Electromagnetic wave absorbing sheet, multilayer electromagnetic wave absorbing sheet, and method for manufacturing electromagnetic wave absorbing sheet

Abstract

The present invention provides a radio wave absorption sheet with an inhibited content of cyclic siloxanes and less color unevenness. The radio wave absorption sheet of the present invention is a resonance type radio wave absorption sheet used in contact with a conductor. The radio wave absorption sheet uses silicone rubber and carbonyl iron powder as the base materials, has a resonance frequency at which the return loss at vertical incidence is maximum in the range of 22 GHz or more and 30 GHz or less, the return loss at the above resonance frequency is 15 dB or more, the total amount of cyclic siloxanes D3 to D20 is 5000 mg / kg or less relative to the mass of the above base materials, and the color difference ΔEgap of the chromaticity within the above radio wave absorption sheet measured by a spectrophotometer is 2.0 or less.

Description

Technical Field

[0001] The present invention relates to an electromagnetic wave absorption sheet, a multilayer electromagnetic wave absorption sheet, and a method for manufacturing the same. Background Art

[0002] With the rapid development of communication, devices that effectively utilize electromagnetic waves in the submillimeter wave to millimeter wave band are widely spreading. For example, in the fifth-generation communication system (5G) that has been used as a commercial service in Japan since 2020, research is being conducted on using frequency bands around 28 GHz or 40 GHz. In addition, in automobiles, with the rapid development of autonomous driving systems, submillimeter wave radars that effectively utilize frequencies around 24 GHz are widely spreading. On the other hand, electromagnetic interference problems in the interior and exterior of such devices have also emerged. Therefore, the use of resonant electromagnetic wave absorption sheets that function at frequencies where electromagnetic wave interference problems occur has increased rapidly. The electromagnetic wave absorption sheet is thin and can cope with the thinning, shortening, and miniaturization of devices that have developed with the rapid development of communication.

[0003] The resonant electromagnetic wave absorption sheet functions by being mounted on a conductor such as metal or by providing a conductive layer such as metal on the back surface of the electromagnetic wave absorption sheet. In the resonant electromagnetic wave absorption sheet, the phase difference between the electromagnetic wave incident on the electromagnetic wave absorption sheet and the electromagnetic wave reflected by the conductive object on the back surface side of the electromagnetic wave absorption sheet is half a wavelength, and the incident wave and the reflected wave cancel each other out, thereby exhibiting electromagnetic wave absorption properties. In the electromagnetic wave absorption sheet, it is known that by adjusting the dielectric constant, magnetic permeability, and thickness of the constituent base material, a peak in the electromagnetic wave absorption amount can be obtained at a specific frequency. In Patent Document 1, it is described that the electromagnetic wave absorption performance changes when the thickness of the resonant electromagnetic wave absorption sheet changes, so it is extremely important to control the thickness of the base material. The resonant electromagnetic wave absorption sheet is made of a material in which loss materials such as carbonyl iron, ferrite, and carbon black are added to rubber such as silicone rubber and EPDM. In particular, silicone rubber has excellent heat resistance and cold resistance, so it is often used as the base material of the electromagnetic wave absorption sheet.

[0004] Prior Art Documents

[0005] Patent Documents

[0006] Patent Document 1: Japanese Patent No. 5481613 Specification.

[0007] However, low molecular weight cyclic siloxanes remain in silicone rubber. This cyclic siloxane volatilizes in the form of siloxane gas at room temperature. Therefore, in the electromagnetic wave absorption sheet using silicone rubber as the base material, there is a risk that the thickness of the electromagnetic wave absorption sheet changes due to the volatilization of the cyclic siloxane, and the electromagnetic wave absorption performance deteriorates. In addition, siloxane gas causes contact failures in electronic devices, so an electromagnetic wave absorption sheet with a reduced residual amount of cyclic siloxane is required.

[0008] The present inventor has found through independent research that the following method is effective: heat-treat a silicone rubber-based radio wave absorption sheet at 100 to 200 °C for a specified period of time in advance to volatilize the cyclic siloxanes remaining in the radio wave absorption sheet. However, during the heat treatment, uneven heating occurs within the radio wave absorption sheet, and due to this uneven heating, deviations in the residual amount of cyclic siloxanes, film thickness, and color tone occur within the radio wave absorption sheet. If the heat treatment temperature is lower than 100 °C, deviations in thickness and color tone caused by uneven heating can be suppressed, but the cyclic siloxanes cannot be sufficiently removed. Summary of the Invention

[0009] In view of this situation, the present invention has been completed, and an object of the present invention is to provide a radio wave absorption sheet in which the content of cyclic siloxanes is suppressed and color unevenness is small.

[0010] Solution to the Problem

[0011] In order to achieve the above problems, the present inventor has repeatedly conducted in-depth research and as a result, obtained the following insights. After forming the radio wave absorption sheet, heat treatment is performed at 100 °C or higher and 200 °C or lower. At this time, by heat-treating the radio wave absorption sheet in a state of being placed on a heat-resistant film, the generation of color unevenness due to uneven heating can be suppressed, and the remaining cyclic siloxanes can be sufficiently volatilized.

[0012] The present invention has been completed based on the above insights. That is, the gist of the present invention is as described below.

[0013] [1] A radio wave absorption sheet, which is a resonance type radio wave absorption sheet used in contact with a conductor,

[0014] The above radio wave absorption sheet is based on silicone rubber and carbonyl iron powder,

[0015] It has a resonance frequency at which the return loss at normal incidence is maximum in the range of 22 GHz or higher and 30 GHz or lower,

[0016] The return loss at the above resonance frequency is 15 dB or more,

[0017] The total amount of cyclic siloxanes D3 to D20 is 5000 mg / kg or less with respect to the mass of the above substrate,

[0018] The difference in chromaticity ΔEgap within the above radio wave absorption sheet measured by a spectrocolorimeter is 2.0 or less.

[0019] Among them, the difference in chromaticity ΔEgap within the above radio wave absorption sheet is the difference between the maximum value and the minimum value of the color difference ΔE within the radio wave absorption sheet, and the color difference ΔE is the difference between the chromaticity of the radio wave absorption sheet and the chromaticity of the radio wave absorption sheet before heat treatment.

[0020] [2] The electromagnetic wave absorption sheet according to [1] above, wherein the total amount of the cyclic siloxanes D3 to D20 is 2,600 mg / kg or less.

[0021] [3] A multilayer electromagnetic wave absorption sheet, which is formed by laminating a silicone rubber film based on silicone rubber and the electromagnetic wave absorption sheet according to [1] or [2] above, and is a multilayer electromagnetic wave absorption sheet used by bringing the electromagnetic wave absorption sheet into contact with a conductor.

[0022] It has one or more resonance frequencies with the maximum return loss at normal incidence in the range of 26 GHz or more and 40 GHz or less.

[0023] The return loss at the above resonance frequency is 15 dB or more.

[0024] [4] An electromagnetic wave absorption sheet, which is a resonance type electromagnetic wave absorption sheet.

[0025] The above electromagnetic wave absorption sheet is based on silicone rubber and carbonyl iron powder.

[0026] It has a conductor layer formed of a conductor on one side.

[0027] It has a resonance frequency with the maximum return loss at normal incidence in the range of 22 GHz or more and 30 GHz or less.

[0028] The return loss at the above resonance frequency is 15 dB or more.

[0029] The total amount of the cyclic siloxanes D3 to D20 is 5,000 mg / kg or less with respect to the mass of the above substrate.

[0030] The color difference ΔEgap in the above electromagnetic wave absorption sheet measured by a spectrocolorimeter is 2.0 or less.

[0031] Among them, the color difference ΔEgap in the above electromagnetic wave absorption sheet is the difference between the maximum value and the minimum value of the color difference ΔE in the electromagnetic wave absorption sheet, and the color difference ΔE is the difference between the color of the electromagnetic wave absorption sheet and the color of the electromagnetic wave absorption sheet without heat treatment.

[0032] [5] A multilayer electromagnetic wave absorption sheet, which is formed by laminating a silicone rubber film based on silicone rubber, the electromagnetic wave absorption sheet according to [1] or [2] above, and a conductor layer formed of a conductor in sequence.

[0033] It has one or more resonance frequencies with the maximum return loss at normal incidence in the range of 26 GHz or more and 40 GHz or less.

[0034] The return loss at the above resonance frequency is 15 dB or more.

[0035] [6] A manufacturing method of a radio wave absorbing sheet, the manufacturing method comprising:

[0036] Molding a pre-heat-treatment radio wave absorbing sheet based on silicone rubber and carbonyl iron powder,

[0037] Placing the pre-heat-treatment radio wave absorbing sheets on a heat-resistant film in a non-contact manner with each other, and performing a heat treatment at 100 °C or higher and 200 °C or lower for 1 hour or longer and 10 hours or shorter to obtain a radio wave absorbing sheet.

[0038] Advantages of the Invention

[0039] According to the present invention, a radio wave absorbing sheet with an inhibited content of cyclic siloxane and less color unevenness can be provided. Description of the Drawings

[0040] Figure 1 It is a diagram showing an example of the radio wave absorption performance of the multilayer radio wave absorbing sheet of Embodiment 2. Detailed Description of the Embodiment

[0041] Hereinafter, embodiments of the present invention will be described. In addition, in this specification, a numerical range represented by "~" means a range including the numerical values described before and after "~" as the lower limit value and the upper limit value.

[0042] <Embodiment 1>

[0043] [Radio Wave Absorbing Sheet]

[0044] First, the radio wave absorbing sheet of Embodiment 1 of the present invention will be described. The radio wave absorbing sheet of one embodiment of the present invention is a resonance type radio wave absorbing sheet used in contact with a conductor,

[0045] The radio wave absorbing sheet is based on silicone rubber and carbonyl iron powder,

[0046] It has a resonance frequency at which the return loss at vertical incidence is maximum in the range of 22 GHz or higher and 30 GHz or lower,

[0047] The return loss at the resonance frequency is 15 dB or higher,

[0048] The total amount of cyclic siloxanes D3 to D20 is 5000 mg / kg or less relative to the mass of the above-mentioned base material,

[0049] The difference in chromaticity ΔEgap in the radio wave absorbing sheet measured by a spectrophotometer is 2.0 or less.

[0050] Here, the difference in chromaticity ΔEgap within the above-described electromagnetic wave absorbing sheet is the difference between the maximum value and the minimum value of the color difference ΔE within the electromagnetic wave absorbing sheet, and the color difference ΔE is the difference between the chromaticity of the electromagnetic wave absorbing sheet and the chromaticity of the electromagnetic wave absorbing sheet before heat treatment.

[0051] The electromagnetic wave absorbing sheet of this embodiment is a resonance type electromagnetic wave absorbing sheet used in contact with a conductor. As described above, the resonance type electromagnetic wave absorbing sheet functions by being mounted on a conductor such as metal or by providing a conductive layer such as metal on the back surface of the electromagnetic wave absorbing sheet.

[0052] The electromagnetic wave absorbing sheet is based on silicone rubber and carbonyl iron powder. In addition, in this specification, being based on silicone rubber and carbonyl iron powder means that 80% by weight or more of the electromagnetic wave absorbing sheet is composed of silicone rubber and carbonyl iron powder.

[0053] As the silicone rubber used for the electromagnetic wave absorbing sheet of the present invention, at least one of a millable state or a liquid state or a combination thereof can be used. Here, an example of using millable silicone rubber will be specifically described. As the silicone rubber, commercially available silicone rubbers for general molding can be used. Most of such commercially available silicone rubbers are pre-added with a vulcanizing agent, and the commercially available silicone rubber can be used in the state at the time of purchase except when intentionally controlling the vulcanization characteristics.

[0054] In addition, among commercially available silicone rubbers, there are also silicone rubbers with a reduced content of cyclic siloxanes. However, in the process of mixing carbonyl iron powder into the silicone rubber and the process of molding the electromagnetic wave absorbing sheet, the silicone rubber is heated and cyclic siloxanes are generated. Therefore, even when using a silicone rubber raw material with a reduced content of cyclic siloxanes, heat treatment must be performed after sheet molding, and there is a problem of providing an electromagnetic wave absorbing sheet with a reduced content of cyclic siloxanes and less color unevenness as described above.

[0055] The hardness of the silicone rubber is not particularly limited, and it is preferably 90 or less as measured by the Shore A durometer hardness. If the hardness of the silicone rubber is 90 or less as measured by the Shore hardness A type, the elongation rate during molding is better, the deviation in thickness becomes smaller, the processability of the final electromagnetic wave absorbing sheet is better, and the electromagnetic wave absorbing sheet can be suitably applied to uneven surfaces. The lower limit of the hardness of the silicone rubber is not particularly limited, and it is preferably 10 or more as measured by the Shore hardness A type.

[0056] Carbonyl iron powder is usually spherical. The average particle size (D50) of the carbonyl iron powder is preferably 1 μm or more and preferably 10 μm or less. If the average particle size is 1 μm or more, the fluidity and dispersibility of the powder are particularly good, and the manufacturability is improved. In addition, if the average particle size is 10 μm or less, the skin effect is suppressed and a good magnetic permeability can be obtained at 22 to 30 GHz. The average particle size of the carbonyl iron powder is more preferably 2 μm or more. In addition, the average particle size of the carbonyl iron powder is more preferably 5 μm or less.

[0057] The type of carbonyl iron powder is not particularly limited. Carbonyl iron powder is classified into soft magnetic and hard magnetic according to magnetism, and either soft magnetic or hard magnetic can be used. In addition, among carbonyl iron powders, in order to improve the affinity with a resin or rubber as a base material or to improve electrical insulation, carbonyl iron powder with a surface coating treatment is commercially available, and carbonyl iron powder with such a surface coating treatment can also be used.

[0058] There is no particular limitation on the addition amount of the carbonyl iron powder. In the resonant type radio wave absorber, the thickness of the radio wave absorber and the magnetic permeability and dielectric constant of the base material are adjusted in combination with the target resonance frequency. Therefore, it is only necessary to determine the particle size and addition amount of the carbonyl iron powder in combination with the target resonance frequency. In one example, the addition amount of the carbonyl iron powder is 40% by weight or more and 90% by weight or less.

[0059] In addition, as needed, additives such as a flame retardant, an extender, a plasticizer, and an antioxidant can be added within a range that does not reduce the radio wave absorption performance. Here, the range that does not reduce the radio wave absorption performance means a range where the content of the additive is 20% by weight or less relative to the base material.

[0060] In the radio wave absorber of the present invention, the total amount of cyclic siloxanes D3 to D20 is suppressed to 5000 mg / kg or less relative to the mass of the base material. Therefore, in this radio wave absorber, the change in the thickness of the radio wave absorber is suppressed, and the reduction in the radio wave absorption performance due to the change in the thickness of the radio wave absorber is suppressed. In addition, the generation amount of siloxane gas is suppressed, so that contact failure of electronic devices due to the generation of siloxane gas can be prevented. Here, the total amount of the above cyclic siloxanes D3 to D20 refers to the total amount of cyclic siloxanes from trimer to eicosamer. In addition, the total amount of cyclic siloxanes D3 to D20 can be quantitatively analyzed and measured by gas chromatography. The total amount of cyclic siloxanes D3 to D20 is preferably 2600 mg / kg or less, and more preferably 1500 mg / kg or less relative to the mass of the base material.

[0061] In addition, in the electromagnetic wave absorbing sheet of the present invention, the difference ΔEgap in chromaticity within the electromagnetic wave absorbing sheet measured by a spectrocolorimeter is suppressed to 2.0 or less. Therefore, an electromagnetic wave absorbing sheet with less color unevenness and high product value can be provided. The difference ΔEgap in chromaticity within the electromagnetic wave absorbing sheet is preferably 1.0 or less, more preferably 0.8 or less. The lower limit of the difference ΔEgap in chromaticity within the electromagnetic wave absorbing sheet is not particularly limited, and can be 0, and in one example, can be 0.01 or more. Here, the above color difference ΔE is the difference in chromaticity between the electromagnetic wave absorbing sheet and the electromagnetic wave absorbing sheet without heat treatment, and the difference ΔEgap in chromaticity within the electromagnetic wave absorbing sheet is the difference between the maximum value and the minimum value of the color difference ΔE within the electromagnetic wave absorbing sheet.

[0062] The electromagnetic wave absorbing sheet of the present invention has a resonance frequency at which the return loss at vertical incidence is maximum in the range of 22 GHz or more and 30 GHz or less, and the return loss at the resonance frequency is 15 dB or more. If the return loss at the resonance frequency is 15 dB or more, 97% of the incident wave can be absorbed, reaching a practical level.

[0063] In addition, since the electromagnetic wave absorbing sheet is used in contact with a conductor such as metal, an adhesive layer or the like can be provided on the back surface of the electromagnetic wave absorbing sheet. Further, since the installation of the adhesive layer or the like causes a difference in electromagnetic wave absorption characteristics compared to the case where only the base material of the electromagnetic wave absorbing sheet is used, in the case of installing the adhesive layer or the like, it is preferable to design the base material of the electromagnetic wave absorbing sheet in combination with the thickness of the adhesive layer or the like and the magnetic permeability / dielectric constant. In addition, when the electromagnetic wave absorbing sheet is not used on a conductor, a conductor layer such as metal can also be provided on the back surface of the electromagnetic wave absorbing sheet.

[0064] [Manufacturing method of electromagnetic wave absorbing sheet]

[0065] Next, the manufacturing process of the electromagnetic wave absorbing sheet will be described. The manufacturing process of the electromagnetic wave absorbing sheet according to one embodiment of the present invention is as follows:

[0066] The electromagnetic wave absorbing sheet before heat treatment based on silicone rubber and carbonyl iron powder is molded.

[0067] The electromagnetic wave absorbing sheet before heat treatment is placed on a heat-resistant film so that the electromagnetic wave absorbing sheets before heat treatment do not contact each other, and heat treatment is performed at 100°C or more and 200°C or less for 1 hour or more and 10 hours or less to obtain an electromagnetic wave absorbing sheet.

[0068] First, silicone rubber and carbonyl iron powder are kneaded to form a base material. Kneading of silicone rubber and carbonyl iron powder can be performed using a pressure kneader or an open roll. At this time, in order not to vulcanize due to heat generation, it is preferable to knead while cooling to 100°C or less.

[0069] Next, the obtained kneaded product is formed into a sheet to obtain a radio wave absorption sheet before heat treatment. The forming method is not particularly limited, and any method such as compression molding, extrusion molding, or calender roll molding can be used. As an example, the case of using compression molding will be described. In one example, the kneaded product is put into a die with a pattern to have a specified thickness after molding. In addition, in one example, compression molding is performed at a temperature of 120 to 200 °C for 5 to 30 minutes during the vulcanization of silicone.

[0070] Next, heat treatment is performed to remove cyclic siloxane from the radio wave absorption sheet before heat treatment obtained by molding. During heat treatment, in the radio wave absorption sheet before heat treatment, when there are parts in contact with air and parts not in contact, there is a risk of uneven volatilization of cyclic siloxane. Therefore, the heat treatment must be performed in a state where the radio wave absorption sheets before heat treatment do not contact each other. For example, for a rectangular radio wave absorption sheet before heat treatment, a mesh shelf that can hold the radio wave absorption sheets one by one and has good air permeability is provided in a heating furnace. In order not to cause discoloration at the contact part between the mesh constituting the shelf and the radio wave absorption sheet before heat treatment, a heat-resistant film is placed on the lower surface where the radio wave absorption sheet before heat treatment is placed, and heat treatment is performed.

[0071] The heat-resistant film is not particularly limited as long as it is a heat-resistant film with high air permeability and heat resistance. In one example, the heat-resistant film is formed of at least one selected from polytetrafluoroethylene (PTFE), polyphenylene sulfide (PPS), polyimide, polyamideimide, and silicone, or a combination thereof. As the heat resistance of the heat-resistant film, it is required not to melt at 100 °C or higher and 220 °C or lower. In one example, the heat-resistant film can be No. 970-2UL manufactured by Nitto Denko Corporation, which is PTFE-impregnated in a glass cloth. When heat treatment is performed in a state where the radio wave absorption sheet before heat treatment is placed on such a heat-resistant film, a radio wave absorption sheet with uniform cyclic siloxane content, thickness, and hue can be obtained.

[0072] The heating temperature of the heat treatment is 100 °C or higher and 200 °C or lower. In order to effectively volatilize cyclic siloxane, the heat treatment is performed at 200 °C or lower. In addition, in order to shorten the treatment time, the heating temperature of the heat treatment is 100 °C or higher. As the treatment time of the heat treatment, it is set to 1 hour or more and 10 hours or less according to the thickness of the radio wave absorption sheet before heat treatment. Through such heat treatment, the total amount of cyclic siloxane D3 to D20 remaining in the final radio wave absorption sheet is 5000 mg / kg or less, preferably 2600 mg / kg or less, and more preferably 1500 mg / kg or less, relative to the mass of the substrate.

[0073] <Embodiment 2>

[0074] Next, Embodiment 2 of the present invention will be described. The multi-layered radio wave absorption sheet of the present embodiment is a multi-layered radio wave absorption sheet formed by laminating a silicone rubber film based on silicone rubber and the above-described radio wave absorption sheet, and is used by bringing the above-described radio wave absorption sheet into contact with a conductor.

[0075] It has one or more resonance frequencies with the maximum return loss at normal incidence in the range of 26 GHz or more and 40 GHz or less.

[0076] The return loss at the above resonance frequency is 15 dB or more.

[0077] The multi-layered radio wave absorption sheet of the present embodiment is formed by laminating a silicone rubber film based on silicone rubber and the radio wave absorption sheet of Embodiment 1. For the silicone rubber film based on silicone rubber, the silicone rubber described in the above Embodiment 1 can be used for production. In addition, the above-mentioned "based on silicone rubber" means that 90% by weight or more of the silicone rubber film is silicone rubber. For the manufacturing method of the silicone rubber film based on silicone rubber, it is the same as the manufacturing method of the radio wave absorption sheet except that carbonyl iron powder is not used.

[0078] After laminating the silicone rubber film based on silicone rubber and the radio wave absorption sheet of Embodiment 1, an adhesive or tape or the like can be interposed between the film and the radio wave absorption sheet for bonding. By forming such a structure, by adjusting the thicknesses of the silicone rubber film and the radio wave absorption sheet, a multi-layered radio wave absorption sheet can be obtained, which has one or more resonance frequencies with return loss at normal incidence in the range of 26 GHz to 40 GHz, and the return loss at the resonance frequency is 15 dB or more, at least 10 dB or more.

[0079] Examples

[0080] Hereinafter, the present invention will be described based on examples, but the present invention is not limited thereto.

[0081] (Example 1)

[0082] Silicone rubber and carbonyl iron were mixed at a weight ratio of 36:64, formed into a pre-heat-treatment radio wave absorption sheet with a thickness of 1.1 mm, placed on a heat-resistant film, and heat-treated at 200 °C for 4 hours to manufacture a radio wave absorption sheet.

[0083] (Example 2)

[0084] The heat treatment was performed for 1 hour, and otherwise, the same procedure as in Example 1 was carried out to manufacture a radio wave absorption sheet.

[0085] (Example 3)

[0086] The heat treatment was performed for 2 hours, and otherwise, the same procedure as in Example 1 was carried out to manufacture a radio wave absorption sheet.

[0087] (Comparative Example 1-1 and Comparative Example 1-2)

[0088] The heat treatment was carried out at 200 °C for 4 hours in a state where 5 pieces of the pre-heat-treatment radio wave absorption sheets formed in the same manner as in Example 1 were overlapped. The sheet in the middle among the 5 overlapped pre-heat-treatment radio wave absorption sheets was Comparative Example 1-1, and the sheet at the top was Comparative Example 1-2.

[0089] (Comparative Example 2)

[0090] For the pre-heat-treatment radio wave absorption sheet formed in the same manner as in Example 1, no heat treatment was carried out as Comparative Example 2.

[0091] The amount of cyclic siloxane in each of the manufactured radio wave absorption sheets (the pre-heat-treatment radio wave absorption sheet in Comparative Example 2) was measured. In the measurement, a gas chromatograph 7890A manufactured by Agilent Technologies was used. The liquid obtained by immersing 0.5 g of each radio wave absorption sheet in 5 ml of acetone and allowing it to stand at room temperature for 24 hours was used. The column used was J&W DW-5ms manufactured by Agilent Technologies. The column temperature was maintained at 50 °C for 5 minutes, heated to 300 °C at a rate of 10 °C / minute, and maintained at 300 °C for 30 minutes for the measurement. Helium was used as the carrier gas, and a flame ionization detector was used as the detector.

[0092] In addition, in the measurement of the radio wave absorption performance of each sample, a vector network analyzer P5008A manufactured by Keysight was used.

[0093] In the colorimetric measurement of the radio wave absorption sheet, a Micro Color measuring device TC-1800MX-II manufactured by Tokyo Denshoku was used. The measurement method was the reflected light measurement method, the colorimetric system was CIELAB, the optical condition was 0°-d, the viewing field was selected as 2-degree viewing field, the standard light was measured with c, and the color difference ΔE of 15 points of each sample was measured one by one based on the average value of 15 points of Comparative Example 2 without heat treatment. In addition, the difference ΔEgap between the maximum value and the minimum value of the color difference ΔE within the sheet was obtained.

[0094] The results of the amount of cyclic siloxane in each sample, the radio wave absorption performance measured after manufacturing the radio wave absorption sheet and the radio wave absorption performance after being placed in a 95 °C environment for 500 hours, the presence or absence of color unevenness based on visual observation, the color difference ΔE, and the difference ΔEgap in the chromaticity within the sheet are shown in Table 1.

[0095] (Example 4)

[0096] Example 4 corresponds to Embodiment 2. First, a pre-heat-treatment film of silicone rubber formed to a thickness of 1.5 mm was manufactured. In addition, silicone rubber and iron carbonyl were mixed at a mixing ratio of 36:64 by weight to manufacture a pre-heat-treatment electromagnetic wave absorption sheet formed to a thickness of 3.0 mm. The pre-heat-treatment film of the silicone rubber and the pre-heat-treatment electromagnetic wave absorption sheet were respectively placed on a heat-resistant film, and heat-treated at 200 °C for 4 hours to obtain a film and an electromagnetic wave absorption sheet. The film was laminated on the electromagnetic wave absorption sheet and bonded using a silicone-based double-sided tape with a thickness of 0.1 mm. The electromagnetic wave absorption performance of Example 4 is as Figure 1 shown.

[0097] [Table 1]

[0098]

[0099] Industrial Applicability

[0100] The electromagnetic wave absorption sheet of the present invention can be installed in electronic devices and can particularly effectively absorb the electromagnetic waves generated in these electronic devices.

Claims

1. A resonance type electromagnetic wave absorbing sheet for use in contact with a conductor, wherein the electromagnetic wave absorbing sheet is based on silicone rubber and carbonyl iron powder, has a resonance frequency at which the return loss at vertical incidence is maximum in the range of 22 GHz or more and 30 GHz or less, has a return loss of 15 dB or more at the resonance frequency, the total amount of cyclic siloxanes D3 - D20 is 5000 mg / kg or less relative to the mass of the base material, the difference in chromaticity ΔEgap within the electromagnetic wave absorbing sheet measured by a spectrocolorimeter is 2.0 or less, Among them, the difference in chromaticity ΔEgap within the electromagnetic wave absorbing sheet is the difference between the maximum and minimum values of the color difference ΔE within the electromagnetic wave absorbing sheet, and the color difference ΔE is the difference between the chromaticity of the electromagnetic wave absorbing sheet and the chromaticity of the electromagnetic wave absorbing sheet without heat treatment.

2. The radio wave absorbing sheet according to claim 1, wherein: The total amount of the cyclic siloxanes D3 - D20 is 2600 mg / kg or less.

3. A multilayer electromagnetic wave absorbing sheet formed by laminating a silicone rubber film based on silicone rubber and the electromagnetic wave absorbing sheet according to claim 1 or 2, for use in contact with a conductor, has one or more resonance frequencies at which the return loss at vertical incidence is maximum in the range of 26 GHz or more and 40 GHz or less, has a return loss of 15 dB or more at the resonance frequency.

4. A multilayer electromagnetic wave absorbing sheet formed by sequentially laminating a silicone rubber film based on silicone rubber, the electromagnetic wave absorbing sheet according to claim 1 or 2, and a conductor layer formed of a conductor, has one or more resonance frequencies at which the return loss at vertical incidence is maximum in the range of 26 GHz or more and 40 GHz or less, has a return loss of 15 dB or more at the resonance frequency.

5. A resonance type electromagnetic wave absorbing sheet, wherein the electromagnetic wave absorbing sheet is based on silicone rubber and carbonyl iron powder, has a conductor layer formed of a conductor on one side, has a resonance frequency at which the return loss at vertical incidence is maximum in the range of 22 GHz or more and 30 GHz or less, has a return loss of 15 dB or more at the resonance frequency, the total amount of cyclic siloxanes D3 - D20 is 5000 mg / kg or less relative to the mass of the base material, the difference in chromaticity ΔEgap within the electromagnetic wave absorbing sheet measured by a spectrocolorimeter is 2.0 or less, Among them, the difference in chromaticity ΔEgap within the electromagnetic wave absorbing sheet is the difference between the maximum and minimum values of the color difference ΔE within the electromagnetic wave absorbing sheet, and the color difference ΔE is the difference between the chromaticity of the electromagnetic wave absorbing sheet and the chromaticity of the electromagnetic wave absorbing sheet without heat treatment.

6. A method for manufacturing an electromagnetic wave absorbing sheet, the manufacturing method comprising: forming a pre - heat - treatment electromagnetic wave absorbing sheet based on silicone rubber and carbonyl iron powder, placing the pre - heat - treatment electromagnetic wave absorbing sheets on a heat - resistant film in a non - contacting manner, and performing heat treatment at 100°C or more and 200°C or less for 1 hour or more and 10 hours or less to obtain the electromagnetic wave absorbing sheet according to claim 1.

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