JP2009088379A - Fire-resistant radio-wave absorbing sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inexpensive and light-weighted fire-resistant radio-wave absorbing sheet which is excellent in workability or flame retarder. <P>SOLUTION: This fire-resistant radio-wave absorbing sheet is configured by dispersing graphite powder in an elastic resin binder, wherein the amounts of radio-wave absorption are equal to or more than 20 dB in the region of frequency 2.4 to 2.5 GHZ. This fire-resistant radio-wave absorbing sheet is configured by dispersing graphite powder and viscosity control materials in the elastic resin binder, wherein the amounts of radio-wave absorption are equal to or more than 20 dB in the region of frequency 2.4 to 2.5 GHZ. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a 2.4 GHz band radio wave absorption sheet, which is a frequency region of a wireless LAN. More specifically, the radio wave absorption layer is made of a mixture of polyester resin and phosphate ester as a binder and special graphite (expanded graphite sheet pulverized powder). The present invention relates to a general-purpose type flame retardant electromagnetic wave absorbing sheet that is lightweight and excellent in workability and economy.

In recent years, road traffic support systems (ETC, collision prevention systems) using radio waves in the GHz band and wireless LANs accompanying diversification of information communication have begun to spread rapidly, and are expected to continue to expand in the future.
In particular, with the development of the information society, wireless communication technology (wireless LAN) is significantly improved, and a large amount of information can be obtained instantaneously.

  However, in the wireless LAN, there is a concern about an error motion due to multiple reflected waves of radio waves from adjacent buildings or radio waves generated from the same building, which leads to a security leakage accident which is currently a problem.

  In such a situation, as one method for avoiding the malfunction, there is a method of absorbing the generated multiple turbulent waves by using a radio wave absorber suitable for the used frequency band.

  However, current radio wave absorbers for wireless LAN are composed of inorganic boards, resistive coatings, inorganic honeycombs, metal plates, etc., and not only have high material costs, but also their processing (cutting) and handling during construction. In addition to paying attention to the above, there are problems such as a long construction time.

In addition, even a sheet-type absorber, which is characterized by excellent workability, uses a large amount of magnetic metal powder in the radio wave absorption layer, and thus has a problem of being heavy, resulting in deterioration of workability.
Under such circumstances, the present inventors have proposed a (5.8 GHz) multiple electromagnetic wave absorbing sheet for ETC, which is excellent in weight reduction using a special elastic binder and pulverized expanded graphite sheet (for example, patent document). See).

However, the high-boiling polar solvent, which is a special elastic binder resin solvent, remains in the sheet and has a problem that antimony trioxide and bromide are used in combination for flame retardancy.
JP 2006-080352 A

  The present invention is a low-cost, lightweight and workable flame retardant by combining a general-purpose resin (using a general-purpose solvent) and special graphite as a malfunction-preventing electromagnetic wave absorbing sheet for wireless LAN (2.4GHZ). The present invention provides a flame retardant electromagnetic wave absorbing sheet using a phosphoric ester.

The present invention relates to the following matters.
(1) A flame retardant radio wave absorption sheet comprising graphite powder dispersed in an elastic resin binder and having a radio wave absorption amount of 20 dB or more in a frequency range of 2.4 to 2.5 GHz.

  (2) A flame retardant radio wave absorption sheet comprising graphite resin and a viscosity adjusting material dispersed in an elastic resin binder, and having a radio wave absorption amount of 20 dB or more in a frequency range of 2.4 to 2.5 GHz.

(3) The flame-retardant radio wave absorption sheet according to the above (1) or (2), wherein the binder resin is a mixture of a polyester resin and a phosphate ester.
(4) The flame-retardant radio wave according to any one of the above (1) to (3), wherein the graphite powder is an expanded graphite sheet pulverized powder and contains 40% by weight or more of particles having a particle size of 60 to 200 μm. Absorption sheet.

(5) The flame-retardant electromagnetic wave absorbing sheet according to (3) or (4), wherein the phosphate ester functions as an elasticity-imparting material and a flame-retardant material.
(6) The flame-retardant radio wave absorbing sheet according to any one of (2) to (5), wherein the viscosity adjusting material is styrene gel beads.
(7) The flame-retardant electromagnetic wave absorbing sheet according to any one of (1) to (6), wherein the bulk density of the sheet is 1.3 g / cm ³ or less.

  ADVANTAGE OF THE INVENTION According to this invention, the flame-retardant electromagnetic wave absorption sheet which uses phosphate ester as a low-cost, lightweight, and workable favorable flame retardant can be provided.

The binder of the radio wave absorbing sheet according to the present invention is preferably a mixture of a general-purpose type polyester resin and a phosphoric ester for imparting flexibility and flame retardancy.
In addition, it uses a graphite powder (expanded graphite sheet pulverized powder) to adjust the dielectric constant, and is a wireless LAN-compatible flame-retardant electromagnetic wave absorbing sheet that can be reduced in weight at a low cost.

The graphite to be used is not particularly limited, but if cost is an important issue, expanded graphite powder using natural graphite or artificial graphite as raw graphite is preferably sheeted and pulverized.
In addition, there is no particular limitation on the particle size of the graphite, but there is no classification step if considering economy, and there is no waste graphite powder generated in the classification step, and the particle size can be pulverized with a normal coarse pulverizer. 40% by weight or more of the particles (dry particle size distribution using a sieve: the pulverized powder is anisotropic, and graphite having a large particle size passes through the sieve through the sieve opening) Graphite powder is preferred. Of course, in consideration of required characteristics, graphites having different particle sizes selected by classification can be mixed and used.

There is no restriction | limiting in particular in the polyester resin to be used, It is preferable to use resin which has an ester bond in the molecular structure which used styrene as a solvent.
Further, the phosphoric acid ester to be used is not particularly limited, and an ester obtained by substituting hydrogen of orthophosphoric acid with an alkyl group or an allyl group is used. The elastic binder resin used in the present invention is uniformly mixed with the polyester resin. Produced.
The mixing ratio of the polyester resin and the phosphate ester is arbitrarily determined depending on the required sheet elastic modulus and the level of flame retardancy to be imparted.

  For example, the optimum blending value of the polyester resin and phosphate ester is preferably in the range of 10 to 30 parts by weight of phosphate ester with respect to 100 parts by weight of the polyester resin used. When the blending amount of the phosphoric acid ester is less than 10 parts by weight, it is difficult to impart flame retardancy, and when the blending amount exceeds 30 parts by weight, stickiness tends to occur on the produced sheet surface.

In the present invention, a viscosity adjusting material may be used for stably mixing the binder resin and the graphite powder (preventing the precipitation of the graphite powder).
As the viscosity adjusting material to be used, various materials such as inorganic powder and organic powder are conceivable, but considering the stabilization of dielectric constant, which is a point of mixing property and radio wave absorption characteristics, the average particle size is in the range of 5 to 100 μm. Styrene beads (styrene gel beads) are preferred.

When the particle size of the styrene beads is less than 5 μm, it is difficult to uniformly disperse the beads because there are many aggregated beads. On the other hand, when the particle diameter exceeds 100 μm, the effect of preventing the precipitation of graphite powder is reduced.
The amount of styrene beads to be used is arbitrarily determined depending on the viscosity (molecular weight / solvent amount / molecular structure) of the polyester resin to be used.

  There is no limitation on the production method of expanded graphite as a raw material of graphite. For example, a 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 to form graphite It can be manufactured by a process of expanding the C-axis direction of the crystal to obtain expanded graphite. As a result, the expanded graphite has a worm-like shape and is intricately intertwined.

The expansion ratio of the expanded graphite is not particularly limited, but is preferably 150 times or more and more preferably 150 to 300 times in consideration of radio wave absorption characteristics.
The expanded graphite is made into a sheet, the density is increased, and the use of pulverized graphite powder makes it possible to suppress the tendency of the graphite powder to collapse, and radio waves are absorbed by a heavy technique to uniformly disperse the graphite powder in the binder resin. Stabilization of characteristics can be achieved.

There is no restriction | limiting in the method to form into a sheet, For example, it can form into a sheet with a roll. The pulverized graphite powder obtained by pulverizing the processed sheet becomes the graphite powder used in the present invention.
Although there is no restriction | limiting in particular as said raw material graphite, The graphite with which the crystal | crystallization developed highly, such as natural graphite, wood graphite, and 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 said natural graphite, Commercially available products, such as F48C (Nippon Graphite Co., Ltd. make, brand name) and H-50 (Nakagetsu Graphite Co., Ltd. brand name), can be used. These are preferably used in the form of scales.

  As the acidic substance used for the treatment of graphite, generally used is an acid substance that can penetrate between graphite layers such as sulfuric acid and generate acidic roots (anions) having sufficient expansion ability. 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 100-1000 weight part with respect to 100 weight part of graphite.

  The oxidizing agent used with the acidic substance may be a peroxide such as hydrogen peroxide, potassium perchlorate, potassium permanganate, or potassium dichromate, or an acid having an oxidizing action such as nitric acid. Hydrogen peroxide is particularly preferable from the viewpoint of being able to obtain good 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 5 to 60 parts by weight of hydrogen peroxide per 100 parts by weight of graphite. It is preferable to mix in a range.

The acidic substance and the oxidizing agent are preferably used in a water-soluble form. 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 method for producing the expanded graphite sheet is not particularly limited, but it is generally preferable to form the expanded graphite obtained above by applying pressure with a press, a roll or the like. The sheet thickness and bulk density when the expanded graphite is made into a sheet are not particularly limited, but those having a range of 0.5 to 1.5 mm and a bulk density of 0.2 to 1.7 g / cm ³ are available. preferable.

  When the thickness is less than 0.5 mm, workability deterioration (handling) in the pulverization step is caused, and when it exceeds 1.5 mm, pulverization tends to be difficult. Further, the sheet density tends to be the same as the above contents. The sheet thickness and density can be arbitrarily changed depending on the gap amount (pressure amount) of the roll.

Although there is no restriction | limiting in particular in the density of the expanded graphite sheet pulverized powder used by this invention, The range of 0.1-0.4 g / cm < ³ > is preferable by a bulk density. When the pulverized powder density is less than 0.1 g / cm ³ , it is difficult to obtain a target radio wave absorption characteristic, and when the density exceeds 0.4 g / cm ³ , there is a tendency that the mixing property with the binder resin is deteriorated. .

  The particle size of the pulverized sheet powder to be used is preferably a pulverized powder in which particles having a particle size of 60 to 200 μm account for 40% by weight or more based on the dry classification method using a sieve as described above. When the ratio is 40% by weight or less, there is a tendency that the elastic strength and radio wave absorption characteristics of the obtained sheet are lowered.

  The blending amount of the above pulverized powder to be used with respect to the binder resin is preferably in the range of 5 to 8 parts by weight with respect to 100 parts by weight of the binder. There is a tendency for the characteristics to deteriorate extremely.

The mixing method of the binder resin and the pulverized powder or the binder resin, the pulverized powder, and the viscosity adjusting material is not particularly limited. However, it is preferable in terms of cost and mixing property to mix using a kneader or a reiki machine.
Although there is no restriction | limiting in particular also about the method of making the said mixture into a sheet, It is preferable in terms of cost to use a heating type simple press. The bulk density of the sheet is preferably 1.3 g / cm ³ or less, and is formed into a sheet by the method as shown in Examples.

Hereinafter, the present invention will be described by way of examples.
Example 1
(1) Manufacture of binder resin to be used Polyester resin (manufactured by Day H. Matialia Co., Ltd., containing 25% by weight of styrene, trade name: Sandoma 1555) and phosphate ester (manufactured by Daihachi Chemical Industry Co., Ltd., trade name) : CR-733S) 185 g was weighed in a 3 L capacity mortar and mixed for 30 minutes by automatic mixing (room temperature). The obtained mixture was homogeneously mixed without separation.

(2) Manufacture of pulverized graphite powder An expanded graphite sheet (made by Hitachi Chemical Co., Ltd., trade name Carbofit: HGP-105) having a plate thickness of 1 mm and a bulk density of 1.0 g / cm ³ is mounted with a 1 mm screen. A product (average particle size: 300 μm) pulverized with a pulverizer (made by Hosokawa Micron Co., Ltd., trade name: Victory Mill VP-2) was used. In the obtained pulverized powder, particles having a particle size of 60 to 200 μm accounted for 55% by weight of the whole.

(3) Mixing of styrene beads and polyester resin curing agent To the mixture obtained in (1), an average particle size of 30 μm styrene beads (manufactured by Soken Chemical Co., Ltd., trade name: SGP-70C) and a polyester curing agent, 18 g of peroxide benzoyl SP (trade name, manufactured by Kawaguchi Pharmaceutical Co., Ltd.) was blended and mixed for 30 minutes at room temperature (25 ° C.).

(4) Mixing of pulverized graphite powder The mixture obtained in (3) is blended with 56.3 g of the pulverized powder prepared in (2), mixed at room temperature for 30 minutes using a raikai machine, defoamed and radio waves. A mixture serving as a base of the absorbent sheet was prepared. The viscosity of the obtained mixture was 122 Pa · s at 25 ° C. (viscosity measuring machine: RE-80U type, manufactured by Toki Sangyo Co., Ltd., trade name: rotor: R14).

(5) Manufacture of radio wave absorbing sheet Place a release-treated film on a 40 cm x 40 cm metal plate, put an 8.5 mm thick metal frame having a space of 35 cm x 35 cm, and place (4 The mixture prepared in (1) was uniformly filled.
A film subjected to a release treatment was placed on the entire surface of the filled mixture, and a metal plate of 40 cm × 40 cm was further placed.

This was placed on the lower stage of a 70-ton automatic press (MB-070 type: manufactured by Marunouchi Iron Works) heated to 85 ° C., and pressed for 30 minutes under conditions of a pressure of 10 MPa.
After completion of pressing, the obtained sheet was cut off from the forming jig and post-cured at 140 ° C. for 1 hour.
Table 1 shows the properties and physical property values of the sheet obtained after cooling. The evaluation results of the radio wave absorption characteristics are shown in FIG. The radio wave absorption measurement conditions are shown in Table 2.

Comparative Example 1
A mixture was prepared in the same manner as in Example 1 except that the amount of the pulverized graphite powder used was 80 g, and a sheet was produced by the same method. Table 3 shows the radio wave absorption characteristics of the sheets produced in Example 1 and Comparative Example 1.

（注）Ｙ方向はＸ方向を９０度回転させた方向に対応する。

(Note) The Y direction corresponds to the direction obtained by rotating the X direction by 90 degrees.

As shown in Table 3, the radio wave absorbing sheet (Example 1) according to the present invention is about 3 at a frequency of 2.45 GHz compared to the Comparative Example 1 sheet with respect to radio wave incidence in the X and Y directions. It is clear that it absorbs twice the amount of radio waves.
In addition, it is clear that the radio wave absorption sheet according to the present invention has no change in the amount of absorption depending on the radio wave incident direction, and is clearly excellent in radio wave absorption characteristics.

It is a graph which shows the evaluation result of a radio wave absorption characteristic.

Claims (7)

  A flame-retardant electromagnetic wave absorbing sheet comprising graphite powder dispersed in an elastic resin binder and having an electric wave absorption amount of 20 dB or more in a frequency range of 2.4 to 2.5 GHZ.   A flame retardant radio wave absorption sheet comprising graphite powder and a viscosity adjusting material dispersed in an elastic resin binder, and having a radio wave absorption amount of 20 dB or more in a frequency range of 2.4 to 2.5 GHz.   The flame-retardant electromagnetic wave absorbing sheet according to claim 1 or 2, wherein the binder resin is a mixture of a polyester resin and a phosphate ester.   The flame-retardant electromagnetic wave absorbing sheet according to any one of claims 1 to 3, wherein the graphite powder is an expanded graphite sheet pulverized powder and contains particles having a particle diameter of 60 to 200 µm in an amount of 40% by weight or more.   The flame-retardant wave absorbing sheet according to claim 3 or 4, wherein the phosphate ester functions as an elasticity-imparting material and a flame-retardant material.   The flame-retardant radio wave absorbing sheet according to any one of claims 2 to 5, wherein the viscosity adjusting material is styrene gel beads. The flame-retardant electromagnetic wave absorbing sheet according to claim 1, wherein the bulk density of the sheet is 1.3 g / cm ³ or less.

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