What are leather dyes today?
The relationship between the dye structure and
its performance properties.
by Dr. Campbell Page and Dr. Jens Fennen
Introduction
The coloration of leather is typically made with azo dyes and it is interesting to take a
deeper look at the factors that allow us to understand these dyes and the process of
dyeing leather. While a considerable amount on dyes and dyeing properties has been
written over time it has often been out-dated due to new legislation or tannery needs in
terms of leather performance. Here we will attempt to bring together the ideas
regarding leather dyes as used today and how the structural properties influence the
performance properties. The theory being that if one needs specific fastness
characteristics one can then use the structure that best meets these needs.
An overview of the future trends in dyes and the properties they will need to fulfil will
also be presented.
Classification of dyes, what are azo dyes?
Colouring agents are those substances that absorb light in the visible wavelength
region of 400 – 700 nm. Hence they appear coloured to our eyes. These colouring
agents can be either organic or inorganic in nature. We will consider only the organic
colouring agents here and from these only those that come under the definition dyes.
That is, those organic compounds which are soluble in the medium they are applied to
the substrate, in this case leather dyes applied in water.
The vast majority of dyes used today for dyeing leather fall into the “azo dyes”
category. Some 70% of all leather (and textile) dyes listed in the literature have the azo
chromophore as the reason for their colour. In practice today more than 90% of all
dyed leather will be coloured with dye(s) containing the azo chromophore.
The “azo dyes” have become a much talked about subject, since the introduction of
legislation in Germany in 1995 banning the use of a small number of azo dyes that
could split under reductive conditions to release toxic aromatic amines. Uninformed
comments, especially in the media, about the toxic nature of azo dyes in general have
in many instances lead to requests from retailers for textile and leather clothes dyed
“without using azo dyes”. Such requests are based on ignorance and are misleading
for the tanner and dye supplier, since they are a wrong interpretation of the actual
situation. It is worth reinforcing again that the German ban on using certain azo dyes
that split to release specific toxic aromatic amines, relates only to a very small number
of dyes and that the majority of the azo dyes can be used without any grounds for
concern. Any reputable dye supplier does not use such amines in their production.
An azo dye is made by coupling an aromatic amine with another aromatic component.
A very simple schematic example for the formation of a mono-azo dye is as follows:
−NH2
H+, NO2-
−N2+
−N=N−
−N2+ +
So it is clear to see from the above scheme that one must have an aromatic amine as
the starting block for an azo dye. Without it the manufacture of an azo dye is not
possible.
Organic compounds with one azo group:
−N=N−
are called mono-azo compounds and the structure of Acid Red 1 below is an example.
O
HO
HN
CH3
N N
HO3S
SO3H
Naturally by making a series of reactions and couplings it is possible to build up more
than one azo group into a dye molecule, these structures are called polyazo dyes. This
concept is the basis for the manufacture of the main leather colours of brown and
black. Using the simple schematic structure, a polyazo dye would look like this:
−N=N−
−N=N−
−N=N−
The more azo chromophores, normally the more the colour tends towards brown or
black, as the individual red, yellow, blue, etc. chromophores start to work together.
Many of the major brown and black dyes for application to leather are polyazo
compounds.
How can we influence the dyeing behaviour?
In the manufacture of polyazo dyes one can influence the dyeing behaviour by altering
the chemistry of the components, especially by changing the substituent on the
aromatic rings. Examples are as follows:
Example 1
−SO3
• the addition of a sulphonic acid group increases markedly the solubility and the
anionic character of the dye molecule, which consequently penetrates into anionic
retanned leather much more.
Example 2
−NO2
• addition of a nitro group can alter the shade of the dye.
So one can see it is possible to influence the end result on leather by choosing a
suitable structure. However, this is a simplified view and many structures prove
unsuitable for other reasons such as solubility, poor light fastness, poor shade
consistency, sensitivity to acid, metals, hard water, etc. Naturally the very practical
aspect of the cost of manufacture (and registration for new structures) is today very
important.
It should be noted at this point that the typical metal complex dyes used for textile and
leather dyeing are mostly in the azo dye category as well. Effectively one complexes
mono-azo dyes with a metal salt to form the metal complex dyes.
For example, the schematic structure for a 1:2 chromium complex dye is as follows:
−N=N−
Cr
−N=N−
What are homogeneous dyes?
It should be understood that a homogeneous dye in terms of leather azo dyes does not
necessarily mean it has only one chemical structure. Especially if it is a polyazo dye
that has undergone several diazotising and coupling steps, it can consist of several
components and/or isomers formed during the manufacture. However, it should be
carefully noted we are not talking here of physical mixtures, which occur after the raw
dye component is manufactured.
A thin layer chromatogram illustrates the nature of 3 homogeneous leather dyes:
However, as it is to be seen in the chromatogram above the dyes can have more than
one chemical component, these are dependent on several factors during manufacture.
Aspects of the reaction itself such as:
•
•
•
•
temperature;
time;
pH;
volume;
can all influence the resultant raw dye.
Additionally the parameters within the manufacturing vessels can strongly influence the
manufacture of the polyazo dyes, parameters such as:
- flow rate between vessels;
- geometry of the vessels;
- speed of stirring in the vessels.
So it is easy to see that one can have quite some difference between dyes of
supposedly the same structure.
The Colour Index system is a useful tool of classifying each of the dye structures of
homogeneous dyes into a specific category appropriate for the application conditions,
such as Acid, Direct, Reactive, Pigment, Disperse, etc. But again the manufacturing
parameters of dyes, as explained above, means that often for dyes with the same
Colour Index classification number they have different properties. Below is a HPLC
comparison of 2 polyazo dyes with the same Colour Index number, Acid Brown 75.
The dyes are from different manufacturers, and the chromatograms show the
difference quite clearly.
HPLC Chromatogram: Acid Brown 75 - Manufacturer 1
HPLC Chromatogram: Acid Brown 75 - Manufacturer 2
What influence does this difference have on the dyeing properties?
For the 2 dyes shown above with different HPLC chromatograms, the dye from
manufacturer 2 has a fastness to migration into PVC of 4 – 5, compared with a much
poorer note of 2 for manufacturer 1. The dye from manufacturer 2 also has a different
shade and somewhat different dye build-up, depending on the anionic charge of the
leather substrate.
This again illustrates the pitfalls of the common assumption that dyes of the same
Colour Index classification number are the same. Too often one sees considerable
differences in their dyeing behaviour.
What are dye performance requirements today?
For quality leathers the leather tanning industry has over recent times tended away
from focussing almost exclusively on chrome tanned leather and specifying just the
leather dyeing properties of light and wet fastness. Today there is a strong demand for
other fastness properties such as migration into PVC and migration into a lacquer
finish, which now often have priority before wet fastness and light fastness. Other new
properties like stability of colour to climate changes and temperature are also gaining
in importance, especially for the car interior requirements.
How does one prepare dyes for these changing performance
requirements?
There is a growing trend in leather for automobiles to make the leather tannage without
using metal salts, one example is the use of aldehyde tanned leather instead of
chrome tanned. For the dye application and fixation this change has a major impact, as
the aldehyde tannage uses the amine sites typically required for the ionic bond
between the dye and leather substrate.
O
HO
OH
NH 2
O
H 2O
O
Cr
O
H2O
H
O
O
H
H 2N
OH 2
Cr
O
O
N
H
N
H
OH 2
chrome tanning
Aldehyde tanning
This results in a considerably lower dye wet fastness for the aldehyde tanned and dyed
leather, the deep dyed shades are especially affected. This is a continuing challenge to
the chemical and dye manufacturers to find ways to improve the fastness of such dyed
leather.
What is migration of colour into PVC?
The migration of colour into an adjacent soft white PVC plate (at 50°C over 16 hours) is
a simple but effective method. It checks the tendency that a dye component will move
from a polar substrate like leather into non-polar adjacent substrate, such as a
synthetic sole of a shoe. This test indicates the polar/non-polar tendencies of the
dye(s) and could be likened to an indication of the attraction towards solvents. Those
dyes that do not migrate into the PVC generally have good water solubility and have
polar structures. Conversely those that migrate into PVC are more non-polar and
generally have lower solubility in water.
This leads to the observation that the dyes with non-polar character have in general
better wet fastness but normally more migration into PVC. Those dyes with a polar
character have in general poorer wet fastness but either no or almost no migration into
PVC.
This overall scheme is represented below:
PVC
(non-polar)
perspiration solution
(polar)
dyed leather
polar dye
non-polar dye
How does the form of the dye molecule influence the performance?
Generally the larger the molecule the poorer the penetration into the leather crosssection, however, increasing the number of sulphonic groups increases the anionic
character of the dye and can improve the penetration for anionic leathers. On the other
hand this will reduce the depth of the dyeing, i.e. less intense, and can decrease the
wet fastness if the dye is not subsequently fixed with cationic agents.
Very small dye molecules, such as for example Acid Black 1, are able to penetrate the
leather cross-section more easily but afterwards are notoriously difficult to fix
successfully. For some leathers this comes down to a decision between either a dark
penetrated cut and poor wet fastness or a lighter cross-section and good wet fastness.
Attempts to correlate the light fastness performance and the dye structure have
involved many skilled chemists and application experts over the years. The clearest
result has been the development and use of the metal complex dyes for reaching high
light fastness on dyed leather. After having found a good solution to the problem, the
tendency in recent years has been to producing metal-free leathers for automotive
uses! This means one is now asking for good light fastness but without using metal
complex dyes.
What about migration of colour into a lacquer finish?
The most important point that is almost always overlooked here when discussing
migration of colour into a lacquer finish is that the dye by itself does not migrate. The
coloured component must be transported. Typical transporting agents are for
example the fatliquor or the slow evaporating solvents from the finishing.
How can we measure the tendency of a dye or coloured component to migrate into the
lacquer finish? For some time the migration into PVC has served as a good guideline
for the performance of dyes, most dye suppliers give these values in their colour cards.
Dyes with good resistance to migration into PVC and finish lacquers are typically those
with a higher polar character.
However, there has been a change towards water-based finishing systems. These
systems are not solvent-free, as assumed by many, but rather are based on water and
water-miscible solvents with high boiling points. Naturally the evaporation rate of these
systems is slower giving more time for the finishing system to interact with the dyed
leather substrate. Comparisons of the migration into PVC and the migration into
lacquer finishes have confirmed the general trend is similar, especially if the migration
colour is different to the dye colour. But enough differences occur to make the results
somewhat puzzling. In some cases, dyes with no record of migration are found to be
involved in some coloration of the finish.
Finally some clarity is appearing as to why some finishes have discoloured and others
not. The typical polyurethane finishes are prepared in alkaline conditions to allow them
to react. Surprisingly though, there is very often a complete lack of pH measurements
of these water-based finishes. Finish preparations of up to pH 11 are not unusual and
any dye technician knows these are ideal conditions for the extraction of leather drum
dyes. The finish formulations are prepared and continually adjusted to give the right
physical properties such as flow, coverage, touch, appearance, etc. and not to meet
any requirement of the underlying dyed substrate.
It is clear to see now that the slower evaporation properties of the high boiling point
solvents and the extremely high pH of the finish preparations could well be the real
reason for the transport of dyes resulting in finish discoloration problems over the
recent years.
How do we get the top performance?
This presentation has pointed out that there is no perfect solution to the problems of
dyeing but as in other areas one has to compromise according the requirements for the
leather being made.
Good wet fastness often means poor values for the migration into PVC, and vice versa.
Good light fastness means preference for metal complex dyes but some automotive
suppliers ask for metal-free leathers. Good resistance to migration into finishes is
helped by choosing polar dyes but the longer exposure to water-miscible solvents at
high pH increases the chance of them being transported to the surface.
Good fixation of the dyes requires sufficient sites for the interaction of the dye molecule
and the ionic groups of the leather substrate. Using metal-free tannages can reduce
the sites available for the dyes making it difficult to get adequate colour build-up for
deep shades and also resulting in poor fastness properties.
What trends are foreseen?
The demand for chrome-free and metal-free leathers especially in the automotive area
will increase, as will the need for high fastness to migration, as well as good light
fastness and wet fastness. Improvements will be made in both the fixing systems as
well as the chemicals used in processing the leather.
Knowing the requirements means the dyes can be focussed more clearly to meet
these needs. However, in most cases it will continue to be a compromise, the aim is
always to improve on specific performance parameters while avoiding the possible
negative effect on other parameters.
Ecological and environmental matters will continue to increase in importance and
additional laws and labelling regulations must be expected.