Chapter 9 covered a class of effect pigments referred to as metallics, which are bronze and copper colored pigments composed of iron oxide or tita- nium dioxide coated natural and synthetic substrates. The metallic pigments discussed here are truly metallic – essentially the pure metals aluminum, copper, and bronze, as represented in Figures 10.1 (bronze and copper) and 10.2 (aluminums). Their mechanism of color display is different from that of both the absorption and the effect pigments.
The absorption colors operate by selectively absorbing and diffusely reflecting different wavelengths of light, as shown in Figure 10.3. The circles represent particles of pigment on the surface of the skin. The long arrows show incident light striking the surface of the pigment and being
Coloring the Cosmetic World: Using Pigments in Decorative Cosmetic Formulations, Second Edition. Edwin B. Faulkner. Edited by Jane C. Hollenberg.
© 2021 John Wiley & Sons Ltd. Published 2021 by John Wiley & Sons Ltd.
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(a) (b)
(c)
Figure 10.1 (a) Bronze pigment. (b) Copper pigment. (c) Copper pigment.
Source: Courtesy of Eckart.
(a) (b)
Figure 10.2 (a) Aluminum pigment. (b) Aluminum pigment (blue).
Source: Courtesy of Eckart.
reflected at diffuse angles. The short arrows penetrating into the pigment demonstrate light being absorbed.
Effect pigments based on metal oxide coated substrates operate by selec- tively absorbing, reflecting, refracting, and transmitting light, as described in detail in Chapter 9 and shown in Figure 10.4. The arrows show incident light being reflected at both the specular and diffuse angles and being transmitted through the crystal.
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Figure 10.3 Absorption pigment light reflection.
Source: Courtesy of Eckart.
Figure 10.4 Effect pigment light interactions.
Source: Courtesy of Eckart.
Figure 10.5 Metallic pigment light reflection.
Source: Courtesy of Eckart.
The metallic pigments, on the other hand, produce their color effects by reflecting the entire wavelength range of incident light, a very high degree of which is at the specular angle. This produces a bright metallic look like that seen in jewelry. Figure 10.5 shows this mechanism, where the flat, plate-like particles are the metallic pigment and the arrows are incoming reflected light, most of which is at the specular angle.
Figures 10.6 and 10.7 are comparisons among two types of metallic pigments and two types of oxide coated effect pigments. Masstones and
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Figure 10.6 Effect and metallic pigment powder comparisons.
Source: Courtesy of Eckart.
reflection colors of all are gold, but composition influences luster, color, and coverage. Readers may find it instructive to see the pigments as they appear in simulated use.
Effects observed by application of the pure dry powders to an inner arm (Figure 10.6) predict performance in powder products, such as eyeshadow, highlighter, or blush.
Metallic: On the left are two golden bronze pigments, composed of bronze powder coated with silica. Their effect is the smooth sheen and high coverage created by the metal flakes. The particle size of the far left sample is the larger of the two, exhibiting stronger reflection and less coverage.
Synthetic Mica Base: The middle pigments consist of oxide coated synthetic mica (fluorophlogopite). Both are composed of fluorophlogopite platelets coated with titanium dioxide, iron oxides, and tin oxide to give a gold masstone combined with a gold reflection. The left middle pigment is an example of a newer technology, showing a more intense color and higher coverage compared to the traditional coated synthetic mica to the right.
Borosilicate Base:The pigment on the far right is composed of calcium sodium borosilicate platelets coated with titanium dioxide, iron oxides, sil- ica, and tin oxide. The borosilicate base allows the highest transparency and light transmission to create a maximum effect.
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Figure 10.7 Effect and metallic pigment drawdown comparisons.
Source: Courtesy of Eckart.
Drawdown of nitrocellulose dispersions (Figure 10.7) specifically pre- dicts performance in nail lacquer, but also shows effect differences to be expected in other formulations in which the powder is wet into a liquid vehi- cle, such as the oils used in lip color. Pigments shown are similar to those in Figure 10.6, only in reverse order.
Borosilicate Base: Sparkle effect and transparency are maximized in the far left of the drawdown.
Synthetic Mica Base:The pigment in the left middle dispersion is an example of traditional oxide coating technology on fluorophlogopite. The more intense color of the newer coating technology to the right shows to good advantage in dispersion form.
Metallic:On the right, golden bronze dispersion has the coverage and polished metal appearance valued in high fashion nail lacquers.
The appearance of metallic pigments, like that of effect pigments, is controlled by several factors, including particle size, shape, distribution,
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and orientation. Of these factors, the influence of particle size dominates, impacting the metallic character and optical properties of the pigment.
With respect to the former, coarser particles have a higher surface to edge ratio, which produces fewer scattering centers, resulting in a higher degree of specular reflection. This gives these pigments a more brilliant metallic character. Finer particle pigments have a lower surface to edge ratio, leading to more scattering centers and thus a higher degree of diffuse reflection, which yields more of a soft, less brilliant metallic character. The brightness of the metallic pigments increases as their particle size increases, while the coverage does the reverse, decreasing as the particle size increases.
The general particle size ranges are as follows:
Fine Grade (5–50𝛍m):Brilliant effect with excellent coverage.
Medium Grade (15–70𝛍m):Bright effect with good coverage.
Large Grade (20–95𝛍m):Sparkle effect with fair coverage.
Aluminum Pigments
Traditionally, aluminum pigments were manufactured by processing alu- minum particles through a ball mill in the presence of a solvent, followed by classification through a sieve, then solvent removal by means of a filter press, before final processing into dry or dispersion forms. This production technique produced particles that are best described as having a cornflake structure, with edges measuring in the 100–500 nm range (Figure 10.8).
More recently, a novel process for the manufacture of aluminum pig- ments was developed, in which aluminum is vaporized and deposited on
Figure 10.8 Cornflake pigment, electron photomicrograph.
Source: Courtesy of Eckart.
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to a polyester film. The pigment is then stripped from the film, classified, and processed into dry or dispersion forms. The particle produced by this method has edges that measure in the 30–50 nm range.
The difference in the edges between the two products makes a dramatic difference in the effect they produce. The conventional cornflake product’s edges cause more light scattering than those of the vapor deposition prod- uct, which results in more diffuse reflection, and therefore a more satin metallic effect. The vapor deposition product produces an almost mirror- like reflection effect. In the electron micrographs in Figures 10.9 and 10.10, the difference between the two types is readily evident; it can be seen how much thinner the vapor deposition pigments are.
Standard Aluminum Pigments
Softer reflection and scattering Brilliant effect
Figure 10.9 Cornflake pigment, photomicrograph.
Source: Courtesy of Eckart.
Uniform reflection Very brilliant mirror-like effect
Figure 10.10 Vapor deposition pigment, photomicrograph.
Source: Courtesy of Eckart.