INTERFACIAL CHEMISTRY AND ADHESION - THE ROLE OF SURFACE-ANALYSIS IN THE DESIGN OF STRONG STABLE INTERFACES FOR IMPROVED ADHESION AND DURABILITY

Durability is a key issue in widening the application of adhesive bonding technology. This paper describes a fundamental study of the adhesion loss in epoxy-steel bonds exposed to water, and the development of steel pretreatments, which significantly enhance durability. XPS studies have revealed two weak links in epoxy/steel bonds, both of which must be improved for optimum performance. They are displacement of the adhesive from the steel and breakdown of the oxide on the metal surface. The former has been addressed by applying monomolecular layers of an aminosilane adhesion promoter to the steel surface. Oxide instability has been remedied by use of a novel and simple treatment that modifies the oxide chemistry to a depth of 2-5 nm. Combining these, results in a joint that loses no strength after 2000 h of water immersion at 50-degrees-C, whereas untreated joints lose 68% of their strength after only 400 h. The study shows how careful identification of failure modes, combined with appropriate modification of the substrate surface, can be used to design strong stable interfaces. These modifications, which affect only a few molecular layers, have a large impact on both bond strength and durability.