Method of making a stiffened permeable resin coated fibrous sheet

Claim 1

IN MAKING A SHEET THAT CONTAINS APPLIED PLASTIC AND IS PERMEABLE TO AIR AND MOISTURE, THE METHOD WHICH COMPRISES MOISTENING WITH WATER A PERMEABLE FIBROUS SHEET PROVIDED WITH FINE PORE SPACES AND BEING NON-SOFTENING AND NONDETERIORATING AT THE SOFTENING TEMPERATURE FOR THE SAID PLASTIC, APPLYING OVER THE MOISTENED SHEET A LAYER OF NON-COHERENT FREE FLOWING SOLID PARTICLES OF A WATER INSOLUBLE ORGANIC THERMOPLASTIC MATERIAL AND OF SIZE ABOUT 20 TO 200 MESH, REMOVING THE APPLIED MATERIAL EXCEPT FOR A SINGLE LAYER OF SPACED PARTICLES THEREOF DIRECTLY IN CONTACT WITH THE MOISTENED SHEET, AND THEN WARMING THE THUS TREATED SHEET TO A TEMPERATURE OF AT LEAST 200* F., TO CAUSE EVAPORATION OF WATER, AND CONTINUING THE WARMING UNTIL THE THERMOPLASTIC MATERIAL IS SOFTENED, TO CAUSE ADHERENCE THEREOF WITHIN THE SURFACE PORTION OF THE SHEET, THE WATER IN THE MOISTENED SHEET AS USED DECREASING THE PENETRATION OF THE PLASTIC MATERIAL ON THE SHEET.