Reflector element for a motor vehicle lighting device and method for its production

Abstract

Reflector element (100) for a motor vehicle lighting device comprising at least: - a basic body (1), - a mirror layer (2) arranged above the base body (1), which contains silver, - a protective layer (3) arranged above the mirror layer (2), which contains siloxane, and - a cover layer (4) arranged above the protective layer (3) which contains titanium dioxide, wherein the protective layer (3) and the cover layer (4) each have a layer thickness (D3, D4) of 45 nm to 55 nm.

Description

[0001] The present invention relates to a reflector element for a motor vehicle lighting device and a method for manufacturing the same. STATE OF THE ART

[0002] Prior art reflector elements for automotive lighting systems comprise a base body, typically made of plastic, onto which a thin reflective layer of aluminum is deposited. To protect against corrosion, a transparent protective layer, usually made of a plasma polymer, is typically applied to the reflective layer. Such reflector elements exhibit reflectances of approximately 85% in the visible light spectrum.

[0003] To achieve higher luminous efficacy, for example in automotive lighting systems with multiple reflector designs, it is known to apply dichroic auxiliary layers to the reflective layer, which increase the reflectance through interference effects. However, the commonly used dichroic auxiliary layers lack corrosion protection, and the typically required high layer thicknesses necessitate long deposition times, which can lead to undesirably high heating of the substrate, potentially resulting in damage or at least discoloration.

[0004] As an alternative to aluminum protective coatings, silver coatings are known in the prior art, which are characterized by a higher reflectance in the visible light spectrum. However, the corrosion resistance of silver coatings is particularly low, especially in the presence of sulfur compounds. Therefore, prior art uses protective coatings with a large thickness, which impairs the reflective properties of the reflector element. In particular, the reflectance of silver drops sharply in the blue region, so that silver mirror coatings inherently have a yellowish appearance, which is further intensified by a thick protective layer.

[0005] DE 10 2015 102 870 A1 discloses a reflector element comprising a plastic substrate, a silver layer, a first barrier layer arranged above the silver layer consisting of an oxide layer at least 15 nm thick, and a second barrier layer arranged above the first barrier layer, which comprises a siloxane, wherein the thickness of the second barrier layer is at least 250 nm and at most 450 nm. The use of a second barrier layer several hundred nm thick results in disadvantageously long process times for the production of such a reflector element. Furthermore, the angular dependence of the reflectance is undesirably increased by the presence of such a thick single layer.

[0006] DE 10 2016 115 921 A1 discloses an optoelectronic component comprising at least one optoelectronic semiconductor chip configured to emit radiation, at least one metallic reflective surface, at least one functional component with a component surface different from the metallic reflective surface, and a barrier layer stack for protection against corrosive media, arranged both on the at least one metallic reflective surface and on the component surface, wherein the barrier layer stack comprises at least one inorganic oxide, oxynitride or nitride layer and at least one plasma-polymerized siloxane layer. REVELATION OF THE INVENTION

[0007] It is therefore the object of the present invention to propose a reflector element for a motor vehicle lighting device which overcomes the aforementioned disadvantages of reflector elements from the prior art and has a high reflectance in the entire visible light spectrum while simultaneously exhibiting high corrosion resistance.

[0008] This problem is solved by a reflector element according to claim 1. Advantageous embodiments of the invention are specified in the dependent claims.

[0009] The invention includes the technical teaching that the reflector element comprises at least: - a basic body, - a mirror layer arranged above the base body, which contains silver, - a protective layer arranged above the mirror layer, which contains siloxane, and - a top layer arranged above the protective layer, which contains titanium dioxide, wherein the protective layer and the top layer each have a thickness of 45 nanometers to 55 nanometers.

[0010] The invention is based on the idea of ​​compensating for the deterioration of the reflection properties caused by the protective layer of a silicon-organic compound necessary for corrosion protection by applying a titanium dioxide top layer, which acts as an optical amplifier layer. Surprisingly, and in contrast to layer structures known from the prior art, it is sufficient according to the invention to make the individual layers of the protective layer and the top layer thinner than 100 nm. The reflector element according to the invention thus exhibits sufficient corrosion and long-term resistance for practical use, and with regard to its functional optical properties, it is characterized by an increase in its reflectance compared to a pure metal layer, while simultaneously optimizing the color neutrality of its appearance.Thanks to the small layer thicknesses according to the invention, the production of the reflector element according to the invention requires only extremely short deposition times, so that the heat input into the usually heat-sensitive base body can be reduced to a minimum.

[0011] In an advantageous embodiment of the reflector element according to the invention, the cover layer comprises titanium dioxide as a binary transition metal oxide with a high refractive index in the visible light spectrum, in particular with a refractive index greater than 2. Due to the high refractive index, the thickness of the cover layer can be thin, i.e., in particular thinner than 100 nm, while still providing sufficient optical amplification with respect to the reflectance of the reflector element according to the invention.

[0012] In a non-inventive embodiment of a reflector element, the mirror layer comprises aluminum and the cover layer comprises titanium dioxide, wherein the protective layer has a thickness of 70 nm to 80 nm, preferably 75 nm, and the cover layer has a thickness of 50 nm to 60 nm, preferably 55 nm. According to the invention, the mirror layer comprises silver and the cover layer comprises titanium dioxide, wherein the protective layer has a thickness of 45 nm to 55 nm, preferably 50 nm, and the cover layer has a thickness of 45 nm to 55 nm, preferably 50 nm. The excellent reflection properties of these embodiments are described in more detail below in connection with the figures.

[0013] Preferably, the reflector element according to the invention has a color-neutral appearance, wherein white light reflected from the reflector element in the normal direction is characterized in the CIELAB color space by color coordinates that have a value less than 2, preferably zero. The CIELAB color space is standardized in EN ISO 11664-4. The three-dimensional CIELAB color space is defined by a color plane spanned by the color coordinates a* and b* and a brightness value L* perpendicular to it. Absolute color neutrality is achieved if both color coordinates a* and b* are identically zero. The reflector element according to the invention is particularly designed such that the intrinsic yellow tint in white light reflection from pure silver is compensated by a coordinated combination of protective and top layers.

[0014] A further advantage of the inventive reflector element is that it exhibits a maximum reflectance of 95% to 99% in the visible light spectrum. In addition to improving color neutrality, the effect of the layer stack applied according to the invention is therefore particularly due to the fact that the reflectance of the inventive reflector element is significantly increased compared to a pure metal mirror layer.

[0015] For example, the base body of the reflector element according to the invention is made of a plastic and in particular comprises a polycarbonate, a polyetherimide, or, for example, a primed bulk-molding compound. These materials are suitable for use in reflector elements of automotive lighting systems with regard to their thermo-mechanical properties, their processability, and economic aspects.

[0016] Furthermore, the reflector element according to the invention, for example, has an optional intermediate layer with a layer thickness of, for example, less than 50 nm, which is arranged between the base body and the mirror layer. Depending on the material used for the base body, this intermediate layer serves as an adhesion promoter layer between the base body and the mirror layer and / or as a diffusion barrier to protect the mirror layer from substances emanating from the base body, such as water or oxygen.

[0017] The invention further relates to a method for manufacturing a reflector element for a motor vehicle lighting device according to one of the aforementioned embodiments, wherein the reflective layer and the top layer are each deposited by sputtering, also known as cathode sputtering, and the protective layer is deposited by plasma-enhanced chemical vapor deposition. The protective layer is preferably formed by plasma polymerization of hexamethyldisiloxane (HMDSO). According to the invention, the reflective layer is deposited by DC sputtering and the top layer by AC sputtering, preferably medium-frequency sputtering at frequencies in the range of, for example, 20 kHz to 70 kHz. PREFERRED EXAMPLES OF THE INVENTION

[0018] Further measures improving the invention are described in more detail below, together with a description of preferred embodiments of the invention, with reference to the figures. The figures show: Fig. 1 a schematic cross-sectional partial view of a reflector element according to the invention, Fig. 2a the spectral reflectance of Al-based layer stacks, Fig. 2b the spectral reflectance of Ag-based layer stacks, Fig. 3a the spectral angle-dependent reflectance of an Al-based layer stack and Fig. 3b the spectral angle-dependent reflectance of an Ag-based layer stack.

[0019] Fig. Figure 1 shows a schematic cross-sectional view of a section of a reflector element 100 according to the invention, which comprises the base body 1 with the thin-film stack of functional layers 2-5 applied thereto. The base body 1 is preferably made of a plastic.

[0020] The reflective layer 2 consists of silver, which, compared to the more commonly used aluminum in the prior art, is characterized by a higher reflectance in the visible light spectrum. Depending on the topography of the substrate, i.e., the surface of the base body 1 and the intermediate layer 5, as well as the deposition conditions, the layer thickness D2 of the reflective layer 2 is preferably selected such that its surface has the lowest possible roughness, i.e., the highest possible reflective effect. The layer thickness D2 is typically between 50 nm and 150 nm.

[0021] The intermediate layer 5, optionally deposited prior to the deposition of the mirror layer 2 onto the substrate, consists, for example, of an HMDSO-based plasma polymer or sputtered titanium. Depending on the material and surface properties of the substrate 1, the intermediate layer 5 can act as an adhesion promoter for the mirror layer 2 and / or represent a diffusion barrier for species escaping from the substrate 1, e.g., water molecules or oxygen, which could damage the mirror layer 2.

[0022] On the reflective layer 2, the protective layer 3, deposited in particular by plasma polymerization of HMDSO, is arranged and serves for corrosion protection. According to the invention, the thickness D3 of the protective layer 3 is 45 nm to 55 nm. The top layer 4 deposited on the protective layer 3 consists of a high-refractive-index material, namely titanium dioxide with a refractive index greater than 2 in the visible light spectrum, and according to the invention also has a low layer thickness D4 of 45 nm to 55 nm.

[0023] Fig. 2a and Fig. Figure 2b shows the experimentally determined spectral reflectance in the visible light spectrum, i.e., at wavelengths between 400 nm and 750 nm, of aluminum- and silver-based layer stacks, comparing a pure reflective layer (Al or Ag), a reflective layer with a protective layer of siloxane plasma polymer, and a reflective layer with a protective layer of siloxane plasma polymer and a top layer of titanium dioxide (TiO2). The thicknesses of the individual layers are given in the figure captions and are all less than 100 nm.

[0024] A comparison of the pure metal layers reveals the higher intrinsic reflectance of silver compared to aluminum. Except for wavelengths in the blue range below 450 nm, the pure silver mirror layer exhibits a reflectance of more than 95%, demonstrating silver's particular suitability for use in reflector elements. The decrease in reflectance in the blue range results in a yellow tint in the appearance of the silver layer. The protective layer reduces the reflectance across the entire spectral range shown, especially in the blue range, which, when combined with silver, adversely intensifies its yellow tint. The layer stacks with a final titanium dioxide topcoat are characterized by a significant increase in reflectance, justifying their designation as optical intensifier layers.Despite the presence of the protective plasma polymer layer, the silver-based layer stack achieves an average reflectance in the visible light spectrum of over 95%. Furthermore, the presence of the top layer advantageously leads to a certain redistribution of the spectral weight from red to blue, which corrects the yellowish appearance. As a result, a reflector element according to the invention with such a layer stack has a color-neutral appearance, in which white light reflected in the normal direction is characterized by color coordinates less than 2 in the CIELAB color space.

[0025] Fig. 3a and Fig. Figure 3b shows, based on simulations, the spectral angle-dependent reflectance in the visible light spectrum of aluminum plasma polymer titanium dioxide and silver plasma polymer titanium dioxide layer stacks, respectively, whose individual layer thicknesses are in the range between 10 nm and 100 nm as provided for in the invention. The reflectance is shown for light incident in the normal direction, i.e., perpendicularly at an angle of 0° to the (local) surface, as well as for grazing light incident at an angle of 80°. For both layer stacks, the reflectance at grazing incidence is characterized by a minimum at a wavelength of approximately 500 nm, which, when illuminated with white light, corresponds to a change in the color appearance compared to normal incidence. It should be emphasized, however, that the average reflectance at the shallow viewing angle of 80° is only insignificantly lower than in the normal direction.This represents a further advantage of reflector elements according to the invention with layer thicknesses of less than 100 nm, since, in contrast, reflector elements with plasma polymer protective layers of higher thickness, particularly greater than 250 nm, known from the prior art, exhibit a significantly greater drop in reflectance at shallow viewing angles. The comparison of the... Fig. 3a and Fig. Figure 3b also demonstrates the advantages of silver-based over aluminum-based layer stacks with regard to the angular dependence of the reflectance. This is due to the intrinsically lower dependence of silver's reflectivity on the polarization of the incident light.

[0026] The invention is not limited in its implementation to the preferred embodiment described above. Rather, a number of variants are conceivable, which utilize the solution presented even in fundamentally different designs. All features and / or advantages arising from the claims, the description, or the drawings, including design details, spatial arrangements, and process steps, can be essential to the invention, both individually and in various combinations. Reference symbol list 100 reflector elements 1 Basic body 2 Mirror layer 3 protective layers 4 Top layer 5 Intermediate layer D2, D3, D4 layer thickness

Claims

[1] Reflector element (100) for a motor vehicle lighting device comprising at least: - a basic body (1), - a mirror layer (2) arranged above the base body (1), which contains silver, - a protective layer (3) arranged above the mirror layer (2), which contains siloxane, and - a cover layer (4) arranged above the protective layer (3) which contains titanium dioxide, wherein the protective layer (3) and the cover layer (4) each have a layer thickness (D3, D4) of 45 nm to 55 nm. [2] Reflector element (100) according to claim 1, characterized by , that the reflector element (100) has a color-neutral appearance, wherein white light reflected in the normal direction from the reflector element (100) is characterized in the CIELAB color space by color coordinates which have a value less than 2, preferably zero. [3] Reflector element (100) according to one of the preceding claims, characterized by , that the reflector element (100) has a maximum reflectance of 95% to 99% in the visible light spectrum. [4] Reflector element (100) according to one of the preceding claims, characterized by , that the base body (1) is made of a plastic and in particular comprises a polycarbonate, a polyetherimide or a bulk molding compound. [5] Reflector element (100) according to any of the preceding claims, characterized by , that the reflector element (100) has an intermediate layer (5) which is arranged between the base body (1) and the mirror layer (2). [6] Method for producing a reflector element (100) for a motor vehicle lighting device according to one of the preceding claims, wherein the mirror layer (2) and the cover layer (4) are each deposited by sputtering and the protective layer (3) is deposited by plasma-enhanced chemical vapor deposition, characterized by, that the mirror layer (2) is deposited by means of DC sputtering and the cover layer (4) is deposited by means of AC sputtering, preferably medium frequency sputtering.

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