Dual-Curable Epoxy-Amine Thermosets: Influence of Stoichiometry and Ratio between Hardeners on Thermal and Thermomechanical Properties

Sequential dual-curable thermosets based on off-stoichiometric and stoichiometric amine-epoxy formulations have been developed, employing different reactivities of two curing agents in terms of temperature. The first curing step is triggered by a self-limiting polycondensation between epoxy and amine groups of a reactive hardener m-xylylenediamine (MXDA) at lower temperatures, and the second one by a reaction of remaining unreacted epoxy groups with amine groups of a latent curing agent dicyanodiamine (DICY) at higher temperatures, respectively. Differential scanning calorimetry (DSC) of reactive dual-curing systems reveals two distinct curing peaks. A reduction of their maxima was achieved by adding AJICURE MY-24 as an accelerator to lower the curing temperature. DSC, rheology, and dynamic mechanical analysis (DMA) demonstrated that a wide range of properties, such as gel time of reactive samples, glass transition, and network density of intermediate and final materials, can be tuned by the proper choice of the ratio between both hardeners. The gel time increases, up to 9 h at room temperature, with increasing DICY fraction. The intermediate materials vary from highly viscous through flexible to rigid with increasing MXDA fraction. However, it was also observed that the stoichiometry of the whole formulations does not influence the shape and glass transition of partially cured samples. Furthermore, the glass transitions of final stoichiometric systems are much higher than those of off-stoichiometric ones. The glass transition temperature of stoichiometric systems, determined by DMA, varied from 91.3 to 100.2 C-degrees. All in all, this study presents a strategy to create stoichiometric dual-curing epoxy resin systems, which opens further possibilities to combine other reactive and latent curing agents in order to obtain the desired properties of partially and fully cured materials. [GRAPHICS]