The challenges regarding the development of new pigments have been fulfilled for many modern effect pigments, which have been the front-runners in the field of inorganic pigments since the s. This applies in particular to special effect pigments (transparent effect pigments, pearlescent pigments, and interference pigments), but also to a lesser extent to metallic effect pigments. Especially with special effect pigments, unique gloss and color effects, e.g., iridescent, glittering, and iridescent colors, can be achieved in the application. It is important that the platelet-shaped pigment particles are oriented parallel to the substrate or surface and parallel to each other in their application system. This is the only way to achieve the desired optical effects in coatings, printing inks, plastics, or cosmetic formulations.
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Typical metal effect pigments consist of thin metal platelets with diameter of up to 100 µm and thickness of less than 1 µm. Suitable metals for such pigments are aluminum and copper, but also zinc/copper alloys [60]. The starting materials for metal effect pigments are the corresponding metals in the form of ingots. These are usually melted and then sprayed in liquid form. The resulting metal grit is then mechanically formed into thin platelets in ball mills in the presence of mineral oils, oleic and stearic acids. Dry grinding is out of the question for safety reasons, as the fresh metal surfaces produced during the grinding process are extremely reactive. In the case of aluminum pigments, which are of greatest technical importance, a differentiation is made today between the following three types in the order of their time of origin: &#;cornflake type,&#; &#;silver dollar type,&#; and &#;VMP type&#; (vacuum metallized pigment). While the cornflake and silver dollar type are produced by the process described above, the VMP type, developed only in recent years, is produced by vacuum evaporation of metallic aluminum and deposition of the vapor onto a polymer foil. The thin aluminum layer formed on the foil after cooling is detached and ground into pigment particles. The pigment particles obtained in this way are very thin. Their thickness is usually well below 50 nm. Figure 7 shows electron micrographs of the three types of aluminum pigment. The metallic gloss effects to be achieved are stronger when the pigment surfaces are smoother and fewer scattering centers are present at the edges and surfaces. Thus, from an optical point of view, the silver dollar type and above all the VMP type represent further developments of the traditional cornflake type [61].
Fig. 7Source: Carl Schlenk AG)
Scanning electron micrographs of the three aluminum types used for effect pigments (
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Typical special effect pigments are based on the layer&#;substrate principle. Thin platelets consisting of natural or synthetic mica, silicon dioxide, aluminum oxide, or glass take over the function of a substrate. All these substrate materials are optically low-refractive. The dimensions are on the order of magnitude mentioned for metal effect pigments. Titanium dioxide, α-iron(III) oxide, and other optically highly refractive metal oxides form thin layers on the substrate platelets. The thickness of the metal oxide layer is mostly in the range of 40&#;250 nm [62, 63]. In the interaction with light, reflection, refraction, and interference play the most important role for these pigments. Depending on the coating material, absorption also comes into play, e.g., in the presence of iron oxide layers. Gloss and color effects are determined by the choice of substrate and coating. Multiple layers can also be deposited on the substrate platelets. In such cases, an alternating arrangement of high- and low-refractive-index layers is usually considered. Interference colors are produced by the superposition of wavelengths of incident light reflected at the interfaces. The color effect thus produced depends on the layer thickness of one metal oxide or more than one metal oxide (if several metal oxide layers are used). Figure 8 shows the typical structure of a layer&#;substrate pigment. As an example of such a pigment, a cross-section through a TiO2&#;mica pigment is included (scanning electron micrograph).
Fig. 8Structure of layer&#;substrate pigments
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The layer&#;substrate principle was first used for effect pigments based on platelets of natural mica. To date, mica platelets coated with TiO2 or α-Fe2O3 are among the most important pigments in this class [2]. The production of such pigments starts with the grinding of natural muscovite mica. The platelets produced during the milling process are classified to produce several grain fractions. The fractions play a decisive role in determining the optical properties of the pigment particles produced in the further manufacturing process, since their size and shape are derived from the mica platelets used. In the next step, the mica particles of a fraction are suspended in water. A titanium salt solution (TiOCl2, TiOSO4) or iron salt solution (FeSO4, FeCl3) is dropped into the suspension at elevated temperature and defined pH with stirring. Under these conditions, titanium dioxide hydrate or iron oxide hydrate nuclei are formed, which are deposited on the mica platelets if the reaction is carried out appropriately. In the course of the reaction, an oxide hydrate layer grows on the platelets, the thickness of which is determined and adjusted by the amount of titanium or iron salt solution added. After the precipitation reaction is complete, the next steps are filtration, washing, drying, and calcination (700&#;950 °C). Only after thermal treatment is titanium dioxide or iron(III) oxide formed from the oxide hydrate on the surface of the mica platelets. This simultaneously achieves the high refractive indices that are so important for the reflection of light at the pigment surface. In addition, the layers adhere firmly to the substrate surface after calcination. In the case of titanium dioxide, the anatase modification is always formed after thermal treatment. To obtain rutile layers, thin tin dioxide layers are first deposited on the mica platelets. The structural relationship between SnO2 and TiO2 (rutile) causes the titanium dioxide formed during calcination to crystallize in the rutile modification. As with the titanium dioxide white pigments, surface coatings are also used for the metal oxide mica pigments for reasons of increased stability and improved compatibility with the application medium [2, 64].
More recent developments in special effect pigments include pigments based on platelet-shaped silica (goniochromatic color effects, i.e., strongly dependent on the viewing angle), aluminum oxide (extremely sparkling color effects, especially in paints, e.g., in automotive coatings), and borosilicate glass (particularly intense and pure interference colors). For some years now, platelets made of synthetic mica have also been available as substrates. A special production process has been developed for each of these substrate materials. The technology ranges from web coating (silicon dioxide) to crystal growth from molten salt (aluminum oxide, synthetic mica) to spraying of liquid glass drops from a glass melt onto a smooth substrate [64].
New developments also include various effect pigments in which iron oxide layers are deposited on aluminum platelets. Such pigments combine the high hiding power of metal effect pigments with strong red, orange, and gold tones resulting from the absorption of the iron oxide in combination with interference phenomena [2].
Inorganic pigments with functional properties such as anticorrosive and magnetic pigments have been available for a long time. However, there is still a need for the development of improved but also completely new products in this segment. This involves functions such as corrosion protection, electrical conductivity, IR reflection, UV absorption, or laser markability. In all functional applications, pigment and application medium must be well matched to achieve the desired property of a surface, e.g., a paint film [65,66,67,68].
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Some newer functional pigments are based on the layer&#;substrate principle already presented for special effect pigments. This principle can also be used in a suitable way for electrically conductive, IR-reflective, and laser-markable pigments [64, 65, 68]. Table 6 contains a list of such pigments with compositions and applications.
Table 6 Functional pigments based on layer&#;substrate principle [2]Full size table
Electrically conductive pigments based on mica platelets coated with a conductive (Sn,Sb)O2 layer exhibit conductivity sufficient for antistatic flooring and electrostatic wet painting. Their advantage over conductive carbon black pigments is the light-gray color of the mica pigments, which offers the user extended coloristic possibilities.
Certain infrared-reflective pigments based on the layer&#;substrate principle are attracting growing interest for various applications. The main focus is on the range of solar thermal radiation (near infrared, 750&#; nm). Mica pigments with TiO2 layers of suitable thickness reflect a large part of solar thermal radiation from their surface while at the same time allowing through the main part of the light visible to the human eye and important for photosynthesis in plants (photosynthetic active radiation (PAR), 400&#;750 nm). Thus, such pigments, embedded in polymers, can be used in a favorable way for greenhouses and agricultural films, but also for transparent roof elements and exterior parts of buildings, where the aim is to avoid temperature rises under solar influence while maintaining the transmission of visible light [64, 65].
Functional pigments based on the layer&#;substrate principle are also suitable for laser marking of polymers. Layers consisting of TiO2 or (Sn,Sb)O2 deposited on mica platelets are particularly suitable for this purpose. When focused laser beams impinge on the pigment particles embedded in the polymer, very strong local heating occurs as a result of absorption of the laser light by the particles. This process can lead to carbonization of the polymer in the area of the absorbing particles and thus to dark-gray to black coloration. Even small amounts of 0.1% pigment in the polymer are sufficient to produce high-contrast markings. Lasers of different wavelengths are used for laser marking of polymers: CO2 laser (10,600 nm), Nd:YAG laser (532 or nm), and excimer laser (193&#;351 nm). The markings created in the polymer are very durable and abrasion resistant. Typical applications for laser marking of polymers are found in the marking of electrical equipment, electronic components, medical devices, automotive components, and traceability codes and serial numbers of various products. Figure 9 shows examples of black and white marking of plastic parts containing laser-markable pigments. Light-colored laser markings are also possible. Here, pigments that lead to local foaming of the partially evaporating polymer when interacting with laser beams are used [2, 68].
Fig. 9Examples of plastic parts with black (left) and white (right) laser marking
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It is ideal for coloring concrete designed to mark, identify, protect, and encase, utility conduit, pipes, wire cables, or fiber optic lines.
Usage It is ideal for coloring concrete designed to mark, identify, protect, and encase, utility conduit, pipes, wire cables, or fiber optic lines.
SCOFIELD® Integral Color Utility uses pigments that are weather resistant, lightfast, and alkali resistant. It contains a minimum of 50 % post-industrial recycled material. It contains no corrosive materials and will not initiate, accelerate, or promote corrosion of reinforcing steel, metal, plastic, or rubber encasements.
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