Improved Adhesion Performance of Soy Protein-Based Adhesives with a Larch Tannin-Based Resin

This study aimed to improve the bonding strength and water resistance of soy protein-based adhesives (SPAs) by modifying with larch tannin-based resins (TRs). This is especially important because of their eco-beneficial effects. The TR was characterized by Fourier Transform Infrared (FTIR) and Thermogravimetric/Derivative Thermogravimetric (TG/DTG) in order to demonstrate the formation of the self-crosslinking structure. Rheological properties, fracture morphology, solubility, and crosslinking density were characterized in detail. Three-ply poplar plywood was fabricated and the wet shear strength was measured. The experimental data showed that the addition of TR improved the moisture uptake, residual rate, and shear strength of SPA. This improvement was attributed to the crosslink reactions of TR with the relevant active functional groups of the side chains of soy protein molecules. The crosslinking structure joined with the TR self-crosslinking structure to form an interpenetrating network, which promoted a uniform and compact cured structure. The 5 wt % TR additions in the SPA was found to yield optimum results by improving the wet shear strength of the plywood by 105.4% to 1.13 MPa, which meets the interior-use plywood requirement. Therefore, the larch tannin could be applied in the modification of soy protein adhesive.