TWI812620B - Electromagnetic wave absorbing sheet - Google Patents

Abstract

The present invention is an electromagnetic wave absorbing sheet that can well absorb high-frequency electromagnetic waves above the millimeter wave range and has elasticity that extends in the plane direction. An electromagnetic wave absorbing sheet having an electromagnetic wave absorbing layer (1) including magnetic iron oxide (1a), an electromagnetic wave absorbing material that generates magnetic resonance in a frequency band above the millimeter wave band, and a rubber binder (1b), wherein in the above The maximum elongation rate of the elastic domain in one direction within the surface of the electromagnetic wave absorbing sheet is 20~200%.

Description

Electromagnetic wave absorbing sheet

The present disclosure relates to an electromagnetic wave absorbing sheet that absorbs electromagnetic waves, in particular, an electromagnetic wave absorbing material that absorbs electromagnetic waves through magnetic resonance, absorbs high frequency electromagnetic waves above the millimeter wave zone, and has elasticity extending in the in-plane direction. .

In order to avoid the influence of external leakage electromagnetic waves released from electrical circuits, etc., or the influence of unintended reflected electromagnetic waves, electromagnetic wave absorbing sheets that absorb electromagnetic waves are used.

In recent years, it has been used in mobile communications such as mobile phones, wireless LAN, automatic charge collection system (ETC), etc., as centimeter waves with a frequency band of several gigahertz (GHz), and has a range of 30 GHz to 300 GHz. Research on technology utilizing electromagnetic waves with a frequency of 1 MHz (THz) is also progressing in the millimeter wave band, electromagnetic waves in frequency bands higher than the millimeter wave band.

In response to the technological trend of utilizing such higher frequency electromagnetic waves, electromagnetic wave absorbers that absorb unnecessary electromagnetic waves or electromagnetic wave absorbing sheets formed into thin sheets are also designed to absorb electromagnetic waves from the gigahertz zone to the megahertz zone. The demand will increase.

As an electromagnetic wave absorber that absorbs electromagnetic waves in high-frequency bands above the millimeter wave band, ε-iron oxide (ε-Fe oxide) has been proposed to exhibit electromagnetic wave absorption properties in a magnetically variable phase in the range of 25 to 100 GHz. ₂ O ₃ ) Electromagnetic wave absorber with a filling structure of crystal particles (see Patent Document 1). In addition, the fine particles of ε-iron oxide are mixed with the binder at the same time. When the binder is dried and hardened, a magnetic field is applied from the outside to improve the magnetic field alignment of the ε-iron oxide particles. For the flake-shaped alignment body proposal (see Patent Document 2).

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

In addition, as a low-cost electromagnetic wave absorbing sheet that can absorb electromagnetic waves in the frequency band of 75 to 77 GHz and is profitable for public use, a matrix resin made of rubber is dispersed with silicon carbide powder on the surface of a metal body. (Refer to Patent Document 4). Furthermore, as an adhesive sheet that is adhered to a flexible printed wiring board and blocks electromagnetic waves from the outside, it is proposed that the repulsive force of the sheet in which a conductive layer and an insulating layer containing conductive fine particles are laminated is maintained in a specific range. It has a flexible printed wiring board that has both bending resistance and heat resistance (see Patent Document 5).

[Prior technical literature] [Patent Document]

Patent Document 1: Japanese Patent Application Publication No. 2008-60484

Patent Document 2: Japanese Patent Application Publication No. 2016-135737

Patent Document 3: Japanese Patent Application Publication No. 2011-233834

Patent Document 4: Japanese Patent Application Publication No. 2005-57093

Patent Document 5: Japanese Patent Application Publication No. 2013-4854

When shielding leakage electromagnetic waves from a source that generates electromagnetic waves, it is necessary to install electromagnetic wave absorbing materials on the frame covering the target circuit component. However, especially when the shape of the installation place is not planar, it is necessary to use an electromagnetic wave absorbing material that can Flexible or elastic electromagnetic wave absorbing sheets extending in the in-plane direction are more convenient than using solid electromagnetic wave absorbers.

However, the electromagnetic wave absorbing sheet described in Patent Document 3, for example, cannot absorb electromagnetic waves with frequencies of tens of gigahertz or more in the millimeter range. In addition, the electromagnetic wave absorbing sheet described in Patent Document 4 is laminated on a non-stretchable metal body, and the adhesive sheet described in Patent Document 5 is heat-compressed to a flexible printed wiring board. It is not a flexible structure.

However, as an electromagnetic wave absorbing member capable of absorbing electromagnetic waves with a frequency of several tens of gigahertz or higher in the millimeter wave region, an elastic sheet-shaped electromagnetic wave absorbing sheet has not yet been realized.

The purpose of this disclosure is to solve past problems, and its purpose is to well absorb electromagnetic waves with high frequencies above the millimeter wave zone and achieve effective An elastic electromagnetic wave absorbing sheet extending in the in-plane direction.

In order to solve the above problems, the electromagnetic wave absorbing sheet disclosed in this application is an electromagnetic wave absorbing sheet having an electromagnetic wave absorbing layer containing an electromagnetic wave absorbing material that generates magnetic resonance in a frequency band above the millimeter wave band and a rubber binder. The sheet is characterized in that the maximum elongation of the elastic domain in one direction within the plane of the electromagnetic wave absorbing sheet is 20 to 200%.

Since the electromagnetic wave absorbing sheet disclosed in this application has an electromagnetic wave absorbing layer as an electromagnetic wave absorbing material and has magnetic iron oxide that generates magnetic resonance in a high frequency band above the millimeter wave band, it can absorb frequencies of tens of gigahertz or more. Electromagnetic waves in high frequency areas are converted into heat and absorbed. In addition, since the rubber binder has a maximum elongation of the elastic domain in the in-plane direction of 20 to 200%, it becomes easy to arrange the desired parts, and it is possible to cover the movable parts. Electromagnetic wave absorbing sheet.

1: Electromagnetic wave absorption layer

1a:ε-iron oxide (magnetic iron oxide)

1b: Rubber binder

2:Add layer

3: Reflective layer

FIG. 1 is a cross-sectional view illustrating the structure of the electromagnetic wave absorbing sheet according to the first embodiment.

FIG. 2 is a diagram illustrating the electromagnetic wave absorption characteristics of ε-iron oxide that partially replaces Fe positions.

FIG. 3 is a diagram illustrating the extension of an electromagnetic wave absorbing sheet having an electromagnetic wave absorbing layer containing a rubber binder when tensile stress is applied from the outside. Figure 3(a) shows the change in the elongation of the electromagnetic wave absorbing sheet that causes fracture when the maximum elongation is exceeded. Figure 3(b) shows the change in the elongation of the electromagnetic wave absorbing sheet that causes plastic deformation when the maximum elongation is exceeded.

FIG. 4 is a graph showing the relationship between the tensile stress applied from the outside and the elongation in the electromagnetic wave absorbing sheet of the Example.

FIG. 5 is a graph showing changes in electromagnetic wave absorption characteristics due to extension of the electromagnetic wave absorbing sheet according to the first embodiment.

FIG. 6 is a graph showing the relationship between the thickness of the electromagnetic wave absorbing sheet and the amount of electromagnetic wave absorption according to the first embodiment.

Fig. 7 is a cross-sectional view explaining the structure of the electromagnetic wave absorbing sheet according to the second embodiment.

FIG. 8 is a graph showing changes in electromagnetic wave absorption characteristics when the elongation of the electromagnetic wave absorbing sheet changes in the electromagnetic wave absorbing sheet according to the second embodiment.

FIG. 9 is a graph showing the relationship between the thickness of the electromagnetic wave absorbing sheet and the amount of electromagnetic wave attenuation in the second embodiment.

The electromagnetic wave absorbing sheet disclosed in this application has the properties of An electromagnetic wave absorbing sheet containing an electromagnetic wave absorbing layer of an electromagnetic wave absorbing material of magnetic iron oxide that generates magnetic resonance in a frequency band above the millimeter wave band and a rubber binder, in which the maximum extension of the elastic domain in one direction within the plane The rate is 20~200%.

As a result, the electromagnetic wave absorbing sheet disclosed in the present application can absorb electromagnetic waves in the millimeter wave zone and high frequency zones above 30 GHz through the magnetic resonance of the magnetic iron oxide as the electromagnetic wave absorbing material. In addition, by using an electromagnetic wave absorbing material and a rubber binder, a highly stretchable electromagnetic wave absorbing sheet with a maximum elongation of 20 to 200% in the in-plane direction can be realized. Therefore, when arranging the electromagnetic wave absorbing sheet in a housing containing electronic circuits to be shielded, the ease of handling the electromagnetic wave absorbing sheet is improved. Especially on complex curved surfaces, it becomes easier to arrange the electromagnetic wave absorbing sheet. . Furthermore, it can cover movable parts of members whose shapes change, such as joint parts such as arm members, to prevent the emission or entry of unintended electromagnetic waves.

In the electromagnetic wave absorbing sheet disclosed in the present application, the magnetic iron oxide is preferably ε-iron oxide. By using ε-iron oxide, which absorbs electromagnetic waves with a frequency higher than 30 GHz, as an electromagnetic wave absorbing material, an electromagnetic wave absorbing sheet that absorbs electromagnetic waves with a high frequency can be realized.

In this case, it is preferable that a part of the Fe position of the ε-iron oxide is replaced with a trivalent metal atom. In this way, the electromagnetic wave absorbing sheet can be realized as an electromagnetic wave absorbing sheet that absorbs electromagnetic waves in a desired frequency band by replacing the material at the Fe position and exhibiting the characteristics of ε-iron oxide with different magnetic resonance frequencies.

In addition, the volume content of the magnetic iron oxide in the electromagnetic wave absorbing layer is preferably 30% or more. By doing this, the value of the imaginary part (μ") of the magnetic permeability of the electromagnetic wave absorbing layer can be increased, thereby realizing an electromagnetic wave absorbing sheet with high electromagnetic wave absorbing properties.

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

Furthermore, the input impedance value of the electromagnetic wave absorbing layer in a state where the maximum elongation rate of the elastic domain is extended by 5 to 75% is preferably integrated with the impedance value in the air. By doing this, the input impedance value can be made close to the impedance value in air over a wide range of the elongation rate of the electromagnetic wave absorbing sheet, and high electromagnetic wave absorption characteristics can be maintained.

In addition, when the electromagnetic wave absorbing layer is extended within the elastic domain, the input impedance value is preferably 360Ω~450Ω. By doing this, even when the electromagnetic wave absorbing sheet expands and contracts within its elastic range, it can be avoided that the input impedance value is greatly separated from the impedance value in the air and the electromagnetic wave absorption characteristics are exerted above a certain level.

Furthermore, in the electromagnetic wave absorbing sheet disclosed in the present application, it is preferable to form a reflective layer that is in contact with one surface of the electromagnetic wave absorbing layer and reflects electromagnetic waves transmitted through the electromagnetic wave absorbing layer. By doing this, it is possible to reliably shield and absorb electromagnetic waves in high-frequency bands above the millimeter wave band, and to realize a so-called reflective electromagnetic wave absorbing sheet.

In addition, it also has a surface that can be attached to the electromagnetic wave absorbing sheet mentioned above. The next layer is better. By doing this, it is possible to realize an electromagnetic wave absorbing sheet that has high electromagnetic wave absorption properties, can be easily placed in a desired place, and is excellent in handling ease.

The second electromagnetic wave absorbing sheet disclosed in the present application includes an electromagnetic wave absorbing layer including an electromagnetic wave absorbing material and a rubber binder, and a reflective layer that is in contact with one surface of the electromagnetic wave absorbing layer and reflects electromagnetic waves transmitted through the electromagnetic wave absorbing layer. , the aforementioned electromagnetic wave absorbing material is a magnetic iron oxide that generates electrical resonance for electromagnetic waves of a specific frequency, and the output impedance value of the aforementioned electromagnetic wave absorbing layer in a state extending in one direction in the plane is equal to the impedance value in the air. Integrate.

The second electromagnetic wave absorbing sheet disclosed in the present application is constituted in such a way that when actually used, the electromagnetic wave absorbing sheet is in a state of being stretched to some extent. As a premise, it is possible to perform air conditioning in a wide range of elongation. The integration of the impedance value and the input impedance value can improve the electromagnetic wave absorption characteristics of the elastic electromagnetic wave absorbing sheet in the actual use state.

Hereinafter, the electromagnetic wave absorbing sheet disclosed in this application will be described with reference to the drawings.

However, since "radio wave" can be understood as a type of electromagnetic wave in a broader sense, in this specification, the term "electromagnetic wave" such as calling an electromagnetic wave absorber an electromagnetic wave absorber is used.

(First Embodiment)

First, as the first electromagnetic wave absorbing sheet disclosed in this application The embodiment will be described with respect to a so-called transmission type electromagnetic wave absorbing sheet that does not include a reflective layer that reflects electromagnetic waves incident on the electromagnetic wave absorbing sheet.

[Sheet composition]

FIG. 1 is a cross-sectional view showing the structure of the electromagnetic wave absorbing sheet according to the first embodiment of the present application.

However, FIG. 1 is a diagram for easy understanding of the structure of the electromagnetic wave absorbing sheet of this embodiment, and the dimensions and thickness of the members shown in the drawings are not representations of actual structures.

The electromagnetic wave absorbing sheet illustrated in this embodiment is provided with an electromagnetic wave absorbing layer 1 including magnetic iron oxide 1a which is a particulate electromagnetic wave absorbing material and a rubber binder 1b. However, the electromagnetic wave absorbing sheet shown in Fig. 1 is formed on the back side of the electromagnetic wave absorbing layer 1 (on the lower side in Fig. 1) so that the electromagnetic wave absorbing sheet can be attached to the inner surface or outer surface of the frame of the electronic device. Adhesion layer 2 at specific locations on the surface, etc.

The electromagnetic wave absorbing sheet according to this embodiment is composed of the magnetic iron oxide 1a contained in the electromagnetic wave absorbing layer 1 which causes magnetic resonance and converts electromagnetic waves into thermal energy and absorbs it through magnetic loss. Therefore, the reflective layer does not need to be provided. One surface of the electromagnetic wave absorbing layer 1 is used as a so-called transmission type electromagnetic wave absorbing sheet that absorbs electromagnetic waves transmitted through the electromagnetic wave absorbing layer 1 .

In addition, the electromagnetic wave absorbing sheet of this embodiment uses various rubber materials as the binder 1b constituting the electromagnetic wave absorbing layer 1. Therefore, especially in the in-plane direction of the electromagnetic wave absorbing sheet, we can obtain to electromagnetic wave absorbing sheets that are easily stretchable. However, since the electromagnetic wave absorbing sheet of this embodiment is formed of an electromagnetic wave absorbing layer using a rubber binder 1b containing magnetic iron oxide 1a, it is an electromagnetic wave absorbing sheet with high elasticity and high flexibility. When processing the electromagnetic wave absorbing sheet, the electromagnetic wave absorbing sheet can be bent, and the electromagnetic wave absorbing sheet can be easily arranged along the curved surface.

Furthermore, the electromagnetic wave absorbing sheet of this embodiment is easily adhered to a desired location on the surface of a member arranged around a source of high-frequency electromagnetic waves, and the adhesive layer 2 is laminated on one surface of the electromagnetic wave absorbing layer 1. . However, having the adhesive layer 2 is not a necessary condition in the electromagnetic wave absorbing sheet according to this embodiment.

[Electromagnetic wave absorbing material]

In the electromagnetic wave absorbing sheet according to this embodiment, as the electromagnetic wave absorbing material, magnetic iron oxide powder such as ε-iron oxide magnetic powder, barium ferrite magnetic powder, strontium ferrite magnetic powder, etc. can be used. Among them, the electrons of ε-iron oxide iron atoms have the highest frequency of precessional motion when rotating, and the effect of absorbing electromagnetic waves with high frequencies of 30 to 300 GHz in the centimeter range or above is the highest. Therefore, it is particularly suitable as an electromagnetic wave absorbing material.

ε-Iron oxide (ε-Fe ₂ O ₃ ) is in iron oxide (Fe ₂ O ₃ ) and appears between α phase (α-Fe ₂ O ₃ ) and γ phase (γ-Fe ₂ O ₃ ). phase, and becomes a magnetic material that can be obtained in a single-phase state through the nanoparticle synthesis method of the reverse microcell method and the sol-gel method.

Although the ε-iron oxide system has fine particles ranging from several nm to several tens of nm, it has the largest coercive force as a metal oxide of approximately 20 kOe at room temperature. Moreover, it has natural magnetic resonance through the gyromagnetic effect based on precessional motion. It is generated in the frequency range of the so-called centimeter wave band above tens of gigahertz.

Moreover, ε-iron oxide is a crystal obtained by replacing part of the Fe site of the crystal with a trivalent metal element such as aluminum (Al), gallium (Ga), rhodium (Rh), indium (In), etc. The magnetic resonance frequency, that is, the frequency of electromagnetic waves absorbed when used as an electromagnetic wave absorbing material, can be made different.

Figure 2 shows the relationship between the coercive force Hc and the natural resonance frequency f of ε-iron oxide in different cases where a metal element is substituted for the Fe position. However, the natural resonance frequency f is consistent with the frequency of the absorbed electromagnetic wave.

It can be understood from Figure 2 that the natural resonance frequency of the ε-iron oxide system that replaces a part of the Fe position is different depending on the type of the replaced metal element. In addition, the higher the value of the natural resonance frequency, the greater the coercive force of the ε-iron oxide becomes.

More specifically, in the case of gallium-substituted ε-iron oxide, that is, ε-Ga _x Fe _2-x O ₃ , by adjusting the substitution amount "x", the frequency range is from about 30 GHz to 150 GHz. The band has an absorption peak, and in the case of aluminum-substituted ε-iron oxide, _that is, _{ε -} Al The frequency band has an absorption peak. Therefore, the natural resonance frequency of the frequency to be absorbed by the electromagnetic wave absorbing sheet determines the type of element that is substituted for the Fe position of ε-iron oxide. Furthermore, by adjusting the amount of substitution with Fe, the absorbed element can be The frequency of the electromagnetic wave serves as the desired value. Furthermore, when the replacement metal is ε-iron oxide of rhodium, that is, ε-Rh _x Fe _2-x O ₃ is from 180 GHz to above, the frequency range of the electromagnetic wave absorbed can be divided into: Displacement to a higher direction.

Since ε-iron oxide has a structure in which a part of the Fe positions are replaced with metal and is commercially available, it can be easily obtained. However, the ideal particle size of the ε-iron oxide powder is about 5 nm to 50 nm as an average particle size, and has a roughly spherical shape or a short rod shape (rod shape).

Barium ferrite magnets (BaFe ₁₂ O ₁₉ ) and strontium ferrite (SrFe ₁₂ O ₁₉ ) systems are both hexagonal ferrite magnets, and have coercive force because of their large magnetic anisotropy.

Barium ferrite magnets or strontium ferrite powder system iron (Fe) and barium or strontium chloride (BaCl ₂ , SrCl ₂ ) are prepared, mixed, and granulated as raw materials according to necessity, containing Ba and Sr The metal oxide is synthesized by calcining it, and then the calcined body is pulverized to produce a powder with a specific particle size. However, the firing conditions are as an example, and the temperature can be 1200 to 1300° C., the firing environment can be the atmosphere, and the firing time can be about 1 to 8 hours.

The size of the powder produced can be adjusted by the load applied during crushing. To obtain relatively large powder, the calcined body can be subjected to impact crushing by a hammer crusher. And wet grinding (grinder, star ball mill, etc.) method, etc. In addition, the particle size can also be adjusted only through impact crushing by a hammer crusher. barium ferrite magnet, or The ideal particle size of strontium ferrite powder is medium (D50), which is 1μm~5μm.

[Electromagnetic wave absorption layer]

As the rubber binder 1b constituting the electromagnetic wave absorbing layer 1, natural rubber (NR), isoprene rubber (IR), butadiene rubber (BR), styrene-butadiene rubber (SBR), Butyl rubber (IIR), nitrile rubber (NBR), ethylene propylene rubber (EPDM), chloroprene rubber (CR), acrylic rubber (ACM), chlorosulfonated polyethylene rubber (CSR), ethylene formate Ester rubber (PUR), silicone rubber (Q), fluorine rubber (FKM), ethylene vinyl acetate rubber (EVA), epichlorohydrin rubber (CO), polysulfide rubber (T) and other rubber materials.

Among these rubber materials, acrylic rubber and silicone rubber can be suitably used when heat resistance is high. In the case of acrylic rubber, even if it is placed in a high temperature environment, it has excellent oil resistance and is relatively cheap and has excellent cost efficiency. In addition, the silicone rubber has high heat resistance and high cold resistance. Furthermore, its physical properties have the least dependence on temperature among synthetic rubbers, and it is also excellent in solvent resistance, ozone resistance, and weather resistance. Moreover, it has excellent electrical insulation properties, covers a wide temperature range and frequency range, and is physically stable.

In the electromagnetic wave absorbing layer 1 of the electromagnetic wave absorbing sheet according to this embodiment, for example, ε-iron oxide powder is used as the electromagnetic wave absorbing material 1a. The ε-iron oxide powder has a particle diameter of several nm to several tens of nm as described above. Because of the fine nanoparticles, when forming the electromagnetic wave absorbing layer 1, it is important to disperse the ε-iron oxide powder well in the binder 1b. because Therefore, in the electromagnetic wave absorbing sheet according to this embodiment, the electromagnetic wave absorbing layer 1 contains arylsulfonic acid such as phenylphosphonic acid and phenylphosphonic acid dichloride, methylphosphonic acid, ethylphosphonic acid, and octylphosphonic acid. Phosphoric acid compounds such as alkylphosphonic acids such as hydroxyethylidenediphosphonic acid and nitrotrimethylenephosphonic acid. These phosphoric acid compounds are flame retardant and function as a dispersant for fine magnetic iron oxide powder, thereby enabling the ε-iron oxide particles in the binder to be well dispersed.

More specifically, as a dispersing agent, phenylphosphonic acid (PPA) manufactured by Wako Pure Chemical Industries, Ltd., or Nissan Chemical Industries, Ltd., or oxidizing agent manufactured by Johoku Chemical Industries, Ltd. Phosphate ester "JP-502" (product name), etc.

However, in thermosetting additional silicone rubber, vulcanization may be hindered by the addition of phosphoric acid compounds. In this case, it is preferable to use a polymer dispersant other than a phosphate compound, silane, or a silane coupling agent. For example, decylsilylsilane "KBM-3103" (trade name: manufactured by Shin-Etsu Chemical Co., Ltd.) can be preferably used.

However, as an example of the composition of the electromagnetic wave absorbing layer 1, for 100 parts of ε-iron oxide powder (magnetic iron oxide), the rubber binder can be 2 to 50 parts, and the phosphate compound content can be 0.1 to 15 parts. Those who share. When the rubber binder is less than 2 parts, the magnetic iron oxide cannot be dispersed well. In addition, it becomes impossible to maintain the shape of the electromagnetic wave absorbing sheet, and it is difficult to extend the electromagnetic wave absorbing sheet. When it is more than 50 parts, the extension of the electromagnetic wave absorbing sheet can be obtained. However, in the electromagnetic wave absorbing sheet, the volume content of magnetic iron oxide becomes smaller and the magnetic permeability becomes lower. Therefore, the electromagnetic wave absorption effect becomes smaller.

When the content of the phosphoric acid compound is less than 0.1 part, the magnetic iron oxide cannot be dispersed satisfactorily using a rubber binder. When it is more than 15 points, the effect of dispersing the magnetic iron oxide well is saturated. In the electromagnetic wave absorbing sheet, the volume content of magnetic iron oxide becomes smaller, and the magnetic permeability becomes lower, so the electromagnetic wave absorption effect becomes smaller.

[Method for manufacturing electromagnetic wave absorbing layer]

Here, a method for manufacturing the electromagnetic wave absorbing layer 1 of the electromagnetic wave absorbing sheet according to this embodiment will be described. In the electromagnetic wave absorbing sheet of this embodiment, magnetic paint containing at least magnetic iron oxide powder and a rubber binder is prepared, coated with a specific thickness, dried and then subjected to a rolling process to form electromagnetic wave absorption. Layer 1. The rolling treatment is not necessary, but it is preferable because it can reduce the gaps in the electromagnetic wave absorbing sheet and improve the filling degree of the magnetic iron oxide powder.

First, make the magnetic paint.

Magnetic paint is obtained by obtaining a mixture of epsilon-iron oxide powder as a magnetic oxide, a phosphoric acid compound as a dispersant, and a rubber binder. The mixture is diluted with a solvent, further dispersed, and then filtered through a filter. As an example, the kneaded material is mixed with a pressurized batch mixer. In addition, the dispersion system of the kneaded product can be obtained as a dispersion liquid by sanding with beads filled with zirconium or the like, as an example. However, at this time, a cross-linking agent can be prepared as necessary.

Put the obtained magnetic paint on a peelable support As an example, the body is coated on a peel-treated polyethylene terephthalate (PET) sheet with a thickness of 38 μm through polysiloxane coating using a flatbed coater or a rod coater.

After that, the wet magnetic paint is dried at 80°C, and a rolling device is used to perform rolling processing at a specific temperature and pressure to form an electromagnetic wave absorbing layer on the support.

As an example, assuming that the thickness of the wet magnetic coating on the coating support is 1 mm, the thickness after drying can be 400 μm, and the thickness of the electromagnetic wave absorbing layer after rolling treatment can be 300 μm.

In this way, the nanometer-level fine ε-iron oxide powder used as the electromagnetic wave absorbing material 1a can form the electromagnetic wave absorbing layer 1 in a state of being well dispersed in the rubber binder 1b.

However, as another method of producing magnetic paint, at least magnetic iron oxide powder, a phosphate compound of a dispersant, and a rubber binder are mixed at high speed with a high-speed mixer to prepare a mixture, and then the mixture is prepared. The obtained mixture is dispersed by sand grinding to obtain magnetic coating.

[Adhering layer]

As shown in FIG. 1 , the electromagnetic wave absorbing sheet according to this embodiment has an adhesive layer 2 formed on the back surface of the electromagnetic wave absorbing layer 1 .

By providing the adhesive layer 2, the electromagnetic wave absorbing layer 1 can be bonded to the inner surface of a frame housing an electrical circuit, or a desired position on the inner or outer surface of an electrical device. In particular, the electromagnetic wave absorption of this embodiment Since the electromagnetic wave absorbing layer 1 of the sheet has an elastic structure, it can be easily adhered to a curved surface through the adhesive layer 2. This improves the ease of handling of the electromagnetic wave absorbing sheet. However, the material of the adhesive layer 2, its thickness, the state of formation, etc. are particular. The adhesive layer 2 can extend so as not to hinder the elastic deformation of the electromagnetic wave absorbing layer 1. For example, an acrylic adhesive with a low glass point temperature (Tg) can be used. Or silicone-based adhesives, rubber-based adhesives, etc. are preferred.

As the adhesive layer 2, well-known materials used as adhesive layers for adhesive tapes, etc., such as acrylic adhesives, rubber adhesives, polysiloxane adhesives, etc. can be used. In particular, when using silicone rubber as the rubber binder, it is preferable to use a silicone-based adhesive as the material of the adhesive layer in order not to reduce the adhesion between the electromagnetic wave absorbing layer and the adhesive layer.

In addition, in order to adjust the adhesion to the adherend and reduce the paste residue, an adhesion-imparting agent or a cross-linking agent can also be used. The adhesion force for the adhered body is preferably 5N/10mm~12N/10mm. If the adhesion force is lower than 5N/10mm, the electromagnetic wave absorbing sheet will be easily peeled off from the adhered body and offset. In addition, when the adhesion force is greater than 12N/10mm, the electromagnetic wave absorbing sheet will not be easily peeled off from the adhered body.

In addition, the thickness of the adhesive layer 2 is preferably 20 μm to 100 μm. When the thickness of the adhesive layer is thinner than 20 μm, the adhesion force becomes smaller, and the electromagnetic wave absorbing sheet is easily peeled off from the adherend and displaced. When the thickness of the adhesive layer is larger than 100 μm, the thickness of the entire electromagnetic wave absorbing sheet becomes thicker, and the flexibility may become smaller. In addition, when the adhesive layer 2 is thick When used, the electromagnetic wave absorbing sheet is not easily peeled off from the attached body. In addition, when the cohesive force of the adhesive layer 2 is small, the electromagnetic wave absorbing sheet is peeled off, which may cause paste to remain on the adherend.

However, in this specification, the adhesive layer 2 may be a non-releasably adhered adhesive layer 2, or the adhesive layer 2 may be releasably adhered.

In addition, when the electromagnetic wave absorbing sheet is adhered to a specific surface, even if the electromagnetic wave absorbing sheet does not have the adhesive layer 2, it can be formed as a surface on the side of the member on which the electromagnetic wave absorbing sheet is arranged, which has adhesiveness and is attached only to the surface. The electromagnetic wave absorbing layer 1 is an electromagnetic wave absorbing sheet. In addition, by using double-sided tape or adhesive, the electromagnetic wave absorbing sheet can be attached to a specific location. In this regard, the adhesive layer 2 is not an essential component of the electromagnetic wave absorbing sheet shown in this embodiment. However, the electromagnetic wave absorbing sheet is provided with the adhesive layer 2 so that it can adhere to electromagnetic waves without using double-sided tape or adhesive. This sheet is ideal because it can be absorbed into specific areas.

[Extension of electromagnetic wave absorbing sheet]

Next, the extension in the in-plane direction of the electromagnetic wave absorbing sheet according to this embodiment will be described.

FIG. 3 is a graph showing the relationship between the stress (tensile stress) applied in the in-plane direction and the elongation of the electromagnetic wave absorbing sheet in the electromagnetic wave absorbing sheet according to this embodiment. Figure 3(a) shows the relationship between stress and elongation of the electromagnetic wave absorbing sheet that breaks when the maximum elongation is exceeded. In addition, Figure 3(b) shows that when the maximum elongation is exceeded, plastic deformation is caused. The relationship between stress and elongation in electromagnetic wave absorbing sheets.

Here, "elongation" refers to the elongation amount of an electromagnetic wave-absorbing sheet that is elongated by applying stress in one direction, and is expressed in % as a value divided by the original length. That is, when the length at zero stress is L1 and the length when a specific stress is applied is L2, the "elongation" when the specific stress is added is expressed as (L2-L1)/L1×100. However, this "elongation" is also called "offset".

As shown in Figure 3(a), in an electromagnetic wave-absorbing sheet that breaks when the maximum elongation is exceeded, when the stress applied from the outside becomes larger until it reaches 170% of the maximum elongation, the electromagnetic wave will appear in a slightly straight line. The elongation of the absorbent sheet increases (part of symbol 11). Then, when stress is applied exceeding 170% of the maximum elongation, the electromagnetic wave absorbing sheet breaks, and the value of the elongation increases from 170% of the maximum elongation (part of symbol 12).

On the other hand, as shown in Figure 3(b), in the electromagnetic wave absorbing sheet that causes plastic deformation when the maximum elongation is exceeded, the stress applied until the elongation reaches 30%, which shows the elongation of the maximum stress. As it becomes larger, the elongation rate rises relatively gently (part of symbol 13). Thereafter, when the electromagnetic wave absorbing sheet is further stretched after reaching the elongation rate of 30% showing the maximum stress, plastic deformation occurs until the elongation rate reaches 230%, and the electromagnetic wave absorbing sheet is elongated (part 14). Therefore, the stress decreases slowly. However, due to plastic deformation, the electromagnetic wave absorbing sheet in the state indicated by reference numeral 14 loses elasticity, and the length of the sheet does not become shorter even if the force to stretch the electromagnetic wave absorbing sheet is released.

However, the elastic deformation range of the rubber material used as the binder 1b can be adjusted by using an appropriately selected vulcanizer. In addition, depending on the intended use, etc., when an electromagnetic wave absorbing sheet that cannot be broken is required, it is considered that a form that is not broken but is plastically deformed is also effective.

In addition, since the input impedance value in the extended size range of the electromagnetic wave absorbing sheet greatly deviates from the impedance value in the air and impedance integration cannot be achieved, the upper limit of the range in which the electromagnetic wave absorbing capability decreases is set to 200%. In addition, if the extension of the electromagnetic wave absorbing sheet is too large, the thickness of the electromagnetic wave absorbing sheet will become thinner, and the density of the electromagnetic wave absorbing material will decrease, so the electromagnetic wave absorbing capability will also decrease. Furthermore, in a state where the electromagnetic wave absorbing sheet is extended by more than 200%, the flexibility or bendability of the electromagnetic wave absorbing sheet decreases.

On the other hand, the elongation of the electromagnetic wave absorbing sheet is smaller than 20%, and when it is attached to a curved substrate, it is not fully elongated, and the workability is reduced. In addition, the electromagnetic wave absorbing sheet of the present embodiment cannot cope with the adhesion of the movable part that changes in shape, and cannot exhibit the elastic characteristics of the electromagnetic wave absorbing sheet of the present embodiment.

Here, the electromagnetic wave absorbing sheet according to this embodiment was actually made with different types of magnetic iron oxide and rubber binders, and the relationship between the tensile stress from the outside and the elongation of the electromagnetic wave absorbing sheet was measured.

The first electromagnetic wave absorbing sheet (Example 1) uses ε-iron oxide as the magnetic iron oxide and acrylic as the rubber binder. Rubber. Table 1 shows the materials used in the production of the first electromagnetic wave absorbing sheet and their proportions.

The second electromagnetic wave absorbing sheet (Example 2) uses ε-iron oxide as the magnetic iron oxide in the same manner as the first electromagnetic wave absorbing sheet, and uses silicone rubber as the rubber binder. Table 2 shows the materials used in the production of the second electromagnetic wave absorbing sheet and their proportions.

The third electromagnetic wave absorbing sheet (Example 3) uses strontium ferrite as the magnetic iron oxide, and uses silicone rubber as the rubber binder in the same manner as the second electromagnetic wave absorbing sheet. Table 3 shows the materials used in the production of the third electromagnetic wave absorbing sheet and their proportions.

Use a pressurized batch mixer to mix the materials of each composition shown in Tables 1 to 3, dilute the resulting mixture with 170 parts of methyl ethyl ketone, and use a filling oxidizer. Zirconium beads are sanded to prepare a dispersion.

The dispersion liquid shown in Table 1 was applied with a blade-type applicator on a polyethylene terephthalate (PET) sheet with a thickness of 38 μm that was peeled off through the polysiloxane coating.

In addition, on a polyethylene terephthalate (PET) sheet with a thickness of 38 μm that was peeled off with a non-polysilicone-based release agent, the above-mentioned Table 2 and Table 2 were coated with a blade-type applicator. The dispersion shown in 3.

The wet paint was dried at 80°C and subjected to a rolling process so that the thickness after rolling became 500 μm, forming an electromagnetic wave absorbing layer.

The electromagnetic wave absorbing layer with a thickness of 500 μm thus produced was stacked in five sheets and thermally compressed using a rolling device to produce a single-film electromagnetic wave absorbing layer of 2500 μm. However, the subsequent layer is not formed and an electromagnetic wave absorbing sheet composed of only the electromagnetic wave absorbing layer is produced.

As each electromagnetic wave absorbing sheet produced, Elongation was measured using a tensile testing machine. Specifically, a TGE-1kN type testing machine (product name) manufactured by Minebea Co., Ltd. was used, and a TT3E-200N load cell was used to measure the elongation of a 20 mm × 50 mm sheet at a tensile speed of 10 mm/min. Time elongation. However, the elongation measurement was performed in an environment with a temperature of 23°C and a humidity of 50% Rh.

The elongation of the three electromagnetic wave absorbing sheets measured in this manner is shown in Figure 4 .

In FIG. 4 , the elongation rate of the first electromagnetic wave absorbing sheet is shown by a solid line (symbol 15), the elongation rate of the second electromagnetic wave absorbing sheet is shown by a dotted line (symbol 16), and the elongation rate of the second electromagnetic wave absorbing sheet is shown by a two-dot chain line (symbol 17). ) shows the elongation of the third electromagnetic wave absorbing sheet.

As shown in Figure 4, the three electromagnetic wave absorbing sheets produced as examples were all electromagnetic wave absorbing sheets that broke when exceeding the maximum elongation shown in Figure 3 (a1), and the maximum elongation was 195 %~200%. As mentioned above, in the electromagnetic wave absorbing sheet of this embodiment, the maximum elongation is preferably 200%, and the three electromagnetic wave absorbing sheets produced are ideal in terms of electromagnetic wave absorbing properties, flexibility, and bendability. Scope person.

When the three electromagnetic wave absorbing sheets are compared, the first electromagnetic wave absorbing sheet 15 has the largest stress required to achieve the same elongation, while the third electromagnetic wave absorbing sheet 17 has the smallest stress. This is considered because the acrylic rubber used has a higher hardness than silicone rubber, and the silicone rubber used for the second electromagnetic wave absorbing sheet has a higher hardness than the silicone rubber used for the third electromagnetic wave absorbing sheet. In addition, in the first electromagnetic wave absorption In the thinning sheet, compared with using ε-iron oxide as the magnetic iron oxide, in the third electromagnetic wave absorbing sheet, using strontium ferrite as the magnetic iron oxide has a larger particle diameter, and the specific surface area becomes smaller and dispersed. It is thought that the hardness of the electromagnetic wave absorbing sheet can be suppressed due to the high resistance.

Next, regarding the electromagnetic wave absorbing sheet according to this embodiment, changes in the electromagnetic wave absorption characteristics when stress is applied to the sheet and the sheet is stretched are measured.

The measurement system uses the free space method to measure the electromagnetic wave absorption amount (electromagnetic wave attenuation amount) of the above-mentioned first electromagnetic wave absorbing sheet. Specifically, the millimeter wave network analyzer ME7838AN5250C (trade name) manufactured by Agilent Technologies Co., Ltd. is used, and the input wave (millimetre wave) of a specific frequency is irradiated on the electromagnetic wave absorbing sheet through a dielectric lens from the transmitting antenna. , measuring the electromagnetic waves transmitted through the receiving antenna arranged on the back side of the electromagnetic wave absorbing sheet. The intensity of the irradiated electromagnetic wave and the intensity of the transmitted electromagnetic wave are each grasped as voltage values, and the amount of electromagnetic wave attenuation is calculated in dB from the intensity difference.

FIG. 5 shows the electromagnetic wave absorption characteristics of the first electromagnetic wave absorbing sheet according to the present embodiment, when no tension is applied to the electromagnetic wave absorbing sheet, and when tension is applied to the electromagnetic wave absorbing sheet, the electromagnetic wave absorbing sheet is stretched. , a diagram of electromagnetic wave absorption characteristics in a state where its thickness is reduced.

In FIG. 5 , the solid line shown as symbol 21 shows the electromagnetic wave absorption characteristics in a state where no tension is applied to the electromagnetic wave absorbing sheet, that is, when the elongation rate is 0%. However, the thickness of the electromagnetic wave absorbing sheet at this time is 2500μm at the time of production.

As shown in Figure 5, the first electromagnetic wave absorbing sheet shows an electromagnetic wave absorption amount (attenuation amount from an incident electromagnetic wave transmitted through the back side) of 26 dB at the resonance frequency of 75.5 GHz of ε-iron oxide, which is an electromagnetic wave absorbing material. High electromagnetic wave absorption properties.

In this regard, when tension is applied to the electromagnetic wave absorbing sheet and the electromagnetic wave absorption characteristics are extended to an elongation rate of 75%, the electromagnetic wave absorption characteristics become a dotted line shown as 22 in FIG. 5 . However, the thickness of the electromagnetic wave absorbing sheet in this case is 1950 μm.

As shown by the dotted line shown as symbol 22 in Figure 5, in the electromagnetic wave absorbing sheet with an elongation rate of 75%, the electromagnetic wave absorption amount at 75.5 GHz is approximately 19 dB, which is the same as the case where the elongation rate is 0% (symbol 21). When compared, it was found that the electromagnetic wave absorption characteristics were reduced. This is thought to be caused by the fact that the thickness of the electromagnetic wave absorbing sheet becomes thinner when stretched in the in-plane direction, and the content of the electromagnetic wave absorbing material in the electromagnetic wave absorbing sheet substantially decreases in the direction through which the electromagnetic wave passes.

That is, in the case of the transmission-type electromagnetic wave absorbing sheet as the first electromagnetic wave absorbing sheet, when the electromagnetic wave absorbing sheet is stretched in the in-plane direction, it is found that the electromagnetic wave absorption characteristics are reduced.

Therefore, the present inventors measured the electromagnetic wave absorption characteristics when the elongation rate of the electromagnetic wave absorbing sheet was larger. In the electromagnetic wave absorbing sheet whose thickness became thinner due to stretching in the in-plane direction, the thickness and electromagnetic wave absorption of the electromagnetic wave absorbing sheet were measured. Quantitative relationship. The measurement results are shown in Figure 6 .

Figure 6 shows the above-mentioned first electromagnetic wave absorbing sheet and The third electromagnetic wave absorbing sheet is the relationship between the thickness of the electromagnetic wave absorbing sheet when the electromagnetic wave absorbing sheet is stretched in one direction in the plane and the electromagnetic wave absorption amount (the attenuation amount of the transmitted electromagnetic wave: the transmission attenuation amount) at the frequency of 75.5 GHz in its state. .

In Figure 6, what is shown by the black circle and the solid line 31 is the change in the electromagnetic wave absorption amount of the first electromagnetic wave absorbing sheet, and what is shown by the black square and the dotted line 32 is the electromagnetic wave absorption of the third electromagnetic wave absorbing sheet. Quantity changes.

As shown in Figure 6, in the first electromagnetic wave absorbing sheet and the third electromagnetic wave absorbing sheet, it can be confirmed that the absorption amount (31, 32) of the electromagnetic wave with a frequency of 75.5 GHz is slightly proportional to the thickness of the electromagnetic wave absorbing sheet, and When the electromagnetic wave absorbing sheet is stretched more strongly and its thickness becomes thinner, the electromagnetic wave absorbing properties decrease.

In this way, in the electromagnetic wave absorbing sheet of this embodiment, when a tensile force is applied to the sheet and the sheet is stretched, the electromagnetic wave absorption characteristics decrease linearly depending on the extent of the elongation at that time. From this point of view, as long as the electromagnetic wave absorbing sheet is in the elastic range within the range where the desired electromagnetic wave absorption amount is obtained and the maximum elongation of the electromagnetic wave absorbing sheet is within the range, the electromagnetic wave absorbing sheet can be stretched and used.

(Second Embodiment) [Reflective electromagnetic wave absorbing sheet]

Next, a specific embodiment of a so-called reflective electromagnetic wave absorbing sheet, which is formed on the back surface of the electromagnetic wave absorbing layer in the second structural example of the electromagnetic wave absorbing sheet disclosed in the present application, will be shown and explained. bright.

FIG. 7 shows the cross-sectional structure of the electromagnetic wave absorbing sheet according to the second embodiment.

However, FIG. 7 is the same as FIG. 1 illustrating the structure of the electromagnetic wave absorbing sheet according to the first embodiment. It is a diagram for easy understanding of the structure. The sizes and thicknesses of the members shown in the figure are not based on reality. Express the composition. In addition, the same components as those constituting the electromagnetic wave absorbing sheet of the first embodiment shown in FIG. 1 are assigned the same reference numerals, and detailed descriptions thereof are omitted.

The electromagnetic wave absorbing sheet disclosed in this application is used as an electromagnetic wave absorbing material and forms an electromagnetic wave absorbing layer together with a rubber binder through the magnetic properties of magnetic iron oxide such as ε-iron oxide, barium ferrite magnets, and strontium ferrite. It resonates and absorbs electromagnetic waves. Therefore, in addition to being configured as a transmissive electromagnetic wave absorbing sheet without the reflective layer shown in the first embodiment, the electromagnetic wave absorbing layer can be used on the surface opposite to the side on which the electromagnetic wave is incident, and has the ability to reflect the electromagnetic wave. layer, as a component of the reflective electromagnetic wave absorbing sheet.

The electromagnetic wave absorbing sheet shown in the second embodiment is in contact with the back side (the lower side in FIG. 7 ) of the electromagnetic wave absorbing layer 1 composed of magnetic iron oxide 1a as an electromagnetic wave absorbing material and a rubber binder 1b. The surface of the electromagnetic wave absorbing layer 1 forms a reflecting layer 3 .

However, in the electromagnetic wave absorbing sheet of the second embodiment shown in FIG. 7, an adhesive layer 2 that can adhere to the electromagnetic wave absorbing sheet at a specific place is formed on the back side of the reflective layer 3. Related to the above-mentioned first embodiment In the case of the electromagnetic wave absorbing sheet, similarly, in the electromagnetic wave absorbing sheet of the second embodiment, the adhesive layer 2 is not an essential component, and an electromagnetic wave absorbing sheet without the adhesive layer 2 can also be formed. However, the electromagnetic wave absorbing sheet is formed by electromagnetic wave absorption. The sheet is preferably composed of an adhesive layer 2 because the electromagnetic wave absorbing sheet can be adhered to a specific location without using double-sided tape or adhesive.

The reflective layer 3 may be a metal layer formed in close contact with the back surface of the electromagnetic wave absorbing layer 1 . However, in the electromagnetic wave absorbing sheet of this embodiment, since the rubber binder 1b is used and the electromagnetic wave absorbing sheet has elasticity, a mesh-shaped conductor or silver nanowire (Ag) is used as the reflective layer 3. -NW), conductive polymer film, etc., even when the electromagnetic wave absorbing layer 1 is stretched, its surface impedance does not rise and maintains an impedance value of approximately 1Ω/□.

As a method of forming the reflective layer on the back side of the electromagnetic wave absorbing layer 1, silver nanowires or conductive polymers can be sprayed or coated on the back side of the electromagnetic wave absorbing sheet. In addition, the same rubber binder as the reflective layer can be used to make the reflective layer 3 with dispersed silver nanowires or conductive polymers, and the elastic reflective layer 3 can be heat-pressed on the electromagnetic wave absorbing layer. Furthermore, A method for forming the electromagnetic wave absorbing layer 1 on the reflective layer 3 by coating the elastic reflective layer 3 with a paint for forming the electromagnetic wave absorbing layer 1 .

However, the type of metal constituting the reflective layer 3 is not particularly limited. In addition to silver used as nanowires, metals such as aluminum, copper, and chromium that have an electrical resistance as small as possible and have high corrosion resistance may also be used. .

In the electromagnetic wave absorbing sheet according to the second embodiment shown in FIG. 7, by providing the reflective layer 3 on the back surface of the electromagnetic wave absorbing layer 1, the electromagnetic wave penetrating the electromagnetic wave absorbing sheet can be reliably avoided. Therefore, it can be optimally used as an electromagnetic wave absorbing sheet that prevents leakage of electromagnetic waves emitted to the outside from electrical circuit components driven at high frequencies.

[Extension of reflective electromagnetic wave absorbing sheet]

In the reflective electromagnetic wave absorbing sheet of the second embodiment, similarly to the electromagnetic wave absorbing sheet of the first embodiment, when the electromagnetic wave absorbing layer 1 is stretched and stretched, the thickness of the electromagnetic wave absorbing layer 1 changes depending on the thickness. Changes in the input impedance value produce changes in electromagnetic wave absorption characteristics due to mismatch in impedance integration, and changes in electromagnetic wave absorption characteristics in which the amount of electromagnetic wave absorbing material in the portion through which electromagnetic waves pass through the electromagnetic wave absorbing layer 1 decreases.

Furthermore, in the case of a reflective electromagnetic wave absorbing sheet, there is a problem that the input impedance value of the electromagnetic wave absorbing sheet must be integrated with the impedance value in the air. The input impedance value of the electromagnetic wave absorbing sheet is greatly different from the impedance value in air of 377Ω (strictly speaking, the impedance value in vacuum). This is because when the electromagnetic wave is incident on the electromagnetic wave absorbing sheet, reflection or scattering occurs, causing damage. And it is the result of the electromagnetic wave absorption characteristics of the reflective electromagnetic wave absorbing sheet, that is, the characteristic of reducing the reflected wave of the incident electromagnetic wave.

Here, in the electromagnetic wave absorbing sheet including magnetic iron oxide as the electromagnetic wave absorbing material, the impedance Z _in of the electromagnetic wave absorbing layer 1 is expressed as the following mathematical formula (1).

In the above formula (1), μ _r is the complex magnetic permeability of the electromagnetic wave absorbing layer 1 , ε _r is the complex dielectric permittivity of the electromagnetic wave absorbing layer 1 , λ is the wavelength of the incident electromagnetic wave, and d is the thickness of the electromagnetic wave absorbing layer 1 . Therefore, when the electromagnetic wave absorbing sheet is extended, the thickness d of the electromagnetic wave absorbing layer 1 becomes smaller and the content of magnetic iron oxide in the electromagnetic wave absorbing material decreases, the magnetic permeability and dielectric constant of the electromagnetic wave absorbing layer 1 change simultaneously. . As a result, the input impedance value (Zin) of the electromagnetic wave absorbing sheet is determined by the thickness of the binder 1b forming the electromagnetic wave absorbing layer 1. This means that when the thickness of the electromagnetic wave absorbing sheet changes due to expansion and contraction of the electromagnetic wave absorbing sheet, the input impedance of the electromagnetic wave absorbing sheet changes. The value (Zin) changes.

According to this situation, the inventors of the present invention are not in a steady state of the electromagnetic wave absorbing sheet, that is, a state in which the elongation rate of the electromagnetic wave absorbing sheet is 0% without any external force, but in the state where the electromagnetic wave absorbing sheet is in By integrating the input impedance value of the electromagnetic wave absorbing sheet with the input impedance value in the air by extending it at a certain degree of elongation, it is thought that optimal absorption can be achieved in a practical state under a wider range of conditions. And the electromagnetic wave thin slice of the incident electromagnetic wave.

Therefore, a reflective electromagnetic wave absorbing sheet was actually produced to verify the effectiveness of absorbing electromagnetic waves in a state of being extended at a certain elongation rate. The effect of integrating the input impedance value of the thin film with the impedance value in the air.

First, as the fourth electromagnetic wave absorbing sheet (Example 4), a reflective electromagnetic wave absorbing sheet was produced.

The reflective electromagnetic wave absorbing sheet of the fourth electromagnetic wave absorbing sheet is the same as the above-mentioned first electromagnetic wave absorbing sheet. The mixed materials having the composition shown in Table 1 are mixed with a pressurized batch mixer. After diluting the resulting mixture with 170% of ethyl ketone, a dispersion liquid was prepared using sand milling filled with zirconia beads.

The above-mentioned dispersion liquid was applied with a leaf-type applicator on a 38 μm-thick polyethylene terephthalate (PET) sheet that had been peeled off through the polysiloxane coating.

The wet paint was dried at 80° C. and subjected to a rolling process so that the thickness after rolling was 410 μm, forming an electromagnetic wave absorbing layer.

Then, a reflective layer is formed on the back side of the electromagnetic wave absorbing layer.

The reflective layer is formed by coating silver nanowires on the back side of the electromagnetic wave absorbing layer.

The electromagnetic wave absorption characteristics of the fourth electromagnetic wave absorbing layer were measured using the free space method in the same manner as the electromagnetic wave absorption measurement of the above-mentioned transmission type electromagnetic wave absorbing sheet. However, in order to measure the characteristics of the reflective electromagnetic wave absorbing sheet, the output of the electromagnetic wave incident on the electromagnetic wave absorbing sheet and the output of the reflected wave released from the electromagnetic wave absorbing sheet were placed in front of the electromagnetic wave absorbing sheet. test sideways Certainly.

However, the electromagnetic wave absorbing sheet produced as the fourth electromagnetic wave absorbing sheet and having an electromagnetic wave absorbing layer containing acrylic rubber as a binder as a main component is shown in Figure 3(b) and determines a specific maximum elongation (specifically, , the case of the fourth electromagnetic wave absorbing sheet is 30%), and in the case of exceeding this, the electromagnetic wave absorbing sheet causes plastic deformation.

In addition, in the fourth electromagnetic wave absorbing sheet, when the elongation is 11%, the thickness becomes 370 μm, and the input impedance value of the electromagnetic wave absorbing sheet in this state of 370 μm thickness becomes 377Ω, which is integrated with the impedance value in air.

Figure 8 shows changes in the electromagnetic wave absorption characteristics of the fourth electromagnetic wave absorbing sheet when the elongation is changed.

In FIG. 8 , the solid line indicated by symbol 41 shows the electromagnetic wave absorption amount (the attenuation amount of the reflected electromagnetic wave) in the steady state of the fourth electromagnetic wave absorbing sheet, that is, the elongation rate is 0%. At this time, the thickness of the fourth electromagnetic wave absorbing sheet was 410 μm. In addition, the dotted line indicated by symbol 42 indicates a state in which the fourth electromagnetic wave absorbing sheet is stretched with an elongation rate of 3%. At this time, the thickness of the electromagnetic wave absorbing sheet is 400 μm. Moreover, a one-point chain line shown by the symbol 43 is the state in which the fourth electromagnetic wave absorbing sheet is extended with an elongation rate of 11%. The thickness of the electromagnetic wave absorbing sheet at this time is 370 μm, and a two-point chain line shown by the symbol 44 is When the fourth electromagnetic wave absorbing sheet is stretched with an elongation rate of 22%, the thickness of the electromagnetic wave absorbing sheet at this time is 335 μm.

As shown in Figure 8, in the fourth electromagnetic wave absorbing sheet, the electromagnetic wave absorbing sheet in a state where the input impedance is integrated is stretched at an elongation rate of 11%. The amount of electromagnetic wave absorption in this state reaches a maximum of about 23dB. In this regard, it is shown that the amount of electromagnetic wave absorption when the elongation is 3% is about 18dB, and when the elongation is 0%, the amount of electromagnetic wave absorption is about 15dB. When the electromagnetic wave absorbing sheet is stretched, the electromagnetic wave absorption decreases, becoming the same as shown in The electromagnetic wave absorption characteristics of the first electromagnetic wave absorbing sheet are shown in Figure 5 and Figure 6, respectively. That is, in the second embodiment, when the elongation becomes higher, the amount of electromagnetic wave absorption becomes higher, which is the opposite result to that in the first embodiment.

This is because in a reflective electromagnetic wave absorbing sheet, as the electromagnetic wave absorbing sheet extends, the amount of electromagnetic wave absorbing material in the part through which the electromagnetic wave passes through the electromagnetic wave absorbing layer decreases. The amount of electromagnetic wave absorption decreases through the electromagnetic wave absorbing layer. It is caused by the change in the input impedance value due to the change in thickness, and the reduction in electromagnetic wave absorption through the mismatch of impedance integration shows a stronger impact.

Accordingly, in the reflective electromagnetic wave absorbing sheet, the input impedance value of the electromagnetic wave absorbing sheet and the impedance value in the air are integrated by taking into account the change in the input impedance value of the electromagnetic wave absorbing layer accompanying the change in the elongation of the electromagnetic wave absorbing sheet. Whichever is better.

Figure 9 is a graph showing the relationship between the thickness of the electromagnetic wave absorbing sheet and the amount of electromagnetic wave absorption in the fourth electromagnetic wave absorbing sheet, and Figure 8 shows the change in the electromagnetic wave absorption characteristics of the fourth electromagnetic wave absorbing sheet in another form of expression. picture.

In Figure 9, symbol 51 shows a state with an elongation of 0% (thickness 410 μm), symbol 52 shows a state with an elongation of 3% (thickness 400 μm), and symbol 52 shows a state with an elongation of 11% (thickness 410 μm). 370 μm), symbol 54 shows the value of electromagnetic wave absorption in the state with an elongation of 22% (thickness 335 μm).

As shown in Figure 9, in the fourth electromagnetic wave absorbing sheet, the electromagnetic wave absorbing sheet is stretched at an elongation rate of 11% and has a thickness of 370 μm. When the input impedance is integrated, the elongation rate is 0%. Until the elongation reaches a state of 22%, the electromagnetic wave absorption amount is maintained below 15 dB, which is considered ideal for practical use. That is, the electromagnetic wave absorption amount can maintain a characteristic of 92% or more.

In this way, the electromagnetic wave absorbing sheet according to the present embodiment is an electromagnetic wave absorbing sheet with extensibility such that the maximum elongation rate in the elastic domain is 20% to 200%. When used in this range, the electromagnetic wave absorption capacity can be maintained while maintaining the electromagnetic wave absorption capacity. flakes. Furthermore, the electromagnetic wave absorbing sheet of this embodiment has a maximum elongation rate of the elastic domain of 20% to 200%, and can maintain a specific electromagnetic wave absorption amount when the elastic domain is extended by 5% to 75% of the maximum elongation rate. Electromagnetic wave absorbing sheet.

As can be understood from the above description, in an electromagnetic wave absorbing sheet including a magnetic member that absorbs electromagnetic waves through magnetic resonance as an electromagnetic wave absorbing material, when the sheet has elasticity and its thickness changes, the electromagnetic wave is stretched to a certain extent. In the state of the absorbing sheet, it is better to integrate the input impedance value of the electromagnetic wave absorbing layer with the impedance value in the air.

This case has been described as the second embodiment, and there is no limitation on the structure of the electromagnetic wave absorbing material that generates magnetic resonance for electromagnetic waves in a high frequency band of millimeter waves or higher. However, the electromagnetic wave absorbing material dispersed in the rubber binder absorbs electricity through the magnetic resonance. Among all elastic reflective electromagnetic wave absorbing sheets, it is better to extend the electromagnetic wave absorbing sheet to a certain extent so that the input impedance of the electromagnetic wave absorbing layer can be integrated with the impedance in the air.

In the fourth electromagnetic wave absorbing sheet as a specific example described in the second embodiment, the electromagnetic wave absorbing sheet with a maximum elongation of 30% has an elongation of 11%, that is, in comparison with the maximum elongation, it is about 37 % elongation state, the input impedance value is integrated. The basis for impedance integration in a state of being stretched by a predetermined amount of elongation is determined by the type of deformation of the rubber material used as a binder when it is stretched beyond the maximum elongation. However, for the maximum elongation Taking the state of 5~75% extension as a standard, in a wide range of practical applications, the difference between the input impedance value of the electromagnetic wave absorbing sheet and the impedance value in the air becomes zero, and the incidence of the electromagnetic wave absorbing sheet can be restricted. Electromagnetic waves are the reduction of electromagnetic wave absorption characteristics caused by reflection or scattering.

In addition, from the opposite point of view, by maintaining the input impedance value of the electromagnetic wave absorbing sheet at a value close to the impedance value in air of 377Ω in the elastic domain, it can be obtained that within the practical range of the electromagnetic wave absorbing sheet, The term of impedance integration above a certain level can avoid the reduction of the electromagnetic wave absorption amount of the electromagnetic wave absorbing sheet. According to the review of the inventors, the value range is 360Ω~450Ω. When the input impedance value of the electromagnetic wave absorbing sheet is smaller than 260Ω or larger than 450Ω, the electromagnetic wave incident on the electromagnetic wave absorbing sheet is generated in the space between the surface of the electromagnetic wave absorbing sheet and the boundary surface of the electromagnetic wave absorbing sheet. Due to large scattering and reflection, the electromagnetic wave cannot be absorbed by the electromagnetic wave possessed by the sheet itself. A player of wave absorption properties.

However, when the input impedance value of the electromagnetic wave absorbing sheet is integrated with the impedance value in the air, it is not a state in which the electromagnetic wave absorbing sheet is not extended with an extension amount of 0%. Instead, the maximum elongation of the electromagnetic wave absorbing sheet is When the state of 5 to 75% elongation is used as a standard, even if the amount of elongation of the electromagnetic wave absorbing sheet changes, the situation in which electromagnetic waves can be absorbed well is not limited to the reflective type electromagnetic wave absorbing sheet explained as the second embodiment. , and the same applies to the transmission type electromagnetic wave absorbing sheet described in the first embodiment.

The transmission type electromagnetic wave absorbing sheet shown in the first embodiment is different from the reflective electromagnetic wave absorbing sheet shown in the second embodiment in that the intensity of the electromagnetic wave scattered and reflected on the side where the electromagnetic wave is incident is not directly affected. It affects the level of electromagnetic wave absorption characteristics (electromagnetic wave attenuation), but for example, while suppressing the external leakage of electromagnetic waves from an electrical circuit that becomes the source of electromagnetic wave radiation, it does not affect the electromagnetic wave as a reflected wave that appears on the surface of the electromagnetic wave absorbing sheet. It is considered that the reflected wave on the electromagnetic wave incident side of the transmission type electromagnetic wave absorbing sheet has not been reduced, such as adverse effects on the electrical circuit. In this case, even if the elongation of the transmissive electromagnetic wave absorbing sheet changes, it is better to keep the input impedance close to the impedance in the air. The maximum elongation of the electromagnetic wave absorbing sheet is 5 to 75%. Using the extended state as a reference, it is better to set the input impedance to 377Ω.

In addition, as shown in FIG. 8 , in the fourth electromagnetic wave absorbing sheet, even if the elongation changes and the thickness changes, in each case Among the electromagnetic wave absorption characteristics of thickness, the frequency of the input electromagnetic wave that shows the maximum amount of absorption remains unchanged at 75.5 GHz. This is because, like the first electromagnetic wave absorbing sheet described in the first embodiment, the electromagnetic wave absorbing sheet disclosed in the present application absorbs the incident electromagnetic waves through the magnetic resonance of the magnetic iron oxide as the electromagnetic wave absorbing material, showing the maximum If the frequency of electromagnetic waves is the same as that of the electromagnetic wave absorbing material, the absorption characteristics are not affected by the thickness of the electromagnetic wave absorbing layer.

In this regard, the electromagnetic wave absorbing sheet currently on the market as an elastic structure that can absorb electromagnetic waves in the millimeter wave region is formed by laminating a dielectric layer and a reflective layer, thereby shifting the phase of the incident electromagnetic wave. 1/2 wavelength attenuates the intensity of the reflected wave, so-called wavelength interference type electromagnetic wave absorbing sheet. In the wavelength interference type electromagnetic wave absorbing sheet, when the thickness of the dielectric layer changes, the wavelength of the absorbed electromagnetic wave changes. When the electromagnetic wave absorbing sheet is elastic and extends at a specific elongation, its thickness changes. At this time, the value of the peak frequency of the absorbed electromagnetic wave also changes. As a result, for example, the electromagnetic wave absorbing sheet arranged to absorb electromagnetic waves of specific wavelengths such as in-vehicle radars may have a problem of degrading the absorption characteristics of electromagnetic waves of a desired frequency due to its elasticity.

In the case of the electromagnetic wave absorbing sheet disclosed in this application, since the frequency of the maximum absorbed electromagnetic wave does not change, no such problem occurs.

(Other composition)

However, in the above-mentioned first and second embodiments, the magnetic iron oxide as the electromagnetic wave absorbing material contained in the electromagnetic wave absorbing layer is mainly explained by exemplifying the structure using ε-iron oxide. As mentioned above, by using ε-iron oxide, it is possible to form an electromagnetic wave absorbing sheet that absorbs electromagnetic waves in the millimeter range of 30 GHz to 300 GHz. In addition, by using rhodium or the like as a metal material that replaces the Fe position, an electromagnetic wave absorbing sheet that absorbs electromagnetic waves of 1 MHz, which is the highest frequency specified for electromagnetic waves, can be realized.

However, in the electromagnetic wave absorbing sheet disclosed in the present application, the magnetic iron oxide used as the electromagnetic wave absorbing material of the electromagnetic wave absorbing layer is not limited to ε-iron oxide.

Barium ferrite magnets, which are hexagonal ferrite magnets as electromagnetic absorbers of the ferrite magnetic system, and strontium ferrite systems, which are also shown in some of the embodiments, play an important role in electromagnetic waves in the range of several gigahertz to tens of gigahertz. Good electromagnetic wave absorption properties. Therefore, in addition to epsilon-iron oxide, it is possible to form an electromagnetic wave absorbing layer by using particles of magnetic iron oxide, which has electromagnetic wave absorption properties in the millimeter range of 30 GHz to 300 GHz, and a rubber binder. An elastic electromagnetic wave absorbing sheet that absorbs electromagnetic waves in the millimeter wave zone.

However, for example, compared with the particles of ε-iron oxide exemplified in the above embodiment, the particle diameter of the hexagonal ferrite magnet is about several μm larger, and the particle shape is not approximately spherical. Platy or needle-shaped crystals. Therefore, when forming a magnetic paint using a rubber binder, it is necessary to use a dispersant or adjust the mixing conditions with the binder to disperse the magnetic paint as uniformly as possible in the state where the magnetic paint is applied. It is preferable to adjust the state of the magnetic iron oxide powder in the electromagnetic wave absorbing layer and adjust the void ratio to be as small as possible.

The electromagnetic wave absorbing sheet described in the above embodiment is made of rubber as a binder constituting the electromagnetic wave absorbing layer, and can realize an elastic electromagnetic wave absorbing sheet. In particular, as an electromagnetic wave absorbing material, magnetic iron oxide that generates magnetic resonance in a high frequency band above the millimeter band absorbs high frequency electromagnetic waves and can realize an elastic electromagnetic wave absorbing sheet.

However, when an electromagnetic wave absorbing sheet using magnetic iron oxide that absorbs electromagnetic waves through magnetic resonance is used as the electromagnetic wave absorbing material, a larger electromagnetic wave can be achieved by increasing the volume content of the electromagnetic wave absorbing material in the electromagnetic wave absorbing sheet. absorption effect. On the other hand, in an electromagnetic wave absorbing sheet having an electromagnetic wave absorbing layer composed of a rubber binder and an electromagnetic wave absorbing material, the electromagnetic wave absorbing material must be included in order to ensure elasticity through the use of the binder. The upper limit of volume content rate. In the electromagnetic wave absorbing sheet disclosed in the present application, when the volume content of the electromagnetic wave absorbing layer of magnetic iron oxide in the electromagnetic wave absorbing material is 30% or more, especially in the case of a reflective electromagnetic wave absorbing sheet, -15 dB can be ensured. The above reflection attenuation amount.

In addition, the volume content of the rubber binder in the electromagnetic wave absorbing layer is preferably 40% to 70%. By setting the volume content of the rubber binder as this range, it is easy to set the maximum elongation of the elastic domain in one direction within the surface of the electromagnetic wave absorbing sheet to the desired range of 20% to 200%.

However, in the above description, as a method of forming the electromagnetic wave absorbing layer, a method of preparing a magnetic paint, applying it, and drying it has been described. As a method for producing the electromagnetic wave absorbing sheet disclosed in the present application, in addition to the method of applying the above-mentioned magnetic paint, it is conceivable to use an extrusion molding method, for example.

More specifically, magnetic iron oxide powder, a rubber binder, and a dispersant are mixed in advance if necessary, and the mixed materials are supplied to the plastic cylinder from the resin supply port of the extrusion molding machine. within. However, as the extrusion molding machine, a normal press having a plastic cylinder, a die installed at the front end of the plastic cylinder, a pusher disposed freely rotatable in the plastic cylinder, and a driving mechanism for driving the pusher can be used. Out of the molding machine. The molten material plasticized by the belt heater of the extrusion molding machine is sent to the front through the rotation of the propeller, and is extruded into a thin sheet from the front end. An electromagnetic wave absorbing sheet with a specific thickness can be obtained by drying, press-forming, and calendering the extruded material.

Furthermore, as a method of forming the electromagnetic wave absorbing layer, a magnetic composite containing magnetic iron oxide powder and a rubber binder is produced, and the magnetic composite is press-molded to a specific thickness.

Specifically, first, a magnetic composite of an electromagnetic wave absorbing composition is produced. The magnetic composite is a mixture of magnetic iron oxide powder and rubber binder, and a cross-linking agent is mixed into the resulting mixture to adjust the viscosity.

As an example, the magnetic composite of the electromagnetic wave absorbing composition obtained in this way is pressed with a temperature using a hydraulic press. 170℃, cross-linked and formed into thin sheets. After that, it is cross-linked twice in a constant temperature bath at a temperature of, for example, 170°C, and can be used as an electromagnetic wave absorbing sheet of a specific shape.

In the above-mentioned embodiment, the electromagnetic wave absorbing sheet constituting the electromagnetic wave absorbing layer is described as a single layer. However, the electromagnetic wave absorbing layer may be formed by laminating a plurality of layers. In the case of the transmission-type electromagnetic wave absorbing sheet shown in the first embodiment, if the electromagnetic wave absorbing layer has a thickness of a certain level or more, the electromagnetic wave absorbing properties are improved. In addition, in the reflective electromagnetic wave absorbing sheet shown as the second embodiment, the electromagnetic wave absorption characteristics can be improved by adjusting the thickness of the electromagnetic wave absorbing layer to adjust the input impedance value and the impedance value in air. Therefore, when the electromagnetic wave absorbing layer cannot be formed with a specific thickness in one layer due to the characteristics of the electromagnetic wave absorbing material or binder forming the electromagnetic wave absorbing layer, it is effective to form the electromagnetic wave absorbing layer as a laminate.

Industrial utilization possibilities

The electromagnetic wave absorbing sheet disclosed in the present application absorbs electromagnetic waves in high-frequency bands above the millimeter band, and is useful as an elastic electromagnetic wave absorbing sheet.

1‧‧‧Electromagnetic wave absorption layer

1a‧‧‧ε-iron oxide (magnetic iron oxide)

1b‧‧‧Rubber binder

2‧‧‧Adhering layer

Claims (8)

An electromagnetic wave absorbing sheet, which is an electromagnetic wave absorbing sheet having an electromagnetic wave absorbing layer composed of an electromagnetic wave absorbing material that generates magnetic resonance in a frequency band above the millimeter wave band and a binder made of magnetic iron oxide and rubber, characterized by the electromagnetic wave absorbing sheet The thickness is 300 μm or more and 2500 μm or less, and the maximum elongation of the elastic domain in one direction within the surface of the electromagnetic wave absorbing sheet is 20 to 200%. For example, in the electromagnetic wave absorbing sheet described in claim 1, the magnetic iron oxide is at least one selected from ε-iron oxide or strontium ferrite magnetic powder. For example, in the electromagnetic wave absorbing sheet described in claim 2, part of the Fe positions of the ε-iron oxide is replaced with a trivalent metal atom. For example, in the electromagnetic wave absorbing sheet described in claim 1 or 2, either acrylic rubber or silicone rubber is used as the rubber binder. For example, the electromagnetic wave absorbing thin film described in item 1 or 2 of the patent application scope piece, wherein the input impedance value of the electromagnetic wave absorbing layer in the state of extending the maximum elongation rate of 5 to 75% of the elastic domain matches the impedance value in the air. For example, in the electromagnetic wave absorbing sheet described in item 1 or 2 of the patent application, the input impedance value when the electromagnetic wave absorbing layer is extended within the elastic domain is 360Ω~450Ω. For example, in the electromagnetic wave absorbing sheet described in claim 1 or 2, a reflective layer that reflects electromagnetic waves transmitted through the electromagnetic wave absorbing layer is formed on one surface in contact with the electromagnetic wave absorbing layer. For example, the electromagnetic wave absorbing sheet described in claim 1 or 2 further includes an adhesive layer that can adhere to the electromagnetic wave absorbing sheet.