Emulsion polymerization heat transfer

Emulsion polymerization also has the advantages of good heat transfer and low viscosity, which follow from the presence of the aqueous phase. The resulting aqueous dispersion of polymer is called a latex. The polymer can be subsequently separated from the aqueous portion of the latex or the latter can be used directly in eventual appUcations. For example, in coatings applications-such as paints, paper coatings, floor pohshes-soft polymer particles coalesce into a continuous film with the evaporation of water after the latex has been applied to the substrate. 

Emulsion Process. The emulsion polymerization process utilizes water as a continuous phase with the reactants suspended as microscopic particles. This low viscosity system allows facile mixing and heat transfer for control purposes. An emulsifier is generally employed to stabilize the water insoluble monomers and other reactants, and to prevent reactor fouling. With SAN the system is composed of water, monomers, chain-transfer agents for molecular weight control, emulsifiers, and initiators. Both batch and semibatch processes are employed. Copolymerization is normally carried out at 60 to 100°C to conversions of - 97%. Lower temperature polymerization can be achieved with redox-initiator systems (51). 

Aqueous media, such as emulsion, suspension, and dispersion polymerization, are by far the most widely used in the acryUc fiber industry. Water acts as a convenient heat-transfer and cooling medium and the polymer is easily recovered by filtration or centrifugation. Fiber producers that use aqueous solutions of thiocyanate or zinc chloride as the solvent for the polymer have an additional benefit. In such cases the reaction medium can be converted directiy to dope to save the costs of polymer recovery. Aqueous emulsions are less common. This type of process is used primarily for modacryUc compositions, such as Dynel. Even in such processes the emulsifier is used at very low levels, giving a polymerization medium with characteristics of both a suspension and a tme emulsion. 

Copolymerization is effected by suspension or emulsion techniques under such conditions that tetrafluoroethylene, but not ethylene, may homopolymerize. Bulk polymerization is not commercially feasible, because of heat-transfer limitations and explosion hazard of the comonomer mixture. Polymerizations typically take place below 100°C and 5 MPa (50 atm). Initiators include peroxides, redox systems (10), free-radical sources (11), and ionizing radiation (12). 

In the paste coating method, a PVC paste, which contains emulsion-polymerized PVC and additives, is appHed onto a substrate and heated to gelation before fusion to produce a coating layer. This method is employed for products with a thin layer, ie, of 0.007—0.05 mm thickness. For foamed vinyl-coated fabrics, a substrate is laminated onto a transfer paper on which a PVC paste containing a foam-blowing agent has been appHed and geUed. After removal of the transfer paper, the paste is blown. 

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). 

Polymerization processes are characterized by extremes. Industrial products are mixtures with molecular weights of lO" to 10. In a particular polymerization of styrene the viscosity increased by a fac tor of lO " as conversion went from 0 to 60 percent. The adiabatic reaction temperature for complete polymerization of ethylene is 1,800 K (3,240 R). Heat transfer coefficients in stirred tanks with high viscosities can be as low as 25 W/(m °C) (16.2 Btu/[h fH °F]). Reaction times for butadiene-styrene rubbers are 8 to 12 h polyethylene molecules continue to grow lor 30 min whereas ethyl acrylate in 20% emulsion reacts in less than 1 min, so monomer must be added gradually to keep the temperature within hmits. Initiators of the chain reactions have concentration of 10" g mol/L so they are highly sensitive to poisons and impurities. 

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, . ... 

Polymers may be made by four different experimental techniques bulk, solution, suspension, and emulsion processes. They are somewhat self-explanatory. In bulk polymerization only the monomers and a small amount of catalyst is present. No separation processes are necessary and the only impurity in the final product is monomer. But heat transfer is a problem as the polymer becomes viscous. In solution polymerization the solvent dissipates the heat better, but it must be removed later and care must be used in choosing the proper solvent so it does not act as a chain transfer agent. In suspension polymerization the monomer and catalyst are suspended as droplets in a continuous phase such as water by continuous agitation. Finally, emulsion polymerization uses an emulsifying agent such as soap, which forms micelles where the polymerization takes place. 

Monomer and initiator must be soluble in the liquid and the solvent must have the desired chain-transfer characteristics, boiling point (above the temperature necessary to carry out the polymerization and low enough to allow for ready removal if the polymer is recovered by solvent evaporation). The presence of the solvent assists in heat removal and control (as it also does for suspension and emulsion polymerization systems). Polymer yield per reaction volume is lower than for bulk reactions. Also, solvent recovery and removal (from the polymer) is necessary. Many free radical and ionic polymerizations are carried out utilizing solution polymerization including water-soluble polymers prepared in aqueous solution (namely poly(acrylic acid), polyacrylamide, and poly(A-vinylpyrrolidinone). Polystyrene, poly(methyl methacrylate), poly(vinyl chloride), and polybutadiene are prepared from organic solution polymerizations. 

Advantages of emulsion polymerization are rapidity and production of high-molecular-weight polymers in a system of relatively low viscosity. Difficulties in agitation, heat transfer, and transfer of materials are minimized. The handling of hazardous solvents is eliminated. The two principal variations in technique used for emulsion polymerization are the redox and the reflux methods. 

Homopolymerization. The free-radical polymerization of VDC has been carried out by solution, slurry, suspension, and emulsion methods. Slurry polymerizations are usually used only in the laboratory. The heterogeneity of the reaction makes stirring and heat transfer difficult consequently, these reactions cannot be easily controlled on a large scale. Aqueous emulsion or suspension reactions are preferred for large-scale operations. The spontaneous polymerization of VDC, so often observed when the monomer is stored at room temperature, is caused by peroxides formed from the reaction of VDC with oxygen, fery pure monomer does not polymerize under these conditions. Heterogeneous polymerization is characteristic of a number of monomers, including vinyl chloride and acrylonitrile. 

The emulsion polymerization system consists of three phases an aqueous phase (containing initiator, emulsifier, and some monomer), emulsified monomer droplets, the monomer-swollen micelles, and monomer-swollen particles. Water is the most important ingredient of the emulsion polymerization system. It is inert and acts as the locus of initiation (the formation of primary and oligomeric radicals) and the medium of transfer of monomer and emulsifier from monomer droplets or the monomer-swollen particle micelles to particles. An aqueous phase maintains a low viscosity and provides an efficient heat transfer. 

While vinyl acetate is normally polymerized in batch or continuous stirred tank reactors, continuous reactors offer the possibility of better heat transfer and more uniform quality. Tubular reactors have been used to produce polystyrene by a mass process (1, 2), and to produce emulsion polymers from styrene and styrene-butadiene (3 -6). The use of mixed emulsifiers to produce mono-disperse latexes has been applied to polyvinyl toluene (5). Dunn and Taylor have proposed that nucleation in seeded vinyl acetate emulsion is prevented by entrapment of oligomeric radicals by the seed particles (6j. Because of the solubility of vinyl acetate in water, Smith -Ewart kinetics (case 2) does not seem to apply, but the kinetic models developed by Ugelstad (7J and Friis (8 ) seem to be more appropriate. 

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. 

Since mixing and good heat transfer are of vital importance in viscous polymerization reactions, a mechanically agitated continuous stirred-tank reactor is widely used in polymerization processes. Solution polymerization, emulsion polymerization, and solid-catalyzed olefin polymerization are all carried out in a mechanically agitated slurry reactor. 

Superficially, emulsion polymerization resembles suspension polymerization, but there are a number of important differences. Water is used as the continuous phase and heat transfer is very good for both suspension and emulsion polymerization. In contrast to suspension polymerization, the polymer particles produced in emulsion polymerization are on the order of 0.1 xm in diameter.33 Another important difference is the presence of an emulsifying agent or soap. At the beginning of polymerization the soap molecules aggregate together in a group of about 50-100 molecules to form what is called a micelle. Some of the... 

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