Electromagnetic wave absorption member

Abstract

To provide an electromagnetic wave absorption member which is excellent in electromagnetic wave absorption property, even if thickness is reduced.SOLUTION: An electromagnetic wave absorption member 1 has a resistance layer 10, a spacer layer 30, and a reflection layer 40, wherein the resistance layer 10, the spacer layer 30 and the reflection layer 40 are stacked in this order, the spacer 30 contains a thermoplastic resin, a high dielectric material and a conductive material, and a specific dielectric constant of the spacer 30 is 5 or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an electromagnetic wave absorbing member. [Background technology]

[0002] Sheet-shaped electromagnetic wave absorbing members that selectively absorb electromagnetic waves of a predetermined frequency are known. The electromagnetic wave absorbing members, for example, include a first frequency selective shielding layer and a second frequency selective shielding layer. In such electromagnetic wave absorbing members, the first frequency selective shielding layer and the second frequency selective shielding layer each have a fine line pattern of FSS (Frequency Selective Surface) elements formed thereon, which causes each layer to absorb electromagnetic waves of a predetermined frequency, thereby selectively shielding electromagnetic waves of two different frequencies as a whole.

[0003] Depending on the application, an electromagnetic wave absorbing material is required to adhere closely to a curved surface when attached to the curved surface, and is required to have excellent electromagnetic wave absorption properties even when the thickness is reduced.

[0004] Patent Document 1 describes that in order to improve the electromagnetic wave absorption of an artificial dielectric, the relative permittivity of the dielectric layer is adjusted by adjusting the shape of multiple nanowires embedded in the dielectric layer of the artificial dielectric or the amount of nanowires mixed.

[0005] Patent Document 2 describes that in order to improve the electromagnetic wave absorption of a λ / 4 type wave absorber, the relative permittivity of the dielectric layer of the λ / 4 type wave absorber is set to 3.0 or less in the 10 GHz band. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-235816 [Patent Document 2] Patent No. 7305392 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the artificial dielectric material described in Patent Document 1 and the wave absorber described in Patent Document 2 have a problem in that when the thickness is reduced, the electromagnetic wave absorption ability is reduced.

[0008] The present invention has been made in view of the above circumstances, and has an object to provide an electromagnetic wave absorbing member that has excellent electromagnetic wave absorbing properties even when its thickness is reduced. [Means for solving the problem]

[0009] The present invention provides the following electromagnetic wave absorbing member. [1] A resistive layer, a spacer layer, and a reflective layer; the resistive layer, the spacer layer, and the reflective layer are laminated in this order; the spacer layer includes a thermoplastic resin, a high-dielectric material, and a conductive material; The spacer layer has a relative dielectric constant of 5 or more. [2] The electromagnetic wave absorbing member according to [1], wherein the content of the high-dielectric material in the spacer layer is 20% by volume or more. [3] The electromagnetic wave absorbing member according to [1] or [2], wherein the content of the conductive material in the spacer layer is 15% by volume or less. [4] The electromagnetic wave absorbing member according to any one of [1] to [3], wherein the spacer layer has a thickness of less than 300 μm. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide an electromagnetic wave absorbing member that has excellent electromagnetic wave absorbing properties even when the thickness is reduced. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a cross-sectional view of a plane along the thickness of an electromagnetic wave absorbing member according to an embodiment of the present invention; [Figure 2]FIG. 2 is a top view showing an example of a resistive layer constituting an electromagnetic wave absorbing member according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0012] An embodiment of the electromagnetic wave absorbing member of the present invention will be described. It should be noted that the present embodiment is specifically described to allow a better understanding of the gist of the invention, and does not limit the present invention unless otherwise specified.

[0013] In this specification, the term "conductor pattern" refers to an object that is a collection of geometrically shaped units and that selectively absorbs electromagnetic waves of a certain frequency. The "conductor pattern" can also be said to have the same function as an antenna. In this specification, "electromagnetic waves in the millimeter wave range" refers to electromagnetic waves with a wavelength of 1 mm to 10 mm. "Electromagnetic waves in the millimeter wave range" can also be said to be electromagnetic waves with a frequency of 30 GHz to 300 GHz. In this specification, the use of "to" to indicate a range of values ​​means that the values ​​before and after it are included as the lower and upper limits.

[0014] [Electromagnetic wave absorbing material] FIG. 1 is a cross-sectional view of a plane along the thickness of an electromagnetic wave absorbing member according to one embodiment of the present invention. As shown in FIG. 1 , the electromagnetic wave absorbing member 1 of this embodiment has a resistance layer 10, a pressure-sensitive adhesive layer 20, a spacer layer 30, and a reflective layer 40. The resistance layer 10, the pressure-sensitive adhesive layer 20, the spacer layer 30, and the reflective layer 40 are laminated in this order. An pressure-sensitive adhesive layer 50 is provided on one surface 40a of the reflective layer 40. The reflective layer 40 is laminated on the spacer layer 30 with the pressure-sensitive adhesive layer 50 interposed therebetween. Hereinafter, the pressure-sensitive adhesive layer 20 may be referred to as the first pressure-sensitive adhesive layer 20, and the pressure-sensitive adhesive layer 50 provided on one surface 40a of the reflective layer 40 may be referred to as the second pressure-sensitive adhesive layer 50.

[0015] The resistive layer 10 may be a single layer, or may include a substrate 11 and a conductive pattern 12 formed on one surface 11a of the substrate 11 as shown in FIG. When the resistive layer 10 is a single layer, the resistive layer 10 is made of the same material as the conductive pattern 12 described later.

[0016] "Resistance layer" The resistive layer 10 is made of a frequency selective surface (FSS), which is a surface that can block or transmit only electromagnetic waves of a specific frequency by forming a continuous structure of a shape smaller than the wavelength using a conductive material or the like.

[0017] Fig. 2 is a top view showing an example of a resistive layer constituting an electromagnetic wave absorbing member according to one embodiment of the present invention. As shown in Fig. 2, resistive layer 10 has, for example, a flat substrate 11 and a conductor pattern 12 formed on one surface 11a of substrate 11. Conductive pattern 12 is made up of, for example, a first conductor pattern 101, a second conductor pattern 102, and a third conductor pattern 103.

[0018] (First conductor pattern) 2, the first conductor pattern 101 is made up of a plurality of first units u1, each of which is a geometrical figure. In other words, the first conductor pattern 101 can be said to be an aggregate of first units u1, which are geometric figures. Each of the first units u1 functions as an antenna. The first conductor pattern 101 may be, for example, a thin line pattern of an FSS element.

[0019] In the first conductor pattern 101, a plurality of first arrays R1 are formed, in which a plurality of first units u1 are arranged along the direction indicated by the double-headed arrow P in Fig. 2. It can also be said that the first conductor pattern 101 has a plurality of first arrays R1. The first conductor pattern 101 can be constructed by forming a plurality of first arrays R1 on the substrate 11 at predetermined intervals along the direction indicated by the double-headed arrow P. The spacing between the multiple first arrays R1 is not particularly limited, and the spacing between the first arrays R1 may be regular or irregular.

[0020] As shown in Figure 2, the first unit u1 has a cross shape that is symmetrical in both the vertical and horizontal directions. Specifically, the first unit u1 has one cross portion S1 and four end portions T1. The cross portion S1 is composed of a straight line portion parallel to the x-axis direction and a straight line portion parallel to the y-axis direction in Figure 2. Linear end portions T1 contact both ends of the straight line portion parallel to the x-axis direction and both ends of the straight line portion parallel to the y-axis direction, respectively, so as to be perpendicular to each straight line portion.

[0021] By adjusting the length of the first unit u1 in the x-axis direction and the length of each of the four ends T1 in the x-axis direction, the electromagnetic wave absorption characteristics of the first unit u1 functioning as a single antenna can be adjusted. Similarly, the electromagnetic wave absorption characteristics can be adjusted in the y-axis direction.

[0022] However, the shape of the first unit is not limited to a cross shape, and is not particularly limited as long as the frequency value at which the amount of electromagnetic wave absorption by first conductor pattern 101 reaches a maximum value is A [GHz]. For example, the shape of the graphic that is the first unit may be a circle, an annular shape, a straight line, a square shape, a polygonal shape, an H-shape, a Y-shape, a V-shape, or the like.

[0023] In the resistive layer 10, the shapes of the multiple first units u1 are the same. However, the shapes of the multiple first units u1 do not have to be the same. In another example of the present invention, the shapes of the multiple first units u1 may be the same or different as long as the absorption characteristics can be adjusted to the target frequency.

[0024] The first conductor pattern 101 improves the transparency of electromagnetic waves having the following frequency A [GHz]. In the resistive layer 10 of this embodiment, the frequency value A is preferably 20 GHz to 110 GHz, more preferably 60 GHz to 100 GHz, even more preferably 65 GHz to 95 GHz, and particularly preferably 70 GHz to 90 GHz. When the frequency value A is within this range, the resulting electromagnetic wave absorbing member 1 can absorb electromagnetic waves in the millimeter wave region, and is easily applicable to automobile parts, road peripheral members, building exterior wall related materials, windows, communication devices, radio telescopes, etc.

[0025] The material of the first unit u1 is not particularly limited as long as it can adjust the absorption characteristics to the desired frequency. Examples of the material of the first unit include thin metal wires, conductive thin films, and conductive paste deposits. Examples of metal materials include copper, aluminum, tungsten, iron, molybdenum, nickel, titanium, silver, gold, and alloys containing two or more of these metals (for example, steels such as stainless steel and carbon steel, brass, phosphor bronze, zirconium-copper alloy, beryllium copper, iron-nickel, nichrome, nickel-titanium, Kanthal, Hastelloy, rhenium-tungsten, etc.). Materials for the conductive thin film include metal oxides such as indium tin oxide (ITO). Examples of materials for the conductive paste include metal particles, carbon nanoparticles, and carbon fibers.

[0026] The distance between the ends of the figure that is the first unit u1 is not particularly limited as long as the absorption characteristics can be adjusted to the desired frequency. For example, the distances between the ends of the first unit u1 figures may be the same or different from each other, but it is preferable that the distances between the ends of the first unit u1 figures be the same, as this makes it easier to design a resistive layer that is less susceptible to the influence of the surrounding environment and improves the accuracy of the frequency band of the transmitted electromagnetic waves during manufacturing.

[0027] (Second conductor pattern) As shown in FIG. 2, the second conductor pattern 102 is made up of a plurality of second units u2. The second conductor pattern 102 is formed in the same manner as the first conductor pattern 101 .

[0028] The second conductor pattern 102 selectively transmits electromagnetic waves with a frequency of B [GHz] that satisfies the following formula (1). The frequency value B [GHz] is the frequency value at which the amount of electromagnetic waves transmitted through the second conductor pattern 102 reaches a maximum. The frequency value B [GHz] satisfies the following formula (1). 1.037×A≦B≦1.30×A...Equation (1)

[0029] As shown in the above formula (1), the second conductor pattern 102 transmits electromagnetic waves with frequencies of 1.037×A [GHz] to 1.30×A [GHz]. The second conductor pattern 102 preferably transmits electromagnetic waves with frequencies of 1.17×A [GHz] to 1.30×A [GHz]. Because the second conductor pattern 102 transmits electromagnetic waves with frequencies of 1.037×A [GHz] or higher, the peak of the amount of electromagnetic wave transmitted by the second conductor pattern 102 fully overlaps with the peak of the amount of electromagnetic wave transmitted by the first conductor pattern 101 in the frequency band higher than A [GHz]. As a result, the frequency band of electromagnetic waves that can be transmitted by the entire resistive layer 10 is expanded to the frequency band higher than A [GHz], compared to when the first conductor pattern 101 is provided alone. Because the second conductor pattern 102 transmits electromagnetic waves with frequencies of 1.30×A [GHz] or less, the frequency difference between the peak of the amount of electromagnetic wave transmitted by the second conductor pattern 102 and the peak of the amount of electromagnetic wave transmitted by the first conductor pattern 101 becomes small in the frequency band higher than A [GHz]. As a result, a single peak is formed where the amount of electromagnetic wave transmitted through the entire resistive layer 10 is at a maximum value. As a result, the second conductor pattern 102 transmits electromagnetic waves with frequencies of 1.037×A [GHz] to 1.30×A [GHz], and therefore the amount of electromagnetic waves transmitted by the entire electromagnetic wave absorbing material 1 is expanded into the higher frequency band.

[0030] The material of the second unit constituting the second conductive pattern 102 is not particularly limited as long as it can transmit electromagnetic waves of B [GHz], and is not particularly limited as long as the transmission characteristics can be adjusted to the desired frequency. The material of the second unit is the same as that described for the material of the first unit u1.

[0031] (Third conductor pattern) As shown in FIG. 2, the third conductor pattern 103 is made up of a plurality of third units u3. The third conductor pattern 103 is formed in the same manner as the first conductor pattern 101 .

[0032] The third conductor pattern 103 selectively transmits electromagnetic waves with a frequency of C [GHz] that satisfies the following formula (2). The frequency value C [GHz] is the frequency value at which the amount of electromagnetic waves transmitted by the third conductor pattern 103 reaches a maximum. The frequency value C [GHz] satisfies the following formula (2). 0.60×A≦C≦0.963×A...Formula (2)

[0033] As shown in the above formula (2), the third conductor pattern 103 transmits electromagnetic waves with frequencies of 0.60×A [GHz] to 0.963×A [GHz]. The third conductor pattern 103 preferably transmits electromagnetic waves with frequencies of 0.60×A [GHz] to 0.83×A [GHz]. Because the third conductor pattern 103 transmits electromagnetic waves with frequencies of 0.60×A [GHz] or higher, the frequency difference between the peak of the amount of electromagnetic wave transmitted by the third conductor pattern 103 and the peak of the amount of electromagnetic wave transmitted by the first conductor pattern 101 becomes small in the frequency band lower than A [GHz]. As a result, a single peak is formed where the amount of electromagnetic wave transmitted through the entire resistive layer 10 is maximized. Because the third conductor pattern 103 transmits electromagnetic waves with frequencies of 0.963×A [GHz] or less, the peak of the amount of electromagnetic wave transmitted by the third conductor pattern 103 fully overlaps with the peak of the amount of electromagnetic wave transmitted by the first conductor pattern 101 in the frequency band lower than A [GHz]. As a result, the frequency band of electromagnetic waves that can be transmitted by the entire resistive layer 10 is expanded to the frequency band lower than A [GHz], compared to a film having the first conductor pattern 101 alone. As described above, the third conductive pattern 103 transmits electromagnetic waves with frequencies of 0.60×A [GHz] to 0.963×A [GHz], so the amount of electromagnetic waves transmitted through the entire resistive layer 10 is expanded to the lower frequency band.

[0034] The material of the third unit u3 constituting the third conductive pattern 103 is not particularly limited as long as it can transmit electromagnetic waves of C [GHz], and is not particularly limited as long as the absorption characteristics can be adjusted to the desired frequency. The material of the third unit u3 is the same as that described for the material of the first unit u1.

[0035] In the resistive layer 10 shown in FIG. 2 , the first array R1, the second array R2, and the third array R3 are arranged adjacent to one another along the direction indicated by the double-headed arrow P. Because the first array R1, the second array R2, and the third array R3 are arranged adjacent to one another on the substrate 11, the frequency band of the electromagnetic waves selectively transmitted by the second conductive pattern 102 and the frequency band of the electromagnetic waves selectively transmitted by the third conductive pattern 103 overlap with each other, based on the frequency value A [GHz] of the peak position of the electromagnetic waves selectively transmitted by the first conductive pattern 101. As a result, the band of the electromagnetic waves transmitted by the entire resistive layer 10 is easily expanded toward both the high-frequency side and the low-frequency side, based on the frequency value A [GHz] of the peak position.

[0036] As shown in FIG. 2, the distance d1 between the first unit u1 and the second unit u2, the distance d2 between the second unit u2 and the third unit u3, and the distance d3 between the third unit u3 and the first unit u1 may be the same as or different from each other. The distance d1 may be, for example, 0.2 mm to 4 mm, 0.3 mm to 3 mm, or 0.5 mm to 2 mm. The distance d2 may be, for example, 0.2 mm to 4 mm, 0.3 mm to 3 mm, or 0.5 mm to 2 mm. The distance d3 may be, for example, 0.2 mm to 4 mm, 0.3 mm to 3 mm, or 0.5 mm to 2 mm. When the distances d1, d2, and d3 are each within the above ranges, the band of electromagnetic waves transmitted through the entire resistive layer 10 tends to be further expanded with the frequency value A [GHz] at the peak position as the reference.

[0037] In the resistive layer 10, the first unit u1, the second unit u2, and the third unit u3 have the same shape. However, the shapes of the first unit u1, the second unit u2, and the third unit u3 do not have to be the same. That is, in another example of the present invention, the shapes of the first unit u1, the second unit u2, and the third unit u3 may be the same or different.

[0038] The substrate 11 is not particularly limited as long as it is flat and has a form that allows the first conductor pattern 101, the second conductor pattern 102, and the third conductor pattern 103 to be formed on one surface 11a. The substrate 11 may have a single-layer structure or a multi-layer structure.

[0039] The thickness of the substrate 11 may be, for example, 5 μm to 500 μm, 15 μm to 200 μm, or 25 μm to 100 μm. The thicknesses of first conductor pattern 101, second conductor pattern 102, and third conductor pattern 103 are not particularly limited. These thicknesses can be changed as desired depending on the desired properties. These three thicknesses may be the same or different from one another, but are preferably the same in consideration of productivity. From the viewpoint of achieving both electromagnetic wave absorption and curved surface conformability in the resulting electromagnetic wave absorbing member 1, the thicknesses of first conductor pattern 101, second conductor pattern 102, and third conductor pattern 103 are preferably 10 nm to 300 μm, more preferably 40 nm to 1 μm, and particularly preferably 80 nm to 400 nm.

[0040] The material of the substrate 11 can be selected appropriately depending on the application of the electromagnetic wave absorbing member 1. For example, the substrate 11 may be made of a transparent material in order to provide transparency to the electromagnetic wave absorbing member 1. Alternatively, the substrate 11 may be made of a flexible material in order to provide conformability to the curved surface of the electromagnetic wave absorbing member 1. The surface of the substrate 11 may be smoothed in order to improve the transparency and three-dimensional formability of the electromagnetic wave absorbing member 1.

[0041] For example, the base material 11 can be made of a resin. The resin may be a thermoplastic resin or a thermosetting resin. However, when the three-dimensional formability of the electromagnetic wave absorbing member 1 is taken into consideration, the base material 11 preferably contains a thermoplastic resin. Examples of thermoplastic resins include polyolefin resins, polyester resins, polyester-polyether resins, polyacrylic resins, polystyrene resins, polyimide resins, polyimideamide resins, polyamide resins, polyurethane resins, polycarbonate resins, polyarylate resins, melamine resins, epoxy resins, urethane resins, silicone resins, and fluororesins. Specific examples of polyolefin resins include polypropylene, polyethylene, etc. Specific examples of polyester resins include polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, etc.

[0042] The substrate 11 may contain optional components within the scope of not impairing the effects of the present invention. Examples of optional components include inorganic fillers, colorants, curing agents, antioxidants, light stabilizers, flame retardants, conductive agents, antistatic agents, and plasticizers.

[0043] In consideration of further improving the electromagnetic wave absorbing performance of the electromagnetic wave absorbing member 1, the thickness, dielectric constant, electrical conductivity, and magnetic permeability of the substrate 11 can be set as appropriate.

[0044] The resistance layer 10 can be produced, for example, by the following method. First, a substrate 11 is prepared. Next, a first conductor pattern 101, a second conductor pattern 102, and a third conductor pattern 103 are formed on one surface 11a of the substrate 11. Each conductive pattern is formed so that the frequency at which the amount of electromagnetic wave transmitted and absorbed by each conductive pattern is at a maximum value is a predetermined value [GHz]. The order in which the respective conductor patterns are formed is not particularly limited, and the respective conductor patterns may be formed in the same process or in separate processes.

[0045] The method for forming each conductor pattern is not particularly limited as long as it can form a predetermined frequency. Examples of the method for forming each conductor pattern include the following methods. A printing method in which each conductor pattern is printed on one surface 11a of the substrate 11 using a conductive paste. A developing method for developing each conductor pattern on one surface 11a of the substrate 11. A method in which a metal thin film is provided on one surface 11a of the substrate 11 by sputtering, vacuum deposition, or lamination of metal foil, and a pattern of the metal thin film is formed on one surface 11a of the substrate 11 by photolithography. A method in which metal wires are arranged on one surface 11a of the substrate 11.

[0046] "Adhesive layer (first adhesive layer)" Examples of adhesives constituting the first adhesive layer 20 include heat-seal type adhesives that bond by heat, adhesives that become adhesive when moistened, and pressure-sensitive adhesives (adhesives) that bond by pressure. Among these, pressure-sensitive adhesives (adhesives) are preferred from the viewpoint of simplicity. Specific examples of the pressure-sensitive adhesive include acrylic pressure-sensitive adhesives, urethane pressure-sensitive adhesives, rubber pressure-sensitive adhesives, polyester pressure-sensitive adhesives, silicone pressure-sensitive adhesives, polyvinyl ether pressure-sensitive adhesives, etc. Among these, at least one selected from the group consisting of acrylic pressure-sensitive adhesives, urethane pressure-sensitive adhesives, and rubber pressure-sensitive adhesives is preferred, and acrylic pressure-sensitive adhesives are more preferred.

[0047] The thickness of the first pressure-sensitive adhesive layer 20 is not particularly limited, but may be, for example, 1 μm to 1000 μm, 5 μm to 500 μm, or 10 μm to 40 μm.

[0048] "Spacer layer" The spacer layer 30 is provided on the other surface 10b side of the resistive layer 10. The spacer layer 30 has two surfaces 30a and 30b. The surface 30a of the spacer layer 30 faces the other surface 10b of the resistive layer 10. The other surface 30b of the spacer layer 30 faces the reflective layer 40. The spacer layer 30 may have a single layer structure or a multi-layer structure.

[0049] The spacer layer 30 includes a thermoplastic resin, a high dielectric material, and a carbon-based conductive material.

[0050] Examples of thermoplastic resins include polyolefin resins, polyester resins, polyester-polyether resins, polyacrylic resins, polystyrene resins, polyimide resins, polyimideamide resins, polyamide resins, polyurethane resins, polycarbonate resins, polyarylate resins, melamine resins, epoxy resins, urethane resins, silicone resins, and fluororesins. Among these, polyester-polyether resins having hard and soft segments are preferred from the viewpoint of achieving both good filler dispersibility and heat resistance.

[0051] The high dielectric material preferably contains at least one selected from the group consisting of barium titanate, titanium oxide, and strontium titanate.

[0052] The content of the high-dielectric material in the spacer layer 30 is preferably 20% by volume or more, more preferably 30% by volume or more, and even more preferably 35% by volume or more. When the content of the high-dielectric material is equal to or greater than the lower limit, the resulting spacer layer can be formed with a high relative dielectric constant. The upper limit of the content of the high-dielectric material is not particularly limited, but may be 60% by volume or less, 50% by volume or less, or 40% by volume or less from the viewpoint of formability.

[0053] Examples of conductive materials include metal-based materials such as gold, silver, copper, and iron, metal oxide-based materials such as tin oxide, indium oxide, and indium tin oxide, and carbon-based materials. Of these, carbon-based conductive materials are preferred from the viewpoint of keeping the spacer layer lightweight. Examples of carbon-based conductive materials include graphite, graphene, carbon nanofibers, carbon nanotubes, and fullerenes. As carbon-based conductive materials, spherical ones are preferred from the viewpoint of improving dispersibility.

[0054] The content of the conductive material in the spacer layer 30 is preferably 15% by volume or less, more preferably 10% by volume or less, and even more preferably 8% by volume or less. When the content of the conductive material is below the upper limit, if an appropriate amount of the conductive material is dispersed in the resin, when electromagnetic waves are irradiated, capacitance components are generated between the conductive materials, and thermal conversion occurs due to resistance components within the conductive material, contributing to improved dielectric properties (relative permittivity). Note that, from the viewpoint of improving the relative permittivity of the spacer layer 30, the lower limit of the content of the conductive material is preferably 1% by volume or more, more preferably 2% by volume or more, and even more preferably 6% by volume or more. Note that, as the amount of conductive material added increases, the distance between the conductive materials decreases, resulting in a decrease in capacitance components, and the conductivity within the system increases, resulting in a decrease in the relative permittivity.

[0055] The spacer layer 30 is obtained by kneading a composition containing a thermoplastic resin, a high-dielectric material, and a carbon-based conductive material, and then hot-pressing the kneaded composition with a hydraulic hot press or the like. The volume resistivity of the composition is 1×10 8 Ω cm or more 1×10 16 It is preferable that the resistance is Ω·cm or less, and 1×10 10 Ω cm or more 5.0×10 15 It is more preferable that the resistance is Ω·cm or less, and 1×10 13 Ω cm or more 1×10 15 It is more preferable that the volume resistivity of the composition is Ω·cm or less. When the volume resistivity of the composition is equal to or greater than the lower limit, the spacer layer 30 can be prevented from becoming a conductor and reducing its relative permittivity. When the volume resistivity of the composition is equal to or less than the upper limit, the range of materials that can be used for the spacer layer 30 is wider.

[0056] The volume resistivity of the composition can be measured by the method described in the examples below.

[0057] The relative dielectric constant of the spacer layer 30 is 5 or more, preferably 10 or more, more preferably 15 or more, and even more preferably 20 or more. When the relative dielectric constant of the spacer layer 30 is equal to or more than the above-mentioned lower limit, the thickness of the spacer layer 30 can be reduced. This allows the electromagnetic wave absorbing member 1 to have better curved surface conformability. The upper limit of the relative dielectric constant of the spacer layer 30 may be 40 or less, 35 or less, or 32 or less, from the viewpoint of preventing the Young's modulus of the spacer layer 30 from becoming too high.

[0058] The relative dielectric constant of the spacer layer 30 can be measured by the method described in the examples below.

[0059] The thickness of the spacer layer 30 is preferably less than 300 μm, more preferably less than 250 μm, and even more preferably less than 220 μm. When the thickness of the spacer layer 30 is less than the above upper limit, the resulting electromagnetic wave absorbing member has excellent curved surface conformability. The lower limit of the thickness of the spacer layer 30 may be 50 μm or more, 100 μm or more, or 150 μm or more.

[0060] When the wavelength shortening effect of the spacer layer 30 is taken into consideration, the thickness of the spacer layer 30 is appropriately changed in accordance with the wavelength of the electromagnetic wave to be absorbed and the relative dielectric constant of the spacer layer 30 . When the wavelength shortening effect of the spacer layer 30 is taken into consideration, the thickness of the spacer layer 30 preferably satisfies the following formula (3). (Thickness of spacer layer 30)=(λ)×(¼) / (ε) 1 / 2 ...Equation (3) In the above formula (3), λ is the wavelength of the incident electromagnetic wave, and ε is the relative dielectric constant of the spacer layer 30. The thickness of the spacer layer 30 may be adjusted appropriately to improve absorption characteristics. For example, it can be changed within a range of 0.1 to 3.0 times the thickness of the spacer layer 30 obtained by formula (3).

[0061] When the relationship between the thickness of the spacer layer 30 and the wavelength λ satisfies the above formula (3), the electromagnetic wave absorbing member 1 has a so-called λ / 4 structure, which further increases the maximum absorption amount of the electromagnetic wave by the electromagnetic wave absorbing member 1.

[0062] The thickness of the spacer layer 30 can be measured by a constant pressure thickness measuring instrument manufactured by Teclock Corporation.

[0063] "Reflective layer" The reflective layer 40 has two surfaces 40a and 40b. A second adhesive layer 50 is provided on one surface 40a of the reflective layer 40. The one surface 40a of the reflective layer 40 faces the other surface 30b of the spacer layer 30. The reflective layer 40 is laminated on the spacer layer 30 via the second adhesive layer 50. A protective layer may be laminated on the other surface 40b of the reflective layer 40.

[0064] The reflective layer 40 is not particularly limited in form as long as it can reflect the electromagnetic waves that arrive at the surface of the electromagnetic wave absorbing member 1 and pass through the electromagnetic wave absorbing member 1. Of the electromagnetic waves that arrive at the electromagnetic wave absorbing member 1, those to be absorbed pass through the resistive layer 10 or are partially absorbed by the resistive layer 10. The electromagnetic waves that have passed through the resistive layer 10 are reflected by the reflective layer 40 toward the resistive layer 10. For example, if the reflective layer 40 has conductivity in the direction of either of the two surfaces 40a, 40b, it can reflect electromagnetic waves that have passed through the resistive layer 10. Specifically, a resin film such as polyethylene terephthalate to which a metal foil such as aluminum foil or copper foil, or a metal plate such as a copper plate is attached may be used as the reflective layer 40. Instead of the metal foil or metal plate, a transparent conductive film such as ITO, or a mesh sheet formed of metal wires, may be used.

[0065] The thickness of the reflective layer 40 is not particularly limited, but may be, for example, 10 nm to 1000 μm, 1 μm to 180 μm, or 10 μm to 80 μm.

[0066] "Adhesive layer (second adhesive layer)" Examples of adhesives constituting the second adhesive layer 50 include heat-seal type adhesives that bond by heat, adhesives that become adhesive when moistened, and pressure-sensitive adhesives (adhesives) that bond by pressure. Among these, pressure-sensitive adhesives (adhesives) are preferred from the viewpoint of simplicity. Specific examples of the pressure-sensitive adhesive include acrylic pressure-sensitive adhesives, urethane pressure-sensitive adhesives, rubber pressure-sensitive adhesives, polyester pressure-sensitive adhesives, silicone pressure-sensitive adhesives, polyvinyl ether pressure-sensitive adhesives, etc. Among these, at least one selected from the group consisting of acrylic pressure-sensitive adhesives, urethane pressure-sensitive adhesives, and rubber pressure-sensitive adhesives is preferred, and acrylic pressure-sensitive adhesives are more preferred.

[0067] The thickness of second pressure-sensitive adhesive layer 50 is not particularly limited, but may be, for example, 1 μm to 1000 μm, 5 μm to 100 μm, or 10 μm to 40 μm.

[0068] The electromagnetic wave absorbing member 1 of this embodiment may include a protective layer formed on the outermost surface (upper surface) 10a of the resistance layer . The protective layer is not particularly limited as long as it can protect the resistance layer 10 .

[0069] According to the electromagnetic wave absorbing member 1 of this embodiment, the spacer layer 30 contains a thermoplastic resin, a high dielectric material, and a conductive material, and the relative dielectric constant of the spacer layer 30 is 5 or more, so that even when the thickness is reduced, the electromagnetic wave absorbing property is excellent. [Example]

[0070] The present invention will be explained in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0071] [Example 1] "Fabrication of electromagnetic wave absorbing materials" A pattern of washable ink was printed on a substrate made of a 50 μm thick PET film (product name: PET50A4160, manufactured by Toyobo Co., Ltd.), and copper was vapor-deposited to a thickness of 100 nm to form a copper thin film. The ink was then washed with water to remove the copper film on the ink, resulting in a conductive pattern as shown in Figure 2, which was used to obtain a resistive layer. Next, a polyester-polyether copolymer (product name: P-55B, manufactured by Toyobo Co., Ltd.) as a thermoplastic resin, barium titanate (product name: BT-UP2, manufactured by Nippon Chemical Industry Co., Ltd.) as a high-dielectric material, and spherical graphite (product name: CGB-6R, manufactured by Nippon Graphite Fiber Co., Ltd., volume average particle size: 6 μm) as a carbon-based conductive material were kneaded at 200°C and 40 rpm for 5 minutes using a Labo Plastomill (model name: 4C150, manufactured by Toyo Seiki Seisakusho Co., Ltd.) to prepare a mixed material (composition) containing 61% by volume of polyester-polyether copolymer, 38% by volume of barium titanate, and 1% by volume of graphite. The above mixed material was pressed at 200° C. for 3 minutes using a hydraulic heating press (model name: SA-302, manufactured by Tester Sangyo Co., Ltd.) to obtain a spacer layer having a thickness of 290 μm. The adhesive layer was made from an acrylic copolymer with a weight-average molecular weight of 800,000, consisting of 70% 2-ethylhexyl acrylate, 29% n-butyl acrylate, 0.5% acrylic acid, and 0.5% 2-hydroxyethyl acrylate. 100 parts by weight (solids equivalent) of the acrylic copolymer was mixed with 1 part by weight (solids equivalent) of an isocyanate crosslinker and 8 parts by weight of an ultraviolet absorber (product name: Tinuvin 477, manufactured by BASF Japan Ltd.), and the mixture was diluted with ethyl acetate to prepare an acrylic adhesive solution. Next, the above acrylic adhesive solution was applied onto a release film, dried at 90° C. for 1 minute, and then cured at room temperature for 1 week to obtain an adhesive layer with a thickness of 20 μm. Next, the above-mentioned adhesive layer was laminated on one surface of the spacer layer, the release film was peeled off, and the resistive layer was laminated so that the conductive pattern-forming surface was in contact with the exposed surface of the adhesive layer. Next, a 50 μm thick aluminum-deposited PET film (manufactured by Toray Advanced Film Co., Ltd., product name Metal Me TS, aluminum layer 100 nm) was prepared as a reflective layer, and an adhesive layer was laminated so as to cover the aluminum-deposited side of the film. The release film was peeled off, and the exposed adhesive layer was attached to the other side of the spacer layer. In this way, an electromagnetic wave absorbing member with a laminated structure as shown in Figure 1 was obtained.

[0072] [Example 2] "Fabrication of electromagnetic wave absorbing materials" The electromagnetic wave absorbing member of Example 2 was obtained in the same manner as Example 1, except that in the above mixed material, the content of polyester-polyether copolymer was 59 volume %, the content of barium titanate was 38 volume %, the content of graphite was 3 volume %, and the thickness of the spacer layer was 240 μm.

[0073] [Example 3] "Fabrication of electromagnetic wave absorbing materials" An electromagnetic wave absorbing member of Example 3 was obtained in the same manner as Example 1, except that in the above mixed material, the content of polyester-polyether copolymer was 57 volume %, the content of barium titanate was 38 volume %, the content of graphite was 5 volume %, and the thickness of the spacer layer was 210 μm.

[0074] [Example 4] "Fabrication of electromagnetic wave absorbing materials" The electromagnetic wave absorbing member of Example 4 was obtained in the same manner as Example 1, except that in the above mixed material, the content of polyester-polyether copolymer was 55 volume %, the content of barium titanate was 38 volume %, the content of graphite was 7 volume %, and the thickness of the spacer layer was 180 μm.

[0075] [Example 5] "Fabrication of electromagnetic wave absorbing materials" The electromagnetic wave absorbing member of Example 5 was obtained in the same manner as Example 1, except that in the above mixed material, the content of polyester-polyether copolymer was 52 volume %, the content of barium titanate was 38 volume %, the content of graphite was 10 volume %, and the thickness of the spacer layer was 240 μm.

[0076] [Comparative Example] "Fabrication of electromagnetic wave absorbing materials" An electromagnetic wave absorbing member of the comparative example was obtained in the same manner as in Example 1, except that in the above mixed material, the content of polyester-polyether copolymer was 62 vol%, the content of barium titanate was 38 vol%, the content of graphite was 0 vol%, and the thickness of the spacer layer was 300 μm.

[0077] [evaluation] The mixed materials, spacer layers, and electromagnetic wave absorbing members of the examples and comparative examples were evaluated as follows. The results are shown in Table 1.

[0078] "Volume resistivity measurement" The spacer layer was placed on a measurement stage using a digital ultra-high resistance / micro current meter (product name: 5451) manufactured by ADC Corporation, and the volume resistivity of the spacer layer was measured in accordance with JIS K 6911: 2006. Specifically, a voltage of 10 V was applied to the spacer layer for 60 seconds, and then the volume resistivity (Ω / sq) was measured.

[0079] "Evaluation of relative permittivity" The relative dielectric constants of the spacer layers obtained in the examples and comparative examples were measured at 55 GHz to 95 GHz using a free space unit (FS-330) manufactured by EMlab.

[0080] "Evaluation of return loss (S11)" The return loss (S11) of the electromagnetic wave absorbing members obtained in the examples and comparative examples was measured at 55 GHz to 95 GHz using a free space unit (FS-330) manufactured by EMlab. The return loss (S11) was measured by the free space method.

[0081] "Evaluation of curved surface conformability" The adhesive layer produced in Example 1 was laminated on the reflective layer side of the electromagnetic wave absorbing members obtained in the Examples and Comparative Examples to obtain measurement samples. The release film was removed from the measurement sample, and the sample was attached to curved surfaces of different diameters to evaluate the curved surface conformability of the electromagnetic wave absorbing member. The minimum diameter (mm) of the curved surface on which the electromagnetic wave absorbing member can be attached without causing appearance defects such as wrinkles or lifted edges was evaluated.

[0082] [Table 1]

[0083] The results shown in Table 1 show that the electromagnetic wave absorbing members of Examples 1 to 5, although thinner than the electromagnetic wave absorbing member of Comparative Example 1, have electromagnetic wave absorption properties and curved surface conformability that are equal to or better than those of the electromagnetic wave absorbing member of Comparative Example 1. On the other hand, it was found that the electromagnetic wave absorbing member of the comparative example was inferior in curved surface conformability. [Industrial Applicability]

[0084] The electromagnetic wave absorbing member of the present invention can be suitably used as an electromagnetic wave absorbing member for transportation equipment such as automobiles. [Explanation of symbols]

[0085] 1. Electromagnetic wave absorbing material 10 resistance layer 11 Base material 12 Conductor pattern 20 adhesive layer 30 spacer layer 40 reflective layer 50 adhesive layer 101 First conductor pattern 102 second conductive pattern 103 Third Conductive Pattern

Claims

1. a resistive layer, a spacer layer, and a reflective layer; the resistive layer, the spacer layer, and the reflective layer are laminated in this order; the spacer layer includes a thermoplastic resin, a high-dielectric material, and a conductive material; The spacer layer has a relative dielectric constant of 5 or more.

2. 2. The electromagnetic wave absorbing member according to claim 1, wherein the content of said high-dielectric material in said spacer layer is 20% by volume or more.

3. 2. The electromagnetic wave absorbing member according to claim 1, wherein the content of said conductive material in said spacer layer is 15% by volume or less.

4. 2. The electromagnetic wave absorbing member according to claim 1, wherein the spacer layer has a thickness of less than 300 [mu]m.

Images