Polystyrene
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- Styrofoam redirects here. For the music band of the same name see Styrofoam (artist).
| 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 MPa |
| Tensile strength(σt) | 46-60 MPa |
| Elongation @ break | 3-4% |
| notch test | 2-5 kJ/m² |
| Glass temperature | 95 °C |
| melting point | - |
| Vicat BTemplate:Ref | 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 |
| # Template:Note Deformation temperature at 10 kN needle load | |
| source: A.K. van der Vegt & L.E. Govaert, Polymeren, | |
| van keten tot kunstof, ISBN 90-407-2388-5 | |
Contents |
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 95% polystyrene and 5% gaseous blowing agent. The solid plastic is expanded into a foam through the use of heat, usually steam. Extruded polystyrene (XPS), which is different than 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 benzene ring.
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, Tm. But metallocene-catalyzed polymerization can produce an ordered syndiotactic polystyrene with the phenyl groups on alternating sides. This form is highly crystalline with a Tm of 270°C.
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. 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 can not be themselves recycled and are thrown away.
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 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.
Polystyrene foam can easily be cut using a hot wire that melts the foam. 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.
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.
See also
External links
- Tutorial on working with styrofoam
- Macrogalleria: Polystyrene
- Society of the Plastics Industry
- DOW.com – Styrofoam
- Bacteria Turns Styrofoam into Biodegradable Plastic
- Polystyrene.org
- Arguments against polystyrenebg:Полистирен
cs:Polystyren de:Polystyrol es:Poliestireno fr:Polystyrène it:Polistirolo nl:Polystyreen id:Polystyrene ja:ポリスチレン pl:Polistyren pt:Isopor fi:Polystyreeni sv:Polystyren zh:聚苯乙烯