JP2006093287A - Wave absorption sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wave absorption sheet which has a superior wave absorbing property regardless of a wave incident angle in a GHz radio frequency band and is light in weight and thin, and low in cost. <P>SOLUTION: The wave absorption sheet uses a binder with a dielectric constant between 2.2 and 3.2 wherein anisotropic graphite is evenly dispersed. It is preferred that the binder has a dielectric dissipation factor (tanδ) of 0.012-0.12 and has a dielectric constant of 0.012-0.12 (s/m), and expanded graphite sheet pulverized powder is used for the anisotropic graphite. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a radio wave absorption sheet for preventing malfunction of a device (for example, ETC: automatic fee payment system: activation frequency: 5.8 GHz) that functions in a radio frequency GHz band used in narrow area communication, and more particularly, radio wave absorption. The present invention relates to an electromagnetic wave absorbing sheet that is excellent in characteristics, economy, light weight, and workability.

  Recently, 5.8 GHz band, 76. The application development of the narrow-band communication (DSRC) system using the radio wave of GHz band is actively performed, and one of the application examples is ETC (non-stop automatic fee payment system, starting frequency: 5.8 GHz), millimeter wave radar. (Startup frequency: 76.5 GHz) has been put into practical use, contributing to comfortable driving and safety for drivers.

  In the future, the above system is expected to expand its application to consumer fields such as general parking lots, drive-throughs, and gas stations. However, DSRC systems including ETC are subject to system malfunction due to multiple turbulence between transmitting and receiving antennas. As the antenna accuracy has been improved, the development of radio wave absorbers that absorb multiple turbulences accurately has been actively conducted.

  The nature of the electromagnetic wave absorber can be broadly divided into panel type, sheet type, paint type, etc., with the diversification of the places where it is used, but the panel type installed near the current ETC passage gate is common, Some sheets are used. Also, millimeter wave radar-mounted absorbers are often in sheet form.

  In particular, the panel-type electromagnetic wave absorber for ETC that is currently used has a radio wave absorber (graphite, Magnetic powder and inorganic powder mixture) are mixed to achieve both weight reduction and radio wave absorption. However, the panel type electromagnetic wave absorber has problems such as large volume, significantly lowering the construction efficiency, difficult to construct the curved part, and high cost because it protects the inside with a tough cover. Yes.

In addition, sheet-type radio wave absorbers have been developed and partially used to improve workability, but because they contain high-cost magnetic powder in the binder, they become heavier and thicker, and the amount of absorption against obliquely incident radio waves is increased. Therefore, it is necessary to stack a plurality of sheets having different performances, and nothing is perfect in terms of economy and workability.
JP 2002-348987 A

  The present invention provides a radio wave absorption sheet that is excellent in radio wave absorption characteristics regardless of the radio wave incident angle in the radio frequency GHz band, is light and thin, and is inexpensive.

The present invention relates to the following.
1. A lightweight electromagnetic wave absorbing sheet using a binder having a relative dielectric constant of 2.2 to 3.2 and having anisotropic graphite uniformly dispersed in the binder.
2. Item 2. The radio wave absorption sheet according to Item 1, wherein the binder has a dielectric loss tangent (tan δ) in the range of 0.012 to 0.12.
3. Item 3. The radio wave absorption sheet according to item 1 or 2, wherein the binder has a conductivity in a range of 0.012 to 0.12 (S / m).
4). Item 2. The radio wave absorption sheet according to Item 1, wherein the anisotropic graphite is expanded graphite.
5. Item 5. The radio wave absorption sheet according to Item 4, wherein the expanded graphite powder is an expanded graphite sheet pulverized powder.

  The radio wave absorption sheet of the present invention is a radio wave absorption sheet that absorbs stable radio waves, absorbs stable radio waves regardless of the radio wave incident angle in the radio frequency GHz band, is light and thin, and is inexpensive. Very suitable.

In the present invention, anisotropic graphite is uniformly dispersed in a binder having electrical characteristics within a certain range, thereby forming a function sharing in which moderately dispersed anisotropic graphite absorbs radio waves having different incident angles. is doing.
In the present invention, a light-weight and thin wave absorbing sheet is further obtained.
In the present invention, it is preferable to use expanded graphite powder as anisotropic graphite.

  Although there is no restriction | limiting in particular about the expanded graphite powder used for this invention, If importance is attached to cost, it is preferable to use natural graphite and artificial graphite as raw material graphite. Although there is no restriction | limiting also about the particle size of the graphite to be used, The graphite from which a particle size differs can be mixed and used in consideration of a required characteristic. The form of expanded graphite powder to be used is not limited, and the obtained expanded graphite powder may be used as it is, and expanded graphite sheet pulverized powder obtained by processing expanded graphite into a sheet shape and pulverizing it can be used.

  In addition, when using a kneader, a mixer, etc. as a method of dispersing the expanded graphite powder to be used as a preferable material in the binder, considering the change in radio wave absorption due to expansion graphite powder destruction during mixing, the strength It is preferable to use an expanded graphite sheet pulverized powder that is excellent in terms of quality.

There is no particular limitation on the method for producing the expanded graphite. For example, the step of immersing raw graphite in a solution containing an acidic substance and an oxidizing agent to form a graphite intercalation compound, and heating the graphite intercalation compound, the C axis of the graphite crystal It can be manufactured by a process of expanding the direction to obtain expanded graphite. As a result, the expanded graphite has a worm-like shape and is intricately intertwined.

  The magnification of the expanded graphite is not particularly limited, but is preferably 100 times or more and more preferably 100 times to 500 times in consideration of radio wave absorption characteristics. If expanded graphite having an expansion ratio of less than 100 times or more than 500 times is used, the radio wave absorption region tends to fluctuate easily.

If necessary, the expanded graphite is heat-treated at a higher temperature to remove impurities contained in the expanded graphite. It is preferable to pulverize and classify the expanded graphite powder and use the desired expanded graphite separately.
When the expanded graphite powder is used with increased purity, it is used after being subjected to a high temperature treatment.

Although there is no restriction | limiting in particular as said raw material, The graphite with which the crystal | crystallization developed highly, such as natural graphite, a bush graphite, and a pyrolytic graphite, is mentioned as a preferable thing. Natural graphite is preferable in consideration of the balance between obtained characteristics and economic efficiency.
There is no restriction | limiting in particular as natural graphite to be used, Commercial products, such as F48C (brand name made from Nippon Graphite Co., Ltd.) and H-50 (brand name made from Chuetsu Graphite Co., Ltd.), can be used. These are preferably used in the form of scaly powder.

  As the acidic substance used for the processing of the raw material graphite, one that can enter an interlayer of graphite such as sulfuric acid and generate an acidic root (anion) having sufficient expansion ability is generally used. There is no restriction | limiting in particular about the usage-amount of an acidic substance, It determines with the target expansion ratio, For example, it is preferable to use 100 weight part-1000 weight part with respect to 100 weight part of graphite.

  As the oxidizing agent used together with acidic substances, peroxides such as hydrogen peroxide, potassium perchlorate, potassium permanganate, potassium dichromate, and acids having an oxidizing action such as nitric acid can be used. Hydrogen peroxide is particularly preferred from the viewpoint of easy obtaining of expanded graphite.

  When hydrogen peroxide is used as the oxidizing agent, it is preferably used as an aqueous solution. At this time, the concentration of hydrogen peroxide is not particularly limited, but is preferably in the range of 20 wt% to 40 wt%. Although there is no restriction | limiting in particular also about the usage-amount, It is preferable to mix | blend in 5 to 60 weight part as hydrogen peroxide with respect to 100 weight part of graphite.

The acidic substance and the oxidizing agent are preferably used in the form of an aqueous solution.
Sulfuric acid as an acidic substance is used at an appropriate concentration, but is preferably 95% by weight or more, and particularly preferably concentrated sulfuric acid.

  The acid-treated graphite obtained by the above method is washed with a large amount of water to remove excess acidic substances, and then dried to remove moisture, whereby the acid-treated graphite as a raw material for expanded graphite is obtained. Then, the acid-treated graphite is heated at a temperature of 1000 ° C. or higher to obtain expanded graphite.

  Expanded graphite obtained by the above method can also be used as the radio wave absorber of the present invention, but considering the miscibility of the binder, destruction of the graphite at the time of mixing, the expanded graphite is once made into a sheet (high density), It is preferable to use a pulverized one.

The method for forming expanded graphite into a sheet is not particularly limited, but in general, the expanded graphite obtained above is preferably formed into a sheet by applying pressure with a press, a roll or the like. There are no particular restrictions on the thickness and bulk density of the expanded graphite sheet, but the thickness is in the range of 0.5 mm to 1.5 mm and the bulk density is 0.2 g / cm ^{3 to} 1.7 g /. Those in the range of cm ³ are preferred. When the thickness is less than 0.5 mm, there is a tendency to cause a reduction in work in the pulverization process (the sheet breaks brittlely during handling), and when it exceeds 1.5 mm, pulverization tends to be difficult.

Moreover, when the bulk density is less than 0.2 g / cm ³ , the radio wave absorption characteristics of the obtained radio wave absorption sheet tend to be lowered, and when it exceeds 1.7 g / cm ³ , the flexibility of the radio wave absorption sheet tends to be lowered. There is.
The magnitude of the bulk density can be adjusted by adjusting the amount of pressurization, the roll gap, and the like. Further, the pulverization of the expanded graphite sheet is preferably performed by coarse pulverization and fine pulverization, and thereafter, classification is performed as necessary.

  The average particle size of the expanded graphite sheet pulverized powder is not particularly limited, but in consideration of the thickness and the amount of radio wave absorption of the obtained radio wave absorbing sheet, the number average particle size is preferably in the range of 20 μm to 100 μm, and 30 μm to 80 μm. A range is further preferred. If the number average particle size is less than 20 μm, the anisotropy of the expanded graphite sheet pulverized powder tends to be small, and there is a tendency that the target radio wave absorption amount cannot be obtained. Tend to occur.

  The binder used in combination with the anisotropic graphite functioning as a radio wave absorber has a relative dielectric constant (real part) in the range of 2.2 to 3.2, preferably 2.3 to 3.0. If a material having a relative dielectric constant deviating from the above range is used, the electrical property balance with anisotropic graphite is lost, and the desired radio wave absorption property cannot be obtained.

  If the relative dielectric constant falls within the above range, the type (molecular structure, molecular weight, solvent used) can be mixed uniformly with anisotropic graphite and the obtained radio wave absorbing sheet exhibits elasticity. Can be used regardless of the presence or absence).

In addition, the binder used in the present invention preferably has a dielectric loss tangent (tan δ) in the range of 0.012 to 0.12, in relation to the balance of electrical properties with anisotropic graphite. The range of is more preferable.
Further, the electrical conductivity, which is related to the value of the relative dielectric constant and the dielectric loss tangent, is also in the range of 0.012 to 0.12 (S / m) in the relationship with the balance of electrical characteristics with the anisotropic graphite as described above. The range of 0.01 to 0.1 (S / m) is more preferable.

  Considering the above problems, it is preferable to use an elastic binder as the binder used in the present invention. Particularly preferred are modified polymer materials that are safe and can be handled by anyone. If necessary, a flame retardant can be added within a range that does not disturb the balance of electrical characteristics, and the radio wave absorbing sheet can be made flame retardant.

  The above-mentioned anisotropic graphite and binder are mixed and dispersed uniformly and processed into a paste form (a radio wave absorbing paste). There is no restriction | limiting in particular about the method of mixing, A general raking machine and a high-viscosity substance mixer can be used. In addition, it is preferable to use a mixer having a device capable of removing air entrained during mixing in order to stabilize characteristics and reduce variations between lots.

The method of processing the paste obtained by mixing into a sheet is not particularly limited, but a coating film feeding device for feeding the coating film, a paste coating device for coating the paste, and drying the solvent If a continuous sheet manufacturing apparatus in which a solvent drying apparatus for winding a sheet and a sheet winding apparatus for winding a sheet are integrated is used, the accuracy (thickness and absorption characteristics) of the resulting coated absorbent sheet is stable, and Since an electromagnetic wave absorption sheet can be manufactured at low cost, it is preferable.
In addition, if the continuous manufacturing apparatus shown above is provided, there will be no restrictions on the shape, dimensions, heat source, etc. of each mechanism.

  The final form of the radio wave absorption sheet is obtained by laminating the coating absorption sheet obtained as described above to a thickness corresponding to each radio wave absorption region (one side becomes a radio wave reflection plate). There are no particular restrictions on the method of laminating the coating absorbent sheet, and there are a method of using a heated roll with a gap adjusted, a method of uniformly applying a load to the top of the laminated coating absorbent sheet, and heating to integrate.

Hereinafter, the present invention will be described by way of examples.
Example 1
(1) Selection of Elastic Binder (Modified Polyamideimide Resin) An elastic binder (made by Hitachi Chemical Co., Ltd., trade name SN-9000CSEN) having a resin content of 46% by weight using γ-butyrolactone as a solvent was selected.

The electrical properties of the elastic binder alone (no heat treatment and no solvent) are as follows: the relative permittivity (real part) is 2.81, the dielectric loss tangent (tan δ) is 0.022, and the conductivity is 0.02 (s / m). there were. The measurement conditions for these electrical characteristics are shown below.
Sample size: 1 mm x 100 mm and 300 mm
Measurement frequency: 5.8 GHz
Measuring method: Propagation delay method CutBack type Measuring instrument: Vector network analyzer Anritsu 37
Measurement temperature: 25 ° C

(2) Production of radio wave absorbing powder (expanded graphite powder) Expanded graphite sheet having a plate thickness of 1.0 mm and a bulk density of 1.0 g / cm ³ (trade name Carbofit HGP-105, manufactured by Hitachi Chemical Co., Ltd.) Were pulverized with a coarse pulverizer and a fine pulverizer, and the obtained pulverized powder was classified to obtain an expanded graphite sheet pulverized powder having a number average particle size of 100 μm.

(3) 1000 g of an elastic binder for producing a mixed paste, 33 g of an epoxy resin (product name YH434L, manufactured by Toto Kasei Co., Ltd.), 110 g of an expanded graphite sheet pulverized powder having a number average particle size of 100 μm obtained in (2) and γ − 200 g of butyrolactone and a small amount of antifoam were weighed into a mixing mortar and mixed lightly with a pestle. This was further mixed for 40 minutes using a biaxial automatic mixer (Ishikawa Stirrer, Model 24). The obtained mixed paste was further placed in a vacuum dryer heated to 70 ° C., decompressed for 20 minutes, and air entrained during mixing was removed to obtain a mixed paste for a radio wave absorbing sheet.

  Next, the mixed paste for a radio wave absorbing sheet obtained above is a tunnel type equipped with a coating film feeding device, a paste coating device (gap roll) having a coating width of 32 cm, and a solvent combustion device having a length of 2 m. Using a continuous sheet manufacturing apparatus having a product winding apparatus using a drying apparatus and a winding roll, a coated continuous absorbent sheet coated on a polyethylene film and dried was obtained.

(4) Production of radio wave absorption sheet After the coating continuous absorption sheet obtained in (3) was cut to a size of 30 cm × 30 cm, the coating film was peeled off, and the coating absorption was cut as described above with the coating direction as the upper part. One sheet is attached to an aluminum plate having a thickness of 0.1 mm, and then, nine coating absorption sheets cut with the coating direction as the upper portion are stacked on the upper surface in the same manner as described above. A radio wave absorbing sheet (elastic sheet) having a mean thickness of 1.9 mm and a sheet density of 1.33 g / cm ³ is passed through a heat roll adjusted to 1 mm at a speed of 0.5 m / min. Obtained.

Comparative Example 1
The same material as in Example 1 was used except that the binder was changed to commercially available styrene-butadiene rubber (SBR: solvent system), and the average thickness of the radio wave absorption layer was 1.92 mm through the same steps as in Example 1. And the electromagnetic wave absorption sheet | seat (elastic sheet) whose sheet density is 1.32 g / cm < ³ > was obtained.
When the electrical properties of the elastic binder alone (without heat treatment and solvent) were measured under the same conditions as in Example 1, the relative dielectric constant (real part) was 3.65, the dielectric loss tangent (tan δ) was 0.09, and the electrical conductivity was It was 0.08 (s / m).
In addition, Table 1 shows working conditions for obtaining the coated continuous absorbent sheet in Example 1 and Comparative Example 1.

  Next, using the radio wave absorption sheets obtained in Example 1 and Comparative Example 1, the radio wave absorption amount at an absorption region of 5.8 GHz was measured. The results are shown in Table 2. The method for measuring the amount of radio wave absorption is as follows.

Measuring machine: Keycom Co., Ltd., electromagnetic wave absorption material measurement system (lens antenna type free space method)
Horn antenna: WR159 type with coaxial waveguide converter Measurement wave: Circular polarization Measurement range: 45 MHz to 20 GHz (vector network analyzer)
Measurement frequency: 5.8 GHz (return loss)

As shown in Table 2, the radio wave absorption sheet obtained in Example 1 showed a larger radio wave absorption amount than the radio wave absorption sheet obtained in Comparative Example 1.

Claims (5)

  An electromagnetic wave absorbing sheet using a binder having a relative dielectric constant of 2.2 to 3.2 and having anisotropic graphite uniformly dispersed in the binder.   The radio wave absorption sheet according to claim 1, wherein the binder has a dielectric loss tangent (tan δ) in a range of 0.012 to 0.12.   The radio wave absorption sheet according to claim 1 or 2, wherein the binder has a conductivity in a range of 0.012 to 0.12 (S / m).   The radio wave absorption sheet according to claim 1, wherein the anisotropic graphite is expanded graphite powder. The radio wave absorption sheet according to claim 4, wherein the expanded graphite powder is an expanded graphite sheet pulverized powder.