TWI756419B - Electromagnetic wave absorbing sheet - Google Patents

Abstract

The present invention relates to an electromagnetic wave absorbing sheet which can well absorb high frequency electromagnetic waves in the millimeter wave band and above, and realizes that the electromagnetic wave absorbing sheet is excellent in flexibility and can be easily arranged in a desired portion. The electromagnetic wave absorbing layer (1) having the magnetic iron oxide (1a) that generates magnetic resonance in the frequency band above the millimeter wave band and the resin-made binder (1b) absorbs the magnetic resonance through the magnetic resonance of the magnetic iron oxide. An electromagnetic wave absorbing sheet for irradiated electromagnetic waves, wherein, in the elastic deformation range, when the belt-shaped sheet is bent, the distance d between the inner side surfaces of the sheets at a position L of 10 mm from the bent portion of the sheet is set as 10 mm. The value obtained by dividing the cross-sectional area D (mm ² ) of the sheet by the value of the flexibility evaluation value F (g/mm ² ) indicated by the value of the necessary weight (g) is greater than 0 and less than 6.

Description

Electromagnetic wave absorbing sheet

The present disclosure relates to an electromagnetic wave absorbing sheet for absorbing electromagnetic waves, and particularly, an electromagnetic wave absorbing sheet having excellent flexibility, a magnetic material that absorbs electromagnetic waves through magnetic resonance, and an electromagnetic wave absorbing sheet that absorbs high-frequency electromagnetic waves above the millimeter wave band.

In order to avoid the influence of leakage electromagnetic waves released to the outside from the electrical circuit or the influence of the electromagnetic waves reflected unexpectedly, an electromagnetic wave absorbing sheet for absorbing electromagnetic waves is used.

In recent years, as a centimeter wave having a frequency band of several gigahertz (GHz), and a frequency band of 30 GHz to 300 GHz, it is used in mobile communication such as mobile phones, wireless LAN, automatic fee collection system (ETC), etc. The millimeter-wave band of frequencies, the electromagnetic waves in the high frequency band exceeding the millimeter-wave band, and the technical research using electromagnetic waves having a frequency of 1 megahertz (THz) are also progressing.

Corresponding to the technical trend of utilizing such higher frequency electromagnetic waves, in the electromagnetic wave absorber that absorbs unnecessary electromagnetic waves or the electromagnetic wave absorbing sheet formed into a sheet shape, it is also possible to absorb electromagnetic waves in the gigahertz band to the megahertz band. Demand has increased.

As an electromagnetic wave absorber that absorbs electromagnetic waves in such a high frequency band, ε-iron oxide (ε-Fe ₂ O ₃ ) having a magnetically changing phase and exhibiting electromagnetic wave absorbing performance in the range of 25 to 100 GHz has been proposed. An electromagnetic wave absorber with a filling structure of crystal particles (refer to Patent Document 1). In addition, as 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. proposal (refer to Patent Document 2). Prior Art Documents Patent Documents

Patent Document 1: Japanese Patent Laid-Open No. 2008-60484 Patent Document 2: Japanese Patent Laid-Open No. 2016-135737

The problem to be solved by the invention

When shielding leakage electromagnetic waves from sources that generate electromagnetic waves, or when protecting specific electronic circuit components from unpredictable reflected waves or electromagnetic waves incident from outside, the electromagnetic wave absorbing member is arranged to cover the shape of the target circuit component. become necessary. In this case, there are many cases where the electromagnetic wave absorbing material is formed to cover the shape of the target circuit member, the housing of the electronic device that accommodates the target circuit member, and the electromagnetic wave absorbing member is attached to the outer surface or inner surface of the existing member. tied as easy.

In this way, when arranging the electromagnetic wave absorbing member around the target circuit member using the existing member, it is extremely important to arrange the electromagnetic wave absorbing material without a gap. In addition, when the shape of the place where the electromagnetic wave absorbing member is arranged is a curved surface, it is more useful than a solid electromagnetic wave absorber, and an electromagnetic wave absorbing sheet that can conform to the shape of the curved surface is more useful. In addition, when the electromagnetic wave absorbing sheet is arranged without a gap, a re-sticking operation may be necessary. However, when the electromagnetic wave absorbing sheet once attached is to be peeled off, it is also considered that the sheet is in a strongly bent state. Therefore, the electromagnetic wave absorbing sheet is required to have high flexibility to withstand such a re-sticking operation. However, conventional electromagnetic wave absorbing members capable of absorbing electromagnetic waves in the millimeter wave band or higher frequency bands cannot be said to have sufficient flexibility even if they are formed in a sheet shape.

The present disclosure is based on the requirements for the above-mentioned electromagnetic wave absorbing sheet, and realizes an electromagnetic wave absorbing sheet that can absorb high-frequency electromagnetic waves in the millimeter wave band and above well, and is excellent in flexibility and can be easily arranged in a desired part. Purpose. means of solving problems

In order to solve the above-mentioned problems, the electromagnetic wave absorbing sheet disclosed in the present application has an electromagnetic wave absorbing layer including a magnetic iron oxide that generates magnetic resonance in a frequency band above the millimeter wave band, and a resin-made binder, and the magnetic iron oxide is passed through the electromagnetic wave absorbing layer. The electromagnetic wave absorbing sheet that absorbs the irradiated electromagnetic wave by magnetic resonance is characterized in that in the elastic deformation range, when the belt-shaped sheet is bent, the space between the inner side surfaces of the sheet at a position of 10 mm from the bending portion of the sheet is formed. d is taken as 10 mm and the value of the flexibility evaluation value F (g/mm ² ) shown by dividing the cross-sectional area D (mm ² ) of the sheet by the necessary weight (g), is larger than 0 and 6 or less. Invention effect

While the electromagnetic wave absorbing sheet disclosed in the present application is provided with magnetic iron oxide that generates magnetic resonance in the periodic number zone above the millimeter wave zone, the flexibility evaluation value F is greater than 0 and less than 6. Therefore, it is possible to realize a highly practical electromagnetic wave absorbing sheet having electromagnetic wave absorbing properties capable of absorbing electromagnetic waves in the high frequency band above the millimeter wave band, and high flexibility.

The electromagnetic wave absorbing sheet disclosed in the present application has an electromagnetic wave absorbing layer containing a resin-made magnetic iron oxide that generates magnetic resonance in a high frequency band above the millimeter wave band, and the electromagnetic wave absorbing layer is formed by the magnetic resonance of the magnetic iron oxide. An electromagnetic wave absorbing sheet for absorbing irradiated electromagnetic waves, wherein, in the elastic deformation range, when a strip-shaped sheet is bent, the distance d between the inner side surfaces of the sheets at a position L of 10 mm from the bent portion of the sheet is set to 10 mm. The value of the flexibility evaluation value F (g/mm ² ) represented by dividing the cross-sectional area D (mm ² ) of the sheet by the necessary weight (g) is greater than 0 and 6 or less.

By doing so, the electromagnetic wave absorbing sheet disclosed in the present application can absorb electromagnetic waves in the millimeter wave band or higher frequency band through the magnetic resonance of the magnetic iron oxide. In addition, when a band-shaped sheet is bent in the elastic deformation range of the electromagnetic wave absorbing sheet using a binder made of magnetic iron oxide and resin, the distance d between the inner sides of the sheets at a position L of 10 mm from the bent portion of the sheet is set as 10mm, and as the value of the flexibility evaluation value F (g/mm ² ) shown by dividing the cross-sectional area D (mm ² ) of the sheet by the necessary weight (g), it is greater than 0 and 6 For the following reasons, it is possible to increase the curvature without plastically deforming the sheet. As a result, even on the outer surface or inner surface of a device housing that accommodates an electronic circuit that is a target of shielding electromagnetic waves, the portion where the electromagnetic wave absorbing sheet is arranged is curved, or the sheet is re-attached, which makes the sheet stronger. In the case of bending, the electromagnetic wave absorbing layer is less likely to have cracks or cracks, and no plastic deformation occurs, and an electromagnetic wave absorbing sheet with high flexibility can be realized.

In the electromagnetic wave absorbing sheet disclosed in the present application, the glass transition temperature (Tg) of the resin binder is preferably below 0 degrees. By doing so, it is possible to realize an electromagnetic wave absorbing sheet having sufficient flexibility in an actual use state.

In addition, the value of the flexibility evaluation value F is preferably 1.5 or more and 3.5 or less. By doing so, while maintaining the electromagnetic wave absorption performance in the millimeter wave band or higher frequency band, it is possible to realize the coexistence of the ease of handling (independence) when the user transports the electromagnetic wave absorbing sheet, and the ability to absorb electromagnetic waves. An electromagnetic wave absorbing sheet with high adaptability (flexibility) of the shape of the placement place of the absorbing sheet and high practicality.

Furthermore, in the electromagnetic wave absorbing sheet disclosed in the present application, in the electromagnetic wave absorbing layer, the content rate of the magnetic iron oxide is 30% by volume or more, and the inorganic filler contained in the binder containing the magnetic iron oxide The content of the entire material powder is preferably 50% by volume or less. By doing so, an electromagnetic wave absorbing sheet having high electromagnetic wave absorption properties and high flexibility can be realized.

In addition, the aforementioned magnetic iron oxide is preferably ε-iron oxide powder. Since it has the largest coercive force as a metal oxide, ε-iron oxide with a natural magnetic resonance frequency of tens of GHz or more can be used as an electromagnetic wave absorbing material that absorbs electromagnetic waves, and can absorb 30~ Electromagnetic waves with a high frequency of 300 GHz or higher.

In this case, the aforementioned ε-iron oxide powder is preferably a powder of ε-iron oxide in which a part of the Fe site is substituted with a trivalent metal atom. By doing so, it is possible to realize an electromagnetic wave absorbing sheet that absorbs electromagnetic waves in a desired frequency range by exercising the properties of ε-iron oxide having different magnetic resonance frequencies by substituting the material for the Fe site.

Moreover, it is preferable that an adhesive layer is formed on the back surface side of the said electromagnetic wave absorption layer. In this way, while having high electromagnetic wave absorption properties, it is possible to realize an electromagnetic wave absorption sheet that can be easily placed in a desired place and is excellent in handling properties.

Furthermore, in the electromagnetic wave absorbing sheet disclosed in the present application, it is preferable to form a reflective layer which is in contact with one of the electromagnetic wave absorbing layers and reflects the electromagnetic waves passing through the electromagnetic wave absorbing layer. By doing so, it is possible to reliably shield and absorb electromagnetic waves in the frequency band of millimeter waves or higher, and it is possible to realize a so-called reflection type electromagnetic wave absorbing sheet.

Moreover, it is preferable that an adhesive layer is formed on the back surface side of the laminated body of the said electromagnetic wave absorption layer and the said reflection layer. By doing so, it is possible to realize a reflection type electromagnetic wave absorbing sheet which can be easily placed in a desired place and has high electromagnetic wave absorption properties and is excellent in handling properties.

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

(Embodiment) [Constitution of Sheet] Fig. 1 is a cross-sectional view illustrating the constitution of an electromagnetic wave absorbing sheet according to the present embodiment.

However, FIG. 1 is a diagram for easy understanding of the configuration of the electromagnetic wave absorbing sheet according to the present embodiment, and the dimensions and thicknesses of the members shown in the drawings are not based on the actual configuration.

The electromagnetic wave absorbing sheet exemplified in this embodiment includes an electromagnetic wave absorbing layer 1 including a magnetic iron oxide 1a and a resin-made binder 1b. However, the electromagnetic wave absorbing sheet shown in FIG. 1 is formed on the back side (lower side in FIG. 1 ) of the electromagnetic wave absorbing layer 1, and is formed so that the electromagnetic wave absorbing sheet can be attached to the inner surface or the outer surface of the frame of the electronic device. Adhesive layer 2 at specific places such as the surface.

The electromagnetic wave absorbing sheet of the present embodiment is composed of the magnetic iron oxide 1a contained in the electromagnetic wave absorbing layer 1, which induces magnetic resonance and converts electromagnetic waves into heat energy through magnetic loss. Absorb electromagnetic waves. Therefore, as shown in FIG. 1, a reflection layer is not provided on one surface of the electromagnetic wave absorbing layer 1, and it can be 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, in the electromagnetic wave absorbing sheet of the present embodiment, the electromagnetic wave absorbing layer 1 is constituted by the resin binder 1b, and in the elastic deformation range, the value of the flexibility evaluation value F (g/mm ² ) is larger than 0 and 6 For the following reasons, it can be easily attached to a curved surface, or once attached, it can be peeled off and attached again, and it can be realized as a highly practical electromagnetic wave absorbing sheet. However, the definition of the flexibility evaluation value F or its measurement method will be described in detail later.

Furthermore, the electromagnetic wave absorbing sheet of the present embodiment is easy to adhere to the surface of the member around the source of high-frequency electromagnetic waves, etc. as desired, 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 an essential condition in the electromagnetic wave absorbing sheet according to the present embodiment.

[Magnetic Oxide] In the electromagnetic wave absorbing sheet according to the present embodiment, the electromagnetic wave absorbing layer 1 contains magnetic iron oxide 1a as a member for absorbing electromagnetic waves. This magnetic iron oxide 1a is constituted to generate magnetic resonance in the frequency band above the millimeter wave band, and ε-iron oxide or strontium ferrite is preferably used. In addition, in the electromagnetic wave absorbing sheet according to the present embodiment, the magnetic iron oxide 1a is contained in the form of particles dispersed in the resin binder 1b, and preferably in the form of particles.

ε-Iron oxide (ε-Fe ₂ O ₃ ) is in iron oxide (Fe ₂ O ₃ ), which occurs between α-phase (α-Fe ₂ O ₃ ) and γ-(γ-Fe ₂ O ₃ ) It becomes a magnetic material that can be obtained in a single-phase state by a nanoparticle synthesis method combining the inverse microcellular method and the sol-gel method.

Although ε-iron oxide is a fine particle of several nanometers to several tens of nanometers, it has the largest coercive force as a metal oxide of about 20kOe at room temperature. Moreover, through the natural magnetic resonance according to the gyromagnetic effect of precession motion Since it is generated in the frequency band of the so-called millimeter-wave band above several tens of gigahertz, it has a high effect of absorbing electromagnetic waves of 30 to 300 gigahertz in the millimeter-wave band or higher frequencies above.

Furthermore, ε-iron oxide is a crystal obtained by substituting a part of the Fe site of the crystal with a trivalent metal element such as aluminum (Al), gallium (Ga), rhodium (Rh), and indium (In). The magnetic resonance frequency, that is, the number of frequencies of electromagnetic waves absorbed in the case of use as an electromagnetic wave absorbing material, can be made different.

FIG. 2 shows the relationship between the coercive force Hc of ε-iron oxide and the natural resonance frequency f in different cases where the metal element substituted into the Fe site is used. However, the natural resonance frequency f is slightly consistent with the frequency of the absorbed electromagnetic wave.

It is understood from FIG. 2 that the natural resonance frequency of the ε-iron oxide system substituted for a part of the Fe site is different by the amount of the type of the substituted metal element. In addition, it is understood that the higher the value of the natural resonance frequency, the larger the coercive force of the ε-iron oxide becomes.

More specifically, for the case of gallium-substituted ε-iron oxide, that is, ε-Ga _x Fe _2-x O ₃ , the frequency range from 30 GHz to 150 GHz is obtained by adjusting the replacement amount "x". The band has an absorption peak, and in the case of aluminum-substituted ε-iron oxide, that is, ε-Al _x Fe _2-x O ₃ , the amount of substitution "x" is adjusted, and it is in the range of 100 GHz to 190 GHz. frequency band with absorption peaks. Therefore, the type of the element to be substituted for the Fe site of ε-iron oxide is determined at the natural resonance frequency which becomes the frequency to be absorbed by the electromagnetic wave absorbing sheet, and furthermore, by adjusting the amount of substitution with Fe, the absorbed element can be The frequency of the electromagnetic wave is taken as the desired value. Furthermore, for the ε-iron oxide with the substituted metal as rhodium, that is, ε-Rh _x Fe _2-x O ₃ from 180 GHz to above, the frequency band of the absorbed electromagnetic wave can be Move to a higher direction.

ε-iron oxide is commercially available because it includes a configuration in which a part of Fe sites is substituted by metal, and can be easily obtained.

In addition, as the magnetic iron oxide 1a contained in the electromagnetic wave absorbing layer 1 of the electromagnetic wave absorbing sheet described in the present embodiment, strontium ferrite can be used. Strontium ferrite system A complex oxide of strontium and iron is generally used as a magnet material. The strontium ferrite system has a hexagonal crystal structure with a size coefficient of μm, which can generate magnetic resonance for tens of GHz electromagnetic waves and absorb them.

[Resin binder] Epoxy resin, polyester resin, urethane resin, acrylic resin, phenol resin, melamine resin, rubber type can be used in the resin binder 1b used for the electromagnetic wave absorbing layer 1 Resin materials such as resin.

More specifically, as the epoxy resin, a compound in which the hydroxyl groups at both ends of bisphenol A are epoxidized can be used. In addition, as the polyurethane-based resin, polyester-based urethane resins, polyether-based urethane resins, polycarbonate-based urethane resins, epoxy-based urethane resins, and the like can be used . As an acrylic resin, a methacrylic resin, an alkyl acrylate and/or an alkyl methacrylate in which the carbon number of the alkyl group is in the range of 2 to 18, and a functional group-containing polymer can be used, The functional group-containing methacrylic acid polymer etc. which can be obtained by copolymerizing with these other modifying polymers which can be copolymerized as necessary.

In addition, as the rubber-based resin, SIS (styrene-isobutylene block copolymer) or SBS (styrene-butadiene block copolymer), which is a styrene-based thermoplastic elastomer, and EPDM, which is a petroleum-based synthetic rubber, can be used. (Ethylene, propylene, diene, rubber), other rubber-based materials such as acrylic rubber or polysiloxane rubber are used as binders.

However, among these various resin binders, polyester-based resins are ideal for achieving high flexibility. In addition, from the viewpoint of taking care of the environment, as the resin used as the binder, it is preferable to use a halogen-free structure that does not contain halogen. These resin materials can be easily obtained because they have a general configuration as a material for a binder of resin sheets.

The electromagnetic wave absorbing sheet according to the present embodiment has flexibility in which the value of the flexibility evaluation value F (g/mm ² ) is larger than 0 and 6 or less in the elastic deformation range. As the electromagnetic wave absorbing sheet has such flexibility, the glass transition point temperature (Tg) of the binder material should be selected as a structure of 0 degrees or less (Celsius), more preferably -5 degrees or less. .

Generally, the glass transition point temperature of the resin material containing metal powder or iron oxide tends to be higher than the glass transition point temperature of the resin material containing no metal powder alone. Therefore, the electromagnetic wave absorbing sheet according to the present embodiment is provided with the electromagnetic wave absorbing layer of the resin binder containing ε-iron oxide powder or strontium ferrite powder containing magnetic iron oxide or more in a specific amount (for example, 30% by volume). In the case of the electromagnetic wave absorbing sheet, in order to ensure the flexibility of the electromagnetic wave absorbing sheet in the actual use state, if the glass transition point temperature of the resin binder is within the above range, the electromagnetic wave absorbing sheet can have good performance. Flexible construction.

However, for the glass transition point temperature (Tg) here, Rheogel-E4000 (product name) manufactured by UBM Co., Ltd. was used. The frequency is 10 Hz, a tensile jig is used as a measurement jig, the sample shape is 2 mm in width x 20 mm in length, and the thickness is the value at the time of thickness measurement with a micrometer.

[Electromagnetic wave absorbing layer] In the electromagnetic wave absorbing layer 1 of the electromagnetic wave absorbing sheet of the present embodiment, the magnetic iron oxide 1a is used as the electromagnetic wave absorbing material, but the magnetic iron oxide 1a is, as described above, ε-iron oxide powder or strontium ferrite. Since the particle size of the bulk powder is several nanometers to several micrometers of fine metal iron oxide particles, it is important to disperse the magnetic iron oxide 1a well in the binder 1b when the electromagnetic wave absorbing layer 1 is formed.

Therefore, in the electromagnetic wave absorbing sheet according to the present embodiment, the electromagnetic wave absorbing layer 1 contains arylsulfonic acid such as phenylphosphonic acid, phenylphosphonic acid dichloride, methylphosphonic acid, ethylphosphonic acid, octylphosphonic acid, etc. Alkylphosphonic acid such as propylphosphonic acid, propylphosphonic acid, etc., or phosphoric acid compounds such as polyfunctional phosphonic acid such as hydroxyethylidene diphosphonic acid and nitrotrimethylenephosphonic acid. These phosphoric acid compounds have flame retardancy and function as a dispersant of the fine magnetic iron oxide 1a, and can disperse the magnetic iron oxide 1a in the binder well.

More specifically, as a dispersant, phenylphosphonic acid (PPA) manufactured by Japan Wako Pure Chemical Industries, Ltd., or phenylphosphonic acid (PPA) manufactured by Japan Nissan Chemical Industry Co., Ltd. Phosphate ester "JP-502" (product name), etc.

However, as an example of the composition of the electromagnetic wave absorbing layer 1, for 100 parts of the ε-iron oxide powder, the resin binder can be 2 to 50 parts, and the content of the phosphoric acid compound can be 0.1 to 15 parts. When the resin binder is less than 2 points, the ε-iron oxide powder cannot be dispersed favorably. In addition, the shape of the electromagnetic wave absorbing sheet cannot be maintained. When it is more than 50 points, the volume content of ε-iron oxide powder in the electromagnetic wave absorbing sheet becomes smaller, and the magnetic permeability becomes lower, so the effect of electromagnetic wave absorption becomes smaller.

When the content of the phosphoric acid compound is less than 0.1 part, it is impossible to disperse the magnetic iron oxide satisfactorily using a resin binder. When more than 15 points, the effect of favorably dispersing the magnetic iron oxide is saturated. In the electromagnetic wave absorbing sheet, the volume content of the magnetic iron oxide becomes smaller, and the magnetic permeability becomes lower, so the effect of electromagnetic wave absorption becomes smaller.

When the content of the resin binder and the phosphoric acid compound is within the above-mentioned range, the dispersibility of the ε-iron oxide powder is improved, and the maximum particle size or the average particle size can be reduced. As a result, an electromagnetic wave absorbing sheet with high flexibility can be realized.

[Manufacturing method of electromagnetic wave absorbing layer] Here, an example of a manufacturing method of the electromagnetic wave absorbing layer 1 of the electromagnetic wave absorbing sheet of the present embodiment will be described. In the electromagnetic wave absorbing sheet of the present embodiment, a magnetic coating material containing at least a magnetic iron oxide 1a and a resin binder 1b is produced, coated with a predetermined thickness, dried and then subjected to a rolling process to form electromagnetic waves Absorber layer 1. However, in the following, the case where ε-iron oxide powder is used as the magnetic iron oxide 1a is exemplified.

First, make the magnetic paint.

The magnetic coating system obtains a mixture of ε-iron oxide powder, phosphoric acid compound of dispersant, and resin binder, which is diluted and further dispersed, and can be obtained by filtration through a filter. As an example, the kneaded product is obtained by kneading through a pressurized batch kneader. In addition, as an example, the dispersion system of the kneaded product can be obtained as a dispersion liquid using sand milling filled with beads of zirconium or the like. However, at this time, a cross-linking agent may be prepared as necessary.

The obtained magnetic coating was coated on a peelable support, as an example, on a polyethylene terephthalate (PET) sheet with a thickness of 38 μm after peeling treatment through polysiloxane coating, and a flatbed coating was used. Coating with cloth machine or bar coater etc.

After that, the magnetic coating material in wet state is dried at 80°C, and a rolling device is used to carry out rolling processing at a specific temperature to form an electromagnetic wave absorbing layer on the support.

As an example, the thickness of the magnetic coating material in the wet state on the coated support is 1 mm, the thickness after drying is 400 μm, and the thickness of the electromagnetic wave absorbing layer after rolling treatment is 300 μm.

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

However, as another method for producing magnetic paint, as magnetic paint components, at least magnetic iron oxide, phosphoric acid compound of dispersant, and binder resin are mixed at a high speed with a high-speed mixer to prepare a mixture, and then the obtained The mixture can be dispersed by sanding to obtain a magnetic coating.

[Adhesive Layer] As shown in FIG. 1, the electromagnetic wave absorbing sheet of the present embodiment is attached to the back surface of the electromagnetic wave absorbing layer 1 to form an adhesive layer 2.

By providing the adhesive layer 2, the electromagnetic wave absorbing layer 1 can be attached to the inner surface of the frame housing the electrical circuit, or the desired position on the inner surface or the outer surface of the electrical equipment. In particular, since the electromagnetic wave absorbing sheet of the present embodiment has a flexible structure, the electromagnetic wave absorbing sheet can be easily attached to a curved surface bent through the adhesive layer 2, and the handling of the electromagnetic wave absorbing sheet is easy. Sex improves.

As the adhesive layer 2, known materials used as adhesive layers such as adhesive tapes, acrylic adhesives, rubber-based adhesives, polysiloxane-based adhesives, and the like can be used. In addition, in order to adjust the adhesive force to the object to be adhered and reduce the paste residue, an adhesion imparting agent or a crosslinking agent can also be used. The adhesive force of the object to be attached is preferably 5N/10mm~12N/10mm. When the adhesive force is smaller than 5N/10mm, the electromagnetic wave absorbing sheet is easy to peel off from the body to be adhered, and it is easy to shift. In addition, when the adhesive force is larger than 12N/10mm, it is difficult to peel off the electromagnetic wave absorbing sheet from the adherend.

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 adhesive force becomes smaller, and the electromagnetic wave absorbing sheet is easily peeled off from the body to which it is attached, as well as deviations. When the thickness of the adhesive layer is larger than 100 μm, the flexibility of the entire electromagnetic wave absorbing sheet may be reduced. In addition, when the adhesive layer 2 is thick, it is difficult to peel off the electromagnetic wave absorbing sheet from the to-be-attached body. In addition, the case where the cohesion force of the adhesive layer 2 is small is the case where the electromagnetic wave absorbing sheet is peeled off, and there is a case where a paste remains on the to-be-attached body.

However, in the present specification, the adhesive layer 2 may be the adhesive layer 2 that is adhered in a non-peelable manner, and the adhesive layer 2 that is adhered releasably may be used.

In addition, when the electromagnetic wave absorbing sheet is attached to a specific surface, even if the electromagnetic wave absorbing sheet does not have the adhesive layer 2, one that has adhesiveness to the surface on the side of the member on which the electromagnetic wave absorbing sheet is arranged, or one that uses a double-sided tape or an adhesive , and can also be attached to the electromagnetic wave absorbing sheet on a specific part. In this regard, it is clearly understood that the adhesive layer 2 is not an essential component of the electromagnetic wave absorbing sheet shown in the present embodiment.

[Flexibility of the electromagnetic wave absorbing sheet] Next, the flexibility of the electromagnetic wave absorbing sheet according to the present embodiment will be described. However, in the following description, as the electromagnetic wave absorbing sheet to be measured, an electromagnetic wave absorbing sheet including only the electromagnetic wave absorbing layer 1 without laminating the adhesive layer 2 will be described.

FIG. 3 is a diagram illustrating a state in which the flexibility evaluation value F of the electromagnetic wave absorbing sheet is measured by adding conditions to the magnitude of the flexibility of the electromagnetic wave absorbing sheet of the present embodiment.

For the measurement system, a strip-shaped electromagnetic wave absorbing sheet with a length of 100 mm and a width of 20 mm was used. Then, as shown in FIG. 3 , the belt-shaped electromagnetic wave absorbing sheet is bent at the center in the longitudinal direction, and the both ends in the longitudinal direction are overlapped, and the external force for maintaining this state is obtained. Furthermore, by dividing the obtained external force by the cross-sectional area of the electromagnetic wave absorbing sheet, the flexibility evaluation value F of the electromagnetic wave absorbing sheet to be measured can be obtained.

For example, as shown in FIG. 3 , the electromagnetic wave absorbing sheet to be measured is placed on the measuring table 11 of the electronic scale, and the self-weight of the electromagnetic wave absorbing sheet in a state where no external force is applied is measured. After that, the weight added to the electronic scale is measured while the external force is applied to deform it, and the weight of the electromagnetic wave absorbing sheet is removed from the obtained measurement result, and the electromagnetic wave absorbing sheet is bent in the state of bending For the sake of maintenance, clearly understand the necessary weight.

In order to maintain the electromagnetic wave absorbing sheet in a specific bending state, a plate member 12 as shown in FIG. 3 is arranged on the upper side of the electromagnetic wave absorbing sheet, and a white arrow 13 is added to the vertical downward side of this plate member 12 . And the external force shown in Figure 3. At this time, the distance L from the end of the outer side of the curved portion of the electromagnetic wave absorbing sheet 1 is in the portion where L = 10 mm, and the distance d between the inner sides of the curved electromagnetic wave absorbing sheet is d = 10 mm, and it is taken as the weight. The magnitude of the external force 13 was measured, and the value obtained by dividing the cross-sectional area D (unit: mm ² ) of the electromagnetic wave absorbing sheet was used as the flexibility evaluation value F (g/mm ² ). However, the flexibility evaluation value F (g/mm ² ) was measured in an environment with a temperature of 23 degrees and a humidity of 50% Rh.

For example, for an electromagnetic wave absorbing sheet with a thickness of 100 μm (=0.1 mm), the external force 13 applied when the state as shown in FIG. 3 is expressed as a weight of 6 grams through an electronic scale is the cross-section of the electromagnetic wave absorbing sheet. Since the area D is 20(mm)×0.1(mm)=2(mm ² ), the flexibility evaluation value F obtained is 6÷2=3g/mm ² . When the value of this flexibility evaluation value F is larger than 0 and 6 or less, it can be said that it has good flexibility as an electromagnetic wave absorbing sheet. In addition, if the numerical value is in the range larger than 0 and 4 or less, it is ideal to have both independence and flexibility, and if the F value is in the range of 1.5 or more and 3.5 or less, it can be said that it is more ideal. The flexible electromagnetic wave absorbing sheet.

However, the reason why the value of the flexibility evaluation value F is larger than 0 is that in the case of the electromagnetic wave absorbing sheet of F=0, the sheet is bent only by its own weight, and cannot be formed from the bent portion as shown in FIG. 3 . A shape in which the parts facing both ends are maintained in parallel. Such an electromagnetic wave absorbing sheet is too soft to lose its independence, and it becomes difficult to handle when a user carries it or when it is attached to a specific place. In addition, when F=6 is exceeded, a large force is required to bend the electromagnetic wave absorbing sheet, and the workability is lowered.

Similarly, as the lower limit value of the flexibility evaluation value F, when it is judged as one significant figure such as 0.1 or 0.01, if the value becomes "0", it is considered that the measured value itself is "0". different situations. As described above, in the case where the state where the electromagnetic wave absorbing sheet is bent only by its own weight is shown as the flexibility evaluation value F=0, even if it is 0.01, if it is a larger value than 0, it means pressing The power of the electromagnetic wave absorbing sheet is necessary. Then, in the present invention, the case where the flexibility evaluation value F of the electromagnetic wave absorbing sheet is larger than 0 means that when the electromagnetic wave absorbing sheet is bent, the distance L from the end portion outside the bent portion is 10 mm In this part, the distance d between the inner side surfaces of the bent electromagnetic wave absorbing sheets is 10 mm, and some external force is required.

However, in the measurement of the flexibility evaluation value F shown in FIG. 3 , it is premised that the electromagnetic wave absorbing sheet to be measured is in the elastic deformation range. That is, after measuring the flexibility evaluation value F, when the plate member 12 on the sheet is removed, it is important that the electromagnetic wave absorbing sheet is reversed to the initial shape. Even if the plate member 12 is removed, plastic deformation occurs due to the application of an external force, the sheet does not return to its initial shape, and there are abnormal appearances such as cracks in the outer portion of the curved portion of the sheet. The electromagnetic wave absorbing sheet is a Those who are judged to have no specific flexibility evaluation value.

In addition, as the initial state of the electromagnetic wave absorbing sheet, the electromagnetic wave absorbing sheet is placed on the measuring table of the electronic scale and the both ends are overlapped. The interval d between the electromagnetic wave absorbing sheets in the portion where the distance L from the outer end of the bent portion is 10 mm is considered to be larger than 10 mm. This flexibility is an initial shape in the case of a relatively large electromagnetic wave absorbing sheet. In addition, in the electromagnetic wave absorbing sheet with lower flexibility, even if the two ends are aligned without overlapping, the interval is wide, and the interval between the inner surfaces of the end portions is larger than that of the curved portions. Furthermore, in the case of the electromagnetic wave absorbing sheet with small flexibility, when the force of the end portion to be overlapped is removed, the electromagnetic wave absorbing sheet returns to a straight shape. Therefore, in this specification, when the electromagnetic wave absorbing sheet is in the elastic deformation range, it is in the state of removing the external force, and each electromagnetic wave absorbing sheet can return to the shape of the initial state corresponding to the size of its flexibility. state.

However, when the electromagnetic wave absorbing sheet is provided with an adhesive layer or a reflective layer described later, the thickness of these layers may be formed to be extremely thin compared with the thickness of the electromagnetic wave absorbing layer. The influence of the flexibility evaluation value F of the layer is small.

In this way, the electromagnetic wave absorbing sheet according to this embodiment is also important in the case of bending with a slight force from the outside and releasing the external force in the state of strong bending, and returning to the original shape with high recovery performance. In order to obtain high recovery in this way, the particle diameter of the magnetic iron oxide contained in the electromagnetic wave absorbing layer is that the magnetic iron oxide is ε-iron oxide and the average particle diameter is 5 to 50 nm, and the magnetic iron oxide is strontium ferrite. In this case, the average particle size is preferably 1 to 5 μm.

FIG. 4 is an image diagram for explaining the influence of the particle size of the magnetic iron oxide contained in the electromagnetic wave absorbing layer on the flexibility of the electromagnetic wave absorbing sheet.

Each of FIG. 4( a ) shows a case where the particle size of the magnetic iron oxide is sufficiently small, and FIG. 4( b ) shows a case where the particle size of the magnetic iron oxide is large. However, as shown in FIG. 4( a ) and FIG. 4( b ), as shown in FIG. 3 , it is shown that the electromagnetic wave absorbing sheet is pressed with a flat plate-shaped member from both sides of the outer side to reduce the interval thereof, and it is shown as the measurement flexibility evaluation value F the shape of the situation.

As shown in Fig. 4(a), when the average particle diameter of the magnetic iron oxide 1a contained in the resin binder 1b is relatively small, the electromagnetic wave absorbing sheet (electromagnetic wave absorbing layer 1) is perfectly curved and self-formed It is a slightly semicircular curved portion, and both ends have a shape extending in parallel straight lines.

On the other hand, as shown in Fig. 4(b), when the particle size of the magnetic iron oxide 1a' contained in the resin binder 1b' is large, the magnetic iron oxides are in contact with each other at the curved portion of the electromagnetic wave absorbing sheet. , and the radius of the curved portion does not become quite small, and the straight portion toward the two end portions does not become parallel, and the end portion side is in an open state. In this state, when the force (external force 13 in FIG. 3 ) is tightened from the outside while the gap between the two ends is reduced, cracks or breaks occur in the resin binder in the bent portion, but the electromagnetic wave absorbing sheet (Electromagnetic wave absorption layer 1) plastic deformation occurs.

However, FIGS. 4( a ) and 4 ( b ) finally represent the state of the magnetic iron oxide in the curved portion as an image, and the ratio of the particle diameter of the magnetic iron oxide to the thickness of the electromagnetic wave absorbing layer is the same as The composition of reality is different.

(Example) Next, the electromagnetic wave absorbing sheet of the present embodiment is actually produced, and the measurement result of measuring the evaluation value of flexibility will be described.

As the electromagnetic wave absorbing sheet of the present embodiment, an electromagnetic wave absorbing sheet using ε-iron oxide as a magnetic oxide and polyester as a binder (Example 1), and an electromagnetic wave absorbing sheet using polyurethane as a binder were produced (Example 2). In addition, an electromagnetic wave absorbing sheet using strontium ferrite as the magnetic oxide and polysiloxane rubber as the binder was produced (Example 3). However, in all the examples, the adhesive layer was not formed, and the electromagnetic wave absorbing sheet was formed only by the electromagnetic wave absorbing layer. The compounding amount in the composition of each electromagnetic wave absorbing sheet was prepared as follows.

○Example 1 Magnetic iron oxide ε-iron oxide powder 40.0g Binder polyester 73.1g Toyobo Co., Ltd. VYLON55SS (product name) Tg: -15°C Solid content: 25.6g Solvent: 47.5g Benzenesulfonic acid (PPA: dispersed agent) 2.0g Methyl ethyl ketone (MEK: solvent) 20.1g ○Example 2 Magnetic iron oxide ε-iron oxide powder 40.0g Binder polyurethane 50.6g Toyobo Co., Ltd. VYLON UR8700 (product name) Tg: -22 ℃ Solid content: 15.2g Solvent: 35.4g Benzenesulfonic acid (PPA: dispersant) 2.0g Methyl ethyl ketone (MEK: solvent) 2.7g ○Example 3 Magnetic iron oxide strontium ferrite powder 100.0g Binder 30.0g of polysiloxane rubber Shin-Etsu Chemical Industry Co., Ltd. KE-510U (product name) Tg: -125℃ Contains 0.9g of C-8A (additive)

For these three electromagnetic wave absorbing sheets, the value of the flexibility evaluation value F was measured by the method shown in FIG. 3 .

In the electromagnetic wave absorbing sheet of Example 1 in which polyester was used for the binder, the value of the flexibility evaluation value F was 1.4 (g/mm ² ). In addition, in the electromagnetic wave absorbing sheet of Example 2 in which polyurethane was used for the binder, the value of the flexibility evaluation value F was 2.7 (g/mm ² ). Furthermore, in the electromagnetic wave absorbing sheet of Example 3 in which polysiloxane rubber was used for the binder, the value of the flexibility evaluation value F was 1.1 (g/mm ² ), and it was confirmed that the value of the flexible evaluation value F was 1.1 (g/mm 2 ). The case of the numerical range of the flexibility evaluation value of the electromagnetic wave absorbing sheet of the form (more than 0 and less than 6).

The electromagnetic wave absorbing sheets of Example 1 and Example 2 are based on the above-mentioned good flexibility. Through the use of ε-iron oxide as the electromagnetic wave absorbing material, as an example, the electromagnetic wave with a frequency of 75 GHz can be absorbed in the millimeter region. . In addition, the electromagnetic wave absorbing sheet of Example 3 can also absorb electromagnetic waves in the millimeter wave band with a frequency of 76 GHz through the strontium ferrite used as the electromagnetic wave absorbing material.

Here, as a comparative example, similarly to the electromagnetic wave absorbing sheets of Example 1, Example 2, and Example 3, the electromagnetic wave absorption of electromagnetic wave absorption of high frequency near 75 GHz was measured by containing ε-iron oxide as magnetic iron oxide. The value of the flexibility of the sheet. The electromagnetic wave absorbing sheet used as a comparative example is all the ones that shift the phase of the reflected wave by 1/2 wavelength, and the incident wave and the reflected wave of the electromagnetic wave absorbing sheet cancel each other out to absorb the electromagnetic wave, so-called electromagnetic wave interference type ( λ/4 type) electromagnetic wave absorbing sheet. The value of the flexibility evaluation value F was measured as a first comparative example, in the case of a rubber-type electromagnetic wave absorbing sheet "SA76 (product name)" manufactured by FDK Co., Ltd., the flexibility measured by removing the metal layer of the reflective layer The value of the evaluation value F was 8.9 (g/mm ² ). In addition, the value of the flexibility evaluation value F was measured as the second comparative example, in the case of the electromagnetic wave absorbing sheet "EC-SORB SF-76.5MB (product name)" manufactured by E&C Engineering Co., Ltd., the metal layer of the reflective layer was removed. On the other hand, the value of the flexibility evaluation value F measured was 7.1 (g/mm ² ). As described above, the existing electromagnetic wave absorbing sheets measured as comparative examples were found to have lower flexibility than the electromagnetic wave absorbing sheets of Example 1, Example 2, and Example 3 described above.

Furthermore, in order to obtain the range of the ideal flexibility evaluation value F, ε-iron oxide powder was used as the magnetic iron oxide, and the electromagnetic wave absorbing sheet of Example 4 and the electromagnetic wave absorbing sheet of the third comparative example (Comparative Example) were produced. 3). The blending amounts in the composition of the electromagnetic wave absorbing sheet of Example 4 and the electromagnetic wave absorbing sheet of Comparative Example 3 are as follows. In addition, each electromagnetic wave absorption sheet was produced by the same production method as the electromagnetic wave absorption sheet of the said Example 1 - Example 3.

○Example 4 Magnetic iron oxide ε-iron oxide powder 40.0g Binder polyester 73.1g Toyobo Co., Ltd. VYLON 50SS (product name) Tg: -3°C Solid content: 15.2g Solvent: 35.4g Benzenesulfonic acid (PPA: Dispersant) 2.0g Methyl ethyl ketone (MEK: solvent) 20.1g ○Comparative example 3 Magnetic iron oxide ε-iron oxide powder 40.0g Binder polyurethane 50.6g Toyobo Co., Ltd. VYLON UR3200 (product name) Tg: 4 ℃ Solid content: 15.2g Solvent: 35.4g Benzenesulfonic acid (PPA: dispersant) 2.0g

The value of the flexibility evaluation value F of the electromagnetic wave absorbing sheet of Example 4 was 3.8 (g/mm ² ), and when the measurement of the flexibility evaluation value F was completed and the external force was removed, it returned to the initial shape. On the other hand, the value of the flexibility evaluation value F of the electromagnetic wave absorbing sheet of Comparative Example 3 is 6.3 (g/mm ² ), and even if an external force is applied, the radius of the curved portion does not decrease considerably, and will When the interval d of the portion where L is 10 mm is measured as 10 mm and then observed, cracks are generated on the surface of the electromagnetic wave absorbing layer 1 outside the bent portion.

In this case, if the value of the flexibility evaluation value F is in the range of 1 or more and 4 or less, it has been confirmed that an electromagnetic wave absorbing sheet with sufficient flexibility and self-supporting properties can be obtained. On the other hand, when the value of the flexibility evaluation value F is larger than 6, the rebound force of the electromagnetic wave absorbing sheet is too large, for example, when the sheet is strongly bent when the sheet is reattached, it can be confirmed that the electromagnetic wave absorbing layer is damaged. Dangerous situation.

However, in Example 1 and Example 2 in which the flexibility evaluation value F was measured, and in the electromagnetic wave absorbing sheet of Example 4 and Comparative Example 3, the content of ε-iron oxide in the electromagnetic wave absorbing layer after completion was The ratio of the ε-iron oxide powder to the binder material is determined so that the ratio becomes 40% by volume. In addition, in the electromagnetic wave absorbing sheet of Example 3, the content rate of strontium ferrite was about 40% by volume, and the mixing ratio of the strontium iron oxide powder and the binder material was determined.

In the case of the electromagnetic wave absorbing sheet of the present embodiment, in order to absorb electromagnetic waves incident through the magnetic resonance of the magnetic iron oxide contained in the electromagnetic wave absorbing layer, when the content ratio of the magnetic iron oxide contained in the electromagnetic wave absorbing layer is low , the electromagnetic wave absorption characteristics are reduced. As electromagnetic wave absorbing properties, the intensity of the transmitted wave transmitted through the electromagnetic wave absorbing sheet is reduced by 15 dB relative to the intensity of the incident wave, that is, when the electromagnetic wave absorbing sheet that absorbs 90% of the electromagnetic wave is obtained, the magnetic iron oxide in the electromagnetic wave absorbing layer If the content ratio is 40 vol% or more, it becomes the standard. In this way, by setting the content ratio of magnetic iron oxide to 40% by volume or more, the value of the imaginary part (μ″) of the magnetic permeability of the electromagnetic wave absorbing layer can be increased, and an electromagnetic wave absorbing sheet having high electromagnetic wave absorbing properties can be realized.

On the other hand, the electromagnetic wave absorbing layer of the electromagnetic wave absorbing sheet is made of an inorganic filler contained as a magnetic iron oxide and a binder material, and the more the filler powder is contained, the more flexible the electromagnetic wave absorbing sheet is. lower. When the inventors examined, when the total content of these inorganic filler powders exceeds 50% by volume, flexibility as an electromagnetic wave absorbing sheet may occur, which may be insufficient. Of course, the degree of flexibility of the electromagnetic wave absorbing layer varies depending on the size and shape of the inorganic filler powder. In addition, when a large amount of magnetic iron oxide is contained through the electromagnetic wave absorption layer, the electromagnetic wave absorption characteristics are improved, and the solid content of fillers other than magnetic iron oxide is preferably as free as possible, and the electromagnetic wave absorption layer is preferably formed.

[Modification of the electromagnetic wave absorbing sheet] Here, a modification of the electromagnetic wave absorbing sheet of the present embodiment will be described.

The electromagnetic wave absorbing sheet disclosed in the present application absorbs electromagnetic waves through the magnetic resonance of the magnetic iron oxide that forms the electromagnetic wave absorbing layer as the electromagnetic wave absorbing material and the resin-made binder at the same time. The surface on the opposite side to the incident side may be constituted by a reflection layer including a reflection layer for reflecting electromagnetic waves such as a metal layer.

FIG. 5 is a cross-sectional view showing the configuration of a modification of the electromagnetic wave absorbing layer according to the present embodiment.

However, FIG. 5 is the same as FIG. 1 for explaining the structure of the electromagnetic wave absorbing sheet according to the present embodiment. In order to make the structure easier to understand, the dimensions and thicknesses of the members shown in the figures are not shown in reality. composition. In addition, about the same member as that which comprises the electromagnetic wave absorption sheet shown in FIG. 1, the same code|symbol is attached|subjected and detailed description is abbreviate|omitted.

The electromagnetic wave absorbing sheet of the modified example is formed on the back side (lower side in FIG. 5 ) of the electromagnetic wave absorbing layer 1 including the magnetic iron oxide 1a that generates magnetic resonance in the frequency band above the millimeter wave band and the resin binder 1b. ), contacting the surface of the electromagnetic wave absorbing layer 1 to form the reflective layer 3 .

However, in the electromagnetic wave absorbing sheet of the modification example shown in FIG. 5, the adhesive layer 2 which can adhere to the electromagnetic wave absorbing sheet at a specific place is formed on the back side of the reflection layer 3.

The reflection 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 the present embodiment, in the elastic deformation range of the electromagnetic wave absorbing sheet, since the value of the flexibility evaluation value F is larger than 0 and 6 or less, a metal plate is used as the reflection layer 3 The situation is difficult. Therefore, the reflection layer 3 can be used as a metal foil arranged in close contact with the back side of the electromagnetic wave absorbing layer 1, or as a metal vapor deposition film deposited on the back surface of the electromagnetic wave absorbing layer 1, or as a metal foil formed on the back surface of the electromagnetic wave absorbing layer 1. It is realized by a metal vapor deposition film on the surface of the electromagnetic wave absorbing layer 1 side surface of a non-metallic sheet member such as resin on the back side of 1.

However, the type of metal constituting the reflective layer 3 is not particularly limited, starting with metal materials generally used for electronic components such as aluminum, copper, and chromium, and various metal materials can be used, but the electrical impedance is limited. It may be small, and it is better to use a metal with high corrosion resistance.

In the electromagnetic wave absorbing sheet according to the modification example shown in FIG. 5, by providing the reflection layer 3 on the back surface of the electromagnetic wave absorbing layer 1, the situation in which the electromagnetic wave penetrates through 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 released from the outside, such as electrical circuit components driven at high frequencies.

As described above, the electromagnetic wave absorbing sheet according to the present embodiment can absorb well in the millimeter wave band or higher frequency band through the magnetic resonance of the magnetic iron oxide contained in the electromagnetic wave absorbing layer as the electromagnetic wave absorbing member. of electromagnetic waves. In addition, since the electromagnetic wave absorbing sheet of the present embodiment is in the elastic deformation range, the value of the flexibility evaluation value F is larger than 0 (g/mm ² ) and 6 (g/mm ² ) or less, and has High flexibility to a practically sufficient degree. Therefore, while the electromagnetic wave absorbing sheet can be easily attached to the curved surface, when the electromagnetic wave absorbing sheet is attached or reattached, the electromagnetic wave absorbing sheet is in a state of strong bending, and it is also possible to achieve no breakage. Or plastic deformable electromagnetic wave absorbing sheet.

However, in the description of the above-mentioned embodiment, as a method of forming the electromagnetic wave absorbing layer, the method of producing a magnetic coating material, applying it, and drying it has been described. The method for producing the electromagnetic wave absorbing layer of the electromagnetic wave absorbing sheet disclosed in the present application is considered to use various molding methods such as extrusion molding in addition to the method of applying the above-mentioned magnetic paint.

More specifically, the magnetic iron oxide powder, the binder, and, if necessary, a dispersant and the like are mixed in advance, and the mixed materials are supplied into the plastic cylinder from the resin supply port of the extrusion molding machine. However, as the extrusion molding machine, a conventional one including a plastic cylinder, a die provided at the front end of the plastic cylinder, a pusher rotatably arranged in the plastic cylinder, and a drive mechanism for driving the stacker can be used. Extrusion molding machine. The molten material plasticized by the belt heater of the extrusion molding machine is conveyed to the front through the rotation of the pusher, and extruded into a sheet from the front end. The electromagnetic wave absorbing layer of a specific thickness can be obtained by drying the extruded material, press molding, and rolling.

In addition, in the above-mentioned embodiment, the electromagnetic wave absorbing sheet which constitutes the electromagnetic wave absorbing layer by one layer has been described, but as the electromagnetic wave absorbing layer, a structure in which a plurality of layers are laminated may be used. In this way, in the case of a reflective electromagnetic wave absorbing sheet having a reflective layer in particular, the input impedance of the electromagnetic wave absorbing layer can be set as a desired value and integrated with the impedance in the air, so that the electromagnetic wave absorbing sheet can be used in the electromagnetic wave absorbing sheet. Enhancer of electromagnetic wave absorption characteristics. Industrial Utilization Possibilities

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

1‧‧‧Electromagnetic wave absorption layer 1a‧‧‧Magnetic iron oxide 1b‧‧‧Binding material 2‧‧‧Adhesive layer

FIG. 1 is a cross-sectional view illustrating the structure of the electromagnetic wave absorbing sheet according to the present embodiment. Fig. 2 is a diagram illustrating the electromagnetic wave absorption characteristics of ε-iron oxide substituted for a part of Fe sites. Fig. 3 is a model diagram illustrating a method of measuring the flexibility characteristic value F from the magnitude of the weight added to the sheet when the sheet is bent. Fig. 4 is a diagram for explaining the relationship between the size of the ε-iron oxide powder contained in the electromagnetic wave absorbing layer and the degree of curvature of the electromagnetic wave absorbing sheet. Fig. 5 is a cross-sectional view illustrating a configuration of a reflection type electromagnetic wave absorbing sheet, which is a modification of the electromagnetic wave absorbing sheet of the present embodiment.

1‧‧‧Electromagnetic wave absorption layer

1a‧‧‧Magnetic iron oxide

1b‧‧‧Binder

2‧‧‧Additional layer

Claims (9)

An electromagnetic wave absorbing sheet, which has an electromagnetic wave absorbing layer containing a magnetic iron oxide and a resin binder that generates magnetic resonance in a frequency band above the millimeter wave band, and absorbs the irradiated electromagnetic waves through the magnetic resonance of the magnetic iron oxide. The electromagnetic wave absorbing sheet is characterized in that in the elastic deformation range, when the strip-shaped sheet is bent, in order to make the distance d between the inner side surfaces of the sheets at a position L of 10 mm from the bent portion of the sheet to be 10 mm, an additional weight is necessary. (g)/The value of the flexibility evaluation value F (g/mm ² ) represented by the cross-sectional area D (mm ² ) of the sheet is greater than 0 and less than 6. The electromagnetic wave absorbing sheet according to claim 1, wherein the glass transition point temperature (Tg) of the resin-made binder is zero. ℃ or lower. The electromagnetic wave absorbing sheet according to claim 1 or claim 2, wherein the value of the flexibility evaluation value F is 1.5 or more and 3.5 or less. The electromagnetic wave absorbing sheet according to claim 1 or claim 2, wherein, in the electromagnetic wave absorbing layer, the content of the magnetic iron oxide is 30% by volume or more, and the magnetic iron oxide is mixed into the resin containing the magnetic iron oxide. The content rate of the total inorganic filler powder of the prepared binder is 50% by volume or less. The electromagnetic wave absorbing sheet according to claim 1 or claim 2, wherein the magnetic iron oxide is ε-iron oxide powder. The electromagnetic wave absorbing sheet according to claim 5, wherein a part of the Fe site of the ε-iron oxide powder is substituted with a trivalent metal atom. The electromagnetic wave absorbing sheet according to claim 1 or claim 2, wherein an adhesive layer is formed on the back side of the electromagnetic wave absorbing layer. The electromagnetic wave absorbing sheet according to claim 1 or claim 2, wherein a reflective layer that reflects electromagnetic waves transmitted through the electromagnetic wave absorbing layer is formed in contact with one of the surfaces of the electromagnetic wave absorbing layer. The electromagnetic wave absorbing sheet according to claim 8, wherein an adhesive layer is formed on the back surface side of the laminate of the electromagnetic wave absorbing layer and the reflecting layer.

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