Development and investigation of a two-component adhesive composed of soybean flour and sugar solution for plywood manufacturing

In this study, soybean flour (SF) and various sugar solutions were utilized as raw materials to prepare eco-friendly plywood adhesives. First, the optimal preparation conditions were investigated by testing the bond performance. When SF was mixed with synthesized sucrose-ammonium dihydrogen phosphate (SADP) solution with 50% solid content in a 1: 4 mass proportion, the wet shear strength of the plywood achieved a maximum value of 0.83 MPa. This value exceeds the minimum limit of China National Standard GB/T 9846-2015. Second, the curing behavior of the SF/SADP adhesive was studied by measuring the insoluble mass proportion and thermal analysis. The results showed that the curing reaction of this adhesive occurred near 170 °C. In addition, the curing mechanism was studied by ATR FT-IR and Py-GC/MS analysis. Based on the results, it was concluded that the cured adhesive formed a network structure via a polycondensation reaction between small-molecule compounds derived from the SADP solution and the products of the caramelization and soy flour. Dimethylene ether bridges were the main linkages of the final polymer. SEM microstructure analysis showed that the cured SF/SADP adhesive exhibited a more compact crosslinked structure than cured SADP.