Colloids and Surfaces A: Physicochemical and Engineering Aspects
Interaction of binders with dispersant stabilised alumina suspensions
Introduction
The rheological properties of concentrated suspensions are complex and much of the early work was phenomenological in nature. Hunter [1], [2], [3] and Goodwin [4], [5], [6], [7] were really the first to relate rheologically determined parameters, the yield value in the first instance and latter the elastic modulus, to the microstructure of the suspension. In this paper, we utilise these ideas to explore the rheological characteristics of ceramic suspensions.
Organic binders are an essential component for the effective processing of many commercial high performance ceramics. These binders are used to provide sufficient strength to the body such that the green bodies can be moulded and retained in the desired shape without breaking or damage, before and during sintering. Many ceramic forming processes make use of a binder, including those of dry pressing, slip casting, tape casting, extrusion, roll forming, thick film printing, injection and compression moulding.
There are various organic substances that have been used, or categorised, as potentially useful binders for ceramics and many useful compilations and descriptions of these different organic binders have been published [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]. They include poly(vinyl) alcohol, cellulose based materials, natural gums, starches, sodium and ammonium alginates etc. The use of polymer latex systems has also been reported recently [18].
The suitability of a binder for a given system depends upon on a number of factors including the particle/solvent interactions with a dispersant, the affinity with the processing liquid, the polarity and the interaction with the particulate material. The ‘ideal’ binder should be compatible with the dispersant, perhaps also function as a stabiliser, induce no interference with the solvent quality, function as a lubricant between the particles and should not produce foaming on air entrainment. Furthermore, an effective burnout profile without the formation of a deleterious residue is also essential. A low glass transition temperature and a high mechanical strength to molecular weight ratio are also desirable. Naturally, cost is also a significant factor. Inevitably, the binders used in practice do not have the properties of the ‘ideal’ binder; a compromise is required. Usually, different substances are mixed in order to obtain, or to approximate to the desired properties. Naturally, the mixing of the different components greatly complicates the rheology of the system.
The binders are introduced in a liquid phase, except in the cases of injection and compression moulding, and are employed in the formulation of a particulate system. The liquid phase provides the vehicle that is necessary for the uniform dispersing of the binder molecules. The dry green strength in the cast body is developed by the evaporation of the supernatant liquid. The residual binder remains in place within the body forming the necessary interparticle bridges required to provide a strong adhesion amongst the ceramic particles.
The addition of the organic binder often affects the rheology of the liquid phase. An increase in the viscosity, as well as a change in flow characteristics, from Newtonian (for water) to shear thinning, are generally the main consequences. The changes in the rheology of the binder/liquid solution directly affect the behaviour of the suspension formed upon the addition of particulates to the solution.
The usefulness of a given binder for a specific system depends very much upon the interactions developed between the different components, e.g. the dispersant, the particles, etc. of the suspension. In this paper, the influence of a poly(vinyl) alcohol (PVA) addition, in the presence of two different dispersants, upon the rheology of an alumina AES-11 aqueous suspension is reported. This binder and these dispersants are frequently used in commercial practice.
The amount of dispersant and PVA used in this paper is expressed on a dry weight of the powder basis (dwb), which means it is equivalent to the wt./wt. basis of the anhydrous solid.
It has been shown, for various particulate systems such as polymer latex particles and surfactant micelles, that when particles are dispersed in a dispersion medium, there are at least four kinds of interactions operating between the different components of the suspensions. Van der Waals attractive forces are the most ubiquitous. The van der Waals attractive forces are very dominant for high surface area particulate materials, and cause aggregation of the particles. If the particulate material has an electrical charge induced, say caused by the adsorption of a charged dispersant, the aggregation of the particles in a suspension can be suppressed through the formation of an electrical double layer. Adsorption, or grafting, of polymer molecules can also prevent aggregation of the particles by the mechanism of steric stabilisation. The addition of a non-adsorbing polymer to a stabilised suspension may cause depletion flocculation, whereby the concentration of non-adsorbing free polymer is effectively reduced in between the particles, and the higher concentration outside the particle–particle approach zone, exerts an osmotic pressure causing the particles to flocculate; Russel et al. [19], Hunter [20], Napper [21], Everett [22] and Fleer et al. [23]. Fig. 1 shows schematically different effects which may cause instability in a stabilised suspension (central region). The arrow direction shows the increasing of that specific effect.
Section snippets
Materials
Alumina AES-11 (Mandoval, Surrey, UK), 99.8% pure, crystalline with a surface area of 8.14 m2 g−1 (BET values), a mean particle size of 0.4 μm was used as received. Two dispersant systems were studied, ‘Darvan C’, an ammonium salt of poly(methacrylate), (∼25% by wt. polymer in water of MW 10 000–16 000) (R.T. Vanderbilt Company, USA), and ‘Tiron’ (Fluke Chemicals), which is sodium salt of 4–5 dihydroxy-1, 3-benzene disulfonic acid (C6H8Na2O8S2, MW 332.2) also called as pyrocatechol-3,
Binders without dispersant
The suspensions prepared from the alumina powder and water, without the addition of any dispersant or binder, were very flocculated and dried very quickly and were therefore not suitable for the production of ‘good’ quality ceramic materials. The addition of external processing agents changes the interparticle interactions appreciably, which changes the rheology of the resulting suspensions [26]. PVA is one of the most frequently used water soluble binders and it has been reported [27] that PVA
Discussion
The increase in the viscosity of the dispersant stabilised suspensions at the shear rate of 1.46 s−1, and the G′, G′′ values as well as the dynamic viscosity at the frequency of 1 Hz., with increasing of the PVA concentration suggests that the addition of the PVA causes a flocculation of the alumina stabilised particles with the 1% dwb ‘Darvan C’ and 0.125% dwb ‘Tiron’ dispersant concentrations. The degree of the flocculation increases as the PVA concentration is increased, and this effect of
Comparison of the two dispersant–poly(vinyl) alcohol combinations
For the two dispersants, ‘Darvan C’ and ‘Tiron’ used to stabilised the alumina AES-11 suspensions, the viscosity at a shear rate of 1.46 s−1 for the respective dispersant’s optimum concentrations, as a function of the PVA concentration, is shown in Fig. 22. This figure shows that as the PVA concentration is increased, the viscosities of the suspension stabilised with the two dispersants start increasing. This increasing of the viscosities, with increasing of the PVA concentrations, is due to
Conclusion
In this paper, selected rheological properties of alumina suspensions in the presence of a PVA binder and two dispersants ‘Darvan C’ and ‘Tiron’ have been described. When the PVA was used without any dispersant, the resulting viscosity of the 40% by volume suspensions remained virtually unaffected for the 0–1.5% dwb PVA concentration range. The behaviour of many different concentration combinations of the PVA and the dispersants has been described. The use of ‘Darvan C’ and ‘Tiron’ as
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