Degradation of epoxy coatings in humid environments: the critical relative humidity for adhesion loss

An abrupt loss of adhesion at interfaces between epoxy and inorganic substrates has been shown to occur whenever the epoxy is equilibrated in air whose relative humidity exceeds a critical value. We report that the critical relative humidity marking the onset of adhesion loss is associated with (1) a sudden increase of water solubility in the epoxy, (2) a corresponding increase in the volume of the epoxy, and (3) a strong decrease in the mobility of absorbed water. Possible mechanisms such as capillary condensation, osmotic cell formation, and a decrease in glass transition temperature induced by water are ruled out on the basis of available data. Instead, it is proposed that the observed behavior is linked to water 'condensation' into traps or clusters in the polymer above the critical humidity. The solubility behavior of a liquid diol that models the diol-terminated epoxy chain tails suggests that the excess sorption above the critical humidity involves trapping of the water by hydroxyl groups which become available as inter-chain hydrogen-bonded structures are broken above the critical relative humidity. Pronounced irreversibility of solubility and swelling is due to a reaction of the absorbed water with residual oxirane groups, leading to the formation of diols. This explains the upward shift of the solubility and swelling curves toward asymptotic behavior upon relative humidity cycling. We also show that osmotic cells that form around artificially introduced water-soluble impurities can cause a loss of adhesion at a predictable critical relative humidity.