Electromagnetic wave absorption member

Abstract

To provide an electromagnetic wave absorption member which is excellent in electromagnetic wave absorption property, even if an angle of a polarization plane of an electromagnetic wave to be measured is shifted.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, a specific dielectric constant of the spacer layer 30 is 5 or more, and a difference between a specific dielectric constant in a length direction and a specific dielectric constant in a width direction of the spacer layer 30 is 0.1 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] Patent Document 1 discloses an electromagnetic wave absorber having a protective layer, an electromagnetic wave absorbing layer, a spacer layer, and a reflective layer in this order. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2022-56768 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the radio wave absorber described in Patent Document 1 has a problem in that its electromagnetic wave absorbing performance deteriorates when the angle of the plane of polarization of the electromagnetic wave to be measured is shifted.

[0006] The present invention has been made in consideration of the above circumstances, and aims to provide an electromagnetic wave absorbing member that has excellent electromagnetic wave absorption properties even when the angle of the polarization plane of the electromagnetic wave to be measured is shifted. [Means for solving the problem]

[0007] 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 has a relative dielectric constant of 5 or more, The spacer layer is an electromagnetic wave absorbing member, wherein the difference between the relative dielectric constant in the length direction and the relative dielectric constant in the width direction is 0.1 or more. [2] The electromagnetic wave absorbing member according to [1], wherein the spacer layer contains a thermoplastic resin and a high-dielectric material. [Effects of the Invention]

[0008] 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 angle of the polarization plane of the electromagnetic wave to be measured is shifted. [Brief explanation of the drawings]

[0009] [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

[0010] 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.

[0011] 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.

[0012] [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.

[0013] 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.

[0014] "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.

[0015] 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.

[0016] (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.

[0017] 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.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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 figures. In another example of the present invention, the shapes of the multiple first units may be the same or different as long as the absorption characteristics can be adjusted to the target frequency.

[0022] 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.

[0023] 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.). The conductive thin film may be made of a metal oxide such as indium oxide (ITO). Examples of materials for the conductive paste include metal particles, carbon nanoparticles, and carbon fibers.

[0024] 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.

[0025] (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 .

[0026] 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)

[0027] 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.

[0028] 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.

[0029] (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 .

[0030] 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)

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] "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.

[0045] 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.

[0046] "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.

[0047] The spacer layer 30 includes a thermoplastic resin and a high dielectric material.

[0048] 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.

[0049] The high dielectric material preferably contains at least one selected from the group consisting of barium titanate, titanium oxide, and strontium titanate.

[0050] The spacer layer 30 can be obtained by a molding method in which a composition containing a thermoplastic resin and a high-dielectric material is kneaded and hot-pressed using a hydraulic hot press or the like, or by molding the composition containing a thermoplastic resin and a high-dielectric material by extrusion molding using a T-die. In order to provide a difference between the dielectric constant in the length direction (MD) and the dielectric constant in the width direction (TD) of the spacer layer 30, it is preferable to mold the spacer layer 30 by extrusion molding using a T-die. That is, the spacer layer 30 is preferably an extrusion molded product of a composition containing a thermoplastic resin and a high-dielectric material. In the extrusion molding method, by adjusting the flow path diameter, die width, and winding speed, it is possible to create a difference between the dielectric constant in the length direction (MD) and the dielectric constant in the width direction (TD) of the spacer layer 30.

[0051] The relative dielectric constant of the spacer layer 30 is 5 or more, preferably 7 or more, more preferably 8 or more, and even more preferably 9 or more. If the relative dielectric constant of the spacer layer 30 is less than the lower limit, the thickness of the spacer layer 30 will be large. By making the relative dielectric constant of the spacer layer 30 5 or more, the thickness of the spacer layer 30 can be made thin. This allows the electromagnetic wave absorbing member 1 to have better curved surface conformability. From the viewpoint of preventing the Young's modulus of the spacer layer 30 from becoming too high, the upper limit of the relative dielectric constant of the spacer layer 30 may be 30 or less, 25 or less, 20 or less, or 15 or less.

[0052] The relative dielectric constant of the spacer layer 30 can be measured by the method described in the examples below.

[0053] The difference between the dielectric constant in the machine direction (MD) and the dielectric constant in the transverse direction (TD) of the spacer layer 30 is 0.1 or more, preferably 0.3 or more, and more preferably 0.6 or more. When the difference in dielectric constant is equal to or greater than the lower limit, the difference in return loss (S11) due to the difference in polarization angle is reduced. The upper limit of the difference in dielectric constant may be 3 or less, 2 or less, 1 or less, or 0.9 or less.

[0054] 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).

[0055] 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. The thickness of the spacer layer 30 can be appropriately set within the range of 100 μm to 1000 μm depending on the wavelength λ of the electromagnetic wave to be absorbed. From the viewpoint of improving the smoothness of the spacer layer surface, the thickness is preferably 200 μm or more, and more preferably 250 μm or more. Furthermore, from the viewpoint of eliminating the angle dependency of the polarization plane, the thickness is preferably 500 μm or less, and more preferably 350 μm or less. The spacer layer 30 may be made of a material with a high dielectric constant. If the spacer layer 30 is a layer with a high dielectric constant, the thickness of the spacer layer 30 can be made relatively thin. When the dielectric constant of the spacer layer 30 is taken into consideration, the spacer layer 30 preferably contains at least one material selected from the group consisting of barium titanate, titanium oxide, and strontium titanate.

[0056] "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.

[0057] 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.

[0058] The thickness of the reflective layer 40 is not particularly limited, but may be, for example, 1 μm to 300 μm, 10 μm to 200 μm, or 30 μm to 100 μm, taking into consideration the ability to follow a curved surface.

[0059] "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.

[0060] 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 500 μm, or 10 μm to 40 μm.

[0061] In the electromagnetic wave absorbing member 1 of this embodiment, the spacer layer 30 has a relative dielectric constant of 5 or more, and the difference between the relative dielectric constant in the length direction (MD) and the relative dielectric constant in the width direction (TD) of the spacer layer 30 is 0.1 or more, so that electromagnetic wave absorption properties independent of the polarization angle are obtained. [Example]

[0062] 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.

[0063] [Example 1] "Fabrication of electromagnetic wave absorbing materials" An electromagnetic wave absorbing member was produced as follows. Copper was vapor-deposited onto a substrate made of a 50 μm thick PET film (product name: PET50A4160, manufactured by Toyobo Co., Ltd.) to form a copper thin film. Thereafter, the copper thin film was patterned into a conductor pattern by photolithography to form the conductor pattern shown in Figure 2, thereby obtaining a resistive layer having a conductor pattern. The thickness of the conductor pattern was set to 100 nm. Next, a polyester-polyether copolymer (product name: P-55B, manufactured by Toyobo Co., Ltd.) as a thermoplastic resin and barium titanate (product name: BT-UP2, manufactured by Nippon Chemical Industry Co., Ltd.) as a high dielectric material were kneaded at 200°C and 40 rpm for 5 minutes using a Laboplastomill (model name: 4C150, manufactured by Toyo Seiki Seisakusho Co., Ltd.) to prepare a mixed material (composition) containing 60% by volume of polyester-polyether copolymer and 40% by volume of barium titanate. The above mixed material was subjected to extrusion molding using a T-die (screw diameter 25 mm, die width 400 m, winding speed 0.8 m / min) to obtain a spacer layer having a thickness of 310 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. Next, a 50 μm thick aluminum-deposited PET film (manufactured by Toray Advanced Film Co., Ltd., product name: Metal Me TS; aluminum-deposited layer thickness: 100 nm) was prepared as a reflective layer, and both sides of the film were laminated with adhesive layers to cover them, resulting in a laminate of release film / adhesive layer / reflective layer / adhesive layer / release film. Next, the release film provided on the adhesive layer laminated on the vapor deposition surface side of the reflective layer was peeled off, and the exposed adhesive layer was laminated on the spacer layer. Next, the resistive layer was laminated on the exposed surface of the spacer layer via the adhesive layer so that the P direction in Figure 2 coincided with the MD direction of the spacer layer and so that the conductive pattern faced the spacer layer, thereby obtaining an electromagnetic wave absorbing member.

[0064] [Example 2] "Fabrication of electromagnetic wave absorbing materials" An electromagnetic wave absorbing member of Example 2 was obtained in the same manner as in Example 1, except that the winding speed in the extrusion molding was changed to 0.65 m / min to obtain and use a spacer layer having a thickness of 325 μm.

[0065] [Comparative Example 1] "Fabrication of electromagnetic wave absorbing materials" An electromagnetic wave absorbing member of Comparative Example 1 was obtained in the same manner as in Example 1, except that a urethane foam sheet with a thickness of 2100 μm was used as the spacer layer.

[0066] Comparative Example 2 "Fabrication of electromagnetic wave absorbing materials" An electromagnetic wave absorbing member of Comparative Example 2 was obtained in the same manner as in Example 1, except that a spacer layer having a thickness of 610 μm and made of a polyester-polyether copolymer (trade name: P-55B, manufactured by Toyobo Co., Ltd.) was used.

[0067] [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.

[0068] "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. The dielectric constants of the spacer layer in the length direction (MD) and the width direction (TD) were measured, and the difference between the dielectric constants in the length direction (MD) and the width direction (TD) was calculated.

[0069] "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 (measurement wave). The return loss was measured by the free space method, in which the measurement wave was irradiated perpendicular to the plane of the electromagnetic wave absorbing material, and in the direction in which the MD direction of the spacer layer and the polarization of the measurement wave overlapped (polarization 0°), or in the direction in which the MD direction of the spacer layer and the polarization of the measurement wave made an angle of 45° (polarization 45°).

[0070] [Table 1]

[0071] The results shown in Table 1 show that the electromagnetic wave absorbing members of Examples 1 and 2 have excellent electromagnetic wave absorption properties even when the polarization angle is changed, because the difference between the relative dielectric constant in the length direction and the relative dielectric constant in the width direction of the spacer layer is 0.7 or more. On the other hand, in the electromagnetic wave absorbing members of Comparative Examples 1 and 2, there was no difference in the relative dielectric constant between the length and width directions of the spacer layer, and it was found that the electromagnetic wave absorption properties depended on the angle of polarization. [Industrial Applicability]

[0072] 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]

[0073] 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 has a relative dielectric constant of 5 or more; The spacer layer has a difference between the relative dielectric constant in the length direction and the relative dielectric constant in the width direction of 0.1 or more.

2. The electromagnetic wave absorbing member according to claim 1 , wherein the spacer layer contains a thermoplastic resin and a high-dielectric material.

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