Gelation and glass transition in thermosetting process of epoxy resin

The curing process of thermosetting resins like an epoxy resin is characterized by gelation and vitrification (glass transition). We attempted to use time domain reflectometry method for dynamic observation of the curing process. From the result of measurements, a marked peak at a frequency of 50 MHz was observed in the power spectra, and as time passed this peak value increased. Furthermore, as temperature increased, this peak value - curing time curve shifted to earlier time and a maximum value of the curve increased. From the comparison of the curves at each temperature with the times of glass transition, the shift of the curve to earlier time is smaller than the shifts of the time of glass transition. For the present, the origin of the peak at 50 MHz is not known. However, the peak at higher frequency is likely due to the relaxation or vibration of short polymeric chains, and the increase of the peak value means that these short chains increased as the. curing process proceeded. The formations of side chains and polymer networks seems to be the most probable, but the possibility of decomposition of harders still remain.