Alumina effect pigment
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The most important class of special effect pigments today exists of natural mica platelets coated with thin films of transparent metal oxides like titanium oxide and iron oxide. To expand the range of achievable color effects, further special effect pigments were developed, which use substrate materials such as aluminum oxide, silicon dioxide or borosilicate instead of mica.
Effect pigments based on aluminum oxide flakes (alumina flakes) are produced using a special, technologically ambitious manufacturing process. In the first step of the production process, Al2O3 (Aluminium oxide) flakes with a very smooth and perfect surface are formed. The flakes are produced using a crystal growth process. They are formed in the corundum structure (α-Al2O3). The desired flake shape is achieved by controlled crystal growth in the axial and equatorial directions. The extraordinary color purity and transparency of the effect pigments obtained by coating the Al2O3 flakes with metal oxides can be attributed to the synthesis procedure yielding single-crystalline thin flakes 
The coating of the alumina platelets with high-refractive metal oxides, especially with titanium dioxide and iron(III) oxide leads to extraordinary strongly reflecting effect pigments. These pigments possess a very intensive glitter effect, the so-called crystal effect or sparkle. The coating process is analogous to that used for metal oxide mica pigments except it starts from an aqueous suspension of Al2O3 flakes. The complete manufacturing process of pigments based on aluminum oxide platelets, consisting of the flake production and the coating with metal oxides is shown in Figure 1 
For outdoor applications they are modified with an additional weather stabilizing surface treatment.
Silverwhite and interference pigments based on Al2O3flakes can be produced—analogous to similar pigments based on mica platelets—in different colors, e.g. gold, red, blue or green, by variation of the titanium dioxide layer thickness.
Depending on the layer thickness, the coating with iron oxide leads to bronze-, copper- or red-colored effect pigments. The pigments obtainable at the market possess a narrow particle size distribution of about 5 to 30 µm as well as a high aspect ratio. The sparkle effect referred to above has its origin in the optimized thickness of all layers in the pigment structure, including the Al2O3substrate. Analytical investigations show that the thickness range of the aluminum oxide platelets and the resultant pigments can be controlled very precisely.
The luster effects of the Al2O3based pigments are additionally intensified by directed illumination, e.g. by sunlight. Previously, the resultant sparkle effect could not be achieved with small-sized effect pigments. This is especially important for automotive coatings, because it was not possible before to obtain a sparkle effect with such intensity with coarser mica pigments without a substantial loss of performance at the same time. An improvement of the application and of the color properties is the result of the process leading typically to minimized superfine pigment particles. Scattering effects are reduced and intensive, pure interference colors can be achieved in combination with the colorless aluminum oxide substrate.
The sparkle effect of the pigments in coating applications is controlled by the quantity of the added pigment. The effect is visible already at concentrations of 0.1% in the paint system. The intensity is steadily increased up to pigment concentrations of about 2%. The single light spots arising from the pigment structure and orientation in the paint system seem to spring back and forth when a painted metal plate is tilted. Also, the color of the other components of the paint has an influence in addition to the concentration of the pigment. A dark color, for example, forms a high contrast to the bright light spots and strengthens the effect. Favorable is also the use of a covering varnish in order to achieve an optimal effect.
The pigments can be applied with all painting techniques established for conventional pearlescent and interference pigments. Effect pigments based on alumina flakes may be applied in all substantial systems, i.e. in coatings, plastics, printing inks and cosmetic formulations. An additional surface treatment is applied to the pigment particles to adjust them to solvent-based and water-based coating systems. The pigments can be used in combination with many conventional pigments as well as with other effect pigments. Nearly unlimited styling possibilities are the result of these combinations.
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