JP2001230588A - Electromagnetic wave absorbing body and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electromagnetic wave absorbing body that can efficiently carry an electromagnetic wave loss material, has superior electromagnetic wave absorbing performance, and measures for weight reduction and fireproof. SOLUTION: Fiber gaps in a porous structure, where inorganic long fibers are stacked in curls, are utilized an for electromagnetic wave loss material.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to construction materials for anechoic chambers and office buildings, and to prevent interference of parabolic antennas.
The present invention relates to an electromagnetic wave absorber that is used for applications such as automobile parts and road wall materials, and particularly absorbs electromagnetic waves in the gigahertz band, and a method of manufacturing the same.

[0002]

2. Description of the Related Art Conventionally, as an electromagnetic wave absorber for absorbing electromagnetic waves in the gigahertz band, a horse-tail hair and coconut fiber are entangled and fixed with an adhesive, which is then covered with a carbon-coated mat-like material or an organic material. Conductive coating is applied to the surface of a polymer fiber to make it conductive (see Japanese Patent Application Laid-Open No. Hei 3-234092), and an aggregate of fibers made of polar polymers coated with carbon and entangled with each other (Japanese Patent Publication No. -10
No. 5616), in which the fiber density is changed in the thickness direction of the fiber aggregate as described above for the purpose of broadening the frequency of the electromagnetic wave to be absorbed (Japanese Patent Publication No. 6-32).
No. 417). However, since these electromagnetic wave absorbers are all based on organic fibers, there is a limit to reducing the fiber diameter, which is an effective means for improving electromagnetic wave absorption characteristics, and the fiber diameter is limited to 100 μm.
m or less. Further, when the number of fibers per unit area is increased instead of reducing the fiber diameter, the weight of the electromagnetic wave absorber may be increased. It is also difficult to deal with nonflammability. further,
In these electromagnetic wave absorbers, the fibers are merely covered with a conductive material, so there is a limit to carrying any more conductive material, and the change in fiber density in the thickness direction and the link Thus, there is a limit to changing the amount of the conductive material carried. Also, Japanese Patent Application Laid-Open No. Sho 60-13630
No. 0 describes an electromagnetic wave absorber made of inorganic fibers coated with carbon. This electromagnetic wave absorber uses an inorganic fiber cloth. Therefore, in order to ensure the thickness of the electromagnetic wave absorber, it is necessary to laminate inorganic fiber cloth, but when the inorganic fiber cloth is laminated, the fiber packing density in the electromagnetic wave absorber increases, and the electromagnetic wave absorption is increased. Performance drops. In addition, this gazette states that
After carrying the substance to be carbonized by heating the fiber, the method of baking and carbon coating is described,
In this method, since the fibers are only coated with carbon, there is a limit in carrying more carbon.

[0003]

SUMMARY OF THE INVENTION The present invention provides an electromagnetic wave absorber in which an electromagnetic wave loss material is efficiently carried, has excellent electromagnetic wave absorption performance, and is reduced in weight and made nonflammable. The purpose is to:

[0004]

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above points, and an electromagnetic wave absorber of the present invention has a porous structure in which inorganic long fibers are laminated in a curled shape as described in claim 1. An electromagnetic wave loss material is carried by utilizing fiber gaps in the structure. An electromagnetic wave absorber according to a second aspect is characterized in that, in the electromagnetic wave absorber according to the first aspect, the average fiber diameter of the inorganic long fibers is 1 μm to 50 μm. The electromagnetic wave absorber according to claim 3 is the electromagnetic wave absorber according to claim 1 or 2,
The porous structure has a fiber density gradient in its thickness direction. An electromagnetic wave absorber according to a fourth aspect is characterized in that, in the electromagnetic wave absorber according to any one of the first to third aspects, a mixed material of graphite and carbon black is used as an electromagnetic wave loss material. Further, the housing for the electromagnetic wave absorber according to the present invention is the same as that in claim 5.
As described above, the electromagnetic wave absorber according to any one of claims 1 to 4 is housed in a bag or a case having no electrical characteristics. Further, in the method for producing an electromagnetic wave absorber of the present invention, as described in claim 6, a porous structure in which inorganic long fibers are laminated in a curl shape is immersed in a dispersion liquid in which an electromagnetic wave loss material is dispersed, and then dried. Thereby, an electromagnetic wave loss material is carried using the fiber gap of the porous structure.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION Since the inorganic long fibers used in the present invention are artificial fibers, each fiber has a uniform fiber diameter, and various fibers having various fiber diameters are adjusted by the production process. Since the fibers can be easily obtained, it is possible to produce a stable electromagnetic wave absorber in quality. Examples of the material of the inorganic long fiber include glass and ceramic, and in view of workability and cost, long glass fiber is preferable.

The average fiber diameter of the inorganic long fibers is 1 μm to 5 μm.
0 μm is preferable, and 10 μm to 30 μm is more preferable. The average fiber diameter is the diameter of a single fiber and is defined by JIS
It is defined by R3420. A porous structure of inorganic long fibers having such a fiber diameter can be manufactured by a known method. For example, a porous structure of long glass fibers is already used as an insulator for an automobile battery or a filter such as a ventilation fan. It is commonly used as The fibers may be loosened in the porous structure if no processing is performed. Therefore, it is preferable to bind the fibers with a binder. As a binding binder, for example,
An organic binder such as an acrylic resin, a melamine resin, a urea resin, or a phenol resin may be used. However, if an inorganic binder is used, it is possible to more cope with nonflammability. The binder may be appropriately selected in consideration of the use environment of the electromagnetic wave absorber.

[0007] The electromagnetic wave absorption characteristics are greatly affected by how the electromagnetic wave loss material is distributed in the electromagnetic wave absorber. In the present invention, the porous structure supporting the electromagnetic wave loss material is formed by laminating inorganic long fibers in a curl shape. By configuring the porous structure in this way,
Since the thickness of the electromagnetic wave absorber can be ensured without increasing the fiber packing density, it is possible to suppress a decrease in electromagnetic wave absorption performance. Also, by forming a fiber density gradient in the thickness direction of the porous structure, it becomes possible to change the electromagnetic wave absorption characteristics. The method of forming the fiber density gradient in the thickness direction of the porous structure is not particularly limited, for example,
Examples include the method described in Examples described later. The thickness of the electromagnetic wave absorber can be arbitrarily set, and two or more electromagnetic wave absorbers may be used depending on the purpose.

As the electromagnetic wave loss material, graphite,
Examples include, but are not limited to, carbon black, titanium oxide, and mixtures thereof. However, considering cost and the like, it is preferable to use graphite or carbon black. In particular, when a mixed material in which about 10% to 40% of carbon black is mixed with graphite is used, the electromagnetic wave absorbing performance can be improved.

In the electromagnetic wave absorber of the present invention, an electromagnetic wave absorbing material is efficiently carried by utilizing fiber gaps in the porous structure. In order to manufacture such an electromagnetic wave absorber, it is preferable to adopt a method in which the porous structure is immersed in a dispersion liquid in which the electromagnetic wave loss material is dispersed, and then dried. By employing this method, it is possible to carry not only the surface of the inorganic long fiber but also the gap between the fibers with the electromagnetic wave loss material, but this is due to the action of the surface tension of the dispersion. it is conceivable that. By supporting the electromagnetic wave loss material in the gap between the fibers, it becomes possible to extremely increase the amount of the loss material carried per number of fibers as compared to the case where the loss material is supported only on the fiber surface. Instead, when the porous structure has a fiber density gradient in the thickness direction, it is possible to form a lossy material carrying gradient (a portion having a large gap between fibers, that is, the fiber density is low). The electromagnetic wave loss material is not easily carried on the sparse portion, and the electromagnetic wave loss material is easily carried on the portion having a small gap, that is, the portion having a high fiber density.

The amount of the electromagnetic wave loss material carried can be adjusted by the solid content concentration of the loss material in the dispersion and the viscosity of the dispersion. When the viscosity of the dispersion is increased, the surface tension increases, and the electromagnetic wave loss material is more likely to be carried in a portion where the gap between the fibers is large. However, the solid content of the loss material in the dispersion is 1% by weight to 30%. % By weight, the viscosity of the dispersion is 50
It is preferable to adjust within the range of mPa · s to 500 mPa · s.

In order to prevent the electromagnetic wave loss material carried on the inorganic long fibers from falling, a binder may be mixed with the dispersion in advance. Examples of the type of the binder include the organic binder and the inorganic binder mentioned as the binder for binding the fibers. These selections may be made appropriately in consideration of the usage environment of the electromagnetic wave absorber. It is preferable that the solid content concentration of the binder in the dispersion is adjusted in the range of 0.1% by weight to 20% by weight.

The electromagnetic wave absorber produced by the above method is
Although it can be used as it is, by storing it in a bag or case having no electrical characteristics, it can be developed for various uses. Bags having no electrical characteristics include bags made of polyvinyl fluoride, polyethylene, polypropylene, and vinyl chloride.
When the electromagnetic wave absorber is used outdoors, weather resistance is required. Therefore, it is preferable to store the electromagnetic wave absorber in a bag made of polyvinyl fluoride. In addition, by housing in a case having no electrical characteristics, for example, a case made of polypropylene, acrylic, or vinyl chloride, it is possible to improve the dimensional accuracy and appearance of the electromagnetic wave absorber. For example, an electromagnetic wave absorber installed in an inner chamber of an anechoic chamber requires strict dimensional accuracy, but the dimensional accuracy of the electromagnetic wave absorber depends on the dimensional performance of the storage case. Since the molding accuracy of the case as described above is relatively good, by housing the electromagnetic wave absorber of the present invention in this case, it is possible to withstand sufficiently strict dimensional accuracy. Further, by coloring the bags and cases to be stored, it is possible to easily respond to various user needs.

[0013]

Next, an embodiment of the electromagnetic wave absorber of the present invention will be described with reference to the drawings. (Example 1) FIG. 1 shows a base material of the electromagnetic wave absorber of the present invention, in which the average fiber diameter is 20 μm, the thickness is 25 mm, and the size is 50.
This figure shows a state in which two porous structures 1 in which long glass fibers of 0 mm × 500 mm are laminated in a curl shape are stacked. The weight of the two-layered porous structure 2 is 60 g. This porous structure has no fiber density gradient in the thickness direction. The manufacturing method is as follows. That is, a condensed mat is obtained by winding the glass long fiber around the drum while moving the nozzle for drawing the molten glass left and right within the width of the winding drum of the glass long fiber that slightly moves left and right. When the long glass fibers are wound around the drum, a binder for binding the fibers is sprayed so that the fibers in the obtained porous structure are not loosened. The condensed mat obtained in this way was cut out of the drum, pulled in the direction perpendicular to the circumferential direction wound around the drum, and the long glass fibers were unfolded to loosen the fibers. The body is obtained. This porous structure is generally used as a filter.

Next, water: graphite: carbon black: acrylic binder: aqueous ammonia: 90.5: 5:
A dispersion mixed at a ratio (weight ratio) of 1: 3: 0.5 was prepared. The viscosity of this dispersion was 150 mPa · s. As shown in FIG. 2, a two-layer porous structure 2 is placed in a container 4 in which the dispersion 3 is filled to a depth of 60 mm.
Soaked for seconds. Then take it out of the dispersion,
It was dried in a drying oven at 100 ° C. for 15 minutes.

FIG. 3 is an enlarged view showing the electromagnetic wave absorber 5 thus obtained and the state of carrying the lossy material in the electromagnetic wave absorber. As shown in the enlarged view, graphite and carbon black, which are electromagnetic wave loss materials, are applied to the fiber surface 6.
And in the gaps 7 between the fibers. The weight of the electromagnetic wave absorber was 95 g, and carried 35 g of the electromagnetic wave loss material.

As shown in FIG. 4, the electromagnetic wave absorber is sandwiched between sheets 8 made of polyvinyl fluoride, and the periphery thereof is heat-sealed 9.
And sealed.

The electromagnetic wave absorbing performance of the container of the electromagnetic wave absorber thus obtained was measured by a measuring system shown in FIG.
In FIG. 5, an electromagnetic wave 11 transmitted from a horn antenna 10 passes through a lens 12 to become a plane wave 13 and reaches a measurement sample 14. The electromagnetic wave reflected from the measurement sample and the metal plate 15 is received again by the horn antenna, and the electromagnetic wave absorption performance of the measurement sample is evaluated from the attenuation (reflection attenuation) of the reception electromagnetic wave with respect to the transmission. FIG. 6 shows the electromagnetic wave absorption characteristics of the electromagnetic wave absorber measured by the measurement system of FIG. As is clear from FIG. 6, the obtained electromagnetic wave absorber exhibited good electromagnetic wave absorption characteristics with a return loss of 20 dB or more in a frequency band of 5 GHz to 40 GHz.

Example 2 FIG. 7 shows another base material of the electromagnetic wave absorber of the present invention, in which the average fiber diameter is 20 μm and the thickness is 50 μm.
1 shows a porous structure 16 in which long glass fibers having a size of 500 mm × 500 mm are laminated in a curled shape. The weight of the porous structure is 60 g. As shown in FIG. 7, this porous structure has a fiber density gradient in the thickness direction. The manufacturing method is the same as the manufacturing method of the porous structure described in Example 1, except that the speed at which the nozzle is moved to the left and right and the number of rotations of the drum are changed instead of being kept constant. A condensed mat having a fiber density gradient is obtained, and is obtained from the condensed mat in the same manner as in the method described in Example 1.

In this porous structure, graphite and carbon black were carried in the same process using the same dispersion liquid as in Example 1 to obtain an electromagnetic wave absorber 17 shown in FIG. The obtained electromagnetic wave absorber has graphite and carbon black, which are electromagnetic wave loss materials, supported on the surface of the fiber and the gap between the fiber and the fiber, and further, has a loading amount gradient of the loss material in the thickness direction. I was The weight of the electromagnetic wave absorber was 95 g, and carried 35 g of the electromagnetic wave loss material.

This electromagnetic wave absorber 17 was sandwiched between polyvinyl fluoride sheets in the same manner as in Example 1, and the periphery was sealed by heat fusion.

FIG. 9 shows the electromagnetic wave absorption characteristics of the electromagnetic wave absorber 17 measured by the measuring system shown in FIG. In addition, the electromagnetic wave was made to enter from the sparse surface 18 having a low fiber density. That is, the electromagnetic wave absorber 17 has a structure in which the carrying amount of the electromagnetic wave loss material gradually increases from the sparse surface 18 which is the incident surface of the electromagnetic wave to the back surface 19. From a comparison between Example 1 and Example 2, it was found that having a supporting gradient of the electromagnetic wave loss material in the thickness direction exhibited more excellent electromagnetic wave absorption characteristics.

Example 3 Water: Graphite: Carbon Black: Acrylic Binder: Ammonia Water 83:
A dispersion mixed at a ratio of 9: 2: 5: 1 (weight ratio) was prepared. The solid concentration of graphite and carbon black, which are electromagnetic wave loss materials, in this dispersion is about twice that of Example 1. The viscosity of this dispersion is 2
It was 00 mPa · s. The same porous structure as in Example 2 was immersed in this dispersion in the same manner as in Example 1, and then dried to obtain an electromagnetic wave absorber. The weight of the obtained electromagnetic wave absorber was 103 g, and carried 43 g of an electromagnetic wave loss material.

FIG. 10 shows the electromagnetic wave absorption characteristics of the electromagnetic wave absorber measured by the measuring system shown in FIG. From a comparison between Examples 2 and 3, it was found that the larger the amount of the electromagnetic wave loss material carried, the better the absorption characteristics especially in a high frequency band.

The electromagnetic wave absorbers of Examples 1 to 3 are all UL (Underwriters Lab).
(oratories) This product has passed the flame retardancy test specified in the 94V0 standard.

(Comparative Example) The same dispersion as in Example 1 was spray-coated on the same porous structure as in Example 1, and then
It was dried in a drying oven at 00 ° C. for 15 minutes. In the obtained electromagnetic wave absorber, an electromagnetic wave loss material was carried on the surface of the fiber, but was not carried in the gap between the fibers.
Its weight was 80 g, and the carrying amount of the electromagnetic wave loss material was only 20 g.

FIG. 11 shows the electromagnetic wave absorption characteristics of the electromagnetic wave absorber measured by the measuring system shown in FIG. Since the amount of the electromagnetic wave loss material carried was small, the electromagnetic wave absorption characteristics of the electromagnetic wave absorbers of Examples 1 to 3 were inferior.

Table 1 shows the return loss at 10 GHz, 20 GHz and 30 GHz of the electromagnetic wave absorbers of Examples 1 to 3 and Comparative Example.

[0028]

[Table 1]

[0029]

As described above, the electromagnetic wave absorber of the present invention
By using an inorganic long fiber whose fiber diameter is constant and whose fiber diameter can be adjusted as a base material, the fiber is stable in quality and can be reduced in weight and incombustibility. Therefore, it can also be used for building materials, road wall materials, and the like.
Since the electromagnetic wave absorber of the present invention is lightweight, it is possible to prevent the work of attaching the electromagnetic wave absorber from being simple, and to prevent an increase in cost for constructing and holding the electromagnetic wave absorber. Further, by coloring the bags and cases to be stored, it is possible to easily respond to various user needs.

[Brief description of the drawings]

FIG. 1 is a view showing a porous structure used in the present invention.

FIG. 2 shows a container filled with a dispersion.

FIG. 3 is a diagram showing an electromagnetic wave absorber of the present invention and an enlarged part thereof.

FIG. 4 is a diagram showing a method for obtaining a housing for an electromagnetic wave absorber of the present invention.

FIG. 5 is a diagram showing a measurement system of electromagnetic wave absorption performance.

FIG. 6 is a view showing the electromagnetic wave absorption characteristics of the electromagnetic wave absorber of the present invention.

FIG. 7 is a view showing another porous structure used in the present invention.

FIG. 8 is a view showing another electromagnetic wave absorber of the present invention.

FIG. 9 is a diagram showing electromagnetic wave absorption characteristics of another electromagnetic wave absorber of the present invention.

FIG. 10 is a diagram showing electromagnetic wave absorption characteristics of another electromagnetic wave absorber of the present invention.

FIG. 11 is a diagram showing electromagnetic wave absorption characteristics of an electromagnetic wave absorber as a comparative example.

[Explanation of symbols]

 1, 2, 16 Porous structure 5, 17 Electromagnetic wave absorber 6 Electromagnetic wave loss material supported on fiber surface 7 Electromagnetic wave loss material supported on gap between fibers

Claims (6)

[Claims] 1. An electromagnetic wave absorber characterized in that an electromagnetic wave loss material is carried by utilizing a fiber gap in a porous structure in which inorganic long fibers are laminated in a curl shape. 2. An inorganic long fiber having an average fiber diameter of 1 μm to 5 μm.
2. The electromagnetic wave absorber according to claim 1, wherein the thickness is 0 μm. 3. The electromagnetic wave absorber according to claim 1, wherein the porous structure has a fiber density gradient in a thickness direction thereof. 4. The material according to claim 1, wherein the electromagnetic wave loss material is a mixture of graphite and carbon black.
4. The electromagnetic wave absorber according to any one of claims 1 to 3. 5. A housing for an electromagnetic wave absorber, wherein the electromagnetic wave absorber according to claim 1 is housed in a bag or case having no electrical characteristics. 6. A porous structure formed by laminating inorganic long fibers in a curl shape is immersed in a dispersion liquid in which an electromagnetic wave loss material is dispersed, and then dried to utilize the fiber gap of the porous structure. A method for producing an electromagnetic wave absorber, wherein an electromagnetic wave loss material is carried on the substrate.