JP2012084664A - Production method of radio wave absorber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To minimize exhibition of anisotropy in the radio wave absorption performance of a radio wave absorber produced by press molding.SOLUTION: In the production method of a radio wave absorber, a molding material M containing thermoplastic resin beads with a radio wave absorbing material adhering to the surface thereof is set in a molding die 10 and press molded into a predetermined shape. When the press conditions are set in the operation, radio wave absorption characteristics of the radio wave absorber can be controlled. Since the thermoplastic resin beads with a radio wave absorbing material adhering to the surface thereof is used as the molding material, orientation in one direction of the radio wave absorbing material is regulated, and anisotropy can be minimized.

Description

  The present invention relates to a method for manufacturing a radio wave absorber.

  Radio wave absorbers have been put into practical use for solving problems such as communication failures due to radio wave interference between electronic devices and malfunctions of electronic devices.

  For example, Patent Document 1 discloses that a radio wave absorber is manufactured by blending a resin emulsion and a dehydration gelation accelerator with a powdered polymer and a radio wave absorbing material, kneading them and then heating and foaming. Yes.

JP-A-11-106538

  By the way, it is very difficult to control the radio wave absorption characteristics of the radio wave absorber because various factors influence each other.

  The subject of this invention is providing the manufacturing method of the electromagnetic wave absorber which can control the electromagnetic wave absorption characteristic of an electromagnetic wave absorber.

  The method for manufacturing a radio wave absorber of the present invention is a method in which a molding material composed of thermoplastic resin beads having a radio wave absorber attached to the surface is set in a mold and press-molded into a predetermined shape.

  According to the present invention, a thermoplastic resin bead having a radio wave absorber attached on its surface is used as a molding material, and the radio wave absorber is manufactured by press molding it. However, it is easily manufactured by setting the press conditions. The radio wave absorption characteristics of the radio wave absorber can be controlled.

It is explanatory drawing which shows the manufacturing method of the electromagnetic wave absorber which concerns on embodiment. It is a graph which shows the relationship between press molding pressure and a complex dielectric constant. It is a graph which shows the relationship between a frequency and a return loss. It is a front view which shows the structure of a radio wave absorption characteristic measuring apparatus. It is a top view which shows the set state of a radio wave absorber sample. It is a top view which shows the set state which the radio wave absorber sample changed. It is a graph which shows the relationship between the frequency of an Example, and a return loss amount. It is a graph which shows the relationship between the frequency and return loss of a comparative example.

  Hereinafter, embodiments will be described in detail.

  The method for manufacturing a radio wave absorber according to the present embodiment is a method in which a molding material composed of thermoplastic resin beads having a radio wave absorber attached to the surface is set in a mold and press-molded into a predetermined shape.

  It is very difficult to control the radio wave absorption characteristics of the production of the radio wave absorber because various factors influence each other. However, according to the manufacturing method of the radio wave absorber according to the present embodiment, the thermoplastic resin beads having the radio wave absorber attached to the surface are used as the molding material, and it is press-molded. It is possible to control the radio wave absorption characteristics of a radio wave absorber that is easily manufactured.

  In addition, the radio wave absorber installed in a state where it is stuck downward on the top surface or hung from above is thin, in order to prevent falling and to reduce secondary damage when dropped. It is required to be light. On the other hand, according to the method for manufacturing a radio wave absorber according to the present embodiment, the radio wave absorber is manufactured by press molding, so that the radio wave absorber can be reduced in thickness and weight.

  Furthermore, in a radio wave absorber manufactured by press molding, it is considered that anisotropy appears in the radio wave absorption performance due to the orientation of the radio wave absorber. However, according to the method of manufacturing a radio wave absorber according to the present embodiment, as a molding material for manufacturing the radio wave absorber by press molding, because the thermoplastic resin beads having a radio wave absorber attached to the surface are used, The orientation of the radio wave absorber attached to the thermoplastic resin beads in one direction is restricted, and therefore, the anisotropy of the radio wave absorption performance can be suppressed.

  The molding material used for manufacturing the radio wave absorber according to this embodiment includes thermoplastic resin beads having a radio wave absorber attached to the surface.

  The thermoplastic resin beads are composed of a resin composition in which a compounding agent is blended using a resin base material as a matrix. The particle diameter of the thermoplastic resin beads is, for example, 0.3 to 5.0 mm.

  Examples of the resin base material for thermoplastic resin beads include flame retardant containing halogen such as polyvinyl chloride, vinylidene chloride, tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer, tetrafluoroethylene / ethylene copolymer, etc. Resins; Polyolefins such as polyethylene, polypropylene, and poly-4-methylpentene-1; Non-halogen-containing resins such as polystyrene, styrene / acrylonitrile copolymer, and polyurethane. Of these, polystyrene and vinylidene chloride are preferred. In particular, vinylidene chlorides are excellent in flame retardancy, weather resistance, and moldability. For example, vinylidene chloride homopolymers, such as vinylidene chloride and vinyl chloride, various acrylic acid esters, and acrylonitrile. A polymer etc. are mentioned. The resin base material may be composed of a single species or a plurality of species.

  The thermoplastic resin beads are general-purpose and easily available, and from the viewpoint of press moldability described later, the foamed beads are preferably foamed with a foaming agent. Examples of the foaming agent contained in the thermoplastic resin beads include inorganic foaming agents such as sodium bicarbonate and ammonium carbonate; and organic foaming agents such as azodicarbonamide and benzenesulfonyl hydrazide. The foaming agent may be composed of a single species or a plurality of species. Content of a foaming agent is 0.5-5.0 mass parts with respect to 100 mass parts of resin base materials, for example, Preferably it is 1.0-3.0 mass parts.

  When the thermoplastic resin beads are foam beads, the thermoplastic resin beads are preferably foamed and expanded 10 to 70 times from the viewpoint of press moldability described later, and foamed 20 to 50 times. More preferably, it is expanded. The thermoplastic resin beads may be primary expanded beads that can be further expanded by secondary expansion, or may be expanded beads that do not have additional expanded expansion performance. In the production of the radio wave absorber according to this embodiment, further foam expansion of the foam beads is unnecessary because press molding is performed, and from the viewpoint of preventing the radio wave absorption performance from being affected by secondary foam, It is preferable that the thermoplastic resin beads are foamed beads. The particle diameter of the expanded beads is, for example, 0.5 to 5.0 mm.

  Examples of the radio wave absorber include conductive carbon black, conductive graphite, and metal powder. Anisotropy of radio wave absorbers manufactured by press molding is often observed when a radio wave absorber having an orientation is used. From this point of view, anisotropy is suppressed by the radio wave absorber manufacturing method according to this embodiment. The effect is particularly remarkable when the electromagnetic wave absorbing material is an acicular electromagnetic wave absorbing material such as acicular magnetic powder. The radio wave absorber may be composed of a single species or a plurality of species. The amount of adhesion of the radio wave absorber to the thermoplastic resin beads is preferably such that the layer average thickness of the radio wave absorber is 0.5 to 10 μm, and more preferably 1 to 5 μm.

  As a method of attaching the radio wave absorber to the thermoplastic resin beads, for example, a conductive liquid in which the radio wave absorber is dispersed in a resin binder paint is prepared, and the conductive liquid is used as a thermoplastic resin bead (in the case of foam beads, unfoamed). Bead or primary foamed beads) in a mixer and stir to attach a radio wave absorber to the surface of the thermoplastic resin beads. Adhesion by applying a very small amount of oil or adhesive on the surface of the thermoplastic resin beads A method of attaching the radio wave absorber to the surface of the thermoplastic resin beads by adding the thermoplastic resin beads and the radio wave absorber to a mixer and stirring the surface treatment after imparting surface properties, oil or adhesive The electromagnetic wave absorbing material, which has been made sticky by adding a very small amount of water, etc., is charged into the thermoplastic resin beads and the mixer, and stirred to add electricity to the surface of the thermoplastic resin beads. Absorber each other and a method of forming a layer that had adhered. In addition, after making a radio wave absorber adhere to the surface of a thermoplastic resin bead, in order to prevent a fall of a radio wave absorber, you may provide an overcoat layer on it.

  In the case of a method using a conductive liquid in which a radio wave absorber is dispersed in a resin binder paint, the resin binder paint may be, for example, a non-crosslinking curable low-viscosity liquid such as a latex of a thermoplastic organic polymer, an ultraviolet curing Cross-linking curable low-viscosity liquids such as curable resin paints and thermosetting enamel varnishes. When the resin binder coating is a non-crosslinking curable low-viscosity liquid, the conductive liquid need only be dried after being applied to the thermoplastic resin beads. When the resin binder paint is a low-viscosity liquid having a crosslinking curable property, if it contains a solvent, the conductive liquid may be applied to the thermoplastic resin beads and dried, followed by a crosslinking treatment. If it is a thing, after apply | coating a electrically conductive liquid to a thermoplastic resin bead, you may perform a crosslinking process immediately, and you may make application | coating liquid and a crosslinking process parallel.

  A thermoplastic adhesive may be further attached to the surface of the thermoplastic resin bead having the radio wave absorber attached to the surface for preventing the radio wave absorber from falling off and bonding the beads. Examples of such thermoplastic adhesives include those whose main component is an ethylene / vinyl acetate copolymer and those whose main component is a modified styrene / butadiene copolymer. The adhesion amount of the thermoplastic adhesive is 5 to 30 parts by mass with respect to 100 parts by mass of the thermoplastic resin beads having the radio wave absorber adhered to the surface. In order to attach the thermoplastic adhesive, the thermoplastic resin beads and the thermoplastic adhesive with the radio wave absorber attached to the surface may be put into a mixer and stirred.

  The molding material may further include adhesive thermoplastic resin beads, coloring beads and the like that do not have a radio wave absorber attached to the surface.

  The molding material can be adjusted by introducing the thermoplastic resin beads having the radio wave absorber adhering to the surface and other components into a mixer and stirring them.

  In the method for manufacturing a radio wave absorber according to the present embodiment, as shown in FIG. 1, a mold 10 composed of an upper mold 11 and a lower mold 12 is heated to a predetermined molding temperature, and the lower mold 12 of the mold 10 is formed. After the mold release agent is applied to the surface of the formed recess 12a, the molding material M is put over the recess 12a, and the upper mold 11 with the mold release agent applied to the molding surface is used as the lower mold 12. Press molding is performed by fitting into the recess 12a and applying a predetermined press molding pressure to the molding material M with the upper mold 11 and holding the state for a predetermined molding time. At this time, the thermoplastic resin beads having the radio wave absorber attached to the surface included in the molding material M are fused to each other or bonded via a thermoplastic adhesive to form a radio wave absorber having a predetermined shape. Further, when the thermoplastic resin beads are expanded beads or primary expanded beads obtained by pre-expanding the beads, elasticity is imparted to the thermoplastic resin beads by the bubbles, the bead filling property is increased, and the moldability is excellent. The gas released from the foam beads is discharged from the mold matching surfaces of the upper mold 11 and the lower mold 12. Here, the molding conditions are, for example, a molding temperature of 100 to 200 ° C., a press molding pressure of 20 to 60 MPa, and a molding time of 5 to 20 minutes.

According to the method for manufacturing a radio wave absorber according to the present embodiment, since it is manufactured by press molding, if it is plate-shaped, the thickness is 3 to 10 mm and the density is 0.2 to 1.0 g / cm. ³ can be produced, and the anisotropy can be suppressed as described above.

  The radio wave absorber manufactured in the present embodiment can be used, for example, in an anechoic chamber, a wireless LAN, and RF / ID.

[Test Evaluation 1]
(Radio wave absorber)
20 times expanded white beads (polystyrene resin) and radio wave absorber were added to the mixer at a ratio of 100 g of the latter to 100 g of the former, and these were stirred, mixed, and dried, so that the radio wave absorber adhered to the surface. Black beads, which are thermoplastic resin beads, were obtained. A conductive material (conductive carbon black mixed solution) was used as the radio wave absorber.

  A thermoplastic adhesive (EVA-based emulsion-based adhesive) was added to the mixer at a ratio of 10 g with respect to 100 g of the black beads obtained above, and they were mixed with stirring and dried.

  By putting 100 g of the black beads with adhesive obtained above into a mold and pressing with a press machine at a molding temperature of 120 ° C., a press molding pressure of 9.81 MPa, and a molding time of 10 minutes, a length of 150 mm A radio wave absorber 1 having a width of 150 mm and a thickness of 6.3 mm was produced.

  Similarly, the radio wave absorber 2 having a length of 150 mm, a width of 150 mm, and a thickness of 5.3 mm was manufactured by changing only the press molding pressure to 29.4 MPa. Moreover, only the press molding pressure was changed to 39.2 MPa, and the radio wave absorber 3 having a length of 150 mm, a width of 150 mm, and a thickness of 5.1 mm was produced.

(Measurement of complex permittivity)
For each of the wave absorbers 1 to 3, a donut-shaped test piece having an outer diameter of 38.4 mm and an inner diameter of 16.5 mm was punched out, and the frequency was measured by the S-parameter method using a network analyzer (model number: HP8722ET manufactured by Hured Packard). And the complex dielectric constant (real part and imaginary part). The S parameter method is a method of measuring the S parameter of the test piece and calculating the magnetic permeability and the dielectric constant from the measured S parameter.

  As a result, when the absorption frequency is 950 MHz (UHF band), the real part of the complex dielectric constant is 13.78 and the imaginary part is 4.25 in the radio wave absorber 1, and the real part of the complex dielectric constant is in the radio wave absorber 2. 14.31 and the imaginary part were 4.40, and in the radio wave absorber 3, the real part of the complex dielectric constant was 15.59 and the imaginary part was 5.74.

From this, the relationship between the press molding pressure and the complex dielectric constant as shown in FIG. 2 was obtained. As can be seen from FIG. 2, either the real part or the imaginary part of the complex dielectric constant is proportional to the press molding pressure. When each approximate expression is found,
Real part of complex permittivity = (5600 x press molding pressure (MPa) / 98067) + 13.08
Imaginary part of complex dielectric constant = (4400 x press molding pressure (MPa) / 98067) + 3.63
It became. This relationship changes depending on the amount of the radio wave absorber attached to the bead surface, the bead diameter, and the like. Such a relationship can be similarly obtained for other absorption frequencies.

(simulation)
As a result of the simulation based on the transmission line theory described in “Development of sheet type electromagnetic wave absorber for ETC / DSRC, Mitsubishi Electric Industrial Co., Ltd. (99), 2002, p69-77” When the pressure is 39.2 MPa, in order to obtain a radio wave absorber having radio wave absorption characteristics at a frequency of 950 MHz from the values of the real part and the imaginary part of the complex dielectric constant at that time, if the thickness is 20 mm I found it good. Note that the real part 15.32 and the imaginary part 5.39 of the complex dielectric constant were calculated by substituting the press forming pressure 39.2 MPa into the above approximate expression as the values of the real part and the imaginary part of the complex dielectric constant.

  FIG. 3 shows the relationship between the frequency of the radio wave absorber and the return loss based on the simulation results. According to this, it can be confirmed that there is a large absorption at a frequency of 950 MHz.

  From the above, when a radio wave absorber is manufactured by using a thermoplastic resin bead with a radio wave absorber attached to the surface as a molding material, and pressing it to produce a radio wave absorber, the radio wave of the radio wave absorber manufactured using the press molding pressure as an index It can be seen that the absorption characteristics can be controlled.

[Test evaluation 2]
(Radio wave absorber)
<Example>
20 times expanded white beads (polystyrene resin) and radio wave absorber were added to the mixer at a ratio of 200 g of the latter to 100 g of the former, and these were stirred, mixed and dried, so that the radio wave absorber adhered to the surface. Black beads, which are thermoplastic resin beads, were obtained. As the radio wave absorber, a liquid in which a conductive material (conductive carbon black mixed liquid) and ferrite magnetic powder were mixed at a mass ratio of 1: 1 was used.

  A thermoplastic adhesive (EVA-based emulsion-based adhesive) was added to the mixer at a ratio of 10 g with respect to 100 g of the black beads obtained above, and they were mixed with stirring and dried.

  100 g of the black beads with adhesive obtained above are put into a mold and press molded under the conditions of a molding temperature of 120 ° C., a press molding pressure of 30 MPa, and a molding time of 10 minutes by a press machine. An electromagnetic wave absorber of an example having a thickness of 300 mm and a thickness of 2 mm was produced. In addition, the radio wave absorber of this embodiment is manufactured with a target of a frequency of 2.45 GHz for wireless LAN.

<Comparative example>
The same radio wave absorber as that used in the examples was added to the chlorinated polyethylene resin powder at a ratio of 200 g of the latter to 100 g of the former, and kneaded and dried to obtain a molding material.

  The molding material obtained above was molded into a sheet shape by a press machine to produce a sheet-like comparative wave absorber having a length and width of 300 mm each. In addition, the radio wave absorber of this comparative example is also produced with the target of a frequency of 2.45 GHz for wireless LAN.

(Test evaluation method)
About each of an Example and a comparative example, the presence or absence of the anisotropy of a radio wave absorption characteristic was test-evaluated by the arch method (reflected power method).

  FIG. 4 shows a radio wave absorption characteristic measuring apparatus (network analyzer, model number: HP8722ET, manufactured by Hured Packard) 20.

  This radio wave absorption characteristic measuring apparatus 20 has a measuring table 21 having a horizontal sample mounting surface, and is arranged on a transmitting antenna 22 in a direction inclined by 10 ° on one side with respect to the normal direction of the center and on the other side. Receiving antennas 23 are provided in directions inclined by 10 °. The transmitting antenna 22 and the receiving antenna 23 are provided such that their rear ends are positioned on a radius R = 2000 mm from the center of the measurement table. The transmitting antenna 22 is connected to the oscillator 24 and the receiving antenna 23 is connected to the receiver 25.

  For each of the example and the comparative example, a mark m is attached to one of the four corners of the radio wave absorber sample S having a square shape in plan view, and the mark m is placed on the measurement table 21. At this time, as shown in FIG. 5, the radio wave absorber sample S was arranged so that the mark m was located on the upper right. Then, a TE wave was emitted from the transmitting antenna 22 to the radio wave absorber sample S, the reflected wave was received by the receiving antenna 23, and the frequency dependence of the return loss was measured. At this time, the measurement frequency range was 1 to 8 GHz.

  Next, as shown in FIG. 6, the arrangement of the radio wave absorber was changed so that the mark was located on the lower right. That is, the radio wave absorber was rotated 90 ° clockwise in plan view. Similarly, the frequency dependence of the return loss was measured.

(Test evaluation results)
FIG. 7 shows the relationship between the frequency of the embodiment and FIG.

  According to FIG. 7, it can be seen that in the example, there is no change even if the extreme value of the return loss is rotated by 90 °, and therefore the radio wave absorption characteristic is isotropic. On the other hand, according to FIG. 8, it can be seen that in the comparative example, when the extreme value of the return loss is rotated by 90 °, both the frequency and return loss change, and thus the radio wave absorption characteristics are anisotropic.

  The present invention is useful for a method of manufacturing a radio wave absorber.

m Marking M Molding material S Radio wave absorber sample 10 Mold 11 Upper mold 12 Lower mold 12a Recess 20 Radio wave absorption characteristic measuring device 21 Measuring base 22 Transmitting antenna 23 Receiving antenna 24 Oscillator 25 Receiver

Claims (3)

  A method for manufacturing a radio wave absorber, comprising setting a molding material including a thermoplastic resin bead having a radio wave absorber attached on a surface thereof to a mold and press-molding the molding material into a predetermined shape. In the manufacturing method of the electromagnetic wave absorber described in claim 1,
A method for producing a radio wave absorber, wherein the thermoplastic resin beads are foam beads containing a foaming agent. In the manufacturing method of the electromagnetic wave absorber described in claim 2,
A method for producing a radio wave absorber, wherein the foam beads are primary foam beads.

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