A new soy flour-based adhesive for making interior type II plywood

In this study, we developed a formaldehyde-free adhesive from abundant, renewable, and inexpensive soy flour (SF). The main ingredients of this adhesive included SF, polyethylenimine (PEI), and maleic anhydride (MA). The optimum formulation of this adhesive and the optimum hot-press conditions for making plywood were investigated. A three-cycle soak test and a boiling water test (BWT) were employed for evaluating the strength and water-resistance of plywood bonded with this adhesive. Results showed that SF, PEI, MA and sodium hydroxide were all essential components for the adhesive and the SF/PEI/MA weight ratio of 7/1.0/0.32 resulted in the highest water-resistance. When the hot-press temperature was in the range of 140-170 degrees C, both water-resistance and shear strength of plywood bonded with the adhesive remained statistically the same, except that the dry shear strength of plywood at 170 degrees C was statistically lower than that at 160 degrees C. When the hot-press time ranged from 2 to 6 min, the plywood panels at 5 min had the highest boiling water test/wet (BWT/w) shear strength. The plywood panels made at 5 min had a higher dry shear strength than those made at 3 min. Plywood panels bonded with this SF/PEI/MA adhesive exceeded the requirements for interior applications.