Abstract: The present invention provides a microporous polyolefin film having a thickness of 1 ?m (inclusive) to 50 ?m (inclusive), a porosity of 30% (inclusive) to 70% (inclusive), a piercing strength of 0.15 N/?m or more in terms of a film thickness of 1 ?m, tensile strengths in the length direction (MD tensile strength) and the width direction (TD tensile strength) of 30 MPa or more each, a thermal shrinkage in the width direction (TD thermal shrinkage) of 1% or less at 65° C., and a thermal shrinkage ratio in the length to width direction (MD/TD thermal shrinkage ratio) of more than 2 at 65° C. A process for producing the microporous film and a nonaqueous electrolyte secondary battery using the film are also provided.

Abstract: An object of the present invention is to provide a polyolefin microporous film that can sufficiently reduce the occurrence of raised edges at a slitting step such as a slitting step at the time of producing the polyolefin microporous film and a slitting step at the time of processing the polyolefin microporous film into a roll. The present invention provides a polyolefin microporous film formed from a polyolefin composition comprising a polyethylene, a first polypropylene having a viscosity average molecular weight of not less than 50,000 and less than 300,000, and a second polypropylene having a viscosity average molecular weight of not less than 300,000.

Abstract: The present invention provides a microporous polyolefin film having a thickness of 1 ?m (inclusive) to 50 ?m (inclusive), a porosity of 30% (inclusive) to 70% (inclusive), a piercing strength of 0.15 N/?m or more in terms of a film thickness of 1 ?m, tensile strengths in the length direction (MD tensile strength) and the width direction (TD tensile strength) of 30 MPa or more each, a thermal shrinkage in the width direction (TD thermal shrinkage) of 1% or less at 65° C., and a thermal shrinkage ratio in the length to width direction (MD/TD thermal shrinkage ratio) of more than 2 at 65° C. A process for producing the microporous film and a nonaqueous electrolyte secondary battery using the film are also provided.