Electromagnetic wave absorbing sheet

Abstract

To provide an electromagnetic wave absorbing sheet that has a high absorption rate and is thin. [Solution] An electromagnetic wave absorbing sheet having a dielectric layer, a support layer, and a metal layer, wherein the dielectric layer contains a conductive filler and a binder resin, the support layer is a resin sheet, the electromagnetic wave absorbing sheet absorbs electromagnetic waves with frequencies of 20 to 90 GHz, the absorption rate at the electromagnetic wave absorption peak in the frequency range of 20 to 90 GHz is -20 dB or less, and the total thickness of the dielectric layer and the support layer is 15% or less of the wavelength of the electromagnetic wave absorption peak.

Description

[Technical Field]

[0001] This disclosure relates to an electromagnetic wave absorbing sheet. [Background technology]

[0002] In recent years, radio waves utilizing the gigahertz band (several GHz to 100 GHz) have been used in mobile communications such as cell phones, automated toll collection systems (ETC), and wireless LANs.

[0003] Furthermore, in order to increase the capacity of data transmitted and received, achieve high-speed communication, and enable simultaneous connections to multiple locations, the practical application of wireless communication using frequency bands of 30 GHz or higher is progressing, and the use of in-vehicle radar is also advancing. Radio waves using such frequency bands interfere with various devices through reflections within the casings, becoming electromagnetic noise that causes device malfunctions. Therefore, there is a need to suppress electromagnetic noise.

[0004] As a means of suppressing electromagnetic noise generated inside electronic devices, it is known that electromagnetic wave absorbers are attached to the housing of electronic devices. Electromagnetic wave absorbing sheets containing magnetic materials are one type of electromagnetic wave absorber. For example, Patent Document 1 describes a reflective electromagnetic wave absorbing sheet having an electromagnetic wave absorbing layer and a reflective layer containing magnetic iron oxide and a rubber binder. Another type of electromagnetic wave absorber is an electromagnetic wave absorbing sheet containing a dielectric material. For example, Patent Document 2 describes a resin composition containing a thermoplastic resin and a conductive substance, and describes forming an electromagnetic wave absorber from the resin composition by injection molding. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2022-115987 [Patent Document 2] Japanese Patent Publication No. 2022-130343 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] In recent years, electronic components have become smaller, limiting the space available for attaching electromagnetic wave absorbing sheets within the enclosure, and thus requiring thinner electromagnetic wave absorbing sheets. Furthermore, the interior of the enclosure is often curved and not always flat. If the electromagnetic wave absorbing sheet is thick, it becomes difficult to apply it smoothly without wrinkles to such curved areas.

[0007] The electromagnetic wave absorbing sheet described in Patent Document 1 is flexible, but its thickness of 2.5 mm is large, making it difficult to fit inside the housing of miniaturized electronic components. The electromagnetic wave absorber described in Patent Document 2 is also molded to a thickness of about 2 mm, making it similarly difficult to fit inside the housing of miniaturized electronic components. Thus, conventional electromagnetic wave absorbing sheets have shortcomings in terms of thickness, and there is a need for an electromagnetic wave absorbing sheet that is thin and has a high absorption rate. [Means for solving the problem]

[0008] This specification provides the following means: [1] An electromagnetic wave absorbing sheet having a dielectric layer and a support layer, wherein the dielectric layer comprises a conductive filler and a binder resin, the support layer is a resin sheet, the electromagnetic wave absorbing sheet absorbs electromagnetic waves with frequencies of 20 to 90 GHz, the absorption rate at the electromagnetic wave absorption peak in the frequency range of 20 to 90 GHz is -20 dB or less, and the total thickness of the dielectric layer and the support layer is 15% or less of the wavelength of the electromagnetic wave absorption peak. [2] The electromagnetic wave absorbing sheet according to [1], wherein the conductive filler is made of carbon material. [3] The electromagnetic wave absorbing sheet according to [1] or [2], wherein the real part of the complex relative permittivity of the dielectric layer is 100 or less, and the ratio of the real part to the imaginary part (imaginary part / real part) of the complex relative permittivity of the dielectric layer is 0.8 or less. [4] An electromagnetic wave absorbing sheet according to any one of [1] to [3], wherein the carbon material comprises a fibrous carbon material. [5] An electromagnetic wave absorbing sheet according to any one of [1] to [4], wherein the carbon material comprises at least one of carbon nanotubes and carbon black. [6] An electromagnetic wave absorbing sheet according to any one of [1] to [5], wherein the ratio of the thickness of the dielectric layer to the thickness of the support layer (thickness of the dielectric layer / thickness of the support layer) is 0.01 or more and 1.5 or less. [7] An electromagnetic wave absorbing sheet according to any one of [1] to [6], wherein the real part of the complex relative permittivity of the support layer is less than 10. [8] An electromagnetic wave absorbing sheet according to any one of [1] to [7], wherein the Shore A hardness of the resin constituting the support layer is 90 or less. [9] An electromagnetic wave absorbing sheet according to any one of [1] to [8], which is used by being laminated on a substrate that reflects electromagnetic waves on the surface facing the support layer. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide an electromagnetic wave absorbing sheet that has a high absorption rate and is thin in thickness. [Modes for carrying out the invention]

[0010] The embodiments of this disclosure will be described in detail below. This disclosure is not limited to the following embodiments. The following embodiments may be modified as appropriate within the scope of the purposes of this disclosure.

[0011] When describing embodiments of this disclosure with reference to the drawings, explanations of redundant components and reference numerals in the drawings may be omitted. Components indicated by the same reference numeral in the drawings are considered to be the same component. Dimensional ratios in the drawings do not necessarily represent actual dimensional ratios.

[0012] In the present disclosure, a numerical range indicated using "~" indicates a range that includes the numerical value described before "~" as the lower limit value and the numerical value described before "~" as the upper limit value. In the numerical ranges described step by step in the present disclosure, the upper limit value or the lower limit value described in a certain numerical range may be replaced with the upper limit value or the lower limit value of the numerical range described in other step-by-step descriptions. Further, in the numerical ranges described in the present disclosure, the upper limit value or the lower limit value described in a certain numerical range may be replaced with the value shown in the examples.

[0013] In the present disclosure, the amount of each component in the composition means the total amount of the corresponding plurality of substances present in the composition when there are a plurality of substances corresponding to each component in the composition, unless otherwise specified.

[0014] In the present disclosure, the term "step" includes not only an independent step but also a step that cannot be clearly distinguished from other steps when the intended purpose is achieved.

[0015] In the present disclosure, "mass%" and "weight%" are synonymous, and "parts by mass" and "parts by weight" are synonymous.

[0016] The thickness is the arithmetic mean value measured at any three points by a cross-sectional photograph of an optical microscope.

[0017] The electromagnetic wave absorbing sheet reduces electromagnetic waves in a so-called 1 / 4λ type method. The outline of the principle of the 1 / 4λ type electromagnetic wave absorbing sheet is as follows. When electromagnetic waves are radiated to the electromagnetic wave absorber, they are divided into electromagnetic waves reflected on the surface and electromagnetic waves that pass through the inside of the electromagnetic wave absorber (that is, the dielectric layer and the support layer), are reflected by the base material or the reflection layer, and are radiated from the electromagnetic wave absorber. If the electromagnetic waves reflected by the base material or the reflection layer have the same level of electromagnetic wave intensity with the phase reversed with respect to the electromagnetic waves reflected on the surface, the two cancel each other out and the reflected electromagnetic waves are reduced. The frequency of the electromagnetic waves to be reduced and the dielectric constant and thickness of the dielectric layer and the support layer are adjusted so as to meet such conditions.

[0018] The electromagnetic wave absorption sheet of the present disclosure is particularly suitable for absorbing electromagnetic waves with frequencies of 20 to 90 GHz. At the electromagnetic wave absorption peak in the frequency range of 20 GHz to 90 GHz (hereinafter, also simply referred to as "electromagnetic wave absorption peak"), the absorption rate is -20 dB or less, preferably -25 dB or less, and more preferably -30 dB or less. The higher the absorption rate, the more preferable it is. Although no particular upper limit value is set, it may be -50 dB or more.

[0019] The total thickness of the dielectric layer and the support layer may be set according to the wavelength of the electromagnetic wave to be absorbed. Generally, the longer the wavelength of the electromagnetic wave to be absorbed, the greater the thickness required for the electromagnetic wave absorption sheet, and the thickness of the electromagnetic wave absorption sheet is determined according to the wavelength of the electromagnetic wave absorption peak. The total thickness of the dielectric layer and the support layer is 15% or less, preferably 10% or less, more preferably 9% or less, and particularly preferably 8% or less of the wavelength of the electromagnetic wave absorption peak in the frequency range of 20 GHz to 90 GHz of the electromagnetic wave absorption sheet. The thinner the total thickness of the dielectric layer and the support layer, the more preferable it is. Although no particular lower limit value is set, it may be 1% or more from the viewpoint of the strength of the electromagnetic wave absorption sheet.

[0020] <Dielectric layer> The dielectric layer includes a conductive filler and a binder resin that binds the conductive fillers. "Conductive" refers to the property of having a volume resistivity of 10 6 Ω·cm or less. The volume resistivity is a value measured by the four-terminal method and can be measured, for example, using a low-resistance meter Loresta GP (MCP-T610) manufactured by Mitsubishi Chemical Analytech Co., Ltd.

[0021] Examples of the conductive filler include metals, metal oxides, ceramics, alloys, carbon materials, and the like. Among these, carbon materials are preferred. By using carbon materials, the electromagnetic wave absorption sheet can be made lighter in weight, and the thickness can be reduced, and as a result, the flexibility can be improved.

[0022] Examples of carbon materials include graphene, graphite, graphene oxide, carbon nanotubes, carbon black, and graphite. Among these, carbon nanotubes and carbon black are particularly preferred because they have a high aspect ratio and are highly effective in increasing the dielectric constant. It is preferable to incorporate at least one of carbon nanotubes and carbon black into the dielectric layer because it is easy to adjust the dielectric constant within an appropriate range, and it is particularly preferable to incorporate a combination of carbon nanotubes and carbon black into the dielectric layer.

[0023] The aspect ratio of the carbon nanotube is preferably 10 to 4000 or less, more preferably 50 to 1000, and particularly preferably 100 to 500.

[0024] The average length (number mean length) of the carbon nanotubes is preferably 0.1 μm to 1000 μm or less, more preferably 1 μm to 500 μm, and particularly preferably 5 μm to 300 μm, and may be in the range of, for example, 50 μm to 200 μm. By using carbon nanotubes with a length within an appropriate range, the dielectric layer can be made thinner.

[0025] The outer diameter of the carbon nanotubes is preferably 0.5 to 200 nm, and more preferably 1 to 100 nm. When the outer diameter of the carbon nanotubes is within this range, they can easily connect to each other, and the dielectric constant can be appropriately adjusted.

[0026] Such carbon nanotubes may be either single-walled carbon nanotubes or multi-walled carbon nanotubes, but multi-walled carbon nanotubes are preferred.

[0027] Carbon black is a type of carbon nanoparticle produced by the thermal decomposition of hydrocarbons. Multiple primary particles in carbon black form a structure, which increases the aspect ratio and allows for the imparting of an appropriate dielectric constant to a dielectric layer, either alone or in combination with other carbon materials such as carbon nanotubes. The average primary particle diameter (number average) of carbon black is preferably 10 nm to 200 nm, more preferably 5 to 100 nm.

[0028] The ratio of carbon nanotubes to carbon black contained in the carbon material is not particularly limited, but is preferably 70% by weight or more, more preferably 80% by weight or more, particularly preferably 90% by weight or more, and may be 100% by weight.

[0029] The ratio of carbon nanotubes to carbon black is not particularly limited and may be changed as appropriate, but adjusting the ratio of carbon nanotubes to carbon black makes it easier to adjust the dielectric constant of the dielectric layer to the designed value. In one embodiment, the weight ratio of carbon nanotubes to carbon black (CNT / CB) is preferably 1 / 50 to 1 / 1, more preferably 1 / 20 to 1 / 2, and particularly preferably 1 / 10 to 1 / 4. In one embodiment, only carbon nanotubes may be used without carbon black, or only carbon black may be used without carbon nanotubes.

[0030] Conductive fillers may be used individually or in combination of multiple types. The conductive filler content is preferably in the range of 1% to 60% by mass, more preferably in the range of 5% to 50% by mass, and even more preferably in the range of 10% to 40% by mass, relative to the total mass of the dielectric layer. By setting the conductive filler content within any of the above ranges, the dielectric constant of the dielectric layer can be adjusted to an appropriate range. In the conductive filler, the carbon material content (particularly the carbon nanotube and carbon black content) is preferably 50% by weight or more, more preferably 80% by weight or more, particularly preferably 90% by weight or more, and may also be 100% by weight.

[0031] The binder resin is not particularly limited and various resins can be used. Examples of binder resins include thermoplastic resins, such as polyester (polyethylene terephthalate (PET), etc.), epoxy resin, phenoxy resin, polyarylene sulfide (polyphenylene sulfide, etc.), (meth)acrylic resin, polyolefin (polyethylene, polypropylene, etc.), polystyrene, acrylic resin, polyamide, polyimide, polyamide-imide, polyethersulfone, polyetheretherketone, and polycarbonate. Among these, polyamide, epoxy resin, phenoxy resin, and (meth)acrylic resin are preferred in terms of their dispersibility with solvents.

[0032] The binder resin may be a thermosetting resin. A specific example of a binder resin is a combination of an epoxy resin and a main resin that reacts with the epoxy resin. When a thermosetting resin is used as the binder resin, a curing accelerator may be further added. Examples of curing accelerators for epoxy resins include imidazole and amines, and these known curing accelerators may be added as appropriate. From the viewpoint of adhesion and heat resistance, the content of the curing accelerator is preferably in the range of 1 to 10 parts by mass, and particularly preferably in the range of 2 to 5 parts by mass, per 100 parts by mass of epoxy resin.

[0033] The binder resin may be used alone or in combination of multiple types. The binder resin content is preferably in the range of 40% to 99% by mass, more preferably in the range of 50% to 95% by mass, and even more preferably in the range of 60% to 90% by mass, relative to the total mass of the dielectric layer.

[0034] Solvents may be used to coat the conductive filler and binder resin. In particular, when a thermosetting resin is used as the binder resin, it is preferable to use a solvent for coating. The solvent is preferably one that uniformly disperses the conductive filler and binder resin, but it is not particularly limited, and various organic solvents and water-based solvents can be used. The solvent is removed by drying after coating and is not usually contained in the dielectric layer, but a small amount may remain. When coating with a solvent, the thickness of the dielectric layer tends to become thinner, so it is preferable to adjust the dielectric constant and thickness of the dielectric layer and support layer so that electromagnetic waves can be absorbed even if the dielectric layer is thin.

[0035] Examples of solvents include alcohols such as methanol, ethanol, isopropyl alcohol, n-propyl alcohol, isobutyl alcohol, n-butyl alcohol, benzyl alcohol, ethylene glycol monomethyl ether, propylene glycol monomethyl ether, diethylene glycol monomethyl ether, and diacetone alcohol; ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, methyl amyl ketone, cyclohexanone, and isophorone; aromatic hydrocarbons such as toluene, xylene, ethylbenzene, and mesitylene; esters such as methyl acetate, ethyl acetate, ethylene glycol monomethyl ether acetate, and 3-methoxybutyl acetate; and aliphatic hydrocarbons such as hexane, heptane, cyclohexane, and methylcyclohexane. These solvents may be used individually or in combination of two or more.

[0036] When using a solvent, from the viewpoint of workability, including film-forming properties, the solvent contained in the coating liquid is used such that the solid content concentration is preferably in the range of 3% to 80% by mass, and more preferably in the range of 10% to 50% by mass.

[0037] The dielectric layer's ability to absorb electromagnetic waves is improved by adjusting its resistivity. The surface resistivity of the dielectric layer is 1.0 × 10⁻⁶. 2 Ω / □~1.0×10 8It is preferable that the ratio is Ω / □, and 1.0 × 10 3 Ω / □~1.0×10 7 A value of Ω / □ is more preferable. Surface resistivity (unit: Ω / □, Ω / sq.) is a value measured by the four-terminal method specified in JIS K7194:1994, and can be measured, for example, using a low-resistance meter Rolester GP (MCP-T610; manufactured by Mitsubishi Chemical Analytec Co., Ltd.).

[0038] The volume resistivity of the dielectric layer is approximately 1.0 Ω·cm ~ 5.0 × 10⁻⁶ 5 It is preferable that the value is Ω·cm, and 10Ω·cm ~ 1.0 × 10 5 A value of Ω·cm is more preferable. Volume resistivity (unit: Ω·cm) is a value measured by the four-terminal method specified in JIS K7194:1994, and can be measured, for example, using a low-resistivity meter Rolester GP (MCP-T610; manufactured by Mitsubishi Chemical Analytec Co., Ltd.).

[0039] From the viewpoint of improving the absorption rate of the electromagnetic wave absorbing sheet, the real part of the complex relative permittivity of the dielectric layer at the wavelength of the electromagnetic wave absorption peak in the frequency range of 20 GHz to 90 GHz is preferably 10 or more, more preferably 30 or more, and particularly preferably 50 or more. From the viewpoint of improving the absorption rate of the electromagnetic wave absorbing sheet, the real part of the complex relative permittivity of the dielectric layer at the wavelength of the electromagnetic wave absorption peak in the frequency range of 20 GHz to 90 GHz is preferably 100 or less, more preferably 90 or less, and particularly preferably 80 or less.

[0040] By appropriately adjusting the ratio of the real and imaginary parts of the complex relative permittivity of the dielectric layer, the total thickness of the dielectric layer and the support layer can be reduced. The ratio of the real and imaginary parts is calculated as (value of imaginary part) / (value of real part), and will hereafter be referred to as "tanδ". From the viewpoint of reducing the thickness of the support layer, the tanδ of the dielectric layer is preferably 0.8 or less, more preferably 0.6 or less, particularly preferably 0.5 or less, and may also be 0.4 or less. From the viewpoint of reducing the thickness of the support layer, the tanδ of the dielectric layer is preferably 10.01 or more, more preferably 0.05 or more, particularly preferably 0.1 or more, and may also be 0.2 or more.

[0041] As the dielectric layer thickens, the electromagnetic wave absorption peak shifts to the lower frequency side. Therefore, from the viewpoint of obtaining an electromagnetic wave absorption peak in the high-frequency band, the thickness of the dielectric layer is preferably 10 μm or more, more preferably 40 μm or more, and particularly preferably 100 μm or more. From the viewpoint of reducing the total thickness of the electromagnetic wave absorption sheet, the thickness of the dielectric layer is preferably 1000 μm or less, more preferably 500 μm or less, and particularly preferably 300 μm or less.

[0042] <Support layer> By providing a support layer, a high absorption rate can be achieved and the thickness of the electromagnetic wave absorbing sheet can be reduced. It is preferable that the support layer be a uniform resin sheet mainly composed of resin, as this facilitates the processing of the electromagnetic wave absorbing sheet. It is preferable that the support layer is laminated over the entire surface of one side of the dielectric layer (the side that is placed on the substrate or reflective layer). The material of the support layer is not particularly limited, but it is preferable that it is flexible, and it is preferable that it is a material that has a low Shore A hardness when made into a sheet. The Shore A hardness is measured by the method specified in JIS K 6253 (measurement temperature 23°C). The Shore A hardness of the resin (or resin composition if it contains multiple resins) constituting the resin sheet is preferably 90 or less, more preferably 80 or less, and particularly preferably 70 or less.

[0043] The support layer may have a lower dielectric constant compared to the dielectric layer. The real part of the complex relative permittivity of the support layer at the wavelength of the electromagnetic wave absorption peak in the frequency range of 20 GHz to 90 GHz is preferably 0.01 to less than 10, more preferably 0.05 to 5, and particularly preferably 0.1 to 3. The imaginary part of the complex relative permittivity of the support layer at the wavelength of the electromagnetic wave absorption peak in the frequency range of 20 GHz to 90 GHz is preferably 0.0001 to 1, and particularly preferably 0.001 to 0.1.

[0044] From the viewpoint of lowering the dielectric constant, the support layer does not need to contain conductive fillers, unlike the dielectric layer. The content of conductive fillers in the support layer is preferably 5% by weight or less, more preferably 1% by weight or less, and may even be 0% by weight.

[0045] The resin component included in the support layer is preferably an electrically insulating resin, such as polyvinyl chloride resin, polyester (polyethylene terephthalate (PET), etc.), epoxy resin, phenoxy resin, polyarylene sulfide (polyphenylene sulfide, etc.), (meth)acrylic resin, polyolefin (polyethylene, polypropylene, etc.), polystyrene, acrylic resin, polyamide, polyimide, polyamide-imide, polyethersulfone, polyetheretherketone, and polycarbonate. The resin included in the resin layer may be the same as or different from the resin included as a binder resin in the dielectric layer.

[0046] A rubbery material with low hardness can preferably be used for the support layer. Specific examples of rubber include acrylic rubber (hereinafter also referred to as ACM), ethylene propylene diene rubber (hereinafter also referred to as EPDM), silicone rubber, chloroprene rubber, acrylonitrile butadiene rubber, styrene butadiene rubber, butadiene rubber, isoprene rubber, butyl rubber, and natural rubber.

[0047] The thickness of the support layer is determined according to the dielectric constant and thickness of the dielectric layer. When the tanδ of the dielectric layer is high, it is preferable to make the support layer thicker in order to obtain a high absorption rate. It is also preferable to make the support layer thicker when the dielectric layer is thin. As a result of the above, the thickness of the support layer becomes larger than the thickness of the dielectric layer. Therefore, the total thickness of the electromagnetic wave absorbing sheet can be reduced by adjusting the thickness of the support layer rather than the dielectric layer. Considering this point, it is preferable to adjust the dielectric constant and thickness of the dielectric layer so that the support layer becomes thinner.

[0048] The thickness of the support layer is preferably 1000 μm or less, more preferably 800 μm or less, and particularly preferably 600 μm or less. From the viewpoint of the strength of the support layer, the thickness of the support layer is preferably 50 μm or more, more preferably 100 μm or more, and particularly preferably 150 μm or more.

[0049] To achieve high absorption and a thin electromagnetic wave absorbing sheet, the ratio of the dielectric layer thickness to the support layer thickness is preferably 0.01 or more, more preferably 0.05 or more, particularly preferably 0.1 or more, and may also be 0.2 or more. The ratio of the dielectric layer thickness to the support layer thickness is preferably 2 or less, more preferably 1.5 or less, particularly preferably 1 or less, and may also be 0.3 or less. The ratio of the dielectric layer thickness to the support layer thickness is calculated as (thickness of the dielectric layer) / (thickness of the support layer).

[0050] The dielectric layer and the support layer can be prepared separately as sheets and laminated by thermal lamination. Alternatively, either the dielectric layer or the support layer can be prepared in advance, and the raw material liquid can be applied to the prepared layer to laminate them. When a thermosetting resin is used for the dielectric layer, it is preferable to disperse the raw material in a solvent, and the latter method is preferred.

[0051] <Reflection layer> An electromagnetic wave absorbing sheet is laminated on a substrate that reflects electromagnetic waves. In one embodiment, the substrate that reflects electromagnetic waves may constitute a part of the electromagnetic wave absorbing sheet as a reflective layer. In another embodiment, a reflective layer may not be provided. For example, an electromagnetic wave absorbing sheet having a dielectric layer and a support layer may be attached to a metal component such as a housing provided for an electronic device as a substrate. In this way, the electromagnetic wave absorbing sheet can absorb electromagnetic waves generated from the electronic device.

[0052] The reflective layer has the function of reflecting electromagnetic waves. There are no particular restrictions on the material used for the reflective layer, and metal plates, molded products, etc., can be used. The reflective layer may be a metal molded product such as a metal plate or metal foil. Examples of metals that can be used for the reflective layer include iron, copper, silver, gold, aluminum or alloys thereof, stainless steel, etc. There are no particular restrictions on the metal content in the reflective layer, and it can be appropriately selected according to the purpose, but 30% by mass or more is preferred, 50% by mass or more is preferred, 80% by mass or more is more preferred, and 90% by mass or more is particularly preferred. There are no particular restrictions on the thickness of the reflective layer, and it may have a thickness in the range of 1 μm to 5 mm, for example.

[0053] There are no particular restrictions on the thickness of the reflective layer; it can be selected appropriately according to the purpose and application. The reflective layer may consist of only one layer or two or more layers.

[0054] <Adhesive layer> An adhesive layer may be provided between the dielectric layer and the support layer. An adhesive layer may be provided between the support layer and the reflective layer. By providing an adhesive layer, even materials with low adhesion can be used. The adhesive used in the adhesive layer is not particularly limited, and various adhesives such as hot melt adhesives, thermosetting adhesives, photocuring adhesives, and adhesives may be used. The adhesive layer may become part of the dielectric layer, support layer, and reflective layer depending on its function, but it is usually made of a non-conductive soft resin similar to the support layer and is part of the support layer. Furthermore, if the resin contained in the dielectric layer or support layer has adhesive properties, an adhesive layer may not be provided.

[0055] Since the electromagnetic wave absorbing sheet of the present disclosure is in the form of a thin film, it can be attached to substrates of various shapes. Therefore, it is particularly preferable to attach the electromagnetic wave absorbing sheet to a curved substrate or a substrate with irregularities so that the characteristics can be utilized.

Examples

[0056] Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples as long as the gist thereof is not exceeded. Unless otherwise specified, "parts" are based on mass.

[0057] (Components for forming the dielectric layer) Details of the components (components shown in Table 1) used for forming the dielectric layer are shown below. The thickness of the film used for the support layer is as described in Table 1. ·Filler CNT<1>: Carbon nanotube Product name: LUCAN BT1003M (multi-walled carbon nanotube, diameter 13 nm, length 50 μm), manufactured by LG Chem Ltd. CNT<2>: Carbon nanotube Multi-walled carbon nanotube, diameter 50 nm, length 10 μm CB: Carbon black Product name: Mitsubishi Carbon Black #3030B (particle size 55 nm), manufactured by Mitsubishi Chemical Corporation Epsilon iron oxide: ε-Ga 0.47 Fe 1.53 O3 Ferrite: Li-Mn ferrite Binder: Resin obtained by adding an amine-based curing agent to cresol novolak type epoxy resin (product name CNE-195XL) and curing it Soft PVC: Soft vinyl chloride sheet (manufactured by Aquis Co., Ltd.), Shore A hardness 60, thickness as described in the following table NR: Natural rubber sheet (manufactured by MonotaLo Co., Ltd.), Shore A hardness 65, thickness as described in the following table Silicone rubber: Silicone rubber sheet (manufactured by Togawa Rubber Co., Ltd.), Shore A hardness 50, thickness as shown in the table below.

[0058] (Example 1) - Fabrication of electromagnetic wave absorbing sheets - The dielectric layer was fabricated as follows. The base material is 75 parts by mass of binder in a toluene solution, with 20 parts by mass of CNT as filler. <2> A coating solution was prepared by mixing 5 parts by mass of CB with the content ratio shown in Table 1. The prepared coating solution was applied onto a release film using a bar coater to form a coating film. The formed coating film was dried at 100°C to produce a dielectric thin film with a thickness of 25 μm. Subsequently, multiple of these dielectric thin films were stacked and hot-pressed at 120°C to produce a dielectric layer with a thickness of 50 μm.

[0059] Next, a sheet of the material listed in Table 1 (thickness: 675 μm) was prepared as the support layer, and a metal plate (aluminum, 120 mm x 120 mm, thickness 0.8 mm) was prepared as the reflective layer. The dielectric layer, support layer, and reflective layer were directly laminated by thermocompression bonding. Electromagnetic wave absorbing sheets with a two-layer laminated structure of dielectric layer / support layer and electromagnetic wave absorbing sheets with a three-layer laminated structure of dielectric layer / support layer / reflective layer were fabricated.

[0060] The evaluation method for the obtained electromagnetic wave absorbing sheet is shown below.

[0061] -Measurement of adhesion retention force on curved surfaces- Two layers of electromagnetic wave absorbing sheets (50mm x 150mm) were fabricated and attached to the side of a cylindrical container with a diameter of 100mm. The support layer was attached with acrylic adhesive tape and left at 25°C. Those that did not peel off after 24 hours were considered good and received a ○ rating. Those that peeled off after 24 hours were considered poor and received a × rating.

[0062] - Measurement of surface resistivity and volume resistivity - The surface resistivity and volume resistivity of the dielectric layer of the fabricated three-layer electromagnetic wave absorbing sheet were measured using a low-resistivity meter, Rolester GP (MCP-T610), manufactured by Mitsubishi Chemical Analytec Co., Ltd., based on the four-terminal method specified in JIS K7194:1994. The measurement results are shown in Table 1.

[0063] -Measurement of dielectric constant, absorption peak intensity, and absorption peak frequency- The dielectric constant was calculated from the measured S-parameters at each frequency in the 18GHz to 110GHz band using a Keysight Technologies N5290A vector network analyzer and an S-parameter method measurement fixture. Furthermore, the electromagnetic wave absorption peak intensity and electromagnetic wave absorption peak frequency were measured using the vector network analyzer and free-space S-parameter method described above to determine the reflection characteristics (S11) of electromagnetic waves at each frequency in the 18 GHz to 110 GHz band. The absorption amount was then calculated from Equation 1 below, and the electromagnetic wave absorption characteristics were determined. Absorption amount (dB) = Input signal - Reflection characteristics (S11) ... Equation 1 Reflection characteristics, also known as the reflection coefficient, refer to the ratio of the reflected wave to the incident wave. The electromagnetic wave absorption characteristics were determined by plotting the amount of absorption at each frequency, and the point where the absorption amount was highest (also called the electromagnetic wave absorption peak maximum) was defined as the electromagnetic wave absorption peak. The absorption rate of the electromagnetic wave absorption peak is the "absorption peak intensity," and the frequency at which the electromagnetic wave absorption peak appears is the "absorption peak frequency." The absorption peak frequency was converted to wavelength to determine the absorption peak wavelength (λ), and then the ratio of the total thickness of the dielectric layer and support layer to λ was calculated. The calculated values ​​are shown in Table 1.

[0064] -comprehensive evaluation- (1) The total thickness of the electromagnetic wave absorbing sheet was 15% or less of the peak absorption wavelength, (2) the absorption rate was -20 dB or less, and (3) the adhesion retention strength to curved surfaces was rated as ○. Sheets that did not meet any one of the above criteria were rated as ×.

[0065] (Examples 2-10 and Comparative Examples 1-4) Electromagnetic wave absorbing sheets were fabricated and evaluated in the same manner as in Example 1, except that the dielectric layer and support layer were changed to those listed in Tables 1 and 2. Comparative Examples 2 to 4 fabricated electromagnetic wave absorbing sheets using magnetic materials for the dielectric layer, but it was necessary to increase the thickness of the dielectric layer to obtain a high absorption rate. For this reason, electromagnetic wave absorbing sheets with a support layer were not fabricated.

[0066] [Table 1]

[0067] [Table 2] In the table, "NA" means not measured (not applicable). "E" represents a power of 10; for example, "2.81E+01" means 2.81 × 10⁻¹⁰ 1 This means 28.1.

Claims

1. An electromagnetic wave absorbing sheet having a dielectric layer and a support layer, The dielectric layer comprises a conductive filler and a binder resin. The support layer is a resin sheet, The electromagnetic wave absorbing sheet absorbs electromagnetic waves with frequencies of 20 to 90 GHz, and its absorption rate is -20 dB or less at the electromagnetic wave absorption peak in the frequency range of 20 to 90 GHz. An electromagnetic wave absorbing sheet in which the total thickness of the dielectric layer and the support layer is 15% or less of the wavelength of the electromagnetic wave absorption peak.

2. The electromagnetic wave absorbing sheet according to claim 1, wherein the conductive filler is a carbon material.

3. The electromagnetic wave absorbing sheet according to claim 1 or 2, wherein the real part of the complex relative permittivity of the dielectric layer is 100 or less, and the ratio of the real part to the imaginary part (imaginary part / real part) of the complex relative permittivity of the dielectric layer is 0.8 or less.

4. The electromagnetic wave absorbing sheet according to claim 1 or 2, wherein the carbon material includes a fibrous carbon material.

5. The electromagnetic wave absorbing sheet according to claim 1 or 2, wherein the carbon material comprises at least one of carbon nanotubes and carbon black.

6. The electromagnetic wave absorbing sheet according to claim 1 or 2, wherein the ratio of the thickness of the dielectric layer to the thickness of the support layer (thickness of the dielectric layer / thickness of the support layer) is 0.01 or more and 1.5 or less.

7. The electromagnetic wave absorbing sheet according to claim 1 or 2, wherein the real part of the complex relative permittivity of the support layer is 10 or less.

8. The electromagnetic wave absorbing sheet according to claim 1 or 2, wherein the Shore A hardness of the resin constituting the support layer is 90 or less.

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