Polyurethane as high-functionality crosslinker for constructing thermally driven dual-crosslinking plant protein adhesion system with integrated strength and ductility

Wood products have gained considerable interest for use in furniture and construction materials; however, adhesives for bonding wood components are mainly formaldehyde-based resins, which release hazardous substances and can cause serious health concerns during service. The development of plant protein adhesives is in high demand to satisfy requirements for green wood-based composites and stringent environmental regulations. However, their practical application is hindered by the trade-off between high mechanical strength and good adhesion toughness. Herein, a biomimetic branched polyurethane is synthesised initially using a blocking agent of dopamine and sodium bisulfite to form a thermally driven dual-functionalised elastomer (PUSD). The prepared PUSD is employed as a novel dual-crosslinker for the improvement of soy protein (SP) adhesives. Because of the deblocking effect of sodium bisulfite under the thermal stimulus during hot pressing, the isocyanate groups are exposed and further cooperates with catechol groups to induce the construction of a thermally responsive dual-crosslinking network within the SP system. These structures not only promote the optimisation of the crosslinking density for a rigid system but also improve the intermolecular interaction patterns, thereby achieving effective energy dissipation. Consequently, the modified SP adhesives exhibit an integrated improvement in adhesive strength, adhesion toughness, and water resistance compared to the untreated adhesive. The outcome of this work enables the utilisation of plant proteins instead of traditional wood adhesives, thus addressing health and environmental concerns.