Patent ID: 8669002

Claim:
A microporous film comprising a thermoplastic resin, which is manufactured by extruding a crystalline thermoplastic resin from a die, further applying heat treatment, and cold stretching and subsequent hot stretching in a machine direction to an extruded film, wherein the microporous film is manufactured according to a method in which a draft ratio during resin extrusion from the die is in the range of 20 to 150, a heat treatment temperature is in the range of a temperature lower by 10° C. than the melting point of the crystalline thermoplastic resin to a temperature less than the melting point, and a period of time of heat treatment is in the range of 3 to 15 minutes, wherein a melt flow rate of the thermoplastic resin in the microporous film is in the range of 0.1 to 2.0 g/10 min, tensile strength in a cross-machine direction is in the range of 5 to 10 MPa and tensile elongation in the cross-machine direction is 300% or more, wherein the crystalline thermoplastic resin used as a raw material of the microporous film is a polyolefin resin, weight average molecular weight/number average molecular weight of the polyolefin resin is in the range of 4 to 15, and an isothermal crystallization time at a temperature lower by 25° C. than the melting point is 200 seconds or less.