Initiators and Emulsifiers

Figure 8, Monomer conversion vs, polymerization time in the helical tubular reactor effect of varying initiator and emulsifier concentrations...

The work reported here used a tubular reactor of approx. 2.5 cm id and 150 meters in length. The reactor, lined with a fluorinated polymer, was coiled in a helical shape. The recipe employed standard concentrations of initiator and emulsifier. 

Figure 1. Conversion-time histories in batch PVC emulsion polymerization for different initiator and emulsifier concentrations.

Describe the microscopic picture of emulsion polymerization according to Harkins, Smith, and Ewart. Where are the monomer, initiator, and emulsifier located Describe the changes that take place as the reaction proceeds to 100% conversion. 

An interesting variation on the use of azo groups-containing polymers is the use of polyacrylamide prepolymers as both initiator and emulsifying agent for the emulsion polymerization of vinyl acetate. The observed reaction kinetics were typical (see Fig. 4.7) for an emulsion polymerization and, in particular, the slope (a = 1) of the plot of lg Rp vs lg CPrep confirms the double function of these prepolymers very clearly. 

Another method to increase the number of polymer particles produced in the first stage reactor with initiator and emulsifier concentrations fixed is to employ a plug flow type reactor such as a tubular reactor for the first stage. The minimum residence time of a plug flow reactor 6 necessary to produce the same number of polymer particles as in E batch reactor is tc. Thus, from Eq.(31) We have ... 

Figures 6, 7 and 8 show experimental verification of Eq.(40) in batch emulsion polymerization of styrene ( 14). The number of polymer particles was measured by electron micrscopy, not at finite but at 1 hour after the start of polymerization. Figure 6 represents the effect of lowering the initial monomer concentration, Mq on the number of polymer particles formed at fixed initial initiator and emulsifier concentrations. The number of polymer particles formed is constant even if M is lowered to the critical value Mc. This is because normal°condition that micelles disappear before the disappearance of monomer droplets is satisfied in the range of monomer concentration above Mc. The value of Mc can be calculated by the following equation obtained by equating XMc, the monomer conversion where micelles disappear, to XMc2, the monomer conversion where monomer droplets disappear.

A small amount of water which just dissolves initiator and emulsifier is fed into the first stage reactor along with all the initiator and emulsifier, and the rest of water into the second stage reactor. 

All three types of emulsion polymerization can be carried out using seeded emulsion polymerization, i.e., by adding monomer, initiator, and emulsifier to a previously-prepared small-particle-size latex, the particles of which grow in size without initiation of a new crop of particles. The purpose of seeded emulsion polymerization is to avoid the uncertainties of the particle initiation stage, obtain better batch-to-batch reproducibility, and give a stable latex of the desired particle size. 

The reactor system used for these experiments is a 190 liter, jacketed/ stainless steel vessel equipped with initiator and emulsifier metering system. The reactor is monitored and controlled by a minicomputer. The computer monitors the reactor temperature and pressure/ the jacket water inlet and outlet temperatures and flow rate, and the initiator and emulsifier flow rates. The computer calculates the amount of heat transferred through the jacket from the process measurements and transmits signals to control the reactor temperature and metering pumps. 

The other variables in equations (2)-(6) are defined in the nomenclature. The series of differential equations above are solved simultaneously with material balances on the initiator and emulsifier concentrations in the reactors ... 

Tubular Reactor Studies. The first run in the tubular reactor was with the same recipe as for Seed I in Table I, but the conversion was very low, and there were two distinct phases. The residence time in the tube was equal to the batch reaction time. Apparently the more nearly constant temperature of the tubular reactor prevented rapid polymerization. In the next run, initiator and emulsifier levels were doubled, but still conversion was low, although phase separation was not so severe. With seed latex and still more emulsifier, Run I shown in Table II, monomer conversions of about 60% were obtained at 50 minutes average residence time in the reactor. No phase separation was evident, but later tests indicated that some phase separation was occurring. 

The first step in the manufacture of fine powder resins is to prepare an aqueous colloidal dispersion by polymerization with initiator and emulsifier present.21 Although the polymerization mechanism is not a typical emulsion type, some of the principles of emulsion polymerization apply here. Both the process and the ingredients have significant effects on the product.22 The solids contents of such disper-... 

Properties of the macromolecule concerning chlorine distribution, chain branching, presence of the double bond, primary oxy groups, and partially polymerized residues of initiating and emulsifying agents. 

This study illustrates a particular use of FT-Raman spectroscopy (Section 2.4.2) to monitor an emulsion polymerization of an acrylic/methacrylic copolymer. There are four reaction components to an emulsion polymerization water-immiscible monomer, water, initiator, and emulsifier. During the reaction process, the monomers become solubilized by the emulsifier. Polymerization reactions were carried using three monomers BA (butyl acrylate), MMA (methyl methacrylate), and AMA (allyl methacrylate). Figure 7-1 shows the FT-Raman spectra of the pure monomers, with the strong vC=C bands highlighted at 1,650 and 1,630 cm-1. The reaction was made at 74°C. As the polymerization proceeded, the disappearance of the C=C vibration could be followed, as illustrated in Fig. 7-2, which shows a plot of the concentration of the vC=C bonds in the emulsion with reaction time. After two hours of the monomer feed, 5% of the unreacted double bonds remained. As the... 

The polymerization rate in the presence of alkyd was slower than that without alkyd. Doubling the initiator and emulsifier concentration increased the reaction rate, but not to the level achieved with the miniemulsion polymerization without alkyd. This retardation (as reported also by Nabuurs) increased with increasing alkyd level. The latexes obtained from the miniemulsion polymerization of the alkyd-acrylate mixtures were uniform emulsions, and no coagulation occurred during polymerization. Macroemulsion polymerization with alkyd resulted in colloidal instabihty, probably due the inabihty of the alkyd to reach the locus of polymerization. 

Figure 9. Influence of initiator and emulsifier concentrations on MM A conversion...

The property balances must he coupled with the balances for the initiator and emulsifier given, respectively, by... 

Details of the particle property model may be found in Kiparissides et al M,2] and Chiang and Thompson [3]. Following an approach used by Dickinson [4J and Gorber [5], the development was based on an age distribution analysis in which the classes of particles born between any time, t and T+dt, were followed through the reactor. The result was a series of differential equations in the total particle size properties (diameter, area and volume), the number of particles, conversion and the initiator and emulsifier levels in the reactor. 

Furthermore, costs can be reduced by use of less initiator and emulsifier in most cases, plus reduction in raw material... 

Formation of latex particles can proceed via the micellar nucleation, homogeneous nucleation and monomer droplet nucleation. The contribution of each particle nucleation mechanism to the whole particle formation process is a complex function of the reaction conditions and the type of reactants. There are various direct and indirect approaches to determine the particle nucleation mechanism involved. These include the variations of the kinetic, colloidal and molecular weight parameters with the concentration and type of initiator and emulsifier. There are some other approaches, such as the dye method where the latex particles generated via homogeneous nucleation do not contribute to the amount of dye detected in the latex particles since diffusion of the extremely hydrophobic dye molecules from the monomer droplets to the latex particles generated in water is prohibited. On the contrary, nucleation of the dye containing monomer droplets leads to the direct incorporation of dye into the polymer product. However, the dye also act as a hydrophobe and enhances the stability of monomer droplets as well as the monomer droplet nucleation. 

Free radical polymerization of an emulsion, consisting of aqueous phase containing an initiator and emulsified oil phase containing the monomer. 

Heterophase polymerization, different monomers, initiators and emulsifiers, 433 reaction rate profiles... 

Vinyl Chloride with Gradual Addition of Initiator and Emulsifying Agent [151]... 

There is an important group of processes where this situation occurs, namely dispersion and emulsion polymerizations. In these processes the reaction starts in the continuous water phase, but after a while another liquid phase begins to form. By carefully selecting the right initiator and emulsifier concentrations, the number of newly formed particles may be controlled. These particles absorb monomer and become the second reaction phase, gradually taking over from the continuous (water) phase. Finally the particles consist mostly of amorphous polymer (see section 13.6). 

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