# Bonding of Selected Hardwoods with PVAc Adhesive
## 3 Results and Discussion
### Adhesion of Hardwoods with PVAc Adhesive  
The values of the adhesion strength (the tensile shear strength of the lap joints) of hardwood specimens bonded with the PVAc Multibond SK8 adhesive are shown in Table 4 and Figure 3.  
The Multibond SK8 adhesive was as a good glue type for the bonding of several hardwoods exposed in dry and water-soaked conditions. This type of PVAc adhesive, for all 13 bonded hardwoods, usually secured the minimum adhesion strengths required by the standard EN 204 [47] (i.e., on average, (a) in the dry state of 13.6 MPa, from 9.5 MPa for bangkirai to 17.2 MPa for bubinga (required minimum = 10 MPa), (b) in the wet state of  
Figure 3: The growth tendency of the adhesion strength from bangkirai (No. 1) to bubinga (No. 13) determined in the dry state of bonded hardwoods (**a**) unequally changed for the individual wood species only in the wet state (**b**), based on the evidently decreased parameter R\({}^{2}\) and reduced significance \(p\) (from 99.9% to 99%) of the linear correlation Adhesion = \(a\) + \(b\)\(\times\) Number of Hardwood, while this tendency returned in the reconditioned state (**c**).  
1.8 MPa, from 0.6 MPa for macroanduba to 2.6 MPa for beech (required minimum = 2 MPa), and (c) in the reconditioned state of 13.0 MPa, from 8.5 MPa for macroanduba to 19.2 MPa for bubinga (required minimum = 8 MPa)) (Table 4, Figure 3).  
In the summary evaluation, using a linear correlation Adhesion = a + b \(\times\) Number of Hardwood (for the numbering of hardwoods, see Figure 1), the adhesion strength of 13 bonded hardwoods determined in the dry state (Figure 3a) unequally decreased in the wet state due to 4 days of soaking in water (Figure 3b). This was based on the coefficient of determination R\({}^{2}\) declining from 0.6 to 0.06, and on the change of the significance level parameter \(p\) from 0.000 to 0.021. However, due to 7 days of reconditioning of the wet samples in a dry environment, the adhesion strength recovered quite equally to the initial values found in the dry state, which was in the linear correlation confirmed by the increasing of R\({}^{2}\) to 0.21 and with \(p\) equal to 0.000 (Figure 3c).  
Partially worsened results were achieved in the wet state of the bonded hardwoods, when the adhesion strength decreased in more cases under the criteria value of 2 MPa (i.e., to 0.6 MPa-2.6 MPa) (Table 4, Figure 3), which could be explained, among other factors, with a different penetration of the PVAc adhesive into the individual hardwood species. For example, the lowest adhesion of 0.6 MPa was determined for the macroanduba wood, the densest species (Table 1) having the lowest roughness (Table 3). On the contrary, the second-highest adhesion of 2.3 MPa was determined for the mernati wood, the least dense species (Table 1) having the highest roughness (Table 3). The highest adhesion of 2.6 MPa was determined for the beech wood (i.e., the species characterized by very good permeability [60; 61]). Generally, different microstructures of the wood-adhesive-wood interfaces, when a probable better penetration of adhesives into less dense and more porous woods, as well as into more permeable woods, could be connected with a higher and more water-stable mechanical adhesion of adhesives with wood surfaces.  
Results achieved in this work for the bonded beech wood were in accordance with the work of [PERSON] and [PERSON] [62]. For this wood species, bonded it at 23 \({}^{\circ}\)C with the PVAC glue Vinavil 2259 L, they determined by the standard EN 205 [46] the adhesion strength in the dry state 15.8 MPa and in the wet state 2.7 MPa.