Interference pigment
Interference pigments are effect pigments whose effect is based primarily or entirely on interference of light on thin, high-refraction layers. They are characterized by the fact that they produce an angle-dependent color impression , which is referred to as a hue flop . The effect of the similar pearlescent pigments is also created by multiple reflections, but is not primarily based on interference.
history
Interference pigments were developed much later than pearlescent pigments and metallic effect pigments . The first relevant patents for coating mica were published in 1942 (Atwood), 1963 and 1964. In particular, the Atwood patent was initially used to manufacture pearlescent pigments. In 1969 the chemical companies DuPont , Mearl (now part of BASF ) and Merck signed cross-licenses for the development of pearlescent and interference pigments.
Since the 1990s, a large number of pigment classes have been developed that take advantage of the phenomenon of interference. Specifically, these are liquid crystal pigments and pigments with a Fabry-Pérot structure (both 1995). With the latter, interference layers are applied to different substrates, such as synthetic mica (1996), aluminum oxide flakes (1999), silicon dioxide flakes (2000) or glass flakes (2002).
Mode of action
In contrast to pearlescent pigments, interference pigments are coated and are not necessarily transparent .
The incident light is partly reflected at the interface between pigment and carrier material . The other part is broken into the interior of the coating . The remaining light is reflected at the interface between the coating and the substrate surface and refracted again at the surface of the coating. Since light rays that leave the surface of the paint in the same place have to cover different path lengths, certain wavelengths are extinguished or amplified by overlapping , so that color is created. Since the difference in the path lengths depends on the angle of incidence, an angle-dependent color impression (color flop) is created.
The difference in path lengths is influenced by the layer thickness . The wavelength with maximum reflection is therefore a function of the layer thickness. The resulting colors can thus be controlled via the layer thickness. The color of the transmitted light corresponds to the complementary color of the reflected light. The complementary color is absorbed on a dark background so that only the interference color can be seen. The complementary color is reflected on a light background so that it adds up with the interference color to form white light .
Manufacturing
To produce interference pigments, different processes are used depending on the type and properties of the base material and its coating (s). In the first step, the platelet-shaped carrier material must be produced, which is then coated with one or more layers to create the interference effect. Depending on the material, a grinding step can follow, and finally the material is classified according to the particle size.
For the preparation of platelet-shaped iron oxide is first FeSO 4 to FeOOH oxidized . In an autoclave is water split off. This results in flaky Fe 2 O 3 .
Platelet- shaped titanium dioxide is produced by converting a TiOCl 2 film to TiO 2 by means of strip coating with simultaneous hydrolysis . Large-area films are created, which are then crushed into flake-shaped particles. In a further step, the resulting film can be coated with metal oxides before comminution. Silicon dioxide and borosilicate flakes are obtained from a melt of pure raw materials. This is followed by grinding and classification, as well as a coating step analogous to the production of mica and Al 2 O 3 pigments by suspension , filtration or calcination .
Metal oxide mica pigments consist of mica flakes that are coated with titanium dioxide. The next step is comminution and classification. Alternatively, titanium salt solutions can be hydrolysed in aqueous mica suspension (homogeneous hydrolysis or titration ). This method was published in 1942, but it was not used commercially until 1960. Production via chemical vapor deposition (CVD) in the gas phase or in a fluidized bed is also possible, but not of industrial importance . Often, zinc oxide is preallocated . This promotes the formation of the rutile modification, which ensures greater stability of the effect pigment.
Pigments with an Fe 2 O 3 base and metal oxide layers can only be produced using the CVD process . 5-layer systems made of Fe 2 O 3 , SiO 2 and Fe 2 O 3 are common . Metallic effect pigments can also be coated with a metal oxide layer using the CVD process . This takes place under a nitrogen atmosphere in a fluidized bed reactor at 450 ° C. Often an intermediate layer made of SiO 2 is produced.
If mica is coated with Fe 2 O 3 , it can be manufactured using the titration or CVD process. In further production steps, the combination with further layers is possible. Multilayer pigments can also be produced in this way or the weather resistance can be increased by coating with silanes .
Using the artificial production of aluminum oxide flakes instead of natural mica is a more demanding process, but it achieves almost perfect surfaces. In this process, an α- corundum structure is generated through controlled crystal growth . In the ideal case, thin , single-crystal flakes are obtained.
Pigments with a so-called Fabry-Perot structure are produced by a roll coater in the vacuum chamber . First, a release film is produced on a moving polymer belt (transfer foil). The first metal layer is then applied, followed by the first dielectric layer. Further metallic and dielectric layers alternate if necessary. Finally, the layer is peeled off and the material ground and classified.
properties
In contrast to classic pigments , whose color effect is based on absorption , mixtures of several interference pigments follow the laws of additive color mixing . Classic pigments, on the other hand, follow the laws of subtractive color mixing .
As with all pigments, the testing of effect pigments takes place in the integrated state, i.e. in a carrier medium. Since the color of the pigments depends on the viewing angle and viewing angle, the assessment must be made from several angles. In the case of visual assessment , this takes place by simply tilting the test item to be assessed. Colorimetrically , the assessment is usually carried out using multi-angle measuring devices. Assessment using a fixed viewing angle and variable lighting is also possible, but is used less frequently. These methods are used, for example, in production control and the quality control of pigments.
Microscopy continues to be the dominant test method for identifying effect pigments . Standard color formulation software can only reproduce one remission curve, but not several. Since all effect pigments require several reflectance curves for correct colorimetric representation, a method for automated recipe calculation is not yet known.
use
Because of their often spectacular appearance, interference pigments are used in vehicle tuning and cosmetics . Since, on the one hand, they are very expensive, but on the other hand, color tones formulated with them are very difficult to reproduce, they are used in the formulation of special printing inks for security features of banknotes . An example of this is the “50” at the bottom right, on the back of the 50 euro note . It can be used in printing inks , plastics and lacquers , but the high raw material costs limit its use in these areas.
literature
- H. Kittel, J. Spille: Textbook of paints and coatings . 2nd Edition. Volume V: Pigments, Fillers and Colorimetry. Hirzel, Stuttgart 2003, ISBN 978-3-7776-1015-3 .
- A. Goldschmidt, H. Streitberger: BASF Handbook Painting Technology . Vincentz Network, Hannover 2002, ISBN 3-87870-324-4 .
- G. Pfaff: Special effect pigments . 2nd Edition. Vincentz Network, Hannover 2007, ISBN 3-86630-895-7 .
Individual evidence
- ↑ a b H. Römpp: Römpp Lexikon Lacke und Druckfarben . Thieme, Stuttgart 1998, ISBN 978-3-13-776001-6 , p. 304 .
- ↑ DIN 55944: Colorants: Classification according to coloristic and chemical aspects