Patent ID: 8906540

Claim:
A biaxially oriented microporous film composed of at least three coextruded layers encompassing an internal cut-off layer and two external layers, wherein all three layers contain a mixture of propylene homopolymer and propylene block copolymer and β-nucleation agent, wherein the propylene block copolymer I of the external layers has a melting point exceeding 140° C. and the propylene block copolymer II of the internal layer has a melting range starting at a temperature ranging from 50 to 120° C. and the melting point of the propylene block copolymer I is greater/higher than the melting point of the propylene block copolymer II and the block copolymer II of the internal layer having a conomonomer content of from 10 to 25% by weight, and the block copolymer I of the external layer has a melting range that starts at a temperature of over 120° C.