Carbon nanotube dispersion

Abstract

To provide a carbon nanotube dispersion which exhibits superior temporal stability in its dispersion state and enables the formation of a molded article having superior conductivity.SOLUTION: A carbon nanotube dispersion contains carbon nanotubes, a solvent, and a dispersant having a weight-average molecular weight of less than 40000, where the carbon nanotubes include single-walled carbon nanotubes, and the carbon nanotubes have a harmonic mean diameter of 600 nm or more and 1240 nm or less.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a carbon nanotube dispersion.

Background Art

[0002] Conventionally, carbon nanotubes (hereinafter sometimes abbreviated as "CNT") have attracted attention as materials excellent in properties such as conductivity, heat conductivity, and strength.

[0003] Here, although the properties of each individual CNT are excellent, since the outer diameter is small, CNTs are likely to be bundled by van der Waals forces (likely to form bundles). Therefore, conventionally, CNTs have been once dispersed in a solvent to prepare a carbon nanotube dispersion (CNT dispersion). And the obtained CNT dispersion is used, for example, in the production of a molded body such as a carbon film formed by aggregation of a plurality of CNTs and an electrode for a secondary battery.

[0004] Therefore, in recent years, attempts have been made to improve various properties of the molded body by improving the CNT dispersion. For example, Patent Document 1 discloses a CNT dispersion containing CNTs, a solvent, and a dispersant, wherein the average outer diameter of the CNTs, the half-value width of the peak at a diffraction angle 2θ = 25° ± 2°, and the G / D ratio are each within a predetermined range. And according to Patent Document 1, by using the CNT dispersion, a resin composition, an electrode film, etc. excellent in conductivity can be obtained. Further, Patent Document 2 discloses a CNT dispersion containing CNTs, a dispersant, and a solvent, wherein the average outer diameter, the BET specific surface area, and the average fiber length of the CNTs are each within a predetermined range. And according to Patent Document 2, by using the CNT dispersion, a resin composition, an electrode film, etc. excellent in conductivity and adhesion can be obtained.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

[0006] However, in recent years, further improvement in various properties of a molded body formed from a CNT dispersion has been demanded. However, the conventional CNT dispersion has room for improvement in terms of further enhancing the conductivity of the molded body. In addition, the conventional CNT dispersion has room for further improvement in the stability of the dispersion state over time.

[0007] Therefore, an object of the present invention is to provide a carbon nanotube dispersion that is excellent in the stability of the dispersion state over time and can form a molded body excellent in conductivity. [Means for Solving the Problems]

[0008] The present inventor has intensively studied for the purpose of solving the above problems. Then, the present inventor has found that in a CNT dispersion containing CNTs containing single-walled CNTs, a solvent, and a dispersant having a weight average molecular weight of less than a predetermined value, if the harmonic mean diameter of the CNTs is within a predetermined range, a CNT dispersion excellent in the stability of the dispersion state over time and capable of enhancing the conductivity of the molded body can be obtained, and the present invention has been completed.

[0009] That is, the present invention is intended to advantageously solve the above problems, and according to the present invention, the CNT dispersions of the following [1] to [4] are provided.

[0010] [1] A carbon nanotube dispersion comprising carbon nanotubes, a solvent, and a dispersant having a weight average molecular weight of less than 40,000, wherein the carbon nanotubes include single-walled carbon nanotubes, and the harmonic mean diameter of the carbon nanotubes is 600 nm or more and 1240 nm or less. A CNT dispersion containing a CNT containing single-walled CNTs, a solvent, and a dispersant having a weight average molecular weight less than the above value, and having a harmonic average diameter of the CNT within the above range is excellent in the temporal stability of the dispersed state and can exhibit excellent conductivity in a molded body. In the present invention, the "harmonic average diameter" refers to the z-average diameter measured by dynamic light scattering (DLS), and can be measured using the method described in the examples of this specification. Further, in the present invention, the "weight average molecular weight" of the dispersant can be measured as a standard polystyrene equivalent value using gel permeation chromatography, and specifically, it can be measured using the method described in the examples of this specification.

[0011] 〔2〕The carbon nanotube dispersion according to 〔1〕 above, wherein the G / D ratio of the carbon nanotube is 0.5 or more and 120 or less. If the G / D ratio of the CNT is within the above range, the temporal stability of the dispersed state of the CNT dispersion can be further improved, and the conductivity of the molded body can be further enhanced. In the present invention, the "G / D ratio" refers to the ratio of the G-band peak intensity to the D-band peak intensity in the Raman spectrum, and can be measured using the method described in the examples of this specification.

[0012] 〔3〕The carbon nanotube dispersion according to 〔1〕 or 〔2〕 above, wherein the dispersant is carboxymethyl cellulose or a salt thereof. If carboxymethyl cellulose or a salt thereof is used as the dispersant, the temporal stability of the dispersed state of the CNT dispersion can be further improved, and the conductivity of the molded body can be further enhanced.

[0013] 〔4〕The carbon nanotube dispersion according to 〔3〕 above, wherein the degree of etherification of the carboxymethyl cellulose is 0.5 or more and 0.9 or less. If the degree of etherification of carboxymethyl cellulose used as a dispersant is within the above-mentioned range, aggregation of CNTs in the CNT dispersion can be suppressed, and the temporal stability of the dispersion state of the CNT dispersion can be further improved. In the present invention, the "degree of etherification" of carboxymethyl cellulose refers to the average value of the number of hydroxyl groups substituted by a substituent (carboxymethyl group) per unit of anhydroglucose constituting carboxymethyl cellulose, and can take a value greater than 0 and less than 3. The larger the degree of etherification, the smaller the proportion of hydroxyl groups in one molecule of carboxymethyl cellulose (i.e., the larger the proportion of substituents), and the smaller the degree of etherification, the larger the proportion of hydroxyl groups in one molecule of carboxymethyl cellulose (i.e., the smaller the proportion of substituents). This degree of etherification (degree of substitution) can be determined by the method described in JP-A-2011-34962.

Advantages of the Invention

[0014] According to the present invention, it is possible to provide a carbon nanotube dispersion that is excellent in the temporal stability of the dispersion state and can form a molded article excellent in conductivity.

Modes for Carrying Out the Invention

[0015] Hereinafter, embodiments of the present invention will be described in detail. Here, the CNT dispersion of the present invention is not particularly limited, and can be used, for example, as a material for forming a molded article such as a carbon film or an elastomer molded article, or an electrode for a secondary battery.

[0016] (Carbon Nanotube Dispersion) The CNT dispersion of the present invention contains at least CNTs, a solvent, and a dispersant, and optionally further contains components other than CNTs, the solvent, and the dispersant (other components). Further, at least a part of the CNTs is single-walled CNTs, and usually, at least a part of the CNTs in the CNT dispersion forms CNT bundles. Note that an CNT bundle is formed by a plurality of CNTs aggregating in a bundle shape (a bundled state) along the long axis direction of the CNTs. Here, the CNT dispersion of the present invention contains a dispersant having a weight average molecular weight of less than 40,000, and is characterized in that the harmonic mean diameter of the CNTs is 600 nm or more and 1240 nm or less.

[0017] And, since the CNT dispersion of the present invention contains a dispersant having a weight average molecular weight less than a predetermined value and the harmonic mean diameter of the CNTs is within the above-described range, the CNT dispersion is excellent in the temporal stability of the dispersed state and can exhibit excellent conductivity in the molded body. Although the reason why the above effects can be obtained by using the CNT dispersion of the present invention is not clear, it is presumed as follows.

[0018] The CNT dispersion of the present invention has a harmonic mean diameter of the CNTs of 600 nm or more and 1240 nm or less. When the harmonic mean diameter of the CNTs is 600 nm or more, it can be said that the CNTs in the CNT dispersion have sufficient length. Further, when the harmonic mean diameter of the CNTs is 1240 nm or less, excessive aggregation of the CNTs is suppressed, and it can be said that a sufficient number of dispersed CNTs are present in the CNT dispersion. That is, when the harmonic mean diameter of the CNTs is 600 nm or more and 1240 nm or less, the length and number of the CNTs present in the CNT dispersion are sufficiently ensured. Further, the CNT dispersion of the present invention contains a dispersant having a weight average molecular weight of less than 40,000. And in the CNT mixture before the dispersion treatment, since the CNTs are likely to form bundle-like aggregates (CNT aggregates) due to the van der Waals force, the dispersant having a small weight average molecular weight penetrates into the gaps of the CNT aggregates and can exhibit its function as a dispersant well. Thus, by the combination of sufficiently ensuring the number and length of the CNTs and the penetration effect of the dispersant into the CNT aggregates, in the CNT dispersion of the present invention, the CNTs can be stably dispersed over time (the temporal stability of the dispersed state can be improved). Further, by using such a CNT dispersion, a good conductive path can be formed by distributing the CNTs having a sufficient length in the molded body. For the above reasons, it is considered that the CNT dispersion of the present invention is excellent in the temporal stability of the dispersed state, and when the CNT dispersion is used, excellent conductivity can be exhibited in the molded body.

[0019] <cnt> The CNT is not particularly limited as long as it contains single-walled CNTs. As the CNT, only single-walled CNTs may be used, or single-walled CNTs and multi-walled CNTs may be used in combination.

[0020] [[Properties]] [Harmonic mean diameter] The harmonic mean diameter of the CNT needs to be 600 nm or more and 1240 nm or less, preferably 650 nm or more, more preferably 700 nm or more, still more preferably 750 nm or more, even more preferably 800 nm or more, particularly preferably 850 nm or more, preferably 1200 nm or less, and more preferably 1150 nm or less. When the harmonic mean diameter of the CNT is less than 600 nm, the conductivity of the molded body decreases. On the other hand, when the harmonic mean diameter of the CNT exceeds 1240 nm, the temporal stability of the dispersion state of the CNT dispersion decreases. The harmonic mean diameter of the CNT can be adjusted, for example, by changing the length and diameter of the raw material CNT, the type and amount of the dispersant, and the conditions of the dispersion treatment. For example, when a wet jet mill is used for the dispersion treatment, the harmonic mean diameter of the CNT becomes smaller by increasing the pressure applied to the mixed liquid passing through the wet jet mill, and becomes larger by decreasing the pressure. Also, the harmonic mean diameter of the CNT becomes smaller by increasing the number of passes (the number of times the total amount of the mixed liquid passes through the wet jet mill) of the dispersion treatment by the wet jet mill, and becomes larger by decreasing the number of passes.

[0021] [G / D ratio] The G / D ratio of the CNT is preferably 0.5 or more, more preferably 1 or more, preferably 120 or less, more preferably 90 or less, still more preferably 60 or less, even more preferably 30 or less, particularly preferably 10 or less. If the G / D ratio of the CNT is within the above range, the temporal stability of the dispersion state of the CNT dispersion can be further improved, and the conductivity of the molded body can be further enhanced.

[0022] [Content ratio] The content ratio of CNTs in the CNT dispersion is preferably 0.01% by mass or more, more preferably 0.05% by mass or more, preferably 2% by mass or less, more preferably 1% by mass or less, and even more preferably 0.5% by mass or less, with the total amount of the CNT dispersion being 100% by mass. If the content ratio of CNTs in the CNT dispersion is 0.01% by mass or more, the conductivity of the molded body can be further enhanced, and the productivity of the molded body can be improved. On the other hand, if the content ratio of CNTs in the CNT dispersion is 2% by mass or less, the temporal stability of the dispersion state of the CNT dispersion can be further improved.

[0023] <Solvent> The CNT dispersion of the present invention may contain only water as the solvent, may contain only an organic solvent (e.g., esters, ketones, alcohols) as the solvent, or the solvent may be a mixture of water and an organic solvent. From the viewpoint of further improving the temporal stability of the dispersion state of the CNT dispersion and further enhancing the conductivity of the molded body, the CNT dispersion of the present invention preferably contains water as the solvent. Note that the CNT dispersion of the present invention may contain one kind of organic solvent or may contain two or more kinds of organic solvents.

[0024] Here, in the CNT dispersion of the present invention, the ratio of water in the solvent is preferably 70% by mass or more, more preferably 90% by mass or more, even more preferably 99% by mass or more, and particularly preferably 100% by mass (i.e., the CNT dispersion of the present invention contains only water as the solvent), with the total amount of the solvent being 100% by mass. If the ratio of water in the solvent in the CNT dispersion is at least the above lower limit, the temporal stability of the dispersion state of the CNT dispersion can be further improved, and the conductivity of the molded body can be further enhanced.

[0025] <Dispersant> The CNT dispersion of the present invention contains a dispersant having a weight average molecular weight of less than 40,000. Examples of such dispersants include water-soluble polymers such as cellulose-based polymers, polycarboxylic acids, polyvinylpyrrolidone, polyvinyl alcohol, polyvinyl methyl ether, poly-N-vinylacetamide, polyacrylamide, and polyimide. In the present invention, when a polymer is "water-soluble", it means that when 0.5 g (in terms of solid content) of the polymer is dissolved in 100 g of water at a temperature of 25 °C, the amount of insoluble matter is less than 10.0% by mass.

[0026] Examples of cellulose-based polymers include carboxymethyl cellulose, methyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl methyl cellulose, and their salts and derivatives. Examples of polycarboxylic acids include polyacrylic acid, polymethacrylic acid, alginic acid, and their salts and derivatives. Here, examples of the above salts include alkali metal salts such as sodium salts and ammonium salts. Examples of the above derivatives include esters such as alkyl esters and ethers.

[0027] The above-mentioned dispersants may be used alone or in combination of two or more in any ratio. Among these, from the viewpoint of further improving the temporal stability of the dispersion state of the CNT dispersion and further enhancing the conductivity of the molded body, a cellulose-based polymer is preferably used as the dispersant, and carboxymethyl cellulose and its salts are more preferred.

[0028] Note that the CNT dispersion may further contain a dispersant having a weight average molecular weight of 40,000 or more. That is, as the dispersant contained in the CNT dispersion, only a dispersant having a weight average molecular weight of less than 40,000 may be used, or a dispersant having a weight average molecular weight of less than 40,000 and a dispersant having a weight average molecular weight of 40,000 or more may be used in combination.

[0029] Here, when a dispersant having a weight-average molecular weight of less than 40,000 and a dispersant having a weight-average molecular weight of 40,000 or more are used in combination, the proportion of the dispersant having a weight-average molecular weight of less than 40,000 is preferably 30% by mass or more, more preferably 40% by mass or more, and still more preferably 50% by mass or more, with the total amount of the dispersants (that is, the total amount of the dispersant having a weight-average molecular weight of less than 40,000 and the dispersant having a weight-average molecular weight of 40,000 or more) being 100% by mass. If the proportion of the dispersant having a weight-average molecular weight of less than 40,000 is 30% by mass or more, the stability over time of the dispersion state of the CNT dispersion can be further improved, and the conductivity of the molded body can be further enhanced.

[0030] <<Weight-average molecular weight>> The CNT dispersion of the present invention needs to contain a dispersant having a weight-average molecular weight of less than 40,000. When the weight-average molecular weight of the dispersant contained in the CNT dispersion is 40,000 or more, it becomes difficult for the dispersant to penetrate into the CNT aggregates, and the stability over time of the dispersion state of the CNT dispersion and the conductivity of the molded body decrease. Also, the viscosity of the CNT dispersion excessively increases. And the CNT dispersion preferably contains a dispersant having a weight-average molecular weight of 36,000 or less, and more preferably contains a dispersant having a weight-average molecular weight of 32,000 or less. If the CNT dispersion contains a dispersant having a weight-average molecular weight of 36,000 or less, the stability over time of the dispersion state of the CNT dispersion can be further improved, and the conductivity of the molded body can be further enhanced. Also, an excessive increase in the viscosity of the CNT dispersion can be suppressed. The lower limit of the weight-average molecular weight of the dispersant is not particularly limited, and can be, for example, 1000 or more, and can be 5000 or more.

[0031] <<Degree of etherification>> Here, carboxymethyl cellulose, which is preferably used as a dispersant, preferably has an etherification degree of 0.5 or more, more preferably 0.6 or more, still more preferably 0.7 or more, preferably 0.9 or less, and more preferably 0.8 or less. If the etherification degree of carboxymethyl cellulose is within the above range, aggregation of CNTs in the CNT dispersion can be suppressed, and the temporal stability of the dispersion state of the CNT dispersion can be further improved.

[0032] <<Content ratio>> The content ratio of the dispersant having a weight average molecular weight of less than 40,000 in the CNT dispersion is preferably 0.05% by mass or more, more preferably 0.1% by mass or more, still more preferably 0.2% by mass or more, preferably 10% by mass or less, more preferably 5% by mass or less, still more preferably 2% by mass or less, and particularly preferably 1% by mass or less, with respect to 100% by mass of the total amount of the CNT dispersion. If the content ratio of the dispersant having a weight average molecular weight of less than 40,000 in the CNT dispersion is within the above range, the temporal stability of the dispersion state of the CNT dispersion can be further improved, and the conductivity of the molded body can be further enhanced.

[0033] The content of the dispersant having a weight average molecular weight of less than 40,000 in the CNT dispersion is preferably 100 parts by mass or more, more preferably 200 parts by mass or more, preferably 500 parts by mass or less, and more preferably 450 parts by mass or less, per 100 parts by mass of CNTs. If the amount ratio of the dispersant having a weight average molecular weight of less than 40,000 to CNTs in the CNT dispersion is within the above range, the temporal stability of the dispersion state of the CNT dispersion can be further improved, and the conductivity of the molded body can be further enhanced.

[0034] <Other components> Examples of other components that the CNT dispersion may optionally contain include conductive materials other than CNTs, polymer components other than the above-described dispersants, antioxidants, resins, and the like. Note that the CNT dispersion may contain only one type of other component or two or more types of other components. The proportion of other components contained in the CNT dispersion is preferably 5% by mass or less, more preferably 1% by mass or less, still more preferably 0.5% by mass or less, even more preferably 0.1% by mass or less, and particularly preferably 0% by mass (that is, the CNT dispersion of the present invention does not contain other components), based on 100% by mass of the total amount of the CNT dispersion.

[0035] <Viscosity at a shear rate of 1000 (1 / s)> The CNT dispersion of the present invention preferably has a viscosity under the condition of a shear rate of 1000 (1 / s) of 1.60 mPa·s or more, more preferably 1.80 mPa·s or more, still more preferably 2.00 mPa·s or more, particularly preferably 2.20 mPa·s or more, and preferably 3.50 mPa·s or less, more preferably 3.20 mPa·s or less. If the viscosity under the condition of a shear rate of 1000 (1 / s) is within the above range, the temporal stability of the dispersion state of the CNT dispersion can be further improved, and the conductivity of the molded body can be further enhanced.

[0036] <Preparation of CNT dispersion> The CNT dispersion of the present invention can be prepared, for example, by subjecting a mixed solution containing the above-described CNT, solvent, dispersant, and any other optional components to a dispersion treatment. The dispersion treatment is not particularly limited, and can be carried out using, for example, a mixer such as a wet jet mill, bead mill, ball mill, sand mill, pigment disperser, attritor, ultrasonic disperser, homogenizer, planetary mixer, or film mixer. Among them, from the viewpoint of further improving the temporal stability of the dispersion state of the CNT dispersion and further enhancing the conductivity of the molded body, it is preferable to use a wet jet mill. Examples of the wet jet mill include "Nanovaitor (registered trademark)" (manufactured by Yoshida Kikai Kogyo Co., Ltd.), "BERYU SYSTEM PRO" (manufactured by Mikari Co., Ltd.), an ultra-high pressure wet atomization device (manufactured by Yoshida Kogyo Co., Ltd.), "Nanomizer (registered trademark)" (manufactured by Nanomizer Co., Ltd.), and "Starburst (registered trademark)" (manufactured by Sugino Machine Ltd.).

[0037] Here, examples of the nozzles provided in the wet jet mill include a straight nozzle, a cross nozzle, etc. The straight nozzle has an I-shaped nozzle cross-section and relatively low processing energy. On the other hand, the cross nozzle has an X-shaped nozzle cross-section and higher processing energy than the straight nozzle. From the perspective of further improving the temporal stability of the dispersion state of the CNT dispersion liquid and further enhancing the conductivity of the molded body, it is preferable to use a straight nozzle as the nozzle provided in the wet jet mill.

[0038] Here, the pressure applied in the dispersion treatment of the mixed liquid by the wet jet mill is preferably 25 MPa or more, preferably 100 MPa or less, more preferably 50 MPa or less, and even more preferably 30 MPa or less. If the pressure applied to the mixed liquid is 25 MPa or more, the temporal stability of the dispersion state of the CNT dispersion liquid can be further improved, and the conductivity of the obtained molded body can be further enhanced. Also, if the pressure applied to the mixed liquid is 100 MPa or less, the conductivity of the obtained molded body can be further enhanced by suppressing the cutting of CNTs by the dispersion treatment.

[0039] Also, the number of passes of the dispersion treatment by the wet jet mill is preferably 5 or more, more preferably 20 or more, even more preferably 45 or more, and preferably 50 or less. If the number of passes of the dispersion treatment by the wet jet mill is 5 or more, the temporal stability of the dispersion state of the CNT dispersion liquid can be further improved, and the conductivity of the obtained molded body can be further enhanced. Also, if the number of passes of the dispersion treatment by the wet jet mill is 50 or less, the conductivity of the obtained molded body can be further enhanced by suppressing the cutting of CNTs by the dispersion treatment.

Example

[0040] Hereinafter, the present invention will be specifically described based on examples, but the present invention is not limited to these examples. In the following description, “%” and “parts” representing amounts are based on mass unless otherwise specified. Then, in the examples and comparative examples, the harmonic mean diameter and G / D ratio of CNTs, the weight average molecular weight of the dispersant, the viscosity at a shear rate of 1000 (1 / s), the particle size change rate and viscosity change rate, and the volume conductivity of the carbon film (formed body) were evaluated by the following methods.

[0041] <Harmonic mean diameter> The CNT dispersions prepared in the examples and comparative examples were diluted with water so that the concentration of CNTs became 10 mass ppm. The obtained diluted solution was measured using a dynamic light scattering measuring instrument (manufactured by Malvern, product name “Zetasizer Nano Nano-ZS”) under the conditions of a temperature of 25° C., a scanning number of 3 times, and a scanning time of 60 seconds to obtain the harmonic mean diameter (z-average diameter). <G / D ratio> Using a microscopic laser Raman spectrophotometer (manufactured by Thermo Fisher Scientific, product name “Nicolet Almega XR”), the Raman spectra of CNTs used in the examples and comparative examples were measured. Then, for the obtained Raman spectra, the intensity of the G-band peak observed in the vicinity of 1590 cm -1 and the intensity of the D-band peak observed in the vicinity of 1340 cm -1 were determined, and the G / D ratio was calculated. <Weight average molecular weight> The weight average molecular weight (Mw) of the dispersant was measured using gel permeation chromatography with an aqueous solution having a dispersant concentration of 1%. Specifically, using a gel permeation chromatograph (manufactured by Tosoh Corporation, product name “HLC-8220”) and using water as the developing solvent, the weight average molecular weight (Mw) of the dispersant was determined as a standard polystyrene conversion value. <Viscosity at a shear rate of 1000 (1 / s)> The viscosity (mPa·s) of the CNT dispersion was measured under the conditions of a shear rate of 1000 (1 / s), a CNT concentration of 0.1%, and a temperature of 20°C. The CNT dispersion immediately after preparation was adjusted to a CNT concentration of 0.1% by adding or removing water as necessary, and the resulting sample was used as the measurement sample. For this measurement sample, a rheometer (manufactured by Antonpaar, product name "MCR-102") was used as the measurement device, a cone plate was used as the measurement jig, and the range of shear rates from 100 to 9000 (1 / s) was measured at 20°C to obtain the viscosity (mPa·s) at a shear rate of 1000 (1 / s). <Temporal stability of the dispersion state> If both the "particle size change rate" and the "viscosity change rate" shown below are evaluated as A or B, it means that the CNT dispersion has excellent temporal stability in the dispersion state. [Particle size change rate] For the CNT dispersions obtained in the examples and comparative examples, the value of the average particle size (D50) immediately after preparation and the value of the average particle size (D50) 10 days after preparation were measured respectively. The average particle size (D50) was measured using a laser diffraction / scattering particle size distribution measuring device (manufactured by HORIBA, product name "Partica LA-960V2") for an aqueous dispersion diluted with water so that the CNT concentration was 0.01%. In the obtained particle size distribution (volume basis), the particle size at which the cumulative volume calculated from the small diameter side was 50% was determined as the average particle size (μm). The measurement conditions were as follows. ·Circulation speed: 3, ultrasonic intensity: 7, ultrasonic time: 1 minute, stirring speed: 1, stirring mode: continuous (operation conditions for circulation / ultrasonic) ·Ultrasonic intensity 7, ultrasonic time 5 seconds (operation conditions for ultrasonic during air bleeding) ·Refractive index of water: 1.333 ·Refractive index of carbon material: 1.92 And the value of the average particle size (D50) immediately after preparation was designated as d 0 (μm), and the value of the average particle size (D50) 10 days after preparation was designated as d 1 (μm), and the particle size change rate (%) = [(d 1 -d 0 ) / d 0 The particle size change rate (%) was calculated by multiplying by 100 and evaluated according to the following criteria. The smaller the value of the particle size change rate, the more difficult it is for the dispersion state of CNTs to change, that is, it means that the dispersion state has excellent stability over time. A: The particle size change rate is less than 1% B: The particle size change rate is 1% or more and less than 15% C: The particle size change rate is 15% or more and less than 30% D: The particle size change rate is 30% or more [Viscosity change rate] For the CNT dispersions obtained in the examples and comparative examples, the viscosity values immediately after preparation and the viscosity values 10 days after preparation were measured respectively. Each viscosity measurement was carried out at a temperature of 20 °C using a rheometer (manufactured by Antonpaar, product name "MCR-102"). Specifically, a cone plate was used as the measurement jig, the range of shear rate from 100 to 9000 (1 / s) was measured, and the viscosity (mPa·s) at a shear rate of 1000 (1 / s) was determined. Then, the viscosity value immediately after preparation was designated as η 0 (mPa·s), and the viscosity value 10 days after preparation was designated as η 1 (mPa·s). The viscosity change rate (%) = [(η 1 - η 0 ) / η 0 ×100 was used to calculate the viscosity change rate (%), and it was evaluated according to the following criteria. The smaller the value of the viscosity change rate, the more difficult it is for the CNT dispersion to thicken over time, that is, it means that the dispersion state has excellent stability over time. A: The viscosity change rate is less than 10% B: The viscosity change rate is 10% or more and less than 15% C: The viscosity change rate is 15% or more and 20% or less D: The viscosity change rate exceeds 20% <Volume conductivity> The volume conductivity (S / cm) of the carbon films (formed bodies) prepared in the examples and comparative examples was measured in accordance with JIS K 7194 using a low resistivity meter (manufactured by Mitsubishi Chemical Analytech Co., Ltd., product name "Roresta-GX"), and evaluated according to the following criteria. The higher the value of this volume conductivity, the more excellent the conductivity of the carbon film (formed body). A: The value of the volume conductivity exceeds 100 S / cm B: The value of the volume conductivity is 50 S / cm or more and 100 S / cm or less C: The value of the volume conductivity is less than 50 S / cm

[0042] (Example 1) (Preparation of CNT dispersion) (Synthesis of CNT) According to the method disclosed in Example 4 of International Publication No. 2022 / 114237, single-walled CNTs (hereinafter referred to as "CNT-a") were produced. The properties of the obtained CNT-a were: G / D ratio: 1.2, tapped bulk density: 0.02 g / cm 3 , average CNT length: 150 μm, BET specific surface area: 600 m 2 / g, average outer diameter: 4.0 nm, and carbon purity: 99.0% by mass. (Preparation of CNT dispersion) To 300 mL of an aqueous solution containing sodium carboxymethyl cellulose (manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd., product name "Cellogen 5A"; weight average molecular weight: 13,000; degree of etherification: 0.7 to 0.8; referred to as "CMC-a") as a dispersant in water as a solvent, 0.3 g of CNT-a as the single-walled CNTs obtained above (G / D ratio: 1.2) was added to obtain a mixed solution (CNT concentration: 0.1%). For this mixed solution, a wet jet mill (manufactured by Yoshida Kikai Kogyo Co., Ltd., model number "NVL-ES008A-D10", product name "Nanovator (registered trademark)") was used to perform a dispersion treatment under the conditions of a pressure of 25 MPa and a number of passes of 30 times. Specifically, while applying a back pressure, a shear force was applied to the mixed solution to disperse the CNTs, and a CNT dispersion was obtained. For the obtained CNT dispersion, the harmonic mean diameter of the CNTs, as well as the particle size change rate and viscosity change rate, were measured. The results are shown in Table 1. (Preparation of carbon film (formed body)) The CNT dispersion prepared above was filtered under the condition of 0.09 MPa using a vacuum filtration device equipped with a membrane filter. After the filtration was completed, the carbon film formed on the membrane filter was washed by passing isopropyl alcohol and water respectively through the vacuum filtration device, and then air was passed through for 15 minutes. Next, the carbon film / membrane filter was immersed in ethanol, and the carbon film was peeled off from the membrane filter to obtain a carbon film as a molded body. The obtained carbon film had the same size as the membrane filter, excellent film-forming properties, and maintained its film state even after being peeled off from the filter, and also had excellent self-supporting properties. The volume conductivity of this carbon film was measured. The results are shown in Table 1.

[0043] (Example 6) When preparing the CNT dispersion, instead of CNT-a as the single-walled CNT, CNT-b (manufactured by Nippon Zeon Co., Ltd., product name "ZEONANO (registered trademark) SG101"; G / D ratio: 5.0) was used, and the preparation of the CNT dispersion and the production of the carbon film were carried out in the same manner as in Example 1 except that the pressure and the number of passes of the dispersion treatment by the wet jet mill were changed as shown in Table 1. Then, the evaluation was carried out in the same manner as in Example 1. The results are shown in Table 1.

[0044] (Example 11) When preparing the CNT dispersion, instead of CMC-a as the dispersant, sodium carboxymethyl cellulose (manufactured by Daiichi Kogyo Seiyaku Co., Ltd., product name "Cellogen 7A"; weight average molecular weight: 30,000; degree of etherification: 0.7 - 0.8; referred to as "CMC-b") was used, and the preparation of the CNT dispersion and the production of the carbon film were carried out in the same manner as in Example 1 except that the pressure and the number of passes of the dispersion treatment by the wet jet mill were changed as shown in Table 2. Then, the evaluation was carried out in the same manner as in Example 1. The results are shown in Table 2.

[0045] (Comparative Example 4) When preparing the CNT dispersion, instead of CMC-a as the dispersant, sodium carboxymethyl cellulose (manufactured by Nippon Paper Industries Co., Ltd., product name "Sunrose MAC (registered trademark) 01LC"; weight average molecular weight: 55,800; degree of etherification: 0.7 to 0.8; referred to as "CMC-c") was used, and the CNT dispersion was prepared and the carbon film was produced in the same manner as in Example 1 except that the pressure and the number of passes of the dispersion treatment by the wet jet mill were changed as shown in Table 3. Then, the evaluation was performed in the same manner as in Example 1. The results are shown in Table 3.

[0046] (Examples 2 to 5, 7 to 10, and 12 to 23, Comparative Examples 1 to 3, and 5 to 6) When preparing the CNT dispersion, the CNT dispersion was prepared and the carbon film was produced in the same manner as in Example 1 except that the types of CNT and the dispersant, and the pressure and the number of passes of the dispersion treatment by the wet jet mill were changed as shown in Tables 1 to 3. Then, the evaluation was performed in the same manner as in Example 1. The results are shown in Tables 1 to 3.

[0047] (Example 24) When preparing the CNT dispersion, instead of CMC-a as the dispersant, a mixture of CMC-a and CMC-c at a ratio of 1:1 (mass ratio) was used, and the CNT dispersion was prepared and the carbon film was produced in the same manner as in Example 1 except that the pressure and the number of passes of the dispersion treatment by the wet jet mill were changed as shown in Table 3. Then, the evaluation was performed in the same manner as in Example 1. The results are shown in Table 3.

[0048] In Tables 1 to 3 shown below, "CNT" indicates single-walled carbon nanotubes, "CMC" indicates sodium carboxymethyl cellulose, "Mw" indicates the weight average molecular weight.

[0049]

Table 1

[0050]

Table 2

[0051]

Table 3

[0052] From Tables 1 to 3, it can be seen that in Examples 1 to 24 using a CNT dispersion liquid containing single-walled CNTs, a solvent, and a dispersant with a weight-average molecular weight less than a predetermined value and having a harmonic mean diameter of CNTs within a predetermined range, a CNT dispersion liquid excellent in the temporal stability of the dispersion state and a carbon film excellent in conductivity can be produced. On the other hand, in Comparative Examples 1 and 3 where the harmonic mean diameter of CNTs is outside the predetermined range, Comparative Example 4 where the harmonic mean diameter of CNTs is outside the predetermined range and the weight-average molecular weight of the dispersant is greater than or equal to the predetermined value, and Comparative Example 5 where the weight-average molecular weight of the dispersant is greater than or equal to the predetermined value, it can be seen that the temporal stability of the dispersion state of the CNT dispersion liquid and the conductivity of the carbon film are lower compared to Examples 1 to 24. Also, in Comparative Example 2 where the harmonic mean diameter of CNTs is outside the predetermined range and Comparative Example 6 where the harmonic mean diameter of CNTs is outside the predetermined range and the weight-average molecular weight of the dispersant is greater than or equal to the predetermined value, it can be seen that the conductivity of the carbon film is lower compared to Examples 1 to 24.

Industrial Applicability

[0053] According to the present invention, it is possible to provide a carbon nanotube dispersion liquid that is excellent in the temporal stability of the dispersion state and can form a molded body excellent in conductivity.< / cnt>

Claims

1. A carbon nanotube dispersion liquid containing carbon nanotubes, a solvent, and a dispersant having a weight average molecular weight of less than 40,000, wherein the carbon nanotubes include single-walled carbon nanotubes, and the harmonic mean diameter of the carbon nanotubes is 600 nm or more and 1240 nm or less. The carbon nanotube dispersion liquid.

2. The carbon nanotube dispersion liquid according to Claim 1, wherein the G / D ratio of the carbon nanotubes is 0.5 or more and 120 or less.

3. The carbon nanotube dispersion liquid according to Claim 1 or 2, wherein the dispersant is carboxymethyl cellulose or a salt thereof.

4. The carbon nanotube dispersion liquid according to Claim 3, wherein the degree of etherification of the carboxymethyl cellulose is 0.5 or more and 0.9 or less.

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