JP2013199419A - Carbon nanotube dispersion - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a carbon nanotube dispersion having excellent stability against high shear force while retaining high dispersibility of carbon nanotubes.SOLUTION: A carbon nanotube dispersion includes a dispersant having a number-average molecular weight of 10,000-150,000 and a solvent, and includes the dispersant in a proportion of 250-2,000 pts.wt. to 100 pts.wt. of carbon nanotubes. The carbon nanotube dispersion is characterized in that the pH of the dispersion is 8-12.

Description

  The present invention relates to a carbon nanotube dispersion. More specifically, the present invention relates to a carbon nanotube dispersion excellent in stability against high shear force.

  Carbon nanotubes are substances that are expected to have various industrial applications due to various properties resulting from their ideal one-dimensional structure, such as good electrical conductivity, thermal conductivity, and mechanical strength. By controlling the diameter, the number of layers, and the length of the carbon nanotube, it is expected to improve the performance and expand the applicability. Carbon nanotubes usually have a high graphite structure when the number of layers is smaller. Single-walled carbon nanotubes and double-walled carbon nanotubes are known to have high properties such as conductivity and thermal conductivity because they have a high graphite structure. Among carbon nanotubes, 2 to 5 carbon nanotubes, which have a relatively small number of layers, have the characteristics of both single-walled carbon nanotubes and multi-walled carbon nanotubes. Therefore, they attract attention as promising materials for various applications. Collecting.

  Applications using the conductivity of carbon nanotubes include, for example, clean room members, display members, automobile members, and the like, which are used for antistatic, conductive, radio wave absorption, electromagnetic wave shielding, and imparting near-infrared cut properties. Since the carbon nanotube has a high aspect ratio and can form a conductive path with a small amount, the carbon nanotube can be a conductive material excellent in light transmittance and drop-off resistance as compared with conventional conductive fine particles such as carbon black. For example, it is known to use a carbon nanotube as a transparent conductive film for optics (Patent Document 1). In order to obtain a conductive film excellent in light transmittance using carbon nanotubes, a bundle of several tens of thick carbon nanotubes and strong aggregation are dissolved and highly dispersed, and the number of carbon nanotubes is efficiently reduced. It is necessary to form a conductive path. As a means for obtaining such a conductive film, for example, a method of applying a dispersion obtained by highly dispersing carbon nanotubes in a solvent to a substrate is known. In order to highly disperse carbon nanotubes in a solvent, there is a technique of dispersing using a dispersant (Patent Documents 1 and 2). Above all, in order to disperse carbon nanotubes to a higher degree, it is preferable to disperse in an aqueous solvent using a dispersant having a hydrophilic group having affinity for water and a hydrophobic group having a high affinity for carbon nanotubes. (Patent Document 2).

  On the other hand, in order to industrially mass-produce such a transparent conductive film, an industrial means is required in which a large amount of carbon nanotube dispersion liquid is continuously applied using a coating machine. In such a process, since a high shearing force is applied to the carbon nanotube dispersion at the time of coating, agglomeration occurs partially depending on the configuration of the carbon nanotube dispersion, and the aggregate is transferred to the film, which becomes a defect. There was a problem. Therefore, there has been a demand for a stable dispersion that does not aggregate even when a high shear force is applied.

JP 2006-269911 A JP 2009-163959 A

  This invention is made | formed in view of the above situations, and makes it a subject to provide the carbon nanotube dispersion liquid excellent in stability with respect to high shear force, maintaining the high dispersibility of a carbon nanotube. .

  As a result of intensive studies, the present inventors have found that carbon nanotubes are dispersed with a specific molecular weight dispersant at a specific ratio, and thus are particularly excellent in stability against high shearing force. It has come.

That is, the present invention is characterized by the following.
<1> A carbon nanotube dispersion comprising 100 parts by weight of carbon nanotubes and 250 parts by weight to 2000 parts by weight of a dispersant having a number average molecular weight of 10,000 to 150,000, and an aqueous solvent, the pH of the dispersion The carbon nanotube dispersion liquid is characterized by being 8-12.
<2> The carbon nanotube dispersion liquid according to <1>, wherein the dispersant is an ionic dispersant.
<3> The carbon nanotube dispersion liquid according to <2>, wherein the dispersant is a polysaccharide.
<4> The carbon nanotube dispersion liquid according to <3>, wherein the dispersant is carboxymethylcellulose or a salt thereof.
<5> The carbon nanotube dispersion liquid according to any one of <1> to <4>, wherein the carbon nanotubes are mainly double-walled carbon nanotubes.
<6> The carbon nanotube dispersion liquid according to any one of <1> to <5>, wherein the dispersant has a molecular weight of 30,000 to 250,000.
<7> The carbon nanotube dispersion liquid according to any one of <1> to <6>, wherein the ratio of the dispersant is 350 parts by weight or more and 1000 parts by weight or less with respect to 100 parts by weight of the carbon nanotubes.
<8> With respect to 100 parts by weight of the carbon nanotube, ultrasonic treatment is performed on a mixture containing 250 to 2000 parts by weight of a dispersant having a number average molecular weight of 10,000 to 150,000 and an aqueous solvent, and the pH is 8 to 12 A method for producing a carbon nanotube dispersion liquid to obtain a dispersion liquid.
<9> A conductive film obtained by applying the carbon nanotube dispersion liquid according to any one of <1> to <7> to a substrate.
<10> The conductive film according to <9>, which has a light transmittance of 70% or more and a surface resistance value of 10 ⁰ to 10 ⁴ Ω / □.
<11> The conductive film according to <10>, having a light transmittance of 80% or more and a surface resistance value of 10 ¹ to 10 ³ Ω / □.
<12> A method for producing a conductive film, wherein the carbon nanotube dispersion liquid according to any one of <1> to <7> is applied to a substrate.
<13> The method for producing a conductive film according to <12>, wherein an excess dispersant is removed after the carbon nanotube dispersion is applied to the substrate.
<14> The method for producing a conductive film according to <13>, wherein the excessive dispersant is removed by washing the conductive film.
<15> The method for producing a conductive film according to <13>, wherein the excess dispersant is removed by adsorption to the dispersant adsorption layer.

  ADVANTAGE OF THE INVENTION According to this invention, the carbon nanotube dispersion liquid excellent in stability with respect to a high shear force is obtained, maintaining the high dispersibility of a carbon nanotube.

  In the present invention, carbon nanotubes are used as the conductive material. The carbon nanotube has a shape in which one surface of graphite is wound into a cylindrical shape, a single-walled carbon nanotube is wound in one layer, a double-walled carbon nanotube is wound in two layers, and is wound in multiple layers. This is called a multi-walled carbon nanotube.

  In the carbon nanotube dispersion of the present invention and the transparent conductive film obtained thereby, any of single-walled, double-walled, and multi-walled carbon nanotubes can be used depending on the required application characteristics. If carbon nanotubes with a single layer to 5 layers and a small number of layers are used, a transparent conductive film having higher conductivity and higher light transmittance can be obtained. A transparent conductive film with less dependency can be obtained. In order to obtain a transparent conductive film with high light transmittance, it is preferable that 50 or more of 100 carbon nanotubes having a single layer to 5 layers are included, and 100 2 to 5 carbon nanotubes are included. It is more preferable that 50 or more of them are contained, and it is particularly preferable that the number of double-walled carbon nanotubes is 50 or more of 100 carbon nanotubes because of their extremely high conductivity and dispersibility. In general, multi-walled carbon nanotubes of 6 or more layers have low crystallinity and low conductivity, and have a large diameter and a small number of contacts per unit amount of carbon nanotubes in the conductive layer, resulting in low transparent conductivity.

  The number of carbon nanotube layers can be measured, for example, by preparing a sample as follows. When the composition is a composition in which carbon nanotubes are dispersed in a medium such as a liquid, when the solvent is aqueous, the composition is appropriately diluted with water to a concentration that can be easily seen, dropped on the collodion film by several μL, air-dried, and then directly transmitted. The carbon nanotubes on the collodion film are examined with a scanning electron microscope. When the solvent is non-aqueous, the solvent is once removed by drying, then dispersed again in water, diluted as appropriate, dropped several μL onto the collodion film, air-dried, and observed with a transmission electron microscope. The number of carbon nanotube layers in the transparent conductive film can be observed in the same manner as the composition before coating. When collecting carbon nanotubes from a transparent conductive film, the transparent conductive film is examined with a transmission electron microscope by observing a section cut into 0.1 μm or less using a microtome after embedding with an epoxy resin. be able to. Alternatively, the carbon nanotubes can be extracted with a solvent and examined by observation with a high-resolution transmission electron microscope in the same manner as in the case of the composition. The concentration of carbon nanotubes in the liquid dropped on the collodion film may be a concentration at which carbon nanotubes can be observed one by one, and is, for example, 0.001% by weight.

  The number of carbon nanotube layers is measured, for example, as follows. Observe at 400,000 magnifications using a transmission electron microscope, and measure the number of layers of 100 carbon nanotubes arbitrarily extracted from the field of view where 10% or more of the field of view is carbon nanotubes in a 75 nm square field of view. To do. When 100 lines cannot be measured in one field of view, measurement is performed from a plurality of fields until 100 lines are obtained. At this time, one carbon nanotube is counted as one if some carbon nanotubes are visible in the field of view, and both ends are not necessarily visible. In addition, even if it is recognized as two in the field of view, it may be connected outside the field of view and become one, but in that case, it is counted as two.

  The diameter of the carbon nanotube is not particularly limited, but the diameter of the carbon nanotube having the number of layers in the above preferable range is 1 nm to 10 nm, and those having a diameter in the range of 1 to 3 nm are preferably used.

  The carbon nanotubes may be modified at the surface or at the ends with functional groups or alkyl groups, or may be doped with alkali metals or halogens. For example, it may be functionalized with a carboxyl group or a hydroxyl group by heating in an acid. Doping is preferable because the conductivity of the carbon nanotube is improved.

  The length of the carbon nanotube is not particularly limited, but is preferably 0.5 μm or more, more preferably 1 μm or more because a conductive path cannot be efficiently formed if it is too short. If the upper limit is too long, the dispersibility tends to decrease, so it is preferably 10 μm or less.

  The length of the carbon nanotube can be examined using a field emission scanning electron microscope as will be described later. In the case of the composition, several μL is dropped on a mica substrate and air-dried, and then examined with a field emission scanning electron microscope. If necessary, it can be observed after exposing the carbon nanotubes by using a solvent, ion sputtering, or firing at 350 ° C. for 30 minutes in an air atmosphere.

  The length of the carbon nanotubes in the transparent conductive film can be observed in the same manner as in the case of the above composition in the composition before coating. The concentration of carbon nanotubes to be dropped is preferably a concentration at which carbon nanotubes can be observed one by one, and may be diluted appropriately. For example, the concentration is 0.01% by weight.

  When carbon nanotubes are collected from the transparent conductive film, the carbon nanotubes can be extracted from the transparent conductive film using a solvent and then observed by the same method as that for the composition.

  Regarding the length of the carbon nanotube, a sample was prepared by the above method and observed with a field emission scanning electron microscope, and a photo was taken where 10 or more carbon nanotubes were included in a 15 μm square field of view. Measure the length. The length of 100 carbon nanotubes arbitrarily extracted from the visual field is measured along the length direction. When 100 lines cannot be measured in one field of view, measurement is performed from a plurality of fields until 100 lines are obtained. By measuring the length of a total of 100 carbon nanotubes, the length and number of carbon nanotubes contained in the 100 can be confirmed. In the present invention, if the number of carbon nanotubes in the range of 0.5 μm or less is 30 or less, the contact resistance can be reduced and the light transmittance can be improved, and the carbon nanotube is preferably 1 μm or less. The number of carbon nanotubes in the range is more preferably 30 or less out of 100. Furthermore, in the present invention, it is preferable that the number of carbon nanotubes in the range of 10 μm or more is 30 or less out of 100 because dispersibility can be improved. When the length of the carbon nanotube is long and the entire length is not visible in the field of view, the length of the carbon nanotube in the field of view is measured. If the length is within 10 μm, it is regarded as the length of the measured value. The number of carbon nanotubes in the range of 0.5 to 10 μm is counted assuming that the length exceeds 10 μm.

  Moreover, in order to obtain a transparent conductive film excellent in transparent conductivity, it is preferable to use high-quality carbon nanotubes with a high degree of crystallinity. A carbon nanotube with a high degree of crystallinity itself is excellent in electrical conductivity. However, such high-quality carbon nanotubes form bundles and aggregates more firmly than carbon nanotubes with low crystallinity, so it is very difficult to disperse them one by one and stably disperse them. It is difficult to. Therefore, in order to obtain a transparent conductive film having higher conductivity using carbon nanotubes having a high degree of crystallinity, a carbon nanotube dispersion technique is very important.

The carbon nanotube used in the present invention is not particularly limited, but a carbon nanotube having linearity and high crystallinity is preferable because of high conductivity. Carbon nanotubes with good linearity are carbon nanotubes with few defects and high carbon nanotube crystallinity. The crystallinity of the carbon nanotube can be evaluated by Raman spectroscopy. Although there are various laser wavelengths used in the Raman spectroscopic analysis method, 532 nm and 633 nm are used here. The Raman shift observed in the vicinity of 1590 cm ⁻¹ in the Raman spectrum is called a graphite-derived G band, and the Raman shift observed in the vicinity of 1350 cm ⁻¹ is called a D band derived from defects in amorphous carbon or graphite. That is, a carbon nanotube having a higher G / D ratio, which is the ratio of the peak height of the G band and the D band, has higher linearity and higher crystallinity and higher quality.

  In the carbon nanotube used in the present invention, a wavelength of 633 nm is used when evaluating the Raman G / D ratio. The higher the G / D ratio, the better. However, if it is 30 or more, it can be said to be a high-quality carbon nanotube. Preferably it is 40 or more, More preferably, it is 50 or more. There is no particular upper limit, but it is usually 200 or less. In addition, solid Raman spectroscopy such as carbon nanotubes may vary depending on sampling. Therefore, at least three places and another place are subjected to Raman spectroscopic analysis, and an arithmetic average thereof is taken.

  The carbon nanotube used in the present invention is produced, for example, as follows.

  A powdery catalyst in which iron is supported on magnesia is made to exist in the entire horizontal cross-sectional direction of the reactor in a vertical reactor, and methane is circulated in the vertical direction in the reactor, and methane and the above catalyst are mixed in 500 to 1200. It is obtained by oxidizing the carbon nanotubes after contacting them at 0 ° C. to produce the carbon nanotubes. That is, carbon nanotubes containing single-walled to five-walled carbon nanotubes can be obtained by the carbon nanotube synthesis method.

  The oxidation treatment of the carbon nanotube is performed by, for example, a firing method. The temperature of the calcination treatment is not particularly limited as long as the carbon nanotube used in the present invention is obtained, but is usually selected in the range of 300 to 1000 ° C. Since the oxidation temperature is affected by the atmospheric gas, it is preferable to perform the baking treatment at a relatively low temperature when the oxygen concentration is high and at a relatively high temperature when the oxygen concentration is low. Examples of the calcination treatment of the carbon nanotubes include a method of calcination in the range of the combustion peak temperature of carbon nanotubes ± 50 ° C. in the atmosphere. If the oxygen concentration is higher than the atmosphere, the calcination treatment is lower than this. Usually, the temperature range is selected, and if it is low, the higher temperature range is selected. In particular, when the firing treatment is performed in the atmosphere, it is preferably performed within the range of the combustion peak temperature ± 15 ° C.

  Further, as an oxidation treatment of carbon nanotubes, treatment with hydrogen peroxide, mixed acid or nitric acid can be mentioned. The treatment of carbon nanotubes with hydrogen peroxide means that the carbon nanotubes are mixed in, for example, commercially available 34.5% hydrogen peroxide water so as to be 0.01 wt% to 10 wt%, and the temperature is set to 0 to 100 ° C. For 0.5 to 48 hours. The treatment of carbon nanotubes with a mixed acid means that the carbon nanotubes are mixed in, for example, a concentrated sulfuric acid / concentrated nitric acid (3/1) mixed solution so as to be 0.01% by weight to 10% by weight. It means to react at temperature for 0.5 to 48 hours. As the mixing ratio of the mixed acid, the ratio of concentrated sulfuric acid / concentrated nitric acid can be set to 1/10 to 10/1 according to the amount of single-walled carbon nanotubes in the generated carbon nanotubes. The treatment of carbon nanotubes with nitric acid means that the carbon nanotubes are mixed in, for example, commercially available nitric acid 40 to 80% by weight so as to be 0.01% by weight to 10% by weight, and the temperature is 60.degree. It means reacting for 5 to 48 hours.

  By performing such an oxidation treatment, it becomes possible to selectively remove impurities such as amorphous carbon and single-walled CNTs having low heat resistance in the product, and the single-layer to five-layer, particularly two- to five-layer The purity of the carbon nanotube can be improved. At the same time, since the surface of the carbon nanotube is functionalized, the affinity with the dispersion medium and additives is improved, so that the dispersibility is improved. Among these oxidation treatments, treatment with nitric acid is particularly preferable.

  These oxidation treatments may be performed immediately after the synthesis of the carbon nanotubes or after another purification treatment. For example, when iron / magnesia is used as the catalyst, the oxidation treatment may be performed after the purification treatment for removing the catalyst with an acid such as hydrochloric acid, or the oxidation treatment may be performed after the oxidation treatment. May be performed.

The carbon nanotube dispersion of the present invention is characterized in that carbon nanotubes are not only highly dispersed in the solution, but are stable even when a high shear force is applied. As a method for evaluating the stability against a high shearing force, for example, there is a method for evaluating the stability of a dispersion when a high shear is continuously applied using a rheometer. As a method for determining the stability against high shear force in the present invention, the method described below is suitable. First, a rheometer (MCR501 manufactured by Anton Paar Co., Ltd.) is used as an apparatus, the stage / rotor gap is set to 0.01 mm when using a smooth rotor (PP25), and 0.3 ml of the dispersion is loaded. The rotor part after applying a high shearing force to this dispersion at a shear rate of 200,000 s ^-1 for 30 minutes is visually observed, and when the carbon nanotube dispersion is not aggregated, it does not aggregate even when a high shearing force is applied. It can be judged that it is stable.

  The applicants have intensively studied the above characteristics, that is, the configuration of the carbon nanotube dispersion liquid that is highly dispersed and stable against high shearing force, and as a result, by using a specific dispersant in a specific ratio, It was found that the nanotubes are highly dispersed and are stable even when a high shear force is applied. That is, it is preferable to use a so-called polymer dispersant as the dispersant used in the present invention. At this time, if the molecular weight is too small, the interaction between the dispersant and the carbon nanotube is weakened, so that the bundle of carbon nanotubes cannot be sufficiently solved. On the other hand, if the molecular weight is too large, the entanglement of the polymer chain with the carbon nanotubes becomes too strong, so that the dispersant cannot be completely removed in the process of removing the excessive dispersant, resulting in deterioration of conductivity. Generally, there are a number average molecular weight and a weight average molecular weight as means for expressing the molecular weight of a polymer. However, in our study, when carbon nanotubes are dispersed using a polymer-based dispersant, the dispersibility of carbon nanotubes is the number average molecular weight. It turns out that there is a correlation. That is, it was found that the interaction between the carbon nanotube and the polymer dispersant is influenced by the number of moles of the polymer existing in the system rather than the size of the polymer itself. Therefore, it is preferable to use the number average molecular weight as an index for defining the molecular weight of the polymer used as the dispersant. And the range of a preferable number average molecular weight is 10,000-150,000, It is especially preferable that it is 20,000-80,000. The range of the preferred weight average molecular weight of the polymer-based dispersant having the preferred number average molecular weight is 10,000 or more and 400,000 or less, particularly preferably 30,000 or more and 250,000 or less, and more preferably 60,000. It is more than 250,000 or less. The number average molecular weight and the weight average molecular weight refer to a molecular weight calculated by using a gel permeation chromatography method and comparing with a calibration curve using polyethylene glycol.

  In order not only to fully unwind the bundle of carbon nanotubes contained in the carbon nanotube dispersion but also to be stable even when a high shear force is applied, an excessive amount of dispersant is required for the carbon nanotubes. Since this excess dispersant is deposited in the form of a film at the interface of the conductive layer when the dispersion is applied to form a conductive layer, when producing a conductive film using the carbon nanotube dispersion of the present invention, It is preferable to have a step of removing excess dispersant. However, when the dispersant is too large to be removed, the conductivity inhibition by the excessive dispersant is too great, and the surface resistance value of the formed carbon nanotube conductive layer is deteriorated. On the other hand, when the amount of the dispersant contained in the dispersion is small, the bundle of carbon nanotubes cannot be completely solved.

  That is, the amount of the dispersant contained in the carbon nanotube dispersion liquid of the present invention is preferably an amount that is excessive beyond that adsorbed on the carbon nanotubes and does not inhibit the conductivity. The dispersant is preferably 250 parts by weight or more and 2000 parts by weight or less, and more preferably 350 parts by weight or more and 1000 parts by weight or less with respect to parts.

  The type of the dispersant is not limited as long as carbon nanotubes can be dispersed, and can be selected from synthetic polymers and natural polymers. The type of polymer is not particularly limited as long as carbon nanotubes can be dispersed, and is preferably selected from synthetic polymers and natural polymers with good dispersibility. Synthetic polymers include, for example, polyether diol, polyester diol, polycarbonate diol, polyvinyl alcohol, partially saponified polyvinyl alcohol, acetoacetyl group-modified polyvinyl alcohol, acetal group-modified polyvinyl alcohol, butyral group-modified polyvinyl alcohol, silanol group-modified polyvinyl alcohol, Ethylene-vinyl alcohol copolymer, ethylene-vinyl alcohol-vinyl acetate copolymer resin, dimethylaminoethyl acrylate, dimethylaminoethyl methacrylate, acrylic resin, epoxy resin, modified epoxy resin, phenoxy resin, modified phenoxy resin, phenoxy Ether resin, phenoxy ester resin, fluorine resin, melamine resin, alkyd resin, phenol resin, polyacryl Bromide, polyacrylic acid, polystyrene sulfonic acid, polyethylene glycol, may be selected from polyvinyl pyrrolidone and derivatives thereof. Natural polymers include, for example, polysaccharides such as starch, pullulan, dextran, dextrin, guar gum, xanthan gum, amylose, amylopectin, alginic acid, gum arabic, carrageenan, chondroitin sulfate, hyaluronic acid, curdlan, chitin, chitosan, cellulose and the like It can be selected from derivatives. A derivative means a conventionally known compound such as an ester, ether, or salt. Among these, it is particularly preferable to use a polysaccharide from the viewpoint of improving dispersibility. These may be used alone or in combination of two or more. In addition, an ionic polymer is preferably used because of its excellent carbon nanotube dispersibility and conductivity. Among these, those having an ionic functional group such as sulfonic acid and carboxylic acid are preferable because of high dispersibility and conductivity. By using a dispersing agent with good dispersibility, it is possible to improve the transparent conductivity by releasing the bundle of carbon nanotubes. Use polystyrene sulfonic acid, chondroitin sulfate, hyaluronic acid, carboxymethyl cellulose and their derivatives. In particular, the use of carboxymethylcellulose, which is a polysaccharide having an ionic functional group, and derivatives thereof is most preferable.

  In the present invention, a carbon nanotube dispersion is prepared using carbon nanotubes, a dispersant and a solvent. The dispersion may further contain other additives, and may be in liquid form or semi-solid form such as paste or gel, but liquid form is preferred. In the present invention, the dispersion refers to a liquid in which the obtained composition is visually free of precipitates and aggregates and is free of sediments and aggregates visually after standing for at least 24 hours. Further, the content of carbon nanotubes with respect to the entire dispersion is preferably 0.01% by weight or more and 20% by weight or less, more preferably 0.05 to 10% by weight.

  The aqueous solvent used in the present invention is not limited as long as the dispersant is dissolved and the carbon nanotubes are dispersed, and water and solvents miscible with water can be used. Solvents include ethers (dioxane, tetrahydrofuran, methyl cellosolve, etc.), ether alcohols (ethoxyethanol, methoxyethoxyethanol, etc.), esters (methyl acetate, ethyl acetate, etc.), ketones (cyclohexanone, methyl ethyl ketone, etc.), alcohols (Ethanol, isopropanol, phenol, etc.), lower carboxylic acids (acetic acid, etc.), amines (triethylamine, trimethanolamine, etc.), nitrogen-containing polar solvents (N, N-dimethylformamide, nitromethane, N-methylpyrrolidone, acetonitrile, etc.) , Sulfur compounds (such as dimethyl sulfoxide) can be used.

  Among these, it is particularly preferable from the viewpoint of dispersibility of carbon nanotubes to contain water, alcohol, ether, and a combination solvent thereof. The pH of the carbon nanotube dispersion liquid is preferably alkaline, more preferably pH 8-12, and particularly preferably pH 9-11. By making the pH of the dispersion alkaline, it is considered that electrostatic repulsion between the dispersant or the carbon nanotubes increases, and the dispersibility and stability against high shear force are improved. The pH can be adjusted by adding an alkaline solution. As the alkaline solution, an ammonia or organic amine solution is used. The organic amine is preferably an organic compound containing nitrogen such as ethanolamine, ethylamine, n-propylamine, isopropylamine, diethylamine, triethylamine, ethylenediamine, hexamethylenediamine, hydrazine, pyridine, piperidine and hydroxypiperidine. Of these ammonia and organic amines, ammonia is most preferred. Water is preferably used as a solvent for dissolving these organic amines and ammonia. The pH is measured by a pH meter (HM-30S, manufactured by Toa Denpa Kogyo Co., Ltd.).

  The carbon nanotube dispersion may be prepared by mixing and dispersing carbon nanotubes, a dispersant and a solvent, which are commonly used in coating production (for example, ball mill, bead mill, sand mill, roll mill, homogenizer, ultrasonic homogenizer, high pressure homogenizer, ultrasonic device, atomizer). A lighter, a dissolver, a paint shaker, etc.) can be mixed to produce a dispersion. Among them, it is preferable to disperse using ultrasonic waves because the dispersibility of the carbon nanotubes is improved. The carbon nanotubes to be dispersed may be in a dry state or a state containing a solvent. However, it is preferable to disperse the carbon nanotubes in a state containing a solvent without being dried after purification because the dispersibility is improved.

  The carbon nanotube dispersion liquid may contain, for example, various polymeric materials such as a surfactant and a conductive or nonconductive polymer as other additives in addition to the dispersant and the carbon nanotube. Moreover, content of an additive can be added in the range which does not inhibit the effect of this invention.

  The carbon nanotube dispersion of the present invention can be applied by the method described later to form a conductive film.

  The substrate used for producing the transparent conductive film obtained by applying the carbon nanotube dispersion liquid of the present invention is not particularly limited in shape, size, and material as long as the carbon nanotube dispersion liquid can be applied and the obtained conductive layer can be fixed. However, it can be selected depending on the intended use, and may be, for example, a film, a sheet, a board, paper, a fiber, or a particle. For example, if the material is an organic material, polycarbonate, polyester, polyamide, acrylic, polyurethane, polymethyl methacrylate, cellulose, triacetyl cellulose, amorphous polyolefin and other resins, and inorganic materials are stainless steel, aluminum, iron, gold It can be selected from metals such as silver, glass and carbon materials. When a resin film is used as the substrate, a conductive film excellent in adhesiveness, stretchability and flexibility can be obtained, which is preferable. The thickness of the preferable base material in that case is not specifically limited, It can take various ranges from moderate thickness to very thin thickness. For example, the substrate can be between about 1 and about 1000 μm thick. In a preferred embodiment, the thickness of the substrate can be from about 5 to about 500 μm. In another preferred embodiment, the thickness of the substrate is from about 10 to about 200 μm.

  The substrate may be subjected to surface hydrophilization treatment such as glow discharge, corona discharge treatment or ozone treatment as necessary. Alternatively, an undercoat layer may be provided. The material for the undercoat layer is preferably a highly hydrophilic material.

  It is also preferable to apply the carbon nanotube dispersion of the present invention to a substrate to form a transparent conductive film, and then overcoat the film with a binder material capable of forming an organic or inorganic transparent film. By overcoating, it is effective for further charge dispersion and movement. In addition, the transparent conductive film contains a binder material capable of forming an organic or inorganic transparent film in the carbon nanotube dispersion liquid, and is applied to the substrate, and then heated as necessary to dry or bake (harden) the coating film. You can also get it. The heating conditions at that time are appropriately set according to the binder type. When the binder is photocurable or radiation curable, the coating film is cured by irradiating the coating film with light or radiation immediately after coating, not by heat curing. As the radiation, ionizing radiation such as electron beam, ultraviolet ray, X-ray and gamma ray can be used, and the irradiation dose is determined according to the binder type.

  The binder material is not particularly limited as long as it is used for conductive paints, and various organic and inorganic binders, that is, transparent organic polymers or precursors thereof (hereinafter sometimes referred to as “organic polymer binders”). Or an inorganic polymer or a precursor thereof (hereinafter sometimes referred to as “inorganic polymer binder”) can be used. The organic polymer binder may be thermoplastic, thermosetting, or radiation curable such as ultraviolet rays or electron beams. Examples of suitable organic binders include polyolefin (polyethylene, polypropylene, etc.), polyamide (nylon 6, nylon 11, nylon 66, nylon 6, 10, etc.), polyester (polyethylene terephthalate, polybutylene terephthalate, etc.), silicone resin, vinyl Resin (polyvinyl chloride, polyvinylidene chloride, polyacrylonitrile, polyacrylate, polystyrene derivatives, polyvinyl acetate, polyvinyl alcohol, etc.), polyketone, polyimide, polycarbonate, polysulfone, polyacetal, fluororesin, phenol resin, urea resin, melamine resin, Organic polymers such as epoxy resins, polyurethane, cellulose polymers, proteins (gelatin, casein, etc.), chitin, polypeptides, polysaccharides, polynucleotides, etc. Chromatography, as well as precursors of these polymers (monomer or oligomer). These can form transparent coatings or matrices simply by evaporation of the solvent, or by thermal curing or curing with light or radiation.

  The organic polymer binder is preferably a compound having an unsaturated bond that can be radically polymerized and cured by radiation or light. This is a monomer, oligomer, or polymer having a vinyl group or a vinylidene group. Examples of this type of monomer include styrene derivatives (such as styrene and methylstyrene), acrylic acid or methacrylic acid or derivatives thereof (such as alkyl acrylate or methacrylate, allyl acrylate or methacrylate), vinyl acetate, acrylonitrile, and itaconic acid. The oligomer or polymer is preferably a compound having a double bond in the main chain or a compound having acryloyl or methacryloyl groups at both ends of the straight chain. This type of radical polymerization curable binder has a high hardness, excellent scratch resistance, and can form a highly transparent film or matrix.

  Examples of inorganic polymer binders include sols of metal oxides such as silica, tin oxide, aluminum oxide and zirconium oxide, or hydrolyzable or thermally decomposable organometallic compounds (organophosphorus compounds, Organic boron compounds, organic silane compounds, organic titanium compounds, organic zirconium compounds, organic lead compounds, organic alkaline earth metal compounds, and the like. Specific examples of hydrolyzable or thermally decomposable organometallic compounds are alkoxides or partial hydrolysates thereof, lower carboxylates such as acetate, and metal complexes such as acetylacetone.

  When these inorganic polymer binders are baked, a glassy inorganic polymer transparent film or matrix made of an oxide or a composite oxide can be formed. The inorganic polymer matrix is generally glassy, has high hardness, excellent scratch resistance, and high transparency.

  The amount of the binder used may be an amount sufficient for overcoating, or an amount sufficient for obtaining a viscosity suitable for coating when mixed in a liquid. If the amount is too small, the coating will not be successful, and if the amount is too large, the conductivity will be hindered.

  In addition, the substrate should be used in combination with a hard coat that has been given the wear resistance, high surface hardness, solvent resistance, contamination resistance, fingerprint resistance, etc. on the opposite side to which the carbon nanotubes are applied. Can do.

  In addition, it is preferable to use a transparent substrate as the substrate because a transparent conductive film excellent in transparency and conductivity can be obtained. A transparent substrate means that the light transmittance at 550 nm is 50% or more.

  The method for applying the carbon nanotube dispersion of the present invention is not particularly limited. Known coating methods such as spray coating, dip coating, roll coating, spin coating, doctor knife coating, kiss coating, slit coating, die coating, slit die coating, gravure coating, micro gravure coating, blade coating, wire bar coating, extrusion Coating, screen printing, gravure printing, inkjet printing, pad printing, and other types of printing can be used. The application may be performed any number of times, and two different application methods may be combined. The most preferable coating method is microgravure coating or wire bar coating.

  Since the coating thickness (wet thickness) also depends on the concentration of the coating solution, it does not need to be specified if the desired conductivity can be obtained. However, among these, the thickness is preferably 0.01 μm to 50 μm. More preferably, it is 0.1 micrometer-20 micrometers.

  The coating thickness (dry thickness) can be measured by observing a cross section of the transparent conductive film. For example, the coating thickness can be observed with a transmission microscope, and can be dyed if necessary. A preferable dry thickness is not specified as long as desired conductivity is obtained, but is preferably 0.001 to 5 μm. More preferably, it is 0.001-1 micrometer.

  When the carbon nanotube dispersion is an aqueous dispersion, a wetting agent may be added to the composition when it is applied on the substrate. By adding a wetting agent such as a surfactant or alcohol to the non-hydrophilic substrate, the composition can be applied to the substrate without being repelled. Among these, alcohol is preferable, and methanol, ethanol, propanol, and isopropanol are preferable among alcohols. Lower alcohols such as methanol, ethanol and isopropanol are highly volatile and can be easily removed when the substrate is dried after coating. In some cases, a mixture of alcohol and water may be used.

  Thus, after apply | coating a carbon nanotube dispersion liquid to a base material, it is preferable to remove an unnecessary solvent by methods, such as air drying, a heating, and pressure reduction, and to dry the formed conductive layer. Thereby, the carbon nanotube forms a three-dimensional stitch structure and is fixed to the base material. Of these, drying by heating is preferred. The drying temperature may be the temperature at which the solvent can be removed and is not higher than the heat resistant temperature of the base material. In the case of a resin base material, it is preferably 0 ° C to 250 ° C, more preferably 15 ° C to 150 ° C.

  Since the conductive layer contains an excessive dispersant, it is preferable to remove the excess dispersant. This operation facilitates the dispersion of charges and improves the conductivity of the conductive composite. As a method for removing the excessive dispersant, there is a method of washing the dispersant by drying the conductive layer and then immersing it in a solvent or spraying the solvent onto the conductive layer. The solvent for washing is not particularly limited as long as it is capable of dissolving a component that lowers the transparent conductivity to be removed, such as an additive or an excessive amount of a dispersant, and does not remove carbon nanotubes. Specific examples include water, alcohols, and acetonitrile.

  As another removal method, there is a method in which a dispersing agent adsorption layer capable of adsorbing an excessive dispersing agent is provided on a substrate in advance. As the dispersant adsorption layer, it is preferable to use a hydrophilic inorganic oxide. More preferred are titania, alumina, and silica. These substances have a hydroxyl group which is a hydrophilic group on the surface and are preferable because high hydrophilicity can be obtained. Further, the dispersant absorbing layer may be a composite of these inorganic oxides and resin, for example, a composite of silica fine particles and polysilicate. The reason why the dispersant can be removed by the dispersant adsorption layer is not clear, but the following may be considered. That is, when a dispersant having a hydrophilic group, such as an ionic dispersant, is used as a dispersant for carbon nanotubes, the dispersant has a higher affinity for the dispersant adsorption layer than the carbon nanotubes themselves. It is thought that it is possible to remove the dispersant locally from the periphery of the carbon nanotubes because the excess dispersant is selectively transferred to the dispersant adsorption layer at the same time as the nanotube dispersion is applied.

  The transparent conductive film obtained by applying the carbon nanotube dispersion liquid of the present invention exhibits a sufficient transparency because the carbon nanotubes in the conductive layer are sufficiently dispersed and exhibit a sufficient conductivity. Have. When a transparent substrate is used, the light transmittance at 550 nm is at least about 50%. In a preferred embodiment, the light transmittance at 550 nm of the transparent conductive film is 60% or more. In another preferred embodiment, the light transmittance at 550 nm of the transparent conductive film is 70% or more. In another preferred embodiment, the light transmittance at 550 nm of the transparent conductive film is 80% or more. Furthermore, in another preferable embodiment, the light transmittance at 550 nm of the transparent conductive film is 90% or more.

Moreover, since the carbon nanotube dispersion liquid of the present invention is highly dispersed, the transparent conductive film obtained by coating the carbon nanotube exhibits excellent conductivity while maintaining high transparency. In a preferred embodiment, the surface resistance value of the transparent conductive film is from 10 ⁰ to 10 ⁴ Ω / □, and in a further preferred embodiment, the surface resistance value of the transparent conductive film is from 10 ¹ to 10 ³ Ω / □.

  In the present invention, the light transmittance is a value obtained by loading a transparent conductive film into a spectrophotometer (Hitachi U-2100) and measuring the light transmittance at a wavelength of 550 nm.

  Further, the surface resistance value is a value obtained by using a four-probe method according to JIS K7194 (established in 1994) and using Loresta EP MCP-T360 (manufactured by Dia Instruments Co., Ltd.).

  In Raman spectroscopic analysis, a powder sample is placed in a resonance Raman spectrometer (INF-300 manufactured by Horiba Joban Yvon), and measurement is performed using a laser wavelength of 532 nm. In the measurement, the analysis is performed at three places and different places, the heights of the G band and the D band are measured, the G / D ratio is obtained by the ratio of the respective heights, and the arithmetic average is expressed.

The coating amount when the carbon nanotube dispersion liquid of the present invention is applied to the substrate can be easily adjusted to achieve various uses that require electrical conductivity. For example, the surface can be increased by increasing the film thickness. Since the resistance is lowered and tends to be increased by reducing the film thickness, the surface resistance value of the transparent conductive film can be easily adjusted by the coating amount. However, the light transmittance and the surface resistance value are increased when the coating amount is decreased to increase the light transmittance, and the light transmittance is decreased when the coating amount is increased to decrease the surface resistance value. Since the values are contradictory, the desired surface resistance value and light transmittance are selected and the coating amount is adjusted. Therefore, by using a dispersion liquid in which the molecular weight and content of the dispersant of the present invention are appropriately adjusted and further removing the excess dispersant, the surface resistance value of the conductive layer is reduced while maintaining the dispersibility of the carbon nanotubes. Therefore, a transparent conductive film that achieves excellent conductivity can be obtained. As a result, it is possible to obtain a transparent conductive film having a surface resistance value of 10 ⁰ to 10 ⁴ Ω / □ or less, a light transmittance of 550 nm of 70% or more, and a surface resistance value of 10 ¹ to 10. It is also possible to obtain a transparent conductive film of ³ Ω / □ or less and a light transmittance of 550 nm of 80% or more.

  The transparent conductive film obtained by applying the carbon nanotube dispersion liquid of the present invention is highly conductive, and it is a member for clean rooms such as antistatic shoes and antistatic plates, electromagnetic shielding, near infrared cut, transparent electrode. Can be used as a touch panel, radio wave absorption display, and automobile parts. In particular, it exhibits particularly excellent performance as a display-related transparent electrode such as a touch panel, a liquid crystal display, organic electroluminescence, and electronic paper, which mainly require surface smoothness.

  Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples.

  In the examples, the light transmittance, the surface resistance value, the G / D of the carbon nanotube, the number of layers of the carbon nanotube, and the molecular weight of the dispersant were measured by the methods described above.

(Reference Example 1)
Carbon nanotubes were obtained as follows.

(Catalyst preparation)
About 24.6 g of iron (III) ammonium citrate (manufactured by Wako Pure Chemical Industries, Ltd.) was dissolved in 6.2 kg of ion-exchanged water. About 1000 g of magnesium oxide (MJ-30 manufactured by Iwatani Corporation) was added to this solution, and after vigorously stirring for 60 minutes with a stirrer, the suspension was introduced into a 10 L autoclave container. At this time, 0.5 kg of ion exchange water was used as a washing solution. In a sealed state, it was heated to 160 ° C. and held for 6 hours. Thereafter, the autoclave container was allowed to cool, the slurry-like cloudy substance was taken out from the container, excess water was filtered off by suction filtration, and a small amount of water contained in the filtered material was dried by heating in a 120 ° C. drier. The obtained solid content was collected on a sieve, and the particle size in the range of 20 to 32 mesh was collected while being finely divided in a mortar. The granular catalyst body shown on the left was introduced into an electric furnace and heated at 600 ° C. for 3 hours in the atmosphere. The bulk density was 0.32 g / mL. Further, when the filtrate was analyzed by an energy dispersive X-ray analyzer (EDX), iron was not detected. From this, it was confirmed that the added iron (III) ammonium citrate was supported on the entire amount of magnesium oxide. Furthermore, from the EDX analysis result of the catalyst body, the iron content contained in the catalyst body was 0.39 wt%.

(Manufacture of carbon nanotubes)
Carbon nanotubes were synthesized using the above catalyst. A catalyst layer was formed by taking 132 g of the solid catalyst and introducing it onto a quartz sintered plate at the center of the reactor installed in the vertical direction. While heating the catalyst layer until the temperature in the reaction tube reaches about 860 ° C., nitrogen gas is supplied from the bottom of the reactor toward the top of the reactor at 16.5 L / min so as to pass through the catalyst layer. Circulated. Thereafter, while supplying nitrogen gas, methane gas was further introduced at 0.78 L / min for 60 minutes and aeration was performed so as to pass through the catalyst body layer to cause a reaction. The introduction of methane gas was stopped, and the quartz reaction tube was cooled to room temperature while supplying nitrogen gas at 16.5 L / min to obtain a carbon nanotube composition with a catalyst. 115 g of this catalyst-attached carbon nanotube composition was stirred in 2000 mL of a 4.8N hydrochloric acid aqueous solution for 1 hour to dissolve iron as the catalyst metal and MgO as the carrier. The obtained black suspension was filtered, and the filtered product was again put into 400 mL of a 4.8N hydrochloric acid aqueous solution, treated with MgO, and collected by filtration. This operation was repeated three times to obtain a carbon nanotube composition from which the catalyst was removed.

(Oxidation treatment of carbon nanotubes)
The above carbon nanotube composition was added to concentrated nitric acid (first grade assay 60 to 61%, manufactured by Wako Pure Chemical Industries, Ltd.) having a weight of about 300 times. Thereafter, the mixture was heated to reflux with stirring in an oil bath at about 140 ° C. for 25 hours. After heating to reflux, a nitric acid solution containing the carbon nanotube-containing composition was diluted with ion-exchanged water three times and suction filtered. The carbon nanotube composition was stored in a wet state containing water after washing with ion-exchanged water until the suspension of the filtered material became neutral. When the average outer diameter of the carbon nanotube composition was observed with a high-resolution transmission electron microscope, it was 1.7 nm. The proportion of the double-walled carbon nanotubes was 90%, the Raman G / D ratio measured at a wavelength of 633 nm was 79, and the combustion peak temperature was 725 ° C.

(Reference Example 2)
(Example of overcoat layer preparation)
In a 100 mL plastic container, 20 g of ethanol was added, and 40 g of n-butyl silicate was added and stirred for 30 minutes. Then, after adding 10 g of 0.1N hydrochloric acid aqueous solution, the mixture was stirred for 2 hours and allowed to stand at 4 ° C. for 12 hours. This solution was diluted with a mixed solution of toluene, isopropyl alcohol and methyl ethyl ketone so that the solid content concentration became 1% by mass.

  This coating solution was applied onto the carbon nanotube layer using the wire bar # 8, and then dried in a 125 ° C. dryer for 1 minute.

(Reference Example 3)
(Dispersant adsorption layer preparation example)
A dispersant adsorbing layer in which hydrophilic silica fine particles having a diameter of 30 nm were exposed was prepared by using the polysilicate as a binder by the following operation.

  Megaaqua hydrophilic DM coat (manufactured by Ryowa Co., Ltd., DM-30-26G-N1) containing about 30 nm of hydrophilic silica fine particles and polysilicate in a solid content concentration of 1% by mass was used as a coating solution for silica film preparation.

  The said coating liquid for silica film preparation was apply | coated on the polyethylene terephthalate (PET) film (Toray Co., Ltd. product (Lumirror U46) using wire bar # 4. After application | coating, it was made to dry for 1 minute in a 120 degreeC dryer. It was.

Example 1
(Transparent conductive film containing carbon nanotubes and carboxymethylcellulose)
15 mg of carbon nanotubes obtained in Reference Example 1 (calculated when dried) in a 20 mL container, carboxymethylcellulose sodium salt (manufactured by Daiichi Kogyo Seiyaku Co., Ltd., number average molecular weight 42,000 (weight average molecular weight 80,000)) 90 mg (Dispersant / carbon nanotube weight ratio = 6) is weighed, distilled water is added to make 10 g, pH is adjusted to 10 using ammonia, ultrasonic homogenizer output is 20 W, and dispersion treatment is performed under ice-cooling for 3 minutes. A liquid was prepared. The obtained liquid was centrifuged at 10,000 G for 15 minutes with a high-speed centrifuge to obtain 9 mL of the supernatant. No precipitate of a size that can be visually observed was found in the remaining liquid after obtaining the supernatant.

  Polyethylene terephthalate (PET) film having a polyester resin surface resin layer (dry thickness 140 nm) subjected to corona discharge treatment by diluting the carbon nanotube dispersion liquid of the supernatant after centrifugation obtained above with ion exchange water 2.5 times (Toray Industries, Inc. 188 μm) Light transmittance 90.2%, 15 cm × 10 cm) was applied using a bar coater. At this time, the coating thickness was adjusted so that the light transmittance at a wavelength of 550 nm was 87%. The coated film was dried in a 120 ° C. dryer for 1 minute to immobilize the carbon nanotubes. Thereafter, the coating film was immersed in a container containing 1 L of ion-exchanged water, and was pulled up and dried after 20 seconds. Further, an overcoat layer was formed on the carbon nanotube layer according to Reference Example 2 to obtain a transparent conductive film. The resulting film had a surface resistance value of 510Ω / □ at a transmittance of 90%.

Next, 0.3 ml of the carbon nanotube dispersion having the concentration used for coating was taken, and a high shearing force was applied for 30 minutes with a rheometer (MCR501, manufactured by Anton Paar) (using smooth rotor PP25, shear rate 200000 s ⁻¹ , gap 0.01 mm). Gave. When the state of the dispersion after 30 minutes was observed, no aggregation occurred.

(Comparative Examples 1 and 2 and Example 2)
(Change ratio of carboxymethylcellulose sodium salt)
The carboxymethylcellulose sodium salt addition amount of Example 1 above was 15 mg (dispersant / carbon nanotube weight ratio = 1) (Comparative Example 1), 30 mg (dispersant / carbon nanotube weight ratio = 2) (Comparative Example 2), 60 mg ( Dispersant / carbon nanotube weight ratio = 4) A carbon nanotube dispersion liquid was obtained in the same manner as in Example 1 except that it was changed to (Example 2). As a result, the dispersion liquid of Comparative Example 1 in which the dispersant / carbon nanotube weight ratio = 1 was observed to be agglomerated and precipitated by centrifugation using a high-speed centrifuge, so it can be said that the dispersibility was insufficient. On the other hand, in the dispersions of Comparative Example 2 and Example 2, no precipitation was observed by centrifugation using a high-speed centrifuge. Therefore, the dispersions of Comparative Example 2 and Example 2 were diluted 2.5 times with ion-exchanged water, applied to a PET film formed in advance with a dispersant adsorption layer according to Reference Example 3, and further overcoated according to Reference Example 2. A layer was formed. The surface resistance values at 90% transmittance of the obtained films were 650Ω / □ (Comparative Example 2) and 550Ω / □ (Example 2), respectively.

  Next, when the dispersions of Comparative Example 2 and Example 2 were evaluated for stability against a high shear force using a rheometer in the same manner as in Example 1, the dispersion of Example 2 showed no aggregation. In the dispersion of Comparative Example 2, agglomeration was observed so as to be visually discernable.

(Example 3)
(Molecular weight change of carboxymethylcellulose sodium salt)
A carbon nanotube dispersion was prepared in the same manner as in Example 2 except that the carboxymethylcellulose sodium salt in Example 2 was replaced with one having a number average molecular weight of 68,000 (weight average molecular weight of 200,000). Next, when a transparent conductive film was produced using these dispersions in the same manner as in Example 2, the surface resistance value at a transmittance of 90% of the obtained film was 600Ω / □. Moreover, when the stability with respect to high shear force was evaluated using the rheometer similarly to Example 1, it turned out that aggregation is not seen but it is stable.

Example 4
(Molecular weight change of carboxymethylcellulose sodium salt)
A carbon nanotube dispersion was prepared in the same manner as in Example 2, except that the carboxymethylcellulose sodium salt of Example 2 was replaced with one having a number average molecular weight of 27,000 (weight average molecular weight of 90,000). Next, when a transparent conductive film was produced using these dispersions in the same manner as in Example 2, the surface resistance value of the obtained film at a transmittance of 90% was 480Ω / □. Moreover, when the stability with respect to high shear force was evaluated using the rheometer similarly to Example 1, it turned out that aggregation is not seen but it is stable.

(Comparative Example 3)
(Molecular weight change of carboxymethylcellulose sodium salt)
A carbon nanotube dispersion was prepared in the same manner as in Example 2 except that the carboxymethyl cellulose sodium salt in Example 2 was replaced with one having a number average molecular weight of 170,000 (weight average molecular weight 450,000). Next, when a transparent conductive film was produced using these dispersions in the same manner as in Example 2, the surface resistance value at 90% transmittance of the obtained film was 6000Ω / □.

(Comparative Example 4)
(Change of dispersant pH)
In a 20 mL container, 15 mg of the carbon nanotubes obtained in Reference Example 1 (converted when dried), polystyrene sulfonate ammonium aqueous solution (manufactured by Aldrich, 30% by weight, number average molecular weight 110,000 (weight average molecular weight 200,000)) 300 mg (polystyrene) The amount of ammonium sulfonate was 90 mg, and the dispersant / carbon nanotube weight ratio was 6), and distilled water was added to make 10 g. The pH at this time was 3.8. Thereafter, a carbon nanotube dispersion was produced in the same manner as in Example 1. Next, when a transparent conductive film was produced using these dispersions in the same manner as in Example 2, the surface resistance value at a transmittance of 90% of the obtained film was 800Ω / □. Moreover, when the stability with respect to high shear force was evaluated using the rheometer similarly to Example 1, the aggregation which can be discriminate | determined visually was observed.

Claims (15)

A dispersion of carbon nanotubes containing 250 to 2000 parts by weight of a dispersant having a number average molecular weight of 10,000 to 150,000 and an aqueous solvent with respect to 100 parts by weight of the carbon nanotubes, wherein the pH of the dispersion is 8 to A carbon nanotube dispersion characterized by being 12. The carbon nanotube dispersion liquid according to claim 1, wherein the dispersant is an ionic dispersant. The carbon nanotube dispersion liquid according to claim 2, wherein the dispersant is a polysaccharide. The carbon nanotube dispersion liquid according to claim 3, wherein the dispersant is carboxymethyl cellulose or a salt thereof. The carbon nanotube dispersion liquid according to any one of claims 1 to 4, wherein the carbon nanotubes are mainly double-walled carbon nanotubes. The carbon nanotube dispersion liquid according to claim 1, wherein the dispersant has a molecular weight of 30,000 to 250,000. The carbon nanotube dispersion liquid according to any one of claims 1 to 6, wherein a ratio of the dispersing agent is 350 parts by weight or more and 1000 parts by weight or less with respect to 100 parts by weight of the carbon nanotubes. A dispersion containing 250 to 2000 parts by weight of a dispersant having a number average molecular weight of 10,000 to 150,000 and an aqueous solvent is sonicated with respect to 100 parts by weight of the carbon nanotube, and a dispersion having a pH of 8 to 12 is obtained. A method for producing a carbon nanotube dispersion obtained. The electroconductive film obtained by apply | coating the carbon nanotube dispersion liquid in any one of Claims 1-7 to a base material. The conductive film according to claim 9, which has a light transmittance of 70% or more and a surface resistance value of 10 ⁰ to 10 ⁴ Ω / □. The conductive film according to claim 10, which has a light transmittance of 80% or more and a surface resistance value of 10 ¹ to 10 ³ Ω / □. The manufacturing method of the electroconductive film which apply | coats the carbon nanotube dispersion liquid in any one of Claims 1-7 on a base material. The manufacturing method of the electroconductive film of Claim 12 which removes an excess dispersing agent after apply | coating a carbon nanotube dispersion liquid to a base material. The method for producing a conductive film according to claim 13, wherein the excessive dispersant is removed by washing the conductive film. The method for producing a conductive film according to claim 13, wherein the excessive dispersant is removed by adsorption to the dispersant adsorption layer.