Patent ID: 8394306

Claim:
A method for producing a microporous thermoplastic resin membrane consisting essentially of the steps of extruding a solution obtained by melt-blending a thermoplastic resin and a solvent (A), cooling the resultant extrudate to form a gel-like product, stretching said gel-like product to form a stretched membrane having oriented molecular chains, and cleansing said stretched membrane to remove said solvent (A) remaining therein, wherein said solvent (A) is removed by two or more cleansing steps, at least one step among the cleansing steps using a nonaqueous solvent (B) in an amount of 300 to 30,000 parts by weight per 100 parts by weight of said gel-like molding, and a solvent (C) causing no depletion of an ozone layer and having a boiling point of 100° C. or lower being used at a final step, and wherein said solvent (C) is (a) hydrofluoroethers, or (b) a mixture of hydrofluoroethers and an alcohol having 3 or 4 carbon atoms, wherein said stretching is biaxial stretching and said biaxial stretching is simultaneous or sequential stretching in longitudinal and transverse directions, wherein said stretching is carried out at a temperature in a range between the crystalline dispersion temperature and under the crystalline melting point of said gel-like product, and wherein said stretching temperature is 110° C. to 120° C., wherein in the at least one step among the cleansing steps using the non-aqueous solvent (B) and in the final step using solvent (C), both non-aqueous solvent (B) and solvent (C) different from each other, wherein in the at least one step using the non-aqueous solvent (B) and in the final step using solvent (C), both solvent (B) and solvent (C) are not used together in the at least one step and the final step.