Patent ID: 8104625

Claim:
A process for producing a microporous membrane which comprises a step of melt-kneading polymer materials comprising at least one polyethylene (PEA), wherein 8 to 60 wt % of said PEA is accounted for by components having a molecular weight of 10,000 or less and in which the ratio of the weight-average molecular weight (Mw) to the number-average molecular weight (Mn), Mw/Mn, is 11 or more and 100 or less and the viscosity-average molecular weight (Mv) is 100,000 or more and 1,000,000 or less, and at least one polyethylene (PEB), which has a Mw/Mn ratio of 1 or more and 10 or less; wherein said PEA shows at least two maximum or shoulder peaks in the molecular weight distribution obtained by gel permeation chromatography and said PEB has a maximum peak between a peak X that is a peak for PEA on the low molecular weight side as a maximum or shoulder peak, and a peak Y that is a peak for PEA on the high molecular weight side as a maximum or shoulder peak, and at least one polypropylene, and a plasticizer, or said polymer materials comprising PEA, PEB and polypropylene, a plasticizer and an inorganic agent, and extruding the resulting mixture; and a step of carrying out stretching and extraction of the plasticizer, and then carrying out thermal fixation to produce a microporous membrane comprising 8-60 wt. % components having a molecular weight of 10,000 or less, a porosity of 20 to 95%, and a degree of thermal shrinkage at 100° C. of 10% or less.