Immobilisation of leather dyes - a vision becomes reality

24 July 2006

Leather has a long tradition as a high-value, hard-wearing material, and leathergoods such as shoes, upholstery and clothing have an excellent reputation with consumers. Nevertheless, leather has to compete with other materials with properties it cannot always match. Demands for innovative new products and processes are quite justified even in the traditional leather sector, and innovation is very important for the future of the industry. It is fair to say, though, that there have been no spectacular innovations in the leather industry over the past ten years. This is certainly true of dyeing, because new developments in the dyeing sector have often been restricted to modifying formulations and optimising conventional dyeing methods based on existing dye molecules. The leather industry no longer develops any far-reaching innovations in the field of dyeing. True innovation can only be achieved by taking a completely fresh approach. In spite of the dramatic developments in the market for leather dyes, which are increasingly becoming mere commodities, BASF are investing in research projects and registering new chemical substances because the company see it as their task to provide their customers with the means to set themselves apart from their competitors and improve their position in the market by producing leather to the highest standards of quality with new innovative products and processes. BASF are able to make use of the scope for innovation that is generated by their large-scale integrated production. Conventional dyeing techniques are limited when it comes to dyeing leather with a deep shade and high wet fastness, because the dye is attached by ionic bonds to the leather fibres or by complex bonds to mineral tanning agents. These bonds are reversible, which makes it practically impossible to obtain leather with a deep shade and high wet fastness with conventional dyes. When leather is exposed to alkaline perspiration or alkaline detergent solutions, the ionic or complex bonds that fix the dye to the leather are weakened and some of the dye is released from the collagen. The dye discolours the washing water and materials that come into contact with the leather, such as textiles, plastics or skin. The consequences for consumers are discoloured socks and faded leather. Advances in leather dyeing, especially improved wet fastness, should ideally lead to improvements along the whole added-value chain that consumers can visibly appreciate and that enhance the image of leather as a high-class material. BASF have developed a completely new reactive dyeing process for leather which ensures that insufficient wet fastness is a thing of the past. Reactive dyes form covalent bonds with collagen, which causes them to become completely immobilised. Covalent bonds are resistant to changes in the electrostatic charge on the surface of the substrate, and the result is that the same excellent wet fastness can be achieved on both wet-blue and wet-white. The resistance of the leather to everyday wear and tear is excellent, and no dye is transferred to other materials it comes into contact with. The key to this new technology is Lugafast Black AN, a novel, innovative dye that is fixed to the leather by a completely new mechanism. Lugafast Black AN is the first reactive dye worldwide that has been specially developed for dyeing leather. It consists of a chromophore group, which imparts colour, and a reactive anchor group that fixes it irreversibly to the collagen. The chemical mechanisms by which the dye is fixed are similar to those that are used to fix reactive dyes to textiles, but both the chromophore group and the anchor group have had to be specially designed to take into account the different structure of collagen. The most important properties of the dye and its practical performance are summarised below. * The reactive dye has a special anchor group that enables it to be fixed to collagen irreversibly by covalent bonding. * It is anionic and homogeneous. * The powder has excellent solubility and performs well in a short float and at temperatures of 35-45°C. * High build-up makes it possible to achieve deep shades, and the coverage of grain defects is excellent. * The dye penetrates easily even through thick wet-blue, and the grain layer can be dyed to deep, level shades. * The dye has no tendency to bronze in deep shades. The excellent fixation of the dye results in much higher wet fastness, which has the following advantages: * Excellent perspiration fastness * Excellent wash fastness * Excellent migration fastness * Excellent resistance to solvents * Improved rub fastness, ie no dye transfer The new dyeing system is very easy to use, because the leather can be neutralised, dyed through the cross-section, top dyed and fixed in a single bath. Only one single dye needs to be used to dye the leather through the cross-section and on the surface. The neutralisation process only takes half an hour, and this is long enough to allow wet-blue with a thickness of 3mm to be dyed through the cross-section with Lugafast Black AN. The dye is added to the bath in powder form, and its excellent solubility ensures that it is dispersed quickly and homogeneously even in dye baths with a temperature as low as 35-45°C. There is no need to dissolve it in advance in hot water, as is often the case with conventional dyes. The dye is fixed by controlling the pH. In principle, the new dyeing system that employs Lugafast Black AN can be used for leather with all types of surface texture. * Smooth, milled nappa leather with a tight grain and without a mill break * Firm shoe upper leather with a tight grain * Floater leather with a coarse milled grain * Milled leather with a fine, pebbled grain * Nubuck and suede * Aniline and semi-aniline leather dyed to deep shades Lugafast Black AN makes it possible to obtain levels of fastness, especially wet fastness, that cannot be matched with traditional dyeing methods for leather. In particular, deep black shades with very high fastness can be obtained that can often match the standards of quality that apply to textile fabrics. Because reactive dyeing does not involve the formation of ionic bonds, reactive dyes perform equally well on wet-blue and wet-white. Up until now, it has not been possible to obtain deep black shades with very high fastness on wet-white. Leather that is used to make products such as unlined shoes, perforated car seats and premium quality sports footwear has to have high perspiration resistance. The new, innovative Lugafast dyeing system makes it possible to produce all these types of leather with excellent perspiration resistance. This has been impossible to achieve up until now, and it represents a quantum leap in dyeing technology. The following figures show the perspiration resistance of perforated automotive seat leather dyed with a conventional dye and leather dyed with the new reactive dye in comparison. It can be seen quite clearly that neither from the flesh side nor from the grain side of the perforated leather can dye be transferred to the contact material. This corresponds to a rating of 5 on the grey scale. Similar results were also obtained for the rub fastness. The high standards of fastness obtained in these examples demonstrate the exceptional performance that can be achieved with the new reactive dyeing system from BASF. Washable leather that has been dyed with Lugafast Black AN has much higher wash fastness and it does not fade even after repeated washing. This type of leather is particularly in demand for garments, sports footwear and leisure footwear. It is easy to produce leather with high resistance to dry cleaning and high resistance to organic solvents, too. These properties are particularly important for garment leather, and the depth of the shade remains constant even after repeated cleaning. The new innovative dyeing system with Lugafast Black AN is also especially recommended for the following types of leather. * Shoe upper leather for unlined shoes * Upper leather for high-performance sports footwear * Upholstery leather and automotive leather * High-quality garment leather that is resistant to dry cleaning * Glove leather with excellent wet fastness * Washable shoe upper leather and garment leather * Nubuck and suede with excellent wet fastness and migration resistance and improved rub fastness * Perforated leather with excellent wet fastness and migration resistance * Perspiration-resistant linings for shoes and garments This new class of dyes and the newly developed dyeing technology now enable products to be manufactured with special effects that were impossible to obtain with conventional dyeing processes. For example, footwear manufacturers and garment manufacturers now have the opportunity to use completely new combinations of materials and innovative designs, such as combinations of jet black leather and white fabrics for riding breeches or washable leather labels for use on jeans dyed to pastel shades. There are now no limits on the creativity of fashion designers. Lugafast Black AN, the first reactive dye for leather, will open up new marketing opportunities for leather manufacturers and manufacturers of leathergoods on account of its excellent fastness, and consumers will benefit from the improved serviceability of the leather. Lugafast Black AN is a world premiere, but BASF intend to introduce more reactive dyes that will enable all the important shades for leather to be matched.

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