Radar compatible coating comprising metal effect pigments on a substrate

Abstract

The present invention relates to a radar compatible coating on a substrate comprising metal effect pigments, to a method for producing such a coating, and to the use of a substrate coated in this way, in particular in a vehicle structure.

Description

[0001] This invention relates to a radar-compatible coating comprising metallic effect pigments on a substrate, a method for producing such a coating, and the use of the substrate coated in this manner, particularly in vehicle structures.

[0002] With the increasing number of vehicles capable of autonomous driving, it has become necessary to integrate radar devices—capable of measuring distances to other vehicles or traffic obstacles, as well as the speeds of other road users—into relevant automotive components to a level previously unimaginable. These radar devices are typically mounted behind the vehicle's bumper to avoid negatively impacting the vehicle's visual appearance.

[0003] For many years, metallic paint, preferably silver metallic paint, has been one of the most popular vehicle paints, especially in the private car industry. However, these metallic coatings present major challenges related to the optical design of the covering components for radar devices installed inside such vehicles. This is because conventional metallic paints containing aluminum-based metallic effect pigments can reflect, attenuate, or absorb radar waves typically in the 76-81 GHz frequency range, to the point that the use of conventional metallic vehicle paints previously used for covering components of radar devices in vehicles would result in an undesirable reduction in the functionality of the radar devices.

[0004] Therefore, there have been numerous attempts to provide solutions for covering vehicle radar devices that do not impair the vehicle's visual appearance and ensure the functionality of the installed radar devices.

[0005] Therefore, the corresponding covering components are designed, for example, as radiator grilles, which have a very large radar wave transparent area and metallized supports that only slightly attenuate the ability of radar waves to pass through.

[0006] For example, such a covering component is described in DE 198 44 021 C2. Here, the outwardly visible metal layer consists of a vapor-deposited indium layer with a thickness in the nanometer range. The visual impression of a radiator grille support coated in this way is claimed to be equivalent to chrome plating.

[0007] Company badges with this type of extremely thin sputtered metal layer can also be provided, such as those described in EP 954 052B1.

[0008] In contrast, in DE 10 201 1 016 683 A1, the black plastic substrate is coated with a silicon layer with a thickness in the nanometer range.

[0009] The covering element is a radiator grille or company emblem, designed to have an externally visible sheen corresponding to chrome in certain areas. However, this type of coating is not suitable for vehicle components that, although located in the beam path of radar devices, are not intended to give observers the visual impression of conventional silver or metallic paint. The challenge here lies in achieving the strong brightness reversal (a noticeable change from bright to dark when lighting or viewing angle changes) typically found in metallic paint, achieving the opacity of metallic paint, and reducing radar wave attenuation to such a degree that radar wave transmission is sufficient to enable the installed radar device to operate in a fully functional manner.

[0010] JP 2004-244516 A discloses a glossy product with high transparency to electromagnetic radiation, which can be used as a radiator grille, but also as an assembly for another vehicle component such as a tailgate. The layer on the polycarbonate sheet may contain metallic particles, such as zinc, tin, or indium, but may also be colored with interference pigments, such as mica coated with titanium dioxide. The particles are applied to the sheet at a concentration of 3 to 8% by weight of the polyurethane layer. A black primer is used as a reverse coating.

[0011] The resulting glossy product, which comprises multiple layers, claims to have high electromagnetic radiation transparency and high gloss.

[0012] Although good transparency of radar radiation can be achieved by using interference pigments containing mica coated with titanium dioxide in such coatings, the opacity of the metallic finish and the strong metallic brightness reversal that can be obtained with the latter are not achievable with this mica-based interference pigment with its simple structure.

[0013] Therefore, JP 2010-030075 A proposes a layer on a plastic substrate containing a low concentration of aluminum pigment in addition to glass flakes or titanium dioxide-coated mica, and can be used, for example, in vehicle bumpers. It is claimed that the low concentration of aluminum pigment in this layer, and the resulting relatively large separation between individual aluminum pigment particles, leads to good radar transparency while maintaining high gloss. However, the small amount of aluminum pigment present does not achieve the hiding power typically found in metallic paints, and glass flakes or TiO2-mica pigments offer virtually no hiding power.

[0014] DE 10 2019 209 893 A1 also discloses a radar-transparent coating for vehicle components (e.g., radiator grilles) comprising a mixture of aluminum-based and silicate-based pigments in a layer on a plastic substrate. The latter pigment can be titanium dioxide-coated mica or titanium dioxide-coated glass. A second layer beneath this layer on the substrate has a low gloss and is preferably black. The resulting radiator grille is said to have a mother-of-pearl-like white color, thus possessing a metallic appearance. However, the visual impression of a metallic finish cannot be achieved using this type of layered structure.

[0015] The purpose of this invention is to provide a radar-wave transparent coating on a substrate, which is suitable for covering components of radar devices, particularly in vehicle structures, and contains conventional metallic effect pigments, especially aluminum pigments, preferably visually as indistinguishable as possible from conventional vehicle metallic finishes, and particularly has a metallic appearance, high opacity and strong brightness reversal, while having good transparency to radar waves.

[0016] Another object of the present invention is to provide a method for producing the above-described coating.

[0017] Furthermore, another object of the present invention is to illustrate the use of this type of coating.

[0018] The object of this invention is achieved through a radar-compatible coating comprising a metallic effect pigment on a substrate, wherein the coating has at least one layer stack consisting of the following:

[0019] A layer (A) comprising at least one pigment with absorption properties and free of metallic effect pigments, and

[0020] A layer (B) comprising a flake-like effect pigment, wherein the flake-like effect pigment is only a metallic effect pigment, and wherein the layer thickness of layer (B) is in the range of 2 to ≤10 μm.

[0021] Furthermore, the object of the present invention is also achieved by a method for producing a radar-compatible coating comprising metallic effect pigments on a substrate, wherein...

[0022] - A layer (A) containing at least one pigment with absorption properties and free of metallic effect pigments is applied to a substrate optionally pre-coated with a plastic sheet or film, and subsequently...

[0023] - Apply layer (B) to layer (A), said layer (B) comprising flake-like effect pigments, wherein the flake-like effect pigments are only metallic effect pigments, and wherein the dry layer thickness of layer (B) is in the range of 2 to ≤10 μm.

[0024] or

[0025] - Apply layer (B) to a substrate optionally pre-coated from a plastic sheet or film, wherein layer (B) comprises flake-like effect pigments, wherein the flake-like effect pigments are only metallic effect pigments, and wherein the dry layer thickness of layer (B) is in the range of 2 to ≤10 μm, and subsequently...

[0026] - A layer (A) containing at least one pigment with absorption properties and free of metallic effect pigments is applied to layer (B).

[0027] Furthermore, the objective of this invention is also achieved by using the aforementioned coating on a substrate as a radar-compatible vehicle topcoat for vehicle components.

[0028] The inventors have unexpectedly discovered that, compared to solutions described in prior art literature, it is possible to provide a coating for radar devices in vehicle structures, having a layer comprising flake-like effect pigments, wherein the flake-like effect pigments consist solely of metallic effect pigments. Thus, this type of layer is very similar to layers in conventional metallic finishes. However, special precautions must be taken to achieve sufficiently high radar wave transmission, which is necessary for the proper functioning of the radar device. Therefore, for example, the coating comprising metallic effect pigments must have only a low layer thickness. According to the invention, this layer thickness is in the range of 2 to ≤10 μm. Within this layer thickness range, the preferred range can be set by the selected concentration of the metallic effect pigments in the coating composition used. Therefore, when the proportion of metallic effect pigments in the coating composition is relatively high, a layer thickness in the range of 4 to 7 μm is preferably sufficient, while when the concentration of metallic effect pigments in the coating composition is relatively low, a layer thickness in the range of >7 to <10 μm is more advantageous. It goes without saying that, in the case of the layer thickness range given herein, variations in layer thickness within the layer due to process technology have no effect. The key factor is the target average layer thickness for each coating operation.

[0029] However, for the desired mode of coating function, it is particularly important that the layer containing the metallic effect pigment is part of a layer stack located on a suitable substrate.

[0030] The layer colored with metallic effect pigments is referred to in this specification as layer (B) of the layer stack. Viewed from the substrate, it is located immediately above and adjacent to layer (A) on the substrate; it can also be located immediately below and adjacent to layer (A) from the substrate perspective. Good radar wave transmission of the coating on the substrate can be achieved in both variations, although the optical appearance of the two variations differs.

[0031] Therefore, the layer stack according to the invention consists of a layer (A) and a layer (B) on a substrate, wherein, in a first embodiment, layer (B) is directly on layer (A) and thus represents the outermost layer of the layer stack on the substrate, or in a second embodiment, layer (A) is directly on layer (B) and thus represents the outermost layer of the layer stack on the substrate.

[0032] According to the invention, the stacked layer (A) contains at least one pigment (of one type) with absorption properties. This can be an organic absorbing pigment, an inorganic absorbing pigment, and / or a flake-like effect pigment with absorption properties. In each case, the pigment with absorption properties may exist in the layer (A) alone, or as a mixture within a substance category (e.g., as a mixture of various inorganic or organic absorbing pigments), as a mixture from different substance categories (e.g., as a mixture of flake-like effect pigments with absorption properties and organic and / or inorganic absorbing pigments), or also as a mixture of organic and / or inorganic absorbing pigments with non-absorbing flake-like non-metallic effect pigments. The layer (A) preferably contains at least one flake-like effect pigment (of one type) with absorption properties, wherein organic and / or inorganic absorbing pigments may optionally be present equally. It is crucial for the invention that the flake-like effect pigment with absorption properties present in the layer (A) cannot be a metallic effect pigment.

[0033] The organic or inorganic pigments used, which possess absorbing properties, can be any of the absorbing pigments commonly used in various industrial coatings. These are preferably present with a particle size in the range of 10 to 500 nm, particularly 10 to <100 nm. Combined with flake-effect pigments, their special effects, such as gloss, shimmer, and interference colors, are particularly visually appealing if the additional absorbing pigments used, whether organic or inorganic, have such small particle sizes that they are transparent to incident light. In this case, the particle size of the absorbing pigment is preferably in the range of 10 to <40 nm. Formulations of absorbing pigments are generally commercially available. Depending on compatibility with the coating system used, for example... W(Heubach,DE), UN(Heubach,DE)、MIPA WBC(Mipa,DE)、 (Standox GmbH, DE) (Standox GmbH, DE) (Arichemie, DE) (Arichemie, DE) or other systems.

[0034] Suitable absorbent pigments include, to name just a few, isoindolineone, benzimidazole, quinacridone, copper phthalocyanine, perylene, carbon black and / or titanium dioxide.

[0035] Preferably applicable to coatings according to the invention, wherein in layer (A) are inorganic and organic absorbing pigments alone or in mixture, inorganic and / or organic absorbing pigments mixed with non-metallic flake effect pigments that do not have absorbing properties, or flake effect pigments with absorbing properties optionally mixed with organic and / or inorganic absorbing pigments.

[0036] In a preferred embodiment, layer (A) of the layer stack of the coating according to the invention comprises a flake-effect pigment with absorption properties.

[0037] In particular, according to the present invention, a sheet-like interference pigment with absorption properties is used.

[0038] The optical effects of flake-like interference pigments typically consist of a combination of reflection and transmission phenomena of light across a series of thin layers, usually on a flake-like carrier material. Very frequently used materials are colorless and highly transparent to visible light, such as flake-like mica pigments coated with titanium dioxide. These pigments may have silver or colored interference colors, but are generally transparent and lack a dominant hue. They can only be used in combination with organic and / or inorganic absorbing pigments in layer (A) of the coating according to the invention.

[0039] If the sheet-like carrier or at least one layer on the sheet-like carrier is composed of a material with an inherent color (i.e., an absorbent color), the interference pigment achieves absorption properties, thereby realizing the main hue. These can be colored metal oxides, metal secondary oxides, mixed metal oxides, or oxygen-depleted metal oxides or metal oxide hydrates.

[0040] Interference pigments also achieve absorption properties due to the presence of organic colored pigments in the layers.

[0041] Also applicable are so-called carbon-containing pigments, which contain a certain proportion of elemental carbon in at least one layer of the thin-film interference pigment.

[0042] Particularly preferred is the use of an interference pigment having one or more interference layers on a transparent carrier sheet, and as a final layer, having a very thin, light-transmitting layer composed of carbon. Such a pigment has been described, for example, in the patent application EP3795645 A1 of the patent holder of this invention.

[0043] The thin-film effect pigment with absorption properties preferably used according to the present invention is an interference pigment, which has at least one layer comprising iron oxide (e.g., Fe2O3, FeO, Fe3O4, FeOOH), titanium secondary oxide (e.g., TiO, Ti2O3, Ti3O5, Ti4O7, Ti2O, Ti3O or Ti6O) or chromium oxide (e.g., Cr2O3) or a layer composed of carbon.

[0044] The sheet-like carrier materials considered are natural or synthetic mica, kaolin, talc, or sericite, as well as glass, calcium aluminum borosilicate, SiO2, TiO2, Al2O3, graphite flakes, or iron oxide flakes. The preferred sheet-like carrier materials are natural or synthetic mica, calcium aluminum borosilicate flakes, glass flakes, SiO2 flakes, or Al2O3 flakes.

[0045] Interference pigments exhibiting both colored interference and colored absorption colors can be readily used in layer (A). Therefore, for example, pigments produced by Merck KGaA and Darmstadt under the trade name... The interference pigment sold by F10-51 Lava Red has proven to be a particularly suitable colored interference pigment. This pigment is based on a flake-like SiO2 substrate coated with Fe2O3.

[0046] Interference pigments with silver-gray absorption are particularly preferred. Such interference pigments are, for example, available from Merck KGaA under the trade name... 9602Silver-Grey SW 9605 Blue Shade Silver SW and 9612 Silver-Grey Fine Satin SW. These are based on mica flakes and have at least one layer containing Fe2O3 or titanium suboxide.

[0047] As mentioned above, interference pigments with absorbing colors, especially silver-gray absorbing colors, can also be mixed with other pigments with absorbing properties, such as carbon black.

[0048] It has been found that interference pigments with silver-gray absorption colors are particularly suitable as absorbing pigments in layer (A) if the coating as a whole is intended to have a silver metallic appearance. Due to the pigment structure in the form of a thin-layer sequence on a flake-like substrate, these interference pigments exhibit a visually perceptible luster when incident light shines on them. Because layer (B) containing metallic effect pigments has a very low layer thickness, its hiding power alone is insufficient to achieve the overall visual impression of an opaque silver metallic finish. Therefore, the hiding power of layer (B) is effectively supplemented by the hiding power of the absorbing pigments in layer (A), especially if interference pigments with silver-gray absorption colors are present in layer (A). The silver-gray absorption color of the interference pigments ensures that layers (A) and (B) of the stacked layers are within the same color gamut, and if layer (B) contains silver metallic effect pigments, such as silver aluminum pigments, the coating formed by the stacked layers leaves an essentially uniform silver-metallic impression with high hiding power, high gloss, and a clear brightness reversal.

[0049] Both the layer (A)-(B) sequence viewed from the substrate and the layer (B)-(A) sequence exhibit an opaque, silvery appearance in the overall layer stack of the coating. The gloss and brightness reversal achievable with the layer (A)-(B) sequence are more pronounced than with the layer (B)-(A) sequence. In contrast, the radar capability of both embodiments is on the same order of magnitude.

[0050] Therefore, in order to achieve the overall silvery impression of the stacked layers, if the stacked layers (A) contain pigments with absorbing properties, then the type and quantity of these pigments make layer (A) appear as a gray shading when considered alone, thus being achromatic and in the medium brightness range (in L). * a * b * In color space, L on black * A 15° angle (within the range of 40 to 90 degrees) is particularly advantageous for the success of this invention. * The determination of the 15 value is described in the Examples section. For this purpose, coloring layer (A) with an interference pigment having a silver-gray absorption color has proven to be particularly suitable. Advantageously, inorganic or organic absorbing pigments having achromatic absorption colors may also be present in the first layer.

[0051] Absorbent pigments with colored absorption colors, especially interference pigments with colored absorption colors, can also be used in layer (A) to increase the opacity of layer (B). In the layer sequence (A)-(B) on the substrate, the overall visual appearance of the stacked layers depends on the metallic effect pigments used. In the case of silver metallic effect pigments in layer (B), the overall silver-metallic impression is slightly shifted towards the absorbed color of the interference pigment with colored absorption colors, which may be the desired specific color nuance. Conversely, if a colored metallic effect pigment in layer (B) is combined with an interference pigment in layer (A) that has an absorbed color within the same color range (e.g., in the orange or red region), a vivid metallic effect with saturated colors can be achieved. In the case of layer sequence (B)-(A) on the substrate, this metallic effect with saturated colors has already occurred when silver metallic effect pigments are used in layer (B). If no other coloring pigments are present in layer (A), the perceptible color of the entire coating corresponds to the absorbed color of the non-metallic interference pigment.

[0052] According to the present invention, the layer (A) containing a pigment having a colored absorption color has an L*15° brightness on black (in the L*, a*, b* color space) in the range of 50 to 100 (measurement conditions as described in the Examples section).

[0053] In contrast, a layer (A) with a black appearance due to the content of pigments with absorption properties would be quite unsuitable for the coating according to the invention, because the overall visual appearance of the coating caused by the low layer thickness of layer (B) in the layer sequence (A)-(B) would have a cloudy or mottled characteristic. Even using the layer sequence (B)-(A), an uneven visual appearance would be obtained. Therefore, according to the invention, a black layer (A) is not preferred.

[0054] Based on the weight of layer (A), the total concentration of pigments with absorption properties in layer (A) is in the range of 10 to 25% by weight, preferably in the range of 15 to 20% by weight.

[0055] If interference pigments with absorption properties are used in the first layer, these pigments typically have a particle size of 1 to 100 μm, particularly 2 to 70 μm, and especially preferably 3 to 50 μm. The thickness of the interference pigment is in the range of 0.1 to 2 μm.

[0056] In contrast, classic absorbent pigments, which may be organic or inorganic, have a particle size in the range of about 10 to <100 nm, preferably 1 to <40 nm.

[0057] In contrast to the layer (A) stacked according to the present invention, layer (B) contains only metallic effect pigments as a sheet-like effect pigment.

[0058] Metallic effect pigments, in the sense of this invention, refer to flake-like effect pigments composed of or having at least one metallic layer. These particularly include aluminum pigments commonly used for metallic finishes, suitable for use in coatings according to the invention in the form of so-called corn flakes or silver dollars. Other aluminum pigments produced by wet milling are also suitable, but aluminum pigments produced by vacuum deposition processes are unsuitable. These aluminum pigments are frequently used in paints and coatings, particularly in automotive finishes. Pure aluminum flakes can be coated with organic and / or inorganic materials to alter or optimize the use or color properties of the pigment.

[0059] Bronze or brass pigments are also suitable as metallic effect pigments, but aluminum pigments are preferred.

[0060] Metallic effect pigments are commercially available from various manufacturers in a wide range of versions and sizes. As a d50 value, suitable particle sizes for metallic effect pigments here range from 5 to 50 μm, preferably from 10 to 35 μm. The particle size is reported by the manufacturer and is therefore optional; thus, separate particle size determination is redundant.

[0061] The size ratio of the metallic effect pigment used in layer (B) of the coating according to the invention is not particularly limited to the range shown, i.e., commonly available commercial metallic effect pigments can be used.

[0062] For example, suitable silver metallic effect pigments are aluminum pigments or pigment formulations from Eckart GmbH. IL Hydrolan 2156 IL Hydrolan 8154 and IL Hydrolan 3580, from Toyal EMR-767E and EMR-1227 or APE-5245-C33 or AQUA from Silberline 5500-C43, to name just a few.

[0063] One non-metallic effect pigment worth mentioning is from Merck KGaA. Series of pigments, F120-30 CWT Taklamakan Gold, F120-51CWT Victoria Red, F120-58 CWT Wahiba Orange and F121-51 CWT Atacama Red.

[0064] Based on the weight of layer (B), metallic effect pigments are present in the layer stack of the coating at an amount of 3 to 25% by weight, particularly 15 to 20% by weight. The amount of metallic effect pigments used is matched with the layer thickness of layer (B).

[0065] In addition to metallic effect pigments, the stacked layers (B) may optionally contain finely segmented absorbent pigments or dyes, but excluding other flake-like effect pigments. Examples of organic and inorganic colored pigments and their size ratios have been described above.

[0066] The thickness of layer (B) is limited to the range of 2 to ≤10 μm so that the attenuation of radar radiation by the metallic effect pigments present therein is not excessive. However, slight attenuation of radar radiation still ensures normal operation of the radar device, allowing the coating on the substrate according to the invention to be placed in the beam path of the radar device. The dependence of the optimal layer thickness of layer (B) on the concentration of metallic effect pigments in the layer has been described above (lower concentrations allow for larger layer thicknesses within the indicated range). Furthermore, the particle size of the metallic effect pigments used should be selected so that layer (B) as a single layer does not appear as an opaque layer. The layer thickness of layer (B) refers to the thickness of the cured and dried layer, i.e., the dry layer thickness.

[0067] In this invention, "radar compatibility" refers to a coating with 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 exhibit a unidirectional transmission attenuation of less than 2 dB when exposed to electromagnetic waves with a peak frequency of 76.5 GHz. Preferably, the unidirectional transmission attenuation is less than 1.5 dB.

[0068] The dielectric constant of the coating and the unidirectional transport attenuation of the coating on the substrate were measured in standard mode using an RMS-D-77 / 79G instrument from perisens GmbH, Germany.

[0069] The layer thickness of layer (A) in the layer system according to the invention is set such that the total layer thickness of the stack of layers consisting of layer (A) and layer (B) is in the range of 10 to 40 μm, preferably 15 to 25 μm.

[0070] The adhesives used for layers (A) and (B) in the layer stack can be any conventional adhesives and adhesive systems that appear transparent in the cured state. All common types of adhesives used in conventional coating processes and compatible with the pigments employed can be used. Solvent-based adhesive systems, water-based adhesive systems, and radiation-cured adhesive systems can be used equally, provided that specific factors in the art regarding pigment selection and coating processes are observed.

[0071] Both layer (A) and layer (B) of the coating according to the present invention may contain other additives commonly used in the art, such as fillers, inhibitors, flame retardants, lubricants, rheology modifiers, dispersants, redispersants, defoamers, flow control agents, film-forming agents, adhesion promoters, drying promoters, photoinitiators, etc.

[0072] Depending on the adhesive system employed, the coating composition used to produce layers (A) and (B) of the layer stack optionally also contains organic solvents and / or water; however, these are no longer present in the coating according to the invention after both layers have cured. Solvent systems commonly used in the art can be used without limitation. The corresponding compositions for the adhesive system, including solvents and additives, are well known to those skilled in the art and, in some cases, are commercially available as finished products in an uncolored state. Those skilled in the art can make appropriate selections based on the corresponding pigments to be used and the desired coating process.

[0073] The possible substrate to which the layered stack according to the invention, consisting of layers (A) and (B), is applied is a sheet or film made of plastic. Plastics commonly used in automotive structures, such as polycarbonate (PC), polypropylene (PP), polyurethane (PUR), polymethyl methacrylate (PMMA), acrylonitrile-butadiene-styrene (ABS), or acrylonitrile-ethylene-styrene (AES) substrates can be used, to name a few. This type of plastic sheet or film exhibits a certain fundamental attenuation of the radar signal, which should only increase slightly due to the coating. Regarding the radar capability of the coating according to the invention, the fundamental attenuation of the radar signal relative to unidirectional transmission due to the corresponding substrate is included in the measurement. The fundamental attenuation of the radar signal in unidirectional transmission caused solely by the substrate is indicated separately in the examples. Due to technical equipment or production-related reasons, it is not possible to measure the radar signal attenuation caused solely by the coating.

[0074] It goes without saying that the substrate can be three-dimensionally shaped, meaning it can have a three-dimensional external shape depending on the application. Thus, for example, a plastic sheet used to form part of a vehicle tailgate naturally has a different three-dimensional external shape than a plastic sheet used for a bumper. Typically, the three-dimensional shape of the substrate is produced by conventional forming processes before the coating according to the invention is applied.

[0075] The fundamental core element of the coating on the substrate according to the invention is the aforementioned layer stack comprising layers (A) and (B) disposed directly on top of each other, wherein, according to an embodiment, layer (A) or layer (B) represents the outermost layer of the layer stack as viewed from the substrate. Furthermore, other layers, which may also be part of the coating according to the invention, may optionally be located between the substrate and the first layer (layer (A) or layer (B, depending on the embodiment)) and / or on top of the second layer (layer (B) or layer (A, depending on the embodiment)).

[0076] This type of additional layer is frequently used in automotive structures to improve the adhesion of the paint layer to the substrate, set color hues, and / or improve the mechanical and chemical strength and weather resistance of the paint layer. These are primer layers, other coloring layers, or an outermost clear coat, typically designed to be transparent and colorless. The coating according to the invention can advantageously have a primer layer and / or a clear coat. According to the invention, all conventional materials widely used in industry and therefore requiring no further explanation can be employed herein.

[0077] The coating on a substrate according to the invention can be advantageously used in all cases where radar devices are to be equipped with coverings that visually have a metallic finish, without adversely affecting the function of the radar devices. This is naturally applicable, in particular, to covering components for vehicle structures. The coating according to the invention is preferably a vehicle topcoat. Due to its good optical properties, it can of course also be used for all types of topcoats that visually correspond to conventional metallic finishes, but consume only a small amount of the metallic effect pigments commonly used. The existing radar beam transparency can also play a secondary role, and the corresponding application areas are not limited to vehicle structures.

[0078] The present invention also relates to a method for producing a radar-compatible coating comprising a metallic effect pigment on a substrate, wherein a layer stack comprising layers (A) and (B) is applied to an optionally pre-coated substrate comprising a plastic sheet or a plastic film, wherein layer (A) comprises at least one pigment having absorption properties and is free of metallic effect pigments, and wherein layer (B) comprises flake-like effect pigments, wherein the flake-like effect pigments are only metallic effect pigments, and wherein layer (B) has a dry layer thickness in the range of 2 to ≤10 μm. The layer stacked layers (A) and (B) are arranged on the substrate in a sequence (A)-(B) or a sequence (B)-(A).

[0079] The foregoing has explained all the material details relating to the composition of the suitable plastic substrate and layers (A) and (B). This is to the extent that the foregoing is referenced here.

[0080] The two stacked layers can be applied to the substrate using conventional coating processes, such as spraying, brushing, in-mold coating, roll coating, coil coating, or curtain coating.

[0081] This type of coating process is common in large-scale industrial applications and can be used skillfully without special adjustments. During application, only the thickness of layer (B) is set, resulting in a final dry layer thickness in the range of 2 to ≤10 μm, which is significantly less than the dry layer thickness of conventional metallic finishes containing similar metallic effect pigments. However, those skilled in the art will be able to set such a dry layer thickness without problem based on their expertise.

[0082] The plastic substrate with predefined radar properties used may optionally be pre-coated, for example, with one or more primers and / or coloring layers. However, if the coating as a whole has radar-compatible properties, it must be ensured that none of the layers optionally present on the corresponding substrate contain metallic effect pigments or other components that would adversely affect the radar transparency required for the coating as a whole.

[0083] Pre-coating the plastic substrate with a primer layer is advantageous because such a primer layer particularly improves the overall mechanical stability of the coating and the adhesion of the first layer of the layer stack to the substrate. Furthermore, it is advantageous to have an outermost transparent coating layer that is typically designed to be colorless and transparent to visible light, especially for the mechanical stability and weather resistance of the coating. In this invention, they are also preferably applied as the outermost layer of the overall coating to the upper layers of the layer stack.

[0084] This invention also relates to the use of the aforementioned coating containing metallic effect pigments as a radar-compatible vehicle topcoat on vehicle components. It can be applied to all vehicle components based on plastic substrates. Metal substrates are unsuitable because they cannot guarantee the required radar capability. The coating can be applied to external body components serving as housings or shielding components for radar devices mounted inside the vehicle, or it can be applied to the entire surface of suitable body components. Body components that may be mentioned include, in particular, bumpers, tailgates, radiator grilles, fenders, or components thereof. Of course, if only the visual appearance of the metallic finish is of interest and radar capability is secondary, the coating according to the invention can also be applied to vehicle components other than those mentioned above. In the latter case, the application of the invention is not limited to vehicle structures.

[0085] The present invention will be explained below with reference to embodiments, but is not limited thereto. Example:

[0086] In each case, the substrate used was a 350 μm thick PET film (Hostaphan RN 350, Mitsubishi Polyester Film GmbH, DE). To determine L... * Values ​​and color separation ΔE were applied to black / white Leneta boards in a manner similar to PET film. This coating was performed using a pneumatic spraying method. The adhesive used was formulation WBC 000 from MIPA SE, DE.

[0087] Example 1 (Reference):

[0088] As a reference for the target visual appearance of the coating, a paint composition colored only with aluminum pigments is applied as a single layer onto the film.

[0089] PMC: Pigment Mass Concentration

[0090] Al: Aluminum pigment ( IL Hydrolan 2156, IL Hydrolan 8154, 1:1 mixture, Eckart)

[0091] DLT: Dry Layer Thickness

[0092] L * : In L * a * b * In a color space, the luminance value L at a certain measurement angle *

[0093] Flip Index: A measure of brightness flip when the viewing angle changes, determined by the following formula:

[0094]

[0095] ΔE * L on a standardized black and white background * a * b * - Color separation of a sample in a color space is determined according to the following formula:

[0096] ΔE * =√(ΔL) *2 +Δa *2 +Δb *2 )

[0097] Examples 2 to 4 (Invention):

[0098] Table 1:

[0099]

[0100]

[0101] In each case, four coating operations were performed, with the first three operations using only 18% by weight. A coating composition colored with 9602 Silver Grey SW (a silver-grey interference pigment with silver-grey absorption color, containing iron oxide, MerckKGaA, and Darmstadt) is applied. Since no temporary drying occurs, three coating operations produce a layer (A) of layer stack according to the invention (technically, three applications are required here).

[0102] As layer (B), a coating composition colored with only 12% by weight (Example 2), 15% by weight (Example 3), or 18% by weight (Example 4) of an aluminum pigment mixture (see above) is applied.

[0103] This table shows the overall dry layer thickness of the coating, consisting of layers (A) and (B). Approximately the same amount of coating composition is applied for each coating operation in each case.

[0104] Colorimetric measurements of the samples were performed using a BYKMac i colorimeter (Byk-Gardner) in SMC5 mode.

[0105] Layer (A) on black L * The 15 value was determined when organic or inorganic absorbing pigments were used alone on a four-zone completely opaque coating on a standardized black / white coated substrate. If a flake-effect pigment with absorbing properties is used in layer (A), the colorimetric measurement of layer (A), particularly L on black, is... * The 15 value was determined by means of a coating on a substrate having a pigment mass concentration of 18% by weight.

[0106] The concentration of the pigment used and the layer thickness of the coating are expressed in each case for a single layer and a stack of layers of the coating according to the invention.

[0107] The black / white board used as the substrate here conforms to ASTM E 1347 standard and is sold by Leneta under the name Metopac T12G board.

[0108] As can be seen from the table, in each case only the increase in aluminum pigment concentration in the second layer, which is about 4 to 5 μm thick, leads to an increase in the flip index and a decrease in the ΔE value, and the visual appearance of the conventional silver metallic finish according to the reference embodiment can be well simulated by Examples 2 to 4.

[0109] Radar wave transmission

[0110] The table below shows the dielectric constant of each layer and the radar signal attenuation in dB for a single beam channel (76.5 GHz).

[0111] Table 2:

[0112]

[0113] Compared to the original metallic finish according to the reference embodiment, embodiments of the present invention show a significant reduction in radar radiation attenuation for a single beam channel. Simultaneously exhibiting good shielding power and excellent brightness reversal, all coatings according to the present invention are, according to technical requirements, significantly more suitable as radar-compatible coatings for vehicle components located in the beam path of radar devices than conventional opaque metallic finishes containing aluminum pigments.

[0114] Examples 5 to 7:

[0115] Using a PET substrate and the coating process as described in Example 1, in each case, a completely opaque coating of RAL shades 7030 (stone gray, Example 5), 7033 (cement gray, Example 6), and 7035 (light gray, Example 7) was applied as layer (A) to the substrate. In each case, 15% by weight of flake aluminum pigment ( IL Hydrolan2156, IL Hydrolan 8154, 1:1 mixture, Eckart) colored layer (B) is applied onto layer (A).

[0116] Individual results regarding the colorimetric properties or radar capability of each coating can be seen in Tables 3 and 4.

[0117] Table 3:

[0118]

[0119] Table 4:

[0120] Example Dielectric constant Attenuation (dB) at 76.5 GHz 5 15.0 1.67 6 15.9 1.75 7 21.1 2.07 PET substrate 3.219 1.05

[0121] The examples demonstrate that a metallic-like visual appearance with high opacity and a satisfactory flip index can be obtained, wherein the coatings in Examples 5 and 6 have unidirectional attenuation of radar signals within the target range, while the coating in Example 7 extends only minimally beyond the target range.

[0122] Example 8:

[0123] According to Example 1, and by means of the spraying method mentioned in Example 1, a coating (B) colored with 18% by weight of aluminum pigment as described above was applied to a PET substrate. The coating was applied in three coating operations using 18% by weight of aluminum pigment. A coating colored with F10-51 Lava Red (Merck KGaA, iron oxide on SiO2 substrate) is used as layer (A).

[0124] Individual results regarding the colorimetric properties or radar capability of each coating can be seen in Tables 5 and 6.

[0125] Table 5:

[0126]

[0127] Table 6:

[0128]

[0129] Example 8 demonstrates that, even with layer structures (B)-(A) on the substrate, a coating exhibiting good light / dark reversal and significantly lower unidirectional radar signal attenuation than standard commercial metallic coatings containing only aluminum pigments was also obtained. The coating according to the invention exhibits a visually appealing red metallic character and good hiding power.

Claims

1. A radar-compatible coating comprising metallic effect pigments on a substrate, characterized in that, The coating comprises at least one layer stack consisting of the following: - A layer (A) comprising one or more interference pigments with absorption properties in a concentration ranging from 10 to 25% by weight, based on the weight of layer (A), and excluding metallic effect pigments, wherein the thickness of the interference pigments is 0.1 to 2 μm; and - Layer (B) comprising a flake-like effect pigment, wherein the flake-like effect pigment is only a metallic effect pigment, the metallic effect pigment being an aluminum pigment, and wherein the layer thickness of layer (B) is in the range of 2 to ≤10 μm, and layer (B) comprises the metallic effect pigment in a concentration in the range of 3 to 25% by weight, based on the weight of layer (B).

2. The coating according to claim 1, characterized in that, The first layer contains organic absorbing pigments, inorganic absorbing pigments, and / or flake-like effect pigments with absorption properties.

3. The coating according to claim 1 or 2, characterized in that, The layer stack consisting of layers (A) and (B) has a total layer thickness in the range of 10 to 40 μm.

4. The coating according to claim 1 or 2, characterized in that, The layer (A) contains an interference pigment with silver-gray absorption color as a flake-effect pigment.

5. The coating according to claim 1 or 2, characterized in that, The layer (A) contains an interference pigment with red absorption color as a flake-like effect pigment.

6. The coating according to claim 1 or 2, characterized in that, Based on the weight of layer (B), layer (B) contains the metallic effect pigment at a concentration in the range of 15 to 20% by weight.

7. The coating according to claim 1 or 2, characterized in that, Based on the weight of layer (A), layer (A) contains one or more pigments with absorption properties in a concentration ranging from 15% to 20% by weight.

8. The coating according to claim 1 or 2, characterized in that, The substrate is a sheet or film of plastic, wherein the sheet or film optionally has a three-dimensional external shape.

9. The coating according to claim 1 or 2, characterized in that, Optionally, other layers may exist between the substrate and the layer stack consisting of layers (A) and (B) and / or on the layer stack.

10. The coating according to claim 9, characterized in that, One or more of the other layers are a primer layer and / or an outermost clear coat layer.

11. The coating according to claim 1 or 2, characterized in that, The coating is the vehicle's topcoat.

12. A method for producing a radar-compatible coating comprising metallic effect pigments on a substrate, characterized in that... - A layer (A) comprising at least one interference pigment with absorption properties, in a concentration ranging from 10 to 25% by weight and free of metallic effect pigments, is applied to a substrate optionally pre-coated with a plastic sheet or film, the thickness of the interference pigment being 0.1 to 2 μm, and subsequently... - A layer (B) is applied to a layer (A), the layer (B) comprising a flake-like effect pigment, wherein the flake-like effect pigment is only a metallic effect pigment, the metallic effect pigment being an aluminum pigment, and wherein the dry layer thickness of the layer (B) is in the range of 2 to ≤10 μm and the layer (B) comprises the metallic effect pigment at a concentration in the range of 3 to 25% by weight, based on the weight of the layer (B). or - Apply layer (B) to a substrate optionally pre-coated from a plastic sheet or film, wherein layer (B) comprises a flake-like effect pigment, wherein the flake-like effect pigment is only a metallic effect pigment, the metallic effect pigment being an aluminum pigment, and wherein the dry layer thickness of layer (B) is in the range of 2 to ≤10 μm and layer (B) comprises the metallic effect pigment at a concentration in the range of 3 to 25% by weight, based on the weight of layer (B), and subsequently - A layer (A) comprising at least one interference pigment with absorption properties and free of metallic effect pigments in a concentration ranging from 10% to 25% by weight based on layer (A) is applied to layer (B), wherein the thickness of the interference pigment is from 0.1 to 2 μm.

13. The method according to claim 12, characterized in that, Layers (A) and (B) are applied by means of spraying, brushing, roller coating, roll coating, curtain coating or in-mold application.

14. The method according to claim 12 or 13, characterized in that, The substrate has been pre-coated with a primer layer.

15. The method according to claim 12 or 13, characterized in that, Apply the clear varnish layer as the outermost layer of the coating onto layer (B) of layer stack (A) (B) or layer (A) of layer stack (B) (A).

16. The use of a coating containing metallic effect pigments on a substrate as a radar-compatible vehicle topcoat on vehicle components, according to any one of claims 1 to 11.

17. A vehicle component comprising a substrate made of a plastic sheet or a plastic film, said substrate having at least one coating according to any one of claims 1 to 11.

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