ADHESION OF NATURAL-RUBBER COMPOUNDS TO PLASMA-POLYMERIZED ACETYLENE FILMS

Plasma-polymerized acetylene films were shown to be novel, highly effe ctive primers for rubber-to-steel bonding. However, the performance of the primers depended strongly on processing variables such as the sub strate pretreatment and the carrier gas. Miniature lap joints were pre pared by using natural rubber as an ''adhesive'' to bond together pair s of pretreated steel adherends primed with plasma-polymerized acetyle ne films which were deposited using various carrier gases. The initial strength of joints prepared from substrates which were mechanically p olished and then coated with plasma-polymerized acetylene films deposi ted using an argon or nitrogen carrier gas was 2000 N for a bonded are a of 64 mm(2) and failure was 100% cohesive in the rubber. Similar res ults were obtained for joints prepared from mechanically-polished bras s substrates. However, the initial strength of joints prepared from po lished substrates which were coated with plasma-polymerized films depo sited using oxygen as a carrier gas was lower by a factor of two and t here was only 30% rubber coverage on the substrate failure surfaces, d emonstrating the importance of the carrier gas. The initial strength o f joints prepared from substrates which were pretreated by alkaline cl eaning, acid etching, or mechanical polishing and then coated with pla sma polymers using argon as the carrier gas was also approximately 200 0 N/64 mm(2) and failure was again 100% cohesive in the rubber. Howeve r, the strength of joints prepared from substrates which were pretreat ed by ultrasonic cleaning in acetone and then coated with plasma polym ers using argon as the carrier gas was lower by a factor of almost two , demonstrating the significance of substrate pretreatment. During exp osure to steam at 121 degrees C, the durability of miniature lap joint s prepared from polished steel substrates primed with plasma-polymeriz ed acetylene films using argon as a carrier gas was excellent. After e xposure for 3 days, the breaking strength of the joints decreased slig htly, from 1740 to 1410 N/64 mm(2). but the locus of failure remained cohesive in the rubber, implying that effect of steam was mostly to re duce the cohesive strength of the rubber. Similar results were obtaine d from joints prepared from polished brass substrates. However. the du rability of joints prepared from polished brass substrates and from po lished steel substrates primed with plasma-polymerized acetylene was p oor during exposure to aqueous salt solutions for three days. Although all of the joints decreased significantly in breaking strength, the s trength of the joints prepared from brass substrates was about 400 N/6 4 mm(2) higher than that of joints prepared from steel primed with pla sma-polymers. Most of the joints prepared from steel primed with plasm a-polymerized acetylene films failed near the interface between the pr imer and the steel substrate although some specimens had 20-40% rubber coverage on the failure surfaces.