Investigation of Steel/Epoxy Adhesion Durability Using Polymeric Coupling Agents. II. Factors Affecting Adhesion Durability

Ethylene mercaptoester (EME) copolymers containing 23-90 wt% mercaptoester units were employed separately as coupling agents in steel/epoxy peel adhesion systems. As coupling agent functionality is increased the initial adhesion strength of, the steel/EME/epoxy peel systems also increased, approaching a ten-fold improvement over controls for the EME 90 coupling agent. However, the degree of corrosion protection decreased significantly with increasing concentration of mercaptoester units. The best corrosion protection was obtained with the EME 23/epoxy resin system which protected the steel adherend for an average of 48 hours in 57°C water baths. Regardless of the coupling agent used, all of the peel specimens exhibited poor adhesion after 5-11 hours exposure to hot water. Information from hydrolysis stability, water absorption, locus of failure and internal stress analyses when pooled show that the observed adhesion loss is attributable to the effect of water on the epoxy resin used; failure occurred within the epoxy.