18 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS divided by the square of their sum. It is written R = (n2 -- nl) 2 -[- k 2 (n• q- nt) 2 q- k Because the absorption coefficient K is very small in the case of white pigments, the second term in numer- ator and denominator has been ignored in calculating the data shown in Table 1. In each medium the covering power of zinc oxide arbitrarily has been designated as 100. Actually tile values decrease progressively from 100 to 37 to 21 as zinc oxide is placed in air, water and petrolatum, respectively, and the values for the other pigments de- crease proportionately. Titanium dioxide has greater covering power per pound than zinc oxide and the latter is superior to chalk and talc. On the basis of cost per unit of cov- ering power, however, zinc oxide is in a more favorable position. PARTICLE SIZE Refractive index, used in the above caldulations, measures the power of a substance to bend light and gather it. into itself. Light- gathering power is important but the light also must be reflected back and this is done at the crystal sur- face. The greater the surface pres- ent, the more light will be reflected. Thus as the particles of a material are made smaller, more surface is created per pound and greater opacity results. Any discussion of covering power would be incomplete 0 0.2 0.4 0.6 0.8 1.0 PARTICLE SiZE IN #ICRON$ Figure 1.--Effect of particle size on light transmission of ZnO in water suspension without reference to the effect of particle size. Generally greater covering power per pound results as the degree of subdivision is in- creased but there are limiting fac- tors, as shown in Fig. 1. This chart is a calibration curve used for routine checking of particle size of zinc oxide in our plant opera- tions. •The turbidimetric method used in this test has been described in the literature (2) and will not be detailed here. It is sufficient to say TASLr. 1--CALcULATED COVERING POWER OF PIGMENTS , Relative Covering Power , Refractive In Air In Water In Petrolatum Pigment Index (n = 1.00) (n = 1.33) (n = 1. 475) TiO= 2.52 166 232 292 Zinc Oxide 2. 008 100 100 100 Chalk 1. 658 55 29 1 $ Talc 1. 589 46 19 6
ZINC OXIDE IN FACE POWDER 19 that particle size increases along the abscissa and opacity decreases along the ordinate. You will note the sharp maximum in opacity that occurs in the neighborhood of .25 micron. This is the particle size for optimum opacity of zinc oxide in water. The optimum size for other pigments varies with refractive in- dex, the higher reft'active index pig- ments having their maximum opac- ity at a finer size. The sharp de- crease in opacity noted below .25 is due to the fact that the particles have become so small in respect to the wavelength of light that scatter- ing becomes more dominant than normal reflection. The particles scattering light act as new light sources, sending light in all direc- tions. Instead of light being re- flected back (as with a coarser pig- ment) enough is scattered in a for- ward direction to make the material appear less opaque. An analogy from nature will clarify the point. During a rain- storm, distant objects are not hidden appreciably because the raindrops are large and do not have much re- flecting surface. On a foggy day, the particle size has b4en reduced to the optimum size and the atmos- phere is really opaque. On a damp humid day, distant objects may be no more obscured than during a rainstorm even though the quantity of water present in the air in all three cases may be identical. In the last case the particles are so infinitely small that they can no longer reflect light. Our line of zinc oxide pigments covers a wide range of particle size. Our U.S.P. zinc oxide falls in the range of .25 to .40 micron and is in the high opacity classification. The product of extremely fine size is used in the rubber industry and for special applications where the chem- ical properties of zinc oxide are de- sired without its attendant opacity. The coarse types are used in ceramics and wherever else a re- duced reaction rate is required. PREVENTING SHINE A_ second desired function of a face powder is to eliminate the skin shininess caused by excessive oily secretion in certain facial areas. The prime requirement of a material for this purpose is a high absorptive capacity. Zinc oxide of the desired particle size for optimum covering power will retain less liquid than some materials specially prepared for this purpose but its oil absorp- tion capacity is in the range of other face powder constituents. In addition to having a capacity to absorb oil or moisture, it is help- ful for the material to have a high- refractive index. All pigments are less opaque in an oil medium than in water, and less in water than in air. Table 2 has been prepared using the data from Table 1 and calculating TABLE 2 RELATIVE COVERING POWER RETAINED AF'rSR WETTING DRY PIGMENT With With Pigment Water, % Petrolatum, TiO•, 51 37 Zinc Oxide 37 21 Chalk 20 5 Talc 15 3
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