Preparation and investigation of a novel plywood eco-friendly adhesive composed with lactic acid and polyvinyl alcohol

In this study, a formaldehyde-free adhesive was developed for plywood, utilizing lactic acid (LA), polyvinyl alcohol (PVA), and polymeric methylene diisocyanate (p-MDI). The optimal adhesive formulation was achieved with a 7:3 mass ratio of LA to PVA, 60% solid content, and synthesized at 100°C for 2 h with 7.5% p-MDI incorporated as the curing agent. Plywood manufactured with this adhesive, under hot pressing conditions at 130°C for 10 min and a spread rate of 200 g/m², exhibited the highest wet shear strength of 0.95 MPa, exceeding the China National Standard GB/T 9846–2015 (wet strength of Class II plywood ≥ 0.7 MPa). Curing behavior was evaluated using insoluble mass proportion (IMP) and thermal analysis. After curing at 130°C for 10 min an IMP content of 51.3% was observed, and thermal analysis revealed that polymerization occurred between 120°C and 170°C. The curing mechanism was further investigated and the results indicated that reactions between the isocyanate groups of p-MDI and the hydroxyl, ester, and carboxyl groups of the prepolymer chains resulted in the formation of a crosslinked polymer network, which contributed to the superior bonding performance. This study provided a novel and eco-friendly approach for the development of formaldehyde-free adhesives for plywood applications.