JPS62123799A - Wave absorbing material - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a radio wave absorbing material that efficiently absorbs radio waves, particularly microwaves and radar waves.

Problems that Future Technologies and Inventions Try to Solve The present age is said to be an advanced information age, and various information media are overflowing and increasing in number, and broadcasting and communications using radio waves are no exception.

That is, in addition to conventional broadcasting and communications such as FM broadcasting, AM broadcasting, VHF television broadcasting, microwave multiplex radio, and ship/fishing boat radar, satellite broadcasting and aviation radio.

BACKGROUND Broadcasting and communication such as personal radio and data communication via satellite relay are becoming popular and increasing. For this reason, not only are radio waves in all frequency bands now being used extensively, but phenomena such as radio wave interference, frequent radio interference, and malfunctions occur, and these phenomena have now become major social and environmental problems.
The so-called EMC (E 1electro Magnet)
ic compatibility) measures are needed.

However, (1) magnetic loss (magnetic permeability μ=μ′+jμ
Radio wave absorbing material made of a sintered body of soft ferrite with a large imaginary part real number μ'' (hereinafter referred to as (1) radio wave absorbing material), (2) radio wave absorbing material made of soft ferrite powder filled in rubber or resin sheet (hereinafter referred to as the radio wave absorbing material in (2)), (3) semiconductive carbon with large dielectric loss including ohmic loss (imaginary part real number ε'' of dielectric constant ε = ε' + iε'') in rubber or resin. Conventional radio wave absorbing materials such as filled radio wave absorbing sheets (hereinafter referred to as (3) radio wave absorbing material) have various problems in terms of performance and construction as described below.

First, the radio wave absorbing material (1) has a problem in that it has a specific gravity of about 5.0, which is too heavy, and since it is a sintered body, it is difficult to process such as cutting, punching, and drilling. There is a problem in that it is difficult to install on site. In addition, the radio wave absorbing material (2) has superior workability compared to the radio wave absorbing material (1) above, and has been used on some bridges in the past due to its advantages in terms of construction. However, it has problems such as being heavy with a specific gravity of 3.5 or more and having insufficient radio wave absorption performance.

It is not currently under construction. Furthermore, although the radio wave absorbing material (3) is relatively lightweight, good radio wave absorption performance cannot be obtained unless the carbon dispersion state and sheet thickness are strictly uniform, making manufacturing and quality control difficult. Due to this problem, it has not been possible to mass-produce products with stable quality, and the product has remained at the stage of prototyping. Moreover, (1) to (3
) radio wave absorbing materials can all essentially show good radio wave absorption characteristics only in a narrow band, and among them, the radio wave absorbing materials (2) and (3) are back-beated with a metal plate and exhibit the λ/4 resonance phenomenon. Since it is a mechanism that absorbs radio waves using Furthermore, sufficient radio wave absorption performance cannot be obtained unless the radio wave absorbing material (1) has a thickness of 3 m or more, and the radio wave absorbing materials (2) and (3) each have a thickness of lllll11 or more. Therefore, the problem is that the radio wave absorbing material is thick and heavy.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a radio wave absorbing material that is flexible, has excellent workability, can be formed into a lightweight and thin film, and has excellent radio wave absorption performance for radio waves in a wide frequency band. With the goal.

Means and Action for Solving the Problems The inventors of the present invention have conducted intensive studies to achieve the above object, and have found that semiconductive ceramic short fibers, semiconductive ceramic short fibers, etc.
Among them, the fiber diameter is 0.1~1oOp and the aspect ratio is 10.
~1000, volume specific resistivity is 1o-2~102Ω・c
When using a planar highly blended mixture of m,
Broad frequency band radio waves +, -'t+ 1. −r ±
, 轡ゆかん ``H Weighing stand hi+-npi also 30 to 30 of
It has been found that a lightweight radio wave absorbing material having excellent radio wave absorbing performance can be obtained by blending a 0 weight part dispersion.

In particular, by molding a polymer material that has film-forming ability, it is possible to create a radio wave absorbing film that is flexible, has excellent processability, is lightweight, and has a thin film thickness, without losing its excellent radio wave absorption ability. I found out that it can be obtained.

In addition, the above-mentioned radio wave absorbing film is produced by first dissolving a polymer substance with film-forming ability in an organic solvent, adding semiconductive ceramic short fibers to the resulting solution, and thoroughly dispersing the semiconductive ceramic fibers. Prepare a dispersion of short fibers,
Next, this dispersion is applied onto a film base such as processing paper or woven fabric, and then the semiconductive ceramic short fibers in the dispersion applied onto this film base are oriented in a plane by passing them between rolling rolls. After that, the organic solvent in the dispersion is evaporated off by heating, etc., so that it can be easily and easily fixed to the movables, and has good manufacturability. I found out.

Therefore, the present invention can be used as a radio wave absorber with a fiber diameter of 0.1 to 1.
00IJXI, semiconductive ceramic staple fibers having an aspect ratio of 10 to 1000 and a specific volume resistivity of 10-2 to 102 Ω·l are dispersed in a polymeric material in an amount of 30 to 300 parts by weight per 100 parts by weight of the polymeric material. The purpose is to provide a blended radio wave absorbing material.

The present invention will be explained in more detail below.

The radio wave absorbing material of the present invention has a fiber diameter of 0.
1 to 100 μm, more preferably 0.1 to 10 μm, and an aspect ratio of 10 to 1000. More preferably 10-3
00, and a volume specific resistivity of 10<-2> to 10<2 >[Omega]*, more preferably 0.5 to 20[Omega]*.

In this case, the fiber diameter of the semiconductive ceramic short fibers is 1oO
If it exceeds Js, it is difficult to obtain a radio wave absorbing material with a smooth surface, no pinholes, high mechanical strength, flexibility, and excellent workability, and the fiber diameter is less than 0.1p or the aspect ratio is is less than 10, the radio wave absorption performance is almost the same as that of powder, and a radio wave absorbing material with excellent radio wave absorption performance cannot be obtained. In addition, if the aspect ratio exceeds 1000, a good film can be formed when forming the radio wave absorbing material into a film, and if the specific volume resistivity is less than 10-2Ω・1 or exceeds 102Ω・■ Good radio wave absorption performance cannot be obtained in these cases, and in any case, those outside the scope of the present invention are unsuitable as radio wave absorbers.

Note that, as described above, the present invention uses semiconductive ceramic short fibers with specific properties as a radio wave absorber, but if other radio wave absorbers, such as metal fibers, are used, the following implementation method may be used. As specifically shown in the example, both radio wave reflectance and specific gravity are large, which is not preferable. In addition, examples of radio wave absorbers other than conductive ceramic short fibers include ferrite powder and semiconductive carbon, but these also have the problems mentioned above, and their use is not covered by the present invention. cannot achieve the purpose.

Regarding the conductivity of the semiconductive ceramic short fibers used in the radio wave absorber according to the present invention, those having semiconductivity that satisfies the scope of the present invention as the ceramic short fibers themselves may be used, or Semiconductivity that satisfies the scope of the present invention is imparted by treating the surface of non-conductive ceramic short fibers by a reduction method or other appropriate surface treatment method, or by coating with a coating agent. These materials may be used, or a mixture thereof may also be used.

Specific examples of semiconductive ceramic short fibers that can be suitably used in the present invention include the following general formula (I) M2O(TiO2)
Examples include alkali titanate short fibers represented by the following formula (where M is an alkali metal and n is an integer of 2 to 12). This alkali titanate short fiber has excellent reinforcing properties, abrasion resistance, surface smoothness, dimensional stability, etc., and is very effective.

In addition, such alkali titanate short fibers include “
Japanese Patent Publication No. 42-27264, Japanese Patent Publication No. 51-49
924, etc., can be used. In addition, as a method for making the surface conductive of alkali titanate short fibers, there is
03204, 1982-135129, 1981-135130, 1982-6235, and the like. Those that have been surface-treated to have electrical conductivity are particularly preferably used as the radio wave absorber according to the present invention.

The radio wave absorbing material of the present invention uses the above-mentioned conductive ceramic short fibers as a radio wave absorber, and this radio wave absorber is dispersed and blended into a polymeric substance, but in this case, the amount of radio wave absorber blended is high. 30 to 30 per 100 parts by weight of molecular substance
0 parts by weight, and particularly preferably 50 to 200 parts by weight for the purposes of the present invention. If the amount of the radio wave absorber blended is less than 30 parts by weight, the radio wave absorption performance will be poor;
If the amount exceeds 300 parts by weight, the radio wave reflectance increases and the mechanical strength of the radio wave absorbing material decreases significantly, and in either case, it is unsuitable for the purpose of the present invention.

In the present invention, there is no particular restriction on the type of polymeric substance in which the radio wave absorber is dispersed and blended, and the radio wave absorber can be formed into various shapes depending on the purpose. When forming a film, it is preferable to use a polymeric substance that has film-forming ability.

In this case, there are no particular restrictions on the polymeric substance that has film-forming ability, but among them natural rubber, synthetic rubber, polyolefin resin, and alkyd resin.

One or more selected from acrylic resins, vinyl resins, epoxy resins, urethane resins, polyamide resins, and polyester resins are preferably used. These polymeric substances are selected mainly based on the mechanical properties required for the radio wave absorbing film, but in addition to mechanical properties, other properties such as flame retardancy are also taken into consideration as appropriate depending on the intended use of the radio wave absorbing film. Selected based on For example, when flexibility and toughness are required, urethane resin is particularly preferably used as the polymer material according to the present invention, and when flame retardancy is required, soft vinyl chloride resin is particularly preferably used. be done. There is no particular limit to the thickness of such a radio wave absorbing film (the thickness of the composition layer having radio wave absorbing ability containing a radio wave absorbing agent and a polymer substance having film forming ability), but the thickness is 10 to 50.
0IJn, particularly in the range of 50 to 15oIm, is preferable for the purpose of the present invention because it does not impair radio wave absorption performance and can make the obtained radio wave absorbing film lightweight and thin.

Furthermore, in addition to the above-mentioned radio wave absorber and polymeric substance, the radio wave absorbing material of the present invention may optionally contain fillers, reinforcing agents, softeners, plasticizers,
Additives such as stabilizers, anti-aging agents, lubricants, leveling agents, pigments, curing agents, crosslinking agents, curing aids, crosslinking accelerators, etc. may be blended.

Furthermore, the radio wave absorbing material of the present invention can be composited with other materials.

For example, when a radio wave absorbing material is formed as a film, a single layer film consisting of the radio wave absorbing material, a polymer substance with film-forming ability, and various additives added as necessary may be used for practical use as is. However, in order to increase mechanical properties, improve durability, improve appearance through coloring, etc., and to increase various functions, overcoating at least one side of the radio wave absorbing film with a resin such as polyurethane, etc. A composite radio wave absorbing film can be used by bonding various films or sheet materials. In other words, such a composite radio wave absorbing film uses, for example, a base material that is effective for multifunctionalization and reinforcement of various functions, and a polymeric substance that has film-forming ability, and various additives are added to this as necessary. It can be produced by dissolving and dispersing a composition in an organic solvent and overcoating the single-layer radio wave absorbing film by coating, dipping, spraying, etc., and a film made of the composition. If it is made of a rubber composition, it can be manufactured by bonding it by applying pressure and heat to perform vulcanization adhesion.

In particular, when the bonding treatment involves bonding organic or inorganic woven fabrics or non-woven fabrics using an adhesive or pressure-sensitive adhesive, the flexibility of the resulting composite radio wave absorbing film is not impaired and the mechanical strength is extremely high. Therefore, it has excellent practical value.JP It is preferable to orient it in a plane. In this case, a radio wave absorbing film in which such a radio wave absorber is oriented in a plane is produced by adding semiconductive ceramic short fibers as a radio wave absorber to a solution in which a polymer substance with film-forming ability is dissolved in an organic solvent. The resulting dispersion is applied onto a film base, and the conductive ceramic short fibers in the dispersion applied onto the film base are oriented in a plane. By removing the solvent by evaporation, it is possible to easily supply a large quantity with stable quality.

That is, to explain this manufacturing method in more detail, first, in order to impart sufficient fluidity to the radio wave absorber so that the radio wave absorber can be oriented in a plane, a polymeric substance having film-forming ability is added to an organic solvent. In the dissolved solution, a semi-conductive ceramic short barrel Kasuri as a radio wave absorber is added as a radio wave absorber. In addition to the fibers, various additives such as the fillers mentioned above may be blended. In addition, in order to impart sufficient fluidity to the dispersion liquid so that the radio wave absorber can be oriented in a plane, 20 to 40 parts by weight of a polymeric substance having film-forming ability is added to 100 parts by weight of the organic solvent. It is preferable to use the ratio of .

Next, the dispersion prepared by the above method is applied onto a film base. The film base used here includes organic or inorganic woven or non-woven fabrics used in the above-mentioned bonding process, engineering paper, mylar film, etc., and all film bases that can be coated with the above-mentioned dispersion can be used. It is possible. Further, as a coating method, a wire bar method, a blade method, etc. can be adopted, and coating conditions such as coating speed and temperature and humidity during coating are appropriately adjusted depending on the viscosity of the dispersion liquid, the thickness of the radio wave absorbing film, etc.

Next, the semiconductive ceramic short fibers in the dispersion applied onto the film base are oriented in a plane.

Methods for orienting these semiconductive ceramic short fibers in a planar manner include a rubbing method in which a film base such as Mylar film is rubbed in one direction with a cotton cloth to impart orientation;
Methods such as the method of orienting the short fibers through rolling rolls can be mentioned, but the method of orienting the short fibers through the rolling rolls is relatively easy and allows for highly planar orientation of the semiconductive ceramic short fibers, and is also easy to mass-produce. It is also suitable because it has excellent properties. In addition, when the above-mentioned method of orienting through rolling rolls is adopted, the semiconductive ceramic short fibers in the dispersion coated on the film base are orientated in a plane, and at the same time, the radio wave absorbing film is usually aligned with a predetermined film. The clearance is adjusted to have a thickness, preferably 10 to 500 voices.

Thereafter, the residual organic solvent in the dispersion layer in which the semiconductive ceramic short fibers on the film base are oriented in a plane is completely evaporated off, and a radio wave absorbing film is manufactured by evaporating and removing the organic solvent. Note that the method for removing organic solvents by evaporation requires different drying conditions such as reduced pressure, heating, and blowing air depending on conditions such as the viscosity of the dispersion, the amount of organic solvent in the dispersion, and the size and length of the radio wave absorbing film. However, a method in which the film is passed through a high-temperature drying chamber under blowing or no wind is advantageous for industrially producing radio wave absorbing films.

Note that the solvent for preparing the above-mentioned dispersion mainly depends on the type of polymeric substance having film-forming ability.
They are selected from the viewpoints of compatibility and workability, but for example, when using vinyl chloride resin or urethane resin as a polymeric substance with film-forming ability, dimethylformamide (DMF) and tetrahydrofuran (THF) are used.
A mixed solvent containing as a main component is advantageous in terms of film-forming properties and workability.

As explained above, the radio wave absorbing material of the present invention has a fiber diameter of 0.1 to 100 tIn, an aspect ratio of 10 to 1000, and a volume resistivity of 10-2 to 102Ω.
・cm of semiconductive ceramic short fiber 9 yen 1 Asahi 9 pattern 9 yen △
By blending 30 to 300 parts by weight of Efune 188 fir summer powder, it is effective against radio waves in the frequency band of 3 GHz to 30 GHz, the so-called X band called SHF, and the X band. It has a high radio wave absorption rate over a wide band and a low radio wave reflectance, so it has excellent radio wave absorption performance, and has a specific gravity of around 1.5, which allows the radio wave absorbing material to be lightweight. Moreover, when made into a film, it has excellent surface properties, mechanical strength, and flexibility, making it easy to process such as cutting, punching, and bonding.Additionally, it can be multifunctionalized and reinforced with various functions through overcoating and bonding treatments. As the appearance, durability, and weather resistance can be easily improved, it can withstand long-term use even under harsh outdoor environmental conditions, and is therefore suitable for ghost countermeasures for VHF and UHF television broadcasting, and for microwave transmission and reception equipment. It is widely used in areas where it is generally required as part of EMC countermeasures, such as radar equipment for ships and fishing boats, countermeasures against radio wave interference around satellite broadcasting receivers, countermeasures against false images in bridges and high-rise buildings.

Hereinafter, specific examples and examples will be briefly described, but the present invention is not limited to the following examples.

In addition, prior to the examples, various measurement methods used in the examples and comparative examples will be explained below.

A. Electromagnetic absorption J body Using a specific resistivity tablet molding machine, the radio wave absorber was heated at room temperature and under pressure conditions of 100
The electrical resistance in the thickness direction was measured using a digital multimeter VP-2661B (Matsushita Tsushin Kogyo Co., Ltd.) for a sample molded into a shape of 20 mm in diameter and 5 Trta in thickness using kg/a+f.
(manufactured by). Based on this measured value, the specific volume resistivity was determined by calculation.

B. Radio wave absorption performance of the radio wave absorbing film The radio wave absorbing film was placed inside the waveguide converter (HP-X281C manufactured by HEWLETT-PACKARD) at 110X.
Insert it as a 22n square shape and use the sweeper (manufactured by the company HP
-8350B) using four frequencies 8.10, 12.
A radio wave with a frequency of 14 GHz and an output of 1 mW was transmitted, and the intensity of the transmitted wave and reflected wave was measured by applying the radio wave to the above radio wave absorbing film. Transfer this measured power signal to a network analyzer (
Radio wave transmittance, radio wave reflectance, and radio wave absorption rate were calculated by inputting and analyzing the data into HP-8756A manufactured by the same company.

[Examples 1 to 9. Comparative Examples 1 to 6] The types of radio wave absorbers shown in Table 1 and vinyl chloride resin (non-plastic type (copolymer type) vinyl chloride resin manufactured by Sekisui Kagaku Co., Ltd.) as a polymer substance having film-forming ability were used. Use the amounts shown in Table 2, and first mix vinyl chloride with tetrahydrofuran (THF) and dimethylformamide (DMF).
After dissolving in 300 parts by weight of a 1/1 weight ratio mixed solvent,
A radio wave absorber was added and thoroughly kneaded to prepare a dispersion.

Next, this dispersion liquid is applied to processing paper and passed through rolling rolls whose clearance is adjusted to obtain a predetermined film thickness, and then passed through a drying room at 120 to 140°C to evaporate and remove the solvent. Accordingly, radio wave absorbing films having film thicknesses shown in Table 2 were manufactured.

The radio wave characteristics shown in Table 2 for the radio wave absorbing films of Examples 1 to 9 and Comparative Examples 1 to 6 thus obtained were 8 to 8.
Transmittance (Tr), reflectance (Rf), and absorptance (Abs) for radio waves at a frequency of 14 GHz were measured.

The above measurement results are also listed in Table 2.

As is clear from the results in Table 2, the radio wave absorbing films of Examples efficiently absorbed all radio waves with a frequency of 8 GHz to 14 GHz, and had a low radio wave reflectance of around 10%. On the other hand, the specific volume resistivity is 10”
The radio wave absorbing film of Comparative Example 1, which uses a radio wave absorber exceeding 104, and the radio wave absorbing film of Comparative Example 2, in which the fiber diameter and aspect ratio are outside the range of the present invention, both have low radio wave absorption rates. Furthermore, although the radio wave absorbing films of Comparative Examples 4 and 5, which used metal fibers such as Ni flakes or brass fibers as radio wave absorbers, showed the same level of radio wave absorption rate for radio waves with a frequency of 14 GHz as the radio wave absorbing films of Examples. , the radio wave absorption rate is inferior to the radio wave absorbing films of Examples for radio waves with a frequency of 12 GHz or less, and the radio wave absorbing films of Comparative Examples 4 and 5 both have a radio wave absorption rate of radio waves with a frequency of 8 to 14 GHz. The reflectance was as high as 60% or more. Furthermore, the radio wave absorbing films of Comparative Examples 3 and 6, which used conductive carbon and soft ferrite as radio wave absorbers, had poor performance with a radio wave absorption rate of 8% or less. The radio wave absorbing films of Comparative Examples 4 and 5 that were prepared had a specific gravity of 2.0 to 2.2, and the radio wave absorbing film of Comparative Example 6 using soft ferrite had a specific gravity of 3.5. The radio wave absorbing films of the examples all had a weight ratio of 1.5 or less, and therefore, it was also recognized that the radio wave absorbing films of the present invention generally have the advantage of being extremely lightweight.

[Example 1 O-y13] A2 in Table 1 was used as the radio wave absorber, and an ether-type urethane resin (Sumitomo Bayer M) was used as the polymeric substance with film-forming ability, and the blending amounts of these were determined as follows. A radio wave absorbing film having the film thickness shown in Table 3 was obtained in the same manner as in the above Examples and Comparative Examples.

In addition to using a part of the above radio wave absorbing film as it is as the radio wave absorbing film of Example 1o, this radio wave absorbing film was overcoated with 30AIn pigmented urethane resin, and a nylon woven fabric or glass fiber fabric was applied using an adhesive. The radio wave characteristics and mechanical strength (tensile strength, tear strength) of the radio wave absorbing films of Examples 10 to 13 obtained by bonding the cloth to obtain the composite radio wave absorbing films of Examples 11 to 13 were evaluated. It was measured.

The above results are also listed in Table 3.

As is clear from the results in Table 3, the radio wave absorbing films of Examples 10 to 13 have almost no difference in radio wave characteristics from the single-layer radio wave absorbing film of Example 10, and have a frequency range of 8 GHz to 14 GHz. It was recognized that the material exhibited excellent radio wave absorption performance for radio waves. However, compared to the single-layer radio wave absorbing film of Example 10, the radio wave absorbing film of Example 11 achieved improvements in appearance by coloring the overcoat layer.
The radio wave absorbing film of No. 3 has far superior mechanical strength in terms of tensile strength and tear strength. Therefore, even if the radio wave absorbing film of the present invention is subjected to composite processing such as overcoat treatment or bonding treatment, it will not absorb radio waves due to composite treatment. It has been found that excellent radio wave absorption performance can be maintained without deterioration in absorption performance.

Claims (1)

[Claims] 1. As a radio wave absorber, the fiber diameter is 0.1 to 100 μm, the aspect ratio is 10 to 1000, and the specific volume resistivity is 10^
- A radio wave absorbing material comprising 30 to 300 parts by weight of semiconductive ceramic short fibers having a diameter of 2 to 102 Ω·cm dispersed in 100 parts by weight of a polymeric substance. 2. The radio wave absorbing material according to claim 1, wherein the radio wave absorbing agent is semiconductive ceramic short fibers made of alkali titanate short fibers. 3. As a polymer substance, one or more types selected from natural rubber, synthetic rubber, polyolefin resin, alkyd resin, acrylic resin, vinyl resin, epoxy resin, urethane resin, polyamide resin, and polyester resin have the ability to form a film. The radio wave absorbing material according to claim 1 or 2, which is formed into a film using a polymeric substance. 4. The radio wave absorbing material according to claim 3, wherein at least one side is overcoated with a layer of a polymeric substance having film-forming ability and not containing semiconductive ceramic short fibers. 5. The radio wave absorbing material according to claim 3, wherein a woven or nonwoven fabric of organic or inorganic fibers is laminated on at least one side. 6. The radio wave absorbing material according to any one of claims 1 to 5, wherein the semiconductive ceramic short fibers are oriented in a plane.