Rheological Behavior of Aqueous Suspensions of Highly-Refined Fibers

Microfibrillated and nanofibrillated celluloses (MFC and NFC) have been researched as additives in papermaking e.g. for improved barrier, strength, surface, and optical properties. The production of MFC and NFC through mechanical fibrillation requires a significant amount of energy. Therefore, producing fibrous materials containing MFC through extensive refining with a conventional refiner can be a more efficient alternative. This approach has the advantage that it is well-established and readily available on an industrial scale. However, unlike MFC suspensions, this material has a very broad size distribution, as it consists of a mixture of fibrillated fibers and fine particles that form simultaneously during refining. These highly-refined fibers could replace MFC in a variety of applications. Another promising strategy is to use them directly for novel cellulose-based applications. However, there is currently little information available on the rheological behavior of the aqueous suspensions of these materials. In this work, we performed a comprehensive rheological analysis for suspensions of highly-refined fibers at four refining levels and five solids contents ranging from 1.2 to 3.0 wt%. Our rheological characterization included amplitude sweep and frequency sweep measurements to analyze the time-dependent viscoelastic behavior of these materials both at small and large deformations, as well as steady-state viscosity measurements to study the flow behavior of these materials at various shear rates.