Glass transition temperature, nanostructure recovery, and physical aging at the surface of crosslinked epoxy coatings

Thermal-mechanical properties and relaxation behavior at the surface of two thermoset epoxy systems were investigated. Mechanically induced surface damage and photodegradation-induced nano-defects show similar trends with annealing, i.e., a characteristic surface transition temperature (10 to 15 degrees lower than the bulk Tg) between two plateaus. A wet coating has a further reduced surface Tg due to moisture plasticization. The addition of pigment enhances the surface Tg and slows down the rate of surface defect relaxation by reinforcing the structure. The surface mechanical properties of coatings were examined by nanoindentation and nano-DMA techniques. A coating surface exhibits a rubbery equilibrium state at temperature around a surface Tg that may be different from the bulk T g. At temperatures lower than the surface Tg, the surface becomes rigid with the occurrence of physical aging. Water transport in epoxies, determined by exposure history, temperature and pigmentation was studied using a gravimetric method. The experimental data was modeled with Fickian diffusion to obtain diffusion coefficients and their activation energy.;The findings in this study contribute to the understanding of the different performance of crosslinked coatings' surface from the bulk properties, and suggest that surface Tg and surface topography should be included in studying the type and sequence of coatings' failure.