Patent ID: 8388878

Claim:
A method of producing a microporous sheet material comprising: a polymeric matrix comprising polyolefin, finely divided substantially water-insoluble inorganic filler distributed throughout the matrix, and a network of interconnecting pores communicating throughout the microporous material, said method comprising: (a) forming a mixture comprising the polyolefin, the inorganic filler and processing plasticizer composition; (b) extruding the mixture to form a continuous sheet; and (c) contacting the continuous sheet with an extraction fluid composition to extract the processing plasticizer composition from the continuous sheet to form the microporous sheet material; wherein the extraction fluid composition consists essentially of an azeotrope of a mixture of trans-1,2-dichloroethylene and 1,1,1,2,3,4,5,5,5-decafluoropentane, a mixture of trans-1,2-dichlorethylene and 1,1,1,3,3-pentafluorobutane, a mixture of 1,1,1,2,2,3,4,5,5,5-decafluoropentane and 1,1,1,3,3-pentafluorobutane, or a mixture of trans-1,2-dichlorethlene, 1,1,1,2,2,3,4,5,5,5-decafluoropentane and 1,1,1,3,3-pentafluorobutane, said extraction fluid composition being non-flammable essentially free of trichloroethylene, and having a boiling point of 75° C. or less, and wherein the microporous sheet material has a tensile strength of at least 800 kPa.