Effective strategy for preparing low-curing-temperature lignin-based non-isocyanate polyurethane wood adhesives

Non-isocyanate polyurethanes (NIPUs) have been identified as a bio-based alternative to hazardous isocyanate-based polyurethanes. NIPU exhibits outstanding performance in polymer bonding, but its application in the plywood industry is questioned due to its relatively high curing temperature, typically above 180 °C. This study explored a unique method for producing NIPU adhesives through amination of lignin (LA), followed by quinone formation via disruption of condensed tannins with sodium periodate, and then methoxycarbonylation with dimethyl carbonate (TOC). LA and TOC were mixed in an alkaline medium to produce LT-NIPU adhesives (control sample) via urethane linkage formation, followed by reinforcement with microcrystalline cellulose (MCC), which effectively engaged the vacant -OH groups and densified the adhesive polymer network (MLT-NIPU). The synergistic interactions between the prepolymers and MCC remarkably enhanced the MLT-NIPU adhesive chemistry. Consequently, MLT-NIPU adhesives at 30% MCC loading exhibited low curing temperatures of 130 °C, outstanding viscoelastic, shear-thinning behavior, and achieved dry and wet shear strength of 1.89 and 0.85 MPa, respectively. Therefore, this study presents a simple yet innovative strategy for constructing low-curing-temperature NIPU adhesives, enhancing bonding integrity, and establishing an alternative pathway for exploring bio-based feedstocks in adhesive development.