Curing Kinetic and Thermomechanical Behaviour of Co-Anhydride Cured Aminoglycidyl Epoxy Resins

Curing behaviour and thermo-mechanical properties of a technical grade of N,N,N’,N’-tetraglycidyl-4,4’-diamino-diphenylmethane (TGDDM-Araldite MY721), cured in the presence of an anhydride hardener mixture consisting of maleic anhydride (MA) and pyromellitic acid dianhydride (PMDA) was studied by calorimetric and dynamic mechanical analysis. Cure kinetics and the influence of varying stoichiometric anhydride to epoxy ratios were evaluated and the apparent activation energy was calculated according to Barrett’s method. High extents of conversion from DSC studies of the MY721-resin were reached after a room temperature cure, without an added catalyst, for 24 hours followed by a post-curing step of 1 hour at 90°C. Ultimate glass transition temperatures for the molar anhydride/epoxy ratio, r=0.8 were close to the decomposition temperature and indications were obtained that the network structure consists of two independent sub-networks. It is suggested that two separate mechanisms contribute to the curing reaction at room temperature. First the tertiary amine structure, intrinsic to aminoglycidyl-resins, may act as an internal catalyst for the anhydride ring opening, and secondly, the unsaturated bond of MA participates in the curing reaction by nucleophilic attack, such as from tertiary amines or carboxylate or alkoxide anions. From a study of a range of different amino-glycidyl resins, this low-temperature curing behaviour is found to be a general phenomenon.