## 2 Materials and Methods  
Poplar plywood (PW) and medium-density fiberboard (MDF) panels, with the initial dimensions of 2000 \(\times\) 1250 mm, were used. These are commercially available panels and were purchased from Compozite S.R.L. Brasov, Romania. The poplar PW consists of seven veneer sheets, glued using urea-formaldehyde resin, with a moisture content of 8%. The MDF had a moisture content 9%. The calculated density for the poplar PW was 671.98 kg/m\({}^{3}\), and for MDF, it was 752.48 kg/m\({}^{3}\). From these, 30 pieces of 300 \(\times\) 300 mm were cut with a circular saw in order to be used for the torrefraction process. Before and after the modification, the samples were weighed with a Kern 250 scale (Dusseldorf, Germany), and their thickness was measured with an electronic caliper. _Torrefraction process._ After conditioning, the samples were placed in a laboratory oven type Memmert (Berlin, Germany) for drying at 105 \(\pm\) 3 \({}^{\circ}\)C for 10 h. Then, they were weighed, and after another hour of drying, the thickness of the samples was determined. The torrefraction process was performed, at temperatures of 170 \({}^{\circ}\)C (T1) and 190 \({}^{\circ}\)C (T2), for 1 h (t1) and 2 h (t2), respectively. Through combining times with temperatures, there were 4 types of thermal hardening regimes, namely T1t1, T1t2, T2t1, and T2t2. Firstly, in order to determine the main parameter of torrefraction process, the mass loss was calculated, using the following equation (Equation (1)):  
\[M_{l}=\frac{m_{i}-m_{f}}{m_{i}}\cdot 100\ [\%] \tag{1}\]  
where \(m_{i}\) is the initial mass of the sample, before torrefraction; \(m_{f}\) is the final mass of the sample. _Water absorption._ Samples measuring 50 \(\times\) 50 mm were cut from different zones of the panel and used to determine the water absorption and thickness swelling. For each sample, 5 replicates were used. The untreated and torrefied samples were dried in an oven at 105 \({}^{\circ}\)C until constant weight was observed. After that, the samples were conditioned at 65% RH and 20 \({}^{\circ}\)C and then placed in an immersion tank with distilled water at room temperature (of 20 \({}^{\circ}\)C), 20 mm below the water level. The samples were placed horizontally on special grids. After 2 and 24 h, the samples were extracted, the excess of water was removed with filter paper, and the mass and their dimensions were measured again. The experiment was realized according to the EN 317:1993 standard [19]. Water absorption was determined using the following equation (Equation (2)):  
\[A_{i}=\frac{m_{i}-m_{0i}}{m_{0i}}\cdot 100\ [\%] \tag{2}\]  
where \(m_{i}\) is the mass of the sample for each period of time (2 or 24 h), \(m_{oi}\) is the initial dry mass of the sample, and \(i\) = 2 or 24 h. Customizing equation (2) for immersion durations of 2 and 24 h, the following two equations are obtained (Equation (3)):  
\[A_{2h}=\frac{m_{2h}-m_{02h}}{m_{02h}}\cdot 100;\ A_{24h}=\frac{m_{24h}-m_{024h}}{m _{024h}}\cdot 100\ [\%] \tag{3}\]  
where \(A_{2h}\) is the water absorption percentage after 2 h of immersion in water; \(m_{2h}\) is the mass of the samples after 2 h of immersion in water; \(m_{02h}\) is the mass of absolutely dry samples for the case of samples immersed for 2 h in water; \(A_{24h}\) is the water absorption percentage after 24 h immersion in water; \(m_{24h}\) is the mass of the samples immersed for 24 h in water; \(m_{024h}\) is the mass of dry samples for the case of immersion in water for a period of 24 h.  
_The thickness swelling_ of the samples was determined according to the variation in the thickness of the samples before and after immersion, using the data obtained during the measurements mentioned above and the following equation (Equation (4)):  
\[S_{i}=\frac{g_{i}-g_{0i}}{g_{0i}}\cdot 100\ [\%] \tag{4}\]  
where \(S_{i}\) represents the swelling percentage in the thickness direction; \(g_{i}\) is the thickness of the samples at time \(i\) (2 or 24 h); \(g_{0i}\) is the initial thickness after drying, at time \(i\) (2 or 24 h).