Abstract: In a porous structure, a macroporous polyethylene substrate defining a network of interconnecting macropores and a microporous matrix of polyvinyl chloride is formed within and completely fills the network of macropores. The microporous matrix is cast within the network of macropores by impregnating the macropores with a solution of a solvent, a polymeric pore former, and polyvinyl chloride. The solution is caused to imbibe water from the atmosphere so as to separate into two liquid phases, one phase being rich in the polyvinyl chloride and the other phase being rich in water. The microporous matrix is coagulated from the first phase and the solvent and pore former are extracted from the macroporous matrix.

Abstract: A porous thermoplastic sheet is formed by adding a material to a polymer latex to passivate the aqueous phase of the latex. An additional material of a preselected particle size, corresponding to the desired pore size of the final product is then added. The latex composition is gelled and is then formed into a sheet. The sheet is dried and cured or sintered and the solubles are extracted therefrom with a solvent. A preferred polymer latex is PTFE.