Effects of Compound Formulation on the Adhesion of Rubber to Brass-Plated Steel Cord

Implications for cord adhesion from this study are as follows: 1) In the low-adhesion compounds containing low cobalt naphthenate and sulfur levels, adhesion increases with decreasing cobalt naphthenate:sulfur ratio, at a given amount of cobalt naphthenate. 2) Under mild curing condition (160°C, 20 min), high-adhesion compounds, containing high cobalt naphthenate and sulfur level, do not show a decrease in adhesion, even if the cobalt naphthenate:sulfur ratio is increased. However, after overcuring (160°C, 60 min), adhesion loss is encountered with increasing cobalt naphthenate:sulfur ratios, at a given cobalt naphthenate level. 3) A high level of unaged adhesion can be obtained if products with a combination of low cobalt and high sulfur content are formed on the brass surface. This condition obtains if the compound contains large amounts of cobalt salt and sulfur and a small ratio of cobalt salt to sulfur. 4) With a high-adhesion compound, the unaged adhesion is rather insensitive to the amount of accelerator. 5) After humidity aging, the adhesion of a high-adhesion compound varies depending upon the amounts of sulfur and accelerator. Compounds containing small amounts of sulfur show a decrease in adhesion with increasing amounts of the accelerator. However, compounds containing large amounts of sulfur maintain high adhesion, even with large amounts of accelerator. 6) For unaged adhesion, only a slight influence is observed by varying the amount of stearic acid. After humidity aging, however, compounds containing larger amounts of stearic acid show a decrease in adhesion, especially in the presence of large amounts of cobalt naphthenate. 7) Compounds which promote the formation of ZnO on the surface during bonding maintain a high adhesion level after humidity aging. This is similar to the effect of using a cord plated with brass containing a higher zinc level.