Enhanced Fire Retardancy and Durability of Wood via Modification with a Bio-based Furfuryl Alcohol–Tannic Acid– Phosphoric Acid System

A bio-based fire-retardant wood modification system was developed through the in-situ co-polymerization of furfuryl alcohol (FA) and tannic acid (TA) catalysed by phosphoric acid (H₃PO₄). Microscopy confirmed pronounced cell-wall bulking and lumen filling in the modified wood, leading to increased density and improved mechanical performance. Shore D hardness increased by 7% for FA-treated wood (FA-W) and 16.5% for tannin–furfurylated wood (TFA-W), while Janka hardness increased by 17% and 56%, respectively, relative to untreated poplar. Both treated materials exhibited excellent durability, retaining over 97% of the polymer mass after EN 84 accelerated leaching. Thermogravimetric analysis showed increased char yield in nitrogen, rising from 15.4% in raw wood to 22.94% in FA-W and 25.6% in TFA-W at 800 °C. Cone calorimetry demonstrated a pronounced condensed-phase flame-retardant effect for TFA-W, including reduced MARHE (160 vs. 173 kW m⁻²), lower CO yields (CO _q 1.47 vs. 3.42 g MJ⁻¹), lower smoke extinction area (SEA _q 2.45 vs. 5.94 m² MJ⁻¹), and substantially higher char residue (23.73% vs. 3.67%) compared to raw wood. The limiting oxygen index increased from 18 (RW) to 25 (TFA-W), indicating a synergistic improvement in flame resistance due to tannin-enhanced char development. Overall, the FA–TA–H₃PO₄ system forms a stable carbon-rich polymer network that provides durable fire retardancy, reduced smoke and toxic gas generation, and improved mechanical performance, offering a sustainable alternative to conventional flame retardants for wood products.