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

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 phy sical aging as a consequence of their glassy non-equilibrium nature. In the first part of this study, the influences of natural exposure and phy sical 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. F or this purpose, the different aging processes have been first separated, i n order to investigate any single effect, and then their simultaneous influ ences have been evaluated. The specimens only physically aged exhibit the highest values of static mod ulus and yield strength, in accordance with current literature. The exposur e to natural weathering (i.e. to chemical and physical agings) determines a fluctuation of mechanical properties, whose average values, however, remai n 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, abl e to erase only physical aging and to remove a limited amount of sorbed wat er, results in an incomplete recovery in modulus and strength. This effect is partially due to the crosslinking reactions taking place in the cold-cur ed 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 ex ternal agent, and that the behavior observed for the selected adhesives can be considered comparable to that of other cold-curing epoxy adhesives.