An illustrative environment where application of the principles of the present invention is particularly advantageous is in electrochemical cells such as, for example, lead-acid cells containing plates made of a grid and a paste applied thereon. The grid, oftimes made of pure lead or an alloy of lead, is used principally to hold the paste in place, the paste for the positive plate is usually comprised of lead oxide, sulfuric acid and usually some type of fiber to increase strength and reduce mud formation. The paste for the negative plate may be comprised of lead oxide and other substances such as barium or strontium sulfate, lampblack and organic additives.
The major function of the separator is to separate the positive plate from the negative plate in a cell. The separator also holds the paste on the plates keeping the paste from falling to the bottom of the cell forming mud. Most battery manufacturers desire to pack as many plates as possible in a given cell to provide maximum energy density per cubic foot. Such packing leaves minimum room for separators.
The material used to form the separator must be an insulator to prevent conduction of electrons between plates and must not introduce impurities into the electrolyte which would be detrimental to the battery's performance. This material should provide low resistance to transfer of electrolyte ions yet be impervious to the corrosive nature of electrolyte and plate materials.
Today, polyolefin materials are of a great interest to manufacturers for use as separator material in battery cells. Surface modification of nonwettable fibers is one of the major approaches used to give polyolefins a wettability characteristic in aqueous battery systems.
U.S. Pat. No. 4,072,802 dated Feb. 7, 1978 to Murata et al discloses a separator for lead-acid batteries comprised of an embossed thin web polypropylene nonwoven material having projecting embossed parts, wherein the web is made wettable after formation with a composition consisting of polyvinyl chloride, silicon oxide and with tetrahydrofuran. This material allegedly provides a suitable acid electrolyte wettability of the polyolefin; however, the embossed parts do not appear to aid in packing plates with separators for maximizing energy density per cubic foot.
U.S. Pat. No. 3,933,525 dated Jan. 20, 1976 to Palmer et al discloses a process for producing a battery separator material suitable for use in a lead acid cell which involves incorporating surfactants having low water solubility in a polymeric resin before the polymeric fibers are extruded. It has been necessary, apparently, to use a second surfactant after the extruded nonwoven material is formed to achieve adequate wetting.
A search for various other means of providing a thin wettable polyolefin separator material that has low electrical resistance, good particle retention, high puncture resistance and that would permit battery manufacturers to pack as many plates as possible in a lead-acid battery cell was initiated. This search was successful and resulted in the present invention.