JP2019134011A - Electromagnetic wave absorption member - Google Patents

Abstract

To provide an electromagnetic wave absorption member that can ensure electromagnetic wave absorptivity at a frequency of 10 GHz or more in the electromagnetic wave absorption member including a carbon-based filler.SOLUTION: An electromagnetic wave absorption member 3 is a composite material having a surface including a plurality of scaly graphite fillers 1 and a matrix material 2 in which the plurality of scaly graphite fillers are dispersed. From among two angles formed by a basal plane C of the scaly graphite filler and the surfaces A and B of the composite material, the average angle θ of the smaller angle is 30 degrees or less.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an electromagnetic wave absorber. In particular, the present invention relates to an electromagnetic wave absorbing material obtained by kneading a carbon material in a matrix material.

  In 2020, it is said that automatic driving of automobiles will be in full swing. The first necessary technique in automatic driving is to detect a distance so as not to collide with other vehicles or people. One method is to transmit electromagnetic waves (radar waves), receive reflected electromagnetic waves, and calculate the distance from the time difference. For this purpose, it is necessary to detect the electromagnetic wave of the own vehicle from the electromagnetic wave emitted by a large number of automobiles and calculate the distance in a short time. That is, if there are few electromagnetic waves to receive, calculation time will also become short and the danger of a collision will also decrease. For this purpose, it is only necessary to shield electromagnetic waves from other vehicles that are not necessary for distance detection and unwanted electromagnetic waves from the vehicle before reaching the antenna. Although unnecessary electromagnetic waves are still shielded at present, metal scattering is mainly used.

  However, if it is scattered, the unnecessary electromagnetic wave flies to another vehicle and hinders distance calculation there. Examples of the scattering of unnecessary electromagnetic waves include a method using a metal plate such as an aluminum plate, a method of applying metal plating, and the like. There are methods for absorbing unnecessary electromagnetic waves, such as a method using a metal fiber metal mesh, a method of covering the surface with a conductive material, and a method of adding a conductive filler to a resin / rubber material, but they cannot be used for absorbing electromagnetic waves of 10 GHz or more. Although it is said that electromagnetic wave absorption of 10 GHz or more can be absorbed by adding a carbon-based filler to a resin / rubber, there has not been enough (for example, see Patent Document 1).

Japanese Patent No. 4746803

  In the case of an electromagnetic wave absorbing material containing a general carbon-based filler, the electromagnetic wave absorptivity is manifested when the filler kneading amount exceeds a certain amount, and further shifts to scattering when it exceeds a certain kneading amount. Therefore, as in Patent Document 1, there is a limit to electromagnetic wave absorption of 10 GHz or more only by dispersing the carbon-based filler in the matrix material, and there is a problem that it is difficult to realize electromagnetic wave absorption of 10 GHz or more.

  This invention solves the said conventional subject, and it aims at providing the electromagnetic wave absorber which implement | achieves the electromagnetic wave absorptivity of 10 GHz or more.

In order to solve the above problems, an electromagnetic wave absorber according to the present invention includes a plurality of scaly graphite fillers,
A matrix material in which a plurality of scaly graphite fillers are dispersed;
An electromagnetic wave absorbing material which is a composite material having a surface,
Of the two angles formed by the basal surface of the scaly graphite filler and the surface of the composite material, the smaller average angle θ is 30 degrees or less.

  According to the electromagnetic wave absorbing material according to the present invention, the scaly graphite filler is dispersed in the matrix material, and the average angle θ of the smaller angles of the two angles formed by the basal surface of the scaly graphite filler and the surface of the composite material. Is 30 degrees or less. As a result, a composite material having high absorptivity with respect to an electromagnetic wave of 10 GHz or more incident on the surface of the composite material substantially perpendicularly can be obtained.

It is sectional drawing cut | disconnected by the surface perpendicular | vertical with respect to the surface of the composite material (electromagnetic wave absorber) which kneaded the scaly graphite filler which concerns on Embodiment 1. FIG. It is a perspective view which shows the shape of a scale-like graphite filler. 2 is a graph showing the major axis d and electromagnetic wave absorptivity of the scaly graphite filler of Example 1. 4 is a graph showing the kneading amount and electromagnetic wave absorptivity of the scaly graphite filler of Example 2. It is a graph which shows the alignment angle (theta) and electromagnetic wave absorptivity of the scaly graphite filler of Example 3.

The electromagnetic wave absorber according to the first aspect includes a plurality of scaly graphite fillers,
A matrix material in which a plurality of scaly graphite fillers are dispersed;
An electromagnetic wave absorbing material which is a composite material having a surface,
Of the two angles formed by the basal surface of the scaly graphite filler and the surface of the composite material, the smaller average angle θ is 30 degrees or less.

  The electromagnetic wave absorbing material according to a second aspect is the above-mentioned first aspect, wherein the average value of the maximum diameter on the basal surface of the scaly graphite filler is d, and the maximum thickness orthogonal to the basal surface of the scaly graphite filler is When the average value is e, d is 5 μm or more and 300 μm or less, the aspect ratio d / e is 10 or more, and the full width at half maximum (FWHM) by X-ray diffraction of the scaly graphite filler is 15 degrees. It may be the following.

  In the electromagnetic wave absorbing material according to the third aspect, in the first or second aspect, a kneading amount of the scaly graphite filler in the composite material may be 2 wt% or more and 30 wt% or less.

  In the electromagnetic wave absorbing material according to the fourth aspect, in any one of the first to third aspects, the matrix material may be a thermoplastic resin or an elastomer.

  The electromagnetic wave absorbing material according to a fifth aspect is the electromagnetic wave absorbing material according to any one of the first to fourth aspects, wherein the electromagnetic wave of 1 GHz or more substantially absorbs when the electromagnetic wave is incident on the surface of the composite material substantially perpendicularly. When the light is incident substantially parallel to the surface of the material, the light may pass through substantially.

  Hereinafter, an electromagnetic wave absorber according to an embodiment will be described with reference to the accompanying drawings. In the drawings, substantially the same members are denoted by the same reference numerals.

(Embodiment 1)

  FIG. 1 is a cross-sectional view taken along a plane perpendicular to the surface A of a composite material (electromagnetic wave absorber) 3 in which scale-like graphite filler according to Embodiment 1 is kneaded. As shown in FIG. 1, the electromagnetic wave absorbing material 3 is a composite material in which scaly graphite fillers 1 are dispersed in a matrix material 2. The composite material 3 has surfaces A and B. Of the two angles formed by the basal plane C of the scale-like graphite filler 1 and the surface A of the composite material, the average angle θ of a small angle is 30 degrees or less. According to the composite material 3, an electromagnetic wave of 1 GHz or more is substantially absorbed when incident on the surface A of the composite material 3 substantially perpendicularly, and is substantially allowed to pass when incident on the surface of the composite material 3 substantially parallel. .

  Each member which comprises this composite material (electromagnetic wave absorber) is demonstrated below.

<Electromagnetic wave absorber>
The electromagnetic wave absorbing material is one of electromagnetic wave shielding materials. The electromagnetic wave shielding material refers to a material in which the electromagnetic wave irradiated on one surface A is not detected on the back surface B. As a realization method, there are reflection / scattering on the surface A, scattering in the shielding material, absorption in the shielding material, and a combination thereof. The electromagnetic wave absorbing material is a material that absorbs the irradiated electromagnetic wave in the shielding material and does not leave it.

  The electromagnetic wave absorbing material has a plate shape, a sheet shape, and the like, and can be molded into a box shape. FIG. 1 shows a case where the electromagnetic wave absorber is plate-shaped. As shown in FIG. 1, the surface A and the surface B of the electromagnetic wave absorber 3 and the basal surface C of the scaly graphite filler 1 kneaded in the electromagnetic wave absorber 3 are aligned in parallel. Here, the basal plane will be briefly described. Graphite has a crystal structure in which six-membered rings of carbon atoms connected in a planar network form a layer. The plane network surface of this six-membered ring is defined as the basal surface. The angle θ formed by the surface A of the electromagnetic wave absorber 3 and the basal plane C of the scaly graphite filler 1 is preferably within 30 degrees. Hereinafter, the angle θ formed by the surface A of the electromagnetic wave absorber 3 and the basal plane C of the scaly graphite filler 1 is defined as “alignment angle θ”. With such a configuration, the electromagnetic wave incident substantially perpendicularly on the surface A of the electromagnetic wave absorber 3 is attenuated by multiple reflection by the scaly graphite filler 1 in the electromagnetic wave absorber 3 and does not come out. When the kneading amount of the scaly graphite filler in the composite material 3 exceeds 30 wt%, the path of the electromagnetic wave is reduced, multiple reflection is difficult to occur, and the electromagnetic wave absorptivity is deteriorated. The energy of the electromagnetic wave confined inside the electromagnetic wave absorbing material 3 by the absorption of the electromagnetic wave becomes heat, and is dissipated and dissipated by the scaly graphite filler having a high thermal conductivity.

  The electromagnetic wave absorption characteristics were evaluated by the waveguide method. As a judgment of pass / fail, it was decided that the electromagnetic wave absorptivity was 10 dB or more. When there is no electromagnetic wave absorptivity of 10 dB or more, electromagnetic waves radiated from other electronic devices may enter the electronic device, which may cause the electronic device to malfunction.

<Definition of alignment angle θ>
Here, the definition of the alignment angle θ will be described with reference to FIG. The smaller angle of the two angles formed by the surface A of the electromagnetic wave absorber 3 and the basal surface C of the scaly graphite filler 1 is the alignment angle θ. In FIG. 1, the surface of the flaky graphite filler 1 and the basal plane C of the flaky graphite filler 1 are schematically shown as the same as shown in FIG. Since the scale-like graphite filler 1 is homogeneously kneaded in the electromagnetic wave absorbing material 3, the cross section may be arbitrary as long as it is perpendicular to the surface of the electromagnetic wave absorbing material 3. The method for obtaining the alignment angle θ is that when an arbitrary cross section is observed as shown in FIG. 1, the surface A of the electromagnetic wave absorbing material and the basal surface C of the scaly graphite filler are related to 20 or more scaly graphite fillers 1. The angle of the extension line is measured, and the averaged value is defined as the alignment angle θ. There are two angles at which the lines intersect, but the smaller acute angle is the alignment angle θ. Note that the angle between the surfaces changes each time a cross section is taken, but here, the alignment angle θ is defined by averaging 20 or more angles that can be seen in an arbitrary cross section.

<Scaly graphite filler 1>
The major axis d of the scaly graphite filler is the longest diameter of the maximum area basal surface. The thickness e of the scaly graphite filler is the thickness of the thickest portion measured perpendicular to the basal plane. The major axis d is 1 μm or more and 500 μm or less on average, and the d / e ratio (aspect ratio) is 10 or more on average. The major axis d is preferably 400 μm or less, and more preferably 300 μm or less.
The scaly graphite filler can be obtained, for example, by pulverizing artificial graphite or natural graphite. Natural scaly graphite powder may be used as it is, and plural kinds of graphite fillers may be mixed and used as long as the major axis d and the d / e ratio (aspect ratio) satisfy the above conditions.

<Evaluation of alignment degree of scaly graphite filler by X-ray>
In the flaky graphite filler, the ordered layered shape as shown in FIG. 2 is schematically shown, and the flaky graphite filler does not consist of one single crystal. That is, the scaly graphite filler is actually composed of crystallite aggregates arranged in substantially the same direction. Therefore, the overall crystallinity can be evaluated by evaluating the degree of alignment of the aggregate of crystallites constituting the scaly graphite filler by X-ray diffraction.
The degree of alignment of the crystallite aggregate is evaluated as follows.
a) Several mg of scaly graphite filler is pressed between flat glass plates and pressed. After removing the glass plate on one side, it is set on a sample holder and irradiated with X-rays. At this time, first, a 2θ-θ scan in which the incident angle and the reflection angle are synchronized is performed to search for an angle at which the peak value of the maximum intensity is obtained.
b) The X-ray source and the detector are fixed at an angle 2θ at which the peak value of the maximum intensity is obtained, and only the θ scan by the rotation of the sample holder is performed to obtain the full width at half maximum (FWHM) with respect to the peak value. This is sometimes called X-ray rocking curve measurement.
The obtained full width at half maximum is used as an index of the degree of alignment of crystallite aggregates in the scaly graphite filler. For example, when the full width at half maximum (FWHM) by X-ray diffraction of the scaly graphite filler is 15 degrees or less, it can be evaluated that the degree of alignment of the aggregate of crystallites is good.

<Method for producing scaly graphite filler 1>
A method for producing the scaly graphite filler will be described. Artificial graphite can be obtained by heat-treating a polymer film at a high temperature of 2400 ° C. or higher, preferably 2600 to 3300 ° C. under an inert gas. The heat treatment may be performed in one step, or may be performed in two or more steps, each changing the temperature. The inert gas is not particularly limited, but nitrogen gas, argon gas, and the like are preferable because they are inexpensive. Although heat processing time is not specifically limited, For example, 2 to 10 hours are preferable.

  The thickness of the polymer film before graphitization may be selected as appropriate, and is, for example, 400 μm or less, and preferably 1 μm or more and 150 μm or less. Even when a relatively thick polymer film is used, peeling occurs between graphite layers when the graphite film is pulverized, so that a thinner scaly graphite filler can be obtained. When the polymer film is thicker than 400 μm, it becomes difficult to apply heat evenly in the film, and the crystallinity of graphite is lowered. On the other hand, if it is thinner than 1 μm, it is destroyed by heat treatment.

  Examples of the polymer film include polyimide, polyamideimide, polyoxadiazole, polybenzothiazole, polybenzobisthiazole, polybenzoxazole, polybenzobisthiazole, poly (p-phenyleneisophthalamide), and poly (m-phenylene). Benzimitazole), poly (phenylene benzobisimitazole), polythiazole, polyparaphenylene vinylene and the like are preferable. The manufacturing method is not limited to the above. These materials may be used individually by 1 type, and may be used in combination of multiple types. For example, a plurality of different types of films may be graphitized and pulverized, and then mixed, or a plurality of types of materials may be combined or alloyed in advance to form a film, and the film may be graphitized for use. Good. A scaly graphite filler is obtained by pulverizing the obtained graphite film. The pulverizing method is not particularly limited, but a jet mill method in which graphite fillers collide with each other or a graphite filler and a substance having high hardness are physically collided is preferable. Examples of other methods include a ball mill method, a nanomizer method, and a freeze pulverization method. What is necessary is just to select the thickness of the graphite film to grind | pulverize suitably according to the thickness e of the desired scaly graphite filler.

<Matrix material 2>
The matrix material 2 can be a thermoplastic resin or an elastomer. You may use the mixture of the thermoplastic resin which does not have elasticity, and the elastomer which has elasticity.
Thermoplastic resins include styrene polymers such as styrene-acrylonitrile copolymers, styrene-maleic anhydride copolymers, (meth) acrylic acid ester-styrene copolymers, and rubber-reinforced resins such as DES resins and DES resins. , Polyethylene, polypropylene, ethylene-vinyl acetate copolymer, ethylene-vinyl alcohol copolymer, olefin polymer such as chlorinated polyethylene, polyvinyl chloride, ethylene-vinyl chloride polymer, polyvinyl chloride such as polyvinylidene chloride Polymers, (meth) acrylic ester polymers such as polymethyl methacrylate, imide polymers such as polyamide, polyimide, polyamideimide, and polyetherimide, polyester polymers such as polyethylene terephthalate and polybutylene terephthalate; polyacetal , Polica Fluorine resins such as boronate, polyarylate, polyphenylene ether, polyphenylene sulfide, polytetrafluoroethylene, and polyvinylidene fluoride; ketone polymers such as polyether ketone and polyether ether ketone; and sulfone polymers such as polysulfone and polyether sulfone , Urethane polymer, polyvinyl acetate and the like. These may be used alone or in combination of two or more, or a plurality of these may be alloyed and used.

  The elastomer is not particularly limited, but chloroprene rubber, isoprene rubber, natural rubber, styrene-butadiene rubber, butadiene rubber, butyl rubber, ethylene-propylene rubber, ethylene-propylene-diene rubber (EPDM), nitrile rubber, urethane rubber, acrylic Examples thereof include rubber, silicone rubber, fluorine rubber, and hydrogenated nitrile rubber. These may be used alone or in combination of two or more.

(Example)
The composite material (electromagnetic wave absorbing material) is produced by the following two steps (i) and (ii).
(I) Production of scale-like graphite filler 1 A polyimide film having a thickness of 25 μm (manufactured by Toray DuPont, Kapton film) was heat-treated at 2600 ° C. for 4 hours in an argon gas atmosphere to obtain a graphite film. The obtained graphite film was pulverized for 15 minutes by a jet mill. By changing the number of rotations of the classification part at the time of pulverization, the average value of the major axis d of the scaly graphite filler can be changed to 1 to 500 μm. The average value e of the maximum thickness e orthogonal to the basal plane of the scaly graphite filler produced by this method is 10 or more, and the FWHM (full width at half maximum, FWHM) by X-ray diffraction is 15 It was less than degree. Even with natural graphite, the filler major axis d can be changed by pulverization.

(Ii) Manufacture of electromagnetic wave absorbing material 3 The obtained scaly graphite filler 1 is sufficiently kneaded with a heat-dissolved polypropylene resin using an 8-inch two-roll kneader, and an injection part having an interval of 1 mm. The composition having a thickness of 1 mm was obtained by further extruding it onto a cooling plate. The scaly graphite filler 1 in the composition was able to change the alignment angle θ of the scaly graphite filler in the composition from 5 degrees to 40 degrees depending on the extrusion speed. Here, a polypropylene resin is used, but a thermoplastic resin and an elastomer can also be used.

Example 1
In Example 1, the major axis d of the scaly graphite filler was made differently from 1 to 500 μm by the method (i). Further, by the method (ii), the same amount of flaky graphite filler was kneaded with polypropylene resin as a matrix material, and compositions having different kneaded flaky graphite filler sizes were obtained. A graph of the electromagnetic wave absorption characteristics of the composition is shown in FIG. Electromagnetic wave absorptivity of 10 dB or more was observed when the major axis d was in the range of 5 to 200 μm.

(Example 2)
In Example 2, a scaly graphite filler having a major axis d of 100 μm was produced by the method (i). In addition, by the method (ii), a scaly graphite filler was kneaded with polypropylene resin to obtain compositions having different kneading amounts. A graph of the electromagnetic wave absorption characteristics of the composition is shown in FIG. Electromagnetic wave absorptivity of 10 dB or more was observed when the kneading amount was 2 wt% or more and 30 wt% or less.

Example 3
In Example 3, a scaly graphite filler having a major axis d of 100 μm was produced by the method (i). In addition, by the method (ii), the same amount of flaky graphite filler is kneaded into the polypropylene resin, and the extrusion angle is changed, so that the alignment angle θ of the flaky graphite filler in the composition is from 5 degrees to 45 degrees. An altered composition was obtained. A graph of the electromagnetic wave absorption characteristics of the composition is shown in FIG. An electromagnetic wave absorptivity of 10 dB or more was observed when the alignment angle θ was 30 degrees or less.

Example 4
In Example 4, a scaly graphite filler having a major axis d of 100 μm was produced by the method (i). Moreover, it knead | mixed with the polypropylene resin by the method of (ii), and obtained the composition whose alignment angle (theta) of the scaly graphite filler in a composition is 5 degree | times. From FIG. 5, when the alignment angle θ is 5 degrees, the absorption characteristic of the electromagnetic wave incident substantially perpendicularly on the surface of the composition is 20 dB. On the other hand, when the composition was irradiated with electromagnetic waves parallel to the surface of the composition, the electromagnetic wave absorption characteristics were 1 dB, which was about 1/20 of the absorption characteristics for electromagnetic waves irradiated from the vertical direction. That is, in this composition, electromagnetic waves incident substantially perpendicularly to the surface of the composition are substantially absorbed, and electromagnetic waves incident substantially parallel to the surface of the composition are allowed to pass substantially.

  In the present disclosure, the size of the scaly graphite filler 1 is discussed as an average value, but when the number is small, one scaly graphite filler 1 is established with the above size.

  It should be noted that the present disclosure includes appropriately combining any of the various embodiments and / or examples described above, and each of the embodiments and / or examples. The effect which an Example has can be show | played.

  The electromagnetic wave absorbing material according to the present invention is suitable for use in shielding unnecessary electromagnetic waves of equipment using an electromagnetic wave of 10 GHz or higher, such as an automobile collision prevention system.

1: scale-like graphite filler 2: matrix material 3: composite material (electromagnetic wave absorbing material)
A: Surface on which electromagnetic wave is applied to electromagnetic wave absorbing material B: Back surface of surface A of electromagnetic wave absorbing material C: Basal surface of scale-like graphite filler d: Average value of maximum diameter of scale-like graphite filler e: Scale-like graphite filler Average thickness θ: Alignment angle

Claims (5)

A plurality of scaly graphite fillers;
A matrix material in which a plurality of scaly graphite fillers are dispersed;
An electromagnetic wave absorbing material which is a composite material having a surface,
The electromagnetic wave absorbing material, wherein an average angle θ of a small angle of two angles formed by the basal surface of the scaly graphite filler and the surface of the composite material is 30 degrees or less.   When the average value of the maximum diameter on the basal surface of the flaky graphite filler is d and the average value of the maximum thickness orthogonal to the basal surface of the flaky graphite filler is e, d is 5 μm or more and 300 μm or less, The electromagnetic wave absorbing material according to claim 1, wherein the aspect ratio d / e is 10 or more, and the full width at half maximum (FWHM) by X-ray diffraction of the scaly graphite filler is 15 degrees or less.   The electromagnetic wave absorbing material according to claim 1 or 2, wherein a kneading amount of the scaly graphite filler in the composite material is 2 wt% or more and 30 wt% or less.   The electromagnetic wave absorbing material according to any one of claims 1 to 3, wherein the matrix material is a thermoplastic resin or an elastomer.   5. The electromagnetic wave according to claim 1, wherein electromagnetic waves of 1 GHz or more are substantially absorbed when incident on the surface of the composite material substantially perpendicularly, and are substantially transmitted when incident on the surface of the composite material substantially parallel to the surface of the composite material. The electromagnetic wave absorber according to one item.

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