Abstract

An investigation has been made of the retannage of chromium tanned hide powder and pelt with hydrolysable or condensed tannins, namely chestnut, myrobalans, mimosa and quebracho. Increasing the amount of chromium in the leather increased the number of potential binding sites for vegetable tannin, and increased the likelihood of sulphate displacement and acidity. Increasing the degree of vegetable retannage increased the tendency of vegetable tannins to enter the chromium complexes, displacing sulphate with release of strong acid; hydrolysable tannins, with their high content of dissociating groups, were especially active in this respect and also tended to cause detannage. Greater reactivity of quebracho compared to mimosa was noted, probably because of the presence of sulphonic acid groups in quebracho, which makes this condensed tannin resemble hydrolysable tannins. Modification of the chromium-collagen complex by adding substances that react preferentially with the chromium and resist the penetration of vegetable tannins into the complex reduced the reactivity and inhibited the development of acidity during ageing.