JP5293451B2 - Radio wave absorbing sheet composition, radio wave absorbing sheet and radio wave absorber using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a composition for a radio wave absorbing sheet for providing the radio wave absorbing sheet which has high radio wave absorption characteristics, and is thin and lightweight, not influenced by a surface shape of a construction surface, and superior in handling property and of reduced cost. <P>SOLUTION: The composition for the radio wave absorbing sheet contains (A) a binder component, (B) a hollow balloon, and (C) graphite powder. In the composition, the binder component contains (a1) thermosetting rubber, (a2) thermoplastic rubber, and (a3) a crosslinking agent for the thermosetting rubber, and the content of the (C) graphite powder is 5 to 20% by mass for the total amount of the composition relative to the radio wave absorbing sheet. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a composition for a radio wave absorption sheet, a radio wave absorption sheet and a radio wave absorber using the same.

  With the development of radio communication systems, the use of automatic fee payment systems (ETC) using GHz band radio waves, inter-vehicle distance automatic measurement systems using millimeter wave radar, wireless LANs and the like has been rapidly expanding in recent years.

  However, in the communication system, a malfunction of the system caused by multiple turbulences of radio waves between the transmitting and receiving antennas is a serious problem. As one of the cheaper malfunction prevention measures, there is a shield method using a conductor (graphite plate or metal plate), but the reflected radio waves may affect other systems. No.

  In addition, as another method, there is a method of using a radio wave absorber and converting the absorbed radio wave into heat, which is a perfect method for preventing malfunction. Common types of electromagnetic wave absorbers include a paint type, a building structure material type, and a sheet type. Although the paint type is simple and can be applied in a short time, it is very difficult to make the film thickness uniform to determine the radio wave absorption region due to the problem of unevenness of the foundation and uniform coating film formation. Moreover, the building structural material type is a heavy object to be constructed on the wall or ceiling of the building, and it is difficult to construct it locally and simply. In order to improve the construction efficiency, a method of using an electromagnetic wave absorbing foam obtained by mixing an electromagnetic wave absorber into a hard foamed resin such as urethane foam has been proposed (Patent Document 1). However, in this method, it is important to maintain a stable foaming ratio and to manage moisture that greatly affects the reaction in the manufacturing process, etc. There is a possibility that it will change greatly and affect the radio wave absorption characteristics.

  Therefore, attention is focused on a sheet-type electromagnetic wave absorber that can be manufactured relatively easily and has elasticity that makes it easy to adjust its thickness. Realized at low cost.

  Currently used radio wave absorption sheets are produced by adjusting dielectric characteristics using a binder component and a conductive material. Such a radio wave absorbing sheet is not only expensive because it uses a very expensive magnetic metal powder as a conductive material, but also has a problem that it is inferior in workability because it is quite heavy and thick. there were.

JP 2002-348987 A

  In view of such circumstances, the present invention provides a low-cost radio wave absorption sheet having high radio wave absorption characteristics, thin and light, excellent in handleability not affected by the surface shape of the construction surface, and low cost. An object of the present invention is to provide a composition for an electromagnetic wave absorbing sheet that can be used. Furthermore, an object of this invention is to provide the electromagnetic wave absorption sheet and electromagnetic wave absorber which use the said composition for electromagnetic wave absorption sheets.

That is, the present invention is a composition for a radio wave absorbing sheet containing (1) (A) a binder component, (B) a hollow balloon and (C) graphite powder,
The binder component comprises (a1) a thermosetting rubber, (a2) a thermoplastic rubber, and (a3) a thermosetting rubber cross-linking agent,
The content of said (C) graphite powder is 5-20 mass% with respect to the total amount of the composition for electromagnetic wave absorption sheets, It is related with the composition for electromagnetic wave absorption sheets characterized by the above-mentioned.

The present invention is also characterized in that (2) the (C) graphite powder has an average particle diameter of 50 to 500 μm and a bulk density of 0.1 to 1.5 g / cm ³ ( It relates to the composition for electromagnetic wave absorption sheets as described in 1).

  Further, the present invention provides (3) (1) or (2), wherein the (C) graphite powder is anisotropic graphite powder, and the shape thereof is a thin needle branch or dendritic shape. It is related with the composition for electromagnetic wave absorption sheets of description.

  Moreover, this invention relates to the composition for electromagnetic wave absorption sheets as described in any one of said (1)-(3) characterized by (4) said (C) graphite powder being expanded graphite.

  The present invention also relates to (5) the composition for an electromagnetic wave absorbing sheet according to (4), wherein the expanded graphite is a pulverized powder obtained by pulverizing an expanded graphite molded sheet.

  In addition, the present invention provides (6) any one of the above (1) to (5), wherein the (a1) thermosetting rubber is a synthetic rubber having a carboxyl group, a hydroxyl group or an amino group. It relates to the composition for electromagnetic wave absorption sheets as described in above.

  In the present invention, (7) the blending amount of the (a2) thermoplastic rubber is 5 to 30 parts by mass with respect to 100 parts by mass of the total amount of (a1) thermosetting rubber and (a2) thermoplastic rubber. It is related with the composition for electromagnetic wave absorption sheets as described in any one of said (1)-(6) characterized by having.

  Moreover, this invention is (8) The radio wave as described in any one of said (1)-(7), wherein the crosslinking agent of said (a3) thermosetting rubber is a polyvalent epoxy compound. The present invention relates to an absorbent sheet composition.

  Moreover, this invention is (9) Said (B) hollow balloon is a shirasu balloon which has an average particle diameter of 10-200 micrometers, It is described in any one of said (1)-(8) characterized by the above-mentioned. The present invention relates to a composition for an electromagnetic wave absorbing sheet.

  Moreover, this invention relates to the composition for electromagnetic wave absorption sheets as described in any one of said (1)-(9) characterized by containing a flame retardant material (10) further.

  Moreover, this invention relates to the electromagnetic wave absorption sheet formed by shape | molding the composition for electromagnetic wave absorption sheets as described in any one of (11) said (1)-(10) in the sheet form.

  The present invention also relates to (12) a radio wave absorber comprising the radio wave absorption sheet according to (11) and a reflector.

  According to the present invention, a low-cost radio wave absorption sheet having high radio wave absorption characteristics, thin and light, excellent in flexibility and workability, and excellent in handleability not affected by the surface shape of the construction surface is obtained. It is possible to provide a composition for an electromagnetic wave absorbing sheet. Furthermore, this invention can provide the electromagnetic wave absorption sheet and electromagnetic wave absorber which use the said composition for electromagnetic wave absorption sheets.

[Composition for radio wave absorbing sheet]
The radio wave absorbing sheet composition of the present invention is a radio wave absorbing sheet composition containing (A) a binder component, (B) a hollow balloon and (C) graphite powder, wherein the binder component is (a1) heat. It includes a curable rubber, (a2) a thermoplastic rubber, and (a3) a crosslinking agent for the thermosetting rubber.

  By using the composition for a radio wave absorption sheet of the present invention, the radio wave absorption sheet having excellent radio wave absorption characteristics regardless of the surface shape of the construction surface, for example, the surface shape such as a smooth type, an uneven type, or a through-hole type. Can be obtained.

  When producing an electromagnetic wave absorbing sheet using the composition for an electromagnetic wave absorbing sheet, if an organic solvent is used, environmental and safety problems occur, and a solvent removal process is required, which increases costs. Prepared in a solvent-free system. The components constituting the radio wave absorbing sheet composition are uniformly mixed using a pressure kneader. (A1) When the molecular weight of the thermosetting rubber is large or the molecular structure is rigid, it occurs during mixing. (A3) The cross-linking agent of the thermosetting rubber mixed with (a1) the thermosetting rubber proceeds at the same time, making it difficult to produce the radio wave absorbing sheet. . As a solution to the above problem, a method of reducing the blending amount of the hollow balloon or the graphite powder to reduce the viscosity of the composition can be considered, but the radio wave absorption characteristics and flexibility, elongation rate, etc. of the final radio wave absorption sheet There is a limit when considering the mechanical property values.

  Therefore, in the present invention, in order to smoothly mix each component, by blending a thermoplastic rubber that does not react with the crosslinking agent (a2), by reducing the viscosity of the composition and lowering the exothermic temperature, (a1 The reaction between the thermosetting rubber and the crosslinking agent of (a3) the thermosetting rubber can be suppressed, and a more uniform mixture can be obtained. In addition, by adjusting the blending amount of the (a2) thermoplastic rubber, it is possible to control mechanical properties such as flexibility and elongation of the radio wave absorbing sheet.

  On the other hand, (a2) when a sheet is produced by pressure-molding a thermoplastic rubber, a so-called spring back occurs in which the mold shrunk by pressure expands and returns to its original state when the pressure is terminated after molding, There is a phenomenon that the thickness increases. In the present invention, by blending (a1) thermosetting rubber and (a3) thermosetting rubber crosslinking agent, (a1) promotes three-dimensional curing of the thermosetting rubber, suppresses the spring bag, The thickness can be controlled.

  Hereinafter, each component used with the composition for electromagnetic wave absorption sheets of this invention is demonstrated.

(A) Binder Component The (A) binder component in the present invention contains (a1) a thermosetting rubber, (a2) a thermoplastic rubber, and (a3) a crosslinking agent for the thermosetting rubber.

(A1) Thermosetting rubber (a1) The thermosetting rubber used in the present invention is a component before curing, and (a3) a composition cured by heat treatment with a crosslinking agent of the thermosetting rubber. .

  In the present invention, (a1) the thermosetting rubber is crosslinked with the crosslinking agent of (a3) the thermosetting rubber, whereby the mechanical strength of the radio wave absorbing sheet is improved and the thickness of the sheet can be controlled. .

  The (a1) thermosetting rubber used in the present invention is not particularly limited, and (a3) any one having a functional group capable of crosslinking with the crosslinking agent of the thermosetting rubber can be used.

  General rubber cross-linking (vulcanization) is achieved by the reaction of a compound such as sulfur, sulfur compounds, and peroxides with a rubber component having a functional group capable of cross-linking with them, but the environment (odor). -From the viewpoint of safety and health and rubber cross-linking management, it is difficult to say that the cross-linking of rubber with the above compound is optimal.

  The cross-linking of the (a1) thermosetting rubber according to the present invention is not simply an increase in the molecular weight of the (a1) thermosetting rubber due to the cross-linking, but the structure and characteristics of the cross-linking agent of the (a3) thermosetting rubber are taken into account. It is cross-linking, and (a1) is intended to reflect the three-dimensional curing between thermosetting rubbers in sheet thickness control. Accordingly, the (a1) thermosetting rubber preferably has a functional group capable of cross-linking with the crosslinking agent of the (a3) thermosetting rubber, and specifically includes a carboxyl group, a hydroxyl group, an amino group, and the like. A synthetic rubber having a functional group is preferred. In particular, acrylonitrile-butadiene rubber (NBR) having a carboxyl group and acrylic rubber having a carboxyl group are preferable in terms of sheet characteristics and cost. Examples of the acrylic rubber having a carboxyl group include a copolymer of (meth) acrylic acid alkyl ester-acrylonitrile- (meth) acrylic acid.

  The (a1) thermosetting rubber is preferably solid at room temperature having a large molecular weight in consideration of the mechanical strength of the sheet, and has, for example, a carboxyl group, a hydroxyl group, an amino group, etc. having a large molecular weight at room temperature. Synthetic rubber is more preferred, carboxyl group-containing synthetic rubber is even more preferred, and carboxy group-containing NBR or carboxy group-containing acrylic rubber is particularly preferred. The molecular weight is not particularly limited, but is preferably 100,000 or more in terms of weight average molecular weight, and more preferably 150,000 to 500,000. Here, normal temperature refers to 15 to 30 ° C. As a commercial product of such (a1) thermosetting rubber, trade name: Nipol 1072 (carboxyl group-containing NBR, weight average molecular weight: 230,000, carboxyl group concentration: 0.75 (KOHmg / g), etc., manufactured by Nippon Zeon Co., Ltd. Can be illustrated.

(A2) Thermoplastic rubber (a2) The thermoplastic rubber used in the present invention reduces the viscosity of the composition when the components constituting the composition for a radio wave absorbing sheet are mixed to produce the composition. It is possible to suppress the exothermic temperature from rising, thereby suppressing the reaction between the (a1) thermosetting rubber and the (a3) thermosetting rubber cross-linking agent, thereby obtaining a more uniform mixture. Moreover, the weather resistance of (a1) thermosetting rubber can be improved by mix | blending (a2) thermoplastic rubber. In addition, by adjusting the blending amount of the (a2) thermoplastic rubber, it is possible to control mechanical properties such as flexibility and elongation of the radio wave absorbing sheet.

  Examples of the thermoplastic rubber (a2) used in the present invention include an acrylic rubber obtained by copolymerization of an acrylate ester with another monomer; an ethylene-propylene copolymer obtained by reacting ethylene and propylene with a catalyst. Rubber (EPM); Butyl rubber obtained by copolymerization of isobutylene and isoprene; Styrene-butadiene copolymer rubber (SBR) obtained by copolymerization of butadiene and styrene; Acrylonitrile-butadiene rubber (NBR) obtained by copolymerization of acrylonitrile and butadiene ) And the like.

  The (a2) thermoplastic rubber exemplified above can be used by mixing with the (a1) thermosetting rubber.

  Although the molecular weight of the (a2) thermoplastic rubber is not particularly limited, the weight average molecular weight is preferably in the range of 200,000 to 2,000,000, and more preferably in the range of 300,000 to 1,500,000. By setting the weight average molecular weight to 200,000 or more, it is possible to prevent the glass transition temperature of the radio wave absorption sheet from being lowered, and to eliminate the stickiness and workability of the surface of the radio wave absorption sheet. On the other hand, by setting the weight average molecular weight to 2 million or less, it is possible to suppress an increase in heat generation temperature when mixing each component with a pressure kneader to produce a composition, and to make the radio wave absorption sheet flexible. Can be granted.

  Specific examples of the (a2) thermoplastic rubber used in the present invention include Nagase ChemteX Corporation trade name: THR-811DR (acrylic rubber, weight average molecular weight: 500,000), Nippon Zeon Corporation trade name: Nipol AR31 (acrylic rubber, Mooney viscosity (central value): 40, glass transition temperature (DSC): −15 ° C.), Nipol AR51 (acrylic rubber, Mooney viscosity (central value): 55, glass transition temperature (DSC): − 14 ° C), Nipol AR71 (acrylic rubber, Mooney viscosity (central value): 50, glass transition temperature (DSC): -15 ° C), Nipol AR42W (acrylic rubber, Mooney viscosity (central value): 33.5, glass transition Temperature (DSC): -26 ° C).

  The blending amount of the (a2) thermoplastic rubber is not limited, but is preferably 5 to 30 parts by mass, more preferably 100 parts by mass with respect to the total amount of (a1) thermosetting rubber and (a2) thermoplastic rubber. Is 5 to 20 parts by mass. When the blending amount is 5 parts by mass or more, a sufficient mechanical property value is obtained, and when it is 30 parts by weight or less, the properties of the thermosetting rubber are not impaired.

(A3) Crosslinking agent for thermosetting rubber (a1) Although a reaction system of a synthetic rubber containing a hydroxyl group as a thermosetting rubber and an isocyanate (NCO) compound as a crosslinking agent is also considered, an isocyanate compound that reacts instantaneously with moisture Is not preferred because it is difficult to handle and control the reaction.

  The crosslinking agent for (a3) thermosetting rubber used in the present invention is not particularly limited. However, (a1) it easily undergoes a crosslinking reaction with a functional group such as a carboxyl group, a hydroxyl group, or an amino group contained in the thermosetting rubber. A compound having a stable physical property is preferable. (A1) In order to promote three-dimensional curing of the thermosetting rubber, (a1) a multifunctional compound having two or more groups that undergo a crosslinking reaction with the functional group of the thermosetting rubber A crosslinking agent is more preferred.

When the (a1) thermosetting rubber is a synthetic rubber having a carboxyl group, a hydroxyl group, or an amino group, the crosslinking agent for the (a3) thermosetting rubber is preferably a polyvalent epoxy compound. The polyvalent epoxy compound is not particularly limited as long as it is a compound having two or more epoxy groups.

Examples of polyvalent epoxy compounds include bisphenol A diglycidyl ether, bisphenol A diβ-methyl glycidyl ether, bisphenol F diglycidyl ether, tetrahydroxyphenylmethane tetraglycidyl ether, 4,4′-biphenol diglycidyl ether, hydroquinone di Glycidyl ether, catechol diglycidyl ether, 1,5-naphthalenediglycidyl ether, 1,6-naphthalenediglycidyl ether, 2,7-naphthalenediglycidyl ether, cyclohexanedimethanol diglycidyl ether, resorcinol diglycidyl ether, brominated bisphenol A diglycidyl ether, chlorinated bisphenol A diglycidyl ether, hydrogenated bisphenol A diglycidyl ether, Diglycidyl ether, novolac diglycidyl ether of phenol A alkylene oxide adduct, polyalkylene glycol diglycidyl ether, glycerin triglycidyl ether, pentaerythritol diglycidyl ether, ethylene glycol diglycidyl ether, diethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether , Propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, neopentyl glycol diglycidyl ether, 1,4-butane diglycidyl ether, 1,5-pentane diglycidyl ether, 1,6-hexane diglycidyl ether, 1,7 -Heptane diglycidyl ether, 1,8-octane diglycidyl ether, 1 9-nonanediglycidyl ether, 1,10-decanediglycidyl ether, sorbitol diglycidyl ether, sorbitol polyglycidyl ether, polyglycerol polyglycidyl ether, spiroglycol diglycidyl ether, polyethylene glycol diglycidyl ether, glycerol polyglycidyl ether, tri Glycidyl ether type compounds such as methylolpropane polyglycidyl ether, diglycerol polyglycidyl ether, pentaerythritol polyglycidyl ether and epoxy urethane resin;
glycidyl ether ester type compounds such as p-glycidyl ether benzoic acid glycidyl ester;
Glycidyl ester types such as diglycidyl phthalate, diglycidyl tetrahydrophthalate, diglycidyl hexahydrophthalate, diglycidyl dimer, diglycidyl adipate, diglycidyl terephthalate and diglycidyl phthalate Compound;
Glycidylamine type compounds such as diglycidylaniline, tetraglycidyldiaminodiphenylmethane, triglycidylisocyanurate, triglycidylaminophenol, triglycidyltris (2-hydroxyethyl) isocyanurate;
Alicyclic epoxy compounds such as bis (3,4-epoxy-6-methylcyclohexyl) adipate, vinylcyclohexene diepoxide, dicyclopentadiene dioxide, bis (2,3-epoxycyclopentyl) ether, limonene dioxide;
Linear short-chain aliphatic epoxy compounds such as 1,2,7,8-diepoxyoctane;
Linear long-chain aliphatic epoxy compounds such as epoxidized polybutadiene and epoxidized soybean oil;
Epichlorohydrin compounds such as polyamide epichlorohydrin;
Phenol novolac type epoxy resin, cresol novolac type epoxy resin, trishydroxyphenylmethane type epoxy resin, dicyclopentadiene phenol type epoxy resin, bisphenol A type epoxy resin, bisphenol F type epoxy resin, bisphenol S type epoxy resin, bisphenol AD type epoxy Epoxy resins such as resins, biphenol type epoxy resins, hydrogenated bisphenol type epoxy resins, polyfunctional epoxy resins, naphthalene skeleton-containing epoxy resins, heterocyclic epoxy resins, halogenated bisphenol type epoxy resins and epoxidized phenol aralkyl resins; Is mentioned.

  Among these polyvalent epoxy compounds, glycidyl ether type compounds are preferable. These polyvalent epoxy compounds may be used alone or in combination of two or more.

  The polyvalent epoxy compound also functions to improve the heat resistance of the radio wave absorption sheet, to improve the adhesion to the construction surface of the radio wave absorption sheet, and to improve the mixing property of the binder component and the graphite powder. A radio wave absorbing sheet using a composition for a radio wave absorbing sheet containing a polyvalent epoxy compound has physical properties partially reflecting the characteristics of the polyvalent epoxy compound. Thus, in the present invention, (a1) crosslinking of thermosetting rubber is (a3) crosslinking in which the structure and characteristics of the crosslinking agent of thermosetting rubber are taken into account, and the characteristics are reflected in the radio wave absorption sheet. .

  The molecular weight and structure of the polyvalent epoxy compound are not particularly limited, but considering the compatibility, physical properties, cost, etc. of (a2) thermoplastic rubber and (a1) thermosetting rubber, the number average molecular weight (Mn) is A compound of 500 or less is preferable, and bisphenol A diglycidyl ether which is a reaction product of epichlorohydrin having a number average molecular weight (Mn) of 500 or less and bisphenol A is more preferable. As a commercially available product of such a polyvalent epoxy compound, trade name: Epicoat 828 (number average molecular weight: 380, bifunctional type, number of epoxy groups (equivalent): 190 (liquid)) manufactured by Shell Chemical Co., Ltd. may be mentioned. .

  (A1) When a carboxyl group-containing synthetic rubber, a hydroxyl group-containing synthetic rubber or an amino group-containing synthetic rubber is used as the thermosetting rubber, the compounding ratio of the polyvalent epoxy compound is (a1) carboxyl contained in the thermosetting rubber. It is determined by the group concentration, hydroxyl group concentration or amino group concentration, the molecular weight of the polyvalent epoxy compound, and the number of epoxy groups (equivalent). Compared to the polyvalent epoxy compound, (a1) the thermosetting rubber has a large molecular weight, so that it is difficult to blend in an equivalent amount. Usually, it is more than 100 parts by mass of carboxyl group-containing synthetic rubber, hydroxyl group-containing synthetic rubber or amino group-containing synthetic rubber. 1 to 30 parts by mass of a monovalent epoxy compound is preferred. By setting the blending amount of the polyvalent epoxy compound to 1 part by mass or more, the crosslink density via the polyvalent epoxy compound is improved, and the spring back after completion of the sheet molding can be suppressed. On the other hand, by setting the blending amount to 30 parts by mass or less, the amount of the polyvalent epoxy compound that does not contribute to crosslinking can be reduced, and the handleability and sheet characteristics can be improved.

  The reaction between the (a3) thermosetting rubber crosslinking agent and the (a1) thermosetting rubber is achieved by heat treatment. There are no particular restrictions on the heat treatment conditions. For example, when the thermosetting rubber (a1) is a carboxyl group-containing synthetic rubber (a1), the complete disappearance of carboxyl groups is absorbed by heating at 180 ° C. for 30 minutes. It is confirmed by analysis (IR) and can be judged to be completely cured. Therefore, when it is desired to reduce the curing strain in the radio wave absorbing sheet, heat treatment is performed at a low temperature for a long time, and heat treatment can be performed at a high temperature for a short time to improve productivity.

(B) Hollow balloon By including a hollow balloon in the composition for a radio wave absorption sheet of the present invention, it is possible to reduce the weight of the radio wave absorption sheet, improve the formability (dimensional accuracy), and reduce the cost. There is no restriction | limiting in particular as (B) hollow balloon used by this invention, At least 1 type of the organic type hollow balloon which made the organic substance the hollow balloon and the inorganic type hollow balloon which made the inorganic substance the hollow balloon can be used.

  Examples of organic balloons include those formed in a hollow balloon shape by applying heat while scattering a synthetic resin powder such as acrylic resin or polystyrene resin in the air. Examples of inorganic balloons include shirasu balloons and silica balloons. Examples include glass balloons. In view of heat treatment and cost in producing the radio wave absorbing sheet, an inorganic hollow balloon is preferable, and a shirasu balloon is more preferable. The Shirasu balloon is made by firing volcanic ash at a high temperature and foaming it into a hollow shape.

  The average particle diameter of the (B) hollow balloon is not particularly limited, but is preferably in the range of 10 to 200 μm. By setting the average particle size to 10 μm or more, the radio wave absorbing sheet can be reduced in weight, and an increase in viscosity when each component is mixed can be suppressed. On the other hand, by setting the average particle size to 200 μm or less, it is possible to prevent the hollow balloon from being broken in the process of mixing the components.

  The content of the (B) hollow balloon is preferably 5 to 55% by mass and more preferably 10 to 45% by mass with respect to the total amount of the composition for a radio wave absorbing sheet. By making the content 5% by mass or more, the wave absorbing sheet can be reduced in weight and flame retardancy, and by making it 55% by mass or less, the viscosity of the composition for the radio wave absorbing sheet is prevented from rapidly increasing. And uniform dispersibility can be obtained. Moreover, the thermal reaction by the temperature of the composition for electromagnetic wave absorption sheets rising can be suppressed.

(C) Graphite powder (C) Graphite powder in the composition for electromagnetic wave absorption sheets of this invention functions as an electromagnetic wave absorption material.

  Content of the said (C) graphite powder is 5-20 mass% with respect to the total amount of the composition for electromagnetic wave absorption sheets, and it is preferable that it is 5-18 mass%. By making the content 5 to 20% by mass, the radio wave absorptivity is excellent.

The average particle diameter (D50) of the (C) graphite powder is not particularly limited, but is preferably in the range of 50 to 500 μm, and more preferably in the range of 40 to 450 μm. By setting the average particle size to 50 μm or more, the anisotropy of the graphite powder that affects the radio wave absorption characteristics increases, the probability that the graphite powders come into contact with each other increases, and the radio wave absorption characteristics are improved. On the other hand, when the average particle diameter is 500 μm or less, it is easy to mix uniformly with the binder component, and variations in radio wave absorption characteristics and physical properties of a radio wave absorption sheet produced using the binder component can be suppressed. Here, the average particle diameter (D50) indicates a volume average particle diameter, and can be measured using a laser diffraction particle size distribution analyzer.
Further, the (C) is not particularly limited with respect to the bulk density of the graphite powder is preferably in the range of 0.1 to 1.5 g / cm ^3, in the range of 0.1 to 1.0 g / cm ³ It is more preferable. The bulk density of the graphite powder is a value obtained by dividing the mass of the graphite powder by the bulk volume occupied, and can be obtained from the volume and the mass when the graphite powder is filled in the container and tapped to such an extent that no volume change is observed. it can.

  The graphite powder used in the present invention may be isotropic graphite powder or anisotropic graphite powder, but anisotropic graphite powder is preferred from the viewpoint of radio wave absorption characteristics. The shape of the anisotropic graphite powder is preferably a thin needle-branch shape or a dendritic shape from the viewpoint of radio wave absorption performance. When the shape of the anisotropic graphite powder is spherical or nearly spherical, the contact between the anisotropic graphite powders is slow, and the radio wave absorbing sheet has the desired excellent radio wave absorption characteristics and is excellent in weight reduction. Cannot be obtained.

The (C) graphite powder used in the present invention may be either natural graphite powder or artificial graphite powder. Examples of natural graphite powder include flaky graphite powder, massive graphite powder, and earthy graphite powder. Examples thereof include expanded graphite powder. Among these, expanded graphite powder is preferable from the viewpoint of the radio wave absorption characteristics because the specific gravity is small and the wave absorbing sheet can be lightened.
The expanded graphite powder may be obtained by pulverizing expanded graphite or by pulverizing an expanded graphite molded sheet obtained by further processing expanded graphite into a sheet shape, but from the viewpoint of mixing with a binder component. An expanded graphite powder obtained by pulverizing an expanded graphite molded sheet is preferred.

  A method for producing an expanded graphite powder by pulverizing an expanded graphite molded sheet includes, for example, the following steps (I) to (IV).

(I) Step of producing expanded graphite (II) Step of producing an expanded graphite molded sheet using the expanded graphite (III) Step of pulverizing the expanded graphite molded sheet to produce a graphite sheet pulverized powder (IV) Step of classifying the pulverized graphite sheet powder to obtain expanded graphite powder In the step (I), there is no particular limitation on the method for producing expanded graphite. For example, raw graphite is placed in a solution containing an acidic substance and an oxidizing agent. The expanded graphite can be produced by dipping to form a graphite intercalation compound, heating the graphite intercalation compound, and expanding the C-axis direction of the graphite crystal. The expanded graphite produced by this method is in a form in which each graphite expands like a worm and the expanded graphite is intertwined in a complicated manner. The expansion ratio of expanded graphite is not particularly limited, but is preferably 10 times or more and more preferably 50 times to 500 times in consideration of radio wave absorption characteristics. The use of expanded graphite having an expansion ratio of 10 times or more improves the strength of the obtained expanded graphite molded sheet, and the use of expanded graphite of 500 times or less facilitates the production of an expanded graphite molded sheet. .

  Although there is no restriction | limiting in particular as raw material graphite used at this process, Graphite with high crystallinity, such as natural graphite, quiche graphite, and pyrolytic graphite, is preferable. Considering the balance between the properties of the obtained expanded graphite and the economical efficiency, natural graphite is more preferable. There is no restriction | limiting in particular as natural graphite, For example, commercial products, such as Nippon Graphite Co., Ltd. brand name: F48C, Chuetsu Graphite Co., Ltd. brand name: H-50, can be used. These are preferably used in the form of scales. Moreover, there is no restriction | limiting in particular also about the particle size of raw material graphite, You may use combining raw material graphite from which a particle size differs.

  The acidic substance used in this step is preferably one that can generate acidic roots (anions) having sufficient expansion ability when entering between graphite layers, and sulfuric acid is generally used. The concentration of sulfuric acid used as the acidic substance is not particularly limited, but is preferably 95% by mass or more, and more preferably concentrated sulfuric acid is used. The amount of the acidic substance used is appropriately determined depending on the target expansion ratio. For example, it is preferable to use 100 to 1000 parts by mass of an acidic substance with respect to 100 parts by mass of raw material graphite.

  The oxidizing agent used in this step is preferably a peroxide such as hydrogen peroxide, potassium perchlorate, potassium permanganate, potassium dichromate, or an acid having an oxidizing action such as nitric acid. From the viewpoint of easily obtaining expanded graphite, hydrogen peroxide is more preferable. When hydrogen peroxide is used, it is preferably used in the form of an aqueous solution, that is, hydrogen peroxide. Although there is no restriction | limiting in particular about the density | concentration of hydrogen peroxide water, The range of 20-40 mass% is preferable. Moreover, although there is no restriction | limiting in particular about the usage-amount of hydrogen peroxide water, It is preferable to use in the range of 5-60 mass parts with respect to 100 mass parts of raw material graphite.

  In this step, if necessary, the produced expanded graphite may be heat-treated at a higher temperature to remove impurities contained in the expanded graphite to increase the purity of the expanded graphite.

  In the step (II), an expanded graphite molded sheet is produced using the expanded graphite obtained in the step (I). There is no restriction | limiting in particular in the method of shape | molding expanded graphite to a sheet | seat, The method of pressure-molding using a roll or a press at normal temperature or a heating is mentioned.

The density of the expanded graphite molded sheet produced in this step is not particularly limited, but is preferably in the range of 0.07 to 1.5 g / cm ³ . By setting the density to 0.07 g / cm ³ or more, the strength of the expanded graphite molded sheet can be improved. By setting the density to 1.5 g / cm ³ or less, the expanded graphite pseudo-aggregate is destroyed during molding. Can be prevented. The density of the expanded graphite molded sheet can be controlled by adjusting the amount of expanded graphite filled and the pressure during molding.

  Although the thickness of the expanded graphite molded sheet produced at this process is not specifically limited, The range of 0.5-1.5 mm is preferable.

  The expanded graphite molded sheet can be obtained by the above steps (I) and (II), but can also be obtained as a commercial product. For example, commercially available products such as trade names: Carbofit HGP-105 and HGP-207 manufactured by Hitachi Chemical Co., Ltd. can be used.

  In the step (III), the expanded graphite molded sheet is pulverized to produce a graphite sheet pulverized powder. The method for pulverizing the expanded graphite molded sheet is not particularly limited, and examples thereof include a method for pulverizing using a general dry pulverizer, for example, trade name: Rotoplex manufactured by Hosokawa Micron Corporation.

The density of the pulverized graphite sheet produced in this step is not particularly limited, but a range of 0.1 to 1.5 g / cm ³ is preferable.

  Although the average particle diameter (D50) of the graphite sheet pulverized powder produced in this step is not particularly limited, it is preferably in the range of 50 to 500 μm, and more preferably in the range of 40 to 450 μm. By setting the average particle size to 50 μm or more, the anisotropy of the graphite powder that influences the radio wave absorption characteristics is increased, the probability that the graphite sheet pulverized powders are in contact with each other increases, and the radio wave absorption characteristics are improved. On the other hand, when the average particle diameter is 500 μm or less, it is easy to mix uniformly with the binder component, and variations in radio wave absorption characteristics and physical properties of a radio wave absorption sheet produced using the binder component can be suppressed. The average particle size of the graphite sheet pulverized powder can be controlled by adjusting the pulverization and classification.

  In the step (IV), the graphite sheet pulverized powder is classified to obtain expanded graphite powder. The method of classifying the pulverized graphite sheet powder is not particularly limited, and examples thereof include a method of classifying using a general dry classifier, for example, a vibration sieve.

  In order to obtain an anisotropic, dendritic or thin needle-branched expanded graphite powder, the above steps (I) to (IV) may be performed.

Bulk density of the expanded graphite powder obtained in this step is not particularly limited, it is preferably in the range of 0.1 to 1.5 g / cm ^3, in the range of 0.1 to 1.0 g / cm ³ More preferably.

  The average particle diameter (D50) of the expanded graphite powder produced in this step is not particularly limited, but is preferably in the range of 50 to 500 μm, and more preferably in the range of 40 to 450 μm. By setting the average particle size to 50 μm or more, the anisotropy of the expanded graphite powder that affects the radio wave absorption characteristics is increased, the probability that the expanded graphite powders are in contact with each other increases, and the radio wave absorption characteristics are improved. On the other hand, when the average particle diameter is 500 μm or less, it is easy to mix uniformly with the binder component, and variations in radio wave absorption characteristics and physical properties of a radio wave absorption sheet produced using the binder component can be suppressed.

(Other ingredients)
The composition for a radio wave absorbing sheet of the present invention contains the components (A) to (C) as essential components, and is an isocyanate or amine compound for the purpose of accelerating the curing reaction of the radio wave absorbing sheet composition. It can contain hardening accelerators, such as. Moreover, a flame retardant can be contained for the purpose of increasing the flame retardancy of the radio wave absorbing sheet.

  Since the flame retardant functions as a part of the (A) binder component, it is preferable to select the flame retardant in consideration of the compatibility with the binder component and the characteristics of the radio wave absorbing sheet. As the flame retardant, a halogen-based compound, a phosphorus-based compound, or the like can be used. However, in consideration of the characteristics of the radio wave absorbing sheet, a phosphate ester-based flame retardant is preferable. For example, an aliphatic phosphate ester such as trimethyl phosphate or triethyl phosphate; Examples thereof include aromatic phosphates such as phenyl phosphate and tricresyl phosphate; aromatic condensed phosphates such as bisphenol A bis (diphenyl phosphate), and the like. These can be used alone or in combination. As a commercial item of aromatic condensed phosphate ester, Daihachi Chemical Co., Ltd. brand name: CR-741 etc. are mentioned. The amount of the flame retardant used is preferably in the range of 100 to 200 parts by mass with respect to 100 parts by mass of the (A) binder component. However, since it varies depending on the amount of (C) graphite powder used, it is appropriately determined in consideration of this point. Is done. By making the amount used 100 parts by mass or more, flame retardancy of UL standard: V-0 can be easily obtained, and by making it 200 parts by mass or less, the flexibility of the obtained radio wave absorption sheet increases rapidly. Prevents stickiness from occurring on the sheet surface.

[Radio wave absorbing sheet]
An electromagnetic wave absorbing sheet can be obtained by molding the composition for an electromagnetic wave absorbing sheet of the present invention into a sheet shape. The thickness of the radio wave absorbing sheet is appropriately selected according to the usage pattern, construction scene, and the like. The density of the radio wave absorbing sheet is appropriately selected according to the use form and construction scene, but is preferably 1.2 to 1.8 g / cm ³ , more preferably 1.3 to 1.7 g / cm ³ . . The density of the radio wave absorbing sheet can be calculated by dividing the weight of the radio wave absorbing sheet by the sheet volume.

  Although the manufacturing method of an electromagnetic wave absorption sheet is not specifically limited, For example, it includes in the following (a) and (b) processes.

(A) The step of obtaining the radio wave absorbing sheet composition by mixing the components (A) to (C) and other components (b) The step of obtaining the radio wave absorbing sheet by molding the radio wave absorbing sheet composition In the step (a), the components (A) to (C) and other components as necessary are mixed to obtain a composition for an electromagnetic wave absorbing sheet. The mixing method of each component is not particularly limited, and examples thereof include a method using a mixing apparatus such as a kneader, a likai machine, or a planetary mixer. A preferable mixing method is a method in which each component can be uniformly mixed in a short time without variation. For example, a mixing method using a pressure type kneader is used. The mixing conditions are arbitrarily determined by (A) the molecular weight of the binder component, (B) the blending amount of the hollow balloon and (C) graphite powder, and the like. The order of adding each component to the mixing apparatus is not particularly limited, but generally, after (a1) thermosetting rubber and (a2) thermoplastic rubber are first charged into the mixing apparatus and kneaded and mixed, a3) A uniform composition for an electromagnetic wave absorbing sheet can be obtained by adding and mixing a thermosetting rubber cross-linking agent, (B) hollow balloon, (C) graphite powder and other components little by little.

  Moreover, the viscosity of the composition for radio wave absorbing sheets is high, frictional heat is generated during mixing, and the crosslinking reaction between (a3) the thermosetting rubber crosslinking agent and (a1) the thermosetting rubber may proceed. In the case where there is, it is preferable to add the crosslinking agent of (a3) thermosetting rubber to the mixing apparatus about 10 to 20 minutes before the end of mixing. Judgment of whether each component is uniformly mixed [(a3) including reaction of crosslinking agent of thermosetting rubber] is to measure the viscosity of the composition for radio wave absorbing sheet with a curast meter or Mooney viscometer To do. In this case, some samples are taken out from the composition for radio wave absorbing sheets, the viscosity is measured, and if the values do not vary greatly, it is determined that the sample is uniform.

Further, if necessary, a small amount of an organic solvent can be added to improve the mixing property, but it is finally necessary to remove the used solvent. The organic solvent used here is preferably one that is excellent in compatibility with (a1) thermosetting rubber and (a2) thermoplastic rubber, hardly evaporates during the mixing operation, and easily evaporates during curing.
In the step (b), the radio wave absorbing sheet composition is molded to obtain a radio wave absorbing sheet. A shaping | molding method will not be specifically limited if it is a method by which the electromagnetic wave absorption sheet of a uniform film thickness is obtained, For example, rolling shaping | molding, press molding, extrusion molding, coating, etc. are mentioned.

  The cross-linking reaction between the (a1) thermosetting rubber and the (a3) thermosetting rubber cross-linking agent is achieved by performing a heat treatment, but the cross-linking reaction can be achieved by carrying out the molding under heating. The crosslinking reaction may be achieved by heat-treating the sheet after completion of the molding. The heat treatment temperature for carrying out the crosslinking reaction varies depending on the type and amount of the (a3) thermosetting rubber crosslinking agent, but is generally achieved by heat treatment at 160 ° C. for about 1 hour.

[Radio wave absorber]
A radio wave absorber can be obtained by applying the radio wave absorption sheet of the present invention to a reflector.

  If a radio wave incident on the radio wave absorbing sheet is not completely absorbed by the radio wave absorbing sheet by installing a reflector on the radio wave absorbing sheet, the unabsorbed radio wave is reflected by the reflector and absorbed by the radio wave absorbing sheet again. Therefore, the reflection loss (return loss) can be increased.

  The reflector is applied to the surface opposite to the surface on which the radio wave of the radio wave absorbing sheet is incident. The construction method is not particularly limited, and for example, a thin adhesive sheet can be laminated on the construction surface of the radio wave absorbing sheet and processed with a laminator device so that the construction can be performed with less air entrainment at the adhesion interface. Moreover, after apply | coating an adhesive agent to the construction surface of a radio wave absorption sheet, you may laminate | stack a reflecting plate.

  The reflection plate is an important member for extracting the radio wave absorption characteristics of the radio wave absorption sheet, and is preferably easy to process and strong, and a metal plate such as an aluminum plate, a copper plate, an iron plate, a galvanized steel plate, or a carbon plate is used. . The thickness of the reflecting plate is appropriately selected according to the type of the reflecting plate, the usage pattern of the radio wave absorber, and the like.

  The radio wave absorbing sheet and radio wave absorber thus obtained are particularly excellent in the radio wave absorption characteristics in the GHz band.

  Hereinafter, the present invention will be described specifically by way of examples. In addition, this invention is not restrict | limited to the following Example.

Example 1
(1) Preparation of expanded graphite powder An expanded graphite molded sheet (trade name: Carbofit HGP-105, manufactured by Hitachi Chemical Co., Ltd.) was pulverized by using a pulverizer (trade name: Rotoplex, manufactured by Hosokawa Micron Corporation). The obtained pulverized powder was classified with a vibrating sieve to produce 2 kg of expanded graphite powder having a bulk density of 0.2 g / cm ³ within a particle size distribution of 50 to 350 μm. It was observed with a microscope (SEM) photograph and confirmed to have an anisotropic dendritic shape, and the average particle diameter (D50) of the expanded graphite powder was 300 μm.

(2) Preparation of composition for radio wave absorbing sheet A kneader (trade name: 1100-S-1 manufactured by Yoshida Seisakusho Co., Ltd.) equipped with a pressurizing jig having a capacity of 1 liter is used at room temperature (25 ° C.). ) NBR (Nippon ZEON Corporation, trade name: Nipol 1072, weight average molecular weight: 230,000, carboxyl group concentration: 0.75 (KOHmg / g)) 204 g (a2) ) 36 g of a thermoplastic rubber (trade name: HTR-811DR, manufactured by Nagase ChemteX Corp., weight average molecular weight: 500,000) was put into a kneader and kneaded and mixed for 10 minutes.

  After the kneading and mixing, (B) Shirasu Balloon (trade name: Shirasu Balloon Five Star Shirasu Balloon, average particle size: 40 μm, manufactured by SK Life Co., Ltd.) as a hollow balloon and aromatic condensed phosphate ester (large) as a flame retardant (Product name: CR-741 manufactured by Hachi Chemical Industry Co., Ltd.) 300 g was alternately added to the kneader in small portions and mixed for 15 minutes after completion of the addition. The temperature of the mixture after mixing was 63 ° C.

  Next, (C) 180 g of expanded graphite powder prepared in (1) above as graphite powder was added to the kneader and mixed for 20 minutes, and then (a3) a polyvalent epoxy compound (Shell Chemical Co., Ltd.) was used as a crosslinking agent for thermosetting rubber. Product name: Epicoat 828, number average molecular weight: 380, bifunctional, epoxy group number (equivalent): 190 (liquid)) 20.4 g was added and mixed for 15 minutes to obtain a composition for an electromagnetic wave absorbing sheet. . The temperature of the obtained composition was 80 ° C.

(3) Production of radio wave absorbing sheet The upper and lower hot plate temperature of a 70-ton automatic press (manufactured by Maruhichi Iron Works: Model “MB-070”) was raised to 180 ° C. One by one was heated. The heated end plate is taken out, and a molding frame (made by SUS, thickness 1.0 mm: frame width 3 cm: molding area 35 cm × 35 cm) is placed on one end plate, and the composition for a radio wave absorption sheet prepared in (2) above A pre-formed 380 g square was placed in the center of the forming frame, and another end plate was gently overlapped and returned to the press hot plate, forming at a forming pressure of 30 MPa for 15 minutes to obtain a sheet. The obtained sheet was further heat-treated at 180 ° C. for 30 minutes to obtain a radio wave absorbing sheet. The radio wave absorbing sheet had an average thickness of 1.83 mm and a sheet density of 1.45 g / cm ³ , indicating great elasticity. Here, the average thickness of the radio wave absorbing sheet was obtained by measuring the thickness of 13 locations on the entire surface of the sheet with a height gauge and calculating the average value. The sheet density was calculated by dividing the weight of the sheet by the sheet volume.
(4) Production of radio wave absorber An acrylic adhesive sheet having a thickness of 0.05 mm is uniformly applied to one side of the radio wave absorption sheet produced in (3), and an aluminum plate having a thickness of 0.3 mm is reflected thereon. The wave absorber 1 was produced by pasting the entire surface as a plate.

(5) Sheet dielectric constant Confirming the dielectric constant of the sheet is an important factor in determining the optimum thickness of the radio wave absorbing sheet.

  The radio wave absorbing sheet prepared in (3) above was used as a dielectric constant measurement sheet. When a dielectric constant of a part of the measurement sheet was measured by a waveguide standing wave method, the real part had a dielectric constant of 60 and the imaginary part had a dielectric constant of 9.1. It was found from the calculation formula that the optimal sheet thickness that the radio wave absorption sheet using the radio wave absorption sheet composition prepared in (2) can absorb 5.8 GHz band radio waves is 1.8 mm.

Example 2
A radio wave absorbing sheet composition was prepared by performing the same operation as in Example 1 except that the amount of the expanded graphite powder was changed to 150 g, and a radio wave absorbing sheet and a radio wave absorber 2 were prepared using the same. The average thickness of the radio wave absorbing sheet was 1.87 mm, and the sheet density was 1.47 g / cm ³ .

Example 3
A radio wave absorbing sheet composition was prepared in the same manner as in Example 1 except that the amount of expanded graphite powder was changed to 128 g, and a radio wave absorbing sheet and a radio wave absorber 3 were manufactured using the same. The average thickness of the radio wave absorbing sheet was 1.90 mm, and the sheet density was 1.50 g / cm ³ .

Comparative Example 1
A radio wave absorbing sheet composition was prepared in the same manner as in Example 1 except that the amount of the expanded graphite powder was 350 g, and a radio wave absorbing sheet and a radio wave absorber 4 were manufactured using the same. The average thickness of the radio wave absorbing sheet was 1.83 mm, and the sheet density was 1.35 g / cm ³ .

<Evaluation of electromagnetic wave absorber>
For the wave absorbers 1 to 4 obtained in Examples 1 to 3 and Comparative Example 1, the radio wave absorption characteristics were measured using the “radio wave absorber and return loss measurement system” of the radio wave absorption characteristic measurement device manufactured by Keycom. evaluated. The results are shown in Table 1.

  As shown in Table 1, it is clear that the radio wave absorbers obtained in Examples 1 to 3 are superior in radio wave absorptivity compared to the radio wave absorber obtained in Comparative Example 1.

Claims (12)

(A) a binder component, (B) a hollow balloon and (C) a composition for a radio wave absorbing sheet containing graphite powder,
The binder component comprises (a1) a thermosetting rubber, (a2) a thermoplastic rubber, and (a3) a thermosetting rubber cross-linking agent,
The composition for a radio wave absorption sheet, wherein the content of the (C) graphite powder is 5 to 20% by mass with respect to the total amount of the radio wave absorption sheet composition. The composition for a radio wave absorbing sheet according to claim 1, wherein the (C) graphite powder has an average particle diameter of 50 to 500 μm and a bulk density of 0.1 to 1.5 g / cm ^3. object.   The composition for a radio wave absorption sheet according to claim 1 or 2, wherein the (C) graphite powder is anisotropic graphite powder, and the shape thereof is a thin needle-branch shape or a dendritic shape.   The said (C) graphite powder is expanded graphite, The composition for electromagnetic wave absorption sheets as described in any one of Claims 1-3 characterized by the above-mentioned.   The composition for an electromagnetic wave absorbing sheet according to claim 4, wherein the expanded graphite is a pulverized powder obtained by pulverizing an expanded graphite molded sheet.   The composition for a radio wave absorbing sheet according to any one of claims 1 to 5, wherein the (a1) thermosetting rubber is a synthetic rubber having a carboxyl group, a hydroxyl group or an amino group.   The blending amount of the (a2) thermoplastic rubber is 5 to 30 parts by mass with respect to 100 parts by mass of the total amount of (a1) thermosetting rubber and (a2) thermoplastic rubber. The composition for electromagnetic wave absorption sheets as described in any one of -6.   The composition for a radio wave absorption sheet according to any one of claims 1 to 7, wherein the crosslinking agent of the (a3) thermosetting rubber is a polyvalent epoxy compound.   The composition for a radio wave absorption sheet according to any one of claims 1 to 8, wherein the (B) hollow balloon is a shirasu balloon having an average particle diameter of 10 to 200 µm.   Furthermore, a flame retardant material is contained, The composition for electromagnetic wave absorption sheets as described in any one of Claims 1-9 characterized by the above-mentioned.   The electromagnetic wave absorption sheet formed by shape | molding the composition for electromagnetic wave absorption sheets as described in any one of Claims 1-10 in a sheet form.   A radio wave absorber comprising the radio wave absorption sheet according to claim 11 and a reflector.