Thermal modification is one of the methods used to improve the properties of wood and extend its durability. It is based on exposing the wood to elevated temperatures in the absence of oxygen. Thermal modification changes the structure of the wood's cell wall, which greatly influences its properties. We selected fir wood that we exposed to temperatures of 170 °C, 180 °C, 190 °C, 200 °C, 210 °C and 220 °C. The thermally modified samples were analysed using thermogravimetry in the temperature range between 25 °C and 600 °C in a nitrogen flow. Based on the analysis results, we constructed calibration curves for several selected temperature ranges. The calibration curves represent the correlation between the mass loss during TG measurement and the mass loss during thermal modification. The calibration curves can be used to determine the degree of thermal modification of unknown samples. The highest correlation coefficient showed the calibration curve for the temperature range between 130 °C and 320 °C, which turned out to be the optimal. We also performed a FTIR analysis of the thermally unmodified sample and the sample that was thermally modified at 210 °C. The spectra were measured in the range between 400 cm$^{−1}$ and 4000 cm$^{−1}$. The results showed characteristic peaks corresponding to the chemical structure of wood. The spectrum of the sample that was not previously thermally modified exhibited more intense peaks than the sample that was thermally modified. We also measured the contact angle of a water droplet for all thermally modified samples of fir wood. The results showed that the contact angles of water droplets on the wood surface increased significantly with the increased degree of thermal modification, indicating an increase in the hydrophobicity of the surface.
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