Development of bio-based hempseed protein adhesives with enhanced properties and performance via glyoxal and zinc chloride modification

Formaldehyde-based adhesives currently dominate the global market, representing over 70 % of the total adhesive usage. However, these synthetic adhesives pose significant health concerns and environmental concerns. In response, this study explores hempseed protein as a viable alternative for creating bio-based adhesives suitable for plywood applications. Hempseed flour was defatted and processed to extract proteins. Adhesive slurries were prepared with 15 % protein concentration through pH adjustment and controlled mixing. The study employed chemical modifiers such as glyoxal and zinc chloride (ZnCl₂), both individually and in combination, to enhance the adhesive's strength, thermal stability, and water resistance. Adhesion tests on cherry wood veneers revealed that all modifiers, particularly glyoxal, significantly improved the dry, wet, and soaked strength, achieving maximum values of 5.69 MPa, 2.68 MPa, and 4.91 MPa, respectively. ZnCl₂ contributed to performance enhancements through ionic crosslinking, yielding moderate improvements in strength and stability. However, combined treatments showed limited synergy effects between covalent and ionic crosslinking mechanisms. Glyoxal notably enhanced thermal stability with onset and denaturation temperatures increasing to 146.34 °C and 147.74 °C, respectively. SEM revealed that glyoxal produced denser, more compact adhesive networks, whereas ZnCl₂ led to more moderate structural improvements. These findings demonstrated the potential of glyoxal-modified hempseed protein adhesives for engineered wood applications, offering a promising avenue toward sustainable technologies. Furthermore, ZnCl₂ allows for flexible formulations, enhancing the overall utility of these bio-based adhesives.