Rheological behavior of cellulose nanofibril suspensions with varied levels of fines and solid content

The rheological behavior of mechanically disk refined cellulose nanofibril (CNF) aqueous suspensions with 50, 60, 70, 80, 90, 100 % fines contents at 1 and 3 wt% solid concentrations was studied by conducting various dynamic and steady-state rheological experiments, including oscillatory shear, flow sweep, startup, flow loop, and three-step oscillation experiments. All the samples exhibited a gel-like behavior in oscillatory shear tests and samples, with 50 % fines showed the highest levels of dynamic moduli for both solid contents. Suspensions with higher concentrations of CNFs showed higher values for viscosity, complex viscosity, and dynamic moduli. A critical shear rate of 10 s- 1 independent of the solid contents of the suspensions was found for varying fines levels, where viscosity measurements above this critical shear rate converged and viscosity measurements below it diverged. All samples exhibited yield in shear flow and yield stresses exhibited a decreasing trend followed by a plateau as fines levels increased. The level of yield stress for 3 wt% suspensions was higher than that of 1 wt% suspensions. The lowest structure recovery was observed for samples containing 50 % fines content. Moreover, a rheopectic-thixotropic transition was observed for samples with high fines content (90 and 100 %) at low solid content (1 wt%).