Interaction of Epoxy/Dicyandiamide Adhesives with Metal Substrates

The molecular structure of the interphase formed by curing a model adhesive system consisting of the diglycidyl ether of bisphenol-A (DGEBA) and dicyandiamide (DDA) against mechanically polished aluminum and electrogalvanized steel (EGS) substrates was determined using reflection–absorption infrared spectroscopy (RAIR) and X-ray photoelectron spectroscopy (XPS). RAIR analysis suggested that DGEBA/DDA mixtures created an interphase with a different molecular structure from the bulk of the adhesive when cured in contact with aluminum. The formation of this unique interphase was mainly due to interactions between DDA and the Al surface. XPS analysis indicated that aluminum ions exposed by heating the substrate surface were necessary for this interaction. DDA was found to adsorb onto the aluminum surface via the lone pair of electrons on the nitrogen atoms of the nitrile groups. A slight decrease in the nitrile stretching frequency suggested an additional back-bonding interaction between aluminum ions and the nitrile groups. Slight back donation of electrons from the metal to DDA resulted in a reduction product that led to the formation of the carbodiimide form of DDA. This specific reaction caused a decrease in the concentration of nitrile groups in the interphase and changed the extent of the reaction between DDA and DGEBA by inhibiting the formation of oxazolidine structures. The interaction of DDA with EGS surfaces followed a similar trend. However, the effects were much more pronounced with EGS and the extent of the curing reaction and the cross-linking rate near the metal surface were strongly affected by EGS/DDA interactions.