## 3 Results and Discussion
### ABC Effect  
The tensile strength perpendicular to the surface of a board is considered a measure of the internal bond (IB) of the particleboard that is strongly correlated with the strength of the adhesive bond between the chips in a panel [18; 19; 20; 21; 22]. Therefore, the IB was involved in building a correlation with the ABC of the chips used as the raw material in the particleboard preparation. Boards made of pine and oak chips, with different amounts of hardener, were prepared. In the case of the pine panels (Table 3), the thickness of the P0 sample was apparently higher, while the structure on the cross-section was rough and loose. In contrast to P0, the structure in the cross-sections of P1 and P2 was more homogeneous and compact. In P3, where the hardener amount was beyond the optimum, a loose structure in the cross-section also occurred. This phenomenon results from the rapid curing of the adhesive with an excess of acidic hardener and subsequent deterioration of the bond line by acid upon storage [1; 5]. In the case of the oak panels (Table 4), it was difficult to observe any significant differences in the structure or thickness of the three boards, regardless of the hardener amount. However, the IB results shown in Figure 5 clearly indicate that the maximum internal bonding is achieved with a hardener addition as small as 0.75 pbw. This is in accordance with the observations reported by [PERSON] and co-workers who found that highly acidic oak chips can be bonded with UF resins without any hardener at all [23]. Figure 5: The correlation between the internal bond (IB) and the amount of added hardener in pine and oak particleboards. \begin{table}
\begin{tabular}{l l l l l} \hline \hline
**Sample** & **P0** & **P1** & **P2** & **P3** \\ \hline Hardener & 0 & 1.1 & 2.2 & 3.3 \\ \hline Species & & & _Pinus silvestris_ L. \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Sample appearance of the boards made of pine. As indicated in Figure 5, different amounts of hardener result in different IBs for particleboards. For both pine and oak particleboards, a second-degree linear correlation (R\({}^{2}>0.99\)) between the IB and the hardener amount were found. Thus, the hardener amount for the highest IB could be predicted. The correlation of \(IB=f\)(hardener amount) calculated for the pine and oak particleboards can be used to predict the optimal amount of hardener for a material with a totally different chemical nature. Hence, the optimal hardener amount was calculated for the ammonia-treated oak (Table 5), taking into account the three different ABC measuring methods (M1-M3). Predicted amounts of hardener were used to produce panels from ammonia-treated oak chips (Table 5). In the case of ammonia-treated oak, the A4 variant, where the amount of hardener was far beyond the typical values, disintegrated during removal from the press (Table 6). It is worth noting that the thickness of specimens A1 and A2, as well as the quality of the chip bonding in the core, were clearly better than those in A3. The obtained results were used to calculate the optimal amount of hardener using a linear correlation of the second degree (Figure 6). It can be seen in Table 5 that the most accurate prediction was obtained from Method M3, i.e., 3.0 g hardener per 50 g resin against 3.2 g hardener per 50 g resin calculated from the model based on the empirical IBs. The results show that Methods M1 and M2 provided erroneous data, which proved them to be inapplicable in the described approach. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & **Q0** & **Q1** & **Q2** \\ \hline Hardener & 0 & 1.1 & 2.2 \\ \hline Species & & _Quercus robur_ L. \\ \hline Surface & & & \\ Cross-section & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Sample appearance of the boards made of oak.