Suspending-phase polymerizations

Many polymerizations use a low viscosity nonsolvent to suspend the polymer phase. Water is the most common suspending phase. Table 13.6 characterizes a variety of reaction mechanisms in which water is the continuous phase. 

FIGURE 13.9 Curved streamlines resulting from a polycondensation in the laminar flow reactor of Example 13.10. 

TABLE 13.6 Classification of Polymerization Mechanisms that Use Water as the Continuous, Suspending Phase 

Process parameter Polymerization type Emulsion - Dispersion Suspension 

The reaction engineering aspects of these polymerizations are similar. Excellent heat transfer makes them suitable for vinyl addition polymerizations. Free radical catalysis is mostly used, but cationic catalysis is used for non-aqueous dispersion polymerization (e.g., of isobutene). High conversions are generally possible, and the resulting polymer, either as a latex or as beads, is directly suitable for some applications (e.g., paints, gel-permeation chromatography beads, expanded polystyrene). Most of these polymerizations are run in the batch mode, but continuous emulsion polymerization is common. 

Emulsion Polymerization. Emulsion polymerization uses soaps and anionic surfactants to create two-phase systems that have having long-term stability. The key steps in a batch emulsion polymerization are the following  

Process Parameter Polymerization Type Emulsion — Dispersion — Suspension 

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 can be avoided by feeding the CSTR with seed particles made in a batch or tubular reactor. 

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This article discusses polymerization reactors where the continuous phase is a solution of a polymer in its own monomer or in a solvent. When the low molecular weight species is primarily monomer, the reaction is a bulk polymerization and when it is a solvent, the reaction is a solution polymerization. This distinction has little practical importance. The important consideration is that a high viscosity polymer solution is the continuous phase and is in contact with the reactor walls and the agitator. In contrast, suspended-phase polymerizations (such as emulsion, dispersion, and suspension) and gas-phase polsrmerizations have a low viscosity continuous phase (see Heterophase Polymerization). 

Suspension Polymerization. Water is the suspending phase. Inorganic salts and vigorous agitation prevent coalescence and agglomeration. The reaction mode is batch. The largest use of suspension polymerization is for the manufacture of expandable polystyrene beads. 

Aqueous solutions can also by microencapsulated in high concentration [6]. To prepare the reverse phase W/0 (Water-in-Oil) emulsions care must be taken to select monomers that will remain in the dispersed water droplets during the emulsion stage. If the monomers diffuse from suspended droplets into the continuous phase polymerization will happen throughout the emulsion and not at the interface as intended. No microcapsules will be formed. This problem has been addressed utilizing carboxy-functional polymers to associate with amine functional reactive monomers dissolved into the water droplets [7]. Shellwalls are formed at the W/0 interface by addition of the oil-soluble monomers to the continuous oil phase. Without the carboxy-functional protective polymers amine monomers would have partitioned out of dispersed water droplets and into the oil phase. Microcapsules would not have been produced. 

Graft polymers containing the carboxyl ligand have also been prepared. For example, acrylic acid can be grafted to an inert support such as polypropylene or polyethylene by two main pathways. In one path the graft polymerization is initiated by radical initiators. For instance, polypropylene is suspended in H2O and grafted with acrylic acid in the presence of water-insoluble radical initiators in water-miscible solvents (14). The other path involves the grafting of acrylic acid to polyethylene by gas phase polymerization (15). 

A nonconventional suspension polymerization process was also investigated by utilizing a water-agarose gel as a suspending phase. Reactor agitation was only at an early stage in the process to fix the droplet size, and stopped when the temper-... 

Typical fully formulated lubricating oil is composed of a combination of mineral or synthetic base oils and specialized dispersed additives designed to improve long-term stability and enhance performance in an aggressive environments. Any commercial lubricant can be viewed as a nonaqueous highly resistive polymeric colloid with low, primarily ionic electrical conductivity. This system is composed of nonpolar base oil and suspended polar molecules and structures. The suspended phase includes both specialized oil additives blended into base oils to enhance the lubricant performance and polar contaminants such as soot, water, and oxidant products that ingress into the lubricant with age and use. The dipolar nature of nonaqueous lubricants and other industrial colloids permits investigation of... 

All FIA atomic absorption methodologies developed for indirect AA determination involve the utilization of flame atomic absorption spectrometry (FAAS) as detector, and are based on oxidation of AA to DHAA and reduction of a metallic specie (Fe(III) to Fe(II), Cr(VI) to Cr(III), Mn(VII) to Mn(II) and Mn(IV) to Mn(II)). Most of the methods apply a microcolumn with a solid phase (polymeric adsorbent Amberlite XAD4 [136], cation-exchange resin Amberlite IR120 [137,138], or poly(aminophosphonic acid) chelating resin [139,140]) to retain the reduced metallic species. The other possibility is the utilization of a solid-phase reactor filled with the substance to be reduced. Thus, Noroozifar et al. [141] propose a reactor filled with Mn02 suspended on silica gel beads. 

Polymerization may be carried out with monomer alone (bulk), in a solvent (solution), as an emulsion in water (emulsion), or as droplets, each one comprising an individual bulk polymerization, suspended in water (suspension). All four methods are commercially applied to radical-initiated chain polymers such as polystyrene. Most ionic and coordination complex systems are inactivated by water, so that only bulk or solution methods can be used. Also, rather few condensations are carried out in emulsion or suspension. However, ethylene dichloride and sodium polysulflde are condensed to give ethylene polysulflde rubber and sodium chloride in an aqueous emulsion. Gas-phase polymerization and interfacial condensation are special techniques, which are mentioned in Section 5.3. 

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