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

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.     

Effect of cationic polymethacrylates on the rheology and flocculation of microfibrillated cellulose

Using two cationic methacrylate polymers: poly([2-(methacryloyloxy)ethyl] trimethyl ammonium iodide) (PDMQ) and poly[(stearyl methacrylate)-stat-([2-(methacryloyloxy)ethyl] trimethyl ammonium iodide)] (PSMA13Q), we modified microfibrillated cellulose (MFC) water suspensions. The aim was to affect the flocculation and rheological behavior of the MFC suspension. PDMQ is a strongly cationic polymer while PSMA13Q, also a cationic polymer, contains hydrophobic segments. We studied the MFC/polymer suspension rheological properties with a rotational rheometer in oscillatory and flow measurements. To observe structural changes in suspensions at different shear rates, we measured flow curves with transparent outer geometry and photographed the sample with a digital camera. The oscillatory measurements showed that a small amount of the cationic PDMQ in the MFC suspension strengthened the gel, whereas a small amount of amphiphilic PSMA13Q weakened it. Increased amounts of either polymer increased the gel strength. PSMA13Q also changed the rheological character of the MFC suspension turning it more fluid-like. When we photographed the flow curve measurement, we saw a clear change in the floc structure. This floc structure rupture coincided with a transient region in the flow curve.     

Analysis of rheology and wall depletion of microfibrillated cellulose suspension using optical coherence tomography

A rheometric method based on velocity profiling by optical coherence tomography (OCT) was used in the analysis of rheological and boundary layer flow properties of a 0.5% microfibrillated cellulose (MFC) suspension. The suspension showed typical shear thinning behaviour of MFC in the interior part of the tube, but the measured shear viscosities followed interestingly two successive power laws with an identical flow index (exponent) and a different consistency index. This kind of viscous behaviour, which has not been reported earlier for MFC, is likely related to a sudden structural change of the suspension. The near-wall flow showed existence of a slip layer of 2–12 μm thickness depending on the flow rate. Both the velocity profile measurement and the amplitude data obtained with OCT indicated that the slip layer was related to a concentration gradient appearing near the tube wall. Close to the wall the fluid appeared nearly Newtonian with high shear rates, and the viscosity approached almost that of pure water with decreasing distance from the wall. The flow rates given by a simple model that included the measured yield stress, viscous behavior, and slip behavior, was found to give the measured flow rates with a good accuracy.     

Model filled rubber. II. Particle composition dependence of suspension rheology

Monodisperse size colloidal particles varying in chemical composition were synthesized by emulsifier‐free emulsion polymerization. Using a stress‐controlled rheometer, the rheological behavior of colloidal suspensions in a low molecular weight liquid polysulfide was investigated. All suspensions exhibited shear thinning behavior. The shear viscosity, dynamic moduli, and yield stress increased as interactions between particles and matrix increased. The rheological properties associated with network buildup in the suspensions were sensitively monitored by a kinetic recovery experiment. We propose that interfacial interactions by polar and hydrogen bonding between particles and matrix strongly promote affinity of matrix polymer to the filler particles, resulting in adsorption or entanglement of polymer chains on the filler surface. A network structure was formed consisting of particles with an immobilized polymer layer on the particle surface with each particle floc acting as a temporary physical crosslinking site. As the interfacial interaction increases, the adsorbed layer thickness on the filler particles, hence, the effective particle volume fraction, increases. As a result, the rheological properties were enhanced in the order PS < PMMA < PSVP. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 815–824, 1999     

Shear-Induced Changes in Rheological Reponses and Neutron Scattering Properties of Hydrophobic Silica Suspensions at Low Silica Concentrations

Rheological responses of hydrophobic fumed silica powders, whose surface silanol groups were modified by hexadecane, suspended in 1,4-dioxane at lower silica concentrations than 6.8 vol% have been investigated as a function of the silica concentration. Transient shear stress behavior before attaining the steady-state shear stress could be classified into three regimes as follows, irrespective of the silica concentration: at the lower shear rates than ca. 0.3 s^?1 a stress overshoot was observed, at the shear rate ranges from 0.3 to 30 s^?1 sustained oscillations in shear stresses were exhibited and these oscillations were first observed for the suspensions at the low particle concentrations, and beyond the shear rate of 30 s^?1 a sigmoid decrease of the shear stress with increasing time, that is, structural breakdown, was observed. At the steady state the silica suspensions showed shear thickening. Small angle neutron scattering (SANS) measurements of the silica suspension under shear flow provided that changes in the SANS intensities were well correlated with the shear thickening behavior. However, shear thinning behavior at higher shear rates did not cause any changes in the SANS intensities.     

Yield Stress Measurement of Dense Pastes with Pipe Transport and Vane

Yield stress is a key rheological parameter of dense paste. Considering practical utility and to estimate the possible slipping flow, yield stress measurements were carried out using the curve extrapolation method and vane method with rheometer in both controlled shear stress (CSS) and controlled shear rate (CSR) model. All measured yield stresses show to increase exponentially with concentration in different scale. The vane method yield stresses are both higher than the extrapolation curve yield stress. For the coal slurry used in this paper, the extrapolation curve yield stress is lower by 17–30% than the dynamic yield stress that obtained under test model CSR. It indicates that wall slip exists in pipeline transportation; meanwhile wall slip can reduce transportation resistance of dense paste to some extent.     

Rheological Behavior of Titanium Dioxide Suspensions

The rheological properties of titanium dioxide dispersed in water are measured over a wide range of powder concentrations, temperatures, and pH values. The value of intrinsic viscosity of titanium dioxide measured with an Ubbelohde capillary viscometer is 3.55, which is useful for determining the shape and aggregation property of the particles. The yield stress and steady shear viscosity of titanium dioxide with broad and narrow particle size distributions were measured over a wide range of solid volume fractions on a Brabender rheometer. It is observed that the rheological properties of the suspensions are quite different due to the difference in particle size distributions. Quemada, Casson, and Zhou's models were used to fit the experimental data and useful parameters were obtained. Calculated data are also in good agreement with the experimental data. As expected, the shear viscosity and yield stress decrease with increasing temperature. But when the temperature is around 50 degrees C, yield stress increases with increasing temperature while shear viscosity exhibits a complex behavior. The phenomena are very interesting and special. The Peclet number was used to analyze the shear thickening behavior. Models were also used to describe the shear viscosity under different temperatures and the master plots of the reduced variables eta/eta(infinity) vs t(c)gamma; at different temperatures are superimposed, which means the agreement is fair and the models are suitable to describe the rheological properties of titanium dioxide suspensions. pH effects were investigated on a Rheometrics RFS-II rheometer and it was found that pH can change the surface charge of the particles, which also affects the rheological behavior. The pH at which maximum shear viscosity and yield stress occur is in concordance with the isoelectric point. Copyright 2001 Academic Press.     

The role of MFC/NFC swelling in the rheological behavior and dewatering of high consistency furnishes

The influence of swelling on the rheological and dewatering properties of high consistency nanocellulose based furnishes is considered. Different consistencies of suspensions (1–4 %) and furnishes (5–15 %) were prepared made of two distinctly different grades of nanocellulose containing, micro fibrillated (MFC) and nanofibrillated (NFC) cellulose, and systematic comparison between the rheological and dewatering parameters was conducted. The characterization of the rheological and dewatering properties was performed with a stress controlled rheometer combined with an immobilization cell in parallel plate geometry, as well as with an independent gravimetric dewatering device. The surface charge of nanofibrillated cellulose was found to influence the rheological and dewatering properties of the evaluated suspensions and furnishes due to its impact on swelling and effectively bound water. Due to the complex behavior of the novel materials, the immobilization times were difficult to determine from the changes in the damping factor, as often used for coating colors. Instead, we propose a modified method for determination of immobilization times based on a rheological analysis adopting the rate of change in viscoelastic loss factor over time, d(tan δ = G′′/G′)/dt, describing the critical point(s) in the ratio of the viscous to elastic stress response moduli. With this approach we show that it is possible to characterize immobilization of these materials incorporating the concept of the combined physical interactions of the components and the non-removable bound water, without requiring a direct measure of the nanocellulose surface swelling. Based on the results, we hypothesize that fibrillar swelling impacts the dewatering of MFC and NFC suspensions, and furnishes containing them, by an interfiber pore connectivity blocking/sealing mechanism, which effectively defines the immobilization of the material matrix at the end point of free water extraction caused by the physical blocking imposed by the remaining bound water.     

A rotational rheometer for rheological studies with prescribed strain or stress history

A rheometer utilizing an eddy-current torque transducer and an air-bearing suspension is described. The rheometer is coupled with a computer-based data acquisition system and permits studies of shear deformation for several strain or stress histories. A sinusoidal stress history is used to determine the shear storage and loss compliances J′(ω) and J″(ω), respectively. Step stress histories are used to determine the shear creep compliance J(t) and the recoverable complaince R(t) or more complicated linear and nonlinear rheological responses related to these. Deformation at constant strain rate is used to determine the stress growth function or the steady-state viscosity. The rheometer may be used over the temperature range – 10–180°C, with torque from 1 to 10⁶ dyn cm, and is adaptabel to use with a variety of sample geometrical shapes (e.g., cone and plate, parallel plate, etc.). Examples of measurements on on viscoelastic fluids and on gels below their yield strain are given.     

Effect of homogenization (microfluidization) process parameters in mechanical production of micro- and nanofibrillated cellulose on its rheological and morphological properties

The rheological properties of microfibrillated cellulose (MFC)/nanofibrillated cellulose (NFC) suspensions have an important role during processing and mixing. In this work, the process parameters for MFC/NFC production within a microfluidizer (i.e., the size of interaction chamber and number of passes) were varied to investigate the influences on morphology, zeta potential, chemical properties and rheological features including viscosity, creep, strain recovery and yield stress behavior. The stability and appropriate viscosity of the fiber suspensions can be controlled by optimizing the processing conditions, resulting in a reduction in fiber diameter and most negative zeta potential value. The viscosity increased with higher amount of fibrillation by using a smaller chamber or higher number of passes, but intermediate plateau values are characteristic for temporary aggregation and breaking-up of the fiber network. The creep response and yield stress have been described by parameters of the Burger model and Herschel–Bulkley model, respectively, showing a more prominent effect on yield stress of chamber size than number of passes. The network formation leads to lower creep compliance and step-like strain recovery. The transition from gel-like to liquid-like behavior as characterized by the dynamic yield point at a specific strain, is almost independent of the processing conditions. Most important, the total number of passes applied in production can be directly related to the rotational Péclet number, which combines rheological and morphological data.     

The influence of shear on the dewatering of high consistency nanofibrillated cellulose furnishes

This paper demonstrates a way to utilize the rheological properties of high consistency microfibrillated and nanofibrillated cellulose (MFC and NFC) based furnishes for improved dewatering. This is relevant to a new manufacturing platform that is being developed to form composite webs from suitable mixtures of MFC or NFC, traditional pulp fibres and pigments. The studied furnishes were evaluated in the consistencies range of 5–15 % with an MCR 300 rheometer and an immobilization cell. This setup enables us to characterize the rheology of the samples before and during the dewatering process. Classical rheological methods are used to characterise MFC and NFC furnishes. Yield stress as an indicator of the flocculated network strength was found to increase with the consistencies, following the increase in elastic moduli, which indicated a gel-like strongly flocculated matrix. The shear thinning properties of furnishes are observed to follow the Oswald’s rheological model on a wide range of shear rates. It was found that when the MFC and NFC furnishes were dewatered under vacuum conditions, the final solids content was increased with application of shear. This behaviour is more pronounced for furnishes which contained the more swollen NFC (higher WRV, i.e. higher zeta potential). This effect is further exemplified by the change of the complex and dynamic viscosities during the dewatering. The shear rate, the fibre content, and the furnish consistencies were also found to influence the dewatering rate.     

Electrorheological behavior of suspensions of a substituted polyaniline with long alkyl pendants

The electrorheological (ER) properties of poly(2-dodecyloxyaniline) (PDOA) suspensions in silicone oil were investigated. The ER behavior of such suspensions of polyaniline particles depends on the type of stabilizer and doping or dedoping level. Here we report on the ER behavior of particles of a substituted polyaniline with long alkyl pendants. Rheological measurements were carried out using a rotational rheometer with high-voltage generator in both constant shear stress and rate modes. Suspensions of the as-synthesized polyaniline particles in silicone oil showed a substantial ER response.     

Preparation and characterization of nanocrystalline cellulose via low-intensity ultrasonic-assisted sulfuric acid hydrolysis

Nanocrystalline cellulose (NCC) was extracted from microcrystalline cellulose via low-intensity ultrasonic-assisted sulfuric acid hydrolysis process. NCC samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), particle size distribution (PSD) analysis, Fourier-transformed infrared spectra (FT-IR), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and rheological measurement. It was found that NCC yield reached 40.4 % under the optimum process of low-intensity ultrasonic-assisted sulfuric acid hydrolysis, while it was only 33.0 % in the absence of ultrasonic treatment. Furthermore, the results showed that the two NCC samples obtained from ultrasonic-assisted hydrolysis and conventional hydrolysis were very similar in morphology, both exhibiting rod-like structures with widths and lengths of 10–20 and 50–150 nm, respectively. XRD result revealed that the NCC sample from ultrasonic-assisted hydrolysis contained a small amount of cellulose II and possessed a Segal Crystallinity Index of 90.38 % and a crystallite size of 58.99 Å, higher than those of the NCC sample from conventional hydrolysis. Moreover, PSD analysis demonstrated that the former exhibited a smaller value in average particle size than the latter. In addition, rheological measurements showed that the NCC suspensions from the ultrasonic-assisted process exhibited a lower viscosity over the range of shear rate from 0.1 to 100 s^?1 in comparison with that prepared in the absence of ultrasonic treatment.     

Equilibrium properties of reversibly flocculated dispersions

The structure of sonic flocculated dispersions can be changed reversibly by means of shearing. Often the changes are not instantaneous. The resulting shear-history effect gives rise to a complex but interesting rheological behaviour. Using non-aqueous suspensions of fumed silica, the rheological equilibrium properties of such systems are investigated. To change the floc structure, the water content of the particles is altered. As well as the steady-state shear viscosity, the equilibrium modulus and the yield stress are measured. Various techniques are compared. The effect of concentration on the equilibrium properties is used to test some structural models. The concentration dependence is best described by a power-law relation, the power being identical for modulus and yield stress. These results compare well with some theoretical predictions. Contrary to the assumptions used in the modelling, the yield stress is often dominated by kinetic phenomena. This shortcoming also shows up in the predictions for the critical strain.     

Rheology and Structure of Cornstarch Suspensions in Water-Poly(propylene glycol) Mixtures

Concentrated aqueous cornstarch (CS) suspensions are often used to demonstrate an extreme example of shear thickening rheological behavior. Here, we describe the increased rheological complexity that occurs on the addition of poly(propylene glycol) (PPG) to an aqueous CS suspension. The appearance of shear thickening/jamming, shear thinning, yield stress and near-Newtonian behaviors is dependent on the PPG:water ratio. Rheological measurements have been complemented by dielectric measurements and optical microscopy. The complex behavior is interpreted in terms of reduced electrostatic stabilization of the CS particles with increased poly(propylene glycol) concentration. The analysis also suggests why cornstarch suspensions in water exhibit particularly good shear thickening characteristics.     

RHEOLOGICAL PROPERTIES OF LIQUID CRYSTALLINE COPOLY (ρ-HYDR0XYBENZ0ATE/BISPHENOL A TEREPHTHALATE)

Various aspects of the rheological behaviour of liquid crystalline copolyesters, i.e.,samples of copoly (p-hydroxybenzoate / bisphenol A terephthalate), were explored by usingInstron capillary rheometer. The experimental results indicated that the apparent viscositywas affected significantly by shear rate, melt temperature and p-hydroxybenzoate unit con-tent. The flow activation energies △E_η are in the range of 205.1 to 74.5 kJ/mol, dependingon the shear rate of 10-1000 s~(-1), at temperature 568-603K. These copolyesters exhibit ayield phenomenon in the shear flow, and the values of yield stress decrease with increasingtemperature. It is quite unusual that the extrudate of the copolyester shows the smallerswelling ratio even than unity at the lower temperature and lower shear rates.     

Synthesis and electrorheological characterization of emulsion-polymerized dodecylbenzenesulfonic acid doped polyaniline-based suspensions

The electrorheological (ER) properties of dodecylbenzenesulfonic acid (DBSA) doped polyaniline suspensions in silicone oil were investigated. In contrast to chemically polymerized polyaniline in an acidic aqueous medium by oxidation polymerization, we adopted an emulsion polymerization technique in which aniline is polymerized in an emulsion of water and a nonpolar (or weakly polar) organic solvent. The effects of electric field strength and particle concentration on the ER properties of DBSA-doped polyaniline suspensions in silicone oil were then examined. Rheological measurements were also carried out using a rotational rheometer with a high-voltage generator in both controlled shear rate and shear stress modes, and the results showed that the ER properties were enhanced by increasing the particle concentration and electric field. Received: 23 August 1999 Accepted: 6 April 2000     

Some applications of the AMPC model of the shear yield stress of particulate fluids

The AMPC structural model of shear yield stress of single component suspensions is extended to three important rheological phenomena encountered in particulate fluids. These are shear yield stress of multi-component systems, time dependent shear yield stress of thixotropic suspensions, and compressive yield stress of particulate fluids. The shear yield stress of multi-component suspensions is modeled by summing the interparticle interactions due to all possible combinations of particle sizes and solid constituents. The time dependent shear yield stress of thixotropic muds is simulated by invoking the proportionality between yield stress and interparticle bond density across the shear plane. The compressive yield stress of particulate fluids is calculated by assuming that consolidation occurs by shear but at a narrower gap between the particles. The ability of the model to describe these diverse phenomena reasonably well seemingly validates its basic premise, namely, a 3-dimensional space-filling network of particles whose mechanical properties can be estimated from the theory of strength of particulate assemblage.     

Synthesis and characterization of cubic cobalt oxide nanocomposite fluids

Narrow size distribution cubic Co₃O₄ nanoparticles were synthesized and rheological properties of suspensions of the cubes in oligomeric polyethylene glycol (PEG) were explored over a range of particle volume fractions and rotational shear flow conditions. At low and high particle volume fractions, the relative viscosity of the suspensions is described by a Krieger–Dougherty formula with an intrinsic viscosity consistent with expectations for suspensions of ideal cuboids. At intermediate to high particle loadings, the suspensions manifest complex rheological behavior, including shear thinning and shear-thickening features. These observations are discussed in terms of the charge carried by the cubes and the shear rate/volume fraction dependency of the transition from shear thinning to shear thickening.     

Rheology of concentrated carbon nanotube suspensions

The rheological properties of non-Brownian carbon nanotube suspensions are measured over a range of nanotube volume fractions spanning the transition from semidilute to concentrated. The polymer-stabilized nanotubes are "sticky" and form a quiescent elastic network with a well-defined shear modulus and yield stress that both depend strongly on nanotube volume fraction with different but related critical exponents. We compare controlled-strain-rate and controlled-stress measurements of yielding in shear flow, and we study the effect of slow periodic stress reversal on yielding and the arrest of flow. Our measurements support a universal scaling of both the linear viscoelastic and steady-shear viscometric response. The former allows us to extract the elastic shear modulus of semidilute nanotube networks for values that are near or below the resolution limit of the rheometers used, while the latter provides a similar extrapolation of the yield stress. A simple scaling argument is used to model the dependence of yield stress and elastic modulus on concentration.