Mass polymerization

In mass polymerization bulk monomer is converted to polymers. In solution polymerization the reaction is completed in the presence of a solvent. In suspension, dispersed mass, pearl or granular polymerization the monomer, containing dissolved initiator, is polymerized while dispersed in the form of fine droplets in a second non-reactive liquid (usually water). In emulsion polymerization an aqueous emulsion of the monomer in the presence of a water-soluble initiator Is converted to a polymer latex (colloidal dispersion of polymer in water). 

Morphology as Polymerized. The principal type of polymerization of PVC is the suspension polymeriza tion route. The morphology formed during polymerization strongly influences the processibiUty and physical properties. Mass-polymerized PVC has a similar morphology to suspension PVC. 

Mass-polymerized PVC also has a skin of compacted PVC primary particles very similar in thickness and appearance to the suspension-polymerized PVC skin, compared in Figure 3. However, mass PVC does not contain the thin-block copolymer membrane (7). 

Polymerization Kinetics of Mass and Suspension PVC. The polymerization kinetics of mass and suspension PVC are considered together because a droplet of monomer in suspension polymerization can be considered to be a mass polymerization in a very tiny reactor. During polymerization, the polymer precipitates from the monomer when the chain size reaches 10—20 monomer units. The precipitated polymer remains swollen with monomer, but has a reduced radical termination rate. This leads to a higher concentration of radicals in the polymer gel and an increased polymerization rate at higher polymerization conversion. 

However, the kinetics of PVC emulsion does not foUow the above theory. The rate shows the same increasing behavior with conversion as mass polymerization (94,95). [N depends on [3], but the relationship varies with the emulsifier type (96,97). However, the rate is nearly independent of [N (95). The average number of radicals per particle is low, 0.0005 to 0.1 (95). The high solubiUty of vinyl chloride in water, 0.6 wt %, accounts for a strong deviation from tme emulsion behavior. Also, PVC s insolubiUty in its own monomer accounts for such behavior as a rate dependence on conversion. 

Table I provides an overview of general reactor designs used with PS and HIPS processes on the basis of reactor function. The polymer concentrations characterizing the mass polymerizations are approximate there could be some overlapping of agitator types with solids level beyond that shown in the tcd>le. Polymer concentration limits on HIPS will be lower because of increased viscosity. There are also additional applications. Tubular reactors, for example, in effect, often exist as the transfer lines between reactors and in external circulating loops associated with continuous reactors.

Figure 19. Schematic of Shell patented continuous mass polymerization process...

In contrast to materials of low molecular mass, polymeric emitters possess the advantage that they can be easily worked into transparent films with a low degree of scattering. In addition, they show a higher morphological stability than that of vapor-deposited low molecular weight compounds (low tendency toward recrystallization). 

Mass market soap manufacture, 22 723 Mass Mean Diameter (MMD), 23.T87 Mass of catalyst, in photocatalysis, 19 77-78 Mass particle diameter, 78 134 Mass polymerization, 70 206 ABS, 7 422... 

The BASF continuous mass polymerization process employed a tower reactor with an upstream continuous stirred tank reactor (16) (Figure 1). 

The emulsion process, however, competed strongly in the initial phase with the continuous mass polymerization process, one reason being the easier heat removal but the main reason being that high molecular weights were obtained in a simple manner. The process first appeared in the patent literature (19, 20) in 1927 and was further improved by H. Fikentscher (21), finding wide application in the whole field of polymer chemistry. 

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... 

Union Carbide (34) and in particular Dow adopted the continuous mass polymerization process. Credit goes to Dow (35) for improving the old BASF process in such a way that good quality impact-resistant polystyrenes became accessible. The result was that impact-resistant polystyrene outstripped unmodified crystal polystyrene. Today, some 60% of polystyrene is of the impact-resistant type. The technical improvement involved numerous details it was necessary to learn how to handle highly viscous polymer melts, how to construct reactors for optimum removal of the reaction heat, how to remove residual monomer and solvents, and how to convey and meter melts and mix them with auxiliaries (antioxidants, antistatics, mold-release agents and colorants). All this was necessary to obtain not only an efficiently operating process but also uniform quality products differentiated to meet the requirements of various fields of application. In the meantime this process has attained technical maturity over the years it has been modified a number of times (Shell in 1966 (36), BASF in 1968 (37), Granada Plastics in 1970 (38) and Monsanto in 1975 (39)) but the basic concept has been retained. 

Because PS is brittle with little impact resistance under normal operating conditions, early work was done to impart impact resistance. The best known material from this work is called HIPS, which is produced by dispersing small particles of BR in with the styrene monomer. Bulk or mass polymerization of the styrene is begun producing what is referred to as... 

In a typical ABS mass polymerization process, styrene and acrylonitrile are copolymerized in the presence of a diene-based rubber. Initially, the rubber is dissolved in the monomers and a continuous homogeneous phase prevails. 

At this point, the phenomenon of phase inversion occurs and the rubber in monomer phase becomes dispersed as discrete particles in a matrix of the polymer in monomer phase. Usually in a mass polymerization process, the rubber will contain occlusions of polymer/monomer, which serve to swell the volume of the rubber particle. In the course of polymerization, monomer is converted to polymer, the viscosity of the mixture increases and greater power is needed to maintain the temperature and the compositional uniformity throughout the polymerized material (8). 

A continuous mass polymerization process for making an extrusion grade ABS resin has been described (11). It would be straightforward to start with a certain feed that runs as such through the reactor cascade. However, it is more advantageous to add between first and second reactor some monomer feed including certain additives. The continuous polymerization is conducted in a cascade of reactors. A typical feed is given in Table 8.5. 

Tough Compositions. The toughness of a thermoplastic, rigid poly(urethane) (PU) can be markedly improved by incorporating ABS resin. The ABS resin is mass-polymerized. 

V.A. Aliberti, R.L. Kruse, and E.M. Valcarce, Mass polymerization process for ABS polyblends, US Patent 4598124, assigned to Monsanto Company (St. Louis, MO), July 1,1986. 

W. Nielinger, H. Kauth, D. Wittmann, D. Freitag, and K.-J. Idel, Production of blend systems by the condensation of oligocarbonates in the presence of mass-polymerized ABS, US Patent5 416161, assigned to Bayer Aktiengesellschaft (Leverkusen, DE), May 16,1995. 

The bulk polymerization effects the special needs to remove the heat of reactions, and moreover, high conversions cannot be reached because the viscosity of the polymer increases drastically with conversion. In order to avoid a high viscosity of the end product before discharging, the mass polymerization is carried out in solution. Ethylbenzene is a common solvent. 

RESINS (Acrylonitrile-Butadiene-Styrene). Commonly referred to as ABS resins, these materials are thermoplastic resins which are produced by grafting styrene and acrylonitrile onto a diene-rubber backbone. The usually preferred substrate is polybutadiene because of its low glass-transition temperature (approximately —80°C). Where ABS resin is prepared by suspension or mass polymerization methods, stereospedfic diene rubber made by solution polymerization is the preferred diene. Otherwise, the diene used is a high-gel or cross-linked latex made by a hot emulsion process. 

Step 3—In a separate step, styrene-acrylonitrile (SAN) resin is prepared by emulsion, suspension, or mass polymerization by free-radical techniques. The operation is carried out in stainless-steel reactors operated at about 75°C (167°F) and moderate pressure for about 7 hours. Tlie final chemical operation is the blending of the ABS graft phase with the SAN resin, plus adding various antioxidants, lubricants, stabilizers, and pigments. Final operations involve preparation of a slurry of fine resin particles (via chemical flocculation), filtering, and drying in a standard fluid-bed dryer at 121-132°C (250-270°F) inlet air temperature. 

For both suspension and mass polymerizations ar less than 2% conversion, PVC precipitates from its monomer as stable primary particles, slightly below 1-p.m dia. 

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