Electromagnetic wave absorbing sheet and method for producing same

Abstract

This electromagnetic wave absorbing sheet is provided with a substrate and a magnetic layer, the magnetic layer containing a binder and a magnetic material, the magnetic material containing flat carbonyl iron, and the half-value width of the peak of the (200) plane of alpha iron detected in powder X-ray diffraction of the magnetic layer being 0.15 or more but less than 0.40. Furthermore, this method for producing an electromagnetic wave absorbing sheet is provided with a step for reducing carbonyl iron, and a step for flattening the reduced carbonyl iron that has been reduced. A step of mixing the reduced carbonyl iron, which has been flattened, with a binder and a solvent to produce a coating material for forming a magnetic layer; and a step of applying the coating material for forming a magnetic layer to a base material and drying the base material.

Description

TECHNICAL FIELD

[0001] The present application relates to an electromagnetic wave absorbing sheet that absorbs electromagnetic waves from the micrometer wave band to the millimeter wave band in a near field. BACKGROUND

[0002] With the development of wireless communication technology represented by mobile phones, various devices and sensors are being connected to the network wirelessly. In addition, in the medical field, from the viewpoint of preventing infection, the wireless of devices is also progressing, and medical devices are beginning to be wirelessly connected. These communications require high-speed and large capacity at a short distance, and the application frequency is high. With the increase in devices that apply such high frequencies, the risk of malfunctions caused by electromagnetic wave noise generated from the devices, interference with applied electromagnetic waves, and the like is increasing in electronic devices and communications. Furthermore, in recent years, millimeter wave radars for the purpose of preventing collision accidents of automobiles are also being mounted. Malfunctions in these medical and automotive devices can affect human life, so there should be no misoperation. Therefore, the necessity of applying electromagnetic wave absorbing sheets as countermeasures against malfunctions caused by electromagnetic wave noise of devices and interference resulting therefrom, so-called EMC (Electromagnetic Compatibility) measures, to circuit elements and transmission paths that transmit electromagnetic waves from the micrometer wave band to the millimeter wave band is increasing.

[0003] In addition, with the recent high-speed and large-capacity of mobile communication systems, the expansion of the fifth generation mobile communication system (5G) and local 5G is being promoted. The frequency band in 5G uses the Sub6 frequency band (3.7 GHz, 4.5 GHz band) and the millimeter wave band of 28 GHz. In correspondence with the trend of technology that utilizes such higher frequency electromagnetic waves, in electromagnetic wave absorbers that absorb unnecessary electromagnetic waves, electromagnetic wave absorbing sheets formed in a sheet shape, the requirement to suppress electromagnetic wave noise in higher frequency bands is also increasing.

[0004] In such a situation, in Patent Literature 1, as a near-field noise suppression sheet, a near-field noise suppression sheet in which the half-value width of the peak of the bcc (200) plane of Fe detected in powder X-ray diffraction is 0.4 or more, and the imaginary part permeability μ" at 10 GHz is 2.0 or more is proposed.

[0005] PRIOR ART DOCUMENTS

[0006] PATENT LITERATURE

[0007] Patent Literature 1: Japanese Patent Application Publication No. 2022-150016 (Japanese Patent No. 7209761) SUMMARY

[0008] PROBLEMS TO BE SOLVED BY THE INVENTION

[0009] However, the evaluation disclosed in Patent Literature 1 is an evaluation result of the imaginary part of permeability μ" at 10 GHz, and does not confirm the high noise suppression effect in the 28 GHz band of the millimeter wave band used in 5G. In addition, the half-value width of the peak of the bcc (200) plane of Fe detected in powder X-ray diffraction varies depending on whether or not the carbonyl iron is subjected to reduction treatment and the degree of flattening processing. The size of the crystallite of the non-reduced carbonyl iron is smaller than that of the reduced carbonyl iron, and the flattening processability is poor, so it is not possible to make the magnetic crystalline anisotropy large, and even if the half-value width of the peak of the bcc (200) plane of Fe is 0.4 or more, it is not possible to improve the noise suppression effect in the 28 GHz band of the millimeter wave band.

[0010] The present application solves the above problems and provides an electromagnetic wave absorbing sheet which can be attached to the concave-convex surface or curved surface of an electronic device in which the influence of electromagnetic wave noise is desired to be prevented, and which has electromagnetic wave absorbing performance in the micron wave band to the millimeter wave band in the near field.

[0011] Method for solving the problem

[0012] The electromagnetic wave absorbing sheet of the present application includes a substrate and a magnetic layer, characterized in that the magnetic layer includes a magnetic material and a binder, the magnetic material includes flat carbonyl iron, and the half-value width of the peak of the (200) plane of alpha iron detected in powder X-ray diffraction of the magnetic layer is 0.15 or more and less than 0.40.

[0013] The manufacturing method of the electromagnetic wave absorbing sheet of the present application is a method of manufacturing the above-described electromagnetic wave absorbing sheet of the present application, characterized by including a step of reducing carbonyl iron, a step of flattening the reduced carbonyl iron subjected to reduction treatment, a step of mixing the reduced carbonyl iron subjected to flattening treatment with a binder and a solvent to produce a magnetic layer-forming paint, and a step of applying the magnetic layer-forming paint to a substrate and drying it.

[0014] Effects of the Invention

[0015] According to the present application, it is possible to provide an electromagnetic wave absorbing sheet which can be attached to the concave-convex surface or curved surface of an electronic device in which the influence of electromagnetic wave noise is desired to be prevented, and which has electromagnetic wave absorbing performance (noise suppression effect) in the micron wave band to the millimeter wave band in the near field. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 is a schematic cross-sectional view showing an example of the electromagnetic wave absorbing sheet of the embodiment.

[0017] Figure 2 is a schematic cross-sectional view showing another example of the electromagnetic wave absorbing sheet of the embodiment. DETAILED DESCRIPTION

[0018] (Electromagnetic wave absorbing sheet)

[0019] An embodiment of the electromagnetic wave absorbing sheet of the present application will be described. The electromagnetic wave absorbing sheet of the present embodiment is characterized by comprising a base material and a magnetic layer, the magnetic layer containing a magnetic material and a binder, the magnetic material containing flat carbonyl iron, and a half-value width of a peak of a (200) plane of alpha iron detected in powder X-ray diffraction of the magnetic layer being 0.15 or more and less than 0.40.

[0020] In the electromagnetic wave absorbing sheet of the present application, the magnetic layer contains flat carbonyl iron as a magnetic material, and a half-value width of a peak of a (200) plane of alpha iron detected in powder X-ray diffraction of the magnetic layer is 0.15 or more and less than 0.40, and thus has high electromagnetic wave absorbing performance in the micro wave band to the millimeter wave band, and in particular, can exert a high noise suppression effect in the 28 GHz band of the millimeter wave band used in 5G. More specifically, it is possible to provide an electromagnetic wave absorbing sheet having a high transmission attenuation rate of 10 dB or more in the 28 GHz band of the millimeter wave band.

[0021] Note that, in the present application, the frequency band of 28.2 to 29.1 GHz that can be used in local 5G is also defined as the "millimeter wave band".

[0022] Hereinafter, the electromagnetic wave absorbing sheet of the present embodiment will be described based on the drawings. Figure 1 is a schematic cross-sectional view showing an example of the electromagnetic wave absorbing sheet of the present embodiment. In Figure 1 , the electromagnetic wave absorbing sheet 10 comprises a base material 11 and a magnetic layer 12 disposed on the base material 11. In Figure 1 , the electromagnetic wave absorbing sheet 10 is a two-layer structure composed of the base material 11 and the magnetic layer 12, but can be further provided with an adhesive layer on the magnetic layer 12 side to be a three-layer structure.

[0023] In addition, Figure 2 is a schematic cross-sectional view showing another example of the electromagnetic wave absorbing sheet of the present embodiment. In Figure 2 , the electromagnetic wave absorbing sheet 10' comprises a base material 11, a metal layer 13 disposed on the base material 11, and a magnetic layer 12 disposed on the metal layer 13. In Figure 2 , the electromagnetic wave absorbing sheet 10' is a three-layer structure composed of the base material 11, the magnetic layer 12, and the metal layer 13, but can be further provided with an adhesive layer on the magnetic layer 12 side to be a four-layer structure.

[0024] The thickness of the electromagnetic wave-absorbing sheet of the present embodiment is not particularly limited and can be set to 20 to 1000 μm, and more preferably 20 to 100 μm. This is because, if the thickness of the electromagnetic wave-absorbing sheet as a whole is too thin, the thickness of the magnetic layer also becomes thin, the electromagnetic wave-absorbing property is reduced, and in addition, the strength of the sheet as a whole is also reduced. On the other hand, if the thickness of the electromagnetic wave-absorbing sheet as a whole is too thick, the softness is reduced, and it is difficult to attach to the concave-convex surface or curved surface of an electronic device and use.

[0025] Next, each component of the electromagnetic wave-absorbing sheet of the present embodiment will be described.

[0026] <Substrate>

[0027] The substrate used in the electromagnetic wave-absorbing sheet of the present embodiment becomes a base of the magnetic layer.

[0028] As the above substrate, as long as it has softness and can ensure adhesion to the magnetic layer, a resin film is generally used. As the resin constituting the above substrate, for example, a substrate composed of a polyolefin-based resin (polyethylene, polypropylene, etc.), a polyester-based resin (polyethylene terephthalate PET, polyethylene naphthalate PEN, polybutylene terephthalate PBT, polybutylene naphthalate PBN, etc.), a polyimide-based resin, a polyamide-based resin, an ethylene-vinyl acetate copolymer, an ionomer-based resin, an ethylene-(meth)acrylic acid copolymer, an ethylene-(meth)acrylate copolymer, an ethylene-butene copolymer, an ethylene-hexene copolymer, a polyurethane-based resin, a polyether ketone-based resin, a polyether-based resin, a polyether sulfone-based resin, a polystyrene-based resin (polystyrene, etc.), a polyvinyl chloride-based resin, a polyvinylidene chloride-based resin, a polyvinyl alcohol-based resin, a polyvinyl acetate-based resin, a vinyl chloride-vinyl acetate copolymer, a polycarbonate-based resin, a fluorine-based resin, a silicone-based resin, a cellulose-based resin, and a crosslinked body of these resin materials can be listed. Among them, polyethylene terephthalate (PET) is more preferable in terms of mechanical properties and price. One or two or more of these resin materials can be used. In addition, the above resin material can have a functional group as needed. In addition, a functional monomer or a modifying monomer can be grafted to the resin material.

[0029] In order to improve adhesion to the adjacent magnetic layer, a known surface treatment can be performed on the surface of the above substrate. As the above surface treatment, specifically, for example, corona discharge treatment, ozone exposure treatment, high-voltage electric shock exposure treatment, ionizing radiation treatment, and the like can be listed. In addition, the above substrate can be subjected to coating treatment based on a primer (silicone treatment, etc.), primer treatment, mat treatment, crosslinking treatment, and the like.

[0030] The substrate can be a single layer or a laminate of two or more layers. In the substrate, a known additive such as a filler, a flame retardant, a deterioration preventive, an antistatic agent, a softening agent, a plasticizer, or the like can be added as needed.

[0031] The thickness of the substrate is not particularly limited, and is preferably 5 to 20 μm, and more preferably 10 to 15 μm. If the thickness of the substrate is within the above range, the strength and the flexibility of the electromagnetic wave absorbing sheet of the present embodiment can be balanced.

[0032] <the magnetic layer>

[0033] The magnetic layer used in the electromagnetic wave absorbing sheet of the present embodiment contains a magnetic material and a binder. The thickness of the magnetic layer is not particularly limited, and is generally set to a range of 5 to 70 μm, because the electromagnetic wave absorbing property is reduced if the thickness is too thin, and the flexibility is reduced if the thickness is too thick. In addition, the surface resistance value of the magnetic layer is preferably 10 5 Ω / sq or more. This is because if the surface resistance value of the magnetic layer is too low, a short circuit can occur when the electromagnetic wave absorbing sheet is attached to a concave-convex surface or a curved surface of an electronic device. The surface resistance value of the magnetic layer is measured using a resistance meter "Hiresta-MCP-HT 450" manufactured by Mitsubishi Analytic Corporation.

[0034] Next, the constituent materials of the magnetic layer are described.

[0035] [binder]

[0036] The binder is not particularly limited, and a resin-based binder can be widely used, but in order to improve the flexibility of the electromagnetic wave absorbing sheet, a polyester resin, an acrylic resin, a polyurethane resin, or the like is preferably used. In particular, a water-soluble polyester resin having high water dispersibility and excellent affinity with carbonyl iron is most preferable.

[0037] [magnetic material]

[0038] As the magnetic material, a flat carbonyl iron is used, and in the case where the magnetic layer is constituted together with the binder, a magnetic material having a half-value width of a peak of an (200) plane of alpha iron detected in powder X-ray diffraction of the magnetic layer of 0.15 or more and less than 0.40 is used. Thereby, an electromagnetic wave absorbing sheet having a high transmission attenuation rate of 10 dB or more in a 28 GHz band of a millimeter wave band can be provided.

[0039] The magnetic material is a soft magnetic material, and has a high initial permeability, and can exhibit electromagnetic wave absorbing properties even if a small amount is contained in the magnetic layer, and thus can exhibit an electromagnetic wave noise suppression effect even if the magnetic layer is made into a thin film.

[0040] Generally, flat magnetic powder has a higher magnetic anisotropy than spherical magnetic powder, and thus, a high noise suppression effect at a high frequency is easily obtained. In addition, when an electromagnetic wave at a high frequency such as a millimeter wave is incident on a soft magnetic material, an alternating current flows in the soft magnetic material. When the soft magnetic material is made flat and the thickness thereof is made to be equal to or less than the skin depth of the alternating current, the value of the imaginary part of the permeability of the soft magnetic material becomes large, and thus, a noise suppression effect is easily obtained.

[0041] The above magnetic material can be obtained by thermally decomposing iron pentacarbonyl represented by Fe(CO)5 to obtain a raw material powder, sequentially performing a grinding (secondary particle deagglomeration) process, a classification (adjustment of particle size distribution) process, and a reduction treatment process on the raw material powder, thereby producing reduced carbonyl iron, and then, performing a flattening process on the reduced carbonyl iron.

[0042] Generally, the size of the crystallite of reduced carbonyl iron becomes larger than that of non-reduced carbonyl iron, and thus, the reduced carbonyl iron is easily flattened. Therefore, when the reduced carbonyl iron is subjected to a flattening process, the strain of the crystallinity of the carbonyl iron increases, and an increase in the magnetic anisotropy or the shape anisotropy can be expected, and thus, an increase in the permeability can be expected.

[0043] The index of the crystallinity of the above carbonyl iron can be represented by the half-value width of the peak of the (200) plane of α-iron detected in powder X-ray diffraction, and in the case where the half-value width is less than 0.15, the flattening of the carbonyl iron does not progress, and in the case where the half-value width is 0.40 or more, the flattening process is excessively performed, and thus, the flat carbonyl iron can be deteriorated. Therefore, as the above magnetic material, in the case where the magnetic layer is formed together with a binder, flat reduced carbonyl iron having a half-value width of the peak of the (200) plane of α-iron detected in powder X-ray diffraction of the magnetic layer of 0.15 or more and less than 0.40 is used. The half-value width of the peak of the (200) plane of α-iron detected in powder X-ray diffraction as the index of the crystallinity of the above carbonyl iron varies depending on the presence or absence of the reduction treatment of the carbonyl iron and the degree of the flattening process.

[0044] The half-value width of the peak of the (200) plane of α-iron detected in powder X-ray diffraction is measured as follows. As a measuring device, an X-ray diffractometer "Smart-Lab" manufactured by Rigaku Corporation is used. The face direction of the magnetic layer side of an electromagnetic wave absorption sheet is set as a measurement face, and measurement is performed using a concentrating optical system. The measurement range is 2θ = 50° to 80°, the scanning speed is 1° / minute or less, the sampling interval is 0.01° or less, and continuous measurement at a constant speed is performed. In the diffraction peak detected in the measurement at 2θ = 65°, the peak width at the time when the value of half of the maximum peak intensity value after the background is subtracted is set as the half-value width.

[0045] As the above magnetic material, a flat carbonyl iron powder is used. Here, regarding the flat carbonyl iron powder, in the present application, the ratio of the short side length of the flat carbonyl iron powder to the thickness of the flat carbonyl iron powder is defined as the aspect ratio (short side length / thickness), and in the case where the aspect ratio is set as the flatness, the flatness is defined as 5.0 or more. Here, the short side length of the flat carbonyl iron powder refers to the length of the short axis of an ellipse assumed when the carbonyl iron powder is visually observed from above. In addition, the thickness of the flat carbonyl iron powder refers to the length of an axis orthogonal to the long axis and the short axis of the above assumed ellipse from the intersection of the long axis and the short axis of the above assumed ellipse.

[0046] The value of the half-value width of the above flat carbonyl iron varies depending on the presence or absence of reduction treatment of the carbonyl iron and the degree of flatness, and thus the thickness of the flat carbonyl iron particle having the half-value width of 0.15 or more and less than 0.40 is not particularly limited, and generally, a carbonyl iron particle having a thickness of 0.1 μm or more and 10 μm or less is used.

[0047] In addition, the maximum diameter of the above flat carbonyl iron particle is also not particularly limited, and a carbonyl iron particle having a maximum diameter of 100 μm or less is preferable. Here, the "maximum diameter" refers to the maximum length in the length direction of the carbonyl iron particle in an observation image obtained by observing the cross section of the magnetic layer using a scanning electron microscope (SEM). By using a carbonyl iron particle having a maximum diameter of 100 μm or less, the increase in the surface area of a carbonyl iron particle having a low resistance can be suppressed, and thus the surface resistance value of the magnetic layer can be 10 5 Ω / sq or more.

[0048] The thickness and the maximum diameter of the above carbonyl iron particle are obtained as the arithmetic average of the thickness and the maximum diameter of 100 arbitrary carbonyl iron particles in an observation image by observing the cross section of the magnetic layer using a SEM.

[0049] The content ratio of the above magnetic material contained in the above magnetic layer is preferably 30 to 70% by volume, and more preferably 50 to 65% by volume. If the above content ratio is less than 30% by volume, there is a tendency that the electromagnetic wave absorbing performance (noise suppression effect) of the magnetic layer becomes insufficient, and if it exceeds 70% by volume, the proportion of the binder in the magnetic layer decreases, the adhesion of the magnetic layer to the substrate decreases, cracks, powdering, and the like are easily generated when the magnetic layer is bent, and in addition, there is a tendency that the surface resistance value of the magnetic layer decreases.

[0050] The specific gravity of the above flat carbonyl iron is not particularly limited, and is generally about 7.2.

[0051] <metal layer>

[0052] As Figure 2As shown, if a metal layer is provided on the electromagnetic wave absorbing sheet of the present embodiment, the electromagnetic wave absorbing sheet can be given an electric field shielding property, and not only magnetic noise but also electric noise can be suppressed. In Figure 2 The metal layer can be provided between the substrate and the magnetic layer, but can also be provided on the outer surface of the magnetic layer.

[0053] The type of metal constituting the metal layer is not particularly limited as long as it has flexibility and adhesion to the magnetic layer, and is preferably aluminum, copper, or the like. This is because aluminum, copper, or the like is inexpensive and can be easily processed into a thin film, and has excellent flexibility.

[0054] The thickness of the metal layer is not particularly limited, and is generally set to a range of 0.1 to 1 μm, because flexibility is reduced when the thickness is too thick.

[0055] The metal layer can be used alone as a metal foil, or can be used by forming a metal thin film on the substrate by a vapor deposition method or a sputtering method.

[0056] [Adhesive layer]

[0057] When the adhesive layer is provided on the electromagnetic wave absorbing sheet of the present embodiment, the thickness of the adhesive layer is preferably 10 to 50 μm, and more preferably 15 to 35 μm. When the thickness is less than 10 μm, it can be difficult to obtain sufficient adhesion. In addition, when the thickness exceeds 50 μm, the adhesion effect of the adhesive layer is saturated, and the total thickness of the electromagnetic wave absorbing sheet becomes large, so the flexibility of the electromagnetic wave absorbing sheet is reduced, the followability to electronic components when the electromagnetic wave absorbing sheet is attached to the electronic components is reduced, and it is difficult to wind the electromagnetic wave absorbing sheet around a wire or the like.

[0058] (Breaking characteristics of the electromagnetic wave absorbing sheet)

[0059] [Breaking elongation]

[0060] The electromagnetic wave absorbing sheet of the present application preferably has a breaking elongation of more than 75% according to JIS (Japanese Industrial Standards) K 7127. When the breaking elongation is more than 75%, the followability of the electromagnetic wave absorbing sheet to an adherend is improved in the case where the electromagnetic wave absorbing sheet of the present application is attached to a concave-convex surface or a curved surface of an electronic device. In particular, even when the electromagnetic wave absorbing sheet of the present application is used by being wound around a cable or a connector, the magnetic layer does not crack, and the magnetic layer does not peel from the substrate. The breaking elongation is more preferably 95% or more, and further preferably 100% or more. The upper limit of the breaking elongation is not particularly limited, and is generally 150% or less.

[0061] [Breaking stress]

[0062] The electromagnetic wave absorbing sheet of the present application preferably has a breaking stress of 30 MPa or more as defined in JIS K7127. If the breaking stress is 30 MPa or more, the electromagnetic wave absorbing sheet has improved followability to the adherend when the electromagnetic wave absorbing sheet of the present application is used by being attached to the uneven surface or curved surface of an electronic device. In particular, even when the electromagnetic wave absorbing sheet of the present application is used by being wound around a cable or a connector, the magnetic layer does not crack, and the magnetic layer does not peel off from the substrate. The breaking stress is more preferably 35 MPa or more. The upper limit of the breaking stress is not particularly limited, and is typically 70 MPa or less.

[0063] Further, the electromagnetic wave absorbing sheet of the present application most preferably has an elongation at break of more than 75% as defined in JIS K7127 and a breaking stress of 30 MPa or more as defined in JIS K7127. Thereby, particularly when the electromagnetic wave absorbing sheet of the present application is used by being wound around a cable or a connector, it is possible to further improve both the strength and the winding property (followability) of the electromagnetic wave absorbing sheet.

[0064] The breaking stress and the elongation at break of the electromagnetic wave absorbing sheet of the present application are measured by the method defined in JIS K7127. Specifically, the electromagnetic wave absorbing sheet produced is stretched at a stretching speed of 50 mm / minute using a tensile compression testing machine, and the test piece is broken. The elongation at break is calculated from (displacement at break / initial distance between marks) x 100 based on the elongation at the time of breaking of the test piece. In addition, the breaking stress is calculated from the breaking test force per unit area of the breaking surface at the time of breaking of the test piece.

[0065] (Method for producing electromagnetic wave absorbing sheet)

[0066] An embodiment of the method for producing the electromagnetic wave absorbing sheet of the present application will be described. The method for producing the electromagnetic wave absorbing sheet of the present application is a method for producing the electromagnetic wave absorbing sheet described above, and includes a step of reducing the carbonyl iron, a step of flat processing the reduced carbonyl iron after the reduction, a step of mixing the flat-processed reduced carbonyl iron with a binder and a solvent to produce a magnetic layer-forming paint, and a step of applying the magnetic layer-forming paint to a substrate and drying it.

[0067] <Carbonyl iron>

[0068] The carbonyl iron before the reduction can be obtained by reacting a raw material iron powder with carbon monoxide (CO gas) to synthesize pentacarbonyl iron represented by Fe(CO)5, and then performing heat decomposition to produce a raw material powder, and then sequentially performing a grinding (deagglomeration of secondary particles) step and a classification (adjustment of particle size distribution) step.

[0069] <Reduction>

[0070] Next, the obtained iron carbonyl is reduced to produce reduced iron carbonyl. This reduction process is typically performed using hydrogen reduction (hydrogen annealing).

[0071] <Flattening Processing>

[0072] Next, the reduced iron carbonyl is subjected to a flattening process to obtain flattened reduced iron carbonyl. The specific method for this flattening process is not particularly limited; for example, mechanical processing using a ball mill, bead mill, or similar equipment can be employed.

[0073] In the flattened reduced carbonyl iron produced in the above process, the half-width at half-maximum (WHM) of the α-iron (200) plane peak detected in powder X-ray diffraction is 0.15 or more and less than 0.40. On the other hand, in spherical reduced carbonyl iron, spherical non-reduced carbonyl iron, and flattened non-reduced carbonyl iron, it is difficult to achieve a WHM of 0.15 or more and less than 0.40 for the α-iron (200) plane peak detected in powder X-ray diffraction.

[0074] Coatings for forming magnetic layers

[0075] The aforementioned coating for forming a magnetic layer can be prepared by mixing reduced carbonyl iron that has undergone flattening processing with a binder and a solvent. In this case, if a water-soluble polyester resin with excellent coating properties is used as the binder, the coating properties of the aforementioned coating for forming a magnetic layer are improved, allowing the magnetic layer to be easily applied to the substrate in a conventional coating process.

[0076] As solvents, for example, water, ethanol, methanol, n-propanol, isopropanol, n-butanol, isobutanol, ethylene glycol, propylene glycol, etc. can be used.

[0077] The content of the solvent is not particularly limited, but it can be set to 50.0% by mass or more and 99.5% by mass or less relative to the total mass of the coating for forming the magnetic layer.

[0078] In the above-mentioned coating for forming magnetic layers, surface conditioners, defoamers, thickeners, etc. can be further added.

[0079] <Formation of the Magnetic Layer>

[0080] As a method for applying the coating for forming the magnetic layer onto the substrate, coating methods such as rod coating, reverse coating, gravure coating, microgravure coating, mold coating, dip coating, spin coating, slot coating, and spray coating can be used.

[0081] The drying after the coating described above can be performed under a condition in which the solvent component of the coating material for forming the magnetic layer described above is evaporated, and is preferably performed at 100 to 150°C for 5 to 60 minutes. If the solvent remains in the magnetic layer, there is a tendency that the strength is poor. As the drying method, for example, hot air drying, heating drying, vacuum drying, natural drying, or the like can be performed.

[0082] Example

[0083] Hereinafter, the present application will be described in detail using examples. However, the present application is not limited to the following examples. In the following description, "parts" means "mass parts" unless otherwise specified.

[0084] (Example 1)

[0085] <Preparation of Coating Material for Forming Magnetic Layer>

[0086] A coating material for forming a magnetic layer was prepared by mixing and dispersing the following components.

[0087] (1) Magnetic material (flat-shaped reduced carbonyl iron powder, manufactured by Tianyi Co., Ltd., trade name "RPZ"): 37.6 parts

[0088] (2) Binder (water-soluble polyester resin solution, manufactured by Kao Chemicals Co., Ltd., trade name "PLAS COAT Z-3310", solid content concentration: 25.0 mass%, solvent: water): 9.6 parts

[0089] (3) Binder (water-soluble polyester resin solution, manufactured by Kao Chemicals Co., Ltd., trade name "PLAS COAT Z-760", solid content concentration: 25.0 mass%, solvent: water): 9.6 parts

[0090] (4) Crosslinking agent (water-soluble polymer containing oxazoline group, manufactured by Nippon Shokubai Co., Ltd., trade name "EPOCROS WS-500"): 4.0 parts

[0091] (5) Thickening agent (water-based rheological additive, manufactured by BYK Co., Ltd., trade name "LAPONITE-RD"): 1.0 part

[0092] (6) Solvent (n-propanol): 10.0 parts

[0093] (7) Purified water: 28.2 parts

[0094] In the coating material for forming the magnetic layer described above, the content ratio of the magnetic material with respect to the total solid content of the coating material for forming the magnetic layer described above was 60 vol%.

[0095] <Formation of Magnetic Layer>

[0096] Next, using a PET film (Toyo Rika Co., Ltd., trade name "Ester Film E 5100") having a thickness of 12 μm as a substrate, the above-mentioned magnetic layer-forming paint was applied to one main surface of the substrate using a bar coater, and then dried at 120°C for 5 minutes, thereby producing an electromagnetic wave absorbing sheet of Example 1 in which a magnetic layer was formed on one main surface. The thickness of the above-mentioned magnetic layer was 64 μm.

[0097] (Example 2)

[0098] A wet-type ball mill was charged with a spherical reduced carbonyl iron powder (trade name "RZE" manufactured by Tainichi Co., Ltd.), a ball for pulverization (zirconia ball φ 5 mm), and a solvent (ethanol), and subjected to a 2-hour flattening treatment. The shape of the flattened treatment powder after drying was observed with an optical microscope, and flattened reduced carbonyl iron powder having a major axis of 10 μm or more was prepared.

[0099] The magnetic material was changed to the above-mentioned prepared flattened reduced carbonyl iron powder, the thickness of the magnetic layer was changed to 68 μm, and otherwise, the electromagnetic wave absorbing sheet of Example 2 was produced in the same manner as in Example 1.

[0100] (Example 3)

[0101] The content ratio of the magnetic material with respect to the total solid components of the magnetic layer-forming paint was changed to 40 vol%, the thickness of the magnetic layer was changed to 56 μm, and otherwise, the electromagnetic wave absorbing sheet of Example 3 was produced in the same manner as in Example 1.

[0102] (Example 4)

[0103] <Preparation of the magnetic layer-forming paint>

[0104] The following components were added to a reflux-type kneader, and mixed at a rotation speed of 35 rpm while being cooled with cold water in such a manner that the temperature did not rise, thereby producing a composite containing a magnetic material.

[0105] (1) Magnetic material (flattened reduced carbonyl iron powder manufactured by Tainichi Co., Ltd., trade name "RPZ"): 88.3 parts

[0106] (2) Binder (silicone rubber, manufactured by Shin-Etsu Chemical Co., Ltd., trade name "KE-951 KU"): 11.7 parts

[0107] Next, the produced composite containing a magnetic material was added to a planetary mixer together with toluene (solvent) and dissolved, thereby preparing a magnetic layer-forming paint. In the above-mentioned magnetic layer-forming paint, the content ratio of the magnetic material with respect to the total solid components of the above-mentioned magnetic layer-forming paint was 60 vol%.

[0108] <Formation of the magnetic layer>

[0109] Next, using a PET film (Toyo Rika Co., Ltd., trade name "Ester Film E 5100") having a thickness of 12 μm as a substrate, the above-mentioned magnetic layer-forming paint was applied to one main surface of the substrate using a doctor blade coater, and then electron beam crosslinking was performed, thereby producing an electromagnetic wave absorber sheet of Example 4 in which a magnetic layer was formed on one main surface. The thickness of the above-mentioned magnetic layer was 64 μm.

[0110] (Example 5)

[0111] <Preparation of magnetic layer-forming paint>

[0112] The following ingredients were added to a reflux kneader, and mixed at a rotation speed of 35 rpm while cooling with cold water so that the temperature did not increase, thereby producing a magnetic material-containing compound.

[0113] (1) Magnetic material (flat-shaped reduced carbonyl iron powder, Taiyo Co., Ltd., trade name "RPZ"): 80.0 parts

[0114] (2) Binder (acrylic rubber, ZEON Co., Ltd., trade name "AR-51"): 20.0 parts

[0115] Using the above-mentioned magnetic material-containing compound, the content ratio of the magnetic material with respect to the total solid components of the magnetic layer-forming paint was changed to 40 vol%, and the thickness of the magnetic layer was changed to 100 μm, and otherwise, an electromagnetic wave absorber sheet of Example 5 was produced in the same manner as Example 4.

[0116] (Comparative Example 1)

[0117] The magnetic material was changed to spherical reduced carbonyl iron powder (trade name "RZE") of Taiyo Co., Ltd., and the thickness of the magnetic layer was changed to 63 μm, and otherwise, an electromagnetic wave absorber sheet of Comparative Example 1 was produced in the same manner as Example 1.

[0118] (Comparative Example 2)

[0119] The magnetic material was changed to spherical non-reduced carbonyl iron powder (trade name "YW3") of Taiyo Co., Ltd., and the thickness of the magnetic layer was changed to 45 μm, and otherwise, an electromagnetic wave absorber sheet of Comparative Example 2 was produced in the same manner as Example 1.

[0120] (Comparative Example 3)

[0121] The magnetic material was changed to flat-shaped non-reduced carbonyl iron powder (trade name "YP") of Taiyo Co., Ltd., and the thickness of the magnetic layer was changed to 41 μm, and otherwise, an electromagnetic wave absorber sheet of Comparative Example 3 was produced in the same manner as Example 1.

[0122] (Comparative Example 4)

[0123] To a wet ball mill, flat-shaped reduced carbonyl iron powder (trade name "RPZ") manufactured by Tenryu Corporation, a ball for pulverization (zirconia ball φ 5 mm), and a solvent (ethanol) were added, and 10 hours of flattening treatment was performed. The shape of the flattened powder after drying was observed with an optical microscope, and flat-shaped reduced carbonyl iron powder having a major axis of 10 μm or more was prepared.

[0124] The magnetic material was changed to the prepared flat-shaped reduced carbonyl iron powder, and otherwise, the electromagnetic wave absorbing sheet of Comparative Example 4 was produced in the same manner as in Example 1.

[0125] For the produced electromagnetic wave absorbing sheets of Examples 1 to 5 and Comparative Examples 1 to 4, the half-value width of the peak of the (200) plane of α-iron detected in the powder X-ray diffraction of the magnetic layer, the surface resistance value of the magnetic layer, the transmission attenuation rate (Rtp) at 28 GHz, the elongation at break, and the breaking stress were measured.

[0126] <Half-value width>

[0127] As described above, the half-value width of the peak of the (200) plane of α-iron detected in the powder X-ray diffraction of the magnetic layer was measured using an X-ray diffractometer "Smart-Lab" manufactured by Rigaku Corporation.

[0128] <Surface resistance value>

[0129] As described above, the surface resistance value of the magnetic layer was measured using a resistance meter "Hiresta-MCP-HT 450" manufactured by Mitsubishi Analytic Corporation.

[0130] <Transmission attenuation rate>

[0131] The transmission attenuation rate (Rtp) at 28 GHz based on the microstrip line method was used to evaluate the magnetic noise suppression effect of the produced electromagnetic wave absorbing sheet. Specifically, the transmission attenuation rate at 28 GHz of the electromagnetic wave absorbing sheet was measured using a near-field noise suppression sheet (noise suppression sheet) evaluation system of Keycom Corporation in accordance with the measurement system (Rtp measurement system) of IEC No.: IEC 62333-1, IEC 62333-2.

[0132] <Elongation at break and breaking stress>

[0133] The breaking elongation and the breaking stress of the produced electromagnetic wave absorbing sheet were measured by the method prescribed in JIS K7127. Specifically, using a tensile compression testing machine "Technograph" manufactured by MinebeaMitsumi Co., Ltd., the produced electromagnetic wave absorbing sheet was stretched at a tensile speed of 50 mm / min to break the test piece. From the elongation at the time of breaking of the test piece, the breaking elongation was calculated by (breaking displacement / interline distance before stretching) x 100. In addition, from the breaking test force per unit area of the breaking surface at the time of breaking of the test piece, the breaking stress was calculated.

[0134] The above results are shown in Table 1 and Table 2 together with the kind of the magnetic material used in the magnetic layer of the produced electromagnetic wave absorbing sheet, the properties thereof, the thickness of the magnetic layer, the magnetic material content rate, and the kind of the adhesive.

[0135] [Table 1]

[0136]

[0137] [Table 2]

[0138]

[0139] According to Table 1, in the electromagnetic wave absorbing sheets of Examples 1 to 5, the half-value width of the peak of the (200) plane of a detected alpha iron in the powder X-ray diffraction of the magnetic layer was 0.15 or more and less than 0.40, the transmission attenuation rate (Rtp) was 10 dB or more at 28 GHz, and a high magnetic noise suppression effect was confirmed.

[0140] In addition, in the electromagnetic wave absorbing sheets of Examples 1 to 3 in which a water-soluble polyester was used as the adhesive, the elastic elongation could be made greater than 75%, and the elastic stress could be made 30 MPa or more. On the other hand, in the electromagnetic wave absorbing sheets of Example 4 in which silicone rubber was used as the adhesive and Example 5 in which acrylic rubber was used, the elastic elongation was 75% or less, and the elastic stress was less than 30 MPa. Therefore, as a result of winding the electromagnetic wave absorbing sheet around a core bar of 3 mm in diameter made of stainless steel according to JIS K5600-5-1, in the electromagnetic wave absorbing sheets of Examples 1 to 3, the winding was not problematic, but in the electromagnetic wave absorbing sheets of Examples 4 and 5, since the elastic elongation was small, a break occurred in the sheet, the breaking stress was small, and thus it was difficult to wind.

[0141] On the other hand, in Comparative Example 1, since the spherical reduced carbonyl iron which was not subjected to the flattening treatment was used, the half-value width was as low as 0.07, and the transmission attenuation rate was reduced to 5.7 dB. In addition, in Comparative Example 2, since the spherical non-reduced carbonyl iron was used, the half-value width was 1.48, which was 0.4 or more, and since the flattening treatment was not performed, the transmission attenuation rate was reduced to 3.4 dB. Further, in Comparative Example 3, since the flattened non-reduced carbonyl iron was used, the half-value width was 2.25, which was 0.4 or more, and since the flattening treatment of the non-reduced carbonyl iron was excessively performed, the transmission attenuation rate was reduced to 6.3 dB. Further, in Comparative Example 4, since the half-value width was 0.50, which was 0.4 or more, the transmission attenuation rate was reduced to 8.0 dB. Thus, it was found that even if the flattened reduced carbonyl iron, if the half-value width is outside the range of 0.15 to 0.40, the noise suppression effect is reduced.

[0142] With respect to the embodiments of the present application including the above Examples 1 to 5, the following supplementary modes are also disclosed.

[0143] Supplementary Mode 1: An electromagnetic wave absorbing sheet comprising a substrate and a magnetic layer, characterized by,

[0144] the magnetic layer comprises a magnetic material and a binder,

[0145] the magnetic material comprises a flattened carbonyl iron,

[0146] a half-value width of a peak of a (200) plane of alpha iron detected in powder X-ray diffraction of the magnetic layer is 0.15 or more and less than 0.40.

[0147] Supplementary Mode 2: The electromagnetic wave absorbing sheet according to Supplementary Mode 1, wherein an elongation at break prescribed in JIS K7127 is more than 75%.

[0148] Supplementary Mode 3: The electromagnetic wave absorbing sheet according to Supplementary Mode 1, wherein the elongation at break prescribed in JIS K7127 is 95% or more.

[0149] Supplementary Mode 4: The electromagnetic wave absorbing sheet according to any one of Supplementary Modes 1 to 3, wherein a breaking stress prescribed in JIS K7127 is 30 MPa or more.

[0150] Supplementary Mode 5: The electromagnetic wave absorbing sheet according to any one of Supplementary Modes 1 to 3, wherein the breaking stress prescribed in JIS K7127 is 35 MPa or more.

[0151] Supplementary Mode 6: The electromagnetic wave absorbing sheet according to any one of Supplementary Modes 1 to 5, wherein the flattened carbonyl iron is a reduced carbonyl iron.

[0152] Note 7: The electromagnetic wave-absorbing sheet according to any one of Notes 1 to 6, wherein the content ratio of the magnetic material contained in the magnetic layer is 30 to 70% by volume.

[0153] Note 8: The electromagnetic wave-absorbing sheet according to any one of Notes 1 to 6, wherein the content ratio of the magnetic material contained in the magnetic layer is 50 to 65% by volume.

[0154] Note 9: The electromagnetic wave-absorbing sheet according to any one of Notes 1 to 8, wherein the binder contains a water-soluble polyester resin.

[0155] Note 10: The electromagnetic wave-absorbing sheet according to any one of Notes 1 to 9, wherein the substrate is a resin film.

[0156] Note 11: The electromagnetic wave-absorbing sheet according to any one of Notes 1 to 10, wherein the surface resistance value of the magnetic layer is 10 5 Ω / sq or more.

[0157] Note 12: The electromagnetic wave-absorbing sheet according to any one of Notes 1 to 11, further comprising a metal layer.

[0158] Note 13: The electromagnetic wave-absorbing sheet according to any one of Notes 1 to 12, wherein the thickness of the whole is 20 to 1000 μm.

[0159] Note 14: A method for manufacturing an electromagnetic wave-absorbing sheet, which is a method for manufacturing the electromagnetic wave-absorbing sheet according to any one of Notes 1 to 13, characterized by comprising:

[0160] a step of reduction-treating carbonyl iron,

[0161] a step of flat-processed reduction-treating reduction carbonyl iron,

[0162] a step of mixing the flat-processed reduction carbonyl iron with a binder and a solvent to produce a magnetic layer-forming paint, and

[0163] a step of applying the magnetic layer-forming paint to a substrate and drying it.

[0164] Note 15: The method for manufacturing an electromagnetic wave-absorbing sheet according to Note 14, wherein the reduction treatment is hydrogen reduction treatment.

[0165] The present application can also be implemented in ways other than those described above. The embodiments disclosed in the present application are merely examples and are not limited thereto. The scope of the present application is to be interpreted in preference to the description in the specification, and all modifications within the scope of the appended claims are included in the claims.

[0166] Explanation of Reference Signs

[0167] 10, 10': electromagnetic wave absorbing sheet, 11: base material, 12: magnetic layer, 13: metal layer.

Claims

1. An electromagnetic wave absorbing sheet, comprising a substrate and a magnetic layer, characterized in that, The magnetic layer comprises magnetic material and adhesive. The magnetic material comprises flattened carbonyl iron. The half-width of the peak of the (200) plane of α iron detected in the powder X-ray diffraction of the magnetic layer is greater than 0.15 and less than 0.

40.

2. The electromagnetic wave absorbing sheet according to claim 1, wherein, The elongation at break specified in JIS K7127 is greater than 75%.

3. The electromagnetic wave absorbing sheet according to claim 1, wherein, The fracture stress specified in JIS K7127 is above 30 MPa.

4. The electromagnetic wave absorbing sheet according to claim 1, wherein, The flattened carbonyl iron is reduced carbonyl iron.

5. The electromagnetic wave absorbing sheet according to claim 1, wherein, The magnetic material contained in the magnetic layer accounts for 30 to 70% by volume.

6. The electromagnetic wave absorbing sheet according to claim 1, wherein, The adhesive comprises a water-soluble polyester resin.

7. The electromagnetic wave absorbing sheet according to claim 1, wherein, The substrate is a resin film.

8. The electromagnetic wave absorbing sheet according to claim 1, wherein, The surface resistivity of the magnetic layer is 10. 5 Ω / sq or higher.

9. The electromagnetic wave absorbing sheet according to claim 1, further comprising a metal layer.

10. The electromagnetic wave absorbing sheet according to claim 1, wherein, The overall thickness is 20~1000μm.

11. A method for manufacturing an electromagnetic wave absorbing sheet, comprising the method for manufacturing the electromagnetic wave absorbing sheet according to any one of claims 1 to 10, characterized in that, include: The process of reducing carbonyl iron, The process of flattening reduced carbonyl iron that has undergone reduction treatment. The process of mixing reduced carbonyl iron that has undergone flattening with binders and solvents to create a coating for forming a magnetic layer, and The process of applying the coating used to form the magnetic layer to a substrate and then drying it.

12. The method for manufacturing an electromagnetic wave absorbing sheet according to claim 11, wherein, The reduction process is a hydrogen reduction process.

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