Enhanced fire retardancy and mechanical performance of wood via modification with a bio-based furfuryl alcohol–tannic acid–phosphoric acid system

A bio-based wood modification approach combining furfuryl alcohol (FA) and tannic acid (TA) was developed to improve both mechanical performance and fire behaviour of poplar wood. The impregnation formulations were catalysed by phosphoric acid (H₃PO₄) to enable in-situ polymerisation and fixation within the wood structure. Optical microscopy confirmed progressive lumen filling and cell-wall impregnation after treatment, with tannin–furfurylated wood (TFA-W) showing a more homogeneous polymer distribution than furfurylated wood (FA-W). The structural densification was reflected in hardness performance: Shore D increased by 7% for FA-W and 16.5% for TFA-W, while Janka hardness increased by 17% and 56%, respectively, compared with untreated wood (RW). To address strength-related performance, three-point bending tests were performed. TFA-W exhibited increased flexural properties (MOR 89.22 MPa; MOE 9.98 GPa) relative to RW (MOR 74.76 MPa; MOE 8.49 GPa). Fire performance was evaluated by limiting oxygen index and cone calorimetry. Both modified systems promoted char formation and reduced flammability, resulting in an upgrade of the predicted reaction-to-fire classification from Euroclass D-s1 for RW to C-s1 for FA-W and TFA-W. Furthermore, the TFA-W demonstrated excellent leach resistance, retaining over 97% of the polymer mass after accelerated leaching tests, indicating high chemical stability. Overall, the results demonstrate that incorporating tannic acid into furfurylation systems is an effective strategy to enhance indentation resistance, maintain or improve flexural performance, and improve fire behaviour of fast-growing poplar wood.