Electromagnetic wave shielding sheet

Abstract

To provide an electromagnetic wave-shielding sheet which is a cured or non-cured sheet formed by impregnating a non-woven fabric with a thickness of 1 mm or less with a bismaleimide resin or stacking the bismaleimide resin on the non-woven fabric, and has excellent electromagnetic shielding property to millimeter waves or terahertz waves.SOLUTION: An electromagnetic wave-shielding sheet has a bismaleimide resin layer on one surface or both surfaces of a carbon nanotube non-woven fabric having a thickness of 1 mm or less and specific resistance of 0.005 Ω cm or less. In the electromagnetic wave-shielding sheet, the bismaleimide resin layer is a non-cured resin layer or a cured resin layer.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an electromagnetic wave shielding sheet, and more specifically to a high-strength electromagnetic wave shielding sheet which is a cured or uncured sheet in which a bismaleimide resin is impregnated and / or laminated onto a carbon nanotube nonwoven fabric having a thickness of 1 mm or less, and which has excellent electromagnetic wave shielding properties against millimeter waves and terahertz waves, as well as adhesiveness, flexibility, and strength. [Background technology]

[0002] In recent years, high-speed communications such as 5G and 6G, which utilize the high-frequency band of electromagnetic waves (1–300 GHz), have become a hot topic. Wireless devices using electromagnetic waves for communication are on the rise. The ever-increasing electromagnetic waves can cause electronic devices to malfunction due to interference from surrounding electromagnetic waves, or even cause information leakage due to their own electromagnetic waves. Furthermore, the rapid advancement of autonomous driving, including automobiles, requires that electromagnetic waves be transmitted and received correctly in a variety of electromagnetic environments, from low-frequency electromagnetic waves to millimeter waves. Therefore, electromagnetic wave shielding has become an important technological challenge, and there is a demand for electromagnetic wave shielding materials with excellent electromagnetic wave shielding performance against microwave, millimeter wave, and terahertz waves. Furthermore, the advancement of electromagnetic wave utilization is driving demand for new functionalities in electromagnetic wave shielding materials, such as thinner, lighter, and larger-area materials.

[0003] In addition to metal materials, many electromagnetic wave shielding technologies have been proposed using conductive polymers, dielectric oxides, etc., such as carbon black, graphene, and carbon nanotubes. Of these, carbon nanotubes made from carbon are attracting attention as a promising electromagnetic wave shielding material. Electromagnetic wave shielding materials using carbon nanotubes include paste materials in which carbon nanotubes are dispersed in resin (Patent Document 1) and water-based paints in which carbon nanotubes are dispersed in an aqueous solution (Patent Document 2), but both are difficult to handle and do not have sufficient electromagnetic wave shielding performance to be practical.The carbon nanotubes used are in the form of fine fibers, so they have a large specific surface area and cannot be dispersed in large quantities in resin, and therefore electromagnetic wave shielding materials using carbon nanotubes also have insufficient electrical conductivity. Furthermore, there is an electromagnetic wave shielding material (Patent Document 3) that uses a carbon nanotube sheet formed by electrospinning, but it has problems such as low strength and difficulty in handling. An electromagnetic wave shielding material (Patent Document 4) has been disclosed in which a protonating agent such as hydronium ions or hydrochloric acid is added to a carbon nanotube sheet, and further ferromagnetic materials such as iron or cobalt are added to improve conductivity. However, the protonating agent is a strongly acidic compound, which poses a problem in handling it. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-144000 [Patent Document 2] Japanese Patent Application Laid-Open No. 2012-174833 [Patent Document 3] Japanese Patent Application Laid-Open No. 2008-218859 [Patent Document 4] Patent No. 6182176 Summary of the Invention [Problem to be solved by the invention]

[0005] Therefore, an object of the present invention is to provide a high-strength electromagnetic wave shielding sheet that has excellent electromagnetic wave shielding properties against millimeter waves and terahertz waves. [Means for solving the problem]

[0006] As a result of extensive research, the inventors have discovered that a cured or uncured electromagnetic wave shielding sheet can be obtained by impregnating and / or laminating a highly conductive carbon nanotube nonwoven fabric having a thickness of 1 mm or less and a resistivity of 0.005 Ω·cm or less with a bismaleimide resin, and that the sheet has high strength, adhesiveness, flexibility, and excellent electromagnetic wave shielding performance, thereby completing the present invention. That is, the present invention provides the following electromagnetic wave shielding sheet.

[0007] [1] An electromagnetic wave shielding sheet having a thickness of 1 mm or less and a bismaleimide resin layer on one or both sides of a carbon nanotube nonwoven fabric with a resistivity of 0.005 Ω·cm or less. [2] The electromagnetic wave shielding sheet according to [1], wherein the bismaleimide resin layer is an uncured resin layer or a cured resin layer. [3] The electromagnetic wave shielding sheet according to [1] or [2], wherein the bismaleimide resin is 10 to 200 parts by mass per 100 parts by mass of the carbon nanotube nonwoven fabric. [4] The electromagnetic wave shielding sheet according to any one of [1] to [3], wherein the carbon nanotube nonwoven fabric is treated with a coupling agent. [5] The electromagnetic wave shielding sheet according to any one of [1] to [4], wherein the bismaleimide resin is a thermosetting resin represented by the following formula (1) and / or formula (2): [ka] [ka] (In formula (2), B's are independently tetravalent organic groups having a cyclic structure, X's are independently divalent hydrocarbon groups having 6 to 200 carbon atoms which may contain a heteroatom, and at least one of them is a hydrocarbon group derived from a dimer acid skeleton, and n is 1 to 100.) [6] The electromagnetic wave shielding sheet according to any one of [1] to [5], wherein an inorganic material is contained in the gaps between the fibers of the carbon nanotube nonwoven fabric. [7] The electromagnetic wave shielding sheet according to [6], wherein the inorganic material is at least one selected from the group consisting of carbon black, carbon nanotubes, silica, zinc oxide, alumina, boron nitride, aluminum nitride, short carbon fibers, and short alumina fibers. [8] [6] The electromagnetic wave shielding sheet according to [6], wherein the inorganic material is copper, iron, silver or gold, or resin particles surface-coated with these metals. [Effects of the Invention]

[0008] The electromagnetic wave shielding sheet of the present invention is a cured or uncured electromagnetic wave shielding sheet in which a highly conductive carbon nanotube nonwoven fabric having a thickness of 1 mm or less and a resistivity of 0.005 Ω cm or less is impregnated with and / or laminated with a bismaleimide resin, and has high strength, adhesiveness, and flexibility, and excellent electromagnetic wave shielding performance. Therefore, the electromagnetic wave shielding sheet of the present invention is useful for applications such as devices compatible with high-speed, large-capacity communication and in-vehicle use. DETAILED DESCRIPTION OF THE INVENTION

[0009] The present invention will be described in detail below.

[0010] <Carbon nanotube nonwoven fabric> The carbon nanotube nonwoven fabric used in the present invention is 1 mm or less in thickness, 50 nm or less in diameter, and 2 mm or less in length, and is composed of entangled single- to multi-layered carbon nanotube fibers. This nonwoven fabric preferably has a high electrical conductivity with a resistivity of 0.005 Ω cm or less, and more preferably 0.003 Ω cm or less. Carbon nanotube nonwoven fabric is made by entangling fibers spun in a plasma furnace using methane gas or carbon nanotube fibers spun from melted carbon nanotubes through pores.

[0011] Carbon nanotube nonwoven fabrics with a resistivity of 0.005 Ω·cm or less, preferably 0.003 Ω·cm or less, have good electrical conductivity and excellent electromagnetic wave shielding performance over a wide frequency range (10 kHz to 100 GHz). However, the carbon nanotube nonwoven fabric itself lacks strength, making it prone to tearing, and it lacks adhesiveness and flexibility to the substrate, making it difficult to use as is. In order to solve these problems, the present invention relates to a B-stage electromagnetic wave shielding sheet obtained by impregnating and / or laminating a carbon nanotube nonwoven fabric with a bismaleimide resin and semi-curing it, and a cured electromagnetic wave shielding sheet with sufficient strength.

[0012] <Bismaleimide resin> The bismaleimide resin used in the present invention is preferably one represented by the following general formula (1) and / or formula (2), because it has excellent heat resistance, low elasticity, toughness, and adhesiveness.

[0013] Bismaleimide resin represented by general formula (1) [ka]

[0014] The dimer acid referred to here is a liquid dibasic acid whose main component is a 36-carbon dicarboxylic acid, produced by the dimerization of 18-carbon unsaturated fatty acids derived from natural sources such as vegetable oils. The dimer acid skeleton is not a single skeleton, but has multiple structures, and several types of isomers exist. Representative dimer acids are classified as linear (a), monocyclic (b), aromatic (c), and polycyclic (d). In this specification, the dimer acid skeleton refers to a group derived from a dimer diamine having a structure in which the carboxy groups of such a dimer acid are substituted with primary aminomethyl groups. That is, the bismaleimide resin represented by general formula (1) or (2) preferably has, as the dimer acid skeleton, a group in which two carboxy groups in each of the dimer acids represented by the following (a) to (d) are substituted with methylene groups. Furthermore, from the viewpoint of the heat resistance and reliability of the cured product, it is more preferable that the hydrocarbon group derived from the dimer acid skeleton in the maleimide compound has a structure in which the carbon-carbon double bond in the hydrocarbon group derived from the dimer acid skeleton is reduced by a hydrogenation reaction. Generally, dimer acids may contain trimers (trimer acids) due to the use of natural products such as vegetable oils and fats as raw materials. However, it is preferable that the proportion of hydrocarbon groups derived from dimer acids among the hydrocarbon groups derived from dimer acids and trimer acids is high, for example, 95 mass % or more, because this tends to result in excellent dielectric properties, excellent moldability due to a tendency for the viscosity to decrease when heated, and reduced effects of moisture absorption. In this specification, the dimer acid (trimer acid) skeleton refers to a group derived from a dimer diamine (trimer triamine) having a structure in which the carboxy group of such a dimer acid (trimer acid) is substituted with a primary aminomethyl group. [ka]

[0015] A representative bismaleimide resin of formula (1) is a resin of the following formula (3) (SLK-6895; trade name, manufactured by Shin-Etsu Chemical Co., Ltd.). [ka]

[0016] Bismaleimide resin represented by general formula (2) [ka] (In formula (2), B's are independently tetravalent organic groups having a cyclic structure, X's are independently divalent hydrocarbon groups having 6 to 200 carbon atoms which may contain a heteroatom, and at least one of them is a hydrocarbon group derived from a dimer acid skeleton, and n is 1 to 100.)

[0017] In formula (2), the organic groups represented by B are independently tetravalent organic groups having a cyclic structure, and are particularly preferably any of the tetravalent organic groups represented by the following structural formulas. [ka] (The bond not bonded to a substituent in the above structural formula is bonded to the carbonyl carbon that forms the cyclic imide structure in formula (2).)

[0018] In formula (2), X's are independently divalent hydrocarbon groups having 6 to 200 carbon atoms, preferably 8 to 100 carbon atoms, and more preferably 10 to 50 carbon atoms, which may contain a heteroatom. Among these, branched divalent hydrocarbon groups in which one or more hydrogen atoms in the divalent hydrocarbon group are substituted with alkyl or alkenyl groups having 6 to 200 carbon atoms, preferably 8 to 100 carbon atoms, and more preferably 10 to 50 carbon atoms, are preferred. The branched divalent hydrocarbon group may be either a saturated aliphatic hydrocarbon group or an unsaturated hydrocarbon group, and may have an alicyclic structure or an aromatic ring structure in the middle of the molecular chain.

[0019] In formula (2), n is 1 to 100, preferably 1 to 60, and more preferably 1 to 50. If n is too large, the solubility and fluidity may decrease, and moldability such as lamination and impregnation may be poor.

[0020] A representative bismaleimide resin of formula (2) is a resin of the following formula (4) (SLK-3000; trade name, manufactured by Shin-Etsu Chemical Co., Ltd.). [ka]

[0021] The bismaleimide resins of formula (1) and formula (2) may be used alone or in combination of two or more kinds. Representative bismaleimide resins include the above-mentioned SLK-6895 (manufactured by Shin-Etsu Chemical Co., Ltd.) and SLK-3000 (manufactured by Shin-Etsu Chemical Co., Ltd.), as well as the SLK-2000 series (manufactured by Shin-Etsu Chemical Co., Ltd.).

[0022] The number average molecular weight (Mn) of the bismaleimide resin is preferably from 800 to 50,000, more preferably from 900 to 30,000, and even more preferably from 1,000 to 25,000. The number average molecular weight (Mn) referred to in this specification refers to the number average molecular weight measured by GPC under the following conditions using polystyrene as the standard substance. [GPC measurement conditions] Developing solvent: tetrahydrofuran (THF) Flow rate: 0.35mL / min Detector: Refractive index detector (RI) Column: TSK Guardcolumn SuperH-L TSKgel SuperHZ4000(4.6mmI.D.×15cm×1) TSKgel SuperHZ3000(4.6mmI.D.×15cm×1) TSKgel SuperHZ2000 (4.6mmI.D.×15cm×2) (All manufactured by Tosoh Corporation) Column temperature: 40℃ Sample injection volume: 5 μL (0.2% by mass THF solution)

[0023] The amount of bismaleimide resin impregnated into and laminated on the carbon nanotube nonwoven fabric is preferably 10 to 200 parts by mass per 100 parts by mass of the carbon nanotube nonwoven fabric.

[0024] <Reaction initiator> A reaction initiator can be added to the bismaleimide resin in order to initiate and promote the crosslinking reaction of the maleimide compound and the reaction between the maleimide group and a reactive group capable of reacting with the maleimide group. The reaction initiator is not particularly limited as long as it promotes the crosslinking reaction, and examples thereof include ionic catalysts such as imidazoles, organic phosphorus compounds, tertiary amines, quaternary ammonium salts, boron trifluoride amine complexes, organophosphines, and organophosphonium salts; organic peroxides such as diallyl peroxide, dialkyl peroxide, peroxide carbonate, and hydroperoxide; and radical polymerization initiators such as azoisobutyronitrile. Among these, organic peroxides and radical polymerization initiators are preferred when promoting the reaction of a bismaleimide resin alone, or when the reactive group of a thermosetting resin other than a bismaleimide resin (described later) having a reactive group capable of reacting with a maleimide group is a group having a carbon-carbon double bond such as a maleimide group, an alkenyl group, or a (meth)acrylic group. Examples of organic peroxides include dicumyl peroxide, t-butyl peroxybenzoate, t-amyl peroxybenzoate, dibenzoyl peroxide, diuraloyl peroxide, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, 1,1-di(t-butylperoxy)cyclohexane, di-t-butyl peroxide, and dibenzoyl peroxide. Furthermore, when the reactive group of the thermosetting resin other than the bismaleimide resin that has a reactive group capable of reacting with a maleimide group is an epoxy group, a hydroxyl group, or an acid anhydride group, basic compounds such as imidazoles and tertiary amines, or organic phosphorus compounds are preferred. Although it is possible to use imidazoles or amines for homopolymerization of the maleimide group, imidazoles and organic phosphorus compounds require extremely high temperatures, and amines may have a very short pot life.

[0025] The reaction initiator is preferably blended in an amount of 0.05 to 10 parts by mass, more preferably 0.1 to 5 parts by mass, per 100 parts by mass of the bismaleimide resin. When other thermosetting resins are blended in the composition, the amount is preferably blended in an amount of 0.05 to 10 parts by mass, particularly 0.1 to 5 parts by mass, per 100 parts by mass of the bismaleimide resin and other thermosetting resin components combined. Amounts outside this range are undesirable because the curing of the bismaleimide resin composition during molding may be very slow or fast. Furthermore, the balance between heat resistance and moisture resistance of the resulting cured product may be poor. The reaction initiator may be used alone or in combination of two or more kinds.

[0026] <Inorganic materials> By filling the gaps between the entangled carbon nanotube fibers in the electromagnetic wave shielding sheet of the present invention with a highly conductive inorganic material, such as inorganic powder, inorganic fiber, or metal particles, it is possible to further increase the electrical conductivity and improve the electromagnetic wave shielding performance in the frequency band of 10 to 300 GHz. Typical inorganic powders include carbon black, carbon nanotubes, graphene, graphite, silica, zinc oxide, alumina, boron nitride, aluminum nitride, short carbon fibers, and short alumina fibers. Metal particles such as copper, iron, silver, and gold, or resin particles coated with these metals, can also be added. The metal particles and / or inorganic powder may be dispersed in a bismaleimide resin and then laminated onto and / or impregnated into the carbon nanotube nonwoven fabric.

[0027] Furthermore, to further increase the thermal conductivity of the electromagnetic wave shielding sheet, inorganic particles or fibers such as silica, zinc oxide, alumina, boron nitride, aluminum nitride, short carbon fibers, short alumina fibers, quartz fibers, glass fibers, etc. may be used. By using these, the thermal conductivity of the electromagnetic wave shielding sheet can be increased to 50 to 80 W / mK.

[0028] The shape of the inorganic material is not particularly limited, but a nearly spherical shape is preferred as it can be easily filled, and an average particle size of 0.5 μm to 30 μm is preferred from the standpoint of heat dissipation and electrical conductivity.

[0029] The inorganic material can be sprayed onto the nonwoven fabric by any method, including, for example, a method of packing the inorganic material into the nonwoven fabric using a press or laminator, a method of dispersing the inorganic material in any solvent, spraying the dispersion onto the nonwoven fabric with a sprayer, and then drying and removing the solvent, or a combination of these methods. Any solvent can be used as the solvent for the dispersion, but highly volatile solvents are preferred, such as water, alcohols such as ethanol and IPA, acetone, toluene, hydrocarbon solvents, and silicone solvents. The concentration of the dispersion is preferably 0.1 to 100 parts by mass relative to 100 parts by mass of the inorganic material.

[0030] The amount of inorganic material to be sprayed onto the nonwoven fabric is preferably 0.01 to 100 parts by mass per 100 parts by mass of the carbon nanotube nonwoven fabric.

[0031] <Electromagnetic wave shielding sheet manufacturing method> The electromagnetic wave shielding sheet of the present invention can be produced by any method, including a wet method in which the bismaleimide resin is dissolved in a (volatile) solvent to reduce the viscosity and then impregnated into a carbon nanotube nonwoven fabric; a melt rolling method in which the bismaleimide resin is heated to reduce the viscosity and then impregnated into a carbon nanotube nonwoven fabric; and a transfer method in which a bismaleimide resin varnish is formed into a film using a coater or the like and then transferred and impregnated into a carbon nanotube nonwoven fabric using a press or laminator. An impregnated carbon nanotube nonwoven fabric is produced by any of these methods, and then a bismaleimide resin film is pressure-laminated on it as needed to semi-cure it into a B-stage or to cure it, thereby producing an electromagnetic wave shielding sheet.

[0032] In the wet method, the bismaleimide resin is impregnated and then the solvent is removed to produce a resin-impregnated electromagnetic wave shielding sheet. If any solvent remains in the electromagnetic wave shielding sheet, it can have adverse effects during molding, leading to problems such as reduced work efficiency. For this reason, the amount of solvent remaining in the electromagnetic wave shielding sheet is set to 1% by mass or less, and preferably 0.5% by mass or less. The method for removing the solvent depends on the boiling point of the solvent used, but heat treatment at 80 to 150° C. for about 10 minutes to 1 hour is preferred, and the solvent can be easily removed by this heat treatment.

[0033] The melt-rolling method is advantageous in various respects, such as not requiring a solvent removal step and having relatively good work efficiency. In the melt-rolling method, the carbon nanotube nonwoven fabric is expanded to the required width using a bar or the like, a film-like bismaleimide resin is sandwiched between release papers from above and below, and the carbon nanotube nonwoven fabric is then nipped between several pairs of heated metal rolls installed at approximately the same height in the direction of travel of the carbon nanotube nonwoven fabric to impregnate the bismaleimide resin into the carbon nanotube nonwoven fabric, thereby producing an electromagnetic wave shielding sheet. In the melt-rolling method, since the nip pressure is a linear pressure, it is preferable to increase the number of nip rolls to achieve sufficient impregnation. Alternatively, pressure molding may be performed using a molding press such as a heatable pressure multi-stage press without using heated metal rolls.

[0034] In the transfer method, a base film is coated with a resin varnish, the solvent is dried to obtain a resin film, and then both sides of the carbon nanotube nonwoven fabric are sandwiched between the resin films and pressed together using a laminator or press, transferring and impregnating the resin film into the carbon nanotube nonwoven fabric. Any film can be used as the base film, but resin films that are easy to peel off, such as PET film, PE film, PP film, Teflon film, and Aflex film, are preferred. If necessary, the surface of the substrate film may be subjected to various surface treatments such as corona treatment, plasma treatment, or silicone treatment. Any solvent can be used for the resin varnish, but highly volatile solvents are preferred, such as alcohols such as ethanol and IPA, acetone, toluene, xylene, anisole, hydrocarbon solvents, and silicone solvents. The concentration of the resin varnish is preferably 0.1 to 200 parts by mass relative to 100 parts by mass of the resin. Any method can be used to coat the base film with the resin varnish, but it is preferable to use a spin coater or bar coater because it is easy to do so. The temperature at which the resin film on the substrate film is dried is preferably a temperature at which the thermosetting resin does not react, and is preferably 30 to 120°C. Furthermore, when sandwiching both sides of the carbon nanotube nonwoven fabric with resin films and pressing them together using a laminator or press to transfer the resin films to the carbon nanotube nonwoven fabric, pressure or heat can be applied as necessary.

[0035] As the electromagnetic wave shielding sheet of the carbon nanotube nonwoven fabric using the bismaleimide resin, it is possible to produce an electromagnetic wave shielding sheet in which the resin is completely cured and an electromagnetic wave shielding sheet in a semi-cured state. By selecting the chemical structure, impregnation amount, lamination amount, and curing method of the bismaleimide resin, it is possible to produce a cured electromagnetic wave shielding sheet that ranges from a hard and tough electromagnetic wave shielding sheet to a flexible and shapeable electromagnetic wave shielding sheet. The semi-hardened electromagnetic wave shielding sheet can be adhered to a case or container that contains communication equipment by applying pressure and heat to the sheet to match the shape of the case or container. The electromagnetic wave shielding sheet of the present invention can be freely manufactured in sheet sizes ranging from small to large, so there is no limit to the area for electromagnetic wave shielding.

[0036] In the present invention, when the carbon nanotube nonwoven fabric is impregnated with the bismaleimide resin, the surface of the carbon nanotube nonwoven fabric is treated with a coupling agent, thereby adhering the carbon nanotube nonwoven fabric to the resin and improving the durability of the electromagnetic wave shielding sheet. As the coupling agent, a silane coupling agent or an alkoxide compound of titanium, aluminum, or the like can also be used. Among them, a silane coupling agent is preferred, and a preferred silane coupling agent is, for example, a compound represented by the general formula Y-Si-X3. Here, Y is an organic group having a functional group such as an amino group, an epoxy group, a hydroxyl group, a carboxyl group, a vinyl group, a methacryl group, or a mercapto group, and X is a hydrolyzable functional group such as an alkoxy group. Representative examples of the compound represented by the general formula Y-Si-X3 include γ-glycidoxypropyltrimethoxysilane, vinyltriethoxysilane, γ-aminopropyltriethoxysilane, γ-mercaptopropyltrimethoxysilane, γ-methacryloxypropyltrimethoxysilane, γ-aminobenzyltriethoxysilane, and γ-aminophenyltriethoxysilane.

[0037] The amount of such coupling agent used is suitably in the range of 0.5 to 20 parts by mass per 100 parts by mass of the bismaleimide resin. The electromagnetic wave shielding sheet of the present invention has an electromagnetic wave shielding property of 50 dB or more, preferably 60 dB or more in the frequency band of 10 to 300 GHz.

[0038] The electromagnetic wave shielding sheet may be formed as a laminate by laminating thermoplastic films such as polyester as protective films on the top and bottom surfaces of the electromagnetic wave shielding sheet. [Example]

[0039] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. In the examples, "parts" means "parts by mass." The materials used in the examples and comparative examples are shown below. The physical properties and characteristic values ​​in the present invention were measured by the following methods.

[0040] (1) Specific resistance The resistivity was calculated using the following formula. Specific resistance (Ω cm) = surface resistivity (Ω / □) x thickness (cm) The surface resistivity was measured using a Loresta-GX MCP-T700 (low resistance resistivity meter, manufactured by Nitto Seiko Analytech Co., Ltd.) and a Hiresta-UX MCP-HT800 (high resistance resistivity meter, manufactured by Nitto Seiko Analytech Co., Ltd.) and the measured values ​​were used to calculate the resistivity.

[0041] (2) Electromagnetic wave shielding properties The electromagnetic wave shielding characteristic (SE) is defined by the following formula: The larger the SE value, the higher the shielding effect.

number

[0042] The values ​​obtained from the above calculation formula at 100 GHz and the values ​​were qualitatively judged according to the following criteria. SE is 50dB or more: Yes SE less than 50dB: ×

[0043] (3) Tensile strength The tensile strength of the electromagnetic wave shielding sheets produced in the examples and comparative examples was measured in accordance with "6.3 Tensile strength and elongation" of JIS L1913:2010 "Test methods for general nonwoven fabrics."

[0044] <90° peel strength test> The adhesive strength of the B-stage electromagnetic wave shielding sheet to an MPI film (polyimide resin; PIXEO SR, manufactured by Kaneka Corporation) and an LCP film (liquid crystal polymer resin; Pellicule LCP0050BXC, manufactured by Chiyoda Integre Co., Ltd.) was evaluated. The electromagnetic wave shielding sheet was cut into a size of 10 mm x 76 mm, and the MPI film or LCP film and glass were bonded together with the electromagnetic wave shielding sheet in between using a vacuum laminator, followed by curing at 180°C for 2 hours to prepare a test piece. The adhesive strength between the electromagnetic wave shielding sheet and the MPI film or LCP film was evaluated visually. High adhesion and sufficient adhesive strength were evaluated as ◯, adhesion but insufficient adhesive strength as △, and no adhesion as ×.

[0045] 1. Carbon nanotube nonwoven fabric (1) CNTM30 Tortech Thickness: 62 μm, resistivity: 1.46E-03 (Ω·cm) (2) CNTM10 Tortech Thickness: 40 μm, resistivity: 2.44E-03 (Ω·cm)

[0046] 2.Inorganic materials (1) Carbon nanotube powder; ZEONAN SG101 manufactured by Zeon Nano Technology Co., Ltd. (average particle size: 100 μm) (2) Silver particles; AgC-HWQ manufactured by Fukuda Metal Foil and Powder Industries Co., Ltd. (average particle size 1.5 μm)

[0047] <Bismaleimide resin> (A-1) A linear alkylene group-containing bismaleimide resin represented by the following formula (3) (SLK-6895, manufactured by Shin-Etsu Chemical Co., Ltd., number average molecular weight 689): [ka] (A-2) A linear alkylene group-containing bismaleimide resin represented by the following formula (4) (SLK-3000, manufactured by Shin-Etsu Chemical Co., Ltd., number average molecular weight 5200) [ka] n≒3 (average value) -C 36 H 70 - represents a hydrocarbon group derived from a dimer acid skeleton. (A-3) Silicone resin (for comparison) Curable silicone resin composition (50% toluene solution) (PLF-100D, manufactured by Shin-Etsu Chemical Co., Ltd., number average molecular weight 5000) [Example]

[0048] A 30 μm thick bismaleimide resin film was prepared containing 100 parts by mass of the bismaleimide resin (A-1) represented by the above formula (3) (SLK-6895, manufactured by Shin-Etsu Chemical Co., Ltd.) and 1 part by mass of a curing catalyst (dicumyl peroxide (trade name: "Percumyl D"), manufactured by NOF Corporation). Both sides of a 62 μm thick carbon nanotube nonwoven fabric CNTM30 were laminated with this resin film at 80°C for 1 minute, and then pressed at 150°C for 15 minutes to prepare a semi-cured solid sheet. Thereafter, it was heat cured at 150° C. for 4 hours to produce electromagnetic wave shielding sheet 1. Table 1 shows the shielding properties and mechanical strength of electromagnetic wave shielding sheet 1 and the peel strength of the semi-cured electromagnetic wave shielding sheet. [Example]

[0049] An electromagnetic wave shielding sheet 2 was produced in the same manner as in Example 1, except that a bismaleimide resin (A-2) (SLK-3000, manufactured by Shin-Etsu Chemical Co., Ltd.) was used instead of the bismaleimide resin (A-1) of Example 1. As in Example 1, the shielding properties, mechanical strength and peel strength of the electromagnetic wave shielding sheet 2 are shown in Table 1. [Example]

[0050] A 5% by mass carbon nanotube powder ethanol dispersion (ZEONANO SG101, manufactured by Zeon Nano Technology Co., Ltd.) was sprayed onto both surfaces of the CNTM30 used in Example 1 as a conductive powder, and the solvent was dried at 100°C for 10 minutes. After that, the sheet was laminated with the resin film of Example 2 and heat-cured to produce electromagnetic wave shielding sheet 3. As in Example 1, the shielding properties, mechanical strength and peel strength of electromagnetic wave shielding sheet 3 are shown in Table 1. [Example]

[0051] Electromagnetic wave shielding sheet 4 was produced in the same manner as in Example 1, except that CNTM10 having a thickness of 40 μm was used instead of the CNT nonwoven fabric CNTM30 used in Example 1, and bismaleimide resin (A-2) was used. As in Example 1, the shielding properties, mechanical strength, and peel strength of electromagnetic wave shielding sheet 4 are shown in Table 1. [Example]

[0052] As in Example 4, a 10% by mass silver particle toluene dispersion (AgC-HWQ, manufactured by Fukuda Metal Foil & Powder Co., Ltd., average particle size 1.5 μm) was sprayed onto both sides as conductive powder, and the solvent was dried at 100° C. for 10 minutes, followed by laminating with a resin film and heat curing to produce electromagnetic wave shielding sheet 5. As in Example 4, the shielding properties, mechanical strength and peel strength of electromagnetic wave shielding sheet 5 are shown in Table 1. Comparative Example 1

[0053] The carbon nanotube nonwoven fabric CNTM30 alone was used as the electromagnetic wave shielding sheet 6. The shielding properties, mechanical strength and peel strength of the electromagnetic wave shielding sheet 6 are shown in Table 1. Comparative Example 2

[0054] The carbon nanotube nonwoven fabric CNTM10 alone was used as the electromagnetic wave shielding sheet 7. The shielding properties, mechanical strength and peel strength of the electromagnetic wave shielding sheet 7 are shown in Table 1. Comparative Example 3

[0055] An electromagnetic wave shielding sheet 8 was produced in the same manner as in Example 1, except that a silicone resin (A-3) was used instead of the bismaleimide resin (A-1) of Example 1. The shielding properties, mechanical strength and peel strength of the electromagnetic wave shielding sheet 8 are shown in Table 1.

[0056] [Table 1]

[0057] By combining carbon nanotube nonwoven fabric with bismaleimide resin, the mechanical strength of the carbon nanotube nonwoven fabric remains unchanged, but the tensile strength is improved, resulting in a highly reliable electromagnetic wave shielding sheet without reducing the electromagnetic wave shielding properties.In addition, the adhesion to the substrate is improved, resulting in a more reliable electromagnetic wave shielding sheet. As described above, it has become clear that the electromagnetic wave shielding sheet of the present invention, which is obtained by impregnating and / or laminating a carbon nanotube nonwoven fabric having a thickness of 1 mm or less and a resistivity of 0.005 Ω·cm or less with a bismaleimide resin, and in particular, one which contains inorganic powder and / or metal particles as inorganic materials in the gaps between the carbon nanotube fibers, provides an electromagnetic wave shielding sheet with superior electromagnetic wave shielding properties against millimeter waves and terahertz waves.

Claims

1. An electromagnetic wave shielding sheet having a thickness of 1 mm or less and a bismaleimide resin layer on one or both sides of a carbon nanotube nonwoven fabric having a specific resistance of 0.005 Ω cm or less, wherein the bismaleimide resin is a thermosetting resin represented by the following formula (1) and / or formula (2): 【Chemistry 1】 【Chemistry 2】 (In formula (2), B's are independently tetravalent organic groups having a cyclic structure; X's are independently divalent hydrocarbon groups having 6 to 200 carbon atoms which may contain a heteroatom, at least one of which is a hydrocarbon group derived from a dimer acid skeleton; and n is 1 to 100.)

2. 2. The electromagnetic wave shielding sheet according to claim 1, wherein the bismaleimide resin layer is a semi-cured resin layer or a cured resin layer.

3. 3. The electromagnetic wave shielding sheet according to claim 1, wherein the bismaleimide resin is contained in an amount of 10 to 200 parts by mass per 100 parts by mass of the carbon nanotube nonwoven fabric.

4. 2. The electromagnetic wave shielding sheet according to claim 1, wherein the carbon nanotube nonwoven fabric is treated with a coupling agent.

5. 2. The electromagnetic wave shielding sheet according to claim 1, wherein the carbon nanotube nonwoven fabric contains an inorganic material in the gaps between the fibers.

6. 6. The electromagnetic wave shielding sheet according to claim 5, wherein the inorganic material is at least one selected from the group consisting of carbon black, carbon nanotubes, silica, zinc oxide, alumina, boron nitride, aluminum nitride, short carbon fibers, and short alumina fibers.

7. 6. The electromagnetic wave shielding sheet according to claim 5, wherein the inorganic material is copper, iron, silver or gold, or resin particles whose surfaces are coated with any of these metals.

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