Sizing agent, carbon fiber, prepreg, carbon fiber reinforced composite material, and method for producing carbon fiber

Abstract

To provide a sizing agent excellent in inhibited foaming, windability, inhibited fuzzing and inhibited powder falling, a carbon fiber, a prepreg, a carbon fiber-reinforced composite material and a method for producing a carbon fiber.SOLUTION: A sizing agent disclosed herein includes particles of a tetrafluoroethylene polymer having a melting temperature of 260°C or higher and including an oxygen-containing polar group, and at least one of a heat-resistant polymer having a 10% weight loss temperature of 320°C or higher and a precursor thereof.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to a sizing agent, a carbon fiber, a prepreg, a carbon fiber reinforced composite material, and a method for producing a carbon fiber. [Background technology]

[0002] Carbon fiber reinforced composite materials (CFRP, CFRTP, etc.), which are made by impregnating carbon fibers into a matrix resin, have excellent strength, elastic modulus, etc., and are lightweight. As a result, they are widely used as composite materials to replace metals in aircraft parts, spacecraft parts, automobile parts, ship parts, etc., as well as in sports applications such as golf shafts and fishing rods, office equipment applications, and computer applications (IC trays, laptop computer housings, etc.) in general industrial fields.

[0003] Carbon fibers are treated with a sizing agent (hereinafter also referred to as "sizing treatment") for the purposes of increasing the affinity between the matrix resin and the carbon fibers, improving interfacial adhesion, and improving the strength of carbon fiber reinforced composite materials, as well as coating and converging the carbon fibers, reducing damage, and making them easier to handle, and various sizing agents have been investigated. Patent Document 1 discloses a prepreg obtained by impregnating carbon fibers to which a sizing agent containing an aliphatic epoxy compound and an aromatic epoxy compound has been attached with a specific epoxy resin composition as a matrix resin. Patent Document 2 discloses carbon fibers coated with a sizing agent containing a polymer having one of ester, urethane, and carbonate bonds in the main chain. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-40566 [Patent Document 2] Japanese Patent Publication No. 2020-23770 Summary of the Invention [Problem to be solved by the invention]

[0005] The polymers contained in the sizing agents described in Patent Documents 1 and 2, such as epoxy resins and polyurethane resins, thermally decompose during heating after the sizing agent is applied to the carbon fibers or during heat molding to produce a carbon fiber reinforced composite material, generating volatile components and foaming, which poses a problem of affecting the appearance, such as smoothness, and the adhesion / peel strength. Furthermore, polyamideimide resin (hereinafter also referred to as "PAI"), polyimide resin (hereinafter also referred to as "PI"), etc. are known as polymers that can be contained in a sizing agent and have excellent heat resistance. Because heat-resistant polymers such as PAI and PA are hard, it is difficult to wind up carbon fiber bundles and carbon fiber sheets sized with the heat-resistant polymers into rolls, leaving room for improvement in productivity. Furthermore, there is a problem that heating after applying a sizing agent containing PAI or PI to carbon fibers promotes imidization, resulting in foaming.

[0006] As a result of extensive research, the present inventors have discovered that the problems of foaming and winding can be solved by incorporating into a sizing agent particles of a tetrafluoroethylene-based polymer having a melting temperature of 260°C or higher and containing an oxygen-containing polar group, and at least one of a heat-resistant polymer and its precursor having a 10% weight loss temperature of 320°C or higher. Furthermore, the present inventors have found that the sizing agent can suppress the occurrence of problems in unwinding the carbon fiber due to powder falling off of the tetrafluoroethylene-based polymer particles from the sized carbon fiber and fluffing. Therefore, the problem to be solved by the present disclosure is to provide a sizing agent, carbon fiber, prepreg, carbon fiber reinforced composite material, and method for producing carbon fiber, which are excellent in foam suppression, winding properties, fuzz suppression, and powder shedding suppression. [Means for solving the problem]

[0007] Means for solving the above problems include the following aspects. <1> A sizing agent containing particles of a tetrafluoroethylene-based polymer having a melting temperature of 260°C or higher and having an oxygen-containing polar group, and at least one selected from the group consisting of a heat-resistant polymer and its precursor having a 10% weight loss temperature of 320°C or higher. <2> The heat-resistant polymer is one or more compounds selected from the group consisting of polyimide resins, polyamide-imide resins, and modified products thereof. <1> The sizing agent according to claim 1. <3> The tetrafluoroethylene polymer particles have an average particle size of 0.1 μm to 200 μm. <1> or <2> The sizing agent according to claim 1. <4> The 10% weight loss temperature of the particles of the tetrafluoroethylene-based polymer is 350°C or higher. <1> ~ <3> 10. The sizing agent according to claim 9, wherein the sizing agent is a sizing agent having a viscosity of 1000 MPa or less. <5> The content of the tetrafluoroethylene polymer particles relative to the total mass of the sizing agent is 1 to 30 mass %. <1> ~ <4> 10. The sizing agent according to claim 9, wherein the sizing agent is a sizing agent having a viscosity of 1000 MPa or less. <6> The content of the heat-resistant polymer relative to the total mass of the sizing agent is 0.5 to 20 mass %. <1> ~ <5> 10. The sizing agent according to claim 9, wherein the sizing agent is a sizing agent having a viscosity of 1000 MPa or less. <7> the ratio of the content of the tetrafluoroethylene-based polymer particles to the sum of the contents of the tetrafluoroethylene-based polymer particles and the heat-resistant polymer contained in the sizing agent is more than 50 / 100 and not more than 99 / 100 by mass; <1> ~ <6> 10. The sizing agent according to claim 9, wherein the sizing agent is a sizing agent having a viscosity of 1000 MPa or less. <8> The composition contains an emulsifier, and the heat-resistant polymer is emulsified by the emulsifier. <1> ~ <7> 10. The sizing agent according to claim 9, wherein the sizing agent is a sizing agent having a viscosity of 1000 MPa or less. <9> The above, containing a surfactant <1> ~ <8> 10. The sizing agent according to claim 9, wherein the sizing agent is a sizing agent having a viscosity of 1000 MPa or less. <10> The above, which contains a liquid dispersion medium. <1> ~ <9> 10. The sizing agent according to claim 9, wherein the sizing agent is a sizing agent having a viscosity of 1000 MPa or less. <11> A carbon fiber having, on its surface, at least one selected from the group consisting of a tetrafluoroethylene-based polymer having a melting temperature of 260°C or higher and having an oxygen-containing polar group, and a heat-resistant polymer and its precursor having a 10% weight loss temperature of 320°C or higher. <12> the above <11> A prepreg comprising the carbon fiber according to claim 1 and a matrix resin. <13> the above <12> A carbon fiber reinforced composite material which is a molded product of the prepreg according to claim 1. <14> the above <1> ~ <10> 1. A method for producing sized carbon fibers, comprising disposing the sizing agent according to any one of the above items on carbon fibers and heat-treating the fibers. [Effects of the Invention]

[0008] According to the present disclosure, there are provided a sizing agent, carbon fiber, prepreg, carbon fiber reinforced composite material, and method for producing carbon fiber, which are excellent in foam suppression, winding properties, fluff suppression, and powder shedding suppression. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, modes for carrying out embodiments of the present disclosure will be described in detail. However, the embodiments of the present disclosure are not limited to the following embodiments. In the following embodiments, components (including element steps, etc.) are not essential unless otherwise specified. The same applies to numerical values ​​and their ranges, and they do not limit the embodiments of the present disclosure.

[0010] In the present disclosure, numerical ranges indicated using "to" include the numerical values ​​before and after "to" as the minimum and maximum values, respectively. In the present disclosure, each component may contain multiple substances corresponding to the component. When multiple substances corresponding to each component are present in the composition, the content or amount of each component means the total content or amount of the multiple substances present in the composition, unless otherwise specified. In the present disclosure, multiple types of particles corresponding to each component may be contained. When multiple types of particles corresponding to each component are present in the composition, the particle size of each component means the value for a mixture of the multiple types of particles present in the composition, unless otherwise specified. In this disclosure, the term "lamination" refers to stacking layers, and two or more layers may be bonded together, or two or more layers may be detachable. In the present disclosure, the "average particle diameter (D50)" refers to the volume-based cumulative 50% diameter of particles determined by laser diffraction / scattering. That is, the particle size distribution is measured by laser diffraction / scattering, and a cumulative curve is calculated with the total volume of the particle population set as 100%. The "average particle diameter (D50)" is the particle diameter at the point on the cumulative curve where the cumulative volume is 50%. The D50 of particles is determined by dispersing the particles in water and analyzing them by the laser diffraction / scattering method using a laser diffraction / scattering particle size distribution measuring device (LA-920 measuring device, manufactured by Horiba, Ltd.). In the present disclosure, the "melting temperature" is the temperature corresponding to the maximum value of the melting peak of a polymer as measured by differential scanning calorimetry (DSC). In the present disclosure, the "10% weight loss temperature" can be measured by a measurement method using a thermogravimetric differential thermal analyzer (TG-DTA) with a temperature rise rate of 10°C / min, a sample weight of 10 mg, and a mixed gas (90% by volume of helium and 10% by volume of oxygen) as the atmospheric gas. For example, when 10 mg of a polymer is heated from 200°C at a rate of 10°C / min in a mixed gas atmosphere (90% by volume of helium and 10% by volume of oxygen) using a thermogravimetric differential thermal analyzer (TG-DTA), the mass loss can be calculated as a percentage value obtained by dividing the value by the heating time (10 minutes) and the polymer sample weight (10 mg). In the present disclosure, the "weight average molecular weight" can be determined in terms of polystyrene using gel permeation chromatography (GPC). In the present disclosure, the term "melt flow rate" refers to the melt mass flow rate of a polymer as defined in JIS K 7210:1999 (ISO 1133:1997). In the present disclosure, the "glass transition temperature (Tg)" is a value measured by analyzing a polymer using a dynamic mechanical analysis (DMA) method. In the present disclosure, "viscosity" is determined by measuring the dispersion using a Brookfield viscometer at 25° C. and a rotation speed of 30 rpm. The measurement is repeated three times, and the average value of the three measured values ​​is used. In the present disclosure, the "thixotropy ratio" is a value calculated by dividing the viscosity η1 of a dispersion measured at a rotation speed of 30 rpm by the viscosity η2 measured at a rotation speed of 60 rpm. Each viscosity measurement is repeated three times, and the average value of the three measurements is used. In the present disclosure, the "average fiber diameter" is the average value of the fiber diameters measured at 100 randomly selected points on a photograph of the cross section of a fiber taken at 1000x magnification using a field emission scanning electron microscope. In the present disclosure, a "polymer" is a compound formed by polymerizing a monomer, i.e., a "polymer" has a plurality of units based on the monomer. In the present disclosure, the term "unit" in a polymer refers to an atomic group based on a monomer formed by polymerization of the monomer. The unit may be a unit formed directly by a polymerization reaction, or may be a unit in which a part of the unit is converted into a different structure by treating the polymer. In the present disclosure, the term "(meth)acrylic" is used as a concept that encompasses both acrylic and methacrylic.

[0011] The sizing agent of the present disclosure contains particles (hereinafter also referred to as "F particles") of a tetrafluoroethylene-based polymer having a melting temperature of 260°C or higher and containing oxygen-containing polar groups (hereinafter, a tetrafluoroethylene-based polymer having a melting temperature of 260°C or higher and containing oxygen-containing polar groups will also be referred to as "F polymer"), and at least one selected from the group consisting of a heat-resistant polymer having a 10% weight loss temperature of 320°C or higher (hereinafter, a heat-resistant polymer having a 10% weight loss temperature of 320°C or higher will also be simply referred to as "heat-resistant polymer") and its precursor. The sizing agent of the present disclosure is excellent in foam suppression, fluff suppression, winding properties, and powder shedding suppression. Although its mechanism of action is unclear, it is generally presumed to be as follows. The F polymer constituting the F particles contained in the sizing agent of the present disclosure has excellent heat resistance, so it is unlikely to decompose even when heated at high temperatures, which is thought to reduce the foaming effect of the heat-resistant polymer and improve the foam-suppressing ability of the sizing agent. Furthermore, the F polymer has excellent flexibility, which is thought to improve the winding ability of carbon fibers produced using the sizing agent. It is presumed that the heat-resistant polymer contained in the sizing agent of the present disclosure functions as a binder for the F particles and carbon fibers, thereby improving the ability to suppress fluffing and powder shedding. Furthermore, since the sizing agent of the present disclosure has excellent foam-suppressing properties, the carbon fibers, prepregs, and carbon fiber-reinforced composite materials produced using the sizing agent have excellent smoothness and good appearance.

[0012] In addition to the F particles and heat-resistant polymer, the sizing agent of the present disclosure may contain a polymer different from the F polymer and the heat-resistant polymer, an emulsifier, a surfactant, a liquid dispersion medium, various additives, and the like. Each component of the sizing agent will be described below.

[0013] The sizing agent of the present disclosure contains F particles, which are particles of an F polymer. One type of F particle may be used, or two or more types may be used. The F polymer is a polymer containing units (hereinafter also referred to as "TFE units") based on tetrafluoroethylene (hereinafter also referred to as "TFE units"). From the viewpoint of suitably expressing the properties due to the TFE units, the content of the TFE units in the F polymer is preferably 50 mol% or more, more preferably 90 mol% or more, based on the total units in the F polymer. The content may be 99 mol% or less, or may be 98 mol% or less. The F polymer preferably has a melt flow rate of 1 to 30 g / min, more preferably 5 to 30 g / min, under a load of 49 N.

[0014] The F polymer has an oxygen-containing polar group. The presence of the oxygen-containing polar group in the F polymer can suppress aggregation of the F particles in the sizing agent and improve affinity with the heat-resistant polymer, resulting in a sizing agent with excellent uniformity and dispersibility. Furthermore, the presence of the oxygen-containing polar group in the F polymer can improve foam suppression. Examples of the oxygen-containing polar group include a hydroxyl group-containing group, a carbonyl group-containing group, and a phosphono group-containing group, with a hydroxyl group-containing group or a carbonyl group-containing group being preferred, and a carbonyl group-containing group being more preferred. The oxygen-containing polar group possessed by the F polymer may be one type or two or more types. The hydroxyl group-containing group is preferably a group containing an alcoholic hydroxyl group, more preferably -CF2CH2OH and -C(CF3)2OH. The carbonyl group-containing group is preferably a carboxyl group, an alkoxycarbonyl group, an amide group, an isocyanate group, a carbamate group (-OC(O)NH), an acid anhydride residue (-C(O)OC(O)-), an imide residue (-C(O)NHC(O)-, etc.) or a carbonate group (-OC(O)O-), and more preferably an acid anhydride residue.

[0015] The number of oxygen-containing polar groups in the F polymer is 1 × 10 6The number of oxygen-containing polar groups per unit is preferably 10 to 5000, more preferably 100 to 3000. The number of oxygen-containing polar groups can be quantified based on the composition of the polymer or by the method described in WO 2020 / 145133. The oxygen-containing polar group may be contained in a unit derived from a monomer in the F polymer, or may be contained in a terminal group of the main chain of the F polymer, the former being preferred. Examples of the latter include tetrafluoroethylene-based polymers having an oxygen-containing polar group as a terminal group derived from a polymerization initiator, a chain transfer agent, etc., and polymers obtained by subjecting tetrafluoroethylene-based polymers to plasma treatment or ionizing radiation treatment.

[0016] As the monomer having a carbonyl group-containing group, itaconic anhydride, citraconic anhydride, and 5-norbornene-2,3-dicarboxylic anhydride (hereinafter also referred to as "NAH") are preferred, and NAH is more preferred from the viewpoint of excellent adhesion to carbon fiber bundles.

[0017] The F polymer is preferably polytetrafluoroethylene (PTFE), a polymer containing TFE units and ethylene-based units (ETFE), a polymer containing TFE units and propylene-based units, a polymer containing TFE units and perfluoro(alkyl vinyl ether) (PAVE)-based units (PAVE units) (PFA), or a polymer containing TFE units and hexafluoropropylene-based units (FEP), more preferably PFA or FEP having oxygen-containing polar groups, and even more preferably PFA having oxygen-containing polar groups.These polymers may further contain units based on other comonomers.

[0018] The F polymer is preferably a polymer having a carbonyl group-containing group containing TFE units and PAVE units, more preferably a polymer containing TFE units, PAVE units, and units based on a monomer having a carbonyl group-containing group, and even more preferably a polymer containing TFE units, PAVE units, and units based on a monomer having a carbonyl group-containing group, in the following order: 90 to 99 mol%, 0.99 to 9.97 mol%, and 0.01 to 3 mol% of these units relative to the total units. Perfluoro(propyl vinyl ether) (PPVE) is preferred as PAVE. Specific examples of such F polymers include the polymers described in WO 2018 / 016644.

[0019] From the viewpoint of further improving the foam suppression property, the melting temperature of the F polymer is preferably 270° C. or higher, more preferably 280° C. or higher, and even more preferably 290° C. or higher. The upper limit of the melting temperature is not particularly limited, but can be 325° C. or lower. From the viewpoint of further improving the foam suppression property, the 10% weight loss temperature of the F polymer is preferably 350° C. or higher, more preferably 360° C. or higher, and even more preferably 370° C. or higher. There is no particular upper limit to the 10% weight loss temperature, but it can be 600° C. or lower. From the viewpoint of further improving the foam suppression property, the glass transition point of the F polymer is preferably 50° C. or higher, more preferably 75° C. or higher. There is no particular upper limit to the glass transition point of the F polymer, but it can be 150° C. or lower. The fluorine content of the F polymer is preferably 70 to 76 mass % from the viewpoint of improving the electrical properties (low dielectric constant, etc.) and heat resistance of the carbon fiber. The fluorine content is determined from the composition of the polymer.

[0020] The average particle size (D50) of the F particles is preferably 0.03 μm to 200 μm, more preferably 0.1 μm to 200 μm, more preferably 0.1 μm to 50 μm, and even more preferably 0.1 μm to 30 μm. F particles having the above D50 have excellent fluidity and tend to be uniformly distributed on the surface of the carbon fiber. In addition, the heat resistance and electrical properties of the F particles are most easily exhibited.

[0021] The F particles may contain polymers other than the F polymer, but preferably contain the F polymer as the main component. The content of the F polymer in the F particles is preferably 80% by mass or more, more preferably 100% by mass.

[0022] From the viewpoint of further improving the foam suppression property and winding property, the content of F particles relative to the total mass of the sizing agent is preferably 1 to 30 mass%, more preferably 3 to 20 mass%, even more preferably 5 to 15 mass%, and particularly preferably 8 to 15 mass%. From the viewpoint of further improving the foam suppression, winding property, fluff suppression, and powder shedding suppression, the ratio of the F particle content to the sum of the F particle and heat-resistant polymer contents contained in the sizing agent (F particle content / sum of F particle and heat-resistant polymer contents) is preferably more than 50 / 100 and not more than 99 / 100, and more preferably 80 / 100 or more and 95 / 100 or less, by mass. From the viewpoint of further improving the foaming suppression, winding property, fluffing suppression, and powder shedding suppression, the sum of the contents of the F particles and the heat-resistant polymer relative to the total mass of the sizing agent is preferably 1 to 50 mass%, more preferably 5 to 50 mass%, and even more preferably 7 to 45 mass%.

[0023] The sizing agent of the present disclosure contains at least one of a heat-resistant polymer and a precursor thereof, the heat-resistant polymer having a 10% weight loss temperature of 320° C. or higher. One type of heat-resistant polymer and a precursor thereof may be used, or two or more types may be used. From the viewpoint of further improving the foam suppression property, the 10% weight loss temperature is preferably 350° C. or higher, and more preferably 370° C. or higher. There is no particular upper limit to the 10% weight loss temperature, but it can be 600° C. or lower. The F polymer constituting the F particles contained in the sizing agent of the present disclosure has excellent flexibility, so that even when a hard heat-resistant polymer is used, the carbon fiber treated with the sizing agent has excellent winding properties.

[0024] Examples of heat-resistant polymers include polyamide resins (hereinafter also referred to as "PA"), PI, PAI, polyetherimide resins, polysulfone resins, polyethersulfone resins, polyetherketone resins, polyetheretherketone resins, polyetherketoneketone resins, polyolefin resins, polyacetal resins, polycarbonate resins, polyester resins, polyphenylene sulfide resins, polyphenylene ether resins, poly(meth)acrylate resins, fluororesins, and modified products thereof. Among the above, PI, PAI, and modified products thereof are preferred from the viewpoints of heat resistance and affinity with F particles. The above-mentioned modified compounds include compounds in which an oxyalkylene group, a sulfo group, a carboxyl group, a 1,3-dioxo-2-oxapropylene group, or the like has been introduced into PI or the like. By modifying PI or the like, adhesion between the carbon fiber and the matrix resin can be improved. Furthermore, the heat-resistant polymer is preferably emulsified with an emulsifier, which will be described later, in the sizing agent. However, by using a modified PI or the like, emulsion stability can be improved and the amount of emulsifier used can be reduced. Hydrosize HP-1632 manufactured by Michelman is known as a commercially available aqueous emulsion containing an emulsion of a heat-resistant polymer.

[0025] When one or more compounds selected from PI, PAI and modified products thereof are used as the heat-resistant polymer, it is preferable that the compound has been completely imidized. Since the compound generates water during imidization, the degree of imidization of the compound can be estimated from the water generation ratio V (mass %) calculated by the following formula (1): The water generation ratio V is preferably 0.05 mass % or less, more preferably 0.03 mass % or less, and even more preferably 0.01 mass % or less. V (mass%)=B / A×100 (1) In formula (1), A represents the mass (g) of the sized carbon fiber measured after leaving the sized carbon fiber standing for 2 hours at 110°C. In formula (1), B represents the difference between the mass (g) of the carbon fiber at 130°C and the mass (g) of the carbon fiber at 415°C measured by thermogravimetric analysis (TGA) after leaving the sized carbon fiber standing for 2 hours at 110°C and then heating the carbon fiber to 450°C at a heating rate of 10°C / min (mass (g) of the carbon fiber at 130°C - mass (g) of the carbon fiber at 415°C).

[0026] The weight average molecular weight of the heat-resistant polymer is not particularly limited, but is preferably 5,000 to 200,000.The glass transition point of the heat-resistant polymer is preferably 10 to 350°C. From the viewpoint of further improving the fuzz suppression and powder shedding suppression, the content of the heat-resistant polymer relative to the total mass of the sizing agent is preferably 0.5% by mass or more, more preferably 0.7% by mass or more, and even more preferably 1% by mass or more. From the viewpoint of further improving the foam suppression and winding property, the content is preferably 20% by mass or less, more preferably 15% by mass or less, and even more preferably 10% by mass or less. The content of the heat-resistant polymer relative to the total mass of the sizing agent is preferably 0.5 to 20% by mass.

[0027] The sizing agent of the present disclosure may contain an emulsifier, which can emulsify the heat-resistant polymer and improve dispersibility in the sizing agent. From the viewpoint of ease of emulsification of the heat-resistant polymer, anionic emulsifiers and nonionic emulsifiers are preferred. Specific examples of emulsifiers include alkylene oxide adducts of alkylphenols, alkylene oxide adducts of arylalkylphenols, Pluronic surfactants, sulfate ester salts of the alkylene oxide adducts of alkylphenols, sulfate ester salts of the alkylene oxide adducts of arylalkylphenols, urethane-joint compounds of the Pluronic surfactants, and urethane-joint compounds of the alkylene oxide adducts of arylalkylphenols and polyethylene glycol. When the sizing agent of the present disclosure contains an emulsifier, the content of the emulsifier relative to the total mass of the sizing agent is preferably 0.01 to 10% by mass from the viewpoint of dispersibility of the heat-resistant polymer.

[0028] The sizing agent of the present disclosure may contain a surfactant, which can improve the dispersibility and handleability of the sizing agent. The surfactant is preferably a nonionic surfactant. The hydrophilic portion of the surfactant preferably has an oxyalkylene group or an alcoholic hydroxyl group. The hydrophobic portion of the surfactant preferably has an acetylene group, a polysiloxane group, or a fluorine-containing organic group (such as a perfluoroalkyl group). In other words, the surfactant is preferably an acetylene-based surfactant, a silicone-based surfactant, or a fluorine-based surfactant. When the sizing agent of the present disclosure contains a surfactant, the content thereof relative to the total mass of the sizing agent is preferably 1 to 15 mass %. However, because the F particles contained in the sizing agent of the present disclosure have excellent dispersibility, a sizing agent with excellent dispersibility and handleability can be produced without necessarily using a surfactant. Therefore, the sizing agent attached to carbon fibers is likely to be uniformly dispersed on the fiber surface, and after heat treatment, the F particles are likely to be densely and uniformly distributed on the fiber surface.

[0029] The sizing agent of the present disclosure may contain a liquid dispersion medium. In other words, a sizing agent of the present disclosure containing a liquid dispersion medium can also be considered a dispersion containing F particles and at least one selected from the group consisting of heat-resistant polymers and their precursors, in which F particles are dispersed in a liquid dispersion medium. In such a dispersion, the F particles are highly and uniformly dispersed in the liquid, making it easier for the above-mentioned mechanism of action to be significantly exhibited. The boiling point of the liquid dispersion medium is preferably in the range of 50 to 240° C. One type of dispersion medium may be used alone, or two or more types may be used in combination. When two or more types of liquid dispersion mediums are used, the two or more types of liquid dispersion mediums are preferably mutually compatible. Examples of the dispersion medium include water, N,N-dimethylformamide, N,N-dimethylacetamide, 3-methoxy-N,N-dimethylpropanamide, 3-butoxy-N,N-dimethylpropanamide, N-methyl-2-pyrrolidone, γ-butyrolactone, cyclohexanone, cyclopentanone, butyl acetate, methyl isopropyl ketone, and methyl ethyl ketone, and among these, water is preferred. When the sizing agent of the present disclosure contains a liquid dispersion medium, the content of the sizing agent relative to the total mass of the sizing agent is preferably 30 to 95 mass %, more preferably 50 to 90 mass %.

[0030] The sizing agent of the present disclosure may further contain a polymer (hereinafter also referred to as a "different polymer") different from the F polymer and the heat-resistant polymer. The different polymer may be thermosetting or thermoplastic. One type of different polymer may be used, or two or more types may be used. The different polymer is not particularly limited as long as it is a polymer other than an F polymer or a heat-resistant polymer, and examples thereof include tetrafluoroethylene polymers, polyester resins (such as liquid crystalline aromatic polyesters), polyimide resins, epoxy resins, maleimide resins, polyurethane resins, polyphenylene ether resins, polyphenylene oxide resins, and polyphenylene sulfide resins. The tetrafluoroethylene-based polymers include PTFE, ETFE, PFA, PFA, and FEP other than F polymer, and non-thermofusible PTFE is preferred from the viewpoint of improving the electrical properties of sized carbon fibers (hereinafter also referred to as "sized fibers"). The non-thermofusible PTFE may be contained in the sizing agent as particles or may be in a non-particulate form. When the sizing agent of the present disclosure contains a different polymer, the content of the sizing agent relative to the total mass of the sizing agent is preferably 0.1 to 5 mass %.

[0031] In addition to the above components, the sizing agent of the present disclosure may further contain other components, such as a thixotropy-imparting agent, a pH adjuster, a pH buffer, a viscosity modifier, an antifoaming agent, a silane coupling agent, a dehydrating agent, a plasticizer, a weathering agent, an antioxidant, a heat stabilizer, a lubricant, an antistatic agent, a brightener, a colorant, a conductive agent, a release agent, a surface treatment agent, a flame retardant, various inorganic fillers, and various organic fillers, within a range that does not impair the effects of the sizing agent.

[0032] When the sizing agent of the present disclosure contains a liquid dispersion medium, the viscosity of the sizing agent of the present disclosure is preferably 75 to 10,000 mPa·s. In this case, not only is the dispersibility excellent, but the handling property and the uniformity of the sizing treatment on the carbon fiber are also likely to be improved. In addition, such a sizing agent has excellent miscibility with varnishes of different resin materials. When the sizing agent of the present disclosure contains a liquid dispersion medium, the thixotropy ratio of the sizing agent of the present disclosure is preferably 1.0 to 2.2, more preferably 1.5 to 2.0. In this case, the sizing agent has excellent dispersibility and good handleability, and the uniformity of the sizing treatment on the carbon fiber is likely to be improved. In addition, such a sizing agent has excellent miscibility with varnishes of different resin materials.

[0033] The sizing agent can be produced by mixing F particles, a heat-resistant polymer, a liquid dispersion medium, and the like. When the sizing agent contains a liquid dispersion medium, the mixing may be performed by adding each component to the liquid dispersion medium all at once or by sequentially adding each component and mixing. The mixing may be performed batchwise or continuously. Examples of devices that can be used for the above mixing include stirring devices equipped with blades (Henschel mixers, pressure kneaders, Banbury mixers, planetary mixers, etc.), grinding devices equipped with media (ball mills, attritors, basket mills, sand mills, sand grinders, Dyno Mills, Dispermats, SC mills, spike mills, agitator mills, etc.), and dispersing devices equipped with other mechanisms (microfluidizers, nanomizers, ultimizers, ultrasonic homogenizers, dissolvers, disper, high-speed impellers, thin film swirling high-speed mixers, etc.).

[0034] The carbon fiber of the present disclosure has, on its surface, at least one selected from the group consisting of a tetrafluoroethylene-based polymer having a melting temperature of 260°C or higher and having an oxygen-containing polar group, and a heat-resistant polymer and its precursor having a 10% weight loss temperature of 320°C or higher. The carbon fibers of the present disclosure tend to have excellent adhesion to matrix resins and also have high impregnation of the matrix resin into carbon fiber bundles, making it easy to obtain prepregs and carbon fiber reinforced composite materials with excellent mechanical properties. The F polymer and the heat-resistant polymer have been described above, and therefore will not be described here. The F polymer may be in the form of particles. The surface of the carbon fiber of the present disclosure may contain other polymers, emulsifiers, surfactants, various additives, and the like, which have been described above, and therefore will not be described here. The F polymer present on the surface of the carbon fiber of the present disclosure has excellent heat resistance and is therefore resistant to decomposition even when heated at high temperatures. Therefore, even when a prepreg is produced using a super engineering plastic such as PA, which requires high-temperature heating when impregnating the carbon fiber, foaming is unlikely to occur. Therefore, the carbon fiber of the present disclosure is suitable for producing a prepreg containing a super engineering plastic. In this disclosure, super engineering plastic refers to a polymer with a continuous use temperature of 150°C or higher.

[0035] The carbon fiber of the present disclosure may be a single carbon fiber or a carbon fiber bundle made up of a plurality of carbon fibers. When the carbon fiber of the present disclosure is a carbon fiber bundle, the number of filaments per bundle is preferably 50 to 100,000, and more preferably 10,000 to 50,000.

[0036] Examples of carbon fibers include pitch-based carbon fibers, rayon-based carbon fibers, acrylonitrile (hereinafter also referred to as "PAN")-based carbon fibers, single-walled carbon nanotubes, multi-walled carbon nanotubes, and carbon nanofibers. Among the above, PAN-based carbon fibers are preferred from the viewpoints of operability, processability, mechanical properties, and the like. PAN-based carbon fibers can be obtained, for example, by subjecting a carbon fiber precursor fiber made of a PAN-based polymer to flame retardation treatment in an oxidizing atmosphere at 200 to 300°C, followed by preliminary carbonization treatment in an inert atmosphere at 500 to 1200°C, and then carbonization treatment in an inert atmosphere at 1200 to 2000°C. The average fiber diameter of the carbon fibers is preferably 3 nm to 300 μm, more preferably 30 nm to 250 μm. The carbon fiber may be in any form such as chopped strands, surfacing, roving, a mat thereof, a woven fabric, or a nonwoven fabric.

[0037] The method for producing sized carbon fibers of the present disclosure is a method for obtaining carbon fibers by disposing the sizing agent on carbon fibers and heat treating them.

[0038] When the sizing agent of the present disclosure contains a liquid dispersion medium, methods for disposing the sizing agent on the carbon fibers include a roller sizing method in which a roller is partially immersed in the sizing agent to adhere the sizing agent to the surface of the roller, and then the carbon fibers are brought into contact with the roller to adhere the sizing agent; a roller immersion method in which the carbon fibers are directly immersed in the sizing agent and then passed through nip rolls as necessary to control the amount of sizing agent attached; and a spray method in which the sizing agent is atomized and sprayed onto the fiber bundle. Among the above methods, the roller immersion method is preferred because it makes it easy to apply the sizing agent uniformly, even to fiber bundles with a large number of single fibers per bundle. The roller immersion method is also preferred because it makes it easy to adjust the amount of sizing agent attached to the carbon fibers by using nip rolls or the like. The application of the sizing agent to the carbon fibers may be carried out by a batch method or a continuous method, but the continuous method is preferred from the viewpoint of easily suppressing uneven application of the sizing agent to the carbon fibers and improving productivity. Note that the carbon fibers may be vibrated with ultrasonic waves when the sizing agent is applied.

[0039] The amount of sizing agent attached to the carbon fiber is preferably adjusted so that the amount of F polymer attached is 0.1 to 10 parts by mass, more preferably 0.2 to 5 parts by mass, per 100 parts by mass of the sized fiber after heat treatment.

[0040] By heat treating carbon fibers on which a sizing agent has been applied, it is possible to obtain sized fibers on which an F polymer and a heat-resistant polymer have been attached to the surface. The F polymer attached to the surface of the carbon fibers is more preferably a baked product of the F polymer. The sized fibers on which a baked product of the F polymer has been attached to the surface can be obtained by heat treating carbon fibers on which a sizing agent has been applied to bake the F polymer. Specifically, when the sizing agent of the present disclosure contains a liquid dispersion medium, the liquid dispersion medium is evaporated from the carbon fibers to which the sizing agent has been applied, and the carbon fibers are then heated to bake the F polymer. The heat treatment for evaporating the liquid dispersion medium is carried out by holding the carbon fiber with the sizing agent at the volatilization temperature of the liquid dispersion medium to dry the liquid coating on the carbon fiber. The heat treatment for baking the F polymer is carried out by holding the dried coating at a temperature above the melting temperature of the F polymer. In this way, a baked F polymer is formed on the surface of the carbon fiber. Note that the temperature during the heat treatment usually means the temperature of the dry atmosphere. During drying, the liquid dispersion medium does not necessarily have to be completely evaporated. Specifically, the amount of liquid dispersion medium to be evaporated is preferably 50% by mass or more of the liquid dispersion medium contained in the sizing agent.

[0041] Heat treatment for baking the F polymer may be carried out using an oven, a ventilation oven, or by irradiating with heat rays such as infrared rays, or may be a combination of infrared heating and hot air heating. The F polymer may be calcined under either normal pressure or reduced pressure. The calcination atmosphere may be any of an oxidizing gas atmosphere, a reducing gas atmosphere, and an inert gas atmosphere. The calcination temperature is preferably equal to or higher than the melting point of the F polymer, and is usually 300 to 380°C. The calcination time is preferably 30 seconds to 30 minutes, and more preferably 1 to 15 minutes. Calcining the F polymer under such conditions increases productivity and also makes it easier to suppress the generation of hydrofluoric acid due to decomposition of the F polymer.

[0042] The prepreg of the present disclosure contains the above-described carbon fibers and a matrix resin, and is typically obtained by impregnating sized fibers with the matrix resin.

[0043] The matrix resin may be a thermosetting resin or a thermoplastic resin. Examples of thermosetting resins include epoxy resins, unsaturated polyester resins, phenolic resins, vinyl ester resins, cyanate ester resins, urethane acrylate resins, phenoxy resins, alkyd resins, urethane resins, prepolymerized resins of maleimide resins and cyanate ester resins, bismaleimide resins, polyimide resins and polyisoimide resins having acetylene terminals, and polyimide resins having Nadic acid terminals. These thermosetting resins may be used alone or in combination of two or more. Furthermore, the thermosetting resin may be used in a thermosetting resin composition containing various additives in addition to a curing agent and a curing accelerator. Examples of thermoplastic resins include polysulfone resins, polyphenylsulfone resins, polyethersulfone resins, polyetherketone resins, polyetheretherketone resins, polyetherketoneketone resins, polyetheretherketoneketone resins, PA, polyester resins, polycarbonate resins, polyetherimide resins, polyarylene oxide-based resins, thermoplastic PA, PAI, polyacetal resins, polyphenylene oxide resins, polyphenylene sulfide resins, liquid crystal polyester resins, poly(meth)acrylate resins, polyacrylonitrile resins, and polybenzimidazole resins. These thermoplastic resins may be used alone or in combination of two or more. Furthermore, the thermoplastic resin may be used in a thermoplastic resin composition containing various additives. When a thermosetting resin or a thermoplastic resin is used in a resin composition, examples of various additives include a plasticizer, a weathering agent, an antioxidant, a heat stabilizer, a lubricant, an antistatic agent, a whitening agent, a colorant, a conductive agent, a mold release agent, a surface treatment agent, a flame retardant, various inorganic fillers, and various organic fillers.

[0044] Among the above, the matrix resin is preferably one or more resins selected from the group consisting of PA, polysulfone resin, polyphenylsulfone resin, polyethersulfone resin, polyetherketone resin, polyetheretherketone resin, polyetherketoneketone resin, polyetheretherketoneketone resin, polyetherimide resin, polyphenylene sulfide resin, and liquid crystal polyester resin. These are classified as ultra-heat-resistant resins known as super engineering plastics, and are more likely to exhibit the effects of sizing-treated fibers. When the matrix resin has polar functional groups such as carbonyl groups, sulfonic groups, or ester groups in its structure, it is presumed that the polar functional groups in the matrix resin interact with the polar functional groups in the F polymer, resulting in high adhesion.

[0045] The carbon fiber content of the prepreg is preferably 40 to 90 mass %, more preferably 50 to 80 mass %, relative to the total mass of the prepreg. Within this range, the mass of the prepreg and the carbon fiber reinforced composite material obtained by molding the prepreg will not be excessively large, and the advantages of the carbon fiber reinforced composite material, which has excellent specific strength and specific modulus, can be utilized.

[0046] The prepreg of the present disclosure can be produced by impregnating sized fibers with a matrix resin or a matrix resin composition, followed by drying and semi-curing. Specifically, it can be produced by a wet method in which the matrix resin or matrix resin composition is dissolved in a solvent such as methyl ethyl ketone or methanol to reduce the viscosity and then impregnated, or a hot melt method in which the viscosity is reduced by heating and then impregnated. In the wet method, the sized fibers are immersed in a liquid containing a matrix resin, then removed, and the solvent is evaporated using an oven or the like to produce a prepreg. In the hot melt method, prepregs can be produced by directly impregnating sized fibers with a matrix resin or matrix resin composition whose viscosity has been reduced by heating, or by first coating a release paper or the like with the matrix resin or matrix resin composition to form a film, then placing the film on one or both sides of the sized fibers and heating and pressurizing the resulting mixture to impregnate the sized fibers into the matrix resin. The hot melt method is preferred because no solvent remains in the prepreg.

[0047] The sizing-treated fibers may be a carbon fiber bundle or a sheet-like carbon fiber substrate. Examples of the sheet-like carbon fiber substrate include carbon fibers aligned in one direction in a sheet form, carbon fibers formed into fabrics such as woven or knitted fabrics and nonwoven fabrics, and multiaxial woven fabrics. The sizing-treated fibers may be continuous fibers or discontinuous fibers. When discontinuous fibers are used, they may be a mixture of fibers in the form of single yarns that have been completely opened and reinforcing fibers in the form of incompletely opened fiber bundles. The fibers may be aligned in the same direction, or a random mat oriented in random directions is also preferably used. The thickness of the carbon fiber sheet is not particularly limited, but for laminate applications, a range of 0.01 mm to 0.2 mm is preferable, and woven fabrics that have been subjected to ultra-opening or tight-weighing treatments are suitable from the standpoint of dimensional stability.

[0048] The carbon fiber reinforced composite material of the present disclosure is a molded product of the above prepreg. The carbon fiber reinforced composite material may be formed by laminating two or more prepregs. The carbon fiber reinforced composite material may also be a laminate formed of a prepreg and a metal substrate. Examples of metals that constitute the metal substrate include iron, stainless steel, aluminum, copper, brass, nickel, zinc, titanium, and alloys of these metals. Furthermore, the carbon fiber reinforced composite material may be a laminate formed by laminating two or more prepregs and a metal substrate. The carbon fiber reinforced composite material may also be one obtained by laminating another substrate such as a heat-resistant resin film with the prepreg or the like.

[0049] In one embodiment, the carbon fiber reinforced composite material of the present disclosure can be produced by laminating two or more prepregs together, heating the laminate while applying pressure to the laminate, and molding it into a laminate. Methods for applying heat and pressure include press molding, autoclave molding, bagging molding, wrapping tape method, and internal pressure molding. In other embodiments, the carbon fiber reinforced composite materials of the present disclosure can be produced by utilizing resin transfer molding, filament winding, sheet winding, and the like. Alternatively, a carbon fiber reinforced composite material (metal-clad laminate) can be produced by arranging a metal substrate on one or both sides of a single prepreg or a laminate of prepregs and laminating the metal substrate.

[0050] For the molding, a multi-stage press, a multi-stage vacuum press, a continuous molding machine, an autoclave molding machine, etc. The molding conditions are not particularly limited, but can be a temperature of 180 to 400°C, a heating time of 100 to 300 minutes, and a press pressure of 1 to 50 MPa.

[0051] The carbon fiber reinforced composite material of the present disclosure is suitable for use in applications requiring properties such as strength, abrasion resistance, chemical resistance, flame retardancy, etc. Examples include exteriors and interiors of transportation equipment such as automobiles, motorcycles, and aircraft, sliding parts typified by gears and bearings, insulating parts, sporting goods such as rackets and bats, industrial machinery, robots, parts of medical equipment, oil drilling equipment, oil transport hoses, hydrogen tanks, hydrogen tank pressure vessels, and wind turbine blades. The carbon fiber reinforced composite material of the present disclosure can also be used in components that require low vibration, such as rotating parts of motors, compressors, and machine tools (such as lathes and milling cutters), as well as the interior and exterior of transportation equipment such as automobiles, motorcycles, and aircraft. Furthermore, since the carbon fiber reinforced composite material of the present disclosure has excellent mechanical properties at low temperatures, it can be used for components to be used at extremely low temperatures, such as liquid hydrogen tanks. [Example]

[0052] Hereinafter, embodiments of the present disclosure will be described in detail with reference to examples, but the embodiments of the present disclosure are not limited to these.

[0053] 1. Preparation of sizing agent, carbon fiber, prepreg, and other components for producing molded bodies [F particle] F particle 1: Contains 97.9 mol%, 0.1 mol%, and 2.0 mol% of TFE units, NAH units, and PPVE units, in that order, and has a carbonyl group-containing group with a main chain carbon number of 1×10 6 Particles (D50: 2.1 μm) made of polymer (melting temperature: 300°C) with 1000 particles per particle. F particle 2: A polymer (D50: 2.0 μm) containing 97.5 mol % of TFE units and 2.5 mol % of PPVE units, in that order, and having no oxygen-containing polar groups (melting temperature: 300° C.). [Emulsion] Emulsion 1: Aqueous emulsion containing a precursor of PAI (10% weight loss temperature: 430°C) and an emulsifier ("Hydrosize HP-1632" manufactured by Michelman). Emulsion 2: Aqueous emulsion containing epoxy resin ("Epicoat 828" manufactured by Japan Epoxy Resins Co., Ltd., 10% weight loss temperature: less than 320°C) and an emulsifier. [Carbon fiber] Carbon fiber 1: Mitsubishi Chemical, "MR50R" [Surfactants] Surfactant 1: Silicone surfactant [film] Film 1: A film (thickness: 15 μm) obtained by melt-extruding polyether ketone ketone (Arkema France, "Kepstan 7003")

[0054] 2. Manufacturing of sizing agents, carbon fiber bundles, prepregs, and carbon fiber reinforced composite materials <Example 1> F particles 1, emulsion 1, surfactant 1, and water were placed in a pot, and zirconia balls were then placed in. The pot was then rolled at 150 rpm for 1 hour to obtain sizing agent 1 containing F particles 1 (9 parts by mass), PAI 1 (1 part by mass), surfactant 1 (0.5 parts by mass), and water (89.5 parts by mass). Sizing agent 1 was applied to carbon fiber 1 by roller immersion, and the fiber was then passed through a drying oven at 120°C for 5 minutes, heated and dried, and then baked in a far-infrared oven at 340°C for 10 minutes to obtain a sized fiber bundle (hereinafter referred to as fiber bundle T1) in which the baked product of F particles 1 and PAI were attached to the surface of carbon fiber 1. Fiber bundle T1 was oriented in one direction to obtain a carbon fiber basis weight of 75 g / m 2 The films 1 were placed on both sides of the sheet-like carbon fiber substrate, and the two films 1 were heated and melted to impregnate the carbon fiber substrate, thereby producing a prepreg 1. Prepreg 1 was cut to a predetermined size and then laminated in a steel mold so that the fiber axis direction of each prepreg was aligned in one direction. The mold with the laminate placed in it was compressed at 380°C and 5 MPa for 30 minutes in a two-stage heating and cooling press (50-ton press, manufactured by Shinto Metal Industries Co., Ltd.), and the temperature was then lowered to 200°C over several minutes to obtain Laminate 1, a carbon fiber reinforced composite material with a thickness of approximately 2 mm. Laminate 1 and copper foil with a thickness of 18 μm were then laminated and compressed under the same conditions to obtain molded product 1, a carbon fiber reinforced composite material.

[0055] <Example 2> A sizing agent, a carbon fiber bundle, a prepreg, and a carbon fiber reinforced composite material were produced in the same manner as in Example 1, except that the content of F particles 1 in sizing agent 1 was changed to 5 parts by mass and the content of the PAI precursor was changed to 5 parts by mass.

[0056] <Example 3> A sizing agent, a carbon fiber bundle, a prepreg, and a carbon fiber reinforced composite material were produced in the same manner as in Example 1, except that F particles 1 were not used and the content of the PAI precursor in sizing agent 1 was changed to 10 parts by mass.

[0057] <Example 4> A sizing agent, a carbon fiber bundle, a prepreg, and a carbon fiber reinforced composite material were produced in the same manner as in Example 1, except that Emulsion 1 was changed to Emulsion 2.

[0058] <Example 5> A sizing agent, a carbon fiber bundle, a prepreg, and a carbon fiber reinforced composite material were produced in the same manner as in Example 1, except that F particles 1 were changed to F particles 2.

[0059] 3. Evaluation <<Dispersibility evaluation>> The sizing agents produced in Examples 1 to 5 were stored in containers in an environment of 25° C. for a long period of time, and then visually inspected and evaluated based on the following evaluation criteria. The results are summarized in Table 1. (Evaluation criteria) A: No agglomerates were found in the sizing agent. B: Aggregates were found adhering to the side walls of the container.

[0060] <<Evaluation of powder shedding prevention>> In Examples 1 to 5, the presence or absence of powder falling off of F particles during the production of the carbon fiber bundles was visually observed, and the results were evaluated based on the following evaluation criteria, and are summarized in Table 1. (Evaluation criteria) A: No powder fall-off of F particles was observed. B: Powder falling off of F particles was confirmed.

[0061] <<Winding performance evaluation>> The carbon fiber bundles produced in Examples 1 to 5 were wound around a roll and evaluated based on the following evaluation criteria. The results are summarized in Table 1. (Evaluation criteria) A: Continuous winding of carbon fiber bundles was easily possible. B: Although lateral slippage occurred during winding, continuous winding of the carbon fiber bundle was possible. C: Continuous winding of the carbon fiber bundle was impossible.

[0062] <<Fuzz suppression evaluation>> The carbon fiber bundles produced in Examples 1 to 5 were visually observed and evaluated for fuzz suppression based on the following evaluation criteria. The carbon fiber bundles were also wound around a roll, unwound, and evaluated based on the following evaluation criteria. (Evaluation criteria) A: No fluffing of the carbon fiber bundle was observed, and the carbon fiber could be easily unwound from the roll. B: Some fluffing of the carbon fiber bundle was observed, but the carbon fiber could be unwound from the roll. C: A lot of fluffing was observed in the carbon fiber bundle, and the carbon fiber was tangled in the roll, making it impossible to unwind it.

[0063] <<Foam suppression evaluation>> The carbon fiber bundles produced in Examples 1 to 5 were visually observed and evaluated based on the following evaluation criteria, and the results are summarized in Table 1. (Evaluation criteria) A: No voids formed by foaming were observed in the carbon fiber bundle, and the carbon fiber bundle was extremely smooth overall. B: Voids were partially observed in the carbon fiber bundle, but the carbon fiber bundle was smooth overall. C: Voids were observed throughout the carbon fiber bundle, the carbon fiber bundle was not smooth, and cracks, peeling, etc. were observed.

[0064] [Table 1]

Claims

1. A sizing agent containing particles of a tetrafluoroethylene-based polymer having a melting temperature of 260°C or higher and having an oxygen-containing polar group, at least one selected from the group consisting of a heat-resistant polymer and a precursor thereof having a 10% weight loss temperature of 320°C or higher, and an emulsifier, the heat-resistant polymer is one or more compounds selected from the group consisting of polyimide resins, polyamide-imide resins, and modified products thereof, emulsified with the emulsifier; A sizing agent, wherein the ratio of the content of the tetrafluoroethylene-based polymer particles to the sum of the contents of the tetrafluoroethylene-based polymer particles and the heat-resistant polymer contained in the sizing agent is, on a mass basis, more than 50 / 100 and not more than 99 / 100.

2. 2. The sizing agent according to claim 1, wherein the average particle size of the tetrafluoroethylene polymer particles is 0.1 μm to 200 μm.

3. 3. The sizing agent according to claim 1, wherein the 10% weight loss temperature of the tetrafluoroethylene-based polymer particles is 350°C or higher.

4. 4. The sizing agent according to claim 1, wherein the content of the tetrafluoroethylene-based polymer particles relative to the total mass of the sizing agent is 1 to 30 mass %.

5. The sizing agent according to any one of claims 1 to 4, wherein the content of the heat-resistant polymer relative to the total mass of the sizing agent is 0.5 to 20 mass%.

6. The sizing agent according to any one of claims 1 to 5, further comprising a surfactant.

7. The sizing agent according to any one of claims 1 to 6, which contains a liquid dispersion medium.

8. A carbon fiber having, on its surface, at least one selected from the group consisting of a tetrafluoroethylene-based polymer having a melting temperature of 260°C or higher and having an oxygen-containing polar group, a heat-resistant polymer having a 10% weight loss temperature of 320°C or higher and a precursor thereof, and an emulsifier, the heat-resistant polymer is one or more compounds selected from the group consisting of polyimide resins, polyamide-imide resins, and modified products thereof; A carbon fiber, wherein the ratio of the amount of the tetrafluoroethylene-based polymer to the sum of the amounts of the tetrafluoroethylene-based polymer and the heat-resistant polymer present on the surface of the carbon fiber is, on a mass basis, more than 50 / 100 and not more than 99 / 100.

9. A prepreg comprising the carbon fiber according to claim 8 and a matrix resin.

10. A carbon fiber reinforced composite material which is a molded product of the prepreg according to claim 9.

11. A method for producing sized carbon fibers, comprising disposing the sizing agent according to any one of claims 1 to 7 on carbon fibers and heat-treating the fibers.

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