WO2024203732A1 - Electromagnetic wave absorber and paste for forming electromagnetic wave absorber - Google Patents

Abstract

Provided are: an electromagnetic wave absorber, the electromagnetic wave absorption characteristics of which are not susceptible to deterioration even when exposed to a high temperature for a long time; and a paste for forming the electromagnetic wave absorber, said paste favorably being used for manufacturing the electromagnetic wave absorber. The electromagnetic wave absorber comprises an electromagnetic wave absorption layer, the electromagnetic wave absorption layer containing a magnetic body, a binder resin, and a non-dielectric material, the binder resin including an amorphous resin that has a glass transition point of 80°C or higher and/or a crystalline resin that has a melting point of 150°C or higher, and the non-dielectric material being different from the magnetic body.

Description

Electromagnetic wave absorber and paste for forming electromagnetic wave absorber

The present invention relates to an electromagnetic wave absorber and a paste for forming an electromagnetic wave absorber.

The use of high-frequency electromagnetic waves is becoming widespread in various information and communication systems, such as mobile phones, wireless LANs, ETC systems, intelligent road transport systems, driving assistance road systems, and satellite broadcasting. However, the expanded use of high-frequency electromagnetic waves has raised concerns that interference between electronic components could lead to breakdowns and malfunctions in electronic devices. As a countermeasure to this problem, a method is being used to absorb unnecessary electromagnetic waves using electromagnetic wave absorbers.

For this reason, even in radars and the like that utilize electromagnetic waves in the high frequency band, electromagnetic wave absorbers are used to reduce the effects of unnecessary electromagnetic waves that should not be received.
In order to meet such demands, various electromagnetic wave absorbers capable of effectively absorbing electromagnetic waves in the high frequency band have been proposed. A specific example is an electromagnetic wave absorbing sheet containing carbon nanocoils and a resin (for example, Patent Document 1).

Among the applications of electromagnetic waves in the high frequency band, research has progressed in the field of automobile driving assistance systems. In such automobile driving assistance systems, electromagnetic waves in the 76 GHz band are used in on-board radar to detect the distance between vehicles, etc. It is predicted that the use of electromagnetic waves in high frequency bands, for example 100 GHz or higher, will spread to a variety of applications, not limited to automobile driving assistance systems. For this reason, there is a demand for electromagnetic wave absorbers that can effectively absorb electromagnetic waves in the 76 GHz band and higher frequency bands.

In order to meet such demands, electromagnetic wave absorbers that can effectively absorb electromagnetic waves over a wide range in the high frequency band have been proposed, such as electromagnetic wave absorbers having an electromagnetic wave absorbing layer containing magnetic crystals made of ε-Fe ₂ O ₃ type iron oxide (e.g., Patent Document 2 and Non-Patent Documents 1 to 4).

JP 2009-060060 A JP 2008-277726 A

A. Namai, S. Sakurai, M. Nakajima, T. Suemoto, K. Matsumoto, M. Goto, S. Sasaki, and S. Ohkoshi, J. Am. Chem. Soc. , 131, 1170-1173 (2009). A. Namai, M. Yoshikiyo, K. Yamada, S. Sakurai, T. Goto, T. Yoshida, T. Miyazaki, M. Nakajima, T. Suemoto, H. Tokoro, and S. Ohkoshi, Nature Communications, 3, 1035/1-6 (2012). S. Ohkoshi, S. Kuroki, S. Sakurai, K. Matsumoto, K. Sato, and S. Sasaki, Angew. Chem. Int. Ed. , 46, 8392-8395 (2007). A. Namai, K. Ogata, M. Yoshikiyo, and S. Ohkoshi, Bull. Chem. Soc. Jpn. , 93, 20-25 (2020).

However, the electromagnetic wave absorbing properties of the electromagnetic wave absorber described in Patent Document 1 may deteriorate if exposed to high temperatures for a long period of time.

The present invention has been made in consideration of the problems with the conventional technology described above, and aims to provide an electromagnetic wave absorber whose electromagnetic wave absorption properties are not easily deteriorated even when exposed to high temperatures for a long period of time, and a paste for forming the electromagnetic wave absorber that is suitable for use in manufacturing the electromagnetic wave absorber.

The inventors discovered that the above problems could be solved by incorporating a magnetic material, a binder resin with specific temperature characteristics, and a non-dielectric material into the electromagnetic wave absorbing layer of the electromagnetic wave absorber, and thus completed the present invention. Specifically, the present invention provides the following:

[1] An electromagnetic wave absorber having an electromagnetic wave absorbing layer,
the electromagnetic wave absorbing layer includes a magnetic material, a binder resin, and a non-dielectric material;
The binder resin contains an amorphous resin having a glass transition point of 80° C. or more and/or a crystalline resin having a melting point of 150° C. or more,
An electromagnetic wave absorber, wherein the non-dielectric material is different from the magnetic material.

[2] The electromagnetic wave absorber described in [1], wherein the electromagnetic wave absorbing layer contains carbon nanotubes.

[3] The electromagnetic wave absorber according to [1] or [2], wherein the magnetic material contains epsilon-type iron oxide.

[4] The electromagnetic wave absorber according to any one of [1] to [3], wherein the amorphous resin and/or the crystalline resin includes at least one selected from the group consisting of polyurethane resin, polyester-urethane resin, urethane polyamide resin, polycarbonate resin, polyester resin, FR-AS resin, FR-ABS resin, AS resin, ABS resin, polyphenylene oxide resin, polyphenylene sulfide resin, polysulfone resin, polyethersulfone resin, polyetheretherketone resin, fluorine-based resin, polyimide resin, polyamideimide resin, polyamide bismaleimide resin, polyetherimide resin, polybenzoxazole resin, polybenzothiazole resin, polybenzimidazole resin, BT resin, polymethylpentene, ultra-high molecular weight polyethylene, FR-polypropylene, cellulose resin, (meth)acrylic resin, and polystyrene.

[5] An electromagnetic wave absorber according to any one of [1] to [4], wherein the non-dielectric material includes at least one selected from the group consisting of barium sulfate, aluminum oxide, aluminum nitride, boron nitride, silicon carbide, silicon dioxide, calcium carbonate, and talc.

[6] The electromagnetic wave absorbing layer is laminated on a base layer,
The electromagnetic wave absorber according to any one of [1] to [5], which is in the form of a film.

[7] A paste for forming an electromagnetic wave absorber, comprising a magnetic material, a binder resin, and a non-dielectric material,
The binder resin contains an amorphous resin having a glass transition point of 80° C. or more and/or a crystalline resin having a melting point of 150° C. or more,
The paste for forming an electromagnetic wave absorber, wherein the non-dielectric material is different from the magnetic material.

The present invention provides an electromagnetic wave absorber whose electromagnetic wave absorption properties are not easily degraded even when exposed to high temperatures for a long period of time, and a paste for forming the electromagnetic wave absorber that is suitable for use in producing the electromagnetic wave absorber.

The following describes in detail the embodiments of the present invention, but the present invention is not limited to the following embodiments and can be modified as appropriate within the scope of the invention.

≪Electromagnetic wave absorber≫
The electromagnetic wave absorber includes an electromagnetic wave absorbing layer. The electromagnetic wave absorber may consist of only the electromagnetic wave absorbing layer, or may include a substrate layer that supports the electromagnetic wave absorbing layer.

From the viewpoint of more reliably absorbing high-frequency electromagnetic waves at or above the millimeter wave band, it is preferable for the electromagnetic wave absorber to absorb electromagnetic waves in a frequency band of 30 gigahertz (GHz) or more, preferably 30 GHz to 300 GHz, and more preferably 40 GHz to 200 GHz. It is also preferable that the return loss of the electromagnetic wave absorber has a peak with an absolute value of 15 dB or more. The return loss is a value measured on the surface where the electromagnetic wave absorbing layer is exposed.

The form of the electromagnetic wave absorber is not particularly limited, but it is preferably in the form of a sheet or film, and more preferably in the form of a film.
When the electromagnetic wave absorber is in the form of a film, the shape of the film may have a curved surface or may be composed of only flat surfaces, and is preferably in the form of a flat plate.
From the viewpoint of making the film thinner and smaller without impairing the effects of the present invention, the thickness of the film as an electromagnetic wave absorber is preferably 1000 μm or less, more preferably 900 μm or less, even more preferably 450 μm or less, and particularly preferably 300 μm or less.
The thickness of the film serving as an electromagnetic wave absorber may be uniform or non-uniform.

<Electromagnetic wave absorbing layer>
The electromagnetic wave absorbing layer includes a binder resin having a predetermined temperature characteristic and a non-dielectric material in addition to the magnetic material. By using a binder resin having a predetermined temperature characteristic and a non-dielectric material in combination in an electromagnetic wave absorbing layer including a magnetic material, an electromagnetic wave absorber whose electromagnetic wave absorbing characteristics are less likely to deteriorate even when exposed to high temperatures for a long period of time can be obtained, compared to when each is used alone. The reason why such an effect is obtained is not necessarily clear, but it is presumed that the combined use of the two can suppress deformation and flow of the electromagnetic wave absorbing layer at high temperatures, and separation of the components in the electromagnetic wave absorbing layer is less likely to occur.

The thickness of the electromagnetic wave absorbing layer is not particularly limited as long as it does not impair the object of the present invention. The thickness of the electromagnetic wave absorbing layer is preferably 150 μm or less, more preferably 100 μm or less, from the viewpoint of the balance between the thinning of the electromagnetic wave absorber and the electromagnetic wave absorbing performance.
The lower limit of the thickness of the electromagnetic wave absorbing layer is not particularly limited as long as it does not impair the effects of the present invention, and examples thereof include 1 μm or more, 10 μm or more, and the like.
The electromagnetic wave absorbing layer may have a uniform or non-uniform thickness.

The required and optional configurations of the electromagnetic wave absorbing layer are explained below.

[Magnetic material (ferromagnetic material)]
The electromagnetic wave absorbing layer contains a magnetic material (ferromagnetic material). The type of magnetic material is not particularly limited as long as the electromagnetic wave absorber exhibits the desired electromagnetic wave absorbing characteristics.
From the viewpoint of being able to absorb high-frequency electromagnetic waves in the millimeter wave band or higher, the magnetic material is preferably a magnetic material that magnetically resonates in a frequency band of 30 GHz or higher, and more preferably a magnetic material that magnetically resonates in a frequency band of 30 GHz or higher and 300 GHz or lower. preferable.

The above-mentioned magnetic resonance includes magnetic resonance based on the precession motion of electrons in atoms undergoing spin motion in frequency bands above the millimeter wave band. Natural magnetic resonance due to the gyromagnetic effect based on the precession motion in frequency bands above the millimeter wave band is preferred.

The magnetic material is not particularly limited as long as it can absorb electromagnetic waves of high frequencies equal to or higher than the millimeter wave band. A preferred magnetic material is a magnetic material containing at least one selected from the group consisting of epsilon-type iron oxide, barium ferrite magnetic material, and strontium ferrite magnetic material. Among them, epsilon-type iron oxide is more preferred.
In terms of providing good electromagnetic wave absorption characteristics to the electromagnetic wave absorber, the ratio of the mass of epsilon-type iron oxide to the total mass of the epsilon-type iron oxide and the magnetic material other than the epsilon-type iron oxide is preferably 70 mass% or more, more preferably 80 mass% or more, even more preferably 90 mass% or more, even more preferably 95 mass% or more, and particularly preferably 100 mass%.
Hereinafter, the epsilon type iron oxide will be described.

(epsilon iron oxide)
The epsilon type iron oxide is preferably at least one selected from the group consisting of ε-Fe ₂ O ₃ crystals and crystals having the same crystal structure and space group as ε-Fe ₂ O ₃ , with part of the Fe sites of the ε-Fe ₂ O ₃ crystals being substituted with an element M other than Fe, and expressed by the formula ε-M _x Fe _2-x O ₃ , where x is 0 or more and 2 or less (preferably 0 or more and less than 2). Since such epsilon type iron oxide crystals are magnetic crystals, in the present specification, the crystals may be referred to as "magnetic crystals".

Any ε-Fe ₂ O ₃ crystal can be used. The crystal structure and space group are the same as those of ε-Fe ₂ O ₃ , and a part of the Fe site of the ε-Fe ₂ O ₃ crystal is replaced with an element M other than Fe, and the crystal is represented by the formula ε-M _x Fe _2-x O ₃ , where x is 0 or more and 2 or less (preferably 0 or more and less than 2), and will be described later.
In this specification, ε-M _x Fe _2-x O ₃ in which part of the Fe sites of the ε-Fe ₂ O ₃ crystal is substituted with the substitution element M is also referred to as "M-substituted ε-Fe ₂ O ₃ ".

The particle size of the particles having ε-Fe ₂ O ₃ crystals and/or M-substituted ε-Fe ₂ O ₃ crystals in the magnetic phase is not particularly limited as long as it does not impair the object of the present invention. For example, the particles having magnetic crystals of epsilon-type iron oxide in the magnetic phase, which are produced by the method described below, have an average particle size measured from a TEM (transmission electron microscope) photograph in the range of 5 nm to 200 nm.
In addition, the coefficient of variation (standard deviation of particle diameter/average particle diameter) of particles having epsilon-type iron oxide magnetic crystals in a magnetic layer, which are produced by the method described below, is in the range of less than 80%, and the particles are relatively fine and have a uniform particle diameter.

In a preferred electromagnetic wave absorbing layer, a powder of such epsilon type iron oxide magnetic particles (i.e., particles having ε-Fe ₂ O ₃ crystals and/or M-substituted ε-Fe ₂ O ₃ crystals in the magnetic phase) is used as a magnetic material in the electromagnetic wave absorbing layer. The "magnetic phase" here is the part that is responsible for the magnetism of the powder.
"Having ε-Fe ₂ O ₃ crystals and/or M-substituted ε-Fe ₂ O ₃ crystals in the magnetic phase" means that the magnetic phase is composed of ε-Fe ₂ O ₃ crystals and/or M-substituted ε-Fe ₂ O ₃ crystals, and includes cases where the magnetic phase contains impurity magnetic crystals that are unavoidable during production.

The magnetic crystals of epsilon-type iron oxide may contain impurity crystals of iron oxides having a space group or oxidation state different from that of ε-Fe ₂ O ₃ crystals (specifically, α-Fe ₂ O ₃ , γ-Fe ₂ O ₃ , FeO, and Fe ₃ O ₄ , as well as crystals in which part of the Fe in these crystals is replaced with other elements).
When the magnetic crystals of epsilon iron oxide contain impurity crystals, it is preferable that the magnetic crystals of ε-Fe ₂ O ₃ and/or M-substituted ε-Fe ₂ O ₃ are the main phase. In other words, it is preferable that the ratio of the magnetic crystals of ε-Fe ₂ O ₃ and/or M-substituted ε-Fe ₂ O ₃ in the magnetic crystals of epsilon iron oxide is 50 mol % or more in terms of molar ratio as a compound.

The abundance ratio of the crystals can be determined by analysis using the Rietveld method based on the X-ray diffraction pattern. Non-magnetic compounds such as silica (SiO ₂ ) formed in the sol-gel process may adhere to the periphery of the magnetic phase.

(M-substituted ε-Fe ₂ O ₃ )
The type of element M in M-substituted ε-Fe ₂ O ₃ is not particularly limited as long as the crystal and space group are the same as those of ε-Fe ₂ O ₃ and a part of the Fe site of the ε-Fe ₂ O ₃ crystal is substituted with an element M other than Fe. The M-substituted ε-Fe ₂ O ₃ may contain multiple kinds of elements M other than Fe.

Suitable examples of the element M include In, Ga, Al, Sc, Cr, Sm, Yb, Ce, Ru, Rh, Ti, Co, Ni, Mn, Zn, Zr, and Y. Among these, In, Ga, Al, Ti, Co, and Rh are preferred. When M is Al, in the composition represented by ε-M _x Fe _2-x O ₃ , x is preferably, for example, in the range of 0 or more and less than 0.8. When M is Ga, x is preferably, for example, in the range of 0 or more and less than 0.8. When M is In, x is preferably, for example, in the range of 0 or more and less than 0.3. When M is Rh, x is preferably, for example, in the range of 0 or more and less than 0.3. When M is Ti and Co, x is preferably, for example, in the range of 0 or more and less than 1.

The frequency at which the amount of electromagnetic wave absorption is maximized can be adjusted by adjusting at least one of the type and the amount of substitution of element M in M-substituted ε-Fe ₂ O ₃ .

Such M-substituted ε-Fe ₂ O ₃ magnetic crystals can be synthesized, for example, by a process combining a reverse micelle method and a sol-gel method, and a firing process, as described below. In addition, M-substituted ε-Fe ₂ O ₃ magnetic crystals can be synthesized by a process combining a direct synthesis method and a sol-gel method, and a firing process, as disclosed in JP-A-2008-174405.

in particular,
Jian Jin, Shin-ichi Ohkoshi and Kazuhito Hashimoto, ADVANCED MATERIALS 2004, 16, No. 1, January 5, p. 48-51,
Shin-ichi Ohkoshi, Shunsuke Sakurai, Jian Jin, Kazuki Hashimoto, JOURNAL OF APPLIED PHYSICS, 97, 10K312 (2005),
Shunsuke Sakurai, Jian Jin, Kazuhito Hashimoto and Shin-ichi Ohkoshi, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, Vol. 74, No. 7, July, 2005, p. 1946-1949,
As described in Asuka Namai, Shunsuke Sakurai, Makoto Nakajima, Tohru Suemoto, Kazuki Matsumoto, Masahiro Goto, Shinya Sasaki, and Shin-ichi Ohkoshi, Journal of the American Chemical Society, Vol. 131, p. 1170-1173, 2009, etc., M-substituted ε-Fe ₂ O ₃ magnetic crystals can be obtained by a process combining the reverse micelle method and the sol-gel method and a firing process.

In the reverse micelle method, two types of micellar solutions containing surfactants, namely micellar solution I (raw micelle) and micellar solution II (neutralizer micelle), are mixed together to allow the precipitation reaction of iron hydroxide to proceed within the micelles. Next, a silica coating is applied to the surface of the iron hydroxide fine particles generated within the micelles by the sol-gel method. After being separated from the liquid, the iron hydroxide fine particles with the silica coating layer are subjected to a heat treatment in an air atmosphere at a predetermined temperature (within the range of 700 to 1300°C). This heat treatment results in fine particles of ε- _{Fe2O3 crystals} .

More specifically, for example, M-substituted ε-Fe ₂ O ₃ magnetic crystals are produced as follows.

First, iron (III) nitrate as the iron source, M nitrate as the M element source to replace part of the iron (aluminum (III) nitrate nonahydrate for Al, gallium (III) nitrate hydrate for Ga, indium (III) nitrate trihydrate for In, titanium (IV) sulfate hydrate and cobalt (II) nitrate hexahydrate for Ti and Co) and a surfactant (e.g. cetyltrimethylammonium bromide) are dissolved in the aqueous phase of micellar solution I, which has n-octane as the oil phase.

An appropriate amount of nitrate of an alkaline earth metal (Ba, Sr, Ca, etc.) can be dissolved in the aqueous phase of the micellar solution I. This nitrate functions as a shape control agent. When an alkaline earth metal is present in the solution, rod-shaped particles of M-substituted ε-Fe ₂ O ₃ magnetic crystals are finally obtained. In the absence of a shape control agent, particles of M-substituted ε-Fe ₂ O ₃ magnetic crystals that are nearly spherical are obtained.

The alkaline earth metal added as a shape control agent may remain in the surface layer of the resulting M-substituted ε-Fe ₂ O ₃ magnetic crystal. The mass of the alkaline earth metal in the M-substituted ε-Fe ₂ O ₃ magnetic crystal is preferably 20 mass% or less, more preferably 10 mass% or less, based on the total mass of the substitution element M and the mass of Fe in the M-substituted ε-Fe ₂ O ₃ magnetic crystal.

An aqueous ammonia solution is used as the water phase of micellar solution II, which uses n-octane as the oil phase.

After mixing micellar solutions I and II, the sol-gel method is applied. That is, silane (e.g., tetraethylorthosilane) is added dropwise to the micellar solution mixture while stirring is continued, and the reaction to produce iron hydroxide or iron hydroxide containing element M within the micelles proceeds. As a result, the particle surfaces of the fine iron hydroxide precipitate produced within the micelles are coated with silica produced by hydrolysis of the silane.

Next, the silica-coated M element-containing iron hydroxide particles are separated from the liquid, washed and dried to obtain a particle powder, which is then charged into a furnace and heat-treated (fired) in air at a temperature range of 700°C or higher and 1300°C or lower, preferably 900°C or higher and 1200°C or lower, and more preferably 950°C or higher and 1150°C or lower.
This heat treatment causes an oxidation reaction to proceed within the silica coating, and the fine particles of M element-containing iron hydroxide are transformed into fine particles of M-substituted ε-Fe ₂ O ₃ .

During this oxidation reaction, the presence of the silica coat contributes to the formation of M-substituted ε-Fe ₂ O ₃ crystals, which have the same space group as ε-Fe ₂ O ₃ , rather than α-Fe ₂ O ₃ or γ-Fe ₂ O ₃ crystals, and also acts to prevent sintering of particles. In addition, when an appropriate amount of alkaline earth metal is present, the particles tend to grow into rod-like shapes.

In addition, as described above, as disclosed in JP-A-2008-174405, the M-substituted ε-Fe ₂ O ₃ magnetic crystal can be synthesized more economically and advantageously by a process combining a direct synthesis method and a sol-gel method, and a firing process.

In simple terms, a precursor consisting of iron hydroxide (which may be partially substituted with another element) is formed by first adding a neutralizing agent such as aqueous ammonia to an aqueous solvent in which a salt of trivalent iron and a salt of the replacement element M (Ga, Al, etc.) are dissolved while stirring.

Then, a sol-gel process is applied to form a silica coating layer on the surface of the precursor particles. After separating the silica-coated particles from the liquid, they are heat-treated (fired) at a predetermined temperature to obtain fine particles of M-substituted ε-Fe ₂ O ₃ magnetic crystals.

In the synthesis of M-substituted ε-Fe ₂ O ₃ as described above, iron oxide crystals (impurity crystals) with different space groups and oxidation states from ε-Fe ₂ O ₃ crystals may be generated. The most common polymorphs with different crystal structures that have the composition of Fe ₂ O ₃ are α-Fe ₂ O ₃ and γ-Fe ₂ O _3. Other iron oxides include FeO, Fe ₃ O ₄ , etc.
The inclusion of such impurity crystals is not preferable in terms of bringing out the properties of the M-substituted ε-Fe ₂ O ₃ crystal as highly as possible, but is permissible within a range that does not impair the effects of the present invention.

In addition, the coercive force _Hc of the M-substituted ε-Fe ₂ O ₃ magnetic crystal changes according to the amount of substitution by the substitution element M. In other words, by adjusting the amount of substitution by the substitution element M in the M-substituted ε-Fe ₂ O ₃ magnetic crystal, the coercive force _Hc of the M-substituted ε-Fe ₂ O ₃ magnetic crystal can be adjusted.
Specifically, when Al _, Ga, or the like is used as the substituting element M, the coercive force _Hc of the M-substituted ε- _Fe2O3 magnetic crystal decreases as the amount of substitution increases. On the other hand, when Rh, or the like is used as the substituting element M, the coercive force _Hc of the M-substituted ε- _Fe2O3 magnetic _crystal increases as the amount of substitution increases.
From the viewpoint that the coercive force _Hc of the M-substituted ε-Fe ₂ O ₃ magnetic crystal can be easily adjusted depending on the amount of substitution by the substitution element M, Ga, Al, In, Ti, Co and Rh are preferable as the substitution element M.

As the coercive force _Hc decreases, the frequency of the peak at which the electromagnetic wave absorption of the epsilon iron oxide is maximized also shifts to the lower or higher frequency side. In other words, the frequency of the peak of the electromagnetic wave absorption can be controlled by the amount of the M element substituted.

In the case of commonly used electromagnetic wave absorbers, if the angle of incidence or frequency of the electromagnetic waves deviates from the designed values, the amount of absorption becomes almost zero. In contrast, when epsilon-type iron oxide is used, electromagnetic wave absorption is exhibited over a wide frequency range and electromagnetic wave incidence angle even if the values are slightly off. This makes it possible to provide an electromagnetic wave absorbing layer that can absorb electromagnetic waves over a wide frequency band.

The particle size of the epsilon-type iron oxide can be controlled, for example, by adjusting the heat treatment (calcination) temperature in the above process.
According to the above-mentioned method of combining the reverse micelle method and the sol-gel method, or the method of combining the direct synthesis method and the sol-gel method disclosed in JP 2008-174405 A, it is possible to synthesize epsilon-type iron oxide particles having an average particle size measured from a TEM (transmission electron microscope) photograph in the range of 5 nm to 200 nm. The average particle size of epsilon-type iron oxide is preferably 10 nm or more, more preferably 20 nm or more.
In addition, when determining the average particle diameter, which is the number average particle diameter, if the epsilon iron oxide particles are rod-shaped, the average particle diameter is calculated by taking the diameter in the major axis direction of the particle observed in the TEM image as the diameter of the particle. The number of particles to be measured when determining the average particle diameter is not particularly limited as long as it is a sufficiently large number for calculating the average value, but it is preferably 300 or more.

In addition, the silica coat formed on the surface of the iron hydroxide fine particles by the sol-gel method may be present on the surface of the M-substituted ε-Fe ₂ O ₃ magnetic crystal after heat treatment (calcination). The presence of a non-magnetic compound such as silica on the surface of the crystal is preferable in terms of improving the handleability, durability, weather resistance, etc. of the magnetic crystal.
Suitable examples of the non-magnetic compound include silica, as well as heat-resistant compounds such as alumina and zirconia.

However, if the amount of the non-magnetic compound attached is too large, the particles may severely aggregate, which is undesirable.
When the non-magnetic compound is silica, the mass of Si in the M-substituted ε-Fe ₂ O ₃ magnetic crystals is preferably 100 mass % or less relative to the total mass of the substituting element M and the mass of Fe in the M-substituted ε-Fe ₂ O ₃ magnetic crystals.
A part or most of the silica attached to the M-substituted ε-Fe ₂ O ₃ magnetic crystals can be removed by immersing them in an alkaline solution. The amount of silica attached can be adjusted to any amount by this method.

The relative permeability of the electromagnetic wave absorbing layer is not particularly limited, but is preferably 1.0 or more and 1.5 or less. The method for adjusting the relative permeability of the electromagnetic wave absorbing layer is not particularly limited. Examples of methods for adjusting the relative permeability of the electromagnetic wave absorbing layer include a method for adjusting the amount of substitution by the substitution element M in the epsilon-type iron oxide, and a method for adjusting the content of epsilon-type iron oxide and other magnetic materials other than epsilon-type iron oxide in the electromagnetic wave absorbing layer.

The content of the magnetic material in the electromagnetic wave absorbing layer is not particularly limited as long as it does not impede the object of the present invention. The content of the magnetic material is preferably 5% by mass or more, more preferably 5% by mass or more and 90% by mass or less, particularly preferably 5% by mass or more and 70% by mass or less, and most preferably 5% by mass or more and 60% by mass or less, relative to the mass of the electromagnetic wave absorbing layer.

[Binder resin]
The electromagnetic wave absorbing layer contains a binder resin. The binder resin contains an amorphous resin having a glass transition point of 80° C. or higher and/or a crystalline resin having a melting point of 150° C. or higher.
In this specification, a crystalline resin refers to a resin having crystalline portions in which molecular chains are regularly oriented and having a glass transition point and a melting point in differential scanning calorimetry (DSC), whereas an amorphous resin refers to a resin having a structure in which molecular chains are randomly entangled and having no melting point in DSC but only a glass transition point.

The glass transition point of the amorphous resin is preferably 101° C. or higher. The upper limit of the glass transition point of the amorphous resin is not particularly limited as long as the effects of the present invention are not impaired, and examples of the upper limit include 300° C. or lower, 200° C. or lower, and 150° C. or lower.
The glass transition point of the amorphous resin can be determined based on the change in viscoelasticity measured, for example, using a dynamic mechanical analyzer (DMA) device, by raising the temperature from 25° C. to 300° C. at a heating rate of 5° C./min under conditions of a frequency of 1 Hz.

The upper limit of the melting point of the crystalline resin is not particularly limited as long as the effects of the present invention are not impaired, and examples thereof include 500° C. or less, 400° C. or less, 300° C. or less, and 200° C. or less.
The melting point of a crystalline resin is, for example, the temperature corresponding to the maximum value on the heat of fusion curve when the temperature is increased at a rate of 10° C./min using a differential scanning calorimetry (DSC) device.

The weight average molecular weight (Mw) of the amorphous resin or the crystalline resin is preferably 5,000 or more and 1,000,000 or less, and more preferably 10,000 or more and 800,000 or less.
The weight average molecular weight (Mw) is a weight average molecular weight measured by GPC and calculated as polystyrene.

Suitable examples of amorphous resins and crystalline resins include polyurethane resins, polyester-urethane resins, urethane polyamide resins, polycarbonate resins, polyester resins, FR-AS resins, FR-ABS resins, AS resins, ABS resins, polyphenylene oxide resins, polyphenylene sulfide resins, polysulfone resins, polyethersulfone resins, polyetheretherketone resins, fluorine-based resins, polyimide resins, polyamideimide resins, polyamide bismaleimide resins, polyetherimide resins, polybenzoxazole resins, polybenzothiazole resins, polybenzimidazole resins, BT resins, polymethylpentene, ultra-high molecular weight polyethylene, FR-polypropylene, cellulose resins, (meth)acrylic resins, and polystyrene. Of these, polyester-urethane resins and fluorine-based resins are more preferred.

Polyester-urethane resin is a copolymer containing an ester bond (-CO-O-) and a urethane bond (-NH-CO-O-). Suitable examples of polyester-urethane resin include aromatic ester-urethane copolymers and aliphatic ester-urethane copolymers. Among these, aromatic ester-urethane copolymers are more preferred.

The aromatic ester-urethane copolymer is a copolymer which contains an ester bond (--CO--O--) and a urethane bond (--NH--CO--O--) and also contains an aromatic group in the main chain skeleton.
The aromatic group in the main chain skeleton may be an aromatic hydrocarbon group or a heterocyclic aromatic group, with an aromatic hydrocarbon group being preferred. The aromatic ester-urethane copolymer may be a random copolymer in which ester bonds and urethane bonds are randomly introduced into the molecular chain, or a block copolymer consisting of one or more ester blocks and one or more urethane blocks.

The method for producing the aromatic ester-urethane copolymer is not particularly limited. The aromatic ester-urethane copolymer can typically be produced by polymerizing one or more monomers selected from the group consisting of a diol component (a1), a dicarboxylic acid (a2), a hydroxycarboxylic acid component (a3), and a diisocyanate component (a4) in one or multiple stages.

The dicarboxylic acid component (a2) and the hydroxycarboxylic acid component (a3) may be used as ester derivatives such as methyl esters and ethyl esters, carboxylic acid halides such as carboxylic acid chlorides, or urethane-forming derivatives.

The monomer used in the production of the aromatic ester-urethane copolymer is preferably a compound in which two functional groups selected from the group consisting of a hydroxyl group, a carboxyl group, and an isocyanate group are bonded to a divalent hydrocarbon group having an unbranched structure.
The divalent unbranched hydrocarbon group may be an alkylene group, an alkenylene group, an alkynylene group, an arylene group, or a combination of these groups. The alkylene group, the alkenylene group, and the alkynylene group are preferably linear.

When the unbranched divalent hydrocarbon group is an alkylene group, an alkenylene group, or an alkynylene group, the number of carbon atoms in these groups is preferably 1 to 8, more preferably 2 to 6, and even more preferably 2 to 4.

When the divalent hydrocarbon group having an unbranched structure is an arylene group, the arylene group is preferably a phenylene group or a naphthylene group, more preferably a phenylene group, and even more preferably a p-phenylene group.

Among the divalent hydrocarbon groups having an unbranched structure described above, alkylene groups, arylene groups, and combinations of alkylene groups and arylene groups are preferred.

Specific preferred examples of the diol component (a1) include ethylene glycol, 1,3-propanediol, 1,4-butanediol, neopentyl glycol, 1,6-hexanediol, 1,4-cyclohexanedimethanol, and 1,5-pentanediol.
Specific preferred examples of the dicarboxylic acid (a2) include terephthalic acid, isophthalic acid, 2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid, 1,4-naphthalenedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, succinic acid, glutaric acid, adipic acid, oxalic acid, and malonic acid.
Specific preferred examples of the hydroxycarboxylic acid component (a3) include 4-hydroxybenzoic acid, 3-hydroxybenzoic acid, 6-hydroxynaphthalene-2-carboxylic acid, glycolic acid, lactic acid, and γ-hydroxybutyric acid.
Specific preferred examples of the diisocyanate component (a4) include ethylene diisocyanate, trimethylene diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, m-xylylene diisocyanate, p-phenylene diisocyanate, tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, and 1,5-naphthalene diisocyanate.

Commercially available aromatic ester-urethane copolymers include the Byron series (product name) (manufactured by Toyobo Co., Ltd.). More specifically, Byron UR-1400, Byron UR-1410, Byron UR-1700, Byron UR-2300, Byron UR-3200, Byron UR-3210, Byron UR-3500, Byron UR-6100, Byron UR-8300, and Byron UR-8700 can be preferably used.

Suitable examples of fluorine-based resins include tetrafluoroethylene/perfluoroalkyl ether copolymer (PFA), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl fluoride (PVF), polychlorotrifluoroethylene (PCTFE), tetrafluoroethylene/hexafluoropropylene copolymer (FEP), ethylene/chlorotrifluoroethylene copolymer (ECTFE), vinylidene fluoride/chlorotrifluoroethylene copolymer, ethylene/tetrafluoroethylene copolymer, chlorotrifluoroethylene/tetrafluoroethylene copolymer, vinylidene fluoride/tetrafluoroethylene copolymer, Examples of the vinylidene fluoride include ethylene/hexafluoropropylene copolymer, ethylene/tetrafluoroethylene/hexafluoropropylene copolymer, vinylidene fluoride/tetrafluoroethylene copolymer, vinylidene fluoride/hexafluoropropylene copolymer, vinylidene fluoride/pentafluoropropylene copolymer, tetrafluoroethylene/hexafluoropropylene/vinylidene fluoride copolymer (THV), vinylidene fluoride/pentafluoropropylene/tetrafluoroethylene copolymer, vinylidene fluoride/perfluoroalkyl vinyl ether/tetrafluoroethylene copolymer, perfluoroalkoxy/tetrafluoromethylene copolymer, etc. Among these, polyvinylidene fluoride is more preferable.

The binder resin may be, for example, an elastic material such as an elastomer or rubber. The binder resin may also be a curable resin. When the binder resin is a curable resin, the curable resin may be a photocurable resin or a thermosetting resin.

Suitable examples of binder resins that are elastic materials include olefin-based elastomers, styrene-based elastomers, polyamide-based elastomers, polyester-based elastomers, and polyurethane-based elastomers.

Suitable examples of the binder resin when it is a thermosetting resin include phenol resin, melamine resin, epoxy resin, and alkyd resin. As the photocurable resin, various vinyl monomers and resins obtained by photocuring monomers having unsaturated bonds such as various (meth)acrylic acid esters can be used.

In the binder resin, the mass ratio of the amorphous resin having a glass transition point of 80°C or higher and the crystalline resin having a melting point of 150°C or higher is preferably 70% by mass or higher, more preferably 80% by mass or higher, even more preferably 90% by mass or higher, even more preferably 95% by mass or higher, and particularly preferably 100% by mass.

The content of the binder resin in the electromagnetic wave absorbing layer is not particularly limited as long as it does not impede the object of the present invention. The content of the binder resin is preferably 5% by mass or more and 30% by mass or less, more preferably 5% by mass or more and 25% by mass or less, based on the mass of the electromagnetic wave absorbing layer.

[Non-dielectric materials]
The electromagnetic wave absorbing layer includes a non-dielectric material different from a magnetic material. The non-dielectric material is not particularly limited as long as it is a material other than a material recognized by a person skilled in the art as a ferroelectric or ferromagnetic material.

An example of a non-dielectric material is an inorganic filler. Suitable examples of such inorganic fillers include barium sulfate, aluminum oxide (alumina), aluminum nitride, boron nitride, silicon carbide, silicon dioxide (silica), calcium carbonate, and talc. Among these, barium sulfate, aluminum oxide, and silicon dioxide are more preferable, and barium sulfate is particularly preferable.

The electromagnetic wave absorbing layer typically contains a powder of a non-dielectric material. The particle size of the powder of the non-dielectric material is not particularly limited as long as it does not impede the object of the present invention. The average particle size of the powder of the non-dielectric material is preferably 1 nm or more and 20 μm or less, and more preferably 5 nm or more and 10 μm or less. Here, the average particle size of the powder of the non-dielectric material is the number average size of the primary particles of the powder of the non-dielectric material observed by an electron microscope.

The content of the non-dielectric material in the electromagnetic wave absorbing layer is not particularly limited as long as it does not impede the object of the present invention. The content of the non-dielectric material is preferably 10% by mass or more and 80% by mass or less, more preferably 20% by mass or more and 75% by mass or less, based on the mass of the electromagnetic wave absorbing layer.

[Dielectrics (ferroelectrics)]
The electromagnetic wave absorbing layer may contain a dielectric (ferroelectric) for the purpose of adjusting the relative dielectric constant of the electromagnetic wave absorbing layer. The relative dielectric constant of the electromagnetic wave absorbing layer can be adjusted by adjusting the content of the dielectric in the electromagnetic wave absorbing layer.
The relative dielectric constant of the electromagnetic wave absorbing layer is not particularly limited, but is preferably 6.5 or more and 65 or less, more preferably 10 or more and 50 or less, and even more preferably 15 or more and 30 or less.

Suitable examples of dielectrics include barium titanate, strontium titanate, calcium titanate, magnesium titanate, bismuth titanate, zirconium titanate, zinc titanate, and titanium dioxide. The electromagnetic wave absorbing layer may contain a combination of powders of multiple types of dielectrics.

The particle size of the dielectric powder used to adjust the relative dielectric constant of the electromagnetic wave absorbing layer is not particularly limited as long as it does not impede the object of the present invention. The average particle size of the dielectric powder is preferably 1 nm or more and 100 nm or less, and more preferably 5 nm or more and 50 nm or less. Here, the average particle size of the dielectric powder is the number average diameter of the primary particles of the dielectric powder observed by an electron microscope.

When adjusting the relative dielectric constant of the electromagnetic wave absorbing layer using dielectric powder, the amount of dielectric powder used is not particularly limited as long as the relative dielectric constant of the electromagnetic wave absorbing layer is within a predetermined range. The amount of dielectric powder used is preferably 0% by mass or more and 20% by mass or less, and more preferably 5% by mass or more and 10% by mass or less, relative to the mass of the electromagnetic wave absorbing layer.

[Carbon nanotubes]
The electromagnetic wave absorbing layer may contain carbon nanotubes. By incorporating carbon nanotubes in the electromagnetic wave absorbing layer, the relative dielectric constant of the electromagnetic wave absorbing layer can be adjusted. The carbon nanotubes may be used in combination with the above-mentioned dielectric powder.

The amount of carbon nanotubes blended into the electromagnetic wave absorbing layer is not particularly limited as long as the relative dielectric constant of the electromagnetic wave absorbing layer is within the above-mentioned range. However, since carbon nanotubes are also a conductive material, if the amount of carbon nanotubes used is excessive, the electromagnetic wave absorbing properties provided by the electromagnetic wave absorbing layer may be impaired.
The amount of carbon nanotubes used is preferably 0% by mass or more and 20% by mass or less, and more preferably 1% by mass or more and 10% by mass or less, based on the mass of the electromagnetic wave absorbing layer.

[Other ingredients]
The electromagnetic wave absorbing layer may contain various additives other than the above components, as long as the object of the present invention is not impaired. Examples of additives that the electromagnetic wave absorbing layer may contain include dispersants, colorants, antioxidants, ultraviolet absorbers, flame retardants, flame retardant assistants, plasticizers, and surfactants. These additives are used in the amounts that are conventionally used, as long as the object of the present invention is not impaired.

By forming a film from the components described above, for example by a method using a paste for forming an electromagnetic wave absorber described below, an electromagnetic wave absorbing layer is obtained that can be used as an electromagnetic wave absorber whose electromagnetic wave absorbing properties are not easily degraded even when exposed to high temperatures for a long period of time.

<Base layer>
The electromagnetic wave absorbing layer may be laminated on a substrate layer. The substrate layer may be a layer containing any substrate as long as it does not impair the effects of the present invention, and examples of the substrate layer include a layer containing a resin.
Examples of the resin include polyethylene terephthalate (PET), polyethylene naphthalate (PEN), acrylic (PMMA), polycarbonate (PC), cycloolefin polymer (COP), polyethersulfone, polyimide, polyamideimide, etc. Among these, PET and PEN are preferred because they have excellent heat resistance and a good balance between dimensional stability and cost.

The shape of the substrate layer may have a curved surface or may be composed of only flat surfaces, and is preferably a flat plate shape.
From the viewpoint of making the film thinner and smaller without impairing the effects of the present invention, the thickness of the base layer is preferably 800 μm or less, more preferably 500 μm or less, even more preferably 300 μm or less, and particularly preferably 150 μm or less.
The lower limit of the thickness of the base layer is not particularly limited as long as the effects of the present invention are not impaired, and examples thereof include 1 μm or more, 10 μm or more, and 50 μm or more.

<Metal Layer>
When the electromagnetic wave absorber includes a base layer, a metal layer may be provided on the surface of the base layer opposite to the surface on which the electromagnetic wave absorbing layer is provided. When the metal layer is provided, the electromagnetic waves reflected by the metal layer can be attenuated. Examples of metals constituting the metal layer include aluminum, titanium, SUS, copper, brass, silver, gold, and platinum.
The thickness of the metal layer is not particularly limited, and from the viewpoint of making the electromagnetic wave absorber thin, it is preferably 600 μm or less, more preferably 400 μm or less, even more preferably 100 μm or less, and particularly preferably 50 μm or less.
The lower limit of the thickness of the metal layer is not particularly limited as long as the effect of the present invention is not impaired, and examples thereof include 0.1 μm or more, 1 μm or more, 5 μm or more, and 10 μm or more.

By combining the electromagnetic wave absorbing layer containing the specified components described above with a base layer, or a base layer and a metal layer as necessary, an electromagnetic wave absorber whose electromagnetic wave absorbing properties are not easily degraded even when exposed to high temperatures for a long period of time can be obtained.

The electromagnetic wave absorber may have an adhesive layer or a pressure-sensitive adhesive layer at any position. Examples of the adhesive layer or the pressure-sensitive adhesive layer include an acrylic pressure-sensitive adhesive layer, a rubber pressure-sensitive adhesive layer, a silicone pressure-sensitive adhesive layer, a urethane pressure-sensitive adhesive layer, etc.

The electromagnetic wave absorber described above can be preferably used as an electromagnetic wave absorbing film for various elements (including vehicle-mounted elements, high-frequency antenna elements, etc.) in various information and communication systems such as mobile phones, wireless LANs, ETC systems, intelligent road transport systems, automobile navigation assistance road systems, and satellite broadcasting.

<Paste for forming electromagnetic wave absorbers>
As a method for forming the electromagnetic wave absorber, a method using a paste for forming an electromagnetic wave absorber is preferred, since it is possible to form an electromagnetic wave absorbing layer with high efficiency without any particular thickness restriction and it is possible to form the electromagnetic wave absorbing layer directly on the base layer.
The paste for forming an electromagnetic wave absorber includes the above-mentioned magnetic material, a binder resin, and a non-dielectric material. The paste for forming an electromagnetic wave absorber may include the above-mentioned substances added for adjusting the relative dielectric constant, relative magnetic permeability, etc., and other components. When the binder resin includes a curable resin, the paste for forming an electromagnetic wave absorber includes a compound that is a precursor of the curable resin. In this case, the paste for forming an electromagnetic wave absorber includes a curing agent, a curing accelerator, a polymerization initiator, etc. as necessary.

In addition, if the paste for forming the electromagnetic wave absorber contains a photopolymerizable or thermally polymerizable compound, the coating film may be exposed to light or heated as necessary to form an electromagnetic wave absorbing layer.

The paste for forming the electromagnetic wave absorber preferably further contains a dispersion medium. As the dispersion medium, water, an organic solvent, or an aqueous solution of an organic solvent can be used. As the dispersion medium, an organic solvent is preferable because it is easy to dissolve organic components, has a low latent heat of vaporization, and is easy to remove by drying.

Suitable examples of organic solvents used as dispersion media include nitrogen-containing polar solvents such as N,N,N',N'-tetramethylurea (TMU), N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMAc), N,N-dimethylisobutyramide, N,N-diethylacetamide, N,N-dimethylformamide (DMF), N,N-diethylformamide, N-methylcaprolactam, 1,3-dimethyl-2-imidazolidinone (DMI), and pyridine; ketones such as diethyl ketone, methyl butyl ketone, dipropyl ketone, and cyclohexanone; alcohols such as n-pentanol, 4-methyl-2-pentanol, cyclohexanol, and diacetone alcohol; ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, diethylene glycol monomethyl ether, and diethylene glycol monoethyl ether; Ether alcohols such as diethylene glycol dimethyl ether and diethylene glycol diethyl ether; saturated aliphatic monocarboxylic acid alkyl esters such as n-butyl acetate and amyl acetate; lactate esters such as ethyl lactate and n-butyl lactate; ketones such as acetone, methyl ethyl ketone, cyclohexanone, acetophenone, and benzophenone; methyl cellosolve acetate, ethyl cellosolve acetate, propylene glycol monomethyl ether acetate, and propylene glycol Examples of ether esters include monoethyl ether acetate, ethyl 3-ethoxypropionate, 2-methoxybutyl acetate, 3-methoxybutyl acetate, 4-methoxybutyl acetate, 2-methyl-3-methoxybutyl acetate, 3-methyl-3-methoxybutyl acetate, 3-ethyl-3-methoxybutyl acetate, 2-ethoxybutyl acetate, 4-ethoxybutyl acetate, 4-propoxybutyl acetate, and 2-methoxypentyl acetate. These can be used alone or in combination of two or more.

The solid content of the paste for forming an electromagnetic wave absorber is adjusted as appropriate depending on the method for applying the paste for forming an electromagnetic wave absorber, the thickness of the electromagnetic wave absorbing layer, etc. Typically, the solid content of the paste for forming an electromagnetic wave absorber is preferably 3% by mass or more and 60% by mass or less, and more preferably 10% by mass or more and 50% by mass or less. The solid content of the paste is a value calculated by taking the total mass of the components not dissolved in the dispersion medium and the mass of the components dissolved in the dispersion medium as the mass of the solids.

(Dispersant)
The paste for forming the electromagnetic wave absorber may contain a dispersant for the purpose of dispersing the above-mentioned magnetic material or a substance used for adjusting the relative dielectric constant and relative magnetic permeability of the electromagnetic wave absorbing layer well in the electromagnetic wave absorbing layer. The dispersant may be mixed uniformly together with the above-mentioned magnetic material and binder resin. The dispersant may be blended in the binder resin. Furthermore, the above-mentioned magnetic material or a substance added for adjusting the relative dielectric constant and relative magnetic permeability, which has been previously treated with a dispersant, may be blended in the material constituting the electromagnetic wave absorbing layer.

The type of dispersant is not particularly limited as long as it does not impede the object of the present invention. The dispersant can be selected from various dispersants that have been used in the past for dispersing various inorganic and organic fine particles.

Suitable examples of dispersants include silane coupling agents (e.g., phenyltrimethoxysilane), titanate coupling agents, zirconate coupling agents, and aluminate coupling agents.

The content of the dispersant is not particularly limited as long as it does not impede the object of the present invention. The content of the dispersant is preferably 0.1% by mass or more and 30% by mass or less, more preferably 1% by mass or more and 15% by mass or less, and particularly preferably 1% by mass or more and 10% by mass or less, based on the solid content mass of the paste for forming an electromagnetic wave absorber.

<Method of manufacturing electromagnetic wave absorber>
The method for producing the above-mentioned electromagnetic wave absorber is not particularly limited as long as it is possible to produce an electromagnetic wave absorber having a predetermined structure.
A preferred method includes a method including an electromagnetic wave absorbing layer forming step in which the above-mentioned paste containing a magnetic material, a binder resin, and a non-dielectric material is applied onto a base layer to form a coating film, and then the coating film is dried to form an electromagnetic wave absorbing layer. As described above, the paste may contain substances, dispersion mediums, dispersants, and other components added to adjust the relative dielectric constant, relative magnetic permeability, etc.

The method of applying the paste for forming an electromagnetic wave absorber onto the base layer is not particularly limited as long as it is possible to form an electromagnetic wave absorber of a desired thickness. Examples of the application method include spray coating, dip coating, roll coating, curtain coating, spin coating, screen printing, doctor blade, slot die, bank coating (knife coating), and applicator methods.
The coating film formed by the above method is dried to remove the dispersion medium, forming an electromagnetic wave absorbing layer on the substrate layer, thereby obtaining an electromagnetic wave absorber. The thickness of the coating film is appropriately adjusted so that the thickness of the electromagnetic wave absorbing layer obtained after drying is a desired thickness.
The drying method is not particularly limited, and examples thereof include (1) a method of drying on a hot plate at a temperature of 80° C. or higher and 180° C. or lower, preferably 90° C. or higher and 160° C. or lower, for 1 minute to 30 minutes, (2) a method of leaving the material at room temperature for several hours to several days, and (3) a method of placing the material in a hot air heater or an infrared heater for several tens of minutes to several hours to remove the solvent.

The method for producing an electromagnetic wave absorber may include a cutting step of cutting the electromagnetic wave absorbing layer, or a laminate comprising a base layer and an electromagnetic wave absorbing layer, obtained in the electromagnetic wave absorbing layer forming step, to obtain an electromagnetic wave absorber of a predetermined size.
As described above, since the electromagnetic wave absorbing layer contains a magnetic material, a binder resin having predetermined temperature characteristics, and a non-dielectric material, the electromagnetic wave absorbing characteristics are unlikely to deteriorate even when exposed to high temperatures for a long period of time.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

Example 1
(Preparation of paste for forming electromagnetic wave absorber)
To 100.0 parts by mass of DMI (1,3-dimethyl-2-imidazolidinone), 34.0 parts by mass of the epsilon-type iron oxide described below, 4.0 parts by mass of carbon nanotubes (CNT) described below, 12.0 parts by mass of binder resin 1, 50.0 parts by mass of granular alumina powder, and 3.4 parts by mass of the dispersant described below were added. Binder resin 1 was added as binder resin solution 1 described below. The mixture was stirred with a rotation/revolution mixer to uniformly dissolve or disperse each component, thereby obtaining a paste for forming an electromagnetic wave absorber.

ε-Al _a Ti _b Co _c Fe _2-(a+b+c) O ₃ was used as epsilon-type iron oxide. a, b, and c were each greater than 0 and less than 2, and a+b+c was greater than 0 and less than 2. The average particle size of the epsilon-type iron oxide was 20 nm or more and 30 nm or less.
As the CNTs, multi-walled carbon nanotubes (product name VGCF-H; manufactured by Showa Denko KK) with a major axis of 150 nm were used.
The alumina powder had an average particle size of 1 μm.
As the dispersant, phenyltrimethoxysilane was used.
As the binder resin solution 1, a polyester-urethane copolymer (manufactured by Toyobo Co., Ltd., product name UR-1400, glass transition point 83° C., weight average molecular weight 40,000, consisting of 30.0 parts by mass of resin, 35.0 parts by mass of methyl ethyl ketone, and 35.0 parts by mass of toluene) was used.

(Manufacture of electromagnetic wave absorbers)
The above-mentioned paste for forming an electromagnetic wave absorber was applied to an aluminum plate (thickness 2 mm) using an applicator. The applied film was then dried under conditions of 90° C. for 10 minutes and 130° C. for 10 minutes to form an electromagnetic wave absorbing layer having a thickness of 35 μm, thereby obtaining an electromagnetic wave absorber.

<Heat resistance test>
Electromagnetic waves of 40 to 120 GHz were incident on the electromagnetic wave absorber before the heat treatment, and the return loss was measured using a terahertz time domain spectrometer (TAS7400 manufactured by Advantest Corporation).
The return loss RL(f) at a frequency f is calculated as RL(f)=-10Log(R(f)/100), where R(f) is the reflectance (%).

Next, each electromagnetic wave absorber was subjected to a heat treatment under any one of the following conditions.
High temperature storage test: 2000 hours at 125°C Heat cycle test: 2000 cycles of 125°C <=> -45°C (1 hour) Pressure cooker test: 192 hours at 121°C, 98% RH

The return loss of each electromagnetic wave absorber after the heat treatment was measured in the same manner as before the heat treatment. At the frequency at which the return loss was maximum before the heat treatment, the change in the return loss after the heat treatment relative to the return loss before the heat treatment was calculated and evaluated according to the following criteria. The results are shown in Table 2.
A: Less than 10% B: 10% to less than 20% C: 20% or more

Example 2
To 100.0 parts by mass of DMI, 34.0 parts by mass of the epsilon-type iron oxide, 4.0 parts by mass of the CNT, 12.0 parts by mass of binder resin 2, 50.0 parts by mass of the alumina powder, and 3.4 parts by mass of the dispersant were added. Binder resin 2 was added as the following binder resin solution 2. The mixture was stirred with a rotation/revolution mixer to uniformly dissolve or disperse each component, thereby obtaining a paste for forming an electromagnetic wave absorber.

A polyester-urethane copolymer (manufactured by Toyobo Co., Ltd., product name UR-4800, glass transition point 106°C, weight average molecular weight 25,000, consisting of 30.0 parts by mass of resin, 35.0 parts by mass of methyl ethyl ketone, and 35.0 parts by mass of toluene) was used as binder resin solution 2.

The above electromagnetic wave absorber forming paste was used to obtain an electromagnetic wave absorber in the same manner as in Example 1. The obtained electromagnetic wave absorber was subjected to a heat resistance test in the same manner as in Example 1. The results are shown in Table 2.

Example 3
(Preparation of paste for forming electromagnetic wave absorber)
To 230.0 parts by mass of DMI, 34.0 parts by mass of the epsilon-type iron oxide, 4.0 parts by mass of the CNT, 12.0 parts by mass of binder resin 3, 50.0 parts by mass of granular silica powder, and 3.4 parts by mass of the dispersant were added. Binder resin 3 was added as the following binder resin solution 3. The mixture was stirred with a rotation/revolution mixer to uniformly dissolve or disperse each component, thereby obtaining a paste for forming an electromagnetic wave absorber.

As the binder resin solution 3, polyvinylidene fluoride (manufactured by Kureha Corporation, product name KF Polymer W#7200, melting point 173° C., weight average molecular weight 630,000, composed of 10.0 parts by mass of resin and 90.0 parts by mass of DMI) was used.
The silica powder had an average particle size of 1 μm.

(Manufacture of electromagnetic wave absorbers)
The paste for forming an electromagnetic wave absorber was applied to the surface opposite to the adhesive layer of a PEN film (thickness 125 μm) laminated with an adhesive layer by an applicator. The applied film was then dried under conditions of 90° C. for 10 minutes and 130° C. for 10 minutes to form an electromagnetic wave absorbing layer having a thickness of 35 μm, and a film-shaped electromagnetic wave absorber was obtained. The film-shaped electromagnetic wave absorber obtained immediately after drying was cut into a square shape of 5 cm on each side to prepare test pieces for the following evaluations.

<Heat resistance test>
A sample of the electromagnetic wave absorber in the form of a film having a square shape of 5 cm on each side was attached to an aluminum plate. The obtained electromagnetic wave absorber was subjected to a heat resistance test in the same manner as in Example 1. The results are shown in Table 2.

Example 4
To 230.0 parts by mass of DMI, 54.5 parts by mass of the epsilon-type iron oxide, 3.5 parts by mass of the CNT, 12.0 parts by mass of the binder resin 3, 30.0 parts by mass of granular barium sulfate powder, and 5.5 parts by mass of the dispersant were added. The mixture was stirred with a rotation/revolution mixer to uniformly dissolve or disperse each component, thereby obtaining a paste for forming an electromagnetic wave absorber.
The average particle size of the barium sulfate powder was 30 nm.

The above electromagnetic wave absorber forming paste was used to obtain an electromagnetic wave absorber in the same manner as in Example 3. The obtained electromagnetic wave absorber was subjected to a heat resistance test in the same manner as in Example 3. The results are shown in Table 2.

Example 5
To 230.0 parts by mass of DMI, 34.0 parts by mass of the epsilon-type iron oxide, 4.0 parts by mass of the CNT, 12.0 parts by mass of the binder resin 3, 50.0 parts by mass of the barium sulfate powder, and 3.4 parts by mass of the dispersant were added. The mixture was stirred with a rotation/revolution mixer to uniformly dissolve or disperse each component, thereby obtaining a paste for forming an electromagnetic wave absorber.

The above electromagnetic wave absorber forming paste was used to obtain an electromagnetic wave absorber in the same manner as in Example 3. The obtained electromagnetic wave absorber was subjected to a heat resistance test in the same manner as in Example 3. The results are shown in Table 2.

Example 6
To 230.0 parts by mass of DMI, 25.0 parts by mass of the epsilon-type iron oxide, 4.0 parts by mass of the CNT, 12.0 parts by mass of the binder resin 3, 59.0 parts by mass of the barium sulfate powder, and 2.5 parts by mass of the dispersant were added. The mixture was stirred with a rotation/revolution mixer to uniformly dissolve or disperse each component, thereby obtaining a paste for forming an electromagnetic wave absorber.

The above electromagnetic wave absorber forming paste was used to obtain an electromagnetic wave absorber in the same manner as in Example 3. The obtained electromagnetic wave absorber was subjected to a heat resistance test in the same manner as in Example 3. The results are shown in Table 2.

Example 7
To 230.0 parts by mass of DMI, 20.0 parts by mass of the epsilon-type iron oxide, 5.0 parts by mass of the CNT, 12.0 parts by mass of the binder resin 3, 63.0 parts by mass of the barium sulfate powder, and 2.0 parts by mass of the dispersant were added. The mixture was stirred with a rotation/revolution mixer to uniformly dissolve or disperse each component, thereby obtaining a paste for forming an electromagnetic wave absorber.

The above electromagnetic wave absorber forming paste was used to obtain an electromagnetic wave absorber in the same manner as in Example 3. The obtained electromagnetic wave absorber was subjected to a heat resistance test in the same manner as in Example 3. The results are shown in Table 2.

Example 8
To 230.0 parts by mass of DMI, 9.0 parts by mass of the epsilon-type iron oxide, 9.0 parts by mass of the CNT, 12.0 parts by mass of the binder resin 3, 70.0 parts by mass of the barium sulfate powder, and 0.9 parts by mass of the dispersant were added. The mixture was stirred with a rotation/revolution mixer to uniformly dissolve or disperse each component, thereby obtaining a paste for forming an electromagnetic wave absorber.

The above electromagnetic wave absorber forming paste was used to obtain an electromagnetic wave absorber in the same manner as in Example 3. The obtained electromagnetic wave absorber was subjected to a heat resistance test in the same manner as in Example 3. The results are shown in Table 2.

Comparative Example 1
To 100.0 parts by mass of DMI, 78.0 parts by mass of the epsilon-type iron oxide, 5.0 parts by mass of the CNT, 17.0 parts by mass of binder resin 4, and 7.8 parts by mass of the dispersant were added. Binder resin 4 was added as the following binder resin solution 4. The mixture was stirred with a rotation/revolution mixer to uniformly dissolve or disperse each component, thereby obtaining a paste for forming an electromagnetic wave absorber.

A polyester-urethane copolymer (manufactured by Toyobo Co., Ltd., product name UR-3210, glass transition point -3°C, weight average molecular weight 40,000, consisting of 30.0 parts by mass of resin and 70.0 parts by mass of methyl ethyl ketone) was used as binder resin solution 4.

The above electromagnetic wave absorber forming paste was used to obtain an electromagnetic wave absorber in the same manner as in Example 3. The obtained electromagnetic wave absorber was subjected to a heat resistance test in the same manner as in Example 3. The results are shown in Table 2.

Comparative Example 2
To 100.0 parts by mass of DMI, 34.0 parts by mass of the epsilon-type iron oxide, 4.0 parts by mass of the CNT, 12.0 parts by mass of the binder resin 4, 50.0 parts by mass of the alumina powder, and 3.4 parts by mass of the dispersant were added. The mixture was stirred with a rotation/revolution mixer to uniformly dissolve or disperse each component, thereby obtaining a paste for forming an electromagnetic wave absorber.

The above electromagnetic wave absorber forming paste was used to obtain an electromagnetic wave absorber in the same manner as in Example 1. The obtained electromagnetic wave absorber was subjected to a heat resistance test in the same manner as in Example 1. The results are shown in Table 2.

Comparative Example 3
To 100.0 parts by mass of DMI, 34.0 parts by mass of the epsilon-type iron oxide, 4.0 parts by mass of the CNT, 12.0 parts by mass of binder resin 5, 50.0 parts by mass of the alumina powder, and 3.4 parts by mass of the dispersant were added. Binder resin 5 was added as the following binder resin solution 5. The mixture was stirred with a rotation/revolution mixer to uniformly dissolve or disperse each component, thereby obtaining a paste for forming an electromagnetic wave absorber.

A polyester-urethane copolymer (manufactured by Toyobo Co., Ltd., product name UR-1350, glass transition point 46°C, weight average molecular weight 40,000, consisting of 30.0 parts by mass of resin, 45.0 parts by mass of methyl ethyl ketone, and 25.0 parts by mass of toluene) was used as the binder resin solution 5.

The above electromagnetic wave absorber forming paste was used to obtain an electromagnetic wave absorber in the same manner as in Example 1. The obtained electromagnetic wave absorber was subjected to a heat resistance test in the same manner as in Example 1. The results are shown in Table 2.

Comparative Example 4
To 100.0 parts by mass of DMI, 34.0 parts by mass of the epsilon-type iron oxide, 4.0 parts by mass of the CNT, 12.0 parts by mass of the binder resin 4, 50.0 parts by mass of the silica powder, and 3.4 parts by mass of the dispersant were added. The mixture was stirred with a rotation/revolution mixer to uniformly dissolve or disperse each component, thereby obtaining a paste for forming an electromagnetic wave absorber.

The above electromagnetic wave absorber forming paste was used to obtain an electromagnetic wave absorber in the same manner as in Example 3. The obtained electromagnetic wave absorber was subjected to a heat resistance test in the same manner as in Example 3. The results are shown in Table 2.

Comparative Example 5
To 100.0 parts by mass of DMI, 34.0 parts by mass of the epsilon-type iron oxide, 4.0 parts by mass of the CNT, 12.0 parts by mass of the binder resin 4, 50.0 parts by mass of the barium sulfate powder, and 3.4 parts by mass of the dispersant were added. The mixture was stirred with a rotation/revolution mixer to uniformly dissolve or disperse each component, thereby obtaining a paste for forming an electromagnetic wave absorber.

The above electromagnetic wave absorber forming paste was used to obtain an electromagnetic wave absorber in the same manner as in Example 3. The obtained electromagnetic wave absorber was subjected to a heat resistance test in the same manner as in Example 3. The results are shown in Table 2.

Comparative Example 6
To 100.0 parts by mass of DMI, 78.0 parts by mass of the epsilon-type iron oxide, 5.0 parts by mass of the CNT, 17.0 parts by mass of the binder resin 1, and 7.8 parts by mass of the dispersant were added. The mixture was stirred with a rotation/revolution mixer to uniformly dissolve or disperse each component, thereby obtaining a paste for forming an electromagnetic wave absorber.

The above electromagnetic wave absorber forming paste was used to obtain an electromagnetic wave absorber in the same manner as in Example 3. The obtained electromagnetic wave absorber was subjected to a heat resistance test in the same manner as in Example 3. The results are shown in Table 2.

Comparative Example 7
To 230.0 parts by mass of DMI, 78.0 parts by mass of the epsilon-type iron oxide, 5.0 parts by mass of the CNT, 17.0 parts by mass of the binder resin 3, and 7.8 parts by mass of the dispersant were added. The mixture was stirred with a rotation/revolution mixer to uniformly dissolve or disperse each component, thereby obtaining a paste for forming an electromagnetic wave absorber.

The above electromagnetic wave absorber forming paste was used to obtain an electromagnetic wave absorber in the same manner as in Example 3. The obtained electromagnetic wave absorber was subjected to a heat resistance test in the same manner as in Example 3. The results are shown in Table 2.

The amounts (parts by mass) of magnetic material, CNT, binder resin, and non-dielectric material in the examples and comparative examples are shown in Table 1.

According to Tables 1 and 2, it can be seen that the electromagnetic wave absorbers of Examples 1 to 8, which contain both a binder resin having predetermined temperature characteristics and a non-dielectric material in an electromagnetic wave absorbing layer containing a magnetic material, are less likely to experience a decrease in electromagnetic wave absorption characteristics even when exposed to high temperatures for a long period of time.
On the other hand, from Comparative Examples 1 to 7, it can be seen that the electromagnetic wave absorption characteristics are significantly reduced when exposed to high temperatures for a long period of time, in any of the cases where the film does not contain both a binder resin having predetermined temperature characteristics and a non-dielectric material, where the film contains only a binder resin having predetermined temperature characteristics, and where the film contains only a non-dielectric material.

Claims (7)

An electromagnetic wave absorber comprising an electromagnetic wave absorbing layer,
the electromagnetic wave absorbing layer includes a magnetic material, a binder resin, and a non-dielectric material;
The binder resin contains an amorphous resin having a glass transition point of 80° C. or more and/or a crystalline resin having a melting point of 150° C. or more,
An electromagnetic wave absorber, wherein the non-dielectric material is different from the magnetic material. The electromagnetic wave absorber according to claim 1, wherein the electromagnetic wave absorbing layer contains carbon nanotubes. The electromagnetic wave absorber according to claim 1, wherein the magnetic material includes epsilon iron oxide. The electromagnetic wave absorber according to claim 1, wherein the amorphous resin and/or the crystalline resin comprises at least one selected from the group consisting of polyurethane resin, polyester-urethane resin, urethane polyamide resin, polycarbonate resin, polyester resin, FR-AS resin, FR-ABS resin, AS resin, ABS resin, polyphenylene oxide resin, polyphenylene sulfide resin, polysulfone resin, polyethersulfone resin, polyetheretherketone resin, fluorine-based resin, polyimide resin, polyamideimide resin, polyamide bismaleimide resin, polyetherimide resin, polybenzoxazole resin, polybenzothiazole resin, polybenzimidazole resin, BT resin, polymethylpentene, ultra-high molecular weight polyethylene, FR-polypropylene, cellulose resin, (meth)acrylic resin, and polystyrene. The electromagnetic wave absorber according to claim 1, wherein the non-dielectric material comprises at least one selected from the group consisting of barium sulfate, aluminum oxide, aluminum nitride, boron nitride, silicon carbide, silicon dioxide, calcium carbonate, and talc. The electromagnetic wave absorbing layer is laminated on a base layer,
The electromagnetic wave absorber according to any one of claims 1 to 5, which is in the form of a film. A paste for forming an electromagnetic wave absorber, comprising a magnetic material, a binder resin, and a non-dielectric material,
The binder resin contains an amorphous resin having a glass transition point of 80° C. or more and/or a crystalline resin having a melting point of 150° C. or more,
The paste for forming an electromagnetic wave absorber, wherein the non-dielectric material is different from the magnetic material.