Constructing a green modifier by using glyoxal-urea resin and chitosan to obtain a modified soy protein adhesive with high bonding strength and excellent water resistance

The manufacturing of soy-based adhesives with high bonding strength, excellent water resistance, and exceptional environmental performance still faces difficulties. In this work, using glyoxal-urea (GU) resin, chitosan (CS), and soy protein isolate (SPI) as the primary raw materials in order to effectively mitigate the release of free formaldehyde commonly found in traditional wood-based panels. Obtaining an adhesive with high strength, excellent water resistance, and a stable cross-linking structure of GU/CS_m/SPI (CS_m represents different mass fractions of chitosan solution). The adhesive's structure, properties, and bonding mechanism were comprehensively investigated. The results demonstrated that the modified soy protein adhesive exhibited variations in solid content, viscosity, and contact angle, with viscosity showing the most significant change. Combined with ¹³C NMR and FTIR analysis findings, it can be inferred that a compact network structure was formed by GU resin, CS, and SPI. When the mass ratio of GU/CS/Distilled water with acetic acid/SPI solution was 50/0.5/9.5/130 (GU/CS₅/SPI), the modified soy protein adhesive displayed improved performance and higher storage modulus after curing. The plywood prepared at a hot pressing temperature of 180 °C achieved cold water strength of 2.14 MPa, hot water strength of 1.47 MPa, and boiling water strength of 1.21 MPa respectively; all exceeding the requirements specified in national standard GB/T 9846-2015 for Class II plywood. This high bonding strength and water resistant soy-based adhesive, providing a new approach to the development of environmentally friendly adhesives for commercial use.