Distinct Roles of Different Nanocellulose Superstructures in Determining Performance of Urea-Formaldehyde Based Adhesives

The inherent ability of cellulose chains to self-assemble at the nanoscale through hydrogen bonding creates hierarchical structures, forming cellulose nanocrystals (CNCs), cellulose nanofibrils (CNFs), and cellulose microfibrils (MFCs). These self-assembled structures have been widely incorporated into petroleum-derived adhesive formulations to mitigate their environmental and health impacts. However, the effects on adhesion strength are often contradictory due to a lack of fundamental understanding of the microstructure-rheology relations. In this study, we investigated the dispersion, stability, phase behavior, rheology, and curing behavior of urea-formaldehyde (UF) adhesives modified with wood-derived cellulose nanoparticles, including CNCs, CNFs, and MFCs. Our results show that CNC- and CNF-containing suspensions were homogeneously distributed in the UF solution, whereas MFCs agglomerated due to a higher degree of entanglements. Rheological measurements revealed that all nanocellulose forms increased the viscosity and moduli of the UF solution, albeit with significant differences in magnitude. Highly viscous composite adhesives with MFCs were unsuitable for spraying and hindered homogeneous spreading and wetting on wood surfaces. In contrast, CNFs caused negligible improvement in rheological properties due to their inherent flexibility and tendency to form isolated bundles. Remarkably, the addition of CNCs to UF resin allowed for precise tuning of the flow properties of the composites with filler content, affecting the properties over several orders of magnitude at concentrations as low as a few percent. Composites with low CNC concentrations (1, 2, and 3 wt%) were homogeneously dispersed in the UF solution, forming a network between negatively charged CNCs and the UF matrix. However, adhesives with higher CNC concentrations (4 and 5 wt%) disrupt the long-range particle network and caused clustering in the UF-CNC mixture and promote gel formation- an undesirable form for practical applications. These physicochemical characteristics reflect well in the adhesion behavior as characterized by lap-shear and tact-tests.<br/><br/>* This work is supported through TUBITAK 2244 Program with grant no: 119C160