# Bonding of Selected Hardwoods with PVAc Adhesive
## 3 Results and Discussion
### Connections between Bonding and Selected Characteristics of Hardwoods  
The adhesion strengths of 13 bonded hardwoods, valued in the dry state, were positively influenced--on the 99% or 95% significance level (\(p<0.01\); \(p<0.05\))--by their higher densities \(\rho\), lower roughness parameters \(Ra\) and \(Rz\) perpendicular to the grain, and lower water contact angles \(\theta_{0}\), \(\theta_{\mathrm{e}}\), and \(\theta_{\mathrm{w}}\), as was documented by the linear correlations (Adhesion = a + b \(\times\) Property of wood) (Table 5 and Figure 4a,d-f).  
On the contrary, the adhesion strengths of the bonded hardwoods were not significantly influenced by the cold water extract, pH value, or roughness parameters \(Ra\), \(Rz\), and \(R_{Sm}\) parallel with the grain (\(p>0.5\)) (Table 5 and Figure 4b,c).  
Generally, the joint quality of bonded timbers depends on several surface characteristics of the wood, including its wetting capacity. For example, beech and bubinga woods were characterized with good wettability values (Table 3), and this property resulted in a positive impact on their adhesion strength with a PVAc adhesive in the dry state (Table 4 and Figures 3a and 4e,f), as well as in the wet state and reconditioned state (Table 4). However, the wettability may not always be a presuming factor. For example, a good bonding quality in the dry state was also determined for ipe, iroko, sapelli, and wenge woods, whose surfaces had evidently higher water contact angles.  
\begin{table}
\begin{tabular}{l c c c c c} \hline \hline
**Property of Hardwood** & **N** & **R\({}^{2}\)** & **T** & **P** & **Adhesion = a + b x Property** \\ \hline Density \(\rho\) (kg/m\({}^{3}\)) & 78 & 0.086 & 2.68 & 0.009 & 8.36 + 0.007 \(\times\)\(\rho\) \\ Cold water extract (\%) & 39 & 0.004 & 0.39 & 0.699 & 12.94 + 0.16 x extract \\ pH & 39 & 0.040 & 1.24 & 0.224 & 10.06 + 0.71 \(\times\) pH \\ Roughness parallel with grain & & & & \\ (\(\mu\)m) & & & & \\ \(Ra\) & 78 & 0.027 & \(-\)1.47 & 0.147 & 14.22–0.097 \(\times\)\(Ra\) \\ \(Rz\) & 78 & 0.015 & \(-\)1.06 & 0.292 & 14.14–0.012 \(\times\)\(Rz\) \\ \(R_{Sm}\) & 78 & 0.039 & 1.75 & 0.085 & 12.43 + 0.002 \(\times\)\(R_{Sm}\) \\ Roughness perpendicular to grain & & & & \\ (\(\mu\)m) & & & & \\ \(Ra\) & 78 & 0.071 & \(-\)2.41 & 0.018 & 14.56–0.087 \(\times\)\(Ra\) \\ \(Rz\) & 78 & 0.081 & \(-\)2.59 & 0.011 & 15.05–0.016 \(\times\)\(Rz\) \\ \(R_{Sm}\) & 78 & 0.006 & \(-\)0.68 & 0.498 & 14.03–0.001 \(\times\)\(R_{Sm}\) \\ Contact angle (\({}^{\circ}\)) & & & & \\ \(\theta_{0}\) & 78 & 0.11 & \(-\)3.03 & 0.003 & 17.52–0.044 \(\times\)\(\theta_{0}\) \\ \(\theta_{e}\) & 78 & 0.10 & \(-\)2.94 & 0.004 & 15.62–0.051 \(\times\)\(\theta_{e}\) \\ \(\theta_{w}\) & 78 & 0.13 & \(-\)3.40 & 0.001 & 15.82–0.043 \(\times\)\(\theta_{w}\) \\ \hline \hline \end{tabular} Note: N is the number of samples. However, several times more measurements were performed and by linear correlations analyzed for the roughness parameters and the contact angles, as for each individual sample, the roughness parameters were determined on 15 spots and the contact angles on 5 spots (see Tables 2 and 3).  
\end{table}
Table 5: Linear correlation analyses between the hardwood characteristics or properties and the adhesion strength, determined for the wood–PVAc adhesive–wood interface in the dry state.  
Figure 4: _Cont._