Interfacial Study of Steel Joints Prepared with a Catechol-Modified Epoxy Adhesive with Enhanced Bonding Performance and Durability

Bonding is widely used in aircraft and vehicles due to its light weight and simple process, but its strength decreases sharply in hot and humid environments. Anodization treatment, used for enhancing aging performance, is environmentally harmful and unsuitable for steel. In this study, a catechol-modified epoxy adhesive (CMEA) was prepared on a hectogram scale. Comparative analysis with phenol-modified epoxy adhesive (PMEA) and pristine epoxy adhesive (EA) revealed that the underwater bonding of CMEA (13.0 MPa) on stainless steel (SS) significantly outperformed the two control groups. Moreover, after 32 days of hydrothermal aging at 50 °C, CMEA preserved 73.9% of its initial bonding strength, while PMEA and EA retained 59.8 and 11.4%, respectively. Furthermore, X-ray photoelectron spectroscopy (XPS) etching at different times to analyze the interface between adhesives and the SS substrate indicated a marked increase in the O–H/O2– value at the interface between CMEA and the SS substrate compared to the two control groups. The above results demonstrated that the catechol-modified adhesive enhanced the bonding and aging properties of the adhesive, possibly due to the formation of a higher density of hydroxyl groups at the interface between the adhesive and the SS substrate. These findings contribute to the understanding of the enhancement mechanism of catechol in improving the bonding and aging properties of adhesives and suggest a feasible direction for designing adhesives with high bonding strength and high durability.