Emulsion semicontinuous

Figure 1. Conversion representation of MMA semicontinuous emulsion polymerization. Curve A results from neglecting lag in sampling while curve B is corrected for 2.7 min of lag.

Figure 2. Rate representation for MMA semicontinuous emulsion polymerization ((-----) the monomer feed rate (right or-...

Synthesis. A series of latexes was prepared by semicontinuous emulsion polymerization of methyl methacrylate. A dialkyl ester of sodium sulfosuccinic acid surfactant yielded the narrow particle size distribution required. An ammonium persulfate/sodium metabisulfate/ferrous sulfate initiator system was used. The initiator was fed over the polymerization time, allowing better control of the polymerization rate. For the smaller size latexes (200 to 450 nm), a seed latex was prepared in situ by polymerizing 10% of the monomer in the presence of the ammonium persulfate. Particle size was adjusted by varying the level of surfactant during the heel reaction. As the exotherm of this reaction subsided, the monomer and the sodium metabisulfate/ferrous sulfate feeds were started and continued over approximately one hour. The... 

Maleate Surfmers were found to outperform methacrylic and crotonic compounds in the copolymerization of styrene, butyl acrylate and acrylic acid in seeded and nonseeded semicontinuous processes [17]. The maleate Surfmer achieved high conversion without homopolymerization in the aqueous phase which can result in emulsion instability. The methacrylate Surfmer was too reactive as opposed to the crotonate which was not sufficiently reactive. The reported dependence of the maleate Surfmer conversion on the particle diameter is consistent with a reaction at the particle surface. 

Emulsion polymerization reactors are made of stainless steel and are normally equipped with top-entry stirrers and ports for addition of reactants. Control of the reaction exotherm and particle size distribution of the polymer latex is achieved most readily by semibatch (also called semicontinuous) processes, in which some or all of the reactants are fed into the reactor during the course of the polymerization. Examples are given in Chapter 8. In vinyl acetate copolymerizations, a convenient monomer addition rate is such that keeps the vinyl acetate/water azeotrope retluxing. at about 70°C. 

The population balance equations are very general and may be applied to batch, semicontinuous, and continuous emulsion polymerizations. Furthermore, both seeded and ab initio polymerizations are comprehended by Eq. (5) in all (or part) of the three commonly considered polymerization intervals. The following sections show how the different possibilities are reflected in different functional forms of the elements of the matrices O and K and of the vector c. It should be remembered, however, that certain conceivable situations are not comprehended by Eq. (5) for example, if the monomer molecules are not freely exchanged between the latex particles so that the monomer concentration inside each latex particle is determined by its growth history. 

Semicontinuous emulsion polymerizations are characterized by the continued addition of monomer to the reaction vessel. This permits the production of latexes with high weight percentage solids while allowing the initial burst of nucleation to be achieved in substantially aqueous surroundings. The theory for semicontinuous systems is substantially that set forth for Interval III of batch polymerizations, except that the materials balance equations [Eq. (17)] must be modified to include the flow of new material into the reactor. The effect of the monomer input is twofold first, the mass of material present in the system is increased and seccmd, the concentration of other reagents may be reduced. 

Three major types of chemical reactor systems are used to produce emulsion polymers batch, semicontinuous, and continuous. Batch reactors usually consist of stirred tanks with various forms of heat removal... 

Pinto, J.C. Dynamic optimization of semicontinu-ous emulsion copolymerization reactions composition and molecular weight distribution. Compos. Chem. Eng. 2001, 25 (4-6), 839-849. 

Snuparek J. Particle coagulation at semicontinuous emulsion polymerization. 

The industrial problems limiting earlier industrial thickeners in the coatings area were overcome with HEUR thickeners. With increasing sales, production limitations were addressed and the batch processes were replaced with semicontinuous techniques. Hydrophobe modification of industry standards (hydroxyethylcellulose and alkali-swellable emulsions) produced by proven industrial processes entered the market and broadened the technology. 

Because the hydrodynamic thickening mechanism of conventional ASTs (ASE and ASNE) is also present in the associative ASTs (HASE and HASNE), common factors affecting the thickening behavior of both types of thickeners would be expected, and are indeed observed. In a previously unpublished study (DeSoto, Inc. internal report), Brizgys and Shay examined the effect of the nonassociative hydrophobic comonomer on the thickening behavior of urethane-functional HASE thickeners prepared under identical conditions of semicontinuous emulsion polymerization. The results (Table I) obtained with respect to Tg and water solubility of the hydrophobic comonomer were generally similar to those observed for conventional ASTs by others cited earlier in this chapter. 

The emulsion copolymerization of vinyl acetate and butyl acrylate has received considerable attention. The butyl acrylate confers improved film forming characteristics to the polymer. The disparities in their water solubilities and of their individual polymerization rates may help to explain the variations in reactivity ratios that have been reported [170,171]. The variation in reactivity ratios may also by related to the following observations The reaction method has an effect on the morphology of the polymer particles. In a batch emulsion process, a butyl acrylate—rich core is formed which is surrounded by a vinyl acetate-rich shell, in a process in which the monomers are fed into the reactor in a semicontinuous manner, particles form with a more uniform distribution of the monomers [172]. The kinetics for a batch process indicates that the initially formed polymer is indeed high in butyl acrylate. As this monomer is used up, eventually a copolymer high in vinyl acetate develops. It is this latter polymer which forms the final shell around the particles. 

The influence of wall shear stress on the droplet size distribution for a 4.8-pjn SPG membrane is shown in Figure 16.17 and Figure 16.18. With increasing the wall shear stress from 1.3 to 30 Pa, the droplet size distribution curve shifts to smaller droplet diameters and becomes narrower and narrower. For the given pore size and experimental conditions, an emulsion with a narrow droplet size distribution (span = 0.43) was even produced at a- = 0.37 Pa, corresponding to Vt = 0.3 m/sec and laminar flow inside the membrane tube (Figure 16.13). Williams et al. [49] obtained a span value of 0.82 at the mean tube velocity of Vt = 0.6 m/sec in a semicontinuous... 

Commercial implementation of emulsion polymerization is mostly carried out in stirred-tank reactors operated semicontinuously. Continuous stirred-tank reactors (CSTRs) are used for the production of some high-tonnage emulsion polymers such as SBR. Batch processes are only used to polymerize monomers with similar reactivities and low heat generation rate (e.g., acrylic-fluorinated copolymers for textile apphcations). 

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. 

Although batch emulsion polymerization is not frequently used, it will be discussed first because it is easier to imderstand as the fundamental processes occur in a sequential way, whereas in the semicontinuous and continuous modes the processes occur simultaneously. 

In a scenario of increasing international competition, margin reduction and public sensitivity to environmental issues, emulsion polymer producers are forced to achieve an efficient production of high-quality materials in a consistent, safe and environmentally friendly way [83]. Most emulsion polymers are produced in semicontinuous stirred-tank reactors. Therefore, this discussion is focussed on these reactors. 

Wu and Zhao studied LIPN systems by a two-stage emulsion polymerization technique (Wu and Zhao 1995). A latex seed (polymer 1) was synthesized first in a semicontinuous emulsion polymerization, swollen by the second-stage monomer or monomer mixture (forming polymer 2), and followed by polymerization to form IPN materials. Six kinds of monomers were used acrylonitrile (AN), vinyl acetate (VAc), n-butyl acrylate (liBA), methyl methacrylate (MMA), ethyl methacrylate (EMA), and ethyl acrylate (EA). The effect of composition, cross-linking level, feeding sequence of polymer 1 and polymer 2 on the IPN miscibility, and... 

Donescu D, Gosa K, Ciupitoiu A (1985) Semicontinuous Emulsion Polymerization of Vinyl Acetate. Part I. Homopol3TTierization with Poly(vinyl alcohol) and Nonionic Coemulsifier. J Macromol. Sd. Part A. 22 931-941. 

Al-Bagoury M, Yaacoub E.J (2003) Semicontinuous Emulsion Copol3rmeiization of 3-0-methacryloyl-l,2 5,6-di-0-isopropylidene-a-D-glucofuranose (3-MDG) and Butyl Acrylate (BA). Monomer Feed Addition. J.Appl. Pol5rm.Sci. 90 091-2102. 

Okaya T, Tanaka T, Yuki K (1993) Study on Physical Properties of Poly (vinyl acetate) Emulsion Films Obtained in Batchwise and in Semicontinuous Systems. J. Appl. Polym. Sci. 50 745. 

El-Aasser M.S, Makgawmata T, VanderHoff, J.W (1983) Batch and Semicontinuous Emulsion Copol3dnerization of Vinyl Acetate-Butyl Acrylate. 1. Bulk, Surface and Colloidal Properties of Copol)nner Latexes. J. Polym. Sci. 21 2363-2382. 

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