Aqueous dispersion for forming electromagnetic wave shielding film

Abstract

To provide a new aqueous dispersion for forming an electromagnetic wave shielding film from which an electromagnetic wave shielding film having good coating property and adhesion to a resin base material and good electrical properties can be obtained.SOLUTION: An aqueous dispersion for forming an electromagnetic wave shielding film contains water, a carbon black, and a urethane dispersion, wherein a ratio of the mass of the particles of the urethane dispersion to the mass of the carbon black is 0.55 to 1.20.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to an aqueous dispersion for forming an electromagnetic wave shielding film. [Background technology]

[0002] In recent years, electronic devices such as personal computers, mobile phones, and tablet terminals have become widespread, and it is now possible to exchange information using these electronic devices not only at home or at work, but also indoors or outdoors. However, while these electronic devices are highly convenient, there is a problem in that unwanted electromagnetic waves (noise) generated by the electronic devices can have adverse effects such as malfunctions on other electronic devices and electrical appliances. In order to solve such problems, various electromagnetic wave suppression sheets have been disclosed.

[0003] Patent Document 1 discloses a composition for forming an electromagnetic wave shielding film used to form a film having an electromagnetic wave shielding effect on a component that generates electromagnetic waves and / or a component that covers the component, the composition being liquid and ejected by an inkjet method, and characterized in that it contains a powder composed of an electromagnetic wave suppression material.

[0004] Patent Document 2 discloses an electromagnetic wave suppression sheet that is made of a resin composition (A) that contains an electromagnetic wave suppression substance, and that is characterized in that the resin composition (A) contains a polyurethane resin (a1) that has a 100% modulus of 1 to 7 MPa.

[0005] Patent Document 3 discloses a radio wave absorbing coating composition containing a coating film-forming resin and pitch-based carbon fiber, in which the content of the pitch-based carbon fiber is 2 to 15 volume % relative to 100 volume % of the solid content of the composition.

[0006] Patent Document 4 discloses an electromagnetic wave shielding paint that contains a resin that is a mixture of a water-dispersible copolymer resin compound and a solvent-soluble copolymer resin compound, a nonionic surfactant, and a conductive material.

[0007] Patent Document 5 discloses an electromagnetic wave absorbing coating agent that contains water, a water-based emulsion or a thickener, and magnetic material-supported coil-shaped carbon fibers.

[0008] Patent Document 6 discloses an inkjet recording ink in which composite particles including a black pigment containing carbon particles having a hollow structure and a resin are dispersed, the resin being an acrylic resin having an acid value of 10 mgKOH / g or more and 150 mgKOH / g or less and a neutralization rate of less than 80%. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] JP 2013-168399 A [Patent Document 2] JP 2017-45782 A [Patent Document 3] JP 2020-111730 A [Patent Document 4] JP 2015-220233 A [Patent Document 5] JP 2015-172152 A [Patent Document 6] Patent No. 5930176 Summary of the Invention [Problem to be solved by the invention]

[0010] The present invention provides a novel aqueous dispersion for forming an electromagnetic wave shielding film, which has good coatability and adhesion to a resin substrate and is capable of producing an electromagnetic wave shielding film having good electrical properties. [Means for solving the problem]

[0011] As a result of intensive research, the present inventors have found that the above problems can be solved by the following means, and have completed the present invention. That is, the present invention is as follows: <Aspect 1> Contains water, carbon black, and a urethane dispersion, the ratio of the mass of the urethane dispersion particles to the mass of the carbon black is 0.55 to 1.30; Aqueous dispersion for forming electromagnetic wave shielding film. <Aspect 2> The aqueous dispersion according to aspect 1, wherein the ratio of the mass of the urethane dispersion particles to the mass of the carbon black is 0.85 to 1.15. <Aspect 3> The aqueous dispersion according to aspect 1 or 2, wherein the average particle size of the particles of the urethane dispersion is 50 nm or less as measured by a laser diffraction method. <Aspect 4> The aqueous dispersion according to any one of aspects 1 to 3, further comprising a volatile base, the content of the volatile base being 0.1 to 3.0 mass %. <Aspect 5> The aqueous dispersion according to any one of Aspects 1 to 4, further comprising a gelling agent. <Aspect 6> The aqueous dispersion according to aspect 5, wherein the gelling agent is an alkali swelling association type gelling agent. Aspect 7: The carbon black has a hollow shell structure and a BET specific surface area of ​​1000 m2 measured by a nitrogen adsorption method. 2 / g or less. Aspect 8: A method for producing an electromagnetic wave-absorbing sheet, comprising applying the aqueous dispersion according to any one of Aspects 1 to 7 to a resin substrate and drying the applied coating to obtain an electromagnetic wave shielding film. Aspect 9: The method according to aspect 8, wherein the surface resistivity of the electromagnetic shielding film measured by a four-probe method is 400 Ω / □ or less. Aspect 10 includes a resin substrate and an electromagnetic wave shielding film on the resin substrate, The electromagnetic wave-shielding film is an electromagnetic wave-absorbing sheet obtained by drying the aqueous dispersion according to any one of aspects 1 to 7. <Aspect 11> The electromagnetic wave-absorbing sheet according to aspect 10, wherein the electromagnetic wave shielding film has a dry thickness of 100 μm or less and a volume resistivity of 1.0 Ω·cm or less. Effect of the Invention

[0012] It is possible to provide a novel aqueous dispersion for forming an electromagnetic wave shielding film, which has good coatability and adhesion to a resin substrate and is capable of producing an electromagnetic wave shielding film having good electrical properties. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a graph showing the relationship between the ratio of the mass of the binder to the mass of carbon black when a urethane dispersion is used as the binder, and the surface resistivity of the resulting electromagnetic wave shielding film. [Diagram 2] FIG. 2 is a graph showing the relationship between the ratio of the mass of the binder to the mass of carbon black and the surface resistivity of the resulting electromagnetic wave shielding film when a styrene-acrylic resin or an acrylic resin is used as the binder. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] <Aqueous dispersion for forming electromagnetic wave shielding film> The aqueous dispersion for forming an electromagnetic wave shielding film of the present invention comprises: It contains water, carbon black, and a urethane dispersion. The ratio of the mass of the urethane dispersion particles to the mass of the carbon black is 0.55 to 1.20.

[0015] The present inventors have found that the above-mentioned composition makes it possible to obtain an electromagnetic wave shielding film that has good coatability onto a resin substrate and good electrical properties.

[0016] The ratio of the mass of the urethane dispersion particles to the mass of the carbon black may be 0.55 or more, 0.60 or more, 0.65 or more, 0.70 or more, 0.80 or more, or 0.85 or more, and may be 1.30 or less, 1.20 or less, 1.15 or less, 1.10 or less, 1.05 or less, or 1.00 or less. Furthermore, at a mass ratio within such a range, the surface resistivity of the resulting electromagnetic wave shielding film becomes low, thereby resulting in good electromagnetic wave absorption. Although not wishing to be bound by theory, it is believed that such good electromagnetic wave absorption can be obtained because the carbon black can be distributed appropriately at such a ratio without covering the surface of the carbon black and inhibiting conductivity.

[0017] The aqueous dispersion for forming an electromagnetic wave shielding film of the present invention may contain an optional dispersant. In this case, the mass ratio of the dispersant to the carbon black may be 0.17 or more, 0.18 or more, or 0.19 or more, and this mass ratio may be 0.40 or less, 0.35 or less, 0.30 or less, 0.28 or less, 0.25 or less, or 0.23 or less.

[0018] The aqueous dispersion for forming an electromagnetic wave shielding film of the present invention preferably contains a volatile base from the viewpoint of suppressing repellency of the coating film.

[0019] It is preferable that the aqueous dispersion for forming an electromagnetic wave shielding film of the present invention further contains a gelling agent, from the viewpoint of obtaining an electromagnetic wave shielding film of a desired thickness, for example, an electromagnetic wave shielding film with a dry film thickness of 5 μm or more, 7 μm or more, 10 μm or more, 15 μm or more, 17 μm or more, 20 μm or more, 25 μm or more, or 30 μm or more.

[0020] Each component of the present invention will now be described.

[0021] <water> As the water, distilled water, ion-exchanged water, etc. can be used.

[0022] Carbon Black As the carbon black, for example, Ketjen black can be used. The carbon black can have a hollow shell-like structure.

[0023] The BET specific surface area of ​​carbon black measured by nitrogen adsorption method is 1000m 2 / g or less, 900m 2 / g or less, 800m 2 / g or less, 700m 2 / g or less, 600m 2 / g or less, 500m 2 / g or less, 400m 2 / g or less, or 300m 2 / g or less. The BET specific surface area is 100 m 2 / g or more, or 200m 2 / g or more.

[0024] The carbon black content may be 5% by mass or more, 6% by mass or more, or 7% by mass or more, and may be 15% by mass or less, 12% by mass or less, 10% by mass or less, or 8% by mass or less, based on the mass of the aqueous dispersion for forming an electromagnetic wave shielding film.

[0025] <Dispersant> As the dispersant, for example, water-soluble resins such as polyvinylpyrrolidone, polyvinyl methyl ether, polyvinyl acetal, polyacrylic acid and its salts, polyethylene oxide, vinyl acetate-polyvinylpyrrolidone copolymer, styrene-acrylic acid copolymer and its salts, isobutylene-maleic anhydride copolymer and its salts, etc. can be used.

[0026] The content of the dispersant can be appropriately selected so as to satisfy the above-mentioned ratio of the dispersant to the carbon black. The content of the dispersant may be, for example, 0.1 mass% or more, 0.3 mass% or more, 0.5 mass% or more, 0.7 mass% or more, 0.9 mass% or more, 1.0 mass% or more, 1.1 mass% or more, 1.3 mass% or more, or 1.5 mass% or more, and may be 6.0 mass% or less, 5.5 mass% or less, 5.0 mass% or less, 4.5 mass% or less, 4.0 mass% or less, 3.5 mass% or less, 3.0 mass% or less, 2.8 mass% or less, or 2.5 mass% or less, based on the mass of the aqueous dispersion for forming an electromagnetic wave shielding film.

[0027] <Urethane dispersion> The urethane dispersion is a dispersion of particles used as a binder for carbon black, and one provided in the form of an aqueous dispersion can be used.

[0028] The average particle size of the particles of the urethane dispersion measured by the laser diffraction method may be 800nm ​​or less, 750nm or less, 700nm or less, 650nm or less, 600nm or less, 550nm or less, 500nm or less, 450nm or less, 400nm or less, 350nm or less, 300nm or less, 250nm or less, 230nm or less, 200nm or less, 170nm or less, 150nm or less, 130nm or less, 110nm or less, 100nm or less, 90nm or less, 80nm or less, 70nm or less, 60nm or less, 50nm or less, or 40nm or less. The average particle size used in this specification is the value of the median diameter (D50) calculated on a volume basis in the laser diffraction method. Measurement by the laser diffraction method can be performed, for example, using a particle size distribution measuring device MT3300II (Microtrack Bell Co., Ltd.).

[0029] The content of particles in the urethane dispersion is preferably 3% by mass or more, 4% by mass or more, or 5% by mass or more relative to the mass of the aqueous dispersion for forming an electromagnetic wave shielding film, from the viewpoint of suppressing repellency when the aqueous dispersion for forming an electromagnetic wave shielding film is applied to a resin substrate. This content may be 15% by mass or less, 13% by mass or less, or 11% by mass or less. Furthermore, with a content in such a range, the surface resistivity of the obtained electromagnetic wave shielding film becomes low, thereby improving the electromagnetic wave absorption.

[0030] <Volatile Bases> As the volatile base, a base that can be volatilized by a drying step, particularly a base having a vapor pressure of more than 1 mmHg at 25° C. can be used. As such a volatile base, for example, ammonia, urea, monoethanolamine, diethanolamine, triethanolamine, aminomethylpropanol, etc. can be used. These may be used alone or in combination.

[0031] The content of the volatile base may be 0.1 mass% or more, 0.3 mass% or more, 0.5 mass% or more, 0.7 mass% or more, 1.0 mass% or more, 1.1 mass% or more, 1.3 mass% or more, 1.5 mass% or more, or 1.7 mass% or more, and may be 3.0 mass% or less, 2.8 mass% or less, 2.5 mass% or less, 2.2 mass% or less, or 2.0 mass% or less, relative to the mass of the aqueous dispersion for forming an electromagnetic shielding film.

[0032] <Gelling Agent> As the gelling agent, for example, an alkali swelling type, particularly an alkali swelling association type, gelling agent is preferably used, from the viewpoint of maintaining the dispersed state of the carbon black in the aqueous dispersion for forming an electromagnetic wave shielding film, and thereby improving the electromagnetic wave absorbing properties of the obtained electromagnetic wave shielding film.

[0033] Here, the term "alkali swelling type" gelling agent refers to an unneutralized acrylic polymer that contains acid groups, mainly carboxyl groups, within the three-dimensional structure of the polymer, which swells when neutralized with an alkali and eventually dissolves when neutralized further.

[0034] In the present invention, among the alkali swelling type gelling agents, it is preferable to use an alkali swelling association type gelling agent. Here, the term "alkali swelling association type" gelling agent refers to a gelling agent that exerts a thickening effect by forming a loose bond between the molecules when the polymer swells, by associating a hydrophobic portion of the molecule with the corresponding portion of another molecule.

[0035] Alkali swelling association type gelling agents are thought to have two main thickening actions. The first thickening action is that they dissolve only in the alkaline region when mixed with water or a polar solvent, and the hydrophilic groups such as the carboxyl groups in the molecule hydrate and expand, causing steric hindrance in the solution and increasing the viscosity. The second thickening action is that the hydrophobic and hydrophilic groups in the molecule each separately associate and adsorb with the hydrophobic and hydrophilic groups of the ink components, such as pigments, solvents, and surfactants, respectively, to form a network-like aggregate, increasing the viscosity.

[0036] The alkali swelling association type gelling agent of the present invention may be a polymer having a carboxyl group and a hydrophobic group. Examples of the hydrophobic group include linear or cyclic hydrocarbon groups, aromatic hydrocarbon groups, halogenated alkyl groups, organosilicon groups (-SiR3), fluorocarbon groups (-C n F 2n+1 Specific examples of the polymer include polyacrylic acid, polymethacrylic acid, polyacrylic acid copolymers, and polymethacrylic acid copolymers.

[0037] The content of the gelling agent is preferably 0.03 mass % or more, 0.05 mass % or more, 0.07 mass % or more, 0.10 mass % or more, or 0.12 mass % or more, relative to the mass of the aqueous dispersion for forming an electromagnetic wave shielding film, from the viewpoint of obtaining an electromagnetic wave shielding film of a desired thickness, and is preferably 0.25 mass % or less, 0.22 mass % or less, 0.20 mass % or less, 0.18 mass % or less, or 0.15 mass % or less, from the viewpoint of the coatability of the aqueous dispersion for forming an electromagnetic wave shielding film.

[0038] <<Method for manufacturing electromagnetic wave absorbing sheet>> The method of the present invention for producing an electromagnetic wave absorbing sheet comprises applying the above-mentioned aqueous dispersion to a resin substrate and drying it to obtain an electromagnetic wave shielding film.

[0039] The surface resistivity of the electromagnetic shielding film thus obtained, measured by a four-probe method, can be 400Ω / □ or less, 380Ω / □ or less, 350Ω / □ or less, or 320Ω / □ or less. This surface resistivity can be, for example, 100Ω / □ or more, 150Ω / □ or more, or 180Ω / □ or more. With such a surface resistivity, good electromagnetic wave absorption properties of the electromagnetic shielding film can be obtained. The probe spacing in the four-probe method is 10 mm.

[0040] The application can be carried out using an applicator, a bar coater, an inkjet printer, a comma coater, a gravure coater, a screen, or the like.

[0041] Examples of resin substrates that can be used include polyolefin resins such as polypropylene, polyester resins such as polyethylene terephthalate, polyamide resins such as nylon and aramid, and polycarbonate.

[0042] 《Electromagnetic wave absorbing sheet》 The electromagnetic wave absorbing sheet of the present invention comprises a resin substrate and an electromagnetic wave shielding film on the resin substrate. The electromagnetic wave shielding film is obtained by drying the aqueous dispersion according to claim 1 or 2, particularly by the method described above.

[0043] The DRY film thickness of the electromagnetic wave shielding film may be 100 μm or less. This DRY film thickness can be 90 μm or less, 80 μm or less, 70 μm or less, 60 μm or less, 50 μm or less, 40 μm or less, 30 μm or less, 20 μm or less, 15 μm or less, 12 μm or less, 10 μm or less, 7 μm or less, or 5 μm or less. This DRY film thickness can be measured using a stylus surface profiler, for example, Dektak by BRUKER.

[0044] The volume resistivity of the electromagnetic wave shielding film can be 1.0 Ω·cm or less, 0.8 Ω·cm or less, 0.5 Ω·cm or less, 0.4 Ω·cm or less, or 0.3 Ω·cm or less. This volume resistivity is measured in accordance with JIS C 2139:2008 at an ambient temperature of 25°C and a relative humidity of 40%RH. EXAMPLES

[0045] The present invention will be specifically described with reference to examples and comparative examples, but the present invention is not limited to these.

[0046] <<Preparation of aqueous dispersion for forming electromagnetic wave shielding film>> Example 1 An aqueous dispersion for forming an electromagnetic wave shielding film of Example 1 was prepared by mixing 60 parts by weight of carbon black toner, 15 parts by weight of urethane dispersion, 1 part by weight of surfactant, 1 part by weight of triethylamine (TEA), and 0.1 parts by weight of preservative.

[0047] The carbon black toner contains 7.8 parts by weight of carbon black (CB), 0.78 parts by weight of aminomethylpropanol (AMP), 1.56 parts by weight of styrene acrylic resin (SA) as a dispersant, and 49.86 parts by weight of water.

[0048] The urethane dispersion used here is a water-based urethane dispersion in which urethane dispersion particles are dispersed in water.

[0049] Examples 2 to 7 and Comparative Examples 1 to 24 Aqueous dispersions for forming an electromagnetic wave shielding film of Examples 2 to 7 and Comparative Examples 1 to 24 were prepared in the same manner as in Example 1, except that the substances and contents used were changed as shown in Tables 1 to 4.

[0050] Details of other substances shown in Tables 1 to 4 are as follows. In addition, where solid content is listed, the mass of the solid content is listed in these tables. Urethane A: Hydran WLS201 (DIC, solid content 35%, average particle size 35 nm) Urethane B: Hydran WLS213 (DIC, solid content 35%, average particle size 27 nm) Urethane C: Hydran HW-920 (DIC, solid content 50%, average particle size 204 nm) Styrene acrylic A: Joncryl 52J (BASF Japan Ltd., Tg 56°C, soluble, solids content 60%) Styrene acrylic B: Joncryl JDX6180 (BASF Japan Ltd., Tg 134°C, soluble, solids content 27%) Acrylic A: Joncryl PDX-7780 (BASF Japan Ltd., Tg 92°C, average particle size 114 nm, solid content 48%) Acrylic B: Joncryl PDX-7734 (BASF Japan Ltd., Tg 40°C, average particle size 73 nm, solid content 41.4%) Acrylic C: Joncryl PDX-7356 (BASF Japan Ltd., Tg 25°C, average particle size 70 nm, solid content 45.5%) Acrylic D: Joncryl PDX-7357 (BASF Japan Ltd., Tg-4°C, average particle size 99 nm, solid content 49.5%) Acrylic E: Joncryl PDX-7430 (BASF Japan Ltd., Tg 30°C, average particle size 752 nm, self-crosslinking, solid content 38%) Acrylic F: Joncryl 352D (BASF Japan Ltd., Tg 56°C, average particle size 99 nm, solid content 45%) Acrylic G: Joncryl 89J (BASF Japan Ltd., Tg 98°C, average particle size 114 nm, solid content 48%) Acrylic H: Ultrasol CMX-235 (Aica Kogyo Co., Ltd., Tg-15°C, average particle size 185 nm, solid content 45%) Acrylic I: Ultrasol LTC-100 (Aica Kogyo Co., Ltd., average particle size 216 nm) Acrylic J: Polysol AP-3720N (Tg 9°C, average particle size 147 nm, solid content 51%) Surfactant: Olfin 4200, Nissin Chemical Industry Co., Ltd. Preservative: Biocide 1700, Taisho Technos Co., Ltd. Dispersant SA: Joncryl 63J (BASF Japan Ltd., Tg 73°C, soluble type, solid content 30%)

[0051] "evaluation" The prepared aqueous dispersion for forming an electromagnetic wave shielding film was applied to the entire surface of a 10 cm x 10 cm polyethylene terephthalate film using an applicator with a wet film thickness set to 50 μm. The applied aqueous dispersion for forming an electromagnetic wave shielding film was then dried to obtain an electromagnetic wave absorbing sheet having an electromagnetic wave shielding film.

[0052] <Paint Repellent> The electromagnetic wave shielding films thus produced were visually evaluated for repelling of the coating film. A: A uniform and flat coating was formed. B: Although some unevenness was observed on the surface, a uniform coating film was formed. C: Repelling occurred to a degree that was visible on the surface of the polyethylene terephthalate film. D: Repelling occurred to the extent that more than half of the surface of the polyethylene terephthalate film was exposed.

[0053] <Adhesion> The adhesion of the electromagnetic shielding film was evaluated by a cross-cut test method. Specifically, a cutter was used to make a number of cuts in a grid pattern at intervals of about 1 mm in the obtained electromagnetic shielding film. Next, cellophane tape (registered trademark) was applied so as to cover the cuts, and this was peeled off at an angle of about 60°, and the adhesion state of the electromagnetic shielding film to the tape was visually confirmed. The evaluation criteria were as follows. A: The electromagnetic shielding film was not attached to the tape. B: Although a small amount of the electromagnetic wave shielding film was attached to the tape, no peeling was observed to the extent that it would affect the appearance of the electromagnetic wave shielding film. C: The electromagnetic shielding film was attached to the tape, resulting in scattered chips in the electromagnetic shielding film. D: Many areas of the electromagnetic shielding film peeled off, but the amount of peeling was less than half. E: The majority of the electromagnetic shielding film was peeled off.

[0054] <Surface resistivity> For the electromagnetic shielding films using the binders "Urethane A," "Urethane B," "Urethane C," "Styrene Acrylic A," "Styrene Acrylic B," and "Acrylic D," which showed sufficient results in terms of adhesion, the surface resistivity was measured using an apparatus consisting of a four-point probe with a probe spacing of 10 mm and a resistivity meter Milliohm HiTester 3227 (HIOKI EE).

[0055] The configurations and evaluation results of the examples and comparative examples are shown in Tables 1 to 4 and FIG. [Table 1]

[0056] [Table 2]

[0057] [Table 3]

[0058] [Table 4]

[0059] The electromagnetic wave shielding films obtained from aqueous dispersions using binders other than "Urethane A," "Urethane B," "Urethane C," "Styrene Acrylic A," "Styrene Acrylic B," and "Acrylic D" did not produce sufficient results, at least in terms of adhesion.

[0060] Furthermore, for the cases where "Urethane A," "Urethane B," and "Urethane C" were used, the ratio of the binder mass to the carbon black mass was plotted on the horizontal axis and the surface resistivity was plotted on the vertical axis, and a quadratic approximation curve obtained from the plotted points is shown in Figure 1. Similarly, quadratic approximation curves were created for the cases where "Styrene Acrylic A" and "Acrylic D" were used, and these are shown in Figure 2.

[0061] FIG. 1 shows that when "Urethane A," "Urethane B," and "Urethane C" were used, low values ​​of surface resistivity were obtained while providing good adhesion when the ratio of the mass of the urethane dispersion particles to the mass of the carbon black was in a relatively small range, specifically in the range of 0.55 to 1.30, and that particularly in the range of 0.85 to 1.15, extremely low values ​​of surface resistivity were obtained.

[0062] Evaluation when using a gelling agent Example 1' The aqueous dispersion for forming an electromagnetic wave shielding film of Example 1 was applied to the entire surface of a 10 cm x 10 cm polyethylene terephthalate film using an applicator with a wet film thickness set to 80 μm. The applied aqueous dispersion for forming an electromagnetic wave shielding film was then dried to obtain an electromagnetic wave absorbing sheet having an electromagnetic wave shielding film. The electromagnetic wave shielding film obtained by changing the wet film thickness setting to 80 μm from the electromagnetic wave shielding film obtained in Example 1 is referred to as Example 1' in Table 5. The actual dry film thickness of the obtained electromagnetic wave shielding film was then measured.

[0063] The dry film thickness was measured using a stylus surface profiler (Dektak, BRUKER). The theoretical dry film thickness was calculated by multiplying the set wet film thickness by the solid content, i.e., the total content of CB, binder, and dispersant.

[0064] Example 8 An aqueous dispersion for forming an electromagnetic shielding film and an electromagnetic shielding film of Example 8 were prepared in the same manner as in Example 1, except that the type and content of each component were changed as shown in Table 5. An alkali swelling association type gelling agent (Primal TT-935, Rohm and Haas Japan, solid content 5%) was used as the gelling agent. Details of other substances are the same as in Tables 1 to 4. Next, actual measurements and theoretical values ​​of the dry film thickness of the electromagnetic shielding film were obtained in the same manner as in Example 1'.

[0065] Examples 1'A, 1'B, 1'C, 8A, 8B and 8C The electromagnetic wave shielding films obtained by further increasing the coating distance of the aqueous dispersion for forming an electromagnetic wave shielding film of Examples 1' and 8 are designated as Examples 1'A and 8A, respectively. Specifically, the aqueous dispersion for forming an electromagnetic wave shielding film was coated over a thickness of 20 m using a comma coater with a wet film thickness setting of 80 μm.

[0066] Moreover, electromagnetic wave shielding films obtained in the same manner as in Examples 1', 8, 1'A and 8A, except that the set value of the wet film thickness was changed to 160 μm, were designated as Examples 1'B, 8B, 1'C and 8C, respectively.

[0067] Next, in the same manner as in Examples 1' and 8, the actual measured values ​​and theoretical values ​​of the dry film thickness of the electromagnetic wave shielding film were obtained.

[0068] "evaluation" In addition to the evaluation of the above repellency, adhesion and surface resistivity, the volume resistivity of each of the electromagnetic wave shielding films obtained was calculated from the following formula. Volume resistivity (Ω cm) = surface resistivity (Ω / □) × DRY film thickness (μm) × 10 -4

[0069] The configurations and evaluation results of Examples 1 and 5 are shown in Table 5.

[0070] [Table 5]

[0071] From Table 5, it can be seen that Example 8, which contains a gelling agent, has similar performance to Example 1', which does not contain a gelling agent, at short coating distances.

[0072] On the other hand, at long coating distances and thick WET thickness settings, it can be seen that Examples 8A, 8B, and 8C have performance similar to that of Example 1' which does not contain a gelling agent, while providing electromagnetic wave shielding films having DRY film thicknesses closer to the theoretical value compared to Examples 1'A, 1'B, and 1'C, respectively.

Claims

1. It contains water, carbon black, and urethane dispersion. the ratio of the mass of the urethane dispersion particles to the mass of the carbon black is 0.55 to 1.30; Aqueous dispersion for forming electromagnetic wave shielding film.

2. 2. The aqueous dispersion according to claim 1, wherein the ratio of the mass of the urethane dispersion particles to the mass of the carbon black is 0.85 to 1.

15.

3. 3. The aqueous dispersion according to claim 1, wherein the particles of the urethane dispersion have an average particle size of 50 nm or less as measured by laser diffraction.

4. 3. The aqueous dispersion according to claim 1, further comprising a volatile base, the content of the volatile base being 0.1 to 3.0% by mass.

5. The aqueous dispersion according to claim 1 or 2, further comprising a gelling agent.

6. The aqueous dispersion according to claim 5 , wherein the gelling agent is an alkali-swelling associative gelling agent.

7. The carbon black has a hollow shell structure and a BET specific surface area of ​​1000 m as measured by a nitrogen adsorption method. ２ The aqueous dispersion according to claim 1 or 2, wherein the molecular weight of the aqueous dispersion is 1 / g or less.

8. A method for producing an electromagnetic wave absorbing sheet, comprising applying the aqueous dispersion according to claim 1 or 2 to a resin substrate and drying the applied coating to obtain an electromagnetic wave shielding film.

9. 9. The method according to claim 8, wherein the surface resistivity of the electromagnetic wave shielding film measured by a four-probe method is 400 Ω / □ or less.

10. a resin substrate and an electromagnetic wave shielding film on the resin substrate, 3. An electromagnetic wave absorbing sheet, wherein the electromagnetic wave shielding film is obtained by drying the aqueous dispersion according to claim 1 or 2.

11. 11. The electromagnetic wave absorbing sheet according to claim 10, wherein the electromagnetic wave shielding film has a dry film thickness of 100 μm or less and a volume resistivity of 1.0 Ω·cm or less.

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