The critical humidity effect in the adhesion of epoxy to glass: role of hydrogen bonding

Abstract

Adhesion between an epoxy [diglycidyl ether of bisphenol F (DGEBF) cured with diethylene triamine] and glass was lowered abruptly when the epoxy was equilibrated with air whose relative humidity (RH) exceeded a critical value of approximately 70% RH. The critical humidity marking the onset of adhesion loss was also associated with a sudden increase of water uptake by the epoxy. In earlier work, it was shown that this 'transition' was not due to capillary condensation, osmotic cell formation or a decrease in the T g of the material. Instead, it was speculated that the critical humidity effect was due to the trapping of water by hydroxyl groups which become available as inter-chain hydrogen-bonded structures are broken. To verify the above hypothesis, two model compounds were synthesized. One closely mimicked the cured DGEBF resin and the other had all of its hydroxyl groups replaced by hydrogen. Comparison of the water sorption isotherms of these two model compounds clearly suggested that hydroxyl groups played a key role in the critical humidity effect. Using molecular simulation software, hydrogen bonding between the various polar sites of the hydroxylated model compound was also studied. In the dry state or at low water concentrations, simulations predicted the formation of hydrogen bonds between polar sites. These hydrogen bonds always involved one or more hydroxyl groups. At higher water concentrations, molecular simulations showed that water tended to displace the hydrogen bond network of the epoxy, and in the process, water-mediated 'bridges' between polar groups were formed. The large decrease in entropy associated with the formation of such macrocyclic conformers is thought to be offset by the decreased enthalpy of condensation of water made possible by multiple hydrogen bonding. This suggests that the critical humidity effect might be an 'order-disorder' transition associated with the formation of ring structures closed by hydrogen-bonded water linkages between polar groups. The first-order energetics of this type of transition is consistent with the abrupt nature of the critical humidity effect.