Patent ID: 7700025

Claim:
A process for producing a microporous polyolefin film which has a molecular weight of 300,000 to 1,500,000 as a whole of the film, a fuse temperature of 120 to 140° C., a film breakage temperature of 150° C. or more, and a ratio of the piercing strength at 140° C. to the piercing strength at 25° of 0.01 to 0.25, the process comprising kneading a mixture comprising a polyolefin composition comprising 5 to 95 wt % of polyethylene (A) having a viscosity average molecular weight (Mv) of 2,000,000 or more, a first melting-peak signal height as determined by DSC (differential scanning calorimetry) of 3.0 mW/mg or more, a specific surface area of 0.7 m 2 /g or more and an average particle size of 1 to 150 μm and 95 to 5 wt % of polyethylene (B) having an Mw of more than 10,000 and less than 200,000, wherein the Mv ratio of the component (A) to the component (B), (A)/(B), is 10 or more, with a plasticizer; extruding the kneaded product and then molding the product into a gel-like sheet to cool and solidify the sheet; drawing the resulting gel-like sheet; extracting the plasticizer to dry the sheet; and thermally fixing the film, wherein the thermal fixation step comprises a step of drawing the film in a TD direction after extracting the plasticizer and a subsequent step of thermally shrinking the film in the TD direction with reference to film width after the drawing step.