Electromagnetic wave absorbing member

Abstract

To provide an electromagnetic wave absorbing part having excellent durability under severe environments where contact with acidic or alkaline splashes is expected.SOLUTION: An electromagnetic wave absorbing part (1) includes: a resistive layer (10) having a conductor pattern (12); an adhesive layer (20); a spacer layer (30); and a reflective layer (40). The resistive layer (10), the adhesive layer (20), the spacer layer (30), and the reflective layer (40) are laminated in this order. The retention strength of the adhesive layer (20) to the spacer layer (30) after an alkali immersion test is at least 5000 seconds or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] 2. Description of the Related Art Sheet-shaped electromagnetic wave absorbing members that selectively absorb electromagnetic waves of a predetermined frequency are known. Electromagnetic wave absorbing members are sometimes used in harsh environments where contact with acidic or alkaline droplets is anticipated.

[0003] Conventionally, a known λ / 4 type electromagnetic wave absorbing member has a multi-layer structure including a conductive layer, a spacer layer, and a reflective layer, stacked in this order (see, for example, Patent Document 1). In such an electromagnetic wave absorbing member, an adhesive layer is often provided to bond the layers together. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2020 / 179349 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the invention described in Patent Document 1 has a problem in that the electromagnetic wave absorbing member has poor durability in an environment where it comes into contact with alkaline droplets.

[0006] The present invention has been made in view of the above circumstances, and has an object to provide an electromagnetic wave absorbing member that is highly durable in harsh environments where contact with acidic or alkaline droplets is expected. [Means for solving the problem]

[0007] The present invention has the following aspects. [1] A resistive layer having a conductive pattern, an adhesive layer, a spacer layer, and a reflective layer, the resistive layer, the adhesive layer, the spacer layer, and the reflective layer are laminated in this order; An electromagnetic wave absorbing member, wherein the holding power of the pressure-sensitive adhesive layer to the spacer layer after an alkali immersion test is 5000 seconds or more. [2] The electromagnetic wave absorbing member according to [1], wherein the spacer layer is made of a resin foam. [3] The electromagnetic wave absorbing member according to [1] or [2], wherein the attenuation rate of the adhesive strength of the adhesive layer to the spacer layer after an alkali immersion test is less than 20% compared to the adhesive strength of the adhesive layer to the spacer layer before the alkali immersion test. [4] The electromagnetic wave absorbing member according to [1] or [2], wherein the adhesive strength of the adhesive layer to the spacer layer is 5 N / 25 mm or more. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide an electromagnetic wave absorbing member that is excellent in durability even in a severe environment where contact with acidic or alkaline droplets is expected. [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.

[0012] 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.

[0013] [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 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. Furthermore, pressure-sensitive adhesive layers 50 are provided on one surface 40a and the other surface 40b of the reflective layer 40. The reflective layer 40 is laminated on 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, the pressure-sensitive adhesive layer 50 provided on one surface 40a of the reflective layer 40 as the second pressure-sensitive adhesive layer 51, and the pressure-sensitive adhesive layer 50 provided on the other surface 40b of the reflective layer 40 as the third pressure-sensitive adhesive layer 52. The electromagnetic wave absorbing member 1 of this embodiment may have a protective layer 60 .

[0014] 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.

[0015] "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.

[0016] 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.

[0017] (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.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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 tin oxide. Examples of materials for the conductive paste include metal particles, carbon nanoparticles, and carbon fibers.

[0025] 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.

[0026] (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 .

[0027] 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)

[0028] 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.

[0029] 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.

[0030] (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 .

[0031] 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)

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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, and 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 1000 nm, and particularly preferably 80 nm to 400 nm.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] "Adhesive layer (first adhesive layer)" The adhesive constituting the first adhesive layer 20 is not particularly limited as long as it provides a predetermined value of holding power to the first adhesive layer 20 after an alkali immersion test described later, but examples include heat-seal type adhesives that bond by heat, adhesives that develop adhesion when moistened, pressure-sensitive adhesives (adhesives) that bond by pressure, etc. Among these, pressure-sensitive adhesives (adhesives) are preferred from the viewpoint of simplicity. Specific examples of pressure-sensitive adhesives include acrylic pressure-sensitive adhesives, epoxy pressure-sensitive adhesives, olefin pressure-sensitive adhesives, urethane pressure-sensitive adhesives, ester pressure-sensitive adhesives, and rubber pressure-sensitive adhesives. Among these, acrylic pressure-sensitive adhesives, epoxy pressure-sensitive adhesives, and olefin pressure-sensitive adhesives are preferred, with acrylic pressure-sensitive adhesives being more preferred. In the case of acrylic pressure-sensitive adhesives, by increasing the amount of acidic groups in the acrylic polymer or adding a certain proportion of a tackifier, it is possible to make the adhesive a so-called highly adhesive pressure-sensitive adhesive, which makes it easier to obtain various adhesive strengths and holding powers described below.

[0046] The thickness of the first pressure-sensitive adhesive layer 20 is not particularly limited, and may be, for example, 1 μm to 500 μm, 10 μm to 100 μm, or 15 μm to 50 μm. When the thickness of the first pressure-sensitive adhesive layer 20 is equal to or greater than the lower limit, the adhesive strength and holding power described below are more easily obtained. When the thickness of the first pressure-sensitive adhesive layer 20 is equal to or less than the upper limit, in-plane variations in thickness that occur when the pressure-sensitive adhesive layer is attached are prevented, and the designed electromagnetic wave absorption properties can be exhibited.

[0047] "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.

[0048] The material of the spacer layer 30 can be appropriately selected depending on the application, and examples thereof include plastic film, paper, cloth, nonwoven fabric, rubber sheet, foam sheet, etc. Among these, foam sheet is preferable from the viewpoint of facilitating weight reduction while increasing thickness. The foamed sheet may be, for example, a resin foam obtained by foaming the resin constituting the plastic film and forming it into a sheet. Specific examples of the resin foam include polyethylene foam, polypropylene foam, and polyurethane foam.

[0049] 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 the spacer layer 30 in the z-axis direction)=(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 the above formula (3).

[0050] 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 set appropriately depending on the wavelength λ of the electromagnetic wave to be absorbed. The thickness of the spacer layer 30 may be, for example, 25 μm to 5000 μm, 300 μm to 4000 μm, or 1000 μm to 3000 μm. 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 may contain a high-dielectric material. Examples of high-dielectric materials include barium titanate, titanium oxide, and strontium titanate.

[0051] "Reflective layer" The reflective layer 40 has two surfaces 40a and 40b. A second adhesive layer 51 is provided on one surface 40a of the reflective layer 40, and a third adhesive layer 52 is provided on the other surface 40b 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 with the second adhesive layer 51 interposed therebetween. The other surface 40b of the reflective layer 40 faces one surface 60a of the protective layer 60. The reflective layer 40 is laminated on the protective layer 60 with the third adhesive layer 52 interposed therebetween. 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.

[0052] The thickness of the reflective layer 40 is not particularly limited, but may be, for example, 0.01 μm to 1000 μm, 0.1 μm to 500 μm, or 1 μm to 100 μm.

[0053] "Adhesive layer (second adhesive layer, third adhesive layer)" Examples of adhesives constituting the second adhesive layer 51 and the third adhesive layer 52 include heat-seal type adhesives that bond by heat, adhesives that become adhesive when moistened, pressure-sensitive adhesives (adhesives) that bond by pressure, etc. 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.

[0054] The thickness of second pressure-sensitive adhesive layer 51 and third pressure-sensitive adhesive layer 52 is not particularly limited, but may be, for example, 1 μm to 1000 μm, 7 μm to 500 μm, or 15 μm to 100 μm.

[0055] "Protective layer" The protective layer 60 is not particularly limited as long as it can protect the reflective layer 40. For example, it may be a release film or a protective film.

[0056] In the electromagnetic wave absorbing member 1 of this embodiment, the holding strength of the first pressure-sensitive adhesive layer 20 to the spacer layer 30 after an alkali immersion test is 5,000 seconds or more, preferably 10,000 seconds or more, and more preferably 50,000 seconds or more. If the holding strength is 5,000 seconds or more, the resistance layer 10 does not partially peel off even after the alkali immersion test, and stable electromagnetic wave absorption properties can be maintained. Here, the holding strength refers to the holding strength of the pressure-sensitive adhesive layer 20 to the spacer layer 30 measured in accordance with JIS Z0237:2009 (ISO 29862-29864 2007).

[0057] The alkali immersion test of the electromagnetic wave absorbing member 1 is carried out by immersing it in a 10N aqueous sodium hydroxide solution for 7 hours, and then drying it for 24 hours.

[0058] In the electromagnetic wave absorbing member 1 of this embodiment, the holding strength of the first pressure-sensitive adhesive layer 20 to the spacer layer 30 after an acid immersion test is preferably 5000 seconds or more, more preferably 10000 seconds or more, and even more preferably 50000 seconds or more. If the holding strength is 5000 seconds or more, there is no partial peeling of the resistance layer 10 even after the acid immersion test, and stable electromagnetic wave absorption properties can be maintained.

[0059] The acid immersion test of the electromagnetic wave absorbing member 1 is carried out by immersing it in a 1N hydrochloric acid aqueous solution for 7 hours, and then drying it for 24 hours.

[0060] To measure the holding strength of the first pressure-sensitive adhesive layer 20 to the spacer layer 30 after the acid or alkali immersion test, the electromagnetic wave absorbing member is cut to a width of 25 mm and a length of 120 mm, a 20 mm length is peeled off at the interface between the spacer layer 30 and the first pressure-sensitive adhesive layer 20, a 1 kg weight is hung from the part of the peeled spacer layer 30 side, the surface of the resistance layer 10 of the electromagnetic wave absorbing member 1 is fixed to a wall perpendicular to the ground, and the time until the spacer layer 30 falls off is measured in an environment of 40°C and 50% RH. Other conditions for the measurement are in accordance with JIS Z0237:2009.

[0061] In the electromagnetic wave absorbing member 1 of this embodiment, the attenuation rate of the adhesive strength of the first pressure-sensitive adhesive layer 20 to the spacer layer 30 after the alkali immersion test relative to the adhesive strength of the first pressure-sensitive adhesive layer 20 to the spacer layer 30 before the alkali immersion test is preferably less than 20%, more preferably less than 10%, and even more preferably less than 5%. When the attenuation rate is less than the upper limit, it is effective in maintaining the radio wave absorbency after the alkali immersion test.

[0062] The adhesive strength of the first adhesive layer 20 to the spacer layer 30 is measured by cutting the electromagnetic wave absorbing member to a width of 25 mm and a length of 100 mm, peeling a 15 mm length at the interface between the spacer layer 30 and the first adhesive layer 20, setting the peeled resistance layer 10 and first adhesive layer 20 in a tensile tester, fixing the spacer layer 30 side, and measuring using the tensile tester under conditions of a peel speed of 300 mm / min and a peel angle of 180°.

[0063] The adhesive strength of the first pressure-sensitive adhesive layer 20 to the spacer layer 30, i.e., the adhesive strength of the first pressure-sensitive adhesive layer 20 to the spacer layer 30 before the alkali immersion test, is preferably 5 N / 25 mm or more, more preferably 6 N / 25 mm or more, and even more preferably 7 N / 25 mm or more. When the adhesive strength is equal to or greater than the lower limit, the adhesion is less likely to decrease even during alkali immersion. When the spacer layer 30 is the aforementioned resin foam, the surface has countless fine holes, which reduces the contact area of ​​the first pressure-sensitive adhesive layer 20, and the adhesive strength of the first pressure-sensitive adhesive layer 20 tends to be significantly lower than when measured on a SUS plate or a glass plate.

[0064] The adhesive strength of the first pressure-sensitive adhesive layer 20 to the spacer layer 30, i.e., the adhesive strength of the first pressure-sensitive adhesive layer 20 to the spacer layer 30 after the alkali immersion test, is preferably 5 N / 25 mm or more, more preferably 6 N / 25 mm or more, and even more preferably 7 N / 25 mm or more. When the adhesive strength is equal to or greater than the lower limit, the adhesion is less likely to decrease even during alkali immersion.

[0065] According to the electromagnetic wave absorbing member 1 of this embodiment, the holding strength of the first adhesive layer 20 to the spacer layer 30 after an alkali immersion test is 5000 seconds or more, and therefore the member has excellent durability in harsh environments where contact with acidic or alkaline droplets is expected. [Example]

[0066] 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.

[0067] [Example 1] "Preparation of adhesive layer" 50 parts by mass (expressed as a solid content equivalent value; the same applies hereinafter) of an acrylic acid ester copolymer (95% by mass of butyl acrylate units, 2% by mass of acrylic acid units, and 3% by mass of methyl methacrylate units) having a weight average molecular weight of 800,000 was blended with 50 parts by mass of a tackifier (manufactured by Arakawa Chemical Industries, Ltd., product name "Pine Crystal KE-359", rosin ester type, softening point 94°C to 104°C, acid value 10mgKOH / g to 20mgKOH / g) and 1 part by mass of an isocyanate crosslinker (manufactured by Toyo Ink Mfg. Co., Ltd., product name "BHS8515"), and the mixture was diluted and mixed with toluene to obtain a pressure-sensitive adhesive composition with a solid content concentration of 20% by mass. The obtained adhesive composition was applied using a knife coater to the release-treated surface of a release sheet (manufactured by Lintec Corporation, product name "SP-PET381031"), one side of which had been release-treated for polyethylene terephthalate film, and then dried at 100°C for 4 minutes. Thereafter, the composition was aged for 7 days in an environment of 23°C and 50% RH to form an adhesive layer A having a thickness of 20 μm.

[0068] "Fabrication of electromagnetic wave absorbing materials" A pattern of washable ink was printed on a substrate made of a 50 μm thick PET film (manufactured by Toyobo Co., Ltd., product name "PET50A4160"), and copper was vapor-deposited to a thickness of 100 nm to form a copper thin film. Thereafter, the ink was washed with water to remove the copper thin film on the ink, thereby patterning the copper thin film into a conductor pattern to obtain a resistive layer. The resulting resistive layer was laminated with the conductive pattern side facing the adhesive layer A, and a 2 mm thick urethane foam sheet (manufactured by Inoac Corporation, trade name "PORON HH-48") was laminated as a 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 the reflective layer was attached with the pressure-sensitive adhesive layer A so that the aluminum-deposited surface side thereof faced the spacer layer. This gave an electromagnetic wave absorbing member of Example 1, in which the substrate / conductor pattern / pressure-sensitive adhesive layer A / spacer layer / pressure-sensitive adhesive layer A / reflective layer were laminated in this order.

[0069] [Example 2] An electromagnetic wave absorbing member of Example 2 was produced in the same manner as in Example 1, except that the thickness of the adhesive layer A between the resistance layer and the spacer layer was changed from 20 μm to 10 μm.

[0070] [Example 3] An electromagnetic wave absorbing member of Example 3 was produced in the same manner as in Example 1, except that the thickness of the adhesive layer A between the resistance layer and the spacer layer was changed from 20 μm to 30 μm.

[0071] [Example 4] An electromagnetic wave absorbing member of Example 4 was produced in the same manner as in Example 1, except that the thickness of the adhesive layer A between the resistance layer and the spacer layer was changed from 20 μm to 40 μm.

[0072] [Comparative Example] An acrylic adhesive (manufactured by Saiden Chemical Co., Ltd., product name "AT-165L") was applied onto the release sheet, dried at 100°C for 4 minutes, and then cured in an environment of 23°C and 50% RH for 7 days to form an adhesive layer B with a thickness of 20 μm. An electromagnetic wave absorbing member of a comparative example was produced in the same manner as in Example 1, except that the obtained pressure-sensitive adhesive layer was used to bond the resistance layer and the spacer layer.

[0073] [evaluation] The electromagnetic wave absorbing members of Experimental Examples 1 to 4 and the Comparative Example were evaluated as follows. The results are shown in Table 1.

[0074] [Acid or alkali immersion test] The electromagnetic wave absorbing members obtained in Experimental Examples 1 to 4 and the Comparative Example were immersed in a 1N hydrochloric acid aqueous solution or a 10N sodium hydroxide aqueous solution for 7 hours, then dried for 24 hours, and subjected to adhesive strength tests, holding strength tests, and radio wave absorption tests.

[0075] [Adhesive strength] The electromagnetic wave absorbing members obtained in Experimental Examples 1 to 4 and the Comparative Example were cut to a width of 25 mm and a length of 100 mm before and after the acid or alkali immersion test, and a 15 mm length was peeled off at the interface between the spacer layer and the adhesive layer. The peeled resistance layer and adhesive layer were set in a tensile tester, and the spacer layer side was fixed, and the adhesive strength was measured using a tensile tester (manufactured by Orientec Co., Ltd., product name "Tensilon") at a peel speed of 300 mm / min and a peel angle of 180°.

[0076] [Holding force] The electromagnetic wave absorbing members obtained in Experimental Examples 1 to 4 and Comparative Example were cut into a width of 25 mm and a length of 120 mm before and after the acid or alkali immersion test. A 20 mm length was peeled off at the interface between the spacer layer and the adhesive layer. A 1 kg weight was hung from the peeled spacer layer side, and the surface of the resistance layer of the electromagnetic wave absorbing member was fixed to a wall perpendicular to the ground. The time until the spacer layer fell off was measured under an environment of 40°C and 50% RH. Measurements were conducted under conditions other than those described here in accordance with JIS Z0237:2009.

[0077] [Electromagnetic wave absorption] The S11 values ​​(reflection coefficients) at 79 GHz of the electromagnetic wave absorbing members obtained in Experimental Examples 1 to 4 and the Comparative Example were obtained using a network analyzer by the free space method, and the transmission characteristics of the electromagnetic wave absorbing members were evaluated. The evaluation was carried out as follows: (wave absorption amount of electromagnetic wave absorbing member after acid or alkali immersion) / (wave absorption amount of sample before acid or alkali immersion)×100(%) was rated as "○" if it was 90% or more and 100% or less, "△" if it was 60% or more and less than 90%, and "×" if it was less than 60%.

[0078] [Table 1]

[0079] The results shown in Table 1 show that the electromagnetic wave absorbing members of Examples 1 to 4 are excellent in adhesive strength, holding power, and electromagnetic wave resistance after immersion in acid or alkali. [Industrial Applicability]

[0080] 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]

[0081] 1. Electromagnetic wave absorbing material 10 resistance layer 11 Base material 12 Conductor pattern 20 Adhesive layer (first adhesive layer) 30 spacer layer 40 reflective layer 50 adhesive layer 51 Second adhesive layer 52 Third adhesive layer 60 protective layer 101 First conductor pattern 102 second conductive pattern 103 Third Conductive Pattern

Claims

1. a resistive layer having a conductive pattern, an adhesive layer, a spacer layer, and a reflective layer; the resistive layer, the adhesive layer, the spacer layer, and the reflective layer are laminated in this order; An electromagnetic wave absorbing member, wherein the holding power of the pressure-sensitive adhesive layer to the spacer layer after an alkali immersion test is 5000 seconds or more.

2. The electromagnetic wave absorbing member according to claim 1 , wherein the spacer layer is made of a resin foam.

3. 3. The electromagnetic wave absorbing member according to claim 1, wherein the attenuation rate of the adhesive strength of the adhesive layer to the spacer layer after an alkali immersion test is less than 20% of the adhesive strength of the adhesive layer to the spacer layer before the alkali immersion test.

4. 3. The electromagnetic wave absorbing member according to claim 1, wherein the adhesive strength of the adhesive layer to the spacer layer is 5 N / 25 mm or more.

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