Systems emulsion polymerization

Hydroperoxides are generally used with reducing agents, eg, iron salts, in redox emulsion polymerization systems. 

Copolymers with butadiene, ie, those containing at least 60 wt % butadiene, are an important family of mbbers. In addition to synthetic mbber, these compositions have extensive uses as paper coatings, water-based paints, and carpet backing. Because of unfavorable reaction kinetics in a mass system, these copolymers are made in an emulsion polymerization system, which favors chain propagation but not termination (199). The result is economically acceptable rates with desirable chain lengths. Usually such processes are mn batchwise in order to achieve satisfactory particle size distribution. 

As a conclusion, the basic kinetic features of the emulsion polymerization system may be summarized as follows ... 

The monomer concentration within the forming latex particles does not change for a long period due to the diffusion of monomer from the droplets to the polymerization loci. Therefore, the rate of the propagation reaction does not change and a constant polymerization rate period is observed in a typical emulsion polymerization system. 

The selection of the polymerization temperature for the emulsion polymerization system is strongly related to the initiation system. A polymerization temperature in which the initiator system exhibits its best performance should be selected. 

Continuous emulsion polymerization systems are studied to elucidate reaction mechanisms and to generate the knowledge necessary for the development of commercial continuous processes. Problems encountered with the development of continuous reactor systems and some of the ways of dealing with these problems will be discussed in this paper. Those interested in more detailed information on chemical mechanisms and theoretical models should consult the review papers by Ugelstad and Hansen (1), (kinetics and mechanisms) and by Poehlein and Dougherty (2, (continuous emulsion polymerization). 

DetaUed Modeling of Multicomponent Emulsion Polymerization Systems... 

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). 

Monomer and initiator must be soluble in the liquid and the solvent must have the desired chain-transfer characteristics, boiling point (above the temperature necessary to carry out the polymerization and low enough to allow for ready removal if the polymer is recovered by solvent evaporation). The presence of the solvent assists in heat removal and control (as it also does for suspension and emulsion polymerization systems). Polymer yield per reaction volume is lower than for bulk reactions. Also, solvent recovery and removal (from the polymer) is necessary. Many free radical and ionic polymerizations are carried out utilizing solution polymerization including water-soluble polymers prepared in aqueous solution (namely poly(acrylic acid), polyacrylamide, and poly(A-vinylpyrrolidinone). Polystyrene, poly(methyl methacrylate), poly(vinyl chloride), and polybutadiene are prepared from organic solution polymerizations. 

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 largest portion of the monomer (>95%) is dispersed as monomer droplets whose size depends on the stirring rate. The monomer droplets are stabilized by surfactant molecules absorbed on their surfaces. Monomer droplets have diameters in the range 1-100 pm (103-105 nm). Thus, in a typical emulsion polymerization system, the monomer droplets are much larger than the monomer-containing micelles. Consequently, while the concentration of micelles is 1019-1021 the concentration of monomer droplets is at most 1012-1014 L 1. A further difference between micelles and monomer droplets is that the total surface area of the micelles is larger than that of the droplets by more than two orders of magnitude. The size, shape, and concentration of each of the various types of particles in the... 

Polymerization of the monomer in solution undoubtedly takes place but does not contribute significantly, since the monomer concentration is low and propagating radicals would precipitate out of aqueous solution at very small (oligomeric) size. The micelles act as a meeting place for the organic (oil-soluble) monomer and the water-soluble initiator. The micelles are favored as the reaction site because of their high monomer concentration (similar to bulk monomer concentration) compared to the monomer in solution. As polymerization proceeds, the micelles grow by the addition of monomer from the aqueous solution whose concentration is replenished by dissolution of monomer from the monomer droplets. A simplified schematic representation of an emulsion polymerization system is shown in Fig. 4-1. The system consists of three types of particles monomer droplets, inactive micelles in which... 

Describe the components of an emulsion polymerization system on a macroscopic level. Compare the pros and cons of emulsion polymerization as a process condition in comparison to bulk and solution polymerization. 

The emulsion polymerization system fundamentally includes hydrophobic monomer, water-soluble initiator, emulsifier, and water. Therefore, the initial system of emulsion polymerization is truly an emulsion. 

Fig. 11.1.2 Illustration of emulsion polymerization system at different steps (I) particleforming step (II) steady step (111) monomer-in-particle consuming step.

The emulsion polymerization system consists of three phases an aqueous phase (containing initiator, emulsifier, and some monomer), emulsified monomer droplets, the monomer-swollen micelles, and monomer-swollen particles. Water is the most important ingredient of the emulsion polymerization system. It is inert and acts as the locus of initiation (the formation of primary and oligomeric radicals) and the medium of transfer of monomer and emulsifier from monomer droplets or the monomer-swollen particle micelles to particles. An aqueous phase maintains a low viscosity and provides an efficient heat transfer. 

Displayed in Figure 3 are the results for the polydispersity index for an emulsion polymerization system in which chain stoppage occurs by a combination of chain transfer ( tr= 1 reciprocal time unit) and disproportionation (c = 100 reciprocal time units). These results were obtained by varying p and thus n. The results suggest how it might be possible to tailor a distribution to some desired polydispersity index. 

The latexes were prepared using a conventional semi-batch emulsion polymerization system modified for power-feed by the addition of a second monomer tank. Polymerization temperatures ranged from 30-85°C using either redox or thermal initiators. Samples were taken periodically during the polymerization and analyzed to determine residual monomer in order to assure a "starved-feed" condition. As used in this study this is a condition in which monomer feed rate and polymerization rate are identical and residual monomer levels are less than 5%. 

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. 

It would appear that these sensors are quite valuable in exploring the dynamics of emulsion polymerization systems both at the laboratory and pilot plant scales. In addition, it appears that these instruments, in more rugged design would have applications for monitoring, alarm, and control functions in industrial-scale installations. Further refinements and applications are under study at present. 

The most common continuous emulsion polymerization systems require isothermal reaction conditions and provide for conversion control through manipulation of initiator feed rates. Typically, as shown in Figure 1, flow rates of monomer, water, and emulsifier solutions into the first reactor of the series are controlled at levels prescribed by the particular recipe being made and reaction temperature is controlled by changing the temperature of the coolant in the reactor jacket. Manipulation of the initiator feed rate to the reactor is then used to control reaction rate and, subsequently, exit conversion. An aspect of this control strategy which has not been considered in the literature is the complication presented by the apparent dead-time which exists between the point of addition of initiator and the point where conversion is measured. In many systems this dead-time is of the order of several hours, presenting a problem which conventional control systems are incapable of solving. This apparent dead-time often encountered in initiation of polymerization. 

Instead of conventional surfactant molecules, amphiphilic water soluble macromonomers, especially PEO macromonomers, have been used extensively as a reactive emulsifier and as steric stabilizer polymer, as summarized in Table 5. Generally speaking, however, the mechanism for the particle nucleation in the emulsion polymerization systems using macromonomers has been poorly established when compared to the dispersion copolymerizations with macromonomers as mentioned earlier. 

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