Effect of Condensed\nWater at an Alumina/Epoxy Resin\nInterface on Curing Reaction

Abstract

The adhesion of epoxy adhesives to aluminum materials\nis an important\nissue in assembling parts for lightweight mobility. Aluminum surfaces\ntypically possess an oxide layer, which readily adsorbs water. In\nthis study, the aggregation states of water and its effect on the\ncuring reaction were examined by placing a water layer between an\namorphous alumina surface and a mixture of epoxy and amine components.\nThis study used molecular dynamics simulations and density functional\ntheory calculations. Before the reaction, water molecules strongly\nadsorbed onto the alumina surface, aggregating excess water. Some\nwater diffused into the epoxy/amine mixture, accelerating the diffusion\nof unreacted substances. This led to faster reaction kinetics, particularly\nin proximity to the alumina surface. The adsorption of water molecules\nonto the alumina surface and the aggregation of excess water were\nsimilarly observed even after the curing process. Subsequently, the\ninteraction between the alumina surface and various functional groups\nof the epoxy/amine mixture was evaluated before and after the reaction.\nEpoxy monomers had little interaction with the alumina surface before\nthe reaction, whereas hydroxy groups formed by the ring-opening reaction\nof epoxy groups exhibited notable interaction. Conversely, sulfonyl\nand amino groups in amine compounds formed hydrogen bonds with OH\ngroups on the alumina surface before the reaction. However, after\nthe reaction, amino groups weakened their interaction with the alumina\nOH groups as they transformed from primary to tertiary during the\ncuring reaction. Both epoxy and amine monomers/fragments similarly\ninteracted with water molecules, both before and after the reaction.\nThe insights gained from this study are expected to contribute to\na better understanding of the impact of moisture absorption on the\napplication of epoxy resins.