Hydrolyzed polyurethane plasticizer

A method for modifying the hardness of an acrylonitrile-butadiene rubber which comprises mixing with the rubber and curing agents a proportion of plasticizer sufficient to modify the hardness of the cured rubber to the desired degree and curing the rubber-plasticizer mix, said plasticizer comprising a mixture, by weight of from 25% to 75% of an ester plasticizer and from 75% to 25% of a hydrolyzed polyester-based polyurethane elastomer produced by autoclaving said elastomer with steam at an elevated temperature and pressure until it becomes liquid, said ester plasticizer having the general miscibility characteristics, with respect to said rubber and said hydrolyzed elastomer, of dioctyl phthalate, dibutyl phthalate, dioctyl adipate, and dioctyl sebacate.