Closed-loop recycling of an intrinsically flame-retardant epoxy/carbon fiber composite toward formaldehyde-free wood adhesives

The development of covalent adaptable networks has addressed the issue of difficult degradation and reprocessing for composite materials. However, the greater severe challenge regarding the utilization of recycled products has not yet been overcome. With the increasing awareness of fire safety and environmental protection, the development of flame-retardant polymers to solve fire safety and recyclability is urgently needed. Epoxy resins typically have a three-dimensional cross-linked network, which requires more complex conditions for their degradation and recovery. Herein, a dual-dynamic cross-linked network composed of hydrazone bonds and ester bonds was proposed to prepare recyclable vanillin-based epoxy with excellent flame retardancy. The introduction of hydrazone bonds greatly enhanced both stiffness and dynamic exchange capability, endowed the materials with good tensile strength and self-healing performance. The prepared resins revealed a high tensile strength of 76.9 MPa and intrinsic flame retardancy UL 94 V0 level. Importantly, driven by the dynamic ester exchange reaction catalyzed by the tris(dimethylaminomethyl)phenol, the resultant carbon fiber composite material was completely degraded in glycol at 120 °C within 4 h. Further, along the theme of sustainable development, the recovered oligomer showed good potential as wood adhesive. Interestingly, the recycled polymer still maintained excellent flame-retardant properties. The further introduction of lignin provided hydrogen bonding and cross-linking capability, endowing the material with a high adhesive strength of 7.3 MPa. This work would provide an approach for the preparation of intrinsic flame-retardant recyclable epoxy resin and realize the conversion of its recycled product to high-value applications.