Dispersion of aluminum‐doped zinc oxide nanopowder in non‐aqueous suspensions

This study proposes appropriate dispersants for dispersing aluminum‐doped zinc oxide (AZO) nanopowder in the commonly used organic solvent‐dimethylacetamide (DMAC). The dispersion efficiencies and stabilization mechanisms of four DMAC‐soluble dispersants, poly(acrylic acid) (PAA), polyethyleneimine (PEI), poly(vinyl alcohol) (PVA), and poly(vinyl pyrrolidone) (PVP), are compared. The non‐polyelectrolyte‐based PVA and PVP surprisingly exhibit greater efficiency than the polyelectrolyte‐based PAA and PEI. This is because the nano AZO is soft‐agglomerated in DMAC and easily de‐agglomerated by the application of ultrasonic power; therefore, the increased viscosity contributed from additions of PVA and PVP efficiently prevent reagglomeration and sedimentation of the nanopowder. This stabilization mechanism is evidenced by an experimental analysis of zeta potentials and rheology and also by theoretical calculations based on Stokes’ law.     

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.     

Efficient Dispersants for TiO2 Nanopowder in Organic Suspensions

This article discusses the appropriate dispersant for titania (TiO₂) nanopowder in organic‐based suspensions. Four types of oleyl‐based dispersants, namely, oleyl alcohol, oleic acid, oleylamine, and oleyl phosphate, which have the functional groups hydroxyl (–OH), carboxyl (–COOH), amino (–NH₂), and phosphorous [–P(=O)(OH)₂], respectively, were compared for their ability to disperse TiO₂. Experimental results for zeta potential, adsorption, FT‐IR spectroscopy, and rheology, as well as theoretical calculations, indicate that dispersants with –P(=O)(OH)₂ and –NH₂ were more efficient than those with –COOH or –OH. The primary reason for this difference is related to the different interactions of TiO₂ with various dispersants and to different dispersion mechanisms. In addition to the major functional groups, –OH in the chemical structure of dispersants was important, as it might have other effects such as destabilization of the suspensions.     

Stabilizing Highly Loaded Silicon Nitride Aqueous Suspensions Using Comb Polymer Concrete Superplasticizers

The stabilization of highly loaded silicon nitride suspensions will afford the processing of complex and near‐net shaped parts using methods such as injection molding or direct write additive manufacturing. In this study, aqueous silicon nitride suspensions up to 45 vol% solids loading were dispersed using commercially available comb‐type copolymer. These copolymers are used as superplasticizers in the concrete industry and are referred to as water‐reducing admixtures (WRAs). Four different WRA dispersants were examined and chemical analysis determined that each was made up of a sodium salt of polyacrylic acid (PAA‐Na) backbone with neutral polyethylene oxide (PEO) side chains that afford steric stabilization. The general structures of the WRAs were compared to each other by measuring the relative areas of their prominent FTIR peaks and calculating a PAA‐Na/PEO peak ratio. Suspensions were made with as‐received silicon nitride powders with 5 wt% aluminum oxide and 5 wt% yttrium oxide added as sintering aids. Three of the four WRA dispersants studied were able to produce suspensions with 43 vol% solids loading and 5 vol% polymer dispersant, while exhibiting a yield‐pseudoplastic behavior up to 30 s^?1. At higher solids loading (45–47 vol%), a shift to shear‐thickening behavior was observed at a critical shear rate for these WRAs. Those WRAs with a lower PAA‐Na/PEO peak ratio displayed better stabilization and diminished shear‐thickening behavior. The vol% of the dispersant was optimized producing yield‐pseudoplastic suspensions containing 45 vol% solids loading with yield stresses less than 75 Pa, no shear‐thickening behavior, and viscosities less than 75 Pa·s for shear rates in the range of 1–30 s^?1.     

Roles of polymeric dispersant charge density on lead zirconate titanate aqueous processing

Poly(acrylic acid) (PAA) and poly(acrylic acid-co-maleic acid) (PACM) were used as dispersants in preparation of lead zirconate titanate (PZT) aqueous suspensions. The effects of dispersant structure on particle stabilization were investigated through properties of the suspensions. Viscosity and sedimentation height measurements showed that addition of the dispersants improved particle stabilization. The dispersant concentrations to obtain the lowest viscosity were 0.4 wt% for PAA and 0.2 wt% for PACM based on powder dried weight basis. Furthermore, effects of pH were studied on the suspensions prepared with 0.2 wt% dispersants. Viscosity and sedimentation behaviors indicated the improvements of particle dispersion and suspension stability with an increasing pH. Particle dispersion revealed by laser light scattering and scanning electron microscopy supported an improvement of particle dispersion at alkaline pHs. Detailed analysis of these data indicated that the PACM exhibited higher dispersant efficiency for PZT aqueous suspension in all conditions. The results were discussed based on the concentrations of anionic –COO^? groups at various pHs and charge density along polymeric backbone of the dispersants.     

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.     

Poly(methacrylate)‐derived diblock dispersant for TiO2 in aqueous suspensions

In this paper, we propose a newly designed dispersant, ammonium poly(methacrylate)‐block‐poly(2‐phenoxyethyl acrylate) (PMA‐b‐PBEA), and our rheological and zeta potential test results verify its superior dispersion efficiency for aqueous suspensions in comparison to the commercial dispersant ammonium polyacrylate (PAA‐NH₄). The extremely high dispersion efficiency of PMA‐b‐PBEA correlates closely to its diblock structure, which simultaneously exhibits a less polar anchoring head group and a water‐dissociable stabilizing moiety. The unique structure of PMA‐b‐PBEA accounts for its high powder adsorption effectiveness, which is demonstrated in its adsorption capability being double that of PAA‐NH₄.     

Colloidal Processing of Silicon Nitride with Poly(acrylic acid): II, Rheological Properties

Poly(acrylic acid) (PAA) was the focus of an investigation in the role and behavior of polyelectrolyte dispersants in aqueous-based colloidal processing of silicon nitride. The steady-shear flow properties of dense suspensions were examined as a function of pH, particle volume fraction, chain length, and polymer concentration. Suspensions exhibited shear-thinning behavior, which was analyzed using the constitutive Bingham equation for plastic flow. The data showed that polyelectrolyte charge and configuration were important in the observed flow behavior. In acidic media, PAA behaved similar to a neutral polymer in a poor solvent, whereas, in alkaline media, PAA behaved similar to a rigid asymmetric particle. Although electrostatic forces dominated flow properties in alkaline media, where PAA was predominantly in the free-polymer form, at lower pH values, PAA interactions with the particle surface resulted in a complex array of suspension effects, including polymer bridging, electrosteric stabilization, and flocculation.     

Effect of Polyelectrolyte Dispersants on the Preparation of Silica-Coated Zinc Oxide Particles in Aqueous Media

Sodium silicate was utilized to obtain a SiO₂ coating on ZnO particles to prevent a photocatalytic reaction between ZnO and phenol. During the coating process, pH control is important to avoid dissolution of the ZnO as well as to obtain a good dispersion. Two kinds of polyelectrolyte dispersants were used to control the surface charge of the ZnO particles in aqueous media. As a result, poly(ethylenimine) (PEI) shifted the isoelectric point of ZnO from pH 9 to pH 10, whereas poly(ammonium acrylate) (PAA) made the surface charge of ZnO negative between pH 6 and pH 11. The change in the ZnO surface charge produced by adding polyelectrolyte dispersants makes it possible to obtain uniform silica–coated ZnO particle in aqueous media. UV–irradiation experiments showed that PEI, which can make the surface charge opposite to that of SiO₂, is more effective in obtaining a thick silica coating on ZnO.     

Selectivity of Hydrophilic and Hydrophobic TiO2 for Organic‐Based Dispersants

The effects of the surface chemistry of TiO₂ powders on the dispersion performance of various dispersants are studied. Four common dispersants (oleic acid, oleylamine, oleyl phosphate, and tris‐(2‐butoxyethyl) phosphate) with different functional groups (carboxyl (–COOH), amino (–NH₂), phosphate (–P(=O)(OH)₂), and –P(=O)) are investigated for their potential to disperse hydrophilic and hydrophobic titania (TiO₂) powders. The outcomes, based on adsorption kinetics, adsorption isotherms, rheologies, and theoretical calculations, indicate that the hydrophilic TiO₂ is more sensitive to the chemistry of dispersants as compared to the hydrophobic TiO₂. However, the relative dispersion efficiencies of the dispersants are not found relevant to the adsorption kinetics, which is dominated by the adsorption amount. In addition, hydrogen bonding between –OH groups of the phosphate‐based dispersants dominates their dispersion ability for TiO₂.     

Stabilization of highly-loaded boron carbide aqueous suspensions

Injection molding of boron carbide (B₄C) slurries affords the production of complex-shaped personal armor. To injection mold, however, requires preparation of a well dispersed, flowable suspension with >45 vol% B₄C loadings to reduce porosity that must be removed during sintering. In the present study, the preparation of highly-loaded B₄C suspensions is investigated using zeta potential and rheological measurements, varying dispersant type, molecular weight, and amount. Of those dispersants investigated, polyethylenimine (PEI) with a molecular weight of 25,000 g/mol was found to produce suspensions with up to 56 vol% B₄C and the requisite rheological properties suitable for injection molding. A PEI concentration of 1.83 mg/m² was established as the appropriate to produce highly-loaded B₄C suspensions. The effect of a prior B₄C powder treatment (ethanol washed or attrition milled) on rheological properties of the suspensions was also investigated. The PEI was completely burned out in argon, nitrogen, and air at 450 °C.     

Effect of Polyacrylic Acid Addition on Rheology of SiC‐Al2O3‐ZrO2(3Y) Mixed Suspensions

A suspension with good rheology and high stability is crucial for slip casting and gelcasting technology. However, a mixed suspension from two or more different powders usually has bad rheology because of the easy agglomeration of mixed powders caused by the attractive force between the powders with heterocharges. We studied the surface modification of the each single‐component powders (SiC, Al₂O₃, ZrO₂(3Y) powders) and the SiC‐Al₂O₃‐ZrO₂(3Y) mixed powders to increase the repulsive force by adjusting the pH value and adding polyacrylic acid (PAA) as dispersant. The PAA addition effects on the SiC‐Al₂O₃‐ZrO₂(3Y) mixture were investigated in terms of zeta potential, pH range for heterocharge region, dispersion of the mixed powders and rheology of the mixed slurry based on the study of each unary suspensions. The results show that before surface modification the SiC‐Al₂O₃‐ZrO₂(3Y) mixed powders were agglomerated severely because they were in the heterocharge region with a broad pH range from 3.5 to 8.25, while after surface modification (pH = 10.5, PAA = 0.8wt%) the heterocharge region was narrowed with a relatively narrower pH range from 2.6 to 3.7. The mixed powders with homocharges were dispersed well because of the great electrostatic repulsive force and steric hindrance offered by PAA and the mixed suspensions had favorable rheology.     

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.     

α‐Alumina nanospheres from nano‐dispersed boehmite synthesized by a wet chemical route

In this work, stable aqueous suspensions of nano‐boehmite were developed through a hybrid wet‐chemical route that uses hyperbranched dendritic poly(ethylene)imine (PEI) as template material for boehmite formation aiming at the development of a deagglomerated α‐alumina nanopowder after calcination. The method involves firstly the interaction between the Al precursor and PEI followed by the hydrolysis and polycondensation reactions. The study was aiming to investigate the effect of solids content and pH during reactions on both the stability of the final suspension and the morphology of the resulting nanocrystals. For this purpose, the suspensions were evaluated through viscosity measurements, zeta potential analyses, FT‐IR, DLS and sedimentation studies, whereas after the proper centrifugation, drying, and calcination steps, the as‐received nanocrystals were evaluated through SEM, TEM and XRD studies. In addition, the boehmite nanopowder was studied using Thermogravimetric and Differential Thermal Analysis, whereas its sinterability was evaluated by dilatometric measurements. The investigation showed that the conditions employed affect greatly both the morphology of nanocrystals as well as the dispersion and the stability of the suspensions. The boehmite suspension with the optimum dispersion and stability can lead, after calcination at 1050°C, to a fine deagglomerated α‐alumina nanopowder with a mean size at about 10 nm.     

Preparation and characterization of waterborne ceramic ink with submicron-sized praseodymium-doped zirconium silicate pigment by water-based diblock polymer dispersants

Two diblock polymer dispersants (i.e., PMAA-b-AMPS and PSSS-b-GMA) were synthesized and used as water-based dispersants for dispersion and stabilization of waterborne ceramic ink with submicron-sized praseodymium-doped zirconium silicate (Pr-ZrSiO₄) pigment. The color property, dispersion, and stability of the pigment particles in aqueous suspension were determined by colorimetry, laser particle size analysis, and sedimentation test. The adsorption mechanism of carbon chains of dispersants on the particle surface was analyzed by Fourier transform infrared spectroscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy and electrokinetic potential measurement. Also, the viscosity, surface tension and stability of waterborne ceramic ink with the pigment particles were measured by rheometry, surface tension analysis and standing settlement. The results show that the submicron-sized pigment particles in waterborne ceramic ink show optimum dispersibility and stability at 5 wt% addition of diblock polymer dispersants PMAA-b-AMPS and PSSS-b-GMA mixed in a mass ratio of 9:1. This could be since the mixed dispersants provide the more interparticle electrostatic repulsion and steric hindrance energies due to the proper adsorption on the particle surfaces. It is indicated that adding 20 wt% of diethylene glycol monobutyl ether into the waterborne ceramic ink can effectively enhance the viscosity and reduce the surface tension, thus satisfying the applied requirements in inkjet printing. In addition, the jettability of waterborne ceramic ink on a simulated ceramic green body was also evaluated based on semi-empirical analysis.     

POEGMA‐b‐PAA comb‐like polymer dispersant for Al2 suspensions

POEGMA‐b‐PAA comb‐like polymer is synthesized through RAFT polymerization, and it is employed as an efficient dispersant for Al₂O₃ suspensions. The POEGMA‐b‐PAA polymer consists of PAA chains and POEGMA comb‐like chains. The former provide electrostatic attraction between Al₂O₃ particles and polymer, while the latter extend to solution and maintain the stability of suspension due to strong steric hindrance. The adsorption is proven and the rheology behaviors of Al₂O₃ suspensions are strongly improved. Different POEGMA‐b‐PAA polymers with different length of side chains have similar but not identical rheological properties. The polymer with the appropriate length of side chain provides the biggest improvement to rheological properties of Al₂O₃ suspensions, such as apparent viscosity and granularity. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43352.     

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.     

Electrosteric stabilization of concentrated cement suspensions imparted by a strong anionic polyelectrolyte and a non-ionic polymer

Strong polyelectrolytes, known as superplasticizers, improve the initial fluidity of concentrated cement suspensions through electrostatic stabilization. These polyelectrolytes do not maintain the initial fluidity, however, primarily due to an increase in the ionic strength of the cementitious suspension. Consequently, non-ionic polymers are often used in conjunction with polyelectrolytes to provide steric stabilization and hence to sustain the desired fluidity over a longer time, and this has lead to the development of copolymers with both electrostatic and steric (electrosteric) functionalities. To design such polymers, it is necessary to optimize the balance between electrostatic and steric stabilization to maximize suspension fluidity. We have quantified the effects of a strong anionic polyelectrolyte, melamine formaldehyde sulfonate (MFS), and a non-ionic polymer, hydroxypropylmethylcellulose (HPMC), on the zeta potential of cement particles and the steady shear and low-amplitude rheological properties of concentrated cement suspensions. While the adsorption of MFS onto the cement particle surfaces leads to a sign inversion in the zeta potential, the adsorption of the non-ionic HPMC has no significant effect on the potential. The addition of HPMC to the suspensions substantially reduces the steady shear viscosity and the storage modulus at constant MFS concentration; in addition, there exists an intermediate HPMC concentration that minimizes fluidity. The resulting suspension fluidity is also maintained over a longer time than in the absence of HPMC. This improvement in the stability and fluidity of cement suspensions is attributed to “complementary electrosteric dispersion/stabilization”, and provides insight to the design of polymers with electrosteric functionality.     

Effect of PAA-g-MPEO Comb Polymer on TiO2 Suspensions

A comb PAA-g-MPEO was prepared. The PAA-g-MPEO underwent two thermal degradation steps. First, the decarboxylation and the dehydration of the carboxyl in the PAA backbone took place in the range of 204–313°C; the weight loss in the range of 313–450°C was from decomposition of MPEO chains and the survivor of the decomposed PAA backbone. By adding comb PAA-g-MPEO as disperser to TiO₂ suspensions, TiO₂/PAA-g-MPEO suspensions were prepared. The PAA-g-MPEO was adsorbed onto the surfaces of the TiO₂ particles, and the zeta potential of TiO₂/PAA-g-MPEO suspensions was higher than that of TiO₂ suspensions. Compared with that of TiO₂ suspensions, the viscosity of TiO₂/PAA-g-MPEO suspensions decreased, and the viscosity further decreased with increasing PAA-g-MPEO content. After adding PAA-g-MPEO, the size distribution of the TiO₂ particles in the suspensions became narrow and the average diameter of the TiO₂ particles decreased due to electrostatic repulsions and the steric hindrances of the PAA-g-MPEO.     

Photopolymerization synthesis of polyacrylic acid dispersant with methoxysilicon end groups and its application in a nano‐SiO2 aqueous system

In order to improve the dispersity and stability of the nano‐SiO₂ aqueous system with high solid content, a kind of polyacrylic acid dispersant with methoxysilicon end groups (KH590‐PAA) was synthesized by photopolymerization of acrylic acid (AA) initiated with (3‐mercaptopropyl)trimethoxysilane (KH590). After adding KH590‐PAA into the nano‐SiO₂ aqueous dispersion system (20 wt% solid content), the viscosity and the curing time of the system were measured with a rotational viscometer and the inverted bottle method. Moreover, the dispersion mechanism of KH590‐PAA for the nano‐SiO₂ aqueous system was researched by measuring the adsorption capacity, the particle size and the zeta potential of the nanoparticles with a conductivity meter, dynamic light scattering, SEM and TEM, respectively. The results showed that the methoxysilicon groups in KH590‐PAA could react with hydroxyl groups on the surface of nano‐SiO₂ in the process of stirring, which enhanced the adsorption capacity of the dispersant and then increased the surface charge of the particles. Therefore, electrostatic repulsion and steric hindrance effects between the SiO₂ nanoparticles could be further enhanced by adding the KH590‐PAA dispersant, and then the nano‐SiO₂ aqueous system exhibited better dispersity and stability. Besides, the dispersion properties of SiO₂ nanoparticles in water were closely related to the addition amount and the molecular weight of the KH590‐PAA dispersant. © 2018 Society of Chemical Industry