The Effect of Tanning Agents on Collagen Structure and Response to Strain in Leather

Abstract

A variety of tanning agents are used to produce leather ofdifferent mechanical performances and properties. Howdifferent tanning agents affect the structure of leather, andconsequently the mechanism underlying the performance of thedifferently tanned leathers, is not well understood. We usedsmall-angle X-ray scattering to investigate the structure of ovineleather tanned with chromium, zirconium, mimosa, oxazolidineor tetrakis hydroxymethyl phosphonium sulfate (THPS), orselected pairs of these agents. Leather tanned using THPS andTHPS in combination with mimosa retained the most orientedcollagen fibril structure. Treatment with chromium, zirconiumor zirconium and mimosa resulted in the least oriented fibrilstructures. The effect of oxazolidine with mimosa wasintermediate between these. A high natural collagen fibrilorientation in a skin is normally associated with high strengthwhile a high degree of crosslinking normally results in a lowerfibril orientation. We suggest that the extent of crosslinking inthe samples is reflected in their fibril orientation, with morecrosslinking producing stronger leathers. This interpretation issupported by the differences in shrinkage temperatures: thoseleathers for which orientation index (a measure of collagen fibrilalignment) changed most had a lower shrinkage temperature(which is normally interpreted as more “tanning”). This providesa greater understanding of the mechanism of action of tanningagents in the development of the properties of leather.