Patent ID: 7435761

Claim:
A method of producing a microporous polyethylene film without the use of irradiation for the purpose of crosslinking, comprising: (a) melt-extruding a resin mixture to form a sheet, the resin mixture comprising: 100 parts by weight of composition containing 20-50 wt % polyethylene with a weight average molecular weight of 5×10 4 -3×10 5 (component I) and 80-50 wt % diluent (component II); 0.1-2 parts by weight of peroxide (component III); and 0.05-0.5 parts by weight of anti-oxidant (component IV), (b) stretching the sheet at a temperature range, where 30-80 wt % of a crystalline portion of the sheet is molten, according to a tenter-type simultaneous stretching manner such that stretching ratios are 3 times or more in machine and transverse directions, respectively and a total stretching ratio is 25-50 times to produce a film; and (c) extracting the diluent from the film, and heat-setting the resulting film, wherein, the microporous polyethylene film has gels of 50 or less per a thickness of 200 μm and area of 2000 cm 2 , a puncture strength of 0.22 N/μm or more and a gas permeability of 1.3 Darcy or more.