Glyoxal-mediated structural modification enhances interfacial adhesion of soy protein adhesive in plywood

Soy protein isolate (SPI) has emerged as a promising bio-based alternative to synthetic wood adhesives due to its renewability and environmental safety. However, its moderate bonding strength has limited its industrial application. In this study, glyoxal was employed as a green crosslinker to improve the adhesive performance of SPI and was compared with conventional formaldehyde-based formulations. Plywood samples were fabricated, and their bonding performance was assessed through dry shear strength testing. The glyoxal-crosslinked SPI adhesive exhibited a significantly higher dry strength of 1.11 MPa, compared to 0.68 MPa for uncrosslinked SPI and 0.66 MPa for SPI crosslinked with formaldehyde. The improvement in dry bonding performance highlights the potential of glyoxal as an effective and safer alternative to formaldehyde. Also, mechanistic insights into the crosslinking reactions were obtained using Fourier transform infrared (FTIR) spectroscopy, which confirmed the formation of covalent linkages between glyoxal and the functional groups of SPI. Interface morphology and failure behavior were further examined using light microscopy, revealing enhanced wood-adhesive interaction in the glyoxal system. Despite the improvement in water resistance is currently underway, our adhesive formulation demonstrated a simple synthesis route and relatively low-cost material production, even when high-protein content of SPI was used, supporting its potential for the further development of formaldehyde-free wood adhesives.