Can polyhydroxyurethane-derived covalent adaptable networks provide environmental benefits in composite applications?

Covalent Adaptable Networks (CAN) and CO2-derived polyhydroxyurethanes (PHU) are often deemed as sustainable alternatives to conventional thermosets, particularly for composites used with epoxy (EP) matrices. However, the sustainability of CAN-based composites has never been assessed, nor has that of thermoset PHUs. Herein, we perform a life cycle assessment of PHU, synergetic hybrid EP-PHU CANs, and epoxy in composite applications with either carbon or natural fibers (NF) in order to address their syntheses, processes, and recycling. We demonstrate that producing cyclic carbonate monomers from epoxy and supercritical CO2 could be advantageous. PHU provides potential environmental benefits to epoxy but is significantly limited by the energy inputs required for curing. Inversely, synergetic EP-PHU demonstrates noticeable environmental gain compared to EP and PHU-based composites and offers ideal recycling pathways. The chemical recovery of carbon fibers by oxidative depolymerization shows substantial benefits compared to virgin material production. When using natural fibers, mechanical recycling of CAN-based matrices is more suited due to the impacts of chemical recycling compared to virgin NF production, highlighting that the viability of a strategy strongly depends on raw material and cannot be generalized easily. Strategies to further enhance the sustainability of composites are also proposed and discussed.