Dielectric monitoring of gelation and cure for fast-curing epoxy resins

Fast curing epoxy resins processed in liquid composite moulding (LCM) enable to shorten the cycle time by accelerating the curing process and therefore provide a high potential for cost-efficient high production volumes in the automotive industry. To meet the target of reducing the in-mould time, a precise monitoring of the curing reaction and transition phenomena such as gelation or vitrification is required. In the present study, simultaneous rheometer measurements and dielectric analysis (DEA) are performed to provide a correlation between rheological and dielectric data to determine gelation time. Subsequently, the transferability of these criteria to in-situ measurements and process-specific pressures up to 10 MPa is investigated. To model the reaction kinetics two different approaches are selected to predict the degree of cure reached at gelation. A simple autocatalytic and a diffusion-controlled approach are selected to overcome premature vitrification before the maximum degree of cure is reached. Model parameters are fitted based on isothermal DEA measurement, as this case is particularly relevant, since deviations can result from cross-linking not included at the beginning of the isothermal measurement. The results show the potential for in-line measurement of process-specific data during the crosslinking of fast-curing epoxy resins and represent an essential prerequisite for DEA based quality control and quality assurance in industrial scale LCM processes.