A sustainable bio-based adhesive derived from defatted soy flour and epichlorohydrin

This study reports on the preparation and application of bio-based adhesive derived from renewable defatted soy flour (DSF) and epichlorohydrin (ECH), aiming at substituting formaldehyde-based adhesives in the wood industry. DSF was treated with Viscozyme® L to hydrolyze plant cell wall carbohydrates and then adjusted to an extremely low pH with hydrochloric acid containing ferric chloride to unfold the tertiary and/or quaternary structure of soy protein and expose its functional groups. The treated DSF was then glycidized with ECH to obtain the bio-based adhesive. The resulting adhesive contained epoxy groups, which were evidenced by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and 1H nuclear magnetic resonance. After curing, the epoxy groups contributed to good thermal stability and gluability of the adhesive. The grafting ratio of ECH decreased by 37.8% as the mass fraction added was increased from 5 to 25%, whereas the viscosity increased sharply by 2889.5% and the solid content increased slowly by 10.5%. Wet shear strength of Pinus massoniana plywood bonded with the bio-based adhesive with an ECH mass fraction of > 10% of the DSF mass was higher than 0.93 MPa, which met the requirements of the Chinese National Standard for plywood for interior/exterior applications. These results support the use of soy flour-based adhesives for plywood and provide data for evaluating their use as sustainable materials for other applications.