Abstract

A procedure has been developed for chrome-tanning hide powder and it has been used to prepare a range of chrome-tanned hide powders with increasing chrome contents. The interaction of these tanned hide powders with the pure dye acid of C.I. Acid Orange 7 has been studied thermodynamically. The saturation capacity of the tanned hide powder for the acid dye increases almost linearly with the amount of chromium present, a chrome oxide content of about 3% leading to a saturation capacity about twice that of untanned hide powder. The saturation capacities can be surprisingly large, rising to almost three molecules dye bound per chromium atom. Chrome-tanned hide powder has been found to have a lower affinity for the acid dye than untanned, which is connected with the much larger saturation capacity and the consequently lower activity in the fibre produced by a given amount of dye on the hide powder. The affinity at 60°C is independent of minor differences in tannage, but decreases with increasing amounts of chromium. The affinity increases with temperature in the range 25-75°C, while the saturation capacity decreases. Ageing chrome-tanned hide powder does not affect the affinity, but decreases somewhat the saturation capacity. Dyeing chrome-tanned hide powder appears to be exothermic and the entropies of dyeing are positive, the process thus being thermodynamically similar to dyeing untanned hide powder.