Fire Performance and Leaching Resistance of Poplar Wood Treated with Tannin-Diammonium Phosphate and Ammonia Fuming

Fast-growing poplar wood is an attractive resource for sustainable construction, but its high flammability and limited durability restrict its use in fire-rated applications. This study evaluates a bio-based flame-retardant treatment combining tannic acid (TA) and diammonium phosphate (DAP) impregnation with a subsequent gas-phase ammonia (NH3) fuming step intended to fix the flame-retardant components within the wood matrix. The combined TA+DAP treatment increased the limiting oxygen index from 23% (untreated) to 77% and reduced peak heat release rate by roughly 4.2x and total heat release by roughly 2.5x, consistent with a stable char layer confirmed by thermogravimetric analysis. Adding tannic acid within this fully fumed system also markedly improved fixation relative to DAP alone, shifting leach retention from a net mass loss (–13.0%) to a positive retention of 28.9%, while modestly increasing hardness and flexural stiffness. Although this leach retention remains below that of covalently fixed systems, and the fire performance and mechanical properties of leached specimens were not directly retested in this study, the results indicate that the combined TA+DAP treatment with ammonia fuming offers a simple, scalable route to enhancing the fire performance and reducing the equilibrium moisture content of low-density plantation wood; both properties were evaluated on unleached specimens only, and whether the fire-performance benefit is retained after leaching remains unconfirmed.