Patent ID: 8802270

Claim:
A biaxially oriented, multilayer, microporous foil with shut-off function whose microporosity is created by converting β-crystalline polypropylene when the foil is stretched, and which comprises at least one shut-off layer I and at least one porous layer II, wherein the shut-off layer contains propylene homopolymer and propylene block copolymer and β-nucleating agent and polyethylene, and the porous layer II contains propylene homopolymer and propylene block copolymer and β-nucleating agent, wherein the foil has a Gurley value of 50 to 5000 s, an e-modulus in the lengthwise direction >300 N/mm 2 to 1,800 N/mm 2 , and >500 N/mm 2 to 3,000 N/mm 2 in the transverse direction, and after exposure to a temperature of 130° C. for 5 minutes the foil exhibits a Gurley value of at least 5000 s, wherein the Gurley value after this temperature treatment is at least 1000 s higher than before.