Degradation Mechanisms of Epoxy Coatings and Their Adhesion to Cementitious Substrates Under Intense Ultraviolet Radiation

To further reveal the degradation mechanism of epoxy coatings under intense ultraviolet radiation in high-altitude environments and clarify its influence on their interfacial adhesion performance, an epoxy coating-cement mortar system was investigated. Ultraviolet (UV) aging tests were conducted, together with attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), surface free energy (SFE) measurements, atomic force microscopy (AFM)-based nano-adhesion force measurements, scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM/EDS), uniaxial tensile tests, and pull-off adhesion strength tests to investigate the evolution of the coating’s characteristic molecular structure, surface polarity, nano-adhesion, coating toughness, macroscopic adhesion performance, and interfacial failure modes at different aging stages. The results showed that the peaks associated with hydroxyl and carbonyl groups in the epoxy coating intensified with increasing UV aging duration. The surface free energy of the coating increased, and its polar component reached 3.9 times the initial value. After 28 d of UV aging, the nano-adhesion force of the coating decreased by 30.1%, its toughness decreased from 2.82 ± 0.052 to 1.21 ± 0.027 MJ·m−3, and the adhesion strength between the epoxy coating and the cementitious substrate decreased by 15.7%. In addition, as aging progressed, the failure path gradually shifted from fracture near the substrate surface toward regions near the coating–cementitious substrate interface and within the coating. Correlation analysis further showed that the decreases in coating toughness and nano-adhesion performance were closely associated with the deterioration of macroscopic adhesion strength. Therefore, greater attention should be paid to the optimization of these two properties in practical applications.