Leather surfaces

The dried rough leather was sold to a currier whose first operation was to dampen the skin and soften it. This was followed by a scouring operation in which the leather surfaces were scrubbed clean. The skins were then pared down to the required thickness after which they were worked on a bench to remove loose tanning materials, flatten and stretch them. They were then partially dried and impregnated with a warm mixture of tallow and fish oils. If a soft, fine product was required, further mechanical operations were undertaken. These had the effect of separating the tanned fibres and allowing... 

Fig. 6.11-16 SEM micrograph of the cross section through a laminated non-woven household product that is composed of synthetic leather and microfiber non-wovens. Layer sequence (from top to bottom) macro-porous synthetic leather surface, non-woven absorbent core, microfiber non-woven [B.107]...

In the finishing process the surface of the leather is treated to obtain special effects and to protect it against mechanical damages, to make it more user-friendly. In the finishing polymer binders, dyestuffs, pigments, waxes and others are sprayed or applied to the leather surface. 

FIGURE 4.127 Lasered leather surface and geometric structure... 

In leather processing, materials have to be introduction between the fibers to smoothen any irregularities on the leather surface. Casein cross-linked with transglutaminase can be used effectively as a filling material (Taylor et al. 2007). 

Its components are pigments, binders and auxiliaries such as waxes and fillers. The pigment coat imparts the desired appearance to the leather and levels out the leather surface. The choice of binders is made according to the finishing effect and fastness profile desired. Generally, polyacrylate dispersions are used in the pigment coat. 

The top coat determines the ultimate appearance and the feel of the leather surface. It further substantially influences the fastness properties of the finish. Instead of organic, solvent-containing lacquers and top coats (e.g., nitrocellulose emulsions), today there is an increasing trend towards waterborne top coat systems. These are usually polyurethane dispersions which, owing to their specific properties (no emulsifier, good film formation despite relatively high hardness), are superior to polyacrylate dispersions in appHcation terms. 

Binders are among the most important components of a finish system, whether it is a pigment finish or a top coat. They bind the color-conferring pigments, which have no inherent affinity for leather, and protect the leather surface through their fihn-forming property. Aqueous finishes generally utilize aqueous polymer dispersions. 

The damage or change in the finish coat and the transfer of color to the rubbing element are assessed after fixed mbbing intervals. This method provides data on the sensitivity of the finished leather surface to mbbing through, abrasion or transfer of color from the pigment coat under both dry and moist conditions. 

Rub-fastnesses are also tested using the SATRA rub-fastness tester. In this test a rotating pad of felt acts on the leather surface under a certain pressure and at a defined speed of rotation. The leather is evaluated after fixed numbers of cycles. This test is likewise carried out with both a dry and a wet felt pad. 

This test determines the adhesion of the finish coat to the leather surface. It also provides evidence of possible inter-adhesion problems within the finish coat These arise in particular when excessive atudliary quantities (especially waxes) are used or the crosslinking of the preceding layer is excessive. Ideally, the adhesion of the finish to the leather is such that, in this test, the finish can only be pulled off together with the grain layer. 

This test is important for finished shoe upper leathers (see above) in particular. 7 iron is moved once back and forth across the leather surface over a shghtly rounded edge as a preliminary test. The damage to the finish and any shift in hue are then evaluated. The test temperatures are increased in intervals of 20 °C. A more sensitive version of this test is carried out on the VESLIC rub-fastness tester using a heatable test punch. Again the temperatures are increased in intervals of 20 °C. The result is assessed in each case after five rubs. 

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