Alkaline Formulation and Heat Treatment Improve the Performance of Lignin-Soy Flour Wood Adhesive

Highlights Kraft lignin incorporation significantly improved the performance of soy flour-based adhesives. Alkaline formulation (pH 8) maximized lignin and protein solubility, reactivity, and network formation. Post-press heat treatment (200°C, 2 h) markedly increased water resistance and wet shear strength. pH shifting (8 to 7) prior to heat treatment yielded substantial wet strength gains compared to untreated controls. ABSTRACT. Traditional soy flour adhesives offer strong dry bonding but poor water resistance, limiting industrial application. This study aimed to develop a heat-resistant, lignin-enhanced soy flour adhesive for engineered wood by addressing three objectives: (1) evaluate the effect of formulation pH (7, 8, and pH shift from 8 to 7) on adhesive solubility, structure, and bonding performance; (2) assess the impact of post-press heat treatment (200°C for 2 h) on adhesive durability; and (3) compare the combined effects of formulation and application treatments on mechanical, thermal, and morphological properties. Kraft lignin (25% by weight) was incorporated into soy flour adhesives, applied to 3-layer plywood via hot pressing, and tested for dry and wet shear strength, water resistance, and thermal and morphological characteristics (SEM, FTIR, TGA, DSC, DMA). Maintaining an alkaline pH (8) maximized lignin solubility, protein unfolding, and reactive site availability, yielding the highest untreated wet strength (0.47 MPa). Heat treatment consistently improved water resistance across all formulations, with the pH 8 adhesive reaching the highest wet strength (1.52 MPa) and lowest film weight loss (5.05%). pH shifting offered modest gains only when combined with heat treatment. These results demonstrate that constant alkaline formulation coupled with high-temperature post-curing delivers superior wet strength, water resistance, and thermal stability without petrochemical crosslinkers, offering a sustainable, high-performance alternative for interior-grade wood adhesives. Keywords: Heat treatment, Lignin, pH shifting, Plywood, Protein-based adhesive, Soy flour adhesive.