Method of treating leather with amphoteric polymers

A method of treating a tanned leather stock comprising:

• retanning said tanned leather stock with particles of an amphoteric polymer dispersed in an aqueous medium to produce a retanned leather stock having improved color expression characteristic, said amphoteric polymer particles being prepared by:
• emulsion polymerizing in said aqueous medium at least one monomer containing an acid group in the range of from 0.5 percent to 20 percent with at least one first stage comonomer in the range of from 99.5 percent to 80 percent to form a first stage copolymer having a Tg varying from -40.degree. C. to +60.degree. C. and a GPC weight average molecular weight varying in the range of from 10,000 to 1,000,000, all percentages being in weight percent based on the total weight of amphoteric polymer solids;
• raising the pH of said aqueous medium sufficiently to avoid coagulation of said amphoteric polymer particles in the presence of an amine group by adding a base to said aqueous medium;
• emulsion polymerizing in said aqueous medium at least one monomer containing said amine group in the range of from 0.5 percent to 20 percent with at least one second stage comonomer in the range of from 99.5 percent to 80 percent to form a second stage copolymer having a Tg varying from -40.degree. C. to +60.degree. C. and a GPC weight average molecular weight varying in the range of from 10,000 to 1,000,000, all percentages being in weight percent based on the total weight of amphoteric polymer solids.