Patent ID: 8680169

Claim:
A porous polypropylene film having a tensile strength in the flow direction of the film, E MD , of 100 MPa or greater; a tensile strength in the direction perpendicular to the flow direction, E TD , of less than 100 MPa; a ratio of E MD and E TD , E MD /E TD , of 1.5 to 10; an air permeability of 700 seconds/100 ml or less; a puncture strength of 1.5 N or greater; and having β activity wherein the porous polypropylene film is obtained under the following stretching conditions of a TD stretching temperature of from 100-160° C., and a MD stretching temperature of from 10 to 120° C. such that a stretch ratio in the flow direction of the film, λ MD , is 3.0 to 8.0 times; a stretch ratio in the direction perpendicular to the flow direction, λ TD , is 1.1 to 2.5 times; and a ratio between λ MD and λ TD , λ MD /λ TD , is 1.5 to 5.0.