Sizing agent composition, sizing agent-adhered carbon fibers, and method for producing same

Abstract

The purpose of the present invention is to provide a sizing agent-adhered carbon fibers having high heat resistance and exhibiting a long residual fiber length. The present invention relates to a sizing agent composition comprising at least (a) at least one polyimide-based resin or a precursor thereof, and (b) at least one non-polyimide-based resin or a precursor thereof, wherein when sizing agent-adhered carbon fibers obtained by applying the sizing agent composition to surfaces of carbon fibers in an adhesion amount of 1.5% by weight with respect to the carbon fibers and heating and drying same in air at 250°C for 5 minutes are used for measurement, the sizing agent-adhered carbon fibers have a surface elastic modulus of 500-1500 MPa; and when the sizing agent composition is heated and dried in air at 250°C for 5 minutes and then subjected to a heat treatment by increasing the temperature to 450°C at a rate of 20°C / minute, the thermal mass reduction rate falls within the range of 5%-40%.

Description

Sizing agent composition, carbon fiber with sizing agent attached, and method for producing same

[0001] The present invention relates to a sizing agent composition, a carbon fiber with a sizing agent attached thereto, and a method for producing the same.

[0002] Carbon fibers have excellent specific strength and specific modulus and are lightweight, and therefore, they are being used as reinforcing fibers for thermosetting resins and thermoplastic resins in a wide range of applications, including not only conventional sports and general industrial applications but also aerospace and automotive applications. As the range of applications expands, even higher performance is being demanded of carbon fiber reinforced plastic composite materials (CFRP, hereinafter also referred to as composites).

[0003] To produce a carbon fiber reinforced composite material, a compound containing a resin and carbon fibers dispersed in the resin can be used. Specifically, for example, a carbon fiber reinforced composite material can be produced by melting a pellet-shaped compound (pellets) and injection molding it. The carbon fiber reinforced composite material formed from the compound has a structure in which the carbon fibers are dispersed in the resin, unlike a material produced by impregnating a sheet of unidirectionally aligned carbon fibers with a resin.

[0004] Such pellets can be produced, for example, by kneading short carbon fibers and a resin and extruding the mixture.

[0005] Conventionally, in the process of producing carbon fibers such as short carbon fibers, a sizing agent is applied to the carbon fibers for the purposes of improving the processability of the carbon fibers and improving the affinity between the carbon fibers and a matrix resin in a composite material, etc. Various studies have been conducted on sizing agents applied to carbon fibers.

[0006] Patent Document 1 describes a sizing agent for inorganic fibers that contains an ester of a specific carboxylic acid component and a specific alcohol component.

[0007] Patent Document 2 describes a sizing agent containing at least a polyoxyalkylene group and an epoxy group in its components.

[0008] US Pat. No. 5,699,499 describes a sizing agent for carbon fiber bundles that contains an aliphatic epoxy resin.

[0009] Patent Document 4 describes a carbon fiber bundle to which a sizing agent is attached, and describes that the sizing agent contains an aromatic thermoplastic resin having a softening point in the range of 80 to 200°C and an aromatic diester.

[0010] Patent Document 5 describes a sizing agent containing an amine adduct, which is a reaction product of an epoxy compound and an amine compound, and polyurethane.

[0011] JP 2002-339246 A JP 2003-3376 A JP 2005-264383 A JP 2005-281955 A JP 2016-3412 A

[0012] During the production of carbon fiber resin composites, particularly during compounding by extrusion and / or injection molding, the carbon fibers can break, leading to reduced physical properties in the resulting composite.

[0013] Although it has been considered to improve the breakage resistance of carbon fibers by coating the surface with a sizing agent, it has been difficult to ensure sufficient heat resistance with conventional sizing agents. If the heat resistance of a sizing agent is low, gas generation may become significant during compounding and / or injection molding, and the amount of sizing agent may be significantly reduced by heating.

[0014] An object of the present invention is to provide a sizing agent composition and sizing-applied carbon fibers that have high heat resistance and exhibit a high residual fiber length (aspect ratio).

[0015] The object of the present invention is solved by the present invention having the following aspects:

[0016] A sizing agent composition comprising: (a) at least one polyimide resin or a precursor thereof; and (b) at least one non-polyimide resin or a precursor thereof, wherein, when measured using sizing-adhered carbon fibers obtained by applying the sizing agent composition to the surface of carbon fibers in a deposition amount of 1.5% by weight relative to the carbon fibers and heating and drying the sizing agent composition in air at 250°C for 5 minutes, the sizing agent-adhered carbon fibers have a surface elasticity of 500 to 1500 MPa, and when the sizing agent composition is heated and dried in air at 250°C for 5 minutes and then heat-treated in a dry air atmosphere by increasing the temperature to 450°C at a rate of 20°C / min, the thermal mass loss rate is in the range of 5 to 40%. A sizing agent composition according to Aspect 1, wherein the at least one polyimide resin or a precursor thereof is selected from polyimide resins, polyetherimide resins, polyamideimide resins, polyphenylimide resins, and precursors thereof. <Aspect 3> The sizing agent composition according to Aspect 1 or 2, wherein the at least one non-polyimide resin or precursor thereof is selected so as to yield a low-urethane bond content resin having 0.1 to 3 weight % of urethane bonds after heat drying in air at 250°C for 5 minutes. <Aspect 4> The sizing agent composition according to any one of Aspects 1 to 3, wherein the at least one non-polyimide resin or precursor thereof contains a blocked isocyanate. <Aspect 5> A method for producing carbon fibers with a sizing agent attached thereto, comprising: applying the sizing agent composition according to any one of Aspects 1 to 4 to surfaces of carbon fibers, and heat drying the carbon fibers with the sizing agent composition attached thereto.Aspect 6: A method for producing carbon fibers with a sizing agent attached thereto, comprising: applying a first sizing agent composition to the surface of carbon fibers and optionally drying by heating; applying a second sizing agent composition to the carbon fibers with the first sizing agent composition attached thereto; and heating and drying the carbon fibers with the first sizing agent composition and the second sizing agent composition attached thereto, wherein the first sizing agent composition comprises (c) at least one selected from the group consisting of a polyurethane resin, a blocked isocyanate, and a precursor thereof, and the second sizing agent composition is the sizing agent composition according to any one of Aspects 1 to 5. Aspect 7: A method for producing carbon fibers with a sizing agent attached thereto, having a sizing agent surface layer formed from the sizing agent composition according to any one of Aspects 1 to 6. Aspect 8: A sizing-agent-attached carbon fiber having a sizing agent attached to the surface of a carbon fiber, the sizing agent comprising: (A) at least one polyimide resin; and (B) at least one non-polyimide resin; the sizing-agent-attached carbon fiber has a surface elastic modulus of 500 to 1500 MPa, and a thermal mass loss rate of the sizing agent when the sizing-agent-attached carbon fiber is heated to 450°C at 20°C / min in a dry air atmosphere is in the range of 5 to 40%. Aspect 9: The sizing-agent-attached carbon fiber according to Aspect 8, wherein the at least one polyimide resin is selected from polyimide resins, polyetherimide resins, polyamideimide resins, and polyphenylimide resins. Aspect 10: The sizing-agent-attached carbon fiber according to Aspect 8 or Aspect 9, wherein the at least one non-polyimide resin comprises a low urethane bond content resin having 0.1 to 3 wt% of urethane bonds. The carbon fiber according to any one of claims 8 to 11, wherein the at least one non-polyimide resin comprises a polymerization product of a blocked isocyanate. The carbon fiber according to any one of claims 8 to 11, wherein the mass ratio of component (A) to component (B) is 100:10 to 10:100.Aspect 13: The sizing-agent-adhered carbon fiber according to any one of Aspects 8 to 12, wherein the sizing-agent-adhered carbon fiber has a first sizing layer disposed on a surface of the carbon fiber and a second sizing layer disposed on the first sizing layer, the first sizing layer being composed of component (C) that is at least one selected from the group consisting of a polymerization product of a polyurethane resin and a blocked isocyanate, and the second sizing layer being composed of components (A) and (B). Aspect 14: The sizing-agent-adhered carbon fiber according to any one of Aspects 8 to 13, wherein the mass ratio of component (A): component (B): component (C) is 50-90:25-5:25-5 when the first sizing layer and the second sizing layer are considered together. Aspect 15: The carbon fiber according to any one of Aspects 8 to 14, wherein the sizing agent adhered to the surface of the carbon fiber has a sea-island structure in which low elastic modulus regions are dispersed within high elastic modulus regions, the high elastic modulus regions have an average elastic modulus of 1.0 GPa to 3.0 GPa, and the low elastic modulus regions have an average elastic modulus of 100 to 600 MPa. Aspect 16: The low elastic modulus regions have a thickness of 0.001 μm. ２ ~0.30 μm ２ Aspect 17: The carbon fiber according to any one of Aspects 8 to 16, wherein the carbon fiber is obtained by a wet spinning method. Aspect 18: A short fiber made of the carbon fiber according to any one of Aspects 8 to 17. Aspect 19: A carbon fiber resin composite material comprising the short fiber according to Aspect 18.

[0017] The present disclosure also includes the following aspects: Aspect 20: A sizing-agent-adhered carbon fiber obtained by applying the sizing agent composition according to any one of Aspects 1 to 4 to the surface of a carbon fiber and optionally heating and drying the fiber, wherein the sizing agent has a surface elastic modulus of 500 to 1500 MPa, and a thermal mass loss rate of the sizing agent is in the range of 5 to 40%.

[0018] The present invention provides a sizing agent composition and sizing-adhered carbon fibers that have high heat resistance and a high residual fiber length (aspect ratio), which can provide high heat resistance during the composite production process and in the resulting composite, as well as a composite with excellent physical properties such as strength.

[0019] FIG. 1 is an image showing the elastic modulus distribution on the surface of a carbon fiber with a sizing agent attached thereto according to one embodiment of the present invention, obtained by measurement using a scanning probe microscope.

[0020] <<Sizing Agent Composition>> The sizing agent composition of the present invention contains at least: (a) at least one polyimide resin or a precursor thereof, and (b) at least one non-polyimide resin or a precursor thereof, and when measured using sizing-adhered carbon fibers obtained by applying the sizing agent composition to the surface of carbon fibers in an amount of 1.5% by weight based on the carbon fibers and heating and drying in air at 250°C for 5 minutes, the sizing-adhered carbon fibers have a surface elastic modulus of 500 to 1500 MPa, and when the sizing agent composition is heated and dried in air at 250°C for 5 minutes and then heat-treated in a dry air atmosphere by increasing the temperature to 450°C at a rate of 20°C / min, the thermal mass loss rate is in the range of 5 to 40%.

[0021] Carbon fiber reinforced resin composite materials (composites) can be produced, for example, by injection molding pellets (carbon fiber-containing resin pellets). Injection molding has the advantages of high productivity and a high degree of molding flexibility.

[0022] Such pellets can be produced, for example, by kneading short carbon fibers (particularly chopped carbon fibers) with a resin and extruding the mixture.

[0023] Such carbon fibers are generally given a sizing agent in order to improve handling, moldability, composite properties, and the like.

[0024] Although there is no intention to be limited by theory, polyimide resins have high heat resistance and are particularly useful in high-temperature applications, but are relatively hard and brittle, and therefore may not be sufficient in terms of mechanically protecting carbon fibers. Furthermore, non-polyimide resins such as polymerization products of blocked isocyanates can provide a relatively soft sizing surface layer and thereby provide resistance to fiber breakage, but because their heat resistance is relatively low, the heat resistance of the resulting sizing-adhered carbon fibers may be insufficient.

[0025] In contrast, the sizing composition of the present invention contains at least (a) at least one polyimide resin or its precursor, and (b) at least one non-polyimide resin or its precursor, and is characterized in that, when measured on a sizing-agent-coated carbon fiber to which the sizing composition is applied, it exhibits a specific surface elastic modulus and thermal mass loss rate. According to the present invention, the composition of the sizing agent is optimized, which is thought to improve resistance to breakage and provide relatively high heat resistance.

[0026] Each component and preferred embodiment of the present invention will be described in more detail below.

[0027] <Sizing Agent Composition> The sizing agent composition of the present invention contains at least: (Component a) at least one polyimide resin or a precursor thereof, and (Component b) at least one non-polyimide resin or a precursor thereof.

[0028] (Polyimide Resin or Precursor Thereof) The at least one polyimide resin or precursor thereof (component a) includes polyimide resins, polyetherimide resins, polyamideimide resins, polyphenylimide resins, and precursors thereof.

[0029] (Polyimide) Polyimide is a polymer containing a repeating unit having an imide bond. Polyimide is particularly an aromatic polyimide resin. Polyimide can be obtained, for example, by a method including polycondensation of tetracarboxylic dianhydride and diamine in a solvent.

[0030] The polyimide according to the present disclosure is preferably a polyimide (particularly an aromatic polyimide) that does not have a melting point and has a thermal decomposition temperature of 400°C or higher (particularly 450°C or higher). The melting point of a resin such as a polyimide can be measured as the temperature at the apex of the melting peak when the temperature is increased at 10°C / min in a nitrogen atmosphere by DSC (differential scanning calorimetry) in accordance with JIS K7121. The thermal decomposition temperature can be measured by TG (thermogravimetry) in accordance with JIS K7120 in a nitrogen atmosphere at a temperature increase rate of 10°C / min.

[0031] In one preferred embodiment according to the present disclosure, the polyimide may be obtained by attaching an aqueous solution containing a polyimide precursor compound (or a mixture thereof) to carbon fibers, followed by imidization by drying at a predetermined temperature.

[0032] Precursors of polyimide resins include polyamic acids, isoimide precursors, and diamine-tetracarboxylic acid precursors.

[0033] (Non-polyimide Resin or Precursor Thereof) The non-polyimide resin or precursor thereof (component b) is a resin or precursor different from the above-mentioned component (a).

[0034] The at least one non-polyimide resin or its precursor may be a blocked isocyanate or a blocked polyisocyanate (which may also be referred to as a "blocked isocyanate" or "blocked polyisocyanate"), which can polymerize with each other to produce a polymerization product.

[0035] In one embodiment, the at least one non-polyimide resin or its precursor is selected to yield a low urethane bond content resin having 0.1 to 3 weight % of urethane bonds after heat drying in air at 250° C. for 5 minutes. Low urethane bond content resins and their precursors include polymers of blocked isocyanates.

[0036] The weight percentage of urethane bonds in the low-urethane-bond-content resin may be, in particular, 0.2% by weight or more, 0.4% by weight or more, 0.6% by weight or more, or 0.8% by weight or more, and / or 2.8% by weight or less, 2.6% by weight or less, 2.4% by weight or less, 2.2% by weight or less, or 2.0% by weight or less.

[0037] (Contents of Component a and Component b) The mass ratio of component a to component b in the sizing agent composition is preferably 10:90 to 90:10, more preferably 20:80 to 80:20, or even 25:75 to 75:25, and still more preferably 30:70 to 70:30.

[0038] The total content of component a and component b in the sizing agent composition may be 0.1% by mass to 30% by mass, preferably 0.5% by mass to 20% by mass, more preferably 1% by mass to 15% by mass, and particularly preferably 1% by mass to 5% by mass.

[0039] (Solvent) The sizing agent composition may contain a dispersion medium and / or a solvent in addition to the above-described components a and b. The method for preparing the sizing agent composition is not particularly limited, and known preparation methods can be used.

[0040] Examples of the dispersion medium or solvent for the sizing agent composition include water and organic solvents (for example, alcohols such as methanol, and acetone).

[0041] The sizing agent composition may be an aqueous dispersion (emulsion) of the sizing agent prepared by emulsifying the composition using a surfactant.

[0042] The surfactant is not particularly limited, and anionic, cationic, nonionic surfactants, etc. can be used. Of these, nonionic surfactants are preferred from the viewpoint of emulsification performance and dispersion stability.

[0043] Examples of nonionic surfactants include polyethylene glycol surfactants (higher alcohol ethylene oxide adducts, alkylphenol ethylene oxide adducts, fatty acid ethylene oxide adducts, polypropylene glycol ethylene oxide adducts, etc.) and polyhydric alcohol surfactants (glycerin fatty acid esters, sorbitol fatty acid esters, fatty acid alkanolamides, etc.). It is particularly preferable to use polyoxyethylene polyoxypropylene block polymers, which can reduce the frictional resistance between the carbon fiber surface and metal.

[0044] Examples of the emulsification method include a method using a batch equipped with stirring blades, a method using a ball mill, a method using a shaker, and a method using a high-shear emulsifier such as a Gaulin homogenizer.

[0045] When the sizing agent composition contains a surfactant, the surfactant is not particularly limited as long as it can emulsify the sizing agent, but it is usually sufficient to add about 0.1 to 30% by mass of the surfactant.

[0046] (Surface Elastic Modulus) The sizing agent composition according to the present invention is characterized by exhibiting the following properties: (i) When measured using sizing-adhered carbon fibers obtained by applying the sizing agent composition to the surface of carbon fibers in an amount of 1.5% by weight based on the carbon fibers and drying by heating in air at 250°C for 5 minutes, the sizing-adhered carbon fibers have a surface elastic modulus of 500 to 1500 MPa.

[0047] The surface elastic modulus may be 550 MPa or more, 580 MPa or more, 650 MPa or more, or 680 MPa or more, and / or 1475 MPa or less, 1450 MPa or less, 1400 MPa or less, 1350 MPa or less, 1300 MPa or less, or 1250 MPa or less. The surface elastic modulus is preferably 600 to 1400 MPa, more preferably 900 MPa to 1300 MPa, and particularly preferably 1000 MPa to 1300 MPa.

[0048] The surface elastic modulus of the carbon fiber with the sizing agent attached thereto can be measured by a scanning probe microscope (SPM).

[0049] That is, the surface elastic modulus can be obtained by using a scanning probe microscope to measure the elastic modulus at 10,000 or more randomly selected locations on the surface of the sizing-adhered carbon fiber and calculating the average value (number average value) of those values.

[0050] (Thermal Mass Reduction Rate) The sizing agent composition according to the present invention is further characterized in that it exhibits the following property: (ii) When the sizing agent composition is heated and dried in air at 250°C for 5 minutes and then heat-treated in a dry air atmosphere by increasing the temperature to 450°C at a rate of 20°C / min, the thermal mass reduction rate is in the range of 5% to 40%.

[0051] The thermal mass loss rate may be 8% or more, 10% or more, 12% or more, 14% or more, or 15% or more, and / or 35% or less, 30% or less, or 25% or less. The surface elasticity modulus is preferably 5% to 35%, more preferably 8% to 30%, and particularly preferably 10% to 25% or even 10% to 20%.

[0052] This thermal mass loss rate (%) at 450°C can be measured using a thermogravimetric differential calorimeter (TG-DTA), and specifically, can be measured as follows: (1-1) Using a thermogravimetric differential calorimeter (TG-DTA), the change in mass of 10 mg of the sizing agent composition is measured under the following conditions: in a dry air atmosphere (flow rate 50 mL / min), the temperature is increased from room temperature to 250°C, and then the temperature is held at 250°C for 5 minutes to heat-dry, and then the temperature is increased from 250°C to 450°C at a rate of 20°C / min. (1-2) Based on the obtained measurement results, the thermal mass loss rate (%) is determined according to the following formula: Thermal mass loss rate (%) = (mass after heat-drying at 250°C - mass after heat treatment at 450°C) / (mass after heat-drying at 250°C) × 100

[0053] <<Carbon Fibers with Sizing Agent>> The present disclosure also includes an invention relating to carbon fibers with a sizing agent attached thereto. Carbon fibers with a sizing agent attached thereto are carbon fibers with a sizing agent attached to the surface thereof.

[0054] The carbon fibers with a sizing agent attached thereto may in particular be short carbon fibers with a sizing agent attached thereto.

[0055] In the present disclosure, "sizing agent" means a component that remains on the carbon fibers after the sizing agent composition is applied to the carbon fibers and dried, and may be a solid or a liquid.

[0056] The sizing-adhered carbon fiber may be produced using the sizing composition according to the present invention. The sizing-adhered carbon fiber may have a sizing surface layer formed from the sizing composition.

[0057] <Sizing-agent-Attached Carbon Fiber (1)> In one embodiment, the sizing-agent-attached carbon fiber according to the present invention has the following characteristics: A sizing agent is attached to the surface of a carbon fiber, and the sizing agent contains (Component A) at least one polyimide resin, and (Component B) at least one non-polyimide resin, and the sizing-agent-attached carbon fiber has a surface elastic modulus of 500 to 1500 MPa, and when the temperature of the sizing-agent-attached carbon fiber is raised to 450°C at 20°C / min in a dry air atmosphere, the thermal mass loss rate of the sizing agent is in the range of 5 to 40%.

[0058] Although there is no intention to be limited by theory, polyimide resins have high heat resistance and are particularly useful in high-temperature applications, but are relatively hard and brittle, and therefore may not be sufficient in terms of mechanically protecting carbon fibers. Furthermore, non-polyimide resins such as polymerization products of blocked isocyanates can provide a relatively soft sizing surface layer and thereby provide resistance to fiber breakage, but because their heat resistance is relatively low, the heat resistance of the resulting sizing-adhered carbon fibers may be insufficient.

[0059] In contrast, the carbon fiber with a sizing agent of the present invention contains at least (A) at least one polyimide resin and (B) at least one non-polyimide resin, and is characterized by exhibiting a specific surface elastic modulus and thermal mass loss rate. According to the present invention, the composition of the sizing agent is optimized, which is thought to improve resistance to breakage and provide relatively high heat resistance.

[0060] (Carbon fiber) The sizing-adhered carbon fiber has carbon fiber. There are no particular limitations on the carbon fiber, and any carbon fiber such as pitch-based, rayon-based, or polyacrylonitrile (PAN)-based may be used. However, in consideration of operability, processability, mechanical strength, etc., acrylonitrile-based carbon fiber is preferred. There are also no particular limitations on the properties of the carbon fiber, such as fineness and strength, and any known carbon fiber can be used without limitation.

[0061] The form of the carbon fiber is not particularly limited, but may be in the form of a carbon fiber bundle composed of a plurality of single fibers (filaments). From the viewpoint of productivity, the number of filaments constituting the carbon fiber bundle is preferably 1,000 to 80,000, and more preferably in the range of 3,000 to 50,000. The single fiber diameter may be 3 μm to 20 μm, preferably 4 μm to 10 μm, and more preferably 4.5 to 6.5 μm. Preferably, when measured according to JIS R 7608, the tensile strength of the carbon fiber is 4000 to 6000 MPa and / or the tensile modulus of elasticity of the carbon fiber is 200 to 400 GPa or 220 to 300 GPa.

[0062] The carbon fiber may be a short fiber (short carbon fiber). Examples of the short fiber include chopped carbon fiber and milled fiber. Chopped carbon fiber can be obtained, for example, by cutting carbon fiber tow. Milled fiber can be obtained, for example, by grinding chopped carbon fiber with a grinder. Milled fiber often has an average length of 0.2 mm or less. The short carbon fiber is particularly chopped carbon fiber. Details of the sizing agent and carbon fiber will be described later.

[0063] The short carbon fibers may have an average length of 1 to 20 mm, preferably 0.5 to 15 mm, or 1 to 12 mm, 1 to 10 mm, 1 to 9 mm, or 1 to 7 mm, more preferably 1 to 6 mm or 1 to 4 mm, and particularly preferably 1 to 3 mm.

[0064] The average length of the short carbon fibers can be determined by measuring the lengths of 30 or more short carbon fibers using a vernier caliper and / or an optical microscope and calculating an average value from the measured values.

[0065] PAN-based carbon fibers can be produced, for example, by the following method.

[0066] (Precursor fiber)

[0067] For example, an acrylic precursor fiber can be produced by preparing a spinning dope containing a polyacrylonitrile polymer, coagulating the spinning dope by dry spinning, wet spinning, or dry-wet spinning (air-gap spinning) to obtain a coagulated fiber, and then washing the coagulated fiber with water, stretching, oiling, drying, and steam stretching. The polyacrylonitrile polymer can be polymerized by, for example, solution polymerization or suspension polymerization. In steam stretching, the total stretch ratio can be set to 5 to 15 times. Furthermore, organic solvents, inorganic solvents, and inorganic salt solvents can be used as the solvent for spinning.

[0068] The spinning method may be, in particular, a dry-wet spinning method or a wet spinning method. In particular, when precursor fibers produced by the dry-wet spinning method are used, carbon fibers having relatively high physical properties (particularly, relatively high tensile strength and tensile modulus) may be obtained. The dry-wet spinning method can produce carbon fibers having excellent surface smoothness.

[0069] The carbon fibers may be carbon fibers obtained by a method based on wet spinning.

[0070] The acrylic precursor fiber is produced by spinning a spinning solution containing preferably 90% by mass or more, more preferably 95% by mass or more, of acrylonitrile and 10% by mass or less of other monomers, either homopolymerized or copolymerized. Examples of other monomers include itaconic acid and (meth)acrylic acid esters. The precursor fiber can be obtained by washing the raw fiber after spinning with water, drying, stretching, and oiling. From the viewpoint of production efficiency, the number of filaments in the precursor fiber is preferably 1,000 or more, more preferably 3,000 or more.

[0071] (Flame-resistant Treatment) The precursor fiber is flame-resistant treated by heating it in heated air at 200 to 300° C. for 10 to 100 minutes. In the flame-resistant treatment, the fiber is preferably stretched at a draw ratio in the range of 0.90 to 1.20.

[0072] (Carbonization Treatment) The flame-retardant treated precursor fiber is carbonized at 300 to 2000°C in an inert atmosphere to obtain a carbon fiber. In order to obtain a carbon fiber bundle having a dense internal structure with higher tensile strength, it is preferable to perform the carbonization treatment through a two-stage carbonization process in which low-temperature carbonization at 300 to 1000°C is followed by high-temperature carbonization at 1000 to 2000°C. When a higher elastic modulus is required, a graphitization treatment may be further performed at a high temperature of 2000 to 3000°C.

[0073] (Surface Oxidation Treatment) The carbon fiber obtained above is preferably subjected to a surface oxidation treatment in order to improve wettability with a sizing agent and / or a matrix resin. The surface oxidation treatment can be performed by any conventionally known method, but electrolytic oxidation is generally used industrially because the apparatus is simple and the process is easy to control.

[0074] The quantity of electricity in the surface oxidation treatment is preferably in the range of 10 to 150 coulombs per 1 g of carbon fiber. By adjusting the quantity of electricity within this range, carbon fiber having excellent mechanical properties as a fiber and improved adhesion to resin can be obtained. Examples of the electrolyte include nitric acid, sulfuric acid, ammonium sulfate, and sodium bicarbonate. The electrolyte concentration of the electrolyte is preferably 0.1 normal or higher, and more preferably 0.1 to 1 normal.

[0075] <O / C Ratio> The carbon fiber may have an O / C ratio (ratio of O atoms to C atoms) on its surface of 15% to 30% (which can also be expressed as 0.15 to 0.30). This O / C ratio is more preferably 18% to 29%, even more preferably 20% to 28%, and most preferably 21% to 27%. In this case, the adhesion of the sizing agent to the carbon fiber can be optimized. The ratio of oxygen atoms to carbon atoms (O / C ratio) on the surface of the carbon fiber can be determined by X-ray photoelectron spectroscopy (XPS) on the carbon fiber before the sizing agent is attached. For more detailed measurement methods, see the description of the examples.

[0076] (Component A: Polyimide Resin) The sizing agent adhered to the sizing-adhered carbon fiber may contain, as component A, at least one polyimide resin.

[0077] The at least one polyimide-based resin (component A) includes polyimide resins, polyetherimide resins, polyamideimide resins, and polyphenylimide resins.

[0078] For the polyimide resin, reference can be made to the above description of the sizing agent composition of the present invention.

[0079] The at least one polyimide-based resin (component A) may be a polyimide resin exhibiting a modulus of elasticity, measured according to standard ISO 527, of 1000 MPa to 5000 MPa.

[0080] (Component B: Non-Polyimide Resin) The sizing agent of the sizing-adhered carbon fiber may contain, as component B, at least one non-polyimide resin.

[0081] The at least one non-polyimide resin may be a polymerization product of a blocked isocyanate.

[0082] In one preferred embodiment, the at least one non-polyimide resin comprises a low urethane bond content resin having 0.1 to 3% by weight of urethane bonds.

[0083] For the blocked isocyanate and the resin with a low urethane bond content, reference can be made to the above description of the sizing agent composition of the present invention.

[0084] The at least one non-polyimide resin (component B) may be a resin exhibiting a modulus of elasticity, measured according to standard ISO 527, of 100 MPa to 600 MPa.

[0085] (Ratio of Component A and Component B) In the sizing agent attached to the sizing-adhered carbon fiber, Component A (at least one polyimide-based resin) and Component B (at least one non-polyimide-based resin) may have a mass content ratio of 100:10 to 10:100. This ratio is preferably 90:10 to 10:90, more preferably 80:20 to 20:80, and particularly preferably 75:25 to 25:75.

[0086] (Amount of sizing agent attached) The amount of sizing agent attached to the carbon fiber may be 0.5 to 3.0% by weight, preferably 0.8 to 2.5% by weight, more preferably 1.0 to 1.8% by weight, based on the carbon fiber. The mass of the sizing agent attached to the carbon fiber can be measured according to JIS R7604 method (Method B).

[0087] (Surface Elastic Modulus) The carbon fiber having a sizing agent attached thereto according to the present invention has the following characteristics: It has a surface elastic modulus of 500 to 1500 MPa.

[0088] The surface elastic modulus of the sizing-adhered carbon fiber can be measured by a scanning probe microscope (SPM). For the method for measuring the surface elastic modulus of the sizing-adhered carbon fiber of the present invention and the preferred range thereof, reference can be made to the above description of the sizing agent composition of the present invention.

[0089] (Thermal Mass Reduction Rate) The sizing-agent-adhered carbon fiber of the present invention further has the following characteristics: When the sizing-agent-adhered carbon fiber is heated to 450°C at a rate of 20°C / min in a dry air atmosphere, the thermal mass reduction rate of the sizing agent is in the range of 5 to 40%.

[0090] The thermal mass loss rate of the sizing agent in the sizing-adhered carbon fiber can be measured using a thermogravimetric differential calorimeter (TG-DTA), and specifically, can be determined by the following method: (2-1) Using a thermogravimetric differential calorimeter (TG-DTA), the mass change of 10 mg of the sizing-adhered carbon fiber is measured: the temperature is raised from room temperature to 450°C at a rate of 20°C / min in a dry air atmosphere (flow rate 50 mL / min). (2-2) Based on the measurement results obtained, the thermal mass loss rate (%) of the sizing agent is determined according to the following formula: Thermal mass loss rate (%) = (mass before heating - mass after heating) / (mass before heat treatment x amount of sizing agent attached) x 100

[0091] <Sea-island structure> In one embodiment of the present invention, the sizing agent attached to the sizing-adhered carbon fiber has an "island-sea structure" in which low elastic modulus regions (islands) are dispersed in a high elastic modulus region (sea). A sizing-adhered carbon fiber having such a structure can provide a sizing-adhered carbon fiber having both the properties of the high elastic modulus region and the low elastic modulus region, and can provide a fiber that is particularly excellent in heat resistance and fiber breakage resistance.

[0092] Such sizing-adhered carbon fibers having an islands-in-a-sea structure can be obtained, for example, by appropriately selecting the compatibility of multiple resins constituting the sizing agent. More specifically, a suitable example of a sizing agent capable of providing such an islands-in-a-sea structure is a combination of a polyimide resin and / or its precursor with a blocked isocyanate and / or its polymerization product. According to this embodiment, for example, high heat resistance derived from the high elasticity region mainly composed of a polyimide resin or the like can be achieved, and high resistance to breakage can be achieved due to the flexibility provided by the low elasticity region.

[0093] In one embodiment, the sizing agent adhered to the surface of the carbon fiber has a sea-island structure in which low elastic modulus regions are dispersed in high elastic modulus regions as described above, and the high elastic modulus regions have an average elastic modulus of 1.0 GPa to 3.0 GPa, and the low elastic modulus regions have an average elastic modulus of 100 to 600 MPa.

[0094] As an example of such an island-in-a-sea structure, reference is made to Figure 1. Figure 1 is an image showing the elastic modulus distribution on the surface of a sizing-agent-coated carbon fiber according to one embodiment of the present invention, obtained using measurements with a scanning probe microscope and image analysis software. The right panel of Figure 1 is an enlarged view of a portion of the left image. As can be seen in Figure 1, the surface of this sizing-agent-coated carbon fiber is largely occupied by high elastic modulus regions (relatively light gray regions), and it can be observed that low elastic modulus regions (dark gray to black regions) are scattered throughout these high elastic modulus regions ("island-in-a-sea structure").

[0095] (High Elastic Modulus Region and Low Elastic Modulus Region) The high elastic modulus region constitutes the "sea" of the sea-island structure in the sizing agent attached to the surface of the carbon fiber.

[0096] The low elastic modulus regions form "islands" in the sea-island structure in the sizing agent adhered to the surface of the carbon fiber. The low elastic modulus regions are dispersed among the high elastic modulus regions in the sizing agent adhered to the surface of the carbon fiber.

[0097] The parameters of the high elastic modulus region and the low elastic modulus region can be measured using image analysis software from data and images showing the elastic modulus distribution obtained by measuring the elastic modulus at more than 10,000 randomly selected locations on the surface of the sizing-agent-attached carbon fiber, obtained using a scanning probe microscope.

[0098] More specifically, the parameters of the high elastic modulus region ("sea") and the low elastic modulus region ("island") can be measured as follows: (1) Based on the data of the elastic modulus distribution on the fiber surface obtained from measurements using a scanning probe microscope, an image is created by performing emphasis processing and binarization processing using image analysis software (e.g., WinROOF2018, Mitani Corporation). In this image, a pixel with a size of 0.001 μm is detected. ２ After removing areas of less than 1 μm, the "island" and "sea" areas are identified in the image. ２ (especially 1 μm of the fiber surface) ２An image of the "island" portion (a portion covering an area of ​​100 to 255 mm) can be used. The conditions for the enhancement and binarization processes can be as follows: Enhancement process conditions: Brightness of all colors 30, contrast 50 Binarization process conditions: H threshold 100 to 330 L threshold 50 to 200 S threshold 100 to 255 (2) The area and average elastic modulus of the "island" portions (low elastic modulus regions) in the image are calculated. The maximum area is also determined. The total area and number of "island" portions are also measured, and the average area of ​​the low elastic modulus regions is calculated based on these values. Note that at least 10 or more "island" portions are analyzed. (3) The average elastic modulus of the "sea" portion (high elastic modulus regions) in the image is calculated from the image. (4) The "coverage" of the "sea" portion (high elastic modulus region) can be determined as the area of ​​the remaining portion after subtracting the total area of ​​the calculated "island" portions.

[0099] (High Elastic Modulus Region) With regard to coverage, the high elastic modulus region may cover most of the surface of the carbon fiber, and may cover more than 50%, or even 55% or more, preferably 60% or more, more preferably 70% or more, even more preferably 80% or more, and particularly preferably 90% or more of the surface of the carbon fiber.

[0100] The high modulus region may have an average modulus of elasticity of 1.0 GPa to 3.0 GPa, preferably 1.1 GPa to 2.5 GPa, more preferably 1.2 GPa to 2.0 GPa, and particularly preferably 1.3 GPa to 1.8 GPa.

[0101] (Low Elastic Modulus Region) The low elastic modulus region may have an average elastic modulus of 50 to 800 MPa, or even 100 to 600 MPa. This average elastic modulus is preferably 110 to 500 MPa, more preferably 120 to 400 MPa, particularly preferably 140 to 300 MPa, and most preferably 150 to 200 MPa.

[0102] In one embodiment, the low elastic modulus region is 0.001 μm ２ ~0.30 μm ２ This average area is preferably 0.002 μm ２ ~0.25 μm ２ , more preferably 0.005 μm２ ~0.20μm ２ , particularly preferably 0.010 μm ２ ~0.100μm ２ or even 0.010 μm ２ ~0.050μm ２ is.

[0103] Regarding the maximum area of ​​the low modulus region, the area of ​​the low modulus region is 1.0 μm ２ The area of ​​the low elastic modulus region is preferably 0.50 μm ２ or less, more preferably 0.20 μm ２ and more preferably 0.10 μm ２ or less, and particularly preferably 0.050 μm ２ The area of ​​the low elastic modulus region can be measured by the above method. The lower limit of the area of ​​the low elastic modulus region is 0.001 μm ２ It may be.

[0104] The method for producing the sizing agent-adhered carbon fiber according to the present disclosure is not particularly limited, but in particular, it can be produced by the production method according to the present invention described below.

[0105] <<Method for producing carbon fibers with a sizing agent attached>> One embodiment of the method for producing carbon fibers with a sizing agent attached according to the present invention includes the following steps: applying the sizing agent composition according to the present invention to the surface of carbon fibers (application step), and heating and drying the carbon fibers with the sizing agent composition attached (heat drying step).

[0106] <Application Step> In the application step, the sizing agent composition according to the present invention is applied to the surface of the carbon fiber. For the sizing agent composition used in this method, the above description of the sizing agent composition according to the present invention can be referred to. For the carbon fiber, the above description of the carbon fiber to which a sizing agent is attached can be referred to. The carbon fiber may be short carbon fiber.

[0107] The sizing agent composition according to the present invention comprises (component a) at least one polyimide resin or a precursor thereof, and (component b) at least one non-polyimide resin or a precursor thereof. When the sizing agent composition having such a composition is applied to carbon fibers and optionally dried by heating, it can produce at least one polyimide resin (component A) and at least one non-polyimide resin (component B).

[0108] In the sizing agent application step, the sizing agent composition can be applied by, for example, contacting carbon fibers (e.g., short carbon fibers) with the sizing agent composition. Specific examples include an immersion method in which carbon fibers (e.g., short carbon fibers) are directly immersed in the sizing agent composition, and a spray method in which the sizing agent composition is sprayed onto the carbon fibers.

[0109] <Drying Step> In the heat drying step, the carbon fibers to which the sizing agent composition is attached are heat dried. Heat drying can improve the degree of adhesion of the sizing agent to the carbon fibers. Heat drying can also remove solvents and / or dispersion media that may be contained in the sizing agent composition. Heat drying can also promote polymerization of the resin precursor contained in the sizing agent composition.

[0110] The conditions for heat drying in this step are not particularly limited, but for example, heat drying can be carried out at a temperature of 240° C. to 280° C. for 1 minute to 10 minutes.

[0111] <Carbon fiber with sizing agent (two-layer structure)>

[0112] The carbon fibers may have a two-layer structure of sizing, such as a first sizing layer disposed on the surface of the carbon fibers and a second sizing layer disposed on the first sizing layer, and these layers may be directly adjacent to each other.

[0113] In a particularly preferred embodiment, the carbon fiber with a sizing agent according to the present invention has the following characteristics: the carbon fiber with a sizing agent has a first sizing layer disposed on the surface of the carbon fiber and a second sizing layer disposed on the first sizing layer, the first sizing layer being composed of at least one selected from the group consisting of polyurethane resins and polymerization products of blocked isocyanates (component C), and the second sizing layer being composed of components (A) and (B).

[0114] In this embodiment, the sizing agent of the sizing-adhered carbon fiber is composed of multiple layers with different compositions (two-layer structure), i.e., the layer adjacent to the carbon fiber (the "undercoat layer") contains a polymerization product of a blocked isocyanate and / or polyurethane, and a layer containing a polyimide-based resin and a non-polyimide-based resin (the "surface layer") is disposed on the undercoat layer. Such sizing-adhered carbon fiber can be obtained, for example, by "double dipping" the sizing agent composition.

[0115] According to this configuration, the undercoat layer has a relatively low elasticity, which improves the flexibility of the sizing-adhered carbon fiber and provides high resistance to breakage, while the surface layer containing a polyimide-based resin can provide high heat resistance.

[0116] (First Sizing Layer) The first sizing layer is composed of at least one material (component C) selected from the group consisting of polyurethane resins and polymerization products of blocked isocyanates.

[0117] The second sizing layer is composed of components (A) and (B). For components (A) and (B), reference can be made to the above description of the sizing agent composition of the present invention.

[0118] When the first sizing layer and the second sizing layer are taken into consideration together, the mass ratio of component (A): component (B): component (C) may be 50-90:25-5:25-5. This ratio is preferably 60-80:20-10:20-10.

[0119] (Production method relating to two-layer structure) The method for producing the above-mentioned sizing-agent-applied carbon fibers having a two-layer structure is not particularly limited, and they can be produced, for example, by the following method according to the present invention: A method for producing sizing-agent-applied carbon fibers, comprising: applying a first sizing agent composition to the surface of carbon fibers and optionally heating and drying (first application step); applying a second sizing agent composition to the carbon fibers to which the first sizing agent composition has been applied (second application step); and heating and drying the carbon fibers to which the first sizing agent composition and the second sizing agent composition have been applied (heat drying step), wherein the first sizing agent composition comprises at least one (component c) selected from the group consisting of polyurethane resins, blocked isocyanates, and precursors thereof, and the second sizing agent composition is the above-mentioned sizing agent composition according to the present invention.

[0120] <First application step> In the first application step, the first sizing agent composition is applied to the surface of the carbon fiber. For the carbon fiber, the above description regarding the sizing agent-attached carbon fiber can be referred to. In particular, the carbon fiber may be short carbon fiber.

[0121] The application method for applying the first sizing agent composition is not particularly limited, and for the application method, reference can be made to the above description regarding the method for producing the sizing-applied carbon fiber of the present invention.

[0122] (First Sizing Agent Composition) The first sizing agent composition contains at least one member (component c) selected from the group consisting of polyurethane resins, blocked isocyanates, and precursors thereof.

[0123] After application of the first sizing agent composition, heat drying may be optionally performed. The conditions for heat drying are not particularly limited, but heat drying can be performed, for example, at a temperature of 240°C to 280°C for 1 minute to 10 minutes.

[0124] <Second application step> In the second application step, a second sizing agent composition is applied to the carbon fibers to which the first sizing agent composition has been attached. The second sizing agent composition is the above-described sizing agent composition according to the present invention. For the sizing agent composition according to the present invention, the above description of the sizing agent composition according to the present invention can be referred to.

[0125] The application method for applying the second sizing agent composition is not particularly limited, and for the application method, reference can be made to the above description of the method for producing the sizing-coated carbon fiber of the present invention.

[0126] <Heat Drying Step> In the heat drying step, the carbon fibers to which the first sizing agent composition and the second sizing agent composition are attached are heated and dried.

[0127] The method of heat drying in this case is not particularly limited, and heat drying can be carried out, for example, at a temperature of 240° C. to 280° C. for 1 minute to 10 minutes.

[0128] <<Short Fiber>> The present disclosure particularly includes short fibers (sizing-agent-attached carbon fiber short fibers) made of the above-described sizing-agent-attached carbon fibers according to the present invention.

[0129] (Average Length) The sizing-adhered carbon fiber short fibers may have an average length of 1 to 20 mm, preferably 0.5 mm to 15 mm, or 1 mm to 12 mm, 1 mm to 10 mm, 1 mm to 9 mm, or 1 mm to 7 mm, more preferably 1 mm to 6 mm or 1 mm to 4 mm, and particularly preferably 1 mm to 3 mm.

[0130] The average length of the sizing-agent-attached carbon fiber short fibers can be determined by measuring the lengths of 30 or more sizing-agent-attached carbon fiber short fibers using a vernier caliper and / or an optical microscope and calculating an average value from the measured values.

[0131] (Single Fiber Diameter) The carbon fibers constituting the sizing-coated carbon fibers (particularly the sizing-coated carbon fiber short fibers) may have a single fiber diameter of 3 to 20 μm.

[0132] The carbon fibers constituting the sizing-adhered carbon fibers (particularly, the sizing-adhered carbon fiber short fibers) preferably have a single fiber diameter of 4 μm to 10 μm, or even 4 μm to 8 μm, and even more preferably 4.5 to 6.5 μm. In this case, when a composite material is produced using the sizing-adhered carbon fibers according to the present invention, a composite material with particularly good physical properties can be obtained. While not intended to be limited by theory, according to the present invention, even when relatively thin carbon fibers are used, fiber breakage is suppressed during the compounding process or the like, ensuring a relatively long remaining fiber length in the composite material. Therefore, when carbon fibers with a single fiber diameter of 4 to 10 μm (or even 4.5 to 6.5 μm) are used, the aspect ratio of the carbon fibers in the composite material can be relatively large, and as a result, it is believed that a composite material exhibiting particularly good tensile strength can be obtained.

[0133] The single fiber diameter can be obtained by averaging the diameters of 30 or more carbon fibers measured using images obtained using an electron microscope or the like.

[0134] The sizing-adhered carbon fiber short fibers according to the present invention are considered to be particularly useful for producing carbon fiber reinforced composite materials having a relatively high fiber content (particularly, a fiber volume content of 20 to 55% by volume). That is, when producing a composite having a relatively high fiber content, it is considered that the fibers are prone to breakage due to interactions between the fibers. With the fibers according to the present invention, even in such cases, fiber breakage can be suppressed and relatively long fibers can remain, thereby achieving excellent composite strength.

[0135] The sizing agent-attached carbon fiber short fibers according to the present disclosure are preferably sizing agent-attached carbon fiber short fibers for use in carbon fiber reinforced composite materials having a fiber volume content of carbon fibers of 20 vol% to 55 vol%, 25 vol% to 50 vol%, or even 30 vol% to 40 vol%.

[0136] <<Pellets>> The present disclosure also encompasses pellets containing the sizing-agent-attached carbon fibers (particularly the sizing-agent-attached carbon fiber short fibers) according to the present disclosure. The pellets according to the present disclosure include, and are preferably composed of, the sizing-agent-attached carbon fiber short fibers according to the present disclosure and a resin.

[0137] Pellets can be produced from the carbon fibers (particularly the short carbon fibers) having a sizing agent attached thereto according to the present disclosure and a resin. Specifically, for example, the carbon fibers and the short carbon fibers having a sizing agent attached thereto according to the present disclosure and a resin are supplied to a twin-screw extruder, and the mixture is optionally heated, kneaded, and extruded to produce pellets.

[0138] The resin contained in the pellets may be a thermosetting resin or a thermoplastic resin.

[0139] Specific 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 resins can be used alone or as a mixture of two or more. Among these, epoxy resins and vinyl ester resins, which have excellent heat resistance, elastic modulus, and chemical resistance, are particularly preferred. These thermosetting resins may contain, in addition to curing agents and curing accelerators, commonly used colorants and various additives.

[0140] Examples of thermoplastic resins include polypropylene (PP) resin, polysulfone (PS) resin, polyethersulfone (PES) resin, polyetherketone resin, polyetheretherketone (PEEK) resin, polyamide (PA) resin, aromatic polyamide resin, polyester (PE) resin, aromatic polyester resin, polycarbonate (PC) resin, aromatic polycarbonate resin, polyetherimide (PEI) resin, polyarylene oxide resin, thermoplastic polyimide resin, polyamideimide resin, polyacetal resin, polyphenylene oxide resin, polyphenylene sulfide resin, polyarylate resin, polyacrylonitrile resin, polyaramid resin, and polybenzimidazole resin.

[0141] Preferably, the resin contained in the pellets has a melting point of 250° C. or higher, 275° C. or higher, 300° C. or higher, 320° C. or higher, or even 340° C. or higher. The melting point of the resin can be measured by DSC (differential scanning calorimetry) in accordance with JIS K7121. The upper limit of the preferred melting point of the resin contained in the pellets is not particularly limited, but may be, for example, 500° C. or lower, or 450° C. or lower.

[0142] Preferably, the resin contained in the pellets is a polyether ketone resin or a polyether ether ketone (PEEK) resin, particularly a polyether ether ketone (PEEK) resin. In this case, a composite obtained by molding the pellets may exhibit particularly good physical properties. Although not intending to be limited by theory, it is believed that the polyimide contained in the sizing agent according to the present invention exhibits particularly good affinity for PEEK, and therefore, in the composite, the adhesion between the carbon fiber and the resin is further improved.

[0143] The content of the resin composition in the pellet or composite may be 10 to 90% by mass, or even 15 to 85% by mass, preferably 20 to 60% by mass, and more preferably 25 to 50% by mass, relative to the mass of the carbon fiber.

[0144] <<Carbon Fiber Resin Composite Material (Composite)>> The present disclosure also encompasses a carbon fiber resin composite material containing short fibers made of the sizing-adhered carbon fibers according to the present disclosure, particularly the sizing-adhered carbon fibers according to the present invention.

[0145] The carbon fiber reinforced composite material contains carbon fibers (particularly short carbon fibers) derived from the sizing agent-attached carbon fibers according to the present disclosure, and a resin.

[0146] In a preferred embodiment, the carbon fiber reinforced composite material according to the present disclosure comprises a resin and carbon fibers dispersed in the resin, and the fiber volume fraction Vf of the carbon fibers is 18 vol% or more, 20 vol% or more, 23 vol% or more, 25 vol% or more, or 30 vol% or more, and / or 60 vol% or less, 55 vol% or less, or 50 vol% or less, particularly 20 to 55 vol%.

[0147] For the carbon fibers (short carbon fibers) contained in the carbon fiber reinforced composite material, the above description of the sizing-adhered short carbon fibers according to the present disclosure can be referenced. Also, for the resin contained in the carbon fiber reinforced composite material, the above description of the compound (particularly the pellet-shaped compound) can be referenced.

[0148] The method for producing the carbon fiber reinforced composite material according to the present disclosure is not particularly limited, and the carbon fiber reinforced composite material can be produced by, for example, a method including the following: forming a compound (particularly a pellet-shaped compound) formed from the sizing-agent-attached carbon fiber according to the present disclosure (particularly the sizing-agent-attached carbon fiber short fiber) and a resin to form a carbon fiber reinforced composite material.

[0149] The above description can be referred to for the method for forming a compound (particularly a pellet-shaped compound) from sizing-adhered carbon fibers (particularly sizing-adhered carbon fiber short fibers) according to the present disclosure and a resin.

[0150] The method for forming a carbon fiber reinforced composite material by molding a compound (particularly a pellet-shaped compound) is not particularly limited, and any known method can be used. The compound (particularly a pellet-shaped compound) can be molded by, for example, injection molding.

[0151] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the descriptions in the examples.

[0152] <<Evaluation Method>> The evaluation method is as follows.

[0153] <Strength and Elastic Modulus of Carbon Fiber> The CF tensile strength (MPa) and tensile modulus (GPa) of the carbon fiber (CF) used to produce the sizing-adhered carbon fiber were measured in accordance with JIS R 7608 standard.

[0154] <O / C Ratio> The ratio of oxygen atoms to carbon atoms (O / C ratio) on the surface of the carbon fiber was determined by X-ray photoelectron spectroscopy (XPS) on the carbon fiber before the sizing agent was attached.

[0155] More specifically, the fibers were cut and spread out on a stainless steel sample support, the photoelectron escape angle was set to 90 degrees, MgKα was used as the X-ray source, and the sample chamber was maintained at a vacuum of 1 x 10-6 [Pa]. To correct for peaks associated with charging during measurement, the binding energy B.E. of the main C1s peak was first adjusted to 284.6 [eV]. The O1s peak area was determined by drawing a straight baseline in the range of 527 to 540 [eV]. The C1s peak area was also determined by drawing a straight baseline in the range of 281 to 297 [eV]. A sensitivity correction factor of 2.6865 was used for the O1s peak relative to the C1s peak. The ratio of the O1s peak area to the C1s peak area was calculated to determine the surface oxygen concentration of the carbon fiber.

[0156] <Diameter of Carbon Fiber (Single Fiber Diameter)> The single fiber diameter of the carbon fiber was obtained by averaging the diameters of 30 or more carbon fibers measured using images obtained using an electron microscope or the like.

[0157] <Amount of Sizing Agent Adhesion> The amount of sizing agent adhering (mass %) is a value relative to the carbon fiber. This was calculated based on the difference in mass before and after chemical decomposition of the sizing-adhered carbon fiber with a mixture of sulfuric acid and hydrogen peroxide in accordance with JIS R 7604.

[0158] <Surface Elastic Modulus> The elastic modulus (MPa) of the surface of the carbon fiber with a sizing agent attached was measured using a scanning probe microscope (SPM) (Jupiter XR manufactured by Oxford Instruments) under the following measurement conditions: Measurement mode: First Force Mapping Probe: FS-1500 manufactured by Oxford Instruments Scan size: 3.5 μm × 3.5 μm Pixel size: 13.7 nm Image size: 256 pixels × 128 pixels Z rate: 200 Hz Set point: 200 nN Force distance: 300 nm

[0159] Then, from the obtained data on the distribution of elastic modulus, the average value (number average) of elastic modulus on the surface of the carbon fiber with the sizing agent attached was calculated. When using a BRUKER SPM, measurement can be performed in peak force tapping mode. The value of elastic modulus is the average value of values ​​obtained from at least 10,000 locations.

[0160] <450°C Decomposition Amount> The thermal mass loss rate (%) at 450°C was measured using a thermogravimetric differential calorimeter (TG-DTA) as follows: (2-1) 10 mg of the sizing agent composition was placed in the thermogravimetric differential calorimeter (TG-DTA), and heat-dried and heat-treated under the following conditions to measure the change in mass: in a dry air atmosphere (flow rate 50 mL / min), the temperature was increased from room temperature to 250°C, and then held at 250°C for 5 minutes to heat-dry, and then the temperature was increased from 250°C to 450°C at a rate of 20°C / min. (2-2) Based on the obtained measurement results, the thermal mass loss rate (%) was determined according to the following formula: Thermal mass loss rate (%) = (mass after heat-drying at 250°C - mass after heat-treatment at 450°C) / (mass after heat-drying at 250°C) × 100

[0161] <Remaining Fiber Length (Composite)> The remaining fiber length in the composite was determined by observing carbon fibers isolated by baking at 585°C for 2 hours under a microscope and measuring the weight average fiber length of approximately 500 fibers.

[0162] <Aspect Ratio (Composite)> The aspect ratio (composite) was calculated from the remaining fiber length and the single fiber diameter in the composite.

[0163] <Production of Composites> The composites (carbon fiber reinforced composite materials) used in the Examples and Comparative Examples were produced as follows: The sizing-adhered carbon fibers according to the Examples or Comparative Examples and a resin (PEEK resin, manufactured by Victrex plc., product name VICTREX PEEK 150P) were supplied to a twin-screw extruder (manufactured by The Japan Steel Works, Ltd., product name TEX30X) at a fiber weight content (Wf) of 40% of the carbon fibers, and the mixture was kneaded and extruded at 380°C to produce a pellet-shaped compound.

[0164] The pellets were then injection molded using an injection molding machine (manufactured by The Japan Steel Works, Ltd., product name J-110AD-180H) at a cylinder temperature of 400°C and a mold temperature of 200°C to produce ISO 20753 A1 type test specimens.

[0165] <Composite Properties> The tensile strength (MPa) and tensile modulus (GPa) of the composite were measured according to JIS K7161, and the flexural strength (MPa) and flexural modulus (GPa) were measured according to JIS K7171.

[0166] <<Examples 1 to 6 and Comparative Examples 1 to 3>> Examples 1 to 6 and Comparative Examples 1 to 3 are experimental examples in which carbon fibers having a single fiber diameter of 7 μm were used.

[0167] Example 1 (Carbon Fiber) Carbon fiber (tensile strength: 4300 MPa, tensile modulus: 240 GPa, single fiber diameter: 7 μm, number of filaments: 24,000) was cut to an average length of 3 mm to obtain chopped carbon fiber.

[0168] (Application and Drying) A sizing agent composition containing the components shown below was applied to the chopped carbon fibers having an average length of 3 mm, and the resulting mixture was dried by heating in air at 250°C for 5 minutes to produce sizing-adhered carbon fibers (short fibers) according to Example 1.

[0169] The components of the sizing agent compositions used in the examples and comparative examples were as follows: Polyimide (PI) (HP-1632, manufactured by MICHELMAN); Blocked isocyanate (BI) (MEIKANATE TP-11, manufactured by Meisei Chemical Industry Co., Ltd.); Polyamideimide (PAI) (HPC-1000-28, manufactured by Resonac Corporation); Polyurethane (PU) (U2022, manufactured by MICHELMAN); Water as a solvent or dispersion medium.

[0170] The polyimide (PI) and blocked isocyanate (BI) used in Example 1 were used in a mass ratio of 50:50.

[0171] The sizing agent-attached carbon fiber according to Example 1 was evaluated according to the above-mentioned evaluation method. The results are shown in Table 1 below.

[0172] In Example 2, except that the mass ratio of PI to BI was changed, sizing agent-adhered carbon fibers were produced and evaluated in the same manner as in Example 1. The results are shown in Table 1 below.

[0173] In Example 3, except that the mass ratio of PI to BI was changed, sizing agent-adhered carbon fibers were produced and evaluated in the same manner as in Example 1. The results are shown in Table 1 below.

[0174] In Example 4, except that polyamideimide (PAI) was used instead of polyimide (PI), production and evaluation of sizing-adhered carbon fibers were carried out in the same manner as in Example 1. The results are shown in Table 1 below.

[0175] Example 5 In Example 5, sizing-adhered carbon fibers were produced through a two-stage sizing application process. Specifically, blocked isocyanate (BI) was applied to the carbon fibers as an undercoat layer, followed by drying. A sizing agent composition containing a polyimide resin (PI) precursor and a non-polyimide resin precursor (BI, blocked isocyanate) was further applied thereto, followed by heating and drying at 250°C for 5 minutes. Other components of the production method (carbon fibers, etc.) were the same as in Example 1. The evaluation results, performed in the same manner as in Example 1, are shown in Table 1 below.

[0176] In Example 6, carbon fibers having a sizing agent attached thereto were produced and evaluated in the same manner as in Example 5, except that a polyurethane resin (PU) was used instead of blocked isocyanate (BI) for the undercoat layer. The results are shown in Table 1 below.

[0177] In Comparative Example 1, sizing-adhered carbon fibers were produced and evaluated in the same manner as in Example 1, except that only polyimide (PI) was used instead of polyimide (PI) and blocked isocyanate (BI), and that carbon fibers with a different O / C were used. The results are shown in Table 1 below.

[0178] In Comparative Example 2, production and evaluation of sizing-adhered carbon fibers were carried out in the same manner as in Example 1, except that only polyimide (PI) was used instead of polyimide (PI) and blocked isocyanate (BI). The results are shown in Table 1 below.

[0179] <Comparative Example 3> In Comparative Example 3, production and evaluation of sizing-adhered carbon fibers were performed in the same manner as in Example 1, except that polyimide (PI) and polyurethane resin (PU) were used instead of polyimide (PI) and blocked isocyanate (BI). The results are shown in Table 1 below.

[0180]

[0181] As can be seen from Table 1, the carbon fibers with sizing agents of Examples 1 to 4 have a surface modulus of 610 to 1230 MPa and exhibit a thermal mass loss rate of the sizing agent of 17 to 40% upon heat treatment at 450° C. The carbon fibers with sizing agents of Examples 5 and 6 have a surface modulus of 1240 to 1270 MPa and exhibit a thermal mass loss rate of the sizing agent of 16 to 18% upon heat treatment at 450° C.

[0182] On the other hand, Comparative Examples 1 and 2 exhibit a relatively high surface elastic modulus (1600 MPa) and a thermal mass loss rate of the sizing agent of 0 to 17% upon heat treatment at 450° C. Comparative Example 3 exhibits an elastic modulus of 760 MPa and a relatively high thermal mass loss rate of the sizing agent (46%) upon heat treatment at 450° C.

[0183] As can be seen from Table 1, the composite materials of Examples 1 to 6 exhibited superior physical properties, at least with respect to tensile strength and flexural strength, to those of Comparative Examples 1 to 3 (the tensile modulus and flexural modulus were equal to or greater than those of Comparative Examples 1 to 3). Furthermore, the composite materials of Examples 1 to 6 had longer aspect ratios of the remaining fibers than those of Comparative Examples 1 to 3.

[0184] Furthermore, as can be seen from Table 1, the composite materials according to Examples 5 and 6 of the "two-layer structure" exhibited superior physical properties, at least in terms of tensile strength and flexural strength, to those of Examples 1 to 4. Furthermore, the composite materials according to Examples 5 and 6 had longer residual fiber aspect ratios than those of Examples 1 to 4.

[0185] <<Examples 7 to 10 and Comparative Examples 4 to 6>> Examples 7 to 10 and Comparative Examples 4 to 6 are experimental examples in which carbon fibers having a single fiber diameter of 5.7 μm were used.

[0186] Example 7 (Carbon Fiber) Carbon fiber (tensile strength: 5100 MPa, tensile modulus: 290 GPa, single fiber diameter: 5.7 μm, number of filaments: 36,000) was cut to an average length of 3 mm to obtain chopped carbon fiber.

[0187] (Application and Drying) A sizing agent composition containing the components shown in Table 2 was applied to the chopped carbon fibers having an average length of 3 mm, and the fibers were dried by heating at 250°C for 5 minutes to produce sizing-adhered carbon fibers (short fibers) according to Example 7.

[0188] The components of the sizing agent compositions used in the examples and comparative examples were as follows: polyimide (PI) (precursor of polyimide resin, manufactured by MICHELMAN, HP-1632); blocked isocyanate (BI) (manufactured by Meisei Chemical Industry Co., Ltd., MEIKANATE TP-11); polyamideimide (PAI) (manufactured by Resonac Corporation, HPC-1000-28); polyurethane (PU) (manufactured by MICHELMAN, U2022); water as a solvent or dispersion medium.

[0189] The polyimide (PI) and blocked isocyanate (BI) used in Example 7 were used in a mass ratio of 50:50.

[0190] The sizing agent-attached carbon fiber according to Example 7 was evaluated according to the above-mentioned evaluation method. The results are shown in Table 2 below.

[0191] <Example 8> In Example 8, except that the mass ratio of PI to BI was changed, production and evaluation of sizing-adhered carbon fibers were performed in the same manner as in Example 7. The results are shown in Table 2 below.

[0192] In Example 8, the sizing agent exhibited an "island-in-sea structure" on the surface of the carbon fiber coated with the sizing agent. The results of evaluating this island-in-sea structure using the method described in the specification were as follows: Average modulus of elasticity (MPa) of the high modulus region = 1400 MPa Coverage (%) of the high modulus region = 63% Average modulus of elasticity (MPa) of the low modulus region = 180 MPa Average area (μm ２ ) = 0.029 μm ２ Maximum area of ​​low elastic modulus region (μm ２ ) = 0.038 μm ２

[0193] Measurements of the "sea-island structure" were carried out as follows: (1) Based on the data on the surface elastic modulus obtained from the measurement using a scanning probe microscope as described above, an image was created by performing enhancement processing and binarization processing under the following conditions using image analysis software (WinROOF2018, Mitani Corporation). In this image, particles with a size of 0.001 μm were identified. ２ After removing areas less than 1 μm from the fiber surface, the "island" and "sea" areas were identified in the image. ２ An image of a portion covering an area of ​​100 to 255 was used. - Enhancement processing conditions: Full color brightness 30, contrast 50 - Binary processing conditions: H threshold 100-330, L threshold 50-200, S threshold 100-255 (2) The area and average elastic modulus of the "island" portions (low elastic modulus regions) in the image were calculated, and the maximum area was determined. In addition, the total area and number of "island" portions were measured, and based on these values, the average area of ​​the low elastic modulus regions was calculated. Note that at least 10 "island" portions were analyzed. (3) The average elastic modulus of the "sea" portions (high elastic modulus regions) in the image was calculated. (4) The "coverage" of the "sea" portions (high elastic modulus regions) was determined as the area of ​​the remaining portion excluding the total area of ​​the calculated "island" portions.

[0194] Example 9: Carbon fibers with a sizing agent attached were produced and evaluated in the same manner as in Example 7, except that polyamideimide (PAI) was used instead of polyimide (PI) in Example 4. The results are shown in Table 2 below.

[0195] Example 10 In Example 10, sizing-adhered carbon fibers were produced through a two-stage sizing agent application process. Specifically, blocked isocyanate (BI) was applied to the carbon fibers as an undercoat layer, followed by drying. A sizing agent composition containing polyimide and blocked isocyanate was then further applied (topcoat layer), followed by heat drying. Other aspects of the production method (carbon fibers, etc.) were the same as in Example 1. The evaluation results, performed in the same manner as in Example 1, are shown in Table 2 below.

[0196] In Comparative Example 4, production and evaluation of sizing-adhered carbon fibers were carried out in the same manner as in Example 7, except that only polyimide (PI) was used instead of polyimide (PI) and blocked isocyanate (BI). The results are shown in Table 2 below.

[0197] Comparative Example 5 is a "two-coat" embodiment. In Comparative Example 5, carbon fibers with a sizing agent attached thereto were produced and evaluated in the same manner as in Example 10, except that the mass ratio of blocked isocyanate (BI) in the undercoat layer was changed and only polyimide (PI) was used in the topcoat layer. The results are shown in Table 2 below.

[0198] <Comparative Example 6> In Comparative Example 6, production and evaluation of sizing-adhered carbon fibers were carried out in the same manner as in Example 7, except that polyimide (PI) and polyurethane (PU) were used instead of polyimide (PI) and blocked isocyanate (BI). The results are shown in Table 2 below.

[0199]

[0200] The carbon fibers with sizing agents of Examples 7 to 9 have moduli of elasticity of 690 to 1170 MPa and exhibit a thermal mass loss rate of the sizing agent of 17 to 31% upon heat treatment at 450° C. The carbon fibers with sizing agents of Example 10 have moduli of elasticity of 1200 MPa and exhibit a thermal mass loss rate of the sizing agent of 16% upon heat treatment at 450° C.

[0201] Comparative Examples 4 and 5 exhibited a relatively high surface elastic modulus (1600 MPa) and a thermal mass loss rate of the sizing agent of 0% after heat treatment at 450° C. Comparative Example 6 exhibited a modulus of elasticity of 760 MPa and a relatively high thermal mass loss rate of the sizing agent of 46% after heat treatment at 450° C.

[0202] As can be seen from Table 2, the composite materials of Examples 7 to 10 exhibited superior physical properties, at least with respect to tensile strength and flexural strength, to those of Comparative Examples 4 to 6. Furthermore, the composite materials of Examples 7 to 10 had longer aspect ratios of the remaining fibers than those of Comparative Examples 4 to 6.

[0203] Furthermore, as can be seen from Table 2, the composite material according to Example 10 exhibited superior physical properties, at least with respect to tensile strength and flexural strength, to those of Examples 7 to 9. Furthermore, the composite material according to Example 10 had a longer aspect ratio of the remaining fibers than those of Examples 7 to 9.

Claims

1. A sizing agent composition comprising: (a) at least one polyimide resin or a precursor thereof; and (b) at least one non-polyimide resin or a precursor thereof; wherein, when the sizing agent composition is applied to the surface of a carbon fiber in an amount of 1.5% by weight of the carbon fiber and then heated and dried in air at 250°C for 5 minutes to obtain a sizing-adhered carbon fiber, the sizing-adhered carbon fiber has a surface elasticity of 500 to 1500 MPa; and when the sizing agent composition is heated and dried in air at 250°C for 5 minutes and then heat-treated in a dry air atmosphere by increasing the temperature to 450°C at a rate of 20°C / min, the thermal mass loss rate is in the range of 5 to 40%.

2. The sizing composition according to claim 1, wherein the at least one polyimide resin or precursor thereof is selected from polyimide resins, polyetherimide resins, polyamideimide resins, and polyphenylimide resins, and precursors thereof.

3. The sizing agent composition according to claim 1 or 2, wherein the at least one non-polyimide resin or precursor thereof is selected so as to yield a low-urethane bond content resin having 0.1 to 3 weight percent urethane bonds after heat drying in air at 250°C for 5 minutes.

4. The sizing agent composition according to claim 1 or 2, wherein the at least one non-polyimide resin or precursor thereof contains a blocked isocyanate.

5. A method for producing carbon fibers with a sizing agent attached thereto, comprising: applying the sizing agent composition according to claim 1 or 2 to the surface of carbon fibers; and heating and drying the carbon fibers with the sizing agent composition attached thereto.

6. A method for producing carbon fibers with a sizing agent attached thereto, comprising: applying a first sizing agent composition to the surface of carbon fibers and optionally heating and drying; applying a second sizing agent composition to the carbon fibers with the first sizing agent composition attached thereto; and heating and drying the carbon fibers with the first sizing agent composition and the second sizing agent composition attached thereto; wherein the first sizing agent composition comprises (c) at least one selected from the group consisting of polyurethane resins, blocked isocyanates, and precursors thereof; and the second sizing agent composition is the sizing agent composition defined in claim 1 or 2.

7. Carbon fibers having a sizing agent-coated surface layer formed from the sizing agent composition according to claim 1 or 2.

8. A sizing-agent-adhered carbon fiber having a sizing agent adhered to the surface of a carbon fiber, wherein the sizing agent contains: (A) at least one polyimide resin; and (B) at least one non-polyimide resin; the sizing-agent-adhered carbon fiber has a surface elastic modulus of 500 to 1500 MPa; and when the sizing-agent-adhered carbon fiber is heated to 450°C at a rate of 20°C / min in a dry air atmosphere, the thermal mass loss rate of the sizing agent is in the range of 5 to 40%.

9. The carbon fiber having a sizing agent attached thereto according to claim 8, wherein the at least one polyimide resin is selected from the group consisting of polyimide resins, polyetherimide resins, polyamideimide resins, and polyphenylimide resins.

10. The carbon fiber with a sizing agent applied thereto according to claim 8 or 9, wherein the at least one non-polyimide resin comprises a resin with a low urethane bond content having 0.1 to 3% by weight of urethane bonds.

11. The carbon fiber having a sizing agent applied thereto according to claim 8 or 9, wherein the at least one non-polyimide resin comprises a polymerization product of a blocked isocyanate.

12. The carbon fiber having a sizing agent applied thereto according to claim 8 or 9, wherein the mass ratio of component (A) to component (B) is 100:10 to 10:

100.

13. The sizing-agent-adhered carbon fiber according to claim 8 or 9, wherein the sizing-agent-adhered carbon fiber has a first sizing layer disposed on the surface of the carbon fiber and a second sizing layer disposed on the first sizing layer, the first sizing layer being composed of component (C), which is at least one selected from the group consisting of a polymerization product of a polyurethane resin and a blocked isocyanate, and the second sizing layer being composed of components (A) and (B).

14. Carbon fibers with a sizing agent applied thereto according to claim 8 or 9, wherein the mass ratio of component (A), component (B), and component (C) is 50-90:25-5:25-5 when the first sizing layer and the second sizing layer are considered together.

15. The carbon fiber with a sizing agent applied thereto according to claim 8 or 9, wherein the sizing agent applied to the surface of the carbon fiber has a sea-island structure in which low elastic modulus regions are dispersed within high elastic modulus regions, the high elastic modulus regions having an average elastic modulus of 1.0 GPa to 3.0 GPa, and the low elastic modulus regions having an average elastic modulus of 100 to 600 MPa.

16. The low elastic modulus region is 0.001 μm ２ ~0.30 μm ２ The carbon fiber according to claim 15, having an average area of 17. The carbon fiber with a sizing agent attached thereto according to claim 8 or 9, wherein the carbon fiber is obtained by a wet spinning method.

18. Short fibers made of carbon fibers having a sizing agent applied thereto according to claim 8 or 9.

19. A carbon fiber resin composite material containing the short fibers according to claim 18.

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