Normal emulsions

Normal emulsion polymerization is sometimes referred to as macroemulsion polymerization because of the large size of monomer droplets (hundreds of microns) compared to those of a microemulsion (tens of nanometer). 

Unsaturated polyesters that are terminated by carboxylic acid groups at both ends of the chain after neutralization are efficient emulsifiers for lipophilic monomers [110] and thus act as self-emulsifying crosslinking agents in the ECP of these systems. Normal emulsions of EUP and comonomers have a white, milky appearance. With an appropriate structure and molar mass of the EUP and within a certain range of EUP/comonomer ratios, however, microemulsions are... 

By first dispersing the EUP in water containing the base for neutralization of the carboxyl acid groups of the EUP and then adding the comonomer with intensive stirring, normal emulsions are obtained. They are favorable because, with multiple emulsions, insoluble polymers are formed, which decrease the yield of microgels. 

In normal emulsion polymerization the diffusion of monomers from droplets allows particles to grow. The polymerization is usually initiated in the aqueous phase and the oligomeric radicals either enter micelles or merge with other growing species. In the crosslinking ECP of EUP the ratio EUP/comonomer and the solubility or insolubility of both components and the initiator in the aqueous and non-aqueous phases respectively are parameters which decide whether diffusion of the reactants in the aqueous phase plays a role and where the initiation takes place. 

Ugelstad et al. (1967) solved tbe simultaneous equations Eqs. (21) and (26) for ft and plotted the calculated value of n against tbe value of a at fixed value of Y, varying tbe value of m as a parameter. Since the radical termination in the water phase is negligible under normal emulsion polymerization conditions as mentioned in Section 1I,A, the condition F = 0 is most important for usual emulsion polymerizations. Figure I shows a plot of... 

The detailed emulsion characterization methods discussed herein can be used to help resolve operational upsets only if a base line of data exists for normal operation. In fact, without a thorough characterization of the normal emulsion properties such as size distribution and mineral and organic composition, the techniques for detailed characterization may actually hinder the understanding and ultimate solution of a particular processing problem by introducing extraneous information. When a base line of data exists, detailed information on the size distribution and the relationship between the dispersed, continuous, and solid phases is invaluable. 

The term microemulsion, which implies a close relationship to ordinary emulsions, is misleading because the microemulsion state embraces a number of different microstructures, most of which have little in common with ordinary emulsions. Although microemulsions may be composed of dispersed droplets of either oil or water, it is now accepted that they are essentially stable, single-phase swollen micellar solutions rather than unstable two-phase dispersions. Microemulsions are readily distinguished from normal emulsions by their transparency, their low viscosity, and more fundamentally their thermodynamic stability and ability to form spontaneously. The dividing line, however, between the size of a swollen micelle ( 10-140 nm) and a fine emulsion droplet ( 100-600 nm) is not well defined, although microemulsions are very labile systems and a microemulsion droplet may disappear within a fraction of a second whilst another droplet forms spontaneously elsewhere in the system. In contrast, ordinary emulsion droplets, however small, exist as individual entities until coalescence or Ostwald ripening occurs. 

The core of a micelle is an exclusion region where substances that are incompatible with the solvent can enter spontaneously in a process called solubilization (4,7). Because of solubilization, micelles become swollen atid may attain the size of a small droplet, i.e., 1000 A or 0.1 nn. If the surfk tant concentration increases well above the CMC, many micelles are formed. If another phase is present, c.g., oil if the solvent is water, and provided that the physicochemical formulation is appropriate, the micelles would solubilize large amounts of this phase and become swollen until ih start interacting in a phenomenon called percolation. Such packed swollen micelle structures that could solubilize large amounts of both oil and water have been called microcmul-sions because they were first thought to be extremely small droplet emulsions (8-11). Actually this is a misnomer for at least two reasons. First, a microemulsion is before all a single-phase system, that is thermodynamically stable. Second, many microemulsions cannot be considered as dispersions of very small droplets, but rather as percolated or bicontinuous structures (12) in which there is no internal or external phase, and no possibility of dilution as in normal emulsions. 

Historically, spray dryers were used because of their ability to produce a constant quality product under full operational control. Normally, emulsion PVC (E-PVC) is water wet in a slurry and dried to a powder in one single-pass operation... 

O Brien showed in his initial analysis (8) that there was a reciprocal relationship between the CVP and the ESA effects so that essentially the same information could be obtained from either. However, it transpires that the information is easier to obtain from the ESA effect because it appears directly. The same information can be obtained from the CVP only if one knows the complex conductivity of the system. This limitation can, however, be overcome by measuring the CVC. O Brien s initial analysis was confined to dilute systems, but was subsequently extended to systems of arbitrary concentration as long as the particles were small compared to the wavelength of the generated sound (10). This condition is always fulfilled in practice for the normal emulsion sizes and for frequencies up to 20 MHZ for which the wavelength is of order 100 pm. The reciprocal relationship between CVP and ESA has been demonstrated for solutions of polyelectrolytes by O Brien etal. (11). 

The analysis of the relationship between the dynamic mobility and the particle properties has been made possible by the development of special procedures for dealing with systems in which the double layer around the particle or droplet is thin compared to the radius of curvature. The double-layer thickness is measured by the Debye-Hiickel parameter k which is related to the ionic strength of the electrolyte (13). For a 1 mM solution of a 1 1 electrolyte, the double-layer thickness, k, is about 10 nm and it decreases as the square root of the concentration, so for a 0.1 M solution it would be about 1 nm. The double layer is regarded as thin if the ratio of radius to thickness (ka) exceeds about 20 and that will be the case for most normal emulsions at most electrolyte concentrations. 

Some experimental facts should be stressed anyway for the present purpose. The delayed inversion of a normal emulsion along a change in composition toward a higher... 

Micro-emulsion is another variant of emulsion polymerisation. Such emulsions are thermodynamically stable systems including swollen monomer micelles dispersed in a continuous phase. In general, they require fairly large concentrations of surfactants to be produced compared with the other dispersed polymerisation systems. Hence, the interfacial tension of the oil/water is generally close to zero. Polymers with ultra-high molecular weight, i.e. above 10 g/mol, can be obtained, as can copolymers with a very well-defined, homogenous composition. Whereas polymerisation can take 24-48 h in the normal emulsion process, it proceeds at a fast rate in micro-emulsion, as total conversion can be obtained in less than 30 min. Polymer particles of very small size (diameter < 100 nm) and narrow distribution can be obtained by this process. 

The reactions in the aqueous phase lead initially to a change in the conductivity and subsequently to the formation of latex particles accompanied by the drop in the transmission (cf. Figure 10). Moreover, the shape of the conductivity curve is qualitatively the same as observed for surfactant-free emulsion polymerizations initiated with potassium peroxodisulfate. The bend of the conductivity curves marks the onset of particle nucleation as conducting species are captured in the diffuse electrical double layer of the particles. These results clearly prove that side reactions of carbon radicals in water lead to conducting species. The zeta-potential of the particles is pH-dependent and negative at pH >4. First hints that such radicals can attack water molecules have been obtained by NMR investigations of polymers made by normal emulsion polymerization (i.e. in the presence of surfactants) initiated with azo-initiators.P Ongoing studies try to clarify the reaction mechanisms. 

For polymerisation in a normal emulsion, the hydrophobic monomer is dispersed in the aqueous phase, in which it is almost completely insoluble, with the help of a surfactant. On the whole it is then located in the form of reservoir droplets of diameter d 1-10 gm, and in surfactant micelles of diameter d 5-10 nm. A small fraction may be solubilised in the continuous aqueous phase. The initiator is generally present in the aqueous phase (see Fig. 6.5). Polymer particles are generated by two simultaneous processes. The first is free radical capture by micelles, called micellar nucleation. There are far fewer droplets than micelles and the total surface area of the micelles is extremely large. Consequently, radicals tend to penetrate micelles rather than monomer droplets. The latter act mainly as monomer reservoirs. The second of the processes mentioned above is the formation of oligomer radicals in the continuous phase, and is referred to as homogeneous nucleation. When radicals reach a certain size, they become insoluble and group together to form polymer particles similar to those formed by micellar nucleation. 

A theoretical treatment of soapless emulsion polymerization of methyl methacrylate in water has shown that the number of particles is determined during the initial stages, and has clarified the relationships between this method, normal emulsion polymerization, and bulk polymerization. Differences between the heterogeneous polymerization of acrylonitrile and vinyl chloride have been discussed, following the development of an elaborate model for the former case in which propagation proceeds in the liquid and, eventually, also in the solid phase. ... 

Normal emulsions (ie, scC02-in-water emulsions, with water as the continuous phase) can also be obtained simply by manipulating the solubility of the surfactant (the phase in which the surfactant is most soluble will be the continuous phase), although examples for the formation of this kind of emulsion in literature are scarce and reverse micelles formed in SCFs have been used with much success as nanoreactors for a wide range of chemical syntheses and synthesizing various nanomaterials. 

Exterior and interior paints - the emulsion has the same function as in normal emulsion paints above... 

---