## 3 Results
### Thickness Swelling after Immersion in Water  
The dimensional stability of composite materials (PW and MDF) was exemplified by the parameter of thickness swelling after total immersion in distilled water. Only the swelling in thickness was used, because the swelling in the longitudinal direction was very small, almost imperceptible. This behavior is very well known from the properties of solid wood, where the ratio between the two directions is 1: (10-20).  
For poplar PW, the thickness swelling after 2 h was lower than the control sample, for the two work regimes (17.48% for T1t1 and 12.02% for T2t1). The same trend was preserved for thickness swelling when the samples were immersed in water for 24 h (6.77% for T1t1  
Figure 4: Water absorption percentage for (**a**) poplar PW and (**b**) MDF.  
and 5.53% for T\({}_{2}\)t\({}_{1}\), respectively) (see Figure 5a). This means that the swelling in thickness is different for the two composite materials studied and is determined by the nature of each composite material, respectively, by the layered nature of the poplar PW and the agglomerated-fibrous nature of the MDF.  
The thickness swelling of the treated MDF in the T\({}_{1}\)t\({}_{1}\) regime after 2 h immersion in water was lower than the values recorded for the control sample (3.74%) and higher than the control sample for the treated MDF in the T\({}_{2}\)t\({}_{1}\) regime (with 1.14%) (Figure 5b). For treated MDF in the T\({}_{1}\)t\({}_{1}\) regime, the thickness swelling after 24 h immersion in water was higher than the recorded values for the control sample (with 6.36%) and lower for the treated MDF in the T\({}_{2}\)t\({}_{1}\) regime (with 17.95%), as Figure 5b shows us.  
Figure 5: Thickness swelling of (**a**) poplar PW and (**b**) MDF; 2 h—2 h immersions in water; 24 h—24 h immersions in water; T\({}_{1}\)t\({}_{1}\)—treatment regime at 170 °C for 1 h; T\({}_{2}\)t\({}_{1}\)—treatment regime at 190 °C for 1 h.  
The variation in thickness swelling in poplar PW and MDF was evidenced by polynomial curves with Pearson coefficient (R) = 1. The maximum thickness swelling for poplar PW related to the control sample was 9.46% after 2 h and 11.77% after 24 h immersion in water. The maximum thickness swelling for treated poplar PW in the T1t1 regime was 10.71% after 2 h and 13.32% after 24 h. The maximum thickness swelling for treated poplar PW in the T2t1 regime was 8.06% after 2 h and 10.34% after 24 h.  
The maximum thickness for MDF related to the control sample was 4.65% after 2 h and 23.14% after 24 h. The maximum thickness swelling for treated MDF in the T1t1 regime was 3.93% after 2 h, while after 24 h, it was 26.95%. The maximum thickness swelling for treated MDF in the T2t1 regime was 3.49% after 2 h, while after 24 h, it was 18.81%.  
For the control poplar PW, the standard deviation for thickness swelling after immersion in water for 2 h was 1.68, and 2.04 after immersion for 24 h. For poplar PW treated at 170 \({}^{\circ}\)C, the standard deviation for the thickness swelling was 1.82 after immersion in water for 2 h, and 1.90 after immersion in water for 24 h, respectively. For poplar PW treated at 190 \({}^{\circ}\)C, the standard deviation for the thickness swelling was 1.73 after immersion in water for 2 h, and 1.98 after immersion in water for 24 h.  
For the control MDF, the standard deviation for the thickness swelling was 1.79 after immersion in water for 2 h and 2.09 after immersion in water for 24 h. For MDF treated at 170 \({}^{\circ}\)C, the standard deviation for the thickness swelling was 1.82 after immersion in water for 2 h and 2.15 after immersion in water for 24 h, respectively. For MDF treated at 190 \({}^{\circ}\)C, the standard deviation for the thickness swelling was 1.36 after immersion in water for 2 h, and 2.75 after immersion in water for 24 h respectively.