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Shellac nail polish, sometimes referred to as gel nail polish, is all the rage these days. They claim to give a chip-free coating to your nails and can last several weeks. It can last even longer than that, but after two or three weeks, your nails have grown out and you can start to see unpolished nail at the base of your polished nail. If you go in for a quick touch up and fill in, your shellac nail polish can I guess last indefinitely!
What is Typical Nail Polish?
To start, let’s talk about typical, boring, old nail polish. At is core, nail polish is nitrocellulose dissolved in a solvent. Nitrocellulose is known for giving a nice, shiny, hard film once the solvent evaporates. Nitrocellulose is also known for exploding. It’s what makes up magicians ‘flash paper,’ and guncotton. Fortunately, our nails don’t explode. I’m pretty sure the manufacturers worked on figuring that one out before mass producing nail polishes.
Plasticizers and other resins are an important component in basic nail polish. Without plasticizers, the nitrocellulose film would be very brittle and would chip and break very easily… well, more easily than it already does. Camphor, stearate esters, other fatty acids, and castor oil are common plasticizers and resins added to nail polish. These allow the polish to both adhere firmly to the nail and allow the polish to have some give to it so it doesn’t chip too easily.
If nothing else is added, you have a nice, clear, colorless nail polish. If you want a colored nail polish, pigments need to be added. Other effects can be made by adding other additives. Micas (ground-up shiny minerals commonly used in cosmetics) or pearl essence (ground-up fish scales) or gold or silver flakes can add nice looking effects to nail polish.
How is Shellac Nail Polish Different?
Shellac nail polish must have some differences from regular nail polish. Just on observation, it’s thicker than regular nail polish and somehow it doesn’t chip or break nearly as easily. Plus, you “cure” it under UV light and it dries immediately. Shellac nail polish is similar to acrylic nail polish in that they are acrylate polymers. Shellac nail polish is a type of methacrylate polymer.
To a first approximation, shellac nail polish is a mixture of methacrylate monomers (not methyl methacrylate, that’s banned for toxicity reasons) and a radical initiator (benzoyl peroxide or some such initiator). When placed under UV light, a classic organic chemistry reaction takes place: radical polymerization. The reaction initiates by the homolytic bond cleavage of benzoyl peroxide to form two oxygen radicals. These decompose further to carbon dioxide plus a phenyl radical. This adds to one side of the methacrylate double bond, leaving a resonance-stabilized carbon-centered radical behind. The reaction propagates by the carbon-centered radical adding to one side of another methacrylate double bond, leaving a new carbon-centered radical. This process repeats indefinitely until all the monomers have polymerized. The reaction terminates when two carbon centered radicals combine.
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Some of the methacrylate is probably in the form of hydroxyethyl methacrylate – a protein specific monomer that will firmly attach itself to the nail plate. This helps the polish adhere to the nail. This, I think, is the key: Once the polymer has formed, it is quite resistant to chipping. The polymer poly(methyl methacrylate) (PMMA) is often a shatter-resistant alternative to glass and is known as Plexiglas. This shatter resistance is what keeps shellac nail polish from chipping over time.
Now, that was just a first approximation. If it was actually a mixture of monomers, the polymerization would not be very efficient. The gel is thick, and it can be difficult to get the UV light deep into the gel. Most of the monomer would not form polymer under these circumstances. Instead, the actual product in shellac nail polish is short methacrylate oligomers, short chains of methacrylate monomer. If the monomer is a liquid and the polymer is a solid, it starts to make sense why the oligomer polish is a gel – intermediate consistency between a liquid and a solid. The polymerization of these oligomers is more efficient than creating the entire polymer from the individual monomer units.
Also, because the gel is rather thick, several thin coatings must be used. If you try to apply one thick coating and just get it over with, the UV light would hardly penetrate into the gel layer. Keeping the layer thin makes for more efficient curing. Several thin layers lead a better effect.
A quick wiping with isopropanol removes the sticky uncured inhibition layer on top and finishes the treatment. To remove, since it’s a more durable polish, you need a stronger nail polish remover. The watered down acetone sold in stores won’t cut it. You really need essentially pure acetone. This will soften the polymer enough to allow it to be removed.
You can now buy home shellac nail kits. You can buy home UV lamps and the shellac nail polish and do all this yourself at home. This is really the only reason I know anything about this. My mom loves this nail polish and bought a home kit. She told me that one day she was enjoying a warm day on the patio and applying her shellac nail polish. By the time she was finished, her brush was completely solidified and unusable! Looking back, this makes a lot of sense. If your outside, you’re in the sunlight, and the UV light from sunlight was enough to polymerize the polish left on the brush and solidify the brush. For this reason, you shouldn’t apply outdoors. Also for this reason, all shellac nail polish (even clear colorless ones) are sold in opaque bottles. Definitely don’t want the polish polymerizing during transit…
Many have asked about the safety of being exposed to UV light. You’re probably exposed to more total UV light during your trip to and from the nail salon than you are during the curing process. Still, broken lamps or extreme over exposure will eventually lead to sunburn.
Bonus: What about Crackle Nail Polish?
As I was presenting this to my organic chemistry class, someone asked about crackle nail polish. How does that work? Going back to nitrocellulose nail polish, to get a nice, smooth, even film the solvent needs to evaporate very slowly. Butyl acetate or ethyl acetate are common solvents for typical nail polish. If you use a solvent that evaporates too quickly, the remaining solvent tends to pool together (bringing the pigment along with it) and leads to a cracked, uneven film finish. This is, technically, a design flaw. Unless you take your design flaw to your boss and say, “Hey! Let’s create a market where there wasn’t one before and market my design flaw as a cool, must-have accessory for nail polish!” Then you get a raise and a promotion.
The solvent in crackle nail polish is typically ethanol, which evaporates more quickly than the acetate solvents used in typical nail polish. If you apply the ethanol-based crackle coat over top of a dried layer of typical nail polish, a cracked, two-toned effect occurs, leading to the crackle finish.
- This page has a really nice, really detailed glimpse into the chemistry and science of manicures
- Here’s a truncated ingredient list for the Shellac system, for you structure junkies out there
- Tetrahydrofurfuryl methacrylate
- PPG5 methacrylate
- Cellulose acetate butyrate
- Acrylates copolymer
- DI-HEMA trimethylhexyl carbamate (close enough, though the structure’s technically wrong)
- Hydroxypropyl methacrylate
- Isopropylidenediphenol (interesting they list it that way instead of its more common name: BPA…)
- Here are the product profile papers for the Base Coat, the Color Coat, and the Top Coat
- Fun fact: German-Swiss chemist Christian Schönbein (1799-1868) sometimes worked on his chemistry at home, against his wife’s wishes. One day, he spilled some nitric acid/sulfuric acid. He mopped it up with his wife’s cotton apron and hung it up to dry. Whereupon it spontaneously ignited and burst into flames – with almost no smoke and no ash. He is credited for accidentally discovering nitrocellulose. At the time (around 1845), when ordinary gunpowder exploded, it produced a thick, black smoke. This is bad in battle for at least three reasons: 1) it gives away your position, 2) it obscures your view of the battlefield, and 3) the soot clogs up the weapon. So the accidentally-discovered nitrocellulose was developed as a useful alternative to gunpowder.
- Since nitrocellulose produces such a nice film (made flexible with plasticizers), it was used as the film base for early motion pictures. It worked pretty well… except for the whole catching fire really easily part. Projector rooms were often fitted with several layers of fire protection, sometimes directly built in to the projector itself, to prevent the inevitable fire from spreading to the auditorium and causing massive damage and casualties.
- Nitrocellulose cinema films were used until the early 1950s when other, safer films (like cellulose triacetate) replaced nitrocellulose film. Around the same time, chemists were discovering that nitrocellulose, dissolved together with some other resins and plasticizers, made nice, high-gloss coatings on things like metals and wood. This allowed nitrocellulose to be used as a lacquer, and many automobiles and instruments were lacquered with fast drying nitrocellulose lacquer. Billiard balls sometimes were also lacquered with nitrocellulose lacquer… except that the high-impact collision with other billiard balls would sometimes cause the billiard balls to explode. In fact, many of these lacquers and early nail polishes got their nitrocellulose from dissolving old nitrocellulose cinema filmstrips.
- Hydroxyethyl methacrylate is a common monomer (the shellac system quotes hydroxypropyl methacrylate in their ingredients list, though). When polymerized by itself, poly(hydroxyethyl methacrylate) is hydrophobic, but is capable of absorbing from 10 – 600% water relative to dry weight and will swell. When the properties were properly refined, the polymer was one of the first materials used to manufacture flexible contact lenses.
So there you go! Enjoy you’re next trip to the salon and see if your manicurist will talk chemistry with you :) If you have ideas for future How Does It Work entries, let me know!