POLYURETHANE MICROPOROUS SHEETS ARE PREPARED BY REACTING AN ORGANIC COMPOUND CONTAINING AT LEAST TWO TERMINAL HYDROXYL GROUPS AND HAVING A MOLECULAR WEIGHT OF FROM ABOUT 500 TO 10,000 AND A CHAIN LENGTHENING AGENT CONTAINING AT LEAST TWO HYDROXYL OR AMINO GROUPS AND HAVING A MOLECULAR WEIGHT OF FROM ABOUT 50 TO ABOUT 600, THE REACTIVE GROUPS OF THIS EXTENDER BEING WITHIN A FACTOR OF 5 AS COMPARED TO THE REACTIVITY OF THE HYDROXYL GROUPS OF THE COMPOUND HAVING A MOLECULAR WEIGHT OF FROM 500 TO 10,000, AND AN ORGANIC POLYISOCYANATE IN AN AMOUNT SUCH THAT THE NCO TO OH OR THE NCO TO AMINO PLUS HYDROXYL RATIO IS FROM ABOUT 0.8 TO 2, THE REACTANTS FORMING A PRODUCT WHICH WHEN IN A HOMOGENEOUS STATE EXHIBITS A SHORE A HARDNESS OF AT LEAST 50 AND A TENSILE STRENGTH OF AT LEAST 50 KG./CM2, THE REACTION BEING CONDUCTED IN A 10 TO 80 PERCENT BY VOLUME SOLUTION AT A TEMPERATURE OF UP TO ABOUT 150* C., THE SOLVENT OR MIXTURE OF SOLVENTS OF THE SOLUTION EXHIBITING THE FOLLOWING CRITERIA:

(A) DISSOLVES THE STARTING MATERIAL AT THE INITIAL REACTION TEMPERATURE, (B) DISSOLVES THE POLYURETHANE BEING FORMED LESS AND LESS READILY AS THE DEGREE IN POLYADDITION INCREASES AND FINALLY PERMITS THE POLYURETHANE TO GEL AFTER NOT MORE THAN 2O MINUTES UNDER THE REACTION CONDITIONS, (C) DOES NOT CAUSE SUBSTANTIAL SWELLING OF THE RESULTING POLYURETHANE, AND (D) HAS AN EVAPORATION NUMBER HIGH ENOUGH TO INSURE ITS PRESENCE IN SUFFICIENT QUANTITY UNDER THE REACTION CONDITIONS UNTIL THE FILM SOLIDIFIES.