shavings differ from leather scrap generated further on in the leather production chrome shavings are hydrophilic in nature whereas other leather scrap contains fats finishing chemicals and similar materials which impart hydrophobic qualities to the final product although it is not fully understood it is known that tanned leather scrap which is in fact treated collagen possesses various pendant functional groups along the polypeptide chains these functional groups include hydroxyl carboxyl thio amine and the like while it is known that these functional groups per se are reactive with isocyanates it is believed that the functional groups in the tanned leather scrap react with the isocyanate to a certain extent although not all functional groups in the tanned leather scrap may be reactive due to the possible stearic hinderance of the polypeptide chains preventing reaction with the isocyanate likewise it is not known whether the water of equilibrium present in tanned leather scrap reacts with the isocyanate or if it is sufficiently bound within the protein structure to make it unavailable for reaction with the isocyanate in any event excess water forming polyureas with reactive polyisocyanates are not necessarily undesirable in that they do in fact build molecular weight and so long as some functional groups in the polypeptide chain react with isocyanate groups in the polyisocyanate structural integrity of the final product can be accomplished the polyisocyanate acts as a binder for the.