Patent ID: 8795900

Claim:
A biaxially oriented, single-layer, microporous foil with shut-off function whose microporosity is created by converting β-crystalline polypropylene when the foil is stretched, and which contains propylene homopolymer and propylene block copolymer and β-nucleating agent and polyethylene, wherein the foil has a Gurley value of 50 to 5000 s, an e-modulus in the lengthwise direction of 300-1800 N/mm 2 , and of 400-3000 N/mm 2 in the transverse direction, and wherein after heat treatment of the foil by exposure to a temperature of 130° C. for 5 minutes the foil exhibits a Gurley value of at least 5000 s and wherein the Gurley value after the heat treatment is at least 1000 s higher than before the heat treatment.