Electromagnetic wave shielding sheet

Abstract

The purpose of the present invention is to provide an electromagnetic wave shielding sheet that excels in shielding performance. The electromagnetic wave shielding sheet of the present invention is characterized by: including monolayer carbon nanotubes, multilayer carbon nanotubes, thin-film graphite, and a polymer; and having a porosity of 1% or higher.

Description

[Technical Field]

[0001] The present invention relates to an electromagnetic wave shielding sheet. [Background technology]

[0002] In recent years, carbon nanotubes (hereinafter sometimes referred to as "CNTs") have been attracting attention as a lightweight material with excellent electrical conductivity and mechanical properties, etc. However, because CNTs are minute structures with diameters on the order of nanometers, they are not necessarily easy to handle or process on their own.

[0003] Therefore, for example, a plurality of CNTs are gathered and formed into a sheet, and the obtained sheet is used in various applications. In particular, a sheet that blocks electromagnetic waves by absorbing and / or reflecting them (electromagnetic wave shielding sheet) has attracted attention as an application of such a sheet.

[0004] For example, in Patent Document 1, an electromagnetic wave shielding sheet is formed by applying a multi-walled carbon nanotube aqueous dispersion coating liquid to a substrate in a predetermined coating amount. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-174833 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the conventional electromagnetic wave shielding sheets described above have room for improvement in terms of further improving their electromagnetic wave shielding performance (shielding performance).

[0007] Therefore, an object of the present invention is to provide an electromagnetic wave shielding sheet having excellent shielding performance. [Means for solving the problem]

[0008] The present inventors have conducted extensive research with the aim of solving the above problems, and have found that an electromagnetic wave shielding sheet that contains single-walled CNTs, multi-walled CNTs, thin film graphite, and a polymer and has a porosity of a predetermined value or more can effectively shield electromagnetic waves, thereby completing the present invention.

[0009] That is, the present invention has an object to advantageously solve the above-mentioned problems, and provides an electromagnetic wave shielding sheet comprising single-walled carbon nanotubes, multi-walled carbon nanotubes, thin film graphite, and a polymer, and having a porosity of 1% or more. Thus, an electromagnetic wave shielding sheet comprising single-walled CNTs, multi-walled CNTs, thin film graphite, and a polymer, and having a porosity of a predetermined value or more, can exhibit excellent shielding performance. In the present invention, the "void ratio" of the electromagnetic wave shielding sheet can be measured by the method described in the examples of this specification.

[0010] Here, the electromagnetic wave shielding sheet of the present invention is such that the surface of the electromagnetic wave shielding sheet is 2 It is preferable that the number of voids having a size of 100 μm or less be 200 or more per 1 mm2 of the surface of the electromagnetic wave shielding sheet. 2 If the number of voids having a size of 100 μm or less among the voids present per unit area is equal to or greater than the above-mentioned predetermined value, the electromagnetic wave shielding performance of the electromagnetic wave shielding sheet can be further improved. In the present invention, the surface of the electromagnetic wave shielding sheet is 2 The number of voids having a size of 100 μm or less among the voids present per unit area can be measured from images obtained by CT (Computed Tomography) of the electromagnetic wave shielding sheet.

[0011] Furthermore, the electromagnetic wave shielding sheet of the present invention has a surface area of ​​1 mm 2In a frequency distribution created with a class width of 5 μm for the size of voids present per unit area, it is preferable that the number of voids in the class with a size of more than 0 μm and not more than 5 μm be the largest. If the number of voids in the class with a size of more than 0 μm and not more than 5 μm be the largest in the above-mentioned predetermined frequency distribution, the shielding performance of the electromagnetic wave shielding sheet can be further improved. In the present invention, the surface of the electromagnetic wave shielding sheet is 2 A frequency distribution with a class interval of 5 μm for the size of voids present per unit area can be created based on the size and number of voids measured from images obtained by CT of the electromagnetic wave shielding sheet.

[0012] Furthermore, the electromagnetic wave shielding sheet of the present invention has a surface area of ​​1 mm 2 It is preferable that the ratio of the number of voids having a size of more than 0 μm and 5 μm or less to the total number of voids present per 1 mm2 of the surface of the electromagnetic wave shielding sheet is 20% or more. 2 If the ratio of the number of voids having a size of more than 0 μm and not more than 5 μm to the total number of voids present per unit area is equal to or greater than the above-mentioned predetermined value, the shielding performance of the electromagnetic wave shielding sheet can be further improved. In the present invention, the surface of the electromagnetic wave shielding sheet is 2 The percentage of voids with a size of more than 0 μm and 5 μm or less to the total number of voids present per unit area can be calculated based on the size and number of voids measured from images obtained by CT of the electromagnetic wave shielding sheet.

[0013] Furthermore, the electromagnetic wave shielding sheet of the present invention preferably contains single-walled carbon nanotubes, multi-walled carbon nanotubes, and thin film graphite in a total content of 40 mass% or more. If the total content of single-walled CNTs, multi-walled CNTs, and thin film graphite in the electromagnetic wave shielding sheet is equal to or greater than the above-mentioned predetermined value, the shielding performance of the electromagnetic wave shielding sheet can be further improved.

[0014] In the electromagnetic wave shielding sheet of the present invention, the mass ratio of multi-walled carbon nanotubes to single-walled carbon nanotubes (multi-walled carbon nanotubes / single-walled carbon nanotubes) preferably exceeds 1: 1. If the mass ratio of multi-walled CNTs to single-walled CNTs (multi-walled CNT / single-walled CNT) in the electromagnetic wave shielding sheet exceeds the above-mentioned predetermined value, the shielding performance of the electromagnetic wave shielding sheet can be further improved. [Effects of the Invention]

[0015] According to the present invention, it is possible to provide an electromagnetic wave shielding sheet having excellent shielding performance. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 2 is a diagram for schematically illustrating an example of an emulsified dispersion state of an emulsified dispersion liquid that can be used to form the electromagnetic wave shielding sheet of the present invention. [Figure 2] 1 is a graph showing the shielding effectiveness (vertical axis) versus the frequency of electromagnetic waves (horizontal axis) for the electromagnetic wave shielding sheets of Example 1 and Comparative Examples 1 and 2. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, embodiments of the present invention will be described in detail.

[0018] (Electromagnetic wave shielding sheet) The electromagnetic wave shielding sheet of the present invention is characterized by containing single-walled carbon nanotubes, multi-walled carbon nanotubes, thin film graphite, and a polymer, and having a porosity of a predetermined value or more. The electromagnetic wave shielding sheet of the present invention can exhibit excellent shielding performance. Note that the electromagnetic wave shielding sheet of the present invention may optionally further contain components other than the above-mentioned single-walled CNTs, multi-walled CNTs, thin film graphite, and polymer.

[0019] <Single-walled carbon nanotubes> Single-walled CNTs are a material that can impart excellent shielding performance to electromagnetic wave shielding sheets. Furthermore, by forming a network within the electromagnetic wave shielding sheet, the electromagnetic wave shielding sheet can maintain good film quality.

[0020] The content of single-walled CNTs in the electromagnetic wave shielding sheet is preferably 5% by mass or more, more preferably 10% by mass or more, and preferably 20% by mass or less, and more preferably 15% by mass or less. If the content of single-walled CNTs in the electromagnetic wave shielding sheet is within the above range, the shielding performance of the electromagnetic wave shielding sheet can be further improved, and the film quality of the electromagnetic wave shielding sheet can also be improved.

[0021] The properties of single-walled CNTs will be explained below, but these properties preferably apply to at least the single-walled CNTs contained in the electromagnetic wave shielding sheet, more preferably apply to at least the single-walled CNTs contained in the electromagnetic wave shielding sheet and the single-walled CNTs in the emulsified dispersion that can be used in producing the electromagnetic wave shielding sheet described below, and even more preferably apply to all of the single-walled CNTs used as a material in preparing the emulsified dispersion, the single-walled CNTs in the emulsified dispersion, and the single-walled CNTs contained in the electromagnetic wave shielding sheet.

[0022] Here, single-walled CNTs are not particularly limited and can be produced using known single-walled CNT synthesis methods such as arc discharge, laser ablation, and chemical vapor deposition (CVD). Specifically, single-walled CNTs can be efficiently produced, for example, by supplying raw material compounds and a carrier gas onto a substrate having a catalyst layer for CNT production on its surface, and synthesizing single-walled CNTs by chemical vapor deposition (CVD) using a method known as the super-growth method (see International Publication No. 2006 / 011655), in which the catalytic activity of the catalyst layer is dramatically improved by adding a trace amount of oxidant (catalytic activator) to the system. Hereinafter, single-walled CNTs obtained by the super-growth method may be referred to as "SGCNTs."

[0023] Single-walled CNTs have a BET specific surface area of ​​600m 2 / g or more is preferable, and 2 / g or more is more preferable, and 1000m 2 / g or more is more preferable, and 2 / g or less, and 1800m 2 / g or less is more preferable, and 1600m 2 / g or less is even more preferable. If the BET specific surface area of ​​the single-walled CNTs is within the above range, the shielding performance of the electromagnetic wave shielding sheet can be further improved. In the present invention, the "BET specific surface area" refers to the nitrogen adsorption specific surface area measured using the BET (Brunauer-Emmett-Teller) method.

[0024] The average diameter of the single-walled CNTs is preferably 1 nm or more, more preferably 3 nm or more, and preferably 60 nm or less, more preferably 30 nm or less, and even more preferably 10 nm or less. Single-walled CNTs with an average diameter within the above range are less likely to aggregate in the emulsified dispersion that can be used in producing the electromagnetic wave shielding sheet described below, and it is possible to obtain an electromagnetic wave shielding sheet that is homogeneous and has even better shielding performance.

[0025] Furthermore, the single-walled CNTs preferably have an average length of 10 μm or more, more preferably 50 μm or more, even more preferably 80 μm or more, and particularly preferably 100 μm or more, and are preferably 600 μm or less, more preferably 500 μm or less, and even more preferably 400 μm or less. Single-walled CNTs having an average length within the above range are less likely to aggregate in an emulsified dispersion that can be used in the production of an electromagnetic wave shielding sheet, as described below, and can produce an electromagnetic wave shielding sheet that is homogeneous and has even better shielding performance.

[0026] Furthermore, single-walled CNTs typically have an aspect ratio (length / diameter) greater than 10. The average diameter, average length, and aspect ratio of single-walled CNTs can be determined by measuring the diameter and length of 100 randomly selected single-walled CNTs using a scanning electron microscope or a transmission electron microscope.

[0027] Furthermore, the single-walled CNTs preferably exhibit an upwardly convex t-plot obtained from the adsorption isotherm.Such single-walled CNTs are more preferably CNTs that have not been subjected to an opening treatment.

[0028] In a material with pores on its surface, the growth of a nitrogen gas adsorption layer can be classified into the following three processes (1) to (3). The slope of the t-plot changes depending on the following processes (1) to (3). (1) The process of forming a monolayer of nitrogen molecules on the entire surface (2) Formation of multi-layer adsorption and the accompanying capillary condensation filling process in the pores (3) The process of multilayer adsorption on an apparently non-porous surface whose pores are filled with nitrogen

[0029] Furthermore, in the t-plot showing an upward convex shape, the plot is located on a straight line passing through the origin in the region where the average thickness t of the nitrogen gas adsorption layer is small, whereas as t increases, the plot shifts downward from the straight line. Single-walled CNTs having such a t-plot shape have a large ratio of internal specific surface area to the total specific surface area of ​​the single-walled CNTs, indicating that numerous openings are formed in the single-walled CNTs. As a result, the single-walled CNTs are less likely to aggregate in the emulsified dispersion liquid that can be used in the production of the electromagnetic wave shielding sheet described below, and a homogeneous electromagnetic wave shielding sheet with even better shielding performance can be obtained.

[0030] The inflection point of the t-plot of the single-walled CNTs is preferably in a range satisfying 0.2≦t(nm)≦1.5, more preferably in the range of 0.45≦t(nm)≦1.5, and even more preferably in the range of 0.55≦t(nm)≦1.0. Single-walled CNTs whose inflection point of the t-plot falls within this range are even less likely to aggregate in the emulsified dispersion that can be used in producing the electromagnetic wave shielding sheet described below. As a result, an electromagnetic wave shielding sheet that is more homogeneous and has excellent shielding performance can be obtained. Here, the "position of the bending point" is the intersection of the approximate straight line A in the above-mentioned process (1) and the approximate straight line B in the above-mentioned process (3).

[0031] Furthermore, the single-walled CNTs preferably have a ratio (S2 / S1) of the internal specific surface area S2 to the total specific surface area S1, obtained from a t-plot, of 0.05 or more and 0.30 or less. Single-walled CNTs having an S2 / S1 value within this range are even less likely to aggregate in the emulsified dispersion liquid that can be used in producing the electromagnetic wave shielding sheet described below. As a result, an electromagnetic wave shielding sheet that is more uniform and has excellent shielding performance can be obtained.

[0032] Here, the total specific surface area S1 and the internal specific surface area S2 can be determined from the t-plot. Specifically, first, the total specific surface area S1 can be determined from the slope of the approximation line in step (1), and the external specific surface area S3 can be determined from the slope of the approximation line in step (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.

[0033] The measurement of the adsorption isotherm of CNT, the creation of t-plots, and the calculation of the total specific surface area S1 and the internal specific surface area S2 based on the analysis of the t-plots can be performed using, for example, a commercially available measuring device, "BELSORP (registered trademark)-mini" (manufactured by BEL Japan Co., Ltd.).

[0034] Furthermore, single-walled CNTs, for which the t-plot obtained from the adsorption isotherm shows an upwardly convex shape, can be efficiently produced by the super-growth method described above, by forming a catalyst layer on the substrate surface using a wet process.

[0035] <Multi-walled carbon nanotubes> Multi-walled CNTs are a material that can improve the shielding performance and film quality of an electromagnetic wave shielding sheet when used in combination with single-walled CNTs.

[0036] Here, the reason why the use of multi-walled CNTs in combination with single-walled CNTs improves the shielding performance and film quality of the electromagnetic wave shielding sheet is not clear, but is presumed to be as follows. First, it is presumed that, in the electromagnetic wave shielding sheet, the multi-walled CNTs interact with the network of single-walled CNTs described above. This is thought to result in the formation of appropriate voids on the surface and inside of the electromagnetic wave shielding sheet. Furthermore, it is presumed that an electromagnetic wave shielding sheet containing both multi-walled CNTs and single-walled CNTs as CNTs has the appropriate voids described above, and therefore can exhibit better shielding performance than an electromagnetic wave shielding sheet containing only single-walled CNTs as CNTs. Furthermore, since an electromagnetic wave shielding sheet containing both multi-walled CNTs and single-walled CNTs as CNTs can form a network suitable for the sheet, it is thought that the film quality will be better than that of an electromagnetic wave shielding sheet containing only single-walled CNTs as CNTs.

[0037] The content of multi-walled CNTs in the electromagnetic wave shielding sheet is preferably 10% by mass or more, more preferably 15% by mass or more, and preferably 30% by mass or less, and more preferably 25% by mass or less. If the content of multi-walled CNTs in the electromagnetic wave shielding sheet is within the above range, the shielding performance of the electromagnetic wave shielding sheet can be further improved, and the film quality of the electromagnetic wave shielding sheet can also be improved.

[0038] Furthermore, the mass ratio of multi-walled CNTs to single-walled CNTs (multi-walled CNTs / single-walled CNTs) in the electromagnetic wave shielding sheet is preferably greater than 1 / 1, more preferably greater than 4 / 3, even more preferably greater than 5 / 3, and preferably equal to or less than 5 / 1, more preferably equal to or less than 4 / 1, and even more preferably equal to or less than 3 / 1. If the mass ratio of multi-walled CNTs to single-walled CNTs in the electromagnetic wave shielding sheet (multi-walled CNTs / single-walled CNTs) is greater than 1 / 1, the shielding performance of the electromagnetic wave shielding sheet can be further improved, and the electromagnetic wave shielding sheet can be improved. On the other hand, if the mass ratio of multi-walled CNTs to single-walled CNTs in the electromagnetic wave shielding sheet (multi-walled CNTs / single-walled CNTs) is 5 / 1 or less, the content of single-walled CNTs in the electromagnetic wave shielding sheet can be ensured to be sufficiently high, thereby maintaining good film quality of the electromagnetic wave shielding sheet.

[0039] The properties of the multi-walled CNTs will be explained below, but these properties preferably apply to at least the multi-walled CNTs contained in the electromagnetic wave shielding sheet, more preferably apply to at least the multi-walled CNTs contained in the electromagnetic wave shielding sheet and the multi-walled CNTs in the emulsified dispersion that can be used in producing the electromagnetic wave shielding sheet described below, and even more preferably apply to all of the multi-walled CNTs used as a material in preparing the emulsified dispersion, the multi-walled CNTs in the emulsified dispersion, and the multi-walled CNTs contained in the electromagnetic wave shielding sheet.

[0040] The number of layers of the multi-walled CNT contained in the electromagnetic wave shielding sheet is not particularly limited, as long as it is two or more layers.

[0041] Here, the multi-walled CNTs can be produced using known methods for synthesizing single-walled CNTs, such as arc discharge, laser ablation, and chemical vapor deposition (CVD), without any particular limitation.

[0042] Multi-walled CNTs have a BET specific surface area of ​​100m 2 / g or more, and 150m 2 / g or more is more preferable, and 250m 2 / g or more is more preferable, and 2 / g or less, and 2 / g or less is more preferable, and 400m 2 If the BET specific surface area of ​​the multi-walled CNTs is within the above range, the shielding performance of the electromagnetic wave shielding sheet can be further improved.

[0043] The average diameter of the multi-walled CNTs is preferably 5 nm or more, more preferably 8 nm or more, and preferably 20 nm or less, and more preferably 15 nm or less. Multi-walled CNTs with an average diameter within the above range are less likely to aggregate in the emulsified dispersion that can be used in producing the electromagnetic wave shielding sheet described below, and it is possible to obtain an electromagnetic wave shielding sheet that is homogeneous and has even better shielding performance.

[0044] Furthermore, the average length of the multi-walled CNTs is preferably 0.5 μm or more, more preferably 1 μm or more, and is preferably 100 μm or less, more preferably 50 μm or less, and even more preferably 15 μm or less. Multi-walled CNTs having an average length within the above range are less likely to aggregate in the emulsified dispersion that can be used in producing the electromagnetic wave shielding sheet described below, and it is possible to obtain an electromagnetic wave shielding sheet that is homogeneous and has even better shielding performance.

[0045] Furthermore, multi-walled CNTs typically have an aspect ratio (length / diameter) of over 100. The average diameter, average length and aspect ratio of multi-walled CNTs can be determined by measuring the diameter and length of 100 randomly selected multi-walled CNTs using a scanning electron microscope or a transmission electron microscope.

[0046] <Thin film graphite> Thin film graphite is a material that can impart excellent shielding properties to an electromagnetic wave shielding sheet. In particular, when an electromagnetic wave shielding sheet contains thin film graphite, it can exhibit excellent shielding performance against low-frequency electromagnetic waves (for example, 1 MHz or less).

[0047] Here, thin film graphite is a material produced by delamination of graphite composed of multiple laminated graphene sheets, and is thinned to an extent that it can surround the surface of a polymer-containing emulsified dispersion material in an emulsified dispersion liquid that can be used in producing an electromagnetic wave shielding sheet, as described below. The number of layers of the graphene sheets that make up the thin film graphite is, for example, one layer (i.e., graphene sheets) to several tens of layers, and preferably one to several layers. More specifically, the number of layers of the graphene sheets that make up the thin film graphite can be, for example, 4 to 30 layers. The average length of the graphite thin film is preferably 10 μm or more and 20 μm or less. The average length of the graphite thin film can be determined by measuring the lengths of 100 randomly selected graphite thin film particles using a scanning electron microscope or a transmission electron microscope.

[0048] The content of thin film graphite in the electromagnetic wave shielding sheet is preferably 5% by mass or more, more preferably 10% by mass or more, and preferably 20% by mass or less, and more preferably 15% by mass or less. If the content of thin film graphite in the electromagnetic wave shielding sheet is equal to or greater than the above-mentioned lower limit, the electromagnetic wave shielding sheet's ability to shield, in particular, low-frequency electromagnetic waves can be further improved. On the other hand, if the content of thin film graphite in the electromagnetic wave shielding sheet is equal to or less than the above-mentioned upper limit, the film quality of the electromagnetic wave shielding sheet can be maintained at a good level.

[0049] Furthermore, the mass ratio of thin film graphite to single-walled CNTs in the electromagnetic wave shielding sheet (thin film graphite / single-walled CNTs) is preferably 1 / 3 or more, more preferably 2 / 3 or more, and preferably 3 / 1 or less, and more preferably 3 / 2 or less. If the mass ratio of thin film graphite to single-walled CNTs in the electromagnetic wave shielding sheet (thin film graphite / single-walled CNTs) is at least the above lower limit, the electromagnetic wave shielding sheet's ability to shield, particularly low-frequency electromagnetic waves, can be further improved. On the other hand, if the mass ratio of thin film graphite to single-walled CNTs in the electromagnetic wave shielding sheet (thin film graphite / single-walled CNTs) is at most the above upper limit, the film quality of the electromagnetic wave shielding sheet can be maintained at a good level.

[0050] The total content of single-walled CNTs, multi-walled CNTs, and thin film graphite in the electromagnetic wave shielding sheet is preferably 40% by mass or more, more preferably 45% by mass or more, and preferably 70% by mass or less, and more preferably 55% by mass or less. If the total content of single-walled CNTs, multi-walled CNTs, and thin film graphite in the electromagnetic wave shielding sheet is equal to or greater than the above-mentioned lower limit, the shielding performance of the electromagnetic wave shielding sheet can be further improved. On the other hand, if the total content of single-walled CNTs, multi-walled CNTs, and thin film graphite in the electromagnetic wave shielding sheet is equal to or less than the above-mentioned upper limit, the film quality of the electromagnetic wave shielding sheet can be maintained at a good level.

[0051] <Polymer> The polymer is a material used to impart good film quality and mechanical strength to the electromagnetic wave shielding sheet. From the viewpoint of improving the film quality and mechanical strength of the electromagnetic wave shielding sheet and further improving the shielding performance, it is preferable to use rubber as the polymer.

[0052] Examples of rubber include natural rubber; fluororubbers such as vinylidene fluoride rubber (FKM) and tetrafluoroethylene-propylene rubber (FEPM); diene rubbers such as butadiene rubber (BR), isoprene rubber (IR), styrene-butadiene rubber (SBR), hydrogenated styrene-butadiene rubber, styrene-isoprene rubber, hydrogenated styrene-isoprene rubber, styrene-isoprene-styrene block copolymer (SIS) and its hydrogenated product (H-SIS), nitrile rubber (NBR), and hydrogenated nitrile rubber (H-NBR); and silicone rubber.

[0053] The above-mentioned polymers such as rubber may be used singly or in combination of two or more kinds in any ratio.

[0054] Furthermore, it is preferable to use a polymer that is insoluble in the dispersion medium (such as water) in the emulsified dispersion that can be used in producing the electromagnetic wave shielding sheet described below. In the present invention, a substance being "insoluble" in a "dispersion medium" such as water means that when 0.5 g of the substance is dissolved in 100 g of the dispersion medium at 25°C, the insoluble content is 90 mass % or more.

[0055] The content of the polymer in the electromagnetic wave shielding sheet is preferably 20% by mass or more, more preferably 25% by mass or more, and preferably 50% by mass or less, and more preferably 40% by mass or less. If the content of the polymer in the electromagnetic wave shielding sheet is equal to or greater than the above-mentioned lower limit, the film quality and mechanical strength of the electromagnetic wave shielding sheet can be improved. On the other hand, if the content of the polymer in the electromagnetic wave shielding sheet is equal to or less than the above-mentioned upper limit, the total content of the single-walled CNTs, multi-walled CNTs, and thin film graphite described above becomes relatively high, and therefore the electromagnetic wave shielding sheet can ensure sufficiently high shielding performance.

[0056] Furthermore, the mass ratio of the polymer to the single-walled CNTs (polymer / single-walled CNTs) in the electromagnetic wave shielding sheet is preferably 1 / 1 or more, more preferably 2 / 1 or more, and preferably 5 / 1 or less, and more preferably 3 / 1 or less. If the mass ratio of the polymer to the single-walled CNTs (polymer / single-walled CNTs) in the electromagnetic wave shielding sheet is at or above the lower limit, the film quality and mechanical strength of the electromagnetic wave shielding sheet can be improved. On the other hand, if the mass ratio of the polymer to the single-walled CNTs (polymer / single-walled CNTs) in the electromagnetic wave shielding sheet is at or below the upper limit, the electromagnetic wave shielding sheet can ensure sufficiently high shielding performance.

[0057] <Other ingredients> The electromagnetic wave shielding sheet of the present invention may further contain components (other components) other than the above-mentioned single-walled CNT, multi-walled CNT, thin film graphite, and polymer.

[0058] Such other components include, for example, thickeners that can be contained in the emulsified dispersion that can be used in producing the electromagnetic wave shielding sheet described below. If a thickener is used in the emulsified dispersion, the single-walled CNTs in the emulsified dispersion are less likely to aggregate, making it possible to obtain an electromagnetic wave shielding sheet that is homogeneous and has even better shielding performance.

[0059] Examples of the thickener include sodium carboxymethylcellulose (CMC), xanthan gum, etc. The thickener may be used alone or in combination of two or more. When the thickener contains water, it is preferable that the thickener is a water-soluble polymer.

[0060] In the present invention, a substance being "water-soluble" means that when 0.5 g of the substance is dissolved in 100 g of water at 25°C, the insoluble content is less than 5 mass %.

[0061] The content of the thickener in the electromagnetic wave shielding sheet is preferably 5% by mass or more, more preferably 10% by mass or more, and preferably 30% by mass or less, and more preferably 25% by mass or less. If the content of the thickener in the electromagnetic wave shielding sheet is within the above range, the electromagnetic wave shielding sheet can be made more homogeneous and the shielding performance of the electromagnetic wave shielding sheet can be further improved.

[0062] Furthermore, the mass ratio of the thickener to the single-walled CNTs in the electromagnetic wave shielding sheet (thickener / single-walled CNTs) is preferably at least 1 / 3, more preferably at least 2 / 3, and is preferably at most 3 / 1, more preferably at most 3 / 2. If the mass ratio of the thickener to the single-walled CNTs in the electromagnetic wave shielding sheet (thickener / single-walled CNTs) is within the above range, the electromagnetic wave shielding sheet can be made more homogeneous and the shielding performance of the electromagnetic wave shielding sheet can be further improved.

[0063] The electromagnetic wave shielding sheet of the present invention may further contain oil such as mineral oil or liquid paraffin, within the range in which the desired effects of the present invention can be obtained. The oil is a component that can be contained as an emulsified dispersion material in the emulsified dispersion liquid that can be used in producing the electromagnetic wave shielding sheet, which will be described later.

[0064] <Porosity> The electromagnetic wave shielding sheet of the present invention must have a porosity of 1% or more, preferably 1.2% or more, more preferably 1.4% or more, even more preferably 1.6% or more, and preferably 2.6% or less, more preferably 2.4% or less, and even more preferably 2.2% or less. If the porosity of the electromagnetic wave shielding sheet is less than 1%, the shielding performance of the electromagnetic wave shielding sheet will be reduced. On the other hand, if the porosity of the electromagnetic wave shielding sheet is 1% or more, the electromagnetic wave shielding sheet will be able to exhibit excellent shielding performance. Furthermore, if the porosity of the electromagnetic wave shielding sheet is 2.6% or less, the shielding performance of the electromagnetic wave shielding sheet can be further improved. The porosity of the electromagnetic wave shielding sheet can be adjusted by the types of components (materials) used in producing the electromagnetic wave shielding sheet, the proportions of each component, the method and conditions for producing the electromagnetic wave shielding sheet, and so on.

[0065] <Gap size> In addition, the surface of the electromagnetic wave shielding sheet of the present invention is 2 Of the voids present per square millimeter on the surface of the electromagnetic wave shielding sheet, the number of voids having a size of 100 μm or less is preferably 200 or more, more preferably 300 or more, and even more preferably 350 or more. 2 If the number of voids having a size of 100 μm or less among the voids present per unit area is 200 or more, the shielding performance of the electromagnetic wave shielding sheet can be further improved. In addition, the surface of the electromagnetic wave shielding sheet is 1mm 2 The number of voids having a size of 100 μm or less per unit area is not particularly limited, but is preferably 1000 or less.

[0066] And the surface of the electromagnetic wave shielding sheet is 1mm 2 In a frequency distribution created with a class width of 5 μm for the size of voids present per unit area, it is preferable that the number of voids in the class with a size of more than 0 μm and not more than 5 μm be the largest. If the number of voids in the class with a size of more than 0 μm and not more than 5 μm be the largest in the above-mentioned predetermined frequency distribution, the shielding performance of the electromagnetic wave shielding sheet can be further improved.

[0067] Furthermore, the surface of the electromagnetic wave shielding sheet is 1mm thick. 2 The proportion of voids having a size of more than 0 μm and 5 μm or less to the total number of voids present per 1 mm of the surface of the electromagnetic wave shielding sheet is preferably 20% or more, and more preferably 25% or more. 2 If the ratio of the number of voids having a size of more than 0 μm and not more than 5 μm to the total number of voids present per unit area is equal to or greater than the above-mentioned predetermined value, the shielding performance of the electromagnetic wave shielding sheet can be further improved. In addition, the surface of the electromagnetic wave shielding sheet is 1mm2 The ratio of the number of voids with a size greater than 0 μm and less than or equal to 5 μm to the total number of voids per hit is not particularly limited, but is preferably 80% or less.

[0068] <BET specific surface area> Moreover, the BET specific surface area of the electromagnetic wave shielding sheet of the present invention is preferably 5 m 2 / g or more, more preferably 6 m 2 / g or more, still more preferably 7 m 2 / g or more, and preferably 20 m 2 / g or less, more preferably 15 m 2 / g or less, still more preferably 10 m 2 / g or less. If the BET specific surface area of the electromagnetic wave shielding sheet is within the above range, the shielding performance of the electromagnetic wave shielding sheet can be further improved.

[0069] <Thickness> The thickness of the electromagnetic wave shielding sheet of the present invention is preferably 5 μm or more, preferably 150 μm or less, and more preferably 100 μm or less. If the thickness is 5 μm or more, the electromagnetic wave shielding sheet can have sufficient mechanical strength and exhibit excellent shielding performance. On the other hand, if the thickness is 150 μm or less, the electromagnetic wave shielding sheet can be thinned and lightened. In the present invention, the "thickness" of the electromagnetic wave shielding sheet can be measured using the method described in the examples of this specification.

[0070] <Method for manufacturing electromagnetic wave shielding sheet> The electromagnetic wave shielding sheet of the present invention is not particularly limited. For example, it can be formed using an emulsion dispersion liquid in which an emulsion dispersion material containing a polymer, single-layer CNT, multi-layer CNT, and thin film graphite are dispersed in a dispersion medium. Specifically, the electromagnetic wave shielding sheet of the present invention can be obtained by removing at least a part of the dispersion medium from the above emulsion dispersion liquid and sheet-forming the solid content in the emulsion dispersion liquid.

[0071] <<Emulsion dispersion liquid>> The emulsified dispersion contains at least a dispersion medium, an emulsified dispersion material containing a polymer, single-walled CNTs, multi-walled CNTs, and thin film graphite, and optionally further contains other components such as a thickener.

[0072] -Dispersion medium- The dispersion medium is not particularly limited, but water and water-soluble organic solvents such as ethanol, methanol, isopropanol, 1-methyl-2-pyrrolidone, dimethylformamide, dimethyl sulfoxide, dimethylacetamide, tetrahydrofuran, acetonitrile, ethylene glycol, and butyl alcohol can be preferably used. The dispersion medium may be used alone or in combination of two or more. From the viewpoint of preparing an emulsion dispersion in which various components are well dissolved or dispersed and further improving the shielding performance of the resulting electromagnetic wave shielding sheet, the dispersion medium is preferably water, ethanol or methanol, and more preferably water.

[0073] -Emulsifying and dispersing material- The emulsified dispersion material to be emulsified and dispersed in the emulsified dispersion liquid contains a polymer, and optionally further contains components other than the polymer.

[0074] The polymer used here may be the same as the polymer contained in the electromagnetic wave shielding sheet described above. The proportion of the polymer content relative to the total solid content in the emulsion dispersion may be appropriately set within the same range as the preferred range of the polymer content in the electromagnetic wave shielding sheet described above.

[0075] The emulsifying and dispersing material may further contain an organic solvent that is not water-soluble (a non-water-soluble organic solvent), such as toluene, cyclohexane, and methyl ethyl ketone (MEK).

[0076] The mass ratio of the polymer to the water-insoluble organic solvent contained in the emulsified dispersion material (polymer / water-insoluble organic solvent) is preferably 1 / 4 or more, more preferably 1 / 3 or more, and preferably 2 / 1 or less, and more preferably 1 / 1 or less. If the mass ratio of the polymer to the water-insoluble organic solvent contained in the emulsified dispersion material (polymer / water-insoluble organic solvent) is within the above range, the polymer can be well dispersed in the emulsified dispersion, thereby making it possible to obtain a homogeneous electromagnetic wave shielding sheet with even better shielding performance.

[0077] The emulsifying and dispersing material may further contain oil such as mineral oil or liquid paraffin in an amount within the range in which the desired effects of the present invention are obtained.

[0078] -Single-walled CNT- The single-walled CNTs may have the same properties as the single-walled CNTs contained in the electromagnetic wave shielding sheet described above. The ratio of the content of the single-walled CNTs to the total solid content in the emulsion dispersion may be set appropriately within the same range as the preferred range of the content of the single-walled CNTs in the electromagnetic wave shielding sheet described above.

[0079] Furthermore, the content of single-walled CNTs in the emulsified dispersion is not particularly limited, but from the viewpoint of obtaining an electromagnetic wave shielding sheet with even better shielding performance, it is preferably 1 part by mass or more, more preferably 5 parts by mass or more, and even more preferably 10 parts by mass or more, and preferably 60 parts by mass or less, more preferably 30 parts by mass or less, and even more preferably 20 parts by mass or less, per 100 parts by mass of the emulsified dispersion material.

[0080] -Multilayer CNT- The multi-walled CNTs may have the same properties as the multi-walled CNTs contained in the electromagnetic wave shielding sheet described above. The content ratio of the multi-walled CNTs to the total solid content in the emulsion dispersion may be appropriately set within the same range as the preferred content ratio of the multi-walled CNTs in the electromagnetic wave shielding sheet described above.

[0081] Furthermore, the content of multi-walled CNTs in the emulsified dispersion is not particularly limited, but from the viewpoint of obtaining an electromagnetic wave shielding sheet with even better shielding performance, it is preferably 2 parts by mass or more, more preferably 7 parts by mass or more, and even more preferably 14 parts by mass or more, and preferably 70 parts by mass or less, more preferably 50 parts by mass or less, and even more preferably 40 parts by mass or less, per 100 parts by mass of the emulsified dispersion material.

[0082] -Thin film graphite- The thin film graphite may be the same as the thin film graphite contained in the electromagnetic wave shielding sheet described above. The content ratio of the thin film graphite to the total solid content in the emulsion dispersion may be set appropriately within the same range as the preferred content ratio of the thin film graphite in the electromagnetic wave shielding sheet described above.

[0083] Furthermore, the content of thin film graphite in the emulsified dispersion is not particularly limited, but from the viewpoint of obtaining an electromagnetic wave shielding sheet with even better shielding performance, it is preferably 1 part by mass or more, more preferably 5 parts by mass or more, and even more preferably 10 parts by mass or more, and preferably 60 parts by mass or less, more preferably 30 parts by mass or less, and even more preferably 20 parts by mass or less, per 100 parts by mass of the emulsified dispersion material.

[0084] -Other ingredients- Other components that can be contained in the emulsified dispersion include, for example, the thickeners that can be contained in the electromagnetic wave shielding sheet described above. The proportion of the thickener content relative to the total solid content in the emulsified dispersion can be appropriately set within the same range as the preferred range of the thickener content in the electromagnetic wave shielding sheet described above.

[0085] Furthermore, the content of the thickener in the emulsified dispersion is not particularly limited, but from the viewpoint of obtaining an electromagnetic wave shielding sheet with even better shielding performance, it is preferably 2 parts by mass or more, more preferably 7 parts by mass or more, and even more preferably 14 parts by mass or more, and preferably 70 parts by mass or less, more preferably 50 parts by mass or less, and even more preferably 40 parts by mass or less, per 100 parts by mass of the emulsified dispersion material.

[0086] -Emulsified dispersion state- The emulsion dispersion that can be used to form the electromagnetic wave shielding sheet of the present invention preferably has an emulsion dispersion material containing a polymer, single-walled CNTs, multi-walled CNTs, and thin film graphite in the following emulsion dispersion state:

[0087] In the emulsion dispersion that can be used to form the electromagnetic wave shielding sheet of the present invention, the polymer-containing emulsion dispersion material is dispersed in the dispersion medium while being surrounded by thin film graphite, and CNTs (single-walled CNTs and multi-walled CNTs) are attached to the surface of the thin film graphite. In this dispersed state, the CNTs present on the surface function as spacers, preventing the thin film graphite surrounding the emulsion dispersion material from contacting each other, making it possible to suppress aggregation of the thin film graphite. The emulsified dispersion state is shown schematically in Fig. 1. In Fig. 1, the surface of particulate emulsified dispersion material 1 is covered with thin film graphite 2, and carbon nanotubes 3 are attached to the surface of thin film graphite 2 (the outer surface of the particle formed by covering the surface of emulsified dispersion material 1 with thin film graphite 2). Note that thin film graphite 2 may cover a part or the entire surface of emulsified dispersion material 1. It is believed that the lipophilicity (hydrophobicity) of thin film graphite contributes to the formation of such an emulsified dispersion state. That is, because thin film graphite has lipophilicity, it easily adheres to the surface of the emulsified dispersion material, and at the same time, the thin film graphite also adheres well to the CNTs. This is thought to be why the above-mentioned emulsified dispersion state can be formed. In other words, it is thought that thin film graphite functions as a so-called emulsifier.

[0088] -Method for preparing emulsion dispersion- The emulsified dispersion having the above-mentioned emulsified dispersion state can be obtained by subjecting a crude dispersion containing the above-mentioned dispersion medium, an emulsified dispersion material containing a polymer, single-walled CNTs, multi-walled CNTs, graphite as a raw material for thin film graphite, and other components added as necessary, to a dispersion treatment that provides a defibration effect. In the dispersion process that produces this defibration effect, shear force is applied to the crude dispersion to defibrate and disperse the aggregates of CNTs (single-walled CNTs and multi-walled CNTs), and then back pressure is applied to the crude dispersion, and the crude dispersion is cooled as necessary, thereby suppressing the generation of bubbles and uniformly dispersing components such as CNTs in the dispersion medium. When a back pressure is applied to the crude dispersion, the back pressure applied to the crude dispersion may be reduced to atmospheric pressure in one go, but it is preferable to reduce the pressure in multiple stages.

[0089] Here, in order to apply shear force to the crude dispersion to further disperse the components such as CNT, for example, a dispersion system having a disperser with the following structure may be used. That is, the disperser is provided with, from the inlet side to the outlet side of the crude dispersion, a disperser orifice with an inner diameter of d1, a dispersion space with an inner diameter of d2, and a terminal portion with an inner diameter of d3 (where d2>d3>d1). In this disperser, the inflowing coarse dispersion at high pressure (for example, 10 to 400 MPa, preferably 50 to 250 MPa) passes through the disperser orifice, whereby the pressure decreases and the coarse dispersion becomes a high-flow-rate fluid and flows into the dispersion space. The high-flow-rate coarse dispersion then flows at high speed within the dispersion space, and is subjected to shear forces during this process. As a result, the flow rate of the coarse dispersion decreases, and the components such as CNTs are dispersed well. Then, from the end, a fluid at a pressure (back pressure) lower than the pressure of the inflowing coarse dispersion flows out as an emulsified dispersion.

[0090] The back pressure of the crude dispersion can be applied to the crude dispersion by applying a load to the flow of the crude dispersion. For example, a desired back pressure can be applied to the crude dispersion by disposing a multistage pressure reducer downstream of the disperser. Furthermore, by reducing the back pressure of the crude dispersion in multiple stages using a multistage pressure reducer, it is possible to prevent bubbles from being generated in the emulsified dispersion when the emulsified dispersion is finally released to atmospheric pressure.

[0091] The disperser may also be equipped with a heat exchanger or a cooling liquid supply mechanism for cooling the crude dispersion, because by cooling the crude dispersion that has been heated to a high temperature by the shear force applied in the disperser, it is possible to further suppress the generation of bubbles in the crude dispersion. Instead of providing a heat exchanger or the like, the generation of bubbles in the emulsified dispersion can also be suppressed by cooling the crude dispersion in advance.

[0092] Examples of distributed systems having the above configuration include the distributed systems described in Patent Nos. 5791142, 59772434, and 6585250, and more specifically, the product name "BERYU SYSTEM PRO" (manufactured by Biryu Co., Ltd.). The dispersion treatment that produces the defibration effect can be carried out by using such a dispersion system and appropriately controlling the dispersion conditions.

[0093] The method for preparing the crude dispersion liquid to be subjected to the above-mentioned dispersion treatment, which contains at least a dispersion medium, an emulsified dispersion material containing a polymer, single-walled CNTs, multi-walled CNTs, and graphite, is not particularly limited. For example, when a thickener is used as another component in addition to the above components, the crude dispersion is preferably prepared by subjecting a mixed liquid containing a dispersion medium, single-walled CNTs, multi-walled CNTs, graphite, and a thickener to a pre-dispersion treatment, and then adding an emulsified dispersion material to the pre-dispersion obtained through the pre-dispersion treatment. In this way, by subjecting the CNTs and the thickener to a pre-dispersion treatment prior to the addition of the emulsified dispersion material, the CNTs are less likely to aggregate in the resulting emulsified dispersion, and a homogeneous electromagnetic wave shielding sheet with even better shielding performance can be obtained. The method of pre-dispersion is not particularly limited, but it is preferable to use a dispersion treatment that can obtain a defibrating effect, similar to the dispersion treatment described above.

[0094] From the viewpoint of obtaining an electromagnetic wave shielding sheet with even better shielding performance, the emulsified dispersion obtained by the above-mentioned method or the like preferably has a solids concentration of 0.5% by mass or more, more preferably 1.0% by mass or more, and preferably 5% by mass or less, more preferably 3% by mass or less.

[0095] <<Removal of dispersion medium (forming an electromagnetic wave shielding sheet)>> Then, at least a part of the dispersion medium is removed from the emulsified dispersion obtained above, and the solid content in the emulsified dispersion is formed into a sheet, thereby forming an electromagnetic wave shielding sheet.

[0096] Here, the method for removing the dispersion medium includes known methods such as filtration and drying.

[0097] The filtration method is not particularly limited, and known filtration methods such as natural filtration, reduced pressure filtration (suction filtration), pressure filtration, and centrifugal filtration can be used. As the drying method, known drying methods such as hot air drying, vacuum drying, heat roll drying, infrared irradiation, etc. can be used. The drying temperature is not particularly limited, but is usually room temperature to 200°C, and the drying time is not particularly limited, but is usually 1 hour or more and 48 hours or less. In addition, drying can be carried out on a known substrate, but is not particularly limited.

[0098] Among these, it is preferable to employ at least drying for removing the dispersion medium, that is, the electromagnetic wave shielding sheet of the present invention is preferably a dried product of the above-mentioned emulsified dispersion. The filtration and drying may be combined. For example, the membrane-like residue (primary sheet) obtained by filtering the emulsion dispersion may be further dried to obtain an electromagnetic wave shielding sheet. [Example]

[0099] The present invention will be described in more detail below using examples, but the present invention is not limited to these examples. In the examples and comparative examples, the thickness, film quality, porosity, BET specific surface area, and shielding performance of the electromagnetic wave shielding sheet were measured or evaluated using the following methods. In the examples and comparative examples, a frequency distribution of the size of voids present on the surface of the electromagnetic wave shielding sheet was also created using the following method.

[0100] <Thickness> The thickness of the electromagnetic wave shielding sheet was measured using a "Digimatic Standard Outside Micrometer" manufactured by Mitutoyo Corporation.

[0101] <Membrane quality> The number of wrinkles and holes visually observed per electromagnetic wave shielding sheet was counted and the film quality was evaluated according to the following criteria: The fewer the number of wrinkles and holes visually observed, the better the film quality of the electromagnetic wave shielding sheet. A: No wrinkles or holes are observed on a 10 cm square. B: Wrinkles are observed in one or more places on a 10 cm square, but no holes are observed on a 10 cm square. C: Wrinkles and holes are observed in one or more places on a 10cm square.

[0102] <Porosity> The porosity of the electromagnetic wave shielding sheet was measured using an image obtained by CT (Computed Tomography) of the electromagnetic wave shielding sheet. Specifically, first, non-destructive observation of the electromagnetic wave shielding sheet was performed using a three-dimensional X-ray CT device (Yamato Scientific Co., Ltd.'s "TDM1000-IS / SP"), and the obtained data was analyzed to obtain a tomographic image. Next, a three-dimensional image was created from the obtained tomographic image using analysis software (three-dimensional volume rendering software: "VG-Studio MAX" manufactured by Volume Graphics). The internal situation was confirmed at an arbitrary cross-section of the created three-dimensional image, and the volume ratio of the void portion was calculated and taken as the porosity of the electromagnetic wave shielding sheet.

[0103] <BET specific surface area> The BET specific surface area of the electromagnetic wave shielding sheet was measured using "BELSORP-Max" manufactured by MicrotracBEL Corporation.

[0104] <Shielding performance> Based on the ASTM standard (ASTM D4935), coaxial waveguides were opposed, an electromagnetic wave shielding sheet was inserted into the joint, and using a vector network analyzer (VNA), the transmission loss (S21 parameter) was obtained from the level difference between when the sample was inserted and when it was not inserted, and the shielding amount was calculated from S21. The S21 when the electromagnetic wave shielding sheet was inserted into the joint and the S21 in the empty state without encapsulation were each measured, and the shielding amount (dB) was calculated from the difference in amplitude between the two expressed in dB. The measurement wavelength range was 100 kHz to 3 GHz.

[0105] <Frequency distribution of void size> Using the same three-dimensional X-ray CT device and analysis software as those used in the above measurement of porosity, CT of the electromagnetic wave shielding sheet was performed to create a three-dimensional image. From the obtained three-dimensional image, the size and number of voids existing on the surface of the electromagnetic wave shielding sheet were measured. The "size" of the void refers to the maximum pore diameter of the void observed on the obtained image. And for the size of the voids existing per 1 mm 2 on the surface of the electromagnetic wave shielding sheet, a frequency distribution was created with a class width of 5 μm.

[0106] Example 1 <Preparation of emulsion dispersion> 99 g of water and 1 g of sodium carboxymethylcellulose (CMC) were stirred using a stirrer to dissolve the CMC in water. 100 g of this CMC aqueous solution was added with SGCNTs (manufactured by Zeon Nano Technology Co., Ltd., "ZEONANO (registered trademark) SG101", no aperture treatment, BET specific surface area: 1,050 m) as single-walled CNTs. 2 / g, average diameter: 3.3 nm, average length: 400 μm, t-plot is convex upward (position of inflection point: 0.6 nm), internal specific surface area S2 / total specific surface area S1: 0.24), and 0.6 g of multi-walled CNT (Nanocyl, product name "NC7000", BET specific surface area: 250 m 2 1 g of cellulose acetate (1.0 g, average diameter: 9.5 nm, average length: 1.5 μm) and 0.6 g of graphite (manufactured by Ito Graphite Co., Ltd., product name "Z-5F") were added, and the mixture was loaded into a high-pressure homogenizer (manufactured by Biryu Co., Ltd., product name "BERYU SYSTEM PRO") equipped with a multistage pressure control device (multistage pressure reducer) that applies back pressure during dispersion, and pre-dispersion treatment was performed at a pressure of 100 MPa. This pre-dispersion treatment was repeated a total of three times. To the resulting pre-dispersion liquid, 180 g of water and 1.68 g of butadiene rubber (manufactured by Nippon Zeon Co., Ltd., product name "Nipol (registered trademark) BR 1250H") and 3.12 g of toluene (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., product name "Wako Special Grade Toluene") were added as emulsifying and dispersing materials, and the mixture was loaded into the same high-pressure homogenizer as above, and dispersion treatment was performed at a pressure of 100 MPa. This dispersion process was repeated a total of three times to obtain an emulsified dispersion (solid content: 1.7% by mass). The obtained emulsified dispersion was observed under a microscope, and it was confirmed that the emulsified dispersion material was dispersed in water surrounded by thin film graphite, and that CNTs (single-walled CNTs and multi-walled CNTs) were attached to the surface of the thin film graphite.

[0107] <Production of electromagnetic wave shielding sheet> The emulsion dispersion obtained as described above was applied to a substrate. The coating film on the substrate was vacuum dried at a temperature of 80° C. for 24 hours. Thereafter, it was peeled off from the substrate to obtain an electromagnetic wave shielding sheet having a thickness of 100 μm. The thickness, film quality, porosity, and BET specific surface area of ​​the obtained electromagnetic wave shielding sheet were measured and evaluated according to the methods described above. The results are shown in Table 1. The shielding performance of the obtained electromagnetic wave shielding sheet was also measured according to the methods described above. The results are shown in Figure 2. Furthermore, a frequency distribution of the size of voids present on the surface of the obtained electromagnetic wave shielding sheet was created according to the methods described above. The created frequency distribution is shown in Table 2.

[0108] (Comparative Example 1) An emulsified dispersion and an electromagnetic wave shielding sheet were prepared and produced in the same manner as in Example 1, except that in preparing the emulsified dispersion of Example 1, 0.6 g of thin film graphite was not added. The thickness, film quality, porosity, and BET specific surface area of ​​the obtained electromagnetic wave shielding sheet were measured and evaluated according to the methods described above. The results are shown in Table 1. The shielding performance of the obtained electromagnetic wave shielding sheet was also measured according to the methods described above. The results are shown in Figure 2. Furthermore, a frequency distribution of the size of voids present on the surface of the obtained electromagnetic wave shielding sheet was created according to the methods described above. The created frequency distribution is shown in Table 2.

[0109] (Comparative Example 2) An emulsified dispersion and an electromagnetic wave shielding sheet were prepared and produced in the same manner as in Example 1, except that in the preparation of the emulsified dispersion of Example 1, 1 g of multi-walled CNTs was not added. The thickness, film quality, and porosity of the obtained electromagnetic wave shielding sheet were measured and evaluated according to the above-mentioned methods. The results are shown in Table 1. An attempt was made to measure the BET specific surface area of ​​the obtained electromagnetic wave shielding sheet, but the value was below the lower limit of measurement, making it impossible to obtain a measured value. The shielding performance of the obtained electromagnetic wave shielding sheet was also measured according to the above-mentioned method. The results are shown in Figure 2. Furthermore, a frequency distribution of the size of voids present on the surface of the obtained electromagnetic wave shielding sheet was created according to the above-mentioned method. The created frequency distribution is shown in Table 2.

[0110] (Comparative Example 3) An emulsified dispersion and an electromagnetic wave shielding sheet were prepared and produced in the same manner as in Example 1, except that in preparing the emulsified dispersion of Example 1, 0.6 g of single-walled CNTs was not added. The thickness, film quality, and porosity of the obtained electromagnetic wave shielding sheet were measured and evaluated according to the above-mentioned methods. The results are shown in Table 1. An attempt was made to measure the BET specific surface area of ​​the obtained electromagnetic wave shielding sheet, but it was below the lower limit of measurement, making it impossible to obtain a measured value. Furthermore, the obtained electromagnetic wave shielding sheet had poor film quality and was full of holes, making it impossible to measure its shielding performance. Furthermore, a frequency distribution of the size of voids present on the surface of the obtained electromagnetic wave shielding sheet was created according to the above-mentioned method. The created frequency distribution is shown in Table 2.

[0111] [Table 1]

[0112] [Table 2]

[0113] 2 shows that the electromagnetic wave shielding sheet of Example 1, which contains single-walled CNTs, multi-walled CNTs, thin film graphite, and a polymer and has a porosity of 1% or more, can exhibit excellent shielding performance (e.g., a shielding amount of 20 dB or more) over a wide frequency range, compared to the electromagnetic wave shielding sheets of Comparative Examples 1 and 2, which do not contain multi-walled CNTs or thin film graphite. The electromagnetic wave shielding sheet of Comparative Example 3, which does not contain single-walled CNTs, had poor film quality and was full of holes, making it impossible to measure the shielding performance. [Industrial Applicability]

[0114] According to the present invention, it is possible to provide an electromagnetic wave shielding sheet having excellent shielding performance. [Explanation of symbols]

[0115] 1 Emulsifying and dispersing material 2 Thin film graphite 3. Carbon nanotubes

Claims

1. single-walled carbon nanotubes, multi-walled carbon nanotubes, thin film graphite, and a polymer; An electromagnetic wave shielding sheet having a porosity of 1% or more and 2.6% or less as measured by the following measurement method. [Method for measuring void ratio] A three-dimensional X-ray CT device is used to perform non-destructive observation of the electromagnetic wave shielding sheet, and the obtained data is analyzed to obtain a tomographic image. Next, a three-dimensional image is created from the obtained tomographic image using analysis software. The internal condition is confirmed at any cross-section of the created three-dimensional image, and the volume fraction of the voids is calculated to obtain the porosity of the electromagnetic wave shielding sheet.

2. The surface of the electromagnetic wave shielding sheet is 1 mm 2 2. The electromagnetic wave shielding sheet according to claim 1, wherein the number of voids having a size of 100 μm or less per unit area is 200 or more.

3. The surface of the electromagnetic wave shielding sheet is 1 mm 2 3. The electromagnetic wave shielding sheet according to claim 1, wherein in a frequency distribution created with a class width of 5 μm for the size of voids present per unit area, the number of voids in the class with a size of more than 0 μm and 5 μm or less is the largest.

4. The surface of the electromagnetic wave shielding sheet is 1 mm 2 4. The electromagnetic wave shielding sheet according to claim 1, wherein the proportion of voids having a size of more than 0 μm and not more than 5 μm to the total number of voids present per unit area is 20% or more.

5. 5. The electromagnetic wave shielding sheet according to claim 1, wherein the total content of the single-walled carbon nanotubes, the multi-walled carbon nanotubes, and the thin film graphite is 40 mass % or more.

6. 6. The electromagnetic wave shielding sheet according to claim 1, wherein the mass ratio of multi-walled carbon nanotubes to single-walled carbon nanotubes (multi-walled carbon nanotubes / single-walled carbon nanotubes) is greater than 1 / 1.

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