JP4684699B2 - Radio wave absorbing sheet material and radio wave absorber using the same - Google Patents

Description

  The present invention relates to a radio wave absorbing sheet material and a radio wave absorber using the same, and more particularly to a radio wave absorbing sheet material having isotropic properties excellent in dielectric loss characteristics and a radio wave absorber using the same.

  The anechoic chamber is used for antenna characteristic measurement tests and electronic device radio wave measurement tests. An anechoic chamber used in this way is equipped with a wave absorber on the wall, ceiling, floor, etc. to shield the intrusion of radio waves from the outside, and the radio waves generated from the internal device under test are radiated to the outside. It is formed not to be.

  Conventionally, many of the radio wave absorbers used for such purposes have been obtained by molding a resin foam such as foamed urethane or foamed styrene impregnated with carbon black, which is a conductive material. However, since the radio wave absorber formed of a resin foam is bulky and brittle, the tip end portion may be damaged by vibration during transportation or collision with other objects. For this reason, the storage space is increased, the storage cost is increased, and a large packing volume is required to protect against damage during transportation, resulting in an increase in transportation cost.

  As a countermeasure for such a problem, a technique has been proposed in which a carbon black-containing plate material is carried up to the construction site and assembled into a radio wave absorber such as a hollow pyramid shape at the construction site (see Patent Documents 1 and 2). ). However, if the thickness of the carbon black-containing plate material is too small, the rigidity of the carbon black-containing plate material is insufficient, which causes distortion and instability in the assembled radio wave absorber. Therefore, it is necessary to increase the thickness of the plate material to about 5 to 20 mm. As a result of thickening the plate material in this way, the weight of the plate material increases, so the on-site workability deteriorates, the transportation cost is not as low as expected, and the amount of carbon black used increases. It was.

  In order to solve the above-described problems, the present inventors previously proposed a radio wave absorbing sheet material having a corrugated cardboard structure in which a core and a planar liner obtained by corrugating an electrically lossy material-containing sheet are laminated. (See Patent Document 3). According to this radio wave absorbing sheet material, it is lightweight because it is based on a corrugated cardboard structure including a hollow portion, and can be easily transported and assembled on site.

However, while the wave-absorbing sheet material having the corrugated cardboard structure has the advantages as described above, it has anisotropy in the dielectric loss characteristic, so that it takes time to assemble the wave-absorbing sheet material into the wave absorber. In addition, since it is necessary to align the direction when shaping from the radio wave absorbing sheet material, there is a problem that the yield is lowered.
JP-A-11-87978 Japanese Patent Laid-Open No. 2000-216584 JP 2004-253760 A

  An object of the present invention is to provide a radio wave absorbing sheet material having an isotropy excellent in dielectric loss characteristics, and a radio wave absorber using the same, which solves the above-described anisotropy problem.

Wave absorbing sheet material of the present invention to achieve the above object, makes electrical loss material containing sheet from cardboard structure obtained by stacking a core and a flat liner in which the waveform processing, the electrical loss material containing sheet when the planar shape the ratio between the maximum dielectric loss dielectric loss in the direction (p) indicating the (ε _"p) and the dielectric loss in the direction (v) perpendicular to the direction (p) (epsilon" in the plane of _v) (ε _"p / Ε ″ _v ) has anisotropy of 1.2 to 4, and the direction (p) showing the maximum dielectric loss is perpendicular to the ridge line direction of the corrugated waveform. To do.

  In addition, the radio wave absorber of the present invention is characterized by using the radio wave absorbing sheet material having the above configuration.

The radio wave absorbing sheet material of the present invention is an electric loss material-containing sheet having a dielectric loss ratio (ε ″ _p / ε ″ _v ) of 1.2 to 4 in the plane when the core of the corrugated cardboard structure is planar. Since the direction (p) showing the maximum dielectric loss is formed so as to be substantially perpendicular to the ridge line direction of the corrugated waveform, the anisotropy and corrugated shape in which the electrical loss material-containing sheet is present are used. The resulting anisotropy cancels each other, and a radio wave absorbing sheet material having excellent isotropic dielectric loss characteristics can be obtained.

  In addition, since the radio wave absorbing sheet material has excellent isotropic dielectric loss characteristics as described above, it is possible to absorb radio waves coming from all directions while shortening the design time when assembling the radio wave absorber. An absorber can be obtained.

The radio wave absorbing sheet material of the present invention has a corrugated cardboard structure in which a corrugated core of a sheet containing an electrical loss material and a planar liner are laminated.

  The cardboard structure is preferably selected from single-sided cardboard, double-sided cardboard, double-sided cardboard, or triple wall in order to obtain a sheet material that is as thin, light, and strong as possible. Here, single-sided corrugated cardboard refers to a corrugated cardboard structure in which a corrugated core is bonded to a single liner. Double-sided cardboard is a cardboard structure in which a corrugated core is bonded between two liners. The double-sided cardboard is a cardboard structure in which a single-sided cardboard is bonded to one side of a double-sided cardboard. The triple wall refers to a cardboard structure in which a single-sided cardboard is further bonded to a double-sided cardboard to form a three-stage structure. Among these, double-sided cardboard is particularly preferable because it has both thinness and appropriate rigidity.

  FIG. 1 illustrates a radio wave absorbing sheet material in the case of double-sided cardboard among the above-mentioned cardboard structures.

An electromagnetic wave absorbing sheet material 1 having a double-sided corrugated cardboard structure has an electrically lossy material-containing sheet sandwiched between corrugated cores 2 having a corrugated height h and an inter-corrugated spacing p, and planar liners 3 on both sides thereof. , 3 are bonded together. As will be described in detail later, the electrically lossy material-containing sheet in the planar shape before corrugation is anisotropic in dielectric loss characteristics, and the dielectric loss in the direction (p) showing the maximum dielectric loss in the plane (epsilon ratio of _"(v and _p), dielectric loss epsilon) in the direction perpendicular to the direction (p) indicating the maximum dielectric loss _{(v)" (ε "p} / ε" v) is 1.2 to What is 4 is used. The anisotropy of the electrical loss material-containing sheet is offset by the anisotropy due to the corrugated shape, and is isotropic as a radio wave absorbing sheet material.

Since the electromagnetic wave absorbing sheet material of the present invention has a structure in which an electrically lossy material-containing sheet is used as a core and the core is protected by a flat liner , the loss or deterioration of the electrically lost material against impact from the outside. Therefore, stable radio wave absorption can be maintained over a long period of time. Moreover, since it consists of the corrugated cardboard structure which encloses a hollow part, it is lightweight and can make it easy to convey. In addition, since the intermediate core has an appropriate rigidity, the application range to various radio wave absorbers can be expanded.

  In the present invention, the electrical loss material refers to a material that performs radio wave attenuation by converting radio wave energy into a minute current and further converting it into thermal energy. Examples of such an electrical loss material include conductive powders such as carbon black, carbon microcoil powder, and graphite powder, and conductive fibers such as carbon fiber, silicon carbide fiber, metal fiber, and metal plating fiber. be able to. In addition, the electrical loss material is usually a carbon fiber or silicon carbide fiber firing temperature of 1000 ° C. or higher, but may be a semiconductor fiber obtained by setting the firing temperature to a low temperature of 500 to 700 ° C.

  As for the aspect of the electrical loss material contained in the electrical loss material-containing sheet, it is particularly desirable that the composite paper is mixed so that the conductive fibers are oriented. This is because by orienting the conductive fibers in the mixed paper, it becomes easy to control the anisotropy of the dielectric loss characteristics of the sheet containing the electrical loss material. That is, if the degree of orientation of the conductive fiber is low, the dielectric loss characteristic of the sheet containing the electrical loss material is low, and if the degree of orientation of the conductive fiber is high, the dielectric loss characteristic of the sheet containing the electrical loss material is low. Anisotropy can be increased. From this viewpoint, it is particularly preferable to use carbon fibers as the conductive fibers. This is because carbon fibers are easy to be mixed with a low specific gravity, and because the fibers themselves are rigid, the fibers are easily oriented.

The fiber length of the conductive fiber is preferably 2 to 20 mm. By setting it to 2 mm or more, it is possible to suppress the fiber orientation from being random and to easily control the orientation. Moreover, by setting it as 20 mm or less, it becomes easy to control an orientation again, preventing a fiber becoming entangled.
As a compounding quantity of electroconductive fiber, 0.1-10 mass% is preferable. When the content is 0.1% by mass or more, a sufficient radio wave absorption effect can be exhibited, and when the content is 10% by mass or less, since there is no excessive conductive fiber content, the orientation is easily controlled.

  Moreover, it is preferable to further mix flame-retardant fibers with the mixed paper. As described above, since radio wave absorption is conversion of radio wave energy into thermal energy, it is preferable to provide the radio wave absorbing sheet with flame retardancy from the viewpoint of safety.

  Examples of the flame retardant fiber include glass fiber, aromatic polyamide fiber, polyether ether ketone fiber, polyparaphenylene benzobisoxazole fiber, and polyphenylene sulfide fiber. Moreover, what made the flame-retardant fiber by impregnating the fiber which consists of non-flame-retardant resin with the resin mixture containing a flame retardant can be used suitably. As such a flame retardant, those which do not contain a halogen element having a large environmental load are preferable. For example, at least one selected from condensed phosphate ester, phosphate ester, aromatic diphosphate, magnesium hydroxide, aluminum hydroxide and red phosphorus is used. Can be mentioned. These flame retardants are preferable because a high flame retardant improvement effect can be obtained even if the amount added is small.

  Moreover, it is preferable that the mixed paper of the electrical loss material-containing sheet further contains 11 to 63.9% by mass of cellulose fibers and 36 to 88.9% by mass of the water-containing inorganic compound in addition to the conductive fibers. . Since the cellulose fiber is hydrophilic and has a property of heat shrinking, an appropriate rigidity can be imparted to the sheet by the action of heat at the time of corrugated cardboard bonding and moisture contained in the paste. However, since the cellulose fiber has a weak point that it easily burns, it is preferable to impart flame retardancy by the combined use with the hydrous inorganic compound. As such a water-containing inorganic compound, aluminum hydroxide or the like can be preferably used.

  By containing 11% by mass or more of cellulose fiber, moderate rigidity can be imparted. Moreover, the upper limit of content of a cellulose fiber is decided by the lower limit of content of the conductive fiber used together and a water-containing inorganic compound.

  Moreover, a flame retardance can be provided by making a water-containing inorganic compound into 36 mass% or more. Moreover, the upper limit of content of a water-containing inorganic compound is decided by the lower limit of content of the cellulose fiber used together.

  Examples of the method for producing a mixed paper include a wet paper making method in which a slurry in which fibers and water are mixed is made, and a dry paper making method in which fibers are stirred and mixed in air and collected in a sheet form. . In both the wet papermaking method and the dry papermaking method, the fiber orientation in the mixed paper can be controlled by the traveling speed of the net conveyor that continuously moves to the paper making means. That is, if the traveling speed of the net conveyor is increased, the orientation of the conductive fibers is advanced, and if the traveling speed is decreased, the orientation of the conductive fibers tends to be suppressed. The reason why such a tendency is obtained is considered to be that the resistance of a medium such as water applied to the conductive fibers changes depending on the traveling speed of the net conveyor.

  In addition, the wet papermaking method and the dry papermaking method are more preferable because the wet papermaking method can be uniformly made while maintaining the properties without damaging the conductive fibers.

  In these papermaking methods, it is preferable to add inorganic binders and organic binders such as starch, polyvinyl alcohol, polyethylene, paraffin, and acrylic fibers. By adding these binders, it is possible to clean the papermaking from the papermaking net.

The basis weight of the electrical loss material-containing sheet used for the core is preferably 120 to 200 g / m ² . By setting it to 120 g / m ² or more, it is possible to prevent sheet breakage or the like that is likely to occur when processing into a corrugated shape and bonding to a planar liner . Moreover, it becomes easy to perform waveform processing by setting it as 200 g / m < ² > or less.

The electric loss material-containing sheet used for the radio wave absorbing sheet material of the present invention includes a dielectric loss (ε ″ _p ) in the direction (p) showing the maximum dielectric loss in the plane when it is planar, and the maximum dielectric The ratio (ε ″ _p / ε ″ _v ) to the dielectric loss (ε ″ _v ) in the direction (v) perpendicular to the direction (p) indicating the loss is 1.2 for radio waves in the frequency range of 3 to 18 GHz. It is necessary to use those having anisotropy of ˜4.

A radio wave having a frequency range of 3 to 18 GHz has an appropriate length of several tens of centimeters to several millimeters, is relatively easy to handle, and a sample size required for measurement may be about 30 cm × 30 cm, so it is easy to handle. Therefore, the dielectric loss ε ″ can be measured with high accuracy. Further, there is a certain degree of correlation with the dielectric loss ratio (ε ″ _p / ε ″ _v ) measured in a frequency range other than 3 to 18 GHz (however, a specific value) The dielectric loss ratio (ε) typically measured at 3 to 18 GHz even when attempting to obtain a radio wave absorbing sheet having absorption in a frequency range other than 3 to 18 GHz, except when resonance specifically occurs at a frequency. This is because the purpose can be achieved by adjusting " _p / ε" _v ) to the same range of 1.2 to 4 as described above.

Here, the dielectric loss is one of the indexes representing the magnitude of the damping effect by energy conversion as described above, and is defined as the imaginary part ε ″ of the relative dielectric constant ε in the following equation.
ε = ε′−jε ″

In the present invention, the reason why it is essential that the electrical loss material-containing sheet has the above-described anisotropy is that the anisotropy inherent in the electrical loss material-containing sheet is processed into a corrugated structure as the core of the corrugated board structure. This is to offset the anisotropy caused by the above. That is, the anisotropy of the dielectric loss due to the shape deformed from the planar shape to the corrugated shape is offset by the anisotropy of the electrically lossy material-containing sheet.

When the dielectric loss ratio (ε ″ _p / ε ″ _v ) is smaller than 1.2, the anisotropy of the dielectric loss due to the shape cannot be absorbed. When the dielectric loss ratio is larger than 4, the anisotropic of the dielectric loss due to the shape It cannot be absorbed by sex. The dielectric loss ratio (ε ″ _p / ε ″ _v ) of the electrically lossy material-containing sheet can be adjusted by controlling the orientation state of the conductive fibers in the paper as described above.

As a method for offsetting the anisotropy of the intrinsic dielectric loss and the anisotropy of the dielectric loss due to the shape of the sheet containing the electrically lossy material, the plane when the planar shape of the sheet containing the electrically lossy material is used. It can be achieved by corrugating the electrically lossy material-containing sheet so that the direction (p) showing the maximum dielectric loss and the corrugated ridge direction are orthogonal to each other.

That is, as a result of detailed studies on the anisotropy of dielectric loss due to the core wave shape, the present inventors have found that the dielectric loss due to the waveform shape is small in the direction of the wave in which the waveform repeats, and is orthogonal thereto. As a result, it has been found that the increase in the ridge line direction of the corrugation, and as a result, the dielectric anisotropy inherent in the sheet containing the electrically lossy material in the planar shape and the anisotropy due to the corrugated shape of the core They have come to know that both anisotropies can be offset by combining the direction of increasing and decreasing loss. Here, the “waveform ridge line direction” in the present invention is a direction orthogonal to the wave direction as described above, and is a direction indicated by an arrow AA in FIG.

To offset the inherent anisotropy in the electrical loss material containing sheet when the planar shape, and anisotropic due to the waveform shape of the central core is electrically loss material containing sheet when the planar shape The direction (p) showing the maximum dielectric loss in the plane of the core and the ridgeline direction of the corrugation of the core do not necessarily have to be orthogonal to the exact 90 °, and a range in which the above-described canceling effect can be obtained. It suffices if the relationship is substantially orthogonal.

  As the shape of the corrugation in the core obtained by corrugating the sheet containing the electrical loss material, when the radio wave absorbing sheet material of the present invention forms a body as a corrugated cardboard, the corrugated height h is 1 mm or more and the interval between adjacent corrugations It is preferable that p is 1 mm or more (see FIG. 1). More preferably, the corrugated height h is 2 to 5 mm, more preferably 2.5 to 5 mm in order to ensure the above-described effect of canceling the anisotropy. Moreover, it is more preferable that the space | interval p between adjacent waveforms is 4-15 mm.

  By setting the height of the waveform to 2 mm or more and the interval between adjacent waveforms to 4 mm or more, it is possible to prevent the anisotropy due to the shape from becoming too large. Moreover, the effect which cancels out the intrinsic anisotropy of an electrical loss material containing sheet | seat can be made easy by making the height of a waveform into 5 mm or less and the space | interval between adjacent waveforms to 15 mm or less. Also, for corrugated cardboard members, it is preferable that the height of the corrugations be within the above range in consideration of both the laminating strength and the laminating workability. Considering both the man-hour and strength required for the process, it is preferable to be within the above range.

  If the corrugated core of the sheet containing the electrical loss material per sheet of the electromagnetic wave absorbing sheet material satisfies the desired electromagnetic wave absorption characteristics and the strength characteristics are to be improved, the corrugated cardboard structure is formed in a multistage structure. It is conceivable to increase the number of cores. In this case, the cores may be laminated so as to be mixed with core corrugated cardboard made of a sheet containing no electrical loss material as the core, thereby improving the strength without increasing the manufacturing cost.

It is preferable to use papermaking as the material of the planar liner . As the papermaking, one containing 12 to 60% by mass of cellulose fibers and 40 to 88% by mass of a water-containing inorganic compound is preferable. Since the cellulose fiber is hydrophilic and has a property of heat shrinking, an appropriate rigidity can be imparted to the sheet by the action of heat at the time of corrugated cardboard bonding and moisture contained in the paste. However, since the cellulose fiber has the weak point which is easy to burn, it is preferable to give a flame retardance by combined use with a water-containing inorganic compound.

The rigidity as a sheet | seat can be provided by making a cellulose fiber into 12 mass% or more. The upper limit value of the cellulose fiber content is determined by the lower limit value of the content of the water-containing inorganic compound used in combination. Moreover, a flame retardance can be provided by making a water-containing inorganic compound into 40 mass% or more. Moreover, the upper limit of content of a water-containing inorganic compound is decided by the lower limit of content of the cellulose fiber used together.
The basis weight of the flat liner, 120~400g / m ² is preferred. By setting it as 120 g / m < ² > or more, it can prevent that a sheet | seat tear etc. generate | occur | produce at the time of adhesion | attachment with a center core. On the other hand, if the basis weight becomes too large, the cost increases, so 400 g / m ² is the upper limit.

As a method for obtaining Oite cardboard structure wave absorbing sheet material of the present invention, as an example, high speed, and it can be produced cost to use a method for manufacturing a cheap known paper cardboard. Specifically, a single-sided cardboard can be produced by corrugating the core with a machine called a corrugator and gluing the front or back liner. Furthermore, with the same corrugator, a method can be used in which a single-sided cardboard and a liner are heated while being in close contact to form a double-sided or double-sided cardboard and sent to a cutter and cut into a predetermined size.
Known adhesives such as starch paste can be used as the adhesive for bonding the members constituting the cardboard such as the core and the planar liner .

  The radio wave absorbing sheet material of the present invention preferably has a plane compressive strength (JIS Z 0403-1) of 40 to 250 kPa. By setting the plane compressive strength to 40 kPa or more, even when subjected to an impact during transportation or assembly, the surface is hardly scratched and excellent handling properties can be obtained. The plane compressive strength is preferably as large as possible, but if it is too large, the weight tends to increase. On the other hand, the handling property is impaired, so 250 kPa is preferably set as the upper limit.

  Moreover, it is preferable that the electromagnetic wave absorbing sheet material of the present invention has a vertical compressive strength (JIS Z 0403-2) of 1.5 to 8 kN / m. By setting the vertical compressive strength to 1.5 kN / m or more, good dimensional stability can be obtained even when a large structure is formed. The higher the vertical compressive strength, the better. However, since the weight increases even if the vertical compressive strength is excessively increased, the handling property is adversely affected. Therefore, the upper limit is preferably 8 kN / m.

  Next, the radio wave absorber of the present invention can be composed of the radio wave absorbing sheet material having the above-described configuration of the present invention.

  As an aspect of the radio wave absorber of the present invention, for example, it can be configured by laminating a plurality of radio wave absorbing sheet materials having the above-described configuration. This is because the frequency of the absorbable radio wave can be adjusted by adjusting the number of laminated cardboard structures in the radio wave absorbing sheet material and the amount of the electrical loss material included in the core. In such a laminated structure, it is also preferable to mix corrugated cardboard having a core that does not include the electrical loss material-containing sheet, in order to adjust the frequency of the absorbable radio wave.

  As another aspect, it is preferable that the radio wave absorbing sheet material is assembled into a hollow solid structure having a wedge shape, a polygonal pyramid shape, or a polygonal column shape. Such a hollow three-dimensional structure can be transported in a cut or folded state before assembling the radio wave absorbing sheet material into the hollow three-dimensional structure and assembled near the installation site, thus reducing transportation and storage costs. This is a preferable aspect. Further, since the cardboard has an appropriate rigidity, it is possible to improve the shape retention of the radio wave absorber after assembly.

  The hollow three-dimensional structure may be erected on a sintered ferrite plate. By combining the hollow three-dimensional structure with a sintered ferrite plate, it is possible to absorb low-frequency radio waves of about 30 MHz to 300 GHz.

  The hollow three-dimensional structure may be erected on a radio wave absorber that is formed by laminating a plurality of the radio wave absorption sheet materials into a flat plate shape. By combining with a flat-plate laminated radio wave absorber, radio waves having a high frequency of about 1 to 100 GHz can be absorbed.

  As described above, the radio wave absorbing sheet material of the present invention can be made into various types of radio wave absorbers in accordance with a desired frequency depending on the mode of assembling the radio wave absorber and the materials used in combination.

  The radio wave absorbing sheet material of the present invention can be used as a constituent member of a radio wave absorber that constitutes the wall surface of the anechoic chamber. It can also be used as a moving body such as a ship or an aircraft, a structure such as a bridge or a steel tower, a device or equipment for wireless communication, a building such as a building, or an interior material such as office supplies. Moreover, it can also be used as an electromagnetic wave shielding wallpaper used in a simple shield room or an unnecessary radio wave suppression sheet attached around an electronic circuit. Thus, it can be used as an electromagnetic environment countermeasure material in various forms for absorbing unwanted reflected waves and preventing radio interference.

  Hereinafter, the present invention will be described in more detail with reference to examples. In addition, the performance value shown in the Example was measured with the following measuring method.

(1) Dielectric loss A double ridge guide horn antenna mounted on an arch-type measuring instrument and a network made by Agilent Technologies, with a sample placed on the front of an aluminum plate 30 cm long × 30 cm wide × 1 mm thick through a polystyrene foam spacer The input impedance at 3-18 GHz was measured using an analyzer. Thereafter, the sample was removed, the input impedance at 3 to 18 GHz was measured with the spacer alone, and the relative dielectric constant of the sample was determined by back calculation from the difference in input impedance with and without the sample.

  For the direction (p) indicating the maximum dielectric loss of the sample, a linear axis was defined on the sample plane, and the defined linear axis was rotated to 90 ° in 10 ° increments to determine a 10-directional linear axis. Ten samples were collected so that one side of the 30 cm × 30 cm sample was parallel to each of the linear axes. A radio wave is applied to each direction axis so that the electric field vibration direction of the radio wave irradiated from the double ridge guide horn antenna is parallel, the input impedance is measured, and among the 10 samples, the relative dielectric constant is the largest. The linear axis direction of the sample was defined as (p).

(2) Radio wave absorptivity Measured using the same network analyzer used for the measurement of the dielectric loss, the reflection level when a radio wave is vertically applied to an aluminum plate 60 cm long x 60 cm wide x 1 mm thick. Similarly, it was obtained from the difference in reflection level shown in the following equation when a radio wave was applied to the area of the wave absorber.
Radio wave absorption (dB)
= Radio wave absorber reflection level (dB) -Aluminum plate reflection level (dB)

(3) Flame retardancy Measured according to UL94 “flammability test of plastic materials for equipment parts”.

(4) Plane compressive strength Measured according to JIS Z 0403-1 "Cardboard-Part 1: Plane compressive strength test method".

(5) Vertical Compressive Strength Measured according to JIS Z 0403-2 “Cardboard—Part 2: Vertical Compressive Strength Test Method”.

[Example 1]
(Core sheet)
The following fibers and water-containing inorganic compounds are mixed in a mass ratio described in water as a medium, wet papermaking is performed at a take-up speed of 100 m / min, and a planar shape electrical material having a thickness of 0.12 mm and a basis weight of 100 g / m ² is obtained. The loss material-containing sheet A was obtained as a core sheet.
PAN-based carbon fiber having an average fiber length of 6 mm and a fiber diameter of 7 μm: 0.8% by mass
Chopped glass fiber having a fiber length of 6 mm and a fiber diameter of 7 μm: 29.2% by mass
Aramid pulp with an average fiber length of 1 mm: 10% by mass
Aluminum hydroxide: 60% by mass.

The direction showing the maximum dielectric loss in the plane of the sheet A was the papermaking flow direction, and the direction showing the minimum dielectric loss was the papermaking width direction. Table 1 shows the dielectric loss (ε ″ _p , ε ″ _v ) and the ratio (ε ″ _p / ε ″ _v ) in these two directions.

As shown in Table 1, the dielectric loss ratio (ε ″ _p / ε ″ _v ) was 1.5 to 1.9 for radio waves of 3 to 18 GHz.

(Sheet for flat liner )
The following fibers and water-containing inorganic compounds are mixed in a mass ratio described in water as a medium, wet papermaking is performed at a take-up speed of 100 m / min, and a planar shape electrical material having a thickness of 0.12 mm and a basis weight of 100 g / m ² is obtained. A sheet B containing no loss material was obtained as a planar liner sheet.

Chopped glass fiber having a fiber length of 6 mm and a fiber diameter of 7 μm: 30% by mass
Aramid pulp with an average fiber length of 1 mm: 10% by mass
Aluminum hydroxide: 60% by mass.

(Radio wave absorbing sheet material)
The electrical loss material-containing sheet A is processed into a waveform by a corrugator so that the direction showing the maximum dielectric loss in the plane of the sheet and the ridge line direction of the waveform are orthogonal to each other. A core with an interval between corrugations of 5 mm was produced.

Subsequently, the core and the flat liner sheet B were adhered to each other with a starch adhesive having a coating amount of 5 g / m ² by the same corrugator to prepare a radio wave absorbing sheet material C having a double-sided cardboard structure. At this time, the feeding direction (longitudinal direction) of corrugated board production was made to be orthogonal to the ridgeline direction of the corrugated core.

  Table 2 shows the results of measuring the dielectric loss when this electric field absorbing sheet material C was applied with an electric field parallel to the longitudinal direction of the corrugated board structure and when the electric field was applied parallel to the width direction.

  As shown in Table 2, the obtained electromagnetic wave absorbing sheet material C had extremely small dielectric loss anisotropy. Further, the radio wave absorbing sheet material C had flame retardancy equivalent to V-0 of UL94, the plane compressive strength was 65 kPa, and the vertical compressive strength was 2 kN / m.

[Example 2]
(Radio wave absorber)
By laminating the two electromagnetic wave absorbing sheet materials C obtained in Example 1 with the direction of the middle core steps aligned, adhering the layers with a starch-based adhesive, and cutting to a size of 60 cm × 60 cm A radio wave absorber was prepared.

  Table 3 shows the results of measuring the radio wave absorptivity when an electric field is applied parallel to the longitudinal direction of the corrugated board structure and when the electric field is applied parallel to the width direction.

  As shown in Table 3, the obtained radio wave absorber had extremely small radio wave absorption anisotropy.

[Example 3]
(Core sheet)
The following fibers and water-containing inorganic compounds are mixed with water as a medium in the mass ratios described below, wet papermaking is performed at a winding speed of 60 m / min, and an electrical loss material having a thickness of 0.15 mm and a basis weight of 120 g / m ² is contained. Sheet D was obtained as a core sheet.
PAN-based carbon fiber having an average fiber length of 12 mm and a fiber diameter of 7 μm: 0.4% by mass
Cellulose fiber having an average fiber length of 2 mm: 20% by mass
Acrylic short fiber having a fiber length of 6 mm and a fiber diameter of 20 μm: 4.6% by mass
Aluminum hydroxide: 75% by mass.

The direction showing the maximum dielectric loss in the plane of the electrically lossy material-containing sheet D was the papermaking flow direction, and the direction showing the minimum dielectric loss was the papermaking width direction. Table 4 shows the dielectric loss (ε ″ _p , ε ″ _v ) and the ratio (ε ″ _p / ε ″ _v ) in these two directions.

As shown in Table 4, the dielectric loss ratio (ε ″ _p / ε ″ _v ) was 1.2 to 1.6 for radio waves of 3 to 18 GHz.

(Sheet for flat liner )
The following fibers and water-containing inorganic compounds are mixed in water at a mass ratio described below as a medium, wet papermaking is performed at a winding speed of 100 m / min, and an electrical loss material having a thickness of 0.18 mm and a basis weight of 160 g / m ² is obtained. Sheet E which does not contain was obtained.
Cellulose fiber having an average fiber length of 2 mm: 20% by mass
Acrylic short fiber having a fiber length of 6 mm and a fiber diameter of 20 μm: 5% by mass
Aluminum hydroxide: 75% by mass.

(Radio wave absorbing sheet material)
The core sheet D is processed into a waveform by a corrugator so that the direction showing the maximum dielectric loss in the plane of the sheet and the ridge line direction of the waveform are orthogonal to each other, and the height of the waveform is 3.5 mm. A core with an interval between waveforms of 8.8 mm was produced.

Subsequently, using the same corrugator, the core and the planar liner sheet E were adhered with a starch adhesive having a coating amount of 5 g / m ² to prepare a radio wave absorbing sheet material F having a double-sided cardboard structure. At this time, the feeding direction (longitudinal direction) of corrugated board production was made to be orthogonal to the ridgeline direction of the corrugated core.

  Table 5 shows the results of measuring the dielectric loss when this electric field absorbing sheet material F was applied with an electric field parallel to the longitudinal direction of the cardboard and when the electric field was applied parallel to the width direction.

  As shown in Table 5, the obtained radio wave absorption sheet material F had extremely small dielectric loss anisotropy. Moreover, the said radio wave absorption sheet material F had the flame retardance equivalent to UL94 VTM-1, the plane compressive strength was 85 kPa, and the vertical compressive strength was 25 kN / m.

[Example 4]
(Radio wave absorber)
Four slices of isosceles triangles having a base of 60 cm and a height of 150 cm were cut out from the radio wave absorbing sheet material F obtained in Example 3. At that time, it was cut so that the height direction of the isosceles triangle section and the longitudinal direction of the cardboard were parallel.

  The four equilateral sides of this isosceles triangle section were bonded together with an adhesive tape to produce a hollow pyramid-shaped hollow three-dimensional structure having a bottom surface of 60 cm × 60 cm.

  Furthermore, this hollow three-dimensional structure radio wave absorber was mounted on a sintered ferrite tile having a size of 60 cm × 60 cm.

  As a result of measuring the radio wave absorption characteristics of this radio wave absorber for radio waves of 30 MHz to 18 GHz, good absorption characteristics of 20 dB or more were obtained over the entire frequency range. Moreover, since the said electromagnetic wave absorber has moderate rigidity, even if it moved the place, the dimension of the structure was hold | maintained favorably.

[Example 5]
(Radio wave absorber)
Four slices of isosceles triangles having a base of 60 cm and a height of 150 cm were cut out from the radio wave absorbing sheet material F obtained in Example 3. At that time, it was cut so that the height direction of the isosceles triangle section and the longitudinal direction of the cardboard were orthogonal to each other.

  The equal sides of the four pieces of the isosceles triangles were pasted together with an adhesive tape to produce a hollow pyramid-shaped hollow three-dimensional structure having a bottom surface of 60 cm × 60 cm.

  Furthermore, this hollow three-dimensional structure radio wave absorber was mounted on a sintered ferrite tile having a size of 60 cm × 60 cm.

  As a result of measuring the radio wave absorption characteristics of this radio wave absorber for radio waves of 30 MHz to 18 GHz, good absorption characteristics of 20 dB or more were obtained over the entire frequency range.

  Examples 4 and 5 show that when the radio wave absorbing sheet material of the present invention is used, radio wave absorbers having the same performance can be produced even when the cutting directions are different.

[Example 6]
(Corrugated cardboard containing no electrical loss material)
The same sheet B as that obtained for the planar liner in Example 1 was processed into a waveform by a corrugator to produce a core having a waveform height of 2.5 mm and an interval between adjacent waveforms of 5 mm.

Subsequently, the same core and the same flat liner sheet B obtained in Example 1 were adhered with the same corrugator with a starch adhesive having a coating amount of 5 g / m ² to produce a single-sided cardboard.

(Radio wave absorber)
The radio wave absorbing sheet material C obtained in Example 1 and the single-sided corrugated cardboard are bonded together with a starch-based adhesive having a coating amount of 5 g / m ² so that the core side of the single-sided corrugated board becomes an adhesive portion. Was made.

  The dielectric loss between the case where an electric field is applied parallel to the longitudinal direction of the corrugated board structure and the case where the electric field is applied parallel to the width direction of the radio wave absorbing sheet material G is the radio wave absorbing sheet material obtained in Example 1. The measurement results were the same as C, and the dielectric loss anisotropy was extremely small. Moreover, the said radio wave absorption sheet material G had the flame retardance equivalent to UL94 VTM-1, and the vertical compressive strength was 3.2 kN / m.

[Comparative Example 1]
(Core sheet)
The same sheet A as obtained in Example 1 was used as the core sheet.

(Sheet for flat liner )
The same sheet B obtained in Example 1 was used as a sheet for a planar liner .

(Radio wave absorbing sheet material)
With respect to the core sheet A, the corrugator processes the corrugator so that the direction of the maximum dielectric loss in the plane of the sheet and the ridge line at the top of the corrugation are parallel, the corrugation height is 2.5 mm, A core with an interval between adjacent waveforms of 5 mm was produced.

Subsequently, using the same corrugator, the core and the flat liner sheet B were adhered with a starch adhesive having a coating amount of 5 g / m ² to prepare a radio wave absorbing sheet material I having a double-sided cardboard structure. At this time, the feeding direction (longitudinal direction) of corrugated board production was made to be orthogonal to the ridgeline direction of the corrugated core.

  Table 6 shows the results of measuring the dielectric loss when the electric field is applied in parallel to the longitudinal direction of the corrugated cardboard and when the electric field is applied in parallel to the width direction.

  As shown in Table 6, the difference between the dielectric losses in both directions was as large as 30 or more, and a large anisotropy occurred.

It is a perspective view which shows an example of the electromagnetic wave absorption sheet material of this invention.

Explanation of symbols

1 Radio wave absorbing sheet material 2 Core 3 Flat liner

Claims (21)

A direction (p) showing a maximum dielectric loss in a plane when the electrically lossy material-containing sheet has a planar shape , comprising a corrugated structure in which a corrugated core of the electrically lossy material-containing sheet and a planar liner are laminated. The ratio (ε ″ _p / ε ″ _v ) of the dielectric loss (ε ″ _p ) in this case to the dielectric loss (ε ″ _v ) in the direction (v) orthogonal to this direction (p) is 1.2-4. An electromagnetic wave absorbing sheet material having a direction and having a direction (p) indicating the maximum dielectric loss orthogonal to a corrugated ridge line direction.   The radio wave absorbing sheet material according to claim 1, wherein the electrical loss material-containing sheet is a mixed paper containing conductive fibers as an electrical loss material.   The radio wave absorbing sheet material according to claim 2, wherein the conductive fiber has a fiber length of 2 to 20 mm.   The electromagnetic wave absorbing sheet material according to claim 2 or 3, wherein a content of the conductive fiber in the mixed paper is 0.1 to 10% by mass.   The electromagnetic wave absorbing sheet material according to any one of claims 2 to 4, wherein the electrical loss material-containing sheet further includes a flame retardant fiber.   The electromagnetic wave absorbing sheet material according to any one of claims 2 to 5, wherein the electrical loss material-containing sheet further contains 11 to 63.9% by mass of cellulose fibers and 36 to 88.9% by mass of a hydrous inorganic compound.   The radio wave absorbing sheet material according to any one of claims 2 to 6, wherein the mixed paper is made by a wet paper making method using water as a medium. The radio wave absorbing sheet material according to claim 1, wherein the basis weight of the electrical loss material-containing sheet is 120 to 200 g / m ² .   The radio wave absorbing sheet material according to any one of claims 1 to 8, wherein the corrugated height of the core is 2 to 5 mm, and an interval between adjacent corrugations is 4 to 15 mm.   The radio wave absorbing sheet material according to claim 9, wherein the height of the corrugated core is 2.5 to 5 mm. The radio wave absorption sheet material according to any one of claims 1 to 10, wherein the planar liner contains 12 to 60% by mass of cellulose fibers and 40 to 88% by mass of a hydrous inorganic compound. The radio wave absorbing sheet material according to claim 1, wherein the planar liner has a basis weight of 120 to 400 g / m ² .   The radio wave absorbing sheet material according to claim 1, wherein the corrugated board structure has a structure in which the core is laminated in a plurality of stages.   The radio wave absorbing sheet material according to claim 13, comprising a configuration in which a corrugated cardboard structure composed of a core made of a sheet not containing an electrical loss material is mixed in the plurality of corrugated cardboard structures.   The radio wave absorbing sheet material according to any one of claims 1 to 14, wherein the plane compressive strength is 40 to 250 kPa.   The radio wave absorbing sheet material according to claim 1, wherein the vertical compressive strength is 1.5 to 8 kN / m.   The electromagnetic wave absorber which consists of an electromagnetic wave absorption sheet material in any one of Claims 1-16.   The radio wave absorber according to claim 17, wherein a plurality of the radio wave absorbing sheet materials are laminated.   The radio wave absorber according to claim 17, wherein the radio wave absorbing sheet material is assembled into a hollow three-dimensional structure having at least one shape selected from a wedge shape, a polygonal pyramid shape, and a polygonal column shape.   The radio wave absorber according to claim 19, comprising a configuration in which the hollow three-dimensional structure is erected on a sintered ferrite plate.   20. The radio wave absorber according to claim 19, wherein the hollow three-dimensional structure is configured to stand on a plate-like body in which a plurality of the radio wave absorption sheet materials are stacked.

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