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-

WATER-BASED NAIL POLISH 107 mers of ethylene oxide, acrylic acid, methacrylic acid, polyvinyl alcohol, and various cellulosics (2). The key event in the formation of a film when an aqueous polymer dispersion is cast on a nail is the process of latex particle coalescence. The film formation process is important in that incomplete film formation can give rise to problems of water sensitivity in the final coating. There are a number of factors that favor the process of particle coalescence and a number that oppose the process. Factors encouraging coales- cence include minimizing surface free energy by minimizing surface area, capillary forces arising from the evaporation of water, and the attractive forces between particles. Those resisting coalescence are viscous deformation of the polymer and the elastic deformation of the polymer. The process proceeds as a result of the balance between these opposing effects, and the final dry film represents an equilibrium between these effects (3,4). Water-based nail polish has a number of special difficulties associated with it. Fre- quently the purpose of a given additive is to overcome or alleviate a given difficulty. It is instructive to consider some of the difficulties and to consider the role of the additives. The rheology of aqueous polymer dispersions is less favorable for application than that of more NewtonJan polymer solutions. The viscosity drops very quickly with increasing shear rate, and consequently low shear rate viscosity tends to be too high to allow good flow and leveling, and the high shear rate viscosity tends to be too low to give good brushability. Addition of water-soluble polymers (thickeners) is vital to increase the high shear rate viscosity to an acceptable level for application of the nail polish and the low shear rate viscosity necessary to effect the flow and leveling characteristics (5). The advent of associative thickeners has been a major advancement since such hydrophobi- cally modified water-soluble polymers provide the opportunity to independently adjust the high shear rate viscosity and the low shear rate viscosity. This control results from the fact that the addition of a cosolvent or a low-molecular-weight surfactant to a system comprising polymer dispersion and hydrophobically modified water-soluble polymer can reduce the viscosity by reducing the magnitude of the hydrophobic association between the thickener hydrophobes and/or between these hydrophobes and the latex particle surface. Common to all associative thickeners is the presence of associating hydrophobic groups that are often located at the end of the polyethylene oxide side groups or end groups. These groups are commonly Cs-C24 hydrocarbon groups. The main examples of water-soluble polymers are homopolymers or copolymers of ethylene oxide, acrylic acid, methacrylic acid, N-vinyl pyrollidene, polyvinyl alcohol, and acrylamide. Although the advantages of aqueous nail polishes are well recognized, there are several other difficulties associated with their formulation. An obvious disadvantage is that water has a high latent heat of evaporation (approx. 540 cal/g) therefore, at high temperature, a high-energy input is required to facilitate drying, while at ambient temperature and/or high relative humidities, drying is slow (6,7). Practically, this slow loss of water gives rise to a number of disadvantages, such as longer drying times. These effects can be alleviated by the use of selected volatile cosolvents. As the principal driving force towards the use of aqueous nail polish is an environmental one, there are strong pressures to either remove the cosolvent altogether or in some instances to move towards more environmentally acceptable cosolvents. Reduction of cosolvent concentra- tion or complete removal may become an insurmountable problem because of the range of functions performed by the cosolvent in a typical aqueous nail coating formulation. A significant problem encountered is that the surface tension of water (approx. 72 dynes/cm) is very high, leading to a number of difficulties, e.g., lack of wetting of most