Hierarchical porous 4A zeolites as a smoke suppressant for ammonium polyphosphate-treated wood composites: Role of mesopore diameter

Abstract Hierarchical porous 4A (H4A) zeolites have shown promising effect curbing the smoke production of wood composites treated with ammonium polyphosphate (APP). However, the mechanisms have not been defined and the effect of pore structure, i.e., the mesopore diameter of H4A zeolites on smoke suppression property, has not been fully understood. In this study, H4A zeolites with varying mesopore diameters (5.6/6.5/9.5 nm) were prepared and combined with APP to minimize the fire risks of wood composites. We found that larger mesopore diameter is favor of improving the smoke suppression properties. Compared with APP-treated samples, the second peak of smoke production rate and total smoke production (PSPR2 and TSP) of APP - 9.5 nm H4A zeolite group were decreased by 63.38% and 18.11%, respectively, showing lowest smoke production. Moreover, it was determined that H4A zeolites played a key role in gas-phase fire-retardant mechanism by minimizing the combustible volatiles and increasing the PO· during the pyrolysis. In addition, the char layer stability was enhanced because of its higher graphitization degree and the formation of more P–O–C and Si–O–C bonds and stable species (such as SiO2 and AlPO4). The enhanced smoke suppression performance could be attributed to the superior adsorption effect of H4A zeolites with larger mesopore diameter on the smoke precursor or nucleus as well the formation of a more protective char layer. This outcome will provide insights into enhancing the fire safety of fire retardant treated materials over hierarchical porous zeolites with proper pore structure.