Thixotropic Flow Behaviour in Chemical Pulp Fibre Suspensions

This paper presents results on the thixotropic behaviour of hardwood and softwood pulp fibre suspensions. Three rheological tests including hysteresis-loops, creep tests, and step-wise experiments were used to investigate the thixotropic rheology. Both suspensions exhibited a plateau in their flow curves where a slight change of the applied shear stress led to a dramatic change in the corresponding shear rates. During creep experiments under controlled stress, they evolved either towards a rapid shear (liquid regime) or stoppage (solid regime), depending on the relative values of the imposed stress, leading to a viscosity bifurcation around a critical stress. The transient response of pulp to step changes in shear rate was marked by a characteristic time, which can be used to understand the rate of structural change for pulp suspensions. Moreover, the yielding and thixotropic behaviour of the pulp suspensions were highly dependent on shear history and the time of rest prior to the measurement.