The formation of epoxy/metal interphases: Mechanisms and their role in practical adhesion

When epoxy/diamine systems are applied onto metallic substrates and cured, an interphase, having chemical, physical, and mechanical properties quite different from bulk polymer is created between the substrate and the polymer. The aim of this work is to understand the interphase formation mechanisms and their role in practical adhesion. Mechanisms were deduced from comparison of behaviors when either epoxy and diamine monomers or epoxydiamine monomer mixtures were applied onto aluminum, titanium, and gold-coated surfaces. Using various analytical techniques (DSC, FTIR, FTIR-RAS, ICP, and POM) we will show both a chemical sorption of the diamine monomers and a partial dissolution of the surface oxide and/or hydroxide metallic layer. Then, metallic ions diffuse through the liquid monomer layer and react with amine groups to form an organo-metallic complex by coordination bonding. When the complex concentration is higher than its solubility limit, these complexes may partially precipitate to form needle-sharp crystals. The liquid part of the organo-metallic complex forms, with the epoxy prepolymer, a new amorphous network having a lower glass transition temperature. This new biphase material can also contain complex crystals which act as short fibers, randomly dispersed in the polymer matrix or oriented in the vicinity of the polymer/metal interface, inducing an increase of the Young's modulus and a decrease of the elongation at break. By using a three-point flexure test, we have determined the effect of the interphase formation on the practical adhesion before and after hydrothermal aging. Results obtained point out that the epoxy/metal interphase significantly affects the initial practical adhesion. However, formation of organo-metallic complexes greatly improve practical adhesion after aging. The created complexes act as corrosion inhibitors.