Clays Coalescence

Suitable protective coUoids for the preparation of acryhc suspension polymers include ceUulose derivatives, polyacrylate salts, starch, poly(vinyl alcohol), gelatin, talc, clay, and clay derivatives (95). These materials are added to prevent the monomer droplets from coalescing during polymerisation (110). Thickeners such as glycerol, glycols, polyglycols, and inorganic salts ate also often added to improve the quahty of acryhc suspension polymers (95). 

Water and Waste Water Treatment. PAG products are used in water treatment for removal of suspended soHds (turbidity) and other contaminants such as natural organic matter from surface waters. Microorganisms and colloidal particles of silt and clay are stabilized by surface electrostatic charges preventing the particles from coalescing. Historically, alum (aluminum sulfate hydrate) was used to neutralize these charges by surface adsorption of Al cations formed upon hydrolysis of the alum. Since 1983 PAG has been sold as an alum replacement in the treatment of natural water for U.S. municipal and industrial use. 

Finely divided solid particles that are wetted to some degree by both oil and water can also act as emulsifying agents. This results from the fact that they can form a particulate film around dispersed droplets, preventing coalescence. Powders that are wetted preferentially by water form O/W emulsions, whereas those more easily wetted by oil form W/O emulsions. The compounds most frequently used in pharmacy are colloidal clays, such as bentonite (aluminum silicate) and veegum (magnesium aluminum silicate). These compounds tend to be adsorbed at the interface and also increase the viscosity of the aqueous phase. They are frequently used in conjunction with a surfactant for external purposes, such as lotions or creams. 

In rubber-plastic blends, clay reportedly disrupted the ordered crystallization of isotactic polypropylene (iPP) and had a key role in shaping the distribution of iPP and ethylene propylene rubber (EPR) phases larger filler contents brought about smaller, less coalesced and more homogeneous rubber domains [22]. Clays, by virtue of their selective residence in the continuous phase and not in the rubber domains, exhibited a significant effect on mechanical properties by controlling the size of rubber domains in the heterophasic matrix. This resulted in nanocomposites with increased stiffness, impact strength, and thermal stability. 

In addition, the clay slip feed nozzles are kept very close to the bed surface (approx. 10 cm or 4 in.) in the second stage to encourage coalescence of the seed particles formed in the first compartment. Maximum production in this type of spray granulator is now of the order of 50,000 kg/h (55 tons/h) of product. 

Fig. 4. SE images of organic polymer binders (a, b) in the primaiy form as dispersion and (c, d) coalesced in silicone resin coatings, a, b Typical particle shape of the primaiy latex particles after rapid diying in a vacuum, c, d Plaster/paint samples etched with 10% HNO3. c Plan view of a coalesced polymer binder of a silicone resin plaster and d cross-section of of a polymer film partly covering a clay mineral particle within a silicone resin emulsion paint.

Colloids can be broadly classified as those that are lyophobic (solvent-hating) and those that are lyophilic and hydrophilic. Surfactant molecules, because of their dual affinity for water and oil and their consequent tendency to associate into micelles, form hydrophilic colloidal dispersions in water. Proteins and gums also form lyophilic colloidal systems. Hydrophilic systems are dealt with in Chapters 8 and 11. Water-insoluble drugs in fine dispersion or clays and oily phases will form lyophobic dispersions, the principal subject of this chapter. While lyophilic dispersions (such as phospholipid vesicles and micelles) are inherently stable, lyophobic colloidal dispersions have a tendency to coalesce because they are thermodynamically unstable as a result of their high surface energy. 

Early diagenetic gypsum has a pyramidal (or hemi-bipyramidal) habit and grows displacively, commonly within siliciclastic sediment from interstitial porewater, and may be termed groundwater gypsum. Coalescing clusters or rosettes of this discoidal form (up to a few centimetres for each blade) lead to desert roses , some of which are indeed pink if clay or iron oxides are incorporated. 

These are by far the most commonly used systems for the formulation of insoluble solids. The solid can be hydrophobic, such as most organic materials that are used in pharmaceuticals, agrochemicals, and paints the solid can also be hydrophilic, such as silica and clays. With some pigments and inks the particles need to be very small - that is, in the nanosize range - and these are referred to as nanosuspensions. Latexes may also be considered as suspensions, particularly if the particles are solid-like at ambient temperatures. With many of the latexes that are used in paints the particles are liquid-like at below and ambient temperature, but when applied to a surface these liquid-like particles coalesce to form a uniform film. The system may then be considered as an emulsion. 

Several industrial systems involve emulsions, of which the following are worthy of mention. Food emulsions include mayonnaise, salad creams, deserts, and beverages, while personal care and cosmetics emulsions include hand creams, lotions, hair sprays, and sunscreens. Agrochemical emulsions include self-emulsifiable oils that produce emulsions on dilution with water, emulsion concentrates with water as the continuous phase, and crop oil sprays. Pharmaceutical emulsions include anaesthetics (O/W emulsions), hpid emulsions, and double and multiple emulsions, while paints may involve emulsions of alkyd resins and latex. Some dry-cleaning formulations may contain water droplets emulsified in the dry cleaning oil that is necessary to remove soils and clays, while bitumen emulsions are prepared stable in their containers but coalesce to form a uniform fihn of bitumen when apphed with road chippings. In the oil industry, many crude oils (e.g.. North sea oil) contain water droplets that must be removed by coalescence followed by separation. In oil slick dispersion, the oil spilled from tankers must be emulsified and then separated, while the emulsification of waste oils is an important process for pollution control. 

Sediment-laden rivers flowing over flat terrain commonly develop extensive floodplains. At times, floodplains coalesce into broad depositional alluvial plains such as the Llanos of South America. The sediments in those deposits weather chemically. Less stable minerals in the sediment are broken down and alluvial soils develop. Eventually, only the most stable minerals such as quartz remain, and the clays are transformed into cation-deficient varieties. Sediment in such rivers, especially the sand, may go through many cycles of deposition, weathering, and erosion before it is transported out of the system. Compositionally, this sediment resembles that derived from transport-limited erosion. Elemental fractionation between the original bedrock and erosion products still occurs because of the permanent burial of some cation-rich material and the uninterrupted transport of much of the fine-grained suspended sediment out of the system (Johnsson etal., 1988 StaUard, 1985,1988). 

Figure 3 summarizes the structures of possible pillaring species described above, based on known structures of species present prior to addition of the clay component. Multiples of these basic units probably constitute the actual pillars in many cases. Coalescence of such clusters is particularly expected during calcination or setting of the cluster to form the pillar, especially in the presence of low water partial pressures. 

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