‘The fact that methods of hide and skin preservation have been thoroughly covered in previous John Arthur Wilson presentations serves to highlights its importance to the leather industry. That salt, in spite of its inherent impact on the environment, is the most widely used preservative today shows how difficult it has been to find a suitable alternative.’
The Wilson lecturer, Dr David Bailey, commented that the only major change in practice in the last few thousand years was using brine curing instead of salt packs and this occurred less than half a century ago.
Salt curing presents environmental challenges for the tanner and the packer that become more difficult to overcome as time goes on. Despite of these shortcomings, no commercially acceptable alternatives have been put into use on a large scale. And Bailey has spent a large part of his career looking for alternatives. In many ways he was successful, but industry felt, and probably still feels, that salt is the easiest, cheapest, and best known preservative for hides. There are, of course, many alternatives and Bailey discussed a few of them in his lecture. He said that in the US these methods could include:
* using hides fresh, but unless the tannery is right on the doorstep this is impractical. However, it may be possible for someone like Tyson to process through to the blue, but there are concerns about whether available markets make the monetary outlay realistic
* chilled, which is expensive, and needs infrastructure, but some of the costs could be offset by other uses of the hides, such as gelatine and collagen cases
* possible chemical preservatives such as biocides, sulfite/acetic acid, and bacitracins, which are anti-bacterial peptides. In some small scale trials, these proved very effective against bacteria at room temperature. However, the work has been halted due to possible patent applications, Bailey noted with regret
The chemically-based one he seemed to like best was the mixture of sulfite and acetic acid, which he trialled in the late 60s. In a 30 day trial, hides were kept in a barrel at elevated temperatures and when tanned produced perfectly good leather. Bailey suggested the mechanism probably involves the release of sulfur dioxide, which acts as the preservative.
On an even simpler level, replacing sodium chloride with the potassium salt is something that could be put in place tomorrow. ‘It’s (KCl) a perfect copy with none of the environmental concerns.’ Bailey cannot understand why this technology hasn’t been developed further. He has done large scale trials and shown that KCl has no adverse effect on leather quality. But as one tanner said to me, the problem is not with the cation, but the chloride, which the Publicly Owned Treatment Works don’t like either.
Another favourite, and one which may eventually become an industry standard is irradiation. Having tried both gamma and electron beam irradiation, Bailey is convinced the latter is a winner. The former was ruled out because: ‘the problem is the radiation comes from spent nuclear fuel rods – try selling that to the environmentalists!’ He said that: ‘now is the right time for e-beam. The FDA have just approved e-beam irradiation of meat, so it is just one step back to the hides.’ He pointed out that e-beam irradiation is not sterilisation of the hide, but that a small initial offer of bactericide ensures the hide is preserved long enough to transport and process.
At times this conference was like going back to my British Leather Confederation days of the late 80s. Sam Mozersky, from the Eastern Regional Research Center, spoke about the quantification of glycosaminoglycans (GAGS), specifically the sulfated polysaccharides in bovine hide. The work is part of USDA’s ongoing investigation of non-collagenous proteins and how they affect the leathermaking process.
Mozersky said that work by Ken Alexander and co-workers at BLC in the late 80s had developed a protease treatment to improve opening up, something I was involved in, and that they needed to quantify GAG removal. By developing a procedure for bovine hide from the work of Bjornsson and Karlsson with Alcian Blue, the BLC were able to do this, and quantify GAG removal.
However, as Mozersky showed, the method, while elegant, is complicated, involving elecrophoresis to separate hyaluronan (or hyaluronic acid as I knew it!) and dermatan sulfate. So, what ERRC have done is to separate the two components chemically using the lack of sulfate on the hyaluronan. Other modifications include pulverisation of the skin under liquid nitrogen, digestion of the skin with collagenase, and various changes to analytical procedures. The result seems to be an excellent method for determining the sulfated GAG content of bovine hides. Mozersky said that the procedure developed yields linear results for 50-300mg samples of skin.
The question of sulfide-free unhairing has also taxed the ERRC for a number of years. Bailey came up with the concentrated sulfide solution which was used to reduce pathogen contamination from hair entering the slaughterhouse. Robert Dudley, from the USDA, presented a paper that looked at three different alkaline oxidative unhairing protocols, two of which were hair burn processes (alkaline hydrogen peroxide amended with either potassium cyanate or urea) and a hair save process (alkaline sodium perborate).
He said leather produced from hides that were unhaired using the sulfide-free unhairing had at least as good physical properties as leather produced from hides unhaired using sulfide, and were more flexible and tougher. An added advantage of the sulfide-free unhairing protocol is that the relime step can be eliminated without adversely affecting the quality of the leather. The work continues.
Liquid tanning
The second flashback to my BLC days came from work reported by Adel Hanna of Elementis, the chrome producers. He discussed the successful trials of Waynetan 2000, a new highly masked liquid chrome tanning agent that eliminates the need for pickling and basifying. Hanna said that customers wanted consistency and uniformity, in ever shorter production times. Processes such as pickling and basification are staging steps, necessary to ensure chrome tanning is completed successfully, but not actually involved directly in giving the final leather its character. And there were always problems of chrome staining if basification was incorrectly done.
Thus, could they be eliminated? To do that the chromium would need to penetrate and fix at pH values at or above neutral. There have been researchers, such as Otto in the 60s, who have masked chromium with multi-functional ligands, but little has been done commercially.
However, Elementis, in collaboration with MK Quimica, have developed the product that can be added after a thorough deliming to pH7.5-8.0. He said that the trials had been very encouraging, and with a final pH after tanning of 3.5-4.0, the tanner can carry on with post-tanning processes as normal. ‘The process eliminates [pickle] acids, with the resulting reduction in sulfate concentrations in effluent and excess salts, and saves time’, Hanna commented.
Fine, I’m sure it works and gives you a leather satisfactory for its intended purpose, yet 40 years ago an ex-colleague of mine at BLC was working at Wades Tannery in Nottingham, chrome tanning elephant hide or, to be more accurate, elephant foreskins, by deliming completely with ammonium sulfate, clear to phenolphthalein. Following a thorough drain, he added 0.15% formic acid, diluted 1:10, no other float, followed by 42 or 50% (25% Cr2O3) chrome powder 10 minutes later – exactly the same effect as with Elementis’ product was achieved, and the hide was tanned after 4 hours. The final pH was about 4.2-4.5 depending on the chrome.
OK, he admitted that if there was a power cut you were stuffed, but that never happened in his time, and it took a degree of leathermaking skill, the art not science of making leather, to do well, but in this day and age, with all the drum technologies available you would think such ‘simple’ technology would be highly useful. Also, of course, there was zero Cr in any waste liquor.
I suggested this to a tanner in the US, and he said a difficulty is that: ‘US tanners are burdened by workers comp assessments, so any time that they can automate a process and reduce handling by people, the risk of injury decreases. Powdered chrome additions to drums of the capacities [most tanners] used would invite back injuries. Graham, you will have to spend time in the US and get more attuned.’
Crisis point
Another problem is, of course, that zero ppm chromium in the waste is not important anymore – many end users don’t want chrome in the leather. Jens Fennen from TFL gave a very interesting lecture on the status of wet-white technology, and pointed out that the technology should not just be seen as the ‘ecological’ alternative. He said that for dashboards in cars, non-chrome leathers are often better. He cited the Porsche Test, which involves cycling leather samples through heat and humidity conditions, and measuring the resulting shrinkage after reconditioning at ambient temperature. Results showed that chrome tanned leathers lost greater area than their non-chrome alternatives. This is important in leathers that are used on dashboards for cars in hot countries, where the temperature in the car can vary dramatically. The fact that the wet shrinkage temperature for the non-chrome leather may be much lower than the chrome equivalent is irrelevant, it is dimensional stability that is important, and the possible recovery after deformation.
In conclusion he said: ‘The major strength of wet-white leathers is their dry shrinkage behaviour, and their lower density, both of which are important to automotive manufacturers.’
Both Hanna’s and Fennen’s papers led to a discussion about how poorly the industry has handled the chrome (VI) ‘crisis’. Practically all views were pessimistic, with comments suggesting that in the automotive industry ‘there would be no chrome tanned leather within ten years’, that the EU’s vehicle directive on totally recyclable cars is a ‘sword of Damocles’ hanging over the industry, and that the industry has done a poor in job in getting our view over to the general public. The main complaint seemed to be that there was too much conflicting information with organisations that are on the periphery of the industry ‘making a quick buck’ on testing for chromium (VI) and sending out inflammatory signals about how much chromium (VI) there supposedly is in leather.
While there was a call for the LIA to take the lead in the US to promote the leather industry’s view, there seemed to be a resigned air of Europe’s chances of still having chrome tanned leather in cars. While some suggested that the industry in the EU needed to agree a limit for chromium (VI) and ensure that no leathers exceeded that, others said there was no point in arguing for a ‘safe’ level as EU laws stated any carcinogen had to have a zero value.
Thus, effectively the limit for Chromium (VI) is the limit of detection of the method used, and as one knows that gets ever lower with increasing use of new detection methods.
An interesting comment came from David Rabinovich: ‘There is no metallurgical need to separate the leather from the car metal that will be melted down for recycling, as substantial amounts of other organic material, wiring insulation, gaskets, seals, paints, plastic etc are always present in scrapped autos and will end up as carbon monoxide gas anyway, and all the chromium, if the leather is allowed to remain in the arc oven, is reduced and alloyed into the resulting metal.’
Given our propensity for leisure and car travel, the need for improved resistance of automotive leather finishing to insect repellent and suntan lotion is a high priority in the leather industry, especially due to the increase in leather-trimmed vehicles being marketed for outdoor activities. A study from TFL (USA), presented by Loyd Burcham, examined the diffusion and thermodynamics of absorption of DEET, a main component in insect repellent and suntan lotion, into a series of polyurethane and polyacrylic coatings. Burcham said that although they wanted to isolate the main component of suntan lotions they couldn’t because: ‘suntan lotions are a real witches brew’.
The sorption behaviour of the products was investigated and the physical changes in the finishes measured using the Veslic test and visually. The samples were also desorped to replicate the effects of ‘cleaning’.
The effect of DEET was a 90% reduction in the number of rubs, but after a period of desorption, values returned to similar predosing levels. The effect of the suntan was less severe, with the crosslinked finishes seeing little or no reduction in integrity. However, following desorption, the Veslic values dropped, which Burcham suggested was due ‘to components of the suntan lotion remaining in the finish.’
Visually, the DEET had no yellowing effect, but the suntan lotion produced a definite change in the finished leather.
Burcham said it was found that the mechanisms of absorption of insect repellent and suntan lotion depend upon the glass transition temperature, polyol type, and crosslinking density of the coatings. He came to the conclusion that finishes are very sensitive to insect repellent and suntan lotion depending on the type of finish used, and this work suggests the tanner needs to talk to the finish suppliers to improve the beach resistance of car leathers. I wonder if the effects of sand mixed with either or both products have been investigated?
Eco-friendly
Other clever chemistry in the congress included the work reported by Gunther Pabst from BASF. When a major chemical company decides to ‘solve’ a problem they have the resources to do just that, and this was a classic example. Pabst said that the degreasing of animal skins is an important step in leather making. A huge quantity of surfactant is used for this process, which leads inevitably to environmental problems. Therefore, the development of eco-friendly degreasing agents and processes is desirable.
For a better understanding of the degreasing process, he and co-workers investigated the fundamental physico-chemical principles of degreasing. Only after this fundamental knowledge had been gathered, did they develop a novel class of eco-friendly degreasing agents. The lecture presented the latest research results on the physicochemical principles of water-based degreasing.
In cooperation with the University of Cologne, they were able to determine that degreasing takes place in two distinct steps. The first step, under the proper conditions, is the formation of a nanophase to solubilise the grease in the skin. The second step is the conversion of that nanophase into a macroemulsion to transport the grease out of, and then away from, the skin. This knowledge led them to the development of a novel class of degreasing agents in a more eco-friendly degreasing process.
Back in 1986, I spent a rather fruitless year trying to emulsify fats and determine their whereabouts in leather. When Pabst said that they used high throughput screening techniques used in the drug discovery industry to investigate, I began to understand why I had limited success! He said that they investigated 400 samples per day, and over 5,000 in total. Encouragingly one conclusion we both came to was the need for a co-emulsifier, something I called a coupling agent, to stabilise the emulsion. In BASF’s case, it is used to convert the nanophase into the macroemulsion to ensure good removal of the grease. But, the lecture just went to show that if you have the money, all the answers are available.
Brian Bolton, vice president of marketing and sales for Optibrand, Fort Collins, Colorado, had an interesting lecture for the audience. He said that the total yield of hides can be severely reduced by the hot iron branding technique, particularly when placed on a high value section of the hide, such as the butt. The requirement to identify cattle by hot iron branding has long been part of the US culture and indeed in many US states branding is the law. The advantages are that it is a permanent method of relating an animal to an owner, and, unlike ear tags, it cannot fall off or easily be altered. Ear tags, whether visual or containing an electronic chip, have been the only other alternative to animal ID. Until now that is.
Optibrand have developed what they claim to be a permanent, secure, painless and low cost method of identifying cattle by capturing an image of the Retinal vascular pattern and linking this image to time, date and place via GPS. The retinal vascular pattern is unique between eyes, is permanent and does not change from birth to death. He claimed the Optibrand system has the power to overcome the shortcomings of both hot iron branding and tags. The result for the industry would be increased hide yield and greater return to the cattle and leather industry.
Back in the Dark Ages, leather was used as body armour to protect against swords. When metal armour came in it went out of fashion, simply because the metal, although far heavier, offered more protection. However, what goes around comes around, and Seshadri Ramkumar, from the Institute of Environmental and Human Health, Texas Tech University, in Lubbock, was on hand to discuss the development of leather clad ballistic proof fabric composite.
He said that the need for multifunctional lightweight ballistic protective fabrics has grown in significance in the current global scenario. A novel methodology has been followed to develop leather based antiballistic composite fabrics.
Ramkumar said that the latest technology has been effectively used to develop a next to the skin nonwoven/ballistic composite with apparel grade bovine leather as the outer layer.
The antiballistic properties were achieved with the help of high-density polyethylene filament fabrics, and the composite imparts the necessary next-to-skin comfort properties due to the nonwoven layer, while the outer leather layer provides the overall fit and abrasion resistance. The leather component also adds to the total strength and more particularly to the abrasion resistance of the composite.
The crux of the work is to develop an antiballistic composite material that can offer protection at Level IIA, which is the protection characteristic of soft body armour, and results showed that the presence of the leather (top) layer and the bottom nonwoven layer added to overall ballistic resistance of the composite. An new and interesting use for leather perhaps?
Conclusion
So, what is the way forward for leather processing? Optibranding the hide, E-beam irradiation, and spray sulfide unhairing, when big business can be bothered to get it right. Follow that with a short deliming and one-pot chrome tanning, assuming, of course, you don’t want to make automotive leather for the EU market.
I understand that tanneries in the US are looking with interest at the e-beam, but while the technology is fine, and it is not intended as a criticism of the work or the method, is it somewhat symptomatic of the world today that simplicity will not do?
I guess not, and perhaps it is simplifying the process. There would be fewer steps, less chemicals in the waste, and reduced handling leading to fewer accident claims compared with the ‘good old days’.
Again a tanner in the US commented to me that e-beam is the way forward: ‘If the hides are for export, they will be encapsulated in a plastic wrap: if for local deliveries then they will be in reusable tubs. Yes, one has to dispose of the plastic, but that is recyclable. The cost is actually no more [for e-beam] than for a conventional brine cured hide, and may well eventually cost less.’ So, roll-on technology!
