Electromagnetic wave shielding laminated sheet

Abstract

The present disclosure provides an electromagnetic wave shielding laminated sheet that is lightweight but capable of suppressing transmission and reflection of electromagnetic waves. This electromagnetic wave shielding laminated sheet (1) includes: an electromagnetic wave absorption layer (10) which comprises a matrix and carbon nanotubes dispersed in said matrix and which has a bulk density of not more than 997 kg / m3; and a metal layer (30) which overlaps the electromagnetic wave absorption layer (10).

Description

Technical Field

[0001] The present disclosure relates to a laminated sheet for electromagnetic wave shielding, and more particularly to a laminated sheet for electromagnetic wave shielding preferably applied to applications for shielding electromagnetic waves.

Background Art

[0002] Patent Document 1 discloses an electromagnetic wave absorber having a laminated structure in which a first layer made of a dielectric, a second layer having conductivity, a third layer made of a dielectric, and a fourth layer having conductivity are laminated in this order, wherein the sheet resistance of the second layer is 100 Ω / sq or more and 300 Ω / sq or less, and the fourth layer is a reflector of electromagnetic waves.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

[0004] An object of the present disclosure is to provide a laminated sheet for electromagnetic wave shielding that is lightweight and can suppress transmission and reflection of electromagnetic waves.

[0005] A laminated sheet for electromagnetic wave shielding according to an aspect of the present disclosure includes an electromagnetic wave absorption layer having a matrix and carbon nanotubes dispersed in the matrix and having a bulk density of 997 kg / m 3 or less, and a metal layer overlapping the electromagnetic wave absorption layer.

Brief Description of the Drawings

[0006]

Figure 1

Figure 2

Best Mode for Carrying Out the Invention

[0007] An outline of the process by which the inventor arrived at the present disclosure will be described.

[0008] In information communication using electronic devices, weight reduction of carriers and the like that carry electronic devices used in the aviation field, space field, and on the ground has become an essential issue from the viewpoint of fuel consumption reduction.

[0009] In the above applications, shielding electromagnetic waves generated from electronic devices is important for preventing malfunction of electronic devices due to noise, controlling the propagation path of electromagnetic waves during communication using electromagnetic waves between electronic devices, etc. In some cases, it is necessary to suppress not only the transmission of electromagnetic waves but also the reflection of electromagnetic waves.

[0010] However, a practical material that is lightweight and can suppress both the transmission and reflection of electromagnetic waves at a high level has not yet been provided.

[0011] Therefore, the inventor has completed the present disclosure in order to provide a laminated sheet for electromagnetic wave shielding that is lightweight and can suppress the transmission and reflection of electromagnetic waves.

[0012] Hereinafter, an embodiment of the present disclosure will be described. Note that the embodiment described below is only one of various embodiments of the present disclosure. The following embodiment can be variously modified according to the design as long as the object of the present disclosure can be achieved.

[0013] The laminated sheet 1 for electromagnetic wave shielding according to this embodiment includes a plurality of laminated layers. The plurality of layers includes at least an electromagnetic wave absorption layer 10 and a metal layer 30. That is, the laminated sheet 1 for electromagnetic wave shielding includes an electromagnetic wave absorption layer 10 and a metal layer 30 that overlaps the electromagnetic wave absorption layer 10. The electromagnetic wave absorption layer 10 has a matrix and carbon nanotubes dispersed in the matrix, and the bulk density is 997 kg / m 3 The following.

[0014] According to this embodiment, the bulk density of the electromagnetic wave absorption layer 10 is 997 kg / m 3 By being below, the weight reduction of the electromagnetic wave shielding laminated sheet 1 is realized. Further, when electromagnetic waves are incident on the electromagnetic wave shielding laminated sheet 1 in the direction from the electromagnetic wave absorption layer 10 side to the metal layer 30 side, the electromagnetic waves are shielded by the metal layer 30, so that it is difficult for the electromagnetic waves to pass through the electromagnetic wave shielding laminated sheet 1. Furthermore, the electromagnetic waves reflected by the metal layer 30 are likely to be multiply reflected between the metal layer 30 and the electromagnetic wave absorption layer 10 within the electromagnetic wave shielding laminated sheet 1, and therefore the electromagnetic waves are likely to be attenuated. For this reason, it is difficult for electromagnetic waves to be reflected from the electromagnetic wave shielding laminated sheet 1.

[0015] Therefore, in this embodiment, it is possible to realize the electromagnetic wave shielding laminated sheet 1 that is lightweight and can suppress the transmission and reflection of electromagnetic waves.

[0016] The configuration of the electromagnetic wave shielding laminated sheet 1 will be described more specifically.

[0017] As described above, the electromagnetic wave shielding laminated sheet 1 includes a plurality of laminated layers, and the plurality of layers include at least the electromagnetic wave absorption layer 10 and the metal layer 30. The plurality of layers may include only the electromagnetic wave absorption layer 10 and the metal layer 30, and in that case, the electromagnetic wave absorption layer 10 and the metal layer 30 may be laminated so as to be in direct contact with each other. The plurality of layers may include, for example, an insulating layer 20, and in that case, the insulating layer 20 may be interposed between the electromagnetic wave absorption layer 10 and the metal layer 30. That is, the electromagnetic wave absorption layer 10, the insulating layer 20, and the metal layer 30 may be laminated in this order.

[0018] The metal layer 30 is, for example, a metal sheet. The material of the metal layer 30 is, for example, aluminum or copper, but is not limited to these. The metal layer 30 may be a thin film formed by, for example, a vapor deposition method or the like, or may be a metal structural material.

[0019] The thickness of the metal layer 30 is appropriately adjusted so as to obtain appropriate electromagnetic wave shielding ability, and is, for example, 1 nm or more. Further, although the thickness of the metal layer 30 is, for example, 100 mm or less, the thickness of the metal layer 30 may be greater than 100 mm.

[0020] As described above, the electromagnetic wave absorption layer 10 has a bulk density of 997 kg / m 3 By being as described below, the electromagnetic wave shielding laminated sheet 1 can be lightened. Further, the electromagnetic wave absorption layer 10 has a matrix and carbon nanotubes dispersed in the matrix as described above, so that multiple reflection of electromagnetic waves can occur between the metal layer 30 and the electromagnetic wave absorption layer 10, and thereby the electromagnetic waves can be attenuated. Here, the matrix means a matrix in the field of composite materials, and the carbon nanotubes are dispersed in the matrix, so that the matrix and the carbon nanotubes are composite.

[0021] The electromagnetic wave absorption layer 10 preferably has a porous structure. In this case, it is easy to realize that the bulk density of the electromagnetic wave absorption layer 10 is 997 kg / m 3 or less. The porous structure is a structure having voids inside. Examples of the porous structure include a structure in which voids that are long in the thickness direction of the electromagnetic wave absorption layer 10 are arranged in a honeycomb shape. In this case, although the electromagnetic wave absorption layer 10 is porous, it is less likely to break when a load is applied in the thickness direction. Note that the porous structure of the electromagnetic wave absorption layer 10 is not limited to this.

[0022] The matrix of the electromagnetic wave absorption layer 10 preferably contains a water-soluble polymer. In this case, by producing the electromagnetic wave absorption layer 10 by a freeze-drying method as described later, it is easy to lighten the electromagnetic wave absorption layer 10, and it is easy to realize that the bulk density of the electromagnetic wave absorption layer 10 is 997 kg / m 3 or less. The bulk density of the electromagnetic wave absorption layer 10 is more preferably 500 kg / m 3 or less, and even more preferably 100 kg / m 3 or less.

[0023] The solubility of the water-soluble polymer in water at 25°C is preferably 1 mg / 1 g-H2O or more. The water-soluble polymer may be any of natural polymers, semi-synthetic polymers, and synthetic polymers. The water-soluble polymer contains at least one selected from the group consisting of, for example, guar gum, carrageenan, sodium alginate, corn starch, xanthan gum, chondroitin sulfate sodium, sodium hyaluronate, carboxymethyl cellulose, sodium carboxymethyl cellulose, methyl cellulose, hydroxyethyl cellulose, hydroxypropyl methyl cellulose, cationized guar gum, carboxyvinyl polymer, polyacrylic acid, polyvinyl pyrrolidone, and polyvinyl alcohol. It is preferable that the water-soluble polymer contains at least one of carboxymethyl cellulose and hydroxypropyl methyl cellulose.

[0024] The matrix preferably further contains a cured product of a thermosetting resin. In this case, when the electromagnetic wave absorption layer 10 comes into contact with water, the water-soluble polymer in the matrix is less likely to elute. The thermosetting resin preferably has the property of forming an aqueous emulsion when dispersed in water. In this case, when the electromagnetic wave absorption layer 10 is produced by the freeze-drying method, the thermosetting resin is easily dispersed in the matrix of the electromagnetic wave absorption layer 10, and therefore the water-soluble polymer in the matrix can be made even less likely to elute into water.

[0025] The thermosetting resin preferably contains an epoxy resin. When the thermosetting resin contains an epoxy resin, the thermosetting resin preferably further contains a curing agent for the epoxy resin, if necessary. In this case, when the electromagnetic wave absorption layer 10 comes into contact with water, the water-soluble polymer in the matrix is even less likely to elute. The epoxy resin, or the epoxy resin and the curing agent preferably have the property of forming an aqueous emulsion when dispersed in water. In this case, when the electromagnetic wave absorption layer 10 is produced by the freeze-drying method, the epoxy resin is easily dispersed in the matrix of the electromagnetic wave absorption layer 10, and therefore the water-soluble polymer in the matrix can be made even less likely to elute into water.

[0026] The compound contained in the epoxy resin is appropriately selected according to the method for manufacturing the electromagnetic wave absorption layer 10 or the like. For example, the epoxy resin contains an emulsion-type epoxy resin called an aqueous epoxy resin or an epoxy resin soluble in an organic solvent.

[0027] The emulsion-type epoxy resin preferably contains at least one selected from the group consisting of bisphenol A-type epoxy resin and bisphenol F-type epoxy resin.

[0028] The epoxy resin soluble in an organic solvent preferably contains, for example, an epoxy resin used for laminate applications. Specifically, for example, it preferably contains at least one selected from the group consisting of bisphenol A-type epoxy resin, bisphenol F-type epoxy resin, brominated bisphenol A-type epoxy resin, naphthalene-type epoxy resin, biphenyl-type epoxy resin, and cresol novolak-type epoxy resin.

[0029] The curing agent may be a general curing agent for epoxy resin, and contains, for example, at least one selected from the group consisting of phenolic curing agents, amine curing agents, and acid anhydride curing agents.

[0030] The ratio of the thermosetting resin to the electromagnetic wave absorption layer 10 is preferably 5% by mass or more and 90% by mass or less. Further, when the thermosetting resin contains an epoxy resin, or an epoxy resin and a curing agent, the ratio of the epoxy resin, or the epoxy resin and the curing agent to the electromagnetic wave absorption layer 10 is preferably 5% by mass or more and 90% by mass or less. When these ratios are 5% by mass or more, the water-soluble polymer in the matrix can be made more difficult to elute into water. Also, when these ratios are 90% by mass or less, the thermosetting resin is less likely to inhibit the weight reduction of the electromagnetic wave absorption layer 10.

[0031] The carbon nanotubes in the electromagnetic wave absorption layer 10 will be described.

[0032] Since the electromagnetic wave absorption layer 10 contains carbon nanotubes, electromagnetic waves can be attenuated well within the laminated sheet 1 for electromagnetic wave shielding, and in particular, electromagnetic waves in the millimeter wave band and the microwave band can be efficiently attenuated.

[0033] The carbon nanotubes (hereinafter also referred to as CNTs) include at least one of, for example, single-walled carbon nanotubes and multi-walled carbon nanotubes.

[0034] The G / D ratio in the Raman spectrum of CNTs is an index generally used to evaluate the quality of CNTs. In the Raman spectrum of CNTs measured by a Raman spectrometer, vibration modes called the G band (around 1600 cm -1 -1) and the D band (around 1350 cm -1 -1) are observed. The G band is a vibration mode derived from the hexagonal lattice structure of graphite, which is the cylindrical surface of CNTs, and the D band is a vibration mode derived from amorphous portions. Therefore, the higher the peak intensity ratio (G / D ratio) of the G band and the D band, the higher the crystallinity of the CNTs can be evaluated. The higher the G / D, the more the CNTs have a conductive graphite structure and the higher the conductivity. For this reason, the electromagnetic wave absorption ability in the high-frequency region of the laminated sheet 1 for electromagnetic wave shielding can be improved with a smaller addition amount of CNTs, which is advantageous from the perspective of cost. Furthermore, since a smaller addition amount of CNTs is sufficient, a laminated sheet 1 for electromagnetic wave shielding with excellent moldability can be obtained. The G / D ratio is preferably 3 or more.

[0035] Also, the CNTs preferably contain carbon nanotubes from single layer to five layers, and more preferably contain single-walled carbon nanotubes. If the CNTs are single-walled carbon nanotubes, the electromagnetic wave absorption ability in the high-frequency region of the laminated sheet 1 for electromagnetic wave shielding can be improved with a smaller addition amount compared to multi-walled carbon nanotubes.

[0036] The average diameter (Av) of the CNT is preferably 0.5 nm or more, more preferably 1 nm or more, preferably 15 nm or less, and more preferably 10 nm or less. If the average diameter (Av) of the CNT is 0.5 nm or more, the electromagnetic wave absorption ability in the high-frequency region of the electromagnetic wave shielding laminated sheet 1 can be further enhanced. Further, if it is 15 nm or less, since it is flexible, the electromagnetic wave absorption ability can be maintained even when the electromagnetic wave shielding laminated sheet 1 is bent.

[0037] Incidentally, the average diameter (Av) can be obtained by measuring the CNT using a transmission electron microscope. The average diameter (Av) may be adjusted by changing the manufacturing method and manufacturing conditions of the CNT, or may be adjusted by combining a plurality of types of CNTs obtained by different manufacturing methods.

[0038] The BET specific surface area of the CNT is preferably 600 m 2 / g or more, more preferably 800 m 2 / g or more, preferably 2500 m 2 / g or less, and more preferably 1200 m 2 / g or less. If the BET specific surface area is 600 m 2 / g or more, the electromagnetic wave absorption ability of the electromagnetic wave shielding laminated sheet 1 in the high-frequency region can be further enhanced. If the BET specific surface area is 2500 m 2 / g or less, the moldability of the electromagnetic wave absorption layer 10 can be enhanced.

[0039] In this specification, the "BET specific surface area" refers to the nitrogen adsorption specific surface area measured using the BET method.

[0040] The CNT can be obtained, for example, according to the super growth method described later, as a CNT aggregate (oriented aggregate) oriented in a direction substantially perpendicular to the substrate on a substrate having a catalyst layer for carbon nanotube growth on the surface. In this case, the mass density of the oriented aggregate is 0.002 g / cm 3 or more and 0.2 g / cm 3The following is preferable. When the mass density is 0.2 g / cm 3 or less, the CNTs can be homogeneously dispersed, and the electromagnetic wave absorption ability in the high-frequency region of the electromagnetic wave shielding laminated sheet 1 can be further enhanced. Further, when the mass density is 0.002 g / cm 3 or more, the dispersion of the CNTs can be suppressed, making handling easier.

[0041] The CNTs preferably exhibit a convex upward shape in the t-plot obtained from the adsorption isotherm. Among them, it is more preferable that the CNTs have not been subjected to opening treatment and the t-plot exhibits a convex upward shape. In this case, the electromagnetic wave absorption ability in the high-frequency region of the electromagnetic wave shielding laminated sheet 1 can be further enhanced.

[0042] Note that the "t-plot" can be obtained by converting the relative pressure measured by the nitrogen gas adsorption method into the average thickness "t" (nm) of the nitrogen gas adsorption layer. That is, it is obtained by plotting the average thickness "t" of the nitrogen gas adsorption layer against the relative pressure P / P0, and determining the average thickness "t" of the nitrogen gas adsorption layer corresponding to the relative pressure from a known standard isotherm and performing the above conversion (t-plot method by de Boer et al.).

[0043] Here, for a substance having pores on its surface, the growth of the nitrogen gas adsorption layer is classified into the following processes (1) to (3). And, due to the following processes (1) to (3), a change occurs in the slope of the t-plot. (1) Process of forming a monomolecular adsorption layer of nitrogen molecules on the entire surface. (2) Process of forming a multimolecular adsorption layer and capillary condensation filling in the pores accompanying it. (3) Process of forming a multimolecular adsorption layer on the apparently non-porous surface where the pores are filled with nitrogen.

[0044] And, the t-plot showing a convex shape upwards is such that in a region where the average thickness “t” of the nitrogen gas adsorption layer is small, the plot is positioned on a straight line passing through the origin, whereas when “t” increases, the plot is shifted downward from the straight line. A structure having such a t-plot shape indicates that the ratio of the internal specific surface area to the total specific surface area is large and that a large number of openings are formed in the CNT.

[0045] Note that the inflection point of the t-plot of the CNT is preferably in the range satisfying 0.2 ≦ t (nm) ≦ 1.5, more preferably in the range of 0.45 ≦ t (nm) ≦ 1.5, and still more preferably in the range of 0.55 ≦ t (nm) ≦ 1.0.

[0046] Note that the “position of the inflection point” is the intersection point of the approximate straight line A in the process (1) described above and the approximate straight line B in the process (3) described above.

[0047] For the CNT, the ratio (S2 / S1) of the internal specific surface area S2 to the total specific surface area S1 obtained from the t-plot is preferably 0.05 or more and 0.30 or less.

[0048] Also, the total specific surface area S1 and the internal specific surface area S2 of the CNT are not particularly limited, but individually, S1 is preferably 600 m 2 / g or more and 1400 m 2 / g or less, more preferably 800 m 2 / g or more and 1200 m 2 / g or less. On the other hand, S2 is preferably 30 m 2 / g or more and 540 m 2 / g or less.

[0049] Here, the total specific surface area S1 and the internal specific surface area S2 of the CNT can be determined from the t-plot of the CNT. Specifically, first, the total specific surface area S1 can be determined from the slope of the approximate straight line in the process (1), and the external specific surface area S3 can be determined from the slope of the approximate straight line in the process (3). Then, the internal specific surface area S2 can be calculated by subtracting the external specific surface area S3 from the total specific surface area S1.

[0050] Incidentally, the measurement of the adsorption isotherm of CNTs, the creation of the t-plot, and the calculation of the total specific surface area S1 and the internal specific surface area S2 based on the analysis of the t-plot can be performed, for example, using a commercially available measuring device, "BELSORP (registered trademark)-mini" (manufactured by Nippon Bell Co., Ltd.).

[0051] The CNT having the above-described properties can be efficiently produced, for example, by supplying a raw material compound and a carrier gas onto a substrate having a catalyst layer for producing carbon nanotubes on its surface and allowing a trace amount of an oxidizing agent (catalyst activating substance) to be present in the system when synthesizing CNTs by chemical vapor deposition (CVD method), thereby dramatically improving the catalytic activity of the catalyst layer (super growth method; see International Publication No. 2006 / 011655). The formation of the catalyst layer on the substrate surface is preferably performed by a wet process. Hereinafter, the carbon nanotubes obtained by the super growth method may be referred to as "SGCNT".

[0052] Note that the CNTs produced by the super growth method may be composed only of SGCNT, or may be composed of SGCNT and a non-cylindrical carbon nanostructure. Specifically, the CNTs may include a single-layer or multi-layer flat cylindrical carbon nanostructure (hereinafter, may be referred to as "graphene nanotape (GNT)") having a tape-like portion where the inner walls are close to or adhered to each other over the entire length.

[0053] In the present disclosure, "having a tape-like portion over the entire length" means "having a tape-like portion continuously or intermittently over 60% or more, preferably 80% or more, more preferably 100% of the length (total length) in the longitudinal direction".

[0054] The shape of the GNT is a flat cylindrical shape, and the presence of a tape-shaped portion in the GNT where the inner walls are close to or adhered to each other can be confirmed, for example, by observing a fullerene-inserted GNT obtained by sealing GNT and fullerene (C60) in a quartz tube and performing heat treatment (fullerene insertion treatment) under reduced pressure with a transmission electron microscope (TEM). It can be confirmed from the presence of a portion where fullerene is not inserted (tape-shaped portion) in the GNT.

[0055] The sheet resistance of the electromagnetic wave absorption layer 10 is preferably 0.01 Ω / sq. or more and 100 Ω / sq. or less. In this case, electromagnetic waves are particularly likely to be attenuated within the electromagnetic wave absorption layer 10.

[0056] The ratio of carbon nanotubes in the electromagnetic wave absorption layer 10 is appropriately set in consideration of the resistivity etc. of the electromagnetic wave absorption layer 10. For example, it is 5 mass% or more and 60 mass% or less with respect to the electromagnetic wave absorption layer 10. If this ratio is 10 mass% or more, it is more preferable, and if it is 20 mass% or more, it is even more preferable. Also, if this ratio is 50 mass% or less, it is more preferable, and if it is 40 mass% or less, it is even more preferable.

[0057] The electromagnetic wave absorption layer 10 may contain amorphous carbon such as carbon black and acetylene black. In this case, the sheet resistance of the electromagnetic wave absorption layer 10 can be adjusted by the amorphous carbon. The ratio of the amorphous carbon is, for example, 10 mass% or more and 50 mass% or less with respect to the electromagnetic wave absorption layer 10.

[0058] The electromagnetic wave absorption layer 10 may contain cellulose nanofibers. In this case, the sheet resistance of the electromagnetic wave absorption layer 10 can be adjusted without impairing the structure and strength of the electromagnetic wave absorption layer 10. The ratio of the cellulose nanofibers is 5 mass% or more and 60 mass% or less with respect to the electromagnetic wave absorption layer 10. If this ratio is 5 mass% or more, the sheet resistance of the electromagnetic wave absorption layer 10 can be adjusted, and if it is 60 mass% or less, it is difficult to inhibit the weight reduction of the electromagnetic wave absorption layer 10.

[0059] The thickness of the electromagnetic wave absorption layer 10 is appropriately set according to the wavelength of the electromagnetic waves to be shielded by the laminated sheet 1 for electromagnetic wave shielding and the like. If the thickness of the electromagnetic wave absorption layer 10 is about 1 / 4 of the wavelength of the electromagnetic waves to be shielded by the laminated sheet 1 for electromagnetic wave shielding, the laminated sheet 1 for electromagnetic wave shielding can efficiently shield the electromagnetic waves. Also, even if the thickness of the electromagnetic wave absorption layer 10 is regulated by the wavelength of the electromagnetic waves in this way, as long as the bulk density of the electromagnetic wave absorption layer 10 is 997 kg / m 3 The following makes it easy to achieve weight reduction of the laminated sheet 1 for electromagnetic wave shielding. The thickness of the electromagnetic wave absorption layer 10 is preferably 0.1 mm or more and 100 mm or less, more preferably 0.5 mm or more and 10 mm or less, and even more preferably 1 mm or more and 5 mm or less.

[0060] The electromagnetic wave absorption layer 10 is produced, for example, by freeze-drying an aqueous dispersion containing the raw material of the electromagnetic wave absorption layer 10. In this case, it is easy to make the electromagnetic wave absorption layer 10 porous and reduce the bulk density.

[0061] For example, the electromagnetic wave absorption layer 10 is a freeze-dried product formed by freeze-drying an aqueous dispersion containing carbon nanotubes and a water-soluble polymer. The aqueous dispersion is prepared, for example, by mixing a water-soluble polymer, carbon nanofibers, and water. The proportion of water in the aqueous dispersion is appropriately set according to the strength and bulk density required for the electromagnetic wave absorption layer 10, etc., but is, for example, 80% by weight or more and 99.9% by weight or less. The conditions for freeze-drying are appropriately set, but for example, by freezing the aqueous dispersion at a temperature of -80°C to -60°C and then drying it under the conditions of a temperature of -50°C or more and -40°C or less and an absolute pressure of 5 Pa or more and 50 Pa or less, the electromagnetic wave absorption layer 10 can be produced.

[0062] When freezing the aqueous dispersion, the aqueous dispersion may be put into a container and the side and bottom surfaces of this container may be covered with a heat insulating material, and then the aqueous dispersion may be frozen. In this case, the aqueous dispersion in the container can be frozen in order from the upper part to the lower part. Then, since the molecules of the water-soluble resin are likely to be oriented in the vertical direction, the electromagnetic wave absorption layer 10 is likely to have a porous structure in which long voids are arranged in a honeycomb shape in the thickness direction.

[0063] When the matrix of the electromagnetic wave absorption layer 10 contains a cured product of a thermosetting resin, the electromagnetic wave absorption layer 10 is, for example, a lyophilized product formed by lyophilizing an aqueous dispersion containing carbon nanotubes, a water-soluble polymer, and a thermosetting resin. The aqueous dispersion is prepared, for example, by mixing a water-soluble polymer, carbon nanofibers, an aqueous emulsion of a thermosetting resin, and water. By lyophilizing this aqueous dispersion in the same manner as described above, the electromagnetic wave absorption layer 10, which is a lyophilized product, can be produced. In this method, when the thermosetting resin contains an epoxy resin, the epoxy resin preferably contains an emulsion-type epoxy resin called the above-described aqueous epoxy resin.

[0064] When the matrix of the electromagnetic wave absorption layer 10 contains a cured product of a thermosetting resin, the electromagnetic wave absorption layer 10 may be formed by impregnating a lyophilized product formed by lyophilizing an aqueous dispersion containing, for example, carbon nanotubes and a water-soluble polymer with a thermosetting resin and then curing the thermosetting resin. In this case, the lyophilized product can be produced in the same manner as in the case of producing the electromagnetic wave absorption layer 10 that does not contain a thermosetting resin. When impregnating the lyophilized product with a thermosetting resin, for example, a varnish containing a thermosetting resin is prepared by impregnating a thermosetting resin into an organic solvent such as methyl ethyl ketone. By impregnating this varnish into the lyophilized product, the thermosetting resin can be impregnated into the lyophilized product. Subsequently, the organic solvent in the varnish is volatilized by drying the lyophilized product impregnated with the varnish. Subsequently, the thermosetting resin is cured by heating the lyophilized product. The conditions for heating the lyophilized product are set to appropriate conditions according to the composition of the thermosetting resin so that the curing reaction of the thermosetting resin proceeds. Thereby, the electromagnetic wave absorption layer 10 can be produced. In this method, when the thermosetting resin contains an epoxy resin, the epoxy resin preferably contains an epoxy resin that is soluble in the above-described organic solvent.

[0065] The electromagnetic wave absorption layer 10 and the metal layer 30 may be in direct contact as described above, or an insulating layer 20 may be interposed between the electromagnetic wave absorption layer 10 and the metal layer 30. The insulating layer 20 is a layer having electrical insulation properties.

[0066] When the electromagnetic wave shielding laminated sheet 1 has the insulating layer 20, the relative permittivity of the insulating layer 20 is preferably lower than the relative permittivity of the electromagnetic wave absorption layer 10. In this case, multiple reflections of electromagnetic waves in the electromagnetic wave shielding laminated sheet 1 are particularly likely to occur, and the inside of the electromagnetic wave shielding laminated sheet 1 is more likely to attenuate electromagnetic waves. Therefore, reflection of electromagnetic waves by the electromagnetic wave shielding laminated sheet 1 is particularly suppressed. The ratio of the relative permittivity of the electromagnetic wave absorption layer 10 at 25°C to the relative permittivity of the insulating layer 20 at 25°C is preferably 1 or more and 50 or less.

[0067] The insulating layer 20 is made of, for example, a resin having electrical insulation properties. The resin having electrical insulation properties preferably contains a fluororesin such as polytetrafluoroethylene. In this case, it is easy to lower the relative permittivity of the insulating layer 20.

[0068] The insulating layer 20 may further contain a filler composed of at least one of resin particles and ceramic particles. In this case, physical properties such as the hardness and coefficient of linear expansion of the insulating layer 20 can be adjusted by the filler. In this case, for example, the insulating layer 20 can be produced by kneading a resin having electrical insulation properties and a filler and forming the kneaded product into a sheet shape.

[0069] The insulating layer 20 may contain an adhesive such as an epoxy resin-based adhesive. In this case, the electromagnetic wave absorption layer 10 and the metal layer 30 can be adhered by the insulating layer 20.

[0070] The insulating layer 20 may contain silicone rubber. In this case, the insulating layer 20 is less likely to inhibit the flexibility of the electromagnetic wave shielding laminated sheet 1, and therefore the electromagnetic wave shielding laminated sheet 1 is less likely to be damaged even when bent.

[0071] The material of the insulating layer 20 is not limited to the above, and the insulating layer 20 can contain an appropriate resin having electrical insulation properties such as polycarbonate.

[0072] It is also preferable that the breakdown voltage of the insulating layer 20 is 5 V / μm or more. In this case, for example, even when the electromagnetic wave shielding laminated sheet 1 is used around a 5G base station, it is possible to make it difficult for the insulating layer 20 to undergo dielectric breakdown.

[0073] Due to the above configuration, the electromagnetic wave shielding laminated sheet 1 according to the present embodiment is lightweight and can suppress the transmission and reflection of electromagnetic waves, and particularly can efficiently suppress the transmission and reflection of electromagnetic waves in the microwave band to the millimeter wave band. Therefore, the electromagnetic wave shielding laminated sheet 1 can be applied to suppress the transmission and reflection of electromagnetic waves, for example, electromagnetic waves in the range of 3 GHz to 10 GHz, which is the communication frequency of ultra-wideband (UWB) wireless communication inside an artificial satellite, and electromagnetic waves in the 28 GHz band and 39 GHz band, etc., which are frequencies around the communication frequency of 5G communication. In these cases, it is possible to make the attenuation amount when the electromagnetic wave passes through the electromagnetic wave shielding laminated sheet 1 and the attenuation amount when the electromagnetic wave is reflected by the electromagnetic wave shielding laminated sheet 1 be -10 dB or less, and it is also possible to make it -20 dB or less.

[0074] For example, the electromagnetic wave shielding laminated sheet 1 can be applied to artificial satellite applications to shield noise generated from a coil for wireless power supply, or to control the signal wave transmission path when performing communication between electronic devices wirelessly instead of wired for weight reduction.

Examples

[0075] Specific examples of the present embodiment are presented below. Note that the present embodiment is not limited only to the following examples.

[0076] 1. Fabrication of the electromagnetic wave absorption layer (Examples 1 to 16) The mixture obtained by adding the materials shown in Tables 1 to 4 to distilled water was sonicated using a sonication horn at an output of 135 W for 3 minutes, and this was repeated three times to prepare an aqueous dispersion with a solid content of 5% by mass. This aqueous dispersion was poured into a mold with dimensions of 180 mm × 180 mm in plan view and a depth of 10 mm, and cooled in a freezer at -80°C for 2 hours to freeze the aqueous dispersion. Subsequently, the frozen aqueous dispersion (hereinafter referred to as the frozen product) was freeze-dried using a freeze dryer. At this time, the absolute atmospheric pressure of the atmosphere around the frozen product was adjusted in the range of 10 Pa to 20 Pa, the temperature was adjusted in the range of -45°C to -50°C, the cold trap temperature was set to -45°C, and the frozen product was freeze-dried over 24 hours. Subsequently, in the case of Examples 13 to 16, in order to cure the epoxy resin in Resin Composition A, the frozen product was further heated at 80°C for 3 hours. Thereby, an electromagnetic wave absorption layer was produced. This electromagnetic wave absorption layer was removed from the mold.

[0077] 2. Preparation of Electromagnetic Wave Absorption Layer (Examples 17 to 20) A substrate having the same structure as the electromagnetic wave absorption layer of Example 2 was prepared under the same conditions as the preparation conditions of the electromagnetic wave absorption layer of Example 2.

[0078] The substrate was impregnated with a varnish by immersing the substrate in a varnish obtained by diluting Resin Composition B with methyl ethyl ketone. Subsequently, the substrate impregnated with the varnish was dried by heating at 100°C for 30 minutes, and then cured by heating at 180°C for 2 hours to produce an electromagnetic wave absorption layer. The ratios of the materials shown in Table 5 indicate the mass ratios of the material of the substrate used to produce the electromagnetic wave absorption layer and Resin Composition B.

[0079] The details of the materials shown in the table are as follows. - Single-walled carbon nanotube 1: Manufactured by ZEON NANO TECHNOLOGY Co., Ltd. Product name ZEONANO (registered trademark) SG101. BET specific surface area 1,050 m 2 / g. G / D ratio 3.7. The average diameter (Av) of 3.3 nm obtained from the results of measuring the diameters of 100 randomly selected single-walled carbon nanotube 1 particles using a transmission electron microscope. The t-plot obtained from the adsorption isotherm shows a convex shape upward. - Single-walled carbon nanotube 2: Manufactured by Meijo Nano Carbon Co., Ltd., product number EC2.0. BET specific surface area 500 m 2 / g. G / D ratio 37. Average diameter (Av) 2.0 nm obtained from the results of measuring the diameters of randomly selected 100 single-walled carbon nanotube 2 particles using a transmission electron microscope. - Cellulose nanofiber: Manufactured by Daiichi Kogyo Seiyaku Co., Ltd., product name Raycrysta I-2SX. - Sodium carboxymethyl cellulose: Manufactured by Fujifilm Wako Pure Chemical Corporation. - Acetylene black: Manufactured by Denka Co., Ltd., product name Denka Black. ― Resin composition A: A mixture obtained by mixing an aqueous epoxy resin (product name 1155R55) manufactured by Mitsubishi Chemical Corporation and a curing agent for aqueous epoxy (product name WD11M60) manufactured by Mitsubishi Chemical Corporation at a mass ratio of 100:53. ― Resin composition B: A mixture obtained by mixing a methyl ethyl ketone solution of a brominated bisphenol A type epoxy resin (product name 153-60M) manufactured by DIC Corporation and an epoxy curing agent (product name dicyandiamide) manufactured by Nippon Carbide Industries Co., Ltd. at a mass ratio of 100:3.

[0080] The bulk density, volume resistivity in the thickness direction, sheet resistance and relative permittivity of the electromagnetic wave absorption layer are shown in the table.

[0081] The bulk density was calculated from the measurement results of the outer dimensions of the electromagnetic wave absorption layer and the measurement results of the mass of the electromagnetic wave absorption layer.

[0082] When measuring the volume resistivity and sheet resistance in the thickness direction, when the resistance value of the electromagnetic wave absorption layer is 10 4 Ω or more and 10 13 Ω or less, a high resistivity meter by the double ring method (manufactured by Mitsubishi Chemical Analytech Co., Ltd., "HiRester (registered trademark) MCP-HT800", probe: URS probe) was used to measure the resistance value (Ω) at room temperature in accordance with JIS K 6911. Also, when the resistance value of the electromagnetic wave absorption layer is 10 -2 Ω or more and 10 4When it is less than Ω, the resistance value (Ω) at room temperature was measured in accordance with JIS K 7194 using a low resistivity meter by the four-probe method (manufactured by Mitsubishi Chemical Analytic Co., Ltd., "Loresta (registered trademark) MCP-610T", probe: ASP probe). Also, in the measurement, three square test pieces of 100 mm × 100 mm were cut out from the electromagnetic wave absorption layer as measurement samples, the probe was pressed against the center position of each measurement sample, and the resistance value (Ω) was measured. The resistance values (Ω) of the three measurement samples were measured, and the average value was taken as the resistance value (Ω) of the electromagnetic wave absorption material. From the obtained resistance value (Ω) and the sample dimensions, the volume resistivity (Ω·cm) and sheet resistance (Ω / sq.) were calculated according to the conventional method.

[0083] The relative permittivity was measured by the coaxial tube method.

[0084] 2. Fabrication of laminated sheet for electromagnetic wave shielding An electromagnetic wave shielding laminated sheet was fabricated by laminating an electromagnetic wave absorption layer and a metal layer, or an electromagnetic wave absorption layer, an insulating layer, and a metal layer in the combinations shown in Tables 1 to 5.

[0085] Note that the volume resistivity and sheet resistance in the thickness direction of each of the insulating layer and the metal layer, and the relative permittivity of the insulating layer were measured in the same manner as in the case of the electromagnetic wave absorption layer.

[0086] 3. Evaluation test (1) Density The measurement results of the density of the electromagnetic wave shielding laminated sheet are shown in the table.

[0087] (2) Flexibility An electromagnetic wave shielding laminated sheet was subjected to a 180° bending test by the cylindrical mandrel method using a mandrel with a radius of 10 mm. As a result, when the electromagnetic wave shielding laminated sheet could be wound around the mandrel and there was no damage to the electromagnetic wave shielding laminated sheet, it was evaluated as "good", and when the electromagnetic wave shielding laminated sheet could be wound around the mandrel but the electromagnetic wave shielding laminated sheet was damaged or the electromagnetic wave shielding laminated sheet could not be wound around the mandrel, it was evaluated as "poor".

[0088] (3) Water resistance The laminated sheet for electromagnetic wave shielding was immersed in purified water at 23°C ± 5°C for 30 minutes and then taken out. As a result, when there was neither dissolution nor breakage in the laminated sheet for electromagnetic wave shielding, it was evaluated as "good", and when at least one of dissolution and breakage was recognized, it was evaluated as "bad".

[0089] (4) Moisture resistance The laminated sheet for electromagnetic wave shielding was placed in a flat-bottomed stainless steel container so that the electromagnetic wave absorption layer was in contact with the bottom surface of the container. The container containing the laminated sheet for electromagnetic wave shielding was left in a thermo-hygrostat maintained at 85°C and 85% RH for 500 hours. Subsequently, when the container was taken out of the thermo-hygrostat and the laminated sheet for electromagnetic wave shielding was further lifted from the bottom surface of the stainless steel container, if the laminated sheet for electromagnetic wave shielding did not break, it was evaluated as "good", and if the laminated sheet for electromagnetic wave shielding was stuck to the bottom surface of the container and broke, it was evaluated as "bad".

[0090] (5) Attenuation characteristics of reflected waves The laminated sheet for electromagnetic wave shielding was fixed to an acrylic resin jig to prepare a test specimen. Also, only the metal layer was fixed to an acrylic resin jig to prepare a reference.

[0091] By the free space method, the S (Scattering) parameter (S21 sample ) at two ports of the test specimen for electromagnetic waves in the frequency range of 1 to 18 GHz was measured. As the measurement system, a vector network analyzer (manufactured by Agilent Technologies, "8720ES"), a transmitting antenna (manufactured by SCHWARZBECK, "BBHA9120-B"), and a receiving antenna (manufactured by SCHWARZBECK, "BBHA9120-B", the same model number as the transmitting antenna) were adopted. In the measurement, the horizontally polarized electromagnetic wave emitted from the transmitting antenna was incident on the test specimen at an incident angle of 45 degrees, and the reflected wave reflected in a direction 90 degrees with respect to the incident direction was received by the receiving antenna.

[0092] The S (Scattering) parameter (S21 reference ) at two ports of the reference was also measured in the same manner.

[0093] The difference between the measured value of the reference and the measured value of the test piece was regarded as the reflection attenuation amount by the electromagnetic wave shielding laminated sheet, and the attenuation amount (dB) of the reflected wave of the electromagnetic wave shielding laminated sheet was calculated according to the following formula (1).

[0094] Reflection attenuation amount (dB) = 20 log |S21 sample |- 20 log |S21 reference |···(1) The frequency indicating the peak of the attenuation amount of the reflected wave and the corresponding attenuation amount in the frequency range of 1 to 18 GHz are shown in a table.

[0095]

Table 1

[0096]

Table 2

[0097]

Table 3

[0098]

Table 4

[0099]

Table 5

Claims

1. It has a matrix and carbon nanotubes dispersed in the matrix, and the bulk density is 997 kg / m ３ An electromagnetic wave absorption layer that is as follows, It includes a metal layer overlapping the electromagnetic wave absorption layer, Further includes an insulating layer between the electromagnetic wave absorption layer and the metal layer, The relative permittivity of the insulating layer is lower than that of the electromagnetic wave absorption layer, A laminated sheet for electromagnetic wave shielding.

2. The matrix contains a water-soluble polymer, The laminated sheet for electromagnetic wave shielding according to Claim 1.

3. The electromagnetic wave absorption layer is porous, The laminated sheet for electromagnetic wave shielding according to Claim 1 or 2.

4. The electromagnetic wave absorption layer is a lyophilized product formed by lyophilizing an aqueous dispersion containing the carbon nanotubes and a water-soluble polymer, The laminated sheet for electromagnetic wave shielding according to any one of Claims 1 to 3.

5. The matrix contains a cured product of a thermosetting resin, The laminated sheet for electromagnetic wave shielding according to any one of Claims 1 to 3.

6. It has a matrix and carbon nanotubes dispersed in the matrix, and an electromagnetic wave absorption layer with a bulk density of 997 kg / m3 or less, It includes a metal layer overlapping the electromagnetic wave absorption layer, The matrix contains a cured product of a thermosetting resin, A laminated sheet for electromagnetic wave shielding.

7. The electromagnetic wave absorption layer is formed by curing the thermosetting resin in a lyophilized product formed by lyophilizing an aqueous dispersion containing the carbon nanotubes, a water-soluble polymer, and the thermosetting resin, The laminated sheet for electromagnetic wave shielding according to Claim 5 or 6.

8. The electromagnetic wave absorption layer is formed by impregnating a freeze-dried body formed by freeze-drying an aqueous dispersion containing the carbon nanotube and a water-soluble polymer with the thermosetting resin and curing the thermosetting resin. The electromagnetic wave shielding laminated sheet according to claim 5 or 6.

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