Dimensionally-stable polyurethane sponge and sponge-forming prepolymer

A foamed sponge having minimal swell when wet and which does not curl upon drying made by mixing together and reacting an aqueous phase and

• a resin phase comprising a prepolymer derived from
• (a) a poly(oxy C.sub.2-4 alkylene) diol having at least about 50% by weight of oxyethylene groups and a nominal number average molecular weight of about at most 1100, said diol having nominally two hydroxyl equivalents per mole,
• (b) a diphenylmethane diisocyanate-containing isocyanate product with a functionality of greater than 2.0 comprising a mixture of a major amount of pure diphenylmethane diisocyanate and minor amounts of carbodiimides and trifunctional cycloadducts of diphenylmethane diisocyanate, and
• (c) a monomeric polyol cross-linking agent having 3 or 4 hydroxyl equivalents per mole,
• said isocyanate product comprising more than 50% by weight of the prepolymer,
• said diol and polyol cross-linking agent being present in a mole ratio in the range of 4:1 to 8:1, and
• the ratio of the isocyanate equivalents to the total hydroxyl equivalents being in the range of 3:1 to 4:1.