Patent ID: 8057718

Claim:
A method of producing a microporous polyethylene film, comprising: (a) melt-extruding a resin composition to form a sheet, the resin composition comprising: 100 parts by weight of a polyethylene composition including 10-50 wt % polyethylene having a weight average molecular weight of from 2Ã—10 5 to less than 5Ã—10 5 (component I) and 90-50 wt % diluent having a kinetic viscosity of 20-200 cSt at 40Â° C. (component II); and 0-150 parts by weight of inorganic powder (component III); (b) stretching the sheet according to a roll-type stretching manner in a machine direction at a stretching ratio of from 3 times to 10 times without using a pinch roll, and then subsequently stretching the stretched sheet according to a tenter-type stretching manner in a transverse direction at a stretching ratio of from 2 times to 10 times to produce a film, both of the roll type stretching and the subsequent tenter type stretching being performed at the temperature that 30-80 wt % of the crystalline portion of the polyethylene is melted; and (c) extracting the diluent from the film, and heat-setting the resulting film, wherein, stretching part of a roll stretching machine used in the roll-type stretching has at least 3 stretching rolls, speed of each of stretching rolls is 1.02 times Ëœ a predetermined value times faster than speed of the immediately preceding roll, and the predetermined value is gained by multiplying a total machine direction stretching ratio by 0.9 and the roll of the stretching part has a surface roughness of 0.2-10 s.