Fabrication of a bio-based flame retardant nitrogen‑silicon doped carbon quantum dots/sodium lignosulfonate and its flame retardant effect in epoxy resin

Epoxy resin (EP) is widely used in various fields due to its excellent adhesion, mechanical strength, chemical resistance, and electrical insulation. However, its low limiting oxygen index (18) makes it highly flammable and generates toxic fumes during combustion, presenting significant fire risks. Consequently, it is crucial to modify epoxy resins to be flame retardant. Bio-based carbon nanomaterial flame retardants offer the advantages of high flame retardant efficiency and balanced performance, providing a novel approach to addressing this issue. This study innovatively develops a bio-based flame retardant through a nitrogen‑silicon synergistic doping strategy, using tetrazole (Tet), tetraethyl orthosilicate (Teos), and glycerol (Gly) as raw materials to synthesize N Si co-doped carbon quantum dots (N-Si-CQDs) via a one-step hydrothermal method. The obtained N-Si-CQDs were further combined with sodium lignosulfonate (Sodi) to prepare N-Si-CQDs/Sodi@EP composites. The composite exhibits remarkable flame retardancy: a limiting oxygen index of 29%, a UL-94 V-0 rating, and significantly enhanced char residue. Peak heat release rate and total smoke production are reduced by 33% and 28%, respectively. The fire growth index decreases by 56%, while the fire performance index increases by 160%. Mechanism analysis confirms that the system acts synergistically in both gaseous and condensed phases - forming a protective char barrier and neutralizing free radicals. This work offers a novel strategy for designing high-performance EP composites.