Abstract

Conclusions drawn from the experimental evidence of a series of publications are discussed and co-ordinated. Chromium pretannage of the collagen increases its reactivity towards vegetable tannins by liberating charged amino groups and other sites not normally available, with the result that vegetable tannins are rapidly absorbed in the surface layers of the skin, and this in turn causes over-loading of the fibre and increased cross-linking, both of which tend to give weak leather. In addition, it has been shown that vegetable tannins and non-tannins form co-ordination compounds with the chromium in the leather, displacing sulphate with a consequent increase in acidity. The possibility exists, especially in retanning with hydrolysable tannins, that protein carboxyl groups may be displaced from the chromium-collagen complex with the result that a degree of detannage occurs. Moreover, on ageing, the reaction of the vegetable tannins with the chromium-collagen complex continues, with the displacement of additional sulphate groups and a further increase in acidity which is thought to promote the hydrolytic degradation of the protein and results in the deterioration of this type of leather. The strength of the leather can be improved by reducing the avidity of the chromium tanned pelt so that a more uniform distribution of the vegetable tannin is obtained, and durability is increased by the inclusion of compounds which react preferentially with the chromium-collagen complex and inhibit the entry of vegetable tannins so that the acidity of the leather is maintained at a satisfactorily low level. Choice of the type of vegetable tanning extract is also of importance; members of the hydrolysable group of tannins should be avoided because of their greater reactivity with chromium compared with the condensed tannins.