| 摘要 |
PROBLEM TO BE SOLVED: To provide a porous membrane which has a shrinkage stress of 5 g or below per 0.1 mm<2> of a cross section upon being heated from 25 to 80 deg.C, a shrinkage of 3% or below after being heated at 60 deg.C for 1 hr and a tensile modulus of 500 kg/cm<2> or above at 25 deg.C and therefore has a low percentage of rejects, a small dispersion of gas permeability and is useful as an improved battery separator safe upon overcharge, particularly a porous membrane for a battery separator having good workability in assembling into a battery, is resistant to scratches or fine breaks causing its inner short circuiting and a process for producing the same. SOLUTION: A laminated precursor film containing a layer containing e.g. a polyethylene having a melting point of 100-140 deg.C and a layer having e.g. a polypropylene having a melting point of 150 deg.C is heat-treated at 50-170 deg.C for 5 sec to 150 hr and stretched at temperature as low as -30 to 100 deg.C at a draw ratio of 10-150% on the basis of the length of the unstretched film, stretched at high temperature at a draw ratio of 10-300% on the basis of the unstretched film, and the obtained laminated porous film is heat-treated for 5 sec to 150 hr at 50-140 deg.C at least twice. |
