Jean-François Baumard

Processing of aqueous α-Al2O3, α-SiO2 and α-SiC suspensions with polyelectrolytes

Abstract The preparation of aqueous α-Al 2 O 3 , α-SiO 2 and α-SiC suspensions with polyelectrolytes, i.e. the NH 4 + salt of polymethacrylic acid (PMA-NH 4 + ) and the acidic form of polyethylene imine (PEI-H + ), is investigated. It is based on the adsorption of these polyelectrolytes which in turn depends mainly on the nature of the polyelectrolyte, and the charge density that develops on the powder surface in water. The efficiency of these dispersants is primarily attributed to the electrostatic interaction between -COO − groups of PMA-NH 4 + and positive sites on the alumina surface at pH 8–9 as well as between -NH 2 + -groups of PEI-H + and negative sites of the silica surface or of the oxidized surface of silicon carbide in acidic media. Good dispersion and stability of dispersions are then obtained through electrostatic and steric stabilization. As an application, the preparation of an Al 2 O 3 -SiC aqueous mixed slurry was investigated as a preliminary step in the processing of Al 2 O 3 /SiC nanocomposites.

Aqueous suspensions for tape-casting based on acrylic binders

Abstract The use of water-based systems represents an interesting alternative to the widespread non-aqueous tape-casting but the low strength of water-based binders generally limits their applicability. A tape-casting slurry is a complex system where each organic component has a substantial effect on the rheological behaviour. In this study the effect of the dispersant and binders in alumina aqueous tape-casting slurries were characterized with electrophoretic mobility and rheological measurements. In aqueous medium, a 4,5-dihydroxy-1,3-benzenedisul-fonic acid, was found to be a very effective dispersant for alumina. The amount of dispersant required to achieve a minimum of viscosity was equal to 0.2 wt%. Two aqueous emulsions of acrylic polymers were used as binders. These binders strongly affect the rheology of the suspensions. The best conditions to obtain a homogeneous stable slurry with a high powder loading suitable for tapecasting were determined in terms of order of component addition, rheological behaviour and ageing of the suspensions. Acrylic binders should act through a cohesive mechanism and lead to green tapes with good mechanical strength.

Catechol derivatives and anion adsorption onto alumina surfaces in aqueous media: influence on the electrokinetic properties

Abstract A high state of dispersion of alumina particles can be achieved by using molecules derived from Catechol 1,2(OH) 2 C 6 H 4 , such as Tiron (OH) 2 C 6 H 2 (SO 3 Na) 2 which allows to obtain stable alumina suspensions. In the field of understanding the dispersion mechanism of Tiron and the contribution of each functional group grafted onto the benzene ring of the Tiron molecule, adsorption experiments and surface charge measurements were conducted by using several compounds, containing a benzene ring, and by varying the nature and the number of substituents. The influence of the size and of the charge of the counter ion, electrostatically attracted by a positive charged alumina surface, onto the suspension stability was also studied. Electrokinetic properties of the suspensions versus pH appear to be very useful to characterize the ion adsorption sequence for an oxide surface. With a high immersion heat, an alumina surface better adsorbs ions with a high structuring power for water molecules. This influences the charge developed onto the oxide surface.

Dispersion of Al2O3 suspension with acrylic copolymers bearing carboxylic groups

Abstract The interaction between acrylic copolymers carrying carboxylic groups and the surface of an alumina powder in water has been studied. The trends of the adsorption isotherms suggest the formation of a monolayer according to the Langmuir model. The quantity absorbed on the plateau increases when the fraction of carboxylic groups on the polymer decreases. The electrokinetic properties of the alumina suspensions were also analysed. Results indicate that the copolymer adsorption leads to a shift in the IEP towards acidic pH. Moreover, the density of charge of the surface of alumina particle at pH 8–9 was found to depend on the quantity of adsorbed carboxylic groups and on the configuration of the adsorbed polymer. The stability of dispersions was investigated through particle size distribution and rheological properties after addition of various amounts of copolymers. The ability of the acrylic copolymer to stabilise alumina suspensions was found to be greatly affected by the content of carboxylic groups contained in the macromolecular chain.

Binder Distribution in Spray-Dried Alumina Agglomerates

Abstract Organic binders, especially those which are water-soluble, tend to migrate with the solvent flow which occurs during the spray-drying process of ceramic suspensions. The adsorption isotherm of PVA on the surface of alumina particles shows a rather low affinity of the polymer when compared to that of the ammonium polymethacrylate. Only 6% of the added PVA (3wt% on a dry weight basis of alumina) was adsorbed onto the alumina surface, and the main fraction of ‘free’, non-adsorbed, binder was able to migrate during drying. Images of the distribution of the polyvinyl alcohol in the alumina agglomerates and in the green compacts indicate an inhomogeneous distribution of the binder. The PVA segregates to the surface of the granules and leads to the formation of a polymer-rich layer. The thickness of this layer, which contains about 50% of the binder introduced, was estimated to be a few percent of the granule radius.

Tape casting using UV curable binders

Abstract High-density tapes were obtained by tape casting of a slurry composed of alumina powder, dispersant, UV curable binder and photoinitiator, and by subsequent photopolymerization. The use of a photopolymerizable binder offers the advantage of eliminating the drying stage, which is a critical step of the tape casting process. The effects of the solid loading, of the dispersant and photoinitiator concentrations and of the temperature on the rheological behaviour of the slurries were investigated. Both addition of dispersant and photoinitiator had a positive effect on the rheology of the slurries. The highly loaded alumina suspensions (82 wt%, i.e. 57 vol%) exhibited a shear-thickening behaviour, which was not detrimental in this solvent-free process because of the possibility of hardening the sheet instantly, by polymerization, just after casting. Nearly theoretical densities (98.9%) were obtained after sintering at 1540 °C of the irradiated alumina tapes.

Influence of the temperature on the stability of aqueous alumina suspensions

Abstract Ceramic process need the preparation of very stable suspensions with high solid loading and the temperature is a fundamental parameter in surface chemistry and at various steps in the production process. The influence of the temperature on the stability of aqueous alumina suspensions dispersed either with the polymethacrylic acid sodium salt either with the Tiron has been studied. The strong adsorption of the two dispersants onto alumina surface is not affected by the variation of the temperature but temperature higher than 40°C strongly influences the amplitude of the repulsive potential created between the particules. By using the potentimetric data fitting software FITEQL 3·2, the electrokinetic properties of the suspensions at 20°C are interpreted and viscosity measurements show that a temperature of 60°C improves the stability of the suspensions and permits to concentrate it.

Interaction of cationic and anionic polyelectrolyte with SiO2 and Al2O3 powders

The adsorption of anionic and cationic polyelectrolytes onto alumina and silica surfaces is studied. The results indicate that the polyelectrolyte-surface interaction is affected by various factors such as (i) polyelectrolyte nature, (ii) distribution and nature of the oxide surface sites, and (iii) medium properties. A strong interaction enhances the formation of a flat conformation of the adsorbed polyelectrolyte onto the surface, whereas a weak interaction leads to a loops conformation. The effect of the adsorbed polyelectrolyte on the electrokinetic behaviour of the two oxides was also investigated using an electrokinetic sonic amplitude analyser.

Binder burnout and evolution of the mechanical strength of dry-pressed ceramics containing poly(vinyl alcohol)

This paper describes the effect of binder removal on the mechanical strength of alumina green samples. These products were prepared by uniaxial pressing of powders spray-dried with poly(vinyl alcohol) used as a binder. Change in mechanical strength of these compacts was studied during thermal debinding. The behaviour was investigated by means of thermogravimetry, DSC and FTIR spectroscopy.

Degradation of dispersant during milling

Abstract Deterioration of the ammonium salt of poly(methacrylic acid) (PMAA-NH 3 ), used as a dispersant during wet ball-milling of an alumina, has been investigated. Rheological behaviour, sedimentation tests, adsorption isotherms, pH and isoelectric point measurements, and infra-red analysis have allowed us to conclude that degradation of the dispersant takes place in two stages: (i) decrease of the charge by dehydration and (ii) complete neutralization of the dispersant by the formation of monodentate COOX groups, which can lead to desorption of the polymer from the alumina surface. This second phenomenon involves m strong increase of viscosity. A low viscosity can be recovered by a subsequent addition of PMAA-NH 3 at the end of milling.