Metal-effect pigment-free radar-compatible coating on substrate

Abstract

The present invention relates to a metallic effect pigment-free radar compatible coating on a substrate, a method for making such a coating, and the use of such a coating, particularly in vehicle manufacturing.

Description

[Technical Field]

[0001] The present invention relates to a metal effect pigment-free radar compatible coating having metallic features on a substrate, a method for producing such a coating on a substrate, and the use of such a coating, particularly in vehicle manufacturing. [Background technology]

[0002] The increase in vehicles capable of autonomous driving has created a previously unimaginable need for radar devices to be installed in the corresponding vehicle components, capable of both measuring the distance to other vehicles and traffic objects and measuring the speed of other traffic participants. Typically, such radar devices are installed behind the bumper of the vehicle so as not to spoil the vehicle's appearance. For many years, metallic paints, preferably silver metallic paints, have been the most popular vehicle paints, especially in the passenger car sector. However, typical metallic paints containing aluminum-based metallic effect pigments reflect, attenuate or absorb radar waves, which are typically in the frequency range of 76-81 GHz, and the use of previously common metallic vehicle paints on cover parts for radar devices in vehicles can result in an undesirable reduction in the functionality of the radar device, posing a major problem for the optical design of cover parts for radar devices mounted inside such vehicles. Therefore, there have been attempts to provide a solution for coatings for vehicle radar devices that do not spoil the appearance of the vehicle and allow good functionality of the mounted radar device. For example, radiator grilles or similar cover parts designed as manufacturer logos and having large radar-transparent areas and metal struts often have a layer of vapor-deposited metal such as indium, which typically gives such components a chrome-like appearance. However, this type of coating is not suitable for vehicle parts that are located in the beam path of radar equipment but are intended to have the visual impression of a conventional silver metallic paint. The challenges are to achieve the strong lightness flop (a clear change in lightness or darkness with changes in lighting or viewing angle) that is typical of metallic paints containing metallic pigments, to achieve the hiding power of this type of metallic paint, and to reduce radar wave attenuation to a level that allows sufficient radar penetration and allows on-board radar equipment to remain fully functional.

[0003] JP 2004-244516 A discloses a photoluminescent product with excellent electromagnetic transparency that can be used as a radiator grille and also as a component of other vehicle parts, such as tailgates. The layer on a polycarbonate panel can contain metal particles, such as zinc, tin, or indium, but can also be colored with interference pigments, such as titanium dioxide-coated mica. The particles are applied to the panel at a concentration of 3 to 8% by weight in a polyurethane-containing layer. A black base coat is applied as a back coat. The resulting multi-layered glitter product is claimed to have excellent electromagnetic transparency and high gloss. Although excellent radar transmission can be achieved by using interference pigments containing titanium dioxide-coated mica in such coatings, the hiding power of metallic finishes containing metal pigments, and the strong metallic brightness flop that can be achieved, cannot be achieved by using this type of simple, transparent, colorless mica-based interference pigment alone. JP 2006-282886 A also discloses a radar-transparent coating for vehicle parts, which contains interference pigments in a layer on a plastic substrate but does not contain metallic effect pigments. It is stated that the interference pigments are based on particularly smooth substrate particles to allow for color variation of the coating. Silicon dioxide or aluminum oxide substrate flakes are suggested as suitable substrate flakes. However, the visual impression of a metallic finish cannot be achieved with such a layer containing interference pigments on the coated plastic substrate.

[0004] It is an object of the present invention to provide a radar-transparent coating on a substrate, which coating is particularly suitable for use in cover parts for radar devices in vehicle manufacturing, which coating does not contain conventional metal effect pigments, in particular aluminum pigments, and which preferably differs as little as possible from conventional silver vehicle metallic finishes, in particular has a silver metallic appearance, high hiding power, and strong brightness flop, while at the same time being highly radar-transparent. It is a further object of the present invention to provide a method for preparing the above-mentioned coating. Additionally, a further object of the present invention is to demonstrate the use of such coatings on substrates.

[0005] The object of the present invention is a radar-compatible coating comprising flake effect pigments on a substrate, the coating not comprising metallic effect pigments and having at least two layers on the substrate in the following order: A) a first base layer, which comprises an absorption pigment and does not comprise a flake effect pigment; and B) a second layer, which is applied to the first layer and which comprises flake effect pigments having absorbing properties in an amount of at least 10% by weight, relative to the weight of the second layer; Furthermore, the second layer is applied in a layer thickness of 14±2 μm over the entire area of ​​the black / white background, and is achieved by a coating having a color separation ΔE* between the coated black background and the coated white background in the range of 0 to 20 when measured spectrophotometrically in the L*, a*, b* color space at an illumination angle of 45 degrees and a viewing angle of 75 degrees. Additionally, an object of the present invention is a method for producing such a metal effect pigment-free radar compatible coating comprising flake effect pigments on a substrate, the method comprising: - applying a first layer comprising at least one absorption pigment and no flake-form effect pigments as a base layer to an optionally pre-coated substrate comprising a plastic plate or plastic film, followed by - applying a second layer to the first layer, the second layer comprising flake effect pigments with absorbing properties in an amount of at least 10% by weight, relative to the weight of the solid layer, The second layer may be applied as a single layer or in two or more partial layers arranged one on top of the other, with drying after each application of the layers. The objects of the present invention are further achieved by the use of the above-described coating on a substrate as a radar-compatible vehicle finish for a vehicle component.

[0006] Surprisingly, the inventors have found that it is possible to provide cover parts for radar devices in vehicle construction with a coating having a layer comprising flake-form effect pigments, the coating as a whole being free of metallic effect pigments but having a visually silvery metallic character. Flake effect pigments that do not have any metallic layer generally do not significantly attenuate radar signals in coatings, but usually exhibit no or little intrinsic absorption and only low hiding power. These properties prevent conventional flake effect pigments, which are usually interference pigments, from producing opaque silver coatings with the high gloss and strong brightness flop characteristic of metallic finishes. The aim was therefore to find conditions under which, for radar device cover parts in vehicle production, coatings that meet the requirements for hiding power, brightness flop, and radar compatibility can be obtained without the presence of metallic effect pigments in the coating, but that still mimic the appearance of a metallic finish to a satisfactory extent. The present inventors have found a layer structure on a substrate that satisfies the above conditions sufficiently.

[0007] The layer structure according to the invention therefore consists of a layer system comprising at least two successive layers on a substrate, the first layer being located directly on the substrate or alternatively on a precoated substrate, in particular on a substrate precoated with a primer layer, and comprising an absorbing pigment and being achromatic. Generally, white, gray, or black are not referred to as colors by experts, since they are achromatic optical phenomena that merely indicate the amount of light absorbed by each surface. In contrast, in the present invention, white, gray, and black are considered to be colors. The first layer of the coating according to the present invention does not contain flake-shaped effect pigments, i.e., it does not contain metallic effect pigments, such as, for example, interference pigments, or other flake-shaped effect pigments. The first layer therefore has a neutral solid colour with no effect, which completely hides the substrate or pre-coating on the substrate, if present. Suitable absorption pigments for the first layer are organic or inorganic pigments with absorbing properties. These are basically standard organic or inorganic absorption pigments. All absorption pigments commonly used in various industrial coatings can be used for this purpose. They are preferably available in particle sizes ranging from 10 to 500 nm, especially from 10 to less than 100 nm. Absorption pigment preparations are generally commercially available. Depending on the suitability of the coating system, coating systems such as Heucotint® W (Heubach, Germany), Heucotint® UN (Heubach, Germany), MIPA WBC (Mipa, Germany), Standoblue® (Standox GmbH, Germany), Standohyd® (Standox GmbH, Germany), Vocaflex® (Arichemie, Germany), and Vocaplast® (Arichemie, Germany) come into consideration. Suitable absorption pigments include, for example, isoindolinone, benzimidazole, quinacridone, copper phthalocyanine, perylene, carbon black, and / or titanium dioxide, to mention a few. Colored absorption pigments can be used in suitable mixtures to obtain neutral neutral colors in the first layer.

[0008] The absorption pigment is present in the first layer in an amount of 1 to 50% by weight, preferably 20 to 40% by weight, based on the weight of the first layer. The absorption pigments can be used alone or in mixtures of two or more. Mixtures are preferably used, especially in the case of monochrome gradations. This figure relates to the weight of the solid layer. The thickness of the first layer is not critical to the success of the present invention. The thickness is selected to completely conceal the substrate or any pre-coatings present on the substrate and to be economically reasonable. Typical thicknesses of the first layer are in the range of 5-15 μm, although deviations therefrom may be made as needed. According to the present invention, the layer thickness of the first layer and the concentration of the absorbing pigment in the first layer are set within the ranges mentioned above so that the first layer has a color difference ΔE* between a coated black background and a coated white background in the range of 0 to 5, preferably 0 to less than 2, and in particular 0 to less than 1, when the first layer is applied as a single layer to the entire area of ​​a standardized black / white background and measured spectrophotometrically in the L*, a*, b* color space at an illumination angle of 45 degrees and a viewing angle of 75 degrees. The first layer serves as the base layer for the coating according to the invention on the substrate. According to the present invention, the second layer is arranged directly on the first layer. The second layer is a pigment containing flake effect pigments, but does not contain metallic effect pigments or other metallic pigments. According to the present invention, the flake effect pigments of the second layer have absorption properties. These are preferably flake interference pigments with a silvery-gray absorption color. The optical effect of flake interference pigments generally consists of a combination of light reflection and transmission phenomena in a series of thin layers that this type of effect pigment typically constitutes on a flake-like support. Very often, only colorless, substantially transparent materials to visible light are used, such as titanium dioxide-coated mica flake pigments. While such pigments can have silvery or chromatic interference colors, they are generally transparent and have no masstone. Interference pigments achieve their absorption properties, and therefore their masstone, when the flake-shaped support, or alternatively, at least one layer located on the flake-shaped support, is made of a material with an intrinsic color, i.e., an absorption color. These may be colored metal oxides, metal suboxides, mixed metal oxides, or oxygen-deficient metal oxides, or metal oxide hydrates. Interference pigments also achieve their absorption properties because they are layers containing organic color pigments.

[0009] In the present invention, an interference pigment having at least one layer containing iron oxide, a mixed oxide containing iron oxide and titanium oxide, or titanium suboxide, or a layer consisting of carbon, is preferably used in the second layer. One or more other layers containing colorless and transparent materials may additionally be disposed on the flake-shaped support. Iron oxides that come into consideration include Fe2O3, FeO, Fe3O4, or FeOOH. Mixed oxides of iron oxide and titanium oxide are often ilmenite (FeTiO3) or pseudobrookite (Fe2TiO5). Suitable titanium suboxides are TiO, Ti2O3, Ti3O5, Ti4O7, Ti2O, Ti3O, or Ti6O. The thickness of the absorption layer comprising iron oxide, a mixed oxide comprising iron oxide and titanium oxide, or titanium suboxide, or the layer consisting of carbon, is set so that the interference pigment has a silvery-gray absorption color, whereas all other layers optionally present on the support do not contribute to the absorption color. Further suitable colorless and transparent layers on the flake-shaped support material are in particular layers comprising colorless metal oxides or metal oxide hydrates, such as tin oxide, titanium dioxide, zirconium oxide, silicon dioxide, silicon oxide hydrate, aluminum oxide, or aluminum oxide hydrate. Flake-like support materials that come into consideration are natural or synthetic mica, kaolin, talc, or sericite, as well as glass, calcium aluminum borosilicate, SiO, TiO, or AlO. The flaky support materials used are preferably natural or synthetic mica, or AlO flakes. Interference pigments of this type are commercially available, for example from Merck KGaA under the trade names Iriodin® 9602 Silver-Grey SW or Iriodin® 9605 Blue Shade Silver SW, and are based on mica flakes and have at least one layer containing iron oxide or titanium suboxide. Particularly preferably, it is also possible to use interference pigments having one or more interference layers and, as a final layer, a very thin light-transmitting layer of carbon on a transparent support flake, as described, for example, in the applicant's EP-A-3795645.

[0010] Interference pigments with a silvery-gray absorption color have been found to be particularly suitable for use as effect pigments with absorption properties in the second layer, since the entire coating is intended to have a silvery metallic appearance. These interference pigments exhibit a visually noticeable luster when struck by incident light, due to their pigment structure in the form of a thin, continuous layer on a flake-like substrate. The silvery-gray absorption color provides a sufficiently high brightness when light is directly incident. These interference pigments with absorption properties generally have a particle size in the range of 1 to 100 μm, in particular in the range of 2 to 70 μm, and particularly preferably in the range of 3 to 50 μm. The thickness of the interference pigments is in the range of 0.1 to 2 μm. The particle size of effect pigments with absorption properties can be measured using laser diffraction. The particle size and particle size distribution over volume are preferably measured using a Malvern instrument (Malvern Mastersizer 3000, APA300, Malvern Instruments Ltd., UK) in standard mode. However, typical particle size ratios can also be found in manufacturer data published product information sheets. This particle size range provides sufficient hiding power for the entire structure when the amount of effect pigment with absorbing properties and the layer thickness of the second layer are set according to the invention.

[0011] According to the invention, the minimum amount of flake-form effect pigments with absorbing properties in the second layer is 10% by weight, relative to the weight of the (solid) second layer. The maximum amount of flake-form effect pigments with absorbing properties in the second layer is 40% by weight, relative to the weight of the second layer. These effect pigments are preferably used in the second layer in a concentration of 15 to 35% by weight, relative to their weight in the second layer. According to the present invention, the thickness of the second layer is in the range of 3 to 25 μm, preferably in the range of 5 to 20 μm. If deemed advantageous, the second layer may also comprise one or more of the absorption pigments described above for the first layer, so long as the optical measurements of hiding power (ΔE*), lightness (L*15), and lightness flop (flop index) are thereby maintained within the specified ranges. The overall optical effect of the opaque, achromatic first layer and the second layer containing an interference pigment with a silvery-gray absorption color creates an overall impression of a homogeneous silvery metallic in the coating according to the invention with high hiding power, high gloss, and a pronounced brightness flop. Hiding power herein is determined from the ΔE* value, which can be determined by spectrophotometric measurement of the coated substrate in the L*, a*, b* color space, where the quantity ΔE* is defined as the color difference of a sample at an illumination angle of 45 degrees and a viewing angle of 75 degrees over standardized black and white backgrounds in the L*a*b* color space, and is determined according to the following formula: ΔE*=√(ΔL* 2 +Δa* 2 +Δb* 2 ) The lower the color difference value, the better the coating hides the background. Complete hiding of the background is generally not achievable with non-metallic effect pigments. The second layer used in the coating according to the invention, when applied to a black / white background in a layer thickness of 14±2 μm and measured under the measurement conditions described above, has a ΔE* value in the range of 0 to 20, preferably in the range of 5 to 20. These values ​​indicate that the hiding power of the second layer is sufficient for the purposes of the present invention.

[0012] The measure used among experts for the lightness of a layer is the L*15 value of a coating, which is determined photometrically in the L*a*b* color space on a standardized black / white background at an illumination angle of 45 degrees. To be suitable as a coating according to the present invention, it must have a minimum lightness that can be obtained on both a white and a black substrate. The use of the above-mentioned effect pigment with a silvery-grey absorption colour in the second layer results in a second layer with a lightness L*15 of at least 105 on both a coated white background and a coated black background, when applied in a layer thickness of 14±2 μm over the entire area of ​​a black / white background and measured spectrophotometrically in the L*, a*, b* colour space at an illumination angle of 45 degrees and a viewing angle of 15 degrees. In addition, good brightness flop is also achieved. This is typically called the flop index, and is determined spectrophotometrically at an illumination angle of 45 degrees and at reflected light resolutions of 15 degrees, 45 degrees, and 110 degrees. Thus, in accordance with the present invention, the flop index is in the range of at least 10 on both white-coated and black-coated backgrounds when the second layer is applied to the entire area of ​​a black / white background in a layer thickness of 14±2 μm and measured spectrophotometrically at an illumination angle of 45 degrees and viewing angles of 45 degrees: 15 degrees, 45 degrees: 45 degrees, and 45 degrees: 110 degrees in the L*, a*, b* color space. The flop index is generally considered in the art to be a measure of brightness flop as the viewing angle changes and is determined according to the following formula:

number

[0013] Surprisingly, it has been found that the brightness flop of the coating according to the invention, as measured by the flop index, can be significantly improved in some cases, especially on a gray or black first undercoat on a substrate, when the second layer consists of two or more, preferably three or four, sub-layers arranged one on top of the other, without significantly impairing the hiding power or brightness of the coating as a whole. In such cases, the total dry layer thickness of the second layer is preferably in the range of only 5 to 15 μm. At least one sub-layer, preferably two or three sub-layers, has a dry layer thickness of 5 μm or less. In particular, the dry layer thickness of at least one sub-layer is 4 μm or less or 3 μm or less, particularly preferably about 2 μm. Preferably, even two or three sub-layers can have such extremely thin layer thicknesses. In order for these thin partial layers to be combined into a visually appealing overall layer as the second layer of the coating according to the invention, the surfaces of the individual partial layers must be smooth. Such a smooth surface is produced by the interfaces between the individual partial layers of the second layer being arranged substantially parallel to the base layer or coated substrate. The interfaces are obtained by intermediate drying after application of each individual partial layer. Due to the intermediate drying, the absorbing flake effect pigments in each partial layer are aligned with their major axes approximately parallel to the surface of the first layer (and the substrate), which results in good reflection of incident light at the individual partial layers. The pigment loading of the individual sub-layers is likewise at least 10% by weight and at most 40% by weight as flake-form effect pigments with absorbing properties, in each case based on the weight of the individual sub-layer. Preferably, 15 to 35% by weight of flake-form effect pigments with absorbing properties are used in each sub-layer.

[0014] "Radar compatible" in the sense of the present invention is understood as a coating that has a dielectric constant of less than 30 when exposed to electromagnetic waves with a peak frequency of 76.5 GHz. Furthermore, the coating on a 350 μm PET substrate must have a one-way transmission attenuation of less than 2 dB when exposed to electromagnetic waves with a peak frequency of 76.5 GHz. Measurements of the dielectric constant of the coating and the one-way transmission attenuation of the coating on the substrate are carried out using an RMS-D-77 / 79G instrument manufactured by perisens GmbH (Germany) in standard mode. All conventional binders and binder systems that appear transparent in the solidified state can be used as binders for the first and second layers of the coating according to the invention. All common types of binders used in conventional coating methods and compatible with the pigments used can be used. Solvent-based binder systems, water-based binder systems, and radiation-curable binder systems can also be used, provided that the pigment selection and coating method conform to the characteristics common in the art. Both the first and second layers of the coating according to the invention may contain further additives common in the art, such as fillers, inhibitors, flame retardants, lubricants, rheological aids, dispersants, redispersants, antifoaming agents, flow control agents, film formers, adhesion promoters, drying accelerators, photoinitiators, and the like. When the second layer consists of two or more sub-layers, the use of a rheological agent is generally required. Rheological agents that can be considered are, for example, BaSO4, polyamide powder, silicates, or other rheological agents known to those skilled in the art, particularly cellulose nanofibers. The latter are particularly preferred. Depending on the binder system used, the coating composition used to prepare the first and second layers of the coating may also optionally contain organic solvents and / or water, which are not present in the coating according to the invention after the two layers have solidified. Solvent systems common in the art can be used without restriction. Similar compositions for binder systems, such as solvents and additives, are well known to those skilled in the art and some are commercially available as finished products in an unpigmented state. Those skilled in the art can make the appropriate selection based on the respective pigments used and the desired coating method.

[0015] The substrate to which the coating according to the present invention, including the first and second layers, may be applied is a plastic plate or film if the coating is intended to be radar-compatible. Plastics commonly used in automotive manufacturing can be used, such as polycarbonate (PC), polypropylene (PP), polyurethane (PUR), polymethylmethacrylate (PMMA), acrylonitrile butadiene styrene (ABS), or acrylonitrile ethylene styrene (AES) substrates, to name a few. While this type of plastic plate or film exhibits some basic attenuation of the radar signal, this should only be slightly increased by the coating placed on top of it. Regarding the radar performance of the coating according to the present invention, the basic attenuation of the radar signal in one-way transmission due to each substrate is included in the measurement. The basic attenuation of the radar signal in one-way transmission due to the substrate alone is shown separately in Example 4. Measuring the attenuation of the radar signal due to the coating alone is not possible due to instrumentation engineering reasons. If the coating according to the invention is intended to be applied to a substrate for purely optical reasons and the radar compatibility of the coating is not an important consideration, it is of course also possible to use metallic or metal-containing substrates. Of course, depending on the application, the substrate can be three-dimensionally shaped, i.e., have a three-dimensional profile. Thus, for example, a plastic sheet intended to form a part of a vehicle tailgate will naturally have a different three-dimensional profile than a plastic sheet intended as a bumper. Generally, the three-dimensional shape of the substrate is created using conventional forming methods, after which the coating according to the present invention is applied.

[0016] The package comprising the above-mentioned first and second layers is the essential core element of the coating according to the invention on a substrate, with the second layer being arranged directly on the first layer as viewed from the substrate. In addition, further layers, which may also be part of the coating according to the invention, may optionally be arranged between the substrate and the first layer and / or above the second layer. Such additional layers are frequently used in automobile manufacturing to improve the adhesion of the paint layer to the substrate and / or to improve the mechanical and chemical strength and weather resistance of the paint layer. These are primer layers or outermost clear coats, which are generally designed to be transparent and colorless. The coating according to the invention may advantageously comprise a primer layer and / or a clear coat. In accordance with the invention, all conventional materials that are widely used in industry and therefore do not require detailed description can be used here. The coating according to the present invention on the substrate can be advantageously used in all cases where a cover with a visually silver-effect finish is applied to a radar device without adversely affecting the functionality of the radar device. This, of course, applies particularly to cover parts used in vehicle structures. The coating according to the present invention is preferably a vehicle finish. Of course, due to its excellent optical properties, it can also be used for any type of finish that is intended to be visually similar to a conventional silver metallic finish. Furthermore, radar wave transparency may play a subordinate role, and the corresponding area of ​​use is not limited to vehicle structures.

[0017] The present invention also relates to a method for producing a metal effect pigment-free radar-compatible coating comprising flake effect pigments on a substrate as described above, in which a first layer comprising at least one absorbing pigment and no flake effect pigments is applied as a base layer to an optionally pre-coated substrate comprising a plastic plate or plastic film, and subsequently a second layer comprising flake effect pigments with absorbing properties is applied to the first layer in an amount of at least 10% by weight, based on the weight of the solid layer, the second layer being applied as a single layer or in two or more partial layers arranged one on top of another, with drying being carried out after the application of each layer. All material details regarding suitable plastic substrates and the construction of the first and second layers have already been described above, and in this regard reference is made thereto. The two layers of coating can be applied to the substrate using conventional coating methods, such as by spraying, brushing, in-mold, roller coating, coil coating, or curtain coating. This type of coating method is common in large scale industry and can be used according to the state of the art. Preferably, a spray method is used. Conventional spraying techniques are suitable for producing coatings according to the invention, which achieve dry layer thicknesses in the range of 5 to 25 μm in a single spraying operation. To produce coatings according to the invention, including a first and a second layer, two coating operations are sufficient, in which the second layer is applied immediately after the first, with or without intermediate drying, and the layers are dried and cured together. However, spraying methods are particularly suitable for applying the second layer, in which superimposed sublayers can be applied successively in multiple work steps to the first layer of the coating according to the invention, with each sublayer having a very thin dry layer thickness. Preferably, two to four sublayers are applied, and in each case, each sublayer is dried after application, ensuring the formation of an interface between the individual sublayers. The temperature for drying the individual layers depends on the binder system and solvent used and is at least 20°C. Temperatures up to 150°C, preferably up to 100°C, can be used.

[0018] The amount of flaky effect pigments with absorbing properties in the second layer is at least 10% by weight, based on the dry layer weight for each sub-layer, but can be in the range of 10 to 40% by weight, in particular in the range of 15 to 35% by weight. The dry layer thickness of at least one of the sub-layers is 5 μm or less, preferably 4 μm or less, and in particular 3 μm or less, or about 2 μm. Preferably, two or three sub-layers have such a low layer thickness. When the dry layer thickness of the partial layers is very thin, the pigment concentration in each partial layer can be set high by significantly reducing the proportion of binder in each coating composition (solids content of about 6-7% by weight) and significantly increasing the proportion of solvent (preferably water). To enable this type of very dilute coating composition to form a continuous film on the substrate, various auxiliaries, especially rheological auxiliaries, are added to the coating composition to provide a suitable viscosity, allowing it to be applied to the background by spraying and to achieve good flowability. During the subsequent drying process, small solid masses containing a very high proportion of effect pigments remain on each background as partial layers, and the effect pigments are also well aligned with their major axes essentially parallel to the surface of each coating. According to the present invention, the rheological aid added here is preferably cellulose nanofibers in an amount of 5 to 20% by mass relative to the mass of the coating composition. The application of multiple partial layers arranged one on top of the other, with intermediate drying of each partial layer, allows for particularly good orientation of the flake-like effect pigments in the second layer, resulting in a high incident light reflection at the surface of the second layer. This allows for a particularly improved lightness flop of the entire coating, while at the same time allowing for a very low total layer thickness of the second layer overall, without significantly impairing the hiding power or lightness of the entire layer structure. Thus, a particularly preferred embodiment of the coating according to the invention, which is visually almost equivalent to a silver metallic finish but has good radar compatibility when applied to plastic substrates, can be realized on the substrate using flake-like effect pigments with absorbing properties in the second layer and without using any kind of metallic pigments in the entire coating.

[0019] The plastic substrates used, having the desired radar properties, can optionally be precoated with, for example, one or more primer and / or pigment layers, but if the overall coating is to have radar-compatible properties, any optional additional layers on each substrate must be free of metallic effect pigments or other ingredients that may adversely affect the requisite radar transparency of the overall coating. Pre-coating a plastic substrate with a primer layer is advantageous because such a primer layer improves, among other things, the mechanical stability of the overall coating and the adhesion of the first layer of the layer package to the substrate. In addition, an outermost clear coat, which is generally designed to be colorless and transparent to visible light, is particularly advantageous for the mechanical stability and weather resistance of the coating. In the present invention, these are also preferably applied as the outermost layer of the overall coating, on top of the layer package including the first and second layers. It goes without saying that the entire coating is subjected to at least one curing operation, which is carried out after the application and drying of the second layer on the substrate and / or after the application of the clear coat. Curing of coatings on substrates, especially in the automotive sector, is a standard activity in the art and does not need to be described in detail. The present invention also relates to the use of the above-mentioned metal-effect pigment-free coating as a radar-compatible vehicle finish for vehicle parts. This can be applied to any vehicle part based on a plastic substrate. Metallic substrates are not suitable, as the desired radar performance cannot be guaranteed. The coating can be applied to external vehicle body parts intended as exterior covers or shielding parts for radar devices installed inside the vehicle, or it can be applied to the entire surface of a suitable vehicle body part. Body parts that may be mentioned include, in particular, bumpers, tailgates, radiator grilles, wings, or components thereof. Naturally, the coating according to the invention can also be applied to vehicle parts other than those mentioned, especially to metal-containing substrates, when only the appearance of the metallic finish is of interest and radar performance is not required. In the latter case, the scope of application of the present invention is not limited to vehicle manufacturing. [Example]

[0020] The present invention will be described below with reference to examples, but is not limited thereto. The substrate, a black / white coated Leneta panel (already with a white and black base layer in each component area), is coated with the second layer. The coating is carried out as a pneumatic spray coating. The binder used is the product WBC000 from MIPA SE (Germany). Finally, all samples are coated with a standard two-component clear coat. The second layer is pigmented with an interference pigment having a silvery grey absorption color in the amounts listed below. Effect pigment A: Mica-based interference pigment with a coating containing SnO2, TiO2, and carbon (C content: 1.14%), particle size 5-25 μm Effect pigment B: an aluminum oxide flake-based interference pigment with a coating containing SnO2, TiO2, and carbon (C content: 0.44%), particle size 5-30 μm; Effect pigment C: Mica-based interference pigment with a coating containing SnO2, TiO2, titanium suboxide, and auxiliaries, particle size 5-40 μm

[0021] Example 1 To determine the hiding power of a coating according to the invention, a coating composition containing pigments A, B, and C with a pigment mass concentration of 18% by weight relative to the mass of the solid second layer is applied in one coating run to standardized black and white coated panels and dried for 5 minutes at 80° C. The smaller the color difference ΔE*75°, the better the hiding power of the effect pigment. When the second layer was applied in four partial layers (in each case 18% by weight PMC, layer thicknesses of 9, 2, 2 and 2 μm, in each case dried at 80° C. for 5 min), only a small change in hiding power occurred compared to the single-layer method. [Table 1] Sublayers: Number of sublayers in the second layer of the coating PMC: pigment mass concentration of the effect pigment in each partial layer DLT: Dry thickness of the entire second layer consisting of x partial layers L*: Lightness value L* in the L*a*b* color space at a viewing angle of 15 degrees and an illumination angle of 45 degrees ΔE*: color difference of a sample over a standardized black and white background (illumination angle 45 degrees, viewing angle 75 degrees) in the L*a*b* color space, determined according to the following formula: ΔE*=√(ΔL* 2 +Δa* 2 +Δb* 2 ) Flop index: An index of brightness flop when the viewing angle is changed (illumination angle 45 degrees, viewing angle 45 degrees: as 15 degrees, 45 degrees: as 45 degrees, 45 degrees: as 110 degrees), determined according to the following formula:

number

[0022] Example 2 To determine the lightness of each coating, the effect pigments A, B, and C of Example 1 are applied to a black or white background, respectively. The same pigments are further applied to a black or white coating background in a three-step process, with a layer thickness of approximately 2 μm per partial layer (drying for 5 minutes at 80° C. after each application) having in each case a pigment mass concentration of 30% by weight relative to the weight of each partial layer. The higher the lightness value L*15, the better the visual imitation of a coating that is simply opaque-colored with aluminum pigments. [Table 2] With each coating variation and each effect pigment in the second layer, high brightness values ​​can be achieved.

[0023] Example 3 All coatings made in Example 2 are re-measured to determine the Flop Index. [Table 3]

[0024] Conventional silver metallic coatings, typically containing aluminum pigments, have a flop index in the range of approximately 12 to 17. This range is achievable with all of the coating variations and with all of the silver-gray interference pigments. In particular, the multilayer coating variations on a black background achieve very high values ​​for the flop index. Colorimetric measurements of the samples are carried out using a BYKMac i model colorimeter (Byk-Gardner) in SMC5 mode. The black / white panels used as substrates herein conform to ASTM E1347 standards and are commercially available from Leneta under the name Metopac T12G panels. As can be seen from the table, the coatings according to the invention, in which each interference pigment used has a silver-gray absorption color and each variation of the coating method was used, achieve good brightness and strong brightness flop while at the same time achieving sufficient hiding power, thereby visually imitating metallic coatings containing aluminum pigments well to very well.Since there are no metal pigments in the coatings, no significant attenuation of radar waves by the coatings on the substrate is expected.

[0025] Example 4 To determine the radar transparency, a 350 μm thick PET film (Hostaphan RN 350, Mitsubishi Polyester Film GmbH, Germany) was used as the substrate in each case. The coating was carried out as a pneumatic spray coating. The binder used was the product WBC000 from MIPA SE (Germany). As first layer a fully opaque layer of RAL colour tint 7037 (Dusty Grey) is used in each case. As a second layer, a layer comprising an interference pigment having a silver-gray absorption color as described in Table 4 is applied in each case in one or four partial layers and dried as described in Example 1. Table 4 shows the dielectric constant of each layer structure and the attenuation (dB) of the radar signal in a single beam path (76.5 GHz) (instrument: RMS-D-77 / 79G manufactured by perisens GmbH, standard mode). The dielectric constant of uncoated PET substrate is approximately 3.2, and the radar wave attenuation is 1.05 dB. In comparison, a coating comprising a single layer on a PET substrate containing a commercially available aluminum pigment, 18% by weight of PMC, and a DLT of approximately 22 μm, had a dielectric constant of approximately 74.9 and a one-way attenuation of the radar signal of approximately 4.5 dB under the same measurement conditions. [Table 4]

Claims

1. 1. A radar-compatible coating comprising flake effect pigments on a substrate, said coating not comprising metallic effect pigments, characterized in that said coating has at least two layers on said substrate in the following order: A) a first base layer, which comprises an absorption pigment and does not comprise a flake effect pigment; and B) a second layer, which is applied to the first layer and which comprises flake-form effect pigments having absorption properties in an amount of at least 10% by weight relative to the weight of the second layer; Furthermore, said second layer is applied in a layer thickness of 14±2 μm over the entire area of ​​the black / white background and has a color difference ΔE* between the coated black background and the coated white background in the range of 0 to 20 when measured spectrophotometrically in the L*, a*, b* color space at an illumination angle of 45 degrees and a viewing angle of 75 degrees. coating.

2. 10. The coating of claim 1, wherein the second layer is applied to a full area of ​​a black / white background in a layer thickness of 14±2 μm and has a lightness L*15 of at least 105 on both the coated white background and the coated black background when measured spectrophotometrically in the L*, a*, b* color space at an illumination angle of 45 degrees and a viewing angle of 15 degrees.

3. 3. The coating according to claim 1 or 2, characterized in that the second layer is applied in a layer thickness of 14±2 μm over the entire area of ​​a black / white background and has a flop index of at least 10 in each case on a coated white background and on a coated black background when measured spectrophotometrically in the L*, a*, b* color space at an illumination angle of 45° and at viewing angles of 45°: as 15°, 45°: as 45°, and 45°: as 110°.

4. Coating according to any one of claims 1 to 3, characterized in that the first layer has a white, grey or black colour and contains organic or inorganic absorption pigments.

5. 5. Coating according to any one of claims 1 to 4, characterized in that the second layer comprises, as flake-shaped effect pigment with absorbing properties, an interference pigment with a silvery-grey absorption color.

6. 6. The coating of claim 5, wherein the interference pigment having a silvery-gray absorption color is a pigment having at least one layer comprising iron oxide or titanium suboxide, or a layer consisting of carbon, on a transparent flake-shaped support.

7. 7. Coating according to any one of claims 1 to 6, characterized in that the second layer comprises the flake-form effect pigments having absorbing properties in a concentration ranging from 10 to 40% by weight relative to the weight of the second layer.

8. Coating according to any one of claims 1 to 7, characterized in that the second layer has a layer thickness in the range of 3 to 25 μm.

9. Coating according to any one of claims 1 to 8, characterized in that the second layer consists of two or more partial layers arranged one on top of the other.

10. Coating according to any one of claims 1 to 9, characterized in that the substrate is a plastic plate or film, the plate or film optionally having a three-dimensional profile.

11. Coating according to any one of claims 1 to 10, characterized in that further layers may be arranged on the substrate below the first layer and / or above the second layer.

12. 12. The coating of claim 11, wherein the one or more further layers are a primer layer and / or an outermost clear coat.

13. A coating according to any one of claims 1 to 12, characterized in that it is a vehicle finish.

14. 10. A method for making a metal effect pigment-free radar compatible coating comprising the flake effect pigments of claim 1 on a substrate, comprising: - applying a first layer comprising at least one absorption pigment and no flake-form effect pigments as a base layer to a substrate, which may be precoated, comprising a plastic plate or a plastic film, followed by - applying a second layer to said first layer, the second layer comprising flake-form effect pigments with absorbing properties in an amount of at least 10% by weight relative to the weight of the second layer, The second layer may be applied as a single layer or in two or more partial layers arranged one on top of the other, with drying after each application of the layers. The method characterized by:

15. 15. The method of claim 14, wherein the second layer is applied with a total dry layer thickness in the range of 3 to 25 μm.

16. 16. The method according to claim 14 or 15, characterized in that the second layer comprises the flake-form effect pigments having absorbing properties in an amount of 10 to 40% by weight, relative to the weight of the second layer.

17. 17. The method according to any one of claims 14 to 16, characterized in that the application of the first and second layers is carried out using a spraying method, a brushing method, a roller coating method, a coil coating method, a curtain coating method, or an in-mold method.

18. 18. The method according to claim 17, characterized in that the application of the second layer is carried out as a spraying method in two to four partial steps by applying two to four partial layers successively and in each case one on top of the other, the amount of the flake-form effect pigments with absorbing properties in each partial layer being at least 10% by weight, based on the dry weight of each partial layer, and drying takes place after the application of each partial layer at a temperature of at least 20°C.

19. 19. The method according to claim 18, characterized in that at least one of the partial layers has a dry layer thickness of 5 μm or less.

20. The method according to any one of claims 14 to 19, characterized in that the substrate is precoated with a primer layer.

21. A method according to any one of claims 14 to 20, characterized in that a clear coat is applied to the second layer as the outermost layer of the coating.

22. Use of a metal effect pigment-free coating comprising flake-form effect pigments with absorbing properties on a substrate according to any one of claims 1 to 13 as a radar compatible vehicle finish for vehicle parts.

23. A vehicle part comprising a substrate comprising a plastic plate or a plastic film, bearing at least one coating according to any one of claims 1 to 13.

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