Electromagnetic wave absorption sheet and method for manufacturing same

Abstract

This electromagnetic wave absorption sheet comprises a base material and a magnetic layer, wherein the magnetic layer contains a magnetic material and a binder, the magnetic material contains flat carbonyl iron, and the half-value width at the peak of the (200) plane of α iron detected in powder X-ray diffraction of the magnetic layer is 0.15 or more and less than 0.40. A method for manufacturing an electromagnetic wave absorbing sheet according to the present application comprises: a step for reducing carbonyl iron; a step for subjecting the reduced carbonyl iron subjected to reduction treatment to a flat processing; a step for mixing the flat processed reduced carbonyl iron, the binder, and a solvent to prepare a coating for forming the magnetic layer; and a step for applying the coating for forming a magnetic layer to a base and drying the coating.

Description

Electromagnetic wave absorbing sheet and its manufacturing method

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

[0002] With the advancement of wireless communication technologies, exemplified by mobile phones, various devices and sensors are now wirelessly connected to networks. Furthermore, in the medical field, cordless devices are becoming increasingly common to prevent infection, and medical devices are beginning to connect wirelessly. These communications require high speed and large capacity over relatively short distances, and therefore use high frequencies. With the increase in devices using such high frequencies, the risk of malfunctions in electronic devices and communications due to malfunctions caused by electromagnetic noise generated by the devices and interference with the electromagnetic waves being used is increasing. Furthermore, in recent years, millimeter-wave radar has begun to be installed in vehicles to prevent automobile collisions. Since malfunctions in these medical and automotive devices could affect human lives, malfunctions must be avoided. Therefore, there is a growing need to apply electromagnetic wave absorbing sheets to circuit elements and transmission lines that emit and receive electromagnetic waves in the microwave to millimeter-wave bands, as a measure to prevent malfunctions caused by electromagnetic noise and the resulting interference in devices, a so-called EMC (Electromagnetic Compatibility) measure.

[0003] Furthermore, in response to the recent increase in speed and capacity of mobile communication systems, the expansion of fifth-generation mobile communication systems (5G) and local 5G is underway. The frequency bands used for 5G are the Sub6 band (3.7 GHz, 4.5 GHz band) and the 28 GHz millimeter wave band. In response to this technological trend toward using higher frequency electromagnetic waves, there is a growing demand for electromagnetic wave absorbers and sheet-shaped electromagnetic wave absorbing sheets that absorb unwanted electromagnetic waves and suppress electromagnetic wave noise in higher frequency bands.

[0004] In this situation, Patent Document 1 proposes a near-field noise suppression sheet in which the half-width of the peak of the bcc(200) plane of Fe detected by powder X-ray diffraction is 0.4 or more and the imaginary part of the magnetic permeability μ″ at 10 GHz is 2.0 or more.

[0005] JP 2022-150016 A (Patent No. 7209761 A)

[0006] However, the evaluation disclosed in Patent Document 1 is the result of evaluating the imaginary part of the magnetic permeability μ″ at 10 GHz, and does not confirm a high noise suppression effect in the 28 GHz millimeter-wave band used in 5G. Furthermore, the half-width at the peak of the bcc (200) plane of Fe detected by powder X-ray diffraction varies depending on whether or not the carbonyl iron has been reduced and the degree of flattening. Unreduced carbonyl iron has smaller crystallite size and poorer flattening processability than reduced carbonyl iron, so it is not possible to increase the crystalline magnetic anisotropy. Therefore, even if the half-width at 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 millimeter-wave band.

[0007] The present application solves the above problem by providing an electromagnetic wave absorbing sheet that can be attached to uneven or curved surfaces of electronic devices that need to be protected from the effects of electromagnetic noise, and that has electromagnetic wave absorbing performance in the near-field from the microwave band to the millimeter wave band.

[0008] The electromagnetic wave absorbing sheet of the present application is an electromagnetic wave absorbing sheet comprising a substrate and a magnetic layer, wherein the magnetic layer comprises a magnetic material and a binder, the magnetic material comprises flat carbonyl iron, and the half-value width of the peak of the (200) plane of α-iron detected by powder X-ray diffraction of the magnetic layer is 0.15 or more and less than 0.40.

[0009] The method for producing an electromagnetic wave absorbing sheet of the present application is a method for producing an electromagnetic wave absorbing sheet of the present application, characterized by comprising the steps of reducing carbonyl iron, flattening the reduced carbonyl iron, mixing the flattened reduced carbonyl iron with a binder and a solvent to prepare a paint for forming a magnetic layer, and applying the paint for forming a magnetic layer to a substrate and drying it.

[0010] According to the present application, an electromagnetic wave absorbing sheet can be provided that can be attached to uneven or curved surfaces of electronic devices where the effects of electromagnetic noise should be prevented, and that has electromagnetic wave absorption performance (noise suppression effect) in the near-field from the microwave band to the millimeter wave band.

[0011] Fig. 1 is a schematic cross-sectional view showing an example of an electromagnetic wave absorbing sheet according to an embodiment, and Fig. 2 is a schematic cross-sectional view showing another example of an electromagnetic wave absorbing sheet according to an embodiment.

[0012] (Electromagnetic wave absorbing sheet) An embodiment of the electromagnetic wave absorbing sheet of the present application will be described. The electromagnetic wave absorbing sheet of this embodiment comprises a substrate and a magnetic layer, the magnetic layer containing a magnetic material and a binder, the magnetic material containing flat carbonyl iron, and the half width of the peak of the (200) plane of α-iron detected by powder X-ray diffraction of the magnetic layer is 0.15 or more and less than 0.40.

[0013] In the electromagnetic wave absorbing sheet of the present application, the magnetic layer contains flat carbonyl iron as the magnetic material, and the half-width at the peak of the α-iron (200) plane detected by powder X-ray diffraction of the magnetic layer is 0.15 or more and less than 0.40, so that the electromagnetic wave absorbing sheet has high electromagnetic wave absorption performance in the microwave to millimeter wave bands, and can exhibit high noise suppression effect in particular in the 28 GHz millimeter wave band used in 5G. More specifically, it can provide an electromagnetic wave absorbing sheet having a high transmission attenuation rate of 10 dB or more in the 28 GHz millimeter wave band.

[0014] In this application, the "millimeter wave band" is defined to include the 28.2 to 29.1 GHz band that can be used for local 5G.

[0015] The electromagnetic wave absorbing sheet of this embodiment will be described below with reference to the drawings. Fig. 1 is a schematic cross-sectional view showing an example of the electromagnetic wave absorbing sheet of this embodiment. In Fig. 1, the electromagnetic wave absorbing sheet 10 includes a substrate 11 and a magnetic layer 12 disposed on the substrate 11. In Fig. 1, the electromagnetic wave absorbing sheet 10 has a two-layer structure consisting of the substrate 11 and the magnetic layer 12, but it may also have a three-layer structure by further disposing an adhesive layer on the magnetic layer 12 side.

[0016] 2 is a schematic cross-sectional view showing another example of the electromagnetic wave absorbing sheet of this embodiment. In Fig. 2, the electromagnetic wave absorbing sheet 10' includes a substrate 11, a metal layer 13 disposed on the substrate 11, and a magnetic layer 12 disposed on the metal layer 13. In Fig. 2, the electromagnetic wave absorbing sheet 10' has a three-layer structure consisting of the substrate 11, the magnetic layer 12, and the metal layer 13, but it may also have a four-layer structure by further disposing an adhesive layer on the magnetic layer 12 side.

[0017] The overall thickness of the electromagnetic wave absorbing sheet of this embodiment is not particularly limited and can be 20 to 1000 μm, but 20 to 100 μm is more preferable. If the overall thickness of the electromagnetic wave absorbing sheet is too thin, the thickness of the magnetic layer will also be thin, reducing the electromagnetic wave absorption properties and the strength of the entire sheet. On the other hand, if the overall thickness of the electromagnetic wave absorbing sheet is too thick, flexibility will be reduced, making it difficult to attach the sheet to uneven or curved surfaces of electronic devices.

[0018] Next, each component of the electromagnetic wave absorbing sheet of this embodiment will be described.

[0019] <Substrate> The substrate used in the electromagnetic wave absorbing sheet of this embodiment is a base on which the magnetic layer is formed.

[0020] The substrate may be any flexible material capable of ensuring adhesion to the magnetic layer, and typically a resin film is used. Examples of resins constituting the substrate include polyolefin resins (polyethylene, polypropylene, etc.), polyester resins (polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polybutylene terephthalate (PBT), polybutylene naphthalate (PBN), etc.), polyimide resins, polyamide resins, ethylene-vinyl acetate copolymers, ionomer resins, ethylene-(meth)acrylic acid copolymers, ethylene-(meth)acrylic acid ester copolymers, ethylene-butene copolymers, ethylene-hexene copolymers, polyurethane resins, polyether ketone resins, polyether resins, polyethersulfone resins, polystyrene resins (polystyrene, etc.), polyvinyl chloride resins, polyvinylidene chloride resins, polyvinyl alcohol resins, polyvinyl acetate resins, vinyl chloride-vinyl acetate copolymers, polycarbonate resins, fluorine-based resins, silicone resins, cellulose resins, and substrates made of crosslinked materials of these resins. Among these, polyethylene terephthalate (PET) is more preferable in terms of mechanical properties and cost. These resin materials can be used alone or in combination. The resin material may have a functional group, if necessary. A functional monomer or a modifying monomer may be grafted onto the resin material.

[0021] The surface of the substrate may be subjected to a known surface treatment to improve adhesion to the adjacent magnetic layer. Specific examples of such surface treatments include corona discharge treatment, ozone exposure treatment, high-voltage shock exposure treatment, and ionizing radiation treatment. The substrate may also be subjected to a coating treatment with an undercoat agent (such as silicone treatment), a primer treatment, a matte treatment, a crosslinking treatment, or the like.

[0022] The substrate may be in the form of a single layer or a laminate of two or more layers. If necessary, known auxiliary agents such as fillers, flame retardants, antidegradants, antistatic agents, softeners, and plasticizers may be added to the substrate.

[0023] The thickness of the substrate is not particularly limited, but is preferably 5 to 20 μm, and more preferably 10 to 15 μm. When the thickness of the substrate is within the above range, the electromagnetic wave-absorbing sheet of the present embodiment can achieve both strength and flexibility.

[0024] <Magnetic Layer> The magnetic layer used in the electromagnetic wave absorbing sheet of this embodiment contains a magnetic material and a binder. The thickness of the magnetic layer is not particularly limited, but if it is too thin, the electromagnetic wave absorption property decreases, and if it is too thick, the flexibility decreases, so it is usually set in the range of 5 to 70 μm. In addition, the surface resistance value of the magnetic layer is 10 5 It is preferably Ω / square or higher. If the surface resistance of the magnetic layer is too low, there is a risk of short-circuiting when the electromagnetic wave absorbing sheet is attached to an uneven or curved surface of an electronic device. The surface resistance of the magnetic layer is measured using a resistance meter "Hiresta-MCP-HT450" manufactured by Mitsubishi Analytech Co., Ltd.

[0025] The materials constituting the magnetic layer will now be described.

[0026] [Binder] The binder is not particularly limited, and a wide variety of resin-based binders can be used, but in order to increase the flexibility of the electromagnetic wave absorbing sheet, it is preferable to use polyester resin, acrylic resin, polyurethane resin, etc. In particular, water-soluble polyester resin, which has high water dispersibility and excellent affinity with carbonyl iron, is most preferable.

[0027] [Magnetic Material] The magnetic material is flat carbonyl iron, which, when used in conjunction with the binder to form a magnetic layer, exhibits a half-width of 0.15 or more and less than 0.40 at the peak of the α-iron (200) plane detected by powder X-ray diffraction of the magnetic layer. This makes it possible to provide an electromagnetic wave absorbing sheet with a high transmission attenuation rate of 10 dB or more in the 28 GHz millimeter wave band.

[0028] The magnetic material is a soft magnetic material with high initial magnetic permeability, and can exhibit electromagnetic wave absorption performance even when contained in a small amount in the magnetic layer, so that electromagnetic wave noise suppression effects can be exhibited even when the magnetic layer is made thin.

[0029] In general, flat magnetic powder has higher magnetic anisotropy than spherical magnetic powder, making it easier to achieve high noise suppression effects at high frequencies. Furthermore, when high-frequency electromagnetic waves, such as those in the millimeter wave band, are incident on a soft magnetic material, an alternating current flows through the soft magnetic material. Flattening a soft magnetic material so that its thickness is equal to or less than the skin depth of the alternating current increases the imaginary part of the magnetic permeability of the soft magnetic material, making it easier to achieve noise suppression effects.

[0030] The magnetic material can be obtained by thermally decomposing pentacarbonyl iron (Fe(CO)5) to obtain raw material powder, which is then subjected to a milling process (deagglomeration of secondary particles), a classification process (adjustment of particle size distribution), and a reduction process in that order to produce reduced carbonyl iron, and then flattening the reduced carbonyl iron.

[0031] In general, reduced carbonyl iron has larger crystallite size than non-reduced carbonyl iron, and is easier to flatten than non-reduced carbonyl iron. Therefore, flattening reduced carbonyl iron increases the crystalline distortion of the carbonyl iron, which can be expected to increase the magnetic anisotropy or shape anisotropy, resulting in a further increase in magnetic permeability.

[0032] The crystallinity index of the carbonyl iron can be expressed by the half-width at the peak of the (200) plane of α-iron detected by powder X-ray diffraction. If the half-width is less than 0.15, the carbonyl iron is not flattened sufficiently. If the half-width is 0.40 or more, there is a concern that the flattening process may be excessive, resulting in deterioration of the flat carbonyl iron. Therefore, when the magnetic layer is formed together with a binder, flat reduced carbonyl iron is used, such that the half-width at the peak of the (200) plane of α-iron detected by powder X-ray diffraction of the magnetic layer is 0.15 or more but less than 0.40. The half-width at the peak of the (200) plane of α-iron detected by powder X-ray diffraction, which is an index of the crystallinity of the carbonyl iron, varies depending on whether or not the carbonyl iron is reduced and the degree of flattening.

[0033] The half-width of the peak of the (200) plane of α-iron detected by powder X-ray diffraction is measured as follows. The measuring device used is a "Smart-Lab" X-ray diffractometer manufactured by Rigaku Corporation. The measurement surface is the surface direction of the magnetic layer side of the electromagnetic wave absorbing sheet, and the measurement is performed using a focused optical system. The measurement range is 2θ = 50° to 80°, the scanning speed is 1° / min or less, and the sampling interval is 0.01° or less, and continuous measurement is performed at a constant speed. For the diffraction peak detected by measurement near 2θ = 65°, the half-width is the peak width at half the maximum peak intensity value after subtracting the background.

[0034] The magnetic material used is a flat carbonyl iron powder. Here, flat carbonyl iron powder is defined as a carbonyl iron powder having a flatness of 5.0 or more, where the aspect ratio (short side length / thickness) is the ratio of the short side length of the flat carbonyl iron powder to the thickness of the flat carbonyl iron powder. The short side length of the flat carbonyl iron powder refers to the length of the minor axis of an imaginary ellipse, assuming that the carbonyl iron powder is an ellipse when viewed from above. The thickness of the flat carbonyl iron powder refers to the length of the axis perpendicular to the imaginary flat ellipse, assuming that the carbonyl iron powder is an ellipse.

[0035] The half-value width of the flat carbonyl iron varies depending on whether or not the carbonyl iron has been subjected to a reduction treatment and the degree of flattening. Therefore, the thickness of the flat carbonyl iron particles having a half-value width of 0.15 or more and less than 0.40 is not particularly limited, but carbonyl iron particles having a thickness of 0.1 μm or more and 10 μm or less are usually used.

[0036] The maximum diameter of the flat carbonyl iron particles is not particularly limited, but carbonyl iron particles having a maximum diameter of 100 μm or less are preferred. Here, "maximum diameter" refers to the maximum length in the longitudinal direction of the carbonyl iron particles in an image of the cross section of the magnetic layer observed with a scanning electron microscope (SEM). By using carbonyl iron particles having a maximum diameter of 100 μm or less, it is possible to suppress an increase in the surface area of ​​carbonyl iron particles with low electrical resistance, and therefore it is possible to prevent the surface resistance of the magnetic layer from increasing beyond 10 5It can be Ω / square or more.

[0037] The thickness and maximum diameter of the carbonyl iron particles are determined by observing the cross section of the magnetic layer with an SEM and calculating the arithmetic mean values ​​of the thickness and maximum diameter of 100 carbonyl iron particles selected from the observed image.

[0038] The content of the magnetic material in the magnetic layer is preferably 30 to 70% by volume, more preferably 50 to 65% by volume. If the content is less than 30% by volume, the electromagnetic wave absorption performance (noise suppression effect) of the magnetic layer tends to be insufficient, while if it exceeds 70% by volume, the proportion of binder in the magnetic layer decreases, reducing the adhesion of the magnetic layer to the substrate, making the magnetic layer more susceptible to cracking and powder shedding when bent, and also tending to reduce the surface resistance of the magnetic layer.

[0039] The specific gravity of the flat carbonyl iron is not particularly limited, but is generally about 7.2.

[0040] <Metal Layer> As shown in Fig. 2, by disposing a metal layer on the electromagnetic wave absorbing sheet of this embodiment, it is possible to impart electric field shielding performance to the electromagnetic wave absorbing sheet and to suppress not only magnetic noise but also electrical noise. In Fig. 2, the metal layer is disposed between the substrate and the magnetic layer, but it may also be disposed on the outer surface of the magnetic layer.

[0041] The type of metal constituting the metal layer is not particularly limited as long as it has flexibility and adhesion to the magnetic layer, but aluminum, copper, etc. are preferred because they are inexpensive, can be easily processed into a thin film, and have excellent flexibility.

[0042] The thickness of the metal layer is not particularly limited, but if it is too thick, flexibility decreases, so it is usually set in the range of 0.1 to 1 μm.

[0043] The metal layer can be used alone as a metal foil, but can also be used by forming a metal thin film on the above-mentioned substrate by vapor deposition or sputtering.

[0044] <Adhesive Layer> When an adhesive layer is disposed on the electromagnetic wave absorbing sheet of this embodiment, the thickness of the adhesive layer is preferably 10 to 50 μm, more preferably 15 to 35 μm. If the thickness is less than 10 μm, sufficient adhesive strength may not be obtained. Furthermore, if the thickness exceeds 50 μm, the adhesive effect of the adhesive layer becomes saturated and the overall thickness of the electromagnetic wave absorbing sheet increases, resulting in a decrease in the flexibility of the electromagnetic wave absorbing sheet, a decrease in its ability to follow when attached to electronic components, and difficulty in wrapping it around wiring, etc.

[0045] (Breaking properties of electromagnetic wave absorbing sheet) [Breaking elongation] The electromagnetic wave absorbing sheet of the present application preferably has a breaking elongation defined in JIS (Japanese Industrial Standards) K7127 of greater than 75%. If the breaking elongation is greater than 75%, the electromagnetic wave absorbing sheet of the present application will have improved conformability to the adherend when attached to an uneven or curved surface of an electronic device. In particular, even when the electromagnetic wave absorbing sheet of the present application is wrapped around a cable or connector, cracks will not occur in the magnetic layer, and the magnetic layer will not peel off from the substrate. The breaking elongation is more preferably 95% or more, and even more preferably 100% or more. The upper limit of the breaking elongation is not particularly limited, but is usually 150% or less.

[0046] [Breaking Stress] The electromagnetic wave absorbing sheet of the present application preferably has a breaking stress as defined in JIS K7127 of 30 MPa or more. If the breaking stress is 30 MPa or more, when the electromagnetic wave absorbing sheet of the present application is attached to an uneven or curved surface of an electronic device, the electromagnetic wave absorbing sheet's ability to conform to the adherend is improved. In particular, even when the electromagnetic wave absorbing sheet of the present application is used by wrapping it around a cable or connector, cracks do not occur in the magnetic layer, and the magnetic layer does not peel off from the substrate. The breaking stress is more preferably 35 MPa or more. There is no particular upper limit for the breaking stress, but it is usually 70 MPa or less.

[0047] Furthermore, it is most preferable that the electromagnetic wave absorbing sheet of the present application has a breaking elongation of more than 75% as defined in JIS K 7127 and a breaking stress of 30 MPa or more as defined in JIS K 7127. This can further improve both the strength and wrapability (conformability) of the electromagnetic wave absorbing sheet, particularly when the electromagnetic wave absorbing sheet of the present application is used by wrapping it around a cable or a connector.

[0048] The breaking elongation and breaking stress of the electromagnetic wave absorbing sheet of the present application are measured by the method specified in JIS K7127. Specifically, the prepared electromagnetic wave absorbing sheet is pulled at a pulling rate of 50 mm / min using a tension-compression tester to break the test piece. From the elongation at which the test piece breaks, the breaking elongation is calculated by (breaking displacement / gauge line distance before pulling) x 100. In addition, the breaking stress is calculated from the breaking test force per unit area of ​​the fracture surface at which the test piece breaks.

[0049] (Method for manufacturing an electromagnetic wave absorbing sheet) An embodiment of the method for manufacturing an electromagnetic wave absorbing sheet of the present application will be described. The method for manufacturing an electromagnetic wave absorbing sheet of this embodiment is a method for manufacturing the electromagnetic wave absorbing sheet of the present application described above, and includes the steps of reducing carbonyl iron, flattening the reduced carbonyl iron, mixing the flattened reduced carbonyl iron, a binder, and a solvent to prepare a coating material for forming a magnetic layer, and applying the coating material for forming a magnetic layer to a substrate and drying it.

[0050] <Carbonyl iron> Carbonyl iron before reduction treatment can be obtained by reacting a raw material iron powder with carbon monoxide (CO gas) to synthesize pentacarbonyl iron represented by Fe(CO)5, which is then thermally decomposed to form raw material powder, which is then subjected to a milling process (deagglomeration of secondary particles) and a classification process (adjustment of particle size distribution) in that order.

[0051] <Reduction Treatment> Next, the carbonyl iron obtained above is reduced to form reduced carbonyl iron. The reduction treatment is usually carried out by hydrogen reduction treatment (hydrogen annealing).

[0052] <Flattening Treatment> The reduced carbonyl iron is then subjected to flattening treatment to obtain flat reduced carbonyl iron. The specific method for the flattening treatment is not particularly limited, but for example, mechanical processing using a ball mill, a bead mill, or the like can be applied.

[0053] In the flat reduced carbonyl iron produced by the above process, the half-width of the peak of the (200) plane of α-iron detected by powder X-ray diffraction can be set to 0.15 or more and less than 0.40. On the other hand, in the spherical reduced carbonyl iron, spherical non-reduced carbonyl iron, and flat non-reduced carbonyl iron, it is difficult to achieve a half-width of the peak of the (200) plane of α-iron detected by powder X-ray diffraction of 0.15 or more and less than 0.40.

[0054] <Magnetic Layer Coating> The magnetic layer coating can be prepared by mixing flattened reduced carbonyl iron, a binder, and a solvent. If a water-soluble polyester resin with excellent coating properties is used as the binder, the coating properties of the magnetic layer coating are improved, and the magnetic layer can be easily applied to the substrate using a normal coating process.

[0055] Examples of the solvent that can be used include water, ethyl alcohol, methyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, ethylene glycol, and propylene glycol.

[0056] The content of the solvent is not particularly limited, but may be 50.0% by mass or more and 99.5% by mass or less relative to the total mass of the coating material for forming the magnetic layer.

[0057] The coating material for forming the magnetic layer may further contain a surface conditioner, an antifoaming agent, a thickener, and the like.

[0058] <Formation of Magnetic Layer> Examples of a method for applying the magnetic layer-forming coating material onto a substrate include bar coating, reverse coating, gravure coating, microgravure (registered trademark) coating, die coating, dipping, spin coating, slit coating, and spray coating.

[0059] The drying after application may be carried out under conditions that allow the solvent component of the magnetic layer-forming coating material to evaporate, and is preferably carried out at 100 to 150°C for 5 to 60 minutes. If the solvent remains in the magnetic layer, the strength tends to decrease. Drying methods include, for example, hot air drying, heat drying, vacuum drying, and natural drying.

[0060] The present application will be described in detail below using examples. However, the present application is not limited to the following examples. Unless otherwise specified, "parts" in the following means "parts by mass."

[0061] Example 1 Preparation of Paint for Forming Magnetic Layer The following components were mixed and dispersed to prepare a paint for forming a magnetic layer. (1) Magnetic material (flat reduced carbonyl iron powder manufactured by Tenichisha, trade name "RPZ"): 37.6 parts (2) Binder (water-soluble polyester resin solution, manufactured by Goo Chemical Co., Ltd., trade name "Plus Coat Z-3310", solid content: 25.0% by mass, solvent: water): 9.6 parts (3) Binder (water-soluble polyester resin solution, manufactured by Goo Chemical Co., Ltd., trade name "Plus Coat Z-760", solid content: 25.0% by mass, solvent: water): 9.6 parts (4) Crosslinking agent (oxazoline group-containing water-soluble polymer, manufactured by Nippon Shokubai Co., Ltd., trade name "Epocross WS-500"): 4.0 parts (5) Thickener (water-based rheology additive, manufactured by BYK, trade name "LAPONITE-RD"): 1.0 parts (6) Solvent (n-propyl alcohol): 10.0 parts (7) Pure water: 28.2 parts

[0062] In the magnetic layer-forming paint, the content of the magnetic material was 60% by volume relative to the total solid content of the magnetic layer-forming paint.

[0063] <Formation of magnetic layer> Next, a 12 μm thick PET film (manufactured by Toyobo Co., Ltd., product name "Ester Film E5100") was used as a substrate, and the above-mentioned magnetic layer-forming paint was applied to one main surface of the substrate using a bar coater, followed by drying 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.

[0064] Example 2: Spherical reduced carbonyl iron powder (trade name "RZE") manufactured by Tenichi Co., Ltd., milling balls (zirconia balls, φ5 mm), and a solvent (ethanol) were placed in a wet ball mill and flattened for 2 hours. The shape of the flattened powder after drying was observed under an optical microscope, and powders with a major axis of 10 μm or more were prepared as flattened reduced carbonyl iron powders.

[0065] An electromagnetic wave absorbing sheet of Example 2 was produced in the same manner as Example 1, except that the magnetic material was changed to the flat reduced carbonyl iron powder prepared above and the thickness of the magnetic layer was changed to 68 μm.

[0066] (Example 3) The electromagnetic wave absorbing sheet of Example 3 was produced in the same manner as Example 1, except that the content of the magnetic material was changed to 40 volume % relative to the total solid content of the paint for forming the magnetic layer, and the thickness of the magnetic layer was changed to 56 μm.

[0067] (Example 4) <Preparation of paint for forming magnetic layer> The following components were added to a palindrome kneader and kneaded at a rotation speed of 35 rpm while cooling with cold water to prevent the temperature from rising, to prepare a magnetic material-containing compound: (1) Magnetic material (flat reduced carbonyl iron powder manufactured by Tenichi Co., Ltd., product name "RPZ"): 88.3 parts (2) Binder (silicone rubber manufactured by Shin-Etsu Chemical Co., Ltd., product name "KE-951KU"): 11.7 parts

[0068] Next, the magnetic material-containing compound thus prepared was dissolved in a planetary mixer together with toluene (solvent) to prepare a magnetic layer-forming paint. The magnetic material content in the magnetic layer-forming paint was 60% by volume relative to the total solid content of the magnetic layer-forming paint.

[0069] <Formation of magnetic layer> Next, a 12 μm thick PET film (manufactured by Toyobo Co., Ltd., product name "Ester Film E5100") was used as a substrate, and the above-mentioned magnetic layer-forming paint was applied to one main surface of the substrate using a comma coater, followed by electron beam crosslinking to produce an electromagnetic wave-absorbing sheet of Example 4 in which a magnetic layer was formed on one main surface. The thickness of the magnetic layer was 64 μm.

[0070] (Example 5) <Preparation of paint for forming magnetic layer> The following components were added to a palindrome kneader and kneaded at a rotation speed of 35 rpm while cooling with cold water to prevent the temperature from rising, to prepare a magnetic material-containing compound. (1) Magnetic material (flat reduced carbonyl iron powder manufactured by Tenichi Co., Ltd., product name "RPZ"): 80.0 parts (2) Binder (acrylic rubber, manufactured by Nippon Zeon Co., Ltd., product name "AR-51"): 20.0 parts

[0071] The electromagnetic wave absorbing sheet of Example 5 was prepared in the same manner as Example 4, except that the magnetic material-containing compound was used, the content of the magnetic material was changed to 40 volume % relative to the total solid content of the coating material for forming the magnetic layer, and the thickness of the magnetic layer was changed to 100 μm.

[0072] (Comparative Example 1) An electromagnetic wave absorbing sheet of Comparative Example 1 was produced in the same manner as in Example 1, except that the magnetic material was changed to spherical reduced carbonyl iron powder (product name "RZE") manufactured by Tenichi Co., Ltd. and the thickness of the magnetic layer was changed to 63 μm.

[0073] (Comparative Example 2) An electromagnetic wave absorbing sheet of Comparative Example 2 was produced in the same manner as in Example 1, except that the magnetic material was changed to spherical non-reduced carbonyl iron powder manufactured by Tenichi Co., Ltd. (product name "YW3") and the thickness of the magnetic layer was changed to 45 μm.

[0074] (Comparative Example 3) An electromagnetic wave absorbing sheet of Comparative Example 3 was produced in the same manner as in Example 1, except that the magnetic material was changed to flat non-reduced carbonyl iron powder (product name "YP") manufactured by Tenichi Co., Ltd., and the thickness of the magnetic layer was changed to 41 μm.

[0075] Comparative Example 4: A flattened reduced carbonyl iron powder (product name "RPZ") manufactured by Tenichi Co., Ltd., milling balls (zirconia balls φ5 mm), and a solvent (ethanol) were placed in a wet ball mill and flattened for 10 hours. The shape of the flattened powder after drying was observed with an optical microscope, and powders with a major axis of 10 μm or more were prepared as flattened reduced carbonyl iron powders.

[0076] An electromagnetic wave absorbing sheet of Comparative Example 4 was produced in the same manner as in Example 1, except that the magnetic material was changed to the flat reduced carbonyl iron powder prepared above.

[0077] For the electromagnetic wave absorbing sheets produced in Examples 1 to 5 and Comparative Examples 1 to 4, the half-width of the peak of the (200) plane of α-iron detected by powder X-ray diffraction of the magnetic layer, the surface resistivity of the magnetic layer, the transmission attenuation rate (Rtp) at 28 GHz, the breaking elongation, and the breaking stress were measured.

[0078] <Half-Width> The half-width of the peak of the (200) plane of α-iron detected in powder X-ray diffraction of the magnetic layer was measured using the X-ray diffractometer "Smart-Lab" manufactured by Rigaku Corporation, as described above.

[0079] <Surface Resistivity> The surface resistance of the magnetic layer was measured using a resistance meter "Hiresta-MCP-HT450" manufactured by Mitsubishi Analytech Co., Ltd., as described above.

[0080] <Transmission attenuation rate> The magnetic noise suppression effect of the prepared electromagnetic wave absorbing sheet was evaluated by the transmission attenuation rate (Rtp) at 28 GHz using the microstrip line method. 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 manufactured by Keycom Co., Ltd., which is in accordance with the measurement system (Rtp measurement system) of IEC No.: IEC62333-1 and IEC62333-2.

[0081] <Elongation at break and stress at break> The elongation at break and stress at break of the prepared electromagnetic wave absorbing sheet were measured by the method specified in JIS K7127. Specifically, the prepared electromagnetic wave absorbing sheet was pulled at a pulling speed of 50 mm / min using a tension / compression testing machine "Technograph" manufactured by MinebeaMitsumi Inc. to break the test piece. From the elongation at the time of break of the test piece, the elongation at break was calculated by (break displacement / gauge line distance before tension) × 100. In addition, the breaking stress was calculated from the breaking test force per unit area of ​​the fracture surface at the time of break of the test piece.

[0082] The above results are shown in Tables 1 and 2 together with the type of magnetic material used in the magnetic layer of the electromagnetic wave absorbing sheet produced, its characteristics, the thickness of the magnetic layer, the content of the magnetic material, and the type of binder.

[0083]

[0084]

[0085] As can be seen from Table 1, in the electromagnetic wave absorbing sheets of Examples 1 to 5, the half-width of the peak of the (200) plane of α-iron detected by powder X-ray diffraction of the magnetic layer was 0.15 or more and less than 0.40, and the transmission attenuation rate (Rtp) was 10 dB or more at 28 GHz, demonstrating a high magnetic noise suppression effect.

[0086] Furthermore, the electromagnetic wave absorbing sheets of Examples 1 to 3, which used water-soluble polyester as the binder, had an elastic elongation greater than 75% and an elastic stress of 30 MPa or more. On the other hand, the electromagnetic wave absorbing sheets of Example 4, which used silicone rubber as the binder, and Example 5, which used acrylic rubber as the binder, had an elastic elongation of 75% or less and an elastic stress of less than 30 MPa. Therefore, when the electromagnetic wave absorbing sheets were wound around a stainless steel mandrel with a diameter of 3 mm in accordance with JIS K5600-5-1, the electromagnetic wave absorbing sheets of Examples 1 to 3 could be wound without any problems, but the electromagnetic wave absorbing sheets of Examples 4 and 5 had small elastic elongations, which caused breakage within the sheet, and small breaking stresses made winding difficult.

[0087] On the other hand, in Comparative Example 1, spherical reduced carbonyl iron that had not been flattened was used, resulting in a low half-width of 0.07 and a low transmission attenuation of 5.7 dB. In Comparative Example 2, spherical non-reduced carbonyl iron was used, resulting in a half-width of 1.48, or more than 0.4, and a low transmission attenuation of 3.4 dB, possibly due to the lack of flattening. Furthermore, in Comparative Example 3, flat non-reduced carbonyl iron was used, resulting in a half-width of 2.25, or more than 0.4, and a low transmission attenuation of 6.3 dB, possibly due to excessive flattening of the non-reduced carbonyl iron. Furthermore, in Comparative Example 4, the half-width was 0.50, or more than 0.4, resulting in a low transmission attenuation of 8.0 dB. This shows that even with flat reduced carbonyl iron, the noise suppression effect decreases if the half-width is outside the range of 0.15 to 0.40.

[0088] The following supplementary embodiments are further disclosed in relation to the embodiments of the present application including Examples 1 to 5 described above. (Supplementary Embodiment 1) An electromagnetic-wave absorbing sheet comprising a substrate and a magnetic layer, wherein the magnetic layer comprises a magnetic material and a binder, and the magnetic material comprises flat carbonyl iron, and wherein the half-width of a peak of the (200) plane of α-iron detected by powder X-ray diffraction of the magnetic layer is 0.15 or more and less than 0.40. (Supplementary Embodiment 2) The electromagnetic-wave absorbing sheet according to Supplementary Embodiment 1, wherein the elongation at break defined in JIS K7127 is greater than 75%. (Supplementary Embodiment 3) The electromagnetic-wave absorbing sheet according to Supplementary Embodiment 1, wherein the elongation at break defined in JIS K7127 is 95% or more. (Supplementary Embodiment 4) The electromagnetic-wave absorbing sheet according to any one of Supplementary Embodiments 1 to 3, wherein the stress at break defined in JIS K7127 is 30 MPa or more. (Additional Form 5) The electromagnetic wave absorbing sheet according to any one of Additional Forms 1 to 3, wherein the breaking stress as defined in JIS K7127 is 35 MPa or more. (Additional Form 6) The electromagnetic wave absorbing sheet according to any one of Additional Forms 1 to 5, wherein the flat carbonyl iron is reduced carbonyl iron. (Additional Form 7) The electromagnetic wave absorbing sheet according to any one of Additional Forms 1 to 6, wherein the content of the magnetic material in the magnetic layer is 30 to 70% by volume. (Additional Form 8) The electromagnetic wave absorbing sheet according to any one of Additional Forms 1 to 6, wherein the content of the magnetic material in the magnetic layer is 50 to 65% by volume. (Additional Form 9) The electromagnetic wave absorbing sheet according to any one of Additional Forms 1 to 8, wherein the binder contains a water-soluble polyester resin. (Additional Form 10) The electromagnetic wave absorbing sheet according to any one of Additional Forms 1 to 9, wherein the substrate is a resin film. (Additional Form 11) The electromagnetic wave absorbing sheet according to any one of Additional Forms 1 to 9, wherein the surface resistance of the magnetic layer is 10 5Ω / square or more. (Additional Form 12) The electromagnetic wave absorbing sheet according to any one of Additional Forms 1 to 11, further comprising a metal layer. (Additional Form 13) The electromagnetic wave absorbing sheet according to any one of Additional Forms 1 to 12, having an overall thickness of 20 to 1000 μm. (Additional Form 14) A method for producing the electromagnetic wave absorbing sheet according to any one of Additional Forms 1 to 13, comprising the steps of reducing carbonyl iron, flattening the reduced carbonyl iron, mixing the flattened reduced carbonyl iron, a binder, and a solvent to prepare a coating material for forming a magnetic layer, and applying the coating material for forming a magnetic layer to a substrate and drying it. (Additional Form 15) A method for producing an electromagnetic wave absorbing sheet according to Additional Form 14, wherein the reduction is a hydrogen reduction treatment.

[0089] The present application may be implemented in other forms than those described above. The embodiments disclosed in the present application are merely examples and are not intended to be limiting. The scope of the present application shall be interpreted in accordance with the appended claims rather than the above description, and all modifications within the scope of the claims are intended to be embraced within the scope of the claims.

[0090] 10, 10' Electromagnetic wave absorbing sheet 11 Substrate 12 Magnetic layer 13 Metal layer

Claims

1. An electromagnetic wave absorbing sheet comprising a substrate and a magnetic layer, wherein the magnetic layer comprises a magnetic material and a binder, the magnetic material comprises flat carbonyl iron, and the half-width of the peak of the (200) plane of alpha iron detected by powder X-ray diffraction of the magnetic layer is 0.15 or more and less than 0.

40.

2. The electromagnetic wave absorbing sheet according to claim 1, which has a breaking elongation as defined in JIS K7127 of more than 75%.

3. The electromagnetic wave absorbing sheet according to claim 1, which has a breaking stress as specified in JIS K7127 of 30 MPa or more.

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

5. The electromagnetic wave absorbing sheet according to claim 1, wherein the content of said magnetic material in said magnetic layer is 30 to 70 volume %.

6. The electromagnetic wave absorbing sheet according to claim 1, wherein the binder contains a water-soluble polyester resin.

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

8. The surface resistance of the magnetic layer is 10 5 2. The electromagnetic wave absorbing sheet according to claim 1, having a resistance of Ω / square or more.

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, which has an overall thickness of 20 to 1000 μm.

11. A method for producing an electromagnetic wave absorbing sheet as defined in any one of claims 1 to 10, comprising the steps of: reducing carbonyl iron; flattening the reduced carbonyl iron; mixing the flattened reduced carbonyl iron, a binder and a solvent to prepare a paint for forming a magnetic layer; and applying the paint for forming a magnetic layer to a substrate and drying it.

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

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