The construction of controllable dynamic bonds based on dynamic ester linkages and steric hindrance effect for efficient recycling of flame-retardant epoxy-based carbon fiber composites

With the increasing demand for carbon peaking and carbon neutrality, the lightweight and high-strength epoxy-based carbon fiber composite materials have become a substitute for structural strength materials. However, there were three core challenges: insufficient flame retardancy of epoxy matrices; difficulty in synergistically optimizing mechanical and flame-retardant properties; and non-recyclability and significant degradation of carbon fiber properties after recycling. To address these issues, two reactive flame-retardant curing agents D1 and D2 with constructed controllable dynamic bonds via caged molecular design were designed and synthesized, using phosphaphenanthrene groups and polyhedral oligomeric silsesquioxane (POSS) as functional units that bring phosphorus-silicon synergistic flame-retardant effects. Epoxy-based carbon fiber composites CFD1 and CFD2 which were prepared via in-situ curing with D1 and D2 achieved UL-94 V0 rating, with 48.2% and 49.2% of limiting oxygen indices (LOI), respectively. The tensile strength of CFD1 and CFD2 reached 3.27 GPa and 3.21 GPa, suggesting the synergistic enhancement of flame retardancy and mechanical properties. CFD1 and CFD2 can be efficiently recycled via the solution method with 94.6% recovery yield. The recycling cost was about RMB 3.0 per kg for recycled carbon fibers and the process greenhouse gas emissions was around 99.9% lower than that of virgin carbon fiber production. The tensile strengths of secondarily molded CFD1-R and CFD2-R remained 1.35 GPa and 1.31 GPa (41.3% of the original properties), their flame-retardant rating remained V0, and over 46.0% LOI. indicating that high-performance recyclable and flame-retardant composites are achieved by the construction of controllable dynamic bonds.