Controlled composite processing based on off‐stoichiometric thiol‐epoxy dual‐curing systems with sequential heat release (SHR)

Control of curing rate and exothermicity during processing of thermosetting composite materials is essential in order to minimize the formation of internal stresses leading to mechanical and dimensional defects in the samples, especially in thick composite samples. It was recently proposed that sequential heat release, an approach based on the kinetic control of the curing sequence of dual‐curing thermosets, would enable a step‐wise release of the reaction heat and therefore a better control of conversion and temperature profiles during the crosslinking stage. In this article, it is shown experimental proof of this concept obtained by means of an instrumented mold that can be used for the processing of small samples with and without carbon fiber reinforcement. Safe processing scenarios have been defined by numerical simulation using a simplified two‐dimensional heat transfer model and validated experimentally. Off‐stoichiometric thiol‐epoxy dual‐curing systems make it possible to process composites in well‐defined curing stages. A temperature overshoot in the first stage is observed, corresponding to the activation of the first reaction (thiol‐epoxy addition), leading to a stable and uniform intermediate material, ungelled or barely crosslinked. The second reaction (epoxy homopolymerization) is activated in a second stage upon heating, producing a crosslinking process with little or no temperature and conversion gradients, therefore enhancing the quality of processed parts.