Abstract:
A photo-reactive nail polish coating composition that cures quickly upon exposure to low levels of ultraviolet radiation. The coating consists of a polymer formed from a composition comprising nitrocellulose, a photo-reactive monomer, a photoinitiator and a reaction inhibitor, resulting in a product compatible with commercially available nail polish of any color and removable by standard acetone-based polish removers. It is also compatible with every-day chores because it is insoluble in water. The composition is not phototoxic and has very low potential for skin irritation or sensitization. The photo-reactive coating is applied over the wet nail polish and then irradiated with safe dosages of ultraviolet radiation, causing the nail polish to dry in a few minutes.

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. application Ser. No. 07/535,596, filed Jun. 11, 1990, now U.S. Pat. No. 5,118,495, which is a continuation-in-part of U.S. application Ser. No. 07/394,200, filed Aug. 15, 1989, now U.S. Pat. No. 5,130,551, and of U.S. application Ser. No. 07/235,349, filed Aug. 23, 1988, abandoned. The sole inventor of the present application is a joint inventor of Ser. No. 07/535,596, now U.S. Pat. No. 5,118,495, whose other joint inventors were also joint inventors of Ser. No. 07/394,200, now U.S. Pat. No. 5,130,551, and Ser. No. 07/235,349, abandoned. All applications have been assigned to the same assignee. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention pertains to the general field of rapidly drying coatings for lacquers and enamels applied to human nails. In particular, the invention concerns a new composition and method for its application which provide a smooth, hard and glossy coating that cures in minutes and is easily removable by commercial nail polish removers. 
     2. Background of the Invention 
     Nail polish, lacquers and enamels comprise a class of products regularly used by modern women as part of their beauty care regimen. Enamels are available in a multitude of colors and it is not uncommon for women to strip and reapply nail enamels several times a week in order to match their wardrobe and makeup. This process is time consuming because three to four different layers of coating must be applied and allowed to air dry. Typically, a first layer of colorless base coat is applied, then two layers of color enamel and, finally, a layer of colorless enamel for gloss and protection. During the drying period, which can take as long as several hours, women have to refrain from tasks that might cause them to mar this painstakingly, and sometimes expensively, applied nail polish. Simple operations, such as opening a car door or extracting keys from a purse, can quickly ruin freshly applied fingernail enamel. Thus, in effect, they are prevented from using their hands in any normal everyday activity while the polish is drying. This is especially burdensome for beauty salon customers who need to be able to function normally immediately after receiving a manicure. 
     Realizing the burden that air drying methods put on people with freshly manicured hands, devices have been developed to try and expedite the drying process. See, for example, U.S. Patent No. 2,262,274 to Fingerlin (1941) and U.S. Pat. No. 2,374,472 to Corbett (1945). These devices consist generally of a box-like dryer that blows or circulates hot or cold air on the nail&#39;s surface for a specified period of time. However, these devices can only dry the surface of the top coating on the nail and do not cause any drying underneath. Consequently, additional exposure to air is required to dry the lower coats of nail polish and in the meantime a manicure can be easily ruined if the nail&#39;s surface comes into contact with any hard surface. As a result, nail polish wearers still have to use their hands cautiously for several hours after application in order not to ruin the product of a manicure. 
     U.S. Pat. Nos. 3,896,014 and 3,928,113 to Rosenberg (1975) disclose a process for coating nails comprising the steps of applying a water soluble base coat to the nails, allowing the base coat to dry, then applying a photocurable nail lacquer and curing the lacquer by exposing it to sufficient amounts of radiation. The inventive purpose behind this patent was to try to develop a nail coating that could be removed by water instead of an acetone based commercially available nail polish remover. Accordingly, the nail lacquer was specifically designed for a water soluble base coat, and commercially available nail polishes could not be used in the process. The water soluble base coat that rendered the photocured composition strippable also made the cured film incompatible with daily human functions such as hand and dish washing, bathing, and all other activities involving the immersion of nails in water. 
     U.S. Patent No. 4,596,260 to Giuliano (1986) discloses a process of applying a photocurable coating to an artificial nail tip whereby upon exposure to suitable radiation the coating hardens to give the appearance of a natural nail. As it consists of a polyfunctional polymer to which the monomer is crosslinkable, the photocurable coating is very difficult to remove if applied on top of commonly used nail polishes. 
     Finally, in U.S. Patent No. 4,126,675 (1978), Boulogne et al. teach a nail polish composition including a copolymer resin based on a mixture of methyl methacrylate and hexyl methacrylate. These two substances are mixed separately and caused to polymerize to form a copolymer under specific conditions before they are added to the balance of the ingredients comprising the nail polish. No polymerization occurs after the copolymer resin is mixed with the balance of the ingredients or after the resulting nail polish is applied on the nails of a user. 
     Thus, while these references are relevant to show the general state of the art, they are not directed to the inventive purpose behind the subject invention, which is to rapidly dry a protective coat that can be applied on any commercially available nail polish, lacquer and enamel. Several commercial products have already been introduced that promise a reduction of the drying time of nail coatings. These products include silicone and mineral oils, which provide a slippery surface to the enamel and thus render it non-tacky; but they leave the solvents in place and do not harden the enamel. Also employed are cyclic siloxanes and other low boiling liquids that dry the top surface of the enamel as they evaporate with some of the solvents near the surface. The problem here is that the bulk of the multilayer enamel coating is left laden with solvent and still takes hours to dry and harden. 
     Infrared lamps, which aid the volatilization of the solvents in the enamel by increasing its temperature, reduce the drying time to about 30 minutes. This is still unsatisfactory, though, for today&#39;s growing number of working women who have to juggle their time between a job and housework. Thus, a method that will further reduce the drying time of multilayer enamel coatings is still needed and it could be expected to have great commercial value in the cosmetics industry. 
     The compositions disclosed in copending application Ser. No. 07/535,596 amount to a great improvement toward the solution of the problems mentioned above. One remaining area for improvement is the reduction of the odor that the cured compositions exhibit for a period of time after application and of the yellowing shade that the initially clear coatings tend to assume after several days of wear. The new matter of this continuation-in-part disclosure is directed at compositions that improve these characteristics. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an ultraviolet photo-reactive nail coating composition that utilizes safe dosages of ultraviolet radiation to polymerize monomer substances in the coating. 
     Another object of the invention is to provide a photo-polymerizable nail coating composition that dries over multiple coats of nail enamels within three minutes of exposure to a low intensity ultraviolet lamp with a long wavelength spectral output. 
     A further object is to provide a photo-polymerizable composition that is compatible with the majority of commercially available nail polishes, lacquers and enamels. 
     Another goal of the invention is a photo-polymerizable composition that is essentially odorless and colorless after application. 
     Yet another object is to provide a photo-polymerizable composition that is not phototoxic and with very low skin irritation and sensitizing potential. 
     Finally, a goal of the invention is to provide a photo-polymerizable composition that after curing by ultraviolet light is still easily removable by means of commercially available nail polish removers. 
     Presently, in beauty salons and homes around the world, nail polish is applied in a three step process. First, a base-coat is used to fill ridges in the nail and to prevent the colored polish or enamel, applied in step two, from staining the natural nail. Second, two coats of colored polish or enamel are applied. Two coats are usually used in order to provide an opaque and colorful finish. Third, a clear top coat of lacquer is applied to protect the lower layers, applied in step two, to give the nail a prominent shine and to provide extended wear. 
     The present invention provides a coating for nails that is applied as the top coat in step three. Like present day top coats, this coating is clear, it imparts a prominent shine, and also provides extended wear. Unlike present day top coats, the coating is photo-reactive. Thus, after this coating has been applied as a top coat, the present invention also requires a source of ultraviolet radiation, in safe dosages, to irradiate the nail and cause the top coat to react, resulting in the nail polish or enamel underneath drying within a few minutes. The required apparatus is described in copending application Ser. No. 394,200. 
     The photocuring of the top coat renders the multilayer enamel system dry to the touch immediately after exposure to ultraviolet light, as described in the copending application. The coat is readily resistant to marring by actions such as rubbing with fingertips and hitting by blunt objects. After 5-15 minutes, it is essentially rock-hard, as demonstrated by its resistance to marring when scraped with fingernails. 
     These and other objects, features and advantages of the present invention, as well as details of the preferred embodiment thereof, will be more fully understood from the following detailed description. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Nail enamels are generally composed of resins, pigments, dyes, plasticizers, and solvents. Mixtures of low and medium boiling solvents are used to control the rate of evaporation. A high boiling solvent slows the drying process but produces a glossier film; therefore, the drying time of an enamel depends to a large extent on the solvent system. It is easily obvious to a person skilled in the art that one can speed up the enamel&#39;s drying time by substituting polymerizable liquid monomers for the solvent system (i.e., the liquid monomers act as the solvent system for the solid components of the enamel). The enamel is then dried and hardened by polymerization of the liquid monomers using a catalytic system that is triggered by heat or light. 
     Such a method of nail coating, formulated into enamels with a variety of dyes and pigments, constitutes a new family of nail enamels that could make current products obsolete. The problem with such systems is that the coatings are difficult to remove. U.S. Pat. No. 3,928,113, described above, offered a solution by the use of a water soluble base coat before applying the photocurable nail enamel. As opposed to that method, the coating of this invention is resistant to the usual tasks of dish washing, hand washing, or other chores involving the contact of fingers with hot water. Furthermore, this prior art is different from the present invention in composition, as well as in function or use. U.S. Pat. No. 3,928,113 claims a method of coating human nails whereby a water soluble or swellable base coat is first applied, followed by a photocurable pigmented or unpigmented polymer composition to provide a multilayer nail enamel that is removable when soaked in hot water. The present invention does not utilize a water soluble or swellable basecoat because, as explained above, that approach is not compatible with normal every day tasks that involve soaking the hands in warm or hot water. In addition, the photocurable composition of this prior art also serves as the principal enamel and therefore is pigmented in most cases. The present invention is clear and unpigmented for use as a topcoat on any commercially available nail enamel. Finally, from a compositional point of view, this prior art discloses a polythiol as an essential ingredient in the claimed composition. The present invention does not require a polythiol. 
     U.S. Pat. No. 4,596,260 utilizes known photopolymerizable compositions for adhering preformed artificial nail tips to natural nails. This composition also differs from the present invention not only in function, but in formulation as well. That patent claims a composition wherein a monomer is crosslinkable with the required polymer component which is claimed to be a low molecular weight acrylated urethane oligomer free of reactive isocyanate groups. Such a reactive polymer is not required in the present invention. 
     Furthermore, that patent describes in detail the ultraviolet lamp which is suitable for photocuring according to the claimed method. Although no specification on the lamp intensity is provided, an example is given with a lamp having 100 watt capacity. The present invention utilizes a lamp that is of low intensity (i.e., less than 10 watts). As a result, the lamp used in the present invention is safer and less annoying to the eyes, as claimed in copending application Ser. No. 394,200. 
     In general, the prior art, which include both the patent literature and polymer texts, is rich with references to photocurable compositions. Typically, these compositions include at least one suitable polyfunctional polymer or oligomer, a photoinitiator and a liquid monomer in which the various other ingredients are soluble, the monomer being crosslinkable with the polyfunctional polymer in the presence of actinic radiation (e.g., ultraviolet radiation) to form a plastic film. As is understood in the art, the degree of firmness is in part dependent upon the degree of crosslinking and one skilled in the art understands that varying the ratio of monomer to polymer can provide greater or lesser firmness or rigidity of the product. Polyfunctional monomers are also used in prior art and the ratio of polyfunctional monomer to monofunctional monomer is varied as well to provide greater or lesser firmness or rigidity to the product. Moreover, the reactivity of the functionality itself affects the curing rate and also determines the firmness or rigidity of the product. For example, acrylates are more reactive and form harder coatings than the corresponding methacrylates. 
     On the bases of prior art knowledge, useable photo-reactive compositions for use in conjunction with an ultraviolet rapid-dry device have been developed and are described in the copending application. As mentioned above, for example, it was found that acrylates react faster than methacrylates and result in harder photocured systems, but methacrylates have lower skin irritation and sensitization potential. Accordingly, compositions based on methacrylate functionality alone are preferable and have become the focus of the present research. In the course of further investigation related to the present invention, a further welcomed but unexpected observation was made that a totally methacrylate monomer system, in the presence of nitrocellulose, cures hard within three minutes of exposure to a low intensity, long wave ultraviolet lamp. Also surprisingly, it was found that a methacrylate system consisting of only monofunctional monomers, instead of the difunctional monomers taught by prior art, cured into a hard coat over three layers of nail enamel within three minutes of exposure to the prototype lamp. As expected, such systems have lower skin irritating properties than a similar system comprised of an acrylate monomer. 
     Accordingly, the photo-reactive coating of this invention is comprised of a primary film-forming resin in a wetting agent and in a solvent carrier, photocurable monomers (mono or difunctional), a photoinitiator and an inhibitor. As these ingredients are commonly found independently in nail coating products, one skilled in the art would readily understand the purpose for each in the photo-reactive coating. Similarly, the method of mixing and preparation of the resulting composition would be obvious in the art. Consequently, these ingredients and the process of preparation of the resulting product need not be discussed in great detail. 
     In the preferred embodiment of the coating, the primary film-forming resin (polymer) is nitrocellulose, which is available commercially in a wetting agent to reduce its reactivity. The particular wetting agent used with nitrocellulose is not critical to the invention, so long as one is present to avoid danger of explosion. A wetting agent is an inherent requirement for being able to use nitrocellulose, which itself alone (i.e., not the wetting agent) is one of the active ingredients in the formulation of the invention. Thus, commercial nitrocellulose such as is sold, for instance, by the Aqualon Company of Wilmington, Del., and by the Wolff Walsroder AG company of Walsrode, Germany, is available in either ethyl, isopropyl, or n-butyl alcohol as wetting agent, according to customer specification. Any of these (or other) nitrocellulose wetting agents would enable one to practice the invention in equivalent fashion. It should be noted that the term wetting agent, as used in the context of nitrocellulose, is not synonymous with the term surfactant, as in the detergent industry. The two terms are distinct and wetting agent simply refers to a means for keeping the nitrocellulose in a wet state. Water is also a suitable wetting agent, but it is not used in the invention because it is incompatible with the other ingredients. On the other hand, the mentioned wetting agents are used not because they are surfactants (although they also lower the surface tension of water), but because they are miscible with the solvents of the invention. 
     The wet nitrocellulose is used in a carrier solvent, such as ethyl acetate or n-butyl acetate, so that it can be mixed with the other ingredients of the invention and stored as a homogeneous solution before use. These solvents (ethyl acetate and n-butyl acetate) are only two of many solvents that could be used as carriers for the wet nitrocellulose, all of which would be obvious to one skilled in the art. These include, for example, methyl acetate, ethyl acetate, n-propyl acetate, isopropyl acetate, n-butyl acetate, isobutyl acetate, sec-butyl acetate, ethyl propionate, acetone, methyl ethyl ketone, methyl n-propyl ketone, methyl isopropyl ketone, methyl n-butyl ketone, methyl isobutyl ketone, methyl sec-butyl ketone, 2-ethyl-n-butyl acetate, and 2-ethoxy ethyl acetate. In addition, hydrocarbon solvents, such as pentanes, hexanes and heptanes, may be used in conjunction with the above listed esters and ketones. As in the case of wetting agents, the solvent is not part of the inventive step of the composition, but it is used only as a carrier for the nitrocellulose ingredient. The particular solvent used as carrier has no effect on the removability of the claimed coating, which depends instead only on the properties of the polymerized composition. It is found that the presence of these solvents also improves the coating&#39;s compatibility with commercially available colored nail enamels and lacquers. 
     Nitrocellulose is used in concentrations from 5 to 30 percent by weight (preferably from 17 to 23 percent) of the total composition. Correspondingly, as a result of the nitrocellulose/isopropanol ratio normally found in commercial products, isopropanol will be present in concentrations varying approximately from 2 to 13 percent by weight. The solvent, whether ethyl acetate or n-butyl acetate, is used in approximately 50 percent greater quantity than nitrocellulose. In any event, the compositions of all ingredients with solvent characteristics may be varied to obtain the desired viscosity of the final product. 
     Photoreactive monomers are used, either alone or in mixture, in total concentrations between 2 and 55 percent by weight. Optimal concentrations are from 5 to 43 percent by weight of the total composition. In general, it was found that dimethacrylate monomers of the following formula are acceptable for practicing this invention: ##STR1## where Y and Y&#39; may be the same or different but can be any hydrocarbyl difunctional radicals with 1-4 carbon atoms; R is any hydrocarbyl difunctional radical with 1-10 carbon atoms; and the Y and Y&#39; radicals may be joined to the R radical by means of any organic group with two free bonds, known to those skilled in the art as linking radicals in which one or two atoms separate R from Y and Y&#39;, such as --O--, --NH--, --S--, --CO--, --CH 2  --, --SO--, --SO 2  --, --SO 2  O--, --NH--CO--, and --NR&#39;--, where R&#39; is an aliphatic monofunctional radical with 1-4 carbon atoms. R may be aliphatic, alicyclic or aromatic. 
     In particular, it was found that the following difunctional methacrylate monomers, together with monofunctional methacrylates according to the total quantities listed above, are particularly suitable in the percentages shown: 
     
         ______________________________________               Weight %   OptimalDifunctional Methacrylate Monomer               Range      Range______________________________________1,4 butanediol dimethacrylate               0.4-10.0   0.5-5.51,3 butanediol dimethacrylate               0.4-10.0   0.5-5.5diurethane dimethacrylate               0.4-6.0    0.4-5.5______________________________________ 
    
     As mentioned above, though, and most significantly for the purposes of this invention, it was also found that monofunctional methacrylate monomers alone produce a rapidly curing product. Some of these methacrylate monomers and the corresponding ranges of use are given below: 
     
         ______________________________________                Weight %   OptimalMonofunctional Methacrylate Monomer                Range      Range______________________________________ethyl methacrylate   2.0-55.0   5.0-43.0cyclohexyl methacrylate                2.0-55.0   2.0-20.02-ethylhexyl methacrylate                2.0-55.0   2.0-20.0isobutyl methacrylate                2.0-55.0   5.0-43.0n-butyl methacrylate 2.0-55.0   5.0-43.0methyl methacrylate  2.0-55.0   5.0-43.0isobornyl methacrylate                2.0-25.0   2.0-6.0furfuryl methacrylate                2.0-25.0   2.0-6.0tetrahydro furfuryl methacrylate                2.0-25.0   2.0-6.0______________________________________ 
    
     The preferred monomers are methacrylic acid ester monomers, such as ethyl methacrylate or cyclohexyl methacrylate or, preferably, a mixture of the two within the total preferred concentrations listed above. In the place of cyclohexyl methacrylate, furfuryl- and tetrahydro furfuryl methacrylate have been found to result in a product with comparable curing characteristics. Similarly, isobutyl and methyl methacrylate can be used instead of ethyl methacrylate. Isobornyl methacrylate has been found to perform well in conjunction with diurethane dimethacrylate, with or without ethyl methacrylate. Since ethyl methacrylate acts as a solvent as well as a reactive monomer, its quantity must be adjusted as a function of the other monomers used to provide the desired product viscosity. 
     Any one of several known photoinitiators of the aryl or the alkyl aryl ketone type may be used to practice this invention. For example, these include photoinitiators such as benzophenone, diethoxyacetophenone, benzil diketal (BDK), as well as other well known products. It was found that 2-hydroxy-2-methyl-1-phenylpropanone, available, for example, as the product sold by EM Industries, Inc. under the trademark &#34;Darocur 1173,&#34; is preferred. The photoinitiator is used in concentrations from 0.5 to 5 weight percent, preferably between 2 and 4 percent of the total composition. 
     Inhibitors to polymerization are a necessary ingredient of the claimed composition in order to provide shelf stability to the formulation. Useful inhibitors are also well known in the art and include hydroquinone and substituted analogs such as 4-methoxyphenol (MEHQ), either singularly or in combination with anti-oxidants to suppress yellowing. For example, MEHQ may be used at a concentration of about 25 to 200 ppm in combination with the anti-oxidant tris(4-t-butyl-3-hydroxy-2,6-dimethylbenzyl)-s-triazine-2,4,6-trione, sold as &#34;Cyanox 1790&#34; (a trademark of American Cyanamid Co.), at 0.25 weight percent of the total composition. 
     While the basic ingredients necessary to practice the invention are nitrocellulose, a photo-reactive monomer, a photoinitiator, and an inhibitor in quantities within the ranges described above, other ingredients may also be added to impart specific desirable characteristics. For example, surfactants like sorbitan trioleate may be added to improve homogeneity; plasticizers like dibutyl phthalate to flexibilize the cured coating; slip agents to give the material a lower coefficient of friction so that it may be applied more easily; and fragrances or masking agents to improve the product&#39;s odor. 
    
    
     The following examples list alternative formulations of the preferred embodiment of the coating according to this invention. 
     EXAMPLE 1 
     
         ______________________________________Ingredient             Weight Percent______________________________________nitrocellulose         19.5isopropanol            8.4ethyl acetate          27.9ethyl methacrylate     26.7cyclohexyl methacrylate                  12.62-hydroxy-2-methyl-1-phenylpropanone                  4.04-methoxyphenol        40 ppmtris(4-t-butyl-3-hydroxy-2,                  0.076-dimethylbenzyl)-s-triazine-2,4,6-trionesorbitan trioleate     0.9______________________________________ 
    
     EXAMPLE 2 
     
         ______________________________________Ingredient             Weight Percent______________________________________nitrocellulose         18.3isopropanol            7.9n-butyl acetate        26.2ethyl methacrylate     28.8cyclohexyl methacrylate                  13.62-hydroxy-2-methyl-1-phenylpropanone                  4.44-methoxyphenol        42 ppmtris(4-t-butyl-3-hydroxy-2,                  0.026-dimethylbenzyl)-s-triazine-2,4,6-trionesorbitan trioleate     1.0______________________________________ 
    
     EXAMPLE 3 
     
         ______________________________________Ingredient             Weight Percent______________________________________nitrocellulose         21.8isopropanol            9.4ethyl acetate          31.2ethyl methacrylate     18.5cyclohexyl methacrylate                  14.42-hydroxy-2-methyl-1-phenylpropanone                  2.84-methoxyphenol        114 ppmtris(4-t-butyl-3-hydroxy-2,                  0.266-dimethylbenzyl)-s-triazine-2,4,6-trionesorbitan trioleate     1.0dibutyl phthalate      0.4fragrance              0.2______________________________________ 
    
     EXAMPLE 4 
     
         ______________________________________Ingredient             Weight Percent______________________________________nitrocellulose         18.3isopropyl alcohol      7.9hexane                 5.5ethyl acetate          16.5ethyl methacrylate     41.31,4 butanediol dimethacrylate                  2.75diurethane dimethacrylate                  2.754-methoxyphenol        165 ppmsorbitan trioleate     0.83fragrance              0.552-hydroxy-2-methyl-1-phenylpropanone                  2.75______________________________________ 
    
     EXAMPLE 5 
     
         ______________________________________Ingredient             Weight Percent______________________________________nitrocellulose         14.1isopropyl alcohol      6.0ethyl acetate          19.7ethyl methacrylate     50.44-methoxyphenol        129 ppm1,4 butanediol dimethacrylate                  2.5diurethane dimethacrylate                  2.62-hydroxy-2-methyl-1-phenylpropanone                  4.0______________________________________ 
    
     EXAMPLE 6 
     
         ______________________________________Ingredient             Weight Percent______________________________________nitrocellulose         18.0isopropyl alcohol      7.7ethyl acetate          58.31,4 butanediol dimethacrylate                  0.5diurethane dimethacrylate                  0.5ethyl methacrylate     12.34-methoxyphenol        148 ppm2-hydroxy-2-methyl-1-phenylpropanone                  2.0______________________________________ 
    
     EXAMPLE 7 
     
         ______________________________________Ingredient             Weight Percent______________________________________nitrocellulose         15.1isopropyl alcohol      6.5diurethane dimethacrylate                  2.41,4 butanediol dimethacrylate                  3.96isobutyl methacrylate  1.94isobornyl methacrylate 4.31ethyl acetate          62.54-methoxyphenol        43 ppm2-hydroxy-2-methyl-1-phenylpropanone                  1.62______________________________________ 
    
     EXAMPLE 8 
     
         ______________________________________Ingredient             Weight Percent______________________________________nitrocellulose         20.0isopropyl alcohol      8.6diurethane dimethacrylate                  2.91,4 butanediol dimethacrylate                  4.7isobornyl methacrylate 2.3cyclohexyl methacrylate                  5.2ethyl acetate          52.04-methoxyphenol        26 ppm2-hydroxy-2-methyl-1-phenylpropanone                  3.1______________________________________ 
    
     EXAMPLE 9 
     
         ______________________________________Ingredient             Weight Percent______________________________________nitrocellulose         15.5isopropyl alcohol      6.7diurethane dimethacrylate                  3.51,4 butanediol dimethacrylate                  5.1isobutyl methacrylate  8.9ethyl acetate          57.74-methoxyphenol        22 ppm2-hydroxy-2-methyl-1-phenylpropanone                  1.7______________________________________ 
    
     EXAMPLE 10 
     
         ______________________________________Ingredient             Weight Percent______________________________________nitrocellulose         20.9isopropyl alcohol      8.9diurethane dimethacrylate                  3.31,4 butanediol dimethacrylate                  5.4ethyl methacrylate     8.6ethyl acetate          50.72-hydroxy-2-methyl-1-phenylpropanone                  2.1______________________________________ 
    
     As seen from the examples above, the preferred photo-reactive monomers are ethyl methacrylate and cyclohexyl methacrylate. These two monomers have very sharp and strong scents that are found to leave a residual odor in the finished manicure that may be objectionable to some people. In addition, the resulting compositions produce a film that is colorless immediately after curing but tends to become yellow after a few days of wear and exposure to sunlight, which is unacceptable for use in conjunction with light-colored polishes. Some commercial nail polishes are also found to react with these compositions to produce blue or purple discolorations. 
     Therefore, in the continuing research effort for improved formulations, it was discovered that when benzil diketal is used the amount of photoinitiator needed for curing can be decreased to approximately 0.05 percent and still obtain complete polymerization by three-minute exposure to low power ultraviolet radiation of the kind emitted by the apparatus described in copending application Ser. No. 394,200, U.S. Pat. No. 5,130,551. As a result of the lower photoinitiator concentration, it is possible to also decrease the amount of inhibitor required for the desired shelf life. Finally, it was found that by combining the lower inhibitor concentration with a higher concentration of anti-oxidant, the resulting cured film did not tend to become yellow from exposure to sunlight and did not display blue or purple discolorations with commercial nail polishes. The amount of the inhibitor required with these lower photoinitiator concentrations is about 6 ppm if an anti-oxidant is present at concentrations greater than 0.1 percent by weight of the total composition. 
     The formulations containing the reduced photoinitiator and inhibitor concentrations are still capable of polymerization within three minutes. As in the prior formulations, the inhibitor in the reduced concentration is effective in keeping the uncured product stable for at least 18 months at 21° C. or for more than one week at 63° C. The stability at the latter temperature is necessary for the product to survive unrefrigerated transportation during the summer months. 
     Moreover, it was found that the residual odor problem associated with the earlier preferred formulations (that is, those containing ethyl methacrylate and cyclohexyl methacrylate monomers) can be greatly corrected by reducing their concentration and by substituting all or part of them with butyl methacrylate monomers. 
     The following examples define the composition of these improved formulations: 
     EXAMPLE 11 
     
         ______________________________________Ingredient          Weight Percent______________________________________nitrocellulose      21.3isopropyl alcohol   9.1ethyl acetate       50.2isobutyl acetate    4.2ethyl methacrylate  11.3cyclohexyl methacrylate               1.4tris(4-t-butyl-3-hydroxy-2,               0.286-dimethylbenzyl)-s-triazine-2,4,6-trionebenzil diketal      0.174-methoxyphenol     12 ppmdibutyl phthalate   0.57sorbitan trioleate  1.13fragrance           0.28______________________________________ 
    
     EXAMPLE 12 
     
         ______________________________________Ingredient          Weight Percent______________________________________nitrocellulose      22.9isopropyl alcohol   9.8ethyl acetate       32.75isobutyl acetate    21.0n-butyl methacrylate               7.14cyclohexyl methacrylate               3.8tris(4-t-butyl-3-hydroxy-2,               0.426-dimethylbenzyl)-s-triazine-2,4,6-trionebenzil diketal      0.294-methoxyphenol     11 ppmdibutyl phthalate   0.42sorbitan trioleate  1.26fragrance           0.2______________________________________ 
    
     In an effort to further improve the coloring of the coating of the invention, which is affected by the yellow nature of nitrocellulose, partial substitutes have been used that result in a lighter and longer lasting clear coating. These include cellulose acetate derivatives, such as cellulose acetate butyrate and cellulose acetate propionate, used in partial replacement of nitrocellulose. Examples of such formulations are given below. In addition, acrylate and/or methacrylate resins (both as polymers and copolymers) are also used as hardeners of the overall formulation. Note Example 14, which includes butyl methacrylate resin to add stiffness and hardness to the hardened coating. 
     EXAMPLE 13 
     
         ______________________________________Ingredient          Weight Percent______________________________________nitrocellulose      17.2isopropyl alcohol   7.4ethyl acetate       32.5isobutyl acetate    30.0cellulose acetate butyrate               2.47butyl methacrylate  0.91n-butyl methacrylate               6.48cyclohexyl methacrylate               2.14tris(4-t-butyl-3-hydroxy-2,               0.106-dimethylbenzyl)-s-triazine-2,4,6-trionebenzil diketal      0.284-methoxyphenol     9 ppmsorbitan trioleate  0.54______________________________________ 
    
     EXAMPLE 14 
     
         ______________________________________Ingredient          Weight Percent______________________________________nitrocellulose      9.3isopropyl alcohol   4.0ethyl acetate       53.2isobutyl acetate    13.3butyl methacrylate resin               0.73cellulose acetate propionate               8.12n-butyl methacrylate               6.65cyclohexyl methacrylate               2.66tris(4-t-butyl-3-hydroxy-2,               0.276-dimethylbenzyl)-s-triazine-2,4,6-trionebenzil diketal      0.214-methoxyphenol     9 ppmdibutyl phthalate   0.33sorbitan trioleate  1.00fragrance           0.22______________________________________ 
    
     EXAMPLE 15 
     
         ______________________________________Ingredient          Weight Percent______________________________________nitrocellulose      23.2isopropyl alcohol   9.9ethyl acetate       45.9isobutyl acetate    12.74n-butyl methacrylate               2.97cyclohexyl methacrylate               2.97tris(4-t-butyl-3-hydroxy,-2,               0.346-dimethylbenzyl)-s-triazine-2,4,6-trionebenzil diketal      0.314-methoxyphenol     6 ppmdibutyl phthalate   0.42sorbitan trioleate  1.27______________________________________ 
    
     It is also to be noted that acrylates and diacrylates may be used, either alone or in combination, as substitutes for the methacrylate and dimethacrylate monomers specified herein, as disclosed in the copending U.S. application Ser. No. 07/235,349 referenced above, filed Aug. 23, 1988, now abandoned. Because monomers of the acrylate and diacrylate families react more efficiently than methacrylates and dimethacrylates, they should be used in smaller concentrations in order to avoid an overly stiff and hard-to-remove coating. Thus, for example, while a formulation containing 55 percent by weight methacrylate and/or dimethacrylate produces a polymerized coating that is still easily removable with regular polish remover, that high percentage of acrylate and/or diacrylate monomer results in a finished coating that is very hard to remove. Therefore, while the percentages disclosed herein are also applicable to these monomers, they would be preferably reduced in embodiments based entirely on them. The exact degree by which the concentration of an acrylate used in substitution of a methacrylate (or a diacrylate used in substitution of a dimethacrylate) should be decreased depends on the desired properties of the final product and on the balance of the ingredients used in conjunction with the monomer. For example, a direct reduction to about 10 to 20 percent (that is, 10 to 20 parts of acrylate or diacrylate are used instead of 100 parts of methacrylate or dimethacrylate) of the amount listed in any formulation will yield acceptable results. In order to maintain the same percentages of the other ingredients, the balance may be made up with solvent. In general, though, I prefer to use acrylates and diacrylates, if at all, only in small quantities together with methacrylates and dimethacrylates because of their higher propensity to cause skin irritation. 
     Since acrylates and diacrylates are more reactive than methacrylates and dimethacrylates under the same conditions, higher amounts of inhibitors are required in order to prevent premature polymerization and ensure a useful shelf life. The proper, higher, percentage of inhibitor is found in the formulations in which the acrylate and diacrylate monomers are available commercially. For example, while a methacrylate is typically available in a solution containing 50 to 100 ppm of MEHQ, the homologous acrylate solution contains 100 to 200 ppm. 
     For the same reasons, as would be obvious to those skilled in the art, less photoinitiator is required to set off the polymerization reaction once the monomer is exposed to ultraviolet radiation. Typically, amounts smaller than those disclosed for methacrylates and dimethacrylates are found to work well with acrylates and diacrylates, respectively. It should be stressed, though, that these amounts are only guidelines for general formulations because different compositions with different percentages of inhibitors and photoinitiators will also work, but will exhibit obvious variations in shelf life and reactivity under UV light. 
     Most importantly, although acrylate and diacrylate monomers are adequate substitutes for methacrylates and dimethacrylates from the point of view of photoinitiated polymerization, I found them to be undesirable from a commercial point of view because of their higher toxicity. They tend to cause skin irritation, and sometime even blisters, in a large number of users. Therefore, I prefer compositions based entirely on methacrylate and dimethacrylate monomers, which were selected, in part, because they are known to be less toxic and generally less photoallergenic. 
     Thus, while the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that changes in form and detail may be made therein without departing from the spirit and the scope of the invention.