Flame retardancy, thermal stability, and hygroscopicity of wood materials modified with melamine and amino trimethylene phosphonic acid

• Impregnation of melamine/phosphoric acid composite on wood surface achieved. • Surface modification imporved fire resistance, durability, and hygroscopicity. • Mechanism of flame-retardance investigated. • Generation of ammonia gas and P/N crosslinked structure formed protective layer. Wood is a widely used as a construction material; however, its inflammability limits its applications in the field of construction. Moreover, high loadings of flame retardants typically required for wood impregnation in fire safety treatments cause increased hygroscopicity and non-durable flame retardance. In this study, melamine (MEL) was impregnated in the porous structure of the wood, in combination with an organic phosphoric acid (AP). Flammability characterization in terms of limiting oxygen index (LOI) and cone calorimetry tests revealed significant improvement in fire resistance of the samples due to the presence of MEL/AP in the porous structure of the wood, synthesized in situ in the wood channel with 2 wt% MEL and 25 wt% AP solutions using a two-step method. The LOI value increased from 21.0% to 68.5%, and the peak-heat-release rate and total-heat-release reduced by 41.7% and 80.2%, respectively, compared with that of the control sample. The MEL/AP synthesized in the porous wood structure improved flame retardancy, durability, and hygroscopicity. The proximity of the ammonia gas-release and wood decomposition temperatures along with the effectively formed cohesive protective and bubble-shaped-char layer enhanced the flame-retardant properties.