JP2018009274A - Carbon fiber bundle - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a carbon fiber bundle hardly providing effect on tensile strength even with increasing defects due to external factors and stable in quality by controlling the amount of defects in a carbon fiber in advance.SOLUTION: There is provided a carbon fiber bundle having a ratio between loop strength σand 2.5 times of strand strength σ, σ/σof 0.150 or more, and strand strength σof 5.5 GPa or more. The loop strength σindicates the product of a ratio between average single fiber diameter d and loop width just before breakage W evaluated by a single fiber loop method, d/W and strand elasticity evaluated in a resin tensile test, E×d/W, and strand strength σindicates tensile strength evaluated by the resin impregnation strand tensile test.SELECTED DRAWING: None

Description

  The present invention relates to a high-performance carbon fiber bundle. More specifically, the present invention relates to a carbon fiber bundle having a stable quality, which does not easily affect the tensile strength even if defects increase due to external factors.

  Since carbon fibers have extremely high specific strength and specific elastic modulus, they have been actively used in recent years in various industrial fields as reinforcing fibers for composite materials. In fields where weight reduction is particularly important, replacement of conventional metal materials with carbon fiber composite materials is accelerating. Specifically, in addition to conventional mainstream sports and aerospace applications, it is widely deployed in general industrial applications such as automobiles, civil engineering / architecture, pressure vessels, and windmill blades. There is a strong demand for both cost reduction. In addition, the carbon fiber market is expected to continue to grow at an annual rate of 10% or more, and improvement of supply capacity is also important. That is, it can be said that a stable supply of high-performance and low-cost carbon fibers is a common issue for carbon fiber manufacturers.

  Since carbon fiber is a brittle material, if there is a slight defect, breakage occurs starting from this. For this reason, since the development of carbon fiber, much effort has been devoted to reducing defects. The mainstream polyacrylonitrile carbon fiber is about 3 GPa in 1971 when industrial production started for the first time. The tensile strength has been raised to a high level of about 7 GPa mainly by suppressing defects. However, since the cause of the defect is not completely clear, when the tensile strength fluctuates due to some external factor, enormous time and labor are often required to understand the factor and take countermeasures.

  As an attempt to suppress the defects of carbon fibers, Patent Document 1 combines high molecular weight of polyacrylonitrile-based polymer, two-stage application of oil agent, densification of precursor fibers, cleaning of the manufacturing process, and refinement of fineness, Patent Document 2 proposes a combination of cleansing, densification, and electrolytic treatment.

  Patent Document 3 proposes improving the fracture toughness by taking specific firing conditions as a means for improving the strength independent of the defects of the carbon fiber.

Japanese Patent Laid-Open No. 11-241230 Japanese Patent Publication No. 8-6210 Patent No. 5907321

  According to Patent Documents 1 and 2, a carbon fiber with very few defects can be obtained, so that the tensile strength as a base is improved, but when defects increase due to some external factor, there are very few defects originally present. Therefore, there is a problem that the tensile strength is likely to fluctuate. Further, according to Patent Document 3, the carbon fiber becomes high strength by improving the fracture toughness, but the ease of fluctuation of the tensile strength when the defect is increased by an external factor is not improved.

  Therefore, an object of the present invention is to provide a carbon fiber bundle having a stable quality, in which a certain amount of defects are included in the carbon fiber in advance, and even if defects increase due to external factors, the tensile strength is hardly affected. Is to provide.

In order to solve the above problem, the carbon fiber bundle of the present invention has a ratio σ _L / σ _S ^2.5 of the 2.5th power of the loop strength σ _L and the strand strength σ _S of 0.150 or more, A carbon fiber bundle having a strand strength σ _S of 5.5 GPa or more. Here, the loop strength σ _L of the present invention means the average single fiber diameter d and the ratio d / W of the loop width W just before break evaluated by the single fiber loop method and the strand elasticity evaluated by the resin impregnated strand tensile test. This refers to the product E × d / W of the rate E, and the strand strength σ _S of the present invention refers to the tensile strength evaluated by the resin-impregnated strand tensile test (in the present invention, unless otherwise specified, tensile strength) Refers to the strand strength).

  The carbon fiber bundle of the present invention includes a controlled fixed amount of defects in advance, so that even if the defects increase due to external factors, the tensile strength is hardly affected, and as a result, the quality of the carbon fiber bundle is stable. It becomes.

In the carbon fiber bundle of the present invention, the ratio σ _L / σ _S ^2.5 of the 2.5th power of the loop strength σ _L and the strand strength σ _S is 0.150 or more, and the strand strength is 5.5 GPa or more. It is a carbon fiber bundle. Here, the loop strength σ _L is the ratio of the average single fiber diameter d to the ratio d / W of the loop width W just before break evaluated by the single fiber loop method and the strand elastic modulus E evaluated by the resin impregnated strand tensile test. It refers to the product E × d / W, and the strand strength σ _S refers to the tensile strength evaluated by the resin-impregnated strand tensile test. The single fiber loop method is a technique for examining the relationship between strain applied to a single fiber by deforming the single fiber into a loop shape and fracture behavior such as single fiber breakage or buckling. When a single fiber is deformed in a loop shape, compressive strain is applied to the inside of the single fiber, and tensile strain is applied to the outside. Since compression buckling occurs before tensile failure, the single fiber loop method has been traditionally used as a test method for carbon fiber single fiber compressive strength, but testing continues even when compression buckling occurs. As a result, it was finally found that tensile fracture occurred. At this time, since the tensile failure occurs near the top of the loop, the tensile failure by the single fiber loop method has the same meaning as the tensile test of an extremely short sample length. When the length of a tensile test is extremely short, the probability that a defect is included in the length is reduced, making it less susceptible to defects and evaluating the strength of the carbon fiber substrate itself, that is, a certain reachable tensile strength. can do. d / W is a value proportional to the tensile strain, and it can be said that the product of this value and the strand elastic modulus E is a value corresponding to the tensile strength. On the other hand, it is known that the strand strength is influenced by the substrate of the carbon fiber itself, but at the same time, it is strongly influenced by defects. For this reason, when the number of defects contained in the carbon fiber increases, the loop strength σ _L is hardly affected, whereas the strand strength σ _S decreases. Therefore, the inventors searched for the most suitable parameter for expressing the amount of defects by trial and error, and found that the ratio σ _L / σ _S ^2.5 was optimal. Such a ratio σ _L / σ _S ^2.5 shows a behavior that the carbon fiber contains a small number of defects and becomes small, and the number of defects becomes large. When the ratio σ _L / σ _S ^2.5 is 0.150 or more, the amount of defects contained in the carbon fiber is large, and even if the number of defects increases due to external factors, the influence on the tensile strength is small. It becomes a stable carbon fiber bundle. The ratio σ _L / σ _S ^2.5 is preferably 0.160 or more, and more preferably 0.165 or more. The reason why the ratio σ _L / σ _S ^2.5 is appropriate as an index for expressing the amount of defects is not completely clarified, but is considered as follows. That is, it is widely known that the single fiber tensile strength of carbon fiber follows the Griffith equation shown in equation (1).

Single fiber tensile strength = fracture toughness value / shape factor / (π × defect size) ^0.5 (1)
Considering that the strand strength is approximately proportional to the single fiber tensile strength, it can be considered from equation (1) that the strand strength σ _S is proportional to the −0.5th power of the defect size. On the other hand, since the loop strength σ _L is not affected by defects, the ratio σ _L / σ _S ^2.5 is σ _L / σ _S ^2.5 ∝1 / (defect size− ^0.5 ) ^2.5 = It is considered that the defect size dependency such as the defect size ^1.25 is exhibited. There is a positive correlation between the number of defects per unit length and the defect size that causes the failure. That is, as the number of defects per unit length increases, the defect size that causes destruction increases. Although the exact relationship is not in a clear, that their relationship is a relationship such as defect size ^{1.25 alpha} defects amount, eventually the ratio σ _{L /} σ _S ^2.5 is sigma _L / σ _S ^2.5比例 Defect amount is considered to be proportional. That is, the ratio σ _L / σ _S ^2.5 is considered as a measure of the amount of defects, and the ratio is 0.150 or more means that the amount of defects contained in the carbon fiber is a normal carbon fiber manufacturing process. Is relatively large with respect to the amount of flaw variation due to external factors assumed in the above, and the rate of change in the total amount of flaws is small. It is done.

Next, a method for introducing defects into the carbon fiber will be described. As a technique for introducing scratches on the surface of the material, a technique such as sandblasting is generally used. However, it is very difficult to introduce defects of just the right size on the surface of each single fiber of carbon fiber having a diameter of several μm, which results in the destruction of the fiber itself. There is generally no known method for introducing defects while controlling a very fine object called carbon fiber. Thus, as a result of investigations by the present inventors, it was found that the defects of the carbon fiber finally obtained show an increasing tendency when the spinning solution is stored at a low temperature for a long time and then subjected to spinning. The reason for this is not clear, but by placing the spinning solution in the environment as described above, the relaxation time of the entanglement between the molecular chains of the polyacrylonitrile polymer becomes longer, and the entanglement becomes difficult to unravel. It is considered that a heterogeneous region is formed, and such a heterogeneous region takes a deformation behavior different from that of the surrounding region in the subsequent process of forming the structure, thereby finally becoming a defect of the carbon fiber. A specific method of storing at the low temperature is to store at a temperature of 25 to 35 ° C. for 24 hours or more in an inert gas atmosphere. At this time, in order to make the whole temperature uniform, it is possible to stir at a low speed up to 5 rpm, or to circulate in a pipe having a heat exchanger. Is obtained. The storage temperature and time vary depending on the composition and molecular weight of the polyacrylonitrile polymer, the solvent used, the concentration of the spinning solution, etc., but the ratio of the loop strength σ _L to the 2.5th power of the strand strength σ _S σ _L / sigma _S ^2.5 may be appropriately adjusted by trial and error so that the scope of the present invention.

In the carbon fiber bundle of the present invention, the ratio σ _L / σ _S of the loop strength σ _L and the strand strength σ _S is preferably 2.3 or more, more preferably 2.4 or more, and 2.5 or more. More preferably. While the loop strength σ _L is not affected by the defect, from the above equation (1), the strand strength σ _S is proportional to the −0.5 power of the defect size, so the ratio σ _L / σ _S is It is considered that the defect size is reflected such that σ _L / σ _S欠 陥 1 / (defect size− ^0.5 ) = defect size ^0.5 . As already mentioned, there is a positive correlation between the number of defects per unit length and the defect size that causes destruction, so when the number of defects per unit length increases, the defect size that causes destruction becomes growing. Therefore, in addition to the ratio σ _L / σ _S ^2.5 , σ _L / σ _S can also be regarded as a scale indicating the amount of defects. Although the relationship between the ratio σ _L / σ _S ^2.5 and the ratio σ _L / σ _S is unclear, considering that the defect size and its fracture strength have distributions, both have different positions in the defect distribution, respectively. It is considered to be an index that reflects in a focused manner. If the ratio σ _L / σ _S is 2.3 or more, the amount of defects contained in the carbon fiber is relatively relative to the amount of defect variation due to external factors assumed in a normal carbon fiber manufacturing process. Since the rate of change in the amount of defects due to large external factors is small, it is considered that the carbon fiber bundle has a stable quality in which the tensile strength is not easily changed. The ratio σ _L / σ _S can be adjusted in the same manner as the control method for the ratio σ _L / σ _S ^2.5 described above.

  The carbon fiber bundle of the present invention preferably has an average single fiber diameter of 4.4 to 6.0 μm. The lower limit of the average single fiber diameter is more preferably 5.0 μm. The upper limit of the average single fiber diameter is more preferably 5.5 μm. In order to reduce the single fiber diameter of the carbon fiber bundle, if the single fiber diameter of the carbon fiber precursor fiber bundle that is the source of the carbon fiber bundle is reduced, the amount of fluff generation in the spinning process is likely to increase, and the fiber made of the carbon fiber precursor There is a tendency for the quality of the bundle to decline. If the single fiber diameter is 4.4 μm or more, the increase in the amount of fluff generated in the spinning process is not significant, and the deterioration in quality can be suppressed. If the single fiber diameter is 5.0 μm or more, an increase in the amount of fluff generation in the spinning process can be effectively suppressed, and productivity can be easily increased. If the single fiber diameter is 6.0 μm or less, a highly uniform carbon fiber bundle can be obtained. If the single fiber diameter is 5.5 μm or less, a carbon fiber bundle having a good balance between quality and physical properties is likely to be obtained. The single fiber diameter d of the carbon fiber bundle can be controlled by a known method such as the stock solution discharge amount from the die and the stretch ratio.

The carbon fiber bundle of the present invention preferably has a loop strength σ _L of 14.6 GPa or more, more preferably 15.0 GPa or more, and even more preferably 15.2 GPa or more. Since the loop strength corresponds to the strength of the carbon fiber substrate itself as described above, the high loop strength means that the carbon fiber substrate is strong and the tensile strength when a certain amount of defects are present is improved. Means that. Therefore, when the defect amount is constant, the strand strength tends to increase as the loop strength increases. When the loop strength is 14.6 GPa or more, the strand strength can be effectively increased. The upper limit of the loop strength is not particularly limited, but it is sufficient that the upper limit is 19.0 GPa. The loop strength can be controlled based on the carbon fiber bundle manufacturing method and known information (for example, Japanese Patent No. 5907321) exemplified later.

The carbon fiber bundle of the present invention has a strand strength σ _S of 5.5 GPa or more and preferably 6.0 GPa or more. Even if the carbon fiber bundle has a stable quality and has a small effect on the tensile strength due to external factors, the industrial utility value is impaired if the original tensile strength is low, but the strand strength should be 5.5 GPa or more. For example, the industrial utility value is high.

  Next, an example of the manufacturing method for obtaining the carbon fiber bundle of this invention is demonstrated.

  The carbon fiber precursor fiber bundle can be obtained by spinning a spinning solution in which a polyacrylonitrile-based polymer is dissolved in a solvent. As the polyacrylonitrile-based polymer, not only a homopolymer obtained only from acrylonitrile but also a polyacrylonitrile-based copolymer using other monomers in addition to acrylonitrile as a main component may be used. Specifically, the polyacrylonitrile-based polymer 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 acrylic acid, methacrylic acid, itaconic acid and their alkali metal salts, ammonium salts and lower alkyl esters, acrylamide and its derivatives, allyl sulfonic acid, methallyl sulfonic acid and Those salts or alkyl esters can be used. The processing conditions (temperature, time) within which the ratio of the above-mentioned loop strength σ _L and strand strength σ _{S to} the power of 2.5 σ _L / σ _S ^2.5 is within the scope of the present invention vary depending on the monomer used. The processing conditions can be adjusted as appropriate so that the ratio is 0.150 or more.

The above polyacrylonitrile polymer is dissolved in a solvent in which the polyacrylonitrile polymer such as dimethyl sulfoxide, dimethylformamide, dimethylacetamide, nitric acid, zinc chloride aqueous solution, and rhodium soda aqueous solution is soluble to obtain a spinning solution. When solution polymerization is used in the production of a polyacrylonitrile-based polymer, if the solvent used for the polymerization and the spinning solvent are the same, the step of separating the obtained polyacrylonitrile-based polymer and re-dissolving in the spinning solvent is performed. It is unnecessary and preferable. The processing conditions (temperature, time) within which the ratio σ _L / σ _S ^{2.5 of the} above-described loop strength σ _L and strand strength σ _S is within the scope of the present invention vary depending on the solvent used. The processing conditions can be adjusted as appropriate so that is 0.150 or more.

  The spinning solution may be filtered through a filter as appropriate. What is necessary is just to implement the filter and filtration conditions to be used in a well-known range.

  The carbon fiber precursor fiber bundle is obtained by introducing the spinning solution into a coagulation bath and coagulating it, and passing the obtained coagulated yarn through a washing step, an in-bath drawing step, an oil agent application step and a drying step. As the spinning method, a known method such as a wet or dry wet spinning method may be used. In particular, the dry and wet spinning method is preferable because the mechanical properties of the obtained carbon fiber bundle can be easily improved. The coagulated yarn may be directly stretched in the bath without the water washing step, or may be stretched in the bath after the solvent is removed by the water washing step. Usually, the stretching in the bath is preferably performed in a single or a plurality of stretching baths adjusted to a temperature of 30 to 98 ° C. Moreover, you may add a dry heat extending process and a steam extending process to said process.

  The average fineness of the single fiber contained in the carbon fiber precursor fiber bundle thus obtained is preferably 0.5 to 1.5 dtex, more preferably 0.5 to 1.1 dtex, 0 More preferably, it is 5-0.8 dtex. By setting the single fiber fineness to 0.5 dtex or more, occurrence of yarn breakage due to contact with a roller or a guide can be suppressed, and the process stability of the yarn making process and the carbon fiber bundle firing process can be maintained. In addition, by setting the single fiber fineness to 1.5 dtex or less, the difference between the inner and outer structures of each single fiber after flame resistance is reduced, the processability in the subsequent carbonization step, and the tensile strength and tensile elasticity of the resulting carbon fiber bundle The rate can be improved. If the single fiber fineness is 1.1 dtex or less, the difference between the inner and outer structures can be easily suppressed without reducing the flameproofing treatment rate so much, and 0.8 dtex or less is sufficient.

  The resulting carbon fiber precursor fiber bundle is usually in the form of continuous fibers. Further, the number of filaments per yarn is preferably 1,000 to 36,000.

  The carbon fiber precursor fiber bundle obtained as described above is subjected to a flameproofing step. In this invention, a flameproofing process means heat-processing a carbon fiber precursor fiber bundle in the oxygen containing atmosphere of the temperature of 200-300 degreeC. In the pre-carbonization step of pre-carbonizing the fiber bundle obtained in the flame-proofing step, the specific weight of the obtained flame-resistant fiber bundle is 1.5 to 1 at a maximum temperature of 500 to 1000 ° C. in an inert atmosphere. It is preferable to heat-treat until .8. What is necessary is just to set suitably the draw ratio of a preliminary | backup carbonization process in the range which the slack of a fiber bundle does not generate | occur | produce or a quality does not fall.

  It is preferable to carbonize the pre-carbonized fiber bundle in an inert atmosphere at a maximum temperature of 1000 to 3000 ° C. The maximum temperature of the carbonization step is preferably high from the viewpoint of increasing the strand elastic modulus of the obtained carbon fiber bundle, but if it is too high, the strength of the high strength region may be lowered, and is set in consideration of both. It is good to do. A more preferable maximum temperature is 1200 to 2000 ° C, and more preferably 1200 to 1600 ° C.

  The carbon fiber bundle obtained as described above is subjected to a surface treatment in order to improve adhesion to the matrix resin, and a functional group containing oxygen atoms is introduced. As the surface treatment method, vapor phase oxidation, liquid phase oxidation and liquid phase electrolytic oxidation are used. From the viewpoint of high productivity and uniform treatment, liquid phase electrolytic oxidation is preferably used. In the present invention, the liquid phase electrolytic oxidation method is not particularly limited, and may be performed by a known method.

  After such a surface treatment, a sizing treatment can also be performed to impart convergence to the obtained carbon fiber bundle. As the sizing agent, a sizing agent having good compatibility with the matrix resin can be appropriately selected according to the type of the matrix resin used in the composite material.

  Hereinafter, the present invention will be described more specifically with reference to examples. The measurement method used in this example will be described next.

<Specific gravity measurement of carbon fiber bundle>
The carbon fiber bundle to be measured was sampled by 1 m and measured by Archimedes method using a specific gravity liquid as o-dichloroethylene. Three measurements were taken and the average value was used.

<Average single fiber diameter d> It was carried out according to JIS R7607 (2000). Specifically, mass A _f (g / m) and specific gravity B _f (−) per unit length were determined for a carbon fiber bundle composed of a large number of carbon filaments to be measured. A filament number C _f of the carbon fiber bundle to the value and the measurement of the obtained A _f and B _f, the average carbon fiber bundle single fiber diameter d (μm), was calculated by the following equation.

Average single fiber diameter (μm) = ((A _f / B _f / C _f ) / π) ^(1/2) × 2 × 10 ³ .

<Single fiber loop method>
Place a single fiber of about 10 cm in length on a glass slide, add 1 to 2 drops of glycerin to the center, and lightly twist both ends of the single fiber in the fiber circumferential direction to create a loop at the center of the single fiber. A cover glass was placed on. This was placed on a microscope stage, and moving images were taken under the conditions of a total magnification of 100 times and a frame rate of 15 frames / second. While adjusting the stage each time so that the loop does not deviate from the field of view, both ends of the looped fiber were pressed in the direction of the slide glass with fingers, and pulled at a constant speed in the opposite direction, and strain was applied until the single fiber broke. . The frame immediately before the break was specified by frame advance, and the horizontal width W (μm) of the loop immediately before the break was measured by image analysis. The average single fiber diameter d (μm) was divided by W to calculate d / W. The test was performed 20 times, an average value of d / W was obtained, and E × d / W was obtained by multiplying the value by the strand elastic modulus (GPa), and this value was taken as the loop strength σ _L (GPa).

<Strand tensile test>
The tensile elastic modulus (strand elastic modulus) and tensile strength (strand strength σ _S ) of the carbon fiber resin-impregnated strand were determined in accordance with JIS R7608 (2008) “Resin-impregnated strand test method”. The strand elastic modulus E was measured in the strain range of 0.1 to 0.6%. The test piece was prepared by impregnating a carbon fiber bundle with the following resin composition and curing conditions of heat treatment at a temperature of 130 ° C. for 35 minutes.

[Resin composition]
・ 3,4-epoxycyclohexylmethyl-3,4-epoxy-cyclohexane-carboxylate (100 parts by mass)
・ Boron trifluoride monoethylamine (3 parts by mass)
-Acetone (4 parts by mass).

  The number of measurements in the tensile test was 6, and the arithmetic average value of each measurement result was taken as the strand elastic modulus and strand strength of the carbon fiber bundle. In the examples and comparative examples described later, as the above 3,4-epoxycyclohexylmethyl-3,4-epoxy-cyclohexane-carboxylate, “BAKELITE (registered trademark)” ERL- manufactured by Union Carbide Co., Ltd. 4221 (or equivalent “Celoxide (registered trademark)” 2021P (manufactured by Daicel Corporation)) was used. The strain was measured using an extensometer.

[Example 1]
A copolymer composed of 99.0% by mass of acrylonitrile and 1.0% by mass of itaconic acid was polymerized by a solution polymerization method using dimethyl sulfoxide as a solvent to obtain a spinning solution containing a polyacrylonitrile-based copolymer. The obtained spinning solution was allowed to stand at a constant temperature of 30 ° C. for 168 hours under a nitrogen atmosphere, and then temporarily discharged from the spinning nozzle into the air without filtering, and introduced into a coagulation bath composed of an aqueous solution of dimethyl sulfoxide. A coagulated yarn was obtained by the dry and wet spinning method. The coagulated yarn was washed with water by a conventional method, and then stretched 3.5 times in two warm water baths. Subsequently, an amino-modified silicone-based silicone oil was applied to the fiber bundle after stretching in the water bath, and a dry densification treatment was performed using a 160 ° C. heating roller. After the number of single fibers is 12,000, the yarn is stretched 3.7 times in pressurized steam, so that the total draw ratio of the yarn is 13 times, and then the entanglement treatment is performed to obtain a crystal orientation degree of 93% and a single fiber number of 12,000. A carbon fiber precursor fiber bundle was obtained. The single fiber fineness of the carbon fiber precursor fiber bundle was 0.7 dtex.

Next, the first flameproofing process is performed using the conditions of a flameproofing temperature of 250 ° C. and a flameproofing time of 11 minutes, and the second flameproofing process is performed using the conditions of a flameproofing temperature of 280 ° C. and a flameproofing time of 6 minutes. While the carbon fiber precursor fiber bundle was stretched at a stretch ratio of 1 in an atmosphere oven, the flame resistant fiber bundle was obtained. Here, with the description of Japanese Patent No. 5907321 as a reference, the ratio of the peak intensity of 1453cm ^-1 to the peak intensity of 1370 cm ^-1 which is evaluated in the infrared spectrum in the fiber bundles during the first oxidation step ends 1. 04, the ratio of the peak intensity of 1453cm ^-1 to the peak intensity of 1370 cm ^-1 which is evaluated in the infrared spectrum in the fiber bundles during the second oxidation step ends 0.70, and the 1370 cm ^-1 in the infrared spectrum The conditions were finely adjusted so that the ratio of 1254 cm ⁻¹ peak intensity to peak intensity was 0.61.

  The obtained flame-resistant fiber bundle was subjected to a pre-carbonization treatment while being drawn at a draw ratio of 1.17 in a nitrogen atmosphere at a maximum temperature of 800 ° C. to obtain a pre-carbonized fiber bundle. The obtained pre-carbonized fiber bundle was carbonized while being stretched at a maximum temperature of 1500 ° C. and a stretch ratio of 0.98 in a nitrogen atmosphere. The evaluation results of the obtained carbon fiber bundle are shown in Table 1. Table 1 also shows the CV value of the strand strength and the determination result when five batches of a series of experimental operations (that is, storage at 30 ° C. to carbonization treatment) excluding polymerization are performed. The determination criteria were defined as ◎ when the CV value was 3.5% or less, ◯ when it was greater than 3.5 and less than 5, and Δ when greater than 5.

[Example 2]
A carbon fiber bundle was obtained in the same manner as in Example 1 except that the storage time at 30 ° C. was 72 hours. The evaluation results are shown in Table 1. Table 1 also shows the CV value of the strand strength when five batches of a series of experimental operations excluding polymerization (that is, storage at 30 ° C. to carbonization treatment) were performed.

[Example 3]
A carbon fiber bundle was obtained in the same manner as in Example 1 except that the storage time at 30 ° C. was 310 hours. The evaluation results are shown in Table 1. Table 1 also shows the CV value of the strand strength when five batches of a series of experimental operations excluding polymerization (that is, storage at 30 ° C. to carbonization treatment) were performed.

[Comparative Example 1]
Without storing the spinning solution at 30 ° C., the spinning solution is filtered through a filter medium having a filtration accuracy of 1 μm prior to spinning, and then immediately subjected to spinning, and the draw ratio in the preliminary carbonization treatment is set to 0. A carbon fiber bundle was obtained in the same manner as in Example 1 except that it was 90. The evaluation results are shown in Table 1. Table 1 also shows the CV value of the strand strength when five batches of a series of experimental operations excluding polymerization (that is, storage at 30 ° C. to carbonization treatment) were performed.

[Comparative Example 2]
A carbon fiber bundle was obtained in the same manner as in Comparative Example 1 except that the draw ratio in the preliminary carbonization treatment was changed to 1.17. The evaluation results are shown in Table 1. Table 1 also shows the CV value of the strand strength when five batches of a series of experimental operations excluding polymerization (that is, storage at 30 ° C. to carbonization treatment) were performed.

[Example 4]
A precursor fiber bundle was obtained in the same manner as in Example 1 except that the draw ratio in the preliminary carbonization treatment was 0.90. Subsequently, a carbon fiber bundle was obtained in the same manner as in Example 1. The evaluation results are shown in Table 1. Table 1 also shows the CV value of the strand strength when five batches of a series of experimental operations excluding polymerization (that is, storage at 30 ° C. to carbonization treatment) were performed.

[Reference Examples 1-3]
Commercially available “Torayca (registered trademark)” T800S (manufactured by Toray Industries, Inc.) and “Torayca (registered trademark)” T1100G (manufactured by Toray Industries, Inc.), “Torayca (registered trademark)” T700S (manufactured by Toray Industries, Inc.) The physical properties of) are shown in Table 1.

Comparing Example 1 and Comparative Example 1, the strand strength is the same at 6.2 GPa, but the ratio σ _L / σ _S ^2.5 of the 2.5th power of the loop strength σ _L and the strand strength σ _S is 0. .164 and 0.129 are significantly different, indicating that Example 1 contains a larger amount of defects. As a result, after the spinning process was repeated 5 times under the same conditions, when 5 batches of carbon fiber were produced, the CV values of the strand strength variation at N = 5 were 3.3% and 5.0%, respectively. It can be seen that in Example 1 in which the ratio is 0.150 or more, the CV value is small. Similar results can be read from Examples 2 to 4 and Comparative Example 2. That is, it was found that by setting the ratio to 0.150 or more, it is possible to obtain a carbon fiber bundle having a stable quality with small variations in strength due to external factors. Furthermore, as can be seen from the reference examples, typical high-strength carbon fibers currently on the market are out of the above-mentioned ratio range of the present invention, and the carbon fiber bundles of the present invention have characteristics that are not found in the prior art. It is shown that.

Claims (5)

A carbon fiber bundle in which the ratio σ _L / σ _S ^2.5 of the 2.5th power of the loop strength σ _L and the strand strength σ _S is 0.150 or more and the strand strength σ _S is 5.5 GPa or more. The carbon fiber bundle according to claim 1, wherein a ratio σ _L / σ _{S of the} loop strength σ _L and the strand strength σ _S is 2.3 or more. The carbon fiber bundle according to claim 1 or 2, wherein an average single fiber diameter is 4.4 to 6.0 µm. The carbon fiber bundle according to any one of claims 1 to 3, wherein the loop strength σ _L is 14.6 GPa or more. Strand strength (sigma) _S is 6.0 GPa or more, The carbon fiber bundle in any one of Claims 1-4.