A complete time–temperature–transformation diagram of the RTM6 epoxy resin used for aerospace applications: integrating cure kinetics, thermal degradation and curing-induced overheating

Abstract

Abstract RTM6 is a monocomponent epoxy resin widely used in the aerospace industry, where it is processed by resin transfer molding and related liquid-composite-molding routes. Its time–temperature–transformation (TTT) diagram maps, as a function of cure temperature and time, the boundaries between the different physical states of the material, and is, therefore, a key tool for the design of the cure cycle. The cure kinetics of RTM6, including the slowdown caused by diffusion as the resin approaches vitrification, was characterized in a previous work by isoconversional methods, which provided the vitrification boundary of the diagram. The present work completes the diagram by adding the two boundaries that close the processing window at high temperature: thermal degradation, whose kinetics is determined here from thermogravimetric measurements analyzed with isoconversional methods, and curing overheating, evaluated with a recently developed analytical model that predicts the maximum temperature excess within a curing laminate and the critical thickness for thermal runaway. Degradation sets the upper temperature limit of the usable range, whereas overheating becomes a critical constraint when curing thick parts, in which the heat released by the exothermic cure reaction cannot be efficiently dissipated. The resulting complete TTT diagram defines the full processing window of RTM6 and provides a practical tool for selecting safe cure temperatures and part thicknesses.