Suspension process

Compositional control ia suspension systems can be achieved with a corrected batch process. A suspension process has been described where styrene monomer is continuously added until 75—85% conversion, and then the excess acrylonittile monomer is removed by stripping with an iaert gas... 

Acrylonitrile—Butadiene—Styrene. ABS is an important commercial polymer, with numerous apphcations. In the late 1950s, ABS was produced by emulsion grafting of styrene-acrylonitrile copolymers onto polybutadiene latex particles. This method continues to be the basis for a considerable volume of ABS manufacture. More recently, ABS has also been produced by continuous mass and mass-suspension processes (237). The various products may be mechanically blended for optimizing properties and cost. Brittle SAN, toughened by SAN-grafted ethylene—propylene and acrylate mbbets, is used in outdoor apphcations. Flame retardancy of ABS is improved by chlorinated PE and other flame-retarding additives (237). 

There are two problems in the manufacture of PS removal of the heat of polymeriza tion (ca 700 kj /kg (300 Btu/lb)) of styrene polymerized and the simultaneous handling of a partially converted polymer symp with a viscosity of ca 10 mPa(=cP). The latter problem strongly aggravates the former. A wide variety of solutions to these problems have been reported for the four mechanisms described earlier, ie, free radical, anionic, cationic, and Ziegler, several processes can be used. Table 6 summarizes the processes which have been used to implement each mechanism for Hquid-phase systems. Free-radical polymerization of styrenic systems, primarily in solution, is of principal commercial interest. Details of suspension processes, which are declining in importance, are available (208,209), as are descriptions of emulsion processes (210) and summaries of the historical development of styrene polymerization processes (208,211,212). 

Vinyhdene chloride copolymerizes randomly with methyl acrylate and nearly so with other acrylates. Very severe composition drift occurs, however, in copolymerizations with vinyl chloride or methacrylates. Several methods have been developed to produce homogeneous copolymers regardless of the reactivity ratio (43). These methods are appHcable mainly to emulsion and suspension processes where adequate stirring can be maintained. Copolymerization rates of VDC with small amounts of a second monomer are normally lower than its rate of homopolymerization. The kinetics of the copolymerization of VDC and VC have been studied (45—48). 

The batch-suspension process does not compensate for composition drift, whereas constant-composition processes have been designed for emulsion or suspension reactions. It is more difficult to design controUed-composition processes by suspension methods. In one approach (155), the less reactive component is removed continuously from the reaction to keep the unreacted monomer composition constant. This method has been used effectively in VT)C-VC copolymerization, where the slower reacting component is a volatile and can be released during the reaction to maintain constant pressure. In many other cases, no practical way is known for removing the slower reacting component. 

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). 

Free-radical copolymerizations have been performed ia bulb (comonomers without solvent), solution (comonomers with solvent), suspension (comonomer droplets suspended ia water), and emulsion (comonomer emulsified ia water). On the other hand, most ionic and coordination copolymerizations have been carried out either ia bulb or solution, because water acts as a poison for many ionic and coordination catalysts. Similarly, few condensation copolymerizations iavolve emulsion or suspension processes. The foUowiag reactions exemplify the various copolymerization mechanisms. 

In suspension processes the fate of the continuous liquid phase and the associated control of the stabilisation and destabilisation of the system are the most important considerations. Many polymers occur in latex form, i.e. as polymer particles of diameter of the order of 1 p.m suspended in a liquid, usually aqueous, medium. Such latices are widely used to produce latex foams, elastic thread, dipped latex rubber goods, emulsion paints and paper additives. In the manufacture and use of such products it is important that premature destabilisation of the latex does not occur but that such destabilisation occurs in a controlled and appropriate manner at the relevant stage in processing. Such control of stability is based on the general precepts of colloid science. As with products from solvent processes diffusion distances for the liquid phase must be kept short furthermore, care has to be taken that the drying rates are not such that a skin of very low permeability is formed whilst there remains undesirable liquid in the mass of the polymer. For most applications it is desirable that destabilisation leads to a coherent film (or spongy mass in the case of foams) of polymers. To achieve this the of the latex compound should not be above ambient temperature so that at such temperatures intermolecular diffusion of the polymer molecules can occur. 

In the suspension process, which was the first method to be commercially developed, propylene is charged into the polymerisation vessel under pressure whilst the catalyst solution and the reaction diluent (usually naphtha) are metered in separately. In batch processes reaction is carried out at temperatures of about 60°C for approximately 1-4 hours. In a typical process an 80-85% conversion to polymer is obtained. Since the reaction is carried out well below the polymer melting point the process involves a form of suspension rather than solution polymerisation. The polymer molecular weight can be controlled in a variety of... 

The disadvantages of the suspension process are that about 70% of the volume of the kettle is taken up by water, the need for a drying stage which could cause discolouration by degradation and the need to convert the small spheres formed into a larger shape suitable for handling. Furthermore, the suspension method cannot easily be converted into a continuous process. 

To produce the Type 2 polymers, styrene and acrylonitrile are added to polybutadiene latex and the mixture warmed to about 50°C to allow absorption of the monomers. A water-soluble initiator such as potassium persulphate is then added to polymerise the styrene and acrylonitrile. The resultant materials will be a mixture of polybutadiene, polybutadiene grafted with acrylonitrile and styrene, and styrene-acrylonitrile copolymer. The presence of graft polymer is essential since straightforwsird mixtures of polybutadiene and styrene-acrylonitrile copolymers are weak. In addition to emulsion processes such as those described above, mass and mass/suspension processes are also of importance. 

In a typical batch suspension process (Figure 12-5), styrene is suspended in water by agitation and use of a stabilizer. The polymer forms beads. The bead/water slurry is separated by centrifugation, dried, and blended with additives. 

Access to Practice. Publications and patents on the batch mass process are limited. Bishop s book CD contains the most detailed description of the polymerization press and mass-suspension processes for PS and HIPS. Fong (16) presents an economic analysis of the press process based on Bishop s description. Patent references are few for the batch-mass process the 1939 Bakelite patent on transfer of prepoly syrup to chambers or containers is of historical interest (17). 

Both Bishop QJ and Fong (] ) give extensive patent reviews of the mass-suspension process for HIPS including the pioneering patents of Stein and Walter (18). 

The advantage of suspension processes over mass processes is the excellent temperature control that can be obtained through the suspending medium, water. This allows for rapid heat removal and shorter polymerization times. It reduces or eliminates hot spots or heat-kicks characteristic of mass reactors. It also allows the polymerization to be driven very close to completion so that no devolatilization step is normally required. 

For multiple liquid phases (e.g. suspension processes) or increasing concentrations of polymers, some more realistic models are desirable (van Laar, Flory-Huggins, Wilson). ... 

C. L. Burdick and J. N. Pullig. Sodium formate fluidized polymer suspensions process. Patent US 5228908, 1993. 

CASE STUDY SITE SELECTION FOR A 150,000,000 IJi/YR POLYSTYRENE PLANT USING THE SUSPENSION PROCESS... 

At the end of Chapters 2 through 11 an application of the material presented in the chapter to a specific exemplary task will be presented. This will be the design of a 150,000,000 lb/yr polystyrene plant, which will use the suspension process. The goal of this example will be to provide just enough information so the board of directors can decide whether the plant should be constructed. 

Polystyrene is made by polymerizing styrene. In the suspension process the styrene is broken up into small droplets which are suspended in water. Various additives aid in controlling this and the reaction rate. These additives amount to about 1% of the styrene added. For high-impact styrene up to 0.15 lb rubber/lb styrene is included. The two major materials needed are water and styrene. 

There are many different ways of making polystyrene using the suspension process. Most producers use a batch process, although there are no technical reasons why a continuous process could not work.10 For this study a batch... 

Typical Formulations Used for the Batch Suspension Process for Polystyrene... 

Figure 6E-6 Plant layout for a 150,000,000 lb / year polystyrene plant using the suspension process.

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