Painting your nails with nail polish might not seem like a particularly complex chemical process, but there’s much more to it than meets the eye. Polymerisation, thixotropic agents, solvents and thermochromism are all terms you might expect to hear more frequently in a lab than in a nail salon, but they can all crop up in relation to nail polish. In this graphic and article, we take a look at the different chemistry that comes together to colour your nails.
Conventional nail polish consists of a polymer, most commonly nitrocellulose, dissolved in a solvent, usually ethyl acetate or butyl acetate. When it is applied the solvent evaporates, leaving the polymer to form a film on the nail. Adhesive polymer resins that are also contained within the formulation help the polymer film to stick to the nail. These so-called film modifiers also impart a glossiness to the polymer finish.
This conventional nail polish is not the only option, however. Gel nail polish is an alternative formulation which consists of methacrylate compounds and photoinitiating compounds such as benzoyl peroxide. Unlike conventional nail polish, these mixtures aren’t simply applied and left to dry. Instead they are applied in layers which are exposed to ultraviolet light; this kicks off a polymerisation process which solidifies the polish. There’s a good more detailed explanation of how this happens here, as well as an explanation for a particular example in this article by Tom Husband.
In both conventional and gel nail polishes, plasticisers are also used. These are compounds added to stop the polish from easily cracking or chipping. They remain behind when the solvents evaporate or when the polish is cured with UV light; in fact they are used in a wide range of plastics, not just polishes, and help to add flexibility.
The plasticisers used in nail polishes are not without a degree of controversy. Dibutyl phthalate (DBP) was a widely used plasticiser but its use has been banned in the EU since 2004 due to concerns over the possibility of of it interfering with the human hormone system. It has also been phased out in the US, and other plasticisers have taken its place, including camphor, glyceryl tribenzoate, and triphenylphosphate (TPPP). The latter, increasingly used as a replacement for DBP, has also been linked to concerns regarding hormone disruption, showing that finding safe replacements for banned ingredients in cosmetics is not always easy.
The key ingredients in nail polishes are the compounds that give them their colours. These tend to be pigments, either inorganic or organic (carbon-based). Inorganic pigments used include chromium oxide for greens, iron oxide for reds and oranges, and ferric ferrocyanide for blues. Organic pigments are similar to those used in food colourings, and come in a range of colours.
More complex coloured effects are also possible. Pearlescence can be achieved through the use of finely ground titanium dioxide or mica mixed in with the nail polish, and small pieces of glitter can also be included. Thickeners, such as stearalkonium hectorite, are added to keep pigments and other additives suspended in the polish.
Thermochromic (temperature-sensitive) and photochromic (light-sensitive) nail polishes are also possible. Thermochromic polishes use compounds called leucodyes contained within microcapsules. These microcapsules also contain a low melting point solvent and an acid. When the temperature is low enough, the dye and the acid molecules are in close proximity, allowing transfer of hydrogen atoms between the molecules and leaving the dye in its coloured form. As temperature increases, the solvent melts, and the molecules move away from each other; with hydrogen transfer no longer occurring, the dye changes to a colourless form.
Photochromic polishes use light-sensitive compounds which experience a structural change when exposed to sunlight. Examples of compounds used include spiropyrans and spirooxazines. The structural change on the absorption of UV light changes the absorption of the compound, causing its colour to change.
UV light can also be a problem for nail polishes – over time exposure to sunlight can cause the colour of the polish to fade. To avoid this, additives such as benzophenone-1 are added into the mixture. These additives absorb UV light and prevent it from bleaching the coloured pigments in the polish.
So there you have it – far from being a simple coloured varnish, nail polish actually touches on a plethora of chemistry topics, and they’re all vital for the final product to be as effective as possible!
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References & Further Reading
- Polished polymers – T Husband, The Mole
- What’s that Stuff? Nail polish – C Drahl, C&EN
- How nail polish works – C Agapakis
- Handbook of cosmetic science and technology – A O Barel and others
- Technology-driven trends in nail polish colour and texture – F C Pagono