| 摘要 |
PROBLEM TO BE SOLVED: To provide a composite film for a battery separator which is excellent in permeability and mechanical strength and has a function of cutting off permeability at a low temperature and is excellent in safety by using a laminated composite film of a polyolefine microporous film and polypropylene nonwoven fabric as a battery separator. SOLUTION: A polyolefine microporous film is composed of a polyolefine composition containing polyolefine whose weight average molecular weight is not less than 5×10<5> or its component, and has specific porosity, an average through hole diameter and tensile rupture strength. A composite film for a battery separator is constituted by laminating this microporous film and polypropylene nonwoven fabric. When a composite film having such constitution is used, in the case where a temperature rises at abnormal time, since the polyolefine microporous film is first softens and melts at a lower temperature and blocks up a pore part, a short circuit current is reduced, and the polypropylene nonwoven fabric acts as a porosity support body, and maintains a function of reducing the short-circuit current up to a high temperature.
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