Michel Schenker

Enhancing the Stability of Aqueous Dispersions and Foams Comprising Cellulose Nanofibrils (CNF) with CaCO3 Particles

In this work, stability of dispersions and foams containing CaCO3-based pigments and cellulose nanofibrils (CNF) was evaluated with the aim to reveal the mechanisms contributing to the overall stability of the selected systems. The utmost interest lies in the recently developed hydrocolloid hybrid CaCO3 pigments and their potential to form bionanocomposite structures when incorporated with CNF. These pigments possess a polyelectrolyte layer deposited on the surface of the particle which is expected to enhance the compatibility between inorganic and organic components. Stability assessment of both dispersions and foams was conducted using turbidity profile scanning. In dispersions, CNF provides stability due to its ability to form a firm percolation network. If surface-modified pigments are introduced, the favourable surface interactions between the pigments and CNF positively influence the stability behaviour and even large macro-size pigments do not interfere with the stability of either dispersions or foams. In foams, the stability can be enhanced due to the synergistic actions brought by CNF and particles with suitable size, shape and wetting characteristics resulting in a condition where the stability mechanism is defined by the formation of a continuous plateau border incorporating a CNF network which is able to trap the inorganic particles uniformly.

Influence of shear rheometer measurement systems on the rheological properties of microfibrillated cellulose (MFC) suspensions

Flow curve and viscoelastic measurements were performed on microfibrillated cellulose (MFC) suspensions of different solids content using both cylinder and cup (smooth and rough) as well as vane in cup geometries. To compare the data quantitatively from amplitude sweep measurements and dynamic flow curves several descriptors were newly introduced to parameterize the observed two-zone behaviour separated by a transition region. It was observed that the cylinder cup geometries are prone to erroneous effects like slip, wall depletion and/or shear banding. However, those effects were not observed when the MFC suspension was not stressed beyond the dynamic critical stress (yield) point, i.e. when still in the linear viscoelastic regime. The vane in cup system on the other hand, seems to be less affected by flow inhomogeneities. By following the rheological properties as a function of the MFC suspension solids content, it could be shown that the global property trends remained alike for all investigated measurement systems, despite the presence of erroneous effects in some geometries. The observed effects were linked to recent model hypotheses in respect to the morphology of MFC suspensions under changing shear situations.