The first commercial production of polystyrene (PS) was carried out in the early 1930s by the Farben Company (Germany) and was soon followed in 1937 by the Dow Chemical Company introducing in the United States a grade called "Styron." Styrene monomer is mainly produced by the dehydrogenation of ethyl-benzene made by reacting ethylene and benzene in a Friedel-Crafts reaction using a catalyst system containing aluminum chloride . Yields in excess of 98% are common in this process. The thermal cracking reaction that produces the dehydrogenation is carried out at 630°C in the presence of a catalyst, commonly a mixture of Fe2O3, Cr2O3, and K2CO3. The reaction yields a mixture of products, but the process conditions can be controlled to obtain about 80% conversion.
The styrene is separated from the product mix, which also contains unreacted ethylbenzene and other impurities, by vacuum distillation. The monomer can easily autopolymerize into a hard solid and is therefore inhibited from polymerization during storage by mixing in a few parts per million of a free-radical reaction inhibitor (generally t-butyl catechol). A relatively small amount of styrene is also made by the oxidation of ethyl benzene in a process introduced by Union Carbide. The ethylbenzene hydroperoxide formed by oxidation is reacted with propylene to form propylene oxide and 2-phenyl ethanol. The latter compound is dehydrated to obtain styrene.
While bulk or emulsion polymerization can also be used for the purpose, the commercial manufacture of polystyrene is mostly carried out in a solution process using a free-radical initiator. The solvent, typically ethylbenzene, used at a level of 2-30%, controls the viscosity of the solution. High-impact-grade polymer used in injection-molding and extrusion is modified with butadiene rubber incorporated during polymerization. The solvent and residual monomer in the crude resin is removed by flash evaporation or in a devolatilizing extruder (at about 225°C). Figure 2.9 is a schematic of the polymerization process.
Expandable polystyrene is extensively used in the production of foam products and extruded foam sheets. These expandable beads contain a nucleating agent and a blowing agent in addition to the high-molecular-weight polystyrene resin. A hydrocarbon blowing agent, typically pentane at a level of 5-8% by weight or a chlorofluorocarbon (CFC) blowing agent, might be used, The latter, however, is now restricted because of its potential for depleting stratospheric ozone, and the U.S. industry phased out the use of ozone-depleting CFCs in expanded foams in the 1980s. With hydrocarbon blowing agents a bicarbonate/citric acid system might be used as the nucleating agent (with CFCs finely divided talc was commonly used for the purpose). The choice and amount of blowing agent determines the density, and therefore the thermal conductivity, of the foam produced, both important considerations in building applications. Densities down to 1 lb/ft3 (16 kg/m3), amounting to a grade of foam that is 97% by volume of air or blowing agent, are possible. The bead is generally expanded with steam or hot air in
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