To chrome or not to chrome

19 May 2005

The abstracts of the following papers on chrome and its alternatives are listed in the same running order as the IULTCS congress programme: New Perspectives in Iron Tanning by N K Chandra Babu, R Karthikeyan, R Ramesh, Usha Ramamurthi and T Ramasami, CLRI India. Due to the similarity in aqueous chemistry, iron (III) salts are often considered potential alternatives for chrome tanning salts. However, although the development of iron tanning is as old as chromium, it has not yet gained foothold in the leather industry due to several disadvantages such as instability of its basic solution, deterioration of leather due to high acidity, lower hydro-thermal stability, darker shade etc. Moreover, iron tanned leathers are also less amenable for the production of soft leathers such as garment and gloving leather. In the present study, an attempt has been made to overcome the problems associated with iron tanning for the production of goat suede and sheep nappa garment leathers. The process technology for the production of iron-tanned leather, which withstands the boil test, has been developed. A judicious combination with chrome has been attempted and a tanning system, which affords good quality leathers while at the same time helping to restrict the amount of chrome and iron in spent liquor to less than 100 ppm, has been standardized. Scanning electron microscope analysis was carried out on the tanned leathers to study the effect on structural characteristics of leathers produced. The various physico-mechanical properties and the dyeing characteristics of tanned leather were also analysed and reported in the paper. In conclusion, the study revealed the possibility of producing a chromium-free fully iron tanned leather with a shrinkage temperature of more than 100°C employing a novel iron tanning process which provides a hydrophobic fibre coating. An iron-BCS combination tanning system has been standardized for the production of soft leathers, goat suede garment leather and sheep nappa garment leathers. The tanning trials show that the exhaustion of iron and chromium could be more than 96% in both cases. Visual assessment data supported by scanning electron microscopic analysis indicate that leathers tanned using an iron and chromium combination tanning system exhibit high degree of softness and smoothness. These results show that replacing chromium partially or fully with iron appears more a possibility now than before. Chrome Free Organic Tannages and Molecular Level Understanding of the Tanning Process for the Production of Leather with High Hydrothermal Stability by Samir DasGupta, New Zealand Leather and Shoe Research Association, and Santanu Deb Choudhury, Massey University. DasGupta has devoted his life to improving the chrome-tanning system. When he was a student, he wondered why chrome tanned leather that stood the boil, did so for only for about 1-2 minutes. Later a product and process for improving the hydrothermal stability of chrome-tanned leather was patented. This showed how chrome tanned leather could be boiled for an hour without area loss. At LASRA they also developed ThruBlu and modified ThruBlu processes to minimise the environmental impact of chrome tanning. LASRA do not believe there is anything wrong with chrome tanning. They say that while being aware that chrome tanning has attracted some concerns about chrome VI, with proper care and management this can be controlled. In fact, studies with New Zealand wet-blue showed that all wet-blue tested had chrome VI below the detection level. Even though the results from dyed leather are not very reliable, a tendency for increased generation of chrome VI in samples subjected to 144 hours of exposure to heat at 100°C and UV light with an increased pH of neutralisation was observed. Most of the reported existence of chrome VI in leather might well be artifacts of the test method (IUC:18) and Pastore et al also showed that chrome VI in irradiated leather when stored in the dark for six months was reduced to below the detection limit. Further, chrome tanning is the most economical and problem-free tanning system that has evolved over many years of experiments and its effluents do not need very sophisticated treatment. However, tanners must provide leather to satisfy the requirements of customers and if they want metal free leather, tanners must produce it. DasGupta stated that he was the first person in the industry to produce metal-free leather that could withstand boiling. He introduced oxazolidines as a new class of tanning agent (British Patent 1481508) to the leather industry in 1973. At that time, it was noticed that oxazolidines reacted strongly with vegetable tannins such as mimosa extract powder; oxazolidine A (4,4-dimethyl-1,3-oxazolidine) being faster reacting than oxazolidine E [1-aza-3,7-dioxabicyclo-5-ethyl (3,3,0) octane]. Similarly with pelts, oxazolidine A reacted faster and fairly independently of pH, and gave a shrinkage temperature (Ts) of about 84-86°C whereas oxazolidine E reacted slowly and with increased pH gave a shrinkage temperature of about 82-84°C at pH 7.5-8.0. In combination tannages they behaved quite differently in imparting characteristics to leather. In combination with vegetable tanning agents, oxazolidine E gave a shrinkage temperature above 100°C easily whereas the too-reactive oxazolidine A gave about 96-97°C. The quality of leather was also different; oxazolidine E gave a mellower and finer grain and a paler coloured leather than oxazolidine A. Up to about 10% mimosa extract the difference in quality was not that significant. On chrome combination tanning, again oxazolidine E gave very good quality leather, with excellent chrome exhaustion, finer grain and a very soft type of leather. Therefore, oxazolidine A was recommended as a pretanning agent for pickled pelt for degreasing purposes whereas oxazolidine E was recommended for combination tannages with chrome or vegetable tanning agent and for wool-on skin tannages. Most of the present so-called wet-white tanning is based on aldehyde chemistry (either oxazolidine or glutaraldehyde) and to a lesser extent tetrakis hydroxy methyl phosphonium sulfate (THPS) and as the patent covering oxazolidines has expired, many companies are marketing oxazolidines under different commercial names. But the leather obtained by these mimosa/oxazolidine combination systems looks and performs like vegetable tanned leather. Earlier a compound derived from vegetable polyphenol and tetrakis hydroxy methyl phophonium salphate (THPS) was synthesised. Oxazolidine was too reactive with polyphenols and the reacted product could not be used. This novel compound gave shrinkage temperatures around 93°C, and a small amount of aluminum salt was required to produce leather of comparable characteristics to chrome-tanned leather. At LASRA they are trying to create a new type of leather that will be in between chrome and vegetable tanned leather by incorporating synthetic resin and reducing the use of mimosa to a minimum. They have been working on the production of chrome-free lambskins with high shrinkage temperature. They concluded that organically tanned leather with a high shrinkage temperature of around 100°C was obtained with 8-10% mimosa and 1-2% oxazolidine. Results of molecular level changes in collagen structure was presented to improve the understanding of tanning of the leather that might confirm the mechanism of oxazolidine tanning as postulated by DasGupta and proposed recently by Lu, Liao and Shi and supported by Vitolo et al.Development of a High Stability Chromium Free Tannage by S Clara and A Hudson, BLC, R J Heather and Y Di, Loughborough University, and H Manock, Industrial Copolymers Ltd. The primary objective was stated as developing a method of converting hides and skins into economically viable, commercially acceptable leather, compatible with current production methods by cross linking the carboxyl groups of collagen through a three-dimensional matrix analogous to chrome tanning. The project is intended to develop a high performance, thermally stable epoxy tannage to be used to produce mineral-free leathers, primarily for the automotive sector within the leather industry. Due to the perceived environmental and commercial advantage of utilising mineral-free products, the leather industry has been increasingly encouraged to consider alternative processes and chemicals. This has led to a resistance to the use of chrome III in tanning, which has been unfortunate in its association with the suspected carcinogen chromium VI. The driving force behind the push towards mineral free tanning is the automotive industry, which is the biggest user of chrome-free leather. Although chrome-free leather only accounts for around 5% of total automotive production, the major automotive leather producers are nearer 30% chrome-free. Taking Audi as an example, around 50% of vehicles produced have leather, all of which is chrome-free. Existing mineral-free tannages tend to yield leathers with low shrinkage temperatures (~80-85°C) compared with chrome tanned leathers (>100°C). This is a measure of the degree of tannage and consequently the thermal dimensional stability of the leather. The commercial mineral-free tanning materials are mostly aldehydic in nature, often based on glutaraldehyde or modified glutaraldehyde, many giving positive results with current formaldehyde testing methods. The issue of formaldehyde is critical. Currently, many car manufacturers impose a very strict limit of 10 ppm for automotive leather which is difficult to achieve. In addition, the use of aldehydes in processing presents health and safety risks to operators. Glutaraldehyde based tanning agents can also cause problems within biological effluent treatment plants where aldehyde can act as an effective biocide. This leads to demands for a mineral free tanning agent which can crosslink the carboxyl groups rather than the amine side chains of the skin's collagen to give a thermal and dimensional stability currently offered by chromium. Studies on leather tanned with multi functional epoxy resin have been found to yield leathers with similar physical properties to chrome and vegetable tanned leathers. However, the tannage gave a poor performance and yielded leathers with a low shrinkage temperature and was reversible. Other researchers showed that the addition reaction of the epoxide with primary amines on collagen is more predominant than that with secondary amines. The cross linkage among collagen fibres is introduced mainly by the reaction between epoxide and primary amines. It is introduced only slightly with secondary amines and carboxyl groups, but not with hydroxyl groups. In later studies, it has been shown that with the use of catalysts such as 2,4,6-tris (dimethylaminomethylphenol) and salicylic acid, the shrinkage temperature can be raised to more than 80°C within the same time-scale as current commercial chrome tanning procedures. It is known that there is a relationship between the rise in the shrinkage temperature and the logarithm of the number of cross linkages introduced by the epoxy tannage of a more conventional nature, but not as a sole tannage. The proposed project is applying and developing the current state epoxy chemistry to an application that shows the potential to be exploited but, for commercial and technical reasons, has remained inadequately investigated in the past. It is being carried out under the Sustainable Technologies Initiative and is being funded by the DTI. A new Pretannage with Glyoxal and N-Thioureidopyromellitamic acid for High Exhaustion Chrome Tannate by Hongru Wang and Xiang Zhou, College of Chemistry and Chemical Engineering, Donghua University, Shanghai. A new compound, N-Thioureidopyromellitamic acid, has been synthesised from pyromellitic diahydride and thiosemicarbazide and used in pre-tanning along with glyoxal to improve chromium uptake. The results show that the pretanned pelt has a higher shrinkage temperature than conventionally processed pelts and subsequent chrome tanning results in a significant increase in chromium uptake. Thus, the chrome offer in conventional tanning can be reduced significantly. Because of the higher shrinkage temperature of the pretanned pelt, the initial temperature of the two-stage chrome tanning process is raised and a much higher chrome exhaustion obtained. The handle and the physical properties of the pretanned crust are not much different from those of unpretanned crust. Organic Tanning in the Production of Leather for Automobile Upholstery by U Sammarco. In 2005 a regulation will come into force in the EU which requires that 95% of abandoned vehicles must be recycled and leather devoid of heavy metals will be more easily biodegraded than that which contains chrome. Another reason which led to the research of new technology is to eliminate the risk of chrome VI formation. The research described in this work has indicated a metal-free process which can be applied at an industrial level. Essentially, this consists of a pretannage using organic, natural and synthetic products. The finished leather gives optimum characteristics of softness, fullness and firm grain together with the mechanico-physico and eco-toxicological standards required by the principal European automotive companies. Studies on Eco-friendly Chrome Tanning Process by Goutam Mukherjee and Sanjoy Chakraborty, Department of Leather Technology, Government College of Engineering and Leather Technology, Salt Lake City, Kolkata, and Gopal Krishna Biswas, Department of Chemical Engineering, Jadavpur University. Conventional chrome tanning methods employed in the leather industry subject the hides and skins to treatment with a wide variety of chemicals and various processing operations while involving an enormous amount of time and, most alarmingly, profuse quantities of water. These contribute to an increase in COD, chlorides, sulfates and other mineral salts in tannery effluent. It is the use of enormous amounts of precious water and the rapid depletion of ground water that this research seeks to address. A process sequence has been investigated whereby deliming, pickle and basification-free chrome tanning can be used for the stabilization of hides and skins. Associated washings which follow liming and deliming in conventional processing have been removed. Limed collagen matrices can be transferred to develop a chrome tanning system which takes place directly after the fleshing operation. This leads to a substantial decrease in chemical consumption by 20% for chrome tanning. Water consumption can be cut by 37-40% and the time for tanning from the raw can be reduced by 12-15%. The performance of the tanned leather is equal to or sometimes better than conventionally processed leathers. Added to this, COD loads are reduced by 80-85%, TS content by 82% and TDS by 84%. The process is also economically viable when compared with conventional processing due to lower power consumption and the use of fewer chemicals. Good quality, fuller, thick tanned leather can be obtained from this formulated process which indirectly results in a better yield. The author said that tanning by this method is currently being carried out in tanneries in Kolkata. Formaldehyde-free Leather: A Realistic Objective? By Renate Meyndt, Heinz-Peter Germann, Lederinstitut Gerberschule Reutlingen (LGR Germany). Recent market investigations reveal an increasing demand for 'formaldehyde free' leather. Probably this situation will become more critical due to the results of the reevaluation of the hazardous potential of formaldehyde and in consequence, its classification as a proven carcinogenic substance. Presently no regulation by law exists for this chemical compound in the EU. But in lots of areas of application the amount of formaldehyde contained in leather is limited by technical specifications or eco-labels. Formaldehyde is used in large quantities as a starting material for the production of synthetic aromatic and resin-tanning agents. During the production of these compounds formaldehyde acts as a condensation agent, helping to get larger molecules. Presenting different hydrolytic stabilities, the resulting condensation products are potential sources of formaldehyde. Some dyeing auxiliaries, fatliquors and finishing products are able to release formaldehyde as well. However, high quality requirements cannot be met by renouncing the use of different tanning and retanning agents and a wide range of dyeing auxiliaries and fatliquors. Therefore, it seems to be necessary to develop a leather producing technology, which meets the high quality requests and at the same time guarantees a minimal potential of formaldehyde release of the resulting product. Attempts to selectively bind free and part hydrolytic releasable formaldehyde in leather with suitable compounds (scavengers) have been already reported. The utilisation of the most recently developed products and the modification of some of the technological steps have also been investigated. Another method of reducing the formaldehyde content of leather is to select the components of the recipe on the basis of their potential to release formaldehyde. The practicability has not been fully given yet because of the lack of an adequate analytical method capable of determining the free formaldehyde in tanning agents and auxiliaries. The development and validation of such a method was an essential part of this research work. Further, investigations were made to see if the interaction of chemicals and leather matrix generate synergistic effects. In this context it was interesting to elucidate the influence of the ageing process on the formaldehyde content of the leather. Additionally, several compounds were tested to reveal their suitability for being used as formaldehyde scavengers. The results of the performed investigations may be summarised as follows: the analytical procedure for the determination of free formaldehyde in products used in leather manufacturing provides reproducible results; the method is suitable to identify formaldehyde sources within the recipe; by selecting recipe chemicals on the basis of free formaldehyde content determined with the LGR-method leathers with considerably low formaldehyde contents were produced. However, the part hydrolytic releasable formaldehyde could not be completely eliminated. Synergistic effects in the leather matrix obviously exist. The efficiency of formaldehyde-scavengers is also depending on the initial formaldehyde content of the leather; the artificial ageing of leathers with low formaldehyde content caused no significant increase of the measurable formaldehyde; the repeated analysis of leathers with a relatively high formaldehyde content stored in standard climate conditions over a period of six months did not result in any significant increase of the analyte (DIN 53 315 B).

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