polyhydric alcohols such as ethylene glycol propylene glycol diethylene glycol neopentyl glycol 1 4 butanediol furan dimethanol cyclohexane dimethanol glycerol trimethylolpropane or pentaerythritol or mixtures thereof with polycarboxylic acids especially dicarboxylic acids or their ester forming derivatives for example succinic glutaric and adipic acids or their methyl esters phthalic anhydride or dimethyl terephthalate polyesters obtained by the polymerization of lactones for example caprolactone in conjunction with the polyol may also be used a particularly suitable polyester polyol is poly 1 6 hexanediol adipate molecular weight about 2000 available as formrez 66 56 from witco corporation mixtures of polyols are often used in the reaction mixture examples of polycarbonate polyols are described in u s pat no 4 692 383 in col 1 line 58 to col 4 line 14 which is incorporated herein by reference the amount of polyol ii used in the reaction mixture generally ranges from about 5 to about 60 percent by weight often about 10 to about 40 percent by weight and more often about 20 to about 30 percent by weight based on total weight of the resin solids used to prepare the polyurethane the reactant iii containing both hydroxyl and carboxylic acid functional groups include carboxy group containing diols and triols for example dihydroxyalkanoic acids of the formula r u2014 c u2014 ch 2 oh 2 cooh wherein r is hydrogen or a c 1 c 10 alkyl group specific examples include 3 hydroxy 2 hydroxymethyl 2 methyl propanoic acid dimethylol propionic acid 2 2 di hydroxymethyl acetic acid 2 2 2 tri hydroxymethyl acetic acid 2 2 di hydroxymethyl butyric acid 2 2 di hydroxymethyl pentanoic.