Incorporating the Heating Stage into Processability Maps for Epoxy Curing Optimization

Real curing cycles combine heating ramps and isothermal holds, producing trajectories that conventional charts fail to capture. Isothermal time-temperature-transformation charts (TTT) neglect dynamic heating, while continuous-heating-transformation diagrams (CHT) oversimplify by omitting isothermal stages, leading to significant deviations from experimental observations. In this paper, we develop a comprehensive time-temperature-transformation diagram that considers an initial heating ramp and subsequent isothermal stage (TTT-β) to assess the processability of the epoxy resins. This chart features the curing, degradation, vitrification, and gelation kinetics evaluated by isoconversional models, thus they allow determining the optimal time and temperature processing. The reliability of these charts has been validated experimentally. Measurements of the curing degree in a typical epoxy-amine resin indicate that, under heating rates, comparable to those used in industrial processes such as autoclave curing, a non-negligible degree of curing is achieved during the heating ramp, before reaching the isothermal stage. Thus, rendering the information from these diagrams unreliable and highlighting the critical importance of the heating phase. We further examined the effects of thermal degradation and vitrification on the interlaminar shear strength (ILSS), compressive strength (σ_c), and the compressive modulus (E _c) of the laminate. Our analysis confirms that degradation significantly affects the composite's mechanical properties and that the proposed charts can support the definition of processing conditions for safe curing.