Cure comprises a complex set of chemical reactions, usually heat activated, which gradually elongate and crosslink the original pre-polymer molecules. This process is accompanied by a gradual and then sudden rise in the viscosity of the resin. The time at which this sudden viscosity rise is noted coincides with gelation of the resin and indicates the formation of a 3D molecular network. Further reactions tighten up this network, increasing its stiffness up to a point where no more reactions can take place (at a given temperature). The network is then said to have vitrified. The regions of gelation and vitrification in the resin are of great practical significance in the processing of fibre reinforced composites in which the thermosetting resin is used as the matrix separating and supporting the fibres.
Many techniques can be considered for monitoring these changes in the resin state, such as DSC, DMA, ultrasound, NMR or mechanical impedance. However, at present, only two types of microsensors are used reasonably widely in both academic research and industrial R&D to monitor the state of cure in real time : dielectric sensors and optical fibre sensors. Both types of sensors can be embedded in selected locations of a composite prior to cure.
Published courtesy of Dr Ivana Partridge, Cranfield University