A plasticiser is an ingredient in nail polish that softens the nitrocellulose film so the dried lacquer bends with the nail and resists chipping, where the film former on its own would set hard and brittle.
Also known as: plasticizer, nail polish plasticiser, weekmaker, acetyl tributyl citrate
A nitrocellulose film is hard and glossy but brittle. A plasticiser is a small molecule mixed into the formula that slips between the polymer chains and lets them move, so the dried coat flexes when the nail bends and does not crack. Without one, polish would chip within a day. Classic solvent polish combines nitrocellulose with plasticisers, resins, pigments and solvents (Personal Care Magazine).
Several plasticisers have a history. Dibutyl phthalate (DBP) was common until concerns about reproductive toxicity; it has been banned in EU cosmetics since 2004. Camphor is a plasticiser that can irritate. Triphenyl phosphate (TPHP) is a plasticiser under scrutiny as a suspected endocrine disruptor and is the ingredient many free-from lists miss (Label Lookout). Modern formulas use citrate esters such as acetyl tributyl citrate, which also appear in breathable polishes (Halalworthy).
Water-based polish needs plasticisers too. It uses permanent plasticisers plus fugitive coalescents, solvents that soften the acrylic particles so they fuse into a film and then evaporate (Personal Care Magazine).
On an ingredient list, look for names ending in citrate, phthalate or phosphate, and for camphor. Under Regulation (EC) 1223/2009, DBP is prohibited, so a 3-free claim in Europe describes a legal minimum, and the plasticiser in an EU polish is a citrate or another permitted substance.