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Nail polish is designed to improve the appearance and condition of nails. It also can protect nails from breakage.
What nail polish does
In its simplest form, a nail polish is a liquid solution of pigments, solvents and film formers. It typically comes in a bottle with a brush applicator. The solution is applied to the nail and then allowed to dry at which point it becomes a smooth, hard, colored film.
How nail polish works
Nail polishes contain a number of ingredients including pigments, film formers, plasticizers, resins, solvents and other additives.
There are two types of pigments used in nail polishes. Mineral pigments like titanium dioxide and colored iron oxides and organic pigments like red #6 and yellow #5. In the US the pigments that are allowed for use in nail polish are strictly limited.
To make the nail polish look right, a film former is needed. The ingredient most frequently used is nitrocellulose. Since it is a dangerous compound to work with there are a limited number of manufacturers who can produce nail polish.
Resins like shellac and acrylic polymers are added to improve the properties of the nitrocellulose film. This makes the film tougher and helps the nail polish last longer. Plasticizers are needed to improve the film flexibility. Camphor and dibutyl phthalate are most often used.
Solvents like alcohols, esters, and ketones are used to help the product spread and dry quickly. Finally, additives like viscosity modifiers and UV protectors may be included.
Nail polish formulation
Here is a standard nail polish formulation (click to enlarge)