Abstract: The present invention relates to a microporous polyolefin film suitable as a separator for batteries and thermal properties thereof. A microporous polyolefin film according to the present invention has a film thickness of 5-40 ?m, a porosity of 30%-60%, a permeability of 2.0×10?5-8.0×10?5 Darcy, a maximum pore size determined by the bubble point method of not more than 0.1 ?m, a puncture strength of 0.20 N/?m or more at room temperature and 0.05 N/?m or more at 120° C., and a maximum shrinkage ratio in the transverse direction (TD) when subjected to a thickness-normalized external force in TMA (thermo-mechanical analysis) of not more than 0%. With excellent thermal stability at high temperature as well as superior puncture strength and gas permeability, the microporous polyolefin film according to the present invention is suitable for high-capacity, and high-power batteries.

Abstract: The present invention relates to a microporous polyethylene film for use as battery separator. The microporous polyethylene film according to the present invention is characterized by having a film thickness of 5-40 ?m, a porosity of 35-55%, a permeability from 2.5×10?5 to 10.0 10?5 Darcy, a puncture strength of at least 0.10 N/?m at 90° C., a puncture angle of at least 30° at 90° C., and a permeability from 2.0 10?5 to 8.0 10?5 Darcy after shrinking freely at 120° C. for 1 hour. The microporous polyethylene film in accordance with the present invention has very superior puncture strength and thermal stability at high temperature and takes place of less decrease of permeability due to low thermal shrinkage at high temperature, as well as superior permeability. Therefore, it can be usefully applied in a high-capacity, high-power battery to improve thermal stability and long-term stability of the battery.