Patent ID: 8338017

Claim:
A microporous membrane comprising (a) from 40 to 60% of a first polyethylene having a weight average molecular weight of from 3×10 5 to 3.5×10 5 and a molecular weight distribution of from 5 to 50, (b) from 20 to 40% of a first polypropylene having a weight average molecular weight of from 0.9×10 6 to 1.1×10 6 , a molecular weight distribution of from 1 to 50, and a heat of fusion of from 80 to 120 J/g, (c) from 10 to 25% of a second polypropylene having a weight average molecular weight of from 5.7×10 5 to 6.6×10 5 , a molecular weight distribution of from 5 to 50, and a heat of fusion of from 80 to 120 J/g, and (d) from 0 to 10% of a second polyethylene having a weight average molecular weight of from 1×10 6 to 3×10 6 and a molecular weight distribution of from 5 to 50, percentages based on the mass of the membrane.