JP2017171773A - Foamed particle molding - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a foamed particle molding which can be suitably used for a radio wave absorber exhibiting an excellent radio wave absorption performance.SOLUTION: A foamed particle molding is provided, which is a thermoplastic resin foamed particle molding including: foamed particles A with a conductive material of 3-30 mass% dispersed therein; and foamed particles B with the content of the conductive material of less than 3 mass% (including 0) as the foamed particles constituting the foamed particle molding, in which the average value of an area ratio (S1/S2) of the total area (S1) of the foamed particles A to the total area (S2) of the foamed particles B in the section of the foamed particle molding is within a range of 0.05-1.0, and a coefficient of variation of the area ratio is 20% or less.SELECTED DRAWING: None

Description

  The present invention relates to a foamed particle molded body that can be suitably used as a radio wave absorber.

  To conduct various tests related to radio waves, such as evaluation tests for conformity to radio waves radiated from electronic equipment, tests for evaluating radio wave reception characteristics of communication equipment such as antennas, etc. In addition, a facility having an electromagnetic wave isolation space called an anechoic chamber is used.

  Standards related to jamming radio waves often specify the allowable value of jamming radio waves in a predetermined radio frequency band. Conventionally, radio wave frequency bands used for such standards are used in televisions, radios, mobile phones, etc. The main frequency band is 30 MHz to 1 GHz. However, with the recent development of wireless LANs, ETCs, smartphones, etc., the frequency band of radio waves used for standards has expanded to a range of 30 MHz to 18 GHz.

  Regarding the evaluation of radio wave reception characteristics, equipment and communication equipment using microwave and millimeter wave radio waves of 10 GHz or higher such as optical communication equipment and radar for preventing car collisions have been developed. There is a need for an anechoic chamber that can evaluate the radio wave reception characteristics of communication devices.

  Therefore, the anechoic chamber is required to be able to measure radio waves in a wide range from radio waves in a low frequency band of less than 1 GHz, that is, in the order of MHz, to radio waves in a high frequency band such as a microwave band and millimeter wave band of 1 GHz or more. Has been.

  By the way, in the anechoic chamber, in order to properly perform various tests related to radio waves, not only does the external radio waves enter the space and the space is isolated from the external radio waves, It is required to be configured to suppress the adverse effects caused by interference between the measurement target radio wave and the reflected wave. In consideration of these demands, a structure called a radio wave absorber is usually provided in the anechoic chamber on the top surface, the surrounding wall surface, and the floor surface as necessary. Therefore, the possibility that the measurement target radio wave is reflected by the wall surface and a reflected wave is generated is suppressed.

  Examples of the radio wave absorber include conductive foam particles, which are foam particles filled with a conductive material such as conductive carbon, and a conductive film obtained by coating the surface of resin foam particles with a coating film made of a graphite-based conductive paint. Expanded particle molded bodies formed by molding expandable expanded particles into a predetermined shape are used. A radio wave absorber composed of such a molded body is excellent in radio wave absorption performance for more effectively absorbing radio waves in a so-called GHz band high frequency band of 1 GHz or higher, which is a relatively high frequency.

  Such a radio wave absorber has excellent radio wave absorption performance in a frequency band of 1 GHz or higher that can be classified as a high frequency band, but radio wave absorption performance in a frequency band of less than 1 GHz that can be classified as a low frequency band. There is a problem that it is difficult to cope with radio wave measurement in a wide frequency band as described above. In response to this problem, attempts have been made to obtain a radio wave absorption characteristic in the MHz band by increasing the length of the foamed particle molded body forming the radio wave absorber. However, if the size of the foamed particle molded body itself is increased, the space occupancy rate of the wave absorber in the anechoic chamber increases, resulting in a problem that the limited space cannot be effectively used.

  To solve this problem, as a radio wave absorber, a mixture of conductive foam particles whose surface was coated with a graphite-based conductive paint and non-conductive foam particles not subjected to such a coating treatment was formed in the mold. It has been proposed to use the obtained expanded particle molded body (see, for example, Patent Documents 1 and 2). Such a radio wave absorber also has a radio wave absorption performance in a frequency band around several hundred MHz. In addition, the unit can be prepared by combining the wave absorber with other low frequency band electromagnetic wave absorbers such as ferrite tiles, and the unit can be installed in the anechoic chamber to increase the size of the wave absorber. At least, an anechoic chamber with improved radio wave absorption performance in the MHz band can be realized.

Japanese Patent Laid-Open No. 10-41684 JP-A-11-209505

  However, the radio wave absorbers alone described in Patent Documents 1 and 2 can only slightly improve the radio wave absorption performance in the several hundred MHz band. The radio wave absorbers described in Patent Documents 1 and 2 have room for improvement because there is a risk that a frequency band having a low radio wave absorbability may be generated from the MHz band to the GHz band.

  The present invention has been made in view of the above problems, and an object thereof is to solve the problem of providing a foamed particle molded body that can be suitably used for a radio wave absorber capable of exhibiting excellent radio wave absorption performance. .

The present invention
(1) A thermoplastic resin foam particle molded body,
As the thermoplastic resin foam particles constituting the foamed particle molded body, the foamed particles A in which the conductive material is dispersed in an amount of 3 to 30% by mass, and the content of the conductive material is less than 3% by mass (including 0). ) Expanded particles B,
The average value of the area ratio (S1 / S2) of the total area (S1) of the expanded particles A and the total area (S2) of the expanded particles B in the cross section of the expanded particle molded body is 0.05 to 1.0. Range,
The expanded particle molded article, wherein the variation coefficient of the area ratio is 20% or less,
(2) The foamed particle molded article according to (1), wherein an average particle diameter of each of the foamed particles A and the foamed particles B is 2 to 8 mm.
(3) The expanded particle molded body according to (1) or (2) above, wherein the expanded particle molded body has a density of 15 to 90 g / L.
(4) The foamed particle molded body according to any one of (1) to (3), wherein the conductive material is conductive carbon black.
(5) The expanded particle molded article according to any one of (1) to (4) above, wherein the resin forming the thermoplastic resin expanded particles is a polyolefin resin.
(6) The expanded particle molded body according to any one of (1) to (4) above, wherein the resin forming the thermoplastic resin expanded particles is a polystyrene-based resin.
Is the gist.

  The foamed particle molded body of the present invention has a small variation in conductive performance between the foamed particle molded bodies when a plurality of foamed particle molded bodies are prepared, and foaming is achieved by changing the mixing ratio of the foamed particles A and B. It is easy to change the conductive performance of the particle compact.

  In particular, when the foamed particle molded body of the present invention is used as a radio wave absorber, the foamed particle molded body is a mixed molded body including a plurality of types of foamed particles A and B containing a specific amount of a conductive material. By changing the mixing ratio of the expanded particles A and B, it is easy to change the dielectric constant of the radio wave absorber, and the radio wave absorber has excellent radio wave absorption performance. In addition, when a plurality of foamed particle molded bodies are manufactured, the variation in radio wave absorption performance between the foamed particle molded bodies is also small. Furthermore, the combination of the radio wave absorber formed of the foamed particle molded body of the present invention and the low frequency absorber can exhibit particularly excellent radio wave absorption performance over a wide frequency range from, for example, the MHz band to the GHz band. .

FIG. 1A schematically shows one of the embodiments in the case where the foamed particle molded body in the present invention is formed in a pyramid shape (square pyramid shape) and the foamed particle molded body is used as a radio wave absorber. It is a schematic perspective schematic diagram for explaining in detail. FIG. 1B shows a case where the foamed particle molded body according to the present invention is formed in a shape in which the inclined surface portion of the pyramid-shaped molded part is cut out from the bottom surface of the foamed particle molded body to a predetermined height. It is a schematic perspective schematic diagram for demonstrating typically one of the Examples in the case of utilizing as a radio wave absorber the structure which combined two or more foamed particle compacts. FIG. 2A schematically shows one of the embodiments when the foamed particle molded body in the present invention is formed in a wedge shape (wedge shape) and the foamed particle molded body is used as a radio wave absorber. It is a schematic perspective schematic diagram for demonstrating. FIG. 2B shows a case where the foamed particle molded body according to the present invention is formed in a shape in which the inclined surface portion of the wedge-shaped molded part is cut out from the bottom surface of the foamed particle molded body to a predetermined height. It is a schematic perspective schematic diagram for demonstrating typically one of the Examples in the case of utilizing as a radio wave absorber the structure which combined two or more foamed particle compacts. FIG. 3A schematically illustrates one example of a composite wave absorber using a foamed particle molded body when the foamed particle molded body of the present invention is formed in a pyramid shape. It is a schematic perspective schematic diagram. FIG. 3B schematically illustrates another example of the composite wave absorber using the foamed particle molded body when the foamed particle molded body of the present invention is formed in a pyramid shape. FIG. FIG. 4A is a view for schematically explaining one example of a composite wave absorber using a foamed particle molded body when the foamed particle molded body in the present invention is formed in a wedge shape. It is a schematic perspective schematic diagram. FIG. 4B schematically illustrates another example of the composite wave absorber using the foamed particle molded body when the foamed particle molded body of the present invention is formed in a wedge shape. FIG. FIG. 5A is a circuit diagram for simplifying and explaining an electrical equivalent circuit relating to radio wave absorption in the present invention. FIG. 5B is a schematic diagram for simplifying and explaining the dielectric loss and the conductive loss generated in the foamed particle molded body composed of the foamed particles A and the foamed particles B in the present invention. FIG. 6 shows the amount of conductive carbon added to each of a mixed foamed particle molded body (in-mold molded body), which is a mixed molded body including the foamed particles A and B, and a molded body consisting only of the foamed particles A (in-mold molded body). It is a graph which shows the relationship between (mass%) and an electrostatic capacitance (pF). FIG. 7 is a schematic perspective schematic view showing the shape of a radio wave absorber prepared for measuring radio wave absorption performance in Examples and Comparative Examples. FIG. 8A is a cross-sectional image of a foamed particle molded body. FIG. 8B is a processed image obtained by performing monotone processing on the cross-sectional image of FIG. 8A. FIG. 9A is a surface photograph of the expanded particle molded body obtained in Example 1. FIG. FIG. 9B is a surface photograph of the foamed particle molded body obtained in Comparative Example 2. FIG. 9C is a surface photograph of the foamed particle molded body obtained in Comparative Example 5. FIG. 10 is a TEM photograph showing a state in which the conductive material is dispersed in the resin constituting the expanded particles A.

(Foamed particle molding)
The molded article of the thermoplastic resin foamed particles of the present invention comprises a first foamed particle (thermoplastic resin foamed particle A) in which 3 to 30% by mass of a conductive material is dispersed while using a thermoplastic resin as a base resin and heat. In-mold molding of a foamed particle mixture comprising a plastic resin as a base resin and a second foamed particle (thermoplastic resin foamed particle B) having a conductive material content of less than 3% by mass (including 0) The foamed particle in-mold product. The thermoplastic resin expanded particles A and B are hereinafter simply referred to as expanded particles A and B, respectively. In the foamed particle molded body which is this in-mold molded body, the average of the area ratio (S1 / S2) of the total area (S1) of the foamed particles A and the total area (S2) of the foamed particles B in the cross section of the foamed particle molded body. The value is in the range of 0.05 to 1.0, and the variation coefficient of the area ratio is 20% or less. In the present specification, the foamed particle molded body of the present invention may be referred to as a thermoplastic resin foamed particle molded body or simply a molded body. In addition, the expanded particles A and the expanded particles B may be collectively referred to as expanded particles.

(dispersion)
In the present specification, the dispersion in the expanded particles A in which the conductive material is dispersed does not necessarily coincide with the meaning of dispersion in mathematical terms shown in mathematical formulas (2) and (3) described later. This means that the functional material is scattered and dispersed in the resin constituting the expanded particles. Specifically, a state in which conductive carbon indicated by granular black dots is scattered and scattered on the photograph of the transmission electron microscope shown in FIG.

(Foamed particle density)
The density of the foamed particle molded body is preferably 15 to 180 g / L, more preferably 15 to 90 g / L, from the viewpoint of the balance between lightness and rigidity. When using the foamed particle molded body as a radio wave absorber, in addition to the above viewpoint of balance, from the viewpoint of exhibiting excellent radio wave absorption performance in the foamed particle molded body obtained using the foamed particle mixture, 15 to 90 g / L, more preferably 20 to 60 g / L, and particularly preferably 25 to 50 g / L.

(Measurement of density of molded foam particles)
The density of the foamed particle molded body can be obtained by dividing the mass (g) of the molded body sample by the volume (L) of the molded body sample. In addition, the volume of the molded body sample is calculated by converting the product of the vertical dimension, horizontal dimension, and height dimension based on the external dimensions (vertical, horizontal, height) of the molded body sample cut out from the molded body into a rectangular parallelepiped shape. The volume (L).

(Foamed particle shape)
The shape of the foamed particle molded body can be appropriately set to a plate shape, a columnar shape, and various three-dimensional shapes according to the purpose, and is not particularly limited, but when the foamed particle molded body is used as a radio wave absorber That is, when preparing a foamed particle molded body radio wave absorber according to the present invention, the foamed particle molded body is preferably formed in a shape that can more effectively realize radio wave absorption. Considering this point, the foamed particle molded body of the present invention is foamed on the plane when assuming a plane having a normal line along the direction from the end on the radio wave arrival side to the other end. The area of the cut surface recognized when the particle compact is cut (the total area of the cut surfaces when there are multiple cut surfaces) is from the end on the radio wave arrival side to the other end along the normal. It is preferable that the shape becomes larger as it goes.

  As specifically illustrated in FIGS. 1A and 2A, the foamed particle molded body 2a preferably has a polygonal pyramid shape such as a pyramid shape or a wedge shape, a polygonal frustum shape, or a wedge shape. By forming the molded body 2a in this way, the pyramid-shaped wave absorber 1 shown in FIG. 1 and the wedge-shaped wave absorber 1 shown in FIG. 2 can be obtained. For example, in the pyramid-shaped wave absorber 1 shown in FIG. 1, the pyramid-shaped top is the end on the radio wave arrival side, the pyramid-shaped bottom is the other end, and the direction from the top to the bottom is along the direction. When the foamed particle molded body is cut on a plane having a normal line and the cut surface is viewed, the area of the cut surface increases from the top to the bottom. The same applies to the wedge-shaped electromagnetic wave absorber 1 shown in FIG. Further, the foamed particle molded body 2a may be formed in a shape having a protruding end as shown in FIGS. 1B and 2B. 1B or 2B can be formed by, for example, forming the inclined surface of a pyramid-shaped or wedge-shaped molded body into a shape in which a portion above a predetermined height from the bottom surface is cut out. .

(Foamed particles A and B)
Foamed particles A are foamed resin particles in which a conductive material is dispersed in the range of 3 to 30% by mass. Furthermore, from the viewpoint of workability and radio wave absorption performance, 5 to 25% by mass, further 7 to 20% by mass is preferable, and 10 to 17% by mass is more preferable. The expanded particles B are resin particles formed without containing a conductive material, or resin particles containing a conductive material in a range of less than 3% by mass (including those coated with a conductive material). Or a resin particle foam containing a conductive material in an amount of less than 3% by mass (the foamed particles B include those coated with a conductive material). Therefore, in the present invention, the foamed particles A are formed from a resin composition containing a thermoplastic resin and a conductive material, and the foamed particles B are formed from a resin composition containing a thermoplastic resin and a conductive material used as necessary. ing. Moreover, about any of the foamed particle A and the foamed particle B, other additives other than an electroconductive material may contain suitably. Examples of the additive include a colorant, a flame retardant, an antistatic agent, and a bubble regulator.

  In both the foamed particles A and the foamed particles B, the resin particles in which the conductive material is dispersed are contained in the resin using a conventionally known kneader such as an extruder or a kneader. It can be obtained by dispersing a conductive material and granulating the resin composition in which the conductive material is dispersed with a pelletizer or the like. In addition, foamed particles, which are foamed resin particles, can be obtained by a conventionally known method for producing extruded foam particles, a method of foaming by releasing foamable resin particles containing a foaming agent from an autoclave, and foaming resin particles containing a foaming agent. It can be produced by a conventionally known foaming method such as a method of foaming by heat softening. Regarding the expanded particles B, resin particles and expanded particles coated with a conductive material can be obtained by applying a conductive material such as conductive paint to the resin particles and expanded particles using a coating device such as a spray coater. Can be obtained.

(Thermoplastic resin)
Examples of the foamed particle molded body of the present invention, the foamed particles constituting the molded body, and the thermoplastic resin constituting the resin particles forming the foamed particles include polyolefin resins such as polyethylene resins and polypropylene resins, polystyrene Resin, polycarbonate resin, polyvinyl chloride resin, polymethacrylic resin, acrylonitrile resin, polyester resin, polyamide resin, polyurethane resin and their blends or copolymers, etc. It is done. When a mixed resin of a polyolefin resin and other resin is used as the thermoplastic resin, the mixed resin preferably contains 50% by mass or more of the polyolefin resin, and 70% by mass or more of the polyolefin resin. More preferably, it is more preferably 90% by mass or more of polyolefin resin.

  Examples of the polyethylene resin that can be used as the thermoplastic resin described above include low density polyethylene, high density polyethylene, linear low density polyethylene, ultra low density polyethylene, ethylene-vinyl acetate copolymer, and ethylene-methyl methacrylate copolymer. Examples thereof include a polymer, an ethylene-methacrylic acid copolymer, and an ionomer resin in which the molecules are crosslinked with metal ions.

  Examples of the polypropylene resin that can be used as the thermoplastic resin described above include a propylene homopolymer, and a propylene copolymer having a structural unit derived from propylene of 50% by mass or more. Propylene copolymers, propylene-butene copolymers, propylene-ethylene-butene copolymers and other propylene and ethylene or α-olefins having 4 or more carbon atoms, propylene-acrylic acid copolymers, propylene- Examples thereof include a maleic anhydride copolymer. These copolymers may be block copolymers, random copolymers, or graft copolymers.

  Among the examples of the thermoplastic resin described above, a polyolefin resin is preferable in terms of excellent toughness, and a polypropylene resin is particularly preferable. In addition, from the viewpoint of improving brittleness and having a good balance between lightness and rigidity, as a thermoplastic resin that constitutes the base resin, it is a modification made by impregnating and polymerizing a polyolefin resin mainly with a polystyrene resin. A polystyrene resin is preferably selected.

  Moreover, it is preferable that the above-mentioned thermoplastic resin is non-crosslinked (not crosslinked) from the viewpoint of recyclability.

(Conductive material)
The conductive material dispersed in the expanded particles A is not particularly limited, and an inorganic material or an organic material can be used.

  When the foamed particle molded body of the present invention is used as a radio wave absorber, the conductive material may be any material that exhibits radio wave absorption performance in a foamed particle molded body containing the conductive material. For example, as an inorganic material that exhibits radio wave absorption performance, conductive carbon black, graphite, graphene, carbon nanotubes, carbon nanofibers, carbon microfibers, carbon microcoils, carbon nanocoils and other carbons, metal fibers, carbon fibers And inorganic oxides such as iron oxide, tin oxide, titanium oxide, zinc oxide, cadmium oxide and iridium oxide. Among these, carbons and metal oxides are preferable as materials that exhibit high radio wave absorption performance. Among them, carbons can be preferably used from the viewpoint of radio wave absorption, and specifically, conductive carbon. Examples thereof include black, graphite, graphene, carbon nanotube, carbon nanofiber, carbon microfiber, carbon microcoil, and carbon nanocoil.

Furthermore, as the conductive material, conductive carbon black can be suitably used among carbons. This conductive carbon black is not limited in production method or composition, and includes oil furnace black, acetylene black, carbon black having a hollow structure, etc., and these carbon blacks are hydrophilized, hydrophobized, oxidized, Reduction, acidification, basification, and organic treatment are also included. In addition, as electroconductive carbon black, the DBP absorption amount measured based on JIS6217-4: 2008 has a preferable thing of 150-700 cm < ³ > / 100g.

  As the conductive material, one type selected from the above-described materials that exhibit radio wave absorption performance may be used, or two or more types may be selected and used in combination. In addition, when foamed particle B contains a conductive material, said conductive material can be illustrated as this conductive material.

(Amount of conductive material)
In the resin particles in which the conductive material for obtaining the foamed particles A is kneaded, if the conductive material is dispersed in the base resin so that the content of the conductive material is 3 to 30% by mass. The foamed particles A having a conductive material content of 3 to 30% by mass can be obtained by foaming the resin particles kneaded with the conductive material. In the radio wave absorber, the content of the conductive material contained in the foam particles A should generally be determined according to the frequency band of the radio wave to be absorbed and the radio wave absorption performance. 3-30 mass% with respect to 100 mass% of particle | grains A, Preferably it is 7-20 mass%, More preferably, it is 10-17 mass%.

  When the content of the conductive material in the foamed particles A is too small, it may be difficult to obtain desired conductive performance and radio wave absorption performance as a foamed particle molded body using the content. Although measures to increase the ratio of the foamed particles A to the foamed particles B even for the conductive foamed particles with a low content of the conductive material are envisaged, the conductive foamed particles coated with the conductive material are As in the case of using the radio wave absorber, a sufficient radio wave absorption performance in the MHz band cannot be obtained. On the other hand, when there is too much content of this electroconductive material, implementation of the process which mixes an electroconductive material with base resin itself becomes difficult, and there exists a possibility that it may become difficult to obtain resin particle itself. In addition, when the content of the conductive material is too large, even if resin particles can be obtained, it is difficult to obtain good expanded particles, and expanded particles with low closed cell ratio or large shrinkage. As a result, it may be difficult to obtain expanded particles having good in-mold moldability. On the other hand, in the resin particle for obtaining said foamed particle B, a conductive material is contained in the range of less than 3 mass% as needed.

(Area ratio)
About the mixed state of the expanded particle A and the expanded particle B in the expanded particle molded body of the present invention, the total area (S1) of the expanded particle A and the total area of the expanded particle B (S2) appearing in the cross section of the expanded particle molded body. ) And the area ratio (S1 / S2) is in the range of 0.05 to 1.0.

  When the average value of the area ratio (S1 / S2) is too small outside the range of 0.05 to 1.0, the upper limit of the content of the conductive material in the expanded particles A is also limited as described above. In addition, since it becomes difficult to disperse the foamed particles A in the foamed particle molded body without large variation, sufficient and reproducible conductive performance and radio wave absorption performance cannot be obtained.

  When the average value of the above-mentioned area ratio (S1 / S2) is too large outside the range of 0.05 to 1.0, the secondary foamability deteriorates when the foamed particles are molded in the mold, and the obtained foamed particle molded body There is a risk that the surface smoothness of the molded article may be poor and the shrinkage rate of the molded article may be increased after the molded foamed article is molded. Furthermore, when the average value of the area ratio is too large, there is a risk that the radio wave absorption performance in the MHz band may be deteriorated in the radio wave absorber made of a foamed particle molded body to be obtained. From the above viewpoint, the mixing ratio of the expanded particles A and the expanded particles B constituting the mixed expanded particles for obtaining the expanded particle molded body of the present invention is a volume ratio (total volume of expanded particles A / total expanded particles B). Volume) in the range of 0.05 to 1.0, preferably 0.08 to 0.7, more preferably 0.1 to 0.5, and particularly preferably 0.1 to 0.3. In addition, the volume ratio of the foamed particle A and the foamed particle B of the mixed foamed particle used in the in-mold molding is the expanded particle A constituting the cross section of the foamed particle molded body obtained by the in-mold molding. And the average value of the area ratio of the expanded particles B. Therefore, in the present invention, the average value of the area ratio is 0.05 to 1.0, preferably 0.08 to 0.7, more preferably 0.1 to 0.5, and particularly preferably 0.1 to 0. .3.

(Area ratio measurement method)
In the present invention, the above-mentioned area ratio (S1 / S2) is the area of the cross section of the plurality of expanded particles A existing within a square of 150 mm in length and 150 mm in width selected on the cross section of the expanded foam molded body. The total (S1) and the total area (S2) of the cross-sections of the plurality of expanded particles B can be obtained, and obtained by dividing S1 by S2. Then, the foamed bead molded article cross-section of 5 points obtained at random, do determine the area ratio (S1 / S2), the area ratio of the obtained 5 points _(SR 1 _{respectively,} SR 2, SR ₃ , SR ₄ , SR ₅ ) is defined as an average value (SV) of the area ratio (S1 / S2).

  In addition, the measurement of S1 and S2 which exist in the range of the square selected on the cross section of the expanded particle molded object can be measured as follows, for example. First, an enlarged photograph of the square area is taken, and the enlarged photograph is converted into image data by a scanner device as shown in FIG. 8A. At this time, a commercially available scanner device can be appropriately selected as the scanner device. Next, a monotone image is prepared as shown in FIG. 8B by applying monotone processing to the enlarged photograph image that has been converted into image data. In FIG. 8B, the expanded particles A are displayed as black portions (portions indicated by reference numeral BL in FIG. 8B). The monotoning process can be realized, for example, by applying an enlarged photograph converted into image data to image analysis software such as NS2K Pro (nano system). The total area (S1) of the expanded particles A is calculated by calculating the area of the blackened portion based on the image data that has been subjected to monotone processing, and the entire image (the portion indicated by the symbol W in FIG. 8B). By subtracting the total area of the expanded particles A from the area, the total area (S2) of the expanded particles B can be calculated.

(Coefficient of variation of area ratio)
The variation coefficient of the area ratio (S1 / S2) in the cross section of the foamed particle molded body of the present invention is 20% or less, preferably 15% or less, more preferably 10% or less. When the coefficient of variation is too large, the dispersion of the expanded state of the expanded particles A between the products of the expanded particle molded product or the product portion is large, leading to the expected performance variations. Therefore, when the foamed particle molded body having a too large coefficient of variation is used as a radio wave absorber, the radio wave absorption performance varies greatly.

(Calculation method of area ratio coefficient of variation (%))
In the present invention, the variation coefficient of the above-mentioned area ratio (S1 / S2) is the area ratio (SR ₁ , SR ₂ , SR ₃ , SR ₄ , SR ₅ ) at five locations on the cross-section of the foamed particle molded body measured as described above. And the average value (SV) of the area ratio are calculated based on the following mathematical formulas (1) to (3).

(Apparent density of expanded particles A and expanded particles B)
In consideration of easily obtaining a foamed particle molded body having the above-described density, the apparent density of the foamed particle A and the foamed particle B is preferably 20 to 150 g / L, and more preferably 30 to 80 g / L, respectively. .

(Measurement method of apparent density of expanded particles)
In the present specification, the apparent density of the expanded particles is measured as follows. The foam particle group of weight W (g) is submerged in a graduated cylinder containing water using a wire mesh, and the volume V (L) of the foam particle group is determined from the rise in water level while taking into account the volume of the tool such as the wire mesh. The value (W / V) obtained by dividing the weight of the expanded particle group by the volume of the expanded particle group is converted to g / L as a unit to determine the true density of the expanded particle. The apparent density of the expanded particles is a value obtained by dividing the true density by 1.6. In addition, the said apparent density becomes a value substantially the same as the density of the foaming particle molded object obtained by normal in-mold shaping | molding, without compressing a foaming particle large.

(Method for producing foamed particle molded body)
As a manufacturing method of the expanded particle molding which concerns on this invention, the following methods can be mentioned, for example.

  First, the expanded particles A and the expanded particles B are prepared as follows.

(Preparation of expanded particles)
The foamed particles B used for forming the foamed particle molded body are obtained by obtaining resin particles from the base resin described above in a state in which substantial use of the conductive material is regulated, and impregnating the resin particles with a foaming agent. Each of the resin particles can be obtained by foaming by a known foaming method. The state in which the substantial use of the conductive material is regulated is a concept that combines a state in which no conductive material is included and a state in which the conductive material is included at less than 3% by mass. To do. As a method of preparing the resin particles for obtaining the expanded particles B, for example, the base resin is put into an extruder and extruded as a strand from a die attached to the tip of the extruder as a molten state, and the extruded strand is cut. And a method of obtaining resin particles (strand cut method). In addition to this method, as a method for preparing the resin particles, a known resin particle production method such as an underwater cut method can be employed.

  Foamed particles A are obtained by obtaining resin particles or resin compositions in which a foaming agent is contained in 3 to 30% by mass of a conductive material dispersed in a thermoplastic resin, and the resin particles or resin compositions are described above. It can be obtained by foaming by a known foaming method. As a method for preparing the resin particles for obtaining the foamed particles A, the conductive material and the thermoplastic resin are largely unevenly distributed in the thermoplastic resin by using a kneader such as a kneader or an extruder. A known resin particle production method such as the strand cut method or the underwater cut method as described above can be employed except that the kneading is performed without being dispersed.

(Preparation of foam particle mixture)
Next, the foamed particle mixture is prepared by mixing the foamed particles A and the foamed particles B using a mixing device or the like. As a mixing device, a paddle type or screw type mixer, a tumbler, or the like can be appropriately selected.

  When the foamed particle molded body of the present invention is used as a radio wave absorber, the mixing state of the foamed particles A and the foamed particles B constituting the foamed particle molded body is ideally uniform. From the standpoint of realizing this, in the process of producing a foamed particle molded body by molding the mixture of foamed particles A and foamed particles B (foamed particle mixture) in the mold, the foam taken out at random from the foamed particle mixture It is preferable that the ratio of the expanded particles A contained in the particle group is in a certain range. From the viewpoint of obtaining a foamed particle mixture in which the foamed particles A and the foamed particles B are uniformly mixed, the relationship between the apparent density and the average particle diameter of the foamed particles A and the foamed particles B is expressed by the following formulas (4) and (5). Is preferably satisfied.

However, in the above formulas (4) and (5),
D1: Apparent density (g / L) of the expanded particles A,
D2: apparent density (g / L) of the expanded particles B,
P1: Average particle diameter (mm) of the expanded particles A,
P2: average particle diameter (mm) of the expanded particles B,
It is.

  The foamed particles satisfying the above formulas (4) and (5) are mixed in a mixing device to prepare a foamed particle mixture, which is included in the mixed foamed particle group randomly extracted from the foamed particle mixture. It becomes easy to make the ratio of the expanded particles A within a certain range.

(Measurement method of average particle diameter of expanded particles)
A graduated cylinder containing water is prepared, and an appropriate amount of foam particles is submerged in water in the graduated cylinder using a tool such as a wire mesh. Then, the volume V1 [L] of the expanded particles read from the rise in the water level is measured while taking into account the volume of a tool such as a wire mesh. By dividing (V1 / N) this volume V1 by the number of foamed particles (N) placed in a graduated cylinder, the average volume per foamed particle is calculated. The diameter of the virtual sphere having the same volume as the obtained average volume is defined as the average particle diameter [mm] of the expanded particles.

(Preparation of foamed particle compact)
A foamed particle molded product is prepared using the foamed particle mixture obtained as described above. The foamed particle molded product can be adjusted based on the in-mold molding method of the foamed particles. As the in-mold molding method, a conventionally known method or the like can be appropriately selected. For example, a foamed particle mixture prepared as described above is filled into a mold formed according to the purpose by a known filling method such as compression filling method, cracking filling method, etc. Is heated with a heating medium such as steam to fuse the foamed particles to each other to obtain a fused product. And a foamed-particles molded object can be obtained by taking out a fused material from a metal mold | die.

(Usage form of foamed particle molded body)
When the foamed particle molded body of the present invention is used as a radio wave absorber, as shown in FIGS. 1A, 1B, 2A, and 2B, a pyramid or wedge shape is mainly adopted, and preferably different. Used in combination with materials that exhibit different electromagnetic wave absorption performance in different frequency bands. However, the shape can be appropriately designed according to the desired target performance, and is not limited to the above shape. For example, the radio wave absorber comprising the foamed particle molded body of the present invention is effectively absorbed in a wider frequency band by using it together with a material that exhibits remarkable radio wave absorption performance mainly in the MHz band such as ferrite tiles. Is possible. Specifically, as a radio wave absorber using a composite of a material exhibiting different radio wave absorption performance in different frequency bands and a foamed particle molded body, a composite mold having the shapes as shown in FIGS. 3A, 3B, 4A and 4B. A radio wave absorber 10 is used, and a low frequency radio wave absorber indicated by reference numeral 3 is used on the bottom surface thereof. In the present specification, the composite wave absorber may be simply referred to as a wave absorber.

  Specific examples of frequency bands absorbed by radio wave absorbers: When ferrite is used as a material exhibiting different radio wave absorption performance in different frequency bands, ferrite tiles mainly absorb radio waves in the low frequency band (30 to 400 MHz). In addition, the foamed particle molded body mainly absorbs radio waves in a high frequency band above it.

By the way, there are two types of radio wave absorption of materials that cause dielectric loss, which are caused by dielectric loss ε ″ ^* and those caused by conductive loss σ, which are expressed by the following formula (6).

However, in the above formula (6),
ε ″ ^* : dielectric loss σ: conductive loss ε ″: imaginary part of complex dielectric constant f: frequency.

  In radio wave absorbers that are a composite of foamed particle compacts and ferrite tiles, the imaginary part of the complex dielectric constant of foamed particle compacts is large in the high frequency band, but mainly because of the absorption effect of ferrite in the low frequency band. It is preferable that the imaginary part of the complex dielectric constant of the foamed particle molded body is small. Therefore, the foamed particle molded body is required to have a small conductive loss σ in the formula (6).

Here, FIG. 5A shows an electrical equivalent circuit of the foamed particle molded body, and FIG. 5B shows a schematic diagram for explaining the conductive loss and dielectric loss in the foamed particle molded body. In FIG. 5A, R is a resistance, and C is a capacitor, and conductive loss and dielectric loss occur in each. In the foamed particle molded body, the foamed particles A exhibiting conductivity can be regarded as a small resistance in the equivalent circuit, and therefore, a conductive loss occurs. Then, as shown in FIG. 5B, the portion where the expanded particles A are spatially separated from each other can be regarded as a small capacitor C, so that dielectric loss occurs there. By the way, in the above formula 6, ε ″ ^* is a dielectric loss caused by the capacitor, and σ is a conduction loss caused by the resistance. Therefore, when the connection of the expanded particles A is too long, the resistance component of the equivalent circuit In this case, the foamed particle molded body is mainly composed of the conductive loss σ in the formula (6), and the foamed particle molded body in this case is not suitable as a radio wave absorber.

  A conventional radio wave absorber made of only conductive foam particles containing a conductive material can increase the amount of conductive material added in order to improve the radio wave absorption performance in the GHz band. However, there is a problem in that the MHz band absorption performance is lowered as the amount of addition is increased. On the other hand, when reducing the addition amount of the conductive material in order to improve the absorption performance in the MHz band, there is a problem that the radio wave absorption performance in the GHz band is lowered. That is, there is a problem that it is difficult to obtain desired radio wave absorption performance in both frequency bands of the GHz band and the MHz band. Also, from a foamed in-mold molded product mixture of conductive film-forming foam particles having a coating layer formed by coating a conventional conductive paint and non-conductive foam particles formed without such a coating layer. However, there is a problem that the radio wave absorber is still in an insufficient level, although the radio wave absorption performance in the MHz band is improved as compared with the above.

  The above-mentioned problem with respect to the radio wave absorber of the in-mold molded body made only of conductive foam particles is that the entire foamed particle molded body constituting the radio wave absorber is composed of a conductive material. Absorption performance is considered to be mainly caused by conductive loss, and is considered to be due to reasons such as easy performance in the high frequency band. In addition, the problem with the radio wave absorber made of an in-mold molded body of conductive film-forming foam particles provided with a coating layer formed by coating the conductive paint is that the conductive foam particles are coated by covering the foam particles with a conductive paint. Therefore, in the conductive film-formed foamed particles, the conductive material is present only in the coating layer of the conductive foamed particles. In addition, an electric circuit serving as a resistance is formed on the surface of the foamed particles, and the conductive loss σ of the conductive film-formed foamed particle molded body is increased. Therefore, such a conductive film-formed foamed particle molded body cannot be said to have sufficient performance as radio wave absorption performance.

  On the other hand, in the present invention, as described above, mixed foaming is performed such that the average value of the area ratio of the foamed particles A and the foamed particles B in the cross section of the foamed particle molded body is in the range of 0.05 to 1.0. By using the particles, the capacitance of the foamed particle molded body is adjusted to an expected value while the foamed particle A is within a small range of 1.0 or less in the area ratio (S1 / S2) in the obtained foamed particle molded body. can do. When the area ratio is within the range, the expanded particles A are preferable because they form single and 2 to 20 and further 2 to 15 connected clusters. As a result, the dielectric loss can be increased while reducing the conductive loss of the equivalent circuit, and the radio wave absorption performance is improved.

  Actually, the capacitances of the foamed particle molded body of the present invention (molded body composed of foamed particles A and foamed particles B) and the molded body composed of single foamed particles (molded body composed only of foamed particles A) were measured. The results are shown in FIG. The amount of conductive carbon black added as the conductive material contained in the foamed particle molded body composed of the foamed particles A and the foamed particles B is the same as the foamed particles A and the foamed particles in which the carbon black is dispersed by 14% by mass. Adjustment was made by changing the mixing ratio with B. Further, the amount of conductive carbon black added as the conductive material contained in the foamed particle molded body composed only of the foamed particles A was adjusted by changing the blending ratio of the conductive carbon black in the foamed particles A. As shown in FIG. 6, it is recognized that the former foamed particle molded body of the present invention can increase the capacitance more efficiently for these foamed particle molded bodies. This means that when the foamed particle molded body is an equivalent circuit, the capacitor component is larger than the resistance component, which indicates that it is desirable as a radio wave absorber.

  The foamed particle molded body of the present invention appropriately has a portion containing a large amount of the conductive material made of the foamed particle A and a portion made of the foamed particle B in a state where substantial use of the conductive material is restricted. Thus, the dielectric loss can be mainly used without mainly conducting loss, so that the property that the radio wave absorption performance in the MHz band is likely to be lowered can be improved. Therefore, the foamed particle molded body of the present invention is excellent in the radio wave absorption performance in the GHz band and can improve the absorption performance in the MHz band. That is, the foamed particle molded body can be used as a radio wave absorber capable of absorbing electromagnetic waves from the MHz band to the GHz band and capable of supporting a wide frequency range.

  In the present invention, since the coefficient of variation of the foamed particle molded body is 20% or less, the foamed particles A in which the conductive material is kneaded are present in a substantially uniformly dispersed state throughout the foamed particle molded body. It can be said. From this, according to the present invention, what can be used as a radio wave absorber that can absorb a radio wave from the MHz band to the GHz band more stably and can cope with a wide frequency range is provided. .

(Low frequency electromagnetic wave absorber)
As the low frequency radio wave absorber 3 used for the composite radio wave absorber, for example, a radio wave absorber made of a ferrite tile or the like is preferably used as a magnetic loss material. Ferrite tiles exhibit extremely good radio wave absorption characteristics in a low frequency range of 400 MHz or less. Therefore, as the composite wave absorber 10, a unit in which the wave absorber formed of the foamed particle molded body of the present invention and a ferrite tile can be exemplified. In such a unit of the composite wave absorber 10, the absorption of the GHz band radio wave is efficiently absorbed by the foamed particle molded body, and the MHz band radio wave is efficiently absorbed by the ferrite tile due to the magnetic loss of the ferrite. As a result, the radio wave absorption performance in a wider frequency band can be exhibited more powerfully.

  When the foamed particle molded body and ferrite tile are used in combination, the conductive material contained in the foamed particle A of the foamed particle molded body is 3 to 30% by mass with respect to 100% by mass of the expanded particle A, as described above. Preferably it is 7-20 mass%, More preferably, it is 10-17 mass%. The reason why there is an upper limit range of the preferable conductive material as described above is that when the content of the conductive material in the foamed particle molded body is too large when the impedance of the ferrite tile and the space (air) is designed to match, This is probably because the impedance is greatly different from that of air, so that the electromagnetic wave is likely to be reflected at the foamed particle molded body due to impedance mismatching, and the absorption characteristics of the ferrite tile responsible for the low frequency range are likely to be hindered.

  On the other hand, if the content of the conductive material of the foamed particle molded body is reduced, the absorption characteristics of the ferrite tile will not be disturbed, but the radio wave absorption performance of the foamed particle molded body itself will be lowered, so that the same performance can be obtained. The size of the electromagnetic wave absorber must be large (if the foamed particle molded body has a polygonal pyramid shape, the height of the polygonal pyramid must be high). As a result, the space occupancy rate such as the anechoic chamber becomes large, and the limited space cannot be used effectively.

  Therefore, the foamed particle molded body of the present invention satisfies the mixing ratio of the specific range between the foamed particle A containing the conductive material in a specific range amount and the foamed particle B that restricts the substantial use of the conductive material. If the ferrite tile is combined and used as a radio wave absorber that forms a unit, the ferrite tile efficiently absorbs radio waves in the low frequency band where the ferrite absorption characteristics are good. In the band, the foamed particle molded body efficiently absorbs radio waves, and particularly excellent radio wave absorption performance can be expected in a wide frequency band.

  Hereinafter, the present invention will be described in more detail with reference to examples.

[Production of expanded particles A]
An extrusion apparatus in which a strand forming die was attached to the outlet side of an extruder having an inner diameter of 65 mm was prepared. As shown in Table 1, the propylene resin shown in Table 1 as a thermoplastic resin as a base resin, and oil furnace black (product name: Ketjen Black (registered trademark) as a conductive inorganic substance as a conductive material. ) EC300J (manufactured by Lion Corporation), DBP absorption ^amount; 360 cm 3/100 g, particle size; 40 nm) were fed to an extruder having an inner diameter of 65 mm, heated at a set temperature 200-240 ° C., molten, kneaded, polypropylene resin After the conductive material is dispersed in, the obtained melt-kneaded product is extruded in the form of a strand from the pores of the die, and 2 mg (the ratio of particle length to diameter (L / D) is 1.3) with a pelletizer. Were cut to obtain cylindrical resin particles. 1 kg of the obtained resin particles, 3 L of water as a dispersion medium, 3 g of kaolin as a dispersant, and 0.02 g of sodium dodecylbenzenesulfonate are charged into a 5 L autoclave, and carbon dioxide as a blowing agent in a sealed container in the dispersion medium. The mixture was heated and heated to the foaming temperature shown in Table 1 under stirring and maintained at the same temperature for 15 minutes to adjust the pressure in the sealed container to 3.0 MPa (G). The foamed particles A were obtained by discharging together with the dispersion medium under atmospheric pressure. The dispersion state of the conductive material in the resin constituting the expanded particles A was as shown in the TEM photograph (magnification 20000 times) in FIG. In the TEM photograph of FIG. 10, the location where the conductive material is present is indicated by a black spot, and the location where the resin is present is indicated by a spot where non-black (white) is present. In addition, Table 1 shows various physical properties of the obtained expanded particles A.

[Production of expanded particles B]
Except not having used an electroconductive material, the process similar to the said foaming particle A was implemented and the foaming particle B was obtained. However, in the production of the expanded particles B, the components such as the base resin and the blending amounts are as shown in Table 2. Moreover, various physical properties of the obtained expanded particles B are shown together in Table 2.

  The physical properties of the expanded particles A and the expanded particles B shown in Tables 1 and 2 above were measured by the following methods.

[Melting point of expanded particles]
Values obtained by a heat flux differential scanning calorimetry method (DSC method) based on JIS K 7122 (1987) were adopted. Using 2 to 4 mg of expanded particles as a sample, the temperature was increased from room temperature to 200 ° C. at a rate of 10 ° C./min by a heat flux differential scanning calorimeter, and then decreased from 200 ° C. to 40 ° C. at a rate of 10 ° C./min, The peak temperature of the maximum endothermic curve peak on the DSC curve obtained by heating from 40 ° C. to 200 ° C. at a rate of 10 ° C./min was taken as the melting point.

The “apparent density” and “average particle diameter” of the expanded particles shown in Tables 1 and 2 were measured by the above-described measuring methods.

(Examples 1-8, Comparative Examples 2-4)
[Production of thermoplastic resin foam particles]
The expanded particles A shown in Table 1 and the expanded particles B shown in Table 2 were mixed with a tumbler at the mixing ratio shown in Table 3 to obtain an expanded particle mixture. The obtained foamed particle mixture was filled in a mold and heated at a steam molding pressure (MPa: gauge pressure) shown in Table 3 to carry out an in-mold molding process, and a bottom surface of 600 mm as shown in FIG. A foamed particle molded body (molded article) comprising a plurality of pyramid-shaped (quadrangular pyramid-shaped) corners of × 600 mm and a height of 300 mm was obtained. In FIG. 7, the symbol E indicates the bottom surface, the symbol F indicates the top, the dimension of the bottom surface E in the foamed particle molded body shown in FIG. 7 is the length of the range indicated by the symbols B and A, and the height is It is specified as the length of the range indicated by the symbol C indicating the difference in height between the bottom surface E and the top F.

  The foamed particle molded bodies obtained in Examples and Comparative Examples were stacked on ferrite tiles (bottom 100 mm × 100 mm, thickness 5.2 mm; manufactured by Riken Environmental Systems Co., Ltd., trade name RF044) and fixed to each other. A unit shaped like 3B was formed. The unit was formed so that the bottom surface of the unit was 600 mm × 600 mm. The unit of the structure composed of the ferrite tile and the foamed particle molded body thus obtained was used as a radio wave absorber (in this case, a composite radio wave absorber).

  For the foamed particle molded bodies obtained in Example 1, Comparative Example 2, and Comparative Example 5 described later, a surface photograph was taken. The results are shown in FIG. FIG. 9A is a surface photograph of the expanded particle molded body obtained in Example 1. FIG. FIG. 9B is a surface photograph of the foamed particle molded body obtained in Comparative Example 2. FIG. 9C is a surface photograph of the foamed particle molded body obtained in Comparative Example 5.

(Comparative Examples 1, 5, 6)
[Production of thermoplastic resin foam particles]
A foamed particle molded body was obtained under the conditions shown in Table 3 in the same manner as in Example 1 except that only the foamed particles A shown in Table 1 were filled in the mold.

  Various physical properties of the expanded foam molded bodies obtained in Examples and Comparative Examples were measured by the following methods, and the results are shown in Table 4. Furthermore, as described above, the composite radio wave absorbers composed of the foamed particle molded bodies of Examples and Comparative Examples were subjected to evaluation of radio wave absorption performance and comprehensive evaluation by the following methods.

[Average particle diameter of each of expanded particles A and expanded particles B constituting the expanded particle molded body (average particle diameter of expanded particles in claim 2 of the present invention)]
The average particle size of the expanded particles A is determined by observing the cross section of the expanded molded product, measuring the maximum diameter of all expanded particles A present within a square of 100 mm length × 100 mm width of the cross section, The arithmetic average value of the diameters was defined as the average particle diameter of the expanded particles A. On the other hand, with respect to the average particle diameter of the expanded particles B, the same expanded particle molded body cross section in which the average particle diameter of the expanded particles A was measured was observed, and was present within the range of a square of 100 mm in length × 100 mm in width. The maximum diameter of all the expanded particles B was measured, and the arithmetic average value of the maximum diameters was defined as the average particle diameter of the expanded particles B. In addition, as shown in Table 4, the average particle diameter of each of the expanded particles A and the expanded particles B was approximately the same value.

[Radio wave absorption performance of radio wave absorber]
100 composite radio wave absorbers were prototyped using the foamed particle molded bodies of Examples and Comparative Examples, and the radio wave absorption was performed at the frequencies of 30 MHz, 100 MHz, 1 GHz, and 18 GHz by the coaxial tube method and the arch method shown in IEEE Std1128. The amount (dB) was measured and the average value was determined.

  In Table 4, “*” indicates that the radio wave absorption performance was unstable.

  In Table 4, the “particle diameter ratio” column and the “density ratio” column describe the ratio of the average particle diameter (mm) of the expanded particles A and B and the ratio of the apparent density of the expanded particles A and B, respectively. In the “area ratio (S1 / S2)” column, the average value of the area ratio is described, and in the “variation coefficient (%)” column, the variation coefficient (%) of the area ratio is described. The ratio of the average particle diameter (mm) of the expanded particles A and B, the ratio of the apparent density of the expanded particles A and B, the average value of the area ratio, and the coefficient of variation (%) of the area ratio are measured by the measurement methods described above, respectively. It was done.

  From the results of Table 4 above, it was confirmed that the radio wave absorbers of Examples 1 to 8 had sufficiently stable radio wave absorption capability in the MHz band and the GHz band. Further, the moldability of the expanded particles during molding in the mold was also good. Therefore, the foamed particle molded body of the present invention has excellent radio wave absorption performance with a wide range of electromagnetic wave absorption characteristics, and is suitable as a radio wave absorber.

  Since the foamed particle molded body of Comparative Example 1 includes only the foamed particles A, the radio wave absorption performance was insufficient in the MHz band, although the absorption performance was excellent in the GHz band.

  In the foamed molded article of Comparative Example 2, the area ratio of the foamed particles A in the mixed foamed particles was too large because the amount of the foamed particles A added was too large, and sufficient radio wave absorption performance was not expressed particularly in the MHz band. .

  The foamed molded products of Comparative Examples 3 and 4 are inferior in moldability when foamed particles are molded in-mold, and the variation coefficient of the area ratio (S1 / S2) in the cross section of the foamed particle molded product is 20% or more. Also, in-mold molding of the foamed particles was difficult, and the variation of the radio wave absorption performance of the obtained foamed particle molded body was more than twice as large as that of Example 1, and stable radio wave absorption performance was not exhibited.

  The foamed molded product of Comparative Example 5 was foamed by adjusting the blending amount of the conductive material so that the content of the conductive material contained in the entire foamed particle molded product was substantially the same as in Example 1. It consists only of particles A. As for the radio wave absorption performance of the foamed particle molded body, satisfactory radio wave absorption performance was exhibited in the MHz band, but sufficient radio wave absorption performance was not expressed in the GHz band.

  The foamed molded product of Comparative Example 6 is composed of only the foamed particles A adjusted in a large amount of the conductive material, and the content of the conductive material in the foamed particle molded product is increased. Since the content of the conductive material in the particles is too large, the impedance is significantly different from that of air, so electromagnetic wave reflection is likely to occur at the foamed particle molded part due to impedance mismatching, and sufficient radio wave absorption performance cannot be obtained. It was.

DESCRIPTION OF SYMBOLS 1 Radio wave absorber 2a Foamed particle molding 3 Low frequency radio wave absorber 10 Composite type radio wave absorber

Claims (6)

A molded thermoplastic resin particle,
As the thermoplastic resin foam particles constituting the foamed particle molded body, the foamed particles A in which the conductive material is dispersed in an amount of 3 to 30% by mass, and the conductive material content is less than 3% by mass (including 0). ) Expanded particles B,
The average value of the area ratio (S1 / S2) of the total area (S1) of the expanded particles A and the total area (S2) of the expanded particles B in the cross section of the expanded particle molded body is 0.05 to 1.0. A foamed particle molded body having a range and a coefficient of variation of the area ratio of 20% or less.   2. The foamed particle molded body according to claim 1, wherein an average particle diameter of each of the foamed particles A and the foamed particles B is 2 to 8 mm.   The density of the said foaming particle molded object is 15-90 g / L, The expanded particle molded object of Claim 1 or 2 characterized by the above-mentioned.   The foamed particle molded body according to any one of claims 1 to 3, wherein the conductive material is conductive carbon black.   The foamed particle molded body according to any one of claims 1 to 4, wherein the resin forming the thermoplastic resin foamed particles is a polyolefin-based resin.   The foamed particle molded body according to any one of claims 1 to 4, wherein the resin forming the thermoplastic resin foamed particles is a polystyrene-based resin.