Crosslinking density regulation and performance optimization of flame retarded lignin/epoxidized soybean oil thermo-reversible polymer network based on Diels-Alder reaction

Modification of sebacic acid and citric acid with furfurylamine produces furfurylated sebacic acid (FS) and furfurylated citric acid (FC). Successfully constructs a high-performance polymer. • FS and FC increase cross-linking sites for DA reactions and elevate cross-linking density. • Raised cross-linking density strengthens synergistic interactions between FS/FC, lignin and flame retardants. In this study, two kinds of furan-terminated small molecules (FS and FC) were synthesized via amidation reactions between furfurylamine and polyacids (sebacic acid or citric acid), respectively, and were incorporated into the furan-functionalized epoxidized soybean oil (FE)/furan-terminated lignin (FL) thermo-reversible polymer network via Diels-Alder (DA) reaction to regulate the crosslinking density of the network. With the incorporation of DOPO-based reactive flame retardants (DE and DF), a structural optimized thermo-reversible/flame-retardant polymer with a reinforced network was fabricated. By introducing linear two-arm (FS) or branched three-arm (FC) structures, especially FC, the crosslinking density of the networks was significantly improved. As a result, the tensile strength, fracture toughness (KIC), and self-healing properties of the polymer were synchronously improved. The increased crosslinking density also enhanced the synergistic effect between FS/FC, lignin, and flame retardants, leading to improved char-forming ability and flame-retardant efficiency. The limiting oxygen index of 75FE/5FL/10FC/5DE/5DF reached 28.9% with the addition of only 5 wt% FL, enabling it to pass the UL-94 V-0 flammability rating. Compared with 100FE, this formulation exhibited reductions in total smoke production (TSP), smoke production rate (SPR), total heat release (THR), and peak heat release rate (pHRR) by 31.4%, 44.4%, 18.06%, and 21.67%, respectively. Additionally, it achieved a self-healing efficiency of 85.20% and retained 71% of its tensile strength after three reprocessing cycles. Through the aforementioned strategy, this work successfully overcomes the common trade-off among mechanical properties, self-healing efficiency, and flame retardancy in biobased dynamic polymers. It offers a practical example for developing sustainable, intelligent advanced polymeric materials for future applications. Download: Download high-res image (171KB) Download: Download full-size image