In-situ synthesized phosphorus–boron–magnesium hybrid flame retardants with core-shell structures in wood microstructures: an effective strategy for realizing durable fire retardancy, smoke suppression, and anti-degradation

Wood, a flammable material, poses safety risks due to heat and smoke release during combustion and is a huge safety hazard. The current approach involves impregnating water-soluble inorganic flame retardants into wood. However, these retardants are susceptible to leaching, negatively impacting the aesthetic appearance and flame retardancy of wood. Herein, a liquid mix of ammonium dihydrogen phosphate, boric acid, and magnesium chloride was vacuum-pressure impregnated into wood. Subsequent concentration and drying produced composite salt particles (named as P-B-Mg). The wood was immersed in sodium silicate solutions of different concentrations. Mg2+ leached from P-B-Mg rapidly combined with SiO32– to precipitate an MgSiO3 layer on the particle surfaces. This in-situ synthesis resulted in P-B-Mg@MgSiO3 hybrid flame retardant with core-shell structure within wood cells. The modified wood displayed superior performance, achieving non-combustible level in limiting oxygen index test and remaining self-extinguishing even after continuous exposure to open flame at 1100 °C for 205 s. Compared with that of untreated wood, the total heat release and total smoke production of the P-B-Mg@40%MgSiO3-modified wood decreased by 94.7 and 86.5%, respectively. Residual char and pyrolysis analysis showed that the flame retarded process of modified wood involved gas, liquid and solid phase synergy effects. Meanwhile, the modified wood exhibited excellent resistance to leaching, durable flame retardancy, smoke suppression after 14-d water immersion cycles. Furthermore, it displayed good anti-degradation properties with pH value close to that of untreated wood. Thus, the modified wood possesses stable material properties suitable for long-term use in both indoor and outdoor applications.