Radio wave absorber and article including radio wave absorber

Abstract

Provided is a radio wave absorber comprising: a first layer which includes a binder and one or more types of particles selected from the group consisting of magnetic particles and dielectric particles; and a second layer which is adjacent to the first layer, wherein the second layer satisfies 1:2.50≤d×√ε≤3.50 or 2:0.60≤d×√ε≤1.40, where d is the thickness of the second layer, the unit of d is mm, and ε is the relative permittivity of the second layer.

Description

Radio wave absorbers and articles containing radio wave absorbers

[0001] The present invention relates to a radio wave absorber and an article including the radio wave absorber.

[0002] In recent years, radio wave absorbers to be incorporated into various articles have been studied (see, for example, Patent Documents 1 to 3).

[0003] JP 2022-082653 A JP 2023-135139 A JP 2023-167269 A

[0004] As an article equipped with a radio wave absorber, for example, radar that recognizes an object by transmitting and receiving radio waves has attracted attention. As an example, an on-board radar transmits radio waves and receives the reflected waves from the object (pedestrian, vehicle, etc.), thereby measuring the direction of the object, the distance to the object, etc. Based on the radar measurement results, an automatic driving control system for an automobile can automatically brake the automobile to a stop as necessary to prevent a collision with the object, or automatically control the speed to maintain a distance from the object.

[0005] In order to increase the reliability of systems that perform various controls based on the results of radar detection as described above, it is desirable to improve the performance of the radar. To this end, in recent years, studies have begun to be conducted on improving the detection accuracy, for example, by installing a radio wave absorber on the front side of the radar's radio wave transmitting and receiving unit (the side on which radio waves enter from outside).

[0006] For example, from the viewpoint of improving the recognition accuracy of radar, it is desirable that the radio wave absorber incorporated in the radar can exhibit excellent radio wave absorbing properties. Also, in various other applications where radio wave absorbers are used, it is desirable that the radio wave absorber can exhibit excellent radio wave absorbing properties.

[0007] Radio wave absorption characteristics include reflection attenuation characteristics and transmission attenuation characteristics. The index of reflection attenuation characteristics is the amount of reflection attenuation, and the index of transmission attenuation characteristics is the amount of transmission attenuation. A radio wave absorber with high reflection attenuation and transmission attenuation is a radio wave absorber with excellent reflection attenuation characteristics and transmission attenuation characteristics.

[0008] Patent Documents 1 to 3 disclose a radio wave absorber having a laminated structure having a dielectric layer (a dielectric substrate in Patent Document 1, and a dielectric layer in Patent Documents 2 and 3). Such a dielectric layer can contribute to the expression of the radio wave absorbing characteristics of the radio wave absorber. Furthermore, a layer containing magnetic particles can also contribute to the expression of the radio wave absorbing characteristics of the radio wave absorber. The present inventors therefore investigated the radio wave absorbing characteristics of a radio wave absorber containing one or more types of particles selected from the group consisting of magnetic particles and dielectric particles, and found that it is not easy to improve the return loss characteristics.

[0009] In view of the above, an object of one aspect of the present invention is to provide a radio wave absorber that can exhibit excellent return loss characteristics.

[0010] One aspect of the present invention is as follows. [1] A radio wave absorber comprising: a first layer containing one or more types of particles selected from the group consisting of magnetic particles and dielectric particles and a binder; and a second layer adjacent to the first layer, wherein the second layer satisfies the following formula 1: 2.50≦d×√ε≦3.50 or the following formula 2: 0.60≦d×√ε≦1.40, where d is the thickness of the second layer, d is expressed in mm, and ε is the relative dielectric constant of the second layer. [2] The radio wave absorber according to [1], wherein the radio wave absorption rate of the second layer is 5.0% or less. [3] The radio wave absorber according to [1] or [2], wherein the ten-point average roughness Rz (hereinafter also referred to as "surface roughness") of a surface of the second layer opposite to the first layer side is 500.0 μm or less. [4] The radio wave absorber according to any of [1] to [3], wherein the particles are magnetic particles, and the magnetic particles are hexagonal ferrite particles. [5] The radio wave absorber according to [4], wherein the hexagonal ferrite is a substituted hexagonal ferrite. [6] The radio wave absorber according to [5], wherein the substituted hexagonal ferrite is an aluminum-substituted hexagonal ferrite. [7] The radio wave absorber according to [6], wherein the aluminum-substituted hexagonal ferrite is a magnetoplumbite-type hexagonal ferrite having a composition represented by the following formula 1: In formula 1, A represents one or more atoms selected from the group consisting of Sr, Ba, Ca, and Pb, and x satisfies 0.50≦x≦8.00. [8] The radio wave absorber according to [7], wherein the atom represented by A in formula 1 includes Sr. [9] The radio wave absorber according to any of [1] to [3], wherein the particles are dielectric particles, and the dielectric particles are carbon-based particles.

[10] The radio wave absorber according to [1], wherein the radio wave absorption rate of the second layer is 5.0% or less, and the ten-point average roughness Rz of the surface of the second layer opposite to the first layer side is 500.0 μm or less, and the particles are magnetic particles, and the magnetic particles are particles of magnetoplumbite-type hexagonal ferrite having a composition represented by the following formula 1: In Formula 1, A represents one or more atoms selected from the group consisting of Sr, Ba, Ca, and Pb, and x satisfies 0.50≦x≦8.00.

[11] The radio wave absorber according to [1], wherein the second layer has a radio wave absorptivity of 5.0% or less, the ten-point average roughness Rz of the surface of the second layer opposite to the first layer side is 500.0 μm or less, the particles are dielectric particles, and the dielectric particles are carbon-based particles.

[12] The radio wave absorber according to any of [1] to

[11] , wherein the absolute value of the difference (A−B) between the reflectance A of the first layer and the reflectance B of the second layer (hereinafter also referred to as “reflectance difference”) is less than 6.0 dB.

[13] The radio wave absorber according to any of [1] to

[12] , wherein the second layer has a radio wave absorptivity of 5.0% or less, and the second layer contains carbon black.

[14] The radio wave absorber according to any one of [1] to

[13] , wherein the first layer is a layer in direct contact with the second layer.

[15] The radio wave absorber according to any one of [1] to

[14] , wherein the ten-point average roughness Rz (hereinafter also referred to as "interface roughness") of the surface of the second layer on the first layer side is 500.0 μm or less.

[16] The radio wave absorber according to any one of [1] to

[15] , wherein the radio wave absorber satisfies the following formula 3: 1.5≦k1 / k2 (hereinafter also referred to as "thermal conductivity ratio"), wherein k1 is the thermal conductivity of the first layer, k2 is the thermal conductivity of the second layer, and the unit of thermal conductivity is W / m k.

[17] An article comprising the radio wave absorber according to any one of [1] to

[16] .

[18] The article according to

[17] , further comprising a radio wave transmission source.

[19] The article according to

[18] , which is a radar.

[0011] According to one aspect of the present invention, it is possible to provide a radio wave absorber that can exhibit excellent return loss characteristics, and an article that includes such a radio wave absorber.

[0012] [Radio Wave Absorber] In the present invention and this specification, "radio waves" refers to electromagnetic waves with a frequency of 3 terahertz (THz) or less. A "radio wave absorber" has radio wave absorption properties for radio waves of one or more frequencies.

[0013] A radio wave absorber according to one embodiment of the present invention has the first layer and second layer described above. In this radio wave absorber, the first layer, which is a layer containing one or more particles selected from the group consisting of magnetic particles and dielectric particles and a binder, contributes to the development of reflection attenuation characteristics and transmission attenuation characteristics, and the second layer contributes to the improvement of reflection attenuation characteristics. Conventionally, laminated radio wave absorbers have been constructed by laminating a metal layer on the surface opposite to the surface on which radio waves are incident (the so-called back surface) of the radio wave absorber (see, for example, Patent Documents 1 to 3 mentioned above). Such radio wave absorbers are called matched radio wave absorbers. Matched radio wave absorbers can improve reflection attenuation characteristics by providing a metal layer and utilizing phase difference absorption. In contrast, the radio wave absorber according to one embodiment of the present invention can exhibit high reflection attenuation without relying on a metal layer. Radio wave absorbers used without laminating a metal layer on the back surface are generally called transmission radio wave absorbers. For example, in the transmission radio wave absorber having only the first layer, attempts to increase transmission attenuation generally tend to result in a decrease in reflection attenuation. In contrast, a radio wave absorber according to one embodiment of the present invention, by including a second layer, can exhibit high return loss and even high transmission loss without relying on a metal layer. The term "metal layer" used herein refers to a layer containing metal and substantially reflecting radio waves. However, if the first layer contains metal, such a first layer does not fall under the category of the metal layer described above. Here, "substantially reflecting radio waves" means, for example, that when radio waves are incident on a radio wave absorber with a metal layer laminated on its rear surface, the radio wave absorber reflects 90% or more of the incident radio waves. Examples of the metal layer include a metal plate and a metal foil. For example, a metal layer formed by vapor deposition on the rear surface of the radio wave absorber can be used without laminating a metal layer on the surface of the first layer opposite the second layer. Being able to use the radio wave absorber without a metal layer is preferable from the perspectives of recycling the radio wave absorber and cost. Furthermore, a radio wave absorber used with a metal layer laminated on its rear surface may experience quality degradation due to deterioration of the metal layer, peeling between the metal layer and the radio wave absorber, etc. Being able to use the device without providing a metal layer on the back surface is also preferable in that such a deterioration in quality does not occur.Furthermore, a radio wave absorber having a metal layer has limitations on the manufacturing method, and it is difficult to use, for example, the injection molding method described below. In contrast, the radio wave absorber according to one aspect of the present invention is preferable in that it can be manufactured without such limitations.

[0014] The radio wave absorber according to one aspect of the present invention will be described in further detail below.

[0015] <First Layer> <<Particles>> The first layer contains one or more types of particles selected from the group consisting of magnetic particles and dielectric particles. The first layer may contain different types of particles selected from the group consisting of magnetic particles and dielectric particles in any ratio. In the present invention and this specification, "magnetic" refers to ferromagnetic property, and "dielectric" refers to a material having a volume resistivity (ρ) at 20°C that is an intrinsic value of the material. ｖ ) is 1 x 10 ５ The particle shape and size are not particularly limited.

[0016] (Dielectric Particles) The dielectric particles that can be contained in the first layer can be particles of various dielectric materials. For example, the dielectric particles can be carbon-based particles. Here, the word "based" is used to mean "including." The carbon-based particles can be particles of various carbon-based materials such as carbon black and graphite.

[0017] (Magnetic Particles) Examples of magnetic particles that can be contained in the first layer include particles of various magnetic materials such as ferrite, iron oxide, cobalt, and chromium oxide. From the viewpoint of radio wave absorption, hexagonal ferrite particles are preferred as magnetic particles. The type of magnetic material constituting the magnetic particles can be confirmed by analysis using X-ray diffraction. For example, magnetic particle powder can be extracted from the first layer-forming composition or the first layer using a known method, and the extracted powder can be analyzed using X-ray diffraction. Alternatively, a portion of the magnetic particle powder used to prepare the first layer-forming composition can be sampled, and the sampled powder can be analyzed using X-ray diffraction. Alternatively, confirmation can be performed using, for example, the following method. Part or all of the first layer is finely chopped, immersed in a solvent (e.g., hexafluoroisopropanol) for 1 to 2 days, and then dried. The dried first layer is further ground into fine powder and analyzed using X-ray diffraction.

[0018] Hexagonal Ferrite Particles In one embodiment, the first layer may contain hexagonal ferrite particles as magnetic particles. In the present invention and this specification, "hexagonal ferrite particles" refers to magnetic particles in which a hexagonal ferrite crystal structure is detected as the main phase by analysis using an X-ray diffraction method. The main phase refers to the structure to which the most intense diffraction peak is assigned in the X-ray diffraction spectrum obtained by the X-ray diffraction method. For example, if the most intense diffraction peak in the X-ray diffraction spectrum obtained by the X-ray diffraction method is assigned to the hexagonal ferrite crystal structure, it is determined that the hexagonal ferrite crystal structure has been detected as the main phase. If only a single structure is detected by the X-ray diffraction method, this detected structure is considered to be the main phase. The hexagonal ferrite crystal structure contains at least iron atoms, divalent metal atoms, and oxygen atoms as constituent atoms. In unsubstituted hexagonal ferrite, the atoms that constitute the hexagonal ferrite crystal structure are only iron atoms, divalent metal atoms, and oxygen atoms. In contrast, substitutional hexagonal ferrite contains one or more other atoms in addition to iron atoms, divalent metal atoms, and oxygen atoms as atoms constituting the crystalline structure of hexagonal ferrite. These one or more other atoms are usually atoms that replace part of the iron in the crystalline structure of hexagonal ferrite. The divalent metal atoms are metal atoms that can become divalent cations as ions, and examples thereof include alkaline earth metal atoms such as strontium atoms, barium atoms, and calcium atoms, and lead atoms. In the present invention and this specification, hexagonal strontium ferrite particles refer to particles in which the main divalent metal atom contained therein is a strontium atom, and hexagonal barium ferrite particles refer to particles in which the main divalent metal atom contained therein is a barium atom. The same applies to particles of other types of hexagonal ferrite. The main divalent metal atom refers to the divalent metal atom that accounts for the largest proportion of the divalent metal atoms contained in the particle on an atomic percentage basis. However, the above divalent metal atoms do not include rare earth atoms. A "rare earth atom" in this invention and specification is selected from the group consisting of scandium atom (Sc), yttrium atom (Y), and lanthanide atom.The lanthanoid atom is selected from the group consisting of lanthanum atom (La), cerium atom (Ce), praseodymium atom (Pr), neodymium atom (Nd), promethium atom (Pm), samarium atom (Sm), europium atom (Eu), gadolinium atom (Gd), terbium atom (Tb), dysprosium atom (Dy), holmium atom (Ho), erbium atom (Er), thulium atom (Tm), ytterbium atom (Yb), and lutetium atom (Lu).

[0019] Regarding the hexagonal ferrite particles, the hexagonal ferrite constituting the particles can be one or more selected from the group consisting of hexagonal strontium ferrite, hexagonal barium ferrite, hexagonal calcium ferrite and hexagonal lead ferrite. From the viewpoint of radio wave absorption in the high frequency band, one or more selected from the group consisting of hexagonal strontium ferrite, hexagonal barium ferrite and hexagonal calcium ferrite are preferred, hexagonal strontium ferrite and / or hexagonal barium ferrite are more preferred, and hexagonal strontium ferrite is even more preferred.

[0020] In one embodiment, the hexagonal ferrite particles can be particles of magnetoplumbite-type (commonly referred to as "M-type") hexagonal ferrite. When magnetoplumbite-type hexagonal ferrite is an unsubstituted type that does not contain atoms substituting for iron, it has the composition formula: AFe １２ O １９ Here, A can represent one or more atoms selected from the group consisting of Sr, Ba, Ca, and Pb, and also includes an embodiment in which two or more of these atoms are contained in any ratio.

[0021] A preferred hexagonal ferrite from the viewpoint of radio wave absorption is a substitutional hexagonal ferrite. A specific example of the substitutional hexagonal ferrite is a substitutional hexagonal ferrite in which some of the iron atoms constituting the crystal structure of the hexagonal ferrite are substituted with aluminum atoms. In the present invention and this specification, such a substitutional hexagonal ferrite is referred to as an "aluminum-substituted hexagonal ferrite." A specific example of the aluminum-substituted hexagonal ferrite is a substitutional magnetoplumbite-type hexagonal ferrite in which some of the iron atoms of the magnetoplumbite-type hexagonal ferrite are substituted with aluminum atoms. One form of such a hexagonal ferrite is a hexagonal ferrite having a composition represented by the following formula 1:

[0022]

[0023] In formula 1, A represents one or more atoms selected from the group consisting of Sr, Ba, Ca, and Pb (hereinafter also referred to as "A atoms"), and may be only one type, or may contain two or more types in any ratio, and from the viewpoint of improving the uniformity of the composition between particles contained in the radio wave absorber, it is preferable that only one type be present. From the viewpoint of radio wave absorption in the high frequency band, A in formula 1 is preferably one or more atoms selected from the group consisting of Sr, Ba, and Ca, more preferably one or more atoms selected from the group consisting of Sr and Ba, still more preferably containing Sr, and even more preferably being Sr.

[0024] In formula 1, x satisfies 0.50≦x≦8.00.

[0025] In one embodiment, x satisfies 1.50≦x≦8.00. From the viewpoint of radio wave absorption in the high frequency band, x is preferably 1.50 or more, more preferably more than 1.50, and even more preferably 1.60 or more. From the viewpoint of magnetic properties, x is 8.00 or less, preferably less than 8.00, more preferably 6.00 or less, and even more preferably less than 6.00.

[0026] A specific example of the magnetoplumbite-type substitutional hexagonal ferrite represented by formula 1, in which x satisfies 1.50≦x≦8.00, is SrFe （９．５８） Al （２．４２） O １９ , SrFe （９．３７） Al （２．６３） O １９ , SrFe （９．２７） Al （２．７３） O １９ , SrFe （９．８５） Al （２．１５） O １９ , SrFe （１０．００） Al （２．００） O １９ , SrFe （９．７４） Al （２．２６） O １９ , SrFe （１０．４４） Al （１．５６） O １９ , SrFe （９．７９） Al （２．２１） O １９ , SrFe （９．３３） Al （２．６７） O １９ , SrFe （７．８８） Al （４．１２） O １９ , SrFe （７．０４） Al （４．９６） O １９ , SrFe （７．３７） Al （４．６３） O １９ , SrFe （６．２５） Al （５．７５） O １９ , SrFe （７．７１） Al （４．２９） O １９ , Sr （０．８０） Ba （０．１０） Ca （０．１０） Fe （９．８３） Al （２．１７） O １９ , BaFe （９．５０） Al （２．５０） O １９ , CaFe （１０．００） Al （２．００） O １９ , PbFe （９．００） Al （３．００） O １９and the like. Specific examples also include the substituted hexagonal strontium ferrite described in the Examples section below. The composition of the magnetic particles can be confirmed, for example, by subjecting a solution in which the magnetic particles have been dissolved to high-frequency inductively coupled plasma atomic emission spectroscopy. Specific examples of confirmation methods include the methods described in the Examples section below. Alternatively, the composition of the magnetic particles contained in the first layer can be confirmed by exposing a cross section by cutting the radio wave absorber or the first layer, for example, and then subjecting the exposed cross section to energy dispersive X-ray analysis, for example.

[0027] In one embodiment, x may satisfy the condition 0.50≦x<1.50. From the viewpoint of radio wave absorption in the high frequency band, x may be 0.50 or more, and preferably 0.70 or more. From the viewpoint of magnetic properties, x is preferably less than 1.50, and more preferably 1.20 or less. An example of a magnetoplumbite-type substitutional hexagonal ferrite represented by Formula 1 in which x satisfies 0.50≦x<1.50 is the substitutional hexagonal strontium ferrite described in the Examples section below.

[0028] In one embodiment, the substitutional hexagonal ferrite may have a single crystalline phase or may contain multiple crystalline phases. A single crystalline phase is preferred, and a magnetoplumbite-type substitutional hexagonal ferrite having a single crystalline phase is more preferred. The term "single crystalline phase" refers to a case where only one diffraction pattern indicating a given crystalline structure is observed in an analysis performed by X-ray diffraction. For example, X-ray diffraction analysis can be performed using the method described in the Examples section below. When multiple crystalline phases are included, two or more diffraction patterns indicating a given crystalline structure are observed in the X-ray diffraction analysis. For the attribution of diffraction patterns, for example, the database of the International Centre for Diffraction Data (ICDD) (registered trademark) can be referenced. For example, for the diffraction pattern of a magnetoplumbite-type hexagonal ferrite containing Sr, reference can be made to "00-033-1340" of the International Centre for Diffraction Data (ICDD). However, if some of the iron atoms are substituted with substituting atoms such as aluminum atoms, the peak position shifts from the peak position when no substituting atoms are included.

[0029] Method for producing hexagonal ferrite particles There are various methods for producing hexagonal ferrite particles, and when the first layer contains hexagonal ferrite particles, the method for producing the hexagonal ferrite particles is not particularly limited. Below, a solid-phase method will be described as an example of a method for producing hexagonal ferrite particles. The solid-phase method is a method for producing hexagonal ferrite particles by mixing multiple solid raw materials and firing the resulting mixture.

[0030] The solid raw materials used in the solid-phase method include compounds of iron atoms and compounds of A atoms. When producing a powder of substitutional hexagonal ferrite, compounds of substitutional atoms can also be used. These compounds can be oxides, carbonates, etc. The A atoms and substitutional atoms are as described above. The mixing ratio of multiple solid raw materials can be determined depending on the desired hexagonal ferrite composition. A raw material mixture can be obtained by mixing multiple solid raw materials simultaneously or sequentially in any order. Furthermore, mixing of the solid raw materials is not limited to dry conditions. Mixing can also be performed under wet conditions, for example, by adding a solvent such as water to form a slurry. Mixing of the solid raw materials can be performed using a commercially available mixer or a mixer with a known configuration. An example of a mixer is a paint shaker. Mixing can be performed, for example, in an atmospheric atmosphere at room temperature.

[0031] After the above mixing, the obtained raw material mixture can be fired. During this firing, crystallization of the raw material mixture can be promoted, thereby forming a hexagonal ferrite crystal structure. As firing conditions, for example, the firing temperature can be in the range of 1000°C to 1500°C. The firing temperature can be, for example, the ambient temperature within the firing device (for example, the temperature within a heating furnace). The firing time can be in the range of 1 hour to 6 hours. However, the above range is merely an example, and firing can be performed under conditions that allow the formation of a hexagonal ferrite crystal structure. Firing can be performed, for example, in an air atmosphere.

[0032] In the firing, a flux can be added to the powder of the raw material mixture before firing. As the flux, various fluxes can be used, for example, SrCl ２ , SrCl ２ ・6H ２ O, CaCl ２ ・2H ２ O, MgCl ２ , KCl, NaCl, BaCl ２ ・2H ２ O, Na ２ B ４ O ７The amount added is preferably 0.1 to 10 parts by mass, and more preferably 0.1 to 8.0 parts by mass, relative to 100 parts by mass of the powder of the raw material mixture.

[0033] The raw material mixture before firing can be subjected to a dry or wet grinding process, and / or the fired product after firing can be subjected to a dry or wet grinding process. By performing the grinding process, the size of the particles constituting the hexagonal ferrite powder can be adjusted. Grinding can be performed using known grinding means such as a mortar and pestle, a grinder (cutter mill, ball mill, bead mill, roller mill, jet mill, hammer mill, attritor, etc.), etc.

[0034] The fired product may be subjected to a process such as pulverization if necessary, and then annealed. The annealing may be carried out, for example, in a heating furnace at a furnace temperature of 500 to 1100°C for about 1 to 10 hours.

[0035] Before and / or after the various steps described above, one or more known steps such as washing and drying may be optionally carried out.

[0036] Filling Rate The first layer contains the particles and a binder. The particle filling rate in the first layer is not particularly limited. In one embodiment, the particle filling rate in the first layer, as a volume-based filling rate (hereinafter also referred to as "volume filling rate"), can be 40 vol% or less or 35 vol% or less. The volume filling rate can be, for example, 15 vol% or more or 20 vol% or more. In another embodiment, the volume filling rate can be greater than 40 vol%. In this case, the volume filling rate can be, for example, greater than 40 vol% to 60 vol% or greater than 40 vol% to 50 vol%. The volume filling rate refers to the volume-based content, with the total volume of the first layer being 100 vol%. Volume % is also referred to as "vol%." "Vol" is an abbreviation for volume. The volume filling rate of the particles in the composition for forming the first layer can also be within the above range. With respect to the composition for forming the first layer, the volume filling rate means the volume-based content, with the total volume of the composition for forming the first layer (excluding the solvent if it contains a solvent; the same applies below) being 100 volume %.

[0037] The particle volume filling rate of the first layer-forming composition can be determined from the composition of the composition. For the first layer, for example, the particles can be collected from the first layer using a known method, and the particle volume filling rate can be calculated by multiplying the collected particle volume by 100. The total volume of the first layer and the particle volume can be determined using a known method. Alternatively, if the composition of the composition used to form the first layer is known, the particle volume filling rate in the first layer can be determined from this known composition. The particle volume filling rate in the first layer can also be determined using a cross-sectional SEM image obtained with a scanning electron microscope (SEM) using the following method. A measurement sample having a square surface with sides of 5 mm is cut out from a randomly determined position on the first layer to be measured. A cross-sectional observation sample is prepared from the cut sample. The cross-sectional observation sample is prepared by FIB (Focused Ion Beam) processing. The prepared cross-sectional observation sample is observed using an SEM, and a cross-sectional image (cross-sectional SEM image) is captured. A field emission scanning electron microscope (FE-SEM) is used as the SEM. Using the FE-SEM, the cross-sectional observation sample is set on the stage with the FIB-processed cross section facing upward, and a cross-sectional SEM image with a field of view of 30 μm × 40 μm is obtained under conditions of an acceleration voltage of 15 kV and an observation magnification of 3000x. The obtained cross-sectional SEM image is then binarized to calculate the proportion (area basis) occupied by the particles. The above operation is performed on five measurement samples cut from different positions on the first layer to be measured, and the volume filling factor of the particles can be calculated as the arithmetic average of the five obtained values. If necessary, the portion of the particles in the cross-sectional SEM image can also be identified by performing elemental analysis of the cross-sectional observation sample. The volume filling rates of the other components described in this specification can be determined in the same manner as above.

[0038] In one embodiment, the particle filling rate in the first layer can be 40% by mass or less or 35% by mass or less, as a mass-based filling rate (hereinafter also referred to as "mass filling rate"). The mass filling rate can be, for example, 15% by mass or more or 20% by mass or more. In another embodiment, the mass filling rate can be greater than 40% by mass. In this case, the mass filling rate can be, for example, greater than 40% by mass and less than 60% by mass, or greater than 40% by mass and less than 50% by mass. The mass filling rate refers to the content based on mass, with the total mass of the first layer being 100% by mass. In other words, in the present invention and this specification, a content expressed in units of % by mass is synonymous with the mass filling rate. % by mass is also referred to as "wt%." "wt" is an abbreviation for weight. The mass filling rate of the particles in the composition for forming the first layer can also be within the above range. With respect to the composition for forming the first layer, the mass filling rate means the mass content, with the total mass of the composition for forming the first layer (excluding the solvent if it contains a solvent; the same applies below) being 100 mass%.

[0039] The mass packing factor of the particles in the first layer-forming composition can be determined from the composition of the composition. For the first layer, for example, the particles can be collected from the first layer using a known method, and the mass packing factor of the particles can be calculated by multiplying the mass of the collected particles by the total mass of the first layer by 100. The total mass of the first layer and the mass of the particles can be determined using a known method. Alternatively, if the composition of the composition used to form the first layer is known, the mass packing factor of the particles in the first layer can be determined from the known composition. The mass packing factors of the other components described herein can also be determined in the same manner.

[0040] <Binder> The first layer contains the particles and further contains a binder. The binder can be, for example, a resin, and examples of the resin include thermoplastic resins and thermosetting resins. Examples of the thermoplastic resin include acrylic resin, polyacetal, polyamide (abbreviated as PA), polyethylene, polypropylene (abbreviated as PP), polyethylene terephthalate, polybutylene terephthalate (abbreviated as PBT), polyethylene terephthalate-1,4-cyclohexanedimethanol terephthalate copolymer, polylactic acid, polycarbonate, polystyrene, polyphenylene sulfide, polyvinyl chloride, ABS (acrylonitrile butadiene styrene) resin obtained by copolymerization of acrylonitrile, butadiene, and styrene, and AS (acrylonitrile styrene) resin obtained by copolymerization of acrylonitrile and styrene. Examples of the thermosetting resin include phenol resin, epoxy resin, melamine resin, urea resin, unsaturated polyester, diallyl phthalate resin, urethane resin, and silicone resin.

[0041] A glass fiber reinforced resin can also be used as the binder for the first layer. An example of a commercially available glass fiber reinforced resin is polyamide (PA) 6 resin (trade name: Nylon 6A1030GFL15) manufactured by Unitika Ltd. In the present invention and this specification, the resin filling rate and content of glass fiber reinforced resin are determined including the glass fiber portion.

[0042] The binder may also be rubber. Examples of rubber that can be used, from the viewpoint of being able to produce a radio wave absorber that has good mixability with magnetic particles and is excellent in durability, weather resistance, and impact resistance, include butadiene rubber, isoprene rubber, chloroprene rubber, halogenated butyl rubber, fluororubber, urethane rubber, acrylic rubber (abbreviated as ACM) obtained by copolymerizing an acrylic acid ester (e.g., ethyl acrylate, butyl acrylate, and 2-ethylhexyl acrylate) with another monomer, ethylene-propylene rubber obtained by coordination polymerization of ethylene and propylene using a Ziegler catalyst, butyl rubber (abbreviated as IIR) obtained by copolymerizing isobutylene and isoprene, styrene-butadiene rubber (abbreviated as SBR) obtained by copolymerizing butadiene and styrene, acrylonitrile-butadiene rubber (abbreviated as NBR) obtained by copolymerizing acrylonitrile and butadiene, and silicone rubber.

[0043] When producing a first layer containing rubber as a binder, the composition for forming the first layer may contain, in addition to rubber, various additives such as a vulcanizing agent, a vulcanization aid, a softener, a plasticizer, etc. Examples of vulcanizing agents include sulfur, an organic sulfur compound, and a metal oxide.

[0044] Examples of the binder include thermoplastic elastomers (TPE), such as olefin thermoplastic elastomers (TPO), styrene thermoplastic elastomers (TPS), amide thermoplastic elastomers (TPA), and polyester thermoplastic elastomers (TPC).

[0045] The first layer may contain only one type of binder, or may contain two or more types. The binder filling rate in the first layer is not particularly limited, and is preferably 60% by volume or more, more preferably 65% ​​by volume or more, even more preferably 65% ​​by volume to 92% by volume, and even more preferably 65% ​​by volume to 85% by volume. When the first layer contains two or more binders, the volume filling rate refers to the total volume filling rate of the two or more binders. This also applies to the filling rates (volume filling rate and mass filling rate) of other components. The volume filling rate of the binder in the first-layer forming composition can also be within the above range. The binder filling rate in the first layer, for example, is preferably 60% by mass or more, more preferably 65% ​​by mass or more, even more preferably 65% ​​by mass to 92% by mass, and even more preferably 65% ​​by mass to 85% by mass. The mass filling rate of the binder in the first-layer forming composition can also be within the above range.

[0046] <Additives> The first layer may optionally contain one or more additives in any proportion. Examples of additives include antioxidants, light stabilizers, dispersants, dispersion aids, antifungal agents, antistatic agents, plasticizers, impact modifiers, nucleating agents, lubricants, surfactants, pigments, dyes, fillers, release agents (fatty acids, fatty acid metal salts, oxyfatty acids, fatty acid esters, partially saponified aliphatic esters, paraffins, low-molecular-weight polyolefins, fatty acid amides, alkylene bisfatty acid amides, aliphatic ketones, fatty acid lower alcohol esters, fatty acid polyhydric alcohol esters, fatty acid polyglycol esters, modified silicones, etc.), processing aids, antifogging agents, anti-drip agents, antibacterial agents, etc. One additive component may perform two or more functions.

[0047] (Antioxidant) In one embodiment, a preferred additive can be an antioxidant. The antioxidant is not particularly limited, and known antioxidants can be used. Examples of antioxidants are described in, for example, "Comprehensive Technology of Polymer Stabilization - Mechanism and Application Development -" edited by Yasukazu Ohkatsu, published by CMC. This description is incorporated herein by reference. Types of antioxidants include phenolic antioxidants, amine-based antioxidants, phosphorus-based antioxidants, sulfur-based antioxidants, etc. As the antioxidant, it is preferable to use a phenolic antioxidant and / or an amine-based antioxidant in combination with a phosphorus-based antioxidant and / or a sulfur-based antioxidant.

[0048] Examples of the phenolic antioxidant include ADK STAB AO-20, ADK STAB AO-30, ADK STAB AO-40, ADK STAB AO-50, ADK STAB AO-60, ADK STAB AO-80, and ADK STAB AO-330 manufactured by ADEKA Corporation, and IRGANOX 1010, IRGANOX 1035, IRGANOX 1076, IRGANOX 1098, IRGANOX 1135, IRGANOX 1330, IRGANOX 1726, IRGANOX 245, IRGANOX 259, IRGANOX 3114, and IRGANOX 565 manufactured by BASF Japan. The above "ADK STAB" and "IRGANOX" are both registered trademarks.

[0049] Examples of amine-based antioxidants include Sanol LS-770, Sanol LS-765, and Sanol LS-2626 from Sankyo Lifetech Co., Ltd.; Adeka STAB LA-77, Adeka STAB LA-57, Adeka STAB LA-52, Adeka STAB LA-62, Adeka STAB LA-63, Adeka STAB LA-67, Adeka STAB LA-68, and Adeka STAB LA-72 from ADEKA Corporation; and TINUVIN 123, TINUVIN 144, TINUVIN 622, TINUVIN 765, and TINUVIN 944 from BASF Japan. Note that "ADK STAB" and "TINUVIN" are both registered trademarks. Furthermore, amine-based compounds capable of quenching radicals can also be used as antioxidants. Examples of such amine compounds include polyethylene glycol bis-TEMPO (Sigma-Aldrich), sebacate bis-TEMPO, etc. "TEMPO" is an abbreviation for tetramethylpiperidine-1-oxyl.

[0050] Examples of phosphorus-based antioxidants include ADK STAB PEP-8, ADK STAB PEP-36, ADK STAB HP-10, and ADK STAB 2112 manufactured by ADEKA Corporation, and IRGAFOS 168 manufactured by BASF Japan Ltd. Both "ADK STAB" and "IRGAFOS" are registered trademarks.

[0051] Examples of sulfur-based antioxidants include Adekastab AO-412S and Adekastab AO-503S manufactured by ADEKA Corporation. Note that "Adekastab" is a registered trademark.

[0052] Among the above, the phenolic antioxidant is preferably one or more selected from the group consisting of ADK STAB AO-20, ADK STAB AO-60, ADK STAB AO-80, and IRGANOX 1010, the amine-based antioxidant is preferably ADK STAB LA-52, the phosphorus-based antioxidant is preferably ADK STAB PEP-36, and the sulfur-based antioxidant is preferably ADK STAB AO-412S. When the first layer contains an antioxidant, it may contain only one type of antioxidant or two or more types.

[0053] When the first layer contains an antioxidant, the content of the antioxidant in the first layer is not particularly limited. For example, from the viewpoint of simultaneously suppressing decomposition of the binder and suppressing bleeding of the antioxidant, the content is preferably 0.1 parts by mass to 10 parts by mass, and more preferably 0.5 parts by mass to 5 parts by mass, relative to 100 parts by mass of the binder.

[0054] (Light Stabilizer) In one embodiment, a preferred additive can be a light stabilizer. Examples of the light stabilizer include HALS (i.e., hindered amine light stabilizer), ultraviolet absorber, singlet oxygen quencher, etc. The HALS may be a high molecular weight HALS, a low molecular weight HALS, or a combination of a high molecular weight HALS and a low molecular weight HALS.

[0055] When the first layer contains a light stabilizer, it may contain only one type of light stabilizer or may contain two or more types of light stabilizers.

[0056] High Molecular Weight HALS In the present invention and this specification, "high molecular weight HALS" refers to a hindered amine-based light stabilizer having a weight-average molecular weight of more than 1,000. Examples of high molecular weight HALS include oligomer-type HALS such as poly[6-(1,1,3,3-tetramethylbutyl)imino-1,3,5-triazine-2,4-diyl][(2,2,6,6-tetramethyl-4-piperidyl)imino]hexamethylene[(2,2,6,6-tetramethyl-4-piperidyl)imino] and polycondensation product of dimethyl succinate-1-(2-hydroxyethyl)-4-hydroxy-2,2,6,6-tetramethylpiperidine. Examples of commercially available high molecular weight HALS include CHIMASSORB 944LD and TINUVIN 622LD manufactured by BASF Japan Ltd. It should be noted that "CHIMASSORB" and "TINUVIN" are both registered trademarks.

[0057] The weight average molecular weight (Mw) in the present invention and this specification is a value measured by gel permeation chromatography (GPC). In measurements using gel permeation chromatography (GPC), an HLC (registered trademark)-8220GPC (manufactured by Tosoh Corporation) can be used as a measuring device, and a series connection of one of the following columns can be used: TSKgel (registered trademark) Super HZM-M (4.6 mm ID x 15 cm, manufactured by Tosoh Corporation), Super HZ4000 (4.6 mm ID x 15 cm, manufactured by Tosoh Corporation), Super HZ3000 (4.6 mm ID x 15 cm, manufactured by Tosoh Corporation), and Super HZ2000 (4.6 mm ID x 15 cm, manufactured by Tosoh Corporation), and THF (tetrahydrofuran) can be used as the eluent. Measurement conditions can be set to a sample concentration of 0.2% by mass, a flow rate of 0.35 mL / min, a sample injection volume of 10 μL, and a measurement temperature of 40° C. A refractive index (RI) detector can be used as the detector. Calibration curves can be created using standard samples manufactured by Tosoh Corporation: "TSK standard, polystyrene," "F-40," "F-20," "F-4," "F-1," "A-5000," "A-2500," and "A-1000."

[0058] When the first layer contains a high-molecular-weight HALS, the content of the high-molecular-weight HALS in the first-layer-forming composition and the first layer is not particularly limited, and is preferably, for example, 0.2% by mass to 10% by mass relative to the total mass of the first layer. From the viewpoint of improving weather resistance, it is preferable that the content of the high-molecular-weight HALS in the first layer be 0.2% by mass or more relative to the total mass of the first layer. When the content of the high-molecular-weight HALS in the first layer is 10% by mass or less relative to the total mass of the first layer, a decrease in mechanical strength and the occurrence of blooming tend to be suppressed.

[0059] Low Molecular Weight HALS In the present invention and this specification, the term "low molecular weight HALS" refers to a hindered amine light stabilizer having a molecular weight of 1,000 or less (preferably 900 or less, more preferably 600 to 900). Examples of low molecular weight HALS include tris(2,2,6,6-tetramethyl-4-piperidyl)benzene-1,3,5-tricarboxylate, tris(2,2,6,6-tetramethyl-4-piperidyl)-2-acetoxypropane-1,2,3-tricarboxylate, tris(2,2,6,6-tetramethyl-4-piperidyl)-2-hydroxypropane-1,2,3-tricarboxylate, tris(2,2,6,6-tetramethyl-4-piperidyl)triazine-2,4,6-tricarboxylate, and tris(2,2,6,6-tetramethyl-4-piperidyl)benzene-1,3,5-tricarboxylate. Examples of commercially available low molecular weight HALS include tetrakis(2,2,6,6-tetramethyl-4-piperidyl)propane-1,1,2,3-tetracarboxylate, tetrakis(2,2,6,6-tetramethyl-4-piperidyl)1,2,3,4-butanetetracarboxylate, tetrakis(1,2,2,6,6-pentamethyl-4-piperidyl)1,2,3,4-butanetetracarboxylate, and bis(1,2,2,6,6-pentamethyl-4-piperidyl)2-(3,5-di-t-butyl-4-hydroxybenzyl)-2-n-butylmalonate. Examples of commercially available low molecular weight HALS include Adeka STAB LA-57 and Adeka STAB LA-52 manufactured by ADEKA Corporation, and TINUVIN 144 manufactured by BASF Japan. The above "ADK STAB" and "TINUVIN" are both registered trademarks.

[0060] When the first layer contains a low-molecular-weight HALS, the content of the low-molecular-weight HALS in the first layer is not particularly limited, and is preferably, for example, 0.2% by mass to 10% by mass relative to the total mass of the first-layer-forming composition or the first layer. From the viewpoint of improving weather resistance, it is preferable that the content of the low-molecular-weight HALS in the first layer be 0.2% by mass or more relative to the total mass of the first layer. When the content of the low-molecular-weight HALS in the first layer is 10% by mass or less relative to the total mass of the first layer, a decrease in mechanical strength and the occurrence of blooming tend to be suppressed.

[0061] Ultraviolet absorbers Examples of ultraviolet absorbers include 2-(2'-hydroxy-3',5'-di-t-butylphenyl)benzotriazole, 2-(3,5-di-t-amyl-2-hydroxyphenyl)benzotriazole, 2-(2'-hydroxy-5'-methyl-phenyl)benzotriazole, 2-(2'-hydroxy-5'-t-octylphenyl)benzotriazole, 2-(2'-hydroxy-3',5'-di-t-amylphenyl)benzotriazole, 2-[2'-hydroxy-3 '-(3'',4'',5'',6''-tetrahydro-phthalimidomethyl)-5'-methylphenyl]benzotriazole, 2,2'-methylenebis[4-(1,1,3,3-tetramethylbutyl)-6-(2H-benzotriazol-2-yl)phenol], 2-[2-hydroxy-3,5-bis(α,α-dimethylbenzyl)phenyl]-2H-benzotriazole, 2-(2-hydroxy-4-octyloxyphenyl)-2H-benzotriazole, 2-( benzotriazole-based ultraviolet absorbers such as 2H-benzotriazol-2-yl)-4-methyl-6-(3,4,5,6-tetrahydrophthalimidylmethyl)phenol, 2-hydroxy-4-methoxybenzophenone, 2,4-dihydroxybenzophenone, 2,2'-dihydroxy-4-methoxybenzophenone, 2,2'-dihydroxy-4,4'-dimethoxybenzophenone, 2-hydroxy-4-n-octoxybenzophenone, 2,2',4,4'-tetrahydroxybenzophenone, benzophenone-based ultraviolet absorbers such as hydroxybenzophenone, 4-dodecyloxy-2-hydroxybenzophenone, 3,5-di-t-butyl-4-hydroxybenzoylbenzoic acid n-hexadecyl ester, 1,4-bis(4-benzoyl-3-hydroxyphenoxy)butane, and 1,6-bis(4-benzoyl-3-hydroxyphenoxy)hexane; and cyanoacrylate-based ultraviolet absorbers typified by ethyl-2-cyano-3,3-diphenylacrylate.Examples of commercially available ultraviolet absorbers include TINUVIN 320, TINUVIN 328, TINUVIN 234, TINUVIN 1577, TINUVIN 622, and the IRGANOX series from BASF Japan Ltd., ADK STAB LA31 from ADEKA Corporation, and SEESORB 102, SEESORB 103, and SEESORB 501 from Shipro Kasei Co., Ltd. Note that the above-mentioned "TINUVIN," "IRGANOX," "ADK STAB," and "SEESORB" are all registered trademarks.

[0062] When the first layer contains an ultraviolet absorber, the content of the ultraviolet absorber in the first layer-forming composition and the first layer is not particularly limited, and is preferably, for example, 0.2% by mass to 10% by mass relative to the total mass of the first layer. From the viewpoint of improving weather resistance, it is preferable that the content of the ultraviolet absorber in the first layer be 0.2% by mass or more relative to the total mass of the first layer. When the content of the ultraviolet absorber in the first layer is 10% by mass or less relative to the total mass of the first layer, a decrease in mechanical strength and the occurrence of blooming tend to be suppressed.

[0063] Singlet oxygen quencher When the first layer contains a singlet oxygen quencher, the content of the singlet oxygen quencher in the first layer is not particularly limited, and is preferably, for example, 0.2% by mass to 10% by mass relative to the total mass of the first layer. From the viewpoint of improving weather resistance, it is preferable that the content of the singlet oxygen quencher in the first layer be 0.2% by mass or more relative to the total mass of the first layer. When the content of the singlet oxygen quencher in the first layer is 10% by mass or less relative to the total mass of the first layer, a decrease in mechanical strength and the occurrence of blooming tend to be suppressed.

[0064] When the first layer contains a light stabilizer, it may contain only one type of light stabilizer or may contain two or more types of light stabilizers.

[0065] The composition for forming the first layer may or may not contain a solvent. When the composition for forming the first layer contains a solvent, the solvent is not particularly limited, and examples thereof include water, an organic solvent, or a mixed solvent of water and an organic solvent. Examples of organic solvents include alcohols such as methanol, ethanol, n-propanol, i-propanol, and methoxypropanol; ketones such as acetone, methyl ethyl ketone, and cyclohexanone; tetrahydrofuran, acetonitrile, ethyl acetate, and toluene. Among these, ketones are preferred, and cyclohexanone is more preferred, from the viewpoint of drying speed. When the composition for forming the first layer contains a solvent, the content of the solvent in the composition is not particularly limited, and may be determined appropriately.

[0066] The composition for forming the first layer can be prepared by mixing the above components. The mixing method is not particularly limited, and examples thereof include a method of mixing by stirring. As a stirring means, a known stirring device can be used. Examples of the stirring device include mixers such as a paddle mixer and an impeller mixer. The stirring time can be set depending on the type of stirring device, the composition of the composition, etc.

[0067] For example, the composition for forming the first layer can be prepared by kneading a mixture of the particles and binder, and, if necessary, a solvent, additives, etc., while heating. The kneaded product can be obtained in any shape, such as a block, pellet, or filament usable for a 3D (three-dimensional) printer. The first layer can be obtained by molding the kneaded product into a desired shape using a known molding method such as extrusion molding, press molding, injection molding, in-mold molding, or 3D printing. The first layer can be formed from only one type of composition, or can be a combination of two or more parts with different compositions (e.g., a laminate). The same applies to the second layer.

[0068] For example, from the viewpoint of ease of handling, the thickness of the first layer is preferably 20.0 mm or less, more preferably 10.0 mm or less, and even more preferably 5.0 mm or less. From the viewpoint of mechanical properties, the thickness is preferably 1.0 mm or more, and more preferably 2.0 mm or more. When the first layer is a laminate, the thickness refers to the total thickness of the first layers constituting the laminate. This also applies to the second layer. The thickness of the first layer can be measured using a digital length measuring device, and can be, for example, the arithmetic average of measurements taken at nine randomly selected locations. The above description regarding thickness measurement also applies to the measurement of the thickness of the second layer and the measurement of the thickness of the radio wave absorber.

[0069] <Second Layer> <<d×√ε>> The second layer is a layer that satisfies Expression 1 (2.50≦d×√ε≦3.50) or Expression 2 (0.60≦d×√ε≦1.40). As a result of intensive research by the present inventors, it has been revealed that a second layer in which "d×√ε" is within the range of 3±0.50 or 1±0.40 allows the "surface reflection of the first layer," the "interfacial reflection at the interface between the first layer and the second layer," and the "reflection from the back surface of the second layer (e.g., the interface between the second layer and air)" to cancel each other out, thereby achieving an improvement in return loss that is difficult to achieve with the first layer alone. Therefore, as described above, the radio wave absorber can exhibit high return loss without relying on a metal layer. Furthermore, the first layer, which includes one or more particles selected from the group consisting of magnetic particles and dielectric particles and a binder, can contribute to the radio wave absorber exhibiting high return loss and high transmission loss.

[0070] In the above formulas 1 and 2, d is the thickness (unit: mm) of the second layer. The method for measuring the thickness of the second layer is as described above.

[0071] In the above formula, ε is the relative permittivity of the second layer. In the present invention and this specification, the relative permittivity of the second layer is the relative permittivity determined at a frequency F in the range of 55.0 GHz (gigahertz) to 95.0 GHz. The frequency F can be, for example, 76.5 GHz or 60.0 GHz. The relative permittivity can be measured by a known method, for example, by the method described in the Examples section below. When only the second layer is available, the relative permittivity of the second layer can be measured. Alternatively, the first layer can be removed by a known method from a laminated wave absorber in which the first and second layers are stacked, to extract only the second layer, and the relative permittivity of the second layer can be measured. The above description also applies to the measurement of the wave absorption rate of the second layer, which will be described later.

[0072] In the present invention and this specification, "d×√ε" is expressed as a unitless value.

[0073] When the second layer satisfies formula 1 (2.50≦d×√ε≦3.50), from the viewpoint of improving the reflection attenuation characteristics of the wave absorber, "d×√ε" of the second layer is 2.50 or more, preferably more than 2.50, and more preferably 2.55 or more, 2.60 or more, 2.65 or more, and 2.70 or more in this order. Furthermore, when the second layer satisfies formula 1 (2.50≦d×√ε≦3.50), from the viewpoint of improving the reflection attenuation characteristics of the wave absorber, "d×√ε" of the second layer is 3.50 or less, preferably less than 3.50, and more preferably 3.45 or less, 3.40 or less, 3.35 or less, 3.30 or less, 3.25 or less, 3.20 or less, 3.15 or less, 3.10 or less, 3.05 or less, and 3.00 or less in this order.

[0074] When the second layer satisfies formula 2 (0.60≦d×√ε≦1.40), from the viewpoint of improving the reflection attenuation characteristics of the wave absorber, "d×√ε" of the second layer is 0.60 or more, preferably 0.63 or more, and more preferably 0.65 or more, 0.70 or more, 0.75 or more, 0.80 or more, 0.85 or more, 0.90 or more, 0.95 or more, and 1.00 or more in that order. Furthermore, when the second layer satisfies formula 2 (0.60≦d×√ε≦1.40), from the viewpoint of improving the reflection attenuation characteristics of the wave absorber, "d×√ε" of the second layer is 1.40 or less, preferably 1.35 or less, and more preferably 1.20 or less, 1.15 or less, and 1.10 or less in that order.

[0075] From the viewpoint of ease of handling, the thickness of the second layer is preferably 10.0 mm or less, more preferably 5.0 mm or less, even more preferably 3.0 mm or less, and even more preferably 2.0 mm or less. From the viewpoint of mechanical properties, the thickness of the second layer is preferably 0.1 mm or more, preferably 0.3 mm or more, and more preferably 0.5 mm or more, 1.0 mm or more, and 1.5 mm or more in that order. In one embodiment, the second layer preferably satisfies Formula 1, more preferably a thickness of 1.0 mm or more that satisfies Formula 1, and even more preferably a thickness of 1.5 mm or more that satisfies Formula 1. In another embodiment, the second layer preferably has a thickness of less than 1.0 mm and satisfies Formula 2.

[0076] In the above-mentioned radio wave absorber, the second layer is a layer adjacent to the first layer. In the present invention and this specification, the term "adjacent" in relation to two layers includes a case where another layer is interposed between the two layers and a case where the two layers are in direct contact without any other layer interposed. Examples of layers that can be interposed between the first and second layers include layers that enhance adhesion between the layers, such as adhesive layers and pressure-sensitive adhesive layers. Such layers can be known adhesive layers, pressure-sensitive adhesive layers, etc. From the viewpoint of further improving the reflection attenuation characteristics of the above-mentioned radio wave absorber, it is preferable that the first layer is a layer that is in direct contact with the second layer. In other words, it is preferable that the first layer and the second layer are in direct contact without any other layer interposed therebetween. From the viewpoint of further improving the radio wave absorption characteristics, it is preferable that the above-mentioned radio wave absorber consists of a first layer and a second layer.

[0077] The relative dielectric constant of the second layer may be, for example, 2.0 or more and 5.5 or less, but is not particularly limited thereto, as long as "d × √ε" falls within the range described above.

[0078] The second layer can be, for example, a layer containing a resin. The relative permittivity of the second layer can be adjusted, for example, by the type of resin. Examples of resins that can be used for the second layer include polyethylenes such as polytetrafluoroethylene (abbreviated as PTFE), polyamide (abbreviated as PA), polypropylene (abbreviated as PP), and high-density polyethylene (abbreviated as HDPE). However, since the "d × √ε" of the second layer can be changed by changing the thickness d, a second layer can be formed in which "d × √ε" satisfies Equation 1 or Equation 2 by changing the thickness d depending on the type of resin. The second layer may contain only one type of resin, or two or more types of resins.

[0079] The resin content of the second layer may be, for example, 70% by mass or more and 100% by mass or less.

[0080] The resin for the second layer may be a glass fiber reinforced resin. An example of a commercially available glass fiber reinforced resin is polyamide (PA) 6 resin (product name: Nylon 6A1030GFL15) manufactured by Unitika Ltd.

[0081] The second layer may be a layer consisting solely of resin, or may contain resin and one or more additives. An example of an additive for the second layer is hollow particles. The dielectric constant of the second layer tends to decrease as the amount of hollow particles added increases. In this invention and this specification, "hollow particles" refers to particles having voids from the surface to the interior of the particle, where no material constituting the particle is present. The shape of the hollow particles is not particularly limited, and the shape, size, and number of voids are also not particularly limited. Commercially available hollow particles can be used, and hollow particles prepared by known methods can also be used. An example of a commercially available product is the Glass Bubbles series manufactured by 3M (e.g., model number iM30K). The hollow particle content of the second layer can be, for example, 30% by mass or less, 20% by mass or less, 10% by mass or less, 5% by mass or less, or 1% by mass or less, or even 0% by mass.

[0082] <<Radio Wave Absorption Rate>> From the viewpoint of further improving the reflection attenuation characteristics of the radio wave absorber, the radio wave absorption rate of the second layer is preferably 10.0% or less, and more preferably 9.0% or less, 8.0% or less, 7.0% or less, or 6.0% or less. 5.0% or less, 4.0% or less, 3.0% or less, 2.0% or less, or 1.0% or less is more preferable in this order. The radio wave absorption rate of the second layer can be, for example, 0.0% or more, more than 0.0%, or 0.1% or more. The lower the radio wave absorption rate of the second layer, the more likely it is that the difference in the intensity ratio between the "surface reflection of the first layer" and the "interface reflection at the interface between the first layer and the second layer" can be reduced, thereby suppressing a decrease in the reflection attenuation characteristics.

[0083] In the present invention and this specification, the radio wave absorptance of the second layer is defined as the radio wave absorptance determined at a frequency (Frequency) F in the range of 55.0 GHz (gigahertz) to 95.0 GHz. Frequency F can be, for example, 76.5 GHz or 60.0 GHz. The radio wave absorptance is determined by measuring the transmittance (unit: %) and reflectance (unit: %) of the second layer using the following formula: Radio wave absorptance (%) = 100 - (Transmittance + Reflectance). The "transmittance" is the percentage value of the S-parameter S21 measured by the free-space method, with an incident angle of 0° and an ambient temperature of 15 to 35°C. The "reflectance" is the percentage value of the S-parameter S11 measured by the above measurement. Measurements can be performed using a known vector network analyzer and horn antenna. Specific examples of measurement methods include those described in the Examples section below.

[0084] The radio wave absorption rate of the second layer tends to decrease as the thickness of the second layer decreases. Furthermore, the addition of a filler to the second layer tends to increase the radio wave absorption rate, and the radio wave absorption rate also tends to increase as the amount of filler added increases. Examples of fillers include carbon-based particles, which were previously exemplified as dielectric particles. From the viewpoint of reducing the radio wave absorption rate of the second layer, the content of the carbon-based particles in the second layer is preferably 2.0% by mass or less, more preferably 1.5% by mass or less, further preferably 1.0% by mass or less, 0.5% by mass or less, and particularly preferably 0% by mass.

[0085] In one embodiment, the second layer may contain carbon black and may further contain a resin. A second layer containing carbon black is black and therefore tends to be less susceptible to color change over time. This is preferable from the viewpoint of maintaining the appearance quality of the wave absorber. Furthermore, when the second layer-forming composition is highly transparent or white, color change is likely to occur during the process of preparing the second layer-forming composition, the process of molding the second layer using the second layer-forming composition, or other processes, due to the adhesion or incorporation of trace amounts of coloring components. While such color change can be suppressed by cleaning the production line, this requires time. In contrast, a second layer-forming composition containing carbon black is black and therefore tends to be less susceptible to color change. This is preferable from the viewpoint of improving production efficiency, as it leads to the elimination of cleaning or the completion of cleaning in a short time. The wave absorptivity of the second layer containing carbon black is also preferably within the above-described range from the viewpoint of further improving the reflection attenuation characteristics of the wave absorber. The carbon black content in the second layer containing carbon black is more than 0% by mass, and from the viewpoint of maintaining a low radio wave absorption rate, it is preferably 2.0% by mass or less, more preferably 1.5% by mass or less, even more preferably 1.0% by mass or less, and even more preferably 0.5% by mass or less.

[0086] <<Reflectance Difference>> The absolute value of the difference (A-B) between the reflectance A of the first layer and the reflectance B of the second layer, i.e., the reflectance difference, can be less than 6.0 dB or greater than 6.0 dB. From the viewpoint of further improving the return loss characteristics, it is preferably less than 6.0 dB, more preferably 5.5 dB or less, and even more preferably 5.0 dB or less, 4.5 dB or less, and 4.0 dB or less in that order. The reflectance difference can be, for example, 0.0 dB or greater, 0.1 dB or greater, or 0.5 dB or greater. The reflectance A of the first layer and the reflectance B of the second layer may have any of the relationships "A=B," "A>B," or "A<B." The absolute values ​​of the reflectance A of the first layer and the reflectance B of the second layer can be, for example, 0.5 or greater and 50.0 or less. However, they are not limited to these ranges. The reflectance of each layer can be controlled by the thickness, composition, etc. of each layer.

[0087] In the present invention and this specification, the reflectance of the first layer and the reflectance of the second layer are reflectances expressed in dB and determined at a frequency F in the range of 55.0 GHz (gigahertz) to 95.0 GHz. The frequency F can be, for example, 76.5 GHz or 60.0 GHz. For a method of measuring the reflectance, the above description of the method of measuring radio wave absorption can be referred to. When only the second layer is available, the reflectance of the second layer can be measured. Alternatively, the first layer can be removed from a laminated radio wave absorber in which the first and second layers are stacked using a known method to extract only the second layer, and the reflectance of the second layer can be measured. The reflectance of the first layer can be determined by measuring the surface on which the first layer is exposed (i.e., the surface of the first layer opposite the side adjacent to the second layer) of the laminated radio wave absorber in which the first and second layers are stacked, facing the incident surface. Alternatively, when only the first layer is available, the reflectance of the first layer can be measured.

[0088] <<Surface Roughness>> In the radio wave absorber, the surface of the second layer opposite the first layer side may be a surface that is irradiated with radio waves transmitted from a radio wave transmission source. For example, when the surface of the second layer opposite the first layer side is the outermost surface of one side of the radio wave absorber, this surface is a surface that is irradiated with radio waves transmitted from the radio wave transmission source. Radio wave irradiation may be exemplified by a form in which radio waves transmitted from the radio wave transmission source directly strike the outermost surface of the radio wave absorber, or a form in which reflected waves of radio waves transmitted from the radio wave transmission source strike the outermost surface of the radio wave absorber. Specific examples include a case in which radio waves transmitted from the radio wave transmission source directly strike the outermost surface of the radio wave absorber, a case in which reflected waves transmitted from the radio wave transmission source and reflected back by an object strike the outermost surface of the radio wave absorber, and a case in which reflected waves reflected back by the object strike a metal plate or the like (e.g., a vehicle body structure, a frame, etc.) disposed behind an object and then reflect therefrom before striking the outermost surface of the radio wave absorber. The surface roughness (ten-point average roughness Rz) of the surface of the second layer opposite to the first layer side, which is the surface that can become the outermost surface, is preferably 700.0 μm or less, from the viewpoint of further improving the reflection attenuation characteristics of the radio wave absorber, and is preferably 650.0 μm or less, 600.0 μm or less, 550.0 μm or less, 500.0 μm or less, 450.0 μm or less, 400.0 μm or less, 350.0 μm or less, 300.0 μm or less, 250.0 μm or less, 200.0 μm or less, 150.0 μm or less The surface roughness is preferably in the order of 100.0 μm or less, 95.0 μm or less, 90.0 μm or less, 85.0 μm or less, 80.0 μm or less, 75.0 μm or less, 70.0 μm or less, 65.0 μm or less, 60.0 μm or less, 55.0 μm or less, 50.0 μm or less, 45.0 μm or less, 40.0 μm or less, 35.0 μm or less, 30.0 μm or less, 25.0 μm or less, 20.0 μm or less, 15.0 μm or less, 10.0 μm or less, 5.0 μm or less, 3.0 μm or less, and 1.0 μm or less. The surface roughness can be 0.0 μm or more, more than 0.0 μm, or 0.1 μm or more.

[0089] The "ten-point average roughness Rz" in this specification and the present invention is the Rz specified in JIS B0601:1994 and is generally referred to as "Rzjis." The Rz of the surface of the second layer opposite the first layer side is measured using a three-dimensional shape measuring instrument. For example, in the measurement of the Rz of the surface of the second layer opposite the first layer side described in the Examples section below, a Keyence VR-3200 three-dimensional shape measuring instrument was used. The Rz of the surface of the second layer opposite the first layer side was measured in a randomly selected area on the surface to be measured. The measurement range was 18 mm x 6 mm, the measurement line length was 18 mm, and measurements were taken on three parallel lines (hence, the number of measurements N = 3), with a 3 mm spacing between two adjacent measurement lines. The arithmetic mean of the measurement results for N = 3 was taken as the Rz (surface roughness) of the surface of the second layer opposite the first layer side. With respect to the three parallel lines described above, "parallel" does not necessarily mean "parallel" in the strict sense, but includes a range of error that is normally acceptable in the technical field to which the present invention pertains. The range of error may mean, for example, a range of ±10° or less from strict parallelism.

[0090] <<Interface Roughness>> The surface of the second layer on the first layer side (hereinafter also referred to as "interface") may have a ten-point average roughness Rz (interface roughness) of 500.0 μm or less or more than 500.0 μm. From the viewpoint of further improving the return loss characteristics, it is preferably 500.0 μm or less, more preferably 450.0 μm or less, 400.0 μm or less, 350.0 μm or less, 300.0 μm or less, 250.0 μm or less, 200.0 μm or less, 150.0 μm or less, 100.0 μm or less, The interface roughness is preferably 0.0 μm or less, 95.0 μm or less, 90.0 μm or less, 85.0 μm or less, 80.0 μm or less, 75.0 μm or less, 70.0 μm or less, 65.0 μm or less, 60.0 μm or less, 55.0 μm or less, 50.0 μm or less, 45.0 μm or less, 40.0 μm or less, 35.0 μm or less, 30.0 μm or less, 25.0 μm or less, 20.0 μm or less, 15.0 μm or less, 10.0 μm or less, and 5.0 μm or less, in that order. The interface roughness can be 0.0 μm or more, more than 0.0 μm, or 0.1 μm or more.

[0091] The Rz of the surface (interface) of the second layer on the first layer side, i.e., the interface roughness, is determined from a cross-sectional SEM image obtained by a scanning electron microscope (SEM). A method for determining the interface roughness is described below.

[0092] (1) Preparation of a sample for cross-section observation A sample for cross-section observation is prepared by cutting out the sample from a randomly determined position of the radio wave absorber to be measured. The preparation of the sample for cross-section observation is performed by FIB (Focused Ion Beam) processing using a gallium ion (Ga+) beam.

[0093] (2) Identification of Interface The prepared cross-sectional observation sample is observed using an SEM, and a cross-sectional SEM image is taken. A field emission scanning electron microscope (FE-SEM) is used as the scanning electron microscope. For example, in the measurement of Rz on the surface of the first layer side of the second layer described in the Examples section below, a Hitachi FE-SEM S4800 was used as the FE-SEM. Cross-sectional SEM images are taken at randomly selected positions on the same cross-sectional observation sample, except that the first and second layers are selected so that the imaged areas do not overlap, resulting in a total of four images. The cross-sectional SEM images are secondary electron images (SE images) taken at an accelerating voltage of 5 kV, a magnification of 180x, and a resolution of 960 vertical pixels by 1280 horizontal pixels. Since each layer included in the radio wave absorber can be identified in the cross-sectional SEM image, for example, direct contact between the first and second layers can also be identified from the cross-sectional SEM image. A method for identifying the surface (interface) of the second layer on the first layer side will be described below, taking the case where the first and second layers are in direct contact as an example. Even when another layer is interposed between the first and second layers, the surface (interface) of the second layer on the first layer side can be identified using a method similar to that described below. The captured cross-sectional SEM image is imported into WinROOF, an image processing software manufactured by Mitani Corporation, and the entire region of the cross-sectional SEM image is selected. The interface between the first and second layers is identified using the following method. The cross-sectional SEM image is digitized to create image brightness data in the thickness direction (consisting of three components: thickness coordinate, width coordinate, and brightness). During digitization, the cross-sectional SEM image is divided into 1280 parts in the width direction, processed at 8-bit brightness to obtain 256 grayscale data, and the image brightness at each divided coordinate point is converted to a predetermined grayscale value. Next, a luminance curve is created by plotting the average luminance in the width direction at each coordinate point in the thickness direction (i.e., the average luminance at each coordinate point divided into 1280) on the vertical axis and the coordinate in the thickness direction on the horizontal axis for the obtained image luminance data. The created luminance curve is differentiated to create a differential curve, and the coordinates of the boundary between the first and second layers are identified from the peak position of the created differential curve.The position corresponding to the identified coordinates on the cross-sectional SEM image is taken as the interface between the first and second layers. Rz measurements are performed in a randomly selected area of ​​this interface, with a measurement range of 18 mm x 6 mm, a measurement line length of 18 mm, and measurements on three parallel lines (hence, the number of measurements N = 3), with a distance of 3 mm between two adjacent measurement lines. The arithmetic mean of the N = 3 measurement results is taken as the Rz (interface roughness) on the surface of the second layer facing the first layer. With regard to the three parallel lines, "parallel" does not necessarily mean strictly parallel, but also includes the range of error normally accepted in the technical field to which the present invention pertains. The range of error can, for example, mean a range of less than ±10° from strict parallelism.

[0094] For example, if the surface of the mold (e.g., a metal mold) used to mold the second layer composition to obtain the second layer is rough, the uneven shape of the roughness will be transferred to the surface of the second layer, resulting in a rough surface of the second layer. Therefore, one example of a method for controlling the surface roughness and interfacial roughness of the second layer is to adjust the uneven shape of the molding surface of the mold used to mold the second layer. For example, multiple irregularities can be formed on the molding surface of the mold by chemical etching. Multiple irregularities can also be formed on the molding surface of the mold by a three-dimensional (3D) printer. The surface roughness and / or interfacial roughness can also be controlled by sandblasting the surface of the molded second layer to form irregularities.

[0095] The second layer can be formed using a second layer-forming composition prepared by mixing the above components. The above description of the first layer can be referenced for the second layer-forming composition and the method for forming the second layer. In one embodiment, a commercially available resin plate itself can be used as the second layer, and the second layer can be cut out into the desired shape and size from a commercially available resin plate.

[0096] The first layer and the second layer can be laminated by a known method. For example, the radio wave absorber can be formed as a laminate in which the first layer and the second layer are laminated without any other layer therebetween by hot pressing, injection molding, or the like. Alternatively, the first layer and the second layer can be laminated by bonding them together via a layer that enhances interlayer adhesion, such as an adhesive layer or a pressure-sensitive adhesive layer. The thickness of the layer that enhances interlayer adhesion can be in the range of 1 μm to 1 mm, for example. The thickness of the layer that enhances interlayer adhesion can be measured by the method described above.

[0097] The shape of the radio wave absorber is not particularly limited, and may be a planar shape, a three-dimensional shape, or a combination of a planar portion and a three-dimensional portion. Examples of planar shapes include a plate shape. A plate shape can also be referred to as a sheet shape, a film shape, or the like. Examples of three-dimensional shapes include a tubular shape (cylindrical shape, square tube shape, etc.), a horn shape, etc.

[0098] The radio wave absorber can be incorporated into various articles to which it is desired to impart radio wave absorbing properties.

[0099] Meanwhile, automotive radar, which has been attracting attention in recent years, is a radar that uses radio waves in the millimeter wave frequency band. Millimeter waves are electromagnetic waves with frequencies between 30.0 GHz and 300.0 GHz. The radio wave absorber described above is suitable as a radio wave absorber to be incorporated into the front side (the side on which radio waves entering from outside) of a radio wave transmitting / receiving unit in an automotive radar, for example, in order to reduce the side lobes of the automotive millimeter wave radar.

[0100] The radio wave absorber can also be incorporated into radio wave absorbing articles used in wireless technology fields, such as motion sensors. The radio wave absorber is suitable as a radio wave absorber for improving recognition accuracy by removing unwanted radio waves in wireless devices, such as internal sensors in mobile phones and biometric sensors. Such a radio wave absorber can be suitably used, for example, in a radio wave absorbing article for the 55.0 to 66.0 GHz band. A radio wave absorbing article is an article that has radio wave absorption properties for radio waves of one or more frequencies, and the radio wave absorption properties can be achieved by incorporating a radio wave absorber as at least a part of the article. A radio wave absorbing article for the 55.0 to 66.0 GHz band is an article that has radio wave absorption properties for radio waves of one or more frequencies in the 55.0 to 66.0 GHz frequency band. Examples of such articles include the various wireless devices mentioned above. By incorporating the radio wave absorber into such a radio wave absorbing article, unwanted radio waves can be removed, improving recognition accuracy.

[0101] Furthermore, depending on the type of radar the radio wave absorber is used for, the bandwidth may be broad. For example, a 60 GHz band radar may be used for a bandwidth of 7.0 GHz, ranging from 57.0 to 64.0 GHz. For such broadband radar applications, a radio wave absorber that can accommodate broadband applications can be produced by forming the first layer by mixing multiple types of particles selected from the group consisting of magnetic particles and dielectric particles.

[0102] <Thermal Conductivity Ratio> In one embodiment, the thermal conductivity ratio (k1 / k2) of the radio wave absorber can satisfy the following formula 3: 1.5≦k1 / k2. In formula 3, k1 is the thermal conductivity of the first layer, k2 is the thermal conductivity of the second layer, and the unit of thermal conductivity is W / m·k. A radio wave absorber that satisfies formula 3, i.e., a radio wave absorber having a thermal conductivity ratio (k1 / k2) of 1.5 or more, is preferred because it has excellent adhesion between the first layer and the second layer. This is presumably because laminating the first layer and the second layer that satisfy formula 3 promotes melting and compatibility at the interface between the two layers, thereby increasing the bonding strength. The thermal conductivity ratio (k1 / k2) can be, for example, 3.0 or less, but is not limited to this value. For example, k1 can be 0.50 W / m·k or more and 1.00 W / m·k or less, and k2 can be 0.10 W / m·k or more and 0.40 W / m·k or less. However, the thermal conductivity of each layer can be controlled by the thickness, composition, etc. of each layer.

[0103] In the present invention and this specification, "thermal conductivity" refers to the thermal conductivity measured by the laser flash method. The first and second layers are removed from the radio wave absorber by a known method. Measurement samples are cut out from each of the removed layers. The thermal conductivity measured by the laser flash method using these measurement samples can be used as the thermal conductivity of each layer. Specific examples of methods for measuring thermal conductivity include the methods described in the Examples section below.

[0104] [Article] One aspect of the present invention relates to an article including the radio wave absorber. The article is not particularly limited as long as it includes the radio wave absorber according to one aspect of the present invention, and other configurations of the article are not particularly limited, and publicly known techniques related to radio wave absorbing articles can be applied.

[0105] In one aspect, the article can include a radio wave emitting source and the radio wave absorber. In one aspect, the frequency F described above can be the frequency of radio waves emitted by the radio wave emitting source.

[0106] An example of an article having a radio wave transmission source is a radar. A radar is a device that has a radio wave transmission source and measures the direction of an object's location, the distance to the object, etc. by receiving reflected waves of radio waves transmitted from the radio wave transmission source. Specific examples of radar include automotive radar, marine radar, and radar for aircraft traffic control. Specific examples of the above article include wireless devices such as internal sensors in mobile phones and biometric sensors. The radio wave transmission source may be disposed, for example, within a radio wave transmitting / receiving unit. Regarding the structure of the above article, details of the radio wave transmission source, etc., known techniques relating to articles having a radio wave transmission source and a radio wave absorber can be applied.

[0107] The present invention will be described below based on examples. However, the present invention is not limited to the embodiments shown in the examples. The steps and evaluations described below were performed in an air atmosphere unless otherwise specified. Furthermore, the steps and evaluations described below were performed at an ambient temperature of 23°C ± 1°C unless otherwise specified.

[0108] [First layer particles] The magnetic particles and dielectric particles shown in the table below are as follows: Magnetic particle 1: In formula 1, A = Sr, x = 1.65 Magnetic particle 2: In formula 1, A = Sr, x = 0.75 Dielectric particle: Carbon black manufactured by Asahi Carbon Co., Ltd. (trade name: Asahi #50)

[0109] <Confirmation of Crystal Structure> The crystal structure of the magnetic material constituting each of magnetic particles 1 and 2 was confirmed by X-ray diffraction analysis. The measurement device used was a powder X-ray diffractometer, X'Pert Pro, manufactured by PANalytical. The measurement conditions are as follows: - Measurement Conditions - X-ray source: CuKα radiation [wavelength: 1.54 Å (0.154 nm), output: 40 mA, 45 kV] Scan range: 20°<2θ<70° Scan interval: 0.05° Scan speed: 0.33° / min

[0110] As a result of the above X-ray diffraction analysis, it was confirmed that magnetic particles 1 and 2 have a magnetoplumbite-type crystal structure and are single-phase magnetoplumbite-type hexagonal ferrite particles that do not contain any crystal structure other than the magnetoplumbite-type.

[0111] <Confirmation of Composition> The composition of the magnetic material constituting each of magnetic particles 1 and 2 was confirmed by high-frequency inductively coupled plasma optical emission spectroscopy. Specifically, confirmation was performed by the following method. A container (beaker) containing 12 mg of magnetic particles and 10 mL of a 4 mol / L hydrochloric acid aqueous solution was held on a hot plate set at 120°C for 3 hours to obtain a solution. 30 mL of pure water was added to the obtained solution, and then filtered using a membrane filter with a filter pore size of 0.1 μm. Elemental analysis of the filtrate thus obtained was performed using a high-frequency inductively coupled plasma optical emission spectroscopy analyzer (Shimadzu Corporation, ICPS-8100). Based on the results of the obtained elemental analysis, the content of each atom relative to 100 atomic % of iron atoms was determined. The composition of the magnetic material was then confirmed based on the obtained content. As a result, it was confirmed that the composition of each of magnetic particles 1 and 2 was a composition in which A in Formula 1 was Sr and x was the value described above.

[0112] [Resin of First Layer] In the table below, the resins listed in the column for resin of the first layer are as follows: PA6: Nylon 6A1030GFL15 manufactured by Unitika Ltd. PP: Polypropylene MA3H manufactured by Japan Polypropylene Corporation PBT: Polybutylene terephthalate 1401X06 manufactured by Toray Industries, Inc.

[0113] [Resin of Second Layer] In the table below, the resins listed in the column for resin of the second layer are as follows: PTFE: Polytetrafluoroethylene sheet, product number CIRAT, manufactured by Misumi Corporation PA6: Nylon 6A1030GFL15, manufactured by Unitika Ltd. PA6(1): Nylon PA6A1030GFL15 SR381, manufactured by Unitika Ltd. PP: Polypropylene MA3H, manufactured by Japan Polypropylene Corporation HDPE: Suntec HDJ340, manufactured by Asahi Kasei Corporation

[0114] PA6(1) (Nylon PA6A1030GFL15 SR381 manufactured by Unitika Ltd.) is a black resin with a carbon black content of approximately 1% by mass, while PA6 (Nylon 6A1030GFL15 manufactured by Unitika Ltd.) is a white resin.

[0115] [Carbon-based particles in second layer] In the tables below, the second layers of Examples in which a value other than 0 mass% is entered in the "CB content" column contain carbon black (abbreviation: CB) (product name: Asahi #50) manufactured by Asahi Carbon Co., Ltd. at the content entered in the "CB content" column.

[0116] [Hollow particles in second layer] In the tables below, the second layer of examples in which a value other than 0 mass% is entered in the "GB content" column contains hollow particles 3M Glass Bubbles (abbreviation: GB) iM30K at the content entered in the "GB content" column.

[0117] [Examples 1-6, 11-13, 24, 25, 27-29, Comparative Examples 1-10, 16, and 17] <Formation of First Layer> A powder of particles (magnetic particles or dielectric particles) shown in the table below was added to a resin shown in the table in an amount such that the powder packing rate of the particles in the first layer to be formed was the value shown in the table. The mixture was kneaded using a twin-screw kneader and then cut using a pelletizer to obtain a composition for forming the first layer (pellets for injection molding). The resulting pellets were molded using an injection molding machine to form a first layer as a flat molded product with the thickness shown in the table. The thickness of the formed first layer was determined as the arithmetic mean of measurements taken at nine randomly selected locations using a digital length measuring device (Mitutoyo Corporation, Litematic® VL-50A). The thickness of each first layer was confirmed to be the value shown in the table below.

[0118] <Formation of Second Layer and Preparation of Laminate (Radio Wave Absorber)> For Examples and Comparative Examples in which "PTFE" is listed in the column for the resin of the second layer in the tables below, a commercially available polytetrafluoroethylene sheet (details as described above) having the thickness listed in the tables was used as the second layer. This second layer was laminated with the first layer by hot pressing. For other Examples and Comparative Examples, the resins listed in the tables below were kneaded using a twin-screw kneader and then cut using a pelletizer to obtain a composition for forming the second layer (pellets for injection molding). The obtained pellets were molded using an injection molding machine to obtain a flat-plate molded product having the thickness listed in the tables, which was then laminated with the first layer. As the injection molding mold for forming the second layer, a mold with no chemical etching applied to the molding surface was used. For Comparative Examples in which "none" is listed in the column for the second layer, the first layer alone was subjected to the following evaluation of the radio wave absorber.

[0119] [Examples 7 to 10] Radio wave absorbers were produced by the method previously described for Example 1, etc., except that a mold in which a plurality of irregularities were provided by chemical etching on the molding surface for molding the surface of the second layer opposite to the first layer side was used as the injection molding mold for forming the second layer. The depths of the recesses of the irregularities in the molds used in Examples 7 to 10 were as follows: Example 7: about 100 μm, Example 8: about 300 μm, Example 9: about 500 μm, Example 10: about 750 μm

[0120] [Comparative Example 11] A first layer was formed by the same method as described above for Example 1, etc., except that carbon fiber (abbreviation: CF) (trade name Plastron PP-CF20, manufactured by PolyPlastics Corporation) was used as the particles for the first layer. CF is a conductive carbon fiber, and does not fall under the category of magnetic particles or dielectric particles. The first single layer formed in this way was subjected to the following evaluation of the wave absorber.

[0121] Comparative Example 12 A first layer was formed by the method described in Comparative Example 11. A second layer was laminated on the first layer thus formed by the method described in Example 1 and the like.

[0122] Comparative Example 13 A metal plate (aluminum plate) having a thickness shown in the table below was used as the first layer. This single first layer was subjected to the following evaluation of the wave absorber.

[0123] Comparative Example 14 A metal plate (aluminum plate) having the thickness shown in the table below was used as the first layer. A second layer was laminated on this first layer by the method described above for Example 1, etc.

[0124] Two wave absorbers having a second layer were produced by the above method. From one wave absorber, the first layer was peeled off by a known method and only the second layer was evaluated as described below, while from the other wave absorber, the first layer was not peeled off and the laminate was used as it was for the following wave absorber evaluation.

[0125] [Evaluation method] <Evaluation of second layer only> (Thickness measurement) The thickness was determined as the arithmetic mean of measurements taken at nine randomly selected points on the second layer using a digital length measuring instrument (Mitutoyo Corporation, Litematic (registered trademark) VL-50A). The thickness of each second layer was confirmed to be the value shown in the table below.

[0126] (Measurement of Relative Dielectric Constant, Calculation of d × √ε) The relative dielectric constant of the second layer was measured by the following method. Using a Keysight vector network analyzer (product name: N5225B) and a Keycom horn antenna (product name: RH12S23) as measuring devices, S parameters were measured by the free space method with an incident angle of 0°, a sweep frequency of 60 GHz to 90 GHz, and the surface of each second layer opposite to the side facing the first layer facing the incident side. The relative dielectric constant at a frequency of 76.5 GHz was calculated using Keysight's N-1500 material constant calculation software. The d × √ε of the second layer was calculated from the relative dielectric constant thus determined and the thickness measured by the method described above.

[0127] (Radio wave absorption rate, reflectance B) The transmittance was calculated as a percentage of the S parameter S21 at a frequency of 76.5 GHz measured using the above method, and the reflectance was calculated as a percentage of the S parameter S11. From the transmittance and reflectance thus calculated, the radio wave absorption rate at a frequency of 76.5 GHz was calculated using the formula described above. The measured value (unit: dB) of the S parameter S11 was taken as the reflectance B of the second layer.

[0128] <Reflectivity A of First Layer> Using a vector network analyzer (product name: N5225B) manufactured by Keysight and a horn antenna (product name: RH12S23) manufactured by Keycom as measuring devices, S parameters were measured by the free space method with an incident angle of 0°, a sweep frequency of 60 GHz to 90 GHz, and the surface of each wave absorber on which the first layer was exposed (i.e., the surface of the first layer opposite to the side adjacent to the second layer) facing the incident surface. The measured value (unit: dB) obtained as the S parameter S11 was taken as the reflectivity A of the first layer.

[0129] <Evaluation of Radio Wave Absorber> (Surface Roughness of Second Layer) For the outermost surface on the second layer side of each radio wave absorber (i.e., the surface of the second layer opposite to the first layer side), the surface roughness (ten-point average roughness Rz) was determined by the method described above using a three-dimensional shape measuring instrument VR-3200 manufactured by Keyence Corporation as the three-dimensional shape measuring instrument.

[0130] (Radio Wave Absorption Characteristics) Using a Keysight vector network analyzer (product name: N5225B) and a Keycom horn antenna (product name: RH12S23) as measuring devices, S parameters were measured by the free space method with an incident angle of 0° and a sweep frequency of 60 GHz to 90 GHz, with the outermost surface of the second layer of each radio wave absorber (i.e., the surface of the second layer opposite the first layer) facing the incident side. In this way, the S parameters S21 and S11 at a frequency of 76.5 GHz were determined. The larger the absolute value of S21, the better the transmission attenuation characteristics. The larger the absolute value of S11, the better the reflection attenuation characteristics. For the comparative radio wave absorber with a single first layer, S parameters were measured with a randomly selected surface of the first layer facing the incident side.

[0131] [Examples 14 to 17] Two wave absorbers were produced for each example by the method previously described for Example 1, etc., except that, when preparing the second layer-forming composition, carbon-based particles (details as described above) were mixed in an amount such that the CB content in the second layer to be formed would be the value shown in the table below. Using each of the two wave absorbers, various evaluations were carried out by the methods previously described.

[0132] [Examples 18 to 21] Two wave absorbers were produced for each example by the method previously described for Example 1, etc., except that, when preparing the second layer-forming composition, hollow particles (details as described above) were mixed in an amount such that the GB content in the second layer to be formed would be the value shown in the table below. Using each of the two wave absorbers, various evaluations were carried out by the methods previously described.

[0133] Example 26 Two wave absorbers were produced by the same method as described above for Example 1, etc., except that PA6(1) (Nylon PA6A1030GFL15 SR381 manufactured by Unitika Ltd.) was used as the resin for the second layer. Using each of the two wave absorbers, various evaluations were carried out by the methods described above.

[0134] [Examples 22 and 23] Two wave absorbers were produced for each of Examples 22 and 23 by the method previously described for Example 1, etc. Using each of the two wave absorbers, various evaluations were carried out by the methods previously described. However, as the S parameters, S parameters at a frequency of 60.0 GHz were obtained.

[0135] [Comparative Example 15] A first layer was formed by the method described in Examples 22 and 23. The single first layer thus formed was used as a radio wave absorber, and the radio wave absorption characteristics of the radio wave absorber were evaluated by the method described above. S parameters were measured with a randomly selected surface of the first layer facing the incident side. The S parameters were determined at a frequency of 60.0 GHz.

[0136] The results are shown in the table below.

[0137]

[0138]

[0139]

[0140]

[0141]

[0142]

[0143]

[0144] Among the examples in the above table, Examples 27 to 29 are examples in which the second layer satisfies Expression 2 (0.60≦d×√ε≦1.40), and Examples 1 to 26 are examples in which the second layer satisfies Expression 1 (2.50≦d×√ε≦3.50). From the results shown in the above table, it can be confirmed that the radio wave absorbers of Examples 1 to 29, which have a first layer containing one or more types of particles selected from the group consisting of magnetic particles and dielectric particles and a binder, and a second layer which satisfies Expression 1 or 2, have an absolute value of S11 of 8.0 dB or more and exhibit excellent reflection attenuation characteristics. Furthermore, from the results shown in the above table, it can also be confirmed that the radio wave absorbers of Examples 1 to 29 exhibit excellent transmission attenuation characteristics.

[0145] Example 30 Two wave absorbers were produced by the same wave absorber production method as in Example 6, except that the first layer and the second layer were bonded together using a commercially available adhesive. The wave absorber produced in this manner had an adhesive layer with a thickness of 0.1 mm between the first layer and the second layer. Using each of the two wave absorbers, various evaluations were carried out using the methods described above.

[0146] [Examples 31 and 32] Two wave absorbers were produced for each Example by the same method for producing a wave absorber as in Example 6, except that a mold in which a plurality of irregularities were formed by chemical etching on the molding surface for molding the surface of the second layer on the first layer side was used as the injection molding mold for the second layer. Using each of the two wave absorbers, various evaluations were carried out by the methods described above. The depths of the recesses in the irregularities in the molds used in Examples 31 and 32 were as follows: Example 31: about 500 μm, Example 32: about 600 μm

[0147] [Interface Roughness] For Example 6 and Examples 30 to 32, the Rz (interface roughness) of the surface of the second layer on the first layer side was also determined to evaluate the radio wave absorber. An FE-SEM S4800 manufactured by Hitachi, Ltd. was used as the FE-SEM, and the interface roughness was determined by the method described above.

[0148]

[0149] [Example 33, Example 34] Except for changing the resin of the first layer, the particle filling rate of the first layer, the resin of the second layer, and the thickness of the second layer as shown in the table below, two wave absorbers were produced for each example by the same wave absorber production method as in Example 6. Using each of the two wave absorbers, various evaluations were carried out by the methods described above.

[0150] For Examples 6, 33 and 34, the interlayer adhesion strength was evaluated and the thermal conductivity ratio (k1 / k2) was measured as evaluations of the radio wave absorbers.

[0151] [Interlayer Adhesion] The interlayer adhesion between the first layer and the second layer was evaluated by the following method. When the blade of a cutter knife was inserted into the interface between the first layer and the second layer of the radio wave absorber and they were peeled off, it was rated as "B." When they were not peeled off or when they were so tightly adhered that the cutter knife could not be inserted into the interface, it was rated as "A."

[0152] [Thermal Conductivity Ratio (k1 / k2)] Each of the first and second layers was extracted from the radio wave absorber using a known method. Measurement samples were cut out from each of the extracted layers. The thermal diffusivity in the thickness direction of the measurement samples cut out from each layer was measured using a NETZSCH "LFA467" by the laser flash method. The specific gravity of the measurement samples cut out from each layer was measured by the Archimedes method (using a "Solid Specific Gravity Measurement Kit") using a Mettler Toledo "XS204" balance. The specific heat of the measurement samples cut out from each layer at 25°C was determined using a Seiko Instruments Inc. "DSC320 / 6200" under a heating rate of 10°C / min. The thermal conductivity of the measurement samples cut out from each layer was calculated by multiplying the obtained thermal diffusivity by the specific gravity and specific heat. The thermal conductivity calculated in this way was used as the thermal conductivity (k1, k2) of each layer.

[0153]

[0154] One aspect of the present invention is useful, for example, in technical fields such as performing various types of automatic driving control, such as automatic driving control of automobiles, and wireless technical fields such as motion sensors.

Claims

1. A radio wave absorber comprising: a first layer containing one or more types of particles selected from the group consisting of magnetic particles and dielectric particles and a binder; and a second layer adjacent to the first layer, wherein the second layer satisfies the following formula 1: 2.50≦d×√ε≦3.50 or the following formula 2: 0.60≦d×√ε≦1.40, where d is the thickness of the second layer, the unit of d is mm, and ε is the relative dielectric constant of the second layer.

2. The radio wave absorber according to claim 1, wherein the radio wave absorption rate of said second layer is 5.0% or less.

3. The radio wave absorber according to claim 1, wherein the ten-point mean roughness Rz of the surface of said second layer opposite to said first layer side is 500.0 μm or less.

4. The radio wave absorber according to claim 1, wherein the particles are magnetic particles, and the magnetic particles are particles of hexagonal ferrite.

5. The radio wave absorber according to claim 4, wherein the hexagonal ferrite is a substitutional hexagonal ferrite.

6. The radio wave absorber according to claim 5, wherein the substituted hexagonal ferrite is aluminum-substituted hexagonal ferrite.

7. The radio wave absorber according to claim 6, wherein the aluminum-substituted hexagonal ferrite is a magnetoplumbite-type hexagonal ferrite having a composition represented by the following formula 1: In formula 1, A represents one or more atoms selected from the group consisting of Sr, Ba, Ca, and Pb, and x satisfies 0.50≦x≦8.

00.

8. The radio wave absorber according to claim 7, wherein the atom represented by A in formula 1 includes Sr.

9. The radio wave absorber according to claim 1, wherein the particles are dielectric particles, and the dielectric particles are carbon-based particles.

10. The radio wave absorber according to claim 1, wherein the radio wave absorption rate of said second layer is 5.0% or less, the ten-point average roughness Rz of the surface of said second layer opposite to said first layer side is 500.0 μm or less, said particles are magnetic particles, and said magnetic particles are particles of magnetoplumbite-type hexagonal ferrite having a composition represented by the following formula 1: In formula 1, A represents one or more atoms selected from the group consisting of Sr, Ba, Ca, and Pb, and x satisfies 0.50≦x≦8.

00.

11. The radio wave absorber according to claim 1, wherein the radio wave absorption rate of said second layer is 5.0% or less, the ten-point average roughness Rz of the surface of said second layer opposite to said first layer side is 500.0 μm or less, said particles are dielectric particles, and said dielectric particles are carbon-based particles.

12. The radio wave absorber according to claim 1, wherein the absolute value of the difference A-B between the reflectance A of said first layer and the reflectance B of said second layer is less than 6.0 dB.

13. The radio wave absorber according to claim 1, wherein the radio wave absorption rate of the second layer is 5.0% or less, and the second layer contains carbon black.

14. The radio wave absorber according to claim 1, wherein the first layer is a layer that is in direct contact with the second layer.

15. The radio wave absorber according to claim 1, wherein the ten-point mean roughness Rz of the surface of said second layer on the side of said first layer is 500.0 μm or less.

16. The radio wave absorber according to claim 1, which satisfies the following formula 3: 1.5≦k1 / k2, where k1 is the thermal conductivity of the first layer, k2 is the thermal conductivity of the second layer, and the unit of thermal conductivity is W / m·k.

17. An article comprising the radio wave absorber according to any one of claims 1 to 16.

18. The article of claim 17, further comprising a radio wave emitting source.

19. The article of claim 18, which is a radar.

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