Surface characterization of novel plasma-polymerized primers for rubber-to-metal bonding

Thin (750 Å) plasma‐polymerized films of acetylene were deposited onto polished steel substrates in an inductively coupled r.f. reactor. The films were characterized by x‐ray photoelectron spectroscopy (XPS), Fourier‐transform infrared spectroscopy (FTIR), Auger electron spectroscopy (AES) and secondary ion mass spectrometry (SIMS) immediately after deposition and after aging in the atmosphere. The FTIR spectra of the as‐deposited films were characterized by bands related to mono‐ and disubstituted acetylene groups and by bands related to methyl and methylene groups. During exposure to the atmosphere, bands related to acetylenic groups decreased in intensity while new bands due to carbonyl groups appeared. When XPS spectra were obtained from films that were exposed to the atmosphere, new components assigned to oxidation products were observed in the C 1s spectra that were not observed for as‐deposited films, verifying that oxidation had occurred. Numerous peaks related to aromatic structures were observed in positive SIMS spectra of as‐deposited films. Results obtained from AES showed that the plasma‐polymerized films were continuous and that the oxide on the substrate surface was partially reduced during deposition. Plasma‐polymerized acetylene films were excellent primers for rubber‐to‐steel bonding. Miniature lap joints were prepared by using rubber as an ‘adhesive’ to bond together pairs of polished steel adherends primed with plasma‐polymerized acetylene films. The force required to break the as‐prepared joints was ∼2000 N for a bonded area of 64 mm2 and failure was 100% cohesive in the rubber. Similar results were obtained for joints prepared using polished brass substrates. Because of the complexity of reactions between rubber and the plasma‐polymerized primer, a model ‘rubber’ consisting of a mixture of squalene, zinc oxide, carbon black, sulfur, stearic acid, cobalt naphthenate, N,N‐dicyclohexylbenzothiazole sulfenamide and diaryl‐p‐diphenyleneamine was used to simulate the cross‐linking reaction. The results obtained using XPS, SIMS, AES and FTIR showed that sulfur diffused through the primer to form a layer of sulfide at the primer/substrate interface. Zinc and cobalt sulfides and perthiomercaptides, which formed at the interface between squalene and the plasma‐polymerized acetylene primer, catalyzed the reaction between squalene and the primer. Cross‐links between squalene and the primer were mostly mono‐sulfidic, although some evidence for di‐