Polymerization continued emulsion

Manufacturing processes have been improved by use of on-line computer control and statistical process control leading to more uniform final products. Production methods now include inverse (water-in-oil) suspension polymerization, inverse emulsion polymerization, and continuous aqueous solution polymerization on moving belts. Conventional azo, peroxy, redox, and gamma-ray initiators are used in batch and continuous processes. Recent patents describe processes for preparing transparent and stable microlatexes by inverse microemulsion polymerization. New methods have also been described for reducing residual acrylamide monomer in finished products. 

The inverse emulsion form is made by emulsifying an aqueous monomer solution in a light hydrocarbon oil to form an oil-continuous emulsion stabilized by a surfactant system (21). This is polymerized to form an emulsion of aqueous polymer particle ranging in size from 1.0 to about 10 pm dispersed in oil. By addition of appropriate surfactants, the emulsion is made self-inverting, which means that when it is added to water with agitation, the oil is emulsified and the polymer goes into solution in a few minutes. Alternatively, a surfactant can be added to the water before addition of the inverse polymer emulsion (see Emulsions). 

Continuous polymerization systems offer the possibiUty of several advantages including better heat transfer and cooling capacity, reduction in downtime, more uniform products, and less raw material handling (59,60). In some continuous emulsion homopolymerization processes, materials are added continuously to a first ketde and partially polymerized, then passed into a second reactor where, with additional initiator, the reaction is concluded. Continuous emulsion copolymerizations of vinyl acetate with ethylene have been described (61—64). Recirculating loop reactors which have high heat-transfer rates have found use for the manufacture of latexes for paint appHcations (59). 

Chloroprene mbber is usually manufactured by either batch or continuous emulsion polymerization and isolated either by freeze coagulation or dmm drying of a polymer film. Figure 1 is a schematic flow sheet of this process. 

Emulsions Emulsions have particles of 0.05 to 5.0 [Lm diameter. The product is a stable latex, rather than a filterable suspension. Some latexes are usable directly, as in paints, or they may be coagulated by various means to produce massive polymers. Figures 23-23d and 23-23 show bead and emulsion processes for vinyl chloride. Continuous emulsion polymerization of outadiene-styrene rubber is done in a CSTR battery with a residence time of 8 to 12 h. Batch treating of emulsions also is widely used. 

Medvedev et al. [57] extensively studied the use of nonionic emulsifiers in emulsion polymerization. The emulsion polymerizations in the presence of nonionic emulsifiers exhibited some differences relative to those carried out with the ionic ones. Medvedev et al, [57] proposed that the size of latex particles remained constant during the reaction period, but their number increased continually with the increasing monomer conversion. The use of nonionic emulsifiers in emulsion polymerization usually results in larger sizes relative to those obtained by the ionic emulsifiers. It is possible to reach a final size value of 250 nm by the use of nonionic emulsifiers in the emulsion polymerization of styrene [58]. 

Continuous Emulsion Polymerization Problems in Development of Commercial Processes... 

Continuous emulsion polymerization systems are studied to elucidate reaction mechanisms and to generate the knowledge necessary for the development of commercial continuous processes. Problems encountered with the development of continuous reactor systems and some of the ways of dealing with these problems will be discussed in this paper. Those interested in more detailed information on chemical mechanisms and theoretical models should consult the review papers by Ugelstad and Hansen (1), (kinetics and mechanisms) and by Poehlein and Dougherty (2, (continuous emulsion polymerization). 

In order to be economically viable, a continuous emulsion polymerization process must be able to produce a latex which satisfies application requirements at high rates without frequent disruptions. Since most latex products are developed in batch equipment, the problems associated with converting to continuous systems can be significant. Making such a change requires an understanding of the differences between batch and continuous reactors and how these differences influence product properties and reactor performance. 

Note References 1. and 2. contain extensive bibliographies on emulsion polymerization kinetics and continuous emulsion polymerization respectively. 

Continuous-Emulsion Polymerization of Styrene in a Tubular Reactor... 

The advantages of continuous tubular reactors are well known. They include the elimination of batch to batch variations, a large heat transfer area and minimal handling of chemical products. Despite these advantages there are no reported commercial instances of emulsion polymerizations done in a tubular reactor instead the continuous emulsion process has been realized in series-connected stirred tank reactors (1, . ... 

A few workers have examined the continuous emulsion polymerization process in a tubular reactor (, 5,, the initial work... 

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. 

Loop A continuous process for polymerizing aqueous emulsions of olefinic compounds such as vinyl acetate. Polymerization takes place in a tubular reactor (the loop) with recycle. Invented by Gulf Oil Canada in 1971 and further developed by several United Kingdom paint companies. It is now used for making copolymers of vinyl acetate with ethylene, used in solvent-free paints and adhesives. 

Manufacturing Processes. The three manufacturing processes already mentioned (continuous mass polymerization, batch suspension and emulsion polymerization) continued to compete with each other after 1945. Whereas the third one gradually decreased in importance, the other two were given preference in... 

A. W. De Graff, Continuous Emulsion Polymerization of Styrene in a One Stirred Tank Reactor. Lehigh Univ. Press, Bethlehem, PA, 1970. 

Deionized water (720 g), sodium lauryl sulfate (4.3 g), dioctanoyl peroxide (40 g), and acetone (133 g) were emulsified using an ultrasonic probe for 10 minutes. The step 1 polystyrene seed (48.0 g seed, 578 g latex) was added to the emulsion together with lauryl sulfate (0.8 g) and acetone (29.6 g). The mixture was transferred to a flask and left to agitate at approximately 25°C for 48 hours. Acetone was then removed and the solution added to a 5-liter double-walled glass reactor. The temperature was increased to 40°C while styrene (336 g) and divinyl benzene (0.88 g) were added drop-wise over approximately 60 minutes. After 4 hours the mixture was treated with deionized water (1200 g), potassium iodide (1.28 g), and polyvinyl pyrrolidone (18.48 g) with the temperature increased to 70°C. The polymerization continued for 6 hours at 70°C and 1 hour at 90°C. Styrene-based oligomer particles with a diameter of 1.7 pm and with a narrow size distribution were obtained. 

Our first attempt to produce fast response, microporous gels of PNIP A Am was to polymerize an emulsion with a monomer-rich continuous phase and a... 

In emulsion polymerization the compartmentalization of reaction loci and the location of monomer in polymer particles favor the growth and slow down termination events. The contribution of solution polymerization in the continuous phase is strongly restricted due to the location of monomer in the monomer droplets and/or polymer particles. This gives rise to greatly different characteristics of polymer formation in latex particles from those in bulk or solution polymerization. In emulsion polymerization, where polymer and monomer are mutually soluble, the polymerization locus is the whole particle. If the monomer and polymer are partly mutually soluble, the particle/water interfacial region is the polymerization locus. 

Block or graft copolymers in a selective solvent can form structures due to their amphiphilic nature. Above the critical micelle concentration (CMC), the free energy of the system is lower if the block copolymers associate into micelles rather than remain dispersed as single chains. Often the micelles are spherical, with a compact core of insoluble polymer chains surrounded by a corona of soluble chains (blocks) [56]. Addition of a solvent compatible with the insoluble blocks (chains) and immiscible with the continuous phase leads to the formation of swollen micelles or polymeric micro emulsion. The presence of insoluble polymer can be responsible for anomalous micelles. 

On the Optimal Reactor Type and Operation for Continuous Emulsion Polymerization... 

Continuous emulsion polymerization processes are presently employed for large scale production of synthetic rubber latexes. Owing to the recent growth of the market for polymers in latex form, this process is becoming more and more important also in the production of a number of other synthetic latexes, and hence, the necessity of the knowledge of continuous emulsion polymerization kinetics has recently increased. Nevertheless/ the study of continuous emulsion polymerization kinetics hasf to datef received comparatively scant attention in contrast to batch kinetics/ and very little published work is available at present/ especially as to the reactor optimization of continuous emulsion polymerization processes. For the theoretical optimization of continuous emulsion polymerization reactors/ it is desirable to understand the kinetics of emulsion polymerization as deeply and quantitatively as possible. 

The present review paper, therefore, refers firstly to the particle formation mechanism in emulsion polymerization, the complete understanding of which is indispensable for establishing a correct kinetic model, and then, deals with the present subject, that is, what type of reactor and operating conditions are the most suitable for a continuous emulsion polymerization process from the standpoint of increasing the volume efficiency and the stability of the reactors. 

Although the early literature described the application of a tubular reactor for the production of SBR latexes(1), the standard continuous emulsion polymerization processes for SBR polymerization still consist of continuous stirred tank reactors(CSTR s) and all of the recipe ingredients are normally fed into the first reactor and a latex is removed from the last one, as shown in Figure 1. However, it is doubtful whether this conventional reactor combination and operation method is the most efficient in continuous emulsion polymerization. As is well known, the kinetic behavior of continuous emulsion polymerization differs very much according to the kind of monomers. In this paper, therefore, the discussion about the present subject will be advanced using the... 

In continuous emulsion polymerization of styrene in a series of CSTR s, it was clarified that almost all the particles formed in the first reactor (.2/2) Since the rate of polymerization is, under normal reaction conditions, proportional to the number of polymer particles present, the number of succeeding reactors after the first can be decreased if the number of polymer particles produced in the first stage reactor is increased. This can be realized by increasing emulsifier and initiator concentrations in the feed stream and by lowering the temperature of the first reactor where particle formation is taking place (2) The former choice is not desirable because production cost and impurities which may be involved in the polymers will increase. The latter practice could be employed in parallel with the technique given in this paper. 

Our final goal in the present paper is to devise an optimal type of the first stage reactor and its operation method which will maximize the number of polymer particles produced in continuous emulsion polymerization. For this purpose, we need a mathematical reaction model which explains particle formation and other kinetic behavior of continuous emulsion polymerization of styrene. 

Figure 1. Flow diagram of typical continuous emulsion polymerization reactor...

Let us determine the value of e by comparing the transient kinetic behavior of monomer conversion in continuous emulsion polymerization of styrene with the model prediction by the Nomura and Harada model. It is reported in the literature that sustained... 

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