Adhesion accelerator

The present invention relates to an adhesion accelerator for enhancing the adhesive strength between rubber and metal. In particular, this invention relates to an adhesion accelerator for enhancing the adhesive strength of rubber to steel cords. In addition, the present invention also relates to a rubber composition containing the above adhesion accelerator.

Heretofore, in order to improve the adhesive strength of natural or synthetic rubber to steel cords used as a reinforcement for a radial tire or a belt of a belt conveyer, there have been used, as an adhesion accelerator, cobalt-containing metal organic compounds such as cobalt stearate, cobalt naphthenate, cobalt-boron complex, etc. Although these cobalt-containing metal organic compounds have a high vulcanization accelerating ability, the presence of cobalt in rubber lowers the storage stability. Besides cobalt-containing organic compounds, some other metal organic compounds have also been investigated. In fact, many patents disclose the use of some specific metal organic compounds as adhesive strength improving agents. For instance, some organic acid cobalt salts are disclosed in the following patents. U.S. Pat. No. 1,919,718 discloses cobalt acetate and cobalt salts of low molecular fatty acids, particularly a cobalt stearate. U.S. Pat. No. 2,912,355 discloses cobalt oleate and cobalt citrate. U.K. patent 1,169,366 discloses cobalt linoleate and cobalt resinate. U.S. Pat. No. 4,076,668 discloses cobalt salts of naphthenic acid, octylic acid, tall oil acid, etc. Japanese Patent Application Laid-Open No. 61-60743 discloses cobalt propionate, calcium propionate, magnesium propionate, nickel propionate, zinc propionate, etc. Japanese Patent Application Laid-Open No. 6-65142 discloses a metallic soap having a structure of naphthalene or biphenyl. Japanese Patent Application Laid-Open No. 60-15444 and Japanese Patent Application Laid-open No. 60-158230 disclose cobalt borate and the like. Japanese Patent Application Laid-Open No. 60-199643 discloses a metal salt of thiosulfuric acid ester. Japanese Patent Application Laid-Open No. 59-207949 and Japanese Patent Application Laid-Open No. 6-329838 disclose a metallic soap having a specific ratio of cobalt to nickel, as an adhesion improving material. Japanese Patent Application Laid-Open No. 6-329840 discloses a metallic soap having a specific ratio of cobalt to zinc, as an adhesion improving material. Although the above substances have a good adhesive property in a specific aspect, they still fail to show a well-balanced adhesive strength as a whole.