Monitoring the epoxy curing by the dielectric thermal analysis method

Evaluation of epoxy cure in magnet impregnation is an important task because the final mechanical and electrical characteristics depend upon how the curing process was done. Usually the evaluation of curing scheme relies on thermal analysis methods, mainly the differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). In spite of their sensitivity and reliability, these methods have difficulties to evaluate fast curing of thermoset polymers. They also do not allow the in-situ monitoring of the cure of epoxy during impregnation of superconducting devices. The dielectric thermal analysis (DETA) is an analytical method commonly used to monitor in-situ the fabrication of polymer-matrix composite materials by the resin transfer molding (RTM) process in which the rheological and calorimetric parameters are continuously evaluated during the curing stage and the response signal is used to control the process. In this work we present the evaluation of dielectric characteristics during the cure of epoxy resins employed in the impregnation of electric and superconducting devices. The dielectric analysis was performed imposing a 2 V amplitude signal with fixed frequency of 120 Hz on a sample connected as a capacitor in a RC equivalent circuit. The epoxy is inserted as a dielectric medium within a parallel plate capacitor. The output signal is measured by a lock-in amplifier and the complex impedance is evaluated against the time and temperature of epoxy cure. The results show a very consistent correlation between the sample impedance signal and the reaction extent as well with viscosity versus temperature evolution in the pre-gelation stage of cure.