Classifications

• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B32/00—Carbon; Compounds thereof
  • C01B32/15—Nano-sized carbon materials
  • C01B32/158—Carbon nanotubes
  • C01B32/168—After-treatment
  • C01B32/174—Derivatisation; Solubilisation; Dispersion in solvents
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J3/00—Processes of treating or compounding macromolecular substances
  • C08J3/20—Compounding polymers with additives, e.g. colouring
  • C08J3/205—Compounding polymers with additives, e.g. colouring in the presence of a continuous liquid phase
  • C08J3/21—Compounding polymers with additives, e.g. colouring in the presence of a continuous liquid phase the polymer being premixed with a liquid phase
  • C08J3/215—Compounding polymers with additives, e.g. colouring in the presence of a continuous liquid phase the polymer being premixed with a liquid phase at least one additive being also premixed with a liquid phase
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08K—Use of inorganic or non-macromolecular organic substances as compounding ingredients
  • C08K3/00—Use of inorganic substances as compounding ingredients
  • C08K3/01—Use of inorganic substances as compounding ingredients characterized by their specific function
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08K—Use of inorganic or non-macromolecular organic substances as compounding ingredients
  • C08K3/00—Use of inorganic substances as compounding ingredients
  • C08K3/02—Elements
  • C08K3/04—Carbon
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08K—Use of inorganic or non-macromolecular organic substances as compounding ingredients
  • C08K5/00—Use of organic ingredients
  • C08K5/16—Nitrogen-containing compounds
  • C08K5/17—Amines; Quaternary ammonium compounds
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
  • C08L101/00—Compositions of unspecified macromolecular compounds
• • C—CHEMISTRY; METALLURGY
  • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
  • C09C—TREATMENT OF INORGANIC MATERIALS, OTHER THAN FIBROUS FILLERS, TO ENHANCE THEIR PIGMENTING OR FILLING PROPERTIES ; PREPARATION OF CARBON BLACK  ; PREPARATION OF INORGANIC MATERIALS WHICH ARE NO SINGLE CHEMICAL COMPOUNDS AND WHICH ARE MAINLY USED AS PIGMENTS OR FILLERS
  • C09C1/00—Treatment of specific inorganic materials other than fibrous fillers; Preparation of carbon black
  • C09C1/44—Carbon
• • C—CHEMISTRY; METALLURGY
  • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
  • C09D—COATING COMPOSITIONS, e.g. PAINTS, VARNISHES OR LACQUERS; FILLING PASTES; CHEMICAL PAINT OR INK REMOVERS; INKS; CORRECTING FLUIDS; WOODSTAINS; PASTES OR SOLIDS FOR COLOURING OR PRINTING; USE OF MATERIALS THEREFOR
  • C09D17/00—Pigment pastes, e.g. for mixing in paints
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
  • H01B1/00—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors
  • H01B1/20—Conductive material dispersed in non-conductive organic material
  • H01B1/24—Conductive material dispersed in non-conductive organic material the conductive material comprising carbon-silicon compounds, carbon or silicon
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
  • H01B13/00—Apparatus or processes specially adapted for manufacturing conductors or cables
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
  • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
  • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
  • H01M4/139—Processes of manufacture
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M4/00—Electrodes
  • H01M4/02—Electrodes composed of, or comprising, active material
  • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
  • Y02E60/10—Energy storage using batteries

Definitions

• the present invention relates to a method for producing a carbon nanotube dispersion paste (also referred to as a conductive pigment paste in the present invention) that has excellent solvent recyclability, pigment dispersibility, and storage stability, a method for producing a composite paste (for lithium ion secondary batteries), and a method for producing a battery electrode layer that has excellent battery performance.
• a carbon nanotube dispersion paste also referred to as a conductive pigment paste in the present invention
• a method for producing a composite paste for lithium ion secondary batteries
• a battery electrode layer that has excellent battery performance.
• paste-like pigment dispersions in which pigments are dispersed in a mixture of pigment dispersion resins and solvents, have been widely used in fields such as paints, battery electrodes, coating materials, electromagnetic shielding, display panels, touch screen panels, colored films, colored sheets, decorative materials, protective materials, magnet modifiers, printing inks, device components, electronic equipment components, printed wiring boards, solar cells, functional rubber components, and resin molding films.
• conductive pigments and conductive polymers are added to these materials to impart functions such as electrostatic paintability, conductivity, electromagnetic shielding, and antistatic properties.
• pigment dispersion resins and pigment pastes are being developed that have excellent pigment dispersion capabilities and excellent pigment dispersion stability that prevents re-agglomeration of pigment particles in the formed pigment dispersion.
• pigment paste When designing pigment paste, it is important to create a pigment paste with a high concentration and uniform dispersion using as little solvent and pigment dispersion resin as possible, so that the pigment dispersion resin does not adversely affect the conductive properties of the final product itself, such as the coating film, and from the perspective of reducing the amount of solvent and pigment dispersion resin used and reducing the energy used during drying.
• waste generated during the manufacturing process there is a demand for waste generated during the manufacturing process to be recyclable, in order to reduce waste, be environmentally friendly, and reduce costs.
• Patent Document 1 discloses a method for producing a slurry for electrodes of a lithium secondary battery, which is characterized by dispersing a solvent containing fibrous carbon with a media (hereinafter sometimes written as "media") type disperser to obtain a slurry, and kneading the slurry with an electrode active material to apply it to a current collector to obtain a slurry.
• media hereinafter sometimes written as "media”
• the object of the present invention is to provide a method for producing a carbon nanotube dispersion paste and a method for producing a composite paste that are excellent in pigment dispersibility, storage stability, and solvent recyclability even in pastes with high pigment concentrations and/or high viscosity, and further to provide a method for producing a coating film (electrode layer for batteries) that is excellent in finish properties, etc.
• the present invention provides the following method for producing a carbon nanotube dispersion paste, a method for producing a composite paste, and a method for producing an electrode layer for a lithium ion secondary battery.
• Item 1 A method for producing a carbon nanotube-dispersed paste obtained by mixing and dispersing components including a pigment dispersion resin (A), carbon nanotubes (B), N-methyl-2-pyrrolidone (C), and an amine compound (D), comprising: the pigment dispersion resin (A) has at least one polar functional group selected from the group consisting of an amide group, an imide group, a hydroxyl group, a carboxyl group, a sulfonic acid group, a phosphate group, a silanol group, a cyano group, a pyrrolidone group, and an amino group, and the concentration of the polar functional group in the pigment dispersion resin (A) is 0.3 mmol/g or more and 23 mmol/g or less; A method for
• Item 2 The method for producing a carbon nanotube dispersion paste according to Item 1, wherein N-methyl-2-pyrrolidone (C) is a recycled product, the moisture content in N-methyl-2-pyrrolidone (C) is controlled to 10,000 ppm or less, and the content of an amine compound in N-methyl-2-pyrrolidone (C) is controlled to 10,000 ppm or less.
• Item 3 The method for producing a carbon nanotube-dispersed paste according to Item 1 or 2, wherein the carbon nanotube-dispersed paste has a moisture content of 10,000 ppm or less.
• Item 5 The method for producing a carbon nanotube-dispersed paste according to any one of Items 1 to 4, wherein the carbon nanotube-dispersed paste further contains polyvinylidene fluoride (E).
• E polyvinylidene fluoride
• a method for producing a composite paste for a lithium ion secondary battery comprising a step of mixing an electrode active material (G) with the carbon nanotube dispersion paste obtained by the method for producing a carbon nanotube dispersion paste according to any one of items 1 to 5.
• a method for producing an electrode layer for a lithium ion secondary battery comprising coating a lithium ion secondary battery composite paste obtained by the method for producing a lithium ion secondary battery composite paste according to item 6 on a current collector.
• Item 8. A step of applying the lithium ion secondary battery composite paste obtained by the method for producing a lithium ion secondary battery composite paste according to item 6 onto a current collector and drying by heating; A step of recovering a vapor containing N-methyl-2-pyrrolidone (C) and an amine compound (D) to produce a mixed solution; a step of separating the amine compound (D) from the mixed solution to produce a recycled product of N-methyl-2-pyrrolidone (C); A method for producing an electrode layer for a lithium ion secondary battery containing N-methyl-2-pyrrolidone (C) and a method for producing a recycled product of N-methyl-2-pyrrolidone (C).
• the method for producing a carbon nanotube-dispersed paste of the present invention is excellent in recyclability, pigment dispersibility, and storage stability (suppression of thickening) even at high pigment concentrations and/or high viscosities, and can sufficiently reduce the viscosity of the paste with a relatively small amount of dispersion resin.
• the coating film (battery electrode layer) has excellent finish and battery performance, etc.
• the present invention is not limited to the following embodiments, but includes various modified examples that are implemented within the scope that does not deviate from the gist of the present invention.
• the paste containing carbon nanotubes is called a "carbon nanotube dispersion paste”, but it can also be called an "conductive pigment paste”.
• the carbon nanotubes can also be abbreviated as "CNT.”
• the paste prepared by further mixing at least one electrode active material and, optionally, other various components in order to coat the carbon nanotube dispersion paste is called a “composite paste.”
• the composite paste that is coated on a substrate and dried is called a “coating film” or a “composite layer.” It can be said that the carbon nanotube dispersion paste is a paste that does not substantially contain an electrode active material.
• the coating film is used as an electrode for a battery, it can also be called an "electrode layer.”
• the carbon nanotube dispersion paste contains a specific pigment dispersion resin (A) and an amine compound (D), which provides good pigment dispersibility and prevents high viscosity and gelation during storage. Furthermore, by keeping the moisture content of the carbon nanotube dispersion paste to 10,000 ppm or less (and the moisture content of N-methyl-2-pyrrolidone (C) to 10,000 ppm or less), high viscosity and gelation during storage can be prevented.
• the acidity of the proton adjacent to the fluorine group in polyvinylidene fluoride is very high due to the electron-attracting property of the fluorine group, and therefore this proton desorption proceeds easily, especially under basic conditions. It is speculated that such proton desorption is particularly likely to proceed when moisture is present in the carbon nanotube dispersion paste and composite paste. After proton desorption, anions are generated on the carbon, which promotes the desorption of the fluorine group, and double bonds are generated in the main chain of the polyvinylidene fluoride molecule.
• the electrode active material in the composite paste may contain lithium hydroxide, which is a relatively strong base, the increase in viscosity and gelation are remarkable in the composite paste.
• the present invention it is believed that by using raw materials with a specified moisture content and by specifying the moisture content of the carbon nanotube dispersion paste and the composite paste, polymerization of the polymer component (polyvinylidene fluoride) is suppressed, and the viscosity increase and gelation of the carbon nanotube dispersion paste or the composite paste can be suppressed. Furthermore, since the moisture is brought in from various raw materials (particularly solvents) and is mixed in from water vapor contained in the air during the manufacturing process, it is practically impossible to reduce the moisture to zero.
• the water content (lower limit) of the carbon nanotube dispersion paste that can be used in the present invention is preferably 100 ppm or more, more preferably 200 ppm or more, and even more preferably 500 ppm or more.
• the water content (lower limit) of N-methyl-2-pyrrolidone (C) is preferably 100 ppm or more, more preferably 200 ppm or more, and even more preferably 500 ppm or more. If the moisture content is within the above lower limit, the product can be produced without excessive moisture content control of the raw materials (reduction of moisture content) or the production process (reduction of moisture contamination).
• the recyclability of the solvent (mainly N-methyl-2-pyrrolidone) discharged during the manufacture of the electrode layer for lithium ion secondary batteries is particularly important in terms of reducing waste and the environment, and if the boiling point of N-methyl-2-pyrrolidone (C) is (Xc)°C and the boiling point of the amine compound (D) is (Xd)°C, then (Xc)-10>(Xd).
• the present invention relates to a method for producing a carbon nanotube dispersion paste obtained by mixing and dispersing components including a pigment dispersion resin (A), carbon nanotubes (B), N-methyl-2-pyrrolidone (C), and an amine compound (D), in which the pigment dispersion resin (A) has at least one polar functional group selected from the group consisting of an amide group, an imide group, a hydroxyl group, a carboxyl group, a sulfonic acid group, a phosphate group, a silanol group, a cyano group, a pyrrolidone group, and an amino group, and the concentration of the polar functional group in the pigment dispersion resin (A) is 0.3 mmol/g or more and 23 mmol/g or less, and when the boiling point of N-methyl-2-pyrrolidone (C) is (Xc) ° C. and the boiling point of the amine compound
• the water content (upper limit) of the carbon nanotube dispersion paste is usually 10,000 ppm or less, preferably 7,500 ppm or less, more preferably 5,000 ppm or less, further preferably 2,500 ppm or less, and particularly preferably 1,000 ppm or less.
• the carbon nanotube dispersion paste used in the present invention can be said to be a substantially non-aqueous paste.
• the moisture content can be measured by Karl Fischer coulometric titration. Specifically, a Karl Fischer moisture meter (manufactured by Kyoto Electronics Manufacturing Co., Ltd., product name "MKC-610”) is used, and the moisture vaporizer (manufactured by Kyoto Electronics Co., Ltd., product name "ADP-611") attached to the device is set at a temperature of 130°C.
• Pigment dispersing resin (A) The pigment dispersion resin (A) has at least one polar functional group selected from the group consisting of an amide group, an imide group, a hydroxyl group, a carboxyl group, a sulfonic acid group, a phosphoric acid group, a silanol group, a cyano group, a pyrrolidone group, and an amino group, and the concentration of the polar functional group in the pigment dispersion resin (A) is 0.3 mmol/g or more and 23 mmol/g or less.
• the acid group may be in the form of a salt.
• the pigment dispersion resin (A) preferably has an alkyl group having 12 or more carbon atoms.
• the alkyl group having 12 or more carbon atoms any known alkyl group (hydrocarbon group) can be used without any particular limitation, and a linear or branched alkyl group is preferable, and a linear alkyl group is more preferable.
• the alkyl group having 12 or more carbon atoms is preferably an alkyl group having 12 or more and less than 30 carbon atoms, more preferably an alkyl group having 15 or more and less than 26 carbon atoms, and even more preferably an alkyl group having 19 or more and less than 24 carbon atoms.
• the alkyl group having 12 or more carbon atoms can be introduced into the resin, for example, by (co)polymerizing a polymerizable monomer containing an alkyl group having 12 or more carbon atoms.
• Examples of polymerizable monomers containing an alkyl group having 12 or more carbon atoms include lauryl (meth)acrylate, stearyl (meth)acrylate, isostearyl (meth)acrylate, behenyl (meth)acrylate, lauryl (meth)acrylamide, stearyl (meth)acrylamide, behenyl (meth)acrylamide, etc. These may be used alone or in combination of two or more. It is believed that when the pigment dispersing resin (A) has a relatively bulky side chain of an alkyl group having 12 or more carbon atoms, the pigment dispersibility and storage stability are improved due to steric repulsion.
• the type of resin is not particularly limited as long as it is a resin other than polyvinylidene fluoride (E) described later.
• acrylic resin polyester resin, epoxy resin, polyether resin, alkyd resin, urethane resin, polyvinyl alcohol, polyvinyl acetal, polyvinylpyrrolidone, polyvinyl acetate, silicone resin, polycarbonate resin, chlorine-based resin, and composite resins thereof can be mentioned. These resins can be used alone or in combination of two or more.
• the pigment dispersing resin (A) preferably contains a vinyl (co)polymer (A1) obtained by polymerizing or copolymerizing a monomer containing a polymerizable unsaturated group-containing monomer of the following formula (1), and in particular, an acrylic resin (co)polymerized with at least one polymerizable unsaturated group-containing monomer that contains a (meth)acryloyl group is preferred.
• the "(co)polymer" of the present invention includes both a polymer obtained by polymerizing one type of monomer and a copolymer obtained by copolymerizing two or more types of monomers.
• R may be the same or different and is a hydrogen atom or an organic group.
• the vinyl (co)polymer (A1) include hydroxyl group-containing vinyl (co)polymers, carboxyl group-containing vinyl (co)polymers, amide group-containing vinyl (co)polymers, sulfonic acid group-containing vinyl (co)polymers, and the like.
• examples of such (co)polymers include vinyl (co)polymers containing phosphoric acid groups, vinyl (co)polymers containing pyrrolidone groups, and vinyl (co)polymers containing amino groups. They can be used alone or in combination of two or more.
• hydroxyl group-containing vinyl (co)polymers examples include polyhydroxyethyl (meth)acrylate, polyvinyl alcohol, vinyl alcohol-fatty acid vinyl copolymer, vinyl alcohol-ethylene copolymer, vinyl alcohol-(N-vinylformamide) copolymer, and copolymers of hydroxyethyl (meth)acrylate and other polymerizable unsaturated monomers.
• the vinyl alcohol units in the (co)polymer may be obtained by (co)polymerizing fatty acid vinyl units and then hydrolyzing them.
• carboxyl group-containing vinyl (co)polymers examples include polymers of (meth)acrylic acid, and copolymers of poly(meth)acrylic acid and other polymerizable unsaturated monomers.
• amide group-containing vinyl (co)polymers examples include (meth)acrylamide polymers and copolymers of (meth)acrylamide and other polymerizable unsaturated monomers.
• sulfonic acid group-containing vinyl (co)polymers examples include polymers of allylsulfonic acid or styrenesulfonic acid, copolymers of allylsulfonic acid and/or styrenesulfonic acid with other polymerizable unsaturated monomers, etc.
• phosphate group-containing vinyl (co)polymers examples include polymers of (meth)acryloyloxyalkyl acid phosphate, and copolymers of (meth)acryloyloxyalkyl acid phosphate with other polymerizable unsaturated monomers.
• amino group-containing vinyl (co)polymers examples include N,N-dimethylaminoethyl (meth)acrylate and copolymers of N,N-diethylaminoethyl (meth)acrylate with other polymerizable unsaturated monomers.
• copolymerizable unsaturated monomers include, for example, vinyl formate, vinyl acetate, vinyl propionate, isopropenyl acetate, vinyl valerate, vinyl caprylate, vinyl caprate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl versatate, and vinyl pivalate; olefins such as ethylene, propylene, and butylene; aromatic vinyls such as styrene and ⁇ -methylstyrene; methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate.
• ethylenically unsaturated carboxylic acid alkyl ester monomers such as dimethyl fumarate, dimethyl maleate, diethyl maleate, and diisopropyl itaconate
• vinyl ether monomers such as methyl vinyl ether, n-propyl vinyl ether, isobutyl vinyl ether, and dodecyl vinyl ether
• halogenated vinyl monomers or vinylidene monomers such as vinyl chloride, vinylidene chloride, vinyl fluoride, and vinylidene fluoride
• allyl compounds such as allyl acetate and allyl chloride
• quaternary ammonium group-containing monomers such as 3-(meth)acrylamidopropyltrimethylammonium chloride.
• the polar functional group concentration of the pigment dispersing resin (A) is preferably 0.3 mmol/g to 23 mmol/g, more preferably 0.3 mmol/g to 15.0 mmol/g, even more preferably 0.3 mmol/g to 8.5 mmol/g, and particularly preferably 0.3 mmol/g to 5.0 mmol/g.
• the above acid groups and amino groups may be in the form of a salt.
• the vinyl (co)polymer (A1) can be produced by a polymerization method known per se.
• a polymerization method known per se.
• solution polymerization it is preferable to use solution polymerization, but this is not limiting, and bulk polymerization, emulsion polymerization, suspension polymerization, etc. may also be used.
• solution polymerization continuous polymerization or batch polymerization may be used, and the monomers may be charged all at once or in portions, or may be added continuously or intermittently.
• the polymerization initiator used in the solution polymerization is not particularly limited, but specific examples include azo compounds such as azobisisobutyronitrile, 2,2'-azobis(2-methylbutyronitrile), azobis-2,4-dimethylparabennitrile, and azobis(4-methoxy-2,4-dimethylparabennitrile); acetyl peroxide, benzoyl peroxide, lauroyl peroxide, acetylcyclohexylsulfonyl peroxide, and 2,4,4-trimethylpentyl-2,4-dimethylparabennitrile.
• azo compounds such as azobisisobutyronitrile, 2,2'-azobis(2-methylbutyronitrile), azobis-2,4-dimethylparabennitrile, and azobis(4-methoxy-2,4-dimethylparabennitrile)
• acetyl peroxide benzoyl per
• -Peroxides such as peroxyphenoxyacetate; percarbonate compounds such as diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, and diethoxyethyl peroxydicarbonate; perester compounds such as t-butyl peroxyneodecanate, ⁇ -cumyl peroxyneodecanate, and t-butyl peroxyneodecanate; and known radical polymerization initiators such as azobisdimethylvaleronitrile and azobismethoxyvaleronitrile can be used.
• percarbonate compounds such as diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, and diethoxyethyl peroxydicarbonate
• perester compounds such as t-butyl peroxyneodecanate, ⁇ -cumyl peroxyneodecanate, and t-butyl
• the polymerization reaction temperature is not particularly limited, but can usually be set in the range of about 30°C to 200°C.
• the vinyl (co)polymer (A1) obtainable as described above has a degree of polymerization of, for example, 100 or more, preferably 150 or more, and, for example, 4,000 or less, preferably 3,000 or less, more preferably 700 or less.
• the weight average molecular weight is, for example, 1,000 or more, preferably 2,000 or more, more preferably 7,000 or more, and, for example, 2,000,000 or less, preferably 1,000,000 or less, more preferably 500,000 or less.
• the weight average molecular weight is a value obtained by converting the retention time (retention volume) measured using a gel permeation chromatograph (GPC) into the molecular weight of polystyrene using the retention time (retention volume) of a standard polystyrene of known molecular weight measured under the same conditions.
• GPC gel permeation chromatograph
• the gel permeation chromatograph is "HLC8120GPC” (product name, manufactured by Tosoh Corporation), and the four columns are “TSKgel G-4000HXL”, “TSKgel G-3000HXL”, “TSKgel G-2500HXL” and “TSKgel G-2000HXL” (all product names, manufactured by Tosoh Corporation), and the measurements can be performed under the following conditions: mobile phase tetrahydrofuran, measurement temperature 40°C, flow rate 1mL/min, and detector RI.
• the vinyl (co)polymer (A1) can be converted into a solid or into a resin solution in which any solvent has been replaced by removing the solvent and/or replacing the solvent.
• the method of desolvation may be by heating at normal pressure or under reduced pressure.
• the method of solvent replacement may be to add a replacement solvent at any stage before, during, or after desolvation.
• the content of the alkyl group having 12 or more carbon atoms in the dispersion resin (A), in the case of the vinyl (co)polymer (A1) is preferably 1 to 100 mass%, more preferably 10 to 90 mass%, even more preferably 20 to 80 mass%, and particularly preferably 30 to 60 mass%, expressed as the mass ratio of the monomer when all monomers are taken as 100 mass%.
• the content of the alkyl group having 12 or more carbon atoms is calculated based on the mass proportion of a compound having a reactive alkyl group having 12 or more carbon atoms added to the resin later.
• the pigment dispersion resin (A) When the pigment dispersion resin (A) is converted from a solid state into a resin solution, it is preferable that, from the viewpoint of solubility in the solvent, the pigment dispersion resin (A) is first mixed and dissolved in a solvent having a liquid temperature of 60° C. or higher (preferably 80° C. or higher) (the upper limit is 200° C. or lower, preferably 100° C. or lower) to convert it into a resin solution, and then it is further mixed with other components [components (B), (C), (D), etc.].
• the "liquid temperature” refers to the temperature of the solvent or resin solution at the time of dissolution.
• the solid pigment dispersion resin (A) may be mixed and dissolved in a solvent at 60° C.
• the solid pigment dispersion resin (A) may be mixed with a solvent and then heated to a temperature of 60° C. or higher.
• the ink may contain components other than the pigment dispersing resin (A) and the solvent.
• the solvent may be used alone or in combination of two or more kinds, and the types of solvents that can be suitably used are those exemplified below as solvents.
• the solid content of the pigment dispersion resin (A) is, based on 100 mass% of the total solid content of the carbon nanotube dispersion paste, for example, 0.1 mass% or more, preferably 1 mass% or more, and more preferably 3 mass% or more, and for example, 40 mass% or less, preferably 30 mass% or less, and more preferably 20 mass% or less.
• the solid content of the pigment dispersion resin (A) is, based on 100 mass% of the total amount of the carbon nanotube dispersion paste, for example, 0.1 mass% or more, preferably 0.4 mass% or more, and more preferably 0.7 mass% or more, and for example, 10 mass% or less, preferably 5 mass% or less, and more preferably 2 mass% or less.
• the solid content of the pigment dispersion resin (A) is, based on the content of the carbon nanotubes (B) being 100% by mass, for example, 0.1% by mass or more, preferably 1% by mass or more, more preferably 5% by mass or more, and for example, 150% by mass or less, preferably 120% by mass or less, more preferably 80% by mass or less.
• Carbon nanotubes (B) As the carbon nanotubes (B), single-walled carbon nanotubes or multi-walled carbon nanotubes can be used alone or in combination. In particular, in terms of viscosity, electrical conductivity, and cost, it is preferable to use multi-walled carbon nanotubes.
• the content of carbon nanotubes (B) is 0.1% by mass or more, for example 0.2% by mass or more, preferably 0.3% by mass or more, more preferably 0.5% by mass or more, even more preferably 1% by mass or more, and even more preferably 2% by mass or more, based on the total amount of the carbon nanotube dispersion paste (100% by mass), and is less than 15% by mass, for example less than 13% by mass, preferably 10% by mass or less, more preferably 7% by mass or less, and even more preferably 6% by mass or less.
• it can be 0.1% by mass or more and less than 15% by mass, 0.2% by mass or more and less than 13% by mass, 0.3% by mass or more and less than 10% by mass, or 0.5% by mass or more and less than 7% by mass.
• it is, for example, 5% by mass or more, preferably 10% by mass or more, and more preferably 20% by mass or more, and for example, 90% by mass or less, preferably 70% by mass or less, and more preferably 50% by mass or less.
• the average outer diameter of the carbon nanotubes (B) is, for example, 1 nm or more, preferably 3 nm or more, more preferably 5 nm or more, and is, for example, 30 nm or less, preferably 28 nm or less, more preferably 25 nm or less.
• the average length of the carbon nanotubes (B) is, for example, 0.1 ⁇ m or more, preferably 1 ⁇ m or more, more preferably 5 ⁇ m or more, and is, for example, 100 ⁇ m or less, preferably 80 ⁇ m or less, more preferably 60 ⁇ m or less.
• the BET specific surface area of the carbon nanotubes (B) is, in consideration of the relationship between viscosity and electrical conductivity, usually 100 m 2 /g or more, preferably 130 m 2 /g or more, more preferably 160 m 2 /g or more, and usually 800 m 2 /g or less, preferably 600 m 2 /g or less, more preferably 400 m 2 /g or less.
• the BET specific surface area of the present invention can be calculated by the BET method using nitrogen adsorption measurement. Specifically, for example, the BET specific surface area (m 2 /g) can be measured using a specific surface area measuring device (BERSORP-MAX (Microtrac-Bell Co., Ltd.)) in accordance with JIS Z8830:2013.
• BERSORP-MAX Microtrac-Bell Co., Ltd.
• the amount of acidic groups in the carbon nanotubes (B) is usually 0.01 mmol/g or more, preferably 0.01 mmol/g or more, and usually 1.0 mmol/g or less, preferably 0.5 mmol/g or less, more preferably 0.2 mmol/g or less, and even more preferably 0.1 mmol/g or less, from the viewpoints of dispersibility and storage property. If the amount of acidic groups is 0.01 mmol/g or more, the dispersibility will be good, and if it is 1.0 mmol/g or less, the storage property will be good.
• the above acidic groups can be imparted to carbon nanotubes by acid treatment as described below.
• the acid treatment method is not particularly limited as long as it can bring the carbon nanotubes into contact with the acid, but a method of immersing the carbon nanotubes in an acid treatment solution (aqueous solution of acid) is preferred.
• the acid contained in the acid treatment solution is not particularly limited, but examples thereof include nitric acid, sulfuric acid, and hydrochloric acid. These can be used alone or in combination of two or more. Among these, nitric acid and sulfuric acid are preferred.
• the amount of acidic groups in the carbon nanotubes can be adjusted by the concentration of the acid treatment solution, the temperature, the treatment time, and the like.
• the excess acid component adhering to the surface is removed by a washing method described below, thereby obtaining acid-treated carbon nanotubes.
• the method for washing the acid-treated carbon nanotubes is not particularly limited, but washing with water is preferred.
• the carbon nanotubes are collected from the acid-treated carbon nanotubes by a known method such as filtration, and then washed with water. After the above washing, the water adhering to the surface can be removed by drying, etc., as necessary, to obtain the acid-treated carbon nanotubes.
• the volume-equivalent median diameter (D50) of the carbon nanotubes (B) is usually 10 ⁇ m or more, preferably 15 ⁇ m or more, more preferably 20 ⁇ m or more, and usually 250 ⁇ m or less, preferably 200 ⁇ m or less, more preferably 150 ⁇ m or less, when measured by the method described in the examples below.
• the median diameter (D50) can be obtained by irradiating a carbon nanotube particle with a laser beam and converting the diameter of the carbon nanotube into a sphere from the scattered light. The larger the median diameter (D50), the more carbon nanotube agglomerates there are, which means that the dispersibility is poor.
• the median diameter (D50) is larger than 250 ⁇ m, there is a high possibility that carbon nanotube agglomerates exist in the electrode, and the conductivity of the entire electrode becomes non-uniform.
• the median diameter (D50) is smaller than 10 ⁇ m, the fiber length is short, so the conductive path is insufficient, and the conductivity decreases.
• the median diameter (D50) is within the range of 10 ⁇ m or more and 250 ⁇ m or less, the carbon nanotubes can be uniformly dispersed within the electrode while maintaining their electrical conductivity.
• the G/D ratio In the Raman spectrum of the carbon nanotube (B), the G/D ratio, where G is the maximum peak intensity in the range of 1560 cm -1 to 1600 cm -1 and D is the maximum peak intensity in the range of 1310 cm -1 to 1350 cm -1 , is usually 0.1 or more, preferably 0.4 or more, more preferably 0.6 or more, and is usually 5.0 or less, preferably 3.0 or less, more preferably 1.0 or less.
• a G/D ratio in the range of 0.1 to 5.0 is preferable because it tends to have high conductivity due to fewer defects and crystal interfaces on the carbon surface.
• carbon nanotubes (B) are fibrous and have a high viscosity, so if the content is 15% by mass or more, the viscosity becomes high and handling becomes difficult.
• the carbon nanotubes (B) can be previously dry-dispersed in a media-type grinder before producing the carbon nanotube dispersion paste.
• the "dry dispersion” of the present invention refers to pulverization (including disintegration) by a pulverizer at a solid content concentration in the pulverized component of 80% by mass or more (preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably 98% by mass or more).
• the content of the carbon nanotubes (B) contained in the solid content of the pulverized component is usually 80% by mass or more, preferably 90% by mass or more, more preferably 95% by mass or more, even more preferably 98% by mass or more, and particularly preferably only the carbon nanotubes (B).
• the components other than the carbon nanotubes (B) in the ground component solvents, resins, pigments other than the carbon nanotubes (B), etc. can be suitably used, but it is preferable that the ground component contains substantially only the carbon nanotubes (B).
• solid content concentration refers to the proportion of solid content (mass %) when 1 g of a sample is dried by heating at 130° C. for 3 hours.
• the dry dispersion is a method of grinding a pigment without substantially containing liquid components, and since energy can be applied directly to the pigment, it is possible to perform highly efficient and powerful grinding (disintegration). In addition, since the grinding surface is activated and interacts with the surrounding substances, good dispersibility and storage stability can be obtained in the paste dispersion process described below, and the coating film can have excellent conductivity and finish.
• the material is pulverized using a pulverizer incorporating pulverizing media such as glass beads, zirconia beads, steel balls, etc.
• the pulverization is carried out by utilizing the pulverizing or destructive force caused by the collision of the pulverizing media with each other and/or the collision of the pulverizer with the pulverizing media.
• Known pulverizing devices such as high-speed impact mills, jet mills, roll mills, attritors, ball mills, vibration mills, and bead mills can be used as the pulverizing device.
• various types of steam or gas can be blown into the grinder during grinding to further activate the surface of the carbon nanotubes (B) or adjust the activity.
• steam acidic or basic compounds are suitable, and as the gas, oxygen, nitrogen, etc. are suitable.
• the outer diameter of the grinding media is preferably 0.1 mm to 5 mm, and more preferably 0.5 mm to 3 mm. Within the above range, the desired grinding force can be obtained, and the pigment can be efficiently ground and crushed without excessively destroying the fiber shape of the carbon nanotubes.
• the carbon nanotube dispersion paste used in the present invention can also use other conductive pigments (B1) other than the carbon nanotubes (B).
• Examples of other conductive pigments (B1) include at least one conductive carbon selected from the group consisting of acetylene black, ketjen black, furnace black, thermal black, graphene, and graphite. Preferably, it is at least one selected from the group consisting of acetylene black, ketjen black, furnace black, and thermal black, more preferably at least one selected from the group consisting of acetylene black and ketjen black, and even more preferably it is acetylene black.
• the average primary particle diameter of the other conductive pigment (B1) is, for example, 10 nm or more, preferably 20 nm or more, and more preferably, for example, 80 nm or less, and more preferably, 70 nm or less.
• the average primary particle diameter refers to the average particle diameter of the primary particles obtained by observing the conductive pigment (B1) under an electron microscope, calculating the projected area of each of 100 particles, calculating the diameter of a circle assuming an area equal to that area, and then averaging the diameters of the 100 particles. Note that if the pigment is in an aggregated state, the calculation is performed using the primary particles that make up the aggregated particles.
• the BET specific surface area of the other conductive pigment (B1) is not particularly limited and is, for example, 1 m 2 /g or more, preferably 10 m 2 /g or more, more preferably 20 m 2 /g or more, and is, for example, 500 m 2 /g or less, preferably 250 m 2 /g or less, more preferably 200 m 2 /g or less, depending on the relationship between viscosity and conductivity.
• the dibutyl phthalate (DBP) oil absorption of the other conductive pigment (B1) is not particularly limited. In relation to pigment dispersibility and conductivity, it is, for example, 60 ml/100 g or more, preferably 150 ml/100 g or more, and, for example, 1,000 ml/100 g or less, preferably 800 ml/100 g or less.
• N-Methyl-2-pyrrolidone (C) The N-methyl-2-pyrrolidone (C) preferably has a water content of 10,000 ppm or less, and is preferably controlled and adjusted before use so that the water content is a certain amount or less. In addition, from the viewpoints of reducing waste, environmental friendliness, and cost reduction, it is preferable to use a regenerated product (recycled product).
• the water content (upper limit) of N-methyl-2-pyrrolidone (C) is preferably 10,000 ppm or less, more preferably 7,500 ppm or less, even more preferably 5,000 ppm or less, particularly preferably 2,500 ppm or less, and even more particularly preferably 1,000 ppm or less.
• the water content (lower limit) is preferably 100 ppm or more, more preferably 200 ppm or more, and even more preferably 500 ppm or more.
• N-methyl-2-pyrrolidone may contain amine components as impurities generated during production or as unreacted raw materials, and in the carbon nanotube dispersion paste of the present invention, the viscosity or tendency to thicken may vary from lot to lot depending on these amine impurities. There is also the issue of odor (work environment), so it is necessary to keep the amount of N-methyl-2-pyrrolidone below a certain level as a raw material.
• the solvent and the like volatilize and do not remain, but it is preferable to recover and reuse the volatilized solvent in order to reduce waste, be environmentally friendly, and/or reduce raw material costs.
• a recycled product as N-methyl-2-pyrrolidone (C).
• This recycled solvent (recycled product) will also contain the amine compounds that were originally contained therein, and similarly, the viscosity or thickening tendency of the paste will differ from lot to lot. Furthermore, amine compounds often have a strong odor.
• N-methyl-2-pyrrolidone (C) N-methyl-2-pyrrolidone
• the amine compound content is preferably 10,000 ppm or less, more preferably 7,500 ppm or less, even more preferably 5,000 ppm or less, particularly preferably 2,500 ppm or less, and even more particularly preferably 1,000 ppm or less. Since the recycled product contains a minimum amount of amine compounds, the lower limit is 1 ppm or more, 5 ppm or more, or 10 ppm or more.
• the content of the amine compound can be quantified by a general analysis such as ion chromatography-mass spectrometry (IC-MS). The content can be quantified by preparing a calibration curve in advance for the peaks of amine species that are expected to be mixed in.
• the N-methyl-2-pyrrolidone (C) used in the present invention contains recycled products at a content of 5% by mass or more (preferably 10% by mass or more, more preferably 30% by mass or more, and even more preferably 50% by mass or more).
• the recycled product it is preferable to use the recycled solvent recovered during the process of heating and drying the composite paste to create the electrode layer, as described below.
• the content of N-methyl-2-pyrrolidone (C) in the carbon nanotube dispersion paste is, for example, 40 mass% or more, preferably 60 mass% or more, and more preferably 80 mass% or more, based on 100 mass% of the total amount of the carbon nanotube dispersion paste, and is, for example, 99 mass% or less, preferably 98 mass% or less, and more preferably 97 mass% or less.
• the solid content of the carbon nanotube dispersion paste is typically 0.1 mass% or more, for example 1 mass% or more, preferably 2 mass% or more, and more preferably 3 mass% or more, based on the total amount of the carbon nanotube dispersion paste being 100 mass%, and is typically less than 80 mass%, for example 60 mass% or less, preferably 40 mass% or less, and more preferably 20 mass% or less.
• the carbon nanotube dispersion paste may also contain solvents other than N-methyl-2-pyrrolidone (C).
• solvents other than N-methyl-2-pyrrolidone (C).
• hydrocarbon solvents such as n-butane, n-hexane, n-heptane, n-octane, cyclopentane, cyclohexane, and cyclobutane
• aromatic solvents such as toluene and xylene
• ketone solvents such as methyl isobutyl ketone
• ether solvents such as n-butyl ether, dioxane, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, and diethylene glycol; ethyl acetate, n-butyl acetate, isobutyl acetate, and ethylene
• solvents include
• Amine compound (D) examples include ammonia, primary amines, secondary amines, and tertiary amines.
• the amine compound (D) of the present invention also includes the amine compounds as impurities of the above-mentioned N-methyl-2-pyrrolidone (C) and the amine compounds contained in the recycled product of N-methyl-2-pyrrolidone (C).
• Examples of common amine compounds include the following amine compounds:
• Primary amines include, for example, ethylamine, n-propylamine, sec-propylamine, n-butylamine, sec-butylamine, i-butylamine, tert-butylamine, pentylamine, hexylamine, heptylamine, octylamine, decylamine, laurylamine, myristyrylamine, 1,2-dimethylhexylamine, 3-pentylamine, 2-ethylhexylamine, allylamine, aminoethanol, 1-aminopropanol, 2-aminopropanol, aminobutanol, aminopentanol, aminohexanol, 3-ethoxypropylamine, 3-propoxypropylamine, 3-isopropoxypropylamine, 3-butoxypropylamine, 3-isopropoxypropylamine, 3-butoxypropylamine, 3-isobutoxypropylamine, 3-(2-ethyl
• Secondary amines include, for example, diethylamine, dipropylamine, di-n-butylamine, di-sec-butylamine, diisobutylamine, di-n-pentylamine, di-3-pentylamine, dihexylamine, dioctylamine, di(2-ethylhexyl)amine, methylhexylamine, diallylamine, pyrrolidine, piperidine, 2,4-leupetidine, 2,6-leupetidine, 3,5-leupetidine, diphenylamine, secondary monoamines such as N,N'-dimethylethylenediamine, N,N'-dimethyl-1,2-diaminopropane, N,N'-dimethyl-1,3-diaminopropane, N,N'-dimethyl-1,2-diaminobutane, N,N'-dimethyl-1,3 ...
• tertiary amines include trimethylamine, triethylamine, tri-n-propylamine, tri-iso-propylamine, tri-1,2-dimethylpropylamine, tri-3-methoxypropylamine, tri-n-butylamine, tri-iso-butylamine, tri-sec-butylamine, tri-pentylamine, tri-3-pentylamine, tri-n-hexylamine, tri-n-octylamine, tri-2-ethylhexylamine, tri-dodecylamine, tri-laurylamine, dicyclohexylethylamine, cyclohexyldiethylamine, tri-cyclohexylamine, N,N-dimethylhexylamine, N-methyldihexylamine, N,N-dimethylcyclohexylamine, N-methyldicyclohexylamine, N,N-diethylethanol
• primary amine compounds are preferred, and monovalent amine compounds (monoamines) are more preferred.
• the above amine compound (D) may be an alkanolamine, an aliphatic amine, an alicyclic amine, an aromatic amine, etc., any of which may be suitably used, but aromatic amines are preferred.
• the weight average molecular weight of the amine compound (D) is preferably less than 1,000, more preferably 800 or less, even more preferably 500 or less, particularly preferably 350 or less, and even more particularly preferably 250 or less. Furthermore, if the boiling point is low, there is a possibility that the liquid will volatilize during production or storage, and furthermore, from the viewpoint of odor, the lower limit of the boiling point is preferably 50° C. or higher, and more preferably 100° C. or higher.
• the amine value of the amine compound (D) is usually 5 mgKOH/g or more, preferably 50 mgKOH/g or more, more preferably 105 mgKOH/g or more, and is usually within the range of 1,000 mgKOH/g or less.
• an amine compound (D) improves pigment dispersibility and storage stability, but many of them have a strong odor, which can worsen the working environment during mixing and drying. In addition, they are generally expensive, which can increase costs. Furthermore, for the sake of recyclability, which will be described later, the content must be kept to the minimum necessary.
• the content of the amine compound (D) is, for example, 0.01% by mass or more, preferably 0.05% by mass or more, and more preferably 0.1% by mass or more, based on 100% by mass of the total amount of the carbon nanotube dispersion paste, and is, for example, 10% by mass or less, preferably 5% by mass or less, and more preferably 1% by mass or less.
• the lower limit is, for example, 1% by mass or more, preferably 2% by mass or more, and more preferably 5% by mass or more
• the upper limit is, for example, 500% by mass or less, preferably 50% by mass or less, and more preferably 15% by mass or less.
• it is, for example, 1% by mass or more, preferably 1.5% by mass or more, and more preferably 2% by mass or more, and is, for example, 600% by mass or less, preferably 300% by mass or less, and more preferably 50% by mass or less.
• the carbon nanotube dispersion paste that can be used in the present invention preferably contains polyvinylidene fluoride (E), which is an essential component of the composite paste described below.
• Polyvinylidene fluoride (E) is a resin intended for forming a film for the electrode layer, and may be modified in various ways. From the viewpoint of adhesion to the substrate, it is preferable that it has a polar functional group.
• the weight average molecular weight of polyvinylidene fluoride (E) is, from the viewpoints of adhesion to the substrate, reinforcement of the film properties, and solvent resistance, for example, 100,000 or more, preferably 500,000 or more, and more preferably 650,000 or more, and for example, 3 million or less, preferably 2 million or less.
• the content of polyvinylidene fluoride (E) is, based on 100% by mass of the solid content of the carbon nanotube dispersion paste, for example, 10.0% by mass or more, preferably 30.0% by mass or more, more preferably 40.0% by mass or more, and for example, 99.0% by mass or less, preferably 80.0% by mass or less, more preferably 60.0% by mass or less. Also, based on 100% by mass of the total amount of the carbon nanotube dispersion paste, for example, 0.1% by mass or more, preferably 0.5% by mass or more, more preferably 1% by mass or more, and for example, 10% by mass or less, preferably 7% by mass or less, more preferably 5% by mass or less.
• the polyvinylidene fluoride (E) in a solid state is made into a resin solution
• a solvent having a liquid temperature of 40° C. or higher preferably 60° C. or higher, and more preferably 80° C. or higher
• the upper limit is 200° C. or lower, and preferably 100° C. or lower
• the "liquid temperature” refers to the temperature of the solvent or resin solution at the time of dissolution.
• Solid polyvinylidene fluoride (E) may be mixed in advance with a solvent at 40° C. or higher and dissolved therein, or solid polyvinylidene fluoride (E) may be mixed with a solvent and then heated to a temperature of 40° C. or higher.
• the composition may contain components other than the polyvinylidene fluoride (E) and the solvent.
• the solvent may be used alone or in combination of two or more kinds. Any solvent that dissolves polyvinylidene fluoride (E) may be suitably used, with N-methyl-2-pyrrolidone (C) being preferred.
• the cooling step is carried out by reacting the resin solution with ... by the following reaction:
• the mixing and dispersing step is a step of mixing and further dispersing components containing at least the pigment dispersion resin (A), the carbon nanotubes (B), N-methyl-2-pyrrolidone (C), and the amine compound (D), and a liquid carbon nanotube dispersion paste can be obtained.
• the upper limit of the solids concentration of the carbon nanotube dispersion paste is usually less than 80% by mass, preferably less than 50% by mass, more preferably less than 20% by mass, and even more preferably less than 10% by mass.
• the lower limit is usually 0.1% by mass or more, preferably 0.5% by mass or more, more preferably 1% by mass or more, and even more preferably 2% by mass or more.
• the components can be uniformly mixed and dispersed using a conventionally known dispersing machine such as a paint shaker, a sand mill, a ball mill, a pebble mill, an LMZ mill, a DCP pearl mill, a planetary ball mill, a homogenizer, a twin-screw kneader, a thin film rotary high-speed mixer (manufactured by Filmix, product name "Clearmix", etc.), etc.
• a conventionally known dispersing machine such as a paint shaker, a sand mill, a ball mill, a pebble mill, an LMZ mill, a DCP pearl mill, a planetary ball mill, a homogenizer, a twin-screw kneader, a thin film rotary high-speed mixer (manufactured by Filmix, product name "Clearmix", etc.), etc.
• the order in which the components are mixed is not particularly limited.
• the mixing and dispersing step further comprises: Step 1: adding a component containing carbon nanotubes (B) in an amount of 70% by mass or less (preferably 50% by mass or less) based on 100% by mass of the total amount of carbon nanotubes (B) contained in the carbon nanotube dispersion paste obtained after dispersion to a dispersing machine and performing a dispersing process; and Step 2: adding carbon nanotubes (B) to a dispersing machine until a desired concentration is reached, and performing a dispersing process. It is preferable that the method includes the steps of sequentially carrying out the steps.
• the dispersion treatment time in step 1 is preferably at least 30 seconds or more (preferably 1 minute or more).
• the aggregation of the carbon nanotubes (B) is alleviated, and a homogeneous paste with good dispersibility is obtained even in a high-concentration paste, and the resulting battery electrode layer (coating film) has excellent finish, conductivity, battery performance, etc.
• the carbon nanotube dispersion paste may further contain other components in addition to the above-mentioned components (A), (B), (C), and (D) and the component (E) which may be contained as necessary.
• Other components include, for example, resins other than the pigment dispersion resin (A) and polyvinylidene fluoride (E), neutralizing agents, defoamers, preservatives, rust inhibitors, plasticizers, pigments other than carbon nanotubes (B), dehydrating agents (F), etc.
• pigments other than carbon nanotubes (B) include the other conductive pigments (B1) described above; white pigments such as titanium white and zinc oxide; blue pigments such as cyanine blue and indanthrene blue; green pigments such as cyanine green and verdigris; organic red pigments such as azo and quinacridone, red pigments such as red iron oxide; organic yellow pigments such as benzimidazolone, isoindolinone, isoindoline and quinophthalone, yellow pigments such as titanium yellow and yellow lead. These pigments can be used alone or in combination of two or more.
• These pigments other than the carbon nanotubes (B) can be used for purposes such as color adjustment and reinforcement of the physical properties of the film, as long as the electrical conductivity is not significantly impaired. They may be dispersed simultaneously with the pigment dispersion resin (A) and the carbon nanotubes (B), or they may be mixed as a pigment or pigment paste after dispersing the pigment dispersion resin (A) and the carbon nanotubes (B) to prepare a paste.
• the content of pigments other than the carbon nanotubes (B) is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 1% by mass or less, based on 100% by mass of all pigments in the carbon nanotube dispersion paste, and it is particularly preferable that they are substantially not contained.
• the viscosity of the carbon nanotube dispersion paste at a shear rate of 2 s ⁇ 1 is, for example, less than 5,000 mPa ⁇ s, preferably less than 2,500 mPa ⁇ s, more preferably less than 1,000 mPa ⁇ s, and is, for example, 10 mPa ⁇ s or more, preferably 50 mPa ⁇ s or more, more preferably 100 mPa ⁇ s or more.
• the viscosity can be measured, for example, using a cone and plate type viscometer (manufactured by HAAKE, trade name "Mars2", diameter 35 mm, 2° inclined cone and plate).
• any known dehydrating agent having a dehydrating effect can be used without any particular restrictions. It may be a solid dehydrating agent that does not dissolve in the solvent of the paste, or a dehydrating agent that dissolves in the solvent.
• solid dehydrating agents such as zeolite, silica gel, calcium oxide, molecular sieve, activated alumina, barium oxide, calcium hydride, and sodium sulfate; phosphate esters such as trimethyl phosphate, tri-2-propyl phosphate, tributyl phosphate, and tetraisopropylethylene phosphonate; phosphine oxides such as tributyl phosphine oxide, trioctyl phosphine oxide, and triphenyl phosphine oxide; orthoformic acid methyl ester, orthoformic acid ethyl ester.
• phosphate esters such as trimethyl phosphate, tri-2-propyl phosphate, tributyl phosphate, and tetraisopropylethylene phosphonate
• phosphine oxides such as tributyl phosphine oxide, trioctyl phosphine oxide, and triphenyl pho
• orthoesters such as methyl orthoacetate, ethyl orthoacetate, and ethyl orthobenzoate
• acid anhydrides such as oxalic anhydride, acetic anhydride, propionic anhydride, butyric anhydride, benzoic anhydride, trifluoroacetic anhydride, disulfuric acid, dinitrogen pentoxide, diphosphoric acid, diphosphorus pentoxide, diphosphorus trioxide, diarsenic pentoxide, diarsenic trioxide, methanesulfonic anhydride, trifluoromethanesulfonic anhydride, and sulfobenzoic anhydride, which may be used alone or in combination of two or more.
• a carbon nanotube dispersion paste having carbon nanotubes (B) is prepared by the above-mentioned method.
• the carbon nanotube dispersion paste and at least one electrode active material (G) are mixed to produce a composite paste (for lithium ion secondary batteries).
• the solid content of the electrode active material (G) is usually 10% by mass or more, preferably 20% by mass or more, based on 100% by mass of the total amount of the composite paste, and is usually 99% by mass or less, preferably 95% by mass or less, which is suitable in terms of battery performance.
• polyvinylidene fluoride (E) which was an optional component in the carbon nanotube dispersion paste, is an essential component in the composite paste and is always contained.
• the solid content of polyvinylidene fluoride (E) is usually 0.05 mass% or more, preferably 0.1 mass% or more, based on 100 mass% of the total amount of the composite paste, and is usually 10 mass% or less, preferably 2 mass% or less, which is suitable in terms of battery performance, paste viscosity, etc.
• the composite paste can be mixed uniformly using a conventionally known mixer and disperser.
• the solid content of the pigment dispersion resin (A) in the solid content of the composite paste is usually 0.01% by mass or more, preferably 0.05% by mass or more, based on 100% by mass of the total amount of the composite paste, and is usually 10% by mass or less, preferably 1% by mass or less, which is suitable in terms of battery performance, paste viscosity, etc.
• the composite paste of the present invention from the viewpoint of storage stability (suppression of thickening) in the composite paste, it is preferable to include a sequence of first mixing the carbon nanotubes (B) with the amine compound (D) by bringing the amine compound (D) into contact with (wetting) the carbon nanotubes (B) and then mixing the electrode active material (G), in order to reduce aggregation between the carbon nanotubes (B) and the electrode active material (G).
• the solid content of carbon nanotubes (B) in the composite paste solids of the present invention is typically 0.01% by mass or more, preferably 0.05% by mass or more, more preferably 0.1% by mass or more, based on 100% by mass of the total composite paste, and is typically 30% by mass or less, preferably 10% by mass or less, more preferably 5% by mass or less, which is preferred in terms of battery performance.
• the content of N-methyl-2-pyrrolidone (C) in the composite paste of the present invention is typically 1% by mass or more, preferably 4% by mass or more, more preferably 7% by mass or more, based on 100% by mass of the total composite paste, and is typically 90% by mass or less, preferably 70% by mass or less, more preferably 50% by mass or less, which is preferred in terms of electrode drying efficiency and paste viscosity.
• the above composite paste is suitable for use as a positive or negative electrode for lithium ion secondary batteries, and is preferably used as a positive electrode.
• the moisture content of the composite paste is usually 10,000 ppm or less, preferably 7,500 ppm or less, more preferably 5,000 ppm or less, even more preferably 2,500 ppm or less, and particularly preferably 1,000 ppm or less, from the viewpoint of suppressing the viscosity increase or gelation of the composite paste described above.
• the composite paste used in the present invention can be said to be a substantially non-aqueous composite paste.
• the moisture content of the composite paste is preferably 100 ppm or more, more preferably 200 ppm or more, and even more preferably 500 ppm or more.
• Electrode active material (G) examples include lithium composite oxides such as lithium nickel oxide (LiNiO 2 ), lithium manganate (LiMn 2 O 4 ), lithium cobalt oxide (LiCoO 2 ), and LiNi 1/3 Co 1/3 Mn 1/3 O 2 ; lithium iron phosphate (LiFePO 4 ); sodium composite oxide; and potassium composite oxide. These electrode active materials (G) can be used alone or in combination of two or more.
• the electrode active material containing lithium iron phosphate is inexpensive and has relatively good cycle characteristics and energy density, and can therefore be used preferably.
• the particle size of the electrode active material is usually 0.5 ⁇ m or more, preferably 10.5 ⁇ m or more, and usually 30 ⁇ m or less, preferably 20 ⁇ m or less.
• the solid content of the electrode active material (G) in the 100% by mass solids of the composite paste for lithium ion secondary battery electrodes of the present invention is usually 50% by mass or more, preferably 60% by mass or more, and is preferably less than 100% by mass in terms of battery capacity, battery resistance, etc.
• the composite paste contains the electrode active material (G), it may thicken during storage.
• the electrode active material (G) has alkali metal hydroxides (e.g., LiOH, KOH, NaOH, etc.) derived from the raw materials on the particle surface, and is thought to aggregate (thicken) due to the conductive pigment having an acidic surface. Therefore, by containing a certain amount or more of the amine compound (D), it is possible to suppress the thickening of the composite paste during storage.
• alkali metal hydroxides e.g., LiOH, KOH, NaOH, etc.
• the water content of the electrode active material (G) is usually 10,000 ppm or less, preferably 7,500 ppm or less, more preferably 5,000 ppm or less, even more preferably 2,500 ppm or less, and particularly preferably 1,000 ppm or less, from the viewpoint of suppressing an increase in viscosity or gelation of the composite paste described above.
• an electrode active material composite (G-1) having at least a part of its surface covered with carbon nanotubes can be suitably used.
• the composite (G-1) can be obtained in advance by mixing the electrode active material (G), the carbon nanotubes, and, if necessary, other components (e.g., a solvent or a dispersion resin). If necessary, a drying step can be added after mixing, so that the carbon nanotubes can be more uniformly adsorbed and/or fixed to the electrode active material (G).
• the electrode active material composite (G-1) produced as described above can form a uniform conductive network around the electrode active material by adsorbing and/or fixing the carbon nanotubes to the surface of the electrode active material.
• any known carbon nanotubes can be used without particular limitation, but the carbon nanotubes exemplified as the carbon nanotubes (B) can be preferably used.
• an electrode layer for a lithium ion secondary battery (also referred to as an electrode mixture layer or a mixture layer) can be produced by applying a mixture paste for a lithium ion secondary battery to a core surface (current collector) of a positive electrode or a negative electrode and drying the applied paste, and is particularly preferably used for a positive electrode.
• the carbon nanotube dispersion paste obtained by the manufacturing method of the present invention can be used not only as a paste for a composite layer (electrode layer), but also as a primer layer (also called a functional layer or adhesive layer) between the electrode core material and the composite layer (electrode layer).
• the method of applying the composite paste for lithium ion secondary batteries can be carried out by a method known per se using a die coater or the like.
• the amount of application of the composite paste for lithium ion secondary batteries is not particularly limited, but can be set so that the thickness of the composite layer after drying is, for example, 0.04 mm or more, preferably 0.06 mm or more, and, for example, 0.30 mm or less, preferably 0.24 mm or less.
• the temperature of the drying step can be appropriately set, for example, 80° C. or more, preferably 100° C. or more, and, for example, 250° C. or less, preferably 200° C. or less.
• the time of the drying step can be appropriately set, for example, 5 seconds or more, and, for example, 120 minutes or less, preferably 60 minutes or less.
• N-methyl-2-pyrrolidone (C) and amine compound (D) volatilize.
• the cycle life is reduced if impurities such as moisture are present in the electrode layer.
• impurities such as moisture are present in the electrode layer.
• the carbon nanotube dispersion paste or composite paste contains more moisture than specified, or if the electrode layer is not dried sufficiently during the manufacturing process, moisture will remain in the electrode layer, which will cause the cycle characteristics of the battery to deteriorate.
• the moisture content in the electrode layer is usually less than 1000 ppm, preferably less than 750 ppm, more preferably less than 500 ppm, even more preferably less than 250 ppm, and particularly preferably less than 100 ppm.
• the composite paste when the composite paste is applied onto a current collector and dried by heating, it is preferable to produce a mixed solution by recovering vapor containing N-methyl-2-pyrrolidone (C), the amine compound (D), etc., and then to remove and separate impurities other than N-methyl-2-pyrrolidone (C) as much as possible by distillation, thereby producing a recycled product of N-methyl-2-pyrrolidone (C).
• the water content and the amount of amine compounds in the N-methyl-2-pyrrolidone (C) recycled product are as described above, and are preferably controlled and adjusted to a certain amount or less. It is also desirable that the product contains as few components as possible other than water and amine compounds, and in that sense, the purity is at least 99.0% or more, preferably 99.9% or more, and more preferably 99.95% or more.
• the method of removing and separating impurities to produce the above-mentioned recycled product can be carried out by a method known per se, such as the method disclosed in JP-A-10-310795 (specifically, filtration, distillation, contact with an acidic substance, etc.). These methods can be carried out alone or in combination of two or more types, and can be repeated multiple times.
• N-methyl-2-pyrrolidone A solvent for the recycled product, made by mixing new material and the recycled product produced in Application Example 1C in a 1:1 ratio.
• the water content is 500 ppm (Note 2) and the amine content is 500 ppm (Note 2).
• Production Example 2 An acrylic resin (A2) having a solids content of 50% was obtained in the same manner as in Production Example 1, except that the monomer types shown in Table 1 below were used. The weight average molecular weights of the resins obtained are shown in Table 1. In the table, “resin A1” means acrylic resin (A1), and “resin A2” means acrylic resin (A2).
• Example 1A Using a continuous dry bead mill “Drystar SDA1" (manufactured by Ashizawa Finetech Co., Ltd.), carbon nanotubes (CNT1 (Table 2)) were pulverized at a feed rate of 0.5 kg/hr using zirconia beads (diameter 3.0 mm), a filling rate of 70%, and a mill peripheral speed of 5.0 m/s.
• Drystar SDA1 manufactured by Ashizawa Finetech Co., Ltd.
• N-methyl-2-pyrrolidone (Note 1), 200 parts of the above-mentioned crushed carbon nanotubes, 80 parts of polyvinylpyrrolidone (40 parts solids) (Note 3) as a dispersion resin, 1,800 parts of KF polymer W#7300 (manufactured by Kureha Corporation, trade name, polyvinylidene fluoride, weight average molecular weight 1,000,000) resin solution (180 parts solids) (Note 4), and 25 parts of benzylamine as an amine were mixed while stirring, and finally adjusted to a total mass of 10,000 parts with N-methyl-2-pyrrolidone (Note 1).
• the mixture was dispersed in a ball mill for 4 hours to produce a carbon nanotube dispersion paste (A-1).
• the water content of the carbon nanotube dispersion paste (A-1) was 800 ppm (Note 2).
• (Note 3) Polyvinylpyrrolidone: heterocycle-containing resin, weight average molecular weight (Mw) 12,000, functional group concentration 9 (mmol/g) (Note 4)
• the polyvinylidene fluoride resin solution was prepared by mixing and dissolving polyvinylidene fluoride and N-methyl-2-pyrrolidone (Note 1) at a temperature of 80° C. Then, the solution was cooled to 30° C. at a cooling rate of about 1° C./min.
• the carbon nanotubes are multi-walled carbon nanotubes.
• the median diameter (D50), G/D ratio, specific surface area (BET specific surface area), and amount of acidic groups in Table 2 were measured by the methods described below.
• Examples 2A to 5A, Comparative Examples 1A to 4A Carbon nanotube dispersion pastes (A-2) to (A-9) were obtained in the same manner as in Example 1A except that the dispersion resin, carbon nanotubes (CNT), amine, and N-methyl-2-pyrrolidone were those shown in Table 3 below and the compositions shown in Table 3 were used.
• Example 3A carbon nanotube dispersion paste (A-3)
• Comparative Example 3A carbon nanotube dispersion paste (A-8)) used only new N-methyl-2-pyrrolidone (water content 100 ppm or less, amine content 100 ppm or less) that was not recycled.
• the water content of the carbon nanotube dispersion paste (Note 2) and the results of the evaluation test described below are shown in Table 3 below.
• the blending amounts of the dispersing resin in Table 3 above are values based on solid content.
• the composition of the dispersing resin (polymethyl methacrylate) in Table 3 above is as follows. Polymethyl methacrylate: weight average molecular weight 20,000, homopolymer of methyl methacrylate, polar functional group concentration 0 (mmol/g)
• the boiling points and molecular weights of the solvents and amine compounds in Table 3 above are as follows.
• N-methyl-2-pyrrolidone 202°C, molecular weight 99 Benzylamine: boiling point 185°C, molecular weight 107 Aminomethylpropanol: boiling point 166°C, molecular weight 89 Phenylethylamine: 195°C, molecular weight 121 Diethanolamine: 217°C, molecular weight 105.
• Examples 6A to 9A Water was added to the carbon nanotube dispersion paste (A-2) (moisture content 800 ppm) obtained in Example 2A so as to have the following moisture content (Note 2), and the mixture was thoroughly stirred to obtain the following carbon nanotube dispersion pastes (A-10) to (A-13).
• the results of the evaluation of the dispersibility of the carbon nanotube dispersion paste are shown in Table 5 below.
• Example 6A Carbon nanotube dispersion paste (A-10), moisture content 2000 ppm
• Example 7A Carbon nanotube dispersion paste (A-11), water content 4000 ppm
• Example 8A Carbon nanotube dispersion paste (A-12), water content 8000 ppm
• Example 9A Carbon nanotube dispersion paste (A-13), water content 12,000 ppm.
• Example 1B 100 parts of the carbon nanotube dispersion paste (A-1) was mixed with 900 parts of electrode active material particles (lithium nickel manganese oxide particles having a spinel structure represented by the composition formula LiNi0.5Mn1.5O4 , average particle diameter 6 ⁇ m, BET specific surface area 0.7 m2 /g, moisture content 100 ppm) using a disperser to produce a composite paste (B-1).
• the moisture content of the composite paste (B-1) was 800 ppm (Note 2).
• the median diameter (D50) was measured using a laser diffraction/scattering type particle size distribution measuring device "LA-960" (trade name, manufactured by HORIBA Co., Ltd.) according to the following procedure.
• aqueous dispersion medium 0.10 g of F10MC (trade name, carboxymethylcellulose sodium (hereinafter also referred to as CMCNa), manufactured by Nippon Paper Industries Co., Ltd.) was added to 100 mL of distilled water and dissolved by stirring at room temperature for 24 hours or more to prepare an aqueous dispersion medium containing 0.1% by mass of CMCNa.
• F10MC carboxymethylcellulose sodium
• CMCNa aqueous solution 2.0 g of F10MC (trade name, sodium carboxymethylcellulose, manufactured by Nippon Paper Industries Co., Ltd.) was added to 100 mL of distilled water and dissolved by stirring at room temperature for 24 hours or more to prepare an aqueous solution of 2.0 mass % CMCNa.
• F10MC trade name, sodium carboxymethylcellulose, manufactured by Nippon Paper Industries Co., Ltd.
• Pre-measurement processing 6.0 mg of carbon nanotubes were weighed into a vial, and 6.0 g of the aqueous dispersion medium was added.
• An ultrasonic homogenizer (Microtec Nithion, "SmurtNR-50") was used for pre-measurement treatment.
• the tip was confirmed to be free of deterioration, and was adjusted so that the tip was immersed 10 mm or more below the surface of the sample to be treated.
• the time set irradiation time was 40 seconds, the power set was 50%, the start power was 50% (output 50%), and the carbon nanotube aqueous dispersion was homogenized by ultrasonic irradiation using auto power operation with a constant output power.
• the prepared carbon nanotube aqueous dispersion was added to the particle size distribution meter so that the relative concentration, which indicates the percentage of light scattered outside the beam by the particles, was 8 to 12%, or the PIDS was 40 to 55%, and ultrasonic irradiation was performed for 2 minutes at 78 W using the particle size distribution meter attachment (measurement pretreatment), and after circulating for 30 seconds to remove air bubbles, the particle size distribution was measured.
• a graph of particle size (particle diameter) versus volume % was obtained, and the presence ratio and median diameter (D50) of dispersed particles of 1 ⁇ m or less were determined.
• D50 median diameter
• ⁇ G/D ratio of carbon nanotubes The Raman spectrum of the carbon nanotube was measured by placing the carbon nanotube in a Raman microscope (manufactured by Horiba, Ltd., product name "XploRA") and using a laser wavelength of 532 nm.
• the G/D ratio of the carbon nanotube was determined by taking the maximum peak intensity G within the range of 1560 cm -1 to 1600 cm -1 in the spectrum and the maximum peak intensity D within the range of 1310 cm -1 to 1350 cm -1 .
• BET specific surface area The BET specific surface area of the carbon nanotubes was measured as a BET specific surface area (m 2 /g) in accordance with JIS Z8830:2013 using a specific surface area measuring device (BERSORP-MAX (Microtrac-Bell Corporation)).
• Evaluation tests were carried out on the carbon nanotube dispersion pastes and composite pastes obtained in the above Examples and Comparative Examples. If even one evaluation result was unacceptable, the evaluation was deemed to be unacceptable.
• the obtained carbon nanotube dispersion paste was evaluated for dispersibility using a grain gauge according to the dispersibility test of JIS K-5600-2-5, based on the following criteria: E is unacceptable.
• A The pigment is dispersed at a particle size of less than 10 ⁇ m. The dispersibility is very good.
• B The pigment is dispersed at a size of 10 ⁇ m or more and less than 20 ⁇ m. The dispersibility is somewhat good.
• C The pigment is dispersed at a particle size of 20 ⁇ m or more, but no aggregates are visible. Dispersibility is normal.
• D A small amount of aggregates was visually observed. Dispersibility was slightly poor.
• E A large number of large aggregates are visually observed. Dispersibility is very poor.
• ⁇ Storage stability> The obtained composite paste was stored at 50°C for 2 weeks, and the initial viscosity was compared with the viscosity after storage.
• the viscosity was measured at a shear rate of 2.0 s -1 using a cone and plate viscometer (manufactured by HAAKE, product name "Mars2", diameter 35 mm, 2° inclination cone and plate), and the viscosity increase rate was calculated using the following formula, and the storage stability was evaluated according to the following criteria. D is failure.
• Viscosity increase rate (%) viscosity after storage (mPa ⁇ s) / initial viscosity (mPa ⁇ s) ⁇ 100 - 100 A: The viscosity increase rate (%) after storage is less than 20%.
• B The viscosity increase rate (%) after storage is 20% or more and less than 100%.
• C The viscosity increase rate (%) after storage is 100% or more and less than 300%.
• D The viscosity increase rate (%) after storage is 300% or more (or gelation makes it impossible to measure).
• a solution was prepared from the composite paste compositions of the Examples and Comparative Examples, excluding the resin components (dispersion resin and polyvinylidene fluoride) and pigment components (CNT and active material).
• 100 g of the above solution was placed in an open container equipped with a thermometer, thermostat, and stirrer, and heated to 200 ⁇ 2°C while stirring the inside of the container, and distilled off until the solution amounted to 95 g, producing a recycled N-methyl-2-pyrrolidone.
• the amine content (Note 2) was measured, and the recyclability of the solvent was evaluated according to the following criteria.
• B and C are failures.
• A: The amine content in the solution is 1000 ppm or less.
• B The amine content in the solution is more than 1000 ppm and not more than 2500 ppm.
• C The amine content in the solution is greater than 2500 ppm.
• the vapor evaporated during the heating and drying in the above step was recovered to obtain a recovered solution (mixed solution).
• the mixed solution was then placed in a flask equipped with a condenser, and the flask was heated to 185°C or higher to distill off the amine. This was continued until the amine content reached 1000 ppm (note 2), producing a regenerated N-methyl-2-pyrrolidone.
• the water content of the regenerated N-methyl-2-pyrrolidone was 1000 ppm (note 2).

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