Polymerization continued recipe

Figure 3.33 lists a recipe for emulsion polymerization of polystyrene in a water dispersion of monomer droplets and soap micelles [20]. The reaction is started by light-sensitive, water-soluble initiators, such as benzoyl peroxide. If one compares the sizes of the dispersed droplets, one notices that the small soap micelles that contain also styrene in their interior are most likely to occasionally initiate a polymerization of the monomer on absorption of a free radical. Once initiated, the reaction continues until a second free radical molecule enters the micelle. Then the reaction is terminated, until a third radical starts another molecule. Monomers continuously add to the micelles, so that the polymerization continues. Keeping the free radical generation constant, a relatively narrow molar mass distribution can be obtained. 

If a linear mbber is used as a feedstock for the mass process (85), the mbber becomes insoluble in the mixture of monomers and SAN polymer which is formed in the reactors, and discrete mbber particles are formed. This is referred to as phase inversion since the continuous phase shifts from mbber to SAN. Grafting of some of the SAN onto the mbber particles occurs as in the emulsion process. Typically, the mass-produced mbber particles are larger (0.5 to 5 llm) than those of emulsion-based ABS (0.1 to 1 llm) and contain much larger internal occlusions of SAN polymer. The reaction recipe can include polymerization initiators, chain-transfer agents, and other additives. Diluents are sometimes used to reduce the viscosity of the monomer and polymer mixture to faciUtate processing at high conversion. The product from the reactor system is devolatilized to remove the unreacted monomers and is then pelletized. Equipment used for devolatilization includes single- and twin-screw extmders, and flash and thin film evaporators. Unreacted monomers are recovered for recycle to the reactors to improve the process yield. 

Table 14. Recipe for Emulsion Polymerization by Continuous Addition ...

The rate of polymerization with styrene-type monomers is directly proportional to the number of particles formed. In batch reactors most of the particles are nucleated early in the reaction and the number formed depends on the emulsifier available to stabilize these small particles. In a CSTR operating at steady-state the rate of nucleation of new particles depends on the concentration of free emulsifier, i.e. the emulsifier not adsorbed on other surfaces. Since the average particle size in a CSTR is larger than the average size at the end of the batch nucleation period, fewer particles are formed in a CSTR than if the same recipe were used in a batch reactor. Since rate is proportional to the number of particles for styrene-type monomers, the rate per unit volume in a CSTR will be less than the interval-two rate in a batch reactor. In fact, the maximum CSTR rate will be about 60 to 70 percent the batch rate for such monomers. Monomers for which the rate is not as strongly dependent on the number of particles will display less of a difference between batch and continuous reactors. Also, continuous reactors with a particle seed in the feed may be capable of higher rates. 

The experimental semibatch apparatus and procedure have been described in several places through the text of Wisseroth s publications ( 1, 7-9). so the details will not be repeated here. For nearly all of his work the reactor volume was one liter, temperature was 80 C, pressure was 30 atm (441 psia), and the feed was polymerization grade I assume that the reactor gas composition was 99% CsHgand 1% inerts. The range of catalyst loading was from 11 to 600 mg of TiCils per batch. The reaction time was varied from 0.5 to 6 hours. The weight ratio of alkyl-to-TiC 3 in the catalyst recipe was varied from 0.5 to 32. No data are reported from a continuous gas phase reactor. 

The experiments were repeated but were not reproducible Acrylamide gels were made anew with various recipes and their swelling curves were determined as a function of acetone concentration, but they were all continuous. It took a couple of months to recognize that the gels that showed the discontinuous transition were old ones, that is, gels prepared a month earlier and left within the tubes in which they were polymerized. Subsequent experiments were all carried out on new gels, and, therefore, underwent a continuous transition. At that time all the old gels were used up, and none were left in the laboratory. 

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... 

Figure 6. Example data acquisition for the continuous emulsion polymerization of MM A showing conversion and surface tension oscillations (Run 15, Recipe 8 T = 40°C initiator (ammonium persulfate) =0.01 gmol/L H20 emulsifier (SLS) = 0.02 gmol/L H20 wt. ratio monomer/water = 0.43)...

The most common continuous emulsion polymerization systems require isothermal reaction conditions and provide for conversion control through manipulation of initiator feed rates. Typically, as shown in Figure 1, flow rates of monomer, water, and emulsifier solutions into the first reactor of the series are controlled at levels prescribed by the particular recipe being made and reaction temperature is controlled by changing the temperature of the coolant in the reactor jacket. Manipulation of the initiator feed rate to the reactor is then used to control reaction rate and, subsequently, exit conversion. An aspect of this control strategy which has not been considered in the literature is the complication presented by the apparent dead-time which exists between the point of addition of initiator and the point where conversion is measured. In many systems this dead-time is of the order of several hours, presenting a problem which conventional control systems are incapable of solving. This apparent dead-time often encountered in initiation of polymerization. 

A recent paper by Kiparissides, et al. (8) details a mathematical model for the continuous polymerization of vinyl acetate in a single CSTR. Operating conditions were shown to exist in which either steady-state operation or sustained conversion oscillations would occur for vinyl acetate. Experimental results for both cases were successfully simulated by their model. In addition, regulatory conversion control policies were considered in which both initiator feed rate and emulsifier feed rate were used as manipulated variables (Kiparissides (9)). The problem of conversion control in the operating region in which sustained conversion oscillations occur is one of significant commercial importance. Most commonly, however, a uniform concentration of emulsifier is required in the emulsion recipe and, hence, emulsifier flow rate cannot be used as a manipulated variable. 

Latexes. Latexes were made in a monomer addition recipe described earlier (10). This is a seeded continuous monomer addition recipe using t-butylhydro-peroxide/hydroxylamine hydrochloride redox couple as initiator. Polymerizations were carried out in stirred glass reactors at 50°C. The only variation in the original recipe was in the surfactants. In the present procedvire, < 1/3 of the soap (- 1.5% based on total monomer) was used in the seed and the remainder fed to the reactor during polymerization. The monomer feed contained styrene and butylacrylate in a 40/60 ratio. This composition was selected because it is readily filmforming and is not affected chemically by the electrodeposition process. The polymer remains soluble and... 

In a single CSTR, monomer and other ingredients of the polymerization recipe are continually fed into the vessel while polymer and the rest of the reaction mixture are removed. The effluent can, of course, serve as the feed to the next CSTR in a series operation. Problems with heat removal are alleviated to some extent because of the beneficial effects of cold monomer feed and the removal of reaction heat with the effluent. CSTR reactors are economically attractive for large-scale production with relatively infrequent changes in product properties. 

A number of factors need to he considered when developing a product that will be produced in a continuous reactor system. The initial development work on most new products is carried out in batch reactors. Bottle polymerizers are often us for this purpose because a large number of experiments can he completed quickly. These early experiments provide a product that can he tested against fundamental standards (molecular weight, particle size, rheology, etc,) and in proposed applications. PreHminary recipes evolve from such tests. 

A typical recipe is shown in Table 7. The solution polymerization is carried out at 140°C by adding the monomer and initiator mixture uniformly for over 4 hr. After the addition of initiator, the reaction is continued over 2 hr. 

A simple recipe for emulsion polymerization would be comprised of hydrophobic monomers (AO to 60 volume percent), a continuous aqueous phase (AO to 60 volume percent), a water-soluble initiator, and an emulsifier or stabilizer. Other minor ingredients such as chain transfer agents, inhibitors or retarders, and buffers may also be present. Emulsion polymerization is characterized by a large number of reaction sites (the polymer particles) that contain a small number of free radicals. These free radicals are isolated because of the water phase between the particles. Typical polymer... 

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