Method for improving the radio wave transmittance of materials

Abstract

To provide a method for improving millimeter-wave electric wave transmittance of a member without changing a material and a design of the member.SOLUTION: A method for improving electric wave transmittance of a member having a base material containing a resin at a frequency of 77 GHz includes stacking a film on the member, wherein the film has thickness of 50 to 10,000 μm, a refractive index of 1.3 to 10 and surface roughness (arithmetic average roughness Ra) of 0.01 to 2.0 μm.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for improving the radio wave transmittance of a component. [Background technology]

[0002] In recent years, the development of communication devices that utilize radio waves in the millimeter-wave range (frequency 30-300 GHz) as an information and communication medium has been expanding, such as millimeter-wave radar installed in automobiles to provide autonomous driving and collision avoidance functions. Components that protect and house these millimeter-wave communication devices require high millimeter-wave transparency.

[0003] As a technology to improve the millimeter-wave transmittance of materials used in components that protect and house millimeter-wave communication equipment, for example, Patent Document 1 describes a technology that achieves both high gloss and high millimeter-wave transmittance in a millimeter-wave transmittance glossy coating obtained by coating a transparent resin substrate with a paint containing aluminum flakes, by setting the average particle size of the aluminum flakes, the area occupancy rate of the aluminum flakes in the coating to a specific range. Furthermore, Patent Document 2 describes a technique for achieving high millimeter-wave transmittance and excellent metallic color coating by using a flake-like pigment having an indium layer and a zinc sulfide layer, and having a cumulative 50% volume particle diameter D50 within a specific range. Patent Document 3 describes a millimeter-wave radar cover that uses a specific (meth)acrylic resin composition as a material and improves millimeter-wave transmittance and weather resistance by setting the dielectric constant and dielectric loss tangent at frequencies of 70 to 90 GHz within a specific range. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Patent No. 6933587 [Patent Document 2] Japanese Patent Publication No. 2022-079861 [Patent Document 3] Japanese Patent Publication No. 2020-147680

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, since the above prior art improves the radio wave transmittance of a member by changing the material of the base material or the coating film, the design and structure design of the member (parts using the member) must be changed according to the change of the material. Therefore, a means for improving the radio wave transmittance of the member (parts using the member) without changing the current design is desired. [[ID=io]]Therefore, an object of the present invention is to provide a method for improving the transmittance of millimeter waves in a member without changing the material and design of the member,

Means for Solving the Problems

[0006] The present invention is as follows. [1] A method for improving the radio wave transmittance at a frequency of 77 GHz in a member having a base material containing a resin, including laminating a film on the member, where the film has a thickness of 50 to 10,000 μm, a refractive index of 1.3 to 10, and a surface roughness (arithmetic mean roughness Ra) of 0.01 to 2.0 μm characterized by the method. [2] The method according to [1], wherein the member has a coating film. [3] The method according to [2], wherein the coating film contains aluminum. [4] The method according to [2] or [3], wherein the coating film is a multi-layer coating film including a primer coating film, a base coating film, and a clear coating film. [5] The base material includes at least one selected from the group consisting of polypropylene (PP), acrylonitrile-butadiene-styrene copolymer (ABS resin), and polycarbonate / ABS alloy (PC / ABS alloy), The method according to any one of [1] to [4], wherein the thickness of the base material is 2 to 3 mm. [6] The method according to any one of [1] to [5], which determines the values of the thickness, refractive index, and surface roughness (arithmetic mean roughness Ra) of the film to be laminated by the following procedures (a) to (d). (a) Obtain the transmittance spectrum of the member (vertical axis: radio wave transmittance (%), horizontal axis: frequency (GHz)), and determine the wavelength at the peak top frequency and the wavelength at the peak valley frequency of the transmittance spectrum. (b) Substitute the thickness of the member, the wavelength at the peak top frequency of the member obtained in (a), and the wavelength at the peak valley frequency of the member into the following formulas (1) and (2) to obtain the value of the constant m. n 部材 ×d 部材 =2m×λ ピークトップ / 4 ···(1) n 部材 ×d 部材 =(2m - 1)×λ ピークバレー / 4 ···(2) (n 部材 : refractive index of the member, d 部材 : thickness of the member (mm), λ ピークトップ : wavelength at the peak top frequency of the member (mm), λ ピークトップ : wavelength at the peak valley frequency of the member (mm), m: integer constant) ピークバレー ピークバレー ピークバレー (c) Substitute the wavelength at the peak top frequency of the member obtained in (a) and the value of the constant m obtained in (b) into the following formula (3) to obtain the relational formula (4) for the values of the film thickness, refractive index, and surface roughness (arithmetic mean roughness Ra). (n フィルム ) (1-(Ra / 4)) ×d フィルム =(2m×|λ ピークトップ -3.896| / 4)×(1±0.7) ···(3) (n フィルム : refractive index of the film, Ra: surface roughness of the film (arithmetic mean roughness Ra) (μm), d フィルム : thickness of the film (mm), m: integer constant, λ フィルム : wavelength at the peak top frequency of the member (mm)) [[ID=⑤1]] ピークトップ (d) Determine the values ​​of the film thickness, refractive index, and surface roughness (arithmetic mean roughness Ra) so as to satisfy the relational equation (4) obtained in (c) above. [7] The method according to any one of [1] to [6], wherein the member is for use in automobile parts. [Effects of the Invention]

[0007] According to the present invention, a method for improving the millimeter-wave transmittance in a component can be provided without changing the material and design of the component. [Modes for carrying out the invention]

[0008] The following describes in detail embodiments for carrying out the present invention (hereinafter also referred to as "this embodiment"). It should be noted that the present invention is not limited to the following embodiments, and can be implemented in various modifications within the scope of its gist.

[0009] The method of this embodiment is a method for improving the radio wave transmittance at a frequency of 77 GHz in a component having a resin-containing substrate (hereinafter also simply referred to as the "improvement method"), and includes laminating a film having a thickness of 50 μm to 10,000 μm, a refractive index of 1.3 to 10, and a surface roughness (arithmetic mean roughness Ra) of 0.01 to 2.0 μm onto the component. The component in question may have a coating in addition to a resin-containing substrate. In this case, the film may be laminated on either the substrate or the coating. That is, it may be laminated in the order of film, substrate, coating, or it may be laminated in the order of substrate, coating, film. The improvement method of this embodiment allows for the improvement of millimeter wave (77 GHz frequency band) transmittance in a component without changing the material or design of the component, by laminating a film having the above-described characteristics onto the component. In particular, when the component has a coating, the transmittance of radio waves decreases due to reflection caused by thin-film interference, but by laminating the above-described film, reflection due to thin-film interference is suppressed, and the transmittance of radio waves in the component at a frequency of 77 GHz can be improved. For example, radio waves in the 77 GHz band (76-77 GHz) are a typical frequency band for medium- and long-range millimeter wave radars mounted on automobiles, and if the component is an automobile part that protects and houses a millimeter wave radar (e.g., radome, bumper, emblem, radiator grille, etc.), applying the method of this embodiment can improve the transmission intensity of millimeter waves that can be transmitted and received by the millimeter wave radar.

[0010] The method for laminating the film onto the component is not particularly limited; for example, the film can be bonded to the component by thermal lamination, dry lamination, cold lamination, etc. When using adhesives or adhesive sheets to bond the film to the component (cold lamination), the effect of the adhesive or adhesive sheet on radio wave transparency is negligible and can therefore be ignored.

[0011] [film] The film used in the improvement method of this embodiment has a thickness of 50 to 10,000 μm, a refractive index of 1.3 to 10, and a surface roughness (arithmetic mean roughness Ra) of 0.01 to 2.0 μm. By adjusting the combination of film thickness, refractive index, and surface roughness (arithmetic mean roughness Ra) to the optimal value for each target component within the above range, the radio wave transmittance at a frequency of 77 GHz can be improved for various components.

[0012] The film is not particularly limited as long as it has a thickness of 50 to 10,000 μm, a refractive index of 1.3 to 10, and a surface roughness (arithmetic mean roughness Ra) of 0.01 to 2.0 μm. For example, it may be a polyolefin film (polyethylene film, polypropylene film, etc.), a polyester film (polyethylene terephthalate film, polycarbonate film, etc.), a polystyrene resin film (polystyrene film, acrylonitrile-styrene copolymer (AS resin) film, acrylonitrile-butadiene-styrene copolymer (ABS resin) film, etc.), a polyurethane film, a urethane nylon film, a polyamide film (nylon 6 film, nylon 66 film, etc.), a polyvinyl chloride film, a polyvinylidene chloride film, an acrylic film (polymethyl methacrylate film, etc.), etc. Among these, polyurethane film is preferred from the viewpoint of elasticity and dielectric constant.

[0013] The film thickness is 50 to 10,000 μm, preferably 100 to 5,000 μm, more preferably 100 to 2,000 μm, even more preferably 200 to 900 μm, and even more preferably 400 to 600 μm. When the film thickness is within the above range, the millimeter-wave transmittance of the component at 77 GHz tends to be 90% or more. The film thickness can be measured using a film thickness gauge (such as the "SDM-miniR" manufactured by Sanko Electronics Laboratory Co., Ltd.), and specifically, it can be measured by the method described in the examples below.

[0014] The refractive index of the film at a measurement wavelength of 15 mm is 1.3 to 10, preferably 1.3 to 5.0, more preferably 1.4 to 3.0, and even more preferably 1.5 to 1.9. When the refractive index of the film is within the above range, reflection due to thin-film interference is suppressed, and the radio wave transmittance of the component at a frequency of 77 GHz tends to be improved. The refractive index of the film can be adjusted, for example, by selecting and adding materials with different dielectric constants. The refractive index of the film can be measured using a refractometer, specifically by the method described in the examples below.

[0015] The surface roughness (arithmetic mean roughness Ra) of the film is 0.01 to 2.0 μm, preferably 0.01 to 1.0 μm, and more preferably 0.01 to 0.5 μm. When the surface roughness (arithmetic mean roughness Ra) of the film is within the above range, the millimeter-wave transmittance of the material at 77 GHz tends to be 90% or more. Furthermore, it is sufficient if the surface roughness (arithmetic mean roughness Ra) of at least the surface of the film opposite to the surface that is adhered to the component is within the above range. The surface roughness (arithmetic mean roughness Ra) of the film can be adjusted, for example, by coating the surface or performing an embossing process. The surface roughness (arithmetic mean roughness Ra) of the film can be measured using a surface roughness measuring instrument in accordance with JIS-B0601, and specifically, it can be measured by the method described in the examples below.

[0016] With the improved method of this embodiment, the radio wave transmittance of the member after film lamination at a frequency of 77 GHz is preferably 85% or more, more preferably 90% or more, and even more preferably 95% or more. The radio wave transmittance at a frequency of 77 GHz can be calculated from the transmitted attenuation (dB) obtained using a radio wave absorption measuring device, and specifically, it can be calculated by the method described in the examples below.

[0017] The combination of film thickness, refractive index, and surface roughness (arithmetic mean roughness Ra) values ​​used in the improvement method of this embodiment can be determined, for example, by the following procedure (a) to (d), depending on the target component. (a) Obtain the transmittance spectrum of the material (vertical axis: radio wave transmittance (%), horizontal axis: frequency (GHz)), and determine the wavelength at the peak top frequency and the wavelength at the peak valley frequency of the transmittance spectrum. (b) The value of the constant m is determined by substituting the thickness of the member and the wavelength at the peak top frequency and the wavelength at the peak valley frequency of the member, which were determined in (a) above, into the following equations (1) and (2). n 部材 ×d 部材 = 2m × λ ピークトップ / 4 ···(1) n 部材 ×d 部材 = (2m-1) × λ ピークバレー / 4 ···(2) (n 部材 :Refractive index of the material, d 部材 : Thickness of the component (mm), λ ピークトップ :Wavelength (mm) at the peak frequency of the component, λ ピークバレー :Wavelength (mm) at the peak-valley frequency of the component, m: an integer constant) (c) By substituting the wavelength at the peak top frequency of the component obtained in (a) above and the value of the constant m obtained in (b) above into the following equation (3), equation (4) relating the film thickness, refractive index, and surface roughness (arithmetic mean roughness Ra) is obtained. (n フィルム ) (1-(Ra / 4)) ×d フィルム =(2m × |λ) ピークトップ -3.896| / 4)×(1±0.7) ···(3) (n フィルム :Refractive index of the film, Ra:Surface roughness of the film (arithmetic mean roughness Ra) (μm), d フィルム : Film thickness (mm), m: integer constant, λ ピークトップ :Wavelength (mm) at the peak frequency of the component. (d) Determine the values ​​of the film thickness, refractive index, and surface roughness (arithmetic mean roughness Ra) so as to satisfy the relational equation (4) obtained in (c) above. Furthermore, when a film is bonded to a component using an adhesive or adhesive sheet (cold lamination), the effect of the adhesive or adhesive sheet on radio wave transparency is negligible compared to the overall effect, and therefore will not be considered in the above relational equation.

[0018] In procedure (a), the transmittance spectrum of the material (vertical axis: radio wave transmittance (%), horizontal axis: frequency (GHz)) is obtained by measuring the transmission attenuation (dB) of the material in a frequency range including 77 GHz where the peak top and peak valley of the material's transmittance can be obtained (e.g., 60 to 90 GHz) using a radio wave absorption measuring device, and calculating the radio wave transmittance from the obtained transmission attenuation. Specifically, it can be obtained by the method described in the embodiments below. In a transmittance spectrum, the peak top refers to the position where the maximum value is observed, while the peak valley refers to the position where the minimum value is observed.

[0019] In procedure (d), multiple combinations of film thickness, refractive index, and surface roughness (arithmetic mean roughness Ra) values ​​that satisfy relation (4) can be obtained. Therefore, by further obtaining the radio wave transmittance (predicted value) at a frequency of 77 GHz using Fresnel's reflection coefficient for several combinations of values, the optimal combination of film thickness, refractive index, and surface roughness (arithmetic mean roughness Ra) values ​​(optimal characteristics of the film) can be determined.

[0020] The following describes the components to which the improvement method of this embodiment applies.

[0021] [Components] The members to which this improvement method is applied (members targeted by the improvement method) are not particularly limited as long as they have a resin-containing substrate. Examples include members commonly used in automobile parts (radomes, bumpers, emblems, radiator grilles, etc.), aircraft parts, infrastructure parts, environmental measurement parts, wireless communication parts, etc. The component may have a coating. Furthermore, the component may have other layers (e.g., an insulating layer) in addition to the base material and coating, but it is preferable to omit the other layers as this makes it easier to improve radio wave transparency.

[0022] The total thickness of the component is preferably 0.5 to 10 mm, more preferably 1.0 to 5.0 mm, and even more preferably 2.0 to 3.0 mm, from the viewpoint of radio wave transparency and strength as a component.

[0023] [[Base material]] The base material constituting the component is not particularly limited as long as it contains resin, and may be made of resin.

[0024] The resin contained in the base material is not particularly limited, but examples include polyolefin resins such as polypropylene (PP); polystyrene resins such as polystyrene, acrylonitrile-butadiene-styrene copolymer (ABS resin), and acrylonitrile-styrene copolymer (AS resin); polycarbonate resins such as polycarbonate; polyamide resins; urethane resins; polyester resins; polyvinyl chloride resins; and alloys thereof (such as polycarbonate / ABS alloy (PC / ABS alloy)). The above resins may be used individually or in combination of multiple types. Among the above resins, it is preferable that the substrate contains at least one selected from the group consisting of PP, ABS resin, and PC / ABS alloy, from the viewpoint of radio wave transparency and adhesion.

[0025] The substrate may contain additives such as curing agents, pigments, surfactants, neutralizing agents, stabilizers, thickeners, defoamers, surface modifiers, leveling agents, pigment dispersants, UV absorbers, antioxidants, inorganic fillers such as silica, conductive carbon, conductive fillers, organic modifiers, and plasticizers, as needed.

[0026] The thickness of the substrate is preferably 1.0 to 5.0 mm, and more preferably 2.0 to 3.0 mm, from the viewpoint of radio wave transparency and strength as a substrate.

[0027] The refractive index of the substrate is preferably 1.3 to 10, more preferably 1.4 to 3.0, and even more preferably 1.5 to 1.9, from the viewpoint of electromagnetic wave transmission. The refractive index of the substrate can be measured using a refractometer.

[0028] When a coating film is formed on a substrate, it is preferable that the substrate undergoes surface treatment such as chemical conversion treatment or degreasing treatment (for example, wiping with isopropyl alcohol) in order to improve adhesion with the coating film.

[0029] [[coating film]] The coating film is not particularly limited and may be formed using a conventionally known coating composition containing a coating film-forming resin.

[0030] The film-forming resin is not particularly limited and may be any resin commonly used for paints, such as acrylic resins, polyamide resins, urethane resins, polyester resins, polyether resins, epoxy resins, alkyd resins, fluororesins, and polyolefin resins. Among these, acrylic resins and urethane resins are preferred from the viewpoint of appearance and adhesion. The above resins may be used individually or in combination of multiple types. The content of the film-forming resin is preferably 20 to 90% by mass, more preferably 30 to 80% by mass, and even more preferably 40 to 70% by mass, based on 100% by mass of the coating film (i.e., the solid content of the paint composition).

[0031] The paint composition may, if necessary, contain other components that are commonly added to paints. These other components include hardeners, luminous pigments, coloring pigments, surfactants, neutralizing agents, stabilizers, thickeners, defoamers, surface modifiers, leveling agents, pigment dispersants, UV absorbers, antioxidants, inorganic fillers such as silica, conductive carbon, conductive fillers, organic modifiers, and plasticizers. The content of other components is preferably 10 to 80% by mass, more preferably 20 to 70% by mass, and even more preferably 30 to 60% by mass, based on 100% by mass of the coating film (i.e., the solid content of the paint composition).

[0032] Examples of curing agents include amino resins such as melamine resin, and blocked isocyanates obtained by adding a blocking agent containing active hydrogen to polyisocyanates such as trimethylene diisocyanate, hexamethylene diisocyanate, xylylene diisocyanate, and isophorone diisocyanate. The above-mentioned hardening agents may be used individually or in combination of multiple types. The hardening agent content is preferably 5 to 60% by mass, more preferably 10 to 50% by mass, and even more preferably 20 to 40% by mass, based on 100% by mass of the coating film (i.e., the solid content of the paint composition).

[0033] Examples of lustrous pigments include metallic lustrous pigments such as aluminum, copper, zinc, iron, nickel, tin, aluminum oxide and their alloys, interference mica pigments, white mica pigments, graphite pigments, and glass flake pigments. Among these, aluminum is preferred because it is readily available. The above-mentioned luminous pigments may be used individually or in combination of multiple types. The content of the luminous pigment is preferably 1 to 40% by mass, more preferably 5 to 30% by mass, and even more preferably 10 to 20% by mass, based on 100% by mass of the coating film (i.e., the solid content of the paint composition).

[0034] The paint composition may be in the form of an aqueous type consisting of a water-soluble, water-dispersible, or emulsion, as well as an organic solvent type or a non-aqueous dispersion type. For organic solvent type and non-aqueous dispersion type paints, conventionally known solvents such as n-butanol, xylene, and toluene can be used.

[0035] A paint composition can be prepared by mixing each component of the paint composition using commonly used mixing methods (for example, a paint shaker, a mixer, or other mixing device).

[0036] The coating may be a single-layer coating or a multi-layer coating of two or more layers. A multi-layer coating may include a base coating, a primer coating, and / or a clear coating. In addition, a clear coating refers to a coating that is formed above the base coating and becomes the uppermost layer (outermost layer), while a primer coating refers to a coating that is applied directly to the surface of the object to be coated (substrate or other layer) and is located below the base coating. When the coating is a multilayer coating containing the above-mentioned lustrous pigment such as aluminum, it is preferable that the lustrous pigment is included in the base coating.

[0037] (Primer coating) The primer coating is not particularly limited and may be formed using a conventionally known primer coating composition containing a coating-forming resin.

[0038] The film-forming resin included in the primer paint composition is not particularly limited, and examples include the resins commonly used for paints as described above. The film-forming resin may be a single type or a combination of multiple types. The content of the film-forming resin is preferably 30 to 95% by mass, more preferably 40 to 80% by mass, and even more preferably 50 to 70% by mass, based on 100% by mass of the primer coating (i.e., the solid content of the primer paint composition).

[0039] The primer coating composition may, if necessary, contain other components that are typically added to primer coatings. Examples of other components include curing agents, brightening pigments, coloring pigments, surfactants, neutralizing agents, stabilizers, thickeners, defoamers, surface modifiers, leveling agents, pigment dispersants, UV absorbers, antioxidants, inorganic fillers such as silica, conductive carbon, conductive fillers, organic modifiers, and plasticizers. The content of other components is preferably 5 to 70% by mass, more preferably 20 to 60% by mass, and even more preferably 30 to 50% by mass, based on 100% by mass of the primer coating film (i.e., the solid content of the primer coating composition).

[0040] The primer coating composition may be in the form of an aqueous type consisting of a water-soluble, water-dispersible, or emulsion, as well as an organic solvent type or a non-aqueous dispersion type. For organic solvent type and non-aqueous dispersion type coatings, conventionally known solvents such as n-butanol, xylene, and toluene can be used.

[0041] A primer paint composition can be prepared by mixing the components constituting the primer paint composition using commonly used mixing means (for example, a paint shaker, a mixer, or other mixing device).

[0042] From the viewpoint of smoothness and adhesion, the thickness of the primer coating film is preferably 5 to 20 μm in dry film thickness, more preferably 6 to 18 μm, and even more preferably 7 to 15 μm. The dry film thickness of the primer coating can be measured using a film thickness gauge (such as the "SDM-miniR" manufactured by Sanko Electronics Laboratory Co., Ltd.).

[0043] (Base coating) The base coating is not particularly limited and may be formed using a conventionally known base coating composition containing a coating-forming resin.

[0044] The film-forming resin included in the base coating composition is not particularly limited, and examples include the resins commonly used for coatings as described above. The film-forming resin may be a single type or a combination of multiple types. The content of the film-forming resin is preferably 40 to 90% by mass, more preferably 50 to 85% by mass, and even more preferably 60 to 80% by mass, based on 100% by mass of the base coating (i.e., the solid content of the base paint composition).

[0045] The base coating composition may, if necessary, contain other components that are typically added to base coatings. Examples of other components include curing agents, luminous pigments, coloring pigments, surfactants, neutralizing agents, stabilizers, thickeners, defoamers, surface modifiers, leveling agents, pigment dispersants, UV absorbers, antioxidants, inorganic fillers such as silica, conductive carbon, conductive fillers, organic modifiers, and plasticizers. The content of other components is preferably 10 to 60% by mass, more preferably 15 to 50% by mass, and even more preferably 20 to 40% by mass, based on 100% by mass of the base coating film (i.e., the solid content of the base paint composition).

[0046] The base coating composition may be in the form of a water-soluble, water-dispersible, or emulsion-based coating, as well as an organic solvent-based or non-aqueous dispersion-based coating. For organic solvent-based or non-aqueous dispersion-based coatings, conventionally known solvents such as n-butanol, xylene, and toluene can be used.

[0047] A base paint composition can be prepared by mixing the components constituting the base paint composition using commonly used mixing means (for example, a paint shaker, a mixer, or other mixing device).

[0048] The thickness of the base coating film is preferably 10 to 30 μm in dry film thickness, more preferably 12 to 27 μm, and even more preferably 15 to 25 μm. When the thickness of the base coating film is 10 μm or more, sufficient opacity tends to be obtained, and when it is 30 μm or less, defects such as sagging and blotches are less likely to occur. The dry film thickness of the base coating can be measured using a film thickness gauge (such as the "SDM-miniR" manufactured by Sanko Electronics Laboratory Co., Ltd.).

[0049] (Clear coating) The clear coating is not particularly limited and may be formed using a conventionally known clear coating composition containing a coating-forming resin.

[0050] The film-forming resin included in the clear coating composition is not particularly limited, and examples include the resins commonly used for coatings as described above. The film-forming resin may be a single type or a combination of multiple types. The content of the film-forming resin is preferably 50 to 85% by mass, and more preferably 55 to 80% by mass, based on 100% by mass of the clear coating (i.e., the solid content of the clear paint composition).

[0051] The clear coating composition may, if necessary, contain other components that are typically added to clear coatings. Examples of other components include hardeners, glossy pigments, coloring pigments, surfactants, neutralizing agents, stabilizers, thickeners, defoamers, surface modifiers, leveling agents, pigment dispersants, UV absorbers, antioxidants, inorganic fillers such as silica, conductive carbon, conductive fillers, organic modifiers, and plasticizers. The content of other components is preferably 15 to 50% by mass, and more preferably 20 to 45% by mass, based on 100% by mass of the clear coating film (i.e., the solid content of the clear coating composition).

[0052] The clear coating composition may be in the form of a water-soluble, water-dispersible, or emulsion-based aqueous type, as well as an organic solvent type or a non-aqueous dispersion type. For organic solvent-based or non-aqueous dispersion type coatings, conventionally known solvents such as n-butanol, xylene, and toluene can be used.

[0053] A clear coating composition can be prepared by mixing the components constituting the clear coating composition using commonly used mixing means (for example, mixing devices such as paint shakers and mixers).

[0054] The thickness of the clear coating is preferably 15 to 50 μm in dry thickness, and more preferably 20 to 30 μm. When the thickness of the clear coating is 20 μm or more, a good appearance and sufficient coating strength tend to be obtained, and when it is 50 μm or less, defects such as sagging and blotches are less likely to occur. The dry film thickness of the clear coating can be measured using a film thickness gauge (such as the "SDM-miniR" manufactured by Sanko Electronics Laboratory Co., Ltd.).

[0055] The thickness of the coating film (or the sum of the thicknesses of each coating film if the coating film is a multi-layer coating) is preferably 50 to 120 μm in dry thickness, more preferably 55 to 100 μm, and even more preferably 60 to 80 μm, from the viewpoint of radio wave transmittance and coating film appearance. The dry film thickness of the coating can be measured using a film thickness gauge (such as the "SDM-miniR" manufactured by Sanko Electronics Laboratory Co., Ltd.).

[0056] From the viewpoint of electromagnetic wave transmission, the refractive index of the coating film is preferably 1.0 to 10, more preferably 1.4 to 3, and even more preferably 1.5 to 1.9. The refractive index of the coating can be measured using a refractometer.

[0057] [[Method for forming a coating film]] When the coating is a single-layer coating, the coating can be formed by applying the coating composition to the object to be coated (substrate or other layer) and then allowing it to dry.

[0058] The method for applying the paint composition to the object to be coated is not particularly limited, and conventionally known methods can be used. Examples include spray painting such as air spray or airless spray, air electrostatic spray painting, bell coating, disc coating, roller coating, brush coating, curtain coating, shower coating, and the like. Conditions such as the discharge rate of the paint composition can be appropriately set according to the desired film thickness, etc.

[0059] If the paint composition contains a curing agent, it is preferable to heat-cur and dry the uncured paint film after applying the paint composition. The heat curing temperature can be, for example, 80 to 140°C. The heat curing time can be, for example, 10 to 60 minutes. In this disclosure, the term "heat curing time" refers to the time from when the target heat curing temperature is reached until that temperature is maintained, without considering the time it takes to reach the target heat curing temperature. The apparatus used for heat curing is not particularly limited, and conventionally known heat curing apparatuses can be used, such as drying ovens that utilize heat sources such as hot air, electricity, gas, or infrared radiation. Furthermore, it is preferable to use a drying oven that uses two or more of these heat sources in combination, as this shortens the drying time.

[0060] If the coating is a multi-layer coating, each coating should be formed in the order of lamination. For example, if the coating is a multi-layer coating consisting of a primer coating, a base coating, and a clear coating, the process includes the steps of: applying a primer coating composition to the object to be coated to form a primer coating; applying a base coating composition on top of the primer coating to form a base coating; and applying a clear coating composition on top of the base coating to form a clear coating.

[0061] The method of applying each paint composition is not particularly limited, and the conventionally known methods described above can be used. The conditions such as the discharge rate of each paint composition can be appropriately set according to the desired film thickness of each coating.

[0062] If each paint composition contains a hardener, each uncured paint film may be formed by heating and curing and drying each applied uncured paint film in each step, or all uncured paint films may be heated and cured and dried simultaneously after all paint compositions have been applied. The heat curing temperature can be, for example, 80 to 120°C. The heat curing time can be, for example, 10 to 60 minutes for each step if heat curing is performed in each step, or 30 to 60 minutes if heat curing is performed all at once at the end. The heat curing apparatus is not particularly limited, and the conventionally known apparatus described above can be used. [Examples]

[0063] The present invention will be described in more detail below with reference to examples, but the present invention is not limited in any way to the following examples.

[0064] The materials and films used in the examples and comparative examples are as follows:

[0065] [Base material] • Base material A: PP base material (350mm x 100mm x 3mm thickness) ·Base material B: Colored PP base material (350mm x 100mm x thickness 3mm)

[0066] [Paint composition of each coating film] • For clear coatings: O-1800 (Macflow O-1800 Clear, manufactured by Nippon Paint Automotive Coatings Co., Ltd.) • For base coats A and C: AR-3020SM (Aquarex AR-3020 Silver Metallic, manufactured by Nippon Paint Automotive Coatings Co., Ltd.) • For base coat B: AR-3020 (manufactured by Nippon Paint Automotive Coatings Co., Ltd., "Aquarex AR-3020") • For primer coating: WB-1200 ("Be-Aqua WB-1200" manufactured by Nippon Paint Automotive Coatings Co., Ltd.)

[0067] [film] Polyurethane films with the thickness, refractive index, and surface roughness (arithmetic mean roughness Ra) shown in Table 1 were purchased from Seedam Co., Ltd.

[0068] The measurement and evaluation methods used in the examples and comparative examples are described below.

[0069] [Thickness of film and coating] The dry film thickness (μm) of the film and coating was measured using a film thickness gauge (SDM-miniR, manufactured by Sanko Electronics Laboratory Co., Ltd.). The coating film was measured after being peeled off the substrate.

[0070] [Refractive index of film] The refractive index of the film was measured at a measurement wavelength of 15 mm using a refractometer (manufactured by the Kyoto Prefectural Small and Medium Enterprise Technology Center). The coating film was measured after being peeled off the substrate.

[0071] [Surface roughness of the film (arithmetic mean roughness Ra)] In accordance with JIS-B0601, the surface roughness (arithmetic mean roughness Ra) of the film surface (the side opposite to the side that is adhered to the material) was measured using a surface roughness measuring instrument (Mitutoyo Corporation's "SURFTEST SJ-201P"). Seven measurements were taken using a sample with a 2.5 mm wide cutoff (5 sections), and the Ra value (μm) was obtained by eliminating upper and lower average measurements.

[0072] [Radio wave transmittance at 77GHz] The radio wave transmittance at 77 GHz for each component (in the example, a component with laminated films) was measured according to the following method. Samples (100mm x 100mm x each thickness) were prepared from each component. The transmission attenuation (dB) of the samples at a frequency of 77GHz was measured using a radio wave absorption meter (manufactured by the Kyoto Prefectural Small and Medium Enterprise Technology Center) under conditions of 25°C / 50%RH. From the transmission attenuation measurement results, the radio wave transmittance (%) was calculated using the following formula.

number

[0073] (Example 1) AR-3020SM was spray-coated onto one side of substrate A, which had been wiped with isopropyl alcohol, using a spray coating machine (Anest Iwata Co., Ltd.'s "Wider 71") in an environment of 25°C / 70%RH. The coating was then dried at 120°C for 30 minutes to form a base coating film A (thickness 20 μm) and obtain the component. After attaching an adhesive sheet to the film, the film was laminated (bonded) to the surface of the base material A of the component obtained above (the side of the component on which the coating film is not formed) via the adhesive sheet, thereby creating a film with the characteristics shown in Table 1. Table 1 shows the measurement results of radio wave transmittance at 77 GHz.

[0074] (Examples 2, 3, 7, 8, 13) The film was laminated onto the component in the same manner as in Example 1, except that the component and film were changed as shown in Table 1. Table 1 shows the measurement results of radio wave transmittance at 77 GHz.

[0075] (Examples 4-6) For a simulation model in which films with the following thickness, refractive index, and surface roughness (arithmetic mean roughness Ra) values ​​were determined and then laminated onto a component, the radio wave transmittance (%) at a frequency of 77 GHz was determined using the Fresnel reflection coefficient. The target component was the same as that used in Examples 1 to 3, having a substrate A (thickness 3 mm) and a base coating A (thickness 20 μm). For the material, the radio wave transmittance (%) at frequencies of 60 to 90 GHz was determined in the same manner as described in [Radio wave transmittance at 77 GHz] above, and the transmittance spectrum of the material was obtained (vertical axis: radio wave transmittance (%), horizontal axis: frequency (GHz)). The wavelength at the peak top frequency (87 GHz) of the transmittance spectrum was 3.45 mm, and the wavelength at the peak valley frequency (73 GHz) was 4.11 mm. Next, by substituting the thickness of the member, the wavelength at the obtained peak top frequency, and the wavelength at the peak valley frequency into the following equations (1) and (2), the refractive index of the member and the value of the constant m were determined, and the refractive index of the member was calculated to be 1.72 and the constant m to be 3. n 部材 ×d 部材 = 2m × λ ピークトップ / 4 ···(1) n 部材 ×d 部材 = (2m-1) × λ ピークバレー / 4 ···(2) (n 部材 :Refractive index of the material, d 部材 : Thickness of the component (mm), λ ピークトップ :Wavelength (mm) at the peak frequency of the component, λ ピークバレー :Wavelength (mm) at the peak-valley frequency of the component, m: an integer constant) Next, by substituting the wavelength at the peak top frequency and the value of the constant m obtained above into equation (3) below, we obtained relation (4). (n フィルム ) (1-(Ra / 4)) ×d フィルム =(2m × |λ) ピークトップ -3.896| / 4)×(1±0.7) ···(3) (n フィルム :Refractive index of film, Ra フィルム : Film surface roughness (arithmetic mean roughness Ra) (μm), d フィルム : Film thickness (mm), m: integer constant, λ ピークトップ :Wavelength (mm) at the peak frequency of the component. (n フィルム ) (1-(Ra / 4)) ×d フィルム = (0.669) × (1 ± 0.7) ... (4) Similar to the films used in Examples 1-3, the film's refractive index was set to 1.75 and its surface roughness (arithmetic mean roughness Ra) to 0.2 μm. The film's thickness range was determined using relational equation (4), resulting in a range of 0.12 to 0.67 mm. Based on the obtained film thickness range, the radio wave transmittance (%) at a frequency of 77 GHz was determined using the above simulation software for simulation models in which films with thicknesses of 200 μm (Example 4), 350 μm (Example 5), and 500 μm (Example 6) were laminated onto a component. Table 1 shows the simulation results of radio wave transmittance at 77 GHz. The radio wave transmittances of Examples 4-6 were equivalent to those of Examples 1-3.

[0076] (Example 9) WB-1200 was spray-coated onto one side of substrate A, which had been wiped with isopropyl alcohol, using a spray coating machine (Anest Iwata Co., Ltd. "Wider 71") in an environment of 25°C / 70%RH, and dried at 120°C for 20 minutes to form a primer coating film. Next, AR-3020SM was spray-painted onto the primer film using a spray painter (ABB's "New Cartridge Bell") under the same environmental conditions (painting conditions: gun distance: 200 mm, gun speed: 900 mm / s, rotation speed: 35000 rpm, shaping air pressure: 0.15 MPa), and dried at 120°C for 30 minutes to form base film A. Next, O-1800 was spray-painted onto base coating A using a spray painter (ABB RoboBell 951) under the same environmental conditions (painting conditions: gun distance: 200 mm, gun speed: 700 mm / s, rotation speed: 25000 rpm, shaping air pressure: 0.07 MPa). After setting for 10 minutes, the material was dried at 120°C for 35 minutes to form a multi-layer coating (total thickness 60 μm: primer coating thickness 10 μm, base coating A thickness 20 μm, clear coating thickness 30 μm), and the component was obtained. After attaching an adhesive sheet to the film, the film was laminated (bonded) to the surface of the base material A of the component obtained above (the side of the component on which the coating film is not formed) via the adhesive sheet, thereby creating a film with the characteristics shown in Table 1. Table 1 shows the measurement results of radio wave transmittance at 77 GHz.

[0077] (Examples 10, 11) As shown in Table 1, the film was laminated onto the component in the same manner as in Example 1, except that the component was changed to one without a coating (substrate only). Table 1 shows the measurement results of radio wave transmittance at 77 GHz.

[0078] (Example 12) WB-1200 was spray-coated onto one side of substrate A, which had been wiped with isopropyl alcohol, using a spray coating machine (Anest Iwata Co., Ltd. "Wider 71") in an environment of 25°C / 70%RH. The mixture was then dried at 120°C for 20 minutes to form a primer coating film (10 μm thick) and obtain the component. After attaching an adhesive sheet to the film, the film was laminated (bonded) to the surface of the base material A of the component obtained above (the side of the component on which the coating film is not formed) via the adhesive sheet, thereby creating a film with the characteristics shown in Table 1. Table 1 shows the measurement results of radio wave transmittance at 77 GHz.

[0079] (Comparative Examples 1-7) The film was not laminated, and only the components shown in Table 2 were used. Table 1 shows the measurement results of radio wave transmittance at 77 GHz.

[0080] [Table 1]

[0081] [Table 2] [Industrial applicability]

[0082] The method of the present invention can improve the radio wave transmittance at a frequency of 77 GHz without changing the material and design of the component, and is therefore particularly suitable for components used in automotive parts, aircraft parts, infrastructure parts, environmental measurement parts, wireless communication parts, etc., that protect millimeter-wave radar.

Claims

1. This is a method for improving the radio wave transmittance at a frequency of 77 GHz in a component having a resin-based substrate. This includes laminating a film onto the aforementioned member, The aforementioned film has a thickness of 50 to 10,000 μm, a refractive index of 1.3 to 10 at a measurement wavelength of 15 mm, and a surface roughness (arithmetic mean roughness Ra) of 0.01 to 2.0 μm. The thickness of the laminated film, the refractive index at a measurement wavelength of 15 mm, and the surface roughness (arithmetic mean roughness Ra) are determined by following the steps (a) to (d) below. A method characterized by the following features. (a) Obtain the transmittance spectrum of the material (vertical axis: radio wave transmittance (%), horizontal axis: frequency (GHz)), and determine the wavelength at the peak top frequency and the wavelength at the peak valley frequency of the transmittance spectrum. (b) The value of the constant m is determined by substituting the thickness of the member and the wavelength at the peak top frequency and the wavelength at the peak valley frequency of the member, which were determined in (a), into the following equations (1) and (2). n member × d member = 2m × λ peak top / 4 ... (1) n member × d member = (2m - 1) × λ peak valley / 4 ... (2) (n: refractive index of the component, d: thickness of the component (mm), λ: peak top: wavelength at the peak top frequency of the component (mm), λ: peak valley: wavelength at the peak valley frequency of the component (mm), m: integer constant) (c) Substitute the wavelength value at the peak top frequency of the component obtained in (a) above and the value of the constant m obtained in (b) above into the following equation (3). The resulting relationship equation (3) is referred to as equation (4) relating the film thickness, the refractive index at a measurement wavelength of 15 mm, and the surface roughness (arithmetic mean roughness Ra). (n film) (1 - (Ra / 4)) × d film = (2m × |λ peak top - 3.896| / 4) × (1 ± 0.7) ... (3) (n film: refractive index of the film at a measurement wavelength of 15 mm, Ra: surface roughness of the film (arithmetic mean roughness Ra) (μm), d film: thickness of the film (mm), m: integer constant, λ peak top: wavelength at the frequency of the peak top of the component (mm)) (d) Determine the film thickness, refractive index at a measurement wavelength of 15 mm, and surface roughness (arithmetic mean roughness Ra) so as to satisfy the relational equation (4) obtained in (c) above.

2. The method according to claim 1, wherein the member has a coating film.

3. The method according to claim 2, wherein the coating film contains aluminum.

4. The method according to claim 2 or 3, wherein the coating film is a multilayer coating film comprising a primer coating film, a base coating film, and a clear coating film.

5. The substrate comprises at least one selected from the group consisting of polypropylene (PP), acrylonitrile-butadiene-styrene copolymer (ABS resin), and polycarbonate / ABS alloy (PC / ABS alloy). The method according to claim 1 or 2, wherein the thickness of the base material is 2 to 3 mm.

6. The method according to claim 1 or 2, wherein the film is a polyolefin film, a polyester film, a polystyrene resin film, a polyurethane film, a urethane nylon film, a polyamide film, a polyvinyl chloride film, a polyvinylidene chloride film, or an acrylic film.

7. The method according to claim 1 or 2, wherein the member is for use in an automobile part.

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