Advances and modern techniques in fatliquoring and retanning

Back in the 70s the standard upper leather was predominantly corrected grain side which required tightness across the hide. For this a fairly standard retannage of 2-4% syntan to level the tannage, 2% resin retanning agent to tighten the flanks and 5% mimosa to give fullness were applied. The rest was left to sperm whale oil and the standard acid dye to provide softness and colour. If the required colour changed and an alternative acid dye was sought, the process of trial and error to determine the exact dye shade was carried out by altering the dyes, not the retannages. The ecological impact caused by such a retannage, which could strip chrome from the leather (in quantities of up to 20%) and the fairly limited uptake, 50-70% of the actual retanning agent, was of minor importance. Concerns from environmentalists over the sperm whale population were also only just coming to a head. And while moves were being made to ban the oil, most companies had sufficient stocks to last several months, if not years. So, what caused the change in the way that leathers are made today? A number of factors can be deduced. Firstly, the change in fashion for high performance, yet aniline leathers meant that extremely smooth, flat grains were required. Post tanning materials on such leathers had to be lightfast and fit for purpose, which now includes many areas that were considered 'impossible'. However, because companies have to compete on the world stage and with a more demanding consumer, processing became more technical. As there was no thick finish to repel the ingress of oil or water and damage the aesthetics of the grain surface, retannages had to change as well. These had to be able to penetrate the leather and become fixed to allow properties such as handle and softness to be modified without making the leather 'loggy' from over use of fatliquor. A good example of this is given below. A pack of leather arrives at the warehouse and is said to be 'too hard'. The decision is made on the next pack to add a further 2% of fatliquor. No adjustment to the retannage, drying or softening processes is made. The new pack arrives at the warehouse softer but looser and loggier! A better option would be to improve the penetration of the retannage, use less fatliquor and make adjustments to the drying and softening processes. This would result in better leather, no hardness, less fatliquor used and perhaps saving in the retannage material through better utilisation. Companies such as Colorantes Industriales have developed new ranges of fatliquoring agents, based on state of the art synthesis and formulation concepts. These take into account the points mentioned above, and consider the fatliquoring process in terms of producing quality leather, and the co-lateral effects that the fatliquor composition in the storage and handling will have an effect on the environmental impact, physical properties and ageing of the finished leathers and manufactured articles. These considerations are embodied in the concept of what Colorantes Industriales call a 'cycle of life' for the fatliquor. This involves the storage stability, the application performance and, not least, the implications and effects on the leathers after fatliquoring. Taking each in turn: * Storage prior to use. The fatliquoring auxiliary must be of constant composition and stable in long term storage in order to confer the desired properties onto the leather. It is thus important that the variability of raw materials is regulated by means of a uniform chemical treatment and a balanced formulation together with the most appropriate synthetic agents in every case. The company's LW range is designed with this first step of the fatliquor life cycle in mind. * Application in the float. A fatliquor incorporates surfactants and other minor substances that guarantee its emulsifying ability and stability to the application conditions. These products, carefully selected in the development of Colorantes Industriales' Eurosintetic LW range, must not affect the performance of the fatliquor once applied, yet they are a key point in getting lower environmental impact by means of good exhaustion, which also means a better economic yield of the product. * The processes after fatliquoring also affect the performance of the finished leather and fatliquoring has a big impact in the drying and further conditioning of the leather prior to the mechanical operations. The LW range improves the physical characteristics, giving higher surface yield and better finish adhesion. In the finished leather, the risk of undesired effects such as spew or colour changes due to migration or oxidation is considerably lower with synthetic products than with the classical products described above. Colorantes Industriales says this is due to their products' simple application, versatility for use on different substrates and their chemical nature giving high affinity to the collagen fibres. A list of the company's products is given in Table 1. Application results The following are the results of several application studies carried out to check the specific properties of the different products in Colorantes Industriales' range. Comparative trials with classic fatliquoring agents were carried out in the laboratories of AIICA (Spanish Leather Research Association). 1. Minimisation of the environmental impact of fatliquoring In Table 2 the analytical results obtained in exhausted fatliquoring floats with an offer of 10% of product on shaved weight are analysed. Comparing LW range products with conventional fatliquors of a similar type, in every case the float was better exhausted, with a corresponding decrease in the COD of the residual float using the LW range. 2. Dyeing yield and colour levelling The LW range fatliquors have an ability to provide uniform, bright and more intense dyeing than the conventional ones, Table 3. The values were obtained in dyeing trials with Direct Red 23, at 1.5% on shaved weight. The intensity was measured using a Win Color colorimeter system. 3. Low extractability A comparison of shoe upper nappa leathers, but with an increased fatliquor offer to get clearer evidence of the differences in extractables performance, is given in Table 4. The standard test method IUC-4 / ISO 4048, extraction with dichloromethane, was used. This is an important property in shoe upper articles, especially when they are intended for vulcanising. The analysis shows that the solvent extractable material can be reduced by 40%. 4. Better physical resistance Tests were performed on garment nappa leathers with a lecithin type fatliquor. The tensile and tear strength were measured according to IUP 6/ ISO 3376 and IUP 8/ ISO 3377, Table 5. The high tear resistance, critical in garment leather performance, is reinforced with a 40% increase in the tensile strength, while the elongation at break increases by 20%. These values illustrate an improvement in the leather elasticity, performing like a softer leather but without increased tendency to deform or bag. 5. Greater surface yield The LW range imparts the leather with good staking and other mechanical properties, which results in a higher surface yield, with the intensity depending on the kind of product and amount used. See Table 6 for the results obtained during comparative trials, based on the application of 10% of product on shaved weight. Conclusion The comparative trials of the LW range products against classic fatliquors show leathers produced with the LW range have a finer grain and more uniform pattern across the hide and gives greater fullness and softness, thanks to their excellent lubricating performance. But, it is tanners such as Pittards that have really led the way in developing new and exciting opportunities in leather sales, by expanding the way in which leathers are produced - much of it in the post-tanning processes. A look at their website shows they have a range of high performance leathers that are, for instance, able to offer 'enhanced breathability for superior hot weather performance, outstanding wet weather performance, superb whiteness and the ultimate in foot comfort'; improve the abrasion resistance of basic glove leather by focusing on the fibril bundles within the leather structure and encasing them with microscopic ceramic 'armour plates' which are more resistant to abrasion, thus increasing the durability of the leather by preventing wear on contact surfaces and materials. Individual fibre lubrication, allowing fibres to move over and against one another, is the key to softness. Traditional lubrication as mentioned above is to look at wet lubrication in the form of oils or fats. However, in a Pittards gloving leather these oils or fats are designed to resist the effects of exposure to moisture or perspiration and at the same time allow controlled fibre extension consistent with the 'fit' required for gloving leathers. There are also cool technology leathers, something that TFL has been promoting recently. Again, Pittards have developed a leather which works by reflecting the sun's rays associated with heat, which has a number of benefits, including increased wearer comfort in hot weather, leathers which will retain original character longer, and stay supple longer. Colours will not degrade in the sun and the leather remains breathable, allowing moisture and heat generated by the body to escape. One of the key benefits of a cool leather system is that it does not require any perforations for breathability and cooling, which is the traditional way motorcycle clothing has been made for use in hot weather. This means that a jacket made of cool leather becomes an all weather jacket as it will keep the rider warm in the winter. TFL have developed a number of post tanning products that exhibit this type of technology. Other technologies such as fire retardant treatment, which chemically bonds to the fibres rather than the surface of the leather, not only prolongs effective life of the product, but once out of reach of a flame, the leather stops burning within two seconds and stops glowing within five seconds. Also of interest is the way in which the changes in post tanning production have been proclaimed. No longer leaving the marketing to its customers, Pittards have advertising tie-ups with many of the companies they supply, such as Nike, Puma and Footjoy, the new breed of post-tanning processes and the desire to produce leathers that are 'fit for purpose' and able to challenge other materials has revolutioned the industry in a way unimaginable 30 years ago.