CARBON FIBER BUNDLE AND PRODUCTION METHOD FOR SAME

Abstract

To provide a carbon fiber bundle capable of suppressing winding due to ring-shaped fuzzes, which occurs when the carbon fiber bundle is rolled out for advanced processing, and a production method for producing the same. Disclosed is a carbon fiber bundle wherein an average single-fiber diameter B is 6.9 to 11.0 μm, a tensile modulus E of resin-impregnated strands is 230 to 310 GPa, the number of fuzzes inherent in the carbon fiber bundle is 40 fuzzes/m or less, and a proportion of fuzzes with a structure having a difference between skin and core is 1 to 25% of fuzzes inherent in the carbon fiber bundle. Such a carbon fiber bundle is preferably obtained by a method including, in a process of heat-treating a polyacrylonitrile-based precursor fiber bundle with a single-fiber fineness of 0.9 to 2.2 dtex in an oxidizing atmosphere at 200 to 300° C., heat-treating the polyacrylonitrile-based precursor fiber bundle so that a heat generation rate Q, which is the left side of the formula (3), is 150 to 500 J/m2/s until the density is 1.22 to 1.24 g/cm3, when q (J/g/s) is the heat generation rate of the single fiber, N is the number of filaments, d (dtex) is a single-fiber fineness of the stabilized fiber bundle and W (mm) is a yarn width, heat-treating the fiber bundle while applying a tension of 1.6 to 4.0 mN/dtex until the density is 1.38 to 1.50 g/cm3 to obtain a stabilized fiber bundle, and heat-treating the stabilized fiber bundle in an inert atmosphere at 1,200 to 1,600° C. Q=q×N×d/W/10  (3)

Description

TECHNICAL FIELD

The present invention provides a carbon fiber bundle which has high load-withstanding property per single fiber and excellent abrasion resistance, and is capable of suppressing winding due to ring-shaped fuzzes, which occurs when the carbon fiber bundle is rolled out for advanced processing, by suppressing specific fuzzes inherent in the carbon fiber bundle even in the case of large single-fiber fineness, and a method for producing the same.

BACKGROUND ART

Composite materials using carbon fiber bundles are used not only for aerospace applications, but also for sports applications such as bicycles and golf clubs. Recently, they have also being used for industrial applications such as automotive components and pressure vessels. In industrial applications, since there is a need to reduce production costs, it is important to improve the processability of these members, such as improving the abrasion resistance against rollers during molding and suppressing fuzzes (single-fiber breakage) when rolling out carbon fiber bundles or running on rollers. In particular, it is important to suppress ring-shaped fuzzes generated when the carbon fiber bundle is rolled out because the surrounding carbon fiber bundle is entangled, leading to an increase in winding around the rollers.

In general, polyacrylonitrile-based carbon fiber bundles are produced by subjecting to a stabilization process of oxidizing a polyacrylonitrile-based precursor fiber bundle in air at 200 to 300° C., a pre-carbonization process of heating in an inert atmosphere at 500 to 1,200° C., and a carbonization process of heating in an inert atmosphere at 1,200 to 3,000° C. To improve the abrasion resistance by increasing the load-withstanding property per single fiber of the carbon fiber bundle, it is effective to increase the weight per single fiber, that is, the single-fiber fineness. To do this, it is effective to increase the yield of the carbon fiber bundle by increasing the amount of heat treatment of the stabilization process or to increase the single-fiber fineness of the polyacrylonitrile-based precursor fiber bundles.

There have hitherto been proposed methods for producing carbon fiber bundles which suppress fuzzes during the production of the carbon fiber bundles (Patent Documents 1 to 4).

Patent Document 1 has proposed that both the tensile modulus E of resin-impregnated strands (hereinafter sometimes abbreviated to strand tensile modulus E) and the compressive strength are improved by increasing the tension of the carbon fiber bundle in the carbonization process while maintaining the carbonization temperature at 1,000 to 1,500° C., reading to reduction of fuzzes when the carbon fiber bundle is rubbed against a roller. Patent Document 2 has proposed that a structure having a difference in structure between skin and core of the carbon fiber bundle can be suppressed by controlling the heat treatment temperature in the stabilization process according to the density of the stabilization fiber bundle in the stabilization process, and also fuzzes can be reduced because of large single-fiber fineness and high knot strength. Patent Document 3 has proposed that a structure having a difference in structure between skin and core of the carbon fiber bundle can be suppressed by controlling the stabilization time so as to satisfy an appropriate stabilized structure in the stabilization process. Patent Document 4 has proposed that a carbon fiber bundle having excellent handleability and processability can be obtained by using hydroxyalkyl methacrylate as a copolymerization component to control the amount of heat generated in the stabilization process, because of high knot strength even in the case of large single-fiber fineness.

PRIOR ART DOCUMENTS

Patent Documents

Patent Document 1: JP 2005-344254 A

Patent Document 2: JP 2017-66580 A

Patent Document 3: JP 2018-178344 A

Patent Document 4: WO 2013/157613 A

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

However, the background art has the following problems.

In Patent Document 1, in addition to not controlling the temperature and time of the stabilization process, the carbon fiber bundle had small single-fiber fineness, leading to low load-withstanding property per single fiber and insufficient abrasion resistance. That proposal is to increase the resistance to rubbing against the roller on average by increasing the average physical properties, and there was a problem that it was impossible to suppress winding due to ring-shaped fuzzes, which occurs only by rolling out the carbon fiber bundle. In Patent Documents 2 and 3, although fuzzes are less likely generated on average, the heat generation rate and the heat removal rate in the stabilization process could not be controlled because of large single-fiber fineness, and thus the cross section of the stabilized fiber bundle could not control specific fuzzes inherent in the carbon fiber bundle due to the temperature unevenness within the bundle. Due to the influence of the inherent fuzzes, there was a problem that it was impossible to suppress winding due to ring-shaped fuzzes, which occurs when the carbon fiber bundle is rolled out for advanced processing. In the Patent Document 4, there was a problem: although the heat generation rate in the stabilization process was controlled, because of not controlling the heat removal in addition to the large single-fiber fineness of the carbon fiber bundle, the specific fuzzes inherent in the carbon fiber bundle could not be suppressed due to the temperature unevenness within the bundle of the stabilized fiber bundles, thus failing to suppress winding due to ring-shaped fuzzes, which occurs when the carbon fiber bundle is rolled out for advanced processing.

As mentioned above, Patent Documents 1 to 2 and 4 have proposed that it is possible to suppress fuzzes generated when running on rollers in the process of producing a carbon fiber bundle and the process of using a carbon fiber bundle by increasing the knot strength, strand tensile modulus E and compressive strength, and Patent Documents 2 to 4 have proposed that a structure having a difference between skin and core can be suppressed by controlling the temperature, time and heat generation rate in the stabilization process. In this way, although there were some proposals which suppressed the structure having a difference between skin and core on average, variation is included and it was not recognized that fuzzes are partially generated to exert an adverse effect. In other words, in all inventions, heat removal in the stabilized fiber bundle in the stabilization process was not taken into consideration, so that when the single-fiber fineness is large, specific fuzzes inherent in the carbon fiber bundle caused by the temperature unevenness could not be suppressed, thus failing to suppress winding due to ring-shaped fuzzes, which occurs when the carbon fiber bundle is rolled out for advanced processing.

An object of the present invention is to provide a carbon fiber bundle which has high load-withstanding property per single fiber and excellent abrasion resistance, and is capable of suppressing winding due to ring-shaped fuzzes, which occurs when the carbon fiber bundle is rolled out for advanced processing, by suppressing specific fuzzes inherent in the carbon fiber bundle even in the case of large single-fiber fineness, and a method for producing the same.

Solutions to the Problems

To solve the problems mentioned above, the present invention has the following constitution.

Namely, the carbon fiber bundle of the present invention is a carbon fiber bundle wherein an average single-fiber diameter B is 6.9 to 11.0 μm, a tensile modulus E of resin-impregnated strands is 230 to 310 GPa, the number of fuzzes inherent in the carbon fiber bundle is 40 fuzzes/m or less, and a proportion of fuzzes with a structure having a difference between skin and core is 1 to 25% of fuzzes inherent in the carbon fiber bundle.

The method for producing a carbon fiber bundle of the present invention is a method for producing a carbon fiber bundle, which includes, in a process of heat-treating a polyacrylonitrile-based precursor fiber bundle with a single-fiber fineness of 0.9 to 2.2 dtex in an oxidizing atmosphere at 200 to 300° C., heat-treating the polyacrylonitrile-based precursor fiber bundle so that a heat generation rate Q obtained by the formula (3) is 150 to 500 J/m²/s until the density is 1.22 to 1.24 g/cm³, when q (J/g/s) is the heat generation rate of the single fiber, N is the number of filaments, d (dtex) is a single-fiber fineness of the stabilized fiber bundle and W (mm) is a yarn width, heat-treating the fiber bundle under tension of 1.6 to 4.0 mN/dtex until the density is 1.38 to 1.50 g/cm³ to obtain a stabilized fiber bundle, and heat-treating the stabilized fiber bundle in an inert atmosphere at 1,200 to 1,600° C. to obtain a carbon fiber bundle.

Q=q×N×d/W/10  (3)

Effects of the Invention

According to the present invention, it is possible to provide a carbon fiber bundle which has high load-withstanding property per single fiber and excellent abrasion resistance, and is capable of suppressing winding due to ring-shaped fuzzes, which occurs when the carbon fiber bundle is rolled out for advanced processing, by suppressing specific fuzzes inherent in the carbon fiber bundle even in the case of large single-fiber fineness, and a method for producing the same.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example of a photograph showing a structure in which an inner layer and an outer layer are present of cross sections of fuzzes inherent in the carbon fiber bundle.

FIG. 2 is an example of a photograph showing fuzzes with a cross section having a hole in the center of fuzzes inherent in the carbon fiber bundle.

FIG. 3 is an example of a photograph showing fuzzes judged to be broken due to bending of fuzzes inherent in the carbon fiber bundle.

FIG. 4 is an example of a photograph showing a deformed cross section of fuzzes inherent in the carbon fiber bundle.

FIG. 5 is an example of a photograph showing a cross section of fuzzes inherent in the carbon fiber bundle, which is vertical and split from the center.

EMBODIMENTS OF THE INVENTION

In producing a carbon fiber bundle which has high load-withstanding property per single fiber and excellent abrasion resistance, and is capable of suppressing winding due to ring-shaped fuzzes, which occurs when the carbon fiber bundle is rolled out for advanced processing, by suppressing specific fuzzes inherent in the carbon fiber bundle even in the case of large single-fiber fineness, the present inventors have found that the amount of heat removal relative to the total heat generation amount of the stabilized fiber bundle can be sufficiently secured, and the temperature unevenness in the stabilized fiber bundle can be reduced even in the case of large single-fiber fineness by appropriately controlling a heat generation rate, the number of filaments N, a single-fiber fineness and a yarn width of a single fiber in a stabilization process, and thus the present invention has been completed.

First, the carbon fiber bundle of the present invention will be described.

The carbon fiber bundle of the present invention has an average single-fiber diameter B of 6.9 to 11.0 μm, preferably 7.0 to 10.0 μm, and more preferably 7.1 to 9.0 μm. If the average single-fiber diameter B is 6.9 μm or more, fuzzes due to abrasion can be suppressed, thus enabling the suppression of fuzzes generated when the carbon fiber bundle is rolled out. If the average single-fiber diameter B is 11.0 μm or less, the structure having a difference between skin and core of the carbon fiber bundle can be suppressed, thus enabling the suppression of winding due to ring-shaped fuzzes, which occurs when the carbon fiber bundle is rolled out for advanced processing. The average single-fiber diameter B can be calculated from the mass and density per unit length of the carbon fiber bundle and the number of filaments N. Such an average single-fiber diameter B can be achieved by controlling the extrusion amount in the production process of a polyacrylonitrile-based precursor fiber bundle, the stretching ratio of each process, and the specific gravity of the stabilized fiber bundle.

The carbon fiber bundle of the present invention has a strand tensile modulus E of 230 to 310 GPa, preferably 245 to 300 GPa, and still more preferably 250 to 290 GPa, in a resin impregnated strand tensile test. If the strand tensile modulus E is 230 GPa or more, it is possible to obtain a satisfactory modulus when used generally for modulus reinforcement. If the strand tensile elastic modulus E is 310 GPa or less, fuzzes due to abrasion can be suppressed, thus enabling the suppression of winding due to ring-shaped fuzzes, which occurs when the carbon fiber bundle is rolled out for advanced processing. The strand tensile elastic modulus E can be determined by the method mentioned in a strand tensile test for carbon fiber bundle mentioned later. At this time, the strain is within a range of 0.1 to 0.6%. The strand tensile modulus E of the carbon fiber bundle can be mainly controlled by applying tension to the fiber bundle in any of heat treatment processes in the production process of the carbon fiber bundle, improving the structure having a difference between skin and core, or changing the carbonization temperature.

The carbon fiber bundle of the present invention preferably has a crystallite size Lc of 1.5 to 2.5 nm, more preferably 1.6 to 2.3 nm, and still more preferably 1.7 to 2.2 nm. The crystallite size Lc is preferably 1.5 nm or more, since it is possible to suppress ring-shaped fuzzes generated when the carbon fiber bundle is rolled out. The crystallite size Lc is preferably 2.5 nm or less, since there is no need to raise the maximum temperature in the carbonization process more than necessary, leading to excellent abrasion resistance, thus enabling the suppression of ring-shaped fuzzes generated when the carbon fiber bundle is rolled out. The crystallite size Lc can be measured by a known method using a wide-angle X-ray diffractometer, and the Scherrer constant in the Scherrer equation mentioned below is 1. Such a crystallite size Lc can be controlled by changing the carbonization temperature.

In the carbon fiber bundle of the present invention, a relationship between the strand tensile modulus E and the crystallite size Lc (nm) preferably satisfies the formula (1), and the intercept on the left side of the formula (1) is more preferably 135, and still more preferably 140. The intercept on the right side of the formula (1) is more preferably 175, and still more preferably 170.

50×Lc+130≤E≤50×Lc+180  (1)

The carbon fiber bundle is a polycrystalline body composed of substantially innumerable graphite crystallites, and raising the maximum temperature of the carbonization process increases the crystallinity of crystallites. In other words, the rearrangement of carbon network planes occurs and the crystal size increases. At the same time, the orientation of the crystals also increases, and thus the strand tensile modulus E of the carbon fiber tends to increase. Therefore, a relationship is found between the strand tensile modulus E and the crystallite size Lc as shown in the formula (1). The intercept on the left side of the formula (1) is preferably 130 or more, since the strand tensile modulus E can be efficiently improved even if the carbonization temperature is low, thus making it possible to obtain high strand tensile modulus E while suppressing fuzzes due to abrasion. The intercept on the left side of the formula (1) is preferably 180 or less, since there is no need to raise the maximum temperature in the carbonization process more than necessary so as to increase the strand tensile modulus E, leading to excellent abrasion resistance, thus enabling the suppression of ring-shaped fuzzes generated when the carbon fiber bundle is rolled out. The strand tensile modulus E and crystallite size Lc can be measured by the above methods. To achieve such a relationship between the strand tensile modulus E and the crystallite size Lc, it is possible to appropriately control by applying tension to the fiber bundle in any of heat treatment processes in the production process of the carbon fiber bundle, improving the structure having a difference between skin and core, or changing the carbonization temperature.

In the carbon fiber bundle of the present invention, the number of fuzzes inherent in the carbon fiber bundle is 40 fuzzes/m or less, preferably 35 fuzzes/m or less, and more preferably 30 fuzzes/m or less. The fuzzes inherent in the carbon fiber bundle are fuzzes existing inside the carbon fiber bundle when the carbon fiber bundle wound around the bobbin is pulled out. If the number of fuzzes inherent in the carbon fiber bundle is 40 fuzzes/m or less, it is possible to sufficiently suppress winding due to ring-shaped fuzzes, which occurs when the carbon fiber bundle is rolled out for advanced processing. The method for measuring the number of fuzzes inherent in the carbon fiber bundle is as follows: 10 m of the carbon fiber bundle is pulled out from the bobbin and the carbon fiber bundle is split into single fibers so that the thickness of the carbon fiber bundle is equivalent to that of two single fibers with enough force to prevent fuzzes from generating, and, if fuzzes exist, the fuzzes are collected, and then the number of fuzzes is measured to calculate the number per meter. At this time, fuzzes generated in the process of dividing each single fiber is excluded. Controlling the number of fuzzes inherent in the carbon fiber bundle within such a range can be achieved by appropriately controlling the heat generation rate of the single fiber, the number of filaments N, the single-fiber fineness and the yarn width in the stabilization process, as mentioned later.

In the carbon fiber bundle of the present invention, the proportion of fuzzes with a structure having a difference between skin and core is 1 to 25%, preferably 1 to 20%, and more preferably 2 to 15%, of fuzzes inherent in the carbon fiber bundle. Of fuzzes inherent in the carbon fiber bundle, fuzzes with a structure having a difference between skin and core refers to fuzzes with a structure in which an inner layer and an outer layer are present, as shown in FIG. 1, and fuzzes with a cross section having a hole in the center, as shown in FIG. 2, when the fuzzes are collected and the cross section is observed by a scanning electron microscope (SEM). However, it is considered that fuzzes judged to be broken due to bending, as shown in FIG. 3, are not fuzzes generated in the production process of the carbon fiber bundle inherent in the carbon fiber bundle, but fuzzes to which bending load is applied when collecting, leading to bending breakage, which shall be excluded from the total number of fuzzes for calculating the proportion of fuzzes with a structure having a difference between skin and core of fuzzes inherent in carbon fiber bundle.

The mechanism by which the cross section of fuzzes inherent in these carbon fiber bundle has a structure having a difference between skin and core is not clearly understood, but it is considered as follows. In other words, the area that became especially high temperature when the temperature unevenness occurred in the stabilization process has a specifically larger structure having a difference between skin and core than the average structure having a difference between skin and core in the stabilized fiber bundle, and thus single-fiber breakage of the carbon fiber bundle occurred due to weak loading, resulting in a cross section with a structure having a difference between skin and core. As a result, it is considered that fuzzes with a structure having a difference between skin and core is particularly likely to occur under weak loading and mainly cause fuzzes inherent in the carbon fiber bundle.

Inclusion of such particularly weak fuzzes in a certain proportion of normally generated fuzzes causes the generation of ring-shaped fuzzes which increase winding while entraining other fuzzes. Therefore, it is considered that the presence of fuzzes with a structure having a difference between skin and core inherent in the carbon fiber bundle in a certain proportion of the entire fuzzes cause winding due to ring-shaped fuzzes, which occurs when the carbon fiber bundle is rolled out for advanced processing.

Therefore, if the proportion of fuzzes with a structure having a difference between skin and core is 25, or less of fuzzes inherent in the carbon fiber bundle, it is possible to sufficiently suppress winding due to ring-shaped fuzzes, which occurs when the carbon fiber bundle is rolled out for advanced processing. If the proportion of fuzzes with a structure having a difference between skin and core is 1% or more of fuzzes inherent in the carbon fiber bundle, since the strand tensile strength of the carbon fiber bundle does not decrease, the proportion of single fibers with low strength affecting fuzzes decrease, thus enabling the suppression of ring-shaped fuzzes generated near the roller when the carbon fiber bundle is rolled out.

Of fuzzes inherent in the carbon fiber bundle, fuzzes with a structure having a difference between skin and core do not exist in commercially available carbon fiber bundles, but the proportion thereof is controlled within such a range by controlling the stabilization process as mentioned later. To determine whether or not the fuzzes are fuzzes with a structure having a difference between skin and core of fuzzes inherent in the carbon fiber bundle, the carbon fiber bundle wound on the bobbin is pulled out by the above-mentioned method, and after collecting fuzzes existing inside the carbon fiber bundle, the cross section is observed by SEM (details are mentioned later). Controlling the proportion of fuzzes with a structure having a difference between skin and core of fuzzes inherent in the carbon fiber bundle within such a range can be achieved by appropriately controlling the heat generation rate of the single fiber, the number of filaments N, the single-fiber fineness and the yarn width in the stabilization process, as mentioned later.

In the carbon fiber bundle of the present invention, the proportion of fuzzes with an area ratio of 50% or less of the cross section perpendicular to the fiber axis is preferably 0 to 3%, more preferably 0.1 to 2.5%, and still more preferably 0.5 to 1.5%, of fuzzes inherent in the carbon fiber bundle.

Here, the cross section of fuzzes inherent in the carbon fiber bundle is a cross section observed when fuzzes existing inside the carbon fiber bundle are collected and a cross section perpendicular to the fiber axis is observed by a scanning electron microscope (SEM). The cross section in which an area ratio perpendicular to the fiber axis of the cross section of fuzzes inherent in the carbon fiber bundle is 50% or less means that the cross section of fuzzes inherent in the carbon fiber bundle is not substantially perpendicular, and means that an original shape of a single fiber is not maintained and is deformed as shown in FIG. 4, or that it is split from the center as shown in FIG. 5.

The proportion of a cross-sectional area of fuzzes inherent in carbon fiber bundles is defined as the proportion of the cross section in which the area ratio perpendicular to the fiber axis to the average cross-sectional area of the cross section cut perpendicular to the single fiber is 50% or less. Therefore, as shown in FIG. 5, those with a vertical cross section of fuzz but split from the center are also included. The cross section perpendicular to the fiber axis is defined as a cross section which makes 85 to 95% with the fiber axis.

The reason why the cross section in which an area ratio perpendicular to the fiber axis of the cross section of fuzzes inherent in the carbon fiber bundle is 50% or less is not clearly understood, but it is considered as follows. In other words, the temperature unevenness in the stabilization process is especially large and the cross section of the above-mentioned fuzzes specifically increases the structure having a difference between skin and core compared to the cross section with a structure having a difference between skin and core, and thus single-fiber breakage of the carbon fiber bundle occurred due to weak loading, leading to deformation of the cross section with a structure having a difference between skin and core of fuzzes. As a result, it is considered that fuzzes with the cross section in which an area ratio perpendicular to the fiber axis of the cross section of fuzzes inherent in the carbon fiber bundle is 50% or less are likely to be generated under weaker load than the cross section of fuzzes with a structure having a difference between skin and core, thus presuming that this leads to a significant increase in winding due to ring-shaped fuzzes, which occurs when the carbon fiber bundle is rolled out for advanced processing. Therefore, the proportion of the cross section with an area ratio of 50% or less perpendicular to the fiber axis of the cross sections of fuzzes inherent in the carbon fiber bundle is preferably 3% or less, since it is possible to sufficiently suppress winding due to ring-shaped fuzzes, which occurs when the carbon fiber bundle is rolled out for advanced processing.

Regarding the fuzzes with the cross section in which an area ratio perpendicular to the fiber axis of the cross section of fuzzes inherent in the carbon fiber bundle is 50% or less, the carbon fiber bundle wound on the bobbin as mentioned above is pulled out and fuzzes existing inside the carbon fiber bundle are collected, and after measuring the angle of the image observed by SEM at an approximate oblique angle of 45° C. of the cross section, using a protractor tool for image analysis software, the area ratio of the cross section, which is the same cross section observed by SEM from the front, at an angle of 85 to 95° with the fiber axis is extracted from image analysis. Furthermore, the cross-sectional area of the single fiber of the carbon fiber bundle can be determined by cutting the carbon fiber bundle vertically with a single blade to afford a vertical cross section and observing the cross section of the single fiber taken out from the front by SEM, and then analyzing the image with image analysis software.

Controlling the cross section in which an area ratio perpendicular to the fiber axis of the cross section of fuzzes inherent in the carbon fiber bundle is 50% or less within such a range can be achieved by appropriately controlling the heat generation rate of the single fiber, the number of filaments N, the single-fiber fineness and the yarn width in the stabilization process, as mentioned later.

The yarn width W of the carbon fiber bundle of the present invention is preferably 5 to 8 mm, more preferably 6 to 8 mm, and still more preferably 7 to 8 mm. The yarn width W of the carbon fiber bundle is the width of the carbon fiber bundle when the carbon fiber bundle is rolled out from the bobbin, and roughly reflects the width of the fiber bundle from the stabilization process unless a fiber opening process is particularly included. The yarn width W is preferably 5 mm or more, since it is possible to suppress fuzzes due to abrasion, thus enabling the suppression of fuzzes generated when the carbon fiber bundle is rolled out. The yarn width W is preferably 8 mm or less, since it is possible to suppress the generation of fuzzes caused by spreading the carbon fiber bundle more than necessary when the carbon fiber bundle is rolled out from the bobbin. The yarn width W of the carbon fiber bundle can be measured by using a ruler or the like after rolling out the carbon fiber bundle from the bobbin. The yarn width W of the carbon fiber bundle can be achieved by the yarn width of the polyacrylonitrile-based precursor fiber bundle and the tension of the stabilized fiber bundle in the stabilization process.

The number of filaments N in the carbon fiber bundle of the present invention is preferably 10,000 to 50,000, more preferably 10,000 to 30,000, and still more preferably 15,000 to 25,000. The number of filaments N of the carbon fiber bundle is the number of single fibers constituting the carbon fiber bundle. The number of filaments N is preferably 10,000 or more, since it is possible to reduce the possibility that the specific fuzzes inherent in the carbon fiber bundle come out on the surface of the carbon fiber bundle, and to sufficiently reduce fuzzes when the carbon fiber bundle is rolled out from the bobbin. The number of filaments N is preferably 50,000 or less, since fuzzes due to abrasion can be suppressed, thus enabling the suppression of fuzzes when the carbon fiber bundle is rolled out. The number of filaments N of the carbon fiber bundle can be determined from the average single-fiber diameter B of the carbon fiber bundle, the specific gravity of the carbon fiber bundle and the basis weight (mass per unit length) mentioned later. The number of filaments N of the carbon fiber bundle can be achieved by adjusting the number of holes of a spinneret in the production process of a polyacrylonitrile-based precursor fiber bundle, and stacking a plurality of polyacrylonitrile-based precursor fiber bundles.

The knot strength A [MPa] of the carbon fiber bundle of the present invention preferably satisfies −88B+1,360≤A, more preferably −88B+1,370≤A, and still more preferably −88B+1,390≤A, in a relationship with the average single-fiber diameter B (μm). The knot strength is an indicator which reflects the mechanical properties of the fiber bundle in directions other than a fiber axis direction, and is a parameter which reflects the strength of bending and compressive loads applied from directions other than the fiber axis when the carbon fiber bundle is rolled out from the bobbin. The knot strength preferably satisfies −88B+1,360≤A, since it is possible to reduce fuzzes when the carbon fiber bundle is rolled out from the bobbin. Such a knot strength can be determined by the method mentioned in the knot strength of the carbon fiber bundle mentioned later. To increase the knot strength of the carbon fiber bundle, the heat generation rate of the single fiber, the number of filaments N, the single-fiber fineness and the yarn width in the stabilization process may be appropriately controlled in the method for producing a carbon fiber bundle of the present invention mentioned later.

The carbon fiber bundle of the present invention has a tensile strength in a resin-impregnated strand tensile test (also simply abbreviated as strand tensile strength) of preferably 5.5 to 7.0 GPa, more preferably 5.8 to 6.8 GPa, and still more preferably 5.9 to 6.7 GPa. The strand tensile strength is strongly related to the average value of the single-fiber strength, and is therefore a parameter which also affects the tensile strength of fuzz, which is a single fiber, and the higher the value, the better. However, the width of strength variation is more important than the average single-fiber strength. The strand tensile strength is preferably 5.5 GPa or more, since it is possible to sufficiently suppress winding due to ring-shaped fuzzes, which occurs when the carbon fiber bundle is rolled out for advanced processing. Although the higher the strand tensile strength, the better, the strand tensile strength is preferably 7.0 GPa or less, since it is possible to sufficiently suppress winding due to ring-shaped fuzzes, which occurs when the carbon fiber bundle is rolled out for advanced processing. The strand tensile strength can be determined by the method mentioned in a strand tensile test for carbon fiber bundle mentioned later. Such parameters can be controlled by using the below-mentioned method for producing a carbon fiber bundle of the present invention.

In the carbon fiber bundle of the present invention, the area ratio of the outer peripheral portion (outer layer) with a structure having a difference between skin and core to the entire cross section perpendicular to the fiber axis of the fiber (hereinafter referred to as outer-layer area ratio) is preferably 85 to 95 area %, more preferably 87 to 94 area %, and still more preferably 89 to 93 area %. Here, the outer-layer area ratio is the area ratio (%) obtained by dividing the area of the outer periphery seen when the cross section perpendicular to the fiber axis of the single fiber is observed by an optical microscope by the entire cross-sectional area perpendicular to the fiber axis of the single fiber.

Since the region inside the outer layer of the single fiber is the region having a low degree of orientation of the crystal portion and a low strand tensile elastic modulus E, the higher the outer-layer area ratio, the more fuzzes (single-fiber breakage) can be suppressed, which is preferable. The outer-layer area ratio is preferably 85 area % or more, since it is possible to suppress specific fuzzes inherent in the carbon fiber bundle. The outer-layer area ratio is preferably 95 area % or less, since it is possible to suppress fuzzes due to abrasion, which becomes easier to occur by excessive heat treatment in the stabilization process.

The outer-layer area can be measured by embedding a carbon fiber bundle in a resin, polishing a cross section perpendicular to the fiber axis direction, and observing the cross section by an optical microscope (details are mentioned later). Such an outer-layer area ratio can be achieved by appropriately controlling the heat generation rate of the single fiber, the number of filaments N, the single-fiber fineness and the yarn width in the stabilization process, as mentioned later.

To solve the problem in the production of a carbon fiber bundle capable of suppressing winding due to ring-shaped fuzzes, which occurs when the carbon fiber bundle is rolled out for advanced processing, according to the method for producing a carbon fiber bundle of the present invention, it has been found that the amount of heat removal relative to the total heat generation amount of the stabilized fiber bundle can be sufficiently secured, and the temperature unevenness in the stabilized fiber bundle can be reduced by appropriately controlling a heat generation rate, the number of filaments N, a single-fiber fineness and a yarn width of a single fiber in a stabilization process. Preferred mode for carrying out the invention will be described in detail below.

The polyacrylonitrile-based polymer is preferably used as a raw material for the production of the polyacrylonitrile-based precursor fiber bundle. In the present invention, the polyacrylonitrile-based copolymer refers to those in which at least acrylonitrile is a main constituent component of a copolymer skeleton. The main constituent component usually refers to a constituent component which accounts for 90 to 100% by mass of the polymer skeleton.

In the production of the polyacrylonitrile-based precursor fiber bundle, the polyacrylonitrile-based polymer preferably contains a copolymerization component such as itaconic acid, acrylamide or methacrylic acid from the viewpoint of improving the spinnability and efficiently performing the stabilization treatment. In the production of the polyacrylonitrile-based precursor fiber bundle, the method for producing a polyacrylonitrile-based polymer can be selected from known polymerization methods.

In the production of the polyacrylonitrile-based precursor fiber bundle suitable for obtaining the carbon fiber bundle of the present invention, the spinning solution is prepared by dissolving the above polyacrylonitrile-based copolymer in a solvent in which polyacrylonitrile such as dimethyl sulfoxide, dimethylformamide and dimethylacetamide, or aqueous solutions of nitric acid, zinc chloride and sodium rhodanide is soluble.

The method for producing a polyacrylonitrile-based fiber bundle used in the present invention is not particularly limited, but the polyacrylonitrile-based fiber bundle can be obtained by subjecting to processes such as stretching, washing with water, application of an oil agent, dry densification treatment and, if necessary, post-stretching. The number of holes in the spinneret in the production process of the polyacrylonitrile-based precursor fiber bundle is not particularly limited, but is preferably 1,000 to 10,000 holes, in view of ease of gathering, in order to achieve the number of filaments N of the above carbon fiber bundle.

In the production of the polyacrylonitrile-based precursor fiber bundle, the coagulation bath preferably contains a solvent such as dimethyl sulfoxide, dimethylformamide or dimethylacetamide used as the solvent for the spinning solution, and a so-called coagulation promoting component. It is possible to use, as the coagulation promoting component, a component which does not dissolve the polyacrylonitrile-based copolymer and is compatible with the solvent used in the spinning solution. Water is preferably used as the coagulation promoting component.

In the production of the polyacrylonitrile-based precursor fiber bundle, it is preferable to wash with a water washing bath composed of a plurality of stages at a water bath temperature of 30 to 98° C. in the water washing process.

The stretching ratio in the water bath stretching process is preferably 2 to 6 times.

After the water bath stretching process, an oil agent made of silicone or the like is preferably applied to the fiber bundle for the purpose of preventing adhesion between single fibers. The silicone oil agent is a modified silicone, and preferably those containing an amino-modified silicone having high heat resistance.

A known method can be used for the dry heat treatment process. For example, the drying temperature is 100 to 200° C.

The dried yarn is preferably further post-stretched in pressurized steam or under dry heating from the viewpoint of the denseness and productivity of the resulting polyacrylonitrile-based precursor fiber bundle. The steam pressure or temperature during post-stretching and the post-stretching ratio are preferably selected appropriately within a range where yarn breakage and the generation of fuzzes do not occur.

The single-fiber fineness of the polyacrylonitrile-based precursor fiber bundle in the method for producing a carbon fiber bundle production of the present invention is 0.9 to 2.2 dtex, preferably 1.0 to 1.8 dtex, and more preferably 1.1 to 1.7 dtex. The single-fiber fineness of the polyacrylonitrile-based precursor fiber bundle means the diameter of a single fiber in the polyacrylonitrile-based precursor fiber bundle. If the single-fiber fineness of the polyacrylonitrile-based precursor fiber bundle is 0.9 dtex or more, the abrasion resistance of the obtained carbon fiber bundle is improved, thus enabling the suppression of fuzzes generated when the carbon fiber bundle is rolled out. If the single-fiber fineness of the polyacrylonitrile-based precursor fiber bundle is 2.2 dtex or less, it is possible to sufficiently secure the amount of heat removal relative to the total heat generation amount of the stabilized fiber bundle in the stabilization process and to reduce the temperature unevenness, and to suppress specific fuzzes inherent in the carbon fiber bundle. The single-fiber fineness of the polyacrylonitrile-based precursor fiber bundle can be calculated from the mass and density per unit length of the polyacrylonitrile-based precursor fiber bundle and the number of filaments N. Such a polyacrylonitrile-based precursor fiber bundle can be achieved by controlling the extrusion rate and the stretching ratio in each process in the production process of the polyacrylonitrile-based precursor fiber bundle.

In the production of the carbon fiber bundle of the present invention, gathering is preferably performed before the stabilization process according to the number of filaments N of the polyacrylonitrile-based precursor fiber bundle, after the production process of the polyacrylonitrile-based precursor fiber bundle. In a preferred mode of gathering, after rolling out the polyacrylonitrile-based precursor fiber bundle from the creel, gathering is performed according to the number of filaments N of the polyacrylonitrile-based precursor fiber bundle so as to achieve the desired number of filaments N of the carbon fiber bundle.

The temperature in the process of heat-treating the polyacrylonitrile-based precursor fiber bundle in an oxidizing atmosphere (stabilization process) in the method for producing a carbon fiber bundle of the present invention is 200 to 300° C., preferably 220 to 290° C., and more preferably 230 to 280° C. If the heat treatment temperature is 200° C. or higher, since the heat treatment temperature is too low to form the non-heat treated portion within the stabilized fiber bundle, the unevenness with a structure having a difference between skin and core is less likely to occur, thus enabling sufficient reduction in fuzzes when the carbon fiber bundle is rolled out from the bobbin. If the heat treatment temperature is 300° C. or lower, since the heat generation rate does not increase more than necessary, it is possible to reduce the temperature unevenness in the stabilized fiber bundle and to suppress specific fuzzes inherent in the carbon fiber bundle. To measure the heat treatment temperature, a thermometer such as a thermocouple is inserted into the heat treatment furnace for the stabilization process to measure the temperature inside the furnace. If there is the temperature unevenness or temperature distribution when the temperature inside the furnace is measured at several points, a simple average temperature is calculated.

In the method for producing a carbon fiber bundle, a heat treatment is performed so that a heat generation rate Q obtained by the formula (3) is 150 to 500 J/m²/s until the density is 1.22 to 1.24 g/cm³, when q (J/g/s) is the heat generation rate of the single fiber in the stabilization process, N is the number of filaments, d (dtex) is a single-fiber fineness of the stabilized fiber bundle and W (mm) is a yarn width.

Q=q×N×d/W/10  (3)

The density of the stabilized fiber bundle is generally used as an indicator of the degree of progress of the stabilization reaction. The density of 1.22 to 1.24 g/cm³ means that it is in the initial stage of the stabilization process, and it is important to control the heat generation rate in the initial stage of the flame resistance process within an appropriate range, because it leads to control the proportion of fuzzes with a structure having a difference between skin and core of fuzzes inherent in the carbon fiber bundle, thus enabling the suppression of winding due to ring-shaped fuzzes, which occurs when the carbon fiber bundle is rolled out for advanced processing.

If the density is 1.22 g/cm³ or more, even if a heat treatment is performed at high temperature in the subsequent stabilization process, a rapid increase in heat generation rate in the stabilized fiber bundle can be suppressed, which leads to the suppression of the temperature unevenness in the stabilized fiber bundle, thus enabling the suppression of the proportion of fuzzes with a structure having a difference between skin and core of fuzzes inherent in the carbon fiber bundle.

If the density is 1.24 g/cm³ or less, fuzzes with a structure capable of sufficiently controlling the structure having a difference between skin and core of the stabilized fiber bundle, thus making it possible to sufficiently enhance the effect of suppressing the proportion of fuzzes with a structure having a difference between skin and core of fuzzes inherent in the carbon fiber bundle when the below-mentioned heat generation rate is controlled.

To confirm that the density for a heat treatment at the below-mentioned heat generation rate Q is within the above range, the fiber bundle is sampled during the stabilization process and the density is measured (the method for measuring the density is mentioned later). For example, if the density of the stabilized fiber bundle is lower than specified, the density can be adjusted by increasing the temperature or lengthening the stabilization time. Here, the oxidizing atmosphere means an atmosphere containing 10% by mass or more of known oxidizing substances such as oxygen and nitrogen dioxide, and an air atmosphere is preferable from the viewpoint of simplicity.

In the stabilization process of the method for producing a carbon fiber bundle of the present invention, the heat generation rate Q until the density is 1.22 to 1.24 g/cm³ is 150 to 500 J/m²/s, preferably 160 to 400 J/m²/s, and more preferably 180 to 350 J/m²/s. After controlling the heat generation rate Q until the density range is 1.22 to 1.24 g/cm³, even if the heat generation rate Q is changed to the density range to be subsequently set, this requirement shall be satisfied. For example, if the heat generation rate Q is controlled within a range of 150 to 500 J/m²/s until the density is 1.23 g/cm³, the heat generation rate Q may fall outside the range of 150 to 500 J/m²/s until the density is more the density of 1.23 g/cm³.

The heat release rate Q in the present invention represents the heat generation rate per unit by dividing the total heat release rate (numerator of the formula (3)) per unit length of the stabilized fiber bundle by the yarn width of the stabilized fiber bundle, and means the relationship between the heat generation and the heat removal of the stabilized fiber bundle. Therefore, the heat release rate Q is a parameter that reflects the temperature unevenness of the stabilized fiber bundle. Therefore, small heat release rate Q means small temperature unevenness of the stabilized fiber bundle.

If the heat generation rate Q is 150 J/m²/s or more, good balance between the heat generation amount of and the heat removal amount prevents the formation of the non-heat treated portion within the stabilized fiber bundle, and the unevenness with a structure having a difference between skin and core is less likely to occur, thus enabling sufficient reduction in fuzzes when the carbon fiber bundle is rolled out from the bobbin.

If the heat generation rate Q is 500 J/m²/s or less, since the heat removal rate is sufficiently larger than the heat generation rate, the temperature unevenness in the stabilized fiber bundle can be reduced and specific fuzzes inherent in the carbon fiber bundle can be suppressed.

To calculate the heat generation rate Q, after measuring the heat generation rate q (J/g/s) of the single fiber and the yarn width W (mm) by the method mentioned later, the heat generation rate Q can be calculated from the formula (3) using the number of filaments N (pieces) and the single-fiber fineness d (dtex). The heat generation rate Q can be controlled by the heat treatment temperature in the stabilization process, the number of filaments N, the single-fiber fineness of the stabilized fiber bundle, and the pitch (width) of the roller grooves.

The final density of the stabilized fiber bundle in the stabilization process in the method for producing a carbon fiber bundle of the present invention is 1.38 to 1.50 g/cm³, and preferably 1.42 to 1.48 g/cm³. If the density of the final stabilized fiber bundle is 1.38 g/cm³ or more, fuzzes due to abrasion of the carbon fiber bundle can be suppressed, thus enabling the suppression of fuzzes generated when the carbon fiber bundle is rolled out. If the density of the final stabilized fiber bundle is 1.50 g/cm³ or less, the heat treatment more than necessary can be prevented, thus enabling the suppression of the proportion of fuzzes with a structure having a difference between skin of fuzzes inherent in the carbon fiber bundle, and the suppression of fuzzes generated when the carbon fiber bundle is rolled out.

To confirm that the density of the final stabilized fiber bundle is within the above range, the stabilized fiber bundle is sampled and the density is measured (the method for measuring the density is mentioned later). For example, if the density of the stabilized fiber bundle is lower than specified, the density can be adjusted by increasing the temperature or lengthening the stabilization time. Here, the oxidizing atmosphere means an atmosphere containing 10% by mass or more of known oxidizing substances such as oxygen and nitrogen dioxide, and an air atmosphere is preferable from the viewpoint of simplicity.

In the stabilization process in the method for producing a carbon fiber bundle of the present invention, a heat treatment is performed so that a heat generation rate Q obtained by the formula (3) is 150 to 500 J/m²/s until the density is 1.22 to 1.24 g/cm³, when q (J/g/s) is the heat generation rate of the single fiber, N is the number of filaments, d (dtex) is a single-fiber fineness of the stabilized fiber bundle and W (mm) is a yarn width, and then a heat treatment is performed so that the heat generation rate Q obtained by the formula (3) is preferably 300 to 1,200 J/m²/s, more preferably 400 to 1,100 J/m²/s, and still more preferably 500 to 1,000 J/m²/s, until the density is 1.32 to 1.35 g/cm³,

For example, this requirement shall be satisfied when a heat treatment is performed so that the heat generation rate Q is 150 to 500 J/m²/s until the density is 1.23 g/cm³, and then a heat treatment is performed so that the heat generation rate Q is 300 to 1,200 J/m²/s until the density is 1.33 g/cm³.

However, at this time, after performing a heat treatment so that the heat generation rate Q is 150 to 500 J/m²/s until the density is 1.23 g/cm³, it is preferable to perform a heat treatment so that the heat generation rate Q is not 150 to 500 J/m²/s but 300 to 1,200 J/m²/s (for example, 800 J/m²/s) until the density is 1.24 g/cm³/s, but it is not preferable to perform a heat treatment so that the heat generation rate Q falls outside the range (for example, 1,500 J/m²/s). Since the stabilized fiber bundle with a density of 1.32 to 1.35 g/cm³ has a moderate degree of progress of the stabilization reaction and the heat generation rate at a moderate degree of progress of the stabilization reaction sometimes affects the structure having a difference between skin and core of the final stabilized fiber and the carbon fiber bundle, it is preferable to control the heat generation rate Q within the above range until the density is 1.32 to 1.35 g/cm³ from the density of 1.22 to 1.24 g/cm³.

If the heat generation rate Q is 300 or more, good balance between the heat generation amount of and the heat removal amount prevents the formation of the non-heat treated portion within the stabilized fiber bundle, and the unevenness with a structure having a difference between skin and core disappears, thus enabling sufficient reduction in fuzzes when the carbon fiber bundle is rolled out from the bobbin.

The heat generation rate Q is preferably 1,200 or less, since the heat removal rate is sufficiently larger than the heat generation rate, the temperature unevenness in the stabilized fiber bundle can be reduced and specific fuzzes inherent in the carbon fiber bundle can be suppressed.

To confirm that the density for a heat treatment at the heat generation rate Q is within the above range, the fiber bundle is sampled during the stabilization process and the density is measured (the method for measuring the density is mentioned later). For example, if the density of the stabilized fiber bundle is lower than specified, the density can be adjusted by increasing the temperature or lengthening the stabilization time.

Here, the oxidizing atmosphere means an atmosphere containing 10% by mass or more of known oxidizing substances such as oxygen and nitrogen dioxide, and an air atmosphere is preferable from the viewpoint of simplicity. To calculate the heat generation rate Q, after measuring the heat generation rate q (J/g/s) of the single fiber and the yarn width W (mm) by the method mentioned later, the heat generation rate Q can be calculated from the formula (3) using the number of filaments N (pieces) and the single-fiber fineness d (dtex). The heat generation rate Q can be controlled by the heat treatment temperature in the stabilization process, the number of filaments N, the single-fiber fineness of the stabilized fiber bundle, and the pitch (width) of the roller grooves.

In the stabilization process in the method for producing a carbon fiber bundle of the present invention, a heat treatment is performed so that a heat generation rate Q obtained by the formula (3) is 150 to 500 J/m/s until the density is 1.22 to 1.24 g/cm³ and a heat treatment is performed so that the heat generation rate Q obtained by the formula (3) is 300 to 1,200 J/m²/s until the density is 1.32 to 1.35 g/cm³, and then a heat treatment is performed so that a heat generation rate Q obtained by the formula (3) is preferably 900 to 1,500 J/m²/s, more preferably 1,000 to 1,400 J/m²/s, and still more preferably 1,100 to 1,300 J/m²/s, until the density is 1.38 to 1.50 g/cm³.

For example, this requirement shall be satisfied when a heat treatment is performed so that the heat generation rate Q is 150 to 500 J/m²/s until the density is 1.23 g/cm³ and a heat treatment is performed so that the heat generation rate Q is 300 to 1,200 J/m²/s until the density is 1.33 g/cm³, and then a heat treatment is performed so that the heat generation rate Q obtained by the formula (3) is 900 to 1,500 J/m²/s until the density is 1.48 g/cm³.

However, at this time, after performing a heat treatment so that the heat generation rate Q is 150 to 500 J/m²/s until the density is 1.33 g/cm³, it is preferable to perform a heat treatment under the conditions where the heat generation rate is not the above preferable heat generation rate Q (300 to 1,200 J/m²/s) but (within a range of 900 to 1,500 J/m²/s, for example, 1,250 J/m²/s) until the density is 1.35 g/cm³, but it is not preferable to perform a heat treatment so that the heat generation rate Q falls outside the range (for example, 1,600 J/m²/s).

Since the density of 1.38 to 1.50 g/cm³ is the final density of the stabilized fiber bundle in the present invention and sometimes affects the structure having a difference between skin and core of the carbon fiber bundle, and when the density is within a range of 1.38 to 1.50 g/cm³, it is preferable to set the heat generation rate Q within a range of 900 to 1,500 J/m²/s. For example, when a heat treatment is performed until 1.50 g/cm³ after setting the heat generation rate Q within a range of 900 to 1,500 J/m²/s (for example, 1,000 J/m²/s) until 1.38 g/cm³, it is preferable to set the heat generation rate Q within a range of 900 to 1,500 J/m²/s (for example, 1,400 J/m²/s).

Since the stabilized fiber bundle with a density of 1.32 to 1.35 g/cm³ has a moderate degree of progress of the stabilization reaction and the heat generation rate at a moderate degree of progress of the stabilization reaction sometimes affects the structure having a difference between skin and core of the final stabilized fiber and the carbon fiber bundle, it is preferable to control the heat generation rate Q within the above range until the density is 1.38 to 1.50 g/cm³ from the density of 1.32 to 1.35 g/cm³.

If the heat generation rate Q is 900 J/m²/s or more, good balance between the heat generation amount of and the heat removal amount prevents the formation of the non-heat treated portion within the stabilized fiber bundle, and the unevenness with a structure having a difference between skin and core disappears, thus enabling sufficient reduction in fuzzes when the carbon fiber bundle is rolled out from the bobbin.

The heat generation rate Q is preferably 1,500 J/m²/s or less, since the heat removal rate is sufficiently larger than the heat generation rate, the temperature unevenness in the stabilized fiber bundle can be reduced and specific fuzzes inherent in the carbon fiber bundle can be suppressed.

To confirm that the density for a heat treatment at the heat generation rate Q is within the above range, the fiber bundle is sampled during the stabilization process and the density is measured (the method for measuring the density is mentioned later). For example, if the density of the stabilized fiber bundle is lower than specified, the density can be adjusted by increasing the temperature or lengthening the stabilization time.

Here, the oxidizing atmosphere means an atmosphere containing 10% by mass or more of known oxidizing substances such as oxygen and nitrogen dioxide, and an air atmosphere is preferable from the viewpoint of simplicity. To calculate the heat generation rate Q, after measuring the heat generation rate q (J/g/s) of the single fiber and the yarn width W (mm) by the method mentioned later, the heat generation rate Q can be calculated from the formula (3) using the number of filaments N (pieces) and the single-fiber fineness d (dtex). The heat generation rate Q can be controlled by the heat treatment temperature in the stabilization process, the number of filaments N, the single-fiber fineness of the stabilized fiber bundle, and the pitch (width) of the roller grooves.

In the present invention, after performing a heat treatment o that the heat generation rate Q obtained by the formula (3) is 150 to 500 J/m²/s until the density is 1.22 to 1.24 g/cm³, the tension applied to the stabilized fiber bundle when a heat treatment is performed until the density is 1.38 to 1.50 g/cm³ is 1.6 to 4.0 mN/dtex, preferably 2.5 to 4.0 mN/dtex, and more preferably 3.0 to 4.0 mN/dtex. For example, after heat treatment to a density of 1.23 g/cm³, there is a need to perform a heat treat under such tension until the density is 1.40 g/cm³. When the tension is 1.6 mN/dtex or more, the orientation of the carbon fiber bundle is sufficiently enhanced and the strand tensile modulus E is improved, thus enabling sufficient reduction in fuzzes when the carbon fiber bundle is rolled out from the bobbin. If the tension is 4.0 mN/dtex or less, fuzzes inherent in the carbon fiber bundle can be suppressed. It is noted that the tension applied to the stabilized fiber bundle in the stabilization process is represented by the value obtained by dividing the tension (mN) measured at the output side of the stabilization furnace by the absolute dry fineness (dtex) of the polyacrylonitrile-based precursor fiber bundle.

In the production of the carbon fiber bundle of the present invention, the roller in the stabilization process preferably have grooves in order to achieve the desired yarn width W of the carbon fiber bundle. The pitch (width) of the roller grooves may be set according to the desired yarn width, and is preferably 5 to 8 mm.

In the production of the carbon fiber bundle of the present invention, pre-carbonization is preferably performed after the polyacrylonitrile-based precursor fiber bundle production process and the stabilization process. In the pre-carbonization process, the obtained stabilized fiber bundle is preferably heat-treated in an inert atmosphere at the maximum temperature of 500 to 1,200° C. until the density is 1.5 to 1.8 g/cm³.

The pre-carbonization is followed by carbonization. In the present invention, in the carbonization process, the resulting pre-carbonized fiber bundle is produced in an inert atmosphere at the maximum temperature of 1,200 to 1,600° C. If the maximum temperature is 1,200° C. or higher, it is possible to suppress winding due to ring-shaped fuzzes, which occurs when the carbon fiber bundle is rolled out for advanced processing. If the maximum temperature is 1,600° C. or lower, it is possible to suppress the generation of fuzzes due to abrasion of the carbon fiber bundle, thus enabling the suppression of fuzzes generated when the carbon fiber bundle is rolled out.

The carbon fiber bundle thus obtained is preferably subjected to an oxidation treatment to introduce oxygen-containing functional groups in order to improve the adhesion to the matrix resin. It is possible to use, as the oxidation treatment, vapor phase oxidation, liquid phase oxidation and liquid phase electrolytic oxidation. From the viewpoint of high productivity and capability of performing uniform treatment, liquid phase electrolytic oxidation is preferably used. There is no particular limitation on the liquid phase electrolytic oxidation method, and a known method may be used.

After the electrolytic treatment, a sizing agent can also be applied to provide converging properties to the obtained carbon fiber bundle. As for the sizing agent, a sizing agent having good compatibility with the matrix resin can be selected as appropriate depending on the type of the matrix resin used in the composite material.

The methods for measuring various physical properties mentioned herein are as follows.

<Measurement of Crystallite Size Lc>

The carbon fibers used in the measurement are aligned, and then the measurement is performed using a wide angle X-ray diffractometer under the following conditions:

X-ray source: CuKα ray (tube voltage of 40 kV, tube current of 30 mA),

Detector: goniometer+monochromator+scintillation counter,

Scan range: 20=10 to 40°

Scan mode: step scan, step unit of 0.01°, scanning speed of 1°/min

In the diffraction pattern thus obtained, with respect to peak appearing around 20=25 to 26°, full width at half maximum is determined, and the crystallite size is calculated from this value by the following Scherrer's equation.

Crystallite size (nm)=Kλ/β₀ cos θ_B

where

• • K: 1.0, λ: 0.15418 nm (wavelength of X-ray)
  • β₀: (β_E²−β₁²)^1/2
  • β₀: apparent full width at half maximum (measured value) rad,
  • β₁: 1.046×10⁻² rad
  • θ_B: Bragg's diffraction angle

<Strand Tensile Strength and Strand Tensile Modulus E of Carbon Fiber Bundle>

The strand tensile strength and strand tensile elastic modulus E of the carbon fiber bundle are determined by the following procedure in conformity with the resin-impregnated strand test method of JIS-R-7608 (2004). Using, 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), and curing conditions used are normal pressure, temperature of 125° C. and time of 30 minutes. Ten resin-impregnated strands of carbon fiber bundles are measured, and the average value is defined as the strand tensile strength. The strain is evaluated using an extensometer. The strain is within a range of 0.1 to 0.6%.

<Number of Fuzzes Inherent in Carbon Fiber Bundle>

The carbon fiber bundle is pulled out 10 m from the bobbin without tension and split into single fibers so that the thickness of the carbon fiber bundle is equivalent to two single fibers by enough force to prevent the generation of fuzzes, and if fuzzes are present, fuzzes are collected and the number of fuzzes are measured. The number of fuzzes per meter is calculated from the number measured as the number of fuzzes inherent in the carbon fiber bundle. The number of fuzzes generated in the process of splitting into single fibers is excluded from the number of fuzzes.

<Proportion of Fuzzes with Structure Having Difference Between Skin and Core of Fuzzes Inherent in Carbon Fiber Bundle>

The carbon fiber bundle is rolled out from the bobbin without tension, and 50 fuzzes inherent in the carbon fiber bundle are collected at random. The tip of the collected fuzzes is observed from the front and observed from approximately 450 diagonally using a scanning electron microscope (SEM) “S-4800” manufactured by Hitachi High-Technologies Corporation. Of the observed cross sections, a cross section with a structure which appears to be two concentric layers as shown in FIG. 1 or a cross section with a hole in the center of fuzzes as shown in FIG. 2 are defined as “cross section with a structure having a difference between skin and core”. The collected fuzzes whose cross section is judged to be broken by bending as shown in FIG. 3 is excluded from the total number of fuzzes, since it is considered that fuzzes generated in the production process of the carbon fiber bundle are not fuzzes inherent in the carbon fiber bundle and was subjected to bending load when fuzzes are collected, resulting in bending breakage. If the total number of excluded fuzzes is 5 or more, 15 fuzzes are further collected so that the total number of fuzzes is 100 or more. The proportion of the total number of “cross section with a structure having a difference between skin and core” to the total number of fuzzes other than those broken by bending obtained in this way is defined as the proportion of fuzzes with a structure having a difference between skin and core of fuzzes inherent in the carbon fiber bundle.

<Proportion of Fuzzes with Area Ratio of 50% or Less of Cross Section Perpendicular to Fiber Axis of Fuzzes Inherent in Carbon Fiber Bundle>

For the cross-sectional area of a single fiber of a carbon fiber bundle, 30 vertical cross sections are obtained by cutting the single fiber using a single blade, and the cross sections thus obtained are imaged from the front using a scanning electron microscope (SEM) “S-4800” manufactured by Hitachi High-Technologies Corporation. The SEM images thus obtained are measured in major axis using a ruler tool of “Image J” as free image analysis software, and the average value of 30 images is defined as the average cross-sectional area of a single fiber of the carbon fiber bundle. Using the protractor tool of “Image J” as free image analysis software, an area of 85 to 95° to the fiber axis in the SEM image of the “cross section of fuzzes inherent in the carbon fiber bundle” mentioned above, which is observed from approximately 45° diagonally, is selected. The area of the cross section perpendicular to the fiber axis is calculated for the selected area by calculating the area of the same cross section imaged from the front using “Image J” as free image analysis software. The ratio of the area of the cross section perpendicular to the fiber axis to the average cross-sectional area of the single fiber of the carbon fiber bundle obtained above is calculated, and the case where the ratio is 50% or less is considered to be corresponding to “fuzzes with an area ratio of 50% or less of the cross section perpendicular to the fiber axis” of fuzzes inherent in the carbon fiber bundle. The proportion of the total number of “fuzzes with an area ratio of 50% or less of the cross section perpendicular to the fiber axis” to the total number of fuzzes other than those broken by bending calculated by the method mentioned above is defined as the proportion of fuzzes with an area ratio of 50% or less of the cross section perpendicular to the fiber axis of fuzzes inherent in the carbon fiber bundle.

<Yarn Width W of Carbon Fiber Bundle>

The carbon fiber bundle is rolled out from the bobbin without tension so that the carbon fiber bundle does not loosen, and the yarn width is measured by a ruler. The measurement was made at three points every 1 m, and the average value is used as the yarn width W of the carbon fiber bundle.

<Knot Strength of Carbon Fiber Bundle>

A grip with a length of 25 mm is attached to both ends of a carbon fiber bundle with a length of 150 mm to produce a test specimen. In the fabrication of the test specimen, a load of 0.1×10⁻³ N/denier is applied to the carbon fiber bundle for alignment. One knot is fabricated at the midpoint of the test specimen, and the test specimen is subjected to a fiber bundle tensile test at a crosshead speed under tension of 100 mm/min. Twelve fiber bundles in total are subjected to the measurement. The average value of 10 fiber bundles excluding two values of the maximum and minimum values is used as the measured value, and the standard deviation of 10 values is used as the standard deviation of the knot strength. As the knot strength, a value obtained by dividing the maximum load value obtained in the bundle tensile test by the average cross-sectional area of the carbon fiber bundles is used.

<Measurement of Density>

The stabilized fiber bundle (1.0 to 3.0 g) is collected and completely dried at 120° C. for 2 hours. After measuring the absolute dry mass C (g), the stabilized fiber bundle is impregnated with ethanol, and after sufficiently defoaming, the fiber mass D (g) in the ethanol solvent bath is measured, and then the fiber specific gravity is measured by fiber specific gravity=(C×ρ)/(C−D). ρ is the specific gravity of ethanol at the measurement temperature.

<Average Single-Fiber Diameter B of Carbon Fiber Bundle>

The mass A_f (g/m) and the density ρ (g/cm³) per unit length of the carbon fiber bundle composed of a large number of carbon filaments to be measured are determined. C_f is the number of filaments N of the carbon fiber bundle to be measured, and the average single-fiber diameter B (μm) of the carbon fiber bundle is calculated by the following formula.

Average single-fiber diameter B (μm) of carbon fiber bundle=((A_f/ρ/C_f)/π)^(1/2)×2×10³

<Outer-Layer Area Ratio to Entire Cross Section Perpendicular to Fiber Axis of Carbon Fiber Single-Fiber>

The carbon fiber bundle to be measured is embedded in a resin and the cross section perpendicular to the fiber axis direction is polished, and then the cross section is observed by an optical microscope using a 100× objective lens at a total magnification of 1,000 times. The outer-layer area of the structure having a difference between skin and core is measured from the cross-sectional microscope image of the polished surface. The analysis is performed using image analysis software Image J. First, in the single-fiber cross-sectional image, black and white regions are divided by binarization. For the luminance distribution in the cross section of a single fiber, the average value of the distribution is set as the threshold value, and binarization is performed. The binarized image thus obtained is measured as the shortest distance from one point on the surface layer to the area with a line from black to white in the direction of the fiber diameter. This is measured for five points within the circumference of the same single fiber, and the average value is calculated as the thickness of the outer layer at that level. From the above, the area ratio (%) of the outer layer to the entire cross section perpendicular to the fiber axis direction of the carbon fiber single fiber is calculated, and the average value of 50 cross sections is defined as the area ratio of the outer layer to the entire cross section perpendicular to the fiber axis of the carbon fiber single-fiber.

<Heat Generation Rate q of Single Fiber>

A polyacrylonitrile-based precursor fiber bundle is dried at 120° C. for 1 hour under reduced pressure conditions of 10 mmHg or less, and then subjected to analysis of the heat generation amount. The dried polyacrylonitrile-based precursor fiber bundle (2 mg) is weighed in an aluminum sample pan. Using a heat flux differential scanning calorimeter (DSC3100SA, manufactured by Bruker AXS), the measurement was made from room temperature to 300° C. under the conditions of a temperature rise rate of 10° C./min and an air supply rate of 100 mL/min, without a lid on the aluminum sample pan. The obtained data are used as the heat generation rate q at a given temperature, with a heat generation rate at 150° C. being zero.

<Quality of Carbon Fiber Bundle During Rolling Out>

The bobbin of the carbon fiber bundle is placed on a creel, pulled by a roller at 10 m/min under a tension of 1.6 mN/dtex, and then wound by a winder. At this time, fuzzes generated between the creel and the roller is counted for 10 minutes and evaluated according to the following indices.

A: 1 to 2/10 minutes

B: 3 to 5/10 minutes

C: 6 or more/10 minutes

EXAMPLES

Hereinafter, the present invention will be described more specifically by way of Examples. However, the present invention is not limited thereto. Each measuring method in the Examples is as mentioned above.

Example 1

A copolymer composed of acrylonitrile and itaconic acid was polymerized by the solution polymerization method using dimethyl sulfoxide as a solvent to produce a polyacrylonitrile-based copolymer, thus obtaining a spinning dope solution containing. A coagulated fiber bundle was obtained by a dry-jet wet spinning method of extruding the obtained spinning dope solution once into the air through a spinneret and introducing the extruded spinning dope solution into a coagulation bath of an aqueous 35% solution of dimethyl sulfoxide controlled to 3° C. The fiber bundle thus obtained was washed with water at 30 to 98° C. by a common method and then stretched. Subsequently, an amino-modified silicone-based oil agent was applied to the fiber bundle obtained after subjecting to the water bath stretching, and the fiber bundle was subjected to a drying densification treatment using a heating roller at 160° C. so that the number of single fibers was 12,000, the fiber bundle was stretched 3.7 times in high pressure steam so that the total stretching ratio was 13 times to obtain a polyacrylonitrile-based precursor fiber bundle comprising 12,000 single-fibers. The amount of the spinning solution extruded from the spinneret was adjusted so that the single-fiber fineness of the polyacrylonitrile-based precursor fiber bundle was as shown in Table 2. For the obtained polyacrylonitrile-based precursor fiber bundle, the heat generation rate q of the single fiber was measured by the method mentioned above. Subsequently, the polyacrylonitrile-based precursor fiber bundle was heat-treated in an oven in an air atmosphere at a stretching ratio of 1 under the conditions of the heat treatment temperature and stabilization time shown in Table 2 to obtain a stabilized fiber bundle.

The stabilized fiber bundle thus obtained was subjected to a pre-carbonization treatment in a nitrogen atmosphere at a temperature of 300 to 800° C. to obtain a pre-carbonized fiber bundle. The pre-carbonized fiber bundle thus obtained was subjected to a carbonization treatment at the maximum temperature of 1,350° C. in a nitrogen atmosphere. The obtained carbon fiber bundle was subjected to a surface treatment and a sizing agent coating treatment to obtain a final carbon fiber bundle.

Table 1 shows the average single-fiber diameter B, the strand tensile modulus E, the crystallite size Lc, the strand tensile strength, the yarn width W, the number of filaments N, the outer-layer area ratio, the knot strength A, the number of fuzzes inherent in the carbon fiber bundle, the cross section of fuzzes inherent in the carbon fiber bundle, and the quality of the carbon fiber bundle during rolling out of the obtained carbon fiber bundle. The number of fuzzes inherent in the carbon fiber bundle was 38 fuzzes/m, the proportion of fuzzes with a structure having a difference between skin and core inherent in the carbon fiber bundle was 24%, and the quality of the carbon fiber bundle during rolling out was satisfactory.

Example 2

The same procedure as in Example 1 was performed, except that the heat treatment temperature until the density was 1.22 to 1.24 g/cm³ was 235° C., and as a result, the number of fuzzes inherent in the carbon fiber bundle was 3 fuzzes/m, the proportion of fuzzes with a structure having a difference between skin and core inherent in the carbon fiber bundle was 4%, and the quality of the carbon fiber bundle during rolling out was very satisfactory. The evaluation results thus obtained are shown in Table 1 and Table 2.

Example 3

The same procedure as in Example 1 was performed, except that the yarn width W was 5 mm, the number of filaments N was 24,000, the heat treatment temperature until the density was 1.22 to 1.24 g/cm³ was 230° C., and the heat treatment temperature until the density was 1.38 to 1.50 g/cm³ was changed to 265° C., and as a result, the number of fuzzes inherent in the carbon fiber bundle was 32 fuzzes/m, the proportion of fuzzes with a structure having a difference between skin and core inherent in the carbon fiber bundle was 12%, and the quality of the carbon fiber bundle during rolling out was very satisfactory. The evaluation results thus obtained are shown in Table 1 and Table 2.

Example 4

The same procedure as in Example 3 was performed, except that the yarn width W was 8 mm and the heat treatment temperature until the density was 1.22 to 1.24 g/cm³ was 235° C., and as a result, the number of fuzzes inherent in the carbon fiber bundle was 2 fuzzes/m, the proportion of fuzzes with a structure having a difference between skin and core inherent in the carbon fiber bundle was 2%, and the quality of the carbon fiber bundle during rolling out was very satisfactory. The evaluation results thus obtained are shown in Table 1 and Table 2.

Example 5

The same procedure as in Example 1 was performed, except that the heat treatment temperature until the density was 1.22 to 1.24 g/cm³ was 235° C., and as a result, the number of fuzzes inherent in the carbon fiber bundle was 4 fuzzes/m, the proportion of fuzzes with a structure having a difference between skin and core inherent in the carbon fiber bundle was 2%, and the quality of the carbon fiber bundle during rolling out was very satisfactory. The evaluation results thus obtained are shown in Table 1 and Table 2.

Example 6

The same procedure as in Example 5 was performed, except that the single-fiber fineness d of the stabilized fiber bundle was 1.2 dtex, and as a result, the number of fuzzes inherent in the carbon fiber bundle was 3 fuzzes/m, the proportion of fuzzes with a structure having a difference between skin and core inherent in the carbon fiber bundle was 4%, and the quality of the carbon fiber bundle during rolling out was very satisfactory. The evaluation results thus obtained are shown in Table 1 and Table 2.

Example 7

The same procedure as in Example 5 was performed, except that the yarn width W was 7 mm and the single-fiber fineness d of the stabilized fiber bundle was 0.9 dtex, and as a result, the number of fuzzes inherent in the carbon fiber bundle was 5 fuzzes/m, the proportion of fuzzes with a structure having a difference between skin and core inherent in the carbon fiber bundle was 1%, and the quality of the carbon fiber bundle during rolling out was very satisfactory. The evaluation results thus obtained are shown in Table 1 and Table 2.

Example 8

The same procedure as in Example 2 was performed, except that the tension of the stabilized fiber bundle, when the heat treatment was performed until the density was 1.38 to 1.50 g/cm³, was 3.8 mN/dtex, and the maximum temperature of the carbonization temperature was 1,600° C., and as a result, the crystallite size Lc was 2.4 nm and the strand tensile modulus E was 300 GPa. Furthermore, the number of fuzzes inherent in the carbon fiber bundle was 2 fuzzes/m, the proportion of fuzzes with a structure having a difference between skin and core inherent in the carbon fiber bundle was 3%, and the quality of the carbon fiber bundle during rolling out was very satisfactory. The evaluation results thus obtained are shown in Table 1 and Table 2.

Example 9

The same procedure as in Example 2 was performed, except that the copolymer was a copolymer made of acrylonitrile, itaconic acid and normal butyl acrylate, and the single-fiber fineness of the polyacrylonitrile-based precursor fiber bundle was 2.2 dtex, and as a result, the average single-fiber diameter B of the carbon fiber bundle was 10.5 μm. Furthermore, the number of fuzzes inherent in the carbon fiber bundle was 35 fuzzes/m, the proportion of fuzzes with a structure having a difference between skin and core inherent in the carbon fiber bundle was 6%, and the quality of the carbon fiber bundle during rolling out was very satisfactory. The evaluation results thus obtained are shown in Table 1 and Table 2.

Example 10

The same procedure as in Example 4 was performed, except that the heat treatment temperature until the density was 1.38 to 1.50 g/cm³ was 285° C., and as a result, the number of fuzzes inherent in the carbon fiber bundle was 16 fuzzes/m, the proportion of fuzzes with a structure having a difference between skin and core inherent in the carbon fiber bundle was 2%, and the quality of the carbon fiber bundle during rolling out was very satisfactory. The evaluation results thus obtained are shown in Table 1 and Table 2.

Example 11

The same procedure as in Example 4 was performed, except that the final temperature of the carbonization temperature was 1,450° C., and as a result, the number of fuzzes inherent in the carbon fiber bundle was 17 fuzzes/m, the proportion of fuzzes with a structure having a difference between skin and core inherent in the carbon fiber bundle was 3%, and the quality of the carbon fiber bundle during rolling out was very satisfactory. The evaluation results thus obtained are shown in Table 1 and Table 2.

Example 12

The same procedure as in Example 4 was performed, except that the yarn width W was 9 mm, the heat treatment temperature until the density was 1.32 to 1.35 g/cm³ was 260° C., the heat treatment temperature until the density was 1.38 to 1.50 g/cm³ was 279° C., and the tension of the stabilized fiber bundle, when the heat treatment was performed until the density was 1.38 to 1.50 g/cm³, was 1.7 mN/dtex, and as a result, the number of fuzzes inherent in the carbon fiber bundle was 2 fuzzes/m, the proportion of fuzzes with a structure having a difference between skin and core inherent in the carbon fiber bundle was 1%, and the quality of the carbon fiber bundle during rolling out was very satisfactory. The evaluation results thus obtained are shown in Table 1 and Table 2.

Comparative Example 1

The same procedure as in Example 1 was performed, except that the yarn width W was 4 mm, and as a result, the number of fuzzes inherent in the carbon fiber bundle was 42 fuzzes/m, the proportion of fuzzes with a structure having a difference between skin and core inherent in the carbon fiber bundle was 27%, and there were a lot of fuzzes when the carbon fiber bundle is rolled out, leading to deterioration of the quality. The evaluation results thus obtained are shown in Table 1 and Table 2.

Comparative Example 2

The same procedure as in Example 2 was performed, except that the yarn width W was 10 mm, and as a result, the number of fuzzes inherent in the carbon fiber bundle was 4 fuzzes/m, and the proportion of fuzzes with a structure having a difference between skin and core inherent in the carbon fiber bundle was 0%. However, since the strand tensile strength was reduced to 5.0 GPa, abrasion occurred near the roller when the carbon fiber bundle is rolled out and thus there were a lot of fuzzes, leading to deterioration of the quality. The evaluation results thus obtained are shown in Table 1 and Table 2.

Comparative Example 3

The same procedure as in Example 2 was performed, except that the number of filaments N was 51,000, and as a result, the number of fuzzes inherent in the carbon fiber bundle was 260 fuzzes/m, the proportion of fuzzes with a structure having a difference between skin and core inherent in the carbon fiber bundle was 70%, and there were a lot of fuzzes when the carbon fiber bundle is rolled out, leading to deterioration of the quality. The evaluation results thus obtained are shown in Table 1 and Table 2.

Comparative Example 4

The same procedure as in Example 2 was performed, except that the number of filaments N was 3,000 and the yarn width W was 3 mm, and as a result, the number of fuzzes inherent in the carbon fiber bundle was 3 fuzzes/m, and the proportion of fuzzes with a structure having a difference between skin and core inherent in the carbon fiber bundle was 0%. However, since the strand tensile strength was reduced to 5.1 GPa, abrasion occurred near the roller when the carbon fiber bundle is rolled out and thus there were a lot of fuzzes, leading to deterioration of the quality. The evaluation results thus obtained are shown in Table 1 and Table 2.

Comparative Example 5

The same procedure as in Example 1 was performed, except that the yarn width W was 5 mm, and the heat treatment temperature until the density was 1.22 to 1.24 g/cm³ was 250° C., and as a result, the number of fuzzes inherent in the carbon fiber bundle was 130 fuzzes/m, the proportion of fuzzes with a structure having a difference between skin and core inherent in the carbon fiber bundle was 30%, and there were a lot of fuzzes when the carbon fiber bundle is rolled out, leading to deterioration of the quality. The evaluation results thus obtained are shown in Table 1 and Table 2.

Comparative Example 6

The same procedure as in Example 1 was performed, except that the heat treatment temperature until the density was 1.22 to 1.24 g/cm³ was 220° C., and the heat treatment temperature until the density was 1.38 to 1.50 g/cm³ was 250° C., and as a result, the number of fuzzes inherent in the carbon fiber bundle was 5 fuzzes/m, and the proportion of fuzzes with a structure having a difference between skin and core inherent in the carbon fiber bundle was 0%. However, since the strand tensile strength was reduced to 5.2 GPa, abrasion occurred near the roller when the carbon fiber bundle is rolled out and thus there were a lot of fuzzes, leading to deterioration of the quality. The evaluation results thus obtained are shown in Table 1 and Table 2.

Comparative Example 7

The heat treatment temperature until the density was 1.22 to 1.24 g/cm³ was 230° C., and the heat treatment temperature until the density was 1.38 to 1.50 g/cm³ was 275° C., and as a result, the density of the final stabilized fiber bundle was 1.36 g/cm³. The same procedure as in Example 1 was performed, except for others, and as a result, the number of fuzzes inherent in the carbon fiber bundle was 64 fuzzes/m, and there were a lot of fuzzes when the carbon fiber bundle is rolled out, leading to deterioration of the quality. The evaluation results thus obtained are shown in Table 1 and Table 2.

Comparative Example 8

The same procedure as in Comparative Example 8 was performed, except that the single-fiber fineness of the polyacrylonitrile-based precursor fiber bundle was 0.7 dtex, and as a result, the average single-fiber diameter B of the carbon fiber bundle was 5.5 μm and fuzzes due to abrasion increased, and thus there were a lot of fuzzes when the carbon fiber bundle is rolled out, leading to deterioration of the quality. The evaluation results thus obtained are shown in Table 1 and Table 2.

Comparative Example 9

The same procedure as in Example 1 was performed, except that, in accordance with Example 2 of JP 2007-314901 A, the number of filaments was 24,000 and stabilization was performed at 240° C. for 130 minutes, and the final carbonization temperature was 1,450° C., and as a result, the density of the final stabilized fiber bundle was 1.35 g/cm³. Therefore, the number of fuzzes inherent in the carbon fiber bundle was 48 fuzzes/m, and there were a lot of fuzzes when the carbon fiber bundle is rolled out, leading to deterioration of the quality. The evaluation results thus obtained are shown in Table 1 and Table 2.

Comparative Example 10

In accordance with Example 1 of JP 2018-145541, the copolymer was a copolymer made of acrylonitrile and 2-hydroxyethyl methacrylate, the single-fiber fineness of the polyacrylonitrile-based precursor fiber bundle was 4.0 dtex and the number of filaments was 3,000, and then a stabilization treatment was performed under the conditions shown in Table 2, and as a result, the density of the stabilized fiber bundle was 1.39 mg/m³, similarly to JP 2018-145541. The same procedure as in Example 1 was performed, except for others, and as a result, the average single-fiber diameter B of the carbon fiber bundle was 13.1 μm. Therefore, the proportion of fuzzes with a structure having a difference between skin and core inherent in the carbon fiber bundle became worse such as 82%, the number of fuzzes inherent in the carbon fiber bundle was 60 fuzzes/m, the proportion of fuzzes with a structure having a difference between skin and core inherent in the carbon fiber bundle was 35%, and there were a lot of fuzzes when the carbon fiber bundle is rolled out, leading to deterioration of the quality. The evaluation results thus obtained are shown in Table 1 and Table 2.

Comparative Example 11

The same procedure as in Example 9 was performed, except that the single-fiber fineness of the polyacrylonitrile-based precursor fiber bundle was 3.0 dtex, and as a result, the average single-fiber diameter B of the carbon fiber bundle was 12 Um and the strand tensile modulus E was reduced to 213 GPa. Furthermore, the number of fuzzes inherent in the carbon fiber bundle was 110 fuzzes/m, the proportion of fuzzes with a structure having a difference between skin and core inherent in the carbon fiber bundle was 45%, and there were a lot of fuzzes when the carbon fiber bundle is rolled out, leading to deterioration of the quality. The evaluation results thus obtained are shown in Table 1 and Table 2.

Comparative Example 12

The same procedure as in Example 2 was performed, except that the maximum temperature of the carbonization temperature was 1,150° C., and as a result, the crystallite size La was 1.4 nm and the strand tensile modulus E was reduced to 215 GPa. Furthermore, the number of fuzzes inherent in the carbon fiber bundle was 4 fuzzes/m, and the proportion of fuzzes with a structure having a difference between skin and core inherent in the carbon fiber bundle was 4%. However, since abrasion occurred near the roller when the carbon fiber bundle is rolled out, there were a lot of fuzzes, leading to deterioration of the quality. The evaluation results thus obtained are shown in Table 1 and Table 2.

Comparative Example 13

The same procedure as in Example 2 was performed, except that the tension of the stabilized fiber bundle in the stabilization process was 1.2 mN/dtex until the density was 1.38 to 1.50 g/cm³, and as a result, the strand tensile modulus was 225 GPa. Furthermore, the number of fuzzes inherent in the carbon fiber bundle was 6 fuzzes/m, and the proportion of fuzzes with a structure having a difference between skin and core inherent in the carbon fiber bundle was 4%. However, since abrasion occurred near the roller when the carbon fiber bundle is rolled out, there were a lot of fuzzes, leading to deterioration of the quality. The evaluation results thus obtained are shown in Table 1 and Table 2.

Comparative Example 14

The same procedure as in Example 2 was performed, except that the tension of the stabilized fiber bundle in the stabilization process was 4.5 mN/dtex until the density was 1.38 to 1.50 g/cm³, and as a result, the number of fuzzes inherent in the carbon fiber bundle was 80 fuzzes/m, and there were a lot of fuzzes when the carbon fiber bundle is rolled out, leading to deterioration of the quality. The evaluation results thus obtained are shown in Table 1 and Table 2.

Comparative Example 15

The same procedure as in Example 2 was performed, except that the maximum temperature of the carbonization temperature was 2,100° C., and as a result, the crystallite size Lc was 2.9 nm and the strand tensile modulus E increased to 320 GPa. However, the number of fuzzes inherent in the carbon fiber bundle was 45 fuzzes/m, and abrasion occurred near the roller when the carbon fiber bundle is rolled out and thus there were a lot of fuzzes, leading to deterioration of the quality. The evaluation results thus obtained are shown in Table 1 and Table 2.

Comparative Example 16

The same procedure as in Example 2 was performed, except that the heat treatment temperature was 305° C. until the density was 1.38 to 1.50 g/cm³, and as a result, the stabilized fiber bundle was broken during the process until the density was 1.38 to 1.50 g/cm³, thus failing to obtain the stabilized fiber bundle and the carbon fiber bundle. The evaluation results thus obtained are shown in Table 1 and Table 2.

	TABLE 1
	Fuzzes inherent in
	carbon fiber bundle
		Proportion
	Proportion of	of fuzzes
	fuzzes with	having area
	structure	ratio of
	Carbon fiber bundle		having	50% or less
									Whether	Whether		difference	of cross	Quality of
	Average single-	Strand		Strand	Yarn			Knot	or not	or not		between	section	carbon fiber
	fiber diameter	tensile	Crystallite	tensile	width	Number of	Outer-layer	strength	satisfying	satisfying		skin and	perpendicular	bundle during
	B	modulus	size Lo	strength	W	filaments	area ratio	A	formula	formula	Number	core	to fiber axis	rolling
	μm	GPA	nm	GPA	mm	N	Area %	MPa	(1)	(2)	Fuzzes/m	%	%	out
Example 1	7.1	235	2.0	5.5	8	12,000	85	760	Satisfy	Satisfy	38	24	0	B
Example 2	7.1	241	2.0	6.4	8	12,000	94	730	Satisfy	Not satisfy	3	4	0	A
Example 3	7.4	243	2.0	6.0	5	24,000	88	730	Satisfy	Satisfy	32	12	0	A
Example 4	7.3	260	2.0	5.9	8	24,000	95	730	Satisfy	Satisfy	2	2	0	A
Example 5	7.2	254	2.0	5.8	8	12,000	94	750	Satisfy	Satisfy	4	2	2	A
Example 6	7.8	242	2.0	5.4	8	12,000	92	700	Satisfy	Satisfy	3	4	0	B
Example 7	6.9	243	2.0	6.6	7	12,000	94	790	Satisfy	Satisfy	5	1	0	A
Example 8	7.1	300	2.4	5.8	8	12,000	93	780	Satisfy	Satisfy	2	3	0	B
Example 9	10.5	232	2.0	4.8	8	12,000	90	450	Satisfy	Satisfy	35	6	3	A
Example 10	7.3	236	2.0	5.4	8	24,000	90	770	Satisfy	Satisfy	16	2	9	A
Example 11	7.5	264	2.0	5.3	8	24,000	90	780	Satisfy	Satisfy	17	3	12	A
Example 12	7.3	250	2.0	6.0	9	24,000	89	780	Satisfy	Satisfy	2	1	0	A
Comparative	7.1	245	2.0	5.5	4	12,000	85	730	Satisfy	Not satisfy	42	27	10	C
Example 1
Comparative	7.1	247	2.0	5.0	10	12,000	98	700	Satisfy	Not satisfy	4	0	0	C
Example 2
Comparative	7.1	233	2.0	5.2	8	51,000	77	700	Satisfy	Not satisfy	260	70	38	C
Example 3
Comparative	7.1	240	2.0	5.1	3	3,000	100	740	Satisfy	Satisfy	3	0	0	C
Example 4
Comparative	7.1	237	2.0	5.3	5	12,000	82	680	Satisfy	Not satisfy	130	30	14	C
Example 5
Comparative	7.1	246	2.0	5.2	8	12,000	96	600	Satisfy	Not satisfy	5	0	0	C
Example 6
Comparative	6.7	248	2.0	5.6	8	12,000	86	680	Satisfy	Not satisfy	64	20	0	C
Example 7
Comparative	5.5	248	2.0	6.0	8	12,000	98	900	Satisfy	Satisfy	20	0	5	C
Example 8
Comparative	6.7	260	2.0	5.7	8	24,000	98	780	Satisfy	Satisfy	48	0	7	C
Example 9
Comparative	13.1	248	2.0	6.0	7	3,000	82	220	Satisfy	Satisfy	60	35	20	C
Example 10
Comparative	12.0	213	2.0	3.6	8	12,000	80	300	Not	Not satisfy	110	45	30	C
Example 11									satisfy
Comparative	7.1	215	1.4	5.2	8	12,000	94	730	Satisfy	Not satisfy	4	4	0	C
Example 12
Comparative	7.1	225	2.0	5.5	8	12,000	94	730	Not	Not satisfy	6	4	0	C
Example 13									satisfy
Comparative	7.1	280	2.0	5.5	8	12,000	94	730	Satisfy	Not satisfy	80	4	0	C
Example 14
Comparative	7.1	320	2.9	5.2	8	12,000	94	730	Satisfy	Not satisfy	45	3	0	C
Example 15
Comparative	Failed to obtain carbon fiber bundle
Example 16

	TABLE 2
						Tension
	Single-					of
	fiber		stabilized		Density (g/cm3)
	fineness of		fiber	Heat generation		1.22 to	1.32 to	1.38 to
	polyacry-	Single-	Stabilized fiber	bundle	rate q of single	Heat generation	Heat treatment	1.24	1.35	1.50
	lonitrile-	fiber	bundle density	having	fiber	rate Q	temperature t	Heat	Heat	Heat
	based	fineness	Density (g/cm3)	density	Density (g/cm3)	Density (g/cm3)	Density (g/cm3)	generation	generation	generation	Final
	precursor	stabilized	1.22	1.32	1.38	of 1.38	1.22	1.32	1.38	1.22	1.32	1.38	1.22	1.323	1.38	rate Q	rate Q	rate Q	temperature of
	fiber	fiber	to	to	to	to 1.50	to	To	To	to	To	to	to	to	to	150 to	300 to	900 to	carbonization
	bundle	bundle	1.24	1.35	1.50	g/cm3	1.24	1.35	1.50	1.24	1.35	1.50	1.24	1.35	1.50	500	1,200	1,500	temperature
	dtex	dtex	g/cm3	mN/dex	J/g/s	J/m2/s	° C.	(J/m2/s)	(J/m2/s)	(J/m2/s)	° C.
Example 1	1.1	1.1	1.23	1.32	1.48	2.2	1.6	2.1	6.0	264	347	990	240	250	285	Satisfy	Satisfy	Satisfy	1,350
Example 2	1.1	1.1	1.23	1.32	1.46	2.2	1.0	2.1	6.0	165	347	990	235	250	285	Satisfy	Satisfy	Satisfy	1,350
Example 3	1.1	1.1	1.23	1.35	1.49	2.2	0.8	2.1	2.9	422	1,109	1,531	230	250	265	Satisfy	Satisfy	Not satisfy	1,350
Example 4	1.1	1.1	1.23	1.32	1.46	2.2	1.0	2.1	2.9	330	693	957	235	250	265	Satisfy	Satisfy	Satisfy	1,350
Example 5	1.1	1.1	1.23	1.32	1.51	2.2	1.0	2.1	6.0	165	347	990	235	250	285	Satisfy	Satisfy	Satisfy	1,350
Example 6	1.2	1.2	1.23	1.32	1.46	2.2	1.0	2.1	6.0	180	378	1,080	235	250	285	Satisfy	Satisfy	Satisfy	1,350
Example 7	0.9	0.9	1.23	1.32	1.45	2.2	1.0	2.1	6.0	154	324	926	235	250	285	Satisfy	Satisfy	Satisfy	1,350
Example 8	1.1	1.1	1.23	1.32	1.46	3.8	1.0	2.1	6.0	165	347	990	235	250	285	Satisfy	Satisfy	Satisfy	1,600
Example 9	2.2	2.2	1.23	1.32	1.46	2.2	1.0	2.1	6.0	330	693	1,980	235	250	285	Satisfy	Satisfy	Not satisfy	1,350
Example 10	1.1	1.1	1.23	1.32	1.46	2.2	1.0	2.1	6.0	330	693	1,980	235	250	285	Satisfy	Satisfy	Not satisfy	1,350
Example 11	1.1	1.1	1.23	1.32	1.46	2.2	1.0	2.3	6.0	330	693	1,980	23	250	285	Satisfy	Satisfy	Not satisfy	1,450
Example 12	1.1	1.1	1.23	1.33	1.46	1.7	1.0	2.6	5.1	293	763	1,496	235	360	279	Satisfy	Satisfy	Satisfy	1,350
Comparative	1.1	1.1	1.23	1.32	1.48	2.2	1.6	2.1	6.0	528	693	1,980	240	250	285	Not satisfy	Satisfy	Not satisfy	1,350
Example 1
Comparative	1.1	1.1	1.23	1.32	1.41	2.2	1.0	2.1	6.0	132	277	792	235	250	285	Not satisfy	Not satisfy	Not satisfy	1,350
Example 2
Comparative	1.1	1.1	1.23	1.34	1.50	2.2	1.0	2.1	6.0	701	1,473	4,208	235	250	285	Not satisfy	Not satisfy	Not satisfy	1,350
Example 3
Comparative	1.1	1.1	1.23	1.35	1.48	2.2	1.0	2.1	6.0	110	231	660	235	250	285	Not satisfy	Not satisfy	Not satisfy	1,350
Example 4
Comparative	1.1	1.1	1.23	1.32	1.50	2.2	2.1	2.1	6.0	554	554	1,584	250	250	285	Not satisfy	Satisfy	Notsatisfy	1,350
Example 5
Comparative	1.1	1.1	1.23	1.32	1.46	2.2	0.4	2.1	2.1	66	347	347	220	250	250	Not satisfy	Satisfy	Notsatisfy	1,350
Example 6
Comparative	1.1	1.1	1.23	1.32	1.36	—	1.0	2.1	—	165	347	—	230	250	—	Satisfy	Satisfy	—	1,350
Example 7
Comparative	0.7	0.7	1.23	1.32	1.36	—	1.0	2.1	—	105	221	—	230	250	—	Not satisfy	Not satisfy	—	1,350
Example 8
Comparative	1.1	1.1	1.23	1.32	1.35	—	1.6	1.6	—	528	528	—	240	240	—	Notsatisfy	Satisfy	—	1,450
Example 9
Comparative	4.0	4.0	1.23	1.32	1.39	10.0	1.0	1.6	6.0	171	274	1,029	230	240	290	Satisfy	Not satisfy	Satisfy	1,350
Example 10
Comparative	3.0	3.0	1.23	1.32	1.46	2.2	1.0	2.1	6.0	450	945	2,700	235	250	285	Satisfy	Satisfy	Notsatisfy	1,350
Example 11
Comparative	1.1	1.1	1.23	1.32	1.46	2.2	1.0	2.1	6.0	165	347	990	235	250	285	Satisfy	Satisfy	Satisfy	1,150
Example 12
Comparative	1.1	1.1	1.23	1.32	1.46	1.2	1.0	2.1	6.0	165	347	990	235	250	285	Satisfy	Satisfy	Satisfy	1,350
Example 13
Comparative	1.1	1.1	1.23	1.32	1.46	4.5	1.0	2.1	5.0	165	347	990	235	250	285	Satisfy	Satisfy	Satisfy	1,350
Example 14
Comparative	1.1	1.1	1.23	1.32	1.46	2.2	1.0	2.1	5.0	165	347	990	235	250	285	Satisfy	Satisfy	Satisfy	2,100
Example 15
Comparative	1.1	1.1	1.23	1.32	—	2.2	1.0	2.1	9.2	—	—	—	235	250	305	—	—	—	—
Example 16

Claims

1. A carbon fiber bundle wherein an average single-fiber diameter B is 6.9 to 11.0 μm, a tensile modulus E of resin-impregnated strands is 230 to 310 GPa, the number of fuzzes inherent in the carbon fiber bundle is 40 fuzzes/m or less, and a proportion of fuzzes with a structure having a difference between skin and core is 1 to 25% of fuzzes inherent in the carbon fiber bundle.

2. The carbon fiber bundle according to claim 1, wherein a proportion of fuzzes with an area ratio of 50% or less of the cross section perpendicular to the fiber axis is 0 to 3% of fuzzes inherent in the carbon fiber bundle.

3. The carbon fiber bundle according to claim 1, wherein the tensile modulus E of resin-impregnated strands and a crystallite size Lc (nm) satisfy the relationship of the formula (1).

50×Lc+130≤E≤50×Lc+180  (1)

4. The carbon fiber bundle according to claim 1, wherein the crystallite size Lc (nm) is 1.5 to 2.5 nm.

5. The carbon fiber bundle according to claim 1, wherein a yarn width W is 5 to 8 mm.

6. The carbon fiber bundle according to claim 1, wherein the number of filaments N is 10,000 to 50,000.

7. The carbon fiber bundle according to claim 1, wherein a knot strength A (MPa) and the average single-fiber diameter B (μm) satisfy the relationship of the formula (2).

−88B+1,360≤A  (2)

8. The carbon fiber bundle according to claim 1, wherein the tensile strength of resin-impregnated strands is 5.5 to 7.0 GPa.

9. The carbon fiber bundle according to claim 1, wherein the area ratio of an outer layer to the entire cross section perpendicular to the fiber axis of the single fiber is 85 to 95 area %.

10. A method for producing a carbon fiber bundle, which comprises, in a process of heat-treating a polyacrylonitrile-based precursor fiber bundle with a single-fiber fineness of 0.9 to 2.2 dtex in an oxidizing atmosphere at 200 to 300° C., heat-treating the polyacrylonitrile-based precursor fiber bundle so that a heat generation rate Q obtained by the formula (3) is 150 to 500 J/m2/s until the density is 1.22 to 1.24 g/cm3, when q (J/g/s) is the heat generation rate of the single fiber, N is the number of filaments, d (dtex) is a single-fiber fineness of the stabilized fiber bundle and W (mm) is a yarn width, heat-treating the fiber bundle under tension of 1.6 to 4.0 mN/dtex until the density is 1.38 to 1.50 g/cm3 to obtain a stabilized fiber bundle, and heat-treating the stabilized fiber bundle in an inert atmosphere at 1,200 to 1,600° C. to obtain a carbon fiber bundle.

Q=q×N×d/W/10  (3)

11. The method for producing a carbon fiber bundle according to claim 10, wherein, in a process of heat-treating in an oxidizing atmosphere, a heat treatment is performed so that the heat generation rate Q obtained by the formula (3) is 150 to 500 J/m2/s until the density is 1.22 to 1.24 g/cm3, and a heat treatment is performed so that the heat generation rate Q obtained by the formula (3) is 300 to 1,200 J/m2/s until the density is 1.32 to 1.35 g/cm3, and then a heat treatment is performed so that the heat generation rate Q obtained by the formula (3) is 900 to 1,500 J/m2/s until the density is 1.38 to 1.50 g/cm3.