Suspensions and Dispersions

Aqueous media, such as emulsion, suspension, and dispersion polymerization, are by far the most widely used in the acryUc fiber industry. Water acts as a convenient heat-transfer and cooling medium and the polymer is easily recovered by filtration or centrifugation. Fiber producers that use aqueous solutions of thiocyanate or zinc chloride as the solvent for the polymer have an additional benefit. In such cases the reaction medium can be converted directiy to dope to save the costs of polymer recovery. Aqueous emulsions are less common. This type of process is used primarily for modacryUc compositions, such as Dynel. Even in such processes the emulsifier is used at very low levels, giving a polymerization medium with characteristics of both a suspension and a tme emulsion. 

Dispersants To keep insoluble combustion and oxidation products in suspension and dispersed Salts of phenolic derivatives polymers containing barium, sulphur and phosphorus calcium or barium soaps of petroleum sulphonic acids... 

R Bodmeier, O Paeratakul. Suspensions and dispersible dosage forms of multiparticulates. In I Ghebre-Sell-assie, ed. Multiparticulate Oral Drug Delivery. New York Marcel Dekker, 1994, pp 143-157. 

Commercial Polymerisation Methods. Aqueous media, such as emulsion. suspension, and dispersion polymerization, are by far [he most widely used in the acrylic fiber industry. Water acts as a convenient hcai-iranslcr... 

Suspensions and colloidal dispersions differ from true solutions in that they are systems with more than one phase. This means that the substances present do not mix very well. The system is said to be heterogeneous and is characterized by interfaces between the phases, for instance between the water and a clay particle in muddy water. However, true solutions are one-phase systems and as a result homogeneous. In addition, they differ because in suspensions and dispersions the solid phase can be separated by means of filtration. 

Subsequent chapters provide many examples of suspensions in industry and everyday life. Solid suspensions and dispersions of one solid in another solid, are not, in general, covered in this book. 

Suspensions and dispersions of particles are characterized by distributions describing their orientation and spatial distribution. Knowledge of these distributions then allow the calculation of bulk mechanical and optical properties. In the following sections, theories are presented for both dilute and semidilute systems. 

In homogeneous free radical polymerization, water is often employed as solvent for water-soluble monomers and polymers with more polar functional substituents such as hydroxyl, amino, oxyethylene, ammonium, and carboxylate groups, along with emulsion, suspension, and dispersion processes. This is also the case for metal-catalyzed living radical polymerization. 

Dispersion, suspension, and heat removal must be simultaneously controlled in dealing with suspension polymerization. In Figure 9.39, the intersection of suspension and dispersion lines represents the largest maximum drop size that can be produced under existing conditions. Selection of different impellers can affect the position of this intersection. In continuous extraction applications, the goal is to create a large interfacial area, yet not make the drops so small that they are difficult to coalesce. 

The preferred methods of disposing of fluo-ropol mers are recycling and landfilling according to the various regulations. In the case of suspensions and dispersions, solids should be removed from the liquid and disposed. Liquid discharge to waste water systems should be according to the permits. None of the polymers should be incinerated unless the in-... 

Emulsions, suspensions, and dispersions are examples of colloidal systems. It is important to mention that these terms are not always used consistently in the literature and that this situation may be confusing for students and nonpolymer scientists [24]. From the point of view of polymer science and engineering, these terms refer to heterogeneous polymerizations, particularly polymerizations in aqueous/alcoholic dispersed media. Thus, the aforementioned terms have connotations that have to do with the initiator, monomer, and polymer solubility in each phase as well as with particle size and the main locus of polymerization. These aspects are treated in detail later for the moment, let us assume that there are no chemical reactions and that such terms are used in the context of colloid science. 

It would be inappropriate not to mention that particle size and particle size distribution are important parameters related to the stability of emulsions, suspensions and dispersions. In general, the stability of these systems decreases with increasing particle size or size distribution. Of course, variables other than particle collisions and sedimentation such as electrokinetics, dispersants, steric stabilizers and others also affect stability. In short, the variables affecting stability can be divided into those factors that cause particles to come together and those factors that make the particles stick together, or stay apart (i.e. factors related to attractive or repulsive forces). 

As the editors of this volume—which represents the first complete coverage of pharmaceutical emulsions and suspensions—we chose the chapter authors on the basis of their experience and high degree of competence. The book should be useful to pharmacists, graduate students in the pharmaceutical sciences, professionals working in industrial research and development, and all those concerned with the health aspects of emulsions, suspensions, and dispersed systems. 

An emulsion has been defined above as a thermodynamically unstable heterogeneous system of two immiscible liquids where one is dispersed in the other. There are two principal possibilities for preparing emulsions the destruction of a larger volume into smaller sub-units (comminution method) or the construction of emulsion droplets from smaller units (condensation method). Both methods are of technical importance for the preparation of emulsions for polymerization processes and will be discussed in more detail below. To impart a certain degree of kinetic stability to emulsions, different additives are employed which have to fulfil special demands in the particular applications. The most important class of such additives, which are also called emulsifying agents, are surface-active and hence influence the interfacial properties. In particular, they have to counteract the rapid coalescence of the droplets caused by the van der Waals attraction forces. In the polymerization sense, these additives can be roughly subdivided into surfactants for emulsion polymerization, polymers for suspension and dispersion polymerization, finely dispersed insoluble particles (also for suspension polymerization), and combinations thereof (cf. below). 

Fluoropolymers are typically synthesized in aqueous polymerization systems (both emulsion and suspension), non-aqueous systems (Freon-113), or Freon-113/aqueous hybrid systems [8]. Such processes require the use of large quantities of water and CFCs (for non-aqueous polymerizations) or fluorinated surfactants (for emulsion polymerization). Aqueous suspension and dispersion poly-... 

Multiphase fluidzed beds are operated usually with continuous flow of gas and liquid phase, whereas the solid (catalyst) phase is either discontinuous or continuous. Here the energy needed for suspension and dispersion is provided by the flowing phases itself. 

Formolcon, Suspension and dispersion resins, Formosa Plastics Corp., USA... 

In-house tests have been collected and correlated by national and international standards organizations to make them universally applicable. ISO has defined tests and standards set up by other organizations such as the ASTM and DIN so that ISO methods are now widely accepted worldwide. ASTM D1755 defines a system for classifying general-purpose (mass and suspension) and dispersion (paste) resins (55). 

Suspension and dispersion polymerization processes for production of high MW PTFE have been widely used since the 1940s but process modification for direct polymerization of low MW PTFE has been more recent. Dispersion polymerization of low MW PTFE is currently most commonly employed. The major difference in polymerization between high and low MW PTFE is the addition of a... 

N.B. We do not differentiate here between the words suspension and dispersion, but some people speak about the latter as made up of small particles, where small might mean sub-micron-sized so that colloidal factors are very significant. ... 

In this chapter, emulsion, miniemulsion, microemulsion, suspension, and dispersion polymerization are discussed. Polyolefins are outside the scope of the chapter. 

In polymerizations in disperse phase (emulsion, suspension, and dispersion polymerizations), the viscosity of the dispersed systems depends on the volume fraction of the dispersed polymer, (j>, particle shape and size, particle size distribution, interparticle interaction, and shear rate [54], For monodisperse particles at low (j>, viscosity is proportional to the volume fraction of the disperse phase as given by the Einstein equation ... 

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