Heterogeneous polymerization suspension

Heterogeneous polymerization is used extensively to control the thermal and viscosity problems. There are three types of heterogeneous polymerization precipitation, suspension, and emulsion. Emulsion polymerization is discussed in Chap. 4. Precipitation polymerizations begin... 

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. 

Heterogeneous polymerizations proceed in two or more phases. Heterogeneity may be caused by the presence of a solid or of a gaseous phase or else the liquid monomer may be dispersed in another liquid with which it does not dissolve. Very important are the systems (a) with a solid initiator and (b) of two practically immiscible liquids. The former is useful for producing stereospecific polymers which are usually formed by a coordination mechanism. The latter makes possible an elegant and efficient removal of the heat of polymerization and it is applied technically with radical polymerizations in suspension or emulsion. 

The reaction mixture is heterogeneous during the entire polymerization because of insolubility of any of the components. Heterogeneous polymerization has been practiced for many years as suspension... 

The modeling of heterogeneous polymerization systems is generally more complicated than that of the homogenous systems because mass and heat transfer effects between two or more immiscible phases must be considered. Industrially important heterogeneous polymerization reactions include emulsion polymerization, suspension polymerization, precipitation polymerization, and solid-catalyzed olefin polymerization. The general polymerization rate equation is represented simply as... 

Product type Small volume specialty polymers heterogeneous polymerization system (e.g., emulsion, suspension, precipitation) Small volume specialty polymers copolymers Large volume commodity polymers engineering polymers... 

Heterogeneous polymerization can be further divided into emulsion polymerization and suspension polymerization. In both types of polymerization, the monomers are dissolved in the dispersed phase. In suspension polymerization, the initiator is dissolved in the dispersed phase as well and nucleation and growth of the beads take place in the droplets. In emulsion polymerization, on the other hand, the initiator is dissolved in the continuous phase, leading to nucleation and bead growth from the continuous phase. 

Suspension Polymerization - Refers to a heterogeneous polymerization regime in which the product of the reaction is a solid forming a suspension in the liquid medium of reaction. Little or no surfactant is added to the reaction medium. Characteristics of the process include high agitation rate and poor particle size control. An advantage of this reaction is high purity of the polymer product as compared to that of the dispersion method. 

Emulsions, suspensions, and dispersions are examples of colloidal systems. It is important to mention that these terms are not always used consistently in the literature and that this situation may be confusing for students and nonpolymer scientists [24]. From the point of view of polymer science and engineering, these terms refer to heterogeneous polymerizations, particularly polymerizations in aqueous/alcoholic dispersed media. Thus, the aforementioned terms have connotations that have to do with the initiator, monomer, and polymer solubility in each phase as well as with particle size and the main locus of polymerization. These aspects are treated in detail later for the moment, let us assume that there are no chemical reactions and that such terms are used in the context of colloid science. 

If possible from the point of view of efficiency, polymerization of the reactants without any use of a dispersing medium is desired. This method is called Bulk Polymerization. However, in order to absorb the heat released in the reaction, other methods like suspension or emulsion polymerization are frequently used. In this case, the water absorbs heat and provides good control of temperature. Another (less popular) method is the solution (homogeneous or heterogeneous) polymerization, which is mostly utilized in ionic initiation. We compare advantages and disadvantages of each method. (Table 2-4 describes the most useful polymerization methods for some commercial polymers). 

The polymeric PVC is insoluble in the monomer therefore, bulk polymerization of PVC is a heterogeneous process. Suspension PVC is synthesized by suspension polymerization. These are suspended droplets approximately 10 to 100 nm in diameter of vinyl chloride monomer in water. Suspension polymerizations allow control of particle size, shape, and size distribution by varying the dispersing agents and stirring rate. Emulsion polymerization results in much smaller particle sizes than snspension polymerized PVC, but soaps used in the emulsion polymerization process can affect the electrical and optical properties. 

Heterophase polymerization systems can be defined as two-phase systems in which the resulting polymer and/or starting monomer are in the form of a fine dispersion in an immiscible liquid medium defined as the polymerization medium , continuous phase , or outer phase . Even if oil-in-water (o/w) systems are greatly preferred on an industrial scale, water-in-oil (w/o) systems may also be envisaged for specific purposes. Heterogeneous polymerization processes can be classified as suspension, dispersion, precipitation, emulsion, or miniemulsion techniques according to interdependent criteria which are the initial state of the polymerization mixture, the kinetics of polymerization, the mechanism of particle formation and the size and shape of the final polymer particles (Fig. 4.2) [18]. 

The final step is termination of chain growth mostly by radical transfer reaction to monomer [306], whereas combination or disproportionation are observed only to a small extent. The monomer radical is able to start a new chain. The most widely used procedures for preparation of commercially PVC resins are, in order of their importance, suspension, emulsion, bulk, and solution polymerization. A common feature of the first three methods is that PVC precipitates in liquid VC at conversions below 1%. The free polymerization of VC in a precipitating medium exhibits an accelerating rate from the beginning of reaction up to high conversion [307]. This behavior is called autoacceleration and is typical for heterogeneous polymerization of halogenated vinyls and acrylonitrile [308]. 

Several general disadvantages of bulk polymerization (removal of the reaction heat, shrinkage, nonsolubility of the resulting polymer in the monomer, side reactions in highly viscous systems such as the Trommsdorff effect or chain transfer with polymer) are responsible for the fact that many polymerization processes are carried out in the presence of a solvent. A homogeneous polymerization occurs when both monomer and polymer are soluble in the solvent. When the polymer is insoluble in the solvent, the process is defined as solution precipitation polymerization. Other heterogeneous polymerization reactions in liquid-solid or liquid-liquid systems such as suspension or emulsion polymerizations are described later. Conventional solution polymerization is compared with solution precipitation polymerization for the synthesis of acrylic resins in Ref. [34]. 

Suspension polymerization (SpP) can be considered the least complicated heterogeneous polymerization technique in terms of its mechanism. 

In general, we can distinguish homogeneous and heterogeneous polymerization processes. Homogeneous processes take place within the substance in question (with the monomer or the formed polymer acting as reaction medium) or in a solvent. Heterogeneous processes include precipitation, slurry phase, suspension, emulsion, or gas-phase polymerizations. 

There have been many heterogeneous polymerizations developed over the decades. The main ones are precipitation, dispersion, suspension, emulsion, slurry, and gas-phase polymerization. In addition to the discussed chain properties, particle sizes and distribution are the important parameters that determine polymer product qrrahty and reactor performance. Among the mentioned heterogeneorrs processes, emulsion polymerization has the hest theoretical development. Scheme 3 shows schematic presentation of an emrrlsion polymerization that consists of three stages. 

In addition to emulsion, dispersion, and suspension polymerization, there are other industrial heterogeneous polymerization processes (Table 2). Of particular importance are slurry and gas-phase processes for polyolefin production, which are also included in Section 4.32.13. 

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