JP7669205B2 - Carbon nanotube dispersion and method for producing electromagnetic wave shielding sheet - Google Patents

Description

The present invention relates to an aqueous dispersion of carbon nanotubes and a method for producing the same.

Carbon nanotubes have a structure similar to a uniform, flat graphene sheet rolled into a cylindrical shape. Because of this unique structure, carbon nanotubes have a variety of properties and are expected to be used in a wide range of fields.

For example, Patent Document 1 describes a method of producing a carbon nanotube dispersion liquid by pressurizing a mixture of carbon nanotube aggregates and a dispersion medium and feeding the mixture into a number of capillary flow paths, causing the mixture to collide and merge. Patent Document 1 also describes that in the method of producing a carbon nanotube dispersion liquid, the concentration of the carbon nanotube aggregates is preferably 0.01% by mass to 1% by mass.

JP 2013-230951 A

In the carbon nanotube aqueous dispersion described above, it is desirable to increase the carbon nanotube content. By increasing the carbon nanotube content, it is possible to improve the properties specific to carbon nanotubes, such as electromagnetic wave shielding performance.

However, when the amount of carbon nanotubes added is simply increased and the tubes are dispersed by repeated collision and merging without using a dispersant such as a surfactant, the viscosity of the resulting carbon nanotube aqueous dispersion becomes too high, making it difficult to produce a carbon nanotube aqueous dispersion with excellent carbon nanotube dispersibility. Furthermore, when the carbon nanotube content is increased, the viscosity of the carbon nanotube aqueous dispersion increases, making it difficult to apply the carbon nanotube aqueous dispersion to a substrate such as paper.

One of the objectives of some aspects of the present invention is to provide a carbon nanotube aqueous dispersion that has a high carbon nanotube content and is easy to apply. Another objective of some aspects of the present invention is to provide a method for producing a carbon nanotube aqueous dispersion that has a high carbon nanotube content and is easy to apply.

One aspect of the carbon nanotube aqueous dispersion according to the present invention is
Carbon nanotubes, sodium carboxymethylcellulose , and water,
The carbon nanotube content is 2.5% by mass or more,
The viscosity measured using a Brookfield viscometer at 25° C. and a rotation speed of 60 rpm is 3,000 cps or less.

In one embodiment of the carbon nanotube aqueous dispersion,
The composition may not contain any dispersing agent other than the sodium carboxymethylcellulose .

In any one of the embodiments of the carbon nanotube aqueous dispersion,
The carbon nanotube content may be 3.0% by mass or more.

In any one of the embodiments of the carbon nanotube aqueous dispersion,
The viscosity may be less than or equal to 2500 cps.

In any one of the embodiments of the carbon nanotube aqueous dispersion,
The viscosity may be 1000 cps or more.

In any one of the embodiments of the carbon nanotube aqueous dispersion,
The content of the sodium carboxymethylcellulose may be 2.5% by mass or more.

In any one of the embodiments of the carbon nanotube aqueous dispersion,
The content of the sodium carboxymethylcellulose may be 3.0% by mass or more.

In any one of the embodiments of the carbon nanotube aqueous dispersion,
It may also be used as a coating agent for electromagnetic wave shielding seeds.

One aspect of the method for producing a carbon nanotube aqueous dispersion according to the present invention is to
preparing a first dispersion liquid containing carbon nanotubes and water, the first dispersion liquid having a carbon nanotube content of 3.0% by mass or more;
preparing a second dispersion containing carbon nanotubes and water, the second dispersion having a carbon nanotube content of less than 3.0% by mass;
mixing the first dispersion and the second dispersion to prepare a third dispersion;
Including,
In the step of preparing the first dispersion, the first dispersion is prepared by performing an underwater head-on collision method with a pass number of a wet-type atomization device set to one,
In the step of preparing the second dispersion, an underwater head-on collision method is performed with the number of passes of a wet-type atomization device set to two or more to prepare the second dispersion.

In one embodiment of the method for producing a carbon nanotube aqueous dispersion,
In the step of preparing the second dispersion, the number of passes through the wet-type atomization device may be set to four.

In any one of the embodiments of the method for producing a carbon nanotube aqueous dispersion,
a step of subjecting the third dispersion to an underwater head-on collision method with a wet-type atomization device being passed through the third dispersion twice or more to prepare a fourth dispersion;
preparing a fifth dispersion containing carbon nanotubes and water, the fifth dispersion having a higher carbon nanotube content than the fourth dispersion;
mixing the fourth dispersion and the fifth dispersion to prepare a sixth dispersion;
Including,
In the step of preparing the fifth dispersion, the fifth dispersion may be prepared by performing an underwater head-on collision method with a single pass through a wet-type atomization device.

In any one of the embodiments of the method for producing a carbon nanotube aqueous dispersion,
In the step of preparing the fourth dispersion, the number of passes through the wet-type atomization device may be four.

In any one of the embodiments of the method for producing a carbon nanotube aqueous dispersion,
a step of subjecting the sixth dispersion to an underwater head-on collision method with a wet-type atomization device being passed through the sixth dispersion twice or more to prepare a seventh dispersion;
preparing an eighth dispersion comprising carbon nanotubes and water, the eighth dispersion having a higher carbon nanotube content than the seventh dispersion;
mixing the seventh dispersion and the eighth dispersion to prepare a ninth dispersion;
Including,
In the step of preparing the eighth dispersion, the eighth dispersion may be prepared by performing an underwater head-on collision method with a single pass through a wet-type atomization device.

In any one of the embodiments of the method for producing a carbon nanotube aqueous dispersion,
In the step of preparing the seventh dispersion, the number of passes through the wet-type atomization device may be set to four.

In any one of the above-mentioned methods for producing an aqueous dispersion of carbon nanotubes,
The content of the carbon nanotubes in the first dispersion is 3.5% by mass or more,
The content of the carbon nanotubes in the second dispersion may be 2.5 mass % or less.

In any one of the embodiments of the method for producing a carbon nanotube aqueous dispersion,
In the step of preparing the third dispersion, the first dispersion and the second dispersion may be mixed such that a ratio of the mass of the first dispersion to a sum of the mass of the first dispersion and the mass of the second dispersion is 0.15 or more and 0.70 or less.

In any one of the embodiments of the method for producing a carbon nanotube aqueous dispersion,
the first dispersion and the second dispersion contain sodium carboxymethylcellulose ;
The content of the sodium carboxymethylcellulose in the first dispersion is 3.0% by mass or more,
The content of the sodium carboxymethylcellulose in the second dispersion may be less than 3.0% by mass.

In any one of the embodiments of the method for producing a carbon nanotube aqueous dispersion,
The content of the sodium carboxymethylcellulose in the first dispersion is 3.5% by mass or more,
The content of the sodium carboxymethylcellulose in the second dispersion may be 2.5% by mass or less.

The carbon nanotube aqueous dispersion according to the present invention contains carbon nanotubes , sodium carboxymethylcellulose, and water, has a carbon nanotube content of 2.5% by mass or more, and has a viscosity of 3000 cps or less measured at 25° C. and 60 rpm using a B-type viscometer. Therefore, the carbon nanotube aqueous dispersion according to the present invention has a high carbon nanotube content and is easy to apply. Furthermore, according to the method for producing a carbon nanotube aqueous dispersion according to the present invention, a carbon nanotube aqueous dispersion having a high carbon nanotube content and being easy to apply can be easily produced.

3 is a flowchart for explaining a method for producing a carbon nanotube aqueous dispersion according to the present embodiment. 6 is a flowchart for explaining a first modified example of the method for producing a carbon nanotube aqueous dispersion according to the embodiment. 10 is a flowchart for explaining a second modified example of the method for producing a carbon nanotube aqueous dispersion according to the embodiment. 1 is a table showing the preparation conditions of carbon nanotube aqueous dispersions of Examples 1 to 11 and Comparative Examples 1 and 2, and the evaluation results of the carbon nanotube content and viscosity.

Below, preferred embodiments of the present invention will be described in detail with reference to the drawings. Note that the embodiments described below do not unduly limit the content of the present invention described in the claims. Furthermore, not all of the configurations described below are necessarily essential components of the present invention.

1. Carbon nanotube aqueous dispersion 1.1. Contents First, the carbon nanotube aqueous dispersion according to this embodiment will be described. The carbon nanotube aqueous dispersion according to this embodiment contains carbon nanotubes (hereinafter also referred to as "CNT"), sodium carboxymethylcellulose (hereinafter also referred to as "CMC"), and water. Each of the contents will be described below.

1.1.1. Carbon nanotubes (CNTs)
Examples of the CNT contained in the CNT aqueous dispersion according to this embodiment include single-walled carbon nanotubes (SWNTs) in which one six-membered ring network (graphene sheet) made of carbon is wound in a cylindrical shape, and multi-walled carbon nanotubes (MWNTs) in which multiple graphene sheets are wound in a concentric shape. The CNT aqueous dispersion may contain only one of SWNTs and MWNTs, or may contain both, but considering the dispersibility of CNTs, it is preferable that the CNTs contain only MWNTs. Both ends of the CNTs may be closed or open.

The above-mentioned CNTs are produced to a desired size by, for example, an arc discharge method, a laser ablation method, or a CVD (Chemical Vapor Deposition) method. The CNTs contained in the CNT aqueous dispersion according to this embodiment may be produced by any method.

The diameter of the CNT is, for example, 1 nm or more and 100 nm or less, preferably 5 nm or more and 50 nm or less, and more preferably 8 nm or more and 15 nm or less. If the diameter of the CNT is 1 nm or more and 100 nm or less, the dispersibility of the CNT can be improved. The diameter of the CNT can be measured by a SEM (Scanning Electron Microscope).

The fiber length of the CNT is, for example, 0.5 μm to 50 μm, and preferably 15 μm to 35 μm. If the fiber length of the CNT is 0.5 μm to 50 μm, the dispersibility of the CNT can be improved. The fiber length of the CNT can be measured by SEM. The "fiber length of the CNT" refers to the length of the CNT when it is bundled by van der Waals forces, and is the length of the CNT before it is dispersed in a solvent.

The BET specific surface area of the CNT is, for example, 50 m ² /g or more and 500 m ² /g or less, and preferably 100 m ² /g or more and 300 ^m 2 /g or less. If the BET specific surface area of the CNT is 50 m ² /g or more and 500 m ² /g or less, the dispersibility of the CNT can be improved. Note that the "BET specific surface area" refers to the specific surface area measured by the BET (Brunauer Emmett Teller) method, and can be measured by an automatic specific surface area measuring device.

In the CNT aqueous dispersion, the CNT content is 2.5% by mass or more, preferably 3.0% by mass or more, and more preferably 3.5% by mass or more. If the CNT content is 2.5% by mass or more, the performance specific to CNT, such as electromagnetic shielding performance, can be improved. The CNT content is, for example, 10.0% by mass or less. If the CNT content is 10.0% by mass or less, the dispersibility of CNT can be improved. The CNT content can be measured by thermal gravimetric analysis (TGA).

1.1.2. Sodium Carboxymethylcellulose (CMC)
In the CNT aqueous dispersion according to this embodiment, CMC functions as a dispersant for dispersing CNT. Only CMC is used as a dispersant for CNT. That is, the CNT aqueous dispersion does not contain any dispersant other than CMC. The term "dispersant" refers to an additive that disperses CNT in water and contributes to preventing the aggregation and sedimentation of CNT. By using only CMC as a dispersant for CNT, it is possible to prevent air bubbles from being mixed into the CNT aqueous dispersion, compared to the case where, for example, an anionic surfactant is added as a dispersant in addition to CMC. Therefore, when the CNT aqueous dispersion according to this embodiment is used as a coating agent for an electromagnetic wave shield, it is possible to prevent air bubbles from being mixed into the coating film to be formed by using only CMC as a dispersant for CNT.

The weight average molecular weight of CMC is, for example, 5,000 to 100,000, preferably 10,000 to 60,000, and more preferably 10,000 to 35,000. If the weight average molecular weight of CMC is 5,000 or more, CMC can easily entangle CNTs, improving the dispersibility of CNTs. However, if the weight average molecular weight is too large, dispersibility will deteriorate, so the molecular weight of CMC is preferably 100,000 or less. The "weight average molecular weight" refers to the weight average molecular weight in terms of polystyrene measured by gel permeation chromatography (GPC).

The degree of etherification of the CMC is, for example, 0.6 or more and 1.2 or less, and preferably 0.6 or more and 0.8 or less. If the degree of etherification of the CMC is 0.6 or more and 1.2 or less, the dispersibility of the CNTs can be improved.

In the CNT aqueous dispersion, the CMC content is 2.5% by mass or more, preferably 3.0% by mass or more, and more preferably 3.5% by mass or more. If the CMC content is 2.5% by mass or more, the dispersibility of the CNT can be improved. The CMC content is, for example, 10.0% by mass or less. If the CMC content is 10.0% by mass or less, the viscosity of the CNT aqueous dispersion can be reduced. The higher the CMC content, the higher the viscosity of the CNT aqueous dispersion. The CMC content can be measured by thermogravimetric analysis.

In the CNT aqueous dispersion, the ratio M _CMC /M _CNT of the mass M _CMC of CMC to the mass M _CNT of CNT is 1/5 or more and 5 or less (CNT:CMC=5:1 to 1:5), and preferably 1/3 or more and 3 or less (CNT:CMC=3:1 to 1:3). If the ratio M _CMC /M _CNT is 1/5 or more, the performance specific to CNT, such as electromagnetic shielding performance, can be improved. If the ratio M _CMC /M _CNT is 5 or less, the dispersibility of CNT can be improved.

1.1.3. Water The CNT aqueous dispersion according to this embodiment contains water as a solvent. Examples of water include pure water such as ion-exchanged water, ultrafiltered water, reverse osmosis water, and distilled water, as well as water from which ionic impurities have been removed as much as possible, such as ultrapure water. Since the CNT aqueous dispersion uses water as a solvent, it is more environmentally friendly than when an organic solvent is used as a solvent. The CNT aqueous dispersion is a dispersion of C
It may contain only NTs, CMC, and water, that is, the CNT aqueous dispersion may be composed of only CNTs, CMC, and water.

1.1.4 Other Additives The CNT aqueous dispersion according to this embodiment may further contain various additives such as a preservative, a pH adjuster, a colorant, and an antifoaming agent, as necessary.

1.2 Viscosity In the CNT aqueous dispersion according to this embodiment, the viscosity measured using a Brookfield viscometer at 25° C. and a rotation speed of 60 rpm is 3000 cps or less, preferably 1000 cps to 2500 cps, and more preferably 1500 cps to 2000 cps. Hereinafter, the "viscosity measured using a Brookfield viscometer at 25° C. and a rotation speed of 60 rpm" is also simply referred to as "viscosity." Note that 1 cps = 1 mPa·s.

If the viscosity of the CNT aqueous dispersion is 3000 cps or less, the CNT aqueous dispersion can be easily applied. Furthermore, the CNT aqueous dispersion can be applied with good uniformity. If the viscosity of the CNT aqueous dispersion is 1000 cps or more, the CNT aqueous dispersion can be applied even more easily.

1.3. Uses The CNT aqueous dispersion according to this embodiment is applied to a predetermined substrate using, for example, a die coater, a gravure coater, a wire bar coater, a knife coater, an air coater, a blade coater, a roll coater, or a reverse roll coater.

CNT aqueous dispersions are used as coating agents for electromagnetic wave shielding sheets to suppress electromagnetic waves. Electromagnetic wave shielding sheets are produced, for example, by coating a substrate such as paper with CNT aqueous dispersion and then drying the coated CNT aqueous dispersion.

The applications of CNT aqueous dispersions are not limited to electromagnetic shielding sheets, but are used in a wide range of fields, including fuel cells, capacitors, semiconductor devices, medical equipment, automobiles, aircraft, and sporting goods.

1.4. Effects and Effects The CNT aqueous dispersion according to this embodiment contains CNT, CMC, and water, has a CNT content of 2.5 mass% or more, and has a viscosity of 3000 cps or less. Therefore, the CNT aqueous dispersion according to this embodiment has a high CNT content and is easy to apply. If the viscosity is higher than 3000 cps, it becomes difficult to apply the CNT aqueous dispersion using the above-mentioned die coater or the like.

2. Method for Producing Carbon Nanotube Aqueous Dispersion 2.1. Overall Flow Next, the method for producing a CNT aqueous dispersion according to this embodiment will be described with reference to the drawings. Fig. 1 is a flow chart for explaining the method for producing a CNT aqueous dispersion according to this embodiment.

As shown in FIG. 1, the method for producing a CNT aqueous dispersion according to this embodiment includes a step (step S1) of preparing a first dispersion having a CNT content of 3.0% by mass or more, a step (step S2) of preparing a second dispersion having a CNT content of less than 3.0% by mass, and a step (step S3) of mixing the first dispersion and the second dispersion to prepare a third dispersion. Each step will be described below.

2.2. Step of preparing first dispersion (Step S1)
2.2.1 Stirring of CMC and Water In the step of preparing the first dispersion, first, CMC and water are stirred. The stirring time is, for example, from 0.1 hours to 3 hours, preferably from 0.5 hours to 1.5 hours.

The CMC content in the first dispersion is 3.0% by mass or more, preferably 3.5% by mass or more, more preferably 4.0% by mass or more, and even more preferably 5.0% by mass or more. If the CMC content is 3.0% by mass or more, the dispersibility of CNT in the first dispersion can be improved. The CMC content in the first dispersion is, for example, 10.0% by mass or less. If the CMC content is 10.0% by mass or less, the viscosity of the mixture of CNT, CMC, and water for preparing the first dispersion does not become too high, and the first dispersion can be prepared by the underwater head-on collision method. If the viscosity of the mixture is too high, the nozzle hole of the wet micronization device will be clogged when performing the underwater head-on collision method.

2.2.2. Stirring of CNT, CMC, and Water Next, CNT is added to the stirred mixture of CMC and water, and the CNT, CMC, and water are stirred. The stirring is performed at a higher speed than the stirring of the CMC and water. The stirring time is, for example, 0.1 hours to 3 hours, preferably 0.5 hours to 1.5 hours.

The CNT content in the first dispersion is 3.0% by mass or more, preferably 3.5% by mass or more, more preferably 4.0% by mass or more, and even more preferably 5.0% by mass or more. If the CNT content in the first dispersion is 3.0% by mass or more, a third dispersion having a large CNT content can be prepared. The CNT content in the first dispersion is, for example, 10.0% by mass or less. If the CNT content in the first dispersion is 10.0% by mass or less, the viscosity of the mixture of CNT, CMC, and water for preparing the first dispersion does not become too high, and the first dispersion can be prepared by the underwater head-on collision method. The greater the CNT content in the mixture, the higher the viscosity of the mixture.

In the first dispersion, the ratio M _CMC /M _CNT of the mass M _CMC of CMC to the mass M _CNT of CNT is 1/5 or more and 5 or less (CNT:CMC=5:1 to 1:5), and preferably 1/3 or more and 3 or less (CNT:CMC=3:1 to 1:3). If the ratio M _CMC /M _CNT is 1/5 or more, a third dispersion having a large CNT content can be prepared. If the ratio M _CMC /M _CNT is 5 or less, the dispersibility of CNT in the first dispersion can be improved.

Next, the stirred mixture of CNT, CMC, and water is subjected to a screening process to remove CNT aggregates, etc. Screening is performed, for example, using a sieve. By performing the screening process, the underwater head-on collision method can be performed smoothly.

2.2.4. Treatment with a homogenizer Next, as a pretreatment for the underwater confrontation collision method, the mixture of CNT, CMC, and water is treated with a homogenizer. The homogenizer may be an ultrasonic type that generates cavitation with ultrasonic waves, an agitation type that agitates the mixture, or a pressure type that applies pressure to the mixture. By performing treatment with a homogenizer, it is possible to reduce aggregates of CNT, and the underwater confrontation collision method can be performed smoothly. The time for treatment with a homogenizer is, for example, 1 minute or more and 30 minutes or less, and preferably 3 minutes or more and 10 minutes or less.

2.2.5. Underwater Head-on Collision Method Next, the underwater head-on collision method is performed on the mixture of CNT, CMC, and water with the number of passes of the wet-type micronization device set to one. In the process of preparing the first dispersion, only CMC is used as the dispersant. Even if only CMC is used as the dispersant, the dispersibility of CNT can be improved by performing the underwater head-on collision method. This makes it possible to prepare a first dispersion with excellent dispersibility of CNT.

In the underwater counter collision method, a mixed liquid containing CNTs is discharged at high pressure from a pair of nozzle holes (first nozzle hole and second nozzle hole) arranged opposite to each other, and the mixed liquid discharged from the first nozzle hole and the mixed liquid discharged from the second nozzle hole are collided to disperse the CNTs. Preferably, in the underwater counter collision method, the CNTs contained in the mixed liquid discharged from the first nozzle hole and the CNTs contained in the mixed liquid discharged from the second nozzle hole are collided to disperse the CNTs. In the underwater counter collision method, as long as the central axis of the first nozzle hole and the central axis of the second nozzle hole intersect with each other, both central axes may be on a straight line or may be inclined to each other. Alternatively, the mixed liquid may be collided from the nozzle hole against a ceramic ball or the like.

In the underwater head-on collision method, the mixed liquid is ejected from a nozzle hole having a diameter of, for example, 50 μm to 200 μm, preferably 80 μm to 120 μm, more preferably 100 μm, and the mixed liquids are caused to collide with each other. If the nozzle hole diameter is 50 μm or more, even a mixed liquid with a relatively high viscosity can be ejected from the nozzle hole. If the nozzle hole diameter is 200 μm or less, the collision energy between the mixed liquids can be increased.

In the underwater head-on collision method, the mixed liquid is discharged at a pressure of, for example, 150 MPa or more and 250 MPa or less, preferably 180 MPa or more and 220 MPa or less, and more preferably 200 MPa, and the mixed liquids are caused to collide with each other. If the pressure is 150 MPa or more, the collision energy between the mixed liquids can be increased. If the pressure is 250 MPa or less, it is possible to prevent the collision energy from being too high and causing the CNT fibers to break.

Specifically, the underwater head-on collision method is carried out using a wet atomization device "Starburst Lab" (model name: HJP-25005) manufactured by Sugino Machine Ltd. This wet atomization device has a higher energy density than, for example, ultrasonic homogenizers or ball mills, and can produce a first dispersion liquid with excellent CNT dispersibility in a short time. Furthermore, this wet atomization device can minimize the inclusion of impurities, making it possible to produce a first dispersion liquid with extremely low impurity inclusion.

In the process of preparing the first dispersion, the underwater head-on collision method is performed with the number of passes through the wet atomization device set to one to prepare the first dispersion. Here, the "number of passes of the mixed liquid in the wet atomization device" refers to the number of times the mixed liquid is circulated through the wet atomization device. For example, "two passes" means that the mixed liquid is circulated twice so that the CNTs that have collided once collide again. In this way, the number of passes corresponds to the number of collisions of the CNTs contained in the mixed liquid. Furthermore, the number of passes is proportional to the processing time in the wet atomization device. If the processing time in the wet atomization device is long, the number of times the mixed liquid is circulated increases.

The first dispersion has a high viscosity because the CNT content is as large as 3.0% by mass or more. The viscosity of the first dispersion is, for example, 9000 cps or more, preferably 9500 cps or more, and more preferably 10000 cps or more. Even if the first dispersion having such a high viscosity is subjected to the underwater facing collision method once more with the wet atomization device (that is, even if the first dispersion is produced by performing the underwater facing collision method with the number of passes set to two), the first dispersion is clogged in the nozzle hole of the wet atomization device, and the underwater facing collision method cannot be performed. Therefore, in the process of producing the first dispersion, it is difficult to set the number of passes of the wet atomization device to two, and the first dispersion produced by performing the underwater facing collision method with the number of passes set to one is used for producing the third dispersion.

By the above steps, a first dispersion liquid can be produced that contains CNT, CMC, and water, and has a CNT content and CMC content of 3.0 mass% or more.

In the above, an example of performing the underwater head-on collision method on a mixture of CNT, CMC, and water has been described, but the first dispersion may also be produced by performing the underwater head-on collision method on a mixture of CNT and water, and then adding CMC to the mixture that has been subjected to the underwater head-on collision method. However, taking into consideration the dispersibility of CNT, it is preferable to perform the underwater head-on collision method on a mixture of CNT, CMC, and water.

2.3. Step of preparing second dispersion (step S2)
2.3.1 Stirring of CMC and Water In the step of preparing the second dispersion, first, CMC and water are stirred. The stirring time is, for example, from 0.1 hours to 3 hours, preferably from 0.5 hours to 1.5 hours.

The CMC content in the second dispersion is less than 3.0% by mass, preferably 2.5% by mass or less, and more preferably 2.3% by mass or less. If the CMC content is less than 3.0% by mass, the viscosity of the second dispersion can be reduced. The CMC content in the second dispersion is, for example, 1.0% by mass or more. If the CMC content is 1.0% by mass or more, the dispersibility of CNTs in the second dispersion can be improved.

2.3.2. Stirring of CNT, CMC, and Water Next, CNT is added to the stirred mixture of CMC and water, and the CNT, CMC, and water are stirred. The stirring is performed at a higher speed than the stirring of the CMC and water. The stirring time is, for example, 0.1 hours to 3 hours, preferably 0.5 hours to 1.5 hours.

The CNT content in the second dispersion is less than 3.0% by mass, preferably 2.5% by mass or less, and more preferably 2.3% by mass or less. If the CNT content is less than 3.0% by mass, the viscosity of the second dispersion can be reduced. The CNT content in the second dispersion is, for example, 1.0% by mass or more. If the CNT content is 1.0% by mass or more, a third dispersion with a high CNT content can be produced.

In the second dispersion, the ratio M _CMC /M _CNT of the mass M _CMC of CMC to the mass M _CNT of CNT is 1/5 or more and 5 or less (CNT:CMC=5:1 to 1:5), and preferably 1/3 or more and 3 or less (CNT:CMC=3:1 to 1:3). If the ratio M _CMC /M _CNT is 1/5 or more, a third dispersion having a large CNT content can be prepared. If the ratio M _CMC /M _CNT is 5 or less, the dispersibility of CNT in the second dispersion can be improved.

2.3.3. Screening Treatment Next, the mixed liquid of CNT, CMC, and water is subjected to a screen treatment in the same manner as in the step of preparing the first dispersion liquid described above (step S1).

2.3.4 Treatment with a Homogenizer Next, the mixture of CNT, CMC, and water is treated with a homogenizer in the same manner as in the step of preparing the first dispersion described above (step S1).

Next, the underwater head-on collision method is performed on the mixture of CNT, CMC, and water by passing the wet-type micro-fine-pulverizing device two or more times. In the step of preparing the second dispersion, the underwater head-on collision method is performed in the same manner as in the step of preparing the first dispersion described above (step S1), except for the number of passes.

The mixture of CNT, CMC, and water in the process of preparing the second dispersion has a low viscosity because the CNT content is less than 3.0% by mass. Therefore, in the process of preparing the second dispersion, the underwater head-on collision method can be performed with the number of passes of the wet micronization device set to two or more. By setting the number of passes to two or more, the viscosity of the second dispersion can be lowered compared to the case where the number of passes is one. As a result, a third dispersion having a viscosity suitable for coating while maintaining a large CNT content can be prepared. The number of passes is preferably three, and more preferably four. The more passes, the lower the viscosity of the second dispersion can be. The number of passes may be five or more, but four is preferable in consideration of shortening the processing time by the wet micronization device.

The viscosity of the second dispersion is, for example, 10 cps or more and 2000 cps or less, preferably 15 cps or more and 1500 cps or less, more preferably 20 cps or more and 1000 cps or less, even more preferably 25 cps or more and 500 cps or less, and even more preferably 30 cps or more and 150 cps or less. If the viscosity is 10 cps or more and 2000 cps or less, a third dispersion having a viscosity suitable for coating can be prepared.

By the above steps, a second dispersion liquid can be produced that contains CNT, CMC, and water, and has a CNT content and CMC content of less than 3.0 mass%.

The order of the process of preparing the first dispersion liquid (step S1) and the process of preparing the second dispersion liquid (step S2) is not particularly limited.

2.4. Step of preparing third dispersion (step S3)
In the step of preparing the third dispersion, the first dispersion and the second dispersion are mixed to prepare the third dispersion. The first dispersion and the second dispersion are mixed, for example, by a high-speed disperser or a homogenizer. When using a high-speed disperser, it may be a stirring type that stirs the mixed liquid, and when using a homogenizer, it may be an ultrasonic type that causes cavitation by ultrasonic waves, or a pressure type that applies pressure to the mixed liquid. The time of the treatment by the high-speed disperser or homogenizer is, for example, 10 minutes or more and 240 minutes or less, preferably 30 minutes or more and 120 minutes or less.

In the step of preparing the third dispersion, the first dispersion and the second dispersion are mixed so that the ratio M ₁ /M _SUM of the mass M ₁ of the first dispersion to the sum M _SUM of the mass of the first dispersion and the mass of the second dispersion is, for example, 0.15 or more and 0.70 or less, preferably 0.20 or more and 0.65 or less, more preferably 0.25 or more and 0.60 or less. If the ratio M ₁ /M _SUM is 0.15 or more and 0.70 or less, a third dispersion having a CNT content of 2.5 mass% or more and a viscosity of 3000 cps or less can be prepared. The CNT content in the third dispersion is smaller than the CNT content in the first dispersion and larger than the CNT content in the second dispersion. The viscosity of the third dispersion is lower than the viscosity of the first dispersion and higher than the viscosity of the second dispersion.

By carrying out the above steps, a third dispersion liquid can be produced as a CNT aqueous dispersion liquid.

2.5. Effects and Effects The method for producing a CNT aqueous dispersion according to this embodiment includes the steps of preparing a first dispersion containing CNT and water and having a CNT content of 3.0% by mass or more, preparing a second dispersion containing CNT and water and having a CNT content of less than 3.0% by mass, and mixing the first dispersion and the second dispersion to prepare a third dispersion. In the step of preparing the first dispersion, the first dispersion is prepared by performing an underwater opposed collision method with one pass through the wet atomization device. In the step of preparing the second dispersion, the second dispersion is prepared by performing an underwater opposed collision method with two or more passes through the wet atomization device.

As described above, the second dispersion has a CNT content of less than 3.0% by mass, and is produced by performing an underwater head-on collision method with two or more passes through the wet atomization device, and therefore has a low viscosity. Furthermore, by mixing the first dispersion, which has a high CNT content, with the second dispersion, which has a low viscosity, a CNT aqueous dispersion can be produced that has a high CNT content and a viscosity suitable for coating. Therefore, the method for producing a CNT aqueous dispersion according to this embodiment makes it possible to easily produce a CNT aqueous dispersion that has a high CNT content and is easy to coat.

2.6. First Modification 2.6.1. Overall Flow Next, a first modification of the manufacturing method of the CNT aqueous dispersion according to this embodiment (hereinafter, simply referred to as the "first modification of the manufacturing method") will be described with reference to the drawings. Fig. 2 is a flow chart for explaining the first modification of the manufacturing method.

As shown in FIG. 2, the first modified manufacturing method includes, after the step of preparing a third dispersion (step S3), a step of performing an underwater head-on collision method on the third dispersion to prepare a fourth dispersion (step S4), a step of preparing a fifth dispersion having a higher CNT content than the fourth dispersion (step S5), and a step of mixing the fourth and fifth dispersions to prepare a sixth dispersion (step S6). Each step will be described below.

2.6.2. Step of preparing fourth dispersion (step S4)
In the step of preparing the fourth dispersion, the third dispersion is subjected to an underwater head-on collision method with the number of passes of the wet atomization device set to 2 or more to prepare the fourth dispersion. In the step of preparing the fourth dispersion, the underwater head-on collision method is performed in the same manner as the step of preparing the first dispersion described above (step S1) except for the number of passes. Since the third dispersion has a low viscosity despite a high content of CNT, the third dispersion can be subjected to an underwater head-on collision method with the number of passes of the wet atomization device set to 2 or more.

By making the number of passes through the wet atomization device two or more times, the viscosity of the fourth dispersion can be made lower than when the number of passes is one. The number of passes is preferably three, and more preferably four.

The viscosity of the fourth dispersion is lower than that of the third dispersion. The viscosity of the fourth dispersion is, for example, 10 cps or more and 2000 cps or less, preferably 15 cps or more and 1000 cps or less, more preferably 20 cps or more and 500 cps or less, and even more preferably 30 cps or more and 300 cps or less. If the viscosity is 10 cps or more and 2000 cps or less, a sixth dispersion having a viscosity suitable for coating can be prepared.

The CNT content in the fourth dispersion is greater than the CNT content in the second dispersion and less than the CNT content in the fifth dispersion. The CNT content in the fourth dispersion is, for example, the same as the CNT content in the third dispersion. The CNT content in the fourth dispersion is, for example, 2.7% by mass or more and 10.0% by mass or less, preferably 3.0% by mass or more and 8.0% by mass or less, and more preferably 3.2% by mass or more and 5.0% by mass or less. If the CNT content is 2.7% by mass or more, a sixth dispersion having a large CNT content can be prepared. If the CNT content is 10.0% by mass or less, the viscosity of the fourth dispersion can be reduced.

2.6.3. Step of preparing fifth dispersion (step S5)
In the step of preparing the fifth dispersion, a fifth dispersion containing CNTs and water and having a higher CNT content than the fourth dispersion is prepared. The explanation of the step of preparing the fifth dispersion can be applied by replacing the "first dispersion" with the "fifth dispersion" in the explanation of the step of preparing the first dispersion (step S1) described above. Therefore, the explanation will be omitted.

The order of the process of preparing the fourth dispersion (step S4) and the process of preparing the fifth dispersion (step S5) is not particularly limited.

2.6.4. Step of preparing sixth dispersion (step S6)
In the step of preparing the sixth dispersion, the fourth dispersion and the fifth dispersion are mixed to prepare the sixth dispersion. The explanation of the step of preparing the sixth dispersion can be applied by replacing the "first dispersion", "second dispersion", and "third dispersion" with the "fourth dispersion", "fifth dispersion", and "sixth dispersion" in the step of preparing the third dispersion (step S3) described above. Therefore, the explanation is omitted.

By carrying out the above steps, the sixth dispersion liquid can be produced as a CNT aqueous dispersion liquid.

2.6.5. Effects In the first modified example of the manufacturing method, in addition to the steps of preparing the first to third dispersions, the method further includes a step of preparing a fourth dispersion by performing an underwater head-on collision method with the third dispersion being passed through the wet atomization device two or more times, a step of preparing a fifth dispersion containing CNT and water and having a higher CNT content than the fourth dispersion, and a step of mixing the fourth and fifth dispersions to prepare a sixth dispersion. In the step of preparing the fifth dispersion, the fifth dispersion is prepared by performing an underwater head-on collision method with the wet atomization device being passed through the device once. Therefore, in the first modified example of the manufacturing method, a sixth dispersion having a higher CNT content than the third dispersion can be prepared as a CNT aqueous dispersion.

2.7. Second Modification 2.7.1. Overall Flow Next, a second modification of the manufacturing method of the CNT aqueous dispersion according to this embodiment (hereinafter, simply referred to as the "second modification of the manufacturing method") will be described with reference to the drawings. Fig. 3 is a flow chart for explaining the second modification of the manufacturing method.

As shown in FIG. 3, the second modified manufacturing method includes, after the step of preparing a sixth dispersion (step S6), a step of preparing a seventh dispersion by performing an underwater head-on collision method on the sixth dispersion (step S7), a step of preparing an eighth dispersion having a higher CNT content than the seventh dispersion (step S8), and a step of mixing the seventh dispersion and the eighth dispersion to prepare a ninth dispersion (step S9). Each step will be described below.

2.7.2. Step of preparing seventh dispersion (step S7)
In the step of preparing the seventh dispersion, the sixth dispersion is subjected to an underwater head-on collision method with the number of passes of the wet atomization device set to two or more to prepare the seventh dispersion. In the step of preparing the seventh dispersion, the underwater head-on collision method is performed in the same manner as in the step of preparing the first dispersion described above (step S1) except for the number of passes. Since the sixth dispersion has a high CNT content but a low viscosity, the sixth dispersion can be subjected to an underwater head-on collision method with the number of passes of the wet atomization device set to two or more.

By making the number of passes through the wet atomization device two or more times, the viscosity of the seventh dispersion can be made lower than when the number of passes is one. The number of passes is preferably three, and more preferably four.

The viscosity of the seventh dispersion is lower than that of the sixth dispersion. The viscosity of the seventh dispersion is, for example, 10 cps or more and 2000 cps or less, preferably 15 cps or more and 1000 cps or less, more preferably 20 cps or more and 500 cps or less, and even more preferably 30 cps or more and 300 cps or less. If the viscosity is 10 cps or more and 2000 cps or less, a ninth dispersion having a viscosity suitable for coating can be produced.

The CNT content in the seventh dispersion is greater than the CNT content in the fourth dispersion and less than the CNT content in the eighth dispersion. The CNT content in the seventh dispersion is, for example, the same as the CNT content in the sixth dispersion. The CNT content in the seventh dispersion is, for example, 3.0% by mass or more and 10.0% by mass or less, preferably 3.2% by mass or more and 8.0% by mass or less, and more preferably 3.5% by mass or more and 5.0% by mass or less. If the CNT content is 3.0% by mass or more, a ninth dispersion having a large CNT content can be prepared. If the CNT content is 10.0% by mass or less, the viscosity of the seventh dispersion can be reduced.

2.7.3. Step of preparing eighth dispersion (step S8)
In the step of preparing the eighth dispersion, an eighth dispersion is prepared that contains CNTs and water and has a higher CNT content than the seventh dispersion. The explanation of the step of preparing the eighth dispersion can be applied by replacing the "first dispersion" with the "eighth dispersion" in the explanation of the step of preparing the first dispersion (step S1) described above. Therefore, the explanation will be omitted.

The order of the process of preparing the seventh dispersion (step S7) and the process of preparing the eighth dispersion (step S8) is not particularly limited.

2.7.4. Step of preparing ninth dispersion (step S9)
In the step of preparing the ninth dispersion, the seventh dispersion and the eighth dispersion are mixed to prepare the ninth dispersion. The explanation of the step of preparing the ninth dispersion can be applied by replacing the "first dispersion", "second dispersion", and "third dispersion" with the "seventh dispersion", "eighth dispersion", and "ninth dispersion" in the step of preparing the third dispersion described above (step S3). Therefore, the explanation is omitted.

By carrying out the above steps, the ninth dispersion liquid, which is a CNT aqueous dispersion liquid, can be produced.

2.7.5. Effects In the second modified example of the manufacturing method, in addition to the steps of preparing the first to sixth dispersions of the first modified example, the second modified example includes a step of preparing a seventh dispersion by performing an underwater head-on collision method with the sixth dispersion having two or more passes through the wet atomization device, a step of preparing an eighth dispersion containing CNT and water and having a higher CNT content than the seventh dispersion, and a step of mixing the seventh dispersion and the eighth dispersion to prepare a ninth dispersion. In the step of preparing the eighth dispersion, the eighth dispersion is prepared by performing an underwater head-on collision method with one pass through the wet atomization device. Therefore, in the second modified example of the manufacturing method, a ninth dispersion having a higher CNT content than the sixth dispersion can be prepared as a CNT aqueous dispersion.

In the above, an example including the steps of preparing the first to ninth dispersions has been described, but the method for producing a CNT aqueous dispersion according to the present invention may further produce a CNT aqueous dispersion by mixing a dispersion (low-concentration dispersion) prepared by performing an underwater head-on collision method with the ninth dispersion with two or more passes, with a dispersion (high-concentration dispersion having a higher CNT concentration than the low-concentration dispersion) prepared by performing an underwater head-on collision method with one pass. In this way, in the method for producing a CNT aqueous dispersion according to the present invention, a CNT aqueous dispersion may be produced by repeatedly mixing a low-concentration dispersion and a high-concentration dispersion. The number of times the mixing is repeated is not particularly limited. The more times the mixing is repeated, the greater the CNT aqueous dispersion that can be produced.

3. Examples and Comparative Examples The present invention will be described in more detail below with reference to the following examples and comparative examples, but the present invention is not limited to these examples and comparative examples.

3.1. Preparation of CNT aqueous dispersion 3.1.1. Example 1
(1) First liquid CMC was added to water and stirred. CMC was added so that the content in the first liquid was 4.0 mass%. The CMC used was "Cellogen 5A" manufactured by Daiichi Kogyo Seiyaku Co., Ltd. The CMC had a molecular weight of 11,000 to 15,000 and an etherification degree of 0.7. Only CMC was used as a dispersant.

Next, CNT was added to the stirred mixture of CMC and water, and stirred for 1 hour. The stirring was performed at a higher speed than that of the mixture of CMC and water. The CNT was added so that the content of the CNT in the first liquid was 4.0 mass %. The CNT used was "K-Nanos-100P" manufactured by Kumho Petrochemical Co., Ltd. The CNT was MWNT, had a diameter of 8 nm to 15 nm, a fiber length of 27 μm (bundle), and a BET specific surface area of 220 m ² /g.

The mixture was then processed using a homogenizer called "Biomixer BM-2" manufactured by Nippon Seiki Seisakusho Co., Ltd. The processing time was set to 5 minutes.

Next, the mixed liquid was subjected to underwater counter impingement. The underwater counter impingement was performed using a wet atomization device "Starburst Lab" (model name: HJP-25005) manufactured by Sugino Machine Co., Ltd. The diameter of the nozzle hole from which the mixed liquid was discharged was set to 100 μm, and the discharge pressure of the mixed liquid was set to 200 MPa. The mixed liquid was passed through the wet atomization device once.

The first liquid was produced by the above process. The viscosity of the first liquid was 10,000 cps or more. The viscosity was measured using a B-type viscometer "BM" manufactured by Tokyo Keiki Co., Ltd. at 25°C and a rotation speed of 60 rpm.

(2) Second liquid CMC was added so that the content in the second liquid was 2.3 mass%. CNT was added so that the content in the second liquid was 2.3 mass%. The mixture of CNT, CMC, and water was passed through the wet-type atomizer four times.

Other than the above, the second liquid was prepared in the same manner as the first liquid described above. The viscosity of the second liquid was 150 cps. The viscosity of the second liquid was measured under the same conditions as the first liquid.

(3) Mixing of the first liquid and the second liquid Next, the first liquid and the second liquid were mixed using a high-speed disperser "Homodisper 2.5 type" manufactured by Primix Corporation to prepare a mixed liquid of the first liquid and the second liquid. The processing time was set to 60 minutes. The ratio of the mass of the first liquid to the mass of the second liquid was adjusted so that the CNT content in the mixed liquid was 3.0%.

By the above process, a mixed liquid was prepared as a CNT aqueous dispersion. Figure 4 is a table showing the preparation conditions of the mixed liquids of Examples 1 to 11 and Comparative Examples 1 and 2. In Figure 4, the "mass ratio of the first liquid" refers to the ratio of the mass of the first liquid to the sum of the mass of the first liquid and the mass of the second liquid.

3.1.2. Examples 2 and 3
In Examples 2 and 3, a mixed liquid of the first liquid and the second liquid was prepared in the same manner as in the above-described first example, except that the CNT content and the CMC content in the first liquid were changed as shown in FIG.

Examples 4 and 5
In Examples 4 and 5, a mixed liquid of the first liquid and the second liquid was prepared in the same manner as in the above-described first example, except that the CNT content and CMC content in the first liquid, and the CNT content and CMC content in the second liquid were changed as shown in Figure 4.

Example 6
In Example 6, a mixed liquid of the first liquid and the second liquid was prepared in the same manner as in the above-mentioned Example 1, except that the mixed liquid of the first example prepared as described above was subjected to an underwater head-on collision method with the number of passes through a wet atomization device set to 4, and the second liquid was used as the second liquid, and the mass ratio of the first liquid was set to 0.40.

Example 7
In Example 7, a mixed liquid of the first liquid and the second liquid was prepared in the same manner as in the above-mentioned Example 1, except that the mixed liquid of Example 6 prepared as described above was subjected to an underwater head-on collision method with the number of passes through a wet atomization device set to 4, and the second liquid was used as the second liquid, and the mass ratio of the first liquid was set to 0.34.

Examples 8 and 9
In Examples 8 and 9, a mixed liquid of the first liquid and the second liquid was prepared in the same manner as in Example 1 described above, except that the CMC content in the first liquid or the second liquid was changed as shown in Figure 4.

Example 10
In Example 10, a mixed liquid of the first liquid and the second liquid was prepared in the same manner as in the above-mentioned Example 1, except that the mixed liquid of Example 9 prepared as described above was subjected to an underwater head-on collision method with the number of passes through a wet atomization device set to 4, and the second liquid was used as the second liquid, and the mass ratio of the first liquid was set to 0.30.

Example 11
In Example 11, a mixed liquid of the first liquid and the second liquid was prepared in the same manner as in Example 1 described above, except that the CMC content in the first liquid and the second liquid was changed as shown in Figure 4.

Comparative Example 1
In Comparative Example 1, except that only water was used as the second liquid, a mixed liquid of the first liquid and the second liquid was prepared in the same manner as in the above-described Example 1. That is, in Comparative Example 1, the first liquid having a CNT content of 4.0 mass % was diluted with water so that the CNT content became 3.0 mass %.

Comparative Example 2
In Comparative Example 2, a mixed liquid of the first and second liquids was prepared in the same manner as in Example 1 described above, except that only water was used as the second liquid, and the ratio of the mass of the first liquid to the mass of the second liquid was adjusted so that the viscosity of the mixed liquid of the first and second liquids was 1500 cps, the same as in Example 1. That is, in Comparative Example 2, the first liquid having a viscosity of 10000 cps or more was diluted with water so that the viscosity was 1500 cps.

3.2. Evaluation Results of CNT Content and Viscosity in Mixtures FIG. 4 shows the evaluation results of the CNT content and viscosity in the mixed solutions of Examples 1 to 11 and Comparative Examples 1 and 2.

As shown in Figure 4, the mixed liquids of Examples 1 to 11 had a CNT content of 2.5 mass% or more and a viscosity of 3000 cps or less. Comparative Example 1 had a CNT content of 2.5 mass% or more, but a viscosity higher than 3000 cps. Comparative Example 2 had a viscosity of 3000 cps or less, but a CNT content of less than 2.5 mass%.

By comparing Example 1 and Comparative Example 1, it was found that even though the CNT content in the mixed liquid was the same 3.0 mass%, Comparative Example 1, which used water as the second liquid, had a significantly higher viscosity.

By comparing Example 1 and Comparative Example 2, it was found that even though the viscosity of the mixed liquid was the same at 1,500 cps, Comparative Example 2, which used water as the second liquid, had a significantly lower CNT content.

By comparing Example 1 with Example 6, it was found that the CNT content can be increased by using the mixed liquid of Example 5 as the second liquid. Furthermore, by comparing Example 6 with Example 7, it was found that the CNT content can be increased even further by using the mixed liquid of Example 6 as the second liquid. In this way, by repeatedly using the mixed liquid of this example prepared as the second liquid, it is possible to bring the CNT content in the mixed liquid as close as possible to 4.0 mass%.

Examples 8, 9, and 11 demonstrated that even if the ratio M CMC /M _CNT of the mass of CMC M _CMC to the mass of _CNT M _CNT is not 1 (CNT:CMC = 1:1), a mixed liquid having a CNT content of 2.5 mass% or more and a viscosity of 3,000 cps or less can be produced.

The above-described embodiment and modified examples are merely examples, and the present invention is not limited to these. For example, each embodiment and each modified example can be combined as appropriate.

The present invention is not limited to the above-described embodiments, and various modifications are possible. For example, the present invention includes configurations that are substantially the same as the configurations described in the embodiments. Substantially the same configurations are, for example, configurations that have the same functions, methods, and results, or configurations that have the same purpose and effect. The present invention also includes configurations in which non-essential parts of the configurations described in the embodiments are replaced. The present invention also includes configurations that have the same effects as the configurations described in the embodiments, or that can achieve the same purpose. The present invention also includes configurations in which publicly known technology is added to the configurations described in the embodiments.

Claims (17)

A carbon nanotube aqueous dispersion used as a coating agent for an electromagnetic wave shielding seed,
Carbon nanotubes, sodium carboxymethylcellulose, and water,
The carbon nanotube content is 2.5% by mass or more,
The viscosity measured using a Brookfield viscometer at 25° C. and a rotation speed of 60 rpm is 2000 cps or less .
A carbon nanotube aqueous dispersion, wherein the ratio of the mass of the sodium carboxymethylcellulose to the mass of the carbon nanotubes is 1/3 or more and 3 or less . The carbon nanotube aqueous dispersion according to claim 1, which does not contain any dispersant other than the sodium carboxymethylcellulose. The carbon nanotube aqueous dispersion according to claim 1 or 2, wherein the carbon nanotube content is 3.0% by mass or more. The carbon nanotube aqueous dispersion according to any one of claims 1 to 3, wherein the viscosity is 2500 cps or less. The carbon nanotube aqueous dispersion according to any one of claims 1 to 4, wherein the viscosity is 1000 cps or more. The carbon nanotube aqueous dispersion according to any one of claims 1 to 5, wherein the content of the sodium carboxymethylcellulose is 2.5% by mass or more. The carbon nanotube aqueous dispersion according to any one of claims 1 to 6, wherein the content of the sodium carboxymethylcellulose is 3.0 mass% or more. A step of producing an aqueous dispersion of carbon nanotubes;
applying the carbon nanotube aqueous dispersion onto a substrate;
drying the applied carbon nanotube aqueous dispersion;
Including,
The step of producing the carbon nanotube aqueous dispersion includes:
preparing a first dispersion liquid containing carbon nanotubes, sodium carboxymethylcellulose, and water, the first dispersion liquid having a carbon nanotube content of 3.0% by mass or more;
preparing a second dispersion containing carbon nanotubes, sodium carboxymethylcellulose, and water, the second dispersion having a carbon nanotube content of less than 3.0% by mass;
mixing the first dispersion and the second dispersion to prepare a third dispersion;
Including,
In the step of preparing the first dispersion, the first dispersion is prepared by performing an underwater head-on collision method with a pass number of a wet-type atomization device set to one,
In the step of preparing the second dispersion, the second dispersion is prepared by performing an underwater head-on collision method with a wet-type atomization device having a pass count of 2 or more;
The carbon nanotube content in the carbon nanotube aqueous dispersion is 2.5% by mass or more,
The viscosity of the carbon nanotube aqueous dispersion measured using a Brookfield viscometer at 25° C. and 60 rpm is 2000 cps or less;
A method for producing an electromagnetic wave shielding sheet, wherein in the carbon nanotube aqueous dispersion, a ratio of the mass of sodium carboxymethylcellulose to the mass of carbon nanotubes is 1/3 or more and 3 or less . The method for producing an electromagnetic shielding sheet according to claim 8 , wherein in the step of preparing the second dispersion, the number of passes through a wet-type atomization device is set to four. a step of subjecting the third dispersion to an underwater head-on collision method with a wet-type atomization device being passed through the third dispersion twice or more to prepare a fourth dispersion;
preparing a fifth dispersion containing carbon nanotubes and water, the fifth dispersion having a higher carbon nanotube content than the fourth dispersion;
mixing the fourth dispersion and the fifth dispersion to prepare a sixth dispersion;
Including,
10. The method for producing an electromagnetic wave shielding sheet according to claim 8 , wherein in the step of preparing the fifth dispersion, the fifth dispersion is prepared by performing an underwater head-on collision method with a pass number of a wet-type atomization device being one . The method for producing an electromagnetic shielding sheet according to claim 10 , wherein in the step of preparing the fourth dispersion, the number of passes through a wet-type atomization device is set to four. a step of subjecting the sixth dispersion to an underwater head-on collision method with a wet-type atomization device being passed through the sixth dispersion twice or more to prepare a seventh dispersion;
preparing an eighth dispersion comprising carbon nanotubes and water, the eighth dispersion having a higher carbon nanotube content than the seventh dispersion;
mixing the seventh dispersion and the eighth dispersion to prepare a ninth dispersion;
Including,
12. The method for producing an electromagnetic wave shielding sheet according to claim 10 , wherein in the step of preparing the eighth dispersion, the eighth dispersion is prepared by performing an underwater head-on collision method with a pass number of a wet-type atomization device being one . The method for producing an electromagnetic shielding sheet according to claim 12 , wherein in the step of preparing the seventh dispersion, the number of passes through a wet-type atomization device is set to four. The content of the carbon nanotubes in the first dispersion is 3.5% by mass or more,
The method for producing an electromagnetic wave shielding sheet according to claim 8 , wherein the content of the carbon nanotubes in the second dispersion liquid is 2.5 mass % or less. 15. The method for producing an electromagnetic wave shielding sheet according to claim 8, wherein in the step of preparing the third dispersion, the first dispersion and the second dispersion are mixed such that a ratio of a mass of the first dispersion to a sum of a mass of the first dispersion and a mass of the second dispersion is 0.15 or more and 0.70 or less. The content of the sodium carboxymethylcellulose in the first dispersion is 3.0% by mass or more,
The method for producing an electromagnetic wave shielding sheet according to claim 8 , wherein the content of the sodium carboxymethyl cellulose in the second dispersion is less than 3.0% by mass. The content of the sodium carboxymethylcellulose in the first dispersion is 3.5% by mass or more,
The method for producing an electromagnetic wave shielding sheet according to claim 16 , wherein the content of the sodium carboxymethyl cellulose in the second dispersion is 2.5 mass% or less.

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