Diels-Alder polymer matrices for fully recyclable fibre-reinforced thermoset composite materials

Fibre-reinforced polymers pose a technological challenge for recycling, mainly due to their intrinsic heterogeneity. This holds true especially for thermoset composites, which, unlike thermoplastics, cannot be directly reprocessed. Indeed, currently proposed technologies for recycling thermoset composites are mostly aimed at reclaiming the fibre content while degrading the matrix component, through pyrolysis or solvolysis. This approach is justified by the fact that carbon reinforcements possess higher economic value than resins. In this work, a fully recyclable thermoset polymer composite is presented, exploiting the thermal reversibility enabled by the Diels-Alder (DA) chemistry. The polymer matrix was obtained by reacting furan-containing linear epoxy polymers with a bismaleimide through DA addition. Thermal reversibility of the crosslinked material, assessed by differential scanning calorimetry, allowed to impregnate different kinds of carbon fibres by compression molding at 120 °C, at which retro-DA reaction is favored. The resulting DA-composites exhibited mechanical properties comparable to composites obtained from conventional epoxy thermosets. Straightforward fibre recovery was possible by dipping composite samples in various solvents (e.g., DMF, DMSO, propylene carbonate or γ-valerolactone) at 120 °C. The purity of reclaimed fibres was evaluated by thermogravimetric analysis and scanning electron microscopy and was found to be > 95%, in line with reported solvolysis processes of conventional thermosets. The reprocessability of the recovered fibres into new composites was also demonstrated. Furthermore, dissolved matrix residues were used for the first time to prepare functional coatings by wet deposition. Such coatings were found to be thermally healable after being scratched, also confirming the full thermal reversibility of the resin. The results of this work provide the first demonstration of complete recyclability of carbon-fibre reinforced composites based on DA polymers with complete recovery of both reinforcing fiber and polymeric matrix and opens the way to the use of stimuli-responsive systems in future circular economy scenarios.