A fully coupled diffusion-reaction scheme for moisture sorption–desorption in an anhydride-cured epoxy resin

Abstract

Thermoset materials frequently display non-classical moisture sorption behaviors. In this paper, we investigated this issue from an experimental point of view as well as in terms of modeling the water transport. We used the gravimetric technique to monitor water uptake by epoxy samples, with several thicknesses exposed to different levels of humidity during absorption and desorption tests. Our results revealed that the polymer displays a two-stage behavior with a residual amount of water that is desorbed progressively. We proposed a phenomenological reaction-diffusion scheme to describe this behavior. The model describes water transport as a competition between diffusion and the reaction, during which the local diffusivity and solubility depend on the local advancement of the reaction. We then implemented our model using COMSOL Multiphysics and identified it using a MATLAB-COMSOL optimization tool and the experimental data. We discussed the relation between the hydrophilicity of the product of the reaction and the diffusion behavior. We examined the reaction-induced modification of the water concentration field. It is worth noting that part of the phenomenology can be explained by the presence of hydrolyzable groups.

Thermomechanical and hygroelastic properties of an epoxy system under humid and cold-warm cycling conditions

“…The kinetics presumably obey Fick's law but this is difficult to confirm owing to the lack of data in the early period of exposure for mass uptake values lower than 60% of the equilibrium value. Nevertheless, Fickian behavior was confirmed by the results of gravimetric experiments reported in a previous paper [27] on the same material.…”

“…We previously showed in Ref. [27] that the studied epoxy system displays non-Fickian behavior. The short-term isothermal sorption experiments revealed two stages in the mass uptake [27].…”

“…In the presence of unreacted epoxy or anhydride groups, hydrolysis can modify the diffusion behavior, which can turn from a Fick to a Langmuir process [26,27]. This has no direct effects on the polymer's mechanical behavior because hydrolysis does not affect the crosslink density.…”

“…After the pioneering work of Golike and Lasoski in the early 1960s [31], there was, to our knowledge, no significant work on diffusion-controlled hydrolysis during the last half century. Recently, our group published a study proposing a new formulation for reactionediffusion coupling kinetics and its application to hygrothermal aging of anhydride-epoxy matrices [27].…”

“…As a first approximation, the whole mass uptake, m w , can be divided into two parts (m w ¼ m 1 w þ m 2 w ). The water uptake, m 2 w , during the second stage, which accounts for the hydrolysis reaction and the hydrophylicity increase [27], can be approximated by the following equation:…”