yisocyanate and a diamine trihydroxy compound having the formula: R (alkylene- 0) 1H \N-(OH2)H-N H O -alkylene) z (alkylene-O) H wherein n is a whole number from 1 to 10, alkylene means a divalent saturated aliphatic hydrocarbon radical containing 2 to 3 carbon atoms, x, y and z are whole numbers and the sum of x, y, z is from 3 to 12, at least two of the groups selected from the groups consisting