Microporous sheet structures

Claim 1

IN A PROCESS FOR THE PRODUCTION OF MICROPROUS SHEET STRUCTURES BY A PROCESS WHICH COMPRISES REACTING AN ORGANIC POLYISOCYANATE WITH AT LEAST ONE ORGANIC COMPOUND CONTAINING AT LEAST TWO HYDROXYL OR AMINO HYDROGEN ATOMS REACTIVE WITH NCO GROUPS AS STARTING MATERIALS TO PRODUCE A POLYADDITION PRODUCT WHICH IN THE HOMOGENEOUS STATE EXHIBITS A SHORE A HARDNESS GREATER THAN 40 AND A SOFTENING RANGE ABOVE ABOUT 100*C, SAID REACTION BEING EFFECTED IN AN ORGANIC LIQUID WHICH BOILS BELOW 250*C. DISSOLVES THE STARTING MATERIALS BUT IS A NONOSOLVENT FOR THE POLYADDITION PRODUCT TO BE FORMED AND HAS NO SIGNIFICANT SWELLING EFFECT UPON THE POLYADDITION PRODUCT, BY APPLYING THE SOLUTION TO A SUBSTRATE AND REMOVING THE ORGANIC LIQUID, THE IMPROVEMENT WHICH COMPRISES, EFECTING SAID REACTION IN A MIXTURE OF SAID ORGANIC LIQUID AND 1 TO 300 PERCENT BY WEIGHT BASED ON THE WEIGHT OF SAID POLYADDITION PRODUCT TO BE FORMED OF A SOLVENT SELECTED FROM THE GROUP CONSISTING OF N,N-DIMETHYL-ACETYL MORPHOLINE, N-ACETYL PYRROLIDIN AMIDE, N,N-DIMETHYL-ACETAMIDE, N-METHYLMORPHOLINE, FORMYL MORPHOLINE, N-ACETYL MORPHOLINE, N-ACETYL PYRROLIDINE, BUTYROLACTONE, N,N''-DIACETYL PIPERAZINE DIETHYL SULPHONE, N,N''-DIFORMYL PIPERAZINE, DIMETHYLNITROAMINE (N,N) DIMETHYL SULPHONE, DIPROPYL SULPHONE, ETHYLENE CARBONATE, METHYL BENZOATE, PROPIOLACTONE, N-METHYL PYRRROLIDONE, HEXAMETHYLPHOSPHORAMIDE, TETRAMETHYL UREA, DIMETHYL SULPHOXIDE, DIMETHYL CYANAMIDE AND CAMPHOR, SAID SOLVENT HAVING AN EVAPORATION NUMBER AT LEAST 1.5 TIMES THAT OF THE AFORESAID ORGANIC NONSOLVENT.