How does phosphoric acid affect the hygroscopicity and chemical components of poplar thermally modified at low temperatures?

Shorter processes and lower temperatures are critical to reducing thermally modified wood costs. In this study, the exogenous H₃PO₄ was infiltrated into poplar (Populus × euramaricana) and then heated at low temperatures of 130–170 °C to speed up the thermal modification process of wood with better performance. The hygroscopicity was analyzed by dynamic vapor sorption detection and its constituents of modified wood were characterized by high-performance liquid chromatography and solid-state CP/MAS ¹³C NMR. The results showed that acid combined with low-temperature thermal modification (acid-LTM) resulted in lower equilibrium moisture content compared with the high-temperature thermal modification (HTM) wood. The addition of H₃PO₄ triggered severe degradation of the carbohydrates in the wood, and the mass loss of cellulose and hemicellulose were 11.9% and 24.1% when modified with 3.0% H₃PO₄ at 150 °C, respectively, thereby reducing the quantities of water sorption sites. Besides, the degradation products of carbohydrates crosslinked with the thermally stable lignin to form “pseudo-lignin” substances, leading to an increase in the lignin content of acid-LTM wood. The increase in the crystalline index and crystallite size of cellulose in acid-LTM wood was also conducive to reducing the wood hygroscopicity. The better hydrophobicity of acid-LTM poplar was further verified by its decrease in the water sorption site density and the theoretical OH content compared with HTM wood and unmodified wood. This study will offer a potential process to manufacture thermal-modified wood at a low cost.