Amphoteric waterborne polyurethane with unsaturated carboxyl group as crosslinker to improve cohesive strength of soybean protein-based adhesive for excellent cold-pressing adhesion

The cold-pressing performance of adhesives is important for the preparation of wood-based panels. Adhesives for wood-based panels must have appropriate viscosity, strong interfacial adhesion to wood substrates with hy-dration layers, and excellent cohesive strength at ambient temperature for cold-pressing adhesion. However, bio-based adhesives synthesized using conventional modification approaches still struggle to meet these standards. Herein, an amphoteric waterborne polyurethane with an unsaturated carboxyl group (AWPU-UC) is prepared as a crosslinker to modify soybean meal (SM) matrix, and prepare a soybean protein-based adhesive with excellent cold-pressing adhesion and mildew resistance. The unsaturated carboxyl groups form electrostatic interactions with tertiary ammonium groups of polyurethane and amino groups of protein molecules in the SM matrix. These bonds enhance both the cohesion of adhesive molecular network and the interaction between the adhesive and wood surface with "hydration layers". Consequently, the cold-pressing bonding strength of the adhesives SM/ AWPU-UC-4 reaches 271% of that of pure SM adhesive. In addition, the quaternary ammonium structure formed by electrostatic interactions in the AWPU-UC system endows the adhesive with mildew resistance. This study provides a simple and effective method for producing high-performance protein-based adhesives with outstanding cold-pressing adhesion and mildew resistance.