| 摘要 |
PROBLEM TO BE SOLVED: To provide a porous film that has excellent air permeability and strength characteristics, to provide a separator for a battery that has excellent air permeability and strength characteristics contributing to battery performance in particular and has a pore size corresponding to a high output of the battery and to provide a method of manufacturing the separator for a battery.SOLUTION: A porous film is composed of a resin composition including (A) a polypropylene resin and (B) one or more kinds of ethylene copolymer selected from the group consisting of an ethylene-α-olefin resin and an ethylene-(meth)acrylic acid ester resin, has a mixing mass ratio (A)/(B) in the resin composition of 88/12 to 70/30, has &bgr;-crystal activity and has an air permeability of less than 500 sec/100 ml. in the porous film, the tensile strength Ein the flow direction (MD) of the porous film is not less than 100 MPa; the tensile strength Ein the vertical direction (TD) with respect to the flow direction is less than 100 MPa; and the ratio (E/E) of the Eto the Eis not less than 3 to less than 10. |
