Pipe rheology of microfibrillated cellulose suspensions

In this paper, we analyse the shear rheology of suspensions of two MFC grades in the consistency ranges of 0.2% to 1.5% and 0.5% to 2.0% with a pipe rheometer combined with ultrasound velocity profiling. In addition to yield stress and shear viscosity, we study the slip behaviour of the MFC suspensions, as this topic has so far been mostly neglected in the existing literature. Although there was a factor ten difference between the particle sizes of the two MFC grades, their rheological behaviour was rather similar. The relation between the yield stress and consistency was for both MFC suspensions a power law with an exponent of 2.4. The MFC suspensions behaved with all consistencies as shear thinning power law fluids. The dependence on consistency of the consistency indexes of both MFC’s was a power law with the same exponent, 2.4, as with yield stress. There, i.e., was a linear relation between the yield stresses and the consistency indexes. The flow indexes of both suspensions followed the same power law as a function of consistency. The slip behaviour was in both cases very strong and contributed up to 95% to the flow rate. When wall shear stress exceeded two times the yield stress slip caused drag reduction with consistencies higher than 0.8%. When slip velocities were plotted as a function of wall shear stress that was scaled with the yield stress, a nice data collapse was obtained.