Pearlescent or nacreous coatings or pigments possess optical effects that not only serve decorative purposes (such as cosmetics, printed products, industrial coatings, or car paints), but also provide important functional roles, such as security printing or optical filters.
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Pearlescent or nacreous pigments have become popular in the creation of luster effects in coatings. This has enabled the generation of new and unique color effects for automotive, industrial, cosmetic and pharmaceutical applications. The pearlescent effect is produced by the specular reflection of light from the many surfaces of the platelets with parallel orientation at various depths within the coating.[1] Light striking the platelets is partially reflected and partially transmitted through the platelets. A pearly luster effect is produced by the dependence of reflection on viewing angle, and the sense of depth is created by reflection from many layers. The platelets must be extremely smooth to maximize reflected light, and any surface roughness diminishes the lustrous effect. Non-fine particles or pigments with rough edges can also negatively affect the lustrous appearance.
Lustrous, iridescent, and angle-dependent optical effects are now commonplace and are found in many products. These optical effects not only serve decorative purposes (such as cosmetics, printed products, industrial coatings, or car paints), but also provide important functional roles, such as security printing or optical filters. In the security field, for example, angle-dependent optical effects obtained from pearlescent pigments cannot be easily copied with photocopier machines or photographic techniques, making forgery much more complicated and expensive.
In decorative applications, several advantages result from the use of special effect pigments. The first is the illusion of optical depth, which is created by the arrangement of a multiplicity of platelet-like semi-transparent particles of a pearlescent pigment. The achieved impression is the result of reflection of light at the different interfaces between pigment and binder and at the boundary layers of the effect pigment itself. The effect is especially strong when extended areas are profiled as in automotive fenders. Currently, pearlescent pigments can be found in the paints of more than 40% of the cars in the United States and 30% in Europe. The second advantage is the subtle to startling eye-catching effect of an angle-dependent color. This greatly enhances aesthetics and promotional eye-catching appeal of articles.
Optical effects such as directed reflection, multiple reflection, interference, and color travel (strong angle-dependent optical effects) cannot only be achieved with special pigments in parallel alignment in an application system, but also by utilizing extended multilayer films. Such films consist of continuous, non-pigmented inorganic or organic polymeric materials, which are manufactured in a thickness of a few to several hundred micrometers. These films are structured typically as multilayer systems with alternating layers of different refractive index. Effect pigments, on the other hand, generate their optical attractiveness in the application system because of the ability of easy parallel orientation of a multitude of platelet-like particles. Also in powder coating applications, where an optimal orientation of the pigment platelets is difficult in some cases, strong lustrous color effects are achievable. Reflection, multiple reflection, and interference is here possible, also under circumstances where the particles are partly disoriented, only the achieved effects can be different.
Pearlescent pigments can basically be classified into two groups:[1][2][3]
Pigments without a layer structure are the well-known metal effect pigments, such as aluminium or copper–zinc (brass) platelets. Flaked transparent effect pigments such as single-crystalline BiOCl and polycrystalline TiO2 also belong to this group. These non-metallic flakes are mostly very thin to achieve a certain interference color. This can lead to a lower mechanical stability compared with flakes based on substrate platelets.
Only a few materials are known to crystallize in thin flakes with suitable size and thickness for use as effect pigments. Substances which do not crystallize in this manner can be used only with thin supporting platelets, the substrates or templates onto which the pigment materials can be deposited. The best-known examples are pearlescent pigments based on platelets of natural or synthetic mica, alumina flakes (Xirallic®) coated with thin layers of TiO2, and Fe2O3. Pigments based on transparent substrates, such as mica-based products, can be easily combined with absorption pigments or with metal effect pigments even in thicker coatings. The advantage of metal effect pigments is their strong hiding power in the application system. Mixtures of transparent pearlescent pigments and more hiding metallic pigments are often used in automotive coatings.
By far the most important effect pigments without layer structure are the metal effect pigments.[4] They consist of flakes, or lamellae, of aluminium ("aluminium bronzes"), copper and copper-zinc alloys ("gold bronzes"), zinc, or other metals. The metallic effect is caused by the reflection of light at the surfaces of the pigment particles. The luster effect decreases as the proportion of light scattered at edges and corners of the particles increases. Larger particles are better reflectors, leading to higher brilliance and brightness. The metallic appearance depends also on the orientation of the metal flakes in the application system, the particle shape, the transparency of the binder matrix, and the presence of other colorants. The required particle size of the pigments depends on the intended use and can vary from a few micrometers (offset printing) to medium grades (10–45 μm, automotive coatings, gravure and flexographic printing) and coarser grades (corrosion-inhibiting systems, plastics). The thickness of the flakes can vary from less than 0.1 to 1 μm.
There are leafing and non-leafing metal effect pigments on the market. Leafing pigments float on the surface of paint or printing ink films because of their high interfacial tension, achieved by the use of stearic acid during the pigment manufacture. On the other hand, non-leafing pigments are completely wetted by the application medium and are dispersed homogeneously throughout the coating. Non-leafing properties are created by using lubricants that consist of branched-chain or unsaturated fatty acids (e.g., oleic acid) or polar substances (e.g., fatty amines).
Metal effect pigments are produced by treating metal granules with stamping machines. Ball mills using dry milling (Hametag process)[1][4] or wet milling (Hall process)[4] are mostly used to produce metal flakes. During the ball milling process, a lubricant is added to prevent cold fusion and to achieve the desired leafing or non-leafing properties.
Standard aluminium pigments are produced in "cornflake" and "silverdollar" types depending on the quality and shape of the starting granules and on the milling conditions. A special type is PVD aluminium, also known as VMP (Vacuum Metallized Pigment), produced by a vacuum process where the aluminium is deposited on a web. After releasing the deposited aluminium from the web, very thin flakes are obtained, with improved mirror-like effects when incorporated into coating systems.
The current world market for metal effect pigments can be estimated at more than 20,000 tonnes per year.[3] Depending on the production process and the application, the pigments are supplied in powder form or as solvent-containing preparations (pastes, granulates). Stabilized aluminium pastes with water or water-miscible solvents are available for waterborne coating or printing systems. Pigments coated with special organic (e.g., acrylics) or inorganic materials (e.g., silica) are achievable for powder coatings.
Natural pearl essence, also called natural fish silver, is a pigment suspension derived from fish scales, skin, or bladder. The pigment particles in the suspension are platelet-shaped with a high aspect ratio. They consist of 75–97% guanine and 3–25% hypoxanthine. The ratio of these two purines depends on the fish species (e.g., herrings, sardines). One ton of fish yields less than 250 g of guanine. An industrial synthetic process for producing purines with this crystal shape does not exist. An aqueous suspension of fish scales is, therefore, extracted with organic solvents to dissolve and remove the proteins. The remaining dispersion contains purine crystals and scale, which are separated from one another by a complicated washing and phase-transfer process.
Pigment platelets tend to agglomerate and are therefore only handled as dispersions. These dispersions are used almost exclusively in cosmetic applications (nail enamels, lotions, shampoos) because of the very high price. The pigment particles of natural pearl essence show a high but soft luster (nD = 1.79 (parallel) to 1.91 (perpendicular)) and a relatively low density of 1.6 g/cm³, which reduces settling in liquid formulations. The world production of natural pearl essence in 2004 is estimated to be less than 50 tonnes.[4]
Basic lead carbonate (Pb(OH)2•2PbCO3) can be synthesized in the form of thin hexagonal platelets by precipitation from aqueous lead acetate solutions. Carbon dioxide is reacted with these solutions under carefully controlled conditions. The resulting platelet-shaped particles are less than 0.05 μm thick and have diameters of about 20 μm, yielding an aspect ratio of about 200. Due to their high refractive index of 2.0 and their even surfaces, the platelets exhibit a very strong luster.[4]
Bismuth oxychloride (BiOCl) effect pigments are produced by hydrolysis of very acidic bismuth salt solutions in the presence of chloride. The crystal quality can be adjusted by the chosen reaction parameters, such as bismuth salt concentration, temperature, pH-value, reactor geometry, and addition of surfactants. The usually tetragonal bipyramidal crystal geometry can be flattened to platelets with high aspect ratio. Pigments with an aspect ratio of 10–15 show low luster and very good skin feeling and are used as fillers in cosmetics. Crystals with higher aspect ratio show strong luster and are mainly used for nail polish. The low light stability, the fast settling behavior caused by a density of 7.73 g/cm³, and the lack of mechanical stability limit the use of bismuth oxychloride in technical applications. Therefore, it is mainly used in cosmetics, but also in buttons and jewelry. The current world market is about 400 t per year.[4][5]
Micaceous iron oxide consists of pure or doped α-iron oxide (α-Fe2O3, hematite). It is found already in nature in form of platelets. This natural product with a density of 4.6–4.8 g/cm³ and a dark gray color with low luster is nearly exclusively used in corrosion protection coatings. Micaceous iron oxide can also be obtained as platelets by hydrothermal reaction in alkaline media. If substantial amounts of doping materials are incorporated, the aspect ratio can be increased up to 100, resulting in much better luster. The color can be also shifted from dull dark to a more attractive reddish brown allowing the use of the platelets for decorative applications.[3] Al2O3, SiO2, and Mn2O3 are the most important doping constituents. Fe(OH)3 or better FeOOH as starting material is heated in an alkaline suspension together with the doping constituents to temperatures above 170 °C, typically 250–300 °C. Platelets of doped micaceous iron oxide are formed after several minutes to hours. In a second reaction phase, the pH-value is further increased leading to the growth of flat basal faces.
Titanium dioxide flakes can be produced breaking down continuous films of TiO2. Such films can be obtained using a web-coating process where TiOCl2 is thermally hydrolyzed on the surface of the web. Substrate-free TiO2 flakes can also be achieved from TiO2-mica pigments by dissolving the substrate in strong acids or hydroxides. The so-obtained titanium dioxide flakes are not single crystals but polycrystalline and slightly porous. They show only limited mechanical stability in most cases and can, therefore, not be used in technical applications where stress is exerted.
Some organic pigments can also be forced to crystallize in form of flakes. Typical examples are 1,4-diketo-3,6-diarylpyrrolo(3,4-c)-pyrrole (DPP), 2,9-dichlorochinacridone, and metal phthalocyanines. However, the difference of their refractive index and that of the typical application media is too small to generate strong interference colors and luster effects. In most cases, the aspect ratio of these crystals is also much smaller than that of the substrate-free and the substrate-based inorganic effect pigments.
Liquid crystal polymers (LCP) can also be used in form of flakes or large films to achieve interference and especially angle-dependent interference color effects. The structure of these materials consist of parallel oriented chiral cholesteric (nematic) liquid crystalline layers having their director rotated by a certain angle with respect to an adjacent layer, building up a helical array. The helical structure is responsible for interference effects because the refractive index is changing from layer to layer. It is also possible to get such optical effects by helical superstructures.[1][5] A number of LCP pigments and films are based on polysiloxanes. The first step of the manufacture is the formation of a thin cross-linked film of a liquid crystalline polymer. After a UV-curing step for polymerization, the so formed solid film is ground to small platelets. These particles can be used to achieve angle-dependent effects when having a thickness of more than 4 μm in most cases. Up to now the application of these pigments is still limited because of the thickness and some stability problems.[5]
Pharmaceutical manufacturers attempt to increase brand awareness and satisfaction through product presentation (for instance, colors incorporated in coatings, encapsulation and other tablet-forming methods).[6] Importantly, this approach could be used to protect pharmaceuticals from counterfeiting and adulteration. Coatings to which pearlescent pigments have been incorporated can further enhance the color appeal of coated tablet and capsule pharmaceutical products. Possible systems for this application utilise polymers like polyvinyl alcohol, hypromellose, Carrageenan, or microcrystalline cellulose, as the carrier materials in levels up to 50%, a pearlescent pigment or combinations, e.g., micacious titanium dioxide in levels between 1–10%, and dispersants like tween, lecithin and polyethylene glycol in levels between 0–15%, to act as stabilisers.