The Modification of Epoxy/Metal Interphases by Adsorbed Contaminants

Abstract

Abstract The interfacial region formed between an epoxy/polyamide adhesive and three metallic substrates has been investigated using x-ray photoelectron spectroscopy. The effect of adsorbed contaminants on the interfacial region has also been studied. The substrates studied were cold-rolled steel, electrogalvanized steel and 2024 aluminum. A “ship-out” oil was used to contaminate the cold-rolled steel and electrogalvanized steel whereas an aerospace hydraulic fluid was used to contaminate the 2024 aluminum. Preferential adsorption of the curing agent by the substrates was observed, the effect being most marked in the case of the electrogalvanized steel substrates. The interaction between the galvanized steel and the 2024 aluminum and the curing agent was primarily via the free amines in the polyamide curing agent. In the case of cold-rolled steel the interaction occurred via the amide component. Cold-rolled steel and 2024 aluminum protonated the curing agents. No protonation was detected in the case of the electrogalvanized steel substrates due to the formation of a zinc/amine coordination compound involving the lone pairs of electrons on the nitrogen atoms. When the contaminated cold-rolled steel substrates were studied, the adhesive displaced the majority of the ship-out oil, but no protonation of the curing agent was observed. This indicated that some residual contaminant remained associated with the cold-rolled steel substrate. The majority of the ship-out oil was not displaced from the electrogalvanized steel because the strong preferential adsorption of the curing agent by the substrates created an adhesive layer enriched in epoxy. As the epoxy resin was incompatible with the oil, this epoxy-rich layer acted as a barrier, preventing the absorption of the oil by the bulk adhesive. In the case of the 2024 aluminum substrate, the level of preferential adsorption of the curing agent was decreased, but protonation of the nitrogen groups was still observed. These results indicated that the hydraulic fluid was displaced from the 2024 aluminum in some areas.