SURFACE CHARACTERIZATION OF NOVEL PLASMA-POLYMERIZED PRIMERS FOR RUBBER-TO-METAL BONDING

Thin (750 Angstrom) plasma-polymerized films of acetylene were deposit ed onto polished steel substrates in an inductively coupled r.f, react or. The films were characterized by x-ray photoelectron spectroscopy ( XPS), Fourier-transform infrared spectroscopy (FTIR), Auger electron s pectroscopy (AES) and secondary ion mass spectrometry (SIMS) immediate ly after deposition and after aging in the atmosphere. The FTIR spectr a of the as-deposited films were characterized by bands related to mon o- and disubstituted acetylene groups and by bands related to methyl a nd methylene groups. During exposure to the atmosphere, bands related to acetylenic groups decreased in intensity while new bands due to car bonyl groups appeared. When XPS spectra were obtained from films that were exposed to the atmosphere, new components assigned to oxidation p roducts were observed in the C 1s spectra that were not observed for a s-deposited films, verifying that oxidation had occurred Numerous peak s related to aromatic structures were observed in positive SIMS spectr a of as-deposited films. Results obtained from AES showed that the pla sma-polymerized films were continuous and that the oxide on the substr ate surface was partially reduced during deposition. Plasma-polymerize d 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-po lymerized acetylene films. The force required to break the as-prepared joints was similar to 2000 N for a bonded area of 64 mm(2) and failur e was 100% cohesive in the rubber. Similar results were obtained for j oints prepared using polished brass substrates. Because of the complex ity of reactions between rubber and the plasma-polymerized primer, a m odel 'rubber' consisting of a mixture of squalene, zinc oxide, carbon black, sulfur, stearic acid, cobalt naphthenate, N,N-dicyclohexylbenzo thiazole sulfenamide and diaryl-p-diphenyleneamine was used to simulat e the cross-linking reaction. The results obtained using XPS, SIMS, AE S and FTIR showed that sulfur diffused through the primer to form a la yer of sulfide at the primer/substrate interface. Zinc and cobalt sulf ides and perthiomercaptides, which formed at the interface between squ alene and the plasma-polymerized acetylene primer, catalyzed the react ion between squalene and the primer. Cross-links between squalene and the primer were mostly mono-sulfidic, although some evidence for di- a nd trisulfidic cross-links was observed.