Reactor seeded emulsion polymerization

The objective of this study was to investigate the feasibility of using a tubular reactor for the seeded emulsion polymerization of vinyl acetate, and to study the effect of process variables on conversion rate and latex properties. 

Manipulation of PSDs is generally attained through modification of surfactant concentrations (mostly in emulsion polymerizations) [48,49], agitation speeds (mostly in suspension polymerizations) [50], and initial catalyst size distributions and reaction times (residence time distributions in continuous reactors, mostly in coordination polymerizations) [51]. Effects of agitation speeds and surfactant concentrations on the PSD of polymer particles produced in suspension and emulsion polymerizations are discussed in detail in Chapters 5 and 6, respectively. When the catalyst is fed into the reactor as a solid material, as in typical polyolefin reactions, then the residence times and the initial PSD of the catalyst particles are used to manipulate the PSD of the final polymer product. Similar strategies are used in seeded emulsion polymerizations, where an initial load of preformed particles can be used to improve the control over the concentration of polymer particles in the latex and over the PSD of the final polymer product. 

There are many variations on this theme. Fed-batch and continuous emulsion polymerizations are common. Continuous polymerization in a CSTR is dynamically unstable when free emulsifier is present. Oscillations with periods of several hours will result, but these can be avoided by feeding the CSTR with seed particles made in a batch or tubular reactor. 

Specific turbidity histories are also plotted vs. dimensionless time for a continuous emulsion polymerization run the samples were withdrawn from the second reactor of a continuous train where the first reactor is a small seeding reactor. Part A of Figure 3 shows the particle size behaviour during start up all monomer, water, initiator and soap feedrates were kept constant until the process reached a steady state. In part B, the soap concentration in the seed reactor was increased a decrease in the particle size was expected and it is clearly shown from the specific turbidity measurements. 

The data on particle size distributions for both PVA and PMMA emulsions suggest that small particles could be quite important in the kinetic scheme, and that the larger particles probably grow by internal polymerization and by flocculation with smaller particles. The experiments with the tubular reactor installed upstream of the CSTR demonstrate a practical way to eliminate uncontrolled transients with continuous systems. We believe that the particles generated in the tube prevent CSTR oscillations by avoiding the unstable particle formation reactions in the CSTR. Berrens (8 ) accomplished the same results by using a particle seed in the feed stream to a CSTR with PVC emulsion polymerizations. 

Polystyrene can be easily prepared by emulsion or suspension techniques. Harkins (1 ), Smith and Ewart(2) and Garden ( ) have described the mechanisms of emulsTon polymerization in batch reactors, and the results have been extended to a series of continuous stirred tank reactors (CSTR)( o Much information on continuous emulsion reactors Ts documented in the patent literature, with such innovations as use of a seed latex (5), use of pulsatile flow to reduce plugging of the tube ( ), and turbulent flow to reduce plugging (7 ). Feldon (8) discusses the tubular polymerization of SBR rubber wTth laminar flow (at Reynolds numbers of 660). There have been recent studies on continuous stirred tank reactors utilizing Smith-Ewart kinetics in a single CSTR ( ) as well as predictions of particle size distribution (10). Continuous tubular reactors have been examined for non-polymeric reactions (1 1 ) and polymeric reactions (12.1 31 The objective of this study was to develop a model for the continuous emulsion polymerization of styrene in a tubular reactor, and to verify the model with experimental data. 

Emulsion polymerization reactions are sometimes carried out with small seed particles formed in another reaction system. A number of advantages can he derived from using seed particles. In a batch reactor seed latex can he helpful hi controlling particle concentration, polymerization rate, particle morphology, and particle size characteristics. In a CSTR the use of a feed stream containing seed particles can also help to prevent conversion and/or surface tension oscillations, which are caused by particle formation phenomena, This factor will be discussed in more detail later in this chapter. 

Process models are also important components of reactor control schemes. Kiparissides et al. [17] and Penlidis et al. [16] have used reactor models for control simulation studies. Particle number and size characteristics are the most difficult latex properties to control. Particle nucleation can be very rapid and a strong function of the concentration of free emulsifier, electrolytes and various possible reagent impurities. Hence the control of particle number and the related particle surface areas can be a difficult problem. Even with on-line light scattering, chromatographic [18], surface tension and/or conversion measurements [19], control of nucleation in a CSTR system can be difficult. The use of a pre-made seed or an upstream tubular reactor can be utilized to avoid nucleation in the CSTR and thereby imjHOve particle number control as well as increase the number of particles formed [20-22]. Figures 8.6 and 8.7 illustrate open-loop CTSR systems for the emulsion polymerization of methyl methacrylate with and... 

H. -C. Lee, Emulsion polymerization in a seed-fed continuous stirred-tank reactor, PhD dissatation, Georgia Institute of Technology, 1985... 

In the semicontinuous process, the reactor is initially charged with a fraction of the formulation (monomers, emulsifiers, initiator and water). The initial charge is polymerized in batch for some time and then the rest of the formulation is added over a certain period of time (typically 3—4 h). The monomers can be fed either as an aqueous pre-emulsion sta-bihzed with some emulsifier or as neat monomers. Monomers contain inhibitors to allow safe storage and they are used without purification. The initiator is fed in a separate stream. The goal of the batch polymerization of the initial charge is to nucleate the desired number of polymer particles. Because particle nucleation is prone to suffer run-to-run irreprodu-cibility, seeded semicontinuous emulsion polymerization is often used to overcome this problem. In this process, the initial charge contains a previously synthesized latex (seed) and eventually a fraction of the formulation (monomers, emulsifiers, initiator and water). Therefore, nucleation of new particles is minimized leading to better reproducibility. 

Temeng, K. O. and Schork, F. J. (1989) Closed-loop control of a seeded continuous emulsion polymerization reactor system, Chem. Eng. Commun. 85,193-19. 

Figure 6.16 shows the evolution of the risk parameters as a function of the polymer/monomer ratio for the emulsion polymerization of VAc/BA/AA (78.5 18.5 3) as obtained in a VSP2 reactor (Fauske Associates). The particle size of the seed and the initiator/monomer ratio were the same in all the experiments. 

FIGURE 7.4 Online monitoring of the seeded semibatch emulsion copolymerization of BA/MMA=50/50 calorimetry versus gravimetry in (a) overall conversion, (b) instantaneous conversion, and (c) and (d) free monomers BA and MMA. Line is calorimetry and dots gravimetry. Reprinted (adapted) with permission from Elizalde O, Azpeitia M, Reis MM, Asua JM, Leiza JR. Monitoring emulsion polymerization reactors calorimetry versus Raman spectroscopy. Ind Eng Chem Res 2005 44 7200-7207. 2005 American Chemical Society. 

Rgure 7.3. A schematic representation for a continuous emulsion polymerization process, in which the relatively monodisperse particle size distribution of seed latex particles introduced into a continuous stirred tank reactor becomes broader at the exit of the reactor. 

To resolve this instability problem, adopting a feed stream of seed latex particles [62] or installing a continuous tubular reactor, which generates seed particles, upstream of the continuous stirred tank reactor [53] have been proved quite effective (Figure 7.4b). For the latter approach, small latex particles form as a seed latex before the reacting stream enters the continuous stirred tank reactor when the monomer conversion at the exit of the tubular reactor is maintained at an adequate level. As a result, the continuous emulsion polymerization system can be operated at a stable steady state. The work of Nomura and Harada [54] also suggests that a tube-stirred tank reactor series... 

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