A NEW GENERATION OF ALUMINIUM-BASED PIGMENTS Dr. Frank J. Maile, André Cabral Martins Schlenk Metallic Pigments GmbH, Germany True Color Pigmentos e Corantes Ltda., Brazil Abstract 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. 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 [1]. This paper provides an insight into our latest effect pigment development which allows the creation of surfaces which exhibit polychromatic light sparks under directed lighting in various coating and printing applications. When used in coil coatings, powder coatings and dispersion paints for interior architecture application, it can be used as a special ingredient which enables the coating formulator to achieve switchable polychromatic effects, i.e. to create semi-smart surfaces [2].
Introduction Every design action causes an emotional reaction. To choose among two similar products, emotional design may influence consumers to make their buying decision. A product must not only have high haptic appeal, i.e. be easy to use (Usability Research), but must also be attractive in terms of design. Just take the equipment developed by Apple as modern-day examples. Aside from their ease-of-use and product design, in colour and marketing circles they are said to have had a significant influence on the trend. Paints which allow the development of high-quality, value-added and customised product design and which, at the same time, make it possible to communicate visually the functional properties of a product, are increasingly sought after in today s market place. They allow products to be positioned in a way that helps them to stand out from the mass. In this respect, effect pigments present unique opportunities. Their application in powder, as well as coil coatings leads to unique optical effects, visible on façades even at a great distance. By bringing them into play, designers and architects are able to create new colour compositions in architecture. Surfaces that change color depending on the angle of view are not smart materials but are of interest in the field of architecture. These types of material include dyes with special-effect pigments [3]. Originally used to direct light through optics, holographic optical elements (HOE) were developed as a potentially new means of directing light. As it is possible to dramatically change the optical appearance of surfaces equipped with diffractive elements, we decided to develop effect pigments incl. these physical properties. MultiFlect effect pigment can e.g. be easily incorporated into coil and powder coatings, two processes frequently used to manufacture coated panels for the use in interior architecture and product design. Other applications include plastics and printing. We consider these surfaces being semi-smart as their diffractive property can be used to switch the effect on and off using the appropriate light source as shown in Fig. 1 Fig.1 Switchable optical effects in interior architecture & design with the use of MultiFlect effect pigments in powder coatings [2] Special Effect Pigments Effect pigments have been widely used for decorative and functional applications in systems such as paints, plastics, printing inks, and cosmetics for several decades. Their unique ability to achieve eye-catching optical effects, angle-dependent interference colors, pearl luster, or multiple reflection, has made them irreplaceable.
Effect pigments show a number of advantages when compared to extended films, e.g. the wide variety of achievable optical effects, the ease of incorporation in all relevant application systems, the possibilities of blending them with other colorants, and the impression of vivid color effects [4]. Effect pigments are made up of either substratefree pigments or substrate-based structures. A property not possessed by conventional organic and inorganic pigments is what is frequently referred to as flop. This is the change in color and/or gloss with the viewing angle. The origin of the effect lies in the almost two-dimensional, anisotropic nature of effect pigments [5]. The anisotropic morphology of the particles explains why their use affects the appearance, particularly, because a change in processing technology results in a modified standard deviati on of flake orientation [6]. Effect pigments can 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 Metal effect pigments The well-known metal effect pigments are materials without a layer structure, for example, aluminum or copper zinc platelets. Similarly, transparent effect pigments such as single- crystalline BiOCl or polycrystalline TiO 2, in the form of flakes, belong to this group. These non-metallic flakes need to be very thin to achieve the interferencecolor effect. This can lead to lower mechanical stability than flakes based on a substrate platelet. Metal effect pigment flakes are produced by treating metal granules with stamping machines. The most common methods employed are ball mills using the dry milling (Hametag [7]) or wet milling (Hall [8) processes. During the treatment, 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 silver-dollar types depending on the quality and shape of the untreated granules and on the milling conditions. One special type is the PVD aluminum, also known as VMP (Vacuum Metallized Pigment, please refer to Fig. 2),. This is 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. When incorporated in coating systems, they give an improved mirror-like effect.
Fig.2 Photograph of a motor-bike helmet: Realization of chrome-like effects without using chromium [9] by using vacuum metallized pigments (VMPs, Decomet ) plus MultiFlect effect pigments in the paint formulation MultiFlect Effect Pigments Angle-dependent optical effects of pigments derive from submicron structures in the pigment and are caused by interference at thin layers. Such effects can also arise from light diffraction on periodically structured surfaces [1] as shown in Fig. 3. Fig. 3 Explanation of the diffraction by a plane grating. A beam of monochromatic light of wavelength l is incident on a grating and diffracted along several discrete paths. The triangular grooves come out of the page; the rays lie in the plane of the page. The sign convention for the angles a and b is shown by the + and signs on either side of the grating normal: A reflection grating: the incident and diffracted rays lie on the same side of the grating [10]
MultiFlect effect pigments are based on a structured polymer film which is metallized and grinded to particles of the desired particle size distribution. Figure 4 shows light microscopic pictures of the structured (diffractive) pigment. The macroscopic appearance of the effect pigments in basecoat layers is shown in Fig. 5. Fig. 4 Light microscopy of MultiFlect pigment particles: bright field (left); dark field (right) at a magnification of 500x Fig. 5 Photograph of car shapes coated with basecoat layers containing MultiFlect effect pigments exhibiting polychromatic light sparks. Conclusion 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 eyecatching 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 [1].
With the introduction of MultiFlect, another generation of effect pigments based on structured surfaces has been introduced to the industry. The pigment allows to create polychromatic light sparks. As MultiFlect contains diffractive elements, it allows to realize semi-smart surfaces. These effects are switchable when using appropriate lighting conditions. It is this property which amplifies the attractivity and use of MultiFlect in modern product design, especially in the field of interior architecture. Literature [1] F.J.Maile, G. Pfaff, P. Reynders, Progr. Org. Coat. 54(2005)150-163 [2] F.J.Maile, A. Kreeratiratanalak, The creation of emotional effect surfaces in architecture and design, in: Proceedings of the Decorative Coatings Conference, Düsseldorf, Germany, Vincentz Network Hannover, June 2015 [3] A. Ritter, Smart Materials in Architecture, Interior Architecture and Design, Birkhäuser 2008, ISBN 978-3-7643-7327-6. [4] F.J.Maile, P. Reynders, APCJ Feb/March (2010) 14. [5] Wheeler, I.R., Metallic Pigments in Polymers, Rapra Technology Ltd., 1999 [6] F. J. Maile, M. Rösler, A. Huber, The Macroscopic Appearance of Effect Coatings and its Relationship to the local Spatial and Angular Distribution of Reflected Light, in: Proceedings of the American Coatings Conference, Charlotte, NC, USA, 2-4 June, 2008 [7] E. Podszus, Hartstoff-Metall AG, GB Patent 204,055, 1923 [8] E.J. Hall, Metals Disintegrating Company Inc., U.S. Patent 1,569,484, 1919; U.S. Patent 2,002,891, 1935 [9] F.J.Maile, A. Cabral Martins, Chrome-like effects without Chromium, Pitture e Vernici European Coatings, Ed. 1/2015. [10] The physics of diffraction gratings, Newport Corporation 2005