Adhesion of Natural Rubber Compounds to Plasma-Polymerized Acetylene Films

Abstract

Plasma-polymerized acetylene films were shown to be novel, highly effective primers for rubber-to-steel bonding. However, the performance of the primers depended strongly on processing variables such as the substrate pretreatment and the carrier gas. Miniature lap joints were prepared by using natural rubber as an “adhesive” to bond together pairs of pretreated steel adherends primed with plasma-polymerized acetylene films which were deposited using various carrier gases. The initial strength of joints prepared from substrates which were mechanically polished and then coated with plasma-polymerized acetylene films deposited using an argon or nitrogen carrier gas 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 from mechanically-polished brass substrates. However, the initial strength of joints prepared from polished substrates which were coated with plasma-polymerized films deposited using oxygen as a carrier gas was lower by a factor of two and there was only 30% rubber coverage on the substrate failure surfaces. demonstrating the importance of the carrier gas.

The initial strength of joints prepared from substrates which were pretreated by alkaline cleaning, acid etching, or mechanical polishing and then coated with plasma polymers using argon as the carrier gas was also approximately 2000 N/64 mm² and failure was again 100% cohesive in the rubber. However, the strength of joints prepared from substrates which were pretreated by ultrasonic cleaning in acetone and then coated with plasma polymers using argon as the carrier gas was lower by a factor of almost two, demonstrating the significance of substrate pretreatment.

During exposure to steam at 121°C, the durability of miniature lap joints prepared from polished steel substrates primed with plasma-polymerized acetylene films using argon as a carrier gas was excellent. After exposure for 3 days, the breaking strength of the joints decreased slightly, from 1740 to 1410 N/64 mm², but the locus of failure remained cohesive in the rubber, implying that effect of steam was mostly to reduce the cohesive strength of the rubber. Similar results were obtained from joints prepared from polished brass substrates. However, the durability of joints prepared from polished brass substrates and from polished steel substrates primed with plasma-polymerized acetylene was poor during exposure to aqueous salt solutions for three days. Although all of the joints decreased significantly in breaking strength, the strength of the joints prepared from brass substrates was about 400 N/64 mm² higher than that of joints prepared from steel primed with plasma-polymers. Most of the joints prepared from steel primed with plasma-polymerized acetylene films failed near the interface between the primer and the steel substrate although some specimens had 20–40% rubber coverage on the failure surfaces.

Key Words:

• Bonding
• durability
• rubber-to-metal
• primers
• plasma-polymerized
• natural rubber
• interphases
• crosslinking
• X-ray photoelectron spectroscopy