Patent ID: 8262973

Claim:
A method of manufacturing microporous polyolefin films having a puncture strength greater than 0.14 N/μm, a Darcy's permeability constant greater than 1.5×10 −5 Darcy, a shut-down temperature lower than 140° C., and a melt-down temperature higher than 160° C. comprising the steps of: melt-extruding a composition comprising 20-50 weight % of a resin composition and 80-50 weight % of a diluent (Component III) to mold in the form of sheets, wherein the resin composition comprises a blend of: (a) 90-98 weight % of polyethylene (Component I) having a weight average molecular weight of 2×10 5 ˜4×10 5 and less than 5 weight % of molecules of which molecular weight is less than 1×10 4 and less than 5 weight % of molecules of which molecular weight is greater than 1×10 6 , and (b) 2-10 weight % of polypropylene (Component II) having a weight average molecular weight of 3.0×10 4 ˜8.0×10 5 and a peak melting point higher than 145° C.; stretching said sheets to mold in the form of films; extracting said diluent from said films; and heat-setting said films.