Mineral tanning mechanisms : a fundamental study

A review across the periodic table of tanning effects of simple inorganic compounds reveals that many elements are capable of being used to make leather. But, if the practical criteria of effectiveness, availability, toxicity and cost are applied, the useful options reduce to chromium (III), the benchmark, or to titanium (IV), zirconium (IV), iron (II/III) and aluminium (III). For mainly environmental reasons, alternative tanning agents to chromium are needed. However, none so far investigated match the all round properties achieved with chromium (III) salts.;In tanning terms, the chemistry of titanium (IV) is dominated by the titanyl ion, TiO2+, limiting its reactivity with collagen. An alternative approach to titanium tanning, using metastable titanium (III) salts as possible tanning agents, was investigated in this study. A method to determine the titanium content in leathers was developed.;Complexes of Ti (III) were investigated using various spectroscopic techniques, including uv/vis spectroscopy, electron spin resonance, electron nuclear double resonance and nuclear magnetic resonance dispersion spectroscopies. The complexes used in subsequent tanning studies were based on titanium (III) citrate, gluconate and tartarte. However, in comparative trials with chromium (III) salts, the study highlighted that titanium (III) salts were not suitable for tanning collagen. Titanium (III) salts were found to be useful in the production of semi-metal tannages.;The research was extended to investigate the hydrothermal shrinking of tanned collagen. Techniques such as extended x-ray absorption - fine structure and x-ray absorption near structure were used to investigate the interaction of chromium (III) tanning salts and titanium (III) complexes with collagen.;The research demonstrated that the shrinking reaction is independent of the tannage. It involves instead the breaking of hydrogen bonds, rather than the breakdown of the tannin-collagen molecule. From this work, and a reappraisal of older work on the shrinking reaction, a new theory of tanning has been formulated based on co-operating units and the role of crystallinity in stabilising the collagen structure. The influence of the solvent is also discussed.