Wet-laid nonwoven fabric for battery separator, its production method and sealed type secondary battery

摘要

An improved wet-laid nonwoven fabric for a battery separator wherein 20 to 95% by weight of thermoplastic staple fibers having a single fiber diameter of 20 rm, or less are three-dimensionally entangled with hot- melt fibers having a melting point lower by 20 C. than that of the thermoplastic staple fibers so that a mean fiber entangling point interval is 300 m or less, wherein at least part of the hot-melt fibers are fused to bond the fibers with each other to fix the nonwoven structure. The nonwoven fabric has a basis weight in a range from 10 to 350 g/m.sup.2, a thickness in a range from 30 to 1000 . mu.m and an apparent density in a range from 0.26 to 0.7 g/cm.sup.3, and is excellent in uniformity and mechanical strength. The wet-laid nonwoven fabric is particularly excellent in air permeability, liquid holding capability, liquid retention capability and liquid absorption rate and satisfies the requirement for a battery separator. Therefore, it is useful for the production of a high performance secondary battery having favorable overcharging characteristics and cycle characteristics. The wet- laid nonwoven fabric according to the present invention is prepared by subjecting a wet-laid web of staple fibers having a ratio L/D of a fiber length L to a single diameter D in a range from 0.5 10.sup.3 to 2. 010.sup.3 to a hydroentangling treatment while taking care not to leave a mark from a water jet on the web.