Processing polystyrene

Polystyrene. There are two types of expandable polystyrene processes expandable polystyrene for molded articles and expandable polystyrene for loose-fill packing materials. 

Thermoplastics. The highest consumption of color concentrates is in thermoplastic resins, such as low and high density polyethylene, polypropylene, PVC, and polystyrene. Processing techniques for thermoplastics are usually based on dry color dispersion in a compatible resin (36). 

Polyphenylene oxide/polystyrene Processability, lower cost... 

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. 

I.G. Farben Process. The first continuous mass polystyrene process was developed in Germany by I.G. 

Figure 14. I. G. Farbenindustrie continuous mass polystyrene process (23)...

The continuous polystyrene process which was commercialized successfully in 1952 (2) is illustrated schematically in Fig. 16. It is characterized by three vertical elongated reactors in series, the contents of which are gently agitated by slowly revolving rods mounted on an axial shaft. Temperature control is provided by horizontal banks of cooling tubes between adjacent agitator rods. Such a reactor, called a "stratifier-... 

A modem polystyrene process consists of a CSTR followed by several stirred tube reactors in series. A description of this typical process is given in... 

Most expandable polystyrene processes involve aqueous suspension systems in which pentane fractions of petroleum are introduced before, during, or after polymerization of styrene. Water-free systems may also be used. Particle size is controlled by suspension polymerization or by chopping fine filaments. The quenched pellet process for expandable polystyrene can consume off-size particles and is a convenient way to add colorants and cell-nucleating additives. 

Figure 8.2 Expandable polystyrene process line starts with preexpanding the PS beads...

Expandable polystyrene process line starts with... 

Probably no part of the polystyrene production plant has changed as much over the last 30 years as the methods of process control. The early polystyrene processes required little process control because they were operated at reaction rates that were inherently stable. For polystyrene, a rule of thumb is that the reaction rate doubles with every increase in temperature of 10 °C. If the reaction is conducted at rates that evolve heat at a rate that requires a temperature... 

Recently, tetrafunctional initiators have also been introduced for styrenics. In 2001, Atofina Chemicals introduced a novel tetrafunctional initiator, Luperox JWEB50, developed specifically for the styrenics industry to produce high molecular weight, high-heat, crystal polystyrene with improved productivity in a cost-effective manner. JWEB50 is a room temperature stable, liquid peroxide with a half-life similar to those of currently used cyclic perketals, appropriate for use in mass polystyrene processes. A unique aspect of... 

I. G. Farbenindustrie in Germany implemented such a concept to produce polystyrene commercially in the 1930s. Two CSTRs in parallel followed by a plug flow reactor were used in their process. During World War II, Union Carbide applied for a patent (US Patent 2496653, 1950) for a continuous polystyrene process. Their process consisted of three cascade CSTR reactors followed by a plug flow reactor. The temperature in the three CSTR reactors is 100, 115-120 and 140 °C, respectively. The conversion at the outflow of the third CSTR reactor is around 85 %. The temperature in the plug flow reactor is between 210 and 215 °C. The final conversion at the plug flow reactor was claimed to be 97 %. 

Figure 7-4 Batch bulk polystyrene process. From Chemical Reactor Theory, p, 543, Copyright 1977, Prentice Hall, Reprinted by permission of Prentice Hall,...

The next step to discovery was the suggestion that we might lessen the cold flow of BR by adding very short end blocks of polystyrene. Because the chain ends do not contribute appreciably to the vulcanized state, we hoped that the abrasion resistance and resilience of polybutadiene would be little affected by the very low molecular weight end blocks of polystyrene. Processability problems with the polybutadiene led us to use a low molecular weight in the polybutadiene portion. Samples of this type were made for evaluation. 

Table 2. Extracted potential incident scenario for extruded polystyrene process.

Operation at the middle steady state gives polymer of a commercial molecular weight and, broadly speaking, is representative of bulk polystyrene processes. The lower steady state gives no polymer, while the upper steady state gives polymer of too low a molecular weight. In fact, thermal depolymerization of polystyrene becomes important above about 300 °C, and runaway reactions become equilibrium limited at about 350 "C. 

For most polsrmerizations starting from monomer, tubular reactors have been avoided because of the various stability problems. They can he used in recycle loops where the per-pass conversion is low, in solution polymerizations with large amounts of solvent, and in post- or finishing reactors intended to drive a polymerization to completion. Shell-and-tube designs with as many as 1000 tubes are used in polystyrene processes where they also serve as devolatilization preheaters. The entering polymer solution has a concentration of about 70%, and its viscosity is high enough to avoid tube-to-tube instabilities. 

Kenics-type static mixers have been used as inserts in tubular reactors. Compared to an open tube operated at the same pressure drop, the static mixer gives about 40% more heat transfer. Stand-alone mixer reactors of the Koch or Sultzer SMR type have been used as post-reactors and devolatilization preheaters. The polymer flows through the shell side of the SMR and the heat transfer fluid flows inside tubes that have been strategically placed to promote radial mixing of the polymer. One bulk polystyrene process used the SMR as in a recycle loop as the first reactor, but the capital cost is high compared to alternatives such as a boiling CSTR or a proprietary stirred-tube reactor. 

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