Bulk polymerization autoclave

The AA-BB polyamides are nearly always prepared using bulk polymerization with a prepolymerization step at higher pressure, although not every laboratory has die facility to carry out a polymerization in an autoclave. Nielinger50 has reported bulk polymerization of PA-6,1 from its salt solution at atmospheric pressure (Example 11). This mediod may also be usable for other laboratory polymerizations like PA-6,6. 

A bnlk polymerization reactor can be as simple as a tube into which the reactants are fed and from which the polymer mixture emerges at the end it can be more of a traditional, continnons stirred-tank reactor (CSTR), or even a high-pressure autoclave-type reactor (see Figure 3.21). A bulk polymerization process need not be continuous, but it should not be confnsed with a batch reaction. There can be batch bnlk polymerizations jnst as there are continnons bulk polymerizations processes. 

PVC bulk polymerizations are two-slage processes in which a very porous PVC seed particle is produced in a vessel provided with very high speed agitation. The wet polymer powder from this stage is fed to horizontal autoclaves in which the polymerization is finished. Reaction heal is removed by cooling the helical ribbon blender-type agitators as well as the vessel jacket. 

PVC is insoluble in VCM. The precipitated polymer tends to coagulate in the conversion range of 1-8%. As the conversion continues, the precipitated polymer absorbs more and more monomer, and at a 15-20% conversion, the reactor contains solid polymer swollen with monomer in a monomer atmosphere. The horizontal autoclave prevents the formation of large polymer blocks by breaking them up. Advantages claimed for this process are the higher bulk density, improved particle size distribution, and more rapid plasticizer absorption. Talamini and coworkers (J ) have demonstrated that the bulk polymerization process is kinetically equivalent to suspension polymerization. 

Rexene Co. and Philips Petroleum Co. first developed the bulk polymerization process with the first-generation TiCU catalyst [8,11,70]. It was then commercialized by Dart Industries in 1964. The reactor feed contains 10-30% propylene in the liquid phase. A mixture of hexane and isopropanol was employed for the removal of catalyst residue as well as the amorphous polypropylene. The process step of removing residual catalyst was later eliminated after the high-efficiency catalyst was adopted, constituting the so-called liquid pool process. Subsequently, Philips and Sumitomo companies further developed the liquid-phase polymerization process. This process enhances the reaction rate, catalyst efficiency, monomer conversion, and therefore results in high productivity. It also eliminates the need for solvent recovery and reduces environmental pollution. However, the process is somewhat complicated by the unreacted monomer, which has to be first vaporized and then liquefied before it is reused. The reaction vessel must be designed to operate under high pressures. In most cases, this process employs autoclaves for batch operation and tubular reactors for continuous operation. 

Since poly(vinyl chloride) is insoluble in its own monomer, the bulk polymerization is a precipitation polymerization. It is carried out in a 12,000 dm cylindrical rotating autoclave to prevent the coagulation of precipitated polymer, which can occur if the heat of the polymerization is not dissipated. A modern process uses two polymerization stages. In the first stage, polymer is produced in relatively low yields. The polymer, which precipitates in the form of a low-density granular material, is then transferred to a second kettle, where further polymerization to high yields occurs on addition of more monomer. This process produces the polymer in particulate form, a form particularly suitable for the assimilation of plasticizers. In addition, this type of plasticizer takeup produces a more homogeneous system than is possible with the older, bulk polymerization method. 

The polymerization process is performed by using one of the following three methods emulsion polymerization, suspension polymerization or bulk polymerization. The first and oldest method is rarely used (around 10% of PVC is produced this way). The production can be periodic or continuous. In the continuous method, there is a negative effect of the polymer being deposited in autoclaves, which can lead to deterioration in heat exchange and an increase in the amoimt of coagulum (non-coUoidal particles of latex). Emulsion polymerization is conducted in the presence of initiators dissolved in water, i.e., non-organic oxides (ammonium, potasium persulfate), emulsifiers (mostly anionic alkylbenzyl sulfonate, fatty acids salts, alkylphenyl ethoxides and fatty acids). The selection and the amount of the emulsifier is crucial since it affects the properties of latex, the particle size, the stability, and the tendency of the powder to cake. Other aids used in the process of polymerization are buffers phosphates and sodium carbonate. 

In Table VII, it will be noted that in a bulk polymerization, at a conversion of 10-20%, secondary particles form. That is, swollen polymer particles collide to form larger particles. As the process proceeds, virtually no free liquid monomer is present. Using a special autoclave, in which polymer lumps could be broken up, bulk polymerization of vinyl chloride could be carried out beyond this low conversion range [58]. This concept was improved upon in the basic patent for the Pechiney-Saint Gobain process [59]. Evidently Produits Chimique Pechiney-Saint Gobain was a... 

PVC can be prepolymerized in bulk to approximately 7-8% conversion. It is then transferred to an autoclave where the particles are polymerized to a solid powder. Most vinyl chloride, however, is polymerized... 

Continuous stirred-tank reactors (CSTRs) are used for large productions of a reduced number of polymer grades. Coordination catalysts are used in the production of LLDPE by solution polymerization (Dowlex, DSM Compact process [29]), of HDPE in slurry (Mitsui CX-process [30]) and of polypropylene in stirred bed gas phase reactors (BP process [22], Novolen process [31]). LDPE and ethylene-vinyl acetate copolymers (EVA) are produced by free-radical polymerization in bulk in a continuous autoclave reactor [30]. A substantial fraction of the SBR used for tires is produced by coagulating the SBR latex produced by emulsion polymerization in a battery of about 10 CSTRs in series [32]. The CSTRs are characterized by a broad residence time distribution, which affects to product properties. For example, latexes with narrow particle size distribution cannot be produced in CSTRs. 

The polymerization of olefins with coordination catalysts is performed in a large variety of polymerization processes and reactor configurations that can be classified broadly into solution, gas-phase, or slurry processes. In solution processes, both the catalyst and the polymer are soluble in the reaction medium. These processes are used to produce most of the commercial EPDM rubbers and some polyethylene resins. Solution processes are performed in autoclave, tubular, and loop reactors. In slurry and gas-phase processes, the polymer is formed around heterogeneous catalyst particles in the way described by the multigrain model. Slurry processes can be subdivided into slurry-diluent and slurry-bulk. In slurry-diluent processes, an inert diluent is used to suspend the polymer particles while gaseous (ethylene and propylene) and liquid (higher a-olefins) monomers are fed into the reactor. On the other hand, only liquid monomer is used in the slurry-bulk pro-... 

Polymerizations are carried out in continuous flow either in stirred reactors (autoclaves) whose volume is in the range of one cubic meter or in less bulky tubular reactors ( 0.4m ) in which the pressure may be higher than in autoclaves. The monomer conversion is only 15-20% for each passage in the autoclaves, but is a little higher (25%) in tubular reactors. It is important to emphasize that ethylene is in a supercritical state under the temperature and the pressure of such polymerizations the corresponding fluid has a density close to 0.6 and is used as solvent for the PE formed. It is thus a polymerization in bulk as defined in Section 8.5.12. 

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