Everything about Polycarbonate totally explained
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 plastics.
Chemistry
Polycarbonates got their name because they're
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 (
BPA). 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's usually referred to as CR-39 plastic — with good optical and mechanical properties, frequently used for eyeglass lenses.
Processing
Polycarbonate has a
glass transition temperature of about 150 C, so softens gradually above this point and is processed from the melt at above 300C.
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 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.
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 molding into ready articles
- extrusion into tubes, rods and other profiles
- extrusion with cylinders 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 (for example 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 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. (External Link)
Being based on bisphenol A (a phenol based on benzene) pricing is largely dependent on phenol and benzene pricing.
Makes
The most common polycarbonate resins are
Lexan from SABIC Innovative Plastics (formerly General Electric Plastics)
Calibre from Dow Chemicals
Makrolon from Bayer
Panlite from Teijin Chemical Limited.
Potential hazards in food contact applications
Polycarbonate may be appealing to manufacturers and purchasers of food storage containers due to its clarity and toughness, being described as lightweight and highly break resistant particularly when compared to silica glass. Polycarbonate may be seen in the form of single use and refillable plastic water bottles.
More than 100 studies have explored the bioactivity of bisphenol A leachates from polycarbonates. Bisphenol A appeared to be released from polycarbonate animal cages into water at room temperature and that 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 while government funded studies tend to find significant effects.
Research by Ana M. Soto, professor of anatomy and cellular biology at Tufts University School of Medicine, Boston, published Dec. 6 in the online edition of Reproductive Toxicology (DOI: 10.1016/j.reprotox.2006.10.002) describes exposure of pregnant rats to bisphenol A at 2.5 to 1,000 µg per kilogram of body weight per day. At the equivalent of puberty for the pups (50 days old), about 25% of their mammary ducts had precancerous lesions, some three to four times higher than unexposed controls. The study is cited as evidence for the hypothesis that environmental exposure to bisphenol A as a fetus can cause breast cancer in adult women.
An expert panel of 12 scientists has found that there's "some concern that exposure to the chemical bisphenol A in utero causes neural and behavioral effects," according to the draft report prepared by The National Toxicology Program (NTP) Center for the Evaluation of Risks to Human Reproduction.
For the general adult population, the expert panel found a "negligible concern for adverse reproductive effects following exposures."
One point of agreement among those studying polycarbonate water and food storage containers may be that using sodium hypochlorite bleach and other alkali cleaners to clean polycarbonate isn't recommended, as they catalyze the release of the bisphenol-A. The tendency of polycarbonate to release bisphenol A was discovered after a lab tech used strong cleaners on polycarbonate lab containers. Endocrine disruption later observed on lab rats was traced to exposure from the cleaned containers.
On April 18, 2008, Health Canada announced that Bisphenol A is "'toxic' to human health". Canada is the first nation to make this designation.
A chemical compatibility chart shows reactivity between chemicals such as polycarbonate and a cleaning agent. Alcohol is one recommended organic solvent for cleaning grease and oils from polycarbonate. For treating mold, borax:H2O 1:96 to 1:8 may be effective.
Synthesis
Polycarbonate can be synthesized from bisphenol A and phosgene (carbonyl dichloride, COCl2). The first step in the synthesis of polycarbonate from bisphenol A is treatment of bisphenol A with sodium hydroxide. This deprotonates the hydroxyl groups of the bisphenol A molecule.
The deprotonated oxygen reacts with phosgene through carbonyl addition to create a tetrahedral intermediate (not shown here), after which the negatively charged oxygen kicks off a chloride ion (Cl-) to form a chloroformate.
The chloroformate is then attacked by another deprotonated bisphenol A, eliminating the remaining chloride ion and forming a dimer of bisphenol A with a carbonate linkage in between.
Repetition of this process yields polycarbonate, a polymer with alternating carbonate groups and groups from bisphenol A.
Interaction with other chemicals
* At room temperature. At temperatures above 60 °C hydrolysis can occur, degrading the plastic. Degradation depends on time and temperature.
Using sodium hypochlorite (bleach) and other alkali cleaners on polycarbonate isn't recommended as they cause the release of bisphenol A, a known endocrine disrupter.
Further Information
Get more info on 'Polycarbonate'.
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