Reprocessable, malleable and intrinsically fire-safe epoxy resin with catalyst-free mixed carboxylate/phosphonate transesterification

Fire hazard and waste crisis are two challenges for thermosets, particularly for epoxy resins (EPs) possessing the largest market share therein; yet it is beset with difficulties to solve the challenges simultaneously. Herein, a commercialized phosphorus-containing diol named diethyl bis(2-hydroxyethyl)aminomethylphosphonate (DAP) with “trinity” structures containing phosphonate ester, primary hydroxyls and tertiary amine was applied in an epoxy-anhydride curing system to achieve the catalyst-free mixed transesterification within the adaptable carboxylate/phosphonate networks. The phosphonate structure in DAP was covalently bonded into the epoxy vitrimers (EVs), endowing the EVs with intrinsic fire safety. In detail, the peak heat release rate (PHRR), total heat release (THR), total smoke production (TSP) and maximum smoke density (Ds max) values were considerably reduced by 62.0%, 73.6%, 78.9% and 70.8%, respectively. Meanwhile, owing to the self-catalytic tertiary amine and highly reactive primary hydroxyls from DAP, the mixed transesterification within the adaptable carboxylate/phosphonate networks was accelerated, endowing facile reprocessibility and malleability of the vitrimers, which were further degraded into low-mass molecules by simple alcoholysis. Killing two birds with one stone, this work provided a promising and feasible way to make sustainable and fire-safe thermosetting polymers with comprehensive properties.