Citric Acid–Urea Modification of Dual-Plasticized Starch Adhesives for Medium-Density Fiberboard Manufacturing

Formaldehyde-free starch adhesives offer a renewable alternative to conventional binders used in medium-density fiberboard (MDF), but their application remains limited by poor moisture resistance and insufficient mechanical performance. The aim of this research work was to investigate and evaluate the combined effects of sorbitol and glycerol plasticizers, citric acid modification, and urea addition on the properties of starch-based adhesives and the resulting MDF panels. Five adhesive formulations were prepared and characterized using Fourier-transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). Laboratory MDF panels were manufactured and evaluated for thickness swelling (TS), water absorption (WA), internal bond strength (IB), modulus of rupture (MOR), and modulus of elasticity (MOE). Among the tested formulations, the hybrid adhesive system, composed of sorbitol/starch/glycerol/urea/citric acid (SSGUC), provided the best overall performance. Panels bonded with SSGUC exhibited the lowest TS and WA after both 2 and 24 h of water immersion, with TS2 values of 22.74 to 24.54% and WA2 and WA24 values of 91.16–101.43% and 132.11–141.29%, respectively. The same panels achieved IB, MOR, and MOE values of 0.59–0.86 MPa, 24.95–26.17 MPa, and 2413–2632 MPa, respectively. FTIR spectral changes were consistent with citric-acid-mediated esterification and modified hydrogen-bonding interactions in the urea-containing formulations, while DSC revealed formulation-dependent differences in thermal behavior. The FTIR interpretation was based primarily on comparative band development and intensity changes rather than on marginal peak-position shifts. Overall, the results indicate that combining sorbitol and glycerol with citric acid and urea can improve the moisture resistance and mechanical performance of starch-bonded MDF. Further optimization, including rheological characterization and comparison with commercial binders, is required to assess industrial applicability.