Abstract

It is now realised that chrome is easily removed from leather under mild alkaline conditions. This results in loss of heat resistance and often in loss of area. In consequence it is desirable to consider how best to produce leather with good resistance to heat and mild alkali (washing and perspiration). Aldehyde tannages have the necessary stability to alkalies if the aldehyde polymer is of the aldol type. This is because the aldehyde is bound to the protein and other aldehyde molecules by stable covalent links. Aldol type aldehyde tannages have the property of reversible shrinkage, so that although shrinkage temperatLires are only 80_85z, higher temperatures cause no permanent loss of area and this property can persist after retannage with other materials. As the leather after aldol type tannage is alkaline, the best method to increase heat resistance is by resin retannage. The resin formed by formal dehyde and mimosa can increase shrinkage temperatures by 30r and being formed by covalent links can be resistant to mild alkali. Metals, particu larly chrome, can be more firmly fixed by such a resin than by collagen. To achieve the best results, conditions must be such that the tannage is achieved by molecules reacting, not by deposition of loosely attached large aggregates, which can often be entangled in and not attached to the fibres.