Process for forming microporous sheet

Claim 1

IN THE PROCESS OF FORMING A MICROPOROUS SHEET OF SOLID POLYURETHANE COMPRISING THE STEPS OF CASTING AN EMULSION OF FINE DROPLETS OF A VOLATILE ORGANIC LIQUID AS THE INTERNAL PHASE IN A CONTINOUS PHASE COMPRISING REACTIVE MATERIAL CONVERTIBLE THROUGH REACTION TO SOLIDIFIED, RESILIENT POLYURETHANE, SAID ORGANIC LIQUID BEING SUBSTANTIALLY NON-SOLVENT FOR AND NONREACTIVE WITH SAID REACTIVE MATERIAL, BEING IMMISCIBLE IN SAID CONTINUOUS PHASE AND BEING PRESENT IN AMOUNT FORM ABOUT 60% TO ABOUT 300% BY WEIGHT BASED ON THE WEIGHT OF THE REACTIVE MATERIAL, SAID ORGANIC LIQUID CONTAINING CATALYTIC OR OTHER NON-VOLATILE MATERIALS WHICH ARE UNDESIRABLE IN THE FINAL SHEET, REACTING SAID MATERIAL AT A TEMPERATURE BELOW THE BOILING POINT OF SAID LIQUID SO THAT THE LIQUID NEVER BOILS TO CAUSE THE EMULSION TO GEL AND SOLIDIFY WITH SAID DROPLETS HELD IN THE SOLIDIFIED MATERIAL AND REMOVING SAID LIQUID WITHOUT EXPANDING THE SOLIDIFIED MATERIAL LEAVING PORES AND DISCONTINUITIES IN THE SOLIDIFIED MATERIAL TO CONSTITUTE PASSAGEWAYS FOR AIR AND VAPOR, THE IMPROVEMENT WHICH COMPRISES FORMING A LAYER OF THE LIQUID EMULSION, REACTING THE MATERIALS OF THE CONTINUOUS PHASE OF THE EMULSION TO A RESILIENT STATE CAPABLE OF RETURN TO SUBSTANTIALLY ITS INITIAL THICKNESS WHEN PRESSED TO 50% OF ITS INITIAL THICKNESS UNDER CONDITIONS PRESERVING THE UNIFORMITY OF THE EMULSION, SUBJECTING THE RESILIENT MATERIAL TO PHYSICAL PRESSURE OF SHORT DURATION PROMPTLY AFTER REACHING SAID RESILIENT CONDITION AND BEFORE COMPLETION OF SAID REACTION TO EXPRESS FROM SAID SHEET A SUBSTANTIAL PORTION OF SAID NON-SOLVENT LIQUID TOGETHER WITH MATERIALS DISSOLVED OR CARRIED IN SAID NON-SOLVENT LIQUID AND TO CLEAR PORES AND PASSAGEWAYS, COMPLETING SAID REACTION TO FORM A STABLE SOLID BODY AND REMOVING FURTHER ORGANIC LIQUID FROM THE SOLIDIFIED MATERIAL.