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.