Patent ID: 8748028

Claim:
A multi-layer microporous membrane comprising: at least two first microporous membrane skin layers comprising; (a) from about 30 to about 70% of a first polyethylene, (b) from about 20 to about 40% of a first polypropylene, the first polypropylene having a weight average molecular weight of from about 0.8×10 6 to about 1.5×10 6 , a molecular weight distribution of from about 1 to about 100 and a heat of fusion of 80 J/g or higher and (c) from about 10 to about 30% of a second polypropylene, the second polypropylene having a weight average molecular weight of from about 4×10 5 to about 7.5×10 5 , a molecular weight distribution of from about 5 to about 100 and a heat of fusion of 80 J/g or higher, percentages based on the weight of the first microporous membrane skin layer; and at least one second microporous membrane layer in planar contact with the first microporous membrane comprising a high density polyethylene and an ultrahigh molecular weight polyethylene; wherein the first and second microporous membrane layers are formed into first layer/second layer/first layer or first layer/second layer/first layer/second layer/first layer structures.