The effect of water sorption on the calorimetric signature of aging in DGEBA epoxies

Abstract

Epoxies hygrothermally aged in the relatively benign environment of a climate controlled experimental laboratory display a broad endothermic signature which extends from as much as 40 °C below the glass transition temperature, Tg, to over 100 °C above Tg upon an initial DSC heating ramp of an open pan. This is in addition to a shift in Tg to lower temperature associated with plasticization and the relatively sharp endothermic peak just above Tg that is characteristic of physical aging. All of these features are erased by the first heating to well above Tg and are not seen in subsequent heating ramps. The association of the broad endothermic peak with desorption of water from the polymer is demonstrated through systematic investigations of hydrothermally aged samples. The area of the desorption peak is proportional to the absorbed mass of water with a cumulative specific heat equal to the heat of vaporization of water (40 ± 4 kJ/mol). For typical equilibrium water saturation in epoxy (∼ 0.04 g water/g epoxy at room temperature and 100% relative humidity), this amounts to ∼80 J/g epoxy and is many times the enthalpy of the physical aging peak in the laboratory aged samples (∼2 J/g). Consequently, moisture desorption is a major contributor to calorimetric thermograms of aged epoxies, and disentangling desorption endotherms from other aging signatures is key to quantifying the role of each mechanism at play. Similarly, plasticization associated with moisture sorption must be considered when aging environments are not dry in order to avoid mis-associating observed Tg depressions with other mechanisms such as chain scission.