JP2017069400A - Foamed particle molding and radio wave absorptive material using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide aesthetic foamed particles for radio wave absorptive material in which variation of radio wave absorption performance is reduced, by using multilayer foamed particles having a unique layer structure, as a raw material for molding, and to provide a radio wave absorptive material using the same.SOLUTION: In a foamed particle molding for use in a radio wave absorptive material 4, the foamed particle 1 composing the foamed particle molding is a multilayer foamed particle having a thermoplastic resin core layer 2, and a thermoplastic resin coated layer 3 in foamed state covering the core layer, mass ratio of the core layer and coated layer is 50:50-1:99, and the core layer contains a dielectric loss material.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a foamed particle molded body for a radio wave absorber and a radio wave absorber using the same.

  An anechoic chamber that is not affected by external radio waves and that does not reflect internal radio waves is used as a facility for evaluating the characteristics of communication devices such as antennas and measuring interference waves emitted from electronic devices. . Conventionally, in the standard concerning electromagnetic interference, the frequency band of radio waves measured in an anechoic chamber has been mainly 30 MHz to 1 GHz band used in televisions, radios, mobile phones, etc. In recent years, however, wireless LAN, ETC Due to the development of smartphones and the like, it is 30 MHz to 18 GHz. Furthermore, with the development of equipment that uses millimeter waves, such as radar for automobile collision prevention and optical communications, there is an evaluation facility that can handle a wide range of frequency bands, including microwaves and millimeter waves above 10 GHz. It has been demanded.

  As an electromagnetic wave absorber used in an anechoic chamber, one using a molded body made of foamed particles highly filled with a dielectric loss material such as conductive carbon is known. Although such a radio wave absorber has excellent radio wave absorption performance in the GHz band, which is a relatively high frequency, the radio wave absorption performance in the MHz band, which is a relatively low frequency, tends to decrease. For this reason, the length of the radio wave absorber is increased to obtain MHz band absorption characteristics. However, there is a problem that the space occupancy rate of the radio wave absorber in the anechoic chamber is increased, and the limited space cannot be effectively used.

  In order to solve such problems, radio wave absorbers that have been miniaturized by combining a foamed particle molded body using foamed particles containing a dielectric loss material and a ferrite tile have been proposed so far. (For example, see Patent Documents 1 and 2). According to these radio wave absorbers, the GHz band radio waves are mainly absorbed by foamed particles containing a dielectric loss material, and the MHz band radio waves are absorbed by a low frequency absorber such as a ferrite tile.

JP-A-4-267596 JP-A-4-56298

  However, in the radio wave absorber described in Patent Document 1, a foamed particle molded body obtained by molding foamed particles kneaded with a dielectric loss material into a mold is used. However, although it has excellent radio wave absorption characteristics in the GHz band, there is a limit to the radio wave absorption performance in the MHz band, leaving problems.

  Moreover, in the radio wave absorber described in Patent Document 2, by forming a foamed particle molded body for a radio wave absorber using foamed particles with a dielectric loss material attached and foamed particles with no dielectric loss material attached, Improves the radio wave absorption performance in the MHz band of the foamed particle compact. However, since it is difficult to completely and uniformly mix these two types of foam particles during molding of the foam particle molded body, there is a possibility that the radio wave absorption performance between the foam particle molded bodies may vary, and production control is difficult. It was difficult and left a manufacturing challenge. In addition, normally, the foamed particles to which the dielectric loss material is attached are black, whereas the foamed particles to which the dielectric loss material is not attached are white, resulting in color spots on the surface of the foamed particle molded body. There was a problem of detracting from aesthetics.

  The present invention has been made in view of the above-described problems, and there is little variation in the radio wave absorption performance between the foam particle molded bodies, and the foam particle molded body for the radio wave absorber, which has excellent aesthetics, and the radio wave absorption using the same. The challenge is to provide a body.

The present invention provides a foamed particle molded body described below and a radio wave absorber using the same.
<1> A foamed particle molded body for a radio wave absorber, wherein the foamed particles constituting the foamed particle molded body have a thermoplastic resin core layer and a foamed thermoplastic resin coating layer covering the core layer. Expanded foam particles, wherein the core layer and the coating layer have a mass ratio of 50:50 to 1:99, and the core layer contains a dielectric loss material. Molded body.
<2> The foamed particles according to <1>, wherein the value of the imaginary part ε ″ of the complex relative dielectric constant at 0.03 to 18 GHz of the foamed particle molded body is 0.01 to 1. Molded body.
<3> The content of the dielectric loss material is 5 to 80% by mass with respect to 100% by mass of the total amount of the thermoplastic resin and the dielectric loss material in the core layer, <1> or <2> The expanded particle molded body according to <2>.
<4> The foamed particle molded article according to any one of <1> to <3>, wherein the dielectric loss material is conductive carbon black.
<5> The foam according to any one of <1> to <4>, wherein the resin constituting the core layer is a polypropylene resin, and the resin constituting the coating layer is a polypropylene resin Particle compact.
<6> A radio wave absorber comprising the foamed particle molded body according to any one of <1> to <5> and a low frequency absorber.

  According to the present invention, the foamed particles constituting the foamed molded article for radio wave absorbers are multilayer foamed particles having a specific layer structure, so that there is little variation in radio wave absorption performance between products and the appearance is excellent. It will be a thing. Moreover, the radio wave absorber including the foamed particle molded body and the low frequency absorber has excellent radio wave absorption performance over a wide frequency range.

It is the schematic perspective view which showed typically one Embodiment of the multilayer expanded particle which comprises the expanded particle molded object for electromagnetic wave absorbers of this invention. It is an electron micrograph corresponding to the cross section of AA of the multilayer expanded particle in FIG. It is an electron micrograph corresponding to the cross section of BB of the multilayer expanded particle in FIG. It is a perspective view which shows the shape of the electromagnetic wave absorber used in an anechoic chamber. (A) is the schematic of the foaming particle molded object comprised from the foaming particle which coat | covered the surface with dielectric loss material, (B) is the equivalent circuit schematic. (A) is the schematic of the foaming particle molded object comprised from the foaming particle of this invention, (B) is the equivalent circuit schematic. It is a graph which shows the relationship between a frequency (MHz) and the imaginary part ((epsilon) '') of a complex dielectric constant.

  Hereinafter, the present invention will be described in more detail with reference to embodiments for carrying out the invention.

  The foamed particles constituting the foamed molded article for a radio wave absorber of the present invention are multilayer foamed particles having a thermoplastic resin core layer and a foamed thermoplastic resin coating layer covering the core layer. In the present invention, a foamed particle molded article having excellent radio wave absorption performance is formed by forming a foamed particle molded article for a radio wave absorber with the multilayer foamed particles.

  Conventionally, when forming foamed particles by kneading dielectric loss materials, it was difficult to make the dielectric loss material unevenly distributed in the foamed particles because only the dielectric loss material was added to the resin constituting the foamed particles. The dielectric loss material is dispersed throughout the foamed particle molded body, and the foamed particle molded body has a property that the value of the conductive loss σ tends to increase and the absorption in the MHz band tends to be hindered.

  Further, in the case of forming a foamed particle molded body using foamed particles containing dielectric loss material and non-containing foamed particles, the presence of foamed particles not containing dielectric loss material, Although the absorption performance in the MHz band is improved, it is difficult to completely and uniformly mix the two types of expanded particles, and there is a problem of stabilizing the quality in production.

  On the other hand, in the present invention, the dielectric loss material can be unevenly distributed in the expanded particles by using the multilayer expanded particles. That is, in the multi-layer foamed particles, a part containing a dielectric loss material and a part not containing can be present in one foamed particle, so it is not necessary to mix two kinds of foamed particles. Manufacturing management is easy. The foamed particle molded body composed of a large number of multilayer foam particles is formed by fusing the multilayer foam particles, and the coating layer portions and the core layer portions are alternately separated and exist. Therefore, the MHz-band absorption performance as a radio wave absorber is improved, and a foamed particle molded article having excellent quality stability in which variations in the radio wave absorption performance hardly occur between products of the radio wave absorber.

  FIG. 1 is a schematic perspective view schematically showing an embodiment of a multilayer expanded particle constituting the expanded particle molded body of the present invention, and FIG. 2 is an actual electron micrograph of a cross section taken along line AA. FIG. 3 shows an actual electron micrograph of the cross section BB. As shown in FIG. 1, the multilayer expanded particle 1 of the present embodiment includes a core layer 2 formed from a thermoplastic resin, and a foamed coating layer 3 formed from a thermoplastic resin.

(Core layer 2)
The core layer 2 of the thermoplastic resin contains a dielectric loss material as an essential component. When the core layer 2 contains a dielectric loss material, the foamed particle molded body is excellent mainly in radio wave absorption performance in the GHz band.

(Thermoplastic resin)
Examples of the thermoplastic resin for the core layer 2 include polyolefin resins such as polyethylene resins and polypropylene resins, polystyrene resins, polycarbonate resins, polyvinyl chloride resins, polymethacrylic resins, acrylonitrile resins, and polyester resins. , Polyamide resins, and blended polymers thereof.

  Among these, it is preferable to use polyolefin resin. Moreover, when using the mixed resin of polyolefin resin and other resin, it is preferable to contain 50 mass% or more of polyolefin resin, it is more preferable to contain 70 mass% or more, and it is further contained 90 mass% or more. preferable.

  Examples of the polyethylene resin include low density polyethylene, high density polyethylene, linear low density polyethylene, ultra low density polyethylene, ethylene-vinyl acetate copolymer, ethylene-methyl methacrylate copolymer, and ethylene-methacrylic acid copolymer. Examples thereof include ionomer-based resins in which coalesced molecules or their molecules are crosslinked with metal ions.

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

  The thermoplastic resin is preferably not crosslinked from the viewpoint of recyclability.

(Dielectric loss material types)
The dielectric loss material contained in the core layer 2 is not particularly limited as long as it exhibits radio wave absorption performance when formed into a foamed particle molded body, and an inorganic material or an organic material can be used.

  Examples of these inorganic materials include conductive carbon black, graphite, graphene, carbon nanotube, carbon nanofiber, carbon microfiber, carbon microcoil, carbon nanocoil and other carbons, glass fiber, metal fiber, carbon fiber, and the like. Fiber, inorganic hydroxide such as aluminum hydroxide, calcium hydroxide, magnesium hydroxide, inorganic carbonate such as calcium carbonate, magnesium carbonate, barium carbonate, inorganic sulfite such as calcium sulfite, magnesium sulfite, calcium sulfate, sulfuric acid Inorganic sulfates such as magnesium and aluminum sulfate, iron oxide, ferrite aluminum oxide, zinc oxide, silicon oxide, lead oxide, magnesium oxide, cobalt oxide, titanium oxide, calcium oxide, antimony oxide and other inorganic oxides, boron Zinc, calcium borate, magnesium borate, borate such as aluminum borate, other talc, clay, can be mentioned kaolin, clay or natural minerals such as zeolite and the like.

  Among these, carbons and metal oxides are preferable as inorganic materials that exhibit high radio wave absorption performance. Among them, carbons can be preferably used from the viewpoint of radio wave absorption. Carbon black, graphite, graphene, carbon nanotube, carbon nanofiber, carbon microfiber, carbon microcoil, carbon nanocoil and the like.

Further, among carbons, conductive carbon black can be suitably used. 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 dielectric loss material, one kind selected from the above-mentioned compounds may be used, or two or more kinds selected from various compounds and the selected compounds may be used in combination.

(Content of dielectric loss material)
The content of the dielectric loss material can be appropriately determined according to the frequency band to be absorbed and its radio wave absorption performance, but is usually a blending ratio with respect to a total of 100% by mass of the resin of the core layer 2 and the dielectric loss material. It is 5 mass% or more and 90 mass% or less, Preferably it is 6 mass% or more and 85 mass% or less, More preferably, it is 7 mass% or more and 80 mass% or less, More preferably, it is the range of 10 mass% or more and 70 mass% or less.

(Structure of core layer 2)
The core layer 2 is required to contain a dielectric loss material that exhibits radio wave absorption performance, but the structure may be in a foamed state or a non-foamed state.

(Coating layer 3)
The covering layer 3 needs to be present in a foamed state, and the foam structure that forms the covering layer 3 is a structure formed by foaming a thermoplastic resin. When the coating layer 3 exists in a foamed state, more electromagnetic waves can be transmitted through the foamed particle molded body, and reflection of electromagnetic waves by the foamed particle molded body can be effectively suppressed. The covering layer 3 is formed by adding a foaming agent, which will be described later, to a thermoplastic resin and foaming it with a predetermined apparatus and predetermined conditions. Furthermore, since the coating layer 3 of the multilayer foamed particles is in a foamed state and excellent in secondary foamability, the fusion property between the foamed particles is improved, so that the surface smoothness is excellent and the mechanical strength is good. A foamed particle molded body having

(Thermoplastic resin)
As the thermoplastic resin forming the coating layer 3, the same thermoplastic resin as that constituting the core layer 2 can be used. In addition, the thermoplastic resin different from the core layer 2 can also be used in the range of the thermoplastic resin illustrated above.

(Existence of dielectric loss material in coating layer 3)
The coating layer 3 may include a dielectric loss material or may not contain a dielectric loss material. However, the coating layer 3 has a blending ratio of the dielectric loss material blended in the coating layer 3. It is preferable that the blending ratio of the dielectric loss material blended in the core layer 2 is smaller, and it is more preferable that the dielectric loss material is not contained. In addition, when the dielectric loss material is included in the coating layer 3, the radio wave absorption performance can be adjusted. From the viewpoint of the moldability of the foamed particle molded body, the dielectric loss material of the coating layer 3 can be adjusted. The content is preferably 30% by mass or less (including 0) as a blending ratio with respect to 100% by mass of the resin forming the coating layer 3 and the dielectric loss material, and 10% by mass or less (including 0). More preferably, it is 5% by mass or less (including 0% by mass).

(Other additives)
The core layer 2 and the coating layer 3 in the multilayer foamed particle 1 can be blended with additives as necessary within a range that does not impair the effects of the present invention. Examples of such additives include antioxidants, ultraviolet absorbers, antistatic agents, flame retardants, metal deactivators, pigments, dyes, crystal nucleating agents and the like.

  When the content of the dielectric loss material in the coating layer 3 is less than the content of the dielectric loss material in the core layer 2 or when the dielectric loss material is not included in the coating layer 3, the multilayer expanded particles are: Compared to single-layer expanded particles containing a dielectric loss material having the same concentration as the core layer 2, the fusion property is excellent. Therefore, it becomes easier to fuse adjacent multilayer foamed particles 1 more reliably than foamed particles containing a single layer type dielectric loss material. Further, the outer layer can be colored or the color tone can be changed by adding an additive or the like. For example, by making the outer layer white or light in color and making it a bright color tone such as white, the illumination efficiency in the anechoic chamber can be increased, and the effect of improving the appearance and color tone can be obtained. .

(Mass ratio of core layer 2 and coating layer 3)
In the multilayer foamed particle 1, the mass ratio of the core layer 2 to the coating layer 3 is 50:50 to 1:99, preferably 40:60 to 2:98, more preferably 20:80. ~ 3: 97. With such a mass ratio, the core layer 2 containing a large amount of dielectric loss material is surely covered with the coating layer 3, and the multi-layer has excellent fusion properties during molding while increasing the content of the dielectric loss material. It becomes foamed particles.

[Method for producing multilayer expanded particle 1]
When producing the thermoplastic resin multilayer foamed particles 1, for example, two extruders are prepared, the thermoplastic resin composition for forming the core layer 2 is kneaded with one extruder, and the other extrusion After kneading the thermoplastic resin composition for forming the coating layer 3 with a machine, the core layer (R) and the coating layer that covers the core layer (R) (R) are coated by coextrusion from a die having a predetermined shape. R) and a sheath-core type string-like composite is obtained, the composite is cut, and 0.1 to 10 mg of a columnar non-foam made of a core layer (R) and a coating layer (R) Of resin particles are obtained.

  Next, the composite resin particles comprising the core layer (R) and the coating layer (R) are dispersed in an aqueous medium (usually water) in a pressurizable sealed container (for example, an autoclave), and a dispersant is added. , Press the required amount of foaming agent, pressurize and stir under heating for the required time to impregnate the foaming agent into the composite resin particles, and then discharge the contents together with the aqueous medium to a pressure lower than the pressure inside the container. Multi-layer expanded particles can be obtained by foaming. Examples of the blowing agent include aliphatic hydrocarbons such as propane, butane, hexane, and heptane, cyclic aliphatic hydrocarbons such as cyclobutane and cyclohexane, chlorofluoromethane, trifluoromethane, methyl chloride, and ethyl chloride. Examples thereof include various organic foaming agents such as organic physical foaming agents such as halogenated hydrocarbons, and so-called inorganic physical foaming agents such as nitrogen, oxygen, air, carbon dioxide, and water. Among these various physical foaming agents, those mainly composed of one or more inorganic physical foaming agents selected from the group consisting of nitrogen, oxygen, air, carbon dioxide, and water are suitable.

(Apparent density)
The multilayer foamed particles preferably have an apparent density of the whole foamed particles of 0.3 g / cm ³ or less from the viewpoint of lightness. Even if the apparent density of the multilayer foamed particles is 0.3 g / cm ³ or less, excellent secondary foamability is obtained and the moldability is also excellent. The apparent density is more preferably from 0.02~0.25g / cm ^3, more preferably from 0.03~0.2g / cm ^3.

(Average bubble diameter)
The average cell diameter of the coating layer 3 portion of the multilayer foamed particles is preferably 40 to 300 μm from the viewpoint of in-mold moldability and mechanical strength of the obtained molded body. Furthermore, the average cell diameter is preferably 45 to 280 μm, more preferably 50 to 250 μm.

(Closed cell rate)
The closed cell ratio of the multilayer foamed particles is preferably 70% or more from the viewpoints of foam moldability, mechanical strength of the foamed particle molded body, surface smoothness, and the like. More preferably, it is 80% or more, More preferably, it is 90% or more. In addition, an average bubble diameter, an apparent density, and a closed cell rate can be calculated | required by the method as described in the Example mentioned later.

(Thermoplastic resin foam particle molding)
The thermoplastic resin expanded particle molded body according to the present invention is composed of a large number of expanded thermoplastic resin particles 1. The foamed particle molded body can be obtained by molding foamed particles in a mold. The shape of the thermoplastic resin expanded particle molded body is not particularly limited, and can be appropriately set to a plate shape, a column shape, or various three-dimensional shapes. Since the expanded particle molded body of the present invention is composed of multilayer expanded particles, the core layer and the coating layer are alternately present in the expanded particle molded body. Therefore, the foamed particle molded body of the present invention has excellent radio wave absorption performance having a wide electromagnetic wave absorption characteristic from the MHz band to the GHz band.

(Apparent density)
Although the apparent density of the thermoplastic resin foam particle molded body is not particularly limited, the apparent density is preferably 0.015 to 0.45 g / cm ³ . The apparent density of the foamed particle molded body can be determined by dividing the mass of the molded body by the volume of the molded body. In view of the above, it is preferably 0.02~0.3g / cm ^3, further preferably 0.03~0.2g / cm ^3. In addition, the volume of a molded object can be calculated | required from the external dimension etc. of a molded object.

(Fusion rate)
The thermoplastic resin foam particle molded body is not particularly limited as long as the integrity can be maintained, but in consideration of obtaining a thermoplastic resin foam particle molded body having excellent mechanical strength The fusion rate of the thermoplastic resin expanded particle molded body is 60% or more, preferably 70% or more, more preferably 80% or more. In the present invention, the multi-layer foamed particles having excellent fusion properties are molded in the mold, so that a foamed particle molded product having excellent fusion properties is obtained.

(Imaginary part ε ″ of complex relative permittivity)
In the foamed particle molded body of the present invention, the imaginary part ε ″ of the complex relative dielectric constant at a frequency of 0.03 to 18 GHz is preferably in the range of 0.01 to 1, More preferably, it is the range of 0.01-0.3. If it is in the said range, it will have the outstanding electromagnetic wave absorption performance as a foaming particle molding for electromagnetic wave absorbers.

[Radio wave absorber]
The radio wave absorber of the present invention comprises the foamed particle molded body and a low frequency absorber, and can absorb a wide range of frequencies by absorbing electromagnetic waves from the MHz band to the GHz band. It becomes. Specifically, the foamed particle molded body and a low frequency absorber made of a magnetic loss material are laminated, the foamed particle molded body is disposed on the electromagnetic wave arrival side, and the low frequency absorber is disposed on the other side. Is preferred. Moreover, it is preferable that the said foaming particle molded object is a shape where the ratio of the volume of the foaming particle molded object which occupies for a unit volume becomes large toward the other edge part from the edge part by the side of electromagnetic wave arrival. For example, the foamed particle molded body preferably has a wedge shape or a polygonal pyramid shape.

  The low-frequency absorber is not particularly limited as long as it can absorb a relatively low-frequency radio wave in the MHz band, and examples thereof include a magnetic loss material such as ferrite. Specifically, a plate-like ferrite is used. There are tiles. By combining the foamed particle molded body of the present invention with a ferrite tile or the like to form a radio wave absorber, a radio wave absorber capable of more stably absorbing electromagnetic waves in a wide frequency band from the MHz band to the GHz band. Can do.

  Hereinafter, a radio wave absorber comprising a foamed particle molded body containing a dielectric loss material and a low frequency absorber containing a magnetic loss material will be described.

  In the composite type electromagnetic wave absorber as described above, for example, an electromagnetic wave absorber composed of a foamed particle molded body containing conductive graphite as a dielectric loss material and a ferrite tile as a magnetic loss material is used. Here, since the ferrite tile shows good absorption characteristics in the low frequency range of 400 MHz or less, radio waves in the low frequency range are absorbed by the magnetic loss of ferrite, and radio waves in the high frequency range of 400 MHz or more are installed on the surface side of the ferrite tile. The foamed particle molded body containing the dielectric loss material thus obtained is absorbed.

  In addition, if the dielectric loss material concentration of the foamed particle molded body is too high when the impedance of ferrite tile and space (air) is designed to match, the impedance is greatly different from that of air. It is considered that electromagnetic waves are likely to be reflected at the body part, and the absorption characteristics of ferrite tiles responsible for the low frequency range are likely to be hindered.

  On the other hand, if the dielectric loss material of the foamed particle compact is reduced in concentration, the absorption characteristics of the ferrite tile will not be disturbed, but the radio wave absorption characteristics per unit length of the foamed particle compact itself will decrease, so the same performance In order to produce the above, it is necessary to lengthen the absorbent body (in the case where the foamed particle molded body has a polygonal pyramid shape, the height of the polygonal pyramid must be increased). As a result, the space occupancy increases and the limited space cannot be used effectively.

Therefore, the foamed particle molded body in the radio wave absorber has a low imaginary part ε ″ of the complex relative dielectric constant in the low frequency range (30 to 400 MHz) where the absorption characteristics of ferrite are good, and the complex ratio in the higher frequency range. It is desirable to have a frequency characteristic that increases the imaginary part ε ″ of the dielectric constant. That is, the ferrite tile absorbs radio waves in the low frequency range, and the foamed particle molded body absorbs radio waves in the high frequency range, so that radio waves can be efficiently absorbed in a wide frequency band.

The electromagnetic wave absorption of the dielectric loss material may be caused by the dielectric loss ε ″ ^* or the conductive loss σ, and is expressed by the following formula (1).
ε ″ = ε ″ ^* + σ / (2πf) (1)
(Where ε ″ ^* is dielectric loss, σ is conduction loss, ε ″ is the imaginary part of the complex relative permittivity, and f is the frequency.)

  From the viewpoint of use in the radio wave absorber in which the foamed particle molded body and the ferrite tile are combined, it is desirable that ε '' of the foamed particle molded body is small because the absorption effect by ferrite is mainly used in the low frequency band. In the frequency range of 30 to 400 MHz, the imaginary part ε ″ of the complex relative dielectric constant is preferably in the range of 0.01 to 1, and more preferably in the range of 0.01 to 0.5.

  When the surface of the expanded particle is coated with a dielectric loss layer as in the conventional example shown in FIG. 5A, a dielectric loss layer is continuously formed between the expanded particles, and an equivalent circuit is shown in FIG. It is thought that it becomes like B). In this case, the influence of the resistance component of the equivalent circuit is large, and the conduction loss σ is mainly used in the equation (1). Therefore, it is not desirable as a radio wave absorber.

  On the other hand, as in the present invention shown in FIG. 6A, in the foamed particle molded body made of multilayer foamed particles, the dielectric loss layer corresponding to the core layer of the foamed particles is completely separated from other dielectric loss layers. Therefore, it is considered that an equivalent circuit is as shown in FIG. In this case, the capacitor component of the equivalent circuit is large, and the dielectric loss ε ″ * is mainly used in the equation (1), so that it is particularly excellent as a radio wave absorber.

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

It adjusted using the following raw materials, and obtained the thermoplastic resin foam particles, the molded object, and the electromagnetic wave absorber of Examples 1-4 and Comparative Examples 1-3.
(material)
Thermoplastic resin: Polypropylene resin shown in Table 1 Dielectric loss material: Conductive carbon black shown in Table 2

[Production of thermoplastic resin foam particles]
Example 1
Using an extruder having a 65 mm inner diameter resin particle core layer extruder and a 30 mm inner diameter resin particle coating layer extruder provided with a die for forming a multilayer strand on the outlet side, the composition shown in Table 3 was obtained. Polypropylene resin of Table 1, conductive carbon black of Table 2 as a dielectric loss material in an extruder for core layer with an inner diameter of 65 mm, and at the same time, the polypropylene resin of Table 1 so as to have the composition shown in Table 3, Carbon black of Table 2 as a colorant was supplied to a coating layer extruder having an inner diameter of 30 mm, and each was heated, melted and kneaded to a set temperature of 200 to 220 ° C., and then supplied to the die. Then, it is coextruded as a multilayer strand in which the coating layer is coated on the side surface of the core layer from the pores of the die that are joined in the die and attached to the tip of the extruder, and is 2 mg (L / D = 1.3) with a pelletizer The columnar resin particles formed into two layers (sheath core structure) were obtained by cutting in such a manner. Further, the mass ratio of the core layer to the coating layer was 10:90 in Example 1. 1 kg of the above resin particles are charged into a 5 L autoclave together with 3 L of water as a dispersion medium, 3 g of kaolin as a dispersant and 0.02 g of sodium dodecylbenzenesulfonate, and carbon dioxide is injected as a blowing agent into a sealed container in the dispersion medium. Then, heat up to the foaming temperature under stirring and adjust it by holding at the same temperature for 15 minutes. After adjusting the pressure in the sealed container to the value shown in Table 3, the autoclave contents are discharged together with water under atmospheric pressure. Expanded particles were obtained.

[Production of thermoplastic resin foamed particle molded body (foamed particle molded body)]
The foamed particles obtained above were filled into a flat plate-shaped mold and subjected to in-mold molding by steam heating to obtain a flat plate-shaped foam molded body.

[Manufacture of electromagnetic wave absorber]
Using the foamed particles obtained in the same manner as described above, a foamed particle molded body was obtained in the same manner as described above, except that the shape was changed to a pyramid shape having a height of 300 mm. The obtained foamed particle molded body and a ferrite tile (bottom 100 mm × 100 mm, thickness 5.2 mm; manufactured by Riken Environment System Co., Ltd., trade name RF044) are laminated to form a molded body shape (the whole A radio wave absorber was obtained with a bottom surface of 600 mm × 600 mm, and the foamed particle molded body was fixed with screws by attaching a wood substrate on the ferrite tile.

(Examples 2 to 5)
Resin particles, expanded particles, expanded particle molded bodies, and radio wave absorbers were obtained in the same manner as in Example 1 except that the conditions shown in Table 3 were changed.

(Comparative Example 1)
Supplied to a single-screw extruder with an inner diameter of 50 mm so that the amount of zinc borate is 0.1 parts by mass with respect to 86 parts by mass of PP1 in Table 1, 14 parts by mass of CB1 in Table 2, and 100 parts by mass of PP1 and CB1. And melt-kneaded at 200 to 220 ° C., extruded into a strand, cooled, cut, and cut with a pelletizer to 2 mg, L / D = 2.4. Resin particles, expanded particles, expanded particle molded bodies, and radio wave absorbers were obtained in the same manner as in Example 1 except that the above resin particles were obtained.

(Comparative Example 2)
Resin particles, expanded particles, expanded particle molded bodies, and radio wave absorbers were obtained in the same manner as in Example 1 except that the conditions shown in Table 3 were changed.

(Comparative Example 3)
In a single screw extruder with an inner diameter of 50 mm, 86 mass% of PP1 in Table 1, 14 mass% of CB1 in Table 2, and 0.1 mass part of zinc borate with respect to 100 mass parts of the total amount of PP1 and CB1. Then, the mixture was melt-kneaded at 200 to 220 ° C., extruded into a strand shape, the strand was cooled and cut, and cut with a pelletizer to give 2 mg and L / D = 2.4 to obtain resin particles 1 .

  Single-screw extrusion with 99.4% by mass of PP2 in Table 1, 0.6% by mass of CB2 in Table 2, 0.1 part by mass of zinc borate with respect to 100 parts by mass of PP2 and CB2, and an inner diameter of 50 mm To the machine, melt kneaded at 200-220 ° C., extruded into a strand, cooled and cut the strand, and cut with a pelletizer to give 2 mg, L / D = 2.4 to obtain resin particles 2 Obtained.

1 kg of the above resin particles 1 are charged into a 5 L autoclave together with 3 L of water as a dispersion medium, 3 g of kaolin as a dispersant and 0.04 g of sodium dodecylbenzenesulfonate, and carbon dioxide is injected as a blowing agent into a sealed container in the dispersion medium. Then, the temperature was raised to 148 ° C. with stirring and the pressure inside the sealed container was maintained at 2.6 MPa, and maintained at the same temperature for 15 minutes, and then the contents of the autoclave were released together with water under atmospheric pressure. Thus, expanded particles 1 having an apparent density of 0.065 g / cm ³ were obtained. Further, in the same manner for the resin particles 2, foamed particles 2 having an apparent density of 0.065 g / cm ³ were obtained. The foamed particles 1 and 2 are mixed with a tumbler so that the mass ratio is 15:85, and using this, molding is performed at a molding pressure of 0.26 (MPa (G)). A foamed particle molded body (apparent density 0.055 g / cm ³ ) and an electromagnetic wave absorber were obtained.

In Table 3, the coating layer / core layer mass ratio indicates the mass ratio (%) of each of the coating layer and the core layer in the total mass of the particles.

  The evaluation method of the expanded particles, the expanded particle molded body, and the radio wave absorber was performed as follows.

[Apparent density of expanded particles]
The apparent density of the expanded particles was determined as follows. First, a 1 L graduated cylinder was prepared, and a group of expanded particles was filled up to a 1 L marked line in the graduated cylinder. The apparent density (g / cm ³ ) of the expanded particles was determined by measuring the mass (g / L) of the expanded expanded particle group per liter filled and converting the unit.

[Closed cell ratio of expanded particles]
The closed cell ratio of the expanded particles was measured as follows. Using the foamed particles left for 10 days in a thermostatic chamber as a measurement sample, the apparent volume Va was accurately measured by the submersion method as described below. After the sample for measurement in which the apparent volume Va was measured was sufficiently dried, it was measured with an “air comparison hydrometer 930” manufactured by Toshiba Beckman Co., Ltd. according to the procedure C described in ASTM-D2856-70. The true volume value Vx of the measurement sample was measured. And based on these volume values Va and Vx, the closed cell rate was calculated by the following formula (2), and the average value of 5 samples (N = 5) was taken as the closed cell rate of the expanded particles.

Closed cell ratio (%) = (Vx−W / ρ) × 100 / (Va−W / ρ) (2)
(Where
Vx: the sum of the true volume of the expanded particles measured by the above method, that is, the volume of the resin constituting the expanded particles and the total volume of bubbles in the closed cell portion in the expanded particles (cm ³ )
Va: The apparent volume of the expanded particles (cm ³ ) measured from the rise in the water level after the expanded particles are submerged in a graduated cylinder containing water.
W: Weight of the foam particle measurement sample (g)
ρ: Density of resin constituting expanded particles (g / cm ³ )
Means)

[Average cell diameter of expanded particles]
The average cell diameter of the expanded particles was measured as follows.
Based on the enlarged photograph which image | photographed the cut surface which divided | segmented expanded particle | grains into the substantially bisected section like FIG. First, in the enlarged photograph of the cut surface of the expanded particle, a perpendicular bisector l is drawn with respect to a line segment that takes the minimum distance from the upper end surface to the lower end surface so as to pass through the center of the expanded surface of the expanded particle, The length of the line segment l from the left end surface to the right end surface of the particle was measured, the length was defined as Lc (μm), the number Nc (number) of bubbles intersecting the straight line l was obtained, and Lc was divided by N The value (Lc / Nc) was defined as the average cell diameter of the core layer 2 portion of one expanded particle.

  Next, a curve passing 100 μm inside from the upper end surface is drawn from the right end surface to the left end surface, the length Ls (μm) and the number Ns (number) of bubbles intersecting the curve are obtained, and Ls is divided by Ns (Ls / Ns) was defined as the average cell diameter of the coating layer 3 portion of one expanded particle. This operation is performed for 10 expanded particles, and the average cell diameter of the core layer 2 and the coating layer 3 portion of each expanded particle is calculated as the average cell diameter of the expanded particle core layer 2 and the coating layer 3 portion. did.

[Apparent density of molded foam particles]
The apparent density of the foamed particle molded body was determined by dividing the mass (g) of the molded body by the volume (cm ³ ) determined from the outer dimensions of the molded body.

[Fusion rate of molded foam particles]
For the measurement of the fusion rate, the ratio of the number of foam particles whose material was destroyed among the foam particles exposed on the fracture surface when the foamed particle molded body was broken was measured. Specifically, a test piece was cut out from the foamed particle molded body, and after cutting a depth of about 5 mm into each test piece with a cutter knife, the foamed particle molded body was broken from the cut portion. Next, the number of foam particles (n) present on the fracture surface of the foam particle molded body and the number of foam particles (b) with material destruction were measured, and the ratio (b / n) of (b) and (n). Was expressed as a percentage and used as the fusion rate (%).
The fusion rates of the foamed particle molded bodies of Examples 1 to 4 and Comparative Examples 1 to 3 were all 100%.

[Imaginary part ε ″ of complex relative permittivity of molded foam particles]
An imaginary part of complex relative permittivity, which is a ratio to the dielectric constant of vacuum, is prepared from a foamed particle molded body formed into a flat plate shape by a S-parameter method using a coaxial waveguide for the sample. ε ″ was measured from 30 MHz to 18 GHz. An example of the measurement result is shown in Table 4 and FIG.

Furthermore, the same measurement was performed for Examples 2 to 5 and Comparative Examples 1 and 2, and the evaluation was made based on the following evaluation criteria.
(Circle): The average value of the measured value of 10 samples in each frequency exists in the range of 0.01-0.5.
X: The average value of the measured values of 10 samples at each frequency is outside the range of 0.01 to 0.5.
The results are shown in Table 3.

[Performance variation among foamed particle compacts]
In Example 1 and Comparative Example 3, five foamed particle molded bodies were produced as prototypes, and ε ″ was measured by the S-parameter method using a coaxial waveguide at a frequency of 1 GHz. Then, (maximum value−minimum value) / 2 / average value of ε ″ was obtained, and the variation in performance was evaluated. The measurement results are shown in Table 5.

[Radio wave absorber performance measurement]
100 radio wave absorbers were made using the expanded particles of Examples and Comparative Examples, and at a frequency of 30 MHz, 100 MHz, 1 GHz, and 18 GHz, according to IEEE Std1128, the coaxial tube method (frequency 30 MHz, 100 MHz) and the arch method (frequency 1 GHz, 18 GHz), the amount of radio wave absorption (dB) was measured, and the average value was obtained. The results are shown in Table 6.

  From the evaluation in Table 3 above, the foams of Examples 1 to 5 have the core layer highly filled with a dielectric loss material, so that the function of the dielectric loss material can be exhibited, and in any part Since the ratio between the core layer and the coating layer is constant, it was confirmed that the core layer has a sufficiently stable radio wave absorption capability.

  On the other hand, since Comparative Example 1 includes only conductive foam particles, the desired radio wave absorption performance could not be obtained.

Further, in Comparative Example 2, since the coating layer was filled with more dielectric loss material than the core layer, sufficient radio wave absorption performance could not be obtained.
In Comparative Example 3, since the blending ratio of the two types of foam particles of the core was not constant depending on the location, the results showed that there were many variations in performance between products.

DESCRIPTION OF SYMBOLS 1 Multi-layer foam particle 2 Core layer 3 Coating layer 4 Electric wave absorber (square pyramid)

Claims (6)

A foamed particle molded body for a radio wave absorber,
The foamed particles constituting the foamed particle molded body are multilayer foamed particles having a thermoplastic resin core layer and a foamed thermoplastic resin coating layer covering the core layer,
The mass ratio of the core layer to the coating layer is in the range of 50:50 to 1:99;
The core layer contains a dielectric loss material.   2. The foamed particle molded body according to claim 1, wherein the foamed particle molded body has a value of an imaginary part ε ″ of a complex relative dielectric constant at 0.03 to 18 GHz of 0.01 to 1. 3.   The foam according to claim 1 or 2, wherein the content of the dielectric loss material is 5 to 80 mass% with respect to 100 mass% of the thermoplastic resin and the dielectric loss material in the core layer. Particle compact.   The foamed particle molded body according to any one of claims 1 to 3, wherein the dielectric loss material is conductive carbon black.   The foamed particle molded body according to any one of claims 1 to 4, wherein the resin constituting the core layer is a polypropylene resin, and the resin constituting the coating layer is a polypropylene resin. . A radio wave absorber comprising the foamed particle molded body according to any one of claims 1 to 5 and a low frequency absorber.