Unraveling the effects of cellulose nanoparticle types on dispersion, rheological behavior, and shear strength in adhesive formulations.

Cellulose chains self-assemble at the nanoscale, forming cellulose nanocrystals (CNCs), cellulose nanofibrils (CNFs), and cellulose microfibrils (MFCs), which have been widely incorporated into petroleum-derived adhesive formulations to mitigate environmental and health impacts. However, the microstructure-rheology-performance relations of different morphologies need to be elucidated. This study 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 were homogeneously distributed in the UF solution, whereas MFCs agglomerated due to a higher degree of entanglement. The addition of CNCs to UF resin allowed 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 (1-3 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%) disrupted the long-range particle network, causing clustering in the UF-CNC mixture and promoting gel formation- an undesirable form for practical applications. These physicochemical characteristics are well reflected in the adhesion behavior characterized by lap-shear tests.