Electromagnetic wave absorbing member

Abstract

An electromagnetic wave absorbing member (1) has a resistance layer (10), a spacer layer (30), and a reflection layer (40). The resistance layer (10), the spacer layer (30), and the reflection layer (40) are stacked in this order. The resistance layer (10) has three or more conductor patterns (12) having differing sizes in plan view. When the difference between the maximum length L1 of the largest pattern in the conductor patterns (12) and the maximum length L3 of the smallest pattern therein is defined as ΔL, there stands ΔL / L1 > 0.34; the thickness of the spacer layer (30) is 400 μm or less; and the dielectric constant of the spacer layer (30) is 5 or more.

Description

Electromagnetic wave absorbing materials

[0001] This application claims priority from Japanese Patent Application No. 2024-025973, 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 therefore, the electromagnetic wave absorbing material is desired to have excellent electromagnetic wave absorption properties even when electromagnetic waves are incident at an oblique angle.

[0004] Patent Document 1 describes a radio wave absorber that, in TM polarization measurement at an incidence angle of 45°, has an absorption range of 4 GHz or more with an absorption performance of 15 dB or more in the frequency band of 75 to 85 GHz, and / or has an absorption range of 4 GHz or more with an absorption performance of 15 dB or more in the frequency band of 75 to 85 GHz, in TE polarization measurement at an incidence angle of 45°.

[0005] Patent Document 2 describes an electromagnetic wave absorbing sheet in which the electromagnetic wave incidence angle showing a return loss of 15 dB or more includes a range of 0° to 20° over a frequency range of 76 GHz to 81 GHz.

[0006] Patent Document 3 describes a radio wave absorber comprising: a first radio wave absorbing section in which the amount of reflection and absorption of radio waves of a specific frequency measured based on Japanese Industrial Standards (JIS) R 1679:2007 is maximized at a first incident angle among incident angles of 0° to 80°; and a second radio wave absorbing section in which the amount of reflection and absorption of the radio waves is maximized at a second incident angle among incident angles of 0° to 80°, wherein the magnitude of the second incident angle is different from the magnitude of the first incident angle, or the type of polarization of the radio waves incident at the second incident angle is different from the type of polarization of the radio waves incident at the first incident angle, and the first radio wave absorbing section and the second radio wave absorbing section are arranged along a predetermined plane.

[0007] JP 2021-103785 A International Publication No. 2022 / 092137 International Publication No. 2022 / 189350

[0008] However, the radio wave absorbers and electromagnetic wave absorbing sheets described in Patent Documents 1 to 3 have the problem that they do not have sufficient absorption performance for electromagnetic waves in the millimeter wave range when the waves are obliquely incident.

[0009] 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 performance in absorbing electromagnetic waves in the millimeter wave range when the waves are obliquely incident.

[0010] The present invention provides the following electromagnetic wave absorbing member. [1] An electromagnetic wave absorbing member comprising 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 resistive layer having three or more conductor patterns of different sizes in a plan view, where ΔL is the difference between the maximum length L1 of the largest pattern and the maximum length L3 of the smallest pattern among the three or more conductor patterns, and ΔL / L1 > 0.34, the thickness of the spacer layer being 400 μm or less, and the dielectric constant of the spacer layer being 5 or more. [2] The electromagnetic wave absorbing member according to [1], wherein the maximum length L1 of the largest pattern among the three or more conductor patterns is 1.5 mm or less.

[0011] According to the present invention, it is possible to provide an electromagnetic wave absorbing member that has excellent performance in absorbing electromagnetic waves in the millimeter wave range when the waves are obliquely incident.

[0012] 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.

[0013] 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.

[0014] 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.

[0015] [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.

[0016] 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.

[0017] "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.

[0018] 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.

[0019] (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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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)

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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)

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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 or different. The distance d1 may be, for example, 0.2 mm to 4 mm, 0.7 mm to 2 mm, or 1.0 mm to 1.8 mm. The distance d2 may be, for example, 0.2 mm to 4 mm, 0.7 mm to 2 mm, or 1.0 mm to 1.8 mm. The distance d3 may be, for example, 0.2 mm to 4 mm, 0.7 mm to 2 mm, or 1.0 mm to 1.8 mm. When the distances d1, d2, and d3 are each within the above-described ranges, the band of electromagnetic waves transmitted through the entire resistive layer 10 is further expanded relative to the peak frequency value A [GHz]. Furthermore, the resistive layer 10 is also more easily adapted to oblique incidence.

[0038] 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.

[0039] In the resistive layer 10, the largest of the three conductor patterns (first conductor pattern 101, second conductor pattern 102, and third conductor pattern 103), i.e., in the conductor pattern 12 shown in FIG. 2, the maximum length L1 of the third unit u3 of the third conductor pattern 103 is preferably 1.50 mm or less, more preferably 1.30 mm or less, and even more preferably 1.18 mm or less. When the maximum length L3 of the third unit u3 is equal to or less than the upper limit, it becomes easier to obtain an electromagnetic wave absorbing member that absorbs obliquely incident electromagnetic waves in the millimeter wave range. The minimum length of the third unit u3 may be 1.00 mm or more, 1.04 mm or more, or 1.08 mm or more.

[0040] In the resistive layer 10, the smallest conductor pattern among the three conductor patterns (first conductor pattern 101, second conductor pattern 102, and third conductor pattern 103), i.e., conductor pattern 12 shown in FIG. 2, has a maximum length L3 of the second unit u2 of the second conductor pattern 102 of preferably 0.8 mm or less, more preferably 0.78 mm or less, and even more preferably 0.74 mm or less. When the maximum length L3 of the second unit u2 is equal to or less than the upper limit, it becomes easier to obtain an electromagnetic wave absorbing member that absorbs obliquely incident electromagnetic waves in the millimeter wave region. The minimum value of the length L2 of the second unit u2 may be 0.50 mm or more, 0.60 mm or more, or 0.70 mm or more.

[0041] When the difference between the maximum length L1 of the largest pattern and the maximum length L3 of the smallest pattern in the conductor patterns 12 is ΔL, it is preferable that ΔL / L1 > 0.34. When ΔL / L1 is equal to or greater than the lower limit, it becomes easier to obtain an electromagnetic wave absorbing member that absorbs obliquely incident electromagnetic waves in the millimeter wave region.

[0042] The ratio of the length in the x-axis direction of the end T3 of the third unit u3 of the third conductor pattern 103 to the maximum length L1 of the third unit u3 of the third conductor pattern 103 is preferably 0.1 to 0.9, more preferably 0.2 to 0.7, and even more preferably 0.3 to 0.5. When the ratio is within the above range, it becomes easier to suppress a decrease in oblique incidence absorption in the millimeter wave region.

[0043] The ratio of the length in the x-axis direction of the end T2 of the second unit u2 of the second conductor pattern 102 to the maximum length L3 of the second unit u2 of the second conductor pattern 102 is preferably 0.3 to 0.9, more preferably 0.4 to 0.8, and even more preferably 0.5 to 0.7. When the ratio is within the above range, it becomes easier to suppress a decrease in oblique incidence absorption in the millimeter wave region.

[0044] The ratio of the length in the x-axis direction of the end T1 of the first unit u1 of the first conductor pattern 101 to the maximum length L2 of the first unit u1 of the first conductor pattern 101 is preferably 0.2 to 0.9, more preferably 0.3 to 0.8, and even more preferably 0.4 to 0.7. When the ratio is within the above range, it becomes easier to suppress a decrease in oblique incidence absorption in the millimeter wave region.

[0045] The ratio of the maximum value of the intervals d1, d2, and d3 to the maximum length L1 of the third unit u3 of the third conductor pattern 103 is preferably 0.5 to 2.0, more preferably 0.8 to 1.7, and even more preferably 1.2 to 1.5. When the ratio is within the above range, it is easy to suppress a decrease in oblique incidence absorption in the millimeter wave region.

[0046] The thickness of the conductor patterns 12 (first conductor pattern 101, second conductor pattern 102, and third conductor pattern 103) is less than 1 mm, preferably 0.1 mm or less, and more preferably 0.001 mm or less. When the thickness of the conductor patterns 12 is less than the upper limit, it becomes easier to obtain an electromagnetic wave absorbing member that absorbs obliquely incident electromagnetic waves in the millimeter wave region. The lower limit of the thickness of the conductor patterns 12 may be 0.000001 mm or more, 0.00001 mm or more, or 0.00005 mm or more.

[0047] 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.

[0048] 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 each other, but are preferably the same in terms of productivity.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] "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.

[0055] 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 100 μm, or 10 μm to 40 μm.

[0056] "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.

[0057] The relative dielectric constant of the spacer layer 30 is 5 or more, preferably 7 or more, more preferably 8 or more, and particularly preferably 9 or more. When the relative dielectric constant of the spacer layer 30 is 5 or more, the thickness of the spacer layer 30 can be made thin. In addition to the thinness of the spacer layer, the high dielectric constant makes it possible to obtain an electromagnetic wave absorbing member with excellent absorption properties for obliquely incident electromagnetic waves in combination with the resistive layer. 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, 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 200 μm or more and 450 μm or less, more preferably 250 μm or more and 400 μm or less, and particularly preferably 300 μm or more and 340 μm or less. When the thickness of the spacer layer 30 is equal to or greater than the lower limit, a spacer layer 30 having a high relative dielectric constant is easily obtained. When the thickness of the spacer layer 30 is equal to or less than the upper limit, bending rigidity is low and curved surface conformability is improved.

[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. This further increases the maximum value of the amount of electromagnetic waves absorbed by the electromagnetic wave absorbing member 1. 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. In order to increase the dielectric constant of the spacer layer 30, it is preferable to include a high-dielectric material in the resin component at a high filling rate. Preferred examples of high-dielectric materials include barium titanate, titanium oxide, and strontium titanate.

[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, 0.001 μm to 1000 μm, 1 μm to 300 μm, or 10 μm to 200 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] 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.

[0070] Example 1 "Preparation of Electromagnetic Wave Absorbing Member" An electromagnetic wave absorbing member was prepared as follows. A water-washable ink was pattern-printed on a substrate made of a 50 μm-thick PET film (product name: PET50A4160, manufactured by Toyobo Co., Ltd.), and a 100 nm-thick copper thin film was formed by vapor deposition. The ink was then washed with water to remove the copper thin film on the water-washable ink, thereby patterning the conductive pattern. As shown in FIG. 2, a resistive layer having three conductive patterns (a first conductive pattern, a second conductive pattern, and a third conductive pattern) was obtained. The maximum length L2 of the first unit u1 of the first conductive pattern was 0.925 mm, the maximum length L3 of the second unit u2 of the second conductive pattern was 0.725 mm, and the maximum length L1 of the third unit u3 of the third conductive pattern was 1.125 mm. The width W1 of the first unit u1 of the first conductor pattern, the width W2 of the second unit u2 of the second conductor pattern, and the width W3 of the third unit u3 of the third conductor pattern were all 0.5 mm. The lengths of the end T1 of the first unit u1, the end T2 of the second unit u2, and the end T3 of the third unit u3 in the direction perpendicular to the x-axis or y-axis were all 0.5 mm. 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 were all 1.6 mm. Next, a polyester-polyether copolymer (product name: P-55B, manufactured by Toyobo Co., Ltd.) as a resin and barium titanate (product name: BT-UP2, manufactured by Nippon Chemical Industry Co., Ltd.) as a filler were kneaded for 6 minutes at 210 °C and 40 rpm using a Laboplastomill (model name: 4C150, manufactured by Toyo Seiki Seisakusho Co., Ltd.) to prepare a mixed material containing 40% by volume of barium titanate. The mixed material was then processed using a high-temperature press (model name: N4046-00, manufactured by NPA Systems Co., Ltd.) at a processing temperature of 210 °C and a processing pressure of 4500 N to form a spacer layer with a thickness of 310 μm. Next, 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 mass (solid content equivalent) of the acrylic copolymer, 1 part by mass (solid content equivalent) of an isocyanate-based crosslinking agent and 8 parts by mass of an ultraviolet absorber (trade 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 on a release film, dried at 90°C for 1 minute, and then cured at room temperature for 1 week to obtain an adhesive layer having a thickness of 20 μm. The adhesive layer was then laminated on one side of a spacer layer. A resistive layer was then laminated on one side of the spacer layer via the adhesive layer so that the conductive pattern was in contact with the adhesive layer. The adhesive layer was then laminated on the other side of the spacer layer, and a 50 μm thick aluminum vapor-deposited film (trade name: Metal Me Namer S Gloss 50B, manufactured by Toray Industries, Inc.) was laminated on the adhesive layer as a reflective layer to obtain an electromagnetic wave absorbing member.

[0071] Example 2 "Preparation of Electromagnetic Wave Absorbing Member" An electromagnetic wave absorbing member was prepared as follows. A water-washable ink was pattern-printed on a substrate made of a 50 μm-thick PET film (product name: PET50A4160, manufactured by Toyobo Co., Ltd.), and a 100 nm-thick copper thin film was formed by vapor deposition. The ink was then washed with water to remove the copper thin film on the water-washable ink, thereby patterning the conductive pattern. As shown in FIG. 2, a resistive layer having three conductive patterns (a first conductive pattern, a second conductive pattern, and a third conductive pattern) was obtained. The maximum length L2 of the first unit u1 of the first conductive pattern was 0.900 mm, the maximum length L3 of the second unit u2 of the second conductive pattern was 0.700 mm, and the maximum length L1 of the third unit u3 of the third conductive pattern was 1.100 mm. The width W1 of the first unit u1 of the first conductor pattern, the width W2 of the second unit u2 of the second conductor pattern, and the width W3 of the third unit u3 of the third conductor pattern were all 0.2 mm. The lengths of the end T1 of the first unit u1, the end T2 of the second unit u2, and the end T3 of the third unit u3 in the direction perpendicular to the x-axis or y-axis were all 0.5 mm. 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 were all 1.6 mm. Next, a polyester-polyether copolymer (product name: P-55B, manufactured by Toyobo Co., Ltd.) as a resin and barium titanate (product name: BT-UP2, manufactured by Nippon Chemical Industry Co., Ltd.) as a filler were kneaded for 6 minutes at 210 °C and 40 rpm using a Labo Plastomill (model name: 4C150, manufactured by Toyo Seiki Seisakusho Co., Ltd.) to prepare a mixed material containing 20% ​​by volume of barium titanate. The mixed material was molded into a spacer layer having a thickness of 370 μm using a high-temperature press (model name: N4046-00, manufactured by NPA Systems Co., Ltd.) at a processing temperature of 210 °C and a processing pressure of 4500 N. Next, the above-mentioned adhesive layer was laminated on one side of the spacer layer. Next, a resistive layer was laminated on one side of the spacer layer via the adhesive layer so that the conductor pattern was in contact with the adhesive layer.Next, the above-mentioned adhesive layer was laminated on the other side of the spacer layer, and a 50 μm thick aluminum vapor deposition film (product name: Metal Me Namer S Gloss 50B, manufactured by Toray Industries, Inc.) was laminated on the adhesive layer as a reflective layer to obtain an electromagnetic wave absorbing member.

[0072] Comparative Example 1 "Preparation of Electromagnetic Wave Absorbing Member" An electromagnetic wave absorbing member was prepared in the same manner as in the Examples, except that the spacer layer was formed using HH-48 (trade name, thickness 2 mm) manufactured by Inoac Corporation, and the conductor pattern was changed as follows. The maximum length L2 of the first unit u1 of the first conductor pattern was set to 1.0 mm, the maximum length L3 of the second unit u2 of the second conductor pattern was set to 0.8 mm, and the maximum length L1 of the third unit u3 of the third conductor pattern was set to 1.2 mm. The width W1 of the first unit u1 of the first conductor pattern, the width W2 of the second unit u2 of the second conductor pattern, and the width W3 of the third unit u3 of the third conductor pattern were all set to 0.15 mm. The length of each of the end T1 of the first unit u1, the end T2 of the second unit u2, and the end T3 of the third unit u3 in the direction perpendicular to the x-axis or y-axis was 0.5 mm. 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 were 2.2 mm.

[0073] Comparative Example 2 "Preparation of Electromagnetic Wave Absorbing Member" An electromagnetic wave absorbing member was prepared in the same manner as in Example 1, except that the spacer layer was formed using HH-48 (trade name, thickness 2 mm) manufactured by Inoac Corporation and the conductor pattern was changed as follows: The maximum length L2 of the first unit u1 of the first conductor pattern was set to 0.925 mm, the maximum length L3 of the second unit u2 of the second conductor pattern was set to 0.725 mm, and the maximum length L1 of the third unit u3 of the third conductor pattern was set to 1.125 mm. The width W1 of the first unit u1 of the first conductor pattern, the width W2 of the second unit u2 of the second conductor pattern, and the width W3 of the third unit u3 of the third conductor pattern were all set to 0.5 mm. The length of each of the end T1 of the first unit u1, the end T2 of the second unit u2, and the end T3 of the third unit u3 in the direction perpendicular to the x-axis or y-axis was 0.5 mm. 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 were 1.6 mm.

[0074] [Evaluation] The electromagnetic wave absorbing members of the examples and comparative examples were evaluated as follows. The results are shown in Table 1.

[0075] "Evaluation of Return Loss (S11)" Using a free space unit (FS-330) manufactured by EMlab, the return loss (S11) was measured for the electromagnetic wave absorbing members obtained in the examples and comparative examples at normal incidence and at oblique incidence (30° to 60°). The return loss (S11) was measured by the free space method.

[0076]

[0077] The results shown in Table 1 indicate that the electromagnetic wave absorbing members of the examples have excellent absorption performance for 76 GHz electromagnetic waves at oblique incidence, whereas the electromagnetic wave absorbing members of comparative examples 1 and 2 have poor absorption performance for 76 GHz electromagnetic waves at oblique incidence.

[0078] 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.

[0079] 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 resistive layer having three or more conductor patterns of different sizes in a planar view, where ΔL is the difference between the maximum length L1 of the largest pattern and the maximum length L3 of the smallest pattern among the three or more conductor patterns, ΔL / L1 > 0.34, the thickness of the spacer layer is 400 μm or less, and the dielectric constant of the spacer layer is 5 or more.

2. The electromagnetic wave absorbing member according to claim 1, wherein the maximum length L1 of the largest of said three or more conductive patterns is 1.5 mm or less.

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