A rapid-curing adhesive based on lactic acid and poly(vinyl alcohol) for plywood

This study focused on optimizing a formaldehyde-free wood adhesive based on lactic acid (LA) and polyvinyl alcohol (PVA). Initially, various modifiers were investigated, revealing that microcrystalline cellulose (MCC) effectively reduced the adhesive's viscosity while enhancing the curing efficiency of plywood. Consequently. the optimized LA-PVA-MCC adhesive was developed. The optimal formulation, with a LA:PVA:MCC mass ratio of 28:11:1 and 60% solid content, was synthesized at 100 °C for 2 h using 10 wt% p-MDI as curing agent. Plywood fabricated under hot-pressing conditions of 120 °C for 6 minutes with a spread rate of 180 g/m² exhibited a wet shear strength of 0.87 MPa, exceeding the requirement of the Chinese National Standard GB/T 9846–2022 (≥0.7 MPa). Assessment of curing behavior via insoluble mass proportion (IMP) and thermal analysis revealed an IMP of 42.3% after cured, while thermal analysis indicated polymerization occurring between 83 and 150 °C. Mechanistic investigation confirmed that cross-linking occurred via reactions between isocyanate groups and the hydroxyl, ester, and carboxyl functionalities on prepolymer chains, forming a robust network. This study provides a viable strategy for formaldehyde-free wood adhesives that addresses the need for rapid curing in sustainable plywood manufacturing.