Effects of a strong polyelectrolyte on the rheological properties of concentrated cementitious suspensions

Strong polyelectrolytes, referred to as superplasticizers, are known to improve the initial fluidity of concentrated cement suspensions. To quantify how the polyelectrolytes affect the fluidity, we have studied the effect of a strong anionic polyelectrolyte, melamine formaldehyde sulfonate (MFS), on the zeta potential of cement particles and on the steady-shear and low-amplitude rheological properties of cement suspensions. Adsorption of low concentrations of MFS onto the cement particles leads to an inversion in the sign of the surface potential, causing the electrostatically flocculated particles to become electrostatically dispersed and giving rise to a corresponding decrease in the steady-shear viscosity and storage modulus. At an intermediate MFS concentration, the steady-shear viscosity and the storage modulus each display a minimum. This concentration corresponds to that at which the zeta potential becomes constant. Larger concentrations of MFS result in an increase in the viscosity and storage modulus, which is attributed to depletion flocculation. These results thus relate the interaction between particles to the suspension fluidity through the analysis on the surface potential of particles and microstructure of suspension.     

Derivatives of biomacromolecules as stabilizers of aqueous alumina suspensions

The stabilization of alumina suspensions is key to the development of high‐performance materials for the ceramic industry, which has motivated extensive research into synthetic polymers used as stabilizers. In this study, mimosa tannin extract and a chitosan derivative, that is, macromolecules obtained from renewable resources, are shown to be promising to replace synthetic polymers, yielding less viscous suspensions with smaller particles and greater fluidity, that is, more homogeneous suspensions that may lead to better‐quality products. The functional groups of tannin present in mimosa extract and N,N,N‐trimethylchitosan (TMC) are capable of establishing interactions with the alumina surface, thus leading to repulsion between the particles mainly due to steric and electrosteric mechanisms, respectively. The stabilization of the suspension induced by either TMC or mimosa tannin was confirmed by a considerable decrease in viscosity and average particle size, in comparison with alumina suspensions without stabilizing agents. The viscosity/average particle size decreased by 49/84% and 52/87% for suspensions with TMC and mimosa tannin, respectively. In addition, the increase in the absolute zeta potential upon addition of either TMC or mimosa tannin extract, especially at high pHs, points to an increased stability of the suspension. The feasibility of using derivatives of macromolecules from renewable sources to stabilize aqueous alumina suspensions was therefore demonstrated. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Dispersion and rheological properties of concentrated kaolin suspensions with polycarboxylate copolymers bearing comb-like side chains

Comb-like polycarboxylate copolymers (PCs) were synthesized and first used as the dispersants for kaolin suspensions. The apparent viscosity, adsorption behavior, zeta potential, rheological behavior, and TEM images of concentrated kaolin slurries deflocculated with these copolymers were systematically investigated. Sodium acrylate homopolymer (SA) was selected as a reference in order to contribute to the knowledge of how the electrostatic interaction and molecular structure of these copolymers influenced the dispersion, adsorption, and rheological properties of colloidal kaolin suspensions. The results showed that the dispersion of kaolin slurries mainly depended on the synergistic effects of electrostatic interaction and steric hindrance for APEG type comb-like copolymers. PC₂ bearing proper carboxylic groups and side chains produced stable dispersion and fluidity in concentrated kaolin slurries. Based on the electrostatic and steric effect of comb-like copolymers, the kaolin slurries displayed a shear thickening behavior, and the Herschel–Bulkley model well described their rheological behaviors.     

Electrostatic and electrosteric stabilization of aqueous suspensions of barite nanoparticles

Electrostatic and electrosteric stabilization of aqueous suspensions of barite nanoparticles were investigated. The state of dispersion was evaluated in terms of zeta potential, apparent viscosity and the mean particle size of solid phase in the solution. Zeta potential, apparent viscosity and the mean particle size as a function of pH were performed in absence of dispersant. The result showed that electrostatic stabilization of the aqueous suspension of barite nanoparticles can be accomplished in low acidic and high basic range of pH. In presence of sodium polyacrylate (PAA-Na) dispersant, the isoelectric point (IEP) of the barite nanoparticles was shifted to lower pH and the negative zeta potential was increased in a large range of pH above the (IEP). The optimum amount of PAA-Na dispersant is discussed in the light of zeta potential and viscosity. It is found that the adsorption of PAA is correlated to the net surface charge of the barite nanoparticles and the fraction of dissociated polymer at pH 4, 8.5 and 10. At pH 4, the state of dispersion was achieved at higher amount of electrolyte due to the low fraction of negatively charged dissociated polymer and the positively charge particles. At basic pH, the fraction of dissociated polymer was high and the surface charge of particle was highly negative, therefore, the lowest viscosity was obtained at a small amount of PAA. In addition, the optimum amount of polymer decreased with the increase in pH of the suspension.     

Effect of dispersion states (electrostatic/electrosteric stabilization) on particles arrangement in the yttria-stabilized zirconia sediments

In the present work, particle arrangement and their packing in the sediment layer of zirconia suspension were studied. To evaluate the particle settling, aqueous suspensions of zirconia nanoparticles were prepared in different dispersion states. In one state, Dolapix CE64 was used as a dispersant to provide electrosteric mechanism. In another state, pH of the suspension was adjusted at 4 to provide electrostatic mechanism. The other state was the combination of dispersant and pH adjustment which resulted in the most stable suspension. First of all, the stability of all dispersion states was evaluated by zeta potential, sediment volume (SV) and height, viscosity, and packing density (PD). Then, the sediment layers of all suspensions were characterized. Incorporation of electrostatic mechanism was resulted in a main decrease in viscosity with high surface charges, while electrosteric mechanism caused lower sedimentation of particles. Fall velocities of particles/agglomerates were estimated, and the influences of dispersion states on the particles fall velocities were characterized. The microstructural observation revealed homogeneous packing of particles in the sediment layer of the stable suspension demonstrating the proper dispersion of particles. Dolapix CE64 and pH adjustment resulted in a uniform arrangement of particles without agglomeration and spherical and regular granules with a uniform shape.     

Dispersing mechanism of carboxymethyl starch as water‐reducing agent

In this article, by using zeta (ζ) potential analyzer, X‐ray photo spectroscopy, and UV–visible adsorption spectrophotometer, the adsorption characteristics and surface electrochemical properties of carboxymethyl starch (CMS) as water‐reducing agent on cement particles and its effects on the fluidity of cement paste were studied in comparison with traditional naphthalene sulfonic acid–formaldehyde condensates, whose dispersion ability mainly depends on electrostatic repulsive force caused by ζ‐potential. The results indicate that the adsorption conformation of CMS is dendritic, generating strong steric hindrance by branched chains and polar side chains of adsorption layer, and the dispersion capacity of CMS mainly comes from steric hindrance effect rather than electrostatic repulsive force. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Stabilization of alumina with polyelectrolyte and comb copolymer in solvent mixtures of water and alcohols

Solvent mixtures of water and ethanol and water and isopropanol have been evaluated for processing of concentrated alumina suspensions. The addition of alcohols may increase the long-term stability of suspensions with soluble ceramic species such as magnesia, which is added as a sintering aid. A poly(acrylic acid) and a hydrophilic comb copolymer were used as dispersants for the different solvent mixtures. The aim was to compare the stabilization efficiency at normal processing conditions, pH 9–10, through rheological measurements and to develop a robust system including magnesia with long-term stability. The electrostatic stabilization of the dispersants in the different solvent mixtures was studied by zeta potential measurements. Highly negative zeta potentials were observed for the poly(acrylic acid) at pH 9–10 in the solvent mixtures. A charge contribution was also seen from the adsorbed comb copolymer, however smaller than for the poly(acrylic acid). Low viscosity was obtained for suspensions stabilized with poly(acrylic acid) in solvent mixtures with either 25 vol% ethanol or isopropanol. Higher alcohol to water ratio led to flocculation of the suspension when poly(acrylic acid) was used as dispersant. Alumina suspensions with added magnesia in isopropanol:water 25:75 and poly(acrylic acid) as dispersant showed long-term stability. The viscosity remained almost constant during 4 days of aging. Suspensions stabilized with the comb copolymer dispersant gave stable systems with ethanol and isopropanol concentrations between 25 and 75 vol%. The superior dispersing efficiency of the comb copolymer at alcohol contents above 25 vol% was believed to originate from steric stabilization in combination with low effective particle size, giving low viscosity through lower apparent solid contents of the suspension.     

Using a Brush Copolymer as Efficient Dispersant for the Preparation of Highly Stabilized Ag Nanoparticles in Aqueous Suspensions

Silver (Ag) nanoparticle has extremely high surface energy and it is difficult to find an efficient dispersant to prevent its agglomeration in suspensions. A new brush copolymer, succinic anhydride modified epoxy-amine poly[(propylene oxide)-co-(ethylene oxide)]-grafted polymer (EPOA), which can efficiently disperse concentrated aqueous suspensions of Ag nanoparticles is revealed. The dispersion efficiency of EPOA for the dispersion of a 60 nm-Ag nanoparticles in aqueous suspension is studied by measuring its sedimentation and rheological behavior, and the results are compared with those of a commercially available dispersant, ammonium poly(acrylic acid) (PAA-NH₄). Interactions between the dispersants and the Ag nanoparticles are characterized by zeta potential and adsorption analyses. Theoretical calculations are conducted to clarify the adsorption and the dominant dispersion stabilization mechanisms of the dispersants. Compared with PAA-NH₄, EPOA obtains a higher stable suspension of Ag nanoparticles with less significant sedimentation over 1 month. The dispersion homogeneity of the suspension remains excellent even at an extremely high solid loading of 30–40 wt%. According to adsorption analysis, it is suggested that both EPOA and PAA-NH₄ adsorb via single-point attachment through the carboxyl group on the Ag surface. Based on theoretical calculations, the Ag nanoparticles are better stabilized by EPOA via an electrosteric dispersion mechanism.     

Hydrolysis and Dispersion Properties of Aqueous Y2Si2O7 Suspensions

The dispersion of aqueous γ-Y₂Si₂O₇ suspensions, which contain only one component but have a complex ion environment, was studied by the introduction of two different polymer dispersants, polyethylenimine (PEI) and polyacrylic acid (PAA). The suspension without any dispersant remains stable in the pH range of 9–11.5 because of electrostatic repulsion, while it is flocculated upon stirring due to the readsorption of hydrolyzed ions on the colloid surface. However, suspensions with 1 dwb% PEI exhibit greater stability in the pH range of 4–11.5. The addition of PEI shifts the isoelectric point (IEP) of the suspensions from pH 5.8 to 10.8. Near the IEP (pH_IEP=10.8), the stability of the suspensions with PEI is dominated by the steric effect. When the pH is decreased to acid direction, the stabilization mechanism is changed from steric hindrance to an electrosteric effect little by little. PAA also has the effect of reducing the hydrolysis speed via a "buffer effect" in the basic pH range, but the lack of adsorption between the highly ionized anionic polymer molecules and the negative colloid particle surfaces shows no positive effect on hydrolysis of colloids and on the stabilization of Y₂Si₂O₇ suspensions.     

Studying surface charge and suspension stability of hydroxyapatite powder in isopropyl alcohol to prepare stable suspension for electrophoretic deposition

Abstract

Suspension stability of hydroxyapatite powder in isopropyl alcohol was investigated by measuring zeta potential. In this study, isopropyl alcohol was used as a suspension media, and nitric acid, polyvinyl butyral and polyethylenimine were the dispersing agents. Fine grain particles with mean particle size and surface area of 2·54 μm and 5·8 m² g^?1 respectively, were prepared. Stable suspensions were made by mixing 40 g L^?1 milled powders in isopropyl alcohol. The zeta potential of the suspensions was measured by using zeta sizer equipment. The results showed that by adding nitric acid and polyvinyl butyral to the system, negative charge was introduced, while polyethylenimine donated positive charge to the surface of particles and induced electrostatic and steric stabilisation. It was concluded that the suspension which was stabilised with 4 g L^?1 polyethylenimine revealed the highest zeta potential and stability which is suitable for cathodic electrophoretic deposition.     

Stability of titania nano-particles in different alcohols

The alcoholic suspensions of titania nano-particles were prepared using the methanol, ethanol, isopropanol and butanol as the solvents as well as triethanolamine (TEA) as an dispersant. The colloidal stability of suspensions, both in the absence as well as presence of TEA, was studied by measuring the zeta potential, sedimentation, pH and electrical conductivity of suspensions, dispersant adsorption and particles size distribution. Results showed that in the absence of TEA, the stability of the suspensions increases with the molecular size of alcohol (from methanol to butanol), while the zeta potential decreases. It was also observed that with the addition of TEA up to the optimum dosage (4 mL/L and 1.33 mL/L for isopropanolic and butanolic suspensions, respectively), the stability of isopropanolic and butanolic suspensions increases; however, its addition into the methanolic and ethanolic suspensions, even at very low concentrations in the order of 0.1 mL/L, results in the quick settling of the particles in the suspension. It was found that the mechanism of TEA action as a dispersant, in the isopropanolic and butanolic suspensions, is its protonation and then adsorption on the titania nano-particles, which yields an electrostatic and some steric stabilization mechanisms.     

Ascorbic acid as a dispersant for concentrated alumina nanopowder suspensions

The stabilization of concentrated nanopowder suspensions is crucial for many industrial applications. Yet, controlling the suspension viscosity is challenging for nanopowder suspension systems. In this study, we examined the adsorption of l-ascorbic acid (Vitamin C) on alumina surfaces and the related reduction in viscosity of the suspensions. Interactions between the ascorbic acid and the alumina surface were investigated by in situ ATR-FTIR and zeta potential measurements. It was shown that ascorbic acid forms complexes with the alumina surface through ligand exchange mechanisms. The optimum concentration of ascorbic acid for minimum suspension viscosity was determined. The maximum achievable solids content could be increased to around 0.35 by the addition of only 1.0 wt.% of dry powder ascorbic acid. Because ascorbic acid is easy to use, inexpensive, and a non-toxic organic additive, it has great potential to be used as a dispersant in a variety of industrial applications, from dilute to concentrated systems of intermediates or products.     

Effects of comb copolymer PAA-g-MPEO on rheological and dispersion properties of aqueous CaCO<Subscript>3</Subscript> suspensions

Summary The comb copolymer poly(acrylic acid) (PAA) grafted methoxyl poly(ethylene oxide) (MPEO) (PAA-g-MPEO) as dispersant was used in aqueous CaCO₃ suspensions. The PAA-g-MPEO was adsorbed onto CaCO₃particle surfaces due to the electrostatic attraction. The adsorbed amount increased with increasing PAA-g-MPEO content. The CaCO₃ adsorbed PAA-g-MPEO displayed negative zeta potential. The zeta potential was more negative with increasing PAA-g-MPEO content. Addition of PAA-g-MPEO, the conductivity of aqueous CaCO₃ suspensions decreased firstly, and then increased with increasing PAA-g-MPEO content. Compared to that of aqueous CaCO₃ suspensions, the viscosity of aqueous CaCO₃/PAA-g-MPEO suspension reduced remarkably, and the liquidity of the suspensions was improved. The dispersion of CaCO₃ particles in aqueous CaCO₃/PAA-g-MPEO suspensions was significantly improved due to electrostatic repulsions and steric hindrance between CaCO₃ particles adsorbed PAA-g-MPEO.     

Polyelectrolyte Effects on the Rheological Properties of Concentrated Cement Suspensions

Polyelectrolyte species, known as superplasticizers, dramatically affect the rheological properties of dense cement suspensions. We have studied the influence of sulfonated naphthalene formaldehyde condensate (SNF) and carboxylated acrylic ester (CAE) grafted copolymers of varying molecular architecture on the surface (e.g., adsorption behavior and zeta potential) and rheological properties of concentrated cement suspensions of white portland cement and two model compounds, β-Ca₂SiO₄ and γ-Ca₂SiO₄. The adsorption of SNF species was strongly dependent on cement chemistry, whereas CAE species exhibited little sensitivity. The respective critical concentrations (Φ*) in suspension required to promote the transition from strongly shear thinning to Newtonian flow (flocculated → stable) behavior were determined from stress viscometry and yield stress measurements. Theoretical analysis of interparticle interactions suggested that only colloidal particles in the size range of ≤1 μm are fully stabilized by adsorbed polyelectrolyte species. Our observations provide guidelines for tailoring the molecular architecture and functionality of superplasticizers for optimal performance.     

Efficient dispersants for the dispersion of gallium zinc oxide nanopowder in aqueous suspensions

Appropriate dispersants for the dispersion of gallium zinc oxide (GZO) nanopowder in aqueous suspensions were identified in this study. The dispersion efficiencies and stabilization mechanisms of water‐based dispersants ammonium poly(acrylic acid) (PAA–NH₄), an anionic polyelectrolyte, and polyethylenimine (PEI), a cationic polyelectrolyte, were compared. The experimental analyses of rheology and sedimentation showed that both PAA–NH₄ and PEI were good dispersants for the dispersion of GZO. Theoretical calculations based on Derjaguin‐Landau‐Verwey‐Overbeek theory revealed that the stabilization mechanism of PEI was mainly related to the steric effect, and a very low molecular weight of 1800 g/mol was insufficient for powder stabilization. GZO was well dispersed by PEI with high molecular weight of 10 000 g/mol, but agglomeration occurred when too much PEI was added. Compared with PEI, PAA–NH₄ was more efficient because of its high contribution to the increase in electrostatic repulsion. Based on theoretical considerations on both steric and electrostatic effects, namely, the electrosteric stabilization mechanism, PAA–NH₄ is optimal for the dispersion of GZO nanopowder in aqueous suspensions.     

ZTA复相陶瓷凝胶注模成形工艺的研究

本文研究了ＺｒＯ２（３Ｙ）－Ａｌ２Ｏ３复相陶瓷的凝胶注模成形工艺,着重研究了低粘度高固相体积分数浓悬浮体的制备。     

Microstructure and phase behavior of concentrated silica particle suspensions

Dispersion stability and microstructural transition of colloidal silica suspensions were examined by rheological measurements under either steady simple shear or oscillatory flow. Monodisperse silica particles were prepared by the so-called modified Stöber method and were stabilized by either steric or electrostatic repulsive force. Depending upon the methods of stabilization, the suspension showed hard-sphere or soft-sphere response. In particular, silica suspensions exhibited hard-sphere response when the silica spheres coated with 3-(trimethoxysilyl)propyl methacrylate (MPTS; (CH₃O)₃Si(CH₂)₃OCOC(CH₃)=CH₂) were dispersed in a refractive-index matching solvent, tetrahydrofurfuryl alcohol. On the other hand, silica particles in aqueous media behaved like soft spheres with long-range electrostatic repulsive interactions when they were coated with steric layer of aminosilane coupling agent, N-[3-(trimethoxysilyl)propyl]ethylenediamine ((CH₃O)₃Si(CH₂)₃NHCH₂CH₂NH₂). In this case, the electrostatic repulsion or equivalently the softness of the silica spheres was controlled by the ionic strength using a symmetric salt KCl. Both the hard-sphere and soft-sphere suspensions showed stable shear-thinning behavior without experiencing shear-induced flocculation. Moreover, the oscillatory shear rheology showed that the electrostatically stabilized soft-sphere suspensions underwent a microstructural transition from liquid-like to solid-like structure when either the particle loading increased or the ionic strength was reduced.     

Electrostatic repulsion between particles in cement suspensions: Domain of validity of linearized Poisson-Boltzmann equation for nonideal electrolytes

To understand the dispersion of cement particles by superplasticizers, proper quantification of the possible mechanisms involved (electrostatic, steric, and depletion forces) is required. This is an important objective to help understand the origin of unexpected incompatibilities between some cement and superplasticizer combinations and more generally predict the rheological behavior. The relative importance of the electrostatic interaction with respect to steric hindrance is currently under much debate. The debate centered on this topic has not fully explored how the nonideal electrolyte found in cement suspensions effects the simplified Debye-Hückel approximation for evaluation of electrostatic repulsion. In this article, the nonideality of the cement aqueous phase—suspension electrolyte—has been taken into account based on solubility equilibria of the possible ionic species present in typical cement suspensions. By replacing the normally assumed symmetric electrolyte (1:1, 2:2, or 3:3) with a noninteger symmetric electrolyte, the simple Debye-Hückel approach has been shown to remain valid for negative potentials down to around 30 mV. Significant deviations are found for positive potentials greater than 10 mV.     

稳定的SrTiO3陶瓷浆料的制备

本采用胶体“电空间稳定机制”，以PMAA-NH4为分散剂，以沉降高度作为衡量浆料稳定性的参数，研究了SrTiO3粉末的悬浮流变特性及分散剂PMAA-NH4加入量对SrTiO3浆料稳定性的影响。在最佳pH值和分散剂加入量条件下，制备了高固相含量（50vol%)，稳定性和分散性好的SrTiO2浆料。     

Ketone-amine based suspensions for electrophoretic deposition of Al2O3 and ZrO2

Stable suspensions based on methylethylketone (MEK), n-butylamine and nitrocellulose were developed for the electrophoretic deposition (EPD) of Al₂O₃ and ZrO₂ powder. Deposits with a high green density, smooth surface and high deposition yield were obtained upon adding 10–15 vol.% n-butylamine in combination with 1 wt.% nitrocellulose. The influence of the reaction between MEK and n-butylamine, forming water and imines, on the electrophoretic deposition behaviour was investigated. Experimental results revealed that the zeta potential is not a straightforward indication of the stability of these suspensions, since the maximum absolute zeta potential did not correspond with a maximum suspension stability, due to the additional electrosteric stabilisation of the adsorbed charged nitrocellulose.