Dispersion of fine particles

Binders in Ceramics, Powder Metallurgy, and Water-Based Coatings of Fluorescent Lamps. In coatings and ceramics appHcations, the suspension rheology needs to be modified to obtain a uniform dispersion of fine particles in the finished product. When PEO is used as a binder in aqueous suspensions, it is possible to remove PEO completely in less than 5 min by baking at temperatures of 400°C. This property has been successfully commercialized in several ceramic appHcations, in powder metallurgy, and in water-based coatings of fluorescent lamps (164—168). 

Soluble polymers are widely used to control the state of dispersion of fine-particle suspensions. Depending on the polymer, and how it is applied, they can serve to enhance stability (dispersants) or to promote aggregation of the particles (flocculants). The topics covered in this chapter are intended as an overview of the use of polymers for stability control in mineral-particle suspensions with particular emphasis on flocculation processes. A brief discussion of stabilisation by polymers is included for completeness. 

Silica sol is a stable dispersion of fine particles, while gel has a three-dimensional continuous structure. Si02 content in sol range between 10-50%, while that in dry sdica gels is between 96.5 to 99.6%. Density of dry gels is 2.22 g/cm3 and sols 2.20 to 2.30 g/cm. Weight loss in sols at 105°C is between... 

VCM and an emulsifier. These components are circulated through a mixing pump (homogeniser) which causes the mixture to disperse into very fine droplets, before being placed in the autoclave. The droplets are coated with surfactant which stabilises them during the reaction. Initiation and polymerisation occur within the droplets. After polymerisation, the autoclave contains a stable dispersion of fine particles of PVC in water. Thereafter the subsequent operations for obtaining the final product are similar to the emulsion polymerisation process. 

In dispersions of fine particles in a liquid (or of particles in a gas) frequent encounters between the particles occur owing to... 

A. J. Rubin, Formation and stability of colloidal dispersions of fine particles in water. Mater. Sci. Res., 17 (1984) 45. 

In general, in dispersions of fine particles in a liquid, frequent encounters between particles occur through Brownian motion. Whether such encounters result in permanent contact or whether the particles rebound and remain free is determined by the forces between them. A dispersion is colloidally stable when its particles remain permanently free. In dilute dispersions it is sufficient to consider only interactions between pairs of particles (Overbeek, 1977). This would be the case for cloudy apple juice (CAJ) where the volume fraction of particles is less than 0.5% (Genovese and Lozano, 2000). 

The emulsion so formed is rapidly decompressed through a suitable device, whereby a gas-borne dispersion of fine particles is formed. 

Commercial ceramic membranes are made by the slip-casting process. This consists of two steps and begins with the preparation of a dispersion of fine particles (referred to as slip) followed by the deposition of the particles on a porous support. 

Roller mills have certain advantages. They can handle viscous materials and produce high-quality dispersions of fine particle size these are necessary in specialty coatings such as inks. Difficult-to-disperse pigments are handled by these mills. However, these mills are costly because the mill base throughput is at best moderate and a premixed paste is required. While the roller mills are used usually for specialty coatings, other mills can handle easy-to-disperse pigments more efficiently. 

The attractive interaction depends largely on the Hamaker constant A, as shown in Eq. (2). The larger the value of A, the greater is the attractive energy between the particles. The net interaction energy is then the sum of Vr and Va-Equations (1) and (2) show that a better dispersion of fine particles must come from systems having large tj/rf and small A. 

When water-soluble initiators and surface-active agents are used, relatively stable latices are formed from which the polymer cannot be separated by filtration. In the case of vinyl acetate, the distinctions are more blurred. Our description of Procedure 3-3 above represents a transitional situation between a solution and a suspension process since the product separated from the reaction medium. Between the true suspension and the true emulsion polymerization, we find, according to Bartl [4], the processes for formation of reasonably stable dispersion of fine particles of poly(vinyl acetate) using reagents which are normally associated with suspension polymerization. The product is described as creme-like. The well-known white, poly(vinyl acetate), household adhesives may very well be examples of these creamy dispersions. The true latices are characterized by low viscosities and particles of 0.005-1 /im diameter. The creme-like dispersions exhibit higher viscosities and particle diameters of 0.5-15 fim. 

At the opposite extreme, it may be required to disperse very fine particles into a highly viscous liquid. For example, the incorporation of carbon black into rubber is such an operation. Here, as with emulsification in liquid-liquid mixing, the product is stable, highly viscous and may well exhibit complex rheology. Such processes often involve surface phenomena and physical contacting only, in contrast to the mass transfer and chemical reactions described in the previous paragraph. The dispersion of fine particles in liquids is considered in detail in Chapter 6. 

Uniform dispersion of fine particles resulting from the intense mixing and agitation imposed by the rotating screw, which cause deaggregation of suspended particles in the molten polymer, is another attribute of this technology. 

The reaction is commonly carried out in water containing the monomer, an emulsifier, or a surface-active agent, and a water-soluble initiator. Initiation may be accomplished through thermal decomposition of the initiator or through a redox reaction. The polymer forms as a colloidal dispersion of fine particles and polymer recovery requires breaking up the emulsion. 

In contrast, colloid chemistry has provided the colloidal dispersion of metal fine particles in water.As early as the 1950 s, colloidal dispersion of fine particles of precious metals was conducted and applied to catalyses. Although they contain fine metal particles, the size of the particles was not so uniform. In addition they were not very stable when used in solution. Thus, there still remained many problems in the reproducibility of the preparation and catalysis using metal nanoparticles. In 1976, the author s group prepared colloidal dispersions of rhodium particles protected by water-soluble polymers by reduction of rhodium(III) ions under mild conditions, i.e., reduction with refluxing alcohol in the presence of water-soluble polymers. They were applied to the catalyst for hydrogenation of olefins. In 1989, we developed colloidal dispersions of Pd/Pt bimetallic nanoclusters by the simultaneous reduction of Pd and Pt ions in the presence of poly(A-vinyl-2-pyrrolidone) (PVP). 

Dust explosions result when there is dispersion of fine particles in air and a heat or flame source ignites the particles. A flame front spreads rapidly through the contaminated air and pressure and temperature increase. Virtually aU organic dusts, some inorganic dusts, and certain metaUic dusts are combustible in air and can explode. In some situations, inert dusts, like limestone, are an extinguishing agent. 

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