## 3 Results and Discussion
### Plywood Test Results
#### 3.4.4 TGA of the Plywood Panels  
The TGA performed on material removed from the middle veneer of each plywood panel manufactured with the adhesive mixtures under study is presented in Figure 7. In every case, the thermal degradation of the adhesive mixtures follows that of the wood, as it was impossible to extract just the adhesive mixture that was cured between the veneers; following this, the mass loss and dTG curves of the plain wood veneer are shown for comparative reasons [45]. A slight mass loss due to humidity that does not exceed 3.5% is reported up to 160 \({}^{\circ}\)C. All the samples present a local maximum in the mass loss rate at about 375-390 \({}^{\circ}\)C due to wood degradation predomination and, in particular, the cellulose decomposition. Plain wood is considered as the most thermally stable one from all the samples under study, as its thermal decomposition maximum is located at 390 \({}^{\circ}\)C. The samples containing adhesive mixtures with olive tree residues, and especially that with residual olive skin (ROS) and olive stone (OS), present a somewhat better thermal stability in comparison with the other glue mixtures, with local degradation maxima above 380 \({}^{\circ}\)C. It should be mentioned that although all the adhesive mixtures--as collected from the wooden panels after their curing--present similar thermal characteristics, the one composed of extracted hemp seeds (HS-E) differs, as it presents two more local degradation maxima, at 310 and 439 \({}^{\circ}\)C. The first one is also observed in other adhesive mixtures, mainly the one with olive leaves (OL-NE), and can be related with the depolymerization of hemicellulose, cellulose, lignin, and pectins, which are associated with the agriculture residues under study [46]. The latter could be associated with lignin pyrolysis or the decomposition of char residues [47]. \begin{table}
\begin{tabular}{l c c c c} \hline \hline \multicolumn{1}{c}{**Sample**} & \multicolumn{1}{c}{**Flexural Stress at Break**} & \multirow{2}{*}{**Flexural Strain (\%)**} & \multicolumn{1}{c}{**Flexural Modulus**} & \multirow{2}{*}{**Flexural Strength (MPa) *} \\  & **(MPa)** & & & **(MPa)** \\ \hline pMDI/SF & 10.20 & 3.00 & 604.6 & 10.60 \\ \hline pMDI/SF/WF & 12.60 & 3.16 & 580.2 & 12.97 \\ \hline pMDI/SF/EOP-NE & 10.08 & 2.31 & 592.1 & 10.17 \\ \hline pMDI/SF/EOP-E & 14.10 & 4.33 & 557.0 & 14.20 \\ \hline pMDI/SF/ROS-NE & 16.52 & 3.81 & 655.2 & 17.25 \\ \hline pMDI/SF/E & 12.10 & 2.70 & 647.3 & 12.41 \\ \hline pMDI/SF/OS-NE & 7.45 & 1.54 & 563.9 & 7.60 \\ \hline pMDI/SF/OS-E & 10.10 & 2.49 & 583.1 & 10.55 \\ \hline pMDI/SF/OL-NE & 8.95 & 2.83 & 494.2 & 9.05 \\ \hline pMDI/SF/OL-E & 7.57 & 2.95 & 467.8 & 9.01 \\ \hline pMDI/SF/HS-E & 9.20 & 3.11 & 536.8 & 9.63 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Flexural data of the prepared plywood panels. Summarizing all the above results, it can be seen that the removal of bio-components from biomasses increases their pH, resulting in less acidic materials and a reduction in their buffer capacity (Table 5). The FTIR analysis of the adhesive mixtures with the agricultural residues before and after their curing shows that they exhibit the exact characteristics with those of the glue mixture with soy flour. Furthermore, it is noted that in the cured adhesive mixtures, the peak corresponding to the isocyanate group is not totally consumed, and isocyanate groups still remain to react during the adhesion stage of the manufacturing of the plywood panels. This is supported by the FPA micro-FTIR analysis of the plywood panels, where the isocyanate groups were totally consumed. Visual observation of the glue lines showed good adhesion in all cases, and mechanical tests revealed that all the panels made from all studied biomaterials exhibit excellent mechanical properties and high wood failure. Notably, the panels with ROS and HSs demonstrated mechanical properties and wood failure performance that are comparable to, or even superior to the reference compositions (Tables 6 and 7).