Carbon nanotube dispersion liquid

Abstract

To provide a carbon nanotube dispersion liquid with suppressed thickening.SOLUTION: A carbon nanotube dispersion liquid disclosed herein includes carbon nanotubes having an average fiber length of 10 μm or more, an aqueous solvent, and a dispersant which is soluble in the aqueous solvent and has a weight-average molecular weight of 600000 or more. The content of the dispersant is 10 pts.mass or more and 500 pts.mass or less relative to 100 pts.mass of the carbon nanotubes.SELECTED DRAWING: Figure 2

Description

Technical Field

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

Background Art

[0002] Conventionally, for example, as a conductive aid for secondary batteries, etc., a CNT dispersion liquid obtained by dispersing carbon nanotubes (hereinafter also referred to as "CNT") in a predetermined solvent has been widely used. As conventional technical documents related to this, Patent Documents 1 and 2 can be cited. For example, Patent Document 1 describes a CNT dispersion liquid containing CNT, a dispersant having a weight average molecular weight of 0.1 to 400,000, a volatile salt, and an aqueous solvent. Further, Patent Document 1 describes that as the above CNT, those having an average fiber length of 10 μm or less are preferable.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] From the viewpoint of improving conductivity, etc., there is a desire to use CNTs with a longer fiber length (for example, an average fiber length of 10 μm or more) in the CNT dispersion liquid. However, according to the studies of the present inventors, CNTs with a long fiber length tend to entangle and aggregate in an aqueous solvent. Therefore, rapid thickening may occur during the dispersion process, making the dispersion itself difficult, or the viscosity of the CNT dispersion liquid may become excessively high, resulting in a decrease in handleability.

[0005] The present invention has been made in view of such points, and its main object is to provide a novel carbon nanotube dispersion liquid containing carbon nanotubes having an average fiber length of 10 μm or more and suppressing thickening.

Means for Solving the Problems

[0006] According to the present invention, there is provided a carbon nanotube dispersion liquid containing carbon nanotubes having an average fiber length of 10 μm or more, an aqueous solvent, and a dispersant that is soluble in the aqueous solvent and has a weight average molecular weight of 600,000 or more, wherein the content of the dispersant is 10 parts by mass or more and 500 parts by mass or less with respect to 100 parts by mass of the carbon nanotubes.

[0007] By using a dispersant having a large weight average molecular weight as described above, the aggregation of carbon nanotubes can be relatively suppressed and the dispersibility of carbon nanotubes can be improved as compared with the case of using a dispersant as described in Patent Document 1, for example. As a result, even when carbon nanotubes having an average fiber length of 10 μm or more are included, thickening of the carbon nanotube dispersion liquid can be suppressed.

Effects of the Invention

[0008] According to the present invention, it is possible to provide a carbon nanotube dispersion liquid containing carbon nanotubes having an average fiber length of 10 μm or more and suppressing thickening.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0010] Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. It should be noted that the embodiments described here are not intended to particularly limit the present invention. Also, members and parts having the same function are denoted by the same reference numerals, and redundant descriptions may be omitted or simplified as appropriate. In addition, in this specification, the notation "X to Y" (X and Y are arbitrary numerical values) indicating a range includes the meaning of "greater than X" and "less than Y" in addition to the meaning of "X or more and Y or less".

[0011] <Carbon nanotube (CNT) dispersion> The CNT dispersion of this embodiment contains (A) CNTs with an average fiber length of 10 μm or more, (B) an aqueous solvent, and (C) a dispersant that is soluble in the aqueous solvent and has a weight average molecular weight of 600,000 or more. The CNT dispersion of this embodiment may further contain other optional components, for example, (D) additives, etc., if necessary.

[0012] <(A) CNT> CNT is a fibrous carbon having a structure in which graphite forming a carbon hexagonal network is rolled into a cylindrical shape. CNT is not particularly limited except that the average fiber length is 10 μm or more, and one or more conventionally known ones can be appropriately used. CNT may be a single-walled carbon nanotube having a structure in which a single layer of graphite is rolled into a cylindrical shape, or a multi-walled carbon nanotube having a structure in which two or more layers of graphite are rolled into a cylindrical shape. CNT may contain impurities (for example, catalysts and amorphous carbon) due to, for example, the manufacturing process.

[0013] In this embodiment, the average fiber length of the CNT is 10 μm or more. The average fiber length of the CNT is preferably 30 μm or more, more preferably 50 μm or more. When the average fiber length is a predetermined value or more, it becomes easier to effectively form a conductive network, and the conductivity can be improved. Furthermore, the average fiber length of the CNT is preferably 100 μm or more, more preferably 125 μm or more, even more preferably 200 μm or more, and particularly preferably 250 μm or more. When the average fiber length is long like this, the CNTs tend to be entangled and aggregate in the aqueous solvent. Therefore, it is particularly effective to apply the technology disclosed herein.

[0014] Although not particularly limited, the average fiber length of the CNT is preferably about 1000 μm or less, more preferably 500 μm or less, and further preferably 400 μm or less, 375 μm or less. When the average fiber length is a predetermined value or less, the CNTs are less likely to be entangled in an aqueous solvent, and the dispersibility of the CNTs can be improved. Therefore, the effect of the technology disclosed herein is easily exhibited at a high level, and dispersibility and conductivity can be combined at a high level. The average fiber length of the CNTs can be calculated by observing a plurality of CNTs with an electron microscope, measuring the length of the long axis direction of each CNT, and averaging the number of the measured values. More specifically, for example, a scanning electron microscope (SEM) is used to observe at a magnification of 10,000 times, and the long axis direction of 50 CNTs randomly extracted from the field of view is measured, and the number average value is calculated.

[0015] The average outer diameter (average diameter) of the CNTs is not particularly limited, but is preferably 5 nm or more, more preferably 5 nm or more, and even more preferably 8 nm or more. When the average outer diameter of the CNTs is a predetermined value or more, even a dispersant with a large weight average molecular weight as in this embodiment can easily penetrate between the bundles of CNTs, so that aggregation between CNTs is easily suppressed, and the effect of the technology disclosed herein is easily exhibited at a high level. Furthermore, excessive stress is not easily applied to the CNTs during dispersion, and the CNTs are not easily cut, so that the above average fiber length is easily maintained.

[0016] In addition, the average outer diameter of the CNTs is preferably 100 nm or less, more preferably 50 nm or less, even more preferably 20 nm or less, and particularly preferably 10 nm or less. When the average outer diameter of the CNTs is a predetermined value or less, the number of CNTs per unit mass increases, and a conductive network can be efficiently formed. Therefore, the conductivity can be improved. The average outer diameter of the CNTs can be calculated by observing a plurality of CNTs with an electron microscope, measuring the length of the short axis direction of each CNT, and averaging the number of the measured values. More specifically, for example, a transmission electron microscope (TEM) is used to observe at a magnification of 400,000 times, and the length of the short axis direction of 50 CNTs randomly extracted from the field of view is measured, and the number of the measured values ​​is averaged.

[0017] The aspect ratio of the CNTs (average fiber length / average outer diameter) is not particularly limited, but is preferably 100 to 100,000, more preferably 1,000 to 50,000, and even more preferably 5,000 to 25,000. When the aspect ratio is within the above range, the effects of the technology disclosed herein can be easily exerted at a high level, and dispersibility and conductivity can both be achieved at a high level.

[0018] CNTs have a Raman spectrum measured by laser Raman spectroscopy using a semiconductor laser, which shows a peak at 1350 cm -1 Intensity of the D band appearing near I D 1580cm-1 The intensity I of the G band that appears in the vicinity G of the ratio (I G / I D ) is preferably generally 10 or less, more preferably, for example, 1 to 8, and even more preferably 1 to 2 in some embodiments. The G band is a peak derived from the crystal structure of the CNT, and the D band is a peak derived from the defect structure of the CNT. Therefore, the larger the above ratio (I G / I D ), the higher the crystallinity and the more achievable higher conductivity. Also, when the above ratio (I G / I D ) is below a predetermined value, the dispersibility of the CNT can be improved better.

[0019] Although not particularly limited, when the total of the CNT dispersion liquid is 100% by mass, the concentration of the CNT is preferably generally 0.01 to 10% by mass. The concentration of the CNT is more preferably 0.1% by mass or more, and even more preferably, for example, 0.2% by mass or more, 0.3% by mass or more, 0.4% by mass or more. When the concentration of the CNT is above a predetermined value, it becomes easier to effectively form a conductive network and the conductivity can be improved. On the other hand, since the CNTs are likely to entangle and aggregate with each other in the aqueous solvent, it is particularly effective to apply the technology disclosed herein. Also, the concentration of the CNT is more preferably 5% by mass or less, and even more preferably, for example, 2% by mass or less, 1% by mass or less. When the concentration of the CNT is below a predetermined value, it becomes difficult for the CNTs to come close to each other, and it becomes easier to exhibit the effects of the technology disclosed herein at a high level, so the dispersibility of the CNT can be improved better.

[0020] <(B) Aqueous solvent> The aqueous solvent is at least a dispersion medium for dispersing CNTs. The aqueous solvent is typically water, but may be a mixed solvent containing one or more water-miscible organic solvents depending on, for example, the use of the CNT dispersion. From the viewpoint of preventing contamination by impurities, ion-exchanged water, distilled water, ultrafiltered water, reverse osmosis permeated water, etc. are preferable as water. As the organic solvent miscible with water, organic solvents that are uniformly mixed with water, such as alcohol, ether, ketone, lower carboxylic acid, etc. can be used. The aqueous solvent preferably has water as the main component (a component occupying 50% by mass or more), more preferably 80% by mass or more is water, still more preferably 95% by mass or more is water, and particularly preferably consists substantially of water (98% by mass or more is water).

[0021] <(C) Dispersant> The dispersant is a component for dispersing CNTs in the aqueous solvent. In the present embodiment, the dispersant is a compound soluble in the aqueous solvent used. The dispersant is not particularly limited as long as it is soluble in the aqueous solvent and satisfies the weight average molecular weight described later (having a weight average molecular weight of 600,000 or more). For example, depending on the use of the CNT dispersion and the type of the aqueous solvent, etc., one or more conventionally known compounds can be appropriately used. The dispersant can typically be a water-soluble polymer having at least one functional group selected from a cationic group, an anionic group, and a nonionic group in the molecule. In this specification, "soluble" means that the solubility in the aqueous solvent used is 1% by mass or more in an environment of 25°C.

[0022] The water-soluble polymer may be any of a homopolymer, a block copolymer, an alternating copolymer, a random copolymer, or a graft copolymer. The molecular structure of the water-soluble polymer may be (1) a linear straight-chain type, (2) a branched-chain type in which one or more side chains (carbon chains branched from the main chain. The same applies hereinafter. For example, a graft chain.) are bonded to a linear main skeleton (the carbon chain having the largest number of carbon atoms. The same applies hereinafter.), or (3) a comb type in which a plurality of side chains are regularly arranged along the main skeleton. However, since the effects of the technology disclosed herein can be exhibited at a high level, the straight-chain type is preferable.

[0023] Water-soluble polymers can have, for example, functional groups such as hydroxy groups, carboxy groups, sulfo groups, amino groups, nitro groups, acyloxy groups, etc., and aromatic ring structures, vinyl structures, ester structures, amide structures, polyoxyalkylene structures, etc. in their repeating units. Among them, those having a hydroxy group that is easily compatible with water are preferred. Specific examples of water-soluble polymers include, for example, cellulose derivatives, starch derivatives, polymers containing oxyalkylene units, vinyl alcohol-based polymers, and the like. Among them, from the viewpoints of safety and the like, cellulose derivatives and starch derivatives are preferred, and cellulose derivatives are particularly preferred.

[0024] Cellulose derivatives include polymers containing β-glucose units as main repeating units and their derivatives, as well as salts thereof in general. Cellulose derivatives are typically linear. Cellulose derivatives can be compounds in which some or all of the hydroxy groups in the β-glucose units are substituted with alkoxy groups and their derivatives. The alkyl group or aryl group (R) in the alkoxy group (RO-) may have some or all of it substituted with an ester group such as a carboxyl group, a nitro group, a halogen, or other organic groups. Examples of the above salts include ammonium salts and alkali metal salts such as lithium salts and sodium salts. Specific examples of cellulose derivatives include, for example, methylcellulose (MC), ethylcellulose (EC), carboxymethylcellulose (CMC), carboxymethylethylcellulose (CMEC), hydroxymethylcellulose (HMC), hydroxyethylcellulose (HEC), hydroxypropylmethylcellulose (HPMC), and their modified products, etc. Among them, CMC is preferred from the viewpoints of easy availability (the point that various grades are available) and versatility, etc.

[0025] Starch derivatives include polymers containing α-glucose units as the main repeating units, their derivatives, and salts thereof in general. Starch derivatives are typically linear. Specific examples of starch derivatives include, for example, α-starch, pullulan, carboxymethyl starch, cyclodextrin, and their modified products, etc.

[0026] Examples of polymers containing oxyalkylene units include, for example, polyethylene oxide (PEO) which is a polymer of ethylene oxide (EO), polypropylene oxide (PPO) which is a polymer of propylene oxide (PO), block copolymers of ethylene oxide (EO) and propylene oxide (PO) or butylene oxide (BO), random copolymers of EO and PO or BO, and their modified products, etc. Also, it may be an esterified product of EO, PO or BO and a compound having a carboxy group.

[0027] Vinyl alcohol-based polymers are typically polymers (PVA) containing vinyl alcohol units (VA units) as the main repeating units and their derivatives. Vinyl alcohol-based polymers are typically linear. It is preferable that the proportion of VA units in all repeating units is 50 mol% or more, and all repeating units may be substantially composed of VA units. Specific examples of vinyl alcohol-based polymers include, for example, polyvinyl alcohol (PVA), polyvinyl butyral (PVB), and their modified products, etc.

[0028] The dispersant preferably has a cellulose derivative (e.g., CMC) as the main component (a component accounting for 50 mass% or more), more preferably 80 mass% or more is a cellulose derivative (e.g., CMC), even more preferably 95 mass% or more is a cellulose derivative (e.g., CMC), and particularly preferably consists substantially of a cellulose derivative (e.g., CMC) (98 mass% or more is a cellulose derivative (e.g., CMC)).

[0029] In this embodiment, the weight-average molecular weight (Mw) of the dispersant is 600,000 or more. The weight-average molecular weight of the dispersant is more preferably 650,000 or more, and even more preferably, for example, 670,000 or more. Although details will be described later, when the weight-average molecular weight is a predetermined value or more, it becomes easier to exhibit the effects of the technology disclosed herein at a high level. Further, the weight-average molecular weight of the dispersant is more preferably 800,000 or more, and even more preferably, for example, 820,000 or more. Thereby, for example, even when the average fiber length of CNTs is particularly long at 100 μm or more, and further 125 μm or more, thickening of the CNT dispersion can be suitably suppressed.

[0030] Also, although not particularly limited, the weight-average molecular weight of the dispersant is preferably 1,000,000 or less, more preferably 900,000 or less, and even more preferably 850,000 or less. When the weight-average molecular weight is a predetermined value or less, it becomes easier for the dispersant to penetrate between the bundles of CNTs, and the dispersibility of CNTs can be improved better. Further, the conductivity of CNTs is less likely to be inhibited, and it becomes easier to exhibit high conductivity. The weight-average molecular weight of the dispersant can be calculated by comparing the measured value by gel permeation chromatography (GPC) with the calibration curve by a standard sample (PEO / PEG). As the weight-average molecular weight, it is more preferable to adopt the average value when measured a plurality of times (for example, twice). Detailed measurement conditions are described in the examples described later.

[0031] The dispersant preferably has a molecular weight distribution (Mw / Mn), which is the ratio of the weight-average molecular weight (Mw) to the number-average molecular weight (Mn), of generally 3 to 6, more preferably 4 to 5, and even more preferably, for example, 4.0 to 4.5. Also, although not particularly limited, the number-average molecular weight (Mn) of the dispersant is preferably generally 100,000 or more, more preferably 150,000 to 300,000, and even more preferably, for example, 170,000 to 200,000. In this specification, as the "number-average molecular weight", the value measured under the same measurement conditions as the weight-average molecular weight is adopted.

[0032] In this embodiment, the content of the dispersant contained in the CNT dispersion is 10 to 500 parts by mass with respect to 100 parts by mass of CNTs. Thereby, the effects of the technology disclosed herein can be appropriately exerted. The content of the dispersant is preferably more than the amount adsorbed on the surface of the CNTs. The content of the dispersant is more preferably 50 parts by mass or more, even more preferably 80 parts by mass or more, and particularly preferably 100 parts by mass or more with respect to 100 parts by mass of CNTs. The content of the dispersant is particularly preferably the same as or more than the content of the CNTs on a mass basis. Thereby, it becomes easier to stably exert the effects of the technology disclosed herein at a high level.

[0033] Also, although not particularly limited, the content of the dispersant is preferably 300 parts by mass or less, more preferably 200 parts by mass or less, and may be, for example, 150 parts by mass or less with respect to 100 parts by mass of CNTs. In the technology disclosed herein, even if the amount of the dispersant used is relatively small, CNTs can be highly dispersed in an aqueous solvent. Therefore, the conductivity of the CNTs is less likely to be inhibited, and it becomes easier to achieve high conductivity.

[0034] The concentration of the dispersant may also vary depending on, for example, the amount and properties of the CNTs used, the type of the aqueous solvent used, the properties of the dispersant, and the like. Therefore, although not particularly limited, in some embodiments, when the total amount of the CNT dispersion is 100% by mass, the concentration of the dispersant is preferably approximately 0.01 to 10% by mass. The concentration of the dispersant is more preferably 0.1% by mass or more, and even more preferably, for example, 0.2% by mass or more, 0.3% by mass or more. When the concentration of the dispersant is equal to or higher than a predetermined value, a conductive network of CNTs can be efficiently formed, and it becomes easier to achieve high conductivity. On the other hand, since CNTs are likely to be entangled and aggregated with each other in the aqueous solvent, it is particularly effective to apply the technology disclosed herein. Also, the concentration of the dispersant is more preferably 5% by mass or less, and even more preferably, for example, 2% by mass or less, 1% by mass or less. When the concentration of the dispersant is equal to or lower than a predetermined value, the dispersibility can be further improved, and it becomes easier to exert the effects of the technology disclosed herein at a high level. Therefore, within the above range, high levels of both dispersibility and conductivity can be achieved.

[0035] (D) As additives, for the purpose of improving various properties of the CNT dispersion, one or more additives known to be conventionally usable for this type of application can be appropriately used. Specific examples of additives include, for example, dispersants with a weight average molecular weight of less than 600,000, organic binders, antioxidants, defoamers, preservatives, plasticizers, colorants (pigments, dyes, etc.) and other organic additives, carbon materials other than CNTs (non-fibrous), such as carbon black and graphite, and metal oxides.

[0036] When the CNT dispersion contains optional components, the content of the optional components (e.g., (D) additives) contained in the CNT dispersion is typically less than the content of CNTs and / or the content of the dispersant. As an example, when the total amount of the CNT dispersion is 100% by mass, the concentration of the additive is preferably 5% by mass or less, more preferably 3% by mass or less, particularly preferably 2% by mass or less, and even more preferably 1% by mass or less.

[0037] Such a CNT dispersion can be prepared by mixing the above (A) CNTs, the above (B) aqueous solvent, the above (C) dispersant, and other optional components, and dispersing or dissolving the CNTs, the dispersant, and other optional components in the aqueous solvent. The CNTs and the dispersant may be added to the aqueous solvent in their entirety at once, or may be added to the aqueous solvent in portions two or more times. For mixing, conventionally known mixing devices such as a disper, a planetary mixer, a kneader, a propeller stirrer, an ultrasonic homogenizer, a magnetic stirrer, a jet mill, a ball mill, a bead mill, and a sand mill can be appropriately used.

[0038] In some embodiments, in the non-aqueous CNT dispersion, when the total solid content is 100% by mass, (A) CNT preferably occupies generally 50% by mass or more, more preferably 80% by mass or more, still more preferably 90% by mass or more, and even more preferably 95% by mass or more. It is particularly preferred that it consists substantially of CNT (98% by mass or more of the total solid content is CNT). In such a case, it is particularly effective to apply the technology disclosed herein. Here, the "solid content" refers to the solid component separated (for example, filtered) from the non-aqueous CNT dispersion by solid-liquid separation under the environment of 25°C. In other words, the solid content is the component dispersed or precipitated in the solvent under the environment of 25°C, and does not include the component dissolved in the solvent.

[0039] The CNT dispersion disclosed herein can be used for various applications. For example, in the application of preparing the electrodes (positive electrode and / or negative electrode) of a secondary battery, a conductive film can be formed on a substrate by applying (typically coating) the CNT dispersion disclosed herein on the substrate and drying it. Therefore, as another aspect of the technology disclosed herein, there is provided a method for manufacturing a conductive film including a step of applying the CNT dispersion and a step of removing the aqueous solvent by drying the carbon nanotube dispersion applied to the substrate. In this case, the CNT dispersion may include an active material (positive electrode active material or negative electrode active material) and a resin binder. As the active material or resin binder, various materials known to be usable in this type of application can be appropriately used.

[0040] As described above, the CNT dispersion of the present embodiment includes (A) CNT having an average fiber length of 10 μm or more, (B) an aqueous solvent, and (C) a dispersant that is soluble in the aqueous solvent and has a weight average molecular weight of 600,000 or more, and the content of the dispersant is 10 parts by mass or more and 500 parts by mass or less with respect to 100 parts by mass of the CNT.

[0041] As a result, for example, when a dispersant that does not satisfy the above weight-average molecular weight (specifically, a dispersant that is insoluble in an aqueous solvent or has a weight-average molecular weight of less than 600,000) coexists with (A) CNTs having an average fiber length of 10 μm or more, aggregation of the CNTs can be relatively suppressed and the dispersibility of the CNTs can be improved. As a result, thickening of the CNT dispersion, particularly, as can be seen from FIG. 2, rapid thickening of the CNT dispersion at the initial stage of the dispersion treatment process can be preferably suppressed. Consequently, efficient dispersion treatment becomes possible, and it is possible to suppress excessive stress being applied to the CNTs during the dispersion treatment, which could cause the CNTs to be shortened or the crystallinity of the CNTs to decrease. From the above, it can be said that the combination of (A) to (C) disclosed herein is more advantageous in terms of achieving excellent dispersion stability and high conductivity.

[0042] Although not intended to be construed as particularly limited, the inventors consider the effects resulting from using the above (C) dispersant as follows. FIG. 1 is a schematic diagram of a CNT dispersion (CNT10 and dispersant 20) for explaining the effects of the technology disclosed herein. That is, as shown in FIG. 1, in the CNT dispersion, the dispersant 20 can take the state 22 of adhering to the surface of the CNT10 and the state 24 of being dissolved (or dispersed) in the aqueous solvent and intervening between the CNT10s. The state 22 and the state 24 may be taken separately by other polymers of the dispersant 20, or a part of the same molecule of the dispersant 20 may adhere to the CNT surface to take the state 22, and the other part may be exposed in the aqueous solvent to take the state 24. The dispersant in the state 24 dissolved or dispersed in the aqueous solvent is more likely to have a spherical shape and a larger three-dimensional size as the degree of polymerization is higher, and thus the weight-average molecular weight becomes larger. That is, the larger the weight-average molecular weight, the more likely the volume in the aqueous solvent is to be large. And when the three-dimensional size of the dispersant is large in this way, as shown in FIG. 1, steric repulsion between the CNT10s is likely to occur. Therefore, it is considered that the CNT10s are less likely to come close to each other, the dispersibility of the CNT10s can be relatively improved, and thickening of the CNT dispersion can be suppressed.

[0043] In the CNT dispersion of this embodiment, the average fiber length of the CNTs is 500 μm or less. As a result, the CNTs are less likely to become entangled with each other, and the dispersibility of the CNTs can be further improved. Therefore, it becomes easier to exhibit the effects of the technology disclosed herein at a high level, and it is possible to have both high dispersibility and conductivity.

[0044] In the CNT dispersion of this embodiment, the average fiber length of the CNTs is 100 μm or more. As a result, it becomes easier to effectively form a conductive network, and the conductivity can be improved. Further, when the average fiber length of the CNTs is long in this way, the CNTs are likely to become entangled and aggregated in an aqueous solvent, so it is particularly effective to apply the technology disclosed herein.

[0045] In the CNT dispersion of this embodiment, the weight average molecular weight of the dispersant is 1,000,000 or less. As a result, the dispersant easily penetrates between the bundles of CNTs, and the dispersibility of the CNTs can be further improved. Furthermore, the conductivity of the CNTs is less likely to be inhibited, and it becomes easier to exhibit high conductivity.

[0046] In the CNT dispersion of this embodiment, the weight average molecular weight of the dispersant is 800,000 or more. As a result, it becomes easier to exhibit the effects of the technology disclosed herein at a high level. For example, even when the average fiber length of the CNTs is particularly long, 100 μm or more, and further 125 μm or more, thickening of the CNT dispersion can be suitably suppressed.

[0047] In the CNT dispersion of this embodiment, the dispersant contains a cellulose derivative. Among them, it is preferable that the dispersant contains carboxymethyl cellulose.

[0048] In the CNT dispersion of this embodiment, the concentration of the above carbon nanotubes is 0.01% by mass or more and 10% by mass or less. When the concentration of CNTs is equal to or higher than a predetermined value, it becomes easier to effectively form a conductive network, and the conductivity can be improved. In addition, since CNTs are likely to entangle and aggregate with each other in an aqueous solvent, it is particularly effective to apply the technology disclosed herein. Furthermore, when the concentration of CNTs is equal to or lower than a predetermined value, it becomes difficult for CNTs to come close to each other, and it becomes easier to exhibit the effects of the technology disclosed herein at a high level, so the dispersibility of CNTs can be improved better.

[0049] In the CNT dispersion of this embodiment, the concentration of the above dispersant is 0.01% by mass or more and 10% by mass or less. When the concentration of the dispersant is equal to or higher than a predetermined value, the dispersibility of CNTs can be improved better, and it becomes easier to exhibit the effects of the technology disclosed herein at a high level. Also, when the concentration of the dispersant is equal to or lower than a predetermined value, the conductivity of CNTs is less likely to be inhibited, and it becomes easier to achieve high conductivity. Therefore, high levels of both dispersibility and conductivity can be achieved.

[0050] Hereinafter, examples related to the present invention will be described, but the present invention is not intended to be limited to those shown in such examples.

[0051] Here, first, as shown in Table 1, four types of CNTs (CNT1 to 4) with different average fiber lengths L, as shown in Table 2, four types of dispersants (dispersants A to D, manufactured by Daicel Miraiz Co., Ltd.) with different molecular weights, and water as an aqueous solvent were prepared.

[0052]

Table 1

[0053]

Table 2

[0054] <Measurement of molecular weight> Using gel permeation chromatography (GPC), the molecular weight of the dispersant was measured under the following conditions. Then, by comparing the measured values with the calibration curve using a standard sample (PEO / PEG), the weight average molecular weight and number average molecular weight were calculated. The results are shown in Table 2. Note that as the standard sample, two types with the same structure of PEG (manufactured by Tosoh Corporation) and PEO (manufactured by Wako Pure Chemical Industries, Ltd.) were mixed and used. Also, Table 2 shows the arithmetic mean value when the number of measurements is two (N = 2). For example, for dispersant C, N1 = 670,000 and N2 = 690,000, and the arithmetic mean is 680,000. Also, for dispersant D, N1 = 840,000 and N2 = 820,000, and the arithmetic mean is 830,000. Column: TSKgel gurdcolumn PW XL (6.0 mm I.D. × 4 cm) + TSKgel GMPWXL (7.8 mm I.D. × 30 cm) × 2 columns Column temperature: 40 °C Mobile phase: 0.1 M NaNO3 Flow rate: 1.0 mL / min Sample concentration: 0.5 mg / mL Detector: RI (refractive index) detector (polarity(+))

[0055] <Dispersion treatment and viscosity measurement> Next, in an environment at 25 °C, CNT and a dispersant were added to the raw material tank of a high-pressure homogenizer (manufactured by Sugino Machine, Starburst HJP-25001V2) in the combinations shown in Tables 3 and 4, and dispersed in an aqueous solvent (here, water) to prepare a CNT dispersion. For each CNT dispersion, the content of the dispersant was 100 parts by mass (equal amount) with respect to 100 parts by mass of CNT, and the concentrations of CNT and the dispersant were each 0.4% by mass. Also, the dispersion treatment was performed using a single nozzle chamber under the conditions of a nozzle diameter of 0.15 mm and a pressure of 150 MPa.

[0056] Then, using a viscometer (manufactured by Toki Sangyo Co., Ltd., TV-200E), the viscosity of the CNT dispersion liquid in the dispersion process was monitored at a rotation speed of 1 rpm, and the peak value of the viscosity (mPa·s, typically the initial viscosity) was recorded. The results are shown in Tables 3 and 4. In the table, the column of "relative value" shows the relative value when the peak value (absolute value) when using dispersant A is set to 100, and the column of "evaluation" shows the results according to the following criteria. · "×": The relative value is greater than 70. · "〇": The relative value is less than or equal to 70 and greater than 55. · "◎": The relative value is less than or equal to 55.

[0057]

Table 3

[0058]

Table 4

[0059] As shown in Tables 3 and 4, when using dispersant A with a weight average molecular weight of 210,000, the thickening of the CNT dispersion liquid was remarkable. Also, when using dispersant B with a weight average molecular weight of 490,000, although a slight decrease in viscosity was observed, the viscosity was still high.

[0060] For these comparative examples, when using dispersants C and D with a weight average molecular weight of 600,000 or more, the relative decrease in viscosity was large, and the thickening of the CNT dispersion liquid was well suppressed. Among them, when using CNTs with an average fiber length L of 125 μm or more, or when using dispersant D with a weight average molecular weight of 800,000 or more, the thickening of the CNT dispersion liquid was particularly well suppressed. Figure 2 shows, as an example, the viscosity of the CNT dispersion liquid when using CNT3 (CNT with an average fiber length L = 250 μm). The number of passes on the horizontal axis represents the number of times the CNT dispersion liquid was circulated and processed by a high-pressure homogenizer. The above results indicate the significance of the technology disclosed herein.

[0061] The preferred embodiments of the present invention have been described above. However, the above-described embodiments are merely examples, and the present invention can be implemented in various other forms.

Explanation of Signs

[0062] 10 CNT 20 Dispersant

Claims

1. Carbon nanotubes with an average fiber length of 10 μm or more, an aqueous solvent, a dispersant that is soluble in the aqueous solvent and has a weight average molecular weight of 600,000 or more, and a carbon nanotube dispersion liquid, wherein the content of the dispersant is 10 parts by mass or more and 500 parts by mass or less with respect to 100 parts by mass of the carbon nanotubes.

2. The carbon nanotube dispersion liquid according to Claim 1, wherein the average fiber length of the carbon nanotubes is 500 μm or less. The carbon nanotube dispersion liquid according to Claim 1.

3. The carbon nanotube dispersion liquid according to Claim 2, wherein the average fiber length of the carbon nanotubes is 100 μm or more. The carbon nanotube dispersion liquid according to Claim 2.

4. The carbon nanotube dispersion liquid according to Claim 1, wherein the weight average molecular weight of the dispersant is 1,000,000 or less. The carbon nanotube dispersion liquid according to Claim 1.

5. The carbon nanotube dispersion liquid according to Claim 4, wherein the weight average molecular weight of the dispersant is 800,000 or more. The carbon nanotube dispersion liquid according to Claim 4.

6. The carbon nanotube dispersion liquid according to any one of Claims 1 to 5, wherein the dispersant contains a cellulose derivative. The carbon nanotube dispersion liquid according to any one of Claims 1 to 5.

7. The carbon nanotube dispersion liquid according to any one of Claims 1 to 5, wherein the dispersant contains carboxymethyl cellulose. The carbon nanotube dispersion liquid according to any one of Claims 1 to 5.

8. The carbon nanotube dispersion liquid according to any one of Claims 1 to 5, wherein the concentration of the carbon nanotubes is 0.01% by mass or more and 10% by mass or less. The carbon nanotube dispersion liquid according to any one of Claims 1 to 5.

9. The carbon nanotube dispersion liquid according to any one of Claims 1 to 5, wherein the concentration of the dispersant is 0.01% by mass or more and 10% by mass or less. The carbon nanotube dispersion liquid according to any one of Claims 1 to 5.

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