Polyacrylonitrile-based flame-resistant fibers, carbon fibers, and methods for producing the same

Abstract

The present invention provides a carbon fiber having a large single fiber diameter of 6.0 μm or more, few internal defects, and excellent tensile strength, and can increase production volume, improve carbon fiber productivity, and also provide cost reduction effects.

Description

[Technical Field]

[0001] This invention is a polyacrylonitrile-based resistant flame This invention relates to synthetic fibers, carbon fibers, and methods for producing them, and more specifically, to polyacrylonitrile-based materials that have high strength without reducing productivity. flame This invention relates to synthetic fibers, carbon fibers, and methods for producing them. [Background technology]

[0002] Carbon fiber possesses excellent physical properties such as lightness, high strength, and high modulus of elasticity, and its applications are expanding further. It is used in sporting goods such as fishing rods, golf clubs, and skis; as a forming material for CNG tanks, flywheels, wind turbines, and turbine blades; as a reinforcing material for structures such as roads and bridge piers; and even as a material for aircraft and space. In particular, carbon fiber can be used in hydrogen fuel tanks, aerospace fuel tanks, and aerospace centrifuges that require high pressures of 700 bar or more, and these require high tensile strength. As the applications of carbon fiber expand in this way, there is a demand for the development of carbon fiber with even higher tensile strength.

[0003] Conventionally, techniques for increasing the tensile strength of carbon fibers by optimizing the coagulation bath to make the undrawn yarn denser have been disclosed in Japanese Patent Publication No. 59-82420 and Japanese Patent Publication No. 6-15722.

[0004] However, this technique for improving the density of undrawn yarn has a drawback: the oxidation process reduces oxygen permeability to the fibers, thereby decreasing the tensile strength of the resin-impregnated carbon fiber strands.

[0005] As another method, Japanese Patent Publication No. 7-37685 describes reducing the average single fiber diameter of carbon fibers to 5.5 μm or less to achieve an average single fiber tensile strength of 530 kg / mm². 2 We are disclosing technologies that further improve upon this.

[0006] However, this technology can only be expected to improve tensile strength when using carbon fibers with a fine denier, where the diameter of the individual carbon fiber is less than 5.5 μm. When the diameter of the individual carbon fiber is thicker than 6.0 μm, there is a limit to how much tensile strength can be improved by this technology. In other words, it is difficult to obtain high tensile strength with non-fine denier fibers, and if the denier is made finer, productivity decreases. [Overview of the project] [Problems that the invention aims to solve]

[0007] The present invention aims to overcome the problems of the prior art described above, and one objective of the present invention is to produce a polyacrylonitrile-based carbon fiber that has high tensile strength and excellent productivity while having a thick denier. flame The objective is to provide synthetic fibers.

[0008] Another object of the present invention is to provide carbon fibers and high-pressure vessels using the same, which have high strength and high elongation properties and can be applied to pressure vessels and the like, while also achieving low cost and high productivity. [Means for solving the problem]

[0009] One aspect of the present invention for solving the above-mentioned problems is a polyacrylonitrile-based resistant fiber having a single fiber diameter of 10.0 μm or more while satisfying the conditions of the following formulas 1 and 2. flame This concerns synthetic fibers.

[0010] [Mathematics 1] Log ( density )≧(α / β) / 100

[0011] [Math 2] 0.120 ≤ (α / β) / 100 ≤ 0.135 Here, density : Flame-resistant fiber density (g / cm³) 3 ) α: Degree of reaction cyclization of flame-resistant fiber, EOR (%) β: Oxygen content (%) within flame-resistant fiber

[0012] Another aspect of the present invention is the polyacrylonitrile-based resistant flame This invention relates to carbon fibers manufactured by carbonizing synthetic fibers, characterized by a single carbon fiber diameter of 6.0 μm or more, a fiber bundle number of 24,000 to 36,000 filaments, a resin-impregnated strand tensile strength of 5.8 GPa to 6.4 GPa, and a tensile elongation of 2.2% to 2.6%.

[0013] A further aspect of the present invention relates to a method for producing carbon fibers, characterized by forming coagulated yarn by dry-wet spinning a polymer solution with a polymer concentration of 20% to 24% by weight in a coagulation bath with a concentration of 30% to 40% by weight containing the same solvent used for polymerization and water; stretching the obtained carbon fiber precursor fibers in multiple water washing baths; applying an oil agent; drying and stretching to obtain carbon fiber precursor fibers having a surface densification degree of 65% to 70% at a wavelength of 380 nm and 82% to 95% at a wavelength of 430 nm; and then carbonizing the carbon fibers at a temperature of 1000°C to 1400°C so that the diameter of the carbon fibers is 6.0 μm or more.

[0014] In the method of the present invention, the weight-average molecular weight (Mw) of the polymer is approximately 120,000 to 180,000, and the weight-average molecular weight is number Polymers with an average molecular weight ratio (Mw / Mn) within the range of 1.6 to 1.8 can be used.

[0015] During spinning, the temperature difference between the spinning solution and the coagulation bath should be kept within 30°C, and the air gap should be kept within 10.0 mm.

[0016] A further aspect of the present invention includes a pressure vessel body and a fiber-reinforced resin layer formed on the surface of the pressure vessel body, wherein the fibers constituting the fiber-reinforced resin layer are polyacrylonitrile-based flameThe present invention relates to a high-pressure vessel characterized in that it is manufactured by carbonizing a chemical fiber, the diameter of a single carbon fiber monofilament is 6.0 μm or more, the number of filaments in the fiber bundle is 24,000 to 36,000, the tensile strength of the resin-impregnated strand is 5.8 GPa to 6.4 GPa, and the tensile elongation is 2.2% to 2.6%. [Effects of the Invention]

[0017] Conventionally, in order to increase the tensile strength of carbon fibers, the fineness of single carbon fiber filaments has been reduced, thereby making the diameter of carbon fibers 6.0 μm or less. As a result, the elastic modulus distribution within each single filament is narrowed and the strength of carbon fibers increases, but there has been a problem that the tensile elastic modulus also increases at the same time. In contrast, according to the present invention, only the tensile strength can be increased without reducing the diameter of single filaments to the fineness of the prior art. Therefore, while increasing the production volume and improving the productivity of carbon fibers, the increase in equipment cost can be suppressed to the maximum extent, and the effect of cost reduction can also be obtained.

[0018] According to the present invention, a molding material using carbon fibers having high strength and high elongation can be provided. The carbon fiber of the present invention is excellent in resin impregnation property and strand diffusibility during molding, and the molding material using the same has a high strength development rate of carbon fibers and becomes a molding material excellent in mechanical properties. The molding material is suitable for use in fiber-reinforced composite materials such as pressure vessels. [Brief Description of the Drawings]

[0019] [Figure 1] It is a cross-sectional micrograph of the carbon fiber of one embodiment of the present invention. [Mode for Carrying Out the Invention]

[0020] Hereinafter, the present invention will be described in more detail.

[0021] In the present specification, the term "fiber" is meant to include a single filament, or a plurality of filaments bound together into a bundle (also referred to as "tow").

[0022] In this specification, the term “precursor fiber” refers to a fiber containing a polymer material that, when sufficiently heated, can be converted into carbon fibers having a carbon content of about 90% by weight or more, specifically about 95% by weight or more. Precursor fibers may include both homopolymers and copolymers of acrylonitrile (AN), and may include copolymers such as methyl acrylate (MA), methacrylic acid (MAA), sodium methylsulfonate (SMAS), itaconic acid (ITA), vinyl bromide (VB), isobutyl methacrylate (IBMA), and combinations thereof.

[0023] One aspect of the present invention is a polyacrylonitrile-based resistant fiber having a single fiber diameter of 10.0 μm or more, while satisfying the conditions of the following formulas 1 and 2. flame This concerns synthetic fibers.

[0024] [Mathematics 1] Log ( density )≧(α / β) / 100

[0025] [Math 2] 0.120 ≤ (α / β) / 100 ≤ 0.135 Here, density : Flame-resistant fiber density (g / cm³) 3 ) α: Degree of reaction cyclization of flame-resistant fiber, EOR (%) β: Oxygen content (%) within flame-resistant fiber

[0026] In this invention, if Log(ρ) is less than 0.01α / β, a problem may arise in which the structure within the fiber cross-section becomes a double structure, preventing the carbon fiber from exhibiting high tensile strength. If Log(ρ) is less than 0.120, it is advantageous for the internal and external structures to be uniform, but insufficient stabilization of the fiber bundle may cause fusion and thread breakage during carbonization. Conversely, if the 0.01α / β value exceeds 0.135, a problem may arise in which the structure within the fiber cross-section becomes a double structure, preventing the carbon fiber from exhibiting high tensile strength.

[0027] Obtaining high-quality carbon fibers requires resistance flameDuring the chemical process, it is necessary to avoid creating a double structure within the fiber cross-section. flame The structure of synthetic fibers is controlled by factors such as the filamentity of the precursor fibers, the temperature in the oxidation furnace, tension, residence time, and airflow rate, and must have a heat-resistant molecular structure. The oxidized fibers in this invention are heat-resistant flame The degree of reaction cyclization (EOR%) of the synthetic fiber can be set to 80-95%, and the oxygen content to 5-8%.

[0028] If the degree of reaction cyclization is less than 80%, the heat resistance is insufficient, the physical properties during carbonization deteriorate, and thread breakage is likely to occur. On the other hand, if the degree of reaction cyclization exceeds 95%, the ladder structure and the molecular structure due to oxygen develop excessively, resulting in a rigid structure, which deteriorates the physical properties during carbonization.

[0029] Endurance flame The oxygen content within synthetic fibers is the total oxygen content relative to the entire interior and exterior of the fiber. Higher oxygen content improves heat resistance, but higher physical properties are achieved when the interior and exterior of the fiber cross-section are more uniform. Therefore, it is important to control the structure to avoid abrupt changes so that oxygen diffuses well into the fiber, thus improving heat resistance. flame The oxygen content within the oxidized fibers of synthetic fibers can be in the range of 5-8%.

[0030] Another aspect of the present invention is the resistance described above. flame This invention relates to a carbon fiber precursor fiber bundle, manufactured by carbonizing synthetic fibers, characterized in that the diameter of the carbon fiber single fiber is 6.0 μm or more, the number of filaments in the fiber bundle is 24,000 to 36,000, the tensile strength of the resin-impregnated strand is 5.8 GPa to 6.4 GPa, and the tensile elongation is 2.2% to 2.6%.

[0031] In addition, in a preferred embodiment, the diameter of the single fiber of the carbon fiber of the present invention may be 9.0 μm or more, the tensile strength of the single fiber may be 9 to 11 g / d, and the elongation of the single fiber may be in the range of 10 to 12%.

[0032] In the carbon fiber bundle of the present invention, the diameter of the individual carbon fibers is 6.0 μm or larger (see Figure 1). The average single fiber diameter can be calculated from the mass, density, and number of filaments per unit length of the carbon fiber bundle, and is measured using an optical microscope or scanning electron microscope (SEM) after polishing a carbon fiber resin tensile test specimen.

[0033] While smaller carbon fiber diameters reduce the difference between internal and external structures, resulting in higher strength properties, they also complicate manufacturing processes and increase production costs. Furthermore, high matrix resin viscosity is unfavorable when producing composite materials, potentially leading to insufficient impregnation and a decrease in the tensile strength of the composite material. A carbon fiber diameter of 6.5 to 8.0 μm is advantageous in terms of manufacturing processes and costs, and is preferable because it reduces the likelihood of thread breakage during composite material production and facilitates resin impregnation.

[0034] The average single fiber diameter of the carbon fiber bundle expands, or the average single fiber diameter of the carbon fiber precursor bundle increases, or it becomes more durable. flame The carbonization yield can be increased by controlling the carbonization conditions, or by lowering the stretching ratio of low-temperature carbonization.

[0035] When using carbon fiber bundles with a small number of filaments, a large number of carbon fiber bundles are required. When using carbon fiber bundles with a large number of filaments, the number of carbon fiber bundles is small, but the number of filaments is large. Therefore, in either case, the interfiber voids become small, resulting in poor resin flow during molding, which can lead to poor impregnation or a long impregnation time.

[0036] In this invention, the carbon fiber precursor fiber is a filament fiber. Furthermore, the number of filaments (single fibers) constituting the fiber bundle is preferably 24,000 to 36,000. While a higher number of single fibers per yarn is preferable for improving productivity, if there are too many, uniformity throughout the bundle may not be achieved. flameIn some cases, chemical treatment may not be possible. The single-fiber density and the number of single fibers can be adjusted as appropriate depending on the purpose. When the number of filaments exceeds 24,000, the number of carbon fiber bobbins can be reduced during the manufacture of high-pressure vessels, which makes the work more convenient. However, if the number of filaments exceeds 36,000, it may not be possible to achieve the sufficient tensile strength required in the field of high-pressure vessels.

[0037] The tensile strength of the carbon fiber precursor fiber bundle of the present invention is 5.4 GPa or higher, preferably 5.4 GPa to 6.4 GPa. If the tensile strength is less than 5.4 GPa, a pressure vessel manufactured using a fiber-reinforced composite material consisting of such carbon fiber bundles cannot have the sufficient tensile strength required for pressure vessels that require high pressures of 700 bar or more.

[0038] The tensile elongation of the carbon fiber precursor fiber bundle of the present invention is preferably 2.2% to 2.6%. If the tensile elongation is lower than 2.2%, the tensile strength of the carbon fiber reinforced composite material using it will be insufficient. The upper limit of the strand tensile elongation is not particularly limited, but 2.6% is appropriate for the purposes of the present invention.

[0039] A further aspect of the present invention relates to a method for producing carbon fibers, characterized by forming carbon fiber precursor fibers by dry-wet spinning a polymer solution with a polymer concentration of 20% to 24% by weight in a coagulation bath with a concentration of 30% to 40% by weight containing the same solvent used for polymerization and water; stretching the obtained carbon fiber precursor fibers in multiple water washing baths; applying an oil agent; drying and stretching to obtain carbon fiber precursor fibers having a surface densification degree of 65% to 70% at a wavelength of 380 nm and 82% to 95% at a wavelength of 430 nm; and then carbonizing the carbon fibers at a temperature of 1000°C to 1400°C so that the diameter of the carbon fibers is 6.0 μm or more.

[0040] In this invention, it is possible to produce carbon fiber precursor fiber bundles that maintain productivity while improving tensile strength even when the diameter of the carbon fiber single fiber is 6.0 μm or more. In order to produce carbon fiber precursor fibers with excellent tensile strength, a spinning stock with few impurities and a uniform molecular weight distribution must be produced by solution polymerization with a dimethyl sulfoxide solvent, and the coagulation bath must be optimized to reduce voids and densify the surface in order to ensure appropriate discharge safety and coagulation stretchability by wet and dry spinning.

[0041] The PAN-based polymer used in this invention has an intrinsic viscosity of 1.5 to 2.0 and a weight-average molecular weight (Mw) of 120,000 to 180,000. Higher molecular weight allows for the production of higher-strength PAN fibers. Generally, high-molecular-weight PAN fibers exhibit relatively higher strength than low-molecular-weight PAN fibers. This means that PAN polymers with a molecular weight above a certain level are required for PAN fiber production.

[0042] In this invention, it is preferable that the molecular weight distribution (PD) (Mw / Mn) of the PAN-based polymer used is within the range of 1.6 to 1.8, because this reduces the content of low-molecular-weight components that are prone to causing structural defects in carbon fibers. A narrower molecular weight distribution (PD) of the PAN-based polymer results in superior spinnability and is advantageous for improving the strength of carbon fibers.

[0043] In this invention, if the molecular weight distribution PD (Mw / Mn) of the PAN-based polymer is less than 1.6, the polymerization process becomes more complicated, requiring more steps and longer processing times, thus reducing economic efficiency. Conversely, if the PD exceeds 1.8, the frequency of gel formation in the polymer increases, leading to more single filaments being generated from the nozzle like drips. This reduces discharge uniformity, increasing the frequency of filament breakage during water washing or steam stretching, and increasing the likelihood of non-uniform precursor fiber properties, resulting in a decrease in the physical properties and quality of the carbon fiber.

[0044] In the present invention, the polymerization method for producing PAN-based polymers can be selected from solution polymerization, suspension polymerization, emulsion polymerization, etc., but for the purpose of uniformly polymerizing AN and copolymer components, solution polymerization is preferred. When polymerization is carried out using solution polymerization, the PAN-based polymer is dissolved in a solvent in which the PAN-based polymer can dissolve, such as dimethyl sulfoxide, dimethylformamide, and dimethylacetamide, to prepare a spinning stock. When using solution polymerization, if the solvent used for polymerization and the spinning solvent are the same, the step of separating the obtained PAN-based polymer and redissolving it in the spinning solvent becomes unnecessary.

[0045] The polymer concentration of the PAN-based polymer solution is preferably in the range of 20 to 24% by weight. If the polymer concentration is less than 20% by weight, the amount of solvent used increases, which is uneconomical, and it can reduce the solidification rate in the solidification bath, causing voids to form inside, which may prevent the acquisition of a dense structure.

[0046] On the other hand, if the polymer concentration exceeds 24% by weight, the viscosity increases, making spinning difficult. The polymer concentration in the spinning solution can be adjusted according to the amount of solvent used.

[0047] In this invention, polymer concentration refers to the weight percentage of the PAN-based polymer contained in the solution of the PAN-based polymer. Specifically, after weighing the solution of the PAN-based polymer, the PAN-based polymer solution is desolvented using a solvent that does not dissolve the PAN-based polymer and is compatible with the solvent used in the PAN-based polymer solution, and then the PAN-based polymer is weighed. The polymer concentration is calculated by dividing the weight of the PAN-based polymer after desolventing by the weight of the PAN-based polymer solution before desolventing.

[0048] To obtain high-strength carbon fibers before spinning the PAN-based polymer solution, it is preferable to remove the polymer raw materials and impurities introduced in each step of the process from the PAN-based polymer solution using a filter with a filtration accuracy of 1 μm or less.

[0049] In this invention, a spinning stock solution with low impurities, a uniform molecular weight distribution, and high viscosity is used, and wet-dry spinning is performed by passing it through a nozzle hole with a diameter of 0.12 to 0.18 mm. The average spinning hole diameter of the spinneret preferably used in this invention is 0.12 mm to 0.18 mm. If the average spinning hole diameter of the spinneret is smaller than 0.12 mm, it is necessary to discharge the polymer solution, which is the spinning solution, from the spinneret under high pressure, which reduces the durability of the spinning apparatus and can cause problems with discharge from the nozzle. On the other hand, if the average spinning hole diameter of the spinneret exceeds 0.18 mm, it may become difficult to obtain coagulated yarn with the desired single-fiber density.

[0050] In wet-dry spinning, the spinning solution discharged from the nozzle is stretched in the air and then solidifies as it passes through the coagulation bath. In this invention, the temperature difference between the spinning solution and the coagulation bath is adjusted to within 30°C, and the distance of the air gap generated from the front of the spinneret to the surface of the coagulation bath can be 10 mm or less, preferably about 2 mm to about 8 mm.

[0051] The lower the temperature of the spinning solution, the higher the viscosity. This can lead to sufficiently high discharge pressure when extruding from the nozzle hole, which is advantageous for spinning safety, but it may be disadvantageous for coagulation and stretching. Conversely, if the temperature is high, the viscosity decreases, which may be disadvantageous for maintaining the air gap.

[0052] The coagulation bath used in the present invention is a mixture of a solvent for the PAN polymer, such as dimethyl sulfoxide, dimethylformamide, dimethylacetamide, aqueous zinc chloride solution, and aqueous sodium thiosulfate solution, and a so-called coagulation promoting component. The coagulation promoting component is preferably one that does not dissolve the PAN polymer and is compatible with the solvent used in the PAN polymer solution. Specific examples of coagulation promoting components include water, methanol, ethanol, and acetone, but water is the most preferred.

[0053] The temperature of the coagulation bath affects the diffusion rate of the solvent into the coagulation bath and the diffusion rate of coagulation-promoting components into the spinning solution. As a result, the lower the temperature of the coagulation bath, the denser the coagulated yarn becomes, and high-strength carbon fibers can be obtained. If the difference between the temperature of the spinning stock solution and the temperature of the coagulation bath exceeds 30°C, the coagulation phase separation units become larger, and voids may increase. If the difference between the temperature of the spinning stock solution and the temperature of the coagulation bath is too small, the coagulation phase separation becomes too small, resulting in a soft surface that is prone to fibril formation due to friction, which can lead to the formation of monofilaments.

[0054] In the present invention, carbon fiber precursor fibers can be obtained by introducing a PAN-based polymer solution into a solidification bath and allowing it to solidify to form solidified yarn, followed by a water washing step, a bath stretching step, an oil application step, and a drying step. Bath stretching can generally be carried out in one or more stretching baths adjusted to a temperature of 30 to 98°C. In this case, the stretching ratio in the bath is preferably 1 to 5 times.

[0055] After the bath stretching process, it is preferable to apply an oil made of silicone or the like to the stretched yarn in order to prevent adhesion between the individual fibers. As the silicone oil, it is preferable to use one that contains modified silicone, such as an amino-modified silicone with high heat resistance. The next drying step can be carried out using a known method. For example, it can be dried at a drying temperature of 70 to 200°C for 10 to 200 seconds.

[0056] Following the drying and heat treatment process, a steam stretching process is carried out. Here, precursor fibers can be produced by stretching them 2 to 6 times in pressurized steam. The single fiber density of the precursor fibers should be 0.95 dtex or higher.

[0057] The carbon fiber precursor fibers produced by the aforementioned method are subjected to a tension of 7-12 cN / dtex in the range of 200-300°C in an oxidation furnace under an air atmosphere. flame By keeping the degree of reaction cyclization (EOR) and oxygen content within a certain range, resistance flame The diameter of the synthetic fiber monofilament is set to 10.0 to 11.5 μm.

[0058] The carbon fiber precursor fiber bundle has a surface densification degree of 87.5-93% at a wavelength of 380-480 nm. If the surface densification degree is less than 87.5%, voids will form defects in the carbon fibers during the firing process, reducing the physical properties and decreasing the tensile strength of the composite material. The upper limit of the surface densification degree is preferable as it is greater, but is not particularly limited. For the purposes of the present invention, 87.5% or more is sufficient. Preferably, the surface densification degree of the carbon fiber precursor fiber bundle is 65-70% at a wavelength of 380 nm and 82-95% at a wavelength of 430 nm.

[0059] Endurance flame It is preferable to obtain carbon fiber bundles by carbonizing synthetic fibers or low-temperature carbonized fiber bundles in an inactive atmosphere at 1000 to 1400°C. If the high-temperature carbonization temperature is below 1000°C, the nitrogen content in the carbon fiber bundle increases, which can lead to the stable development of strand strength. If the carbonization temperature exceeds 1400°C, it may be difficult to obtain a satisfactory carbonization yield.

[0060] In this invention, low-temperature carbonization and high-temperature carbonization are carried out in an inert atmosphere. Carbonization is performed so that the diameter of the carbon fibers is 6.0 μm or more. Examples of gases used in the inert atmosphere include nitrogen, argon, and xenon, and nitrogen is preferably used from an economic standpoint.

[0061] Sizing treatment is performed to impart bundle properties to the carbon fibers.

[0062] As a sizing agent, one can select an agent that has good compatibility with the matrix resin, depending on the type of matrix resin used.

[0063] A further aspect of the present invention relates to a pressure vessel having a container body and a fiber-reinforced resin layer formed on the surface of the container body.

[0064] The fiber-reinforced resin layer of the pressure vessel of the present invention has a fiber-reinforced resin in which reinforcing fibers are impregnated with resin. As such reinforcing fibers, a polyacrylonitrile-based resin having a single fiber diameter of 10.0 μm or more and satisfying the conditions of formulas 1 and 2 is available. flame This product contains carbon fibers manufactured by carbonizing synthetic fibers, with a single carbon fiber diameter of 6.0 μm or more, a fiber bundle containing 24,000 to 36,000 filaments, a resin-impregnated strand tensile strength of 5.8 GPa to 6.4 GPa, and a tensile elongation of 2.2% to 2.6%.

[0065] The carbon fibers obtained in this invention can be suitably used in aircraft parts, pressure vessels, automotive products, and sporting goods by various molding methods, such as autoclave molding as a prepreg, resin transfer molding as a preform such as a woven fabric, and filament winding.

[0066] In particular, since the tensile strength can be improved according to the present invention, the pressure vessel of the present invention can be suitably used as a fuel tank for various transportation equipment such as automobiles.

[0067] The present invention will be described more specifically below based on examples. Such examples are for illustrative purposes only and are not intended to limit the scope of protection of the present invention.

[0068] Examples Comparative Example 1 The polymerization reaction was carried out by adding DMSO solvent to a mixture containing 99 wt% acrylonitrile, 1 wt% itaconic acid, 0.01 wt% chain transfer agent, and 20-22 wt% monomer. After adding 0.5 wt% initiator (AIBN) under a nitrogen atmosphere, the polymerization reaction was carried out. The mixture was mixed at 65°C for 4 hours under an inert atmosphere, and then the temperature was increased to 80°C at equal intervals over 3 hours, and maintained at that temperature for 6 hours.

[0069] After the reaction was complete, the mixture was degassed, and then neutralized with ammonia gas against the carboxyl groups of the polymerized itaconic acid to prepare a 22 wt% spinning solution. The temperature of the spinning solution was set to be 20°C lower than the temperature of the coagulation bath, and wet-dry spinning was carried out using a 6-8K nozzle so that the DMSO aqueous solution was discharged into a 30-40 wt% coagulation bath.

[0070] After passing through the coagulation bath, the coagulated yarn was washed in multiple stages, then stretched in hot water, and an amino-modified silicone-based silicone oil was applied. The resulting stretched fiber bundles were then sequentially brought into contact with multiple heated rollers and run through them to perform a drying heat treatment.

[0071] Next, polyacrylonitrile-based precursor fiber bundles were produced by stretching them to 80% of the maximum stretch ratio in pressurized steam at a temperature of 140-185°C. These precursor fibers were then placed in an oxidation furnace, and temperature and shrinkage control were performed to ensure that the oxidation tension in air was 20 cN / tex or less and the oxidized fiber density was 1.35 or less.

[0072] Subsequently, carbon fiber bundles were obtained by performing low-temperature carbonization in a nitrogen atmosphere at 300-700°C, followed by high-temperature carbonization at 1100-1400°C in a nitrogen atmosphere so that the carbonization tension was 15 cN / tex or less.

[0073] The diameter of the individual fibers in the carbon fiber bundle was adjusted by changing the diameter and plasticity conditions of the precursor fiber individual fibers, and the diameter of the individual fibers in the precursor fiber bundle was changed by adjusting the discharge rate of the spinning solution.

[0074] Comparative Example 2 Polyacrylonitrile precursor fiber bundles were produced by polymerizing and spinning in the same manner as in Comparative Example 1, except that the amount of spinning solution discharged was set to 70% of that in Comparative Example 1, and the spinning conditions were changed as shown in Table 1 below.

[0075] Example 1 In the same polymerization and spinning process as in Comparative Example 1, precursor fiber bundles were produced by extruding the spinning solution at 90% of the level used in Comparative Example 1 and then spinning. Subsequently, carbon fiber bundles were produced by oxidation and carbonization.

[0076] Comparative Example 3 Polyacrylonitrile precursor fiber bundles were produced by polymerizing and spinning in the same manner as in Comparative Example 1, except that the amount of spinning solution discharged was set to 90% of that of Comparative Example 1, and the spinning conditions were changed as shown in Table 1 below.

[0077] Examples 2 and 3 Carbon fiber precursor fiber bundles and carbon fiber bundles were produced in the same manner as in Example 1, except that the difference (△T) between the temperature of the spinning solution and the temperature of the coagulation bath was changed as shown in Table 2 below.

[0078] Examples 4-6 and Comparative Example 4 Carbon fiber precursor fiber bundles and carbon fiber bundles were produced in the same manner as in Example 1, except that the polymerization was carried out with a molecular weight distribution of 1.6, and the difference (△T) between the temperature of the spinning stock solution and the temperature of the coagulation bath was changed as shown in Table 1 below.

[0079] Comparative Examples 5 and 6 Except for the fact that the polymerization was carried out with a molecular weight distribution of 2.0, and the difference (△T) between the temperature of the spinning stock solution and the temperature of the coagulation bath was changed as shown in Table 1 below, carbon fiber precursor fiber bundles and carbon fiber bundles were produced in the same manner as in Example 1.

[0080] Examples 7-9 and Comparative Example 7 Carbon fiber precursor fiber bundles and carbon fiber bundles were produced in the same manner as in Example 1, except that the molecular weight distribution of the polymer was set to 1.6 and the difference (△T) between the temperature of the spinning stock solution and the temperature of the coagulation bath was changed as shown in Table 2 below.

[0081] Comparative Example 8 Polyacrylonitrile-based precursor fiber bundles were produced by polymerization and spinning in the same manner as in Comparative Example 7, except that the amount of spinning solution discharged was set to 70% of that in Comparative Example 1.

[0082] Test example The molecular weights, spinning conditions, and tensile properties of the PAN-based polymers of Examples 1-9 and Comparative Examples 1-8 were measured using the following methods, and the results are shown in Tables 1 and 2 below.

[0083] <Measurement of polymer molecular weight and molecular weight distribution> A liquid polymer was thinly cast to form a film, washed with running water for one day, and then dried to prepare the sample. The sample was then dissolved in dimethylformamide to a concentration of 0.1% by weight, and the molecular weight of the obtained liquid sample was measured using a GPC instrument. From the measured GPC curve, a molecular weight distribution curve was obtained, and the weight-average molecular weight (Mw) and number-average molecular weight (Mn) were calculated.

[0084] <Density measurement> Take a 1.0-3.0 g sample of chloride-resistant fiber bundle and dry it completely at 120°C for 2 hours. Then, measure the mass (A) (g) of the completely dried fiber, immerse it in ethanol to remove air bubbles, and measure the fiber mass (B) (g) in the ethanol solvent bath. The density = (A × ρ) / (A - The density is determined by method B). ρ is the specific gravity of ethanol at the measurement temperature.

[0085] <Degree of surface densification of precursor fibers> Using methoxybenzene as a reference solution, the UV transmittance is measured in the 380-480 nm range using an ultraviolet spectrophotometer to indicate the degree of surface densification (%). For the precursor fiber bundles, oil is removed, and the bundles are dried. Ten samples are prepared by spreading the bundles thinly to a width of 9 mm, and the degree of surface densification (%) is indicated from the average value of the ten samples. A higher degree of surface densification indicates greater density.

[0086] <Degree of reaction and cyclization of chlorinated fiber (EOR%), [Extent of Reaction] )> In salt-resistant fibers, polymer cyclization progresses as the temperature increases in an air atmosphere. On the surface of the oxidized fiber obtained at this time, Analysis using FT-IR after infrared irradiation. the degree of progression of molecular structure cyclization is determined by obtaining the peak intensity corresponding to the functional groups (C=N, C≡N), and the cyclization degree can be calculated by the following formula.

[0087] EOR(%)[Extent of Reaction]=I(C=N) / [I(C≡N)+I(C=N)] <Internal oxygen content (%) of flame oxidized fiber> The oxygen content is measured for flame oxidized fiber using an Elemental Analysis instrument.

[0088] <Average single fiber diameter of carbon fiber bundle> For a carbon fiber bundle composed of a plurality of carbon filaments to be measured, let the mass per unit length be A f (g / m) and the density be B f (g / cm 3 ), which are obtained. Let the number of filaments in the carbon fiber bundle to be measured be C f , then the average single fiber diameter (μm) of carbon fiber can be obtained by the following formula: average single fiber diameter of carbon fiber (μm)=((A f / B f / C f ) / π (1 / 2) ×2×10 3 ; alternatively, after preparing a resin tensile specimen, it can be polished and observed with an optical microscope.

[0089] <Measurement of tensile properties of carbon fiber resin-impregnated strand> The carbon fiber bundles produced in the examples and comparative examples were untwisted, and the tensile properties of the strand carbon fiber precursor fiber bundles were measured. The strand strength is measured by impregnating an epoxy resin in accordance with ISO 10618, stretching the cured carbon fiber strand, and evaluating the tensile properties of the carbon fiber. Here, measurement is performed on 10 carbon fiber precursor fiber bundles, the minimum value and maximum value are removed, and the average value is expressed as the strand tensile strength and the strand tensile elongation.

[0090] [Table 1]

[0091] [Table 2]

[0092] As shown in Tables 1 and 2 above, the carbon fiber bundles of Examples 7 to 9 exhibit a tensile strength of 5.8 GPa or more and a tensile elongation of 2.2% or more even when the diameter of the individual carbon fibers is 6.0 μm or more, whereas the carbon fiber bundles of Comparative Examples 7 and 8 have reduced tensile strength or tensile elongation, making them unsuitable for use in pressure vessels and other applications requiring high pressure.

[0093] Although the present invention has been described in detail above based on preferred embodiments, it will be apparent to the ordinary person of the art that a variety of variations or modifications to the present invention are possible. Therefore, the present invention is not limited to any particular embodiment, but is considered to include all embodiments within the scope of the appended claims and equivalent thereto.

Claims

1. The diameter of the single fiber is 10.0 μm or more, and the conditions of the following equations 1 and 2 are met. Polyacrylonitrile-based flame-retardant fiber. Log (density) ≧ (α / β) / 100 (1) ０．１２０≦（α／β）／１００≦０．１３５ （２） Here, density: density of flame-resistant fiber (g / cm³) ３ ) α: Degree of reaction cyclization of flame-resistant fiber, EOR (%) β: Oxygen content (%) within flame-resistant fiber (The density mentioned above was calculated using equation 3 below: Density=(A×ρ) / (AB) (3) (A)(g): Mass of the completely dried flame-resistant fiber (A)(g) (B)(g): After impregnating the completely dried flame-resistant fibers in ethanol and thoroughly degassing them, the mass of the flame-resistant fibers in the ethanol solvent bath (B)(g) ρ: Specific gravity of ethanol at the measurement temperature.

2. A carbon fiber manufactured by low-temperature carbonization of the polyacrylonitrile flame-retardant fiber described in claim 1 in a nitrogen atmosphere at 300 to 700°C, and then high-temperature carbonization at a temperature of 1100 to 1400°C in a nitrogen atmosphere to carbonize the fiber to a carbon tension of 15 cN / tex or less, wherein the diameter of the carbon fiber single fiber is 6.0 μm or more, the number of filaments in the fiber bundle is 24,000 to 36,000, the tensile strength of the epoxy resin-impregnated strand is 5.8 GPa to 6.4 GPa and the tensile elongation is 2.2% to 2.6% according to ISO 10618.

3. A high-pressure vessel comprising a pressure vessel body and a fiber-reinforced resin layer formed on the surface of the pressure vessel body, wherein the fibers constituting the fiber-reinforced resin layer are carbon fibers as described in claim 2.

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