The long-term failure of seemingly intact corrosion resistant organic coatings is thought to occur via the development of ionic transport channels, which spontaneously evolve from hydrophilic regions on immersion, i.e., as a result of localized water uptake. To this end, we investigate water uptake characteristics for industrial epoxy-phenolic can coatings after immersion in deionized water and drying. Moisture sorption and the changing nature of polymer-water interactions are assessed using FTIR for dry and pre-soaked films. More water is found to be absorbed by the pre-soaked coatings on exposure to a humid environment, with a greater degree of hydrogen-bonding between the polymer and water. Furthermore, morphological changes are then correlated to localized water uptake using the AFM-IR technique. Nanoscale softened regions develop on soaking, and these are found to absorb a greater proportion of water from a humid environment.

a b s t r a c tThermoset coatings commonly rely on high cross-linking density to provide enhanced barrier properties. Hence it is surprising that for the industrial epoxy-phenolic network investigated, equilibrium moisture uptake is found to increase with respect to cure time, i.e., with greater cross-linking. Molecular interactions between absorbed water and the resin are characterised using infrared spectroscopy, and water uptake is correlated to network polymer features such as polarity and free volume.

Water sorption in epoxy networks is associated with deleterious physical effects such as swelling, hydrolysis, lowering of the Tg, cracking and crazing.Nonetheless, water uptake in epoxy coatings is poorly understood in relation to macromolecular structure. In this contribution, we study the effect of cure time (closely related to cross-linking density and free volume) on water uptake for a model epoxy-phenolic coating. Localised water uptake is then mapped with nanoscale lateral resolution using AFM-IR, and correlated to cross-linking density.