Relevant content on wood and its thermal analysis

　　The reaction system exhibits a pronounced thermal effect, which, as confirmed by the thermal analysis of the wood‑treatment agent components, arises from their vigorous thermal decomposition. Based on the extraction rate of the wood‑treatment agent, the characteristics of the products, and the extraction mechanism, a model of the subcritical and supercritical extraction process has been developed. Below 120°C, the process involves the physical dissolution of low‑molecular‑weight substances in the wood‑treatment agent.

　　The semi‑continuous, non‑isothermal dynamic extraction of wood was conducted using a supercritical fluid extraction apparatus for solid materials manufactured in Germany. The entire system comprises three major components: the solvent supply system, the material extraction system, and the product processing system. The lignin content in the extract was determined by UV spectrophotometry at 205 nm.

　　Thermal analysis of wood and its constituents was conducted on a PCT‑1 differential thermal analyzer (manufactured by Beijing Optical Instrument Factory), with a heating rate of 10 °C/min, a sample mass of approximately 5 mg, and a static air atmosphere.

　　The thermal effects of the extraction process were investigated using a supercritical fluid extraction apparatus for solid materials manufactured in Germany. The experiment employed a non-isothermal approach, with a constant heating rate and an advanced temperature-control system. However, during the study, it was observed that within the 320–360 °C range, the heating rate exhibited significant instability; at times, it was difficult to maintain control, resulting in abrupt temperature spikes followed by slow declines—abnormal behavior indicative of pronounced exothermic or endothermic effects in this temperature interval.

　　For anhydrous ethanol extraction, the exothermic peak temperature is 355–359°C; as pressure increases, the duration of the exotherm tends to shorten. When aqueous ethanol is used for extraction, the exothermic peak shifts to lower temperatures: at a water mole fraction of 0.22, the peak occurs at 340°C, and at a water mole fraction of 0.39, it shifts to 306°C, with a corresponding reduction in duration. These observations suggest that elevated pressure and the addition of water can suppress exothermic reactions. Furthermore, during the exothermic process, both the rate of extract formation and the gas evolution rate reach their respective maxima.

　　The aforementioned experimental observations can be scientifically and reasonably explained by the differential scanning calorimetry (DSC) analysis of wood. Within the temperature range of 214–415 °C, wood exhibits a thermal peak with a heat effect of 5.2 kJ/g and a peak temperature of 341 °C. During extraction, an exothermic peak is observed at 340–359 °C, which is in good agreement with the DSC data. Thus, it is evident that both wood itself and its various constituents display relatively pronounced exothermic effects in the 300–360 °C range, consistent with the DSC curve of wood shown in Figure 1 under a static air atmosphere.