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Abstract

A microporous film of high molecular weight polyolefin useful in the prepration of filter mediums or non-aqueous battery separators and a process for producing the film are disclosed. The microporous film of high molecular weight polyolefin contains leaf vein-like fibrils as a main constituent on each of which fibrils indeterminate-form crystallites of not more than 1 .mu.m in size flocculate and has an intrinsic viscosity, ?.eta.!, of 4 dl/g or more and, preferably, has: (a) a void content of 25% or more; (b) a gas permeability of 1900 sec/100 cc or less; (c) a tensile strength of 0.05 GPa or more; and (d) a gas impermeability-acquiring temperature of 140.degree. C. or lower. The microporous film is produced by subjecting a high molecular weight, biaxially oriented polyolefin film of 60% or more in crystallinity to a heat treatment and, if necessary, to a stretch treatment to thereby melt or dissolve the indeterminate-form portion of the film, then crystallizing it onto the fibrils as crystallites.

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First Claim