Manufacturing method for carbon nanotube dispersion

Abstract

To provide a manufacturing method for a carbon nanotube dispersion that can improve the dispersibility of carbon nanotubes.SOLUTION: The manufacturing method disclosed herein includes: a preparation step S1 of preparing a predetermined amount of carbon nanotubes and a solvent; and a mixing step S2 of dividing the predetermined amount of carbon nanotubes into at least three portions, adding each portion into the solvent, and mixing each time the portion is added.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for producing a carbon nanotube dispersion liquid. [Background technology]

[0002] Conventionally, a CNT dispersion liquid obtained by dispersing carbon nanotubes (CNTs) in a specific solvent has been widely used as, for example, a conductive assistant for secondary batteries. Related prior art documents include Patent Documents 1 and 2. For example, Patent Document 1 describes the preparation of a CNT dispersion liquid by weighing a specific amount of CNTs, adding a solvent (e.g., toluene), and dispersing the CNTs with a dispersing device (e.g., an ultrasonic homogenizer). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2022 / 075387 [Patent Document 2] JP 2019-81676 A Summary of the Invention [Problem to be solved by the invention]

[0004] According to the study by the present inventors, for example, depending on the type and properties of CNTs, when a certain amount of CNTs is put into a solvent all at once, the viscosity may increase excessively, making the dispersion process difficult, or aggregates may form in the solvent, making it difficult to disperse uniformly. This tendency is particularly strong for CNTs with long fiber lengths, which tend to be easily entangled and aggregated. If the dispersion becomes uneven, the performance of the product (e.g., secondary battery) may vary, or the resistance may increase, causing the performance of the product to deteriorate.

[0005] The present invention has been made in view of the above-mentioned circumstances, and a main object of the present invention is to provide a method for producing a carbon nanotube dispersion liquid capable of improving the dispersibility of carbon nanotubes. [Means for solving the problem]

[0006] The present invention provides a method for producing a carbon nanotube dispersion liquid, the method including: a preparation step of preparing a predetermined amount of carbon nanotubes and a solvent; and a mixing step of adding the predetermined amount of carbon nanotubes to the solvent in at least three separate additions and mixing each addition.

[0007] According to the manufacturing method of the present invention, the viscosity of the mixture is prevented from increasing too much compared to the case where a predetermined amount of carbon nanotubes is mixed in a solvent at once, and the dispersion process is facilitated. In addition, the generation of aggregates is suppressed, and the dispersibility of carbon nanotubes is improved. As a result, a homogeneous carbon nanotube dispersion can be stably prepared. Effect of the Invention

[0008] According to the present invention, it is possible to provide a method for producing a carbon nanotube dispersion liquid capable of improving the dispersibility of carbon nanotubes. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a flow chart illustrating a manufacturing method according to one embodiment. [Diagram 2] FIG. 2 is a graph showing the change in viscosity of the mixed liquids according to Examples 1 to 4. [Diagram 3] FIG. 3 is a graph showing the change in viscosity of the mixed liquids according to Comparative Examples 1 and 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

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

[0011] <Method for producing carbon nanotube (CNT) dispersion liquid> FIG. 1 is a flow chart showing a method for producing a carbon nanotube (hereinafter, sometimes simply referred to as "CNT") dispersion according to one embodiment. As shown in FIG. 1, the production method according to this embodiment includes a preparation step S1 and a mixing step S2. The mixing step S2 is a step in which CNTs are added in three steps, and includes first to third mixing steps S21 to S23. The production method according to this embodiment can be particularly suitably adopted when producing a CNT dispersion having a CNT concentration of about 0.1 to 3 mass%, for example, 0.5 to 1 mass%. In addition, as will be described in detail later, when producing a CNT dispersion having a CNT concentration of more than about 3 mass%, for example, it is preferable to have the mixing step S2 include more stages.

[0012] The preparation step S1 is a step of preparing at least a predetermined amount of (A) CNT and (C) solvent. In this embodiment, it is a step of preparing a predetermined amount of (A) CNT, a predetermined amount of (B) dispersant, and a predetermined amount of (C) solvent. In this step, other materials, such as (D) additives, may be further prepared as necessary.

[0013] (A) CNT is a fibrous carbon having a structure in which graphite forming a carbon hexagonal network is rolled into a cylindrical shape. There is no particular limitation on the CNT, and one or more types of conventionally known CNTs can be appropriately used. The 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. The CNT may contain impurities (e.g., catalysts and amorphous carbon) derived from, for example, the manufacturing process.

[0014] The average diameter of the CNTs is not particularly limited, but is preferably 5 nm or more, more preferably 8 nm or more, and even more preferably 10 nm or more. When the average diameter of the CNTs is a predetermined value or more, the aggregation of the CNTs is suppressed, and the dispersibility can be improved. In addition, the average diameter of the CNTs is preferably 100 nm or less, more preferably 50 nm or less, and even more preferably 20 nm or less. When the average diameter of the CNTs is a predetermined value or less, the number of CNTs per unit mass increases, and a conductive network can be efficiently formed. Therefore, the conductivity can be improved. The average diameter of the CNTs can be calculated by observing multiple CNTs with a transmission electron microscope, measuring the length of the short axis, and taking the number average value.

[0015] The average fiber length of the CNT is not particularly limited, but is preferably 0.1 μm or more, more preferably 1 μm or more, and further preferably, for example, 5 μm or more, 10 μm or more. When the average fiber length is a predetermined value or more, it becomes easy to effectively form a conductive network, and the conductivity can be improved. The average fiber length of the CNT may be, for example, 100 μm or more, or even 200 μm or more. When the average fiber length is long like this, the CNTs tend to be entangled and aggregate in the solvent. Therefore, it is particularly effective to apply the technology disclosed herein. In addition, the average fiber length of the CNT may be, for example, 500 μm or less, 300 μm or less, or 250 μm or less. When the average fiber length is a predetermined value or less, the CNTs are less likely to be entangled, and a good conductive network is easily formed. The average fiber length of the CNTs can be calculated by observing a plurality of CNTs with a transmission electron microscope, measuring the length of the major axis, and taking the number average value.

[0016] The aspect ratio of the CNTs (average fiber length / average diameter) is not particularly limited, but is preferably 100 to 100,000, more preferably 1,000 to 50,000, and even more preferably 5,000 to 25,000. When the aspect ratio is within the above range, it is possible to achieve both high levels of dispersibility and electrical conductivity.

[0017] CNTs have a Raman spectrum measured by laser Raman spectroscopy using a semiconductor laser, which shows a peak at 1350 cm -1 Intensity of the D band appearing near I D 1580cm -1 G-band intensity appearing near I G The ratio (I G / I D ) is preferably about 10 or less, for example, more preferably 1 to 8, and in some embodiments, even more preferably 1 to 2. 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 above ratio (I G / I D The larger the ratio (I G / I D When the ratio (R) is equal to or less than a predetermined value, aggregation of CNTs is suppressed, and dispersibility can be improved.

[0018] The mass of CNTs prepared in this step (in other words, contained in the CNT dispersion) is a design item that should be appropriately changed depending on, for example, the application of the CNT dispersion, the type and amount of the solvent used, etc. In one example, it is preferable to prepare an amount of CNTs such that the CNT concentration in the final CNT dispersion is about 0.1 to 3 mass %, for example 0.5 to 1 mass %.

[0019] (B) The dispersant is a component for stabilizing the dispersion of CNTs. The dispersant is not particularly limited, and one or more conventionally known dispersants can be appropriately used depending on, for example, the purpose of the CNT dispersion liquid and the type of solvent. The dispersant may be a surfactant type consisting of a hydrophilic group and a hydrophobic group, or may be a polymer type (resin type). The dispersant is preferably one that is soluble or dispersible in the solvent used, and more preferably one that is soluble in the solvent used. In this specification, "soluble" means that the solubility in the solvent used is 1% by mass or more in an environment of 25°C.

[0020] The surfactant-type dispersant may be anionic, cationic, nonionic, or amphoteric. Examples of polymer-type (resin-type) dispersants include cellulose derivatives, polyvinyl alcohol, polyvinyl butyral, polyvinylpyrrolidone, polyacrylonitrile, poly(ethylene maleic anhydride), and poly(styrene maleic anhydride).

[0021] Among them, when an aqueous solvent is used as the solvent, a water-soluble polymer is preferred as the dispersant, and in particular, water-soluble cellulose derivatives, polyvinyl alcohol (PVA), polyvinyl butyral (PVB), etc. are preferred. Examples of water-soluble cellulose derivatives include cellulose such as carboxymethyl cellulose (CMC), methyl cellulose (MC), ethyl cellulose (EC), hydroxyethyl cellulose (HEC), hydroxypropyl methyl cellulose (HPMC), etc., and salts thereof. Examples of the salts include ammonium salts, and alkali metal salts such as lithium salts and sodium salts. Among them, CMC and its salts are preferred, and ammonium salts of CMC are particularly preferred because of their large viscosity reducing effect.

[0022] In addition, when a non-aqueous solvent (organic solvent) is used as the solvent, the dispersant is preferably a polymer containing a maleic anhydride moiety, polyvinylpyrrolidone (PVP), polyacrylonitrile, etc. Examples of polymers containing a maleic anhydride moiety include poly(ethylene maleic anhydride), poly(isobutylene maleic anhydride), poly(styrene maleic anhydride), poly(methyl vinyl ether maleic anhydride), and derivatives thereof. Among them, poly(styrene maleic anhydride) is preferred.

[0023] The weight-average molecular weight of the dispersant is not particularly limited, but is preferably 1000 to 1,000,000, more preferably 10,000 to 700,000, and even more preferably 30,000 to 500,000. If the weight-average molecular weight is within the above range, it becomes easier to entangle with the CNT in the mixing step S2 described below, and the dispersibility of the CNT can be improved to a high level. Furthermore, it is possible to suppress the aggregation of the CNT in the dispersion liquid, so that it is easier to exhibit excellent conductivity. The weight-average molecular weight of the dispersant can be calculated by comparing the measured value by gel permeation chromatography (GPC) with a calibration curve by polyethylene glycol.

[0024] The viscosity of the dispersant is not particularly limited, but the viscosity of a 1% by mass aqueous solution measured at 25°C with a B-type viscometer at a rotor speed of 60 rpm is preferably about 1000 mPa·s or less, for example, 5 to 500 mPa·s, more preferably 10 to 100 mPa·s, and even more preferably 20 to 50 mPa·s. This makes it easier to obtain a uniform CNT dispersion liquid by suppressing viscosity increase. In addition, the handleability and workability of the obtained CNT dispersion liquid can be improved.

[0025] The mass of the dispersant prepared in this step (in other words, contained in the CNT dispersion) is not particularly limited because it may vary depending on, for example, the amount and properties of the CNTs used, the type of solvent used, the properties of the dispersant, etc. It is preferable that the amount of the dispersant is an amount that does not inhibit the conductivity of the CNTs. As an example, it is preferable that the amount of the dispersant is equal to or less than the amount of the CNTs. In the technology disclosed herein, even if a relatively small amount of dispersant is used, the CNTs can be highly dispersed in the solvent, so that a homogeneous CNT dispersion can be prepared.

[0026] The (C) solvent is a dispersion medium for dispersing at least CNT. The solvent is not particularly limited, and one or more of conventionally known solvents can be appropriately used depending on the application of the CNT dispersion liquid. The solvent may be an aqueous solvent containing water, or a non-aqueous solvent made of an organic solvent. The aqueous solvent is typically water, but may be a mixed solvent mainly made of water (a component that accounts for 50% by mass or more). As the solvent other than water that constitutes the mixed solvent, an organic solvent that can be uniformly mixed with water, such as alcohol, ether, ketone, etc., can be used. As the water, from the viewpoint of preventing the inclusion of impurities, ion-exchanged water, distilled water, ultrafiltered water, reverse osmosis water, etc. are preferable. The aqueous solvent is preferably 80% by mass or more of water, more preferably 95% by mass or more of water, and even more preferably substantially made of water (98% by mass or more of water).

[0027] Examples of non-aqueous solvents include aprotic polar solvents such as N-methyl-2-pyrrolidone (NMP), alcohol solvents such as ethanol and methanol, ketone solvents such as acetone and methyl ethyl ketone, ester solvents such as methyl acetate, ethyl acetate, butyl acetate, methyl butyrate, ethyl butyrate, and butyl butyrate, and hydrocarbon solvents such as toluene, xylene, cyclohexane, and heptane. Among these, NMP is preferred.

[0028] As the (D) additive, for example, for the purpose of improving various properties of the CNT dispersion liquid, one or more of those known to be usable for this type of application can be appropriately used. Specific examples of the additive include organic additives such as antioxidants, defoamers, preservatives, plasticizers, and colorants (pigments, dyes, etc.), and (non-fibrous) carbon materials other than CNTs, such as carbon black and graphite.

[0029] In the mixing step S2, the CNTs prepared in the preparation step S1 are added to the solvent in at least three separate portions, and the CNTs are mixed each time they are added. In the embodiment of FIG. 1, the mixing step S2 adds a predetermined amount of CNTs in three stages (three separate portions), and includes a first mixing step S21, a second mixing step S22, and a third mixing step S23. In addition, when producing a CNT dispersion liquid with a high CNT concentration (for example, a CNT concentration exceeding 3 mass%), it is preferable to further include a fourth mixing step, a fifth mixing step, and the like. The number of times T of divided addition is 3 here. The number of times T of divided addition is not particularly limited, but is generally 100 or less, preferably 50 or less, for example 20 or less. By setting the number of times T to a predetermined value or less, the time required for this step can be shortened, and workability and productivity can be improved. In addition, it is easy to achieve excellent conductivity by suppressing the shortening of the CNTs.

[0030] For mixing, a conventionally known dispersion device such as a disperser, planetary mixer, kneader, propeller stirrer, ultrasonic homogenizer, magnetic stirrer, roll mill, jet mill, ball mill, bead mill, sand mill, etc. can be appropriately used. Among them, a device that does not use a medium (media-less) is preferable from the viewpoint of reducing contamination of the medium. In addition, a circulation type dispersion device is preferable because dispersion is likely to proceed uniformly. In addition, a device that utilizes the shear force of high-speed stirring is preferable because it can shorten the processing time.

[0031] An example of a device that utilizes the shear force of high-speed stirring is a high-pressure homogenizer. Among them, a configuration having a pair of nozzles and a chamber located between the pair of nozzles is preferable. In a high-pressure homogenizer having such a configuration, the mixed liquids (slurries) pressurized by a pressure booster are discharged from a pair of nozzles at high speed, respectively, and collide obliquely at the center of the chamber. Although not particularly limited, in some embodiments, the pressure when using the high-pressure homogenizer is preferably 50 to 200 MPa, more preferably 100 to 150 MPa. In addition, in the case of a configuration having the above nozzle, the diameter of the nozzle is, for example, preferably 0.05 to 0.5 mm, more preferably 0.1 to 0.2 mm.

[0032] The temperature environment in this step is preferably 10° C. or higher, and more preferably 20° C. or higher, from the viewpoint of, for example, increasing the solubility of the dispersant in the solvent, etc. On the other hand, from the viewpoint of, for example, suppressing the evaporation of the solvent, a temperature below the boiling point of the solvent, for example, 100° C. or lower, preferably 50° C. or lower, more preferably 35° C. or lower, is preferred.

[0033] The first mixing step S21 is a step of dispersing a first amount M1 of CNTs among the CNTs prepared in the preparation step S1 in a solvent to obtain a first mixed solution. In this embodiment, for example, in a stirring tank of a dispersion device, the first amount M1 of (A) CNTs, the (B) dispersant, and the (C) solvent are charged and mixed. Although not particularly limited, in this step, it is preferable to charge the entire amount of the (B) dispersant and the (C) solvent prepared in the preparation step. Note that in the embodiment of FIG. 1, the (B) dispersant is first charged into the (C) solvent, dissolved in the solvent, and then the (A) CNTs are charged, but the order of charging is not particularly limited. For example, the (A) CNTs and the (B) dispersant may be mixed first to form a mixture, and then charged into the (C) solvent.

[0034] In this step, it is preferable to select multiple types of devices from among the conventionally known dispersing devices as described above, and use them in combination to obtain the first mixed liquid. For example, from the viewpoint of promoting wetting of CNTs (compatibility with the solvent) and dissolving coarse particles and agglomerations of CNTs, it is more preferable to first prepare a preliminary mixed liquid using a propeller stirrer in the initial stage of dispersion, and then obtain the first mixed liquid using a high-pressure homogenizer.

[0035] Although not particularly limited, in preparing the preliminary mixture, the rotation speed of the propeller of the propeller stirrer is preferably 10 to 1000 rpm, more preferably 100 to 500 rpm. The preliminary mixture may be mixed until the D50 particle size (median diameter) as confirmed by, for example, a particle size distribution meter is about 1000 μm. This makes it easier to disperse the CNTs in the high-pressure homogenizer, and prevents, for example, the CNTs from clogging the nozzle. The conditions of the high-pressure homogenizer (for example, pressure and nozzle diameter) are as described above. Mixing with the high-pressure homogenizer is preferably performed until the first mixture becomes homogenous.

[0036] Although it is not intended to be particularly limited, according to the study by the present inventors, after the CNTs are added, the CNTs are defibrated and a three-dimensional structure is formed as the dispersion progresses. When the solvent molecules enter this three-dimensional structure, the mixture becomes gelled and the viscosity of the mixture increases rapidly. The viscosity of the mixture then reaches a maximum value (peak value). If the dispersion is continued from there, the CNTs become shorter and more isolated and dispersed, and the viscosity of the mixture decreases.

[0037] The peak value of the viscosity of the mixture in the first mixing step S21 is not particularly limited because it depends on the type of dispersion device used, but the viscosity measured by an E-type viscometer at a rotor speed of 1 rpm in an environment of 25°C is preferably not more than about 50,000 mPa·s, more preferably not more than 40,000 mPa·s, even more preferably not more than 30,000 mPa·s, and particularly preferably not more than 20,000 mPa·s, for example, 5,000 to 15,000 mPa·s. This results in a so-called thick kneaded state, and a large shear force can be applied to the mixture. In addition, the viscosity (particularly the peak value) of the mixture can be prevented from becoming too high in the second mixing step S22 to the third mixing step S23 described later.

[0038] The first amount M1 is the total amount M of CNTs prepared in the preparation step S1. all The first amount M1 is preferably an amount such that the peak viscosity measured after the CNTs are added using an E-type viscometer at a rotor speed of 1 rpm in an environment of 25°C does not exceed 50,000 mPa·s, more preferably an amount such that the peak viscosity does not exceed 40,000 mPa·s, even more preferably an amount such that the peak viscosity does not exceed 30,000 mPa·s, or even more preferably an amount such that the peak viscosity does not exceed 20,000 mPa·s. This prevents the viscosity of the mixture from becoming too high even when the viscosity reaches a peak value, and reduces the load on the dispersing device.

[0039] In some embodiments, it is preferable that the first amount M1 is larger than the amounts of CNTs added after the second time (here, the second amount M2 in the second mixing step S22 and the third amount M3 in the third mixing step S23) described below. That is, it is preferable to make the amount of CNTs added for the first time the largest (in this embodiment, M1, M2, and M3 satisfy M2 < M1 and M3 < M1). Thereby, in the mixing steps after the second time (in this embodiment, the second mixing step S22 to the third mixing step S23 described below), it is possible to suppress the viscosity of the mixed liquid (especially the peak value) from becoming too high.

[0040] In some embodiments, the first amount M1 is the total amount M of CNTs prepared in the preparation step S1 all and the number of divided additions T satisfy the following relationship: (M all / T) < M1; It is preferable to satisfy. That is, here, since the number of divided additions T is 3, it is preferable to satisfy (M all / 3) < M1. Thereby, the number of divided additions T can be reduced, and CNTs can be efficiently dispersed.

[0041] In some embodiments, the first amount M1 is the total amount M of CNTs prepared in the preparation step S1 all When taken as 100% by mass, it is preferably approximately 10% by mass or more, for example, 20% by mass or more, 30% by mass or more, 35% by mass or more of the total amount M all . By setting the first amount M1 to a predetermined value or more, the number of divided additions T can be reduced, and CNTs can be efficiently dispersed. The first amount M1 is the total amount M of CNTs all When taken as 100% by mass, it is preferably approximately 99% by mass or less, for example, 80% by mass or less, 70% by mass or less, 50% by mass or less. By setting the first amount M1 to a predetermined value or less, it is possible to suppress the viscosity of the first mixed liquid (especially the peak value) from becoming too high.

[0042] The first mixture is preferably mixed until the viscosity of the mixture after the CNTs are added exceeds the peak value and falls to a predetermined threshold value, and more preferably, the mixture is mixed until the viscosity of the mixture after the CNTs are added exceeds the peak value and falls to a predetermined threshold value, and more preferably, the mixture is mixed until the viscosity of the mixture after the CNTs are added exceeds the peak value and falls to a predetermined threshold value, and more preferably, the mixture is mixed until the viscosity of the mixture after the CNTs are added exceeds the peak value and falls to a predetermined threshold value, and more preferably, the mixture is mixed until the viscosity of the mixture after the CNTs are added exceeds the peak value and falls to a predetermined threshold value, and more preferably, the mixture is mixed until the viscosity of the mixture after the CNTs are added falls to a peak value and falls to a predetermined threshold value. This allows the CNTs to be added in a state where the three-dimensional structure of the previously added CNTs is sufficiently disintegrated, and therefore, the viscosity can be suitably prevented from increasing too much in the second and subsequent mixing steps (in this embodiment, the second mixing step S22 to the third mixing step S23 described later).

[0043] The value of n is not particularly limited as long as it is 2 or more, but is generally 50 or less, preferably 20 or less, more preferably 10 or less, and even more preferably 5 or less. By setting the value of n to a predetermined value or less, it is possible to suppress shortening of the CNTs and realize excellent conductivity. In addition, the time required for this process can be shortened, and workability and productivity can be improved.

[0044] In this case, the timing of the second and subsequent CNT additions (in other words, the timing of the start of the second mixing step S22 to the third mixing step S23) is determined based on the peak viscosity value, but is not limited to this. In other embodiments, the timing of the second and subsequent CNT additions may be, for example, after a predetermined time has elapsed based on the start point of this step.

[0045] The viscosity of the first mixed liquid is not particularly limited, but the viscosity measured by an E-type viscometer at a rotor speed of 1 rpm and 25° C. is preferably about 10,000 mPa·s or less, more preferably 8,000 mPa·s or less, and even more preferably, for example, 3,000 mPa·s or less. This makes it possible to prevent the viscosity (particularly the peak value) of the mixed liquid from becoming too high in the second mixing step S22 to the third mixing step S23 described below. Furthermore, from the viewpoint of improving the conductivity and productivity, the viscosity of the first mixed liquid is preferably about 100 mPa·s or more, more preferably 500 mPa·s or more, and even more preferably, for example, 1,000 mPa·s or more.

[0046] The second mixing step S22 is a step of obtaining a second mixed liquid by adding and dispersing a second amount M2 of CNTs among the CNTs prepared in the preparation step S1 into the first mixed liquid prepared as described above. In this step, it is preferable to obtain the second mixed liquid using a high-pressure homogenizer. The conditions of the high-pressure homogenizer (e.g., pressure and nozzle diameter) are as described above. The conditions of the high-pressure homogenizer may be the same as or different from those of the first mixing step S21.

[0047] Although not particularly limited because it depends on, for example, the type of dispersion device used, the peak value of the viscosity in the second mixing step S22 is typically larger than that in the first mixing step S21, and preferably does not exceed 40,000 mPa·s, more preferably does not exceed 30,000 mPa·s, and may be, for example, 10,000 to 25,000 mPa·s. This results in a so-called thick kneaded state, and a large shear force can be applied to the mixed liquid. In addition, it is possible to prevent the viscosity (particularly the peak value) of the mixed liquid from becoming too high in the third mixing step S23 described below.

[0048] The second amount M2 is the total amount M of CNTs prepared in the preparation step S1. allThe second amount M2 is an amount smaller than the first amount M1. As with the first amount M1, the second amount M2 is an amount such that the peak value of the viscosity measured at 1 rpm in an environment of 25°C after the CNTs are added does not exceed 50,000 mPa·s, more preferably does not exceed 40,000 mPa·s, and further preferably does not exceed 30,000 mPa·s. This makes it possible to prevent the viscosity of the mixture from becoming too high even when the viscosity reaches a peak value, and reduces the load on the dispersing device.

[0049] In some embodiments, the second amount M2 is preferably less than the first amount M1, which can prevent the viscosity (particularly the peak value) of the second mixture from becoming too high.

[0050] In some embodiments, the second amount M2 is equal to or less than the total amount M of CNTs. all When the total amount M is 100% by mass, all The second amount M2 is preferably about 10% by mass or more, for example, 20% by mass or more, 30% by mass or more. By setting the second amount M2 to a predetermined value or more, the number of divided additions T can be reduced, and the CNTs can be efficiently dispersed. The second amount M2 is the total amount M all is taken as 100 mass%, it is preferably about 50 mass% or less, for example 40 mass% or less, 30 mass% or less. By setting the second amount M2 to a predetermined value or less, it is possible to prevent the viscosity (particularly the peak value) of the second mixture from becoming too high.

[0051] In this embodiment, the (B) dispersant and (C) solvent prepared in the preparation step are all added in the first mixing step S21, so that the (B) dispersant and (C) solvent are not added in this step. However, in other embodiments, a part of the (B) dispersant or (C) solvent may be additionally added in this step, for example, so that the peak value of the viscosity falls within the above range.

[0052] The viscosity of the second mixed liquid is not particularly limited, but the viscosity measured by an E-type viscometer at a rotor speed of 1 rpm and 25° C. is typically higher than that of the first mixed liquid, and is preferably about 20,000 mPa·s or less, more preferably 10,000 mPa·s or less, and even more preferably, for example, 5,000 mPa·s or less. This makes it possible to prevent the viscosity (particularly the peak value) of the mixed liquid from becoming too high in the third mixing step S23 described below. Furthermore, from the viewpoint of improving the electrical conductivity and productivity, the viscosity of the second mixed liquid is preferably about 100 mPa·s or more, more preferably 500 mPa·s or more, and even more preferably, for example, 1,000 mPa·s or more.

[0053] The third mixing step S23 is a step of adding a third amount M3 of CNTs prepared in the preparation step S1 to the second mixed liquid prepared as described above and dispersing them to obtain a CNT dispersion liquid of a desired concentration. In this step, it is preferable to obtain the third mixed liquid using a high-pressure homogenizer. The conditions of the high-pressure homogenizer (e.g., pressure and nozzle diameter) are as described above. The conditions of the high-pressure homogenizer may be the same as or different from those of the first mixing step S21 and / or the second mixing step S22.

[0054] The third amount M3 is the total amount M of CNTs prepared in the preparation step S1. all The third amount M3 is now the total amount M all minus the first amount M1 and the second amount M2 (i.e., M3=M all -M1-M2).

[0055] The third amount M3, similar to the first amount M1 to the second amount M2, is preferably an amount such that the peak value of the viscosity measured at a rotor rotation speed of 1 rpm in an environment of 25°C does not exceed 50,000 mPa·s after introducing CNTs, and more preferably does not exceed 40,000 mPa·s. Thereby, even when the viscosity reaches the peak value, it is possible to suppress the viscosity of the mixed liquid from becoming too high and reduce the load on the dispersion device. Also, although not particularly limited because it depends on, for example, the type of the dispersion device used, the peak value of the viscosity in the third mixing step S23 is typically larger than that in the second mixing step S22 and preferably does not exceed 50,000 mPa·s, more preferably does not exceed 45,000 mPa·s, and may be, for example, 15,000 to 40,000 mPa·s. Thereby, a so-called solidified state is achieved, and a large shearing force can be applied to the mixed liquid.

[0056] In some embodiments, the third amount M3 is preferably less than the first amount M1 (M3 < M1). Thereby, it is possible to suppress the viscosity (especially the peak value) of the CNT dispersion liquid from becoming too high.

[0057] In some embodiments, the third amount M3 may be the same as the second amount M2 (M3 = M2), or may be less than the second amount M2 (M3 < M2). That is, after the second time, it is preferable to introduce CNTs such that the input amount is equal or gradually decreases according to the number of divided inputs T. Thereby, in the mixing steps after the third time (in this embodiment, the third mixing step S23), it is possible to suppress the viscosity (especially the peak value) of the mixed liquid from becoming too high.

[0058] In some embodiments, the third amount M3 is the total amount M of CNTs all When taken as 100% by mass, it is preferably approximately 10% by mass or more, for example, 20% by mass or more, 30% by mass or more of the total amount M all By setting the third amount M3 to a predetermined value or more, the number of divided inputs T can be reduced, and CNTs can be efficiently dispersed. The third amount M3 is the total amount M of CNTs allWhen taking it as 100% by mass, it is preferably approximately 50% by mass or less, for example 40% by mass or less, 30% by mass or less. By making the third amount M3 not exceed a predetermined value, it is possible to suppress the viscosity (especially the peak value) of the CNT dispersion liquid from becoming too high.

[0059] In this embodiment, since the (B) dispersant and the (C) solvent prepared in the preparation step are all added in the first mixing step S21, the (B) dispersant and the (C) solvent are not added in this step. However, in other embodiments, for example, in order for the peak value of the viscosity to fall within the above range, a part of the (B) dispersant or the (C) solvent may be additionally added in this step. Also, when adding the (D) additive, it is preferably added after adding all of the CNTs.

[0060] <CNT dispersion liquid> The CNT dispersion liquid can be used for various applications. For example, in the application of preparing electrodes (positive electrodes and / or negative electrodes) of secondary batteries, at least (A) CNTs can be made into a dispersion liquid dispersed in water or NMP as the (C) solvent. When taking the total solid content as 100% by mass, the CNTs preferably account for approximately 50% by mass or more, preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more of the CNT dispersion liquid, and it is particularly preferable that it consists substantially of CNTs (98% by mass or more of the total solid content is CNTs). In such a case, it is particularly effective to apply the technology disclosed herein. Here, the "solid content" refers to the solid component separated (for example, filtered) from the CNT dispersion liquid by solid-liquid separation under the environment of 25°C. In other words, the solid content is the component dispersed or precipitated in the solvent under the environment of 25°C and does not include the component dissolved in the solvent.

[0061] The viscosity of the CNT dispersion is not particularly limited, but the viscosity measured with an E-type viscometer at a rotor speed of 1 rpm and 25°C is typically higher than the viscosity of the second mixture, and is preferably about 30,000 mPa·s or less, more preferably 15,000 mPa·s or less, and even more preferably, for example, 10,000 mPa·s or less. The viscosity of the CNT dispersion is preferably about 100 mPa·s or more, more preferably 500 mPa·s or more, and even more preferably, for example, 2,000 mPa·s or more. This can improve workability and handling.

[0062] In some embodiments, when the entire CNT dispersion is taken as 100% by mass, the ratio of (A) CNT is preferably about 0.1% by mass or more, more preferably 0.5% by mass or more, and even more preferably 1% by mass or more. According to the technology disclosed herein, even a high-concentration CNT dispersion in which the ratio of CNT is 1% by mass or more can be suitably produced. Furthermore, when the entire CNT dispersion is taken as 100% by mass, the ratio of (A) CNT is preferably 20% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less, from the viewpoint of dispersion stability.

[0063] When the CNT dispersion contains the dispersant (B), the proportion of the dispersant (B) is preferably about 0.1% by mass or more, more preferably 0.5% by mass or more, and even more preferably 1% by mass or more, when the entire CNT dispersion is taken as 100% by mass, from the viewpoint of improving the dispersibility and dispersion stability of the CNTs. Also, the proportion of the dispersant (B) is preferably 20% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less, when the entire CNT dispersion is taken as 100% by mass, from the viewpoint of improving the electrical conductivity.

[0064] In some embodiments, the proportion of the dispersant (B) is preferably 10 parts by mass or more, more preferably 50 parts by mass or more, and more preferably 100 parts by mass or more, from the viewpoint of improving the dispersibility and dispersion stability of the CNTs, when the CNTs (A) are taken as 100 parts by mass. Also, the proportion of the dispersant (B) is preferably 500 parts by mass or less, more preferably 200 parts by mass or less, and even more preferably 150 parts by mass or less, from the viewpoint of improving the electrical conductivity, when the CNTs (A) are taken as 100 parts by mass.

[0065] When the CNT dispersion contains the (D) additive, when the entire CNT dispersion is taken as 100 mass%, the proportion of the (D) additive is preferably 10 mass% or less, more preferably 5 mass% or less, even more preferably 3 mass% or less, particularly preferably 2 mass% or less, and 1 mass% or less.

[0066] As described above, the manufacturing method of this embodiment includes a preparation step S1 of preparing a predetermined amount of carbon nanotubes (CNTs) and a solvent, and a mixing step S2 of adding the predetermined amount of carbon nanotubes to the solvent in at least three separate times and mixing each time. This makes it possible to prevent the viscosity of the mixture from increasing too much, and to facilitate dispersion processing, compared to the case where a predetermined amount of CNTs is added to the solvent all at once and mixed. In addition, the generation of aggregates is suppressed, and the dispersibility of CNTs can be improved. As a result, a homogeneous CNT dispersion can be stably prepared. Furthermore, a dispersion with a high CNT concentration can be suitably prepared, and the CNT concentration in the dispersion can be improved.

[0067] In the manufacturing method of this embodiment, the amount of carbon nanotubes added in the first mixing step S2 is the largest, which makes it possible to prevent the viscosity (especially the peak value) of the mixture from becoming too high in the second and subsequent mixing steps S22 and S23.

[0068] In the manufacturing method of the present embodiment, in the mixing step S2, the carbon nanotubes are added in an even or gradually decreasing amount depending on the number of divided additions from the second time onwards. This makes it possible to prevent the viscosity (especially the peak value) of the mixture from becoming too high in the mixing steps S22 and S23 from the second time onwards.

[0069] In the manufacturing method of this embodiment, in the mixing step S2, the amount of carbon nanotubes added each time is set so that the peak viscosity value measured with an E-type viscometer at a rotor speed of 1 rpm in an environment of 25° C. does not exceed 50,000 mPa s after one addition of the carbon nanotubes. This makes it possible to prevent the viscosity of the mixture from becoming too high even when the viscosity reaches the peak value, and reduces the load on the dispersing device.

[0070] In the manufacturing method of this embodiment, in the mixing step S2, the second or subsequent addition of the carbon nanotubes is performed when the viscosity after the previous addition of the carbon nanotubes has decreased to 1 / n (an integer 2≦n) of the peak value. This allows the carbon nanotubes to be added in a state in which the three-dimensional structures of the previously added carbon nanotubes are sufficiently disintegrated, making it possible to suitably prevent the viscosity from increasing too much in the second or subsequent mixing steps S22 and S23.

[0071] In the manufacturing method of this embodiment, the above n is 2 or more and 5 or less. This makes it possible to suppress shortening of the CNTs and achieve excellent electrical conductivity. In addition, the time required for this step can be shortened, improving workability and productivity.

[0072] In the manufacturing method of the present embodiment, the amount of the carbon nanotubes added in the first mixing step S2 is 50% by mass or less of the predetermined amount, which makes it possible to prevent the viscosity (especially the peak value) of the first mixture from becoming too high.

[0073] In the manufacturing method of the present embodiment, in the mixing step S2, when the carbon nanotubes are first introduced, the entire amount of the solvent prepared in the preparation step S1 is added. Thereby, workability and productivity can be improved.

[0074] In the manufacturing method of the present embodiment, in the preparation step S1, a dispersant soluble in the solvent is further prepared, and in the mixing step S2, when the carbon nanotubes are first introduced, the entire amount of the dispersant prepared in the preparation step S1 is added. Thereby, the dispersion stability of the carbon nanotube dispersion can be improved. Also, workability and productivity can be improved.

[0075] In the manufacturing method of the present embodiment, in the preparation step S1, carbon nanotubes having an average fiber length of 100 μm or more are prepared. Thereby, it becomes easier to effectively form a conductive network and the conductivity can be improved. Also, when the average fiber length of the carbon nanotubes is long, the carbon nanotubes are likely to be entangled and aggregated in the solvent. Therefore, it is particularly effective to apply the technology disclosed herein.

[0076] In the manufacturing method of the present embodiment, in the carbon nanotube dispersion, when the total solid content is 100% by mass, the carbon nanotubes occupy 50% by mass or more. In such a case, it is particularly effective to apply the technology disclosed herein.

[0077] Hereinafter, examples of the present invention will be described, but the present invention is not intended to be limited to those shown in such examples.

[0078] <Preparation of CNT Dispersion> In Examples 1 to 4, a predetermined amount of CNT (average diameter 10 nm, average fiber length 250 μm), CMC (CMC Daicel Ammonium DN10L manufactured by Daicel Miraize) as a dispersant, and water as a solvent were prepared, and a predetermined amount of CNT was divided into three portions and put into the solvent to prepare a CNT dispersion liquid with 1.0 mass % of CNT. FIG. 2 is a graph showing the transition of viscosity in Examples 1 to 4. In FIG. 2, the number of passes on the horizontal axis represents the number of times the mixed liquid was circulated in the circulation type dispersing device. In each example, an asterisk is attached to the location where CNT was additionally added, and an arrow is attached to the peak value of the viscosity (peak top viscosity) in each of the first to third mixing steps.

[0079] Example 1 In Example 1, in the second and third mixing steps, CNTs were added after the viscosity of the mixed liquid became half of the peak value. Specifically, first, CNTs, CMC as a dispersant, and water as a solvent were added to a polypropylene container in a ratio of 0.4 mass% CNT, 1.0 mass% CMC, and 98.6 mass% water, and the mixture was stirred at a rotation speed of 340 rpm for 30 minutes using a propeller stirrer to prepare a preliminary mixed liquid. As a result, the D50 particle size (median diameter) of the CNTs in the preliminary mixed liquid was reduced to about 1000 μm.

[0080] Next, the preliminary mixed liquid was put into the raw material tank of a high-pressure homogenizer (Starburst HJP-25001V2, manufactured by Sugino Machine), and the mixed liquid was subjected to a circulation type dispersion process. The dispersion process was carried out using a single nozzle chamber, with a nozzle diameter of 0.15 mm and a pressure of 150 MPa. At this time, a viscometer (TV-200E, manufactured by Toki Sangyo) was used to monitor the viscosity of the mixed liquid during the dispersion process, and the dispersion process was continued until the viscosity of the mixed liquid measured at a rotation speed of 1 rpm was half the peak value (6500 mPa s in this case) (first mixing process). This resulted in the first mixed liquid being obtained.

[0081] Next, 0.3 mass% of CNTs were added to the first mixed liquid in the raw material tank, and the circulation-type dispersion process was carried out again using the high-pressure homogenizer. At this time, the viscosity of the mixed liquid was measured using an E-type viscometer, and the dispersion process was continued until the viscosity of the mixed liquid was half the peak value after the additional CNTs were added (11,070 mPa s in this case) (second mixing process). This produced a second mixed liquid.

[0082] Next, 0.3 mass% of CNTs was further added to the second mixed liquid in the raw material tank, and a circulation-type dispersion process was again performed using the high-pressure homogenizer. At this time, the viscosity of the mixed liquid was measured using an E-type viscometer, and the dispersion process was continued until the viscosity of the mixed liquid was half the peak value after the second additional CNT addition (here, 13030 mPa s) (third mixing step). In this manner, a CNT dispersion liquid containing 1.0 mass% CNTs (Example 1) was obtained.

[0083] Example 2 In Example 2, a CNT dispersion liquid (Example 2) was obtained in the same manner as Example 1, except that the timing of adding additional CNTs and ending the dispersion process in the circulation-type dispersion process were changed to one-fifth of the peak viscosity value.

[0084] Example 3 In Example 3, a CNT dispersion liquid (Example 3) was obtained in the same manner as Example 1, except that the timing of adding additional CNTs and ending the dispersion process in the circulation-type dispersion process were changed to one-tenth of the peak viscosity value.

[0085] Example 4 In Example 4, a CNT dispersion liquid (Example 4) was obtained in the same manner as in Example 1, except that the timing of adding additional CNTs and ending the dispersion process in the circulation-type dispersion process were changed to 1 / 30 of the peak viscosity value.

[0086] Comparative Example 1 In Comparative Example 1, a predetermined amount of CNT was all put in at once to prepare a CNT dispersion liquid with 1.0 mass% CNT. Specifically, first, CNT, CMC (dispersant), and water (solvent) were put into a polypropylene container in a ratio of 1.0 mass% CNT, 1.0 mass% CMC, and 98.0 mass% water, and a preliminary mixture liquid was prepared in the same manner as in Example 1. Next, the preliminary mixture liquid was put into the raw material tank of a high-pressure homogenizer, and a circulation-type dispersion process was performed under the same conditions as in Example 1 (single nozzle chamber, nozzle diameter 0.15 mm, pressure 150 MPa). However, due to the effect of thickening, the viscosity of the mixture liquid exceeded the upper limit of the processing capacity of the dispersion device, making it impossible to process, and a CNT dispersion liquid could not be obtained. The viscosity of the mixture liquid at the stage where dispersion processing became impossible was 50,500 mPa·s.

[0087] Comparative Example 2 In Comparative Example 2, a predetermined amount of CNT was divided into two and put in, and an attempt was made to prepare a CNT dispersion liquid with 1.0 mass% CNT. Specifically, first, CNT, CMC (dispersant), and water (solvent) were put into a polypropylene container in a ratio of 0.5 mass% CNT, 1.0 mass% CMC, and 98.5 mass% water, and a preliminary mixture liquid was prepared in the same manner as in Example 1. Next, the preliminary mixture liquid was put into a raw material tank of a high-pressure homogenizer, and a circulation-type dispersion treatment was performed under the same conditions as in Example 1 (single nozzle chamber, nozzle diameter 0.15 mm, pressure 150 MPa). At this time, as in Example 2, the viscosity of the mixture liquid during the dispersion treatment process was monitored using a viscometer, and the dispersion treatment was performed until the viscosity of the mixture liquid measured at a rotation speed of 1 rpm became one-fifth of the peak value (first mixing step). As a result, a first mixture liquid was obtained.

[0088] Next, 0.5% by mass of CNTs was added to the first mixed liquid in the raw material tank, and the circulation-type dispersion process was carried out again using the high-pressure homogenizer. However, due to the effect of thickening, the viscosity of the mixed liquid exceeded the upper limit of the processing capacity of the dispersion device, making processing impossible, and a CNT dispersion liquid could not be obtained. The viscosity of the mixed liquid at the stage where dispersion processing became impossible was 50,380 mPa s.

[0089] <Evaluation of CNT Dispersibility> The CNT dispersions of Examples 1 to 4 were dropped onto a slide glass, covered with a cover glass, observed at 200 times magnification using an optical microscope (manufactured by Keyence, VHX-8000), and the dispersion state was evaluated according to the following criteria. The results are shown in Table 1. · "◎": No aggregates · "〇": Slight aggregates · "×": Many aggregates

[0090] <Evaluation of Surface Resistivity of CNT Film> The 1.0 mass% dispersions of Examples 1 to 4 were diluted with a 5 mass% aqueous CMC solution to prepare 0.08 mass%, 0.1 mass%, 0.12 mass%, 0.16 mass%, 0.2 mass%, and 0.4 mass%, respectively. 0.3 g of the diluted dispersion was collected onto a slide glass, uniformly coated over the entire surface of the slide glass using a film applicator (manufactured by Allgood, with film thickness adjustment function), and then dried by heating at 100 °C for 30 minutes to prepare a CNT film. Then, a copper foil was overlaid on the CNT film so that the distance was 30 mm and the width was 25 mm, and the surface resistance of the CNT film was measured using a resistance meter (manufactured by HIOKI, RM3548). From the obtained surface resistance values, the following formula (1): Surface resistivity (Ω / sq.) = Surface resistance value × (25 mm / 30 mm) Formula (1) was used to calculate the surface resistivity. The results are shown in Table 1 and Figure 4. In Table 1, the evaluation results are shown according to the following criteria. · "◎": The surface resistivity at a CNT concentration of 0.16 mass% is 10000 Ω / sq. or less. · "〇": The surface resistivity at a CNT concentration of 0.16 mass% exceeds 10000 Ω / sq. and is 50000 Ω / sq. or less. · "×": The surface resistivity at a CNT concentration of 0.16 mass% exceeds 50000 Ω / sq.

[0091]

Table 1

[0092] As shown in Table 1, the CNT dispersions of Examples 1 to 4 had fewer aggregates and were more dispersible than those of Comparative Examples 1 and 2. In particular, in Examples 2 to 4, the generation of aggregates was highly suppressed. The reason for this is thought to be that by dispersing the CNTs at a relatively low concentration, it was possible to suppress thickening of the mixed liquid while efficiently applying shear to the CNTs in the dispersing device, thereby defibrating the CNTs. These results demonstrate the significance of the technology disclosed herein.

[0093] In addition, as shown in Table 1 and Fig. 4, when the CNT dispersions of Examples 1 and 2 were used among Examples 1 to 4, the surface resistivity of the CNT coating film was kept low. The reason for this is considered to be that in Examples 1 and 2, the number of passes was kept relatively small, so that the CNT fiber length in the CNT dispersion could be kept relatively long, and as a result, a conductive path could be efficiently formed in the coating film. In contrast, in Examples 3 and 4, the number of passes was relatively large, so that the CNTs became shorter, and it was considered to be difficult to form a conductive path in the coating film.

[0094] Although the preferred embodiments of the present invention have been described above, the above-described embodiments are merely examples, and the present invention can be embodied in various other forms. [Explanation of symbols]

[0095] S1 Preparation process S2 mixing process S21 1st mixing process S22 2nd mixing process S23 3rd mixing process

Claims

[Claim 1] preparing a predetermined amount of carbon nanotubes and a solvent; a mixing step of adding the predetermined amount of carbon nanotubes to the solvent in at least three separate additions and mixing each addition; The method for producing a carbon nanotube dispersion liquid includes the steps of:

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