Processing of emulsions

ASTM D3346, 2003. Processibility of emulsion SBR (styrene-butadiene Rubber) with the Mooney viscometer (Delta Mooney). 

The manufacturing process of emulsions is apparently more cumbersome and time-consuming than that for the conventional formulations. This adds to the cost of the product. 

The process of emulsion droplets floating upwards under gravity or in a centrifugal field to form a concentrated emulsion (cream) quite distinct from the underlying dilute emulsion. This is not the same as the breaking of an emulsion. See also Sedimentation. 

Fig. 3 Schematic representation of the various processes of emulsion breakdown.

Processing of Emulsions and Dispersions by Homogenization, APV Homogenizers Wilmington, MA, 1996 1-23. 

The processes of emulsion breaking are similar in their nature and mechanisms to those of foam breaking [59]. Sedimentation of droplets takes place in dilute emulsions. Depending on the difference in densities of dispersed phase and dispersion medium, the sedimentation may proceed either in downward or upward directions. At lower density of dispersed phase the emulsion droplets float in an upward direction (the so-called creaming, typical for most direct emulsions [1]), while at higher density of dispersed phase sedimentation proceeds in a downward direction. 

Research sponsored by the government and private corporations has contributed much to the understanding of fundamentals that govern the complex process of emulsion polymerization. Emulsifiers play the triple role of emulsifsdng the monomers, furnishing micelles which are the sites of polymer initiation, and stabilizing the latex during the polymerization and afterward. To obtain satisfactory rates of polymerization at low tem-... 

Wasan and his research group focused on the field of interfacial rheology during the past three decades [15]. They developed novel instruments, such as oscillatory deep-channel interfacial viscometer [20,21,28] and biconical bob oscillatory interfacial rheometer [29] for interfacial shear measurement and the maximum bubble-pressure method [15,29,30] and the controlled drop tensiometer [1,31] for interfacial dilatational measurement, to resolve complex interfacial flow behavior in dynamic stress conditions [1,15,27,32-35]. Their research has clearly demonstrated the importance of interfacial rheology in the coalescence process of emulsions and foams. In connection with the maximum bubble-pressure method, it has been used in the BLM system to access the properties of lipid bilayers formed from a variety of surfactants [17,28,36]. 

The small difference between densities of drops and the ambient liquid, as well as small size of drops in the emulsion, result in a low sedimentation rate of drops in gravitational field. Thus the main challenge in the process of emulsion separation is to increase the drop size. This problem can be addressed by intensifying the coalescence of drops. The factors utilized to enhance the rate of drop integration may include the apphcation of electric field and turbulization of the flow. Before we proceed to describe these effects, consider in general the process of drop coalescence in the emulsion. 

The behavior of emulsions is considered in Section V in connection with the process of oil dehydration. Actual problems of drop integration in emulsions are discussed. It is shown that this process occurs most effectively if the emulsion is subjected to an electric field. In this context, the behavior of conducting drops in emulsions, the interaction of drops in an electric field, and the coalescence of drops in emulsions are examined in detail. In terms of applications, processes of emulsion separation in settling tanks, electro dehydrators, and electric filters are considered. 

In terms of its understanding of emulsions and emulsification, the oil-spill industry has not kept pace wifli flie petroleum production industry and colloid science generally. Workers in the spill industry often revert to old papers published in oil-spill literature, which is frequently incorrect and reflects very old knowledge. A basic understanding of the formation, stability, and processes of emulsions is now evident in literature in both the colloid science and oil-spill fields, although some new papers still appear with references to 15-year-old literature and no newer literature. 

Figure 18 Schematic illustration of the putative process of emulsion breakdown during the measurement of cef in W/O emulsions (see Refs 20 and 85-92 for detailed discussion of the method and phenomenon Ref. 93 describes our application).

FIGURE 10.3. In the process of emulsion polymerization, the incipient latex particle begins as a free-radical-initiated dimer or oligomer in solution (a). As polymerization proceeds, the growing chain precipitates and continues to grow, fed by new monomer taken from the reservoir of emulsified material (b). Potymerization continues until all... 

Polycarboxylates with high molecular weights are obtained by the process of emulsion polymerization, which also results in high polymerization rates. The heat of polymerization is easily removed through the aqueous phase since the viscosity is typically low. This production method typically results in polymers with up to 60 wt% of solids and molecular mass in the range 10 -10 . These are... 

The most important consequence of micelle formation in the process of emulsion polymerisation is the solubilisation of organic compounds in aqueous media. Since the association of lipophilic groups inside a micelle leads to a formation of centres of attraction for organic compounds, it is possible to dissolve appreciably higher proportions of sparingly water-soluble monomers in micellar solutions than in water alone. Monomers, which are essentially non-polar in character, may be expected to dissolve only inside the hydrocarbon portion of the micelle. Compounds with polar groups can at least partially be accommodated in the water phase or at the micelle surface so that the requirements for the micelle dimensions are less critical. 

The process of emulsion polymerisation begins when the free radicals derived from the, usually water-soluble, polymerisation initiator enter the monomer-saturated micelles where they find a sufficient number of solubilised molecules to start a rapid chain reaction (Elgood and Gilbekian, 1973). Each polymer radical first exhausts the monomer contained in the micelle and then captures additional supplies from 50 or more other micelles before the chain reaction is terminated. Some of the depleted micelles then break up and the released emulsifier molecules are adsorbed at the surface of the newly formed primary polymer particles (Dunn, 1971). The remainder are replenished by diffusion from the emulsified monomer droplets, which act essentially as reservoirs. 

The theory and process of emulsion polymerization have been the subject of a number of reviews (Alexander and Napper, 1971 Blackley, 1975 Ugelstad and Hansen, 1976) whilst its application to SBR has also been surveyed (Uraneck, 1968). 

The industrial reaction to this situation is one of compliance, with an increasing use of waterborne paint formulations based on the use of synthetic latices as binders especially in paints for domestic application. Such latices are usually polymeric colloids, of volume fraction from 0.2 to 0.5, dispersed in aqueous surfactant solution, prepared by a process of emulsion polymerization. These dispersions have a slightly turbid appearance, often with a low viscosity of order 1 mPa s. The latices can be readily prepared as near-monodisperse colloids. 

As we have seen, the process of emulsion polymerization is complete when micelles are absent from the aqueous dispersion. The aqueous phase surfactant concentration present in latices must therefore be less than or equal to the CMC. However, significant amounts of surfactant must be present but adsorbed at the polymer surface— indeed, estimates of (1.3-1.5) x 10 M m- on a styrene-acrylate copolymer have been made for SDS and aerosol OT at bulk phase concentrations close to the CMC [17]. In preparing complete formulations of paints and varnishes, further amounts of surfactant are usually added as wetting agents. This may well mean that surfactant solutions in the aqueous phase of latex paints are micellar. 

The process of emulsion formation is determined by the property of the interface, in particular the interfacial tension which is determined by the concentration and type of the emulsifier. This is illustrated as follows. Consider a system in which an oil is represented by a large drop 2 of euea A1 immersed in a liquid 2, which is now subdivided into a large number of smaller droplets with total area A2 (A2 > Al) as shown in Fig. 1.7. The interfacial tension is the same for the large and smaller droplets since the latter are generally in the region of 0.1 to few pm. 

Shear viscosities of emulsions are significantly important for structure preservation in spray processing of emulsions as a result of their impact on the dispersion of secondary as well as tertiary droplets [53, 54]. Shear viscosity / as a function of shear rate y is exemplarily demonstrated in Fig. 23.2 for the emulsion SE-lb (Table 23.1) with 40 % w/w disperse phase (here oil) fraction and different disperse mean drop sizes of 2, 4, and 10 pm. 

In the process of emulsion polymerization as initiating system one uses often a mixture of potassium persulphate and sodium thiosulfate ... 

As processing aids in the process of emulsion polymerization one uses emulsifiers, emulsion stabilizers, initiators, and regulating substances. 

Resin-water emulsions can be produced by the post-reaction emulsification of a condensation or addition polymer, by the use of surfactants or emulsifiers to form an oil-in-water emulsion of the polymer in the aqueous phase. This is simply a method of obtaining a polymer in an aqueous medium. This type of emulsion is different from a tme emulsion polymer made by the process of emulsion polymerisation. 

The process of emulsion polymerisation consists of dispersing a water insoluble acrylic monomer phase in an aqueous phase with the aid of surfactants. 

Concerning the barrier effect, Capek [69], in a review where the destabilization processes of emulsions is extensively analyzed, explains that the presence of a fatty alcohol can form with an ionic surfactant a denser inter facial layer that can increase the resistance to transfer of the dispersed component from small droplets to the continuous phase and from the continuous phase to the big droplets. 

Nitrile and polychloroprene latices are made by the industrial process of emulsion polymerization, in which the polymerization reaction and the formation of an aqueous emulsion occur simultaneously. In this process, reaction temperature and control of the polymer molecular weight are important in order to obtain the desired final glove properties. In the case of the nitrile latex, the ratio of the three monomers can also be used by the polymer chemist as an important tool in tailoring the final product properties. 

Early practical theories of emulsion stability recognized the importance of additives such as surfactants, polymers, and particulates to the processes of emulsion preparation, the type of emulsion produced, and the overall stability of the final system. However, a reasonably sound theoretical picture began to evolve only once an understanding of the concepts and principles of interfaces and monolayers began to become clear. Studies of oriented amphiphilic monolayers at interfaces led to the conclusion that such structures, in which each portion of the adsorbed molecules showed a strong preference for association with one of the two liquid phases, offered the best explanation for observed experimental results. As a result, it became possible to schematically represent the emulsion droplet as shown in Figure 9.2d. 

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