# Bonding of Selected Hardwoods with PVAc Adhesive
## 3 Results and Discussion
### Density, Cold Water Extract, pH Value, Roughness, and Wettability of Hardwoods
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Wood Species**} & \multirow{2}{*}{**Scientific Name**} & \multicolumn{2}{c}{**Density (kg/m\({}^{3}\))**} & \multicolumn{2}{c}{**Cold Water Extract (\%)**} & \multicolumn{1}{c}{**pH**} \\ \cline{3-6}  & & **EN 350 [54]** & **Obtain** & & \\ \hline Bangkirai & _Shorea obtusa_ & 700-930-1150 & 834 & 2.70 & 3.82 \\ Beech & _Fagus sylvatica_ & 690-710-750 & 705 & 2.13 & 5.79 \\ Black locust & _Robinia_ & 720-740-800 & 726 & 4.56 & 4.65 \\  & _Pseudoacacia_ & & & & \\ Bubinga & _Guibourtia_ & 700-830-910 & 887 & 3.15 & 4.12 \\  & _deneusii_ & 900-1050-1150 1 & 957 & 2.08 & 3.94 \\ Ipe & _Nilicia excels_ & 630-650-670 & 641 & 3.51 & 5.60 \\  & _Manilkara_ & 1000-1100-1150 1 & 1105 & 3.66 & 4.60 \\ Macaranduba & _Sborea curtisii_ & 600-680-730 & 636 & 1.55 & 3.38 \\ Oak & _Quercus robur_ & 670-710-760 & 779 & 4.33 & 3.83 \\ Palisander & _Macharium_ & 700-900-1000 1 & 818 & 4.21 & 5.84 \\ Sapelli & _Entandrophraga_ & 640-650-700 & 693 & 1.62 & 5.23 \\  & _cylindricalcium_ & 780-830-900 & 881 & 1.53 & 4.32 \\ Zebrano & _Microbrilia_ & 700-770-850 1 & 777 & 0.91 & 5.62 \\ \hline \hline \end{tabular} Notes: Mean values of density are from six measurements, and the cold water extract and pH value are from three measurements. 1 By [PERSON] [55].  
\end{table} 
Table 1: Densities, cold water extracts and pH values of the hardwoods. The roughness parameters \(Ra\) and \(Rz\) determined for 13 hardwoods exhibited, on average, 48.2% and 79.1% higher values, respectively, measured perpendicularly to the grains (on average: \(Ra=10.4\)\(\mu\)m, \(Rz=85.6\)\(\mu\)m) than those determined parallel with the grains (on average: \(Ra=7.0\)\(\mu\)m, \(Rz=47.8\)\(\mu\)m). Generally, these represented differences from 8% to 114%, according to the relevant wood species (Table 2). However, the parameter \(R_{Sm}\) was approximately comparable in both measured directions of the wood (Table 2). The surfaces of the densest macaranduba wood were characterized by the lowest roughness parameters \(Ra\) and \(Rz\). On the contrary, the highest roughness parameters were exhibited by the least dense wood, the meranti wood, but at the same time by the bangkirai and oak woods also, which had a medium density (Tables 1 and 2). In these circumstances, an unexpectedly higher surface roughness of the bangkirai and oak, belonging to the ring-porous wood species [55], can be explained by their naturally more porous morphological structure. The water's potential for surface wetting of the tested hardwood species was assessed based on the water contact angles \(\theta_{0}\), \(\theta_{e}\), and \(\theta_{w}\) (Table 3). Iroko had the highest initial and abstract contact angles (\(\theta_{0}=112.5^{\circ}\), \(\theta_{w}=78.5^{\circ}\)), and black locust had the lowest equilibrium and abstract contact angles (\(\theta_{e}=19.7^{\circ}\), \(\theta_{w}=25.4^{\circ}\)), while bubinga had the lowest initial contact angle (\(\theta_{0}=54.8^{\circ}\)) (Table 3). According to [PERSON] et al. [56], the values of the contact angles \(\theta_{0}\) and \(\theta_{e}\) depend on the wood surface roughness and chemistry. These authors supposed that the calculated contact angle value \(\theta_{w}\), valid for an ideal smooth surface, depends exclusively on the wood chemistry. Consequently, the different values of the contact angle \(\theta_{w}\) in Table 3 indicate species-related differences in the wood surface chemistry. This concerns not only the different types and contents of extractives, but also the chemical composition of the lignin and polysaccharides in tested hardwoods. The wettability of wood is a substantial parameter which gives basic information on the interaction between the solid wood surface and liquids, such as adhesives and paints (e.g., how easily and efficiently the liquids spread over a solid surface) [57; 58].