Polystyrene

Polystyrene
Density	1050 kg/m³
Electrical conductivity (σ)	10^-16 S/m
Thermal conductivity	0.08 W/(m·K)
Young's modulus (E)	3000-3600 M Pa
Tensile strength (σ_t₎	46–60 M Pa
Elongation at break	3–4%
Notch test	2–5 kJ/m²
Glass temperature	95 °C
Melting point	240 °C
Vicat B	90 °C
Heat transfer coefficient (λ)	0.17 W/(m·K)
Linear expansion coefficient (α)	8 10^{-5^/K}
Specific heat (c)	1.3 kJ/(kg·K)
Water absorption (ASTM)	0.03–0.1
source: A.K. van der Vegt & L.E. Govaert, Polymeren,
van keten tot kunstof, ISBN 90-407-2388-5

Polystyrene is a polymer made from the monomer styrene, a liquid hydrocarbon that is commercially manufactured from petroleum. At room temperature, polystyrene is normally a solid thermoplastic, but can be melted at higher temperature for molding or extrusion, then resolidified. Styrene is an aromatic monomer, and polystyrene is an aromatic polymer.

Polystyrene was accidentally discovered in 1839 by Eduard Simon, an apothecary in Berlin, Germany. From storax, the resin of Liquidambar orientalis, he distilled an oily substance, a monomer which he named styrol. Several days later Simon found that the styrol had thickened, presumably due to oxidation, into a jelly he dubbed styrol oxide ("Stryroloxyd"). 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 I.G. Farben company 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.

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. It is economical and is used for producing plastic model assembly kits, plastic cutlery, CD "jewel" cases, and many other objects where a fairly rigid, economical plastic of any of various colors is desired.

Standard bulk form

For architectural and engineering modelling, polystyrene is extruded into forms of standard modelling scale with the cross-sections of a miniature I-beam as well as rods and tubes. It is also formed into sheets with various patterns for this purpose as well. The blank sheets of polystyrene are referred to as "plasticard" in Britain, after the vulgarization of a trademark, but are called "sheet styrene" in the US.

Polystyrene fabricated into a sheet can be stamped (formed) into economic, disposable cups, glasses, bowls, lids, and other items, especially when high strength, durability, and heat resistance are not essential. A thin layer of transparent polystyrene is often used as an infra-red spectroscopy standard.

Solid foam

Polystyrene's most common use, however, is as expanded polystyrene (EPS). Expanded polystyrene is produced from a mixture of about 5-10% gaseous blowing agent and 90-95% polystyrene by weight, most commonly pentane or carbon dioxide. The solid plastic is expanded into a foam through the use of heat, usually steam. Extruded polystyrene (XPS), which is different from expanded polystyrene, is commonly known by the trade name Styrofoam. Expandable polystyrene is the lightweight material of which coffee cups and takeaway food containers are made. The voids filled with trapped air give expanded polystyrene low thermal conductivity. This makes it ideal as a construction material and it is used in structural insulated panel building systems. It is also used as insulation in building structures, as molded packing material for cushioning fragile equipment inside boxes, as packing "peanuts", as non-weight-bearing architectural structures (such as pillars), and also in crafts and model building, particularly architectural models. Foamed between two sheets of paper, it makes a more-uniform substitute for corrugated cardboard, tradenamed Fome-Cor.

Expanded polystyrene used to contain CFCs, but other, more environmentally-safe blowing agents are now used. 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.

Production methods include sheet stamping (PS) and injection molding (both PS and HIPS).

The chemical makeup of polystyrene is a long chain hydrocarbon with every other carbon connected to a Phenyl group (an aromatic ring similar to benzene.

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 isn't 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 isn't 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.

Standard markings

. Unfortunately, the majority of polystyrene products are currently not recycled due to a lack of suitable recycling facilities. Furthermore, when it is "recycled," it is not a closed loop — polystyrene cups and other packaging materials are usually recycled into fillers in other plastics, or other items that cannot themselves be recycled and are thrown away.

The Unicode character is U+9848, which will appear here if you have a suitable font installed: ♸.

Toughening

Pure polystyrene is brittle, but hard enough that a fairly high-performance product can be made by giving it some of the properties of a stretchier material, such as polybutadiene rubber. The two materials cannot normally be mixed due to the amplified effect of intermolecular forces on polymer solubility (see plastic recycling), but if polybutadiene is added during polymerization it can become chemically bonded to the polystyrene, forming a graft copolymer which helps to incorporate normal polybutadiene into the final mix, resulting in high-impact polystyrene or HIPS, often called "high-impact plastic" in advertisements. Common applications include use in toys and product casings. HIPS is usually injection molded in production.

Acrylonitrile butadiene styrene or ABS plastic is similar to HIPS: a copolymer of acrylonitrile and styrene, toughened with polybutadiene. Most electronics cases are made of this form of polystyrene, as are many sewer pipes.

Styrene can be copolymerized with other monomers; for example, divinylbenzene for cross-linking the polystyrene chains.

Cutting and shaping

Expanded polystyrene is very easily cut with a hot-wire foam cutter, which is easily made by a heated and taut length wire, usually nichrome due to nichrome's resistance to oxidation at high temperatures and its suitable electrical conductivity. The hot wire foam cutter works by heating the wire to the point where it can vaporize foam immediately adjacent to it. The foam gets vaporized before actually touching the heated wire, which yields exceptionally smooth cuts.

Polystyrene, shaped and cut with hot wire foam cutters, is used in architecture models, actual signage, amusement park and movie sets, airplane construction, and much more. Such cutters may cost just a few dollars (for a completely manual cutter) to tens of thousands of dollars for large CNC machines that can be used in high-volume industrial production.

Polysterene can also be cut with a traditional cutter, in order to do this without ruining the sides one must first dip the blade in water and cut with the blade at an angle of about 30º, the procedure has to be repeated multiple times for best results.

Finishing

In the United States, environmental protection regulations prohibit the use of solvents on polystyrene (which would dissolve the polystyrene and de-foam most of foams anyway).

Some acceptable finishing materials are

Water-based paint (artists have created paintings on polystyrene with gouache)
Mortar or acrylic/cement render, often used in the building industry as a weather-hard overcoat that hides the foam completely after finishing the objects.
Cotton wool or other fabrics used in conjunction with a stapling implement.

Fire Hazard

Polystyrene is classified according to DIN4102 as a "B3" product, meaning highly flammable or "easily ignited". Consequently, though it is an efficient insulator at low temperatures, it is prohibited from being used in any exposed installations in building construction. It must be concealed behind drywall, sheet metal or concrete. Foamed plastic materials have been accidentally ignited and caused huge fires and losses. Examples include the Düsseldorf airport, the Channel tunnel, where it was inside a railcar and caught on fire, and the Browns Ferry nuclear plant, where fire reached through a fire retardant, reached the foamed plastic underneath, inside a firestop that did not consider bounding.

Explosives

Polystyrene is used in some polymer-bonded explosives:

Some Polystyrene PBX Examples
Name	Explosive Ingredients	Binder Ingredients	Usage
PBX-9205	RDX 92%	Polystyrene 6%; DOP 2%
PBX-9007	RDX 90%	Polystyrene 9.1%; DOP 0.5%; rosin 0.4%

External links

Gourt Home::Gourt :: Polystyrene