ALKALINE PRIMARY BATTERY WITH IMPROVED SEAL AND METHOD OF MANUFACTURING SAME

Abstract: The leakage of electrolyte from alkaline primary batteries is substantially reduced by improving the seal in such batter­ ies. The novel seal is formed by applying a thin adherent plastic film to the surface of the metal container which forms a bond between the metal container and the sealant covered seal member normally used in such batteries.

Description: The invention relates to alkaline primary batteries and, in particular, to the seal which prevents the leakage of electrolyte and a method of manufacturing such alkaline batteries. Alkaline primary batteries and, in particular, alkaline manganese dioxide/zinc dry cells (MnO₂/Zn), have been a successful commercial development. When manufactured in a cylindrical configuration, the battery seal is typically made by compressing a plastic member within a metal can by crimping the open end of the can as disclosed by Ralston and KO in U.S. Patent No. 3 663 301. More particularly, the typical commercial use of this type seal employs a nickel plated steel can as the positive current collector, a plastic seal member, typically of nylon or polypropylene, and a sealant, such as bitumen, between the metal can and the plastic seal member. While such seals performs satisfactorily when the steel can is plated with nickel, if the steel can is unplated, such seals are very poor. The poor seals are probably due to differences in bond strength of bitumen and steel and bitumen and nickel plated steel. From "Patent Abstracts of Japan", Vol.7, No.243(E-207) (1388), Oct. 23, 1983, it is known to apply one of a number of different tacky alkali resistant coatings to nickel-plated positive metal cans in order to improve close adhesion between a nylon or polyethylene seal ring of L-shaped section and a seal plate and thus provide a leakproof battery. It is an object of the invention as defined in the appendant claims to provide for an even stronger seal for an alkaline cylindrical cell and a method of manufacture of such cell, especially wherein the metal cans, i.e., the positive current collectors, are unplated. A polymeric resin, which upon drying becomes a thin adherent plastic film, when applied to the surface of the metal can of an alkaline dry cell, in the area of the seal greatly improves the strength of the seal, thereby reducing leakage problems associated therewith. The advantages of the present invention are achieved by applying a polymeric resin to the surface of the unplated metallic container (positive current collector) in the area of the battery seal. The resulting thin plastic film forms a bond between the metallic container and the sealant covered seal member, normally used in battery seals. By practicing the present invention, the positive current container can be unplated steel, which reduces the cost of manufacturing alkaline cylindrical cells. In accordance with a preferred embodiment of the invention, the interior surface of the metal can of an alkaline dry cell is coated with a polymeric primer. This primer, which may be applied by various techniques, including painting, spraying or dipping, contains an alkaline resistant organic binder dissolved in a compatible solvent. Spraying is the preferred methof of applying the organic primer. After being applied to the metal can, the primer is dried at elevated temperatures, which permits the evaporation of the solvent and allows for the adhesion of a thin, resin coating to the metallic container, The resulting coating is typically 15 to 20 µm (0.0006" to 0.0008") in thickness, but may range from 25 to 50 µm (0.0001" to 0.0002") in thickness. The binder is a film forming polymer which is compatible with alkaline battery components. Film forming binders which hydrolyze or oxidize in the presence of the electrolyte cannot be used in practicing the present invention. The present invention can be successfully practiced with a wide range of polymeric binders including ABS (acrylonitrile butadiene styrene), PVC (polyvinyl chloride), epoxies, fluorcarbons, nylons, polypropylene, polybutylene, polystyrenes and neoprenes. The present invention may also be practiced with binders which are rubbers and/or elastomers, such as, isobutylene, isoprene, chloroprene, polysulfide, ethylene propylene, chlorinated and chlorosulfonated polyethylene, fluorosilicone and propylene oxide. However, materials which are soluble in KOH, the usual electrolyte found in alkaline battery systems, such as CMC, should not be used in practicing the present invention. The solvent portion or the resin must wet the surface of the metal container. It must also be compatible with alkaline battery components and with the binder. The present invention can be successfully practiced with solvents such as ethyl acetate, butanol, methyl ethyl ketone, methyl isobutyl ketone and paraffinic hydrocarbon liquids. The polymeric resin, which may itself be conductive, may contain other ingredients such as fillers. Particularly, a non-conductive polymeric resin may contain small amounts of conductive filler materials such as carbon or metal powder. When carbon is added as the conductive filler, the weight percent of carbon in the film after the evaporation of the solvent should be less than 40%. Increasing the amount of carbon, which reduces the amount of binder in the plastic film, decreases the mechanical integrity of the plastic film and increases the probability that the conductive film will not remain adhered to the surface of the metal container. In order to provide the advantages disclosed herein the plastic film must be impervious to the alkaline electrolyte. Therefore, the plastic film must be continous in the area of the seal/sealant and metal container interface. Figure 1 is a drawing of an alkaline cylindrical battery incorporating this invention. Figure 1 is a drawing of an alkaline cylindrical battery constructed according to the present invention. The positive current collector, a drawn steel container 2, open on one end and about 0.010"(0,25 mm)thick, has a coating 14 applied to its interior surfaces. Two cathode annular rings 5, formed such that their outside diameters are greater than the inside diameter of the positive current collector, are forced into the positive current collector, forming a pressure contact therewith. A bead 10 is rolled into the container near the open end to support the sealing disk. A separator 4 and an anode 3 are placed inside of the cathode rings. A sealing disk 6 containing a negative current collector 1 is placed into the open end of the container and in contact with the bead. The open end of the container is crimped over the sealing disk thus compressing it between the crimp and the bead to seal the cell. An insulation washer 7 with a central aperture is placed over the crimped end of the cell such that the end of the negative current collector 1 protrudes through the aperture. A contact spring 8 is affixed to the end of the negative current collector 1. Terminal caps 9 and 13 are placed into contact with the contact spring 8 and the positive current collector 2, respectively, and an insulating tube 12 and steel shell 11 are placed around the cell and crimped on their ends to hold the terminal caps in place. Examples of the utility of the present invention will now be explained. Example 1 Three sets of adhesion test samples were prepared. The first set used a substrate of nickel-plated steel, the second set used a substrate of unplated steel, and the third set used a substrate of unplated steel coated with P-70 primer ( a conductive resin of PVC and carbon black dissolved in solvent manufactured by Pervel Industries). On to each substrate a metal washer with a 0.5" (12,7 mm) diameter opening was placed and a molten bitumen, Pioneer 135 (Witco Chemical Corp.), was poured inside the washer. The 5/8" (15,9 mm) diameter head of a bolt was then placed on the molten bitumen. These samples were then arranged in a tensile testing machine so that the force required to separate the bitumen from the substrate could be measured. The results in Table I show that the bond stregth was improved by use of the P-70 primer. In Table 1, the failure mode designation indicates the location of the separation. Cohesive failure is desired as this indicates that the weakest point of the bond is within the bitumen sealant itself, not at the substrate surface. Example 2 Two sets of alkaline manganese D-size cells were constructed using unplated steel cans as the positive current collector. The cans for the first set of batteries were not treated, while the metal containers used for the second set of batteries had their inner surfaces sprayed with P-70 primer. Batteries were then manufactured identically from the two sets of cans according to Figure 1. The cells were leakage tested by subjecting them to a thermal shock cycle consisting of 8 hours at 130°F (55°C) followed by 16 hours at 0°F (-12,5°C) for a total of three cycles. The outer wrap of each cell, consisting of the two terminal caps, the paper insulating tube, the insulating washer, the contact spring and the steel shell were then removed, and the number of cells with leakage between the sealing disks and the metal cans were counted. The data shown in Table II indicates that the present invention greatly improves the seal in alkaline cylindrical cells. From the results in the foregoing examples add the referenced draining, it is evident that the alkaline primary cells of this invention are superior to conventional alkaline primary batteries. While the foregoing examples used the alkaline Mn₂O/Zn electrochemical system in a commercial cylindrical configuration, the present invention includes other alkaline electrochemical systems which use an electrolyte which is not corrosive to the positive current collector.

Claims: An alkaline electrochemical cell having a sealing means comprised of a sealant disposed between the interior surface of an unplated metallic container(2), which functions as a positive current collector, and a plastic seal member (6), characterized in that the interior unplated surface of the metallic container (2) is provided with an alkali-resistant conductive plastic coating (14). An alkaline electrochemical cell according to claim 1, wherein the sealant is bitumen. An alkaline electrochemical cell according to claim 1 or 2, wherein the the plastic seal member (6) is comprised of nylon or polypropylene. An alkaline electrochemical cell as in claim 1, 2 or 3, wherein the said alkali-resistant conductive plastic coating (14) comprises conductive fillers. An alkaline electrochemical cell as in claim 4, wherein the conductive fillers are carbon particles selected from the group consisting of acetylene black, carbon black, and graphite. An alkaline electrochemical cell as in claim 1 or 5, wherein the weight percent of said fillers ranges up to 40. An alkaline electrochemical cell as in claim 1 wherein the metallic container (2) is unplated steel. Alkaline electrochemical cell according to any of claims to 7, characterized in that the plastic coating is provided only in the area where the seal/sealant and metal container contact. An alkaline electrochemical cell according to any of claims 1 to 8, characterized in that the plastic coating is 2.5 to 50 µm thick, preferably 15 to 20 µm thick. An alkaline electrochemical cell according to claim 1 wherein the said alkali-resistant conductive coating is not soluble in the alkaline electrolyte. An alkaline electrochemical cell according to claim 1 wherein the said alkali-resistant conductive coating is not hydrolyzed or oxidized in the alkaline electrolyte. Method of manufacture of an alkaline electrochemical cell according to claim 1, characterized by comprising the following steps: (a) coating the interior unplated surface of the unplated metallic container, serving as the positive current collector with an alkaline-resistant, conductive polymeric resin; (b) drying the polymeric resin coating, thereby forming an alkaline-resistant conductive plastic film on the surface of said metallic container; (c) placing alkaline battery components within said metallic container; and (d) disposing a sealant between a plastic seal member and between the interior surface of said metallic container. A method of manufacturing an alkaline electrochemical cell according to claim 12, characterized in that the metallic container is unplated steel. A method of manufacturing an alkaline electrochemical cell according to claim 12, characterized in that the sealant is bitumen. A method of manufacturing an alkaline electrochemical cell according to claim 12, characterized in that the plastic seal member is comprised of nylon or polypropylene. A method of manufacturing an alkaline electrochemical cell according to claim 12 which further comprises the step of: sealing said alkaline cell by crimping the open end of said metallic container. The method according to claim 12 characterized in that the metallic container is coated with an alkali-resistant polymeric resin comprising a conductive polymeric binder dissolved in a solvent. The method according to claim 12 or 17 characterized in that the polymeric binder does not dissolve in the alkaline electrolyte. The method according to claim 12, 17 or 18 characterized in that the polymeric binder is not hydrolyzed or oxidized by the alkaline electrolyte. The method according to any of claims 12, 17, 18 or 19 characterized in that the solvent wets the surface of the metallic container. The method according to any of claims 12 to 20, characterized in that the alkali-resistant conductive polymeric resin comprises fillers which are carbon particles selected from the group consisting of acetylene black, and graphite. The method according to claim 21, characterized in that the fillers are present in such an amount which range in weight percent up to 40. The method according to any of claims 12 or 17 to 22 characterized in that the alkali-resistant conductive coating is applied in such an amount to form a plastic film that is 2,5 µm to 50 µm thick, preferably 15 µm to 20 µm thick. The method according to claim 12, or 17 or 23, characterized in that the polymeric resin is applied to the metal container only in the area where the seal/sealant and metal container contact.

Inventors: KENYON, KENNETH HARRY, KULIBERT, GREGORY STEPHEN
Applicants: RAYOVAC CORPORATION
Application year: 1985
Main classification: H01M2
IPC codes: H01M2