Adhesion of natural rubber to steel substrates: The use of plasma polymerized primers

Plasma-polymerized acetylene films are excellent primers for bending natura l rubber (NR) to steel substrates. The purpose of this research was to dete rmine the mechanisms responsible for adhesion at the NR/primer interface. I nteractions between natural rubber and plasma-polymerized acetylene films w ere simulated using model systems containing squalene (C30H50) or squalane (C30H62), carbon black, sulfur, stearic acid, N, N-dicyclohexyl-benzothiazo le-sulfenamide (DCBS), cobalt naphthenate, and diaryl-p-phenylene-diamine. Analysis of plasma-polymerized acetylene films before and after reaction wi th model rubber systems was accomplished using Fourier transform infrared s pectroscopy (FTIR). The importance of the unsaturation in the reaction was demonstrated by comparing the behavior of the two model systems. In the squ alane-based system, squalane itself was stable throughout the reaction, wit h only a slight reaction with the antioxidant and sulfur. No significant re actions occurred between the squalane-based model system and plasma-polymer ized acetylene films with the exception of slight adsorption of antioxidant and zinc and/or cobalt stearate onto the films. Reactions between plasma polymerized primers and the squalene-based model s ystem were complex. Squalene itself went through double-bond migration. ZnO and cobalt naphthenate reacted with stearic acid to form zinc and cobalt s tearates which then reacted with DCBS and sulfur to form zinc and cobalt ac celerator perthiomercaptide complexes. These complexes reacted with squalen e and with the primer film to form accelerator terminated, perthiomercaprid e pendant groups and, eventually, short polysulfide crosslinks. Since there was little reaction of squalane-based model rubber compounds with plasma p olymerized primers but extensive reaction of squalene-based models, it was concluded that an intermediate formed in that reaction was responsible for crosslinking between squalene and the primer in the model system and for ad hesion at the NR/primer interface in an actual bond.