Patent ID: 8486555

Claim:
A method for producing a porous laminate configured to be a battery separator having micro-pores interconnected with each other in a thickness direction of the laminate, the method comprising: (A) forming a laminate comprising (i) an interlayer comprising polypropylene as a hard segment and ethylene-propylene rubber as a soft segment, and excluding homopolymers of polyethylene and a filler, and (ii) a first pore-unformed outer layer and (iii) a second pore-unformed outer layer, each comprising polyethylene and a filler, which are disposed on both outermost surfaces of the interlayer (i); (B) making the interlayer (i) porous by forming micro-pores through the interlayer (i) after the laminate obtained in (A) is impregnated with a fluid in a supercritical state or a subcritical state, the fluid is relieved from the supercritical state or the subcritical state to vaporize the fluid; and (C) stretching the laminate to separate an interface between the filler and the polyethylene in the first (ii) and second (iii) outer layers, to form micro-pores through the first (ii) and second (iii) outer layers, and to make the first (ii) and second (iii) outer layers porous, thereby interconnecting the micro-pores of the interlayer (i) with micro-pores formed through the first (ii) and second (iii) outer layers after (B), wherein micro-pores are not formed through the first (ii) and second (iii) outer layers when the fluid which has impregnated the laminate in the supercritical state or the subcritical state is relieved from the supercritical state or the subcritical state, and wherein the density of the polyethylene in the first (ii) and second (iii) outer layers is 0.92 g/cm 3 .