New aspects of ecology in finishing

The most important step was taken during the 1980s, when the basis for aqueous finishing was developed₁. Water dilutable top coats have been replaced by waterborne top coats, which have been used in leather finishing since the 1990s.

The consequent development of ecological leather finishing is being driven by the following forces:

* The society consumes more carefully. Traces of toxic substances in food or products for the daily life are under everyday discussion

* Public discussion about toxic residues in products does harm to the (chemical) industry. The consequent removal of risky substances is an opportunity for the chemical industry for better marketing in the sense of responsible care

* Further development and chemical analysis has set down limiting values

* New toxicological research leads to a new valuation of previously widely used chemicals. In the case of a new evaluation as a toxic substance, a complicated process will follow. For example, the discussion about the carcinogen aryl amines in leather dyes has resulted in reducing the limits to the lowest possible analytical levels.

What effects has this for the ecology of (leather finishing) chemicals?

* A result of globalisation is that national legislation covering ecology can influence worldwide legislation and vice versa

* Each industry branch creates its own lists for banned substances. Well known is the list of declarable materials in automobile manufacturing – substances in components and construction materials (previous VDA-List 232-101) – which is additionally expanded by the individual automobile manufacturers. Background information is also seen in the ‘End of-life vehicles’ (ELV) directive of the EU for achieving the recycling level of 95% by 2015. The list contains more than 100 substances and substance categories, which have a potential risk. Several of these substances are relevant for leather (finishing) chemicals

* The shoe making industry, especially the sport shoe producers, create similar lists. As standard there is the Öko-Tex Standard 100, mainly used by the textile industry. Depending on the textile class and use, different limits for selected hazardous substances are defined. The EU has created a similar label, the EU eco-label textile

* For leather, the LGR has developed the label ‘leather produced in an environmentally acceptable way and tested for hazardous substances’

New critical substances always lead to a massive workload for the tanneries. They have to analyse the leathers and the leather chemical producers, in turn, have to analyse their own products.

The main problem here is the relationship between substance levels in the leather and the concentration of the leather chemical. If the analysis of the leather is not to ascertain concentrations of substances in leather, but a test for leather emissions, the relationship is even more complicated.

In any case, the final result for the hazardous substance is demanded for the leather. An additional problem is the absence of (concerned) analytical methods for (leather) chemicals.

New developments for several products and substances are described as follows:

Phthalates

Phthalates such as dibutylphthalate (DBP) and di-iso-octylphthalate (DEHP, DOP) have been used as universal plasticisers for many decades. In leather finishing, these substances were related to the success of water dilutable nitrocellulose lacquer emulsions as top coats in the 1970s, but because of their reproductive toxicological potential, (leather finishing) chemicals with more than 0.5% of these phthalates have had to be labelled with T within the EU since July 30, 2002.

But the replacement by ecological non-risky materials such as castor oil was successful. Important was the maintenance of the typical properties of these lacquer emulsions: gloss, touch, feel, softness and elasticity.

Nonylphenol (ethoxylates)

In June 2003, the EU commission decided to incorporate nonylphenol and nonylphenolethoxylates into the directive 76/769/EEC (Restrictions on the marketing and use of certain dangerous substances and preparations).

Now the EU members have to include it in their national legislation by July 2004 and this will become active in January 2005.

It will then be forbidden to use and sell products with more than 0.1% nonylphenol(ethoxylates) within the EU. Background is the high persistence of nonylphenol in the environment because of its difficult degradability.

Additionally the nonylphenol part shows a hormone (oestrogen) like behaviour.

The discussion about it is not new. It is well known from other industries. For example the detergent producers stopped using it many years ago. Also for leather (finishing) chemicals no new developments using nonylphenol(ethoxylates) have been made. They have been used before and now replaced in

* acrylic resin emulsions as stabilising emulsifiers

* pigment preparations as wetting agents

* lacquer emulsions as emulsifiers

* impregnation additives as penetrators

Ecologically useful alternatives are fatty alcohol ethoxylates, especially the C12-C14 alcohol ethoxylates. More often combinations of them are successfully used to replace nonylphenolethoxylates.

Biocides

Biocides are used for the microbiological protection of mainly water based finishing chemicals. The EU has decided on a directive on biocides.

The producers of biocides had until March 2002 to notify all products for this kind of application. Enormous examinations and data were necessary for this approval and this process has in turn reduced the number of products on the market.

Additionally the EU has classified the often used combination of methylisothiazolinone (MIT) and chlorisothiazolinone (CIT) as an irritant. Products with more than 15ppm of this combination have to be labelled with Xi (R 43) since July 2002.

Another restriction has been made by Proposition 65 of the California Office of Environmental Health Hazard Assessment (OEHHA). The OEHHA has seen the fungicide o-phenylphenol (OPP) as carcinogenic, but meanwhile set a high safe harbour level.

The use of OPP is restricted for leather finishing chemicals in the future if the Öko Tex Label is required. For all kinds of articles there exists here a limit for OPP.

All of these new regulations had an influence in the preservation of finishing chemicals. New combinations had to be selected achieving the same high preservation level as before. The reliability of the preservation is especially important for regions with a humid and hot climate such as Asia.

N-Methylpyrrolidone (NMP)

The most important new classification during recent years was made by the California OEHHA in June 2001: NMP is classified as a reproductive toxin. All products sold in California containing NMP have to be labelled now.

There was no limit set for the concentration of NMP in the products. This local decision had a worldwide influence, so a big demand for NMP-free products has been created. The EU is now also proofing NMP for a possible reproductive toxicity.

NMP is used mainly in polyurethane dispersions (PUD):

* as an additive for producing PUDs; using the transfer process it reduces the viscosity of the prepolymer melt; normal NMP contents for PUDs are 1-5%

* for aqueous pu top coat systems it works as a coalescence additive, plasticiser, levelling agent, wetting agent and as a swelling material giving a better adhesion on the base coat.

NMP has a boiling point of around 203°C and a very low evaporation value. It is aprotic, so it cannot react with isocyanate crosslinkers and strongly polar substances, so it is miscible with water in any ratio.

The result of this is a high permanence in finished leathers. Only a small quantity evaporates during the finishing process. Depending on the leather article, concentrations of 200-3,000ppm can be found₂.

The lack of low risk alternatives leads to a basic rethinking in finishing. Moreover, the removal of NMP and other similar solvents results in a further reduction of the solvent content in finishing systems overall. A simple reduction of NMP is no way to overcome the problem.

An alternative for PUD production could be the acetone process, where the production additive acetone is distilled off at the end of the production. The disadvantage is the energy consuming distillation process with additional higher costs for acetone and recycling costs for the distillate. But the result is a solvent-free PUD.

NMP was often used for top coat formulations and now they also have to be changed. By using tailor made PUDs with solvent free levelling and wetting agents dull and deep black top coats are achievable. With the new technology, heavily crosslinked base coats can also be oversprayed without adhesion problems or grey break.

As a conclusion, the single decision of California’s OEHHA has further accelerated the development of low VOC top coat systems. Also the development of high wear top coat acrylics, being free of solvents, has been refreshed.

Formaldehyde

The discussion on formaldehyde containing syntans in the wet-end area has now also influenced finishing.

Formaldehyde has two functions in leather finishing:

* as a crosslinker for casein top coats

* as a biocide, direct or by formaldehyde releasing biocides for preservation

Apart from the highly reactive aziridine, there are no other crosslinkers for casein as aldehydes like formaldehyde. All of them have a toxic potential. But a reduction of the absolute formaldehyde content is possible by using formaldehyde releasing chemicals.

As a biocide, other chemicals have the same effectiveness, so there is no need for it anymore.

Finishing chemicals may contain formaldehyde even when no formaldeyde was used for production. This is because many raw materials are still preserved by a formaldehyde base. The only way to overcome this is the exchange of formaldehyde-free raw materials with additional consequent analysis. As a method of analysis, the adoption of the method for leather (DIN 53315) is appropriate. The aqueous finishing chemical is directly analysed by using the photometric or HPLC method (for coloured chemicals).

So far only small quantities could be found. The limit set mostly by the leather manufacturing industry was 75ppm and most chemicals have less than 30ppm. Another way of preventing high formaldehyde levels may be the use of formaldehyde scavengers, as is used for some wet-end chemicals.

Heavy metals

The analysing of leather for extractable heavy metals is a concern for several labels (VDA list, Öko Tex Standard, EU eco-label textile). The list of heavy metals contains chromium (III), chromium (VI), lead, mercury, arsenic, antimony, cadmium, copper, nickel and cobalt.

Similar to the testing methods for Öko Tex Standard 100 and EN ISO 105-E04 (colour fastness to perspiration), the leather is extracted with an artificial sweat solution. The extract is then analysed for heavy metals using the atom absorption spectroscopy (AAS) method.

The discussion about heavy metals before was more concentrated on replacing the inorganic heavy metal pigments such as lead chromate, cadmium sulfide and others by organic pigments. The inorganic pigments had a higher covering power, fastness and brilliance than organic pigments. But because of their toxic properties, high fastness organic pigments are mainly used now. These are, of course, free of carcinogenic aryl amines.

Another source for heavy metals are organic green and blue pigments based on copper-phthalocyanine complexes. They can have residues of soluble copper salts. But analysis of leather containing these pigments had lower extractable heavy metal values, than the limits for, say, Öko Tex Standard 100.

Also iron oxide pigments, which are the basis for most brown pigment paste preparations, may have residues of other heavy metals but, because these residues are low soluble oxides, no contribution to extractable heavy metals in leather could be found.

Organotin compounds

Organotin compounds were under public discussion in spring 2000 because high quantities of tribuyl tin (TBT) compounds had been found in several sports textiles. TBT is mainly used in anti-fouling ship paints but is also used as an antimicrobial additive in textiles.

It is highly toxic and has hormone-like behaviour. So far the demand for tin-free leather chemicals has come from the sport shoe producers.

In leather finishing, only dibutyl tin (DBT) is in use as a catalyst for the production of PUDs. DBT contains some TBT as an impurity. For producing tin-free PUDs, other catalysts or other process technologies are now in use.

Summary

Stronger consumer consciousness on one side and stricter regulations by the worldwide authorities require products with the lowest possible risk for the environment. The chemical industry has accepted this new challenge with the development of new, eco friendly products. Together with modern finishing processes, an environmentally friendly leather process is possible.