Radar-compatible coatings on substrates without metal effect pigments

Abstract

The invention relates to a radar-compatible coating on a substrate which is free of metal effect pigments, a process for producing such a coating, and the use of such a coating, in particular in vehicle construction.

Description

[0001] The invention relates to a radar-compatible coating having metallic properties on a substrate, free of metal effect pigments, a method for producing such a coating on a substrate, and the use of such a coating, in particular in vehicle construction.

[0002] With the increase in vehicles capable of autonomous driving, it is necessary to integrate radar devices into the corresponding automotive parts, which radar devices enable both the measurement of the distance to other vehicles or traffic obstacles and the measurement of the speed of other traffic participants to a consistent degree until then. Such radar devices are usually installed behind the bumper of the vehicle in order not to adversely impair the visual appearance of the vehicle.

[0003] For many years, metallic paints, preferably silver metallic paints, have counted among the most popular vehicle paints, in particular for the private vehicle sector. However, these metallic paints represent a major challenge in connection with the optical design of covering parts of radar devices installed in the interior of such vehicles, since conventional metallic paints containing aluminum-based metallic effect pigments can reflect, attenuate or absorb radar waves, typically in the frequency range 76-81 GHz, to such an extent that the use of previously conventional metallic vehicle paints for covering parts of radar devices in vehicles would lead to an undesirable reduction in the functionality of the radar devices.

[0004] There is therefore no lack of attempts to provide solutions for covering vehicle radar devices which do not impair the visual appearance of the vehicle and which enable good functionality of the installed radar device.

[0005] Corresponding covering parts, which are designed, for example, as radiator grids or company logos and have very substantially radar-transparent areas and metallized struts, often have a layer of vapor-deposited metals, such as indium. Such components often exhibit a chrome-like visual appearance.

[0006] However, coatings of this type are not suitable for vehicle parts which, although located in the beam path of a radar device, are intended to give the observer the visual impression of a conventional silver metallic paint. The difficulty here is to achieve the strong lightness flop (a noticeable change from light to dark when the lighting or the viewing angle changes) which is usually the case with metallic paints containing metallic pigments, to achieve the hiding power of metallic paints of this type, and to reduce the attenuation of radar waves to such an extent that the transmission of radar waves is sufficient to be able to operate the installed radar device in a fully functional manner.

[0007] JP 2004-244516 A discloses a glossy product with high transparency to electromagnetic radiation, which can be used as a radiator grid, but also as a component of another vehicle part, such as a tailgate. The layer on the polycarbonate panel here can contain metal particles, such as zinc, tin or indium, but can also be colored with interference pigments, such as, for example, titanium dioxide-coated mica. The particles are applied to the panel in a concentration of 3% to 8% by weight in a polyurethane-containing layer. A black primer is applied thereto as a reverse coating.

[0008] The obtained glossy product comprising a plurality of layers is said to have high transparency to electromagnetic radiation and high gloss.

[0009] Although good transparency to radar waves can be achieved in such coatings with interference pigments comprising titanium dioxide-coated mica, the hiding power of metallic paint layers containing metallic pigments and the strong metallic brightness reversal that can be achieved with the latter cannot be achieved with transparent and colorless mica-based interference pigments of this type having a simple structure.

[0010] JP 2006-282886 A also discloses a radar-transparent coating for vehicle parts, which contains interference pigments in the layer on the plastic substrate and omits metal effect pigments. In order to achieve color differentiation of the coating, the interference pigments are said to be based on particularly smooth substrate particles. Silicon dioxide or aluminum oxide substrate flakes are suggested as suitable substrate flakes. However, layers of this type containing interference pigments on the plastic substrate to be coated also cannot achieve the visual impression of a metallic paint layer.

[0011] The object of the present invention is to provide a radar-transparent coating on a substrate which is suitable for covering parts of radar devices, in particular in vehicle construction, and which dispenses with conventional metal effect pigments, in particular aluminum pigments, preferably visually differs as little as possible from conventional silver-colored vehicle metallic paint layers and has, in particular, a silver-metallic appearance, high hiding power and a strong brightness reversal while having good radar transparency.

[0012] Another object of the present invention is to provide a method for preparing the above coating.

[0013] Furthermore, a further object of the invention is to indicate the use of such a coating on a substrate.

[0014] The object of the present invention is achieved by a radar-compatible coating comprising flake-form effect pigments on a substrate, wherein the coating is free of metallic effect pigments and has at least two layers on the substrate in the following order:

[0015] A) - a first layer, which refers to the bottom layer, comprises absorbing pigments and is free of flake-form effect pigments, and

[0016] B) a second layer, which is applied to the first layer and comprises flake-form effect pigments having absorbing properties in an amount of at least 10% by weight, based on the weight of the second layer,

[0017] and wherein when the second layer is applied with a layer thickness of 14±2 μm over the entire area of ​​the black / white background and at an illumination angle of 45° and a viewing angle of 75° at L * 、a * , b * The second layer has a color separation ΔE between a coated black background and a coated white background in the range of 0 to 20 when measured spectrophotometrically in color space * .

[0018] Furthermore, the object of the present invention is also achieved by a method for producing a radar-compatible coating free of metal effect pigments comprising flake-form effect pigments on a substrate, wherein

[0019] - applying a first layer comprising at least one absorbing pigment and free of flake-form effect pigments as a primer to an optionally precoated substrate comprising a plastic sheet or a plastic film, and subsequently

[0020] - applying a second layer onto the first layer which comprises flake-form effect pigments having absorbing properties in an amount of at least 10 wt.-%, based on the solid layer weight,

[0021] The second layer is applied as a single layer or in two or more layered layers arranged one above the other and drying is carried out after application of each layer.

[0022] Furthermore, the object of the invention is also achieved by using a coating as described above on a substrate as a radar-compatible vehicle paint layer on a vehicle part.

[0023] The inventors have surprisingly found that it is possible to provide a covering part of a radar device in vehicle construction with a coating having a layer comprising flake-form effect pigments, wherein the coating as a whole is free of metallic effect pigments but visually has a silvery metallic character.

[0024] Although flake-form effect pigments without any metallic layer generally do not lead to a strong attenuation of radar signals in the coating, they generally exhibit little or no intrinsic absorption and have only low hiding power. These properties have the effect that opaque silver coatings with high gloss and strong brightness reversal, as characteristic of metallic paint layers, cannot be achieved with conventional flake-form effect pigments, which are generally interference pigments. The aim was therefore to find out the conditions under which coatings which meet the requirements for hiding power, brightness reversal and radar compatibility can be obtained on cover parts of radar devices in vehicle production without the presence of metallic effect pigments in the coating, but which can imitate the visual appearance of a metallic paint layer to a suitable extent.

[0025] The inventors have found a layer structure on a substrate which satisfies the conditions very well.

[0026] The layer structure according to the invention therefore consists of at least a layer system comprising two consecutive layers on a substrate, wherein the first layer, which is located directly on the substrate or alternatively also on a precoated substrate, in particular a substrate precoated with a primer layer, contains absorbing pigments and has an achromatic color.

[0027] Usually, white, gray or black are not referred to as colors in professional circles because they are achromatic optical phenomena, which only represent the amount of light absorbed by the corresponding surface. However, in the present invention, white, gray and black will be regarded as colors in contrast.

[0028] The first layer of the coating according to the invention comprises no flake-form effect pigments, ie neither metallic effect pigments nor other flake-form effect pigments, such as, for example, interference pigments.

[0029] The first coat thus has a solid, non-effect, neutral coloration. It completely hides the substrate or, if present, the pre-coat on the substrate.

[0030] Suitable absorbing pigments for the first layer are organic or inorganic pigments with absorbing properties. These are essentially the classic organic absorbing pigments or inorganic absorbing pigments. All absorbing pigments commonly used for various industrial coatings can be used for this purpose. These preferably have a particle size ranging from 10 to 500 nm, in particular 10 to <100 nm. Formulations of absorbing pigments are generally commercially available. Depending on the compatibility with the paint 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.

[0031] Suitable absorbing pigments are, for example, isoindolinones, benzimidazoles, quinacridones, Cu phthalocyanines, perylenes, carbon black and / or titanium dioxide, to mention only a few. In order to obtain a neutral, achromatic coloration of the first layer, colored absorbing pigments can be employed in suitable mixtures.

[0032] The absorbing pigments are present in the first layer in an amount of 1% to 50% by weight, preferably in a range of 20% to 40% by weight, based on the weight of the first layer. They can be used alone or in a mixture of two or more. Mixtures are preferably used, especially in the case of gray shades. The figures relate to the weight of the solid layer.

[0033] The layer thickness of the first layer is not critical for the success of the invention. The layer thickness is set so that the substrate or any pre-coating located on the substrate is completely covered and is economically justified. Typical layer thicknesses of the first layer are in the range of 5 to 15 μm, but can also deviate therefrom if desired.

[0034] According to the invention, the layer thickness of the first layer and the concentration of the absorbing pigments in the first layer are set within the above-mentioned limits so that when the first layer is applied as a single layer over the entire area of ​​a standardized black / white background and at an illumination angle of 45° and a viewing angle of 75° at L * 、a * , b * The first layer has a color separation ΔE between a coated black background and a coated white background in the range of 0 to 5, preferably 0 to <2 and in particular 0 to <1 when measured spectrophotometrically in color space * .

[0035] The first layer represents the bottom layer of the coating according to the invention on the substrate.

[0036] According to the invention, the second layer is arranged directly on the first layer. The second layer is pigmented with flake-shaped effect pigments, but contains no metal effect pigments and no further metal pigments. According to the invention, the flake-shaped effect pigments in the second layer have absorption properties. They are preferably flake-shaped interference pigments with a silver-grey absorption color.

[0037] The optical effect of flake-form interference pigments usually consists of a combination of reflection and transmission phenomena of light at a series of thin layers in which effect pigments of this type are usually on, usually consisting of, flake-form support materials. Frequently, only materials which are colorless and very substantially transparent to visible light are used here, such as, for example, flake-form mica pigments coated with titanium dioxide. Such pigments can have silvery interference colors or colored interference colors, but are generally transparent and have no primary color tone.

[0038] If the flake-shaped support or at least one of the layers located on the flake-shaped support alternatively consists of a material with an intrinsic color (i.e., an absorption color), the interference pigments acquire absorption properties and thus a primary color tone. These can be colored metal oxides, metal suboxides, mixed metal oxides or oxygen-deficient metal oxides or metal oxide hydrates. The interference pigments also acquire absorption properties due to the layer containing organic colored pigments.

[0039] In the present invention, interference pigments are preferably used in the second layer, which have at least one layer comprising iron oxide, a mixed oxide comprising iron oxide and titanium oxide, or a titanium suboxide, or a layer consisting of carbon. One or more further layers comprising colorless, transparent materials may additionally be located on the flake-shaped support material.

[0040] The iron oxides considered are Fe2O3, FeO, Fe3O4 or FeOOH. The mixed oxides of iron oxide and titanium oxide are usually ilmenite (FeTiO3) or tialite (Fe2TiO5). Suitable titanium suboxides are TiO, Ti2O3, Ti3O5, Ti4O7, Ti2O, Ti3O or Ti6O.

[0041] The layer thickness of the absorbing layer comprising iron oxide, a mixed oxide comprising iron oxide and titanium oxide, or a titanium suboxide, or a layer consisting of carbon is set so that the interference pigment has a silvery grey absorption color. In contrast, all other layers optionally present on the support material do not contribute to the absorption color.

[0042] Suitable further colorless transparent layers on the flake-form 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.

[0043] Contemplated flake-shaped support materials are natural or synthetic mica, kaolin, talc or sericite, and also glass, calcium aluminum borosilicate, SiO2, TiO2 or Al2O3. The flake-shaped support materials employed are preferably natural or synthetic mica or Al2O3 flakes.

[0044] Interference pigments of the type mentioned are commercially available. They are available, for example, under the trade names 9602Silver-Grey SW or 9605 Blue Shade Silver SW is available from Merck KGaA. These are based on mica flakes and have at least one layer comprising iron oxide or titanium suboxide.

[0045] Particularly preferably, 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 can also be used. Such pigments are described, for example, in the patent application EP3795645 A1 of the applicant.

[0046] It has been found that interference pigments with a silver-grey absorption color are particularly suitable for use as effect pigments with absorbing properties in the second layer, since the coating as a whole is intended to have a silver-metallic appearance. Due to the sequential pigment structure of the thin layers on the flake-form substrate, interference pigments of this type exhibit a visually perceptible luster when incident light strikes them. The silver-grey absorption color produces a sufficiently high brightness in the case of direct incidence of light.

[0047] These interference pigments having absorption properties generally have a particle size in the range from 1 to 100 μm, in particular from 2 to 70 μm and particularly preferably in the range from 3 to 50 μm. The thickness of the interference pigments is in the range from 0.1 to 2 μm.

[0048] The particle size of the effect pigments having absorption properties can be determined by means of laser diffraction. The particle size and the particle size distribution relative to the volume are preferably determined using a Malvern instrument (Malvern Mastersizer 3000, APA300, a product from Malvern Instruments Ltd., UK) in standard mode. However, typical particle size ratios can also be found in the manufacturer's data in publicly available product information sheets.

[0049] Within this size range, sufficient hiding power can be achieved in the overall structure if the amount of effect pigments having absorption properties and the layer thickness of the second layer are set according to the invention.

[0050] According to the invention, the minimum amount of flake-form effect pigments having absorption properties in the second layer is 10% by weight, based on the weight of the (solid) second layer. The maximum amount of flake-form effect pigments having absorption properties in the second layer is 40% by weight, based on 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, based on their weight in the second layer.

[0051] According to the invention, the layer thickness of the second layer is in the range from 3 to 25 μm, preferably in the range from 5 to 20 μm.

[0052] If it seems advantageous, the second layer may also contain one or more of the absorbing pigments described above for the first layer, as long as the hiding power (ΔE * ), brightness (L * 15) and optical measurements of brightness flip (Dynamic Index) remain within specified limits.

[0053] The overall optical effect from the opaque, achromatic first layer and the second layer comprising interference pigments having a silver-grey absorption color results in a homogeneous silver-metallic overall impression of the coating according to the invention with high hiding power, high gloss and a pronounced brightness reversal.

[0054] Here the hiding power is given by ΔE * The value of ΔE * The value can be in L * 、a * , b * The quantity ΔE is determined spectrophotometrically in color space of the coated substrate. * It is defined here as the intensity at L on a standardized black and white background at an illumination angle of 45° and a viewing angle of 75°. * a * b * The colors of the samples are separated in color space and determined according to the following equation:

[0055] ΔE * =√(ΔL *2 +Δa *2 +Δb *2 ).

[0056] The lower the value of the resulting color separation, the better the coating hides the background. Complete hiding of the background is usually not achievable with non-metallic effect pigments. The second layer employed in the coating according to the invention has a ΔE in the range of 0 to 20, preferably in the range of 5 to 20, if the second layer is applied with a layer thickness in the range of 14±2 μm to a black / white background and measured under the above-mentioned measuring conditions. * These values ​​indicate the hiding power of the second layer which is sufficient for the purposes of the present invention.

[0057] The measure used in professional circles for the brightness of a layer is the L of the coating. * 15 value, which is at L on a standardized black / white background at a lighting angle of 45° * 、a * , b * Determined photometrically in color space. In order to be suitable as a coating according to the invention, it should have a minimum brightness that is achieved on a white substrate and also on a black substrate.

[0058] When the above-described effect pigments with a silvery grey absorption colour are used in the second layer, a second layer is obtained which, when applied with a layer thickness of 14±2 μm to the entire area of ​​a black / white background and at an illumination angle of 45° and a viewing angle of 15°, is observed at L * 、a * , b * The second layer has a brightness L of at least 105 on both a coated white background and a coated black background when measured spectrophotometrically in color space * 15.

[0059] In addition, a good brightness reversal can also be achieved. As a standard, this is quoted as a dynamic index and is determined spectrophotometrically at an illumination angle of 45° and at aspecular intervals of 15°, 45° and 110° from the specular angle. Therefore, according to the invention, when the second layer is applied to the entire area of ​​the black / white background with a layer thickness of 14±2 μm and at an illumination angle of 45° and viewing angles of 45°:15°, 45°:45° and 45°:110° at L * 、a * 、b * The dynamic index is within the range of at least 10 on both a white-coated background and a black-coated background when measured spectrophotometrically in color space.

[0060] The dynamic index is generally considered in the art as a measure of brightness flipping under varying viewing angles and is determined according to the following formula:

[0061]

[0062] The upper limit values ​​quoted do not apply to the brightness L * 15 or dynamic index, since both quantities have an open upper value and a measurement above the stated minimum value in each case has a positive influence on the overall optical result of observing a hiding power within the stated range.

[0063] Details of the spectrophotometric methods and instrumentation are described in the Examples section.

[0064] Surprisingly, it has been found that, in particular, the brightness reversal of the coating according to the invention quoted by the dynamic index can be significantly improved in some cases, especially on a grey or black first basecoat on a substrate, without significantly impairing the hiding power or brightness of the coating as a whole which must be accepted, if the second layer consists of two or more, preferably three or four, sublayers arranged one above the other. In these cases, the total dry layer thickness of the second layer is preferably only in the range of 5 to 15 μm.

[0065] At least one of the sub-layers, preferably two or three of the sub-layers, has a dry layer thickness of ≤5 μm. In particular, the dry layer thickness of at least one of the sub-layers is ≤4 μm or ≤3 μm, particularly preferably about 2 μm. These very low layer thicknesses can also preferably be present in two or three of the sub-layers.

[0066] In order to be able to combine such thin layerings to give a visually appealing overall layer as the second layer of the coating according to the invention, a smooth surface of these individual layerings is necessary. These interfaces between the individual layerings originating from the second layer arranged substantially parallel to the bottom layer or the coated substrate are obtained by intermediate drying after the application of each individual layering. Due to the intermediate drying, the flake-shaped effect pigments with absorption properties in each layering are aligned with their main axis approximately parallel to the surface of the first layer (and the substrate), and thus a good reflection of the incident light is achieved in each individual layering.

[0067] The pigment loading of the individual layers is likewise at least 10% by weight and a maximum of 40% by weight of absorbing flake-form effect pigments, based in each case on the weight of the individual layers. Preferably 15% to 35% by weight of absorbing flake-form effect pigments are used in each layer.

[0068] "Radar compatible" in the sense of the present invention means a coating having a dielectric constant of <30 when exposed to electromagnetic waves with a peak frequency of 76.5 GHz. Furthermore, it is necessary that the coating on a 350 μm PET substrate has a one-way transmission attenuation of <2 dB when exposed to electromagnetic waves with a peak frequency of 76.5 GHz.

[0069] The measurements of the dielectric constant of the coatings and the unidirectional transmission attenuation of the coatings on the substrate were performed using an RMS-D-77 / 79G instrument from perisens GmbH, Germany, in standard mode.

[0070] The binders used for the first and second layers of the coating according to the invention may be all conventional binders and binder systems which are transparent in the cured state. All common types of binders which are used in conventional coating methods and are compatible with the pigments used may be used here. Solvent-based binder systems, water-based binder systems and radiation-curing binder systems may be used equally, provided that the usual characteristics in the art with respect to the choice of pigment and with respect to the coating method are observed.

[0071] Both the first layer and the second layer of the coating according to the invention may contain further additives commonly used in the art, such as, for example, fillers, inhibitors, fire retardants, lubricants, rheology aids, dispersants, redispersants, defoamers, flow control agents, film formers, adhesion promoters, drying promoters, photoinitiators, etc.

[0072] If the second layer consists of two or more layered layers, the use of a rheological aid is generally indicated. The rheological aids considered are substances familiar to the person skilled in the art, such as, for example, BaSO4, polyamide powder, silicates or other rheological aids, but in particular cellulose-based nanofibers. The latter are particularly preferably used.

[0073] Depending on the binder system used, the coating composition for preparing the first layer and the second layer of the coating optionally also comprises an organic solvent and / or water, however, after the two layers are cured, the organic solvent and / or water are no longer present in the coating according to the present invention. The solvent system commonly used in the art can be adopted without restriction. The corresponding composition (including solvents and additives) for the binder system is fully known to those skilled in the art and is also commercially available as a finished product in an uncolored state in some cases. Those skilled in the art can make corresponding selections based on the corresponding coloring adopted and the desired coating method.

[0074] If the coating is intended to be radar-compatible, possible substrates to which the coating according to the invention comprising the first and second layers is applied are plastic sheets or films. Plastics commonly used in automobile construction, such as polycarbonate (PC), polypropylene (PP), polyurethane (PUR), polymethyl methacrylate (PMMA), acrylonitrile-butadiene-styrene (ABS) or acrylonitrile-ethylene-styrene (AES) substrates, to mention only a few, can be used. Plastic sheets or plastic films of this type have a certain basic attenuation of radar signals, which should only experience a slight increase due to the coating located thereon. With regard to the radar capability of the coating according to the invention, the value of the basic attenuation of the radar signal relative to the unidirectional transmission that is present due to the corresponding substrate is included in the measured values. The basic attenuation of the unidirectional transmission of the radar signal caused solely by the substrate is indicated separately in Example 4. For reasons related to the technical equipment, the measurement of the radar signal attenuation caused solely by the coating is not possible.

[0075] If the coating according to the invention is intended to be applied to a substrate for purely optical reasons and if the focus is not on radar compatibility of the coating, it is of course also possible to use metallic or metal-containing substrates.

[0076] It goes without saying that, depending on the application, the substrate may be three-dimensionally shaped, i.e. may have a three-dimensional outer shape. Thus, for example, a plastic sheet intended to form a part of a vehicle tailgate naturally has a different three-dimensional outer shape than a plastic sheet used as a bumper. Usually, the three-dimensional shape of the substrate is produced by means of conventional shaping methods before the coating according to the invention is applied.

[0077] The basic core element of the coating according to the invention on a substrate is a stack comprising the first layer and the second layer described above, wherein the second layer is arranged directly on the first layer as seen from the substrate. In addition, further layers which can also be part of the coating according to the invention can also optionally be located between the substrate and the first layer and / or on the second layer.

[0078] Additional layers of this type are often used in automobile construction in order 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 usually 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 which are widely used in industry and therefore do not require further explanation can be used here.

[0079] The coating according to the invention on a substrate can be advantageously used in all cases in which a radar device has a covering with a visually silver-effect paint layer without the function of the radar device being adversely affected. This is naturally particularly suitable for covering parts used in vehicle construction. The coating according to the invention is preferably an automotive paint layer. Due to its good optical properties, this can of course also be used for all types of paint layers that are intended to correspond very significantly to conventional silver metallic paint layers visually. The existing transparency to radar waves can also play a subordinate role, and the corresponding area of ​​use is not limited to vehicle construction.

[0080] The present invention also relates to a method for producing a radar-compatible coating comprising flake-form effect pigments on a substrate as described above, wherein a first layer comprising at least one absorbing pigment and being free of flake-form effect pigments is applied as a primer layer to an optionally precoated substrate comprising a plastic sheet or a plastic film, and subsequently a second layer comprising flake-form effect pigments having absorbing properties is applied to the first layer in an amount of at least 10 wt.-%, based on the solid layer weight, wherein the second layer is applied as a single layer or in two or more layered layers arranged one above the other and drying is carried out after the application of each layer.

[0081] All material details about suitable plastic substrates and the composition of the first layer and the second layer have been explained above. In this sense, reference is made here to them.

[0082] The two layers of the coating can be applied to the substrate by means of customary coating methods, for example by spraying, brushing, in-mold, roller coating, coil coating or curtain coating.

[0083] This type of coating process is common in large-scale industry and can be employed according to the art.

[0084] Preferably, the spraying method is used.

[0085] Conventional spraying techniques are suitable for producing the coatings according to the invention, in which dry layer thicknesses in the range of 5 to 25 μm can be obtained with a single spraying operation. To produce the coating according to the invention comprising a first layer and a second layer, two coating operations are sufficient, wherein the second layer is applied immediately after the first layer, one or more layers are dried with or without intermediate drying, and the two layers are cured in combination.

[0086] However, for the application of the second layer, particularly suitable spraying methods are those which allow the successive application of layers arranged one on top of the other to the first layer of the coating according to the invention in a plurality of processing steps, wherein the individual layers have very low dry layer thicknesses. In each case, preferably two to four layers are dried after the application of each individual layer, so that interfaces are formed between the individual layers. The temperature for drying the individual layers depends on the respective binder system and the solvent employed and is at least 20° C. Temperatures of up to 150° C., preferably up to 100° C., can be employed.

[0087] The amount of flake-form effect pigments having absorbing properties in the second layer is at least 10% by weight for each individual sublayer, based on the dry layer weight, 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 sublayers is ≤5 μm, preferably ≤4 μm and particularly preferably ≤3 μm or about 2 μm. Preferably, two or three sublayers have such a low layer thickness.

[0088] In the case of very low dry layer thicknesses of the strata, a high pigment concentration in the respective strata can be set by greatly reducing the proportion of binder in the respective coating composition (solids content of about 6% to 7% by weight) and greatly increasing the proportion of solvent (preferably water). In order that very dilute coating compositions of this type can form a continuous coating film on the substrate, various auxiliaries, in particular rheological auxiliaries, are added, which ensure that a suitable viscosity of the coating composition is set so that this can be applied to the background by means of a spraying method and exhibits good flow properties. In the course of the subsequent drying process, small solid substances with a very high proportion of effect pigments remain as strata on the respective background, wherein the effect pigments are also well aligned with their main axis essentially parallel to the respective coating surface.

[0089] According to the present invention, the rheological additive added here is preferably cellulose-based nanofibers in an amount of 5 to 20 wt % based on the weight of the coating composition.

[0090] Due to the multiple application of the layering arranged one above the other and the corresponding intermediate drying of the layering, the flake-form effect pigments in the second layer can be particularly well oriented, so that a high reflection of the incident light is obtained at the surface of the second layer. This improves the brightness reversal of the coating as a whole, while the total layer thickness of the second layer as a whole is extremely low, without significantly impairing the hiding power or the brightness of the layer structure as a whole. A particularly preferred embodiment of the coating according to the invention, which visually corresponds very substantially to a silver-colored metallic paint layer, but has good radar compatibility if it is applied to plastic substrates, can thus be achieved on substrates with flake-form effect pigments having absorption properties in the second layer, and without using all types of metallic pigments in the coating as a whole.

[0091] The plastic substrates employed which have predetermined radar properties may optionally be pre-coated, for example, with one or more primer layers 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 additionally present on the respective substrate contains metallic effect pigments or other components which could adversely affect the desired radar transparency of the coating as a whole.

[0092] Pre-coating the plastic substrate with a primer layer is advantageous, since such a primer layer improves, in particular, the mechanical stability of the coating as a whole and the adhesion of the first layer of the layer stack to the substrate. In addition, an outermost transparent coating layer, which is usually designed to be colorless and transparent to visible light, is advantageous, in particular for the mechanical stability and weather resistance of the coating layer. In the present invention, they are also preferably applied as the outermost layer of the coating layer as a whole to the upper layer of the layer stack comprising the first layer and the second layer.

[0093] It goes without saying that the coating as a whole is subjected to at least one hardening operation, which is carried out after the second layer has been applied to the substrate and dried and / or after the clearcoat has been applied. The hardening of coatings on substrates, in particular on substrates in the automotive sector, is a standard activity in the art and does not need to be described in more detail.

[0094] The invention also relates to the use of the above-described coating without metal effect pigments as a radar-compatible vehicle paint layer on vehicle parts. It can be applied to all vehicle parts based on a plastic matrix (substrate). Metal substrates are unsuitable because they do not guarantee the desired radar capability. The coating can be applied to external body parts that serve as external coverings or shielding parts for radar devices installed in the interior of the vehicle, or also to the entire surface of suitable body parts. Body parts that may be mentioned are in particular bumpers, tailgates, radiator grilles, wings or parts thereof. If only the visual appearance of the metallic paint layer is of interest and radar capability is not necessary, the coating according to the invention can of course also be applied to vehicle parts other than those mentioned, and in particular also to metal-containing substrates. In the latter case, the field of application of the invention is also not limited to vehicle construction.

[0095] The following is intended to explain the present invention with reference to examples, but is not limited thereto. Example:

[0096] A black / white coated Leneta plate as substrate was coated with a second layer (white and black base layers were already present on the corresponding partial areas). The coating was carried out as pneumatic spraying. The binder used was the formulation WBC 000 from MIPA SE, DE. Finally, all samples were coated with a standard 2-component clearcoat.

[0097] The second layer was pigmented with an interference pigment having a silver-grey absorption color in the amounts indicated in the table.

[0098] Effect pigment A: mica-based interference pigment with a coating comprising SnO2, TiO2 and carbon (C content 1.14%), particle size 5-25 μm;

[0099] Effect pigment B: interference pigment based on aluminum oxide flakes with a coating comprising SnO2, TiO2 and carbon (C content 0.44%), particle size 5-30 μm;

[0100] Effect Pigment C: Mica-based interference pigment with a coating comprising SnO2, TiO2, titanium suboxide and additives, particle size 5-40 μm.

[0101] Embodiment 1:

[0102] To determine the hiding power of the coatings according to the invention, a coating composition comprising pigments A, B and C in a pigment mass concentration of 18 wt.-%, based on the weight of the solid second layer, was applied in a single coating operation to a standardized black and white coated panel and dried at 80° C. for 5 minutes. The resulting color separation ΔE * The smaller the 75°, the better the hiding power of the effect pigment.

[0103] If the second layer is applied in four layers (in each case 18% by weight PMC, layer thicknesses 9 μm, 2 μm, 2 μm, 2 μm, dried in each case for 5 minutes at 80° C.), only a slight change in hiding power occurs compared to the one-layer process.

[0104] Table 1: Hiding power

[0105] pigment Layering PMC(%) DLT(μm) <![CDATA[ΔE * 75°]]> A 1 18 15 7 B 1 18 15 20 C 1 18 15 18 A 4 18 15 9 B 4 18 15 25 C 4 18 15 20

[0106] Delamination: The number of delaminations of the second layer of the coating

[0107] PMC: Pigment mass concentration of effect pigments in each layer

[0108] DLT: dry layer thickness of the entire second layer consisting of x layers

[0109] L * : At an observation angle of 15° and an illumination angle of 45°, L * a * b * Brightness value L in color space *

[0110] ΔE * : Sample in L * a * b * The color separation in color space on a standardized black and white background (illumination angle 45°, viewing angle 75°) is determined according to the following formula:

[0111] ΔE * =√(ΔL *2 +Δa *2 +Δb *2 )

[0112] Dynamic index: A measure of brightness flip at different viewing angles (illumination angle 45°, viewing angle 45°:15°, 45°:45°, 45°:110°), determined according to the following formula:

[0113]

[0114] Baseline: Color chromaticity of the first layer

[0115] Embodiment 2:

[0116] To determine the brightness of the respective coatings, the effect pigments A, B and C as in Example 1 were applied to the respective black or white backgrounds. In addition, the same pigments were applied in a three-step process with a layer thickness of about 2 μm / layer (drying after each application: 80° C., 5 minutes) to a black or white coated background, wherein the pigment mass concentration in each case was 30% by weight, based on the weight of each layer. The resulting brightness values ​​L* 15 The larger the value, the better the opaque tinted coating with only aluminum pigments can be visually imitated.

[0117] Table 2: Brightness

[0118]

[0119] High brightness values ​​can be achieved with each coating variant and each effect pigment in the second layer.

[0120] Embodiment 3:

[0121] To determine the dynamic index, all coatings prepared in Example 2 were remeasured.

[0122] Table 3: Dynamic Index

[0123]

[0124] Conventional silver metallic coatings, which usually contain aluminum pigments, have a dynamic index in the range of about 12 to 17. This range can be achieved with all of the coating variants described and with all of the silver-grey interference pigments described. In particular, the multilayer coating variants on a black background achieve very high dynamic index values.

[0125] The colorimetric measurements of the samples were carried out using a model BYKMac i colorimeter (Byk-Gardner) in SMC5 mode. The black / white plates used as substrates here conform to the ASTM E 1347 standard and are sold by Leneta under the name Metopac T12G plates.

[0126] As can be seen from the table, the coating according to the invention, in which each interference pigment used has a silvery grey absorption color and each coating method variant used, achieves good brightness and a strong brightness reversal with satisfactory hiding power and is therefore able to visually imitate metallic coatings containing aluminum pigments in a good to very good manner. Due to the absence of metallic pigments in the coating, a significant attenuation of radar waves by the corresponding coating on the substrate is not to be expected.

[0127] Embodiment 4:

[0128] To determine the radar transparency, a PET film (Hostaphan RN 350, Mitsubishi Polyester Film GmbH, DE) with a thickness of 350 μm was used as substrate in each case. The coating was carried out as pneumatic spraying. The binder used was the formulation WBC 000 from MIPA SE, DE.

[0129] As first layer, a completely opaque layer of RAL colour shade 7037 (earth grey) was applied in each case.

[0130] As second layer, a layer of interference pigments having the silvery grey absorption color listed in Table 4 was applied in each case in one or four layers and dried as described in Example 1.

[0131] Table 4 shows the dielectric constant (permittivity) of the corresponding layer structures for a single beam channel (76.5 GHz) and the attenuation of the radar signal in dB (instrument: RMS-D-77 / 79G from perisens GmbH, DE, standard method)

[0132] The uncoated PET substrate has a dielectric constant of about 3.2 and a radar wave attenuation of 1.05 dB.

[0133] A coating comprising a single layer on a PET substrate comprising a commercially available aluminum pigment having 18 wt. % PMC and a DLT of about 22 μm had, for comparison, a dielectric constant of about 74.9 and a unidirectional attenuation of radar signals of about 4.5 dB under the same measurement conditions.

[0134] Table 4: Radar wave transparency

[0135]

Claims

1. A radar-compatible coating comprising flake-form effect pigments on a substrate, wherein the coating is free of metallic effect pigments, characterized in that The coating has at least two layers on the substrate in the following order: A) a first layer, which refers to the bottom layer, comprises absorbing pigments and is free of flake-form effect pigments, and B) a second layer, which is applied onto the first layer and comprises interference pigments as flake-form effect pigments with absorption properties, which are present in an amount of 15% by weight to 40% by weight, based on the weight of the second layer, the interference pigments having a silvery-grey absorption color, and the interference pigments having a silvery-grey absorption color are pigments having at least one layer comprising titanium suboxides or having a layer consisting of carbon on a transparent flake-form support material, The interference pigment has a particle size in the range of 1 to 100 μm; The second layer has a layer thickness in the range of 3 to 25 µm; The substrate is a plastic plate or a plastic film; and wherein when the second layer is applied with a layer thickness of 14±2 µm over the entire area of ​​the black / white background and at an illumination angle of 45° and a viewing angle of 75° at L * 、a * 、b * The second layer has a color separation ΔE between a coated black background and a coated white background ranging from 0 to 20 when measured spectrophotometrically in color space * .

2. The coating according to claim 1, characterized in that When the second layer is applied with a layer thickness of 14±2 µm over the entire area of ​​the black / white background and at an illumination angle of 45° and a viewing angle of 15° at L * 、a * 、b * The second layer has a brightness L of at least 105 on a coated white background and on a coated black background when measured spectrophotometrically in color space. * 15.

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

4. The coating according to claim 1 or 2, characterized in that The first layer has a white, grey or black color and contains organic or inorganic absorbing pigments.

5. The coating according to claim 1 or 2, characterized in that The second layer consists of two or more layered layers arranged one above the other.

6. The coating according to claim 1 or 2, characterized in that The plastic sheet or plastic film optionally has a three-dimensional outer shape.

7. The coating according to claim 1 or 2, characterized in that Further layers are located on the substrate, optionally beneath the first layer and / or above the second layer.

8. The coating according to claim 7, characterized in that One or more of the further layers is a primer layer and / or an outermost clear coat layer.

9. The coating according to claim 1 or 2, characterized in that It is the vehicle paint layer.

10. A method for preparing a coating according to claim 1, characterized in that - applying a first layer comprising at least one absorbing pigment and free of flake-form effect pigments as a primer to an optionally precoated substrate, which is a plastic sheet or a plastic film, and subsequently - applying a second layer comprising flake-form effect pigments with absorbing properties in an amount of 15 wt.-% to 40 wt.-%, based on the solid layer weight, onto the first layer, the second layer comprising interference pigments as flake-form effect pigments with absorbing properties, the interference pigments having a silver-grey absorption color, and the interference pigments having a silver-grey absorption color are pigments having at least one layer comprising titanium suboxides or having a layer consisting of carbon on a transparent flake-form support material, wherein the second layer is applied as a single layer or in two or more layered layers arranged one above the other and drying is carried out after application of each layer; The interference pigment has a particle size in the range of 1 to 100 μm; The second layer is applied in a total dry layer thickness in the range from 3 to 25 μm.

11. The method according to claim 10, characterized in that The first and second layers are applied by means of spraying, brushing, rolling, coiling, curtain coating or in-mold methods.

12. The method according to claim 11, characterized in that The application of the second layer is carried out by spraying in two to four partial steps in such a way that two to four layer layers are applied successively and each on top of each other, wherein the amount of the flake-form effect pigments having absorption properties in each of the layer layers is at least 10% by weight, based on the dry weight of the respective layer layer, and wherein drying is carried out after the application of each layer layer at a temperature of at least 20° C.

13. The method according to claim 12, characterized in that At least one of the layered layers has a dry layer thickness of ≤ 5 µm.

14. The method according to claim 10 or 11, characterized in that: The substrate has been pre-coated with a primer layer.

15. The method according to claim 10 or 11, characterized in that: A clear lacquer layer is applied onto the second layer as the outermost layer of the coating.

16. Use of a coating according to any one of claims 1 to 9 as a radar-compatible vehicle paint layer on vehicle parts. 17 . A vehicle part comprising a plastic sheet substrate or a plastic film substrate, the plastic sheet or the plastic film having at least one coating according to claim 1 .

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