Sizing agent-adhered carbon fiber bundle and its manufacturing method, carbon fiber-reinforced composite material, and pressure vessel

Abstract

The present invention relates to a carbon fiber bundle with a sizing agent applied to improve the morphological stability, abrasion resistance, and resin impregnation properties of the carbon fiber bundle, having 6,000 to 50,000 filaments, a thickness of 0.10 mm to 0.20 mm, a coefficient of variation of thickness of 6.5% or less, a drape value of 6 cm to 9 cm, and a strand tensile strength (strand tensile strength A') evaluated according to JIS R7608 (2007) using the following resin formulation A, wherein the sizing agent-coated carbon fiber bundle has a sizing agent applied to it, has a thickness of 0.10 mm to 0.20 mm, a coefficient of variation of thickness of 6.5% or less, a drape value of 6 cm to 9 cm, and a strand tensile strength (strand tensile strength A') of 4.9 GPa to 6.7 GPa.

Description

[Technical Field]

[0001] This invention relates to carbon fiber bundles that are suitable for use in aircraft components, automobile components, and ship components, as well as in sports applications such as golf shafts and fishing rods, and other general industrial applications. [Background technology]

[0002] Carbon fiber possesses excellent mechanical properties, particularly high specific strength and specific modulus. Therefore, it is widely used in aerospace, leisure, and general industrial applications such as automobiles, and various molding methods have been developed. Among these, filament winding (FW) molding is becoming increasingly widely applied to carbon fiber due to its excellent moldability and the properties of the resulting composite materials. In particular, fuel containers for natural gas vehicles, which have attracted attention in recent years, are beginning to use FW-molded containers with carbon fiber as a reinforcing fiber to achieve lightweight and high-performance properties. Furthermore, in recent years, there has been a growing demand for carbon fiber suitable for FW molding in applications using higher pressures than before, such as compressed hydrogen gas containers intended for filling fuel cells with hydrogen gas. For example, compressed hydrogen gas containers operate at pressures of 50-100 MPa, which is higher than the approximately 20-30 MPa used for conventional compressed natural gas containers. Especially for compressed hydrogen gas containers for automotive applications, weight reduction is desired to increase the vehicle's driving range. Therefore, weight reduction is achieved by using high-strength carbon fiber reinforced composite materials (CFRP) to reduce the amount of material used. In the FW molding method, carbon fibers are opened and impregnated with resin by passing them through multiple 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 rub against the fixed bars, causing friction and fuzzing of the carbon fiber bundles. Insufficient impregnation (void formation) can occur due to uneven thickness caused by fiber opening and differences in resin impregnation properties caused by sizing agents. If voids are formed in the resin-impregnated material, it ultimately leads to a decrease in the quality and grade of the molded product. Therefore, the application of carbon fiber bundles with uniform thickness that have abrasion resistance and resin impregnation properties is desired. Thus, it is desirable to efficiently express the mechanical properties of carbon fibers in CFRP used in such applications. Furthermore, it is desired to improve the uniformity of CFRP and enhance quality stability by making the carbon fibers suitable for the FW molding method.

[0003] Several proposals have been made to improve the strength development when using CFRP. Patent documents 1 to 4 propose improving the moldability of CFRP by preventing fiber splitting and improving the fiber opening properties and resin impregnation of the carbon fiber bundles during molding, thereby improving strength development by eliminating unimpregnated and excess resin portions. Patent document 1 proposes a technique that achieves both abrasion resistance, fiber breakage resistance, and resin impregnation by applying a sizing agent to the inner layer of the carbon fiber bundle and then blowing gas to remove the sizing agent from the outer layer of the carbon fiber bundle. Patent document 2 proposes a technique that improves fiber opening properties during molding and produces a homogeneous molded body by flattening the shape of the carbon fiber bundle. Patent documents 3 and 4 propose improving the abrasion resistance of carbon fiber bundles by modifying the sizing agent applied, mainly from the viewpoint of improving the convergence properties of the carbon fiber bundles and controlling the mechanical properties of the molded product. Patent Document 3 discloses a technique to reduce the coefficient of friction with a fixed bar and suppress abrasion fuzz by adding a higher fatty acid ester to a sizing agent mainly composed of epoxy resin. Patent Document 4 describes how using an alkylene oxide adduct of bisphenol A as a sizing agent reduces the coefficient of friction with a fixed bar and reduces abrasion fuzz. Patent Documents 5 and 6 propose ways to improve the mechanical properties of carbon fiber bundles. Patent Document 5 describes how the crystalline structure in carbon fibers is controlled to a desirable state by adjusting the firing conditions in the flame-retardant and carbonization process, making it easier to achieve 0° tensile strength in CFRP. Patent Document 6 proposes a technique in which the surface layer of a carbon fiber bundle obtained from a precursor fiber bundle whose density has been increased by wet-dry spinning is electrolytically oxidized in an aqueous electrolyte solution containing nitrate ions as an essential component. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2018-9280 [Patent Document 2] Japanese Patent Publication No. 2002-294568 [Patent Document 3] Japanese Patent Publication No. 2002-317382 [Patent Document 4] Japanese Patent Application Publication No. 07-009444 [Patent Document 5] Japanese Patent Publication No. 2020-73737 [Patent Document 6] Japanese Patent Application Publication No. 2-104767 [Overview of the project] [Problems that the invention aims to solve]

[0005] However, the technologies described in Patent Documents 1 and 2 are merely techniques to improve the moldability of carbon fiber bundles, and further improvement in mechanical properties cannot be expected if good moldability has already been established. Patent Document 1 proposes a technique to improve uneven adhesion inside and outside the bundle by adhering the sizing agent to the inside of the yarn bundle, but it does not mention controlling the thickness of the yarn bundle or the impregnation of the resin into the carbon fiber bundle by the properties of the sizing agent. Patent Document 2 proposes a technique to create a homogeneous molded body by improving the fiber-opening properties of the carbon fiber bundle, but it does not mention controlling the thickness of the yarn bundle or the impregnation of the resin into the carbon fiber bundle by the properties of the sizing agent. Patent Documents 3 and 4 certainly show an effect of suppressing fraying, but they do not mention controlling the thickness of the yarn bundle or the impregnation of the resin into the carbon fiber bundle by the properties of the sizing agent or the method of applying the sizing agent, or giving the carbon fiber bundle uniform bundleness to improve the quality stability of the molded product. Furthermore, while the technology described in Patent Document 5 shows that the 0° tensile strength of CFRP is high relative to the resin-impregnated strand strength of carbon fiber bundles, similar to Patent Documents 1-4, it does not mention controlling the thickness of the yarn bundle or the impregnation of the resin into the carbon fiber bundle by the properties of the sizing agent. In the technology described in Patent Document 6, although an improvement in strand strength is observed when measured under either of the two resin formulations before and after nitric acid treatment, post-treatment involving nitric acid treatment, drying, and inactivation at 700°C in a nitrogen atmosphere for several minutes is required. Therefore, especially for carbon fibers with small single fiber fineness, there are challenges in terms of quality reduction due to the generation of fluff, as well as productivity and cost due to the post-treatment.

[0006] Therefore, an object of the present invention is to provide a carbon fiber bundle that enables stable quality when formed into a carbon fiber reinforced composite material by optimally designing the manufacturing method of the carbon fiber bundle, the morphological stability of the sizing agent-attached carbon fiber bundle, the abrasion resistance, and the resin impregnation property into the carbon fiber bundle.

Means for Solving the Problems

[0007] In order to achieve the above object, the present invention has the following configuration. (1) A carbon fiber bundle having 6,000 or more and 50,000 or less filaments, to which a sizing agent is applied, having a thickness of 0.10 mm or more and 0.20 mm or less, a coefficient of variation in thickness of 6.5% or less, a drape value of 6 cm or more and 9 cm or less, and a strand tensile strength (strand tensile strength A') evaluated based on JIS R7608 (2007) using the following resin formulation A of not less than 4.9 GPa and not more than 6.7 GPa, a sizing agent-attached carbon fiber bundle. Resin formulation A: containing 97% by mass or more of (3',4'-epoxycyclohexane)methyl-3,4-epoxycyclohexyl carboxylate, and having a composition of epoxy resin / boron trifluoride monoethylamine / acetone = 100 / 3 / 4 (parts by mass) with an epoxy equivalent of 130 g / eq and a viscosity of 240 mPa·s at 25°C, and using normal pressure, a temperature of 125°C, and a time of 30 minutes as its curing conditions. (2) The sizing agent-attached carbon fiber bundle according to (1) above, having a strand tensile strength A' of 5.6 GPa or more and 6.7 GPa or less, and a strand tensile strength (strand tensile strength B') evaluated based on JIS R7608 (2007) using the following resin formulation B of 5.3 GPa or more and 6.4 GPa or less.

[0008] Resin formulation B: The main agent contains 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. 2,2'-Dimethyl-4,4'-methylenebis(cyclohexylamine) is used as the curing agent, and the ratio of the main agent to the curing agent is 100 / 35 (parts by mass). As the curing conditions, normal pressure, a temperature of 80°C for 120 minutes, and then normal pressure, a temperature of 110°C for 240 minutes are used. (3) The sizing agent-attached carbon fiber bundle according to (2) above, wherein the strand tensile strength A' is 5.9 GPa or more and 6.7 GPa or less, and the strand tensile strength (strand tensile strength B') evaluated based on JIS R7608 (2007) using resin formulation B is 5.6 GPa or more and 6.4 GPa or less. (4) The sizing agent-attached carbon fiber bundle according to any one of (1) to (3) above, wherein the resin incorporation ratio is 0.45 or more and 0.64 or less. (5) The sizing agent-attached carbon fiber bundle according to any one of (1) to (4) above, wherein the single fiber diameter is 6.6 μm or more and 7.3 μm or less. (6) The sizing agent-attached carbon fiber bundle according to (3) above, wherein the strand tensile strength A' is 6.3 GPa or more and 6.7 GPa or less, and the strand tensile strength B' is 6.0 GPa or more and 6.4 GPa or less. (7) The sizing agent-attached carbon fiber bundle according to any one of (1) to (6) above, wherein the strand tensile modulus evaluated based on JIS R7608 (2007) using resin formulation A is from 240 GPa to 275 GPa. (8) The sizing agent-attached carbon fiber bundle according to any one of (1) to (7) above, wherein the sizing agent attachment amount is 0.7% by mass or more and 1.0% by mass or less based on 100% by mass of the sizing agent-attached carbon fiber bundle. (9) The sizing agent-attached carbon fiber bundle according to any one of (1) to (8) above, wherein the sizing agent does not contain a cyclic oxygen-containing compound. (10) The sizing agent-attached carbon fiber bundle according to any one of (1) to (9) above, wherein the viscosity of the sizing agent at 30°C is 500 mPa·s or more and 1,400 mPa·s or less. (11) A carbon fiber bundle with a sizing agent attached, as described in any of (1) to (10) above, having a coefficient of variation of thickness of 4.3% or less and a drape value of 6.5 cm or more and 7.5 cm or less. (12) A carbon fiber reinforced composite material using a carbon fiber bundle with a sizing agent attached as described in any of (1) to (11) above. (13) A pressure vessel using a carbon fiber bundle with a sizing agent attached as described in any of (1) to (12) above. (14) A process of flame-retardantizing, pre-carbonizing, and carbonizing a polyacrylonitrile carbon fiber precursor fiber bundle having 6,000 to 50,000 filaments, A method for producing carbon fiber bundles with a sizing agent attached, comprising the steps of: oxidizing the fiber bundles obtained in the carbonization step to obtain carbon fiber bundles; and immersing the carbon fiber bundles in a sizing agent-containing liquid to set the amount of sizing agent attached to 100% by mass of the carbon fiber bundles with the sizing agent attached to 0.6% by mass or more and 1.1% by mass or less, wherein the sizing agent has a proportion of cyclic oxygen-containing compound components of 50% by mass or less in the total amount of sizing agent, and a viscosity of 200 mPa·s or more and 2,000 mPa·s or less at 30°C, and is substantially untwisted. (15) A polyacrylonitrile carbon fiber precursor fiber bundle having a single fiber fineness of 0.58 dtex or more and 1.20 dtex or less, and a filament count of 18,000 or more and 42,000 or less, is subjected to flame retardation in two stages: a first flame retardation step and a second flame retardation step. In the second flame retardation step, the infrared spectrum at 1,370 cm⁻¹ is measured. -1 1,453 cm⁻¹ for peak intensity -1 The ratio of peak intensities is in the range of 0.70 to 0.75, and the infrared spectrum is 1,370 cm⁻¹. -1 1,254 cm⁻¹ for peak intensity -1The method for producing carbon fiber bundles with a sizing agent attached, as described in (14) above, is to make the fiber bundles flame-retardant until the ratio of the peak intensities is in the range of 0.50 to 0.65, and includes a pre-carbonization step in which the fiber bundles obtained in the second flame-retardant step are stretched to a ratio of 1.00 to 1.15 in an inert atmosphere with a maximum temperature of 600 to 800°C, and a carbonization step in which the fiber bundles obtained in the pre-carbonization step are carbonized in an inert atmosphere with a maximum temperature of 1000 to 1600°C at a heating rate of 0.4 to 1.1°C / second. (16) A method for producing a carbon fiber bundle with a sizing agent attached, wherein in the step of immersing the carbon fiber bundle in a sizing agent-containing liquid, the amount of sizing agent attached is 0.7% by mass or more and 1.0% by mass or less, and the sizing agent does not contain cyclic oxygen-containing compounds and has a viscosity of 500 mPa·s or more and 1,400 mPa·s or less at 30°C, as described in (15) or (15) above. [Effects of the Invention]

[0009] According to the present invention, the carbon fiber bundle has good morphological stability, abrasion resistance, and resin impregnation properties, making it possible to stably maintain a high tensile strength in the carbon fiber reinforced composite material. [Brief explanation of the drawing]

[0010] [Figure 1] Figure 1 shows how to set up a sample for drape value measurement. [Figure 2] Figure 2 shows the method for measuring drape values. [Modes for carrying out the invention]

[0011] The thickness of the carbon fiber bundle with sizing agent attached in this invention is 0.20 mm or less. The thickness of the carbon fiber bundle with sizing agent attached is evaluated by the method described in the Examples section. When impregnating with resin using the filament winding molding method, the resin impregnation depends on the thickness, so a thinner thickness allows for the efficient production of composite materials. If the thickness of the carbon fiber bundle with sizing agent attached is 0.20 mm or less, the quality when made into CFRP tends to be good. The thickness of the carbon fiber bundle with sizing agent attached is preferably 0.19 mm or less, and more preferably 0.18 mm or less. Thus, from the viewpoint of quality, the smaller the thickness of the carbon fiber bundle, the better. However, if the thickness is less than 0.10 mm, the thickness of the carbon fiber bundle with sizing agent attached becomes too small, and in the FW molding process, the fiber opening becomes uneven, gaps are created within the carbon fiber bundle, and the resin impregnation decreases, which can lead to a decrease in quality and mechanical properties when made into CFRP. Therefore, in this invention, the thickness of the carbon fiber bundle with sizing agent attached is controlled to be 0.10 mm or more. In other words, the thickness of the carbon fiber bundle with sizing agent attached is 0.10 mm or more and 0.20 mm or less, preferably 0.10 mm or more and 0.19 mm or less, and more preferably 0.10 mm or more and 0.18 mm or less. The thickness of the carbon fiber bundle with sizing agent attached can be controlled by the number of filaments in the carbon fiber precursor bundle, the properties of the sizing agent, and the amount of sizing agent attached. The thickness of the carbon fiber bundle with sizing agent attached in this invention can be evaluated by the method described in the Examples section.

[0012] The carbon fiber bundles with sizing agent attached according to the present invention have a coefficient of variation of thickness of 6.5% or less, preferably 6.0% or less, more preferably 5.5% or less, and even more preferably 4.3% or less. While there is no particular lower limit, it should be 0% or greater. A small coefficient of variation of the carbon fiber bundle thickness allows for the production of smooth CFRP, resulting in a greater effect in suppressing deformation of the component. If the coefficient of variation of the carbon fiber bundle thickness exceeds 6.5%, even with a small carbon fiber bundle thickness, the gaps between carbon fiber bundles when they are laminated increase, significantly reducing the smoothness of the CFRP. In other words, in addition to a small carbon fiber bundle thickness, it is important that the coefficient of variation of the carbon fiber bundle thickness is below a certain level. The coefficient of variation of the carbon fiber bundle thickness can be controlled not only by reducing the thickness of the carbon fiber bundle itself, but also by the surface temperature of the hot roller used to attach the sizing agent, the processing time, the properties of the sizing agent, and the amount of sizing agent attached. The thickness of the carbon fiber bundles with sizing agent attached according to the present invention can be evaluated by the method described in the Examples section.

[0013] The carbon fiber bundle with sizing agent attached according to the present invention has a drape value of 6 cm or more and 9 cm or less, preferably 6.5 cm or more at the lower limit, preferably 8.5 cm or less at the upper limit, and more preferably 7.5 cm or less. The drape value is an index representing the stiffness of the carbon fiber bundle; the larger the drape value, the stiffer the carbon fiber bundle. The drape value can be evaluated by the method described in the Examples section.

[0014] When the drape value of a carbon fiber bundle is less than 6 cm, there is often little or no crosslinking between the single filaments due to the sizing agent, and even if there is crosslinking, it is often weak. As a result, the fiber bundle is soft, making it prone to bending and twisting when the bundle is pulled from the bobbin and used as a guide, such as a comb, during the manufacturing of the composite material. When bending and twisting occur, those parts become difficult to open, resulting in uneven opening, which is undesirable. Also, when the drape value of a carbon fiber bundle is greater than 9 cm, the fiber bundle is stiff, which can easily cause fuzzing when it deforms in contact with a guide such as a comb, and can also result in poor resin impregnation. A drape value of 6 cm to 9 cm is preferable because it suppresses both uneven opening and fuzzing when in contact with a guide, and also allows for good resin impregnation.

[0015] The carbon fiber bundle of the present invention preferably has a resin incorporation ratio of 0.45 to 0.64. More preferably, the lower limit is 0.47 or higher, and more preferably, the upper limit is 0.60 or lower, and even more preferably 0.58 or lower. The resin incorporation ratio is an index representing the resin impregnation properties of the carbon fiber bundle during filament winding. A higher resin incorporation ratio indicates that the carbon fiber bundle exhibits better resin impregnation properties during filament winding. The resin incorporation ratio can be evaluated by the method described in the Examples section.

[0016] When the resin incorporation ratio of carbon fiber bundles is 0.45 or higher, the carbon fiber bundles tend to spread widely or uniformly, making it less likely for voids to be present in the pressure vessel when it is manufactured. Reducing the void content helps prevent a decrease in the strength of the pressure vessel. Furthermore, when the resin incorporation ratio of carbon fiber bundles is 0.64 or lower, it becomes less likely for localized areas with high resin content to be present between layers in the pressure vessel. Suppressing localized areas of high resin content helps prevent a decrease in the strength of the pressure vessel. In other words, a resin incorporation ratio of 0.45 or higher and 0.64 or lower is preferable because it suppresses both the occurrence of voids and areas of high resin content, resulting in good strength for the pressure vessel.

[0017] The sizing agent-attached carbon fiber bundle of the present invention has a filament count of 6,000 to 50,000. Preferably, it is 18,000 to 42,000, more preferably 24,000 to 36,000, and even more preferably 30,000 or more. The filament count refers to the number of single fibers contained in one sizing agent-attached carbon fiber bundle. If the filament count is 18,000 or more, the number of cracks occurring between the sizing agent-attached carbon fiber bundles when made into CFRP will be reduced. If the filament count is 50,000 or less, the thickness of the carbon fiber bundle can be kept small, and a CFRP with a small thickness can be obtained. The filament count can be arbitrarily determined during the manufacturing process of the polyacrylonitrile-based carbon fiber precursor fiber bundle.

[0018] The carbon fiber bundles with sizing agent attached according to the present invention have a strand tensile strength (hereinafter, the strand tensile strength with this resin formulation A may be referred to as "strand tensile strength A'") which is evaluated using the resin formulation A described below, which is a combination of resins described in JIS R7608 (2007), in accordance with the tensile test method for resin-impregnated strands described in JIS R7608 (2007), preferably 4.9 GPa or more and 6.7 GPa or less, more preferably 5.6 GPa or more and 6.7 GPa or less, even more preferably 5.9 GPa or more and 6.6 GPa or less, and most preferably 6.3 GPa or more and 6.6 GPa or less.

[0019] Resin formulation A: Contains 97% by mass or more of (3',4'-epoxycyclohexane)methyl-3,4-epoxycyclohexyl carboxylate, with an epoxy equivalent of 130 g / eq and a viscosity of 240 mPa·s at 25°C. The composition is 100 / 3 / 4 (parts by mass) of epoxy resin / boron trifluoride monoethylamine / acetone, and is cured under normal pressure and temperature of 125°C in an oven for 30 minutes.

[0020] Furthermore, for the epoxy resin used in resin formulation A (hereinafter referred to as resin composition A), which contains 97% by mass or more of (3',4'-epoxycyclohexane)methyl-3,4-epoxycyclohexylcarboxylate, and has an epoxy equivalent of 130 g / eq and a viscosity of 240 mPa·s at 25°C, "Celoxide (registered trademark)" 2021P may be used.

[0021] Strand tensile strength A' is an index that indicates the resistance of carbon fibers to fracture when a load is applied, and represents the strength of the carbon fiber bundle itself. If the strand tensile strength A' is 4.9 GPa or higher, it is easier to increase the strength when it is made into CFRP. The strand tensile strength A' can be controlled by manufacturing according to the carbon fiber manufacturing method described later.

[0022] The carbon fiber bundles with sizing agent attached according to the present invention preferably have a strand tensile strength (hereinafter, the strand tensile strength in this resin formulation B may be referred to as "strand tensile strength B'") of 5.3 GPa or more and 6.4 GPa or less, as evaluated according to the tensile test for resin-impregnated strands described in JIS R7608 (2007) using the resin formulation B described below, and more preferably 5.6 GPa or more and 6.3 GPa or less. In particular, carbon fiber bundles having a strand tensile strength A' of 5.6 GPa or more and a strand tensile strength B' within the above range are preferably used. Another preferred embodiment is a carbon fiber bundle having a strand tensile strength A' of 6.3 GPa or more and 6.7 GPa or less and a strand tensile strength B' of 6.0 GPa or more and 6.4 GPa or less.

[0023] Resin formulation B: The main component is 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, and 2,2'-dimethyl-4,4'-methylenebis(cyclohexylamine) is used as the curing agent, with a ratio of 100 / 35 (parts by mass) of the main component to the curing agent. The curing conditions are as follows: curing in an oven set to atmospheric pressure and temperature 80°C for 120 minutes, followed by curing in an oven set to temperature 110°C for 240 minutes.

[0024] Furthermore, “Araldite®” LY1564 SP CI may be used as the main agent in resin formulation B. Also, “Baxxodur®” EC331 may be used as the curing agent.

[0025] The resin composition used in resin formulation B (hereinafter referred to as resin composition B) is a practical resin for filament winding using the wet method. It has a viscosity suitable for preparing strand test specimens using the impregnation method described in JIS R7608 (2007). Furthermore, resin composition B has more functional groups than resin composition A, which is a combination of resins described in JIS R7608 (2007), and exhibits higher adhesion to carbon fiber bundles. Therefore, the strand tensile strength B' can be used as an index to model the strength of a given carbon fiber bundle when it is made into CFRP. If the strand tensile strength B' is 5.3 GPa or higher, it is easier to increase the strength when actually making CFRP. Also, if the strand tensile strength B' is 6.0 GPa or higher, it is often sufficient to produce CFRP that is suitable for practical use.

[0026] The strand tensile strength B' can be improved by increasing the strand tensile strength A' of the carbon fiber bundle according to the carbon fiber manufacturing method described later, and by adjusting the properties of the sizing agent present between the carbon fiber and the resin, thereby enabling highly efficient control of the physical properties.

[0027] The single fiber diameter of the carbon fiber bundle with the sizing agent attached according to the present invention is preferably 5.2 μm or more. The single fiber diameter of the carbon fiber bundle can be evaluated by the method described in the Examples section.

[0028] When manufacturing prepregs, the impregnation of the matrix resin into the carbon fiber bundle depends on the diameter of the individual fibers. Therefore, a larger individual fiber diameter improves resin impregnation and allows for the efficient production of composite materials. Furthermore, since the breaking load per fiber is determined by the strand tensile strength and the cross-sectional area of ​​the individual fibers, the individual fiber diameter affects the breaking load per fiber. In addition, a larger individual fiber diameter tends to reduce fuzzing due to friction during the process, thus affecting the quality.

[0029] If the single fiber diameter is 5.2 μm or larger, the quality tends to be good when producing carbon fiber bundles or CFRP. Preferably, the single fiber diameter is 6.3 μm or larger, and more preferably 6.6 μm or larger. If the single fiber diameter is too large, the reaction within the single fiber may become non-uniform during the firing process, which can reduce the strand tensile strength and strand tensile modulus. Therefore, it should be controlled to be 7.3 μm or less. In other words, the single fiber diameter is preferably 5.2 μm or larger and 7.3 μm or smaller, more preferably 6.3 μm or larger and 7.2 μm or smaller, and even more preferably 6.6 μm or larger and 7.1 μm or smaller.

[0030] The diameter of a single fiber can be controlled by factors such as the amount of carbon fiber precursor fiber bundle extruded from the spinneret during spinning and the draw ratio during each process.

[0031] In the carbon fiber bundle with sizing agent attached in this invention, if the strand tensile modulus, evaluated according to JIS R7608 (2007) using resin formulation A, is too high, it becomes difficult to improve the strand tensile strength using the same resin formulation A. Therefore, it is preferable to set the strand tensile modulus to 230 GPa or more and 290 GPa or less, more preferably 235 GPa or more and 280 GPa or less, and even more preferably 240 GPa or more and 275 GPa or less. This strand tensile modulus can be controlled by adjusting the firing conditions according to the carbon fiber manufacturing method described later. Hereafter, unless otherwise specified, strand tensile strength and strand tensile modulus refer to the properties obtained by evaluation according to JIS R7608 (2007) using resin formulation A.

[0032] The sizing agent attached to the carbon fiber bundle with the sizing agent attached in this invention has a viscosity of 200 mPa·s or more and 2,000 mPa·s or less at 30°C. Viscosity in this invention refers to the value obtained by measurement using a B-type viscometer. Details of the conditions can be found in the Examples section. By having a viscosity of 200 mPa·s or more and 2,000 mPa·s or less at 30°C of the sizing agent, good resin impregnation properties can be imparted to the carbon fiber bundle. The viscosity of the sizing agent at 30°C is preferably 500 mPa·s or more and 1,400 mPa·s or less, more preferably 600 mPa·s or more and 1,300 mPa·s or less, and even more preferably 800 mPa·s or more as the lower limit. If the viscosity of the sizing agent at 30°C is 500 mPa·s or more, sufficient bundling properties can be imparted to the carbon fiber bundle with the sizing agent attached, and the thickness and coefficient of variation of the thickness of the carbon fiber bundle with the sizing agent attached can be reduced. If the viscosity of the sizing agent at 30°C becomes too high, the resin impregnation into the carbon fiber bundles to which the sizing agent is attached may become uneven during the FW molding process, potentially degrading the quality of the CFRP. Therefore, it is preferable to control the viscosity to 2000 mPa·s or less. The viscosity of the sizing agent can be measured using a B-type viscometer if the sizing agent is available. Alternatively, the sizing agent can be extracted from the carbon fiber bundles to which it is attached using a solvent, and in some cases, separated using a known method, after which it can be measured with a B-type viscometer. If the sizing agent is a mixture, its viscosity (η) is calculated additively by taking the respective viscosities (ηa, ηb) of compound a and compound b according to their respective mass ratios (Ra, Rb).

[0033] In the present invention, the sizing agent attached to the carbon fiber bundle is preferably such that the total amount of cyclic oxygen-containing compounds in 100% by mass of the total sizing agent is 50% by mass or less. There are no particular restrictions on the above cyclic oxygen-containing compounds, but compounds containing a cyclic oxygen-containing structure that can react with carboxyl groups or hydroxyl groups include epoxy group-containing compounds and oxazoline group-containing compounds. Sizing agents containing the above cyclic oxygen-containing compounds tend to have a high intermolecular crosslink density due to the reaction of the compounds, which increases viscosity, reduces the impregnation of the resin, and the layer formed by the sizing agent becomes brittle, leading to a decrease in the tensile strength of the CFRP. The total amount of compounds containing a cyclic oxygen-containing structure in 100% by mass of the total sizing agent is preferably 30% by mass or less, and it is more preferable that the sizing agent does not contain any compounds containing a cyclic oxygen-containing structure.

[0034] The proportion and presence or absence of cyclic oxygen-containing compounds in 100% by mass of the total sizing agent can be determined from the structural formula of the compound if the structural formula and composition of the sizing agent are known. If they are unknown, the sizing agent can be extracted from a carbon fiber bundle containing the sizing agent, and its structure can be identified by known methods such as proton NMR, carbon NMR, mass spectrometry, and TOF-SIMS.

[0035] In the present invention, the amount of sizing agent attached to the carbon fiber bundle is preferably in the range of 0.6% by mass or more and 1.1% by mass or less per 100% by mass of the carbon fiber bundle attached with the sizing agent. The amount of sizing agent attached in the present invention is determined by taking 2.0 ± 0.5 g of the carbon fiber bundle attached with the sizing agent and measuring the change in mass before and after heat treatment at 450°C in a nitrogen atmosphere for 15 minutes, and dividing the change in mass by the mass before heat treatment (by mass%). When the sizing agent content is 0.6% by mass or more, sufficient bundling properties can be imparted to the carbon fiber bundle attached with the sizing agent, the accumulation of fuzz can be suppressed and the thickness of the carbon fiber bundle attached with the sizing agent can be reduced, and the coefficient of variation of thickness can also be reduced. When the sizing agent content is 1.1% by mass or less, the resin impregnation during higher-order processing is excellent and the development of strand tensile strength is stable. If the amount of sizing agent attached is too high, the resin impregnation properties decrease and the development of strand tensile strength becomes unstable, and as a result the carbon fiber bundle attached with the sizing agent may have poor quality stability. Therefore, the sizing agent content is preferably 0.7% by mass or more and 1.0% by mass or less, and more preferably 0.8% by mass or more and 0.9% by mass or less. The amount of sizing agent attached to the carbon fiber bundle with the sizing agent attached in the present invention can be evaluated by the method described in the Examples section.

[0036] In the present invention, it is preferable to process the carbon fiber bundles impregnated with a sizing agent using a hot roller with a surface temperature of 100°C to 180°C. If the surface temperature of the hot roller is 100°C or higher, the sizing agent applied to the carbon fiber bundles will dry sufficiently, and as a result, the thickness of the carbon fiber bundles will not increase easily. If the surface temperature of the hot roller is 180°C or lower, the sizing agent applied to the carbon fiber bundles will not decompose and the bundle-forming ability will not deteriorate easily, the quality of the carbon fiber bundles themselves can be maintained, and the occurrence of fuzzy wrapping on the rollers during carbon fiber bundle manufacturing will not increase, so the number of untangled fuzzy fibers will not increase and the coefficient of variation of the thickness of the carbon fiber bundles will not increase easily. It is preferable to process the carbon fiber bundles using a hot roller surface temperature corresponding to the viscosity of the sizing agent. If the viscosity of the sizing agent at 30°C is 500 mPa·s or higher and 2000 mPa·s or lower, it is preferable to process them using a hot roller with a temperature of 150°C to 170°C. Because sizing agents tend to flow easily and unevenly, leading to cracking after drying, it is preferable to fix the sizing agent before it becomes uneven. A temperature of 150°C or higher is preferable because it can sufficiently suppress unevenness, and a temperature of 170°C or lower is preferable because it can suppress the decomposition of low-viscosity sizing agents. If the viscosity of the sizing agent at 30°C exceeds 2000 mPa·s, it is preferable to process it with a hot roller at a temperature of 120°C or higher and below 150°C. It is preferable to dry at a low temperature to allow time for penetration into the interior, as this facilitates film formation during drying. A temperature of 120°C or higher is preferable because it can sufficiently fix the sizing agent before it becomes thick, and a temperature of 150°C or lower is preferable because it can sufficiently penetrate into the interior.

[0037] The carbon fiber bundles with sizing agent attached according to the present invention are preferably substantially untwisted. A substantially untwisted carbon fiber bundle means either no twist at all, or, if twisted, 0.5 turns or less per meter. When carbon fiber bundles are untwisted, they exhibit excellent spreading properties when used as reinforcing fibers for CFRP, often resulting in superior physical properties and quality of the CFRP.

[0038] Next, the method for producing carbon fiber bundles with sizing agent attached according to the present invention will be described.

[0039] In the production of carbon fiber bundles, a polyacrylonitrile-based precursor fiber bundle is first obtained. It is preferable to use a polyacrylonitrile copolymer as the raw material for producing the polyacrylonitrile-based precursor fiber bundle. In this invention, a polyacrylonitrile copolymer refers to one in which acrylonitrile is the main component of the copolymer. The main component usually refers to a component that accounts for 90% to 100% by mass of the polymer. As monomers that can be used as copolymer components other than the main component, monomers containing one or more carboxylic acid groups or amide groups are preferably used from the viewpoint of promoting flame resistance. For example, monomers containing carboxylic acid groups include acrylic acid, methacrylic acid, itaconic acid and their alkali metal salts, and ammonium salts. A monomer containing an amide group is acrylamide.

[0040] In the production of polyacrylonitrile precursor fiber bundles, the method for producing the polyacrylonitrile copolymer can be selected from known polymerization methods.

[0041] In producing polyacrylonitrile precursor fiber bundles, either a wet-dry spinning method or a wet spinning method may be used for spinning, but it is preferable to use a wet-dry spinning method which is advantageous for the strand tensile strength of the resulting carbon fiber bundles. The spinning process consists of a spinning step in which a spinning solution is discharged from a spinneret into a coagulation bath and spun; a washing step in which the fibers obtained in the spinning step are washed in a water bath; a water bath stretching step in which the fiber bundles obtained in the washing step are stretched in a water bath; and a drying heat treatment step in which the fiber bundles obtained in the water bath stretching step are dried and heat treated. If necessary, a steam stretching step is included in which the fiber bundles obtained in the drying heat treatment step are steam stretched. The order of each step can be changed as appropriate.

[0042] The spinning solution is prepared by dissolving the polyacrylonitrile copolymer described above in an organic solvent such as dimethyl sulfoxide, dimethylformamide, or dimethylacetamide, or in an aqueous solution such as nitric acid, zinc chloride, or sodium rhodane, in which the polyacrylonitrile copolymer is soluble.

[0043] The above-mentioned coagulation bath preferably contains a solvent such as dimethyl sulfoxide, dimethylformamide, or dimethylacetamide, which was used as the solvent for the spinning stock solution, and a coagulation-promoting component. As the coagulation-promoting component, a component that does not dissolve the above-mentioned polyacrylonitrile copolymer and is compatible with the solvent used in the spinning solution can be used. Specifically, it is preferable to use water as the coagulation-promoting component.

[0044] In the washing process described above, it is preferable to use a washing bath consisting of multiple stages with a temperature of 30°C to 98°C. Furthermore, the stretching ratio in the water bath stretching process is preferably between 2 and 6 times.

[0045] After the water bath stretching process, it is preferable to apply an oil made of silicone or the like to the fiber bundle to prevent the individual fibers from fusing together. It is preferable to use a modified silicone oil, and more preferably one that contains highly heat-resistant amino-modified silicone.

[0046] The drying heat treatment process can utilize known methods. For example, a drying temperature of 100-200°C is exemplified.

[0047] After the above-described water washing, water bath stretching, oil application, and drying heat treatment steps, a polyacrylonitrile-based precursor fiber bundle suitable for obtaining the carbon fiber bundle of the present invention can be obtained by steam stretching as needed. Steam stretching is preferably performed in pressurized steam with a stretching ratio of 2 to 6 times.

[0048] The fineness of a single fiber of the polyacrylonitrile-based precursor fiber bundle is preferably 0.58 dtex or more and 1.20 dtex or less from the viewpoint of increasing the strand tensile strength of the carbon fiber bundle. Such a single fiber fineness is preferably 0.70 dtex or more and 1.15 dtex or less, and more preferably 1.05 dtex or more and 1.10 dtex or less.

[0049] In the method for producing a carbon fiber bundle, a polyacrylonitrile-based precursor fiber bundle is subjected to a flameproofing step, a pre-carbonization step, and a carbonization step to obtain a carbon fiber bundle.

[0050] In the present invention, the flameproofing step means heat-treating a polyacrylonitrile-based precursor fiber bundle at 200°C or more and 300°C or less in an atmosphere containing oxygen.

[0051] The flameproofing of the polyacrylonitrile-based precursor fiber is preferably controlled such that the density of the flameproofed fiber bundle falls within the range of 1.25 g / cm 3 or more and 1.35 g / cm 3 or less. Specifically, the flameproofing temperature is preferably carried out in air at 200°C or more and 300°C or less, more preferably in air at 210°C or more and 285°C or less, and even more preferably in air at 230°C or more and 280°C or less. The preferred ranges of these flameproofing treatment times and flameproofing temperatures vary depending on the properties of the polyacrylonitrile-based precursor fiber bundle and the copolymer composition of the polyacrylonitrile copolymer. The densities of the flameproofed fiber bundle and the carbon fiber bundle are evaluated by the method described in the Examples section.

[0052] In order to increase the strand tensile strength of the carbon fiber, in the flameproofing step, the obtained flameproofed fiber has a ratio of the peak intensity at 1,453 cm[[ID=二十一]] -1 to the peak intensity at 1,370 cm -1 in the infrared spectrum within the range of 0.60 or more and 0.75 or less, and a ratio of the peak intensity at 1,254 cm -1 to the peak intensity at 1,370 cm -1It is preferable to control the ratio of the peak intensities so that it is in the range of 0.50 to 0.65. In the infrared spectrum, 1,453 cm⁻¹ -1 The peak is of alkene origin and decreases as flame resistance progresses. 1,370 cm -1 The peak is 1,254 cm. -1 The peak is due to the flame-retardant structure and increases as flame resistance progresses. Note that a standard flame-retardant fiber with a specific gravity of 1.35 has a specific gravity of 1,370 cm². -1 1,453 cm⁻¹ for peak intensity -1 The ratio of peak intensities is approximately 0.63 to 0.69, and in the flame-retardant process of the present invention, it is preferable that the resulting flame-retardant fiber contains a larger amount of alkene-derived structure. 1,370 cm -1 1,254 cm⁻¹ for peak intensity -1 The ratio of peak intensities decreases as flame resistance progresses, with a particularly large decrease in the initial stages. However, depending on the flame resistance conditions, the peak intensity ratio may not fall below 0.65 even with increased time, so it is advisable to adjust the conditions.

[0053] To achieve a balance between these two peak intensity ratios within the desired range, the basic principle is to set the conditions by focusing on the following: a low amount of copolymer components in the polyacrylonitrile polymer constituting the precursor fiber, a high degree of crystal orientation of the precursor fiber, a low single-fiber fineness of the precursor fiber, and a high flame-retardant temperature in the later stages. Specifically, flame-retardant treatment is performed in two stages: a first flame-retardant treatment and a second flame-retardant treatment. In the second flame-retardant treatment, the fiber obtained in the first flame-retardant treatment is subjected to infrared spectroscopy at 1,370 cm⁻¹. -1 1,453 cm⁻¹ for peak intensity -1 The ratio of peak intensities is in the range of 0.60 to 0.75, and the infrared spectrum is 1,370 cm⁻¹. -1 1,254 cm⁻¹ for peak intensity -1 The flame-retardant treatment is performed until the peak intensity ratio falls within the range of 0.50 to 0.65. In addition, in the first flame-retardant treatment process, the infrared spectrum at 1,370 cm⁻¹ is measured. -1 1,453 cm⁻¹ for peak intensity -1It is preferable to make the material flame-resistant until the ratio of the peak intensities is in the range of 0.98 to 1.10.

[0054] In the first flame-retardant treatment step, the flame-retardant temperature is preferably 200°C to 250°C, more preferably 230°C to 250°C, in order to control the infrared spectrum within the range described above.

[0055] The flame-retardant temperature in the second flame-retardant process is usually higher than that of the first flame-retardant process. To shorten the flame-retardant time in the second flame-retardant process, the flame-retardant temperature should be adjusted to be higher, but the appropriate flame-retardant temperature depends on the properties of the precursor fiber. It is preferable to set the flame-retardant temperature to 265°C to 295°C, more preferably 265°C to 285°C, and even more preferably 270°C to 275°C in order to control the infrared spectrum within the range described above. The flame-retardant temperature does not need to be constant, and a multi-stage temperature setting is also acceptable. To increase the strand tensile strength of the resulting carbon fiber, it is preferable to set a high flame-retardant temperature and a short flame-retardant time.

[0056] In this invention, flame-retardant treatment refers to heat treatment of the precursor fiber in an oxygen-containing atmosphere at a temperature of 200°C to 310°C.

[0057] The flame-retardant time referred to here means the time the fibers remain in the flame-retardant furnace. Flame-retardant fibers refer to fibers after the flame-retardant process but before the pre-carbonization process. The peak intensity referred to here is the absorbance at each wavelength after baseline correction of the spectrum obtained by measuring the infrared spectrum of a small sample of flame-retardant fibers; no peak splitting is performed. The sample concentration for infrared spectrum measurement is diluted with potassium bromide (KBr) to 0.67 mass%. In this way, the infrared spectrum should be measured each time the flame-retardant conditions are changed, and the conditions should be investigated according to the preferred manufacturing method described later. By appropriately controlling the infrared spectral peak intensity ratio of the flame-retardant fibers, the strand tensile strength of the resulting carbon fibers can be controlled.

[0058] In the pre-carbonization step, which pre-carbonizes the fiber bundles obtained in the flame-retardant step, the obtained flame-retardant fibers are typically heat-treated in an inert atmosphere at a maximum temperature of 500°C to 1,000°C until the specific gravity is preferably 1.5 to 1.8. If the maximum temperature of the pre-carbonization step is 500°C or higher, the pre-carbonized fiber bundles can be carbonized in the subsequent carbonization step without breaking due to thermal decomposition. There is no particular upper limit to the maximum temperature of the pre-carbonization step, but it is preferable to be 1,000°C or lower in order to keep it below the carbonization temperature in the subsequent carbonization step. The stretch ratio in the pre-carbonization step is preferably 1.00 to 1.15. More preferably 1.02 to 1.13. If the stretch ratio in the pre-carbonization step is 1.00 or higher, the strand tensile modulus tends to increase, and the strand tensile strength tends to increase. If the stretch ratio in the pre-carbonization step is 1.15 or lower, the strand tensile modulus tends to be suppressed to 280 GPa or lower.

[0059] In the carbonization process of pre-carbonized fibers, it is preferable to carbonize in an inert atmosphere at a maximum temperature of 1,000°C to 1,600°C. More preferably, the maximum temperature is 1,250°C to 1,550°C, and even more preferably 1,300°C to 1,500°C. From the viewpoint of increasing the strength of the resulting carbon fibers, a lower maximum temperature in the carbonization process is preferable, but if it is too low, the strand tensile strength may decrease, so it is preferable to set the temperature taking both into consideration. A carbonization temperature of 1,000°C is sufficient to allow carbonization to proceed sufficiently and increase the crystallite size, while a temperature of 1,600°C or lower is sufficient to maintain a balance between the strand tensile strength and the strand tensile modulus of the carbon fiber bundle.

[0060] The heating rate in the carbonization process is preferably 0.2°C / second or more and 1.1°C / second or less, more preferably 0.3°C / second or more and 1.0°C / second or less, and even more preferably 0.4°C / second or more and 0.6°C / second or less. The heating rate in the carbonization process affects the decomposition gas desorption rate and therefore affects the strand tensile strength. In the present invention, the heating rate is defined as the average temperature increase per second when a fiber passes through a region where the temperature of a section exceeds 1,000°C, as the fiber passes through a plurality of sections in the carbonization furnace, where the temperature is controlled to increase in stages. Specifically, for example, if a fiber passes through a section with a temperature of 1,000°C in the carbonization furnace and then through to the next section with a temperature of 1,100°C in 100 seconds, the heating rate is 1.0°C / second. In another example, if the fibers pass through the carbonization furnace from the 950°C section to the next 1,150°C section in 200 seconds, the heating rate is 1.0°C / second. Furthermore, if the maximum temperature of the carbonization process is set below 1,100°C, the heating rate up to the maximum temperature is used. That is, if the maximum temperature is 1,050°C, and the fibers pass through the carbonization furnace from the 1,000°C section to the next 1,050°C section in 50 seconds, the heating rate is 1.0°C / second. The temperature of the first section in the carbonization furnace is preferably 1,000°C or lower. A heating rate of 0.2°C / second or higher makes it easier to obtain a stable strand tensile modulus. A heating rate of 1.1°C / second or less makes it easier to suppress the decrease in strand tensile strength.

[0061] The carbon fiber bundle obtained as described above is preferably subjected to an oxidation treatment on its surface. For example, oxygen-containing functional groups can be introduced to the surface of the carbon fibers by electrolytic surface treatment. For the electrolytic surface treatment of the present invention, gas-phase oxidation, liquid-phase oxidation, and liquid-phase electrolytic oxidation can be used, but liquid-phase electrolytic oxidation is preferably used from the viewpoint of high productivity and uniform treatment. In the present invention, there are no particular restrictions on the method of liquid-phase electrolytic oxidation, and any known method may be used.

[0062] In the present invention, examples of electrolytes used in liquid-phase electrolytic oxidation include acidic electrolytes 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; or salts such as ammonium sulfate and ammonium bisulfate.

[0063] 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.

[0064] Next, the means for applying the sizing agent constituting the present invention to the carbon fiber bundle will be described. In the present invention, it is preferable to use the sizing agent as a sizing agent-containing solution obtained by diluting the sizing agent component with a solvent. Examples of solvents include water, methanol, ethanol, isopropanol, acetone, methyl ethyl ketone, dimethylformamide, and dimethylacetamide, but among these, an aqueous solution or an aqueous dispersion emulsified with a surfactant is preferably used because it is easy to handle and advantageous from the viewpoint of safety.

[0065] Methods for applying the sizing agent to the carbon fiber bundle include, for example, immersing the carbon fibers in a sizing agent-containing liquid via a roller (immersion method), bringing the carbon fibers into contact with a roller to which the sizing agent-containing liquid has adhered, and spraying the sizing agent-containing liquid onto the carbon fibers in a mist form. However, in manufacturing the sizing agent-containing carbon fiber bundle of the present invention, the immersion method is preferably used.

[0066] Furthermore, the sizing agent can be applied in either a batch or continuous manner, but a continuous method is preferred because it offers better productivity and reduces variability. In this case, it is preferable to control the concentration of the sizing agent solution, temperature, and yarn tension so that the sizing agent active ingredient is uniformly applied to the carbon fibers within an appropriate range. It is also preferable to vibrate the carbon fibers with ultrasound when applying the sizing agent.

[0067] Furthermore, in the present invention, it is preferable to apply the sizing agent to the carbon fiber bundle and then pass it through a heated roller as a pre-drying step. The heating temperature in the pre-drying step is preferably 125°C or higher, which is the temperature at which water is sufficiently dried, and the drying time should be as short as possible. If this step is considered the first drying step, a second drying step may be added, which involves further heat treatment. In this step, it is preferable to perform the heat treatment at a temperature range of 100 to 260°C for 10 to 600 seconds. In addition, the heat treatment in the second drying step can also be performed by microwave irradiation and / or infrared irradiation.

[0068] By using the sizing agent-adhered carbon fiber bundles of the present invention as described above, the quality and grade of carbon fiber reinforced composite materials can be improved when combined with a matrix resin. In particular, the quality and grade of pressure vessels manufactured using methods such as filament winding molding, where carbon fiber bundles are wound at high speed and the accumulation of fuzz during the process tends to be a problem, can be improved. The carbon fiber reinforced composite material described above preferably contains sizing agent-adhered carbon fiber bundles and a matrix resin, and may contain other components as needed. Specifically, it can be manufactured by impregnating sizing agent-adhered carbon fiber bundles with a matrix resin, or by curing the same, and the same applies to the pressure vessels described above.

[0069] The upper and lower limits of the numerical ranges described above can be combined in any way. Furthermore, the present invention will be specifically described below with reference to examples. [Examples]

[0070] The methods for measuring various physical properties used in this invention are as follows.

[0071] <Strand tensile test of carbon fiber bundles> <Tensile strength and tensile modulus of carbon fiber bundles> The strand tensile strength and strand tensile modulus of carbon fiber bundles were determined according to the resin-impregnated strand strength test method of JIS-R-7608 (2007), using each resin formulation (strand tensile strength was determined using resin formulation A, resin formulation B, and strand tensile modulus was determined using resin formulation A). The resin compositions A and B used in each resin formulation are described below. The curing conditions performed using resin composition A or resin composition B are as shown for each resin formulation A and B described above. Ten strands of carbon fiber bundles were measured, and their arithmetic mean values ​​were taken as the strand tensile strength and strand tensile modulus. The strand tensile modulus was measured within a strain range of 0.1 to 0.6%.

[0072] [Resin Composition A] The compounds (i) to (iii) were used to form resin composition A in the following proportions. (i) (3',4'-Epoxycyclohexane)methyl-3,4-epoxycyclohexylcarboxylate (100 parts by mass) (ii) Boron trifluoride monoethylamine (3 parts by mass) (iii) Acetone (4 parts by mass).

[0073] As the compound in (i), Celoxide P2021P (manufactured by Daicel Corporation) was used.

[0074] [Resin Composition B] The compounds (i) to (iii) were used to create resin composition B in the following proportions. (i) 2,2'-[(1-methylethylidene)bis(4,1-phenyleneoxymethylene)]bisoxiran (80 parts by mass) (ii) 1,4-bis(2,3-epoxypropoxy)butane (20 parts by mass) (iii) 2,2'-dimethyl-4,4'-methylenebis(cyclohexylamine) (35 parts by mass) "Araldite®" LY1564 SP CI was used as the mixture of (i) and (ii), and "Baxxodur®" EC331 (35 parts by mass) was used as the compound of (iii).

[0075] <Resin Incorporation Ratio> The gap between the blade and roller (100 mm in diameter) of the filament winder's kiss roller is set to 0.13 mm. A resin with a viscosity of 450 mPa·s is placed in the kiss roller bath. The carbon fiber bundle is fed out at a speed of 60 m / min and the creel tension is set to 10 N. The carbon fiber bundle is run using the filament winder, passes through the kiss roller bath to coat it with resin, and then 20 m of the carbon fiber bundle is wound onto a weighed paper tube. After winding, the amount of resin incorporated into the carbon fiber bundle (hereinafter referred to as "resin incorporation amount") is weighed. The weight of the carbon fiber bundle before resin impregnation of 20 m is calculated from the basis weight (g / m) of the carbon fiber bundle, and the resin incorporation ratio is calculated using the following formula. The basis weight (g / m) of the carbon fiber bundle is measured by weighing a carbon fiber bundle cut to 1 m. Three samples are used for measurement, and the arithmetic mean is used. Resin incorporation ratio = Amount of resin incorporated (g) / Weight of 20m carbon fiber bundle before resin impregnation (g) <Single fiber diameter of carbon fiber bundle> For a carbon fiber bundle consisting of numerous carbon filaments to be measured, the mass A per unit length is f (g / m³) and density B f (g / cm 3 The density is determined by the density measurement method described below. The number of filaments in the carbon fiber bundle to be measured is C f Assuming the cross-section of the carbon fiber is a perfect circle, the single fiber diameter (μm) of the carbon fiber is calculated using the following formula. Single fiber diameter of carbon fiber (μm) = ((A f / B f / C f ) / π) (1 / 2) ×2×10 3 .

[0076] <Density measurement of flame-resistant fiber bundles and carbon fiber bundles> Density B of flame-resistant yarn fiber bundles and carbon fiber bundles f (g / cm 3 The specific gravity (%) is calculated using the Archimedes method with o-dichlorobenzene as the specific gravity solution. The measurement is performed with 3 samples, and the arithmetic mean is used.

[0077] <Method for measuring viscosity> The compounds were placed in plastic bottles and measured using a Type B viscometer (Tokyo Keiki Seisakusho, Model: BL) while the bottles were heated to 30°C in a water bath. For compounds with a viscosity lower than 10,000 mPa·s, 160cc plastic bottles were used; for compounds with a viscosity of 10,000 mPa·s or higher, 100cc bottles were used. For the Type B viscometer, the rotor and rotation speed were set as follows: No. 1, 30 rpm for compounds predicted to have a viscosity less than 1,000 mPa·s; No. 3, 30 rpm for compounds predicted to have a viscosity between 1,000 mPa·s and 10,000 mPa·s; and No. 4, 6 rpm for compounds predicted to have a viscosity of 10,000 mPa·s or higher.

[0078] <Amount of sizing agent applied> A 2.0±0.5g carbon fiber bundle coated with sizing agent is taken, weighed (W1) (read to the fourth decimal place), and then heated in an electric furnace (capacity 120cm³) set to 450°C in a nitrogen stream of 50 ml / min. 3 The sizing agent was left in the sizing solution for 15 minutes to completely decompose. Then, the carbon fiber bundle was transferred to a container with a 20 liters / min dry nitrogen stream and cooled for 15 minutes. The carbon fiber bundle was then weighed (W2) (read to the fourth decimal place), and the weight loss due to heating was calculated using W1-W2. This weight loss due to heating was converted to a mass percentage (rounded to the third decimal place) with the carbon fiber bundle coated with the sizing agent set to 100 mass%, and this value was taken as the amount of sizing agent attached (mass%). The measurement was performed twice, and the arithmetic mean was taken as the amount of sizing agent attached.

[0079] <Thickness of carbon fiber bundles with sizing agent attached and its coefficient of variation> A sizing agent-coated carbon fiber bundle is positioned so that the horizontal direction is the longer side, and the dimensions are 0.9 × 10 -4 The carbon fiber bundles are aligned by applying a load of mN / dtex, and the thickness is measured using a micrometer along a line segment that passes through the center of the circumscribed circle in the cross-section of the sizing agent-attached carbon fiber bundle and is perpendicular to the long side within that cross-section. The thickness of the above line segment is measured at 50 points at 0.2m intervals in the long side direction, and the arithmetic mean of these 50 points is used as the thickness of the carbon fiber bundle with the sizing agent attached, and the standard deviation of these 50 points is used as the standard deviation of the thickness of the carbon fiber bundle with the sizing agent attached. The coefficient of variation of the thickness of the carbon fiber bundle with the sizing agent attached is the ratio of the thickness of the carbon fiber bundle with the sizing agent attached and the standard deviation of the thickness, expressed as a percentage ([standard deviation] / [arithmetic mean]×100).

[0080] <Measurement of drape value> As shown in Figure 1, a 50cm carbon fiber bundle 2 was suspended vertically from a fixing bar 1, and a weight 3 was attached with a load of 0.0375 [g / tex] in an atmosphere of 25°C. The bundle was left for more than 30 minutes to release the twist. A 30cm length of the carbon fiber bundle was sampled from the longitudinal center, extending 15cm above and below. As shown in Figure 2, the carbon fiber bundle 2 was placed on a rectangular horizontal stand 4, supporting it to prevent it from bending, so that 25cm of the carbon fiber bundle extended beyond the edge of the stand. The carbon fiber bundle 2 on the horizontal stand 4 was then secured with tape. After that, the support for the carbon fiber bundle 2 extending beyond the horizontal stand 4 was removed, allowing it to hang down. The horizontal distance L from the fulcrum was measured after 1 second. The measurement was performed once for each of the five carbon fiber bundles 2, for a total of five measurements, and the arithmetic mean was taken as the drape value.

[0081] <Process passability of carbon fiber bundles> Two stainless steel metal bars, each 50 mm in diameter and with a surface roughness Rmax of 0.3 μm, are placed horizontally at a 150 mm interval, with a height difference between them, so that when the carbon fiber bundle is viewed from the side, it passes through the bars while in contact with them at an angle of 0.3925π(rad) ± 0.04π(rad) or -0.3925π(rad) ± 0.04π(rad) with respect to the horizontal plane of each metal bar. The carbon fiber bundle is then passed from the higher metal bar towards the lower metal bar. When a carbon fiber bundle with 24,000 filaments is stretched across the metal bars, the unwinding tension from the package is set to 1600 gf, and the metal bars are pulled at 4 m / min by a drive roll for 1 minute. The number of fluffs generated on the carbon fiber bundle during the 1 minute after passing through the second metal bar is counted, and the number of fluffs in the process is calculated using the following formula as an indicator of process passability.

[0082] Number of fibers per meter in a carbon fiber bundle = Number of fibers counted / 4 (meters) S: Process fiber count is 1.0 fibers / m or less A: The number of fibers in the process exceeds 1.0 fibers / m and is 1.5 fibers / m or less. B: The number of fibers in the process exceeds 1.5 fibers / m and is 5.0 fibers / m or less. C: Process lint count exceeds 5.0 pieces / m Furthermore, when a carbon fiber bundle with 36,000 filaments was stretched across a metal bar, the release tension was set to 2,400 gf, and the number of fuzz strands obtained by counting and using the above formula was divided by √1.5 (1.5 to the power of 1 / 2) to standardize the evaluation criteria before determining the passability through the process. A and B are preferred results in this invention, with A being the more preferred result.

[0083] <Indicators of uniformity of carbon fiber bundles in a pressure vessel> A 7.5L polyethylene liner was placed in a filament winding machine, and resin composition B, which had been uniformly mixed beforehand at 25°C, was fed onto carbon fiber bundles that had been treated with a sizing agent, impregnating them with the resin composition B at a rate of 22-28% by mass relative to the total mass of the carbon fiber bundles and resin composition B. By feeding the yarn in the above manner, a hoop layer forming an angle of +89° with respect to the axial direction of the liner and a hoop layer forming an angle of -89° with respect to the axial direction of the liner were wound as the first layer, with a thickness of 1.4 mm. Next, a helical layer forming an angle of +20° with respect to the axial direction of the liner and a helical layer forming an angle of -20° with respect to the axial direction of the liner were fed and wound as the second layer, with a thickness of 2.2 mm. Furthermore, as a third layer, a hoop layer forming an angle of +89° with respect to the axial direction of the liner and a hoop layer forming an angle of -89° with respect to the axial direction of the liner were fed and wound to a thickness of 0.6 mm to obtain an intermediate. After winding the carbon fiber bundle, the intermediate was rotated at a speed of 7 rpm and held in a 20°C environment for 15 minutes. The intermediate was cured under the conditions of resin formulation B to obtain a pressure vessel for pressure resistance testing.

[0084] The pressure vessel was cut perpendicular to the hoop layer, and the cross-section of the carbon fiber bundle was observed to evaluate the stability of the carbon fiber bundle (uniformity of thickness, presence or absence of voids).

[0085] Observations were performed using an optical microscope on 20 fiber bundles in the hoop layer. The thickness was defined as the line segment passing through the center of the cross-section of the carbon fiber bundle and perpendicular to the long side of the cross-section. The standard deviation of the thicknesses at these 20 points was calculated, and the coefficient of variation of the thickness of the carbon fiber bundles in CFRP was determined as a percentage by dividing the thickness by the standard deviation of the thickness ([standard deviation] / [arithmetic mean] × 100).

[0086] A coefficient of variation of thickness of 10% or less was judged as good uniformity, and a coefficient of variation between 10% and 20% was judged as acceptable uniformity. In addition, the void content was confirmed by observing the innermost layer in a cross section perpendicular to the hoop layer of the pressure vessel, within a range of 3 mm in the thickness direction and 30 mm in the radial direction, using an optical microscope. The uniformity was evaluated according to the following criteria and recorded in the table "Uniformity of carbon fiber bundles in the pressure vessel". A and B are preferred results in this invention, with A being the more preferred result. S: Void content: Less than 2.5%, good uniformity A: Void content: Less than 4.0%, good uniformity B: Void content: less than 4.0%, uniformity acceptable. C: Void content: Less than 4.0% D: Void content: 4.0% or more The present invention will be described in more detail below with reference to examples.

[0087] The compounds used in each example and comparative example are as follows. Viscosities are summarized in Table 1.

[0088] [Table 1]

[0089] a1: Bisphenol A ethylene oxide adduct (average number of moles of ethylene oxide added: 10) a2: Polyethylene glycol (Mn=600) a3: Propylene oxide / ethylene oxide block copolymer (Mn=3,750) b1: Polyglycerol polyglycidyl ether (Ex-521) b2: Bisphenol A epoxy (jER834) c1: 2-ethylhexyl stearate (Example 1) A polyacrylonitrile copolymer was prepared by polymerizing a polyacrylonitrile copolymer copolymerized with itaconic acid using a solution polymerization method with dimethyl sulfoxide as the solvent. The spinning solution obtained from the prepared polyacrylonitrile copolymer was extruded into the air from a spinneret and introduced into a coagulation bath consisting of an aqueous solution of dimethyl sulfoxide to obtain coagulated yarn using a wet-dry spinning method. After washing this coagulated yarn with water by a conventional method, it was stretched 3.5 times in a hot water bath. Subsequently, an amino-modified silicone-based silicone oil was applied to the fiber bundle after stretching in the water bath, and a drying and densification treatment was performed using a heated roller at 160°C. The fiber bundle was then stretched 3.7 times in pressurized steam to achieve a total stretch ratio of 13 times, and after entanglement treatment, a polyacrylonitrile-based precursor fiber bundle with a crystal orientation of 93%, a single fiber fineness of 1.11 dtex, and 24,000 single fibers was obtained. Next, setting the flame-retardant temperature to 235°C to 267°C, the density of the resulting flame-retardant fiber is 1.30 g / cm³. 3 The flame-retardant treatment time was adjusted to obtain flame-retardant fiber bundles by stretching polyacrylonitrile-based precursor fiber bundles at a stretch ratio of 1 in an oven under an air atmosphere. The obtained flame-retardant fiber bundles were then run in a nitrogen atmosphere at a temperature of 300-800°C, and the stretch ratio was controlled to obtain pre-carbonized fiber bundles. The obtained pre-carbonized fiber bundles were then subjected to carbonization treatment in a nitrogen atmosphere, with the maximum temperature and heating rate controlled. The flame-retardant treatment conditions, pre-carbonization conditions, and carbonization conditions are summarized in Table 2.

[0090] [Table 2]

[0091] The obtained carbon fiber bundles were subjected to liquid-phase electrolytic oxidation and sizing agent coating to obtain the final carbon fiber bundles with sizing agent attached. For liquid-phase electrolytic oxidation, an aqueous sulfuric acid solution was used as the electrolyte, and the electrolytic surface treatment was performed at an electric charge of 10 coulombs per gram of carbon fiber. The carbon fiber bundles subjected to this liquid-phase electrolytic oxidation were then washed with water and dried in heated air at 150°C to remove water, thereby obtaining the carbon fiber bundles.

[0092] The composition of A1 in Table 1 (100% by mass of a1) was used as a sizing agent in an aqueous solution. The sizing agent was applied to the carbon fiber bundles by immersion, and then, as a first drying step, the bundles were pre-dried for 15 seconds on a hot roller at a surface temperature of 160°C. Subsequently, as a second drying step, the bundles were heat-treated in heated air at 230°C for 35 seconds to obtain carbon fiber bundles with the sizing agent attached. The amount of sizing agent attached was adjusted to 0.9% by mass relative to 100% by mass of the total amount of surface-treated carbon fiber bundles with the sizing agent attached.

[0093] The carbon fiber bundles with the sizing agent attached were evaluated according to the strand tensile tests for each resin formulation described above, as well as the evaluation methods for abrasion fuzz, drape value, thickness, resin incorporation ratio, and uniformity of yarn width after resin impregnation. As summarized in Table 2, carbon fiber bundles with high mechanical properties, high abrasion resistance, and stable uniformity of the yarn bundle after FW molding were obtained.

[0094] (Example 2) Except for changing the hot roller temperature in the sizing agent process, carbon fiber bundles were obtained in the same manner as in Example 1, and various evaluations were performed. The results are summarized in Table 2, and carbon fiber bundles were obtained that had high mechanical properties, good abrasion resistance, and stable uniformity of the yarn bundles after FW molding.

[0095] (Example 3) Except for changing the sizing agent applied in the sizing agent process to A3, carbon fiber bundles were obtained in the same manner as in Example 1, and various evaluations were performed. The results are summarized in Table 2, and carbon fiber bundles were obtained that had high mechanical properties, good abrasion resistance, and stable uniformity of the yarn bundles after FW molding.

[0096] (Example 4) Except for changing the heating rate in the carbonization process as shown in Table 2, carbon fiber bundles were obtained in the same manner as in Example 1, and various evaluations were performed. The results are summarized in Table 2, and carbon fiber bundles with high mechanical properties, high abrasion resistance, and stable uniformity of the yarn bundles after FW molding were obtained.

[0097] (Example 5) Except for changing the sizing agent applied in the sizing agent process to A4, carbon fiber bundles were obtained in the same manner as in Example 1, and various evaluations were performed. The results are summarized in Table 2, and carbon fiber bundles with stable uniformity of the yarn bundles after FW molding and high mechanical properties were obtained.

[0098] (Example 6) Except for changing the amount of sizing agent applied in the sizing agent process as shown in Table 2, carbon fiber bundles were obtained in the same manner as in Example 4, and various evaluations were performed. The results are summarized in Table 2, and carbon fiber bundles were obtained that had high mechanical properties, good abrasion resistance, and stable uniformity of the yarn bundles after FW molding.

[0099] (Example 7) Except for changing the sizing agent applied in the sizing agent process to A5, carbon fiber bundles were obtained in the same manner as in Example 4, and various evaluations were performed. The results are summarized in Table 2, and carbon fiber bundles were obtained that had high mechanical properties, good abrasion resistance, and stable uniformity of the yarn bundles after FW molding.

[0100] (Example 8) Except for changing the sizing agent applied in the sizing agent process to A3, carbon fiber bundles were obtained in the same manner as in Example 4, and various evaluations were performed. The results are summarized in Table 2, and carbon fiber bundles were obtained that had high mechanical properties, good abrasion resistance, and stable uniformity of the yarn bundles after FW molding.

[0101] (Example 9) Except for changing the heating rate in the carbonization process as shown in Table 2, carbon fiber bundles were obtained in the same manner as in Example 1, and various evaluations were performed. The results are summarized in Table 2, and carbon fiber bundles were obtained that had good mechanical properties, high abrasion resistance, and stable uniformity of the yarn bundles after FW molding.

[0102] (Example 10) Except for changing the heating rate in the carbonization process as shown in Table 2, carbon fiber bundles were obtained in the same manner as in Example 1, and various evaluations were performed. The results are summarized in Table 2, and carbon fiber bundles were obtained that had good mechanical properties, high abrasion resistance, and stable uniformity of the yarn bundles after FW molding.

[0103] (Example 11) Except for changing the stretching ratio in the preliminary carbonization process and the maximum temperature in the carbonization process as shown in Table 2, carbon fiber bundles were obtained in the same manner as in Example 1, and various evaluations were performed. The results are summarized in Table 2, and carbon fiber bundles were obtained that had high mechanical properties, high abrasion resistance, and stable uniformity of the yarn bundles after FW molding.

[0104] (Example 12) Except for changing the maximum temperature in the carbonization process as shown in Table 2, and changing the sizing agent applied in the sizing agent process to A3, carbon fiber bundles were obtained in the same manner as in Example 11, and various evaluations were performed. The results are summarized in Table 2, and carbon fiber bundles were obtained that had high mechanical properties, good abrasion resistance, and stable uniformity of the yarn bundles after FW molding.

[0105] (Example 13) Except for changing the maximum temperature in the carbonization process as shown in Table 2, carbon fiber bundles were obtained in the same manner as in Example 11, and various evaluations were performed. The results are summarized in Table 2, and carbon fiber bundles were obtained that had good mechanical properties, high abrasion resistance, and stable uniformity of the yarn bundles after FW molding.

[0106] (Example 14) Except for changing the single fiber diameter, number of filaments, flame-retardant temperature, and IR peak intensity ratio of the flame-retardant fiber bundle as shown in Table 2, carbon fiber bundles were obtained in the same manner as in Example 4, and various evaluations were performed. The results are summarized in Table 2, and carbon fiber bundles with high mechanical properties, good abrasion resistance, and good uniformity of the yarn bundle after FW molding were obtained.

[0107] (Example 15) Except for changing the single fiber diameter as shown in Table 2, carbon fiber bundles were obtained in the same manner as in Example 14, and various evaluations were performed. The results are summarized in Table 2, and carbon fiber bundles were obtained that had high mechanical properties, high abrasion resistance, and stable uniformity of the yarn bundle after FW molding.

[0108] (Example 16) Except for changing the heating rate in the carbonization process as shown in Table 2, carbon fiber bundles were obtained in the same manner as in Example 15, and various evaluations were performed. The results are summarized in Table 2, and carbon fiber bundles were obtained that had high mechanical properties, high abrasion resistance, and stable uniformity of the yarn bundles after FW molding.

[0109] (Example 17) Except for changing the stretching ratio in the preliminary carbonization process, the maximum temperature in the carbonization process, and the heating rate in the carbonization process as shown in Table 2, carbon fiber bundles were obtained in the same manner as in Example 16, and various evaluations were performed. The results are summarized in Table 2, and carbon fiber bundles were obtained that had high mechanical properties, high abrasion resistance, and stable uniformity of the yarn bundles after FW molding.

[0110] (Example 18) Except for changing the maximum temperature in the carbonization process as shown in Table 2 and changing the sizing agent applied in the sizing agent process to A3, carbon fiber bundles were obtained in the same manner as in Example 17 and various evaluations were performed. The results are summarized in Table 2, and carbon fiber bundles were obtained that had high mechanical properties, high abrasion resistance, and stable uniformity of the yarn bundles after FW molding.

[0111] (Example 19) Except for changing the flame-retardant temperature, the IR peak intensity ratio of the flame-retardant fiber bundles, and the single fiber diameter as shown in Table 2, carbon fiber bundles were obtained in the same manner as in Example 4, and various evaluations were performed. The results are summarized in Table 2, and the obtained carbon fiber bundles showed good abrasion resistance and uniformity of the yarn bundles after FW molding. (Example 20) Except for changing the temperature in the flame-retardant process, the IR peak ratio of the flame-retardant fiber bundle, the stretching ratio in the pre-carbonization process, and the heating rate in the carbonization process as shown in Table 2, carbon fiber bundles were obtained in the same manner as in Example 1 and various evaluations were performed. The results are summarized in Table 2, and the obtained carbon fiber bundles showed high abrasion resistance and uniformity of the yarn bundles after FW molding.

[0112] (Comparative Example 1) Except for changing the hot roller temperature in the sizing agent process, carbon fiber bundles were obtained in the same manner as in Example 3, and various evaluations were performed. The results are summarized in Table 3. The mechanical properties were high and the abrasion resistance was good, but the void content in the yarn bundles after FW molding was high.

[0113] [Table 3]

[0114] (Comparative Example 2) Except for changing the amount of sizing agent applied during the sizing agent process as shown in Table 3, carbon fiber bundles were obtained in the same manner as in Example 4, and various evaluations were performed. The results are summarized in Table 3. The mechanical properties were high, but the abrasion resistance and the uniformity of the yarn bundles after FW molding were poor.

[0115] (Comparative Example 3) Except for changing the amount of sizing agent applied during the sizing agent process as shown in Table 3, carbon fiber bundles were obtained in the same manner as in Example 4, and various evaluations were performed. The results are summarized in Table 3. While abrasion resistance and mechanical properties were good, the uniformity of the yarn bundles after FW molding was poor.

[0116] (Comparative Example 4) Except for changing the sizing agent applied in the sizing agent process to A2, carbon fiber bundles were obtained in the same manner as in Example 4, and various evaluations were performed. The results are summarized in Table 3. The mechanical properties were high, but the abrasion resistance and the uniformity of the yarn bundles after FW molding were poor.

[0117] (Comparative Example 5) Except for changing the hot roller temperature in the sizing agent process and changing the sizing agent applied in the sizing agent process to A6, carbon fiber bundles were obtained in the same manner as in Example 4, and various evaluations were performed. The results are summarized in Table 3. The mechanical properties and uniformity of the yarn bundles after FW molding were good, but the abrasion resistance was poor.

[0118] (Comparative Example 6) Except for changing the sizing agent applied in the sizing agent process to A7, carbon fiber bundles were obtained in the same manner as in Example 3, and various evaluations were performed. The results are summarized in Table 3. The mechanical properties were high and the abrasion resistance was good, but the void content of the yarn bundles after FW molding was high.

[0119] (Comparative Example 7) Except for changing the hot roller temperature in the sizing agent process, carbon fiber bundles were obtained in the same manner as in Comparative Example 6, and various evaluations were performed. The results are summarized in Table 3. The mechanical properties were high and the abrasion resistance was good, but the void content in the yarn bundle after FW molding was high.

[0120] (Comparative Example 8) Except for changing the stretch ratio and single fiber diameter in the preliminary carbonization process as shown in Table 3, carbon fiber bundles were obtained in the same manner as in Example 1, and various evaluations were performed. The results are summarized in Table 3. The mechanical properties were high, but the abrasion resistance and uniformity of the yarn bundle after FW molding were poor.

[0121] (Comparative Example 9) Except for changing the amount of sizing agent applied during the sizing agent process as shown in Table 3, carbon fiber bundles were obtained in the same manner as in Example 15, and various evaluations were performed. The results are summarized in Table 3. The mechanical properties were high, but the abrasion resistance and the uniformity of the yarn bundles after FW molding were poor. [Explanation of symbols]

[0122] 1 Fixed bar 2. Carbon fiber bundle 3 weights 4 Horizontal platform

Claims

1. A carbon fiber bundle having 6,000 to 50,000 filaments, to which a sizing agent has been applied, The thickness is 0.10 mm or more and 0.20 mm or less, and the coefficient of variation of the thickness is 6.5% or less. The drape value is between 6 cm and 9 cm. A carbon fiber bundle with a sizing agent attached, wherein the strand tensile strength (strand tensile strength A') evaluated according to JIS R7608 (2007) using the following resin formulation A is between 4.9 GPa and 6.7 GPa. Resin formulation A: Contains 97% by mass or more of (3',4'-epoxycyclohexane)methyl-3,4-epoxycyclohexylcarboxylate, and has an epoxy equivalent of 130 g / eq and a viscosity of 240 mPa·s at 25°C. The composition is 100 / 3 / 4 (parts by mass) of epoxy resin / boron trifluoride monoethylamine / acetone, and the curing conditions are atmospheric pressure, temperature of 125°C, and time of 30 minutes.

2. A carbon fiber bundle with a sizing agent attached, according to claim 1, wherein the strand tensile strength A' is 5.6 GPa or more and 6.7 GPa or less, and the strand tensile strength (strand tensile strength B') evaluated according to JIS R7608 (2007) using the following resin formulation B is 5.3 GPa or more and 6.4 GPa or less. Resin formulation B: The main component is 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, and 2,2'-dimethyl-4,4'-methylenebis(cyclohexylamine) is used as a curing agent, with a ratio of 100 / 35 (parts by mass) of the main component to the curing agent. The curing conditions are atmospheric pressure, temperature 80°C, time 120 minutes, followed by atmospheric pressure, temperature 110°C, time 240 minutes.

3. The carbon fiber bundle with sizing agent attached according to claim 2, wherein the strand tensile strength A' is 5.9 GPa or more and 6.7 GPa or less, and the strand tensile strength (strand tensile strength B') evaluated according to JIS R7608 (2007) using resin formulation B is 5.6 GPa or more and 6.4 GPa or less.

4. A sizing agent-adhered carbon fiber bundle according to claim 1 or 2, wherein the resin incorporation ratio is 0.45 or more and 0.64 or less.

5. A sizing agent-adhered carbon fiber bundle according to claim 1 or 2, wherein the single fiber diameter is 6.6 μm or more and 7.3 μm or less.

6. The carbon fiber bundle with sizing agent attached according to claim 3, wherein the strand tensile strength A' is 6.3 GPa or more and 6.7 GPa or less, and the strand tensile strength B' is 6.0 GPa or more and 6.4 GPa or less.

7. A carbon fiber bundle with a sizing agent attached, according to claim 1 or 2, wherein the strand tensile modulus, as evaluated using resin formulation A in accordance with JIS R7608 (2007), is 240 GPa or more and 275 GPa or less.

8. The carbon fiber bundle with sizing agent attached according to claim 1 or 2, wherein the amount of sizing agent attached is 0.7% by mass or more and 1.0% by mass or less of the carbon fiber bundle with sizing agent attached, relative to 100% by mass of the carbon fiber bundle with sizing agent attached.

9. A carbon fiber bundle with a sizing agent attached, according to claim 1 or 2, wherein the sizing agent does not contain a cyclic oxygen-containing compound.

10. The sizing agent is a carbon fiber bundle to which a sizing agent is attached, wherein the sizing agent has a viscosity of 500 mPa·s or more and 1,400 mPa·s or less at 30°C, according to claim 1 or 2.

11. A carbon fiber bundle with a sizing agent attached, according to claim 1, wherein the coefficient of variation of thickness is 4.3% or less, and the drape value is 6.5 cm or more and 7.5 cm or less.

12. A carbon fiber reinforced composite material using a sizing agent-adhered carbon fiber bundle according to claim 1 or 2.

13. A pressure vessel using a carbon fiber bundle with a sizing agent attached, as described in claim 10.

14. A process of flame-retardantizing, pre-carbonizing, and carbonizing a polyacrylonitrile-based carbon fiber precursor fiber bundle having 6,000 to 50,000 filaments, A step of oxidizing the fiber bundle obtained in the carbonization step to obtain a carbon fiber bundle, The process includes a step of immersing a carbon fiber bundle in a sizing agent-containing solution to ensure that the amount of sizing agent adhering to 100% by mass of the carbon fiber bundle is 0.7% by mass or more and 1.0% by mass or less. The sizing agent does not contain cyclic oxygen-containing compounds and has a viscosity of 500 mPa·s or more and 1,400 mPa·s or less at 30°C. Next, the process includes treating the carbon fiber bundle with the sizing agent attached with a hot roller at a surface temperature of 150°C or higher and 170°C or lower. A method for producing a carbon fiber bundle with sizing agent attached that is essentially untwisted.

15. A polyacrylonitrile-based carbon fiber precursor fiber bundle having a single fiber fineness of 0.58 dtex or more and 1.20 dtex or less, and a filament count of 6,000 to 42,000, is subjected to flame-retardant treatment in two stages: a first flame-retardant treatment and a second flame-retardant treatment. In the second flame-retardant treatment process, the infrared spectrum at 1,370 cm⁻¹ －１ 1,453 cm⁻¹ relative to the peak intensity －１ The ratio of peak intensities is in the range of 0.70 to 0.75, and the infrared spectrum is at 1,370 cm⁻¹. －１ 1,254 cm⁻¹ relative to the peak intensity －１ The flame resistance is increased until the ratio of the peak intensity is in the range of 0.50 to 0.

65. The fiber bundle obtained in the second flame-retardant treatment process is subjected to a preliminary carbonization process in which the stretch ratio is increased to 1.00 or more and 1.15 or less in an inert atmosphere with a maximum temperature of 600°C to 800°C, A method for producing a carbon fiber bundle with a sizing agent attached, according to claim 14, comprising a carbonization step of carbonizing the fiber bundle obtained in a preliminary carbonization step in an inert atmosphere at a maximum temperature of 1000°C to 1600°C at a heating rate of 0.4°C / second to 1.1°C / second.

Images