JP2021163838A - Molding material for radio wave absorbing member and radio wave absorber - Google Patents

Abstract

To obtain a molding material for a radio wave absorbing member whose radio wave absorbing characteristics do not change significantly even after undergoing a process such as a molding process or a kneading process in which high mechanical shearing force is applied to a radio wave absorbing composition.SOLUTION: A molding material for a radio wave absorbing member includes a magnetic iron oxide 1a that magnetically resonates with radio waves with a frequency of 20 GHz to 300 GHz, and a binder 1b of an organic material, and is molded into a predetermined shape, and the magnetic iron oxide is a hexagonal ferrite, and in a case in which, in a hysteresis curve, a point magnetized by applying a positive magnetic field of +10 kOe or more and +15 k or less is a point A, and a point obtained by decreasing the applied magnetic field strength from the point A and magnetized by applying a negative magnetic field of -10 kOe or more and -15 kOe or less, when the apex of the maximum peak in the positive direction is P1 and the apex of the peak having the extreme value in the positive direction is P2 in a curve in which the applied magnetic field obtained by differentiating the hysteresis curve from the point A to the point B twice is in the range of 0 to -10 kOe, the value of P2/P1 is 0.15 or less.SELECTED DRAWING: Figure 1

Description

The present disclosure relates to a molding material for a radio wave absorbing member that absorbs radio waves and a radio wave absorber manufactured using this material, and in particular, a magnetic iron oxide powder as a member that absorbs radio waves is provided from several tens of gigahertz to millimeters. The present invention relates to a molding material for a radio wave absorbing member and a radio wave absorber capable of absorbing radio waves having a high frequency in a wave band.

A radio wave absorber that absorbs radio waves is used in order to avoid the influence of leaked radio waves emitted to the outside from an electric circuit or the like and radio waves that are undesirably reflected. The radio wave absorbing body is formed by molding and coating a material having radio wave absorbing material according to the application such as a block shape or a sheet shape.

In recent years, in mobile communications such as mobile phones, wireless LANs, automatic toll collection systems (ETCs), etc., centimeter waves with a frequency band of several gigahertz (GHz) and milliwave bands with a frequency of 30 gigahertz to 300 gigahertz. Furthermore, research on a technique for using a radio wave having a frequency of 1 terahertz (THz) as a radio wave in a high frequency band exceeding the milliwave band is also in progress.

In response to such technological trends that use higher frequency radio waves, even as a radio wave absorber that absorbs unnecessary radio waves, we have changed to one that can absorb radio waves with high frequencies in the millimeter wave band from several tens of gigahertz. The demand is increasing.

As a radio wave absorber capable of absorbing such high frequency radio waves, by regulating the distribution degree of the particle size of the hexagonal ferrite powder and using a powder having a large proportion of particles close to the primary particles, 1 GHz or more. A radio wave absorber capable of absorbing the radio waves of the above with a thinner sheet thickness than the conventional one has been proposed (see Patent Document 1).

Further, as a radio wave absorber having high absorption characteristics of radio waves in the GHz band and having no anisotropy in radio wave absorption, a hexagonal ferrite sintered body in which the degree of orientation of ferrite crystal particles is limited and a manufacturing method thereof are available. It has been proposed (see Patent Document 2).

Japanese Unexamined Patent Publication No. 2012-216865 JP-A-2018-154541

By including magnetic iron oxide as a radio wave absorber, high frequency radio waves can be absorbed by magnetic resonance of magnetic iron oxide. As such magnetic iron oxide, hexagonal ferrite and epsilon iron oxide, which have also been used in the above-mentioned conventional techniques, have already been put into practical use.

For the radio wave absorber containing hexagonal ferrite as a radio wave absorbing material, a composition which is a molding material in which powder which is a sintered body particle of hexagonal ferrite is mixed in a binder of an organic material is prepared, and this composition is injected. It is desired to perform molding processing such as molding or extrusion molding, or to form a sheet having a predetermined thickness by applying and drying the composition by a coater method or the like and then appropriately undergoing a calendering process. Create a radio wave absorber with a shape.

As a result of confirmation by the inventors, when the composition which is a molding material is molded, or when the radio wave absorber after processing is subjected to a stirring process by a kneader or the like, the radio wave absorption characteristics of the processed radio wave absorber are obtained in advance at the time of the molding material. It was found that the radio wave absorption characteristics are different from those grasped in, and more specifically, undesired situations such as a shift in the frequency of the absorbed radio waves and a decrease in the radio wave absorption capacity at a specific frequency occur. rice field. As described above, if the design absorption frequency and the actual absorption frequency of the processed radio wave absorber are different, it is not possible to stably manufacture a radio wave absorber having a desired radio wave absorption characteristic.

The present disclosure has solved the above-mentioned conventional problems and has a material for molding even after undergoing a step such as a molding process or a kneading step in which a high mechanical shearing force is applied to the radio wave absorbing composition. An object of the present invention is to obtain a molding material for a radio wave absorbing member whose radio wave absorbing characteristics do not change significantly, and a radio wave absorber manufactured by using the molding material for the radio wave absorbing member.

In order to solve the above problems, the molding member for a radio wave absorbing material disclosed in the present application contains a magnetic iron oxide that magnetically resonates with a radio wave having a frequency of 20 GHz to 300 GHz and a binder made of an organic material, and is molded into a predetermined shape. A molding material for an absorbent member, wherein the magnetic iron oxide is hexagonal ferrite, and in the hysteresis curve, the points magnetized by applying a positive magnetic field of +10 kOe or more and + 15 k or less are the points A and the applied magnetic field strength from the point A. When a point magnetized by applying a negative magnetic field of -10 kOe or more and -15 kOe or less is defined as point B, the applied magnetic field obtained by differentiating the hysteresis curve from point A to point B twice. In the curve in the range from 0 to -10 kOe, the value of P2 / P1 is 0 when the peak of the maximum peak in the positive direction is P1 and the peak of the peak having the polar value in the positive direction is P2. It is characterized by being .15 or less.

Further, the radio wave absorber disclosed in the present application is characterized in that it is manufactured by molding any of the molding materials for radio wave absorbing members disclosed in the present application.

The molding material for a radio wave absorbing member disclosed in the present application has a value of P2 / P1 of 0.15 or less when the vertices of two peaks in the curve obtained by differentiating the hysteresis curve twice are P1 and P2. be. By using such a molding material for a radio wave absorbing member, changes in radio wave absorption characteristics before and after molding are suppressed within a predetermined range, and radio waves suitably used for applications in which radio waves of a specific frequency are used. An absorber can be made.

It is sectional drawing explaining the structure of the radio wave absorption sheet which is the radio wave absorber made from the molding material for the radio wave absorption member which concerns on this embodiment. It is a figure explaining the hysteresis curve of the molding material for the 1st radio wave absorption member which concerns on this embodiment. It is a figure which shows the curve obtained by differentiating the hysteresis curve of the molding material for the 1st radio wave absorption member which concerns on this embodiment. It is a figure which shows the curve obtained by differentiating the hysteresis curve of the molding material for the 1st radio wave absorption member which concerns on this embodiment twice. It is a figure which shows the radio wave absorption characteristic before and after the molding process in the molding material for the 1st radio wave absorption member which concerns on this embodiment. It is a figure which shows the curve obtained by differentiating the hysteresis curve of the molding material for the 2nd radio wave absorption member which concerns on this embodiment twice. It is a figure which shows the radio wave absorption characteristic before and after the molding process in the molding material for the 2nd radio wave absorption member which concerns on this embodiment. It is a figure explaining the hysteresis curve of the 4th radio wave absorption member molding material which concerns on this embodiment. It is a figure which shows the curve obtained by differentiating the hysteresis curve of the 4th radio wave absorption member molding material which concerns on this embodiment twice. It is a figure which shows the radio wave absorption characteristic before and after the molding process in the 4th radio wave absorption member molding material which concerns on this embodiment. It is a figure explaining the hysteresis curve of the molding material for a radio wave absorption member of the 1st comparative example. It is a figure which shows the curve obtained by differentiating the hysteresis curve of the molding material for a radio wave absorption member of the 1st comparative example twice. It is a figure which shows the radio wave absorption characteristic before and after the molding process in the molding material for a radio wave absorption member of the 1st comparative example. It is a figure which shows the curve obtained by differentiating the hysteresis curve of the molding material for a radio wave absorption member of the 2nd comparative example twice. It is a figure which shows the radio wave absorption characteristic before and after the molding process in the molding material for a radio wave absorption member of the 2nd comparative example.

The molding material for a radio wave absorbing member disclosed in the present application is a molding material for a radio wave absorbing member that contains magnetic iron oxide that magnetically resonates with a radio wave having a frequency of 20 GHz to 300 GHz and a binder of an organic material, and is molded into a predetermined shape. Therefore, the magnetic iron oxide is hexagonal ferrite, and in the hysteresis curve, the points magnetized by applying a positive magnetic field of +10 kOe or more and + 15 k or less are point A, and the applied magnetic field strength is reduced from point A to -10 kOe. When the point magnetized by applying a negative magnetic field of -15 kOe or less is defined as point B, the applied magnetic field obtained by differentiating the hysteresis curve from point A to point B twice is from 0 to -10 kOe. When the peak of the maximum peak in the positive direction is P1 and the peak of the peak having the polar value in the positive direction is P2 in the curve in the range of, the value of P2 / P1 is 0.15 or less.

By doing so, the molding material for the radio wave absorbing member disclosed in the present application can suppress the change in the radio wave absorbing characteristic before and after the molding process within a predetermined range. Therefore, it is possible to manufacture a radio wave absorber that suitably absorbs undesired radio waves from equipment used in a specific field in which the range of frequencies used is regulated.

In the molding material for a radio wave absorbing member disclosed in the present application, it is preferable that the magnetic iron oxide is strontium ferrite.

Further, as the molding material for the radio wave absorbing member disclosed in the present application, a material in which a part of the Fe site of the magnetic iron oxide is replaced with a trivalent metal atom can be used.

Further, as the molding material for the radio wave absorbing member disclosed in the present application, any one of a thermosetting rubber, a thermoplastic elastomer, and a thermoplastic resin can be used as the binder.

The radio wave absorber disclosed in the present application is manufactured by molding any of the molding materials for radio wave absorbing members disclosed in the present application.

By manufacturing using the molding material for the radio wave absorbing member disclosed in the present application, the change in the radio wave absorbing characteristics before and after the molding process can be suppressed within a predetermined range, so that the radio wave of the radio wave absorber after molding can be suppressed. Absorption characteristics can be sufficiently controlled at the stage of molding material. For this reason, there is no big difference between the design absorption frequency and the absorption frequency of the molded body after processing, and it is adopted for electronic devices in a specific field where the range of frequencies used is regulated, and undesired radio waves. It is possible to easily manufacture a radio wave absorber capable of preventing the emission of radio waves and the incident of undesired radio waves on the electronic device.

Hereinafter, the molding material for the radio wave absorbing member disclosed in the present application and the radio wave absorber manufactured by molding the molding material will be described with reference to the drawings.

(Embodiment)
[Radio wave absorber]
First, as an embodiment of the radio wave absorber disclosed in the present application, a radio wave absorbing sheet which is a radio wave absorber composed of a radio wave absorbing layer containing a granular magnetic iron oxide and a binder of an organic material will be described.

FIG. 1 is a cross-sectional view showing a configuration of a radio wave absorbing sheet which is a radio wave absorber described in the present embodiment.

FIG. 1 shows a state in which an electromagnetic wave absorbing composition is applied onto a resin sheet 2 as a base material and dried to form an electromagnetic wave absorbing sheet 1.

It should be noted that FIG. 1 is a diagram drawn to make it easier to understand the configuration of the electromagnetic wave absorbing sheet according to the present embodiment, and the size and thickness of the members shown in the figure are realistically represented. It's not a thing.

The electromagnetic wave absorbing sheet 1 illustrated in the present embodiment is formed as an electromagnetic wave absorbing layer containing strontium ferrite powder 1a as magnetic iron oxide powder and a resin binder 1b.

In FIG. 1, a radio wave absorbing sheet is illustrated as the radio wave absorber disclosed in the present application, but as the radio wave absorber disclosed in the present application, a molding material for a radio wave absorbing member is injection-molded using a predetermined mold (mold). Alternatively, by extrusion molding, it is possible to manufacture a radio wave absorber having an arbitrary shape corresponding to the shape of the block or the shape of the place where the radio wave is arranged.

The radio wave absorbing sheet 1 illustrated in the present embodiment contains strontium ferrite magnetic iron oxide 1a as magnetic iron oxide that magnetically resonates with radio waves of 20 GHz to 300 GHz in the binder 1b of the organic material.

[Magnetic iron oxide]
In the radio wave absorbing sheet 1 according to the present embodiment, strontium ferrite is used as the particulate magnetic iron oxide 1a.

The resonance frequency of strontium ferrite _{can be set to 60 GHz to 80 GHz by using a system in which Al is added to SrFe 12} O ₁₉ , and a radio wave absorber compatible with a wireless LAN in the 60 GHz band can be manufactured. By adding Al, the frequency indicating radio wave absorption shifts to the high frequency side, which is considered to correspond to the increase in the value of the _{anisotropic magnetic field (HA).}

Further, as the hexagonal ferrite magnetic powder used in the radio wave absorbing sheet disclosed in the present application, barium ferrite magnetic powder can be used. In M-type (magneticplumpite-type) ferrite containing barium ferrite, the imaginary part (μr'') of the complex magnetic permeability related to radio wave absorption becomes high at the frequency at which resonance occurs when the magnetic material is magnetized at high frequency. since the magnetic resonance frequency f which is proportional to the anisotropy field (H _a) having the material, the value of the magnetic resonance frequency f higher the anisotropy field (H _a) material increases. Magnetic resonance frequency f of BaFe ₁₂ O ₁₉ is a M-type ferrite, the value of the H _A is calculated as 48GHz from 1.35 mA / m, it is possible to absorb radio waves high GHz band. In addition, the magnetic resonance frequency f is in the range of 5 GHz to 150 GHz by controlling the value of the anisotropic magnetic field ( _HA ^{) by substituting a part of Fe 3+} with (TIMn) ³⁺ , Al ^{3+, or the like.} Can be controlled with.

[binder]
The radio wave absorbing sheet 1 according to the present embodiment has flexibility as a whole because the magnetic iron oxide particles 1a are dispersed in the binder 1b of the organic material.

A thermosetting plastic, a thermoplastic plastic, or an elastomer can be used as the binder 1b of the organic material used for the molding material for the radio wave absorbing member for producing the radio wave absorbing sheet 1.

Use phenol resin (PF), urea resin (UF), melamine resin (MF), unsaturated polyester (UP), epoxy resin (EP), silicon resin (SI), polyurethane (PUR) as the thermosetting plastic. Can be done.

Thermoplastics include general purpose plastics, engineering plastics, and super engineering plastics. As general-purpose plastics, polyvinylidene chloride (PVC), polyethylene (PE), polypropylene (PP), polystyrene (PS), acrylonitrile-butadiene-styrene (ABS), acrylonitrile-styrene (AS), polymethylmethacrylic (PMMA), Polyvinyl alcohol (PVA), polyvinylidene chloride (PVDC), and polystyrene terephthalate (PET) can be used.

Engineering plastics include polyamide (PA), polyacetal (POM), polycarbonate (PC), polyfilene ether (PPE), polybutylene terephthalate (PBT), ultra high molecular weight polyethylene (U-EP), polyvinylidene fluoride (PVDF). Can be used.

As superengineering plastics, polysulfone (PSU), polyethersulfone (PES), polyphenylene sulfide (PPS), polyallylate (PAR), polyamideimide (PAI), polyetherimide (PEI), polyetheretherketone (PEEK), Polyamide (PI), liquid crystal polymer (LCP), and polytetrafluoroethylene (PTFE) can be used.

Elastomers include thermosetting rubber (vulcanized rubber) and thermoplastic elastomers. Styrene butadiene rubber (SBR), nitrile rubber (NBR), butadiene rubber (BR), chlorobrene rubber (CR), ethylene propylene diene rubber (EPDM), butyl rubber (IIR), as thermosetting rubber (vulverized rubber), Isobutylene rubber (IR), acrylic rubber (ACM), and silicone rubber (Q) can be used.

Thermoplastic elastomers include softened polyvinyl chloride, heat-resistant polyvinyl chloride, ethylene-vinyl acetate copolymer (EVA), ethylene propylene rubber (EPR), styrene-based TPE (TPS), olefin-based TPE (TPO), and urethane-based TPE. (TPU), polyester-based (TPPE), chlorinated polyethylene and the like can be used.

From the viewpoint of consideration for the environment, it is preferable to use a halogen-free organic material that does not contain halogen as the organic material used as the binder. Since these resin materials are common as binder materials for resin sheets, they can be easily obtained.

Here, the term "flexible" as used herein means that the radio wave absorbing sheet 1 can be bent to a certain extent, that is, when the radio wave absorbing sheet 1 is bent and then returned to its original state, it breaks or the like. It shows a state in which the original shape is restored without plastic deformation. Further, even when the radio wave absorber is not a sheet shape but a block body having a certain thickness or more, for example, when an external force is applied while holding both ends of the radio wave absorber, the radio wave absorber is curved and the radio wave absorber is curved. It is shown that when this external force is released, the radio wave absorption can be easily restored to the original shape.

As a molding material for a radio wave absorbing member for forming the radio wave absorber 10, it is important to satisfactorily disperse magnetic iron oxide powder in a binder made of an organic material. Therefore, as a dispersant in the binder 2, aryl phosphonic acid such as phenylphosphonic acid and phenylphosphonic acid dichloride, alkylphosphonic acid such as methylphosphonic acid, ethylphosphonic acid, octylphosphonic acid and propylphosphonic acid, or hydroxyethanedi. Phosphonates, polyfunctional phosphonic acids such as nitrotrismethylenephosphonic acid and the like can be included. Since these phosphoric acid compounds have flame retardancy and function as a dispersant for magnetic iron oxide powder, magnetic iron oxide in the binder can be satisfactorily dispersed.

More specifically, as the dispersant, phenylphosphonic acid (PPA) manufactured by Wako Pure Chemical Industries, Ltd. or Nissan Chemical Industry Co., Ltd., and oxidized phosphoric acid ester "JP-502" manufactured by Johoku Chemical Industry Co., Ltd. (Product name) etc. can be used.

As an example, the composition of the molding material for the radio wave absorbing member is such that the binder of the organic material is 2 to 50 parts and the content of the phosphoric acid compound is 0.1 to 15 parts with respect to 100 parts of the magnetic iron oxide powder. can do. If the amount of the binder of the organic material is less than two parts, the magnetic iron oxide cannot be dispersed well. In addition, it becomes impossible to maintain a predetermined shape as a radio wave absorber. If it is more than 50 parts, the volume content of magnetic iron oxide in the radio wave absorber becomes small, and the magnetic permeability becomes low, so that the effect of radio wave absorption becomes small.

If the content of the phosphoric acid compound is less than 0.1 part, the magnetic iron oxide cannot be satisfactorily dispersed using the resin binder. If it is more than 15 parts, the effect of satisfactorily dispersing magnetic iron oxide is saturated. The volume content of magnetic iron oxide in the radio wave absorption layer becomes small, and the magnetic permeability becomes low, so that the effect of radio wave absorption becomes small.

[Manufacturing method of radio wave absorber]
Here, an example of a method for manufacturing a radio wave absorber according to the present embodiment will be described.

The molding material for a radio wave absorbing member according to the present embodiment is produced as a magnetic compound containing magnetic iron oxide powder, a binder made of an organic material such as rubber, and an appropriate filler. The magnetic compound can be obtained by kneading magnetic iron oxide powder and a binder and mixing a cross-linking agent with the obtained kneaded product to adjust the viscosity.

As shown in FIG. 1, the radio wave absorbing sheet 1 as a radio wave absorber can be produced by applying a magnetic compound to a predetermined thickness, drying the material, and then performing a calendar treatment.

When the radio wave absorbing sheet 1 is produced, as shown in FIG. 1, the magnetic compound produced above is applied onto the resin sheet 2. As the resin sheet 2, as an example, a polyethylene terephthalate (PET) sheet having a thickness of 38 μm whose surface has been peeled off by a silicon coat can be used. The magnetic compound is applied onto the resin sheet 2 by using a coating method such as a table coater method or a bar coater method. Then, the magnetic compound in the wet state is dried and further subjected to a calendar treatment. The thickness of the radio wave absorbing sheet 1 can be controlled by the coating thickness, calendar treatment conditions, and the like. The electromagnetic wave absorbing sheet 1 after the calendar treatment is peeled off from the resin sheet 2 to obtain an electromagnetic wave absorbing sheet 1 having a desired thickness. The calendar treatment may be performed as necessary, and if the volume content of the magnetic iron oxide powder is within a predetermined range in a dried state of the magnetic compound, the calendar treatment may not be performed. No.

If the formed radio wave absorbing sheet 1 is too thin to obtain the predetermined strength required for the radio wave absorbing sheet 1, the resin sheet 2 used as the equipment is used as the base film, and the radio wave absorbing sheet 1 is used. It can be laminated on the resin base film 2. When the resin sheet 2 is used as the base film, a paper member such as rubber or Japanese paper can be used in addition to various resin films such as the PET film described above. Since the material and thickness of the base film do not affect the radio wave absorption characteristics of the radio wave absorption sheet 1, it is an appropriate material and appropriate from the practical viewpoints such as the strength and ease of handling of the radio wave absorption sheet. A base film having a large thickness can be selected.

Next, a case where a radio wave absorber having a desired shape is produced will be described.

As an example, when a rubber binder is used, 80 parts by weight of strontium ferrite is used as magnetic iron oxide powder, and 18 parts by weight of silicone rubber KE-510-U (trade name: manufactured by Shin-Etsu Chemical Co., Ltd.) is pressed as a rubber binder. Knead with a batch kneader. 2 parts by weight of 2.5 dimethyl-2.5 bishexane C-8A (trade name: manufactured by Shin-Etsu Chemical Co., Ltd.) is mixed with this kneaded product as a cross-linking agent.

The molding material for the radio wave absorbing member thus obtained is vulcanized and molded using a press molding machine to be molded into a desired shape such as a sheet and an O-ring (packing). As described above, the radio wave absorber disclosed in the present application is based on silicone rubber, and therefore has deformability such as being curved as a whole or absorbing some unevenness. For example, it is mounted on a circuit board. By arranging it so as to cover a specific circuit component that becomes a noise source, it is possible to wrap the periphery of the electronic component of the noise source without a gap and enhance the shield of unwanted leaked radio waves.

When producing a block-shaped radio wave absorber such as a rectangular parallelepiped, for example, a magnetic compound, which is a molding material for a radio wave absorbing member, is crosslinked and molded into a sheet at a temperature of 150 ° C. using, for example, a hydraulic press. do. Then, in a constant temperature bath, for example, a secondary cross-linking treatment is performed at a temperature of 170 degrees to obtain a radio wave absorber having a predetermined shape. As described above, when a binder made of a rubber-based material is used, it can be produced by direct molding by heat vulcanization by a press.

When a thermoplastic resin is used as the binder, 80 parts by weight of strontium ferrite is added as the magnetic iron oxide powder, and 20 parts by weight of nylon 6 1013B (trade name: manufactured by Ube Kosan Co., Ltd.) is added as the thermoplastic resin. Knead with a pressure type batch kneader.

The molding material for the radio wave absorbing member thus obtained is injection-molded using an injection molding machine to be molded into a desired shape such as a bracket and a cover. Since the radio wave absorber disclosed in the present application is based on nylon 6 as described above, it is tough and strong as a whole. Therefore, for example, it is suitably used as a radio wave absorber to be arranged in an in-vehicle collision prevention millimeter-wave radar accompanied by vibration, and among the radio waves radiated from the radar, unnecessary radio waves that are diffusely reflected can be absorbed and suppressed.

[Adhesive layer]
Although not shown in FIG. 1, an adhesive layer can be formed on the surface of at least a part of the radio wave absorber so that the radio wave absorber according to the present embodiment can be easily fixed at a predetermined position. .. In particular, in the case of a thin block-shaped radio wave absorber, by providing an adhesive layer on at least one of its main surfaces, the radio wave absorber can be used on the inner surface of a housing for accommodating an electric circuit or on electricity. It can be easily attached to a desired position on the inner or outer surface of the device. In particular, since the radio wave absorber of the present embodiment has elasticity by using a rubber binder, it can be easily attached even on a curved surface curved by the adhesive layer 2, and the radio wave absorber can be easily handled. Sex improves.

As the adhesive layer, a known material used as an adhesive layer such as an adhesive tape, an acrylic pressure-sensitive adhesive, a rubber-based pressure-sensitive adhesive, a silicon-based pressure-sensitive adhesive, or the like can be used. Further, a tackifier or a cross-linking agent can be used to adjust the adhesive force to the adherend and reduce the adhesive residue. The adhesive force to the adherend is preferably 5N / 10mm to 12N / 10mm. If the adhesive strength is less than 5N / 10mm, the radio wave absorber may be easily peeled off or displaced from the adherend. Further, when the adhesive strength is larger than 12N / 10mm, it becomes difficult to peel the radio wave absorber from the adherend.

The thickness of the adhesive layer is also preferably a predetermined thickness, for example, about 20 μm to 1 mm or more so that the radio wave absorber does not easily peel off. When the radio wave absorber is re-tensioned as appropriate, the adherend may not be damaged when it is peeled off from the adherend, the radio wave absorber may be damaged, or adhesive residue may not be generated. It is necessary to consider the adhesive strength and thickness of the adhesive layer.

Further, when the radio wave absorber is attached to a predetermined surface, even if the radio wave absorber does not have an adhesive layer, the surface on the side of the member on which the radio wave absorber is arranged is provided with adhesiveness to provide the radio wave absorber. Can be pasted. Further, by using a double-sided tape or an adhesive, the radio wave absorber can be attached to a predetermined portion. In this respect, it is clear that the adhesive layer is not an essential component of the radio wave absorber shown in this embodiment.

[Radio wave absorption characteristics]
Hereinafter, the magnetic properties defined as the molding material for the radio wave absorbing member disclosed in the present application will be specifically described.

In measuring the magnetic characteristics, a radio absorber containing a predetermined magnetic iron oxide was cut to prepare a disk-shaped sample having a diameter of 8 mmφ and a thickness of 0.1 mm or more and 10 mm or less. The applied magnetic field was measured in the range of -15 kOe to + 15 kOe using a vibrating sample magnetometer VSM-P7 (product name) manufactured by Co., Ltd. The time constant Tc of the measurement was 0.03 sec, the drawing step was 40 bits, and the weight time was 0.2 Sec.

The average particle size of magnetic iron oxide was measured as follows.

Using a scanning electron microscope (SEM) "S-4800 (trade name)" manufactured by Hitachi, Ltd., acceleration voltage: 2 kV, magnification: 10000 times, observation conditions: oxidation in one field from a photograph taken with U-LA100. The maximum diameter of 100 particles of iron powder was measured, and the simple average value of the measured particle diameters was defined as the average particle diameter.

FIG. 2 is a diagram showing a magnetization curve of the first molding material for a radio wave absorbing member according to the present embodiment.

The first molding material for the radio wave absorbing member (hereinafter, appropriately referred to as “molding material of Example 1”) is 79 parts by weight of strontium ferrite as magnetic iron oxide powder, and nylon 6 (manufactured by Ube Kosan Co., Ltd.) as a binder. 17 parts by weight of 1013B (trade name)) and 4 parts by weight of a low volatile carboxylic acid derivative as a dispersant were used. The average particle size of strontium ferrite was 3.6 nm.

After stirring the magnetic iron oxide powder, the binder material, and the dispersant with a tumbler, the twin-screw extruder (15 nmφ, L / D = 45) was continuously kneaded at a temperature of 235 ° C to 270 ° C and a screw rotation speed of 100 rpm. .. The kneaded product on the strand extruded from the nozzle was water-cooled and cut into a length of 3 mm with a pelletizer to obtain a molding material, and a molded product having a length of 120 mm □ and a thickness of 2 mm was prepared by an injection molding machine.

A disk-shaped sample of Example 1 having a diameter of 8 mmφ and a thickness of 1 mm was prepared from this molded body.

As shown in FIG. 2, the magnetization curve showing the strength of the magnetization remaining in the magnetic iron oxide when a magnetic field whose strength changes from the outside is applied draws a so-called hysteresis curve.

The relationship shown in the following equation (1) holds between the value of the anisotropic magnetic field ( _{HA) and the magnetic resonance frequency fr of the magnetic material.}

fr = ν / 2π * _HA (1)
Here, ν is a gyro magnetic constant, which is a value determined by the type of magnetic material.

As described above, in the gyro magnetic resonance type magnetic material, a proportional relationship is established between the value _{of the anisotropic magnetic field (HA) and the magnetic resonance frequency fr.}

For example, when a magnetic material contains magnetic materials having different anisotropic magnetic field ( _HA ) values, magnetic resonance occurs at different frequencies to convert radio waves of that frequency into heat and attenuate them. .. As a result, it is possible to absorb radio waves of predetermined different frequencies. Further, it is known that the magnitude of the anisotropic magnetic field ( _HA ) of magnetic iron oxide changes depending on its particle size. Therefore, by analyzing the hysteresis curve of magnetic iron oxide, the particle size distribution of magnetic iron oxide contained in the sample can be grasped.

FIG. 3 is obtained by differentiating the hysteresis curve of the molding material of Example 1 shown in FIG. 2 from the point A where the applied magnetic field is +15 kOe to the point B where the applied magnetic field is -15 kOe. It is a figure which shows the curve (SFD: Switching Field Distribution). Note that FIG. 3 shows an SFD curve in which the applied magnetic field is from 0 kOe to −10 kOe.

More specifically, in the measuring device described in the present embodiment, since the points A to B of the hysteresis curve shown in FIG. 2 are drawn by the data of 637, the points from the 11th point to the 627th point are drawn. A linear least squares approximation was performed at 10 points before and after the measurement points, for a total of 21 points, and the slope of the obtained approximation formula was used as the differential value (numerical value obtained by one-time differentiation) at the measurement points.

FIG. 4 shows that in the hysteresis curve of the molding material of Example 1 shown in FIG. 2, the point A where the applied magnetic field is +15 kOe is differentiated twice from the point B where the applied magnetic field is -15 kOe. It is a figure which shows the obtained curve. FIG. 4 also shows a double differential curve in which the applied magnetic field is from 0 kOe to -10 kOe.

Specifically, the differential values at each measurement point from the 11th point to the 627th point of the hysteresis curve shown in FIG. 2 are the measurement points from the 21st point to the 617th point and 10 points before and after each measurement point. The slope of the straight line is obtained by approximating the differential values at the 21 measurement points in total again to the minimum square, and this value is the value obtained by differentiating the hysteresis curve at the measurement point twice (the value obtained by differentiating twice). ).

In the molding material of Example 1 of the present embodiment, as shown in FIG. 4, the applied magnetic field is the first positive peak P1 (3.12 × 10 ^-6 ) in the vicinity of -4000 kOe, and the applied magnetic field is −. A second positive peak P2 (4.33 × 10 ^-7 ) appears near 9500 kOe. From these P1 and P2, it can be seen that P2 / P1 is 0.14.

FIG. 5 shows the radio wave absorption characteristics of the molding material of Example 1 and the radio wave absorption characteristics of the first radio wave absorber obtained by injection molding the molding material of Example 1.

As the radio wave absorption characteristic, the radio wave absorption amount (radio wave attenuation) was measured using the free space method. Specifically, using a millimeter-wave network analyzer ME7838A (product name) manufactured by Anritsu Co., Ltd., a predetermined frequency is applied to each of the sample 1 or the radio wave absorber produced from the sample 1 from the transmitting antenna via the dielectric lens. The input wave (millimeter wave) was irradiated, and the radio wave transmitted by the receiving antenna arranged on the back side was measured. The intensity of the irradiated radio wave and the intensity of the transmitted radio wave were grasped as voltage values, and the amount of radio wave attenuation was obtained in dB from the difference in intensity.

As shown in FIG. 5, the absorption peak frequency in the radio wave absorption characteristic (reference numeral 5a) of the molding material of Example 1 and the radio wave absorption characteristic (reference numeral 5b) of the radio wave absorber molded using the molding material of Example 1. The deviation was as small as 0.1 GHz, and the curves showing the two radio wave absorption characteristics almost overlapped, and it can be confirmed that the change in the radio wave absorption characteristics before and after the molding process was small.

Next, the molding material for the radio wave absorbing member of Example 2 which also used strontium ferrite as the magnetic iron oxide, and the molding material for the radio wave absorbing member of Example 3 which also used strontium ferrite as the magnetic iron oxide were used. Made.

Further, a molding material for a radio wave absorbing member of Example 4 using barium ferrite as the magnetic iron oxide and a molding material for a radio wave absorbing member of Example 5 using barium ferrite as the magnetic iron oxide were similarly produced. The details of the molding materials of Examples 2 to 5 are as follows.

[Example 2]
The same procedure as in Example 1 was carried out except that strontium ferrite having an average particle size of 4.1 nm was used as the magnetic iron oxide powder.

[Example 3]
The same procedure as in Example 1 was carried out except that strontium ferrite having an average particle size of 2.3 nm was used as the magnetic iron oxide powder.

[Example 4]
The same as in Example 1 was used except that pallium ferrite having an average particle size of 4.8 nm was used as the magnetic iron oxide powder.

[Example 5]
The same as in Example 1 was used except that pallium ferrite having an average particle size of 5.0 nm was used as the magnetic iron oxide powder.

Further, as a comparative example, the radio wave absorption of Comparative Example 1 and Comparative Example 2 which are molding materials for a radio wave absorbing member using strontium ferrite as the magnetic iron oxide, and Comparative Example 3 using barium ferrite as the magnetic iron oxide. A molding material for a member was produced. The details of the molding materials for the radio wave absorbing member of Comparative Example 1, Comparative Example 2, and Comparative Example 3 are as follows.

[Comparative Example 1]
As the magnetic iron oxide powder, strontium ferrite having an average particle diameter of 4.1 nm was used, the magnetic iron oxide powder was 82 parts by weight, the nylon 6 was 18 parts by weight, and a low volatile carboxylic acid derivative was not used. Other than these, it was the same as in Example 1.

[Comparative Example 2]
The same procedure as in Example 1 was carried out except that strontium ferrite having an average particle size of 49.4 nm was used as the magnetic iron oxide powder.

[Comparative Example 3]
The same procedure as in Example 1 was carried out except that pallium ferrite having an average particle size of 5.5 nm was used as the magnetic iron oxide powder.

The magnetic properties of the molding materials of Examples 2 and 4, and the molding materials of Comparative Examples 1 and 2 as the molding materials for the radio wave absorbing member, as in the case of the molding materials of Example 1 described above. Was measured, and the obtained hysteresis curve was differentiated twice from point A to point B.

FIG. 6 shows a curve obtained by differentiating the hysteresis curve of the molding material of Example 2 twice.

FIG. 7 is a diagram showing the radio wave absorption characteristics of the molding material of Example 2 in a state as a molding material for a radio wave absorbing member and the radio wave absorption characteristics of a radio wave absorber obtained by injection molding the molding material.

^{As shown in FIG. 6, the first positive peak P1 (3.99 × 10 -6} ) is also applied to the curve obtained by differentiating the hysteresis curve twice in the vicinity of the applied magnetic field of -4000 kOe. A second positive peak P2 (6.02 × 10 ^-7 ) appears near the magnetic field of -9500 kOe. From these P1 and P2, it can be seen that P2 / P1 is 0.15.

Further, comparing the radio wave absorption characteristic (reference numeral 7a) of the molded member of Example 2 shown in FIG. 7 with the radio wave absorption characteristic (reference numeral 7b) of the radio wave absorber molded using the molded member of Example 2, the molding is performed. By using a radio wave absorber, it can be confirmed that the absorption curve is shifted to the low frequency side as compared with that before molding. However, the peak absorption frequencies are 77 GHz and 76 GHz, respectively, and the amount of deviation of the absorption peak frequency remains at 0.9 GHz. Further, it can be confirmed that both of them have almost the same absorption characteristics as about 14.5 dB in terms of the amount of transmission attenuation indicating the absorption capacity.

FIG. 8 is a drawing showing a hysteresis curve of the molding member of the fourth embodiment.

Since the magnetic iron oxide of the molding member of Example 4 is barium ferrite, hysteresis is compared with the case of the molding material for the radio wave absorbing member of Example 1 in which strontium ferrite is used as the magnetic iron oxide shown in FIG. It can be seen that the shape of the curve is different.

Also in this case, as shown in FIG. 8, a curve is obtained by differentiating twice from the point A where the applied magnetic field is +15 kOe to the point B where the applied magnetic field is −15 kOe.

FIG. 9 is a diagram showing a curve obtained by differentiating points A to B of the hysteresis curve of the molding member of Example 4 twice. Note that also in FIG. 9, as in FIGS. 4 and 6, a double differential curve in which the applied magnetic field is from 0 kOe to −10 kOe is shown.

^{As shown in FIG. 9, the first positive peak P1 (1.33 × 10 -5} ) is also applied to the curve obtained by differentiating the hysteresis curve twice in the vicinity of −1000 kOe. A second positive peak P2 (5.84 × 10 ^-7 ) appears near the magnetic field of −9000 kOe. From these P1 and P2, it can be seen that P2 / P1 is 0.04.

Further, comparing the radio wave absorption characteristic (reference numeral 10a) of the molded member of Example 4 shown in FIG. 10 with the radio wave absorption characteristic (reference numeral 10b) of the radio wave absorber molded using the molded member of Example 4, the molding is performed. The amount of deviation of the absorption peak frequency before and after is almost 0 GHz, and it can be seen that the radio wave absorption characteristics have hardly changed.

Next, the case of the molding material for the radio wave absorbing member of the comparative example will be examined.

FIG. 11 shows the hysteresis curve of the molding material of Comparative Example 1, which is a molding material for a radio wave absorbing member, which is a sample prepared as a comparative example.

FIG. 12 is a diagram showing a curve obtained by differentiating points A to B of the hysteresis curve of the molding material of Comparative Example 1 twice. Note that also in FIG. 12, similarly to FIGS. 4, 6 and 9, a double differential curve in which the applied magnetic field is from 0 kOe to −10 kOe is shown.

As shown in FIG. 12, the curve obtained by differentiating twice the hysteresis curve of the molding material of Comparative Example 2, applied magnetic field in the vicinity -4000kOe first positive peak P1 (3.54 × 10 ^{- 6} ), and the second positive peak P2 (6.14 × 10 ^-7 ) appears in the vicinity of the applied magnetic field of -9500 kOe. From these P1 and P2, it can be seen that P2 / P1 is 0.17.

FIG. 13 is a diagram showing the radio wave absorption characteristics of the molding member of Comparative Example 1 in a state as a molding material for a radio wave absorption member and the radio wave absorption characteristics of the radio wave absorber after injection molding.

Comparing the radio wave absorption characteristics (reference numeral 13a) of the molding material of Comparative Example 1 shown in FIG. 13 with the radio wave absorption characteristics (reference numeral 13b) as the radio wave absorber after molding, it is molded into a radio wave absorber. Therefore, it can be confirmed that the absorption characteristics are shifted to the low frequency side as compared with those before molding. As shown in FIG. 13, the peak frequency of absorption in the radio wave absorption characteristic (reference numeral 13a) as the molding member of Comparative Example 1 is about 77.3 GHz, whereas the molding member of Comparative Example 1 is used. It can be seen that the peak frequency of absorption in the radio wave absorption characteristic (reference numeral 13b) of the molded radio wave absorber is about 76.1 GHz, and the peak frequency of absorption of about 1.2 GHz is deviated.

FIG. 14 is a diagram showing a curve obtained by differentiating points A to B of the hysteresis curve of the molding material of Comparative Example 2 twice. In addition, also in FIG. 14, similarly to FIG. 4, FIG. 6, FIG. 9, and FIG. 12, a double differential curve in which the applied magnetic field is from 0 kOe to −10 kOe is shown.

^{As shown in FIG. 14, the first positive peak P1 (3.44 × 10 -6} ) is also applied to the curve obtained by differentiating the hysteresis curve twice in the vicinity of the applied magnetic field of -4000 kOe. A second positive peak P2 (11.16 × 10 ^-6 ) appears near the magnetic field of -9500 kOe. From these P1 and P2, it can be seen that P2 / P1 is 0.34.

FIG. 15 is a diagram showing the radio wave absorption characteristics of the molding material of Comparative Example 2 in a state as a molding material for a radio wave absorbing member and the radio wave absorption characteristics of the radio wave absorber after injection molding.

Comparing the radio wave absorption characteristics (reference numeral 15a) of the comparative example 2 molding material shown in FIG. 15 with the radio wave absorption characteristics (reference numeral 15b) of the radio wave absorber after molding using the comparative example 2, the radio wave absorption characteristics of the molded material are compared. By using the body, the peak frequency of absorption in the radio wave absorption characteristics (reference numeral 15a) as a molding material is about 77.5 GHz, whereas the radio wave absorption characteristics of the radio wave absorber molded using Comparative Example 2 It can be seen that the absorption peak frequency at (reference numeral 15b) is about 75.6 GHz, and the absorption peak frequency deviates from about 1.9 GHz.

In addition to the above measurement results, for the molding member of Example 3, the molding member of Example 5, and the molding member of Comparative Example 3, the values of P1, P2, P2 / P1 and the measurement result of the deviation amount of the peak frequency are shown. It is summarized in Table 1 below.

In addition, in the molding material of Comparative Example 3, the amount of deviation of the absorption peak frequency is larger than that in the case of using the molding materials of Examples 4 and 5 in which barium ferrite magnetic powder is also used as the magnetic iron oxide. This is because, like the molding material of Comparative Example 2 described above, the average particle size of the barium ferrite magnetic powder was large, so the particle size changed due to the shearing force applied to the particles in the injection molding process, and before and after injection molding. It is considered that the cause is that _{the anisotropic magnetic field (HA} ) of magnetic iron oxide has changed as a result of the difference in the substantial average particle size of.

For example, in an in-vehicle radar or the like, the frequency of radio waves is defined according to the purpose of use, such as 76.5 GHz ± 0.5 GHz. Therefore, the peak absorption frequency grasped at the stage of the molding material for the radio wave absorbing member, such as the molding member of Comparative Example 1 and the molding member of Comparative Example 2, is greatly shifted in the state of the radio wave absorber after the molding process. If this happens, the performance as a radio wave absorber used in the originally intended technical field will be significantly reduced.

Therefore, if it is found that the shift amount of the peak frequency of the absorbed radio wave before and after the molding process is small at the stage of the molding material for the radio wave absorbing member, the design absorption frequency and the absorption frequency of the molded body after the processing are set. Since there is no big difference, the radio wave absorber can be manufactured with peace of mind, and the yield of the radio wave absorber can be improved and the cost can be reduced by not using a material having a large shift amount.

Therefore, in the molding material for the radio wave absorbing member disclosed in the present application, in the hysteresis curve, the points magnetized by applying a positive magnetic field of +10 kOe or more and + 15 k or less are point A, and the applied magnetic field strength is reduced from the point A to -10 kOe. When the point magnetized by applying a negative magnetic field of -15 kOe or less is defined as point B, the applied magnetic field obtained by differentiating the hysteresis curve from point A to point B twice is in the range of 0 to -10 kOe. When the peak of the maximum peak in the positive direction is P1 and the peak of the peak having the polar value in the positive direction is P2, the value of P2 / P1 is defined as 0.15 or less. By defining in this way, the molding material for a radio wave absorbing member disclosed in the present application is a radio wave absorber that suppresses changes in radio wave absorbing characteristics before and after molding into a radio wave absorber and absorbs radio waves of a predetermined frequency satisfactorily. Can be realized.

In the above measurement, the amount of deviation of the absorption peak frequency before and after injection molding was measured, but magnetic iron oxide powder was kneaded with a resin binder to prepare a radio wave absorbing paste, which was used to form a sheet. Even when the radio wave absorbing sheet is produced, as in the case of injection molding, if the value of P2 / P1 is 0.15 or less, the absorption peak frequency in the state of the molding material and after molding into a sheet It was confirmed that the amount of deviation of the absorption peak frequency of was suppressed to 1 GHz or less.

In addition to the strontium ferrite and barium ferrite exemplified in the above embodiment, other hexagonal ferrites such as Pb ferrite are considered to exhibit similar radio wave absorption characteristics, but since Pb ferrite contains lead. Not suitable for practical use.

Further, in the above embodiment, it has been described that the peak of the absorption frequency can be shifted by using a hexagonal ferrite in which a part of the Fe site is substituted with Al, but the metal to be substituted is gallium in addition to aluminum. Ti-Cu-Zn, rhodium and the like can be used.

As described above, the molding material for the radio wave absorbing member disclosed in the present application and the radio wave absorber obtained by molding the molding material have two peak values P1 of a curve obtained by differentiating the hysteresis curve of the molding material twice. And P2 have a value of P2 / P1 of 0.15 or less, so that the shift amount of the radio wave absorption characteristics before and after the molding process, particularly the absorption peak frequency, can be suppressed within a certain range (about 1 GHz). ..

The strength of the external magnetic field for measuring the hysteresis curve is set to 15 kOe to -15 kOe, which means that a good hysteresis curve can be obtained by applying an external magnetic field in at least this range. .. Therefore, there is no problem even if the magnitude of the applied external magnetic field is made larger than the absolute value of 15 kOe, and the hysteresis curve in the range where the magnitude of the external magnetic field is in the range of 15 kOe to -15 kOe is measured. The values of P1 and P2 may be obtained from the differential curve.

The molding material for a radio wave absorbing member disclosed in the present application is useful as a molding material that satisfactorily absorbs radio waves of 20 GHz to 300 GHz and suppresses changes in radio wave absorption characteristics before and after molding. Further, a radio wave absorber molded using this molding material can be manufactured at low cost as a radio wave absorber in which fluctuations in its radio wave absorption characteristics are suppressed within a certain range, and is therefore extremely useful in practical use. ..

1a Magnetic iron oxide particles 1b Binder 10 Radio wave absorber

Claims (5)

A molding material for a radio wave absorbing member, which contains magnetic iron oxide that magnetically resonates with radio waves having a frequency of 20 GHz to 300 GHz and a binder of an organic material, and is molded into a predetermined shape.
The magnetic iron oxide is hexagonal ferrite,
In the hysteresis curve, the point magnetized by applying a positive magnetic field of + 10 kOe or more and + 15 k or less is magnetized by applying a negative magnetic field of -10 kOe or more and -15 kOe or less by reducing the applied magnetic field strength from the point A. When the point B is taken as the point B, the maximum peak in the positive direction in the curve in which the applied magnetic field obtained by differentiating the hysteresis curve from the point A to the point B twice is in the range of 0 to -10 kOe. A molding material for a radio wave absorbing member, wherein the value of P2 / P1 is 0.15 or less, where P1 is the apex and then P2 is the apex of the peak having an extreme value in the positive direction. The molding material for a radio wave absorbing member according to claim 1, wherein the magnetic iron oxide is strontium ferrite. The molding material for a radio wave absorbing member according to claim 1 or 2, wherein a part of the Fe site of the magnetic iron oxide is replaced with a trivalent metal atom. The molding material for a radio wave absorbing member according to any one of claims 1 to 3, wherein the binder is any one of a thermosetting rubber, a thermoplastic elastomer, and a thermoplastic resin. A radio wave absorber produced by molding the molding material for a radio wave absorbing member according to any one of claims 1 to 4.

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