Thin epoxy layers on native aluminium oxide: Specific ageing processes in mild conditions

Abstract

Post-cured epoxy layers (EP, approx. 100 nm and 1 µm) of three amine hydrogen – oxirane ratios are prepared in dried air on natively oxidised aluminium substrate. A less crosslinked interphase forms as the consequence of adhesive interactions. Even though the layers exhibit very good durability in moist-warm air for up to 100 d, two stages of chemical ageing are revealed. During a short initial stage, some water absorbs firmly in the EP networks. Residual oxirane rings and secondary amine groups are shown to hydrolyse, while network chains remain intact. Marking the 2nd stage, dedicated autoxidation reactions are proven to simultaneously attack tertiary amine crosslinks and α-CH2 groups at phenylene ether units. These reactions destroy network chains, and the release of network fragments was established. Moreover, network destruction proceeds more effectively in 100 nm layers as the surface erodes slightly, even under the given environmentally mild conditions. That observation proves a specifically reduced durability for the interphase at the contact with native aluminium oxide.