Method of producing microporous high density polyethylene film

1. A method of producing a microporous high density polyethylene film, comprising:

(a) melt-extruding a composition, including 20-50 wt % high density polyethylene with a weight average molecular weight of 2×10 5 -5×10 5 (component I), containing 5 wt % or less molecule with a molecular weight of 1×10 4 or less; and 80-50 wt % diluent (component II); to form a sheet;

(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;

(c) extracting the diluent from the film; and

(d) heat-setting the resulting film at a temperature range where 10-30 wt % of a crystalline portion of the film is molten,

the microporous high density polyethylene film having tensile strengths of 1,100 kg/cm 2 or more in transverse and machine directions respectively, a puncture strength of 0.22 N/μm or more, a gas permeability (Darcy's permeability constant) of 1.3×10 −5 Darcy or more, and shrinkages of 5% or less in transverse and machine directions, respectively.

2. The method as set forth in claim 1 , wherein the component I contains 2 wt % or less comonomer, the comonomer being selected from the group consisting of propylene, butene-1, hexene-1, and 4-methylpentene-1.

3. The method as set forth in claim 1 , wherein the component II includes a paraffin oil with a kinetic viscosity of 20-200 cSt at 40° C.

4. The method as set forth in claim 1 , wherein a content of the diluent remaining in the film is 1 wt % or less in the step of (c).

5. The method as set forth in claim 4 , wherein a heat-setting time is 1-20 min in the step of (d).