JPH0756919B2 - Radio wave absorber and method of manufacturing the same - Google Patents

Description

The present invention relates to a radio wave absorber made of pre-expanded particles made of polystyrene or a copolymer thereof, and a method for producing the same.

[Conventional technology]

Conventionally, some anechoic chamber absorbers include those using foamed styrene and those using polyurethane foam as a base material. As a radio wave absorber using expanded styrene, a high-structure carbon black on the surface of spherical pre-expanded particles with a diameter of several mm obtained by pre-expanding a polystyrene polymer containing a foaming agent under pressure steam. A polystyrene-based electromagnetic wave absorber is known in which a polyhedron is integrally molded by coating (hereinafter abbreviated as CB), an adhesive, and a mixture, and then secondary-foaming this polymer in a mold. This radio wave absorber has good weather resistance, but since the above pre-expanded particles to which CB is fixed are secondary-foamed and fused in a mold, they are not flexible and have the drawback of poor impact resistance. there were. Therefore, it is difficult to form a radio wave absorber having a pyramid shape or the like having good radio wave absorption performance. In addition, since the secondary foaming is caused by the heat of the steam in the mold, the thermal conductivity is different between the location near the mold surface and the interior, and the secondary foaming does not occur sufficiently to the inside, so the variation of the complex dielectric constant is large. It was Moreover, since it is necessary to coat the surface of the pre-expanded particles with a mixture of high-structure carbon and an adhesive as CB, it takes time to mold, and it is molded by the pre-foaming process. The cost had to be expensive.

On the other hand, there is known a radio wave absorber in which polyurethane foam is forcibly impregnated with CB-containing rubber latex and dried. However, since this radio wave absorber is made of polyurethane as a base material, it is flexible and has good impact resistance, but it has a drawback that it has poor weather resistance as compared with the polystyrene radio wave absorber. Also, since the open cells of polyurethane foam are forcibly impregnated with CB-containing rubber latex,
Impregnation tends to be non-uniform, and in the case of a high-viscosity, high-structure CB-containing rubber latex, impregnation itself was extremely difficult. For this reason, the complex dielectric constant of the electromagnetic wave absorber is large, the types of CB that can be used are limited, and no one having good electromagnetic wave absorption performance compared to the polystyrene type electromagnetic wave absorber was obtained.

[Problems to be Solved by the Invention]

An object of the present invention is to solve the above-mentioned drawbacks of conventional radio wave absorbers and to provide a radio wave absorber having excellent impact resistance and radio wave absorption performance based on a polystyrene foam having excellent weather resistance, and a method for producing the same. Is to provide.

[Means for Solving the Problems]

Such an object of the present invention is a bulk specific gravity of 0.010 to 0.030,
Dibutyl phthalate (hereinafter abbreviated as DBP) oil absorption of 100-400 cc / 100g with a large number of pre-expanded particles made of polystyrene-based polymer having an average particle size of 2.0-20 mm without secondary expansion of the pre-expanded particles. CB or average particle size 0.5 to 20
This can be achieved by a wave absorber bonded and integrated with a rubber latex containing at least one of micron graphite.

The radio wave absorber is prepared by pre-mixing, in a solid state, at least one of polystyrene pre-expanded particles having the bulk specific gravity and average particle size and the above-mentioned CB or graphite to obtain a surface of the pre-expanded particles. At least one of CB and graphite is uniformly adsorbed on the above, and then rubber latex is added to this to directly fix at least one of CB and graphite to the surface of the pre-expanded particles with the rubber in the rubber latex. It can be manufactured by a method of pouring this mixture into a mold and drying and dehydrating. Alternatively, after a mixed solution of at least one of CB or graphite and rubber latex is fixed to the surface of the pre-expanded particles having the bulk specific gravity and the average particle size, this is poured into a mold and dried. It can also be produced by a dehydration method.

In the present invention, the polystyrene-based polymer may be any one that can be foamed in the presence of a foaming agent, and in addition to polystyrene, a styrene-acrylonitrile copolymer obtained by substituting a part of styrene with another monomer such as acrylonitrile, etc. The styrene-based copolymer of is also included.

In the present invention, the pre-expanded particles are secondary foams (molded foams) obtained by pre-expanding expandable pellets or beads made of a styrene copolymer at a temperature of 90 ° C to 110 ° C. Also called) diameter that can be 2.0-2
Refers to 0 mm spherical foam particles.

Further, the secondary expansion means that the pre-expanded particles are further expanded by heating and are softened to be fused and integrated with each other.

Steam is mainly used as a heat source in the secondary foaming. Further, as the secondary molding method, a method in which pre-expanded particles are placed in a steam chest and steam is directly blown, a method in which a shell mold having holes is heated in an autoclave, a method in which steam is blown in a mold and the like are used. is there.

Since the electromagnetic wave absorber of the present invention is bonded and integrated with the pre-expanded particles by the rubber latex having the rubber elasticity without secondary foaming, it has the elongation and flexibility to easily follow the deformation such as bending. It is possible to exhibit excellent impact resistance as a radio wave absorber. Also, on the surface of the pre-expanded particles,
DBP Oil absorption of 100-400cc / 100g CB of high structure or graphite with an average particle size of 0.5-20 microns is pre-uniformly adsorbed and fixed with rubber latex,
Alternatively, by adhering a mixed solution of at least one of CB and graphite and rubber latex to the surface of the pre-expanded particles, excellent electromagnetic wave absorption performance can be imparted to the electromagnetic wave absorber.

The present invention, by setting the bulk specific gravity of the pre-expanded particles to 0.010 or more, it is possible to stably maintain a desired shape without easily deforming with a slight force during processing into a radio wave absorber. It is possible to make it difficult for rubber latex to be sucked into the inside. Further, by setting the bulk specific gravity to 0.030 or less, the weight of the radio wave absorber can be reduced and the weight can be reduced, and the complex dielectric constant close to that of air desired for the radio wave absorber can be easily obtained.

The pre-expanded particles of the present invention are generally spherical particles,
The shapes are not uniform, and one expanded particle itself cannot be said to be completely spherical. Therefore, the pre-expanded particles of the present invention have an average particle diameter [d] of 2 mm or more and 20 mm or less.

In the present invention, the average particle diameter [d] means that when the average value of the major axis and the minor axis of the pre-expanded particles is d, the d is measured for 20 pre-expanded particles, and the arithmetic mean of this d [ d]
A value defined by.

By setting the average particle diameter [d] of the pre-expanded particles to 2 mm or more, it is possible to prevent the specific surface area per unit volume of the radio wave absorber from becoming too large and the dielectric loss from decreasing. Further, by setting the average particle diameter [d] to 20 mm or less, for example, on the high frequency side of the operating frequency band of the anechoic chamber, the average particle diameter [d] is 1/10 to 1/20 of the wavelength.
Therefore, the size of the chain formed by CB can be prevented from approaching the wavelength.

The CB used in the present invention has a DBP oil absorption of 100 to 400 cc / 100 g. When the DBP oil absorption is 100 cc / 100 g or more, it is possible to give a high dielectric loss rate by adding a small amount, and it is possible to give sufficient electromagnetic wave absorption performance. In addition, when the load is 400 cc / 100 g or less, it is possible to prevent the electric wave absorption performance from becoming unstable due to a slight load.

Further, by setting the average particle diameter of graphite in the range of 0.5 to 20 μm, adsorption and fixation to the surface of the pre-expanded particles can be facilitated, and it is possible to make it difficult to fall off.

The amount of CB or graphite added to the radio wave absorber, that is, the concentration of CB or graphite is not particularly limited and can be appropriately selected and determined according to the radio wave absorption performance of the radio wave absorber.

The rubber latex of the present invention binds and integrates pre-expanded particles with each other, fixes CB or graphite on the surface of the pre-expanded particles, and improves the flexibility and impact resistance of the electromagnetic wave absorber by its elasticity as an elastomer. be able to.

The electromagnetic wave absorber of the present invention can be formed into various shapes such as a pyramid type, a wedge type, a box type, a cubic type, and a rectangular parallelepiped type. Depending on the frequency band to be absorbed, the thickness is relatively thin for high frequencies and thick for low frequencies. In the case of the relatively low frequency band MHz,
It can be used in combination with a ferrite tile or the like.

Such a method for producing a radio wave absorber of the present invention comprises, as a first step, at least one of pre-expanded particles composed of the polystyrene-based polymer having the bulk specific gravity and the average particle diameter and the high oil absorption CB or graphite. The seeds are mixed in the solid state, preferably in the closed state. By mixing both in a solid state in advance, the CB or graphite can be uniformly adsorbed around the pre-expanded particles.

Then, as a second step, rubber latex is added to the pre-expanded particles on which the CB or graphite is adsorbed,
CB or graphite is fixed directly on the surface of the pre-expanded particles, and the mixture is poured into a mold and dried and dehydrated, so that the rubber latex solidified in the film form is interposed between the pre-expanded particles and CB or graphite is applied to the surface. The fixed pre-expanded particles are bonded and integrated.

Further, the radio wave absorber of the present invention comprises, on the surface of the pre-expanded particles, at least one of carbon black having the dibutyl phthalate oil absorption of 100 to 400 cc / 100 g or graphite having an average particle size of 0.5 to 20 μm and a rubber latex. It can also be produced by fixing a mixed solution consisting of, and then injecting it into a mold and drying and dehydrating.

Of the carbon blacks, those with a large DBP oil absorption increase the viscosity when mixed with rubber latex and make the mixing operation difficult, so increase the amount of rubber latex to reduce this viscosity. become. But,
Increasing the amount of rubber latex is not preferable because it increases the weight of the radio wave absorber. On the other hand, graphite is advantageous in reducing the weight of the radio wave absorber because it has a small thickening effect as described above.

Examples of the rubber latex include styrene / butadiene copolymer rubber, acrylonitrile / butadiene copolymer rubber, chloroprene rubber, natural rubber, and latexes of other rubber-like substances obtained by emulsion polymerization and the like. These rubber latexes contain 40% rubber solids.
It is preferable that the content is in the range of not less than 75% by weight and not more than 75% by weight. If the rubber solid content exceeds 75%, the rubber latex becomes unstable and difficult to use. On the other hand, if the rubber solid content is less than 40%, the water content is too high, which requires a long time for drying and dehydration and makes molding difficult, which is not preferable.

The rubber latex may contain a crosslinking agent, a vulcanizing agent and / or a vulcanization accelerator. Further, in order to improve the weather resistance of the uncrosslinked or crosslinked rubber film formed from the rubber latex, an amine-based, phenol-based antiaging agent or the like can be appropriately added.

〔Example〕

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

The DBP oil absorption in the examples is a value measured by the method defined in ASTM-D2414-78.

The following four types of electromagnetic wave absorbers were measured for impact resistance and electromagnetic wave absorption performance.

Inventive product I: Specific gravity 0.015, average particle size 2.5 mm polystyrene pre-expanded particles 40 wt% and DBP oil absorption 178 cc / 100 g CB (Cabot XC-72R) 2.3 wt%, sealed state In the solid state, CB was uniformly adsorbed around the pre-expanded particles. Next, 57.7% by weight of styrene / phthaldiene copolymer rubber latex (# 4850A manufactured by Nippon Zeon Co., Ltd., solid content 70% by weight) was added to fix the CB to the surface of the pre-expanded particles. Next, this mixture was poured into a mold, dried and dehydrated at room temperature, and molded. The CB concentration of the obtained electromagnetic wave absorber I was 1 g / l.

Inventive product II: 44.8% by weight of the same polystyrene pre-expanded particles as in the inventive product I, and an earth-like graphite S-having an average particle diameter of 3.5 microns
3 (manufactured by Chuetsu Graphite Co., Ltd.) and styrene / butadiene copolymer rubber latex (LX-20 manufactured by Nippon Zeon Co., Ltd.)
(9, solid content 45% by weight) 55.2% by weight of a mixed solution consisting of 100 parts by weight was gradually added and uniformly adhered to the surface of the pre-expanded particles. Next, this mixture was poured into a mold, dried and dehydrated at room temperature, and molded. The radio wave absorber II thus obtained had a graphite concentration of 5 g / l.

Conventional product I: The same polystyrene pre-expanded particles as the product I of the present invention and CB were mixed in the same manner, and then the mixture was put into a mold and heated to secondary-expand the pre-expanded particles,
It is a commercially available electromagnetic wave absorber having the same shape as the created product I of the present invention.

Conventional product II: A commercially available electromagnetic wave absorber made by impregnating polyurethane foam with CB-containing rubber latex.

The impact resistance of each of the present invention products I and II and the conventional product I was measured by the following flexural fracture resistance.

As a result, the conventional product I was cut when the tip was bent by 90 °, whereas the products I and II of the present invention did not cut even when bent by 140 °, and the cutting progressed little by little at 160 °. That is, it is shown that the products I and II of the present invention are significantly more excellent in impact resistance than the conventional product I.

[Measurement method for flexural fracture resistance]

The present invention products I and II and the conventional product I are cut into a shape having a length of 23 cm, a thickness of 3 cm, and a width of 5 cm, and the long side (length direction) end portion of 3 cm is fixed,
The other end was bent slowly and the extent to which the tip bends was measured.

Next, regarding the present invention product I and the conventional product II, 30 MHz and 100
Measuring the complex permittivity (E ^* = E′−jE ″) at MHz,
The results are shown in FIG.

From FIG. 1, the product I of the present invention has a dielectric loss E ″ at any frequency when the dielectric constant E ′ is constant as compared with the product II of the related art.
It is understood that has a large and excellent electromagnetic wave absorption performance.

Further, with respect to the above-mentioned products I and II of the present invention and the conventional product II, the electromagnetic wave absorbers in which ferrite tiles are combined,
Reflection loss of radio waves when the frequency (GHz) is changed (Refle
ction loss) (dB) was measured. The results are shown in Fig. 2.

From the figure, it can be seen that the products I and II of the present invention have equal or greater reflection loss in a wider frequency range than the conventional product II,
It can be seen that the electromagnetic wave absorption performance is excellent.

〔The invention's effect〕

According to the present invention, the radio wave absorber is composed of pre-expanded particles made of a polystyrene polymer having a specific bulk specific gravity and an average particle diameter, and at least one of CB or graphite having a high oil absorption amount is provided on the surface thereof. Since the pre-expanded particles are bonded and integrated with each other by the rubber latex without secondary foaming in the adsorbed state, a radio wave absorber excellent in impact resistance and radio wave absorption performance can be obtained.

Therefore, the electromagnetic wave absorber of the present invention can be assembled not only in a anechoic chamber having a complicated shape such as a pyramid type but also in a box type, a cubic type, a rectangular parallelepiped type, etc., and the anechoic chamber is manufactured by forming the front surface on a flat surface. In this case, the electromagnetic wave absorber is easy to assemble, has excellent stability after construction, and the thickness of the electromagnetic wave absorber can be made thinner than that of the pyramid type, so the anechoic chamber itself can be downsized and the object to be measured comes into contact. However, there is an effect that the electromagnetic wave absorber is not damaged and there is no feeling of pressure in the room like the pyramid type.

[Brief description of drawings]

Fig. 1 shows the relationship between the dielectric constant E'and the dielectric loss E "of the electromagnetic wave absorber at 30MHz and 100MHz, and Fig. 2 shows the relationship between the frequency (GHz) of the electromagnetic wave absorber and the electromagnetic wave reflection loss (dB). FIG.

Claims (3)

[Claims] 1. A bulk specific gravity of 0.010 to 0.030 and an average particle size of 2.0.
In the state where the pre-expanded particles are not secondary-expanded between a large number of pre-expanded particles composed of a polystyrene polymer of about 20 mm,
Dibutyl phthalate An electromagnetic wave absorber integrally bonded by a rubber latex containing at least one of carbon black having an oil absorption of 100 to 400 cc / 100 g and graphite having an average particle size of 0.5 to 20 microns. 2. A bulk specific gravity of 0.010 to 0.030 and an average particle size of 2.0.
~ 20 mm pre-expanded polystyrene polymer particles and at least one of carbon black with dibutyl phthalate oil absorption of 100-400 cc / 100 g or graphite with an average particle size of 0.5-20 microns in advance in a solid state By at least one of the carbon black and graphite is uniformly adsorbed on the surface of the pre-expanded particles, and then rubber latex is added to the surface of the pre-expanded particles to add at least one of carbon black and graphite. A method of manufacturing a radio wave absorber in which the mixture is directly fixed, the mixture is poured into a mold, and the mixture is dried and dehydrated. 3. A bulk specific gravity of 0.010 to 0.030 and an average particle size of 2.0.
On the surface of pre-expanded particles of polystyrene polymer of ~ 20mm, at least one of carbon black with dibutyl phthalate oil absorption of 100-400cc / 100g or graphite of average particle size 0.5-20 microns and rubber latex A method of manufacturing a radio wave absorber, which comprises fixing a mixed solution, injecting it into a mold, and drying and dehydrating.