Interactions of a latent curing agent and metal oxides – relation of pH of the varnish and isoelectric point of surfaces

Description

Epoxy coatings are found in several industrial applications such as adhesives. For delayed crosslinking of epoxy varnishes, dicyandiamide (DICY) is often used as latent curing agent. Evidently, the amine-based hardener of the coating formulation is highly reactive and can also interact with metal surfaces. Hence, this study focuses on the relevance of the used substrate for the interaction between the DICY and metallic surfaces. Therefore, six different metal oxides with differing isoelectric point were investigated. In a first experiment, immersion tests in a boiling solution of DICY in pure water were performed. The products of the reaction were investigated with X-ray photoelectron spectroscopy (XPS). It was confirmed that an isoelectric point of the metal oxide below, above or close the ph of the used solution results in a different outcome. While a relatively low isoelectric point caused the amine groups to protonate, a higher value increases the peak intensity of the imine group and less reaction occurred for the remaining case. Furthermore, under certain conditions also nitro groups formed, as it was observed for titanium oxide as a substrate. In a next step, epoxy coated metal sheets were investigated, by the means of cryo ultra-low-angle microtomy (cryo-ULAM), a preparation technique which has proven to be successful for depth profiling of polymeric samples without destroying the chemistry. Differences in the interaction between the more complex model epoxy coatings and the various metal oxide passivation layers were ascertained. Furthermore, an enrichment of the hardener at the exposed metal/polymer interface was deduced for some of the substrate surfaces. Consequently, the results of the ULAM depth analysis were compared with the immersion experiment in a comprehensive manner. The products are similar as the change in the XPS spectra was comparable. Hence, it was confirmed that mostly the hardener reacts with the metal oxide surface.

Period	14 Jun 2024
Event title	European Conference on Applications of Surface and Interface Analysis (ECASIA 2024)
Event type	Conference
Location	Göteborg, SwedenShow on map