EVALUATION OF NAIL ENAMEL 45 proportion and pattern of any coating removed within the scribed area. Instead of a lambda, a cross-hatch pattern may be made in the coating. Tape is applied and removed as above. An estimate of the percent of coating removed (from 100% to 0%) is an indication of the adhesion of the nail coating. The Hoffman Scratch-Hardness Tester, consists of a four-wheeled carriage, a scale arm graduated from 0 to 20 that is attached permanently to the carriage in a counter poised condition about the pivot axis, and a scratching tool with a sharp circular rim mounted at 40 ø to the flat test surface. Riders are attached to the scale arm at the numbered positions. The carriage is held down firmly by hand and moved in the opposite direction to cause a trailing scratch. The large standard rider loads 100 g. per division, while the small quarter rider loads only 25 g. per division. This small rider may be used for making low-range measurements involving small increments of pressure, or it may serve as a vernier with the large rider in making more precise medium-range measurements. Scratch-hardness of a coating is expressed as the force necessary to cut through the film to the substrate while adhesion is expressed as the force required to scrape a path through the film when the stylus begins its motion on the uncoated portion of the panel. A third test method involves the use of an Arco Microknife (3). The adhesion of a lacquer is tested by the parallel-groove method. Successive cuts to the substrate are made with the distance between cuts decreasing by increments of one mil until lateral thrust dislodges the coating. Adhesion is expressed as the widest groove spacing where film removal occured. Determining the force necessary to cut through the film to the substrate constitutes a hardness set. Finally, a practical test for measuring film characteristics of nail lacquer for adhesion and hardness may be done by using a clear glass panel and a Bird Film Applicator (Bird & Son, Inc., East Walpole, MA 02032) to apply 3-mil wet films of the standard and production batch. After 4-5 hours the films are scraped with a knife which has a slight curve toward the end of the blade. The knife is used to scrape a groove through the film. The scraping is done, holding the knife 90 ø to the glass panel. Both the resistance to the knife and the appearance of the groove in the lacquer film differentiates the films. A smooth groove that is produced with little resistance indicates the film is too soft. A groove that is produced with finely "feathered" edges along the length of it is normal, as it shows good adhesion. A groove that shows jagged edges, and the material that is scraped off is rather powdery, indicates a film that is too hard or brittle. It is advisable to change standards about once each year since nail enamel loses some of its hardness over a long period of time. The practical test is the one most often employed in the nail enamel laboratory but lacks the sophistication necessary to compare different formulas for adhesion and hardness. It is best to utilize 2 of the 3 methods for accurate results. APPLICATION Application to one's nails is the most reliable test method in this case. Checking for evenness and smoothness of brushing as well as streaking should be examined. Before applying a nail lacquer, moisture and traces of oil or soil should be removed from the nail surface to ensure good adhesion and gloss. The thickness of the film governs its wear resistance and gloss. It is desirable to obtain in a single application as thick a film

46 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS as is practical, from the view of ease of application and rate of drying. Most lacquer requires two applications to obtain a film of required thickness. The film thickness for a single application will depend on the non-volatile components and the viscosity of the lacquer as applied. When one coat has been applied, the first film must be completely dry before the second coat is put on. Some marketers feel that a low viscosity, easy applying enamel is preferred whereby two or three thinner coats are required in order to provide a chip resistant colored film. Application of the standard on the middle finger and batch on immediate fingers to the right and left is done with two coats. Note comparisons. The batch should not be too thin or thick either characteristic could cause brush marks. COLOR A simple check of color and opacity can be made by a pour of both batch and standard onto a glass plate (opal for creams, clear for pearl shades). The plate should then be set vertically so that both pours flow down with edges touching each other. Samples of standard and batch should be compared for color in current nail enamel bottles. Comparison of colors on the nail is similar to the application test previously mentioned. Color should be checked in North daylight or under a Macbeth Daylight Lamp (the Macbeth Corporation, Box 950, Newburgh, N.Y. 12550). An alternative method is to compare the colors on the two thumbnails, holding them side by side, moving the thumb with the standard first on the right, and then on the left. Artificial acrylic nails have been utilized as well for matching comparative shades. Color strength is evaluated by comparing the standard and batch applied in a 3-mil-thick wet film on a black and white Morest Chart (4). A 3-mil Bird Film Applicator is used to apply nail enamels to the Morest Chart (The Morest Company, Inc., 101 Broad Ave., Fairview, NJ. 07022). A liquid puddle, approximately 3/4 in in diameter, of both the standard and the batch are poured on the upper area of the Morest Chart, side by side, with the standard on the right, and then the Bird Film Applicator is pulled across the surface with a smooth continuous motion, to spread both liquids, at once, into two 3-mil films, side by side. A vacuum suction plate may be used to prevent charts from curling, and causing uneven films. Similar colors should have similar coverage, or hiding power or opacity. Color stability test may be performed and accelerated aging and weathering effects is simulated by storing samples in a 140øF oven. Color is measured against the Padin Color Standard (ASTM Method D 365). The method employing the Bird Film Applicator and Morest type Chart is more definitive and critical than the glass plate or artificial nail method. DENSITY The density of nail enamel, in English units, is determined conveniently by use of the weight-per-gallon cup. This metal cup, calibrated to contain 83.2 ml is filled with test material. When the cover is put into place, a volume in excess of 83.2 ml is forced through a small hole in the top. This excess is wiped off and the vessel weighed. The density in pounds/gallon is calculated as one-tenth of the difference between the cup weight in grams when full and when empty.