Developing an antifungal and high-strength soy protein-based adhesive modified by lignin-based polymer

Soy protein adhesive is readily deteriorated by microorganism, which greatly limits its practical application in the plywood industry. In this research, a soybean meal (SM)-based adhesive with excellent antifungal property, thermal stability and bonding strength was successfully prepared using epoxy polymer of lignin (EPL) as a crosslinking agent. The results confirmed that the lignin-based polymer can notably improve the antifungal ability of a soy-based adhesive. The initial microbial attack of SM/EPL adhesive was better than that of pure SM adhesive (on 3rd day), delaying to 22th day under high humidity conditions. Moreover, the fungal growth on the modified SM adhesives was Aspergillus flavus. The mycelia only existed on the surface of the modified SM adhesives, but not observed inside, and the modified SM adhesives had no significant structural change, and still exhibited a good thermal stability. Furthermore, no microbial attack observed on the surface and glueline of plywood bonded by modified SM adhesives. These results may be due to its phenolic hydroxyl groups and dense network structure hindering the erosion of fungi. On the other hand, the wet shear strength of SM/EPL adhesive reached 1.02 MPa, which was a 363.6 % increase compared with the neat SM adhesive (0.22 MPa). Also, the adhesion strength still met the requirements of interior-use plywood (≥ 0.7 MPa) after being treated for a month. This work not only highlights the application potential of lignin-based polymer as an effective antifungal material, but also broadens the application of soy-based adhesives.