Wet-white for shoe uppers

Only modern leathers, which are manufactured in accordance with ecological demands and can be recycled after use, have a long-term chance in the future. They should be in line with the health-centred, sporting life-style of the new millenium. New prescriptions on full-recycling systems and the desire to use renewable resources speed up this development. In this market segment, TFL note a growing need for so-called FOC (free of chrome) leather, normally produced from wet-white. In actual fact, theproduction of FOC leather is as old as tanning with vegetable extracts. It was not until the dawn of the 20th century that the advent of chrome tanning created a universally traded commodity in the form of wet-blue. An intermediate product, wet-blue is bought and sold around the globe and provides the basis for more than 80% of total leather production. But chrome-free leather has many demonstrable advantages: * no chromium in the effluent * fully recyclable shavings and end-products (shavings as nitrogenous fertiliser or components for leather fibres, FOC leather combined with natural latex as sole material for fully recyclable footwear) * no risk of Cr(VI) formation * excellent shrinking behaviour * brilliant dyeings, especially for fashion items * leather free of heavy metals for allergic persons * improved scope for sorting at the pretanned stage The key factor in the manufacture of chrome-free leather is pretanning the skins into wet-white. For this, TFL provide a variety of pretanning systems based on modified glutardialdehyde. This mode of pretanning has now been largely accepted in the industry. After pickling, 2.5-3% glutardialdehyde¹ (25% modified) is added gradually at pH2.8-3.1. Key factors in ensuring 100% through tanning, with unsplit raw material in particular, is complete deliming and an adequate pickle. The Sellatan Check is an indicator which has recently been developed by TFL as a dependable, rapid means to check for through-tanning (see Figure 1). The final pH after basification has a marked effect on the colour of the wet-white. TFL recommend raising the pH gradually to no higher than 4.2. This is enough to permit complete fixation of the glutardialdehyde and optimise the shrinking temperature. Decisive advantages can be gained by pretanning in combination with selected syntans and polymeric tanning agents. Combination pretanned wet-whites: * are lighter in colour and outstandingly suited for conversion into pastel shade leathers * achieve shrinking temperatures of at least 70°C * allow faster tanning and retanning with reduced amounts of chemicals * make possible more uniform final thickness and greater shaving accuracy through improved hydroextraction in samming and reduced plumping of the leathers in final tanning because the fibres are more firmly fixed * ensure greater reliability in item-specific sorting * enhance fullness and grain pattern² For conventionally and combination pretanned wet-white, the basic differences as against wet-blue or chrome crust are retained: * the shrinking temperature of the pretanned wet-white is between 68 and 78°C, and therefore below that of wet-blue * improved shrinking behaviour and dimensional stability versus chrome tanned leather * dried wet-white is more difficult to wet back than wet-blue * wet-white requires suitable preservation There are also a number of important differences between wet-white and wet-blue in terms of dyeing properties. The reflectance curve for undyed wet-white is more linear than that for wet-blue (the whiter the material, the greater its reflectance). Wet-white tanned and retanned with synthetic products therefore yields clearer, whiter leathers for dyeing; this results in more brilliant shades. Particular care must be taken to select chrome-free dyes and fixatives with a low formaldehyde content. FOC leathers have a higher anionic charge, hence the fastness properties of the dyes should be accurately tested. Wet-white can be tanned and retanned by various methods. Retanning with chrome or chrome syntans gives so-called semi-chrome leathers. The advantages are chrome-free shavings and reduced consumption of tanning and retanning chemicals. Retanning can also be performed with vegetable extracts (mimosa, chestnut, quebracho etc). When heavily topped with vegetable tanning agents, semi-vegetable tanned wet-white acquires properties closely similar to those of conventional full vegetable tanned split hide. Conventional soft upper, saddle and fancy leathers can be produced efficiently in this way. Pretanning with glutardialdehyde normally results in softer types of leather. Greater firmness can be obtained by adding selected polyphosphates³ and polymeric tanning agents⁴. It is very important here to choose suitable fatliquors. In upper leather production from wet-white, fatliquors based on animal fats such as lanolin and lard oil in combination with selected synthetic fatliquors have been found to give the best results. Because final tanning with vegetable and synthetic tanning agents gives a fairly dry fibre, great care should be taken to conserve the tensile strength. The remedy is to add products containing phosphoric acid esters. Combination fatting with highly concentrated fatliquors containing lecithin leaves nubuck or suede with a silky feel⁵. Retanning with just syntans converts wet-white into very pale coloured crust leather which can be dyed to brilliant shades. Wet-white is an all-round product which can be turned into a wide variety of finished leathers and leathergoods. Given the increased demand for chrome-free leather on the part of both consumers and processors, leather from wet-white could also have a crucial role to play in footwear manufacture. Upper leather of this type can be: * upper leather for leisure footwear, full pigmented/embossed * floaters/drawn grain leather for leisure footwear * tight grain, natural colour semi-aniline leather for shoes and handbags * upper leather in brilliant shades for children's shoes * fashionable upper and fancy leathers with various effects and embossing * but also conventional leather for fancy goods Currently the amount of shoe upper leather produced from wet-white is fairly small. Fifteen years ago, however, the automobile industry among others could not have imagined the present importance of such leathers. Degradable materials obviously have an enormous advantage for industry and the consumer. An ecological marketing concept and the advantageous properties unquestionably characteristic of leather from wet-white have given a tremendous boost to demand. Wet-white yields a leather which differs in character from that of wet-blue origin and comes with its own specification and fastness properties. In marketing, these differences should be emphasised, not concealed. Appropriate appreciation by the consumer can only be gained through optimum positioning of these different leathers.