In order to promote adhesion between rubber and ferrous metals it is known to employ a variety of metallic salts as coatings to the metal or as an ingredient in a rubber composition. It is also known to add various resins as tackifiers and/or adhesion promoters and, in other instances, to employ both a metal salt and a resin. Typical of the latter type of art is U.S. Pat. No. 3,628,038 which discloses a method for bonding EPDM rubbers to a metal wherein sulfur, vulcanization accelerators, and a halo-genated alkylphenol-formaldehyde resin as a vulcanizing agent are added to the rubber which is then placed upon the metal and bonded thereto during vulcanization. Data presented in the patent established an increase in adhesion where all three components were employed as compared to control stocks containing only sulfur or the vulcanizing agent. Apart from the fact that rubber stocks normally carry an accelerator to shorten cure time and that the patent disclosed an accelerator, there is no teaching that the accelerator alone can improve rubber-to-metal adhesion, particularly in rubbers other than EPDM.
The use of metallic salts is exemplified by U.S. Pat. No. 3,905,947 which is directed toward a method for improving rubber-to-metal adhesion by adding an organo-nickel salt to the rubber and then vulcanizing it while in contact with the metal surface. The organo-nickel salts include a plurality of compounds such as nickel bis-[(3,5-di-t-butyl-4-hydroxybenzyl)-benzenephosphinate]; nickel bis(o-butyl-3,5-di-t-butyl-4-hydroxybenzyl-phosphonate); nickel bis(3,5-di-t-butyl-4-hydroxybenzoate); nickel n-octylthioacetate; [2,2'-thiobis(4-t-octylphenolato)]n-butylamine nickel and the like. None of these compounds is disclosed as having any activity as a vulcanization accelerator.
Finally, U.K. Pat. No. 914,787 discloses the use of cobalt-mercaptobenzothiazole complexes in natural and synthetic rubbers as both an accelerator and to permit the rubber to be glued or bonded directly to ferrous metal during vulcanization. The exemplary rubber stocks reported in the patent also contain pine tar, which is added to increase the strength of the adherence of rubber to the metal. The patent also states that the effect of the cobalt complex to promote adhesion is improved by adding lead dioxide.
While others have sought to enhance adhesion between rubber compositions and metals by employing various combinations of cobalt and other metal salts with resins, the art of which has been presented herein has not disclosed the exclusive use of an accelerator comprising an organic salt of nickel with mercaptobenzothiazole to increase adhesion properties between rubber and brass-plated metallic reinforcement.