Flocculation behavior of cellulose nanofibrils under different salt conditions and its impact on network strength and dewatering ability

Flocculation of cellulose nanofibrils (CNF) was induced by charge neutralization with different types and concentrations of salt, and its effects on the network strength and dewatering ability of the CNF suspension were investigated. Aggregation of the CNF suspension was evaluated by measurement of light transmission using Turbiscan equipment. This procedure enabled us to characterize the aggregation and sedimentation behavior of nanofibrils under different salinity conditions. Aggregation and sedimentation of CNF occurred on salt addition because the degree of compression of the electrical double layer was changed by adsorption of cations onto the CNF, depending on the type and concentration of salt. The changes in network strength of the CNF suspension due to flocculation were evaluated using a conventional oscillatory rheometer. The viscosity, storage modulus, and yield stress of the CNF suspension increased with an increase in the ionic strength of the suspension. Microrheology measurements using the dynamic light scattering technique were also adopted to characterize the viscoelastic properties of the CNF suspension, revealing that CNF suspension containing a high concentration of salt showed more solid-like behavior. In addition, the aggregation degree of the CNF affected the dewatering ability of the CNF suspension. Bivalent cations were more effective for increasing the network strength and dewatering for a small amount of added salt compared with monovalent cations.