Lustrous world of effect pigments

Effect pigments are colorants that give additional color effects, such as angular color dependence or texture, when applied in an application medium. They are classified simply as metal effect pigments or pearl luster pigments.

Pigments are substances consisting of particles which are practically insoluble in the application medium. They are used to color but in addition they can provide corrosion inhibiting, magnetic, electrical or electromagnetic properties. The applications vary from paints, coatings, plastics, printing inks, construction materials, ceramic products, glass, enamel, to cosmetic formulations.
Pigments are differentiated and classified with respect to the way they interact with light. The most important pigment classes are:

• white pigments,
• colored pigments,
• black pigments, and
• effect pigments.

Fundamentally, all these pigments are employed because of their inherent ability to generate color.

The difference between effect pigment categories, metal effect pigments and pearl luster pigments, is that metal effect pigments are luster pigments consisting of metallic particles only. In applications, with a parallel aligned orientation in their application, they show a metal-like luster by reflection of light at the surface of the metal platelets. Pearl luster pigments on the other hand are luster pigments consisting of transparent or semitransparent platelets of metal oxides or other materials with high refractive index. They can also be applied with a parallel orientation to give a characteristic pearl luster generated by multiple reflections. Pearl luster pigments showing additional colors generated by interference of light are also designated as nacreous or interference pigments. Interference pigments are by definition effect pigments whose color is generated completely or predominantly by the phenomenon of interference of light. 

Various new effect pigments consisting of platelets of transparent materials have been developed in recent years. These pigments exhibit effects arising from multiple reflections followed by light interaction, but at the same time they do not have a characteristic pearl luster. In addition, the class of effect pigments has been broadened by the development of interference pigments based on platelets of non-transparent materials. This has made the classification more complex and therefore the new term “special effect pigments” was created. This includes all platelet-like effect pigments, which can not be categorized as metal effect pigments. According to this categorization, special effect pigments are pearl luster pigments and transparent and non-transparent interference pigments, which independent of their composition and structure lead to pearl lustrous or non pearl lustrous effects in combination with interference phenomena in the application medium.

Special effect pigments are natural or synthetic pigments, characterized by high luster, brilliance and iridescent color phenomena from optically thin films. The visual impression has its origin in reflection and refraction of the light at thin single and multiple layers. It is not limited in nature to pearls and clamshells as it can be seen in many fascinating examples in the world of birds, fishes, gemstones, minerals and insects. Fundamental investigations into the optical principles of the luster of natural pearls for instance shows that"the brilliant colors are derived from layered and structured biopolymers formed by bio-mineralization." 

Light reflectance of pigments

White, colored and black pigments typically have particle diameters of 0.1 to 1 μm. This encompasses the wavelength range of visible light. Some carbon black pigments have diameters even below 0.1 μm. Effect pigments on the other hand have particle sizes predominantly in the range 5 to 100 μm, in some cases also above these values. Therefore they have an average platelet diameter significantly larger than the wavelength of visible light. The thickness of the platelets, with some minor exceptions, is typically below 1 μm, which is of the order to interfere with visible light. Effect pigments therefore have high aspect ratios (ratio of diameter to thickness), with values of up to 200. White, colored and black pigments are mostly irregularly formed and predominantly interact with light as follows.

White pigments: The pigment particles completely scatter the incident light equally in all directions

Colored pigments: The pigment particles absorb certain wavelengths of visible light and scatter the remaining wavelengths, depending on the color

Black pigments: The pigment particles absorb all wavelengths of visible light

Metal effect pigments consist of thin metal (mostly aluminum) platelets; they are not transparent and reflect all incident light in one direction. The pigment particles act like small mirrors leading to a reflecting metal luster when applied to give an aligned parallel orientation.

Transparent interference pearl luster pigments selectively reflect part of the light that is directly incident on the smooth platelet surface. The other part of the light enters into the transparent particles and is partially reflected either at interfaces within the pigments or at the bottom of the platelets. The internally reflected light leaves the platelets leading to interactions with adjoining pigment particles to create further reflections. The partial reflection of light and optical superimposition create interference phenomena, which together with multiple internal reflections lead to the characteristic appearance of pearl luster pigments. 

"Pearl luster pigments can imitate the luster of natural pearls when the pigment layer and the paint layer structure are chosen in a suitable manner." 

Pearls and mother-of-pearl consist of a multitude of alternating transparent layers with different refractive indices. They have in most cases much larger lateral dimensions than in pearl luster pigments. The layers consist of calcium carbonate (CaCO3) with a relatively high refractive index and proteins with a relatively low refractive index. The optical effects seen in pearls, mother-of-pearl and pearl luster pigments are similarly created by the interference of light rays, which are at least partly reflected at different interfaces.

Differences in the refractive indices of adjoining thin layers, particularly between pigment layers and the surrounding medium are a prerequisite for all iridescent colors arising from interference effects. Such effects are also well-known from the interfaces between air and an oil film or an oil film and a water surface. Similar differences in refractive index arise in the use of special effect pigments where the high refractive index platelet layers border upon the low refractive optically thinner application systems such as lacquers, printing inks or plastics. In applications, with a parallel aligned orientation of the platelets in the medium, by selective choice of optical principles, manufacture, properties and types of special effect pigments, the layer thicknesses of the pigments, the interference color and effects can be controlled in a manner suited to the wavelength of the incident light.

Algol Chemicals pigment portfolio contains Carbon black, TiO2 and a broad spectrum of Iron oxides, but also pearlescent pigments and metal effect pigments. All products are of the highest quality and sourced from responsible and reliable partners all over the world.