Polystyrene
From Freepedia
| Polystyrene | |
|---|---|
| Density | 1050 kg/m3 |
| Electrical conductivity(σ) | 10-16 S/m |
| Thermal conductivity | 0.08 W/(m·K) |
- Styrofoam redirects here. For the music band of the same name see Styrofoam (artist).
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 first manufactured in 1839 by Eduard Simon, a German apothecary. While Eduard isolated the substance from natural resin, he did not realize what he had discovered. It took another German, organic chemist, Hermann Staudinger, to realize that Simon's discovery contained long chains of styrene molecules, which was a plastic polymer. Pure solid polystyrene is a colorless, harder plastic with limited flexibility which 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.
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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, which is a mixture of about 5% polystyrene and 95% gaseous blowing agent. This 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 is used in structural insulated panel building systems. It is also used as insulation in building structures, as packing material for cushioning inside boxes, 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 Foamcore.
Expanded polystyrene used to contain CFCs but other, more environmentally-safe blowing agents are now used. Because it is an aromatic hydrocarbon, polystyrene is flammable and 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 for polystyrene includes a long chain of hydrocarbons, where every other carbon is connected to a benzene ring.
Standard markings
The resin identification code symbol for polystyrene, developed by the Society of the Plastics Industry so that items can be labeled for easy recycling, is Image:Recycle-resin-logos-lr 06.png.
The Unicode character is ♸, 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 its thermal conductance. Hot wire foam cutters work by heating special wire (usually nichrome or stainless steel) 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.
Hot wire foam cutters are available for anywhere from under a hundred dollars to tens of thousands of dollars for large CNC machines.
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
- Mortar, often used in the building industry as a weather-hard overcoat that makes the foam disappear completely after finishing the objects.
- Cotton wool or other fabrics used in conjunction with a stapling implement.
External links
- Macrogalleria: Polystyrene
- Society of the Plastics Industry
- Dart Container
- [1]
- Dow Chemical Plastics
- [2]
- Arguments against polystyrene
