The changes in modern leather processing, to meet environmental regulations and increasing improvements in the performance of leather, put a continuing pressure on the chemical components used in leather manufacture.

In the case of dyes, the special needs of the automotive industry in particular have altered the selection criteria considerably. A range of new aspects have come forward, such as metal-free leathers, good stability to high humidities and temperatures as well as no colour migration. Drs Campbell Page and Jens Fennen, from TFL, gave a presentation entitled: ‘What are leather dyes today? — the relationship between the dye structure and its performance properties.’

The relationship between the dye structure and the selection criteria today for dyes has been part of an on-going study and these aspects were highlighted. The compatibility between dyes, the wet fastness of the dye and the propensity for colour to migrate are several of the properties, which can be related directly to the dye characteristics. Changes that are often said to come from the easily seen dye can be shown to have been due to other colourless chemicals in the processing chain. They suggested that a better understanding of the dye criteria for each application purpose was possible.

Catherine Money, head of CSIRO’s leather department, was lecturing on a favourite subject: ‘The need for improved fungicides for wet-blue.’ Money has carried out much work on this subject and she said that mould protection of wet-blue continues to be a major cost and concern to tanners. There is a need for more effective fungicides with low human and fish toxicity and minimum environmental impact.

The advantages and disadvantages of currently used fungicides were reviewed. She said there were problems associated with reduced effectiveness of some fungicides in the presence of sulfide, bisulfite, oxidising agents or grease. Systems used to overcome these problems were compared and she discussed fungicides that could control difficult moulds, such as cladosporium.

Results were presented from both laboratory and industrial trials that compared a range of fungicides and showed the benefits of some combinations, see figure 1. The results give the number of weeks of incubation at 30°C for the various treatments without mould growth on the grain or split. Money commented: ‘There is now a range of fungicides available to give good control of mould in wet-blue provided care is taken in their choice and application.’

‘Zebra among the horses – using robotics, high throughput screening and combinatorial synthesis to improve fungicide discovery for leather’ was the title of Dr Velma King’s lecture. Dr King is part of Buckman Laboratories’ discovery group. She said: ‘New technology and product needs in the leather industry raises new problems and applications, which occasionally call for new products.’

However, there are many problems related to the registration and regulatory needs. While, not costing the many millions of dollars needed to bring a drug to market, the outlay for chemical companies such as Buckman is high.

Dr King said that ‘a changing world and market place encourages an invention of new products, even in the area of biologically active substances. Closeness to the customer is a necessary ingredient to ascertain the needs a new product has to fill.

‘Because time is money, microbicide discovery research must be done in the most efficient and effective manner possible.’

Both synthesis and screening processes have witnessed tremendous technological advances, which include combinatorial synthesis and high throughput screening. To show that these procedures are producing actives, she said that new fungicide actives were currently under development.

Professor Tony Covington, from the British School of Leather Technology, UK, spoke about: ‘Advances in tanning theory.’ He said that a model of the tanning reaction has been developed, which unifies all types of tanning chemistry, inorganic and organic.

The theory is based on the creation of matrices around the collagen triple helix. All tanning processes have an entropic interaction with collagen, that introduces order into the structure, ie it has a destabilising effect, indeed some tannages can have a negative effect. But, there are tannages that also introduce some enthalpically controlled structure: the net effect is to control the shrinkage temperature and to limit the values to about 85°C. It is only those tannages that impose significant structure that can raise the shrinkage temperature to values above 100°C. The relationship between groups of tanning reactions is shown in figure 2. The matrix created can be synthetic or incorporate the supramolecular water of solvation or, perhaps, vegetable tannins can replace the water; the effect is seen with a disproportionate increase in shrinkage temperature when condensed vegetable tannins are retanned with metal ions. Covington said that with this unifying theory, all known tanning chemistries can be rationalised, in terms of their effects on hydrothermal stability.

He then went on to say that an extreme expression of tanning is mineralisation, which provides a highly stable matrix, within which the collagen is embedded. The new theory also sheds light on the breakdown of collagen. ‘Thermal deterioration advances at different rates, depending on the nature of the cross-linking matrix, in an accelerating or decelerating mode. But, initiation of deterioration is linked to the porosity of the material, variable in leather, but minimised in mineralised materials, such as bone’, he said.

In another paper (Future Tanning Chemistries), Covington spoke further about thermodynamic effects. He said: ‘It has always been assumed that tanning processes are controlled by the chemistry of the reaction alone; partitioning of reagents between the solvent and the substrate, which incorporates the notion of transport between media, has not been considered.’ He said this factor could be expressed as follows:

[substrate]solvated + [reagent]solvated <-> [reagent-substrate]solvated <-> [product]solvated

The reagent is partitioned between the solvent and the substrate and in a heterogeneous reaction such as tanning, the partition coefficient depends on the relative thermodynamics of the solvation and the affinity of the reagent for the substrate. Thus, if the solvation is made less favourable, the reagent is forced to interact with the substrate.

This idea is already used in dyeing, with the hydrophilic-hydrophobic balance (HHB) values used to match dyes and substrates. The Japanese have investigated HHB with respect to fatliquors and the notion of HHB is widely used in the surfactant industry – so applying this ‘technology’ to other leathermaking processes such as tanning, or indeed, beamhouse processes, cannot be far away.

‘Pickle-free high stability organic tanning for the production of high-tech speciality leather’ was a paper from Elkede in Greece and Conciaricerca Italia, Italy. Part of a European sponsored project, the groups investigated the benefits of a novel pickle-free tanning system using oxazolidine as the pre-tanning agent followed by condensed vegetable tannins for the tanning reaction.

The results, while encouraging, were nothing special if shrinkage temperature is taken as the criteria. While we may not like this parameter to be always at the forefront, it is important if the leather is to be fit-for-purpose. However, they came to the following conclusions:

* residual salt could be significantly reduced

* there was a reduction of 30% in total dissolved solids

* because the system eliminates a pickle step, there is a reduction in strong mineral and organic acids use

* they claimed that the tanning process could be reduced by up to five hours

* the shrinkage temperatures ranged from 98-104°C, depending on the substrate

There are essentially two mechanisms of hair removal: breaking down the structure, to a sufficient degree that it appears to dissolve (hair burning), or to detach the hair at its anchoring point in the follicle, so that at least part of the structure is apparently removed intact (hair saving).

Vikki Addy, BLC Leathersellers’ Research Centre, UK, showed some stunning electron micrographs of hair having undergone these processes. The microscopy studies of the morphology of hair structure, during typical hair burning with lime and sulfide, with and without the presence of proteolytic enzymes, demonstrated different rates of decomposition of the various components of hair: cuticle, cortex, medulla and root zone.

They have shown that the decomposition mechanism is the dissolution of the cortex, the main constituent of the internal structure of the hair shaft. Whether or not the hair is cut prior to applying the hair burning agents, the cortex is dissolved, together with the medulla if there is a proteolytic enzyme present.

Within the conditions and time period of typical hair burning, the outer part of the hair structure, the cuticle, is not dissolved. It may either remain intact, but collapse inwards, due to the hollowing mechanism, or it may break down into its scaly components: this is the scud of residual hair.

The alternative approach is to detach the hair at its base, familiar in current hair saving techniques. A safer approach, avoiding over-opening up, especially by proteolytic enzymes, is to attack the anchoring mechanism, the collagenous proteins of the basement membrane. This is something that Laurie Stephens, formerly at CSIRO, suggested in the first Heidemann symposium in 1989 at the Philadelphia IULTCS Congress.

In an associated poster, her colleague, Gordon Paul, discussed the idea further. ‘The use of dispase in enzymatic unhairing’ showed how the use of enzymatic removal of hair during beamhouse processing offers a suitable alternative, at least on the non-industrial scale, to destructive sulfide unhairing.

One approach to enzymatic hair removal involves the degradation of the basement membrane to bring about epidermal sloughing with concomitant hair loosening. To be successful in this, the enzyme used must be able to degrade the basement membrane, type IV collagen in particular, while leaving the fibrous collagen of the dermis unaffected.

The neutral protease dispase has such specificity and was consequently used in a series of studies to assess its effectiveness in hair removal from bovine hides. The enzyme caused loosening of the hair and associated hair loss, without damaging the fibrous collagen of the dermis. The elastin network of the grain layer was significantly affected by enzyme treatment, as were the physical properties of the resultant leather when compared with that of conventionally processed hide.

‘The influence of enzymatic action on the speed of collagen destruction in bovine hides’ was discussed by Dr Thomas Feigel from Trumpler. He set out to show under which conditions hide could be destroyed by bacterial action. Fresh bovine hide was stored under different conditions (time, addition of bactericide etc) and was treated afterwards with different soaking procedures (time, enzymes, wetting agents, bactericide etc).

He came to a number of conclusions:

* an uncontrolled bacterial growth during storage is the most dangerous factor for the collagen destruction. Depending on the storage time, significant amounts of collagen are broken down

* tailor-made enzymes remove predominantly non-collageneous proteins from the hide structure and surpass the action of wetting agents in their cleaning effect. Even though a hide may have already undergone bacterial damage, enzymatic soaking agents will not break down more collagen than a traditional non-enzymatic soaking procedure

* the use of a suitable bactericide will negate the possibility of further collagen degradation and is recommended if the storage time and conditions of the hides prior to arrival at the tannery are not clear


The ‘Evaluation of the performance of silico-aluminates in leather tanning’ was a poster which followed up the work of Dr Costantini, who presented his work at the last ALCA meeting. It is well known that tanning pollution is mainly related to the use of chromium-based chemicals, resulting in the presence of chromium in wastewaters and thereby in the treated sludges as well as in solid residues.

In order to reduce chromium concentrations in tannery wastes chromium conversion and consequently float exhaustion should be enhanced. In this work the authors, from SSIP in Naples, have evaluated the chromium exhaustion properties of some suitable silico-aluminates auxiliaries with different Si/Al ratio and compared them with respect to a conventional chromium tanning process.

Using calfskins, in laboratory drums, samples were treated with basic chromium sulfate in the presence of different silico-aluminates and both the leather and residual float were analysed, as a function of time, for the amount of Cr, Si and Al present. They were determined by Inductive Coupled Plasma – Atomic Emission Spectroscopy. Furthermore, the hydrothermal stability of the skins was evaluated by DSC and TGA.

They found that the use of silico-aluminates results in both higher float exhaustion of Cr when compared with a conventional chromium tannage and higher initial shrinkage temperatures (> 100°C after approximately half an hour).

The performance of the silico-aluminates was only slightly affected by their structure and chemical composition. Due to their pretanning and filling properties, the silico-aluminates were tested in combination with oxazolidines, as vegetable tannin substitutes, with the aim of achieving a chromium-free tannage. The results obtained with an oxazolidine/silico-aluminate combined tannage were very promising.

Dr Gupta, from CLRI, India, discussed some work the organisation had carried out with Atofina, involving ‘fluorinated acrylic copolymers (FAC) for water and oil repellency of suede leathers.’ He said that suede leathers are basically non-finished leathers. Thus, they are very sensitive to water and oil ingression. In order to upgrade suede leathers and impart them with water and oil resistant properties without modifying their aspect, the impact of fluorinated acrylic copolymer was studied.

The products were sprayed on the leather surface and, in addition to the fluorinated acrylic copolymer, different cross linkers were mixed with the sprayed solution to enhance the intrinsic repellency properties of the fluorinated compounds. AATCC 118 and Water-Isopropyl Alcohol (IPA) standard procedures were used to evaluate the oil and water repellency. Some of the results are set out in table 1.

They found that the use of fluorinated acrylic copolymers enhanced the repellency properties of suede leathers. Moreover, these repellency properties were enhanced with the addition of cross-linkers. The best results were obtained with polyfunctional nitrogen-based cross-linkers. Fine spraying of polymeric solution does not affect the quality of the nap.

Tim Allsop, from the New Zealand Leather and Shoe Research Association, discussed the ‘scavenging of analysable formaldehyde in woolskins tanned with phosphonium compounds.’ He said that there are increasingly stringent limits being placed on the amounts of analysable formaldehyde permissible in wool-on skins, particularly those used for clothing or for infant bedding.

Some effective white tanning agents, notably those based on tetrakishydroxymethyl phosphonium (THPX) compounds produce some formaldehyde as a byproduct of the tanning reaction. The work identified compounds that could be applied post tanning to remove most residual formaldehyde from the final product tanned with phosphonium compound.

He said that oxidising and conjugating agents were evaluated for their effects on analysable formaldehyde in solution and on woolskins tanned with THPX. Some results are shown in table 2.

The most effective of these formaldehyde scavenging agents were then evaluated for their effects on wool and leather quality over a range of application concentrations.

The results showed that treatment of woolskins at the end of wet processing could be used to lower the residual levels of analysable formaldehyde to within tolerance limits with no significant detrimental effects on the wool or leather. He also showed that holding THPX tanned skins in the wet for a time reduced the levels of residual formaldehyde. ‘Continuing the reaction between the formaldehyde and the woolskin caused a decline in formaldehyde levels whereas drying woolskins stabilised the reactivity, so the formaldehyde levels remained constant’, he said.

The problems of formaldehyde concentrations in leather have been highlighted recently and Dr Gerhard Wolf, from BASF, spoke on the subject. In a paper entitled ‘the formation of formaldehyde in leather – sources and methods for avoidance’ he said that increasing efforts are being made to eliminate formaldehyde from leather. These efforts are being encouraged by manufacturers of leathergoods, and especially by the automotive industry.

Although there is no good reason for this, the automotive manufacturers are demanding that levels of formaldehyde should be well below those expected by cosmetics producers. Putting a cream on your face containing a few hundred ppm of formaldehyde is OK, while sitting on leather containing the same concentration could be banned in the next few years.

BASF have made a critical comparison between the various methods that are employed to measure the concentration of formaldehyde in leather and the products that are applied to leather. The gas phase method is derived from a test devised by Audi/VW and has been adopted by many car manufacturers. It is based on the assumption that only formaldehyde emitted from leather into the interior of the vehicle needs to be considered.

An extraction method, DIN53315, is used mainly for shoe and garment leathers. The philosophy here is that the transfer of formaldehyde follows the sequence ‘leather/water/contact with human skin’.

The states in which formaldehyde is formed in leather were discussed. These, Wolf said, were ‘free’ molecular formaldehyde, reversibly bound formaldehyde, capable of being released and irreversibly bound formaldehyde. Preventive measures, such as mechanical processes and the use of special products, were described and some guidelines for leather manufacturers suggested. A correlation was found between the formaldehyde content of leather and the formaldehyde contents of the products employed in the production process.

There were a number of papers from Spanish sources. Jaime Cot, the new IULTCS vice-president, spoke about a pilot plant which processed chromed collagen residues and turned them into bioproducts. The work is a collaboration between academia, CSIC and IQS in Barcelona, the university of Lleida and industry, Pielcolor and Tabercolor and the Incusa tannery from Valencia.

Cot said the group were concerned with the processing of chromed collagen residues produced at the different stages of production. He highlighted five distinct groups but said the majority came from the group that included the trimmings from industrial manufacturers of leather products such as footwear, clothing and upholstery. He said that the work had shown that 98% of chromed residues could be industrially reused. Uses included as a substitute for casein or as a pretanning/ retanning agent.

Luisa Cabeza and colleagues presented work on the recovery of chrome from sheepskin tanning baths using an anionic ion exchange process. The work is on-going and this poster was a follow up of the paper presented at IULTCS XXV in Chennai, India. This poster showed that tanning trials using the recovered baths were very successful. The ovine leather produced had excellent physicochemical properties and were visually acceptable.

Dr Morera from Igualada presented a paper on the use of ultrasonic waves in vegetable tanning and came to the conclusion that ‘superior quality’ leather could be produced using the technique.

Using partial least squares regression techniques, Dr Ruis from the Igualada tanning school, has developed a model that can determine the concentration of chromium in tanning baths. He said that the results obtained when the model was supplied are statistically comparable with those obtained by a reference method and a good accuracy is obtained.

As with many conferences, there were papers which presented work in the area of effluent control. Michel Aloy, CTC, France, spoke about the ‘Reuse of tannery treated effluent through reverse osmosis.’

In arid countries, facing the low availability and quality of water, it seems useful to evaluate every area for the reuse of treated effluents being discharged to natural waters. Regardless of the quality of waste treatment, most tannery effluents contain a high level of total dissolved solids (TDS) preventing any possibility of reuse for post tanning processes. Thus, it was considered worthwhile to test the viability of reverse osmosis for this purpose.

Under a Unido supported project, a pilot plant was installed in a tannery in Tamil Nadu, India, to process effluent following full physicochemical and biological treatment. It was possible to reduce TDS from 6,000 mg/1 to 400mg/1 with a recovery rate of 70-75% of the initial volume processed.

The cost of treatment is quite comparable with that of fresh water bought by tanners and brought to the tannery by tanker. The treatment and/or disposal of the concentrate (25 to 30 g/l of TDS) and the lasting life of the membrane are the limiting factors for successful application of this technology.

Another paper investigating the problems of TDS was ‘Accelerated evaporation of saline streams in solar pans.’ This was discussed by Jakov Buljan from Unido. He said that the ultimate disposal of fully treated tannery effluent poses a serious problem in arid regions, especially in locations where the effluent cannot be mixed with domestic wastewater.

Given its high content of Total Dissolved Solids (TDS), primarily chlorides and sulfates, treated tannery effluent is not only of no use, but it can also constitute an environmental hazard. The proposals for dealing with the issue include a series of prevention methods and end-of-pipe technologies, including membrane treatment, yet they have proven to have only limited success for a variety of reasons, such as lack of infrastructure or high investment and operation costs.

In some countries environmental authorities require that soaking and pickling liquors be segregated and evaporated in solar evaporation pans. However, given the low natural evaporation rates, especially in regions with high air humidity, large areas of land are needed.

To overcome these problems a specially designed pilot unit for the accelerated evaporation of saline streams was installed adjacent to a fully-fledged tannery effluent treatment plant. After removing the suspended solids by means of a simple clarification process, the saline effluent is pumped onto the concrete platform and its temperature increased by having it run down a steeply inclined flat-plate collector painted black.

The effluent is then pumped back through a sprinkler system. After a number of cycles and close to saturation point, the effluent is transferred to conventional shallow evaporation pans.

For more than a year a study was made of the impact on the rate of evaporation of various parameters, such as wind velocity, air humidity, ambient temperature, collector angle, sprinkler type, nozzle diameter and intensity of solar radiation.

It has been found that by adopting the improved system, it was possible to increase the evaporation rate more than fourfold and, thus, reduce the area and time needed for evaporation.