Electromagnetic wave absorbing 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 particularly to an electromagnetic wave absorbing sheet that has an electromagnetic wave absorbing material that absorbs electromagnetic waves by magnetic resonance, absorbs electromagnetic waves of a high frequency equal to or higher than the millimeter wave band, and has elasticity extending in the in-plane direction.

Background Art

[0002] An electromagnetic wave absorbing sheet that absorbs electromagnetic waves is used to avoid the influence of leakage electromagnetic waves emitted to the outside from an electric circuit or the like and electromagnetic waves reflected undesirably.

[0003] In recent years, in mobile communications such as mobile phones, wireless LANs, and toll collection systems (ETC), centimeter waves having a frequency band of several gigahertz (GHz), and further, millimeter wave bands having frequencies from 30 gigahertz to 300 gigahertz, and electromagnetic waves in a high frequency band exceeding the millimeter wave band, research on technologies using electromagnetic waves having a frequency of 1 terahertz (THz) has also been progressing.

[0004] In response to such a technological trend of using electromagnetic waves of a higher frequency, there is an increasing demand for electromagnetic wave absorbers and electromagnetic wave absorbing sheets formed in a sheet shape that can absorb electromagnetic waves in the gigahertz band to the terahertz band.

[0005] As an electromagnetic wave absorber that absorbs electromagnetic waves in a high frequency band equal to or higher than the millimeter wave band, an electromagnetic wave absorber having a filling structure of particles having ε-iron oxide (ε-Fe2O3) crystals that exhibit electromagnetic wave absorption performance in the range of 25 to 100 gigahertz in the magnetic phase has been proposed (see Patent Document 1). Further, a proposal has been made for a sheet-shaped oriented body in which fine particles of ε-iron oxide are kneaded together with a binder, and a magnetic field is applied from the outside when the binder is dried and cured to enhance the magnetic field orientation of the ε-iron oxide particles (see Patent Document 2).

[0006] Furthermore, as an electromagnetic wave absorbing sheet having elasticity, an electromagnetic wave absorbing sheet capable of absorbing centimeter waves in which carbon nanotubes are dispersed in silicone rubber has been proposed (see Patent Document 3).

[0007] In addition, as a low-cost electromagnetic wave absorbing sheet capable of absorbing electromagnetic waves in the frequency band of 75 to 77 GHz and having profitability for civilian 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 adhered to a flexible printed wiring board and shields electromagnetic waves from the outside, by maintaining the repulsive force of a sheet in which a conductive layer containing conductive fine particles and an insulating layer are laminated within a predetermined range, a sheet having bend resistance and heat resistance that can be bent together with the flexible printed wiring board has been proposed (see Patent Document 5).

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[0009] When shielding leaked electromagnetic waves from a source that generates electromagnetic waves, it is necessary to arrange an electromagnetic wave absorbing material on a housing that covers the target circuit components. In particular, when the shape of the arrangement location is not a planar shape, it is more convenient and preferable to use an electromagnetic wave absorbing sheet having flexibility and elasticity that extends in the in-plane direction rather than using a solid electromagnetic wave absorber.

[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 in the millimeter wave band. Further, 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 none of them has elasticity.

[0011] As described above, an elastic sheet-like electromagnetic wave absorbing sheet that can absorb electromagnetic waves with frequencies of several tens of gigahertz or more in the millimeter wave band has not been realized as an electromagnetic wave absorbing member.

[0012] In order to solve the conventional problems, the present disclosure aims to realize an electromagnetic wave absorbing sheet that can favorably absorb electromagnetic waves with high frequencies of millimeter wave band or higher and has elasticity extending in the in-plane direction.

Means for Solving the Problems

[0013] The electromagnetic wave absorbing sheet disclosed in the present application to solve the above problems is an electromagnetic wave absorbing sheet having an electromagnetic wave absorbing layer containing magnetic iron oxide, which is an electromagnetic wave absorbing material that resonates magnetically, and a rubber binder, including 2 to 50 parts of the rubber binder with respect to 100 parts of the magnetic iron oxide, and characterized in that an input impedance value when the electromagnetic wave absorbing layer is stretched within the elastic range is 360Ω to 450Ω.

Effects of the Invention

[0014] The electromagnetic wave absorbing sheet disclosed in the present application includes magnetic iron oxide that resonates magnetically in a high frequency band of millimeter wave band or higher as an electromagnetic wave absorbing material in the electromagnetic wave absorbing layer, so that electromagnetic waves in a high frequency band of several tens of gigahertz or more can be converted into heat and absorbed. Further, since it is provided with a rubber binder and the maximum elongation rate in the elastic range in the in-plane direction is 20 to 200%, it is easy to arrange it at a desired portion, and furthermore, an electromagnetic wave absorbing sheet that can also cover a movable portion can be realized.

Brief Description of the Drawings

[0015]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

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Figure 7

Figure 8

Figure 9

Modes for Carrying Out the Invention

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

[0017] By doing so, the electromagnetic wave absorption sheet disclosed in the present application can absorb electromagnetic waves in a high-frequency band of 30 gigahertz or more, which is a millimeter-wave band, by the magnetic resonance of magnetic iron oxide, which is an electromagnetic wave absorption material. Further, by using an electromagnetic wave absorption material and a rubber binder, an electromagnetic wave absorption sheet having a high stretchability with a maximum elongation rate in the in-plane direction of 20 to 200% can be realized. Therefore, when arranging the electromagnetic wave absorption sheet on a housing or the like in which an electronic circuit to be shielded is accommodated, the ease of handling of the electromagnetic wave absorption sheet is improved. In particular, it becomes easy to arrange the electromagnetic wave absorption sheet on a complicatedly curved surface. Furthermore, it is possible to prevent the radiation and entry of undesired electromagnetic waves by covering movable parts of members whose shapes change, such as joint parts of arm members.

[0018] In the electromagnetic wave absorption sheet disclosed in the present application, it is preferable that the magnetic iron oxide is epsilon iron oxide. By using epsilon iron oxide that absorbs electromagnetic waves having a frequency higher than 30 gigahertz as an electromagnetic wave absorption material, an electromagnetic wave absorption sheet that absorbs high-frequency electromagnetic waves can be realized.

[0019] In this case, it is preferable that a part of the Fe sites of the epsilon iron oxide is substituted with trivalent metal atoms. By doing so, it is possible to realize an electromagnetic wave absorption sheet that absorbs electromagnetic waves in a desired frequency band by taking advantage of the characteristics of epsilon iron oxide having different magnetic resonance frequencies depending on the material that substitutes the Fe sites.

[0020] Further, it is preferable that the volume fraction of the magnetic iron oxide in the electromagnetic wave absorption layer is 30% or more. By doing so, the value of the imaginary part (μ'') of the magnetic permeability of the electromagnetic wave absorption layer can be increased, and an electromagnetic wave absorption sheet having high electromagnetic wave absorption characteristics can be realized.

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

[0022] Furthermore, it is preferable that the input impedance value of the electromagnetic wave absorption layer in a state where it is stretched by 5 to 75% of the maximum elongation rate in the elastic region matches the impedance value in air. By doing so, the input impedance value can be made close to the impedance value in air over a wide range of elongation rates of the electromagnetic wave absorption sheet, and high electromagnetic wave absorption characteristics can be maintained.

[0023] Also, it is preferable that the input impedance value when the electromagnetic wave absorption layer is stretched within the elastic region is 360 Ω to 450 Ω. By doing so, even when the electromagnetic wave absorption sheet expands and contracts within its elastic region, it is possible to avoid the input impedance value being significantly different from the impedance value in air, and to exhibit electromagnetic wave absorption characteristics above a certain level.

[0024] Furthermore, in the electromagnetic wave absorption sheet disclosed in the present application, it is preferable that a reflection layer is formed in contact with one surface of the electromagnetic wave absorption layer to reflect the electromagnetic wave transmitted through the electromagnetic wave absorption layer. By doing so, it is possible to realize a so-called reflection-type electromagnetic wave absorption sheet that can surely shield and absorb electromagnetic waves in a high-frequency band of millimeter wave band or higher.

[0025] Also, it is preferable to further provide an adhesive layer that enables the electromagnetic wave absorption sheet to be adhered. By doing so, it is possible to realize an electromagnetic wave absorption sheet that has excellent electromagnetic wave absorption characteristics and is easy to handle, and can be easily arranged at a desired location.

[0026] The second electromagnetic wave absorption sheet disclosed in the present application includes an electromagnetic wave absorption layer containing an electromagnetic wave absorption material and a rubber binder, and a reflection layer in contact with one surface of the electromagnetic wave absorption layer to reflect the electromagnetic wave transmitted through the electromagnetic wave absorption layer. The electromagnetic wave absorption material is magnetic iron oxide that resonates magnetically with electromagnetic waves of a predetermined frequency, and the input impedance value of the electromagnetic wave absorption layer in a state where it is stretched in one direction in the plane matches the impedance value in air.

[0027] By adopting such a configuration, the second electromagnetic wave absorption sheet disclosed in the present application can match the impedance value in the air with the input impedance value in a wide range of elongation rates on the premise that the electromagnetic wave absorption sheet is stretched to a certain extent during actual use, and can improve the electromagnetic wave absorption characteristics in the actual use state of the elastic electromagnetic wave absorption sheet.

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

[0029] Since "radio wave" can be understood as a kind of electromagnetic wave in a broader sense, in this specification, terms such as "electromagnetic wave" will be used, for example, referring to a radio wave absorber as an electromagnetic wave absorber.

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

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

[0032] Note that FIG. 1 is a diagram described to facilitate understanding of the configuration of the electromagnetic wave absorption sheet according to the present embodiment, and the sizes and thicknesses of the members shown in the figure are not represented in accordance with reality.

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

[0034] The electromagnetic wave absorption sheet according to this embodiment is such that the magnetic iron oxide 1a contained in the electromagnetic wave absorption layer 1 causes magnetic resonance, and thus converts electromagnetic waves into thermal energy by magnetic loss and absorbs them. Therefore, without providing a reflection layer on one surface of the electromagnetic wave absorption layer 1, it can be used as a so-called transmission type electromagnetic wave absorption sheet that absorbs electromagnetic waves transmitted through the electromagnetic wave absorption layer 1.

[0035] Also, in the electromagnetic wave absorption sheet of this embodiment, various rubber materials are used as the binder 1b that constitutes the electromagnetic wave absorption layer 1. Therefore, in particular, in the in-plane direction of the electromagnetic wave absorption sheet, an electromagnetic wave absorption sheet that easily expands and contracts can be obtained. Note that the electromagnetic wave absorption sheet according to this embodiment has a high elasticity and high flexibility at the same time because the magnetic iron oxide 1a is contained in the rubber binder 1b to form the electromagnetic wave absorption layer. When handling the electromagnetic wave absorption sheet, it can be rolled up, and it can be easily arranged along a curved surface.

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

[0037] [Electromagnetic Wave Absorbing Material] In the electromagnetic wave absorption sheet according to this embodiment, as the electromagnetic wave absorbing material, powders of magnetic iron oxides such as epsilon iron oxide magnetic powder, barium ferrite magnetic powder, and strontium ferrite magnetic powder can be used. Among these, epsilon iron oxide is particularly suitable as the electromagnetic wave absorbing material because the precession frequency when the electrons of iron atoms perform spin motion is high, and it has a high effect of absorbing electromagnetic waves in the millimeter wave band of 30 to 300 gigahertz or higher frequencies.

[0038] Epsilon iron oxide (ε-Fe2O3) is a phase that appears between the alpha phase (α-Fe2O3) and the gamma phase (γ-Fe2O3) in ferric oxide (Fe2O3), and it is a magnetic material that can be obtained in a single-phase state by a nanoparticle synthesis method combining the reverse micelle method and the sol-gel method.

[0039] Although epsilon iron oxide is composed of fine particles several nanometers to dozens of nanometers in size, it has the largest coercive force among metal oxides, about 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 with a frequency of dozens of gigahertz or more.

[0040] Furthermore, by making epsilon iron oxide into a crystal in which some of the Fe sites in the crystal are substituted with trivalent metal elements such as aluminum (Al), gallium (Ga), rhodium (Rh), indium (In), etc., the magnetic resonance frequency, that is, the frequency of the electromagnetic wave absorbed when used as an electromagnetic wave absorbing material, can be varied.

[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 used to substitute the Fe sites. Note that the natural resonance frequency f coincides with the frequency of the absorbed electromagnetic wave.

[0042] From Figure 2, it can be seen that epsilon iron oxide with some of the Fe sites substituted has different natural resonance frequencies depending on the type and amount of the substituted metal element. Also, it can be seen that the higher the value of the natural resonance frequency, the greater the coercive force of the epsilon iron oxide.

[0043] More specifically, in the case of gallium-substituted epsilon iron oxide, that is, ε-Ga x Fe 2-x O3, by adjusting the substitution amount "x", it has an absorption peak in the frequency band from about 30 GHz to 150 GHz. For aluminum-substituted epsilon iron oxide, that is, ε-Al x Fe 2-xIn the case of O3, by adjusting the substitution amount “x”, it has an absorption peak in a frequency band of about 100 GHz to 190 GHz. Therefore, in order to obtain the natural resonance frequency of the frequency to be absorbed by the electromagnetic wave absorption sheet, determine the type of element to substitute the Fe site of epsilon iron oxide, and further adjust the substitution amount with Fe to make the frequency of the absorbed electromagnetic wave a desired value. Furthermore, epsilon iron oxide with rhodium as the substituting metal, that is, ε-Rh x Fe 2-x In the case of O3, it is possible to shift the frequency band of the absorbed electromagnetic wave higher, from 180 GHz and above.

[0044] Epsilon iron oxide is commercially available including those in which some Fe sites are metal-substituted, so it can be easily obtained. The preferred particle size of epsilon iron oxide powder is 5 nm to 50 nm as the average particle size, and it has a substantially 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 a large coercive force due to their large magnetic anisotropy.

[0046] The powder of barium ferrite or strontium ferrite can be synthesized by blending iron (Fe) and barium or strontium chloride (BaCl2, SrCl2), and if necessary, further metal oxides containing Ba and Sr as raw materials, mixing, granulating, and then firing this, and the fired body can be pulverized to obtain a powder having a predetermined particle size. As an 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 h.

[0047] The size of the powder to be produced can be adjusted by the magnitude of the load applied during pulverization. When obtaining a relatively large powder, methods such as subjecting the fired body to impact pulverization by 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 absorption layer] For the rubber binder 1b constituting the electromagnetic wave absorption layer 1, 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), polysulfide rubber (T) can be used.

[0049] Among these rubber materials, acrylic rubber and silicone rubber can be preferably used because of their high heat resistance. In the case of acrylic rubber, it has excellent oil resistance even in a high-temperature environment, is relatively inexpensive, and has excellent cost performance. Also, in the case of silicone rubber, it has high cold resistance in addition to heat resistance. Furthermore, the temperature dependence of its physical properties is the lowest among synthetic rubbers, and it also has excellent solvent resistance, ozone resistance, and weather resistance. Moreover, it has excellent electrical insulation and is materially stable over a wide temperature range and frequency range.

[0050] In the electromagnetic wave absorption layer 1 of the electromagnetic wave absorption sheet according to this embodiment, when epsilon iron oxide powder is used as the electromagnetic wave absorption material 1a, since the epsilon iron oxide powder is fine nanoparticles with 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 absorption layer 1. For this reason, in the electromagnetic wave absorption sheet according to this embodiment, the electromagnetic wave absorption layer 1 contains phosphoric acid compounds such as arylsulfonic acids such as phenylphosphonic acid and phenylphosphonic acid dichloride, alkylphosphonic acids such as methylphosphonic acid, ethylphosphonic acid, octylphosphonic acid, and propylphosphonic acid, or polyfunctional phosphonic acids such as hydroxyethanediphosphonic acid and nitrilotrimethylenephosphonic acid. These phosphoric acid compounds have flame retardancy and function as dispersants for fine magnetic iron oxide powder, so that the epsilon iron oxide particles in the binder can be dispersed well.

[0051] More specifically, as the dispersant, phenylphosphonic acid (PPA) manufactured by Wako Pure Chemical Industries, Ltd. or Nissan Chemical Industries, Ltd., or phosphoric acid ester oxide "JP-502" (product name) manufactured by Johoku Chemical Industry Co., Ltd. can be used.

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

[0053] As an example of the composition of the electromagnetic wave absorption layer 1, the content of the rubber binder can be 2 to 50 parts and the content of the phosphate compound can be 0.1 to 15 parts with respect to 100 parts of epsilon iron oxide powder (magnetic iron oxide). If the rubber binder is less than 2 parts, it is impossible to disperse the magnetic iron oxide well. In addition, the shape as an electromagnetic wave absorption sheet cannot be maintained, and it becomes difficult to obtain the elongation of the electromagnetic wave absorption sheet. If it is more than 50 parts, the elongation of the electromagnetic wave absorption sheet can be obtained, but the volume fraction of the magnetic iron oxide in the electromagnetic wave absorption sheet becomes small and the magnetic permeability becomes low, so the electromagnetic wave absorption effect becomes small.

[0054] If the content of the phosphate compound is less than 0.1 part, it is impossible to disperse the magnetic iron oxide well using the rubber binder. If it is more than 15 parts, the effect of dispersing the magnetic iron oxide well saturates. The volume fraction of the magnetic iron oxide in the electromagnetic wave absorption sheet becomes small and the magnetic permeability becomes low, so the electromagnetic wave absorption effect becomes small.

[0055] [Manufacturing method of electromagnetic wave absorption layer] Here, the manufacturing method of the electromagnetic wave absorption layer 1 of the electromagnetic wave absorption sheet according to this embodiment will be described. In the electromagnetic wave absorption sheet of this embodiment, a magnetic paint containing at least magnetic iron oxide powder and a rubber binder is prepared, applied with a predetermined thickness, dried, and then calendered to form the electromagnetic wave absorption layer 1. The calendering process is not essential, but it is preferable to perform the calendering process because the voids in the electromagnetic wave absorption sheet can be reduced and the filling degree of the magnetic iron oxide powder can be improved.

[0056] First, prepare the magnetic paint.

[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 this with a solvent, further dispersing it, and then filtering it through a filter. The kneaded product can be obtained, for example, by kneading in a pressure-type batch kneader. Also, the dispersion of the kneaded product can be obtained as a dispersion liquid, for example, using a sand mill filled with beads such as zirconia. At this time, a crosslinking agent can be blended as necessary.

[0058] The obtained magnetic paint is applied onto a support having releasability, for example, a sheet of polyethylene terephthalate (PET) with a thickness of 38 μm that has been subjected to a release treatment by a silicone coat, using a table coater, a bar coater, or the like.

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

[0060] For example, by setting the thickness of the wet magnetic paint applied onto the support to 1 mm, the thickness after drying can be made 400 μm, and the thickness of the electromagnetic wave absorption layer after calendering can be made 300 μm.

[0061] In this way, an electromagnetic wave absorption layer 1 in a state where fine epsilon iron oxide powder on the nm order used as the electromagnetic wave absorption material 1a is well dispersed in the rubber binder 1b can be formed.

[0062] In addition, as another method for producing the magnetic paint, as the magnetic paint components, at least magnetic iron oxide powder, a phosphoric acid compound as a dispersant, and a rubber binder are rapidly mixed using a high-speed stirrer to prepare a mixture, and then the obtained mixture is dispersed using a sand mill to also obtain the magnetic paint.

[0063] [Adhesive layer] As shown in FIG. 1, in the electromagnetic wave absorption sheet according to this embodiment, an adhesive layer 2 is formed on the back surface of the electromagnetic wave absorption layer 1.

[0064] By providing the adhesive layer 2, the electromagnetic wave absorption layer 1 can be adhered to a desired position on the inner surface of the housing for accommodating the electric circuit, or the inner or outer surface of the electric device. In particular, since the electromagnetic wave absorption layer 1 of the present embodiment has elasticity, the electromagnetic wave absorption sheet can be easily adhered even to a curved surface curved by the adhesive layer 2, improving the ease of handling of the electromagnetic wave absorption sheet. Note that, by devising the material, formed thickness, formed state, etc. of the adhesive layer 2, the adhesive layer 2 does not prevent the elongation due to the elastic deformation of the electromagnetic wave absorption layer 1. For example, it is preferable to use an acrylic-based adhesive, a silicone-based adhesive, a rubber-based adhesive, etc. having a low glass transition temperature (Tg).

[0065] As the adhesive layer 2, known materials used as an adhesive layer such as an adhesive tape, an acrylic-based adhesive, a rubber-based adhesive, a silicone-based adhesive, etc. can be used. In particular, when silicone rubber is used as the rubber binder, it is preferable to use a silicone-based adhesive as the material of the adhesive layer in order not to reduce the adhesion between the electromagnetic wave absorption layer and the adhesive layer.

[0066] Also, in order to adjust the adhesive force to the adherend and reduce the adhesive residue, an adhesion promoter or a crosslinking agent can be used. The adhesive force to the adherend is preferably 5 N / 10 mm to 12 N / 10 mm. If the adhesive force is less than 5 N / 10 mm, the electromagnetic wave absorption sheet may be easily peeled off or displaced from the adherend. Also, if the adhesive force is greater than 12 N / 10 mm, it becomes difficult to peel the electromagnetic wave absorption sheet from the adherend.

[0067] Also, the thickness of the adhesive layer 2 is preferably 20 μm to 100 μm. If the thickness of the adhesive layer is less than 20 μm, the adhesive force becomes small, and the electromagnetic wave absorption sheet may be easily peeled off or displaced from the adherend. If the thickness of the adhesive layer is greater than 100 μm, the thickness of the entire electromagnetic wave absorption sheet becomes thick, so there is a risk that the flexibility will decrease. In addition, when the adhesive layer 2 is thick, it becomes difficult to peel the electromagnetic wave absorption sheet from the adherend. Also, when the cohesive force of the adhesive layer 2 is small, when the electromagnetic wave absorption sheet is peeled off, there may be adhesive residue on the adherend.

[0068] In addition, in the present specification, the adhesive layer 2 may be an adhesive layer 2 that is adhered inseparably, or may be an adhesive layer 2 that performs peelable adhesion.

[0069] Also, when attaching the electromagnetic wave absorption sheet to a predetermined surface, even if the electromagnetic wave absorption sheet does not include the adhesive layer 2, the surface on the side of the member where the electromagnetic wave absorption sheet is disposed is provided with adhesiveness, and only the electromagnetic wave absorption layer 1 is formed. The electromagnetic wave absorption sheet can be pasted. Also, by using a double-sided tape or an adhesive, the electromagnetic wave absorption sheet can be attached to a predetermined site. In this regard, the adhesive layer 2 is not an essential component in the electromagnetic wave absorption sheet shown in this embodiment, but the configuration in which the electromagnetic wave absorption sheet includes the adhesive layer 2 is preferable because the electromagnetic wave absorption sheet can be attached to a predetermined site without using a double-sided tape or an adhesive.

[0070] [Elongation of Electromagnetic Wave Absorption Sheet] Next, the elongation of the electromagnetic wave absorption sheet according to this embodiment in the in-plane direction 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 rate of the electromagnetic wave absorption sheet in the electromagnetic wave absorption sheet according to this embodiment. FIG. 3(a) shows the relationship between the stress and the elongation rate in the electromagnetic wave absorption sheet that breaks when the maximum elongation rate is exceeded. Also, FIG. 3(b) shows the relationship between the stress and the elongation rate in the electromagnetic wave absorption sheet that undergoes plastic deformation when the maximum elongation rate is exceeded, respectively.

[0072] Here, the "elongation rate" is a numerical value expressed in % obtained by dividing the amount of elongation of the electromagnetic wave absorption sheet elongated by applying stress in one direction by the original length. That is, if the length at stress 0 is L1 and the length when a predetermined stress is applied is L2, the "elongation rate" when this predetermined stress is applied is expressed as (L2 - L1) / L1 × 100. Note that this "elongation rate" is also referred to as "strain".

[0073] As shown in Fig. 3(a), in the electromagnetic wave absorption sheet that breaks when the maximum elongation rate is exceeded, until it reaches the maximum elongation rate of 170%, as the stress applied from the outside increases, the elongation rate of the electromagnetic wave absorption sheet increases almost linearly (the part of reference numeral 11). After that, when stress is applied exceeding the maximum elongation rate of 170%, the electromagnetic wave absorption sheet breaks, and the value of the elongation rate does not increase from the maximum elongation rate of 170% (the part of reference numeral 12).

[0074] On the other hand, in the electromagnetic wave absorption sheet that undergoes plastic deformation when the maximum elongation rate is exceeded, as shown in Fig. 3(b), until it reaches the elongation rate of 30% which is the elongation rate indicating the maximum stress, as the applied stress increases, the elongation rate rises relatively gently (the part of reference numeral 13). After that, when the electromagnetic wave absorption sheet is further pulled after reaching the elongation rate of 30% indicating the maximum stress, the electromagnetic wave absorption sheet elongates until it undergoes plastic deformation and reaches an elongation rate of 230% (the part of reference numeral 14). For this reason, the stress gradually decreases. Note that because plastic deformation is occurring, the electromagnetic wave absorption 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 absorption sheet is released.

[0075] Note that the elastic deformation region of the rubber material used as the binder 1b can be adjusted by using a vulcanizing agent selected as appropriate. Also, considering the relationship such as the intended use, when an electromagnetic wave absorption sheet that must not break is required, it is considered effective to adopt a form that undergoes plastic deformation without breaking.

[0076] In addition, as the range of the elongation of the electromagnetic wave absorption sheet, the upper limit is set to 200% as the range in which the input impedance value deviates significantly from the impedance value in air and the impedance matching cannot be achieved, resulting in a decrease in the electromagnetic wave absorption ability. Also, if the elongation of the electromagnetic wave absorption sheet is too large, the thickness of the electromagnetic wave absorption sheet becomes thin and the density of the electromagnetic wave absorption material decreases, so the electromagnetic wave absorption ability also decreases. Furthermore, when the elongation of the electromagnetic wave absorption sheet exceeds 200%, the flexibility and bendability of the electromagnetic wave absorption sheet decrease.

[0077] On the other hand, if the elongation of the electromagnetic wave absorption sheet is less than 20%, it cannot be sufficiently stretched when attaching it to a curved adherend, resulting in a decrease in workability. Also, it becomes impossible to cope with attachment to a movable part whose shape changes, and the characteristics of the electromagnetic wave absorption sheet according to this embodiment having elasticity cannot be utilized.

[0078] Here, for the electromagnetic wave absorption sheet according to this embodiment, those with different types of magnetic iron oxide and rubber binders were actually manufactured, and the relationship between the tensile stress from the outside and the elongation rate of the electromagnetic wave absorption sheet was measured.

[0079] The first electromagnetic wave absorption sheet (Example 1) used epsilon iron oxide as the magnetic iron oxide and acrylic rubber as the rubber binder. Table 1 shows the materials used for manufacturing the first electromagnetic wave absorption sheet and their ratios.

[0080]

Table 1

[0081] The second electromagnetic wave absorption sheet (Example 2) used epsilon iron oxide as the magnetic iron oxide, similar to the first electromagnetic wave absorption sheet, and silicone rubber as the rubber binder. Table 2 shows the materials used for manufacturing the second electromagnetic wave absorption sheet and their ratios.

[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 in the same manner as the second electromagnetic wave absorbing sheet. Table 3 shows the materials used for producing the third electromagnetic wave absorbing sheet and their ratios.

[0084]

Table 3

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

[0086] The dispersion shown in Table 1 above was coated on a 38-μm-thick polyethylene terephthalate (PET) sheet subjected to release treatment with a silicone coat using a sheet-fed coater.

[0087] Also, the dispersions shown in Tables 2 and 3 above were coated on a 38-μm-thick polyethylene terephthalate (PET) sheet subjected to release treatment with a non-silicone-based release agent using a sheet-fed coater.

[0088] The wet paint was dried at 80 °C, and an electromagnetic wave absorption layer was formed by calendering so that the thickness after calendering was 500 μm.

[0089] The 500-μm-thick electromagnetic wave absorption layer thus produced was stacked five layers and hot-compressed by a calendar device to produce a 2500-μm single-film electromagnetic wave absorption layer. Note that an electromagnetic wave absorbing sheet consisting only of the electromagnetic wave absorption layer was obtained without forming an adhesive layer.

[0090] For each of the produced electromagnetic wave absorption sheets, the elongation rate was measured using a tensile testing machine. Specifically, the elongation rate was measured when a sheet sized 20 mm × 50 mm was stretched under the condition of a tensile speed of 10 mm / min using a TGE-1kN type testing machine (product name) manufactured by Minebea Co., Ltd. and TT3E-200N as a load cell. The elongation rate measurement was carried out in an environment with a temperature of 23°C and a humidity of 50%Rh.

[0091] The elongation rates of the three electromagnetic wave absorption sheets measured in this way are shown in Fig. 4.

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

[0093] As shown in Fig. 4, the three electromagnetic wave absorption sheets produced as examples are all of the type that break when exceeding the maximum elongation rate shown in Fig. 3(a), and the maximum elongation rate was 195% - 200%. As described above, in the electromagnetic wave absorption sheet according to this embodiment, it is preferable that the maximum elongation is 200%, and the three electromagnetic wave absorption sheets produced are within a preferable range in terms of electromagnetic wave absorption characteristics, flexibility, and bendability.

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

[0095] Next, regarding the electromagnetic wave absorption sheet according to this embodiment, the change in the electromagnetic wave absorption characteristics when stress was applied to the sheet and it was stretched was measured.

[0096] The measurement was performed by measuring the electromagnetic wave absorption amount (electromagnetic wave attenuation amount) using the free space method for the above-described first electromagnetic wave absorption sheet. Specifically, using a millimeter-wave network analyzer ME7838AN5250C (product name) manufactured by Anritsu Corporation, an input wave (millimeter wave) of a predetermined frequency was irradiated onto the electromagnetic wave absorption sheet from a transmitting antenna through a dielectric lens, and the electromagnetic wave transmitted by a receiving antenna arranged on the back side of the electromagnetic wave absorption sheet was measured. The intensity of the irradiated electromagnetic wave and the intensity of the transmitted electromagnetic wave were grasped as voltage values, respectively, and the electromagnetic wave attenuation amount was obtained in dB from the intensity difference.

[0097] FIG. 5 is a diagram showing the electromagnetic wave absorption characteristics in the state where no tension is applied to the first electromagnetic wave absorption sheet according to this embodiment, and the electromagnetic wave absorption characteristics in the state where tension is applied to the electromagnetic wave absorption sheet, elongation occurs in the electromagnetic wave absorption sheet, and its thickness decreases.

[0098] In FIG. 5, the solid line indicated by reference numeral 21 is the electromagnetic wave absorption characteristics in the state where no tension is applied to the electromagnetic wave absorption sheet, that is, the state where the elongation rate is 0%. At this time, the thickness of the electromagnetic wave absorption sheet was 2500 μm at the time of production.

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

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

[0101] As shown by the dotted line labeled 22 in Fig. 5, in the electromagnetic wave absorption sheet with an elongation rate of 75%, the electromagnetic wave absorption amount at 75.5 GHz is approximately 19 dB. It can be seen that compared with the case where the elongation rate is 0% (label 21), the electromagnetic wave absorption characteristics have deteriorated. This is presumably due to the fact that when the electromagnetic wave absorption sheet is pulled in its in-plane direction, its thickness decreases, and the content of the electromagnetic wave absorbing material in the direction in which the electromagnetic wave penetrates within the electromagnetic wave absorption sheet substantially decreases.

[0102] That is, in the case of a transmissive electromagnetic wave absorption sheet such as the first electromagnetic wave absorption sheet, it was found that the electromagnetic wave absorption characteristics deteriorate when the electromagnetic wave absorption sheet is pulled in its in-plane direction.

[0103] Therefore, the inventors measured the electromagnetic wave absorption characteristics when the elongation rate of the electromagnetic wave absorption sheet was further increased, and measured the relationship between the thickness of the electromagnetic wave absorption sheet and the electromagnetic wave absorption amount in the electromagnetic wave absorption sheet whose thickness decreased due to being pulled in the in-plane direction. The measurement results are shown in Fig. 6.

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

[0105] In Fig. 6, the change in the electromagnetic wave absorption amount of the first electromagnetic wave absorption sheet is indicated by black circles and solid line 31, and the change in the electromagnetic wave absorption amount of the third electromagnetic wave absorption sheet is indicated by black squares and dotted line 32.

[0106] As shown in Fig. 6, in both the first electromagnetic wave absorption sheet and the third electromagnetic wave absorption sheet, the absorption amount (31, 32) of the electromagnetic wave at a frequency of 75.5 GHz is approximately proportional to the thickness of the electromagnetic wave absorption sheet. It was confirmed that the stronger the electromagnetic wave absorption sheet is pulled and the thinner its thickness becomes, the more the electromagnetic wave absorption characteristics deteriorate.

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

[0108] (Second Embodiment) [Reflective Electromagnetic Wave Absorption Sheet] Next, a so-called reflective electromagnetic wave absorption sheet in which a reflective layer is formed on the back surface of the electromagnetic wave absorption layer, which is the second configuration example of the electromagnetic wave absorption sheet disclosed in the present application, will be described while showing a specific embodiment.

[0109] FIG. 7 shows a cross-sectional configuration of the electromagnetic wave absorption sheet of the second embodiment.

[0110] Note that FIG. 7 is a diagram described for easy understanding of the configuration, similar to FIG. 1 that explained the configuration of the electromagnetic wave absorption sheet according to the first embodiment, and the sizes and thicknesses of the members shown in the figure are not represented in accordance with reality. Also, the same members as those constituting the electromagnetic wave absorption sheet according to the first embodiment shown in FIG. 1 are given the same reference numerals and detailed descriptions thereof are omitted.

[0111] The electromagnetic wave absorption sheet disclosed in the present application absorbs electromagnetic waves by magnetic resonance of magnetic iron oxides such as iron oxide epsilon, barium ferrite, and strontium ferrite, which form an electromagnetic wave absorption layer together with a rubber binder as an electromagnetic wave absorption material. Therefore, in addition to being configured as a transmissive electromagnetic wave absorption sheet without a reflective layer shown as the first embodiment, a configuration as a reflective electromagnetic wave absorption sheet provided with a reflective layer that reflects electromagnetic waves on the surface of the electromagnetic wave absorption layer opposite to the side where the electromagnetic waves are incident can be adopted.

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

[0113] In the electromagnetic wave absorbing sheet of the second embodiment shown in FIG. 7, an adhesive layer 2 is formed on the further back side of the reflection layer 3 to enable the electromagnetic wave absorbing sheet to be adhered to a predetermined location. Similar to the case of the electromagnetic wave absorbing sheet according to the above-described first embodiment, in the electromagnetic wave absorbing sheet according to the second embodiment, the adhesive layer 2 is not an essential component, and it is also possible to form an electromagnetic wave absorbing sheet without the adhesive layer 2. However, by configuring the electromagnetic wave absorbing sheet to include the adhesive layer 2, it is preferable because the electromagnetic wave absorbing sheet can be adhered to a predetermined site without using a double-sided tape or an adhesive.

[0114] The reflection 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 the present embodiment, since the electromagnetic wave absorbing sheet has elasticity by using the rubber binder 1b, as the reflection layer 3, a mesh-like conductor, silver nanowire (Ag-NW), a conductive polymer film, etc. are used so that even when the electromagnetic wave absorbing layer 1 stretches, its surface resistance value does not increase and a resistance value of about 1 Ω / square can be maintained.

[0115] As a method of forming a reflection layer on the back surface of the electromagnetic wave absorbing layer 1, a method of spraying or coating silver nanowire or a conductive polymer on the back side of the electromagnetic wave absorbing sheet can be adopted. Also, a reflection layer 3 in which silver nanowire or a conductive polymer is dispersed in a rubber binder similar to the reflection layer is produced, and the elastic reflection layer 3 is thermocompression bonded to the electromagnetic wave absorbing layer. Furthermore, a method of applying a paint for producing the electromagnetic wave absorbing layer 1 to the elastic reflection layer 3 and forming the electromagnetic wave absorbing layer 1 on the reflection layer 3 can be adopted.

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

[0117] In the electromagnetic wave absorption sheet according to the second embodiment shown in FIG. 7, by providing the reflection layer 3 on the back surface of the electromagnetic wave absorption layer 1, it is possible to surely avoid the situation where electromagnetic waves penetrate the electromagnetic wave absorption sheet. Therefore, it can be suitably used as an electromagnetic wave absorption sheet for preventing the leakage of electromagnetic waves emitted from electrical circuit components driven particularly at high frequencies to the outside.

[0118] [Elongation of the reflective electromagnetic wave absorption sheet] Also in the reflective electromagnetic wave absorption sheet according to the second embodiment, similar to the electromagnetic wave absorption sheet of the first embodiment, when the electromagnetic wave absorption layer 1 is pulled and elongated, due to the change in the thickness of the electromagnetic wave absorption layer 1 resulting in a change in the input impedance value, there occurs a change in the electromagnetic wave absorption characteristics due to impedance matching mismatch, and a change in the electromagnetic wave absorption characteristics due to a decrease in the amount of the electromagnetic wave absorption material in the portion of the electromagnetic wave absorption layer 1 through which the electromagnetic wave passes.

[0119] Furthermore, in the case of a reflective electromagnetic wave absorption sheet, there is a problem that it is necessary to match the input impedance value of the electromagnetic wave absorption sheet with the impedance value in air. If the input impedance value of the electromagnetic wave absorption 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 occur when electromagnetic waves are incident on the electromagnetic wave absorption sheet, resulting in impairment of the electromagnetic wave absorption characteristics as a reflective electromagnetic wave absorption sheet, that is, the characteristic of reducing the reflected wave of the incident electromagnetic wave.

[0120] Here, the impedance Z of the electromagnetic wave absorption layer 1 in the electromagnetic wave absorption sheet provided with magnetic iron oxide as the electromagnetic wave absorption material in is expressed by the following mathematical formula (1).

[0121] [Number]

[0122] In the above formula (1), μ r is the complex permeability of the electromagnetic wave absorption layer 1, ε r is the complex permittivity of the electromagnetic wave absorption layer 1, λ is the wavelength of the incident electromagnetic wave, and d is the thickness of the electromagnetic wave absorption layer 1. Therefore, when the electromagnetic wave absorption sheet is stretched, the thickness d of the electromagnetic wave absorption layer 1 becomes smaller, and the content of magnetic iron oxide, which is the electromagnetic wave absorption material, decreases, so that both the permeability and the permittivity of the electromagnetic wave absorption layer 1 change. As a result, the input impedance value (Zin) of the electromagnetic wave absorption sheet depends on the thickness of the binder 1b forming the electromagnetic wave absorption layer 1, which means that when the thickness fluctuates with the stretching and shrinking of the electromagnetic wave absorption sheet, the input impedance value (Zin) of the electromagnetic wave absorption sheet fluctuates.

[0123] Based on this, the inventors of the present invention considered that, instead of the steady state of the electromagnetic wave absorption sheet, that is, the state where the elongation rate of the electromagnetic wave absorption sheet is 0% without external force, but the state where the electromagnetic wave absorption sheet is stretched at a certain elongation rate, by matching the input impedance value of the electromagnetic wave absorption sheet with the input impedance value in the air, an electromagnetic wave sheet that can preferably absorb electromagnetic waves incident under a wider range of conditions in the practical state can be realized.

[0124] Therefore, an actual reflection-type electromagnetic wave absorption sheet was manufactured, and the effect of matching the input impedance value of the electromagnetic wave absorption sheet with the impedance value in the air in the state of being stretched at a certain elongation rate was verified.

[0125] First, as the fourth electromagnetic wave absorption sheet (Example 4), a reflection-type electromagnetic wave absorption sheet was manufactured.

[0126] The reflective electromagnetic wave absorption sheet, which is the fourth electromagnetic wave absorption sheet, is kneaded with the mixed material having the composition shown in Table 1 in a pressure-type batch kneader in the same manner as the above-described first electromagnetic wave absorption sheet. After diluting the obtained kneaded product with 170 parts of methyl ethyl ketone, a dispersion was prepared using a sand mill filled with zirconia beads.

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

[0128] The wet paint was dried at 80°C, and an electromagnetic wave absorption layer was formed by calendering so that the thickness after calendering was 410 μm.

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

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

[0131] The measurement of the electromagnetic wave absorption characteristics of the fourth electromagnetic wave absorption sheet was performed using the free space method in the same manner as the electromagnetic wave absorption measurement in the above-described transmissive electromagnetic wave absorption sheet. However, in order to measure the characteristics of the reflective electromagnetic wave absorption sheet, the output of the electromagnetic wave incident on the electromagnetic wave absorption sheet and the output of the reflected wave emitted from the electromagnetic wave absorption sheet were measured by arranging a transmitting antenna and a receiving antenna on the front surface side of the electromagnetic wave absorption sheet.

[0132] Note that the electromagnetic wave absorption sheet provided with an electromagnetic wave absorption layer having acrylic rubber as a main component as a binder, which was produced as the fourth electromagnetic wave absorption sheet, had a predetermined maximum elongation rate determined as shown in Fig. 3(b) (specifically, 30% in the case of the fourth electromagnetic wave absorption sheet), and was an electromagnetic wave absorption sheet that caused plastic deformation when it exceeded this value.

[0133] In addition, in the fourth electromagnetic wave absorption sheet, when the elongation rate is 11%, the thickness becomes 370 μm, and the input impedance value of the electromagnetic wave absorption sheet in this state with a thickness of 370 μ matches the impedance value in air, which is 377 Ω.

[0134] FIG. 8 shows the change in the electromagnetic wave absorption characteristics in the fourth electromagnetic wave absorption sheet with the elongation rate changed.

[0135] In FIG. 8, the solid line indicated by reference numeral 41 shows the electromagnetic wave absorption amount (attenuation amount by the reflected electromagnetic wave) in the steady state of the fourth electromagnetic wave absorption sheet, that is, the state where the elongation rate is 0%. At this time, the thickness of the fourth electromagnetic wave absorption sheet was 410 μm. Also, the dotted line indicated by reference numeral 42 is the state where the fourth electromagnetic wave absorption sheet is stretched at an elongation rate of 3%, and the thickness of the electromagnetic wave absorption sheet at this time was 400 μm. Further, the dashed-dotted line indicated by reference numeral 43 is the state where the fourth electromagnetic wave absorption sheet is stretched at an elongation rate of 11%, and the thickness of the electromagnetic wave absorption sheet at this time is 370 μm. The double-dashed-dotted line indicated by reference numeral 44 is the state where the fourth electromagnetic wave absorption sheet is stretched at an elongation rate of 22%, and the thickness of the electromagnetic wave absorption sheet at this time was 335 μm.

[0136] As shown in FIG. 8, in the fourth electromagnetic wave absorption sheet, the electromagnetic wave absorption amount in the state where the electromagnetic wave absorption sheet with the input impedance matched is stretched at an elongation rate of 11% is the largest, about 23 dB. In contrast, the electromagnetic wave absorption amount when the elongation rate is 3% is about 18 dB, and the electromagnetic wave absorption amount when the elongation rate is 0% is about 15 dB, indicating that the electromagnetic wave absorption amount decreases as the electromagnetic wave absorption sheet stretches. This is a different result from FIGS. 5 and 6 showing the electromagnetic wave absorption characteristics of the first electromagnetic wave absorption sheet. That is, in the second embodiment, the result is opposite to that of the first embodiment, where the electromagnetic wave absorption amount increases as the elongation rate increases.

[0137] This indicates that in a reflective electromagnetic wave absorption sheet, the change in the input impedance value due to the change in the thickness of the electromagnetic wave absorption layer has a stronger impact on the reduction of the electromagnetic wave absorption amount caused by the impedance mismatch of impedance matching than the reduction of the electromagnetic wave absorption amount due to the decrease in the amount of the electromagnetic wave absorption material in the portion where the electromagnetic wave of the electromagnetic wave absorption layer passes due to the elongation of the electromagnetic wave absorption sheet.

[0138] Therefore, in a reflective electromagnetic wave absorption sheet, it is preferable to match the input impedance value of the electromagnetic wave absorption sheet with the impedance value in the air in consideration of the change in the input impedance value of the electromagnetic wave absorption layer accompanying the change in the elongation rate of the electromagnetic wave absorption sheet.

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

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

[0141] As shown in FIG. 9, in the fourth electromagnetic wave absorption sheet, by performing impedance matching in the state where the electromagnetic wave absorption sheet is elongated at an elongation rate of 11% and the thickness becomes 370 μm, the electromagnetic wave absorption amount can be maintained at 15 dB or less, which is considered practically preferable, that is, 92% or more as the electromagnetic wave absorption amount, from the state where the elongation rate is 0% to the state where the elongation rate is 22%.

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

[0143] As is clear from the above description, in an electromagnetic wave absorption sheet provided with a magnetic member that absorbs electromagnetic waves by magnetic resonance as an electromagnetic wave absorption material, when the sheet has elasticity and its thickness changes, it is preferable that the input impedance value of the electromagnetic wave absorption layer matches the impedance value in air in a state where the electromagnetic wave absorption sheet is stretched to some extent.

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

[0145] In the fourth electromagnetic wave absorption sheet described with a specific example in the second embodiment, in the electromagnetic wave absorption sheet with a maximum elongation rate of 30%, the elongation rate was 11%, that is, in a state of about 37% of the elongation rate compared with the maximum elongation rate, the input impedance value was matched. As a criterion for impedance matching in a state of being elongated by a predetermined amount of elongation in advance, it depends on the type of deformation when the rubber material used as a binder is stretched particularly beyond the maximum elongation rate. By using as a criterion a state of being stretched by 5 to 75% with respect to the maximum elongation rate, in a wide practical range, the difference between the input impedance value of the electromagnetic wave absorption sheet and the impedance value in the air disappears, and it is possible to limit the decrease in the electromagnetic wave absorption characteristics due to reflection and scattering of the electromagnetic wave incident on the electromagnetic wave absorption sheet.

[0146] Also, from the reverse perspective, by maintaining the value of the input impedance of the electromagnetic wave absorption sheet close to 377 Ω, which is the impedance value in the air, within the range of its elastic region, in the practical range of the electromagnetic wave absorption sheet, a term of impedance matching of a certain level or more can be obtained, and a reduction in the electromagnetic wave absorption amount of the electromagnetic wave absorption sheet can be avoided. According to the study by the inventors, the numerical range is 360 Ω to 450 Ω. When the value of the input impedance of the electromagnetic wave absorption sheet is less than 360 Ω or greater than 450 Ω, at the interface between the space that is the surface of the electromagnetic wave absorption sheet and the electromagnetic wave absorption sheet, the electromagnetic wave incident on the electromagnetic wave absorption sheet is greatly scattered and reflected, so that the electromagnetic wave absorption characteristics inherent in the electromagnetic wave absorption sheet itself cannot be exhibited.

[0147] Note that when matching the input impedance value of the electromagnetic wave absorption sheet with the impedance in the air, by using as a criterion a state in which the electromagnetic wave absorption sheet is stretched by 5 to 75% with respect to its maximum elongation rate instead of a state of 0% elongation amount in which the electromagnetic wave absorption sheet is not stretched, the fact that the electromagnetic wave can be well absorbed even when the elongation amount of the electromagnetic wave absorption sheet changes is not limited to the reflective electromagnetic wave absorption sheet described as the second embodiment, but is the same in the transmissive electromagnetic wave absorption sheet shown as the first embodiment.

[0148] In the transmissive electromagnetic wave absorption sheet shown in the first embodiment, unlike the reflective electromagnetic wave absorption sheet shown in the second embodiment, the intensity of the electromagnetic wave scattered and reflected on the side where the electromagnetic wave is incident does not directly affect the level of the electromagnetic wave absorption characteristics (electromagnetic wave attenuation amount). However, for example, when suppressing the leakage of electromagnetic waves from an electric circuit serving as an electromagnetic wave radiation source to the outside and preventing the reflected wave on the surface of the electromagnetic wave absorption sheet from adversely affecting the electric circuit, there may be a case where it is desired to reduce the reflected wave on the electromagnetic wave incident side of the transmissive electromagnetic wave absorption sheet. In such a case, even for a transmissive electromagnetic wave absorption sheet, it is preferable to make its input impedance approach the impedance in air even when the elongation rate changes. Based on the state where the sheet is stretched by 5 to 75% with respect to the maximum elongation rate of the electromagnetic wave absorption sheet, it is preferable to set its input impedance to 377 Ω.

[0149] Also, as shown in FIG. 8, in the fourth electromagnetic wave absorption 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 does not change at 75.5 gigahertz. This is the same as the first electromagnetic wave absorption sheet described in the first embodiment. In the electromagnetic wave absorption sheet disclosed in the present application, since the incident electromagnetic wave is absorbed by the magnetic resonance of the magnetic iron oxide which is the electromagnetic wave absorption material, the frequency of the electromagnetic wave showing the largest absorption characteristics is not affected by the thickness of the electromagnetic wave absorption layer as long as the electromagnetic wave absorption material is the same, which is an indication of this fact.

[0150] On the other hand, an electromagnetic wave absorption sheet that is currently commercially available and can absorb electromagnetic waves in the millimeter-wave band and has elasticity is a so-called wavelength interference type electromagnetic wave absorption sheet formed by laminating a dielectric layer and a reflection layer, which attenuates the intensity of the reflected wave by shifting the phase of the incident electromagnetic wave by half a wavelength. In a wavelength interference type electromagnetic wave absorption sheet, when the thickness of the dielectric layer changes, the wavelength of the absorbed electromagnetic wave changes. Therefore, when the electromagnetic wave absorption sheet has elasticity and its thickness changes when it stretches at a predetermined elongation rate, the value of the peak frequency of the absorbed electromagnetic wave also changes. As a result, there is a risk that a problem may occur in that an electromagnetic wave absorption sheet arranged to absorb electromagnetic waves of a specific wavelength, such as an in-vehicle radar, has elasticity but instead deteriorates the absorption characteristics of electromagnetic waves of a desired frequency.

[0151] In the case of the electromagnetic wave absorption sheet disclosed in the present application, since the frequency of the electromagnetic wave that is maximally absorbed does not change, such a problem does not occur.

[0152] (Other configurations) In the above first and second embodiments, as the electromagnetic wave absorbing material contained in the electromagnetic wave absorbing layer, magnetite iron oxide mainly using epsilon iron oxide has been exemplified and described. As described above, by using epsilon iron oxide, an electromagnetic wave absorption sheet that absorbs electromagnetic waves in the millimeter-wave band of 30 gigahertz to 300 gigahertz can be formed. Further, by using rhodium or the like as the metal material for substituting the Fe site, an electromagnetic wave absorption sheet that absorbs electromagnetic waves of 1 terahertz, which is the highest frequency defined as an electromagnetic wave, can be realized.

[0153] However, in the electromagnetic wave absorption sheet disclosed in the present application, the magnetite iron oxide used as the electromagnetic wave absorbing material in 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 shown as some examples exhibit good electromagnetic wave absorption characteristics for electromagnetic waves in the frequency band of several gigahertz to several tens of gigahertz. Therefore, in addition to epsilon iron oxide, by forming an electromagnetic wave absorption layer using magnetic iron oxide particles having electromagnetic wave absorption characteristics in the millimeter wave band of 30 gigahertz to 300 gigahertz and a rubber binder, an electromagnetic wave absorption sheet having elasticity that absorbs electromagnetic waves in the millimeter wave band can be realized.

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

[0156] The electromagnetic wave absorption sheet described in the above embodiment can realize an electromagnetic wave absorption sheet having elasticity by using a rubber binder as the binder constituting the electromagnetic wave absorption layer. In particular, as an electromagnetic wave absorption material, by providing magnetic iron oxide that resonates magnetically in a high frequency band of millimeter wave band or higher, an electromagnetic wave absorption sheet that absorbs high frequency electromagnetic waves and has elasticity can be realized.

[0157] In the case of an electromagnetic wave absorption sheet using magnetic iron oxide that absorbs electromagnetic waves by magnetic resonance as the electromagnetic wave absorption material, a greater electromagnetic wave absorption effect can be achieved by increasing the volume fraction of the electromagnetic wave absorption material in the electromagnetic wave absorption sheet. However, on the other hand, in an electromagnetic wave absorption sheet provided with an electromagnetic wave absorption layer composed of a rubber binder and an electromagnetic wave absorption material, an upper limit of the volume fraction of the electromagnetic wave absorption material is inevitably determined in order to ensure the elasticity due to the use of the binder. In the electromagnetic wave absorption sheet disclosed in the present application, by setting the volume fraction of magnetic iron oxide, which is the electromagnetic wave absorption material, in the electromagnetic wave absorption layer to 30% or more, a reflection attenuation amount of -15 dB or more can be ensured, particularly in the case of a reflection-type electromagnetic wave absorption sheet.

[0158] Also, it is preferable that the volume fraction of the rubber binder in the electromagnetic wave absorption layer be 40% to 70%. By setting the volume fraction of the rubber binder within this range, it becomes easier to set the maximum elongation rate of the elastic region in one direction in the plane of the electromagnetic wave absorption sheet within the desired range of 20% to 200%.

[0159] In the above description, as a method of forming the electromagnetic wave absorption layer, a method of preparing a magnetic paint, applying it, and drying it was described. As a method of manufacturing the electromagnetic wave absorption sheet disclosed in the present application, in addition to the method of applying the above magnetic paint, for example, an extrusion molding method can be considered.

[0160] More specifically, magnetic iron oxide powder, a rubber binder, and, if necessary, a dispersant or the like are blended in advance, and the blended materials are supplied into a plastic cylinder from the resin supply port of an extrusion molding machine. As the extrusion molding machine, a normal extrusion molding machine equipped with a plastic cylinder, a die provided at the tip of the plastic cylinder, a screw rotatably disposed in the plastic cylinder, and a drive mechanism for driving the screw can be used. The molten material plasticized by the band heater of the extrusion molding machine is sent forward by the rotation of the screw and extruded in a sheet shape from the tip. By performing drying, pressure molding, calendering, etc. on the extruded material, an electromagnetic wave absorption layer with a predetermined thickness can be obtained.

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

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

[0163] As an example, the magnetic compound obtained in this way as an electromagnetic wave absorbing composition is crosslinked and formed into a sheet at a temperature of 170°C using a hydraulic press. Then, in a constant temperature bath, for example, a secondary crosslinking treatment is performed at a temperature of 170°C to obtain an electromagnetic wave absorption sheet having a predetermined shape.

[0164] In addition, in the above embodiment, an electromagnetic wave absorption sheet in which the electromagnetic wave absorption layer is composed of a single layer has been described, but a structure in which a plurality of layers are laminated as the electromagnetic wave absorption layer can be adopted. In the case of the transmission type electromagnetic wave absorption sheet shown as the first embodiment, the electromagnetic wave absorption characteristics are improved when the electromagnetic wave absorption layer has a thickness of a certain degree or more. Also, in the reflection type electromagnetic wave absorption sheet shown as the second embodiment, the electromagnetic wave absorption characteristics can be further improved by adjusting the thickness of the electromagnetic wave absorption layer to match its input impedance value with the impedance value in air. Therefore, when a predetermined thickness of the electromagnetic wave absorption layer cannot be formed with a single layer depending on the characteristics of the electromagnetic wave absorption material and the binder for forming the electromagnetic wave absorption layer, it is effective to form the electromagnetic wave absorption layer as a laminate.

Industrial Applicability

[0165] The electromagnetic wave absorption sheet disclosed in the present application absorbs electromagnetic waves in a high frequency band of millimeter wave band or higher, and is further useful as an electromagnetic wave absorption sheet having elasticity.

Explanation of Symbols

[0166] 1 Electromagnetic wave absorption layer 1a Iron oxide epsilon (magnetic iron oxide) 1b Rubber binder 2 Adhesive layer 3 Reflective layer

Claims

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

2. The electromagnetic wave absorption sheet according to Claim 1, wherein the volume fraction of the magnetic iron oxide in the electromagnetic wave absorption layer is 30% or more.

3. The electromagnetic wave absorption sheet according to Claim 1 or 2, wherein the magnetic iron oxide is at least one selected from epsilon iron oxide, barium ferrite magnetic powder, or strontium ferrite.

4. The electromagnetic wave absorption sheet according to any one of Claims 1 to 3, wherein the input impedance value in a state where the electromagnetic wave absorption layer is stretched at any elongation rate in the range of 5 to 75% with respect to the maximum elongation rate in the elastic range matches the impedance value in air.

5. The electromagnetic wave absorption sheet according to any one of Claims 1 to 4, wherein a reflection layer that reflects the electromagnetic wave transmitted through the electromagnetic wave absorption layer is formed in contact with one surface of the electromagnetic wave absorption layer.

Images