Understanding Clean Technology
Understanding Clean Technology
by Dr. Campbell Page
TFL Ledertechnik AG
CH-4106 Basel
Version 2 , December 2004
2
Contents:
1.
2.
3.
4.
5.
6.
7.
8.
9.
Introduction
Leather production pathway
Solid wastes
3.1
Solid wastes in effluent
3.2
Suspended solids in effluent
Liquid wastes
4.1
Waste water treatment
4.2
Improvements possible using optimised processes
Contaminants in waste water
5.1
pH value
5.2
Oxygen demand
5.2.1 Chemical oxygen demand (COD)
5.2.2 Biochemical oxygen demand (BOD5)
5.3
Nitrogen
5.4
Sulphide (S2-)
5.5
Sulphates (SO42-)
5.6
Chlorides (Cl- )
5.7
Oils and grease
5.8
Metals from the tannage
5.8.1 Chromium salts (chromium III, trivalent chrome)
5.8.2 Other metals
5.9
AOX chemicals and APEO surfactants
5.10
Toxicity of effluent components
Air wastes
Finished leather - unwanted contaminants
7.1
Formaldehyde in leather
7.2
Chromium (VI) in leather
7.3
Aromatic amines in leather
7.4
Heavy metals in leather
7.5
NMP-free polyurethane finishing products
7.6
EU Directive restricting NPE and NP products
Wet-white, wet-blue, vegetable extract?
Trends, future laws and restrictions
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3
1.
Introduction
The raw hide or skin is a by-product of the meat industry and from this raw
material the leather tanning industry converts approximately 25% by weight
into leather. The production of leather from the raw hides and skins involves
an intensive use of water as well as many mechanical and chemical
processing steps. Consequently the tanning industry generates considerable
amounts of solid, liquid and gaseous wastes. Well planned clean technology
practices through the use of minimising, re-cycling, and re-use of water and
solids following the best available technologies (BAT), can allow tanners to
comply with tough environmental pressures.
The modern leather industry has a responsibility to operate in a manner that is
compatible with the best ecological and environmental practices. Worldwide
there is a lot of emphasis on these aspects, which is requiring many tanneries
and supply industries to have a better understanding of the whole
environmental picture from the start to the end.
Consequently many new developments in the field of application processes,
chemicals and dyes, as well as the process control and mechanical
developments are concerned with clean technology. This means ecological
factors like reducing the load in the wastewater, exhaust emissions and
lowering solid waste production have become the focus. Often it is asked if
the new environment-friendly processes and products are also feasible for the
tannery from the point of view of economy. There are many very positive and
impressive examples, which prove that economical and ecological
considerations are not irreconcilable; to mention but a few:
•
•
•
•
•
•
safe and reliable hair-save processes;
ammonium-free deliming;
dust-free enzymes;
high-exhaust chrome tannage;
low salt liquid syntans and dyes;
solvent-free aqueous finishes.
Clean technology processes can have up-front additional costs but these
have to be balanced against large savings from lower wastewater charges
and reduced sludge disposal costs. The regulations relating to our
environment will become stricter in the coming years and clean technology
has to be understood and implemented.
The idea of this brochure is to help the lay person understand the factors
involved and provide in a clear and uncomplicated way some of the options
that one has.
4
Schematic overview of the clean technology inputs and
outputs for leather production
raw hide + chemicals
(1000 kg)
wastewater, 25 - 50 m3
water, 25 - 50 m3
tannery
solid wastes, 600 kg
finished leather, 250 kg
5
2.
Leather production pathway
Main chemical wastes
salt (manual removal)
Solid/air wastes
Raw hides
↓
salt (TDS)
Soaking
↓
Green fleshing
fleshings
↓
sulphides, lime
Unhairing, liming
H2S
↓
Lime fleshing
trimmings and fleshings
↓
ammonia N
Deliming, bating
salt, chrome and
vegetable tanning
agents
Pickling, tanning
↓
↓
Chrome splitting,
shaving
shavings (chrome)
trimmings (chrome)
↓
vegetable extracts,
chromium salts, salt
(sulphate)
Retanning, dyeing,
fatliquoring
↓
Drying
↓
Buffing, trimming
leather trimmings, dust
↓
liquid finishing residues,
solvents
Finishing
VOC
↓
Leather
6
3.
Solid wastes
•
•
•
chrome shavings / gluestock and fleshings / fat
solids in effluent
sludge
The solid wastes like shavings, fleshing and trimming wastes and natural fat
are normally reasonably efficiently removed in mechanical operations. They
can be readily collected for further processing and a number of possible uses
for these wastes are available, such as converting to gelatine, etc.
3.1
Solid wastes in effluent
Gross solids are those larger than an effluent sampling devices can handle,
hence they are not measured. The waste components that give rise to this
problem are:
•
•
•
•
•
trimmings and gross shavings;
fleshing residues;
solid hair debris;
large pieces of leather cuttings;
other solid contaminants like paper/plastic bags.
They can be removed by means of coarse bar screens set in the wastewater
flow. If not removed these materials can block the pipes causing severe
problems.
3.2
Suspended solids in effluent
Main source:
Most of the suspended solids are protein residues from the beamhouse
operations - mainly from the liming process. However, large quantities are
also produced owing to non-exhausted vegetable tannins, another source
being poor uptake during retanning.
Problems:
If the wastewater is to be treated on-site, the main problems that arise are due
to the large volume of sludge that forms as the solids settle. Sludge often
contains up to 97% water, giving rise to very large quantities of 'light' sludge.
Even viscous sludge has a water content of around 93%, and can easily block
sludge pumps and pipes. All this sludge has to be removed, transported, dewatered, dried and disposed of.
7
Analysis:
The suspended solids component of an effluent is defined as the quantity of
insoluble matter contained in the wastewater and can be determined by
filtration or by centrifugation.
The majority of these solids settle within 5 to 10 minutes, although some fine
solids require more than an hour to settle. Semi-colloidal solids are very fine
solids that will not settle even after a considerable period of time. They can,
however, be filtered from solutions. Together with the more readily settleable
solids, they thus comprise the suspended solids of an effluent that can be
measured.
4.
Liquid wastes
Wet-end
19%
Beamhouse
81%
Sources of liquid wastes
4.1 Waste water treatment
The major part of the waste in a tannery is diluted in the effluent. This sewage
water cannot be discharged into surface waters or even communal
wastewater without intensive treatment in purification plants.
Legal limits are given as concentrations of the relevant pollutant in the
wastewater, usually in milligrams/litre (mg/l), sometimes as parts per million
(ppm). Peaks of concentrations of certain toxic pollutants above these limits
must be avoided to protect the environment. Most of the communal fees are
calculated according to the sum parameter; where the total amount of a
discharged pollutant per day or per month is determined. However, the
disposal charges can also include penalty fees if the peak limits are
exceeded. Typically holding tanks are used to even out fluctuations over a
24h period.
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Primary treatment of waste water
As a simple overview the process is as follows:
tannery effluent
flocculation
unhairing liming
pickling / tanning
balancing
tank
sulphide
oxidation
(24h)
primary
settling tank
(4h)
chrome
precipitation
sludge
Primary treatment (precipitation and flocculation treatments):
• - removed
50 – 90% total suspended solids
40 – 70% COD
approx. 60% BOD
•
- not removed
inorganic salts
Many tanneries undertake this primary treatment process either on-site or in a
collective water treatment facility. Through flocculation and precipitation it
removes most of the suspended organic matter and anionic organic products
like syntans, fatliquors and dyes. Prior to this the concentrated sulphide waste
stream is separated and converted by aeration to sulphates, as well the
chromium waste stream can be separated and in modern treatment plants the
chromium salts can be relatively easily precipitated and recycled.
9
Secondary (biological) treatment of waste water
A simple overview the secondary or biological process that follows the primary
treatment is as follows:
40 % return
secondary
treated waste
biological digestion
water
settling tank
sludge
Secondary treatment (biological treatment):
• removed
80% total suspended solids
70% COD
99% BOD
•
not removed inorganic salts
The standard aerobic biological treatment plant will readily degrade the typical
effluents of leather processing. Once the bacteria in the active sludge have
digested the organic components they will settle out and be removed as
sludge. To avoid depleting the bacteria in the active sludge it is important that
some 40% of the liquid flow-through is returned to the digestion tank.
The nitrogenous compounds can be broken down by combining intensive
aerobic and anoxic biological treatments. The oxygen demand is very high,
thus leading to correspondingly high operational and energy costs, (40% of
the oxygen demand).
An efficiently run secondary treatment plant can meet relatively stringent limits
for COD and BOD and allow discharge to surface water. Tanners are often
reluctant to re-use water as they are not certain what contaminants remain in
it. However, this water, which will contain residual salts, can be considered for
re-use, for example in the pickle float.
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4.2 Improvements possible using optimised processes
As has been indicated it is difficult to remove inorganic salts from waste water
by waste water treatment plants. So logically it is best to try and minimise the
amount used in producing leather. A number of reduced salt processes have
been developed and the results of one study using an optimised (low-salt)
procedure are presented. If we take a specific parameter such as total
dissolved salt (TDS), which is a considerable problem in locations where the
water supplies are limited and are not close to the sea, there can be
significant reductions in the amount of TDS in the waste water. Naturally the
major effect of implementing an optimised process is to be had in the
Beamhouse.
kg / ton raw hides
400
Standard
Optimised
300
200
100
0
Beamhouse
Wet-End
Change in TDS parameter when using a low-salt optimised process
The individual components in the effluents were also measured in order to
have a better overall idea of how the effluent parameters have changed. As
expected the salts like sulphate and chloride are noticeably reduced.
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400
Standard beamhouse
Optimised beamhouse
kg / ton raw hides
350
300
250
200
150
100
50
0
COD
BOD
chloride sulfide
sulfate
TDS
kg / ton raw hides
Waste water analysis in the whole Beamhouse process
35
Standard wet-end
30
Optimised wet-end
25
20
15
10
5
0
COD
BOD
chloride
sulphate
TDS
Waste water analysis in the whole Wet-End process
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5.
Contaminants in waste water
The effect on the environment of excessive pollutant levels commonly found
in untreated tannery effluents can be severe. So it is important that waste
water parameters can be measured and monitored.
The individual components that are typically measured in effluents and their
impact are described below in a simplified manner for guidance. The main
problems presented by those components are summarised together with an
outline of quantitative analytical methods.
5.1
pH value
Main source:
Most discharged floats.
Problem:
Acceptable limits for the discharge of wastewaters to both surface waters and
sewers vary, ranging between from pH 5.5 to 10.0. If the surface water pH
shifts too far away from the pH range of 6.5 - 7.5, sensitive fish and plant life
are susceptible to loss.
Municipal and common treatment plants prefer discharges to be slightly
alkaline as it reduces the corrosive effect on concrete and helps compensate
for the domestic wastewater that tends to be slightly acid. When biological
processes are included as part of the treatment, the pH is lowered to more
neutral conditions by the carbon dioxide so evolved.
Analysis:
pH-meter; titration with acid / alkali.
5.2
Oxygen demand
Main source:
Surfactants, all non-exhausted organic auxiliaries, hide substance from the
liming process, ammonium and sulphide.
Problem:
All these components in effluents are broken down by bacterial action into
more simple components. Oxygen is required for both the survival of these
bacteria (aerobic bacteria) and the breakdown of the components. Depending
on their composition, this breakdown can be quite rapid or may take a very
long time.
If effluent with a high oxygen demand is discharged directly into surface
water, the sensitive balance maintained in the water becomes overloaded.
Oxygen is stripped from the water causing oxygen dependent plants, bacteria,
and fish to die. The outcome is an environment populated by non-oxygen
dependent (anaerobic) bacteria leading to toxic water conditions.
A healthy river can tolerate substances with low levels of oxygen demand.
The load created by tanneries can be excessive, so the effluent requires
treatment prior to discharge.
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Analysis:
This can be achieved in two different ways:
5.2.1 Chemical oxygen demand (COD)
This method measures the oxygen required to oxidise the effluent sample
completely. It gives a value for all the contaminants, which means the
materials that would normally be digested in the BOD5 analysis (within 5
days), the longer term biodegradable products, as well as the chemicals that
remain unaffected by bacterial activity.
The method is fast and very aggressive. A suitable volume of effluent is boiled
with an oxidiser (potassium dichromate) and sulphuric acid. As the effluent
components oxidise, they use oxygen from the potassium dichromate. The
amount used is determined by titration.
5.2.2 Biochemical oxygen demand (BOD5)
This method is more complex. Essentially, the effluent sample is diluted in
water, the pH is adjusted and it is seeded with bacteria (often settled sewage
effluent). The samples are then incubated in the dark for five days at 20°C.
Bacteria use the oxygen dissolved in the water while the organic matter in the
sample is broken down. The oxygen remaining is determined and the BOD5
can be calculated by comparison to the oxygen in the effluent-free sample.
The results of the BOD5 are always lower than those obtained using the COD
analysis. As a rule of thumb, the ratio between COD: BOD is 2.5:1, although
in untreated effluent samples variations can be found as great as 2:1 and 3:1.
This depends on the chemicals used in the different leather making processes
and their rate of biodegradability.
5.3
Nitrogen
Main source:
The most common sources are ammonia (from deliming materials) and the
nitrogen contained in proteinaceous materials (from liming/unhairing
operations).
Problem:
Nitrogen is a key nutrition factor for plants. High levels released by
substances containing nitrogen over-stimulate growth. Water-based plants
and algae grow too rapidly, thereby waterways become clogged and flows are
impaired.
The nitrogen released through protein breakdown and the deliming process is
in the form of ammonia. Large volumes of oxygen are needed so bacteria can
convert it into water and nitrogen gas. If oxygen demand is greater than the
level supplied naturally by the water stream, toxic anaerobic conditions can
rapidly develop.
Both processes lead to an eutrophication of the surface water.
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Analysis:
Ammonia is released from the wastewater by boiling it with sodium hydroxide,
and subsequently trapping it in a boric acid solution. The level of ammonia
released is determined by titration and its value calculated as ammoniacal
nitrogen.
The other method, the Total Kjeldahl Nitrogen (TKN) method, analyses all the
nitrogenous matter including proteins. This is broken down first by boiling the
sample with sulphuric acid to form ammonium compounds. These are then
analysed according to the method above.
5.4
Sulphide (S2-)
Main source:
Sodium sulphide, sodium hydrosulphide and the breakdown of hair in the
unhairing process.
Problems:
The problem here arises when the pH of the effluent drops below 9.5, very
toxic hydrogen sulphide gas, H2S, evolves from the effluent: Characterised by
a smell of rotten eggs. Even a low concentration causes headaches, nausea,
and eye irritation. At higher levels it is lethal.
Hydrogen sulphide is readily soluble in water and causes rapid corrosion of
metal pipes, fittings and building materials. If discharged to surface water,
even low concentrations can be a toxicological hazard.
Analysis:
The most accurate methods rely on the acidification of effluent to generate
hydrogen sulphide. It is trapped and converted into zinc sulphide. The amount
of sulphide is determined by titration.
5.5
Sulphates (SO42-)
Main source:
Sulphuric acid, chromium sulphate tanning and retanning agents and sodium
sulphate, used as a standardising salt in many powdered products like bating
enzymes, synthetic retanning agents and dyes.
An additional source is created by oxidation of sulphide from in the effluent
treatment process.
Problem:
Problems arise with soluble sulphates for two main reasons:
1. Sulphates cannot be removed completely from a solution by chemical
means. Under certain biological conditions, it is possible to remove the
sulphate from a solution and bind the sulphur into micro-organisms.
Generally, however, the sulphate either remains as sulphate or is broken
down by anaerobic bacteria to produce malodorous hydrogen sulphide. If
effluents remain static this bacterial conversion process occurs very rapidly in
effluent treatment plants, sewage systems and watercourses. This results in
the corrosion of metal parts and concrete.
2. The total concentration of salts (TDS) in the surface water is increased.
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Analysis:
Adding barium chloride solution to a sample of filtered effluent.
5.6
Chlorides (Cl- )
Main source:
Common salt (sodium chloride) used in hide and skin preservation or the
pickling process.
Problem:
Chlorides are highly soluble and stable, and cannot be removed by effluent
treatment and nature. They remain as a problem in surface waters since
chlorides inhibit the growth of plants, bacteria and fish. If the effluent water is
used for irrigation purposes, surface salinity increases through evaporation.
High salt contents are only acceptable if the effluents are discharged into
tidal/marine environments.
Analysis:
Titrating of effluent with a silver nitrate solution.
5.7
Oils and grease
Main source:
Natural oils and grease released from the skin structure, non-exhausted
fatliquors.
Problem:
Grease and fatty particles tend to float and agglomerate, they bind to other
materials causing potential blockage problems especially in effluent treatment
systems. Grease or thin layers of oil on the water surface can reduce the
oxygen transfer from the atmosphere. If these fatty substances are in
emulsions, they can create a very high oxygen demand on account of their
slow bio-degradability.
Analysis:
Extraction of the effluent sample with a suitable solvent and evaporation of the
organic phase. The residual grease can be weighed and calculated.
5.8
Metals from the tannage
Metal compounds are not biodegradable. They can thus be regarded as longterm environmental features. Heavy metals are the subjects of close attention
since they can also have accumulative properties.
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5.8.1 Chromium salts (chromium III, trivalent chrome)
Main source:
Non-exhausted floats from the tanning and retanning.
Problem:
Chrome tanning is carried out in form of greenish chromium (III) sulphate
salts. (It should be clearly differentiated from the highly toxic and oxidising
chromium (VI) chromate salts, which are not used for tanning.)
Excess chromium sulphate from the tanning process float is typically
discharged to a separate tank, where it can be easily precipitated under
alkaline conditions and collected by filtration for re-use. This procedure is a
common practice worldwide and efficiently removes most of the soluble
chromium salts from the effluent.
A small amount of the chromium salts can also be washed out from the
leather during retanning, dyeing and acidification processes, so together with
proteins it finishes up in the sludge. Depending on the amount of chromium
the sludge it may be required to be disposed of separately as a hazardous
waste.
Analysis:
Oxidation of the sample by nitric acid to form the soluble chromate. Several
analytical techniques are possible, for example, atomic absorption; titration as
barium chromate or colorimetric measurement at 670 nm.
5.8.2 Other metals
Main source:
Non-exhausted floats from chrome-free aluminium or zirconium based tanning
and retanning processes.
Problem:
Depending on the chemical species, these metals have differing toxicity that is
also affected by the presence of other organic matter, complexing agents and
the pH of the water. Aluminium, in particular, appears to inhibit the growth of
green algae and crustaceans are sensitive to it in low concentrations.
Analysis:
Complexometric titration methods using chelating ligands like EDTA and
specific indicators.
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5.9
AOX chemicals and APEO surfactants
Main source:
Degreasing of small skins and finishing operations.
Problems:
Organic halogen containing chemicals (AOX) and alkyl phenol ethoxylated
surfactants (APEO), e.g. nonylphenyl ethoxylate (NPE), can be difficult to
break down. Thus they can remain in the eco-system for extended periods of
time and can accumulate in the food chain.
These substances with low bio-degradability in effluents discharged to surface
waters can contribute significantly to the COD/BOD load.
Analysis:
Highly specialised methods often based on HPLC (high performance liquid
chromatography) or GC (gas chromatography).
5.10 Toxicity of effluent components
Main source:
Non-exhausted bactericides and fungicides, tanning agents.
Problems:
Biocides by their very nature are toxic or they would not function. They are
used to protect the partially processed hides from bacterial and fungal
damage. Often processes using these substances cannot be avoided. If they
are insufficiently exhausted during the process or spilled by accident, they end
up in the wastewater and can cause problems in the sewage plant and in
surface waters.
Analysis:
A measure of toxicity of a chemical in water can be expressed as LD50,
representing the dose, which will kill 50 per cent of a sample species. Not
every species reacts to the same degree to a given exposure, and the type of
response to an equal dose of a chemical may differ widely. When values are
given, the species under test should be stated and the time period taken for
evaluation should normally be either 24 hours, 96 hours or 14 days.
Highly specialised analytical methods often based on HPLC (high
performance liquid chromatography) or GC (gas chromatography).
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6.
Air wastes
In the water treatment processes like tanning and post-tanning it is unlikely
any significant air emissions will be released if good operating procedures are
followed. With sulphides from the liming / unhairing process as long as the pH
is kept above 8.5 then the formation of H2S gas is avoided.
In the finishing process the introduction of water-based technologies has
reduced the amount of VOC emissions to the air. The exhaust air emissions
from spray finish applications are typically passed through a wet scrubbing
procedure to keep this from becoming a problem.
7.
Finished leather - unwanted contaminants
What the consumer buys is the end product, finished leather. With a growing
demand for more information about consumer products and their preparation
it is logical that the end article leather is also subject to an array of comments
about its ecological and toxicological properties. The media is quick to
highlight comments like “some 20% of leather consists of chemicals and
therefore it could be harmful”, without considering the real truth of the
comment in more depth. The overwhelming majority of products found in
leather are harmless and offer no danger to users of the natural product,
leather.
The development of analytical methods to determine very low levels of
contaminants in consumer products like textiles has also been applied to
leather. So now a list of unwanted substances found at trace levels have been
developed and are widely circulated. A full risk analysis to determine the real
toxic limits has in most cases not been undertaken.
Analyses for trace levels of the following substances are now common:
•
•
•
•
•
•
formaldehyde
chromium (VI)
certain organic amines derived from azo dyes
organotin compounds
nickel, cadmium, lead and other heavy metals
pentachlorophenol and chlorinated phenols
Additionally tests for NPE, extractable chromium (III), extractable organic
substances, biocides, organohalogen compounds are requested by some
ecological labels.
This list can look very daunting at first when you are asked to ensure none of
them are your leather. In most cases they have been taken from lists for other
purposes and products, therefore some of the listed contaminants are not
found in leather.
We cover some of the more important ones here in some detail to give a good
background to the problem and the methods used for testing them.
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7.1
Formaldehyde in leather
Introduction
Formaldehyde is widely used in the manufacture of chemicals. In the case of
leather chemicals it is used to join together molecules to form larger molecular
structures, the so-called condensation products.
Once this chemical reaction has occurred the formaldehyde is no longer
available and cannot be detected. The only free-formaldehyde that is
detectable originates from the small excesses used in manufacture that have
not reacted completely. Some of this formaldehyde is released (also called
reversibly bound) by water extraction, as well some formaldehyde is enclosed
in the matrix of the leather and can be emitted in gas form as free
formaldehyde.
In our environment the main sources of formaldehyde are from incomplete
combustion processes. So it can be found outside in the streets from motor
exhaust fumes and inside probably from cigarette smoke, which can contain
levels of 100 ppm. Trace levels of formaldehyde are commonly found as
emissions from some building materials, but also in many natural products, for
example, fruit like apples can contain 10 – 20 ppm. Additionally up to 2000
ppm in cosmetic items and 1000 ppm in toothpaste is allowed in the EU.
Under mild alkaline conditions formaldehyde reacts readily with protein and
collagen and this has been used in the past for formaldehyde tannages of
leather.
In combined tannery effluents formaldehyde can normally not be detected
even in trace levels. It rapidly reacts with other compounds of the effluent and
is readily degradable in wastewater treatment plants.
Test Methods
There are several methods for quantitative analysis of formaldehyde in leather
and care must be taken in determining which method is being used, what
limits are required and in interpreting the results. The 2 main methods used
are as follows:
1) Free formaldehyde and releasable formaldehyde by water
extraction method (Test Methods: CEN ISO/TS 17226, IUC 19, DIN
53315)
This is the traditional method of analysis, extracting the leather at 40°C for 1h
in a deionised water solution containing a small amount of wetting agent. The
extracted solution is analysed either by liquid chromatography (preferred
method) or colourimetrically for formaldehyde (or more correctly aldehyde)
content. Since the analytical conditions for preparing these samples for the 2
procedures are not the same they will give different results.
20
Additionally the colourimetric method is subject to interference from extracted
dyes and additionally measures all aldehyde substances, so it is not specific
for formaldehyde. BLC has reported that using this colourimetric method,
some 80% of results obtained from coloured leathers must be considered
incorrect.
2) Free-formaldehyde by gas phase method
(Test Methods: VDA 275, PV 3925 - VW/Audi, EN 717-3)
A gas phase method was originally developed for building materials and is
now used especially by the automotive industry. This method determines just
the free-formaldehyde content in leather. A leather sample is suspended over
a deionised water solution in a closed bottle. This container is heated at 60°C
for 3 hours and after cooling for 1 hour the amount of formaldehyde
transferred via the gas phase into the water solution is measured
colourimetrically.
The leather sample is not in direct contact with the water, so the method is
only measuring the formaldehyde that can be released into the air by warming
the sample.
Limits allowed
In Europe there are currently no legal limits for formaldehyde in leather.
In leather the limit values normally requested are set by commercial
companies promoting their ecological label, for example Oeko-Tex and SG.
For shoes an additional ecological label covering the whole life cycle of the
shoe is available from the EU.
Typically requested eco-label limits for “free-formaldehyde” in leather using
the traditional water extraction method are:
•
•
•
for leather with indirect skin contact, such as shoes, limits of 150 or
300 ppm, are required depending on the ecological label;
for leather items with direct skin contact a level of 75 ppm is required;
for children’s shoes the level is lower at 50 ppm.
The automotive industry normally recommends the gas phase method of
analysis, but note carefully, the limits for this different method are much lower,
typically 10 ppm.
What differences are there between the water extraction and gas phase
methods?
It is not possible to compare results between the two methods. The traditional
water extraction method will typically give considerably higher results
compared with the gas phase method; some comparisons indicate that values
5 to 10 times higher could be expected. The water extraction is measuring
both types of formaldehyde, namely the free-formaldehyde as well as the
releasable formaldehyde.
It should be noted that changes in the extraction temperature and times could
have a considerable influence on the result, so these parameters in the test
method must be followed closely.
21
What level of formaldehyde in leather is achievable?
Modern chemical products and application processes enable a limit of
300ppm (water extraction method) to be complied with. With appropriate
selection of products the lower levels can also be complied with.
Where does the formaldehyde in leather come from?
The most common and therefore the most critical sources are formaldehydereleasing products such as:
•
•
•
organic tanning agents;
resin-type retanning agents;
fixing agents used at the end of the drum application process.
There are several other possible sources, such as, syntans, fatliquors and
retanning/dyeing auxiliaries, but generally they only become a problem if one
uses high amounts or has to reach very low limits.
How can you avoid having formaldehyde in leather?
Select only those products that are either formaldehyde-free or have low
formaldehyde content. Basically in the normal tannery situation this means
avoid using those few products such as resin-based retanning agents, fixing
agents and organic tanning agents which could release high levels of
formaldehyde.
It is very difficult to try reducing the formaldehyde level during or after leather
production. For example, processes that include the use of products to react
with formaldehyde can reduce the level of formaldehyde in leather but can not
eliminate it.
7.2
Chromium (VI) in leather
Introduction
Chromium can exist in several oxidation states. For leather tanning the most
important chemical used is basic chromium sulphate, it has the oxidation state
chromium (III). This oxidation state is a natural trace mineral that we need in
our bodies for our everyday life.
Chromium (III) can be oxidised in some special situations to the much more
toxic form, namely chromium (VI). It should be clearly stated that this Cr (VI)
oxidation state does not tan leather and does not form organic complexes,
e.g. organic chromium complex dyes cannot be chromium (VI) based.
Test Method
The standard method for chromium (VI) in leather is:
1) Cr (VI) in leather
(Test Methods, CEN/TS 14495, IUC 18, DIN 53314)
The traditional method of analysis is to shake the leather in a deaerated pH 8
buffer solution at room temperature for 3h. The solution is analysed
colourimetrically for chromium (VI) content. The analytical conditions require it
to be made under an inert atmosphere to avoid the oxidation of the Cr (III)
during the analytical procedure.
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The colourimetric method can be subject to interference from organic
substances such as extracted dyes and possibly vegetable tanning agents.
Therefore it is important that these components are removed by using small
chromatographic cartridge clean-up procedures before adding the
diphenylcarbizide colour-developing agent.
a)
Limits allowed
In Europe since Cr (VI) is listed as a carcinogen the legal limit is quite simple:
it must not be detectable. Hence the actual limit is de facto the detection limit
for the analytical procedures. The DIN 53314 method stated 3 ppm as the
detection limit but inter lab trials showed wide variations in results when
making analyses on leather samples. Subsequently it was shown the DIN
method is unsuitable for many coloured leathers as any extracted dye caused
an interference to the analytical result. Therefore the DIN method is only
suitable for analysing for Cr(VI) in undyed leathers. The new CEN/TS 14495
method has a clean-up procedure to remove the extracted dye. So this
method and various other eco-labels have recognised the complex matrix that
leather is in terms of analysis and they set 10 ppm as a level, which can be
detected with reliability over a wide range of leather samples.
Where does the chromium (VI) come from?
The main sources of Cr (VI) are likely to be from the oxidation of trace levels
of Cr (III) through fatliquors, moisture, heat and light.
How can you avoid having chromium (VI) in leather?
Select products that allow reductive conditions during the retanning,
fatliquoring and dyeing process. Avoid the use of unsaturated fats in situations
where oxidation could occur. Avoid storing for extended periods chrome
tanned leathers in moist and warm situations.
7.3
Aromatic amines in leather
Introduction
In 1994 the Health Ministry in Germany introduced a regulation, which forbids
certain aromatic amines that can result from azo dye cleavage in those
consumer goods with skin contact. The regulation allowed a period of time
before it came into affect. The amines forbidden were those on the German
MAK list of chemicals known to cause cancer or suspected to cause cancer.
Since at that time no official analytical procedures existed for these amines, a
period of uncertainty existed.
Over the following few years, analytical methods were established with
reasonable detection limits to minimise the chances of false positives and
some other European countries introduced similar regulations. In order to
make a uniform regulation in the EU, it made a directive in 2002 that was very
similar to the German regulation and the list of forbidden aromatic amines was
increased to 22. At this time all except one of the 22 amines can be analysed
using standard sophisticated HPLC analytical techniques, although the
interpretation of the results needs to be made with care, as there is still the
possibility of false positive results.
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Test Method
The standard method for forbidden amines in leather is:
1)
Dyed leather- test for aromatic amines from the break down of azo
dyes (Test Methods, CEN/TS 17234, IUC 20, DIN 53316.)
The leather is digested at pH 6 and 70°C in the presence of a reducing agent,
which splits the azo bonds in the azo dyes. The aromatic amines formed are
extracted and analysed by HPLC.
The method sets 30 ppm per amine as the minimum detection level. Results
above this level would indicate it is possible an azo dye has been used which
can split to release the forbidden amine.
Where do the amines come from?
Aromatic amines are an essential raw material for making azo dyes, without
these amines it is not possible to make azo dyes. Today the dye suppliers do
not use any of the small number of forbidden amines in the manufacture of
azo dyes.
How can you avoid having forbidden amines from azo dyes?
Purchase the dyes from reliable suppliers who are prepared to back up their
service with a guarantee of compliance with the regulations when leather
samples are analysed.
7.4
Heavy metals in leather
Many eco-labels require the absence of heavy metals such as nickel, tin,
mercury. These are not present in the normal preparation of leather and have
come from other industries, e.g. allergic reactions to nickel in jewellery.
Traces of organotin compounds like tributyl tin (TBT) were found in some
textile garments and received a lot of media attention. Since then the ecolabels have required that there should be no detectable amounts of TBT in the
consumer article. To our knowledge there has been no problem with TBT in
leather products.
7.5
NMP-free polyurethane finishing products
In 2001 the solvent N-methyl-pyrrolidone (NMP) was included in the
Californian Proposition 65, the common name for a Californian regulation,
which lists chemicals that could cause health risks to the citizens or affect its
water sources. Consumer articles containing any Proposition 65 listed
substances must contain a warning label when sold in California. If no
minimum safe limit has been set then any detectable amount must be
labelled.
The solvent NMP is used in many polyurethane dispersions for improving the
flow and film formation during finishing. Automotive leathers in particular are
dependent on polyurethane finishes so there has been a need to develop
alternative finishing formulations.
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7.6 EU Directive restricting the use of nonylphenol ethoxylate
(NPE) and nonylphenol (NP) products
Background
An EU risk assessment was made under the Existing Substances Regulation
93/793/EEC because of the large quantity of nonylphenol (NP) manufactured
and used, as well as its toxicity to aquatic organisms and concerns about its
biodegradability. The nonylphenol ethoxylated (NPE) products were also
assessed as they are the main pathway of nonylphenol into the environment.
For NP the risk assessment showed that the main concern is the high aquatic
toxicity and that it did not break down readily in ecosystems. No adverse
human exposure risks during use were identified.
The environmental risk assessment indicated the need to reduce the risks
associated with the production, formulation into products and end-uses of
NPE and NP. It is estimated that the proposed restrictions will decrease
emissions to the aquatic environment by about 80%.
EU Directive
The EU passed in July 2003 the directive 2003/53/EC, which will restrict the
marketing and use in Europe of products and product formulations that
contain more than 0.1% of NPE or NP. This applies to many industries
including the textile and leather industries, except in the case of closed
application systems where no release into waste waters occurs.
The EU European countries have until January 2005 to implement in their
own country the necessary legislation to make this EU directive into law.
So in summary, the sale and use of products containing more than 0.1%
NPE or NP will not be allowed in Europe from Jan. 2005.
It should be clearly understood that in this EU Directive there is no restriction
on the import of goods like leather, which have been treated with NPE
products outside of Europe.
The above explains the official chemical regulations in EU, some leather
specifiers and users have interpreted the Directive differently by forbidding the
use of NPE during leather processing. They check their leather deliveries from
all over the world by analysing the leather for residues of the NPE surfactants.
This has resulted in an EU restriction on the use of NPE surfactants now
being implemented in many parts of the world.
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8.
Wet-white, wet-blue, vegetable extract?
Several ecological balance studies have been undertaken comparing the
various methods of preparing leather and their uses. The oldest tanning
process using vegetable extracts to tan leathers has traditionally been looked
at as the “natural” leather. Chrome tanned (wet blue) leather has for over a
hundred years proved it’s suitability and superiority as a material for the
manufacture of shoes, garments and upholstery leather. Wet blue tanned
leathers have come under criticism because of the difficulties that many
people have understanding chromium. In the last years a rapidly growing
market has developed for automotive leather based on wet white, that is
aldehyde tanned, free-of-chrome leather. For the automotive industry there
are advantages in the dry heat stability of wet white leather compared with
chrome tanned leather. Additionally the absence of chromium(III) salts in the
shavings, waste water and end leather article has resulted in comments that
wet white processes are ecologically better.
Interestingly the ecological balance studies showed that when all aspects are
evaluated there was little between the leather processes. For example, the
natural vegetable tanning processes had disadvantages with high wastewater
COD loading, as to a lesser extent does the wet white. Wet blue leathers
needed less chemicals and often showed superior performance properties.
Overall the results indicated that when using best available technologies there
is little difference in the life cycle ecological balances between the methods of
preparing leather. It depends on the leather article desired by the customer
and the weighting given to each aspect.
9.
Trends, future laws and restrictions
The Waste Management, Sustainable Development and Environmental units
within the EU are quite active in leading with new environmental regulations,
which over time other countries often implement. A number of projects and
directives are currently being worked on, such as:
•
IPPC, Intergrated Pollution Prevention and Control, setting emission
limits EU wide according to the implementation of best available
technologies;
•
Directives on restrictions on marketing and use of dangerous
substances and preparations
o
o
o
o
(nonylphenol ethoxylate)
(azo colorants)
(organostannic compounds)
(short chain chlorinated paraffins);
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•
Environmental agreements within EU countries – simplification and
improvement of regulations
•
Directive on Dangerous Substances – for transporting Dangerous
Goods including waste materials
•
Directive on Sewerage Sludge – restrictions on heavy metals for
sludge applied to agricultural land
•
Directive on Landfill – reduce landfilling of biodegradable waste
•
Project RESTORM, Radically Environmentally Sustainable Tannery
Operation by Resource Management, to assist the tanning industry
change to production methods to ensure a sustainable manufacturing
industry for the future.
Our environment and eco-system is something we all have a responsibility to
protect. So we must expect that further restrictions will be implemented to
avoid polluting, as well as ecological and toxicological harm. By using the best
available technologies and optimised systems the leather industry can meet
these challenges.
The request for information about consumer products is a growing demand
and this will certainly lead to more laws and restrictions. Leather as a natural
based product can abide with all reasonable requests.
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