Polymerization continued semicontinuous

Effects of starvation feeding in semicontinuous or continuous polymerization systems. 

Semicontinuous polymerization experiments were carried out in a stainless steel enclosure (dry box) under a dry nitrogen atmosphere in three neck flasks (11) equipped with overhead stirrer and an inlet for the continuous introduction of precooled inifer/isobutylene/ solvent feeds to stirred, dilute BCI3/solvent charges. Experiments with binifer were performed at -80°C by the use of CH3Cl/n-hexane solvent mixtures (80/20 v/v), with trinifer at -40°C using CH3CI... 

The semicontinuous polymerization was carried out in a 500-ml four-neck flask immersed in a constant temperature bath at 60° C, and equipped with a reflux condenser, a two-bladed stainless steel stirrer and a graduated dropping funnel. The water and surfactant were introduced into the flask, then nitrogen gas was bubbled with agitation for at least 20 minutes. The initiator solution was added, and after 5 minutes the mixture of monomer addition started and continued for a total of three hours. The polymerization was continued for at least one hour past this point. Under these conditions the rate of monomers addition was less than 1/10 of their maximum rate of polymerization (Rpmax). 

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... 

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. 

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). 

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. 

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. 

ProCGdures. From a technological point of view heterophase polymerizations can be carried out either batchwise, semicontinuous (or semibatch), or continuous. In the batchwise case the reactor is filled with all ingredients before the polymerization is started and the reactor content is removed at the end of the polymerization. In a semibatch procedure, at the start of the poljunerization the reactor is filled only partially and a stream of either neat monomers or monomer emulsion with constant or deliberately changed composition is fed continuously until the reactor is filled. After a final post-feeding batch reaction period, the reactor is emptied. A continuous procedure means that all necessary ingredients are fed and final latex is removed continuously. In all three cases the poljunerization can be carried out in the absence or presence of preformed particles (so-called seed particles). Reactions in the absence of seed particles are frequently called ab initio polymerizations and require that particle nucleation takes place. Table 7 is an... 

Semibatch or semicontinuous polymerization, with the monomer added in stages or continuously as reaction proceeds... 

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... 

The predicted variation in composition with conversion for a terpolymer prepared by batch and by semicontinuous polymerization is shewn in Figure 5, The average ccitposition on a mole basis, for the terpolymer is approximately 74% monomer A, 21% moncmer B> and 5% monomer C, The nearly uniform conpositicn of the semi-ocsitinuous polymer is evident. Monomer C is soluble in water as well as in the monomer mixture, whereas the other two monomers are sparingly soluble in water. Because the relative amount of monomer mixture to water is low during the initial portion of the somi-continuous reaction, practically edl of C resides in the aqueous phase and little is incorporated in polymer formed in the first part of the reacticxi. 

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)). 

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]. 

Although batch processes are the workhorse in research laboratory environments, continuous (and semicontinuous) reactors predominate for commercial PE production. In a continuous polymerization reactor, all monomers and reagents are constantly fed into the reactor, and the polymer is isolated from the effluent. Flows are adjusted to achieve the desired steady-state conditions as measured by online analytical instruments and polymer analysis. 

On the industrial scale known are also the semicontinuous or continuous bulk polymerization methods. 

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