Impact of polar groups concentration and free volume on water sorption in model epoxy free films and coatings

The water sorption characteristics have been determined for two model epoxy systems based on the DGEBA resin and two amine hardeners (TETA and DAMP). A particular attention was paid to the curing protocol to control the macromolecular architectures. Using FTIR, DSC and DMTA measurements, it was demonstrated that both network microstructures differ mainly in the polar groups density, the potential crosslinking nodes and the free volume. DGEBA/DAMP system presents a higher free volume fraction and a lower polar groups concentration compared to the DGEBA/TETA system. Gravimetric measurements on free films and EIS experiments onto coated steel Q-panels were carried out to follow the water sorption kinetics at different temperatures (30, 40, 50 and 60 degrees C). For both epoxy systems, the diffusion processes follow a pseudo-fickian behaviour with two diffusive stages. Solubility and diffusion processes are discussed in relation with the microstructural specificities of the polymer networks using a thermodynamic framework. The main result is that in DGEBA/DAMP and DGEBA/TETA systems, the water diffusion and solubility are mainly governed by a water polar group interaction mechanism. (C) 2016 Elsevier B.V. All rights reserved.