Bulk suspension process

Commercial product of syndiotactic polypropene utilizes a silica supported metallocene in a bulk suspension process at 50-70 °C and a pressure of 30 kg/cm2 [157]. 

As a means to improve the rubber utilization, a bulk/suspension process evolved, whereby polybutadiene rubber was dissolved in styrene monomer and polymerized in bulk beyond phase inversion before being dropped into suspension. The HIPS produced this way had two distinct advantages over the compounded version styrene to rubber grafting and discrete rubber spheres or particles uniformly dispersed in a polystyrene matrix. This improved the impact strength dramatically per unit of rubber and gave better processing stability, because the rubber phase was dispersed instead of being co-continuous with the polystyrene. 

Today, HIPS is produced by two basic variants the batch process and the continuous process. Pre-polymerization, i.e. the polymerization phase up to completion of phase inversion, is identical in the two process variants. After completion of the pre-polymerization, the polymerization is continued in suspension in the batch process and in solution in the continuous process. The batch process is, therefore, also referred to as the bulk suspension process and the continuous variant as the solution process. The continuous process is a refinement of the original I.G. Farben process for standard polystyrene, which The Dow Chemical Company has adapted to the needs of rubber-containing styrene solutions. A number of modifications are now practiced. 

The bulk suspension process for HIPS was developed by Monsanto. Batch preparation and pre-polymerization are equivalent to the solution process. 

The advantages of the bulk suspension process are its high flexibility, reliable heat dissipation and complete conversion of the styrene. 

High Impact Polystyrene (HIPS) HIPS is a heterogeneous material produced by continuous bulk or bulk-suspension processes, in which a butadiene-based elastomer (polybutadiene (PB), or a block copolymer of styrene-butadiene) is first dissolved in styrene monomer (St) and the resulting mixture is then heated so that the polymerization proceeds either thermally or with the aid of a chemical initiator. At the molecular level, the product is a mixture of free polystyrene (PSt) chains and elastomer chains grafted with PSt side chains. The process yields a continuous (free) PSt matrix containing... 

High-impact polystyrene and acrylonitrile-butadiene-styrene copolymer are often prepared in a combined bulk-suspension process. This begins with a solution of polybutadiene in styrene or styrene/acrylonitrile. Subsequently, the polymerization of styrene or styrene/acrylonitrile is initiated and continues under stirring until phase inversion occurs (i.e., polybutadiene is dispersed in a continuous PS matrix - Chapter 4). In the final stage, water and dispersant are added to the system and the polymerization is completed in suspension. 

Polymethacrylates. Poly(methyl methacrylate) [9011-14-7] is a thermoplastic. Itis the acryUc resin most used in building products, frequendy as a blend or copolymer with other materials to improve its properties. The monomer is polymerized either by bulk or suspension processes. Eor glazing material, its greatest use, only the bulk process is used. Sheets are prepared either by casting between glass plates or by extmsion of pellets through a sHt die. This second method is less expensive and more commonly used. Peroxide or azo initiators are used for the polymerization (see Methacrylic polymers). 

Polymerization of vinyl chloride occurs through a radical chain addition mechanism, which can be achieved through bulk, suspension, or emulsion polymerization processes. Radical initiators used in vinyl chloride polymerization fall into two classes water-soluble or monomer-soluble. The water-soluble initiators, such as hydrogen peroxide and alkali metal persulfates, are used in emulsion polymerization processes where polymerization begins in the aqueous phase. Monomer-soluble initiators include peroxides, such as dilauryl and benzoyl peroxide, and azo species, such as 1,1 -azobisisobutyrate, which are shown in Fig. 22.2. These initiators are used in emulsion and bulk polymerization processes. 

Bulk and suspension polymerisation are the most commonly used techniques. In bulk polymerisation styrene is heated to 80°C for about 2 days to get a viscous solution of polymer in styrene. The solution is then fed to a tower wherein polymerisation is completed at 100°C, 150C° and 180C° stagewise. In suspension process, styrene is suspended in dimineralised water in presence of suspending agent and initiator like benzoyl peroxide and heated to 20°C. The product is washed with acid, water and dried. 

The second large-scale process was the batch mass suspension process. Monsanto did the pioneer work on this (41). In this process, prepolymerization is carried out in bulk and main polymerization in suspension the latter is taken to conversions of over 99%. In contrast to the continuous mass process, peroxide starters are used in order to achieve a high conversion at tolerable reaction times. Figure 3 shows a basic flow diagram of such a plant. A detailed discussion of advantages and disadvantages of the two processes can be found in R. Bishop s monograph published in 1971 (42), and it is continued in a paper by Simon and Chappelear in 1979 (43). It was a decisive factor for the economic success of impact polystyrene that these processes had been completely developed and mastered in theory and practice. 

The already low cost of converting VCM to PVC probably precludes any revolutionary new process replacing the current suspension and emulsion processes. The Pechiney-St. Gobain bulk polymerization process is being tested thoroughly within the next year it will be evident if the process offers advantages, either in product quality or conversion cost. 

An elegant way of removing the heat of reaction occurs in suspension or emulsion polymerizations. Suspension polymerization is kinetically simpler. It really proceeds in bulk, as every monomer-polymer drop of the suspension is an individual reactor . These particles are small (100-150 pm), they have a large surface area, and the heat is effectively transferred by water to the cooling jacket. The polymer is contaminated by the tenside used for suspension stabilization. Therefore it must be washed, and even so it is sometimes less suitable for high-performance electrotechnical applications than a polymer prepared in bulk. For the suspension process, the initiator must be soluble in the monomer. 

Each year, hundres of thousands of tons of vinyl chloride are polymerized in the world. Commensurate attention is thus paid to studies of its polymerization. Vinyl chloride is one of those monomers that are transformed to polymer by a complicated mechanism. Poly(vinyl chloride) is soluble neither in its own monomer nor in the common solvents. Its formation is therefore connected with the appearance of a solid phase the process has the character of precipitation polymerization. This greatly complicates the kinetics of solution and bulk (suspension) polymerization. 

Commercial polystyrene manufacturing techniques are based either on a suspension process if the material is to be foamed or on a bulk polymerization process for GPPS and IPS. ABS-type polymers can also be produced via emulsion polymerization. Figure 2.5 shows the differences in emulsion and mass polymerization processes and the resulting morphology. Typically,... 

However, the probability for the reaction progression greatly depends on the monomer conversion. Because the viscosity of the dispersed phase, in the first stage, is fairly low and the quantity of styrene is sufficiently high, the decomposition process (Figure 9.4) occurs only up to the benzoyloxy radical, which can directly start the kinetic chain. The purely thermal start of chains with reactive dimers of styrene, as a result of Diels-Alder reaction, can be ignored at fairly low temperatures of suspension polymerization, in contrast to the conditions for the bulk styrene process [4-7]. 

The preparation of ASA in bulk or bulk-suspension polymerization processes has been described by McKee et al. [14-18]. The system is similar to that used in the preparation of high-impact polystyrene (HIPS) and in bulk-produced ABS. Thereby the rubber was prepared using free radical polymerization, dissolved in the SAN monomers which were then polymerized using free radicals. Phase separation between the rubber and PSAN occurred, followed by phase inversion. 

Both the emulsion and suspension processes use water as a heat sink. Polymerization reactions are easier to control in both these processes than in bulk or solution systems because stirring is easier and removal of the exothermic heat of polymerization is facilitated. 

Polymers can be prepared by many different processes. Free radical polymerization can be accomplished in bulk, suspension, solution, or emulsion. Ionic and other nonradical polymerizations are usually produced in solution polymerizations. Each technique has characteristic advantages and disadvantages. 

The computer program simulates the batch polymerization of styrene and has been applied to the relatively low temperature peroxide-initiated polymerization typical of suspension processes and to higher temperature bulk, thermal conditions. It has been useful in the design of new suspension processes and for more general process analysis. 

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