polycarbonates

 


For polycarbonate at a glance, click here.


Polycarbonate, or specifically polycarbonate of bisphenol A, is a clear plastic used to make shatterproof windows, lightweight eyeglass lenses, and such. General Electric makes this stuff and sells it as Lexan.
 

 

Polycarbonate gets its name from the carbonate groups in its backbone chain. We call it polycarbonate of bisphenol A because it is made from bisphenol A and phosgene. This starts out with the reaction of bisphenol A with sodium hydroxide to get the sodium salt of bisphenol A.
 

 

The sodium salt of bisphenol A is then reacted with phosgene, a right nasty compound which was a favorite chemical weapon in World War I, to produce the polycarbonate.
 

 

What? You want the gritty details of the reaction? Then click here and you will not be disappointed.

Another polymer used for unbreakable windows is poly(methyl methacrylate).

 

Seeing Another Polycarbonate More Clearly

Up until now, we've been talking about only one polycarbonate, polycarbonate of bisphenol A. But there's another polycarbonate out there, that some of us look at all the time. In fact, some of us, like me, never look at anything without the help of this polycarbonate. This is the polycarbonate that is used to make ultra-light eyeglass lenses. For people with really bad eyesight, like me, if the lenses were made out of glass, they would be so thick that they'd be too heavy to wear. I know. I used to have glass lenses. My glasses were so heavy that wearing them gave me a headache. But this new polycarbonate changed all that. Not only is it a lot lighter than glass, but it has a much higher refractive index. That means it bends light more than glass, so my glasses don't need to be nearly so thick.

So what is this wonderful new polycarbonate? It's very different from polycarbonate of bisphenol A. We make it by starting with this monomer:

 

You can see that it has two allyl groups on the ends. These allyl groups have carbon-carbon double bonds in them. This means they can polymerize by free radical vinyl polymerization. Of course, there are two allyl groups on each monomer. The two allyl groups will become parts of different polymer chains. In this way, all the chains will become tied together to form a crosslinked material that looks like this:

 

As you can see, the carbonate-containing groups (shown in blue) for the crosslinks between the polymer chains (shown in red). This crosslinking is makes the material very strong, so it won't break nearly as easily as glass will. This is really important for kids' glasses! If only this stuff had been invented when I was a kid!

There is a fundamental difference in the two types of polycarbonate described here that I should point out. Polycarbonate of bisphenol A is a thermoplastic. This means it can be molded when it is hot. But the polycarbonate used in eyeglasses is a thermoset. Thermosets do not melt, and they can't be remolded. They are used to make things that need to be really strong and heat resistant.

Other polymers used as plastics include:
 

Other polymers used as thermosets include:
 

    Epoxy Resins
    Polyimides
    Polydicyclopentadiene
    courtesy:pslc.ws/macrog/pc.htm
    Chemistry

    Polycarbonates received 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 polycarbonates can be divided 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 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 larger temperature range but also 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.

     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.

    Image:Bisphenol A plus NaOH.PNG

    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.

    Image:Bisphenolate A plus Phosgene.PNG

    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.

    Image:Adding Bisphenolate A to Chloroformate.PNG

    Repetition of this process yields a polycarbonate with alternating carbonate groups and groups from bisphenol A.

     

     Processing

    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.

    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:

    • extrusion into tubes, rods and other profiles
    • extrusion with cylinders into sheets (0.5–15 mm (0.020–0.59 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

    Typical injected applications:

    Typical sheet/film application:

    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.[1] However, scientific research indicates possible problems with biocompatibility. Dow Chemical strictly limits all its plastics with regard to medical applications.[2][3]

    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.[4]

    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 stress corrosion cracking, its use is limited to low stress applications.

    Being based on bisphenol A (a phenol based on benzene) pricing is largely dependent on phenol and benzene pricing.

    Trade names

    Other trade names for polycarbonate include:

    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.[5]

    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.[6]

    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[7] 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.[8]

    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."[9]

    A study of cross-sectional data from the 2003-2004 U.S. National Health and Nutritional Examination Survey published in the September 17, 2008 edition of the Journal of the American Medical Association (JAMA) demonstrated positive and statistically significant correlations between the concentration of bisphenol A in the urine and self-reported histories of cardiovascular disease and diabetes.[10]

    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.[11][12]

    On April 18, 2008, Health Canada announced that bisphenol A is "toxic to human health".[13] Canada is the first and only nation to make this designation.

     Chemical compatibility

    A chemical compatibility chart shows reactivity between chemicals such as polycarbonate and a cleaning agent.[14] 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]

    Compatibility with various chemicals[14]
    Will damage polycarbonate Require caution Considered safe
    † At room temperature. Above 60 °C hydrolysis degradation can occur. Degradation also depends on time.[citation needed]

    Using sodium hypochlorite (bleach) and other alkali cleaners on polycarbonate is not recommended as they cause the release of bisphenol A, a known endocrine disruptor.

courtesy:en.wikipedia.org/wiki/Polycarbonate


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