Isocyanate-Functionalized Collagen Hydrolysates as Pretanning Agents for Organic Wet-White Leather

The present paper aims to synthesize and test a pretanning agent based on isocyanate-functionalized collagen hydrolysates of low molecular mass (0.9 ÷ 3.6 kDa; polypeptides that include 8 to 32 amino acids), obtained starting from hides wet wastes, and used without further purification. Raw colloidal suspension resulted after hydrolysis was centrifuged to separate insoluble particles, and then was repeatedly filtered to retain coarse particulate and fatty maters. The clarified solution was concentrated by vaporization up to 35 % w/v dry matter, mixed with 10 % v/v dimethyl sulfoxide (DMSO), and maturated overnight under efficient mixing, in a hermetically closed reaction vessel, at ambient temperature. Using a solution of diisocyanate in DMSO, the molecular mass of the product was further increased by cross-linking, in parallel with the functionalization in the virtue of a small amount of free isocyanate groups that remain unreacted. The functionalized collagen hydrolysate was characterized by chemical (total nitrogen content, the amount of free carboxyl, amino, and isocyanate groups) and instrumental methods (infrared spectroscopy). The fraction of increased molecular mass after cross-linking was determined by comparative dialysis through membranes with 3.5 and 12 kDa. Pretanning ability of the functionalized hydrolysates was estimated by gelatin precipitation, and tested on sheep pelts. An increase of 6 ºC was measured for the shrinkage temperature. Pretanned pelts were gradually dehydrated (preventing the local drying) and drum-tumbled, and then was treated once again with the same functionalized product, in a short concentrated float. A supplemental increase of shrinkage temperature with 8 ºC was measured.