| 摘要 |
PROBLEM TO BE SOLVED: To obtain a porous film which, when used as a battery separator, has proofness against release charge and excellent resistance to pin separation and allows one to smoothly pour an electrolyte into the battery by protruding part of the particles from the surface of particle-containing porous film. SOLUTION: A mixture containing a film-forming material (e.g. polyolefin resin) and particles having a mean particle diameter of 0.05-10μm is molded into a film. This film is heat-treated at 100-160 deg.C for 1-100hr. Next, the heat- treated film is stretched in the machine direction at a temperature as low as -20 to 80 deg.C at a draw ratio of 10-200% and further stretched in the machine direction at 80-130 deg.C at a draw ratio of 50-200%. The film is then shrunk at 80-130 deg.C so that the size in the direction of stretch may diminish by 5-40% and then heat-set at 80-130 deg.C for 0.1-24hr to protrude a part (100100,000 protrusions on the average per mm<2> of the surface area of the film) of the particles from the surface of the porous film at a height of 0.02-3μm.
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