Effect of diluents on fusion temperature of the crystalline regions in plain and tanned cowhide

Abstract

AbstractMelting (shrinkage) phenomena in plain cowhide were investigated by measuring the temperature at which contraction first occurred in specimens containing varying amounts of ethylene glycol, formamide, phenol, and water as diluents. Similar measurements were made on cowhide specimens which had been tanned with formaldehyde and with a vegetable tannin and contained water as the diluent. In all cases the temperature at which contraction first occurred was found to increase as the diluent concentration was decreased in general accord with the theory of melting‐point depression. The melting temperature of the crystallites in undiluted cowhide was found, by extrapolation of the data of each system to zero diluent content to be about 145°C. The melting temperatures of undiluted tanned cowhide, however, depended on the tanning agent employed. A value of 120°C. was obtained for the vegetable‐tanned cowhide, and 151° for formaldehyde‐tanned cowhide. The former value was verified experimentally by direct measurement. This evidence for a decrease in crystallite stability indicates that vegetable tannin acts as a diluent. The observations indicates that tanning reduces the availability of the amorphous regions to diluents either through a type of crosslinking and/or a reduction in the number of active sites. This accounts for the observed elevation of the commonly measured shrinkage temperature of hide specimens after tanning. In addition, the tanning agents can markedly modify the mechanical properties of the amorphous regions by acting as a plasticizer or stiffening agent. The heats of fusion, ΔHu, for cowhide diluted with glycol, formamide, and phenol were 2200, 1600, and 1800 cal./mole, respectively. However, with water as diluent for plain hide, ΔHu was 7200 cal./mole. Similar values were obtained with the tanned hide. Entropies of fusion, ΔSu, for cowhide diluted with glycol, formamide, and phenol (assuming two bonds per residue freed on melting) were 2.6, 1.9, and 2.2 cal./deg./bond, respectively, while for plain hide, vegetable‐tanned hide, and formaldehyde‐tanned hide diluted with water, the values were 8.5, 9.2, and 9.5 cal./deg./bond. In order to bring the entropy values for the water systems into agreement with the other diluent systems it is necessary to assume that eight bonds per residue are freed on melting.