Other Emulsion Polymerization Systems

In the conventional emulsion polymerization, a hydrophobic monomer is emulsified in water and polymerization initiated with a water-soluble initiator. Emulson polymerization can also be carried out as an inverse emulsion polymerization [Poehlein, 1986]. Here, an aqueous solution of a hydrophilic monomer is emulsified in a nonpolar organic solvent such as xylene or paraffin and polymerization initiated with an oil-soluble initiator. The two types of emulsion polymerizations are referred to as oil-in-water (o/w) and water-in-oil (w/o) emulsions, respectively. Inverse emulsion polymerization is used in various commerical polymerizations and copolymerizations of acrylamide as well as other water-soluble monomers. The end use of the reverse latices often involves their addition to water at the point of application. The polymer dissolves readily in water, and the aqueous solution is used in applications such as secondary oil recovery and flocculation (clarification of wastewater, metal recovery). 

Nonionic surfactants such as sorbitan monooleate yield more stable emulsions than do ionic surfactants, However, the latices from inverse emulsion polymerizations are generally less stable than those from conventional emulsion polymerizations, and flocculation is a problem. 

Microemulsion polymerization is an emulsion polymerization with very much smaller monomer droplets, about 10-100 nm compared to 1-100 pm. Micelles are present because the surfactant concentration is above CMC. The final polymer particles generally have diameters of 10-50 nm. Although many of the characteristics of microemulsion polymerization parallel those of emulsion polymerization, the details are not exactly the same [Co et al., 2001 de Vries et al., 2001 Lopez et al., 2000 Medizabial et al., 2000]. Water-soluble initiators are commonly used, but there are many reports of microemulsion polymerization with 

Blackley, D. C., Emulsion Polymerization, Applied Science, London, 1975. 

Kolthoff, A. I. Medalia, and E. J. Meehan, Emulsion Polymerization, Interscience, New York, 1955. 

---

Because of their amphiphilic character, alkali resinates have been exploited both as polymer latex stabilizers and as surfactants in emulsion polymerization from the early development of these techniques, as in the pre-Second World War industrial example of the polymerization of 2-chloro-l,3-butadiene, to produce neoprene [68]. In the following decades, other emulsion polymerizations systems, like the synthesis of styrene-butadiene copolymers [68, 69], also called upon these surfactants, which are still being envisaged today, for example, for the polymerization of styrene [70] and chloroprene [71]. However, the reactivity of the conjugated double bond towards free radicals has made it more profitable to use hydrogenated or dehydrogenated rosins rather than their natural forms [68, 72]. 

Two of the most comprehensive discussions of these models were presented by Min and Ray (5) and by Poehlein and Dougherty (6). Min and Ray (5) gave a very general model framework which should be capable of modelling most emulsion polymerization systems. Of course, decisions must be made on the relative importance of the various phenomena occurring in a particular system. Other, more recent efforts on the modelling of emulsion reactors include the ones of Table I. Further details can also be found in (30). 

The physical picture of emulsion polymerization is based on the original qualitative picture of Harkins [1947] and the quantitative treatment of Smith and Ewart [1948] with subsequent contributions by other workers [Blackley, 1975 Casey et al., 1990 Gao and Penlidis, 2002 Gardon, 1977 Gilbert, 1995, 2003 Hawkett et al., 1977 Piirma, 1982 Poehlein, 1986 Ugelstad and Hansen, 1976]. Table 4-1 shows a typical recipe for an emulsion polymerization [Vandenberg and Hulse, 1948]. This formulation, one of the early ones employed for the production of styrene-1,3-butadiene rubber (trade name GR-S), is typical of all emulsion polymerization systems. The main components are the monomer(s), dispersing medium, emulsifier, and water-soluble initiator. The dispersing medium is the liquid, usually water,... 

The available data from emulsion polymerization systems have been obtained almost exclusively through manual, off-line analysis of monomer conversion, emulsifier concentration, particle size, molecular weight, etc. For batch systems this results in a large expenditure of time in order to sample with sufficient frequency to accurately observe the system kinetics. In continuous systems a large number of samples are required to observe interesting system dynamics such as multiple steady states or limit cycles. In addition, feedback control of any process variable other than temperature or pressure is impossible without specialized on-line sensors. This note describes the initial stages of development of two such sensors, (one for the monitoring of reactor conversion and the other for the continuous measurement of surface tension), and their implementation as part of a computer data acquisition system for the emulsion polymerization of methyl methacrylate. 

The basic concept of the present study was to show, other things being equal, that the rate of polymerization is affected by the size of the micelles and not by the total surfactant concentration as expressed by Equation (l). This micellar size effect was believed to be the reason why a nonlinear, i.e., a convex curve, relationship between In Rp and In Cg was obtained with emulsion polymerization systems of changing surfactant... 

For emulsion polymerization systems where polymerization takes place exclusively in the particles, tbe rate of emulsion polymerization can be expressed in the same way as in other radical polymerizations... 

If one assumes the absolute propagation rate constant, kp (measured by means of the rotating-sector technique or other absolute methods in block or solution polymerization systems), to be independent of the physical properties of the system (temperature excepted), it is possible to use these values for emulsion polymerization systems too. 

Monomer droplet nucleation plays an important role in both the DMA (1) and SM A (2) containing mini-emulsion polymerization of St initiated by AIBN [116]. On the other hand, increasing [SPS] promotes the homogeneous nucleation in the St mini-emulsion polymerization system (see above). For example, for the SMA series, the degree of homogeneous nucleation decreases in the series ... 

Other Components Other components of emulsion polymerization systems include electrolytes and sequestering agents. Sometimes electrolytes are added to act as buffers and to avoid the hydrolysis of monomers containing the ester group or the acceleration of persulfate initiators decomposition as well [79, 111], It has to be kept in mind that the addition of electrolytes has an influence on the colloidal stability of the latex, CMC, micellar aggregation number, and adsorption of surfactant, as well as on other physicochemical phenomena [25],... 

Other aspects of the subject which have received considerable attention in recent years include the preparation of mono-disperse latices by emulsion polymerization, emulsion polymerization in reaction systems to which no surfactant has been added, the special features which attend the emulsion polymerization of polar monomers such as acrylic esters, the preparation of latices of carboxyla-tcd polymers by emulsion polymerization, and the characteristics of emulsion polymerization systems to which new reactants are continuously being added and from which the product is continuously being removed. 

An important characteristics of emulsion polymerization is that, unlike other polymerization processes, the polymerization rate and molecular weight can be increased at the same time. This is due to the compartmentalization of the system that reduces the probability of mutual termination of propagating radicals. The behavior of this compartmentalized system dqiends on the rate of exchange of species between the elements of the system. The main ingredients of an emulsion polymerization system include monomer, dispersant, emulsifier, and initiator. Water is commonly used as the dispergant. A water-insoluble monomer can be dispersed in water by means of an oil-in-water emulsifier and polymerized with a water-soluble initiator. 

Sutterlin [46] studied the effect of the polarity of various monomers (styrene, acrylate ester monomers, and methacrylate ester monomers see Table 3.1) on the particle nucleation mechanisms involved in emulsion polymerization. When the surfactant concentration is above its CMC, the emulsion polymerization of styrene follows the Smith-Ewart theory (Npj 5o ) except those experiments with relatively low levels of surfactant. The exponent x in the relationship Npj So decreases with increasing monomer polarity when the surfactant concentration is above its CMC. This trend is attributed to the increased tendency of agglomeration of particle nuclei with monomer polarity. The emulsion polymerizations of less polar monomers deviate significantly from the Smith-Ewart theory (x 0.6) if the surfactant concentration is reduced to a level just below its CMC. This implies that some mechanisms other than micellar nucleation (homogeneous nucleation or coagulative nucleation) must operate in these emulsion polymerization systems. 

The basic framework of emulsion polymerization mechanisms and kinetics is primarily built on the aforementioned pioneering studies, and many other excellent contributions appeared thereafter. Several very useful empirical or approximate equations for calculating n were also developed for the emulsion polymerization system in the absence of the bimolecular termination of free radicals in the continuous aqueous phase (i.e., Y = 0). For example ... 

The concentration of monomer in particle nuclei in the particle nucleation stage is generally assumed to be the saturated concentration involved in the Smith-Ewart Interval II. On the other hand, the concentration of monomer in latex particles in the absence of monomer droplets (Smith-Ewart Interval III) continues to decrease to the end of polymerization the concentration of monomer in latex particles is linearly proportional to (1 - X), where X is the fractional conversion of monomer. To minimize residual monomer in latex products is essential for the successful product development because of the potential hazard to end-users. An initiator pair of reducer and oxidant is usually post-added to the emulsion polymerization system to achieve this goal. 

In summary, formation of particle nuclei from emulsified monomer droplets is almost certain to occur in any emulsion polymerization system in which these droplets are present. As mentioned earlier, however, monomer droplets containing polymer will primarily serve as reservoirs to provide monomer to the much more numerous and smaller latex particles formed by other particle nucleation mechanisms. Polymerization in monomer droplets can be eliminated or at least minimized by using seed polymer particles and slowly adding monomer (neat or as an emulsion) to supply the growing seed particles (i.e., seeded semibatch emulsion polymerization under the monomer-starved condition). 

---