| 摘要 |
PURPOSE:To obtain the title membrane having a high void content and excellent mechanical strengths, by forming in a membrane micropores of an elliptic shape having a specified ratio of the major axis to the minor axis when observed from the membrane surface. CONSTITUTION:100pts.vol. polyolefin resin such as polypropylene of an intrinsic viscosity of 1.8-3.3dl/g, an isotactic index of 93% or higher, a weight-average MW/number-average MW of 5 or less, a melt crystallization temp. of 106 deg.C or higher, and an m.p. of 120 deg.C or higher and 80-180pts.vol. org. solid (e.g., dicyclohexyl phthalate) are compounded and melt-extruded. 95% or more of the org. solid is extracted with a good solvent for the org. solid (e.g., trichloromethane). Then, the film is drawn in one direction 1.5-6-fold at a temp. in a range from the glass transition temp. of polyolefin to the m.p. -100 deg.C in one direction and heat-set to obtain a microporous polyolefin membrane of a void content of 50-85% and an average void diameter of 0.05-5mum, wherein the content of the micropores having an elliptical shape wherein the ratio of the major axis to the minor axis is 1.5-7 when observed from the membrane surface accounts for 60% or more of the whole micropores. |
