A Soybean Meal-Based Adhesive with Improved Hot-Pressing Efficiency and Water Resistance Fabricated from Polyamidoamine-Epichlorohydrin, Low-Cost Melamine-Urea-Glyoxal, and Blocked Polyurethane Resins

Abstract Soybean meal (SM), an agricultural byproduct, exhibits promising renewable resources for developing biobased adhesives. However, its undesirable hot-pressing efficiency and poor water resistance restricted industrial applications of SM and SM-based materials. Herein, an efficient SM adhesive was fabricated by incorporating blocked polyurethane (BPU) as a reinforcement into the cocrosslinked polyamidoamine-epichlorohydrin (PAE) with melamine-urea-glyoxal (MUG) resin. The addition protocol of epichlorohydrin (ECH) tailored the PAE structure by avoiding ECH hydrolysis, by which a PAE resin (PAEc) with promoted cocrosslinking efficiency was obtained. Adding BPU resin into the cocrosslinked SM adhesive formed denser multicrosslinked networks, which increased the hot-pressing efficiency of plywood production. The optimal SM/PAEc/MUG30/BPU5 adhesive exhibited a 12.5% lower hot-pressing temperature of 105 °C, a 20% shorter hot-pressing time of 4 min, and a wet shear strength of 0.97 MPa. Therefore, this work provides a potential strategy for valorizing agricultural byproduct SM into sustainable biomaterials.