Advantages of Emulsion Polymerization

Free-radical polymerization is a very exothermic process, liable to suffer thermal runaways. Compared with bulk and solution polymerization, emulsion polymeriza- 

Some valuable products with applications in paper coating, leather treatment, binders for nonwoven fabrics, additives for paper, textiles and construction materials, impact modifiers for plastic matrices, and diagnostic tests and drug delivery systems, can only be produced by emulsion polymerization. In addition, when needed (for example, for rubber for tires) latexes are easy to process into dry polymer. The main disadvantage of emulsion polymerization is that the product contains emulsifier and residues of initiator, which give it water sensitivity. 

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As mentioned earlier, the necessary components for emulsion polymerization are monomer, emulsifier, initiator, and water. The monomer is insoluble in water in principle, although a small amount of water-soluble monomer can be used as a comonomer in some practical cases. On the other hand, the initiator is usually water soluble. These relations mean that the loci for radical formation and radical polymerization are different, that is, in aqueous and organic phases, respectively. This might look irrational, but the unique advantages of emulsion polymerization lie in it. When radicals are formed from a water-soluble initiator in an aqueous medium in Figure 11.1.2, top, left, each radical undergoes one of the following reactions ... 

Advantages of emulsion polymerization are rapidity and production of high-molecular-weight polymers in a system of relatively low viscosity. Difficulties in agitation, heat transfer, and transfer of materials are minimized. The handling of hazardous solvents is eliminated. The two principal variations in technique used for emulsion polymerization are the redox and the reflux methods. 

When a water-soluble monomer dispersed in a continuous oil phase is polymerized with an initiator soluble in the continuous phase, we speak of inverse emulsion polymerization [159]. This system has all the advantages of emulsion polymerizations (rapid polymerization, high degree of polymeriza-... 

Emulsion polymerization is considered the generally accepted technique to produce dispersion (plastisol) resins and vinyl latexes today. The monomers are made dispersible by emulsifiers in a continuous water phase. The initiators or catalysts are water soluble while the emulsifying agents stabilize the emulsion formed when the system is agitated. Gellner (2 ) cites the following advantages of emulsion polymerization over other methods ... 

ABS is also produced via bulk polymerization, with similar characteristics as noted above for HIPS. However, commercial ABS is primarily produced via emulsion polymerization, where SAN is polymerized in the presence of prepolymerized rubber (PB or SBR) emulsions. SAN is grafted to the rubber particles and also crosshnks the rubber particles, providing melt processing particle size stability. The advantage of emulsion polymerization versus bulk polymerization involves the ability to attain lower particle size and thus improved gloss of the injection molded or extruded articles of mamrfacture. ABS can tolerate lower rubber particle size than HIPS without loss of impact strength. In addition to PB and SBR, EPDM and acrylate rubbers have also been employed in the impact modification of PS and SAN. The primary advantage of these rubber modifiers involves the improved weatherabihty and thermal (oxidative) stability. Styrene-maleic anhydride (SMA) copolymers have been impact modified with similar rubber and processes as HIPS and ABS. 

Among aU these various polymerization processes, one major advantage of emulsion polymerization techniques is that according to the selected recipe, it is really possible to carefully adjust both macromolecular and colloidal properties of the obtained latexes, which is quite versatile in view of the variety of applications. For specialty ones, the control of particle size, particle size distribution, surface morphology, surface chemistry and functionality etc. is indeed of paramount importance. In that purpose, emulsion polymerization has long been proved to be appropriate in the synthesis of functional latex particles (Arshady, 1999 Kawaguchi etaL, 2003). 

The advantages of continuous tubular reactors are well known. They include the elimination of batch to batch variations, a large heat transfer area and minimal handling of chemical products. Despite these advantages there are no reported commercial instances of emulsion polymerizations done in a tubular reactor instead the continuous emulsion process has been realized in series-connected stirred tank reactors (1, . ... 

On the other hand, however, it is not straightforward to calculate the MWDs for intermediate cases using the conventional approach. A notable advantage of using an MC simulation technique is that it can be applied to virtually any type of emulsion polymerization, and can account for the chain-length-dependent bimolecular termination reactions in a straightforward manner [265]. Sample simulation results for instantaneous MWDs were shown [265] that were obtained using parameters for styrene polymerization that were reported by Russell [289]. 

Abstract The subject of miniemulsion polymerization is reviewed. The approach taken is one that combines a review of the technology with historical and tutorial aspects. Rather than developing an absolutely exhaustive review, a tutorial approach has been taken, emphasizing the critical features and advantages of miniemulsion polymerization. In keeping with this tutorial approach, a discussion of conventional emulsion polymerization is included in order to be able to compare and contrast miniemulsion polymerization and conventional emulsion polymerization later in the review. Areas where miniemulsion polymerization has been adopted commercially, or where it is likely to be adopted are highlighted. 

Emulsion polymerization is a powerful technique for developing products for a wide range of industry. The process can be used for the development of structured products in the nano- to microscales. Some of the key advantages of the process are the use of mild conditions, the near-complete conversion of monomers, the minimization of separation and recycling, the improved heat and mass transfer, and the improved environmental benefits due to the use of a water medium. However, the process is complex and requires careful modeling and scale-up. The prediction of process behavior, the key product properties, and the control of emulsion polymerization systems in particu-... 

Water is a chief ingredient in both suspension and emulsion polymerization. As the continuous phase, although inert, it acts to maintain a low viscosity and provides for good heat transfer. In addition, it serves to isolate the polymerization loci. Termed compartmentalization, this is a particular advantage in emulsion polymerization as will be described later in terms of rates of polymerization and molar masses. The water also acts as the medium of transfer of monomer from... 

Polymerizations performed with an emulsifier above its critical micelle concentration with all the monomer solubilized within the micelles and without any monomer present as emulsion droplets may be described as micellar polymerizations [62]. Although such systems can never produce a high yield of polymer per unit volume they are advantageous if it is desired to use photochemical initiation, these being transparent whereas emulsions are opaque. Micellar polymerizations can help to elucidate the mechanism of emulsion polymerizations. They are useful practically when it is desired to copolymerize hydrophilic and hydrophobic monomers to synthesize associative thickeners [63,64]. 

If the product of a polymerization is a latex then the process of polymerization can be considered, either in part or in full, as an emulsion polymerization. The advantage of doing so is that the physical chemistry of emulsion polymerization, combined with the appropriate polymer chemistry allows, in many cases, fuller understanding of the unconventional emulsion polymerizatiotL... 

Most, if not all, of the large-toimage grades of emulsion-polymerized styrene-butadiene rubbers are produced by continuous emulsion polymerization. The advantages over batch and setni[Pg.684]

As alluded to earlier, one of the key advantages of miniemulsion polymerizations ova conventional emulsion polymerizaticxis was seen to be the direct ccHitrol over the resulting number of particles which coirld be achieved by controlling the initial numbo of monomer droplets. If the latter could be controlled in a reproducible fashion, then the irreprodudbilities associated with the creation of latex particles might be eliminated. In conventional emulsion polymerizations, the number of particles is chiefly controlled by the concentrations of surfactant and initiator. 

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