Emulsion Polymerization in Continuous Reactors

School of Chemical Engineering Georgia Institute of Technology Atlanta, GA 30332 USA 

Polymer colloids are produced in batch, semi-batch and continuous reactors of a wide variety of designs. All of these reactors have assets and liabilities and every commercial process design involves compromises among concerns for product quality, manufacturing simplicity, costs, and potential future alternatives. 

The primary purpose of this paper is to provide information on continuous emulsion polymerization reactors that will help the reader to make rational decisions on the possible use of such reactors in the commercial production of polymer latexes. A second purpose is to demonstrate the utility of continuous reactors as a tool for studying reaction kinetics. 

Styrene-butadiene elastomers were among the first commercial products to be manufactured in continuous reactor systems. Today continuous processes are used to produce commercial quantities of a wide variety of products including elastomers, engineering plastics, coatings and adhesives. The advantages usually cited for continuous reactors are (1) high productivity, (2) constant product quality, (3) uniform heat load and (4) low operating costs. Capital costs for continuous processes can also be less than would be required for a batch or semi-batch processes of equivalent production capacity. This is not always true, however, especially for processes that must produce a variety of products or for processes which are intended to produce smaller quantities of material 

Lack of flexibility can be a major disadvantage of continuous reactors,. Continuous processes are often designed for a single product or for a family of closely related products. Hence, it is 

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Semicontinuous and Continuous Emulsion Polymerization In senticontinuous reactors, monomers, surfactant, initiator, and water are continuously fed into the reactor. Monomer droplets form if the rate at which the monomer is fed into the reactor exceeds the polymerization rate. This is not a desirable situation because the presence of free monomer in the system lowers the capability for controlling the polymer characteristics [8]. 

The use of a precision digital density meter as supplied by Mettler Instruments (Anton Paar, Ag.) appeared attractive. Few references on using density measurements to follow polymerization or other reactions appear in the literature. Poehlein and Dougherty (2) mentioned, without elaboration, the occasional use of y-ray density meters to measure conversion for control purposes in continuous emulsion polymerization. Braun and Disselhoff (3) utilized an instrument by Anton Paar, Ag. but only in a very limited fashion. More recently Rentsch and Schultz(4) also utilized an instrument by Anton Paar, Ag. for the continuous density measurement of the cationic polymerization of 1,3,6,9-tetraoxacycloundecane. Ray(5) has used a newer model Paar digital density meter to monitor emulsion polymerization in a continuous stirred tank reactor train. Trathnigg(6, 7) quite recently considered the solution polymerization of styrene in tetrahydrofuran and discusses the effect of mixing on the reliability of the conversion data calculated. Two other references by Russian authors(8,9) are known citing kinetic measurements by the density method but their procedures do not fulfill the above stated requirements. 

Emulsion Polymerization in a CSTR. Emulsion polymerization is usually carried out isothermally in batch or continuous stirred tank reactors. Temperature control is much easier than for bulk or solution polymerization because the small (. 5 Jim) polymer particles, which are the locus of reaction, are suspended in a continuous aqueous medium as shown in Figure 5. This complex, multiphase reactor also shows multiple steady states under isothermal conditions. Gerrens and coworkers at BASF seem to be the first to report these phenomena both computationally and experimentally. Figure 6 (taken from ref. (253)) plots the autocatalytic behavior of the reaction rate for styrene polymerization vs. monomer conversion in the reactor. The intersection... 

The stirred-tank reactor and the tubular reactor are two basic reactors used for continuous processes, so much of the experimental and theoretical studies pubhshed to date on continuous emulsion polymerization have been conducted using these reactors. The most important elements in the theory of continuous emulsion polymerization in a stirred-tank reactor or in stirred-tank reactor trains were presented by Gershberg and Longfleld [330]. They started with the S-E theory for particle formation (Case B), employing the same assumptions as stated in Sect. 3.3, and proposed the balance equation describing the steady-state number of polymer particles produced as ... 

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. 

Batch polymerization reactors are ideal to manufacture small volume polymers, specialty polymers, and polymers that are difficult to make in continuous reactors. Emulsion polymers, suspension polymers, and precipitation polymers are mostly made by batch polymerization processes. One of the disadvantages of a batch reactor is that the ratio of heat transfer surface area to reactor volume decreases as the reactor size is increased. For many polymer products made in batch reactors, the process economy improves with an increase in reactor size. Therefore, effective heat removal becomes a critical factor in designing and controlling a large-scale batch polymerization reactor. 

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

Continuous stirred-tank reactors (CSTRs) are used for large productions of a reduced number of polymer grades. Coordination catalysts are used in the production of LLDPE by solution polymerization (Dowlex, DSM Compact process [29]), of HDPE in slurry (Mitsui CX-process [30]) and of polypropylene in stirred bed gas phase reactors (BP process [22], Novolen process [31]). LDPE and ethylene-vinyl acetate copolymers (EVA) are produced by free-radical polymerization in bulk in a continuous autoclave reactor [30]. A substantial fraction of the SBR used for tires is produced by coagulating the SBR latex produced by emulsion polymerization in a battery of about 10 CSTRs in series [32]. The CSTRs are characterized by a broad residence time distribution, which affects to product properties. For example, latexes with narrow particle size distribution cannot be produced in CSTRs. 

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. 

Nomura, M. and Harada, M. (1981) On the optimal reactor type and operations for continuous emulsion polymerization, in Emulsion Polymers and Emulsion Polymerization, (eds D. R. Bassett and A. E. Hamielec), ACS, Washington, pp. 121-144. 

Other recent contributions to this aspect of the subject include those of Thompson et concerning a numerical technique for analysing particle-size distribution data for latices produced by continuous emulsion polymerization reactors, of Azizyan et alJ on the calculation of the number of steps in the continuous emulsion polymerization of vinyl acetate and chloroprene, of Lukhovitskii and Listratov on continuous emulsion polymerization in an ideal mixing-reactor, and of Brooks et al. on the emulsion polymerization of styrene in a continuous stirred reactor. The results presented in this last paper are particularly interesting in that the polymerization rates were Initially very high, but declined subsequently and did not always attain a steady value, but sometimes oscillated with time. 

Poehlein GW. Emulsion polymerization and copolymeiization in continuous reactor systems. Polym Int 1993 30 243-251. 

M ass Process. In the mass (or bulk) (83) ABS process the polymerization is conducted in a monomer medium rather than in water. This process usually consists of a series of two or more continuous reactors. The mbber used in this process is most commonly a solution-polymerized linear polybutadiene (or copolymer containing sytrene), although some mass processes utilize emulsion-polymerized ABS with a high mbber content for the mbber component (84). If a linear mbber is used, a solution of the mbber in the monomers is prepared for feeding to the reactor system. If emulsion ABS is used as the source of mbber, a dispersion of the ABS in the monomers is usually prepared after the water has been removed from the ABS latex. 

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