Effect of tannin on the bonding performance and mildew resistance of soybean meal-based adhesive

For improving the water resistance, bonding performance, and anti-mildew property of the soy protein-based adhesive (SPA), in this research, based on cross-linking enhancement theory, soybean meal flour as the main raw material, self-synthesizing triglycidylamine (TGA), and larch tannin (LT) were used to build efficient crosslinking water-resistant structure and develop SPA. The effect of LT addition on the physical properties, bonding strength, and anti-mildew property of the resultant adhesive was investigated. The results showed that the residual rate of SPA with 4 % LT and 3 % TGA was improved by 2.1 % and the corresponding moisture absorption rate was reduced by 20.1 % compared with the SPA only modified by 3 % TGA. In addition, the SPA with 4 % LT and 3 % TGA had a 42.3 % increase in bonding strength, reaching 1.11 MPa, which satisfied the standard requirements of interior use plywood (≥ 0.7 MPa). The improved water-resistant bonding performance is mainly because interaction forces, such as hydrogen bonds, ionic bonds, and covalent bonds, formed between tannin and protein, and under the cross-linking effect of TGA, the whole cross-linking system forms a dense network structure. LT not only had a reinforcing effect on the SPA but also could prolong the pot life of the adhesive for 60 h due to the phenolic hydroxyl groups of tannin. Compared with the commercial anti-mildew agents, the mildew resistance before and after curing is the following: 1,2-benzisothiazole in-3-one > LT > sodium tetraborate > potassium sorbate.