## 2 Materials and Methods
Customizing the previous relationship (4) for immersion periods of 2 h and 24 h, the following two calculation formulas were obtained (Equation (5)):  
\[S_{2h}=\frac{g_{2h}-g_{02h}}{g_{02h}}\cdot 100\ [\%]\ S_{24h}=\frac{g_{24h}-g_{024h}}{g_{024h}} \cdot 100\ [\%] \tag{5}\]  
where \(S_{2h}\) is the percentage of swelling of the thickness of the samples after 2 h of immersion in water; \(g_{2h}\) is the thickness recorded after 2 h immersion; \(g_{02h}\) is the thickness of the absolutely dry samples before immersion in water for a period of 2 h; \(g_{24h}\) is the swelling percentage of the thickness of the samples after 24 h of immersion in water; \(g_{24h}\) is the thickness of wet samples, after immersion for a period of 24 h; \(g_{024h}\) is the thickness of the absolutely dry samples before immersion in water for a period of 24 h.  
_Modulus of rupture (MOR)_. These tests were performed according to EU standard EN 310: 1993 [44]. To determine the modulus of rupture, samples measuring \(50\times 360\) mm (width \(\times\) length) were processed from poplar PW, and measuring \(50\times 400\) mm (width \(\times\) length) from MDF. From each type of wooden composites (PW and MDF), 10 replicates were used, including the addition of 10 new control replicates. Before the measurements, the samples were conditioned at 20 \({}^{\circ}\)C and 65% relative humidity. To measure the deformation, a minimum force of 200 N and a maximum one of 600 N were applied. The MOR was calculated using the following equation (Equation (6)):  
\[MOR=\frac{3}{2}\frac{P_{max}\cdot l}{b\cdot g^{2}}\ [N/mm^{2}] \tag{6}\]  
where _MOR_ (N/\(mm^{2}\)) is the modulus of rupture; \(P_{max}\) (\(N\)) is the maximum breaking force of the sample; \(l\) (\(mm\)) is the distance between supports; \(b\) (\(mm\)) is the width of the samples of each type of wood-based composite; \(g\) (\(mm\)) is the thickness of the samples. A schematic representation of the determination of modulus of rupture is presented in Figure 1.  
_Roughness of PW and MDF_. To determine the roughness, 30 samples with dimensions of 50 \(\times\) 50 mm (width \(\times\) length) from poplar PW and MDF were used. The roughness was expressed through three parameters: the arithmetic mean of irregularities (R\({}_{\rm a}\)), the mean height of irregularities (R\({}_{\rm z}\)), and the mean square root (R\({}_{\rm q}\)). Figure 1: Determination of modulus of rupture. For the measurements, the Mitutoyo Surftest SJ-301 instrument (Mitutoyo company, Kawasaki, Japan) was used. The cut-off length was 2.5 mm, the sampling length was 12.5 mm, and the detection tip radius was 5 \(\upmu\)m in the surface roughness measurements. _Color measurements of PW and MDF._ A portable colorimeter type Tes-135 A, manufactured by Tes Electrical Electronic Corp (Taipei, Taiwan), was used to determine the color, using the CIELab system. The L* axis represents lightness and has the value 0 for black and 100 for white, with values in between being a series of shades of grey. The a* axis refers to the green-red opposition, with negative values towards green and positive values towards red. The b* axis quantifies the blue-yellow opposition, with negative values towards the blue zone and positive values towards the yellow zone [45]. Figure 2 defines the entire CIELab colorimetric space with the 3 components L, a, and b. From the L*, a*, and b* values, the color uniformity was calculated as the difference in the lightness and chromaticity parameters between heat-treated and control (untreated) poplar PW and MDF samples. Based on the direct observations on the analyzed parts, it is predicted that after the heat treatment of torrefraction, the luminance will migrate towards black (i.e., it will decrease), the coefficient a* will migrate towards green (i.e., it will decrease slightly), while the coefficient b* will migrate towards yellow (i.e., it will increase slightly).