JP2023129145A - Radio wave absorption sheet for millimeter wave band - Google Patents

Abstract

To provide a radio wave absorption sheet that has all of excellent radio wave absorption performance in the millimeter wave band, thinness, high surface resistance, excellent heat resistance, moisture resistance, flame retardance, and environment adaptability.SOLUTION: A radio wave absorption sheet 100 for a millimeter wave band according to an embodiment of the present invention includes an insulating layer 10, a dielectric loss layer 20, a pressure-sensitive adhesive layer 30, and a conductive layer 40 in order from the radio wave incident side, and has a radio wave absorption amount of 10 dB or more in the frequency band of 76 to 81 GHz. The surface resistance value of the insulating layer 10 is 1010 Ω/square or more. The ratio B/C of the thickness B of the dielectric loss layer 20 to the thickness C of the insulating layer 10 is 3.0 or more. The dielectric loss layer 20 includes base material, loss material made of carbon powder dispersed in the base material, and a non-halogen flame retardant of 5% by volume or more with respect to the dielectric loss layer.SELECTED DRAWING: Figure 1

Description

The present invention relates to a radio wave absorbing sheet for millimeter wave bands.

Electromagnetic waves are used as a means of connecting things and detecting obstacles in many industries, including the electronic communication industry such as smartphones and tablet terminals, the automobile industry, and the home appliance industry, creating a situation where many electromagnetic waves are flying around. There is. Under such circumstances, functional sheets for rectifying electromagnetic waves, such as noise suppression sheets and radio wave absorption sheets, are increasingly being used to avoid malfunctions due to mutual interference and self-poisoning problems, and to improve product performance. It's coming.

Particularly notable is the millimeter wave band. For example, there is an increase in frequency due to the change in communication standards from 4G (4th generation mobile communication) to 5G (5th generation mobile communication). Until now, communication at around 1 GHz has been mainstream, but there has been a shift to the Sub6 (3 to 6 GHz) band, 28 GHz band, and 40 GHz band. Furthermore, in the automotive field, millimeter-wave radars are increasingly being installed in vehicles to prevent collisions and as equipment for autonomous driving, which is expected to increase in the future.

Millimeter-wave radar is a device that recognizes obstacles and measures their distance by emitting millimeter-wave electromagnetic waves and receiving the electromagnetic waves reflected from objects. Because they have a long detectable distance and are not affected by weather conditions, they are used as safety devices for obstacle recognition and as key devices in autonomous driving technology. Millimeter waves are electromagnetic waves that generally have a wavelength of 1 to 10 mm and a frequency of 30 to 300 GHz, but the 76 to 81 GHz band is mainly used for millimeter wave radar applications. One of the factors that affects the reliability of millimeter-wave radar is that it does not falsely detect objects that are unrelated to the target. For example, if an electromagnetic wave is received that is reflected from a road surface or the like that is not the intended object, detection accuracy will decrease. As a result, false detections occur, leading to a decrease in safety. To solve this problem, a radio wave absorbing sheet is provided inside the millimeter wave radar.

The following prior art exists as a millimeter wave band radio wave absorbing sheet that can be installed inside a radar.

Patent Document 1 discloses a radio wave absorbing sheet for millimeter wave bands in which a radio wave reflecting layer, a radio wave absorbing layer, and a protective layer are sequentially laminated, and the radio wave absorbing layer is made of ferrite powder on a base material made of urethane resin and EPDM rubber. It is a dielectric loss layer added with , and a protective layer made of vinyl chloride resin or the like is described.

Patent Document 2 has a laminated structure consisting of at least a resin composition layer and an electromagnetic wave reflecting layer, and the resin composition layer contains conductive acicular titanium oxide in a cured product obtained by curing an acrylic polymeric resin. A matching electromagnetic wave absorber is described in which aluminum and aluminum hydroxide are dispersed and fixed.

Patent Document 3 discloses a flexible electromagnetic wave absorbing sheet formed in a sheet shape by sequentially laminating a protective layer, a resistive film, a dielectric layer, and an electromagnetic shielding layer, in which the resistive film has a surface resistance. It is described that the resistance film is adjusted to around 377Ω/□, and the total thickness of the resistive film and dielectric layer is relatively thin, 300 to 500 μm.

Patent Document 4 describes an electromagnetic wave absorber formed by sequentially laminating a resin layer, a resistance layer, a dielectric layer, a conductive layer, and a resin layer, in which the resistance layer has a surface resistance adjusted to around 377Ω/□. It is described that the total thickness of the resistive layer and the dielectric layer is relatively thin at 400 to 600 μm.

International Publication No. 2019/077808 International Publication No. 2017/090623 JP 2019-140404 Publication JP2018-98367A

It goes without saying that radio wave absorbing sheets for millimeter wave bands are required to have excellent radio wave absorption performance in the millimeter wave band, but are also required to have various other properties such as the following.

In recent years, as radars have become smaller, radio wave absorbing sheets have to be installed in smaller spaces. Although it depends on the installation space inside the radar, the total thickness of the radio wave absorbing sheet without a conductive layer (reflection layer) is 1000 μm or less, or even as thin as possible, the more versatile it is, and the more demand there is.

In addition, since the radio wave absorbing sheet is placed near the circuit, if the radio wave absorbing sheet falls off from the mounting part and the surface resistance of the radio wave absorbing sheet is less than 10 ¹⁰ Ω/□, the radio wave absorbing sheet will not connect to the circuit. There is a risk of contact and short circuit. Therefore, radio wave absorbing sheets are required to have high surface resistance.

Furthermore, radars installed in vehicles require high reliability. Specifically, in terms of heat resistance, a temperature of 85 to 125°C is required, and in terms of humidity resistance (high temperature and high humidity), a temperature of 85°C and 85% RH is required. It is required that the characteristics do not deteriorate significantly. In terms of flame retardancy, a level of at least UL94-HB or higher is required.

In addition, not only in the automotive field, but in recent years there has been a demand for greater consideration for the environment, and radio wave absorbing sheets are also required to be free of halogen substances such as chlorine.

However, the radio wave absorbing sheets of Patent Documents 1 to 4 do not satisfy all of the above requirements as described below.

In Patent Document 1, there are no problems with the thickness or surface resistance of the radio wave absorbing sheet, but the dielectric layer does not contain a flame retardant, so it has poor flame retardancy. Furthermore, vinyl chloride resin is sometimes used for the surface protective layer, which is not preferable from an environmental standpoint.

In Patent Document 2, although there is no problem in thickness or flame retardancy, the surface resistance is less than 10 ¹⁰ Ω/□. Furthermore, the filler added to exhibit radio wave absorbing properties is a needle-like substance, and orientation of the filler occurs during the process of manufacturing the radio wave absorbing sheet. As a result, anisotropy occurs in radio wave absorption, which is unfavorable for practical use.

In Patent Document 3, the surface protective layer can play a role in increasing insulation, but the conductive organic polymer containing PEDOT that constitutes the resistive film has poor heat resistance, and the resistance decreases when left in an environment of 85°C. The surface resistance value of the film will be significantly altered, and the radio wave absorption will be significantly reduced. Furthermore, the flame retardance of the radio wave absorbing sheet is also poor.

In Patent Document 4, the resin layer can play a role in improving insulation, but the conductive material mainly composed of ITO that constitutes the resistance layer is not a pure inorganic material, but contains an organic polymer with poor heat resistance. If the resistive layer is left in an environment of 85° C., the surface resistance value of the resistive layer will be significantly altered, and the radio wave absorption will be significantly reduced. Furthermore, the flame retardance of the radio wave absorbing sheet is also poor.

In addition to those listed in Patent Documents 1 to 4, radio wave absorbing sheets for millimeter wave bands include those using ε iron oxide for the dielectric layer and those using a mesh metal film for the resistive film. A product that satisfies all of the above requirements has not been obtained.

In view of the above problems, the present invention aims to provide a radio wave that has excellent radio wave absorption performance in the millimeter wave band, thinness, high surface resistance, excellent heat resistance, moisture resistance, flame retardance, and environmental adaptability. The purpose is to provide absorbent sheets.

In order to solve the above problems, the present inventors conducted extensive research and obtained the following knowledge. That is, the basic structure of a radio wave absorbing sheet includes: an adhesive layer for pasting the radio wave absorbing sheet onto a conductive material, a dielectric loss layer provided in contact with the adhesive layer, and a dielectric loss layer provided in contact with the dielectric loss layer. and an insulating layer forming a radio wave incident surface. that time,
(I) The surface resistance value of the insulating layer is 10 ¹⁰ Ω/□ or more,
(II) The ratio B/C of the thickness B of the dielectric loss layer to the thickness C of the insulating layer is 3.0 or more,
(III) The dielectric loss layer shall include a base material, a loss material made of carbon powder dispersed in the base material, and a non-halogen flame retardant in an amount of 5% by volume or more with respect to the dielectric loss layer. By this method, it is possible to obtain a radio wave absorbing sheet that has excellent radio wave absorption performance in the millimeter wave band, thinness, high surface resistance, excellent heat resistance, moisture resistance, flame retardance, and environmental adaptability. The present invention has been completed. In particular, in the present invention, in order to achieve both radio wave absorption performance and flame retardancy, the ratio B/C of the thickness B of the dielectric loss layer to the thickness C of the insulating layer and the amount of flame retardant added to the dielectric loss layer are adjusted. It is characterized by its balance.

The gist of the present invention, which was completed based on the above findings, is as follows.
[1] A radio wave absorbing sheet for millimeter wave bands, which has a radio wave absorption amount of 10 dB or more in the frequency band of 76 to 81 GHz when attached to a conductive material,
an adhesive layer for pasting the radio wave absorbing sheet onto the conductive material;
a dielectric loss layer provided on and in contact with the adhesive layer;
an insulating layer provided in contact with the dielectric loss layer and forming a radio wave incident surface;
has
The surface resistance value of the insulating layer is 10 ¹⁰ Ω/□ or more,
The ratio B/C of the thickness B of the dielectric loss layer to the thickness C of the insulating layer is 3.0 or more,
The dielectric loss layer is characterized in that it includes a base material, a loss material made of carbon powder dispersed in the base material, and a non-halogen flame retardant in an amount of 5% or more by volume with respect to the dielectric loss layer. Radio wave absorption sheet for millimeter wave band.

[2] The radio wave absorption for millimeter wave band according to [1] above, wherein the total thickness A+B+C of the thickness A of the adhesive layer, the thickness B of the dielectric loss layer, and the thickness C of the insulating layer is 100 μm or more and 700 μm or less. sheet.

[3] The radio wave absorbing sheet for the millimeter wave band according to [1] or [2] above, wherein the thickness C of the insulating layer is 12 μm or more and 100 μm or less, and the thickness B of the dielectric loss layer is 50 μm or more and 400 μm or less. .

[4] The radio wave absorbing sheet for the millimeter wave band according to any one of [1] to [3] above, wherein the adhesive layer has a thickness A of 30 μm or more and 150 μm or less.

[5] The radio wave absorbing sheet for millimeter wave bands according to any one of [1] to [4] above, wherein the insulating layer is made of either polyethylene terephthalate (PET) or polyethylene naphthalate (PEN). .

[6] The radio wave absorbing sheet for millimeter wave bands according to any one of [1] to [5] above, wherein the base material of the dielectric loss layer is made of either silicone rubber or acrylic resin.

[7] The insulating layer has a real part of a dielectric constant of 2.0 or more and 4.0 or less at 76 to 81 GHz,
The dielectric loss layer has a real part of a dielectric constant of 5.9 or more and 12.5 or less, an absolute value of an imaginary part of the dielectric constant of 1.4 or more and 10.2 or less at 76 to 81 GHz, and has a dielectric constant of 1.4 to 10.2. The radio wave absorbing sheet for millimeter wave bands according to any one of [1] to [6] above, having a tangent of 0.23 or more and 0.82 or less.

[8] The radio wave absorption for millimeter wave band according to any one of [1] to [7] above, wherein the non-halogen flame retardant is composed of one or more of a nitrogen compound, a hydroxide compound, and a phosphorus compound. sheet.

[9] The millimeter wave band according to any one of [1] to [8] above, wherein the conductive material includes a conductive layer made of aluminum or copper and having a thickness of 10 nm or more and provided in contact with the adhesive layer. radio wave absorption sheet.

[10] Any of the above [1] to [9], comprising a second adhesive layer provided in contact with the conductive layer on the side opposite to the adhesive layer for attaching the radio wave absorbing sheet to an object. The radio wave absorbing sheet for the millimeter wave band according to item (1).

The radio wave absorbing sheet of the present invention has excellent radio wave absorption performance in the millimeter wave band, thinness, high surface resistance, excellent heat resistance, moisture resistance, flame retardance, and environmental adaptability.

FIG. 1 is a schematic cross-sectional view of a millimeter wave band radio wave absorbing sheet 100 according to an embodiment of the present invention.

(Radio wave absorption sheet for millimeter wave band)
Referring to FIG. 1, a millimeter wave band radio wave absorbing sheet 100 according to an embodiment of the present invention is formed by sequentially laminating an insulating layer 10, a dielectric loss layer 20, an adhesive layer 30, and a conductive layer 40. . In the case of this embodiment, a second adhesive layer (not shown) for attaching the radio wave absorbing sheet 100 to an object is provided on the opposite side of the conductive layer 40 from the adhesive layer 30. The radio wave absorbing sheet 100, described from the side closer to the object, includes a second adhesive layer (not shown), a conductive layer 40 provided in contact with the second adhesive layer, and a conductive layer 40 provided in contact with the conductive layer 40. an adhesive layer 30, a dielectric loss layer 20 provided in contact with the adhesive layer 30, and an insulating layer 10 provided in contact with the dielectric loss layer 20, and the insulating layer 10 receives radio waves. form a surface.

In the radio wave absorbing sheet 100 according to the present embodiment, (A) radio wave absorption by the lossy material included in the dielectric loss layer 20 (absorption by converting radio waves into heat), and (B) radio wave absorption by the lossy material included in the dielectric loss layer 20, and (B) radio wave absorption by the lossy material included in the dielectric loss layer 20. Radio wave absorption is performed in two ways: the radio wave and the radio wave transmitted through the dielectric loss layer 20 and reflected on the surface of the conductive layer 40 cancel each other out. The radio wave absorbing sheet 100 according to the present embodiment is not limited to a conductive material, and can be attached to an object made of any material including a non-conductive material such as a resin plate.

However, in the present invention, the conductive layer 40 and the second adhesive layer have any configuration. For example, when the object is a conductive object, that is, when the radio wave absorbing sheet is attached to the conductive object, the conductive layer 40 and the second adhesive layer are not necessary. In this case, the radio wave absorbing sheet of the present invention has a configuration in which the conductive layer 40 is removed in FIG. It has a dielectric loss layer 20 and an insulating layer 10 provided in contact with the dielectric loss layer 20, and this insulating layer 10 forms a radio wave incident surface. In this case, the radio wave absorption according to (B) above is caused by the radio waves reflected on the surface of the dielectric loss layer 20 and the radio waves reflected on the surface of the electrically conductive object canceling each other out.

[Insulating layer]
The insulating layer 10 is a layer for ensuring surface insulation of the radio wave absorbing sheet 100 and capturing millimeter waves incident on the radio wave absorbing sheet 100.

Although there are no restrictions on the material of the insulating layer 10, a flexible resin film is generally used. Examples of the type of resin include polypropylene, polyethylene, polyethylene terephthalate, polyethylene naphthalate, polyimide, and polyamideimide, but from a practical point of view, either polyethylene terephthalate or polyethylene naphthalate is preferable. . Note that if higher moisture resistance is required, a resin film made of a moisture-resistant material or a resin film that has been subjected to moisture-resistant treatment can also be used. Furthermore, when higher flame retardance is required, flame-retardant PET that has been imparted with flame retardancy can also be used, and flame-retardant PET that does not contain halogens is more preferred.

The thickness C of the insulating layer 10 is preferably 12 μm or more, more preferably 25 μm or more, from the viewpoint of ensuring the surface insulation of the radio wave absorbing sheet. Further, from the viewpoint of flexibility and flame retardancy of the radio wave absorbing sheet, the thickness C of the insulating layer 10 is preferably 100 μm or less, more preferably 75 μm or less.

From the viewpoint of ensuring the surface insulation of the radio wave absorbing sheet, the surface resistance value of the insulating layer 10 needs to be 10 ¹⁰ Ω/□ or more. Although the upper limit of the surface resistance value of the insulating layer 10 is not limited, in this embodiment, the surface resistance value of the insulating layer 10 is 10 ¹⁴ Ω/□ or less.

The insulating layer 10 is a layer to which millimeter wave radio waves are first incident. Therefore, the real part of the dielectric constant of the insulating layer 10 at 76 to 81 GHz is preferably 2.0 or more and 4.0 or less. This suppresses the reflection of radio waves at the interface between the air layer and the insulating layer 10, and also allows the radio waves to propagate inside the insulating layer 10 before entering the dielectric loss layer 20, thereby reducing the wavelength of the radio waves by 1. /√ε. Next to the insulating layer 10, a radio wave with a wavelength shortened to 1/√ε is incident on the dielectric loss layer 20, into which the millimeter wave radio waves are incident, so that the resonant radio wave exhibits an absorption peak at a predetermined frequency. In the absorbent sheet, the dielectric loss layer 20 can be made thinner than when the insulating layer 10 is not provided. If the real part of the relative dielectric constant is less than 2.0, the above effect will be weakened, and the dielectric loss layer 20 will have to be made thicker. In addition, if the real part of the relative dielectric constant exceeds 4.0, the millimeter wave radio waves will be reflected on the surface of the insulating layer 10, and the radio wave absorption sheet will not be able to take in the millimeter wave radio waves, causing the radio wave absorption The amount will decrease.

[Dielectric loss layer]
The dielectric loss layer 20 is a layer disposed in contact with the insulating layer 10 without an adhesive layer interposed therebetween, and is a layer in which radio wave absorption in the above-mentioned aspects (A) and (B) is performed.

The dielectric loss layer 20 includes a base material, a loss material made of carbon powder dispersed in the base material, and a non-halogen flame retardant in an amount of 5% by volume or more based on the dielectric loss layer 20.

The base material is preferably made of rubber or resin that is flexible, can be closely attached to the insulating layer 10 without intervening an adhesive layer, and has high heat resistance. Specifically, the base material is preferably made of either silicone rubber or acrylic resin.

The loss material added to and dispersed in the base material of the dielectric loss layer 20 is a carbon powder containing carbon as a main component, and is generally a carbon black-based material such as Ketjen black, acetylene black, furnace black, etc. It is a material made of graphite-based materials such as spherical graphite, flaky graphite, and expanded graphite, and combinations thereof. The amount of the loss material added may be set so that the dielectric loss layer 20 has a dielectric constant as described below.

The flame retardant added to the base material of the dielectric loss layer 20 needs to be a non-halogen flame retardant in consideration of the environment. The non-halogen flame retardant is preferably composed of one or more of nitrogen compounds, hydroxide compounds, and phosphorus compounds. Examples of nitrogen compounds include melamine cyanurate and ammonium carbonate. Examples of hydroxide compounds include aluminum hydroxide, magnesium hydroxide, and calcium hydroxide. Examples of phosphorus compounds include red phosphorus and phosphoric acid esters. From the viewpoint of obtaining flame retardancy of the UL94-HB level required for radio wave absorbing sheets for vehicle-mounted anti-collision radars, the content of flame retardant in the dielectric loss layer 20 needs to be 5% by volume or more, and 10% by volume. % or more. On the other hand, from the viewpoint of preventing poor moldability due to a decrease in the base material ratio, the content of the flame retardant in the dielectric loss layer 20 is preferably 25% by volume or less, more preferably 20% by volume or less. preferable.

The thickness B of the dielectric loss layer 20 is preferably 50 μm or more, more preferably 100 μm or more, from the viewpoint of ensuring sufficient radio wave absorption in the frequency band of 76 to 81 GHz. On the other hand, if the dielectric loss layer 20 is too thick, there are concerns that an absorption peak cannot be obtained at 76 to 81 GHz, that the desired amount of radio wave absorption cannot be obtained, and that the dielectric loss layer cannot be stably formed. . From this viewpoint, the thickness B of the dielectric loss layer 20 is preferably 400 μm or less, more preferably 300 μm or less.

In the dielectric loss layer 20, the real part of the relative permittivity at 76 to 81 GHz is preferably 5.9 or more and 12.5 or less, and the absolute value of the imaginary part of the relative permittivity at 76 to 81 GHz is preferably 1.4 or more and 10 The dielectric loss tangent (tan δ) at 76 to 81 GHz is preferably 0.23 or more and 0.82 or less. Note that the dielectric loss tangent is defined as the absolute value of the imaginary part of the dielectric constant/the real part of the dielectric constant. If each parameter is below a predetermined lower limit value, not only will the lossy material not be able to sufficiently absorb radio waves, but the thickness of the dielectric loss layer required for canceling radio waves will increase. If each parameter exceeds a predetermined upper limit value, the reflection of radio waves on the surface of the dielectric loss layer becomes strong, and the amount of radio waves taken into the inside of the dielectric loss layer decreases, causing radio wave absorption by the loss material and radio waves. The radio wave absorption effect due to cancellation is weakened.

Note that the relative permittivity of each of the insulating layer 10 and the dielectric loss layer 20 at a frequency of 76 to 81 GHz can be continuously measured by the free space method (S parameter method). It can be calculated from the measured values of the S parameters at 76 to 81 GHz using a measurement jig for free space evaluation (manufactured by Keycom Co., Ltd.) and a permittivity calculation program (software manufactured by Keycom Co., Ltd.).

[Adhesive layer]
The adhesive layer 30 is a layer for integrating the dielectric loss layer 20 and the conductive layer 40, with the dielectric loss layer 20 and the conductive layer 40 disposed in contact with each other on both surfaces thereof. When the electromagnetic wave absorbing sheet of the present invention does not have the conductive layer 40, the electroconductive layer 40 is a layer for pasting the electromagnetic wave absorbing sheet onto a conductive object as an object.

The material of the adhesive layer 30 is not particularly limited, but it is an organic adhesive or an organic pressure-sensitive adhesive, and an organic pressure-sensitive adhesive is particularly preferable. Examples of organic adhesives include synthetic adhesives such as acrylic, silicone, urethane, and rubber adhesives, which can be used depending on the object to be bonded.

The thickness A of the adhesive layer 30 is preferably 30 μm or more and 150 μm or less. If the thickness A is 30 μm or more, sufficient adhesive force can be exhibited. If the thickness A is 150 μm or less, the total thickness of the radio wave absorbing sheet can be suppressed without adversely affecting the radio wave absorption performance.

[Total thickness and thickness ratio]
In this embodiment, it is preferable that the total thickness A+B+C of the thickness A of the adhesive layer 30, the thickness B of the dielectric loss layer 20, and the thickness C of the insulating layer 10 is 100 μm or more and 700 μm or less. When the total thickness is less than 100 μm, since the total thickness is small, it is necessary to increase the imaginary part of the dielectric constant of the dielectric loss layer in order to efficiently absorb radio waves. At the same time, the real part of the dielectric constant becomes large. As a result, the difference in the real part of the relative permittivity of two adjacent layers (insulating layer and dielectric loss layer, dielectric loss layer and adhesive layer) becomes large, so the radio wave reflection at the boundaries of each layer becomes stronger, and 76 It is not possible to obtain sufficient radio wave absorption performance at ~81 GHz. If the total thickness exceeds 700 μm, it may be possible to obtain the desired radio wave absorption performance by adjusting the dielectric constant or optimizing the thickness of each layer, but this is not preferable due to space issues and moldability issues. .

In this embodiment, it is important that the ratio B/C of the thickness B of the dielectric loss layer 20 to the thickness C of the insulating layer 10 is 3.0 or more, and preferably 4.0 or more. When the thickness ratio B/C is less than 3.0, sufficient flame retardancy cannot be obtained. The upper limit of the thickness ratio B/C is not particularly limited, but from the viewpoint of stably forming the dielectric loss layer 20 in contact with the insulating layer 10, it is preferably 30 or less, and more preferably 20 or less. .

In addition, in the present invention, the thickness of each layer is determined by observing the cross section of the radio wave absorbing sheet with a scanning electron microscope (SEM), measuring the thickness at any 10 points in the obtained image, and adopting the average value. shall be taken as a thing.

[Conductive layer]
The conductive layer 40 is a layer for reflecting millimeter wave radio waves that have passed through the insulating layer 10 , the dielectric loss layer 20 , and the adhesive layer 30 . It serves to cancel out radio waves reflected on the surface of the conductive layer 40. Note that when the radio wave absorbing sheet is attached to a conductive material such as an aluminum casing, it functions as a radio wave reflecting layer, so there is no need to provide a conductive layer on the radio wave absorbing sheet.

Although there are no particular restrictions on the material of the conductive layer 40, metal is generally used. Examples of metal types include brass, copper, iron, nickel, stainless steel, and aluminum. Furthermore, the conductive layer is not made of a single metal, but may also be made of aluminum deposited on a film, for example.

The thickness of the conductive layer 40 is preferably 10 nm or more, and preferably 50 nm or more, from the viewpoint of reliably reflecting millimeter wave radio waves on the surface of the conductive layer 40 and sufficiently obtaining the above-mentioned radio wave cancellation effect. is more preferable. On the other hand, the thickness of the conductive layer 40 is preferably 300 μm or less, more preferably 100 μm or less, from the viewpoint of suppressing the total thickness of the radio wave absorbing sheet and ensuring flexibility of the radio wave absorbing sheet.

[Second adhesive layer]
The second adhesive layer, not shown in FIG. 1, is provided in contact with the conductive layer 40 on the side opposite to the adhesive layer 30, and has the function of attaching the radio wave absorbing sheet 100 to an object.

The material of the second adhesive layer is not particularly limited, but it is an organic adhesive or an organic pressure-sensitive adhesive, and an organic pressure-sensitive adhesive is particularly preferable. Examples of organic adhesives include synthetic adhesives such as acrylic, silicone, urethane, and rubber adhesives, which can be used depending on the object to be bonded.

The thickness of the second adhesive layer is preferably 30 μm or more and 150 μm or less. If the thickness is 30 μm or more, sufficient adhesive force can be exhibited. If the thickness is 150 μm or less, the total thickness of the radio wave absorbing sheet can be suppressed, so the space in which it can be installed can be expanded.

[effect]
The radio wave absorbing sheet of the present invention described above has a radio wave absorption amount of 10 dB or more in the frequency band of 76 to 81 GHz when attached to a conductive material (the conductive layer 40 in the radio wave absorbing sheet 100 according to the present embodiment), Demonstrates excellent radio wave absorption performance. The higher the amount of radio wave absorption, the better, so the upper limit is not limited.

(Production method of radio wave absorption sheet for millimeter wave band)
An example of a method for manufacturing the radio wave absorbing sheet 100 according to an embodiment of the present invention will be described.

First, a resin film that will become the insulating layer 10 is prepared. Hereinafter, a case will be described in which a polyethylene terephthalate (PET) film is used as the resin film.

A dielectric loss layer 20 is formed by kneading silicone rubber or acrylic resin as a base material, carbon powder as a loss material, a flame retardant, and an organic solvent (for example, toluene, methyl ethyl ketone (MEK), etc.) in a ball mill. Prepare slurry for

When the base material is silicone rubber, use a general PET film for carriers that has been treated with silicone using a coating method to a film thickness of 1 μm or less (mold release treatment) on one side of the PET film, and on the PET film, The dielectric loss layer 20 is formed by directly applying the slurry and drying it to evaporate the organic solvent. After drying, the insulating layer 10 made of a resin film and the dielectric loss layer 20 are in close contact with each other without using an adhesive layer. This is presumably because the silicone used in the PET and the silicone used in the dielectric loss layer are the same material, so they react and adhere to each other. In addition, when the base material is silicone rubber, the insulating layer 10 is not limited to a PET film, and any film can be selected from the various resin films described above. By performing mold release treatment on one side of the resin film, the silicone treated on the resin film and the silicone used for the dielectric loss layer become the same material. can be attached (without any intervention).

When the base material is an acrylic resin, the dielectric loss layer 20 is formed by using a PET film that has not been subjected to mold release treatment, applying the above slurry directly onto the PET film, and drying it to evaporate the organic solvent. Form. After drying, the insulating layer 10 made of a resin film and the dielectric loss layer 20 are in close contact with each other without using an adhesive layer. This is presumed to be because the acrylic resin serving as the base material is similar to the constituent elements of PET, so that they are in close contact through elemental bonds.

In this embodiment, the insulating layer 10 and the dielectric loss layer 20 can be directly attached (without intervening an adhesive layer). Therefore, the sheet thickness can be reduced by the amount of the adhesive layer (approximately 100 μm), contributing to thinning of the film.

As the adhesive layer 30, a commercially available double-sided tape can be used, and this can be attached to the dielectric loss layer 20. When selecting a double-sided tape, choose one that suits the substrate you want to adhere to. When the base material of the dielectric loss layer 20 is silicone rubber, the base sheet of the double-sided tape is the center, and the dielectric loss layer 20 side uses a silicone adhesive, and the conductive layer 40 side uses an acrylic adhesive. Preferably, tape is used.

If the object to which the radio wave absorbing sheet is attached is a non-conductive object such as a resin plate, the conductive layer 40 can be formed by laminating the metal foil that will become the conductive layer 40 to the adhesive layer 30 using a laminator or the like. can. Furthermore, a second adhesive layer for attaching the radio wave absorbing sheet 100 to an object may be attached to the side of the conductive layer 40 opposite to the adhesive layer 30. Note that when the object is a conductive object, the conductive layer 40 and the second adhesive layer are not necessary as described above.

(Preparation of radio wave absorbing sheet)
It contains an insulating layer with the film type, dielectric constant, and thickness listed in Table 1, and the base material, loss material (absorbing filler), and flame retardant listed in Table 1, and has the dielectric constant and thickness listed in Table 1. A dielectric loss layer having a dielectric loss layer, an adhesive layer having the adhesive type and thickness listed in Table 1, and a conductive layer having the metal type and thickness listed in Table 1 were sequentially formed to produce various radio wave absorbing sheets. It was subjected to characteristic evaluation.

(Characteristics evaluation)
[Radio wave absorption amount]
The amount of radio wave absorption was evaluated by evaluating the amount of radio wave absorption at normal incidence using a horn antenna (manufactured by Keycom Co., Ltd.) compatible with 60 GHz to 90 GHz and a dielectric lens. The measurement frequency was 60 to 90 GHz, and Table 1 shows the radio wave absorption characteristics in the range of 76 to 81 GHz.

[Surface insulation]
The surface resistance value of the insulating layer was measured using a surface resistance meter and is shown in Table 1.

[Flame retardance]
Flame retardancy was determined by the HB test according to the UL94 combustion standard. The results are shown in Table 1 as ``conforming'' when the combustion rate is within the specified value, and as ``non-conforming'' when it is above the specified value.

[Heat-resistant]
Heat resistance is evaluated by exposing the radio wave absorbing sheet in a refrigerator set at 125℃ for 1000 hours and evaluating it according to the following evaluation contents.If all standards are cleared, it is evaluated as "〇", and if even one criterion is not met, it is evaluated as "×". ” as shown in Table 1.
・Radio wave absorption: 10dB or more (76-81GHz)
・Surface insulation: 10 ¹⁰ Ω/□ or more ・Flame retardancy: UL94-HB compliant

[Moisture resistance]
Moisture resistance is evaluated by exposing the radio wave absorbing sheet in a refrigerator set at 85°C and 85% RH for 1000 hours and evaluating it using the following evaluation criteria. was marked as “×”.
・Radio wave absorption: 10dB or more (76-81GHz)
・Surface insulation: 10 ¹⁰ Ω/□ or more ・Flame retardancy: UL94-HB compliant

The radio wave absorbing sheet of the present invention has excellent radio wave absorption performance in the millimeter wave band, thinness, high surface resistance, excellent heat resistance, moisture resistance, flame retardance, and environmental adaptability, so it can be used in vehicles. It is suitable for installation in various millimeter wave radars such as anti-collision radars.

100 Radio wave absorption sheet 10 Insulating layer 20 Dielectric loss layer 30 Adhesive layer 40 Conductive layer

Claims (10)

A radio wave absorbing sheet for millimeter wave band, which has a radio wave absorption amount of 10 dB or more in the frequency band of 76 to 81 GHz when attached to a conductive material,
an adhesive layer for pasting the radio wave absorbing sheet onto the conductive material;
a dielectric loss layer provided on and in contact with the adhesive layer;
an insulating layer provided in contact with the dielectric loss layer and forming a radio wave incident surface;
has
The surface resistance value of the insulating layer is 10 ¹⁰ Ω/□ or more,
The ratio B/C of the thickness B of the dielectric loss layer to the thickness C of the insulating layer is 3.0 or more,
The dielectric loss layer is characterized in that it includes a base material, a loss material made of carbon powder dispersed in the base material, and a non-halogen flame retardant in an amount of 5% or more by volume with respect to the dielectric loss layer. Radio wave absorption sheet for millimeter wave band. The radio wave absorbing sheet for the millimeter wave band according to claim 1, wherein a total thickness A+B+C of the thickness A of the adhesive layer, the thickness B of the dielectric loss layer, and the thickness C of the insulating layer is 100 μm or more and 700 μm or less. The radio wave absorbing sheet for millimeter wave band according to claim 1 or 2, wherein the thickness C of the insulating layer is 12 μm or more and 100 μm or less, and the thickness B of the dielectric loss layer is 50 μm or more and 400 μm or less. The radio wave absorbing sheet for millimeter wave bands according to any one of claims 1 to 3, wherein the adhesive layer has a thickness A of 30 μm or more and 150 μm or less. The radio wave absorbing sheet for millimeter wave bands according to any one of claims 1 to 4, wherein the insulating layer is made of either polyethylene terephthalate (PET) or polyethylene naphthalate (PEN). The radio wave absorbing sheet for millimeter wave bands according to any one of claims 1 to 5, wherein the base material of the dielectric loss layer is made of silicone rubber or acrylic resin. The insulating layer has a real part of a dielectric constant of 2.0 or more and 4.0 or less at 76 to 81 GHz,
The dielectric loss layer has a real part of a dielectric constant of 5.9 or more and 12.5 or less, an absolute value of an imaginary part of the dielectric constant of 1.4 or more and 10.2 or less at 76 to 81 GHz, and has a dielectric constant of 1.4 to 10.2. The radio wave absorbing sheet for millimeter wave band according to any one of claims 1 to 6, having a tangent of 0.23 or more and 0.82 or less. The radio wave absorbing sheet for millimeter wave bands according to any one of claims 1 to 7, wherein the non-halogen flame retardant comprises one or more of a nitrogen compound, a hydroxide compound, and a phosphorus compound. The radio wave absorbing sheet for millimeter wave bands according to any one of claims 1 to 8, comprising a conductive layer made of aluminum or copper and having a thickness of 10 nm or more and provided in contact with the adhesive layer as the conductive material. 10. The electroconductive layer according to claim 1, further comprising a second adhesive layer provided on a side opposite to the adhesive layer of the conductive layer for attaching the radio wave absorbing sheet to an object. Radio wave absorption sheet for millimeter wave band.

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