Assessing the Impact of the Off-Stoichiometric Epoxy/Amine Ratio on the Curing Kinetics and Properties of Space-Grade Disulfide-Based Vitrimer Resins

Carbon fiber-reinforced polymer composites are widely used in space structures due to their lightweight nature and excellent thermal stability. However, repairing manufacturing defects in cross-linked thermosets remains a significant challenge. In this regard, vitrimers offer a promising alternative by combining the mechanical strength of thermosets with the reprocessability of glass. These materials feature a dynamic network structure, allowing them to behave like traditional thermosets at operational temperatures while exhibiting fluid-like properties at elevated temperatures. In this study, a space-grade epoxy thermoset matrix was transformed into a high-Tg vitrimer (Tg ≈ 200 °C) for potential use in composite applications. The vitrimer chemistry relies on disulfide exchange reactions facilitated by an industrially available reagent (4-AFD). Rheological and reactivity characterization tests were performed to ensure that the cross-linking kinetics in the presence of dynamic bonds remain compatible with existing composite manufacturing processes. These measurements enabled the development of time–temperature transformation diagrams, providing precise insights into the polymerization process. To enhance repairability, off-stoichiometric formulations (epoxy/amine = 1/1.2) were explored to optimize both the curing process and the vitrimeric properties. The applicability of kinetic models to off-stoichiometric systems was also assessed. Notably, even under off-stoichiometric conditions, the resin maintained a high Tg of 175 °C while exhibiting excellent mechanical properties and improved reprocessability. Additionally, the outgassing properties remained identical to those of the stoichiometric reference. These findings highlight the potential of off-stoichiometric epoxy/4-AFD formulations as viable matrix materials for composite applications. By offering improved reprocessability without compromising thermal or mechanical performance, these materials present a compelling alternative to traditional thermosets for high-performance aerospace applications.