EVALUATION OF NAIL ENAMEL 49 Sward Rocker or the Tukon Microhardness Tester should be used. Either method requires some experience to obtain consistent results. Lastly, due to varying amounts of pigments and pearls present in different shades, the hardness as determined by the Sward Rocker will vary from shade to shade. A range between 8 and 11 oscillations is normal. NONVOLATILE CONTENT OR SOLIDS Solvents are used in nail lacquers to dissolve the solid components and subsequently to deposit even films. Although the solvents leave the wet film by evaporation and are usually not recovered, they significantly affect the application, appearance and durability of the film. Despite this they add essentially nothing to the volume of the film deposited. The solid components play the primary role. The concentration of solids in the coatings is the factor which determines to the largest extent the amount of coverage that can be obtained. There are many methods for determination of the solids but the "dish method" is generally accepted. A laboratory analytical balance sensitive to .01 g is required, as well as a ventilated laboratory oven. Pour approximately 1 to 2 g of the nail enamel into a tared weighing dish, approximately 2• in. in diameter. The exact weight of the sample is then determined by rapidly weighing the dish and sample. The dish with the sample is then placed in a ventilated laboratory oven at a constant temperature of 105øC _+ 2øC for 2 h. The dish is removed, placed in a dessicator, allowed to cool to 25øC. Weigh the dish with the residue and subtract the weight of the empty dish to determine the amount of non-volatile solids. Divide the residue weight by the weight of the sample to obtain the percent solids content. The solids, as formulated, is approximately 29% for both creme and pearl nail enamels. Since some of the plasticizers in the enamel evaporate in this test, laboratory determination of solids should range between 27-29% for a 29% solids formulation. Samples should be analyzed in duplicate or triplicate on each batch tested. SETTLING TEST A quick test to determine whether a suspension nail enamel will settle in a relatively short period of time is to subject a firmly sealed sample to temperatures of 40øC and 50øC for 24 h prior to release from the manufacturing plant. Samples should be analyzed in duplicate. There should be no visible settling of solid material in the bottom of the tubes. This test is especially important for enamels containing bismuth oxychloride and mica synthetic pearls. Tests may be run at these elevated temperatures for many weeks to determine stabilities on nail enamels. The correlation between oven and storage room temperature stabilities are often poor due to evaporation of solvents, decomposition of nitrocellulose, and other phenomena which occur at elevated temperatures. Still comparative results may be obtained among different formulations and some predictions on storage stability of samples may be determined. Nail enamels exhibit stability at room temperature anywhere from three months to one year depending upon the amount and type of colored pigments present as well as the manufacturer.

50 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS VISCOSITY The measurement of viscosities of suspension nail enamels is not as precise as the old style non-suspending type. Variations in the technique and procedure will produce large variations in results. No one procedure is correct, but the procedure must be standardized. The following suggested procedure is based on ASTN-D-2198-58: "Standard Method of Test for Rheological Properties of Non-Newtonian Materials." This test method has been modified for suspension type nail enamel so as to provide a standard test method which is not too time consuming. The instrument preferred is a Brook field Viscometer, (Brookfield Engineering Laboratories, Inc. 240 Cushing Street, Stroughton, MA 02172) Model LVT or Model LVF, Spindle //3, equipped with a helipath unit, so that the rotation is read in cps, while the spindle is being lowered into the enamel. The sample size should be 200 g in an 8-oz closed jar. Jar should be covered as much as possible during the running of the viscosity to minimize solvent evaporation. Sample should be aged at least 8 h at 25øC before performing the out lined test. At 25øC, shake very vigorously ten times. Start timer, insert the spindle into the sample to the scored line, with the motor running at 60 RPM. Have the spindle in the correct position in less than one minute. Read the instrument at the end of ten minutes, then switch the speed control to 6 RPM and read the instrument again at the end of another ten minutes. In order to convert the dial readings to centipoise, multiply by the following factors: 60 rpm x 20 6 rpm x 200. The viscosity-thixotropy relationship of cream nail enamels should be about 375-500 cps. at 60 rpm,//3 spindle, at 25øC. The high speed reading may be considered the viscosity while the second viscosity reading is determined at 6 rpm. The difference in both readings indicates the thixotropy of the enamel--the property that enables the lacquer to suspend pigments and pearlescent materials. The viscosity of pearl nail enamels should be approximately 400-600 cps, at 60 rpm,//3 spindle at 25øC. The reading at 6 rpm for cream and pearl enamels based on guanine should be at least 150 cps higher and at least 200 cps higher for synthetic pearl essence. Base coats, top coats and clears are virtually newtonian liquids and have the same viscosity at both speeds. Often times a Ford viscosity cup run according to ASTMD 1200 is used for the aforementioned products. To obtain a measurement, the cup first is placed on a level plane, and the orifice is closed with the finger. The sample, approximately 100 ml, is poured into the cup until it overflows into the rim provided. A level fill is obtained by drawing a straight edge across the top of the cup to remove excess liquid. A timer is started when the finger is removed and is stopped at the first break in the stream of liquid flowing from the cup orifice. This interval of time, to the nearest 0.2 s, is reported as a measure of the liquid viscosity. The No. 4 cup, for viscous liquid, has an orifice of diameter 0.162 in. and is most often utilized. Busch, et al. found a significant relationship between the application properties of a nail enamel and its rheological requirements (5). It was found that application to the nails involves very low shear rates and low shear forces. WATER CONTENT If water is present in substantial quantity as an impurity in solvents and diluents, the shelf life and performance of a finished lacquer can be affected seriously. Water content is determined chiefly by the Karl Fischer procedure and modified by ASTM D 1364. A sample of suitable size is dissolved in a flask containing pyridine and ethylene