Method for producing carbon fiber precursor fiber

Abstract

The purpose of the present invention is to provide a production method for carbon fiber precursor fibers that have superior quality and production properties, by using a spinning dope that has superior stringiness and extensibility. The present invention uses, as a spinning dope, an acrylonitrile polymer in which: the area ratio (B / A) of the peak area A in the range in which the polystyrene equivalent molecular weight as measured by a gel permeation chromatography (GPC) method is at least 3,000,000 but less than 5,000,000 and the peak area B in the range in which the polystyrene equivalent molecular weight is 5,000,000-15,000,000, inclusive, is 0.8 or greater; and the area ratio (D / C) of the peak area C in the range in which the polystyrene equivalent molecular weight as measured by the gel permeation chromatography (GPC) method is less than 3,000,000, and the peak area D in the range in which the polystyrene equivalent molecular weight is between 3,000,000 and 15,000,000, inclusive, is 0.025 or less.

Description

[Technical Field]

[0001] The present invention relates to a method for producing a precursor fiber for carbon fiber, and more specifically to a method for producing a precursor fiber for carbon fiber in which the molecular weight distribution of an acrylonitrile-based polymer constituting the precursor fiber for carbon fiber is controlled as much as possible. [Background technology]

[0002] Carbon fiber has excellent mechanical properties, especially high specific strength and specific modulus, and is therefore widely used as a reinforcing material in the aerospace, automotive, and leisure goods industries. Its excellent mechanical properties also make it possible to reduce the weight of aircraft and automobiles, and it has attracted attention as a means of reducing carbon dioxide emissions in aircraft and automobile operations.

[0003] Carbon fiber is produced by subjecting fibers prepared from organic polymer precursors to a flame retardant treatment in the presence of oxygen, followed by carbonization. Examples of precursors include cellulose, phenolic resin, polyvinyl alcohol, vinylidene chloride, pitch, and polyacrylonitrile (hereinafter simply referred to as PAN). Of these, carbon fiber obtained from PAN-based fibers has excellent mechanical properties such as specific strength and specific modulus, and can be produced with consistent and stable quality and performance, leading to industrial mass production.

[0004] PAN-based fibers are generally produced by wet spinning or dry-wet spinning. In either spinning method, a spinning dope in which the raw material acrylonitrile polymer is dissolved in a solvent is generally used. To improve the performance of carbon fibers, it is effective to improve the quality and toughness of carbon fiber precursor fibers. In particular, it is necessary to suppress single-fiber irregularities and defects that may occur due to various treatments during the spinning process. To solve this problem, several technologies have been disclosed.

[0005] For example, there have been proposed a technology for improving the spinnability of a spinning dope by controlling the molecular weight distribution of an acrylonitrile-based polymer constituting the spinning dope, thereby improving production stability (Patent Document 1), and a technology for improving the toughness of a carbon fiber precursor fiber by adjusting the molecular weight distribution of an acrylonitrile-based polymer constituting a carbon fiber precursor fiber, thereby improving its stretchability during baking (Patent Document 2).

[0006] However, in these conventional techniques, in order to control the molecular weight distribution of the acrylonitrile-based polymer, two types of acrylonitrile-based polymers having different weight-average molecular weights, including an acrylonitrile-based polymer having a weight-average molecular weight of 1,000,000 or more, which is considered to be an ultrahigh molecular weight, are allowed to coexist, and an excessive amount of an acrylonitrile-based polymer having an ultrahigh molecular weight is included in order to improve the spinnability of the spinning dope and the toughness of the carbon fiber precursor fiber. Therefore, the spinnability and drawability in the production of the carbon fiber precursor fiber are impaired, and there is a problem in that it is difficult to achieve both high quality and high productivity of the carbon fiber precursor fiber. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-248219 [Patent Document 2] International Publication No. 2009-125832 DISCLOSURE OF THE INVENTION [Problem to be solved by the invention]

[0008] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for producing a carbon fiber precursor fiber that is excellent in quality and productivity by using a spinning dope that is excellent in both spinnability and drawability. [Means for solving the problem]

[0009] As a result of intensive studies to solve the above problems, the present inventors have found that, in the above-mentioned conventional techniques, when the content of an acrylonitrile-based polymer having a weight-average molecular weight of 1,000,000 or more, which is considered to be an ultrahigh molecular weight substance, is skillfully controlled, an acrylonitrile-based polymer solution having a controlled molecular weight distribution can be obtained, and thus a carbon fiber precursor fiber excellent in quality and productivity can be obtained, and have completed the present invention.

[0010] That is, according to the present invention, 1. A method for producing a carbon fiber precursor fiber, characterized in that an acrylonitrile polymer is used as a spinning dope, in which the area ratio (B / A) between the peak area A in the range of 3 million to less than 5 million in terms of polystyrene as measured by gel permission chromatography (GPC) and the peak area B in the range of 5 million to 15 million in terms of polystyrene as measured by gel permission chromatography (GPC) is 0.8 or more, and the area ratio (D / C) between the peak area C in the range of less than 3 million in terms of polystyrene as measured by gel permission chromatography (GPC) and the peak area D in the range of 3 million to 15 million in terms of polystyrene as measured by gel permission chromatography (GPC) is 0.025 or less. 2. The method for producing a carbon fiber precursor fiber according to claim 1, wherein the reduced viscosity of the acrylonitrile polymer is 1.0 to 2.5, and the concentration of the acrylonitrile polymer in the spinning dope is 18% by mass or more and 25% by mass or less. and, 3. The method for producing a carbon fiber precursor fiber according to claim 1, wherein the acrylonitrile-based polymer is an acrylonitrile-based polymer obtained by removing a polymerization inhibitor contained in acrylonitrile and polymerizing it by a solution polymerization method. is provided. [Effects of the Invention]

[0011] According to the present invention, a spinning dope having excellent spinnability and drawability is used, whereby a carbon fiber precursor fiber having excellent quality can be provided. BEST MODE FOR CARRYING OUT THE INVENTION

[0012] The present invention will be described in detail below.

[0013] [Acrylonitrile polymer] The acrylonitrile polymer used in the present invention is characterized in that, in the polystyrene-equivalent molecular weight measured by gel permeation chromatography (GPC), the ratio (B / A) of the peak area B of 5 million to 15 million to the peak area A of 3 million to less than 5 million (hereinafter, this may be referred to as the ultra-high molecular weight component index) is 0.8 or more. If the ultra-high molecular weight component index is less than 0.8, the effect of the ultra-high molecular weight component in improving the spinnability of the spinning dope becomes insufficient, and in order to achieve high spinnability, an acrylonitrile polymer having an excess of the ultra-high molecular weight component is required. However, this results in a decrease in drawability, which not only reduces the quality of the resulting carbon fiber precursor fiber but also reduces the quality and strength of the carbon fiber obtained by calcination.

[0014] The ultra-high molecular weight component index is preferably 1.00 or more, and more preferably 1.10 or more. When the ultra-high molecular weight component index is 1.00 or more, excellent spinnability can be exhibited, and single-fiber breakage during the coagulation process can be suppressed. Furthermore, when the ultra-high molecular weight component index is 1.10 or more, excellent spinnability can be exhibited, and the spinning speed can be improved.

[0015] Furthermore, the acrylonitrile polymer used in the present invention is characterized in that the ratio (D / C) of the peak area C in the range of less than 3 million to the peak area D in the range of 3 million to 15 million in terms of polystyrene molecular weight measured by gel permeation chromatography (GPC) (hereinafter this may be referred to as the ultra-high molecular weight ratio) is 0.025 or less. If the ultra-high molecular weight ratio exceeds 0.025, the amount of ultra-high molecular weight component becomes excessive, resulting in a decrease in stretchability, which not only decreases the quality of the obtained carbon fiber precursor fiber but also decreases the quality and strength of the carbon fiber obtained by calcination.

[0016] The ultra-high molecular weight ratio is preferably 0.020 or less. When the ultra-high molecular weight ratio is 0.020 or less, a precursor fiber for carbon fiber having good drawability and superior quality can be obtained.

[0017] Representative average molecular weights measured by gel permeation chromatography (GPC) include number average molecular weight (Mn), weight average molecular weight (Mw), z average molecular weight (Mz), and z+1 average molecular weight (Mz+1), but these values ​​are more sensitive to the influence of high molecular weight components than Mn, Mz than Mw, and Mz+1 than Mz. Therefore, by comparing (Mz+1) / Mw with Mw, the molecular weight and amount of ultra-high molecular weight components in an acrylonitrile polymer can be expressed.

[0018] The acrylonitrile polymer used in the present invention preferably has a weight average molecular weight (Mw) and z+1 average molecular weight (Mz+1) measured by gel permeation chromatography (GPC) that satisfy either X or Y shown below. X: Mw is 350,000 or more and less than 450,000, and (Mz+1) / Mw is 15 or more Y: Mw is 450,000 or more and less than 600,000, and (Mz+1) / Mw is 17 or more When the weight-average molecular weight (Mw) is 350,000 or more and less than 450,000, and (Mz+1) / Mw is less than 15, the effect of the ultra-high molecular weight component in improving the spinnability of the spinning solution becomes insufficient, and in order to achieve high spinnability, it is necessary to use an acrylonitrile-based polymer having an excess of the ultra-high molecular weight component. However, on the other hand, this results in a decrease in drawability, which not only reduces the quality of the obtained carbon fiber precursor fiber but also may cause problems such as a decrease in the quality and strength of the carbon fiber obtained by calcination.

[0019] Similarly, when the weight-average molecular weight (Mw) is 450,000 or more and less than 600,000, and (Mz+1) / Mw is less than 17, the effect of the ultra-high molecular weight component in improving the spinnability of the spinning dope becomes insufficient, and in order to achieve high spinnability, it is necessary to use an acrylonitrile-based polymer having an excess of the ultra-high molecular weight component. However, this results in a decrease in drawability, which not only reduces the quality of the obtained carbon fiber precursor fiber but also may cause problems such as a decrease in the quality and strength of the carbon fiber obtained by calcination.

[0020] Increasing the weight-average molecular weight is desirable because it improves the toughness of the carbon fiber precursor fiber, but increasing the amount of the ultra-high molecular weight component in this case is undesirable because it impairs spinnability. Therefore, when the weight-average molecular weight is set to 450,000 or more and less than 600,000, it is desirable to increase the molecular weight of the ultra-high molecular weight component and suppress the amount of the component in order to maintain sufficiently high spinnability, and therefore it is preferable to set (Mz+1) / Mw to 17 or more.

[0021] When the weight average molecular weight (Mw) is 350,000 or more and less than 450,000, the value of (Mz+1) / Mw is preferably 15 or more and 30 or less, and more preferably 15 or more and 25 or less.

[0022] Furthermore, when the weight-average molecular weight (Mw) is 450,000 or more and less than 600,000, the value of (Mz+1) / Mw is preferably 17 or more and 30 or less, and more preferably 17 or more and 25 or less. When the weight-average molecular weight (Mw) is 450,000 or more and 500,000 or less and (Mz+1) / Mw is 30 or less, industrial handling is easy in terms of the viscosity characteristics of the spinning dope, which is preferable. When (Mz+1) / Mw is 25 or less, better stretchability can be obtained.

[0023] In the present invention, the polymerization method for obtaining the acrylonitrile polymer can be any known polymerization method such as solution polymerization, suspension polymerization, or emulsion polymerization, but solution polymerization is preferred.

[0024] When solution polymerization is selected, known acrylonitrile polymerization methods such as radical polymerization and ionic polymerization can be selected, but radical polymerization is preferred from the viewpoint of productivity. In this case, in order to control the molecular weight distribution of the acrylonitrile-based polymer, the concentrations and addition timings of initiators, chain transfer agents, solvents, etc. can be adjusted based on the reaction rate of acrylonitrile as a monomer.

[0025] For example, the weight-average molecular weight of the acrylonitrile-based polymer to be produced can be increased by decreasing the ratio of initiator to acrylonitrile or by lowering the reaction temperature. Furthermore, the weight-average molecular weight of the acrylonitrile-based polymer can be decreased by increasing the ratio of chain transfer agent to acrylonitrile. By changing the reaction conditions while checking the transition of the acrylonitrile reaction rate according to these trends, it becomes possible to produce an acrylonitrile-based polymer having a desired molecular weight distribution.

[0026] In the method for producing an acrylonitrile-based polymer of the present invention, it is preferable to first prepare an ultrahigh molecular weight acrylonitrile-based polymer, and then add an initiator or chain transfer agent to the reaction solution to react with unreacted acrylonitrile and adjust the molecular weight distribution. The ultrahigh molecular weight ratio can be increased or decreased by increasing or decreasing the reaction time of the ultrahigh molecular weight acrylonitrile-based polymer. Since increasing the ratio of acrylonitrile to solvent tends to increase the molecular weight, when preparing an ultrahigh molecular weight acrylonitrile-based polymer, the ratio of solvent to acrylonitrile may be decreased and the solvent may be added after the ultrahigh molecular weight polymer is prepared. Furthermore, since removing the polymerization inhibitor contained in acrylonitrile tends to increase the weight-average molecular weight of the ultrahigh molecular weight acrylonitrile-based polymer, the polymerization inhibitor contained therein may be removed before subjecting the acrylonitrile to the reaction. Methods for removing the polymerization inhibitor include known methods such as vaporization followed by coagulation and recovery. In this context, "removal" also includes the presence of 5 ppm or less of the polymerization inhibitor.

[0027] It is also possible to prepare an ultrahigh molecular weight acrylonitrile polymer by suspension polymerization or emulsion polymerization and dissolve it in an acrylonitrile polymer solution obtained by solution polymerization. When suspension polymerization or emulsion polymerization is selected, two types of acrylonitrile polymers with different weight average molecular weights can be prepared by dissolving them in a solvent in order to control the molecular weight distribution of the acrylonitrile polymer. In this case, a known dissolution method can be selected. Solution polymerization is preferred from the viewpoint of obtaining a spinning solution in which the acrylonitrile polymer is uniformly dissolved.

[0028] When solution polymerization is selected, a known solvent that can sufficiently grow the molecular weight in a polymerization reaction using acrylonitrile as the main monomer component can be used. For example, an aqueous zinc chloride solution or dimethyl sulfoxide can be used. When preparing a spinning solution by dissolving two types of acrylonitrile-based polymers with different weight-average molecular weights in a solvent, a known solvent that has good solubility for acrylonitrile-based polymers can be used, such as an aqueous zinc chloride solution, dimethylacetamide, dimethylsulfoxide, or dimethylformamide.

[0029] Known initiators can be used as initiators for radical polymerization. Azobisisobutyronitrile is preferred from the viewpoints of economy and reaction rate. A chain transfer agent may be added to control the molecular weight. Known compounds can be used as chain transfer agents, but thiol compounds such as 1-octanethiol (also known as octyl mercaptan) and 1-butanethiol are preferred from the viewpoint of reaction controllability. 1-octanethiol is more preferred from the viewpoints of handling and economy.

[0030] The acrylonitrile polymer contains acrylonitrile as the main component and can be copolymerized with a known monomer copolymerizable with acrylonitrile. The acrylonitrile content is preferably 93% by mass or more, more preferably 95% by mass or more and 99.5% by mass or less, and even more preferably 98% by mass or more and 99.5% by mass or less.

[0031] Known monomers copolymerizable with acrylonitrile include, for example, acrylic acid, methacrylic acid, methyl methacrylate, ethyl methacrylate, acrylamide, itaconic acid, vinylsulfonic acid, and the like, and one or more of these can be selected. From the viewpoint of maintaining a sufficiently high carbonization rate during carbon fiber production and maintaining high production efficiency in the flame-retardant treatment, it is preferable to include at least one selected from itaconic acid and methacrylic acid. From the same viewpoint, the amount of the copolymerized monomer is preferably less than 7% by mass, more preferably less than 5% by mass, and even more preferably less than 2% by mass.

[0032] The reduced viscosity of the acrylonitrile polymer is preferably 1.0 or more and 2.5 or less. More preferably, it is 1.3 or more and 2.3 or less. If the reduced viscosity is less than 1.0, the viscosity of the spinning dope may become too low, resulting in a decrease in spinnability. If the reduced viscosity is more than 2.5, the viscosity of the spinning dope may become too high, resulting in a decrease in spinnability.

[0033] [Spinning dope] The spinning dope in the present invention refers to a polymer solution containing an acrylonitrile polymer and a solvent as main components, which is subjected to a spinning process, and to which known additives can be added for the purpose of achieving various improvements. In particular, from the viewpoint of coagulation control, it is desirable to add a basic compound that neutralizes the acid component in the acrylonitrile polymer solution. As the basic compound, from the viewpoint of suppressing defects in the carbon fiber precursor fiber, a non-metallic compound is preferred, and ammonia is more preferred.

[0034] The concentration of the acrylonitrile polymer in the spinning dope is preferably 18% by mass or more and 25% by mass or less, and more preferably 19% by mass or more and 23% by mass or less. If the concentration of the acrylonitrile polymer is less than 18% by mass, not only will spinnability decrease, but it may also be difficult to obtain a carbon fiber precursor fiber with a dense structure. If the concentration of the acrylonitrile polymer exceeds 25% by mass, it is necessary to reduce the weight-average molecular weight of the acrylonitrile polymer to achieve good extrusion stability, which may result in decreased stretchability and a decrease in the toughness of the carbon fiber precursor fiber.

[0035] 〔spinning〕 The spinning dope is passed through a filter medium to remove gel-like foreign matter and insoluble components, and then subjected to the spinning process. From the viewpoints of productivity and the performance of the resulting carbon fiber, it is preferable that the spinning dope is highly defoamed or defoamed and does not contain bubbles. Defoaming or defoaming can be promoted by applying pressure or reducing pressure.

[0036] In the present invention, the spinning dope is discharged into a coagulation bath and coagulated to form threads. The coagulation liquid in the coagulation bath is preferably an aqueous solution or an alcohol solution in which a solvent capable of dissolving an acrylonitrile polymer is dissolved in water. The solvent contained in the coagulation liquid can be the same as the solvent used for the spinning dope described above, and preferably the same solvent as that used for the spinning solution used is used. From the viewpoints of coagulation properties and spinning stability, the solvent concentration in the coagulation liquid is preferably 10 to 70% by mass, more preferably 15 to 40% by mass. The temperature of the coagulation liquid is preferably 0 to 60°C. A lower temperature of the coagulation liquid makes it easier to obtain precursor fiber bundles and carbon fiber bundles with high circularity.

[0037] The spinneret for extruding the spinning dope preferably has 1,000 to 100,000 spinning holes. The hole diameter of the spinning holes is preferably 0.02 to 0.5 mm. A hole diameter of 0.5 mm or less makes it difficult for the extruded threads to adhere to each other, which is preferable, as it makes it easier to obtain a carbon fiber precursor fiber bundle with excellent homogeneity. A hole diameter of 0.02 mm or more is preferable, as it makes it easier to suppress the occurrence of spun thread breakage and makes it easier to maintain spinning stability. The temperature when extruding the spinning dope is preferably 25°C or higher. If this temperature is lower than 25°C, the viscosity of the spinning dope increases, and extrusion stability decreases.

[0038] The obtained yarn is subjected to a drying process after being washed with water and oiled. The washing is carried out for the purpose of removing the solvent and can be carried out by a known method. The oiling is carried out for the purpose of imparting convergence to the yarn bundle and can be carried out by a known method using a known oil.

[0039] The yarn may be drawn in a bath before oiling. In this case, the drawing is preferably carried out in a single or multiple baths adjusted to a temperature of 30° C. or higher and 98° C. or lower. The draw ratio is preferably 1 to 5 times, more preferably 1.1 to 4 times.

[0040] Drying is carried out for the purposes of removing water retained inside and outside the yarn after water washing and oiling, softening the acrylonitrile polymer constituting the yarn, reducing voids inside each single yarn constituting the yarn, and improving the denseness of each single yarn, and can be carried out by a known method.

[0041] The yarn is then subjected to a drawing step to produce a carbon fiber precursor fiber. A known method, such as a dry heat method or a pressurized steam method, can be used for the drawing. The draw ratio is preferably 1.1 times or more and 7 times or less, and more preferably 1.1 times or more and 6 times or less. After the drawing step, the yarn may be subjected to a heat treatment step. This heat treatment is performed for the purposes of controlling the crystal orientation of the carbon fiber precursor fiber and relaxing tension within the yarn, and can be performed by a known method. [Example]

[0042] The present invention will be described in detail below with reference to examples and comparative examples, but the scope of the present invention is not limited to the following examples and comparative examples. In addition, each physical property in the examples was measured by the following methods.

[0043] (1)Molecular weight When synthesizing an acrylonitrile-based polymer by solution polymerization, the acrylonitrile-based polymer was completely precipitated with water from the acrylonitrile-based polymer solution after the completion of the polymerization reaction, and dried to prepare a specimen. Specifically, the acrylonitrile-based polymer was obtained by the following procedure.

[0044] 10 g of an acrylonitrile-based polymer solution with a known concentration of acrylonitrile-based polymer was weighed out, and water was added in an amount at least 50 times the volume of the acrylonitrile-based polymer solution. The mixture was then pulverized using a mixer to completely precipitate the acrylonitrile-based polymer. The liquid was then removed by filtration, and the mixture was dried in an electric dryer to obtain a sample for GPC measurement. Care was taken to prevent the acrylonitrile-based polymer from deteriorating due to heat or oxidation. For example, the mixture was dried by heating under a nitrogen atmosphere to prevent degeneration.

[0045] The obtained acrylonitrile polymer was dissolved in dimethylformamide to a concentration of 0.1% by mass to obtain a sample solution. At this time, the dimethylformamide used had lithium bromide added to it to a concentration of 10 mmol / L. The heating temperature during dissolution was set to 60°C or less to prevent the acrylonitrile polymer from denaturing.

[0046] The GPC device used was a Waters e2695, and the refractive index detector used was a Waters 2414. The columns used were two Tosoh TSKgel-α-M columns and a Tosoh TSKgel-guard-column-α column, and the column temperature was set to 70°C.

[0047] Monodisperse polystyrene (14 types: molecular weights 6,870,000, 3,750,000, 2,630,000, 1,740,000, 991,000, 728,000, 508,000, 277,000, 118,000, 46,400, 18,000, 6,660, 3,090, and 1,300) was used as polymer standards for creating the calibration curve. The sample solution was filtered through a membrane filter (pore size 0.45 μm) before measurement.

[0048] A molecular weight distribution curve was obtained from the obtained GPC curve, and various average molecular weight values ​​(Mw, Mz), ultra-high molecular weight component index, and ultra-high molecular weight ratio were calculated.

[0049] The ultra-high molecular weight component index was calculated from the obtained molecular weight distribution curve by calculating the peak area A in the range of polystyrene-equivalent molecular weight of 3 million or more but less than 5 million, and the peak area B in the range of 5 million or more but less than 15 million, based on the following formula: The ultra-high molecular weight ratio was calculated from the obtained molecular weight distribution curve by calculating the peak area C in the range of polystyrene-equivalent molecular weight of less than 3 million, and the peak area D in the range of 3 million or more but less than 15 million, based on the following formula: Ultra-high molecular weight component index=B / A Ultra high molecular weight ratio=D / C (2) Reduced viscosity (ηred) When synthesizing an acrylonitrile-based polymer by solution polymerization, the acrylonitrile-based polymer was completely precipitated with water from the acrylonitrile-based polymer solution after the completion of the polymerization reaction, and dried to prepare a specimen. Specifically, the acrylonitrile-based polymer was obtained by the following procedure.

[0050] 10 g of an acrylonitrile polymer solution with a known concentration of acrylonitrile polymer was weighed out, and water was poured into the solution in an amount 50 times or more the volume of the acrylonitrile polymer solution. The mixture was then pulverized using a mixer to completely precipitate the acrylonitrile polymer. The liquid was then removed by filtration, and the mixture was dried in an electric dryer to obtain a sample for GPC measurement. Care was taken to prevent the acrylonitrile polymer from being altered by heat or oxidation.

[0051] The obtained acrylonitrile polymer was dissolved in dimethylformamide to a concentration of 0.5 g / 100 mL to obtain a sample solution. The heating temperature was set to 60° C. or less to prevent denaturation of the acrylonitrile polymer during dissolution.

[0052] An Ubbelohde viscometer was used as the viscometer. After the sample solution was placed in a water bath adjusted to 35°C and held for 30 minutes, the drop time between the marked lines was measured three times with an accuracy of 1 / 100 seconds, and the average value was taken as t (seconds). Similarly, measurements were also taken for dimethylformamide in which no acrylonitrile polymer was dissolved, and the average value was taken as t (seconds). The reduced viscosity (ηred) was calculated based on the following formula. Reduced viscosity = (t-t0) / (0.5×t0) (3) Acrylonitrile polymer concentration in spinning solution 10 g of the spinning solution was weighed out, and water was poured into the spinning solution in an amount 50 times or more the amount of the spinning solution, followed by pulverization using a mixer to completely precipitate the acrylonitrile polymer. The liquid was then removed by filtration, and the mixture was dried in an electric dryer. Care was taken to prevent the acrylonitrile polymer from being altered by heat or oxidation. The acrylonitrile polymer concentration was calculated from the mass of the resulting acrylonitrile polymer and the mass of the spinning solution.

[0053] (4) Maximum spinning draft The spinning solution was passed through a filter with a mesh size of 5 μm, and then discharged into the air (process length: 5 mm) at a discharge speed of 1 m / min using a spinneret with 3,000 holes and a hole diameter of 0.15 mm at a temperature of 35°C. The coagulated yarn was then wound up. During this process, the winding speed was gradually increased, and the maximum spinning draft was calculated from the ratio of the winding speed to the discharge speed when yarn breakage occurred.

[0054] (5) Amount of fluff (number of fluffs detected per 1,000 m) A laser sensor (Omron ZX-LT005) was installed just before the precursor fiber winding device, and a 10 mm-wide laser beam projected from the laser projector was irradiated onto the running yarn passing between the projector and receiver, and the fluctuations in the received laser width were continuously measured. An amplifier unit (Omron ZX-LDA11-N) was installed so that when the fluctuations in the laser width reaching the receiver exceeded a threshold, it was counted as fluff, and the fluff amount was measured using a fluff detector. The threshold was 0.03 mm. The fluff amount was measured in units of the number of fluff detections per 1000 m of precursor fiber (times / 1000 m). Note that keeping the fluff amount at 7 times / 1000 m or less is preferable because it reduces the amount of winding around the roller when the carbon fiber precursor fiber is calcined, resulting in high-quality carbon fiber.

[0055] (6) Amount of polymerization inhibitor contained in acrylonitrile The amount of polymerization inhibitor contained in acrylonitrile was measured by adding 5 ml of nitric acid to 10 ml of acrylonitrile and stirring the mixture to prepare a measurement sample, which was then filled into a quartz cell and measured for absorbance at a wavelength of 400 nm using a spectrophotometer (U-3010 manufactured by Hitachi, Ltd.) In the same manner, a calibration curve was prepared using acrylonitrile with a known concentration of polymerization inhibitor, and the concentration of the polymerization inhibitor in the measurement sample was calculated from the calibration curve. Example 1 Nitrogen was introduced into a polymerization vessel equipped with a stirring blade. 129 parts by weight of dimethyl sulfoxide, 100 parts by weight of acrylonitrile, and 1 part by weight of itaconic acid were charged and stirred until homogeneous. The mixture was then heated to 65°C. The acrylonitrile was first vaporized and then coagulated and recovered, after which the polymerization inhibitor added was removed until it reached 0 ppm. After reaching 65°C, 0.006 parts by weight of azobisisobutyronitrile was added to initiate the solution polymerization reaction. The temperature was controlled at 65°C for 50 minutes after the start of the reaction. Then, 210 parts by weight of dimethyl sulfoxide was added, and the temperature was raised to 60°C. 0.4 parts by weight of azobisisobutyronitrile and 0.17 parts by weight of octyl mercaptan were added, and the reaction temperature was controlled to 60°C and maintained for 4 hours. The temperature was then raised at a rate of 10°C / hour for 2 hours. For the next 6 hours, the reaction temperature was controlled to 80°C, and an acrylonitrile polymer solution was obtained. At this time, the ultra-high molecular weight component index of the acrylonitrile polymer was 1.45, the ultra-high molecular weight ratio was 0.018, and the reduced viscosity was 1.68.

[0056] The obtained acrylonitrile-based polymer solution was decompressed to distill off unreacted acrylonitrile. Subsequently, ammonia gas was blown into the acrylonitrile-based polymer solution and mixed uniformly to obtain a spinning dope. At this time, the acrylonitrile-based polymer concentration in the spinning dope was 20.9%.

[0057] The resulting spinning solution was transferred to a storage tank and continuously spun to obtain carbon fiber precursor fibers. The spinning solution was passed through a filter with a mesh size of 5 μm and then extruded into the air (5 mm length) using a nozzle with a pore size of 0.15 mm. After passing through the air, the solution was introduced into a dimethyl sulfoxide aqueous solution with a concentration of 35% by mass and a bath temperature of 3°C to obtain coagulated fibers. The spinning draft was set to 12.

[0058] The coagulated yarn was desolvated by washing with water, stretched twice in hot water, coated with silicone oil, and dried using heated rollers with gradually increasing temperatures. It was then stretched twice in pressurized steam. It was then heat-treated using a heat-setting roller to obtain a carbon fiber precursor fiber. The maximum spinning draft of this spinning dope was 100. The amount of fuzz measured by a fuzz detector after passing through the pressurized steam stretching device was 7 times / 1000m. Example 2 Nitrogen was introduced into a polymerization vessel equipped with a stirring blade. 129 parts by weight of dimethyl sulfoxide, 100 parts by weight of acrylonitrile, and 1 part by weight of itaconic acid were charged and stirred until homogeneous. The mixture was then heated to 65°C. The acrylonitrile was first vaporized and then coagulated and recovered, after which the polymerization inhibitor added previously was removed until it reached 1 ppm. After reaching 65°C, 0.006 parts by weight of azobisisobutyronitrile was added to initiate the solution polymerization reaction. The temperature was controlled at 65°C for 40 minutes after the start of the reaction. Then, 200 parts by weight of dimethyl sulfoxide was added, and the temperature was raised to 60°C. 0.4 parts by weight of azobisisobutyronitrile and 0.17 parts by weight of octyl mercaptan were added, and the reaction temperature was controlled to 60°C and maintained for 4 hours. The temperature was then raised at a rate of 10°C / hour for 2 hours. For the next 6 hours, the reaction temperature was controlled to 80°C, and an acrylonitrile polymer solution was obtained. At this time, the ultra-high molecular weight component index of the acrylonitrile polymer was 1.20, the ultra-high molecular weight ratio was 0.008, and the reduced viscosity was 1.55.

[0059] The obtained acrylonitrile-based polymer solution was decompressed to distill off unreacted acrylonitrile. Subsequently, ammonia gas was blown into the acrylonitrile-based polymer solution and mixed uniformly to obtain a spinning dope. At this time, the acrylonitrile-based polymer concentration in the spinning dope was 21.5%.

[0060] The resulting spinning solution was transferred to a storage tank and continuously spun to obtain carbon fiber precursor fibers. The spinning solution was passed through a filter with a mesh size of 5 μm and then extruded into the air (5 mm length) using a nozzle with a pore size of 0.15 mm. After passing through the air, the solution was introduced into a dimethyl sulfoxide aqueous solution with a concentration of 35% by mass and a bath temperature of 3°C to obtain coagulated fibers. The spinning draft was set to 12.

[0061] The coagulated yarn was desolvated by washing with water, stretched twice in hot water, coated with silicone oil, and dried using heated rollers with gradually increasing temperatures. It was then stretched twice in pressurized steam. It was then heat-treated using a heat-setting roller to obtain a carbon fiber precursor fiber. The maximum spinning draft of this spinning dope was 100. The amount of fuzz measured by a fuzz detector after passing through the pressurized steam stretching device was 7 times / 1000m. Example 3 Nitrogen was introduced into a polymerization vessel equipped with a stirring blade. 129 parts by weight of dimethyl sulfoxide, 100 parts by weight of acrylonitrile, and 1 part by weight of itaconic acid were charged and stirred until homogeneous. The mixture was then heated to 60°C. The acrylonitrile was first vaporized and then coagulated and recovered, after which the polymerization inhibitor added previously was removed until it reached 0 ppm. After reaching 60°C, 0.006 parts by weight of azobisisobutyronitrile was added to initiate the solution polymerization reaction. The temperature was controlled to 60°C for 60 minutes after the start of the reaction. Then, 210 parts by weight of dimethyl sulfoxide was added, and the temperature was raised to 60°C. 0.4 parts by weight of azobisisobutyronitrile and 0.15 parts by weight of octyl mercaptan were added, and the reaction temperature was controlled to 60°C and maintained for 4 hours. The temperature was then raised at a rate of 10°C / hour for 2 hours. For the next 6 hours, the reaction temperature was controlled to 80°C, and an acrylonitrile polymer solution was obtained. At this time, the ultra-high molecular weight component index of the acrylonitrile polymer was 1.55, the ultra-high molecular weight ratio was 0.016, and the reduced viscosity was 1.69.

[0062] The obtained acrylonitrile-based polymer solution was decompressed to distill off unreacted acrylonitrile. Subsequently, ammonia gas was blown into the acrylonitrile-based polymer solution and mixed uniformly to obtain a spinning dope. At this time, the acrylonitrile-based polymer concentration in the spinning dope was 20.8%.

[0063] The resulting spinning solution was transferred to a storage tank and continuously spun to obtain carbon fiber precursor fibers. The spinning solution was passed through a filter with a mesh size of 5 μm and then extruded into the air (5 mm length) using a nozzle with a pore size of 0.15 mm. After passing through the air, the solution was introduced into a dimethyl sulfoxide aqueous solution with a concentration of 35% by mass and a bath temperature of 3°C to obtain coagulated fibers. The spinning draft was set to 12.

[0064] The coagulated yarn was desolvated by washing with water, stretched twice in hot water, coated with silicone oil, and dried using heated rollers with gradually increasing temperatures. It was then stretched twice in pressurized steam. It was then heat-treated using a heat-setting roller to obtain a carbon fiber precursor fiber. The maximum spinning draft of this spinning dope was 120. The amount of fuzz measured by a fuzz detector after passing through the pressurized steam stretching device was 3 times / 1000 m. Example 4 Nitrogen was introduced into a polymerization vessel equipped with a stirring blade. 129 parts by weight of dimethyl sulfoxide, 100 parts by weight of acrylonitrile, and 1 part by weight of itaconic acid were charged and stirred until homogeneous. The mixture was then heated to 65°C. The acrylonitrile was first vaporized and then coagulated and recovered, after which the polymerization inhibitor added was removed until it reached 0 ppm. After reaching 65°C, 0.006 parts by weight of azobisisobutyronitrile was added to initiate the solution polymerization reaction. The temperature was controlled at 65°C for 50 minutes after the start of the reaction. Then, 220 parts by weight of dimethyl sulfoxide was added, and the temperature was raised to 60°C. 0.4 parts by weight of azobisisobutyronitrile and 0.15 parts by weight of octyl mercaptan were added, and the reaction temperature was controlled to 60°C and maintained for 4 hours. The temperature was then raised at a rate of 10°C / hour for 2 hours. For the next 6 hours, the reaction temperature was controlled to 80°C, and an acrylonitrile polymer solution was obtained. At this time, the ultra-high molecular weight component index of the acrylonitrile polymer was 1.30, the ultra-high molecular weight ratio was 0.016, and the reduced viscosity was 1.75.

[0065] The obtained acrylonitrile-based polymer solution was decompressed to distill off unreacted acrylonitrile. Subsequently, ammonia gas was blown into the acrylonitrile-based polymer solution and mixed uniformly to obtain a spinning dope. At this time, the acrylonitrile-based polymer concentration in the spinning dope was 20.3%.

[0066] The resulting spinning solution was transferred to a storage tank and continuously spun to obtain carbon fiber precursor fibers. The spinning solution was passed through a filter with a mesh size of 5 μm and then extruded into the air (5 mm length) using a nozzle with a pore size of 0.15 mm. After passing through the air, the solution was introduced into a dimethyl sulfoxide aqueous solution with a concentration of 35% by mass and a bath temperature of 3°C to obtain coagulated fibers. The spinning draft was set to 12.

[0067] The coagulated yarn was desolvated by washing with water, stretched twice in hot water, coated with silicone oil, and dried using heated rollers with gradually increasing temperatures. It was then stretched twice in pressurized steam. It was then heat-treated using a heat-setting roller to obtain a carbon fiber precursor fiber. The maximum spinning draft of this spinning dope was 120. The amount of fuzz measured by a fuzz detector after passing through the pressurized steam stretching device was 3 times / 1000 m. Example 5 Nitrogen was introduced into a polymerization vessel equipped with a stirring blade. 129 parts by weight of dimethyl sulfoxide, 100 parts by weight of acrylonitrile, and 1 part by weight of itaconic acid were charged and stirred until homogeneous. The mixture was then heated to 65°C. The acrylonitrile was first vaporized and then coagulated and recovered, after which the polymerization inhibitor added previously was removed until it reached 2 ppm. After reaching 65°C, 0.006 parts by weight of azobisisobutyronitrile was added to initiate the solution polymerization reaction. The temperature was controlled at 65°C for 45 minutes after the start of the reaction. Then, 230 parts by weight of dimethyl sulfoxide was added, and the temperature was raised to 60°C. 0.4 parts by weight of azobisisobutyronitrile and 0.15 parts by weight of octyl mercaptan were added, and the reaction temperature was controlled to 60°C and maintained for 4 hours. The temperature was then raised at a rate of 10°C / hour for 2 hours. For the next 6 hours, the reaction temperature was controlled to 80°C, and an acrylonitrile polymer solution was obtained. At this time, the ultra-high molecular weight component index of the acrylonitrile polymer was 1.29, the ultra-high molecular weight ratio was 0.013, and the reduced viscosity was 1.69.

[0068] The obtained acrylonitrile-based polymer solution was decompressed to distill off unreacted acrylonitrile. Subsequently, ammonia gas was blown into the acrylonitrile-based polymer solution and mixed uniformly to obtain a spinning dope. At this time, the acrylonitrile-based polymer concentration in the spinning dope was 20.0%.

[0069] The resulting spinning solution was transferred to a storage tank and continuously spun to obtain carbon fiber precursor fibers. The spinning solution was passed through a filter with a mesh size of 5 μm and then extruded into the air (5 mm length) using a nozzle with a pore size of 0.15 mm. After passing through the air, the solution was introduced into a dimethyl sulfoxide aqueous solution with a concentration of 35% by mass and a bath temperature of 3°C to obtain coagulated fibers. The spinning draft was set to 12.

[0070] The coagulated yarn was desolvated by washing with water, stretched twice in hot water, coated with silicone oil, and dried using heated rollers with gradually increasing temperatures. It was then stretched twice in pressurized steam. It was then heat-treated using a heat-setting roller to obtain a carbon fiber precursor fiber. The maximum spinning draft of this spinning dope was 120. The amount of fuzz measured by a fuzz detector after passing through the pressurized steam stretching device was 3 times / 1000 m. Example 6 Nitrogen was introduced into a polymerization vessel equipped with a stirring blade. 262 parts by weight of dimethyl sulfoxide, 100 parts by weight of acrylonitrile, and 1 part by weight of itaconic acid were charged and stirred until homogeneous. The mixture was then heated to 65°C. The acrylonitrile was first vaporized and then coagulated and recovered, after which the polymerization inhibitor added previously was removed until it reached 5 ppm. After reaching 65°C, 0.015 parts by weight of azobisisobutyronitrile was added to initiate the solution polymerization reaction. The temperature was controlled at 65°C for 50 minutes after the start of the reaction. Then, 65 parts by weight of dimethyl sulfoxide was added, and the temperature was raised to 60°C. 0.4 parts by weight of azobisisobutyronitrile and 0.15 parts by weight of octyl mercaptan were added, and the reaction temperature was controlled to 60°C and maintained for 4 hours. The temperature was then raised at a rate of 10°C / hour for 2 hours. For the next 6 hours, the reaction temperature was controlled to 80°C, and an acrylonitrile polymer solution was obtained. At this time, the ultra-high molecular weight component index of the acrylonitrile polymer was 0.91, the ultra-high molecular weight ratio was 0.021, and the reduced viscosity was 1.82.

[0071] The obtained acrylonitrile-based polymer solution was decompressed to distill off unreacted acrylonitrile. Subsequently, ammonia gas was blown into the acrylonitrile-based polymer solution and mixed uniformly to obtain a spinning dope. At this time, the acrylonitrile-based polymer concentration in the spinning dope was 21.5%.

[0072] The resulting spinning solution was transferred to a storage tank and continuously spun to obtain carbon fiber precursor fibers. The spinning solution was passed through a filter with a mesh size of 5 μm and then extruded into the air (5 mm length) using a nozzle with a pore size of 0.15 mm. After passing through the air, the solution was introduced into a dimethyl sulfoxide aqueous solution with a concentration of 35% by mass and a bath temperature of 3°C to obtain coagulated fibers. The spinning draft was set to 12.

[0073] The coagulated yarn was desolvated by washing with water, stretched twice in hot water, coated with silicone oil, and dried using heated rollers with gradually increasing temperatures. It was then stretched twice in pressurized steam. It was then heat-treated using a heat-setting roller to obtain a carbon fiber precursor fiber. The maximum spinning draft of this spinning dope was 110. The amount of fuzz measured by a fuzz detector after passing through the pressurized steam stretching device was 8 times / 1000m. Comparative Example 1 Nitrogen was introduced into a polymerization vessel equipped with a stirring blade. 262 parts by weight of dimethyl sulfoxide, 100 parts by weight of acrylonitrile, and 1 part by weight of itaconic acid were charged and stirred until homogeneous. The mixture was then heated to 65°C. The acrylonitrile was first vaporized and then coagulated and recovered, after which the polymerization inhibitor added previously was removed until it reached 0 ppm. After reaching 65°C, 0.022 parts by weight of azobisisobutyronitrile was added to initiate the solution polymerization reaction. The temperature was controlled at 65°C for 70 minutes after the start of the reaction. Then, 105 parts by weight of dimethyl sulfoxide was added, and the temperature was raised to 60°C. 0.4 parts by weight of azobisisobutyronitrile and 0.15 parts by weight of octyl mercaptan were added, and the reaction temperature was controlled to 60°C and maintained for 4 hours. The temperature was then raised at a rate of 10°C / hour for 2 hours. For the next 6 hours, the reaction temperature was controlled to 80°C, and an acrylonitrile polymer solution was obtained. At this time, the ultra-high molecular weight component index of the acrylonitrile polymer was 0.74, the ultra-high molecular weight ratio was 0.034, and the reduced viscosity was 2.00.

[0074] The obtained acrylonitrile-based polymer solution was decompressed to distill off unreacted acrylonitrile. Subsequently, ammonia gas was blown into the acrylonitrile-based polymer solution and mixed uniformly to obtain a spinning dope. At this time, the acrylonitrile-based polymer concentration in the spinning dope was 19.6%.

[0075] The resulting spinning solution was transferred to a storage tank and continuously spun to obtain carbon fiber precursor fibers. The spinning solution was passed through a filter with 5 μm openings and extruded into the air (5 mm length) using a nozzle with a diameter of 0.15 mm. After passing through the air, the solution was introduced into a dimethyl sulfoxide aqueous solution with a concentration of 35% by mass and a bath temperature of 3°C to obtain coagulated fibers. The spinning draft was set to 12.

[0076] The coagulated yarn was desolvated by washing with water, stretched twice in hot water, coated with silicone oil, and dried using heated rollers with gradually increasing temperatures. It was then stretched twice in pressurized steam. It was then heat-treated using a heat-setting roller to obtain a carbon fiber precursor fiber. The maximum spinning draft of this spinning dope was 70. The amount of fuzz measured by a fuzz detector after passing through the pressurized steam stretching device was 15 times / 1000 m. Comparative Example 2 Nitrogen was introduced into a polymerization vessel equipped with a stirring blade. 262 parts by weight of dimethyl sulfoxide, 100 parts by weight of acrylonitrile, and 1 part by weight of itaconic acid were charged and stirred until homogeneous. The mixture was then heated to 65°C. The acrylonitrile was first vaporized and then coagulated and recovered, after which the polymerization inhibitor added was removed until it reached 3 ppm. After reaching 65°C, 0.018 parts by weight of azobisisobutyronitrile was added to initiate the solution polymerization reaction. The temperature was controlled to 65°C for 65 minutes after the start of the reaction. Then, 90 parts by weight of dimethyl sulfoxide was added, and the temperature was raised to 60°C. 0.4 parts by weight of azobisisobutyronitrile and 0.17 parts by weight of octyl mercaptan were added, and the reaction temperature was controlled to 60°C and maintained for 4 hours. The temperature was then raised at a rate of 10°C / hour for 2 hours. For the next 6 hours, the reaction temperature was controlled to 80°C, and an acrylonitrile polymer solution was obtained. At this time, the ultra-high molecular weight component index of the acrylonitrile polymer was 0.74, the ultra-high molecular weight ratio was 0.026, and the reduced viscosity was 1.89.

[0077] The obtained acrylonitrile-based polymer solution was decompressed to distill off unreacted acrylonitrile. Subsequently, ammonia gas was blown into the acrylonitrile-based polymer solution and mixed uniformly to obtain a spinning dope. At this time, the acrylonitrile-based polymer concentration in the spinning dope was 20.3%.

[0078] The resulting spinning solution was transferred to a storage tank and continuously spun to obtain carbon fiber precursor fibers. The spinning solution was passed through a filter with 5 μm openings and extruded into the air (5 mm length) using a nozzle with a diameter of 0.15 mm. After passing through the air, the solution was introduced into a dimethyl sulfoxide aqueous solution with a concentration of 35% by mass and a bath temperature of 3°C to obtain coagulated fibers. The spinning draft was set to 12.

[0079] The coagulated yarn was desolvated by washing with water, stretched twice in hot water, coated with silicone oil, and dried using heated rollers with gradually increasing temperatures. It was then stretched twice in pressurized steam. It was then heat-treated using a heat-setting roller to obtain a carbon fiber precursor yarn. The maximum spinning draft of this spinning dope was 70. The amount of fuzz measured by a fuzz detector after passing through the pressurized steam stretching device was 15 times / 1000 m. Comparative Example 3 Nitrogen was introduced into a polymerization vessel equipped with a stirring blade. 262 parts by weight of dimethyl sulfoxide, 100 parts by weight of acrylonitrile, and 1 part by weight of itaconic acid were charged and stirred until homogeneous. The mixture was then heated to 65°C. The acrylonitrile was first vaporized and then coagulated and recovered, after which the polymerization inhibitor added previously was removed until it reached 0 ppm. After reaching 65°C, 0.018 parts by weight of azobisisobutyronitrile was added to initiate the solution polymerization reaction. The temperature was controlled to 65°C for 55 minutes after the start of the reaction. Then, 85 parts by weight of dimethyl sulfoxide was added, and the temperature was raised to 60°C. 0.4 parts by weight of azobisisobutyronitrile and 0.19 parts by weight of octyl mercaptan were added, and the reaction temperature was controlled to 60°C and maintained for 4 hours. The temperature was then raised at a rate of 10°C / hour for 2 hours. For the next 6 hours, the reaction temperature was controlled to 80°C, and an acrylonitrile polymer solution was obtained. At this time, the ultra-high molecular weight component index of the acrylonitrile polymer was 0.95, the ultra-high molecular weight ratio was 0.027, and the reduced viscosity was 1.84.

[0080] The obtained acrylonitrile-based polymer solution was decompressed to distill off unreacted acrylonitrile. Subsequently, ammonia gas was blown into the acrylonitrile-based polymer solution and mixed uniformly to obtain a spinning dope. At this time, the acrylonitrile-based polymer concentration in the spinning dope was 20.6%.

[0081] The resulting spinning solution was transferred to a storage tank and continuously spun to obtain carbon fiber precursor fibers. The spinning solution was passed through a 5 μm filter and extruded into the air (5 mm length) using a 0.15 mm diameter nozzle. After passing through the air, the solution was introduced into a 35% by mass aqueous solution of dimethyl sulfoxide at a concentration of 35% by mass and a bath temperature of 3°C to obtain coagulated fibers. The spinning draft was set to 12.

[0082] The coagulated yarn was desolvated by washing with water, stretched twice in hot water, coated with silicone oil, and dried using heated rollers with gradually increasing temperatures. It was then stretched twice in pressurized steam. It was then heat-treated using a heat-setting roller to obtain a carbon fiber precursor fiber. The maximum spinning draft of this spinning dope was 70. The amount of fuzz measured by a fuzz detector after passing through the pressurized steam stretching device was 15 times / 1000 m. Comparative Example 4 Nitrogen was introduced into a polymerization vessel equipped with a stirring blade. 262 parts by weight of dimethyl sulfoxide, 100 parts by weight of acrylonitrile, and 1 part by weight of itaconic acid were charged and stirred until homogeneous. The mixture was then heated to 65°C. The acrylonitrile was first vaporized and then coagulated and recovered, after which the polymerization inhibitor added previously was removed until it reached 5 ppm. After reaching 65°C, 0.015 parts by weight of azobisisobutyronitrile was added to initiate the solution polymerization reaction. The temperature was controlled at 65°C for 30 minutes after the start of the reaction. Then, 80 parts by weight of dimethyl sulfoxide was added, and the temperature was raised to 60°C. 0.4 parts by weight of azobisisobutyronitrile and 0.15 parts by weight of octyl mercaptan were added, and the reaction temperature was controlled to 60°C and maintained for 4 hours. The temperature was then raised at a rate of 10°C / hour for 2 hours. For the next 6 hours, the reaction temperature was controlled to 80°C, and an acrylonitrile polymer solution was obtained. At this time, the ultra-high molecular weight component index of the acrylonitrile polymer was 0.59, the ultra-high molecular weight ratio was 0.007, and the reduced viscosity was 1.71.

[0083] The obtained acrylonitrile-based polymer solution was decompressed to distill off unreacted acrylonitrile. Subsequently, ammonia gas was blown into the acrylonitrile-based polymer solution and mixed uniformly to obtain a spinning dope. At this time, the acrylonitrile-based polymer concentration in the spinning dope was 20.7%.

[0084] The resulting spinning solution was transferred to a storage tank and continuously spun to obtain carbon fiber precursor fibers. The spinning solution was passed through a filter with 5 μm openings and then extruded into the air (5 mm process length) using a nozzle with a diameter of 0.15 mm. After passing through the air, the solution was introduced into an aqueous dimethyl sulfoxide solution with a concentration of 35% by mass and a bath temperature of 3°C. However, fiber breakage occurred and coagulated fibers could not be obtained. The spinning draft was set to 12. The maximum spinning draft of this spinning solution was 10. Comparative Example 5 Nitrogen was introduced into a polymerization vessel equipped with a stirring blade. 262 parts by weight of dimethyl sulfoxide, 100 parts by weight of acrylonitrile, and 1 part by weight of itaconic acid were charged and stirred until homogeneous. The mixture was then heated to 65°C. The acrylonitrile was first vaporized and then coagulated and recovered, after which the polymerization inhibitor added previously was removed until it reached 0 ppm. After reaching 65°C, 0.022 parts by weight of azobisisobutyronitrile was added to initiate the solution polymerization reaction. The temperature was controlled to 65°C for 60 minutes after the start of the reaction. Then, 70 parts by weight of dimethyl sulfoxide was added, and the temperature was raised to 60°C. 0.4 parts by weight of azobisisobutyronitrile and 0.24 parts by weight of octyl mercaptan were added, and the reaction temperature was controlled to 60°C and maintained for 4 hours. The temperature was then raised at a rate of 10°C / hour for 2 hours. For the next 6 hours, the reaction temperature was controlled to 80°C, and an acrylonitrile polymer solution was obtained. At this time, the ultra-high molecular weight component index of the acrylonitrile polymer was 0.79, the ultra-high molecular weight ratio was 0.030, and the reduced viscosity was 1.71.

[0085] The obtained acrylonitrile-based polymer solution was decompressed to distill off unreacted acrylonitrile. Subsequently, ammonia gas was blown into the acrylonitrile-based polymer solution and mixed to become uniform, thereby obtaining a spinning dope. At this time, the acrylonitrile-based polymer concentration in the spinning dope was 21.2%.

[0086] The resulting spinning solution was transferred to a storage tank and continuously spun to obtain carbon fiber precursor fibers. The spinning solution was passed through a 5 μm filter and extruded into the air (5 mm length) using a 0.15 mm diameter nozzle. After passing through the air, the solution was introduced into a 35% by mass aqueous solution of dimethyl sulfoxide at a concentration of 35% by mass and a bath temperature of 3°C to obtain coagulated fibers. The spinning draft was set to 12.

[0087] The coagulated yarn was desolvated by washing with water, stretched twice in hot water, coated with silicone oil, and dried using heated rollers with gradually increasing temperatures. It was then stretched twice in pressurized steam. It was then heat-treated using a heat-setting roller to obtain a carbon fiber precursor fiber. The maximum spinning draft of this spinning dope was 120. The amount of fuzz measured by a fuzz detector after passing through the pressurized steam stretching device was 15 times / 1000 m. Comparative Example 6 Nitrogen was introduced into a polymerization vessel equipped with a stirring blade. 262 parts by weight of dimethyl sulfoxide, 100 parts by weight of acrylonitrile, and 1 part by weight of itaconic acid were charged and stirred until homogeneous. The mixture was then heated to 65°C. The acrylonitrile was first vaporized and then coagulated and recovered, after which the polymerization inhibitor added was removed until it reached 0 ppm. After reaching 65°C, 0.022 parts by weight of azobisisobutyronitrile was added to initiate the solution polymerization reaction. The temperature was controlled at 65°C for 45 minutes after the start of the reaction. Then, 90 parts by weight of dimethyl sulfoxide was added, and the temperature was raised to 60°C. 0.4 parts by weight of azobisisobutyronitrile and 0.14 parts by weight of octyl mercaptan were added, and the reaction temperature was controlled to 60°C and maintained for 4 hours. The temperature was then raised at a rate of 10°C / hour for 2 hours. For the next 6 hours, the reaction temperature was controlled to 80°C, and an acrylonitrile polymer solution was obtained. At this time, the ultra-high molecular weight component index of the acrylonitrile polymer was 0.68, the ultra-high molecular weight ratio was 0.021, and the reduced viscosity was 1.85.

[0088] The obtained acrylonitrile-based polymer solution was decompressed to distill off unreacted acrylonitrile. Subsequently, ammonia gas was blown into the acrylonitrile-based polymer solution and mixed uniformly to obtain a spinning dope. At this time, the acrylonitrile-based polymer concentration in the spinning dope was 20.4%.

[0089] The resulting spinning solution was transferred to a storage tank and continuously spun to obtain carbon fiber precursor fibers. The spinning solution was passed through a filter with 5 μm openings and then extruded into the air (5 mm process length) using a nozzle with a diameter of 0.15 mm. After passing through the air, the solution was introduced into an aqueous dimethyl sulfoxide solution with a concentration of 35% by mass and a bath temperature of 3°C. However, fiber breakage occurred and coagulated fibers could not be obtained. The spinning draft was set to 12. The maximum spinning draft of this spinning solution was 10. Example 7 The same procedure as in Example 1 was carried out, except that the spinning draft was set to 4, the fiber was drawn to a draw ratio of 3 in hot water, and then drawn to a draw ratio of 4 in pressurized steam, to obtain a carbon fiber precursor fiber. After passing through the pressurized steam drawing device, the amount of fluff measured by a fluff detector was 3 times / 1000 m. Example 8 The same procedure as in Example 2 was carried out to obtain a carbon fiber precursor fiber, except that the spinning draft was set to 4, the fiber was drawn to a draw ratio of 3 in hot water, and then drawn to a draw ratio of 4 in pressurized steam. After passing through the pressurized steam drawing device, the amount of fluff measured by a fluff detector was 3 times / 1000 m. Example 9 The same procedure as in Example 3 was carried out, except that the spinning draft was set to 4, the fiber was drawn to a 3-fold stretch in hot water, and then drawn to a 4-fold stretch in pressurized steam, to obtain a carbon fiber precursor fiber. The number of detections by a fluff detector after passing through the pressurized steam drawing device was 3 times / 1000 m. Example 10 The same procedure as in Example 4 was carried out to obtain a carbon fiber precursor fiber, except that the spinning draft was set to 4, the fiber was drawn to a draw ratio of 3 in hot water, and then drawn to a draw ratio of 4 in pressurized steam. After passing through the pressurized steam drawing device, the amount of fluff measured by a fluff detector was 3 times / 1000 m. Example 11 The same procedure as in Example 5 was carried out to obtain a carbon fiber precursor fiber, except that the spinning draft was set to 4, the fiber was drawn to a draw ratio of 3 in hot water, and then drawn to a draw ratio of 4 in pressurized steam. After passing through the pressurized steam drawing device, the amount of fluff measured by a fluff detector was 3 times / 1000 m. Example 12 The same procedure as in Example 6 was carried out, except that the spinning draft was set to 4, the fiber was drawn to a draw ratio of 3 in hot water, and then drawn to a draw ratio of 4 in pressurized steam, to obtain a carbon fiber precursor fiber. After passing through the pressurized steam drawing device, the amount of fluff measured by a fluff detector was 8 times / 1000 m. Comparative Example 7 The same procedures as in Comparative Example 1 were carried out except that the spinning draft was set to 4, the drawing was performed to a 3-fold ratio in hot water, and then the drawing was performed to a 4-fold ratio in pressurized steam. However, since fiber breakage occurred during drawing in pressurized steam, a carbon fiber precursor fiber could not be obtained. Comparative Example 8 The same procedures as in Comparative Example 2 were carried out except that the spinning draft was set to 4, the fiber was stretched to a draw ratio of 3 in hot water, and then stretched to a draw ratio of 4 in pressurized steam. However, because fiber breakage occurred during stretching in pressurized steam, a carbon fiber precursor fiber could not be obtained. Comparative Example 9 The same procedures as in Comparative Example 3 were carried out except that the spinning draft was set to 4, the drawing was performed to a 3-fold ratio in hot water, and then the drawing was performed to a 4-fold ratio in pressurized steam. However, since fiber breakage occurred during drawing in pressurized steam, a carbon fiber precursor fiber could not be obtained. Comparative Example 10 The same procedure as in Comparative Example 5 was carried out except that the spinning draft was set to 4, the fiber was stretched to 3 times its original size in hot water, and then stretched to 4 times its original size in pressurized steam. However, since fiber breakage occurred during stretching in pressurized steam, a carbon fiber precursor fiber could not be obtained. The results of the above examples and comparative examples are shown in Tables 1 and 2.

[0090] [Table 1]

[0091] [Table 2] [Industrial Applicability]

[0092] According to the present invention, a spinning dope having excellent spinnability and drawability can be obtained, and by using the spinning dope, a carbon fiber precursor fiber having excellent quality can be provided. Therefore, the present invention has high industrial applicability and extremely great industrial value.

Claims

1. A method for producing a carbon fiber precursor fiber, comprising using as a spinning dope an acrylonitrile polymer in which the area ratio (B / A) between the peak area A in the range of 3 million to less than 5 million in terms of polystyrene as measured by gel permission chromatography (GPC) and the peak area B in the range of 5 million to 15 million in terms of polystyrene as measured by gel permission chromatography (GPC) is 0.8 or more, and the area ratio (D / C) between the peak area C in the range of less than 3 million in terms of polystyrene as measured by gel permission chromatography (GPC) and the peak area D in the range of 3 million to 15 million in terms of polystyrene as measured by gel permission chromatography (GPC) is 0.025 or less.

2. 2. The method for producing a carbon fiber precursor fiber according to claim 1, wherein the reduced viscosity of the acrylonitrile polymer is 1.0 to 2.5, and the concentration of the acrylonitrile polymer in the spinning dope is 18% by mass or more and 25% by mass or less.

3. 2. The method for producing a precursor fiber for carbon fiber according to claim 1, wherein the acrylonitrile polymer is an acrylonitrile polymer obtained by removing a polymerization inhibitor contained in the acrylonitrile and polymerizing the acrylonitrile polymer by a solution polymerization method.

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