Patent ID: 8003261

Claim:
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., wherein tensile elongations in the length direction (MD tensile elongation) and the width direction (TD tensile elongation) are each 10% (inclusive) to 200% (inclusive); and the total of the MD tensile elongation and TD tensile elongation is 20% (inclusive) to 250% (inclusive).