Stability and Size Control of Drops

For many applications, the size range of the final product particles is very important. For example, bead diameters affect flow rates through ion-exchange columns. But particle size can also be important when the polymer is to be converted to a macroscopic object. Heat transfer rates to polymer particles during extrusion and mass transfer rates of plasticizers in particulate polymers both depend on particle size. 

During suspension polymerization, drop size depends on the physical properties of the two phases, the phase ratio, the nature of the suspension flow, and the condition of the phase interface. Interfacial tension and drop stability depend largely on the nature of the drop stabilizer. If no stabilizer were used to protect the drops, the suspension would be unstable and the final polymer particles would reach an undesirable size. 

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Control of Drop Size w/o Emulsions. Although the control of the drift of herbicide sprays was the initial reason for renewed interest in water-in-oil emulsion sprays, there are other interesting and novel ways of applying pesticides. The physical properties of w/o emulsions are considerably different from those of more conventional types of spray liquids, and the emulsifiers required to form and stabilize w/o emulsions are different from the usual materials employed in oil-in-water and wettable powder sprays. These differences can affect all aspects of spray performance, and a thorough study is required to appreciate the advantages and disadvantages which w/o emulsions may possess over aqueous-based sprays. 

The procedure to fabricate colloidal silver, (Ag°) , spherical nanoparticles is similar to that already described (see Section 9.3.3) The Cu( AOT)2 is replaced by the silver derivative. The relative concentration of Na(AOT), Ag(AOT)2, and the reducing agent remain the same. Control of the particle size is obtained from 2 nm to 6 nm (67). To stabilize the particles and to prevent their growth, 1 p.l/mL of pure dodecanethiol is added to the reverse micellar system containing the particles. This induces a selective reaction at the interface, with covalent attachment, between thio derivatives and silver atoms (68). The micellar solution is evaporated at 60°C, and a solid mixture of dodecanethiol-coated nanoparticles and surfactant is obtained. To remove the AOT and excess dodecanethiol surfactant, a large amount of ethanol is added and the particles are dried and dispersed in heptane. A slight size selection occurs, and the size distribution drops from 43% to 37%. The size distribution is reduced through the size selected precipitation (SSP) technique (38). 

Coalescence is also controlled by the condition of drop surfaces. Surfactants reduce the interfacial tension and help preserve drop stability, therefore affecting drop sizes. Surface-active materials are important in suspension/emulsion polymerization processes. 

Heat and mass transfer coefficients can be used to interrogate the importance of external transport phenomena and how to choose reactor size. The latter controls (i) pressure drop, (ii) residence time and thus reactant conversion or flow rate and thus power generated, (iri) the effective reaction rate and thus the process efficiency, (iv) the temperature and (v) whether a system is kinetically controlled and thus ideal for extraction of catalytic kinetics. Another application of Nu and Sh is that a 2D or 3D problem can be reduced to a computationally tractable problem by approximating the transverse transport phenomena using overall transport correlations. Such pseudo-2 D models (also called heterogeneous ID models for catalytic systems) have been used to explore the stability and performance of microbumers with a significantly lower computational effort than CFD models (e.g. [23-25]). 

When a water-miscible polymer is to be made via a suspension process, the continuous phase is a water-immiscible fluid, often a hydrocarbon. In such circumstances the adjective inverse is often used to identify the process [118]. The drop phase is often an aqueous monomer solution which contains a water-soluble initiator. Inverse processes that produce very small polymer particles are sometimes referred to as inverse emulsion polymerization but that is often a misnomer because the polymerization mechanism is not always analogous to conventional emulsion polymerization. A more accurate expression is either inverse microsuspension or inverse dispersion polymerization. Here, as with conventional suspension polymerization, the polymerization reaction occurs inside the monomer-containing drops. The drop stabilizers are initially dispersed in the continuous (nonaqueous phase). If particulate solids are used for drop stabilization, the surfaces of the small particles must be rendered hydrophobic. Inverse dispersion polymerization is used to make water-soluble polymers and copolymers from monomers such as acrylic acid, acylamide, and methacrylic acid. These polymers are used in water treatment and as thickening agents for textile applications. Beads of polysaccharides can also be made in inverse suspensions but, in those cases, the polymers are usually preformed before the suspension is created. Physical changes, rather than polymerization reactions, occur in the drops. Conventional stirred reactors are usually used for inverse suspension polymerization and the drop size distribution can be fairly wide. However, Ni et al. [119] found that good control of DSD and PSD could be achieved in the inverse-phase suspension polymerization of acrylamide by using an oscillatory baffled reactor. 

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