Free/bound water absorption in an epoxy adhesive

The classic Fickian approach for describing the kinetics of liquid diffusion/absorption in solids was supplemented some 25 years ago by the 2-phase model of Carter and Kibler, referred to by them (modestly) as ‘Langmuir-type’. Their theory assumes the existence of ‘mobile’ and ‘bound’ diffusant molecules. In this contribution, we have studied water uptake by an epoxy resin, using gravimetry, and found a good, overall, empirical agreement with the Carter and Kibler description. The ‘mobile’ and ‘bound’ components have also been considered separately. In addition, deuterium NMR analysis has been employed to study the mobility of water absorbed. Analysis and decomposition of spectra permitted us to assign two signals to the fractions of ‘mobile’ and ‘bound’ water. We suggest that the ‘mobile’ phase corresponds to diffusing molecules, whereas the ‘bound’ phase corresponds to ‘clusters’.