Polycarbonate

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 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.

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 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. Dow Plastics Medical Application Policy. MAKROLON® Polycarbonate Biocompatibility Grades.

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.Future Weapons : F-22 Raptor : Discovery Channel

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.Howdeshell, KL; Peterman PH, Judy BM, Taylor JA, Orazio CE, Ruhlen RL, Vom Saal FS, Welshons WV (Jul 2003). "Bisphenol A is released from used polycarbonate animal cages into water at room temperature". Environmental Health Perspectives 111 (9): 1180-7. PMID 12842771. Retrieved on 2006-06-07. 

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.vom Saal, FS; Hughes C (Aug 2005). "An extensive new literature concerning low-dose effects of bisphenol A shows the need for a new risk assessment". Environmental Health Perspectives 113 (8): 926-33. PMID 16079060. Retrieved on 2006-06-07. 

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.http://pubs.acs.org/cen/news/84/i50/8450bisphenol.html Retrieved on 2007-02-26

An expert panel of 12 scientists has found that there is "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."Julie\'s Health Club - Where alternative and mainstream health meet | Chicago Tribune | Blog | Julie\'s Health Club

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 is not 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. Hunt, PA; Kara E. Koehler, Martha Susiarjo, Craig A. Hodges, Arlene Ilagan, Robert C. Voigt, Sally Thomas, Brian F. Thomas and Terry J. Hassold (April 2003). "Bisphenol A Exposure Causes Meiotic Aneuploidy in the Female Mouse". Current Biology 13 (7): 546-553. 

A chemical compatibility chart shows reactivity between chemicals such as polycarbonate and a cleaning agent.Polycarbonate 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.^{[citation needed]}

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 is not recommended as they cause the release of bisphenol A, a known endocrine disrupter.

References

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