Fully recyclable, flame-retardant and high-performance carbon fiber composites based on vanillin-terminated cyclophosphazene polyimine thermosets

Abstract Carbon fiber reinforced polymer composites (CFRPs) have been widely applied in various high-tech fields because of their superior performance. However, with the increasing consumptions of CFRPs, the nondestructive recycling of expensive carbon fibers (CFs) and the reduction of fire-hazards are highly desirable for the sustainable and safe utilization of CFRPs. Herein, a fully recyclable CFRPs with high flame retardance was newly designed by incorporating the vanillin-terminated phosphazene monomer (HVP) into the polyimine binder material. The six-functional HVP monomer makes the polyimine thermosets have a high crosslinking density and high P/N content, thereby ensuring the excellent flame retardancy and mechanical performances of the resulted composites. And the preparation of this kind of composites just need simple hot-pressing at lower temperature and shorter time, which is more energy-saving and efficient. Owing to the degradable imine bonds, the nondestructive closed-loop recycling of CFs and HVP monomers from the composites are achieved under mild conditions, which is extremely vital to preserve the structure of virgin CFs and HVP functional monomer. The regenerated CFRPs displayed same performance with that of fresh ones, and it also could be reprocessed and repaired within few minutes at elevated temperatures. More importantly, due to the rational combination of cyclophosphazene and imine bond, the excellent synergistic effect of gas phase and condensed phase during combustion endows the composites with superior flame retardancy. This work prepared bio-based recyclable, flame-retardant CFRPs, offering a green, sustainable and fire-safe strategy for designing eco-friendly high-performance composites.