Electromagnetic wave absorption member

Abstract

An electromagnetic wave absorption member (1) having a resistive layer (10), a spacer layer (30), and a reflective layer (40), wherein the resistive layer (10), the spacer layer (30), and the reflective layer (40) are laminated in that order, the spacer layer (30) includes a thermoplastic resin, a high-permittivity material, and a conductive material, and the relative permittivity of the spacer layer (30) is 5 or greater.

Description

Electromagnetic wave absorbing materials

[0001] This application claims priority from Japanese Patent Application No. 2024-026020, filed on February 22, 2024, the contents of which are incorporated herein by reference.

[0002] A sheet-like electromagnetic wave absorbing member that selectively absorbs electromagnetic waves of a predetermined frequency is known. The electromagnetic wave absorbing member, for example, includes a first frequency selective shielding layer and a second frequency selective shielding layer. In such an electromagnetic wave absorbing member, each layer absorbs electromagnetic waves of a predetermined frequency by means of fine line patterns of FSS (Frequency Selective Surface) elements formed in the first frequency selective shielding layer and the second frequency selective shielding layer, and as a whole, the electromagnetic wave absorbing member selectively blocks electromagnetic waves of two different frequencies.

[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 radio wave absorber, the relative dielectric constant of the dielectric layer of the λ / 4 type radio wave absorber is set to 3.0 or less in the 10 GHz band.

[0006] JP 2013-235816 A Japanese Patent No. 7305392 A

[0007] However, the artificial dielectric material described in Patent Document 1 and the radio 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.

[0009] The present invention provides the following electromagnetic wave absorbing members. [1] An electromagnetic wave absorbing member having a resistive layer, a spacer layer, and a reflective layer, the resistive layer, the spacer layer, and the reflective layer being laminated in this order, the spacer layer containing a thermoplastic resin, a high-dielectric material, and a conductive material, and the spacer layer having 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 of [1] to [3], wherein the thickness of the spacer layer is less than 300 μm.

[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.

[0011] 1 is a cross-sectional view of an electromagnetic wave absorbing member according to an embodiment of the present invention, taken along a thickness direction thereof, and FIG. 2 is a top view of an example of a resistive layer constituting the electromagnetic wave absorbing member according to an embodiment of the present invention.

[0012] An embodiment of the electromagnetic wave absorbing member of the present invention will be described. Note that this embodiment is specifically described to provide 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 aggregate of geometrically shaped units, and refers to an object that selectively absorbs electromagnetic waves of a certain frequency. A "conductor pattern" can also be said to have the same function as an antenna. In this specification, "electromagnetic waves in the millimeter wave region" refers to electromagnetic waves with a wavelength of 1 mm to 10 mm. "Electromagnetic waves in the millimeter wave region" can also be said to be electromagnetic waves with a frequency of 30 GHz to 300 GHz. In this specification, the symbol "to" indicating a numerical range means that the numerical values ​​before and after it are included as the lower and upper limits.

[0014] [Electromagnetic Wave Absorbing Member] Fig. 1 is a cross-sectional view of an electromagnetic wave absorbing member according to one embodiment of the present invention, taken along the thickness direction. As shown in Fig. 1, the electromagnetic wave absorbing member 1 of this embodiment includes 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 adhesive layer 50 is provided on one surface 40a of the reflective layer 40. The reflective layer 40 is laminated with the spacer layer 30 via the pressure-sensitive adhesive layer 50. 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] 1, the resistive layer 10 may be a single layer, or may include a substrate 11 and a conductor pattern 12 formed on one surface 11a of the substrate 11. When the resistive layer 10 is a single layer, the resistive layer 10 is made of the same material as the conductor pattern 12 described below.

[0016] "Resistive Layer" The resistive layer 10 is made of a frequency selective surface (FSS). A frequency selective surface is a surface that can block or transmit only electromagnetic waves of a specific frequency by forming a continuous structure with 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, the resistive layer 10 has, for example, a flat substrate 11 and a conductor pattern 12 formed on one surface 11a of the substrate 11. The conductor 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) As shown in FIG. 2, the first conductor pattern 101 is composed of a plurality of first units u1. Each of the first units u1 is a geometric figure. In other words, the first conductor pattern 101 can be said to be a collection 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, each of which has a plurality of first units u1 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 configured 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 intervals between the plurality of first arrays R1 are not particularly limited. The intervals 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. The shape of the first unit is not particularly limited as long as the frequency value at which the amount of electromagnetic waves absorbed by the first conductor pattern 101 reaches a maximum value is A [GHz]. For example, the shape of the figure that is the first unit may be a circle, an annular shape, a straight line, a square, a polygon, an H-shape, a Y-shape, a V-shape, etc.

[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 a target frequency.

[0024] The first conductor pattern 101 improves the transmittance of electromagnetic waves having the following frequency A [GHz]. In the resistance 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 the above numerical range, the obtained electromagnetic wave absorbing member 1 can absorb electromagnetic waves in the millimeter wave region, making it easier to apply to automobile parts, road peripheral members, building exterior wall-related materials, windows, communication equipment, radio telescopes, etc.

[0025] The material of the first unit u1 is not particularly limited as long as its absorption characteristics can be adjusted to the desired frequency. Examples of materials for 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 (e.g., steels such as stainless steel and carbon steel, brass, phosphor bronze, zirconium-copper alloys, beryllium copper, iron-nickel, nichrome, nickel-titanium, Kanthal, Hastelloy, and rhenium-tungsten). Examples of 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 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 may all be the same or may be different from each other. However, it is preferable that the distances between the ends of the first unit u1 are 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] 2, the second conductor pattern 102 is composed 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 having 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 exhibits a maximum value. The frequency value B [GHz] satisfies the following formula (1): 1.037×A≦B≦1.30×A...formula (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 peaks of the amount of electromagnetic wave transmitted by the second conductor pattern 102 and the peaks of the amount of electromagnetic wave transmitted by the first conductor pattern 101 sufficiently overlap 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 a 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 at which the amount of electromagnetic wave transmitted through the entire resistive layer 10 reaches a maximum value. As described above, because the second conductor pattern 102 transmits electromagnetic waves with frequencies of 1.037×A [GHz] to 1.30×A [GHz], the amount of electromagnetic wave transmitted through the entire electromagnetic wave absorbing member 1 is expanded into the higher frequency band.

[0030] The material of the second unit constituting the second conductor pattern 102 is not particularly limited as long as it can transmit electromagnetic waves of B [GHz] and can adjust the transmission characteristics 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] 2, the third conductor pattern 103 is composed 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 having 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 exhibits a maximum value. The frequency value C [GHz] satisfies the following formula (2): 0.60×A≦C≦0.963×A (2)

[0033] As shown in the above formula (2), the third conductor pattern 103 transmits electromagnetic waves having a frequency of 0.60×A [GHz] to 0.963×A [GHz]. The third conductor pattern 103 preferably transmits electromagnetic waves having a frequency of 0.60×A [GHz] to 0.83×A [GHz]. Because the third conductor pattern 103 transmits electromagnetic waves having a frequency 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 smaller in the frequency band lower than A [GHz]. As a result, a single peak is formed at which 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 waves transmitted by the third conductor pattern 103 and the peak of the amount of electromagnetic waves transmitted by the first conductor pattern 101 sufficiently overlap in the frequency band lower than A [GHz]. As a result, the frequency band of electromagnetic waves that can be transmitted through the entire resistive layer 10 is expanded to a frequency band lower than A [GHz] compared to a film having only the first conductor pattern 101. As described above, because the third conductor pattern 103 transmits electromagnetic waves with frequencies of 0.60×A [GHz] to 0.963×A [GHz], the amount of electromagnetic waves transmitted through the entire resistive layer 10 is expanded to a frequency band lower in frequency.

[0034] The material of the third unit u3 constituting the third conductor pattern 103 is not particularly limited as long as it can transmit electromagnetic waves of C [GHz] and 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 conductor pattern 102 and the frequency band of the electromagnetic waves selectively transmitted by the third conductor 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 conductor 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] 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 or different. 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-mentioned ranges, the band of electromagnetic waves transmitted through the entire resistive layer 10 is likely to be further expanded relative to the frequency value A [GHz] at the peak position.

[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 the first conductor pattern 101, the second conductor pattern 102, and the third conductor pattern 103 are not particularly limited. These thicknesses can be changed as desired depending on the desired characteristics. Furthermore, 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 the first conductor pattern 101, the second conductor pattern 102, and the 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 a 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 substrate 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 substrate 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 and polyethylene. Specific examples of polyester resins include polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate.

[0042] The substrate 11 may contain optional components within the scope of not impairing the effects of the present invention. Examples of the 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 absorption 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 fabricated, 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. When forming each conductor pattern, the conductor patterns are formed so that the frequency value at which the amount of electromagnetic wave transmission absorbed by each conductor pattern is at a maximum value is a predetermined value [GHz]. The order in which each conductor pattern is formed is not particularly limited. Each conductor pattern may be formed in the same process, or may be formed 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: A printing method in which each conductor pattern is printed on one surface 11a of the substrate 11 using a conductive paste. A development method in which each conductor pattern is developed 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 a 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 a metal wire is 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 develop stickiness by wetting; 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 adhesives include acrylic adhesives, urethane adhesives, rubber adhesives, polyester adhesives, silicone adhesives, polyvinyl ether adhesives, and the like. Among these, at least one selected from the group consisting of acrylic adhesives, urethane adhesives, and rubber adhesives is preferred, with acrylic adhesives being 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. One 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 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 segments 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 (dielectric constant). From the viewpoint of improving the dielectric constant 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. 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 dielectric constant.

[0055] The spacer layer 30 is obtained by kneading a composition containing a thermoplastic resin, a high-dielectric material, and a 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 preferably 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 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 considering the wavelength shortening effect of the spacer layer 30, the thickness of the spacer layer 30 is appropriately changed according to the wavelength of the electromagnetic wave to be absorbed and the relative dielectric constant of the spacer layer 30. When considering the wavelength shortening effect of the spacer layer 30, it is preferable that the thickness of the spacer layer 30 satisfy the following formula (3): (Thickness of the spacer layer 30) = (λ) × (¼) / (ε) １／２ ...Equation (3) In the above equation (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 obtain absorption characteristics. For example, the thickness can be changed within a range of 0.1 to 3.0 times the thickness of the spacer layer 30 obtained by equation (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 as long as it can reflect 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 either pass through the resistive layer 10 or are partially absorbed by the resistive layer 10. The electromagnetic waves that pass through the resistive layer 10 are reflected by the reflective layer 40 toward the resistive layer 10. For example, if the reflective layer 40 is conductive in the surface direction of either of the two surfaces 40a, 40b, it can reflect the electromagnetic waves that pass through the resistive layer 10. Specifically, the reflective layer 40 may be formed by laminating a metal foil such as aluminum foil or copper foil, or a metal plate such as a copper plate, to a resin film such as polyethylene terephthalate. Instead of the metal foil or metal plate, a transparent conductive film such as ITO, or a mesh sheet formed of metal wire, 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 develop stickiness by wetting; 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 adhesives include acrylic adhesives, urethane adhesives, rubber adhesives, polyester adhesives, silicone adhesives, polyvinyl ether adhesives, and the like. Among these, at least one selected from the group consisting of acrylic adhesives, urethane adhesives, and rubber adhesives is preferred, with acrylic adhesives being more preferred.

[0067] The thickness of the 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 10. 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.

[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] "Preparation of electromagnetic wave absorbing member" A pattern of water-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 thin copper film. Thereafter, the ink was washed away with water, and the thin copper film on the water-washable ink was removed, thereby patterning the conductive pattern as shown in FIG. 2 to obtain a resistive layer. Next, polyester-polyether copolymer (trade name: P-55B, manufactured by Toyobo Co., Ltd.) as a thermoplastic resin, barium titanate (trade name: BT-UP2, manufactured by Nippon Chemical Industry Co., Ltd.) as a high dielectric material, and spherical graphite (trade name: CGB-6R, volume average particle size 6 μm, manufactured by Nippon Graphite Fiber Co., Ltd.) as a carbon-based conductive material were kneaded for 5 minutes at 200 ° C. and 40 rpm using a Labo Plastomill (model name: 4C150, manufactured by Toyo Seiki Seisakusho Co., Ltd.) to prepare a mixed material (composition) containing 61 vol% polyester-polyether copolymer, 38 vol% barium titanate, and 1 vol% graphite. The mixed material was pressed for 3 minutes at 200 ° C. 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. An acrylic copolymer with a weight-average molecular weight of 800,000, consisting of 70% by weight of 2-ethylhexyl acrylate, 29% by weight of n-butyl acrylate, 0.5% by weight of acrylic acid, and 0.5% by weight of 2-hydroxyethyl acrylate, was prepared as the adhesive layer material. To 100 parts by weight (solids content equivalent) of the acrylic copolymer, 1 part by weight (solids content equivalent) of an isocyanate-based crosslinking agent and 8 parts by weight of an ultraviolet absorber (product name: Tinuvin 477, manufactured by BASF Japan Ltd.) were added, and the mixture was diluted with ethyl acetate to prepare an acrylic adhesive solution. The acrylic adhesive solution was then coated onto a release film, dried at 90°C for 1 minute, and then aged at room temperature for 1 week to obtain an adhesive layer with a thickness of 20 μm. Next, the adhesive layer was laminated on one side 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 Metalme 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 having a laminated structure as shown in FIG. 1 was obtained.

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

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

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

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

[0076] Comparative Example "Preparation of Electromagnetic Wave Absorbing Member" 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] "Measurement of Volume Resistivity" Using a digital ultra-high resistance / micro current meter (product name: 5451) manufactured by ADC Corporation, the spacer layer was placed on a measurement stage 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 Dielectric Constant" Using a free space unit (FS-330) manufactured by EMlab, the relative dielectric constants of the spacer layers obtained in the examples and comparative examples were measured in the range of 55 GHz to 95 GHz.

[0080] "Evaluation of Return Loss (S11)" Using a free space unit (FS-330) manufactured by EMlab, 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. The return loss (S11) was measured by the free space method.

[0081] "Evaluation of curved surface conformability" Measurement samples were obtained by laminating the pressure-sensitive adhesive layer produced in Example 1 on the reflective layer side of the electromagnetic wave absorbing members obtained in the Examples and Comparative Examples. The release film was peeled off from the measurement samples, and the measurement samples were attached to curved surfaces of different diameters to evaluate the curved surface conformability of the electromagnetic wave absorbing members. The smallest curved surface diameter (mm) at which the electromagnetic wave absorbing members could be attached to the curved surfaces without appearance defects such as wrinkles or lifted edges was evaluated.

[0082]

[0083] The results shown in Table 1 show that the electromagnetic wave absorbing members of Examples 1 to 5 were equal to or better in electromagnetic wave absorption and curved surface conformability than the electromagnetic wave absorbing member of Comparative Example 1, despite being thinner than the electromagnetic wave absorbing member of Comparative Example 1. On the other hand, it was found that the electromagnetic wave absorbing member of Comparative Example was inferior in curved surface conformability.

[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.

[0085] REFERENCE SIGNS LIST 1 electromagnetic wave absorbing member 10 resistive layer 11 substrate 12 conductive pattern 20 adhesive layer 30 spacer layer 40 reflective layer 50 adhesive layer 101 first conductive pattern 102 second conductive pattern 103 third conductive pattern

Claims

1. An electromagnetic wave absorbing member having a resistive layer, a spacer layer, and a reflective layer, the resistive layer, the spacer layer, and the reflective layer being laminated in this order, the spacer layer containing a thermoplastic resin, a high dielectric material, and a conductive material, and the spacer layer having a relative dielectric constant of 5 or more.

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. 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. The electromagnetic wave absorbing member according to claim 1, wherein the thickness of said spacer layer is less than 300 μm.

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