Carbon Fiber Bundle

The present invention relates to a carbon fiber bundle having excellent strength, and excellent process stability during further processing, while having high total fineness.

Since carbon fiber bundles have high specific strength and high specific elastic modulus, they have been widely used as reinforcing fibers for composite materials in the field of aerospace and the like. In recent years, carbon fiber bundles have been used also for industrial applications such as automotive parts and wind power generation. In particular, since wind power generation requires lightweightness and rigidity, carbon fiber bundles having excellent specific elastic modulus are often used therefor. Thus, in recent years, there has been an increasing demand for carbon fiber bundles for wind power generation.

In industrial applications, reduction of the cost of the final composite material product is strongly demanded. Therefore, emphasis is placed not only on reduction of the cost of the carbon fiber bundles, but also on processability of further processing in the production of intermediate substrates such as prepregs, towpregs, woven fabrics, and sheet molding compounds (SMCs), and carbon fiber-reinforced composites such as pultrusions. In order to increase the processability of further processing, it is especially important for a carbon fiber bundle to hardly generate fuzz, to have excellent spreadability, and not to cause fracture of the entire carbon fiber bundle or single carbon fibers during the course of unwinding from a bobbin and running through the production process, and to show favorable process stability.

In industrial applications for which reduction of the cost is strongly demanded, the so-called large-tow carbon fiber bundles, which have a single fiber fineness of not less than 0.6 dtex and whose number of filaments is not less than 40,000, are often used. In an attempt to reduce the cost of large-tow carbon fiber bundles, for example, polyacrylonitrile-based precursor fibers prepared by application of the wet spinning method, which is highly productive, are used to increase the processing unit and the processing density, to thereby increase the productivity, or simple equipment for acrylic fibers for clothing is applied. Compared to regular-tow carbon fiber bundles, whose number of filaments is 12,000 to 24,000, large-tow carbon fiber bundles are advantageous in terms of the cost. However, at present, large-tow carbon fiber bundles are inferior to regular-tow carbon fiber bundles in terms of the strand strength, fuzz number, and processability of further processing of carbon fiber bundles, so that further improvement that can be achieved without deterioration of the productivity has been demanded.

In order to address such an issue, Patent Document 1 proposes a technique for a large-tow carbon fiber bundle in which a polyacrylonitrile-based precursor fiber bundle having a dynamic viscoelastic property and a silicon content within particular ranges is subjected to heat treatment and drawing under particular conditions, to produce a high-quality large-tow carbon fiber bundle with high productivity.

Patent Documents 2, 3, and 4 propose techniques in which the composition of an oil agent to be applied to, and the amount of such an oil agent to be attached to, a polyacrylonitrile-based precursor fiber bundle are controlled to improve the processability in the stabilization process, to thereby improve the strand strength and the quality of the carbon fibers obtained.

Patent Documents 5 and 6 propose techniques in which a particular defect that forms a fracture origin of the resulting carbon fiber bundle is controlled within a certain range, to improve the strand strength and the quality of the resulting carbon fiber bundle.

However, the prior art has the following problems.

Patent Document 1 discloses that improved strand strength is achieved in a large-tow carbon fiber bundle, and that generation of fuzz during its production process can be effectively suppressed. However, there is neither disclosure nor suggestion for improvement of the process stability during further processing of the fiber bundle. Further, although the kinematic viscosity of an oil agent is important for suppressing voids of not less than 100 nm and hence for effectively suppressing fuzz generation during further processing, the kinematic viscosity of the oil agent (which was, for example, 450 mm/sec in Example 1) is insufficient, leading to insufficiency of the improving effect by the oil agent, which has been problematic.

Patent Document 2 discloses that improved strand strength is achieved in a regular-tow carbon fiber bundle, and that generation of fuzz during its production process is effectively suppressed. However, there is neither disclosure nor suggestion for improvement of the process stability during further processing of the fiber bundle. Further, although the kinematic viscosity of an oil agent is important for suppressing voids of not less than 100 nm and hence for effectively suppressing fuzz generation during further processing, the kinematic viscosity of the oil agent is insufficient (not more than 5000 mm/sec), leading to insufficiency of the improving effect by the oil agent, which has been problematic.

Patent Document 3 discloses that improved strand strength is achieved in a regular-tow carbon fiber bundle, and that generation of fuzz during its production process is effectively suppressed. However, there is neither disclosure nor suggestion for improvement of the process stability during further processing of the fiber bundle. Moreover, although the document discloses that the kinematic viscosity of an oil agent, important for suppressing voids of not less than 100 nm and hence for effectively suppressing fuzz generation during further processing, is 3500 to 20,000 mm/sec, there is no specific disclosure on the void state of the polyacrylonitrile-based precursor fibers to which the oil agent is applied, and the draw ratio in warm water, which is important for reduction of voids, is insufficient (the ratio was, for example, 3.5 in Example 1), so that the improving effect is insufficient, which has been problematic.

Furthermore, this invention is based on the use of a polyacrylonitrile-based precursor fiber bundle containing only a small number of filaments, obtained by the dry-jet wet spinning method. Therefore, in cases where the method is applied to a large-tow carbon fiber bundle in which fibrils are present on the fiber surface, and which has a large processing unit and a high processing density, an excessive silicon content leads to insufficiency of the effect that suppresses voids of not less than 100 nm, so that fuzz generation during further processing cannot be effectively suppressed, which has been problematic.

Patent Document 4 discloses that improved strand strength is achieved in a regular-tow carbon fiber bundle, and that generation of fuzz during its production process is effectively suppressed. However, there is neither disclosure nor suggestion for improvement of the process stability during further processing of the fiber bundle. Moreover, the kinematic viscosity of the oil agent, important for suppressing voids of not less than 100 nm, and hence for effectively suppressing fuzz generation during further processing, is insufficient. In addition, there is neither disclosure nor suggestion about the void state of the poly acrylonitrile-based precursor fibers to which the oil agent is applied, and the draw ratio in warm water, which is important for reduction of voids, is insufficient (the ratio was, for example, 3.5 in Example 1), so that the improving effect is insufficient, which has been problematic.

Furthermore, this invention is based on the use of a polyacrylonitrile-based precursor fiber bundle containing only a small number of filaments, obtained by the dry-jet wet spinning method. Therefore, in cases where the method is applied to a large-tow carbon fiber bundle in which fibrils are present on the fiber surface, and which has a large processing unit and a high processing density, an excessive silicon content leads to insufficiency of the effect that suppresses voids of not less than 100 nm, so that fuzz generation during further processing cannot be effectively suppressed, which has been problematic.

Patent Documents 5 and 6 disclose that a particular defect that appears on a fracture surface of a carbon fiber bundle resulting from a single fiber tensile test at a gauge length of 10 mm is controlled to improve the strand strength of a regular-tow carbon fiber bundle and to effectively suppress generation of fuzz during its production process. However, there is neither disclosure nor suggestion for improvement of the process stability during further processing of the fiber bundle. Further, although the defect that appears on the fracture surface resulting from the single fiber tensile test of the carbon fiber bundle at a gauge length of 10 mm showed a good correlation with the achievement of the strand strength, the above defect is different from the defect that causes the generation of fuzz during further processing in terms of the type and the existence probability, so that the above defect does not contribute to identification or improvement of the cause of the generation of fuzz, which has been problematic.

Thus, although the prior art has proposed techniques for improvement of the strand strength and suppression of the generation of fuzz during the carbon fiber bundle production process, there has been neither a disclosure on a technique for improving the generation of fuzz during further processing of a carbon fiber bundle, nor a disclosure on the defect that causes the generation of fuzz during further processing of a carbon fiber bundle or on a technique for identification of such a defect. Therefore, the fundamental reduction of the generation of fuzz during further processing of a carbon fiber bundle has been difficult.

Further, regarding suppression of voids and adhesion, which suppression is effective for suppressing the generation of fuzz during further processing, there has been no technique based on a large-tow carbon fiber bundle that comprehensively proposes control of the surface shape and voids of polyacrylonitrile-based precursor fibers, and the composition of an oil agent suitable therefor and control of the amount of such an agent to be attached. As a result, improvement of the generation of fuzz during further processing of a large-tow carbon fiber bundle has been insufficient.

In order to solve the above problems, the carbon fiber bundle of the present invention has the following constitution.

The present invention enables production of a carbon fiber bundle having excellent strength, and excellent process stability during further processing, while having high total fineness, wherein mechanical properties are likely to be achieved when the carbon fiber bundle is prepared into a carbon fiber-reinforced composite material.

The carbon fiber bundle of the present invention comprises fibers having a surface on which a fibril(s) is/are present along the fiber axis direction, wherein in a single fiber tensile test at a gauge length of 50 mm in accordance with JIS R7606 (2000), the ratio of the number of fibers having a fracture surface where a fibrillar substance(s) with an aspect ratio of 3.0 to 10.0 is/are present in the fracture origin is 1 to 20%, and the ratio of the number of fibers having a fracture surface where a void(s) of not less than 100 nm is/are present in the fracture origin is 1 to 14%, and wherein the number of filaments is 48,000 to 60,000.

In the carbon fiber bundle of the present invention, fibrils need to be present along the fiber axis direction on fiber surfaces. The fibrils have a width of preferably 100 to 600 nm, more preferably 200 to 400 nm. By the presence of the fibrils along the fiber axis direction on the carbon fiber surfaces, the coefficient of friction can be within an appropriate range. As a result, generation of fuzz during further processing can be reduced, and the carbon fiber bundle can have favorable spreadability. Further, by the presence of the fibrils, adhesion of fineness to each other especially in the early stage of stabilization can be prevented, so that the amount of a silicone-containing oil agent attached, which leads to formation of voids, can be reduced to allow reduction of voids. The presence and the width of the fibrils can be confirmed by observation of the fiber surfaces using a scanning electron microscope. The fibril width can be determined by observing 10 fibers at a magnification of ×25,000 to measure the width in the direction perpendicular to the fiber axis at 10 positions per fiber, and then calculating the arithmetic average of the measured values. The presence and the width of the fibrils can be controlled, for example, by employing wet spinning as the spinning method for the polyacrylonitrile-based precursor fiber bundle, by the coagulation conditions, or by the draw ratio in warm water.

In the carbon fiber bundle of the present invention, in a single fiber tensile test at a gauge length of 50 mm in accordance with JIS R7606 (2000), the ratio of the number of fibers having a fracture surface where a fibrillar substance(s) with an aspect ratio of 3.0 to 10.0 is/are present in the fracture origin is 1 to 20%, and the ratio of the number of fibers having a fracture surface where a void(s) of not less than 100 nm is/are present in the fracture origin is 1 to 14%.

The ratio of the number of fibers having a fracture surface where a fibrillar substance(s) with an aspect ratio of 3.0 to 10.0 is/are present is preferably 1 to 15%, more preferably 1 to 13%, still more preferably 1 to 10%. The ratio of the number of fibers having a fracture surface where a void(s) of not less than 100 nm is/are present is preferably 1 to 10%, more preferably 1 to 6%, still more preferably 1 to 4%. For either type of fracture surface, the smaller the ratio of the number of fibers having such a fracture surface, the more easily the effect of the present invention can be obtained. However, in the industrial production scale, a sufficient effect can be obtained by decreasing the ratio to 1% in most cases.

In cases where the ratio of the number of fibers having a fracture surface where a fibrillar substance(s) with an aspect ratio of 3.0 to 10.0 is/are present is not more than 20%, and the ratio of the number of fibers having a fracture surface where a void(s) of not less than 100 nm is/are present is not more than 14%, generation of fuzz during further processing can be suppressed, so that favorable process stability can be obtained.

In the carbon fiber bundle of the present invention, when the ratio of the number of fibers having a fracture surface where a fibrillar substance(s) with an aspect ratio of 3.0 to 10.0 is/are present is A (%), and the ratio of the number of fibers having a fracture surface where a void(s) of not less than 100 nm is/are present is B (%), A and B preferably satisfy the relationship of the following Formula (1).

A and B more preferably satisfy the following Formula (2), still more preferably satisfy the following Formula (3).

In cases where A and B satisfy the preferred relationship described above, both the ratio of the number of fibers having a fracture surface where the fibrillar substance(s) is/are present and the ratio of the number of fibers having a fracture surface where the void(s) is/are present are low. Therefore, the number of defects in the carbon fibers is small, and hence generation of fuzz during further processing can be suppressed so that favorable process stability can be obtained.

The strength of single fibers of the carbon fibers is dependent on the sizes, the types, and the existence probabilities of defects. A change in the gauge length results in changes in the sizes and the types of the defects included along the gauge length, so that the strength changes. The strand strength is commonly used as an index of the strength of a carbon fiber bundle, and shows a good correlation with the single-fiber strength at a gauge length of about 10 mm. On the other hand, in a study by the present inventors, the process stability during further processing was found to be correlated with the ratio of a particular defect at a gauge length of 50 mm. The reason why the process stability during further processing is correlated with a longer gauge length than that in the case of the strand strength is not necessarily unclear. However, this could be due to the fact that a serious defect with a relatively low existence probability causes fiber fracture upon application of tension or abrasion during the further processing.

The fibrillar substance with an aspect ratio of 3.0 to 10.0 is a defect that is thought to be generated in the course of production of the carbon fiber bundle, due to adhesion of single fibers to each other followed by their peeling. A polyacrylonitrile-based precursor fiber is an aggregate of fibrils along the fiber axis direction, and the above-described adhesion and peeling tend to cause destruction in the unit of fibrils. The result of a study by the present inventors indicates that fibrillar substances representing defects that are thought to be derived by the destruction in such a unit often have an aspect ratio of 3.0 to 10.0. The ratio of the number of fibers having a fracture surface where the fibrillar substance(s) is/are present can be calculated according to the method described later.

For controlling the ratio of the number of fibers having a fracture surface where a fibrillar substance(s) with an aspect ratio of 3.0 to 10.0 is/are present in the fracture origin in a single fiber tensile test at a gauge length of 50 mm in accordance with JIS R7606 (2000), it is important to suppress adhesion of single fibers to each other during the process of production of the carbon fiber bundle. In a generally known method for suppressing adhesion of single fibers to each other, a silicone-based oil agent is applied to polyacrylonitrile-based precursor fibers. However, a study by the present inventors showed that the simple use of a silicone-based oil agent is insufficient. A preferred control method is described later as a preferred production method for the carbon fiber bundle.

The void of not less than 100 nm is a defect that is thought to be formed in a case where a void present in a poly acrylonitrile-based precursor fiber remains without disappearance even after the subsequent stabilization process, the pre-carbonization process, and the carbonization process. Examples of the cause why the void present in the polyacrylonitrile-based precursor fiber does not disappear include the fact that the size of the void present in the polyacrylonitrile-based precursor fiber before the application of the oil agent is large, and the fact that the oil agent infiltrates into the void upon the application of the oil agent, to inhibit densification. Based on the result of a study by the present inventors, the size of the void generated by the above mechanism is often not less than 100 nm. The ratio of the number of fibers having a fracture surface where a void(s) of not less than 100 nm is/are present can be calculated according to the method described later.

For controlling the ratio of the number of fibers having a fracture surface where a void(s) of not less than 100 nm is/are present in the fracture origin in a single fiber tensile test at a gauge length of 50 mm in accordance with JIS R7606 (2000), it is important, for example, to reduce the size of voids present in the polyacrylonitrile-based precursor fibers, and to suppress infiltration of the oil agent into the voids. A preferred control method is described later as a preferred production method for the carbon fiber bundle.

The number of filaments in the carbon fiber bundle of the present invention is 48,000 to 60,000, preferably 50,000 to 55,000. The number of filaments is the number of single fibers constituting the carbon fiber bundle. The larger the number, the higher the productivity of the carbon fiber-reinforced composite material. However, in cases where the number of filaments is too large, spreadability of the carbon fiber bundle may decrease, and mechanical properties of the obtained carbon fiber-reinforced composite material may decrease from the viewpoint of the resin impregnating property. In cases where the number of filaments is 48,000 to 60,000, excellent productivity can be achieved in the molding of the composite material, so that the carbon fiber bundle can be favorably used in industrial applications. The number filaments can be controlled based on the number of holes of the spinneret in the fiber production process of the polyacrylonitrile-based precursor fiber bundle, and dividing and combining of fibers.

The strand strength of the carbon fiber bundle of the present invention is preferably 4.5 to 6.0 GPa, more preferably 4.6 to 6.0 GPa, still more preferably 4.8 to 6.0 GPa. The strand strength can be measured by the method described later and, in cases where the strand strength is 4.5 to 6.0 GPa, the carbon fiber bundle can be favorably used for industrial applications such as blade materials for windmills, reinforcing materials for pressure vessels, and structural parts for automobiles.

A method of producing a carbon fiber bundle preferred for obtaining the carbon fiber bundle of the present invention is described below.

The carbon fiber bundle of the present invention is preferably produced by

In the method of producing the polyacrylonitrile-based precursor fiber bundle described above, the polyacrylonitrile-based polymer means a polymer containing at least acrylonitrile as a major component of the polymer unit, wherein the major component means a component that accounts for 90 to 100% by mass of the polymer unit.

The polyacrylonitrile-based polymer preferably contains a copolymer component such as itaconic acid, acrylamide, or methacrylic acid, for example, from the viewpoint of improvement of the fiber production efficiency, and from the viewpoint of efficiently carrying out the stabilization. In the production of the polyacrylonitrile-based precursor fiber bundle, the method of producing the polyacrylonitrile-based polymer can be selected from known polymerization methods such as solution polymerization and aqueous suspension polymerization. The polyacrylonitrile-based polymer is provided as a spinning dope solution in which the polymer is dissolved in a solvent, for the production of the polyacrylonitrile-based precursor fibers. The solvent used for the spinning dope solution can be selected from known solvents in which polyacrylonitrile is soluble, such as dimethyl sulfoxide, dimethylformamide, and dimethylacetamide, aqueous nitric acid solutions, aqueous zinc chloride solutions, and aqueous sodium rhodanide solutions.

The method of producing a polyacrylonitrile-based precursor fiber bundle described above comprises a step of subjecting a polyacrylonitrile-based polymer to wet spinning. The wet spinning herein means a spinning method in which the poly acrylonitrile-based polymer is directly discharged into a coagulation bath through a spinneret. By the application of the wet spinning, a fiber surface shape having fibrils suitable for the production of the carbon fiber bundle of the present invention can be obtained.

In the method of producing the polyacrylonitrile-based precursor fiber bundle described above, the number of holes of the spinneret is preferably 3000 to 200,000 from the viewpoint of achievement of the above-described number of filaments of the carbon fiber bundle. By dividing and combining of the fibers, a polyacrylonitrile-based precursor fiber bundle with a predetermined number of filaments can be obtained.

In the method of producing the polyacrylonitrile-based precursor fiber bundle described above, the composition of the coagulation bath preferably contains a solvent used as the solvent of the spinning dope solution, such as dimethyl sulfoxide, dimethylformamide, or dimethylacetamide, and the so-called coagulation-promoting component. The solvent is more preferably dimethyl sulfoxide or dimethylformamide from the viewpoint of allowing the formation of appropriate fibrils on the surface of the polyacrylonitrile-based precursor fibers without deteriorating the productivity. As the coagulation-promoting component, a component in which the poly acrylonitrile-based polymer is insoluble, and which is compatible with the solvent used for the spinning dope solution, can be used. Water is preferably used.

The method of producing a polyacrylonitrile-based precursor fiber bundle described above comprises a step of drawing the fibers at a ratio of 5.0 to 8.0 in warm water at 30 to 99° C. The fibers obtained by the wet spinning of the poly acrylonitrile-based polymer are drawn in warm water while the solvent is washed away therein. The washing and the drawing may be carried out either at the same time or separately as long as the fibers are drawn at a ratio of 5.0 to 8.0 in warm water at 30 to 99° C. In cases where the fibers are drawn in warm water, the drawing is preferably carried out stepwise in a plurality of warm water baths. The temperature of the warm water is preferably 50 to 99° C., more preferably 70 to 99° C. The higher the temperature of the warm water, the more easily the fibers can be drawn, but the more likely the fibers are to adhere to each other. Therefore, it is preferred to use a plurality of warm water baths to increase the temperature of the warm water in a stepwise manner. The draw ratio in the warm water is preferably 5.5 to 8.0, more preferably 6.0 to 8.0. The higher the draw ratio, the smaller the number of voids present in the obtained polyacrylonitrile-based precursor fiber bundle, which is preferred for the production of the carbon fiber bundle of the present invention. In cases where the draw ratio is not more than 8.0, breakage of fibers due to the drawing can be suppressed to enable stable production of a polyacrylonitrile-based precursor fiber bundle with high quality.

The method of producing a polyacrylonitrile-based precursor fiber bundle described above comprises a step of applying an oil agent containing a silicone having a kinematic viscosity at 25° C. of 6000 to 20,000 mm/sec. The kinematic viscosity of the silicone at 25° C. is preferably 10,000 to 20,000 mm/sec, more preferably 15,000 to 18,000 mm/sec. In cases where the kinematic viscosity of the silicone at 25° C. is not less than 6000 mm/sec, when the oil agent is applied to a fiber bundle in which the number of voids is small, drawn at a ratio of 5.0 to 8.0 in warm water at 30 to 99° C., adhesion of fibers to each other can be suppressed, and moreover, infiltration of the oil agent into the voids can be effectively suppressed. In cases where the kinematic viscosity of the silicone at 25° C. is not more than 20,000 mm/sec, uneven attachment can be suppressed, so that a stable strand strength can be achieved in the obtained carbon fiber bundle. The kinematic viscosity at 25° C. can be measured according to JIS-Z-8803 (2011) or ASTM D 445-46T using, for example, an Ubbelohde viscometer.

The silicone used in the method of producing a polyacrylonitrile-based precursor fiber bundle described above is preferably an amino-modified silicone from the viewpoint of its uniform attachment. The amino-modified silicone is a silicone containing polydimethylsiloxane as a basic structure wherein side-chain methyl groups are partially modified with amino groups. The amino-modified silicone used may contain other modifying groups added thereto in addition to the amino groups. Although the amino groups as modifying groups may be either of a monoamine type or a polyamine type, polyamine-type amino groups are preferred from the viewpoint of promoting cross-linking. Diamine-type amino groups are more preferably used.

The amino equivalent, which is an index of the amount of amino groups (NH), in the amino-modified silicone is preferably 1000 to 14,000 g/mol, more preferably 1500 to 6000 g/mol, still more preferably 2000 to 4000 g/mol. In cases where the amino equivalent is not less than 1000 g/mol, uneven attachment due to excessive progress of cross-linking can be suppressed, and a stable strand strength can be achieved in the obtained carbon fiber bundle as a result. In cases where the amino equivalent is not more than 14,000 g/mol, the silicone can be sufficiently cross-linked, and a stable strand strength can be achieved in the obtained carbon fiber bundle as a result. The amino equivalent can be measured by a known method such as neutralization titration. The amino equivalent can be controlled, for example, by the amount of amine added during the polymerization of the amino-modified silicone.

The oil agent used in the method of producing a polyacrylonitrile-based precursor fiber bundle described above may contain a surfactant, an antioxidant, an antistatic agent, a lubricant, or the like in addition to the silicone having a kinematic viscosity at 25° C. of 6000 to 20,000 mm/sec.