WO2019244830A1

WO2019244830A1 - Carbon fiber and method for producing same

Info

Publication number: WO2019244830A1
Application number: PCT/JP2019/023851
Authority: WO
Inventors: 奥田治己; 渡邉潤; 松本直浩; 田中文彦
Original assignee: 東レ株式会社
Priority date: 2018-06-18
Filing date: 2019-06-17
Publication date: 2019-12-26
Also published as: KR20210019029A; US20210115597A1; EP3808880A1; JPWO2019244830A1; CN112368432A; JP6702511B1; EP3808880A4; MX2020013140A; CN112368432B; TW202006201A

Abstract

The present invention addresses the problem of providing a carbon fiber for a carbon fiber reinforced composite material which is not easily damaged during a molding process and exhibits an excellent elastic modulus. The carbon fiber has a strand elastic modulus of at least 360 GPa, a strand strength of at least 3.5 GPa, and a single fiber diameter of at least 6.0 µm, and satisfies one or more of the requirements below. (A) When one end is a fixed end and the other end is a free end that can rotate about the axis of a fiber bundle, the remaining twist number is at least 2 turns/m. (B) The total fineness, which is the product of the single fiber fineness (g/km) as a carbon fiber and the number of filaments (counts), is at least 740 g/km. In addition, the single fiber elastic modulus Es (Gpa) and the loop breaking load A (N) of the carbon fiber satisfy the relationship of expression (1). (1): A≥-0.0017×Es+1.02. Furthermore, the single fiber diameter of the carbon fiber is at least 6.0 µm, the relationship between the strand elastic modulus E (GPa) of the carbon fiber and the knot strength B (MPa) of the carbon fiber as evaluated at a heating loss rate of 0.15% or less at 450°C satisfies expression (2), and the twist number of the carbon fiber is 20-80 turns/m. (2): B≥6.7×10⁹×E^-2.85

Description

Carbon fiber and method for producing the same

The present invention relates to a carbon fiber and a method for producing the same.

Carbon fiber has excellent specific strength and specific elastic modulus, and by using it as a reinforcing fiber of carbon fiber reinforced composite material, it is possible to significantly reduce the weight of members, making it an indispensable material for realizing a society with high energy use efficiency. One of them is used in a wide range of fields. In recent years, applications have been advanced in fields where there is a strong demand for cost reduction, such as automobiles and electronic equipment housings, and there is a strong demand for reduction of final member costs including molding costs.

In order to effectively reduce the cost of final components, it is important not only to reduce the cost of the carbon fiber itself, but also to reduce the required amount by improving the performance of the carbon fiber and reduce the molding cost by improving the moldability. is there.

However, for example, when aiming to reduce the amount of carbon fiber used while maintaining rigidity, which is one of the important properties as a final member, simply applying an existing high-modulus carbon fiber does not necessarily require the cost of the final member. Often it does not go down. This is because the productivity of existing high-modulus carbon fibers is low and tends to be expensive, and the low processability tends to increase the total processing cost up to the final member. The formability of carbon fiber is, for example, good handling properties as a yarn bundle, difficulty in fluffing, and ease of thread connection required when switching carbon fiber bobbins when continuously producing carbon fiber reinforced composite materials. It is determined by the ease of handling in various processes until the final member and the processability.

In recent years, the use of carbon fibers as discontinuous fibers has been increasing, especially for applications that emphasize cost reduction. Generally, when carbon fibers are used as discontinuous fibers, the fiber length of the carbon fibers tends to be short due to shearing or bending in the molding process. Existing high modulus carbon fibers are particularly prone to this tendency, and even if the tensile modulus of the carbon fibers is high, the rigidity of the final member is not effectively improved accordingly.

The most widely used polyacrylonitrile-based carbon fiber is an oxidizing step in which a carbon fiber precursor fiber is converted into an oxidizing fiber under an oxidizing atmosphere at 200 to 300 ° C, and a carbon fiber under an inert atmosphere at 300 to 2000 ° C. It is industrially manufactured through a carbonization process. Polyacrylonitrile-based high modulus carbon fibers are industrially manufactured through a graphitization step of further graphitization in an inert atmosphere at a maximum temperature of 3000 ° C. Such a graphitization step can effectively increase the tensile modulus of the carbon fiber, but requires equipment corresponding to a high temperature, or promotes crystal growth in the carbon fiber, thereby obtaining the carbon fiber obtained. Tends to have low tensile strength and compressive strength. And such a high modulus carbon fiber tends to have low productivity as a carbon fiber as described above, and low moldability in obtaining a carbon fiber reinforced composite material, and when used as a discontinuous fiber, the fiber length is short. Easy to be short.

いくつか Several methods other than graphitization have been proposed to increase the tensile modulus of carbon fibers. As one of them, a method of applying high tension in a carbon fiber manufacturing process has been proposed.

Patent Documents 1 and 2 propose a technique capable of controlling the generation of fluff by controlling the molecular weight of the polyacrylonitrile copolymer even when a high tension is applied in the carbonization step.

Patent Document 3 proposes a technique for increasing the strand elastic modulus by performing high stretching in the flame-proofing step and the preliminary carbonization step.

Further, Patent Documents 4 to 7 propose a technique for improving the processability in a carbonization step by adding entanglement to a carbon fiber precursor fiber bundle, and Patent Documents 8 and 9 by adding twist.

Patent Document 10 discloses that the carbon fiber and the matrix are formed by increasing the strand elastic modulus of the obtained carbon fiber by controlling the test length dependence of the pre-carbonized fiber bundle by entanglement or twisting and carbonizing with high tension. There has been proposed a technique for suppressing a decrease in the adhesiveness with the adhesive.

Patent Document 11 proposes a carbon fiber having a high knot strength even with a large single fiber fineness and excellent moldability by controlling the copolymer composition of the carbon fiber precursor fiber bundle.

Similarly, Patent Document 12 proposes a carbon fiber in which a decrease in mechanical properties is suppressed even when the diameter of a single fiber is large.

International Publication No. WO2008 / 047745 JP 2009-256833 A International Publication No. WO2008 / 063886 JP-A-2001-49536 JP-A-10-195718 JP-A-2000-160436 Japanese Patent Publication No. 47-026964 JP-A-56-091015 JP 2002-001725 A JP 2014-141761 A International Publication No. WO2013 / 157613 International Publication No. WO2013 / 157612

従来 However, the conventional technology has the following problems.

In Patent Documents 1 and 2, the molecular weight of the polyacrylonitrile copolymer is controlled. However, the effect of increasing the critical stretching tension in the carbonization step is small, and a large improvement in strand elastic modulus cannot be expected.

In Patent Document 3, although the stretching ratio up to the preliminary carbonization step is set to be high, the stretching ratio in the carbonization step that easily improves the strand elastic modulus of the carbon fiber is low, and a large improvement in strand elasticity cannot be expected. Was.

In Patent Documents 4 to 9, no attention was paid to increasing the draw ratio in the carbonization step, and there was no idea to pay attention to them.

Patent Document 10 shows that the strand elastic modulus, the adhesiveness to the matrix, and the strand strength can be compatible at a high level, and that the passability in the carbonization step is also good. However, no attention has been paid to the formability at the time of obtaining a carbon fiber reinforced composite material, or fiber breakage when used as a discontinuous fiber, and there is no idea to pay attention to them.

Patent Documents 11 and 12 do not pay special attention to the draw ratio in the carbonization step. In Examples, the strand elastic modulus is increased to 343 GPa at maximum by increasing the carbonization temperature. Although not described, conventional approaches to increasing the carbonization temperature tend to result in poor moldability in obtaining carbon fiber reinforced composites, as with commercially available high modulus grade carbon fibers. In addition, no attention was paid to fiber breakage when used as discontinuous fibers, and there was no idea to pay attention to them.

In summary, the prior art describes a method for achieving a high level of compatibility between the tensile modulus of the carbon fiber and the formability, and the ease of maintaining the fiber length when used as a discontinuous fiber. In order to realize a total cost reduction as a final member, it has been an issue to obtain a method for achieving these at a high level.

In order to achieve the above object, a first aspect of the carbon fiber of the present invention is a carbon fiber having a strand elastic modulus of 360 GPa or more, a strand strength of 3.5 GPa or more, and a single fiber diameter of 6.0 μm or more. And a carbon fiber satisfying the following requirements (A) or (B).
(B) When one end is a fixed end and the other end is a free end rotatable with respect to the axis of the fiber bundle, the number of twists remaining is 2 turns / m or more. The total fineness, which is the product of the fiber fineness (g / km) and the number of filaments (lines), is 740 g / km or more.

Further, a second aspect of the carbon fiber of the present invention is a carbon fiber in which the single fiber elastic modulus Es (GPa) and the loop breaking load A (N) satisfy the relationship of the formula (1).
A ≧ −0.0017 × Es + 1.02 Formula (1)
In the third aspect of the carbon fiber of the present invention, the knot strength evaluated at a single fiber diameter of 6.0 μm or more, a strand elasticity E (GPa) and a loss on heating at 450 ° C. of 0.15% or less. A carbon fiber whose relationship with B (MPa) satisfies the expression (2) and has a twist number of 5 to 80 turns / m.
B ≧ 6.7 × 10 ⁹ × E ^-2.85 Formula (2)
Further, in the method for producing carbon fibers according to the present invention, the carbon fiber precursor fiber bundle is subjected to an oxidizing treatment in an air atmosphere at a temperature range of 200 to 300 ° C., and the obtained oxidized fiber bundle is subjected to an inert atmosphere. Pre-carbonization at a maximum temperature of 500 to 1000 ° C. until the density becomes 1.5 to 1.8 g / cm ³ , and the obtained pre-carbonized fiber bundle is heat-treated in an inert atmosphere. A method for producing carbon fiber for carbonization, wherein the single fiber fineness of a carbon fiber precursor fiber bundle is 0.9 dtex or more, and the tension during the carbonization treatment is controlled to 5 mN / dtex or more, and the following (c) ) Or (d).
(C) The number of twists of the fiber bundle to be subjected to the carbonization treatment is 2 turns / m or more. (D) The total fineness which is the product of the single fiber fineness (g / km) of the obtained carbon fiber and the number of filaments (number) is 740 g / km or more

炭素 The carbon fiber of the present invention is a carbon fiber that has both excellent tensile modulus and processability into a composite material, and easily maintains the fiber length even when used as a discontinuous fiber. The carbon fiber of the present invention is effective for reducing the required amount of carbon fiber and improving the productivity and mechanical properties of the composite material.

において In the present invention, a single fiber of carbon fibers and an aggregate thereof are simply referred to as carbon fibers. The aggregate of single fibers of carbon fibers in the present invention includes various forms such as a bundle form, a web form, and a composite form thereof. The method for producing the carbon fiber of the present invention will be described later.

引張 In the present invention, the tensile modulus is a generic term indicating a single fiber modulus evaluated by a single fiber tensile test of carbon fibers and a strand modulus evaluated by a method described later. The relationship between the single fiber elastic modulus and the strand elastic modulus will be described later.

A first aspect of the carbon fiber of the present invention is a carbon fiber having a strand elastic modulus of 360 GPa or more, a strand strength of 3.5 GPa or more, and a single fiber diameter of 6.0 μm or more. ) Or (b). It is more preferable that both (a) and (b) are satisfied.
(B) When one end is a fixed end and the other end is a free end rotatable with respect to the axis of the fiber bundle, the number of twists remaining is 2 turns / m or more. The total fineness, which is the product of the fiber fineness (g / km) and the number of filaments (lines), is 740 g / km or more.
Hereinafter, each requirement will be described.

において In the first aspect of the carbon fiber of the present invention, the strand elastic modulus is 360 GPa or more. The strand elastic modulus is preferably 370 GPa or more, more preferably 380 GPa or more, even more preferably 400 GPa or more, and even more preferably 440 GPa or more. The higher the strand elastic modulus, the greater the effect of improving the rigidity of the carbon fiber reinforced composite material, and the easier it is to obtain a highly rigid carbon fiber reinforced composite material. If the strand elastic modulus is 360 GPa or more, the rigidity of the carbon fiber reinforced composite material can be greatly increased, and thus the industrial value is large. From the viewpoint of increasing the rigidity of the carbon fiber reinforced composite material, the strand elastic modulus of the carbon fiber is preferably as high as possible. However, conventionally, if the strand elastic modulus is too high, the molding processability when obtaining the carbon fiber composite material is reduced. When used as discontinuous fibers, the fiber length was easily reduced. The strand elastic modulus can be evaluated according to the tensile test of the resin-impregnated strand described in JIS R7608: 2004. The details of the method for evaluating the strand elastic modulus will be described later. Although the strand elastic modulus can be controlled by various known methods, in the present invention, it is preferable to control the strand elasticity by the tension in the carbonization treatment.

において In the first aspect of the carbon fiber of the present invention, the strand strength is 3.5 GPa or more. The strand strength is preferably at least 3.7 GPa, more preferably at least 3.9 GPa, even more preferably at least 4.3 GPa. Generally, the higher the strand strength, the higher the tensile strength of the carbon fiber reinforced composite material, and thus a higher performance carbon fiber reinforced composite material can be obtained. A carbon fiber having an extremely low strand strength may lead to a reduction in the formability of a carbon fiber-reinforced composite material, but if it is 3.5 GPa or more, it often does not pose a major problem. The strand strength can be evaluated according to the tensile test of a resin-impregnated strand described in JIS R7608: 2004. Details of the method for evaluating the strand strength will be described later. The strand strength can be controlled by various known methods. However, in the usual method of increasing the carbonization temperature, the strand strength tends to decrease as the strand elastic modulus increases. Even if the strand elastic modulus is high, a carbon fiber having a strand strength of 3.5 GPa or more can be obtained by the carbon fiber manufacturing method of the present invention described later.

において In the first aspect of the carbon fiber of the present invention, the single fiber diameter is 6.0 μm or more. The single fiber diameter is preferably at least 6.5 μm, more preferably at least 6.9 μm. As the diameter of the single fiber is larger, it is often difficult to achieve both the strand elastic modulus and the strand strength at a high level in many cases. However, according to the first aspect of the carbon fiber of the present invention, the single fiber diameter is Even when the thickness is 6.0 μm or more, both can be achieved at the above-mentioned high level. In addition, as the diameter of the single fiber is larger, when forming a carbon fiber reinforced composite material, fuzzing due to friction between carbon fibers when unwinding from a bobbin, friction with a guide member such as a roller, and accumulation of fluff on the guide member. It is easy to be suppressed, and it is easy to enhance moldability. In the first aspect of the carbon fiber of the present invention, the upper limit of the single fiber diameter is not particularly limited, but if it is too large, the strand strength and the strand elastic modulus are likely to be reduced. Further, the single fiber diameter is preferably 7.4 μm or less from the viewpoint that the strand elastic modulus and the strand strength are easily compatible at a high level. The method for evaluating the diameter of the single fiber will be described later, but it may be calculated from the specific gravity, the basis weight, and the number of filaments of the fiber bundle, or may be evaluated by observation with a scanning electron microscope. As long as the evaluation device used is correctly calibrated, an equivalent result can be obtained by any method. When the cross-sectional shape of the single fiber is not a perfect circle when evaluated by observation with a scanning electron microscope, a circle equivalent diameter is used instead. The equivalent circle diameter refers to the diameter of a perfect circle having a cross-sectional area equal to the measured cross-sectional area of a single fiber. The single fiber diameter can be controlled by the discharge amount from the die during spinning of the carbon fiber precursor fiber bundle, the draw ratio in each step, and the like.

A first aspect of the carbon fiber of the present invention is a carbon fiber that satisfies one or more of the following requirements in addition to the requirements regarding the above-described strand elastic modulus, strand strength, and single fiber diameter.
(B) When one end is a fixed end and the other end is a free end rotatable with respect to the axis of the fiber bundle, the number of twists remaining is 2 turns / m or more. The total fineness, which is the product of the fiber fineness (g / km) and the number of filaments (number), is 740 g / km or more. Strand elastic modulus is satisfied by satisfying either or both of these requirements (a) and (b). Is high, the decrease in molding processability can be effectively suppressed, and the industrial value is great.

In the first aspect of the carbon fiber of the present invention, the number of remaining twists is preferably 2 turns / m or more, more preferably 5 turns / m or more, and further preferably 10 turns / m or more. It is more preferably at least 16 turns / m, more preferably at least 20 turns / m, even more preferably at least 30 turns / m, even more preferably at least 46 turns / m.

In the present invention, the fixed end is an arbitrary portion on the fiber bundle that is fixed so as not to rotate around the longitudinal direction of the fiber bundle, and restricts the rotation of the fiber bundle using an adhesive tape or the like. Can be realized by In the present invention, the free end refers to an end that appears when a continuous fiber bundle is cut in a cross section perpendicular to the longitudinal direction, is not fixed to anything, and has an axis in the longitudinal direction of the fiber bundle. It is an end that can be rotated. In the present invention, when one end is a fixed end and the other is a free end, the remaining number of twists refers to the number of twists per meter of permanent twists of a fiber bundle of carbon fibers. A semi-permanent twist refers to a twist that cannot be unwound without the action of an external force. In the present invention, one end is a fixed end, the other is a free end, the twist that remains without unraveling after being left undisturbed in the specific arrangement described in the examples for 5 minutes, semi-permanent twist, That is, it is defined as a remaining twist. If the number of twists remaining is 2 turns / m or more, it is easy to maintain high moldability even if the strand elastic modulus is high. The reason for this has not been clarified quantitatively, but is qualitatively understood as follows. That is, since the relative position of the single fiber in the fiber bundle is easily fixed due to the twisting of the carbon fiber having the remaining number of twists of 2 turns / m or more, the single fiber inside the fiber bundle is It is considered that the material is easily preserved without being damaged by friction with a guide member or the like. Further, if the number of remaining twists is 5 turns / m or more, fluff is suppressed, so that a high tension can be applied in the carbonization step, and the strand elastic modulus is easily increased effectively. If the number of remaining twists is 20 turns / m or more, the alignment of the fiber bundles is controlled with less fluff, and as a result, the stress transmission between the fiber bundles becomes smooth, and the knot strength described later tends to increase. When one end is a fixed end and the other is a free end, the number of twists remaining can be controlled by a known method. Specifically, the number of twists remaining can be controlled by adjusting the number of twists of the fiber bundle in the carbonization process.

As described above, in the first aspect of the carbon fiber of the present invention, the total fineness is preferably 740 g / km or more, more preferably 850 g / km or more, and still more preferably 1300 g / km or more. 1600 g / km or more, more preferably 2,000 g / km or more. If the total fineness is 740 g / km or more, it is easy to maintain high moldability even if the strand elastic modulus is high. The reason for this has not been clarified quantitatively, but is qualitatively understood as follows. That is, in the carbon fibers having a total fineness of 740 g / km or more, the proportion of the single fibers present in the outermost layer of the fiber bundle easily damaged by the friction with respect to the total number of the single fibers constituting the fiber bundle is reduced. Therefore, it is considered that the above-described damage due to friction is easily reduced in the entire fiber bundle. The total fineness is a product of the single fiber fineness (g / km) and the number of filaments (number), and can be controlled by changing the single fiber fineness and the number of filaments.

A second aspect of the carbon fiber of the present invention is a carbon fiber in which the single fiber elastic modulus Es (GPa) and the loop breaking load A (N) satisfy the relationship of the formula (1).
A ≧ −0.0017 × Es + 1.02 Formula (1)
The constant term in the formula (1) is preferably 1.04, more preferably 1.06, further preferably 1.08, and particularly preferably 1.10. The loop breaking load corresponds to a load at which a break occurs when a single fiber is bent in a loop shape, and is evaluated by a method described later. The single fiber elastic modulus is a tensile elastic modulus of carbon fiber as a single fiber, and has a certain correlation with the strand elastic modulus. In the present invention, the single fiber elastic modulus is described in detail later, but a single fiber tensile test is performed with a plurality of test lengths, the slope of the stress-strain curve at each test length is calculated, and the test length dependency is considered. By doing so, it can be obtained by removing the influence of the compliance of the device system. Generally, when the single fiber elastic modulus is increased, the loop breaking load tends to decrease. When the loop breaking load is low, the carbon fiber is easily broken by the force in the bending direction during the forming process as the discontinuous fiber, and the effect of improving the rigidity of the carbon fiber reinforced composite material is reduced by shortening the fiber length. The higher the loop breaking load, the more difficult it is to break even when a force in the bending direction is applied to the single fiber.Since the fiber length tends to be maintained during processing such as a discontinuous fiber in which a large bending direction force is applied, carbon It is easy to increase the rigidity of the fiber reinforced composite material. When the loop breaking load A and the single fiber elastic modulus Es satisfy the relationship of the formula (1), the carbon fiber becomes a carbon fiber which is hard to be broken by a force in a bending direction in spite of a high single fiber elastic modulus, and is used as a discontinuous fiber. In addition, the rigidity of the carbon fiber reinforced composite material can be efficiently increased. The carbon fiber satisfying the relationship of the formula (1) can be obtained by the method for producing a carbon fiber of the present invention described later. It is preferable that the carbon fiber according to the first aspect of the present invention also satisfies the second aspect at the same time. Even if the carbon fiber has a high strand elasticity, it can not only effectively suppress the reduction in the formability, but also easily maintain the fiber length when used as a discontinuous fiber. Easy to get material.

In the second aspect of the carbon fiber of the present invention, the single fiber elastic modulus is preferably 360 GPa or more, more preferably 370 GPa or more, still more preferably 380 GPa or more, and further preferably 400 GPa or more. It is more preferably 440 GPa or more. Conventionally, the higher the single fiber elastic modulus, the lower the loop breaking load, and the shorter the fiber length during molding as a discontinuous fiber, the shorter the fiber length. However, in the second aspect of the carbon fiber of the present invention, the single fiber elastic Since the loop breaking load is higher than the modulus, the rigidity of the carbon fiber reinforced composite material can be effectively increased even when the single fiber elastic modulus is increased. The method of improving the single fiber elastic modulus is the same as that of the strand elastic modulus.

In a third aspect of the carbon fiber of the present invention, the knot strength B (evaluated at a single fiber diameter of 6.0 μm or more, a strand elasticity E (GPa) and a loss on heating at 450 ° C. of 0.15% or less) is used. MPa) satisfies the relationship of the formula (2), and the number of twists is 5 to 80 turns / m.
B ≧ 6.7 × 10 ⁹ × E ^-2.85 Formula (2)
In the third aspect of the carbon fiber of the present invention, the single fiber diameter is 6.0 μm or more. The single fiber diameter is preferably at least 6.5 μm, more preferably at least 6.9 μm. As the diameter of the single fiber increases, it is often difficult to achieve both the strand elastic modulus and the knot strength at a high level in many cases. However, according to the third aspect of the carbon fiber of the present invention, the single fiber diameter increases. Even when the thickness is 6.0 μm or more, both can be compatible at a high level. Also, the larger the single fiber diameter, the more the carbon fiber reinforced composite material, the more the fuzzing due to the friction between the carbon fibers when unwinding from the bobbin and the friction with the guide member such as a roller can be further suppressed, and the forming process Can be enhanced. In the third aspect of the carbon fiber of the present invention, the upper limit of the diameter of the single fiber is not particularly limited, but if it is too large, the knot strength and the strand elastic modulus are likely to be reduced. Further, from the viewpoint that the elastic modulus of the strand and the knot are easily compatible at a high level, it is also preferable that the single fiber diameter is 7.4 μm or less.

In the third aspect of the carbon fiber of the present invention, the strand elastic modulus E (GPa) and the knot strength B (MPa) evaluated at a rate of loss on heating at 450 ° C. of 0.15% or less have a relationship represented by the formula (2). Fulfill.
B ≧ 6.7 × 10 ⁹ × E ^-2.85 Formula (2)
In the present invention, the heating loss rate at 450 ° C. is calculated from a change in mass before and after heating when the carbon fiber is heated in an oven in a nitrogen atmosphere at a temperature of 450 ° C. for 15 minutes, which will be described in detail later. The knot strength is an index that reflects the mechanical properties of the fiber bundle other than the fiber axis direction. When manufacturing a composite material, a bending stress other than the fiber axis direction is applied to the carbon fiber bundle, and the knot strength affects the generation of fluff, which is a fiber break generated in the process of manufacturing the composite material. In order to efficiently produce a composite material, fluffing occurs when the traveling speed of the fiber bundle during production of the composite material is increased.However, by increasing the knot strength, the composite material can be produced with good quality even under conditions where the traveling speed of the fiber bundle is high. Obtainable. Such a knot strength tends to increase when a sizing agent is applied to the carbon fiber bundle. On the other hand, such as when using a matrix having a high molding temperature, when there is a concern about a decrease in the adhesive strength between the carbon fiber and the matrix due to the thermally decomposed product of the sizing agent, it is preferable not to add a sizing agent from the viewpoint of improving the adhesive strength is there. Therefore, in the present invention, the knot strength of the carbon fiber bundle in a state where sizing is not provided is used as an evaluation index. That is, the evaluation that the loss on heating at 450 ° C. is 0.15% or less means that the sizing material is not provided, or the sizing material is provided and the loss on heating at 450 ° C. exceeds 0.15%. In this case, the evaluation is performed after removing the sizing material. The sizing agent may be removed by a known method, such as a method of removing the sizing agent with a solvent in which the sizing agent is soluble. When the knot strength is low, fluff is likely to be generated during the forming process into the carbon fiber reinforced composite material, and the forming processability tends to decrease. In general, the higher the strand modulus, the lower the knot strength. When the strand elastic modulus and the knot strength satisfy the relationship of the formula (2), the strand elastic modulus and the knot strength can be compatible with a high balance. The proportional constant in the equation (2) is preferably 6.9 × 10 ⁹ , more preferably 7.2 × 10 ⁹ . The carbon fiber in which the strand elastic modulus and the knot strength satisfy the relationship of the formula (2) can be obtained by the carbon fiber manufacturing method of the present invention described later.

It is preferable that the carbon fiber according to the first aspect of the present invention also satisfies the third aspect and / or the second aspect at the same time. Such a carbon fiber can effectively suppress a decrease in moldability even if the strand elastic modulus is high. In particular, when yarn joining is required at the time of forming, the yarn joining portion is less likely to break, which is advantageous for continuous production.

において In the third aspect of the carbon fiber of the present invention, the number of twists is 5 to 80 turns / m. If the number of twists is within the above range, the alignment of the fiber bundles can be controlled with less fluff, and as a result, the stress transmission between the fiber bundles becomes smooth, and the knot strength tends to increase. From the viewpoint of improving the handleability during molding, the number of twists in the third embodiment is preferably from 20 to 80 turns / m.

炭素 When the carbon fiber of the present invention takes the form of a carbon fiber bundle, the twist angle of the surface layer of the carbon fiber bundle is preferably 2.0 to 30.5 °. The twist angle of the surface layer of the carbon fiber bundle is the angle that the fiber axis direction of the single fiber present in the outermost layer of the carbon fiber bundle forms with the long axis direction of the bundle of carbon fiber bundles. However, it can be calculated with higher accuracy from the number of twists, the number of filaments, and the diameter of a single fiber as described later. If the twist angle is controlled within the above range, fluff is suppressed, so that a high tension can be applied in the carbonization step, and the strand elastic modulus is easily increased effectively. The twist angle of the carbon fiber bundle surface layer in the present invention is preferably from 4.8 to 30.5 °, more preferably from 4.8 to 24.0 °, and from 4.8 to 12.5 °. More preferably, the angle is 4.8 to 10.0 °. The carbon fiber bundle having a twist angle satisfying the above range can be produced according to the method for producing carbon fiber of the present invention described later. Specifically, the twist angle of the carbon fiber bundle surface layer can be controlled by adjusting the number of filaments and the diameter of a single fiber in the carbonization step in addition to adjusting the number of twists of the fiber bundle. The larger the number of filaments of the carbon fiber bundle and the diameter of the single fiber, the larger the twist angle can be maintained for the same number of twisted fiber bundles, so that the effect of twisting can be further enhanced.

In the carbon fiber of the present invention, it is preferable that the crystallite size Lc (nm) and the degree of crystal orientation π ₀₀₂ (%) satisfy the relationship of the expression (3).
π ₀₀₂ ≧ 4.0 × Lc + 73.2 Equation (3)
The crystallite size Lc is an index representing the thickness of the crystallite existing in the carbon fiber in the c-axis direction. Usually, evaluation is often made by wide-angle X-ray diffraction of a fiber bundle. However, evaluation is performed on one single fiber by micro-beam wide-angle X-ray diffraction, and an average of measured values for three single fibers is taken. The child size Lc (s) may be used. When the size of the microbeam is equal to or less than the diameter of the single fiber, the average crystallite size Lc (s) is a value obtained by averaging values evaluated at a plurality of points in the diameter direction of the single fiber as an evaluation value of the single fiber, The average value of the evaluation values similarly obtained for the three single fibers is adopted. A detailed evaluation method will be described later. Note that the wide-angle X-ray diffraction data of a single fiber and the generally known wide-angle X-ray diffraction data of a fiber bundle are equivalent, and the average crystallite size Lc (s) and the crystallite size Lc take substantially the same value. The inventors have studied and found that as the crystallite size Lc increases, the degree of crystal orientation π ₀₀₂ tends to increase. Equation (3) empirically shows the upper limit of the relationship from known carbon fiber data. I have. Usually, as the crystallite size Lc increases, the strand elastic modulus increases, but the strand strength, the knot strength, the loop breaking load, and the moldability of the carbon fiber reinforced composite material tend to decrease. The degree of crystal orientation π ₀₀₂ strongly affects the strand elastic modulus, and the higher the degree of crystal orientation, the higher the strand elastic modulus. The fact that the degree of crystal orientation π ₀₀₂ satisfies the relationship of the expression (3) means that the degree of crystal orientation π ₀₀₂ is large for the crystallite size Lc. The knot strength, loop rupture load, and reduction in moldability can be effectively suppressed, and the industrial value is great. In the present invention, the constant term in Expression (3) is more preferably 73.5, and further preferably 74.0. The carbon fiber satisfying the relationship of the formula (3) can be obtained by increasing the drawing tension in the carbonization step.

In the carbon fiber of the present invention, the crystallite size Lc is preferably 2.2 to 3.5 nm, more preferably 2.4 to 3.3 nm or more, and more preferably 2.6 to 3.1 nm. More preferably, it is particularly preferably 2.8 to 3.1 nm. When the crystallite size Lc is 2.2 nm or more, the stress inside the carbon fiber is effectively applied, so that the single fiber elastic modulus is easily increased, and when the crystallite size Lc is 3.5 nm or less, the cause of stress concentration is Therefore, strand strength, knot strength, loop breaking load, and formability are likely to be high. The crystallite size Lc can be controlled mainly by the processing time of the carbonization step and the maximum temperature.

In the carbon fiber of the present invention, the degree of crystal orientation π ₀₀₂ is preferably 80.0 to 95.0%, more preferably 80.0 to 90.0%, and 82.0 to 90.0%. Is more preferable. The degree of crystal orientation π ₀₀₂ is an index indicating the orientation angle of the crystallites present in the carbon fiber with respect to the fiber axis. Similarly to the crystallite size, one single fiber may be evaluated by microbeam wide-angle X-ray diffraction, and the average of the measured values for three single fibers may be taken as the average degree of crystal orientation π ₀₀₂ (s). When the size of the microbeam is equal to or smaller than the diameter of the single fiber, the average degree of crystal orientation π ₀₀₂ (s) is obtained by averaging the values evaluated at a plurality of points in the diameter direction of the single fiber as the evaluation value of the single fiber. The average value of the evaluation values obtained in the same manner for the three single fibers is adopted. A detailed evaluation method will be described later. The wide-angle X-ray diffraction data of a single fiber and the generally known wide-angle X-ray diffraction data of a fiber bundle are equivalent, and the average degree of crystal orientation π ₀₀₂ (s) and the degree of crystal orientation π ₀₀₂ are almost the same. Take. If the degree of crystal orientation is 80.0% or more, the strand elastic modulus tends to be high. The degree of crystal orientation π ₀₀₂ (s) can be controlled by the stretching tension in addition to the temperature and time in the carbonization step.

In the carbon fiber of the present invention, it is preferable that the strand elastic modulus E (GPa) and the crystallite size Lc (nm) satisfy the relationship of the expression (4).
E × Lc ^−0.5 ≧ 200 (GPa / nm ^0.5 ) Equation (4)
As a result of the study by the present inventors, it has been found that when the carbon fiber satisfies the expression (4), the strand elastic modulus and the formability are easily compatible at a particularly high level. Although it is not completely clear why the strand elastic modulus and the formability are easily compatible at a high level by satisfying the expression (4), it is considered as follows. That is, as can be seen from the Hall-Petch equation widely used in the field of polycrystalline materials, the −0.5 power of the crystallite size Lc is an index indicating a certain strength of a material. It can be interpreted that the larger Lc- ^0.5 is, the stronger the material is, and the smaller Lc- ^0.5 is, the more brittle the material is. Therefore, satisfying the expression (4) means that the product of the strand elastic modulus and the toughness of the material is equal to or more than a certain value, and the strand elastic modulus and the toughness of the material are compatible at a high level. It is thought to mean that. The carbon fiber satisfying the formula (4) can be obtained by increasing the drawing tension in the carbonization step.

において In the carbon fiber of the present invention, the surface oxygen concentration O / C is preferably 0.05 to 0.50. The surface oxygen concentration is an index indicating the amount of a functional group containing an oxygen atom introduced into the surface of the carbon fiber, and can be evaluated by photoelectron spectroscopy described later. The higher the surface oxygen concentration, the better the adhesion between the carbon fiber and the matrix, and the easier it is to improve the mechanical properties of the carbon fiber reinforced composite material. The surface oxygen concentration O / C is more preferably 0.07 to 0.30. When the surface oxygen concentration O / C is 0.05 or more, the adhesion to the matrix is at a sufficient level. When the surface oxygen concentration is 0.50 or less, peeling of the carbon fiber surface due to excessive oxidation is suppressed. The mechanical properties of are improved. A method for keeping the surface oxygen concentration O / C within the above range will be described later.

(4) When the carbon fiber of the present invention takes the form of a carbon fiber bundle, the number of filaments is preferably 10,000 or more. The number of filaments is more preferably 15,000 or more, and even more preferably 20,000 or more. If the number of twists is the same, the larger the number of filaments, the greater the distance between the center axis of the twist and the outer circumference of the fiber bundle, so that the twist is easy to stabilize, and fuzzing or breakage occurs even when high tension is applied in the carbonization process. Can be easily suppressed, the strand elastic modulus can be effectively increased, and the moldability can be improved.

Hereinafter, the method for producing carbon fiber of the present invention will be described.

炭素 The carbon fiber precursor fiber bundle which is the basis of the carbon fiber of the present invention can be obtained by spinning a spinning solution of a polyacrylonitrile copolymer.

As the polyacrylonitrile copolymer, not only a homopolymer obtained from acrylonitrile alone, but also other monomers in addition to acrylonitrile as a main component may be used. Specifically, the polyacrylonitrile copolymer preferably contains 90 to 100% by mass of acrylonitrile and less than 10% by mass of a copolymerizable monomer.

Examples of monomers copolymerizable with acrylonitrile include, for example, acrylic acid, methacrylic acid, itaconic acid and their alkali metal salts, ammonium salts and lower alkyl esters, acrylamide and its derivatives, allylsulfonic acid, methallylsulfonic acid and Their salts or alkyl esters can be used.

ポリ The above-mentioned polyacrylonitrile copolymer is dissolved in a solvent in which the polyacrylonitrile copolymer is soluble, such as dimethylsulfoxide, dimethylformamide, dimethylacetamide, nitric acid, an aqueous solution of zinc chloride, and an aqueous solution of rhoda soda, to obtain a spinning solution. When using solution polymerization for the production of polyacrylonitrile copolymer, if the solvent used for the polymerization and the spinning solvent are the same, the step of separating the obtained polyacrylonitrile copolymer and re-dissolving it in the spinning solvent is performed. It is unnecessary and preferable.

炭素 By spinning the spinning solution obtained as described above by a wet or dry-wet spinning method, a carbon fiber precursor fiber bundle can be produced.

The spinning solution is introduced into a coagulation bath for coagulation, and the obtained coagulated fiber bundle is passed through a washing step, a drawing step in a bath, an oil agent applying step, and a drying step, whereby a carbon fiber precursor fiber bundle is obtained. . The coagulated fiber bundle may be stretched directly in the bath without the washing step, or may be stretched in the bath after the solvent is removed in the washing step. The in-bath stretching is usually preferably performed in a single or a plurality of stretching baths whose temperature is controlled at a temperature of 30 to 98 ° C. Further, a dry heat stretching step or a steam stretching step may be added to the above steps.

単 The single fiber fineness of the carbon fiber precursor fiber bundle is preferably 0.9 dtex or more, more preferably 1.0 dtex or more, and still more preferably 1.1 dtex or more. The higher the single fiber fineness of the carbon fiber precursor fiber bundle, the more the fiber bundle breakage caused by contact with rollers and guides is suppressed, and the process stability of the spinning process and the flame resistance of the carbon fiber as well as the preliminary carbonization and carbonization processes Easy to maintain. When the single fiber fineness of the carbon fiber precursor fiber bundle is 0.9 dtex or more, process stability is easily maintained. If the single fiber fineness of the carbon fiber precursor fiber bundle is too high, it may be difficult to uniformly treat the fiber in the flame-proofing step, and the manufacturing process may become unstable or the resulting carbon fiber bundle and carbon fiber dynamics Or the characteristic may be deteriorated. The single fiber fineness of the carbon fiber precursor fiber bundle can be controlled by a known method such as a discharge amount of a spinning solution from a die and a draw ratio.

炭素 The obtained carbon fiber precursor fiber bundle is usually in the form of continuous fiber. Further, the number of filaments per yarn is preferably 1,000 to 80,000. In the present invention, the carbon fiber precursor fiber bundle may be twisted as necessary to adjust the number of filaments per filament of the obtained carbon fiber.

炭素 The carbon fiber of the present invention can be obtained by sequentially performing a preliminary carbonization treatment and a carbonization treatment after the carbon fiber precursor fiber bundle described above is subjected to a flame-proof treatment.

炎 The oxidizing treatment of the carbon fiber precursor fiber bundle is preferably performed in an air atmosphere at a temperature in the range of 200 to 300 ° C. The carbon fiber precursor fiber bundle is subjected to a flame-resistant treatment, and becomes a flame-resistant fiber bundle.

In the present invention, the carbonization of the oxidized fiber bundle is performed subsequent to the oxidization. In the preliminary carbonization step, the oxidized fiber bundle obtained by the oxidization treatment is heat-treated in an inert atmosphere at a maximum temperature of 500 to 1000 ° C. until the density becomes 1.5 to 1.8 g / cm ^3. Is preferred. The oxidized fiber bundle is subjected to a pre-carbonization treatment to form a pre-carbonized fiber bundle.

Further, following the pre-carbonization, the pre-carbonized fiber bundle is carbonized. In the carbonization step, the preliminary carbonized fiber bundle obtained by the preliminary carbonization treatment is subjected to a carbonization treatment in an inert atmosphere. The maximum temperature of the carbonization treatment is preferably 1500 ° C. or higher, more preferably 2300 ° C. or higher. The highest temperature in the carbonization step is preferably higher from the viewpoint of increasing the strand elastic modulus and single fiber elastic modulus of the obtained carbon fiber, and if it is 1500 ° C or higher, the strand elastic modulus, single fiber elastic modulus and knot strength, and the loop A carbon fiber having a high level of breaking load can be obtained. On the other hand, if the carbonization temperature is too high, the knot strength and loop breaking load tend to decrease, so the maximum temperature in the carbonization process is the required strand elastic modulus and single fiber elastic modulus, and the knot strength and loop breaking load. It is better to decide in consideration of balance. The carbon fiber of the present invention can easily maintain the balance of these properties even when the maximum temperature in the carbonization step is 2300 ° C.

In the present invention, the tension in the carbonization step is 5 mN / dtex or more, preferably 5 to 18 mN / dtex, more preferably 7 to 18 mN / dtex, and more preferably 9 to 18 mN / dtex. Is particularly preferred. The tension in the carbonization step is the tension (mN) measured on the exit side of the carbonization furnace, and the total fineness (dtex) which is the product of the single fiber fineness (dtex) of the used carbon fiber precursor fiber bundle and the number of filaments. Shall be removed. By controlling the tension within the above numerical range, the degree of crystal orientation π ₀₀₂ can be controlled without significantly affecting the crystallite size Lc of the obtained carbon fiber, and the aforementioned equation (1) or / And a carbon fiber satisfying the relationship of equation (2) is obtained. From the viewpoint of increasing the strand elastic modulus and single fiber elastic modulus of the carbon fiber, the tension is preferably higher, but if too high, the permeability of the carbonization step and the quality of the obtained carbon fiber may be reduced, It is good to set in consideration of both.

In the method for producing a carbon fiber of the present invention, a method for producing a carbon fiber that further satisfies the following requirement (c) or (ii) is more preferable. It is more preferable that both (c) and (d) are satisfied.
(C) The number of twists of the fiber bundle to be subjected to the carbonization treatment is 2 turns / m or more. (D) The total fineness which is the product of the single fiber fineness (g / km) of the obtained carbon fiber and the number of filaments (number) is By satisfying (c) or (ii) above 740 g / km or more, even if the strand elastic modulus is high, a carbon fiber having excellent moldability can be obtained.

炭素 In the carbon fiber of the present invention, the number of twists of the fiber bundle during the carbonization treatment is 2 turns / m or more. The number of twists is preferably 5 turns / m or more, more preferably 10 turns / m or more, further preferably 16 turns / m or more, and still more preferably 30 turns / m or more. , 46 turns / m or more. Although the upper limit of the number of twists is not particularly limited, it is effective to reduce the number of twists to approximately 60 turns / m or less in order to increase the productivity and the stretching limit in the carbonization step. By controlling the number of twists in the above range, the generation of fluff is suppressed in the carbon fiber production process, so that it is possible to apply a high tension, and a carbon fiber having a high strand elastic modulus and a high single fiber elastic modulus can be obtained. Easy to get. The number of twists of the fiber bundle during the carbonization treatment is the number of twists of the fiber bundle that has been carbonized. If the tension in the carbonization step is increased without imparting twist, single fiber breakage occurs, and the fluff increases, thereby reducing the passability of the carbonization step or causing the entire fiber bundle to break, In some cases, the tension cannot be maintained. The number of twists is such that the carbon fiber precursor fiber bundle or the oxidized fiber bundle, the pre-carbonized fiber bundle is once wound on a bobbin, and then, when the fiber bundle is unwound, the surface orthogonal to the unwinding direction of the bobbin. Can be controlled by, for example, a method of turning the roller bundle or a method of applying a twist by bringing a rotating roller or belt into contact with a running fiber bundle without winding it on a bobbin.

において In the present invention, the number of filaments of the fiber bundle during the carbonization treatment is preferably 10,000 or more, more preferably 15,000 or more, and even more preferably 20,000 or more. If the number of twists of the fiber bundle during the carbonization treatment is the same, the larger the number of filaments, the greater the distance between the center axis of the twist and the outer periphery of the fiber bundle. The single fiber elastic modulus of the obtained carbon fiber can be effectively increased. The upper limit of the number of filaments is not particularly limited, and may be set according to the intended use.

において In the present invention, as the inert gas used in the inert atmosphere, for example, nitrogen, argon, xenon and the like are preferably exemplified, and nitrogen is preferably used from an economic viewpoint.

炭素 The carbon fiber bundle obtained by the above manufacturing method may be subjected to additional graphitization treatment in an inert atmosphere up to 3000 ° C., and the elastic modulus of the single fiber may be appropriately adjusted according to the application.

The carbon fiber bundle obtained as described above is preferably subjected to a surface treatment after the carbonization treatment to introduce a functional group containing an oxygen atom in order to improve the adhesive strength between the carbon fiber and the matrix. As the surface treatment method, gas phase oxidation, liquid phase oxidation, and liquid phase electrolytic oxidation are used, but liquid phase electrolytic oxidation is preferably used from the viewpoint of high productivity and uniform treatment. In the present invention, the method of liquid phase electrolytic oxidation is not particularly limited, and may be a known method. The amount of current at the time of electrolytic surface treatment for performing liquid phase electrolytic oxidation is preferably 2 to 100 c / g, and more preferably 2 to 80 c / g. When the amount of current at the time of electrolytic surface treatment is 2 c / g or more, a sufficient oxygen-containing functional group is introduced into the carbon fiber surface, adhesion to resin is easily obtained, and a decrease in the elastic modulus of the composite material can be suppressed. If it is not more than g, the formation of defects on the carbon fiber surface due to the electrolytic surface treatment can be suppressed, and the decrease in loop breaking load can be suppressed.

表面 By performing such surface treatment such as electrolytic surface treatment, a functional group containing an oxygen atom can be introduced into the carbon fiber bundle, and the surface oxygen concentration O / C of the carbon fiber bundle can be adjusted. In order to control the surface oxygen concentration O / C within a preferable range of the present invention, the current amount and the treatment time in the surface treatment may be adjusted by a known method.

After the electrolytic treatment, a sizing agent can be attached to the carbon fiber bundle so as to further improve the handleability and high-order workability, or to increase the adhesive strength between the carbon fiber and the matrix. The sizing agent can be appropriately selected according to the type of matrix used for the carbon fiber reinforced composite material. In addition, the amount of adhesion and the like may be finely adjusted from the viewpoint of handleability and higher workability. Furthermore, when there is a concern that the adhesive strength between the carbon fiber and the matrix may be reduced due to the thermal decomposition product of the sizing agent, such as when using a matrix with a high molding temperature, the sizing adhesion amount should be reduced as much as possible, and the sizing treatment should be performed. Need not be performed.

測定 The methods for measuring various physical properties described in this specification are as follows. In addition, what was not described in particular was evaluated by measurement n number 1.

<Strand strength and strand elastic modulus of carbon fiber>
The strand strength and the strand elastic modulus of the carbon fiber are determined according to the following procedure in accordance with the resin impregnated strand test method of JIS R7608: 2004. However, in the case where the fiber bundle of carbon fibers has a twist, it is evaluated after untwisting by giving the same number of twists in reverse rotation as the number of twists. As the resin formulation, “CELLOXIDE (registered trademark)” 2021P (manufactured by Daicel Chemical Industries, Ltd.) / Boron trifluoride monoethylamine (manufactured by Tokyo Chemical Industry Co., Ltd.) / Acetone = 100/3/4 (parts by mass) is used. As the curing conditions, normal pressure, a temperature of 125 ° C. and a time of 30 minutes are used. Ten strands of the carbon fiber bundle are measured, and the average value is defined as the strand strength and the strand elastic modulus. Note that the strain range for calculating the strand elastic modulus is 0.1 to 0.6%.

<Average single fiber diameter of carbon fiber>
A cross section of a single fiber of the carbon fiber to be evaluated is observed with a scanning electron microscope to evaluate a cross sectional area. The diameter of a perfect circle having the same cross-sectional area as this cross-sectional area is calculated and defined as a single fiber diameter. The N number for calculating the single fiber diameter is set to 50, and the average value is adopted. The acceleration voltage is 5 keV.

In this example, a scanning electron microscope (SEM) “S-4800” manufactured by Hitachi High-Technologies Corporation was used as the scanning electron microscope.

<Number of twists remaining when one end is fixed end and the other is free end>
A guide bar is installed at a height of 60 cm from the horizontal plane, and an arbitrary position of the carbon fiber bundle is fixed to the guide bar with a tape, and then the carbon fiber bundle is separated at a position 50 cm away from the fixed end. Cut to form free ends. The free end is sealed so as to be sandwiched between tapes, and processed so as not to unravel into single fiber units. In order to eliminate the twists that return temporarily or over time other than the semi-permanent twists, leave them in this state for 5 minutes, and then rotate the free end while counting the number of turns until they are completely untwisted. The number of turns n (turn) is recorded. The number of remaining twists is calculated by the following equation. The average of the above three measurements is taken as the number of remaining twists in the present invention.

数 Number of remaining twists (turns / m) = n (turns) /0.5 (m).

<Elastic modulus of single fiber of carbon fiber>
The single fiber elastic modulus of the carbon fiber is determined as follows with reference to JIS R7606: 2000. First, a bundle of carbon fibers of about 20 cm is divided into approximately four equal parts, and single fibers are sampled sequentially from the four bundles, and the whole bundle is sampled as evenly as possible. The sampled single fiber is fixed on a perforated backing of 10, 25, 50 mm. For fixing, an epoxy adhesive “Araldite (registered trademark)” manufactured by Nichiban Co., Ltd. is used, which is fast-curing type. The backing on which the single fibers were fixed was attached to a tensile tester, and a tensile test was performed at a strain rate of 40% / min and 15 samples at each of the gauge lengths of 10, 25, and 50 mm. From the stress (MPa) -strain (%) curve of each single fiber, the apparent single fiber elastic modulus is calculated from the slope (MPa /%) in the range of strain 0.3-0.7% by the following equation. .

Apparent single fiber elastic modulus (GPa) = Slope in the range of 0.3 to 0.7% strain (MPa /%) / 10
Next, for each of the gauge lengths of 10, 25, and 50 mm, the average value E _app (GPa) of the apparent single fiber elastic modulus was calculated, and the reciprocal 1 / E _app (GPa ⁻¹ ) was plotted on the vertical axis (Y axis). The reciprocal 1 / L ₀ (mm ⁻¹ ) of the gauge length L ₀ (mm) is plotted on the horizontal axis (X axis). The Y intercept in such a plot is read, and the reciprocal of the Y intercept is the single fiber elastic modulus after compliance correction, and this value is used as the single fiber elastic modulus in the present invention.

In this example, a tensile tester "Tensilon RTF-1210" manufactured by A & D Corporation was used as a tensile tester.

<Loop breaking load>
Place a single fiber about 10cm long on a glass slide, drop 1-2 drops of glycerin at the center and twist both ends of the single fiber lightly in the circumferential direction to form a loop at the center of the single fiber. Put the cover glass on. This is set on the stage of a microscope, and a moving image is shot under the conditions that the total magnification is 100 times and the frame rate is 15 frames / second. While adjusting the stage each time so that the loop does not deviate from the field of view, the both ends of the looped fiber are pulled at a constant speed in the opposite direction while being pressed with a finger toward the slide glass, whereby strain is applied until the single fiber breaks. The frame immediately before the break is specified by frame feed, and the width W of the loop immediately before the break is measured by image analysis. The diameter d of the single fiber is divided by W to calculate d / W. The number of n in the test is 20, and the loop strength Es × d / W is obtained by multiplying the average value of d / W by the single fiber elastic modulus Es. Furthermore, multiplied by the cross-sectional area [pi] d ^2/4 obtained from a single fiber diameter, and a loop breaking load of πEs × ^d 3 / 4W.

<Loss on heating of carbon fiber bundle at 450 ° C.>
Those of carbon fiber bundle to be evaluated was cut to be a mass 2.5g to hank wound having a diameter of about 3 cm, is weighed mass w ₀ before heat treatment _(g). Next, the mixture is heated in an oven in a nitrogen atmosphere at a temperature of 450 ° C. for 15 minutes, allowed to cool to room temperature in a desiccator, and then weighed after heating, w ₁ (g). The heating loss rate at 450 ° C. is calculated by the following equation. Evaluation is performed three times, and the average value is adopted.
Loss on heating at 450 ° C. (%) = (w ₀ −w ₁ ) / w ₀ × 100 (%).

<Knotting strength of carbon fiber bundle>
For measurement of knot strength, a carbon fiber bundle having a weight loss rate of 0.15% or less when heated at 450 ° C. was used. When evaluating the carbon fiber bundle to which the sizing has been applied, the sizing agent is removed by washing in acetone, and the dried carbon fiber bundle is used. After drying, the weight loss rate of the carbon fiber bundle when heated at 450 ° C. is evaluated, and the carbon fiber bundle is repeatedly washed until it becomes 0.15% or less.

When the carbon fiber bundle has a twist, it is evaluated after untwisting by imparting the same number of twists in reverse rotation as the number of twists. The carbon fiber bundle having a length of 150 mm is divided or combined so that the total fineness of the carbon fiber bundle is 7000 to 8500 dtex, to obtain a carbon fiber bundle to be used for measurement. The total fineness of the carbon fiber bundle is a product of the average fineness (dtex) of the single fiber of the carbon fiber bundle and the number of filaments. A grip having a length of 25 mm is attached to both ends of the carbon fiber bundle as a test specimen, and when preparing the test specimen, the carbon fiber bundle is aligned by applying a load of 0.1 × 10 ⁻³ N / denier. One knot is formed at the midpoint of the test body, and a bundle tension test is performed with the crosshead speed during tension at 100 mm / min. The measurement is performed on a total of 12 fiber bundles, and an average value of ten fibers obtained by dividing two values of the maximum value and the minimum value is used as a measured value, and the standard deviation of ten fibers is used as a standard deviation of knot strength. As the knot strength, a value obtained by dividing the maximum load value obtained in the tensile test by the average sectional area value of the carbon fiber bundle is used.

<Twist angle of carbon fiber bundle surface layer>
After calculating the diameter (μm) of the entire carbon fiber bundle from the single fiber diameter (μm) and the number of filaments by the following formula, the twist of the surface layer of the carbon fiber bundle is calculated by the following formula using the number of twists (turns / m). Calculate the angle (°).

Diameter (μm) of entire carbon fiber bundle = {(diameter of single fiber) ² × number of filaments} ^0.5
Twist angle (°) of surface layer of carbon fiber bundle = atan (diameter of entire fiber bundle × 10 ⁻⁶ × π × number of twists).
<Crystallite size Lc and degree of crystal orientation π _{002 of} carbon fiber bundle>
A carbon fiber bundle to be subjected to measurement is aligned and solidified using a collodion-alcohol solution to prepare a square pillar measurement sample having a length of 4 cm and a side of 1 mm. The prepared measurement sample is measured using a wide-angle X-ray diffractometer under the following conditions.

1. Measurement of crystallite size Lc ・ X-ray source: CuKα ray (tube voltage 40 kV, tube current 30 mA)
・ Detector: Goniometer + Monochromator + Scintillation counter ・ Scanning range: 2θ = 10-40 °
Scan mode: step scan, step unit 0.02 °, counting time 2 seconds.

(5) In the obtained diffraction pattern, a half-value width is obtained for a peak appearing near 2θ = 25 to 26 °, and a crystallite size is calculated from this value by the following Scherrer equation.

Crystallite size (nm) = Kλ / β ₀ cos θ _B
However,
K: 1.0, λ: 0.15418 nm (wavelength of X-ray)
β ₀ : (β _E ² −β ₁ ² ) ^1/2
β _E : apparent half width (measured value) rad, β ₁ : 1.046 × 10 ⁻² rad
θ _B : Bragg diffraction angle.

2. Measurement of crystal orientation degree π ₀₀₂ The crystal orientation degree π ₀₀₂ is calculated from the half width of the intensity distribution obtained by scanning the above-mentioned crystal peak in the circumferential direction using the following equation.
π ₀₀₂ = (180−H) / 180
However,
H: Apparent half width (deg)
The above measurement is performed three times, and the arithmetic average is defined as the crystallite size and the degree of crystal orientation of the carbon fiber bundle.

In the examples and comparative examples described below, XRD-6100 manufactured by Shimadzu Corporation was used as the wide-angle X-ray diffractometer.

<Average crystallite size Lc (s) and average crystal orientation degree π ₀₀₂ (s) of carbon fiber single fiber>
Single fibers are randomly extracted from the carbon fiber bundle, and wide-angle X-ray diffraction measurement is performed using an apparatus capable of using an X-ray μ-beam. The measurement is carried out using a microbeam having a wavelength of 0.1305 nm arranged in a shape of 3 μm in the fiber axis direction and 1 μm in the fiber diameter direction while scanning the single fiber in steps of 1 μm in the fiber diameter direction. The irradiation time for each step is 2 seconds. The camera length, which is the distance between the detector and the sample, is set to fall within the range of 40 to 200 mm. The camera length and the coordinates of the beam center are determined by measuring cerium oxide as a standard sample. By subtracting the two-dimensional diffraction pattern measured by removing the sample from the detected two-dimensional diffraction pattern, dark noise due to the detector and scattering noise due to air are canceled, and a corrected two-dimensional diffraction pattern is obtained. . By adding the corrected two-dimensional diffraction patterns at each position in the fiber diameter direction of the single fiber, an average two-dimensional diffraction pattern in the fiber diameter direction of the single fiber is obtained. In such an average two-dimensional diffraction pattern, a sector integral is performed at an angle of ± 5 ° around the direction orthogonal to the fiber axis to obtain a diffraction intensity profile in the 2θ direction. The least square fitting of the diffraction intensity profile in the 2θ direction using two Gaussian functions is performed, and the angle 2θ _m (°) of 2θ at which the diffraction intensity is maximum and the full width at half maximum FWHM (°) of the combined function of the two Gaussian functions are obtained. calculate. Further, the circumference integral is performed at a width of ± 5 ° around the angle 2θ _m (°) at which the diffraction intensity profile in the 2θ direction becomes the maximum, and the diffraction intensity profile in the circumferential direction is obtained. The full width at half maximum FWHM _β (°) is calculated by performing least-squares fitting of the diffraction intensity profile in the circumferential direction using one Gaussian function. The crystallite size Lc (s) and the degree of crystal orientation π ₀₀₂ (s) of the single fiber are obtained by the following formulas, and the results for each of the three single fibers are averaged to obtain the average crystallite size Lc (s) and the average crystal size. The degree of orientation π ₀₀₂ (s) is calculated.

Lc (s) (nm) = Kλ / FWHMcos (2θ _m / 2)
Here, Scherrer coefficient K is 1.0, the X-ray wavelength λ is 0.1305Nm, full width half maximum FWHM and 2 [Theta] _m is used to convert the units from the angle (°) in radian (rad).

π ₀₀₂ (s) (%) = (180−FWHM _β ) / 180 × 100 (%).

In this embodiment, the SPring-8 beam line BL03XU (FSBL) second hatch is used as an apparatus capable of using the X-ray μ-beam, and the flat panel detector “C9827DK-10” (manufactured by Hamamatsu Photonics KK) is used as a detector. A pixel size of 50 μm × 50 μm) was used.

<Surface oxygen concentration of carbon fiber O / C>
The surface oxygen concentration O / C of the carbon fiber is determined by X-ray photoelectron spectroscopy according to the following procedure. First, the carbon fiber from which dirt attached to the surface has been removed using a solvent is cut into about 20 mm and spread on a copper sample support. Next, the sample support is set in the sample chamber, and the inside of the sample chamber is maintained at 1 × 10 ⁻⁸ Torr. Subsequently, using AlK _{1, 2} as an X-ray source and measures the photoelectron escape angle as 90 °. Note that combined the binding energy value of the main peak (peak top) of the _{C 1s} as a correction value of the peak due to the measurement time of the charged _{286.1EV, C 1s} peak area drawing a linear base line in a range of 282 ~ 296eV We ask by doing. Further, the O _1s peak area is determined by drawing a straight base line in the range of 528 to 540 eV. Here, the surface oxygen concentration is calculated as the atomic number ratio from the above ratio of the O _1s peak area to the C _1s peak area using a sensitivity correction value unique to the apparatus. In this example, ESCA-1600 manufactured by ULVAC-PHI was used as the X-ray photoelectron spectroscopy apparatus, and the sensitivity correction value unique to the apparatus was 2.33.

<Driving stability>
As a model evaluation of the formability, the running stability is evaluated as follows. A running stability evaluation unit is prepared in which five V-groove rollers having a diameter of 50 mm, a groove width of 10 mm, and a groove depth of 10 mm are linearly fixed at 300 mm intervals. The carbon fiber bundle to be evaluated is passed through each V-groove roller of the running stability evaluation unit in a zigzag manner so as to contact the upper surface, lower surface, upper surface, lower surface, and upper surface in a state where the sizing agent is not applied, and the dancer weight is The vehicle is run at a linear velocity of 10 m / min for 30 minutes while applying a tension of 1 kg with. Thereafter, the five V-groove rollers after removing the carbon fiber bundles are graded as follows according to the state of the rollers at the time of visual inspection.
A: No adhesion of carbon fiber to the roller is observed. In addition, among A, those in which the carbon fibers did not adhere to the roller even after running for 150 minutes are particularly designated as AA.
B: Slight wrapping of the carbon fiber around the rollers is observed (winding is observed on one or two of the five rollers).
C: Winding of the carbon fiber around the roller is observed. (Three or four out of five rollers can be wrapped)
D: The winding of the carbon fiber around the roller is remarkable. (Winding is seen on all five rollers)

Hereinafter, the present invention will be described in detail with reference to Examples, but the present invention is not limited thereto.

実施 Examples 1 to 11 and Comparative Examples 1 to 16 described below were carried out in the manner described in the following comprehensive examples using the conditions described in Table 1 or Table 2.

[Comprehensive embodiment]
A monomer composition comprising acrylonitrile and itaconic acid was polymerized by a solution polymerization method using dimethyl sulfoxide as a solvent to obtain a spinning solution containing a polyacrylonitrile copolymer. After filtering the obtained spinning solution, the spinning solution was once discharged into the air from a spinneret and introduced into a coagulation bath comprising an aqueous solution of dimethyl sulfoxide to obtain a coagulated yarn by a dry-wet spinning method. After the coagulated yarn was washed with water, it was stretched at a draw ratio of 3 times in hot water of 90 ° C. in a bath, further applied with a silicone oil agent, and dried using a roller heated to a temperature of 160 ° C., Pressurized steam drawing was performed at a draw ratio of 4 times to obtain a carbon fiber precursor fiber bundle having a single fiber fineness of 1.1 dtex. Next, the obtained carbon fiber precursor fiber bundles are combined into four filaments to make the number of single fibers 12,000, and heat treatment is performed in an oven at 240 to 280 ° C. in an air atmosphere with a draw ratio of 1, and heat treatment is performed. Turned into

[Example 1]
After obtaining the oxidized fiber bundle by the method described in the comprehensive example, the obtained oxidized fiber bundle is twisted to give a twist of 75 turns / m, in a nitrogen atmosphere at a temperature of 300 to 800 ° C. , A preliminary carbonization treatment was performed at a draw ratio of 0.97 to obtain a preliminary carbonized fiber bundle. Next, the carbonized fiber bundle is subjected to a carbonization treatment under the conditions shown in Table 1, and then subjected to an electrolytic surface treatment using an aqueous sulfuric acid solution as an electrolytic solution at an electric quantity of 30 coulombs per gram of carbon fiber to obtain a surface oxygen concentration. A carbon fiber bundle with (O / C) of 0.09 was obtained. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. The formability grade was AA, a very high level. Table 1 shows the evaluation results of the obtained carbon fibers.

[Example 2]
A carbon fiber bundle was obtained in the same manner as in Example 1 except that the number of twists was set to 50 turns / m and the tension during the carbonization treatment was set to 5.2 mN / dtex. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. The formability grade was AA, a very high level. Table 1 shows the evaluation results of the obtained carbon fibers.

[Example 3]
A carbon fiber bundle was obtained in the same manner as in Example 2, except that the tension during the carbonization treatment was set to 10.2 mN / dtex. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. The formability grade was AA, a very high level. Table 1 shows the evaluation results of the obtained carbon fibers.

[Example 4]
A carbon fiber bundle was obtained in the same manner as in Example 1, except that the number of twists was 20 turns / m and the tension during the carbonization treatment was 10.3 mN / dtex. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. The formability grade was AA, a very high level. Table 1 shows the evaluation results of the obtained carbon fibers.

[Example 5]
A carbon fiber bundle was obtained in the same manner as in Example 3 except that the number of twines of the precursor fiber bundle was set to 8 and the number of single fibers was set to 24,000 in the comprehensive example. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. The formability grade was AA, a very high level. Table 1 shows the evaluation results of the obtained carbon fibers.

[Example 6]
A carbon fiber bundle was obtained in the same manner as in Example 2, except that the maximum temperature of the carbonization treatment was 2350 ° C and the tension during the carbonization treatment was 6.5 mN / dtex. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. The formability grade was A, a high level. Table 1 shows the evaluation results of the obtained carbon fibers.

[Example 7]
A carbon fiber bundle was obtained in the same manner as in Example 6, except that the tension during the carbonization treatment was set to 9.1 mN / dtex. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. The formability grade was A, a high level. Table 1 shows the evaluation results of the obtained carbon fibers.

Example 8
A carbon fiber bundle was obtained in the same manner as in Example 6, except that the tension during the carbonization treatment was set to 11.6 mN / dtex. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. The formability grade was A, a high level. Table 1 shows the evaluation results of the obtained carbon fibers.

[Example 9]
A carbon fiber bundle was obtained in the same manner as in Example 5, except that the number of twists was 20 turns / m and the tension during the carbonization treatment was 11.0 mN / dtex. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. The formability grade was AA, a very high level. Table 1 shows the evaluation results of the obtained carbon fibers.

[Example 10]
A carbon fiber bundle was obtained in the same manner as in Example 9 except that the number of twists was changed to 5 turns / m. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. The formability grade was AA, a very high level. Table 1 shows the evaluation results of the obtained carbon fibers.

[Example 11]
In the comprehensive example, a carbon fiber bundle was obtained in the same manner as in Example 3, except that the number of twined yarns of the precursor fiber bundle was changed to 2, and the number of single fibers was changed to 6,000. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. The formability grade was A, a high level. Table 1 shows the evaluation results of the obtained carbon fibers.

[Comparative Example 1]
A carbon fiber bundle was obtained in the same manner as in Example 1, except that the number of twists was set to 0 turn / m and the tension during the carbonization treatment was set to 5.3 mN / dtex. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. Since the number of remaining twists was out of the range of the present invention, the grade of the formability was B and was lower than that of Example 1. Table 2 shows the evaluation results of the obtained carbon fibers.

[Comparative Example 2]
A carbon fiber bundle was obtained in the same manner as in Example 3, except that the number of twists was 0 turn / m, the tension during the carbonization treatment was 5.4 mN / dtex, and the maximum temperature was 1400 ° C. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. Since the number of remaining twists was out of the range of the present invention, the grade of the formability was B and was lower than that of Example 1. Table 2 shows the evaluation results of the obtained carbon fibers.

[Comparative Example 3]
A carbon fiber bundle was obtained in the same manner as in Example 2, except that the tension during the carbonization treatment was set to 1.0 mN / dtex. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. Further, although the grade of the formability was A, which was a high level, the elastic modulus of the obtained carbon fiber was lower than that of Example 1 because the tension at the time of carbonization was out of the range of the present invention. did. Table 2 shows the evaluation results of the obtained carbon fibers.

[Comparative Example 4]
A carbon fiber bundle was prepared in the same manner as in Example 2 except that a carbon fiber precursor fiber bundle having a single fiber fineness of 0.8 dtex was used, the tension during the carbonization treatment was 10.3 mN / dtex, and the maximum temperature was 1400 ° C. Got. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. Since the carbon fiber precursor fiber bundle having a small single fiber fineness was used, the grade of the moldability was B, which was lower than that of Example 2. Table 2 shows the evaluation results of the obtained carbon fibers.

[Comparative Example 5]
A carbon fiber bundle was obtained in the same manner as in Example 2, except that the tension during the carbonization treatment was set to 1.0 mN / dtex, and no twist was applied. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. The grade of moldability was B, which was slightly lower. Table 2 shows the results of the evaluation of the obtained carbon fiber bundle.

[Comparative Example 6]
A carbon fiber bundle was prepared in the same manner as in Example 2 except that a carbon fiber precursor fiber bundle having a single fiber fineness of 0.8 dtex was used, the tension during the carbonization treatment was 10.3 mN / dtex, and the maximum temperature was 1900 ° C. Got. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. Since the number of remaining twists is out of the range of the present invention, the grade of the formability is B and lower than that of Example 2. Table 2 shows the results of the evaluation of the obtained carbon fiber bundle.

[Comparative Example 7]
A carbon fiber bundle was obtained in the same manner as in Example 6, except that the tension during the carbonization treatment was set to 1.6 mN / dtex. The carbonization process passability was good, and the quality of the obtained carbon fiber bundle was also good. The grade of moldability was B, which was slightly lower. Table 2 shows the evaluation results of the obtained carbon fibers.

[Comparative Example 8]
Except that the number of twists was set to 0 turns / m, carbon fibers were formed in the same manner as in Example 3. In the carbonization step, a phenomenon in which the yarn being processed was broken repeatedly occurred, and it was difficult to collect a carbon fiber bundle.

[Comparative Example 9]
A carbon fiber bundle was obtained in the same manner as in Example 2, except that the number of twists was set to 0 turns / m. Although some fluff was observed in the carbonization step, a carbon fiber bundle could be collected. The obtained carbon fiber bundle had fluff, and the quality was low. Since the number of remaining twists is out of the range of the present invention, the grade of the formability is B and lower than that of Example 2. Table 2 shows the evaluation results.

[Comparative Example 10]
A carbon fiber bundle was obtained in the same manner as in Comparative Example 9, except that the tension during the carbonization treatment was set to 3.4 mN / dtex. The passability of the carbonization step was good, and the quality of the obtained carbon fiber bundle was also good. Since the tension during the carbonization treatment was out of the range of the present invention, the elastic modulus of the obtained carbon fiber was lower than that of Example 2. In addition, since the number of remaining twists was out of the range of the present invention, the grade of the formability was B and was lower than that of Example 2. Table 2 shows the evaluation results.

[Comparative Example 11]
In the comprehensive example, the number of plied yarns of the precursor fiber bundle was 2, the number of single fibers was 6,000, the number of twists was 0 turns / m, and the tension during the carbonization treatment was 3.4 mN / dtex. A carbon fiber bundle was obtained in the same manner as in Example 2 except that the above conditions were satisfied. The passability of the carbonization step was good, and the quality of the obtained carbon fiber bundle was also good. Since the tension during the carbonization treatment was out of the range of the present invention, the elastic modulus of the obtained carbon fiber was lower than that of Example 2. Since the number of remaining twists and the total fineness were out of the range of the present invention, the grade of the formability was C, which was lower than that of Example 2. Table 2 shows the evaluation results.

[Comparative Example 12]
A carbon fiber bundle was obtained in the same manner as in Comparative Example 11, except that the number of twists was changed to 50 turns / m. The passability of the carbonization step was good, and the quality of the obtained carbon fiber bundle was also good. Since the tension during the carbonization treatment was out of the range of the present invention, the elastic modulus of the obtained carbon fiber was lower than that of Example 2. Since the total fineness was out of the range of the present invention, the grade of the formability was B and was lower than that of Example 2. Table 2 shows the evaluation results.

[Comparative Example 13]
In the comprehensive example, a carbon fiber bundle was obtained in the same manner as in Example 2, except that the single fiber fineness of the precursor fiber bundle was set to 0.8 dtex and the tension during the carbonization treatment was set to 3.4 mN / dtex. Was. The passability of the carbonization step was good, and the quality of the obtained carbon fiber bundle was also good. Since the tension during the carbonization treatment was out of the range of the present invention, the elastic modulus of the obtained carbon fiber was lower than that of Example 2. Since the carbon fiber precursor fiber bundle having a small single fiber fineness was used, the grade of the moldability was B, which was lower than that of Example 2. Table 2 shows the evaluation results.

[Comparative Example 14]
A carbon fiber bundle was obtained in the same manner as in Comparative Example 13 except that the number of twists was set to 0 turns / m. The passability of the carbonization step was good, and the quality of the obtained carbon fiber bundle was also good. Since the tension during the carbonization treatment was out of the range of the present invention, the elastic modulus of the obtained carbon fiber was lower than that of Example 2. Since the carbon fiber precursor fiber bundle having a small single fiber fineness was used and the number of remaining twists was out of the range of the present invention, the grade of the moldability was D, and the stability was lower than that of Example 2. Further decline. Table 2 shows the evaluation results.

[Comparative Example 15]
In the comprehensive example, a carbon fiber bundle was obtained in the same manner as in Comparative Example 13 except that the number of yarns of the precursor fiber bundle was two and the number of single fibers was 6,000. The passability of the carbonization step was good, and the quality of the obtained carbon fiber bundle was also good. Since the tension during the carbonization treatment was out of the range of the present invention, the elastic modulus of the obtained carbon fiber was lower than that of Example 2. Since the carbon fiber precursor fiber bundle having a small single fiber fineness was used and the total fineness was out of the range of the present invention, the grade of the formability was C, which was lower than that of Example 2. Table 2 shows the evaluation results.

[Comparative Example 16]
A carbon fiber bundle was obtained in the same manner as in Comparative Example 15 except that the number of twists was set to 0 turns / m. The passability of the carbonization step was good, and the quality of the obtained carbon fiber bundle was also good. Since the tension during the carbonization treatment was out of the range of the present invention, the elastic modulus of the obtained carbon fiber was lower than that of Example 2. Since the carbon fiber precursor fiber bundle having a small single fiber fineness was used, and the number of twists remaining and the total fineness were out of the range of the present invention, the formability was D, which was more stable than that of Example 2. Sex was further reduced. Table 2 shows the evaluation results.

[Reference Example 1]
Table 2 shows the evaluation results of "Trayca (registered trademark)" T700S manufactured by Toray Industries, Inc. In addition, the knot strength in a state where the sizing was given was 826 MPa. The grade of moldability was B, which was slightly lower.

[Reference Example 2]
Table 2 shows the results of the evaluation of "Torayca (registered trademark)" M35J manufactured by Toray Industries, Inc.

[Reference Example 3]
Table 2 shows the evaluation results of Torayca M40J manufactured by Toray Industries, Inc.

[Reference Example 4]
Table 2 shows the evaluation results of "Torayca (registered trademark)" M46J manufactured by Toray Industries, Inc.

[Reference Example 5]
Table 2 shows the evaluation results of "Torayca (registered trademark)" M40 manufactured by Toray Industries, Inc.

(4) The present invention relates to a carbon fiber which has both excellent tensile modulus and processability into a composite material, and can easily maintain a fiber length even when used as a discontinuous fiber, and a method for producing the same. The carbon fiber bundle obtained in the present invention is suitably used for general industrial applications such as aircraft, automobiles and marine members, and sports applications such as golf shafts and fishing rods, utilizing such features.

Claims

A carbon fiber having a strand elastic modulus of 360 GPa or more, a strand strength of 3.5 GPa or more, a single fiber diameter of 6.0 μm or more, and further satisfying the following requirements (a) or (b).
(B) When one end is a fixed end and the other end is a free end rotatable with respect to the axis of the fiber bundle, the number of twists remaining is 2 turns / m or more. The total fineness, which is the product of the fiber fineness (g / km) and the number of filaments (lines), is 740 g / km or more.
2. The carbon fiber according to claim 1, wherein the single fiber elastic modulus Es (GPa) and the loop breaking load A (N) satisfy the relationship of Expression (1). 3.
A ≧ −0.0017 × Es + 1.02 Formula (1)
The relationship between the single fiber diameter is 6.0 μm or more, the strand elastic modulus E (GPa) and the knot strength B (MPa) evaluated at a loss rate of heating at 450 ° C. of 0.15% or less satisfies the formula (2). The carbon fiber according to claim 1, wherein the number of twists is 20 to 80 turns / m.
B ≧ 6.7 × 10 9 × E -2.85 Formula (2)
The carbon fiber according to any one of claims 1 to 3, wherein the total fineness is 850 g / km or more.
The carbon fiber according to any one of claims 1 to 4, wherein a strand elastic modulus is 440 GPa or more.
The carbon fiber according to any one of claims 1 to 5, wherein the twist angle of the surface layer of the carbon fiber bundle is 2.0 to 30.5 °.
The carbon fiber according to claim 6, wherein the twist angle of the surface layer of the carbon fiber bundle is 4.8 to 10.0 °.
The carbon fiber according to any one of claims 1 to 7, wherein a single fiber diameter is 6.5 μm or more.
The carbon fiber according to any one of claims 1 to 8, wherein a single fiber diameter is 7.4 μm or less.
The carbon fiber according to any one of claims 1 to 9, wherein the crystallite size Lc (nm) and the degree of crystal orientation π 002 (%) satisfy the relationship of Expression (3).
π 002 ≧ 4.0 × Lc + 73.2 Equation (3)
The carbon fiber according to any one of claims 1 to 10, wherein the crystallite size Lc is 2.2 to 3.5 nm.
The carbon fiber according to any one of claims 1 to 11, wherein the strand elastic modulus E (GPa) and the crystallite size Lc (nm) satisfy the relationship of Expression (4).
E × Lc −0.5 ≧ 200 (GPa / nm 0.5 ) Equation (4)
The carbon fiber according to any one of claims 1 to 12, wherein the surface oxygen concentration O / C is 0.05 to 0.50.
The carbon fiber bundle according to any one of claims 1 to 13, wherein the number of filaments is 10,000 or more.
A carbon fiber in which the single fiber elastic modulus Es (GPa) and the loop breaking load A (N) satisfy the relationship of Expression (1).
A ≧ −0.0017 × Es + 1.02 Formula (1)
The relationship between the single fiber diameter is 6.0 μm or more, the strand elastic modulus E (GPa) and the knot strength B (MPa) evaluated at a loss rate of heating at 450 ° C. of 0.15% or less satisfies the formula (2). And carbon fibers having a twist number of 5 to 80 turns / m.
B ≧ 6.7 × 10 9 × E -2.85 Formula (2)
The carbon fiber according to claim 15 or 16, wherein a single fiber elastic modulus or a strand elastic modulus is 360 GPa or more.
The carbon fiber precursor fiber bundle is subjected to an oxidizing treatment in an air atmosphere at a temperature range of 200 to 300 ° C., and the obtained oxidized fiber bundle is densified at a maximum temperature of 500 to 1000 ° C. in an inert atmosphere. A method for producing carbon fibers, comprising performing preliminary carbonization by heat-treating to 1.5 to 1.8 g / cm 3 and heat-treating the obtained preliminarily carbonized fiber bundle in an inert atmosphere. The single fiber fineness of the carbon fiber precursor fiber bundle is 0.9 dtex or more, the tension during the carbonization treatment is controlled to 5 mN / dtex or more, and the following (c) or (d) is satisfied. A method for producing a carbon fiber having 360 GPa or more.
(C) The number of twists of the fiber bundle to be subjected to the carbonization treatment is 2 turns / m or more. (D) The total fineness which is the product of the single fiber fineness (g / km) of the obtained carbon fiber and the number of filaments (number) is 740 g / km or more
The method for producing carbon fibers according to claim 18, wherein the number of twists of the fiber bundle to be subjected to the carbonization treatment is 16 turns / m or more.
The method for producing carbon fibers according to claim 18, wherein the maximum temperature of the carbonization treatment is 1500 ° C. or higher.
The method for producing carbon fibers according to claim 20, wherein the maximum temperature of the carbonization treatment is 2300 ° C or higher.
The method for producing carbon fibers according to any one of claims 18 to 21, wherein an electrolytic surface treatment is performed at a current amount of 2 to 100 c / g after the carbonization treatment.