Abstract

Conclusions drawn from the experimental evidence of the preceding five sections of this series are discussed and coordinated. It appears that covalent bonds or quinone type linkages are not a factor in normal vegetable tannage and that oxidation and ageing of wattle tanned leathers under normal pH conditions have very little tan fixing effects.

Vegetable tannage appears to be a solvent reversible multipoint crosslinking mechanism in which hydrogen bonds provide the dominant forces and the peptide links are probably the most powerful bonding sites. While sulphonic acid and carboxyl groups attached to the tannin molecule may provide a reinforcing factor at low pH values, they increase tannin removal by solvents at pH 70. At normal tanning pH conditions there seems to be no clear distinction between the mechanisms of tannage of condensed and hydrolysable tannins, and the important factors appear to be molecular size, shape and distributijn of polar groups.

Page’s observation of breaks in the aqueous elution curve have been disproved, and it is considered that the terms “fixed tans” and “combined water solubles” are purely arbitrary, and are equilibrium functions of pH, temperature, time, degree of subdivision of leather, and nature of solvent. T

he role of aqueous organic solvents in increasing the elution of vegetable tans from leather is elegantly demonstrated by chromatography using sheepskin strips and appears to be attributable to the effective “blocking” of the reactive hydrogen bonding sites on the tannin by interaction with the solvent molecules, the interaction being increased by the shielding effect of local hydrophobic structure and the degree of polarity in the organic solvent used.