In order to promote adhesion between rubber and ferrous metals it is known to employ a variety of organometallic salts as coatings to the metal or as an ingredient in a rubber composition. One early patent embodying the former technique is U.S. Pat. No. 2,240,805 which discloses that rubber stock adheres more readily to a variety of metals including brass after they have been plated with pure or alloyed cobalt. The invention requires the coating of the metal with cobalt, rather than adding cobalt or an inorganic salt thereof to the rubber stock.
Addition of an organo-cobalt compound to a rubber compound is disclosed by U.S. Pat. No. 2,912,355 wherein 0.1% to 10% by weight of a partially calcined organo-cobalt compound, based on the weight of rubber is incorporated into a vulcanizable rubber composition to increase the adhesion of the rubber, when vulcanized, to contacted metal surfaces, preferably copper, zinc, or their alloys, among others. The cobalt compounds recited by the patentees are primarily cobalt salts of organic acids, preferably aliphatic fatty acids, which are heated under oxidizing conditions prior to incorporation into the rubber composition. The patent discloses that total oxidation of the organo-cobalt compound to cobalt oxide results in over a fifty percent drop in the adhesive properties of the treated rubber composition, although the cobalt oxide sample exhibited nearly twice the adhesion of the control rubber composition containing no cobalt compound. The patent also states that rosins and resins are among known rubber compounding ingredients, however, there is no recognition or indication that adhesion between rubber and metallic reinforcements could be improved with any of these ingredients.
U.S. Pat. No. 3,897,583 is directed toward the adhesion of metal to rubber by incorporating an organo-cobalt salt in a rubber stock which contains an adhesive resin forming system based on a methylene donor which is a methylolated nitroalkane in combination with a resorcinol type methylene acceptor. Cobalt salts disclosed include those of aliphatic or alicyclic carboxylic acids having 6-30 carbon atoms.
U.S. Pat. No. 3,936,536 discloses the method of adhering rubber to metal by coating the metal with rubber containing a small amount of Co(NH.sub.3).sub.2.Cl.sub.2.
U.S. Pat. No. 4,148,769 is directed toward a sulfur-containing rubber composition having an improved adhesion with brass-plated steel cords comprising, as unconventional ingredients, litharge and a cobalt salt of a fatty acid. The patentees further state their belief that adhesive strength is improved by the addition of a phenolic resin such as resorcinol, or a prereacted soluble resorcinol resin. Resorcinol resin has long been known to increase the adhesion between rubber and brass. The data in the patent indicates that even more satisfactory results obtain when the cobalt salt and litharge are incorporated together in the rubber composition.
Thus, while others have sought to enhance adhesion between rubber compositions and metals by employing phenolic resins and certain cobalt compounds, particularly organic salts thereof, the combined effect of certain rosin-derived resins with inorganic salts of cobalt as additives to unvulcanized rubber compositions to improve adhesion with metals has not been described in any patents or publications of which we are aware.