A film former is the ingredient in nail polish or liquid eyeliner that turns from a liquid into a solid, continuous coating as the solvent evaporates, and it decides how hard, glossy and long-wearing the finished coat is.
Also known as: film-forming polymer, film-forming agent, filmvormer, acrylates copolymer
Every product that dries to a coat has a film former. In classic nail polish it is nitrocellulose, dissolved in butyl acetate and ethyl acetate. As the solvents leave, the polymer chains pack together into a hard, glossy film that grips the nail plate. The film dries from the bottom up in a solvent polish and from the top down in a water-based one (Personal Care Magazine).
Water-based polish uses an acrylic polymer dispersion as its film former. The particles are suspended in water and fuse into a film as the water evaporates, helped by coalescing solvents. It avoids flammable solvents but dries more slowly and has historically worn less well (Personal Care Magazine).
Secondary film formers and resins improve adhesion and gloss. Tosylamide/formaldehyde resin and ethyl tosylamide did that job in older formulas; both appear on 5-free and 7-free lists, and ethyl tosylamide is restricted in the EU (Label Lookout).
Liquid eyeliner works the same way with different chemistry. Pigment is dispersed in water with film-forming polymers such as acrylates copolymer or styrene/acrylates copolymer that dry to a flexible, water-resistant line. A higher film-former load resists transfer better but flakes more and is harder to remove (Zena Makeup). A plasticiser is added in both cases so the film bends with the nail or the lid.