Dielectric studies of water absorption and desorption in epoxy resins: influences of cure process on behaviour

Little is known about the way in which the chemical structure of an epoxy resin influences its ability to absorb and desorb moisture. This issue is addressed in a study of dicyandiamide- and amine-cured epoxy resins. The dicyandiamide-cured material will have a significantly lower preponderance of pendant hydroxyl groups than the amine-cured material and may exhibit different behaviour when exposed to moisture. The uptake and loss of moisture was monitored gravimetrically, using broad band dielectric, dynamic mechanical thermal analysis and thickness measurements performed as a function of time at various temperatures. A comparison of the uptake and loss profiles for the first and subsequent cycles indicated significant differences in behaviour attributed to the way in which water can plasticise the matrix. Stresses frozen into the matrix during the cure process are allowed to relax as a consequence of the water hydrating the matrix and create voids and also allow matrix densification. These processes occur during the first hydration cycle and are not reversible. Subsequent hydration and dehydration appear to be reversible after the first hydration cycle. Water in the polymer is distributed between free water which is to be found in microvoids and bound water which is attached to the polymer chain. The amine-cured epoxy resin which contains pendant hydroxyl groups has a greater capacity for water absorption than the ether-containing backbone of the dicyandiamide-cured material