A Phosphorus–Nitrogen Synergistic Flame Retardant System for Wood Coatings: Enhanced Fire Resistance and Smoke Suppression

ABSTRACT The development of environmentally friendly and effective flame retardants is critical for improving the fire safety of wood materials. In this study, a bio‐based flame retardant system composed of sodium lignosulfonate, chitosan, and ammonium polyphosphate was incorporated into a melamine‐modified urea–formaldehyde resin to enhance the flame retardancy of wood surfaces. Combustion behavior was evaluated using cone calorimetry. Compared to untreated wood, the treated samples exhibited a 78.89% reduction in peak heat release rate, a 68.68% decrease in total heat release, and a 75.32% reduction in the flame growth index. The synergistic action of carbon‐, nitrogen‐, and phosphorus‐containing components promoted the formation of a dense char layer, effectively suppressing oxygen ingress and heat release during combustion. This work demonstrates a sustainable and efficient strategy for improving the thermal stability and fire resistance of wood via resin‐based surface modification and offers theoretical insights for the design of high‐performance flame retardant coatings.