Carbon nanotube dispersion and method for producing the same

Abstract

To provide a carbon nanotube dispersion which exhibits superior temporal stability in its dispersion state and offers a high storage elastic modulus.SOLUTION: A carbon nanotube dispersion comprises a plurality of carbon nanotubes, a solvent, and a dispersant. The carbon nanotubes have a G / D ratio of 0.5 or more to 20 or less. The plurality of carbon nanotubes include single-walled carbon nanotubes, and at least some of the plurality of carbon nanotubes form carbon nanotube bundles. When observed with an optical microscope, the carbon nanotube bundles have an average length of 20 μm or more; and among 100 carbon nanotube bundles, the count of those whose diameter exceeds 2 μm is 5 or fewer.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a carbon nanotube dispersion liquid and a method for producing the same.

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, due to its small outer diameter, it is easily bundled (prone to forming bundles) by van der Waals forces. Therefore, conventionally, CNT has been once dispersed in a solvent to prepare a carbon nanotube dispersion liquid (CNT dispersion liquid). The obtained CNT dispersion liquid is used, for example, in the production of molded articles such as carbon films formed by aggregation of a plurality of CNTs and electrodes for secondary batteries.

[0004] Therefore, in recent years, attempts have been made to improve various properties of molded articles by improving CNT dispersion liquids. For example, Patent Document 1 discloses a CNT dispersion liquid containing CNT, a solvent, and a dispersant, wherein the average outer diameter of CNT, the half-value width of the peak at diffraction angle 2θ = 25° ± 2°, and the G / D ratio are each within a predetermined range. According to Patent Document 1, by using the CNT dispersion liquid, a resin composition, an electrode film, etc. excellent in conductivity can be obtained. Further, Patent Document 2 discloses a CNT dispersion liquid containing CNT, a dispersant, and a solvent, wherein the average outer diameter, BET specific surface area, and average fiber length of CNT are each within a predetermined range. According to Patent Document 2, by using the CNT dispersion liquid, 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, according to the study by the present inventor, when the above conventional CNT dispersion is mixed with an electrode active material or the like to prepare a slurry composition for a secondary battery electrode, due to the low storage elastic modulus of the CNT dispersion, it has been clarified that there is a problem that it is difficult to uniformly mix each component. In addition, there is still room for further improvement in the temporal stability of the dispersion state of the above conventional CNT dispersion.

[0007] Therefore, an object of the present invention is to provide a carbon nanotube dispersion having excellent temporal stability of the dispersion state and a high storage elastic modulus, and a method for producing the same. [Means for Solving the Problems]

[0008] The present inventor has intensively studied for the purpose of solving the above problems. Here, the present inventor focused on a bundle (hereinafter, may be referred to as a "CNT bundle") formed by aggregation of a plurality of CNTs in the CNT dispersion. And the present inventor found that in a CNT dispersion containing single-walled CNTs and having a G / D ratio within a predetermined range, a solvent, and a dispersant, when the average length of the CNT bundles when observed with an optical microscope is set to a predetermined value or more and the ratio of CNT bundles having a diameter exceeding 2 μm is set to a predetermined value or less, a CNT dispersion having excellent temporal stability of the dispersion state and a high storage elastic modulus can be obtained, and thus completed the present invention.

[0009] That is, this invention aims to advantageously solve the above problems, and according to the present invention, there are provided the CNT dispersions of the following [1] to [5], and the methods for producing the CNT dispersions of the following [6] to [7].[[]END]]

[0010] 〔1〕A carbon nanotube dispersion liquid containing a plurality of carbon nanotubes, a solvent, and a dispersant, wherein the G / D ratio of the carbon nanotubes is 0.5 or more and 20 or less, the plurality of carbon nanotubes include single-walled carbon nanotubes, at least a part of the plurality of carbon nanotubes forms a carbon nanotube bundle, when the carbon nanotube bundle is observed with an optical microscope, the average length of the carbon nanotube bundle is 20 μm or more, and the number of carbon nanotube bundles with a diameter exceeding 2 μm among 100 carbon nanotube bundles is 5 or less. A plurality of CNTs containing single-walled CNTs, a solvent, and a dispersant, wherein the G / D ratio of the CNTs is within the above range, the average length of the CNT bundle when observed with an optical microscope (hereinafter, may be simply abbreviated as "average length of CNT bundle").) is not less than the above value, and the number of CNT bundles with a diameter exceeding 2 μm among 100 CNT bundles when observed with an optical microscope (hereinafter, may be simply abbreviated as "number of CNT bundles with a diameter exceeding 2 μm among 100 CNT bundles").) is not more than the above value. The CNT dispersion liquid has excellent temporal stability of the dispersion state and a high storage elastic modulus. In the present invention, the "average length of CNT bundle" and the "number of CNT bundles with a diameter exceeding 2 μm among 100 CNT bundles" can be measured using the methods described in the examples of this specification, respectively. 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.

[0011] 〔2〕The carbon nanotube dispersion liquid according to the above 〔1〕, wherein the harmonic mean diameter of the carbon nanotubes is 1700 nm or more and 7000 nm or less. If the harmonic mean diameter of the CNTs is within the above range, the temporal stability of the dispersion state of the CNT dispersion liquid can be further improved, and the storage elastic modulus of the CNT dispersion liquid can be further increased. In the present invention, the "harmonic mean diameter" refers to the z-average diameter measured by the dynamic light scattering method (DLS), and can be measured using the method described in the examples of this specification.

[0012] 〔3〕The carbon nanotube dispersion liquid according to the above 〔1〕 or 〔2〕, wherein the weight average molecular weight of the dispersant is 10,000 or more and 100,000 or less. If the weight average molecular weight of the dispersant is within the above range, the temporal stability of the dispersion state of the CNT dispersion liquid can be further improved, and the storage elastic modulus of the CNT dispersion liquid can be further increased. In the present invention, the "weight average molecular weight" of the dispersant can be measured as a standard polystyrene conversion value using gel permeation chromatography, and specifically, it can be measured using the method described in the examples of this specification.

[0013] 〔4〕The carbon nanotube dispersion liquid according to any one of the above 〔1〕 to 〔3〕, 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 dispersion state of the CNT dispersion liquid can be further improved, and the storage elastic modulus of the CNT dispersion liquid can be further increased. In addition, the conductivity of the molded body obtained from the CNT dispersion liquid can be increased.

[0014] 〔5〕The carbon nanotube dispersion liquid according to the above 〔4〕, 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 the carboxymethyl cellulose used as the dispersant is within the above range, the aggregation of CNTs in the CNT dispersion liquid can be suppressed, and the temporal stability of the dispersion state of the CNT dispersion liquid 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 glucose anhydride 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.

[0015] 〔6〕A method for producing a carbon nanotube dispersion liquid according to any one of the above 〔1〕 to 〔5〕, including a step of subjecting a mixed liquid containing the plurality of carbon nanotubes, the solvent, and the dispersant to a dispersion treatment by a wet jet mill. According to the method for producing a CNT dispersion liquid including the above-described steps, a CNT dispersion liquid of the present invention having excellent temporal stability of the dispersed state and a high storage elastic modulus can be produced.

[0016] 〔7〕The method for producing a carbon nanotube dispersion liquid according to the above 〔6〕, wherein the pressure applied to the mixed liquid in the dispersion treatment is 1 MPa or more and 30 MPa or less, and the number of passes in the dispersion treatment is 10 times or more and 80 times or less. If the pressure and the number of passes in the dispersion treatment by a wet jet mill are within the above-described ranges, respectively, the temporal stability of the dispersed state of the CNT dispersion liquid can be further improved, and the storage elastic modulus of the CNT dispersion liquid can be further increased. In the present invention, the "number of passes" means the number of times the total amount of the mixed liquid passes through the wet jet mill.

Advantages of the Invention

[0017] According to the present invention, it is possible to provide a carbon nanotube dispersion liquid having excellent temporal stability in a dispersed state and a high storage elastic modulus, and a method for producing the same.

Embodiments for Carrying Out the Invention

[0018] Hereinafter, embodiments of the present invention will be described in detail. Here, the CNT dispersion liquid of the present invention is not particularly limited, and for example, it can be used as a material for forming a molded body such as a carbon film or an elastomer molded body, an electrode for a secondary battery, and the like. And the CNT dispersion liquid of the present invention can be produced using the method for producing the CNT dispersion liquid of the present invention.

[0019] (Carbon Nanotube Dispersion Liquid) The CNT dispersion liquid of the present invention contains at least CNT, a solvent, and a dispersant, and optionally further contains components other than CNT, the solvent, and the dispersant (other components). Note that at least a part of a plurality of CNTs is a single-walled CNT, and CNT bundles are formed in the CNT dispersion liquid. Here, the CNT dispersion liquid of the present invention is characterized in that the G / D ratio of CNT is 0.5 or more and 20 or less, the average length of CNT bundles is 20 μm or more, and the number of CNT bundles having a diameter exceeding 2 μm among 100 CNT bundles is 5 or less. 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 CNT.

[0020] And since the G / D ratio of CNT, the average length of CNT bundles, and the number of CNT bundles having a diameter exceeding 2 μm among 100 CNT bundles of the CNT dispersion liquid of the present invention each satisfy predetermined conditions, the CNT dispersion liquid of the present invention is excellent in temporal stability of the dispersed state and has a high storage elastic modulus. Although the reason for obtaining the above effects in such a CNT dispersion liquid of the present invention is not clear, it is presumed as follows.

[0021] In order to improve the dispersibility of the CNT dispersion, the inventor focused on the CNT bundles composed of multiple CNTs in the CNT dispersion. The smaller the number of CNT bundles with a diameter exceeding 2 μm among 100 CNT bundles, the larger the number of CNT bundles present in the CNT dispersion. In the CNT dispersion of the present invention, by setting the number of CNT bundles with a diameter exceeding 2 μm among 100 CNT bundles to 5 or less, the number of CNT bundles present in the CNT dispersion is sufficiently ensured. In addition, in the dispersion of the present invention, since the average length of the CNT bundles is 20 μm or more, the length of the CNT bundles present in the CNT dispersion is sufficiently ensured. In the CNT dispersion in which the number and length of the CNT bundles are sufficiently ensured in this way, the CNTs can be stably dispersed over time (the temporal stability of the dispersed state can be improved). Furthermore, it has been clarified by the study of the inventor that the storage elastic modulus of the CNT dispersion depends on the dispersion state of the CNTs in the CNT dispersion. And when the G / D ratio of the CNTs is 0.2 or more and 20 or less, the average length of the CNT bundles is 20 μm or more, and the number of CNT bundles with a diameter exceeding 2 μm among 100 CNT bundles is 5 or less, it has been clarified that the storage elastic modulus of the CNT dispersion becomes sufficiently high. 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 has a high storage elastic modulus.

[0022] <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.

[0023] <<Properties>> [Average length of CNT bundles] The average length of the CNT bundles needs to be 20 μm or more, preferably 30 μm or more, and more preferably 50 μm or more. When the average length of the CNT bundles is less than 20 μm, the storage elastic modulus of the CNT dispersion decreases. Also, the upper limit of the average length of the CNT bundles is not particularly limited and can be, for example, less than 200 μm, less than 150 μm, or less than 100 μm. Note that the average length of the CNT bundles in the CNT dispersion can be adjusted, for example, by changing the average length of the raw material CNT (the CNT used as the raw material when preparing the CNT dispersion), the type and amount of the dispersant, and the conditions of the dispersion treatment. Specifically, in the dispersion step of the method for producing the CNT dispersion of the present invention described later, increasing the pressure applied to the mixture decreases the average length of the CNT bundles, and decreasing the pressure increases the average length. Also, increasing the number of passes of the dispersion treatment by a wet jet mill decreases the average length of the CNT bundles in the CNT dispersion, and decreasing the number of passes increases the average length.

[0024] [Number of CNT bundles with a diameter exceeding 2 μm] Out of 100 CNT bundles, the number of bundles with a diameter exceeding 2 μm should be 5 or less, preferably 4 or less, and more preferably 3 or less. If the number of bundles with a diameter exceeding 2 μm exceeds 5, the temporal stability of the dispersion state of the CNT dispersion decreases, and the conductivity of the molded body obtained from the CNT dispersion decreases. Also, the lower limit of the number of CNT bundles with a diameter exceeding 2 μm out of 100 CNT bundles is not particularly limited, and for example, it can be 0 or more, or 1 or more. Note that the CNT dispersion of the present invention may not contain CNT bundles with a diameter exceeding 2 μm, and the number of CNT bundles with a diameter exceeding 2 μm out of 100 CNT bundles may be 0. Note that the number of CNT bundles with a diameter exceeding 2 μm can be adjusted, for example, by changing the average diameter of the raw material CNT, the type and amount of the dispersant, and the conditions of the dispersion treatment. Specifically, in the dispersion step of the method for producing the CNT dispersion of the present invention described later, increasing the pressure applied to the mixed liquid decreases the number of CNT bundles with a diameter exceeding 2 μm, and decreasing the pressure increases the number. Also, increasing the number of passes of the dispersion treatment using a wet jet mill decreases the number of CNT bundles with a diameter exceeding 2 μm, and decreasing the number increases the number.

[0025] [G / D ratio] The G / D ratio of the CNT should be 0.5 or more and 20 or less, preferably 1 or more, preferably 15 or less, and more preferably 10 or less. If the G / D ratio of the CNT is less than 0.5, the storage elastic modulus of the CNT dispersion decreases due to a decrease in the purity of the CNT, and the conductivity of the molded body obtained from the CNT dispersion decreases. On the other hand, if the G / D ratio of the CNT exceeds 20, the CNT bundles are not sufficiently defibrated and the number of CNT bundles decreases, so the storage elastic modulus of the CNT dispersion decreases.

[0026] [Harmonic mean diameter] The harmonic mean diameter of the CNTs is preferably 1700 nm or more, more preferably 2000 nm or more, still more preferably 2500 nm or more, even more preferably 3000 nm or more, particularly preferably 3500 nm or more, preferably 7000 nm or less, more preferably 6750 nm or less, and still more preferably 6500 nm or less. If the harmonic mean diameter of the CNTs is 1700 nm or more, the storage elastic modulus of the CNT dispersion can be further increased. On the other hand, if the harmonic mean diameter of the CNTs is 7000 nm or less, the temporal stability of the dispersion state of the CNT dispersion can be further improved. Incidentally, the harmonic mean diameter of the CNTs can be adjusted, for example, by changing the average length and average diameter of the raw material CNTs, the type and amount of the dispersant, and the conditions of the dispersion treatment. Specifically, in the dispersion step of the method for producing the CNT dispersion of the present invention described later, the harmonic mean diameter of the CNTs becomes smaller by increasing the pressure applied to the mixed solution, and becomes larger by decreasing the pressure. Also, the harmonic mean diameter of the CNTs becomes smaller by increasing the number of passes of the dispersion treatment using a wet jet mill, and becomes larger by decreasing the number of passes.

[0027] And the average length of the raw material CNTs is preferably 50 μm or more, and more preferably 100 μm or more. If the average length of the raw material CNTs is the above value or more, it becomes easy to adjust the average length of the CNT bundles in the obtained CNT dispersion to the above-mentioned predetermined value or more. Also, the upper limit of the average length of the raw material CNTs is not particularly limited, and can be, for example, 1000 μm or less, and can be 600 μm or less. In the present invention, the average length of the raw material CNTs can be obtained by measuring the lengths of 100 raw material CNTs with a scanning electron microscope and taking the average value thereof.

[0028] <<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, still more preferably 0.1% by mass or more, preferably 2% by mass or less, more preferably 1% by mass or less, and still 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 storage elastic modulus of the CNT dispersion can be further increased, and the conductivity 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.

[0029] <Solvent> The CNT dispersion of the present invention may contain only water as the solvent, may contain only an organic solvent (for example, esters, ketones, alcohols) as the solvent, or the solvent may be a mixture of water and an organic solvent. From the viewpoint of further increasing the storage elastic modulus of the CNT dispersion and further improving the temporal stability of the dispersion state, 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.

[0030] 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, still more preferably 99% by mass or more, and particularly preferably 100% by mass (that is, 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 storage elastic modulus of the CNT dispersion can be further increased. In addition, the conductivity of the molded body obtained from the CNT dispersion can be increased.

[0031] <Dispersant> As the dispersant, known dispersants can be used without particular limitation. Examples of the dispersant 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, the polymer being "water-soluble" 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 undissolved matter is less than 10.0% by mass.

[0032] Examples of the cellulose-based polymer include carboxymethyl cellulose, methyl cellulose, hydroxypropyl methyl cellulose, hydroxyethyl methyl cellulose, and their salts and derivatives. Examples of the polycarboxylic acid 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.

[0033] 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 increasing the storage elastic modulus of the CNT dispersion while further improving the temporal stability of the dispersed state and increasing the conductivity of the molded body, as the dispersant, a cellulose-based polymer is preferable, and carboxymethyl cellulose and its salts are more preferable.

[0034] <<Weight average molecular weight>> The weight average molecular weight of the dispersant is preferably 10,000 or more, more preferably 15,000 or more, still more preferably 20,000 or more, preferably 100,000 or less, more preferably 85,000 or less, and still more preferably 70,000 or less. If the weight average molecular weight of the dispersant is within the above range, the temporal stability of the dispersion state of the CNT dispersion can be further improved, and the storage elastic modulus of the CNT dispersion can be further increased.

[0035] <<Degree of etherification>> Here, the carboxymethyl cellulose preferably used as the dispersant preferably has a degree of etherification 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 degree of etherification of the 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.

[0036] <<Content ratio>> The content ratio of the dispersant in the CNT dispersion is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, still more preferably 0.4% 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 the total amount of the CNT dispersion being 100% by mass. If the content ratio of the dispersant 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 storage elastic modulus of the CNT dispersion can be further increased. In addition, the conductivity of the molded body obtained from the CNT dispersion can be increased.

[0037] The content of the dispersant in the CNT dispersion is preferably 50 parts by mass or more, more preferably 100 parts by mass or more, preferably 400 parts by mass or less, more preferably 300 parts by mass or less, and still more preferably 200 parts by mass or less per 100 parts by mass of CNT. If the amount ratio of the dispersant to CNT 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 storage elastic modulus of the CNT dispersion can be further increased. In addition, the conductivity of the molded body obtained from the CNT dispersion can be increased.

[0038] <Other components> Examples of other components that the CNT dispersion may optionally contain include conductive materials other than CNT, polymer components other than the above-described dispersant, 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), assuming that the total amount of the CNT dispersion is 100% by mass.

[0039] <Viscosity at a shear rate of 1000 (1 / s)> The viscosity of the CNT dispersion of the present invention under the condition of a shear rate of 1000 (1 / s) is preferably 55 mPa·s or more, more preferably 60 mPa·s or more, preferably 85 mPa·s or less, and more preferably 80 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 storage elastic modulus of the CNT dispersion can be further increased. In addition, the conductivity of the molded body obtained from the CNT dispersion can be increased.

[0040] (Method for producing carbon nanotube dispersion) The above-described CNT dispersion of the present invention can be prepared, for example, using the method for producing a CNT dispersion of the present invention. The method for producing a CNT dispersion of the present invention includes a step of subjecting a mixed solution containing a plurality of CNTs, a solvent, and a dispersant to a dispersion treatment using a wet jet mill (dispersion step). Note that the method for producing a CNT dispersion of the present invention may include steps other than the above-described dispersion step (other steps).

[0041] According to the method for producing a CNT dispersion of the present invention, a CNT dispersion of the present invention having excellent temporal stability of the dispersed state and a high storage elastic modulus can be obtained.

[0042] <Dispersion step> In the dispersion step, a mixed solution containing a plurality of CNTs, a solvent, and a dispersant is subjected to a dispersion treatment using a wet jet mill. Note that as the CNTs, the solvent, and the dispersant, those described above in the section of "CNT dispersion" of the present invention can be used.

[0043] <<Wet jet mill>> Examples of the wet jet mill that can be used for the dispersion treatment in the dispersion step include "Nanovaita (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.).

[0044] Here, examples of the nozzle 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 treatment energy. On the other hand, the cross nozzle has an X-shaped nozzle cross-section and higher treatment energy than the straight nozzle. And it is preferable to use a straight nozzle as the nozzle provided in the wet jet mill.

[0045] [Pressure] In the dispersion treatment of the mixed liquid by a wet jet mill, the pressure applied is preferably 1 MPa or more, more preferably 2.5 MPa or more, still more preferably 5 MPa or more, preferably 30 MPa or less, more preferably 20 MPa or less, and still more preferably 10 MPa or less. If the pressure applied to the mixed liquid is 1 MPa or more, the temporal stability of the dispersion state of the CNT dispersion liquid can be further improved. Also, if the pressure applied to the mixed liquid is 30 MPa or less, the storage elastic modulus of the CNT dispersion liquid can be further increased.

[0046] [Number of passes] The number of passes of the dispersion treatment by the wet jet mill is preferably 10 or more, more preferably 20 or more, still more preferably 30 or more, preferably 80 or less, more preferably 70 or less, and still more preferably 60 or less. If the number of passes of the dispersion treatment by the wet jet mill is 10 or more, the temporal stability of the dispersion state of the CNT dispersion liquid can be further improved. Also, if the number of passes of the dispersion treatment by the wet jet mill is 80 or less, the storage elastic modulus of the CNT dispersion liquid can be further increased.

[0047] <Other steps> Examples of other steps include a step (mixing step) of mixing a plurality of CNTs, a solvent, and a dispersant before the above-described dispersion step to obtain a mixed liquid.

Examples

[0048] 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 average length of the CNT bundles, the number of CNT bundles with a diameter exceeding 2 μm out of 100 CNT bundles, the G / D ratio and the harmonic mean diameter of the CNTs, the weight average molecular weight of the dispersant, the viscosity of the CNT dispersion at a shear rate of 1000 (1 / s), the particle size change rate, the viscosity change rate and the storage modulus, and the volume conductivity of the carbon film (formed body) were evaluated by the following methods.

[0049] <Average length> The CNT dispersions prepared in the examples and comparative examples were diluted with pure water so that the CNT concentration became 0.1%. After dropping one drop of the obtained diluted solution onto a slide glass with a dropper, it was sandwiched between slide glasses and observed at a magnification of 700 times using a digital microscope (manufactured by Keyence Corporation, product name "VHX-7000"). The lengths of all CNT bundles observed in one field of view were measured, and the arithmetic mean value was taken as the average length (μm) of the CNT bundles. <Number of CNT bundles with a diameter exceeding 2 μm> The lengths of 100 CNT bundles were measured as described above, and the number of those with a length exceeding 2 μm was determined. The same measurement was performed 10 times, and the arithmetic mean value was taken as the number of CNT bundles with a diameter exceeding 2 μm out of 100 CNT bundles. <G / D ratio> Using a microscopic laser Raman spectrophotometer (manufactured by Thermo Fisher Scientific, product name "Nicolet Almega XR"), the Raman spectra of the 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. <Harmonic mean diameter> The CNT dispersions prepared in the examples and comparative examples were diluted with water so that the CNT concentration 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). <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 0.4%, and a temperature of 20°C. The CNT dispersion immediately after preparation was adjusted to a CNT concentration of 0.4% by adding or removing water as necessary, and the adjusted sample was used as the measurement sample. For this measurement sample, using a rheometer (manufactured by Anton Paar GmbH, product name "MCR-102") as the measuring device and a cone plate as the measuring jig, the range of shear rate from 100 to 9000 (1 / s) was measured at 20°C, and the viscosity (mPa·s) at a shear rate of 1000 (1 / s) was determined. <Storage modulus> The storage modulus of the CNT dispersions (CNT concentration: 0.4%) prepared in the examples and comparative examples was measured using a dynamic viscoelasticity measuring device (manufactured by Anton Paar GmbH, product name "MCR102", cone plate with a diameter of 25 mm) under the conditions of a temperature of 20°C, a frequency of 1 Hz, and a strain of 0.01 to 1000%, and evaluated according to the following criteria. A: The storage modulus at a strain of 1% exceeds 200 Pa B: The storage modulus at a strain of 1% is 100 Pa or more and 200 Pa or less C: The storage modulus at a strain of 1% is 50 Pa or more and less than 100 Pa D: The storage modulus at a strain of 1% is less than 50 Pa <Temporal stability of the dispersion state> If both the "particle size change rate" and the "viscosity change rate" shown below are evaluated as A to C, it means that the CNT dispersion has excellent temporal stability of the dispersion state. [Particle size change rate] For the CNT dispersion liquids obtained in the examples and comparative examples, the values of the average particle diameter (D50) immediately after preparation and the values of the average particle diameter (D50) 10 days after preparation were measured respectively. The average particle diameter (D50) was measured with a laser diffraction / scattering particle size distribution measuring device (manufactured by HORIBA, product name "Partica LA-960V2") for an aqueous dispersion liquid diluted with water so that the CNT concentration became 0.01%. In the obtained particle size distribution (volume basis), the particle diameter at which the cumulative volume calculated from the smaller diameter side became 50% was determined as the average particle diameter (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 of circulation / ultrasonic) ·Ultrasonic intensity 7, ultrasonic time 5 seconds (operation conditions of ultrasonic during air bleeding) ·Refractive index of water: 1.333 ·Refractive index of carbon material: 1.92 And the value of the average particle diameter (D50) immediately after preparation was taken as d 0 (μm), and the value of the average particle diameter (D50) 10 days after preparation was taken as d 1 (μm). The particle diameter change rate (%) = [(d 1 - d 0 ) / d 0 × 100 was used to calculate the particle diameter change rate (%). Evaluation was carried out according to the following criteria. The smaller the value of the particle diameter change rate, the more difficult it is for the dispersion state of CNT to change, that is, it means that the dispersion state has excellent temporal stability. A: Particle diameter change rate is less than 1% B: Particle diameter change rate is 1% or more and less than 15% C: Particle diameter change rate is 15% or more and less than 30% D: Particle diameter change rate is 30% or more [Viscosity change rate] For the CNT dispersion liquids 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 performed at a temperature of 20 °C using a rheometer (manufactured by Anton Paar, 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. And 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 the evaluation was carried out according to the following criteria. The smaller the value of the viscosity change rate, the less likely the CNT dispersion liquid is to thicken over time, that is, it means that the dispersion state has excellent temporal stability. A: Viscosity change rate is less than 10% B: Viscosity change rate is 10% or more and less than 15% C: Viscosity change rate is 15% or more and 20% or less D: 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 the evaluation was carried out 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: Value of volume conductivity exceeds 100 S / cm B: Value of volume conductivity is 50 S / cm or more and 100 S / cm or less C: Value of volume conductivity is less than 50 S / cm

[0050] (Example 1) <Preparation of CNT dispersion liquid> [Synthesis of CNT] According to the method disclosed in Example 4 of International Publication No. 2022 / 114237, a single-layer CNT (hereinafter referred to as "CNT-a") was produced. The properties of the obtained CNT-a were as follows: 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 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 7A"; weight average molecular weight: 30,000; degree of etherification: 0.7 to 0.8; hereinafter referred to as "CMC-b") as a dispersant in water as a solvent, 1.2 g of CNT-a (G / D ratio: 1.2) as the single-layer CNT obtained above was added to obtain a mixed solution (CNT concentration: 0.4%). For this mixed solution, a wet jet mill (manufactured by Yoshida Kikai Kogyo Co., Ltd., model number "NVL-ES008A-D10", product name "Nano Veyta (registered trademark)") was used to perform a dispersion treatment under the conditions of a pressure of 5 MPa and a number of passes of 30 times. Specifically, while applying back pressure, a shearing force was applied to the mixed solution to disperse the CNTs, and a CNT dispersion was obtained. Regarding the obtained CNT dispersion, the average length of the CNT bundles, the number of CNT bundles with a diameter exceeding 2 μm out of 100 CNT bundles, the harmonic mean diameter of the CNTs, the viscosity at a shear rate of 1000 (1 / s), the storage modulus, the particle size change rate, and the 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 through the vacuum filtration device respectively, 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.

[0051] (Example 8) When preparing the CNT dispersion, instead of CNT-a as single-walled CNTs, CNT-b (manufactured by Nippon Zeon Co., Ltd., product name "ZEONANO (registered trademark) SG101"; average length: 400 μm; G / D ratio: 5.0) was used, and the pressure and number of passes of the dispersion treatment by a wet jet mill were changed as shown in Table 1. Otherwise, the preparation of the CNT dispersion and the production of the carbon film were carried out in the same manner as in Example 1. Then, the evaluation was carried out in the same manner as in Example 1. The results are shown in Table 1.

[0052] (Example 29) When preparing the CNT dispersion, instead of CMC-b as a dispersant, sodium carboxymethyl cellulose (manufactured by Daiichi Kogyo Seiyaku Co., Ltd., product name "Cellogen 5A"; weight average molecular weight: 13,000; degree of etherification: 0.7 - 0.8; referred to as "CMC-a") was used, and the pressure and number of passes of the dispersion treatment by a wet jet mill were changed as shown in Table 3. Otherwise, the preparation of the CNT dispersion and the production of the carbon film were carried out in the same manner as in Example 1. Then, the evaluation was carried out in the same manner as in Example 1. The results are shown in Table 3.

[0053] (Example 31) When preparing the CNT dispersion, instead of CMC-b 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; hereinafter referred to as "CMC-c") 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 3. Then, the evaluation was carried out in the same manner as in Example 1. The results are shown in Table 3.

[0054] (Examples 2 to 7, 9 to 28, 30 and 32 to 34, Comparative Examples 1 to 5) When preparing the CNT dispersion, 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 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 4. Then, the evaluation was carried out in the same manner as in Example 1. The results are shown in Tables 1 to 4.

[0055] (Comparative Example 6) When preparing the CNT dispersion, instead of the wet jet mill, an ultrasonic disperser (manufactured by Sinky Co., Ltd., product name "Ultrasonic Nanotaro") was used, and the dispersion treatment was carried out under the conditions of an output of 140 W and a treatment time of 60 minutes. The preparation of the CNT dispersion and the production of the carbon film were carried out in the same manner as in Example 1. Then, the evaluation was carried out in the same manner as in Example 1. The results are shown in Table 4. Incidentally, the dispersibility of CNT in the CNT dispersion was extremely low, and the harmonic mean diameter of CNT could not be measured.

[0056] (Comparative Example 7) When preparing the CNT dispersion, instead of the wet jet mill, a homogenizer (manufactured by Primix Corporation, product name "Neo Mixer (registered trademark)") was used, and the dispersion treatment was carried out under the conditions of a rotation speed of 5000 rpm and a treatment time of 60 minutes. The preparation of the CNT dispersion and the production of the carbon film were carried out in the same manner as in Example 1. Then, the evaluation was carried out in the same manner as in Example 1. The results are shown in Table 4. Incidentally, the dispersibility of CNT in the CNT dispersion was extremely low, and the harmonic mean diameter of CNT could not be measured.

[0057] (Comparative Example 8) When preparing the CNT dispersion, CNT-c (manufactured by OCSiAl, product name "Tuball (registered trademark)"; G / D ratio: 109) was used instead of CNT-a as the single-walled CNT, and the pressure and number of passes of the dispersion treatment by a wet jet mill were changed as shown in Table 4, and the CNT dispersion was prepared and the carbon film was produced in the same manner as in Example 1. Then, the evaluation was performed in the same manner as in Example 1. The results are shown in Table 4. Note that the dispersibility of CNTs in the CNT dispersion was extremely low, and the harmonic mean diameter of CNTs could not be measured.

[0058] In Tables 1 to 4 shown below, "CNT" indicates single-walled carbon nanotubes, "CMC" indicates sodium carboxymethyl cellulose, "Mw" indicates weight average molecular weight, "Number of CNT bundles with a diameter exceeding 2 μm" indicates the number of CNT bundles with a diameter exceeding 2 μm out of 100 CNT bundles, "Jet mill" indicates a wet jet mill, "Ultrasonic" indicates an ultrasonic disperser.

[0059] [Table 1]

[0060] [Table 2]

[0061] [Table 3]

[0062] [Table 4]

[0063] From Tables 1 to 4, it can be seen that the CNT dispersions of Examples 1 to 34, which contain single-walled CNTs with a G / D ratio within a predetermined range, a solvent, and a dispersant, have an average length of CNT bundles of a predetermined value or more and a number of CNT bundles with a diameter exceeding 2 μm among 100 CNT bundles of a predetermined value or less, are excellent in the temporal stability of the dispersion state and have a high storage elastic modulus. On the other hand, in Comparative Examples 1 and 6 to 7 where the number of CNT bundles with a diameter exceeding 2 μm among 100 CNT bundles exceeds a predetermined value, it can be seen that the temporal stability of the dispersion state of the CNT dispersion is lower than that of Examples 1 to 34. Also, in Comparative Example 2 where the number of CNT bundles with a diameter exceeding 2 μm among 100 CNT bundles exceeds a predetermined value, it can be seen that both the storage elastic modulus and the temporal stability of the dispersion state of the CNT dispersion are lower than those of Examples 1 to 34. And in Comparative Examples 3 to 5 where the average length of the CNT bundles is less than a predetermined value, it can be seen that the storage elastic modulus of the CNT dispersion is lower than that of Examples 1 to 34. Furthermore, in Comparative Example 8 where none of the G / D ratio, the average length of the CNT bundles, and the number of CNT bundles with a diameter exceeding 2 μm among 100 CNT bundles satisfy the predetermined conditions, it can be seen that both the temporal stability of the dispersion state and the storage elastic modulus of the CNT dispersion are lower than those of Examples 1 to 34.

Industrial Applicability

[0064] According to the present invention, it is possible to provide a carbon nanotube dispersion excellent in the temporal stability of the dispersion state and having a high storage elastic modulus, and a method for producing the same.< / cnt>

Claims

1. A carbon nanotube dispersion liquid containing a plurality of carbon nanotubes, a solvent, and a dispersant, wherein the G / D ratio of the carbon nanotubes is 0.5 or more and 20 or less, the plurality of carbon nanotubes include single-walled carbon nanotubes, at least a part of the plurality of carbon nanotubes forms a carbon nanotube bundle, when the carbon nanotube bundle is observed with an optical microscope, the average length of the carbon nanotube bundle is 20 μm or more, and the number of carbon nanotube bundles with a diameter exceeding 2 μm among 100 carbon nanotube bundles is 5 or less. Carbon nanotube dispersion liquid.

2. The carbon nanotube dispersion liquid according to claim 1, wherein the harmonic mean diameter of the carbon nanotubes is 1700 nm or more and 7000 nm or less.

3. The carbon nanotube dispersion liquid according to claim 1, wherein the weight average molecular weight of the dispersant is 10,000 or more and 100,000 or less.

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

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

6. A method for producing a carbon nanotube dispersion liquid according to any one of claims 1 to 5, comprising: a step of subjecting a mixed liquid containing the plurality of carbon nanotubes, the solvent, and the dispersant to a dispersion treatment by a wet jet mill A method for producing a carbon nanotube dispersion liquid.

7. In the dispersion treatment, the pressure applied to the mixed liquid is 1 MPa or more and 30 MPa or less, The method for producing a carbon nanotube dispersion liquid according to claim 6, wherein the number of passes in the dispersion treatment is 10 or more and 80 or less.

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