j. Cosmet. sci., 50, 105-109 (March/April 1999) Additives for water-based nail polish MITCHELL SCHLOSSMAN, Kobo Products and Presperse, Inc., West Orange, NJ. Accepted for publication March 31, 1999. Synopsis Water-based nail polish systems are reviewed. These may be described as complex mixed colloidal disper- sions of polymer(s) and pigment in water. These frequently also contain water-soluble polymers. Polymers are used as both binders and additives for water-based coatings. Typical of their role as additives is their use as thickeners and pigment dispersants. The many polymer types used are discussed in a systematic manner since it can be vital to the formulator to have a clear understanding of the main features and essential differences between the various polymer types in use. INTRODUCTION There are four components of organic coatings: binders, solvents, pigments, and addi- tives. Each component serves a different function: Binder: Polymer (resin) and cross-linker both react to form cross-linked networks. Solvent: To provide proper application and alter certain properties. Pigment: To provide color and enhance other properties. Additives: To control application and protective properties. Depending upon the content and binder/solvent make-up, nail polishes are classified into the following three categories: (a) Solvent-borne: Low solids (binder) content high solvent content high volatile organic content. (b) Water-borne: Binder/cross-linker dispersed in water low volatile organic con- tent. (c) Latex: Synthesized polymer particles suspended in water low volatile organic content. DISCUSSION One fundamental difference between a nail polish based on aqueous polymer dispersions and those based on solutions of polymers in non-aqueous solvents lies in the mechanism of the film formation process which occurs as the volatile continuous phase is lost from a freshly coated nail. The particle coalescence process that occurs in an aqueous disper- 105

106 JOURNAL OF COSMETIC SCIENCE sion will be discussed because it has a major bearing on the performance of the final system, and because many of the formulation additives (e.g., cosolvents, surfactants, plasticizers) and many of the polymer binder variables (e.g., Tg, particle size, stabili- zation) profoundly affect this process. The question plaguing most cosmetic chemists and marketing personnel is what can be done with a water-base nail polish to make it more durable? The key to its durability is to have the correct polymer properties such as gloss, flexibility, hardness, and adhe- sion. Next is to have these properties in a stable form. The choice of resin backbone is therefore a critical first step. Additionally, the molecular weight should be large enough to maximize chain entanglement. Finally, the cross-linking of the film will also promote strength, abrasion resistance, and general durability and stain resistance. Water-base nail polish based on latex systems consists of a resin emulsion in water to which pigment, thickeners, freeze-thaw stabilizers, coalescing aids, and various other products have been added. The properties of the dried film depend to a large extent on the properties of the resin film that acts as a binder for the coating ingredients. In order for a continuous resin film to be formed from the emulsion, after the water has evaporated, it is necessary for the resin emulsion particles to coalesce. This ability is related to the glass transition temperatures, Tg, of the polymer: a soft resin with a low Tg will coalesce into a continuous film at a lower temperature than will a harder, high-Tg polymer. Generally, 1ow-Tg polymers of the type used in resin emulsion coatings are softer and have poorer physical properties than high-Tg polymers (1). Coalescing aids act to reduce the Tg of the emulsion polymer so that efficient film formation can occur at the drying tempera- ture after the water has evaporated and the film has coalesced. The aid should evaporate from the resin film so that the Tg increases and improved physical properties can develop. Coalescing aids are usually very slow-evaporating solvents such as glycol ethers, glycol ether esters, and ester-alcohols. They soften the emulsion polymer particles, allowing them to fuse together. In addition to lowering the minimum film formation temperature of the water-based enamel, a coalescing aid improves weatherability, du- rability, and color development. The main types of polymers used in water-based nail polish formulations are aqueous polymer dispersions and water-soluble polymers. Aqueous dispersions of water-insoluble polymers are frequently prepared by aqueous emulsion polymerization, in which case they are generally described as emulsion polymers or latexes. Such polymers are mainly based on water-insoluble comonomers such as methyl methacrylate, butyl acrylate, styrene, and vinylidene chloride these polymers have a low degree of water solubility. Water-soluble comonomers such as acrylic acid and methacrylic acid are often present at low concentrations for reason of adhesion, colloidal stability, or cross-linkability. Ther- mosetting acrylic resins are those that contain functional groups capable of chemically reacting with a cross-linking agent that is incorporated into the formulation. These systems have an advantage over a thermoplastic system because the build-up of mo- lecular weight is achieved by cross-linking reactions after application. Thus it is possible to apply these resins at a fairly low molecular weight and still obtain a nail polish with good film properties. Water-soluble polymers that give genuine water solutions over a broad pH range are rarely used as binders for aqueous nail polish because of the obvious problem of water sensitivity of the final product. Even after cross-linking, they are rarely successful in eliminating this problem. Instead, they are often used as additives, chiefly thickeners, colloidal stabilizers, and pigment dispersants. Main examples arc homopoly-