Adhesion of Natural Rubber to Steel Substrates: The Use of Plasma Polymerized Primers

Plasma-polymerized acetylene films are excellent primers for bonding natural rubber (NR) to steel substrates. The purpose of this research was to determine the mechanisms responsible for adhesion at the NR/primer interface. Interactions between natural rubber and plasma-polymerized acetylene films were simulated using model systems containing squalene (C30H50) or squalane (C30H62), carbon black, sulfur, stearic acid, N, N-dicyclohexyl-benzothiazole-sulfenamide (DCBS), cobalt naphthenate, and diaryl-p-phenylene-diamine. Analysis of plasma-polymerized acetylene films before and after reaction with model rubber systems was accomplished using Fourier transform infrared spectroscopy (FTIR). The importance of the unsaturation in the reaction was demonstrated by comparing the behavior of the two model systems. In the squalane-based system, squalane itself was stable throughout the reaction, with only a slight reaction with the antioxidant and sulfur. No significant reactions occurred between the squalane-based model system and plasma-polymerized 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 system were complex. Squalene itself went through double-bond migration. ZnO and cobalt naphthenate reacted with stearic acid to form zinc and cobalt stearates which then reacted with DCBS and sulfur to form zinc and cobalt accelerator perthiomercaptide complexes. These complexes reacted with squalene and with the primer film to form accelerator terminated, perthiomercaptide pendant groups and, eventually, short polysulfide crosslinks. Since there was little reaction of squalane-based model rubber compounds with plasma polymerized 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 adhesion at the NR/primer interface in an actual bond.