Patent ID: 8313678

Claim:
A process for preparing microporous high-density polyethylene having puncture strength of 0.22 N/μm or more, Darcy's permeability constant of 1.8×10 −5 Darcy or more, and shrinkage of 4% or less in machine and transverse direction, respectively, which comprises the steps of melt-extruding a composition consisting of 20 to 50 wt % of high-density polyethylene with weight average molecular weight of 2×10 5 to 4×10 5 , containing not more than 5 wt % of molecules with molecular weight of 1×10 4 or less and not more than 5 wt % of molecules with molecular weight of 1×10 6 or more (Component I), and 80 to 50 wt % of diluent (Component II) to form a sheet; stretching the sheet at a temperature range where 30 to 80 wt % of the crystalline portion of the sheet melts, by using roll type or tenter type sequential or simultaneous stretcher in a stretching ratio of 4 times or more in machine and transverse direction, respectively, with total stretching ratio of 25 to 50 times to form a film; cooling the film, just upon the completion of stretching, under the condition of retaining the tension applied during the stretching, wherein a cooling temperature is higher than ambient temperature but lower than a crystallization temperature of the stretched film; extracting the diluent from the resultant film; and heat-setting the film at a temperature range where 10 to 30 wt % of the crystalline portion of the film melts.