Surface morphology and chemistry of epoxy-based coatings after exposure to ultraviolet radiation

The effects of ultraviolet (UV) radiation on the surface morphology and the surface chemistry of an epoxy-resin-based coating were characterized with both scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The techniques were applied to follow the chemical changes that occur after different UV-exposure periods. SEM images indicated the generation of “silver cracks” on the surface of the coating as a result of UV-exposure. The shape and width of the cracks were highly related to the UV-exposure period. XPS revealed that the main carbon peak had an unsymmetrical shape due to the large variation in the type of the carbon bonds in the chemical structure of the epoxy resin. Evidence of an oxidation degradation process was observed on the coating surface. The longer the exposure of the coating to UV radiation, the more oxygen bonds were formed, causing the oxygen content to increase and the carbon concentration to decrease. XPS-peak fittings for four carbon bonds were conducted. Results indicated the cleavage of aliphatic carbon–hydrogen (CH) and hydroxyl (COH) bonds side by side with the formation of carbonyl (CO) and carboxyl (COO) bonds. These changes that occurred on the surface of the epoxy-based coating were stopped after a specific UV-exposure period of time.