WO2024236425A1 - Effect pigments composed of a three-layered stack, method of manufacture and use - Google Patents

Abstract

This invention deals with flaky effect pigment having as only optical active layers a three-layered stack consisting of planar discrete layers of i) a semiconductor layer ii) a central dielectric layer with refractive index < 1.80 iii) a semiconductor layer wherein the semiconductor layers i) and iii) are mady by a semiconductor material with a band gap in a range of 0.1 to 2.5 eV and independently have an average atomic composition of: a) Si_(1-x)Ge_x, wherein 0 ≤ x < 1.00 or b) Si_(1-y)Sn_y, wherein 0 < y < 0.90 or c) Ge_(1-z)Sn_z, wherein 0 < z ≤ 0.60 or d) Si_(1-m-n)Ge_mSn_n, wherein 0 < m < 1.00, 0 < n < 1.00 with the provisos that x < 1.00; y < 1.00, z < 1.00 and m + n < 1.00.

Description

Effect pigments composed of a three-layered stack, method of manufacture and Use The present invention relates to three-layered effect pigments composed of a stack of a central dielecric layer coated with semiconductor layers on both main sites with silvery interference color and radar transparency. The use of radar is becoming ubiquitous in modern transportation including passenger vehicles with advanced driver assistance systems (ADAS), such as adaptive cruise control (ACC), automatic breaking, and the like. The use of radar will likely further increase as higher levels of autonomous driving are implemented. Radar performance can be hindered by unwanted radar signal loss, which may result from the use of metallic pigments, such as aluminum flakes, commonly used in coatings to achieve a certain luster, sparkle, flop and/or a metallic color. Coatings, films, and articles of manufacture that minimize interference with radar while providing the desired appearance are desired. US 2010/0022696 A1 disclosed a radar transparent mixture of metal effect pigments and pearlescent pigments. Aluminum effect pigments obtained from milling processes or from PVD deposition processes were disclosed. The aluminum pigments have to be used in such concentrations that they have a certain distance to each other in order to not act as “large” particles which can adversely reflect radar microwaves. The attenuations obtained were in the order of less than 1.0 dB. Such effect pigments mixtures, however, did not obtain good hiding power as the pearlescent pigments used were rather transparent. Furthermore, the coatings systems described therein do not match current demands of coatings thicknesses or effect pigments concentrations. This drawback was overcome by US 2022/0145082 A1 which disclose a mixture of milled aluminum effect pigments and silvery pearlescent pigments of special kind with high opacity. With such mixtures existing automotive full-tone silver formulations could be matched with rather low concentrations of effect pigments and a radar transparency of below 3.0 dB was reached. A mixture of metallic aluminum pigments with pearlescent pigments was also disclosed in US 2021/0040329 A1. Another solution without the use of metallic pigments was disclosed in WO 2022/064018 A1. Herein, a first base-coat layer pigmented without effect pigments but with absorbing pigments and thereon a second base-coat layer pigmented with effect pigments was disclosed. The effect pigments in the second base-coat are pearlescent pigments and also silvery absorbing pearlescent pigments were used. However, such two-coat setup would increase costs of the base coats and also the resulting coatings had rather low brightness. Another two base coat system was disclosed in WO 2022/049041 A1, wherein a first base coat without effect pigments but with absorbing pigments and thereon a second base-coat layer pigmented solely with metallic effect pigments was disclosed. Again, such two-coat system solution will be to cost intensive for automotive industry. WO 2022/011131 A1 discloses coatings with a compound pigment comprising a nonconductive composite comprising a semiconductor and/or a dielectric, and a metal dispersed in and/or on the semiconductor and/or dielectric, wherein the pigment has an aspect ratio of at least 5, such as, at least at least 10, at least 50, at least 100, at least 500, or at least 1000 wherein the aspect ratio is an average lateral size of the pigment divided by an average thickness of the pigment. These composites were produced with PVD methods. However, such composites are difficult to reproduce. Furthermore, the coating industry has less possibilities of coloristic variations as in these compounds the ratio of the semiconductor or the dielectric and the metallic parts are fixed. US 2005/132929 A1 discloses dichroic effect pigments with strog Chroma color flops. The effect pigments are most preferably five-layer stacks of alternating high and low refractive index layers which imply rather high total pigment thicknesses. Such effect pigments are usually used for security printing applications due to their unusual color flops, but are not used in automotive industry. There is still a need to further improve the radar transparency of metallic-like coatings and to provide effect pigments having optical properties similar to metallic pigments with improved hiding power against pearlescent pigments to enable transparency for longer than visible light wavelength like especially radar transparent coatings with metallic look using these effect pigments as replacement of metallic aluminum pigments or in admixture with little amounts of aluminum effect pigments. The metallic appearance with respect to flop and brightness should be comparable to metallic pigments. Such effect pigments should be applicable in a single base coat. A further object is to provide a simple method of manufacturing such effect pigments. The object is solved by providing a flaky effect pigment having as only optical active layers a three-layered stack consisting of planar discrete layers of i) a semiconductor layer ii) a central dielectric layer with refractive index < 1.80 iii) a semiconductor layer wherein the semiconductor layers i) and iii) are mady by a semiconductor material with a band gap in a range of 0.1 to 2.5 eV and independently have an average atomic composition of: a) Si(1-x)Gex, wherein 0 ≤ x < 1.00 or b) Si(1-y)Sny, wherein 0 < y < 0.90 or c) Ge(1-z)Snz, wherein 0 < z ≤ 0.60 or d) Si(1-m-n)GemSnn, wherein 0 < m < 1.00, 0 < n < 1.00 with the provisos that x < 1.00; y < 1.00, z < 1.00 and m + n < 1.00. Preferred embodiments are contained in dependent claims 2 to 13. The object is further solved by providing a method of manufacture of the effect pigments comprising the steps: a) providing a flexible substrate coated with a release agent, b) evaporating under ultra high vacuum conditions a first semiconductor material i) with a band gap in a range of 0.1 to 2.5 eV onto the flexible substrate a), then evaporating a dielectric layer ii) with refractive index < 1.80 onto the first semiconductor layer i) and finally evaporating a second semiconductor material iii) onto the dielectic layer ii) to yield a three-layered stack, c) stripping the three-layered stack film from step b) from the flexible substrate in a suitable solvent and comminuiting the particles in the dispersion to obtain effect pigment flakes, d) separating the effect pigment flakes from the solvent and e) optionally conducting further steps like any of further size classifying of the effect pigment flakes or dispersing the effect pigment flakes in a different solvent or conducting further surface treatment steps of the effect pigment flakes. Finally the object of the present invention is solved by providing coating systems comprising a binder and the flaky effect pigments. In a preferred embodiment the band gap of the semiconductor layers i) and iii) is in a range of 0.2 to 1.4 eV and more preferred in a range of 0.4 to 1.2 eV. Such bandgaps are typical for semiconductor materials. In this invention the semiconductor layers i) and iii) have independently an average atomic composition of: a) Si(1-x)Gex, wherein 0 < x < 1.00 or b) Si(1-y)Sny, wherein 0 < y < 0.90 or c) Ge(1-z)Snz, wherein 0 < z ≤ 0.60 or d) Si(1-m-n)GemSnn, wherein 0 < m < 1.00 and 0 < n < 1.00 and with the provisos that x < 1.00; y < 1.00, z < 1.00 and m + n < 1.00. The x, y, n and m are mole fractions. In further preferred embodiments the semiconductor layers i) and iii) according to a) have a composition of 0.01 < x < 0.9, more preferably 0.02 ≤ x ≤ 0.8 and most preferably 0.05 ≤ x ≤ 0.65. These materials are alloys of silicon and germanium. Germanium adds interesting color effects as this material is absorbing in the visible wavelength region. This also enhances the opacity compared to pure silicon flakes. Due to the high costs of this material the content of germanium is preferably as low as possible. The layers i) or iii) may be of different composition or the same composition. A symmetrical stack with the same composition of layers i) and iii) is preferred. In further preferred embodiments the semiconductor layers i) and iii) according to b) have a composition of 0.02 ≤ y ≤ 0.75 and more preferred of 0.05 ≤ y ≤ 0.55. These materials are alloys of silicon and tin. In further preferred embodiments the semiconductor layers i) and iii) according to c) have a composition of 0.02 ≤ z ≤ 0.5 and more preferred a composition of 0.05 ≤ z ≤ 0.4. These materials are alloys of germanium and tin. In further preferred embodiments the semiconductor layers i) and iii) according to d) have a composition characterized by 0.02 ≤ m ≤ 0.8 and 0.02 ≤ n ≤ 0.75 and more preferred a composition characterized by 0.05 ≤ m ≤ 0.65, 0.05 ≤ n ≤ 0.55. The platelet semiconductor layers i) and iii) may further contain usual impurities occurring by the manufacture of the materials such as carbon, nitrogen or oxygen. These materials are not included into the formulas mentioned above. Impurities of other metals or other semiconductor materials not contained into the formulas above are typically less than 0.1 wt.-%, preferably less than 0.05 wt.-%, more preferably less than 0.005 wt.-% of the platelet semiconductor material and are also not included into the formulas above. The semiconductor layers i) and iii) may further contain some amounts of oxygen due to surface oxidation. For example, a platelet alloy semiconductor flake may be oxidized on it`s surface. This kind of oxygen is also not included in the formulas for the sake of clarity. Preferably the semiconductor layers i) and iii) do not contain any noticeable amount of oxygen in their interior. The dielectric layer ii) has a low refractive index in the visible electromagnetic spectrum (380 to 770 nm) below of 1.80 and more preferably below 1.65 to ensure a maximum of difference to the semiconductor layers I and iii). Most preferably the dielectric layer ii) has a bandgap > 3.0 eV which means transparency in the visible electromagnetic spectrum. Preferred materials for the central dielectric layer ii) are SiO2, Al2O3, MgO, MgF2, AlF3, B2O3, CeF3, LaF3, Na3AlF6, NdF3, SmF3, BaF2, CaF2, LiF and polymer, combinations thereof, or any other low index material having an index of refraction of about 1.80 or less. As low index polymer materials acrylates (e.g., methacrylate), perfluoroalkenes, polytetrafluoroethylene (Teflon), fluorinated ethylene propylene (FEP), combinations thereof and the like may be utilized.. More preferred with respect to costs and versatility are SiO2 or MgF2 and most preferred is SiO2. The formulas “SiO2” or Al2O3” also include certain understochiometric species like SiO(2-y), wherein y = 0.00 to 0.40, more preferred y = 0.00 to 0.30 and most preferred y = 0.00 to 0.20 which is typical for layers deposited by PVD techniques. The effect pigments do not contain metal layers or metal nanoparticles. Therefore, the effect pigments are completely gassing stable which is a prerequisite for use in automotive industry . The three-layer stack has descrete three layers which has the advantage that the thickness and thus optical properties can be easily adjusted. All layers are planar as typical for layers obtained by PVD methods. No diffraction patterns exist as this would lead to not desired color flops. The use of descrete and planar surfaces enables the development of best interference phenomena. The three-layer stack of the flaky effect pigments have a solid constitution with a low or without porosity in it`s inner structure. The porosity as determined by mercury porosity measurements is either essentially zero or cannot be determined at all because of the lack of porosity. The effect pigments are preferably produced by PVD methods. Their main surfaces (top and below) are rather flat and smooth as typical for PVD effect pigments. Such smooth structures and the absence of noticeable inner porosity enable the platelets to act with optimal reflectance. Due to the high refractive index of the semiconductor materials i) and iii) in the visible wavelength region and the low refractive index of the central layer ii) the flaky effect pigments exhibit a rather strong reflection due to interference phenomena. Depending on the thickness of the semiconductor platelets various colors may be produced. In preferred embodiments with neutral or slightly bluish tone the maximum reflection range of the effect pigments is located in a wavelength region of 425 to 550 nm and more preferably in a wavelength region of 425 to 525 nm in order to deliver a color neutral pigment with a slightly bluish hue, which is similar to aluminium. For a further preferred blue embodiment the maximum reflection range of the effect pigments is located in a wavelength region of 400 to 500 nm and more preferably in a wavelength region of 415 to 475 nm. The visible wavelength region is the range of 380 nm to 770 nm as also used in the CieLab color system. In this invention the semiconductor layers i) or iii) independently have a preferred thickness in a range of 15 to 50 nm, a more preferred in a range of 18 to 35 nm, and most preferred in a range of 20 to 30 nm. Preferably, the thickness of layers i) and iii) is about the same with deviations of a maximum of 25% and more preferred of 10% with respect to the thicker layer. These embodiments thus correspond to a A-B-A stack. Below of 15 nm thickness the reflectivity and hiding power of the effect pigment may not be high enough and above a thickness of 50 nm a substantial color flop may occur in the visible light range. The central dielectric layer ii) preferably has an average thickness of a range of 45 to 140 nm, more preferred a range of 60 –to120 nm, and most preferred a thickness range of 80 to 105 nm. Above a thickness of 140 nm or below 45 nm more color flopping effect pigments are achieved which is not the object of this invention. The flaky effect pigment preferably has an average total stack thickness in a range of 100 – 200 nm, a more preferred stack thickness in a range of 105 – 190 nm, and most preferred a stack thickness in a range of 110 – 180 nm. Above a thickness of 200 nm or below 100 nm more again color flopping effect pigments are achieved. The total stack thickness of the three-layered effect pigment is thus in the same range as aluminum effect pigments obtained by milling of silverdollar type used in the automotive industry. Therefore, a good plane-parallel orientation is achieved in a coating and the effect pigments are easy to be produced as a stack of three layers is still feasible to be produced with PVD methods within reasonable time and costs. When not mentioned otherwise in this invention the term “thickness” denotes to the geometrical thickness of a layer. The thicknesses may be determined from the three- layered effect pigments by using SEM on cross-sections of suitable draw-downs or coatings of the effect pigments in a coating system suitable for such kind of analysis. The effect pigments shall be well oriented plane-parallel to the substrate in such applications to minimize errors due to misalignment. All thicknesses of any of the three-stack layers as well as the overall thickness are intended to provide effect pigments with a high reflection in the visible spectrum and without high chroma color flops. Preferred embodiments denote to color neutral or bluish three-layer stacked effect pigments. Preferably the overall stack optical thickness pigments (meaning the sum of all products of the refractive index multipled with the geometrical thickness of each respective layer) is in a range of 240 to 440 nm, more preferred in a range of 260 to 420 nm and most preferred in a range of 330 to 400 nm. Herein the literature refractive index of the materials might be used at a wavelength ^=500 nm which is recognized as a point for color neutrality. Outside of the range of 220 to 440 nm overall stack optical thickness color flopping effect pigments and(or effect pigments with color tones in the yellow to red region are achieved. In preferred embodiments the overall stack optical thicknessof the effect pigmentis designed for color neutral or slightly bluish effect is in a range of 300 to 440 nm, more preferred in a range of 310 to 420 nm and most preferred in a range of 320 to 390 nm. In other embodiments blue effect pigments can be obtained with an overall stack optical thickness in the range of 240 to 330 nm and more preferably in a range of 250 to 320 nm. In preferred embodiments the average optical thickness of the semiconductor layers i) or iii) of the effect pigments are in a range of 64 to 150 nm, preferably in an range of 75 to 135 nm and most preferably in an range of 90 to 125 nm. In preferred embodiments the average optical thickness of the central dielectric layer ii) is in a range of 65 to 205 nm, preferably in an range of 80 to 180 nm, and most preferably in an range of 110 to 150 nm. Specifically the literature values for the refractive index used at ^=500 nm are 4.29 for silicon and 1.47 for SiO2. In a most preferred embodiment the semiconductor layers i) and iii) are made from silicon and are most preferred of about equal thickness (within a maximum of 10% deviation) while the central dielectric layer ii) is made from SiO2. The preferred geometrical thickness for the silicon layers are in a range of 15 to 35 nm and more preferred in a range of 18 to 32 nm and a most preferred range of 20 to 30 nm. Above a thickness 35 nm a yellowish color tone is achieved while below of 15 nm either color flops or a too violet tone is obtained for most embodiments. The preferred thickness range for the SiO2 layer is in a range of 45 to 140 nm, more most preferred in a range of 60 to 130 nm and most preferred in a range of 85 to 105 nm. The desired “low chroma color flop” can be evaluated by color measurements of a draw- down of the effect pigment in a nitrocellulose binder at a pigment concentration of 5.5 wt.% and a pigment:binder wt. ratio in a range of about 1.0:0.4 to 1.0:2.0… When the a* and b* values are determined by using a BYK Mac i colorimeter apparatus within the common CieLab system such low color flop may be expressed preferably by displaying a difference between the maximum and minimum values of a* referring to the set of angles of 15°, 25°, 45°, 75° and 110° in trans configuration, and the same for b* values. Using this metric, a ^C* = C*max - C*min may be defined, wherein C*max denotes to the maximum of and C*min denotes to the minimum of C* at any of the angles of observation mentioned before. Effect pigments with low chroma color flop and neutral to bluish color tones are defined in this invention as having a ^C* of < 10.0 and more preferably a ^C* < 8.0 and most preferably a ^C < 5.0. Regarding the sizes and size distributions of the flaky effect pigments typical size ranges of coatings in the automotive industry or of industrial coatings are chosen. Preferably the flaky effect pigment have a d50 of the particle size distribution is in a range of 5 to 100 µm, more preferably in a range of 6 to 40 µm, further more preferred in a range of 7 to 35 µm and most preferably in a range of 8 to 30 µm. The pigment size is typically indicated using quantiles (d values) from the volume averaged particle size distribution. Here, the number indicates the percentage of particles smaller than a specified size contained in a volume-averaged particle size distribution. For example, the d50 value indicates the size where 50% of the particles are smaller than this value. These measurements are conducted e.g. by means of laser granulometry using a particle size analyzer manufactured by Horiba and is a Horiba LA 950 instrument. The measurements are conducted using Fraunhofer approximation for equivalent spheres and suitable parameters according to informations from the manufacturer. The d10-values characterize the amount of fine particles and typically range from 2 to 20 µm and preferably from 4 to 15 µm. The d90-values characterize the amount of coarse particles and typically range from 15 µm to 140 µm and preferably from 20 µm to 50 µm. The width of the particle size distribution can be characterized by the span defined as (d90-d10)/d50 and preferably this span is in a range of 1.50 to 2.2 and more preferably in a range of 1.6 to 2.0. The flaky effect pigments according to this invention preferably have an aspect ratio defined as d50/h50 in a range of 30 to 350, more preferred in a range of 40 to 300 and most preferred in a range of 50 to 200. Within this invention the only optically active layer of the flaky effect pigments consist of three-layered stack described before. A further advantage of these effect pigments compared to metal flakes, especially to widly used aluminum flakes is their excellent gassing stability. Usually these platelets do not need to be coated with further corrosion inhibition layers. Furthermore, these effect pigments exhibit essentially no or very low attenuation of radar radiation. Compared to pearlescent pigments and especially to silvery pearlescent pigments described in US 2022/0145082 A1 which have quite significant absorption in the visible spectrum the three-layered effect pigments of this invention have a rather higher opacity. However, in some cases further coatings might be necessary. More often, certain coatings with optically non-active materials might be useful. Such coatings might be also necessary to further facilitate the bonding of organofunctional silanes onto the surface of the effect pigments in order to modify the surface properties of the effect pigments. Therefore, in further embodiments the three-layeres effect pigment is further encapsulated with transparent not optically active metal oxides of refractive index n < 1.8, preferably a refractive index of < 1.6. Typically such non-active layers have a mean refractive index in the visible wavelength region of less than 1.7, more preferably less than 1.6. Such layers are called as “opticaly non-active layer” if they have an optical thickness of less than 34 nm, more preferably less than 20 nm and most preferably less than 15 nm in the visible wavelength region. Herein, the refractive index refers to literature bulk values of the respective material rather than the effective refractive index of the layer. In preferred embodiments the optically non-active layer encapsulates essentially the whole three-layered stack effect pigments and consists of a layer of SiO2, Al2O3, B2O3 or mixtures thereof. If not used for further enhancing gassing stability a typical optically non- active layer are surface modifiers like organofunctional silanes, titanates, aluminates or zirconates, phosphate ester, phosphonate esters, phosphite esters, alcohol or amine based additives and combinations thereof. Such surface modifiers are used as top- coating to adjust the chemical compatibility of the effect pigment to the binder medium of the final application as described in e.g. EP 1084198 A1. They can be coated either directly on the three-layered stack effect pigments or on the optically non-active layer. Most preferred as surface modifiers are organofunctional silanes. In another preferred ambodiment the semiconductor platelet is coated first by a thin layer or SiO2 and then coated with suitable surface modifiers, most preferably organofunctional silanes. The SiO2 layer here is primarly used to enhance the adhesion of the organofunctional silanes to the surface of the semicoductor platelet. Suitable organofunctional silanes are available commercially and are produced, for example, by Evonik, Rheinfelden, Germany and sold under the trade name "Dynasylan^®". Further products can be purchased from OSi Specialties (Silquest^® silanes) or from Wacker (Genosil^® silanes). Examples of suitable organofunctional silanes are 3-methacryloxypropyl trimethoxy silane (Dynasylan MEMO), vinyl tri(m)ethoxy silane (Dynasylan VTMO or VTEO), 3- mercaptopropyl tri(m)ethoxy silane (Dynasylan MTMO or 3201), 3-glycidyloxypropyl trimethoxy silane (Dynasylan GLYMO), tris(3-trimethoxysilylpropyl) isocyanurate (Silquest Y-11597), gamma-mercaptopropyl trimethoxy silane (Silquest A-189), bis(3- triethoxysilylpropyl) polysulfide (Silquest A-1289), bis(3-triethoxysilyl) disulfide (Silquest A-1589), beta(3,4-epoxycyclohexyl) ethyltri-methoxysilane (Silquest A-186), gamma- isocyanatopropyl-trimethoxsilane (Silquest A-Link 35, Genosil GF40), (methacryloyloxymethyl) trimethoxysilane (Genosil XL 33) and (isocyanatomethyl)trimethoxysilane (Genosil XL 43). In one preferred embodiment the organofunctional silane mixture that modifies the SiO2 layer comprises at least one amino-functional silane. The amino function is a functional group which is able to enter into chemical interactions with the majority of groups present in binders. This interaction may involve a covalent bond, such as with isocyanate or carboxylate functions of the binder, for example, or hydrogen bonds such as with OH or COOR functions, or else ionic interactions. It is therefore very highly suitable for the purpose of the chemical attachment of the effect pigment to different kinds of binder. The following compounds are employed preferably for this purpose: aminopropyl trimethoxy silane (Dynasylan AMMO), aminopropyl triethoxy silane (Dynasylan AMEO), N-(2-aminoethyl)-3-aminopropyl trimethoxy silane (Dynasylan DAMO), N-(2-aminoethyl)-3-aminopropyl triethoxy silane, triamino-functional trimethoxy silane (Silquest A-1130), bis(gamma-trimethoxysilylpropyl)amine (Silquest A-1170), N- ethyl-gamma-aminoisobutyl trimethoxy silane (Silquest A-Link 15), N-phenyl-gamma- diaminopropyl trimethoxy silane (Silquest Y-9669), 4-amino-3,3-dimethylbutyltrimethoxy- silane (Silquest Y-11637), (N-cyclohexylaminomethyl)-triethoxy silane (Genosil XL 926), (N-phenylaminomethyl)-trimethoxy silane (Genosil XL 973) and mixtures thereof. In another embodiment pre-hydrolysed and pre-condensated organofunctional silanes may be used as described in EP 3080209 B1. Method of manufactoring the flaky effect pigment: A method of manufactoring the flaky effect pigment comprises the steps: a) providing a flexible substrate coated with a release agent, b) evaporating under ultra high vacuum conditions a first semiconductor material i) onto the flexible substrate a), then evaporating a dielectric layer ii) with refractive index < 1.80 onto the first semiconductor layer i) and finally evaporating a second semiconductor material iii) onto the dielectic layer ii) to yield a three-layered stack, wherein the semiconductor layers i) and iii) independently have an average atomic composition of: Si_(1-x)Ge_x, wherein 0 < x < 1.00 or Si(1-y)Sny, wherein 0 < y < 0.90 or Ge(1-z)Snz, wherein 0 < z ≤ 0.60 or Si(1-m-n)GemSnn, wherein 0 < m < 1.00, 0 < n < 1.00 with the provisos that x < 1.00; y < 1.00, z < 1.00 and m + n < 1.00, c) stripping the three-stacked film from the flexible substrate in a suitable solvent and comminuiting the particles in the dispersion to obtain semiconductor flakes, d) separating the semiconductor flakes from the solvent and e) optionally conducting further steps like any of further size classifying of the effect pigment flakes or dispersing the effect pigment flakes in a different solvent or conducting further surface treatment steps of the effect pigment flakes. Step a): This step is conducted essentially in the same manner than known from the manufacture of PVD metal pigments, especially aluminum effect pigments. The flexible subtrate is usually a webb made from polymers and most preferably a PET polymer. As release agents those common in the art can be used. Usually the release agents are polymers like for axample acrylics, methacrylics or polystyrol. They can be also other organic materials as described e.g. in US 2004/0131776 A1 or in US 20100062244 A1. In a preferred embodiment all the evaporation steps of step b) are done by a roll-to-roll process. In step b) in one embodiment semiconductor alloys of a predetermined composition are used as bulk materials for layers i) and/or iii) which are evaporated by suitable means to produce respective gas molecules which are transferred to the flexible substrate coated with a release layer under ultra high vacuum conditions. In another embodiment two or three suitable bulk semiconductor materials of a predetermined purity are used wherein their vapor clouds are allowed to overlap before reaching the substrate. Step b) can be conducted as an electron beam process, magneton sputtering, resitive evaporation or inductive heating. Most preferred is evaporation of the semiconductor bulk material of layers i) and iii) as well as the dilectric layer ii) by an electron beam process. Steps c), d) and e) are again well known in the art. Preferred solvents for stripping of the material stack from the flexible substrate in step e) are acetone, ethyl acetate, propylene glycol methoxy ether, isopropyl alcohol, ethanol, or water. In further preferred embodiments the first semiconductor layer i) and the second semiconductor layer iii) are composed of the same material. In further preferred embodiments the first and the second semiconductor layers essentially have the same thickness. With “essentially the thickness” it meant that the deviations of the thickness of the first and the second semiconductor layers are less than 10%. If additionally the first and second semiconductor layer are made from the same material the coloration on will be the same on both sides of the central dielectric layer. The effect pigments according to the present invention can be used of a broad range of applications, typically for metallic effect pigments, such as coatings, printing inks, cosmetic formulations or plastics. Another embodiment of the present invention is concerned with a coating system comprising a binder and the flaky effect pigments of this invention. The binder systems can be acrylics, polyesters, polyurethanes, polyepoxides and copolymers frome these. Preferably the coating systems are automotive basic coats. Such coating system additionally can also comprise other pigments like color pigments, pearlescent pigments or metal effect pigments. Furthermore the coating systems comprise solvents or solvent mixtures. Preferably they are water-based coating systems. Additionally they may contain fillers or additives as common in the art. The effect pigment volume concentration in such coatings is preferably 0.1 to 100%, more preferably 1 to 20% and most preferably 1.5 to 15%.

EXAMPLES Example 1: Si-SiO₂-Si A silicon-silica-silicon tri-layer material was deposited on a 30 cm wide clear polyester film coated with polyurethane polymer as releasing agent using ebeam PVD evaporation. The silicon and silica were deposited in 3 separate layers using an ebeam sources positioned 20 cm below the web during process and conditions were modified to achieve a desired thicknesses of the individual layers. Optical density sensors were utilized to target specific layer thicknesses. The ebeam source accelerating voltages were held at a constant 10 kV throughout the runs. The materials obtained in Example 1 were all stripped from the polyester film and homogenized to a particles size of ~14 ^m (D50 value). Pigments were prepared with a 22 wt.% non-volatile content (NVC) in propylene glycol methoxy ether. The average particle thickness of the effect pigment, obtained via SEM analysis, is 165 +/- 15 nm, with average thicknesses of the silicon layers and of the SiO2 layer of 32 nm +/- 5 nm and 101 nm +/- 5 nm, respectively. Drawdown inks were prepared in a binder system, composed of Hagedorn H7 Nitrocellulose binder (obtainable from Hagedorn AG, Osnabrück, Germany) in a solvent blend of ethyl acetate and propylene glycol methoxy ether. Formulations were based on a 0.6:1 weight ratio of binder to effect pigment content . The inks were drawn down with a wire-wound rod at 40 ^m wetfilm thickness on both BYK opacity charts and Leneta polyester charts. A pigment sample was diluted to 9.5% NVC with propyl glycol methyl ether acetate for spray application in Deltron DBC500 Color Blender. The spray system was adjusted to a binder:pigment ratio of approximately 7.4:1 and applied to coverage in two coats over an ABS plastic substrate. The substrate was dried for 15 minutes at 60°C between coats. The dried panels were then cut in half, after which one segment was clear-coated using Deltron DC4000 and cured for 60 minutes at 60°C. Color data were collected using a BYK Mac colorimeter. Optical measurements for the polyester drawdowns were collected from the face (coated side) of the film. The flop was calculated according to the common formula: Flop index

The results of these measurements are summarized in Tables 1a-1c below. Commercially available Symic OEM Fine Opaque Silver is used as Comparative Example 1. This is a silvery pearlescent pigment of Eckart according to US 2022/0145082 A1. The comparative example was excluded from the spray application due to inadequate coverage. Opacity data were collected using an X-rite 341C transmission densitometer by averaging 6 collection points along the coated polyester film. The average optical densities of the silicon-silica-silicon tri-layer material (Sample 1a) and Symic OEM Fine Opaque Silver (Comp. Ex.1a) were found to be 1.51 and 0.15, respectively. Therefore, the effect pigment of Example 1 exhibits a much higher opacity than the pearlescent pigment. The radar transparency measurements were done with microwave radiation with a frequency of 76.5 GHz using as a measurement system an RMS -D-77/79G apparatus from Perisens GmbH, Germany. RADAR data was collected on Sample 1a. Radar data has been background corrected to account for loss produced by the uncoated substrate. A background-corrected RADAR attenuation of 0.3 dB was measured on sample 1a. Table 1a: Sample Application Flop L*_15° L*_25° L*_45° L*_75° L*_110° ΔC* Type (max- min) 1a Polyester 32.6 135.5 76.4 28.6 13.3 8.4 4.95 Comp. Polyester Ex.1a 20.1 108.7 65.7 31.6 20.7 20.1 7.92 1b Drawdown 32.3 134.4 75.3 28.7 13.1 8.0 4.37 Comp. Drawdown 23.8 104.1 59.9 27.0 16.4 12.8 Ex.1b 8.9 1c Spray w/out 19.0 126.9 90.2 47.0 21.7 12.0 clear 1.96 1d Spray w/ 15.3 117.1 91.0 52.5 25.9 14.3 clear 1.47 Table 1b: Sample Application a*_15° a*_25° a*_45° a*_75° a*_110° Type 1a Polyester -3.00 1.54 3.78 3.38 2.01 Comp. Polyester Ex.1a -1.38 -0.93 -0.34 -0.02 0.54 1b Drawdown -2.89 1.55 3.42 3.22 1.72 Comp. Drawdown -2.14 -1.44 -0.93 -0.59 -0.34 Ex.1b 1c Spray w/out -5.06 -1.94 0.71 1.79 1.42 clear 1d Spray w/ -4.43 -2.41 -0.30 0.92 0.92 clear Table 1c: Sample Application b*_15° b*_25° b*_45° b*_75° b*_110° Type 1a Polyester -1.67 4.08 7.47 5.93 3.84 Comp. Polyester Ex.1a -10.00 -4.95 -2.33 -2.33 -2.11 1b Drawdown -1.66 4.22 6.90 5.69 3.53 Comp. Drawdown -12.39 -6.29 -4.12 -3.62 -3.89 Ex.1b 1c Spray w/out 0.37 2.45 4.00 4.28 3.75 clear 1d Spray w/ 1.66 2.47 3.25 3.61 3.37 clear As can be seen in the above data, the example effect pigments are brighter as they exhibit significantly larger L15° values and have substantially higher opacity than the commercially available comparative example. Furthermore, the example effect pigments display a rather neutral color tone with slightly bluish shade and a low color flop over the range of measured angles. Gassing tests conducted in several test systems yielded no gassing.

Claims

Claims: 1. A flaky effect pigment having as only optical active layers a three-layered stack consisting of planar discrete layers of i) a semiconductor layer ii) a central dielectric layer with refractive index < 1.80 iii) a semiconductor layer wherein the semiconductor layers i) and iii) are mady by a semiconductor material with a band gap in a range of 0.1 to 2.5 eV and independently have an average atomic composition of: a) Si(1-x)Gex, wherein 0 ≤ x < 1.00 or b) Si(1-y)Sny, wherein 0 < y < 0.90 or c) Ge(1-z)Snz, wherein

0.60 or d) Si(1-m-n)GemSnn, wherein 0 < m < 1.00, 0 < n < 1.00 with the provisos that x < 1.00; y < 1.00, z < 1.00 and m + n < 1.00. 2. A flaky effect pigment according to claim 1, wherein the band gap of semiconductor layers i) and iii) is in a range of 0.2 to 1.4 eV. 3. A flaky effect pigment according to claim 1 or 2, wherein the semiconductor layers i) and iii) independently have an average atomic composition of: a) Si(1-x)Gex, wherein 0.01 < x < 0.9 and preferably 0.02 ≤ x ≤ 0.8 or b) Si(1-y)Sny, wherein 0.02 ≤ y ≤ 0.75 or c) Ge(1-z)Snz, wherein 0.02 ≤ z ≤ 0.5 or d) Si(1-m-n)GemSnn, wherein 0.02 ≤ m ≤ 0.8, 0.02 ≤ n ≤ 0.75. 4. A flaky effect pigment according to claims 1 to 3, wherein the semiconductor layers i) and iii) independently have an average atomic composition of: a) Si(1-x)Gex, wherein 0.05 ≤ x ≤ 0.65 or b) Si(1-y)Sny, wherein 0.05 ≤ y ≤ 0.55 or c) Ge(1-z)Snz, wherein 0.05 ≤ z ≤ 0.4 or d) Si(1-m-n)GemSnn, wherein 0.05 ≤ m ≤ 0.65, 0.05 ≤ n ≤ 0.55. 5. A flaky effect pigment according to any of the preceding claims, wherein the central dielectric layer is composed of the group consisting of SiO2, Al2O3, MgO, MgF2, AlF3, B2O3, CeF3, LaF3, Na3AlF6, NdF3, SmF3, BaF2, CaF2, LiF and polymer and combinations thereof. 6. A flaky effect pigment according to any of the preceding claims, wherein the overall stack optical thickness is in a range of 240 to 440 nm, more preferred in a range of 260 to 420 nm and most preferred in a range of 330 to 400 nm, wherein these values denote to the wavelength ^ = 500 nm and literature values of bulk materials are used for the refractive index. 7. A flaky effect pigment according to any of the preceding claims, wherein the average optical thickness of the semiconductor layers i) or iii) are in a range of 64 to 150 nm, preferably in an range of 75 to 135 nm and most preferably in an range of 90 to 125 nm. 8. A flaky effect pigment according to any of the preceding claims, wherein the average optical thickness of the central dielectric layer ii) is in a range of 65 to 205 nm, preferably in an range of 80 to 180 nm, and most preferably in an range of 110 to 150 nm. 9. A flaky effect pigment according to any of the preceding claims, wherein the average thickness of the whole three layered stack is in a range of 100 to 200 nm, preferably in an range of 105 to 190 nm, and most preferably in an range of 110 to 180 nm. . 10. A flaky effect pigment according to any of the preceding claims, wherein the layers i) and iii) are composed of silicon with a thickness in a range of 15 to 35 nm and the central dielectric layer ii) is composed of SiO2 with a thickness in a range of 45 to 140 nm. 11. A flaky effect pigment according to any of the preceding claims, wherein the three-layered stack is further coating or encapsulated with a transparent not optically active metal oxide of refractive index n < 1.8, preferably SiO2. 12. A flaky effect pigment according to any of the preceding claims, wherein the effect pigment is further coated with surface modifiers such as organofunctional silanes, titanates, aluminates or zirconates, phosphate ester, phosphonate esters, phosphite esters and combinations thereof. 13. A flaky effect pigment according to any of the preceding claims, wherein the effect pigments when applied in a draw-down at a concentration of 5.5 wt.% and a wt. ratio of effect pigment:binder in a range of about 1.0:0.4 to 1.0:2.0 and determined by using a BYK Mac i apparatus with an angle of incidence of 45° and color coordinates at angles of observation of 15°, 25°, 45°, 75° and 110°, to display a ^C* = C*max - C*min of < 10.0. 14. A method of manufacture of the effect pigments of claims 1 to 13 comprising the steps: a) providing a flexible substrate coated with a release agent, b) evaporating under ultra high vacuum conditions a first semiconductor material i) onto the flexible substrate a), then evaporating a dielectric layer ii) with refractive index < 1.80 onto the first semiconductor layer i) and finally evaporating a second semiconductor material iii) onto the dielectic layer ii) to yield a three- layered stack, c) stripping the three-layered stack film from step b) from the flexible substrate in a suitable solvent and comminuiting the particles in the dispersion to obtain effect pigment flakes, d) separating the effect pigment flakes from the solvent and e) optionally conducting further steps like any of further size classifying of the effect pigment flakes or dispersing the effect pigment flakes in a different solvent or conducting further surface treatment steps of the effect pigment flakes. 15. Method of manufactoring the flaky effect pigment according to claim 14), wherein the evaporation steps of step b) are done by a roll-to-roll process. 16. Method of manufactoring the flaky effect pigment according to any of claims 14) or 15), wherein the evaporation steps of step b) are done with an electron beam process. 17. Coating system comprising a binder and the flaky effect pigments according to claims 1 to 13.