Metal laminates based on waterborne epoxy varnishes - chemical interactions and effect on fatigue crack growth kinetics

Epoxy coatings cured with the latent curing agent dicyandiamide (DICY) are well-established. During coating of waterborne epoxy varnishes onto the metal substrate, chemical interactions take place, which were evaluated in this paper for different metal substrates. Moreover, their effect on the fatigue crack growth kinetics of bonded laminates was assessed. First, different pure metal sheets were immersed in a boiling solution of DICY in distilled water. Metals with a lower isoelectric point (IEP) (pH < 5) revealed the formation of a higher amount of protonated amine. In contrast, for metals with a higher IEP (pH > 8) the formation of imine was preferred. Furthermore, epoxy coated metal sheets were prepared by cryo ultra-low-angle microtomy (cryo-ULAM) and the reaction products were elucidated by X-ray photoelectron spectroscopy (XPS). Close to the metal/epoxy interface, reaction products were ascertained, which corroborated the results of the immersion test. Finally, the effect of the IEP of the substrate on the crack growth kinetics of bonded laminates was evaluated by cyclic fatigue experiments on double cantilever beam specimens. The higher the IEP of the metal substrate, the better was the crack growth resistance in the threshold and stable crack growth regime. Overall, this study clearly confirmed that chemical interactions are taking place during manufacturing of metal/waterborne epoxy laminates, which have a significant impact on the adherence and crack growth resistance. • Chemical interactions of waterborne DICY with metal substrates were confirmed. • Formation of protonated amine on lower isoelectric point (IEP) metal surfaces. • Imine species was formed on higher electric point metal surfaces. • Metal ion diffusion into epoxy coating for high IEP substrates. • Better fatigue crack growth resistance of high IEP metal/epoxy laminates.