JP5043082B2 - Electromagnetic wave absorbing material - Google Patents

Description

  The present invention relates to an electromagnetic wave absorbing material for absorbing electromagnetic waves.

In recent digital electronic devices such as personal computers and mobile phones, unnecessary noise generated from the devices has been increased in frequency from the MHz band to the GHz band due to the increase in the speed and frequency of the electrical signals. The band extends to 100 MHz to 3 GHz, and further higher frequencies are expected in the future. On the other hand, there is a great demand for downsizing and weight reduction for electronic devices, and circuit components are highly integrated. Under these circumstances, multiple reflections of radiated noise in a metal casing, mutual interference between circuit boards, and the like are becoming more frequent. Nevertheless, there is a problem that the space for installing the radiated noise countermeasure component is very narrow, and the unnecessary noise countermeasure component is also required to have high performance and further thinning.

  Furthermore, with the development of information and communication technologies in recent years, wireless LANs are widely used for computer network formation indoors. The frequency of the radio wave used for the wireless LAN is 2.4 to 2.5 GHz for the medium speed wireless LAN and 5.15 to 5.25 GHz for the high speed wireless LAN. In particular, the 2.4 to 2.5 GHz band has a high frequency of use for industrial applications, and is a band in which electromagnetic waves are complicated.

  As described above, with serious countermeasures against unwanted radiation noise and increasing demand in the field of wireless communication, problems such as crosstalk of radio waves between systems, mutual interference of electromagnetic waves, interference due to delay dispersion, malfunction, and inability to communicate arise. The fear is increasing.

  Usually, an electromagnetic shielding material is used for radio waves entering from the outside. However, since the electromagnetic wave shielding material reflects incident radio waves, it is not effective for self-radio waves reflected on the wall surface and floor surface of the room. For this reason, there exists a problem that communication quality will fall under the influence of the reflected wave of a self-radio wave.

For this reason, various electromagnetic wave absorbing materials that absorb unnecessary radio waves and unnecessary radiation noise have been proposed. For example, Patent Document 1 below discloses a radio wave absorbing coating composition in which a predetermined amount of ferrite and carbon are dispersed in a coating vehicle. Patent Document 2 listed below describes a thin-film electromagnetic wave absorber composition containing scaly graphite having a fixed carbon content of 80% or more, a particle diameter of 200 to 800 μm, and an aspect ratio of 50 or more.
However, with the technologies disclosed in these Patent Documents 1 and 2, the intended electromagnetic wave absorption performance cannot be obtained. In other words, Patent Documents 1 and 2 disclose means for freely controlling the electromagnetic wave absorption performance. Absent.

Furthermore, Patent Documents 3 and 4 below describe electromagnetic wave shielding paints in which graphite powder is mixed into a resin paint, but graphite is used alone, and means for freely controlling electromagnetic wave absorption performance is described below. Not disclosed.
Patent Document 5 below describes an electromagnetic wave erasing plate in which metal powder and graphite powder are mixed into an acrylic resin and formed into a plate shape. However, although the reflectivity of electromagnetic waves is described, the absorption performance is described. Is not described, and means for freely controlling electromagnetic wave absorption performance is not disclosed.

  In general, the performance of an electromagnetic wave absorber is determined by frequency, thickness, and material constants (ε ′, ε ″, μ ′, μ ″). Accordingly, it is necessary to control six parameters. In particular, material constants determined as material characteristics are difficult parameters for individual control because each constant varies in a related manner.

ここで、電波吸収体の材料として誘電体のみ（カーボンブラックなど）を用いる場合には、複素透磁率は１となる。そして、ｄ／λの変化させて、下記式で求められる複素誘電率の実部ε'と虚部ε''とを求め、横軸を実部ε'とし、縦軸を虚部ε''としたグラフにプロットして設計チャート（無反射曲線）を作成する。

That is, the conditional expression for satisfying the matching condition that the electromagnetic wave absorption amount becomes maximum, that is, the input impedance Z _in seen from the surface of the electromagnetic wave absorbing material is equal to the wave wave (characteristic) impedance Z _{0 of the} plane wave is expressed by the following expression. The

Here, when only a dielectric (such as carbon black) is used as the material of the radio wave absorber, the complex magnetic permeability is 1. Then, by changing d / λ, the real part ε ′ and the imaginary part ε ″ of the complex permittivity obtained by the following equation are obtained, the horizontal axis is the real part ε ′, and the vertical axis is the imaginary part ε ″. A design chart (non-reflection curve) is created by plotting on the graph.

  In order to design an electromagnetic wave absorbing material that absorbs an electromagnetic wave having a specific frequency λ, for example, d / λ = 0.1, the real part ε ′ corresponding to (d / λ = 0.1) from the design chart, An imaginary part ε ″ is obtained and substituted into the [Equation 2] to determine the complex dielectric constant. Further, the thickness of the electromagnetic wave absorbing material can be obtained from d / λ = 0.1. As a result, it can be seen that a dielectric having a complex dielectric constant of the above value is used and the thickness is determined from (d / λ = 0.1).

  However, when carbon black is used as the dielectric, there is a characteristic that the real part ε ′ is increased and the imaginary part ε ″ is also increased (that is, tan δ = ε ″ / ε ′ is increased). An increase in the imaginary part ε ″ or tan δ means that the conductivity of the material is improved and the resistance is lowered. On the other hand, the mechanism of electromagnetic wave absorption is to convert the energy of incident electromagnetic waves into thermal energy inside, so such an improvement in conductivity (decrease in resistance) is negative for electromagnetic wave absorption, rather, reflection of electromagnetic waves. The characteristics will be improved.

  On the other hand, titanium oxide is also known as a dielectric. However, when titanium oxide is used, it is possible to reduce tan δ, but titanium oxide has a large specific gravity of 4.2 and the real part ε ′ cannot be increased unless a large amount is filled. There is a problem that the obtained electromagnetic wave absorbing material becomes heavy and further the workability is lowered.

Further, the electromagnetic wave absorbing material is required to have a reduced thickness in addition to the realization of the electromagnetic wave absorbing characteristics as intended. For this reason, the actual situation is that the design of the electromagnetic wave absorbing material is very difficult.
Although the above description is for the case where a dielectric is used, the same problem occurs when a magnetic material such as ferrite is used. When a dielectric material and a magnetic material are used in combination, the variation parameter in the material constant increases, making it more difficult to satisfy the matching condition.

JP-A-54-61239 Japanese Patent Laid-Open No. 1-230299 JP 62-109866 A JP 62-111499 A JP-A-61-2250501

Osamu Hashimoto, “The story of the electromagnetic wave absorber”, Nikkan Kogyo Shimbun, June 29, 2001, p. 45-50

The main subject of this invention is providing the electromagnetic wave absorption material with a high design freedom.
Another object of the present invention is to provide an electromagnetic wave absorbing material having a thin thickness and high electromagnetic wave absorbing performance.

  In the process of intensive research to solve the above-mentioned problems, the present inventors can freely adjust the material constant by using graphite among the dielectrics, and can achieve high ε ′ and low ε ″. I got new knowledge. Therefore, by incorporating graphite as a dielectric constant adjusting agent into an electromagnetic wave absorbing material containing a dielectric and / or magnetic material excluding graphite, the material constant can be freely adjusted, so that the matching condition can be satisfied and high electromagnetic waves can be achieved. Since the absorption performance can be achieved and the thickness can be reduced, the inventors have found a new fact that the degree of freedom in designing the electromagnetic wave absorbing material is widened, and have completed the present invention.

That is, the electromagnetic wave absorbing material of the present invention has the following configuration.
(1) with a dielectric (except graphite) and / or magnetic material as the dielectric constant adjusting agent, fixed carbon content 97%, ash content of 3% or less, volatiles contained 3% of graphite, 1 The real part ε ′ of the complex dielectric constant in the 25 GHz band is 170 to 392, the imaginary part ε ″ is 39 to 120, and tan δ (= ε ″ / ε ′) is 0.17 to 0.47. An electromagnetic wave absorbing material.
(2) The electromagnetic wave absorbing material according to (1), wherein the dielectric is carbon black.
(3) The electromagnetic wave absorbing material according to (1), wherein the magnetic body is ferrite or iron.
(4) The electromagnetic wave absorbing material according to (3), wherein the iron is a Fe—Al—Si alloy, a Fe—Cr—Si alloy, or a Fe—Ni—Cr—Si alloy.
(5) The electromagnetic wave absorbing material according to (3) or (4), wherein the magnetic material has a flat shape having a major axis of 5 to 400 μm and an aspect ratio of 5 to 100.
(6) The electromagnetic wave absorbing material according to any one of (1) to (5), wherein at least one of the dielectric, magnetic material and graphite is subjected to insulation treatment.
(7) The electromagnetic wave absorbing material according to any one of (1) to (6), wherein the volume resistivity is 10 ⁶ Ω · cm or more.
(8) The electromagnetic wave absorbing material according to any one of (1) to (7), which contains 1 to 50% by volume of a heat dissipation material.
(9) The electromagnetic wave absorbing material according to any one of (1) to (8), wherein the heat dissipation material is boron nitride or aluminum nitride.
(10) The electromagnetic wave absorbing material according to any one of (1) to (9), which is a rubber sheet or a synthetic resin sheet.
(11) The electromagnetic wave absorbing material according to (10), wherein the sheet has a thickness of 0.01 to 10 mm.
(12) The electromagnetic wave absorbing material according to the above (10) or (11), comprising 10 to 1000 parts by weight of the dielectric and / or magnetic substance and 3 to 150 parts by weight of graphite with respect to 100 parts by weight of rubber or synthetic resin. .
(13) The electromagnetic wave absorbing material according to (10) or (11), wherein the dielectric and / or magnetic substance is contained in an amount of 10 to 90% by volume and graphite is contained in an amount of 1 to 30% by volume in the electromagnetic wave absorbing material.
(14) Any one of the above (1) to (13) containing at least one flame retardant selected from a phosphoric flame retardant, a hydroxide flame retardant, a bromine flame retardant, and a metal compound flame retardant Electromagnetic wave absorbing material.
(15) The electromagnetic wave absorbing material according to the above (14), wherein the flame retardant is decabromodiphenyl oxide and / or antimony trioxide.

According to the electromagnetic wave absorbing material of the present invention, the material constant can be freely adjusted by including specific graphite as a dielectric constant adjusting agent together with the dielectric and / or magnetic material, and high ε ′ and low ε ″ can be obtained. Therefore, the degree of freedom in design is improved, high electromagnetic wave absorption performance can be realized in a desired frequency band, and the thickness can be reduced.
Further, as described in (8) and (9) above, by including a heat dissipation material, thermal conductivity can be imparted in addition to the electromagnetic wave absorbing performance. Furthermore, as described in (14) and (15) above, by containing a flame retardant, flame retardancy can also be imparted.

It is a graph which shows the relationship between the real part (epsilon) 'and imaginary part (epsilon)' 'of the complex dielectric constant of various dielectrics. It is a graph which shows the relationship between real part (epsilon) 'and imaginary part (epsilon)' 'for demonstrating the design method of electromagnetic wave absorption material. It is the schematic which shows the shape of the microstrip line used in Example 7 and Comparative Example 8. It is a graph which shows the transmission loss of the microstrip line of Example 7 and Comparative Example 8.

  The electromagnetic wave absorbing material of the present invention is a material in which specific graphite is contained as a dielectric constant adjusting agent together with a dielectric material and / or a magnetic material.

  Examples of the material include polymer materials such as rubber and synthetic resins (thermoplastic elastomers and various plastics). Examples of the rubber include natural rubber, isoprene rubber, butadiene rubber, styrene-butadiene rubber, ethylene-propylene rubber, butyl rubber, chloroprene rubber, nitrile rubber, acrylic rubber, epichlorohydrin rubber, fluorine rubber, urethane rubber, chlorine. And synthetic rubbers such as hydrogenated nitrile rubber, hydrogenated nitrile rubber, and silicon rubber, or those modified by various modification treatments. These rubber materials can be used alone or in admixture of two or more, and can also be used by blending a plurality of other rubbers and resins. In addition to vulcanizing agents, rubbers may be appropriately mixed with vulcanization accelerators, anti-aging agents, softeners, plasticizers, fillers, coloring agents, and the like that have been conventionally used as rubber compounding agents. it can.

  Examples of the thermoplastic elastomer include various thermoplastic elastomers such as polystyrene, polyolefin, polyvinyl chloride, polyurethane, polyester, and polyamide.

In addition, polyethylene, polypropylene, AS resin, ABS resin, polystyrene, polyvinyl chloride, polyvinylidene chloride, polyvinyl acetate, fluororesin, acrylic resin, nylon, polycarbonate, polyethylene terephthalate, alkyd resin, unsaturated polyester, polysulfone, polyurethane Thermoplastic resins such as phenol resins, urea resins, and epoxy resins, or thermosetting resins can also be used as the material.
In addition to the polymer material, a material that can contain a filler, such as a gypsum material and a cement material, can also be used as appropriate.

Examples of the dielectric include carbon black and titanium oxide. Examples of the carbon black include furnace black and channel black. Specifically, it is preferable to use ketjen black manufactured by Lion Akzo Corporation. Carbon fibers and carbon coils may be used as carbon black.
In particular, the combined use of low ε "graphite and high ε" carbon black allows the complex permittivity to be freely controlled on the non-reflective curve, thus improving the degree of design freedom to meet the matching conditions. There is.

  Examples of the magnetic body include ferrite, iron alloy, and pure iron particles. These magnetic materials have a characteristic that the real part μ ′ of the complex permeability is increased and the imaginary part μ ″ is also increased (that is, tan δ = μ ″ / μ ′ is increased). In addition, since the magnetic material itself has an inherent complex dielectric constant (real part ε ′ and imaginary part ε ″), four parameters (ε ′, ε ″, μ ', Μ' ') will fluctuate. Therefore, it is not easy to satisfy the matching condition under the conditions of the thin film. By including graphite as a dielectric constant adjusting agent in the electromagnetic wave absorbing material containing such a magnetic substance, the matching condition can be satisfied, and the electromagnetic wave absorbing performance can be improved.

  Examples of the ferrite include soft ferrite such as Mn—Zn ferrite, Ni—Zn ferrite, Mn—Mg ferrite, Mn ferrite, Cu—Zn ferrite, and Cu—Mg—Zn ferrite, or hard ferrite that is a permanent magnet material. . Examples of iron alloys include magnetic stainless steel (Fe—Cr—Al—Si alloy), sendust (Fe—Si—Al alloy), perm alloy (Fe—Ni alloy), silicon copper (Fe—Cu—Si alloy), Fe -Si-B (-Cu-Nb) alloy, Fe-Cr-Si alloy, Fe-Ni-Cr-Si alloy, etc. are mentioned. In addition, in these alloys, you may use a flat thing. Examples of the pure iron particles include carbonyl iron powder. It is preferable to use soft ferrite powder having a low magnetic permeability and high cost. As a countermeasure against unwanted noise in the near electromagnetic field, it is preferable to use an Fe—Al—Si alloy, an Fe—Cr—Si alloy, or an Fe—Ni—Cr—Si alloy.

  Moreover, regarding the particle size of a magnetic body, 0.01 micrometer-30 micrometers are preferable. Particularly preferred is 0.1 μm to 10 μm. If the particle size of the magnetic material is less than 0.01 μm, it is difficult to disperse because the cohesive force is high. On the other hand, if it exceeds 30 μm, the complex permeability decreases and the workability also deteriorates. Regarding the shape, when carbonyl iron is used, the carbonyl iron is preferably close to a true sphere.

  When the magnetic material is processed into a flat shape, the strength of the magnetic material itself is inferior and easily distorted, making it difficult to orient in one direction in the polymer, but the magnetic permeability (real part μ ′ and imaginary part μ ″) The flat magnetic body preferably has a major axis of 5 to 400 μm, more preferably a major axis of 5 to 300 μm, and still more preferably a major axis of 10 to 250 μm. If the major axis of the magnetic material is 5 μm or less, the magnetic permeability is low, and conversely, if the magnetic material is 400 μm or more, the workability deteriorates, and the aspect ratio of the flat magnetic material is preferably 5 to 100. It is particularly preferably 80 to 80. When the aspect ratio of the flat shape is 5 or less, the magnetic permeability is low, whereas when it is 100 or more, it is difficult to maintain the shape of the magnetic material.

As a means for reducing the thickness of the electromagnetic wave absorbing material, the blending amount of the magnetic material may be increased. However, since iron alloys (soft magnetic metals, etc.) and pure iron particles have high electrical conductivity, they can cause contact between fillers in the raw material when mixed in large quantities, resulting in radio waves. The conductivity of the absorbing material increases and the imaginary part ε ″ tends to increase. As described above, when the imaginary part ε ″ increases, the radio wave absorption performance is impaired. On the other hand, in the case of a radio wave absorbing material using only a dielectric, the same problem can occur if the filling amount is increased.
Since the electromagnetic wave absorbing material tends to have a lower ε ″ as the volume resistivity is higher, the volume resistivity is preferably 10 ⁶ Ω · cm or more, and more preferably 10 ⁸ Ω · cm or more.

  As a means for solving this problem, a method in which at least one of a dielectric material, a magnetic material, and graphite is subjected to an insulating treatment in advance or simultaneously with kneading is effective. As a result, even when a large amount of fillers such as dielectrics, magnetic substances, and graphite are blended, it is possible to suppress conduction due to contact, to suppress an increase in imaginary part ε ", and to easily satisfy the matching conditions Insulation treatment methods include plasma treatment method, alkoxide method, water glass treatment method, treatment method with silane coupling agent or titanate coupling agent, oxide film formation, coating with epoxy resin, coating with plating of ferrite, etc. Etc.

  The electromagnetic wave absorber can be used after imparting heat dissipation characteristics. Examples of the heat dissipation material include at least one selected from the group consisting of metal oxide, aluminum nitride, boron nitride, silicon nitride, and silicon carbide. Particularly preferred are boron nitride and aluminum nitride. High purity grade shows good characteristics as heat dissipation material. Magnetic powders that are highly conductive additives, such as Fe-based alloys such as Fe-Ni-Cr-Si based alloys, or graphite as a dielectric also contribute to improving heat dissipation characteristics. It is necessary to determine the heat conduction characteristics. For example, since both Fe-based alloys and graphite have high flatness, anisotropy tends to be exhibited in thermal conductivity when dispersed in a polymer. In this case, it is difficult to secure a path through which heat diffuses well in the polymer in the direction of poor thermal conductivity, that is, in the direction substantially perpendicular to the oriented powder. Therefore, the thermal conductivity in the direction almost perpendicular to the oriented powder will be improved by blending the thermal conductive fine particles with low conductivity and small particle size so as to be dispersed between the magnetic material and graphite particles. .

The blending amount of the heat dissipation material is 1 to 50% by volume, preferably 10 to 50% by volume. If the heat dissipating material is 50% by volume or more, workability deteriorates. In addition, the addition amount, conductivity, size, shape, and the like are determined so that the above-described volume resistance does not fall below 10 ⁶ due to the addition of the heat dissipation material.

  In order to impart flame retardancy, a flame retardant may be contained in the electromagnetic wave absorbing material. As such a flame retardant, for example, at least one selected from a phosphoric flame retardant, a hydroxide flame retardant, a bromine flame retardant, and a metal compound flame retardant is used.

  Examples of the phosphoric acid flame retardant include phosphoric acid ester, ammonium polyphosphate, and red phosphorus. Examples of the hydroxide flame retardant include magnesium hydroxide, aluminum hydroxide, zirconium hydroxide, calcium sulfate hydrate, zinc borate hydrate and the like. Examples of brominated flame retardants include decabromodiphenyl oxide, 2,2-bis (3,5-dibromo-4-hydroxyphenyl), 1,2,5,6,9,10-hexabromocyclododecane, hexabromo Examples thereof include benzene, tetrabromophthalic anhydride, pentabromotoluene and the like. Examples of the metal compound flame retardant include antimony trioxide, molybdenum oxide, manganese oxide, chromium oxide, and iron oxide.

  A particularly preferable flame retardant is a flame retardant added with decabromodiphenyl oxide and antimony trioxide, and a high flame retardant effect can be obtained in a small amount. In this case, you may use the flame retardant product which mixed decabromo diphenyl oxide and antimony trioxide, for example, the brand name "poly safe FCT-5" marketed by Ajinomoto Fine Technology.

  Graphite used as a dielectric constant adjuster has a fixed carbon content representing purity of 97% or more, preferably 99% or more, ash content of 3% or less, preferably 1% or less, and volatile content of 3% or less, preferably 1 % Or less. If the fixed carbon content of the graphite is less than 97%, the complex permittivity will not be increased unless a large amount of the corresponding graphite is blended, so that the workability is deteriorated. If the ash content of graphite is less than 3%, the complex dielectric constant will not increase unless a large amount of the corresponding graphite is blended, resulting in poor workability. Furthermore, if the volatile content of graphite is less than 3%, the complex dielectric constant will not increase unless a large amount of the graphite is blended, so that the workability will deteriorate.

Further, graphite having an average particle size (maximum particle size in the case of long piece-like graphite) of 0.1 to 650 μm, preferably 5 to 100 μm can be used. When the thickness is less than 0.1 μm, dispersion is difficult, and when it exceeds 650 μm, the deviation from the particle size of the magnetic material is large and the performance tends to become unstable. The apparent density of graphite is 0.03 to 1.0 g / cm ³ , preferably 0.03 to 0.5 g / cm ³ . The apparent density of graphite is measured by the method described in JIS M8511.
Examples of such graphite include artificial graphite; natural graphite such as scaly graphite, flaky graphite, massive graphite, earthy graphite, spherical graphite, and expanded graphite. Moreover, you may use a graphite fiber. When expanded graphite is used, the expanded graphite itself has properties as a flame retardant, and flame retardancy can be imparted at the same time.

  As described above, graphite has the property that high ε ′ and low ε ″ can be achieved simultaneously. Since such characteristics are not affected by the shape and particle size of graphite, any graphite may be used. In the present invention, however, it is preferable to use scaly graphite which can obtain a high effect in a small amount, and since it is a small amount, it is processed. Work and formulation design become easy.

  The electromagnetic wave absorbing material of the present invention contains a predetermined amount of dielectric and / or magnetic material, and further graphite according to a pre-designed compounding prescription, and has a predetermined thickness of sheet or film or any other shape. It is manufactured by molding. The arbitrary shape is an IC package, a substrate, an FPC, a cable coating layer, or the like. In addition, when the electromagnetic wave absorbing material is thin (for example, 0.3 mm or less), it can be produced using a known technique such as coating, spraying, four rolls, and knife coating.

  The formulation design is performed as follows. As a design target, frequency, thickness of electromagnetic wave absorbing material, and electromagnetic wave absorbing performance (dB) are determined. When the complex magnetic permeability (the real part μ ′ and the imaginary part μ ″) is fixed with the blending amount of the magnetic material constant, the target electromagnetic wave absorption performance is shown based on the design formula shown in the above [Equation 1]. A range (preferred range of the real part ε ′ and the imaginary part ε ″) is determined. Therefore, by adjusting the amount of graphite, the complex dielectric constant (real part ε ′ and imaginary part ε ″) can be taken as intended.

  Even when a dielectric (carbon black, etc.) is used instead of a magnetic material, the complex dielectric constant (real part ε ′ and imaginary part ε ″) can be obtained as intended by adjusting the amount of graphite. can go.

  Since the unnecessary noise countermeasure sheet is used for the purpose of reducing noise in a wide frequency range from 100 MHz to 3 GHz, a material design that satisfies the matching condition referred to in the present invention is not necessarily taken. However, when receiving countermeasures against high-frequency noise such as microwaves in the space, the concept of electromagnetic wave absorption by matching is effective, and a noise absorbing sheet having a matching frequency at an arbitrary frequency is maintained while maintaining a high imaginary part μ ''. It can be provided. That is, the high real part μ ′ and imaginary part μ ″ of the noise absorbing sheet are determined by the type, amount, and orientation state of the magnetic substance to be added, but the complex dielectric constant (particularly the real part ε ′) is controlled by graphite. Thereby, it can be set as the noise absorption sheet which gave consistency to the arbitrary frequencies (for example, 1-5 GHz) in arbitrary thickness.

  The compounding amount of graphite as a dielectric constant adjuster is appropriately determined so as to satisfy the matching condition, but is usually 3 to 150 parts by weight, preferably 5 to 80 parts by weight with respect to 100 parts by weight of the material material. It should be contained. The dielectric and / or magnetic substance may be contained in an amount of 10 to 1000 parts by weight, preferably 80 to 600 parts by weight, based on 100 parts by weight of the material material. In other words, it is preferable to use 10 to 90% by volume of dielectric and / or magnetic material and 1 to 30% by volume of graphite. If the blending amount of graphite and dielectric and / or magnetic material exceeds the above range, workability may be deteriorated. Further, if the amount of graphite is less than the above range, the complex dielectric constant may not be adjusted as designed.

  The electromagnetic wave absorbing material of the present invention can be formed into an arbitrary shape such as a sheet. In this case, according to the present invention, since the dielectric constant can be adjusted with graphite, the lower limit of the thickness can be reduced to 0.01 mm, preferably about 0.03 mm. The upper limit of the thickness is about 10 mm, preferably about 5 mm. Here, the lower limit value of the thickness indicates a lower limit value in manufacturing when a noise countermeasure wave absorber for a near electromagnetic field is manufactured by coating.

  When rubber is used as the material material, the electromagnetic wave absorbing material of the present invention is obtained by adding various compounding agents including dielectric materials such as carbon black, carbonyl iron, and ferrite and magnetic materials and graphite to the raw material rubber, kneading, sheets, etc. And is vulcanized into a predetermined shape. The electromagnetic wave absorbing material of the present invention is a liquid at room temperature, and can be used in a form that is cured at room temperature or by applying heat.

  In the case of thermoplastic elastomers and plastics, the electromagnetic wave absorbing material of the present invention is obtained by adding various compounding agents including dielectric materials such as carbon black, carbonyl iron and ferrite, magnetic materials, and graphite to the thermoplastic elastomers and plastics as raw materials. It is manufactured by molding into a predetermined shape by any molding means such as injection molding or extrusion molding.

  The electromagnetic wave absorbing material according to the present invention can be suitably used for, for example, indoor wireless LAN-compatible flooring materials, wall materials and ceiling materials, OA, FA related applications, outdoor wireless base stations, etc. It can also be applied as a field noise countermeasure wave absorber. The electromagnetic wave absorbing material of the present invention may be used alone as it is in the form of a sheet or the like, or may be used in a form laminated on a substrate and laminated by coating or the like.

  Hereinafter, although the electromagnetic wave absorbing material of the present invention will be described in detail with reference to Reference Examples, Examples and Comparative Examples, the present invention is not limited to the following Examples. In addition, all the compounding quantities shown to the following table | surface are a weight part.

[Reference Example 1]
(Characteristics of graphite)
In the formulation shown in Table 1 below, graphite (CB100 manufactured by Nippon Graphite Industry Co., Ltd.) was added to chlorinated polyethylene (Elastylene 302NA manufactured by Showa Denko KK), which is a thermoplastic elastomer, and kneaded. Samples 1 to 9 having a thickness of 2.0 mm were obtained.
In addition, graphite and various compounding agents were added to chloroprene rubber (Showaden GW manufactured by Showa Denko KK) with the composition shown in Table 2 below, kneaded, molded into a sheet, vulcanized, and a thickness of 2.0 mm Samples 10 to 12 were obtained.

For the obtained samples 1 to 12, the complex dielectric constant (real part ε ′ and imaginary part ε ″) at a frequency of 2.45 GHz was determined by the S-parameter method using a circular coaxial tube, and tan δ (= ε ′) '/ Ε') was calculated. The results are shown in Tables 1 and 2 together with the specific gravity of each sample.

On the other hand, for comparison, Comparative Samples 1 to 6 were obtained in the same manner as Samples 1 to 6 in Table 1 except that carbon black (IP1000 manufactured by Showa Cabot Corporation) was used instead of graphite. Table 3 shows the blending amounts of Comparative Samples 1 to 6 and the complex dielectric constant and tan δ determined in the same manner as described above.
Further, Comparative Samples 7 to 11 were obtained in the same manner as the samples in Table 1 except that titanium oxide (R310 manufactured by Sakai Chemical Industry Co., Ltd.) was used as the dielectric and was used in the blending amounts shown in Table 4. Table 4 shows the blending amounts of Comparative Samples 7 to 11 and the complex dielectric constant and tan δ determined in the same manner as described above.

  The relationship between the real part ε ′ and the imaginary part ε ″ of each sample obtained in Tables 1 to 4 is shown in FIG. From FIG. 1, carbon black increases imaginary part ε ″ abruptly as real part ε ′ increases, whereas graphite hardly increases imaginary part ε ″ even when real part ε ′ increases. It can be seen that the low value is maintained. Titanium oxide also has a low imaginary part ε ″ like graphite, but has a high density because it needs to be blended in a large amount as shown in Table 4.

[Reference Example 2]
(Formulation design)
It aims at achieving electromagnetic wave absorption of 20 dB or more at a frequency of 2.45 GHz using an electromagnetic wave absorbing material having a thickness of 5 mm.

  First, when the amount of ferrite (KNS415 manufactured by Toda Kogyo Co., Ltd.) as a magnetic material is fixed to 100 parts by weight, the complex permeability is 1.31 for μ ′ and 0.34 for μ ″. By substituting the values of μ ′ and μ ″ and the numerical value of the frequency of 2.45 GHz and the thickness of 5 mm into the design formula shown in the above [Equation 1], a condition for achieving electromagnetic wave absorption of 20 dB or more was calculated. . As a result, it was found that the range A shown in FIG. 2 is the real part ε ′ and imaginary part ε ″ of the complex dielectric constant necessary to achieve electromagnetic wave absorption of 20 dB or more.

図２から明らかなように、黒鉛はカーボンブラックよりも虚部ε''が小さいために目標を容易に達成できることがわかる。 Next, the compounding amount of the dielectric is adjusted so that the complex dielectric constant (real part ε ′, imaginary part ε ″) falls within the target range A. At this time, graphite (CB100 manufactured by Nippon Graphite Co., Ltd.) or carbon black (Ketjen Black EC manufactured by Lion Corp.) was used as the dielectric, and the blending amount of graphite and carbon black was changed within the range shown in Table 5. The obtained complex dielectric constant (real part ε ′, imaginary part ε ″) is shown in Table 6 and FIG.
In addition, the flame retardance of the electromagnetic wave absorbing material having the composition shown in Table 5 was measured according to the method described in the flame retardancy test standard (UL94) of plastic materials. The results are as shown in Table 5, and an evaluation of “UL94VO” indicating good flame retardancy was obtained.

As can be seen from FIG. 2, graphite has a smaller imaginary part ε ″ than that of carbon black, so that the target can be easily achieved.

リン酸エステル：味の素ファインテクノロジー社製のレフォロスＲＤＰ
赤燐：燐化学工業社製のノーバエクセル１４０
水酸化マグネシウム：堺化学工業社製のＭＧＺ−２
ＥＶＡ：東ソー社製のエチレン−酢酸ビニル共重合体（商品名：ウルトラセン７５０）
また、黒鉛を添加した実施例７のシート状電磁波吸収材料（厚さ約１００μｍ）について伝送損失を測定した。測定にはインピーダンスＺ＝５０Ωのマイクロストリップラインを使用した。マイクロストリップライン線路は、面実装部品の実装に適した構造と作成のしやすさによって、広く使われている近傍ノイズの伝送損失測定方法である。図３は、使用したマイクロストリップラインの形状を示す。このものは、絶縁体基板１の表面に直線状の導体路２を設け、この導体路２上にシート状の電磁波吸収材料４を載置したものである。導体路２の両端はネットワークアナライザー（図示せず）に接続される。そして、矢印Ａで示す入射波に対して、電磁波吸収材料４の載置部位からの反射量（ｄＢ）（矢印Ｓ11で示す）および透過量（ｄＢ）（矢印Ｓ21で示す）を測定し、それらの差をロス量とし、伝送損失を下記式から求めた。

マイクロストリップラインの伝送損失は電磁波吸収材料の厚みが厚くなるほど高くなる。一般的には、厚みが薄く、しかし高伝送損失の電磁波吸収材料が望まれている。
試験結果を図４に示す。図４から、実施例７の電磁波吸収材料は、厚さが１１０μｍと薄いにもかかわらず、高い伝送損失を示していることがわかる。
[Examples 1 and 2 ]
In the blending amounts shown in Table 7 , H-NBR (hydrogenated acrylonitrile-butadiene rubber, the same applies hereinafter) to flat soft magnetic material (JEM powder manufactured by Mitsubishi Materials Corporation) and graphite (CB100 manufactured by Nippon Graphite Co.) or carbon black (Showa) IP1000) manufactured by Cabot Corp. was added, and another compounding agent and a solvent were further blended to prepare a coating material. A sheet-like electromagnetic wave absorbing material having a thickness of about 100 μm was obtained by a bar coater. Complex magnetic permeability at 1.25GHz band of the electromagnetic wave absorbing material, the complex dielectric constant and the electromagnetic wave absorption determined meth. The results are also shown in Table 7 . The complex permeability and complex permittivity were measured by the S parameter method using a circular coaxial tube. The amount of electromagnetic wave absorption was calculated using the complex permeability and complex dielectric constant.

Phosphate ester: Rephoro RDP manufactured by Ajinomoto Fine Technology
Red phosphorus: Nova Excel 140 manufactured by Rin Chemical Industry Co., Ltd.
Magnesium hydroxide: MGZ-2 manufactured by Sakai Chemical Industry
EVA: ethylene-vinyl acetate copolymer (trade name: Ultrasen 750) manufactured by Tosoh Corporation
Moreover, the transmission loss was measured for the sheet-like electromagnetic wave absorbing material of Example 7 (thickness: about 100 μm) to which graphite was added. For the measurement, a microstrip line having an impedance Z = 50Ω was used. The microstrip line is a method for measuring transmission loss of nearby noise that is widely used due to its structure suitable for mounting surface-mounted components and ease of production. FIG. 3 shows the shape of the microstrip line used. In this device, a linear conductor path 2 is provided on the surface of the insulating substrate 1, and a sheet-like electromagnetic wave absorbing material 4 is placed on the conductor path 2. Both ends of the conductor path 2 are connected to a network analyzer (not shown). Then, with respect to the incident wave indicated by the arrow A, the reflection amount (dB) (indicated by the arrow S11) and the transmission amount (dB) (indicated by the arrow S21) from the placement site of the electromagnetic wave absorbing material 4 are measured. The transmission loss was calculated from the following equation.

The transmission loss of the microstrip line increases as the thickness of the electromagnetic wave absorbing material increases. In general, an electromagnetic wave absorbing material having a small thickness but high transmission loss is desired.
The test results are shown in FIG. FIG. 4 shows that the electromagnetic wave absorbing material of Example 7 shows high transmission loss even though the thickness is as thin as 110 μm.

[Example 3 and Comparative Example 1 ]
In the compounding amounts shown in Table 8 , H-NBR, boron nitride (SP-2 manufactured by Denki Kagaku Kogyo Co., Ltd.), flat soft magnetic material (JEM powder manufactured by Mitsubishi Materials Corporation), graphite (CB100 manufactured by Nippon Graphite Co., Ltd.) or carbon Black (IP1000 manufactured by Showa Cabot Co., Ltd.) was added, and another paint and solvent were further blended to prepare a paint, and a sheet-like electromagnetic wave absorbing material having a thickness of about 100 μm was obtained using a bar coater. The complex magnetic permeability, complex dielectric constant, and electromagnetic wave absorption in the 1.25 GHz band of this electromagnetic wave absorbing material were determined in the same manner as in Examples 1 and 2 . The results are also shown in Table 8 .

[Example 4 and Comparative Example 2 ]
H-NBR with aluminum nitride (Toyalnite UM manufactured by Toyo Aluminum Co., Ltd.) and flat soft magnetic material (JEM powder manufactured by Mitsubishi Materials Co., Ltd.) and graphite (CB100 manufactured by Nippon Graphite Co.) or carbon black with the compounding amounts shown in Table 9 (IP1000 manufactured by Showa Cabot Co., Ltd.) was added, and another paint and solvent were further blended to prepare a paint, and a sheet-like electromagnetic wave absorbing material having a thickness of about 100 μm was obtained by a bar coater. The complex magnetic permeability, complex dielectric constant, and electromagnetic wave absorption in the 1.25 GHz band of this electromagnetic wave absorbing material were determined in the same manner as in Examples 1 and 2 . The results are also shown in Table 9 .

Claims (14)

With magnetics, as the dielectric constant adjusting agent, fixed carbon content 97%, ash content of 3% or less, volatiles contained 3% of graphite, the real part of the complex dielectric constant ε at 1.25GHz band 'is 170-392, electromagnetic wave absorption materials imaginary part epsilon '' is characterized in that it is a 39 to 12 0.   The electromagnetic wave absorbing material according to claim 1, wherein the magnetic body is ferrite or iron. The electromagnetic wave absorbing material according to claim 2, wherein the iron is an Fe-Al-Si alloy, an Fe-Cr-Si alloy, or an Fe-Ni-Cr-Si alloy. The electromagnetic wave absorbing material according to claim 2 or 3, wherein the magnetic body has a flat shape having a major axis of 5 to 400 µm and an aspect ratio of 5 to 100. Electromagnetic wave absorbing material according to any one of claims 1-4 wherein at least one of the previous Ki磁 material elements and graphite although those insulated. Wave absorber according to any one of claims 1 to 5 vol resistivity is 10 ⁶ Ω · cm or more. The electromagnetic wave absorbing material according to any one of claims 1 to 6 , comprising 1 to 50% by volume of a heat dissipating material. The electromagnetic wave absorbing material according to claim 7 , wherein the heat dissipating material is boron nitride or aluminum nitride. The electromagnetic wave absorbing material according to any one of claims 1 to 8 , which is a rubber sheet or a synthetic resin sheet. The electromagnetic wave absorbing material according to claim 9 , wherein the sheet has a thickness of 0.01 to 10 mm. Rubber or synthetic the resin 100 parts by weight, the electromagnetic wave absorbing material prior Ki磁 material element 10 to 1000 parts by weight, claim 9 or 10, wherein containing graphite 3 to 150 parts by weight. Electromagnetic wave absorbing material prior Ki磁 material elements 10 to 90 vol%, claim 9 or 10, wherein containing graphite 1-30% by volume. The electromagnetic wave absorbing material according to any one of claims 1 to 12 , comprising at least one flame retardant selected from phosphoric flame retardants, hydroxide flame retardants, bromine flame retardants, and metal compound flame retardants. The electromagnetic wave absorbing material according to claim 13, wherein the flame retardant is decabromodiphenyl oxide , antimony trioxide , or a flame retardant in which decabromodiphenyl oxide and antimony trioxide are mixed .

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