Polycarbonates

Polycarbonates are a particular group of thermoplastic polymers. They are easily worked, moulded, and thermoformed; as such, these plastics are very widely used in the modern chemical industry. Their interesting features (temperature resistance, impact resistance and optical properties) position them between commodity plastics and engineering

Chemistry
Polycarbonates got their name because they are polymers having functional groups linked together by carbonate groups (-O-(C=O)-O-) in a long molecular chain. Also carbon monoxide was used as a C1-synthon on an industrial scale to produce diphenyl carbonate, being later trans-esterified with a diphenolic derivative affording poly (aromatic carbonate) s. Taking into consideration the C1-synthon we can divide polycarbonates into poly(aromatic carbonate)s and poly(aliphatic carbonate)s. The second one, poly(aliphatic carbonate)s are a product of the reaction of carbon dioxide with epoxides, which owing to the thermodynamical stability of carbon dioxide requires the use of a catalyst.

The working systems are based on porphyrins, alkoxides, carboxylates, salens and beta-diiminates as organic, chelating ligands and aluminium, zinc, cobalt and chromium as the metal centres. Poly(aliphatic carbonate)s display promising characteristics, have a better biodegradability than the aromatic ones and could be employed to develop other specialty polymers.

One type of polycarbonate plastic is made from bisphenol A. This polycarbonate is a very durable material, and can be laminated to make bullet-proof "glass", though “bullet-resistant” would be more accurate. Although polycarbonate has high impact-resistance, it has low scratch-resistance and so a hard coating is applied to polycarbonate eyewear lenses. The characteristics of polycarbonate are quite like those of polymethyl methacrylate (PMMA; acrylic), but polycarbonate is stronger and more expensive.

This polymer is highly transparent to visible light and has better light transmission characteristics than many kinds of glass. CR-39 is a specific polycarbonate material — although it is usually referred to as CR-39 plastic — with good optical and mechanical properties, frequently used for eyeglass lenses.

Applications
Polycarbonate is becoming more common in housewares as well as laboratories and in industry, especially in applications where any of its main features—high impact resistance, temperature resistance, optical properties—are required.

Main transformation techniques for polycarbonate resins:

• injection moulding into ready articles
• extrusion into tubes, rods and other profiles
• extrusion with calenders into sheets (0.5-15 mm) and films (below 1 mm), which can be used directly or manufactured into other shapes using thermoforming or secondary fabrication techniques, such as bending, drilling, routing, laser cutting etc.

Typical injected applications:

• lighting lenses, sunglass/eyeglass lenses, safety glasses, automotive headlamp lenses
  compact discs, DVDs
• lab equipment, research animal enclosures
• drinking bottles
• iPod/Mp3 player cases

Typical sheet/film application:

• Industry: machined or formed, cases, machine glazing, riot shields, visors, instrument panels
• Advertisement: signs, displays, poster protection
• Building: domelights, flat or curved glazing, sound walls,
• Computers: Apple, Inc.'s MacBook, iMac, and Mac mini

For use in applications exposed to weathering or UV-radiation, a special surface treatment is needed. This either can be a coating (e.g. for improved abrasion resistance), or a coextrusion for enhanced weathering resistance.

Some polycarbonate grades are used in medical applications and comply with both ISO 10993-1 and USP Class VI standards (occasionally referred to as PC-ISO). Class VI is the most stringent of the six USP ratings. These grades can be sterilized using steam at 120 °C, gamma radiation or the ethylene oxide (EtO) method. See Medical Applications of Polycarbonate for more information. However, scientific research indicates possible problems with biocompatibility. Dow Chemical strictly limits all its plastics with regard to medical applications.

The most common resins are LEXAN® from General Electric, CALIBRE® from DOW Chemicals, MAKROLON® from Bayer and PANLITE® from Teijin Chemical Limited. Being based on bisphenol A—a phenol based on benzene—pricing is largely dependent on phenol and benzene pricing.

The cockpit canopy of the F-22 Raptor is made from a piece of high optical quality polycarbonate, and is the largest piece of its type formed in the world.