## 3 Results and Discussion
### Physicochemical and Electrical Properties of PVAc-Bioadditive IFR Adhesive Blends
#### 3.3.2. Delamination of Wood Joints and Penetration of Modified Commercial PVAc  
Adhesive Using IFR Bioadditive  
Delamination is a defect in the adhesive bond caused by the weakening of the glue line due to aging, exposure to moisture and temperature, or factors such as inadequate substrate preparation, grammage, properties of the adhesive resin, and incompatibility of the adhesive with the conditions of use [79].  
The results of the average delamination in wood specimens glued with PVAc-bioadditive IFR adhesive blends (Figure 5a) showed variations between 0% and 70%. The pure PVAc adhesive presented the lowest delamination (0%), followed by the PVAc-2.5% F1 (3%) and PVAc-5% F2 (5%) adhesive blends, meeting the requirement \(\leq\) 5%, according to ISO 12580. Based on the statistical analysis, no statistically significant differences were found between these samples.  
According to the results obtained by the shear test and delamination evaluation, five formulations with the highest performance concerning bond line quality were selected to measure the adhesive penetration and fire resistance, which were: PVAc-2.5% F1; PVAc-12.5% F1; PVAc-2.5% F2; PVAc-5% F2; PVAc-12.5% F2, and as a comparative base, the standard PVAc adhesive.  
It was found that the effective penetration depth (EP) and average penetration depth (AP) values ranged between 18.80 and 73.25 \(\upmu\)m and 23.11-197.95 \(\upmu\)m, respectively. The highest EP and AP values were observed in joints with standard PVAc adhesive, followed by PVAc-5% F2 and PVAc-2.5% F1 adhesive blends, meeting the ISO 12580 standard (delamination \(\leq\) 5%). On the other hand, the lowest penetrations and highest delamination (68% and 62%) occurred in joints with PVAc-12.5% F1 and PVAc-12.5% F2 (Figure 5b).  
The low penetration depth in adhesive blends with higher bioadditive concentration could be due to their higher viscosity, as in the 12.5% F2 formulation (8106.67 cP) compared  
Figure 5: Quality of the bonding line: (**a**) Delamination (%) of all glued-laminated timber joints; (**b**) Penetration (\(\upmu\)m), of the five selected PVAc-bioadditive IFR adhesive blends.  
to the standard adhesive (5823.33 cP). This is reflected in the morphology of the glue lines: the PVAc standard adhesive showed higher penetration into the substrate (Figure 6a), while PVAc-12.5% F2 showed the lowest penetration (Figure 6b).  
A greater penetration of the adhesive into the pore spaces of the wood such as lumens and cell walls favors the formation of solid bonds between the resin and the substrate, which improves the quality and strength of the adhesive bond [80]. Adhesive penetration depends on factors such as resin properties, wood characteristics, and gluing process conditions [81]. Adhesive viscosity is key: high viscosity limits penetration, while low viscosity can cause over-penetration, affecting the adhesive line's quality and the bond's mechanical performance [82; 83]. In addition, high concentrations of bioadditive result in stiff but brittle bonds, with weak adhesion that can result in significant delamination.