Importance

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Abstract

A separator for battery which exhibits a low electrical resistivity at normal time, good SD characteristics and a high mechanical strength is disclosed. A high molecular weight polypropylene having a weight average molecular weight of 500,000 or more and a polyethylene having a melting point of from 100.degree. to 140.degree. C. are extruded through a T-die film-forming machine to prepare a laminate film comprising a laminate of the high molecular polypropylene layer and the polyethylene layer. The laminate film is stretched at a temperature as low as from -20.degree. C. to 80.degree. C. by 10% to 100% based on the length of the original film, stretched at a temperature as high as 80.degree. C. to 130.degree. C. by 60% to 300% based on the length of the unstretched film, and then shrunk at the same temperature by 5% to 30% based on the length of the stretched film to render the film porous. Thus, a porous laminate film having a Gurley value of from 200 to 1,500 is prepared as a separator for battery.

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First Claim