Sizing agent-containing carbon fiber bundle, carbon fiber-reinforced composite material, pressure container, and sizing agent-containing carbon fiber bundle for pressure container

Abstract

This pressure container is obtained by impregnating a sizing agent-containing carbon fiber bundle containing a sizing agent containing 50 mass% or more of a compound (A) not containing an epoxy group with a thermosetting resin, and curing the result. The sizing agent-containing carbon fiber bundle has a strand strength (strand strength B') evaluated on the basis of JIS R 7608 (2007) according to resin curing formulation B of 4.9 GPa or more. The ratio (strength utilization ratio) of strand strength (strand strength A') evaluated on the basis of JIS R 7608 (2007) according to resin curing formulation A to strand strength B' is 90% or more, and the amount of accumulated fluff is 0.80 mg or less. A pressure container that stably exhibits high burst strength is provided. The present invention also provides a sizing agent-containing carbon fiber bundle that suppresses accumulation of fluff, has good bundling properties, and is stable in quality.

Description

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

[0001] The present invention relates to a sizing-agent-containing carbon fiber bundle, a carbon fiber reinforced composite material, a pressure vessel, and a sizing-agent-containing carbon fiber bundle suitable for pressure vessel applications.

[0002] In recent years, the demand for pressure vessels has increased due to the growing global demand for energy and the ongoing search for sustainable energy sources. Traditionally, pressure vessels have been made of metal, but they are heavy and labor-intensive to move and transport. For automobiles that run on natural gas or hydrogen gas, their weight also impacts fuel economy. Furthermore, to extend the distance a vehicle can travel without refueling, pressure vessels that can hold more fuel are needed. To achieve this, the pressure resistance (burst strength) of pressure vessels must be improved. Against this background, pressure vessels made from lightweight, high-strength carbon fiber reinforced composites (CFPCs), which combine carbon fiber bundles with a matrix resin, have recently attracted attention. Carbon fiber bundles have excellent tensile strength, and generally, the higher the tensile strength of the carbon fiber bundles used, the higher the burst strength of the pressure vessel. For this reason, by applying carbon fiber bundles with higher tensile strength, even among carbon fiber bundles, higher burst strength can be expected, and if a required burst strength value is required for the pressure vessel, the burst strength can be satisfied with a smaller amount of carbon fiber bundle, which can also lead to a weight reduction of the pressure vessel.

[0003] As such, it is important to increase the burst strength of pressure vessels. However, it is known that the burst strength of pressure vessels is affected not only by the tensile strength of the carbon fiber bundles, but also by the sizing agent and matrix resin applied to the carbon fiber bundles.

[0004] Patent Document 1 describes that the burst strength of a pressure vessel can be increased by applying a sizing agent component that controls the interface between the carbon fiber bundles and the matrix resin. In other words, it is important not only to simply apply a carbon fiber bundle having a high tensile strength, but also to design the sizing agent.

[0005] The burst strength of a pressure vessel can also be increased by controlling the matrix resin. Patent Document 2 discloses a technique for fully utilizing the strain utilization rate of a pressure vessel, i.e., the tensile strength of the carbon fiber bundle, by controlling the structure of the matrix resin.

[0006] Furthermore, because carbon fiber bundles are lightweight yet have excellent strength and elastic modulus, they are used as composite materials in combination with various matrix resins in a wide range of fields, including aerospace and space applications, sports applications, and general industrial applications such as automobiles, ships, civil engineering and construction, pressure vessels, and wind turbines.

[0007] During the production of prepreg, which is a type of composite material using carbon fiber bundles, or during a filament winding process, which is a molding method, the carbon fiber bundle passes through a plurality of guide members such as rollers and fixed bars (hereinafter referred to as fixed bars). In particular, when passing through the fixed bars, the carbon fiber bundles are rubbed by contact with the fixed bars, causing fluffing in the carbon fiber bundles (hereinafter referred to as abrasion fluff), which ultimately leads to a deterioration in the quality of molded products, and therefore, a carbon fiber bundle having abrasion resistance is generally desired.

[0008] Furthermore, the abrasion characteristics of the carbon fiber bundle are also an important factor in determining the burst strength of a pressure vessel. In recent years, the application of the filament winding molding method has been increasing as a molding method for pressure vessels in order to improve productivity. This molding method, in particular, involves a process in which the carbon fiber bundle passes through multiple guide members such as fixed bars and rollers. Therefore, it is important that the carbon fiber bundle does not abrade as it passes through the fixed bars, and that the carbon fiber bundle has a bundling property to prevent it from winding around the rollers.

[0009] In order to improve the abrasion resistance of carbon fiber bundles, it is generally considered to impart abrasion resistance by devising sizing agents that are applied mainly from the viewpoint of improving the bundling ability of the carbon fiber bundles and controlling the mechanical properties of the molded products.

[0010] Patent Document 3 discloses a technology in which a higher fatty acid ester is added to a sizing agent mainly composed of an epoxy resin to reduce the coefficient of friction with a fixed bar and suppress abrasion fuzz. Patent Document 4 describes that a polyurethane-based sizing agent is applied, and abrasion fuzz is reduced as the polyurethane content increases. Patent Document 5 describes that an alkylene oxide adduct of bisphenol A is used as a sizing agent to reduce the coefficient of friction with a fixed bar and reduce abrasion fuzz.

[0011] JP 2022-58846 A JP 2017-120127 A JP 2002-317382 A JP 10-266076 A JP 07-009444 A

[0012] As described above, in order to control the burst strength of a pressure vessel, not only the tensile strength of the carbon fiber bundle but also other factors, particularly the design of the sizing agent, are important.

[0013] With regard to the matrix resin described in Patent Document 2, although the strain utilization rate can be increased by controlling the structure of the matrix resin, there is still room for further improvement in this effect, and the inventors understand that it is important to consider the combination with the carbon fiber bundle to be applied.

[0014] Regarding sizing agents, although sizing agents that suppress abrasion fuzz have been proposed, it has been found that the fuzz (carbon fibers cut and separated from the carbon fiber bundle, hereinafter referred to as accumulated fuzz) that adheres to and accumulates on another new fixed bar or roller after passing through the fixed bar cannot necessarily be suppressed even if known abrasion fuzz suppression techniques are applied.

[0015] The amount of accumulated fluff gradually increases depending on the running time of the carbon fiber bundle, and when the amount of accumulated fluff exceeds a certain level, it is pulled into the carbon fiber bundle and becomes mixed in as a large fluff mass during the process, which may cause a decrease in the burst strength of the pressure vessel. Furthermore, if there is a lot of accumulated fluff, it becomes necessary to increase the frequency of cleaning when molding the pressure vessel, which also leads to a decrease in productivity.

[0016] According to the studies of the present inventors, it has been found that the reason why it has been difficult to reduce accumulated fuzz in conventional techniques is that it is difficult to correctly evaluate the likelihood of this accumulated fuzz occurring using known abrasion fuzz evaluation methods. In the abrasion fuzz evaluation methods described in Patent Documents 3 and 5, a carbon fiber bundle abraded by a fixed bar is sandwiched between sponges, and the trapped carbon fiber fuzz is recovered and weighed to evaluate the amount of abrasion fuzz. With this method, fuzz other than accumulated fuzz, i.e., fuzz other than carbon fibers cut and separated from the carbon fiber bundle (carbon fibers cut on only one side), is recovered, making it difficult to evaluate only accumulated fuzz. As a result, it is difficult to say that abrasion fuzz can be correctly evaluated and reflected in the optimization of the sizing agent.

[0017] In Patent Document 4, fluff protruding from a carbon fiber bundle rubbed with a fixed bar is detected by a laser, but this method also detects fluff other than accumulated fluff, and is therefore not suitable for evaluating accumulated fluff. In other words, no evaluation system has been established that focuses on accumulated fluff and confirms its suppression effect, and as a result, no sizing agent that sufficiently suppresses accumulated fluff has been proposed up to now.

[0018] Furthermore, it is desirable for a sizing agent to not only suppress the accumulation of fuzz but also satisfy other required properties. For example, it is also required to suppress winding of carbon fibers around rollers and to provide bundling properties to improve the handleability of carbon fibers. If the carbon fibers have poor bundling properties, part of the carbon fiber bundle may wind around the roller, which may cause a decrease in burst strength or may require more frequent cleaning of the roller, resulting in a decrease in productivity. The inventions described in Patent Documents 4 and 5 certainly have an effect of suppressing abrasion fuzz, but the bundling properties are not sufficient.

[0019] Furthermore, when carbon fiber bundles are stored as inventory, one of the important required properties is that the quality does not change over time and the burst strength of the pressure vessel is stably exhibited. The present inventors have recognized that although the sizing agent used in Patent Document 1 is somewhat effective in improving the burst strength, the sizing agent mainly contains a compound having an epoxy group, and therefore, there is a problem that the quality stability is poor due to the progress of hydrolysis of the epoxy group, and the burst strength of the pressure vessel is not stably exhibited. The present inventors have recognized that the sizing agent used in Patent Document 3 also has a recognized effect of suppressing abrasion fuzz, but since an epoxy-based sizing agent is used, there is also a problem that the quality stability is poor and the burst strength of the pressure vessel is not stably exhibited.

[0020] As described above, in the prior art, technological development for stably increasing the burst strength of pressure vessels has not progressed sufficiently. Furthermore, in the case of carbon fiber bundles containing a sizing agent, a method for analyzing all factors that affect the burst strength of pressure vessels, such as the tensile strength, bundle strength, abrasion fuzz suppression, and quality stability of the carbon fiber bundle, has not been established. As a result, pressure vessels that stably exhibit high burst strength have not been obtained. Furthermore, a carbon fiber bundle containing a sizing agent that is optimal for improving the burst strength of pressure vessels, suppressing accumulated fuzz and combining bundle strength and quality stability, has not been obtained.

[0021] The present invention for solving the above-mentioned problems is as follows: (1) A pressure vessel obtained by impregnating a sizing-agent-containing carbon fiber bundle containing a sizing agent containing 50 mass % or more of the following compound (A) that does not contain an epoxy group with a thermosetting resin and curing the impregnated carbon fiber bundle, wherein the sizing-agent-containing carbon fiber bundle has a strand strength (strand strength B') of 4.9 GPa or more as evaluated in accordance with JIS R7608 (2007) according to the following resin curing prescription B, a strength utilization factor which is the ratio of the strand strength (strand strength A') to the strand strength B' as evaluated in accordance with JIS R7608 (2007) according to the following resin curing prescription A, and an amount of accumulated fluff of 0.80 mg or less: Compound (A): a compound having an SP value of 4.0 to 9.5 as calculated by the Fedors method and a viscosity at 30°C of 1,400 mPa s or more; Resin curing formulation A: A resin containing 80% by mass of 2,2'-[(1-methylethylidene)bis(4,1-phenyleneoxymethylene)]bisoxirane and 20% by mass of 1,4-bis(2,3-epoxypropoxy)butane was used as the base resin, and 2,2'-dimethyl-4,4'-methylenebis(cyclohexylamine) was used as the curing agent, with the ratio of the base resin to the curing agent being 100 / 35 (parts by mass). The curing conditions were normal pressure, curing in an oven set at a temperature of 80°C for 120 minutes, and then curing in an oven set at a temperature of 110°C for 240 minutes; Resin curing formulation B: An epoxy resin containing 97% by mass or more of (3',4'-epoxycyclohexane)methyl-3,4-epoxycyclohexylcarboxylate and having an epoxy equivalent of 130 g / eq and a viscosity at 25°C of 240 mPa s, containing boron trifluoride monoethylamine and acetone in a ratio of 100 / 3 / 4 (parts by mass), was used, and the curing conditions were set at atmospheric pressure and 125°C in an oven for 30 minutes.(2) A pressure vessel obtained by impregnating a sizing-agent-containing carbon fiber bundle containing a sizing agent containing 50 mass % or more of the following compound (A) that does not contain an epoxy group with a thermosetting resin and curing the impregnated carbon fiber bundle, wherein the sizing-agent-containing carbon fiber bundle has a strand strength (strand strength B') of 4.9 GPa or more as evaluated in accordance with JIS R7608 (2007) according to the following resin curing prescription B, a ratio (strand strength A') to strand strength B' as evaluated in accordance with JIS R7608 (2007) according to the following resin curing prescription A (strand strength A') of 90% or more, and an amount of accumulated fluff of 0.80 mg or less. Compound (A): A compound having an SP value of 4.0 to 9.5 as calculated by the Fedors method and a viscosity at 30°C of 1,400 mPa s or more. Resin curing formulation A: "Araldite" (registered trademark) LY1564 SP CI / "Baxodur" (registered trademark) EC331 = 100 / 35 (parts by mass) was used, and the curing conditions were normal pressure and curing in an oven set at a temperature of 80°C for 120 minutes, followed by curing in an oven set at a temperature of 110°C for 240 minutes. Resin curing formulation B: "Celloxide (registered trademark)" 2021P / boron trifluoride monoethylamine / acetone = 100 / 3 / 4 (parts by mass) was used, and the curing conditions were normal pressure and curing in an oven set at a temperature of 125°C for 30 minutes. (3) The pressure vessel according to (1) or (2), wherein the sizing agent contains a compound (B) having a viscosity at 30°C of 0.1 mPa·s or more and 200 mPa·s or less. (4) The pressure vessel according to any one of (1) to (3), wherein the product of X and Y is 0.00 meq. / g or more and 0.20 meq. / g or less, when 30 g of the sizing-agent-containing carbon fiber bundle is immersed in 150 ml of N,N-dimethylformamide at a temperature of 25°C and subjected to ultrasonic treatment three times for 30 minutes at a frequency of 40 kHz, and the epoxy value of the sizing agent eluted is X meq. / g and the content of the sizing agent applied to the carbon fiber bundle is Y mass %. (5) The pressure vessel according to any one of (1) to (4), wherein the sizing-agent-containing carbon fiber bundle is obtained by subjecting the carbon fiber bundle to electrolytic surface treatment at 2 to 20 C / g and then applying a sizing agent.(6) A sizing-agent-containing carbon fiber bundle for pressure vessels, which contains a sizing agent containing 50% by mass or more of the following compound (A) that does not contain an epoxy group, wherein the strand strength (strand strength B') evaluated in accordance with JIS R7608 (2007) according to the following resin curing prescription B is 4.9 GPa or more, the ratio (strand strength A') evaluated in accordance with JIS R7608 (2007) according to the following resin curing prescription A to strand strength B' (strand strength utilization factor) is 90% or more, and the amount of accumulated fluff is 0.80 mg or less. Compound (A): A compound having an SP value calculated by the Fedors method of 4.0 to 9.5, and a viscosity at 30°C of 1,400 mPa s or more. Resin curing formulation A: A resin containing 80% by mass of 2,2'-[(1-methylethylidene)bis(4,1-phenyleneoxymethylene)]bisoxirane and 20% by mass of 1,4-bis(2,3-epoxypropoxy)butane was used as the base resin, and 2,2'-dimethyl-4,4'-methylenebis(cyclohexylamine) was used as the curing agent, with the ratio of the base resin to the curing agent being 100 / 35 (parts by mass). The curing conditions were normal pressure and curing in an oven set at a temperature of 80°C for 120 minutes, followed by curing for 240 minutes in an oven set at a temperature of 110°C. Resin curing formulation B: An epoxy resin containing 97% by mass or more of (3',4'-epoxycyclohexane)methyl-3,4-epoxycyclohexylcarboxylate and having an epoxy equivalent of 130 g / eq and a viscosity at 25°C of 240 mPa s, containing boron trifluoride monoethylamine and acetone in a ratio of 100 / 3 / 4 (parts by mass), was used, and the curing conditions were set at atmospheric pressure and 125°C in an oven for 30 minutes. (7) A sizing-agent-containing carbon fiber bundle for pressure vessels, which contains a sizing agent containing 50% by mass or more of the following compound (A) that does not contain an epoxy group, wherein the strand strength (strand strength B') evaluated in accordance with JIS R7608 (2007) according to the following resin curing prescription B is 4.9 GPa or more, the ratio (strand strength A') of the strand strength evaluated in accordance with JIS R7608 (2007) according to the following resin curing prescription A to the strand strength B' (strand strength utilization factor) is 90% or more, and the amount of accumulated fluff is 0.80 mg or less.Compound (A): A compound having an SP value calculated by the Fedors method of 4.0 or more and 9.5 or less, and a viscosity at 30°C of 1,400 mPa·s or more. Resin curing formulation A: "Araldite" (registered trademark) LY1564 SP CI / "Baxodur" (registered trademark) EC331 = 100 / 35 (parts by mass) is used, and the curing conditions are normal pressure and curing in an oven set at a temperature of 80°C for 120 minutes, followed by curing in an oven set at a temperature of 110°C for 240 minutes. Resin curing formulation B: "Celloxide (registered trademark)" 2021P / boron trifluoride monoethylamine / acetone = 100 / 3 / 4 (parts by mass) is used, and the curing conditions are normal pressure and curing in an oven set at a temperature of 125°C for 30 minutes. (8) The sizing-agent-containing carbon fiber bundle according to (6) or (7), wherein the sizing agent contains a compound (B) having a viscosity of 0.1 mPa·s or more and 200 mPa·s or less at 30° C. (9) The sizing-agent-containing carbon fiber bundle for pressure vessels according to any one of (6) to (8), wherein 30 g of the sizing-agent-containing carbon fiber bundle is immersed in 150 ml of N,N-dimethylformamide at a temperature of 25° C. and subjected to ultrasonic treatment three times for 30 minutes at a frequency of 40 kHz, and the product of X and Y is 0.00 meq· / g or more and 0.20 meq· / g or less, where X meq· / g is the epoxy value of the sizing agent eluted, and Y mass % is the content of the sizing agent applied to the carbon fiber bundle. (10) A sizing-agent-containing carbon fiber bundle for pressure vessels according to any one of (6) to (9), which is obtained by applying a sizing agent to a carbon fiber bundle after electrolytic surface treatment at 2 to 20 C / g. (11) A sizing-agent-containing carbon fiber bundle having a strand strength (strand strength B') of 4.0 GPa or more as evaluated in accordance with JIS R7608 (2007) in accordance with the following resin curing prescription B, a sizing agent content of 0.4% by mass or more and 2.0% by mass or less relative to the total mass of the carbon fiber bundle, the sizing agent containing 50% by mass or more of molecule A having a structural unit derived from ethylene oxide (EO) and a structural unit derived from propylene oxide (PO) relative to the total mass of the sizing agent, and an epoxy value of the sizing agent eluted from the sizing-agent-containing carbon fiber bundle of 1.0 meq / g or less.Molecule A: a molecule other than polyoxyethylene bisphenol A ether that is in a liquid state at 1 atmosphere and 25°C, and in which the combined mass of EO-derived structural units and PO-derived structural units is 65 mass% or more of the total mass of the molecule. Resin curing formulation B: An epoxy resin containing 97 mass% or more of (3',4'-epoxycyclohexane)methyl-3,4-epoxycyclohexylcarboxylate and having an epoxy equivalent of 130 g / eq and a viscosity of 240 mPa·s at 25°C is used in a ratio of boron trifluoride monoethylamine / acetone = 100 / 3 / 4 (parts by mass), and cured for 30 minutes in an oven set at normal pressure and a temperature of 125°C. (12) A sizing-agent-containing carbon fiber bundle according to (11), in which the number-average molecular weight of the molecule A eluted from the sizing-agent-containing carbon fiber bundle is 3,000 or more. (13) The sizing-agent-containing carbon fiber bundle according to (11) or (12), wherein the content of PO-derived structural units relative to the EO-derived structural units in the molecule A is 10% by mass or more and 100% by mass or less. (14) The sizing-agent-containing carbon fiber bundle according to any one of (11) to (13), wherein the number of filaments in the sizing-agent-containing carbon fiber bundle is 20,000 or more. (15) The sizing-agent-containing carbon fiber bundle according to any one of (11) to (14), wherein the sizing agent is water-soluble. (16) A carbon fiber reinforced composite material using the sizing-agent-containing carbon fiber bundle according to any one of (11) to (15). (17) A pressure vessel using the sizing-agent-containing carbon fiber bundle according to any one of (11) to (15). (18) A sizing-agent-containing carbon fiber bundle, in which the sizing agent content is 0.1 to 3.0% by mass relative to 100% by mass of the carbon fiber bundle containing the sizing agent, the sizing agent contains a compound (A) that does not contain an epoxy group, the compound (A) is contained in an amount of 50% by mass or more relative to 100% by mass of the total amount of the sizing agent, and the compound (A) satisfies the following conditions: Compound (A): The SP value calculated by the Fedors method is 4.0 to 9.5, and the viscosity at 30°C is 1,400 mPa·s or more. (19) The carbon fiber bundle according to (18), in which the viscosity at 30°C of the compound (A) is 2,000 mPa·s or more. (20) The sizing-agent-containing carbon fiber bundle according to (18) or (19), in which the content of the compound containing an epoxy group is less than 9% by mass relative to 100% by mass of the total amount of the sizing agent.(21) The sizing-agent-containing carbon fiber bundle according to any one of (18) to (20), wherein the compound (A) is water-soluble. (22) The sizing-agent-containing carbon fiber bundle according to (21), wherein the compound (A) is completely miscible with water. (23) The sizing-agent-containing carbon fiber bundle according to any one of (18) to (22), wherein the sizing agent contains a compound (B) having a viscosity at 30°C of 0.1 mPa·s to 200 mPa·s. (24) The sizing-agent-containing carbon fiber bundle according to (23), wherein the compound (B) has an SP value of 4.0 to 11.0, as calculated by the Fedors method. (25) The sizing-agent-containing carbon fiber bundle according to (23) or (24), wherein the compound (B) has an SP value of 4.0 to 9.5, as calculated by the Fedors method. (26) A sizing-agent-containing carbon fiber bundle according to any one of (23) to (25), wherein the compound (B) is contained in a total amount of 5 mass% or more relative to 100 mass% of the total amount of the sizing agent. (27) A sizing-agent-containing carbon fiber bundle according to any one of (23) to (26), wherein the compound (A) and the compound (B) have a mixed viscosity of 20,000 mPa s or more. (28) A carbon fiber reinforced composite material using the sizing-agent-containing carbon fiber bundle according to any one of (18) to (27). (29) A pressure vessel using the sizing-agent-containing carbon fiber bundle according to any one of (18) to (27).

[0022] According to the present invention, a pressure vessel that stably exhibits high burst strength can be obtained. Alternatively, according to the present invention, a sizing-agent-containing carbon fiber bundle that suppresses accumulation of fluff and has both good bundling ability and stable quality can be obtained.

[0023] 1 is a schematic diagram of an apparatus for carrying out a pressure test on a pressure vessel.

[0024] Hereinafter, embodiments for carrying out the present invention will be described. A sizing-agent-containing carbon fiber bundle, a carbon fiber reinforced composite material, a pressure vessel, and a sizing-agent-containing carbon fiber bundle for a pressure vessel according to the present invention will be described.

[0025] A pressure vessel in a first aspect of the present invention is a pressure vessel obtained by impregnating a sizing-agent-containing carbon fiber bundle containing a sizing agent containing 50 mass % or more of the following compound (A) which does not contain an epoxy group with a thermosetting resin and curing the impregnated carbon fiber bundle, wherein the sizing-agent-containing carbon fiber bundle has a strand strength (strand strength B') of 4.9 GPa or more as evaluated in accordance with JIS R7608 (2007) according to the following resin curing prescription B, a strength utilization factor which is the ratio of the strand strength (strand strength A') to the strand strength B' as evaluated in accordance with JIS R7608 (2007) according to the following resin curing prescription A, and an amount of accumulated fluff of 0.80 mg or less. Compound (A): A compound having an SP value of 4.0 to 9.5 as calculated by the Fedors method and a viscosity at 30°C of 1,400 mPa s or more. Resin curing formulation A: A base resin containing 80% by mass of 2,2'-[(1-methylethylidene)bis(4,1-phenyleneoxymethylene)]bisoxirane and 20% by mass of 1,4-bis(2,3-epoxypropoxy)butane was used, and 2,2'-dimethyl-4,4'-methylenebis(cyclohexylamine) was used as a curing agent, with the ratio of the base resin to the curing agent being 100 / 35 (parts by mass), i.e., a mass ratio of 100:35. The curing conditions were normal pressure, curing for 120 minutes in an oven set at a temperature of 80°C, and then curing for 240 minutes in an oven set at a temperature of 110°C. Resin curing formulation B: An epoxy resin containing 97% by mass or more of (3',4'-epoxycyclohexane)methyl-3,4-epoxycyclohexylcarboxylate and having an epoxy equivalent of 130 g / eq and a viscosity at 25°C of 240 mPa s, containing boron trifluoride monoethylamine and acetone in a ratio of 100 / 3 / 4 (parts by mass), was used, and the curing conditions were set at atmospheric pressure and 125°C in an oven for 30 minutes.

[0026] In addition, "Araldite (registered trademark)" LY1564 SP CI may be used as the "main component containing 80 mass % of 2,2'-[(1-methylethylidene)bis(4,1-phenyleneoxymethylene)]bisoxirane and 20 mass % of 1,4-bis(2,3-epoxypropoxy)butane" in Resin Formulation A. Furthermore, "Baxodur (registered trademark)" EC331 may be used as the "2,2'-dimethyl-4,4'-methylenebis(cyclohexylamine)".

[0027] Furthermore, "Celloxide (registered trademark)" 2021P may be used as the "epoxy resin containing 97% by mass or more of (3',4'-epoxycyclohexane)methyl-3,4-epoxycyclohexylcarboxylate and having an epoxy equivalent of 130 g / eq and a viscosity at 25°C of 240 mPa s" in Resin Formulation B.

[0028] The resin composition in Resin Curing Formula A is a practical resin for filament winding molding, and has a viscosity suitable for preparing strand test pieces by the impregnation method described in JIS R7608 (2007). On the other hand, Resin Curing Formula A has more functional groups than the resin composition described in JIS R7608 (2007), and has high adhesiveness to sizing-agent-containing carbon fiber bundles. Therefore, the inventors have found that the strand strength (Strand Strength A') measured according to Resin Curing Formula A can be used as an index that represents, in a model manner, the burst strength of a pressure vessel when the sizing-agent-containing carbon fiber bundle to be measured is used as the pressure vessel.

[0029] Resin curing formulation B is a resin impregnated strand tensile test described in JIS R7608 (2007) that contains 97% by mass or more of (3',4'-epoxycyclohexane)methyl-3,4-epoxycyclohexylcarboxylate, and uses an epoxy resin with an epoxy equivalent of 130 g / eq and a viscosity at 25°C of 240 mPa s in a ratio of boron trifluoride monoethylamine / acetone = 100 / 3 / 4 (parts by mass), and is cured for 30 minutes in an oven set at normal pressure and a temperature of 125°C.

[0030] The strand strength B' evaluated in accordance with JIS R7608 (2007) in accordance with resin curing prescription B is an index showing the resistance to breakage when a load is applied to the sizing-agent-containing carbon fiber bundle, and is used as an index showing the strength of the sizing-agent-containing carbon fiber bundle itself. The higher the strand strength B', the higher the strength of the sizing-agent-containing carbon fiber bundle itself, and therefore the greater the potential for increasing the burst strength of the resulting pressure vessel. In the present invention, it is necessary that the strand strength B' be 4.9 GPa or more.

[0031] However, the strand strength B' that has been used conventionally may deviate from the burst strength of the pressure vessel, and therefore there is a limit to how much the burst strength of the pressure vessel can be increased by designing only the strand strength B' as an index.

[0032] In a first aspect of the present invention, the sizing-agent-containing carbon fiber bundle used for the pressure vessel has a ratio (hereinafter also referred to as strength utilization factor) of strand strength A' evaluated in accordance with resin curing formula A based on JIS R7608 (2007) to strand strength B' in resin curing formula B, of 90% or more, preferably 94% or more, and more preferably 97% or more. A strength utilization factor of 90% or more can sufficiently increase the burst strength of the pressure vessel. A strength utilization factor of 90% or more can be achieved by controlling the interface between the sizing-agent-containing carbon fiber bundle and the matrix resin.

[0033] The interface between the sizing-agent-containing carbon fiber bundle and the matrix resin can be controlled by the sizing agent or surface treatment of the carbon fiber bundle. The strength utilization rate is affected by the amount of epoxy groups contained in the sizing agent. When the sizing agent contains a compound containing epoxy groups, the strength utilization rate tends to decrease, possibly due to reaction with the surface functional groups of the carbon fiber bundle and the matrix resin. The amount of epoxy groups contained in the sizing agent is expressed by controlling the product of the epoxy value (X) of the sizing agent and the sizing agent content (Y) in the sizing-agent-containing carbon fiber bundle. 30 g of sizing-agent-containing carbon fiber bundle was immersed in 150 ml of N,N-dimethylformamide at 25°C and subjected to ultrasonic treatment three times for 30 minutes at a frequency of 40 kHz to measure the epoxy value of the eluted sizing agent. When the obtained epoxy value is X meq. / g and the content of sizing agent applied to the carbon fiber bundle is Y mass%, the product of X and Y is preferably 0.00 meq. / g or more and 0.20 meq. / g or less, as this tends to increase the strength utilization rate.

[0034] The epoxy value is an index representing the amount of epoxy groups contained per gram of sizing agent, and the larger the epoxy value, the more epoxy groups are contained per gram of sizing agent. The sizing agent content is expressed as a percentage (mass%) of the total mass of the sizing agent contained in the carbon fiber bundle when the mass of the carbon fiber bundle containing the sizing agent is taken as 100 mass%. The measurement method will be described later.

[0035] The interface between the sizing-agent-containing carbon fiber bundle and the matrix resin can also be controlled by surface treatment of the carbon fiber bundle. By subjecting the carbon fiber bundle to oxidation treatment, oxygen-containing functional groups are introduced onto the surface, thereby controlling the strength utilization rate. Oxidation treatment methods include gas-phase oxidation, liquid-phase oxidation, and liquid-phase electrolytic oxidation. Liquid-phase electrolytic oxidation is preferred from the viewpoints of high productivity and uniform treatment. Examples of electrolytes used in liquid-phase electrolytic oxidation include acidic and alkaline electrolytes. Examples of acidic electrolytes include inorganic acids such as sulfuric acid, nitric acid, hydrochloric acid, phosphoric acid, boric acid, and carbonic acid; organic acids such as acetic acid, butyric acid, oxalic acid, acrylic acid, and maleic acid; and salts such as ammonium sulfate and ammonium hydrogen sulfate.

[0036] 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.

[0037] The surface treatment amount of the carbon fiber bundles, expressed as the quantity of electricity per gram of carbon fiber bundles, is preferably 2 to 20 C / g, and more preferably 2 to 10 C / g in terms of increasing the strength utilization factor, since too low a quantity of electricity, coulomb (C), may occur at the interface between the carbon fiber bundles and the matrix resin, leading to breakage of the entire carbon fiber reinforced composite material, whereas too high a quantity of electricity, coulomb (C), may lead to breakage of the entire carbon fiber reinforced composite material.

[0038] Furthermore, the sizing-agent-containing carbon fiber bundle used in the pressure vessel of the present invention has a deposited fluff amount of 0.80 mg or less. The deposited fluff amount is measured according to the <Absolute evaluation method for deposited fluff amount> described below. The inventors have found that by keeping the deposited fluff amount to 0.80 mg or less, it is possible to suppress a decrease in the burst strength of the pressure vessel caused by the deposited fluff generated during filament winding molding, and to achieve a burst strength that corresponds to a certain degree to the strength utilization rate. The smaller the deposited fluff amount, the higher the burst strength of the pressure vessel can be, so the deposited fluff amount is preferably 0.50 mg or less, more preferably 0.30 mg or less, and even more preferably 0.10 mg or less. The deposited fluff amount can be adjusted by controlling the sizing agent described below.

[0039] In the sizing-agent-containing carbon fiber bundle used in the pressure vessel of the present invention, the sizing agent contains 50% by mass or more of the following compound (A) which does not contain an epoxy group: Compound (A) has an SP value calculated by the Fedors method of 4.0 or more and 9.5 or less, and a viscosity at 30°C of 1,400 mPa s or more.

[0040] The SP value mentioned above is a commonly used solubility parameter that serves as an index of solubility and polarity, and is a value calculated by the method (Fedors method) described in R.F. Fedors, Polym. Eng. Sci., 14(2), 147-154 (1974). It is generally difficult to imagine that this SP value, which serves as an index of solubility, is related to the amount of accumulated fluff, but the present inventors have newly discovered that it is one of the major governing parameters that affect the amount of accumulated fluff. That is, the amount of accumulated fluff is suppressed by setting the SP value of compound (A) to 4.0 or more and 9.5 or less. The smaller the SP value, the more the amount of accumulated fluff is suppressed, so the SP value is set to 9.5 or less. However, compounds with an SP value of less than 4.0 are mainly gases or low-boiling point compounds and are not suitable as sizing agents. Considering the structure of suitable compounds described below, preferred examples of compound (A) include compounds containing alkylene oxide, and particularly compounds containing propylene oxide or ethylene oxide. Polymers or copolymers of propylene oxide or ethylene oxide are also possible, and more preferred examples include copolymers of propylene oxide and ethylene oxide. From the viewpoints of boiling point and water solubility, the SP value of compound (A) is preferably 8.5 to 9.3, more preferably 8.8 to 9.2.

[0041] The SP value of compound (A) can be calculated from the structural formula when the structural formula of compound (A) is known. When the structural formula is unknown, the SP value can be calculated by extracting the sizing agent from a carbon fiber bundle containing the sizing agent and identifying the structure by a known method such as proton NMR, carbon NMR, mass spectrometry, or TOF-SIMS.

[0042] Furthermore, compound (A) has a viscosity of 1,400 mPa·s or more at 30°C. The viscosity in the present invention is a value obtained by measurement using a Brookfield viscometer. The rotor and rotation speed of the Brookfield viscometer are No. 1 and 30 rpm for compounds having a viscosity at 30°C of less than 1,000 mPa·s, No. 3 and 30 rpm for compounds having a viscosity of 1,000 mPa·s or more but less than 10,000 mPa·s, and No. 4 and 6 rpm for compounds having a viscosity of 10,000 mPa·s or more. When compound (A) has a viscosity at 30°C of 1,400 mPa·s or more, the sizing-agent-containing carbon fiber bundle can have good collimation properties. Poor collimation properties can cause the sizing-agent-containing carbon fiber bundle to partially or entirely wind around rollers or the like during molding of the pressure vessel, making molding difficult and reducing the burst strength of the pressure vessel. Therefore, the viscosity of the compound (A) at 30° C. is preferably 2,000 mPa·s or more, more preferably 5,000 mPa·s or more, even more preferably 10,000 mPa·s or more, particularly preferably 30,000 mPa·s or more, and most preferably 100,000 mPa·s or more. On the other hand, the upper limit of the viscosity is preferably 10,000,000 mPa·s or less.

[0043] The inventors have found through their studies that when the viscosity of compound (A) is high, the amount of accumulated fluff tends to increase, but they have discovered that by controlling the SP value, it is possible to impart sizing properties while suppressing an increase in the amount of accumulated fluff. The viscosity of compound (A) can be measured using a B-type viscometer. When measuring the viscosity of a sizing-agent-containing carbon fiber bundle, the sizing agent can be extracted from the sizing-agent-containing carbon fiber bundle with a solvent or the like, and in some cases, compound (A) can be separated and then measured using a B-type viscometer.

[0044] The compound (A) is contained in an amount of 50% by mass or more relative to 100% by mass of the total amount of the sizing agent. If the content of the compound (A) is less than 50% by mass, the amount of accumulated fluff may increase. Furthermore, if the content of the compound (A) is less than 50% by mass, the bundling ability may be significantly reduced, possibly because a network structure such as interaction or entanglement between the compounds (A) cannot be formed. The content of the compound (A) is preferably 55 to 95% by mass, more preferably 60 to 90% by mass. Furthermore, when a plurality of compounds (A) are contained, the total content thereof is taken as the content of the compound (A). The content (% by mass) of the compound (A) contained in 100% by mass of the total amount of the sizing agent can be calculated by extracting the sizing agent from a sizing-agent-containing carbon fiber bundle and using a known method such as proton NMR, carbon NMR, mass spectrometry, TOF-SIMS, or thermogravimetry.

[0045] Compound (A) must not contain an epoxy group. According to the findings of the present inventors, although the detailed mechanism is unknown, compounds containing epoxy groups may cause hydrolysis, and a large amount of such compounds leads to a change in the quality of sizing-agent-containing carbon fiber bundles over time during storage. The change in quality of sizing-agent-containing carbon fiber bundles affects the quality stability of the burst strength of pressure vessels. Furthermore, compounds containing epoxy groups may actually reduce the burst strength of pressure vessels. For the same reason, the total amount of epoxy group-containing compounds in 100% by mass of the total amount of sizing agent is preferably 9% by mass or less. The total amount of epoxy group-containing compounds in 100% by mass of the total amount of sizing agent is more preferably 5% by mass or less, and even more preferably 3% by mass or less. On the other hand, the preferred lower limit of the total amount of epoxy group-containing compounds in 100% by mass of the total amount of sizing agent is 0% by mass or more.

[0046] The presence or absence of a compound containing an epoxy group and the proportion of the compound containing an epoxy group in 100% by mass of the total amount of the sizing agent can be calculated from the structural formula and composition of the compound when the structural formula of the compound and the composition of the sizing agent are known. When the structural formula of the compound and the composition of the sizing agent are unknown, the sizing agent can be extracted from a carbon fiber bundle containing the sizing agent, and the structure and composition can be identified and calculated by a known method such as proton NMR, carbon NMR, mass spectrometry, or TOF-SIMS.

[0047] As described above, by using a specific sizing agent containing a carbon fiber bundle having a strength utilization rate of 90% or more and an accumulated fluff amount of 0.8 mg or less, a carbon fiber bundle containing a sizing agent having excellent bundling properties and stable quality can be obtained, thereby making it possible to obtain a pressure vessel that stably exhibits high burst strength.

[0048] The sizing agent constituting the present invention preferably contains, in addition to compound (A), compound (B) having a viscosity of 0.1 mPa·s or more and 200 mPa·s or less at 30° C. The present inventors have found that by deliberately combining compound (A) having a high viscosity of 1,400 mPa·s or more with compound (B) having a low viscosity of 200 mPa·s or less, it is possible to further suppress the amount of accumulated fluff while maintaining sizing properties.

[0049] That is, by applying a sizing agent containing, in addition to compound (A), compound (B) having a viscosity at 30°C of 0.1 mPa·s or more and 200 mPa·s or less, the amount of accumulated fluff can be further suppressed. 0.1 mPa·s is the minimum viscosity of a compound that can be used as a sizing agent. If the viscosity exceeds 200 mPa·s, the effect of suppressing the amount of accumulated fluff may be insufficient. The viscosity of compound (B) at 30°C is preferably 0.1 mPa·s or more and 150 mPa·s or less, more preferably 0.1 mPa·s or more and 100 mPa·s or less, and even more preferably 0.1 mPa·s or more and 50 mPa·s or less. The viscosity of compound (B) can be measured in the same manner as compound (A).

[0050] The compound (B) preferably has an SP value calculated by the Fedors method of 4.0 or more and 11.0 or less. Taking into consideration water solubility, water dispersibility, and boiling point, the SP value of the compound (B) calculated by the Fedors method is preferably 8.0 to 9.5, more preferably 8.3 to 9.1, and even more preferably 8.3 to 8.7.

[0051] From the viewpoint of suppressing the amount of accumulated fluff, the content of compound (B) is preferably 5% by mass or more relative to 100% by mass of the total amount of the sizing agent. It is more preferably 10 to 50% by mass, even more preferably 20 to 50% by mass, and particularly preferably 30 to 50% by mass. The mass % of compound (B) relative to 100% by mass of the total amount of the sizing agent can be calculated by extracting the sizing agent from a carbon fiber bundle containing the sizing agent, as with compound (A), and using a known method such as proton NMR, carbon NMR, mass spectrometry, TOF-SIMS, or thermogravimetry.

[0052] The compound (B) is preferably mixed with the compound (A) so that the mixed viscosity (η) with the compound (A) is 20,000 mPa·s or more. ａ ) and the viscosity (η ｂ ) is calculated by the mass ratio (R ａ ) and the mass ratio of compound (B) (R ｂ Taking into consideration the case where a plurality of compounds (A) and (B) are present, the mixed viscosity is calculated by the following formula:

[0053]

[0054]

[0055]

[0056]

[0057] Here, m ｔｏｔａｌ is the total mass calculated as the sum of the mass of compound (A) and the mass of compound (B), m ａ and m ｂare the masses of compounds (A) and (B), respectively. The variable i in the formula represents a component of compound (A) and is an integer of 1 or greater. When i is 2 or greater, this means that the epoxy resin composition contains multiple compounds (A). Similarly, the variable j represents a component of compound (B) and is an integer of 1 or greater. m ａ１ , m ａ２ , m ａ３ ... and η ａ１ , η ａ２ , η ａ３ ... corresponds to the mass and viscosity of each component of the compound (A). ｂ１ , m ｂ２ , m ｂ３ ... and η ｂ１ , η ｂ２ , η ｂ３ ... corresponds to the mass and viscosity of each component of compound (B).

[0058] By making the mixed viscosity of compound (A) and compound (B) 20,000 mPa·s or more, high sizing ability can be maintained, so a viscosity of 30,000 mPa·s or more is preferred, and 35,000 mPa·s or more is more preferred, and a viscosity of 10,000,000 mPa·s or less is usually used.

[0059] The content of the sizing agent in the sizing-agent-containing carbon fiber bundle constituting the present invention is preferably in the range of 0.1 to 3.0% by mass based on 100% by mass of the sizing-agent-containing carbon fiber bundle. The content of the sizing agent in the present invention is determined by measuring the change in mass before and after heat treatment when 2.0±0.5 g of the sizing-agent-containing carbon fiber bundle is sampled and subjected to heat treatment at 450°C in a nitrogen atmosphere for 15 minutes, and dividing the amount of change in mass by the mass before heat treatment (mass %).

[0060] When the sizing agent content is 0.1% by mass or more, the amount of accumulated fluff is suppressed and the bundleability is improved. When the sizing agent content is 3.0% by mass or less, the resin impregnation during advanced processing is excellent and the strand strength is stable. Therefore, the sizing agent content is preferably 0.1 to 2.0% by mass, more preferably 0.2 to 1.0% by mass.

[0061] Next, a method for applying the sizing agent to the carbon fiber bundle constituting the present invention will be described. In the present invention, the sizing agent is preferably used as a sizing-agent-containing liquid in which the sizing agent components are diluted with a solvent. Examples of the solvent include water, methanol, ethanol, isopropyl alcohol, acetone, methyl ethyl ketone, dimethylformamide, and dimethylacetamide. Among these, an aqueous solution or an aqueous dispersion emulsified with a surfactant is preferably used because of its ease of handling and its safety advantages.

[0062] Examples of means for applying the sizing agent to the carbon fiber bundle include a method of immersing the carbon fiber bundle in the sizing-agent-containing liquid via a roller, a method of bringing the carbon fiber bundle into contact with a roller to which the sizing-agent-containing liquid has been attached, and a method of spraying the sizing-agent-containing liquid in the form of a mist onto the carbon fiber bundle. In producing the sizing-agent-containing carbon fiber bundle used in the present invention, the method of immersing the carbon fiber bundle in the sizing-agent solution via a roller is preferably used.

[0063] The means for applying the sizing agent may be either a batch method or a continuous method, but a continuous method is preferably used because it has good productivity and small variation. In this case, it is preferable to control the concentration of the sizing agent-containing solution, temperature, yarn tension, etc. so that the content of the active ingredient of the sizing agent relative to the carbon fiber bundle is uniform within an appropriate range. In addition, it is also a preferred embodiment to vibrate the carbon fiber bundle with ultrasonic waves when applying the sizing agent.

[0064] After the application of the sizing agent, the carbon fiber bundle may be passed through heated rollers as a preliminary drying step, and then further heat-treated as a second 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.

[0065] The carbon fiber bundle used in the present invention is not particularly limited, but from the viewpoint of mechanical properties, polyacrylonitrile-based carbon fiber bundles are preferably used. The polyacrylonitrile-based carbon fiber bundle used in the present invention can be obtained by subjecting a carbon fiber precursor fiber bundle 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.

[0066] In the present invention, there is no particular limitation on the number of filaments in the sizing-agent-containing carbon fiber bundle, but it is preferably 1,000 to 100,000.

[0067] The sizing agent-containing carbon fiber bundle according to the second embodiment of the present invention will be described.

[0068] The sizing-agent-containing carbon fiber bundle of the present invention has a strand strength (strand strength B') of 4.0 GPa or more, evaluated in accordance with JIS R7608 (2007) in accordance with the following resin curing prescription B, a sizing agent content of 0.4% by mass or more and 2.0% by mass or less relative to the total mass of the carbon fiber bundle, the sizing agent contains 50% by mass or more of molecule A having a structural unit derived from ethylene oxide (EO) and a structural unit derived from propylene oxide (PO) relative to the total mass of the sizing agent, and the epoxy value of the sizing agent eluted from the sizing-agent-containing carbon fiber bundle is 1.0 meq / g or less. Molecule A: a molecule other than polyoxyethylene bisphenol A ether that is in a liquid state at 25°C under 1 atmosphere and in which the total mass of the EO-derived structural unit and the PO-derived structural unit is 65% by mass or more of the total mass of the molecule. Resin curing formulation B: An epoxy resin containing 97% by mass or more of (3',4'-epoxycyclohexane)methyl-3,4-epoxycyclohexylcarboxylate and having an epoxy equivalent of 130 g / eq and a viscosity at 25°C of 240 mPa s, containing boron trifluoride monoethylamine and acetone in a ratio of 100 / 3 / 4 (parts by mass), was used, and the curing conditions were set at atmospheric pressure and 125°C in an oven for 30 minutes.

[0069] Examples of molecule A include block copolymers and random copolymers of EO and PO, as well as multi-branched copolymers such as triblock copolymers and hyperbranched copolymers. Also usable are EO / PO adducts of glycerin, EO / PO adducts of trimethylolpropane, EO / PO adducts of pentaerythritol, EO / PO adducts of sorbitol, and EO / PO adducts of diglycerin or polyglycerin. However, polyoxyethylene bisphenol A ether is not included in molecule A. There are no limitations on the polymerization mode of EO / PO in each EO / PO adduct, and either a block copolymer or a random copolymer may be used.

[0070] In addition, "Celloxide (registered trademark)" 2021P may be used for the "epoxy resin containing 97 mass% or more of (3',4'-epoxycyclohexane)methyl-3,4-epoxycyclohexylcarboxylate and having an epoxy equivalent of 130 g / eq and a viscosity at 25°C of 240 mPa s" in Resin Formulation B.

[0071] The strand strength B' evaluated in accordance with JIS R7608 (2007) in accordance with resin curing prescription B is an index showing the resistance to breakage when a load is applied to the sizing-agent-containing carbon fiber bundle, and is used as an index showing the strength of the sizing-agent-containing carbon fiber bundle itself. A higher strand strength B' indicates a higher strength of the sizing-agent-containing carbon fiber bundle itself, and therefore a higher potential for improving the burst strength of the resulting pressure vessel. The strand strength B' needs to be 4.0 GPa or more, and preferably 6.0 GPa or more. The present inventors have first discovered that when the strand strength B' is less than 4.0 GPa, the single yarn strength during processing decreases, increasing the rate of single yarn breakage due to tension during processing, resulting in an excessive increase in fuzz, and significantly reducing the burst strength of the pressure vessel beyond the potential of the carbon fiber bundle strength; and that when the strand strength B' is 6.0 GPa or more, single yarn breakage during processing is suppressed, resulting in a better amount of accumulated fuzz.

[0072] However, the strand strength B' that has been used conventionally may deviate from the burst strength of the pressure vessel, and therefore there is a limit to how much the burst strength of the pressure vessel can be increased by designing only the strand strength B' as an index.

[0073] Therefore, the present inventors have discovered resin curing formula A, which serves as an index for pressure vessels. The resin composition in resin curing formula A is a practical resin for filament winding molding, and has a viscosity suitable for producing strand test pieces by the impregnation method described in JIS R7608 (2007). On the other hand, resin curing formula A has more functional groups than the resin composition described in JIS R7608 (2007), and has high adhesiveness to sizing-agent-containing carbon fiber bundles. Therefore, the inventors have found that the strand strength (strand strength A') measured according to resin curing formula A can be used as an index that represents, in a model manner, the burst strength of a pressure vessel when the sizing-agent-containing carbon fiber bundle to be measured is used as the pressure vessel.

[0074] The content of the sizing agent relative to the entire carbon fiber bundle in the present invention is 0.4% by mass or more and 2.0% by mass or less, preferably 0.4% by mass or more and 1.5% by mass or less, and more preferably 0.7% by mass or more and 1.5% by mass or less.

[0075] The sizing agent content of the entire carbon fiber bundle is generally adjusted. However, because the strand strength B', which is generally used as an index representing the strength of the sizing-containing carbon fiber bundle itself, is only slightly affected by the sizing agent content, it is often optimized in terms of other properties (e.g., bundling ability). For example, as another known document, Japanese Patent Laid-Open No. 7-9444, which uses an EO adduct of bisphenol A, specifies a preferred range from the viewpoints of moldability during advanced processing, fluff, and yarn breakage, and states that the sizing agent content should be 0.1 to 0.5% by mass or less. Furthermore, Japanese Patent Laid-Open No. 7269781, which uses an EO / PO copolymer as a sizing agent for carbon fiber bundles, states that the sizing agent content should be 0.05 to 5% by mass, as this will further improve the strength of the molded product. The present inventors conducted an investigation using strand strength A', which can be used as an index to model the burst strength when a sizing-agent-containing carbon fiber bundle is used as a pressure vessel. As a result, they confirmed that, as with strand strength B', the amount of sizing agent attached does not have a significant effect on strand strength A'. However, they found that when a specific sizing agent is used, strand strength A' varies greatly depending on the amount of sizing agent attached. Furthermore, they noticed that the content of sizing agent also affects other factors related to the manifestation of the burst strength of a pressure vessel, such as abrasion resistance and cohesiveness, and therefore, adjustment of the sizing agent content is extremely important for the manifestation of the burst strength of a pressure vessel.

[0076] With the structure of the sizing agent used in the present invention, if the sizing agent content is 0.4% by mass or more, the sizing agent can sufficiently cover the surface of the carbon fiber bundle, so that the abrasion resistance and bundling effects of the sizing agent can be obtained, and the quality and burst strength of the pressure vessel become good. If the sizing agent content is 2.0% by mass or less, it is possible to prevent a significant decrease in strand strength A'. As to why a sizing agent content exceeding 2.0% by mass can significantly decrease strand strength A', the detailed mechanism is unknown, but it is presumed that this is because poor curing occurs during the resin curing process due to an increase in the sizing agent content. The sizing agent content relative to the entire carbon fiber bundle can be measured using the measurement method described in the Examples below.

[0077] The sizing agent of the present invention contains 50% by mass or more, preferably 70% by mass or more, and more preferably 90% by mass or more, of molecule A having a structural unit derived from ethylene oxide (EO) and a structural unit derived from propylene oxide (PO) relative to the total mass of the sizing agent. By containing 50% by mass or more of molecule A in the sizing agent, the properties of molecule A can be fully exhibited.

[0078] The sizing agent of the present invention has an epoxy value of 0.0 to 1.0 meq. / g, preferably 0.0 to 0.7 meq. / g, measured using the epoxy value measurement method described in the Examples below. According to the findings of the present inventors, although the detailed mechanism is unknown, compounds containing epoxy groups may cause hydrolysis, and a large amount of such compounds leads to deterioration in the quality of sizing-agent-containing carbon fiber bundles during storage. Because epoxy groups affect the quality stability of the burst strength of pressure vessels, while improving adhesiveness, they may also be a factor in reducing the burst strength of pressure vessels. If the epoxy value of the sizing agent is 0.0 to 1.0 meq. / g, it is possible to sufficiently minimize the effect of changes in the epoxy value over time. The epoxy value can be measured using a reaction that ring-opens the epoxy groups in the sizing agent extracted from the sizing-agent-containing carbon fiber bundles. Specifically, the measurement method described in the Examples below can be used.

[0079] In the sizing-agent-containing carbon fiber bundle of the present invention, the number-average molecular weight of molecule A eluted from the sizing-agent-containing carbon fiber bundle, as measured using the method for measuring the number-average molecular weight of the eluted sizing agent described in the Examples below, is preferably 3,000 or more, more preferably 8,000 or more, and even more preferably 13,000 or more. The present inventors have found for the first time that adhesion of a sizing agent to the surface of a carbon fiber bundle greatly accelerates the thermal decomposition of the sizing agent, and that even in a temperature range where thermal decomposition is hardly observed in an undiluted sizing agent solution, thermal decomposition occurs in the drying step after application of the sizing agent. Consequently, thermal decomposition changes the molecular weight of the sizing agent, which in turn changes the viscosity of the sizing agent on the surface of the carbon fiber bundle, thereby affecting the bundleability and abrasion resistance of the sizing-agent-containing carbon fiber bundle. Therefore, in the present invention, it is not the number-average molecular weight of the sizing agent before application that is important, but rather the number-average molecular weight of the sizing agent adhered to the sizing-agent-containing carbon fiber bundle. If the number average molecular weight of the extracted molecule A is 3,000 or more, sufficient viscosity can be obtained, and therefore sufficient convergence and abrasion resistance can be obtained in the sizing-agent-containing carbon fiber bundle to exhibit the burst strength of a pressure vessel. Measurement of the number average molecular weight can be performed using a method commonly used for molecular weight measurement, such as GPC. When the sizing agent is composed of multiple components, the number average molecular weight of molecule A can be measured using a known method, such as preparative GPC. The number average molecular weight here is measured using the method described in the Examples. The columns and detectors used during measurement are not particularly limited, as long as they are commonly used. The number average molecular weight of the eluted sizing agent can be adjusted by keeping the temperature in the drying process as low as possible or shortening the drying time.

[0080] In the present invention, the mass ratio of the PO-derived structural units to the EO-derived structural units in molecule A is preferably 10 parts by mass or more and 100 parts by mass or less, more preferably 10 parts by mass or more and 75 parts by mass or less, and even more preferably 15 parts by mass or more and 50 parts by mass or less, per 100 parts by mass of the EO-derived structural units. The inventors have recognized that any sizing agent having EO structural units and PO structural units can be used as a suitable sizing agent for pressure vessels, but have also recognized the phenomenon that fuzz increases at the roller section during processing only when the proportion of EO structural units in the molecule is high. When the PO-derived structural units are 10 parts by mass or more and 100 parts by mass or less relative to the EO-derived structural units, there is no adverse effect of the EO structural units, and stable burst strength of the pressure vessel can be obtained.

[0081] The number of filaments in the sizing-agent-containing carbon fiber bundle of the present invention is preferably 20,000 or more, more preferably 24,000 or more. When producing a composite material by filament winding, productivity depends on the yarn speed and the number of filaments, so a large number of filaments allows for efficient production of the composite material. A filament number of 20,000 or more is satisfactory from the standpoint of productivity. On the other hand, a smaller number of filaments is advantageous in terms of resin impregnation during advanced processing, which is advantageous in terms of strength development of the resulting carbon fiber reinforced composite material. Therefore, this trade-off must be taken into consideration when determining the number of filaments. When the number of filaments is large, productivity can be improved without a decrease in burst strength by applying a sizing agent with particularly good resin impregnation properties. The present inventors have newly discovered that the sizing agent of the present invention has excellent resin impregnation properties and can maintain burst strength even when the number of filaments is increased.

[0082] The sizing agent in the present invention is preferably water-soluble. The means for applying the sizing agent to the carbon fiber bundle is preferably to use a sizing-agent-containing liquid diluted with a solvent. Examples of solvents include water, methanol, ethanol, isopropyl alcohol, acetone, methyl ethyl ketone, dimethylformamide, and dimethylacetamide. Among these, water is preferably used because it is easy to handle and has advantages from the viewpoint of safety. Therefore, the sizing agent is preferably water-soluble.

[0083] Furthermore, a sizing agent-containing carbon fiber bundle according to a third embodiment of the present invention will be described.

[0084] The sizing-agent-containing carbon fiber bundle of the present invention is a sizing-agent-containing carbon fiber bundle in which the sizing agent contains a compound (A) that does not contain an epoxy group, the compound (A) is contained in an amount of 50 mass% or more relative to 100 mass% of the total amount of the sizing agent, and the compound (A) satisfies the following conditions: the compound (A) has an SP value calculated by the Fedors method of 4.0 or more and 9.5 or less, and a viscosity at 30°C of 1,400 mPa s or more.

[0085] The compound (A) in the present invention has an SP value calculated by the Fedors method of 4.0 or more and 9.5 or less. The SP value is a commonly used solubility parameter that serves as an indicator of solubility and polarity, and is a value calculated by the method (Fedors method) described in R.F. Fedors., Polym. Eng. Sci., 14(2), 147-154 (1974). It was generally difficult to imagine that this SP value, which serves as an indicator of solubility, would be related to accumulated fluff, but the present inventors have newly discovered that the SP value is one of the major governing parameters that affect accumulated fluff. That is, accumulated fluff is suppressed by setting the SP value of the compound (A) to 4.0 or more and 9.5 or less. The smaller the SP value, the more the accumulated fluff is suppressed, so the SP value is set to 9.5 or less. However, compounds with an SP value of less than 4.0 are mainly gases or low-boiling point compounds and are not suitable as sizing agents. Considering the structure of suitable compounds described below, preferred examples of compound (A) include compounds containing alkylene oxide, and particularly compounds containing propylene oxide or ethylene oxide. Polymers or copolymers of propylene oxide or ethylene oxide are also possible, and more preferred examples include copolymers of propylene oxide and ethylene oxide. From the viewpoints of boiling point and water solubility, the SP value of compound (A) is preferably 8.5 to 9.3, more preferably 8.8 to 9.2.

[0086] The SP value of compound (A) can be calculated from the structural formula when the structural formula of compound (A) is known. When the structural formula is unknown, the SP value can be calculated by extracting the sizing agent from a carbon fiber bundle containing the sizing agent and identifying the structure by a known method such as proton NMR, carbon NMR, mass spectrometry, or TOF-SIMS.

[0087] The compound (A) has a viscosity of 1,400 mPa·s or more at 30°C. The viscosity in the present invention is a value obtained by measurement using a Brookfield viscometer. When the viscosity of the compound (A) at 30°C is 1,400 mPa·s or more, good sizing properties can be obtained. The viscosity of the compound (A) at 30°C is preferably 2,000 mPa·s or more, more preferably 5,000 mPa·s or more, even more preferably 10,000 mPa·s or more, particularly preferably 30,000 mPa·s or more, and most preferably 100,000 mPa·s. On the other hand, the upper limit of the viscosity is preferably 10,000,000 mPa·s or less.

[0088] The inventors have found through their studies that when the viscosity of compound (A) is high, the amount of accumulated fuzz tends to increase, but they have discovered that by controlling the SP value, it is possible to impart sizing properties while suppressing the increase in accumulated fuzz. The viscosity of compound (A) can be measured using a B-type viscometer. When measuring the viscosity of a carbon fiber bundle containing a sizing agent, the sizing agent can be extracted from the carbon fiber bundle containing the sizing agent using a solvent or the like, and in some cases, compound (A) can be separated and then measured using a B-type viscometer.

[0089] The compound (A) is contained in an amount of 50% by mass or more relative to 100% by mass of the total amount of the sizing agent. If the content of the compound (A) is less than 50% by mass, the amount of accumulated fuzz may increase. Furthermore, if the content of the compound (A) is less than 50% by mass, the bundling ability may be significantly reduced, possibly because a network structure such as interaction or entanglement between the compounds (A) cannot be formed. The content of the compound (A) is preferably 55 to 95% by mass, more preferably 60 to 90% by mass. Furthermore, when a plurality of compounds (A) are contained, the total content thereof is taken as the content of the compound (A). The content (% by mass) of the compound (A) contained in 100% by mass of the total amount of the sizing agent can be calculated by extracting the sizing agent from a carbon fiber bundle containing the sizing agent and using a known method such as proton NMR, carbon NMR, mass spectrometry, TOF-SIMS, or thermogravimetry.

[0090] The compound (A) must not contain an epoxy group. According to the findings of the present inventors, although the detailed mechanism is unknown, compounds containing epoxy groups may cause hydrolysis, and if the amount is large, this leads to deterioration in the quality of sizing-agent-containing carbon fiber bundles over time during storage. For the same reason, the total amount of compounds containing epoxy groups in 100% by mass of the total amount of sizing agent is preferably less than 9% by mass. The total amount of compounds containing epoxy groups in 100% by mass of the total amount of sizing agent is more preferably 5% by mass or less, and even more preferably 3% by mass or less. On the other hand, the preferred lower limit of the total amount of compounds containing epoxy groups in 100% by mass of the total amount of sizing agent is 0% by mass or more.

[0091] The compound (A) in the present invention is preferably water-soluble. Water-soluble in the present invention means that the compound (A) does not separate into two or more layers after adding the compound (A) to 100% by mass of water so that the compound (A) is 0.1% by mass, stirring, and leaving to stand for 24 hours. When the compound (A) is not water-soluble, a method using an organic solvent or a method of applying the compound as an aqueous emulsion can be used. However, the use of an organic solvent may require an increase in the size of the production equipment, and the application of an aqueous emulsion may result in a decrease in the uniformity of adhesion to the carbon fiber bundle, affecting the quality, so the compound (A) is preferably water-soluble.

[0092] Furthermore, it is more preferable that the compound (A) is completely miscible with water. "Completely miscible with water" means that, regardless of the mixing ratio of water and compound (A), compound (A) does not separate into two or more layers after adding compound (A) to water, stirring, and leaving to stand at 25°C for 24 hours. Normally, sizing agents are stored for long periods of time at a specific concentration according to the conditions of use, and in some cases transported. However, in the case of complete miscibility, there is no limit to the concentration setting, and storage and transport at a high concentration are possible, which leads to compact storage facilities and reduced transportation costs, and these benefits are extremely significant.

[0093] 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 surface of the carbon fiber bundles by subjecting the carbon fiber bundles to an oxidation treatment. Gas-phase oxidation, liquid-phase oxidation, and liquid-phase electrolytic oxidation are used as oxidation treatment methods. Liquid-phase electrolytic oxidation is preferably used from the viewpoints of high productivity and enabling uniform treatment.

[0094] By using the sizing-agent-containing carbon fiber bundle of the present invention, a fiber-reinforced composite material with excellent quality and grade can be obtained. In addition, the quality and grade of pressure vessels obtained by the filament winding molding method, in which the carbon fiber bundle is wound at high speed and the accumulation of fluff during the process is likely to be an issue, can be improved.

[0095] The upper and lower limits of the above-described ranges of values ​​can be combined in any manner.

[0096] A known method can be used to prepare a carbon fiber reinforced composite material using the carbon fiber bundle of the present invention. The carbon fiber bundle of the present invention and a carbon fiber reinforced composite material using the same can be used in many fields, such as aerospace, automobiles, railway vehicles, ships, civil engineering and construction, and sporting goods. In particular, they can be suitably used to manufacture hollow containers such as pressure vessels, and cylinders.

[0097] The present invention will be specifically explained below with reference to Examples, Comparative Examples and Reference Examples.

[0098] <Method for measuring sizing agent content (Y)> The mass of 2.0±0.5 g of a carbon fiber bundle containing a sizing agent was weighed (W １ ) (read to four decimal places), and then place in an electric furnace (capacity 120 cm ) set to a temperature of 450°C in a nitrogen gas flow of 50 ml / min. ３ ) for 15 minutes to completely decompose the sizing agent. Then, the carbon fiber bundle was transferred to a container in a dry nitrogen gas flow of 20 liters / minute, and after cooling for 15 minutes, the mass of the carbon fiber bundle was weighed (W ２ ) (read to the fourth decimal place) and W １ -W ２ The sizing agent content was calculated by W １ -W ２ W１ The result was divided by 1 / (1 / 2), rounded off to two decimal places, and converted into mass % when the entire sizing-containing carbon fiber bundle was taken as 100 mass %, thereby calculating the content (Y) (mass %) of the sizing agent. The measurement was carried out three times, and the average value was taken as the content (mass %) of the sizing agent.

[0099] <Viscosity Measurement Method> The compound to be measured was placed in a plastic bottle and heated to 30°C in a water bath. Viscosity was measured using a Brookfield viscometer (Tokyo Keiki Seizosho, Model: BL). Plastic bottles of 160 cc capacity were used for compounds with viscosities lower than 10,000 mPa·s, and 100 cc capacity for compounds with viscosities of 10,000 mPa·s or higher. The rotor and rotation speed of the Brookfield viscometer were No. 1 and 30 rpm for compounds with viscosities lower than 1,000 mPa·s, No. 3 and 30 rpm for compounds with viscosities between 1,000 mPa·s and 10,000 mPa·s, and No. 4 and 6 rpm for compounds with viscosities of 10,000 mPa·s or higher.

[0100] <Method for Relative Evaluation of Accumulated Fuzz Amount> The accumulated fluff amount was evaluated by weighing the carbon fibers cut and separated from the carbon fiber bundle when the carbon fiber bundle was rubbed against the stainless steel rod. When a single stainless steel rod having a smooth surface and a circular bottom surface of 5 mm in diameter was placed horizontally and the bottom surface was viewed from the side, two flat stainless steel rollers with smooth surfaces were placed in front of and behind the stainless steel rod as rollers for conveying the carbon fiber bundle so that the carbon fiber bundle passed under the stainless steel rod while contacting it at an angle of 20°. In this case, both of the flat stainless steel rollers were located at a higher position than the stainless steel rod.

[0101] During the evaluation, a carbon fiber bundle having 12,000 filaments was passed through the stainless steel rod at a speed of 10 m / min for 10 minutes while being in contact with the rod and under an initial tension of 1,000 gf. Since the deposited fluff occurs when carbon fibers cut and separated from the carbon fiber bundle are deposited on rollers, regulating guides, etc. in a subsequent process, an evaluation method is required in which only the carbon fibers cut and separated by contact with the stainless steel rod are collected.

[0102] The present inventors have found that it is difficult to evaluate only the cut and separated carbon fibers using known methods (a method of collecting the fibers by sandwiching them between sponges or a method of detecting them with a laser), and that these methods cannot serve as an index of the amount of accumulated fluff. Therefore, they have tried a method of collecting the cut and separated carbon fibers by suction at a position 2 cm above the carbon fiber bundle after it has passed through a stainless steel rod, and have found that the mass of the carbon fibers collected by this method can be used as an index of the amount of accumulated fluff.

[0103] Therefore, the mass of the carbon fibers recovered and weighed by this method was taken as the amount of accumulated fluff, and the results were relatively evaluated on a 7-point scale. A higher value of 7 indicates a smaller amount of accumulated fluff, and a lower value of 1 indicates a larger amount of accumulated fluff. A value of 3 or more was rated as pass, and a value of 2 or less was rated as fail. The relative evaluation of the amount of accumulated fluff was performed using the evaluation result of a carbon fiber bundle containing 2.0% of the following compound a'2 (Comparative Example 3 below) as the standard. 1 means an amount of accumulated fluff that is 2 times or more of the standard, 2 means an amount of accumulated fluff that is 1 time or more but less than 2 times the standard, 3 means an amount of accumulated fluff that is 0.6 times or more but less than 1 time the standard, 4 means an amount of accumulated fluff that is 0.4 times or more but less than 0.6 times the standard, 5 means an amount of accumulated fluff that is 0.3 times or more but less than 0.4 times the standard, 6 means an amount of accumulated fluff that is 0.2 times or more but less than 0.3 times the standard, and 7 means an amount of accumulated fluff that is less than 0.2 times the standard.

[0104] In Table 7, the mass of the carbon fibers recovered and weighed in the same manner except that the number of filaments was 24,000 was taken as the amount of accumulated fluff, and the results are shown as being evaluated on the following four-point scale: SS: less than 0.01 mg, S: 0.01 mg to less than 0.1 mg, A: 0.1 mg to less than 0.7 mg, and B: 0.7 mg or more. SS, S, and A were considered pass, and B was considered fail.

[0105] <Method for absolute evaluation of accumulated fluff amount> The absolute evaluation of the accumulated fluff amount was performed by weighing the carbon fiber bundles that were cut and separated from the carbon fiber bundle when the carbon fiber bundle was rubbed against the stainless steel rod. As in the above <Method for relative evaluation of accumulated fluff amount>, flat stainless steel rollers with smooth surfaces were placed before and after a single stainless steel rod with a diameter of 5 mm and a mirror-finished smooth surface, as rollers for conveying the carbon fiber bundle so that the carbon fiber bundle passed under the stainless steel rod while contacting the stainless steel rod at an angle of 20°.

[0106] In the evaluation, a carbon fiber bundle having 12,000 filaments was passed through the stainless steel rod at a speed of 10 m / min for 10 minutes while being in contact with the rod while applying an initial tension of 1,000 gf. Using a suction machine having suction holes, suction was performed at a position 2 cm above the carbon fiber bundle after passing through the stainless steel rod so that the suction air speed was 7 m / s at the position of the carbon fiber bundle, and the cut and separated carbon fiber bundles accumulated on a nonwoven fabric attached to the suction holes were recovered, and the mass of the weighed carbon fiber bundle was defined as the amount of deposited fluff.

[0107] <Method for Evaluating Bundling Ability> 1 m of sizing-agent-containing carbon fiber bundle was cut into 1 cm increments, placed in a 50 cc glass bottle, and the glass bottle was shaken to disintegrate the bundle. The dispersed carbon fiber bundles were removed from the glass bottle, and the thickness of each dispersed carbon fiber bundle was measured to evaluate the bundleability. Relative evaluation was performed using the following four levels of SS, S, A, and B. Using the carbon fiber bundle of Reference Example 12 as a reference, the thickness of the carbon fiber bundle was determined as follows: SS: a bundle thickness that is at least twice that of Reference Example 12; S: a bundle thickness that is 1.7 to less than 2.0 times that of Reference Example 12; A: a bundle thickness that is 1.3 to less than 1.7 times that of Reference Example 12; and B: a bundle thickness that is less than 1.3 times that of Reference Example 12.

[0108] Tables 2, 5, and 7 show the results of relative evaluation using four levels: SS, S, A, and B. The thicker the carbon fiber bundle, the less likely the fiber bundle is to come apart and the better the bundling ability. Conversely, the thinner the carbon fiber bundle, the poorer the bundling ability. SS, S, and A were considered pass, and B was considered fail.

[0109] <Evaluation of Strength Utilization Rate> The strand strength A' of a sizing-containing carbon fiber bundle using a practical resin was determined according to the following procedure in accordance with the resin-impregnated strand strength test method of JIS-R-7608 (2007). The resin curing formula used here was "Araldite (registered trademark)" LY1564 SP CI / "Baxodur (registered trademark)" EC331 = 100 / 35 (parts by mass), and the curing conditions were 120 minutes of curing in an oven set at a temperature of 80°C, followed by 240 minutes of curing in an oven set at a temperature of 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 taken as the strand strength. This resin curing formula is a practical resin curing formula for filament winding molding, and has a viscosity suitable for preparing strand test pieces using the impregnation method described in JIS R7608 (2007).

[0110] The strand strength B' of the sizing-agent-containing carbon fiber bundle using a general resin was determined according to the following procedure in accordance with the resin-impregnated strand strength test method of JIS-R-7608 (2007). The resin curing formulation used was "Celloxide (registered trademark)" 2021P / boron trifluoride monoethylamine / acetone = 100 / 3 / 4 (parts by mass), and the curing conditions were normal pressure and curing for 30 minutes in an oven set at a temperature of 125°C. Ten strands of the sizing-agent-containing carbon fiber bundle were measured, and the average value was taken as strand strength B'. The strength utilization rate was evaluated by determining the ratio of strand strength A' to the obtained strand strength B'.

[0111] <Method for Evaluating Quality Stability> Quality stability was evaluated by the degree of change in strand strength A' over time. Strand strength A' was evaluated in the same manner as above. The degree of change in strand strength A' over time was evaluated by measuring and comparing the strand strengths described below for a sizing-agent-containing carbon fiber bundle immediately after production and a sizing-agent-containing carbon fiber bundle after storage for 200 hours in an environment of 60°C and a relative humidity of 95% (storage under high temperature and high humidity). The difference in strand strength between immediately after production and after storage under high temperature and high humidity was divided by the strand strength immediately after production, and the converted value was converted into a strength change rate (%) when the strand strength immediately after production was taken as 100%. The degree of change over time was defined as A when the degree of change over time was less than 5%, B when it was 5% or more but less than 10%, and C when it was 10% or more. A means the most excellent quality stability, and C means the poorest quality stability. A and B were evaluated as passing.

[0112] <Measurement of Epoxy Value (X)> 30 g of a sizing-agent-containing carbon fiber bundle was immersed in 150 ml of N,N-dimethylformamide at a temperature of 25°C, and ultrasonic treatment was performed three times at a frequency of 40 kHz for 30 minutes each, thereby eluting the sizing agent from the sizing-agent-containing carbon fiber bundle to obtain a sizing agent solution. Using the obtained sizing agent solution, the epoxy groups were ring-opened with hydrochloric acid, and the epoxy value (X) was determined by acid-base titration. Note that when the sizing agent is eluted from the sizing-agent-containing carbon fiber bundle, it is preferable that the amount of sizing agent remaining in the carbon fiber bundle be 0.20 mass% or less.

[0113] <Evaluation of Burst Strength of Pressure Vessel> A pressure vessel H obtained by the method described in the Examples section was placed in a pressure vessel hydraulic burst testing device shown in Figure 1 and fixed with a fixing mechanism F, and a safety cover G was installed. Water was supplied to and pressurized by a water pressure pump C connected to a factory compressed air line A and a pressure booster B via a liquid supply hose E into the pressure vessel H. While monitoring the condition using a data logger D, the burst strength when the vessel burst was measured and relatively evaluated based on the following criteria. The relative evaluation of burst strength was performed using the evaluation result of a carbon fiber bundle containing 1.0 mass% of the following compound a'2 (Comparative Example 2 below) as the standard. S: Burst strength 15% or more higher than the standard A: Burst strength 10% or more but less than 15% higher than the standard B: Burst strength 5% or more but less than 10% higher than the standard C: Burst strength 1% or more but less than 5% higher than the standard D: Burst strength less than +1% higher than the standard

[0114] <Method for Evaluating Resin Impregnation> Resin impregnation was evaluated by impregnating a sizing-containing carbon fiber bundle with a base resin used in filament winding molding and measuring the amount of the base resin attached. Specifically, the sizing-containing carbon fiber bundle was impregnated with "Araldite (registered trademark)" LY1564 SP CIN as the base resin, and then measuring the weight of the entire carbon fiber bundle while the base resin was impregnated into the carbon fiber bundle. The weight of the carbon fiber bundle before impregnation was calculated by measuring the length of the carbon fiber bundle whose weight was measured. The weight of only the base agent impregnated into the carbon fiber bundle was calculated by calculating the difference between the weight of the carbon fiber bundle impregnated with the base agent and the weight of the carbon fiber bundle before impregnation. The weight of the base agent attached to a single fiber per unit length was calculated by dividing the calculated weight of only the base agent by the length of the carbon fiber bundle and the number of filaments. A larger amount of the base agent attached indicates that more base agent is attached to the carbon fiber bundle, indicating better impregnation properties during resin impregnation. The results were evaluated on the following four-point scale: Example 26, which had the best impregnation among the Examples of the present invention and Comparative Examples, was used as the standard, and from Example 26, the weight of the main agent attached to a single yarn per unit length was evaluated as S when it was 1.0 to 0.9 times, A when it was less than 0.9 to 0.8 times, B when it was less than 0.8 to 0.7 times, and C when it was less than 0.7 to 0.6 times.

[0115] <Measurement of number average molecular weight of eluted sizing agent> 5 ml of tetrahydrofuran was added to 1 g of a sizing-agent-containing carbon fiber bundle, and the bundle was subjected to ultrasonic treatment for 10 seconds and shaken for 30 minutes in an incubator at 40° C. to elute the sizing agent from the sizing-agent-containing carbon fiber bundle. The tetrahydrofuran solution of the eluted sizing agent was then filtered using a 0.45 μm filter, and then subjected to gel permeation chromatography under the conditions shown below to measure the number average molecular weight of the sizing agent in terms of polystyrene.

[0116] Measurement equipment: manufactured by Shimadzu Corporation Columns used: TSKgel G4000HXL (1 tube), G3000HXL (1 tube), and G2000HXL (1 tube), manufactured by TOSOH BIOSCIENCE Solvent: tetrahydrofuran Flow rate: 1.0 ml / min Column temperature: 23°C Injection volume: 0.2 ml Standard substance: polystyrene (manufactured by Tosoh Corporation) Detector: differential refractive index detector (RI-8020, manufactured by Tosoh Corporation).

[0117] The compounds used in each of the Examples, Comparative Examples, and Reference Examples are as follows. The SP value and viscosity of each compound are summarized in Table 1. a1: Propylene oxide / ethylene oxide block copolymer (M ｎ = 14,000) a2: propylene oxide / ethylene oxide block copolymer (M ｎ = 3,750) a3: propylene oxide / ethylene oxide block copolymer (M ｎ = 40,000) a'1: propylene glycol (M ｎ = 1,000) a'2: Bisphenol A ethylene oxide adduct (average number of moles of ethylene oxide added: 10) a'3: Sorbitol-based polyether polyol a'4: Polyglycerol polyglycidyl ether b1: Polyethylene glycol (M ｎ= 600) b2: ethylene oxide fatty acid diester (fatty acid ester adduct of b1) b3: 2-ethylhexyl stearate jER828: bisphenol A-type epoxy [Reference Example 1] An acrylonitrile copolymer was spun and baked to obtain a carbon fiber bundle having a total filament count of 12,000, a total fineness of 800 tex, a strand tensile strength of 4.9 GPa, and a strand tensile modulus of 240 GPa. Next, the carbon fiber bundle was subjected to an electrolytic surface treatment using an aqueous sulfuric acid solution as the electrolyte. The carbon fiber bundle that had been subjected to this electrolytic surface treatment was then washed with water and dried in heated air as a first drying step to obtain a carbon fiber bundle that would serve as a raw material.

[0118] The water-soluble compound a1 was used as compound (A), and water was added to obtain an aqueous solution in which a1 was uniformly dissolved. a1 is completely miscible with water at 25°C, which offers great advantages in terms of production and transportation management. This aqueous solution was used as a sizing agent solution, and the sizing agent was applied to the carbon fiber bundle by a dipping method, followed by pre-drying. Subsequently, as a second 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 content of the sizing agent was adjusted to 0.5% by mass relative to 100% by mass of the total amount of the surface-treated sizing-agent-containing carbon fiber bundle. The sizing-agent-containing carbon fiber bundle was evaluated according to the above-mentioned relative evaluation methods for accumulated fluff amount, and evaluation methods for bundleability and quality stability.

[0119] The results are summarized in Table 2. The amount of accumulated fluff was 4, the bundling ability was S, and the quality stability was A, and a carbon fiber bundle was obtained that was excellent in all properties. In Table 2, the "content" in the sizing agent composition represents the content of each compound in 100% by mass of the total amount of the sizing agent.

[0120] [Reference Example 2] A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Reference Example 1, except that the composition of the sizing agent was changed as shown in Table 2, and various evaluations were performed. The results are summarized in Table 2, with the accumulated fluff amount being 5, the sizing ability being S, and the quality stability being A. By containing compound b1 having a viscosity at 30°C of 0.1 mPa·s or more and 200 mPa·s or less as compound (B), a carbon fiber bundle was obtained that had a higher effect of suppressing the accumulated fluff amount than Reference Example 1. In addition, since the mixed viscosity of the sizing agent was 20,000 mPa·s or more, high sizing ability was exhibited.

[0121] [Reference Example 3] A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Reference Example 1, except that the composition of the sizing agent was changed as shown in Table 2, and various evaluations were performed. The results are summarized in Table 2, with the accumulated fluff amount being 6, the sizing ability being S, and the quality stability being A. By containing, as compound (B), compound b2 having a viscosity at 30°C of 0.1 mPa·s or more and 200 mPa·s or less and an SP value calculated by the Fedors method of 4.0 or more and 9.5 or less, a carbon fiber bundle with an even higher effect of suppressing the accumulated fluff amount than in Reference Example 2 was obtained. In addition, because the mixed viscosity of the sizing agent was 20,000 mPa·s or more, high sizing ability was exhibited.

[0122] [Reference Example 4] A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Reference Example 1, except that the composition of the sizing agent was changed as shown in Table 2, and various evaluations were performed. The sizing agent solution was prepared by mixing an oil-in-water emulsion prepared by phase inversion emulsification using b2 and b3 with an aqueous solution in which a1 was uniformly dissolved. The results are summarized in Table 2, with the accumulated fluff amount being 6, the sizing ability being S, and the quality stability being A. By containing compounds b2 and b3 as compound (B), each of which has a viscosity at 30°C of 0.1 mPa·s or more and 200 mPa·s or less and an SP value calculated by the Fedors method of 4.0 or more and 9.5 or less, a carbon fiber bundle with an even higher effect of suppressing the accumulated fluff amount than Reference Example 2 was obtained. In addition, since the mixed viscosity of the sizing agent was 20,000 mPa·s or more, high sizing ability was exhibited.

[0123] [Reference Example 5] A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Reference Example 4, except that the composition of the sizing agent was changed as shown in Table 2, and various evaluations were performed. The results are summarized in Table 2, with the accumulated fluff amount being 7, the sizing ability being S, and the quality stability being A. Compared to Reference Example 4, by changing the proportion of compound (B) to 20% by mass or more and less than 50% by mass in 100% by mass of the total amount of the sizing agent, a carbon fiber bundle with an even higher effect of suppressing the amount of accumulated fluff was obtained. In addition, since the mixed viscosity of the sizing agent was 20,000 mPa s or more, high sizing ability was exhibited.

[0124] [Reference Example 6] A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Reference Example 4, except that the composition and content of the sizing agent were changed as shown in Table 2, and various evaluations were performed. The results are summarized in Table 2, with the accumulated fluff amount being 7, the bundling ability being S, and the quality stability being A. By increasing the sizing agent content to 1.0% by mass compared to Reference Example 4, a carbon fiber bundle was obtained that had a higher effect of suppressing the accumulated fluff amount while maintaining the bundling ability and quality stability.

[0125] [Reference Example 7] A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Reference Example 1, except that the composition and content of the sizing agent were changed as shown in Table 2, and various evaluations were performed. The results are summarized in Table 2, with the accumulated fluff amount being 3, the bundling ability being S, and the quality stability being A. By reducing the sizing agent content to 0.3% by mass compared to Reference Example 1, a carbon fiber bundle was obtained that was inferior in the effect of suppressing the accumulated fluff amount compared to Reference Example 1, but was good in all of the properties of the accumulated fluff amount, the bundling ability, and the quality stability.

[0126] Reference Example 8 A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Reference Example 1, except that the composition of the sizing agent was changed as shown in Table 2 and dimethylformamide was used as the solvent for the sizing agent solution, and various evaluations were performed. The results are summarized in Table 2, with the amount of accumulated fluff being 4, the bundling ability being S, and the quality stability being B. By making the total amount of the compounds containing epoxy groups 9% by mass or more relative to 100% by mass of the total amount of the sizing agent, a carbon fiber bundle was obtained that was similar in amount of accumulated fluff and bundling ability to those of Reference Example 1 in which the total amount of the compounds containing epoxy groups was less than 9% by mass, but was slightly inferior in quality stability.

[0127] [Reference Example 9] A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Reference Example 1, except that water-soluble a2 was used as compound (A), and various evaluations were carried out. a2 is completely miscible with water at 25°C, and has great advantages in terms of production and transportation management. The results are summarized in Table 2, with the amount of accumulated fluff being 5, the sizing ability being A, and the quality stability being A. A carbon fiber bundle was obtained that was inferior in sizing ability to that of Reference Example 1, but was good in all properties.

[0128] [Reference Example 10] A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Reference Example 1, except that water-soluble a3 was used as compound (A), and various evaluations were carried out. a3 is completely miscible with water at 25°C, and has great advantages in terms of production and transportation management. The results are summarized in Table 2, with the amount of accumulated fluff being 4, the bundleability being SS, and the quality stability being A. A carbon fiber bundle was obtained that had even better bundleability than Reference Example 1 and was good in all properties.

[0129] Reference Example 11 A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Reference Example 4, except that water-soluble a3 was used as the compound (A) and the composition of the sizing agent was changed as shown in Table 2, and various evaluations were performed. The results are summarized in Table 2, with the amount of accumulated fluff being 6, the sizing ability being SS, and the quality stability being A. A carbon fiber bundle having even better sizing ability than Reference Example 2 and good properties in all respects was obtained.

[0130] Reference Example 12 A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Reference Example 1, except that the composition of the sizing agent was changed as shown in Table 2, and various evaluations were performed. The sizing agent solution used was an oil-in-water emulsion prepared by phase inversion emulsification using b2 instead of b3. The results are summarized in Table 2, with the amount of accumulated fluff being 7, the bundling ability being B, and the quality stability being A. The effect of suppressing the amount of accumulated fluff was superior to Reference Example 1, and the quality stability was equivalent to that of Reference Example 1; however, by using as a sizing agent only a compound whose viscosity at 30°C did not satisfy the requirement of 1,400 mPa s or more, a carbon fiber bundle with poor bundling ability was obtained.

[0131] Reference Example 13 A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Reference Example 1, except that the composition of the sizing agent was changed as shown in Table 2, and various evaluations were performed. The results are summarized in Table 2, with the accumulated fluff amount being 7, the sizing ability being B, and the quality stability being A. The effect of suppressing the accumulated fluff amount being better than in Reference Example 1, and the quality stability was equivalent to that of Reference Example 1, but by using compound a'1, which did not satisfy the viscosity at 30°C of 1,400 mPa s or more, as a substitute for compound (A), a carbon fiber bundle with poor sizing ability was obtained.

[0132] Reference Example 14 A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Reference Example 1, except that the composition of the sizing agent was changed as shown in Table 2, and various evaluations were performed. The results are summarized in Table 2, and the accumulated fluff amount was 2, the colligability was B, and the quality stability was A. The quality stability was equivalent to that of Reference Example 1, but by using compound a'2, which did not satisfy the range of 4.0 or more and 9.5 or less in SP value calculated by the Fedors method and did not satisfy the viscosity at 30°C of 1,400 mPa s or more, as a substitute for compound (A), a carbon fiber bundle with poor effect of suppressing the accumulated fluff amount and poor colligability was obtained.

[0133] Reference Example 15 A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Reference Example 1, except that the composition of the sizing agent was changed as shown in Table 2, and various evaluations were performed. The results are summarized in Table 2, and the accumulated fluff amount was 1, the bundling ability was S, and the quality stability was A. The bundling ability and quality stability were equivalent to those of Reference Example 1, but by using compound a'3, which did not satisfy the range of 4.0 or more and 9.5 or less in SP value calculated by the Fedors method, as a substitute for compound (A), a carbon fiber bundle with a poor effect of suppressing the accumulated fluff amount was obtained.

[0134] Reference Example 16 A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Reference Example 1, except that the composition of the sizing agent was changed as shown in Table 2, and various evaluations were performed. The results are summarized in Table 2, with the accumulated fluff amount being 2, the bundling ability being A, and the quality stability being C. By using compound a'4, which contains an epoxy group and whose SP value calculated by the Fedors method does not satisfy the range of 4.0 or more and 9.5 or less, as a substitute for compound (A), a carbon fiber bundle poor in the effect of suppressing the accumulated fluff amount and in the quality stability was obtained.

[0135] Reference Example 17 A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Reference Example 4, except that the composition of the sizing agent was changed as shown in Table 2, and various evaluations were performed. The results are summarized in Table 2, with the accumulated fluff amount being 6, the sizing ability being B, and the quality stability being A. The accumulated fluff amount and the quality stability were equivalent to those of Reference Example 4, but the amount of compound (A) contained in 100% by mass of the total amount of the sizing agent was less than 50% by mass, and therefore a network structure such as interaction or entanglement between the compounds (A) could not be formed, and the carbon fiber bundle had poor sizing ability.

[0136] [Example 1] A carbon fiber bundle T700S-12k (manufactured by Toray Industries, Inc.) was heat-treated in a nitrogen atmosphere at 550°C for 15 minutes to obtain a carbon fiber bundle from which the sizing agent had been removed. The carbon fiber bundle from which the sizing agent had been removed was then subjected to an electrolytic surface treatment using an aqueous sulfuric acid solution as the electrolyte, with a surface treatment amount of 2 C / g. The carbon fiber bundle subjected to this electrolytic surface treatment was then washed with water and dried in heated air as a first drying step.

[0137] The water-soluble a1 was used as compound (A), and water was added to obtain an aqueous solution in which a1 was uniformly dissolved. This aqueous solution was used as a sizing agent solution, and the sizing agent was applied to the carbon fiber bundle by a dipping method, followed by pre-drying. Subsequently, as a second 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 was adjusted to 0.5% by mass relative to 100% by mass of the total amount of the sizing-agent-containing carbon fiber bundle. The obtained sizing-agent-containing carbon fiber bundle was evaluated according to the above-mentioned absolute evaluation methods for the amount of accumulated fluff, the bundling ability, the quality stability, the epoxy value, and the strength utilization rate.

[0138] A polyethylene winding core was placed in the filament winding machine, and the resin composition of the resin curing formula A, which had been uniformly mixed in advance at 25°C, was fed to the winding core while impregnating each of the sizing-containing carbon fiber bundles. Before the sizing-containing carbon fiber bundles were impregnated with the resin curing formula A, the running direction of the sizing-containing carbon fiber bundles was restricted by multiple fixed guides, resulting in abrasion of the sizing-containing carbon fiber bundles due to contact with the fixed bars. The sizing-containing carbon fiber bundles were wound around the winding core at a winding angle of ±83° relative to the axial direction of the winding core over a 60 mm width, and stacked to a thickness of 1 mm to obtain a molded intermediate. After winding the fiber bundles, the molded intermediate was rotated at a speed of 7 rpm and held in a 20°C environment for 15 minutes. The molded intermediate was then heated at 80°C for 120 minutes and at 110°C for 240 minutes to cure the resin, yielding a pressure vessel. The obtained pressure vessel was evaluated according to the evaluation of the burst strength of the pressure vessel described above.

[0139] The evaluation results are shown in Table 4. The results of sizing ability are those of Reference Example 1, which used a sizing agent of the same composition. Both the strength utilization rate and the amount of accumulated fluff were good, and the pressure vessel exhibited high burst strength.

[0140] [Example 2] A sizing-agent-containing carbon fiber bundle and a pressure vessel were obtained in the same manner as in Example 1, except that the composition of the sizing agent was changed as shown in Table 3, and various evaluations were performed. The evaluation results are shown in Table 4. The results of sizing ability are those of Reference Example 8, which used a sizing agent of the same composition. Both the accumulated fluff and the strength were good. On the other hand, the strength utilization rate was lower than in Example 1, and the burst strength of the pressure vessel was better in Example 1 than in Example 2. In addition, Example 1, which did not contain a compound containing an epoxy group, also had better quality stability.

[0141] [Example 3] A sizing-agent-containing carbon fiber bundle and a pressure vessel were obtained in the same manner as in Example 1, except that the composition of the sizing agent was changed as shown in Table 3, and various evaluations were carried out. The evaluation results are shown in Table 4. The results of sizing ability are those of Reference Example 5, which used a sizing agent of the same composition. Both the strength utilization rate and the amount of accumulated fluff were good. Because the amount of accumulated fluff was even smaller than in Example 1, the burst strength of the pressure vessel was higher than in Example 1.

[0142] [Example 4] A sizing-agent-containing carbon fiber bundle and a pressure vessel were obtained in the same manner as in Example 3, except that the surface treatment amount of the electrolytic surface treatment before applying the sizing agent was changed to 20 C / g, and various evaluations were performed. The evaluation results are shown in Table 4. The bundleability, strength utilization rate, and amount of accumulated fluff were all good. The strength utilization rate was lower than in Example 3, and the burst strength of the pressure vessel was superior in Example 3.

[0143] [Example 5] A sizing-agent-containing carbon fiber bundle and a pressure vessel were obtained in the same manner as in Example 1, except that water-soluble a3 was used as compound (A), and various evaluations were carried out. The evaluation results are shown in Table 4. The results of sizing ability are those of Reference Example 10, which used a sizing agent of the same composition. The results were superior to those of Example 1, and the burst strength of the pressure vessel was higher than that of Example 1.

[0144] [Example 6] A sizing-agent-containing carbon fiber bundle and a pressure vessel were obtained in the same manner as in Example 5, except that the composition of the sizing agent was changed as shown in Table 3, and various evaluations were performed. The evaluation results are shown in Table 4. The results of sizing ability are those of Reference Example 11, which used a sizing agent of the same composition. In addition to being as excellent as in Example 5, accumulated fluff was further suppressed, and the burst strength of the pressure vessel was higher than in Example 5.

[0145] Comparative Example 1 A sizing-agent-containing carbon fiber bundle and a pressure vessel were obtained in the same manner as in Example 4, except that the composition of the sizing agent was changed as shown in Table 3, and various evaluations were carried out. The evaluation results are shown in Table 4. The results of sizing ability are those of Reference Example 12, which used a sizing agent of the same composition. The strength utilization rate and amount of accumulated fluff were good, but the sizing ability was poor and the burst strength of the pressure vessel was low.

[0146] [Comparative Example 2] A sizing-agent-containing carbon fiber bundle and a pressure vessel were obtained in the same manner as in Comparative Example 1, except that the composition of the sizing agent was changed as shown in Table 3, and various evaluations were performed. The evaluation results are shown in Table 4. The results of sizing ability are those of Reference Example 14, which used a sizing agent of the same composition. The sizing ability and strength utilization rate were good, but the amount of accumulated fluff was large and the burst strength of the pressure vessel was low.

[0147] Comparative Example 3 A sizing-agent-containing carbon fiber bundle and a pressure vessel were obtained in the same manner as in Comparative Example 1, except that the composition of the sizing agent was changed as shown in Table 3 and the surface treatment amount was changed to 30 C / g, and various evaluations were performed. The evaluation results are shown in Table 4. The results of sizing ability show the results of Reference Example 8, which used a sizing agent of the same composition. The sizing ability and amount of accumulated fluff were good, but the strength utilization rate was low and the burst strength of the pressure vessel was low.

[0148]

[0149]

[0150]

[0151]

[0152] The compounds used in the following Examples, Comparative Examples and Reference Examples are as described above. The SP value and viscosity of each compound are summarized in Table 1.

[0153] [Example 7] An acrylonitrile copolymer was spun and calcined to obtain a carbon fiber bundle having a total filament count of 12,000, a total fineness of 800 tex, a strand tensile strength of 4.9 GPa, and a strand tensile modulus of 240 GPa. The carbon fiber bundle was then subjected to an electrolytic surface treatment using an aqueous sulfuric acid solution as an electrolyte. The carbon fiber bundle subjected to this electrolytic surface treatment was then washed with water and dried in heated air to obtain a carbon fiber bundle as a raw material.

[0154] The water-soluble compound a1 was used as compound (A), and water was added to obtain an aqueous solution in which a1 was uniformly dissolved. a1 is completely miscible with water at 25°C, which offers great advantages in terms of production and transportation management. This aqueous solution was used as a sizing agent solution, and the sizing agent was applied to the carbon fiber bundle by a dipping method, followed by pre-drying. Subsequently, as a second drying step, the bundle was heat-treated in heated air at a temperature of 230°C for 35 seconds, thereby obtaining a sizing-agent-containing carbon fiber bundle. The content of the sizing agent was adjusted to 0.5% by mass relative to 100% by mass of the total amount of the surface-treated sizing-agent-containing carbon fiber bundle.

[0155] The sizing-agent-containing carbon fiber bundle was evaluated according to the above-mentioned evaluation methods for accumulated fluff, sizing ability, and quality stability. The results are summarized in Table 5. In Table 5, the "content" in the sizing agent composition represents the content of each compound in 100% by mass of the total amount of the sizing agent. The accumulated fluff was 4, the sizing ability was S, and the quality stability was A, and a carbon fiber bundle with good properties was obtained.

[0156] [Example 8] A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 7, except that the composition of the sizing agent was changed as shown in Table 5, and various evaluations were performed. The results are summarized in Table 5, with the accumulated fluff being 5, the sizing ability being S, and the quality stability being A. By containing compound b1 having a viscosity at 30°C of 0.1 mPa·s or more and 200 mPa·s or less as compound (B), a carbon fiber bundle having a higher effect of suppressing accumulated fluff than in Example 7 was obtained. In addition, since the mixed viscosity of the sizing agent was 20,000 mPa·s or more, high sizing ability was exhibited.

[0157] [Example 9] A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 7, except that the composition of the sizing agent was changed as shown in Table 5, and various evaluations were performed. The results are summarized in Table 5, with the accumulated fluff being 6, the sizing ability being S, and the quality stability being A. By containing, as compound (B), compound b2 having a viscosity at 30°C of 0.1 mPa·s or more and 200 mPa·s or less and an SP value calculated by the Fedors method of 4.0 or more and 9.5 or less, a carbon fiber bundle with an even higher effect of suppressing accumulated fluff than in Example 8 was obtained. In addition, because the mixed viscosity of the sizing agent was 20,000 mPa·s or more, high sizing ability was exhibited.

[0158] [Example 10] A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 7, except that the composition of the sizing agent was changed as shown in Table 5, and various evaluations were performed. The sizing agent solution was prepared by mixing an emulsion prepared by phase inversion emulsification using b2 and b3 with an aqueous solution in which a1 was uniformly dissolved. The results are summarized in Table 5, with the accumulated fluff score being 6, the sizing ability being S, and the quality stability being A. By including compounds b2 and b3 as compound (B), each of which has a viscosity at 30°C of 0.1 mPa·s or more and 200 mPa·s or less and an SP value calculated by the Fedors method of 4.0 or more and 9.5 or less, a carbon fiber bundle with an even higher effect of suppressing accumulated fluff than in Example 8 was obtained. Furthermore, since the mixed viscosity of the sizing agent was 20,000 mPa·s or more, high sizing ability was exhibited.

[0159] [Example 11] A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 10, except that the composition of the sizing agent was changed as shown in Table 5, and various evaluations were performed. The results are summarized in Table 5, with the accumulated fluff being 7, the sizing ability being S, and the quality stability being A. Compared to Example 10, by changing the proportion of compound (B) to 20% by mass or more and 50% by mass or less in 100% by mass of the total amount of the sizing agent, a carbon fiber bundle with an even higher effect of suppressing accumulated fluff was obtained. In addition, since the mixed viscosity of the sizing agent was 20,000 mPa s or more, high sizing ability was exhibited.

[0160] [Example 12] A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 10, except that the composition and content of the sizing agent were changed as shown in Table 5, and various evaluations were performed. The results are summarized in Table 5, with the accumulated fluff being 7, the bundling ability being S, and the quality stability being A. By increasing the sizing agent content to 1.0% by mass compared to Example 10, a carbon fiber bundle was obtained that had an even higher effect of suppressing accumulated fluff while maintaining the bundling ability and quality stability.

[0161] [Example 13] A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 7, except that the composition and content of the sizing agent were changed as shown in Table 5, and various evaluations were performed. The results are summarized in Table 5, with the accumulated fluff being 3, the bundling ability being S, and the quality stability being A. By reducing the sizing agent content to 0.3% by mass compared to Example 7, a carbon fiber bundle was obtained that was inferior in the effect of suppressing accumulated fluff compared to Example 7, but was good in all of the properties of accumulated fluff, bundling ability, and quality stability.

[0162] [Example 14] A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 7, except that the composition of the sizing agent was changed as shown in Table 5 and dimethylformamide was used as the solvent for the sizing agent solution, and various evaluations were performed. The results are summarized in Table 5, with the accumulated fluff being 4, the sizing ability being S, and the quality stability being B. By making the total amount of the compounds containing epoxy groups 9% by mass or more relative to 100% by mass of the total amount of the sizing agent, a carbon fiber bundle was obtained that had similar levels of accumulated fluff and sizing ability but slightly inferior quality stability compared to Example 7, in which the total amount of the compounds containing epoxy groups was less than 9% by mass.

[0163] [Example 15] A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 7, except that water-soluble a2 was used as compound (A), and various evaluations were performed. a2 is completely miscible with water at 25°C, and has great advantages in terms of production and transportation management. The results are summarized in Table 5, with the accumulated fluff being 5, the sizing ability being A, and the quality stability being A. A carbon fiber bundle was obtained that was inferior in sizing ability to that of Example 7, but was good in all properties.

[0164] [Example 16] A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 7, except that the sizing agent content was changed as shown in Table 5, and various evaluations were performed. The results are summarized in Table 5, with the accumulated fluff being 5, the bundling ability being S, and the quality stability being A. By increasing the sizing agent content to 1.0% by mass compared to Example 7, the effect of suppressing accumulated fluff was improved compared to Example 7, and a carbon fiber bundle having good bundling ability and quality stability was obtained.

[0165] [Example 17] A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 7, except that the sizing agent content was changed as shown in Table 5, and various evaluations were performed. The results are summarized in Table 5, with the accumulated fluff being 5, the bundling ability being S, and the quality stability being A. By increasing the sizing agent content to 3.0% by mass with respect to Example 7, the effect of suppressing accumulated fluff was improved compared to Example 1, but was equivalent to that of Example 10 in which the sizing agent content was 1.0% by mass. A carbon fiber bundle with good properties in both bundling ability and quality stability was obtained.

[0166] [Example 18] A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 7, except that water-soluble a3 was used as compound (A), and various evaluations were performed. a3 is completely miscible with water at 25°C, which offers significant advantages in terms of production and transportation management. Although a3 had such a high viscosity that it was difficult to quantify, it was confirmed that the viscosity was 100,000 mPa s or more. The results are summarized in Table 5, with the accumulated fluff being 4, the bundleability being SS, and the quality stability being A. Because the viscosity was higher than in Example 7, a carbon fiber bundle with better bundleability was obtained.

[0167] Example 19 A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 11, except that water-soluble a3 was used as the compound (A) and the composition was as shown in Table 5, and various evaluations were performed. The results are summarized in Table 5, with the accumulated fluff being 6, the bundleability being SS, and the quality stability being A. A carbon fiber bundle was obtained that showed an excellent effect of suppressing accumulated fluff, equivalent to that of Example 11, and also had excellent bundleability.

[0168] [Comparative Examples 4 and 5] A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 7, except that the composition of the sizing agent was changed as shown in Table 5, and various evaluations were performed. In Comparative Example 4, an emulsion prepared by phase inversion emulsification using b2 instead of b3 was used as the sizing agent solution. The results are summarized in Table 2, with the accumulated fluff being 7, the bundling ability being B, and the quality stability being A. The effect of suppressing accumulated fluff was superior to that of Example 7, and the quality stability was equivalent to that of Example 7, but the use of only a compound whose viscosity at 30°C did not satisfy the requirement of 1,400 mPa s or more as a sizing agent resulted in a carbon fiber bundle with poor bundling ability.

[0169] Comparative Example 6 A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 7, except that the composition of the sizing agent was changed as shown in Table 5, and various evaluations were performed. The results are summarized in Table 5, with the accumulated fluff being 2, the bundling ability being B, and the quality stability being A. The quality stability was equivalent to that of Example 7, but by using compound a'2, which did not satisfy the range of 4.0 or more and 9.5 or less in SP value calculated by the Fedors method and did not satisfy the viscosity at 30°C of 1,400 mPa s or more, as a substitute for compound (A), a carbon fiber bundle with poor accumulated fluff suppression effect and poor bundling ability was obtained.

[0170] Comparative Example 7 A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 7, except that the composition of the sizing agent was changed as shown in Table 5, and various evaluations were performed. The results are summarized in Table 5, with the accumulated fluff being 1, the bundling ability being S, and the quality stability being A. The bundling ability and quality stability were equivalent to those of Example 7, but by using compound a'3, which did not satisfy the range of 4.0 or more and 9.5 or less in SP value calculated by the Fedors method, as a substitute for compound (A), a carbon fiber bundle with a poor effect of suppressing accumulated fluff was obtained.

[0171] Comparative Example 8 A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 7, except that the composition of the sizing agent was changed as shown in Table 5, and various evaluations were performed. The results are summarized in Table 5, with the accumulated fluff being 2, the bundling ability being A, and the quality stability being C. By using compound a'4, which contains an epoxy group and whose SP value calculated by the Fedors method does not satisfy the range of 4.0 or more and 9.5 or less, as a substitute for compound (A), a carbon fiber bundle poor in the effect of suppressing accumulated fluff and in the quality stability was obtained.

[0172] Comparative Example 9 A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 10, except that the composition of the sizing agent was changed as shown in Table 5, and various evaluations were performed. The results are summarized in Table 5, with the accumulated fluff being 6, the sizing ability being B, and the quality stability being A. The accumulated fluff and the quality stability were equivalent to those of Example 10, but the amount of compound (A) contained in 100% by mass of the total amount of the sizing agent was less than 50% by mass, and therefore a network structure such as interaction or entanglement between the compounds (A) could not be formed, resulting in a carbon fiber bundle with poor sizing ability.

[0173] [Comparative Example 10] A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 7, except that the sizing agent content was changed as shown in Table 5, and various evaluations were performed. The results are summarized in Table 5, with the accumulated fluff being 5, the bundling ability being S, and the quality stability being C. Compared to Example 7, by increasing the sizing agent content to 6.0% by mass, the resin impregnation property deteriorated, and the strand strength became unstable, resulting in a decrease in the quality stability.

[0174]

[0175] The conditions of the carbon fiber bundles and sizing agents used in the following Examples, Comparative Examples, and Reference Examples are as shown in Table 6.

[0176] Example 20: An acrylonitrile copolymer was spun and calcined to obtain a carbon fiber bundle having a total filament count of 24,000, a strand tensile strength B' of 4.9 GPa, and a strand tensile modulus of elasticity of 240 GPa. The carbon fiber bundle was then electrolytically surface-treated 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 in a first drying step to obtain a carbon fiber bundle as a raw material. An EO / PO block copolymer (Mn = 14,000 (number average molecular weight before application)) was used as molecule A, and water was added to uniformly dissolve the copolymer to obtain an aqueous solution. This aqueous solution was used as a sizing agent solution, and the sizing agent was applied to the carbon fiber bundle by a dipping method, followed by thermal drying to obtain a sizing-agent-containing carbon fiber bundle. The content of the sizing agent relative to the sizing-agent-containing carbon fiber bundle was adjusted to the amount shown in Table 6. The sizing agent-containing carbon fiber bundle was evaluated according to the evaluation methods for the epoxy value, number average molecular weight, strand strength A', amount of accumulated fluff, bundling ability, resin impregnation ability, and quality stability of the sizing agent after solvent extraction. By drying the sizing agent at a temperature equal to or lower than the thermal decomposition temperature of molecule A, there was no difference between the undiluted solution of molecule A and the eluted sizing agent.

[0177] [Examples 21 to 23, Comparative Examples 11 and 12] A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 20, using an EO / PO block copolymer (Mn=14,000 (number average molecular weight before application)) as molecule A, except that the content of sizing agent relative to the sizing-agent-containing carbon fiber was as shown in Table 6. Within the range of the Examples, compared to Example 20, a tendency was observed that a higher sizing agent content improved the abrasion resistance and bundling ability, and a lower sizing agent content improved the impregnation ability, and the sizing agent content tended to affect the strand strength A', which is an index of the burst strength of a pressure vessel.

[0178] Example 24, Comparative Example 13 A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 20, except that bisphenol A-type epoxy was mixed with molecule A as a sizing agent, the content of molecule A in the sizing agent was as shown in Table 6, and then the mixture was diluted with DMF and used as a sizing agent. Bisphenol A-type epoxy was used as the sizing agent because, from the perspective of adhesiveness, compounds containing epoxy groups are commonly used as sizing agents for carbon fiber bundles. The inclusion of epoxy groups in the sizing agent improved bundling ability while decreasing impregnation and abrasion resistance. However, it was confirmed that these required properties were not a problem within the ranges described in Example 24. Furthermore, in Comparative Example 13, the inclusion of a large amount of epoxy groups decreased stability over time, and the strand strength A' tended to change over time.

[0179] [Example 25] A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 20, except that the temperature in the second drying step was set high. The number average molecular weight of the sizing agent eluted from the sizing-agent-containing carbon fiber bundle was significantly reduced to the number average molecular weight (Mn = 1,400) shown in Table 6, and even if the same molecule as in Example 20 was used as molecule A, it was confirmed that the abrasion resistance and colligating ability tended to decrease due to thermal decomposition during the drying step.

[0180] [Example 26] A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 20, except that the number of filaments used in the carbon fiber bundle was 12,000. Compared to Example 20, increasing the number of filaments to 12,000 was disadvantageous in terms of productivity, but tended to improve in terms of impregnation.

[0181] [Example 27] A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 20, except that the sizing-agent-containing carbon fiber bundle was changed to one having strand strength B' shown in Table 6. By using a carbon fiber bundle having better strand strength B', the strand strength A' was improved compared to Example 20.

[0182] [Example 28] A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 20, except that the EO / PO block copolymer (Mn = 3,800) shown in Table 6 was used as molecule A in the sizing agent. Although Example 20 was superior in abrasion resistance and collectibility due to the difference in the number average molecular weight of molecule A, it was confirmed that the molecule A described in Example 28 was sufficient to provide the desired effect.

[0183] Example 29 A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 20, except that the EO / PO block copolymer (Mn=10,000) shown in Table 6 was used as molecule A in the sizing agent. Although Example 20 was superior in abrasion resistance and collectibility due to the difference in the number average molecular weight of molecule A, it was confirmed that molecule A described in Example 29 was effective enough.

[0184] Comparative Example 14 A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 20, except that the sizing-agent-containing carbon fiber bundle had a strand strength B' shown in Table 6. A large decrease in the carbon fiber bundle strength led to an increase in fuzz and a deterioration in bundle colligability, resulting in a decrease in strand strength A' that was even greater than the strength potential of the carbon fiber bundle.

[0185] Comparative Examples 15 to 17 A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 20, except that a bisphenol A-ethylene oxide adduct (average number of moles of ethylene oxide added: 10) was used as the sizing agent instead of molecule A, and the content of the sizing agent relative to the sizing-agent-containing carbon fiber bundle was adjusted to the amount shown in Table 6. In Table 6, the form of the bisphenol A-ethylene oxide adduct is shown in the column for molecule A. Comparison with Examples 21 to 23 and Comparative Examples 11 and 12 revealed that when the sizing agent was a bisphenol A-ethylene oxide adduct, the content had almost no effect on strand strength A', whereas when the sizing agent was an EO / PO copolymer, the effect was significant, and that adjustment of the content of the EO / PO copolymer was particularly important for strength development.

[0186] Comparative Example 18 A sizing-containing carbon fiber bundle was obtained in the same manner as in Example 20, except that the number of filaments was 12,000 and the sizing agent content was set to the proportion described in Comparative Example 18. By comparing with Example 26, it was found that even under conditions of a small number of filaments and excellent impregnation, when an EO / PO copolymer was used, the strand strength A' decreased with an increase in the sizing agent content, and that the decrease in strength due to the EO / PO block copolymer content was not due to a decrease in impregnation or the like, but was the effect of the EO / PO block copolymer itself.

[0187] [Comparative Examples 19 and 20] In Comparative Example 19, a sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 20, except that bisphenol A-type epoxy and polyethylene glycol (Mn = 400) instead of molecule A were mixed in the ratio shown in Comparative Example 19 in Table 6 (note that in Table 6, polyethylene glycol is shown as molecule A), diluted with DMF, and used as a sizing agent. In Comparative Example 20, a sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Comparative Example 19, except that the number of filaments was 12,000. By comparing Comparative Examples 19 and 20 with Examples 20 and 26, it was found that the impregnation ability of the sizing agents used in Comparative Examples 19 and 20 was significantly reduced when the number of filaments was increased from 12,000 to 24,000, whereas the impregnation ability of the sizing agents used in Examples 20 and 26 was maintained good regardless of the number of filaments.

[0188] [Comparative Example 21] A sizing-agent-containing carbon fiber bundle was obtained in the same manner as in Example 20, except that the carbon fiber bundle used had the same strand strength B' as the strand strength B' of Example 27 shown in Table 6, and polyethylene glycol (Mn = 400) was mixed in place of bisphenol A-type epoxy and molecule A in the ratio shown in Comparative Example 21 in Table 6 (note that in Table 6, polyethylene glycol is shown as molecule A), diluted with DMF, and used as a sizing agent. By comparing with Example 27, it was found that Example 27 was superior in terms of the expression of strand strength A' relative to strand strength B', and also that, as for accumulated fluff, since the abrasion resistance of the sizing agent in Comparative Example 21 was low, there was no change even when a high-strength carbon fiber bundle was used, whereas Example 27 was superior in terms of the amount of accumulated fluff.

[0189]

[0190]

[0191] A: Factory compressed air line B: Pressure booster C: Water pressure pump D: Data logger E: Liquid supply hose F: Fixing mechanism G: Safety cover H: Pressure vessel

Claims

1. A pressure vessel obtained by impregnating a sizing-agent-containing carbon fiber bundle containing a sizing agent containing 50 mass % or more of the following compound (A) which does not contain epoxy groups with a thermosetting resin and curing the impregnated carbon fiber bundle, wherein the sizing-agent-containing carbon fiber bundle has a strand strength (strand strength B') of 4.9 GPa or more as evaluated in accordance with JIS R7608 (2007) according to the following resin curing prescription B, a strength utilization factor which is the ratio of the strand strength (strand strength A') to the strand strength B' as evaluated in accordance with JIS R7608 (2007) according to the following resin curing prescription A, and an amount of accumulated fluff of 0.80 mg or less: Compound (A): A compound having an SP value of 4.0 to 9.5 as calculated by the Fedors method and a viscosity at 30°C of 1,400 mPa s or more; Resin curing formulation A: A resin containing 80% by mass of 2,2'-[(1-methylethylidene)bis(4,1-phenyleneoxymethylene)]bisoxirane and 20% by mass of 1,4-bis(2,3-epoxypropoxy)butane was used as the base resin, and 2,2'-dimethyl-4,4'-methylenebis(cyclohexylamine) was used as the curing agent, with the ratio of the base resin to the curing agent being 100 / 35 (parts by mass). The curing conditions were normal pressure, curing in an oven set at a temperature of 80°C for 120 minutes, and then curing in an oven set at a temperature of 110°C for 240 minutes; Resin curing formulation B: An epoxy resin containing 97% by mass or more of (3',4'-epoxycyclohexane)methyl-3,4-epoxycyclohexylcarboxylate and having an epoxy equivalent of 130 g / eq and a viscosity at 25°C of 240 mPa s, containing boron trifluoride monoethylamine and acetone in a ratio of 100 / 3 / 4 (parts by mass), was used, and the curing conditions were set at atmospheric pressure and 125°C in an oven for 30 minutes.

2. The pressure vessel according to claim 1, wherein the sizing agent further contains a compound (B) having a viscosity at 30°C of 0.1 mPa·s or more and 200 mPa·s or less.

3. The pressure vessel according to claim 1 or 2, wherein 30 g of the sizing-agent-containing carbon fiber bundle is immersed in 150 ml of N,N-dimethylformamide at 25°C and subjected to ultrasonic treatment three times for 30 minutes at a frequency of 40 kHz, and the epoxy value of the sizing agent eluted is X meq. / g, and the content of the sizing agent applied to the carbon fiber bundle is Y mass %, the product of X and Y is 0.00 meq. / g or more and 0.20 meq. / g or less.

4. A pressure vessel according to claim 1 or 2, wherein the sizing-containing carbon fiber bundle is obtained by subjecting a carbon fiber bundle to electrolytic surface treatment at 2 to 20 C / g and then applying a sizing agent.

5. A sizing-agent-containing carbon fiber bundle for pressure vessels, which contains a sizing agent containing 50 mass % or more of the following compound (A) that does not contain an epoxy group, wherein the strand strength (strand strength B') evaluated in accordance with JIS R7608 (2007) according to the following resin curing prescription B is 4.9 GPa or more, the ratio (strand strength A') to strand strength B' (strand strength utilization factor) evaluated in accordance with JIS R7608 (2007) according to the following resin curing prescription A is 90% or more, and the amount of accumulated fluff is 0.80 mg or less. Compound (A): A compound having an SP value of 4.0 to 9.5 calculated by the Fedors method and a viscosity at 30°C of 1,400 mPa s or more. Resin curing formulation A: A resin containing 80% by mass of 2,2'-[(1-methylethylidene)bis(4,1-phenyleneoxymethylene)]bisoxirane and 20% by mass of 1,4-bis(2,3-epoxypropoxy)butane was used as the base resin, and 2,2'-dimethyl-4,4'-methylenebis(cyclohexylamine) was used as the curing agent, with the ratio of the base resin to the curing agent being 100 / 35 (parts by mass). The curing conditions were normal pressure and curing in an oven set at a temperature of 80°C for 120 minutes, followed by curing for 240 minutes in an oven set at a temperature of 110°C. Resin curing formulation B: An epoxy resin containing 97% by mass or more of (3',4'-epoxycyclohexane)methyl-3,4-epoxycyclohexylcarboxylate and having an epoxy equivalent of 130 g / eq and a viscosity at 25°C of 240 mPa s, containing boron trifluoride monoethylamine and acetone in a ratio of 100 / 3 / 4 (parts by mass), was used, and the curing conditions were set at atmospheric pressure and 125°C in an oven for 30 minutes.

6. A sizing-agent-containing carbon fiber bundle for pressure vessels according to claim 5, wherein the sizing agent contains a compound (B) having a viscosity at 30°C of 0.1 mPa·s or more and 200 mPa·s or less.

7. The sizing-agent-containing carbon fiber bundle for pressure vessels according to claim 5 or 6, wherein 30 g of the sizing-agent-containing carbon fiber bundle is immersed in 150 ml of N,N-dimethylformamide at 25°C and subjected to ultrasonic treatment three times for 30 minutes at a frequency of 40 kHz, and the epoxy value of the sizing agent eluted is X meq. / g and the content of the sizing agent applied to the carbon fiber bundle is Y mass %, and the product of X and Y is 0.00 meq. / g or more and 0.20 meq. / g or less.

8. A sizing-agent-containing carbon fiber bundle for pressure vessels according to claim 5 or 6, which is obtained by subjecting a carbon fiber bundle to electrolytic surface treatment at 2 to 20 C / g and then applying a sizing agent.

9. The strand strength (strand strength B') evaluated in accordance with JIS R7608 (2007) in accordance with the following resin curing prescription B is 4.0 GPa or more, the content of the sizing agent relative to the total mass of the carbon fiber bundle is 0.4 mass% or more and 2.0 mass% or less, the sizing agent contains 50 mass% or more of molecule A having a structural unit derived from ethylene oxide (EO) and a structural unit derived from propylene oxide (PO) relative to the total mass of the sizing agent, and the epoxy value of the sizing agent eluted from the sizing-agent-containing carbon fiber bundle is 1.0 meq. / g or less sizing-agent-containing carbon fiber bundle: Molecule A: a molecule other than polyoxyethylene bisphenol A ether that is in a liquid state at 25°C under 1 atmosphere, and in which the total mass of EO-derived structural units and PO-derived structural units is 65 mass% or more of the total mass of the molecule; Resin curing formulation B: an epoxy resin containing 97 mass% or more of (3',4'-epoxycyclohexane)methyl-3,4-epoxycyclohexylcarboxylate and having an epoxy equivalent of 130 g / eq and a viscosity of 240 mPa·s at 25°C, in a ratio of boron trifluoride monoethylamine / acetone = 100 / 3 / 4 (parts by mass), is used, and the curing conditions are set at normal pressure and 125°C for 30 minutes in an oven.

10. A sizing-agent-containing carbon fiber bundle according to claim 9, wherein the number-average molecular weight of the molecules A eluted from the sizing-agent-containing carbon fiber bundle is 3,000 or more.

11. A sizing-agent-containing carbon fiber bundle according to claim 9 or 10, wherein the content of PO-derived structural units relative to EO-derived structural units in molecule A is 10% by mass or more and 100% by mass or less.

12. A sizing-agent-containing carbon fiber bundle according to claim 9 or 10, wherein the number of filaments in the sizing-agent-containing carbon fiber bundle is 20,000 or more.

13. The sizing agent-containing carbon fiber bundle according to claim 9 or 10, wherein the sizing agent is water-soluble.

14. A carbon fiber reinforced composite material using the sizing agent-containing carbon fiber bundle according to claim 9 or 10.

15. A pressure vessel using the sizing agent-containing carbon fiber bundle according to claim 9 or 10.

16. A sizing-agent-containing carbon fiber bundle having a sizing agent content of 0.1 to 3.0% by mass relative to 100% by mass of the carbon fiber bundle containing the sizing agent, wherein the sizing agent contains a compound (A) that does not contain an epoxy group, and wherein the compound (A) accounts for 50% by mass or more of the total amount of the sizing agent (100% by mass), and wherein the compound (A) satisfies the following conditions: Compound (A): The SP value calculated by the Fedors method is 4.0 or more but 9.5 or less, and the viscosity at 30°C is 1,400 mPa s or more.

17. The sizing-agent-containing carbon fiber bundle according to claim 16, wherein the compound (A) has a viscosity at 30°C of 2,000 mPa·s or more.

18. A sizing-agent-containing carbon fiber bundle according to claim 16 or 17, wherein the content of the compound containing an epoxy group is less than 9 mass % in 100 mass % of the total amount of the sizing agent.

19. The sizing agent-containing carbon fiber bundle according to claim 16 or 17, wherein the compound (A) is water-soluble.

20. The sizing agent-containing carbon fiber bundle according to claim 19, wherein the compound (A) is completely miscible with water.

21. A sizing-agent-containing carbon fiber bundle according to claim 16 or 17, wherein the sizing agent contains a compound (B) having a viscosity at 30°C of 0.1 mPa·s or more and 200 mPa·s or less.

22. The sizing-agent-containing carbon fiber bundle according to claim 21, wherein the compound (B) has an SP value calculated by the Fedors method of 4.0 or more and 11.0 or less.

23. The sizing-agent-containing carbon fiber bundle according to claim 21, wherein the compound (B) has an SP value calculated by the Fedors method of 4.0 or more and 9.5 or less.

24. A sizing-agent-containing carbon fiber bundle according to claim 21, wherein the compound (B) is contained in an amount of 5 mass % or more based on 100 mass % of the total amount of the sizing agent.

25. The sizing-agent-containing carbon fiber bundle according to claim 21, wherein the mixed viscosity of said compound (A) and said compound (B) is 20,000 mPa·s or more.

26. A carbon fiber reinforced composite material using the sizing agent-containing carbon fiber bundle according to claim 16 or 17.

27. A pressure vessel using the sizing agent-containing carbon fiber bundle according to claim 16 or 17.

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