Sizing agent dispersion for carbon fiber bundle, sizing agent-containing carbon fiber bundle, composite material, and pressure vessel

Abstract

To provide a sizing agent-containing carbon fiber bundle having superior tensile strength and bundling performance in a composite material, capable of suppressing deposition fluff generated during processing, while also ensuring quality stability.SOLUTION: The present invention provides a sizing agent dispersion for carbon fiber bundle satisfying the following (i) and (ii): (i) a bisphenol A type ethylene oxide adduct (A) represented by the following chemical formula 1 is comprised in the sizing agent, an unsaturated polyester (B) being a condensate of an unsaturated dibasic acid and an alkylene oxide adduct of a bisphenol is comprised, and an epoxy compound (C) is comprised (where m+n is the number of moles of added ethylene oxide, ranging from 14 to 22); (ii) when the sizing agent content in the sizing agent dispersion is 0.1 mass%, the ultraviolet-visible absorbance of the sizing agent dispersion at a wavelength of 500 nm is 0.05 or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a sizing agent dispersion for carbon fiber bundles and a sizing agent-containing carbon fiber bundle that are suitable for use in aerospace components, automobile components, and ship components, as well as in sports applications such as golf shafts and fishing rods, and other general industrial applications. [Background technology]

[0002] Carbon fiber bundles are lightweight yet have excellent strength and elastic modulus, and are therefore used in a wide range of fields, including aerospace and sports applications, general industrial applications such as automobiles, ships, civil engineering and construction, pressure vessels, and wind turbines, as composite materials combined with various matrix resins.

[0003] Carbon fiber bundles are typically coated with a sizing agent to form sizing-containing carbon fiber bundles. Sizing agents have a variety of functions, including providing bundles of carbon fiber with a uniform bundling property to improve handling; reducing the occurrence of fuzzing, which is particularly pronounced during the production of prepregs using carbon fiber bundles, especially in the filament winding process, a molding method for pressure vessels, when the fiber bundles pass through multiple guide members such as rollers and fixed bars; and enhancing the mechanical properties of composite materials (tensile strength, flexural strength, interlaminar shear strength, etc.). These functions must also be stable over time and universally independent of the matrix resin. While various sizing agents have been proposed to meet these requirements, designing a sizing agent that balances these diverse requirements is challenging.

[0004] Patent Document 1 describes that by applying a sizing agent that is a mixture of an epoxy compound, an unsaturated dibasic acid, and a condensate of an alkylene oxide of a bisphenol (unsaturated polyester), bundling properties are exhibited and the flexural strength and interlaminar shear strength of the composite material are increased, even when the matrix resin is an unsaturated polyester resin rather than the commonly used epoxy resin. Furthermore, a sizing agent composition has been proposed in which both the epoxy compound and the unsaturated polyester have a bisphenol A skeleton, and it is believed that increasing the affinity with the matrix resin leads to the expression of the flexural strength and interlaminar shear strength of the composite material.

[0005] Other sizing agents based on a bisphenol A skeleton have also been proposed. Patent Document 2 describes that by using an ethylene oxide adduct with an added mole number of 25 or 30 to a bisphenol A skeleton as a sizing agent, the coefficient of friction is reduced, fluffing during processing is suppressed, and the interlaminar shear strength of the composite material is increased.

[0006] Patent Document 3 describes that by applying an ethylene oxide adduct with an added mole number of 10 or 30 to a bisphenol A skeleton, the generation of fluff is reduced, the matrix resin is more easily penetrated to the interior, and the interlaminar shear strength of the composite material is increased.

[0007] Patent Document 4 describes that the interlaminar shear strength of a composite material can be increased by applying an ethylene oxide adduct having an added mole number of 10 or 12 to a bisphenol A skeleton.

[0008] Patent Document 5 describes that by applying a sizing agent containing an ethylene oxide adduct with an addition mole number of 60 to bisphenol A and an epoxy compound, it is possible to obtain good resin impregnation properties while maintaining sizing properties.

[0009] Patent Document 6 describes that by applying a sizing agent containing an ethylene oxide adduct with an addition mole number of 8, 10, or 17.5 to bisphenol A and a specific polyester, it is possible to achieve both sizing, adhesiveness, and abrasion resistance (resistance to pilling when abraded). [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Publication No. 53-52796 [Patent Document 2] Japanese Patent Application Publication No. 1-272867 [Patent Document 3] Japanese Patent Application Publication No. 7-9444 [Patent Document 4] Japanese Patent Application Publication No. 6-212565 [Patent Document 5] Japanese Patent Application Laid-Open No. 2008-280623 [Patent Document 6] International Publication No. 2023 / 026674 Summary of the Invention [Problem to be solved by the invention]

[0011] As mentioned above, sizing agents based on a bisphenol A skeleton have been proposed. Patent Document 1 describes the effects of bundling and the expression and universality of mechanical properties as a composite material, while Patent Documents 2 to 4 describe the expression of mechanical properties as a composite material and the suppression of fuzzing. Patent Document 5 describes the effect of enhancing the expression of mechanical properties as a composite material and bundling. Furthermore, with regard to the expression of mechanical properties, the effect of improving bending strength and interlaminar shear strength, which are related to the affinity between the matrix resin and carbon fiber, is described. While there is no mention of tensile strength, tensile strength is also one of the important mechanical properties related to the affinity between the matrix resin and carbon fiber. The above proposed sizing agents each have the following issues.

[0012] The present inventors have found that although the sizing agent of Patent Document 1 has some effect in suppressing fluffing of carbon fiber bundles when they pass through a fixed bar, it cannot suppress fluff that accumulates on the fixed bar (carbon fibers that have been cut and separated from the carbon fiber bundle and adhere to and accumulate on the solid bar, hereinafter referred to as accumulated fluff. Accumulated fluff can cause reduced productivity and quality problems.) Furthermore, the sizing agent is designed to contain a large amount of epoxy compounds, and there are issues with quality stability due to epoxy hydrolysis.

[0013] Furthermore, the sizing agent of Patent Document 2 had poorer bundling properties and was not easy to handle compared to Patent Document 1. Furthermore, the present inventors have recognized that when the number of moles of ethylene oxide added to the bisphenol A skeleton is 25 or 30, the melting point is near room temperature, and therefore there is a risk that the properties will change depending on the temperature of use, resulting in a decrease in quality stability.

[0014] Patent Document 3 also discloses that the number of moles of ethylene oxide added to the bisphenol A skeleton is 10 in addition to 30, and the inventors have found that when the number of moles added is 10, the suppression of accumulated fuzz is somewhat effective. On the other hand, as with Patent Document 2, the sizing ability is insufficient compared to Patent Document 1, and it has been found that both accumulated fuzz and sizing ability cannot be achieved by optimizing the number of moles added alone. Patent Document 4 also discloses that the number of moles of ethylene oxide added to the bisphenol A skeleton is 10 or 12, and similarly the sizing ability is insufficient.

[0015] In Patent Document 5, the number of moles of ethylene oxide added to the bisphenol A skeleton is as large as 60, and since it is solid at room temperature, it can improve to some extent the sizing ability that is a problem of bisphenol A-type ethylene oxide adducts, but the proportion of the bisphenol A skeleton is reduced, which reduces the affinity with the matrix resin, which is its original advantage. To compensate for this reduction, it has been proposed to mix it with various epoxy compounds, but the inventors have found that a design that includes a large amount of epoxy compound is strongly affected by the deterioration of the epoxy compound over time (hydrolysis of the epoxy group), resulting in poor stability over time.

[0016] Patent Document 6 describes that a sizing agent containing an ethylene oxide adduct of bisphenol A with an added mole number of 8, 10, or 17.5 and a specific polyester is excellent in abrasion resistance, but the present inventors have found that although there is some effect in suppressing fuzzing of carbon fiber bundles when they pass through a fixed bar, etc., the aforementioned accumulated fuzz cannot be sufficiently suppressed unless an ethylene oxide adduct of bisphenol A with a specific added mole number is used. Furthermore, they have found that the specific polyester (not containing a bisphenol A skeleton) described in Patent Document 6 cannot sufficiently increase the tensile strength of a composite material, and may also have insufficient bundling ability.

[0017] As described above, it is important for a composite material to satisfy all of the requirements for mechanical properties, bundle property, accumulated fuzz, and quality stability (changes over time and changes due to temperature during use). However, Patent Document 1 has issues with accumulated fuzz and quality stability (changes over time), Patent Document 2 has issues with bundle property and quality stability (changes due to temperature during use), Patent Documents 3 and 4 have issues with bundle property, Patent Document 5 has issues with quality stability (changes over time), and Patent Document 6 has issues with accumulated fuzz, bundle property, and mechanical properties.

[0018] Thus, although many sizing agents with a bisphenol A skeleton have been investigated, known technologies have not yet achieved a balance of all properties. As in Patent Document 5, increasing the number of moles of ethylene oxide added to improve bundling, which is a problem with bisphenol A ethylene oxide, reduces affinity with the matrix resin, and adding an epoxy compound to compensate for this reduced affinity reduces quality stability. Thus, there is a trade-off between the various properties, and simple mixing has not yielded a sizing agent that satisfies the mechanical properties, bundling, accumulated fuzz, and quality stability (changes over time and with use temperature) of composite materials. [Means for solving the problem]

[0019] In order to solve the above-mentioned problems, the present inventors have discovered that by controlling not only the composition of the bisphenol A compound and the mixing ratio with other components but also the dispersion state, an excellent sizing agent can be obtained that balances all properties. (1) A sizing agent dispersion for carbon fiber bundles that satisfies the following (i) and (ii): (i) The sizing agent contains a bisphenol A-type ethylene oxide adduct (A) represented by the following chemical formula 1, an unsaturated polyester (B) which is a condensate of an unsaturated dibasic acid and an alkylene oxide adduct of a bisphenol, and an epoxy compound (C).

[0020] [ka]

[0021] (where m+n is the number of moles of ethylene oxide added, and is 14 to 22.) (ii) When the sizing agent content in the sizing agent dispersion is 0.1% by mass, the sizing agent dispersion has an ultraviolet-visible absorbance of 0.05 or more at a wavelength of 500 nm. (2) The sizing agent dispersion for carbon fiber bundles according to (1), wherein the bisphenol A ethylene oxide adduct (A) is contained in the sizing agent in an amount of 30 to 90% by mass. (3) The sizing agent dispersion for carbon fiber bundles according to (1) or (2), wherein the sizing agent contains the bisphenol A ethylene oxide adduct (A) in an amount of 60 to 80% by mass. (4) The sizing agent dispersion for carbon fiber bundles according to (1), which contains 3 to 30% by mass of the unsaturated polyester (B). (5) The sizing agent dispersion for carbon fiber bundles according to (1), which contains 10 to 30% by mass of the unsaturated polyester (B). (6) The sizing agent dispersion for carbon fiber according to (1), which contains 3 to 30 mass % of the epoxy compound (C). (7) The sizing agent dispersion for carbon fiber according to (1), which contains 3 to 15 mass % of the epoxy compound (C). (8) The sizing agent dispersion according to any one of (1) to (7), wherein the dispersion medium of the sizing agent dispersion is water. (9) A sizing agent dispersion according to any one of (1) to (8), which contains a solution containing a bisphenol A-type ethylene oxide adduct (A) and an emulsion containing an unsaturated polyester (B) and an epoxy compound (C). (10) A sizing-agent-containing carbon fiber bundle containing the sizing agent according to any one of (1) to (9). (11) The sizing agent-containing carbon fiber bundle according to (10), wherein the sizing agent content is 0.6 to 1.2% by mass. (12) A composite material comprising the sizing-agent-containing carbon fiber bundle according to either (10) or (11) above and a matrix resin. (13) A pressure vessel comprising the sizing-agent-containing carbon fiber bundle according to either (10) or (11) above and a matrix resin. [Effects of the Invention]

[0022] According to the present invention, it is possible to obtain a sizing-agent-containing carbon fiber bundle that is excellent in the tensile strength and bundling strength of a composite material, suppresses the generation of accumulated fluff that occurs during the process, and also has stable quality. DETAILED DESCRIPTION OF THE INVENTION

[0023] Hereinafter, embodiments for carrying out the present invention will be described. The sizing agent dispersion for carbon fiber in the present invention is a sizing agent dispersion for carbon fiber bundles that satisfies the following (i) and (ii): (i) The sizing agent contains a bisphenol A-type ethylene oxide adduct (A) represented by the following chemical formula 1, an unsaturated polyester (B), and an epoxy compound (C).

[0024] [ka]

[0025] (where m+n is the number of moles of ethylene oxide added, and is 14 to 22.) (ii) When the sizing agent content in the sizing agent dispersion is 0.1% by mass, the sizing agent dispersion has an ultraviolet-visible absorbance of 0.05 or more at a wavelength of 500 nm.

[0026] The bisphenol A ethylene oxide adduct (A) in the present invention is a compound (chemical formula 1) in which an ethylene oxide group is added to bisphenol A, and m+n, which is the number of moles of ethylene oxide added, is 14-22.

[0027] The inventors have found that by setting m+n to 14 to 22, the tensile strength of the composite material can be improved while the effect of suppressing accumulation of fuzz can be enhanced, and the melting point is not near room temperature, resulting in excellent quality stability (the properties are less likely to be affected by the temperature used when processing into a composite material).

[0028] Here, deposited fluff will be described in detail. Deposited fluff occurs when carbon fibers that have been cut and separated from a carbon fiber bundle using a fixed bar or the like adhere to and accumulate, so it is necessary to evaluate the amount of cut and separated carbon fibers. The inventors of the present invention have found that it is difficult to evaluate only the cut and separated carbon fibers using known methods (methods of collecting the fibers by sandwiching them between sponges and methods of detecting them using a laser), and that this cannot serve as an indicator of the amount of deposited fluff. Therefore, they have tried a method of collecting the cut and separated carbon fibers by suctioning the carbon fiber bundle after it has passed through a fixed bar, and have found that the mass of the carbon fibers collected by this method can be used as an indicator of the amount of deposited fluff.

[0029] In the present invention, if the m+n value of the bisphenol A-type ethylene oxide adduct is less than 14, the proportion of the bisphenol A skeleton increases, possibly resulting in increased viscosity and increased fuzz accumulation. If the m+n value is greater than 22, the melting point will be near room temperature, and the properties will be affected by the temperature at which the adduct is used.

[0030] Furthermore, when m+n becomes as large as 60 as in JP 2008-280623 A (Patent Document 5), the proportion of bisphenol A skeleton decreases, which reduces affinity with the matrix resin, resulting in poor impregnation and a decrease in the tensile strength of the composite material. From the above perspectives, m+n is preferably 16 to 20.

[0031] The bisphenol A ethylene oxide adduct (A) is preferably contained in an amount of 30 to 90% by mass relative to the total mass of the sizing agent (100% by mass), since this achieves a balance between the effect of suppressing accumulated fuzz and the effect of developing tensile strength in the composite material. The bisphenol A ethylene oxide adduct (A) is more preferably contained in an amount of 50 to 90% by mass relative to the total mass of the sizing agent (100% by mass), since this enhances the effect of suppressing accumulated fuzz, and more preferably contained in an amount of 60 to 80% by mass, and may even be contained in an amount of 70 to 90% by mass.

[0032] The unsaturated polyester (B) in the present invention is a condensate of an unsaturated dibasic acid and an alkylene oxide adduct of a bisphenol. Examples of the unsaturated dibasic acid include fumaric acid, maleic acid, citraconic acid, and itaconic acid. Examples of the alkylene oxide adduct of a bisphenol include an alkylene oxide (e.g., ethylene oxide, propylene oxide, or butylene oxide) adduct of a bisphenol (a random or block adduct in the case of two or more alkylene oxide adducts). Among these, a condensate of fumaric acid or maleic acid with a bisphenol A-type ethylene oxide adduct having an ethylene oxide addition mole number of 2 to 10 is preferred from the viewpoint of affinity with the matrix resin. The use of an unsaturated polyester (B), which is a condensate of an unsaturated dibasic acid and a bisphenol, effectively controls resin impregnation and adhesive strength, thereby enhancing the tensile strength of the composite material. It also effectively improves the bundling ability of fiber bundles.

[0033] In the present invention, the unsaturated polyester (B) must be contained in the sizing agent. The unsaturated polyester (B) not only enhances the tensile strength of the composite material even when the unsaturated polyester is used as the matrix resin, but also has a strong effect of imparting bundling properties to the carbon fiber bundles. However, if the amount is too much, the effect of inhibiting the accumulation of fluff of the bisphenol A-type ethylene oxide adduct is weakened, so the content of the unsaturated polyester (B) is preferably 3 to 30 mass %, more preferably 10 to 30 mass %, based on the total mass of the sizing agent (100 mass %).

[0034] The sizing agent of the present invention must contain an epoxy compound (C). The epoxy compound (C) is a compound containing at least one epoxy group in the molecule. Its affinity with the matrix resin improves the tensile strength of the composite material and provides bundling properties. However, excessive amounts can lead to unstable properties due to epoxy degradation over time (hydrolysis of the epoxy groups) or even a failure to improve the tensile strength of the composite material. Therefore, the amount of the epoxy compound (C) is preferably 3 to 30% by mass, more preferably 3 to 20% by mass, and even more preferably 3 to 15% by mass, based on the total mass of the sizing agent (100% by mass). While the epoxy compound is not particularly limited, bisphenol A-type epoxy is preferred because it can easily improve the tensile strength of the composite material. Representative bisphenol A-type epoxies, varying in degree of polymerization, include jER828 (degree of polymerization: approximately 0.1), jER834 (degree of polymerization: approximately 0.5), and jER1001 (degree of polymerization: approximately 2.0) (manufactured by Mitsubishi Chemical). Lower degrees of polymerization are preferred because they reduce the amount of accumulated fluff.

[0035] The mass concentration of the bisphenol A ethylene oxide adduct (A), the unsaturated polyester (B), or the epoxy compound (C) relative to the total mass of the sizing agent can be calculated from the structural formula and composition of the compound if the structural formula and composition of the sizing agent are known; if these are unknown, the mass concentration can be calculated by extracting the sizing agent from a carbon fiber bundle containing the sizing agent and identifying the structure and composition using a known method such as proton NMR, carbon NMR, mass spectrometry, or TOF-SIMS.

[0036] Next, the sizing agent dispersion of the present invention will be described. The sizing agent dispersion refers to a liquid in which a sizing agent is dispersed in a dispersion medium, and the dispersion medium here refers to a medium in which the bisphenol A ethylene oxide adduct (A) is dissolved and the unsaturated polyester (B) and the epoxy compound (C) are dispersed.

[0037] When the sizing agent content in the sizing agent dispersion of the present invention is 0.1% by mass, the sizing agent dispersion has an ultraviolet-visible absorbance at a wavelength of 500 nm of 0.05 or more. A ultraviolet-visible absorbance at a wavelength of 500 nm of 0.05 or more means that the sizing agent dispersion is not a complete solution but an emulsion or a mixture of an emulsion and a solution. Not creating a complete solution is an important factor for balancing the various properties. The present inventors have discovered that applying the sizing agent in a dispersed state rather than completely dissolved in the solvent can suppress the generation of accumulated fuzz and provide high bundling ability. Although uniform application of the sizing agent is generally preferred, the present inventors have discovered that deliberately creating a certain degree of uneven adhesion can increase the number of bonding points between carbon fibers, improve bundling ability, and suppress accumulated fuzz. When the sizing agent dispersion is completely in solution, light is not scattered, resulting in a UV-visible absorbance of approximately 0 at a wavelength of 500 nm. However, if the sizing agent is dispersed to a certain extent, light is scattered, resulting in a UV-visible absorbance of 0.05 or higher. While there is no theoretical upper limit for UV-visible absorbance, it depends on the detection accuracy of the analytical device used, and the upper limit for measurement is generally 2 or 3. Furthermore, the dispersion medium for the sizing agent dispersion of the present invention is not particularly limited and may be a common solvent, although water is preferred from an industrial standpoint. The method for measuring UV-visible absorbance is described below in the Examples section, and can be performed using a UV-visible spectrophotometer.

[0038] As described above, the sizing agent of the present invention is a bisphenol A-ethylene oxide adduct with m + n within a specific range. This ensures compatibility with the resin, enhancing the tensile strength of the composite material while providing excellent quality stability (less susceptibility to temperature changes during use). Furthermore, suppressing accumulated fuzz is one key to balancing these properties. Ensuring compatibility with the resin allows for a relative reduction in the amount of epoxy compound added, thereby suppressing quality stability (deterioration over time due to epoxy hydrolysis). Furthermore, by intentionally adding unsaturated polyester and epoxy compounds as emulsions, which not only enhance the tensile strength of the composite material but also impart sizing properties, the number of binding sites is increased, improving the sizing properties that have been a problem with bisphenol A-ethylene oxide adducts. This allows for the production of a sizing agent that achieves both tensile strength, reduced fuzz accumulation, quality stability (changes over time and with temperature changes), and sizing properties for the composite material.

[0039] The sizing agent dispersion in the present invention preferably contains a solution containing a bisphenol A-type ethylene oxide adduct (A) and an emulsion containing an unsaturated polyester (B) and an epoxy compound (C).

[0040] The sizing agent dispersion of the present invention is preferably prepared by mixing a solution containing a bisphenol A-type ethylene oxide adduct (A) with an emulsion containing an unsaturated polyester (B) and an epoxy compound (C). In this case, the emulsion is typically present in the solution containing the bisphenol A-type ethylene oxide adduct (A). The unsaturated polyester (B) and the epoxy compound (C) have the effect of enhancing or imparting sizing properties to the tensile strength of the composite material, while the bisphenol A-type ethylene oxide adduct (A) not only enhances the tensile strength of the composite material but also suppresses accumulated fuzz. The inventors have found that intentionally applying the unsaturated polyester (B) and the epoxy compound (C), which have the effect of imparting sizing properties, unevenly increases the number of bonding points between the carbon fibers, improves sizing properties, and suppresses accumulated fuzz. Therefore, these two components are applied as an emulsion. Furthermore, when the bisphenol A ethylene oxide adduct (A), which has the effect of suppressing accumulated fuzz, is dissolved in a solution and uniformly adhered to the carbon fibers, the bundling ability is improved and accumulated fuzz can be suppressed without reducing the strength of the bonding points between the carbon fibers. Patent Document 5 describes mixing an aqueous emulsion of an epoxy compound with an aqueous solution of a bisphenol A ethylene oxide adduct, but this is thought to be from the perspective of industrial feasibility and safety when using water, and there is no description of the technical idea of ​​suppressing bundling ability and accumulated fuzz by controlling the adhesion state of the sizing agent.

[0041] The solution containing the bisphenol A ethylene oxide adduct (A) can be prepared by dissolving the bisphenol A ethylene oxide adduct in a solvent.

[0042] The emulsifier for emulsifying the unsaturated polyester (B) and the epoxy compound (C) is not particularly limited, but preferred examples include polyalkylene oxides such as polyethylene oxide and polypropylene oxide, compounds in which polyalkylene oxides such as polyethylene oxide and polypropylene oxide are added to higher alcohols, polyhydric alcohols, alkylphenols, and styrenated phenols, and nonionic surfactants such as block copolymers of ethylene oxide and propylene oxide.

[0043] Next, a method for applying the sizing agent dispersion to the carbon fiber bundle according to the present invention will be described.

[0044] Examples of methods for applying a sizing agent to a carbon fiber bundle include a method of immersing carbon fibers in a sizing-agent-containing liquid via a roller (immersion method), a method of contacting carbon fibers with a roller to which the sizing-agent-containing liquid has been applied, and a method of spraying a mist of the sizing-agent-containing liquid onto the carbon fibers. When applying the sizing agent, it is preferable to control the concentration of the sizing-agent-containing liquid, temperature, and yarn tension so that the content of the active ingredient of the sizing agent is uniformly contained within an appropriate range relative to the carbon fibers. Another preferred embodiment is to ultrasonically vibrate the carbon fibers when applying the sizing agent.

[0045] In the present invention, after passing the film through heated rollers as a preliminary drying step, a further heat treatment may be carried out as a drying step, and the heat treatment is preferably carried out for 10 to 600 seconds at a temperature in the range of 100 to 260° C. The heat treatment can also be carried out by microwave irradiation and / or infrared irradiation.

[0046] The components constituting the sizing-agent-containing carbon fiber bundle of the present invention will be described. There are no particular limitations on the carbon fiber bundle used in the present invention, but polyacrylonitrile-based carbon fibers are preferably used from the viewpoint of mechanical properties. The polyacrylonitrile-based carbon fiber bundle used in the present invention can be obtained by subjecting a carbon fiber precursor fiber made of a polyacrylonitrile-based polymer to flame retardation treatment in an oxidizing atmosphere at a maximum temperature of 200 to 300°C, followed by preliminary carbonization treatment in an inert atmosphere at a maximum temperature of 500 to 1,200°C, and then carbonization treatment in an inert atmosphere at a maximum temperature of 1,200 to 2,000°C. There are no particular limitations on the number of filaments in the sizing-agent-containing carbon fiber of the present invention, but it is preferably 1,000 to 100,000.

[0047] In the present invention, in order to improve the adhesion between the carbon fiber bundles and the matrix resin, it is preferable to introduce oxygen-containing functional groups onto the surfaces of the carbon fiber bundles by subjecting the carbon fiber bundles to an oxidation treatment. As the oxidation treatment method, gas phase oxidation, liquid phase oxidation, and liquid phase electrolytic oxidation are used, but liquid phase electrolytic oxidation is preferably used from the viewpoints of high productivity and enabling uniform treatment.

[0048] In the present invention, the electrolyte used in the liquid-phase electrolytic oxidation may be an acidic electrolyte or an alkaline electrolyte, for example, an inorganic acid such as sulfuric acid, nitric acid, hydrochloric acid, phosphoric acid, boric acid, or carbonic acid, an organic acid such as acetic acid, butyric acid, oxalic acid, acrylic acid, or maleic acid, or a salt such as ammonium sulfate or ammonium hydrogen sulfate.

[0049] Specific examples of alkaline electrolytes include aqueous solutions of hydroxides such as sodium hydroxide, potassium hydroxide, magnesium hydroxide, calcium hydroxide, and barium hydroxide; aqueous solutions of carbonates such as sodium carbonate, potassium carbonate, magnesium carbonate, calcium carbonate, barium carbonate, and ammonium carbonate; aqueous solutions of bicarbonates such as sodium bicarbonate, potassium bicarbonate, magnesium bicarbonate, calcium bicarbonate, barium bicarbonate, and ammonium bicarbonate; and aqueous solutions of ammonia, tetraalkylammonium hydroxide, and hydrazine.

[0050] The sizing agent content of the sizing-agent-containing carbon fiber bundle of the present invention is preferably in the range of 0.6 to 1.2% by mass based on 100% by mass of the sizing-agent-containing carbon fiber. The sizing agent content of the present invention is determined by measuring the change in mass before and after heat treatment when 2.0±0.5 g of sizing-agent-containing carbon fiber is sampled and subjected to heat treatment at 450°C in a nitrogen atmosphere for 15 minutes, and dividing the mass change by the mass before heat treatment (mass %). By setting the sizing agent content to 0.6% by mass or more, more preferably 0.75% by mass or more, the suppression of fuzz accumulation and the bundling ability are improved, while adding more than 1.2% by mass may reduce the tensile strength of the composite material. The sizing agent content can be adjusted by appropriately changing the concentration of the sizing agent dispersion.

[0051] By using the sizing-agent-containing carbon fiber bundle of the present invention described above, it is possible to improve the quality and grade of a composite material combined with a matrix resin. Furthermore, it is particularly possible to improve the quality and grade of a pressure vessel produced using a filament winding molding method in which carbon fiber bundles are wound at high speed and fluff accumulation during the process is likely to be an issue. The above-mentioned composite material contains a sizing-agent-containing carbon fiber bundle and a matrix resin, and may contain other components as necessary. Specifically, the composite material can be produced by impregnating a sizing-agent-containing carbon fiber bundle with a matrix resin, or by curing the same, and the same is true for the above-mentioned pressure vessel. [Example]

[0052] The present invention will be specifically described below with reference to examples.

[0053] <Method for measuring sizing agent content> After weighing (W1) (reading to four decimal places) 2.0±0.5 g of the sizing agent-containing carbon fiber bundle, the bundle was placed in an electric furnace (capacity 120 cm) set at 450°C in a nitrogen gas flow of 50 ml / min. 3) for 15 minutes to completely thermally decompose the sizing agent. Then, the carbon fiber bundle is transferred to a container in a dry nitrogen gas flow of 20 liters / minute and cooled for 15 minutes. The mass of the carbon fiber bundle is weighed (W2) (read to four decimal places), and the sizing agent content is calculated by W1 - W2. This sizing agent content is divided by W1 to convert it to mass% (rounded to two decimal places) when the entire sizing-agent-containing carbon fiber bundle is taken as 100 mass%, and this value is taken as the content (mass%) of the sizing agent contained. The measurement was performed three times, and the average value was taken as the content (mass%) of the sizing agent.

[0054] <Method for measuring UV-visible absorbance> A UV-visible spectrophotometer (JASCO, V-550) was used to measure UV-visible absorbance. A sizing agent dispersion was prepared with a sizing agent content of 0.1% by mass, and the UV-visible absorbance at a wavelength of 500 nm was measured.

[0055] <Evaluation method for accumulated fluff> The evaluation of accumulated fuzz was carried out by weighing the carbon fibers cut and separated from the carbon fiber bundle when the carbon fiber yarn was rubbed against the stainless steel rod. A stainless steel rod having a smooth surface and a circular bottom surface of 5 mm in diameter was placed horizontally, and two smooth-surfaced stainless steel flat rollers were placed in front of and behind the stainless steel rod so that the carbon fiber yarn passed under the stainless steel rod while contacting it at an angle of 20° when the bottom surface was viewed from the side. In this case, both stainless steel flat rollers were positioned higher than the stainless steel rod.

[0056] An initial tension of 1,000 gf was applied to 12,000 carbon fiber strands, which were passed through a stainless steel rod at a speed of 10 m / min for 10 minutes. After passing through the stainless steel rod, the carbon fiber bundle was suctioned at a position 2 cm above the top, and the cut and separated carbon fibers were collected.

[0057] The mass of the carbon fibers recovered and weighed in this manner was taken as the amount of accumulated fluff, and the results were relatively evaluated on a four-level scale of A, B, C, and D. When the amount of accumulated fluff in Comparative Example 5 was taken as 1.0, a relative value of less than 0.1 was rated as A, 0.1 or more but less than 0.8 as B, 0.8 or more but less than 1.0 as C, and 1.0 or more as D, with A, B, and C being considered pass, and D being considered fail.

[0058] <Method for evaluating focusing ability> A 1m length of carbon fiber bundle containing a sizing agent was cut into 1cm increments and placed in a 50cc glass bottle and shaken. The loose carbon fiber bundles were removed from the glass bottle, and the bundle thickness was measured to evaluate the bundleability. A relative evaluation was performed on a four-level scale of A, B, C, and D, where A means the bundle is thick, i.e., the fiber bundle is less likely to loosen and has good bundleability, B means the bundle is thinner than A, C means the bundle is thinner than B, and D means the bundle is even thinner than C, i.e., has poor bundleability. A, B, and C were considered pass, and D was considered fail.

[0059] <Evaluation of tensile strength of composite materials> The strand strength of the sizing-containing carbon fiber bundle was measured to evaluate the tensile strength of the composite material. Strand strength was determined according to the following procedure, in accordance with JIS-R-7608 (2007) Resin-Impregnated Strand Strength Test Method. The resin formulation used was "Araldite®" LY1564 SP CI / "Baxxodur®" EC331 = 100 / 35 (parts by mass). The curing conditions included 120 minutes of curing in an oven set at 80°C, followed by 240 minutes of curing in an oven set at 110°C. Note that "normal pressure" refers to standard atmospheric pressure. Ten strands of the sizing-containing carbon fiber bundle were measured, and the average value was used as the strand strength. A strength of 4.5 GPa or more was considered acceptable, and a strength of less than 4.5 GPa was considered unacceptable.

[0060] <Quality stability evaluation method> Quality stability was evaluated by the degree of change in strand strength over time. The degree of change in strand strength over time was measured and compared according to the above <Evaluation of Tensile Strength of Composite Materials> for carbon fiber bundles containing a sizing agent immediately after production and for carbon fiber bundles containing a sizing agent after 200 hours of storage in an environment of 60°C and 95% relative humidity. A bundle with a small difference in strand strength between immediately after production and after storage under high temperature and humidity was considered to have excellent quality stability, and a relative evaluation was performed on a three-level scale of A, B, and C. A strength difference of 0.0 GPa or more but 0.2 GPa or less was rated A, a strength difference of more than 0.2 GPa but 0.4 GPa or less was rated B, and a strength difference of more than 0.4 GPa was rated C, with A and B being considered pass and C being fail.

[0061] The compounds used in each example and comparative example are as follows. a1: Bisphenol A ethylene oxide adduct (molar number of additions m + n = 16) a2: Bisphenol A ethylene oxide adduct (molar number of additions m + n = 20) a'1: Bisphenol A ethylene oxide adduct (number of moles added m + n = 10) a'2: Bisphenol A ethylene oxide adduct (number of moles added m + n = 25) b1: Unsaturated polyester that is a condensation product of 5 moles of bisphenol A-type ethylene oxide adduct (the number of moles of ethylene oxide added is 10) and 6 moles of maleic acid b2: Unsaturated polyester that is a condensation product of 4 moles of bisphenol A-type ethylene oxide adduct (the number of moles of ethylene oxide added is 2) and 3 moles of maleic acid b'1: Aliphatic polyester (condensation product of polyoxyethylene glycol and adipic acid) b'2: Bisphenol A diglycidyl ether acrylic acid adduct c1: Bisphenol A type epoxy (jER828) manufactured by Mitsubishi Chemical.

[0062] [Example 1] An acrylonitrile copolymer was spun and calcined to obtain a carbon fiber bundle with a total filament count of 12,000. The carbon fiber bundle was then subjected to an electrolytic surface treatment using an aqueous sulfuric acid solution as the electrolyte. The carbon fiber bundle subjected to this electrolytic surface treatment was then washed with water and dried in heated air to obtain the raw carbon fiber bundle. An aqueous solution in which a1 was uniformly dissolved in water and an aqueous emulsion in which the mass ratio of b1, c1, and an emulsifier (polyoxyethylene distyrenated phenyl ether) was 4:4:2 were prepared. The concentrations of the sizing agent other than water in this aqueous solution and aqueous emulsion were each adjusted to 10% by mass or less, and the aqueous solution and aqueous emulsion were mixed to prepare a sizing agent dispersion. The obtained sizing agent dispersion was cloudy, and the UV-visible absorbance at a wavelength of 500 nm of the 0.1% by mass sizing agent dispersion was 0.7.

[0063] The component ratio of the sizing agent in the sizing agent dispersion is as shown in Table 1. Using the obtained sizing agent dispersion, the sizing agent was applied to the carbon fiber bundle by a dipping method, followed by pre-drying. Subsequently, as a drying step, the bundle was heat-treated in heated air at a temperature of 230°C for 35 seconds to obtain a sizing-agent-containing carbon fiber bundle. The sizing agent content in the sizing agent dispersion was adjusted so that the sizing agent content was 0.8% by mass relative to 100% by mass of the total amount of the surface-treated, sizing-agent-containing carbon fiber bundle. The evaluation results for the accumulated fluff, bundling ability, strand strength, and quality stability of the sizing-agent-containing carbon fiber bundle are shown in Table 1. The accumulated fluff and quality stability were improved compared to Comparative Example 5, which did not contain a bisphenol A-type ethylene oxide adduct. The strand strength was 4.9 GPa, which was higher than that of Comparative Example 5, and the mechanical properties were also fully satisfactory.

[0064] [Example 2] A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 1, except that the component ratio of the sizing agent in the sizing agent dispersion was changed as shown in Table 1. By increasing the mass proportion of the bisphenol A-type ethylene oxide adduct, the amount of accumulated fluff was improved compared to Example 1. The strand strength was 4.6 GPa, and the mechanical properties were also fully satisfactory.

[0065] [Example 3] A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 1, except that the component ratio of the sizing agent in the sizing agent dispersion was changed as shown in Table 1. By increasing the mass proportion of the bisphenol A-type ethylene oxide adduct, the accumulated fuzz was improved compared to Example 2. Furthermore, the amount of epoxy compound added could be relatively reduced, improving quality stability. Furthermore, despite the same component ratio of the sizing agent, the sizing-agent-containing carbon fiber bundle was superior in terms of accumulated fuzz and bundling ability to that of Comparative Example 6, in which the sizing agent was applied as a transparent solution in dimethylformamide. The strand strength was 4.7 GPa, and the mechanical properties were also fully satisfactory. Controlling the adhesion state of the sizing agent is extremely important for achieving both properties.

[0066] [Example 4] A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 3, except that the content of the sizing agent in the sizing-agent-containing carbon fiber bundle was set to 0.6% by mass. The sizing ability was lower than in Example 3. The strand strength was 4.7 GPa, and the mechanical properties were also fully satisfactory.

[0067] [Example 5] A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 1, except that the component ratio of the sizing agent in the sizing agent dispersion was changed as shown in Table 1. As the mass proportion of the bisphenol A-type ethylene oxide adduct increased, the bundling ability decreased, but the strand strength and quality stability were good.

[0068] [Example 6] A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 5, except that the content of the sizing agent in the sizing-agent-containing carbon fiber bundle was set to 1.2% by mass. The strand strength was slightly lower than that in Example 5, but was 4.7 GPa, which was fully satisfactory in terms of mechanical properties.

[0069] [Example 7] A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 5, except that the content of the sizing agent in the sizing-agent-containing carbon fiber bundle was 1.6% by mass. Because the amount of adhesion was too large, the strand strength was slightly reduced, but it was 4.6 GPa, and the mechanical properties were also fully satisfactory.

[0070] [Example 8] A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 3, except that b2 was used instead of b1. As in Example 3, the results were excellent in terms of accumulated fluff, bundling ability, strand strength, and quality stability.

[0071] [Example 9] Except for using a2 instead of a1, a sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 3. As in Example 3, the results were excellent in terms of accumulated fluff, bundling ability, strand strength, and quality stability.

[0072] [Comparative Example 1] Using a1, water was added to obtain an aqueous solution in which a1 was uniformly dissolved. A sizing agent-containing carbon fiber bundle was obtained in the same manner as in Example 1, except that the sizing agent was applied to the carbon fiber bundle using this aqueous solution. Although the sizing ability was poor, it was found that the accumulated fluff was less likely to occur than in Comparative Example 3, which uses the same bisphenol A-type ethylene oxide adduct.

[0073] Comparative Example 2 Using a2, water was added to obtain an aqueous solution in which a2 was uniformly dissolved. A sizing agent-containing carbon fiber bundle was obtained in the same manner as in Example 1, except that the sizing agent was applied to the carbon fiber bundle using this aqueous solution. Although the sizing ability was poor, it was found that the accumulated fluff was less likely to occur than in Comparative Example 3, which uses the same bisphenol A-type ethylene oxide adduct.

[0074] Comparative Example 3 Using a'1, water was added to obtain an aqueous solution in which a'1 was uniformly dissolved. A sizing agent-containing carbon fiber bundle was obtained in the same manner as in Example 1, except that a sizing agent was applied to the carbon fiber bundle using this aqueous solution. The accumulated fuzz was suppressed more than in Comparative Example 5, but not as effectively as in Comparative Examples 1 and 2. Even with the same bisphenol A-type ethylene oxide adduct, controlling the number of moles added within an appropriate range is effective in suppressing accumulated fuzz.

[0075] Comparative Example 4 Although a'2 was used, it was found that its melting point was 20°C, and it was found that it lacked quality stability (its properties are less affected by the temperature at which it is used when processed into a composite material).

[0076] Comparative Example 5 A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 1, except that b1, c1, and an emulsifier (polyoxyethylene distyrenated phenyl ether) were mixed in a mass ratio of 4:4:2 to form an aqueous emulsion, and the sizing agent was applied to the carbon fiber bundle using the emulsion. Although the bundling ability was excellent, there was a lot of accumulated fluff, and the strand strength was insufficient at 4.3 GPa due to the excessive amount of epoxy compound, and the quality stability was poor.

[0077] Comparative Example 6 A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 3, except that a sizing agent dispersion prepared by dissolving all of a1, b1, c1, and an emulsifier (polyoxyethylene distyrenated phenyl ether) in dimethylformamide was used to apply the sizing agent to the carbon fiber bundle. Despite the same sizing agent composition, more accumulated fuzz was generated than in Example 3, and the bundleability was also poor. Controlling the adhesion state of the sizing agent is also important for balancing the properties.

[0078] Comparative Example 7 Except for using a'1 instead of a1, a sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 3. a'1 alone had little effect in suppressing accumulated fluff, but in addition, mixing the unsaturated polyester with the epoxy resin resulted in even more accumulated fluff.

[0079] [Comparative Example 8] A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 3, except that b'1 was used instead of b1. The bundle had insufficient sizing ability and tensile strength.

[0080] Comparative Example 9 A sizing agent-containing carbon fiber bundle was obtained in the same manner as in Example 3, except that b'2 was used instead of b1. The tensile strength was not sufficient.

[0081] [Table 1]

Claims

1. A sizing agent dispersion for carbon fiber bundles that satisfies the following (i) and (ii): (i) The sizing agent contains a bisphenol A-type ethylene oxide adduct (A) represented by the following chemical formula 1, an unsaturated polyester (B) which is a condensate of an unsaturated dibasic acid and an alkylene oxide adduct of a bisphenol, and an epoxy compound (C). 【Chemical 1】 (where m+n is the number of moles of ethylene oxide added, and is 14 to 22.) (ii) When the sizing agent content in the sizing agent dispersion is 0.1% by mass, the sizing agent dispersion has an ultraviolet-visible absorbance of 0.05 or more at a wavelength of 500 nm.

2. 2. The sizing agent dispersion for carbon fiber bundles according to claim 1, wherein the bisphenol A-type ethylene oxide adduct (A) is contained in the sizing agent in an amount of 30 to 90% by mass.

3. 3. The sizing agent dispersion for carbon fiber bundles according to claim 1, wherein the sizing agent contains the bisphenol A-type ethylene oxide adduct (A) in an amount of 60 to 80% by mass.

4. 2. The sizing agent dispersion for carbon fiber bundles according to claim 1, containing 3 to 30 mass % of the unsaturated polyester (B).

5. 2. The sizing agent dispersion for carbon fiber bundles according to claim 1, comprising 10 to 30 mass % of the unsaturated polyester (B).

6. The sizing agent dispersion for carbon fiber according to claim 1, containing 3 to 30 mass% of the epoxy compound (C).

7. The sizing agent dispersion for carbon fiber according to claim 1, containing 3 to 15 mass% of the epoxy compound (C).

8. 2. The sizing agent dispersion for carbon fiber bundles according to claim 1, wherein the dispersion medium of the sizing agent dispersion is water.

9. 2. The sizing agent dispersion for carbon fiber bundles according to claim 1, comprising a solution containing a bisphenol A-type ethylene oxide adduct (A) and an emulsion containing an unsaturated polyester (B) and an epoxy compound (C).

10. A sizing-agent-containing carbon fiber bundle comprising the sizing agent according to claim 1.

11. The sizing agent-containing carbon fiber bundle according to claim 10, wherein the sizing agent content is 0.6 to 1.2 mass%.

12. A composite material comprising the sizing-agent-containing carbon fiber bundle according to claim 10 or 11 and a matrix resin.

13. A pressure vessel comprising the sizing-agent-containing carbon fiber bundle according to claim 10 or 11 and a matrix resin.

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