Low molar ratio urea-formaldehyde adhesives are considered environmentally preferable due to their lower formaldehyde content and reduced emissions. However, reducing the formaldehyde-to-urea (F/U) molar ratio often leads to decreased bonding strength. This study addresses this limitation by modifying a low-molar-ratio urea-formaldehyde (UF) adhesive (F/U = 0.8) through the in-situ incorporation of citric acid during the condensation stage to influence resin formation and curing behavior. Citric acid was added at concentrations of 0%, 2.5%, and 5%, with commercial UF adhesive as a reference. Adhesive properties were evaluated based on pH, solid content, viscosity, and gelation time, and further analyzed using FTIR, XRD, TGA, and Py-GCMS. Plywood panels manufactured from sengon, jabon, and karet veneers were tested for physical and mechanical properties. The results showed that 2.5% citric acid significantly improved shear strength while reducing water absorption and thickness swelling. FTIR confirmed ester bond formation, indicating enhanced cross-linking and stronger adhesive-wood interactions. These findings demonstrate that in-situ citric acid modification effectively compensates for the reduced bonding performance of low F/U adhesives, offering a promising approach for developing low-formaldehyde adhesive systems.
Syukur et al. (Sun,) studied this question.