US Patent 8003261 Polyolefin microporous membrane

The present invention provides a microporous polyolefin film having a thickness of 1 μm (inclusive) to 50 μm (inclusive), a porosity of 30% (inclusive) to 70% (inclusive), a piercing strength of 0.15 N/μm or more in terms of a film thickness of 1 μm, 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.