Polycarbonate

Polycarbonate
File:Lexan.png Repeating chemical structure unit of Polycarbonate made from bisphenol A
Physical Properties
Density (ρ)	1.20–1.22 g/cm3
Abbe number (V)	34.0
Refractive index (n)	1.584–1.586
Flammability	V0-V2
Limiting oxygen index	25–27%
Water absorption - Equilibrium(ASTM)	0.16–0.35%
Water absorption - over 24 hours	0.1%
Radiation resistance	Fair
Ultraviolet (1-380nm) resistance	Fair
Mechanical Properties
Young's modulus (E)	2.0–2.4 GPa
Tensile strength (σt)	55–75 MPa
Compressive strength (σc)	>80 MPa
Elongation (ε) @ break	80–150%
Poisson's ratio (ν)	0.37
Hardness - Rockwell	M70
Izod impact strength	600–850 J/m
Notch test	20–35 kJ/m2
Abrasive resistance - ASTM D1044	10–15 mg/1000 cycles
Coefficient of friction (μ)	0.31
Speed of sound	2270 m/s
Thermal Properties
Melting temperature (Tm)	267 °C*
Glass transition temperature(Tg)	150 °C
Heat deflection temperature - 10 kN (Vicat B)	145 °C
Heat deflection temperature - 0.45 MPa	140 °C
Heat deflection temperature - 1.8 MPa	128–138 °C
Upper working temperature	115–130 °C
Lower working temperature	–40 °C
Linear thermal expansion coefficient (α)	65–70 × 10−6/K
Specific heat capacity (c)	1.2–1.3 kJ/(kg·K)
Thermal conductivity (k) @ 23 °C	0.19–0.22 W/(m·K)
Electrical Properties
Dielectric constant (εr) @ 1 MHz	2.9
Permittivity (ε) @ 1 MHz	2.568 × 10−11 F/m
Relative permeability (μr) @ 1 MHz	0.866(2)
Permeability (μ) @ 1 MHz	1.089(2) μN/A2
Dielectric strength	15–67 kV/mm
Dissipation factor @ 1 MHz	0.01
Surface resistivity	1015 Ω/sq
Volume resistivity (ρ)	1012–1014 Ω·m
Near to Short-wave Infrared Transmittance Spectrum
File:Polycarbonate IR transmission.png IR transmittance of polycarbonate. Also, polycarbonate is almost completely transparent throughout the entire visible region until 400 nm, blocking UV light.
Chemical Resistance
Acids - concentrated	Poor
Acids - dilute	Good
Alcohols	Good
Alkalis	Good-Poor
Aromatic hydrocarbons	Poor
Greases & Oils	Good-fair
Halogenated Hydrocarbons	Good-poor
Halogens	Poor
Ketones	Poor
Gas permeation @ 20 °C
Nitrogen	10 – 25 cm3·mm/(m2·day·Bar)
Oxygen	70 – 130 cm3·mm/(m2·day·Bar)
Carbon dioxide	400 – 800 cm3·mm/(m2·day·Bar)
Water vapour	1–2 gram·mm/(m2·day) @ 85%–0% RH gradient)
Economic Properties
Price	5–9 €/kg

Polycarbonates, commonly known by the trademarked name Lexan, are a particular group of thermoplastic polymers. They are easily worked, moulded, and thermoformed. Because of these properties, polycarbonates find many applications. Polycarbonates do not have a unique plastic identification code and are identified as Other, 7.

Structure

Polycarbonates received their name because they are polymers containing carbonate groups (-O-(C=O)-O-). Most polycarbonates of commercial interest are derived from rigid monomers. A balance of useful features including temperature resistance, impact resistance and optical properties position polycarbonates between commodity plastics and engineering plastics.

Production

The main polycarbonate material is producted by the reaction of bisphenol A and phosgene (COCl₂). The first step involves treatment of bisphenol A with sodium hydroxide, which deprotonates the hydroxyl groups of the bisphenol A.

(HOC₆H₄)₂CMe₂ + 2 NaOH → (NaOC₆H₄)₂CMe₂ + 2 H₂O

The diphenoxide ((NaOC₆H₄)₂CMe₂) reacts with phosgene to give a chloroformate, which subsequently is attacked by another phenoxide. The net reaction from the diphenoxide is:

(NaOC₆H₄)₂CMe₂ + COCl₂ → 1/n [OC(OC₆H₄)₂CMe₂]_n + 2 NaCl

In this way, approximately one billion kilograms of polycarbonate is produced annually. Many other diols have been tested in place of bisphenol A, e.g. 1,1-bis(4-hydroxyphenyl)cyclohexane and dihydroxybenzophenone including some, e.g. tetramethylcyclobutanediol, that are unlikely endocrine disruptors.

An alternative route to polycarbonates entails transesterification from BPA and diphenyl carbonate:

(HOC₆H₄)₂CMe₂ + (C₆H₅O)₂CO → 1/n [OC(OC₆H₄)₂CMe₂]_n + 2 C₆H₅OH

The diphenyl carbonate was derived in part from carbon monoxide, this route being greener than the phosgene method.

Properties and processing

Polycarbonate derived from BPA is a very durable material. Although it has high impact-resistance, it has low scratch-resistance and so a hard coating is applied to polycarbonate eyewear lenses and polycarbonate exterior automotive components. The characteristics of polycarbonate are quite like those of polymethyl methacrylate (PMMA, acrylic), but polycarbonate is stronger, usable in a wider temperature range but more expensive. This polymer is highly transparent to visible light and has better light transmission characteristics than many kinds of glass.

Polycarbonate has a glass transition temperature of about 150 °C (302 °F), so it softens gradually above this point and flows above about 300 °C (572 °F). Injection moulding is more difficult than other common thermoplastics owing to its non-Newtonian fluid flow behaviour. Tools must be held at high temperatures, generally above 80 °C (176 °F) to make strain- and stress-free products. Low molecular mass grades are easier to mould than higher grades, but their strength is lower as a result. The toughest grades have the highest molecular mass, but are much more difficult to process.

Unlike most thermoplastics, polycarbonate can undergo large plastic deformations without cracking or breaking. As a result, it can be processed and formed at room temperature using sheet metal techniques, such as forming bends on a brake. Even for sharp angle bends with a tight radius, no heating is generally necessary. This makes it valuable in prototyping applications where transparent or electrically non-conductive parts are needed, which cannot be made from sheet metal. Note that PMMA/Plexiglas, which is similar in appearance to polycarbonate, is brittle and cannot be bent at room temperature.

Main transformation techniques for polycarbonate resins:

• extrusion into tubes, rods and other profiles
• extrusion with cylinders into sheets (0.5–15 mm (0.020–0.591 in)) and films (below 1 mm (0.039 in)), which can be used directly or manufactured into other shapes using thermoforming or secondary fabrication techniques, such as bending, drilling, routing, laser cutting etc.
• injection molding into ready articles

File:Polycarbonate water bottle.JPG

A bottle made from polycarbonate

Applications

Electronic components

Polycarbonate is mainly used for electronic applications that capitalize on its collective safety features. Being a good electrical insulator and having heat and flame resistant properties, it is used in myriad products associated with electrical and telecommunications hardware. They are used as dielectric in high stability capacitors.

Construction materials

The second largest consumer of polycarbonates is the construction industry, e.g. for domelights, flat or curved glazing, and sound walls.

Data storage

A major application of polycarbonate is the production of compact discs, DVDs, and Blu-ray Discs. The blanks are produced by injection molding.Typical products of sheet/film production include applications in advertisement (signs, displays, poster protection).

Automotive and aircraft components

In the automotive industry, injection moulded polycarbonate can produce very smooth surfaces that make it well suited for direct (without the need for a basecoat) metalised parts such as decorative bezels and optical reflectors. Its uniform mould shrinkage results in parts with greater accuracy than those made of polypropylene. However, due to its susceptibility to environmental stress cracking, its use is limited to low stress applications. It can be laminated to make bullet-proof "glass", although “bullet-resistant” would be more accurate.

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

Niche applications

Polycarbonate, being a versatile material with attractive processing and physical properties, has attracted myriad smaller applications. The use of injection molded drinking bottles and glasses and food containers has stirred serious controversy (see below).

Many kinds of lenses are manufactured from polycarbonate, including automotive headlamp lenses, lighting lenses, sunglass/eyeglass, lenses, and safety glasses. Other miscellaneous items: MP3/Digital audio player cases, Ocarinas, computer cases, riot shields, visors, instrument panels. Many toys and hobby items are made from polycarbonate parts, e.g. fins, gyro mounts, and flybar locks for use with radio-controlled helicopters.

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.

Medical applications

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 by the ethylene oxide (EtO) method. However, scientific research indicates possible problems with biocompatibility. Dow Chemical strictly limits all its plastics with regard to medical applications.

Potential hazards in food contact applications

Main article: Bisphenol A

Main article: Endocrine disruptor

The use of polycarbonate containers for the storage of food storage is controversial. The basis of this controversy is their hydrolysis (degradation by water, often referred to as leaching) releases bisphenol A:

1/n [OC(OC₆H₄)₂CMe₂]_n + H2O → (HOC₆H₄)₂CMe₂ + CO₂

More than 100 studies have explored the bioactivity of bisphenol A derived from polycarbonates. Bisphenol A appeared to be released from polycarbonate animal cages into water at room temperature and it may have been responsible for enlargement of the reproductive organs of female mice.

An analysis of the literature on bisphenol A leachate low-dose effects by vom Saal and Hughes published in August 2005 seems to have found a suggestive correlation between the source of funding and the conclusion drawn. Industry funded studies tend to find no significant effects whereas government funded studies tend to find significant effects.

Sodium hypochlorite bleach and other alkali cleaners catalyze the release of the bisphenol A from polycarbonate containers. A chemical compatibility chart shows that polycarbonate is incompatible with many typical cleaning agent, e.g. ammonia, acetone. Alcohol is one recommended organic solvent for cleaning grease and oils from polycarbonate.

See also

• Bioplastic
• CR-39
• Organic electronics

References

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