Patent ID: 8354185

Claim:
A method for preparing a microporous polyethylene film comprising: (a) melting, compounding and extruding a mixture comprising 20-50 wt % polyethylene and 80-50 wt % diluents, which occurs liquid-liquid phase separation from polyethylene at 160-280° C., above the temperature of liquid-liquid phase separation in an extruder to form a thermodynamic single phase, (b) carrying out phase separation of the resultant of molten material in a single phase and forming it into a sheet, (c) stretching the sheet by sequential or simultaneous biaxial stretching at a stretching ratio of at least 5.0 times in transverse and machine directions, respectively, (d) extracting the diluents from the stretched film, and then shrinking the film by 15-30% while drying, and (e) heat-setting the dried film to remove residual stress from the stretched film at a heat-setting tenter temperature at which 30-50% of the film is melted, such that the film that has been stretched to at least 1.2 times is finally shrunken to 1.1 times or less, based on the film width provided to the heat-setting tenter, wherein the microporous polyethylene film has a film thickness of 5-40 μm, a porosity of 35-55%, a permeability from 2.5×10 −5 to 10.0×10 −5 Darcy, a puncture strength of at least 0.10 N/μm at 90° C., a puncture angle of at least 30° at 90° C., and a permeability from 2.0×10 −5 to 8.0×10 −5 Darcy after shrinking freely at 120° C. for 1 hour.