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

108 JOURNAL OF COSMETIC SCIENCE surfaces (especially low-energy ones such as nails), crawling, cratering, and pigment wetting. While the surface tension can be readily reduced by the addition of cosolvents and surfactants, the addition of the former have unfavorable VOC implications as previously noted. The addition of a surfactant to aid surface wetting and/or to help stabilize the dispersed component in an aqueous system (polymer and pigments) can itself cause difficulties because such additives are, by necessity, surface-active and may cause foam and pinhole formation by stabilizing the water-air interface of entrained air bubbles by increasing surface elasticity and/or surface viscosity. The adhesion strength of the aqueous coating may be adversely effected. It is generally more difficult to achieve a good pigment dispersion in an aqueous nail polish system than in the case of a corresponding solvent system (8). Good pigment dispersion is necessary for a number of reasons, e.g., achieving good gloss, good opacity and hiding powder, and obtaining good barrier properties when pigmenting barrier polymers. Thickeners can act as flocculants for dispersed pigment particles, and in some instances, for latex particles. Ionic impurities may also act as fiocculants. The freezing point of water is high compared to that of most organic solvents therefore, it is frequently necessary to add freezing point depressants to protect the formulations from freezing. This is by no means easy to achieve during transportation and storage. It is possible to have a colloid-stable formulation during several freeze-thaw cycles, but achieving such colloid stability for all the dispersed components in fully formulated nail polish is very difficult. Finally, bacterial growth requires an aqueous environment. It is frequently necessary to add a preservative to an aqueous formulation to prevent bacterial growth in the wet formulation, thus further increasing the number of components. CONCLUSION Water-based formulations have improved over the past five years. Further advancements are needed for them to succeed against nitrocellulose-based products. In order to over- come the problems associated with water-based nail polish, it is necessary to exert extreme care in the selection of additives. The pigment dispersant has a vital role to play in protecting pigments from fiocculation, and the choice of the pigment dispersant type and concentration is a crucial consideration if good pigment dispersion is to be achieved. The actual choice of pigment dispersant(s) is in practice determined not only by pigment dispersion and stabilization considerations, but in many instances by the need to avoid the use of dispersant(s), which impart water sensitivity to the final dry nail polish. There is an important need for surfactants to stabilize the latex particles and for dispersants to disperse and stabilize the pigments present in a water-based system, but these additives can cause foam and pinhole formation. Additionally, it is frequently necessary to apply high-shear mixing techniques to attain the desired degree of pigment dispersion, which by nature greatly increases the tendency to give foaming. Foam can result in bubbles in the final film, which in turn can reduce the effectiveness and the wear. The most effective way of avoiding the initial creation of foam is to use an effective antifoam, to minimize shear processes during formulation. Typically polysiloxanes, silanes, metallic soaps, am- ides, fatty acids, and their esters have been used as defoamers. One should search for the availability of certain surfactants that are capable of lowering the surface tension of