To provide a stretched laminated porous polyolefin film having uniform micropores, proper in pore closing temp. by heat, high in closed pore keeping upper limit temp., wide in a closed pore keeping temp. region, excellent in air permeability and having a proper heat shrinkage factor.
A high m.p. polyolefin film and a low m.p. polyolefin film different in m.p. by 20°C or higher are prepared and the high m.p. polyolefin film is heat-treated at temp. lower than its m.p. by 35-55°C while the low m.p. polyolefin film is heat-treated at a temp. lower than its m.p. by 30-55°C and both polyolefin films are bonded within a temp. range from the m.p. of the low m.p. polyolefin film to temp. higher than the m.p. by 10°C under pressure to obtain a laminated film consisting of three or more layers of the high and low m.p. polyolefin films. This laminated film is stretched by 5-200% within a temp. range from -20°C to 50°C and subsequently stretched by 100-400% at 70-130°C to be perforated and subsequently thermally fixed.
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SHIMADA JUNICHI
FUJII TERUAKI
