JP6706108B2 - Expanded particle molding - Google Patents

Description

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

To carry out various tests related to radio waves, such as standard conformity evaluation tests for radio waves radiated from electronic devices and tests for evaluating radio wave reception characteristics of communication devices such as antennas. A facility equipped with a radio wave isolation space called an anechoic chamber is used.

Interference radio standards often specify the allowable value of interfering radio waves in a predetermined radio frequency band, and the radio frequency bands used in these standards are conventionally used in televisions, radios, mobile phones, etc. The main frequency band was 30 MHz to 1 GHz. However, with the recent development of wireless LANs, ETCs, smartphones, etc., the frequency band of radio waves used in the standard has expanded to the range of 30 MHz to 18 GHz.

Regarding the evaluation of radio wave reception characteristics, since equipment and communication devices using radio waves in the microwave band and millimeter wave band of 10 GHz or more, such as optical communication equipment and radars for preventing vehicle collision, have been developed, There is a demand for an anechoic chamber capable of evaluating the radio wave reception characteristics of communication devices.

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

By the way, in the anechoic chamber, in order to properly carry out various tests related to radio waves, not only is the state of being isolated from external radio waves by blocking the intrusion of external radio waves into the space, It is required to be configured to suppress the adverse effects of the interference between the measurement target radio wave and its reflected wave. In consideration of these requirements, structures called radio wave absorbers are usually installed in the anechoic chamber on the top surface, surrounding wall surfaces, and, if necessary, the floor surface. Thus, it is possible to suppress the possibility that the measurement target radio wave is reflected on the wall surface to generate a reflected wave.

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

Although such a radio wave absorber has excellent radio wave absorption performance in a frequency band of 1 GHz or more that can be classified as a high frequency band, it has a radio wave absorption performance in a frequency band of less than 1 GHz that can be classified as a low frequency band, a so-called MHz band. However, there is a problem that it is difficult to support radio wave measurement in a wide frequency band as described above. To solve this problem, it has been attempted to obtain the electromagnetic wave absorption characteristics in the MHz band by increasing the length dimension of the expanded particle molded body forming the electromagnetic wave absorber. However, if the size of the foamed particle molded body is increased, the space occupancy rate of the radio wave absorber in the radio wave darkroom increases, which causes a problem that the limited space cannot be effectively utilized.

In response to this problem, as a radio wave absorber, a mixture of conductive foamed particles whose surface is coated with a graphite-based conductive paint and non-conductive foamed particles not subjected to such a coating treatment are formed in a 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 of around several hundred MHz. In addition, the unit is prepared by combining the electromagnetic wave absorber with other low frequency band electromagnetic wave absorbing members such as ferrite tiles, and the unit can be installed in an anechoic chamber to increase the size of the electromagnetic wave absorber. Even without this, it is possible to realize an anechoic chamber with improved electromagnetic wave absorption performance in the MHz band.

JP, 10-41674, A JP-A-11-209505

However, the radio wave absorbers described in Patent Documents 1 and 2 alone 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 possibility that a frequency band with weak radio wave absorption may occur in 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 as a radio wave absorber capable of exhibiting excellent radio wave absorption performance. ..

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

INDUSTRIAL APPLICABILITY The expanded-particle molded product of the present invention has a small variation in the conductive performance among the expanded-particle molded products when a plurality of expanded-particle molded products are prepared, and foaming is achieved by changing the mixing ratio of the expanded particles A and B. It is easy to change the conductive performance of the particle compact.

In particular, when the expanded particle molded product of the present invention is used as a radio wave absorber, the expanded particle molded product is a mixed molded product containing a plurality of types of expanded particles A and B containing a specific amount of a conductive material. By changing the mixing ratio of the expanded particles A and B, the dielectric constant of the radio wave absorber can be easily changed, and the radio wave absorber has excellent radio wave absorption performance. Further, when a plurality of foamed particle molded bodies are manufactured, variations in the radio wave absorption performance among the foamed particle molded bodies are small. Further, a combination of the radio wave absorber made of the expanded particle molded article 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 is a schematic view of one example of the case where the expanded particle molded body according to the present invention is formed in a pyramid shape (quadrangular pyramid shape) and the expanded particle molded body is used as a radio wave absorber. FIG. 3 is a schematic perspective schematic view for explaining in detail. FIG. 1B shows a case where the expanded-particle molded product according to the present invention has a shape in which the inclined surface portion of the pyramid-shaped molded portion is cut out from the bottom surface of the expanded-particle molded product to a position of a predetermined height. FIG. 3 is a schematic perspective schematic view for schematically explaining one of the examples in which a structure in which a plurality of foamed particle molded bodies are combined is used as a radio wave absorber. FIG. 2A is a schematic diagram showing one example of the case where the expanded particle molded body of the present invention is formed in a wedge shape (wedge shape) and the expanded particle molded body is used as a radio wave absorber. It is a schematic perspective schematic diagram for explaining. FIG. 2B shows a case where the expanded bead molded article 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 expanded bead molded article to a position of a predetermined height. FIG. 3 is a schematic perspective schematic view for schematically explaining one of the examples in which a structure in which a plurality of foamed particle molded bodies are combined is used as a radio wave absorber. FIG. 3A is a view for schematically explaining one example of the composite type electromagnetic wave absorber using the expanded particle molded body in the case where the expanded particle molded body according to the present invention is formed in a pyramid shape. It is a schematic perspective schematic diagram. FIG. 3B schematically illustrates one of other examples of the composite type electromagnetic wave absorber using the expanded particle molded body in the case where the expanded particle molded body according to the present invention is formed in a pyramid shape. It is a schematic perspective schematic view for. FIG. 4A is a view for schematically explaining one example of the composite type electromagnetic wave absorber using the expanded particle molded body in the case where the expanded particle molded body according to the present invention is formed in a wedge shape. It is a schematic perspective schematic diagram. FIG. 4B schematically illustrates one of other examples of the composite type electromagnetic wave absorber using the expanded particle molded body in the case where the expanded particle molded body according to the present invention is formed in a wedge shape. It is a schematic perspective schematic view for. FIG. 5A is a circuit diagram for simplifying and explaining an electrical equivalent circuit relating to electromagnetic wave absorption in the present invention. FIG. 5B is a schematic diagram for simplifying and explaining the dielectric loss and the conductive loss that occur in the expanded-particle molded body including the expanded particles A and the expanded particles B in the present invention. FIG. 6 shows the amount of conductive carbon added in each of a mixed expanded particle molded product (in-mold molded product) which is a mixed molded product containing expanded particles A and B and a molded product (in-mold molded product) consisting only of expanded particles A. It is a graph which shows the relationship between (mass %) and 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 subjecting the cross-sectional image of FIG. 8A to monotone processing. FIG. 9A is a surface photograph of the expanded bead molded article obtained in Example 1. FIG. 9B is a surface photograph of the expanded bead molded product obtained in Comparative Example 2. FIG. 9C is a surface photograph of the expanded bead molded product obtained in Comparative Example 5. FIG. 10 is a TEM photograph showing a state in which a conductive material is dispersed in the resin forming the expanded beads A.

(Expanded particle molding)
The thermoplastic resin expanded particle molded body of the present invention contains a thermoplastic resin as a base resin and a first expanded particle (thermoplastic resin expanded particle A) in which a conductive material is dispersed in an amount of 3 to 30% by mass. In-mold molding of a foamed particle mixture containing a thermoplastic resin as a base resin and second expanded particles (thermoplastic resin expanded particles B) having a conductive material content of less than 3% by mass (including 0). It is a foamed particle in-mold molded article. The thermoplastic resin expanded particles A and B are hereinafter simply referred to as expanded particles A and B, respectively. In the expanded particle molded article which is the in-mold molded article, the average 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 article. The value is in the range of 0.05 to 1.0, and the coefficient of variation of the area ratio is 20% or less. In addition, in this specification, the expanded particle molded article of the present invention may be referred to as a thermoplastic resin expanded particle molded article or simply a molded article. Further, 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 mean the dispersion in the mathematical terms shown in the mathematical expressions (2) and (3) described later, It means that the conductive material is dispersed in the resin forming the expanded beads. Specifically, a state in which conductive carbon indicated by granular black dots is scattered on the photograph of the transmission electron microscope shown in FIG. 10 is exemplified.

(Expanded particle compact density)
The density of the foamed particle molded product is preferably 15 to 180 g/L, and more preferably 15 to 90 g/L from the viewpoint of a balance between lightness and rigidity. In the case of using a foamed particle molded body as a radio wave absorber, in addition to the above viewpoint, from the viewpoint of exhibiting excellent radio wave absorption performance in a foamed particle molded body obtained by 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 foamed particle molded body)
The density of the foamed particle molded body can be determined by dividing the mass (g) of the molded body sample by the volume (L) of the molded body sample. The volume of the molded body sample is calculated by calculating the product of the vertical dimension, horizontal dimension, and height dimension based on the external dimensions (length, width, height) of the molded body sample cut from the molded body into a rectangular shape. Let the volume (L).

(Foam particle molded body shape)
The shape of the foamed particle molded body can be appropriately set to a plate shape, a columnar shape, or 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, in the case of preparing the foamed particle molded body radio wave absorber according to the present invention, it is preferable that the foamed particle molded body is formed in a shape that can more effectively realize the absorption of radio waves. In consideration of this point, when the foamed particle molded body of the present invention is assumed to be a plane whose normal is a straight line along the direction from the end on the radio wave arrival side to the other end, foaming is performed on that plane. The area of the cut surface (when there are multiple cut surfaces, the total area of the cut surface) that is recognized when cutting the particle compact 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, it is preferable that the foamed particle molded body 2a 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 manner, the pyramid-shaped radio wave absorber 1 shown in FIG. 1 and the wedge-shaped radio wave absorber 1 shown in FIG. 2 can be obtained. For example, in the pyramid-shaped radio wave absorber 1 shown in FIG. 1, the pyramid-shaped top portion is the end portion on the incoming side of the radio wave and the pyramid-shaped bottom portion is the other end portion, and the pyramid-shaped bottom portion is arranged along the direction from the top portion to the bottom portion. When the foamed particle molded body is cut along 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 radio 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 FIG. 1B or FIG. 2B. The expanded particle molded body 2a of FIG. 1B or FIG. 2B can be formed, for example, by forming the pyramid-shaped or wedge-shaped molded body into a shape in which the inclined surface is cut out at a portion above a predetermined height from the bottom position. ..

(Expanded particles A and expanded particles B)
The expanded particles A are resin particles in which the conductive material is dispersed in the range of 3 to 30% by mass. Further, from the viewpoint of workability and radio wave absorption performance, 5 to 25% by mass is preferable, 7 to 20% by mass is more preferable, and 10 to 17% by mass is particularly 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 covered with a conductive material). Or a resin particle foam containing a conductive material in a range of less than 3% by mass (the foam particles B include those covered 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. Further, both the expanded particles A and the expanded particles B may appropriately contain other additives in addition to the conductive material. Examples of the additive include a colorant, a flame retardant, an antistatic agent, a bubble control agent, and the like.

In both the foamed particles A and the foamed particles B, the resin particles in which the above-mentioned conductive material is dispersed are prepared by mixing the base resin and the conductive material in a resin by using a conventionally known kneader such as an extruder or a kneader. It can be obtained by dispersing a conductive material in the above and granulating the resin composition in which the conductive material is dispersed with a pelletizer or the like. Further, the foamed particles which are foams of the resin particles are conventionally known methods for producing extruded foamed particles or a method of releasing foamable resin particles containing a foaming agent from an autoclave to foam, foamable resin particles containing a foaming agent. It can be produced by a conventionally known foaming method such as a method of softening by heating to foam. Regarding the foamed particles B, the resin particles or the foamed particles coated with the conductive material can be obtained by applying a conductive material such as a conductive paint to the resin particles or the foamed particles by using a coating device such as a spray coater. Obtainable.

(Thermoplastic resin)
As the thermoplastic resin forming the expanded particle molded product of the present invention, the expanded particles forming the molded product, and the resin particles forming the expanded particles, a polyolefin resin such as a polyethylene resin or a polypropylene resin, or polystyrene. Resins, polycarbonate-based resins, polyvinyl chloride resins, polymethacryl-based resins, acrylonitrile-based resins, polyester-based resins, polyamide-based resins, polyurethane-based resins and blends thereof, or thermoplastic resins such as copolymers Be done. When a mixed resin of a polyolefin resin and another 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. It is more preferable that the content of the polyolefin resin is 90% by mass or more.

Examples of the polyethylene-based 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 polymers, ethylene-methacrylic acid copolymers, and ionomer resins in which the molecules are crosslinked with metal ions.

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

Among the above-mentioned examples of the thermoplastic resin, a polyolefin resin is preferable in terms of excellent toughness, and a polypropylene resin is particularly preferable. Further, from the viewpoint of improving the brittleness and having an excellent balance between lightness and rigidity, as a thermoplastic resin that constitutes the base resin, polystyrene resin is mainly used as a modified resin obtained by impregnating a styrene monomer into a polyolefin resin and polymerizing it. It is preferable that a polystyrene resin is selected.

Further, the above-mentioned thermoplastic resin is preferably 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 expanded particle molded body of the present invention is used as a radio wave absorber, the conductive material may be any material that exhibits a radio wave absorbing performance in the expanded particle molded body containing the conductive material. For example, as the inorganic material exhibiting radio wave absorption performance, carbon such as conductive carbon black, graphite, graphene, carbon nanotube, carbon nanofiber, carbon microfiber, carbon microcoil, carbon nanocoil, metal fiber, carbon fiber And the like, 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 the material exhibiting high radio wave absorption performance. Among them, carbons can be preferably used from the viewpoint of radio wave absorption, and specifically, conductive carbons can be used. Examples include black, graphite, graphene, carbon nanotubes, carbon nanofibers, carbon microfibers, carbon microcoils, and carbon nanocoils.

Further, as the conductive material, conductive carbon black among carbons can be preferably used. The conductive carbon black is not limited in production method and composition, and includes oil furnace black and acetylene black, carbon black having a hollow structure, etc., and hydrophilizes, oxidizes, and hydrophilizes those carbon blacks. Also included are those that have been subjected to reduction, acidification, basification, or organic treatment. The conductive carbon black preferably has a DBP absorption amount of 150 to 700 cm ³ /100 g measured according to JIS 6217-4:2008.

As the conductive material, one kind selected from the above-mentioned materials exhibiting electromagnetic wave absorption performance may be used, or two or more kinds may be selected and used in combination. When the expanded particles B contain a conductive material, the above-mentioned conductive material can be exemplified as the conductive material.

(Amount of conductive material blended)
In the resin particles in which the conductive material for kneading the expanded 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. Well, by foaming the resin particles in which the conductive material is kneaded, it is possible to obtain the expanded particles A in which the content of the conductive material is similarly 3 to 30% by mass. In the electromagnetic wave absorber, the content of the conductive material contained in the expanded particles A should be determined according to the frequency band of the electromagnetic wave to be generally absorbed and its electromagnetic wave absorption performance. It is 3 to 30% by mass, preferably 7 to 20% by mass, and more preferably 10 to 17% by mass, relative to 100% by mass of the particle A.

When the content of the conductive material in the expanded beads A is too small, it may be difficult to obtain desired conductive performance and radio wave absorption performance even when using the expanded material as a foamed particle molded body. Even if the conductive foamed particles have a small content of the conductive material, a measure for increasing the ratio of the foamed particles A to the foamed particles B can be considered, but the conductive foamed particles coated with the conductive material are As in the case of use, the electromagnetic wave absorber does not have sufficient electromagnetic wave absorption performance in the MHz band. On the other hand, if the content of the conductive material is too large, it may be difficult to perform the step of mixing the conductive material with the base resin, and it may be difficult to obtain the resin particles themselves. Further, when the content of the conductive material is too large, even if it is possible to obtain resin particles, it is difficult to obtain good expanded particles, expanded particles having a low closed cell rate or expanded particles having large shrinkage. Therefore, it may be difficult to obtain expanded particles having good in-mold moldability. On the other hand, in the resin particles for obtaining the expanded beads B, the conductive material is contained in a range of less than 3% by mass, if necessary.

(Area ratio)
Regarding the mixed state of the expanded particles A and the expanded particles B in the expanded particle molded product of the present invention, the total area of the expanded particles A (S1) and the total area of the expanded particles B (S2) appearing in the cross section of the expanded particle molded product. The average value of the area ratio (S1/S2) is 0.05 to 1.0.

When the average value of the above area ratio (S1/S2) is out of the range of 0.05 to 1.0 and is too small, 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 is difficult to disperse the expanded beads A in the expanded beads molded article without a large variation, sufficient and reproducible conductive performance and electromagnetic wave absorption performance cannot be obtained.

When the average value of the above area ratio (S1/S2) deviates from the range of 0.05 to 1.0 and is too large, the secondary foamability of the expanded particles at the time of in-mold molding is decreased, and the obtained expanded particle molded body is obtained. There is a risk of poor surface smoothness and an increase in shrinkage rate of the molded product after molding of the expanded particle molded product. Further, if the average value of the area ratio is too large, the radio wave absorbing performance in the MHz band of the obtained foamed particle molded article radio wave absorber may be deteriorated. From the above viewpoint, the mixing ratio of the expanded particles A and the expanded particles B forming the mixed expanded particles for obtaining the expanded particle molded article of the present invention is such that the volume ratio (total volume of expanded particles A/total expanded particles B). The volume) 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. The volume ratio of the foamed particles A and the foamed particles B of the mixed foamed particles used in the in-mold molding is the foamed 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 foamed particles B can be regarded as the same. 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 measuring method)
In the present invention, the above-mentioned area ratio (S1/S2) is the area of a plurality of expanded particles A cross sections existing within a range of a square of 150 mm in length and 150 mm in width at a plurality of locations selected on the cross section of the expanded particle molded article. (S1) and the total area (S2) of the cross-sections of the plurality of expanded particles B, and then S1 is divided by S2. Then, an operation for obtaining this area ratio (S1/S2) was performed on the cross section of the expanded-particle molded article at 5 locations obtained at random, and the area ratios for 5 locations obtained (SR ₁ , SR ₂ , SR _{3 respectively).} , SR ₄ , SR ₅ ) is the average value (SV) of the area ratio (S1/S2).

The measurement of S1 and S2 existing in the range of the selected square on the cross section of the expanded particle molded article can be performed as follows, for example. First, an enlarged photograph of the square range 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 performing a monotone process on the enlarged image of the image data. In FIG. 8B, the foamed particles A are displayed as a black portion (a portion indicated by reference symbol BL in FIG. 8B). The monotone processing can be realized by applying the enlarged photograph converted into image data to image analysis software such as NS2K Pro (nanosystem). The total area (S1) of the expanded particles A is calculated by calculating the area of the portion that appears black based on the image data that has been subjected to the monotone process, and the entire image (the portion indicated by the symbol W in FIG. 8B) is calculated. 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 coefficient of variation of the area ratio (S1/S2) in the cross section of the expanded bead molded product of the present invention is 20% or less, preferably 15% or less, more preferably 10% or less. If the coefficient of variation is too large, the dispersion of the expanded particles A between the products of the expanded particle molded product or the product part is large, which leads to the expected performance dispersion. Therefore, when the expanded particle molded body having an excessively large coefficient of variation is used as a radio wave absorber, the radio wave absorption performance varies greatly.

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

(Apparent density of expanded particles A and expanded particles B)
Considering that a foamed particle molded body having the above-described density is easily obtained, the apparent density of the foamed particles A and the foamed particles B is preferably 20 to 150 g/L, and further preferably 30 to 80 g/L. ..

(Method of measuring apparent density of expanded particles)
In this specification, the apparent density of expanded particles is measured as follows. The foamed particle group of weight W (g) is submerged in a graduated cylinder containing water using a wire netting or the like, and the volume V (L) of the foamed particle group is calculated from the rise of the water level while considering the volume of the tool such as the wire netting. And the true density of the expanded beads is calculated by converting the value (W/V) obtained by dividing the weight of the expanded beads by the volume of the expanded beads into a unit of g/L. The apparent density of the expanded particles is a value obtained by dividing the true density by 1.6. The apparent density is approximately the same value as the density of a foamed particle molded body obtained by ordinary in-mold molding without significantly compressing the foamed particles.

(Method for producing foamed particle molded body)
Examples of the method for producing the expanded bead molded article according to the present invention include the following methods.

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

(Preparation of expanded particles)
The expanded particles B used for forming the expanded particle molded body are obtained by obtaining resin particles from the above-mentioned base resin in a state in which the use of the conductive material is substantially restricted, and impregnating the resin particles with a foaming agent, It can be obtained by foaming the resin particles by a known foaming method. It should be noted that the state in which the use of the conductive material is substantially regulated is a concept including a state in which the conductive material is not contained at all and a state in which the conductive material is contained in less than 3% by mass. To do. As a method for preparing resin particles for obtaining the foamed particles B, for example, a base resin is put into an extruder, melted, extruded in a strand form from a die attached to the tip of the extruder, and the extruded strand is cut. To obtain resin particles (strand cut method). In addition to this method, as a method for preparing resin particles, a well-known resin particle manufacturing method such as an underwater cut method can be adopted.

The expanded particles A are obtained by obtaining a resin particle or a resin composition in which a conductive material is dispersed in a thermoplastic resin in an amount of 3 to 30% by mass and containing a foaming agent, and the resin particle or the resin composition is described above. It can be obtained by foaming by a known foaming method. As a method of preparing the resin particles for obtaining the expanded beads 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. Other than kneading so as to disperse the resin particles without using any of the above methods, well-known methods for producing resin particles such as the strand cutting method and the underwater cutting method described above can be adopted.

(Preparation of expanded particle mixture)
Next, the expanded particles A and the expanded particles B are mixed using a mixing device or the like to prepare an expanded particle mixture. As the mixing device, a paddle type or screw type mixer, a tumbler, or the like can be appropriately selected.

When the expanded particle molded product of the present invention is used as a radio wave absorber, it is ideal that the mixed state of the expanded particles A and the expanded particles B constituting the expanded particle molded product is uniform. From the viewpoint of achieving this, in the process of in-mold molding a mixture of expanded particles A and expanded particles B (expanded particle mixture) to produce an expanded particle molded article, foams randomly extracted from the expanded particle mixture. It is preferable that the ratio of the expanded particles A contained in the particle group is within 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 represented by the following formulas (4) and (5). Is preferably satisfied.

However, in the above formulas (4) and (5),
D1: apparent density (g/L) of 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,
Is.

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 a group of mixed foamed particles randomly extracted from the foamed particle mixture. It becomes easy to keep 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 the expanded particle group is submerged in water in the graduated cylinder using a tool such as a wire net. Then, the volume V1 [L] of the expanded particles read from the water level rise is measured while considering the volume of a tool such as a wire net. This volume V1 is divided (V1/N) by the number (N) of expanded particles put in the graduated cylinder to calculate the average volume per expanded particle. The diameter of a 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 molded body)
An expanded particle molded product is prepared using the expanded particle mixture obtained as described above. The foamed particle molded product can be prepared based on the in-mold molding method of the foamed particles. As the in-mold molding method, a conventionally known method can be appropriately selected. For example, in a mold formed into a shape according to the purpose, the foamed particle mixture prepared as described above is filled by a known filling method such as a compression filling method or a cracking filling method, and the foamed particles in the die are filled. Is heated with a heating medium such as steam to fuse the foamed particles to each other to obtain a fusion product. Then, the expanded particle molded body can be obtained by taking out the fused material from the mold.

(Use form of foamed particle molded body)
When the foamed particle molded body according to the present invention is used as a radio wave absorber, as shown in FIGS. 1A, 1B, 2A, and 2B, a pyramid-shaped or wedge-shaped shape is mainly adopted, and preferably different. It is used in combination with materials that show different electromagnetic 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. Note that, for example, the electromagnetic wave absorber made of the expanded particle molded body of the present invention can be efficiently absorbed in a wider frequency band by being used together with a material that exhibits remarkable electromagnetic wave absorption performance mainly in the MHz band such as ferrite tile. Is possible. 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, specifically, a composite type having a shape as shown in FIGS. 3A, B and 4A, B. The 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 type electromagnetic wave absorber may be simply referred to as an electromagnetic wave absorber.

To give a concrete example of the frequency bands absorbed by the radio wave absorber, when ferrite is used as a material exhibiting different radio wave absorption performance in different frequency bands, the ferrite tile mainly absorbs radio waves in the low frequency band (30 to 400 MHz). However, the expanded particle molded body mainly absorbs radio waves in the high frequency band above it.

By the way, the electromagnetic wave absorption of a material that causes a dielectric loss includes one caused by a dielectric loss ε″ ^* and one caused by a conductive loss σ, which is expressed by the following mathematical expression (6).

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

In a radio wave absorber that combines a foamed particle molded body and a ferrite tile, since the imaginary part of the complex dielectric constant of the foamed particle molded body is large in the high frequency band, the absorption effect of ferrite is mainly 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 expanded particle molded body is required to have a small conductive loss σ in the formula (6).

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

In the case of the conventional electromagnetic wave absorber of the in-mold molded article containing only the conductive foam particles containing the conductive material, it is possible to increase the amount of the conductive material added in order to improve the electromagnetic wave absorption performance in the GHz band. However, there is a problem that the MHz band absorption performance deteriorates as the added amount increases. On the other hand, when the amount of the conductive material added is reduced 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 deteriorates. That is, there is a problem in that it is difficult to obtain desired radio wave absorption performance in both the GHz band and the MHz band. Further, from a mixed foamed particle type in-molded product of a conductive film-forming foamed particle having a coating layer formed by coating a conventional conductive paint and a non-conductive foamed particle formed without such a coating layer. In this radio wave absorber, the radio wave absorption performance in the MHz band is improved as compared with the above, but there is a problem that it is still at an insufficient level.

The above-mentioned problem regarding the electromagnetic wave absorber of the in-mold molded body composed only of conductive foamed particles is caused by the fact that the entire foamed particle molded body forming the electromagnetic wave absorber is composed of a conductive material. It is considered that the absorption performance is mainly caused by the conduction loss, and it is considered that the absorption performance is easily exhibited in the high frequency band. Further, the problem of the radio wave absorber made of the in-mold molded body of the conductive film-forming foamed particles having the coating layer formed by coating the conductive paint is that the conductive foamed particles are formed by coating the foamed particles with the conductive paint. Therefore, in the conductive film-forming expanded particles, the conductive material is present only in the coating layer of the conductive expanded particles. Further, an electric circuit that becomes a resistance is formed on the surface of the expanded beads, and the conductive loss σ of the expanded film-formed molded particles having a conductive film increases. Therefore, such a conductive film-forming expanded particle molded article cannot be said to have a sufficient radio wave absorption performance.

On the other hand, in the present invention, as described above, mixed foaming in which the average value of the area ratio of the expanded particles A and the expanded particles B in the cross section of the expanded particle molded product is in the range of 0.05 to 1.0. By using the particles, while adjusting the area ratio (S1/S2) of the expanded particles A in the resulting expanded particle molded body to a small range of 1.0 or less, the capacitance of the expanded particle molded body is adjusted to a desired value. can do. When the area ratio is within the above range, the expanded particles A are preferable because they form a single cluster 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 electromagnetic wave absorption performance is improved.

Actually, the capacitances of the expanded-particle molded product of the present invention (molded product composed of expanded particles A and expanded particles B) and the molded product composed of single expanded particles (molded product composed of only expanded particles A) were measured. Results are shown in FIG. The added amount of conductive carbon black as a conductive material contained in a foamed particle molded body composed of foamed particles A and foamed particles B is as follows. It was adjusted by changing the mixing ratio with B. In addition, the amount of conductive carbon black added as a conductive material contained in the expanded particle molded body consisting of expanded particles A was adjusted by changing the compounding ratio of the conductive carbon black in expanded particles A. Regarding these expanded-particle molded products, as shown in FIG. 6, it is recognized that the former expanded-particle molded product of the present invention can efficiently increase the electrostatic capacity. This means that when the foamed particle molded body is used as an equivalent circuit, its capacitor component is larger than the resistance component, which is desirable as a radio wave absorber.

The foamed-particle molded article of the present invention appropriately has a portion containing a large amount of the conductive material composed of the foamed particles A and a portion composed of the foamed particles B in a state in which the use of the conductive material is substantially restricted. As a result, the dielectric loss can be the main component without the conduction loss being the main component, so that the property that the electromagnetic wave absorption performance in the MHz band is likely to deteriorate can be improved. Therefore, the expanded bead molded product of the present invention is excellent in the electromagnetic 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 in 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 expanded particle molded article is 20% or less, the expanded particles A in which the conductive material is kneaded are present substantially uniformly dispersed throughout the expanded particle molded article. Can be said. From this, according to the present invention, what is available as a radio wave absorber capable of more stably absorbing radio waves from the MHz band to the GHz band and capable of supporting a wide frequency range is provided. ..

(Low frequency wave absorber)
As the low-frequency wave absorber 3 used for the composite type wave absorber, for example, a wave absorber made of ferrite tile or the like is preferably used as a magnetic loss material. The ferrite tile exhibits extremely good electromagnetic wave absorption characteristics in the low frequency region of 400 MHz or less. Therefore, as the composite type electromagnetic wave absorber 10, a unit in which an electromagnetic wave absorber made of the expanded particle molded body of the present invention and a ferrite tile are combined can be exemplified. In such a unit of the composite type electromagnetic wave absorber 10, the absorption of the radio wave in the GHz band is efficiently absorbed by the expanded particle molded body, and the radio wave in the MHz band is efficiently absorbed by the ferrite tile due to the magnetic loss of ferrite. As a result, it becomes possible to more strongly exert the electromagnetic wave absorption performance in a wider frequency band.

When the expanded particle molded body and the ferrite tile are used in combination, the conductive material contained in the expanded particle A of the expanded particle molded body is 3 to 30% by mass relative to 100% by mass of the expanded particle A as described above. It is preferably 7 to 20% by mass, more preferably 10 to 17% by mass. The reason why the preferable upper limit range of the conductive material exists is that the content of the conductive material of the expanded particle molded body is too large when the ferrite tile and the space (air) are designed to be matched in impedance. It is considered that since the impedance is largely different from that of air, electromagnetic waves are likely to be reflected at the foamed particle molded body portion due to impedance mismatching, and the absorption characteristics of the ferrite tile responsible for the low frequency region are easily impaired.

On the other hand, if the content of the conductive material in the expanded particle molded article is reduced, the absorption characteristics of the ferrite tile will not be hindered, but the radio wave absorption performance of the expanded particle molded article itself will decrease, so in order to achieve the same performance. Requires that the electromagnetic wave absorber be made large (the height of the polygonal cone is increased when the expanded particle molded body has a polygonal cone shape). As a result, the space occupancy of the anechoic chamber and the like becomes large, and it becomes impossible to effectively utilize the limited space.

Therefore, the expanded particle molded article of the present invention satisfies the mixing ratio of the specific range of the expanded particles A containing the conductive material in the specific range and the expanded particles B in which the use of the conductive material is substantially restricted. When used as a radio wave absorber that is combined with a ferrite tile to form a unit, the ferrite tile efficiently absorbs radio waves in the low frequency band with good ferrite absorption characteristics, and 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 beads A]
An extruder having an inner diameter of 65 mm and a strand forming die attached to the exit side of the extruder was prepared. The propylene-based resin shown in Table 1 is used as the thermoplastic resin as the base resin so as to have the composition shown in Table 1, and the oil furnace black (product name; Ketjen Black (registered trademark) as the conductive inorganic substance that is the conductive material). ) EC300J (manufactured by Lion Corporation), DBP absorption amount; 360 cm ³ /100 g, particle size; 40 nm) are supplied to an extruder having an inner diameter of 65 mm, heated at a set temperature of 200 to 240° C., melted, kneaded, and in a polypropylene resin. After the conductive material is dispersed in the mixture, the obtained melt-kneaded product is extruded in a strand form from the pores of the die, and 2 mg (the ratio of particle length to diameter (L/D) is 1.3) with a pelletizer. 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 were charged into a 5 L autoclave, and carbon dioxide was used as a foaming agent in a closed container in the dispersion medium. Was charged under pressure 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 inside the closed container to 3.0 MPa (G). The particles were discharged together with the dispersion medium under atmospheric pressure to obtain expanded particles A. The dispersed state of the conductive material in the resin forming the expanded particles A was as shown in the TEM photograph (magnification: 20000 times) of FIG. 10. In the TEM photograph of FIG. 10, a black portion indicates a location of the conductive material, and a non-black (white) location indicates a location of the resin. Further, various physical properties of the obtained expanded beads A are also shown in Table 1.

[Production of expanded beads B]
Except for not using a conductive material, the same steps as those for the expanded particles A were performed to obtain expanded particles B. However, in the production of the expanded beads B, the components such as the base resin and the compounding amounts are as shown in Table 2. Further, various physical properties of the obtained expanded beads B are also shown in Table 2.

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

[Melting point of expanded particles]
The value obtained by the heat flux differential scanning calorimetry (DSC method) based on JIS K 7122 (1987) was adopted. Using foamed particles of 2 to 4 mg as a sample, the temperature was raised from room temperature to 200° C. at a rate of 10° C./min by a heat flux differential scanning calorimeter, and then the temperature was lowered from 200° C. to 40° C. at a rate of 10° C./min, and again. The apex temperature of the maximum endothermic curve peak on the DSC curve obtained by raising the temperature 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-mentioned measuring methods.

(Examples 1 to 8, Comparative Examples 2 to 4)
[Manufacture of expanded thermoplastic resin particles]
The expanded beads A shown in Table 1 and the expanded beads B shown in Table 2 were mixed in a tumbler at the mixing ratio shown in Table 3 to obtain an expanded beads mixture. The resulting foamed particle mixture was filled in a mold and heated at the steam forming pressure (MPa: gauge pressure) shown in Table 3 to carry out the in-mold forming step, and the bottom surface 600 mm as shown in FIG. A foamed particle molded body (molded article) having a plurality of pyramid-shaped (quadrangular pyramid-shaped) corners having a size of 600 mm and a height of 300 mm was obtained. In FIG. 7, reference numeral E indicates the bottom surface, reference numeral F indicates the top portion, the dimension of the bottom surface E in the expanded particle molded body shown in FIG. 7 is the length in the range indicated by reference numerals 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 portion F.

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

Surface photographs were taken of the foamed particle molded bodies obtained in Example 1, Comparative Example 2 and Comparative Example 5 described later. The results are shown in Fig. 9. FIG. 9A is a surface photograph of the expanded bead molded article obtained in Example 1. FIG. 9B is a surface photograph of the expanded bead molded product obtained in Comparative Example 2. FIG. 9C is a surface photograph of the expanded bead molded product obtained in Comparative Example 5.

(Comparative Examples 1, 5, 6)
[Manufacture of expanded thermoplastic resin particles]
Except for filling only the expanded beads A shown in Table 1 into the mold, the expanded beads molded article was obtained under the conditions shown in Table 3 in the same manner as in Example 1.

Various physical properties of the expanded bead molded products obtained in Examples and Comparative Examples were measured by the following methods, and the results are shown in Table 4. Furthermore, as described above, Table 4 also shows the results of the evaluation and comprehensive evaluation of the electromagnetic wave absorption performance of the composite type electromagnetic wave absorbers formed of the expanded particle molded products of Examples and Comparative Examples by the following method.

[Average particle diameter of each of expanded particles A and expanded particles B constituting the expanded particle molded article (average particle diameter of expanded particles in claim 2 of the present invention)]
The average particle diameter of the expanded particles A is obtained by observing the cross section of the expanded particle molded article and measuring the maximum diameter of all the expanded particles A present within the range of 100 mm in length×100 mm in 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, regarding the average particle diameter of the expanded particles B, the same expanded particle molded article cross section in which the average particle diameter of the expanded particles A was measured was observed, and the particles exist within the range of a square of 100 mm length×100 mm width. The maximum diameters of all the foamed particles B were measured, and the arithmetic mean value of the maximum diameters was defined as the average particle diameter of the foamed particles B. As shown in Table 4, the average particle diameters of the expanded particles A and the expanded particles B were approximately the same value.

[Radio wave absorption performance of radio wave absorber]
100 composite type electromagnetic wave absorbers were prototyped using the foamed particle molded bodies of Examples and Comparative Examples, and electromagnetic waves were absorbed by the coaxial tube method and the arch method shown in IEEE Std 1128 at frequencies of 30 MHz, 100 MHz, 1 GHz and 18 GHz. The amount (dB) was measured and the average value was calculated.

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

Further, in Table 4, in the "particle diameter ratio" column and the "density ratio" column, 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 are described, The "area ratio (S1/S2)" column describes the average value of the area ratio, and the "variation coefficient (%)" column describes the variation coefficient (%) of the area ratio. The ratio of the average particle diameter (mm) of the foamed particles A and B, the ratio of the apparent density of the foamed particles A and B, the average value of the area ratio, and the coefficient of variation (%) of the area ratio are measured by the above-described measurement methods. Was done.

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

Since the expanded bead molded article of Comparative Example 1 contained only the expanded particles A, it had excellent absorption performance in the GHz band, but had poor radio wave absorption performance in the MHz 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 added amount of the foamed particles A was too large, and sufficient electromagnetic wave absorption performance was not exhibited particularly in the MHz band. ..

The foamed molded articles of Comparative Examples 3 and 4 were inferior in moldability during in-mold molding of the foamed particles, and the coefficient of variation of the area ratio (S1/S2) in the cross section of the foamed particle molded article was 20% or more. In-mold molding of expanded particles was also difficult, and the variation in the electromagnetic wave absorption performance of the obtained expanded particle molded article was more than twice as large as that of Example 1, and stable quality electromagnetic wave absorption performance was not expressed.

The foamed molded article of Comparative Example 5 was foamed in which the compounding amount of the conductive material was adjusted to be small so that the content of the conductive material contained in the entire foamed particle molded article was substantially the same as that of Example 1. It is composed only of particles A. Regarding the electromagnetic wave absorption performance of this foamed particle molded article, it showed good absorption characteristics in the MHz band, but did not exhibit sufficient electromagnetic wave absorption performance in the GHz band.

The foamed molded article of Comparative Example 6 was composed of only the foamed particles A in which the amount of the conductive material was adjusted to a large amount, and although the content of the conductive material in the foamed particle molded body was large, the conductive foamed material was used. Since the content of the conductive material in the particles is too large, its impedance is significantly different from that of air, so that electromagnetic wave reflection easily occurs at the foamed particle molded body part due to impedance mismatch, and sufficient electromagnetic wave absorption performance cannot be obtained. It was

1 Radio Wave Absorber 2a Expanded Particle Molded Body 3 Low Frequency Radio Wave Absorber 10 Composite Wave Absorber

Claims (6)

A thermoplastic resin expanded particle molded article,
As the thermoplastic resin expanded particles forming the expanded particle molded body, expanded particles A in which a conductive material is dispersed in an amount of 3 to 30% by mass, and a conductive material content of less than 3% by mass (including 0) ) Foamed particles B,
The average value of the area ratio (S1/S2) between 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. And the coefficient of variation of the area ratio is 20% or less. The expanded particle molded body according to claim 1, wherein an average particle diameter of each of the expanded particles A and the expanded particles B is 2 to 8 mm. The expanded particle molded product according to claim 1 or 2, wherein the expanded particle molded product has a density of 15 to 90 g/L. The expanded particle molded body according to any one of claims 1 to 3, wherein the conductive material is conductive carbon black. The expanded foam molded article according to any one of claims 1 to 4, wherein the resin forming the expanded thermoplastic resin particles is a polyolefin resin. The expanded particle molded body according to any one of claims 1 to 4, wherein the resin forming the expanded thermoplastic resin particles is a polystyrene resin.