Mechanisms of Formation of the Interphase in Epoxy-Amine/Aluminium Joints

Detailed investigations of the interaction of 1,2- diaminoethane with aluminium surfaces have been performed to understand the mechanisms of interphase formation in epoxy-amine/aluminium joints. In particular, it has been shown that both metal bond surface complexes (O-Al … N bonds) and hydrogen-bonded surface complexes (Al-OH … N and CxOyHz … N bonds) can be formed on aluminium surfaces covered with a partly contaminated (hydr)oxide film. However, surface dissolution can only be induced by mononuclear bidentate metal bond surface complexes (chelates), which result from a ligand exchange mechanism between specific hydroxyl sites (η1- and µ2-OH) of the surface and the amino terminations of the molecule. The formation of these chelates weakens the trans Al-O bonds and detachment of the ligand-metal complexes can occur. This mechanism is enhanced by the presence of moisture. In practical epoxy-amine/aluminium joints, the interphase can, thus, be formed by migration of these complexes in the liquid polymeric phase before curing is achieved.