Flocculated flow of microfibrillated cellulose water suspensions: an imaging approach for characterisation of rheological behaviour

Abstract

Our aim was to characterise the suspension rheology of microfibrillated cellulose (MFC) in relation to flocculation of the cellulose fibrils. Measurements were carried out using a rotational rheometer and a transparent cylindrical measuring system that allows combining visual information to rheological parameters. The photographs were analyzed for their floc size distribution. Conclusions were drawn by comparing the photographs and data obtained from measurements. Variables selected for examination of MFC suspensions were degree of disintegration of fibres into microfibrils, the gap between the cylinders, sodium chloride concentration, and the effects of changing shear rate during the measurement. We studied changes in floc size under different conditions and during network structure decomposition. At rest, the suspension consisted of flocs sintered together into a network. With shearing, the network separated first into chain-like floc formations and, upon further shear rate increase, into individual spherical flocs. The size of these spherical flocs was inversely proportional to the shear rate. Investigations also confirmed that floc size depends on the geometry gap, and it affects the measured shear stress. Furthermore, suspension photographs revealed an increasing tendency to aggregation and wall depletion with sodium chloride concentration of 10 -3 M and higher.

Flocculation of microfibrillated cellulose in shear flow

“…The floc structure is to some extent similar to pulp suspensions (Saarikoski et al 2012); although MFC fibers have significantly smaller dimensions and a higher aspect ratio than pulp fibers. The flocculated structure of MFC (Saarikoski et al 2012) and pulp suspensions (Beghello 1998;Wågberg and Nordqvist 1999;Chen et al 2002;Björkman 2003a, b) have been studied by imaging. At rest, the flocs can be described as areas of higher fiber density within a network (Björkman 2000;Kerekes 2006).…”

“…At rest, the flocs can be described as areas of higher fiber density within a network (Björkman 2000;Kerekes 2006). Under shear, the network breaks gradually, and at sufficiently high shear rates, the suspension flows as individual flocs (Björkman 2006;Saarikoski et al 2012), and the floc size diminishes with increasing shear rate (Björkman 2005;Saarikoski et al 2012). The floc strength is determined by number of contacts and cohesive forces present between the fibers (Kerekes et al 1985).…”

“…In addition to these, flocculation can be affected by non-DLVO interactions like steric and electrosteric interactions caused by polymers and polyelectrolytes in the system (Hubbe and Rojas 2008). All these forces and thus the floc structure can be changed for example by salt addition (Ono et al 2004;Saarikoski et al 2012), polyelectrolyte addition (Wågberg and Nordqvist 1999;Hubbe 2007;Karppinen et al 2011) or by changing the pH (Beghello 1998). The suspension structure and the related rheological properties are important for the processing of MFC, e.g., pumping or storage, and in some applications, e.g., use as a stabilizer in dispersions or rheology modifier in paints and coatings.…”

Forming and Dewatering of a Microfibrillated Cellulose Composite Paper

“…As expected, the furnish with higher consistency exhibited higher dynamic viscosity throughout the shear range. The shear thinning behavior for both consistencies indicate that the structure was broken as more shear was applied, which is typical for MFC containing furnishes (Lasseuguette et al 2008;Iotti et al 2011;Karppinen et al 2011;Saarikoski et al 2012;Dimic-Misic et al 2013b). This result shows the potential of tuning the viscosity by applying shear to the furnish, which is relevant for any pumping or mixing operations in a manufacturing process.…”

An Efficient Dispersive Agent — KCl for Ultrasonic Preparation of Microfibrillated Cellulose (MFC)

“…In the case of the KCl dispersive agent, the shear viscosity of MFC suspension remained rather low, at only 135 cP even after 70 min of ultrasonication. Because the high viscosity of MFC suspension is primarily caused by the strongly entangled and disordered gel networks formed by inherently connected cellulose fibrils, the reduction in the viscosity of the MFC suspension with KCl solution as a dispersive agent could be attributed to the weakening or even full breakage of this network by breaking the balance between repulsive and attractive forces between the fibrils with the salt addition (Saarikoski et al 2012).…”