Rheological Properties of Silica Particle Suspensions in Mineral Oil

The rheological properties of particles suspended in a non‐polar mineral oil have been investigated as a function of volume fraction of particles, particle size, surface properties and shear rate. Three different types of particles were investigated; glass microspheres, monodisperse silica particles and fumed silica. The suspensions showed shear thinning behavior at higher volume fractions, and the viscosity increased with decreasing particle size. The hydrophobic particles display lass shear thinning effects. The relative viscosity of all the suspensions was well fitted to the Krieger and Dougherty model.     

Effects of the degree of surface modification on the rheological responses of precipitated silica suspensions in benzyl alcohol

Two types of precipitated silica powders modified by poly (dimethylsiloxane) (PDMS) were suspended in benzyl alcohol and their rheological properties were investigated as a function of silica volume fraction, φ. The suspensions were classified into sol, pre-gel, and gel states based on the increase in φ. An increase in the degree of surface modification by PDMS caused gelation at higher φ. Plots of apparent shear viscosity against shear rate in the sol and pre-gel states of highly modified silica suspensions showed weak shear thickening behavior, while the same plots for silica suspensions with a low modification level exhibited shear thinning behavior. The dynamic moduli of hydrophobic suspensions in the pre-gel and gel states were dependent on the surface modification: the storage modulus G′ was larger than the loss modulus G″ in the linear region and these moduli increased with increasing φ, irrespective of the silica powder. The linear region of the φ range for the precipitated silica suspensions was wider than that for the fumed silica powders modified by PDMS suspended in benzyl alcohol, while the G′ value in the linear region for the precipitated silica suspensions was less than those for the fumed silica suspensions.     

Application Relevant Characterization of Aqueous Silica Nanodispersions

This work describes the application relevant characterization of eight commercially available silica powders dependent on dispersing procedure using different instruments: photon correlation spectroscopy, nitrogen adsorption, and particle charge detector. The particle size of silica was found to be different in dispersion from that of all types of fumed silica, where, after proper dispersion, the measured average particle size is similar and much higher than the primary particle size. The different properties of these two silica types will be discussed.     

Control of particle cluster dispersion using responsive polymeric additives

The dispersion behavior of clusters of hydrophilic and hydrophobic silica treated with a thermoresponsive polymer has been investigated. The influence of the thermoresponsive interfacial chemistry was assessed by performing dispersion studies in poly(dimethylsiloxane) (PDMS) at 25 and 68 degrees C for agglomerates of powders incorporating poly(N-isopropyl acrylamide) (PNIPAM), a thermoresponsive polymer, and water. While the dispersion of clusters of hydrophilic silica was found to be temperature independent, dispersion of hydrophobic silica exhibited a temperature dependency. The response of the polymer upon temperature variation brings about modifications in the interparticle interactions of the PNIPAM-treated powders thus leading to changes in the cluster cohesivity. The dependence of the cluster cohesivity on temperature was evidenced through changes in the dispersion mechanism and kinetics of the treated silica clusters at both tested temperatures. In addition, optical microscopy of silica suspensions with and without PNIPAM at 25 and 60 degrees C, illustrated the effect of the polymer presence and configuration on the particle-particle interaction forces. While micrographs of the particles at 25 degrees C show less evidence of particle aggregation, the micrographs at 60 degrees C illustrate an increase in aggregation suggesting an enhancement in the attractive forces between particles.     

Effects of Surface Properties on Rheological and Interfacial Properties of Pickering Emulsions Prepared by Fumed Silica Suspensions Pre-Adsorbed Poly(N-Isopropylacrylamide)

The hydrophobic fumed silica suspensions physically pre-adsorbed poly(N-isopropylacrylamide) (PNIPAM) in water could prepare oil dispersed in water (O/W) Pickering emulsion by mixing of silicone oil. The resulting Pickering emulsions were characterized by the measurements of volume factions of emulsified silicone oil, adsorbed amounts of the silica suspensions, oil droplet size, and some rheological responses, such as stress-strain sweep curve and dynamic viscoelastic moduli as a function of the added amount of PNIPAM. Moreover, their characteristics were compared with those of the O/W Pickering emulsions prepared by the hydrophilic fumed silica suspensions pre-adsorbed PNIPAM. For the emulsions prepared by the hydrophobic silica suspensions, an increase in the added amount of PNIPAM led to (1) a decrease in the volume fraction of the emulsified oil in the emulsified phase, (2) both the size of oil droplets and the adsorbed amount of the corresponding silica suspensions being almost constant, except for the higher added amounts, and (3) both the storage modulus (G′) and the yield shear strain being constant. The term of 1 is the same for the emulsions prepared by the hydrophilic silica suspensions, whereas both the adsorbed amount of the corresponding silica suspension and the G′ value increase and both the droplet size and the yield shear strain decrease with an increase in the added amount of PNIPAM. The differences between the rheological properties of the emulsions prepared by the hydrophilic silica suspensions and those by the hydrophobic ones are attributed to the hydrophobic interactions of the flocculated silica particles in the Pickering emulsions.     

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.     

Rheological behavior of silica suspensions in aqueous solutions of associating polymer

Associating polymers are hydrophilic long-chain molecules containing a small amount of hydrophobic groups. The aqueous solutions show viscoelastic responses above some critical concentrations because a three-dimensional structure is formed by association of hydrophobic groups. When the associating polymers are added to silica suspensions at low concentrations, the flocculation is induced by bridging mechanisms, and the flow of suspensions become shear-thinning. For suspensions prepared with polymer solutions in which the associating network is developed, the viscosity decreases, shows a minimum, and then increases with increasing particle concentration. The viscosity decrease may arise from the breakdown of associating network due to adsorption of polymer chains onto the silica surfaces. As the particle concentration is increased, the polymer concentration in solution is decreased, and finally, all polymer chains are adsorbed on the surfaces. Beyond this point, the partial coverage of particle surfaces takes place and strong interactions are generated between particles by polymer bridging. Since the stable suspensions are converted to highly flocculated systems, the viscosity is increased and the flow becomes shear-thinning. The concentration effect of silica particles on the viscosity behavior of suspensions can be explained by a combination of viscosity decrease in solution due to polymer adsorption and viscosity increase due to flocculation.     

Stability and Flow-Induced Flocculation of Fumed Silica Suspensions in Mixture of Water-Glycerol

A systematic investigation has been performed to relate the effect of glycerol composition to the rheological properties of aqueous suspensions of hydrophilic fumed silica at pH far from the isoelectric point. Steady state/dynamic rheology and electrophoresis measurements are compared to correlate the stability of the suspension with particle-particle and particle-solvent interactions. Although the extent of electrostatic stability is reduced by addition of glycerol, the rheological properties show a transition from a highly flocculated gel to stable dispersions containing no microstructures. This is attributed to a high degree of hydrogen-bonding between glycerol and the Aerosil surface silanol groups. Small dissociation of NaCl and particles reduce the effect of ion exchange and particle bridging mechanisms when the suspensions destabilise in the presence of glycerol. The high viscosity of glycerol is important with respect to the formation of a thick solvation layer around the particles. These parameters give rise to short-range, non-DLVO repulsive solvation forces, which stabilise the dispersion. At intermediate concentrations of glycerol (30–60 wt%) the apparent viscosity increase abruptly and irreversibly as both the extent and time of shearing are increased. The shear rate for the onset of the shear thickening is found to be retarded by decreasing the particle and salt concentration as well as by increasing the glycerol concentration. It is postulated that at intermediate glycerol concentration, where the height of the energy barrier is small, mechanical forces can activate the particles to overcome the energy barrier to enter the region where attractive forces dominate. Here, domination of the hydrodynamic forces to the colloidal forces under the shear results in formation of irreversible gels which does not relax to its initial condition.     

Effect of electrolyte in silicone oil-in-water emulsions stabilised by fumed silica particles

Partially hydrophobised fumed silica particles are used to make silicone oil-in-water emulsions at natural pH of the aqueous phase. The stability and rheological properties of the emulsions and suspensions are studied at NaCl concentrations in the range 0-100 mM. It is found that all emulsions are very stable to coalescence irrespective of the NaCl concentration. However, a strong effect of electrolyte on the creaming and rheological properties is observed and linked to the particle interactions in aqueous suspensions. The creaming rate and extent are large at low electrolyte concentrations but both abruptly decrease at salt concentrations exceeding the critical flocculation concentration of the suspension (approximately 1 mM NaCl). The drastic improvement of the stability to creaming is attributed to the formation of a visco-elastic three-dimensional network of interconnected particles and emulsion droplets.     

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     

Hard structured particles: suspension synthesis, characterization, and compressibility

Hard interactions are developed on three grades of fumed silica by eliminating interparticle forces and sterically stabilizing the particles by attaching an organic coating to the surface of the particles, suspending them in an index-matching solvent and screening the electrostatics. These hard-structured particles are studied to understand the effects of the particle's microstructure on suspension properties without the influence of interparticle forces other than volume exclusion, Brownian, and hydrodynamic interactions. Light and X-ray scattering studies of low-volume-fraction suspensions suggest that the fumed silicas consist of primary particle of radius of gyration R(g1) approximately equals 16 nm and aggregate size R(g2) approximately 50 nm and mass fractal dimension D(f) approximately equals 2.2. Osmotic compressibilities of these suspensions are measured as a function of particle concentration exploring the packing mechanism of fumed silica. While there is minimal detectable change in the primary particle size, R(g2) varies by approximately 15%, providing insight into how suspension properties are related to particle size. As expected of hard particles with the same microstructure, the concentration dependence on the osmotic pressure superimposes with volume fraction of solids. The comparison of fumed-silica-suspension measurements to the known behavior of hard-sphere suspensions demonstrates the effects of particle geometry on suspension properties with indications of interpenetration of the fumed silica due to their open geometry.     

Viscosity behavior of silica suspensions flocculated by associating polymers

Associating polymers are hydrophilic long-chain molecules containing a small number of hydrophobic groups, and act as flocculants in aqueous suspensions. The effects of associating and nonassociating polymers on viscosity behavior are studied for silica suspensions. Since flocculation is induced by polymer bridging, the viscosity behavior is converted from Newtonian to shear-thinning profiles. The additions of surfactant cause an increase in viscosity for suspensions prepared with associating polymer, whereas the flow behavior of suspensions with nonassociating polymer is not significantly influenced. In adsorption of associating polymers onto silica particles, the chain may adopt a conformation with a water-soluble backbone attached to the particle surfaces. The hydrophobic groups extending from the chains adsorbed onto different particles can form a micelle by association with surfactant. Therefore, the bridging flocculation is enhanced by surfactant. The cooperative micellar formation between associating polymer and surfactant is responsible for viscosity increase in suspensions.     

Modification of the rheological properties of polyurethanes by adding fumed silica: influence of the preparation procedure

The way of addition of fumed silica determined the rheological properties of polyurethane (PU) adhesives. The higher the shear rate during preparation of fumed silica containingPU adhesives, the higher viscosity and improved plasticity and thixotropy in the solutions. The improved properties of these adhesive solutions were ascribed to the creation of interactions between the silanol groups on the fumed silica, the polar groups in the soft segments of the polyurethane and/or the solvent. However, the way of incorporate the fumed silica in the polyurethane did not affect the rheological properties of fumed silica-PU composites (obtained by solvent removal from the solutions), indicating the key role of the solvent in the rheology of PU adhesive solutions.     

Rheological studies of fumed silica–polydimethylsiloxane suspensions

In this study, dynamic mechanical measurements and transmission electron microscopy are used to examine the behavior of fumed silica/polydimethylsiloxane (PDMS) suspensions. The work focuses on three aspects: (i) the modulation of the interactions between silica and PDMS through a controlled silylation of the silica surface; (ii) the effect of the process used to graft the silica surface; (iii) the effect of the silica volume fraction used in the dispersions. A relation between the silica grafting ratio, the aggregate size and the rheological properties has been established and the results have interpreted through a model of fractal clusters.     

Study of the magnetorheological response of aqueous magnetite suspensions stabilized by acrylic acid polymers

In this paper we describe the magnetorheological (MR) behavior of aqueous suspensions consisting of magnetite particles stabilized by poly(acrylic acid) polymers (PAA). A previous work on the colloidal stability of the same systems for different pH values and polymer concentrations demonstrated that the addition of PAA polymers has a very significant effect on the stability. In the present contribution, we study the MR effect of the suspensions stabilized by two different commercial polymers, as a function of pH, magnetic field strength and magnetite volume fraction. All the results are discussed in terms of the interfacial properties of the systems. It is demonstrated that for a given concentration of micrometer particles, the rheological response strongly depends on pH, on the volume fraction of magnetite particles, on the type of polymer added for increasing the stability and on the magnetic field strength. Changing the polymer used provokes clear rheological differences for the same sample conditions (field strength, volume fraction and pH). This is suggested to be due to the hydrophobic/hydrophilic balance of the polymer affecting the magnetic field ability to form magnetic structures by aggregation of the magnetized particles. The results are compared to the predictions of the so-called standard chain model, based on the assumption that the MR effect is the result of the balance between the magnetic interactions (tending to establish some degree of order in the suspension by formation of particle chains in the direction of the field) and hydrodynamic ones (tending to destroy the formed structures by viscous stress on the chains). It is found that the behavior of the yield stress does not agree well with the predictions of the model when the relative proportion of both particle and polymer confers optimum stability to the dispersions. This is likely due to the fact that the presence of the stabilizing polyelectrolyte provokes that the magnetic field is not as effective in structuring the suspension as deduced from the chain model.     

Rheological Properties of Particle-Stabilized Emulsions

We have studied the rheological properties of fumed silica particle-stabilized emulsions. Two particles of different polarity were considered, the first more hydrophilic “Aerosil R7200,” the second more hydrophobic “Aerosil R972.” These particles flocculate and probably form a network at the investigated concentration. The flow curves of emulsions stabilized by a single type of particles exhibit yield stress, shear-thinning behavior and thixotropy. Moreover they display rheological features typical of gels. These features are attributed to strengthening of the particle network by droplets. Moreover the rheological properties of w/o emulsions stabilized by hydrophobic are similar to the ones of o/w emulsions stabilized by hydrophilic particles. The rheological properties of o/w emulsions stabilized by mixtures of hydrophilic and hydrophobic particles have then been studied by keeping the total particle concentration constant and varying the mass ratio between particles. The results show that when the hydrophobic particle concentration increases, the viscosity and stability of emulsions decrease establishing evidence that the network is weakened due to preferential orientation of hydrophobic particles towards the oil phase.     

Fumed silica – rheological additive for adhesives,resins, and paints

Fumed silica, a synthetic silicon dioxide, is a powerful rheological additive for resins and paints to introduce thixotropy or even a yield point. The rheological effectiveness of fumed silica is based on its ability to form percolating networks which immobilize large volumes of liquid. By a combination of advanced rheological experiments, spectroscopical investigations, and quantum chemical calculations it could be demonstrated that the formation and stability of the silica network is strongly influenced by particle-resin interactions. The results can be used to develop comprehensive models, which explain the rheological performance of different grades of fumed silica in different resins.     

Structure and effective interactions in polymer nanocomposite melts: an integral equation theory study

The polymer reference interaction site model from integral equation theory is used to investigate the structure and effective interactions in polymer nanocomposite melts where strong nanoparticle-monomer interactions are principally considered in this work. For finite particle volume fraction, the compromise for the interference between polymers and nanoparticles results in an optimum particle volume fraction for nanoparticle dispersion in polymer melts. At constant particle volume fraction, the effects of degree of polymerization become insignificant when it reaches a threshold value, below which quantitative effects on the organization states of polymer nanocomposite melts are found and help nanoparticles to well disperse in polymer. The aggregation of large nanoparticles decreases with the increase of the nanoparticle-monomer attraction strength. These observations may provide useful information for the development of new polymer materials.     

Hydrophobicity modification of woven cotton fabric by hydrophobic fumed silica coating

Water repellency of woven cotton fabric was achieved by coating with the aqueous dispersion containing organosilane agent (HDTMS) and fumed silica. The coating agents were applied using padding method and then followed by batching the coated fabric at the ambient temperature for 24 h to allow the condensation reaction between HDTMS silanol group and fumed silica silanol group, rendering silica particles hydrophobic. An ultrasonicator was employed to prepare the homogenous coating dispersion. The water repellency evaluated by water contact angle determination which showed the contact angle over 110° was obtained with low amount of applied HDTMS of 1 wt%. The effect of fumed silica addition on an increase in fiber surface roughness geometry showed the influential result in improving the water contact angle. From durability to washing test, the hydrophobic coatings evidenced from SEM and ATR/FTIR remained adhering to fiber surface, indicating the durability. After washing, the coating on the fabric with fumed silica addition appeared to be scatter particles which made a contribution to the higher contact angle value when compared to sheet-like layer coating in case of HDTMS coating alone.     

Effect of associating polymer on the viscosity behavior of suspensions consisting of particles with different surface properties

Associating polymers which consist of water-soluble long-chain molecules containing a small fraction of hydrophobic groups (hydrophobes) behave as flocculants in aqueous suspensions. The effects of associating polymers on the rheological behavior are studied for single suspensions of particles with hydrophilic and hydrophobic surfaces, and their mixtures. For particles with hydrophilic surfaces, the suspensions are highly flocculated by a bridging mechanism, because the water-soluble chains adsorb onto hydrophilic surfaces. On the other hand, the particles with hydrophobic surfaces cannot be dispersed in water without polymer and the additions of a small amount of polymer are required for preparation of homogeneous suspensions. The associating polymer acts as a dispersant at low concentrations. However, further additions of polymer lead to a drastic increase in viscosity. Since the hydrophobes on one end of molecules adsorb onto hydrophobic surfaces and other hydrophobes tending from the particles can form micelles, the particles are connected by linkage of interchain associations. By mixing two suspensions of particles with hydrophilic and hydrophobic surfaces, the viscosity is substantially reduced and the flow becomes nearly Newtonian. The associating polymer in complex suspensions acts as a binder between the hydrophilic and hydrophobic surfaces. The hetero-flocculation which leads to the formation of composite particles may be responsible for the viscosity reduction of complex suspensions.