Rheological Measurement of Concentrated Pulp Fiber Suspensions in Oscillatory Shear Using a Novel Device

This paper presents a novel device based on an oscillating torsion resonator with continuously varying frequency capability to characterize the rheological properties of pulp fiber suspensions in a concentrated regime. Fiber suspensions made from unbleached wheat straw pulp at concentrations ranging from 5 wt% to 15 wt% were used. Pulp suspensions exhibit shear-thinning behavior up to a limited frequency value, after which Newtonian behavior prevails. Effects of frequency, fiber concentration, and beating process on suspension viscoelastic properties are discussed. The suspensions at different concentrations are structured in a similar way, leading to a weak gel-like structure. The storage modulus (G') of the suspension can be determined by a two-region (shear increasing – shear decreasing) profile, while loss modulus (G'') keeps increasing for the whole frequency range investigated. At the same frequency, G' and G'' increase nonlinearly with fiber concentration. The beating process brings a decrease in both G' and G''. The power-law model is used for data fitting.