Cellulose nanomaterial (CNM), which includes cellulose nanocrystal (CNC) and cellulose nanofiber (CNF), is a renewable and sustainable class of nanomaterials that are produced from abundant cellulose sources, such as wood and plant. CNC are rod-like colloidal particles, and TEMPO-mediated oxidized cellulose nanofibril (TEMPO-CNF) are flexible fibers. Their high strength and biodegradability make it attractive to many applications such as composites, coatings and rheological modifiers. To expand the production capacity to industrially relevant scales and to ensure consistently high-quality CNM, one of the most urgent issues to be addressed is the lack of standardized, rapid and reliable characterization methods for quality control during the manufacturing process. Current characterization techniques, such as electron microscopy and light scattering, are expensive and time-consuming, and cannot easily be integrated into an industrial manufacturing process for online monitoring. Moreover, these methods suffer from potential bias because they only probe a small portion of the CNM sample. Rheology provides a fast and cost-effective way to characterize nanocelluloses in large volume. Although previous studies have characterized the CNM viscosity, studied the concentration effect and attempted model fitting to estimate the CNC aspect ratio, a knowledge gap still exists when it comes to translating lab-scale rheological characterization to industry-scale quality control.
In this work, we will show the potential of rheology as a quality control tool for nanocellulose materials. We developed detailed test protocols to obtain reliable steady-state shear viscosity data. We studied the influence of concentration on rheological properties of CNC and TEMPO-CNF, and formulated a rheological model to accurately capture the viscosity across shear rates and concentrations. The model can be used to estimate the concentration of an uncharacterized CNM sample by measuring its viscosity relative to a series of characterized samples serving as calibration, which is relevant for quality control. Moreover, we developed a flow index that condenses large sets of data into one single value. We will demonstrate the indexâs capability to describe the viscosity behaviors across shear rates and concentrations, and works as a fingerprint of the material. We will also show that the processing conditions, including the effect of pressure homogenization (pressure and number of passes) and the carboxylate content of TEMPO-CNF, can be identified through the flow index.