EVALUATION OF NAIL ENAMEL 47 DRYING TIME The application and performance properties of a nail coating depend greatly on the volatility characteristics of its solvent system and therefore on its drying time. A thin layer of lacquer is spread or flowed out on a clean and clear glass panel and observed. The time taken to dry is measured with a stop watch, and checked by pressing the film with a finger, until no mark remains on the surface. This may be done by application of wet films comparing standard against batch using a 3-mil Bird Film Applicator. The two films should be checked by finger tips after about 5 min. The two films should be dry to touch at approximately the same time (within 45 s of each other). Dry-to-touch is the condition at which the film may be touched with a clean fingertip without the resultant transfer of any material to the finger. The total time for achievement should be in the range of 8-10 min, depending upon temperature, humidity, air movement and total solids. Films should be applied under controlled temperature and humidity conditions at 25øC and 50% relative humidity to a completely non-porous surface, in order to standardize this technique. A smoother coat of enamel may be brushed on the nails and dried at room temperature. The film must be dry with no blushing within a specified time ie. three minutes maximum. There are a few marketers that require the enamel to dry quick enough so that the second coat may be applied immediately after the last finger of the second hand is brushed with the first coat. FLEXIBILITY Flexibility of the nail enamel film can be measured on a mandril set in accordance with ASTM Method D-1737-62. Briefly, a coated steel panel is placed on its back against a cone or cylinder and then is bent to conform to this shape over a mandril rod of known diameter in a circular arc. The coating is examined for cracking, flaking and loss of adhesion to the substrate against a suitable standard which has undergone the same tests. This method will usually point out inflexible coatings but since the shape of individual nails vary this test alone should not be used to evaluate nail lacquer films flexibility. FLOW One of the most important characteristics that a coating should possess is flow or film smoothness. Flow is dependent on the solvency and volatility of the solvent system used. Proper solvency and balanced volatility rate insures a smooth dry film. Flow evaluations are usually subjective when comparisons are made against a standard. Application of enamel is made by pouring from about 1.5 in. to a glass plate and then raising the plate to vertical. The film is observed with a magnifying glass of approximately 5 power. The film should reveal no presence of foreign matter, coarse particles or pinholes. GLOSS Gloss comparisons may be made by the flow pour method previously described. This is a visible observation. Gloss comparisons may be made visually by comparing

48 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS standard and batch on a Morest Chart as in the color tests previously described. Finally, more critical determination of gloss can be evaluated on a glossmeter such as the Glossgar © portable glossmeter (Gardner Laboratories, Inc. Box 5728, Bethesda, MD 20014). Gloss depends upon pigment concentration, solids composition and solvent composition of the nail enamel. An incandescent light is directed at a 60 ø angle toward the test specimen which has been applied to a Morest Chart. ^ photosensitive device is provided to measure the light reflected from the test coating at a 60 ø angle. The light flux is then indicated numerically on a galvanometer. The difference between 100% reflection and the reading obtained represents the incidence light absorbed by the coating or scattered at angles other than 60 ø by irregularities of the coating surface. Gloss is often defined as the percent of luminous reflectance from a specimen, so gloss readings must lie between zero and one hundred. Instruments must be calibrated to this condition with known standards. Use of the glossmeter is the most preferred. It is the only way that small differences in gloss may be measured. Visual comparisons may only determine if a sample has good or poor gloss when compared to a standard. HARDNESS In addition to the methods reviewed under the section on adhesion a simple comparative test is one in which a pour on a glass plate can be scratched with the thumbnail. A harder coating is generally considered to be superior in general performance providing that the flexibility, impact resistance, adhesion, and other properties are equal. Two methods of film surface hardness most commonly used in evaluation of nail enamels are the Sward Rocker and the Tukon Microhardness Tester. Films of 0.0006 in. should be cast on a glass plate and dried for 48 h at 25øC and an additional 2 h at 7IøC. The films should be conditioned for an additional 48 h at 25øC before testing with the Rocker. The Rocker consists of two 4-in. metal rings spaced 1 in. apart, a gravity bob for regulating the oscillation frequency or period of the moving system at 1.2 s and a rider on the horizontal nameplate for adjusting the zero point of the balanced Rocker. In order to make a hardness test, the Rocker is placed on the leveled surface and set in motion. The total time required for the amplitude of oscillation to decay by a fixed amount corresponding to a certain number of complete oscillations (not swings), is a measure of the hardness of the test panel or specimen. A greater number of rocks will be recorded for a harder film than for a softer one. The Tukon Microhardness Tester consists of a load applicator, and indenter, and a microscope fitted with a movable stage. To conduct a test, a coating of specified film thickness is applied to a rigid substrate. After the film is dried for a given period, a load is placed on the surface of the coating for approximately eighteen seconds through a pyramid-shaped diamond. Temperature and relative humidity during the test run are kept constant at 25øC and 50%, respectively. The resultant permanent impression made by the diamond is measured with a filar micrometer and the length of the identification is converted to a Knoop Hardness Number (KHN). This number relates the applied load in grams to the unrecovered (approximate) projected area in square millimeters. Knoop Hardness Numbers increase in magnitude with increasing film hardness. Again, as in the adhesion test previously described the thumbnail test can only determine comparative hardness. In order to obtain accurate measurements a