Enzymatic hydrolysis-assisted protein adhesive with enhanced strength and toughness via polyamide-polyurethane resin integration

To develop a high-strength and high-toughness wood adhesive derived from high-temperature soy protein meal (HSM), a low-cost industrial byproduct, and addressing the inherent challenges of HSM protein aggregation, poor dispersion, weak intermolecular interactions, and incomplete crosslinking systems, a polyurethane (PU)-blocked polyamidoamine-epichlorohydrin (PAE) resin featuring a microphase-separated structure and cationic high-reactivity sites was self-synthesized. This resin was then used to co-modify the HSM-based adhesive with the assistance of protein hydrolysis enzymes. On the one hand, the enzymatic reaction significantly unfolded aggregated HSM protein molecules, exposing more active groups and facilitating more efficient and stable crosslinking of proteins with other components. On the other hand, the PU-blocked PAE resin (PUE) retained highly reactive cationic azetidinium, enabling effective covalent and non-covalent interactions, while the microphase-separated structure imparted by the PU segments further facilitated rapid stress transfer and dissipation during the curing process, enhancing the toughness of the cured adhesive layer. This enhanced resin strengthened the water resistance and bonding strength of the HU3 sample (PUE solution was 20% of HSM protein), reaching a wet bonding strength of 1.21 MPa, with an increase of 706.67% compared to the enzymatic control sample. Additionally, this sample demonstrated exceptional toughness, with a 200% increase in the work of adhesion compared to the control. As a result, the PUE-modified protein adhesive exhibited both superior bonding strength and excellent toughness, demonstrating significant potential for high-performance, protein-based adhesive applications in wood-based materials. • A protein adhesive with enhanced strength and toughness was developed. • Enzymatic reaction broken down aggregated HSM protein molecules. • Polyurethane-block-modified polyamidoamine-epichlorohydrin resin was prepared. • Modified adhesives have practical application potential.