Electromagnetic wave absorber and electromagnetic wave-blocking composite

Abstract

The purpose of the present invention is to solve the problem of variation in electromagnetic wave absorption characteristics that occurs in an electromagnetic wave absorber that is capable of absorbing electromagnetic waves in a high-frequency band region.　The problem is solved by an electromagnetic wave absorber that contains fibrous carbon in a matrix that contains at least one component selected from a rubber, a resin, and an elastomer, wherein: the electromagnetic wave absorber has a Shore A hardness of 60 or less; and the electromagnetic wave absorber satisfies, at each frequency of 77 GHz, 24 GHz, and 40 GHz, the relationships 2≤xCPr≤14, 2≤yCPr≤14, 1.5≤xCPi≤6, 1.5≤yCPi≤6, and |xCPr-yCPr|≤2, where CPr is the real part of a complex dielectric constant, CPi is the imaginary part of the complex dielectric constant, xCPr and xCPi are respectively CPr and CPi in the X direction, and yCPr and yCPi are respectively CPr and CPi in the Y direction, in any X direction and Y direction that are perpendicular to each other.

Description

Electromagnetic wave absorber and electromagnetic wave shielding composite 【0001】 This invention relates to an electromagnetic wave absorber capable of absorbing electromagnetic waves. It also relates to an electromagnetic wave shielding composite equipped with an electromagnetic wave absorber. 【0002】 In recent years, the frequency range of electromagnetic waves used in communication equipment and radar has been increasing at an accelerating pace. For example, millimeter-wave radar used in automobile collision avoidance systems utilizes high-frequency electromagnetic waves in the 76-79 GHz range, and with the future spread of autonomous driving technology, the number of millimeter-wave radars installed for omnidirectional surveying and other purposes is expected to increase dramatically. 【0003】 In the field of communication equipment, expectations are rising for the fifth-generation mobile communication system (5G) to achieve higher capacity and faster communication than the conventional fourth-generation mobile communication system (4G). 5G mainly uses frequency bands of 28 GHz and above. To prevent malfunctions in such high-frequency band regions, electromagnetic shielding materials are used to suppress malfunctions caused by electromagnetic interference between devices or between circuits within devices. As an electromagnetic wave absorber that corresponds to the high-frequency band region, for example, Patent Document 1 discloses an electromagnetic wave absorber containing fibrous carbon. 【0004】 WO2023 / 228258 【0005】 The technology disclosed in Patent Document 1 makes it possible to provide an electromagnetic wave absorber having high electromagnetic wave absorption characteristics in the high-frequency band region exceeding 18 GHz. However, variations in the electromagnetic wave absorption characteristics of the electromagnetic wave absorber sometimes occurred. The present invention solves the problem of variations in electromagnetic wave absorption characteristics that occur in electromagnetic wave absorbers capable of absorbing electromagnetic waves in the high-frequency band region. 【0006】 The inventors investigated the above problem and found that the cause of the variation in electromagnetic wave absorption characteristics is that the fibrous carbon incorporated as a filler in the matrix of the electromagnetic wave absorber is oriented within the matrix, causing anisotropy in the complex dielectric constant of the electromagnetic wave absorber. The inventors then completed the invention by improving the orientation of the fibrous carbon in the matrix and reducing the anisotropy in the complex dielectric constant. 【0007】 One embodiment of the present invention is an electromagnetic wave absorber containing fibrous carbon in a matrix comprising at least one selected from rubber, resin, and elastomer, wherein the Shore A hardness is 60 or less, and when the real part of the complex dielectric constant is CPr, the imaginary part of the complex dielectric constant is CPi, and in any mutually perpendicular X and Y directions, CPr and CPi in the X direction are xCPr and xCPi, respectively, and CPr and CPi in the Y direction are yCPr and yCPi, respectively, at a frequency of 77 GHz, the electromagnetic wave absorber satisfies 2 ≤ xCPr ≤ 14, 2 ≤ yCPr ≤ 14, 1.5 ≤ xCPi ≤ 6, 1.5 ≤ yCPi ≤ 6, and |xCPr - yCPr| ≤ 2. 【0008】 Another embodiment of the present invention is an electromagnetic wave absorber comprising fibrous carbon in a matrix comprising at least one selected from rubber, resin, and elastomer, wherein the Shore A hardness is 60 or less, and the real part of the complex dielectric constant is CPr, the imaginary part of the complex dielectric constant is CPi, and in any mutually perpendicular X and Y directions, CPr and CPi in the X direction are xCPr and xCPi, respectively, and CPr and CPi in the Y direction are yCPr and yCPi, respectively, the electromagnetic wave absorber satisfies 2 ≤ xCPr ≤ 14, 2 ≤ yCPr ≤ 14, 1.5 ≤ xCPi ≤ 6, 1.5 ≤ yCPi ≤ 6, and |xCPr - yCPr| ≤ 2 at a frequency of 24 GHz. 【0009】 A further embodiment of the present invention is an electromagnetic wave absorber comprising fibrous carbon in a matrix comprising at least one selected from rubber, resin, and elastomer, wherein the Shore A hardness is 60 or less, and the real part of the complex dielectric constant is CPr, the imaginary part of the complex dielectric constant is CPi, and in any mutually perpendicular X and Y directions, CPr and CPi in the X direction are xCPr and xCPi, respectively, and CPr and CPi in the Y direction are yCPr and yCPi, respectively, the electromagnetic wave absorber satisfies 2 ≤ xCPr ≤ 14, 2 ≤ yCPr ≤ 14, 1.5 ≤ xCPi ≤ 6, 1.5 ≤ yCPi ≤ 6, and |xCPr - yCPr| ≤ 2 at a frequency of 40 GHz. 【0010】 Furthermore, the tensile stress at the point of 10% elongation is S １０ Let S in the X direction １０ to xS１０ is defined as S in the Y direction １０ is defined as yS １０ When it is set as S １０ S ≤ 1 MPa, (|xS １０ − yS １０ |) / [(xS １０ + yS １０ ) / 2] ≤ 0.25 is preferably satisfied. Let the elongation at break be E, the E in the X direction be xE, and the E in the Y direction be yE. When E ≥ 200%, (|xE − yE|) / [(xE + yE) / 2] ≤ 0.15 is preferably satisfied. 【0011】 Also, the content of the fibrous carbon is preferably 0.2% by mass or less, the fibrous carbon preferably has an average length of 1 mm or more, and in the length direction of the fibrous carbon, among the angles formed by the virtual straight line connecting one end and the other end of the fibrous carbon observed on the observation surface of the scanning electron microscope of the cut surface of the electromagnetic wave absorber and the fibrous carbon, the smallest angle is preferably 5° or more. 【0012】 In the cross-sectional observation, the number of fiber bundles of the fibrous carbon having a maximum diameter of 100 nm or more is preferably 10 or less per 10 μm × 10 μm region, and it is preferable that a 10 μm × 10 μm region where no fiber bundle of the fibrous carbon having a maximum diameter of 100 nm or more exists can be selected. Also, the fibrous carbon is preferably a carbon nanotube. Also, in another form, it is an electromagnetic wave shielding composite having the electromagnetic wave absorber and an electromagnetic wave reflection layer disposed in contact with the electromagnetic wave absorber. 【0013】 According to the present invention, an electromagnetic wave absorber that maintains sufficient absorption characteristics for electromagnetic waves in the high-frequency band region, is flexible, and solves the problem of variations in electromagnetic wave absorption characteristics can be provided. 【0014】 It is a graph showing the reflection attenuation amount of the electromagnetic wave absorber manufactured in the example. 【0015】One embodiment of the present invention is an electromagnetic wave absorber containing fibrous carbon in a matrix comprising at least one selected from rubber, resin, and elastomer. <Matrix> The matrix comprises at least one selected from rubber, resin, and elastomer. Examples of rubber include known materials such as natural rubber (NR), styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber (NBR), nitrile rubber, hydrogenated nitrile rubber, polyisoprene rubber (IR), butadiene rubber (BR), butyl rubber (IIR), chloroprene rubber (CR), acrylic rubber (ACM), fluororubber (FKM, PTFE), and silicone rubber. 【0016】 The resin may be a thermoplastic resin or a thermosetting resin. Examples of known resins include polyester resin, polyether resin, polyolefin resin (polyethylene resin, polypropylene resin, etc.), polystyrene resin, polyamide resin, polycarbonate resin, acrylic resin, polyvinyl chloride resin, polyphenylene sulfide resin, polyphenylene ether resin, polytetrafluoroethylene resin, polyimide resin, polyamideimide resin, polyetherimide resin, polysulfone resin, polyethersulfone resin, polyketone resin, polyetherketone resin, polyetheretherketone resin, polyarylate resin, polyethernitrile resin, phenol resin, phenoxy resin, fluororesin, urea resin, melamine resin, benzoguanamine resin, alkyd resin, epoxy resin, silicone resin, urethane resin, furan resin, xylene resin, and the like. 【0017】 Examples of elastomers include well-known materials such as polystyrene elastomers, polyolefin elastomers, polyurethane elastomers, polyester elastomers, polyamide elastomers, polybutadiene elastomers, polyisoprene elastomers, fluorine-based elastomers, and silicone elastomers. 【0018】The weight-average molecular weight of the polymer material used as the matrix is ​​preferably 5,000 to 15,000,000, more preferably 8,000 to 13,000,000, and even more preferably 10,000 to 10,000,000. In this specification, the weight-average molecular weight of the polymer material refers to the weight-average molecular weight in polystyrene terms measured by gel permeation chromatography (GPC). The matrix is ​​preferably at least one selected from silicone resin, silicone elastomer, and silicone rubber, and more preferably silicone rubber, for reasons of heat resistance, chemical resistance, and durability. 【0019】 <Fibrous Carbon> Examples of fibrous carbon include carbon nanotubes, carbon nanofibers, carbon nanohorns, and carbon fibers, with carbon nanotubes being preferred. Carbon nanotubes are materials in which graphene sheets are formed into single-layer or multi-layer tubular structures. In this specification, carbon nanotubes also include cone-shaped materials called carbon nanohorns. Carbon nanotubes may be single-layer carbon nanotubes or multi-layer carbon nanotubes. 【0020】 The average diameter of the fibrous carbon used is preferably 1 to 200 nm, more preferably 1 nm or more, even more preferably 3 nm or more, even more preferably 150 nm or less, and even more preferably 100 nm or less. 【0021】 The average length of the fibrous carbon used is preferably 1 mm or more. There is no particular upper limit, but it is preferably 5 mm or less, and more preferably 3 mm or less. By having an average length of fibrous carbon of 1 mm or more, a large number of capacitors with high capacitance can be formed within the matrix, and the loss due to fiber resistance can also be increased by forming a fiber network, so that the desired electromagnetic wave absorption characteristics can be achieved with a small amount of fibrous carbon. 【0022】In this specification, the average diameter and average length of fibrous carbon are calculated by measuring the diameter (outer diameter) and length of 30 randomly selected fibrous carbons using a transmission electron microscope (TEM) or scanning electron microscope (SEM), and then calculating the average value of these measurements. Furthermore, it is preferable that the fibrous carbon used forms a wave in the longitudinal direction, with an average period of 0.5 μm to 2.0 μm observed on the observation surface of the scanning electron microscope. Here, "wave" refers to the form in which the fibrous carbon forms an undulation in its longitudinal direction, and "average period" refers to the average length (average wavelength) corresponding to the wavelength when the undulation formed in the longitudinal direction of the fibrous carbon vibrates back and forth in a direction perpendicular to the longitudinal direction. The average period (average wavelength) of fibrous carbon is obtained by observing the fibrous carbon with an SEM, arbitrarily selecting 30 wavelengths along the longitudinal direction of one fibrous carbon, and dividing the total distance of the 30 wavelengths by 30 to obtain the average value. By using such fibrous carbon, the fibrous carbon becomes more easily deformable in the composite material. When using such fibrous carbon, it is preferable that the smallest angle between the imaginary line connecting one end and the other end of the fibrous carbon and the fibrous carbon itself within the electromagnetic wave absorber is 5° or more. 【0023】The fibrous carbon forming the above wave is more deformable than straight, rigid fibrous carbon, and can take on a gently curved structure within the electromagnetic wave absorber. This makes it easier for the fibrous carbon to follow the matrix during the mixing process, improving its dispersibility. As a result, it becomes possible to uniformly create a capacitor-like structure within the electromagnetic wave absorber, increasing the loss of AC radio waves and thus enabling efficient noise absorption. The degree of curvature of the fibrous carbon in the absorber can be confirmed by observing the angle between a virtual straight line connecting one end and the other end of the fibrous carbon observed with a scanning electron microscope on the cross-section of the electromagnetic wave absorber, and the tangent in the fiber direction from one end to the other end. The above angle is preferably 5° to 80°, more preferably 7° to 75°, and even more preferably 10° to 70°. An angle of 80° or more is undesirable because the fibers tend to aggregate during the mixing process, reducing absorption efficiency. 【0024】 The fibrous carbon may have acidic or basic groups introduced to its surface by oxidation treatment or modification with a polymer. Examples of acidic groups include carboxyl groups, and examples of basic groups include amine groups. 【0025】 The content of fibrous carbon in the electromagnetic wave absorber is not particularly limited as long as it satisfies the electromagnetic wave absorption characteristics described later, but it is preferably 0.2% by mass or less, and may be 0.15% by mass or less. In this embodiment, because the fibrous carbon content is small, the electromagnetic wave absorber is flexible, the value of the elongation at break described later is large, resulting in an electromagnetic wave absorber that is difficult to break and can be arranged to follow the surface shape of the electromagnetic wave reflecting layer. Furthermore, to exhibit absorption characteristics, 0.05% or more is preferable, and 0.07% or more is more preferable. 【0026】<Electromagnetic Wave Absorber> The electromagnetic wave absorber of this embodiment preferably has a Shore A hardness of 60 or less, more preferably 55 or less, and even more preferably 50 or less. Because of the low Shore A hardness of this embodiment, the electromagnetic wave absorber is flexible and less prone to breakage, and can be positioned to conform to the surface shape of the electromagnetic wave reflection layer. 【0027】 In this embodiment of the electromagnetic wave absorber, at frequencies of 77 GHz, 24 GHz, and 40 GHz, the real part of the complex permittivity is CPr, the imaginary part of the complex permittivity is CPi, and in any mutually perpendicular X and Y directions, when CPr and CPi in the X direction are xCPr and xCPi respectively, and CPr and CPi in the Y direction are yCPr and yCPi respectively, the following conditions are satisfied: 2 ≤ xCPr ≤ 14, 2 ≤ yCPr ≤ 14, 1.5 ≤ xCPi ≤ 6, 1.5 ≤ yCPi ≤ 6, and |xCPr - yCPr| ≤ 2. Note that the X and Y directions are mutually orthogonal directions on the surface of the sheet when the electromagnetic wave absorber is in sheet form, and mutually orthogonal directions in any cross-section within the block when the electromagnetic wave absorber is in block form. 【0028】 This type of electromagnetic wave absorber can be implemented by placing it on the surface of a material that reflects electromagnetic waves, such as metal, and controlling the amplitude and phase of the reflected waves from the metal surface and the front surface of the electromagnetic wave absorber to attenuate the reflected waves. Therefore, by placing this type of electromagnetic wave absorber on the surface of a material that reflects electromagnetic waves, such as metal, electromagnetic waves can be absorbed efficiently. Electromagnetic wave absorbers that reduce electromagnetic waves through this mechanism are also called resonant electromagnetic wave absorbers. In a resonant electromagnetic wave absorber, theoretically, if the following equation, known as the non-reflection condition, is satisfied, there will be no reflected electromagnetic waves. In equation (1), "tanh" is an operator with a hyperbolic tangent function, "ε" is the complex permittivity, "j" is the imaginary unit, "d" is the thickness of the electromagnetic wave absorber (dielectric layer), and "λ" is the wavelength of the electromagnetic wave. As shown in the above equation, the absorption characteristics of a resonant electromagnetic wave absorber are greatly affected by the complex permittivity of the composite material. By having the real and imaginary parts of the complex permittivity within the above range, it is possible to obtain an electromagnetic wave absorber that has high electromagnetic wave absorption characteristics in the high-frequency band region, and also improves the anisotropy of the complex permittivity of the electromagnetic wave absorber in the X and Y directions, resulting in an electromagnetic wave absorber with small variations in electromagnetic wave absorption characteristics. 【0029】 The 77 GHz frequency is primarily used in millimeter-wave radar for automotive collision avoidance systems. Because the real and imaginary parts of the complex permittivity fall within the specified range, the conditions are close to the non-reflection condition at 77 GHz, allowing for efficient absorption of electromagnetic waves and the creation of an electromagnetic wave absorbing sheet with non-anisotropic absorption characteristics. The 24 GHz frequency is used in millimeter-wave radar for automotive collision avoidance systems as well as in vital signs sensors. Because the real and imaginary parts of the complex permittivity fall within the specified range, the conditions are close to the non-reflection condition at 24 GHz, allowing for efficient absorption of electromagnetic waves and the creation of an electromagnetic wave absorbing sheet with non-anisotropic absorption characteristics. The 40 GHz frequency, due to its wide bandwidth, is used in high-definition video transmission systems over extremely short distances, such as within venues. Since the real and imaginary parts of the complex permittivity described above fall within the specified range, the conditions become close to the non-reflection condition at 40 GHz, allowing for efficient absorption of electromagnetic waves and enabling the creation of an electromagnetic wave absorbing sheet with no anisotropy in its absorption characteristics. In this embodiment, since the real and imaginary parts fall within the specified range at the above frequency, an electromagnetic wave absorber with small variations in electromagnetic wave absorption characteristics across a wide frequency band can be obtained. 【0030】At each of the above frequencies, |xCPr - yCPr| is the absolute value of the difference between the X and Y directions of the real part. The smaller this value is, the smaller the variation in electromagnetic wave absorption characteristics. Preferably, it is 1.5 or less, and more preferably 1 or less. The absolute value of the difference between the X and Y directions of the real part of the complex dielectric constant can be adjusted to the desired range by widening the roll gap in roll rolling when forming the electromagnetic wave absorber into a sheet shape, adding a masterbatch containing fibrous carbon at a high concentration and performing kneading and forming, etc. Also, by stacking a plurality of sheet-shaped electromagnetic wave absorbers in a direction where the orientation directions of the fibrous carbon are substantially orthogonal and pressing them into one sheet, the desired range can be achieved. 【0031】 For the electromagnetic wave absorber of this embodiment, the tensile stress at the 10% elongation point is S １０ Let xS １０ be the S in the X direction, １０ and let yS １０ be the S in the Y direction. １０ When this is done, it is preferable that S １０ ≤ 1 MPa and (|xS １０ - yS １０ |) / [(xS １０ + yS １０ ) / 2] ≤ 0.25 are satisfied. When S １０ is 1 MPa or less, the electromagnetic wave absorber has flexibility and is difficult to break. More preferably, S １０ is 0.2 MPa or less. The tensile stress at the 10% elongation point of the electromagnetic wave absorber is the value obtained by dividing the absolute value of the difference between the stress values in the X and Y directions by the average value of the stress values in the X and Y directions, that is, (|xS １０ - yS １０ |) / [(xS １０ + yS １０ ) / 2] being 0.25 or less means that the difference in flexibility between the X and Y directions of the electromagnetic wave absorber is small, and the anisotropy in the ease of breaking of the electromagnetic wave absorber is small. More preferably, this value is 0.2 or less. By making the difference in tensile stress in the X direction / Y direction fall within the above range, the electromagnetic wave absorber can be uniformly attached to the curved surface without wrinkles. 【0032】For the electromagnetic wave absorber of this embodiment, when the elongation at break is E, the elongation at break in the X direction is xE, and the elongation at break in the Y direction is yE, it is preferable that E≥200% and (|xE - yE|) / [(xE + yE) / 2]≤0.15. When the elongation at break E is 200% or more, the electromagnetic wave absorber has flexibility and is difficult to break. More preferably, E is 300% or more. The elongation at break of the electromagnetic wave absorber is the value obtained by dividing the absolute value of the difference in elongation at break between the X direction and the Y direction by the average value of the elongation at break in the X direction and the Y direction, that is, (|xE - yE|) / [(xE + yE) / 2]. When this value is 0.15 or less, the difference in flexibility between the X direction and the Y direction of the electromagnetic wave absorber is small, and the anisotropy in the ease of breakage of the electromagnetic wave absorber is small. More preferably, this value is 0.1 or less. 【0033】 The electromagnetic wave absorber of this embodiment functions because the metal layer is on the back side of the absorber with respect to the incident wave. When the elongation at break is within the above range, even when the metal layer forms a curved surface, it can follow and adhere through the adhesive layer. Also, it can be adhered to a material that expands and contracts. The electromagnetic wave absorber is adhered to the metal layer through the adhesive layer, and this metal layer can be further adhered to a substrate such as a plastic plate through the adhesive layer. Of course, when the substrate is metal, the electromagnetic wave absorber can be directly adhered to the substrate through the adhesive layer for use. When the elongation at break is within the above range, the electromagnetic wave absorber can adhere following the curved surface, so it can be adhered without wrinkles, and it can also be adhered to a material that expands and contracts. 【0034】 For the electromagnetic wave absorber of this embodiment, in the observation of the cross-section, it is preferable that the number of fiber bundles of the fibrous carbon with a maximum diameter of 100 nm or more is 10 or less per 10 μm×10 μm region. Thereby, the dispersibility of the fibrous carbon in the electromagnetic wave absorber becomes good, and higher electromagnetic wave absorption characteristics can be obtained. More preferably, the number of fiber bundles of the fibrous carbon is 8 or less, and even more preferably 6 or less. 【0035】In this embodiment of the electromagnetic wave absorber, it is preferable to be able to select a 10 μm × 10 μm region in cross-section observation in which no fiber bundles of fibrous carbon with a maximum diameter of 100 nm or more are present. This improves the dispersion of fibrous carbon within the electromagnetic wave absorber, resulting in higher electromagnetic wave absorption characteristics. 【0036】 The electromagnetic wave absorber of this embodiment can measure the absorption capacity of electromagnetic waves with frequencies from 18 GHz to 110 GHz by measuring the attenuation of incident electromagnetic waves while it is attached to an electromagnetic wave reflective layer such as aluminum. The minimum peak of the reflection attenuation of the electromagnetic wave absorber measured in this way is preferably -15 dB or less, and more preferably -20 dB or less. 【0037】 There are no particular limitations on the shape of the electromagnetic wave absorber, and it can be appropriately selected according to the purpose and application. For example, it can be in the form of a sheet, plate, or rod. In the case of a sheet, the thickness is not particularly limited, but it is preferably 0.1 mm or more and 2.0 mm or less, more preferably 1.8 mm or less, even more preferably 1.5 mm or less, even more preferably 0.15 mm or more, and even more preferably 0.2 mm or more. The thickness of the electromagnetic wave absorber is determined by the thickness d of the electromagnetic wave absorber calculated by the above formula (1) depending on the design frequency. 【0038】 <Manufacturing Method> The manufacturing method is described below. The manufacturing method for this embodiment of the electromagnetic wave absorber involves compounding a rubber or resin matrix with fibrous carbon and molding the composite into a desired shape. When forming the composite, it is preferable to first prepare a masterbatch containing fibrous carbon at a high concentration in the matrix, and then further dilute this masterbatch with rubber or resin. By preparing a masterbatch with a high concentration of fibrous carbon, aggregation of low-concentration fibrous carbon can be suppressed even after dilution, resulting in an electromagnetic wave absorber with excellent dispersibility of fibrous carbon. The content of fibrous carbon in the high-concentration masterbatch is preferably 2 to 10% by mass, more preferably 3 to 8% by mass, and even more preferably 4 to 6% by mass. 【0039】A masterbatch is prepared by directly mixing fibrous carbon powder with a matrix, for example, using a pressurized kneader for rubber, or by kneading with a twin-screw extruder for thermoplastic resins. If the fibrous carbon is difficult to disperse in the matrix, the masterbatch can be prepared by mixing a fibrous carbon dispersion containing fibrous carbon and a solvent with a matrix material-containing solution (mixture preparation step), adding a coagulant to the mixture to solidify it and produce a solidified product containing fibrous carbon and matrix (solidification step), and drying the solidified product (drying step). For information on the preparation of a masterbatch, see, for example, Patent Document 1 (WO2023 / 228258). 【0040】 The resin composition is molded into the desired shape of the electromagnetic wave absorber. When forming a sheet, methods such as vacuum forming, pressure forming, press forming, roll rolling, and blow forming can be used. In this embodiment of the electromagnetic wave absorber, the orientation of fibrous carbon in the matrix can be suppressed by widening the roll gap during roll rolling. When forming a sheet by roll rolling, the roll gap is preferably 2 mm or more, and more preferably 3 mm or more. 【0041】 <Other Additives> The electromagnetic wave absorber may contain additional additives as needed. Examples of additives include antioxidants, heat stabilizers, light stabilizers, ultraviolet absorbers, crosslinking agents, pigments, colorants, foaming agents, antistatic agents, flame retardants, lubricants, softeners, tackifiers, plasticizers, release agents, deodorants, and fragrances. The content of additives in the electromagnetic wave absorber is preferably 30% by mass or less, more preferably 25% by mass or less, even more preferably 20% by mass or less, even more preferably 10% by mass or less, and particularly preferably 5% by mass or less. The lower limit may be 0% by mass or more, 0.01% by mass or more, or 0.1% by mass or more. 【0042】Electromagnetic wave absorbers can be used by placing them on the surface of an electromagnetic wave reflecting layer, such as a metal layer. Specifically, they can be used for countermeasures against radiated noise in electronic equipment such as communication equipment, computers, home appliances, automotive electrical equipment, and medical electrical equipment, as well as for countermeasures against electromagnetic wave reflection in ETC (Electronic Toll Collection System), radar, etc. 【0043】 <Electromagnetic Wave Shielding Composite> Another embodiment of the present invention is an electromagnetic wave shielding composite. The electromagnetic wave shielding composite comprises the electromagnetic wave absorber described above and an electromagnetic wave reflecting layer placed in contact with the electromagnetic wave absorber, and is generally called a "resonant electromagnetic wave absorber". The electromagnetic wave reflecting layer may be attached to the electromagnetic wave absorber during manufacturing, or if there is a material that reflects electromagnetic waves, such as metal, in a place where electromagnetic wave absorption is required, it may be attached to the electromagnetic wave absorber via an adhesive layer or a bonding layer to form an electromagnetic wave shielding composite. Furthermore, if the electromagnetic wave absorber is in the form of a sheet, the electromagnetic wave reflecting layer may be provided on one side of the electromagnetic wave absorber by methods such as sputtering or vapor deposition. There are no particular limitations on the type of electromagnetic wave reflecting layer in the electromagnetic wave shielding composite. Examples include a metal layer made of metal such as aluminum foil, a woven fabric made of carbon fiber, a nonwoven fabric, a coating film, etc. 【0044】 The thickness of the electromagnetic wave reflective layer is preferably 0.005 to 1 mm. The lower limit is preferably 0.01 mm or more. The upper limit is preferably 0.9 mm or less, and more preferably 0.5 mm or less. When the electromagnetic wave reflective layer is attached, the thickness of the electromagnetic wave reflective layer is not particularly limited. The minimum peak of the reflection attenuation of the electromagnetic wave absorber is preferably -10 dB or less, and more preferably -15 dB or less. 【0045】 The present invention will be described in more detail below with reference to examples. The materials, amounts used, proportions, processing content, and processing procedures shown in the following examples can be modified as appropriate, as long as they do not depart from the spirit of the present invention. Therefore, the scope of the present invention is not limited to the specific examples shown below. 【0046】<Carbon Nanotube Production> A supported film was formed on one side of the substrate by sputtering with aluminum as the target. Subsequently, a catalyst film was formed on the supported film by sputtering with iron as the target. Next, the substrate with the catalyst film formed was placed upright in the reaction vessel of the CVD apparatus. Next, the reaction vessel was closed, and heating of the reaction vessel was started by simultaneously reducing the pressure to 1 Pa and energizing the heater. Next, when the temperature inside the reaction vessel reached 600°C to 700°C, nitrogen gas was supplied into the reaction vessel, and the reaction vessel was continuously evacuated using a pump to maintain the pressure inside the reaction vessel at 90 kPa. Next, the reaction vessel was heated to 750°C, and both nitrogen and hydrogen gases were supplied to the reaction vessel. The pressure inside the reaction vessel was maintained at 30 kPa. After the temperature inside the reaction vessel reached 750°C, nitrogen gas, hydrogen gas, and acetylene gas were supplied along the catalyst film surface from the top to the bottom of the substrate inside the reaction vessel while preheating with the heater. Carbon nanotubes were synthesized on the substrate while maintaining the pressure inside the reaction vessel at 30 kPa, resulting in carbon nanotubes with a length of 0.1 to 2.2 mm, an average length of 2.0 mm, 3 to 8 layers, a diameter of 5 to 12 nm, and a G / D ratio of 0.8. 【0047】 The length of the carbon nanotubes was evaluated from SEM images taken with a scanning electron microscope (JEOL Ltd., model: JSM-7800F), either on the substrate or after the carbon nanotubes had been harvested from the substrate. 【0048】 The number of carbon nanotube layers and their diameters were evaluated from TEM images taken with a transmission electron microscope (Hitachi High-Tech Corporation, model: HF2200) either on the substrate or after the carbon nanotubes had been harvested from the substrate. 【0049】 The G / D ratio of carbon nanotubes was determined using a Raman microscope (RENISHAW, model: inVia). Specifically, the G-band (1590 cm⁻¹) of the Raman spectrum measured using the Raman microscope was used. －１ The peak intensity of ) is measured using D-band (1350 cm). －１ The value obtained by dividing by the peak intensity of ) was calculated. 【0050】 <Manufacturing of Electromagnetic Wave Absorbers> (Examples 1-6 and Comparative Examples 1-4) Using a Laboplast Mill (Model 4C150, Toyo Seiki Co., Ltd.), the carbon nanotubes and matrix manufactured above were kneaded with the formulations shown in Table 1. For the matrix, a mixture of 100 parts by mass of silicone rubber (product name KE-951-U, Shin-Etsu Chemical Co., Ltd.) and 2 parts by mass of vulcanizing agent (product name O8, Shin-Etsu Chemical Co., Ltd.) was used. In resonant electromagnetic wave absorbers, it is necessary to adjust the dielectric constant of the absorber to a predetermined dielectric constant and thickness. To adjust the dielectric constant, acetylene black (product name Denka Black, manufactured by Denka Co., Ltd.), barium titanate (product name HPBT, manufactured by Fuji Titanium Industry Co., Ltd.), and aluminum oxide (product name DW-03, manufactured by Denka Co., Ltd.) were blended in the proportions shown in Table 1. The resulting mixture was further kneaded in a roll mill (two-roll mill, manufactured by Daihan Co., Ltd.), then press-formed using a hydraulic heating and cooling press (model 1811, manufactured by Imoto Seisakusho), and finally vulcanized to produce a sheet-like electromagnetic wave absorber with the thickness shown in Table 1, measuring 25 cm in length and 25 cm in width. In the preliminary step before press-forming, the material was rolled into a sheet using the above-mentioned roll mill to facilitate press-forming. In Examples 1 to 6 and Comparative Examples 2 to 4, the sheet was formed with a 3 mm gap in the roll mill, and then two sheets were laminated in a direction where the orientation direction of the carbon fibers was approximately perpendicular to each other, and then press-formed to create the sheet. In Comparative Example 1, the sheet was formed with a 1 mm gap in the roll mill, and the sheet was formed without lamination. In Table 1, the carbon nanotube content in the electromagnetic wave absorber is indicated in the "CNT" column. 【0051】 <Measurement of Complex Permittivity> The obtained electromagnetic wave absorber was set in a free-space measuring device (model FS-110, EM Lab Co., Ltd.) so that the film thickness direction was the direction of electromagnetic wave incidence. The transmission and reflection coefficients at each design frequency of 77 GHz, 24 GHz, and 40 GHz were measured using a network analyzer (model PNA N5290, KEY SIGHT Co., Ltd.), and the real and imaginary parts of the complex permittivity at each design frequency were derived. The X and Y directions were arbitrarily set on the surface of the sheet-like electromagnetic wave absorber so that they were orthogonal to each other. 【0052】<Measurement of Return Loss> A metal plate was attached to one side of the electromagnetic wave absorber, and the absorber was set in a free-space measuring device (model FS-110, EM Lab Co., Ltd.) so that it was in the direction of incidence of the electromagnetic wave, similar to the measurement of the complex permittivity. The return loss of electromagnetic waves near the design frequency was measured using a network analyzer (model PNA N5290, KEY SIGHT Co., Ltd.) in accordance with JIS R 1679. Figure 1 shows the return loss of electromagnetic wave absorbers for Examples 1 to 6. 【0053】 <Evaluation of Fiber Bundles> The obtained electromagnetic wave absorber was cut in the thickness direction. The cross-section of the electromagnetic wave absorber was observed with a scanning electron microscope, and the number of fiber bundles with a maximum diameter of 100 nm or more present per 10 μm × 10 μm area was counted. The counts listed in the table are the average values ​​of the counts obtained for 10 squares of 10 μm × 10 μm. The curvature of the fiber bundles was determined by measuring the smallest angle between the fiber bundle and a virtual curve formed by connecting one end and the other end of the fiber observed with the scanning electron microscope, and this was defined as the fiber bundle curvature. 【0054】 <Measurement of elongation at break> The elongation at break of the electromagnetic wave absorber was measured in accordance with JIS K 6251:2017. These evaluation results are also shown in Table 1. By using the electromagnetic wave absorbers of Examples 1 to 6 in communication equipment and the like, malfunctions caused by electromagnetic interference between circuits inside the equipment can be effectively suppressed. 【0055】

Claims

1. An electromagnetic wave absorber comprising fibrous carbon in a matrix containing at least one selected from rubber, resin, and elastomer, wherein the Shore A hardness is 60 or less, and the real part of the complex dielectric constant is CPr, the imaginary part of the complex dielectric constant is CPi, and in any mutually perpendicular X and Y directions, CPr and CPi in the X direction are xCPr and xCPi, respectively, and CPr and CPi in the Y direction are yCPr and yCPi, respectively, the electromagnetic wave absorber satisfies the following conditions at a frequency of 77 GHz: 2 ≤ xCPr ≤ 14, 2 ≤ yCPr ≤ 14, 1.5 ≤ xCPi ≤ 6, 1.5 ≤ yCPi ≤ 6, and |xCPr - yCPr| ≤ 2.

2. An electromagnetic wave absorber comprising fibrous carbon in a matrix comprising at least one selected from rubber, resin, and elastomer, wherein the Shore A hardness is 60 or less, and when the real part of the complex dielectric constant is CPr, the imaginary part of the complex dielectric constant is CPi, and in any mutually perpendicular X and Y directions, CPr and CPi in the X direction are xCPr and xCPi respectively, and CPr and CPi in the Y direction are yCPr and yCPi respectively, at a design frequency of 24 GHz, the electromagnetic wave absorber satisfies 2 ≤ xCPr ≤ 14, 2 ≤ yCPr ≤ 14, 1.5 ≤ xCPi ≤ 6, 1.5 ≤ yCPi ≤ 6, and |xCPr - yCPr| ≤ 2.

3. An electromagnetic wave absorber comprising fibrous carbon in a matrix comprising at least one selected from rubber, resin, and elastomer, wherein the Shore A hardness is 60 or less, and when the real part of the complex dielectric constant is CPr, the imaginary part of the complex dielectric constant is CPi, and in any mutually perpendicular X and Y directions, CPr and CPi in the X direction are xCPr and xCPi respectively, and CPr and CPi in the Y direction are yCPr and yCPi respectively, at a design frequency of 40 GHz, the electromagnetic wave absorber satisfies 2 ≤ xCPr ≤ 14, 2 ≤ yCPr ≤ 14, 1.5 ≤ xCPi ≤ 6, 1.5 ≤ yCPi ≤ 6, and |xCPr - yCPr| ≤ 2.

4. Let the tensile stress at the 10% elongation point be S １０ , １０ and the S in the X direction １０ be xS １０ and the S in the Y direction １０ be yS １０ When this is the case, S １０ ≤ 1 MPa, (|xS １０ − yS １０ |) / [(xS １０ + yS １０ ) / 2] ≤ 0.

25. The electromagnetic wave absorber according to any one of claims 1 to 3, which satisfies these conditions.

5. An electromagnetic wave absorber according to any one of claims 1 to 3, wherein when the elongation at break is E, E in the X direction is xE, and E in the Y direction is yE, E ≥ 200% and (|xE - yE|) / [(xE + yE) / 2] ≤ 0.

15.

6. The electromagnetic wave absorber according to any one of claims 1 to 3, wherein the content of the fibrous carbon is 0.2% by mass or less.

7. The electromagnetic wave absorber according to any one of claims 1 to 3, wherein, in the longitudinal direction, the smallest angle between the fibrous carbon and a virtual straight line connecting one end and the other end of the fibrous carbon observed on the observation surface of a scanning electron microscope is 5° or more.

8. An electromagnetic wave absorber according to any one of claims 1 to 3, wherein, in cross-sectional observation, the number of fiber bundles of the fibrous carbon having a maximum diameter of 100 nm or more is 10 or less per 10 μm × 10 μm area.

9. An electromagnetic wave absorber according to any one of claims 1 to 3, wherein, in cross-sectional observation, a region of 10 μm × 10 μm in which no fiber bundles of fibrous carbon with a maximum diameter of 100 nm or more are present can be selected.

10. The electromagnetic wave absorber according to any one of claims 1 to 3, wherein the fibrous carbon is a carbon nanotube.

11. A composite for electromagnetic wave shielding, comprising an electromagnetic wave absorber according to any one of claims 1 to 3, and an electromagnetic wave reflecting layer disposed in contact with the electromagnetic wave absorber.

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