Electromagnetic wave absorption sheet

Abstract

To provide an electromagnetic wave absorption sheet which can desirably absorb electromagnetic waves of higher frequencies in at least a milli-wave zone, is highly flexible, and can be easily arranged in a desired part.SOLUTION: The electromagnetic wave absorption sheet has an electromagnetic wave absorption layer including a magnetic oxide iron as an electromagnetic wave absorption material which resonates with magnetism and a rubber binder. The electromagnetic wave absorption sheet includes 2 to 50 parts of the rubber binder to 100 parts of the magnetic oxide iron. The input impedance value when the electromagnetic wave absorption layer is extended in the range of an elastic area is 360 Ω to 450 Ω.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to an electromagnetic wave absorbing sheet that absorbs electromagnetic waves, and in particular to an electromagnetic wave absorbing sheet that has an electromagnetic wave absorbing material that absorbs electromagnetic waves by magnetic resonance, absorbs electromagnetic waves at high frequencies above the millimeter wave band, and has elasticity that stretches in an in-plane direction. [Background technology]

[0002] 2. Description of the Related Art In order to avoid the influence of leakage electromagnetic waves emitted to the outside from electric circuits and the like, and undesirably reflected electromagnetic waves, electromagnetic wave absorbing sheets that absorb electromagnetic waves are used.

[0003] In recent years, research has been progressing on technologies that utilize centimeter waves, which have a frequency band of several gigahertz (GHz), as well as millimeter waves, which have frequencies from 30 gigahertz to 300 gigahertz, and electromagnetic waves with a frequency of 1 terahertz (THz) as electromagnetic waves in a high-frequency band beyond the millimeter wave band, for use in mobile communications such as mobile phones, wireless LAN, and automated toll collection systems (ETC).

[0004] In response to this technological trend toward utilizing electromagnetic waves of higher frequencies, there is an increasing demand for electromagnetic wave absorbers and sheet-shaped electromagnetic wave absorbing sheets that absorb unwanted electromagnetic waves and are capable of absorbing electromagnetic waves from the gigahertz to terahertz bands.

[0005] As an electromagnetic wave absorber that absorbs electromagnetic waves in the millimeter wave band and higher, epsilon iron oxide (ε-Fe 2 O 3 An electromagnetic wave absorber having a packed structure of particles having epsilon iron oxide crystals in the magnetic phase has been proposed (see Patent Document 1). Also, a sheet-shaped oriented body has been proposed in which fine particles of epsilon iron oxide are kneaded with a binder, and an external magnetic field is applied when the binder is dried and hardened to enhance the magnetic field orientation of the epsilon iron oxide particles (see Patent Document 2).

[0006] Furthermore, as an elastic electromagnetic wave absorbing sheet, an electromagnetic wave absorbing sheet capable of absorbing centimeter waves, which is made of silicone rubber with carbon nanotubes dispersed therein, has been proposed (see Patent Document 3).

[0007] Also, as a low-cost electromagnetic wave absorbing sheet capable of absorbing electromagnetic waves in the 75 to 77 GHz frequency band and having profitability for consumer use, a sheet in which silicon carbide powder is dispersed in a rubber matrix resin on the surface of a metal body has been proposed (see Patent Document 4).Furthermore, as an adhesive sheet that is attached to a flexible printed wiring board to shield electromagnetic waves from the outside, a sheet in which a conductive layer containing conductive particles and an insulating layer are laminated is laminated, and the repulsive force of the sheet is kept within a predetermined range, thereby providing bending resistance and heat resistance that allows bending together with the flexible printed wiring board (see Patent Document 5). [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2008-60484 A [Patent Document 2] JP 2016-135737 A [Patent Document 3] JP 2011-233834 A [Patent Document 4] JP 2005-57093 A [Patent Document 5] JP 2013-4854 A Summary of the Invention [Problem to be solved by the invention]

[0009] When shielding against leakage electromagnetic waves from a source that generates electromagnetic waves, it is necessary to place an electromagnetic wave absorbing material on a housing or the like that covers the target circuit components. However, particularly when the shape of the placement location is not flat, it is more convenient and preferable to use an electromagnetic wave absorbing sheet that has flexibility and elasticity that stretches in an in-plane direction rather than using an electromagnetic wave absorber that is a solid body.

[0010] However, for example, the electromagnetic wave absorbing sheet described in Patent Document 3 cannot absorb electromagnetic waves with frequencies of several tens of gigahertz or more, which is the millimeter wave band. In addition, the electromagnetic wave absorbing sheet described in Patent Document 4 is laminated on a non-stretchable metal body, and the adhesive sheet described in Patent Document 5 is thermocompression bonded to a flexible printed wiring board, so neither of them has elasticity.

[0011] Thus, no elastic sheet-shaped electromagnetic wave absorbing sheet has been realized as an electromagnetic wave absorbing member capable of absorbing electromagnetic waves at frequencies of several tens of gigahertz or more, which is the millimeter wave band.

[0012] In order to solve the problems of the related art, the present disclosure aims to realize an electromagnetic wave absorbing sheet that can effectively absorb electromagnetic waves at high frequencies above the millimeter wave band and has elasticity that stretches in the in-plane direction. [Means for solving the problem]

[0013] In order to solve the above problems, the electromagnetic wave absorbing sheet disclosed in the present application has an electromagnetic wave absorbing layer containing magnetic iron oxide, which is an electromagnetic wave absorbing material that magnetically resonates, and a rubber binder, and is characterized in that the electromagnetic wave absorbing layer contains 2 to 50 parts of the rubber binder per 100 parts of the magnetic iron oxide, and has an input impedance value of 360 Ω to 450 Ω when stretched within its elastic range. Effect of the Invention

[0014] The electromagnetic wave absorbing sheet disclosed in the present application has an electromagnetic wave absorbing layer that contains magnetic iron oxide as an electromagnetic wave absorbing material, which resonates magnetically in a high frequency band of tens of gigahertz or more, and can convert electromagnetic waves in a high frequency band of several tens of gigahertz or more into heat and absorb them. In addition, the electromagnetic wave absorbing sheet has a rubber binder and a maximum elongation rate of 20 to 200% in the elastic region in the in-plane direction, which makes it easy to arrange the sheet in a desired location, and furthermore, can realize an electromagnetic wave absorbing sheet that can cover movable parts. [Brief description of the drawings]

[0015] [Figure 1] 1 is a cross-sectional view illustrating a configuration of an electromagnetic wave absorbing sheet according to a first embodiment. [Diagram 2] FIG. 1 is a diagram illustrating the electromagnetic wave absorption characteristics of epsilon iron oxide in which a portion of the Fe site is substituted. [Diagram 3] This is a diagram for explaining the elongation of an electromagnetic wave absorbing sheet having an electromagnetic wave absorbing layer containing a rubber binder when an external tensile stress is applied. Fig. 3(a) shows the change in elongation of the electromagnetic wave absorbing sheet that causes breakage when the maximum elongation is exceeded. Fig. 3(b) shows the change in elongation of the electromagnetic wave absorbing sheet that causes plastic deformation when the maximum elongation is exceeded. [Figure 4] FIG. 13 is a diagram showing the relationship between an externally applied tensile stress and elongation in an electromagnetic wave absorbing sheet of an example. [Diagram 5] FIG. 3 is a diagram showing changes in electromagnetic wave absorbing characteristics due to elongation of the electromagnetic wave absorbing sheet according to the first embodiment. [Figure 6] FIG. 3 is a diagram showing the relationship between the thickness and the amount of electromagnetic wave absorption of the electromagnetic wave absorbing sheet according to the first embodiment. [Figure 7] FIG. 4 is a cross-sectional view illustrating the configuration of an electromagnetic wave absorbing sheet according to a second embodiment. [Figure 8] FIG. 11 is a diagram showing changes in the electromagnetic wave absorbing characteristics when the elongation rate of the electromagnetic wave absorbing sheet according to the second embodiment is changed. [Figure 9] FIG. 11 is a diagram showing the relationship between the thickness and the amount of electromagnetic wave attenuation of an electromagnetic wave absorbing sheet according to a second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] The electromagnetic wave absorbing sheet disclosed in the present application has an electromagnetic wave absorbing layer that contains magnetic iron oxide, which is an electromagnetic wave absorbing material that magnetically resonates in a frequency band equal to or higher than the millimeter wave band, and a rubber binder, and has a maximum elongation rate in the elastic region in one in-plane direction of 20% to 200%.

[0017] In this way, the electromagnetic wave absorbing sheet disclosed in the present application can absorb electromagnetic waves in the high frequency band of 30 gigahertz or more, which is the millimeter wave band, by the magnetic resonance of the magnetic iron oxide, which is the electromagnetic wave absorbing material. In addition, by using the electromagnetic wave absorbing material and a rubber binder, an electromagnetic wave absorbing sheet having high elasticity with a maximum elongation rate of 20 to 200% in the in-plane direction can be realized. Therefore, when the electromagnetic wave absorbing sheet is arranged in a housing or the like that houses an electronic circuit to be shielded, the ease of handling of the electromagnetic wave absorbing sheet is improved, and it becomes easy to arrange the electromagnetic wave absorbing sheet on a complexly curved surface in particular. Furthermore, it is possible to cover the movable parts of a member whose shape changes, such as the joint parts of an arm member, to prevent undesired radiation or intrusion of electromagnetic waves.

[0018] In the electromagnetic wave absorbing sheet disclosed in the present application, the magnetic iron oxide is preferably epsilon iron oxide. By using epsilon iron oxide, which absorbs electromagnetic waves with frequencies higher than 30 gigahertz, as the electromagnetic wave absorbing material, it is possible to realize an electromagnetic wave absorbing sheet that absorbs electromagnetic waves of high frequencies.

[0019] In this case, it is preferable that a part of the Fe site of the epsilon iron oxide is substituted with a trivalent metal atom, thereby making it possible to realize an electromagnetic wave absorbing sheet that absorbs electromagnetic waves in a desired frequency band by utilizing the characteristics of epsilon iron oxide, which has a different magnetic resonance frequency depending on the material substituting the Fe site.

[0020] It is also preferable that the volume content of the magnetic iron oxide in the electromagnetic wave absorbing layer is 30% or more. In this way, the imaginary part of the magnetic permeability (μ″) of the electromagnetic wave absorbing layer can be increased, and an electromagnetic wave absorbing sheet with high electromagnetic wave absorbing properties can be realized.

[0021] Furthermore, it is preferable to use either acrylic rubber or silicone rubber as the rubber binder. By using a rubber material with high heat resistance, a highly reliable electromagnetic wave absorbing sheet can be realized.

[0022] Furthermore, it is preferable that the input impedance value of the electromagnetic wave absorbing layer when stretched by 5 to 75% of the maximum elongation in the elastic region matches the impedance value in air. In this way, the input impedance value can be made close to the impedance value in air over a wide range of elongation of the electromagnetic wave absorbing sheet, and high electromagnetic wave absorbing properties can be maintained.

[0023] It is also preferable that the input impedance value when the electromagnetic wave absorbing layer is stretched within the elastic region is 360Ω to 450Ω. By doing so, even when the electromagnetic wave absorbing sheet expands and contracts within its elastic region, the input impedance value is prevented from being significantly different from the impedance value in air, and a certain level of electromagnetic wave absorbing characteristics can be exhibited.

[0024] Furthermore, in the electromagnetic wave absorbing sheet disclosed in the present application, it is preferable that a reflective layer is formed in contact with one surface of the electromagnetic wave absorbing layer to reflect the electromagnetic waves transmitted through the electromagnetic wave absorbing layer, thereby realizing a so-called reflective type electromagnetic wave absorbing sheet that can reliably block and absorb electromagnetic waves in a high frequency band equal to or higher than the millimeter wave band.

[0025] It is also preferable that the electromagnetic wave absorbing sheet further comprises an adhesive layer that enables the electromagnetic wave absorbing sheet to be attached. In this way, it is possible to realize an electromagnetic wave absorbing sheet that has high electromagnetic wave absorbing properties and is easy to handle and can be easily placed in a desired location.

[0026] A second electromagnetic wave absorbing sheet disclosed in the present application comprises an electromagnetic wave absorbing layer containing an electromagnetic wave absorbing material and a rubber binder, and a reflective layer in contact with one side of the electromagnetic wave absorbing layer for reflecting electromagnetic waves that have passed through the electromagnetic wave absorbing layer, wherein the electromagnetic wave absorbing material is magnetic iron oxide that magnetically resonates with electromagnetic waves of a predetermined frequency, and the input impedance value of the electromagnetic wave absorbing layer when stretched in one in-plane direction matches the impedance value in air.

[0027] The second electromagnetic wave absorbing sheet disclosed in the present application has such a configuration, and on the premise that the electromagnetic wave absorbing sheet is stretched to a certain extent during actual use, it is possible to match the impedance value in air and the input impedance value over a wide range of elongation, thereby improving the electromagnetic wave absorption characteristics of the elastic electromagnetic wave absorbing sheet during actual use.

[0028] Hereinafter, the electromagnetic wave absorbing sheet disclosed in the present application will be described with reference to the drawings.

[0029] In addition, since "radio waves" can be understood in a broader sense as a type of electromagnetic wave, in this specification, the term "electromagnetic waves" will be used, for example, when referring to a radio wave absorber as an electromagnetic wave absorber.

[0030] (First embodiment) First, as a first embodiment of the electromagnetic wave absorbing sheet disclosed in the present application, a so-called transmission type electromagnetic wave absorbing sheet that does not include a reflective layer that reflects electromagnetic waves incident on the electromagnetic wave absorbing sheet will be described.

[0031] [Seat configuration] FIG. 1 is a cross-sectional view showing the configuration of an electromagnetic wave absorbing sheet according to a first embodiment of the present application.

[0032] Note that FIG. 1 is a diagram provided to facilitate understanding of the configuration of the electromagnetic wave absorbing sheet according to this embodiment, and the sizes and thicknesses of the components shown in the diagram are not depicted based on reality.

[0033] The electromagnetic wave absorbing sheet exemplified in this embodiment includes an electromagnetic wave absorbing layer 1 containing magnetic iron oxide 1a, which is a particulate electromagnetic wave absorbing material, and a rubber binder 1b. Note that the electromagnetic wave absorbing sheet shown in Fig. 1 has an adhesive layer 2 formed on the back side (lower side in Fig. 1) of the electromagnetic wave absorbing layer 1, for enabling the electromagnetic wave absorbing sheet to be attached to a predetermined location such as the inner surface or outer surface of the housing of an electronic device.

[0034] The electromagnetic wave absorbing sheet of this embodiment converts electromagnetic waves into thermal energy through magnetic loss due to magnetic resonance caused by the magnetic iron oxide 1a contained in the electromagnetic wave absorbing layer 1, and absorbs the electromagnetic waves. Therefore, the electromagnetic wave absorbing sheet of this embodiment can be used as a so-called transmission type electromagnetic wave absorbing sheet that absorbs electromagnetic waves that pass through the electromagnetic wave absorbing layer 1 without providing a reflective layer on one surface of the electromagnetic wave absorbing layer 1.

[0035] Moreover, in the electromagnetic wave absorbing sheet of this embodiment, various rubber materials are used as the binder 1b constituting the electromagnetic wave absorbing layer 1. Therefore, it is possible to obtain an electromagnetic wave absorbing sheet that easily expands and contracts, particularly in the in-plane direction of the electromagnetic wave absorbing sheet. Note that, in the electromagnetic wave absorbing sheet of this embodiment, the electromagnetic wave absorbing layer is formed by containing magnetic iron oxide 1a in the rubber binder 1b, so that the electromagnetic wave absorbing sheet is highly elastic and at the same time highly flexible, and can be rolled up when handling the electromagnetic wave absorbing sheet, and can be easily arranged along a curved surface.

[0036] Furthermore, in the electromagnetic wave absorbing sheet of this embodiment, an adhesive layer 2 is laminated on one surface of the electromagnetic wave absorbing layer 1 so that the electromagnetic wave absorbing sheet can be easily attached to a desired location, such as the surface of a member arranged around a source of high-frequency electromagnetic waves. Incidentally, the inclusion of the adhesive layer 2 is not an essential requirement for the electromagnetic wave absorbing sheet of this embodiment.

[0037] [Electromagnetic wave absorbing materials] In the electromagnetic wave absorbing sheet according to the present embodiment, magnetic iron oxide powder such as epsilon iron oxide magnetic powder, barium ferrite magnetic powder, strontium ferrite magnetic powder, etc. can be used as the electromagnetic wave absorbing material. Among these, epsilon iron oxide is particularly suitable as the electromagnetic wave absorbing material because it has a high precession frequency when the electrons of iron atoms spin and is highly effective in absorbing high-frequency electromagnetic waves in the millimeter wave band of 30 to 300 gigahertz or higher.

[0038] Epsilon-iron oxide (ε-Fe 2 O3 ) is ferric oxide (Fe 2 O 3 ) in the alpha phase (α-Fe 2 O 3 ) and gamma phase (γ-Fe 2 O 3 ) and is a magnetic material that can be obtained in a single phase state by a nanoparticle synthesis method that combines the reverse micelle method and the sol-gel method.

[0039] Although epsilon iron oxide is a microscopic particle measuring a few to tens of nanometers, it has the largest coercive force of any metal oxide, approximately 20 kOe, at room temperature. Furthermore, natural magnetic resonance due to the gyromagnetic effect based on precession occurs in the so-called millimeter wave frequency band, which is tens of gigahertz or more.

[0040] Furthermore, by replacing some of the Fe sites of epsilon iron oxide with trivalent metal elements such as aluminum (Al), gallium (Ga), rhodium (Rh), and indium (In), it is possible to change the magnetic resonance frequency, i.e., the frequency of the electromagnetic waves that it absorbs when used as an electromagnetic wave absorbing material.

[0041] Figure 2 shows the relationship between the coercive force Hc and the natural resonance frequency f of epsilon iron oxide when different metal elements are substituted for the Fe site. The natural resonance frequency f coincides with the frequency of the electromagnetic waves absorbed.

[0042] As shown in Figure 2, the natural resonance frequency of epsilon iron oxide, in which part of the Fe site is substituted, varies depending on the type and amount of the substituted metal element. It can also be seen that the higher the natural resonance frequency, the greater the coercive force of the epsilon iron oxide.

[0043] More specifically, gallium-substituted epsilon iron oxide, or ε-Ga x Fe 2-x O 3In the case of ε-Al, the absorption peak is in the frequency band from 30 GHz to 150 GHz by adjusting the substitution amount “x”. x Fe 2-x O 3 In the case of epsilon iron oxide, by adjusting the amount of substitution "x", it has an absorption peak in the frequency band of about 100 GHz to 190 GHz. Therefore, by determining the type of element to substitute for the Fe site of epsilon iron oxide so that the natural resonance frequency of the frequency to be absorbed by the electromagnetic wave absorbing sheet is achieved, and further by adjusting the amount of substitution with Fe, it is possible to set the frequency of the electromagnetic waves to be absorbed to the desired value. Furthermore, epsilon iron oxide with rhodium as the substituting metal, i.e., ε-Rh x Fe 2-x O 3 In this case, it is possible to shift the frequency band of electromagnetic waves absorbed in the higher direction, from 180 GHz and above.

[0044] Epsilon iron oxide is readily available on the market, including those in which some of the Fe sites are substituted with metals. The preferred particle size of the epsilon iron oxide powder is 5 nm to 50 nm as the average particle size, and it has a roughly spherical or short rod shape (rod shape).

[0045] Barium ferrite (BaFe 12 O 19 ), strontium ferrite (SrFe 12 O 19 ) are both hexagonal ferrites and have large magnetic anisotropy and therefore large coercive force.

[0046] Barium ferrite or strontium ferrite powder is a compound of iron (Fe) and barium or strontium chloride (BaCl 2 , SrCl 2), and if necessary, a metal oxide containing Ba and Sr is further blended as a raw material, mixed, granulated, and then fired to synthesize the material, which is then pulverized to produce a powder having a desired particle size. For example, the firing conditions can be a temperature of 1200 to 1300°C, a firing atmosphere of air, and a firing time of about 1 to 8 hours.

[0047] The size of the powder produced can be adjusted by the magnitude of the load applied during pulverization. To obtain a relatively large powder, a method of subjecting the sintered body to impact pulverization using a hammer mill and wet pulverization (attritor, planetary ball mill, etc.) can be used. It is also possible to adjust the particle size only by impact pulverization using a hammer mill. The preferred particle size of the barium ferrite or strontium ferrite powder is 1 μm to 5 μm in median diameter (D50).

[0048] [Electromagnetic wave absorbing layer] The rubber binder 1b constituting the electromagnetic wave absorbing layer 1 can be made of various rubber materials, such as natural rubber (NR), isoprene rubber (IR), butadiene rubber (BR), styrene-butadiene rubber (SBR), butyl rubber (IIR), nitrile rubber (NBR), ethylene-propylene rubber (EPDM), chloroprene rubber (CR), acrylic rubber (ACM), chlorosulfonated polyethylene rubber (CSR), urethane rubber (PUR), silicone rubber (Q), fluororubber (FKM), ethylene-vinyl acetate rubber (EVA), epichlorohydrin rubber (CO), and polysulfide rubber (T).

[0049] Among these rubber materials, acrylic rubber and silicone rubber are preferably used due to their high heat resistance. Acrylic rubber has excellent oil resistance even in high temperature environments, is relatively inexpensive, and offers excellent cost performance. Silicone rubber is not only heat resistant, but also has high cold resistance. Furthermore, among synthetic rubbers, its physical properties have the least dependency on temperature, and it also has excellent solvent resistance, ozone resistance, and weather resistance. Furthermore, it has excellent electrical insulation properties and is physically stable over a wide temperature range and frequency range.

[0050] In the electromagnetic wave absorbing layer 1 of the electromagnetic wave absorbing sheet according to this embodiment, when epsilon iron oxide powder is used as the electromagnetic wave absorbing material 1a, since the epsilon iron oxide powder is a fine nanoparticle having a particle size of several nm to several tens of nm as described above, it is important to disperse the epsilon iron oxide powder well in the binder 1b when forming the electromagnetic wave absorbing layer 1. For this reason, in the electromagnetic wave absorbing sheet according to this embodiment, the electromagnetic wave absorbing layer 1 contains a phosphoric acid compound such as an arylsulfonic acid such as phenylphosphonic acid or phenylphosphonic acid dichloride, an alkylphosphonic acid such as methylphosphonic acid, ethylphosphonic acid, octylphosphonic acid, or propylphosphonic acid, or a polyfunctional phosphonic acid such as hydroxyethanediphosphonic acid or nitrotrismethylenephosphonic acid. These phosphoric acid compounds have flame retardancy and function as dispersants for the fine magnetic iron oxide powder, so that the epsilon iron oxide particles in the binder can be dispersed well.

[0051] More specifically, examples of dispersants that can be used include phenylphosphonic acid (PPA) manufactured by Wako Pure Chemical Industries, Ltd. or Nissan Chemical Industries, Ltd., and oxidized phosphoric acid ester "JP-502" (product name) manufactured by Johoku Chemical Industry Co., Ltd.

[0052] However, in the case of thermosetting addition-type silicone rubber, the addition of a phosphoric acid compound may cause inhibition of vulcanization. In such cases, it is preferable to use a polymer dispersant, silane, or silane coupling agent other than a phosphoric acid compound. For example, decylsilylmethoxysilane "KBM-3103" (product name: manufactured by Shin-Etsu Chemical Co., Ltd.) can be suitably used.

[0053] As an example of the composition of the electromagnetic wave absorbing layer 1, 2 to 50 parts of rubber binder and 0.1 to 15 parts of phosphoric acid compound can be used per 100 parts of epsilon iron oxide powder (magnetic iron oxide). If the rubber binder is less than 2 parts, the magnetic iron oxide cannot be dispersed well. In addition, the shape of the electromagnetic wave absorbing sheet cannot be maintained and it becomes difficult to obtain elongation of the electromagnetic wave absorbing sheet. If the rubber binder is more than 50 parts, the electromagnetic wave absorbing sheet can be elongated, but the volume content of the magnetic iron oxide in the electromagnetic wave absorbing sheet becomes small, and the magnetic permeability becomes low, resulting in a small electromagnetic wave absorbing effect.

[0054] If the content of the phosphate compound is less than 0.1 parts, the magnetic iron oxide cannot be dispersed well using a rubber binder. If it is more than 15 parts, the effect of dispersing the magnetic iron oxide well is saturated. The volume content of the magnetic iron oxide in the electromagnetic wave absorbing sheet becomes small, and the magnetic permeability becomes low, so the effect of absorbing electromagnetic waves becomes small.

[0055] [Method of manufacturing the electromagnetic wave absorbing layer] Here, a method for manufacturing the electromagnetic wave absorbing layer 1 of the electromagnetic wave absorbing sheet according to this embodiment will be described. In the electromagnetic wave absorbing sheet of this embodiment, a magnetic paint containing at least magnetic iron oxide powder and a rubber binder is prepared, applied to a predetermined thickness, dried, and then calendered to form the electromagnetic wave absorbing layer 1. Although calendering is not essential, it is preferable to perform calendering because it can reduce voids in the electromagnetic wave absorbing sheet and improve the filling level of the magnetic iron oxide powder.

[0056] First, a magnetic paint is prepared.

[0057] The magnetic paint can be obtained by obtaining a kneaded product of epsilon iron oxide powder as a magnetic oxide, a phosphoric acid compound as a dispersant, and a rubber binder, diluting the kneaded product with a solvent, dispersing the product, and then filtering the product. The kneaded product can be obtained, for example, by kneading the product with a pressurized batch kneader. The kneaded product can also be dispersed, for example, by using a sand mill filled with beads such as zirconia to obtain a dispersion liquid. At this time, a crosslinking agent can be added as necessary.

[0058] The obtained magnetic paint is applied onto a support having releasability, for example a 38 μm-thick polyethylene terephthalate (PET) sheet that has been treated for release with a silicone coat, using a table coater or bar coater.

[0059] Thereafter, the wet magnetic paint is dried at 80° C., and then calendered at a predetermined temperature and pressure using a calender to form an electromagnetic wave absorbing layer on the support.

[0060] As an example, by applying the magnetic paint to a support to a thickness of 1 mm in a wet state, the thickness after drying can be made 400 μm, and the thickness of the electromagnetic wave absorbing layer after calendaring can be made 300 μm.

[0061] In this manner, the electromagnetic wave absorbing layer 1 can be formed in which the nm-order fine epsilon iron oxide powder used as the electromagnetic wave absorbing material 1a is well dispersed in the rubber binder 1b.

[0062] As another method for preparing magnetic paint, at least the magnetic iron oxide powder, the dispersant phosphoric acid compound, and the rubber binder are mixed at high speed in a high-speed mixer to prepare a mixture, and then the resulting mixture is dispersed in a sand mill to obtain magnetic paint.

[0063] [Adhesive layer] As shown in FIG. 1, the electromagnetic wave absorbing sheet according to this embodiment has an electromagnetic wave absorbing layer 1 and an adhesive layer 2 formed on the back surface thereof.

[0064] By providing the adhesive layer 2, the electromagnetic wave absorbing layer 1 can be attached to a desired position on the inner surface of a housing that stores an electric circuit, or on the inner or outer surface of an electric device. In particular, since the electromagnetic wave absorbing layer 1 of the electromagnetic wave absorbing sheet of this embodiment has elasticity, the adhesive layer 2 can easily attach the electromagnetic wave absorbing sheet to a curved surface, improving the ease of handling of the electromagnetic wave absorbing sheet. Note that it is preferable to use, for example, an acrylic adhesive, a silicone adhesive, or a rubber adhesive having a low glass point temperature (Tg) by devising the material, formation thickness, formation state, etc. of the adhesive layer 2 so that the adhesive layer 2 does not hinder the elongation due to elastic deformation of the electromagnetic wave absorbing layer 1.

[0065] Known materials used as adhesive layers for adhesive tapes, acrylic adhesives, rubber adhesives, silicone adhesives, etc. can be used as the adhesive layer 2. In particular, when silicone rubber is used as the rubber binder, it is preferable to use a silicone adhesive as the material for the adhesive layer in order not to reduce the adhesive strength between the electromagnetic wave absorbing layer and the adhesive layer.

[0066] A tackifier or crosslinking agent can also be used to adjust the adhesive strength to the adherend and reduce adhesive residue. The adhesive strength to the adherend is preferably 5N / 10mm to 12N / 10mm. If the adhesive strength is less than 5N / 10mm, the electromagnetic wave absorbing sheet may easily peel off or shift from the adherend. If the adhesive strength is more than 12N / 10mm, the electromagnetic wave absorbing sheet becomes difficult to peel off from the adherend.

[0067] The thickness of the adhesive layer 2 is preferably 20 μm to 100 μm. If the thickness of the adhesive layer is thinner than 20 μm, the adhesive strength is weak, and the electromagnetic wave absorbing sheet may easily peel off or shift from the adherend. If the thickness of the adhesive layer is thicker than 100 μm, the overall thickness of the electromagnetic wave absorbing sheet is thick, and there is a risk of the flexibility being reduced. If the adhesive layer 2 is thick, the electromagnetic wave absorbing sheet is difficult to peel off from the adherend. If the cohesive strength of the adhesive layer 2 is weak, adhesive residue may be left on the adherend when the electromagnetic wave absorbing sheet is peeled off.

[0068] In the present specification, the adhesive layer 2 may be an adhesive layer 2 that adheres in an unremovable manner, or an adhesive layer 2 that adheres in a removably manner.

[0069] Furthermore, when adhering the electromagnetic wave absorbing sheet to a predetermined surface, even if the electromagnetic wave absorbing sheet does not have the adhesive layer 2, it is possible to provide adhesiveness to the surface of the member on which the electromagnetic wave absorbing sheet is to be placed, and to attach the electromagnetic wave absorbing sheet formed only of the electromagnetic wave absorbing layer 1. Furthermore, the electromagnetic wave absorbing sheet can be attached to a predetermined site by using a double-sided tape or adhesive. In this respect, although the adhesive layer 2 is not an essential component of the electromagnetic wave absorbing sheet shown in this embodiment, a configuration in which the electromagnetic wave absorbing sheet has the adhesive layer 2 is preferable because the electromagnetic wave absorbing sheet can be attached to a predetermined site without using a double-sided tape or adhesive.

[0070] [Stretching of electromagnetic wave absorbing sheet] Next, the elongation in the in-plane direction of the electromagnetic wave absorbing sheet according to this embodiment will be described.

[0071] Fig. 3 is a diagram showing the relationship between the stress (tensile stress) applied in the in-plane direction and the elongation of the electromagnetic wave absorbing sheet according to this embodiment. Fig. 3(a) shows the relationship between the stress and the elongation of an electromagnetic wave absorbing sheet that breaks when the maximum elongation is exceeded. Fig. 3(b) shows the relationship between the stress and the elongation of an electromagnetic wave absorbing sheet that undergoes plastic deformation when the maximum elongation is exceeded.

[0072] Here, "elongation" refers to the amount of elongation of an electromagnetic wave absorbing sheet caused by applying stress in one direction, divided by the original length, expressed as a percentage. In other words, if the length when there is no stress is L1 and the length when a certain stress is applied is L2, then the "elongation" when this certain stress is applied is expressed as (L2-L1) / L1×100. This "elongation" is also called "strain."

[0073] 3(a), in an electromagnetic wave absorbing sheet that breaks when the maximum elongation is exceeded, as the externally applied stress increases, the elongation of the electromagnetic wave absorbing sheet increases almost linearly until it reaches the maximum elongation of 170% (part indicated by reference numeral 11). After that, when stress exceeding the maximum elongation of 170% is applied, the electromagnetic wave absorbing sheet breaks, and the value of the elongation does not increase beyond the maximum elongation of 170% (part indicated by reference numeral 12).

[0074] On the other hand, in the case of an electromagnetic wave absorbing sheet that undergoes plastic deformation when it exceeds its maximum elongation rate, as shown in FIG. 3(b), the elongation rate increases relatively slowly as the applied stress increases until it reaches an elongation rate of 30%, which is the elongation rate that indicates the maximum stress (part indicated by reference numeral 13). If the electromagnetic wave absorbing sheet is subsequently pulled further after it reaches an elongation rate of 30%, which indicates the maximum stress, it undergoes plastic deformation and stretches until it reaches an elongation rate of 230% (part indicated by reference numeral 14). As a result, the stress gradually decreases. Note that, since plastic deformation has occurred, the electromagnetic wave absorbing sheet in the state indicated by reference numeral 14 has lost its elasticity, and the length of the sheet does not shorten even when the force pulling the electromagnetic wave absorbing sheet is released.

[0075] The elastic deformation range of the rubber material used as the binder 1b can be adjusted by using an appropriately selected vulcanizing agent. In addition, when an electromagnetic wave absorbing sheet that must not break is required due to the usage application, it is considered effective to make it in a form that can plastically deform without breaking.

[0076] The upper limit of the range of the degree of elongation of the electromagnetic wave absorbing sheet is 200%, which is the range in which the input impedance value deviates significantly from the impedance value in air, making it impossible to achieve impedance matching and reducing the electromagnetic wave absorbing capacity. If the electromagnetic wave absorbing sheet elongates too much, the thickness of the electromagnetic wave absorbing sheet becomes thin and the density of the electromagnetic wave absorbing material decreases, reducing the electromagnetic wave absorbing capacity. Furthermore, if the electromagnetic wave absorbing sheet elongates more than 200%, the flexibility and bendability of the electromagnetic wave absorbing sheet decrease.

[0077] On the other hand, if the elongation of the electromagnetic wave absorbing sheet is less than 20%, it will not be possible to stretch it sufficiently when attaching it to a curved adherend, resulting in reduced workability. In addition, it will not be possible to attach it to a movable part whose shape changes, and it will not be possible to take advantage of the characteristic of the electromagnetic wave absorbing sheet according to this embodiment, that is, elasticity.

[0078] Here, electromagnetic wave absorbing sheets according to this embodiment were actually produced using different types of magnetic iron oxide and rubber binder, and the relationship between the external tensile stress and the elongation of the electromagnetic wave absorbing sheet was measured.

[0079] The first electromagnetic wave absorbing sheet (Example 1) used epsilon iron oxide as the magnetic iron oxide and acrylic rubber as the rubber binder. Table 1 shows the materials and their proportions used in producing the first electromagnetic wave absorbing sheet.

[0080] [Table 1]

[0081] The second electromagnetic wave absorbing sheet (Example 2) used epsilon iron oxide as the magnetic iron oxide as in the first electromagnetic wave absorbing sheet, and silicone rubber as the rubber binder. Table 2 shows the materials and their proportions used in producing the second electromagnetic wave absorbing sheet.

[0082] [Table 2]

[0083] The third electromagnetic wave absorbing sheet (Example 3) used strontium ferrite as the magnetic iron oxide and silicone rubber as the rubber binder, similar to the second electromagnetic wave absorbing sheet. Table 3 shows the materials and their proportions used in producing the third electromagnetic wave absorbing sheet.

[0084] [Table 3]

[0085] The materials having the respective compositions shown in Tables 1 to 3 were kneaded in a pressure batch kneader, and the resulting kneaded mixture was diluted with 170 parts of methyl ethyl ketone, and then a dispersion was prepared using a sand mill filled with zirconia beads.

[0086] The dispersion liquid shown in Table 1 above was applied by a sheet-feed coater onto a 38 μm-thick polyethylene terephthalate (PET) sheet that had been subjected to a release treatment by silicone coating.

[0087] Furthermore, the dispersions shown in Tables 2 and 3 were applied by a sheet-feed coater onto a 38 μm-thick polyethylene terephthalate (PET) sheet that had been subjected to a release treatment using a non-silicone release agent.

[0088] The wet paint was dried at 80° C. and calendered to form an electromagnetic wave absorbing layer having a thickness of 500 μm after calendering.

[0089] Five of the electromagnetic wave absorbing layers thus produced, each having a thickness of 500 μm, were stacked and thermally compressed using a calendar device to produce a single electromagnetic wave absorbing layer having a thickness of 2500 μm. Note that no adhesive layer was formed, and the electromagnetic wave absorbing sheet was made of only the electromagnetic wave absorbing layer.

[0090] The elongation of each of the electromagnetic wave absorbing sheets produced was measured using a tensile tester. Specifically, a TGE-1kN type tester (product name) manufactured by Minebea Co., Ltd. and a TT3E-200N load cell were used to measure the elongation of a sheet measuring 20 mm x 50 mm when stretched at a tensile speed of 10 mm / min. The elongation measurement was performed in an environment of a temperature of 23°C and a humidity of 50% Rh.

[0091] The elongation rates of the three electromagnetic wave absorbing sheets measured in this manner are shown in FIG.

[0092] In FIG. 4, the elongation rate of the first electromagnetic wave absorbing sheet is shown by a solid line (reference number 15), the elongation rate of the second electromagnetic wave absorbing sheet is shown by a dotted line (reference number 16), and the elongation rate of the third electromagnetic wave absorbing sheet is shown by a two-dot chain line (reference number 17).

[0093] As shown in Fig. 4, all of the three electromagnetic wave absorbing sheets produced as examples are of the type that break when they exceed the maximum elongation shown in Fig. 3(a), and the maximum elongation is 195% to 200%. As described above, in the electromagnetic wave absorbing sheet according to this embodiment, the maximum elongation is preferably 200%, and the three produced electromagnetic wave absorbing sheets have electromagnetic wave absorbing properties, flexibility, and bendability within preferred ranges.

[0094] Comparing the three electromagnetic wave absorbing sheets, the stress required to achieve the same elongation was the largest for the first electromagnetic wave absorbing sheet 15 and the smallest for the third electromagnetic wave absorbing sheet 17. This is believed to be because the acrylic rubber used is harder than the silicone rubber, and the silicone rubber used in the second electromagnetic wave absorbing sheet is harder than the silicone rubber used in the third electromagnetic wave absorbing sheet. Also, compared to the epsilon iron oxide used as the magnetic iron oxide in the first electromagnetic wave absorbing sheet, the particle diameter of the strontium ferrite used as the magnetic iron oxide in the third electromagnetic wave absorbing sheet is larger, so the specific surface area is smaller and the dispersibility is higher, which is believed to suppress the hardness of the electromagnetic wave absorbing sheet.

[0095] Next, the electromagnetic wave absorbing sheet according to this embodiment was measured for changes in electromagnetic wave absorbing properties when stress was applied to the sheet to stretch it.

[0096] The measurement was carried out by using the free space method to measure the amount of electromagnetic wave absorption (electromagnetic wave attenuation) of the first electromagnetic wave absorbing sheet described above. Specifically, using a millimeter wave network analyzer ME7838AN5250C (product name) manufactured by Anritsu Agilent Technologies, an input wave (millimeter wave) of a specific frequency was irradiated from a transmitting antenna to the electromagnetic wave absorbing sheet through a dielectric lens, and the electromagnetic wave transmitted by a receiving antenna arranged on the back side of the electromagnetic wave absorbing sheet was measured. The intensity of the irradiated electromagnetic wave and the intensity of the transmitted electromagnetic wave were grasped as voltage values, and the electromagnetic wave attenuation was calculated in dB from the intensity difference.

[0097] FIG. 5 is a diagram showing the electromagnetic wave absorbing characteristics of the first electromagnetic wave absorbing sheet according to this embodiment when no tension is applied to the electromagnetic wave absorbing sheet, and when tension is applied to the electromagnetic wave absorbing sheet, causing the electromagnetic wave absorbing sheet to elongate and reduce in thickness.

[0098] 5, the solid line indicated by reference numeral 21 represents the electromagnetic wave absorption characteristics when no tension is applied to the electromagnetic wave absorbing sheet, i.e., when the elongation rate is 0%. The thickness of the electromagnetic wave absorbing sheet at this time was 2500 μm as produced.

[0099] As shown in Figure 5, the first electromagnetic wave absorbing sheet exhibited high electromagnetic wave absorption characteristics, with an electromagnetic wave absorption amount (attenuation of the electromagnetic wave from the incident wave that transmits to the back side) of 26 dB at 75.5 GHz, which is the resonant frequency of epsilon ferric oxide, an electromagnetic wave absorbing material.

[0100] In contrast, the electromagnetic wave absorbing characteristics when tension is applied to the electromagnetic wave absorbing sheet and it is stretched to an elongation rate of 75% are as shown by the dotted line indicated by reference numeral 22 in Fig. 5. The thickness of the electromagnetic wave absorbing sheet in this case was 1950 µm.

[0101] As shown by the dotted line and reference numeral 22 in Fig. 5, the electromagnetic wave absorption amount at 75.5 GHz in the electromagnetic wave absorbing sheet with an elongation rate of 75% is about 19 dB, and it is found that the electromagnetic wave absorption characteristics are deteriorated compared to the case where the elongation rate is 0% (reference numeral 21). This is considered to be due to the fact that the electromagnetic wave absorbing sheet is pulled in its in-plane direction, thereby reducing its thickness, and the content of the electromagnetic wave absorbing material in the electromagnetic wave transmission direction in the electromagnetic wave absorbing sheet is substantially reduced.

[0102] That is, in the case of a transmission type electromagnetic wave absorbing sheet such as the first electromagnetic wave absorbing sheet, it was found that the electromagnetic wave absorbing characteristics are reduced when the electromagnetic wave absorbing sheet is pulled in the in-plane direction.

[0103] Therefore, the inventors measured the electromagnetic wave absorption characteristics when the elongation rate of the electromagnetic wave absorbing sheet was further increased, and measured the relationship between the thickness of the electromagnetic wave absorbing sheet and the amount of electromagnetic wave absorption in the electromagnetic wave absorbing sheet that was thinned by being stretched in the in-plane direction. The measurement results are shown in Figure 6.

[0104] FIG. 6 shows the relationship between the sheet thickness when the electromagnetic wave absorbing sheet is pulled in one in-plane direction and the electromagnetic wave absorption amount (attenuation of transmitted electromagnetic waves: transmission attenuation) at a frequency of 75.5 GHz in that state for the first electromagnetic wave absorbing sheet and the third electromagnetic wave absorbing sheet described above.

[0105] In FIG. 6, the black circles and solid line 31 indicate the change in the amount of electromagnetic wave absorption of the first electromagnetic wave absorbing sheet, and the black squares and dotted line 32 indicate the change in the amount of electromagnetic wave absorption of the third electromagnetic wave absorbing sheet.

[0106] As shown in FIG. 6, in the first electromagnetic wave absorbing sheet and the third electromagnetic wave absorbing sheet, the amount of absorption of electromagnetic waves with a frequency of 75.5 GHz (31, 32) was approximately proportional to the thickness of the electromagnetic wave absorbing sheet, and it was confirmed that the electromagnetic wave absorption characteristics deteriorate as the electromagnetic wave absorbing sheet is pulled more strongly and its thickness becomes thinner.

[0107] In this way, in the electromagnetic wave absorbing sheet of this embodiment, when the sheet is stretched by application of a tensile force, the electromagnetic wave absorption characteristics decrease linearly according to the magnitude of the elongation rate at that time. From this, it can be said that the electromagnetic wave absorbing sheet can be used while being stretched as long as it is within a range in which a desired amount of electromagnetic wave absorption is obtained, and within the range of the maximum elongation rate of the electromagnetic wave absorbing sheet and in the elastic region.

[0108] (Second embodiment) [Reflective electromagnetic wave absorbing sheet] Next, a so-called reflective type electromagnetic wave absorbing sheet in which a reflective layer is formed on the back surface of an electromagnetic wave absorbing layer, which is a second configuration example of the electromagnetic wave absorbing sheet disclosed in the present application, will be described with reference to a specific embodiment.

[0109] FIG. 7 shows a cross-sectional configuration of an electromagnetic wave absorbing sheet according to the second embodiment.

[0110] 7 is a diagram for making the configuration of the electromagnetic wave absorbing sheet according to the first embodiment easier to understand, similar to Fig. 1 which explains the configuration of the electromagnetic wave absorbing sheet according to the first embodiment, and does not represent the sizes and thicknesses of the members shown in the diagram based on reality. Also, the same members as those constituting the electromagnetic wave absorbing sheet according to the first embodiment shown in Fig. 1 are given the same reference numerals, and detailed explanations thereof will be omitted.

[0111] The electromagnetic wave absorbing sheet disclosed in the present application absorbs electromagnetic waves by magnetic resonance of magnetic iron oxides such as epsilon iron oxide, barium ferrite, strontium ferrite, etc., which form an electromagnetic wave absorbing layer together with a rubber binder as an electromagnetic wave absorbing material. Therefore, in addition to being configured as a transmission type electromagnetic wave absorbing sheet not provided with a reflection layer as shown in the first embodiment, it can also be configured as a reflection type electromagnetic wave absorbing sheet provided with a reflection layer that reflects electromagnetic waves on the surface of the electromagnetic wave absorbing layer opposite to the side where the electromagnetic waves are incident.

[0112] The electromagnetic wave absorbing sheet shown in the second embodiment has a reflective layer 3 formed on the back side (lower side in Figure 7) of an electromagnetic wave absorbing layer 1 containing magnetic iron oxide 1a, which is an electromagnetic wave absorbing material, and a rubber binder 1b, in contact with the surface of the electromagnetic wave absorbing layer 1.

[0113] 7, an adhesive layer 2 that enables the electromagnetic wave absorbing sheet to be attached to a predetermined location is formed on the rear side of the reflective layer 3. As in the case of the electromagnetic wave absorbing sheet according to the above-mentioned first embodiment, the adhesive layer 2 is not an essential component of the electromagnetic wave absorbing sheet according to the second embodiment either, and it is possible to form an electromagnetic wave absorbing sheet without the adhesive layer 2. However, a configuration in which the electromagnetic wave absorbing sheet includes the adhesive layer 2 is preferable because the electromagnetic wave absorbing sheet can be attached to a predetermined location without using double-sided tape or adhesive.

[0114] The reflective layer 3 may be a metal layer formed in close contact with the back surface of the electromagnetic wave absorbing layer 1. However, in the electromagnetic wave absorbing sheet of this embodiment, since the electromagnetic wave absorbing sheet has elasticity due to the use of the rubber binder 1b, the reflective layer 3 is made of a mesh-shaped conductor, silver nanowires (Ag-NW), a conductive polymer film, or the like, so that the surface resistance of the electromagnetic wave absorbing layer 1 does not increase even when it is stretched, and a resistance of about 1 Ω / □ is maintained.

[0115] A method of spraying or applying silver nanowires or a conductive polymer to the back side of the electromagnetic wave absorbing sheet can be used as a method of forming a reflective layer on the back side of the electromagnetic wave absorbing layer 1. A method of preparing a reflective layer 3 in which silver nanowires or a conductive polymer is dispersed in a rubber binder similar to that of the reflective layer, and then thermocompressing the elastic reflective layer 3 to the electromagnetic wave absorbing layer, or a method of applying a paint for preparing the electromagnetic wave absorbing layer 1 to the elastic reflective layer 3 to form the electromagnetic wave absorbing layer 1 on the reflective layer 3 can also be used.

[0116] The type of metal constituting the reflective layer 3 is not particularly limited, and in addition to the silver used for the nanowires, metals with as low an electrical resistance as possible and high corrosion resistance, such as aluminum, copper, and chromium, can be used.

[0117] 7, the electromagnetic wave absorbing sheet according to the second embodiment can reliably prevent electromagnetic waves from penetrating the electromagnetic wave absorbing sheet by providing a reflective layer 3 on the back surface of the electromagnetic wave absorbing layer 1. Therefore, the electromagnetic wave absorbing sheet can be suitably used as an electromagnetic wave absorbing sheet for preventing leakage of electromagnetic waves emitted to the outside from electric circuit components driven by high frequency waves.

[0118] [Stretching of reflective electromagnetic wave absorbing sheet] In the reflective electromagnetic wave absorbing sheet according to the second embodiment, similarly to the electromagnetic wave absorbing sheet according to the first embodiment, when the electromagnetic wave absorbing layer 1 is stretched by pulling, the thickness of the electromagnetic wave absorbing layer 1 changes, causing a change in the input impedance value, resulting in a change in the electromagnetic wave absorbing characteristics due to impedance mismatching, and the amount of electromagnetic wave absorbing material in the portion of the electromagnetic wave absorbing layer 1 through which the electromagnetic wave passes decreases, resulting in a change in the electromagnetic wave absorbing characteristics.

[0119] Furthermore, in the case of a reflective type electromagnetic wave absorbing sheet, there is a problem that the input impedance value of the electromagnetic wave absorbing sheet must be matched to the impedance value in air. If the input impedance value of the electromagnetic wave absorbing sheet is significantly different from the impedance value in air, which is 377 Ω (strictly speaking, the impedance value in a vacuum), reflection and scattering will occur when the electromagnetic wave is incident on the electromagnetic wave absorbing sheet, resulting in a loss of the electromagnetic wave absorption characteristics of the reflective type electromagnetic wave absorbing sheet, i.e., the characteristic of reducing the reflected wave of the incident electromagnetic wave.

[0120] Here, the impedance Z of the electromagnetic wave absorbing layer 1 in the electromagnetic wave absorbing sheet having magnetic iron oxide as the electromagnetic wave absorbing material is in is expressed as the following equation (1).

[0121]

number

[0122] In the above formula (1), μ r is the complex permeability of the electromagnetic wave absorbing layer 1, ε r is the complex dielectric constant of the electromagnetic wave absorbing layer 1, λ is the wavelength of the incident electromagnetic wave, and d is the thickness of the electromagnetic wave absorbing layer 1. For this reason, when the electromagnetic wave absorbing sheet stretches, the thickness d of the electromagnetic wave absorbing layer 1 decreases, and the content of the magnetic iron oxide, which is the electromagnetic wave absorbing material, decreases, causing changes in both the magnetic permeability and dielectric constant of the electromagnetic wave absorbing layer 1. As a result, the input impedance value (Zin) of the electromagnetic wave absorbing sheet depends on the thickness of the binder 1b that forms the electromagnetic wave absorbing layer 1, which means that if the thickness of the electromagnetic wave absorbing sheet fluctuates as the electromagnetic wave absorbing sheet stretches and shrinks, the input impedance value (Zin) of the electromagnetic wave absorbing sheet will fluctuate.

[0123] Taking this into consideration, the inventors of the present invention have come up with the idea that by matching the input impedance value of an electromagnetic wave absorbing sheet to the input impedance value in air when the electromagnetic wave absorbing sheet is in a steady state, i.e., when no external force is applied and the electromagnetic wave absorbing sheet has an elongation rate of 0%, but when the electromagnetic wave absorbing sheet is stretched at a certain elongation rate, it is possible to realize an electromagnetic wave sheet that can suitably absorb incident electromagnetic waves under a wider range of conditions in practical use.

[0124] Therefore, a reflective electromagnetic wave absorbing sheet was actually produced and the effect of matching the input impedance value of the electromagnetic wave absorbing sheet with the impedance value in air while it was stretched at a constant elongation rate was verified.

[0125] First, a reflective type electromagnetic wave absorbing sheet was produced as a fourth electromagnetic wave absorbing sheet (Example 4).

[0126] The fourth electromagnetic wave absorbing sheet, a reflective type electromagnetic wave absorbing sheet, was prepared in the same manner as the first electromagnetic wave absorbing sheet described above, by kneading the mixed materials having the composition shown in Table 1 in a pressurized batch kneader, diluting the resulting mixture with 170 parts of methyl ethyl ketone, and then preparing a dispersion using a sand mill filled with zirconia beads.

[0127] The above dispersion was applied by a sheet-feed coater onto a 38 μm-thick polyethylene terephthalate (PET) sheet that had been subjected to a release treatment with a silicone coat.

[0128] The wet paint was dried at 80° C. and calendered to a thickness of 410 μm after calendering to form an electromagnetic wave absorbing layer.

[0129] Subsequently, a reflective layer was formed on the rear surface of the electromagnetic wave absorbing layer.

[0130] The reflective layer was formed by applying silver nanowires to the back side of the electromagnetic wave absorbing layer.

[0131] The electromagnetic wave absorption characteristics of the fourth electromagnetic wave absorbing sheet were measured using the free space method, similar to the electromagnetic wave absorption measurement of the transmission type electromagnetic wave absorbing sheet. However, in order to measure the characteristics of the reflection type electromagnetic wave absorbing sheet, the output of the electromagnetic wave incident on the electromagnetic wave absorbing sheet and the output of the reflected wave emitted from the electromagnetic wave absorbing sheet were measured by arranging a transmitting antenna and a receiving antenna on the front side of the electromagnetic wave absorbing sheet.

[0132] The fourth electromagnetic wave absorbing sheet, which has an electromagnetic wave absorbing layer containing acrylic rubber as a binder, has a predetermined maximum elongation percentage shown in Figure 3(b) (specifically, 30% in the case of the fourth electromagnetic wave absorbing sheet), and when this percentage is exceeded, the electromagnetic wave absorbing sheet undergoes plastic deformation.

[0133] Furthermore, in the fourth electromagnetic wave absorbing sheet, the thickness is 370 μm when the elongation rate is 11%, and the input impedance value of the electromagnetic wave absorbing sheet at this thickness of 370 μm is 377 Ω, which matches the impedance value in air.

[0134] FIG. 8 shows the change in electromagnetic wave absorbing characteristics when the elongation rate is changed in the fourth electromagnetic wave absorbing sheet.

[0135] In Fig. 8, the solid line indicated by reference numeral 41 indicates the electromagnetic wave absorption amount (attenuation amount of reflected electromagnetic waves) of the fourth electromagnetic wave absorbing sheet in a steady state, i.e., when the elongation rate is 0%. At this time, the thickness of the fourth electromagnetic wave absorbing sheet was 410 µm. The dotted line indicated by reference numeral 42 indicates the state in which the fourth electromagnetic wave absorbing sheet was elongated at an elongation rate of 3%, and the thickness of the electromagnetic wave absorbing sheet at this time was 400 µm. The dashed line indicated by reference numeral 43 indicates the state in which the fourth electromagnetic wave absorbing sheet was elongated at an elongation rate of 11%, and the thickness of the electromagnetic wave absorbing sheet at this time was 370 µm, and the two-dot dashed line indicated by reference numeral 44 indicates the state in which the fourth electromagnetic wave absorbing sheet was elongated at an elongation rate of 22%, and the thickness of the electromagnetic wave absorbing sheet at this time was 335 µm.

[0136] As shown in Fig. 8, in the fourth electromagnetic wave absorbing sheet, the electromagnetic wave absorption amount is the largest at about 23 dB when the electromagnetic wave absorbing sheet is stretched at an elongation rate of 11%, which is the state where the input impedance is matched. In contrast, the electromagnetic wave absorption amount is about 18 dB when the elongation rate is 3%, and about 15 dB when the elongation rate is 0%, which is different from the results in Figs. 5 and 6 showing the electromagnetic wave absorption characteristics of the first electromagnetic wave absorbing sheet, which show that the electromagnetic wave absorption amount decreases as the electromagnetic wave absorbing sheet stretches. In other words, in the second embodiment, the electromagnetic wave absorption amount increases as the elongation rate increases, which is the opposite result to that of the first embodiment.

[0137] This shows that in a reflective type electromagnetic wave absorbing sheet, the decrease in the amount of electromagnetic wave absorption is more strongly influenced by impedance mismatching caused by a change in the input impedance value due to a change in the thickness of the electromagnetic wave absorbing layer, than by a decrease in the amount of electromagnetic wave absorption due to a decrease in the amount of electromagnetic wave absorbing material in the part of the electromagnetic wave absorbing layer through which the electromagnetic waves pass, caused by the stretching of the electromagnetic wave absorbing sheet.

[0138] Therefore, in a reflective type electromagnetic wave absorbing sheet, it is preferable to match the input impedance value of the electromagnetic wave absorbing sheet with the impedance value in air, taking into account the change in the input impedance value of the electromagnetic wave absorbing layer that accompanies a change in the elongation rate of the electromagnetic wave absorbing sheet.

[0139] FIG. 9 is a diagram showing the relationship between the thickness of the fourth electromagnetic wave absorbing sheet and the amount of electromagnetic wave absorption, and shows the change in the electromagnetic wave absorption characteristics of the fourth electromagnetic wave absorbing sheet shown in FIG. 8 in a different form.

[0140] In FIG. 9, reference numeral 51 indicates the electromagnetic wave absorption value when the elongation rate is 0% (thickness 410 μm), reference numeral 52 indicates the electromagnetic wave absorption value when the elongation rate is 3% (thickness 400 μm), reference numeral 53 indicates the electromagnetic wave absorption value when the elongation rate is 11% (thickness 370 μm), and reference numeral 54 indicates the electromagnetic wave absorption value when the elongation rate is 22% (thickness 335 μm).

[0141] As shown in Figure 9, in the fourth electromagnetic wave absorbing sheet, by matching the input impedance when the electromagnetic wave absorbing sheet is stretched at an elongation rate of 11% to a thickness of 370 µm, it is possible to maintain the electromagnetic wave absorption rate of 15 dB or less, which is considered to be practically preferable, in other words, an electromagnetic wave absorption rate of 92% or more, from a state where the elongation rate is 0% to a state where the elongation rate is 22%.

[0142] Thus, the electromagnetic wave absorbing sheet according to this embodiment has extensibility with a maximum elongation rate in the elastic region of 20% to 200%, and is an electromagnetic wave absorbing sheet that can maintain its electromagnetic wave absorption amount even when stretched within this range for use. Furthermore, the electromagnetic wave absorbing sheet according to this embodiment has a maximum elongation rate in the elastic region of 20% to 200%, and is an electromagnetic wave absorbing sheet that can maintain a predetermined electromagnetic wave absorption amount when stretched by 5 to 75% of the maximum elongation rate in the elastic region.

[0143] As is clear from the above explanation, in an electromagnetic wave absorbing sheet having, as an electromagnetic wave absorbing material, a magnetic member that absorbs electromagnetic waves by magnetic resonance, if the sheet has elasticity and its thickness changes, it is preferable that the input impedance value of the electromagnetic wave absorbing layer be matched to the impedance value in air when the electromagnetic wave absorbing sheet is stretched to a certain extent.

[0144] This is not limited to the electromagnetic wave absorbing material that magnetically resonates with electromagnetic waves in a high frequency band of millimeter waves or higher as explained in the second embodiment. Therefore, in all elastic reflective electromagnetic wave absorbing sheets that absorb electromagnetic waves by magnetic resonance of an electromagnetic wave absorbing material dispersed in a rubber binder, it can be said that it is preferable to match the input impedance value of the electromagnetic wave absorbing layer with the impedance value in air when the electromagnetic wave absorbing sheet is stretched to a certain extent.

[0145] In the fourth electromagnetic wave absorbing sheet, a specific example of which was described in the second embodiment, the input impedance value was matched when the elongation rate was 11%, i.e., about 37% compared to the maximum elongation rate, in an electromagnetic wave absorbing sheet with a maximum elongation rate of 30%. The standard for performing impedance matching in a state stretched to a predetermined elongation rate depends on the type of deformation of the rubber material used as the binder, particularly when stretched to the maximum elongation rate or more, but by using a state stretched by 5 to 75% of the maximum elongation rate as the standard, the difference between the input impedance value of the electromagnetic wave absorbing sheet and the impedance value in air is eliminated in a wide practical range, and it is possible to limit the deterioration of the electromagnetic wave absorption characteristics caused by the reflection and scattering of the electromagnetic waves incident on the electromagnetic wave absorbing sheet.

[0146] From the opposite perspective, by maintaining the input impedance value of the electromagnetic wave absorbing sheet at a value close to 377Ω, which is the impedance value in air, within the elastic range, a certain level of impedance matching can be obtained within the practical range of the electromagnetic wave absorbing sheet, and a reduction in the amount of electromagnetic wave absorption by the electromagnetic wave absorbing sheet can be avoided. According to the inventors' study, the numerical range is 360Ω to 450Ω. When the input impedance value of the electromagnetic wave absorbing sheet is smaller than 360Ω or larger than 450Ω, the electromagnetic waves incident on the electromagnetic wave absorbing sheet are largely scattered and reflected at the boundary between the space, which is the surface of the electromagnetic wave absorbing sheet, and the electromagnetic wave absorbing sheet, and therefore the electromagnetic wave absorbing characteristics of the electromagnetic wave absorbing sheet itself cannot be exhibited.

[0147] When matching the input impedance value of the electromagnetic wave absorbing sheet with the impedance in air, by using a state in which the electromagnetic wave absorbing sheet is stretched by 5 to 75% of its maximum elongation as the reference rather than a state in which the electromagnetic wave absorbing sheet is not stretched (0%), the electromagnetic waves can be absorbed well even when the elongation of the electromagnetic wave absorbing sheet changes. This is not limited to the reflective type electromagnetic wave absorbing sheet described as the second embodiment, but is also true for the transmissive type electromagnetic wave absorbing sheet shown as the first embodiment.

[0148] In the transmission type electromagnetic wave absorbing sheet shown in the first embodiment, unlike the reflection type electromagnetic wave absorbing sheet shown in the second embodiment, the strength of the electromagnetic wave scattered / reflected toward the electromagnetic wave incident side does not directly affect the level of the electromagnetic wave absorption characteristics (electromagnetic wave attenuation), but there may be cases where it is desired to reduce the reflected wave on the electromagnetic wave incident side of the transmission type electromagnetic wave absorbing sheet, for example, when suppressing the leakage of electromagnetic waves from an electric circuit that is an electromagnetic wave radiation source to the outside and preventing the reflected wave on the surface of the electromagnetic wave absorbing sheet from adversely affecting the electric circuit. In such cases, even in the case of a transmission type electromagnetic wave absorbing sheet, it is preferable to make its input impedance close to the impedance in air even if the elongation rate changes, and it is preferable to set its input impedance to 377Ω based on a state where the electromagnetic wave absorbing sheet is stretched by 5 to 75% from the maximum elongation rate of the electromagnetic wave absorbing sheet.

[0149] 8, in the fourth electromagnetic wave absorbing sheet, even when the elongation rate changes and the thickness changes, the frequency of the input electromagnetic wave showing the largest absorption amount in the electromagnetic wave absorption characteristics at each thickness remains unchanged at 75.5 GHz. This indicates that, like the first electromagnetic wave absorbing sheet described in the first embodiment, the electromagnetic wave absorbing sheet disclosed in the present application absorbs the incident electromagnetic wave by magnetic resonance of the magnetic iron oxide, which is the electromagnetic wave absorbing material, and therefore the frequency of the electromagnetic wave showing the largest absorption characteristic is not affected by the thickness of the electromagnetic wave absorbing layer if the electromagnetic wave absorbing material is the same.

[0150] On the other hand, electromagnetic wave absorbing sheets currently available on the market that can absorb electromagnetic waves in the millimeter wave band and have elasticity are so-called wavelength interference type electromagnetic wave absorbing sheets that are formed by laminating a dielectric layer and a reflective layer and attenuate the intensity of the reflected wave by shifting the phase of the incident electromagnetic wave by 1 / 2 wavelength. In wavelength interference type electromagnetic wave absorbing sheets, the wavelength of the electromagnetic wave absorbed changes when the thickness of the dielectric layer changes, so if the electromagnetic wave absorbing sheet has elasticity and its thickness changes when it is stretched at a predetermined elongation rate, the value of the peak frequency of the electromagnetic wave absorbed also changes. As a result, there is a risk that a problem will occur in which an electromagnetic wave absorbing sheet arranged to absorb electromagnetic waves of a specific wavelength, such as an on-vehicle radar, has elasticity and therefore deteriorates the absorption characteristics of electromagnetic waves of a desired frequency.

[0151] In the case of the electromagnetic wave absorbing sheet disclosed in the present application, the frequency of the electromagnetic wave that is absorbed to a maximum extent does not change, so such a problem does not occur.

[0152] (Other configurations) In the above first and second embodiments, the magnetic iron oxide as the electromagnetic wave absorbing material contained in the electromagnetic wave absorbing layer is mainly epsilon iron oxide. As described above, by using epsilon iron oxide, it is possible to form an electromagnetic wave absorbing sheet that absorbs electromagnetic waves in the millimeter wave band of 30 gigahertz to 300 gigahertz. In addition, by using rhodium or the like as a metal material that substitutes for the Fe site, it is possible to realize an electromagnetic wave absorbing sheet that absorbs electromagnetic waves of 1 terahertz, which is the highest frequency defined as electromagnetic waves.

[0153] However, in the electromagnetic wave absorbing sheet disclosed in the present application, the magnetic iron oxide used as the electromagnetic wave absorbing material of the electromagnetic wave absorbing layer is not limited to epsilon iron oxide.

[0154] Barium ferrite, which is a hexagonal ferrite as a ferrite-based electromagnetic absorber, and strontium ferrite, which is also shown as one of the examples, exhibit good electromagnetic wave absorbing properties for electromagnetic waves in the band from several gigahertz to several tens of gigahertz. Therefore, by forming an electromagnetic wave absorbing layer using magnetic iron oxide particles other than epsilon iron oxide that have electromagnetic wave absorbing properties in the millimeter wave band of 30 gigahertz to 300 gigahertz and a rubber binder, it is possible to realize an electromagnetic wave absorbing sheet that absorbs electromagnetic waves in the millimeter wave band and has elasticity.

[0155] For example, the particle diameter of the hexagonal ferrite particles is larger than that of the epsilon iron oxide particles exemplified in the above embodiment, being several μm, and the particle shape is not substantially spherical but is a plate-like or needle-like crystal. Therefore, when forming a magnetic paint using a rubber binder, it is preferable to use a dispersant and adjust the kneading conditions with the binder so that, when applied as a magnetic paint, the magnetic iron oxide powder is dispersed as uniformly as possible in the electromagnetic wave absorbing layer, and the void ratio is adjusted as small as possible.

[0156] The electromagnetic wave absorbing sheet described in the above embodiment can be made elastic by using a rubber binder for the electromagnetic wave absorbing layer. In particular, by using magnetic iron oxide that resonates magnetically in a high frequency band equal to or higher than the millimeter wave band as the electromagnetic wave absorbing material, it is possible to realize an electromagnetic wave absorbing sheet that absorbs high frequency electromagnetic waves and has elasticity.

[0157] In the case of an electromagnetic wave absorbing sheet using magnetic iron oxide, which absorbs electromagnetic waves by magnetic resonance, as the electromagnetic wave absorbing material, a greater electromagnetic wave absorbing effect can be achieved by increasing the volume content of the electromagnetic wave absorbing material in the electromagnetic wave absorbing sheet. However, in an electromagnetic wave absorbing sheet having an electromagnetic wave absorbing layer composed of a rubber binder and an electromagnetic wave absorbing material, an upper limit of the volume content of the electromagnetic wave absorbing material is inevitably determined in order to ensure the elasticity due to the use of the binder. In the electromagnetic wave absorbing sheet disclosed in the present application, by setting the volume content of the magnetic iron oxide, which is the electromagnetic wave absorbing material, in the electromagnetic wave absorbing layer to 30% or more, a return loss of -15 dB or more can be ensured, particularly in the case of a reflective type electromagnetic wave absorbing sheet.

[0158] The volume content of the rubber binder in the electromagnetic wave absorbing layer is preferably 40% to 70%. By setting the volume content of the rubber binder in this range, it becomes easier to set the maximum elongation percentage of the elastic region in one in-plane direction of the electromagnetic wave absorbing sheet to the desired range of 20% to 200%.

[0159] In the above description, the electromagnetic wave absorbing layer is formed by preparing a magnetic paint, applying the paint, and drying the paint. In addition to the above method of applying a magnetic paint, the electromagnetic wave absorbing sheet disclosed in the present application may be produced by, for example, an extrusion molding method.

[0160] More specifically, magnetic iron oxide powder, a rubber binder, and, if necessary, a dispersant are blended in advance, and the blended materials are fed into a plastic cylinder from a resin feed port of an extruder. As the extruder, a normal extruder equipped with a plastic cylinder, a die provided at the tip of the plastic cylinder, a screw rotatably arranged in the plastic cylinder, and a drive mechanism for driving the screw can be used. The molten material plasticized by a band heater of the extruder is sent forward by the rotation of the screw and extruded into a sheet shape from the tip. The extruded material is dried, pressurized, calendered, etc. to obtain an electromagnetic wave absorbing layer of a predetermined thickness.

[0161] Furthermore, as a method for forming the electromagnetic wave absorbing layer, a method can be adopted in which a magnetic compound containing magnetic iron oxide powder and a rubber binder is prepared and this magnetic compound is press-molded to a predetermined thickness.

[0162] Specifically, first, a magnetic compound, which is an electromagnetic wave absorbing composition, is prepared. This magnetic compound can be obtained by kneading magnetic iron oxide powder with a rubber binder, and then mixing the resulting kneaded product with a crosslinking agent to adjust the viscosity.

[0163] The magnetic compound thus obtained as the electromagnetic wave absorbing composition is crosslinked and molded into a sheet at a temperature of 170° C., for example, using a hydraulic press. Thereafter, a secondary crosslinking treatment is performed in a thermostatic chamber at a temperature of, for example, 170° C., to form an electromagnetic wave absorbing sheet of a predetermined shape.

[0164] In the above embodiment, the electromagnetic wave absorbing sheet is described as being composed of a single layer, but a laminate of multiple layers can be used as the electromagnetic wave absorbing layer. In the case of the transmission type electromagnetic wave absorbing sheet shown as the first embodiment, the electromagnetic wave absorbing layer having a certain thickness or more improves the electromagnetic wave absorbing characteristics. In the reflection type electromagnetic wave absorbing sheet shown as the second embodiment, the electromagnetic wave absorbing characteristics can be further improved by adjusting the thickness of the electromagnetic wave absorbing layer to match its input impedance value with the impedance value in air. For this reason, when an electromagnetic wave absorbing layer of a predetermined thickness cannot be formed in a single layer due to the characteristics of the electromagnetic wave absorbing material or binder that forms the electromagnetic wave absorbing layer, it is effective to form the electromagnetic wave absorbing layer as a laminate. [Industrial Applicability]

[0165] The electromagnetic wave absorbing sheet disclosed in the present application is useful as an electromagnetic wave absorbing sheet that absorbs electromagnetic waves in a high frequency band equal to or higher than the millimeter wave band and further has elasticity. [Explanation of symbols]

[0166] 1. Electromagnetic wave absorbing layer 1a Epsilon iron oxide (magnetic iron oxide) 1b Rubber binder 2 Adhesive layer 3 reflective layer

Claims

1. An electromagnetic wave absorbing sheet having an electromagnetic wave absorbing layer containing magnetic iron oxide, which is an electromagnetic wave absorbing material that resonates with magnetic waves, and a rubber binder, The rubber binder is contained in an amount of 2 to 50 parts per 100 parts of the magnetic iron oxide, An electromagnetic wave absorbing sheet, characterized in that the input impedance value of the electromagnetic wave absorbing layer when stretched within the range of its elastic region is 360Ω to 450Ω.

2. 2. The electromagnetic wave absorbing sheet according to claim 1, wherein a volume content of said magnetic iron oxide in said electromagnetic wave absorbing layer is 30% or more.

3. 3. The electromagnetic wave absorbing sheet according to claim 1, wherein the magnetic iron oxide is at least one selected from the group consisting of epsilon iron oxide, barium ferrite magnetic powder, and strontium ferrite.

4. 4. The electromagnetic wave absorbing sheet according to claim 1, wherein the input impedance value of the electromagnetic wave absorbing layer when stretched at any elongation rate in a range of 5 to 75% of the maximum elongation rate in the elastic region matches the impedance value in air.

5. 5. The electromagnetic wave absorbing sheet according to claim 1, further comprising a reflective layer formed in contact with one surface of the electromagnetic wave absorbing layer for reflecting electromagnetic waves transmitted through the electromagnetic wave absorbing layer.

Images