## 3 Results and Discussion  
### Wettability of Spruce Wood Effected by Inorganic Preservatives  
The wettability of the spruce wood specimens treated with the 2% aqueous solutions of inorganic preservatives was evaluated by measuring the contact angles of the drops of redistilled water on their surface two times, i.e., \(\theta_{0}\)--initial contact angle, and \(\theta_{e}\)--equilibrium contact angle (Table 1). The specimens treated with ZnCl\({}_{2}\) showed a larger initial contact angle \(\theta_{0}\) by 28.2% than the control untreated specimens, while on the contrary, in the specimens treated with (NH\({}_{4}\))\({}_{2}\)SO\({}_{4}\), the contact angle \(\theta_{0}\) was smaller by 22.9%. The significant differences in the initial contact angle \(\theta_{0}\) of the specimens ranged from 57.96\({}^{\circ}\) (at using (NH\({}_{4}\))\({}_{2}\)SO\({}_{4}\)) to 96.43\({}^{\circ}\) (at using ZnCl\({}_{2}\)), i.e., the difference in \(\theta_{0}\) was as much as 38.47\({}^{\circ}\). When determining the equilibrium contact angles \(\theta_{e}\), no more evident effects of individual preservatives were observed, whereby the highest value, even 18.3% higher than in the case of untreated spruce wood showed that in the wood treated with ZnCl\({}_{2}\), the angle \(\theta_{e}\) increased from 39.25\({}^{\circ}\) to 46.45\({}^{\circ}\). The maximal difference of an angle \(\theta_{e}\), i.e., between the highest value (in presence of ZnCl\({}_{2}\)) and the lowest value (in presence of (NH\({}_{4}\))\({}_{2}\)SO\({}_{4}\)), was 10.31\({}^{\circ}\). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Spruce Wood Treated with 2\% Inorganic Preservative** & \(\theta_{0}\) & \(\theta_{e}\) \\ \hline Without preservative & 75.20 (8.03) & 39.25 (7.58) \\ CuSO\({}_{4}\).5H\({}_{2}\)O & 71.05 (9.61) d & 37.32 (7.86) d \\ ZnCl\({}_{2}\) & 96.43 (9.21) a & 46.45 (5.70) b \\ H\({}_{3}\)BO\({}_{3}\) & 62.54 (5.98) a & 40.06 (5.06) d \\ (NH\({}_{4}\))\({}_{2}\)SO\({}_{4}\) & 57.96 (5.47) a & 36.14 (9.61) d \\ \hline \hline \end{tabular} Notes: Mean values are from 18 measurements (6 different measuring spots using 3 replicates). Standard deviations are in parentheses. Duncan tests are documented by the indexes (a, b, d), comparing the contact angles \(\theta_{0}\) and \(\theta_{e}\) of the treated specimens to the control ones without preservative. (Significance of indexes: a—very significant change > 99.9%, b—significant change > 99%, d—significant change < 95%).  
\end{table} 
Table 1: Wettability of spruce wood surfaces treated with 2% inorganic wood preservatives—characterized by the \(\theta_{0}\) “initial contact angle” and the \(\theta_{e}\) “equilibrium contact angle”. Figure 2: Preparation of the micro-plate (6 mm \(\times\) 5 mm \(\times\) 5 mm) for microscopic analysis of the “wood—PUR” interphase. Lower values of contact angles indicate improved wettability and better glue penetration into the wood structure [57]. Following, the results of the Duncan tests can conclude that the wood wettability was not significantly affected only by CuSO\({}_{4}\)-SH\({}_{2}\)O. It was significantly and most apparently decreased by zinc chloride; however, such treated wood retained good wettability with polar water when \(\theta_{\rm e}\) did not exceed 47\({}^{\circ}\) (Table 1). This result also indicates, that due to the presence of ZnCl\({}_{2}\) in the wood, the strength of glued joints could be decreased (see Section 3.2). On the contrary, the wood wettability was significantly improved with ammonium sulfate (Table 1), and subsequently, the strength of the glued joints should not be more evidently worsened (see Section 3.2). Spruce wood treated with H\({}_{3}\)BO\({}_{3}\) had an \"initial\" contact angle about 16.8% lower than the reference, and an \"equilibrium\" contact angle already higher about 2.1% than the reference wood.