a component of household detergents. Unfortunately, it is also expensive and its main use
is probably as an FBA for nylon, on which fibre it gives better value for money. Its
preparation is shown in Scheme 11.7.
A more soluble derivative of compound 11.17, the tetrasulphonated analogue 11.18, has
been recommended for application to cotton in combination with a resin finish. Unlike DASTtype FBAs under these conditions, compound 11.18 is compatible with resin formulations
containing zinc nitrate as latent acid catalyst. The brightness achieved is not high, however.
Many other products of a variety of structures have been patented for the brightening of
cellulosic substrates. The reader is referred to the reviews mentioned earlier for further
information.
11.7 BRIGHTENERS FOR CELLULOSE ACETATE AND TRIACETATE FIBRES
Cellulose acetate and triacetate fibres are brightened with disperse-type FBAs, including
derivatives of 1,3-diphenylpyrazoline (11.19). These form a commercially important group of
FBAs. If suitably substituted they can be applied to substrates other than acetate and
triacetate. The commercially more important products of this type are used to brighten
nylon and acrylic fibres. Their preparation and other aspects of pyrazoline chemistry are
discussed in section 11.8. Examples of pyrazolines used to brighten acetate and triacetate
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FLUORESCENT BRIGHTENING AGENTS
H2N
SO3H
HC
CH
11.7
O
C
HO3S
H3C
NH2
Alkyl nitrite
(acid or alkoxide)
1. Diazotise
2. Na2SO3, then acid
H2N
NH
O
SO3H
HO
2
+
C
N
HC
CH
CH
CH3COONa
HO3S
HN
NH2
HC
N
OH
N
NH
SO3Na
HC
N
N
N
CH
NaO3S
SO3Na
HN
N
HO
HC
CH
N
CH
NaO3S
CH3OH / H2O / H2NCONH2 / (CH3CO)2O
11.17
N
N
N
Scheme 11.7
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BRIGHTENERS FOR CELLULOSE ACETATE AND TRIACETATE FIBRES
783
fibres include the sulphonamide 11.20 and the sulphone 11.21, the former giving greenish
tones and the latter violet effects.
A pyrene derivative (11.22), a naphthalimide (11.23) and benzoxazoles of smaller
molecular size are also used and these are discussed in more detail in section 11.10. The
naphthalimide ring system is highly stable, leading to products with good light fastness and
stability to chlorite. Their main disadvantage, however, is relatively low fluorescence
efficiency, which is primarily a result of low molar extinction coefficients. Stronger
fluorescence arises when there is an electron-donating group such as methoxy (as in 11.23)
or alkylamino in the 4-position, in which case the absorption and emission properties are
associated with intramolecular charge transfer involving the donor group and the electronwithdrawing peri-carbonyl groups.
NaO3S
N
N
N
SO3Na
N
N
HC
11.18
CH
11.19
NaO3S
1,3-Diphenylpyrazoline
N
N
N
SO3Na
N
Cl
N
SO2NH2
N
Cl
11.20
N
SO2CH3
11.21
OCH3
N
N
N
CH3
OCH3
O
N
O
11.22
11.23
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15/11/02, 15:46
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FLUORESCENT BRIGHTENING AGENTS
Several interesting analogues of structure 11.23 were synthesised recently. These
derivatives of 4-methoxynaphthalimide contained a triazine ring with an unsaturated
polymerisable substituent capable of addition copolymerisation with other vinyl or acrylic
monomers. Such brighteners can be incorporated into the synthesis of polymeric finishes
and show exceptional durability to organic solvents and wet treatments [42–44].
11.8 BRIGHTENERS FOR NYLON
Disperse brighteners of the types used to brighten cellulose acetate, triacetate or polyester
fibres can be used to brighten nylon. In practice, however, disperse types are little used and
nylon is usually brightened with sulphonated compounds akin to acid dyes. Silk and
polyurethane fibres can also be brightened with the FBAs normally used on nylon. Chief
amongst these are products of the DAST type already discussed in section 11.6.3. Two series
of DAST derivatives of the general structure 11.5 were evaluated recently, in which the R1
groups were morpholino and the R2 groups either arylurea or arylthiourea (11.24; Ar =
aryl). These products showed high substantivity for both cotton and nylon, the ureido (X =
O) or thioureido (X = S) residues providing scope for hydrogen bonding with polar groups
in these substrates [45].
X
C
Ar
NH
NH
N
N
NH
N
SO3Na
N
HC
O
CH
O
NaO3S
N
N
HN
11.24
N
N
HN
HN
Ar
C
X
Exhaust brightening of nylon is usually combined with a reduction bleach based on
sodium dithionite. Important DAST-type FBAs suitable for this process are exemplified by
the four most cotton-substantive compounds listed in Table 11.1. The methoxy-substituted
derivative can be applied to nylon successfully at pH 6–6.5 but those with three NH groups
attached to each triazine ring are best applied at pH 4–5. All these products show a light
fastness rating of only about 3 on nylon. If a reductive bleach is unnecessary the DAST
brightener 11.25 can be applied to nylon from an alkaline scouring bath. Good whites are
achieved but the light fastness of this tetrakis(anilino) derivative is suspect on nylon.
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BRIGHTENERS FOR NYLON
785
NH
N
N
NH
N
SO3Na
NH
HC
CH
HN
NaO3S
N
HN
N
11.25
N
HN
The premium FBAs 11.15 and 11.17 already mentioned are of importance on nylon
because of their superior fastness properties. As on cotton, the distyryldiphenyl structure
11.15 has slightly higher light fastness than the DAST-type brighteners. In contrast to its
performance on cotton, however, it has excellent fastness to washing on nylon.
The light fastness of the vic-triazole 11.17 on nylon is 4–5; as on cotton this is
significantly superior to that of the DAST derivatives. Unlike the DAST types, the victriazole is also stable towards a sodium chlorite bleach. Applied to nylon in combination
with sodium chlorite, compound 11.17 can give exceptionally high whiteness and excellent
fastness properties.
Nylon can also be brightened using an anionic derivative of 1,3-diphenylpyrazoline, such
as FBA 11.26, 11.27 or 11.28. Although these pyrazolines give excellent whites when
applied to nylon by exhaustion, they are usually less cost-effective than the DAST
brighteners. For continuous application by pad–thermosol or pad–acid shock methods the
situation is reversed, however, and the pyrazoline FBAs are commercially important in this
sector.
N
Cl
11.26
N
SO2CH2CH2SO3Na
CH3
N
Cl
SO2CH2
N
11.27
CH3
CH3
N
Cl
N
SO2CH2CH2SO3Na
11.28
Cl
chpt11(2).pmd
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C SO3Na
15/11/02, 15:46
786
FLUORESCENT BRIGHTENING AGENTS
On nylon these three pyrazolines (11.26–11.28) have light fastness values in the range 3–
4, slightly superior to the DAST types. Light fastness in the wet state is generally lower,
however, and the pyrazolines suffer more in this respect than the DAST brighteners.
Pyrazolines 11.26 and 11.27 in particular have very poor light fastness values of 1–2 in the
wet state. The two electron-withdrawing chloro substituents in compound 11.28 have the
effect of improving light fastness, especially in the wet state, but at the expense of a loss in
solubility and slightly more difficult formulation and application. All three pyrazolines give
violet-toned whites on nylon, compound 11.28 being slightly less violet than the other two.
Electron withdrawal by the 3,4-dichloro substituents and the lactone ring in the 3′,4′positions of the diphenylpyrazoline derivative 11.29 enhances light fastness in both the dry
and wet states. Thus the fastness rating of compound 11.29 is slightly higher than those of
compounds 11.26–11.28 in the dry state and significantly higher on wet nylon. Pyrazoline
11.29 is also capable of giving brilliant whites of a pleasing bluish hue. It is complicated to
manufacture, however. Increasing substitution also reduces solubility and thus adversely
affects pad liquor stability, although this problem can be solved by suitable formulation. Pad
liquor formulation can be especially important in the pad–thermosol process, where the
brightened nylon fabric is often intended as a prepared white ground for colour printing with
acid dyes. If too much surfactant has been applied together with the FBA in the pad liquor,
‘bleeding’ from the fabric at the printing stage can give an unacceptably blurred appearance
to the printed design.
CH3
O
N
Cl
11.29
N
Cl
O
CH2SO3Na
The general method for the preparation of diphenylpyrazolines is shown in Scheme 11.8,
in which X is a suitable leaving group, usually chloro but sometimes dialkylamino. This
reaction normally proceeds readily, although pH control may be important. Preparation of
the substituted ketone and hydrazine intermediates needed for the synthesis may involve
lengthy and complicated sequences. Further reactions are often required to modify the
substitution in ring B after formation of the pyrazoline ring. The preparation of compound
11.26 shown in Scheme 11.9 illustrates one of the simpler instances.
Derivatives of 1,3-diphenylpyrazoline have been used to brighten cellulose acetate
(section 11.7) and acrylic fibres (section 11.11.1) as well as nylon. There has been much
study of the effects of substituents on application properties and some general rules can be
formulated:
(1) The greater the electron-withdrawing character of substituents in ring A, the greener
the hue of brightening.
(2) The greater the electron-withdrawing character of substituents in ring B, the more
violet the hue of brightening.
(3) An electron-donating group in the 4-position of the pyrazoline ring has a slight
hypsochromic effect, but an electron-withdrawing group has a bathochromic effect.
(4) Light fastness is improved by the introduction of electron-withdrawing substituents into
ring A, but is adversely affected by electron-donating substituents.
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BRIGHTENERS FOR NYLON
H2N
O
C
A
HN
+
CH
CH
N
B
A
N
B
base
X
Scheme 11.8
O
Cl
+
Cl
C
CH2CH2Cl
Chlorobenzene
AlCl3
O
Cl
H2N
C
+
SO2CH2CH2OH
HN
CH2CH2Cl
base
N
N
Cl
SO2CH2CH2OH
1. H2SO4
2. Na2SO3
N
Cl
N
Scheme 11.9
SO2CH2CH2SO3Na
11.26
Theoretical explanations for the effects of substituents on the hue of diphenylpyrazoline
brighteners have been published by Güsten and co-workers [46,47]. In practice almost all
commercially important diphenylpyrazoline FBAs have the general structure 11.30, in which
SO2R is a sulphone or sulphonamide grouping.
CH3
N
Ar
N
SO2R
Ar = Cl
or
Cl
Cl
11.30
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FLUORESCENT BRIGHTENING AGENTS
Since pyrazoline FBAs tend to stain cotton they cannot be used to brighten nylon/cotton
blends, which require an FBA of the DAST type, the distyryldiphenyl 11.15 or the victriazole 11.17. The two most important of these are probably the methoxy-substituted
DAST product in Table 11.1 and the distyry1diphenyl derivative. By careful adjustment of
the dyebath pH these products will exhaust onto both nylon and cotton to give a good solid
white. Nylon can also be brightened by incorporation of a thermally stable FBA in the melt,
the FBA being added to the polymer before extrusion or shaping. Structure 11.31 is typical
of the compounds selected for this purpose. The benzoxazole ring system is particularly
important in intensifying the fluorescence of the stilbene system and conferring high fastness
to light.
H3C
C
CH3
N
H3C
HC
O
CH
11.31
11.9 BRIGHTENERS FOR WOOL
Wool is naturally yellower than other textile substrates and bleached wool gradually becomes
yellow again when exposed to sunlight or other ultraviolet sources. The effect of light on
wool is complex and depends on the conditions of exposure. The problem of yellowing is
accentuated if the wool is left wet in sunlight. Since FBAs absorb ultraviolet light they
accelerate this photo-initiated yellowing. The rate of yellowing of bleached wool increases
with decrease in wavelength of the incident radiation, 300 nm being the approximate lower
limit of ultraviolet reaching the earth from the sun. In contrast, the most important
wavelengths for yellowing FBA-treated wool are within the range 340–420 nm. Maximum
free-radical formation and acceleration of yellowing occurs in the region 350–410 nm, where
brightener excitation takes place [48].
For satisfactory whiteness on wool, it is essential for the fibre to be well scoured and
bleached, either oxidatively with hydrogen peroxide or by reduction using stabilised sodium
dithionite. Brightener is usually applied together with the dithionite bleach. To achieve the
highest possible whiteness, the wool should first be scoured to remove natural waxes and
other contaminants, then bleached with peroxide and finally treated with FBA during a
second bleach with dithionite.
If wool containing a reducing agent is exposed to irradiation, the rate of yellowing is
slower than on untreated wool. Thiourea dioxide is particularly effective in this regard,
especially when used in conjunction with formaldehyde. Although thiourea dioxide retards
the yellowing of wool treated with an FBA, it does not prevent destruction of the latter. It
was shown that this reducing agent minimises yellowing by reducing the coloured products
formed on photodegradation of the FBA and certain amino acid residues in the substrate
[49].
The FBAs used to brighten wool are mainly DAST types and pyrazolines of the acid
dyeing type already discussed in section 11.8. Examples include the three most cottonsubstantive DAST brighteners listed in Table 11.1, although on wool these give light
chpt11(2).pmd
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BRIGHTENERS FOR WOOL
789
fastness ratings of only approximately 2. The pyrazolines 11.26–11.28 have light fastness of
3–4 on dry wool, but very poor light fastness in the wet state. The coumarin derivative 11.32
is sometimes used on wool and can give exceptional brilliance, but unfortunately its light
fastness is only 1. The important fluorescent coumarin derivatives almost invariably contain
an electron-donating substituent in the 7-position, for example a dialkylamino group as in
compound 11.32. Furthermore, an electron-withdrawing substituent in the 3- or 4-position,
such as a cyano group, leads to shifts of the absorption and emission bands to longer
wavelengths [50].
CH3
(CH3)2N
O
O
11.32
Although wool is most often brightened using FBAs containing stilbene or pyrazoline
fluorescent systems, such compounds degrade rather quickly on exposure to sunlight and
also sensitise photodegradation of the wool. Degradation on the surface of wool appears to
involve interaction of the stilbene with wool keratin and this is not necessarily a photooxidative process. There is good evidence that adsorbed stilbene derivatives can sensitise
the formation of singlet oxygen, which then reacts with indole rings (tryptophan residues) in
wool. Like the pyrazolines, stilbenes have the facility to self-destruct in photochemical
processes [51].
Novel fluorescent anionic surfactants of the types 11.33 and 11.34, where R represents
alkyl groups of various lengths, have been applied to wool in order to study their distribution
and effects on the physical and chemical properties of the fibre. Sections of the treated fibres
were examined under a fluorescence microscope. The intercellular and cell remnant regions
appeared to be the preferred locations of the adsorbed surfactants, but the distribution
pattern was dependent on the length of the R chain of the surfactant and the conditions of
application to wool [52].
CH3
KO3S
R
N
R
HO
O
11.33
N
SO3NH4
O
11.34
Fluorescence quenching studies of these alkylated FBAs in aqueous solution were carried
out using spectroscopic techniques. For the higher members of each series, plots of
fluorescence quantum yield against agent concentration showed a sharp decrease in
fluorescence intensity at a specific concentration, in contrast to the smoothly decreasing
curve characteristic of the lower alkyl or unsubstituted FBAs. The kinetics of fluorescence
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790
FLUORESCENT BRIGHTENING AGENTS
quenching were consistent with the formation of micelles from the more surface-active
higher alkyl derivatives [53].
Long-lived transient species have been detected during laser flash photolysis of solutions
of the disulphonated distyryldiphenyl derivative 11.15. These transients are readily
quenched by reducing agents but their yield is enhanced in the presence of oxidising ions.
Such species are believed to be radical cations formed following monophotonic photoionisation. Transient quenching is observed in the presence of indole, tryptophan and
tryptophyl peptides. These results are indicative of the sensitised photodegradation of wool
keratin in the presence of FBAs of the distyrylarene class [54].
11.10 BRIGHTENERS FOR POLYESTER FIBRES
Much research has focused on the development of better brighteners for application to
polyester. Huge numbers of patents have appeared and it is impossible to cover all the
chemical variations in this chapter. Many of the more important commercial products and
chemical types are discussed here but the reader is referred to published reviews [5,6,10,11]
for more detail.
Although polyester is always brightened with disperse-type products, the methods of
application vary. FBAs are marketed for incorporation in the polymer mass, for exhaust
application with or without carrier and for use in the pad–thermosol process at a
temperature within the range 160–220 °C. Most products are applicable by more than one
method, although none can be applied satisfactorily by all methods and cost-effective
products introduced in the 1950s still remain important today.
In general and as expected, brighteners of relatively small molecular size are most suitable
for application by exhaustion. Less volatile compounds of larger molecular size tend to be
preferred for pad–thermosol application or for incorporation in the polymer mass.
Commercially important for exhaust application are the previously mentioned pyrene
derivative 11.22, the naphthalimide 11.23, the bis(benzoxazolyl)ethene 11.35, the
bis(benzoxazolyl)thiophene 11.36, the distyrylbenzene 11.37 and the stilbene bis(acrylic
ester) derivative 11.38. Products of the 11.35 type show excellent light fastness but only
moderate fastness to sublimation. In view of this volatility they can be used in the transfer
printing of polyester.
H3C
N
HC
O
O
CH
11.35
N
CH3
N
CN
O
HC
CH
HC
O
S
11.36
CH
11.37
chpt11(2).pmd
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NC
15/11/02, 15:46
N
791
BRIGHTENERS FOR POLYESTER FIBRES
O
CH3CH2O
CH
C
HC
HC
CH
CH
C
OCH2CH3
HC
O
11.38
Polyester is brightened more effectively by exhaustion either in pressurised equipment at
125–130 °C or at the boil in the presence of a carrier. Small amounts of carrier may be added
to assist levelling in the high-temperature process but the use of carriers is increasingly
deprecated for environmental reasons. Commercially satisfactory results are obtained at the
boil in the absence of carrier using rapidly diffusing FBAs of relatively small molecular size,
such as the naphthalimide 11.23 or the benzoxazole derivatives 11.35 and 11.36. Polyester
FBAs that are suitable for exhaust application are normally stable to sodium chlorite
bleaching, although the pyrene derivative 11.22 and the bis-ester 11.38 are exceptions.
Most of the FBAs used to brighten polyester by exhaustion may also be applied
successfully by the pad–thermosol method at a baking temperature up to 190 °C. At
temperatures higher than this the more volatile brighteners sublime and give poor yields.
Compounds suitable for use with a baking temperature that exceeds 190 °C include the
pyrene derivative 11.22, the benzoxazole 11.31, the distyrylbenzene 11.37 and the stilbene
bis-ester 11.38. The temperature used during the baking stage in this process depends
largely on the equipment available to the finisher. Thus FBAs showing optimum
performance at temperatures ranging from 160 to 220 °C all have a niche in the market.
Shorter baking times, leading to greater throughput of brightened fabric, are possible at the
higher temperatures, although energy costs are greater. Products capable of giving good
whiteness at relatively low thermosol temperatures appear to be gaining in importance [12].
Many other compounds have been marketed as polyester brighteners for application by
exhaustion or in the pad–thermosol process. No account would be complete without
mention of the important class of coumarin disperse FBAs, of which structure 11.39 is a
typical example. Many commercial brighteners for polyester contain one or two benzoxazole
groups, including compounds 11.31, 11.35, 11.36, 11.40 and 11.41.
N
N
O
O
N
CH3
11.39
H3C
N
H3C
O
HC
OCH3
CH
11.40
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C
O
792
FLUORESCENT BRIGHTENING AGENTS
H3C
N
O
O
N
CH3
C
CH3
H3C
OH
CH3
C
H3C
11.41
CH3
11.42
Polyester brighteners typically show excellent fastness properties. Light fastness is usually
5–6, the pyrene derivative 11.22 being an exception with light fastness of only 2–3.
Although this compound gives a distinctly greenish brightening tone it is capable of
producing remarkably brilliant whites and remains an important commercial product.
Yellowing of white textiles in the presence of gas fumes (nitrogen oxides) has become
important in recent years. The yellowing is often attributable to the formation of quinonoid
compounds arising from reaction between the oxides of nitrogen and antioxidants such as
di-t-butyl-p-cresol 11.42 present in packaging materials. Reinehr and Schmidt have shown
that several polyester FBAs yellow in the presence of exceptionally high concentrations of
nitrogen oxides, but they were unable to detect any significant yellowing at concentrations
likely to be approached in practice [55].
Synergistic effects can often be observed with polyester brighteners and formulated
mixtures of brighteners are increasing in importance. For example, mixture products
containing the pyrene derivative 11.22 with either the naphthalimide 11.23 or the
benzoxazole 11.35 have been marketed. This property may be exploited either to increase
the maximum whiteness achievable or to attain a desired level of whiteness by applying a
lower concentration of the synergistic mixture. The subject has been discussed by Martini
and Probst [56], but the mechanism by which the synergy operates is not completely
understood.
In a recent evaluation of this phenomenon, the whiteness indices given by eleven
individual brighteners on polyester were compared with those of their binary mixtures in
various ratios. In many cases the whiteness performance of a mixture was markedly superior
to that shown by the individual components [57]. A more specific investigation was
confined to a series of benzoxazole FBAs. Their fluorescence spectra and fluorescence
lifetimes were determined individually and in mixtures. The relationships between
molecular structure and photophysical properties were discussed [58].
Much polyester fibre intended for curtain net or other white goods is sold in prebrightened form. The brightener is added to the polymer melt before or during extrusion, so
it must exhibit high thermal stability. Two important FBAs used in this way are the
triazolylcoumarin 11.43 and the bis(benzoxazolyl)stilbene 11.44. The latter structure
provides brilliant whiteness with a violet tone, more pleasing to most observers than the
slightly greenish hue given by compound 11.43. The coumarin, however, is stable under the
conditions of condensation polymerisation and can be incorporated before polymer
manufacture. The benzoxazole is not entirely stable and this has to be added to the polymer
granules immediately before extrusion. Polyester brightened with the coumarin has a light
fastness rating greater than 7, making it very important commercially. The presence of the
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BRIGHTENERS FOR POLYESTER FIBRES
793
phenyl and naphthotriazolyl groupings, contributing π-electron mobility in positions 3 and 7
respectively, markedly amplifies the fluorescence intensity, as well as conferring exceptional
photostability.
N
N
O
O
N
11.43
N
HC
O
O
CH
11.44
N
The photochemical fading of disperse dyeings on polyester is retarded if an FBA has been
applied by mass pigmentation before extrusion. The higher the concentration of FBA
present, the greater is the protective effect on the light fastness of the disperse dyes [59].
Studies of the mechanism of photochemical decomposition of DAST-type brighteners have
shown that stilbenes may act not only as sensitisers to produce singlet oxygen, but also as
physical and chemical quenchers [60]. Owing to their capacity to self-destruct by such
mechanisms, certain types of FBA can be preferentially destroyed in the presence of the
photochemically more stable disperse dye molecules [59].
Since the structures of polyester FBAs are so varied, the reactions employed in their
synthesis are also diverse. The organic chemistry can be complex and the intermediates
required are often difficult to prepare. A full discussion is beyond the scope of this chapter.
The reader is referred, in the first place, to the reviews mentioned in the introduction for
further information [3–13]. A summary of the more important methods of manufacture
follows.
Those polyester FBAs containing a benzoxazole group are usually prepared from the
appropriate o-aminophenol and carboxylic acid (11.45; Y = OH) or one of its derivatives, as
shown in Scheme 11.10. The reaction proceeds via an intermediate amide and it can be
advantageous to start from an acid derivative such as the acid chloride (11.45; Y = Cl) or
ester (11.45; Y = OEt), which are both more effective acylating agents. The preparation of
compound 11.36, shown in Scheme 11.11, illustrates this process, but the optimum
conditions for ring closure vary considerably from one structure to another. The article by
Gold contains a valuable and detailed summary [4].
Formation of the oxazole ring is not always the last step in synthesis of the brightener.
Unsymmetrical compounds that contain both a benzoxazole group and an ethene linkage
can be prepared by the anil synthesis [51], in which a compound possessing an activated
methyl group reacts with a Schiff base. The preparation of brightener 11.31 is an illustration
of this method (Scheme 11.12).
chpt11(2).pmd
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FLUORESCENT BRIGHTENING AGENTS
NH2
Y
NH
R
+
OH
R
C
C
O
O
N
R
O
OH
11.45
2-Aminophenol
Acylating agent
Benzoxazole
Amide
Scheme 11.10
O
HO
C
C
S
O
OH
N
H3BO3
+
trichlorobenzene
150–220 °C
NH2
O
S
O
N
11.36
2
OH
Scheme 11.11
(CH3)3C
N
CH3
+
N
CH
O
Benzoxazole intermediate
Schiff base
KOH/dimethylformamide
40–60 °C
(CH3)3C
N
HC
CH
O
+
NH2
11.31
Aniline
Scheme 11.12
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BRIGHTENERS FOR POLYESTER FIBRES
795
Most of the important class of coumarins used as polyester FBAs are made via 7-amino-3phenylcoumarin (11.46), which can be prepared by the Pechmann procedure from maminophenol. Conversion of intermediate 11.46 to FBAs may be achieved in various ways,
two of which are shown in Scheme 11.13.
The naphthalimide 11.23 is manufactured from acenaphthene by sulphonation, oxidation
to the naphthalic anhydride derivative and conversion to 4-methoxy-N-methylnaphthalimide as outlined in Scheme 11.14.
A Michaelis-Arbusov rearrangement followed by a Wittig-Horner reaction is involved in
preparation of the distyrylbenzene derivative 11.37, as shown in Scheme 11.15. Precautions
must be taken in the first stage to minimise formation of the carcinogenic by-product
bis(chloromethyl) ether 11.16. The stilbene bis-ester 11.38 can be made by a similar
procedure, or alternatively by the reaction of ethyl acrylate with 4,4′-dibromostilbene in the
presence of a palladium-based catalyst (Scheme 11.16), a synthesis that yields the required
trans form of the brightener.
The important bluish mixing component 11.22 for whitening polyester is made by
Friedel-Crafts acylation of pyrene (Scheme 11.17). This tetracyclic hydrocarbon is not
unlike anthracene in its susceptibility to substitution reactions. The most stable bond
arrangement in pyrene appears to be that shown as form 11.47a, which contains three
benzenoid (b) rings. Canonical form 11.47b, containing only two such rings, contributes to a
lesser extent (Scheme 11.18). In all monosubstitutions, pyrene is attacked initially at the
3-position, corresponding to the α-positions in anthracene or naphthalene.
Brightener structures of only moderate molecular size are of interest for white grounds in
the transfer printing of polyester fabrics. Derivatives of 6-acetamidoquinoxaline with an
electron-donating substituent (X) in the 2-position (11.48) were prepared by converting
quinoxalin-2-one to 2-chloro-6-nitroquinoxaline and condensation with amines (X =
RNH), alcohols (X = RO) or phenols (X = PhO), followed by reduction and acetylation
(Scheme 11.19). The nitro intermediates (11.49) are also of interest as low-energy disperse
dyes for polyester [61].
FBAs for incorporation in the polymer melt are usually sold as the pure brightener
without diluents. Most polyester FBAs, however, are supplied in the form of an aqueous
dispersion. Considerable care is required in formulating these dispersions; not only must the
dispersion be stable in transportation and storage, but the dispersing agents selected must
not adversely affect the properties (such as light fastness) of the goods treated with the
brightener. An FBA must be correctly formulated if it is to succeed commercially.
White polyester/cotton fabrics represent a substantial segment of the market. Such blends
can be brightened either by exhaustion or continuously by pad–thermosol or pad–steam
processes. Suitable brighteners are selected from those intended for use on polyester or
cellulosic substrates. Most polyester/cotton fabrics are woven constructions and it is
essential to desize them before application of an FBA. Fabrics produced for sale as white
goods must be chemically bleached before, during or after FBA treatment. In order to
achieve the most solid white effects both fibre components of the blend require a brightener.
The disperse and anionic brighteners selected must be compatible in hue. It is common
practice, however, to brighten only one of the blend components for less critical end-uses.
In a recent detailed evaluation of CI Fluorescent Brightener 393 on polyester, this
product was incorporated into the polymer melt. The prebrightened fibre was blended with
cotton and fabric knitted from these yarns was scoured and bleached. It was demonstrated
chpt11(2).pmd
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15/11/02, 15:46
796
FLUORESCENT BRIGHTENING AGENTS
CH
O
+
H2N
CH
C
OH
CH3CH2O
O
Lewis acid
catalyst, heat
Inert solvent
1. Diazotisation
2. Tobias acid
H2N
O
O
N
N
O
O
11.46
NH2
1. Diazotisation
2. Na2SO3,
then acid
Copper-containing catalyst
Inert solvent, air
N
H2N
NH
O
N
O
O
O
O
N
O
11.43
+
O
C
C
H3C
N
OH
Ring closure
N
N
N
Scheme 11.13
chpt11(2).pmd
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796
11.39
15/11/02, 15:46
BRIGHTENERS FOR POLYESTER FIBRES
797
CH3
O
O
O
Na2Cr2O7 / H2O
O
N
1. CH3NH2
2. CH3OH / NaOH
heat in an
autoclave
SO3H
SO3H
OCH3
11.23
Scheme 11.14
HCHO
ClCH2
CH2Cl
HCl / ZnCl2
Benzene
2 P(OCH2CH3)3
Triethyl phosphite
OCH2CH3
O
P
OCH2CH3
CH2
CH2
OCH2CH3
P
O
+ 2 CH3CH2Cl
OCH2CH3
Ethyl chloride
CN
CHO
2
base
CN
HC
CH
HC
CH
11.37
NC
Scheme 11.15
chpt11(2).pmd
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O
15/11/02, 15:46
798
FLUORESCENT BRIGHTENING AGENTS
O
Br
HC
+
CH
CH
2
Br
C
OCH2CH3
H2C
Ethyl acrylate
Ph3P/Pd(OAc)2/NaOAc
dimethylformamide
O
CH
CH3CH2O
C
HC
HC
CH
CH
HC
C
OCH2CH3
O
11.38
Scheme 11.16
OCH3
N
OCH3
N
AlCl3
N
N
N
OCH3
+
Cl
N
OCH3
11.22
11.47
Pyrene
Scheme 11.17
b
b
b
b
b
11.47a
11.47b
Scheme 11.18
that if an effective cotton bleaching process is applied in combination with producerbrightened polyester, the final degree of whiteness is sufficiently high to avoid the use of a
cotton-substantive fluorescent brightener [62].
If a padding process is used to brighten a polyester/cotton blend, both the disperse and
anionic brighteners may be applied from the same pad bath, even when a resin finish is
applied simultaneously to the cellulosic component of the blend. Similarly, both types of
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BRIGHTENERS FOR ACRYLIC FIBRES
N
HNO3
N
O2N
N
O
N
H
POCl3
O2N
799
N
N
O
Cl
H
HX
Quinoxalin-2-one
H2N
O2N
N
N
reduction
N
N
X
X
11.49
acetylation
CH3CONH
N
N
X
11.48
Scheme 11.19
FBA may be applied by exhaustion from the same bath. If the polyester portion of the blend
is to be bleached with sodium chlorite, the cotton is usually brightened in a second step
since most FBAs for cotton are destroyed by sodium chlorite. Both types of FBA are
normally compatible with a hydrogen peroxide bleaching process.
11.11 BRIGHTENERS FOR ACRYLIC FIBRES
At one time disperse-type FBAs, such as pyrazoline, coumarin or naphthalimide derivatives,
were commonly used to brighten acrylic fibres. Today all the important brighteners for these
fibres are cationic in character and can be divided into two main categories:
– type A: products that are oxidised by sodium chlorite
– type B: products that are stable to sodium chlorite.
Type B brighteners can be applied in the absence of bleach, of course, but show their best
results when applied simultaneously with sodium chlorite, where they are capable of giving
exceptionally high whiteness.
Acrylic fibres are usually brightened from an exhaust bath in the presence of dilute
organic acid. Application of brightener by a padding method, such as pad–roll or pad–steam,
can be used but is uncommon. When these fibres are brightened from an exhaust bath,
careful control of application conditions is necessary. Most acrylic fibres have a glasstransition temperature in the region of approximately 80 °C and the rate of absorption
accelerates rapidly with increase in temperature over this critical region. Too rapid an
increase in temperature can lead to unlevel absorption of the FBA.
As an alternative to oxidative bleaching with sodium chlorite, acrylic fibres may be given
a reductive bleach using sodium bisulphite in the presence of oxalic acid. This method is
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800
FLUORESCENT BRIGHTENING AGENTS
necessary with Courtelle (Courtaulds) because this fibre would be damaged by chlorite
bleaching. Both types of acrylic brightener can be applied with a bisulphite bleach.
11.11.1 Type A products
This category mainly consists of derivatives of 1,3-diphenylpyrazoline, such as compounds
11.50–11.52. None of these substituted pyrazolines shows significant resistance to oxidative
bleaching. The fluorescence stemming from the central ring disappears owing to
dehydrogenation to the corresponding pyrazole (Scheme 11.20). The sulphones 11.50 and
11.51 are marketed as aqueous solutions of their formate salts. They produce violet
brightening effects of light fastness 4 and are capable of producing excellent whiteness. The
sulphonamide 11.52 gives greener effects and is incapable of producing the levels of
whiteness attainable using either 11.50 or 11.51. In order to suppress the violet tone slightly
and to achieve a higher visual level of whiteness from a given amount of FBA, the sulphone
types are sometimes formulated with a small amount of a shading dye.
The general method for the preparation of diphenylpyrazoline FBAs has already been
discussed (Scheme 11.8). As a further illustration, the synthesis of the sulphone 11.51 is
shown in Scheme 11.21.
N
Cl
N
H2C
X
CH2
oxidative bleach
N
N
Cl
HC
Scheme 11.20
X
+ H2O
CH
CH3
X=
11.50
SO2CH2CH2OCH
+
CH2NH(CH3)2
Cationic
grouping
X=
_
HCOO
Anion
+
SO2CH2CH2CONHCH2CH2NH(CH3)2
Cationic
grouping
11.51
X=
+
SO2NHCH2CH2CH2N(CH3)3
11.52
chpt11(2).pmd
Cationic
grouping
800
_
Cl
Anion
15/11/02, 15:46
_
HCOO
Anion
BRIGHTENERS FOR ACRYLIC FIBRES
801
H2N
O
+
Cl
SO3H
HN
CH2CH2Cl
base
N
Cl
SO3Na
N
SOCl2
N
Cl
SO2Cl
N
Na2SO3
N
N
Cl
SO2Na
+
H2C
CHCONHCH2CH2N(CH3)2
HX
N
N
Cl
SO2CH2CH2CONHCH2CH2N(CH3)2
HCOOH
N
Cl
N
+
SO2CH2CH2CONHCH2CH2NH(CH3)2
_
HCOO
11.51
Scheme 11.21
11.11.2 Type B products
The benzimidazoles 11.53 and 11.54, both of which gave greenish brightening effects, were
formerly used widely in the chlorite bleaching of acrylic fibres. The first of them to be
introduced, the bis(benzimidazolyl)furan 11.53, gave excellent whites of light fastness 4 but
a strongly acidic dyebath was recommended to give the best results. More recently,
compounds 11.53 and 11.54 were supplanted by the improved benzofuranyl (11.55) and
benzoxazolyl (11.56) benzimidazole derivatives, which give neutral shades of white with
light fastness ratings slightly higher than the bis-benzimidazole 11.53. They are also easier to
apply and are capable of producing a higher level of whiteness.
For a time the parent compound 11.57, easier to prepare than its methylsulphonyl
derivative 11.55, was also marketed. This was capable of producing brilliant whites on
acrylic fibres that were exceptionally violet in tone. If violet brightening effects of good light
fastness are required they can be achieved in combination with sodium chlorite using the
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FLUORESCENT BRIGHTENING AGENTS
CH3
CH3
N
+
NH
_
X
N
N
N
+
O
N
N
CH3
CH3
11.53
N
O
_
X
11.54
CH3
CH3
N
N
N
O
N
+
H3CO
O
+
N
_
CH3
11.55
H3CO
SO2CH3
X
CH3
11.56
N
N
CH3
N
N
+
H3CO
O
CH3CH2
N
CH3
11.57
N
O
_
X
+
N CH3
_
X
O
N
_
X
CH3
O
11.58
CH3
CH3
_
N X
CH3
N
N
H3CO
O
CH3
11.59
coumarin 11.58, which has been well-established for some years. Naphthalimide derivatives
such as compound 11.59 can be used to obtain greenish shades of white on acrylic fibres in
combination with a sodium chlorite bleach, but the effects are generally inferior to those
produced by the preferred benzimidazoles 11.55 and 11.56.
Acrylic fibres can also be brightened during manufacture by gel application during the
wet spinning process. Special FBAs have not been developed for this purpose. Products such
as the pyrazoline sulphone 11.50 and the benzofuranyl-benzimidazole 11.55 are suitable for
this application.
Some interesting organic chemistry is involved in the synthesis of chlorite-resistant
brighteners for acrylic fibres. None of these compounds is easy to make and methods for
preparation of the starting materials can be complex. Much manufacturing know-how is
involved. One route for introduction of the benzimidazole nucleus into structure 11.55 is
shown in Scheme 11.22. Preparation of the chemically rather simpler benzoxazole grouping
in product 11.56 is shown in Scheme 11.23.
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BRIGHTENERS IN DETERGENT FORMULATIONS
803
Synthesis of the coumarin derivative 11.58 containing two isomeric triazolyl rings is
indicated in Scheme 11.24. The substituted pyrazolyl derivative of naphthalimide 11.59 is
prepared by a reaction sequence somewhat similar in principle to that already shown in
Scheme 11.14, using 4-amino-1,3,5-trimethylpyrazole in the penultimate step followed by
quaternisation.
11.12 BRIGHTENERS IN DETERGENT FORMULATIONS
The largest single commercial use of FBAs is in domestic detergents. Detergent technology
is continually changing; modified, improved or even chemically new FBAs are still appearing
on the market. The combination of large-volume sales by a limited number of detergent
manufacturers with several suppliers of FBAs competing for the available business ensures
that prices remain low.
In the 1960s, FBAs for both cotton and nylon were incorporated into household
detergents. Today FBAs for nylon are of negligible significance for the detergent industry.
FBAs that are capable of effectively brightening polyester from a household wash at an
acceptable laundering temperature (≤60 °C) remain undiscovered. Fibre types other than
the cellulosics are essentially ignored from the viewpoint of FBA selection in the context of
household detergents.
There are fundamental differences in approach to the selection of FBAs for either
household detergents or textile finishing. Brighteners in detergent formulations are intended to
preserve the whiteness of fabrics that already contain FBAs during many successive wash and
H2N
Cl
+
C
O
H3CO
O
H2N
SO2CH3
base
NH
H3CO
O
N
SO2CH3
(CH3)2SO4
CH3
N
+
H3CO
O
11.55
N
CH3
SO2CH3
_
CH3SO4
Scheme 11.22
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804
FLUORESCENT BRIGHTENING AGENTS
NH2
NH
+
H3CO
Cl3C
N
OH
base
H3CO
N
NH
O
N
(CH3)2SO4
CH3
N
N
O
N
+
H3CO
_
CH3SO4
CH3
Scheme 11.23
N
CH3CH2
N
OH
+
C
CH3
N
N
C
O
H2N
NH
O
O
N
CH3CH2
N
OH
N
NH
N
C
C
CH3
O
1. Ring closure
2. (CH3)2SO4
N
N
N
CH3CH2
N
O
+
N
CH3
_
CH3SO4
O
N
11.58
CH3
Scheme 11.24
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O
N
BRIGHTENERS IN DETERGENT FORMULATIONS
805
wear cycles, whereas textile finishers apply FBAs to unbrightened material. If too much FBA is
present during household washing then most of it is wasted. Using excessive amounts could
even lead to deterioration in whiteness, as an excess of FBA builds up after several washes. If
too little FBA is used, the goods will gradually suffer a loss in whiteness although it could be
several months before this is noticed by the detergent user. Typically a household detergent
powder contains 0.02–0.05% FBA, although the trend is towards the use of less FBA with
activated oxidative bleaching agents that are effective at lower washing temperatures.
The washing conditions and the type of surfactant present in household detergents vary
from one part of the world to another. In some countries washing temperatures can be as low
as 30 °C, whereas in others they can be as high as 90 °C. Chlorine-containing bleaches are
routinely added to the wash in some countries but in others very rarely, if at all. The
intensity of sunlight to which the washed goods are exposed during drying greatly influences
the rate of fading of the FBA and this obviously varies considerably throughout the world.
Since household detergents are marketed directly to the public, much attention has been
given to packaging, physical appearance and handling of the various formulations available.
Discoloration or development of odour on storage of the product, for example, would inhibit
sales whatever the actual performance of the product in the wash. All these considerations
influence the choice of type and quantity of FBA to be incorporated into a formulation.
Furthermore, wash loads normally contain a variety of textile articles and an FBA designed
to brighten cellulosic fibres must not adversely affect other fibre types present under the
conditions of use of the detergent into which it has been incorporated.
Safety in use and environmental impact are increasingly important factors for the
selection of FBAs used in household detergents. The waste water from a household wash is
discharged directly into the municipal drainage system. There is no opportunity for
treatment of this effluent before disposal, in contrast to washing processes operated under
factory conditions. The total quantity of waste waters from domestic washing is a major
contribution to the effluent load on municipal effluent treatment facilities. Accordingly, the
FBA and other components in a household detergent must be innocuous in the
environment and free from toxic hazard.
Evaluation of an FBA for use in household detergents is a lengthy and costly procedure.
The information obtained from a standard washing test on unbrightened cotton is valuable
but does not go far enough. Products must also be screened by measuring the build-up of
whiteness during a series of successive washes using a detergent formulation containing the
small proportion of FBA usually present in practice. Further tests continue on prebrightened textiles in machine washing cycles and in field trials. Extensive toxicity and
environmental test protocols must also be followed. Although many different FBAs have
been found acceptable for use in household detergent formulations, only a few products
remain important today.
The structures of typical examples have been mentioned already, such as the substantive
DAST-type product 11.60, the distyryldiphenyl 11.15 and the vic-triazole 11.17. Certain
FBAs, including these three compounds, can exist in relatively purer, near-colourless βcrystal modifications or in less pure yellowish crystalline forms. They are most easily
prepared in the yellower form but incorporation of this material into a detergent formulation
leads to unacceptable discoloration of the powder. Today, these products are supplied in a
near-colourless form that may be prepared, for example, by heating an aqueous alkaline
suspension of the yellowish material together with a co-solvent.
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806
FLUORESCENT BRIGHTENING AGENTS
NH
N
N
NH
N
HOCH2CH2
SO3Na
N
CH3
HC
CH
H3C
N
NaO3S
CH2CH2OH
N
HN
N
N
11.60
HN
The DAST brightener 11.61 is the most important FBA used in detergent formulations
and is probably the cheapest to manufacture. It shows excellent performance at
temperatures of 60 °C and above but relatively poor solubility in cold water compared with
the alternatives specified above. If it is to perform satisfactorily in low-temperature washing,
it must be supplied in a finely divided form so that it will dissolve adequately during a typical
household washing treatment. The necessary particle size can be achieved in various ways,
one of which is wet milling in the presence of excess salt.
NH
N
N
NH
N
SO3Na
N
O
HC
O
CH
N
NaO3S
N
HN
11.61
N
N
HN
DAST-type FBAs may contain by-products (such as 11.13) derived from hydrolysis of one
or more of the chloro substituents in cyanuric chloride (11.10). One such troublesome byproduct is 2,4-bis(anilino)-6-hydroxy-s-triazine (11.62). Not only is this compound
environmentally undesirable, it may also interact with certain bleaching agents and its
presence can lead to the development of unpleasant odours on storage of a detergent
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BRIGHTENERS IN DETERGENT FORMULATIONS
807
powder. The proportion of this triazine present as an impurity in a brightener such as 11.61
can be kept to a minimum by careful control of the reaction conditions during manufacture.
Alternatively, it can be extracted from the FBA using hot alkali.
HN
N
N
NH
N
OH
11.62
The instability of DAST-type brighteners towards chlorine-containing bleaches has been
mentioned already. They also show limited stability towards per-acids. As recommended
washing temperatures have tended to fall in recent years, a bleach consisting of sodium
perborate activated by addition of tetra-acetylethylenediamine (11.63) has become an
important component of household detergent formulations. This system is effective at
temperatures as low as 40–50 °C. Since the FBA may be sensitive to the activated oxidant,
however, in some formulations it is necessary to protect compounds such as 11.60 or 11.61
by encapsulating either the brightener or the activator, if adequate shelf-life is to be
maintained.
O
O
CH3
C
N
CH3
CH2CH2
C
O
C
CH3
C
CH3
N
O
11.63
The so-called ‘super brighteners’ 11.15 and 11.17 are generally much more stable towards
activated per-acid bleaching systems. Both products, and especially the vic-triazole 11.17,
offer higher light fastness than the DAST brighteners. The distyryldiphenyl 11.15 is an
effective FBA when applied from a wash liquor at a temperature below 50 °C, but shows
poor performance at higher temperatures and poor washing fastness in soft water. The victriazole 11.17 is effective at all temperatures but is expensive. Nevertheless, it may prove
cost-effective in tropical climates where washed textiles can fade severely during drying.
Compound 11.15 is particularly effective in enhancing the brightness of the detergent
powder itself, although this in no way indicates its performance in the wash. Table 11.5
summarises the advantages and drawbacks of the four major FBAs discussed above.
The poor performance of the distyryldiphenyl derivative 11.15 at higher washing
temperatures is a serious drawback in some countries. In an attempt to overcome this
disadvantage, product 11.15 has been marketed in admixture with an analogous FBA
(11.64) derived from 4-chlorobenzaldehyde-3-sulphonic acid (see Scheme 11.5). This much
less soluble variant is highly effective at high washing temperatures.
Where resistance to chlorine bleaches such as sodium hypochlorite is required, the
naphthotriazole 11.65 can be used. Formerly, this FBA was extremely important for use in
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808
FLUORESCENT BRIGHTENING AGENTS
Table 11.5 Advantages and disadvantages of FBAs in detergent formulations
Product
Advantages
Disadvantages
11.61
Low price
Effective at all temperatures
Unstable towards hypochlorite and activated perborate
11.60
Effective at all temperatures
Unstable towards hypochlorite and activated perborate
11.15
Stable in bleaching
Good light fastness
Poor performance above 50 °C
Poor wash fastness
11.17
Effective at all temperatures
Stable in bleaching
Excellent light fastness
High price
Cl
HC
SO3Na
HC
CH
NaO3S
Cl
CH
11.64
O
N
SO3Na
Cl
N
N
N
OCH3
HC
N
C
CH
11.66
11.65
detergents but today it is much less so. It has the advantage of brightening both cotton and
nylon from the wash bath.
In the absence of hypochlorite bleach, pyrazolines such as the sulphonamide 11.20 and
the ester 11.66 give brighter whites than the naphthotriazole 11.65 on nylon garments in the
wash. Especially brilliant whites with a somewhat greenish tone are given by compound
11.66 but this FBA tends to stain polyester goods under wash bath conditions. The
sulphonamide 11.20 gives less intense whites but stains polyester less. Neither pyrazoline
derivative is effective on cotton, so they are not much used in detergent formulations
nowadays.
Domestic detergents in liquid form have become increasingly popular in recent years.
This trend has created problems in the choice of suitable FBAs, as it is more difficult to
devise liquid formulations of adequate storage stability. Liquid detergents containing FBAs
can cause yellow ‘specking’ faults on the washed goods, which can be a serious problem. It
has been claimed [63] that the use of an FBA such as 11.65 or 11.67, which contain only
one sulphonic acid group in their stilbene residue, ameliorates the ‘specking’ problem.
Classical approaches to the preparation of stilbenes or their symmetrical disulphonates are
not applicable to the synthesis of their unsymmetrical monosulphonated analogues. Several
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ANALYSIS OF FBAs
809
condensation routes provide viable opportunities to introduce a single sulphonic acid group
into the stilbene nucleus but these procedures do add substantially to the cost of
manufacture [64].
NH
N
N
NH
N
N
SO3Na
O
HC
O
CH
N
N
11.67
HN
N
N
HN
The disulphonated DAST derivative 11.25 containing four anilino groups per molecule is
effective in liquid detergent formulations and much cheaper to manufacture than the
monosulphonated DAST brightener 11.67, which was withdrawn from the market in the
late 1980s. It has been necessary to purify compound 11.25 specially for use in detergents, in
order to eliminate traces of residual unreacted aniline as far as possible, owing to the toxic
properties of this impurity.
11.13 ANALYSIS OF FBAs
Qualitative analysis of FBAs is best carried out by thin-layer chromatography (TLC). Silica
gel is the usual stationary phase for the TLC of FBAs. Various eluants are available and
these can be chosen according to the chemical nature of the FBA under test. Suitable
standards are required. The plethora of possible brighteners of the DAST type, together
with the various impurities present in such products, can cause difficulties in their
identification by TLC. The technique can be used quantitatively, although costly
instrumentation is required and considerable care must be taken in preparing and handling
chromatograms. Chapters by Theidel and Anders in the book edited by Anliker and Müller
[7] contain valuable information on the analysis of FBAs by TLC. More recently, Lepri and
Desideri have described methods for the TLC identification of FBAs in detergent
formulations [65].
If suitable standards are unavailable (for example, if the FBA has not been encountered
previously) the active agent must first be isolated and purified. The pure compound can be
characterised by the usual techniques, including elemental analysis and infrared, n.m.r. and
mass spectroscopy. Final proof of structure demands synthesis of the FBA indicated by the
analytical data. Once again, difficulties may be encountered with compounds of the DAST
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FLUORESCENT BRIGHTENING AGENTS
class. Optical densitometry using a Shimadzu CS 9000 flying spot scanner has been
evaluated for the analysis of anionic DAST-type brighteners [66].
Once the FBA has been identified, ultraviolet absorption spectroscopy affords a rapid and
accurate method of quantitative analysis. Care must be taken when interpreting the spectra
of stilbene-type compounds, since trans to cis isomerisation is promoted by ultraviolet
radiation. Usually, however, a control spectrum of the trans isomer can be obtained before
the compound undergoes any analytically significant isomerisation. FBAs are often marketed
on the basis of strength comparisons determined by ultraviolet spectroscopy.
FBAs can also be estimated quantitatively by fluorescence spectroscopy, which is much
more sensitive than the ultraviolet method but tends to be prone to error and is less
convenient to use. Small quantities of impurities may lead to serious distortions of both
emission and excitation spectra. Indeed, a comparison of ultraviolet absorption and
fluorescence excitation spectra can yield useful information on the purity of an FBA.
Different samples of an analytically pure FBA will show identical absorption and excitation
spectra. Nevertheless, an on-line fluorescence spectroscopic method of analysis has been
developed for the quantitative estimation of FBAs and other fluorescent additives present
on a textile substrate. The procedure was demonstrated by measuring the fluorescence
intensity at various excitation wavelengths of moving nylon woven fabrics treated with
various concentrations of an FBA and an anionic sizing agent. It is possible to detect
remarkably small differences in concentrations of the absorbed materials present [67].
High-performance liquid chromatography (HPLC) is being used increasingly to identify
FBAs, to investigate product purity and for process control. HPLC has many attributes that
TLC lacks, including greater sensitivity, better resolution and discriminatory power.
Quantitative analysis can be carried out conveniently and rapidly using HPLC, providing
the constitution of the FBA is known and a pure sample is available for calibration.
Drawbacks of this approach, however, include the fact that samples have to be run
sequentially rather than in parallel, substantially increasing the time for analysis. Care is
needed to minimise the risk of cross-contamination caused by carry-over from one sample to
the next.
Although HPLC quickly became established for the analysis of organic compounds in
many fields, the development of test procedures for textile dyes and FBAs took place more
slowly. This is attributable to the number and variety of chemical classes represented and
the fact that these structures may be anionic, cationic or nonionic. Attempts to devise
general eluant systems to cope with this diversity in solubility characteristics met with
considerable difficulties. Gradient elution systems using mixed solvents in various
proportions could be used but the techniques were complicated and time-consuming.
Multichannel detection of peak wavelengths using a variable wavelength UV-Vis detector is
effective in enabling simultaneous monitoring of different components in mixtures [68].
Chromatographic problems associated with the separation of anionic dyes and FBAs were
attributed to undesirable properties of silica-based packing materials in the column and
better results were found using a chemically inert copolymer of styrene and divinylbenzene
[69]. The addition of di-t-butyl-p-cresol (11.42) as an antioxidant was useful in ensuring
chemical stability of oxidation-sensitive dyes and FBAs during extraction and analysis [70].
Established techniques of HPLC analysis are available for estimation of the relatively few
FBAs that are widely used in detergent formulations [71,72].
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811
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812
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CHAPTER 12
Auxiliaries associated with main dye classes
Terence M Baldwinson
12.1 INTRODUCTION
The aim in this chapter is to summarise the properties of auxiliaries normally used with each
of the main dye classes. Where these agents have been dealt with earlier, the emphasis here
is on application behaviour. Chemical details are included, however, for those auxiliaries
that have not yet been mentioned; emphasis is given to the auxiliaries used rather than to
processing details.
12.2 ACID DYES
Only the products associated with acid and premetallised dyes are dealt with in this section.
The auxiliaries used with mordant dyes are covered in section 5.8. Anionic acid dyes,
applied principally to wool and nylon, vary widely in their fastness and level-dyeing
properties (section 3.2.2); in general, the higher the wet fastness of a dye the more difficult
it is to apply evenly. Hence it is not surprising that the use of auxiliaries with acid dyes is
related mainly to level-dyeing properties. There are two basic aspects:
(1) controlling the pH to give a satisfactory dyeing rate and ultimate exhaustion
(2) using auxiliaries to give additional levelling, either through a competitive mechanism that
exerts further control on absorption or through the promotion of migration and diffusion.
Temperature provides another means of control although this is rarely the only technique
employed. The control of pH is of particular importance, as the optimal pH varies with
different types of acid dyes. This can be seen in Table 12.1, which shows the pH values
generally required to give 80–85% exhaustion [1]. However, in some cases, either by
modification of the dye type or by addition of certain auxiliaries, different pH values from
those listed may be used.
Table 12.1 Dyebath pH values to give 80–85% exhaustion [1]
Dye class
pH
1:1 Metal-complex dyes
Levelling acid dyes
Chrome dyes
Milling acid dyes
Disulphonated 1:2 metal-complex dyes
‘Super-milling’ acid dyes
Monosulphonated 1:2 metal-complex dyes
Unsulphonated 1:2 metal-complex dyes
2.0–2.5
2.5–3.5
4.0–5.0
4.5–5.5
4.5–5.5
5.0–6.0
5.0–6.0
5.5–6.5
813
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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES
Levelling acid dyes and particularly 1:1 metal-complex types generally require an
exceptionally low pH in order to promote exhaustion and levelling; up to 3% o.w.f. sulphuric
acid is most commonly used for levelling acid dyes, although hydrochloric, formic and
phosphoric acids are also effective. In the case of conventional 1:1 metal-complex dyes it is
essential to use a sufficient excess of acid over and above the typical 4% o.w.f. sulphuric acid
normally absorbed by the wool, otherwise there may be a tendency towards tippy dyeings
and lower wet fastness. The actual excess required depends on applied depth and liquor
ratio [2]; typical recommendations are given in Table 12.2.
Table 12.2 Amounts of sulphuric acid used with conventional 1:1 metal-complex dyes [2]
Sulphuric acid (96% solution)
(% o.w.f.)
Liquor ratio
<1% dye
>1% dye
10:1
20:1
30:1
40:1
50:1
60:1
4.7
5.4
6.1
6.8
7.5
8.2
5
6
7
8
9
10
Such high concentrations of strong acid may cause fibre damage at the boil. After dyeing
it is essential to ensure that the acid in the fibre is adequately neutralised. Hence formic acid
(8–10% o.w.f.) is sometimes used instead, a further advantage being that it leads to less
chromium in the effluent. If the dyes are modified to have one or more of the three water
ligands replaced by colourless inorganic complex anions such as hexafluorosilicate (SiF6)2–
ligands, their dyeing behaviour is markedly altered. This facilitates dyeing at pH 3.5–4.0
with formic acid and an amphoteric auxiliary (a mixture, said to be synergistic, of quaternary
and esterified fatty amine ethoxylates, polyaddition compounds of fatty amine ethoxylates
and fluorosilicates) [3,4].
The use of sulphamic acid (12.1) has been recommended, resulting in a shift of pH from
1.8 to between 3.0 and 3.5 as the temperature approaches the boil, thus giving rise to less
fibre damage. Typically, 6% o.w.f. sulphamic acid is added, together with an auxiliary and
sodium sulphate. The change in pH arises as a result of hydrolysis of the sulphamic acid to
give ammonium bisulphate (Scheme 12.1)[2,5].
NH2SO3H + H2O
NH4HSO4
12.1
Scheme 12.1
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ACID DYES
815
With the so-called ‘half-milling’ or intermediate levelling dyes, values in the range 1.8–
3.5 would lead to too rapid a rate of exhaustion with consequent risk of unlevel dyeing. For
these dyes, the optimal pH is 4.0–5.5, generally achieved using up to 2% o.w.f. acetic acid.
Milling acid dyes and 1:2 metal-complex types are highly responsive to acid. Hence the
tendency with these dyes is to use a pH-shift system (section 10.1), starting from neutral or
slightly alkaline conditions and progressively decreasing the pH to the required level as
dyeing proceeds. A hydrolysable organic ester or a latent-acid salt, such as ammonium
sulphate or ammonium acetate, may be used, often with ammonia to give a higher initial
pH. Whilst such techniques do not damage nylon, initially alkaline conditions can lead to
some degradation of wool. Hence for wool it is preferable to choose dyes showing low
substantivity at pH 7–8 but exhausting well at pH values of 6.2 or lower [2].
Figure 12.1 shows the ease with which wool is damaged in highly acidic, neutral or
alkaline dyebaths [3,4]. The least damage occurs at pH 3.5–4.0, slightly lower than the
isoelectric point of pH 4.5–5.0 (section 3.2.2). These considerations have led to the
development of processes by which milling dyes and 1:2 metal complexes can be applied at
pH values close to the isoelectric range. An effective surfactant-type levelling or retarding
agent must then be used to counteract the high rate of exhaustion promoted by this degree
of acidity [1–4,6–10].
Increasing hydrolysis
of amide groups
Increasing hydrolysis
of cystine links
Dyeing with 1:1 metal-complex dyes
Region of least wool damage
Isoelectric range
1
2
3
4
5
6
7
8
9
10
pH
Figure 12.1 Wool hydrolysis and region of least damage as a function of dyebath pH at the boil [3,4]
In general, rather less acidity is required on nylon than on wool for application of the
same combination of dyes. However, there has been some discussion regarding the best
means of controlling pH [11]. In some cases, starting at pH 7 or higher with ammonium
sulphate or acetate can lead to variations in pH at the end of the process, with
consequential variations in performance. Better end-point control is achieved by starting at
pH 6.0–6.5 using a sodium dihydrogen phosphate/disodium hydrogen phosphate buffer and
ensuring a slow rise of temperature. The improved consistency of dyeing may offset the
higher cost of the phosphate buffer. In some regions, however, the use of phosphates is
regarded as environmentally sensitive.
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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES
A neutral electrolyte, usually 10–20% o.w.f. sodium sulphate or sodium chloride, is often
added with acid dyes to aid levelling. This action results from the competition for the dyeing
sites in the fibre provided by this high concentration of inorganic anions. Ultimately these
are replaced by the dye anions as a result of their higher affinity. Electrolytes are less
effective as levelling agents in near-neutral dyebaths, however, since under these conditions
the cationic charge on the fibre is too low to attract simple inorganic anions and dye
sorption is generally through nonpolar rather than electrostatic forces. Nevertheless, it is
still common to add an electrolyte when applying these dyes, although it functions primarily
to boost exhaustion through a common-ion mechanism rather than as a levelling agent.
The use of surfactant-type levelling agents is of importance with acid dyes on wool and
nylon, especially with dyes of relatively high wet fastness. Anionic surfactants act by
competing for the cationic sites and are mainly used to counteract fibre-oriented
unlevelness due to physical and chemical irregularities in the fibre. Strongly cationic
quaternary compounds readily form complexes with acid dyes, but may precipitate them if
used alone. Weakly cationic ethoxylated tertiary amines do not suffer from this disadvantage
and are of great importance in minimising unlevelness associated with rapid dye uptake.
Combinations of anionic and weakly cationic types, carefully chosen according to the
principles described in section 10.7, are of particular importance since they counteract both
types of unlevelness. An incompatible combination of dyes is one that does not build up on
tone because of the sequential sorption of individual components. A well-chosen levelling
agent, or a combination of suitable agents, can effectively convert such a mixture into a
compatible one.
Amphoteric levelling agents, combining the properties of anionic and weakly cationic
agents in the same molecule, have attained increasing importance [6–10]. Originally
developed for the application of reactive dyes on wool, amphoteric agents have been
exploited with acid dyes, particularly for dyeing at pH 4–5. They are especially suitable for
the ranges of 1:2 metal-complex dyes containing ionic solubilising groups (carboxyl or
sulpho) rather than the nonionised but polar groups (such as sulphonamide or sulphone) in
traditional dyes (sections 3.2.2 and 5.1). These are often cheaper to manufacture and offer
better wet fastness; their development and exploitation owed much to the use of amphoteric
betaine levelling agents [12,13]. Although the behaviour of amphoteric agents has been
studied with metal-complex and acid dyes on both wool and nylon, their main focus of
interest has remained the application of reactive dyes to wool. For this reason, therefore,
their application and mechanism of action are considered in more detail in section 12.7.2.
Suffice to say here that the mechanisms observed are generally applicable to both reactive
and acid dyes, including metal-complex types.
By using more than the optimal amount of dye-complexing agent required for effective
levelling, some of these products can be used as stripping aids either alone for partial nondestructive stripping or in combination with oxidising agents (such as sodium dichromate
and sulphuric acid) or reducing agents (such as sodium formaldehyde-sulphoxylate or
sodium dithionite) for more drastic destructive stripping.
There has been long-standing interest in the so-called low-temperature dyeing of wool
and, to some extent, of nylon. Generally this implies dyeing at 60–90 °C, most commonly at
80–85 °C, rather than at the traditional boil. This approach results in energy savings with
both fibre types but is particularly attractive to wool dyers because it results in less damage
to the fibre than when dyeing at the boil. Although some earlier methods involved the use
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ACID DYES
817
of solvents such as benzyl alcohol or chlorinated hydrocarbons, now unacceptable on
environmental, health and safety grounds, methods involving surfactants continue to
generate interest, even though this does not result in widespread commercial use. Nonionic
surfactants have been favoured [2,13], including ethoxylated alcohols, ethoxylated
nonylphenol [2] and polyglycol esters and ethers [14]. Amphoteric auxiliaries have also
given effective results. On nylon dyed at 75 °C with acid dyes, the best results were obtained
[15] with the lauryl-substituted member of the fatty acylamidoethyl-N,N-dimethylglycine
betaine series indicated in structure 12.2. Compounds of general formula 12.3 have been
found effective with acid dyes on wool [16].
O
O
R
R1
C
HN
CH2CH2
CH3
_
+
N CH2COO
CH3
12.2
R = lauric CH3(CH2)10 or palmitic CH3(CH2)14
C
HN
R3
R2 N
R4
12.3
R1
= long-chain alkyl
R2
= short-chain alkylene
R3, R4 = short-chain alkyl
The most effective auxiliaries for low-temperature dyeing processes are generally those
that result in accumulation at the fibre surface of an auxiliary-rich phase of high dye
concentration. This implies the formation of an auxiliary-dye complex at a concentration as
close as possible to the critical micelle concentration and hence usually close to the limit of
solubility for the system. This complex then migrates into the fibre; depending on the
applied depth, dyeing time may need to be extended. In order to meet these criteria, the
nonionic types selected should have a low degree of ethoxylation, typically 4–10 ethylene
oxide units per molecule. This can lead to a high degree of auxiliary-dye specificity with
consequent implications for the compatibility of dyes in mixtures, as well as possible
problems of precipitation as a result of low cloud point phenomena. In spite of the potential
attractions, such low-temperature dyeing methods have attained only limited commercial
use.
It is interesting to note that products effective in the low-temperature dyeing of wool and
nylon tend to be effective in overcoming dyeability variations attributable to fibre
irregularities, such as tippy or skittery dyeing in wool and barry dyeing in nylon. The
amphoteric agents have proved to be particularly efficaceous in this respect, playing an
important part in facilitating level dyeing with sulphonated 1:2 metal-complex dyes, which
are otherwise rather prone to tippy or skittery dyeing. The phenomenon of barriness in nylon
fabric dyeing has been reviewed [17,18] and the factors discussed in section 10.7 in regard
to the action of levelling agents are pertinent here. Mixtures of auxiliaries are particularly
effective, such as a sulphonated anionic with a fibre-substantive cationic type based on an
aliphatic amine [18]. Care should be taken that such mixtures are compatible, either
through the use of ethoxylated components and/or the addition of a solubilising ethoxylated
nonionic agent.
In a study of ethoxylated ethylenediamine derivatives (12.4) in the application of acid dyes
to nylon, covering a range of ethoxylation from 40 to 180 units per molecule (average
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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES
n = 10–45), the best initial restraining effect together with the highest uptake of dye at
equilibrium was obtained with 180 ethylene oxide units (average n = 45) [19]. This highly
ethoxylated ethylenediamine was found to increase dye uptake when incorporated into nylon 6
granules as an antistatic agent [20].
H
(OCH2CH2)n
N
H
CH2CH2
(OCH2CH2)n
(CH2CH2O)n
H
(CH2CH2O)n
H
N
12.4
The use of liposomes as complexing agents in the application of premetallised acid dyes to
wool has been investigated [21–24]. Liposomes are lipid structures containing aqueous
compartments surrounded by bilayer membranes. However, the methods as yet available for
the preparation of these agents are hardly practical in dyehouse terms (section 10.3.4).
It is possible to increase colour yields on wool by the use of protease or hydrolase enzymes
(section 10.4.2). Although some improvement in yield was observed at temperatures as low
as 50 °C, the increase was insufficient to be of commercial interest, but yields with enzyme
at 85 °C were close to those obtained without enzyme at 100 °C [25].
The presence in dyehouse effluents of typical dye-complexing metal ions is an environmentally sensitive issue, such metallic contamination arising mostly from the decomposition
of metal-complex dyes [26]. The synthetic complexing agent cucurbituril (section 10.3.2)
can be used to selectively extract such metal ions from the effluent.
Continuous dyeing with acid dyes is most frequently carried out on loose fibre, tow or
slubbing before yarn manufacture. Resilient fibres such as wool can cause problems at the
immersion stage and during subsequent steaming, leading to unlevel results characterised
chiefly by tippy or frosty effects. Similar effects can be observed with pile fabrics such as
carpets owing to differing degrees of penetration of the pile. These defects are usually
overcome using a hydrotropic agent such as urea with surfactant auxiliaries [9,12,27,28].
Certain anionic surfactants are claimed to be effective, particularly sodium dioctylsulphosuccinate and its 2-ethylhexyl and 1-methylheptyl isomers [27,28]. The mechanism involves
formation of an agent-dye complex that wets the fibres evenly and forms a uniform film
around them. The surfactant creates a foam during steam fixation, thus assisting the
uniform transport of the dye throughout the fibre; the complex subsequently breaks down
and the dye is then uniformly fixed.
Detailed accounts of the printing of wool with acid dyes and metal-complex types are
available [2,29]. Typical formulation details for print pastes are given in Table 12.3. A
hydrotrope such as urea or thiourea is used to increase solvation of the dyes and to act as a
humectant, thereby enhancing fixation. Additional solvents, such as thiodiethylene glycol
(12.5) or sec-butylcarbinol (12.6), may also be added [2]. Locust bean or guar derivatives
are used as thickening agents, either alone or in combination with water-soluble British
gum; high solids content is preferred for fine line effects and low solids content for larger
blotch prints, because of better levelling and freedom from crack marks.
For generation of acidic conditions, a non-volatile acid such as citric acid (12.7), or an
acid donor such as ammonium tartrate (12.8) or ammonium sulphate, is preferred. An acid
or acid donor is not used with 1:2 metal-complex dyes of high neutral-dyeing affinity,
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ACID DYES
819
Table 12.3 Typical formulation for the printing of wool with
acid dyes [2]
Concentration (g/l)
Dye
Urea
Thiodiethylene glycol
Wetting agent
Antifoam
Acid or acid donor
Thickener (10–12%)
Water to give
x
50–100
50
5–10
1–5
10–30
500
1000
CH2CH3
HOCH 2CH2SCH2CH2OH
HOCH2HC
12.5
12.6
CH3
O
HO
O
C
C
CH2
HO
C
CH2
HO
O
HO
CH
HO
CH
ONH4
12.8
C
OH
C
C
O
ONH4
O
12.7
however, since this may lead to destabilisation of the print paste, aggregation, specking and
unlevelness. Wool or the modified natural thickeners present may tend to promote
reduction of certain sensitive azo dyes; to counteract this a small amount of sodium chlorate
may be added to the print paste. Defoamers and surfactants to prevent frosting may also be
required.
In discharge printing a reducing agent is also required. The most widely used is zinc
formaldehyde-sulphoxylate (CI Reducing Agent 6), since this functions in the weakly acidic
pH range and thus gives less fibre damage. There can be problems in washing out the
unsulphonated arylamines produced by reduction of certain azo dyes [2]. Sodium
formaldehyde-sulphoxylate (CI Reducing Agent 2) may give excessive fibre damage since it
requires an alkaline medium. The water-insoluble calcium formaldehyde-sulphoxylate (CI
Reducing Agent 12) may cause screen blockage and inadequate penetration, although
commercial formulation as a 30% dispersion is said to give better results. The calcium salt
may be applied in admixture with the sodium salt. Thiourea dioxide is rarely chosen because
of its low solubility (only 37 g/l at 20 °C) [2].
The washing-off of prints is best carried out with anionic polycondensation products of
arylsulphonic acids [29] since these can improve the wet fastness of anionic dyes.
chpt12(2).pmd
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820
AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES
12.3 AZOIC COMPONENTS
There are three main demands for auxiliaries in the application of azoic components [30–
32]:
(a) the composition of the naphtholate solution
(b) the composition of the diazo solution (developing bath)
(c) aftertreatments to develop hue and maximum fastness.
These will be considered first in relation to batchwise application, followed by variations
pertinent to continuous dyeing and printing. The discussion relates solely to cotton, by far
the most important substrate for these dyes; application to other cellulosic substrates follows
generally similar principles, the main difference being in product concentrations.
12.3.1 Composition of the naphtholate solution
A primary requirement for naphtholate preparation is soft water; otherwise, insoluble
calcium or magnesium naphtholates will be formed. If soft water is not available then a
sequestering agent must be added, the sodium hexametaphosphate, EDTA or NTA types
(section 10.2) being suitable. A little alcohol is generally added during the initial pasting and
dissolving of the naphthol. Given water of suitable quality, the naphtholate bath in
batchwise dyeing then usually contains the following additions:
– alkali, most often sodium hydroxide, although in certain circumstances (particularly with
regenerated cellulosic or bast fibres) sodium carbonate or trisodium orthophosphate may
be used [31]
– a protective colloid (dispersing agent) and perhaps a wetting agent
– formaldehyde
– electrolyte, either sodium chloride or sodium sulphate.
The purpose of the alkali is to convert the insoluble free naphthol into its colloidally soluble
sodium salt. An excess of sodium hydroxide is generally needed but too much will tend to
promote hydrolysis of the amide groups present in most azoic coupling components. The
actual amount required varies with the naphthol and processing conditions; the
manufacturer’s detailed literature must be consulted.
The protective colloid/wetting agent may be a single anionic agent; Turkey Red Oil, for
example, combines both functions but is prone to form a precipitate in hard water. Only
anionic types are suitable, since nonionic and cationic types generally cause precipitation
[31]. Most protective colloids are of the following types:
(a) lignosulphonates
(b) protein-fatty acid condensates
(c) sulphonated condensates of aromatic compounds, especially of phenols and naphthols
with formaldehyde.
The chemistry of these product types has been described previously (section 10.6.1). The
anionic polyelectrolyte helps to stabilise the colloidal solution of the naphtholate, through a
mechanism similar to that already described. Where the protective colloid itself does not give
adequate wetting of the fabric a suitable wetting agent, which in batchwise dyeing must
function well in the cold, should be added; the alkylnaphthalenesulphonate types are suitable.
chpt12(2).pmd
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AZOIC COMPONENTS
821
The formaldehyde plays an important role in counteracting the tendency of amidecontaining naphtholates to hydrolyse at high pH values to the o-carboxynaphthol, which
couples to give coloured by-products of inferior fastness. Its protective action is in addition
to that provided by the excess alkali and its use is recommended with most naphthols,
exceptions being yellow acetoacetarylamides where coupling is inhibited. Formaldehyde
operates through the reversible formation of a 1-methylol derivative at 40–50 °C, but at
temperatures above 50 °C this derivative reacts with a second molecule of naphtholate to
give a non-coupling dinaphthylmethane compound (Scheme 12.2) [30].
HO
CH2
OH
OH
HCHO
R
R
CONH
CONH
Naphtholate
Methylol derivative
High temperature
Naphtholate
R
CONH
OH
CH2
OH
R
CONH
Scheme 12.2
Insoluble methylene compound
The naphthols used in batchwise dyeing are moderately substantive and their exhaustion
is improved by electrolyte addition, with consequent improvement in yield and fastness
properties. The amount of electrolyte required varies with the substantivity of the naphthol,
the depth applied, liquor ratio and substrate quality, but generally ranges from 10 to 40 g/l
sodium chloride or sodium sulphate. Higher amounts are required for heavier depths of lowsubstantivity naphthols in long liquors. With high-substantivity naphthols on substrates that
are difficult to penetrate and short liquor ratios, treatment is begun in a salt-free
naphtholate solution and electrolyte is added later. After application of naphthols by
batchwise techniques, excess surface naphthol is usually minimised or removed by
hydroextraction, suction, squeezing or by rinsing in 10–50 g/l electrolyte and 0.3–0.6 g/l
sodium hydroxide solution.
Batchwise application of naphthols is generally carried out at 20–30 °C. Although a
higher temperature may be chosen to improve the penetration of difficult substrates, it
chpt12(2).pmd
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15/11/02, 15:46
822
AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES
should not be allowed to rise above 50 °C; substantivity decreases with increasing
temperature. In continuous dyeing, however, in order to ensure uniformity of uptake from
the pad bath, the requirement is for minimal substantivity. Hence application temperatures
are generally high (80–95 °C) and naphthols of low to medium inherent substantivity are
used. These factors indicate a need for the following modifications to the auxiliary
formulations used:
(a) formaldehyde should be omitted due to formation of the non-coupling
dinaphthylmethane derivative at temperatures higher than 50 °C
(b) electrolyte should be omitted
(c) the amount of wetting agent can be reduced, or it may be omitted if the higher
temperature together with the protective colloid provide sufficient wetting.
Naphtholates in the form of ready-to-use solutions are now available [31,32]. These are
formulated to contain the essential auxiliaries, are quite stable on storage and offer the
following advantages:
– no dissolving or boiling necessary
– no dust during weighing and preparation and therefore cleaner working conditions
– shorter times for setting up the process.
12.3.2 Composition of the diazo solution or developing bath
This bath is essentially a dilute solution of a diazonium salt produced either by the
diazotisation of an arylamine (Fast Colour Base) or by simply dissolving a stabilised
diazonium compound (Fast Colour Salt). Soft water is desirable but not essential. General
additions for batchwise dyeing with Fast Colour Bases include acid, sodium nitrite and
possibly ice, together with a dispersing agent. Hydrochloric acid is the most widely used acid
to effect dissolution of the base and activation of the sodium nitrite so as to bring about
diazotisation. Temperatures must be kept low (5–15 °C) to avoid decomposition of the
relatively unstable diazonium salt (section 4.3.1); hence ice is often added to the solution. A
dispersing agent is used to ensure the fine and uniform dispersion of the azoic dye as it is
formed during coupling. Only nonionic types such as fatty alcohol ethoxylates (section 9.6)
are suitable, as anionic or cationic types may cause precipitation [31].
Once diazotisation is complete the excess hydrochloric acid must be neutralised before
the diazonium salt is coupled with the naphthol, usually by addition of an alkali-binding
agent. The agent most commonly used is sodium acetate, which by reaction with the
hydrochloric acid produces acetic acid, so that the resultant mixture of acetic acid and
sodium acetate acts as a buffer. The acetic acid/sodium acetate balance must be adjusted to
suit specific needs related to the reactivity or coupling energy of the system (section 4.4),
giving a pH varying from 4 to 5.5 for those having high coupling energy to 6–7 for those
with low coupling energy. Examples of azoic diazo components and their relative coupling
energies are given in Table 7.2 of reference [30]. Sometimes buffer systems utilising sodium
dihydrogen phosphate and disodium hydrogen phosphate or sodium bicarbonate are
preferred.
When Fast Colour Salts are used hydrochloric acid and sodium nitrite are obviously not
required, although some Fast Colour Salts do need an addition of acetic or formic acid. The
nonionic dispersing agent is still necessary but as most Fast Colour Salts contain an alkali-
chpt12(2).pmd
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AZOIC COMPONENTS
823
binding agent (aluminium sulphate, zinc sulphate, magnesium sulphate or, in a few cases,
chromium acetate) to give the required pH, the only electrolyte additions to the developing
bath are to correct any local variations in pH.
In some cases, as in the batchwise application of diazo components, it may be advisable to
add electrolyte to the developing bath to inhibit bleed-off of low-substantivity naphthols.
Otherwise the auxiliaries for batchwise and continuous application of diazo components are
essentially the same.
Fast Colour Bases in the form of ready-to-use solutions or dispersions are now available
[31,32]. As can be seen from their advantages listed below, their use has implications
regarding the addition of auxiliaries:
– the formulations are resistant to freezing temperatures and stable in use
– all except one product can be diazotised without the addition of ice provided the bath
temperature does not exceed 20 °C
– diazotisation is complete in 2–5 min compared with up to 30 min for conventional Fast
Bases
– no nitrous gases are formed during diazotisation
– these formulations already contain a dispersing agent and no further addition is needed
– no dusting or formation of lumps as during dissolution of conventional Fast Bases.
12.3.3 Aftertreatments to develop hue and maximum fastness
After the coupling (development) process is complete the goods are rinsed, acidified and
given an alkaline soaping treatment. This not only substantially removes surface dye but also
brings about a process of aggregation of dye molecules within the fibre, thus developing the
full potential of hue and fastness. A combination of Marseilles (olive oil) soap (3–5 g/l) with
sodium carbonate (batchwise 1–2 g/l, continuous 2–3 g/l) is traditionally used. A
polyphosphate sequestering agent is needed if the water is hard. A second wash with a
nonionic surfactant is also required.
The conventional technique in printing is to apply the naphthol by padding as described
for continuous dyeing, followed by printing with the diazo componenents using cellulose
ether, locust bean or guar derivatives as thickening agents. In other respects the auxiliaries
and general processing requirements are similar to those described above. A different system
involves application of the naphthol coupling component and a stabilised diazonium salt in
the same print paste followed by neutral steaming to effect development; a starch ether
thickening agent is recommended for this process [29]. In certain resist styles aluminium
sulphate is applied by printing onto naphthol-treated fabric; this brings about a localised
reduction in pH that inhibits coupling during subsequent application of the diazo
component, thus giving rise to a resist effect [29].
Overall, the application procedures for azoic dyeing are quite complex since many factors
must be taken into account, such as:
– the specific naphthol and diazo components selected, with regard to molecular
characteristics, substantivity and applied depth
– the application method, e.g. continuous or batchwise, paying attention particularly to
liquor ratio and substantivity in the latter case
– the substrate, including unmercerised or mercerised cotton, causticised regenerated
cellulosics or bast fibres.
chpt12(2).pmd
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824
AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES
All the above criteria influence the concentrations of the various components required.
Some indication of how they are affected can be gleaned from reference [30]. It is important
therefore to consult detailed information from the supplier of the naphthols and bases. Such
information available on disc for use on a personal computer has been provided for the
batchwise dyeing of cellulosic yarns [31] and the continuous dyeing of cellulosic fabrics [32].
The stripping of fully developed azoic dyeings can often be carried out using a hot
solution of sodium hydroxide (1.5–3 g/l), sodium dithionite (3–5 g/l) and a surfactant;
addition of anthraquinone (0.5–1 g/l) generally increases the effectiveness of the process.
Yellow azoic dyeings are resistant, however, and can only be partially stripped [30]. On the
other hand, stripping of naphtholated material before it has been coupled with the diazo
component can be done quite effectively in boiling alkali.
12.4 BASIC DYES
There are two major characteristics of basic dyes applied by exhaustion techniques to acrylic
fibres:
(1) below the glass-transition temperature (about 80 °C) exhaustion is very slight,
becoming much more rapid at temperatures only a little above this
(2) very little, if any, migration occurs at temperatures up to 100 °C.
Consequently the rate of dyeing, and hence levelness, are very difficult to control; the
degree of difficulty varies from fibre to fibre, generally tending to a maximum for readily
dyeable fibres with a high glass-transition temperature. Owing to the sensitivity of some
basic dyes to alkaline hydrolysis, these dyes vary in their response to dyebath pH, again
depending on fibre type (section 3.2.4). The pH must be controlled to within 4.0 to 5.5 in
order to obtain reliable, reproducible results across the range of dyes and fibres. Hence in
the conventional batchwise application of basic dyes to acrylic fibres, auxiliaries must fulfil
two functions:
(1) to give the required pH
(2) to control the rate of sorption in the critical temperature region and, as far as possible,
to promote migration.
A buffer system is preferred for the control of pH, the most common one being the relatively
cheap acetic acid/sodium acetate system, although a simple addition of acetic acid may be
adequate with water that does not show a significant pH shift on heating.
The major variables are undoubtedly the rate of temperature rise and the use of retarding
agents to control level dyeing. A review of acrylic fibres and their processing is available
[33]; here we are concerned only with the essential chemistry of the auxiliaries used,
particular emphasis being given to retarding agents.
Cationic types of retarding agent are especially important. These function essentially as
colourless cationic moieties competing for the anionic sorption sites in the fibre. Quaternary
ammonium compounds (section 9.5) largely predominate; their fundamental structure
(12.9) offers the possibility of varying up to four substituent groups around a quaternary
nitrogen atom, and hence the variety of possible structures is enormous. A range of these
compounds examined for their retarding effect in the application of basic dyes [34] gives
some idea of the possibilities (Table 12.4). The selection of a retarder depends on several
chpt12(2).pmd
824
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BASIC DYES
825
factors, however, of which the most important are the rate and extent of sorption of the
retarder compared with those of the dyes. The dyeing kinetics of basic dyes in mixtures are
now universally denoted by compatibility values covering the range from 1 to 5 [35–38].
Simply varying the substituents in otherwise structurally similar dyes can change their
compatibility values [37]. The sorption properties of quaternary ammonium compounds can
be varied and characterised similarly, as seen from the examples shown in Table 12.5 [39],
which were obtained using a titration-spectrophotometric method [40].
The type of associated anion has only a minor effect on the properties of a cationic
retarder. General practical experience [39,41] suggests that optimal control is achieved if the
Table 12.4 Structures of some typical retarding agents [34]
Substituents in quaternary ammonium compound 12.9
R
R1
R2
R3
Anion X
C12H25 (dodecyl)
Coco*
C16H33 (hexadecyl)
C18H37 (octadecyl)
Tallow**
Coco*
Coco*
Coco*
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3CH2
C6H5CH2
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
Cl
Cl
Cl
Cl
Cl
CH3SO4
CH3SO4
Cl
* Consisted of approximately 47% C12 and 18% C14 with lesser amounts of C8, C10 ,
C16 and C18 hydrophobes.
** Consisted of approximately 48% oleyl, 27% cetyl and 13% stearyl, with minor
quantities of others.
R
R1
_
+
N R2 X
R3
12.9
Table 12.5 Compatibility values of retarding agents [39]
Substituents in quaternary
ammonium compound 12.9
chpt12(2).pmd
R
R1
R2
R3
C14H29
C6H5CH2
CH3
C6H5CH2
C6H5CH2
CH3
CH3
CH3
CH3
CH3
C14H29
C14H29
CH3
C7–9H15–19
C6H5CH2
CH3
CH3
C14H29
CH3
CH3
825
Compatibility
value
assigned by
experiment
1.0
2.5
3.0
5.0
>5.0
15/11/02, 15:46
826
AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES
retarder has a compatibility value equal to or slightly lower than that of the dyes, so that it
will tend to be absorbed by the fibre either at the same rate as the dyes or somewhat more
quickly. If the compatibility value of the retarder is significantly lower than that of the dyes,
then there is a very real tendency for it to act as a blocking agent (with attendant problems),
whilst if its compatibility value is much higher its efficacy is impaired.
Once the substituents in the quaternary ammonium compound have been selected,
consideration must be given to the applied concentration of retarder. Acrylic fibres vary
significantly in the number of anionic sites available for sorption of cations but it is generally
assumed that maximum likelihood of level dyeing occurs when the number of cations in the
system (retarder as well as dyes) is just enough to saturate the anionic sites in the fibre.
Thus the amount of retarder needed to achieve this theoretical saturation will vary from
fibre to fibre, and also depends on the applied concentrations of the dyes. More retarder will
be needed for fibres of high saturation value and for lower applied depths; the actual
quantities required to satisfy the given conditions are generally specified by the dye
manufacturers. However, the use of these theoretical quantities can lead to lower degrees of
dye exhaustion within normal dyeing times. In any case level dyeing is not just simply a
function of the ionic dye–fibre system but involves many other aspects, especially physical
factors such as substrate form and machinery efficiency. It may well be that in a given
practical situation there may be little or no level dyeing problem, so why use any more
retarder than is necessary to ensure a level dyeing under practical conditions? Experience
suggests that much less than the theoretical amount of retarder will often be adequate and
this will help to alleviate any problems due to saturation if subsequent reprocessing for shade
correction is needed.
In addition to having an effect on the rate of dyeing, cationic retarders will assist
migration to an extent that depends on the fibre and the substantivity of the dyes. Retarders
tend to diffuse more quickly than dyes and to be absorbed at lower temperatures (typically
65–70 °C, compared with 80–85 °C), although the magnitude of these effects will depend on
the structure and properties of both retarder and dye. In some cases, such as hank dyeing on
machines with poor circulation or inadequate temperature control, it may be preferable to
use a retarder that almost totally restrains the uptake of dye until the top temperature has
been reached, after which dye sorption takes place gradually.
A useful general classification of cationic retarders according to their properties has been
given [42]:
(a) strongly cationic with a strong blocking effect
(b) moderately cationic with a weak blocking effect
(c) weakly cationic with no blocking effect
(d) products with little or no retarding effect but giving some levelling.
Products in groups (b) and (c) allow for greater safety margins and give optimal exhaustion
curves in bulk practice, although they may be more expensive than products in group (a).
The main need for retarding activity arises during the critical exhaustion phase as the
temperature increases from about 80 °C to the boil. Therefore some cationic retarders have
been designed to hydrolyse progressively in this temperature region, so reducing the
retarding activity in the later stages of dyeing and safeguarding against blocking effects.
Subsequent shading and redyeing are then less problematical. Furthermore, the amounts of
hydrolysing retarder used are perhaps less critical than with their non-hydrolysing
chpt12(2).pmd
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BASIC DYES
CH2CH2OH
RCOCO
N
RCOCO
CH2CH2OH
12.10
CH3
_
+
N CH2COO
CH3
RCOCO
CH3
12.11
827
N
O
CH3
12.12
counterparts, although more may be needed initially to obtain an equivalent retarding effect.
A combination of hydrolysing and non-hydrolysing types may be used in some
circumstances.
On 100% acrylic materials the quaternary ammonium retarders are used almost
exclusively. Other types have been evaluated, however. For example, saturated alkylamines
(RNH2; R = C10, C12, C14 and C16 hydrophobes) were found to be just as effective as the
quaternary types although other factors, such as aqueous solubility at the optimal dyebath
pH and resistance to subsequent discoloration, favour the quaternary compounds [34]. On
the other hand, bis(hydroxyethyl)cocoamine (12.10) had relatively little effect and the
amphoteric carboxymethyldimethylcocoamine (12.11) none at all, although dimethylcocoamine oxide (12.12) was quite an effective retarder [34]. Other cationic compounds
used [43,44] have included alkylpyridinium salts, imidazoles and imidazolinium salts,
alkyldiamines, alkylpolyamines, as well as sulphonium and phosphonium derivatives.
Polymeric cationic retarders that contain up to several hundred cationic groups per
molecule have been proposed [45–47]. The early types [45,46] were described as
quaternised polyamines (section 9.5) of relative molecular mass 1000–20 000, as compared
with 300–500 for conventional quaternary ammonium compounds. Polyacrylamides of
molecular mass 2500–780 000 have been evaluated more recently [47].
These agents, because of their large molecular size, do not diffuse into the fibre but are
strongly adsorbed at the fibre surface, reducing its anionic potential. They retard the dyeing
rate far more than does an equal concentration of a conventional quaternary agent, but do
not assist migration. Some of these products can adversely affect the compatibility of dyes as
a result of selective behaviour but are said to be free from blocking effects. They do not
interfere with crimp development as conventional retarders sometimes do and are
particularly effective in giving superior coverage of bicomponent fibres. Since they are not
absorbed into the fibre but concentrate their activity at the surface, these polymers are
effective at much lower concentrations than are conventional types and this favours their
cost-effectiveness. Their retarding action is sustained throughout the dyeing cycle.
Nevertheless, it does appear that the molecular mass of the polymer needs to be optimised
in relation to the type of dyes used [46,47] With polyacrylamides [47] for example, a dye of
small molecular size responds best to a retarder of large molecular size, and vice versa. These
effects have been explained on the basis of the relative ease of diffusivity of the dyes through
the polymeric retarder to reach the fibre surface. Thus a dye of small molecular size needs the
extra resistance to diffusion of a retarder of great molecular mass, whilst such a retarder would
present too effective a barrier to diffusion of a dye of larger molecular size. This relationship
clearly has major implications for the selection of dyes to achieve close compatibility in
mixture recipes. Despite the claimed advantages and the prediction as long ago as 1973 that
polymeric retarders would rapidly become the preferred choice for dyeing acrylic fibres with a
high content of dye sites [45], they have not attained a commercially significant role.
chpt12(2).pmd
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828
AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES
Retarders of opposite ionic charge to the dyes can be used [33,36,48–51]. Anionic
retarders function by forming a thermally labile complex with the dye and thus lowering the
substantivity of the dye for the fibre. Undesirable precipitation of this complex, which is one
of the drawbacks of the system, can be inhibited:
(a) by using excess anionic agent
(b) by using an anionic agent that contains two or more sulphonate groups so that the
resultant 1:1 complex retains solubility
(c) by incorporating a nonionic agent as an antiprecipitant.
Examples include sodium dinaphthylmethanesulphonates (section 10.6.1) and
polyethoxylated alkylarylsulphates (section 9.4). Polymeric types, such as polystyrene
sulphonate, have been tried but do not seem to offer any advantages.
The advantages and disadvantages of anionic retarders can be summarised as follows.
The advantages include:
(a) the system is compatible with anionic dyes and anionic dispersing agents in the dyeing
of fibre blends
(b) there is no blocking of dyeing sites in the fibre
(c) they have no adverse effects on the bulkiness of certain bicomponent fibres
(d) they promote good migration of dyes
(e) they can be used as stripping agents to reduce the depth of colour in reprocessing.
The disadvantages include:
(a) to prevent precipitation the quantity of anionic retarder should increase with increasing
quantity of dye (the opposite of the situation with cationic retarders) and this conflicts
with requirements for promoting exhaustion; hence exhaustion of dye when applying
medium or heavy depths is poor
(b) they show less levelling during the exhaustion stage
(c) the use of cationic softeners in the dyebath is not possible.
In practical terms the disadvantages outweigh the advantages, thus limiting the importance
of anionic retarders.
Electrolytes such as sodium chloride and sodium sulphate tend to retard dyeing
[36,52,53] through preferential adsorption and subsequent displacement by the dye of the
more mobile sodium ions, although the effect is relatively weak even compared with the
weaker cationic retarders. Nevertheless, the use of up to 10% o.w.f. sodium sulphate in
combination with a cationic retarder may enable the amount of the latter to be reduced by
up to 20–30% [36]. The limitations of electrolytes, apart from this lower effectiveness, are
that they reduce the final uptake of dye, their effectiveness decreases with increase in
temperature and their effect is greatest with fibres containing weakly anionic groups such as
carboxylate, rather than stronger ones such as sulphonate. Cationic softeners for acrylic
fibres are sensitive to the presence of electrolytes, although sulphate-tolerant softeners may
be used.
The retarding effect of electrolytes in the application of basic dyes to acrylic fibres
increases with increasing concentration of salt up to a certain level. Increasing the
concentration beyond this point has no further effect on exhaustion with certain univalent
anions, whilst with multivalent types there is an increase in dye sorption (Figure 12.2)
chpt12(2).pmd
828
15/11/02, 15:46
BASIC DYES
829
[54,55]. These results have led to the conclusion that ionic mechanisms alone do not
entirely explain the complex interactions that occur between basic dyes and acrylic fibres.
Hydrophobic interaction also plays an important part and it has been demonstrated [55]
that multivalent anions such as sulphate or phosphate can enhance the hydrophobic
interaction, thereby also increasing dye sorption in some circumstances. Whilst such results
are of interest in terms of dyeing theory, it is extremely doubtful whether there will ever be
practical interest in exploiting the use of electrolytes at such high concentrations.
A Represents sulphates
and phosphates
B Represents chlorides,
bromides and nitrates
Dye sorption on fibre
A
B
Salt concentration in dyebath
Figure 12.2 The effect of salt on the equilibrium sorption of basic dyes on acrylic fibres [54,55]
The use of dye-solubilising agents such as urea or thiourea is more usually associated with
continuous dyeing or printing. However, such compounds have also been investigated for their
effects in exhaust dyeing [56,57]. These compounds increase the rate of exhaustion and exert
their maximal accelerating effect at low temperatures (e.g. 80 °C) and low dye concentrations.
Despite the higher acceleration factor at 80 °C as compared with 100 °C, the ultimate yields at
80 °C are lower than at 100 °C (Figure 12.3) [57]. When dyeing at 100 °C or above it is
difficult to see any commercial reason for making these additions, particularly in view of
environmental concerns regarding such compounds.
Continuous dyeing of acrylic fibres with basic dyes [50,58,59] generally requires the use
of saturated steam for fixation. As in batchwise dyeing there is a need to maintain an
optimum pH of 4.5–5.0. If a sodium acetate/acetic acid buffer were to be used the acetic
acid may volatilise in the steam, leading to development of alkalinity; hence it is usual to
utilise a non-volatile acid such as citric acid (12.7) or tartaric acid (12.14). The thickening
agent for use with basic dyes must not be anionic, a useful choice being galactomannanbased locust bean gum. Hydrotropes and fibre-swelling agents assist dye solubilisation and
fixation; compounds used include thiodiethylene glycol (12.5), dicyanoethylformamide
(12.15) and potassium thiocyanate (KSCN). A nonionic wetting and solubilising agent may
also be useful.
In general, for the pad–steam application of basic dyes to acrylic fibres, the auxiliaries are
selected to increase the solubility of the dyes with minimal retardation and maximal
improvement of fixation. In a detailed statistical study along these lines [60], it was
chpt12(2).pmd
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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES
Dyeings at 80 oC
100
Dyeings at 100 oC
100
Dye concn/
% owf
90
1.5
2.0
2.5
3.0
80
Dye exhaustion/%
Dye exhaustion/%
90
Dye concn/
% owf
70
60
50
1.5
2.0
2.5
3.0
80
70
60
50
0.4
0.8
1.2
1.6
0.4
Thiourea concentration/g l–1
0.8
1.2
1.6
Thiourea concentration/g l–1
Figure 12.3 Effect of thiourea on exhaustion of CI Basic Yellow 21 (12.13) by acrylic fibres [57]
H3C
CH3
CH
N
CH
N
O
OH
C
Cl
CH2CH2CN
H
CH3
CH3
HO
CH
HO
CH
12.13
C
12.14
N
CH2CH2CN
O
C
CI Basic Yellow 21
O
OH
12.15
concluded that the following represented the optimal composition of a mixed auxiliary
formulation:
50% of a solubilising agent based on a branched-chain fatty alcohol with 8 units ethylene
oxide per molecule
32% of a short-chain fatty alcohol with 2.5 units ethylene oxide per molecule
14% of a plasticising agent for acrylic fibres
12% of an ethoxylated long-chain alcohol (C10–C18) with 15 units ethylene oxide per
molecule
12% of a short-chain fatty alcohol that increases the fixation of basic dyes.
In addition to the conventional dyeing of acrylic fibres, there is considerable interest in socalled gel dyeing of acrylic filaments during the manufacturing process after extrusion. From
the viewpoint of auxiliary usage this is outside the scope of the present work, but a useful
account of the factors involved is available [61].
Basic dyes are used to dye acid-modified polyester fibres, in which case there is usually less
need for a retarding agent. Glauber’s salt is often added, however, to guard against hydrolytic
degradation of these fibres [62]. Cotton modified according to Scheme 12.3 can then be dyed
with basic dyes, although commercial exploitation of such a process is unlikely [63].
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BASIC DYES
831
O
C
O
CH2OH
CH
CH2
C
O
O
CH
O
Cl
CH
CH
CH
CH
OH
OH
CH
CH
acylation
CH
CH
OH
OH
oxidation NaIO4
O
C
O
C
HO
CH2
O
CH
O
CH2
O
CH
NaHSO3
CH
CH
CH
CH
SO3Na
SO3Na
O
CH
OH
CH
CH
CH
O
O
Scheme 12.3
O
OH
O
C
O
(CH2)7CH3
C
O
(CH2)7CH3
C
HN
O
12.16
H2C
CH2
CH2
CH2
OH
CH2
12.17
12.18
In direct printing of acrylic fabrics a typical stock print paste [29] may contain the
following components by mass:
0.5% citric acid (12.7)
1–2%
dicyanoethylformamide (12.15)
3%
thiodiethylene glycol (12.5)
7%
acetic acid (30%)
50–60%
locust bean thickener
Dioctyl phthalate (12.16), caprolactam (12.17) and urea together with resorcinol (12.18)
are also said to act as fixation assistants [64].A further addition of an anionic thickening
agent such as carboxymethylcellulose [29] can act as a levelling agent when printing large
blotches. A wash-off with anionic surfactant is usually given.
chpt12(2).pmd
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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES
Discharge white styles are obtained using either formaldehyde-sulphoxylate or the weaker
tin(II) chloride as reducing agent; crystal gum or British gum are recommended thickening
agents, together with potassium thiocyanate as a fibre-swelling agent [29]. For coloured
discharges tin(II) chloride is the recommended reducing agent, since formaldehydesulphoxylate would reduce the illuminant basic dyes; other additions are generally as for
direct printing. Discharge styles, after steaming and rinsing, are given a clearing treatment at
40 °C in 1 ml/l ammonia (25%) and 1 g/l sodium dithionite [29], followed by rinsing and
soaping with anionic detergent at 60–70 °C.
Non-destructive partial stripping techniques for basic dyes on acrylic fibres are carried out
at 100 °C (or higher if possible) using, for example, 1–10% o.w.f. anionic retarder and 1 g/l
acetic acid (60%), or 1–5 g/l Marseilles (olive oil) soap. Destructive stripping requires
acidified (pH 5.5–6.0) sodium hypochlorite, followed by an antichlor treatment in sodium
dithionite or sodium bisulphite. In some cases a preliminary boiling treatment in 5 g/l
monoethanolamine and 5 g/l sodium chloride is said to improve the effect of the stripping
treatment.
12.5 DIRECT DYES
Direct dyes represent one of the simplest dyeing systems, usually requiring only an
electrolyte as an essential auxiliary for their application. Nevertheless a surfactant may
sometimes be added to assist wetting and levelling, as well as a sequestering agent, since
many direct dyes are sensitive to hard water. Control of pH may also be desirable. Certain
traditional dyes require aftercoppering as part of their application procedure, whilst it is
usual to aftertreat direct dyeings to improve their wet fastness properties. The dyeing of
polyester/cellulosic blends with direct and disperse dyes requires application at temperatures
higher than 100 °C.
An up-to-date account of the application of direct dyes is available [30]. The main area
to be considered in the batchwise application of these dyes is the use of either sodium
chloride or sodium sulphate to promote exhaustion, although the sulphate can give rise to
calcium sulphate deposits in hard water. Direct dyes vary enormously in their response to
electrolyte; in general the more highly sulphonated dyes require greater amounts of salt.
This is in line with the behaviour of dyes according to the universally used SDC
classification [30,65,66], whereby dyes are allocated to three application classes. Class A
dyes are generally the most soluble and least sensitive to salt, hence necessitating substantial
additions of electrolyte to boost their low exhaustion values. It is advisable with class A dyes
to add electrolyte to the rinsing water to inhibit the otherwise copious bleed-off of dye into
the water. For this purpose magnesium sulphate may be more efficient than sodium salts
since it can form the less soluble magnesium salt of the dye, but the acceptability of this will
depend on whether magnesium can be tolerated in subsequent processing. Dyes in classes B
and C are generally less soluble and are so responsive to electrolyte that salt must be added
gradually over the dyeing cycle as otherwise the rate of strike will be so rapid as to give
unlevel, poorly penetrated dyeings and there may even be salting out of the dye in the
dyebath. More salt is needed in longer liquors, and for heavier depths.
Dyes having the same CI generic name but made by different manufacturers may also
require different amounts of electrolyte to be added to the dyebath, according to the amount
of electrolyte present in the commercial formulation. A typical instruction is to use from 0 to
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DIRECT DYES
833
20 g/l salt depending on the factors described above. Electrolyte may influence migration as
well as exhaustion [67], an optimal concentration of electrolyte being found for maximal
migration of class A and B dyes, whilst the migration of class C dyes decreases with
increasing amounts of salt. As mentioned above, sodium chloride and sodium sulphate are
the electrolytes most commonly used in practice and it is generally accepted that they exert
their effect by means of the common ion effect. There is another aspect, however;
electrolytes also modify the structure of water around the hydrophobic groups in dye
molecules and around the surface of the fibre, creating a new order in solution as a result of
solvation. This enables dye molecules to approach more closely to the fibre surface within
the influence of short-range interactive forces [68].
Numerous electrolytes have been investigated in fact, although some of the research
work is seriously limited by having been carried out with only a few dyes, sometimes just
one. In an investigation of the relative effects of Zn, Mn, Cd, Sr, Al and Ce nitrates [68], it
was found that the size of the cation, as well as its charge, played a part in the sorption
process: saturation values and sorption rates increased with increasing size of the cation. A
similar effect has been observed for the series: LiCl < NaCl < KCl [69], whilst in binary
mixtures of these electrolytes [70] the larger cation has a strongly promotional effect on the
activity of the smaller cation (Figure 12.4). It has to be admitted, however, that these
experiments were carried out at temperatures lower than is typical in commercial dyeing
with direct dyes.
Mention has been made above of the use of magnesium sulphate to prevent bleeding of
class A dyes during rinsing. With carefully selected dyes [71], magnesium salts can
effectively replace the conventional sodium salts during dyeing. An optional mixture of
Equilibrium absorption at 34 oC
Rate of dyeing at 40 oC
0.30
8
0.25
0.20
6
Mt /M∞
Concentration of dye
on fibre/(mol/kg) × 102
10
0.15
4
0.10
2
0.05
2
6
10
Concentration of dye
in solution/(mol/l) × 104
KCl (0.1 mol/l)
NaCl (0.1 mol/l)
LiCl (0.1 mol/l)
14
2
6
10
1
t1/ (min /2)
2
KCl (0.05 mol/l) + NaCl (0.05 mol/l)
LiCl (0.05 mol/l) + NaCl (0.05 mol/l)
Figure 12.4 Equilibrium absorption isotherms at 34 °C and rate of dyeing curves at 40 °C for CI Direct
Blue 1 on viscose in the presence of electrolytes singly and in binary mixtures [70]
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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES
organic magnesium salts has been formulated. Since precipitation could be a problem, this
proprietary mixture also contains a polymeric sequestrant. The rationale behind this process
was to eliminate the conventional inorganic electrolytes from the dyeing process, since these
have chronic toxic effects on freshwater organisms above certain concentrations. In
addition, the usual electrolytes are quite difficult to remove from effluent. It is claimed that
magnesium salts are used at lower concentrations and are easily removed from the effluent
by precipitation.
For environmental reasons, other attempts have been made to reduce the amount of
conventional electrolyte added. Lowering the liquor ratio will in itself reduce the amount of
electrolyte required. In one commercially feasible system [72], a range of direct dyes was
successfully screened to select members that could be applied efficiently to give 95–100%
exhaustion using significantly less electrolyte than usual. Thus at applied depths up to 2–3%,
only 2–5 g/l salt is required; navy and black dyeings can be produced with only 7.5–10 g/l
salt compared with the conventional 25 g/l addition.
Ultrasonic irradiation has been shown in laboratory studies [73] to increase dye
exhaustion, enabling salt levels to be reduced. However, it seems doubtful whether the
higher effectiveness is sufficient to merit development to overcome the problems involved in
scaling-up the ultrasonic equipment to bulk-scale processing. For example, in one
experiment using 5% salt at 65 °C, ultrasound treatment increased the dye exhaustion from
77% to 82%.
A sequestering agent is usually necessary in hard water to prevent the formation of
sparingly soluble calcium and/or magnesium salts. These can lead to uneven deposits of
lower fastness on the surface of the fabric as well as reduced yields due to precipitation in
the dyebath. Polyphosphates are particularly useful in this respect. Organic sequestering
agents such as EDTA must be avoided with metal-complex direct dyes as they tend to
extract the metal from the dye molecules, resulting in a change in hue and a significant
lowering of fastness, although they can be used safely with unmetallised dyes. In a rather
unorthodox approach to the use of sequestering agents, ethylenediaminetetramethylphosphonic acid (EDTMP; 12.19) applied as a pretreatment for cotton at ambient
temperature was shown to increase the exhaustion of CI Direct Red 79 (12.20) applied at
100 °C in the presence of salt [74]. The maximum effect was achieved with 3 mg of
sequestering agent per g of cotton. On dyeings carried out for 30 minutes, this gave
surprising improvements in exhaustion from 35% to 45% (for 10 mg dye per g cotton) and
from 17% to 29% (for 40 mg dye per g cotton). Rate of dyeing was apparently increased, too.
Only this one dye and one sequestering agent were examined, however. It was found that
about 35% of the tetraphosphonate was absorbed by the cotton. It was postulated that the
phosphonate groups are only partially ionised in the neutral dyebath and that the presence
of the nitrogen atoms favours hydrogen bonding between nonionised phosphonate groups
and anionic sulpho groups in the dye molecule (12.21). Increased fastness to washing was
also claimed [74].
Some direct dyes are sensitive to reduction or hydrolysis under alkaline conditions,
particularly if temperatures above 100 °C are used (section 3.1.3); pH 6 is frequently
favoured for stability and this can usually be achieved using ammonium sulphate. A few dyes
give optimal results under alkaline conditions, using sodium carbonate or soap; the tetraamino dye CI Direct Black 22 (12.22) is an example. Whether or not an addition is needed
will depend on whether alkali is already present in the commercial brand.
chpt12(2).pmd
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DIRECT DYES
O
O
HO
P
P
CH2
CH2
OH
OH
HO
835
N
CH2CH2
OH
OH
OH
N
CH2
CH2
P
P
OH
O
O
12.19
EDTMP
H3C
CH3
O
NaO3S
SO3Na
C
HN
N
NH
N
N
N
H3CO
OCH3
OH
HO
12.20
NaO3S
SO3Na
CI Direct Red 79
H
P
S
[dye]
O
O
O
NaO
O
O
N
H
H2N
NH2
12.21
NH2
N
H2N
N
N
SO3Na
O
H
H
N
N
O
N
N
N
NaO3S
SO3Na
NaO3S
12.22
CI Direct Black 22
Levelling and wetting agents for direct dyeing are mostly ethoxylated adducts, such as
alkylaryl ethoxylates, although anionic types such as alkylarylsulphonates, phosphate esters
and alkylbenzimidazoles are also marketed. Care should be exercised in the use of such
agents; there is spectrophotometric evidence [75] that they interact with dyes, leading to
lower exhaustion. Since this interaction is dye-specific, there may be problems with mixtures
of dyes. Such auxiliaries also add quantitatively to the COD value of the effluent [76].
Various other auxiliaries have been investigated but their commercial use is either limited or
chpt12(2).pmd
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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES
non-existent: urea, pyridine, amyl acetate, gelatin, carboxymethylcellulose [77].
Cyclodextrins [75,78] may have some potential for the future (section 10.3.1), although
their action with direct dyes is dye-specific [75].
There is research interest in the potential to pretreat cotton with reactive compounds
(section 10.9.1) followed by application of direct dyes or nucleophilic amino-containing dyes
to give increased fixation and/or fastness. Attempts have also been made to apply such
reactive compounds simultaneously with direct dyes [79]. Despite the inroads made by
reactive dyes, there still remains considerable interest in the application of direct dyes, as
evidenced by the introduction of the Optisal system by Clariant [80–82]. This is a carefully
designed package of selected metal-free direct dyes, that require little salt to give high
exhaustion and are stable up to 130 °C, together with a cationic formaldehyde-free fixation
agent applied as an aftertreatment. The dyes may also be applied isothermally [81].
Environmental advantages are claimed, including high exhaustion, less pollution of effluent
and low salt usage.
Various techniques are available for the application of direct dyes by semi-continuous and
continuous methods, such as pad–jig, pad–batch, pad–steam, pad–dry and pad–thermofix.
The major problem arises from the high substantivity of direct dyes for cellulosic substrates,
making it very difficult to avoid tailing problems. Hence concentrated brands of dyes having
minimal electrolyte content are preferred; of these, the class B dyes offer better operating
properties. The main methods of controlling uniform uptake remain careful selection of dyes
for compatibility, speed of padding and the rate of supply of padding liquor. Low solubility of
the dyes may also be a problem; use of a hydrotropic agent such as urea improves the
solubility of certain dyes and may also improve fixation, particularly in dry fixation processes.
The washing-off process after fixation may be combined with an aftertreatment to improve
the wet fastness and avoid undesirable bleeding of dye. Treatment with durable-press resins
and with cationic products, particularly of the multifunctional reactant type, is especially
useful here. The aftertreatment of conventional direct dyeings to improve fastness to light
and particularly to wet treatments, using copper(II) sulphate, formaldehyde, diazotisation
and coupling techniques or cationic fixing agents, has been described in section 10.9.5 and
will not be discussed further here.
An interesting, if little-used, method of overcoming dye substantivity problems at the
padding stage involved the use of certain amines, particularly those containing carboxyl or
hydroxy groups such as structure 12.23 [83], in combination with 1:1 copper-complex direct
dyes. A low-stability amine–copper–dye complex was formed. The complex diffused readily
into the fibre and reverted to dye and amine during steam fixation, the amine being
subsequently removed during washing-off. Careful selection of the amine, or mixture of
amines, was necessary to achieve the desirable balance of properties. It is interesting to
compare this use of an amine and 1:1 metal-complex direct dyes with the similar requirement
for such dyes in the much more recent Indosol (Clariant) process (section 10.9.5).
HOCH2CH2NHCH2CH2NHCH2CH2OH
12.23
Direct dyes are of limited interest for printing because of their restricted wet fastness,
resulting in cross-staining of whites or pastel-dyed grounds when the prints are subsequently
washed. Somewhat better results can be achieved by treating the prints after steam fixation
chpt12(2).pmd
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DISPERSE DYES
837
with, for example, a cationic fixing agent, durable-press resin or, in the case of chelatable
dyes, copper(II) sulphate as already described (section 10.9.5). Most azo direct dyes will
discharge easily with reducing agents and can therefore be applied as dyed grounds for
discharge print styles, although the limitations described above with regard to wet fastness
are especially pertinent here.
Partial non-destructive stripping of untreated direct dyeings can be accomplished with an
alkaline solution of soap or synthetic detergent. Destructive stripping using a reducing agent
such as sodium dithionite is also effective except with stilbene-type dyes. Aftertreated dyeings
may additionally require a treatment to counteract the aftertreatment; for example, coppered
dyeings can be treated with a sequestering agent such as EDTA and dyeings treated with a
simple, non-reactive cationic agent may respond to treatment with an anionic detergent.
12.6 DISPERSE DYES
Disperse dyes as a class are peculiarly sensitive to the influence of auxiliary agents, both as
regards the quality and stability of the dispersion and the response of the dyes during the
various coloration processes. Essential auxiliaries in batchwise dyeing include dispersing
agents and chemicals to control the pH. Supplementary auxiliaries termed ‘carriers’ may be
needed under certain circumstances to accelerate the otherwise inadequate rate of dyeing.
Aftertreatment of the dyeings to remove surface dyes is important in many cases, as are the
conditions of drying and finishing since these can influence fastness properties.
12.6.1 Dispersing agents
The essential chemistry of dispersing agents has been discussed in section 10.6.1, where it
was noted that different considerations may apply at the comminution stage of dye
formulation compared with maintaining the stability of the dispersion during subsequent
coloration processes. The dyeing of polyester at a temperature in the region of 120–135 °C
in beam and package dyeing machines places severe demands on initial dispersion quality
and subsequent stability under adverse conditions. Jig dyeing with a high concentration of
dye in a very short liquor (as for navy blues and blacks) can also be the source of dispersion
stability problems.
The crux of the problem lies in the inherent thermodynamic instability of all dye
dispersions, there being an overall tendency of fine particles to undergo Ostwald ripening
with the consequent formation of larger particles. Although disperse dyes are generally
considered to be virtually insoluble in water they are, in colloidal terms, sparingly soluble;
indeed a low degree of solubility seems to be a necessary prerequisite for dyeing to take place
from an aqueous medium. It is this limited solubility that favours Ostwald ripening. The
detailed colloid chemistry of dispersions with particular reference to these phenomena has
been thoroughly discussed [84]. The solubility of disperse dyes normally increases with
temperature and dispersing agent concentration, although these effects vary enormously
from agent to agent and from dye to dye.
Most dispersing agents are of the anionic polyelectrolyte type, comprising various
sulphonated condensation products of aromatic compounds and lignosulphonates (section
10.6.1). Increasing understanding of lignin chemistry with consequent improvements in
manufacture, enabling lignins to be more economically and reliably ‘tailored’ for specific
chpt12(2).pmd
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838
AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES
end-uses, currently favours the use of these products, although by no means exclusively.
Solid brands of disperse dyes contain a significant proportion of dispersing agent added
during formulation; liquid brands contain rather less as they do not have to withstand the
thermal and mechanical rigours of spray drying [85] and do not require redispersing at the
dyebath preparation stage. Despite this, it is still advisable to add extra dispersing agent at
the dyeing stage, more being required with liquid dyes to compensate for their lower
intrinsic content. At the grinding stage of dye manufacture lignosulphonates with a high
degree of sulphonation generally perform better. Less sulphonated types tend to give better
stability at high dyeing temperatures [86], since they are more readily adsorbed onto and
retained by the hydrophobic surfaces of the dye particles.
The complexity of the relationships within a disperse dye system is well illustrated in
Figure 12.5. The particle size distribution in a disperse dyebath and any transformations
taking place during dyeing, including the three successive phases of heating up, maintaining
top temperature and subsequent cooling, can exert critical effects on rate of dyeing, final
degree of sorption and levelness. Whilst microscopy techniques are undoubtedly useful in
studying dispersions, much more detailed information is obtained from photon correlation
spectroscopy using a Coulter counter [88]. Figure 12.6 shows an example of particle size
Molecular solution of dye
Micellar
solubilisation
Dispersing agent system
Hydrotropy
Molecules
Micelle
Displacement
Dissolving
Crystallisation
Agglomeration
Crystallites with
dispersant
Crystal formation
from dispertion
Aggregation
Crystal formation
from solution
Crystal
Figure 12.5 Model of the disperse dye system [87,88]
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DISPERSE DYES
50
839
A
Volume/%
40
30
B
20
10
C
0.4
0.8 1.2
2
3.2
5
8
12.6
20
Particle size/µm
Treatment A 70 oC
5 min
100 oC
10 min
100 oC
10 min
70 oC
oC
20 min
130
oC
60 min
130
oC
15 min
70 oC
Treatment C 70 oC
20 min
130
oC
180 min
130
oC
15 min
70 oC
Treatment B 70
Figure 12.6 Effect of temperature changes on the particle size distribution in a dyebath containing
0.6% CI Disperse Orange 13 [88]
transformations taking place in a specific dispersion of CI Disperse Orange 13 on exposure
to three different temperature profiles.
The addition of auxiliaries, such as additional dispersing agents, levelling agents, carriers
and electrolytes, brings about further changes that may be beneficial or otherwise,
depending on circumstances. A method based on a ternary diagram (Figure 12.7)
representing the relationship between dyeing properties and the concentrations of three
dispersing agents used simultaneously in the dyebath has been described [89]. Of course,
any ranges of concentration can be chosen that are appropriate for the dispersing agents
present. The scheme is suitable not only for studying particle size distribution using a laser
particle sizer as in this instance, but also for examining effects such as solubilisation, rate of
dissolution, diffusion coefficient, dyeing rate and colour difference. Figure 12.8 shows how
these various factors can be compared in a single diagram [89].
An alternative approach to evaluating the efficiency of dispersing agents depends on the
partition effect [90]. In this simple and practical method a sample of the aqueous dispersion
is extracted with a water-immiscible solvent (e.g. chloroform, methylene dichloride,
monochlorobenzene, dimethyl phthalate, tetrachloroethylene) in which the dye but not the
dispersing agent is soluble. Unprotected disperse dye particles dissolve immediately in the
solvent layer. Dye particles protected by a sheath of dispersing agent are more hydrophilic
and therefore favour the aqueous layer. The rate of extraction of a disperse dye from the
aqueous layer into the solvent depends on the stability of the dispersion and the extraction
conditions. However, there must be limits to the applicability of this method to the study of
phase transitions during the dyeing process.
The influence of dispersant structure on the thermal stability of dye dispersions has been
illustrated as in Figure 12.9. Thermal stability appears to be related to the relative numbers
and strengths of the adsorbing and stabilising groups in the dispersing agent. Partial blocking
chpt12(2).pmd
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840
AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES
Agent 1
1
0.5
0.1
8
0.4
0.2
0.3
0.3
2
6
7
0.4
0.2
10
9
0.1
0.5
4
3
0.5
Agent 3
0.4
5
0.3
0.2
0.1
Agent 2
Figure 12.7 Basic points of the ternary diagram in which individual parameters were determined
experimentally; agent concentrations are quoted in g/l [89]
Agent 1
Rate of dissolution (mg/l min)
36
Rate of dyeing (mg/g min)
25
5
43
1
100
Solubilisation at 60 min (mg/1)
BASF diffusion test (m2/s)
51
Colour difference (∆E)
5
0
25
42
0
7
2
100
28
63
15
25
Agent 3
0
54
7
9
17
100
6
45
0
80
43
83
78
10
61
15
59
96
3
62
68
0
3
56
44
47
36
25
61
8
59
100
57
4
57
58
100
68
5
57
100
Agent 2
Figure 12.8 Overall dependence of observed parameters in dyeing with CI Disperse Orange 21 in the
presence of a mixture of three agents [89]. The value within each symbol represents a percentage of
the maximal effect (= 100%) for that factor
chpt12(2).pmd
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DISPERSE DYES
x
x
841
x
x
x
x
x
Dye crystal
Dye crystal
Very good thermal stability
Moderate thermal stability
x Dispersant stabilising group
Dispersant adsorbing group
Figure 12.9 Dependence of dispersion thermal stability on dispersing agent structure [91]
of phenolic groups in a lignosulphonate, for example, reduces the thermal stability by an
amount that corresponds quite well with the lower concentrations of residual phenolic and
carboxyl groups. The degree of adsorption can be determined by equilibriating a known mass
of dye with a surfactant solution of appropriate concentration at constant temperature. The
dye particles are then separated by filtration or centrifugation, followed by UV analysis of
the supernatant liquid to determine the concentration of dispersant remaining. The amount
of surfactant adsorbed per gram or per unit area of the dye particles is then calculated from
the difference between the initial and final concentrations of the surfactant [91].
From the environmental viewpoint there are two important problems associated with
dispersing agents:
(1) dustiness of powder brands
(2) inadequate and slow biodegradability.
Since disperse dye powders supplied to the dyer have been treated already to render them
essentially non-dusting, dustiness problems are mainly of concern during dye manufacture
[92]. Nevertheless, some dyes remain inadequately treated, or after an initially adequate
treatment they may deteriorate during storage, thus giving rise to hazards in handling
associated with excessive dustiness. Methods of assessing dustiness have been reviewed [92].
Since dispersing agents are not significantly absorbed by the fibre, they remain in the
exhaust dyebath and are discharged to effluent. As a result of their polymeric nature and the
presence of stable benzenoid rings, most lignosulphonate and formaldehydenaphthalenesulphonate dispersing agents are only bioeliminated to about 30% [92–94].
They may also contain small amounts of residual starting materials that are toxic to fish
[92]. More complete elimination can be achieved by precipitation with heavy metal salts or
cationic surfactants, but this leads to problems of disposal of solid wastes. Dispersing agents
based on mixtures of sodium salts of arylcarboxylates are claimed to offer superior
bioeliminability (70%) and to show markedly improved application properties compared
with traditional dispersing agents [93,94].
Dyebath pH exerts a marked influence on the efficacy of lignosulphonate dispersing agents,
since this factor determines the degree of dissociation of phenolic and carboxylic acid groups,
influencing the extent to which they are able to interact with the dye molecule. In general, the
lowest pH that can be tolerated by the system (dye, fibre and auxiliaries) tends to give the
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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES
greatest dispersion stability during heating of sodium lignosulphonates [91]. The situation is
somewhat different with amine salts of lignosulphonates, since these differ from the sodium
salts in their degree of ionisation. For example, when the pH is lowered from pH 7 to 4, more
amino groups are protonated, thus increasing the proportion of ionised sulphonate groups
present. The solubility of the dispersant is increased and so the adsorption of agent by the
disperse dye particles becomes more difficult. However, the overall situation is complex
because lignin derivatives vary in their content of phenolic and carboxylic acid groups and
these exhibit a range of pKa values according to their location within the macromolecule.
Some azo dyes are susceptible to reduction under unfavourable conditions [95]. The least
stable dyes tend to be those containing electron-withdrawing groups, such as nitro, chloro or
cyano, ortho to the azo linkage. This instability to reduction is minimised by dyeing at the
optimal pH, usually pH 4–5, in the presence of air, and by minimising the dyeing time at
high temperature. Hence, under appropriate conditions, instability is not a serious problem.
Decomposition is favoured, however, by various factors:
(1) pH values greater than 6
(2) the absence of air (anaerobic dyeing conditions)
(3) the presence of fibres containing reducing groups, such as wool or cellulose
(4) the presence of reducing metal ions, such as copper(I) or iron(II), in the water supply
(5) dispersing agents containing phenolic groups
(6) conditions that tend to maintain the dye for longer periods in the liquor, such as slowerdyeing substrates (low-porosity sewing threads, for example) and auxiliaries that tend to
solubilise the dye too much.
Lignosulphonate dispersing agents tend to promote this reduction of sensitive dyes, much
more so than the naphthalenesulphonic acid condensation types, probably owing to the
presence in lignin of catechol residues and other easily oxidised functional groups [95]
(structures 10.104 and 10.105). Commercial lignins vary considerably in their detailed
constitution, however, and consequently in their reducing power. In certain cases the
problem can be ameliorated by adding an oxidising agent (such as sodium dichromate) to
the dyebath, but the effects can be variable and difficult to control. In printing applications,
where steam fixation can have a pronounced reductive effect, stronger oxidising agents such
as sodium chlorate are often added to the print paste. In theory the reductive tendency of
lignosulphonates can be counteracted by chemical blocking of the active phenolic groups
but this impairs the dispersing properties of the product [91,95]. Significant improvements
can be achieved by replacing the conventional sodium ion in the lignosulphonate salts by
other cations [91]; lithium is effective in this respect, but the most promising salt appears to
be that of triethanolamine. This compound additionally acts as a chelating agent and so
protects against the catalytic influence of iron(II) and copper(I) ions.
Since high concentrations of electrolyte can adversely affect dispersion stability, low-salt
formulations of dispersing agents have been developed [95]. These also help to minimise the
lowering of viscosity by electrolytes with certain synthetic thickening agents in printing
applications.
In some cases it is necessary to choose dispersing agents that give minimal staining of the
substrate. This applies particularly when dyeing nylon since anionic dispersing agents have
significant substantivity for this fibre under acidic conditions. In general, lignosulphonates
have a greater propensity to stain than have the naphthalenesulphonic acid condensation
products.
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12.6.2 pH control and sequestering agents
Although many disperse dyes give good results over an extensive pH range (pH 2–9 for
example), some will only give satisfactory results over a narrower acidic range (pH 2–6) and
a few require careful control to within pH 4 to 5.5. Since practically all dyes give good
results at pH 5, this region tends to be regarded as the standard for exhaust dyeing
conditions. A simple addition of acetic acid will be satisfactory where water quality permits;
otherwise a buffered system is preferred. EDTA (section 10.2.1) is widely used to counteract
the effects of metallic impurities, which not only affect the hue and fastness of a few
susceptible dyes but may also catalyse dye reduction and promote deterioration of dispersion
properties, as described above.
In spite of the traditional preference for dyeing at about pH 5, the past decade has seen
the promotion of polyester dyeing methods under alkaline conditions at pH 9.0–9.5 [96–98].
Various advantages are claimed for alkaline dyeing conditions. These include the benefits of
economy and convenience that arise from dyeing at a pH closer to those used in
preparation, including bleaching, mercerising of polyester/cotton and caustic weight
reduction of polyester, as well as the clearing of surface dye after dyeing. This approach may
eliminate the need for neutralisation or slight acidification after alkaline treatments. Further
advantages claimed include improved handle of the substrate, more effective solubilisation
and removal of oligomer, less frequent and easier cleaning of machinery and possible
avoidance of reduction clearing. Alkaline conditions facilitate the simultaneous application
of disperse and reactive dyes to polyester/cellulosic blends.
Apart from the essential primary requirement of selecting disperse dyes that are stable to
pH 9.5 at least, or preferably higher to ensure a safety margin, the choice of auxiliaries for
this process is critical. The main requirement is a buffer system having sufficient reserve
capacity to maintain pH 9 throughout most of the dyeing process. This is more difficult than
might be assumed, since the polyester fibre and the oligomers present are partially
hydrolysed by alkali at a high dyeing temperature to form carboxyl and other groups that
cause a gradual lowering of the pH. Consequently, dye manufacturers have introduced
alkaline dyeing ‘packages’ comprising a selected range of stable disperse dyes together with a
purpose-designed auxiliary system to maintain the required pH. Little has been published
about the detailed composition of such systems. However, they are claimed to contain more
than just a suitable buffering system. For example, one such auxiliary is claimed [96] to be
designed to (a) stabilise the dyes, (b) provide adequate buffering, (c) chelate metal ions and
(d) assist dissolution of oligomers, whilst a second auxiliary is offered for use where there is
an unusually high content of oligomer.
Attention must be given to dispersion stability and to oligomer control. This is because
certain types of dispersing agent have inferior efficiency under alkaline conditions and more
oligomer is released into alkaline liquors. Unless the oligomers are adequately dispersed or
solubilised they may contribute to dye dispersion problems and may be deposited onto the
fibre or machinery. Hence, careful thought must be given to the selection of dispersing or
solubilising agents.
With regard to the buffer system, an extensive range of amino acid derivatives applied in
combination with an alkali have been claimed [99]. From this extensive list, primary
preference is given to N,N-bis(hydroxyethyl)glycine (12.24) in combination with sodium
hydroxide. However, N,N-dimethylglycine, N-methylglycine and N-methylalanine are also
listed as preferred compounds, whilst other possible alkalis include sodium carbonate,
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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES
sodium bicarbonate or borax. Another system depends on the formulation of a mixture of a
phosphonate and a polycarbonic acid in combination with sodium hydroxide, borax, sodium
carbonate or bicarbonate [100]. Whichever buffer system is used, extensive empirical trials
are required to determine the balance and concentration level needed to ensure stability of
pH during the specific conditions of use.
CH2CH2OH
O
C
CH2
N
CH2CH2OH
HO
12.24
12.6.3 Electrolytes
Electrolytes are unnecessary for the application of disperse dyes alone. Nevertheless,
electrolytes will be present when applying disperse dyes together with direct or reactive dyes
in the dyeing of fibre blends. In particular, the high concentration of salt often used with
reactive dyes can have an adverse effect on dispersion stability and may also interfere with
the stability and/or efficacy of other auxiliaries, particularly those based on emulsion
systems. These effects are often attributed to the destabilising influence of inorganic ions on
the forces of attraction between disperse dye particles and dispersing agents. Manufacturers
take these effects into consideration in marketing ‘electrolyte-stable’ formulations of
disperse dyes or auxiliaries.
12.6.4 Levelling agents
It is necessary to distinguish clearly between levelling agents and dispersing agents. The
primary function of a dispersing agent is to maintain a stable dispersion. Since most of these
agents enhance the low water solubility of disperse dyes they may improve level dyeing,
although they vary significantly in this effect. Maximal dispersion stability is usually attained
with agents that maintain dye particles at constant size and minimal solubility. Hence
primary dispersing agents seldom enhance levelling; different auxiliaries are added where
levelling action is needed. These are invariably anionic or nonionic surfactants and they
tend to solubilise the dye much more effectively. Some anionic levelling agents are able to
promote dispersion stability but nonionic types have a destabilising effect and great care is
therefore required in selection.
It is useful to consider how levelling agents can adversely affect dispersion stability. As
dyebath temperature increases, thermal effects tend to cleave the film of dispersing agent
protecting the dye particles. High shear rates in jet dyeing machines and additives such as
electrolytes, fibre lubricants or sizes, as well as oligomers from the fibre, can contribute to
this effect. Commercial batches of the same dye brand may behave differently according to
the initial dispersion quality of the dye. A dispersion of a vulnerable dye then tends to
deteriorate, resulting in crystallisation and agglomeration. The types of precipitation that
can occur have been described [87,88,101] and are illustrated diagrammatically in Figure
12.5 (section 12.6.1).
Suspended dye crystallites tend to agglomerate, eventually forming larger crystals. The
dissolved dye molecules are able to diffuse into the fibre, but under adverse conditions
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DISPERSE DYES
845
Relative dye uptake after 120 min
crystallisation is favoured. Once seeded, the crystals may grow in size whilst retaining their
original form, or they may undergo a transformation from the original thermodynamically
metastable form to a more stable but less soluble form. On the other hand, such crystals may
not form until the incompletely exhausted dyebath is cooled after the dyeing process; this
problem may be avoided by blowing off the dye liquor at 125–130 °C. Precipitation by
agglomeration tends to predominate with those dispersing agents that do not enhance dye
solubility significantly, whereas crystallisation is more prevalent with levelling agents or
dispersing agents having greater solubilising power. It is interesting that surfactant additions
may be used during dye synthesis in order to obtain the dye in the optimal form for isolation
and subsequent milling; for example, very fine crystals may clog filters, whilst thin needlelike crystals tend to mill more easily than platelets.
Anionic levelling agents, especially the polyelectrolyte dispersing agents described
previously, are generally preferred as the primary addition [102], particularly where it is desired
to promote level dyeing by control of exhaustion during the heating phase of dyeing; higher
concentrations have a greater retarding effect. Few of these anionic products promote dye
migration, a characteristic that is useful if a more powerful levelling action is required.
Nonionic surfactants, on the other hand, tend to solubilise the dye much more effectively
and thus contribute to level dyeing both by a retarding effect and through the promotion of
migration. Consequently they are generally more powerful levelling agents than anionic
products although their effects are much more dye-specific. The dye-specific effects on
retarding and restraining have been well-publicised [103–107], although the full extent of
the variations is often overlooked in the industry. The restraining effects of a typical
nonionic agent, a nonylphenol with an average of 20 ethylene oxide units per molecule, on
five commercially important disperse dyes are illustrated in Figure 12.10. The spread of
results between CI Disperse Blue 56 (only slightly affected) and CI Disperse Yellow 42
(much more affected) should be noted. Other nonionic auxiliaries would yield different
effects and dyes may behave differently in mixtures compared with their response when
tested in isolation. The retarding effects of the same agent on CI Disperse Blue 56 and
Yellow 42 applied as a green 1:1 mixture are shown in Figure 12.11.
1.0
0.8
0.6
0.4
0.2
CI Disperse
Blue 56
Yellow 5
Red 60
Red 82
Yellow 42
1
5
10
Agent concentration/g l–1
Figure 12.10 Relative dye uptake values for five disperse dyes on polyester at various concentrations
of a nonylphenol 20 EO levelling agent [107]
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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES
Dye uptake/mg dye per g polyester
CI Disperse Yellow 42
3
Agent/g l–1
1
5
10
2
1
CI Disperse Blue 56
3
Agent/g l–1
1
5
10
2
1
Temperature 90
Time
100
110
120
130
30
60
90
130 oC
120 min at 130 oC
Figure 12.11 Rate of dyeing curves for CI Disperse Blue 56 and Yellow 42 in admixture on polyester at
various concentrations of a nonylphenol 20 EO levelling agent [107]
This complexity of response has critical implications for reproducibility of dyeing,
particularly within the context of right-first-time dyeing. Careful prior evaluation and
optimisation of each recipe, followed by consistent bulk use and monitoring, are essential for
good reproducibility. Instrumental colour difference measurements are particularly useful for
evaluating and monitoring responses [107,108]. On the other hand, a study of four different
polyethoxylated sorbitan esters indicated that there were no significant differences between
them in terms of desorption of disperse dyes from polyester [109]. Desorption, however, is
only one aspect of level dyeing and migration.
A major problem with nonionic agents arises from their inverse solubility. Thus an agent
with a low cloud point may increase dye precipitation, although once again the effect is dyespecific. Published data suggest that a nonylphenol with a low degree of ethoxylation, having
a cloud point of about 40 °C, should not be used as a disperse dye levelling agent [110]. A
product of type 12.25, having a cloud point of about 105 °C, should be satisfactory for dyeing
at any temperature up to 100 °C but should be avoided at higher temperatures. On the other
hand, a carefully selected nonionic agent may be mixed with an anionic agent to raise its
cloud point. For example, a mixture of the fatty acid ethoxylate 12.26 with 7–10% sodium
dodecylbenzenesulphonate [110] has a cloud point of about 150 °C and is suitable for use in
high-temperature dyeing.
O
CH3(CH2)15(OCH2CH2)17OH
12.25
CH3(CH2)7CH
CH(CH2)7
C
(OCH2CH2)14OH
12.26
Surprisingly, other investigators were unable to confirm the adverse effect of nonionic
surfactants of low cloud point in the high-temperature dyeing of polyester, even in the
presence of electrolytes [111]. This was probably because of the rather low concentrations
used. Adducts containing a C18–C20 hydrophobe and a decaoxyethylene hydrophile, as well
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DISPERSE DYES
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as sorbitan ester ethoxylates were shown to be particularly effective levelling agents. Certain
dyes were found to be sensitive to nonionic additives under the stringent conditions of the
laboratory dispersion tests carried out in the absence of fibre, but it was pointed out that
these systems still performed satisfactorily in actual dyeings [111].
An alternative type of levelling system contains a mixture of ethoxylates with aliphatic
esters [112]. This combination exerts a retarding effect on many dyes during heating up to
about 100–110 °C, especially if the dyes are present in low concentration. At higher
temperatures this retarding effect is increasingly offset by the accelerating effect of the
aliphatic esters. This temperature-dependent interaction is said to improve the compatibility
of combinations of dyes applied with this system.
The adverse effect of nonionic adducts of low cloud point can be avoided by the use of
hybrid agents of the ethoxylated anionic type, variously and confusingly referred to as
‘modified nonionic’, ‘modified anionic’ or ‘weakly anionic’ types. Thus Mortimer [113] has
proposed the use of products of the ethoxylated phosphate type (12.27). In this structure, R,
as well as the degree of ethoxylation (n) may be varied to optimise the overall HLB value.
The numerous ether groups are said to enhance the dye-solubilising and levelling capacity,
whilst the polyphosphate grouping exerts several useful effects [113]. These compounds:
(1) are sufficiently anionic to avoid most of the disadvantages of conventional nonionic
agents with regard to high-temperature instability and the lack of an electrical double
layer of value in dispersion stability
(2) behave similarly to more orthodox polyphosphate sequestrants, thus offering some
protection from hard water and other trace metal impurities
(3) maintain effective stability in high concentrations of electrolyte
(4) offer possibilities for pH control by varying the nature of M
(5) are fully effective at pH 4–5, the most useful pH range for application of disperse dyes,
whereas conventional nonionic types are said to become less effective as levelling
agents at pH values less than 7.
O
R
(OCH2CH2)n
O
P
O
O
M
M
x
12.27
R
n
x
M
=
=
=
=
hydrophobe
typically 10–20
typically 1–3
H, alkali metal or organic base
Hence these agents are sophisticated multifunctional auxiliaries, which can take the place of
separate additions of levelling agent, sequestering agent and a buffer to control the pH.
Nonionic surfactants can be beneficial in minimising the redeposition of the sparingly
soluble polyester oligomers that are released from polyester fibres during high-temperature
dyeing.
Ethoxylated multi-ester compounds (so-called ‘oligo-soaps’) have been promoted recently
as dispersing/levelling agents [114]. These contain a multi-branched hydrophobe with
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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES
pendant carboxyl groups that are esterified with poly(ethylene glycol). Thus they are similar
in structural class to a mono-ester (so-called ‘mono-soap’) of structure 12.28, a
conventional condensate of a fatty acid of high molecular mass with poly(ethylene glycol)
but the multi-ester has a much higher relative molecular mass (Table 12.6). A micelle
formed from an ethoxylated mono-ester is an aggregate of several molecules, whereas the
individual molecule of an ethoxylated multi-ester is said to behave as a micelle, thus
exhibiting a much lower critical micelle concentration. The thermal stability of a multi-ester
is much greater because this macromolecular structure resists thermal agitation to a much
greater degree. This maintains a more stable dye dispersion at high temperature under
conditions of high shear. Further advantages include:
(1) solubilisation of the dye takes place at a lower temperature
(2) strike rates at lower temperatures in the dyeing cycle are much slower
(3) solubilisation of oligomer and acrylic size
(4) low foaming.
O
R
C
(OCH2CH2)n
OH
12.28
R = fatty alkyl group
Table 12.6 Comparison of characteristic properties of mono-ester and
multi-ester compounds [114]
Relative molecular mass
Critical micelle concentration
Foaming tendency
Oligomer solubilisation
Size solubilisation
Mono-ester
Multi-ester
Small
0.400 g/1
High
No
No
Large
0.004 g/1
Low
Yes
Yes
12.6.5 Carriers
Although polyester or cellulose triacetate fibres are normally dyed at high temperatures,
their blends with wool are still dyed at or near the boil. In such cases an auxiliary termed a
carrier must be used to promote adequate exhaustion of disperse dyes by the ester fibre
within a commercial dyeing time. Even in high-temperature dyeing, there are occasions
when the usual maximum temperature (around 130 °C for polyester) cannot be used, as
when dyeing qualities of texturised polyester that suffer loss of crimp at 130 °C. Carriers are
then used to assist more rapid and complete exhaustion, using smaller amounts than at or
near the boil. Carriers are sometimes employed to promote migration of unlevel dyeings.
The active component of a carrier formulation is generally a nonionic compound of Mr
150–200 containing a benzenoid ring system. A comprehensive review listed the classes of
compounds used together with their general properties, ideal requirements and the
mechanisms that have been proposed for carrier action [115]. Carrier compounds fall into
four main classes: phenols, primary arylamines, aryl hydrocarbons and aryl esters. Major
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representatives of these in commercial use include o-phenylphenol, biphenyl, methylnaphthalene, trichlorobenzene, methyl cresotinate, methyl salicylate (sometimes mixed with
phenyl salicylate), butyl benzoate, ethers of 2,4-dichlorophenol, diethyl or diallyl phthalate
and N-alkylphthalimide derivatives. Benzaldehyde has been used for the dyeing of aramid
fibres [116,117].
Over the last decade the use of carriers has declined markedly and continues to do so,
essentially for health, safety and environmental reasons [118–121]. In some countries these
products are now virtually banned. Nearly all carrier compounds exhibit all or some of the
following: toxicity, physiological irritancy or poor biodegradability (Table 12.7). Typical
pollution loads for comparable high-temperature and carrier methods are given in Table
12.8.
Table 12.7 Chemical and biochemical oxygen demand
data for various types of carrier chemical [118]
Carrier type
COD (mg/l)
BOD5 (mg/l)
o-Phenylphenol
N-Alkylphthalimide
Arylcarbonate ester
Methyl cresotinate
Dichlorobenzene
Trichlorobenzene
1000–2000
1000–2100
900–1900
800–1700
500–1000
300–1000
200–800
100–200
700–800
200–800
0
0
Table 12.8 Chemical and biochemical oxygen demand data for hightemperature and carrier dyeing methods [120]
Polyester
dyeing method
Liquor
ratio
BOD5
(mg/l)
High-temp.
jet dyeing
40:1
40:1
165
Carrier dyeing
on the winch
40:1
20:1
200
189
COD
(mg/l)
BOD5:
COD
Harmful
factor*
584
722
1:4.4
140.2
72.2
2043
1888
1:10.2
1:10
408.6
188.8
* Harmful factor = g COD per kg of dyed goods
Harmful effects from carrier dyeing can arise in three ways:
(1) residual carrier in the dyebath contributes to effluent pollution and may be
environmentally harmful
(2) carrier that is volatilised during dyeing or subsequent heat setting becomes an
atmospheric contaminant
(3) residual carrier in the fibre can be a health hazard, as well as causing an unpleasant
odour on heating or during storage.
The degree of carrier action is important in practice and varies considerably. For example,
the phthalates have little action on polyester but are efficient on cellulose triacetate, for
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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES
which they are the most widely used compounds. Ortho-phenylphenol and the chlorinated
benzenes are generally powerful carriers for polyester, whilst methylnaphthalene and
particularly butyl benzoate are less powerful, Although all carriers tend to promote the
exhaustion of dyes, some degree of dye-specific behaviour results from the respective
hydrophobic/hydrophilic balance of dye and carrier [102,109].
Some carriers, such as o-phenylphenol, tend to lower the light fastness of many dyes if
carrier residues remain in the dyed fibre; others, such as the chlorobenzenes, have no effect
on this property. Similarly, carrier residues differ considerably in odour. A dry heat treatment
at 160–180 °C after dyeing, to volatilise the residual carrier, is the best method of minimising
problems with light fastness and odour. The steam volatility of a carrier and its toxicity to
human and plant life need careful consideration. For example, o-phenylphenol has relatively
low volatility in steam and traditionally has been used in machines open to the atmosphere.
The chlorinated benzenes, on the other hand, are readily steam-volatile and are toxic, so
should not be used in machines where volatilised carrier is likely to condense (for example,
on a cooler lid) since drops of condensate may cause ‘carrier spots’ if they fall onto the
fabric. Biphenyl is relatively non-toxic to river life but is not readily biodegradable;
methylnaphthalene, also of low toxicity, is moderately biodegradable, but halogenated
benzenes are both toxic and difficult to biodegrade. Some carriers such as chlorinated
benzenes and butyl benzoate are relatively efficient in promoting migration; others, such as
o-phenylphenol, are less so. When dyeing a blend such as polyester/wool it is useful to
consider the extent to which the carrier will promote migration of dye to polyester so as to
minimise staining of the wool.
All the carrier compounds mentioned above have little or no solubility in cold water.
They are therefore used in the form of emulsions, many being marketed as ‘self-emulsifiable’
liquids that form stable emulsions on dilution in the dyebath. The choice of emulsifying
system is very important, not only from the viewpoint of emulsifying the active carrier
component, but also to ensure stability of the emulsion under dyebath conditions and
compatibility with dyes and dispersing agents, as well as efficacy of carrier action. Thus two
carriers of identical active components but with different emulsifying systems may well differ
appreciably in behaviour. Two typical formulations [122] are given in Table 12.9, both being
completely solubilised concentrates that on dilution in the dyebath give stable emulsions of
good dyebath compatibility. The weakly anionic ethoxysulphates and ethoxyphosphates are
especially useful emulsion bases for carriers. A small amount of a simple organic solvent
such as ethanol may also be added to improve stability.
Most commercial carriers are used in the dyebath at concentrations within the range 1–8
g/l depending on active strength of the carrier concentrate, applied depth, liquor ratio and
Table 12.9 Typical examples of carrier emulsions [122]
chpt12(2).pmd
Formulation 1
Formulation 2
90% Diethyl phthalate
10% Ethoxylated castor oil
(40 mol ethylene oxide)
40% Phenyl salicylate
40% Methyl salicylate
20% Ethoxylated nonylphenol
(20 mol ethylene oxide)
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other dyeing conditions. Although carriers exhibit dye-specific properties to some extent a
particular carrier will generally have an optimal concentration in the dyebath to give
maximum dye yield; higher concentrations will tend to solubilise the dye to such an extent
that colour yield is depressed.
12.6.6 Aftertreatments and thermomigration
Cellulose acetate and nylon dyed with disperse dyes are usually given a simple rinse with or
without a synthetic detergent (anionic or nonionic) after dyeing. Most cellulose triacetate is
similarly treated; however, some full depths are given a clearing treatment to remove surface
dye so as to improve the fastness properties. Such a clearing treatment is more generally
important with polyester dyeings. It most frequently takes the form of a reduction clear using
1–2 g/l sodium dithionite in alkaline solution. For triacetate the preferred alkali is ammonia
(1–2 ml/l of s.g. 0.800) at temperatures up to 60 °C. Polyester will tolerate more severe
conditions; hence the alkali is usually 1–2 g/l sodium hydroxide used at temperatures up to
70 °C, or even higher in continuous ‘short-dwell’ processes. This treatment works mostly by
reductive fission of azo dyes and by converting anthraquinone dyes to their soluble leuco
forms. It is also advantageous to use 1–2 g/l of a nonionic surfactant in the reduction clear
to assist solubilisation of the reduction products and in some cases their thorough removal is
ensured by a subsequent treatment with a nonionic detergent alone. Fatty acid ethoxylates
of the type mentioned earlier (structure 12.26) are excellent nonionic agents for use in
reduction clearing.
This process, however, is not only expensive in itself, but creates additional expense
through the need to deal with an environmentally unacceptable effluent. There is also the
cost and inconvenience in carrying out two changes of pH: first from the acidic dyebath to
the alkaline reduction clear, followed by neutralisation of the substrate after the reduction
clear. It is not surprising, therefore, that reduction clearing is nowadays avoided as much as
possible. One possibility is to use specialised dyes that can be cleared with alkali alone
(section 4.9.2): this avoids the environmental nuisance of the reducing agent but still leaves
alkali and the need for two pH changes.
Alternative reducing agents are still sometimes proposed and evaluated. A detailed
comparison of five reducing agents has been reported: sodium dithionite, thiourea dioxide,
iron(II) chloride/gluconic acid, sodium hydroxymethanesulphinate and hydroxyacetone
[123]. Results of fastness tests on black polyester dyeings variously aftertreated are given in
Table 12.10.
Hydroxyacetone must be used at temperatures above 80 °C on account of its sluggish
action. Nevertheless it did not give adequate improvement of fastness to washing. It gives
high COD values and has an unpleasant smell. The reducing power of sodium
hydroxymethanesulphinate, even with anthraquinone as activator, is insufficient under these
conditions. It did not give adequate improvement of washing fastness. Iron(II) chloride has
the environmental advantage that it does not contain sulphur but the gluconic acid
complexing agent results in relatively high COD values. Improvement of washing fastness
was inadequate. Only thiourea dioxide gave results as good as sodium dithionite. It is three
times more expensive but causes only half the sulphur pollution of dithionite. The relative
usefulness of these two reducing agents really depends on the dyeing process. In the winch,
the slow production of active species from thiourea dioxide is a disadvantage when working
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Table 12.10 Fastness of black polyester dyeings after various reduction clearing
treatments [123]
Fastness properties
Perspiration
Reduction clear
Washing at 60 °C
Acidic
Alkaline
Untreated control
Hydroxyacetone
Sodium hydroxymethanesulphinate
Same, with anthraquinone activator
Iron(II) chloride/gluconic acid
Thiourea dioxide
Sodium dithionite
1–2
2–3
2–3
3
3
4–5
4–5
2–3
4–5
4–5
4–5
4–5
4–5
4–5
2–3
4–5
4–5
4–5
4–5
4–5
4–5
with the counterflow. On the other hand it could be an advantage in jet dyeing machines,
although this only arises if reduction of the dye occurs relatively quickly. In closed
machines, sodium dithionite is more effective.
Several reduction clearing auxiliaries have been introduced under commercial brand
names, their composition in many cases not being revealed. At least one of these is designed
to be used under acidic conditions [124]. Advantages claimed for this product include: no
pH changes needed, low COD, high biodegradability, low toxicity, with further savings of
time and water consumption. Moreover, since the agent is added directly to the exhaust
dyebath any residual dye present is decolorised before discharge to effluent. Although highly
effective with the majority of dyes, in a few cases (e.g. CI Disperse Yellow 29, Violet 35 or
Blue 56) a higher concentration is needed.
Polyester dyed with disperse dyes generally shows excellent fastness to wet treatments and
rubbing after thorough reduction clearing and drying at low temperature (below 120 °C),
irrespective of the dyes used. However, cost-effective production and the requirements for
fabric dimensional stability demand the use of a combined drying and heat setting treatment
at temperatures in the range 150–210 °C, most frequently at 180 °C. This causes some dyes
to migrate from the core of the fibre to the surface, thus tending to negate the effect of
reduction clearing. This surface dye is a potential source of lower fastness to rubbing and wet
treatments [125–127], although the extent to which this occurs depends greatly on the dye
and its applied depth. This phenomenon has been termed ‘thermomigration’, its effect on
fastness varying considerably because of the generally adverse influence of surfactants,
lubricants, softeners, antistats and so on. Similar problems occur on cellulose triacetate. A
method for assessing the influence of auxiliaries on thermomigration has been published
[128]. This is carried out with 1/1 standard depth CI Disperse Blue 56 (or other suitable
dyeings) and the fastness to an ISO C02 washing test is determined after reduction clearing
and stentering at specified temperatures. A detergent-based, rather than soap-based,
washing test would be more critical [129].
The mechanisms operating during thermomigration in the presence of a surfactant have
been evaluated experimentally [130]. The overall mechanism consists of four main processes:
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(1) extremely rapid attainment of equilibrium between dye in the surfactant layer and dye
in the surface zone of the fibre
(2) rapid diffusion of the dye molecules from the interior of the fibre towards the surface
(3) slower diffusion of surfactant molecules into the substrate phase
(4) eventual formation of a composite dye–fibre–surfactant phase in the surface region.
In fact thermomigration readily takes place in the absence of surfactant, albeit usually to a
lesser extent. In this case only the second of the above processes takes place. The term
‘surfactant’ in this model can be interpreted broadly to include any residual surfactant,
reduction clearing assistant or applied finish, such as an antistat, lubricant or softener.
No general correlations exist between the degree of thermomigration and the structure of
a dye, its molecular mass, diffusion coefficient or fastness to sublimation. Nevertheless, to a
limited extent, in a series of disperse dyes of closely related constitution there does appear to
be some relation between the hydrophobic–hydrophilic balance of the dye molecule and its
susceptibility to thermomigration. In such restricted series of related dyes, thermomigration
decreases with increasing hydrophobicity [130]. This could be related to the strength of
dye–fibre hydrophobic bonding, stronger bonding tending to limit migration of dye to the
fibre surface. There seems to be a relationship between thermomigration and the degree of
interaction between dye and surfactant, more specific interaction leading to greater
thermomigration [130]. The degree of interaction in this work was deduced from specific
conductivity measurements.
It is important to recognise that the degree of thermomigration in itself is not necessarily
indicative of any practical implications that may show up in fastness tests [129]. For
example, CI Disperse Blue 60 thermomigrates to an appreciable extent, as measured by
solvent extraction of the dyed fibre after stentering. Nevertheless, in most wet fastness tests
it still gives excellent results simply because it has relatively low substantivity for adjacent
fibres (especially nylon) in wet fastness tests. Conversely, a dye may show very little actual
thermomigration yet give poor wet fastness after stentering on account of its high
substantivity for nylon under the conditions of test. Direct measurements of dye diffusion
behaviour generally have little practical significance in themselves, unless they can be
related to the effects on fastness properties. Problems arising from thermomigration are best
avoided by selecting dyes that show acceptable washing fastness after heat setting treatment
and by ensuring that all surfactants from dyeing and afterclearing are completely rinsed out.
Careful choice of finishing agents and finishing conditions is also important.
In heat setting and curing, for example, temperature has a greater effect than time in
promoting thermomigration [131]. Thus improved fastness to rubbing and wet treatments
may be achieved using a selected durable press/softener finish (incorporating a rapidly
reacting resin/catalyst system) giving the required finish effect at 140–160 °C. The longer
curing time required at a lower temperature has a less deleterious effect than a higher
temperature (such as 180 °C) for a shorter time. Another means of minimising the effects of
thermomigration is to apply certain mildly reducing chemicals after dyeing. The application
by padding of a polysiloxane and an organotin catalyst along with any other finishing agents
[131,132] gives rise to a reducing effect during subsequent dry heat treatment that is
capable of decomposing many dyes brought to the surface by thermomigration. Such
products do not work successfully with all dyes and finishes, however, and can confer a
degree of water repellency that is not always desirable or even acceptable.
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The presence in polyester fibres of polymerisation by-products (oligomers) can give rise to
problems, particularly if their concentration is greater than normal. With polyesters based on
ethylene glycol and terephthalic acid, the amount of oligomer is normally between 1.4 and
1.7%, consisting mainly of the cyclic trimer of ethylene terephthalate with smaller amounts of
a pentamer, a dimer containing diethylene glycol residues and traces of other compounds.
Significant migration of such oligomers from within the fibre can occur at dyeing temperatures
of 110–135 °C, leading to deposits on the fibre and/or machine surfaces and sometimes also to
interference with dispersion stability, since dispersed oligomer particles form potential nuclei
for the crystallisation and agglomeration of disperse dyes. Discharge of the spent hot dye
liquors without prior cooling is the best way of avoiding oligomer problems. Reduction clearing
will normally remove any deposits from the fibre surface. Deposits on machine surfaces must
be removed by regular cleaning at high temperature with strong solutions (5 g/l) of sodium
hydroxide together with thermally stable surfactants and solvents.
12.6.7 Continuous dyeing
The conventional method of continuous dyeing with disperse dyes is the pad–thermofix
process [133,134], most frequently used for polyester/cellulosic blends although it can be
used with 100% polyester (or cellulose triacetate) materials. The auxiliaries normally used at
the padding stage include a thickening agent as migration inhibitor and a wetting agent.
Alginates and other polyelectrolytes such as polyacrylamides are popular as migration
inhibitors. Anionic sulphosuccinates are suitable wetting agents; since cloud point problems
do not arise in continuous dyeing to the same extent as in batchwise processes, nonionic
ethoxylates may also be used, often fulfilling a dual role as wetting and levelling agents. An
addition of acetic acid to give pH 5–6 is usually adequate when applying disperse dyes alone,
although certain processes may demand selection of dyes stable at higher pH, as in the
combined alkaline application of disperse and reactive dyes to polyester/cellulosic blends. In
some processes, too, the use of hydrotropes such as polyglycols and their esters, as well as of
urea and related compounds, can be useful to enhance the degree of fixation.
12.6.8 Printing
Printing with disperse dyes is generally carried out using a thickening agent and an acid
donor to maintain a low pH during steam fixation. High-solids thickeners such as crystal
gum or British gum give optimal sharpness of outline but suffer from the disadvantage of
forming brittle films [29]. Hence low-solids thickeners such as alginates and locust bean
ethers, which form more elastic films and are more easily removed in subsequent washingoff, are preferred. Further additions may include a fixation accelerator (hydrotropes such as
urea, thiodiethylene glycol, cyclohexanol, dicyanoethylformamide) or a carrier and an
oxidising agent, such as sodium chlorate or sodium m-nitrobenzenesulphonate, to inhibit the
possible reduction of susceptible dyes during steaming. The dispersing agent present in the
disperse dye formulation can have an influence on printing problems involving loss of
viscosity of the print paste and reduction of some azo dyes. Loss of print paste viscosity is
particularly associated with synthetic thickeners. Nonionic formulations of disperse dyes
were developed to counteract this problem; in these brands the usual anionic dispersing
agent was wholly or substantially replaced by a nonionic system.
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Freedom from electrolytes is also desirable for much the same reasons. The nature of the
cations associated with the sulphonate groups in lignosulphonate dispersing agents is
important, since it affects the hydrophilicity of the agent, this aspect having the primary
influence on the absorptive behaviour of the dispersant. The concentration of ionised
functions affected by the nature of these cations correlates directly with conductivity,
printing paste viscosity and azo dye reduction [91]. A detailed investigation of three
inorganic (Na, K, NH4) and six amine salts of lignosulphonates showed the triethanolamine
salt in particular to offer the greatest benefits. The results of these conductivity and print
paste viscosity measurements are shown in Table 12.11. Apart from the beneficial effects of
the triethanolamine salt on print paste viscosity, this agent showed the least sensitivity to
pH, did not promote reduction of azo dyes and was an effective sequestrant for dissolved
iron and copper ions.
Table 12.11 Conductivity and print paste viscosity data for various salts of lignosulphonate dispersant [91]
Lignosulphonate
cationic salt
Conductance
(m.mhos) at 5%
dispersant
concentration
Print paste*
viscosity (cps)
at 25 °C
Nonionic control
Conventional low-sulphonated Na salt
Low-electrolyte (Na) lignosulphonate
Dimethylamine salt
Trimethylamine salt
Triethanolamine salt
–
9.80
5.26
4.75
4.34
3.31
71 000
1 800
29 000
23 000
27 500
41 000
* Viscosity sample: 8g dispersant in 970 ml water at pH 7 added to 30 g synthetic thickening
For polyester, the washing-off process to remove unfixed dye and thickening agent is
generally a reduction clear as described in section 12.6.6. A simple wash-off with nonionic
surfactant must be used on cellulose acetate or triacetate, although a mild reduction clear
may be preferable on triacetate.
Discharge effects on acetate are carried out by overprinting dyed grounds with a thickened
paste containing the reducing agent thiourea dioxide and thiodiethylene glycol; a disperse dye
stable to these reducing conditions may be added to a similar paste to give a contrast
illuminated effect. Similar effects may be produced on polyester by printing a discharge
(reducing) paste onto fabric that has been padded with dye; reduction of the discharge areas
and simultaneous fixation of dye in the undischarged areas then takes place during subsequent
steaming. The discharge paste may contain a reducing agent such as zinc formaldehydesulphoxylate or tin(II) chloride, although special ranges of alkali-dischargeable dyes are
available that require only alkali (section 4.9.2). Reduction-stable dyes may be added to the
discharge paste to create illuminated effects. More detailed recipes are available elsewhere.
12.6.9 Stripping
Non-destructive stripping can be carried out at dyeing temperature with surfactants, a
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nonionic type with a high cloud point being particularly effective. In the case of polyester
the stripping effect can be increased markedly by adding a carrier that has migrationpromoting properties, the chlorobenzenes and butyl benzoate being particularly effective,
although this is now much restricted on health, safety and environmental grounds. The
efficiency of destructive stripping depends on the fibre and dye types. The most usual
method is to use a reduction process (alkaline sodium dithionite) together with a nonionic
surfactant and, where possible, a carrier, the temperature being varied to suit the fibre. In
some cases, particularly with anthraquinone dyes, an oxidation treatment (chlorite,
hypochlorite or permanganate) may be more efficient. Occasionally a sequential
combination of oxidation and reduction treatments may have to be used.
12.7 REACTIVE DYES
12.7.1 Cellulosic fibres
As described in Chapter 7, the various ranges of reactive dyes for cellulosic fibres differ
considerably in their reactivity and the number of application procedures is bewilderingly
large, including numerous variants within each category of batchwise, continuous, semicontinuous and printing methods. Even a specific method may require modifications to suit
a particular quality or form of substrate, dyeing machine or the specific dyes selected from a
given range. Fortunately certain general principles are applicable to the great majority, if not
all, of these methods. The characteristics of each range of reactive dyes and details of their
application methods are fully described elsewhere [30], but in the working situation it is
especially important with reactive dyes to consult the dye manufacturer’s literature.
Recent years have seen considerable research into the modification of cellulose and
reactive dyes, specifically to overcome some of the drawbacks of this dye–fibre system,
including the limited degree of fixation in full depths, the need for alkali and relatively high
concentrations of electrolyte. This research, which is driven by environmental
considerations, was discussed in sections 7.10 and 10.9.1. Thus it need not be considered
further here.
Exhaust dyeing
The critical importance of substantivity and its overriding influence on application
properties has been described in sections 3.2.1, 3.3.2 and 7.5, as well as elsewhere [30]. The
essential auxiliaries used to control reactive dyes in batchwise dyeing are electrolyte and
alkali. Secondary auxiliaries may include sequestering agents, mild oxidising agents to
prevent reduction of certain sensitive dyes and wetting or levelling agents. The classic
procedure for dyeing with reactive dyes involves application under substantially non-reactive
conditions by exhaustion with electrolyte at a temperature selected according to the
reactivity of the particular dyes used, followed by addition of alkali to enhance absorption
and, more particularly, to create the conditions through which the dyes can react covalently
with the fibre. In the so-called ‘all-in’ process electrolyte and alkali are present together
throughout to bring about simultaneous sorption and reaction, though this inevitably
increases the opportunity for hydrolysis of the dye in the dyebath. Application temperatures
vary from room temperature to the boil or even higher.
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There is as yet no official method of classifying reactive dyes according to their dyeing
properties, unlike the situation with direct or vat dyes. Nevertheless, a promising preliminary
scheme has been proposed [135] and it is worthwhile presenting the rationale here, since
this scheme has a bearing on the use of auxiliaries insofar as they affect levelling properties.
Group 1 – Alkali-controllable reactive dyes
Dyes that have their optimal fixation temperature between 40 and 60 °C belong to this
group. These dyes are characterised by relatively low exhaustion in neutral solution before
the addition of alkali. This type of dye has high reactivity and careful addition of alkali must
be made in order to obtain level dyeing. For these reasons, the name ‘alkali-controllable
reactive dyes’ has been chosen.
Typical examples of dyes belonging to this group are dichlorotriazine, dichloroquinoxaline, difluoropyrimidine and vinylsulphone dyes.
Group 2 – Salt-controllable reactive dyes
This group includes dyes that have their optimal fixation temperature between 80 and 95
°C. Such dyes show comparatively high exhaustion before fixation so it is important to
ensure that dyeings are level. Salt should be added portionwise at specified stages during the
exhaustion process, hence they are termed ‘salt-controllable reactive dyes’.
Typical examples of dyes belonging to this group are aminochlorotriazine,
bis(aminochlorotriazine) and trichloropyrimidine dyes.
Group 3 – Temperature-controllable reactive dyes
This group includes dyes that react with the fibre at 100 °C or above, without alkali present.
Dyes in this group have self-levelling properties so there is no need to exercise control by
means of dyeing auxiliaries. Good results can be obtained by controlling the rate of
temperature rise.
At present only the Kayacelon React (KYK) range of bis(aminonicotinotriazine) dyes
represent this group.
Sodium chloride is undoubtedly the most widely used electrolyte, a particular advantage
being its ease of dissolution. Certain dyes, such as brilliant blue phthalocyanines and
anthraquinones, are susceptible to aggregation and sometimes even precipitation in its
presence, however, and in these cases sodium sulphate, which has a lesser aggregating effect,
is preferred. The electrolyte should be free from calcium and magnesium salts and from
alkali to avoid premature fixation in a two-stage process. The electrolyte functions with
reactive dyes in a manner similar to that with direct dyes, but as reactive dyes are more
highly sulphonated and hence less substantive than direct dyes, more salt is required to
attain equivalent exhaustion. As with direct dyes, the higher the concentration of salt the
greater the uptake of dye, provided over-aggregation and precipitation do not occur. The
primary objective is to achieve maximal exhaustion over an optimal dyeing period, taking
care to ensure level uptake since this is the only phase during which the rapidly diffusing
reactive dyes can migrate. Hence electrolyte may be added in portions over the whole
dyeing period.
As might be expected with highly soluble dyes, liquor ratio has a pronounced effect on
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exhaustion and thus on the amount of electrolyte needed, especially with dyes of low
substantivity. Decreasing the liquor ratio from 30:1 to 5:1 would justify lowering the
electrolyte concentration to one-sixth but in fact even less than this can be used because of
the marked effect on exhaustion of the reduction in liquor ratio. There is another important
effect of electrolyte: initially the internal pH of the fibre is lower than that of the dyebath, a
state that tends to favour hydrolysis of the absorbed dyes; adding electrolyte tends to
equalise these pH values, thus protecting against hydrolysis. In general, the longer the liquor
ratio, the lower the dye substantivity and the greater the applied depth, then the higher is
the concentration of electrolyte required.
Not long ago a rather cavalier attitude existed towards the consumption of electrolyte,
based on the prevailing philosophy that ‘salt is cheaper than dye’ [136]. This outlook led
many dyers to use as much as 20–30% more salt than recommended by dye manufacturers
(100–150 g/l being not unknown) in an effort to secure maximum exhaustion. A graphic
illustration of the total amount of salt used in reactive dyeing worldwide has been provided
[137]: this is 1.8 million tons p.a., equating to 80 000 loaded rail wagons that would stretch
for 1000 km from Paris to Berlin. This tendency towards excessive salt usage has been
turned on its head, on both ecological and economic grounds. Considerable effort is now
devoted to defining application conditions whereby the minimal amount of salt can be used,
sometimes supported by computer programs supplied by dye manufacturers [136]. The trend
towards lower liquor ratios evinced by machinery developments and the increasing use of
bifunctional dyes with their higher average levels of fixation have both contributed to
lowering of the amounts of salt used. Special ranges of ‘low-salt’ reactive dyes have been
marketed, giving further emphasis to this important and worthwhile trend [137,138]. In
spite of these advances, however, it has been suggested [139] that such developments have
not proven to be totally satisfactory.
Consequently, investigations were carried out to explore the potential of cationic
surfactants, Groups IA, IIA and IIIA chlorides and carboxylate salts as alternatives to
conventional electrolytes [139]. Cationic surfactants proved unsuitable as they promoted only
surface deposition with low fixation, most of the dye being easily removed by washing. Groups
IIA and IIIA chlorides were precipitated as hydroxides under alkaline conditions and were
thus also unsuitable, although amongst Group IA salts potassium and caesium chlorides gave
increased exhaustion and fixation of CI Reactive Red 180 (12.29) with increasing salt
concentration (Figure 12.12). Fixation increased with the atomic size of the cation (Cs+ > K+
> Na+ > Li+) and equivalent fixation was achieved with about 20 g/l less of KC1 and about
40 g/l less of CsCl compared with a conventional concentration of NaCl [139]. Nevertheless,
although potassium and caesium chlorides promoted higher exhaustion and fixation, it is
difficult to agree that they are viable commercial replacements for sodium chloride. Significant
amounts would still be left in the effluent; they will always be more costly and less readily
available in commercial quantities, both short- and long-term.
More promising results were observed with potassium salts of di-, tri- and tetra-carboxylic
acids (Figures 12.13–12.15). Multicarboxylate salts facilitate much higher levels of dye
exhaustion and fixation than sodium chloride, sodium citrate being particularly effective
(Figure 12.16). Although sodium citrate is a chelating agent, it does not appear to affect
metal-containing reactive dyes under these conditions [139].
Depending on the reactivity of the dyes used and the applied depth, the pH required for
reaction with the fibre varies from 8 to 12 and in practice falls mainly between 9 and 11.
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Dye fixation/mg per g fibre
4
LiCl
NaCl
KCl
CsCl
3
2
1
0
0.342
0.684
1.027
1.369
1.711
Salt concentration/N
Figure 12.12 Fixation of CI Reactive Red 180 with various concentrations of Group IA chlorides [139]
O
NaO3SO
CH2CH2
SO2
C
SO3Na
H
HN
O
N
N
NaO3S
12.29
CI Reactive Red 180
4
O
Dye fixation/mg per g fibre
KCl
K oxalate (12.30)
OK
C
K tartrate (12.31)
K phthalate (12.32)
C
3
O
OK
O
OK
C
2
H
C
OH
H
C
OH
12.30
C
O
1
12.31
OK
O
C
OK
C
OK
0
0.342
0.684
1.027
1.369
1.711
Salt concentration/N
O
12.32
Figure 12.13 Fixation of CI Reactive Red 180 with various concentrations of potassium salts of
dicarboxylic acids [139]
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O
C
4
Dye fixation/mg per g fibre
KCl
K citrate (12.33)
K B3CA (12.34)
C
3
OK
H2C
O
C
OH
KO H2C
C
OK
O
2
12.33
O
1
0
0.342
0.684
1.027
1.369
1.711
KO
C
O
Salt concentration/N
C
OK
C
OK
O
12.34
Figure 12.14 Fixation of CI Reactive Red 180 with various concentrations of potassium salts of
tricarboxylic acids [139]. K B3CA = Tripotassium benzene-1,2,4-tricarboxylate
O
C
4
Dye fixation/mg per g fibre
KCl
K BTCA (12.35)
K B4CA (12.36)
O
C
3
CH
KO
HC
CH2
KO
2
O
KO
C
KO
C
OK
C
O
C
O
1
OK
H2C
12.35
O
C
OK
C
OK
0
0.342
0.684
1.027
Salt concentration/N
1.369
1.711
O
O
12.36
Figure 12.15 Fixation of CI Reactive Red 180 with various concentrations of potassium salts of
tetracarboxylic acids [139]. K BTCA = Tetrapotassium butane-1,2,3,4-tetracarboxylate; K B4CA =
Tetrapotassium benzene-1,2,4,5-tetracarboxylate
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80
Na citrate
NaCl
Exhaustion/%
60
40
20
0
20
40
60
80
Dyeing time/min
Figure 12.16 Exhaustion profiles for CI Reactive Red 180 in the presence of 1.711N sodium chloride
or 1.711N sodium citrate [139]
The most widely used alkali is sodium carbonate, although sodium bicarbonate and sodium
hydroxide are also used; these reagents may be used singly or in mixtures. Sodium
bicarbonate, for example, can be used as a pH-shift agent, since the pH slowly increases on
heating as sodium carbonate is formed (Scheme 12.4). The alkali induces ionisation of the
cellulosic hydroxy groups, enabling the dye to react with these nucleophilic anionic sites to
form covalent bonds with the fibre (section 7.3.1). It might be thought that increasing
quantities of alkali would favour maximal reaction between dye and fibre, but in practice an
optimum level of alkali, rather than a maximum, has to be sought. This is because the
cellulosate anion tends to repel the reactive dye anion, thus decreasing the efficiency of dye
uptake and so increasing the tendency towards hydrolysis of the dye. Consequently, the aim
must be to keep the pH as low as possible consistent with maintaining complete reaction
within a commercially acceptable dyeing time; however, fixation conditions vary widely
according to the dye type and process used.
2 NaHCO3
Na2CO3 + CO2 + H2O
Scheme 12.4
In certain applications sodium silicate is preferred. Replacing sodium carbonate by
sodium silicate can increase yields by 10–30%, as well as giving an improvement in fastness
to washing. The increase in yield was higher with cold-dyeing than with hot-dyeing types.
Cost savings were estimated at 15–35%, or 50–70% in some cases [140]. Occasionally,
however, the lower alkalinity of silicate could result in greater hydrolysis with corresponding
reduction in yield.
It is opportune at this point to illustrate the combined effects of electrolyte and alkali
(Figure 12.17). The initial stage with electrolyte alone at neutral pH approaches equilibrium
primary exhaustion of dye; there is no fixation during this stage. Once the alkali is added (in
this case after 20 minutes), fixation of the dye begins to take place; at the same time, the
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alkali contributes to further exhaustion, which is termed secondary exhaustion. The
difference between equilibrium exhaustion and fixation at the end of the process represents
unfixed dye that requires washing off in order to maximise fastness properties. The profiles
of these curves differ according to how the individual dyes respond to the conditions (type
and concentration of auxiliaries, pH, temperature). Once the dye is fixed, it cannot migrate.
Hence the difference between the exhaustion and fixation curves represents the potential
for levelling during the alkaline treatment.
Primary
exhaustion
Secondary
exhaustion
Exhaustion or fixation/%
100
Exhaustion curve
Fixation curve
50
S
F
P
0
20
40
60
80
Dyeing time/min
P Degree of primary exhaustion
S Degree of secondary exhaustion
F Degree of dye fixation
Figure 12.17 Dye exhaustion and fixation profiles in exhaust dyeing with a typical reactive dye [141]
The traditional requirement for alkalinity can be a disadvantage on both environmental
and cost grounds. Furthermore, this treatment is not fully compatible with dyeing
requirements for other fibres used in blends with cellulosic fibres, particularly polyester, thus
complicating the dyeing of such blends. This has favoured the development of so-called
‘neutral-dyeing’ reactive dyes. These do not require the addition of alkali for fixation but
still need electrolyte for exhaustion [142–144]. With these dyes reaction does not occur at
neutral pH below 100 °C but fixation becomes optimum at 120–130 °C and pH 6.5–7.5. A
pH of 8 is recommended for the hank dyeing of yarn and jig dyeing of fabrics, where dyeing
at high temperature is not feasible. This range of dyes is the only one to fall into Group 3 of
the classification mentioned previously; hence their levelling is controlled by temperature
rather than by additions of alkali or electrolyte.
As mentioned at the beginning of this section, the so-called ‘all-in’ method may be
adopted, treatment time being saved by adding the electrolyte and alkali together, albeit at
the expense of lower fixation and (usually) inferior reproducibility. Studies of
dichlorotriazine dyes applied with various alkalis or combinations of alkalis (NaHCO 3,
Na 2CO 3/NaOH or Na2CO 3/NaHCO3) have shown recently that this process can be
optimised to give enhanced dye fixation by buffering to give pH 8 or 9 [145].
Reactive dyes in general are not unusually sensitive to hard water. Nevertheless, the alkali
used in most reactive dyeing processes may precipitate calcium or magnesium hydroxide on
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the substrate, to cause problems in later processes. Ideally, soft water with a pH not greater
than 7 is preferred. Where the use of hard water is unavoidable, a sodium
hexametaphosphate sequestering agent may be used in the minimal amount needed to
overcome the hardness, since excessive quantities may bring about a significant reduction in
dye yield. Organic sequestering agents of the EDTA type (section 10.2.1) are generally best
avoided because they often result in colour changes and reduced light fastness, although
they can occasionally be used successfully in minimal quantities [30]. It has been shown, at
least with some reactive dyes, that hue changes due to traces of iron or copper in modal
fibres (Table 12.12) can be prevented by the use of dimethylaminomethane-1,1diphosphonate (12.37), 1-hydroxyethane-1,1-diphosphonate (12.38), EDTA (12.39) or
certain water-soluble polymers as sequestering agents [146]. However, as discussed later, the
potential for trace metals to cause problems at the washing-off stage should not be
overlooked [30].
As mentioned previously, once reactive dyes have reacted with the fibre no levelling is
possible; hence all levelling must be achieved before the reaction has reached equilibrium.
The preferred means is by controlled additions of alkali (Group 1) and electrolyte (Group 2)
rather than by using a surfactant-type levelling agent. The latter causes restraining, although
such products may be added in minimal amounts to aid wetting and to safeguard against
rope marks through lubricating action [30]. A particularly detailed study of the effects of a
range of nonionic aromatic and aliphatic ethoxylates confirmed the negative influence of
these agents on the fixation of reactive dyes [147–149]. Figure 12.18 shows typical results
Table 12.12 Metal content of regenerated cellulosic fibres [146]
Metal content (ppm)
Metal
Modal
fibre
Schwarza
modal fabric
Copper
Iron
Zinc
Magnesium
Lead
Manganese
4.1
46.0
15.0
–
27.0
2.2
6.6
63.0
20.0
22.0
15.0
1.5
Ash content (%)
0.27
Lenzing
modal fabric
2.0
13.0
38.0
9.0
<0.1
0.2
0.37
0.25
O
O
C
HO
H3C
N
CH3
O
HO
P
HO
O
CH
OH
P
CH3 O
P
C
P
OH
OH
OH
N
OH
HO
OH
C
CH2CH2
CH2
863
OH
N
H2C
O
OH
12.37
chpt12(2).pmd
O
C
H2C
CH2
OH
C
O
12.38
12.39
HEDP
EDTA
15/11/02, 15:47
864
AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES
obtained with a nonylphenol ethoxylate (20 mols ethylene oxide) in dyeings of CI Reactive
Red 2 (12.40).
80
No agent
Exhaustion/%
60
40
5 g/l agent
20
20
40
60
80
Time/min
Fixation/mg per g fibre
0.8
No agent
0.6
5 g/l agent
0.4
0.2
20
40
60
80
Time/min
Figure 12.18 Exhaustion and fixation curves for CI Reactive Red 2 in the presence and absence of a
nonionic surfactant at 30 °C [147]
Cl
N
N
Cl
N
HN
H
O
SO3Na
N
N
NaO3S
12.40
CI Reactive Red 2
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REACTIVE DYES
865
However, the decrease in yield varied considerably with dye and surfactant. The
magnitude of the effect increased with increasing degree of ethoxylation of the surfactant
and varied considerably from dye to dye. Interestingly, this loss of yield occurred even
though kinetic evidence indicated that the presence of the nonionic agent decelerated the
rate of dye hydrolysis [149]. Spectrophotometric studies [148] showed positive evidence of
interaction between the dye and surfactant. This interaction shifts the equilibrium between
adsorbed dye and dissolved dye in favour of the dyebath [147]. Electrolyte and temperature
also influenced the interaction [148]; higher temperatures tended to destabilise the dye–dye
and dye–surfactant interactions, although these interactive effects were partially maintained
at normal dyeing temperatures. Thus the possibility of using surfactants as levelling agents
for reactive dyeing is best avoided. Similarly, any surfactants added during preparation
should be completely rinsed out before dyeing. Provided goods have been prepared
thoroughly, it should be unnecessary to add wetting or levelling agents to the dyebath [30].
Mention of the importance of thorough rinsing after preparation applies not only to the
need to remove surfactants but also to prevent the considerable number of dyeing faults
resulting from residual unevenly distributed alkali and residual peroxide [150].
Various attempts have been made to optimise the exhaust dyeing of reactive dyes. For
example, some methods depend on pH control via precisely metered alkali dosing systems
[151,152]. The curves in Figure 12.19 show how the fixation rate can be modified
considerably by dye-specific alkali metering in order to increase the scope for levelling.
Detailed attention to the precise effects of all dyeing parameters with a view to ultimate dyespecific control enables the minimal amounts of auxiliaries to be used that will give safe and
effective control. Further advantages claimed include reduced water, steam and energy
consumption together with substrate quality improvements [151–153].
Ideal
fixation
profile
Fixation
Conventional
fixation
curve
Metered
alkali addition
Dyeing time
Figure 12.19 Control of reactive dye fixation rate by Remazol automet (DyStar) alkali addition [151]
Neps in cotton fabrics can pose a problem for reactive dyeing. Effective coverage of such
dead or immature cotton can be achieved by pretreatment with poly(ethylene imine) on the jig
at 10:1 liquor ratio before the dyeing process. The cationic polymer not only gave good
coverage of the neps but also improved the colour yield generally. In addition, wet fastness
could be improved by a low-temperature curing treatment with a cationic polymer emulsion
[154].
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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES
Viscose fibres are particularly suitable for dyeing with reactive dyes, the differences in
colour yield between dyed viscose and cotton varying between individual dyes and
depending on the chromogen present [155]. Salt and alkali requirements are generally lower
on viscose than on cotton, although most turquoise and green hues based on
phthalocyanines are applied according to special recommendations [30]. A comparison of
three types of reactive dye applied to cotton, viscose and lyocell showed little difference in
colour between these fibres [156]. The colour strength was lowest on cotton, slightly higher
on lyocell and marginally the highest on viscose. It should be borne in mind that the quality
of today’s viscose is somewhat different from that produced in the 1970s: as regards response
to preparation and dyeing parameters modern viscose is generally superior [157]. Hence
some care is needed in relating current data to earlier results and many of the older
recommendations should not be followed today. Certain dyes are susceptible to reduction,
particularly under anaerobic dyeing conditions. An addition of 1–2 g/l sodium mnitrobenzenesulphonate is a useful palliative.
Continuous and semi-continuous dyeing
There are numerous variations of procedure for the continuous or semi-continuous dyeing
of cellulosic fabrics with reactive dyes, viable reasons being evinced for the promotion of
most of the major ranges of reactive dyes. We are concerned here only with rationale in the
selection of auxiliaries; details of the many processing routes can be found elsewhere
[30,158–163]. Low substantivity at the padding stage is often preferred in order to minimise
tailing, but in fact dyes covering the whole range of reactivities can be used. Highly reactive
dyes can be fixed in shorter times at lower pH and are often easier to wash off, whilst those
of low reactivity offer greater stability in the pad liquor. The bifunctional types give highly
efficient fixation and excellent fastness performance [30].
Many processes require only dye and alkali; for example, on cotton equal concentrations of
dye and alkali over the range 5–30 g/l may be used with a pick-up of 60–80%, whilst on viscose
the pick-up is usually higher (90–100%) and the alkali reduced to half the quantity of dye.
With some aminochlorotriazine dyes both salt and alkali are used – perhaps up to 30 g/l
sodium chloride and 10–15 g/l alkali. In some processes sodium silicate is preferred as the main
alkali, in order to alleviate the problem of white selvedges. This fault can occur with sodium
hydroxide, caused by neutralisation of the sodium hydroxide by carbon dioxide or other acidic
atmospheric agents. A wetting agent is generally required in continuous processes to aid rapid
wetting at the padding stage. A hydrotrope such as urea may be added, particularly for deep
shades, to boost the solubility of less sulphonated dyes and to aid fixation through the
mechanism of retaining moisture by hydrogen bonding, particularly in fully continuous
thermofixation techniques. If a thickening agent is needed to minimise migration, sodium
alginate is preferred since it does not interact with reactive dyes; electrolyte addition may serve
the same purpose. An advantage of fully continuous procedures is that minimal quantities of
alkali and water are consumed. Two-stage continuous processes, comprising two wet-on-wet
paddings with the second pad applying the alkali, are applicable in certain circumstances.
The paramount importance of efficient preparation in producing goods of thorough and
uniform absorbency for continuous dyeing cannot be over-emphasised [157,158]; all
continuous dyeing systems are heavily dependent on this prerequisite. Typical build-up
curves on mercerised and unmercerised cotton fabrics in Figure 12.20 illustrate the
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REACTIVE DYES
867
significantly higher colour yields attained in a two-bath pad–steam process using sodium
silicate (variant B) with vinylsulphone dyes. Recommended variant A and B chemical pad
formulations are shown in Tables 12.13 and 12.14, for a steaming treatment of 30–60
seconds at 102–105 °C. In this process, important factors for the control of dye migration
include the lowest possible pick-up of dye liquor in padding, minimum addition of wetting
agent, sufficient electrolyte in the chemical pad liquor and selection of vinylsulphone dyes of
high substantivity. If a one-bath, pad–steam process is used, the recommended variant A
and B chemical formulations are as presented in Tables 12.15 and 12.16 respectively.
Steaming treatment is again 30–60 seconds at 102–105 °C.
Variant
B Bleached
100
and mercerised
A cotton
Relative colour yield/%
80
B Bleached
cotton
A fabric
60
40
20
20
40
60
80
100
Dye concentration/g l–1
Figure 12.20 Typical dye build-up curves for pad–steam process variants A and B on mercerised and
unmercerised cotton [158]
Table 12.13 Chemical pad additions for two-bath salt variant A[158]
Dye concentration (g/l)
<20
20–40
>40
Steaming time (s)
30
60
30
60
30
60
Sodium chloride or sulphate (g/l)
Sodium carbonate (g/l)
Caustic soda 32.5% (ml/l)
250
20
10
250
20
5
250
20
15
250
20
7.5
250
20
20
250
20
10
Table 12.14 Chemical pad additions for two-bath silicate variant B [158]
chpt12(2).pmd
Silicate
strength (°Be)
Mass ratio
Na2O:SiO2
Silicate
addition (ml/l)
Caustic soda
solution
32.5% (ml/l)
37–40
40–42
1:3.3
1:3.3
900
760
100
100
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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES
Table 12.15 Chemical formulation for one-bath salt
variant A[158]
30 g/l
20 g/l
5 ml/l
10 ml/l
Sodium chloride or sulphate
Sodium carbonate
Caustic soda 38°Be if <15 g/l Dye
Caustic soda 38°Be if >15 g/l Dye
Table 12.16 Chemical formulation for one-bath silicate
variant B[158]
95 ml/l Sodium silicate 37–40°Be
8 ml/l Caustic soda 38°Be (32.5%) if <15 g/l dye
15 ml/l Caustic soda 38°Be (32.5%) if >15 g/l dye
For the pad–batch fixation of heterobifunctional Cibacron C (Ciba) dyes four alkali
variants have been suggested [159]:
(1) Rapid fixation in 5–8 hours using a mixture of sodium hydroxide and sodium silicate, a
metering pump being necessary. A high concentration of alkali gives rapid fixation but
still allows excellent bath stability at temperatures up to 30 °C.
(2) Fixation in 6–12 hours with a mixture of sodium hydroxide and trisodium
orthophosphate, a metering device being necessary. This method is recommended for
regenerated cellulosic fibres. This formulation contains the same total amount of alkali
as method (1) with the same bath stability, but may be preferred where some buffering
capacity is required and sodium silicate is undesirable.
(3) A different combination of sodium hydroxide (reduced amount) and sodium silicate, a
metering pump not being necessary. This method demands a longer fixation time (12–
24 hours). Bath stability is greater than two hours but the total amount of alkali is
inadequate for application to grey goods, where the raw cotton consumes a great deal of
alkali.
(4) A mixture of sodium hydroxide and sodium carbonate, a metering pump being
necessary. This method avoids the use of either silicate or phosphate and is popular for
woven goods and in circumstances where silicate would pose problems. Ideally the
carbonate should be free from bicarbonate. This system has less buffering capacity and
gives slightly lower bath stability than methods (1) and (2).
Methods (1) and (2) are the most widely used because they offer the most reliable results in
bulk-scale processing.
In pad–batch dyeing with the highly reactive chlorodifluoropyrimidine Drimarene R or K
(Clariant) brands, it is equally possible to use either a weak alkali (sodium carbonate) for a
long batching time or a strong alkali (sodium hydroxide) for rapid fixation. It is claimed that
the versatility of monofunctional dyes of this established type makes the more expensive
bifunctional types unnecessary [160]. Detailed studies of each dye in the range led to the
generation of a series of three-dimensional graphs from which a computer-based optimised
system has been developed and made available to dyers.
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REACTIVE DYES
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The Levafix E/EA/EN (DyStar) system based on computer-centred optimisation is also
available [161]. Two main alkali formulations are involved:
(1) Sodium carbonate with or without sodium hydroxide
(2) Sodium silicate with or without sodium hydroxide.
In general the prospect of working without silicate is regarded as a major advantage because
the padding and washing-off of sodium silicate places considerable stresses on the machinery
and on effluent treatment. However, the use of silicate in no way impairs the performance of
Levafix dyes. Six optimised alkali recipes for this system are shown in Table 12.17, whilst
Table 12.18 shows the relation between pad liquor pH, batching time and pad liquor
stability.
Continuous dyeing with Sumifix Supra (NSK) heterobifunctional dyes is claimed [162] to
give reproducible dyeings exhibiting a high degree of fixation and good wet fastness. The
dyed fabric has an attractive handle and good appearance. The high fixation contributes
advantageously to low effluent loading. Typical pad liquor formulations and fixation
conditions, in comparison with those for aminochlorotriazine and vinylsulphone dyes, are
given in Table 12.19.
There is a general tendency to abandon the ‘classical’ continuous reactive dyeing
processes in favour of shorter pad–dry processes that is mainly driven by environmental and
economic factors [163,164]. Particularly pertinent to this trend is the marked decrease in
average length of run to a given shade. The latest developments in dyeing machinery allow
greater control of the dyeing parameters and eliminate the former need for lengthy and
Table 12.17 Optimised alkali recipes for Levafix (DyStar) dyes at various applied depths [161]
chpt12(2).pmd
Recipe
Dye conc (g/l)
A
Sodium carbonate (g/l)
pH 11.3
5
B
Sodium carbonate (g/l)
Caustic soda 45% (ml/l)
pH 11.5–12.1
C
Sodium carbonate (g/l)
Caustic soda 45% (ml/l)
pH 11.8–13.2
D
Sodium carbonate (g/l)
Caustic soda 45% (ml/l)
pH 12.6–13.5
E
Sodium silicate 37°Be (g/l)
Caustic soda 45% (ml/l)
pH 11.5–13.0
40
2.0
F
Sodium silicate 37°Be (g/l)
pH 11.0
25
869
5
10
20
40
60
10
20
40
10
0.2
20
0.35
20
0.65
20
1.3
20
2.15
10
0.35
20
0.65
20
1.3
20
2.6
20
4.0
20
5.3
20
7.9
17
2.0
13.5
4.3
10.5
6.3
7
8.6
4
10.6
40
3.3
40
5.6
80
8.9
100
11.7
100
13.2
100
14.9
25
25
40
15/11/02, 15:47
80
120
870
AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES
Table 12.18 Relation between pad liquor pH, batching time and pad liquor stability for five optimised
alkali recipes and Levafix (DyStar) dyes [161]
Batching time (hours)
Recipe
pH
Pad liquor
stability (min)
Bleached
cotton
Mercerised
cotton
Sodium carbonate
without caustic soda
A
11.3
142
21
14
Sodium carbonate
with a little caustic soda
B
11.9
41
14
8
Sodium carbonate with
increasing caustic soda
C
12.2
18
10
6
Increasing caustic soda,
decreasing sodium carbonate
D
12.7
8
5
3
Sodium silicate with
increasing caustic soda
E
12.0
37
5
3
Table 12.19 Pad liquor formulations and fixation conditions for application of monofunctional and
heterobifunctional reactive dyes by three continuous dyeing methods [162]
One-bath pad–thermofix
Reactive
system Temp. / time
ACT
200 °C 30 s
One-bath pad–steam
Two-bath pad–steam
Alkali
Temp. / time
Alkali
Temp. / time
Alkali
Na2CO3
10–20 g/l
103 °C 5–10 min
Na2CO3
10 g/l
103°C 60 s
NaOH
40°Be
20 ml/l
NaCl
300 g/l
103 °C 3–8 min
Na2CO3
6–12 g/l
103°C 30 s
NaOH
40°Be
20 ml/l
NaCl
250 g/l
103 °C 1–5 min
NaHCO3
10–20 g/l
103°C 30–60 s NaOH
40°Be
10 ml/l
Na2CO3
20 g/l
NaCl
100–150 g/l
Urea
100–200 g/l
VS
150 °C 60 s
Na2CO3
10–30 g/l
Urea
50–100 g/l
ACT-VS
180 °C 30–60 s Na2CO3
10–20 g/l
Urea
50–100 g/l
ACT
VS
chpt12(2).pmd
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Vinylsulphone
870
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REACTIVE DYES
871
expensive conditioning of machinery at the beginning of each run. Liquor wastage is a major
factor both economically and environmentally in continuous dyeing. Downtimes up to 60
minutes and waste liquor volumes up to 120 litres have been typical for traditional long runs
to a shade [164]. Developments have reduced downtimes to as brief as 5 minutes and
computerised systems inform management of the quantity and composition of pad liquors so
that wastage is minimised to about 20 litres. By adopting pad–dry–bake or simple pad–dry
methods chemical padding and steaming stages are eliminated, thus benefitting the
environment by avoiding the demand for large amounts of salt.
A typical development in this area is the Econtrol (Monforts and Zeneca) system
[163,164], which consists of padding with dye, 1–2 g/l wetting agent and the alkali. With
high–reactivity dichlorotriazine dyes this can be sodium bicarbonate (10 g/l). No urea,
sodium silicate, electrolyte or other chemicals are required. After a short air passage the
uniformly squeezed fabric is passed through a hot flue in which the carefully controlled
relative humidity (25% moisture by volume) brings about fixation in two minutes. The
humidity fulfils the function that otherwise would be provided by an environmentally
problematical hydrotrope such as urea. This reduction of chemical consumption is indeed a
major economic and environmental benefit.
Acrylamide-based migration inhibitors are claimed to give more efficient fixation [165].
A comparison of the differences between cotton and viscose has revealed [157] that viscose
requires significantly longer immersion times. For example, immersion time for thoroughly
prepared cotton can be less than one second on today’s high-speed ranges, whereas 1–1.5
seconds is preferable for viscose. Contrary to some views, the addition of urea does not
shorten immersion times and can in fact lengthen them [157].
Printing
Similar general considerations apply to direct printing processes as to continuous dyeing. A
single all-in or a two-stage, pad–steam process may be used. Alginates are the preferred
thickeners because other carbohydrates react with the dyes. The non-reactivity of alginates,
in spite of their hydroxy groups, is thought to be due to the presence on each mannuronic
unit of a carboxyl group that tends to repel the dye anions. As discussed in section 10.8.1,
difficulties in obtaining alginates have led to the evaluation of alternatives, in particular the
synthetic anionic poly(acrylic acid) types that give higher colour yields but can be more
difficult to wash off. These products have been slow to replace the alginates apparently
because the synthetic acrylic thickeners show varying sensitivity to electrolytes. However,
detailed comparative rheological studies [166] have indicated that even if electrolyte
sensitivity were controlled the acrylic polymers might still prove unsuitable despite the
greater fixation that they give. At the slow printing speeds necessary in these laboratory
studies the amount of paste absorbed by the fabric was always greater with the synthetic
thickener, but this effect did not give higher colour yields because penetration was greater. It
was concluded, therefore, that synthetic thickeners should not be adopted because of the
higher dye consumption that outweighs any price advantage offered by the synthetic
thickener. This is expected to be even more relevant at the high printing speeds necessary in
commercial production, because of the greater shear-thinning of the synthetic thickener
under these conditions of application [166].
Emulsion thickenings, either oil-in-water or water-in-oil and including half-emulsions,
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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES
have been important in the past but are now much less popular on environmental grounds.
Hydrotropes and a mild oxidising agent such as sodium m-nitrobenzenesulphonate are
commonly added, the latter to protect azo dyes from reductive decomposition. Choice of
alkali depends on the reactivity of the dye. Sodium bicarbonate is usually preferred, being
cheap and offering high print paste stability with all but the most reactive dyes, but for dyes
of sufficiently high stability the stronger alkalis (sodium carbonate or sodium hydroxide) may
be chosen to provide higher colour yields under more alkaline conditions. The amount of
bicarbonate may be reduced with highly reactive dyes. A pH-shift agent such as sodium
trichloroacetate can be selected; this hydrolyses during steaming to release sodium
carbonate (Scheme 12.5). However, another product of this hydrolysis reaction is
chloroform. As well as being a volatile AOX-generating compound, chloroform is
hepatotoxic and its use may be severely restricted.
Cl
2 Cl
C
Cl
Cl
O
+ H2O
C
ONa
2 Cl
CH
+ Na2CO3 + CO2
Cl
Scheme 12.5
In the two-stage process the dyes are first applied without alkali, using a thickening agent
such as sodium alginate that gels on subsequent application of alkali, usually together with
electrolyte. High concentrations of sodium silicate (Na2O/SiO2 = 1:2.1, 47°Bé) or mixed
alkali solutions (for example, 185 g/kg sodium carbonate + 185 g/kg potassium carbonate +
30 g/kg sodium hydroxide 32.5% or 38°Bé) are often used.
Undoubtedly of greatest concern in the printing of cellulosic fibres with reactive dyes is the
essential demand for large quantities of the hydrotrope urea (typically 80–200 g per 1000 g of
print paste). This auxiliary forms hydrogen bonds with water molecules, assisting in swelling of
the fibre and the thickener, dissolution of the dyes and promotion of interaction between dye
and fibre [167]. The most obvious benefits provided by urea are improved levelness, diffusion
and colour yield but this compound is now environmentally suspect. Various attempts to
replace urea over the last decade have led to contradictions, ranging from claims of complete
success to the viewpoint that it has not yet been possible to find a satisfactory alternative and
the prognosis for doing so is not particularly good.
Claims have been made [168] that the two-phase flash age process is the only way to
avoid urea problems. In this process, no urea or alkali is present when printing is carried out
initially, followed by padding with alkali and perhaps electrolyte (to limit migration) prior to
steaming. However, even this approach has its ecological drawbacks due to the alkali (which
may include silicate) and electrolyte. Furthermore, not all dyes give a satisfactory response,
in particular phthalocyanine turquoise blues [167]. It is possible to improve the versatility of
the process by including small amounts (up to 50 g/l) of urea; whilst this does not attain the
environmental objective of totally avoiding urea, it does give an improvement over more
conventional procedures. Suitable recipes are given in Table 12.20.
In conventional processes, the moisture content of the fabric at the fixation stage is about
20% or even less. However, in the flash age process it is significantly greater than this and
the extent to which urea usage can be minimised is closely related to the amount of
moisture present. Whilst traditional flash agers operated at around 50% humidity, more
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REACTIVE DYES
873
Table 12.20 Print paste and fixation pad liquor formulations
for two-phase flash age printing with reactive dyes [29]
Print paste recipe
Urea
Sodium m-nitrobenzenesulphonate
Sodium alginate thickener
Water
Fixation pad recipe
Potassium hydroxide
Sodium carbonate
Sodium hydroxide 38°Be
Water
0–50
10
400–500
440–590
1000
g
g
g
g
g
185
185
30
600
1000
g
g
g
g
g
recent attempts to eliminate urea have seen increases in this level. For example, the
inclusion of humidifying or moisturising equipment in front of an ager to increase moisture
content by 30%, aided by adjustment of the viscosity of the thickener, gave satisfactory
yields in the absence of urea, even with phthalocyanine turquoise blues [167]. One means of
increasing moisture content in the single-phase system of printing is to pre-wet by means of
foam [169]. This enables the complete elimination of urea, giving additional advantages of
better flow properties of the print paste, shorter fixation times, reduced water consumption
and savings of thickener. In another approach, the concentration of urea could be
considerably reduced by minor additions to the print paste of cyclodextrin, chitosan
derivatives or so-called ‘superabsorbers’ based on acrylates [170].
However, after careful consideration of the various roles played by urea, it was concluded
that the prospects of finding an adequate substitute are not promising [171]. This echoes
earlier views [172] resulting from a study of the fixation of reactive prints with various
concentrations of urea by saturated (102 °C) or high-temperature (120 °C) steaming. It is
important to remember that urea functions not just by attracting moisture but also by
entering into various interactions with dyes, thickener and substrate. As regards
environmental aspects, the use of print paste preservatives, as discussed in section 10.8.6,
should be borne in mind.
Conventional reductive discharge prints are applicable on grounds dyed with reactive
dyes, especially if vinylsulphone dyes have been used, but there are difficulties with those
non-azo blue and turquoise dyes that cede preference to resist processes [29]. Resists can be
achieved by printing with a thickened paste containing a non-volatile acid (tartaric or citric
acid, for example) or with an acidic salt such as sodium dihydrogen orthophosphate. The
thickening agent used must be stable to acid; hydroxyethyl or methoxyethyl cellulose ethers,
locust bean gum or tragacanth are suitable. The pre-printed and dried resist areas are then
overprinted or padded with a solution of a highly reactive dye together with the minimum
amount of sodium bicarbonate.
The washing-off process
An especially important and critical aspect of the application of reactive dyes, whether by
dyeing or printing, is the washing-off process necessary to remove unfixed or hydrolysed dyes,
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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES
as well as other products such as alkali, electrolyte and thickening agents. After rinsing to
remove the more readily soluble products such as alkali and electrolyte, the goods are soaped
at a temperature close to the boil to remove the unfixed active or hydrolysed dyes; a high
temperature is necessary to remove hydrolysed dye from the interior of the fibres. Surfactants,
especially nonionic types, are often added to the soaping liquor and a sequestering agent is
added if the water is hard. Where thickening agents have been used, sufficient time must be
allowed during the initial rinsing for the thickener to become hydrated and easier to remove.
This seemingly straightforward washing-off process has enormous environmental impact
and has generated much research over the last few years. Washing-off accounts for as much
as 50% of the processing cost of a reactive dyeing in terms of the washing-off process itself
and subsequent treatment of the effluent. In addition to these environmental and economic
consequences, the efficiency of washing-off has a critical influence on the fastness levels
attained. Herein lies the primary rule: the extent of the washing-off sequence should be just
sufficient to achieve the target fastness properties, since any further washing treatment only
leads to unnecessary expenditure and greater effluent volumes. Seven parameters have been
listed [173] as pertinent to the effects and efficiency of washing-off:
(1) number of washing baths
(2) duration of washing, varying in practice from a few seconds (continuous) to many
minutes (batchwise)
(3) temperature, varying from rinsing at about 20 °C up to soaping at the boil, or higher in
special circumstances
(4) liquor ratio, varying from 5:1 (continuous) to 100:1 (batchwise)
(5) mechanical movement of liquor and/or goods is of major importance in influencing
liquor interchange
(6) composition of wash liquor: the electrolyte and chemical content of dyeings varies
widely and these contaminants, together with any auxiliaries added to the washing
baths, will considerably influence the result
(7) the substantivity of the reactive dyes present markedly influences their washing-off
behaviour.
As a general illustration of the main effects, Table 12.21 shows the effect of a single washing
treatment of varying time and temperature on an exhaust dyeing of the monoazo J acid
scarlet CI Reactive Red 123 and a continuous dyeing of the copper formazan CI Reactive
Blue 104. Both dyes contain a chlorodifluoropyrimidine reactive group, but consistently
more dye was removed from the continuous dyeing, especially for shorter times at the lower
temperatures. Even after six successive washes at 25 °C, the removal of unfixed dye from the
exhaust dyeing only increased to 56, 62 and 74% respectively from the 24, 36 and 52%
shown for one wash [173]. In general, these results emphasise the importance of longer
times and higher temperatures. Further results demonstrated that electrolyte additions up to
10 g/l did not markedly influence the removal of dye but showed significantly increased
effects at 20 g/l and especially 50 g/l. Mild alkaline washing was just as effective as mild
acidic washing and the addition of a nonionic agent had a favourable influence on dye
removal. These results are specific to the two dyes evaluated. However, it is abundantly clear
from these trends and comments, bearing in mind the wide variety of reactive dyes,
substrates and machinery, that optimising the washing-off procedure in each case for
maximum economy and minimum environmental impact is no easy matter.
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Table 12.21 Effect of a single washing treatment for various times at various
temperatures on the removal of unfixed dye from exhaust or continuous dyeings
with chlorodifluoropyrimidine reactive dyes [173]
Removal (%) of unfixed dye from exhaust or continuous dyeing
Washing
Exhaust dyeing of
CI Reactive Red 123
on bleached cotton
Continuous dyeing of
CI Reactive Blue 104
on mercerised cotton
Temperature
25 °C
70 °C
98 °C
25 °C
70 °C
98 °C
Time 15 s
1 min
5 min
24
36
52
64
74
90
84
88
92
38
52
74
80
90
92
86
90
94
These difficulties have been thoroughly discussed in an exhaustive study [174].
Theoretical considerations were thoroughly explored and the economic and technical merits
of four different washing systems were analysed:
(1) individual washing baths
(2) dyeing machines operating with continuous liquor exchange
(3) continuous rope-washing machines
(4) open-width washing machines.
Three key stages applicable to most washing processes were identified:
(1) In the first stage, unfixed dyes, salt and alkali present in the liquor phase must be
removed and this is best done by replacing this liquor with fresh water. Sorption,
desorption and diffusion processes play only subordinate roles in this stage, the key
factors being liquor flow, mechanical action and liquor exchange. The dilution laws are
generally applicable.
(2) In the second stage, substantial amounts of alkali and unfixed dyes are desorbed and
diffuse from the fibre pores into the liquor phase. This is the diffusion stage and the
rate-determining step is diffusion of the labile dye molecules out of the fibre phase.
This takes time and is accelerated by higher temperatures and perhaps by mechanical
agitation of the substrate.
(3) In the final stage, the electrolyte and unfixed dye concentrations are low but further
changes of liquor must take place until almost all the unfixed dye molecules are
desorbed.
These three processes are not necessarily distinct consecutive stages but are major
identifiable events that may be more or less separated or interlinked by transitional steps. It
is argued that the washing-off requirements are different in each stage. In the first stage of
liquor exchange, cold rinsing is preferred because higher temperatures offer only a slight
advantage and alkalinity may hydrolyse dye–fibre bonds. Empirically, this stage may be
regarded as completed when the electrolyte content reaches about 1 g/l; the number of
liquor changes required is related to liquor ratio. A shorter liquor ratio demands more
changes and involves only slightly lower water consumption, so is without advantage at this
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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES
stage. Continuous methods are more effective, since these give direct displacement of
contaminated liquor by fresh water, rather than mixing of them. In the second stage of
diffusion from the fibre, hot washing is necessary to ensure the major advantage of more
rapid diffusion. Individual dye properties, such as substantivity and diffusion coefficient, are
particularly pertinent here. In contrast to earlier conclusions [173], these studies involving
several surfactants showed no particular advantage from the use of detergents [174]. The
addition of a detergent is best avoided if possible, since it further contaminates the effluent.
It is useful to compare the economics and fastness performance, particularly in heavy
depths, of a thorough washing-off against a less than complete extraction of unfixed dyes
followed by a cationic aftertreatment.
To minimise effluent problems by treatment of liquors before discharge to effluent, it is
preferable to concentrate the residual dyes in as small a volume as possible. This implies that
removal of unfixed dyes should be restricted to as few baths as possible. In this context dye
substantivity is an important consideration [175]. Cold initial rinsing removes much of the
low-substantivity dye present but dyes of high intrinsic substantivity are difficult to remove
in this way. If the initial rinse is hot, however, even these high-substantivity products are
effectively removed, thus achieving concentration of these dyes in the initial stage and
ensuring easier subsequent removal. Thus it was proposed that washing-off could be
improved by using a hot rather than a cold initial wash [175]. This was substantiated in
further optimisation studies which enabled the residual dyes to be confined to two or three
baths, surfactant additions not being necessary for these water-soluble contaminants. These
results were confirmed in bulk processing. It was found that lower liquor ratios were
beneficial in washing-off [176] and similar control parameters were identified in continuous
washing-off [177]. The benefits of hot initial washing (60–70 °C) have been demonstrated
in yarn package dyeing, emptying of the vessel after dyeing being unnecessary [178].
From the viewpoint of energy conservation, the washing temperature should be chosen
according to the dyes present [179]. Since dyes of low substantivity are desorbed more
readily under mild conditions, a lower temperature should be selected for these dyes and
higher temperatures for dyes of higher substantivity. In an evaluation of the washing-off
behaviour of dichlorotriazine dyes from cotton at temperatures ranging from 20 to 98 °C
with water alone, and at 98 °C with a surfactant, the surfactant-aided method was found to
be the most effective [180] and the benefits of aftertreating with cationic agents were
confirmed.
A detailed comparison [181] of three vinylsulphone dyes included a low-substantivity
monoazo N-acetyl H acid derivative (CI Reactive Red 35), a monoazo N-acetyl J acid type
of higher substantivity (CI Reactive Orange 82) and a phthalocyanine turquoise somewhat
prone to aggregation (CI Reactive Blue 21). Dyeings of these individual products were
subjected to three wash-off procedures:
(1) conventional sequence: cold, cold, warm, hot, cold
(2) cold/hot sequence: 30 °C, followed by several baths at 95 °C
(3) hot sequence: repeated treatments at 95 °C.
Typical results are given in Table 12.22. The objective was to examine the relationship
between the substantivity of the unfixed dyes and their response to changes in the
conditions of the washing sequence.
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Table 12.22 Effects of various washing–off sequences on the rate of desorption of dye
in successive washing baths and the degree of staining of adjacent material in washing
tests [181]
*Colour value of wash liquor
Fastness to washing
(1)
(1)
(2)
(3)
CI Reactive Red 35 (low substantivity) W = 510 nm
Washing bath
1
16
15
18
2
2
2
1
3
2
1
0
4
1
1
0
2
3
4
4–5
2
4
4–5
4–5
3
4
4–5
4–5
CI Reactive Orange 82 (moderate substantivity) W = 490 nm
Washing bath
1
17
23
56
2
10
31
4
3
11
2
0
4
13
0
0
5
2
0
0
1
1–2
2
3–4
4–5
1
3
4
4–5
4–5
2
3–4
4–5
4–5
4–5
CI Reactive Blue 21 (phthalocyanine type) W = 664 nm
Washing bath
1
14
15
33
2
3
15
6
3
2
2
1
4
6
0
0
5
3
0
0
2
2–3
2–3
3
4
2
3
4
4–5
4–5
2–3
4
4–5
4–5
4–5
Washing sequence
(2)
(3)
* Colour value given by 1000 E/d, where E is extinction at the peak wavelength (W nm) and d mm is
the cell thickness
The low-substantivity red dye was desorbed relatively quickly, mainly in the initial cold
rinse, and the acceptable fastness rating of 4–5 was reached after three or four washing
baths. The greater difficulty of desorbing the more substantive orange dye was clearly
evident. Only after the first hot bath of the sequence was most of the unfixed dye desorbed.
A fastness rating of 4–5 was reached after the 5th, 4th and 3rd baths respectively of the
washing sequences (1), (2) and (3). With CI Reactive Blue 21 seven baths were needed in
the conventional sequence (1) to give a fastness rating of 4–5. Sequences (2) and (3) were
more effective, giving acceptable fastness after four and three baths respectively. In all cases
it is the hot washing sequence (without surfactant) that enables this target to be reached
quickly and with the lowest water consumption. It was confirmed that the final bath need
not be absolutely colourless after washing, nor is complete removal of unfixed dye necessary
to attain the target fastness level.
Similar results have been observed in the washing-off of reactive prints, in which it is
necessary to monitor the removal of the alginate thickener as well as desorption of the
unfixed dyes [182]. Batchwise washing showed that the thickener was rapidly washed out
and that elevated temperatures increased the rate of removal. No significant dwell time for
thickener swelling seemed to be necessary. Significantly more effective removal of thickener
with increasing temperature was observed in continuous washing and most of the thickener
was eliminated in the first wash.
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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES
Confirmation of earlier work [180] on the temperatures of washing-off of dichlorotriazine
reactive dyeings on cotton revealed that surfactant and sodium carbonate together were
more effective than water alone, although there was little difference in performance between
the six surfactants examined [183]. However, the addition of surfactant did not appear to be
essential in order to achieve adequate dye desorption and fastness to washing, 2–5 g/l
sodium carbonate being sufficient. These results were later substantiated with
dichlorotriazine dyeings on lyocell [184] and for aminochlorotriazine dyeings on cotton
[185].
Further studies involved different alkalis (ammonia, sodium bicarbonate, potassium
hydroxide and sodium hydroxide) and unspecified buffers covering the range pH 7–13 with
aminochlorotriazine dyeings on cotton. Although all these alkalis and buffer solutions were
capable of removing unfixed dyes, the effectiveness of washing-off of the systems varied;
potassium hydroxide and sodium bicarbonate were the most effective and the pH 8 buffer
solution was the least. In general, effectiveness increased with increasing pH but the
outstanding behaviour of potassium hydroxide and sodium bicarbonate could not be
explained in terms of pH alone. Despite varying effectiveness in terms of washing-off, all of
the washed dyeings were virtually identical in fastness to washing. When multiple alkaline
washes were evaluated on aminochlorotriazine dyeings, potassium hydroxide and sodium
carbonate were the most effective, although sodium bicarbonate was more attractive in
terms of low cost, environmental impact and fastness to washing. However, some of these
multiple alkaline treatments showed evidence that dye–fibre bond cleavage may have taken
place in addition to removal of hydrolysed dye [185].
When washing-off in a jet machine, a combination of liquor exchange and continuous
overflow rinsing is advantageous [186]. Optimal rinsing procedures depend on machine
parameters and the cost structure of the plant, but in general the most economical system
appears to be:
(1) liquor exchange rinsing initially (salt removal)
(2) continuous rinsing at a higher temperature (diffusion of unfixed dye)
(3) liquor exchange rinsing finally (removal of dye liquor).
The objective of stage (1) is to lower the salt concentration to 1–2 g/l and this requirement
determines the number of initial baths given before subjecting the dyeing to a higher
temperature. Salt removal is accelerated by liquor changes and dilution before commencing
continuous rinsing. The duration of continuous rinsing is adjusted according to the depth of
shade and the known diffusion properties of the dyes present. Calcium and magnesium ions
present when washing in hard water make the unfixed dye anions more difficult to remove;
poly(acrylic acid) derivatives are effective sequestering agents in mildly alkaline liquors
[187].
Stripping
The stripping of cellulosic materials dyed with reactive dyes is carried out by alkaline
reduction followed by hypochlorite oxidation, preceded by a boiling treatment with EDTA if
metal-containing dyes have been used. For example, a treatment with 5 g/l sodium
carbonate or sodium hydroxide and 5g/l sodium dithionite at the boil is followed by a
treatment in 0.5–1 °Tw hypochlorite, an antichlor and thorough rinsing.
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12.7.2 Wool
Although wool can be dyed with typical reactive dyes produced essentially for cellulosic
substrates, the adoption of reactive dyes on wool has depended on the development of
special ranges of dyes capable of covalent bonding under slightly acidic conditions. The use
of surfactant auxiliaries is essential. The types of dyes used and the mechanisms of reaction
have been discussed in section 7.7 and elsewhere [2]. These dyes are generally similar in
response to dyebath pH to milling acid dyes, medium or full depths being applied at pH 5.0–
5.5 and pale depths at pH 5.5–6.0. At lower pH values adsorption and fixation may be
irregular, whilst at higher values exhaustion is poor. Even at optimal pH fixation may still be
incomplete within normal dyeing times, particularly when dyeing navy or black shades.
Hence the bath is adjusted to a weakly alkaline state, which ensures complete fixation as
well as helping to remove hydrolysed dye. A typical procedure is to dye at the boil and pH 5,
followed by cooling to 80 °C and adjustment of the pH to 8.0–8.5 with ammonia, after
which treatment is continued for some 20–30 minutes at 80 °C.
A surfactant auxiliary is necessary to prevent tippy or skittery dyeing. Cationic or nonionic
products have been used but the most useful have been the amphoteric N-alkylbetaines and
alkylamidobetaines (described in section 9.7) and ethoxylated amphoteric types represented by
structure 12.41 [188]. Compounds of this type can interact under slightly acidic conditions
with both fibre and dye. The mechanisms have been investigated in detail [188–192]. The
initial step appears to be the formation of an auxiliary–dye complex by interaction between the
quaternary group in the surfactant and the dye anion, such a complex being less soluble than
the dye alone. As the concentration of the auxiliary is increased to an excess over that
required for complexing with the dye anions present, surfactant micelles are formed that tend
to solubilise the auxiliary–dye complex. Unlike most levelling agents these amphoteric
products often increase the rate of wool dyeing, to an extent dependent on the chemistry and
concentration of the auxiliary and of the dye. Conversely, they may have a retarding effect if
the concentration of auxiliary is so high as to solubilise the auxiliary–dye complex completely.
These auxiliaries also tend to improve dye exhaustion at equilibrium.
H3C(H2C)10H2C
(CH2CH2O)m
_
SO3 NH4+
(CH2CH2O)n
H
N
+
H3C
_
CH3SO4
12.41
(m+n = 10–80)
The accepted explanation for this behaviour is that the zwitterionic auxiliary–dye
complex is less electronegative than the dye anion; hence it exhibits more hydrophobic
characteristics with increased molecular size and lower aqueous solubility. Consequently, the
affinity of the dye for the undamaged roots of the wool fibres is enhanced relative to that for
the more hydrophilic damaged tips. The auxiliary–dye complex is less sensitive to root-tip
differences and thus gives more level dyeing. Furthermore, adsorption of the auxiliary by the
fibre increases the electropositive charge on the fibre, thus increasing the attraction for
anionic dyes. This will tend to further increase the rate of dyeing and is also the mechanism
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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES
whereby the total amount of dye absorbed is increased. On the whole, there is little evidence
that these agents increase migration and indeed no migration can take place once the dye
has reacted covalently with the fibre.
The levelling action of betaines increases with concentration and with increasing length
of the alkyl chain [188]; the effects on rate and total absorption disappear with products
containing a very long alkyl chain (about C16) at a high concentration, although the
levelling properties are maintained irrespective of the rate effects. In general, the betaines
bring about a greater change in sorption characteristics than do the ethoxylated amphoteric
compounds of similar alkyl chain length [188]. Thus higher concentrations of the latter are
necessary to produce a similar effect; this can be an advantage, since the effects are not then
so critically sensitive to the concentration of auxiliary. Furthermore, the manufacturer can
vary the balance of properties of the auxiliary by varying the length of the oxyethylene
chains as well as of the alkyl group.
Using laser Raman spectroscopic techniques, it was confirmed that the primary
mechanism involves ionic interaction between the anionic sulphonate groups of the dye and
the cationic quaternary group of the auxiliary, although hydrophobic interactions were also
significant [190]. However, investigations of the coverage of damaged wool indicated that
interaction of the auxiliary with the fibre is mainly responsible for the improvement in
levelness, both the surface and the interior of the fibre being involved [191]. Dye–auxiliary
interaction does not seem to play a major role in levelling out root-to-tip variations. Studies
with variously modified wools, including wool pretreated with an amphoteric agent,
demonstrated that these auxiliaries mainly accelerate the rate of exhaustion on normal wool,
whereas the effects on modified wools are minimal [192]. Although these agents promote
dye uptake and fixation on normal wool, they do not enhance dye uptake to the same extent
on Hercosett-treated wool [2].
Traditionally, the alkaline treatment given after reactive dyeing has been with ammonia
at pH 8.0–8.5 and this is still the favoured method of removing unfixed dyes, although
sodium bicarbonate is occasionally used. Indeed, it is claimed that the lower basicity of
bicarbonate results in less fibre damage and no detrimental effect on fastness properties
[193]. A specific disadvantage of ammonia is that it can cause uneven treatment in different
parts of a yarn package because of irregular swelling of the fibre [2].
Several other agents have been suggested as potential aftertreating agents for reactivedyed wool:
(1) Hexamethylenetetramine: this ammonia precursor does not cause fibre swelling and the
unfixed dyes are removed efficiently at pH 6.5, compared with pH 8.5 with ammonia,
thus causing less damage to the wool. However, the hydrolysis of this compound
(Scheme 12.6) results in the formation of formaldehyde and this can modify the hue of
certain dyes [2].
(2) Sodium bisulphite: this additive reacts by nucleophilic addition to the vinylsulphone
group of dyes of this type, decreasing the substantivity and increasing the aqueous
solubility of the unfixed dyes (Scheme 12.7).
(3) A commercially branded product believed to be sodium trichloroacetate: this is added
at a concentration of 5% to the dyebath 20 minutes before the end of dyeing. Sodium
carbonate is formed by hydrolysis of the trichloroacetate (Scheme 12.5), accompanied
by a change of pH from 5.0–6.0 to 6.7–6.9 [2]. This reaction also releases the volatile
AOX-generating chloroform, however.
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REACTIVE DYES
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(4) Various amine salts, such as hydrochlorides of 1,6-diaminohexane, 1,12-diaminododecane, 1-aminododecane and a bisulphate adduct of 1,6-di-isocyanatohexane. This
was an attempt to introduce a final reactive step to form crosslinks between unfixed dye
molecules on the fibre [193].
N
H2C
CH2
H2C
4 NH3 + 6 HCHO
N
CH2
Scheme 12.6
[dye]
100 °C
+ 6 H2O
N
H2C
N
CH2
SO2CH
CH2
+ NaHSO3
[dye]
SO2CH2CH2SO3Na
Scheme 12.7
Although all of these compounds were applicable in principle, treatment with sodium
bicarbonate offers a much more economical and reliable option.
The mechanism of reaction of α-bromoacrylamide dyes with wool has been investigated
[194]. The application to wool of a range of heterobifunctional dyes normally used on
cellulosic fibres has been promoted recently [195]. These dyes offer high exhaustion and
fixation, as well as exceptional fastness. They are relatively insensitive to dyeing variables
and suitable for dyeing wool–cellulosic blends by a one-bath method, with savings in energy
costs and less wool damage. The choice of auxiliaries is essentially the same as when
applying these dyes to cotton, although the pH is varied according to whether wool or wool–
cotton is to be dyed. An ammonia aftertreatment is recommended for wool dyeings. The
reactive dyeing of wool at 110 °C and pH 4.5 (ammonium sulphate and acetic acid) in the
presence of a betaine auxiliary has been described [196].
Reactive dyes can be applied to wool by printing [2,29]; suitable ranges include
chlorodifluoropyrimidine, α-bromoacrylamide, sulphatoethylsulphone and aminochlorotriazine. A typical print paste recipe is shown in Table 12.23. Derivatives of locust bean or guar
gum, either alone or in combination with water-soluble British gum, are the preferred
thickeners. Humectants, particularly urea, are essential to aid solubilisation and penetration as
well as swelling of the wool. A wetting agent (5–10 g/kg) may also be needed if chlorination of
the goods has been less than optimum and where complete penetration of the print design is
desirable. Antifoam addition is generally necessary in machine printing. A non-volatile acid
such as citric acid or an acid donor such as ammonium tartrate or ammonium sulphate is
normally required to give optimum pH conditions; however, vinylsulphone dyes are applied
under neutral to slightly alkaline conditions using sodium acetate (40 g/kg). The oxidising
agent sodium chlorate is often added to counteract the reducing effect of the wool, although if
vinylsulphone dyes are printed under alkaline conditions sodium m-nitrobenzenesulphonate is
effective in protecting any reduction-sensitive dyes.
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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES
Table 12.23 Typical print paste recipe for the
printing of wool with reactive dyes [29]
Dye
Urea
Thiodiethylene glycol
Thickener (10–12%)
Acid or acid donor
Antifoam
Water to
xg
50 g
50 g
500 g
10–30 g
1–5 g
1000 g
The most important reducing agents used for discharge printing on wool are the
formaldehyde-sulphoxylates [2]. These agents are restricted to the minimum effective
concentration within the range 30–180 g/kg. The print is washed in several baths containing
2 g/l disodium hydrogen orthophosphate, ammonia (to give pH 9) and an anionic surfactant
in successive baths at temperatures increasing from 40 to 80 °C. The printing of silk with
reactive dyes, using sodium silicate as alkali and sodium alginate as thickener, has been
described [197]. Problems arising from the contamination of effluents with reactive dyeing
and printing auxiliaries represent an important issue that has been fully discussed elsewhere
[198–201].
12.8 SULPHUR DYES
The application of sulphur dyes to cellulosic substrates involves conversion to the
substantive leuco form by alkaline reduction, followed by oxidation on the fibre to the
insoluble disulphide form (section 1.6.2). Consequently, reducing and oxidising agents are
essential auxiliaries. Secondary auxiliaries include wetting agents, sequestering agents,
antioxidants, electrolytes and hydrotropes. Useful reviews of developments in sulphur dyes
and their application are available [30,202,203]. The concern here is with the essentials of
the auxiliary agents used in their application. It is convenient to deal separately with the
aspects of reduction and oxidation. The discussion is confined to application by dyeing since
the use of sulphur dyes in printing nowadays is restricted mainly to sulphur blacks applied by
techniques similar to those used in vat printing [29].
12.8.1 Reduction
Sulphur dyes in the insoluble disulphide form and the CI Solubilised Sulphur brands are
reduced by the dyer as part of the application procedure. In the case of the CI Leuco
Sulphur brands reduction has already been carried out by the manufacturer, so that they are
substantially in a form suitable for immediate application (section 1.6.2) The chemistry of
the reduction of sulphur dyes is complex, as is the chemistry of the dyes themselves; it has
been well described elsewhere [204]. It is possible to describe the state of a reduced sulphur
dye in alkaline sulphide or polysulphide solution by the general formula 12.42, but there are
certain complications. In many cases the chromogen is not itself reduced, but in others,
notably reddish browns, blues and navy blues based on indophenols, the chromogenic
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883
quinonimine grouping can be reduced (Scheme 12.8). Additionally, the value of n in
structure 12.42 varies, even for a given dye. It is not surprising, therefore, that the amount
of reducing agent (and alkali) required varies from dye to dye, depending on the chemistry
of the dye as well as on the concentration in the formulation. Hence the manufacturer’s
literature must always be consulted for the amounts of auxiliaries to be used with particular
dyes in the various batchwise, semi-continuous and continuous processes.
D
Sn SX
12.42
m
D = chromogen
n = 0–5 m ≥ 1
X = H or Na, depending on pH
reduction
N
O
oxidation
H
N
OH
Scheme 12.8
The reduction process is invariably carried out in an alkaline medium, partly because of
the instability of most reducing agents at low pH and partly because the acidic thiol groups
react with alkali to give the much more soluble anionic thiolate form [204]. Traditionally the
most widely used reducing agents have been sodium sulphide (Na2S) and sodium hydrogen
sulphide (NaHS). Technically these are still the most widely preferred, not only for their
efficacy but also because they are relatively inexpensive. Nowadays, however, they are
increasingly subject to scrutiny on environmental grounds. At least 12 g/l sodium sulphide is
required to dissolve the water-insoluble CI Sulphur dyes. The quantity added to the dyebath
varies from dye to dye but is generally proportional to the amount of dye, with a minimum of
1.5–3.0 g/l [30]. When using sodium hydrogen sulphide the quantity is generally 0.6 times
that of sodium sulphide, but it is also necessary to add alkali (10 g sodium hydroxide or 5 g
sodium carbonate per 7 g sodium hydrogen sulphide). The pre-reduced CI Leuco Sulphur
dyes usually contain a mixture of sodium sulphide and sodium hydrogen sulphide, together
with hydrotropic and dispersing agents such as 2-ethoxyethanol, sodium 1,3-xylene-4sulphonate, sodium p-toluenesulphonate and sodium tetralinsulphonate [202]. The
quantities of sodium sulphide and sodium hydrogen sulphide referred to above relate to the
full-strength concentrated products and must obviously be proportionately adjusted if
weaker commercial brands are available.
In some applications, particularly in jet and winch dyeing, there is a danger that the
reducing agent may be prematurely oxidised by air. Antioxidants, added along with the dyes
and the primary reducing agent at the beginning of dyeing, can be used as palliatives.
Polysulphides of general formula 12.43, such as disodium tetrathionite, have been widely
used for this purpose and provide improved dyebath stability. These additives can be used
with other reducing agents described below but are not compatible with dithionite. Another
approach is to add a relatively more stable alkaline reducing agent such as sodium
dithiodiglycolate 12.44 [205].
Environmental concerns are gradually curtailing the use of sulphides as reducing agents
[203,206], although it has been indicated [207] that as late as 1995 some 90% of all sulphur
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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES
XO3S
Sn
SO3X
O
12.43
O
C
X = Na, K, H or NH4
n = 1–4
CH2
S
S
NaO
CH2
C
ONa
12.44
dyes applied worldwide were still reduced by sulphides. The environmental problems arising
from sulphides include the toxicity of hydrogen sulphide, corrosion of the effluent drainage
system, damage to the treatment works and the often associated high pH and unpleasant
odours [208]. Sulphides cause no odour nuisance above pH 9 but at neutral or acidic pH
values gaseous hydrogen sulphide is liberated. Neutralisation or acidification can occur in
the dyehouse or during waste stream mixing. The MAK value of hydrogen sulphide is 15
mg/m3 (10 ppm), this being the maximum allowable concentration in the workplace at a
continual contamination level to avoid impairment of health, the odour threshold being
0.035–0.14 mg/m3 of air [207]. Polysulphides yield free sulphur on acidification and this can
lead to odours of sulphur dioxide on the dyed substrate. Nevertheless, sulphides can be
quantitatively removed before discharge to the effluent, albeit at a substantial cost
[208,209].
Several reducing agents have been suggested as environmentally more acceptable
alternatives to the alkali sulphides. All are more expensive and exhibit other disadvantages.
For example, the reduction may be more difficult to control, or a particular agent may be
effective only with a limited range of dyes. Even then the alternatives may be less effective
than the alkali sulphides in terms of colour yield. Nor is the fact that such compounds do
not generate hydrogen sulphide a guarantee of freedom from environmental problems; for
example, some give quite high COD levels. The most obvious alternative is sodium
dithionite (12.45) with alkali, the reducing agent most widely used with vat dyes (section
12.9). When used with sulphur dyes, however, the process is difficult to control and some
dyes may be partly destroyed through over-reduction. Nevertheless, dithionite is effective
with CI Solubilised Sulphur and sulphurised vat dyes.
NaO
O
O
S
S
ONa
12.45
CI Reducing Agent 1
Sodium hydroxide is the alkali usually used in conjunction with dithionite. Sodium
carbonate is a possible alternative when CI Solubilised Sulphur dyes are used but is
insufficiently alkaline for the CI Sulphur brands, requiring careful control if over-reduction
and the associated lower yields are to be avoided [30]. Typical concentrations are given in
Table 12.24. The system of sodium carbonate and sodium dithionite used to reduce blue and
black CI Solubilised Sulphur dyes is particularly suitable for flame-retardant viscose fibres
that are sensitive to strong alkalis, since it preserves a satisfactory level of flame retardancy
[30]. It is also possible to use a mixture of dithionite with sodium sulphide in alkaline media.
Sodium formaldehyde-sulphoxylate (12.46; sodium hydroxymethanesulphinate) and
alkali, although more stable than alkaline dithionite, tends to share the same disadvantages
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Table 12.24 Typical applied concentrations of sodium dithionite and
alkali for exhaust dyeing [30]
Liquor ratio 10:1
1% Dye
6% Dye
Caustic soda flakes
Sodium dithionite
3.5 g/l
2.5 g/l
7.5 g/l
7.0 g/l
These amounts are decreased by 30–40% at liquor ratio 20:1 or
increased correspondingly at liquor ratio 5:1
Sodium carbonate
Sodium dithionite
0.5 –1.5 g per g of dye
0.25–0.75 g per g of dye
HOCH2
O
S
ONa
12.46
CI Reducing Agent 2
and is rarely, if ever, used with sulphur dyes owing to associated handling difficulties,
inadequate cost-effectiveness and poor efficiency [207]. Sodium dithiodiglycolate (12.44)
was mentioned above as an antioxidant. Such compounds may be used as the primary
reducing agent in conjunction with alkali. Although they do not give rise to
environmentally undesirable inorganic sulphides in the effluent, their chemical stability
results in a high chemical oxygen demand, often causing more problems than those arising
with sodium sulphide [202]. This system is rarely used for much the same reasons as sodium
formaldehyde-sulphoxylate [207].
The most promising alternative to sulphides, from an environmental point of view, is the
use of the reducing sugar glucose with sodium hydroxide or sodium carbonate. This system
does not satisfactorily reduce all dyes, however. It is reasonably effective with CI Solubilised
Sulphur brands [210], with which it may be used either as the sole reducing agent or in
conjunction with sodium polysulphide, usually resulting in increased dye yields. It can be
used as an additional reducing agent with CI Leuco Sulphur dyes, thus giving a lower
sulphide content in the dyebath, or together with sulphide or polysulphide in the reduction
of the traditional water-insoluble CI Sulphur brands [30]. The system is pH- and
temperature-sensitive; hence performance may be good on jet machines but poor on the
more temperature-sensitive jigs. Typical quantities recommended for a batchwise dyeing
method [30] at liquor ratios of 10:1 to 20:1 are 3–8 g/l glucose, 4–10 g/l sodium carbonate
and 2–6 g/l sodium hydroxide, depending on applied depth, a pH of 11–12 and a minimum
dyeing temperature of 90–95 °C being necessary [206].
The glucose reducing system has a characteristic odour of burning sugar that many
people consider preferable to the odour of an alkaline sulphide bath, although others dislike
it, finding it excessively sweet and nauseous [30]. Nevertheless, the versatility of glucosebased binary systems has been emphasised [207]. The major problem with an alkaline
glucose system is that it is gradually transformed into various decomposition products, thus
losing its reducing action. The intermediate by-products possess some reducing action but
are not sufficiently stable. More stable decomposition products are formed if dithionite is
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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES
added to the system, this being the subject of patents more than sixty years ago [207].
Scheme 12.9 has been suggested to explain this effect.
O
S
ONa
CHOH
CHOH
H
CH2OH
O
C
H
CHOH
CHOH
CHOH
sodium
hydroxide
Sodium
glyderylsulphinate
O
C
2 CHOH
sodium
+
dithionite
CH2OH
CHOH
O
Glyceraldehyde
O
CH2OH
S
ONa
CHOH
Glucose
CHOH
CH2OH
Sodium
glycerylsulphonate
Scheme 12.9
The reduction potentials of various reducing agents listed in Table 12.25 show that at 50 °C
the dithionite/glucose system has a potential that is only slightly lower than that of dithionite
alone, even though glucose has the lowest potential in this series. The addition of glucose
reduces the potential of sodium dithionite to the point where full colour yield is obtained
without the risk of over-reduction. Dyeing tests have confirmed that although sodium
dithionite alone is exceptionally concentration-sensitive, the addition of glucose gives a more
stable system. Optimal colour yield and good reproducibility are obtained even if dyeing
temperature, time and concentrations of chemicals fluctuate within certain narrow limits.
Similar results have been obtained with other glucose binary systems, including hydroxyacetone or sodium formaldehyde-sulphoxylate as stabiliser.
Table 12.25 Reduction potentials of various reducing systems at
50 °C [207]
Reducing system
Reduction potential
(mV) at 50 °C
(platinum electrode)
Sodium formaldehyde-sulphoxylate
Sodium dithionite
Sodium dithionite/glucose
Hydroxyacetone
Sodium sulphide
Sodium polysulphide
Glucose
–900
–850
–700
–680
–650
–500
–200
5 g/l Reducing agent
10 ml/l Caustic soda 38°Be
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The environmentally innocuous properties of reducing sugars have been claimed for the
dyeing of jute with sulphur dyes [211]. This claim is based on the influence of citric acid
addition on the hydrolysis of cane sugar, giving a higher proportion of reducing sugars (Table
12.26). The optimum addition is 0.04% citric acid and treatment of the coarse brown sugar
from the sap of palm trees (so-called ‘jaggery’) is for 24 hours at ambient temperature, giving
a product termed ‘liquid jaggery’. This is added at 2.5 times the mass of sulphur dye in the
dyeing of jute at 100 °C for 1 hour in a liquor containing 50 g/l sodium chloride. This
product can also be utilised in printing, a typical print paste formulation being shown in
Table 12.27.
Table 12.26 Effect of citric acid addition on the composition of
cane sugar after hydrolysis [211]
Composition of product
Citric acid
concentration (%)
pH
Reducing
sugars (%)
Non-reducing
sugars (%)
0
0.01
0.02
0.03
0.04
0.05
5.9
5.8
5.5
5.3
5.3
5.0
21
30
33
36
39
43
40
38
36
32
28
24
Table 12.27 Typical print paste formulation for
the application of liquid jaggery as a reducing
system for sulphur dyes [211]
Sulphur dye
Sodium carbonate
Liquid jaggery
Thickening agent
Water (if necessary) to
3–5 g
2–3 g
7.5–12.5 g
80 g
100 g
2-Mercaptoethanol (12.47) with alkali has been suggested as an alternative to sulphides
[210], offering the advantages of no sulphides in the effluent and no odour from the
dyebath, although the product itself can give off unpleasant and highly toxic fumes. This
process is relatively expensive, with a tendency towards lower yields and a more restricted
range of suitable dyes than when using traditional sulphides, so it has not achieved
significant commercial use.
HSCH2CH2OH
12.47
2-Mercaptoethanol
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CH3COCH2OH
12.48
Hydroxyacetone
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AUXILIARIES ASSOCIATED WITH MAIN DYE CLASSES
Hydroxyacetone (12.48), originally introduced for vat dyeing, has proved moderately
successful with sulphur dyes. This compound requires strongly alkaline conditions, the
concentrations being critical. Colour yields are somewhat lower than with sulphide, the
product is flammable and the dyebaths have an odour characteristic of acetone. Nevertheless,
when used with CI Solubilised Sulphur dyes the effluent is free from sulphide. Mention has
been made already of its possible use in a binary system with glucose [207