JP2009054983A - Radio wave absorbing material and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a radio wave absorbing material which can be obtained at a relatively low cost, is flexible, and has high shieldability for electromagnetic waves, particularly ≤3 GHz. <P>SOLUTION: Flaky graphite fine powder whose average particle size is ≥1 μm and ≤7 μm is mixed for 60 parts or more to 60 parts of the resin solid portion of a flexible resin and is agitated to uniformly disperse the graphite fine powder. It is applied with a prescribed thickness onto a PET film, on the surface of which a remover is executed. A solvent is vaporized by heating, the resin is hardened as well, and a hardened object is removed from a base material to attain the radio wave absorbing material. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a more improved flexible radio wave absorber suitable as a material for noise suppression sheets used as noise countermeasures for electronic devices such as computers, and a method for manufacturing the same, and more particularly, to radio waves in a low frequency (3 GHz or less) region. The present invention relates to a radio wave absorber excellent in absorption and a method for manufacturing the same.

2. Description of the Related Art Conventionally, various radio wave absorbers are used in order to prevent interference due to external radio waves to electronic devices and the like. Normally, a method of blocking external radio waves with a metal box or the like has been adopted, but there is a drawback in that it lacks flexibility and is heavy. Therefore, a method has been adopted in which particles of conductive metal or the like are kneaded into a resin or the like and used to block radio waves. Among them, there has been proposed a radio wave absorber including graphite, which is obtained at a relatively low cost and is lightweight. (For example, see Patent Documents 1 and 2 below)
However, both have a problem of low radio wave shielding because the amount of graphite contained in the resin is small.

  The above-described technique is for preventing interference from outside as described above. When a large-scale integrated circuit used in many electronic devices in recent years is energized, the circuit itself may be interfered by electromagnetic waves generated from the circuit itself. In the above-described technology, in order to prevent interference due to an electromagnetic wave of the electronic device itself to be protected, no contrivance has been made so as to absorb radio waves and not cause reflection.

  In the following Patent Document 3, a plate-like material called graphite sheet, which is mainly made of graphite, is bonded to a material such as a resin having flexibility, so as to have a certain flexibility and a radio wave blocking property. Attempts have been made.

  In the first place, since the graphite sheet is expensive and lacks flexibility, a radio wave absorber that is truly soft and excellent in radio wave blocking properties has not been obtained.

  The above-described radio wave absorber is excellent in absorption of radio waves mainly on the high frequency side exceeding 3 GHz. At present, a radio wave absorber having excellent radio wave absorption of 3 GHz or less and having flexibility is not yet implemented in the market.

JP-A 61-276399 JP 2004-99675 A JP 2005-166893 A

  Accordingly, an object of the present invention is to provide a radio wave absorber that is obtained at a relatively low cost, has flexibility, and has high shielding properties against electromagnetic waves, particularly electromagnetic waves of 3 GHz or less, and a method for producing the same.

  According to the present invention, a flexible resin and a scaly graphite powder uniformly dispersed in the resin (hereinafter simply referred to as graphite powder) are contained, and the graphite powder is 60 parts by weight or more with respect to 60 parts by weight of the resin. An electromagnetic wave absorber included is provided. The average particle size of the graphite powder is 1 μm or more and 50 μm or less, more preferably 1 μm or more and 7 μm or less. Alternatively, the graphite powder is a mixture of graphite having a particle diameter of 1 μm or more and 7 μm or less and graphite having a particle size exceeding 7 μm and 50 μm or less.

  By containing the graphite powder in an amount of 60 parts by weight or more, preferably 70 parts by weight or more, more preferably 110 parts by weight or more with respect to 60 parts by weight of the resin, it has high electromagnetic wave shielding properties, particularly high electromagnetic wave shielding properties of 3 GHz or less. A radio wave absorber is obtained.

  The average particle diameter of the graphite powder is 1 μm or more and 50 μm or less. If a part or all of the graphite powder is 7 μm or less, the radio wave absorption performance is improved.

  If it is less than 1 μm, the specific surface area becomes large and it is difficult to add a large amount of graphite. If it exceeds 50 μm, it is difficult to obtain a smooth sheet.

The specific surface area of the graphite powder is desirably 30 m ² / g or less. If it exceeds 30 m ² / g, the viscosity of the dispersion before curing will increase too much, making molding difficult. In the case of graphite having an average particle size of 7 μm or less, it is preferably 17 m ² / g or less.

  The graphite powder is preferably spherical graphite. As the shape of the graphite, a massive spherical graphite is preferable because it has a smaller specific surface area than a leaf shape.

  About adding 60 parts by weight or more of scaly graphite powder to 60 parts by weight of resin, conventionally, as described in Patent Documents 1 and 2, the flexibility (or adhesion) of a product (radio wave absorber) or It was shunned that the smoothness of the surface was impaired.

  However, unless 60 parts by weight or more of graphite is mixed with 60 parts by weight of resin, absorption on the high frequency side is not performed well. In addition, the above-mentioned amounts cannot be kneaded into the resin with the leaf-like graphite of the tree.

  The type A durometer hardness defined in JIS_K6253_1997 of the radio wave absorber is preferably 20 or more and 70 or less. More preferably, it is 20 or more and 60 or less. Those with less than 20 are difficult to produce, and the drawbacks are that the change with time is remarkable, while those with more than 70 have poor elasticity and do not lose against electronic parts, etc., and damage the substrate containing electronic parts. The defect that it cannot be deformed so as to cover the substrate (has poor adhesion) appears.

  As the resin, one type and / or two or more types having flexibility are selected. It is preferable to have an elastomer having a glass transition temperature of −20 ° C. or lower and an elastic modulus of 23 ° C. (0.1 Hz) of 1 MPa or lower in order to ensure the mechanical strength and appropriate elongation of the product. The resin can be selected from ordinary elastomers, adhesives, liquid rubbers and the like. The resin used as an auxiliary component is preferably compatible with the main component resin, and the glass transition temperature of the mixture may be higher than −20 ° C. or the elastic modulus at 23 ° C. (0.1 Hz) may shift to 1 MPa or more. However, it is preferable that the blending ratio is 30% by mass or less of the entire resin.

  More preferably, the resin is selected from pressure-sensitive adhesives including at least an addition reaction type or peroxide-crosslinking type silicone pressure-sensitive adhesive, an acrylic pressure-sensitive adhesive, and a urethane-based pressure-sensitive adhesive, and a cured product thereof is selected. Is desirable.

  Silicone adhesives basically have the properties of a silicone resin, with excellent heat resistance, excellent flexibility, and little change over time, making them suitable as base materials for this radio wave absorber. . The silicone adhesive before curing includes an addition reaction type and a peroxide curing type. However, since no decomposition product is generated, an addition reaction type that can avoid problems such as odor and corrosion is preferred.

  There are two types of acrylic pressure-sensitive adhesives: an emulsion type and an emulsion type, but it is preferable to select a solvent type with a high degree of polymerization.

  The urethane-based adhesive is preferably a urethane resin obtained by reacting an isocyanate group-terminated urethane prepolymer with a chain extender, a terminal terminator, and a polyisocyanate compound, and a solvent using the same solvent. It is preferable to use a mixture with a type acrylic adhesive or the like.

  According to the present invention, 60 parts by weight or more of graphite fine powder including at least graphite having an average particle diameter of 1 μm or more and 7 μm or less and a solvent as necessary are mixed with 60 parts by weight of the resin before curing. The graphite fine powder is uniformly dispersed in the resin by stirring, and the mixture in which the graphite fine powder is uniformly dispersed is heated to volatilize the added solvent and to cure the resin. The manufacturing method of the electromagnetic wave absorber containing is provided.

(Examples 1-2)
100 parts of silicone adhesive (SD4580 (solid content 60%) manufactured by Toray Dow Corning Silicone Co., Ltd.) and curing agent (SRX212 manufactured by Toray Dow Corning Silicone Co., Ltd.) 0.6 parts, 160 parts of toluene, and 60 to 120 parts of graphite fine powder (manufactured by Nippon Graphite Industry Co., Ltd.) are mixed and dispersed by stirring using a reciprocating rotary stirrer agitator (manufactured by Shimazaki Seisakusho Co., Ltd.). I let you. This was coated on the surface of a PET film treated with a fluorine release agent using an applicator. It was put in an oven at 100 ° C. to volatilize toluene, and the silicone adhesive was subjected to a curing reaction. The heating time varies depending on the formulation, but is 3 to 5 minutes. The cured product was peeled from the PET film to obtain a noise suppression sheet.

(Example 3)
Acrylic adhesive (SK Dyne 1717 (solid content 45%) manufactured by Soken Chemical Co., Ltd.), 100 parts (ie 45 parts of resin solid content) and a curing agent (Coronate L manufactured by Nippon Polyurethane Co., Ltd.) 0.5 Part, 140 parts of toluene and 90 parts of graphite fine powder (manufactured by Nippon Graphite Industry Co., Ltd.) were mixed, and the following obtained a noise suppression sheet in the same process as in Examples 1-2.

Example 4
Urethane pressure-sensitive adhesive (PA16035 manufactured by Asahi Glass Co., Ltd. (solid content 50%)) 100 parts (that is, resin solid content 50 parts) 1.0 part of curing agent (Japan Polyurethane Co., Ltd. Coronate L), toluene 140 Part and 90 parts of graphite fine powder (manufactured by Nippon Graphite Industry Co., Ltd.) were mixed, and the following obtained a noise suppression sheet in the same process as in Examples 1-2.

(Example 5)
60 parts of urethane adhesive (Asahi Glass Co., Ltd. PA16035 (solid content 50%)), 40 parts of the acrylic adhesive (namely, 30 + 18 parts of solid resin mold) and a curing agent (Coronate L manufactured by Nippon Polyurethane Co., Ltd.) ) 1.0 part, 140 parts of toluene and 100 parts of graphite fine powder were mixed, and a noise suppression sheet was obtained in the same process as in Examples 1 and 2 below.

  Table 1 shows the specific surface area, average particle diameter, crystallinity, and particle size of the graphite fine powder used and carbon black (Ketjen Black EC manufactured by Lion Corporation) used as carbon powder instead of graphite fine powder as Comparative Example 1. The shape of is shown. Table 2 shows the compounding ratio, the thickness after drying and curing, and the type A durometer hardness in each example and comparative example.

  The average particle diameter was measured by a laser diffraction method. Using a “Coulter N4 plus submicron particle size distribution measuring device” manufactured by Beckman Coulter Co., Ltd., a sample solution in which a measurement target is stirred and dispersed in water (ion-exchanged water) in a polystyrene transparent cell in a 25 ° C. environment. Was measured.

  The specific surface area was measured by a gas adsorption method. Using “BELSORP 28SA” manufactured by Nippon Bell Co., Ltd., a sample was measured in an environment of 25 ° C. with an adsorbate N 2 gas, an air thermostat temperature of 40 ° C., an adsorption temperature of 77 K, and an initial introduction pressure of 0 Torr.

  The type A durometer hardness was measured according to the procedure as defined in JIS_K6253-1997. The measurement temperature was 25 ° C.

  With respect to the obtained noise suppression sheet (50 mm × 100 mm), the amount of absorption and the reflection loss were measured in the frequency range of 0.12 to 1.81 GHz by a transmission attenuation power ratio measurement system TF-3B manufactured by Keycom. The results and the surface state of the sheet are shown in Table 3.

  As is clear from the results shown in Table 3, the samples of Examples 1 and 2 are excellent in electromagnetic wave absorption at 3 GHz or less and have a good reflection loss.

(Examples 6 to 7)
100 parts of silicone adhesive (KR3701 manufactured by Shin-Etsu Chemical Co., Ltd. (solid content 60%)) and curing agent (CAT-PL-50T manufactured by Shin-Etsu Chemical Co., Ltd.) 0. Of 5 parts, 160 parts of toluene, and graphite fine powder of Table 4 (manufactured by Nippon Graphite Industry Co., Ltd.), 2 types each containing graphite C having an average particle diameter of 6 μm are selected, a total of 120 parts are mixed, and a reciprocating rotary type The mixture was stirred and dispersed using an agitator agitator (manufactured by Shimazaki Seisakusho). This was coated on the surface of a PET film treated with a fluorine release agent using an applicator. It was put in an oven at 100 ° C. to volatilize toluene, and the silicone adhesive was subjected to a curing reaction. The heating time varies depending on the formulation, but is 3 to 5 minutes. The cured product was peeled from the PET film to obtain a noise suppression sheet.

(Example 8)
Acrylic adhesive (SK Dyne 1717 (solid content 45%) manufactured by Soken Chemical Co., Ltd.) 140 parts (namely resin solid content 63 parts) and hardener (Nihon Polyurethane Co., Ltd. Coronate L) 0.6 parts In addition, 140 parts of toluene and graphite fine powder of Table 4 (manufactured by Nippon Graphite Industry Co., Ltd.) were selected from 2 types including graphite C having an average particle diameter of 6 μm, and a total of 120 parts were mixed. The noise suppression sheet was obtained in the same process as 7.

Example 9
130 parts urethane adhesive (Asahi Glass Co., Ltd. PA16035 (solid content 50%)), 130 parts (that is, 65 parts resin solid content) Curing agent (Coronate L, Nippon Polyurethane Co., Ltd.) 1.0 part, Toluene 140 2 parts of graphite fine powder (manufactured by Nippon Graphite Industry Co., Ltd.) of Table 4 and graphite C having an average particle diameter of 6 μm are selected, a total of 120 parts are mixed, and the following is the same as in Examples 6-7 A noise suppression sheet was obtained in the process.

(Example 10)
In Example 6-7, only 1 type of graphite fine powder C (manufactured by Nippon Graphite Industry Co., Ltd.) in Table 4 was selected for the silicone-based pressure-sensitive adhesive, curing agent and toluene, and 120 parts were mixed. The noise suppression sheet | seat was obtained in the same process as ~ 2.

(Examples 11 to 13)
Except for the graphite-based fine powder C shown in Table 4 to the silicone-based pressure-sensitive adhesive, curing agent, and toluene of Examples 6 to 7, as one of graphite A, graphite B, or graphite D, the number of blending parts is the same. The noise suppression sheet was obtained in the same process as 6-7.

(Example 14)
The acrylic pressure-sensitive adhesive, curing agent, and toluene of Example 8 were made of only graphite A1 and the number of blending parts was the same. A noise suppression sheet was obtained in the same process as in Example 8 below.

(Example 15)
The urethane-based pressure-sensitive adhesive, curing agent, and toluene of Example 9 were made of only graphite B1, and the number of blending parts was the same.

(Comparative Examples 2 and 3)
The silicone-based pressure-sensitive adhesive, curing agent, and toluene of Examples 6 to 7 are one of the graphite E or graphite F of Table 4 and the number of blending parts is the same. A sheet was obtained.

(Comparative Example 4)
As the silicone-based pressure-sensitive adhesives and curing agents of Examples 6 to 7, only the carbon black of Table 4 and 140 parts of toluene were used, and the noise suppression sheets were obtained in the same steps as in Examples 6 to 7 below.

(Comparative Example 5)
120 parts of nickel particle powder was mixed with the silicone-based pressure-sensitive adhesives, curing agents, and toluene of Examples 6 to 7, and the following obtained noise suppression sheets in the same process as Examples 6 to 7.

  Table 4 shows the properties of the graphite fine powder used in each Example and Comparative Example.

  Table 5 shows the compounding ratio, the thickness after drying / curing, the surface state of the sheet, and the type A durometer hardness in each example and comparative example. Moreover, about the obtained noise suppression sheet | seat (50 mm x 100 mm), the electromagnetic wave absorption loss and reflection loss measured by Keycom transmission attenuation power ratio measurement system TF-3B are shown.

  The average particle diameter, specific surface area, and type A durometer hardness are measured in the same manner as in Examples 1 to 5 and Comparative Example 1 described above.

  As is clear from the results shown in Table 5, the samples of Examples 6 to 15, particularly Examples 6, 7, and 10, are excellent in electromagnetic wave absorption at 3 GHz or less, and the reflection loss is at a level with no particular problem.

Claims (15)

  A resin having flexibility and a scaly graphite powder uniformly dispersed in the resin, the graphite powder being contained in an amount of 60 parts by weight or more with respect to 60 parts by weight of the resin, and the average particle diameter of the graphite powder being 1 μm or more An electromagnetic wave absorber that is 50 μm or less.   The radio wave absorber according to claim 1, wherein the graphite powder has an average particle size of 1 μm or more and 7 μm or less. The radio wave absorber according to claim 1 or 2, wherein the specific surface area of the graphite powder is 30 m ² / g or less. The radio wave absorber according to claim 1, wherein the specific surface area of the graphite powder is 17 m ² / g or less.   The radio wave absorber according to any one of claims 1 to 4, wherein the graphite powder is spherical graphite.   6. The radio wave absorber according to claim 1, wherein the resin is mainly composed of an elastomer having a glass transition temperature of −10 ° C. or lower, an elastic modulus of 1 MPa or lower at 23 ° C. and 0.1 Hz.   The radio wave absorber according to any one of claims 1 to 6, wherein a type A durometer hardness defined in JIS_K6253-1997 is 20 or more and 70 or less.   The radio wave absorber according to any one of claims 1 to 7, wherein the resin is obtained by curing an addition reaction type or peroxide-crosslinking type silicone adhesive.   The radio wave absorber according to claim 1, wherein the resin is obtained by curing an acrylic adhesive.   The radio wave absorber according to claim 1, wherein the resin is obtained by curing an adhesive containing at least a urethane-based adhesive.   The radio wave absorber according to claim 1, wherein the radio wave absorber has a tensile strength of 0.3 MPa or more and a tensile elongation of 50% or more.   60 parts by weight or more of the resin before curing, graphite fine powder having an average particle size of 1 μm or more and 50 μm or less, and a solvent as necessary are mixed with 60 parts by weight or more, and the mixture is stirred into the resin. Radio wave absorption including uniformly dispersing the graphite fine powder, volatilizing the solvent added as necessary by heating the mixture in which the graphite fine powder is uniformly dispersed, and curing the resin A method of manufacturing the material.   The method according to claim 12, wherein the resin before curing is an addition reaction type or peroxide cross-linked type silicone adhesive.   The method according to claim 12, wherein the resin before curing is an acrylic pressure-sensitive adhesive.   The method according to claim 12, wherein the resin before curing is an adhesive containing at least a urethane-based adhesive.