NAIL ENAMEL TECHNOLOGY 33 plasticizer. No one plasticizer possesses all the desirable properties necessary to obtain permanent extensibility and flexibility in the resulting films. Since nitrocellulose is far too brittle to be used in nail enamels alone, it is necessary to incorporate plasticizers. Plasticizers act by modifying the Van der Waals forces between the molecular chains of the polymer. Because of their high boiling points, they do not volatilize when the nitrocellulose film dries, but remain in it and render it pliable. In addition, small amounts may improve gloss and flow properties. Commonly used solvent-type plasticizers are dibutyl phthalate (bp 340 ø C), dioctyl phthalate, diphenyl phthalate, camphor, dibutoxy ethyl phthalate, tricresyl phosphate (241-255 ø C), triphenyl phos- phate, and citrate plasticizers. The phthalate esters are widely used because they act as solvents for many modifying resins. Non-solvent types such as castor oil act more like softeners or lubricants rather than true plasticizers and are always used in conjunction with the solvent-types. Colorants for nail enamel are usually confined to those non-bleeding in lacquer solvents. All must be FDA certified or approved inorganics must have low heavy metal content. Modern creme nail enamels contain insoluble colors together with small proportions of titanium dioxide. The use of soluble dyes have been limited to tinting of clears for the most part. The most widely used pigments in opaque enamels are red organic pigments, D & C Red if4,//7,//9,//10, #30, #33, and //34 Lakes, D & C Yellow//5 Lakes, titanium dioxide, iron oxides used to produce brown and tan shades and iron black and iron blue. Basic Violet #1 and #3, D & C Red//17, D & C Violet//2, and D & C Red//19 dyes are the most widely used soluble dyes for transparent systems. The colors selected for nail enamels need to be relatively light fast. Organic colors are responsible for producing the bright clear shades. Inorganics are duller, but have good light fastness. Pigments with relatively high specific gravities such as titanium dioxide and the iron oxides are most widely used in creme shades and most responsible for pigment settling problems. The degree of dispersion of the pigment has a major influence on the gloss of the enamel, its smoothness, and the ability of the pigment to remain in suspension. Nail enamel colors are usually processed on two-roll mill equipment. The pigment is mixed with nitrocellulose and plasticizer and passed through a two-roll mill. The final dispersion form of the color is that of a hard plastic chip which at any convenient time may be dissolved in a clear base or solvents to serve as a concentrate from which a range of nail enamel shades may be formulated. The total pigment-color concentration in nail lacquer varies, but usually contains no more than 3-5%. This level is needed for gloss and wear resistance. Nacreous pigments are important colorants for nail enamel. Both natural pearl essence or guanine (2-amino-6-hydroxy purine) and synthetic grades such as bismuth oxychloride and micas are used. They are supplied in nitrocellulose/butyl acetate lacquer at 11 and 22% concentrations for the guanine and 11 and 25% concentrations for the bismuth oxychloride. The guanine is obtained from the scales and bodies of various fish. The brilliance and luster of the guanine crystals take place during the coating of the nail surface and the drying of the film. There are two types of guanine used: plate and needle pearl. The plate pearl is brighter, more brilliant in the bottle and on the nail, being slightly more transparent on the nail. The needle type has a soft luster and is slightly better in coverage. Needle pearl is used primarily to formulate creme shades. Synthetic pearl such as bismuth oxychloride has a bright, mirror-like effect. It produces a very high luster and brilliance at a much lower cost than guanine. It is denser and is more difficult to suspend than guanine. Bismuth

34 .JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS oxychloride tends to have poor light stability and U-V absorbers are usually included in nail lacquer in which it is contained. Titanium coated mica is the second type and has a softer luster, similar to needle-type guanine. They are available in various colors. In general, opaque pigments should be used sparingly in pearlescent nail enamels. The viscosity and thixotropy of pearlescent nail enamels are usually higher than for creme types. Pearl enamels run approximately 400-600 cps at 60 RPM, //3 spindle, at 25 ø C (Brookfield Viscometer, Brookfield Engineering Laboratories, Inc., 240 Cushing Street, Stoughton, MA 02072) while creme shades run approximately 375-500 cps. In general, pearl essence appears to improve wear resistance of lacquers and guanine has been noted to have slightly better adhesion than the synthetic pearls. Settling, plating on the bottle, and difficulties in re-dispersion of pearlescent pigments have been aided by placing two stainless steel balls in each bottle to allow for ease of re-dispersion after shaking. All pigments and colorants used in enamels have specific gravities higher than that of the lacquer itself. As a result, these pigments and colorants settle in the bottle. Production of nail lacquer which is thixotropic in nature has slowed the precipitation of the pigments. Most "nonsettling" nail enamels today are produced with cation modified montmorillonites as fellants or suspending agents. The term nonsettling is a comparative term and the degree of settling of pigments and colorants depends, for the most part, upon the manufacturer and shade. The cation modified montmorillonite is usually introduced into the nail enamel formulation in the form of nitrocellulose chips analogous to the color pigment chips previously discussed, although a gelled nitrocellulose paste may also be utilized. Ranges from 0.75-3% of the montmorillonite, or %tearalkonium hectorite" as it is referred to, may be added to produce the proper suspension system. Occasionally, inorganic or organic acids have been used as additives to alter thixotropy and thus produce a shorter recovery for this '•nonsettling" system. When utilized, these swelling agents must be added judiciously. The aforementioned pigment suspending system is described in a patent granted in 1975 to Busch (Chesebrough-Pond's, Inc.) (5) Most recently, the possible use of microcrystal- line cellulose/gum tragacanth mixtures as a potential suspending agent in nail enamel has been studied. In the manufacture of nail enamel, a mechanical stirrer is used and in practice the diluent is added first, followed by solvent, plasticizer, and resin. Agitation is continued in stainless steel or aluminum vessels, until solution is complete. The clear lacquer is then passed through a filter press to remove any insolubles. This will improve its clarity and brilliance, and ensure a glossier film. Pigmented enamels are prepared by adding under agitation the appropriate amount of pigment paste or concentrate, to the clear lacquer in the mixing tank and color matched to a standard lab master. Laboratory evaluations of formulations will involve the examination of the following properties of nail enamel: gloss, flow characteristics, compatibility of constituents during drying, drying rate, and wear resistance. To obtain consistent results, it is necessary to follow exact quality control procedures. The viscosity of enamel is determined at two different spindle speeds, to determine the thixotropic nature of the lacquer. The reading of the lower speed should be at least 150 cps higher than at the higher initial speed. Standardization of this procedure usually involves vigorously shaking the enamel for a predetermined number of times before running viscosity-thixotropy relationships. This property enables the lacquer to properly suspend pigments and pearlescence and is also an important factor for brushability.