Polystyrene

Expanded polystyrene tray with tomato seedlings

Polystyrene IPA: /ˌpɒliˈstaɪriːn/ (IUPAC polyphenylethene), 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 a thermoplastic substance, normally existing in solid state at room temperature, but melting if heated (for molding or extrusion), and becoming solid again when cooling off. Polystyrene can be recycled, and has the number "6" as its recycling symbol. Polystyrene is one of the most widely used kinds of plastic. 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 drinks cups. Discarded polystyrene, which does not biodegrade, is often abundant in the outdoor environment, particularly along shores and waterways.

History

Polystyrene was discovered in 1839 by Eduard Simon,^[1] 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 leading 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 (C₈H₉)_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.

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.

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

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 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.^[4]

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

Expanded polystyrene foam (EPS) is usually white and made of expanded polystyrene beads. Familiar uses include packing "peanuts" and molded packing material for cushioning fragile items inside boxes. It is commonly packaged as rigid panels (size 4 by 8 or 2 by 8 square feet in the United States), which are also known as "bead-board". Thermal resistivity is usually about 28 m·K/W (or R-4 per inch in American customary units). Some EPS boards have a flame spread of less than 25 and a smoke-developed index of less than 450, which means they can be used without a fire barrier according to US building codes.

Extruded polystyrene foam

Extruded polystyrene foam (XPS) has air inclusions which gives it moderate flexibility, a low density, and a low thermal conductivity. XPS is sometimes abbreviated "EPS" - not to be confused with expanded polystyrene foam.

Extruded polystyrene material is also in crafts and model building, particularly architectural models. Foamed between two sheets of paper, it makes a more uniform substitute for corrugated cardboard. Thermal resistivity is usually about 35 m·Kelvin/W (or R-5 per inch in American customary units).

Trade names for XPS include "Styrofoam" and "Foamcore". ("Styrofoam" is often also used as a generic name for all polystyrene foams.)

Copolymers

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 such materials can never normally be mixed because of the amplified effect of intermolecular forces on polymer insolubility (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. One commercial name for HIPS is Bextrene. Common applications of HIPS include toys and product casings. HIPS is usually injection molded in production. Autoclaving polystyrene can compress and harden the material.

Several other copolymers are also used with styrene. 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. ABS pipes may become brittle over time. SAN is a copolymer of styrene with acrylonitrile, and SMA one with maleic anhydride. Styrene can be copolymerized with other monomers; for example, divinylbenzene for cross-linking the polystyrene chains.

Disposal and environmental issues

Polystyrene is not easily recycled because of its light weight (especially if foamed) and its low scrap value. It is generally not accepted in kerbside (curbside) collection recycling programs.

On the other hand, great advances have been made in recycling expanded polystyrene at an industrial level. Many different methods of densification have been developed. This increase in density, usually greater than 15#/cubic foot makes clean polystyrene a good profit center in recycling operations. Some industrial polystyrene manufacturers accept post consumer EPS for recycling. As an example Dart Container Corporation in Mason, Michigan has an ongoing post consumer recycling operation as well as an industrial EPS scrap recycling operation.

Environmental impact

Discarded polystyrene does not biodegrade and is resistant to photolysis^[5]. Since the foamed kinds not only float on water, but also blow in the wind, it is often abundant in the outdoor environment, particularly along shores and waterways. According to the California Coastal Commission, it is now a principal component of marine debris. The plastic may also be harmful to wild animals if it is ingested by them.

Polystyrene foams are produced using blowing agents that form bubbles and expand the foam. In expanded polystyrene, these are usually hydrocarbons such as pentane, which may pose a flammability hazard in manufacturing or storage of newly manufactured material, but have relatively mild environmental impact. However, extruded polystyrene is usually made with HCFC blowing agents with have effects on ozone depletion and on global warming. Their ozone depletion potential is greatly reduced relative to CFCs which were formerly used, but their global warming potential can be on the order of 1000 or more, meaning it has 1000 times greater effect on global warming than does carbon dioxide.

Recycling

The resin identification code symbol for polystyrene

Currently, the majority of polystyrene products are not recycled because of a lack of consumer awareness regarding suitable recycling facilities and methods. Expanded polystyrene can be used to make park benches, flower pots, and toys. However, polystyrene "recycling" is not a closed loop, producing more polystyrene; polystyrene cups and other packaging materials are instead usually used as fillers in other plastics, or in other items that cannot themselves be recycled and are thrown away.

Recently though, manufacturers in the United States have begun making and marketing recycled EPS as building materials such as crown molding, trim, and decking lumber substitutes. Recycled EPS is used in many metal casting operations. It can be combined with cement to be used as an insulating amendment in the making of concrete foundations.

Incineration

If polystyrene is properly incinerated at high temperatures, the only chemicals generated are water, carbon dioxide, some volatile compounds, and carbon soot^[6]. If properly burned, one ton of foam cups produces 0.2 ounces of ash. Paper cups, when incinerated, produce an average of 200 pounds of ash per ton. Recently, great advances have been made in using condensed EPS as fuel in the Pacific Rim.

When burned without enough oxygen or at lower temperatures (as in a campfire or a household fireplace), polystyrene can produce polycyclic aromatic compounds, carbon black, and carbon monoxide, as well as styrene monomers.^[6][7]

Burial

Foam cups and other polystyrene products can be safely buried in landfills, since it is as stable as concrete or brick. No plastic film is required to protect the air and underground water.

Reducing

Restricting the use of foamed polystyrene takeout food packaging is a priority of many solid waste environmental organizations. A campaign to achieve the first ban of polystyrene foam from the food & beverage industry in Canada has been launched in Toronto as of January 2007, by local non-profit organization NaturoPack.^[8]

See also: Environmental issues in the United States#Polystrene

Finishing

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

Health and fire hazards

There is concern about the trace presence of polystyrene's production chemicals in the final plastic product, most of which are toxic if not removed. For instance benzene, which is used to produce ethylbenzene for styrene, is a known carcinogen. As well, unpolymerized styrene may pose health risks. Nevertheless, the EPA states:

Polystyrene is classified according to DIN4102 as a "B3" product, meaning highly flammable or "easily ignited." Consequently, although it is an efficient insulator at low temperatures, its use is prohibited in any exposed installations in building construction if the material is not flame retardant, e.g., with hexabromocyclododecane. It must be concealed behind drywall, sheet metal or concrete. Foamed polystyrene plastic materials have been accidentally ignited and caused huge fires and losses, for example at the Düsseldorf International Airport, the Channel tunnel (where polystyrene was inside a railcar that caught on fire), and the Browns Ferry Nuclear Power Plant (where fire reached through a fire retardant and reached the foamed plastic underneath, inside a firestop that had not been tested and certified in accordance with the final installation).

In addition to fire hazard, polystyrene can be dissolved by substances that contain acetone (such as most aerosol paint sprays), and by cyanoacrylate glues.

See also

• Structural insulated panel
• ThermaSAVE
• Greatpac Sdn. Bhd.
• Geofoam
• Styrofoam

References

External links

• Polystyrene - The University of Southern Mississippi
• SPI resin identification code - Society of the Plastics Industry
• Bacteria Turns Styrofoam into Biodegradable Plastic - Scientific American, February 27, 2006
• Polystyrene Data Sheet
• Polystyrene (packaging) facts