## 3 Results and Discussion
### Physicochemical and Electrical Properties of PVAc-Bioadditive IFR Adhesive Blends
#### 3.3.1 Shear Strength Test  
Table 7 presents the results of shear strength and wood failure under normal, elevated temperature, and three-cycle conditions compared to the requirements of ASTM D5751. In dry conditions, the shear strength of specimens bonded with PVAc-bioadditive IFR adhesive blends ranged from 6.77 MPa to 12.21 MPa. The PVAc-5% F2, PVAc-2.5% F2, and PVAc-12.5% F2 blends exhibited the highest strengths, outperforming the standard PVAc adhesive by 22%, 16%, and 4%, and exceeding the 6.69 MPa requirements of ASTM D5751 by 83%, 73%, and 56%, respectively. Wood failure ranged from 55% to 92%, being higher in the same mixes, with increases of 10%, 8%, and 7% over the standard adhesive and exceeding the standard requirement by more than 200% (30%). The results indicate that the modified adhesive with IFR bioadditive provided similar or superior shear strength and wood failure to the standard adhesive, and it was significantly higher than the required by the standard. In particular, the F2 formulation at low concentrations (2.5% and 5%) showed significantly higher strength and better wood failure than the commercial PVAc adhesive. This suggests that less intervention in the commercial adhesive optimizes its mechanical properties. However, an excess of bioadditive can generate brittleness due to stresses, defects, or excess additives that exceed the reaction capacity of the polymer matrix [59]. The bioadditive with high concentrations of Mg(OH)\({}_{2}\) (formulation F2) significantly strengthened the adhesive system, in line with the study of [PERSON] et al. (2021) [62] on Mg(OH)\({}_{2}\)-reinforced PVAc. This showed improvements in shear load carrying capacity, reaching its maximum at 7.5 phr, outperforming the standard adhesive by 11%. However,  
\begin{table}
\begin{tabular}{c c c c c c c} \hline
**PVAc-** & \multicolumn{2}{c}{**Cured (dry) Condition**} & \multicolumn{2}{c}{**Elevated Temperature Condition**} & \multicolumn{2}{c}{**Three-Cycle Soak Condition**} \\ \cline{2-7}
**Bioadditive IFR** & **Shear Strength** & **Wood Failure** & **Shear Strength** & **Wood Failure** & **Shear Strength** & **Wood Failure** \\
**Adhesive Blends** & **(MPa)** & **(\%)** & **(MPa)** & **(\%)** & **(MPa)** & **(MPa)** \\ \hline PVAc & 10.01 \({}^{\text{bc}}\) \(\pm\) 1.72 & 84 \({}^{\text{bc}}\) \(\pm\) 16 & 5.21 \({}^{\text{a}}\)\(\pm\) 1.17 & 50 \({}^{  
higher concentrations (10 phr) reduced the reinforcing capacity due to inefficient interactions between Mg(OH)\({}_{2}\) and the polymer. This highlights the importance of determining specific limits in the modification of PVAc adhesives since small amounts of additive can achieve significant improvements, but excess amounts can impair their mechanical properties [76; 77]. Under elevated temperature conditions, the shear strength of the samples ranged from 5.21 MPa to 8.90 MPa. The PVAc-12.5% F1, PVAc-5% F2, PVAc-12.5% F2, and PVAc-2.5% F1 adhesive blends showed the highest strengths, exceeding the standard adhesive by 71%, 59%, 52%, and 31%, respectively. They also exceeded the ASTM D5751 requirements (4.46 MPa) by 100%, 86%, 76%, and 53%. As for wood failure, values ranged from 13% to 75%, with the highest values for PVAc-5% F2, which exceeded the standard by 50%, and PVAc-2.5% F1, which equaled the standard result. Samples PVAc-5% F2 and PVAc-2.5% F1 exceeded the standard requirement for wood failure (40%) by 88% and 25%, respectively, while samples PVAc-5% F1 and PVAc-12.5% F2 met the established limit. The IFR bioadditive, composed of a PM, Mg(OH)\({}_{2}\), and LS, improved the thermal stability and mechanical properties of the PVAc adhesive, especially in adhesive blends such as PVAc-5% F2. This increased the bond stiffness under high temperatures (104 \({}^{\circ}\)C for 6 h), exceeding the performance of the standard adhesive. However, this reinforcing effect may have reduced the ductility and penetration of the adhesive blend, resulting in rips lower than those required by the standard.