Nail polishes: substrate characteristics, physicochemistry and formulation

As early as 3,000 BC, the Babylonians painted their nails with kohl, while the Chinese, especially the royal family, covered them with a mixture of gum arabic, egg white, gelatin, beeswax and sometimes plant extracts. The art of manicure came back into vogue in 18th-century Europe, where nails were filed, polished and even bleached with vinegar or lemon juice, then colored with fragrant red oil in the early 19th century. Gradually, polishing powders and colored creams were used to obtain a smooth, shiny surface. Since then, nail polishes have undergone a singular evolution impacted by three factors: the availability of new raw materials, fashion and, more recently, regulations. Color pigments, waxes and resins were successively added to gloss powders to produce pasty varnishes, solvents to obtain a liquid formulation and then a film after application and evaporation, film-forming agents and then rheological agents. Nail varnishes gradually became complex formulations, on the borderline between cosmetics and coatings, making up the nail by forming a supple, glossy film on its surface. After the First World War, the use of nitrocellulose, with its remarkable film-forming properties, and covering pigments such as titanium dioxide, made it possible to obtain the first quality varnishes similar to those of today. Nail varnish became a common make-up accessory, often associated with lipstick, whose functional properties were soon imposed by consumers. Ideally, nail polish should be easy to apply, quick-drying and odorless. However, it's the properties of the film that are most important; in particular, it must be glossy, have good staying power, be water-resistant and color-stable.

The need to obtain a product that respects the environment and human health, and the numerous technical requirements have made the formulator's task even more complex. All the more so since the 1976 and 2009 cosmetics directives, among others, have reduced the number of raw materials available. Nevertheless, the nail polish market, estimated at over 5 billion euros in 2013, with growth of 36% in three years, continues to motivate manufacturers to innovate in one of the most buoyant cosmetics markets.

In order to address these complex formulations and understand their operating mechanisms, the anatomy, composition and physicochemistry of the nail are first described. The composition and manufacturing process of solvent-based nail polishes are then detailed. Nail varnishes are systematically tested and evaluated in liquid form or after film application, using performance tests and according to the regulations commented on in this article. The physicochemical mechanisms involved are also described. Finally, biobased varnishes and alternatives (aqueous, semi-permanent) to solvent-based varnishes...