Effect pigments—past, present and future

Progress in Organic Coatings 54 (2005) 150–163

Review

Effect pigments—past, present and future Frank J. Maile, Gerhard Pfaff, Peter Reynders ∗ Merck KGaA, Pigments R&D, Frankfurter Str. 250 D-64271 Darmstadt, Germany Received 3 January 2005; received in revised form 16 June 2005; accepted 20 July 2005

Abstract Driven by trends in fashion, automotive and other consumer markets, pigments that generate special effects like angle-dependent color or decorative texture have a growing economic significance and can be found in various industrial products and end-user applications [G. Pfaff, K.D. Franz, R. Emmert, K. Nitta, R. Besold, Ullmann’s Encyclopedia of Industrial Chemistry: Pigments, Inorganic, Section 4.3, sixth ed., VCH Verlagsgesellschaft, Weinheim, Germany, 1998 (electronic release)]. In decorative uses, special effect pigments provide three major advantages: (a) they can create the illusion of optical depth, which is for example be observed when applying pearlescent pigments in car paints; (b) they can generate subtle to startling angle-dependent eye-catching color effects, which can for example be used in car paints or decorative printing; (c) the have the ability to imitate the effect of natural pearls in buttons, plastic bottles, and many other decorative objects. Pearlescent pigments have been reviewed in a number of publications [L.M. Greenstein, in: P.R. Lewis (Ed.), Pigment Handbook, vol. I, second ed., John Wiley & Sons, New York, 1998, p. 829; R. Maisch, M. Weigand, Pearl Luster Pigments, Verlag Moderne Industrie, Landsberg/Lech, Germany, 1991; R. Glausch, M. Kieser, R. Maisch, G. Pfaff, J. Weitzel, in: U. Zorll (Ed.), Special Effect Pigments, Vincentz Verlag, Hannover, Germany, 1998; G. Pfaff, P. Reynders, Chem. Rev. 99 (1999) 1963; G. Pfaff, Chem. unserer Zeit 31 (1997) 6]. This paper provides additional information on the latest developments related to effect pigments and their production technology, the orientation behavior of effect pigment particles and the analytical methods used for the investigation of inorganic layers in effect pigments. © 2005 Elsevier B.V. All rights reserved. Keywords: Effect pigments; Effect pigment technology; Optical films; Analytical methods (SEM, TEM, etc.); Pigment particle orientation

Contents 1. 2. 3. 4.

∗

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Comparison of effect pigments with extended optical films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Types and manufacture of extended optical films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effect pigments without a layer structure—substrate-free pigments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Metal effect pigments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Natural pearl essence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Basic lead carbonate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Bismuth oxychloride . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5. Micaceous iron oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6. Titanium dioxide flakes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7. Flaky organic pigments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.8. Pigments based on liquid crystal polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Corresponding author. Tel.: +49 6151726004; fax: +49 615172916004. E-mail address: [email protected] (P. Reynders).

0300-9440/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2005.07.003

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5. 6.

7. 8.

9. 10.

Multilayer structures of the Fabry–Perot type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Substrate-based effect pigments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. Pigments based on mica platelets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2. Pigments based on alumina flakes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3. Pigments based on silica flakes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4. Pigments based on glass flakes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5. Pigments based on iron oxide flakes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6. Pigments based on graphite flakes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.7. Pigments based on aluminum flakes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pigments with structured surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Analytical methods used for analysis of inorganic layers in effect pigments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1. Particle size distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2. Light and electron microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3. Methods with depth profiling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4. Wettability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5. Isoelectric point and zeta potentials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Orientation of effect pigments in applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Lustrous, iridescent, and angle-dependent optical effects can be found in various industrial products and end-user applications. These effects are used for functional purposes, such as security printing or optical filters, and for decorative purposes, such as cosmetics, plastics, printed products, industrial coatings, or car paints. In the security field, for example, angle-dependent optical effects cannot be easily copied with copier machines or photographic techniques. Forging is only possible if similar products are utilized, which makes forging much more complicated and expensive. Consequently, pearlescent and optical multilayer pigments are used on banknotes by many countries. Specially designed optical multilayer polymer films as well as parallel oriented effect pigments in an application medium can reflect a certain portion of the visible light. They can act as decorative or functional optical system. An example for a functional application is the use of special effect pigments in greenhouses where they take an influence on the transmitted and reflected part of the light and, hence, take an influence on the plant growth. In decorative applications, three major advantages are seen 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. Such an effect is especially strong when extended areas are profiled as in automotive fenders. Pearlescent pigments can be found in the paints of more than 40% of the cars in the United States and 30% in Europe.

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The second advantage is the subtle to startling eyecatching effect of an angle-dependent color. A consequence of this is that special effect pigments are often used in application for their aesthetic and promotional eye-catching appeal. The third field is the ability to imitate the effect of natural pearls in buttons, plastic bottles, and many other objects [5]. Optical effects such as directed reflection, multiple reflection, interference, and color travel (strong angle-dependent optical effects) cannot only be achieved by using specially designed effect pigments that are incorporated in parallel alignment in an application system but also by utilizing extended multiplayer films. Such films consist basically 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 likewise possible, also under circumstances where the particles are partly disoriented, only the achieved effects can be different. In this review, a special focus is set on the comparison of effect pigments in their application systems with those of extended optical films, the description of the different effect pigment types, the analytical methods for their analysis, and the relationship of particle geometry and orientation of effect pigments.

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2. Comparison of effect pigments with extended optical films Effect pigments show several advantages in decorative and functional applications compared with extended films: • easy to incorporate in all relevant application systems; • many possibilities to blend effect pigments with other pigments leading to a broad variety of different color effects; • achievable effects are more “vivid” because of the single particles, which are resolved to a certain degree by the human eye and therefore showing various textural effects, which are easy be obtain; • possibilities to enlarge the variety of optical and functional effects by different particle sizes of the pigment platelets; • easy to manufacture with acceptable costs; • environmentally save to produce; • high flexibility to equip surfaces with the desired effects by easy to handle application procedures, e.g., spraying, printing, extruding. Extended films, on the other hand, can in most of these cases be used only in a very limited manner or their utilization is practically impossible. Therefore, the development of new materials for the discussed decorative and also for functional effects (e.g., electrical conductivity, infrared reflectivity) is clearly oriented on platelet-shaped effect pigment particles.

3. Types and manufacture of extended optical films Extended interference films can be created in three different ways: • gas phase deposition (for inorganic films); • liquid phase deposition (for inorganic and organic films); • casting and extrusion techniques (for organic films); Gas phase deposition can be carried out in two main processes, the physical vapor deposition (PVD) and the chemical vapor deposition (CVD). Commercially used PVD methods are high-vacuum evaporation (thermal evaporation by electrical current or with electron guns), magnetron sputtering, ion plating, and ion-assisted evaporation. Practically used CVD methods are low-pressure CVD and plasma-enhanced CVD. The MOCVD process is using metal organic precursors, which are decomposed or brought to reaction with other gaseous compounds. Most of the substrates used for the formation of extended optical films show a low refractive index, e.g., glass or polymers. The coating must therefore possess a higher refractive index than the substrate in order to achieve reflection and interference phenomena. Titanium dioxide and zirconium dioxide are often used as coating materials, but also other compounds like zinc sulfide and tantalum pentoxide or metals are deposited on the substrate. In many cases multilayer films are produced containing for example silicon dioxide, aluminum oxide, or magnesium

fluoride as low-refractive thin layers. These layers are combined with layers of high-refractive materials in alternating order. Most of these layers are transparent; some are semitransparent (e.g., iron oxide, metals). Mixed oxide layers are also feasible when the evaporation conditions of the materials are similar. Cost reasons are mainly responsible that gas phase deposition processes are only limited used for the formation of extended films. Optical lenses, filters, laser mirrors, eyeglasses, and elements for communication systems are the most important applications. Besides these, electro-chromic and transparent conductive coatings formed by gas phase deposition play an increasing role. Liquid phase deposition is very often used to generate extended optical films. There are three important methods for liquid phase deposition: • dipping processes (for large surfaces, most economic, easy to apply); • spreading out the liquid film by centrifugally spinning the wetted surfaces (spin coating, only applicable for small circular objects); • spraying processes. For the manufacture of homogeneous layers by liquid phase deposition, the coating solutions need the following properties: • suitable solubility of the starting compounds together with an adequate tendency toward crystallization during the evaporation of the solvent; • sufficiently small contact angles between substrate and solution to achieve a good wetting behavior; • easy transformation of the deposited gel film into a homogeneous solid film layer without the formation of cracks and surface roughness. Liquid phase deposition processes are already used over several decades for the production of extended optical and functional films. Glass is acting in most cases as the substrate for these films. Optical effects, especially such with angle-dependent color impressions, can be generated by organic multilayer films. Refractive indices of organic materials differ much less than those of inorganic. Therefore, such films usually contain more than 70 layers, in some cases up to 1000 layers to achieve a strong interference effect. The difference of the refractive indices should be at least 0.06 [5]. The single layers of such multilayer arrangements are usually 50–400 nm thick. The quantity of the reflected light and the chroma depends on the difference of the refractive indices, the number of layers, the ratio of the optical thicknesses of the layers, and the uniformity of the thicknesses. Organic multilayer films can be manufactured by a chillroll casting technique using a conventional single manifold flat film in combination with a feed block that collects the polymer melts from two or more extruders (co-extrusion) and arranges them into the desired layer design. Polymers acting

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as a low-refractive index layer are usually polymethacrylate, polymethacrylate-polyester, polypropylene, ethylene vinyl acetate, or polyether glycol (refractive indices of 1.48–1.50). The high-refractive index polymers are mostly poly(ethylene terephthalate) and poly(butylene terephthalate) and their copolymers (1.55–1.61), polystyrene (1.60), or polycarbonate (1.59). The polymer layers can contain colorants and/or pigments. It is still difficult in industrial scale to achieve a uniform interference color effect throughout the complete film. The manufacture generates a slight thickness variation leading to different color patches, for example, green and blue, which appear very close to each other. Therefore, the application of these films is mostly limited to gift-wrapping materials and specialty packaging.

4. Effect pigments without a layer structure—substrate-free pigments Effect pigments can basically be classified into two groups: • platelets that consist of only one optically homogeneous material (substrate-free pigments); • platelets that have a layered structure and consist of at least two optically different layer materials (pigments with layer-substrate structure or multilayer pigments without a substrate). Pigments without a layer structure are the well-known metal effect pigments, such as aluminum or copper–zinc platelets. Also transparent effect pigments like singlecrystalline BiOCl or polycrystalline TiO2 in flaky shape 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 a substrate platelet. Only a few materials are known to crystallize as thin flakes suitable in respect of size and thickness for effect pigments. Substances, which do not crystallize in this manner, can be used for effect pigments only by working with thin supporting platelets, the so-called substrates or templates onto which the interesting materials can be deposited. The best-known examples are the pearlescent pigments based on platelets of natural or synthetic mica coated with thin layers of TiO2 or Fe2 O3 . Pigments based on transparent substrates, such as the 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. For outdoor use, for example in automotive coatings and architecture, the weathering stability of many effect pigments can be improved by an additional surface treatment [1–6].

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4.1. Metal effect pigments The most important effect pigments without a layer structure are by far the metal effect pigments. They consist of flakes or lamellae of aluminum (“aluminum bronzes”), copper and copper-zinc alloys (“gold bronzes”), zinc or other metals. The metallic effect is caused by the reflection of light at the surface of the pigment particles. The so observed luster effect is decreased when the part of the 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 few micrometers (offset printing) to medium grades (10–45 ␮m, automotive coatings, gravure and flexographic printing) and coarser grades (corrosion-inhibiting systems, plastics). The thickness of the flakes can vary from smaller than 0.1–1 ␮m. 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 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. The non-leafing properties are obtained by using lubricants that consist of branchedchain or unsaturated fatty acids (e.g., oleic acid) or polar substances (e.g., fatty amines). Metal effect pigments are produced treating metal granules with stamping machines. Ball mills using dry milling (Hametag process [7]) or wet milling (Hall process [8]) are mostly used to produce the 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 aluminum pigments are produced as “cornflake” and “silverdollar” types depending on the quality and shape of the starting granules and on the milling conditions. A special type is the PVD aluminum, also known as VMP (Vacuum Metallized Pigment), produced by a vacuum process where the aluminum is deposited on a web. After releasing the deposited aluminum from the web, very thin flakes are obtained showing improved mirror-like effects when incorporated in coating systems. Electron micrographs of the three different aluminum pigments are shown in Fig. 1. The current world market for metal effect pigments can be estimated at more than 20,000 t per year. Depending on the production process and the application, the pigments are supplied in powder form or as solvent-containing preparation (pastes, granulates). Stabilized aluminum 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.

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Fig. 1. Scanning electron micrographs of the aluminum pigments (a) Alucar® (cornflake-type), (b) Alushine® (silverdollar-type), and (c) Decomet® (PVD-type); all pigments from Schlenk Metallpulver GmbH & Co. KK.

4.2. Natural pearl essence

4.3. Basic lead carbonate

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. The 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 t [4,5].

Basic lead carbonate (Pb(OH)2 ·2PbCO3 ) can be synthesized in form of thin hexagonal platelets by precipitation from aqueous lead acetate solutions. Carbon dioxide is reacting with these solutions under carefully controlled conditions. The resulting platelet-shaped particles are less than 0.05 ␮m thick and show diameters of about 20 ␮m, yielding an aspect ratio of about 200. Due to their high refractive index of 2.0 and their even surface, the platelets exhibit a very strong luster. Increasing of the particle thickness by modified reaction conditions can lead to pigments with interference colors. The crystals are very fragile and their high density of 6.4 g/cm3 results in fast settling in suspensions. Basic lead carbonate is handled as stabilized dispersion due to its agglomeration tendency. The use of the pigment is more and more limited in view of toxicological risks. Less than 1000 t of basic lead carbonate effect pigments are produced annually worldwide [4,5]. 4.4. Bismuth oxychloride 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

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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]. 4.5. Micaceous iron oxide Micaceous iron oxide consists of pure or doped ␣-iron oxide (␣-Fe2 O3 , 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 [9]. Al2 O3 , SiO2 , and Mn2 O3 are the most important doping constituents. SiO2 forces the formation of thin small platelets, Al2 O3 yields thin larger flakes, and Mn2 O3 reduces the thickness. The use of ZrO2 , B2 O3 , P2 O5 , SiO2 , or additional Al2 O3 reduces the agglomeration in the application system. 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. 4.6. Titanium dioxide flakes 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. 4.7. Flaky organic pigments Some organic pigments can also be forced to crystallize in form of flakes. Typical examples are 1,4-diketo-3,6-diaryl-

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pyrrolo(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. 4.8. Pigments based on liquid crystal polymers [5] 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 [10]. 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 [11]. 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 ␮m in most cases. Up to now the application of these pigments is still limited because of the thickness and some stability problems.

5. Multilayer structures of the Fabry–Perot type Structural arrangements consisting of alternating thin metal and dielectric layers can be used to achieve strong angle-dependent optical effects, e.g., in form of so-called optically variable pigments (OVP) [5,12]. Different color shifts can be produced by precisely controlled thickness of the multilayers. The metal layers consist in most cases of chromium (semitransparent absorber layers on the top and the bottom of a five-layer system) and of aluminum (opaque reflector layer in the center of the layer structure). The dielectric layers in between the chromium and aluminum layers consist mostly of magnesium fluoride or silicon dioxide. Such layer systems are the basis for an optical interference phenomenon called Fabry–Perot effect, which is different from interference effects of transparent layer systems because of the complete reflection of the light at the opaque reflector layer. Symmetrical arrangements of at least five layers are necessary to achieve strong color-shifting effects. In the case of pigments, only the five-layer systems play a role for practical use. The pigment flakes are typically produced using a physical vapor deposition process in a series of specific coating machines. The different layers are coated one after another on a polymer web. In a next step, these layers are removed

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from the web and crushed to form flaky particles with dimensions of 0.2–2 ␮m in thickness and 1–100 ␮m in diameter. The ready pigments have found an application especially in preventing counterfeiting of documents such as banknotes, stock certificates, visas, or passports [12].

6. Substrate-based effect pigments As explained above, non-substrate based, platelet-like effect pigments are in some cases brittle and often mechanically not stable. In addition, they are limited by their chemical composition. Consequently, effect pigments were developed which are based on a substrate as the mechanical support of a thin optical layer. The substrate also acts as a template for the formation of the thin layer. The material of this layer can be chosen from a much larger group than for the nonsubstrate based effect pigment. If the thickness distribution of the substrate becomes narrower, the substrate will start to act as an optical layer and becomes part of an optical three-layer or multilayer system [13]. Special effect pigments based on mica, alumina, silica, and metal flakes, have been extensively reviewed in 1999 [5] and 2003 [14]. The most important optical layers consist of titania (both rutile and anatase), iron(III) oxide, mixed titanium–iron oxides, silica (as low-refractive layer in multilayer systems), and chromium(III) oxide. 6.1. Pigments based on mica platelets Mica-based effect pigments were first described in 1942 [15,16]. Their commercial success started in the 1970s due to improved reproducibility of production and got accelerated in the mid 1980s by the introduction of weather-resistant types for outdoor application. The latest significant development was the synthesis of optical multilayer-systems on mica at the end of the 1990s. Compared to the synthetic substrates, natural muscovite mica is rather inexpensive and available in large quantities in nature. Due to its crystal structure as a layered silicate, it can be cleaved to thinner flakes of a mean thickness of typically 200–500 nm. The diameters of the mica flakes are mostly in the range from 5–200 ␮m. The advantages of the metal oxide-mica pigments lead to the fact that this group of transparent effect pigments accounts for more than 90% of the world market. Synthetic fluorophlogopite mica has reduced iron content compared with natural muscovite and, consequently, a somewhat whiter masstone. However, this advantage is rather subtle and has not lead to any significant market success yet. Mica-based pigments with single layers show an interference color that “turns on and off” because they consist of just one optical layer [5]. When looking closely at the angle dependence of the interference color, one can find a few degrees of a color shift. Effect pigments on mica are typically produced by the deposition of metal oxide layers on the mica in aqueous

Fig. 2. Transmission electron micrograph of a TiO2 -mica pigment; the cross section shows the mica platelet and the closed TiO2 layer with its typical grain structure covering the mica. FIB cross sectional preparation, diffraction contrast image.

suspension followed by a calcination process [1–6]. Titanium dioxide-mica pigments are manufactured starting from TiOSO4 (homogeneous hydrolysis) or TiOCl2 (titration). TiO2 can be formed as anatase (TiO2 direct on the mica) or rutile (using an intermediate SnO2 layer directly on the mica). The interference color of these pigments is dependent on the thickness of the TiO2 layers, which is typically in the range of 50–300 nm on both sides of the mica platelets. Therefore, the control of this thickness is one of the most important factors for the reproducible manufacture of the metal oxide-mica pigments. Iron oxide layers can be formed on the mica by a comparable process starting from iron(II) sulfate or iron(III) chloride. The transmission electron micrograph in Fig. 2 shows a cross section through a TiO2 -mica pigment depicting the mica platelet and the TiO2 layer thickness, which is precisely controlled. Mica-based pigments with multilayers can show a pronounced angle dependent color effect, if the optical thicknesses of the layers are carefully chosen. However, the mica-multilayer pigments are much thicker than the later discussed silica flake based pigments (e.g., twice the TiO2 –SiO2 –TiO2 stack plus the optically inactive mica thickness) and also much heavier leading to a higher pigment load required for certain color strength. 6.2. Pigments based on alumina flakes Alumina (␣-Al2 O3 , corundum) flakes can be produced in a good optical quality using a controlled crystal growth process in molten sodium sulfate [17,18]. After washing, very thin flakes are found which consist of corundum and show a high aspect ratio, a narrow thickness distribution, and very smooth surfaces. The thickness can be controlled by doping and special reaction conditions. When coated with rutile or iron(III) oxide, the flakes exhibit a distinct directed reflection, often described as crystal luster. The reason for this is the smooth surface combined with the relatively homogeneous thickness of the particles and the adjusted metal oxide layer thickness. Therefore, the pigments based on this substrate often show a texture appearance (looking like sparkle) different from the other pigments discussed here. Fig. 3 gives an impression of the shape and

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Fig. 3. Scanning electron micrographs of (a) pure alumina flakes and (b) alumina flakes coated with TiO2 (rutile) as used for pearlescent pigments.

size of alumina flakes as well as the quality of a TiO2 coating applied thereon. The narrow thickness distribution of the alumina substrate in the commercially available products is also leading to color effects between mica- and silica-flake based pigments. As a specialty, silver-white pearl luster on an alumina substrate is created via blending batches of different substrate thickness in order to yield a strong sparkle composed of colored light flashes. 6.3. Pigments based on silica flakes Thin silica flakes (SiO2 ) with a very uniform and controllable thickness can be manufactured using a specially designed web-coating process [18]. The flakes can also be used as substrate particles for effect pigments to achieve improved chromatic strength and purity as well as color travel

effects by coating with high refractive metal oxide layers (titanium dioxide, iron(III) oxide). Scanning electron micrographs of silica flakes and iron oxide coated silica flakes are shown in Fig. 4. The thickness of the silica flakes in the existing commercially available products is chosen to be so narrow that it leads together with the adjusted metal oxide layers to the desired effects of optimized optical threelayer systems. The very homogeneous thickness of the SiO2 flakes used in practice is in the order of 400 nm and is therefore comparable to that of the average thickness of the mica particles. Some of the silica-based pigments show color travel effects such as violet-green, red-gold, green-red or gold-blue. They can be used amongst others in automotive effect coatings, cosmetic formulations, security printings and decorative plastics. A detailed description of the SiO2 flakes and the effect pigments based thereon can be found in [5,18,19].

Fig. 4. Scanning electron micrographs of (a) pure silica flakes and (b) a silica flake coated with ␣-Fe2 O3 ; the iron oxide layer is well sintered but still grainy.

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6.4. Pigments based on glass flakes Pearlescent pigments consisting of metal oxide-coated glass flakes are known since 1963 [16]. However, the available glass flake substrates have been rather thick, typically 10 ␮m, making of the use of such pigments impractical in layers like automotive paints and printing inks. After improvements of the glass manufacturing technology [20,21], effect pigments based on thinner glass flakes were introduced to the market during the last 5 years [22,23]. Compared to alumina and mica flakes, the glass flake substrates are more transparent and are in this aspect comparable to the silica flakes. However, they have a much broader thickness distribution than the latter. Therefore, color travel effects are not achieved by using single layer coatings on glass flakes. A large variety of silver metal-coated glass flakes are also commercially available. Very coarse sparkle fractions can be used in applications like plastic housings of household goods. Nickel-coated glass flakes are not as lustrous and play only a very minor role in the market. 6.5. Pigments based on iron oxide flakes The iron oxide flakes described before can also be used as a substrate for metal oxide layers. There is a first product on the market having a layer structure consisting of silica and iron oxide on the iron oxide flakes. The pigment with this layer arrangement shows an angle-dependent color changing from purple to gold [24]. 6.6. Pigments based on graphite flakes Flaky graphite can be used a substrate for effect pigments [25]. If thin enough for this use, e.g., <3 ␮m, the substrate is not complete hiding. A few products are commercially available. In comparison to the physical mixture of micabased pearl pigments and carbon black, the graphite-based products have an improved luster due to the lack of scattering at the carbon black surfaces. 6.7. Pigments based on aluminum flakes Some metal flakes, especially aluminum platelets, can be coated with iron oxide in a CVD process [26]. These metal flakes are fluidized in nitrogen gas atmosphere at temperatures around 450 ◦ C. Then the reagents Fe(CO)5 and oxygen are injected into the fluidized bed. They must be highly diluted in the inert gas to achieve a proper coating. The thickness of the formed iron oxide layers on the aluminum is controlled by the reaction time. The so-obtained pigments show golden, orange and reddish metal-like effects. The pigments based on aluminum can be modified using an additional silicon dioxide layer, which is wet-chemically deposited. The iron oxide layer is formed with the CVD process after the formation of this SiO2 layer. These pigments show strong color travel effects [24].

Fig. 5. Transmission electron micrographs of a structured (diffractive) pigment consisting of the layer order MgF2 /Al/MgF2 . FIB cross sectional preparation, diffraction contrast image.

7. Pigments with structured surfaces While the color effects of all other pigments discussed in this paper are caused by interference at thin layers, such effects can also arise from light diffraction on periodically structured surfaces. Two types of structured pigments are commercially available: a structured polymer films is metallized and grinded to particles [27,28]. Due to lack of availability of thinner polymer films, the minimum thickness of these products is 10 ␮m, which limits the use in printing and paint layer applications. The other type of products uses a structured polymer film as a template for vacuum deposition a MgF2 /Al/MgF2 or SiO2 /Al/SiO2 layer sequence, which is then separated from the polymer film by dissolving a supporting release layer [29]. The resulting particles are separated into size fractions. A typical thickness is below 1 ␮m, the periodicity of the structure is in the order of 1 ␮m and its depth is several hundreds of nanometers. Fig. 5 shows a transmission electron micrograph of a structured (diffractive) pigment of this type. MgF2 or SiO2 acts as a mechanical support of the thin structured aluminum film and not as high-refractive interference layer. The evaporation technique and the oneway use of the expensive structured film lead to extremely high production costs, which limits the use of this second type of structured pigments to high-end applications like banknotes.

8. Analytical methods used for analysis of inorganic layers in effect pigments 8.1. Particle size distribution Normal light or laser scattering methods assume a spherical particle shape for the measurement as well as for the calculation of particle size distributions (PSD). This assumption is not valid for effect pigments. If these methods are used nevertheless, the calculated PSD is often not in agreement with the particle sizes seen via light or electron microscopy.

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The method of choice is the image analysis of light microscopy pictures, which results in particle diameter distributions as well as in other parameters that characterize the shape of the particles such as circularity. Because of the broad diameter distribution several thousand to 50,000 particles need to be evaluated. 8.2. Light and electron microscopy Light microscopy is a decisive tool to evaluate the quality of optical layers because it is directly related to the visible color effect, the most important property the customer expects. Bright and dark field methods are often combined. The diffuse dark field, using stray light from the side, is the most valuable choice for transparent effect pigments for detecting internal features of the pigment particles. The lateral resolution of light microscopy is generally limited to a half of the illumination wavelength. Special techniques, such as near-field scanning optical microscopy (NSOM [30] or confocal laser microscopy [31]) have extended the usable resolution down to 50 nm. The thickness of the optical layers is normally in this range too, like the 60 nm thick TiO2 layer of silverwhite pearlescent pigments. Therefore, defects in the layers, such as cracks and pores, [14] are difficult to examine via these techniques [32]. Although being smaller than the resolution of the visible light, these small defects have nevertheless an influence on the optical properties: They scatter light via the Rayleigh mech-

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anism and are often the origin of more extended structural damage during further processing steps. In addition, they can cause problems in application, such as reduced weatherability. Therefore, electron microscopy is often used for the fine structure examination of layered pigments. The use of scanning electron microscopy (SEM) for effect pigments is not complicated since these samples do not change under vacuum and most of the interesting features are accessible via this technique [14]. Simple cut-through samples for transmission electron microscopy (TEM) are difficult to obtain because of the brittleness of their layered structure. One can overcome these difficulties by using special optimized ion-thinning method, like focused ion-beam (FIB), in the sample preparation (Figs. 2 and 5). An alternative method is atomic force microscopy (AFM) with its different variants, such as scanning tunneling microscopy (STM). Although its resolution can reach the atomic level, the scanning tip is too large to reach into the pores of the metal oxide layer of the typical pearlescent pigments. Consequently, one of the most important morphological features is not properly detected. AFM, however, can be used to characterize the usually relatively flat substrates (Fig. 6). One can easily see from Fig. 6 that the surface of the mica and the alumina substrate is relatively flat with some nm-sized steps where as the other substrates show longer range unevenness that is for the SiO2 flake caused by the production process and for the aluminum flake caused by the ductile character.

Fig. 6. Atomic force microscopy pictures of substrates for pearlescent pigments: (a) mica, (b) SiO2 flake and (c) alumina flake.

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8.3. Methods with depth profiling [33] For the characterization of layered systems depth profiling is obviously desirable. Some techniques, e.g., EDX, show a penetration that extent well through the whole pigment particle and are, therefore, not feasible for the task. SIMS, SNMS, Auger, and XPS techniques are applicable when combined with suitable ion beam sputtering. The sample preparation remains difficult, because all flakes in the sample spot need to be accurately aligned parallel to the sample holder. Otherwise, the layer boundaries appear less defined in the measurement that in reality. The usual restrictions of these methods are, of course, valid, such as matrix effects and change of the sample during the ablation via the ion beam. 8.4. Wettability Effect pigment particles are too small for placing a drop of fluid on them and, hence, measuring the contact angle directly. On the other side, they are also too large for the porous-powder approximation [34], e.g., leading to large interparticle pores in a pressed powder, which then prohibit the placement of a stable drop onto the powder. The attempt to determine Hansen parameters [35] via the dispersibility in a series of solvents failed because of the fast sedimentation of the rather large effect pigment particles [36]. Techniques based on the penetration of pigment beds by solvents failed because of irreproducible structural changes in the bed upon penetration. Techniques that are based on the penetration of pigment columns have turned out to be more reproducible [37]. But no results have been published yet. 8.5. Isoelectric point and zeta potentials Problems arise during the measurement because of the relatively large size, the weight, the shape and the charge anisotropy of the effect pigment particles. When the typical electrophoretic measurements [37] are applied, the particles settle and, due to friction, the mobility data become unreliable. When methods based on titration with polyelectrolytes [38] are used, preferential absorption of the polymers are observed yielding misleading results. The most reliable but still unpublished results were obtained with electroacoustical methods [39]. It is well known that the basal faces of mica have a different isoelectric point than the edges. Consequently, different electroacoustical responses were found for laminar and turbulent flow through the measurement cell [37].

9. Orientation of effect pigments in applications As described above, effect pigments provide unique visual effects, derived from their chemical composition and physical characteristics, whereas particle morphology/size as well

as flake orientation play a crucial role for the development of color and effect in different applications (e.g., mass pigmentation, powder/solvent/waterborne coatings, printing, cosmetics). When comparing particle size, specific gravity and particle geometry of effect and in-/organic absorption pigments significant differences are found. In comparison to in-/organic colorants effect pigments consist of particles that are 102 –103 times larger, this implies problems in printing processes, as the applied layer here is very thin (e.g., in offset printing around 2 ␮m). Another problem in connection with the size and specific gravity of effect pigments is sedimentation in coating formulations. According to Stokes’ law heavier particles tend to settle faster, a phenomenon influenced by the weight of the particles and the viscosity of the surrounding media. Additionally, the dielectric constant of the solvent used and colloidal interactions have to be taken into consideration. In this context it is predominantly important to circumvent the formation of hard sediments that cannot be redispersed [36]. A property not available from conventional in-/organic pigments is frequently called “flop”, it is the change of color and/or gloss with the viewing angle at which the origin of the effect lies in the almost two-dimensional, anisotropic nature of effect pigments [40]. This anisotropic morphology of the particles is the reason why their incorporation affects appearance, because a change in processing technology results in a mutated standard deviation of flake orientation. Fig. 7 provides an example and depicts cross-sections of an automotive basecoat layer incl. 2.5% of interference pigment (green type) as well as a digital photograph of the identical samples which have been applied in a pneumatic and electrostatic application process, respectively. The microscopic investigation of the cross-sections (Fig. 7b) displays the flake orientation of the effect pigments in the coating layer and confirms, that the application process has a major influence on the particle orientation in the polymer matrix (Fig. 7c), leading to apparent color deviations (Fig. 7a). They can be explained considering the following points: (1) a separation of particles during and after application (darker = electrostatic), and (2) a higher amount of “disoriented” platelets in the “electrostatic” layer, leading to an altered angle dependency or “flop”. Due to the huge differences in lightness of the panels their reflectance data should differ significantly. In order to verify this assumption, a reflectance curve of each surface was determined using an ETA Multi-FX 10 instrument, the expected discrepancy is readily seen in Fig. 7c. Industrial (liquid) paint application processes always make allowance for parameters like air pressure, pistol type, nozzle size, spraying pistol to object distance as they control flake orientation within the dried paint layer as well. They account for the geometry of the paint droplets, the impact speed and the spraying pattern generated on the substrate. The development of the paint film layer or coalescence of these polymer droplets loaded inter alia with flaky pigments is biased, another factor which affects the standard deviation

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Fig. 7. Digital photograph of steel panels painted (automotive OEM paint incl. 2.5% of interference pigment (green type)) under use of different application techniques (a) electrostatic (left) and pneumatic (right), (b) cross-sections (pneumatic and electrostatic, respectively), and (c) measured reflectance data (geometry: 45◦ /120◦ ) of both panels (red: pneumatic blue: ESTA).

of particle orientation within the polymer matrix. Finally, also the evaporation of (co-) solvents in solvent-/waterborne paints and the stratification of the polymer film should be mentioned for the sake of completeness. All these factors explain, why for example car body parts are being repaired by experienced body painters, as the repair of effect pigmented car surfaces is a real challenge and reproducibility of all application steps is the key [41]. One influencing factor for the orientation of the lamellar particles is the use of special additives in the paint formulation, like polymer beads (Fig. 8). This tool is frequently being used by refinish paint producers for “flop” and “sparkle” adjustment, whereas the objective of their use is the obtainment of a perfect match for the repaired parts and the areas which have been coated in an OEM application process.

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Regarding the term “sparkle”, it describes the ability of flakes to reflect light in a non-uniform manner and is determined in turn by particle size and orientation. Flakes that all lie parallel to one another, as in the case of leafing pigments (please read below) in coatings, will reflect incident light into the same direction. They therefore appear continuously, uniformly brilliant and exhibit no sparkle if the concentration is high enough. Where flakes make a more random angle with the substrate, as for non-leafing pigments (read below) in coatings or a formulation where the polymer additive (beads) are used (Fig. 8), a proportion will be oriented in such a way as to reflect light falling on their flat surfaces into the eye. Such flakes appear brighter than their neighbors and it is these pinpoints of light that collectively provide “sparkle” [40]. Fig. 8 depicts the orientation behavior of pearlescent pigments in a solvent-borne (acrylic-melamine) car refinish basecoat with/without the polymeric additive (beads), their influence on the orientation of the effect pigment particles is readily seen in the cross-sections. It could be shown [42], that a correlation between disorientation, realised in this case by a variation of the additive concentration, and the sparkle strength exists (Fig. 8). Another option to exert influence on the orientation of effect pigments is to take advantage of non-/leafing pigment properties. Leafing pigments cover the surface of a suitable vehicle with layers of overlapping particles, light scattering from the edges is reduced as the flakes are well oriented. Non-leafing effect pigments are equipped with oleophilic lubricants. In contrast to leafing pigments, they show good wetting in the medium they are incorporated, hence the flakes are dispersed uniformly within the coating. Solvent evaporation during coalescence and subsequent shrinking of the coating layer will tend to orient the flakes more parallel to the substrate. As this process is generally incomplete, a portion of light tends to be scattered and the visual effect is rather sparkling than uniformly reflective. In comparison to a solvent/waterborne coating, a solventfree powder coating has limited flake orientation (Fig. 9). As the mechanism to improve alignment of the flakes parallel to the polymer surface is missing, light reflection from these randomly oriented flakes is lower. In mass pigmented polymers, an incorporation of the pigment in a molten polymer mass during processing into compound or masterbatch, the terms leafing and non-leafing have no meaning. Here, the melt viscosity of polymers is too high to allow true leafing to take place [40]. As advantages of a coating over a mass pigmented system one should mention reflectivity and the “uniformity” of the coating. The influence of application described above can lead to surface defects and disrupts the flawlessness/spotlessness of the surface. The appearance of effectpigmented coatings is, as discussed above, influenced by the manner in which the flakes lie down in the dry film. If this is not uniform, the overall effect can appear mottled, sometimes called “cloudy”. In mass pigmented systems localised differences in the concentration of effect pigments may contribute

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Fig. 8. Orientation behavior of pearlescent pigments in a solvent-borne (acrylic–melamine) car refinish basecoat without (left, up) and with (left, down) polymer additive (beads), right: sparkle strength (arbitrary unit, calculated from the evaluation of five experienced lab technicians) against concentration of polymer beads and standard deviation σ of the flake orientation (0◦ = platelet lies parallel to the substrate surface, ±x◦ = tilt of the pigment platelets (anti-/clockwise)).

to so called flow lines (please read below), where the areas of lower concentration appear darker. Flow and weld lines can be visible in moldings colored by in-/organic pigments as well, however effect pigment flakes do tend to make the effect more prominent. Much of the reason lies in the flake shape. Non-flake pigment particles have three broadly similar dimensions. Therefore, their orientation in the polymer has relatively little effect on their perceived color. The same is not true for flake pigments [40]. Of course there are advantages for mass pigmentation, e.g., a coloration throughout a mass pigmented article is preferable to a relatively thin surface coating that can chip or delaminate to expose the often contrasting color of the polymer substrate. This is a particularly important consideration in the manufacture of automotive lower body parts such as wheel and body trims. Mass pigmentation may also have advantages for complex molded shapes, for example, where a sprayed coating cannot evenly fill recesses. The reverse is the case if the part requires tool inserts or sudden changes of thickness. Here, the presence of flow and weld lines will disfigure the surface and a more uniform effect can be achieved by coating [40].

Fig. 9. SEM image (FIB cross sectional preparation) showing the orientation of effect pigment particles in a “Primid” powder coating layer of approximately 80 ␮m (5% of silver-white pearlescent pigment in RAL formulation 7037).

10. Conclusion and outlook Effect pigments have found in the last decades a broad application for decorative and functional purposes in systems like paints, plastics, printing inks, and cosmetics. With their unique possibilities for the achievement of optical impressions such as eye-catching effects, angle-dependent interference colors, pearl luster, or multiple reflection, they are meanwhile irreplaceable in many application systems. Effect pigments show several advantages in decorative and functional applications in comparison to extended films, e.g., the broad variety of achievable optical effects, the ease of incorporation in all relevant application systems, the possibilities to blend pigments with other colorants, and the impression of “vivid” color effects. Effect pigments consist either of substrate-free pigments or of layered structures. There are diverse new developments in the last 5–10 years especially for the substrate-based types, e.g., multilayers on mica or pigments based on alumina, silica, and glass flakes. Pigments with structured surfaces like the diffractive types belong also to the actual developments. A continuation of the development of innovative effect pigments can be expected in the future. A direction of new developments is that the effect pigments are offered not only as free-flowing powders, but also in the form of preparations (granulates, chips, pastes, color concentrates). These preparations contain the pigment in as high as possible concentration [43]. In addition to the pigment, the preparations consist of binder components or binder mixtures based on solvent or waterborne systems. The advantages of such pigment–binder combinations are for example better pigment dispersibility, non-dusting introduction of the pigment in the application system, optimized wetting behavior, or improved color effects in the final products. Another important research field is the improvement of optical and non-optical performance

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by better flake orientation and better pigment–binder interaction. Specially designed effect pigments are used due to their functional properties such as electrical conductivity, reflection of infrared light, or ability to mark polymers by a laser beam [44].

Acknowledgement The authors would like to thank Michael R¨osler for the fruitful discussion and Helmut Plamper and Heinrich Opfermann for the SEM pictures.

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