Patent ID: 8012622

Claim:
A three-layer microporous membrane, comprising: a first microporous layer containing a first layer material, a third microporous layer containing the first layer material, and a second microporous layer containing a second layer material, the second microporous layer being located between the first and third microporous layers, wherein the first layer material comprises a first polyethylene and a first polypropylene and the second layer material comprises a second polyethylene and a second polypropylene, the second polypropylene having (1) a weight-average molecular weight of 6×10 5 or more, (2) a heat of fusion of 90 J/g or more, and (3) a fraction having a molecular weight of 5×10 4 or less with the fraction being 5% or less by mass of the second polypropylene, and the first polyethylene is present in the first layer material in a first polyethylene amount in the range of from about 50 wt. % to about 99 wt. % based on the weight of the first layer material, the first polypropylene is present in the first layer material in a first polypropylene amount in the range of from about 1 wt. % to about 50 wt. % based on the weight of the first layer material, the second polyethylene is present in the second layer material in a second polyethylene amount in the range of from about 5 wt. % to about 95 wt. % based on the weight of the second layer material, and the second polypropylene is present in the second layer material in a second polypropylene amount in the range of from about 5 wt. % to about 95 wt. % based on the weight of the second layer material.