A polyurethane of claim 1 wherein the polyester is a polyester of a dicarboxylic acid of the formula HOOC(CH ),COOH wherein x is a number from 2 to 8 10, a glycol of the formula HO(CH OH wherein x is 2 to 10, the aliphatic diol contains 5 to 8 carbon atoms and terminal hydroxyl groups and the diisocyanate is an alicyclic diisocyanate. 3.