Dry and wet shear adhesion strength of OSA-modified SPA sam-ples are summarized in Table 2. The dry shear strength of soy protein was not significantly affected by OSA; all samples displayed 100% WCF. The wet strength of SPA was considerably improved at OSA concentration up to 3.5%, from 1.8 MPa to 3.1 MPa, then
leveled off as OSA content increased further. Partial WCF was also observed in terms of that the fiber was pulled out from the glued wood surface at OSA concentration higher than 2% OSA could be grafted onto some soy protein functional groups, such as NH2 and OH, through succinylation reaction, thus inducing protein conformation changes that would increase contact surface area of protein with wood substrate, which should be beneficial to the protein adhesion strength. Meanwhile, succinylation not only generated newly formed hydrophilic carboxyl groups, but also introduced the hydrophobic alkyl long chains to the protein complex, which enhanced the hydrophobic interactions through reacting with hydrophobic groups within the protein. Therefore, it is possible that the increased hydrophilic carboxyl groups’ detriment to water resistance might be negligible. In addition, OSA is an oil-like material in nature; it cannot dissolve in water. When the assembled wood sample was soaked in water, the hydrophilic groups of protein dissolved into the water and cavities were generated between the protein and wood surface, thus weakening the adhesive strength. On the other hand, when SPA was modified with OSA, the introduced hydrophobic groups to protein molecules and the hydrophobic nature of OSA prevented the water from penetrating the interfacial surface of adhesive and wood surface, thus hindering the formation of hollow cavities. This could be the main reason for significant improvement of water resistance of SPA samples.
The OSA-modified SPA samples lost flowability (continuous protein phase) when OSA concentration was higher than 3.5%. At this point, the maximum electrostatic interaction around the isoelectric point and enhanced hydrophobic force could promote the formation of larger-size protein aggregates; as a result, the wetting ability between protein and wood surface would decrease. Poor wetting ability counteracted increased protein-protein interactions, leading to limited improvement in wet strength as OSA increased from 3.5% to 6.5%