Importance

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Abstract

A microporous polyolefin membrane made of a polyolefin composition containing 1 weight % or more of an ultra-high-molecular-weight polyolefin having a weight-average molecular weight of 7.times.10.sup.5 or more and having a weight-average molecular weight/number-average molecular weight ratio of 10-300, the microporous membrane having a thickness of 0.1-25 .mu.m, a porosity of 35-95%, an average pore diameter of 0.001-0.2 .mu.m and a breaking strength of 0.2 kg or more per 15 mm width is produced by (a) preparing a solution comprising 10-50 weight % of the polyolefin composition and 50-90 weight % of a solvent; (b) extruding the solution through a die; (c) cooling the extruded solution to form a gel-like article; (d) stretching the gel-like article at a temperature equal to or lower than a melting point of the polyolefin composition +10.degree. C; and (e) removing the remaining solvent.

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First Claim