Styrofoam Panels

Polystyrene (pronounced /ˌpɒliˈstaɪriːn/ ) (IUPAC Poly(1-phenylethane-1,2-diyl) ), sometimes abbreviated PS , is an aromatic polymer made from the aromatic monomer styrene, a liquid hydrocarbon that is commercially manufactured from petroleum by the chemical industry. Polystyrene is one of the most widely used kinds of plastic.

Polystyrene is a thermoplastic substance, which is in solid (glassy) state at room temperature, but flows if heated above its glass transition temperature (for molding or extrusion), and becoming solid again when cooling off. Pure solid polystyrene is a colorless, hard plastic with limited flexibility. It can be cast into molds with fine detail. Polystyrene can be transparent or can be made to take on various colors.

Solid polystyrene is used, for example, in disposable cutlery, plastic models, CD and DVD cases, and smoke detector housings. Products made from foamed polystyrene are nearly ubiquitous, for example packing materials, insulation, and foam drink cups.

Polystyrene can be recycled, and has the number "6" as its recycling symbol. Unrecycled polystyrene, which does not biodegrade, is often abundant in the outdoor environment, particularly along shores and waterways, and is a form of pollution.

History

Polystyrene was discovered in 1839 by Eduard Simon, an apothecary in Berlin. From storax, the resin of the Turkish sweetgum tree (Liquidambar orientalis ), he distilled an oily substance, a monomer which he named styrol. Several days later, Simon found that the styrol had thickened, presumably from oxidation, into a jelly he dubbed styrol oxide ("Styroloxyd"). By 1845 English chemist John Blyth and German chemist August Wilhelm von Hofmann showed that the same transformation of styrol took place in the absence of oxygen. They called their substance metastyrol. Analysis later showed that it was chemically identical to Styroloxyd. In 1866 Marcelin Berthelot correctly identified the formation of metastyrol from styrol as a polymerization process. About 80 years went by before it was realized that heating of styrol starts a chain reaction which produces macromolecules, following the thesis of German organic chemist Hermann Staudinger (1881–1965). This eventually led to the substance receiving its present name, polystyrene.

The company I. G. Farben began manufacturing polystyrene in Ludwigshafen, Germany, about 1931, hoping it would be a suitable replacement for die-cast zinc in many applications. Success was achieved when they developed a reactor vessel that extruded polystyrene through a heated tube and cutter, producing polystyrene in pellet form.

In 1959, the Koppers Company in Pittsburgh, Pennsylvania, developed expanded polystyrene (EPS) foam.

Structure and properties

The chemical makeup of polystyrene is a long chain hydrocarbon with every other carbon connected to a phenyl group (the name given to the aromatic ring benzene, when bonded to complex carbon substituents). Polystyrene's chemical formula is (C ₈ H ₈ ) _n ; it contains the chemical elements carbon and hydrogen. Because it is an aromatic hydrocarbon, it burns with an orange-yellow flame, giving off soot, as opposed to non-aromatic hydrocarbon polymers such as polyethylene, which burn with a light yellow flame (often with a blue tinge) and no soot. Complete oxidation of polystyrene produces only carbon dioxide and water vapor.

This addition polymer of styrene results when vinyl benzene styrene monomers (which contain double bonds between carbon atoms) attach to form a polystyrene chain (with each carbon attached with a single bond to two other carbons and a phenyl group).

Let us consider polystyrene's properties based on its structure shown above. Polystyrene is chemically unreactive (this is why it is used to create products such as containers for chemicals, solvents and foods). This stability is the result of the transformation of carbon-carbon double bonds into less reactive single bonds. Structurally, the unsaturated alkene monomers have been transformed into less saturated structures with carbon alkane backbones. A molecule is considered saturated when its carbons are bonded to the maximum number of hydrogen atoms possible. The strong bonds within the molecule make styrene very stable.

Polystyrene is generally flexible and can come in the form of moldable solids or viscous liquids. The force of attraction in polystyrene is mainly due to short range van der Waals attractions between chains. Since the molecules and long hydrocarbon chains that consist of thousand of atoms, the total attractive force between the molecules is large. However, when the polymer is heated (or, equivalently, deformed at a rapid rate, due to a combination of viscoelastic and thermal insulative properties), the chains are able to take on a higher degree of conformation and slide past each other. This intramolecular weakness (versus the high intermolecular strength due to the hydrocarbon backbone) allows the polystyrene chains to slide along each other, rendering the bulk system flexible and stretchable. The ability of the system to be readily deformed above its glass transition temperature allows polystyrene (and thermoplastic polymers in general) to be readily softened and molded with the addition of heat.

A 3-D model would show that each of the chiral backbone carbons lies at the center of a tetrahedron, with its 4 bonds pointing toward the vertices. Say the -C-C- bonds are rotated so that the backbone chain lies entirely in the plane of the diagram. From this flat schematic, it is not evident which of the phenyl (benzene) groups are angled toward us from the plane of the diagram, and which ones are angled away. The isomer where all of them are on the same side is called isotactic polystyrene, which is not produced commercially.

Ordinary atactic polystyrene has these large phenyl groups randomly distributed on both sides of the chain. This random positioning prevents the chains from ever aligning with sufficient regularity to achieve any crystallinity, so the plastic has no melting temperature, T _m . But metallocene-catalyzed polymerization can produce an ordered syndiotactic polystyrene with the phenyl groups on alternating sides. This form is highly crystalline with a T _m of 270 °C (518 °F).

Extruded polystyrene is about as strong as unalloyed aluminium, but much more flexible and much lighter (1.05 g/cm ³ vs. 2.70 g/cm ³ for aluminium).

Forms produced

Polystyrene is commonly produced in three forms: extruded polystyrene, expanded polystyrene foam, and extruded polystyrene foam, each with a variety of applications. Polystyrene copolymers are also produced; these contain one or more other monomers in addition to styrene. In recent years the expanded polystyrene composites with cellulose and starch have also been produced.

Extruded polystyrene foam insulation is sold under the trademark Styrofoam by Dow Chemical. This term is often used informally for other foamed polystyrene products.

Polystyrene is used in some polymer-bonded explosives:

It is also a component of napalm and a component of most designs of hydrogen bombs.

Extruded polystyrene

Extruded polystyrene (PS) is economical, and is used for producing plastic model assembly kits, plastic cutlery, CD "jewel" cases, smoke detector housings, license plate frames, and many other objects where a fairly rigid, economical plastic is desired. Production methods include stamping and injection molding.

Polystyrene Petri dishes and other laboratory containers such as test tubes and microplates play an important role in biomedical research and science. For these uses, articles are almost always made by injection molding, and often sterilized post-molding, either by irradiation or treatment with ethylene oxide. Post-mold surface modification, usually with oxygen-rich plasmas, is often done to introduce polar groups. Much of modern biomedical research relies on the use of such products; they therefore play a critical role in pharmaceutical research.

Foams

Polystyrene foams are good thermal insulators, and are therefore often used as building insulation materials, such as in structural insulated panel building systems. They are also used for non-weight-bearing architectural structures (such as ornamental pillars).

Expanded polystyrene foam

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