Cold-curing epoxy resins: Aging and environmental effects. II - Mechanical properties.

Thermosetting adhesives, suitable to join different materials in structural applications, are influenced during their service life by various effects which might cause them damage and that are not deeply experienced. They can be, in fact, exposed to atmospheric influences as well as subjected to physical aging as a consequence of their glassy non-equilibrium nature. In the first part of this study, the influences of natural exposure and physical aging, occurring separately or concurrently, on thermal properties of cold-curing epoxy adhesives have been examined. This second part analyzes weathering influences on the mechanical properties of the same adhesives. For this purpose, the different aging processes have been first separated, in order to investigate any single effect, and then their simultaneous influences have been evaluated. The specimens only physically aged exhibit the highest values of static modulus and yield strength, in accordance with current literature. The exposure to natural weathering (i.e. to chemical and physical agings) determines a fluctuation of mechanical properties, whose average values, however, remain close to the initial ones. The effects of chemical aging have been found to be particularly severe on the filled adhesive. A de-aging treatment, able to erase only physical aging and to remove a limited amount of sorbed water, results in an incomplete recovery in modulus and strength. This effect is partially due to the crosslinking reactions taking place in the cold-cured adhesives. It is thought that the results of this qualitative study can be extended to longer periods of natural exposure, even in the absence of any critical external agent, and that the behavior observed for the selected adhesives can be considered comparable to that of other cold-curing epoxy adhesives.