The Use of Organic Coupling Agents to Improve the Durability of Steel/epoxy Joints in the Presence of Moisture.

Despite the unique combination of properties epoxy resins possess, their versatility for structural bonding of metal substrates has not been fully utilized. The use of epoxy resins (and in general all adhesives) has been limited to non-load bearing or low load bearing applications. One of the primary reasons is the bond sensitivity of these resins to water or to high humidity. Recognizing this limitation, this study was undertaken with the goal of developing organic coupling agents which will chemically bond to the surface of steel via a chelate bond. B-diketone coupling agents were synthesized and tested. The coupling agents were applied from organic solution to the surface of a modified napkin ring torsion joint. The joints were bonded with epoxy resin adhesives and water immersed. The bond durability of the adhesive system was determined by subjecting the joints to a shear load. An ammonium citrate pretreatment was developed for use with the B-diketone agents. Optimization studies of the citrate pretreatment and coupling agent concentration were undertaken. The B-diketone coupling agents significantly improved the bond durability of low temperature cured epoxy systems. Polyfunctional mercaptoester coupling agents were also synthesized and evaluated. The effect of functionality, ring size, citrate pretreatment, and atmosphere on bond durability were examined. The mercaptoesters were found to be far superior to the B-diketones in improving bond durability. Low temperature and high temperature cured resins were examined. XPS analysis of 1018 steel treated with the mercaptoesters indicates the formation of polymeric coupling agent layers, approximately 100 (ANGSTROM) thick, on the steel surface. The results also indicate chelate bonding to the steel surface. A two step strength decay curve is observed for the mercaptoester treated systems, irrespective of the mercaptoester or the curing agent used. Plasticization of the mercaptoester layer at the early stages of strength loss is proposed as an explanation of the linear loss of shear strength of the torsion joints with immersion time. Substrate corrosion is thought to be a controlling factor of strength loss at extended immersion times.