Battery separator

This invention relates to a battery separator and, more particulary, to such a separator which includes at least one non-woven sheet of organic fibers, particularly polyolefin fibers, and may include, in addition, at least one non-woven sheet of glass fibers. A method for producing a battery separator is disclosed. The method comprises applying a coating of an ethylenically unsaturated monomer to the fibers of a non-woven sheet and polymerizing the monomer in situ on the fiber surfaces. The non-woven sheet is from 50 to 3000 microns thick, and is composed of polyolefin fibers having an average fiber diameter from 0.5 to 5 microns and a surface area from 0.2 to 30 square meters per gram. The ethylenically unsaturated monomer is polymerizable to a thermoplastic polymer which is hydrophilic as a consequence of containing COOH, OH, sulfonyl, sulfonic acid or carbonyl groups, or a monomer which has one of the following formulas: In the foregoing formulas, R is an aliphatic or aromatic hydrocarbon group having from 2 to 8 carbon atoms, R4 is hydrogen or methyl, n is an integer from 2 to 15, and Xis a cation selected from the group consisting of hydrogen, alkali metal cations, alkaline earth cations, cations of the transition metals Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn, and ammonium cations of the following formula:each of RR, Rand Ris hydrogen, alkyl, alkenyl, cycloalkyl or cycloaklkenylene, and not more than one is a bivalent group bonded to Nand by removing two hydrogens from an alkane, an alkene, a cycloalkane or a cycloalkene; the total number of carbons in RR, Rand Rdoes not exceed