Tannin-modified ultra-low-molar-ratio urea-formaldehyde adhesive for plywood from tropical wood species

Tannin has been investigated as a renewable material modifier for urea-formaldehyde (UF) adhesives; however, its influence on ultra-low-molar-ratio UF systems and their performance across diverse wood substrates remains inadequately characterised. This investigation assessed UF adhesive with an F/U ratio of 0.8 post-modified with 0, 2.5%, and 5% tannin to clarify concentration-dependent alterations in adhesive properties and their correlation with plywood performance. The adhesives underwent characterisation via physicochemical analysis, FTIR, TGA, DSC, and Py-GC/MS, whereas three-layer plywood manufactured from mahogany, jabon, and rubberwood was assessed for physical and bonding characteristics. Tannin substantially reduced gelation time, enhanced phenolic and aromatic constituents, displaced the principal exothermic reaction by approximately 20 °C toward lower temperatures, and diminished apparent enthalpy by 30-36%. The 5% formulation additionally demonstrated enhanced high-temperature residue generation. Nevertheless, these molecular and thermal modifications did not yield uniform enhancement in plywood performance. Physical characteristics fluctuated according to tannin concentration and wood species, whereas tensile shear strength remained largely unaffected overall. These results indicate that tannin constitutes an efficacious modifier of ultra-low-molar-ratio UF adhesive, yet its plywood performance is contingent upon both tannin concentration and wood substrate composition.