## 2 Materials and Methods
### Methods
#### 2.2.3. Testings of Surface and Burning Properties  
* Surface roughness test Surface roughness measurements were conducted in accordance with the DIN 4768 (1990) standard [49], employing a stylus-type profilometer (Mitutoyo SJ-301, Mitutoyo Corp., Kawasaki, Japan). The measurement was taken on the wood surfaces in parallel to the grain direction (\(\parallel\)). The roughness values were captured with a sensitivity of 0.5 \(\upmu\)m, ensuring precision in the measurements. The key instrument parameters included a measuring speed of 10 mm/min, a pin diameter of 4 \(\upmu\)m, and a pin top angle set at 90 degrees. The points selected for roughness measurement were deliberately marked in a random manner across the sample surface. All measurements were conducted parallel to the direction of the wood fibers. Three standard roughness parameters were determined: average roughness (Ra); mean peak-to-valley height (Rz); and maximum roughness (Rmax). Ra values were specifically employed for statistical evaluations. Additionally, measurements were repeated whenever the scanning needle's tip entered the cell spaces within the wood sample.
* Water contact angle test According to GB/T 30693 (2014), the water contact angle of the surface of wood was calculated. KRUSS DSA30 water contact angle meter (KRUSS, Hamburg, Germany) was used. The size of the water drop was 4 \(\upmu\)L. The data were obtained 15 s after the water droplet made contact with the wood surface. The contact angles were obtained at five separate sites on the same sample surface using five replicates for each group, and the mean value was calculated.
* FTIR analysis The samples were finely ground, falling within the 40-100 mesh size range, in preparation for their utilization in Fourier-transform infrared (FTIR) spectroscopy and thermogravimetric analyses (TGA). Following the grinding process, pellets were created by subjecting 10 mg of wood flour and KBr to a 1:100 (\(w\)/\(w\)) ratio for each sample group. These pellets were formed by applying a pressure of 602 N/mm\({}^{2}\). For the FTIR analysis, a Perkin Elmer BX FTIR spectrometer instrument (PerkinElmer U.S. LLC, Shelton, CT, USA) was employed, operating at ambient temperature. The instrument covered a wavenumber range of 4000 to 400 cm\({}^{-1}\), with a spectral resolution of 4 cm\({}^{-1}\).
* DTG/TGA analysis To assess the thermal stability of the samples, thermogravimetric analysis (TGA) was conducted using an Exstar SII TG DTA 7200 (Exstar, SII NanoTechnology In., Tokyo, Japan) instrument. The analysis was carried out under a nitrogen gas atmosphere, with the samples experiencing a gradual temperature increase at a rate of 10 \({}^{\circ}\)C per minute, spanning from 25 to 600 \({}^{\circ}\)C. Each sample was weighed at approximately 5 mg.
* LOI analysis A flammability test to determine the limiting oxygen index (LOI) of wood samples was carried out using a flammability tester (S.C. Dey Co., Kolkata, India). This test was conducted in accordance with the ASTM D-2863 method [50]. The LOI value expresses the minimum amount of oxygen required to start combustion. In the LOI apparatus, the wood sample was positioned vertically and subjected to ignition for a minimum duration of 30 s. Throughout the ignition process, the ratio between nitrogen and oxygen in the environment was carefully monitored and recorded.
* SEM analysis A scanning electron microscope (FESEM, Zeiss Gemini Sigma 300, Jena, Germany) equipped with an energy dispersive spectrometer (EDS) system (Bruker XFlash 6I100) was employed to reveal the effect of impregnation on the particles.
* Statistical analysis Statistical analysis was performed on the study's findings using SPSS(r) 20.0 for Windows(r) software. The data were subjected to an analysis of variance (ANOVA) test. A Duncan test was used to identify the various groups in cases where the ANOVA test revealed statistical differences via SPSS(r) 20.0 for Windows(r) (IBM Corp., Armonk, NY, USA).