METHOD FOR PRODUCING MICROPOROUS FILM, AND BATTERY SEPARATOR

PROBLEM TO BE SOLVED: To provide a method for producing a microporous film which can efficiently produce a microporous film with high pore density and porosity.SOLUTION: The method for producing microporous film includes the steps of (A) drawing a polypropylene resin composition in a molten state at a drawing ratio of 2-500 to obtain a film; (B) thermally-processing the film obtained in the step (A) at a temperatures of 100°C or higher and 160°C or lower; (C) cold-stretching the film obtained in the step (B) at a temperatures of -20°C or higher and lower than 90°C; and (D) thermally-stretching the film obtained in the step (C) at a temperatures of 90°C or higher and lower than 160°C, wherein the polypropylene resin composition contains 70-30 pts.mass of polypropylene resin having an MFR of 0.1 or more and less than 4.0 and 30-70 pts.mass of polypropylene resin having an MFR of 4.0 or more and 10.0 or less and isotacticity exceeding 95% in such a manner that the total of these two polypropylene resin becomes 100 pts.mass.