Semicontinuous emulsion polymerization

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... 

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. 

Snuparek J. Particle coagulation at semicontinuous emulsion polymerization. 

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. 

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. 

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. 

Donescu D, Go a K, Lang uri J (1990) Semicontinuous Emulsion Polymerization of Vinyl Acetate. Vin. Copol3rmerization with Di-2-ethylhexyl Maleate. Acta Pol3rm. 41 210-214. 

Donescu D, Fusulan L (1994) Semicontinuous Emulsion Polymerization of Vinyl Acetate X. Kinetics of Homopol5rmerizations, Copolymerizations, and Initiator Decomposition in the Presence of Sulfosucdnate-type Surfactants. 15 543-560. 

The aqueous emulsion polymerization is conducted by batch, semicontinu-ous, or a continuous process (Fig. 5). In a simple batch process, all the ingredients are charged to the reactor, the temperature raised, and the poljnnerization nm to completion. 

Shaffer et al [365] have continued to modify staining techniques for TEM of latex particles. Recent work on structured latex particles prepared by seeded emulsion polymerization focused on the effects of changes in polymerization variables, such as batch versus semicontinuous, core-shell ratio, shell thickness and shell composition. In this system the core was poly(n-butyl acrylate) and the shell was poly(benzyl methacrylate-styrene). A few drops of the latex was combined with a few drops of a 2% uranyl acetate solution which serves as a negative stain. A drop of that mixture was deposited on a stainless steel formvar-coated grid. After drying it was stained in ruthenium tetroxide vapor to differentiate the rubbery core, which is not... 

Typically, emulsion polymerization is carried out in stirred-tank reactors, which commonly operate in a semicontinuous mode, although both batch and continuous operations are also used. 

In a semicontinuous reactor in which monomers, surfactant, initiator, and water may be continuously fed into the reactor, emulsion polymerization does not follow the sequence of events described above. Thus, slow monomer feed and fast surfactant feed may lead to a system composed of polymer particles and micelles (Figure 6.3(a)). The system will contain only monomer-swoDen polymer particles if both monomer and surfactant are fed slowly (Figure 6.3(b)). On the other hand, a fast monomer feed and a low surfactant feed will lead to a system containing monomer droplets and polymer particles (Figure 6.3(c)). 

Figure 8. Dynamic mechanical spectra (loss modulus) of vinyl acetate (A)-butyl acrylate (B) co polymers prepared by batch ) and semicontinuous (o) emulsion polymerization copolymers with [A]=89%(I), 71%(II), and 49%(III). (Pichot, Llamo, Pham, Ref. 25)...

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]. 

---