Abstract

Comparative small scale tannages over the pH range 30 to 70 have been carried out on sheepskin using a selection of organic tanning materials.

The condensed tannins (wattle and quebracho) and the replacement syntan (condensed cresol sulphonic acid type) showed only slight shrinkage temperature—pH dependence; the hydrolysable tannins (chestnut, myrtan, and myrobalans) yielded leathers of lower hydro-thermal stability generally with optimum values in the vicinity of pH 5. p-benzoquinone contributed no stability at pH 3 and approximately 20°c at pH 5. Enhanced stability and leathery properties were conferred by the auxiliary syntan (naphthalene sulphonic acid type) at low pH values only. After urea extraction and washing, the p-benzoquinone and formaldehyde tannages were relatively unaffected, in contrast with drastic reductions of shrinkage temperatures of all the other tannages to the neighbourhood of, or even below, the pelt control, depending on urea concentration and the type of tannin.

The observations suggest that the shrinkage temperature elevation and leather-like character conferred by vegetable tannins and syntans differ radically from that produced by tannage with typically covalently reacting materials such as p-benzoquinone and formaldehyde, and is attributable in the main to association of the tannins through hydrogen bonds and/or secondary valence forces. In cases where the presence of strongly dissociating groups imparts anionic character under acid tannage conditions, electrostatic forces play an important role. The possibility of a significant contribution to shrinkage temperature elevation under normal tannage conditions due to covalent type bonding, particularly of the type implicated in quinone tannage, would appear to be excluded both by the low reactivity of p-benoquinone at p1-I 3 and the high degree of reversibility in stability following extraction in urea solution.