Polyurethane (PU) adhesive derived from lignin-based polyol can be used as a substitute for urea–formaldehyde (UF) adhesive for plywood because it can be used for exterior application, it is more environmentally friendly, and does not produce formaldehyde emissions. This research work aimed to investigate and evaluate the adhesive performance of lignin-based PU adhesive for bonding plywood, synthesized by mixing methylene diphenyl diisocyanate (MDI), polyol, polyvinyl alcohol (PVOH), and lignin, then applied to the rubberwood (Hevea brasiliensis) veneers and hot-pressed at a temperature of 120 °C and a pressure of 2.5 MPa for 4 min. The lignin yield obtained was 36.17%, moisture content 5.07 ± 0.71%, ash content 0.31 ± 0.19%, and lignin purity 95.32 ± 0.61%. The increased addition level of PVOH/lignin resulted in decreased solids content of the adhesive, while viscosity, cohesion, torque, storage modulus, loss modulus, and tan delta increased. Based on the Fourier-transform infrared spectroscopy (FTIR) analysis results, urethane bonds were identified in both the adhesive and plywood. Lignin-based PU adhesive formulas with 10%, 20%, and 30% PVOH/lignin fulfilled the minimum requirements for delamination, tensile strength, and wood failure, as specified by the Japanese Agricultural Standard (JAS) No. 233:2003. Markedly, plywood bonded lignin-based PU adhesive with 30% PVOH/lignin met the standards of JAS No. 233:2003 for exterior grade plywood.
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