Thermal degradation, flame retardancy, and char analysis of ammonium phytate-based flame retardant treated loblolly pine wood

To improve the flame retardancy of wood, an ammonium phytate-based flame retardant (APFR) system was prepared via the reaction between phytic acid and urea without organic solvent purification. Loblolly pine (Pinus taeda) wood was impregnated by the APFR (APFR-W), and the flame retardancy and thermal degradation behaviors of APFR-W were studied. Compared with untreated wood (W), the total heat release and total smoke production of APFR-W (weight% gain = 27.8 ± 0.7%) were decreased by 81.5% and 43.2%, respectively. During decomposition, APFR produced noncombustible gases and phosphoric acid, which diluted the combustible volatiles and facilitated the charring reaction of cellulose. The excess urea in the system melted to absorb heat and might act as a solvent or medium to catalyze the phosphorylation of cellulose, and accordingly reduced the formation of levoglucosan and further increased the char yield. An intumescent structure was formed to protect the underlying wood. More importantly, analysis of the chemical structure of char residue indicated that a graphitized char with nitrogen and phosphorus doping was formed. This char layer acted as a highly thermally stable barrier and effectively impeded the transfer of heat and mass, leading to improved flame retardancy and smoke suppression performance.