Catalytic heat treatment of wood with exogenous phosphoric acid: Enhancing thermo-chemical modification efficiency and fire resistance

Heat treatment is an eco-friendly technique for improving wood properties without chemical additives. However, conventional heat treatment (CHT) typically requires high temperatures and long durations, offering limited enhancement in fire resistance. This study investigates an efficiency-enhanced acid-heat treatment (AHT), examining the effects of exogenous phosphoric acid concentration, temperature, and holding time. Results reveal that exogenous acid serves as a catalyst, accelerating wood degradation at 120–180°C and promoting carbonization at 285–325°C. AHT significantly reduces moisture absorption and enhances fire resistance. The mass loss of AHT 0.5–180–1 wood (0.5 mol/L acid pretreated and 180°C heat-treated for 1 h) was 5.67 %, nine times greater than that of CHT 180–2 wood (180°C heat-treated for 2 h), despite a 50 % shorter duration. Increased reducing sugar content during AHT confirms acid-catalyzed hemicellulose degradation, while higher residual mass after pyrolysis suggests improved thermal stability. Under mild AHT conditions (0.05 mol/L, <180°C), mechanical strength was preserved, while the limiting oxygen index increased and smoke density decreased. In contrast, under severe AHT conditions (0.5 mol/L, 180°C), the heat release rate, total heat release, and fire growth index decreased by 35.55 %, 35.57 %, and 56.76 %, respectively, compared to CHT 180–2 . Among influencing factors, acid concentration had the greatest impact on mechanical and fire-resistant properties, followed by temperature and holding time. Overall, mild AHT conditions are recommended to balance fire performance enhancement with structural integrity and energy efficiency. These findings provide insight into AHT mechanisms and demonstrate its potential as a sustainable and effective wood modification approach for enhancing fire performance while minimizing structural compromise. ● Acid-induced heat treatment improving modification efficiency and wood fire resistance was proven. ● Both acid impregnation and acid-heat treatment significantly enhanced the thermal stability of wood. ● The catalytic effect of exogenous phosphoric acid on the wood heat treatment and pyrolysis processes was demonstrated.