Suspension polymerization scaling

Suspension polymerization of VDE in water are batch processes in autoclaves designed to limit scale formation (91). Most systems operate from 30 to 100°C and are initiated with monomer-soluble organic free-radical initiators such as diisopropyl peroxydicarbonate (92—96), tert-huty peroxypivalate (97), or / fZ-amyl peroxypivalate (98). Usually water-soluble polymers, eg, cellulose derivatives or poly(vinyl alcohol), are used as suspending agents to reduce coalescence of polymer particles. Organic solvents that may act as a reaction accelerator or chain-transfer agent are often employed. The reactor product is a slurry of suspended polymer particles, usually spheres of 30—100 pm in diameter they are separated from the water phase thoroughly washed and dried. Size and internal stmcture of beads, ie, porosity, and dispersant residues affect how the resin performs in appHcations. 

The criterion of maintaining equal power per unit volume has been commonly used for dupHcating dispersion qualities on the two scales of mixing. However, this criterion would be conservative if only dispersion homogeneity is desired. The scale-up criterion based on laminar shear mechanism (9) consists of constant > typical for suspension polymerization. The turbulence model gives constant tip speed %ND for scale-up. 

Processes that are essentially modifications of laboratory methods and that allow operation on a larger scale are used for commercial preparation of vinyhdene chloride polymers. The intended use dictates the polymer characteristics and, to some extent, the method of manufacture. Emulsion polymerization and suspension polymerization are the preferred industrial processes. Either process is carried out in a closed, stirred reactor, which should be glass-lined and jacketed for heating and cooling. The reactor must be purged of oxygen, and the water and monomer must be free of metallic impurities to prevent an adverse effect on the thermal stabiUty of the polymer. 

Free-radical polymerization of alkenes has been carried out in aqueous conditions.115 Aqueous emulsion and suspension polymerization is carried out today on a large scale by free-radical routes. Polymer latexes can be obtained as products (i.e., stable aqueous dispersions... 

Since cross-linked polymers caruiot be re-formed or re-shaped it is necessary to synthesize them in the final physical form appropriate for each particular application. Particles in the size range 50-1000 pm are suitable for laboratory scale chemistry, while larger particles have advantages in large scale continuous processes. Irregularly shaped particles are susceptible to mechanical attrition and breakdown to fines , whereas the process of suspension polymerization [13] yields uniform spherical cross-linked polymer particles often referred to as beads, pearls or resins. These are much more mechanically robust and are widely exploited on both a small and large scale e. g. as the basis of ion exchange resins [14]. 

Fig. 1.3 S mall scale oscillatory baffled reactor (OBR) for gram scale suspension polymerization.

On the industrial scale, suspension polymerizations are not only carried out in the aqueous phase, but also in aliphatic hydrocarbons using Ziegler-Natta catalysts, as for example, in the polymerization of ethylene and propylene (see also Sect. 3.3.1). 

Wieme,)., de Roo, T Marin, G., Heynderickx, G., Simulation of pilot- and industrial-scale vinyl chloride batch suspension polymerization reactors, Ind. Eng. Chem. Res., 2007, 46,1179-1196... 

Formation of a scale of polymer on the reactor walls is normally less than in corresponding bulk or solution polymerizations. Scale formation is troublesome in PVC suspension polymerizations, however, because the polymer is not soluble in its monomer, and a deposit formed on the wall will not be washed off by fresh monomer. This build-up has to be removed in order to maintain satisfactory heat transfer and prevent inclusion of gelled polymer ( fish eyes ) in the product. Cleanliness of the reactor walls is very important because the productivity of the equipment is enhanced by longer intervals between shutdowns for cleaning. To this end, some phenolic coatings have been designed that inhibit polymer buildup by terminating free-radical reactions on the walls (cf. Section 6.9). 

As mentioned earlier, both chemical (catalyst, surfactants, stabilizers) and physical (fluid dynamics, energy dissipation rates, circulation time and so on) factors control the performance of the suspension polymerization reactor. It is first necessary to examine the available experimental data to clearly understand the role of these chemical and physical factors. The available data indicates that the yield of usable polymer beads in laboratory scale reactor is more than 85%. Laboratory experiments were then planned to examine the sensitivity of the yield to various parameters of the polymerization recipe under the same hydrodynamic conditions. These experiments showed that the yield is relatively insensitive to small deviations in the chemical recipe. Analysis of the available data on pilot and plant scale indicated a progressive decrease in the yield of usable polymer beads from laboratory to pilot to plant scale. This analysis and some indirect evidence suggested that it may be possible to re-design the plant-scale reactor hardware to generate better fluid dynamics and mixing to increase the yield of particles in the desired size range. 

As a reaction medium for transition rnetal-catalyzed polymerizations, water will, most likely, not be the first choice. The extreme water sensitivity of Ziegler or Phillips catalysts is well known. However, carrying out polymerization reactions in aqueous systems offers unique advantages. Thus, traditional free-radical emulsion and suspension polymerization are carried out on a large scale industrially. A brief review of these established reactions demonstrates some specific properties of polymerizations in aqueous systems. 

Aqueous dispersions of poly(vinyl acetate) and vinyl acetate-ethylene copolymers, homo- and copolymers of acrylic monomers, and styrene-butadiene copolymers are the most important types of polymer latexes today. Applications include paints, coatings, adhesives, paper manufacturing, leather manufacturing, textiles and other industries. In addition to emulsion polymerization, other aqueous free-radical polymerizations are applied on a large scale. In suspension polymerization a water-irnrniscible olefinic monomer is also polymerized. However, by contrast to emulsion polymerization a monomer-soluble initiator is employed, and usually no surfactant is added. Polymerization occurs in the monomer droplets, with kinetics similar to bulk polymerization. The particles obtained are much larger (>15 pm) than in emulsion polymerization, and they do not form stable latexes but precipitate during polymerization (Scheme 7.2). 

Equation (9.48) was tested commercially for scale-up using geometrically similar vessels and for drop sizes ranging from 300 to 1200 jm. Noncoalescing suspension polymerization experiments enabled sizes to be determined for finished beads using screen-analysis measurements. Equation... 

By comparison with the intensively investigated syntheses of low molecular weight compounds by biphasic catalysis, catalytic polymerization in aqueous systems has received less attention. This is somewhat surprising, as polymerization in aqueous systems offers unique advantages, as illustrated by the large-scale applications of free-radical emulsion and suspension polymerization. 

The fact that alcohols, too, induce the formation of large pores in molded rods is well consistent with the same observation for beaded polymers prepared by suspension polymerization. In general, suspension polymerization is considered to be bulk polymerization on a mini scale. Hence, all dependences that are characteristic of bulk polymerization would have to be valid for both the molded rods and the beads. Indeed, poly(gIycidyl methacrylate-co-ethylene dimethacrylate) obtained under identical conditions in the form of rod or beads have similar surface area and pore volume values. Both polymers can be expected to have more or... 

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