JP2005150461A - Wave absorber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wave absorber exhibiting excellent wave absorption characteristics even when the amount of fine carbon fibers added thereto is extremely small. <P>SOLUTION: The wave absorber is a molding of a resin composite comprising a resin layer and a metal layer, wherein the resin layer contains a resin component (A), and fine carbon fibers (B) having an average fiber diameter of ≤50 nm, with an aspect ratio (fiber length/fiber diameter) not larger than 10. The proportion of the fine carbon fibers (B) to the total of the resin component (A) and the fine carbon fibers (B) falls in the range of 0.01 to 7 wt.%, and the volume resistivity (ρ<SB>VC</SB>) inside the molding falls in the range of 8×10<SP>0</SP>to 1×10<SP>5</SP>Ω/cm. The wave absorber is efficiently manufactured by extrusion or injection, and may be formed into any shape as desired, with a high degree of freedom in the designing process. The wave absorber may be easily controlled for absorbing desired wavelengths, and fabricated to be thinner. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a radio wave absorber useful as a casing material or the like that absorbs radio waves in personal computers, mobile phones, road traffic information systems, car navigation systems, other information communication devices, medical electronic devices, and the like.

  In recent years, the use of radio waves has been increasing rapidly, mainly in the field of mobile communications such as mobile phones. If PHS (Personal Handyphone System) and wireless LAN, which have already been put into practical use, become even more widespread, this trend is expected to accelerate further.

  On the other hand, consumer computers equipped with personal computers and microcomputers are already widespread, and electromagnetic waves generated by these devices are added to electromagnetic waves generated by wireless devices, making the radio wave environment more diversified and complicated, and even high-frequency bands. Accordingly, there is a concern that so-called electromagnetic interference such as malfunction of electronic devices, leakage of information, noise of televisions and radios due to the above devices will increase further in the future.

  Conventionally, electromagnetic wave shielding materials and radio wave absorbers are used for such countermeasures against radio wave interference. The electromagnetic wave shielding material prevents intrusion of electromagnetic waves into the inside and radiation to the outside by reflection, and generally a metal coating material or conductive coating is used. However, in the electromagnetic shielding material, the confined electromagnetic waves are likely to cause interference inside the device, and there is also a problem of leakage from a slight gap such as a joint or a joint, resulting in a deterioration of the electromagnetic shielding effect. ing.

  For this reason, a radio wave absorber that has the effect of reducing reflected waves and transmitted waves with absorption has attracted attention. The radio wave absorber converts incident radio waves into thermal energy and reduces reflected waves and transmitted waves. Conventionally, as a radio wave absorber, a molded resin composition to which a magnetic component (ferrite or the like), a dielectric component or a conductive component (carbon black, carbon fiber or the like) is added has been used. For example, JP-A-10-27986 and JP-A-2001-223494 propose a radio wave absorber made of a molded product of a resin composition in which carbon black and carbon fiber are blended.

  However, the additive components used in the prior art need to be added in a large amount in order to obtain radio wave absorption characteristics. As a result, there are problems such as an increase in the weight of the radio wave absorber obtained and a decrease in strength. Occurs. Moreover, since the resin composition which mix | blended these additional components in large quantities is inferior to a moldability, there also exists a problem that there are few freedom degrees of a shape.

  By the way, the radio wave absorber is generally a composite structure in which a conductive layer made of a metal foil or a metal plate and a dielectric layer made of a radio wave absorber are laminated, and the non-reflection conditional expression at that time is expressed as follows. The

  For a certain wavelength λ, the real part ε ′ and the imaginary part ε ″ (dielectric loss) of the complex permittivity satisfying the above-described non-reflection conditional expression with d / λ as a parameter are plotted on the complex plane as “none”. This is called a “reflection curve”, for example, as shown by a broken line in FIG.

  Therefore, in order to obtain good radio wave absorption performance, the real part ε ′ and the imaginary part ε ″ of the complex dielectric constant of the material constituting the radio wave absorber are on this antireflection curve or It is necessary to be close.

  Conventionally, the balance between the real part ε ′ and the imaginary part ε ″ of the complex dielectric constant of a radio wave absorber has been determined by the type and amount of additive components. Therefore, the balance of ε ′ and ε ″ Since the thickness of the absorber and the peak of the absorption wavelength are determined, the degree of freedom in designing the thickness is small. For example, in carbon black, as the amount of addition increases, ε ″ increases remarkably. As a result, there are significant restrictions on the design of the complex dielectric constant and the thickness of the material.

On the other hand, in JP-A-10-27986 and JP-A-2001-223494, different components of carbon black and carbon fiber are used in combination as an additive component to adjust the balance between ε ′ and ε ″. However, in these technologies, as described above, it is necessary to add a large amount of carbon black or carbon fiber. As a result, the toughness of the molded body is reduced or the flexibility is lowered. Problems arise.
JP-A-10-27986 JP 2001-223494 A

Accordingly, an object of the present invention is to provide a radio wave absorber that exhibits excellent radio wave absorption characteristics with an extremely small addition amount of fine carbon fibers as an additive component.
The present invention also makes it possible to adjust the real part ε ′ and the imaginary part ε ″ of the complex permittivity by optimizing the manufacturing conditions, and to increase the degree of freedom in designing the radio wave absorber to approximate the non-reflection curve. The purpose is to spread.

The radio wave absorber of the present invention comprises a composite of a resin layer and a metal layer. The resin layer has (A) a resin component, (B) an average fiber diameter of 50 nm or less, and an aspect ratio (fiber length / A resin composition in which the ratio of (B) fine carbon fibers is 0.01 to 7% by weight with respect to the total of (A) resin component and (B) fine carbon fibers. A radio wave absorber made of a molded body made of a product, wherein the volume resistivity (ρ _VC ) inside the molded body is 8 × 10 ⁰ to 1 × 10 ⁵ Ω · cm.

The fine carbon fiber having the above-mentioned specific average fiber diameter and aspect ratio used in the present invention has a structure much finer than that of conventionally used carbon black and carbon fiber, so that the total amount with the resin component can be reduced. Excellent radio wave absorptivity is manifested with a small addition of 0.01 to 7% by weight. Thus, if it is an electromagnetic wave absorber with a volume resistivity (ρ _VC ) controlled to 8 × 10 ⁰ to 1 × 10 ⁵ Ω · cm with a small amount of fine carbon fiber blended, excellent radio wave absorption characteristics and high toughness And as a result, the radio wave absorber can be made thinner and lighter.

  Since the resin composition containing a small amount of fine carbon fibers according to the present invention is excellent in moldability, it can be continuously molded by extrusion molding or injection molding of the thermoplastic resin composition.

In particular, in the present invention, as will be described later, the volume resistivity (ρ _VC ) inside the molded body is 8 × 10 ⁰ to 1 × 10 ⁵ Ω · cm, preferably 8 by controlling the dispersion state of the fine carbon fibers. Excellent radio wave absorption characteristics can be obtained by adjusting to x10 ^{0 to} 5 × 10 ⁴ Ω · cm, more preferably 1 × 10 ¹ to 1 × 10 ⁴ Ω · cm.

If this volume resistivity (ρ _VC ) is larger than the above range (low conductivity), the value of the imaginary part ε ″ of the complex dielectric constant becomes too small, and conversely smaller than the above range (high conductivity). And the imaginary part ε ″ of the complex dielectric constant becomes too large. In either case, the radio wave absorption characteristics are impaired because the conditions greatly deviate from the non-reflection condition.

By the way, the volume resistivity is generally measured by a resistance value between the front surface and the back surface through the surface of the molded body. However, especially when the radio wave absorber of the present invention is manufactured by extrusion molding or injection molding, the volume resistivity by such a measuring method cannot reflect radio wave absorption characteristics. Therefore, in the present invention, the volume resistivity (ρ _VC ) inside the molded body is used as an index of the radio wave absorption characteristics, and the volume resistivity (ρ _VC ) is controlled in the range of 8 × 10 ⁰ to 1 × 10 ⁵ Ω · cm. To do.

Moreover, the radio wave absorber of the present invention, particularly when molded by extrusion molding or injection molding, has a volume resistivity (ρ _VS ) measured through the surface of the molded body and a volume resistivity (ρ _VC ) inside the molded body. )) (Ρ _VS ) / (ρ _VC ) is preferably 100 or more, more preferably 1000 or more, the conductivity of the surface is lower than that of the inside of the molded body, and the difference is large. This is desirable not only because the dielectric constant balance is good, but also because the surface appearance is good. Further, it is desirable in that the absorption characteristics can be expected to be improved by reducing the reflection of radio waves on the surface by increasing the resistance value of the surface.

In the present invention, the volume resistivity (ρ _VC ) inside the molded body is a volume resistivity measured by providing an electrode inside the molded body without passing through the surface of the molded body. That is, for example, a fracture sample (width W, length L, thickness t) 20 as shown in FIG. 2 is cut out from the flat portion of the molded body, and the fracture surface of the fracture sample 20 is cut as shown in FIG. Electrodes 21A and 21B are formed on 20A and 20B, and the resistance value between the electrodes 21A and 21B is measured. As will be described later, since the surface layer of the fractured sample 20 is a high resistance layer having a resistance value R _S due to the orientation of fine carbon fibers, and the inside is a low resistance layer having a resistance value R _C , the surface resistance value R _S Is sufficiently larger than the internal resistance value _RC , the resistance value R _VC measured in the parallel equivalent circuit shown in FIG. 3B is substantially equal to the resistance value _RC of the internal low resistance layer. Actually, it is considered that the resistance value is gradually inclined from the surface of the molded body to the inside, but in any case, the resistance value R _VC is dominated by the internal low resistance layer.

Further, as shown in FIG. 4A, when the electrodes 22A and 22B are formed on both surfaces of the fracture sample 20 and the resistance value between the electrodes 22A and 22B is measured, the resistance value R measured through the surface is measured. _As shown in FIG. 4 (b), _VS is the resistance value _RS of the high resistance layer on one surface, the resistance value _RC of the internal low resistance layer, and the resistance value _RS of the high resistance layer on the other surface. It is the resistance value of the serial equivalent circuit that is configured, that is, it corresponds to the sum of these resistance values. Accordingly, the resistance value R _VS is substantially equal to the high resistance value R _S near the surface.

From the measured resistance value, the volume resistivity is
Since volume resistivity = (measured value of resistance value) × electrode area / distance between electrodes, the volume resistivity (ρ _VC ) inside the molded body and the volume resistivity (ρ measured through the surface of the molded body) _VS ) can be obtained as follows.
Volume resistivity inside molded body (ρ _VC ) = (W × t) / L × (R _VC )
Volume resistivity (ρ _VS ) measured via the surface of the molded body = A / t × (R _VS )
(However, A is the area of electrodes 22A and 22B)

The ratio (ρ _VS ) / (ρ _VC ) of the volume resistivity (ρ _VS ) measured through the surface of the molded body to the volume resistivity (ρ _VC ) inside the molded body is 100 or more. The resistance value R _S of the surface layer is sufficiently larger than the resistance value R _{C of the} inner layer, that is, the conductivity is high because there are many contact points between the fine carbon fibers described later in the molded body. The fine carbon fibers are oriented on the surface of the body, and the conductivity is low because there are few contact points between the fine carbon fibers. However, the orientation of such fine carbon fibers has good fluidity and excellent surface smoothness of the resin composition. Represents that.

  As described above, the second object of the present invention is to optimize the manufacturing conditions by using the resin composition of the present invention, thereby realizing the real part (real part) ε ′ and the imaginary part (imaginary part) of the complex dielectric constant. This is to enable the balance adjustment of ε ″ and to increase the degree of freedom in designing the radio wave absorber.

  That is, the present inventor has found that the resin composition within the scope of the present invention greatly changes the balance of ε ′ and ε ″ depending not only on the amount of fine carbon fibers added but also on the dispersion state of the fine carbon fibers in the molded body. Furthermore, by optimizing the viscosity and molding conditions of the resin composition, the dispersion structure of the fine carbon fibers can be controlled, the absorption wavelength can be controlled, and the wave absorber can be made thinner. I found it possible.

  Hereinafter, the balance control mechanism of the real part ε ′ and the imaginary part ε ″ of the complex dielectric constant by optimizing the manufacturing conditions in the present invention will be described.

  When a resin composition in which fine carbon fibers are dispersed is molded by a molding method involving flow such as extrusion molding or injection molding, the fine carbon fibers are oriented along the flow direction. As shown in FIG. 1, the orientation of the fine carbon fibers is large near the surface of the molded body 10, and the orientation on the inner side is small. And, by this orientation, the contact between the fine carbon fibers 2 and 2 is reduced, the conductivity is impaired, the surface layer portion of the molded body 10 becomes a high resistance layer 10A having a relatively high resistance value, and the inside is a fine carbon fiber. The low resistance layer 10B that retains conductivity by contact between two and two is obtained. However, the oriented fine carbon fibers 2 agglomerate again by attractive force between fibers or entanglement between fibers until the resin is cooled and solidified, that is, while the resin has a low melt viscosity. As a result, contact between the fine carbon fibers occurs again, and the conductivity is improved. Therefore, the dispersion state (contact state) of the fine carbon fibers in the finally obtained molded body is determined by the initial flow rate, the resin It changes according to manufacturing conditions, such as temperature, a viscosity, and a cooling rate.

  The conductivity due to the contact state between the fine carbon fibers greatly affects the dielectric constant, but particularly the dielectric loss ε ″. Therefore, the contact condition between the fine carbon fibers is adjusted by adjusting the manufacturing conditions of the molded body. It is possible to change the balance between ε ′ and ε ″ by controlling. As a result, even if the blending amount of the same fine carbon fiber is used, it becomes possible to easily approximate the non-reflection curve by controlling the dispersion state of the fine carbon fiber. That is, for example, by increasing ε ′ and decreasing ε ″, it becomes possible to approach the non-reflection curve in a region where d / λ is small, the thickness of the wave absorber is reduced, and the absorption wavelength characteristic is reduced to a low frequency. It is also possible to shift to the side.

In order to control the balance of ε ′ and ε ″ by such a mechanism,
1) Because the conductive component is fine and has a fiber shape, a large number of contact points occur even with a small amount of addition.
2) The attractive force between the conductive components is large, and it is necessary to satisfy that the contact point is agglomerated in the resin cooling process and the contact point is generated again. The average fiber diameter used in the present invention is 50 nm or less and the aspect ratio is 10 or more. The fine carbon fiber sufficiently satisfies such conditions.

  On the other hand, generally used carbon fibers have a large fiber diameter of 6 to 12 μm, and therefore a large amount of addition is necessary. In addition, in the molded body of the resin composition to which a large amount of thick fibers are added, the fibers are difficult to be oriented, particularly in the portion where the shear rate is low inside the molded body, and the resistance due to orientation or the conductivity due to contact is stably controlled. It is difficult. Although vapor-grown carbon fibers as disclosed in JP-A No. 2001-223494 have a relatively small fiber diameter (0.1 to 1 μm), they are insufficient to obtain the above effects of the present invention. Further, in the above carbon fiber and vapor grown carbon fiber, contact points are not easily generated due to re-aggregation during cooling during molding.

  As described above, the radio wave absorber of the present invention can ensure radio wave absorption characteristics with a small amount of fine carbon fiber, and can also control radio wave absorption characteristics by controlling its dispersion state. In addition, it can be produced efficiently and inexpensively by injection molding, and it is excellent in the degree of freedom in designing the radio wave absorber.

  ADVANTAGE OF THE INVENTION According to this invention, the electromagnetic wave absorber which has the outstanding electromagnetic wave absorption characteristic with the very small compounding quantity of a fine carbon fiber can be provided. In addition, the wave absorber can be efficiently manufactured by extrusion molding or injection molding, and the shape of the wave absorber itself is arbitrary, and the degree of freedom in designing the wave absorber is wide. Wavelength control and thinning are easy.

Hereinafter, embodiments of the radio wave absorber of the present invention will be described in detail.
First, the components of the resin composition, which is a molding material for the radio wave absorber of the present invention, will be described.

<Constituents>
(A) Resin component In the present invention, the resin is a broad meaning resin including rubber, and the type thereof is not particularly limited, and physical properties such as strength, heat resistance, moldability, etc. are taken into consideration according to the application. As appropriate. For example, chloroprene rubber, chlorosulfonated polyethylene rubber, chlorinated polyethylene rubber, ethylene / α-olefin rubber, ethylene / propylene rubber, silicone rubber, acrylic rubber, fluorine rubber, styrene / butadiene rubber, isoprene rubber, etc. Examples of the thermoplastic resin, silicone resin, phenol resin, urea resin, epoxy resin, and other thermoplastic resins, urethane-based, amide-based, and ester-based thermoplastic elastomers can be used. Moreover, these 2 types or more can also be mixed and used as needed.

  Examples of the thermoplastic resin include ABS resin, AS resin, polycarbonate, polyarylate, polyphenylsulfone, polyethersulfone, polyetherimide, polyoxymethylene, polystyrene, alicyclic polyolefin, polyethylene, polypropylene, and polymethyl. Examples include pentene, polyethylene terephthalate, polybutylene terephthalate, polyphenylene sulfide, polyamide, and polyether ether ketone.

  As the component (A), among others, thermoplastic elastomers having polar groups such as urethane, amide, and ester, polycarbonate, polyarylate, polyoxymethylene, polyethylene terephthalate, polybutylene terephthalate, polyphenylene sulfide, polyamide, poly A resin having a polar group such as ether ether ketone, ABS resin, or polyether imide is desirable because of its good affinity with fine carbon fibers and excellent moldability in extrusion molding and injection molding. Among these, a radio wave absorber obtained by injection molding a composition using a resin having a flexural modulus (ASTM D790) of 1500 MPa or more has a high degree of freedom in strength, rigidity, and shape, and is excellent as a structure. desirable.

(B) Fine carbon fiber As a fine carbon fiber, a carbon nanotube can be used, for example. Carbon nanotubes are generally produced by a vapor phase growth method such as an arc discharge method, a laser evaporation method, or a thermal decomposition method, and are hollow tubular bodies each having a substantially continuous graphite surface rounded into a cylindrical shape.

  The carbon nanotube may be single-walled, double-walled, or multi-walled, and the average fiber diameter (that is, the average tube outer diameter) may be 50 nm or less. When the average fiber diameter exceeds 50 nm, a large amount of addition is required to obtain radio wave absorption characteristics, which not only leads to a decrease in strength and moldability, but also makes it difficult to control the balance of complex permittivity depending on the manufacturing conditions. Therefore, the average fiber diameter is 50 nm or less, preferably 30 nm or less, particularly preferably 20 nm or less. The lower limit of the average fiber diameter of fine carbon fibers such as carbon nanotubes is preferably 0.1 nm or more, and particularly preferably 0.5 nm or more. If the average fiber diameter is smaller than this, the production is extremely difficult.

  Further, the aspect ratio (length to diameter ratio) of the fine carbon fiber is 10 or more, desirably 20 or more, and more desirably 100 or more. If the aspect ratio of the fine carbon fiber is smaller than this, not only a large amount of addition is required to obtain radio wave absorption characteristics, but also it becomes difficult to control the balance of the complex dielectric constant depending on the manufacturing conditions. Further, the upper limit of the aspect ratio of the fine carbon fiber is 100,000 or less, preferably 10,000 or less. If it is larger than this, the entanglement of the fine carbon fibers becomes too large, making it difficult to disperse in the resin.

  The wall thickness of the carbon nanotube (wall thickness of the tubular body) is usually about 0.1 to 0.5 nm. This usually corresponds to 2 to 500 times the outer diameter of the carbon nanotube.

  When at least a part of the fine carbon fiber is in the form of an aggregate, the resin composition as a raw material contains a fine carbon fiber aggregate having a diameter measured on an area basis of about 50 μm, particularly greater than 10 μm. It is desirable that the amount added is small in order to obtain predetermined radio wave absorption characteristics, and the mechanical properties and the like of the resulting molded article are not deteriorated.

  In addition, if the fine carbon fiber has a meandering fiber shape, the cohesive force in the cooling process during extrusion molding or injection molding increases due to the entanglement effect in the resin, making it easy to control the complex dielectric constant. Desirable in terms. Accordingly, it is preferable that the fine carbon fiber has a bending degree of 5 ° or more, particularly 20 ° or more.

  The degree of bending of the fine carbon fiber can be determined, for example, by removing the resin component of the radio wave absorber of the present invention with a solvent or ion sputtering to expose the fine carbon fiber or by cutting an ultrathin slice cut from the molded body into an electron. It can be measured by observing under a microscope. In this case, fine carbon fibers in the surface layer within 50 μm from the surface of the molded body are measured.

As shown in FIG. 5, the degree of bending is observed by observing fine carbon fibers 2 such as carbon nanotubes with a microscope, and on the same fiber, the fiber diameter is measured five times (fiber diameter (part d in FIG. 5)). in any two points a to the process according to) apart such measure along the fiber, select B, the tangent L _a to each _point, pulling the L _B, the exterior angle of the tangent line L _a, Q point of intersection of the L _B (An angle indicated by α in FIG. 5) is measured. The average value of 10 points is taken and this is defined as the degree of bending.
This angle is 0 ° if the fiber is straight, 180 ° if it is a semicircle, and 360 ° if it is a circle.

  In the present invention, as the fine carbon fibers, commercially available products such as carbon nanotubes manufactured by Hyperion Catalysis International, Inc. can be used.

  In addition, you may perform the process by (A) resin component or various surface treatments, and a dispersing agent to the fine carbon fiber used by this invention. As the treating agent in this case, for example, a coupling agent such as a silane coupling agent, a titanate coupling agent or an aluminum coupling agent, or a block or graft copolymer of a nonpolar segment and a polar segment is used. be able to.

  In the present invention, the content of the fine carbon fibers in the resin composition is 0.01 to 7% by weight, preferably 0.05 to the total of the fine carbon fibers of the (A) resin component and the (B) component. -6% by weight, more preferably 0.1-5% by weight. If the content of the fine carbon fiber is less than this range, the complex dielectric constant is too low to exhibit radio wave absorptivity. On the other hand, if it exceeds this range, the imaginary part ε ″ of the complex dielectric constant becomes too large, thereby impairing the radio wave absorption characteristics, and it becomes extremely difficult to control the balance of the complex dielectric constant depending on the manufacturing conditions.

(C) Additive component Various additive components can be added to the resin composition according to the present invention as long as the effects of the present invention are not impaired.

  Such additive components include, for example, metallic fillers such as aluminum, silver, copper, zinc, nickel, stainless steel, brass, titanium, and the like, carbon black, activated carbon, graphite, pitch-based carbon, etc. Fiber, PAN-based carbon fiber, carbon-based filler such as graphite whisker, whisker and particles such as potassium titanate, barium titanate, titanium oxide, zinc oxide, silica, silica alumina, tin oxide, indium oxide, aluminum borate, glass One type or two or more types of fibers, glass flakes, glass beads, aramid fibers, polyimide fibers, fluororesin fibers, etc. may be mentioned. Note that, among metal oxide fillers, in the case where surplus electrons are generated due to the presence of lattice defects and show conductivity, a filler added with a dopant to increase conductivity may be used. For example, aluminum is used for zinc oxide, antimony for tin oxide, tin for indium oxide, and the like as dopants. In addition, a composite conductive filler in which carbon fiber or the like is coated with a metal or carbon, metal, or conductive tin oxide is formed on the surface of a potassium titanate whisker can be used.

  Furthermore, the resin composition according to the present invention includes inorganic fillers such as talc, calcium carbonate, mica, glass powder and glass balloon, solid lubricants such as fluororesin powder and molybdenum disulfide, plasticizers such as paraffin oil, Antioxidants, heat stabilizers, light stabilizers, UV absorbers, neutralizers, lubricants, compatibilizers, antifogging agents, antiblocking agents, slip agents, dispersants, colorants, antibacterial agents, fluorescent whitening Various additives such as an agent can also be added.

  Among them, among conductive materials such as carbon fillers and metal fillers, the particle diameter is 0.05 μm or more, desirably 0.5 to 200 μm, the particle diameter is 0.1 μm or more, desirably 0.5 Of the flaky filler having an aspect ratio (particle diameter / thickness) of 20 or less and a fiber filler such as carbon fiber having an aspect ratio (fiber length / fiber diameter) of 20 or less, the complex dielectric constant It is suitable as an additive for adjusting the balance. This is because the filler contributes little to improving the conductivity. Therefore, when the filler is added, the real part ε ′ is increased without largely increasing the imaginary part ε ″ of the complex dielectric constant. Because it can.

  Moreover, a high dielectric constant filler such as barium titanate can be added as an additive component for adjusting the balance of the complex dielectric constant. As the shape of such a high dielectric constant filler, particles, fibers, and scales can be used.

  The above-mentioned additive component for adjusting the balance of the complex dielectric constant is 0.1 to 200 parts by weight, desirably 1 to 100 parts by weight of the resin composition containing fine carbon fibers (the total of the fine carbon fibers and the resin component). Up to 30 parts by weight can be added.

  Below, the method of shape | molding such a resin composition and manufacturing the electromagnetic wave absorber of this invention is demonstrated.

<Manufacturing method>
The resin composition according to the present invention can be produced by an ordinary resin processing method. For example, (A) a thermoplastic resin as a resin component and (B) fine carbon fiber and (C) an additive component to be blended as necessary are mixed in advance, and then Banbury mixer, roll, Brabender, single It can be produced by melt kneading with a shaft kneading extruder, a biaxial kneading extruder, a kneader or the like.

  Moreover, the molded object which comprises the electromagnetic wave absorber of this invention can be manufactured by shape | molding such a thermoplastic resin composition using various melt-molding methods. Specific examples of the molding method include press molding, extrusion molding, injection molding, blow molding, and vacuum molding. Of these, extrusion molding or injection molding is desirable not only because of excellent productivity, but also because the balance between the real part and the imaginary part of the complex dielectric constant can be controlled well.

  In addition, it is known that the radio wave absorber has a multilayer structure (lower dielectric constant at the surface) in order to suppress the reflection of radio waves on the surface, but it is complex depending on the concentration of fine carbon fibers and molding conditions. A multilayer wave absorber can also be manufactured by stacking layers having different dielectric constants. In this case, for example, it can be molded by multilayer laminate extrusion molding, insert injection molding, multicolor injection molding (core back molding, rotational mold molding), or the like.

  The radio wave absorber of the present invention is put to practical use by pasting a metal plate as a reflection layer on the back side of the radio wave incident surface of the molded body as described above, or performing plating. The electromagnetic wave absorber having such a laminated structure is produced by laminating a metal film at the time of extrusion molding or by metal film in-mold molding at the time of injection molding when the molded body according to the present invention is manufactured by extrusion molding or injection molding. It can be manufactured at low cost. In addition, a radio wave absorber having a complicated shape can be manufactured by injection molding. In that case, the obtained molded body may be plated to form a metal film. This metal film can be formed by, for example, wet plating of a metal such as copper or nickel, vacuum deposition, sputtering, or the like.

  For example, manufacturing a structure having radio wave absorptivity by manufacturing a case such as a personal computer or a mobile phone or an antenna cover with the radio wave absorber of the present invention and applying metal plating such as copper or nickel on one side. Can do.

<Volume resistivity of molded body>
In the present invention, the volume resistivity (ρ _VC ) inside the molded body is 8 × 10 ⁰ to 1 × 10 ⁵ Ω · cm, preferably 8 × 10 ^{0 to} 5 × 10 ⁴ Ω · cm, more preferably 1 × 10. ^{By making it 1} to 1 × 10 ⁴ Ω · cm, excellent radio wave absorption characteristics are obtained.

If this volume resistivity (ρ _VC ) is larger than the above range (low conductivity), the value of the imaginary part ε ″ of the complex dielectric constant becomes too small, and conversely smaller than the above range (high conductivity). And the imaginary part ε ″ of the complex dielectric constant becomes too large. In either case, the radio wave absorption characteristics are impaired because the conditions greatly deviate from the non-reflection condition.

Further, the radio wave absorber of the present invention has a ratio (ρ _VS ) / (ρ _VC ) of the volume resistivity (ρ _VS ) measured through the surface of the molded body and the volume resistivity (ρ _VC ) inside the molded body. ) Is preferably 100 or more, more preferably 1000 or more, and the conductivity of the surface is lower than that of the inside of the molded body and the difference is large, thereby improving the complex dielectric constant balance. A molded article having excellent surface smoothness and a good surface appearance can be obtained. Furthermore, it is desirable in that the absorption characteristic can be expected to be improved by reducing the reflection of radio waves on the surface by increasing the resistance value of the surface.

An example of the volume resistivity measurement method according to the present invention is as follows.
In order to measure the volume resistivity (ρ _VC ) inside the molded body, it is necessary to measure by forming an electrode inside the molded body. For this purpose, for example, measurement may be performed according to the following procedure.

  (i) The molded body is fractured so that the two fracture surfaces face each other to obtain a fracture sample 20 as shown in FIG. At this time, when the molded body sample is cooled to a low temperature (preferably cooled in liquid nitrogen (−147 ° C.)) and fractured, the dispersion state of the fine carbon fibers is maintained, and the resistance value can be accurately measured. .

  (ii) Electrodes 21A and 21B are formed on the fractured surfaces 20A and 20B of the fracture sample 20 by applying a conductive paste or depositing metal as shown in FIG. The electrode needs to be made of a material that is sufficiently higher than the conductivity of the sample 20 (having a low resistance value, preferably a resistance value that is at least one order lower). Therefore, it is desirable to form an electrode by evaporating a metal such as silver.

(iii) The resistance value between the fractured surfaces of the fractured sample 20 (between the electrodes 21A and 21B) is measured, and the volume resistance value is calculated from the following formula.
(Ρ _VC ) = A / L × (R _VC )
Where A: electrode area (= sample thickness t × width W)
L: Distance between electrodes (= sample length L)
R _VC ; measured value

Further, for the measurement of the volume resistivity (ρ _VS ) measured through the surface of the molded body, electrodes 22A and 22B are provided so as to face the back surface and the surface of the fracture sample 20 as shown in FIG. Since the volume resistivity (ρ _VS ) is a relatively high value, the electrode may be applied with a conductive paste, or directly measured with a conductive rubber or metal probe. However, it is desirable to form an electrode by vapor-depositing silver or the like in that a sufficient contact area between the sample surface and the electrode can be secured. The volume resistivity (ρ _VS ) is similarly calculated from the above formula, but in this case, the distance between the electrodes corresponds to the molded body thickness t.

  Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples.

<Combination raw material>
The raw materials used in Examples and Comparative Examples are as follows.
Polycarbonate resin: Mitsubishi Engineering Plastics product name “Novaret”
7022 "(flexural modulus (ASTM D790) 2300
MPa)
Fine carbon fiber: Carbon Nanochu manufactured by Hyperion Catalysis International
Graphite: Nippon Graphite Co., Ltd. Trade name “CP B” (flaky graphite)
Vapor growth carbon fiber: trade name “VGCF” (average fiber diameter 150 nm, manufactured by Showa Denko KK)
(Aspect ratio 10-500)
Carbon Black: Product name “DENKA BLACK” (Acetylene) manufactured by Denki Kagaku Kogyo
Black, average particle size 42nm)

<Preparation of resin composition>
Each component was mixed with the ingredients shown in Table 1, and a barrel temperature of 300 was used using a twin-screw extruder ("PCM45" manufactured by Ikekai Steel Co., Ltd., L / D = 32 (L; screw length, D: screw diameter)). The mixture was melt-kneaded at a temperature of 160 ° C. and a screw rotation speed of 160 rpm to obtain pellets of a polycarbonate resin composition. In addition, the compounding kneading | mixing of the fine carbon fiber produced the masterbatch which added 15 weight% of fine carbon fibers to the polycarbonate resin previously, diluted this with the remaining component, and was set as the predetermined compounding quantity.

<Molding>
Using the obtained pellets of each composition, each was molded by the following molding method.

(injection molding)
Using a 75 TON injection molding machine, a sheet sample was molded using a 100 × 100 × 2 mm thick sheet mold having a film gate. The molding pressure was set to 1800 Kg / cm ² or less, and molding was performed under the conditions shown in Table 2 while optimizing the pressure and speed so as to achieve a predetermined filling time according to the fluidity of the composition.
However, when it could not be filled even at 1800 kg / cm ² , the molding was interrupted as “poor filling”.

(Press molding)
Using a 25 TON press molding machine, a sheet sample was molded under the following conditions using a spacer of 100 × 100 × 2 mm thickness.
Press temperature: 300 ° C
Press pressure: 30 kg / cm ²
Press time: 2 min

(Extruded molding)
A T-die having a width of 750 mm was attached to an extruder having a diameter of 65 mm (L / D = 28), and a sheet sample having a thickness shown in Table 3 was molded under the following conditions.
Die temperature: 250 ° C
Chill roll temperature: 80 ° C
Extrusion amount: 170 kg / hr

<Measurement and evaluation>
(1) Measurement of average fiber diameter, aspect ratio, and bending degree of fine carbon fiber From sheet sample A-2, an ultrathin section was cut out along the flow direction of the resin, observed with a transmission electron microscope, and a molded body. The fiber diameter, aspect ratio, and bending degree of the internal fine carbon fiber were measured at 10 points, and the average value was calculated. The results are shown below.
Average fiber diameter: 10.2 nm
Aspect ratio: 56 or more Flexion degree: 46 ° Here, with respect to the aspect ratio, since a part of the fiber is cut when an ultrathin section is produced, an accurate fiber length cannot be measured. Is at least within the desirable range of the present invention.

(2) Measurement of particle diameter and aspect ratio of graphite The particle diameter of graphite before kneading was previously measured at 150 points with an optical microscope, and the average value was calculated. As a result, it was 16.3 μm.
Moreover, the fracture surface which fractured | ruptured sheet | seat sample G-1 along the flow direction was observed with an electron microscope, the thickness of graphite was measured at 20 points, and the average value was calculated. As a result of calculating the aspect ratio by calculating the ratio, it was 36.

(3) Measurement of volume resistivity
(1) A test piece cut into a strip shape having a width of 15 mm from the center portion of each sheet sample was cooled in liquid nitrogen, then broken to a length of about 30 mm, and a width W = 15 mm as shown in FIG. A fracture sample 20 having a length L = about 30 mm was produced (thickness t is the thickness of the sheet sample).
(2) Silver was vapor-deposited with a thickness of 1500 mm on the fractured surfaces 20A and 20B of the fracture sample 20, and electrodes 21A and 21B were formed as shown in FIG.
(3) The resistance value between the electrodes 21A and 21B was measured, and the volume resistivity (ρ _VC ) was calculated by actually measuring the width W, length L, and thickness of the sample.
(4) Next, as shown in FIG. 4A, silver is deposited in the same position on the front and back plate surfaces of the fractured specimen 20 in a size of 10 mm × 10 mm and a thickness of 1500 mm to form electrodes 22A and 22B. did.
(5) The resistance value between the electrodes 22A and 22B was measured, the thickness of the sample was measured, and the volume resistivity (ρ _VS ) was calculated.
In addition, the following measuring devices were used for the measurement of the resistance value.
When the resistance value is 1 × 10 ⁴ Ω · cm or more: “Hiresta I” manufactured by Dia Instruments Co., Ltd.
P "
Applied voltage 10V
BP probe (2 probes)
When the resistance value is less than 1 × 10 ⁴ Ω · cm: “Loresta SP manufactured by Dia Instruments Co., Ltd.
"
BSP probe (4 probes)
Table 3 shows the measurement results.

(4) Measurement of complex dielectric constant As shown in FIG. 6A, a sample is cut out in a ring shape along the broken line from the center of each sheet sample 30 to obtain a ring-shaped sample 31 shown in FIG. 6B. (In FIG. 6 (b), d corresponds to the thickness of the sheet sample). The ring-shaped sample 31 has an outer diameter of 7 mm and an inner diameter of 3 mm, and upper and lower end surfaces thereof are molding surfaces. As shown in FIG. 7C, the ring-shaped sample 31 is inserted into a coaxial tube sample holder (APC connector, inner diameter 7 mm) 32 having an outer conductor 32A and a center conductor 32B, and an optical component analyzer (Agilent Technology Co., Ltd.) 8703A), the complex dielectric constant was measured by the S-parameter method using a coaxial tube structure. Prior to insertion, a silver paste was applied to the outer peripheral side surface 31A and the inner peripheral side surface 31B of the ring-shaped sample 31 so that no gap was generated between the sample 31 and the conductor contact surface.
Table 3 shows the measurement results.

(5) Measurement of reflection loss As shown in FIG. 6 (d), the coaxial tube sample holder 32 used in the measurement of the complex inductivity in (4) is connected to a metal terminal connector (trade name “Agilent 85050D 7mm ECONOMY CALIBRATION KIT”. SHORT CIRCUIT 85050-80007 ") 33, and the ring-shaped sample 31 was brought into contact with the terminal connector 33 to measure reflection loss.
Table 3 shows the measurement results.

(6) Measurement of 10-point average roughness (Rz) As an index of surface appearance, Rz (10-point average roughness) of the resistance value measurement part was measured under the following conditions using the following surface roughness meter.
Surface roughness meter “Surfcom 480A” manufactured by Tokyo Seimitsu Co., Ltd.
Cut-off wavelength: 2.5mm
Measurement length: 3mm
Measurement speed: 0.3mm / sec

The 10-point average surface roughness (Rz) is the absolute value of the altitude of the highest peak from the highest peak measured at a cutoff wavelength of 2.5 mm from the average line of the roughness curve in the direction of the vertical magnification. And the sum of the average value of the absolute values of the elevations of the bottom valley from the lowest valley bottom to the fifth. Therefore, the smaller the value of Rz, the smoother the surface and the better the appearance. However, in the case of a very smooth surface, calculation is impossible unless there are five or more peaks and valleys in the measurement range. In such a case, it can be replaced by the sum of the maximum peak and the maximum valley, that is, R _max .

In addition, it confirmed that the metal mold _| die surface of the used molding machine was Rmax0.2micrometer or less. Therefore, when Rz (or R _max ) is larger than this, it means that the mold transferability on the surface is poor and the appearance is impaired.
Table 3 shows the measurement results.

<Discussion>
FIG. 7 shows sheet samples A-3 and B-4 obtained by press-molding a composition to which fine carbon fibers have been added, sheet sample E-1 to which vapor-grown carbon fibers have been added, and sheet sample F- to which carbon black has been added. As an example of the complex permittivity of 1, a complex permittivity at 10 GHz is shown.
As is apparent from the complex dielectric constant of the sheet sample A-3, the addition of only 1.5% by weight of fine carbon fibers reaches a level close to an antireflection curve.
In contrast, carbon black (sheet sample E-1) and vapor-grown carbon fiber (sheet sample F-1) do not increase ε ″ even when added by 3 wt. Yes.

  On the other hand, the reflection loss of these samples, as shown in FIGS. 8 to 11, the broken line is the theoretical value calculated from the complex dielectric constant and the thickness, and the solid line is the actually measured value, and it can be confirmed that the actually measured value and the theoretical value are substantially the same (see FIGS. 13 to 13 described later). The same applies to 16).

In FIG. 7, the sheet sample B-4 added with 3% by weight of fine carbon fibers has a volume resistivity (ρ _VC ) that is too low, so that ε ″ becomes too large and deviates from the non-reflection curve. As a result, the amount of absorption also decreases as shown in FIG.

Next, in FIG. 12, the complex dielectric constants of the sheet samples B-1, B-2, B-4, and B-5 obtained by molding the resin composition B added with 3% by weight of fine carbon fibers under various conditions. Indicates.
In the sheet samples B-2 and B-5, the complex dielectric constant is not balanced by optimizing the molding conditions of the injection molding and the extrusion molding and setting the volume resistivity (ρ _VC ) inside the molded body within the range of the present invention. It was possible to get close to the reflection curve.
On the other hand, in sheet sample B-1, since the molding conditions were inappropriate, ε ″ was excessively decreased.

As shown in FIGS. 13 to 16 showing reflection loss (radio wave absorption characteristics) (FIG. 15 is the same as FIG. 9), in these sheet samples, the molding conditions can be controlled even with the same amount of fine carbon fiber added. Thus, the volume resistivity (ρ _VC ) is adjusted within the range of the present invention, and as a result, excellent radio wave absorption characteristics are exhibited.

Next, FIG. 17 shows the complex dielectric constants of the sheet samples A-3, B-2, C-2, and G-1. From FIG. 17, by adjusting the addition amount of fine carbon fibers and the molding conditions (volume resistivity (ρ _VC )), the complex dielectric constant can be brought close to a wide range on the non-reflection curve. It can be seen that d / λ (absorber thickness / absorption wavelength) can be set.

  For example, as shown in FIG. 18 which shows actual measurement values of the reflection loss of these sheet samples, the frequency characteristics to be absorbed can be adjusted over a wide range even with the same thickness (2 mm), and radio waves of a specific frequency can be absorbed. Therefore, the absorber thickness can be designed to be thin.

  The radio wave absorber of the present invention is excellent in radio wave absorption characteristics, and has a wide degree of freedom in designing the shape of the radio wave absorber and its radio wave absorption characteristics, such as personal computers, mobile phones, road traffic information systems, car navigation systems, and other information communications. Industrially, it is extremely useful as a housing material that absorbs radio waves and prevents leakage in devices and medical electronic devices. Further, in the antenna field, it is also useful as an absorbing component for improving antenna performance including directivity.

It is a schematic diagram which shows the dispersion | distribution state of the surface of a molded object, and an internal fine carbon fiber. It is a perspective view which shows the fracture | rupture sample for resistance value measurement. FIG. 3A is a sectional view of a fractured sample showing a method for measuring the volume resistivity (ρ _VC ) inside the molded body, and FIG. 3B is an equivalent circuit diagram of the resistance value at this time. FIG. 4A is a cross-sectional view of a fractured sample showing a method of measuring volume resistivity (ρ _VS ) measured through the surface of the molded body, and FIG. 4B is an equivalent circuit diagram of the resistance value at this time. It is. It is explanatory drawing of the measuring method of the bending degree of the fine carbon fiber which concerns on this invention. FIGS. 6A and 6B are explanatory diagrams showing measurement methods of complex dielectric constant and reflection loss in Examples and Comparative Examples, in which FIG. 6A is a plan view of a sheet sample, FIG. 6B is a perspective view of a ring-shaped sample, and FIG. (C) is sectional drawing which shows the measuring method of complex permittivity, FIG.6 (d) is sectional drawing which shows the measuring method of reflection loss. It is a graph which shows the balance of the complex dielectric constant (10 GHz) (epsilon) 'and (epsilon) "of sheet | seat sample A-3, B-4, E-1, and F-1. It is a graph which shows the reflection loss of sheet sample A-3. It is a graph which shows the reflection loss of sheet sample B-4. It is a graph which shows the reflection loss of sheet sample E-1. It is a graph which shows the reflection loss of the sheet sample F-1. It is a graph which shows the balance of the complex dielectric constant (10 GHz) (epsilon) ', (epsilon) "of sheet | seat sample B-1, B-2, B-4, B-5. It is a graph which shows the reflection loss of sheet sample B-1. It is a graph which shows the reflection loss of sheet sample B-2. It is a graph which shows the reflection loss of sheet sample B-4. It is a graph which shows the reflection loss of sheet | seat sample B-5. It is a graph which shows the balance of the complex dielectric constant (10 GHz) (epsilon) ', (epsilon) "of sheet | seat sample A-3, B-2, C-2, G-1. It is a graph which shows the reflection loss of sheet | seat sample A-3, B-2, C-2, G-1.

Explanation of symbols

2 Fine carbon fiber 10 Molded body 10A High resistance layer 10B Low resistance layer 20 Broken sample 21A, 21B, 22A, 22B Electrode 30 Sheet sample 31 Ring sample 32 Coaxial tube sample holder 33 Termination connector

Claims (3)

It consists of a composite of a resin layer and a metal layer, and the resin layer is
(A) a resin component;
(B) including an average fiber diameter of 50 nm or less and fine carbon fibers having an aspect ratio (fiber length / fiber diameter) of 10 or more,
(A) A radio wave absorber made of a molded product of a resin composition in which the ratio of (B) fine carbon fibers to the total of resin components and (B) fine carbon fibers is 0.01 to 7% by weight,
A radio wave absorber characterized by having a volume resistivity (ρ _VC ) inside the molded body of 8 × 10 ⁰ to 1 × 10 ⁵ Ω · cm.   2. The radio wave absorber according to claim 1, wherein the resin component is a thermoplastic resin, and the molded body is an extrusion molded body or an injection molded body. 3. The ratio (ρ _VS ) / (ρ _VC ) according to claim 1 or 2, wherein the volume resistivity (ρ _VS ) measured through the surface of the molded body and the volume resistivity (ρ _VC ) inside the molded body. Is a radio wave absorber characterized by being 100 or more.

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