Polystyrene prepolymerization

Process flow for a typical batch-mass polystyrene process(1) is shown in Figure 1. Styrene monomer is charged to the low conversion prepolymerization reactor with catalyst and other additives, and the temperature is increased stepwise until the desired conversion is reached. It is then transferred into the press. Polycycles are 6 to 14 hours in the low conversion reactor, and 16 to 24 hours in the press. At completion, the cakes are then cooled with water and removed from the press to be ground and then (usually) extruded into pellets. 

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. 

Today, a large part of the more than one billion lbs/year of impact polystyrene and 500 million lbs/year of ABS produced domestically is made by graft copolymerization. Impact polystyrene may be synthesized by dissolving a diene rubber in styrene monomer, in the presence or absence of another solvent, prepolymerizing the solution, and completing the polymerization in bulk, solution, or suspension. R. B. dejong describes a process wherein he prepolymerizes in emulsion with styrene as the continuous phase and the water as the dispersed phase and completes polymerization in aqueous suspension. 

High Impact Polystyrene by Prepolymerization in a Water-in-Oil Emulsion Followed by Suspension Polymerization... 

The rubber particle size in the final product increases several fold if the prepolymerization is carried out in the presence of a dilute aqueous solution of an alkane sulfonate or polyvinyl alcohol in place of pure water. The addition of a surface-active agent converts the coarsely dispersed oil-water mixture—obtained as above in the presence of pure water—into an oil-in-water emulsion. In this case even prolonged stirring during prepolymerization does not decrease the rubber particle size appreciably in the final product. The stabilization of the droplets of the organic phase in water by the emulsifier obviously impedes or prevents agitation within the polymeric phase. Figure 1 shows the influence of these three prepolymerization methods (under otherwise equal reaction conditions) on the dispersion of rubber particles in polystyrene. 

As explained above the rubber particle size in the final product is a measure for the rate of agitation—under otherwise equal reaction conditions—within the rubber-polystyrene-styrene solution during prepolymerization. Figure 1 shows that agitation is least effective if the organic... 

Figure I. Interference phase contrast micrographs of rubber particles in polystyrene. Prepared by prepolymerization (A) in bulk, (B) in the presence of water, and (C) in an o/w emulsion.

High impact polystyrene can be made by prepolymerization in a w/o emulsion with ensuing suspension polymerization. The processes which... 

Polymer in solution Any (e.g., a condensation product or another polymer phase) Heterogeneous bulk or solution polymerization Salt precipitating from a condensation reaction. Prepolymerized rubber precipitating from a solution of polystyrene in styrene monomer... 

Other chemical companies have also designed their own continuous process to produce high-impact polystyrene (HIPS), such as the Dow process, which consists of three elongated reactors in series (US Patent 2727 884, 1955) the BASF process, which consists of a prepolymerization CSTR followed by cascade of three CSTRs (US Patent 3 658 946, 1972) the Shell process, which consists of three CSTRs followed by a plug flow reactor (US Patent 4011 284, 1977) and the Monsanto process, which consists of a CSTR followed by a horizontal plug flow reactor (US Patent 3 903 202, 1975). 

Very recently, an aqueous olefin polymerization using an early transition metal catalyst has also been reported [84]. A toluene solution of styrene is prepolymerized briefly by a catalyst prepared by combination of [(CsMesjTifOMe),] with a borate and an aluminum-alkyl as activators. The reaction mixture is then emulsified in water, where further polymerization occurs to form syndiotactic polystyrene stereoselectively. It is assumed that the catalyst is contained in emulsified droplets and is thus protected from water, with the formation of crystalline polymer enhancing this effect. Cationic or neutral surfactants were found to be suitable, whereas anionic surfactants deactivated the catalyst. The crystalline polystyrene formed was reported to precipitate from the reaction mixture as relatively large particles (500 pm). 

Impact polystyrene is produced commercially in three steps solid polybutadiene rubber is cut up and dispersed as small particles in styrene monomer mass prepolymerization and completion of the polymerization either in mass or aqueous suspension. During the prepolymerization step, styrene starts to polymerize by itself by forming droplets of polystyrene upon phase separation. When equal phase volumes are attained, phase inversion occurs (15). The droplets of polystyrene become the continuous phase in which the rubber particles are dispersed. Impact strength increases with rubber particle size and concentration, whereas gloss and rigidity decrease. 

Simultaneous IPNs are formed by homogeneously mixing together monomers, prepolymers, linear polymers, initiators, and crosslinkers, The monomers and prepolymers are simultaneously polymerized by independent reactions that differ enough to avoid interfering with each other. For example, a polyure-thane/polymethacrylate and a polyurethane/polystyrene were made in a process in which both monomers were prepolymerized, dissolved together, and reacted to form an IPN. Another urethane system was made from castor oil reacted with toluene diisocyanate and sebacic acid polyesters. The resultant urethane prepolymer was then mixed with polystyrene to form an IPN. 

In contrast (Fig, 2E), when the oil was prepolymerized (but not to the point of gelation) prior to addition of the styrene, such PS domains were evident, as found previously for a castor-oil-based SIN (9) and high-impact polystyrene (HIPS) (16). It is evident from Figs. 2A and 2B that when the oil is not prepolymerized prior to adding the styrene-DVB the resulting elastomer phase exhibits a lower rubber-phase volume fraction (RPVF) than is the case when the oil is prepolymerized. 

Initially, styrene monomer is completely miscible with the oil prepolymer, but as the styrene polymerizes to high molecular weight polystyrene, the two components phase separate from each other. At this point, it is thought that the oil-rich phase is continuous, and the polystyrene-rich phase discontinuous. If the oil is not prepolymerized, a phase inversion will occur, as in Figures 4-A and 4-B. For extensive prepolymerization, the oil remains continuous, but the domains are smaller. As described below, the morphology of Figure 4-C yielded the highest impact resistance. 

The pure polystyrene phase in the highly prepolymerized system should have no shift in Tg, since no miscibility is expected between the high molecular weight oil and polystyrene, which is observed (Figure 5, A.V. = 47). The oil-polyester phase in this case contains more polystyrene than in the non-prepolymerized case, causing a more pronounced shift in the oil-polyester Tg. This is due to the trapping effect of the gel, since some of the polystyrene to polymerize inside of the oil-polyester phase may not be able to nucleate a phase domain, or diffuse to an already formed domain. 

Figure 4. Effect of oil prepolymerization on morphology of 15/85 epoxidized lunaria oil-dimer acid/polystyrene network SINs. Acid values A, 98 B, 75 C, 55.

Impact polystyrene can be prepared by suspension polymerization. However, there is no shearing agitation within the individual polymer particles. Thus, a prepolymerization with shearing agitation needs to be carried out before suspension polymerization to obtain good polymer properties. 

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