High Shear Flow Properties of Nanocellulose

Abstract

Nanocellulosics, often found in the form of cellulose nanocrystals (CNCs) and nanofibrillated cellulose (NFCs), provide promise as rheological modifiers and reinforcement fillers for composite materials. The biological origin of CNCs promises a bio-renewable resource with the potential to expedite degradation times compared to synthetic polymer species. Additionally, the surface functional groups provide a route for both hydrogen bonding and further chemical modification. While much research is currently investigating the possible uses of these materials, they offer limited aid if their use is not scalable to industrial processing techniques. Common processing techniques such as injection molding subject materials to high temperatures and strain rates upwards of 100000 s-1. Thermal stability is a known challenge that can be increased via chemical modifications, but little is known about the effects of high or extended shear stresses typical of those experienced during typical polymer processing. High shear rates, which proportionally result in high shear stresses, have the potential to influence the alignment, degradation, and overall usability of these materials when employed in consumer applications. In this work, we investigate the rheology and processing of aqueous CNC suspensions at concentrations up to 12.1 wt% and of aqueous NFC suspensions at concentrations up to 20 wt% under capillary shear stresses. Traditional capillary rheology corrections, including the Weissenberg-Rabinowitsch-Mooney (WRM) correction for non-Newtonian fluids, and the Bagley correction for entrance pressure effects, have been applied to determine the true rheological behaviors of these suspensions. Additional analysis using atomic force microscopy (AFM), wide-angle x-ray scattering (WAXS), and conductometric titration assist identification of morphological and chemical changes that affect the CNMs after they have been subjected to industry-relevant shear rates. These studies demonstrate that processing conditions can significantly affect the size and shape of the post-processed nanomaterials by fracturing the CNCs and unwinding the larger bundles of the NFCs. Given the importance of the final aspect ratio of filler and reinforcement materials, the impact of this discovery will substantially influence how these materials are used and processed to create consumer products.