## 3 Results and Discussion
### Strength of joints \"Spruce Wood--PUR Glue\" Effected by Inorganic Preservatives
The Duncan test for the second group of test sets (created by bonding two chemically treated spruce wood specimens) proved that the inorganic preservatives localized between the layers of PUR glue in both specimens have a statistically significant effect on a decrease in the strength of glued joints--more significantly in comparison to the first group of test sets (Table 2). Specifically, in the case of a 2% concentration of preservatives, there was a decrease in the strength of glued joints when using biocides CuSO\({}_{4}\cdot\)5H\({}_{2}\)O, ZnCl\({}_{2}\) and H\({}_{3}\)BO\({}_{3}\) at a level of significance of 99.9%; in the case of fire retardant (NH\({}_{4}\))\({}_{2}\)SO\({}_{4}\) the decrease was at a level of significance of 99%. A negative effect of this fire retardant on the strength of the glued joint was apparently more modest compared to biocides, and also in the case of the use of lower 0.5 and 1% concentrations. The test sets treated with 2% ZnCl  
Figure 3: Linear dependencies between the shear strength of glued joints (\(\sigma\)) and the concentration of a preservative (c) in the first group of test sets “Treated spruce and Untreated spruce”. showed the most significant decrease in the strength of the glued joint to 8.44 MPa, i.e., by 20.83%. The strength of the glued joint under 9 MPa was shown for all other test sets consisting of two spruce wood specimens treated with 2% solutions of CuSO\({}_{4}\cdot\)5H\({}_{2}\)O (by 17.64%), H\({}_{3}\)BO\({}_{3}\) (by 18.57%), and (NH\({}_{4}\))\({}_{2}\)SO\({}_{4}\) (by 17.17%). In general, it can be stated that the quality of adhesion joints between PUR glue and spruce wood specimens was negatively influenced by the primary treatment of specimens with inorganic preservatives CuSO\({}_{4}\cdot\)5H\({}_{2}\)O, ZnCl\({}_{2}\), H\({}_{3}\)BO\({}_{3}\), and (NH\({}_{4}\))\({}_{2}\)SO\({}_{4}\). The minimum strength values given by the standard EN 204 [62] (minimum value 10 MPa) were not met in the case of most test sets prepared from the chemically treated specimens (Figures 3 and 4). Similar results were also mentioned by [PERSON] et al. [63] and [PERSON] [30], whereby a decrease in the strength of glued joints was explained by a reduced penetration of an adhesive into the lumens and intercellular spaces in wood and reduced adhesion. Due to impregnating the wood with preservatives with a low pH-value, the lignin-saccharide matrix in wood can degrade and individual wood cells can be defibrated which results in the loss of wood mechanical properties [64; 65]. Subsequently, a decrease in the mechanical properties of wood can affect the strength of glued joints. It was stated by [PERSON], [PERSON] [66]. In our study, the most significant decrease in the strength of glued joints, by 17% to as much as 21%, occurred in the case when both specimens of spruce wood were treated with the highest 2% concentration of inorganic preservatives used. Figure 4: Linear dependencies between the shear strength of glued joints (\(\sigma\)) and the concentration of a preservative (c) in the second group of test sets “Treated spruce and Treated spruce”.