Nonaqueous carbon nanotube dispersion liquid

Abstract

To provide a nonaqueous carbon nanotube dispersion liquid with suppressed thickening.SOLUTION: A nonaqueous carbon nanotube dispersion liquid disclosed herein includes carbon nanotubes having an average fiber length of 100 μm or more, a nonaqueous solvent, and a dispersant which is soluble in the nonaqueous solvent and has a weight-average molecular weight of 70000 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 non-aqueous carbon nanotube dispersion.

Background Art

[0002] Conventionally, for example, as a conductive assistant for secondary batteries, etc., a CNT dispersion obtained by dispersing carbon nanotubes (hereinafter also referred to as "CNT") in a predetermined solvent has been widely used. As conventional technical documents related thereto, Patent Documents 1 to 4 can be cited. For example, Patent Document 1 describes an aqueous CNT dispersion containing CNT, a dispersant having a weight average molecular weight of 0.1 to 400,000, a volatile salt, and an aqueous solvent. Patent Document 1 describes that as the above CNT, those having an average fiber length of 10 μm or less are preferable.

[0003] In addition, as the dispersion medium, there is a desire to use a non-aqueous solvent (organic solvent system not containing water) instead of an aqueous solvent because it is easy to dry, etc. Patent Document 3 describes a CNT dispersion solution composed of non-fluorinated CNT, an amide-based polar organic solvent, polyvinylpyrrolidone (PVP), and not containing a nonionic surfactant.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0005] From the viewpoint of improving conductivity and the like, there is a desire to use CNTs with a longer fiber length (for example, an average fiber length of 100 μm or more) in the CNT dispersion. However, according to the studies of the present inventors, CNTs with a long fiber length tend to entangle and aggregate in a non-aqueous solvent. Therefore, during the dispersion process, rapid thickening may occur, making dispersion itself difficult, or the viscosity of the CNT dispersion may become excessively high, resulting in a decrease in handleability.

[0006] The present invention has been made in view of such points, and its main object is to provide a novel non-aqueous carbon nanotube dispersion containing carbon nanotubes with an average fiber length of 100 μm or more and a non-aqueous solvent, and having suppressed thickening.

Means for Solving the Problems

[0007] According to the present invention, there is provided a non-aqueous carbon nanotube dispersion containing carbon nanotubes with an average fiber length of 100 μm or more, a non-aqueous solvent, and a dispersant that is soluble in the non-aqueous solvent and has a weight average molecular weight of 70,000 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.

[0008] By including carbon nanotubes with an average fiber length of 100 μm or more, it becomes relatively easier to form a conductive network and improve conductivity compared to the case of using carbon nanotubes with a short fiber length as described in Patent Document 1, for example. Further, by coexisting the carbon nanotubes with a dispersant having a weight average molecular weight of 70,000 or more in a non-aqueous solvent, aggregation of the carbon nanotubes can be suppressed and the dispersibility of the carbon nanotubes can be improved. As a result, thickening of the dispersion can be suppressed.

Effects of the Invention

[0009] According to the present invention, it is possible to provide a non-aqueous carbon nanotube dispersion containing carbon nanotubes with an average fiber length of 100 μm or more and a non-aqueous solvent, and having suppressed thickening.

Brief Description of the Drawings

[0010]

Figure 1

Figure 2

DETAILED DESCRIPTION OF THE INVENTION

[0011] 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 limit the present invention in particular. Also, members and parts having the same function are denoted by the same reference numerals, and duplicate explanations 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" as well as the meaning of "X or more and Y or less".

[0012] <Non-aqueous carbon nanotube (CNT) dispersion> The non-aqueous CNT dispersion of this embodiment contains (A) CNT having an average fiber length of 100 μm or more, (B) a non-aqueous solvent, and (C) a dispersant that is soluble in the non-aqueous solvent and has a weight average molecular weight of 70,000 or more. The non-aqueous CNT dispersion of this embodiment may further contain other optional components, for example, (D) additives, etc., if necessary.

[0013] <(A) CNT> CNT is 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 100 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 one 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), for example, derived from the manufacturing process.

[0014] In this embodiment, the average fiber length of the CNTs is 100 μm or more. The average fiber length of the CNTs is preferably 120 μm or more, and more preferably 125 μm or more. When the average fiber length is equal to or greater than a predetermined value, it becomes easier to effectively form a conductive network and the conductivity can be improved. Also, when the average fiber length is long in this way, the CNTs are likely to become entangled and aggregated in the non-aqueous solvent. Therefore, it is particularly effective to apply the technology disclosed herein. The average fiber length of the CNTs is preferably generally 1000 μm or less, more preferably 500 μm or less, and even more preferably, for example, 400 μm or less, 375 μm or less. The average fiber length of the CNTs may be 250 μm or less. When the average fiber length is equal to or less than a predetermined value, the CNTs are less likely to become entangled in the non-aqueous solvent, and the dispersibility of the CNTs can be improved better. 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-level dispersibility and conductivity.

[0015] Note that the average fiber length of the CNTs can be calculated by observing a plurality of CNTs with an electron microscope, measuring the length in the major axis direction of each CNT, and taking the number average value thereof. More specifically, for example, using a Scanning Electron Microscope (SEM), observations are made at a magnification of, for example, 10,000 times, and the lengths in the major axis direction are measured for 50 CNTs arbitrarily extracted from the field of view, and the number average value can be calculated therefrom.

[0016] 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 equal to or greater than a predetermined value, even a dispersant having a large weight average molecular weight as in this embodiment can easily penetrate between the bundles of CNTs, so that the aggregation of the CNTs is likely to be suppressed, and it becomes easier to exhibit the effects of the technology disclosed herein at a high level. As a result, excessive stress is less likely to be applied to the CNTs during dispersion, and the CNTs are less likely to be cut, so that it becomes easier to maintain the above average fiber length.

[0017] In addition, the average outer diameter of the CNT is preferably 100 nm or less, more preferably 50 nm or less, still more preferably 20 nm or less, and particularly preferably 10 nm or less. When the average outer diameter of the CNT is equal to or less than a predetermined value, the number of CNTs per unit mass increases, and a conductive network can be efficiently formed. Therefore, the conductivity can be improved more effectively. The average outer diameter of the CNT can be calculated by observing a plurality of CNTs with an electron microscope, measuring the length in the short-axis direction of each CNT, and taking the numerical average value. More specifically, for example, using a transmission electron microscope (TEM), observation is performed at a magnification of, for example, 400,000 times, and the length in the short-axis direction is measured for 50 CNTs arbitrarily extracted from the field of view, and the numerical average value can be calculated therefrom.

[0018] The aspect ratio (average fiber length / average outer diameter) of the CNT is not particularly limited, but is preferably from 100 to 100,000, more preferably from 1000 to 50,000, and still more preferably from 5000 to 25,000. When the aspect ratio is within the above range, the effects of the technology disclosed herein are likely to be exhibited at a high level, and high levels of dispersibility and conductivity can be achieved simultaneously.

[0019] In the Raman spectrum measured by laser Raman spectroscopy using a semiconductor laser, the CNT has a D band intensity I -1 appearing near 1350 cm D and a G band intensity I -1 appearing near 1580 cm G The ratio (I G / I D ) is preferably generally 10 or less, more preferably, for example, from 1 to 8, and still more preferably from 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 higher the conductivity that can be achieved. Also, the above ratio (IG / I D When ) is equal to or less than a predetermined value, the dispersibility of CNTs can be improved better.

[0020] Although not particularly limited, when the total amount of the non-aqueous CNT dispersion is 100% by mass, the concentration of CNTs is preferably generally 0.01 to 10% by mass. The concentration of CNTs is more preferably 0.1% by mass or more, and further 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 CNTs is equal to or more than a predetermined value, it becomes easier to effectively form a conductive network and the conductivity can be improved. On the other hand, since CNTs are likely to entangle and aggregate with each other in a non-aqueous solvent, it is particularly effective to apply the technology disclosed herein. Also, the concentration of CNTs is more preferably 5% by mass or less, and further preferably, for example, 2% by mass or less, 1% by mass or less. When the concentration of CNTs is equal to or less 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 that the dispersibility of CNTs can be improved better.

[0021] <(B) Non-aqueous solvent> The non-aqueous solvent is at least a dispersion medium for dispersing CNTs. The non-aqueous solvent substantially does not contain water. The water content in the non-aqueous solvent is preferably 1000 ppm or less, more preferably 500 ppm or less, and particularly preferably 100 ppm or less. The water content in the non-aqueous solvent can be measured, for example, by the Karl Fischer titration method (JIS K0068:2001).

[0022] The type of non-aqueous solvent is not particularly limited, and for example, depending on the use of the non-aqueous CNT dispersion liquid or the like, one kind of organic solvent or a combination of two or more kinds of organic solvents can be appropriately used. Examples of the organic solvent include aprotic polar solvents such as N-methyl-2-pyrrolidone (NMP), N,N-dimethylformamide, N,N-dimethylacetamide, and dimethyl sulfoxide; alcohol solvents such as methanol, ethanol, n-propanol, isopropanol, and cyclohexanol; ester solvents such as methyl acetate, ethyl acetate, butyl acetate, methyl butyrate, ethyl butyrate, butyl butyrate, methoxybutyl acetate, methyl cellosolve acetate, ethyl cellosolve acetate, propylene glycol monomethyl ether acetate, and γ-butyrolactone; carbonate solvents such as dimethyl carbonate, diethyl carbonate, ethylene carbonate, and propylene carbonate; ketone solvents such as acetone, methyl ethyl ketone, and cyclohexanone; hydrocarbon solvents such as toluene, xylene, cyclohexane, and heptane; and the like. Among them, aprotic polar solvents are preferred, and NMP is particularly preferred.

[0023] The non-aqueous solvent preferably contains an aprotic polar solvent (for example, NMP) as the main component (a component accounting for 50% by mass or more). More preferably, 80% by mass or more is an aprotic polar solvent (for example, NMP). Even more preferably, 95% by mass or more is an aprotic polar solvent (for example, NMP). Particularly preferably, it consists substantially of an aprotic polar solvent (for example, NMP) (98% by mass or more is an aprotic polar solvent (for example, NMP)). The aprotic polar solvent (for example, NMP) can dissolve many kinds of organic substances, particularly the (C) dispersant described later. Therefore, by using an aprotic polar solvent (for example, NMP) of a predetermined value or more, the range of selection of the dispersant can be widened.

[0024] <(C) Dispersant> The dispersant is a component for dispersing CNTs in a non-aqueous solvent. In the present embodiment, the dispersant is a compound soluble in the non-aqueous solvent to be used. The dispersant is not particularly limited as long as it is soluble in the non-aqueous solvent and satisfies the weight average molecular weight described below (having a weight average molecular weight of 70,000 or more). For example, depending on the use of the non-aqueous CNT dispersion and the type of non-aqueous solvent, one or more conventionally known compounds can be appropriately used. The dispersant can typically be a polymer compound 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 non-aqueous solvent to be used is 1 mass% or more under an environment of 25°C.

[0025] This polymer compound may be any of a homopolymer, a block copolymer, an alternating copolymer, a random copolymer, or a graft copolymer. The molecular structure of the polymer compound may be any of (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 shall apply 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 shall apply 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 preferred. Specific examples of the polymer compound include, for example, vinyl pyrrolidone-based polymers, vinyl alcohol-based polymers, styrene-maleic acid-based polymers, polyacrylonitrile-based polymers, and the like. Among them, vinyl pyrrolidone-based polymers are preferred. Since it is known that vinyl pyrrolidone-based polymers preferably exhibit a so-called wrapping effect of adsorbing on the surface of CNTs and wrapping the CNTs, including a vinyl pyrrolidone-based polymer makes it easier to exhibit the effects of the technology disclosed herein at a high level.

[0026] Vinyl pyrrolidone polymers are typically polymers (PVP) containing vinyl pyrrolidone units (VP units) as the main repeating units and their derivatives. Vinyl pyrrolidone polymers are typically linear. It is preferable that the proportion of VP units in all repeating units is 50 mol% or more, and all repeating units may be substantially composed of VP units. Specific examples of vinyl pyrrolidone polymers include, for example, polyvinyl pyrrolidone (PVP), copolymers of vinyl pyrrolidone and vinyl acetate, copolymers of vinyl pyrrolidone and dimethylaminoethyl methacrylate, copolymers of vinyl pyrrolidone and dimethylaminoethyl methacrylate, copolymers of vinyl pyrrolidone and vinyl alcohol, and modified products thereof. Among them, PVP is preferable from the viewpoints of easy availability, etc.

[0027] Vinyl alcohol polymers are typically polymers (PVA) containing vinyl alcohol units (VA units) as the main repeating units and their derivatives. Vinyl alcohol 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 polymers include, for example, polyvinyl alcohol (PVA), polyvinyl butyral (PVB), and modified products thereof.

[0028] Maleic acid polymers are polymers containing maleic acid units as the main repeating units and their derivatives. Maleic acid polymers include maleic acid-based (open-ring state) polymers and maleic anhydride-based (closed-ring state) polymers. Specific examples of maleic acid polymers include, for example, poly(ethylene maleic acid), poly(isobutylene maleic acid), poly(styrene maleic acid), poly(methyl vinyl ether maleic anhydride), and modified products thereof.

[0029] The dispersant preferably contains a vinyl pyrrolidone polymer (such as PVP) as the main component (a component accounting for 50% by mass or more), more preferably 80% by mass or more is a vinyl pyrrolidone polymer (such as PVP), still more preferably 95% by mass or more is a vinyl pyrrolidone polymer (such as PVP), and particularly preferably consists substantially of a vinyl pyrrolidone polymer (such as PVP) (98% by mass or more is a vinyl pyrrolidone polymer (such as PVP)).

[0030] In this embodiment, the weight average molecular weight (Mw) of the dispersant is 70,000 or more. The weight average molecular weight of the dispersant is more preferably 72,000 or more, and still more preferably 74,000 or more. In some embodiments, the weight average molecular weight of the dispersant is more preferably 81,000 or more, for example, still more preferably 82,000 or more. As will be described in detail later, when the weight average molecular weight is a predetermined value or more, it is easier to exhibit the effects of the technology disclosed herein at a high level. Also, the weight average molecular weight of the dispersant is preferably 100,000 or less, more preferably 95,000 or less, and still more preferably 90,000 or less. When the weight average molecular weight is a predetermined value or less, the dispersant can easily penetrate between the bundles of CNTs, and the dispersibility of CNTs can be improved better. Furthermore, the conductivity of CNTs is less likely to be inhibited, and it is 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 multiple times (for example, 2 times). The detailed measurement conditions are described in the examples below.

[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 10,000 or more, more preferably 20,000 to 300,000, and even more preferably, for example, 30,000 to 200,000. In this specification, the "number average molecular weight" shall be the value measured under the same measurement conditions as the weight average molecular weight.

[0032] In this embodiment, the content of the dispersant contained in the non-aqueous CNT dispersion is 10 to 500 parts by mass with respect to 100 parts by mass of CNT. 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 CNT. 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 CNT. The content of the dispersant is particularly preferably the same as or more than the content of CNT on a mass basis. Thereby, the effects of the technology disclosed herein can be stably exerted at a high level and easily.

[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 CNT. In the technology disclosed herein, even if the amount of the dispersant used is relatively small, the CNT can be highly dispersed in the non-aqueous solvent, so the conductivity of the CNT is hardly inhibited, and it is easy to achieve high conductivity.

[0034] The concentration of the dispersant can vary depending on, for example, the amount and properties of the CNTs used, the type of non-aqueous solvent used, the properties of the dispersant, etc. Therefore, although not particularly limited, in some embodiments, when the total of the non-aqueous CNT dispersion is 100% by mass, the concentration of the dispersant is preferably generally 0.01 to 10% by mass. The concentration of the dispersant is more preferably 0.1% by mass or more, and further 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 entangle and aggregate with each other in an 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 further 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 improved better, and it becomes easier to exhibit 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 example, for the purpose of improving various properties of the non-aqueous CNT dispersion, one or more kinds known to be conventionally usable for this type of application can be appropriately used. Specific examples of additives include, for example, dispersants having a weight average molecular weight of less than 80,000, organic binders, antioxidants, defoamers, preservatives, plasticizers, coloring agents (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 non-aqueous CNT dispersion contains optional components, the content of the optional components (for example, (D) additives) contained in the non-aqueous CNT dispersion is typically less than the content of CNTs and / or the content of the dispersant. As an example, when the total of the non-aqueous 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, and particularly further preferably 2% by mass or less, 1% by mass or less.

[0037] Such a non-aqueous CNT dispersion can be prepared by mixing the above (A) CNT, the above (B) non-aqueous solvent, the above (C) dispersant, and other optional components to disperse or dissolve the CNT, the dispersant, and other optional components in the non-aqueous solvent. The CNT and the dispersant may be entirely introduced into the non-aqueous solvent at once, or may be introduced into the non-aqueous solvent in portions two or more times. For mixing, for example, 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. Among them, from the viewpoint of reducing contamination of the medium, a device that does not use a medium (mediumless) is preferable. Also, from the viewpoints that dispersion easily proceeds homogeneously, shortening of the CNT can be suppressed, and it is easy to maintain CNT having a long average fiber length, a circulation-type dispersion device is preferable. Further, since the processing time can be shortened, a device that utilizes the shearing force of high-speed stirring is preferable. As an example of such a mixing device, a high-pressure homogenizer can be mentioned.

[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, preferably 80% by mass or more, more preferably 90% by mass or more, and still more preferably 95% by mass or more, and it is particularly preferable 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 a solid component separated (for example, filtered) from the non-aqueous CNT dispersion by solid-liquid separation in an environment of 25°C. In other words, the solid content is a component dispersed or precipitated in the solvent in an environment of 25°C, and does not include a component dissolved in the solvent.

[0039] The non-aqueous CNT dispersion disclosed herein can be used for various applications. For example, in the application of creating electrodes (positive electrodes and / or negative electrodes) of secondary batteries, a conductive film can be formed on a substrate by applying (typically coating) the non-aqueous 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 a non-aqueous CNT dispersion and a step of removing the non-aqueous solvent by drying the carbon nanotube dispersion applied to the substrate. In this case, the non-aqueous CNT dispersion may contain 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 kind of application conventionally can be appropriately used.

[0040] As described above, the non-aqueous CNT dispersion of the present embodiment contains (A) CNTs having an average fiber length of 100 μm or more, (B) a non-aqueous solvent, and (C) a dispersant that is soluble in the non-aqueous solvent and has a weight average molecular weight of 70,000 or more, and is characterized in that 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 CNTs.

[0041] By including carbon nanotubes having an average fiber length of 100 μm or more, it becomes easier to form a conductive network and the conductivity can be improved. Further, by coexisting the carbon nanotubes with a dispersant having a weight average molecular weight of 70,000 or more in a non-aqueous solvent, for example, compared with the case of coexisting with a dispersant that does not satisfy the above weight average molecular weight (specifically, a dispersant that is insoluble in the non-aqueous solvent or has a weight average molecular weight of less than 80,000), the aggregation of CNTs can be relatively suppressed and the dispersibility of CNTs can be improved. As a result, thickening of the dispersion, particularly rapid thickening of the 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 the CNTs from being shortened or the crystallinity of the CNTs from being reduced due to excessive stress being applied to the CNTs during the dispersion treatment. From the above, it can be said that the combination of (A) to (C) disclosed herein is more advantageous in terms of excellent dispersion stability and the ability to achieve high conductivity.

[0042] Although it is not intended to be particularly limited, the present inventors consider the effect produced by the combination of the above (A) to (C) as follows. FIG. 1 is a schematic diagram of a non-aqueous CNT dispersion (CNTs 10 and a dispersant 20) for explaining the effect of the technology disclosed herein. That is, as shown in FIG. 1, in a non-aqueous CNT dispersion, the dispersant 20 can be in a state 22 attached to the surface of the CNTs 10, and in a state 24 dissolved (or dispersed) in a non-aqueous solvent and interposed between the CNTs 10. The states 22 and 24 may be taken by different polymers of the dispersant 20 alone, or a part of the same molecule of the dispersant 20 may be attached to the CNT surface and take the state 22, and the other part may be exposed to the aqueous solvent and take the state 24. The dispersant in the state 24 dissolved or dispersed in the non-aqueous solvent is more likely to have a spherical shape as the degree of polymerization increases, and the three-dimensional size increases, resulting in a larger weight-average molecular weight. In other words, the larger the weight-average molecular weight, the larger the volume in a non-aqueous solvent. If the three-dimensional size of the dispersant is large in this way, steric repulsion between the CNTs 10 is likely to occur, as shown in Figure 1. This makes it difficult for the CNTs 10 to approach each other, which is thought to improve the dispersibility of the CNTs 10 relatively and suppress the thickening of the dispersion liquid.

[0043] In the non-aqueous CNT dispersion of this embodiment, the average fiber length of the CNT is 500 μm or less. In particular, the average fiber length of the CNT is preferably 125 μm or more and 375 μm or less. This makes it difficult for the CNTs to become entangled, and the dispersibility of the CNTs can be improved. Therefore, the effect of the technology disclosed herein can be easily exerted at a high level, and dispersibility and conductivity can be combined at a high level.

[0044] In the non-aqueous CNT dispersion of this embodiment, the weight average molecular weight of the dispersant is 100,000 or less. This allows the dispersant to easily penetrate between the bundles of CNTs, and improves the dispersibility of CNTs. Furthermore, the conductivity of CNTs is less likely to be hindered, and CNTs are more likely to exhibit high conductivity.

[0045] In the non-aqueous CNT dispersion of the present embodiment, the non-aqueous solvent is an aprotic polar solvent. As a result, the effects of the technology disclosed herein are likely to be exhibited at a high level. In addition, since aprotic polar solvents can dissolve many types of organic substances, particularly the (C) dispersant, the range of choices for the dispersant can be widened.

[0046] In the non-aqueous CNT dispersion of the present embodiment, the dispersant includes a vinylpyrrolidone-based polymer containing a vinylpyrrolidone unit as a repeating unit. Among them, it is preferable that the dispersant contains polyvinylpyrrolidone. Since vinylpyrrolidone-based polymers (for example, polyvinylpyrrolidone) are known to preferably exhibit a so-called wrapping effect in which they adsorb on the surface of CNTs and wrap the CNTs, including a vinylpyrrolidone-based polymer makes it easier to exhibit the effects of the technology disclosed herein at a high level.

[0047] In the non-aqueous CNT dispersion of the present embodiment, the concentration of the carbon nanotubes is 0.01% by mass or more and 10% by mass or less. When the concentration of CNTs is at a predetermined value or more, 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 the non-aqueous solvent, it is particularly effective to apply the technology disclosed herein. Furthermore, when the concentration of CNTs is at a predetermined value or less, it becomes difficult for CNTs to approach 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 further improved.

[0048] In the non-aqueous CNT dispersion of the present embodiment, the concentration of the dispersant is 0.01% by mass or more and 10% by mass or less. When the concentration of the dispersant is at a predetermined value or more, the dispersibility of CNTs can be further improved, and it becomes easier to exhibit the effects of the technology disclosed herein at a high level. In addition, when the concentration of the dispersant is at a predetermined value or less, 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.

[0049] 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.

[0050] 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, three types of dispersants (Dispersants A to C, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.) with different molecular weights, and NMP as a non-aqueous solvent were prepared.

[0051]

Table 1

[0052]

Table 2

[0053] <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 the 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) 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 2 times (N = 2). For example, for Dispersant C, N1 = 74,000 and N2 = 90,000, and the arithmetic mean is 82,000. 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(+))

[0054] <Dispersion treatment and viscosity measurement> Next, in an environment of 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 a non-aqueous solvent (here, in NMP) to prepare a non-aqueous CNT dispersion. For each non-aqueous 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. The dispersion treatment was carried out using a single nozzle chamber under the conditions of a nozzle diameter of 0.15 mm and a pressure of 150 MPa.

[0055] Then, using a viscometer (manufactured by Toki Sangyo, TV-200E), the viscosity of the non-aqueous CNT dispersion in the dispersion treatment 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 column of "relative value" in the table, the relative value is shown 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 70 or less

[0056]

Table 3

[0057]

Table 4

[0058] As shown in Table 3, in the test example containing CNT1 with an average fiber length L of 50 μm, there were cases where the viscosity was low when using dispersant A with a weight average molecular weight of 11,000. When the weight average molecular weight of the dispersant increased, instead, the dispersion thickened, and the effects of the technology disclosed herein were not recognized.

[0059] As shown in Tables 3 and 4, in the test examples containing CNT2 to 4 with an average fiber length L of 100 μm or more, when dispersant A with a weight average molecular weight of 11,000 was used, the thickening of the dispersion liquid was remarkable. Also, even when dispersant B with a weight average molecular weight of 28,000 was used, almost no change in viscosity was observed. In contrast to these comparative examples, when dispersant C with a weight average molecular weight of 70,000 or more was used, the relative decrease in viscosity was large, and the thickening of the dispersion liquid was well suppressed. Figure 2 shows, as an example, the viscosity of a non-aqueous CNT dispersion liquid when CNT3 (CNT with an average fiber length L = 250 μm) was used. The number of passes on the horizontal axis represents the number of times the non-aqueous CNT dispersion liquid was circulated and treated with a high-pressure homogenizer. The above results indicate the significance of the technology disclosed herein.

[0060] 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

[0061] 10 CNT 20 Dispersant

Claims

1. Carbon nanotubes with an average fiber length of 100 μm or more, a non-aqueous solvent, a dispersant that is soluble in the non-aqueous solvent and has a weight-average molecular weight of 70,000 or more, comprising, a non-aqueous carbon nanotube dispersion liquid in which 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 non-aqueous carbon nanotube dispersion liquid according to claim 1, wherein the average fiber length of the carbon nanotubes is 500 μm or less. The non-aqueous carbon nanotube dispersion liquid according to claim 1.

3. The non-aqueous carbon nanotube dispersion liquid according to claim 2, wherein the average fiber length of the carbon nanotubes is 125 μm or more and 375 μm or less. The non-aqueous carbon nanotube dispersion liquid according to claim 2.

4. The non-aqueous carbon nanotube dispersion liquid according to claim 1, wherein the weight-average molecular weight of the dispersant is 100,000 or less. The non-aqueous carbon nanotube dispersion liquid according to claim 1.

5. The non-aqueous carbon nanotube dispersion liquid according to any one of claims 1 to 4, wherein the non-aqueous solvent is an aprotic polar solvent. The non-aqueous carbon nanotube dispersion liquid according to any one of claims 1 to 4.

6. The non-aqueous carbon nanotube dispersion liquid according to any one of claims 1 to 4, wherein the dispersant contains a vinyl pyrrolidone-based polymer containing a vinyl pyrrolidone unit as a repeating unit. The non-aqueous carbon nanotube dispersion liquid according to any one of claims 1 to 4.

7. The non-aqueous carbon nanotube dispersion liquid according to any one of claims 1 to 4, wherein the dispersant contains polyvinyl pyrrolidone. The non-aqueous carbon nanotube dispersion liquid according to any one of claims 1 to 4.

8. The non-aqueous carbon nanotube dispersion liquid according to any one of claims 1 to 4, wherein the concentration of the carbon nanotubes is 0.01% by mass or more and 10% by mass or less. The non-aqueous carbon nanotube dispersion liquid according to any one of claims 1 to 4.

9. The non-aqueous carbon nanotube dispersion liquid according to any one of claims 1 to 4, wherein the concentration of the dispersant is 0.01% by mass or more and 10% by mass or less. The non-aqueous carbon nanotube dispersion liquid according to any one of claims 1 to 4.

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