Wet adhesion of epoxy–amine coatings on 2024-T3 aluminum alloy

• Wet adhesion of coatings significantly influenced by molar epoxide-to-amine (E/NH) ratio.
• No clear correlation between the coating adhesion strength and their cross-link density and water-uptake.
• Thermal expansion coefficient (CTE) of substrate and coatings lead to the presence of a tensile internal stress (TIS).
• Water induced compressive residual stress was confirmed by immersed sample adhesion tests.
• Samples possessing excess epoxide functional groups show a higher degree of recovery.

To explore the influence of the molar epoxide-to-amine ratio E/NH on the coating adhesion strength, samples with E/NH ratios ranging from 0.6 to 1.4 were prepared and fully cured. Coatings with an E/NH value of 1.0 appeared to have the highest Tg and cross-link density, and the lowest water absorption ability, which was proved to be independent from solubility or free volume effects. Being verified by the classic “cross-hatch” and the more quantitative pull-off tests, the wet adhesion of coatings was found to be significantly influenced by the epoxide content. Excess epoxide groups highly likely to favor the formation of stronger adhesive bonding at coating–metal interface. No clear correlation could be established between the coating adhesion strength and their cross-link density and water-uptake. In addition, the measurements of the thermal expansion coefficients of substrate and coatings revealed the presence of a tensile internal stress, which is more pronounced for stoichiometric samples. The existence of compressive internal stress induced by water uptake was also confirmed by the adhesion tests on samples which were immersed in water for two days. In these samples, a partial recovery of adhesion strength was found, after the samples were dried for two days, indicating that the compressive internal stress deteriorated the load-bearing properties of adhesive bonds and contributed to debonding. This influence becomes larger as the coating thickness increases as well as the E/NH off-stoichiometry increases. Interestingly, for the samples possessing excess epoxide functional groups, a higher degree of recovery was obtained. This was not the case for coatings with excess amine, confirming that specifically the epoxide groups facilitate the formation of strong bonds against water at the polymer–metal interface.

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