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

WATER-BASED NAIL POLISH 109 aqueous systems without significantly increasing the tendency to foam. As previously mentioned, drying difficulties at low temperatures and/or high relative humidities is an inherent problem. It is therefore far less easy to control the drying rate for a water-based system than for a solvent-borne one where there is a large choice of volatile solvents. Nevertheless, cosolvents such as propylene glycol and volatile silicones can be used to provide some control over evaporation rate and film-formation temperature. It is clear from the foregoing discussion that the formulation of aqueous nail polish is generally more complex than is the case for solvent systems. This very complexity, i.e., the need for more formulation additives, can itself be regarded as a disadvantage. Additives introduced to achieve one effect can be damaging in other respects (e.g., wetting versus foaming). Developments that allow formulation simplification, such as low-foaming dispersions and latexes with good wetting ability and film-forming char- acteristics, would be beneficial. REFERENCES (1) J. M. Friel, E•opean patent 0,466,409 (1992). (2) C. A. Finch, Chemistry and Technology q[ Water Solule Polymers, (Plenum Press, New York, 1983). (3) J. C. Padget, in Additives for Water Soluble Coatings, D. R. Karsa, Ed. (Royal Society of Chemistry, 1988), Special publ. 76, p. 1. (4) K. Kendall and J. C. Padget, J. Adhesion and Adhesives, 7, 142, (1982). (5) J. E. Glass, Ed., The Inj7•ence of A•sociative Thickeners and Rheology on Coatings Performance, (North Dakota State University, Fargo, 1983). (6) P. W. Dillon, J. Coatings Technol., 49(634), 38, (1977). (7) A. L. Rocklin, J. Coatings Technol., 58(732), 61, (1986). (8) G. D. Parfitt, Dispersions of Powders in Liquids, (Applied Science Publishers, 1981).