Rheology of microfibrillated cellulose (MFC) suspensions: influence of the degree of fibrillation and residual fibre content on flow and viscoelastic properties

The influence of the degree of fibrillation (DoF), i.e. the fibril width distribution, on the rheological properties of microfibrilated cellulose (MFC) suspensions was investigated. To extend the understanding of the dominating effect of either fibril diameter alone or diameter size distribution, flow curves (viscosity against shear rate) and viscoelastic measurements were performed on single, double and ternary component mixtures of medium and highly fibrillated MFCs and pulp fibres across a range of solids content. The data were quantified using classical and recently introduced descriptors, and presented in comprehensive 3D/ternary contour plots to identify qualitative trends. It was found that several rheological properties followed the trends that are generally described in the literature, i.e. that an increasing DoF increases the MFC suspension network strength. It was, however, also found that coarse pulp fibres can have additional effects that cannot be explained by the increased fibril widths alone. It is hypothesised that the increased stiffness (directly caused by the larger fibril width) as well as the reduced mobility of the pulp fibres are additional contributors. The data are discussed in relation to recent findings in the field of rheology and related morphological models of MFC suspension flow behaviour.