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Plastics are a girl's best friend.

If you were a fashionable lady in the 1950s, you sported shoes with heels made of acrylic or, to be more specific, polymethyl methacrylate. The common name for this plastic, depending on the company that manufactured it, was Lucite, Perspex or Plexiglas. Acrylic is clear and resembles glass, so shoes that featured a see-through heel looked unusual and appealing. In a classic scene from the movie How to Marry a Millionaire, Marilyn Monroe shows off her shapely legs as she sun bathes wearing Lucite shoes. Don't get the wrong mental image, though. Monroe is also wearing a bathing suit.

As early as 1877, German chemist Wilhelm Rudolph Fittig helped polymerize methyl methacrylate, a colourless liquid, into polymethyl methacrylate, a solid. But it wasn't until 1936 that commercial production began under the name Plexiglas, with both the process and name patented by the German chemist Otto Rohm. During the Second World War this novel plastic, being clear and stronger than glass, found all sorts of applications ranging from submarine periscopes to fighter plane canopies and gun turret enclosures on bombers. And that had an interesting spin-off. Airmen who got shards of Plexiglas in their eye from shattered airplane canopies fared better than those who were injured by glass splinters. Acrylics turned out to be more compatible with human tissue than glass and did not cause as much inflammation. This observation led to the use of acrylics in the first hard contact lenses.

Acrylics also turned out to be ideal for dentures and found a use in composite dental fillings. Hockey was also a beneficiary; the protective Plexiglas around the rink was far better than netting. But because the plastic doesn't have much give, it has mostly been replaced by tempered glass. Acrylic paints also appeared, consisting of pigments and polymethyl methacrylate suspended in water, so there was no worry about solvent vapours.

And then along came acrylic fingernails. There are two types. One is made of polymethyl methacrylate and is glued on to the fingernail with cyanoacrylate, another type of acrylic polymer. The gel type is painted on the fingernail and hardened by exposure to ultraviolet light. That involves some interesting chemistry with the polymerization actually taking place on the finger.

This kind of a reaction is initiated by the formation of free radicals. Once formed, a free radical adds to a molecule of methyl methacrylate, which then becomes very reactive and adds to another molecule to form a dimer that then latches on to another monomer. Soon the original monomers are zipped into a long chain. You now have a hardened acrylic nail. The whole process begins with the acrylic-covered nail being exposed to an ultraviolet lamp. Ultraviolet light is energetic enough to break chemical bonds, which is exactly what it does to a photoinitiator that is incorporated into the mix. Under the effect of UV it breaks apart into free radicals. These then start the cascade of reactions resulting in a polymer. Of course ultraviolet light is energetic enough to break other chemical bonds as well, including those in DNA. That's why excessive exposure to the sun causes skin cancer. And that brings up an interesting question. Is there a risk of cancer by exposing the skin on the hands to ultraviolet light while waiting for the acrylic gel to harden?

The risk, statistically, is very low. Ultraviolet light-cured acrylic nails have been popular for some 20 years with millions of women using them. Any significant risk of skin cancer on the hands would have already been noted epidemiologically. Calculations show that exposure from a nail lamp is equivalent to spending an extra 1.5 to three minutes a day in sunlight between salon visits, the time depending on whether the lamp has one or two bulbs. The time spent under the lamps just isn't long enough to present a significant risk. So there's no need to fret about acrylic fingernails, at least not because of UV exposure. However, in addition to the acrylic monomers and photoinitiators, there are cross-linking agents, reaction accelerators, plasticizers and pigments. So irritation and allergic reactions are always a possibility. Of course Lucite heels can be worn safely. And they're still kicking around, in more shapes and styles than ever.

Joe Schwarcz is the director of McGill University's Office for Science and Society. Read his blog at
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Title Annotation:CHEMFUSION
Author:Schwarcz, Joe
Publication:Canadian Chemical News
Date:Feb 1, 2012
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