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Cold-Curing Epoxy Resins: Aging and Environmental Effects. I - Thermal Properties

The effects of physical and chemical aging have been investigated on two cold-curing epoxy adhesives. One of the two has been physically aged at different temperatures below its glass transition (i.e. 10, 23, 30 and 40°C) and then subjected to enthalpy relaxation measurements in a differential scanning calorimeter (DSC). According to recent literature, DSC studies have suggested that enthalpy relaxation gradually increases with aging time to a limiting value, depending on the temperature, until structural equilibrium is reached. Specimens of both adhesives, previously cured at room temperature for 10 days, have been aged in a dry-dark atmosphere or in a freezer (-20°C) for a prolonged time, measuring Tg and enthalpy relaxation at different time spans. When the structural equilibrium is reached, the samples aged in the dark-dry atmosphere exhibit Tg and relaxation peak values similar to those found in the earlier analysis for samples isothermally cured at 23°C. Samples aged at -20°C, the curing time suggested by suppliers, which was not sufficient to cure the adhesives completely, have glass transition temperatures that are always lower. The cured adhesives have also been exposed to natural weather for up to 36 months. Some samples, taken at different periods of time, have been analyzed in DSC. Other samples have been subjected to a de-aging procedure (24 hours at 50°C) before the thermal analysis. The natural exposition, regarded as chemical aging, determines a cycling change of Tg of both adhesives around average values slightly lower than the initial ones. This behavior cannot be erased by the de-aging procedure, which is able to delete only the effects due to physical aging. Enthalpy relaxation peaks, characteristics of physical aging, are observed only for exposed samples not subjected to de-aging procedure. These samples, finally, are subjected during the natural exposition to aging and de-aging processes taking place in non-isothermal conditions and, therefore, not predictable. The presented study must be regarded as a qualitative analysis of the aging phenomena taking place in cold curing epoxy resins.