Radio wave absorbing material and radio wave absorber

Abstract

Provided is a radio wave absorbing material containing a composition comprising water and a polymer. The polymer accounts for 8 wt% or more, with the combined total of the water and the polymer taken as 100 wt%. The content of the water is 10 wt% or more relative to the total weight of the composition.

Description

Radio wave absorbing materials and radio wave absorbers

[0001] The present invention relates to a radio wave absorbing material having low radio wave reflectivity and excellent radio wave absorption performance, and to a radio wave absorber using such a radio wave absorbing material.

[0002] It is known that water blocks radio waves, making underwater radio communication impossible, and attempts are being made to use water as a radio wave control material.

[0003] As a technology for using water as a radio wave control material, for example, Patent Document 1 proposes an electromagnetic wave shielding material comprising a gel containing water, a natural water-soluble polymer, and a metal salt compound, wherein the concentration of the natural water-soluble polymer is 0.4 to 5% by mass in the electromagnetic wave shielding material, and the concentration of the metal salt compound is in the range of 0.004 to 2 moles / kg in the electromagnetic wave shielding material. Furthermore, Patent Document 2 proposes an electromagnetic wave suppressor having an electromagnetic wave suppression sheet comprising a first moisture-proof film, a second moisture-proof film, and a specific acrylate-based polymer gel sealed between the first and second moisture-proof films.

[0004] Japanese Patent Publication No. 2018-189250 Japanese Patent Publication No. 2009-259905

[0005] Conventional radio wave absorbers generally use metals, and most of them are opaque or have low transparency. However, in recent years, there has been a demand for transparent radio wave absorbers to ensure natural light and visibility in living environments and to avoid causing a feeling of oppression to people. In contrast, water is 10 ３ ~10 ２１ Water is known to shield electromagnetic waves across a wide bandwidth of Hz, and therefore, its use as a radio wave absorber is expected to lead to the application of transparent radio wave absorbers. On the other hand, the inventors have found that water has high radio wave reflectivity on its surface. According to the inventors' findings, high radio wave reflectivity can cause undesirable effects from radio waves reflected from the radio wave absorber depending on the application, and therefore, lower radio wave reflectivity is preferable.

[0006] This invention has been made in view of these circumstances, and aims to provide a radio wave absorbing material that has low radio wave reflectivity and excellent radio wave absorption performance.

[0007] In order to achieve the above objective, the inventors of the present invention conducted research and discovered that by using a composition containing water and a polymer in specific proportions as a radio wave absorbing material, it is possible to achieve low radio wave reflectivity and excellent radio wave absorption performance, thus completing the present invention.

[0008] In other words, the present invention provides the following radio wave absorbing materials and radio wave absorbers: [1] A radio wave absorbing material comprising a composition containing water and a polymer, wherein the proportion of the polymer is 8% by weight or more of the total 100% by weight of the water and polymer, and the proportion of water to the whole composition is 10% by weight or more. [2] The radio wave absorbing material according to [1], wherein the composition contains a salt compound. [3] The radio wave absorbing material according to [1] or [2], wherein the polymer is a synthetic polymer. [4] The radio wave absorbing material according to [3], wherein the polymer is an acrylamide polymer or a polyalkylene oxide polymer. [5] A radio wave absorber comprising the radio wave absorbing material according to any one of [1] to [4]. [6] A radio wave absorber comprising the radio wave absorbing material according to any one of [1] to [4] and an outer covering having a water content of less than 10% by weight, wherein at least a part of the radio wave absorbing material is covered by the outer covering. [7] The radio wave absorber according to [5] or [6], wherein at least a part of the surface has an uneven shape with a height difference of 0.5 mm or more. [8] A radio wave absorber according to any one of [5] to [7], wherein a member made of rubber material is disposed on at least a part of the surface of the radio wave absorbing material. [9] A radio wave absorber according to any one of [5] to [8], wherein a transparent material having a fine uneven structure is disposed on at least a part of the surface of the radio wave absorbing material.

[10] A radio wave absorber according to any one of [5] to [9], wherein a resistive film layer is disposed on at least a part outside the radio wave absorbing material.

[11] A radio wave absorber according to any one of [5] to

[10] that absorbs radio waves with a frequency of 1 GHz or more and 220 THz or less.

[12] A radio wave absorber according to any one of [5] to

[10] that absorbs radio waves with a frequency of 5.6 GHz or more and 110 GHz or less.

[13] A radio wave absorber according to any one of [5] to

[10] that absorbs radio waves with a frequency of 20 GHz or more and 110 GHz or less.

[14] A radio wave absorber according to any one of [5] to

[10] that absorbs radio waves with a frequency of 100 GHz or more and 110 THz or less.

[15] A radio wave absorber according to any one of [5] to

[10] , having a radio wave reflectance surface that is 20% or less of radio waves with a frequency of 100 GHz or more and 220 THz or less.

[0009] According to the present invention, it is possible to provide a radio wave absorbing material that has low radio wave reflectivity and excellent radio wave absorption performance, and a radio wave absorber using such a radio wave absorbing material.

[0010] Figure 1 is a graph of the return loss and transmission loss of the test specimen (3-8') in Example 3 at 5.6 to 110 GHz. Figure 2 is a graph of the absorptivity of the test specimen (3-8') in Example 3 at 5.6 to 110 GHz.

[0011] <Radio Wave Absorbing Material> The radio wave absorbing material of the present invention comprises a composition containing water and a polymer, wherein the proportion of the polymer is 8% by weight or more of the total 100% by weight of water and polymer, and the proportion of water relative to the whole composition is 10% by weight or more. The radio wave absorbing material of the present invention contains water and a polymer in the above predetermined proportions, thereby achieving excellent radio wave absorption performance while keeping the radio wave reflectivity low. In particular, the radio wave absorbing material of the present invention can reduce the reflectivity of radio waves from the GHz band to the THz band, and moreover, it has excellent radio wave absorption and shielding characteristics.

[0012] In recent years, with the advent of the 5G and 6G era, there has been a demand for materials that control radio waves. Conductive carbon materials and magnetic ceramic materials are known as materials that absorb and shield radio waves, while metals are known as materials that reflect and shield radio waves. However, these are all opaque, hard, rigid, and heavy materials. Therefore, although it is possible to improve their properties by making them into extremely thin layers, fibers, or fine particles such as powders, there are limits to the degree of improvement and the shapes that can be realized. As a result, there is a demand for flexible, transparent, and lightweight materials as materials that control radio waves. In particular, transparent materials have been in demand in recent years to ensure natural light and visibility in living environments and to avoid causing a feeling of oppression to people.

[0013] On the other hand, although attempts have been made to use water as a radio wave control material, the inventors have found that water has high radio wave reflectivity on its surface, and therefore, depending on the application, the radio waves reflected from the radio wave absorber may have an undesirable effect due to this high radio wave reflectivity. In response to this, the inventors have conducted diligent research and have found that by applying a composition containing water and a polymer in the above-mentioned predetermined proportions to a radio wave absorbing material, it is possible to achieve excellent radio wave absorption performance while keeping the radio wave reflectivity low.

[0014] Furthermore, to enhance radio wave shielding performance, a structure called a λ / 4 type is sometimes used. In a λ / 4 type structure, radio wave shielding performance is achieved by the phase difference between incident radio waves and reflected radio waves. For example, Japanese Patent Publication No. 2022-165989 describes a polymer material with radio wave shielding properties that utilizes a λ / 4 type structure. According to the technology of Japanese Patent Publication No. 2022-165989, since it is a technology that utilizes polymer materials, it is thought that it can be lightweight and have excellent flexibility. However, when adopting a λ / 4 type structure such as the technology of Japanese Patent Publication No. 2022-165989, it is necessary to make it into a sheet shape in order to achieve high radio wave shielding performance, and there are constraints on that shape. Furthermore, when a λ / 4 type structure is adopted, absorption is achieved by shifting the phase of the reflected wave by 1 / 2 wavelength relative to the incident wave and canceling it out. As a result, it exhibits frequency specificity, meaning that while it shows high absorption performance for the target frequency, it cannot absorb other frequencies. Moreover, it is basically designed for radio waves incident from the vertical direction, and its absorption performance decreases for radio waves incident from an oblique direction. In other words, radio wave absorbing materials using a λ / 4 type structure cannot achieve radio wave shielding over a wide frequency range or shielding against radio waves from a wide range of angles. In addition, with radio wave absorbing materials using a λ / 4 type structure, the wavelength that can be absorbed changes with thickness, so the absorption performance fluctuates with variations in thickness during manufacturing. This presents a challenge in obtaining radio wave absorbing materials that show good shielding performance for the desired wavelength. In particular, when using a λ / 4 type structure to achieve absorption characteristics for frequencies above gigahertz, the thickness of the radio wave absorbing material needs to be around several hundred micrometers, which makes these problems even more pronounced. Furthermore, scratches or dirt from dust during use can change the thickness, potentially reducing absorption performance. In addition, a conductive layer is necessary to create a λ / 4 type structure, and since metal foil or similar materials are used to form this conductive layer, transparency cannot be guaranteed in most cases.

[0015] In contrast, the present invention employs a configuration containing water and a polymer, and the water contained in the radio wave absorbing material itself exhibits excellent radio wave absorption and radio wave blocking properties. As a result, it is a material with excellent transparency and light weight, and since it does not utilize a λ / 4 type structure, the reduction in radio wave absorption performance due to thickness variations can be suppressed.

[0016] In this invention, radio waves are a concept that includes electromagnetic waves. ３ Hz to 10 １６ This includes radio waves or electromagnetic waves in the Hz frequency band.

[0017] The radio wave absorbing material of the present invention is capable of suitably reflecting and / or absorbing radio waves with frequencies of 1 GHz to 220 THz, more preferably with frequencies of 5.6 GHz to 110 GHz, even more preferably with frequencies of 20 GHz to 110 GHz, and even more preferably with frequencies of 100 GHz to 110 THz. Furthermore, the radio wave absorbing material of the present invention may also be capable of suitably absorbing radio waves with frequencies of 100 GHz to 220 THz.

[0018] In the composition constituting the radio wave absorbing material of the present invention, the proportion of polymer may be 8% by weight or more out of 100% by weight of the total of water and polymer. However, from the viewpoint of reducing radio wave reflectivity while achieving superior mechanical strength, the proportion of polymer is preferably 10% by weight or more, more preferably 15% by weight or more, and even more preferably 20% by weight or more out of 100% by weight of the total of water and polymer. Furthermore, from the viewpoint of further improving radio wave absorption performance such as transmission attenuation performance, the proportion of polymer is preferably 90% by weight or less, more preferably 80% by weight or less, even more preferably 70% by weight or less, and particularly preferably 60% by weight or less out of 100% by weight of the total of water and polymer.

[0019] The water content relative to the total composition of the radio wave absorbing material of the present invention may be 10% by weight or more, but from the viewpoint of further improving the radio wave absorption performance, the water content relative to the total composition is preferably 12 to 90% by weight, more preferably 15 to 80% by weight, and even more preferably 20 to 75% by weight.

[0020] The radio wave absorbing material of the present invention is flame-retardant because it contains a relatively large amount of water. For example, conventional radio wave absorbing materials are mainly composed of polymers, so once ignited, they burn until they are completely consumed. However, the radio wave absorbing material of the present invention has self-extinguishing properties, meaning that combustion ceases when it is removed from the source of the fire. In particular, radio wave absorbing materials generate heat by absorbing radio waves, and in some cases there is a possibility of ignition, but the radio wave absorbing material of the present invention effectively solves this problem.

[0021] The polymer can be any polymer with a molecular weight (weight-average molecular weight (Mw)) of 1000 or more, and is not particularly limited; it may be a synthetic polymer or a natural polymer. However, since the radio wave absorbing material of the present invention contains water, it may be prone to spoilage. From the viewpoint of further suppressing spoilage, synthetic polymers and polymers obtained by modifying natural polymers are preferred, and synthetic polymers are more preferred. As synthetic polymers, neutral polymers that do not contain salt structures in their structure and cation group-containing polymers can be suitably used. By using a cation group-containing polymer, the antibacterial properties of the radio wave absorbing material can be enhanced.

[0022] Furthermore, the polymer may be a crosslinked polymer (three-dimensional crosslinked polymer) that is crosslinked (three-dimensional crosslinked) in order to improve dimensional stability and thereby improve the stability of radio wave absorption performance. Alternatively, it may be an uncrosslinked polymer, but in the case of an uncrosslinked polymer, from the viewpoint of shape stability, the viscosity at 25°C is preferably 50,000 cp or more, more preferably 100,000 cp or more, even more preferably 500,000 cp or more, and even more preferably 1,000,000 cp or more.

[0023] The cation group-containing polymer can be any polymer having a cation group in its polymer chain, and is not particularly limited. It may be a polymer having a cation group in its main chain, or a polymer having a cation group in its side chain, or even a polymer having cation groups in both its main chain and side chain.

[0024] The cationic groups contained in the cationic group-containing polymer are not particularly limited, but include cationic groups containing a nitrogen atom, phosphorus atom, sulfur atom, or oxygen atom as the central atom. Among these, cationic groups containing a nitrogen atom as the central atom are preferred from the viewpoint of being able to further enhance antibacterial properties. Preferred cationic structures are the following structures (I) or (II), specifically including ammonium, iminium, imidazolium, pyridinium, etc. Furthermore, it is preferable that the substituents of the cationic group consist of two or more groups with two or more atoms bonded to them. In addition, the cationic group-containing polymer may also have anionic groups in the polymer chain in addition to the cationic group, for example, it may have a betaine structure.

[0025] A cation-containing polymer preferably has polar atoms other than cations in addition to the cation group. The presence of such polar atoms neutralizes the positive charge of the cation, improving the anion's dissociation ability, thereby enhancing radio wave absorption performance, transparency, and flexibility. Examples of such polar atoms include oxygen atoms and nitrogen atoms. Examples of groups having such polar atoms include, but are not limited to, ether bonds, ketone groups, carboxyl groups, hydroxyl groups, amine groups, amide bonds, urethane bonds, and urea bonds. In addition, the imidazolium structure has another nitrogen atom in the imidazolium ring in addition to the cationic nitrogen atom, and such another nitrogen atom can also be considered a polar atom other than a cation.

[0026] Examples of cationic group-containing polymers include addition polymers of vinyl compounds, polyethers, polyethyleneimines, and polyoxazolines, which have cationic groups in their side chains, and ionenes, epichlorohydrin-amine condensates, and polyamide polyamine epichlorohydrins, which have cationic groups in their main chains. These may have any other substituents, or some hydrogen atoms may be substituted with halogen atoms such as fluorine. Furthermore, cationic group-containing polymers may be copolymerized with structural units derived from monomers that do not have cationic groups, in addition to structural units that have cationic groups, and the pH may be adjusted by introducing structural units derived from acidic monomers or structural units derived from basic monomers.

[0027] A cation group-containing polymer has a cation group in its polymer chain. The counter anions for such cation groups are not particularly limited, but include, for example, imides such as fluorosulfonylimide, bistrifluoromethylsulfonylimide, and bispentafluoroethylsulfonylimide; halogens such as chlorides and bromides; tetrafluoroboric acid, hexafluorophosphate, dicyanoamide, tetracyanoborate, carbonate, alkyl carbonate, triflate, perchloric acid, nitric acid, sulfuric acid, alkyl sulfuric acid, sulfonic acid, phosphoric acid, and alkyl phosphoric acid; and are not particularly limited.

[0028] The cation group-containing polymer is not particularly limited, but as a polymer having cation groups in its side chains, for example, a cation group-containing polyether (A) containing repeating units represented by the following general formula (1) can be mentioned.

[0029] (In the above general formula (1), A ＋ represents a nitrogen-containing cation group. Also, in the above general formula (1), X － (This represents an anion.)

[0030] A ＋Examples of the nitrogen-containing cationic group represented by include an amino group, a nitrogen-containing cationic aromatic group, and a nitrogen-containing cationic aliphatic group.

[0031] A ＋ As the nitrogen-containing cationic aromatic group as , a group containing a cationic nitrogen-containing aromatic heterocyclic ring is preferable. The nitrogen-containing aromatic heterocyclic ring in the cationic nitrogen-containing aromatic heterocyclic ring in the group containing a cationic nitrogen-containing aromatic heterocyclic ring may have a nitrogen atom in the ring and have aromaticity, and may have heteroatoms other than nitrogen atoms such as an oxygen atom and a sulfur atom, and also, a part of the atoms constituting the heterocyclic ring may be substituted by a substituent. Further, it may have a polycyclic structure in which two or more rings are condensed. Examples of the structure of such a nitrogen-containing aromatic heterocyclic ring include five-membered heterocyclic rings such as an imidazole ring, a pyrrole ring, a thiazole ring, an oxazole ring, a pyrazole ring, and an isoxazole ring; six-membered heterocyclic rings such as a pyridine ring, a pyrazine ring, a pyrimidine ring, a pyridazine ring, and a triazine ring; condensed heterocyclic rings such as a quinoline ring, an isoquinoline ring, a quinoxaline ring, a quinazoline ring, a cinnoline ring, a purine ring, an indole ring, an isoindole ring, a benzimidazole ring, a benzoxazole ring, and a benzoisoxazole ring; and the like. Among these, five-membered heterocyclic rings and six-membered heterocyclic rings are preferable, and an imidazole ring is more preferable.

[0032] The substituent of the nitrogen-containing aromatic heterocyclic ring is not particularly limited, and examples thereof include an alkyl group; a cycloalkyl group; an alkenyl group such as a vinyl group; an aryl group such as a phenyl group; an arylalkyl group; an alkylaryl group; an alkoxyl group; an alkoxyalkyl group; an aryloxy group; an alkanol group; a hydroxyl group; a carbonyl group; an alkoxycarbonyl group; an amino group; an imino group; a nitrile group; an alkylsilyl group; a halogen atom; and the like. The number of carbon atoms of these substituents is preferably 0 to 12, more preferably 1 to 8, and even more preferably 1 to 6.

[0033] A ＋ The nitrogen-containing cationic aliphatic group as may be linear or branched, and may have a non-aromatic ring structure.

[0034] A＋ Specific examples of nitrogen-containing cationic groups represented by ammonium include: ammonium groups; monosubstituted ammonium groups containing a cationic nitrogen atom such as methylammonium group, butylammonium group, cyclohexylammonium group, anilinium group, benzylammonium group, and ethanolammonium group; disubstituted ammonium groups containing a cationic nitrogen atom such as dimethylammonium group, diethylammonium group, dibutylammonium group, and nonylphenylammonium group; trimethylammonium group, triethylammonium group, n-butyldimethylammonium group, stearyldimethylammonium group, tributylammonium group, trivinylammonium group, triethanolammonium group, N,N-dimethylethanolammonium group, and tri(2-ethoxyethyl)ammonium group. Examples include trisubstituted ammonium groups containing a cationic nitrogen atom such as an ammonium group; and heterocyclic groups containing a cationic nitrogen atom such as a piperidinium group, 1-methylpyrrolidinium group, 1-butylpyrrolidinium group, imidazolium group, 1-methylimidazolium group, 1-ethylimidazolium group, 1-butylimidazolium group, benzimidazolium group, pyrorium group, 1-methylpyrorium group, oxazolium group, benzoxazolium group, pyrazolium group, isoxazolium group, pyridinium group, 2,6-dimethylpyridinium group, pyrazinium group, pyrimidinium group, pyridazinium group, triazinium group, N,N-dimethylanilinium group, quinolinium group, isoquinolinium group, indolinium group, quinoxalium group, and isoquinoxalium group. Among these, trisubstituted ammonium groups containing a cationic nitrogen atom and heterocyclic groups containing a cationic nitrogen atom are preferred.

[0035] In the above general formula (1), X － The anion represented by A ＋ This is the counteranion of the nitrogen-containing cationic group represented by X. － For example, as a monovalent anion, F － , Cl － , Br － , I － Halide ions such as (FSO ２ )２ N － (CF ３ SO ２ ) ２ N － (CF ３ CF ２ SO ２ ) ２ N － Sulfonylimidide ions such as CH ３ COO － C3H7COO － CF ３ COO － PhCOO － (Ph indicates the phenyl group.) Carboxylate ions such as CH ３ SO ３ － CF ３ SO ３ － Sulfonoxide ions such as OH － BF ４ － , PF ６ － , ClO ４ － , B(CN) ４ － SCN － (NC) ２ N － These are some examples. － The anion may be a polyvalent anion, or it may be a polyanion having two or more monovalent anionic groups in its molecule. For example, a polyvalent anion is the sulfate ion (SO4). ４ ２－ ) and carbonate ions (CO2) ３ ２－ Examples include: － O ３ SCF2CF2CF2SO ３ － , － O ３ SCF2CF2SO ３ － CF ３ SO ２ N － SO ２CF2CF2OCF2CF2OCF2CF2SO ２ N － SO ２ CF ３ etc. Among them, from the viewpoint of radio wave absorption performance, Cl － , Br － , sulfonylimide ion, carboxylate ion, BF ４ － are preferred, and Cl － , Br － , sulfonylimide ion, CH ３ COO － , BF ４ － are more preferred.

[0036] In the cation group-containing polyether (A), the units represented by the above general formula (1) are each independent, and two or more units represented by the general formula (1) may be present in the cation group-containing polyether (A). For example, in the entire repeating unit represented by the general formula (1) in the cation group-containing polyether (A), all of the nitrogen-containing cationic groups represented by A ＋ may be the same kind of nitrogen-containing cationic group, or different kinds of nitrogen-containing cationic groups may be mixed. Also, in the entire repeating unit represented by the general formula (1) in the cation group-containing polyether (A), all of the anions represented by X － may be the same kind of anion, or different kinds of anions may be mixed.

[0037] Examples of the repeating unit represented by the above general formula (1) include the repeating unit represented by the following general formula (2). The repeating unit represented by the following general formula (2) is an oxirane unit containing an imidazolium structure. (In the above general formula (2), R １ to R ４ each independently represent a hydrogen atom or a substituent, and R ２ and R ３ may be bonded to each other. Also, in the general formula (2), X － represents an anion.)

[0038] In the above general formula (2), R１ to R ４ each independently represents a hydrogen atom or a substituent. Examples of the substituent include the same ones as those described above as the substituent of the nitrogen-containing aromatic heterocyclic ring. R １ to R ４ as the substituent may be linear or branched, and may also have a ring structure. R １ to R ４ as the substituent is preferably linear.

[0039] In the above general formula (2), R １ may be a hydrogen atom or a substituent, and is not particularly limited, but is preferably a hydrogen atom or a hydrocarbon group, more preferably a hydrocarbon group, further preferably an alkyl group or an alkenyl group, particularly preferably an alkyl group or a vinyl group, and most preferably an alkyl group. The carbon number of R １ is preferably 0 to 12, more preferably 0 to 8, further preferably 1 to 6, even more preferably 1 to 4, particularly preferably 1 to 3, and most preferably 1 to 2.

[0040] In the above general formula (2), R ２ to R ４ each independently may be a hydrogen atom or a substituent, and is not particularly limited, but each independently is preferably a hydrogen atom or a hydrocarbon group, more preferably a hydrogen atom, an alkyl group or a vinyl group, further preferably a hydrogen atom or an alkyl group, and particularly preferably a hydrogen atom. The carbon number of R ２ to R ４ is each independently preferably 0 to 8, more preferably 0 to 6, further preferably 0 to 4, even more preferably 0 to 3, particularly preferably 0 to 2, and most preferably 0 to 1.

[0041] In the above general formula (2), it is preferable that 1 to 3 of R ２ to R ４ represent hydrogen atoms, and more preferably 2 to 3 represent hydrogen atoms. Also, R ２ to R ４It is preferable that 0 to 2 of these represent substituents such as hydrocarbon groups, and it is more preferable that 0 to 1 of them represent substituents such as hydrocarbon groups.

[0042] X in the general formula (2) above － The anion represented by is X in general formula (1). － Examples of anions similar to those represented by can be cited, and preferred embodiments are also similar.

[0043] The repeating unit represented by the above general formula (2) preferably contains an imidazolium group, a 1-methylimidazolium group, a 1-butylimidazolium group, a 1-hexylimidazolium group, or a 1-vinylimidazolium group, and more preferably contains a 1-methylimidazolium group.

[0044] The cationic group-containing polyether (A) may contain repeating units other than the repeating unit represented by the general formula (1) above. The repeating units other than the repeating unit represented by the general formula (1) above are not particularly limited, but may be any units derived from monomers copolymerizable with the monomer that gives the repeating unit represented by the general formula (1) above. Examples include alkylene oxide monomer units such as ethylene oxide units, propylene oxide units, 1,2-butylene oxide units, and 1,2-octylene oxide units; aromatic oxirane monomer units such as styrene oxide units, epihalohydrin monomer units such as epichlorohydrin units, epibromohydrin units, and epiiodohydrin units; alkenyl group-containing oxirane monomer units such as allyl glycidyl ether units; aromatic ether group-containing oxirane monomer units such as phenyl glycidyl ether units; and (meth)acryloyl group-containing oxirane monomer units such as glycidyl acrylate units and glycidyl methacrylate units. Among these, alkylene oxide monomer units, epihalohydrin monomer units, and (meth)acryloyl group-containing oxirane monomer units are preferred, and ethylene oxide units, propylene oxide units, epichlorohydrin units, and glycidyl methacrylate units are more preferred. The cationic group-containing polyether (A) may contain one repeating unit other than the repeating unit represented by the general formula (1) above, or it may contain two or more repeating units.

[0045] The cation group-containing polyether (A) may contain two or more repeating units, in which case the distribution pattern of these multiple repeating units is not particularly limited, but it is preferable that it has a random distribution.

[0046] The chain structure of the cation group-containing polyether (A) is not particularly limited and may be linear, or it may be a chain structure having branching such as graft or radial.

[0047] The terminal group of the cationic group-containing polyether (A) is not particularly limited and can be any monovalent group. Specific examples of terminal groups include hydrogen atoms, halogen groups, alkyl groups, haloalkyl groups, hydroxyl groups, azide groups, etc. Furthermore, the terminal group may be a nitrogen-containing cationic group (A) possessed by the repeating unit represented by general formula (1). ＋ ) and anion (X － ) may be a base consisting of the following.

[0048] The content of the repeating units represented by the above general formula (1) in the cationic group-containing polyether (A) is not particularly limited, but is preferably 1 to 100,000 on average per molecule, more preferably 3 to 50,000, even more preferably 10 to 30,000, and particularly preferably 30 to 10,000.

[0049] The weight-average molecular weight (Mw) of the cationic group-containing polyether (A) is not particularly limited, but is preferably 1,000 to 2,000,000, more preferably 2,000 to 1,500,000, even more preferably 4,000 to 1,000,000, and particularly preferably 10,000 to 500,000.

[0050] The molecular weight distribution (Mw / Mn) of the cationic group-containing polyether (A) is not particularly limited, but is preferably 1.0 to 4.0, more preferably 1.0 to 2.0, and even more preferably 1.0 to 1.5.

[0051] The weight-average molecular weight and molecular weight distribution of the cation-containing polyether (A) can be measured as polyethylene oxide equivalent values ​​by gel permeation chromatography (GPC) using tetrahydrofuran as the solvent. The molecular weight distribution of the cation-containing polyether (A) can be treated as unchanged from the molecular weight distribution of the base polymer (polyether without cation groups) before the introduction of the cation groups.

[0052] The proportion of the repeating units represented by the above general formula (1) in the cationic group-containing polyether (A) is not particularly limited, but is preferably 5 to 100 mol%, and more preferably 10 to 100 mol%, relative to the total repeating units of the cationic group-containing polyether (A).

[0053] The method for synthesizing the cation group-containing polyether (A) is not particularly limited, and any synthesis method can be used as long as it yields the desired polyether compound. For example, the method described in International Publication No. 2023 / 042748 can be used.

[0054] Furthermore, as an example of a cationic group-containing polymer, a polymer (B) containing a repeating unit represented by the following general formula (3) is also an example of a polymer having cationic groups in its side chains, specifically a side-chain ammonium group-containing polymer. (In the above general formula (3), Z is a divalent linking group, and R ５ ~R ７ Each of these independently represents a hydrogen atom or a substituent, R ５ ~R ７ They may be joined to each other. Also, in general formula (3), X － (This represents an anion.)

[0055] Z is a divalent linking group and is preferably an alkylene group which may contain a heteroatom. Examples of heteroatoms include oxygen, nitrogen, and sulfur atoms, and examples of groups containing heteroatoms include amide groups (-CO-NH-), ester groups (-COO-), ether groups (-O-), and thioether groups (-S-). Among these, Z is preferably an alkylene group containing an amide group, such as -CO-NH-(CH ２ ) ｐ It is more preferable that the base be represented by -(where p is an integer from 1 to 5, preferably n=3).

[0056] R ５ ~R ７Each of these may independently be a hydrogen atom or a substituent, and is not particularly limited, but each may independently be a hydrogen atom or a hydrocarbon group, more preferably a hydrogen atom, an alkyl group or a vinyl group, even more preferably a hydrogen atom or an alkyl group, and particularly preferably an alkyl group. ５ ~R ７ The number of carbon atoms in each group is preferably 1 to 8, more preferably 1 to 3, even more preferably 1 to 4, and particularly preferably 1. In other words, it is preferably a methyl group.

[0057] In the above general formula (3), X － The anion represented by is not particularly limited, but examples include those similar to those in the general formula (1) above.

[0058] Furthermore, the side-chain ammonium group-containing polymer (B) may also contain repeating units other than the repeating unit represented by general formula (3). The repeating units other than the repeating unit represented by general formula (3) are not particularly limited, but may be any units derived from monomers copolymerizable with the monomer that gives the repeating unit represented by general formula (3). Examples include unsaturated carboxylic acids and their salts such as acrylic acid and methacrylic acid, unsaturated carboxylic acid esters such as methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, n-butyl acrylate, n-butyl methacrylate, and methoxyethoxyethoxyethyl acrylate, acrylamide, substituent-containing acrylamide, styrene, vinylpyridine, vinylpyrrolidone, etc.

[0059] In the side-chain ammonium group-containing polymer (B), the units represented by the general formula (3) are independent of each other, and two or more units represented by the general formula (3) may be present in the side-chain ammonium group-containing polymer (B). When the side-chain ammonium group-containing polymer (B) contains two or more repeating units, the distribution pattern of these multiple repeating units is not particularly limited, but it is preferable that they have a random distribution.

[0060] Specific examples of side-chain ammonium group-containing polymers (B) include poly{[3-(methacryloylamino)propyl]trimethylammonium chloride}, poly[(3-acrylamidopropyl)trimethylammonium chloride], poly{[2-(methacryloyloxy)ethyl]trimethylammonium chloride}, poly{[2-(acryloyloxy)ethyl]trimethylammonium chloride}, poly[2-(dimethylamino)ethyl acrylate-benzyl chloride quaternary salt], and poly[2-dimethylaminoethyl methacrylate ethyl sulfate]. Examples include poly[vinylbenzyltrimethylammonium chloride], poly[N-(2-acryloyloxyethyl)-N-benzyl-N,N-dimethylammonium chloride], poly[2-methacryloyloxyethyl phosphorylcholine], and poly[2-(N-3-sulfopropyl-N,N-dimethylammonium)ethyl methacrylate], among which poly[(3-acrylamidopropyl)trimethylammonium chloride] and poly[2-(dimethylamino)ethyl acrylate-benzyl chloride quaternary salt] are preferred. These may also be copolymerized with a crosslinkable monomer such as diallyldimethylammonium chloride to have crosslinkable groups. In this case, the content of the crosslinkable monomer units is preferably 0.0001 to 5% by weight.

[0061] The weight-average molecular weight (Mw) of the side-chain ammonium group-containing polymer (B) is not particularly limited, but is preferably 750 to 2,000,000, more preferably 1,000 to 1,500,000, even more preferably 2,000 to 1,000,000, and particularly preferably 4,000 to 500,000. The weight-average molecular weight of the side-chain ammonium group-containing polymer (B) can be determined using GPC on a standard polyethylene oxide basis.

[0062] Furthermore, as a cationic group-containing polymer, a side-chain cyclic ammonium group-containing polymer (C) is also included, which has a cationic group in the side chain of the polymer and contains a repeating unit represented by the following general formula (4-1) or the following general formula (4-2). (In the above general formulas (4-1) and (4-2), R ８ , R ９ Each of these independently represents a hydrogen atom or a substituent, R ８ , R ９ They may be combined with each other. Also, in general formulas (4-1) and (4-2), X － (This represents an anion.)

[0063] R ８ , R ９ Each of these may independently be a hydrogen atom or a substituent, and is not particularly limited, but each may independently be a hydrogen atom or a hydrocarbon group, more preferably a hydrogen atom, an alkyl group or a vinyl group, even more preferably a hydrogen atom or an alkyl group, and particularly preferably an alkyl group. ８ , R ９ The number of carbon atoms in each group is preferably 1 to 8, more preferably 1 to 3, even more preferably 1 to 4, and particularly preferably 1. In other words, it is preferably a methyl group.

[0064] In the above general formulas (4-1) and (4-2), X － The anion represented by is not particularly limited, but examples include those similar to those in the general formula (1) above.

[0065] Furthermore, the side-chain cyclic ammonium group-containing polymer (C) may also contain repeating units other than those represented by general formulas (4-1) and (4-2). The repeating units other than those represented by general formula (4) may be any units derived from monomers copolymerizable with the monomers that give the repeating units represented by general formulas (4-1) and (4-2), and are not particularly limited. Examples of such monomers include N-vinylpyrrolidone, acrylamide, (meth)acrylic acid, (meth)acrylate salts, and (meth)acrylates having ethylene oxide units.

[0066] In the side-chain cyclic ammonium group-containing polymer (C), the units represented by the general formula (4) are independent of each other, and two or more units represented by the general formula (4) may be present in the side-chain cyclic ammonium group-containing polymer (C). When the side-chain cyclic ammonium group-containing polymer (C) contains two or more repeating units, the distribution pattern of these multiple repeating units is not particularly limited, but it is preferable that they have a random distribution.

[0067] Specific examples of the side-chain cyclic ammonium group-containing polymer (C) include poly(diallyldimethylammonium chloride). Copolymers of diallyldimethylammonium chloride and crosslinkable monomers such as diallyldimethylammonium chloride can also be suitably used. In this case, the content ratio of the crosslinkable monomer units is preferably 0.0001 to 5% by weight.

[0068] The weight-average molecular weight (Mw) of the side-chain cyclic ammonium group-containing polymer (C) is not particularly limited, but is preferably 1,000 to 2,000,000, more preferably 2,000 to 1,500,000, even more preferably 4,000 to 1,000,000, and particularly preferably 10,000 to 500,000. The weight-average molecular weight of the side-chain cyclic ammonium group-containing polymer (C) can be determined using GPC on a standard polyethylene oxide basis.

[0069] Furthermore, as a cation group-containing polymer, an example is a main-chain ammonium group-containing polymer (D) that contains repeating units represented by the following general formula (5), as a polymer having a cation group in the main chain of the polymer. (In the above general formula (5), R １０ , R １１ Each of these independently represents a hydrogen atom or a substituent, R １０ , R １１ They may be joined to each other. Also, in general formula (5), X － (This represents an anion.)

[0070] R １０ , R １１Each of these may independently be a hydrogen atom or a substituent, and is not particularly limited, but each may independently be a hydrogen atom or a hydrocarbon group, more preferably a hydrogen atom, an alkyl group or a vinyl group, even more preferably a hydrogen atom or an alkyl group, and particularly preferably an alkyl group. １０ , R １１ The number of carbon atoms in each group is preferably 1 to 8, more preferably 1 to 3, even more preferably 1 to 4, and particularly preferably 1. In other words, it is preferably a methyl group.

[0071] In the above general formula (5), X － The anion represented by is not particularly limited, but examples include those similar to those in the general formula (1) above.

[0072] The main-chain ammonium group-containing polymer (D) can be obtained by reacting a secondary amine with an epihalohydrin to form a repeating unit represented by general formula (5).

[0073] Secondary amines are compounds in which one nitrogen atom is bonded to two hydrocarbon groups and one hydrogen atom, as well as compounds having a nitrogen atom in a heterocyclic ring, where the nitrogen atom has one hydrogen atom. Examples of such secondary amines include aliphatic secondary amines, aromatic secondary amines, alicyclic secondary amines, heterocyclic secondary amines, and two or more of these may be used in combination.

[0074] Examples of aliphatic secondary amines include dimethylamine, diethylamine, diisopropylamine, dibutylamine, methylethylamine, methylpropylamine, methylbutylamine, methyloctylamine, methyllaurylamine, and dibenzylamine.

[0075] Examples of aromatic secondary amines include N-alkylanilines such as N-methylaniline, N-ethylaniline, N-propylaniline, N-butylaniline, N-pentylaniline, N-hexylaniline, N-octylaniline, N-decylaniline, N-laurylaniline, and N-benzylaniline; and N-methyltoluidine, N-ethyltoluidine, N-propyltoluidine, N-butyltoluidine, N-pentyltoluidine, N-hexyltoluidine, and N-octyltoluidine. Examples include N-alkyltoluidines such as idine, N-decyltoluidine, N-lauryltoluidine, and N-benzyltoluidine; and N-alkylnaphthylamines such as N-methylnaphthylamine, N-ethylnaphthylamine, N-propylnaphthylamine, N-butylnaphthylamine, N-pentylnaphthylamine, N-hexylnaphthylamine, N-octylnaphthylamine, N-decylnaphthylamine, N-laurylnaphthylamine, and N-benzylnaphthylamine.

[0076] Examples of alicyclic secondary amines include N-alkylcyclohexylamines such as N-methylcyclohexylamine, N-ethylcyclohexylamine, N-propylcyclohexylamine, N-butylcyclohexylamine, N-hexylcyclohexylamine, N-octylcyclohexylamine, N-decylcyclohexylamine, and N-laurylcyclohexylamine; N-alkylcyclooctylamines such as N-methylcyclooctylamine, N-ethylcyclooctylamine, N-propylcyclooctylamine, N-butylcyclooctylamine, N-hexylcyclooctylamine, N-octylcyclooctylamine, N-decylcyclooctylamine, and N-laurylcyclooctylamine; dicyclohexylamines and dicycloalkylamines such as dicyclooctylamine; and the like.

[0077] Examples of heterocyclic secondary amines include piperidine, pyrrolidine, 2-methylpiperidine, and 4-methylpiperidine.

[0078] As the secondary amine, an aliphatic secondary amine is preferred, with dimethylamine and diethylamine being preferred, and dimethylamine being particularly preferred.

[0079] Examples of epihalohydrins include epichlorohydrin, epibromohydrin, methylepichlorohydrin, and methylepibromohydrin. Among these, epichlorohydrin is particularly preferred.

[0080] Furthermore, the main-chain ammonium group-containing polymer (D) may be obtained by reacting an amine other than a secondary amine with a secondary amine and an epihalohydrin. Examples of amines other than secondary amines include compounds having two or more amino groups, such as ethylenediamine, propylenediamine, diethylenetriamine, hexylenediamine, triethylenetetramine, tetraethylenepentamine, isophoronediamine, piperazine, diphenylmethanediamine, hydrazine, and hydrazides such as adipic acid dihydrazide. Among these, ethylenediamine is preferably used.

[0081] The main-chain ammonium group-containing polymer (D) is produced, for example, by mixing and stirring a secondary amine, an epihalohydrin, and other amines other than the secondary amine, as needed, under heating conditions, and then by addition polymerization using a known and conventional method.

[0082] The weight-average molecular weight (Mw) of the main-chain ammonium group-containing polymer (D) is not particularly limited, but is preferably 1,000 to 2,000,000, more preferably 2,000 to 1,500,000, even more preferably 4,000 to 1,000,000, and particularly preferably 10,000 to 500,000. The weight-average molecular weight of the main-chain ammonium group-containing polymer (D) can be determined using GPC on a standard polyethylene oxide basis.

[0083] Furthermore, a crosslinked polymer (three-dimensional crosslinked body) may be obtained by crosslinking (three-dimensional crosslinking) a cation group-containing polymer. In this case, a crosslinkable polymer capable of forming a crosslinked structure may be present separately from the cation group-containing polymer, and the crosslinked polymer (three-dimensional crosslinked body) may be crosslinked (three-dimensional crosslinked). Examples of crosslinking methods include ultraviolet irradiation, visible light irradiation, heating, electron beam irradiation, and radiation irradiation. Urethane reactions, epoxy reactions, oxazoline group reactions, dimerization reactions of cinnamic acid, and azetidinium ring reactions can also be utilized.

[0084] Furthermore, while the neutral polymer is not particularly limited, it is preferably a polymer that exhibits affinity for water, more preferably a polymer having a polar group, and especially preferably a polymer having a -OH, -NH, alkylene oxide chain structure. Examples of neutral polymers include acrylic acid polymers, acrylamide polymers, polyalkylene oxide polymers, polyvinyl alcohol polymers, and polymers having a sugar backbone. Among these, acrylic acid polymers, acrylamide polymers, and polyalkylene oxide polymers are preferred, with polyalkylene oxide polymers being particularly preferred.

[0085] Polyalkylene oxide polymers can be any polymer having a polyalkylene oxide skeleton, such as polyethylene oxide, polypropylene oxide, and polybutylene oxide.

[0086] Furthermore, polyalkylene oxide polymers may be obtained by polymerizing polyalkylene oxide compounds having polymerizable functional groups such as acrylate groups or methacrylate groups. Examples of polyalkylene oxide compounds having polymerizable functional groups include monofunctional polyalkylene oxide compounds such as polyethylene glycol acrylate, polypropylene glycol acrylate, polyethylene glycol monomethyl ether acrylate, polypropylene glycol monomethyl acrylate, polyethylene glycol methacrylate, polypropylene glycol methacrylate, polyethylene glycol monomethyl ether methacrylate, and polypropylene glycol monomethyl methacrylate; and polyfunctional polyalkylene oxide compounds such as polyethylene glycol dimethacrylate, polyethylene glycol diacrylate, polypropylene glycol dimethacrylate, polypropylene glycol diacrylate, polytetramethylene glycol dimethacrylate, polytetramethylene glycol diacrylate, polybutylene glycol dimethacrylate, and polybutylene glycol diacrylate. These can be used individually or in combination of two or more. Furthermore, by using polyfunctional polyalkylene oxide compounds, polyalkylene oxide polymers can be made crosslinkable (three-dimensional crosslinking). Examples of crosslinking methods include ultraviolet irradiation, visible light irradiation, heating, electron beam irradiation, and radiation irradiation. In addition to polyalkylene oxide compounds having polymerizable functional groups, polymerizable compounds that do not have a polyalkylene oxide structure may also be copolymerized.

[0087] The acrylamide polymer can be any polymer obtained by polymerizing acrylamide monomers. Examples of acrylamide monomers include acrylamide; N-alkylacrylamides such as N-methylacrylamide, N-ethylacrylamide, N-n-propylacrylamide, N-isopropylacrylamide, N-n-butylacrylamide, and N-tert-butylacrylamide; N-cycloalkylacrylamides such as N-cyclohexylacrylamide; dialkylacrylamides such as N,N-dimethylacrylamide and N,N-diethylacrylamide; dialkylaminoalkylacrylamides such as dimethylaminoethylacrylamide and dimethylaminopropylacrylamide; hydroxyalkylacrylamides such as N-(hydroxymethyl)acrylamide and N-(hydroxyethyl)acrylamide; N-arylacrylamides such as N-phenylacrylamide; diacetoneacrylamide; and N,N'-methylenebisacrylic. Examples include N,N'-alkylene bisacrylamides such as amides; methacrylamides; N-alkyl methacrylamides such as N-methyl methacrylamide, N-ethyl methacrylamide, N-n-propyl methacrylamide, N-isopropyl methacrylamide, N-n-butyl methacrylamide, and N-tert-butyl methacrylamide; N-cycloalkyl methacrylamides such as N-cyclohexyl methacrylamide; dialkyl methacrylamides such as N,N-dimethyl methacrylamide; dialkylaminoalkyl methacrylamides such as dimethylaminoethyl methacrylamide and dimethylaminopropyl methacrylamide; hydroxyalkyl methacrylamides such as N-(hydroxymethyl) methacrylamide and N-(hydroxyethyl) methacrylamide; N-aryl methacrylamides such as N-phenyl methacrylamide; diacetone methacrylamide; and N,N'-alkylene bismethacrylamides such as N,N'-methylene bismethacrylamide. These can be used individually or in combination of two or more.Furthermore, by using polyfunctional acrylamide monomers such as N,N'-alkylenebisacrylamide or N,N'-alkylenebismethacrylamide, acrylamide polymers can be made crosslinkable (three-dimensional crosslinking). Examples of crosslinking methods include ultraviolet irradiation, visible light irradiation, heating, electron beam irradiation, and radiation irradiation. In addition to acrylamide monomers, polymerizable compounds other than acrylamide monomers may also be copolymerized.

[0088] Examples of acrylic acid-based polymers include polyacrylic acid. Alternatively, acrylic acid-based polymers may be polymers having ionic groups, such as salts of acrylic acid-based polymers, including sodium polyacrylate and potassium polyacrylate.

[0089] The composition constituting the radio wave absorbing material of the present invention may contain a salt compound. The salt compound is not particularly limited, but it is preferable to use an organic ammonium salt, as using an organic ammonium salt can improve corrosion resistance. Furthermore, a deliquescent salt compound may be used as the salt compound. Here, deliquescent refers to the phenomenon in which a substance absorbs moisture from the air and spontaneously becomes an aqueous solution, and a deliquescent salt compound refers to a salt compound that has such a characteristic. Specifically, a deliquescent salt compound is a compound that absorbs moisture from the air, undergoes ionic dissociation, and becomes an aqueous solution. The deliquescent salt compound may be any salt compound that exhibits deliquescent phenomena at 80% RH, but it is preferable that it is a salt compound that exhibits deliquescent phenomena at 25°C and 80% RH, and more preferably that it is a salt compound that exhibits deliquescent phenomena at 25°C and 50% RH.

[0090] Examples of organic ammonium salts include tetramethylammonium chloride, methyltriethylammonium chloride, tetraethylammonium chloride, tetrabutylammonium chloride, trimethylbenzylammonium chloride, choline chloride, cetylpyridinium chloride, benzethonium chloride, and benzalkonium chloride.

[0091] Furthermore, examples of hygroscopic chloride compounds include LiCl, CsCl, and CaCl. ２ MgCl ２ ZnCl ２ Salts of chlorine with alkali metals, salts of chlorine with alkaline earth metals; LiBr, MgBr ２ Salts of bromine with alkali metals, salts of bromine with alkaline earth metals; salts of acetic acid with alkali metals, salts of acetic acid with alkaline earth metals; Li(CF) ３ SO ２ ) ２ N, Na(CF ３ SO ２ ) ２ (CF) ３ SO ２ ) ２ N － and alkali metal salts, (CF ３ SO ２ ) ２ N － Salts of alkaline earth metals; Li(FSO) ２ ) ２ N, Na(FSO) ２ ) ２ N, K (FSO) ２ ) ２ N etc. (FSO) ２ ) ２ N － and alkali metal salts, (FSO ２ ) ２ N － Salts of alkaline earth metals; NH ４ Examples include inorganic ammonium halides such as Cl.

[0092] The salt compounds can be used individually or in combination of two or more. The content of the salt compound in the composition constituting the radio wave absorbing material of the present invention is preferably 5 parts by weight or more, more preferably 10 parts by weight or more, even more preferably 15 parts by weight or more, and even more preferably 20 parts by weight or more, per 100 parts by weight of water, with no particular upper limit, but preferably 300 parts by weight or less.

[0093] <Radio wave absorber> The radio wave absorber of the present invention is made of the radio wave absorbing material of the present invention described above.

[0094] The shape of the radio wave absorber of the present invention is not particularly limited, but it is preferably in the form of a sheet or a plate. In this case, the radio wave absorbing material formed in the form of a sheet or a plate may be used as a single layer, or multiple layers may be stacked. By using multiple layers, the number of reflections can be increased, thereby allowing the radio wave absorption performance to be adjusted. When stacking multiple layers, the radio wave absorbing materials of the present invention may be stacked on top of each other, or a sheet or plate different from the radio wave absorbing material of the present invention may be sandwiched between two or more radio wave absorbing materials of the present invention. The radio wave absorbing material may be a gradient material in which the composition changes in stages. Furthermore, the radio wave absorbing material of the present invention can also be used as a bulk material (=block), and may be given any shape like clay modeling, or may be used to fill gaps like putty.

[0095] For example, when the radio wave absorbing material of the present invention is in the form of a sheet, plate, or bulk body, the molding method is not particularly limited, but it may be formed by injecting a fluid monomer aqueous solution into the desired area and polymerizing it in place, thereby forming a sheet, plate, or bulk body. Alternatively, when the radio wave absorbing material of the present invention is a crosslinked body formed by crosslinking polymers, it may be formed by injecting a fluid monomer aqueous solution or a fluid polymer aqueous solution into the desired area and polymerizing it in place, thereby forming a sheet, plate, or bulk body.

[0096] The hardness of the radio wave absorber of the present invention is not particularly limited, but it is preferably 15 or higher, more preferably 20 or higher, and even more preferably 30 or higher, with no particular upper limit, but preferably 98 or lower, as measured in accordance with JIS K 7312. By having the Asker C hardness within the above range, deformation such as crushing can be effectively prevented even when some external force is applied, and fluctuations in radio wave absorption performance caused by deformation can be effectively suppressed.

[0097] The thickness of the radio wave absorber of the present invention is preferably 50 μm or more, more preferably 100 μm or more, even more preferably 150 μm or more, even more preferably 200 μm or more, preferably 10 cm or less, more preferably 7 cm or less, even more preferably 4 cm or less, and even more preferably 2 cm or less. By setting the thickness of the radio wave absorber within the above range, it is possible to achieve excellent radio wave absorption performance without excessively increasing the weight, while also ensuring sufficient flexibility. Furthermore, to ensure ease of bending, a part of the radio wave absorber may be cut or notched.

[0098] The radio wave absorber of the present invention may have an uneven surface on at least one surface, and it is preferable that the surface that absorbs radio waves has an uneven surface. By having an uneven surface on the surface that absorbs radio waves, the radio wave reflectivity can be further reduced. The uneven surface may be a regular repeating structure or an irregular shape. From the viewpoint of reducing radio wave reflectivity, the height difference of the uneven surface is preferably 0.3 mm or more, more preferably 0.5 mm or more, even more preferably 1 mm or more, even more preferably 2 mm or more, and particularly preferably 3 mm or more, and there is no particular upper limit, but it is preferably 70 mm or less. The height difference of the uneven surface can be determined by adding the value of the largest deviation in the upward direction from the average surface of the radio wave absorber and the value of the largest deviation in the opposite direction. Furthermore, if the radio wave absorber has an outer casing, the uneven surface may be formed on at least one surface of the radio wave absorber by using an outer casing that has an uneven surface on at least one surface. Furthermore, the uneven shape is not particularly limited, but may include structures in which multiple grooves are formed, or structures having a pattern of square pyramidal protrusions (pyramid-shaped protrusions).

[0099] In particular, conventionally, a method has been known to suppress reflection by making radio wave absorbers, such as those used in anechoic chambers, which use a dielectric material in which carbon is dispersed in expanded polystyrene as an absorbent, into a square pyramidal shape. However, in such techniques, since opaque carbon material is used, visible light transmittance cannot be ensured. In contrast, according to the present invention, since there is no need to use opaque carbon material, it is possible to realize a radio wave absorber that ensures high visible light transmittance. Furthermore, commercially available square pyramidal radio wave absorbers are several tens of centimeters tall, and some are even taller than 1 meter, making them bulky in terms of space. In contrast, according to the present invention, by keeping the height difference of the uneven shape within the above range, it is possible to effectively reduce radio wave reflectivity while contributing to space saving.

[0100] Furthermore, while the radio wave absorber of the present invention may consist solely of the radio wave absorbing material of the present invention, it may also be equipped with an outer casing in order to further reduce radio wave reflectivity. More specifically, at least a portion of the radio wave absorbing material of the present invention may be covered by an outer casing.

[0101] For example, if the radio wave absorber of the present invention is in the form of a sheet or a plate, a pair of sheet or plate-shaped outer coverings can be used as the outer covering, and the absorber can be sandwiched between the pair of outer coverings.

[0102] As for the exterior material, any material with a water content of less than 10% by weight is acceptable, but from the viewpoint of further reducing radio wave reflectivity, a water content of 1% by weight or less is preferred, and although there is no particular lower limit, it is preferably 0.00001% by weight or more.

[0103] For the exterior, it is preferable to use organic materials such as plastics and rubber, or inorganic materials such as glass. In particular, since the radio wave absorbing material of the present invention has high visible light transmittance and light such as sunlight easily penetrates into the interior of the material, it is desirable to select a material with high weather resistance for the exterior. Specifically, suitable exterior materials include polyacrylic acid ester resins, polyacrylamide resins, polyvinyl alcohol resins, polycarbonate resins, polyurethanes, acrylic resins, vinyl chloride resins, chlorinated vinyl chloride resins, polyamide resins, silicone resins, silicone rubbers, fluororesins, fluororubber, cycloolefin polymer resins, ionomer resins, glass, and quartz. From the viewpoint of low flammability, polycarbonate resins, polyurethanes, vinyl chloride resins, chlorinated vinyl chloride resins, silicone resins, silicone rubbers, fluororesins, fluororubber, glass, and quartz. Furthermore, from the viewpoint of combining flexibility, transparency, and low flammability, polyacrylamide resins, vinyl chloride resins, and silicone rubbers are suitable. Rubber materials are also suitable because they are difficult to crush and easy to bend. Furthermore, if the exterior is made of resin or rubber, it may be cross-linked for durability, or it may be a thermoplastic elastomer. In addition, a transparent film made by thinly coating the resin with an inorganic oxide such as alumina (for example, a non-metallic, transparent moisture-proof sheet used for packaging retort foods) can be used, and such a film can effectively achieve both radio wave absorption performance and moisture resistance.

[0104] As for the outer casing, it is preferable to use a material with relatively high strength from the viewpoint of suppressing deformation of the radio wave absorbing material. By using a material with relatively high strength, it is possible to effectively suppress changes in radio wave absorption performance due to the crushing of the radio wave absorber. On the other hand, if the strength is too high, it becomes difficult to bend, so it is preferable to select a material taking these points into consideration. When the radio wave absorber of the present invention is equipped with an outer casing, the hardness of the radio wave absorber is preferably 5 or higher on the Asker C hardness scale, more preferably 10 or higher, and even more preferably 15 or higher. There is no particular upper limit, but it is preferably 80 or lower.

[0105] Furthermore, regarding the outer casing, from the viewpoint of suppressing changes in the moisture content of the radio wave absorbing material constituting the radio wave absorber, a material with low water vapor permeability is preferable, and a water vapor permeability of 10 g / (m³) is preferable. ２ • A daily dose of 5 g / m is preferred, and more preferably 5 g / m ２ - day) or less, more preferably 1 g / (m ２ - day) or less, more preferably 0.1 g / (m ２ - (day) or less. Within the range where the water vapor permeability of the exterior is applied, fluctuations in moisture content of the radio wave absorbing material can be suppressed, thereby suppressing fluctuations in the reflection, absorption, and shielding performance of radio waves.

[0106] The thickness of the outer casing is preferably 50 μm or more, more preferably 100 μm or more, even more preferably 150 μm or more, and even more preferably 200 μm or more, and preferably 5 cm or less, more preferably 3 cm or less, even more preferably 2 cm or less, and even more preferably 1 cm or less. The outer casing may also be multilayered by stacking multiple layers, in which case the thickness of the multilayered structure should be within the above range.

[0107] As the outer casing, one having an uneven surface may be used. Using an outer casing with an uneven surface makes it easier to bend, so for example, the radio wave absorber of the present invention can be effectively applied to curved surfaces. When using an outer casing with an uneven surface on at least a part of its surface, the height difference of the uneven surface is preferably 0.3 mm or more, more preferably 0.5 mm or more, and even more preferably 1 mm or more. There is no particular upper limit, but it is preferably 70 mm or less. The height difference of the uneven surface can be calculated by adding the value of the largest deviation in the upward direction from the average surface of the outer casing and the value of the largest deviation in the opposite direction. By setting the height difference of the uneven surface of the outer casing within the above range, it is possible to more effectively suppress the crushing of the radio wave absorber, which would otherwise change its radio wave absorption performance.

[0108] Furthermore, in the radio wave absorber of the present invention, an outer covering having a fine uneven surface, such as an outer covering having a fine uneven surface known as a moth-eye structure, may be used. As such a material having a fine uneven surface known as a moth-eye structure, commercially available materials such as Mosmite (manufactured by Mitsubishi Chemical Corporation) can be used. As such an outer covering having a fine uneven surface, a height difference of preferably 10 μm or less, more preferably 5 μm or less, even more preferably 1 μm or less, and even more preferably 0.5 μm or less can be used, thereby further lowering the radio wave reflectivity. The lower limit of the height difference of the outer covering having a fine uneven surface is not particularly limited, but is preferably 0.01 nm or more. In addition, multiple outer coverings having a fine uneven surface may be used in stacks.

[0109] By providing an exterior with a finely textured surface, it is possible to prevent raindrops and other water droplets from adhering to the surface, effectively preventing the formation of a reflective surface due to the adhesion of water droplets, and thus enabling more effective use in such environments. Furthermore, as described above, the exterior with a finely textured surface prevents dust from adhering, effectively preventing changes in radio wave absorption performance due to dust adhesion, and thus enabling more effective use in environments where dust adhesion occurs.

[0110] Furthermore, the radio wave absorber of the present invention may have a resistive film layer on at least a portion of the surface outside the radio wave absorbing material, or on at least one surface, and it is preferable to have a resistive film layer on the surface that becomes the radio wave absorbing surface. By having a resistive film layer on the surface that becomes the radio wave absorbing surface, the radio wave reflectivity can be reduced. While a configuration with a resistive film layer is known for λ / 4 type radio wave absorbers in which a good electrical conductor such as metal is used as the radio wave reflective layer, the finding that the radio wave reflectivity can be reduced by forming a resistive film layer when using a polymer such as the radio wave absorber of the present invention is a new finding discovered by the inventors. In addition, if the radio wave absorber of the present invention has an outer casing on the surface that becomes the radio wave absorbing surface, a resistive film layer may be formed on the surface of the outer casing. Furthermore, in addition to forming a resistive film layer, it is preferable to use an outer casing with a fine uneven surface shape, as this further enhances the effect of reducing radio wave reflectivity.

[0111] The materials constituting the resistive film layer are not particularly limited, but examples include thin metal films, metal films with partial openings, conductive inorganic compounds such as ITO, FTO, and AZO known as transparent conductive films, conductive polymer films such as polythiophene, polyacetylene, and polyaniline, and carbon materials such as CNTs (carbon nanotubes), CNBs (carbon nanobuds), and graphene. The resistive film layer only needs to be thin enough to transmit electromagnetic waves, and this eliminates the need to use opaque metals, thus ensuring high visible light transmittance, lighting, and visibility.

[0112] From the viewpoint of transparency, conductive inorganic oxides such as ITO and carbon materials are more preferred, and conductive inorganic compounds such as ITO are even more preferred. From the viewpoint of ease of manufacturing a film with the desired sheet resistivity and ease of keeping variations in sheet resistivity low, conductive polymer films and carbon materials are preferred, and carbon materials are even more preferred.

[0113] From the viewpoint of further lowering the radio wave reflectivity, the sheet resistivity of the resistive film layer is preferably 10 Ω / □ or higher, more preferably 100 Ω / □ or higher, even more preferably 200 Ω / □ or higher, even more preferably 350 Ω / □ or higher, preferably 10,000 Ω / □ or lower, more preferably 5,000 Ω / □ or lower, even more preferably 1,000 Ω / □ or lower, and even more preferably 400 Ω / □ or lower, with a particularly preferred value of 377 Ω / □, which is the impedance value of air, or near 377 Ω / □. Since the resistive film layer inhibits the transmission of visible light, from the viewpoint of more sufficiently ensuring the transparency of the radio wave absorber, the sheet resistivity is preferably 50 Ω / □ or higher, more preferably 150 Ω / □ or higher, even more preferably 300 Ω / □ or higher, and even more preferably 400 Ω / □ or higher. If it is 50 Ω / □ or higher, a visible light transmittance of 65% or higher can be ensured; if it is 150 Ω / □ or higher, a visible light transmittance of 70% or higher can be ensured; if it is 300 Ω / □ or higher, a visible light transmittance of 75% or higher can be ensured; and if it is 400 Ω or higher, a visible light transmittance of 80% or higher can be ensured.

[0114] Furthermore, the radio wave absorber of the present invention may contain additives such as antioxidants, ultraviolet absorbers, colorants, preservatives, flame retardants, transparent fillers, transparent fibers, low molecular weight organic compounds, surfactants, and salts, to the extent that they do not impair visible light transmittance. If the radio wave absorber has an outer casing, these additives may be contained in either the radio wave absorber or the outer casing, or in both.

[0115] Furthermore, when the radio wave absorber of the present invention is used in applications where visible light transmittance is not required, it may contain inorganic fillers such as ceramics, particularly MHz-band electromagnetic wave absorbing ceramics, conductive carbon, metal powder, or opaque fibers. If the radio wave absorber has an outer casing, these inorganic fillers can be contained in either the radio wave absorber or the casing, or both.

[0116] The radio wave absorber of the present invention can be used in various forms. For example, by being in the form of a sheet or plate, it can be used in the form of partitions, screens, wall materials (walls, wallpaper), road signs, humidity control materials, moisture collection materials, moisture release materials, heat shielding materials, curtains, windows, aprons, hats, glasses, clothing, coats, and so on. Furthermore, the radio wave absorber of the present invention may also be used in the form of a three-dimensional object having a predetermined shape. For example, it may be used as an ornament placed in a room, and by being an ornament, it can absorb unwanted radio waves in the room.

[0117] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples. Unless otherwise specified, "parts" refers to mass.

[0118] <Example 1-1> Eight parts of polyethylene glycol diacrylate (Sigma-Aldrich, number-average molecular weight (Mn) = 700) containing 1% by weight of Omnirad 651 (IGM Resins, photopolymerization initiator) were mixed with 92 parts of water to obtain a precursor solution for the radio wave absorbing material. Separately, an acrylic resin container with an internal space of 15 cm × 15 cm × 0.3 cm was prepared. The acrylic resin container had a thickness of 2 mm and an opening of 15 cm × 15 cm at the top. The precursor solution prepared above was poured into this acrylic resin container, and under nitrogen, 365 nm UV light was irradiated from above (through the opening) to promote a crosslinking reaction inside the container, obtaining a test specimen with a thickness of 3 mm. The radio wave reflectance, transmission attenuation, and radio wave absorption rate at 24.15 GHz were measured using this specimen. The results are shown in Table 1. The obtained test specimen was cloudy and scattered light, but it was still possible to collect light.

[0119] The radio wave reflectivity, transmission attenuation, and radio wave absorption rates were measured using the following method. Specifically, using the test specimen prepared above, the specimen was positioned so that the surface exposed to a 15 cm x 15 cm opening was perpendicular to the ground. By irradiating the surface exposed to the opening with 24.15 GHz radio waves, the radio wave reflection attenuation (dB) and transmission attenuation (dB) were measured, and the radio wave reflectivity, transmission attenuation, and radio wave absorption rates were determined from these measurement results. Specifically, using the free-space method, the test specimen prepared above was irradiated with 24.15 GHz radio waves from a perpendicular direction to the surface exposed to the opening, and the S (Scattering) parameters (S11) and (S21) of the reflected and transmitted waves were measured. A smaller reflection attenuation indicates lower radio wave reflectivity, and a larger transmission attenuation indicates better radio wave absorption performance. The reflection attenuation can be calculated using the following formula. Reflection loss (dB) = 20 log | S11 |

[0120] The measurement conditions were as follows: (Equipment configuration) ・Free Space Microwave Measurement System (HVSFS, Mac Systems Co., Ltd.) ・Network analyzer: Keysight Technologies (Agilent Technologies), N5227N 10MHz to 67GHz ・Wave Guide T / R Module: Keysight Technologies (Agilent Technologies), N5260-60004 67GHz to 110GHz ・Millimeter wave controller: Keysight Technologies (Agilent Technologies), 8510XF ・Transmitting and receiving antenna (C band): HVS Technology FSS-01 with dielectric lens ・Transmitting and receiving antenna (X band): HVS Technology FSS-04 with dielectric lens ・Transmitting and receiving antenna (Ku-W band): HVS Technology FSS-07 Dielectric lens and coaxial cable: Keysight Technologies (Agilent Technologies), 11500-60002 1mm Test Cable (Measurement conditions) - Focal length: 30.5 cm Antenna-sample distance reference - Polarization plane: Linear polarization (Horizontal / Vertical) - Measurement frequency: 24.15 GHz

[0121] <Examples 1-2 to 1-8> Test specimens were obtained in the same manner as in Example 1-1, except that the amount of polyethylene glycol diacrylate (referred to as "polymer" in Table 1) and the amount of water were changed as shown in Table 1. The radio wave reflectance, transmission attenuation, and radio wave absorptive rate at 24.15 GHz were then measured. The results are shown in Table 1. In Examples 1-2 to 1-8, the obtained test specimens were cloudy when the amount of polymer was small, but became transparent when the amount of polymer was sufficiently large, ensuring light collection and visibility in visible light. To improve transparency, it is preferable to change the photopolymerization initiator from a water-insoluble one (e.g., Omnirad 651) to a water-soluble one (e.g., Omnirad 2959 (manufactured by IGM Resins, a photopolymerization initiator)). In that case, Examples 1-1 to 1-3 become slightly cloudy. To further improve transparency, it is advisable to use a polymer containing an acrylamide monomer or a monomer having an ammonium group. In Examples 1-1 to 1-4, when Omnirad 2959 (manufactured by IGM Resins, a photopolymerization initiator) was used as the photopolymerization initiator and dimethylacrylamide or acrylamidopropyltrimethylammonium chloride was used as the polymer, all test specimens became clear. Asker C hardness was also measured. As the amount of polymer increased, the Asker C hardness improved, and an effect of reduced deformation under external force was observed. For test specimens with an amount of polymer of 8 parts by weight or less, measurement of Asker C hardness could not be performed because holes were created in the test specimens during the measurement. It is preferable to provide such radio wave absorbers with a strong outer casing to prevent deformation and breakage.

[0122] <Comparative Example 1-1> A test specimen was obtained in the same manner as in Example 1-1, except that the amount of polyethylene glycol diacrylate (referred to as "polymer" in Table 1) and the amount of water were changed as shown in Table 1. When attempts were made to measure the radio wave reflectance, transmission attenuation, and radio wave absorption, when the test specimen was placed upright with the surface exposed to the opening perpendicular to the ground, the water-containing polymer portion collapsed, making it impossible to perform the measurements.

[0123] <Comparative Example 1-2> A test specimen was obtained in the same manner as in Example 1-1, except that a radio wave absorbing material containing 90 parts water and 10 parts sodium chloride was used. When attempts were made to measure the radio wave reflectivity, transmission attenuation, and radio wave absorption, when the test specimen was placed upright with the surface exposed to the opening perpendicular to the ground, the liquid state of the test specimen caused it to flow out, making it impossible to perform the measurements.

[0124]

[0125] <Evaluation of Examples 1-1 to 1-8 and Comparative Examples 1-1 and 1-2> As shown in Table 1, a radio wave absorbing material in which the proportion of polymer is 8% by weight or more and the proportion of water is 10% by weight or more out of a total of 100% by weight of water and polymer exhibits low radio wave reflectivity and excellent radio wave absorption performance. In particular, the radio wave reflectivity was reduced compared to the radio wave reflectivity of water (literature value: Ueda, Miyata, Kamitori, Tanizaki, Kamakura: Method for distinguishing between water and ice on a road surface using quasi-millimeter waves, Transactions of the Society of Instrument and Control Engineers, Vol. 54, No. 3, 331 / 339 (2018)), which is 77%. On the other hand, the transmission attenuation performance tends to decrease as the proportion of water decreases. When used as a radio wave shielding material, it is preferable to keep the radio wave transmittance below 10% by a transmission attenuation performance of -10 dB or less. Therefore, it is more preferable to have a water content of more than 10% by weight.

[0126] <Example 2-1> A precursor solution was prepared in the same manner as in Example 1-1. The prepared precursor solution was poured into an acrylic resin container having an internal space of 15 cm × 15 cm × 0.3 cm, and a cross-linking reaction was carried out in the container to obtain a test specimen. Then, a 2 mm thick acrylic resin plate was placed over the test specimen from above through a 15 cm × 15 cm opening formed at the top, and bonded to it, thereby creating a test specimen in which the entire circumference of the radio wave absorbing material was covered with an outer covering. Then, the radio wave reflectivity, transmission attenuation, and radio wave absorption rate at 24.15 GHz were measured in the same manner as in Example 1-1, except that the surface covered with the acrylic resin plate was irradiated with radio waves instead of the surface exposed to the opening. The results are shown in Table 2. In Example 2-1 as well, the test specimen covered with the outer covering was cloudy and scattered light, but it was still possible to collect light.

[0127] <Examples 2-2 to 2-8> Except for changing the amount of polyethylene glycol diacrylate (referred to as "polymer" in Table 2) and the amount of water as shown in Table 2, test specimens covered with an outer coating were obtained in the same manner as in Example 2-1, and the radio wave reflectance, transmission attenuation, and radio wave absorption rate at 24.15 GHz were measured. The results are shown in Table 2. In Examples 2-2 to 2-8, as in Examples 1-1 to 1-8, the obtained test specimens were cloudy when the amount of polymer was small, but became transparent when the amount of polymer was sufficiently large, ensuring light collection and visibility in visible light.

[0128] <Comparative Example 2-1> A test specimen covered with an outer coating was obtained in the same manner as in Example 2-1, except that the amount of polyethylene glycol diacrylate (referred to as "polymer" in Table 1) and the amount of water were changed as shown in Table 1, and the radio wave reflectance, transmission attenuation, and radio wave absorption rate were measured. The results are shown in Table 2.

[0129] <Comparative Example 2-2> A test specimen covered with an outer casing was obtained in the same manner as in Example 2-1, except that a radio wave absorbing material containing 90 parts water and 10 parts sodium chloride was used, and the radio wave reflectance, transmission attenuation, and radio wave absorption rate were measured. The results are shown in Table 2.

[0130] <Comparative Example 2-3> A test specimen covered with an outer casing was obtained in the same manner as in Example 2-1, except that only 100 parts of water were used as the radio wave absorbing material. Radio wave reflectance, transmission attenuation, and radio wave absorption rate were measured. The results are shown in Table 2.

[0131]

[0132] <Evaluation of Examples 2-1 to 2-8 and Comparative Examples 2-1 to 2-3> As shown in Table 1, a radio wave absorbing material in which the proportion of polymer is 8% by weight or more and the proportion of water is 10% by weight or more out of a total of 100% by weight of water and polymer, when combined with an exterior, can achieve excellent radio wave absorption performance while further lowering the radio wave reflectivity. This effect is also evident from the comparison between Examples 2-1 to 2-8 and Comparative Example 2-3, which uses water alone. Furthermore, in Comparative Example 2-2, which uses sodium chloride instead of polymer, the radio wave reflectivity increased, contrary to Examples 2-1 to 2-8. From this, it can be confirmed that the effect of the present invention is not due to a relative decrease in water content by including polymer.

[0133] <Example 3> In order to produce a radio wave absorber having a pattern of square pyramidal protrusions (regular square pyramids with a square base), a concave mold with regularly spaced square pyramidal holes was fabricated using a 3D printer (dimensions will be described later). Then, a solution prepared by adding 1 part of methylenebisacrylamide and 1 part of Omnirad 2959 (IGM Resins, photopolymerization initiator) to 98 parts of an aqueous solution of diallyldimethylammonium chloride (Sigma-Aldrich, 60 wt% aqueous solution), stirring thoroughly, and dissolving the solution was poured into the obtained concave mold, and polymerization and crosslinking were carried out by photopolymerization by irradiation with 365 nm UV light under nitrogen, to obtain a flexible and transparent hydrogel-like radio wave absorber. The obtained radio wave absorber was obtained in the form of a flat plate shape with 15 cm square and 3 mm thick, with numerous square pyramidal protrusions formed in a pattern on one side. The radio wave absorber was peeled from the mold and attached to a 2 mm thick acrylic plate to serve as a sample for measuring radio wave absorption performance. The radio wave reflectivity was measured in the same manner as in Example 1-1, except that it was irradiated with 77 GHz radio waves. The results are shown in Table 3. In addition to the radio wave absorption performance, to evaluate the ease of crushing, a 100 g weight was placed on the formed protrusion, left for 5 seconds, and then removed. The height was measured 5 seconds after removal. The results, along with the initial height before placing the weight, the height after placing the weight, and the height reduction rate, are shown in Table 3. In Table 3, height is defined as "thickness + protrusion height".

[0134] In Example 3, the following test specimens were prepared and evaluated: (3-1) without a protrusion, (3-2) with a protrusion height H = 0.3 mm and a side length of the protrusion base (side length of the base of the square pyramid) L = 2 mm, (3-3) with a protrusion height H = 0.5 mm and a side length of the protrusion base L = 2 mm, (3-4) with a protrusion height H = 1 mm and a side length of the protrusion base L = 2 mm, (3-5) with a protrusion height H = 2 mm and a side length of the protrusion base L = 2 mm, (3-6) with a protrusion height H = 4 mm and a side length of the protrusion base L = 2 mm, and (3-7) with a protrusion height H = 4 mm and a side length of the protrusion base L = 4 mm.

[0135] In Example 3, separately from the above, a 20 cm square, 200 μm thick ionomer resin sheet (product name "Hymiran," manufactured by Mitsui Dow Chemical Co., Ltd.) was prepared and vacuum-formed to manufacture an exterior with a structure in which square pyramidal holes corresponding to square pyramidal protrusions (regular square pyramids with a square base) with a protrusion height H = 4 mm and a side length L = 4 mm of the base of the protrusions were regularly spaced apart from each other. Then, instead of the concave mold made using a 3D printer, this exterior was used, and without removing the exterior, it was attached to a 2 mm thick acrylic plate on the side opposite to the side where the protrusions were formed. Except for this, the procedure was the same as for test specimen (3-7), and test specimen (3-8) was obtained. This was used as a sample for measuring radio wave absorption performance, and the radio wave reflectivity was measured in the same manner as above. Test specimen (3-8) was also evaluated for its ease of crushing using weights in the same manner as above. The results are shown in Table 3.

[0136] Furthermore, in the above example, test specimen (3-8) was obtained by attaching it to a 2 mm thick acrylic plate. However, by replacing the acrylic plate with a 15 cm square, 200 mm thick transparent silicone rubber, and attaching it to the side opposite to the side where the protrusions were formed, test specimen (3-8') was obtained. Test specimen (3-8), having an acrylic plate exterior on one side, had excellent flatness, but could hardly be bent even when force was applied, and returned to its original shape when the force was removed. Test specimen (3-8'), with flexible silicone rubber on one side and a protruding, airtight surface on the other, was very easy to bend and could conform to curved surfaces. For example, it was possible to wrap it around a 1 cm diameter pipe, and no damage was observed after removing the wrapped object. When test specimens (3-8) and (3-8') were attached to the outside of a sunny window in the laboratory, no changes, including discoloration, were observed even after one year. Although test specimens (3-8) and (3-8') are transparent, the pyramidal protrusions scatter visible light, making it difficult to see far into the distance. Therefore, when used in screens or windows, privacy can be ensured while still allowing light in. Furthermore, when directly attached to boards with writing, such as road signs, the written text was still legible. In addition, the radio wave absorption performance of test specimen (3-8') in the 5.6 to 110 GHz range was measured in the same manner as described in Example 1-1 above, and the results are shown in Figures 1 and 2.

[0137]

[0138] <Evaluation of Example 3> As can be seen from the results in Table 3, the radio wave reflectivity can be lowered by providing protrusions such as a square pyramidal shape on the surface of the radio wave absorber. Furthermore, from the viewpoint of more effectively suppressing the occurrence of crushing, it is preferable that the height of the protrusions be 0.5 mm or more.

[0139] <Example 4> A solution was prepared by adding 1 part methylenebisacrylamide and 1 part Omnirad 2959 (IGM Resins, photopolymerization initiator) to 98 parts of an aqueous solution of diallyldimethylammonium chloride (Sigma-Aldrich, 60% by weight aqueous solution), and stirring thoroughly until dissolved. This solution was then cast into a 15 cm × 15 cm × 0.3 cm plate. This was then irradiated with 365 nm UV light under nitrogen to polymerize and crosslink, obtaining a flexible and transparent sheet. This was attached to a 2 mm acrylic plate, and then a 500 μm PET film with adhesive was attached on top of it. Furthermore, an 80 μm PET film having a resistive film adjusted to 377 Ω / □ by coating it with the conductive polymer PEDOT-PSS was attached so that the resistive film was the bonding surface, thereby obtaining a test specimen (4-1) with a resistive film (the outer structure consisted of PET film / resistive film / PET film / acrylic plate / radio wave absorbing material from the surface side). The transmittance of this radio wave absorber at 800 nm (approximately 375 THz) was 79%, and the transmittance at 1450 nm (approximately 207 THz) was 3%, indicating that it was transparent to visible light and absorbed infrared rays. For comparison, test specimen (4-2) was prepared in the same manner as test specimen (4-1), except that a 2 mm acrylic plate was attached to the surface instead of using a multilayer structure of PET and a resistive film. Then, the radio wave reflectance was measured using these obtained test specimens in the same manner as in Example 1-1, except that they were irradiated with 77 GHz radio waves. The results were 10% for test specimen (4-1) and 19% for test specimen (4-2). The results showed that a lower surface reflectance could be obtained by using an outer casing with a resistive film.

[0140] <Example 5> 99 parts of an aqueous solution of acrylamidopropyltrimethylammonium chloride (75% by weight aqueous solution, manufactured by Sigma-Aldrich) were mixed with 1 part of Omnirad 2959 (a photopolymerization initiator, manufactured by IGM Resins), and the solution was prepared by thoroughly stirring and dissolving it. This solution was then cast into a 15 cm × 15 cm × 0.3 cm plate. Under nitrogen, 365 nm UV light was irradiated to polymerize and crosslink the plate to obtain a flexible and transparent sheet. This sheet was used as the test specimen (5-1), and the radio wave reflectance was measured in the same manner as in Example 1-1, except that it was irradiated with 77 GHz radio waves. The results are shown in Table 4.

[0141] Next, a 140 μm thick PET film (product name "Mosmite", manufactured by Mitsubishi Chemical Corporation) with fine irregularities was attached to the test specimen (5-1) obtained as described above, with the surface forming the fine irregularities facing outwards, to create test specimen (5-2). The radio wave reflectivity was measured in the same manner as in Example 1-1, except that it was irradiated with 77 GHz radio waves. The results are shown in Table 4.

[0142] Separately from the above, a solution was prepared by adding 1 part methylenebisacrylamide and 1 part Omnirad 2959 (IGM Resins, photopolymerization initiator) to 98 parts of an aqueous solution of diallyldimethylammonium chloride (Sigma-Aldrich, 60% by weight aqueous solution), stirring thoroughly to dissolve the solution. This solution was then cast into a 15 cm × 15 cm × 0.3 cm flat plate on the surface of a PET film with micro-textures (product name "Mosmite," Mitsubishi Chemical Corporation). This was then irradiated with 365 nm UV light under nitrogen to polymerize and crosslink, obtaining a flexible and transparent sheet. Next, another PET film with micro-textures (product name "Mosmite," Mitsubishi Chemical Corporation) was attached to this sheet with the surface of the micro-textures facing outwards to form test specimen (5-3). The radio wave reflectance was measured in the same manner as in Example 1-1, except that it was irradiated with 77 GHz radio waves. The results are shown in Table 4. Furthermore, the transmittances of test specimens (5-1), (5-2), and (5-3) at 800 nm (approximately 375 THz) were 84%, 88%, and 92%, respectively. All were transparent in visible light, and those with PET films featuring minute irregularities showed higher transparency.

[0143]

[0144] <Evaluation of Example 5> From Table 4, it can be confirmed that the radio wave reflectivity can be further reduced by providing an exterior with minute irregularities, and furthermore, increasing the number of exteriors with minute irregularities enhances the effect of reducing radio wave reflectivity. It is not known that such minute irregularities on the order of μm can suppress reflection in the GHz band. In addition, the installation method of facing the irregular surface inward is also unusual, and an unusual effect was confirmed. On the other hand, since the reflectivity hardly changes even when an exterior with fine irregularities is provided on the acrylic resin, the combination of this radio wave absorbing material and an exterior with minute irregularities produces an unexpected effect.

Claims

1. A radio wave absorbing material comprising a composition containing water and a polymer, wherein the proportion of the polymer is 8% by weight or more of the total weight of water and polymer, and the proportion of water relative to the whole composition is 10% by weight or more.

2. The radio wave absorbing material according to claim 1, wherein the composition contains a salt compound.

3. The radio wave absorbing material according to claim 1 or 2, wherein the polymer is a synthetic polymer.

4. The radio wave absorbing material according to claim 3, wherein the polymer is an acrylamide-based polymer or a polyalkylene oxide-based polymer.

5. A radio wave absorber made of the radio wave absorbing material described in any one of claims 1 to 4.

6. A radio wave absorber comprising a radio wave absorbing material according to any one of claims 1 to 4 and an outer casing having a water content of less than 10% by weight, wherein at least a portion of the radio wave absorbing material is covered by the outer casing.

7. The radio wave absorber according to claim 5 or 6, wherein at least a portion of its surface has an uneven surface with a height difference of 0.5 mm or more.

8. The radio wave absorber according to any one of claims 5 to 7, wherein a member made of rubber material is arranged on at least a portion of the surface of the radio wave absorbing material.

9. The radio wave absorber according to any one of claims 5 to 8, wherein a transparent material having a fine uneven structure is disposed on at least a portion of the surface of the radio wave absorbing material.

10. The radio wave absorber according to any one of claims 5 to 9, wherein a resistive film layer is disposed on at least a portion of the area outside the radio wave absorbing material.

11. A radio wave absorber according to any one of claims 5 to 10, which absorbs radio waves with a frequency of 1 GHz or more and 220 THz or less.

12. A radio wave absorber according to any one of claims 5 to 10, which absorbs radio waves with a frequency of 5.6 GHz or more and 110 GHz or less.

13. A radio wave absorber according to any one of claims 5 to 10, which absorbs radio waves with a frequency of 20 GHz or more and 110 GHz or less.

14. A radio wave absorber according to any one of claims 5 to 10, which absorbs radio waves with a frequency of 100 GHz or more and 110 THz or less.

15. A radio wave absorber according to any one of claims 5 to 10, having a radio wave reflectance surface that is 20% or less for radio waves with a frequency of 100 GHz or more and 220 THz or less.

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