An Innovative Approach for Enhancing Wood Heat Treatment Efficiency: Exploring the Mechanistic Influence of Exogenous Phosphoric Acid on Thermal-Induced Discoloration and Dimensional Stability of Poplar Wood

Conventional heat treatment is an industrialized and environmentally friendly wood modification method but is considered a self-catalysis-reacted modification and energy-intensive consumption process. This research uses exogenous phosphoric acid (EPA) to facilitate the thermal reaction of wood and thoroughly investigates the catalytic mechanism of acid treatment during the heat treatment process. Poplar wood was first impregnated with a 0.05 and 0.5 mol/L EPA solution and then subjected to 120∼180 °C for 1 and 2 h. The results revealed that EPA enlarged the reaction of wood to heat, acid-heat-treated (AHT) wood’s color deepened, and dimensional stability enhancement was superior to conventional heat-treated (CHT) wood. The antiswelling rate of the AHT wood (AHT0.05–180–2 and AHT0.5–180–2) was 2.72- and 4.26-fold compared to CHT180–2, and the color difference was 8.8% and 614.2% higher. The AHT0.5–120–1 wood’s antiswelling rate reached the same level as that of CHT180–2, with a 1/3 reduction of treatment temperature and a 50% decline in holding time. Pyrolysis kinetics analysis revealed that the hemicellulose’s activation energy (Ea) declined, whereas the Ea of lignin changed according to EPA concentration. Interestingly, the EPA increased the Ea of wood and cellulose because of its carbonization effect at a higher temperature, which endowed a potential fire resistance capacity to the AHT wood. EPA reduced the initial thermal degradation temperature of wood and wood components, providing an efficient way to induce discoloration and improve wood dimensional stability. This novel modification technique requires less energy consumption and endows multifunctionality in wood. The outcomes offer a new sustainable and high-efficiency chemical combined thermal processing technology for wood.