Developing carbon dots as green modifiers for improving the bonding performance of low-molar-ratio urea-formaldehyde resin

Reducing formaldehyde-to-urea molar ratio is conducive to improving environmental-friendliness of urea-formaldehyde (UF) resin-bonded wooden products, but at the expense of bonding performance, such contradictoriness is intrinsically due to the decreased cohesion of resin polymer structures. In this study, we proposed a novel strategy to enhance the bonding performance of low-molar-ratio UF resins by use of carbon dots (CDs) as green modifiers. Taking advantages of surface-contained plentiful carboxyl and hydroxyl groups, the synthesized CDs could generate strong physicochemical interactions with polymer segments and further improve cohesion of resin matrix. The optical microrheology demonstrated that the microscopic viscoelasticity of CDs-modified UF resin was increased as increasing of CDs content due primarily to the formation of strong hydrogen bonding interactions between CDs and resin polymers. Based on the analysis for curing behavior, the polycondensation reactivity of resin oligomers was almost free from the interaction or connection between CDs and polymer segments. Furthermore, as confirmed by nanoindentation technology, the physicochemical interactions formed between oxygenous functional groups of CDs and polymer matrices could improve the cohesion of resin after curing reaction, thus resulting in the enhanced micromechanical strength of wood-resin interphase and further bonding strength of resin at macroscopic level. Remarkably, this reinforcing effect imparted by CDs resulted in the tensile shear strength of plywood increased to 0.96 MPa, which was 77.8% increase in comparison to the panels bonded with unmodified resin and largely exceed the requirement given by national standard (0.7 MPa for plywood type Ⅱ). Taken together, this work demonstrated that the synthesized CDs are efficient and provided a potential platform for improving the bonding performance of low-molar-ratio UF resins.