Water diffusion with temperature enabling predictions for sorption and transport behavior in thermoset materials

Abstract

Water desorption kinetics for a number of epoxy materials (five amine and one anhydride cured), with glass transitions from below room temperature to 245 °C, were studied using thermogravimetry and near-infrared spectroscopy between ambient and 200 °C. Computational models based on 1D finite difference or 3D finite element Fickian diffusion processes were then applied to determine the underlying diffusivity. Classical Fickian diffusion with a constant D was sufficient to describe the water desorption behavior for all evaluated materials and conditions, with water induced swelling affecting the diffusion process no more than through small dimensional changes. The temperature dependence for diffusivity displayed mostly a single activation energy (Ea) between 44 and 62 kJ mol−1 for all materials except one, which showed a change from 75 kJ mol−1 below Tg (∼65 °C), to 53 kJ mol−1 above. The obtained diffusivities and their Ea were then used to predict and illustrate the sorption behavior for other geometries and temperatures. The trends for these materials were discussed within the existing theories for fractional free volume, effective crosslink density and chemical composition. Unexpectedly, this study shows that Tg has no or only limited influence on water diffusivity in contrast with the Cohen-Turnbull model. Water sorption (solubility in the material) is of similar magnitude independent of temperature (i.e. low Ea) as anticipated. Only a fraction of the water uptake was found to be associated with free volume utilization, with the majority of water resulting in material volume changes, which as a consequence means that dynamic water uptake and drying will result in changing stress conditions (expansion and contraction) for glassy thermosets even under ambient conditions.