PS ( Polystyrene )

 

Polystyrene

Density 1050 kg/m
Electrical conductivity (σ) 10-16 S/m
Thermal conductivity 0.08 W/(mK)
Young's modulus (E) 3000-3600 MPa
Tensile strength (σt) 4660 MPa
Elongation at break 34%
Notch test 25 kJ/m
Glass temperature 95 C
Melting point[citation needed] 240 C
Vicat B[citation needed] 90 C
Heat transfer coefficient (λ) 0.17 W/(mK)
Linear expansion coefficient (α) 8 10-5 /K
Specific heat (c) 1.3 kJ/(kgK)
Water absorption (ASTM) 0.030.1

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 ("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 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.

Solid foam

Polystyrene packaging materialPolystyrene's most common use, however, is as expanded polystyrene (EPS). Expanded polystyrene is produced from a mixture of about 90-95% polystyrene and 5-10% gaseous blowing agent, most commonly pentane or carbon dioxide [citation needed]. 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. The voids filled with trapped air give it 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, 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 270C.


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 . 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+2678, which will appear here if you have a suitable font installed.

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 due to 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 include use in toys and product casings. HIPS is usually injection molded in production. Autoclaving polystyrene can compress and harden the material.

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 parks, 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.

Polystyrene 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 30, the procedure has to be repeated multiple times for best results.

Polystyrene is also able to be cut on 3 and 5-axis routers, enabling large scale prototyping and model making to be accomplished. Special polystyrene cutters are available that look more like large cylindrical rasps.


Use in biology
Petri dishes and other containers such as test tubes, made of polystyrene, 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 therfore play a critical role in pharmaceutical research. Major manufactueres include Corning/Costar, Nalgene/Nunc, Greiner and BD/Falcon. The web sites of these companies contain a wealth of information.

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.

Dangers and Fire hazard
The health effects caused by consuming Polystyrene (PS) when it migrates from food containers (primarily due to a leaching caused by heat exchange) into food is under serious investigation. Although the EPA has not yet come to a definitive conclusion on the direct carcinogenic effects of PS, the evidence is mounting. Benzene (a material used in the production of PS) is a known human carcinogen. Moreover, Butadiene and Styrene (basic building blocks of PS), when combined, they become benzene-like in both form and function. For this reason PS is highly suspected to be carcinogenic in the same manner as benzene, although the jury is still out due to a lack of controlled studies.

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 Dsseldorf International 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.

In addition to fire hazard, substances that contain Acetone (such as most Aerosol paint sprays) and Cyanoacrylate glues can cause polystyrene foam to melt.

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%