Polystyrene (pol-ee-STYE-reen) is a thermoplastic polymer made from styrene. A thermoplastic polymer is a material that can be repeatedly softened and hardened by alternately heating and cooling. Styrene is a hydrocarbon derived from petroleum with the formula C6H5CH=CH2. The presence of the double bond in the styrene molecule makes it possible for styrene molecules to react with each other in long chains that constitute the polymer polystyrene.
Polystyrene is a hard, strong, transparent solid highly resistant to mechanical impact. It is an excellent thermal (heat) and electrical insulator, is easily shaped and molded in the liquid state, and takes dyes readily. It can be produced in a wide variety of shapes and forms, including sheets, plates, rods, beads, and foams.
Varies widely; ranges from 190°C-260°C (370°F-500°F)
The history of polystyrene dates to 1839 when a German apothecary (druggist) named Eduard Simon discovered styrene in petroleum. Later scientists attempted to incorporate styrene into some of the commercial products they made, such as rubber tires. But a polymer based on styrene was not actually produced until 1930 when researchers at the German chemical firm of I. G. Farben discovered how to make the material. Farber's parent company, BASF, shortly made the product commercially available and in 1937, Dow Chemical first made the compound available in the United States. During World War II, polystyrene was used for the manufacture of synthetic rubber products. After the war, the number of commercial and industrial uses expanded rapidly. Today, it is virtually impossible to avoid polystyrene products in one's daily life.
HOW IT IS MADE
Compounds like styrene with double bonds often polymerize easily. The double bond on a styrene molecule breaks open and a hydrogen atom from a second styrene molecule adds to one side of the double bond, while the rest of the second styrene molecule adds to the second side of the double bond. The product of this reaction still has a double bond. So the reaction can be repeated a second time; and a third time; and a fourth time; and so on. One goal of research on polystyrene has been to determine how the size of the polystyrene affects its properties (and, therefore, its uses) and how to stop the polymerization reaction at some desired point.
All that is needed to start the polymerization of styrene is a material that will cause the first double bond to break. Such materials are known as polymerization initiators. One of the most common initiators used in the polymerization of styrene is benzoyl peroxide (C6H5COOOCOC6H5). Once the polymerization reaction begins, it tends to release enough energy for the reaction to continue on its own.
An especially popular form of polystyrene is known as expanded polystyrene. It is made by blending air with molten polystyrene to make a lightweight foam sold under the trade name of Styrofoam®.
- Polystyrene is sold commercially under more than a hundred trade names, the most famous of which is probably Styrofoam®.
- Only about 5 percent of a styrofoam cup is polystyrene. The rest is air.
- One of the innovative uses for polystyrene is as a building material for the construction of new houses. Scientists suggest that it is perfect for the purpose: lightweight, inexpensive, strong, a good insulator, and available all over the world. One of the first applications suggested for polystyrene as a building material is in the construction of houses in Afghanistan, where many families have lost their homes after two decades of wars and earthquakes.
COMMON USES AND POTENTIAL HAZARDS
Polystyrene is the fourth largest thermoplastic polymer made in the United States by production volume. It is used in the manufacture of hundreds of commercial, industrial, household, and personal articles. Some examples include:
- Plastic model kits and toys;
- Containers with lids; disposable cups, plates, knives, forks, and spoons;
- "Jewel" cases for compact discs and cases for audiocassettes;
- Plastic coat hangers and plastic trays;
- Refrigerator doors and air conditioner cases;
- Housing for machines; and
- Cabinets for clocks, radios, and television sets.
Some uses of expanded polystyrene include:
- In all kinds of containers to keep foods either hot or cold (such as ice chests);
- Egg cartons;
- Fillers in shipping containers;
- Packages for carry-out foods;
- Insulation for buildings;
- In the construction of boats; and
- For the construction of some types of furniture.
Polystyrene dust and powder formed during production can be a mild irritant to the eyes, skin, and respiratory system. But even for workers in the field, the risk is regarded as being very low. A more serious problem posed by the compound is the risk it poses for the environment. About half of all the polystyrene produced in the United States is used for packaging and "one-time use" purposes. That is, someone uses the product and then throws it away. Since polystyrene does not readily decompose, it tends to accumulate in landfills and dumps. Some environmentalists point out that large volumes of discarded polystyrene contribute significantly to the nation's solid waste disposal problems. Industry spokespersons, however, point out that polystyrene accounts for less than one percent of all solid wastes. In any case, a number of industries and companies have attempted to reduce the amount of polystyrene used in their products in order to cut back on their contribution to the solid waste disposal problem.
Words to Know
- a compound consisting of carbon and hydrogen.
- a compound consisting of very large molecules made of one or two small repeated units called monomers.
- can be repeatedly softened and hardened by alternately heating and cooling.
FOR FURTHER INFORMATION
Boyd, Clark. "Polystyrene homes planned for Afghans." BBC News. http://news.bbc.co.uk/2/hi/technology/3528716.stm (accessed on October 26, 2005).
"Energy & Waste—Landfilling." Energy Information Administration, Department of Energy. http://www.eia.doe.gov/kids/energyfacts/saving/recycling/solidwaste/landfiller.html (accessed on October 26, 2005).
"Polystyrene." U.S. Environmental Protection Agency, Emissions Factors and Policy Applications Center. http://www.epa.gov/ttn/chief/ap42/ch06/final/c06s06-3.pdf (accessed on October 26, 2005).
"Polystyrene Packaging Delivers!" Polystyrene Packaging Council. http://www.polystyrene.org/ (accessed on October 26, 2005).
Sims, Judith. "Polystyrene." In Environmental Encyclopedia. 3rd ed. Edited by Marci Bortman and Peter Brimblecombe. Detroit, Mich.: Gale, 2003.
Polystyrene is a lightweight transparent plastic derived from petroleum by-products and natural gas . It is widely used in the packaging industry, but most polystyrene is used to make durable goods such as television cabinets, appliances, and furniture. Polystyrene also has excellent insulating properties. As packaging, polystyrene is used both in foam and solid forms. Solid polystyrene is used in yogurt, sour cream, and cottage cheese containers, cutlery, clear clamshells used at salad bars, and video and audio cassette containers. As foam, polystyrene is used in cups, bowls, plates, trays, clamshell containers, meat trays, egg cartons, and packaging for electronics and other delicate items. Polystyrene foam is manufactured by processing solid polystyrene resin pellets with a gaseous expansion agent. In the past, about 30% of polystyrene foam products were made with chlorofluorocarbons (CFCs), which were identified as contributors to the deterioration of the ozone layer in the Earth's upper atmosphere . By 1990, manufacturers had phased out the use of CFCs as a polystyrene expansion agent. Some manufacturers switched to the use of hydrochlorofluorocarbon-22 (HCFC-22), which reduced ozone layer depletion potential by about 95% over CFCs. By 1994, as required by federal law, polystyrene foam manufacturers phased out the use of HCFC-22 and by 1997 were using alternative expansion agents, most commonly pentane gas. Pentane does not affect the ozone layer, but because it can contribute to smog formation some manufacturers recycle pentane emissions. Carbon dioxide (CO2) is also used by manufacturers as a polystyrene expansion agent. CO2 is nontoxic, nonflammable, does not contribute to smog, and has no atmospheric ozone depletion potential, but it has been implicated in global warming.
Polystyrene packaging accounts for about 1.2 % (by weight) of the total solid waste stream in the United States, and polystyrene makes up from about 2-10% of materials thrown away as litter. However, polystyrene can be recycled and used to make new polystyrene products such as wall insulation, packing filler, and cafeteria trays. Polystyrene has an energy content of 17,000 Btu/pound, about four times that of average municipal solid waste , and can be burned with other solid wastes in an incinerator that has appropriate emission controls to contain potentially toxic combustion products. Another potential environmental concern is the release of toxic metals such as cadmium and lead from combustion of inks used to tint the polystyrene. The polystyrene manufacturing industry is developing source reduction techniques, including improving resin properties to make stronger products with less material and making lighter-weight foam products.
Polystyrene is manufactured from styrene , which is a hazardous chemical. When styrene is heated to 392°F (198°C), it is converted into the polymer polystyrene. Health concerns have been raised about styrene residues remaining in polystyrene products, especially those that are used to contain food for consumption. Styrene is soluble in oil and in ethanol , substances commonly found in foods and alcoholic beverages. A U.S. EPA National Human Adipose Tissue Survey in 1986 identified styrene in 100% of the 46 human fat samples collected.
[Judith Sims ]
Expansion Agents for Polystyrene Foam. Washington, DC: Polystyrene Packaging Council, 1997.
Polystyrene in the Solid Waste Stream. Washington, DC: Polystyrene Packaging Council, 1997.
Uses and Benefits of Polystyrene. Washington, DC: Polystyrene Packaging Council, 1997.