j. Soc. Cosmet. Chem., 43, 331-337 (November/December 1992) Advances in nail enamel technology MITCHELL L. SCHLOSSMAN and ERIC WIMMER, Tevco, Inc., South Plainfield, NJ 07080 (M.L.S.), and SNPE, Bergerac, France (E.W.). Received May 15, 1992. Synopsis Nail enamel is a very esoteric area of cosmetic formulating, related for the most part to paint and lacquer technology. Non-settling or suspension nail enamel was and is a development in cosmetic formulation that represents an excellent example of the application of rheological principles. Suspension nail enamel has undergone many changes from the formulations of the last 30 years. This paper will review some of the past formulations, manufacturing procedures, and evaluation methods, and attempt to discuss the technology of the 1990s. Packaging innovations and advances in nail care lacquer areas such as base and top coat, hardness and strengtheners will be outlined. The presentation will address environmental and governmental issues and the impact they may have on future nail enamel formulations, namely toluene-free, formaldehyde-free, and water-based systems. Nail enamel is a very esoteric area of cosmetic formulating, related for the most part to paint and lacquer technology. There is a remarkable amount of technology packed into that little bottle. Nail enamel is only nominally related to the cosmetic industry in meeting that industry's unique requirements as to governmental regulations and the strange requirement of having a paint product look nice in a tiny glass bottle. Nail enamel formulations are virtually anhydrous, and inherently bactericidal. The manu- facturing process is capital intensive, and involves handling large quantities of hazardous raw materials that are unfamiliar to most cosmetic operations. Nail enamel plants must meet the strict electrical codes for explosion-proof wiring, and the buildings must be designed for explosion venting. For these reasons, cosmetic companies generally do not manufacture their own nail enamel. The vast majority of nail enamel is based on nitrocellulose the structural unit for cellulose is shown in Figure 1. The most important attributes of a nitrocellulose lacquer are that it dries quickly and has a high gloss. Various materials have been suggested as alternative film formers, but none have so far matched nitrocellulose in terms of hard- ness, toughness, and resistance to abrasion. Furthermore, nitrocellulose does not oxidize and polymerize, and it produces films that are waterproof and stable to atmospheric conditions. Finally, nitrocellulose is an extremely good polymer for the wetting of pigments. This is important in effecting good color dispersion. A small pigment load is able to produce clean, bright color with good opacity. Several primary film formers for nail enamel have been tried, namely polyurethanes, polyamides, polyesters, and cel- 331

332 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS H H OH CH 2 CH OH H H 0 H H OH H CH20H H OH Figure 1. Structural unit of cellulose. lulosics such as cellulose acetate isobutyrate (CAB), cellulose acetate, cellulose acetate propionate (CAP), and ethyl cellulose (EC). Generally the aforementioned polymers exhibited poor adhesion to the nails or soft films. The advantages of these replacements are low flammability, no discoloration, and solubility in low-cost solvents'. Another development was a nitrocellulose-free nail lacquer (1) comprising copolymers of hydro- phobic and hydrophilic monomers such as isobornyl methacrylate and diacetone acry- lamide, respectively, and applied to nails. A waterproof film having a high gloss was provided. A more recent patent discloses a nail lacquer free from water and nitrocellu- lose, containing alkyl(meth)acrylate and hydroxyalkyl(meth)acrylate in solution (2). A nail lacquer having good gloss and adhesion is claimed. Secondary resins are added to nitrocellulose lacquers to improve application, wear, and gloss properties. The most widely used are the toluene sulfonamide formaldehyde (TSFR) type, as shown in Figures 2 and 3. TSFR is a low-molecular-weight material, used as a filling resin in nail lacquers for improving the quality of the film (adhesion, gloss, flexibility...). There has been some controversy about the use of TSFR because there is a small amount of residual formaldehyde. TSFR has been identified as a potential sensitizer for a small number of nail lacquer users. Polyesters (3), nylon, acrylic ester oligomers, alkyds, etc., have been used as replacements (4). None of the aforementioned have replaced TSFR resin as a secondary resin for nitrocellulose lacquers. In an attempt to identify in the TSFR molecule what kind of function is responsible for its attributes, a study was conducted (5), the purpose of which was to help design a new improved resin for nail enamel. In summary, the analysis of TSFR showed clearly that its commonly accepted representation of a polymeric structure is at best an oversimplification of what is a mixture of oligomers having functional end groups. From that study, the synthesis of a new family of resins, the aryl alkyl sulfonyl urethanes (6), were developed. A structural formula is shown in Figure 4. Another secondary resin derived from para toluene sulfonamide is toluene sulfonamide epoxy resin (7). Plasticizers are used to make the polymer film compatible with the nail. They make the film more flexible. Dibutyl phthalate and camphor are the most widely used plasticizers for nitrocellulose films. There has been a question as to whether or not phthalates act as sensitizers for some nail enamel wearers. A recent patent discusses nail enamels con- taining glycerol, glycol, or citrate esters (9). Glycerol tribenzoate is an example of a