JP4957632B2 - Method for producing carbon fiber precursor fiber - Google Patents

Description

  The present invention relates to a method for producing a carbon fiber precursor fiber by spinning a polymer solution for a carbon fiber precursor fiber having a low foreign matter content.

  Since carbon fiber has higher specific strength and specific modulus than other fibers, it can be used as a reinforcing fiber for composite materials in addition to conventional sports applications, aerospace applications, automobiles, civil engineering / architecture, pressure vessels and Widely used in general industrial applications such as windmill blades, there is a strong demand for both higher performance and lower costs.

  Among the carbon fibers, the most widely used polyacrylonitrile (hereinafter sometimes referred to as PAN) carbon fiber is a spinning solution composed of a PAN polymer as a precursor, and wet spinning or dry spinning. Alternatively, after carbon fiber precursor fiber is obtained by dry and wet spinning, it is heated in an oxidizing atmosphere at a temperature of 200 to 400 ° C. to convert to a flame-resistant fiber, and in an inert atmosphere at a temperature of at least 1000 ° C. It is manufactured industrially by heating and carbonizing at Since carbon fiber is a brittle material and causes a decrease in strength due to slight surface defects and intrinsic defects, delicate attention has been paid to the generation of defects. Foreign matter in the spinning solution is taken into the fiber, and the influence is particularly large, but the foreign matter is not only foreign matter such as dust in the raw material, but also a gel made by part of the spinning solution and thermal degradation . The fact that this gel is not discharged from the die has been carried out not only for reducing the strength but also for reducing yarn breakage in the yarn making / firing process during ejection from the die. In order not to discharge the gel from the die, a method of filtering before discharging is also performed, but it is also necessary not to generate the gel. As a method for obtaining a stable spinning solution that does not generate a gel, for example, a technique of adding formic acid or nitric acid to a spinning solution is disclosed (Patent Document 1), but a PAN-based polymer solution containing formic acid or nitric acid is Not only is it difficult to separate formic acid and nitric acid by separating it from the solvent in the process of recovering the solvent, but the thermal stability effect is low, and the PAN-based polymer becomes acidic. There is a problem that the degree of ionization is lowered, the hydrophilicity of the PAN-based polymer is lowered, voids are easily generated during solidification, and the characteristics of the carbon fiber to be produced are lowered. Therefore, a method has been disclosed in which ammonia, hydrazine, amine, or the like is mixed into a PAN-based polymer and the terminal hydrogen of the carboxyl group is replaced with ammonia, hydrazine, quaternary ammonium ions, or the like (Patent Document 2). In addition, a method of adding water to the spinning solution is disclosed (Patent Document 3), but the thermal stability effect by water is low, and water is a strong coagulant of PAN-based polymer and coagulates in the piping. There is a problem that the PAN-based polymer adheres and blocks the piping. Furthermore, although a spinning solution containing 0.01 to 3 wt% of an ethylenic double bond compound is disclosed (Patent Document 4), not only the gelation inhibitory power is not so high, but also disclosed in, for example, Examples. Acrylonitrile is considered to be a Class 1 designated substance in the Act on Promotion of Improvement of Management (PRTR Law), etc., ascertaining the amount of specified chemical substances released into the environment, and there are concerns about adverse effects on the human body. It is not preferable that the spinning solution contains a large amount.

  Various methods for reducing the residual acrylonitrile in the spinning solution have been studied. For example, a method of dispersing a polymer solution in a degassing tank filled with a filler and depressurized is disclosed (Patent Document 5). However, the spinning solution in which only the remaining acrylonitrile is reduced decreases in thermal stability, and the spinning solution staying in the pipe for a long time gels and mixes with a part of the spinning solution that flows, leading to yarn breakage. There is a problem that the amount of residual acrylonitrile is small, and it is desired to suppress the formation of gel.

  In addition, when the production speed per hour is increased by increasing the yarn production speed, the spinnability that indicates the limit take-up speed that can be taken up by the driving roller out of the coagulation bath without breaking the yarn is the limit of the yarn production speed. In dry-wet spinning that facilitates increasing the spinning speed, the yarn length is important for spinnability, but as shown in FIG. 1 of Patent Document 6, the yarn length is about 100 mm at most, When the yarn was wound at high speed, the yarn length was drastically reduced. Such a yarn length is a general level of those skilled in the art, and the property of increasing the spinning speed as a polymer solution was not satisfactory.

In order to reduce the production cost of carbon fibers, the present inventors have proposed a technique that can increase the spinning speed and the spinning draft rate by using a PAN-based polymer having a specific molecular weight distribution. (See Japanese Patent Application No. 2007-269822). In order to obtain higher strength and less fuzz when using such a technique as carbon fiber, it is desired that the spinning solution be free from gel when discharged from the die.
Japanese Patent Laid-Open No. 9-3722 JP-A-53-2126325 JP-A-8-246230 JP 2002-249924 A JP 2002-363214 A Japanese Patent Laid-Open No. 11-152619 (FIG. 1)

  An object of the present invention is to develop a polymer solution for a carbon fiber precursor fiber with a small amount of residual acrylonitrile and a small amount of foreign matter that suppresses the formation of gel, in view of the background of such conventional technology, and stably produces a carbon fiber precursor. It aims at proposing the method which can manufacture a body fiber.

In order to solve this problem, the present invention has the following configuration. That is, poly having a ratio Mz / Mw of 2 or more, which is a ratio of the weight average molecular weight Mw obtained by copolymerizing at least one acid monomer selected from the group consisting of acrylic acid, methacrylic acid and itaconic acid to the Z average molecular weight Mz The acrylonitrile-based polymer is dissolved in a solvent, and the content ratio of acrylonitrile is 0.001 to 0.1% by weight, and at least one requirement selected from the following [a] to [c] A method for producing a carbon fiber precursor fiber in which a spinning solution to be filled is spun to obtain a coagulated yarn, which is then washed with water to form a carbon fiber precursor fiber.
[A] In the polyacrylonitrile-based polymer, the hydrogen ion of the carboxylic acid group derived from the acid monomer is 0 to 0.00% of the contained carboxyl group by at least one ion selected from the group consisting of ammonium ion and sodium ion. 3 equivalents are replaced.
[B] The spinning solution contains 0.01 to 3% by weight of an alkylating agent based on the weight of the spinning solution.
[C] The spinning solution contains 0.01 to 3% by weight of an organic carboxylic acid having a boiling point of 200 ° C. or more based on the weight of the spinning solution.

  According to the present invention, it is possible to obtain a carbon fiber precursor fiber polymer solution with little residual acrylonitrile and less foreign matter with suppressed gel formation, and carbon fiber precursor fibers and carbon fibers having good quality and quality are obtained. Manufacturable.

  In the present invention, spinning is performed using a spinning solution in which a PAN-based polymer in which Mz / Mw, which is a ratio of the Z average molecular weight Mz to the weight average molecular weight Mw, is 2 or more is dissolved in a solvent. This PAN polymer contains a polymer component with a large molecular weight, and after the preferential molecular weight reduction of the polymer component with a large molecular weight occurs once during the process of thermal degradation, it gels, reducing the residence in the piping. The inventors have found that the formation of gel is suppressed and have reached the present invention.

  First, the PAN polymer that can be suitably used in the present invention will be described. In the present invention, a PAN-based polymer having a weight average molecular weight Mw of 100,000 to 1,000,000, preferably 200,000 to 650,000, more preferably 300 to 500,000 is suitably used. In addition, the PAN-based polymer has a polydispersity Mz / Mw represented by a ratio of the Z average molecular weight Mz to Mw of 2.5 to 10, preferably 2.5 to 6, and more preferably 3 to 6. Is good.

  In the present invention, various average molecular weights are measured by a gel permeation chromatograph (hereinafter abbreviated as GPC) method and are obtained as polystyrene equivalent values. The polydispersity Mz / Mw has the following meaning. That is, the number average molecular weight (hereinafter abbreviated as Mn) is sensitively influenced by the low molecular weight substance contained in the polymer compound. On the other hand, Mw is sensitively influenced by the high molecular weight substance, and Mz is more sensitively influenced by the high molecular weight substance. Therefore, the spread of the molecular weight distribution can be evaluated by using Mw / Mn as the molecular weight distribution and Mz / Mw as the polydispersity. It is monodisperse when Mw / Mn is 1, and shows that the molecular weight distribution becomes broader around the low molecular weight side as it becomes larger, whereas the molecular weight distribution becomes higher as Mz / Mw becomes larger. It shows that it becomes broad around.

  As described above, since Mw / Mn and Mz / Mw are different from each other, even if Mw / Mn is large, Mz / Mw is not necessarily 2.5 or more.

  By using the PAN-based polymer as described above, a mechanism capable of producing a high-quality carbon fiber precursor fiber with less fuzz while achieving both improvement in productivity and stabilization is not necessarily clear. It is not a translation, but it is thought as follows. When the PAN-based polymer is stretched and deformed immediately after the mouthpiece hole, the ultrahigh molecular weight substance and the high molecular weight substance are entangled, and the molecular chain between the entanglement is tensioned around the ultrahigh molecular weight substance, resulting in a rapid increase in elongational viscosity. That is, strain hardening occurs. As the PAN polymer solution is thinned, the elongational viscosity of the thinned portion is increased and the flow is stabilized, so that the spinning speed can be increased, the spinning draft rate can be increased, and the spinning speed can be increased. By using a solution in which a PAN-based polymer as described above is dissolved in a solvent, the yarn will not break even if the yarn is wound at a high speed of 20000 mm / min, and the length of the yarn cannot be measured. Can do.

  Furthermore, although the mechanism which suppresses the production | generation of a gel by using the above PAN-type polymers is not necessarily clarified, it thinks as follows. When the polymer solution is sent to the spinneret via the pipe after the polymerization, it is very difficult to eliminate the staying portion. In the staying part, the molecular weight lowering and the cross-linking reaction with adjacent molecules proceed simultaneously by heat. Since the preferential molecular weight reduction of a polymer component having a large molecular weight occurs before the influence of the crosslinking reaction appears, the polymer solution not containing the polymer component having a high molecular weight becomes strong in viscosity. On the other hand, what flows around the staying part is a polymer solution containing a polymer component having a large molecular weight, which is relatively strong and easily pushes the viscous polymer solution in the staying part. If the conditions other than the molecular weight distribution are the same, the speed of the cross-linking reaction is the same, but the residence time is short, so it is considered that the gel formation is suppressed. Since the conventional polymer solution is quickly gelled by the cross-linking reaction in the staying part, or even if the molecular weight is reduced, the viscoelasticity does not change just by lowering the overall molecular weight. The polymer solution in the stagnation part is washed away, and the stagnation hardly occurs. And when gelation begins and elasticity becomes stronger, it is impossible to push away with a polymer solution with strong viscosity and no deterioration, and retention and gelation are repeated, and a part of the hard gel falls off and is washed away. thinking.

  It is preferable that Mz / Mw is larger. When Mz / Mw is less than 2.5, strain hardening is weak and the discharge stability of the PAN polymer is insufficiently improved. Moreover, when Mw is less than 100,000, the strength of the precursor fiber is insufficient, and when Mw is greater than 1 million, ejection becomes difficult.

  In addition, Mw / Mn is preferably as small as possible because the smaller the content of low molecular components that are likely to be structural defects of the carbon fiber, and Mw / Mn is preferably smaller than Mz / Mw. That is, even if it is broad on both the high molecular weight side and the low molecular weight side, there is little decrease in ejection stability, but the low molecular weight side is preferably as sharp as possible, and Mz / Mw is higher than Mw / Mn. It is preferably 1.5 times or more, and more preferably 1.8 times or more. According to the study by the present inventors, in radical polymerization such as aqueous suspension or solution method, which is usually performed in the polymerization of acrylonitrile (hereinafter abbreviated as AN), the molecular weight distribution has a lower molecular weight side. Therefore, Mw / Mn becomes larger than Mz / Mw. Therefore, when performing polymerization under special conditions such as the type and ratio of the polymerization initiator and sequential addition, or when using general radical polymerization, there is a method of mixing two or more PAN-based polymers. The method of mixing is simple. The smaller the number of types to be mixed, the easier and the two types are often sufficient from the viewpoint of ejection stability.

  The Mw of the polymer to be mixed is a PAN-based polymer having a large Mw as the A component, and a PAN-based polymer having a small Mw is the B component. Is 1 million to 5 million, and Mw of the B component is preferably 50,000 to 900,000. The larger the difference between the Mw of the A component and the B component, the greater the Mz / Mw of the mixed polymer tends to increase. This is a preferred embodiment, but when the Mw of the A component is greater than 15 million, production of the A component When the Mw of the B component is less than 150,000, the strength of the precursor fiber may be insufficient, and it is realistic that Mz / Mw is 10 or less.

  Specifically, the ratio of the weight average molecular weight of the A component and the B component is preferably 4 to 45, and more preferably 20 to 45.

  The weight ratio of the A component and the B component is preferably 0.003 to 0.3, more preferably 0.005 to 0.2, and 0.01 to 0.1. Further preferred. When the weight ratio of the A component to the B component is less than 0.003, strain hardening may be insufficient, and when it is greater than 0.3, the discharge viscosity of the polymer solution may increase so that the discharge may become difficult.

  When mixing the polymer of component A and component B, a method of mixing both polymers and then diluting with a solvent, a method of mixing each of the polymers diluted in a solvent, and a high molecular weight material that is difficult to dissolve A method in which the B component is mixed and dissolved after diluting the A component in the solvent, and a solution in which the monomer constituting the B component is mixed with a solution obtained by diluting the high molecular weight A component in the solvent. It is possible to employ a mixing method. For mixing, a method of stirring in a mixing tank, a method of quantifying with a gear pump or the like and mixing with a static mixer, a method of using a twin screw extruder, or the like can be preferably employed. From the viewpoint of uniformly dissolving the high molecular weight product, a method of first dissolving the component A which is a high molecular weight product is preferable. In particular, in the case of producing a carbon fiber precursor, the dissolved state of the high molecular weight component A is extremely important, and even if a slight amount of undissolved material exists, it is recognized as a foreign substance. When it is filtered by the filter medium or small enough not to be filtered, a void may be formed inside the carbon fiber.

  Specifically, the polymer concentration of the component A with respect to the solvent, that is, when assuming a solution consisting only of the component A and the solvent, the polymer concentration of the component A in the solution is preferably 0.1 to 5% by weight. Then, the B component is mixed, or the monomers constituting the B component are mixed and polymerized. The polymer concentration of the component A is more preferably 0.3 to 3% by weight, and further preferably 0.5 to 2% by weight. More specifically, the polymer concentration of the component A is preferably a quasi-dilute solution in which the polymers are slightly overlapped as an aggregate state of the polymer, and the component B is mixed or the component B is mixed. When the constituent monomers are mixed and polymerized, the mixed state is likely to be uniform, so that a dilute solution that is in an isolated chain state is a more preferable embodiment. The concentration of the dilute solution is considered to be determined by the intramolecular exclusion volume determined by the polymer molecular weight and the solubility of the polymer in the solvent. It is less likely to aggregate and deposit in the filter media. When the polymer concentration exceeds 5% by weight, an undissolved product of component A may be present. When the polymer concentration is less than 0.1% by weight, it is effective because it is a dilute solution depending on the molecular weight. Is often saturated.

  As described above, the polymer concentration with respect to the solvent of the component A is preferably 0.1 to 5% by weight, and then the method of mixing and dissolving the component B may be used. It is preferable to employ a method of mixing a product obtained by diluting a product with a solvent and a monomer constituting the component B and mixing the monomers by solution polymerization.

  The method for adjusting the polymer concentration of the component A to the solvent to 0.1 to 5% by weight may be a method using dilution or a method using polymerization. When diluting, it is important to stir until it can be diluted uniformly, and the dilution temperature is preferably 50 to 120 ° C. The dilution time varies depending on the dilution temperature and the concentration before dilution, and may be appropriately set. When the dilution temperature is less than 50 ° C, it may take time to dilute, and when it exceeds 120 ° C, the component A may be altered. In addition, from the viewpoint of reducing the process of diluting the overlapping of the polymers and mixing them uniformly, from the production of the A component to the start of mixing of the B component, or the polymerization of the monomer constituting the B component. Meanwhile, the polymer concentration with respect to the solvent of the component A is preferably controlled in the range of 0.1 to 5% by weight. Specifically, when the component A is produced by solution polymerization, the polymerization is stopped when the polymer concentration is 5% by weight or less, and the component B is mixed therewith, or the monomer constituting the component B is mixed. This is a method of polymerizing the monomer. Usually, when the ratio of the charged monomer to the solvent is small, it is often difficult to produce a high molecular weight product by solution polymerization, so the ratio of the charged monomer is increased. At the stage of 5% by weight or less, the polymerization rate is low and a large amount of unreacted monomer remains. The B component may be mixed after removing the unreacted monomer by volatilization, but from the viewpoint of omitting the process, it is preferable to solution polymerize the B component using the unreacted monomer.

  The component A preferably used in the present invention desirably has compatibility with PAN, and is preferably a PAN-based polymer from the viewpoint of compatibility. As the composition, AN is preferably 93 to 100 mol%, and copolymerization may be carried out if the monomer copolymerizable with AN is 7 mol% or less, but the chain transfer constant of the copolymerization component is smaller than AN. When it is difficult to obtain the required Mw, it is preferable to reduce the amount of the copolymer component as much as possible.

  Examples of monomers copolymerizable with AN include acrylic acid, methacrylic acid, itaconic acid and their alkali metal salts, ammonium salts and lower alkyl esters, acrylamide and its derivatives, allyl sulfonic acid, methallyl sulfonic acid and Those salts or alkyl esters can be used.

  The polymerization method for producing the PAN-based polymer as component A can be selected from a solution polymerization method, a suspension polymerization method, an emulsion polymerization method, etc., and the purpose of uniformly polymerizing AN and copolymer components It is preferable to use a solution polymerization method. In the case of polymerizing using the solution polymerization method, as the solvent, for example, a solvent in which PAN is soluble, such as an aqueous zinc chloride solution, dimethyl sulfoxide, dimethylformamide, and dimethylacetamide, is preferably used. If it is difficult to obtain the required Mw, a solvent having a large chain transfer constant, that is, a solution polymerization method using a zinc chloride aqueous solution or a suspension polymerization method using water is also preferably used.

  As the composition of the PAN-based polymer, which is the B component suitably used in the present invention, AN is preferably 93 to 100 mol%, and a monomer copolymerizable with AN is copolymerized if it is 7 mol% or less. However, as the amount of the copolymerization component increases, molecular breakage due to thermal decomposition at the copolymerization portion becomes more prominent in the flameproofing step, and the tensile strength of the resulting carbon fiber decreases.

  Examples of the monomer copolymerizable with AN include, for example, acrylic acid, methacrylic acid, itaconic acid and alkali metal salts thereof, ammonium salts and lower alkyl esters, acrylamide and derivatives thereof, allyl, from the viewpoint of promoting flame resistance. Sulfonic acid, methallylsulfonic acid and their salts or alkyl esters can be used, but in the present invention, it is selected from the group consisting of acrylic acid, methacrylic acid and itaconic acid for the purpose of promoting flame resistance. At least one acid monomer is used. If these acid monomers are not used, the gelation rate may be reduced and may not cause problems. However, as a polymer that forms carbon fiber precursor fibers that require a flame-resistant reaction to proceed with high productivity. It is important to copolymerize these acid monomers.

  In the present invention, when the content ratio of AN exceeds 0.1% by weight of the spinning solution, AN is released out of the spinning system, so that not only the working environment is deteriorated but also the influence on the environment is deteriorated. A smaller AN is preferable, but the minimum value is about 0.001% by weight in consideration of the removal efficiency. More preferably, it is 0.001 to 0.01 weight%.

  Usually, in radical polymerization, the polymerization rate is rarely 100% and contains an unreacted monomer. Therefore, in order to control the content ratio of AN, in addition to not adding AN after polymerization, unreacted monomers are removed and the amount thereof is controlled. In the present invention, the removal of the unreacted monomer can be carried out by a method such as JP-B-48-29797 and JP-A-2002-363214.

  Further, the concentration of the polymerization initiator remaining after the completion of the polymerization is preferably 10 ppm or less of the spinning solution. If the residual polymerization initiator concentration exceeds 10 ppm of the spinning solution, radicals may be gradually generated in the pipe to the spinneret to promote gelation. The polymerization initiator has a certain half-life, and if it is attempted to generate radicals completely, the polymerization time becomes longer, so the polymerization temperature is raised and radicals are generated while unreacted AN remains. Is good. It is also preferable to mix a radical scavenger after polymerization from the viewpoint of suppressing radical generation in the pipe to the spinneret.

  The polymerization method for producing the B component PAN-based polymer can be selected from a solution polymerization method, a suspension polymerization method, an emulsion polymerization method, and the like. It is preferable to use a solution polymerization method. In the case of polymerizing using the solution polymerization method, as the solvent, for example, a solvent in which PAN is soluble, such as an aqueous zinc chloride solution, dimethyl sulfoxide, dimethylformamide, and dimethylacetamide, is preferably used. Among these, dimethyl sulfoxide is preferably used from the viewpoint of PAN solubility. The raw materials used for these polymerizations are preferably used after passing through a filter medium having a filtration accuracy of 1 μm or less.

  The above-mentioned PAN-based polymer is dissolved in a solvent in which the PAN-based polymer is soluble, such as dimethyl sulfoxide, dimethylformamide, dimethylacetamide, thiocyanate aqueous solution and zinc chloride aqueous solution to obtain a spinning solution. In the case of using solution polymerization, if the solvent used for polymerization and the spinning solvent are the same, a step of separating the obtained PAN-based polymer and re-dissolving in the spinning solvent becomes unnecessary.

  Moreover, you may mix water in a solvent from a viewpoint of suppressing gelation. The mixing amount of water is preferably 0.01 to 5% by weight with respect to the polymer solution. If it is less than 0.01% by weight, the effect of suppressing gelation is small, and if it exceeds 5% by weight, the polymer may be difficult to dissolve.

  The polymer concentration in the spinning solution is preferably in the range of 5 to 30% by weight, more preferably 14 to 25% by weight, and most preferably 18 to 23% by weight. If the polymer concentration is less than 5% by weight, the amount of solvent used increases, which is not economical, and the coagulation rate in the coagulation bath is lowered, voids are generated inside, and a dense structure may not be obtained. On the other hand, when the polymer concentration exceeds 30% by weight, the viscosity increases and spinning tends to be difficult. The polymer concentration of the spinning solution can be adjusted according to the amount of solvent used.

  In the present invention, the polymer concentration is the weight percent of the PAN polymer contained in the PAN polymer solution. Specifically, after weighing the PAN-based polymer solution, the measured PAN-based polymer solution is removed to a solvent that does not dissolve the PAN-based polymer and is compatible with the solvent used for the PAN-based polymer solution. After solvent, the PAN polymer is weighed. The polymer concentration is calculated by dividing the weight of the PAN polymer after desolvation by the weight of the PAN polymer solution before desolvation.

  Further, the viscosity of the spinning solution at a temperature of 45 ° C. is preferably in the range of 15 to 200 Pa · s, more preferably in the range of 20 to 150 Pa · s, and more preferably in the range of 30 to 100 Pa · s. Most preferably. When the solution viscosity is less than 15 Pa · s, since the formability of the spun yarn is lowered, the speed at which the yarn taken out from the die is taken, that is, the spinnability tends to be lowered. Moreover, when solution viscosity exceeds 200 Pa * s, it will become easy to gelatinize and the tendency for a filter medium to become obstruct | occluded easily will be shown. The viscosity of the spinning solution can be controlled by the amount of polymerization initiator or chain transfer agent.

  The viscosity of the PAN polymer solution at a temperature of 45 ° C. can be measured with a B-type viscometer. Specifically, the PAN-based polymer solution placed in a beaker is immersed in a hot water bath adjusted to a temperature of 45 ° C. to adjust the temperature, and as a B-type viscometer, for example, B8L manufactured by Tokyo Keiki Co., Ltd. Using a viscometer, rotor No. 4 is used, and the viscosity of the PAN polymer solution is in the range of 0 to 100 Pa · s. p. m. The viscosity of the PAN-based polymer solution is in the range of 100 to 1000 Pa · s. p. m. Measure with

  The spinning solution preferably has a storage elastic modulus G ′ of 0.1 to 2 Pa when the measurement temperature is 35 ° C. and the measurement angular velocity is 0.05 rad / s. The storage elastic modulus of the viscoelastic body depends on the measurement temperature and the measurement angular velocity. Here, the reason why the measurement angular velocity is set to 0.05 rad / s is that a large shear rate is not applied when flowing in a pipe or the like, and its representative value is shown. If the storage elastic modulus G ′ is less than 0.1 Pa, the effect of pushing away the staying spinning solution is small, and if it is more than 2 Pa, abnormal retention may be caused.

  In the present invention, the AN content ratio in the spinning solution is low in consideration of the environmental aspect, and the acid monomer is copolymerized with the PAN polymer in consideration of the promotion of flame resistance, so that the crosslinking reaction is promoted. Easy conditions. Although the molecular weight lowering and the cross-linking reaction with adjacent molecules proceed simultaneously in the staying part of the piping due to heat, the effect of the present invention cannot be obtained if the cross-linking reaction is fast and gelation occurs before the molecular weight lowers. Therefore, it is important that the spinning solution satisfies at least one requirement selected from the following [a] to [c] that suppresses the crosslinking reaction. However, conventionally, these requirements alone have often lacked the effect of suppressing gelation, but exhibit high effects when combined with the polymer in the present invention.

  The requirement [a] in the present invention is that, in the PAN-based polymer, the hydrogen ion of the carboxylic acid group derived from the acid monomer is contained in at least one ion selected from the group consisting of an ammonium ion and a sodium ion. Of 0 to 0.3 equivalents.

  Ammonia, sodium hydroxide, etc. are added to the polymer solution to neutralize the PAN-based polymer to become a spinning solution. The hydrogen ions of the carboxylic acid group derived from the acid monomer are ammonium ions and sodium ions. Since the hydrophilicity of the PAN-based polymer is improved by substitution with at least one ion selected from the group consisting of the above, the denseness when the spinning solution is discharged and solidified is improved. In the present invention, even if the requirement [b] and the requirement [c] described later are not provided, the amount to be substituted is 0 to 0.3 equivalent of the acid monomer used for the PAN polymer, preferably 0 to By reducing the amount to 0.1 equivalent, the effect of the present invention appears. When the amount is higher than 0.3 equivalent, gelation is promoted by cyclization of the PAN-based polymer, and the smaller the amount, the better. Further, when the requirement [a] and / or the requirement [b] described later is provided, 0.3 to 1.2 equivalents is also a preferable range. The larger the amount to be substituted, the better the hydrophilicity of the PAN-based polymer, and it is preferable, but when the amount is higher than 1.2 equivalents, gelation tends to be promoted. In addition, the process to replace should just be discharged from a nozzle | cap | die, and should just be mixed enough with a static mixer etc.

  The requirement [b] in the present invention is that the spinning solution contains an alkylating agent in an amount of 0.01 to 3% by weight based on the weight of the spinning solution.

  The alkylating agent in the present invention is an organic compound in which an alkyl group and an electronegative atom or substituent are bonded, and when mixed with an amine compound, alcohol compound, or thiol compound, a hydrogen atom on the nitrogen atom of the amine compound, A compound in which a hydrogen atom on an oxygen atom of an alcohol compound and a hydrogen atom on a sulfur atom of a thiol compound are substituted with an alkyl group. Preferred examples of the alkylating agent include sulfuric acid ester, sulfonic acid ester, phosphoric acid triester, phosphoric acid diester, phosphoric acid monoester, phosphonic acid diester, phosphonic acid monoester, phosphinic acid ester, and carbonate ester. More preferably, the alkylating agent is dimethyl methylphosphonate, diethyl methylphosphonate, dimethyl ethylphosphonate, diethyl ethylphosphonate, dimethyl 2-oxopropylphosphonate, diethyl cyanophosphonate, dimethyl allylphosphonate, dimethyl hydroxyethylphosphonate, phosphoric acid. Examples include trimethyl, triethyl phosphate, trimethyl phosphonoacetate, triethyl phosphonoacetate, methyl methanesulfonate, methyl ethanesulfonate, ethyl methanesulfonate, methyl propanesulfonate, dimethyl sulfate, diethyl sulfate, dimethyl carbonate, diethyl carbonate, and ethylene carbonate. . In particular, as the alkylating agent, trimethyl phosphate, triethyl phosphate, dimethyl carbonate, diethyl carbonate, and ethylene carbonate are preferably used from the viewpoint of alkylation reaction activity and handling safety. Among them, trimethyl phosphate is preferable from the viewpoint of alkylation reaction activity. Is more preferable. The mechanism by which the gelation is suppressed by the alkylating agent is not always clear, but the alkylating agent reacts with the amino group end generated by the cyclization of the PAN polymer in the spinning solution to produce an alkylamine, and the adjacent molecule. This is considered to suppress the crosslinking reaction with the nitrile group. Furthermore, since the alkylamine produced | generated here contributes to molecular weight fall, it suppresses the production | generation of a gel more effectively.

  The alkylating agent in the present invention is preferably soluble in the solvent used as the PAN polymer solution in order to be uniformly mixed with the PAN polymer in the spinning solution. Moreover, it is preferable that it does not remain in the fiber after spinning, and it is preferable that it is soluble in a coagulant. Examples of the coagulant include water and ethanol, but water is preferable from the viewpoint of safety.

  In requirement [b], the content of the alkylating agent is preferably 0.02 to 2% by weight, more preferably 0.03 to 1% by weight, based on the weight of the spinning solution. If the content is less than 0.01% by weight and neither the requirement [a] nor the requirement [c] is satisfied, the effect of thermal stability is reduced, and the component having a large molecular weight in the PAN-based polymer of the present invention is reduced. Gelation occurs before the molecular weight is reduced, and the effect of suppressing gel formation is reduced. When the content exceeds 3% by weight, the acid after the alkylation reaction reduces the hydrogen ion dissociation of the acid monomer. The hydrophilicity of the PAN-based polymer is lowered, the physical properties of the precursor fiber are affected, and the quality of the precursor fiber is lowered.

  In the present invention, when an alkylating agent is contained in the spinning solution, the alkylating agent may be added anywhere after polymerization until discharge from the spinneret, but there is a possibility that retention will occur immediately after polymerization. It is added so that it becomes 0.01 to 3% by weight in the spinning solution at a place closest to the polymerization reaction other than the place where the polymerization reaction is performed. In order to mix the alkylating agent, only the alkylating agent may be directly mixed with the polymer solution, or an acrylating agent solution once dissolved in the same solvent as the solvent used for the polymer solution is mixed. However, when the alkylating agent is solid at room temperature, that is, powder, it is preferable from the viewpoint of uniformity to mix an acrylating agent solution previously dissolved in a solvent. When the alkylating agent is mixed with the polymer solution, for example, a mixing device such as a static mixer, an extruder or a kneader is preferably used.

  The requirement [c] in the present invention is that the spinning solution contains 0.01 to 3% by weight of an organic carboxylic acid having a boiling point of 200 ° C. or more per weight of the spinning solution. When such boiling point is low, separation of the organic carboxylic acid and the solvent becomes difficult in the solvent recovery step usually employed in solution spinning, which affects the polymerization. It is important to use an acid. When a mineral acid is used, the mineral acid remains in the carbon fiber, which may reduce the carbon content of the carbon fiber and cause a defect. Moreover, in order to make it easy to wash with water, it is preferable to use water-soluble organic carboxylic acid. Examples of organic carboxylic acids used in the requirement [c] include (anhydrous) maleic acid, (anhydrous) fumaric acid, malic acid, succinic acid, citric acid and the like, and particularly from the number of carboxylic acid groups per molecular weight, Citric acid is preferably used. The mechanism for suppressing gelation by acid is presumed as follows. That is, it is considered to neutralize the terminal end of the amino group generated by cyclization of the PAN polymer in the spinning solution and suppress the crosslinking reaction with the nitrile group of the adjacent molecule.

  In the requirement [c], the content of the organic carboxylic acid having a boiling point of 200 ° C. or higher is preferably 0.02 to 2% by weight, more preferably 0.03 to 1% by weight, based on the weight of the spinning solution. If the content is less than 0.01% by weight, if neither requirement [a] nor [b] is satisfied, the effect of thermal stability is reduced, and the molecular weight reduction of the component having a large molecular weight in the PAN-based polymer is reduced. Gelation occurs first, and the effect of suppressing gel formation is reduced. If the content exceeds 3% by weight, the physical properties of the precursor fiber are affected, and the quality of the precursor fiber is lowered.

  In the present invention, when an organic carboxylic acid is contained in the spinning solution, the organic carboxylic acid may be added anywhere from after polymerization until discharge from the spinneret, but there is a possibility that retention will occur immediately after polymerization. For example, it is added to the tank to be used before the polymerization reaction or during the reaction, so that it is added as much as possible in an amount of 0.01 to 3% by weight in the spinning solution. In order to mix the organic carboxylic acid, only the organic carboxylic acid may be mixed directly with the polymer solution, or an acidic solution once dissolved in the same solvent as the solvent used for the polymer solution may be mixed. Although it is good, when the organic carboxylic acid to be used is a solid at room temperature, that is, a powder, it is preferable from the viewpoint of uniformity to mix an acidic solution previously dissolved in a solvent. When the organic carboxylic acid is mixed with the polymer solution, for example, a mixing device such as a static mixer, an extruder or a kneader is preferably used.

  Moreover, it is preferable to substantially eliminate oxygen in the solution by deoxygenation treatment. In the presence of oxygen, gelation by crosslinking is promoted. One specific method of deoxygenation is, for example, a method of substituting oxygen with an inert gas by pressurizing or depressurizing an inert gas. While stirring the spinning solution in a well-sealed container, reduce the pressure until the solution boils or until it boils lightly, and then introduce an inert gas such as nitrogen, helium, or argon into the stirring container. The pressure is set to 0.1 to 0.2 MPa. Again, pressure reduction until boiling and pressurization with an inert gas are repeated under stirring conditions. This pressurization and depressurization with an inert gas under the stirring condition is repeated twice or more, preferably four times or more. When deoxygenation is performed in this manner, the present invention defines that there is substantially no oxygen. Deoxygenation treatment includes not only inert gas replacement but also a method using a chemical reaction with oxygen and a method of adsorbing oxygen, and can be used according to the purpose. However, as the deoxygenation method, the method of repeating the above-described pressurization and depressurization with an inert gas a plurality of times under the stirring conditions is most preferable in terms of workability and completeness of deoxygenation. In particular, it is effective when including a step of dissolving the polymer in a solvent.

  In the present invention, prior to spinning the spinning solution as described above, it is also preferable to remove the polymer raw material and impurities mixed in each step through a filter device. The filter device in the present invention means a facility for filtering and removing foreign matters present in the spinning solution, an inflow path for guiding the spinning solution to be filtered into the filter device, and for filtering the spinning solution. Filter medium, an outflow path for guiding the filtered spinning solution to the outside of the filter device, and a container for storing these. Here, the filter medium is a means for filtering the spinning solution stored in the filter device. The filtration method preferably used in the present invention will be described below.

Foreign substances are roughly classified into solid and gel-like substances, but the influence of the gel-like substances is large, and it is preferable to remove them by filtration before reaching the die, but the gel-like substances are easily deformed by filtration pressure and easily pass through the filter medium. . When the filtration pressure is high, the gel-like material is deformed and easily passes through the filter medium. Therefore, a method for accurately filtering the gel-like material with a low filtration pressure is required. In the method of filtering with a filter medium having a large thickness and fineness, the pressure applied to the polymer solution during filtration is high, and the gel-like substance may flow out, which is not preferable. Specifically, the filter medium has a filtration guarantee layer with a basis weight A (g / m ² ) and material density B (g / m ³ ) satisfying the following formula, and the filtration resistance coefficient C is 5-30. By using a filter medium having a size of × 10 ⁵ cm ⁻¹ , it is preferable to accurately filter the gel-like material while preventing deformation of the gel-like material with a low filtration pressure.
0.01 ≦ A / B × 1000 ≦ 0.06
Some filter media have a uniform wire diameter and filling rate along an axis perpendicular to the filter surface, but many are obtained by laminating a plurality of layers having different wire diameters and filling rates. The filtration guarantee layer refers to a layer having the finest pore diameter among layers constituting the filter medium, that is, a layer having the highest filtration accuracy. In the case where there are intermittently a plurality of filtration guarantee layers having the same filtration accuracy, the integrated value is used. The basis weight of the filtration guarantee layer is shown as a mass (g) per 1 m ^{2 of the} filtration guarantee layer, that is, g / m ² . The filtration guarantee layer generally has a mechanical strength by fixing the structure through a process of sintering that promotes crystallization at the contact portion of the metal fiber. Even when the filtration guarantee layer is produced by sintering, the change in weight and area before and after sintering is negligible. Use. Although the value obtained by taking out only the filtration assurance layer portion after sintering may be used, when the basis weight of the filtration assurance layer before and after sintering is different, the value before sintering is preferentially used. The filtration resistance coefficient C is a value indicating the difficulty of passing the fluid through the filter medium, which can be calculated from the air flow per unit time.

  In order to reduce the outflow due to deformation or pulverization of the gel-like material existing in the spinning solution on the filter medium, it is necessary to reduce the shear stress applied to the gel-like material. Since the shear stress has a positive correlation with the pressure loss, it is effective to reduce the pressure loss in order to avoid the outflow of the gel. For this purpose, it is preferable to reduce the filtration resistance coefficient C which is proportional to the pressure loss. The filtration resistance coefficient depends on the pore size, filling rate, thickness, etc. of the filter medium.

  In particular, the filtration resistance coefficient can be effectively reduced by adjusting the filling rate of the filter medium as described later. There is a positive correlation between the filling rate of the filter medium and the filtration resistance coefficient. The higher the filling rate, the larger the filtration resistance coefficient. There is a problem that simply lowering the filling rate in order to reduce the filtration resistance coefficient causes a decrease in filtration accuracy. Therefore, by reducing the wire diameter of the metal fiber forming the filter medium to 0.5 to 2.5 μm. The filtration resistance coefficient may be reduced.

  A / B corresponds to the thickness (unit: m) when it is assumed that the filtration guarantee layer is compressed so that the aperture is zero, and hereinafter referred to as filtration guarantee layer compression thickness. A / B × 1000 is obtained by converting the filtration guarantee layer compression thickness into mm units. The compression guaranteed layer compression thickness can be used as a constant standard for the thickness regardless of the aperture diameter of the filtration guaranteed layer.

When the compression guarantee layer compression thickness is less than 0.01 mm, the filter medium may not be able to secure the strength against the pressure loss that acts during filtration, and the filter medium may be broken, and the gel material may flow out. However, if it exceeds 0.06 mm, the filtration pressure may become too high. When the filtration guarantee layer is made of stainless steel, B is about 8 × 10 ⁶ g / m ^3, so the compression guarantee layer compression thickness is adjusted within the above range by controlling A to be 80 to 500 g / m ^2. Good. Moreover, when the filtration resistance coefficient C is less than 5 × 10 ⁵ cm ⁻¹ , the pore diameter is large, and a fine gel-like material easily passes through the eye. When the filtration resistance coefficient C exceeds 30 × 10 ⁵ cm ⁻¹ , Pressure loss is high and gelled material may flow out.

  Specifically, as a filter medium that satisfies the above-mentioned conditions, a wire having a wire diameter of 10 to 25 μm is used in a twill woven, warp yarn is 500 to 800 mesh, preferably 635 to 800 mesh, and weft yarn is 3500 to 6000 mesh, preferably 4300. Examples thereof include a wire mesh that is woven as ˜6000 mesh, and a single layer or a plurality of layers are laminated and sintered. Moreover, it is also possible to use a sintered metal nonwoven fabric as a filter medium that satisfies the above conditions. Since a nonwoven fabric with a small wire diameter can be used, there is an advantage that the aperture diameter can be easily reduced, which is preferable. On the other hand, in order to compensate for the wide distribution of pore diameters that are common in non-woven fabrics, the thickness may be increased, but the number of filtration stages can be increased by reducing the wire diameter to 0.5 to 2.5 μm without increasing the thickness. Therefore, it is often solved. For non-woven fabrics, metal fibers and powders with an aspect ratio of 1 to 1000 can be used according to the purpose, and the pore diameter can be controlled. Mixing metal fibers and powders with different aspect ratios is also an effective means. It is.

  As the sintering, a conventional method may be used, and it is preferable to control the pore diameter of the woven fabric or the nonwoven fabric by adjusting the sintering time, the sintering temperature, or the number of times of sintering.

  The filtration accuracy of the filter medium, which is closely related to the aperture diameter, is preferably 1 to 10 μm, more preferably 1 to 5 μm, and still more preferably 1 to 3 μm. If the filtration accuracy becomes too high, foreign matter in the spinning solution after passing through the filter medium increases, which is not preferable. Therefore, the smaller the better, the smaller is 1 μm. The filtration accuracy of the filter medium in the present invention is defined by the particle diameter (diameter) of spherical particles capable of collecting 95% while passing through the filter medium. The measurement method conforms to JIS B8356, and a standard material is used for the particles.

The filtration rate S per unit filtration area (m ² ) of the filter medium is preferably 1 to 150 L / m ² / hour, more preferably 1 to 40 L / m ² / hour, and still more preferably 1 to 20 L / m ^2. Filter as / hour. When the speed is less than 1 L / m ² / hour, the filtration area becomes too large and gelation may occur due to polymer solution retention, and when it exceeds 150 L / m ² / hour, the spinning solution contains If a component having a high molecular weight is contained, it is filtered. By reducing the filtration rate, the filtration pressure can be reduced.

In addition, the filtration rate S per unit filtration area (m ² ) of the filter medium is often closely related to the filtration accuracy. There is a relationship between the filter filtration accuracy and the aperture diameter, and it is common to increase the filtration accuracy by narrowing the aperture diameter. When the pore diameter is narrow, the shear rate when passing through the spinning solution increases and the filtration pressure increases, so the spinning solution filtration rate S (L / m ² / hour) per unit filtration area (m ² ) of the filter medium and the filtration accuracy F (Μm) preferably satisfies the relationship of the following formula.

1 × F ≦ S ≦ 15 × F
That is, when a filter medium with high filtration accuracy is used, it is preferable to lower the filtration rate and lower the shear rate. When the filtration rate is slower than such a relational expression, the filtration area becomes excessively large and excessive specifications are often obtained, and when the filtration speed is faster than the relational expression, the filtration pressure is often increased.

  The value of the residence time V / W in the filter medium indicated by the ratio between the effective volume V (L) of the filter medium and the polymer solution filtration flow rate W (L / min) may be set to 0.01 to 10 minutes. Preferred for filtering the product. Here, the effective volume V of the filter medium represents the spatial volume of the filter medium. Such a volume is calculated from the filter filtration area and the thickness of the filter medium. Although the filter medium may have a layer structure, it refers to the thickness of the narrowest part of the filter medium with the pore diameter. The polymer solution filtration flow rate W represents the volume flow rate of the spinning solution passing through the filter medium per minute, and the residence time V / W in the filter medium represents the average residence time of the spinning solution passing through the filter medium. The V / W may be controlled by controlling the thickness of the filter medium and the filtration speed. When the V / W exceeds 10 minutes, a gel-like material is often generated due to retention, and conversely, the V / W is If it is less than 0.01 minutes, the filtration pressure increases, which is not preferable.

  The effect of the present invention is improved by using a filtration method capable of accurately filtering the gel with respect to the spinning solution used in the present invention, in which the generation of the gel is unlikely to occur.

  In the present invention, the carbon fiber precursor fiber is produced by spinning the spinning solution filtered as described above by a dry, wet, or dry-wet spinning method. In particular, the dry and wet spinning method is preferably used because it exhibits the characteristics of the PAN-based polymer having the specific molecular weight distribution described above.

  The diameter of the die hole used for spinning is preferably 0.05 mm to 0.3 mm, and more preferably 0.1 to 0.15 mm. When the diameter of the nozzle hole is smaller than 0.05 mm, shearing stress is applied to the spinning solution, which may block the nozzle. On the other hand, when the diameter of the die hole exceeds 0.3 mm, it may be difficult to obtain a fiber having a single fiber fineness of 1.5 dtex or less.

  In the present invention, the coagulation bath preferably contains a solvent such as dimethyl sulfoxide, dimethylformamide, and dimethylacetamide used as a solvent for the PAN polymer solution and a so-called coagulation promoting component. As the coagulation accelerating component, a component that does not dissolve the PAN-based polymer and is compatible with the solvent used in the PAN-based polymer solution is preferable. Specifically, it is preferable to use water. The conditions as the coagulation bath are preferably controlled so that the cross section of the single fiber in the coagulated yarn is as close to a perfect circle as possible, and the concentration of the solvent is preferably not more than a concentration called a critical bath concentration. If the concentration of the solvent is high, the subsequent solvent washing step becomes long and the productivity is lowered. For example, when dimethyl sulfoxide is used as the solvent, the concentration of the dimethyl sulfoxide aqueous solution is preferably 5 to 55% by weight, and more preferably 5 to 30% by weight. The temperature of the coagulation bath is preferably controlled so that the fiber side surface is as smooth as possible. Specifically, the temperature is preferably −10 to 30 ° C., more preferably −5 to 5 ° C.

  Further, it is preferable to dissolve ammonia in the coagulation bath and increase the pH of the coagulation bath in terms of increasing the hydrophilicity of the PAN-based polymer, and it is preferable to control the pH to be 8 to 13. The pH of the coagulation bath is preferably 9-12, more preferably 9-11.

  The spinning solution is introduced into a coagulation bath and coagulated to form a yarn to obtain a coagulated yarn, which is then taken up by a roller having a drive source, and the take-up speed of the coagulated yarn is 50 to 500 m / min. Is preferable from the viewpoint of exhibiting the characteristics of the PAN-based polymer having the specific molecular weight distribution described above. When the take-up speed is less than 50 m / min, the productivity is lowered, and when the take-up speed exceeds 500 m / min, the liquid level fluctuation of the coagulation bath becomes remarkable and the resulting fineness tends to be uneven.

  In the present invention, the single fiber fineness of the dried coagulated yarn is determined by the polymer concentration of the polymer solution containing the PAN polymer, the spinning draft, and the pore diameter of the spinneret used. The spinning draft is represented by the following formula.

Spinning draft = (coagulated yarn take-up speed) / (discharge line speed)
The single fiber fineness of the dried coagulated yarn of the present invention is preferably 0.4 to 2.3 dtex, more preferably 0.4 to 1.8 dtex, still more preferably 0.4 to 0.00. 9 dtex. The single fiber fineness of the dried coagulated yarn corresponds to the fiber diameter of the single fiber constituting the coagulated yarn at the time of coagulation, and the larger the single fiber diameter, the difference in solidification state between the surface layer and the center of the coagulated yarn during coagulation When the carbon fiber is baked to produce a carbon fiber, a difference in physical properties due to it is observed, and the physical properties are often lowered. In addition, when any of the above requirements [a] to [c] is satisfied, the hydrophilicity of the PAN-based polymer is lowered and the solidification state difference may be promoted. The smaller the fineness, the smaller the influence of the decrease in hydrophilicity of the PAN-based polymer, which is preferable. When the single fiber fineness of the dried coagulated yarn exceeds 2.3 dtex, the larger the single fiber diameter, the greater the difference in the coagulation state between the surface layer portion and the center portion of the coagulated yarn during coagulation, and firing into carbon fibers In some cases, a difference in physical properties due to the difference is observed, and the physical properties are often lowered. Moreover, when the single fiber fineness of the dried coagulated yarn is less than 0.4 dtex, the single fiber fineness of the precursor fiber as the precursor fiber is less than 0.4 dtex. When the fiber reinforced composite material is used, the matrix resin is hardly impregnated, and the physical properties of the carbon fiber are not expressed, and the physical properties of the carbon fiber reinforced composite material are reduced for the manufacturing cost.

  In the present invention, the dried coagulated yarn having a single fiber fineness of 0.4 to 2.3 dtex is obtained by averaging a PAN polymer solution having a PAN polymer concentration of 6 to 25% by weight on an average basis. A spinneret having a pore diameter of 0.05 to 0.18 mm can be suitably obtained by discharging and spinning so as to control the spinning draft to 8 to 100.

  The single fiber fineness (dtex) of the dried coagulated yarn in the present invention means that the coagulated yarn taken up by a roller having a drive source that first contacts after leaving the spinneret is washed with running water for 1 hour or more, and single fiber The dry weight (g) per 10,000 m.

  Thereafter, the taken-up coagulated yarn is usually subjected to a water washing step, an in-bath drawing step, an oil agent application step, and a drying step to obtain a carbon fiber precursor fiber. Moreover, you may add a dry heat extending process and a steam extending process to said process. The solidified yarn may be directly stretched in the bath without the water washing step, or may be stretched in the bath after removing the solvent by the water washing step. Usually, the stretching in the bath is preferably performed in a single or a plurality of stretching baths adjusted to a temperature of 30 to 98 ° C. The draw ratio at that time is preferably 1 to 5 times, and more preferably 1 to 3 times.

  After the stretching step in the bath, it is preferable to apply an oil agent made of silicone or the like to the stretched fiber yarn for the purpose of preventing adhesion between single fibers. As the silicone oil, it is preferable to use a silicone oil containing a modified silicone such as amino-modified silicone having high heat resistance.

  As the drying step, for example, a drying condition in which a drying temperature is 70 to 200 ° C. and a drying time is 10 seconds to 200 seconds gives preferable results. As an improvement in productivity and an improvement in the degree of crystal orientation, it is preferable to stretch in a heating heat medium after the drying step. As the heating heat medium, for example, pressurized steam or superheated steam is suitably used in terms of operational stability and cost, and the draw ratio is usually 1.5 to 10 times.

  The single fiber fineness of the carbon fiber precursor fiber thus obtained is preferably 0.01 to 1.5 dtex, more preferably 0.05 to 1.0 dtex, 0.1 to 0 More preferably, it is .8 dtex. If the single fiber fineness is too small, the process stability of the yarn making process and the carbon fiber firing process may decrease due to the occurrence of yarn breakage due to contact with a roller or a guide. On the other hand, if the single fiber fineness is too large, the difference between the inner and outer structures of each single fiber after flame resistance will increase, and the processability in the subsequent carbonization process and the tensile strength and tensile modulus of the resulting carbon fiber will decrease. There is.

  The obtained carbon fiber precursor fiber is usually in the form of a continuous fiber (filament). The number of filaments per one yarn (multifilament) is preferably 1,000 to 3,000,000, more preferably 12,000 to 3,000,000, and still more preferably. The number is 24,000 to 2,500,000, and most preferably 24,000 to 2,000,000. Since the obtained carbon fiber precursor fiber has high drawability, the single fiber fineness can be made smaller than before, while the number of filaments per yarn is larger for the purpose of improving productivity. Although it is preferable, if it is too much, it may not be possible to uniformly treat the inside of the bundle.

  The carbon fiber precursor fiber obtained as described above can be made into carbon fiber according to a conventional method. That is, the carbon fiber precursor fiber obtained as described above is subjected to flame resistance treatment in air at a temperature of 200 to 300 ° C., and then pre-carbonized in an inert atmosphere at a temperature of 300 to 800 ° C. Then, carbonization is performed in an inert atmosphere at a temperature of 1,000 to 3,000 ° C. In particular, from the viewpoint of increasing the tensile modulus of carbon fiber, the tension in carbonization is preferably 5.9 to 13.0 mN per unit fineness (dtex) of the precursor. Since a carbon fiber precursor fiber is obtained, problems such as fluffing are unlikely to occur in the firing process.

Hereinafter, the present invention will be described more specifically with reference to examples. The measurement method used in this example will be described next.
<Various molecular weights: Mz, Mw, Mn>
A sample solution is prepared by dissolving 0.1% by weight of the polymer to be measured in dimethylformamide (0.01N-lithium bromide added). For the prepared specimen solution, a molecular weight distribution curve is obtained from a GPC curve measured under the following conditions using a GPC apparatus, and Mz, Mw, and Mn are calculated. The measurement was performed three times, and the values of Mz, Mw, and Mn were averaged and used.
-Column: Column for polar organic solvent GPC-Flow rate: 0.5 ml / min-Temperature: 75 ° C
・ Sample filtration: Membrane filter (0.45μm cut)
・ Injection volume: 200 μl
・ Detector: Differential refractive index detector Mw uses at least six types of monodispersed polystyrenes with different molecular weights and known molecular weights to create an elution time-molecular weight calibration curve, and corresponds to the corresponding elution time on the calibration curve. It is obtained by reading the molecular weight in terms of polystyrene.

In this example, CLASS-LC2010 manufactured by Shimadzu Corporation as a GPC device, and TSK-GEL-α-M (× 2) manufactured by Tosoh Corporation as a column and TSK-guard Column α manufactured by Tosoh Corporation, Calibration curves were manufactured by Wako Pure Chemical Industries, Ltd. as dimethylformamide and lithium bromide, 0.45 μm-FHLP FILTER manufactured by Millipore Corporation as a membrane filter, and RID-10AV manufactured by Shimadzu Corporation as a differential refractive index detector. As the monodisperse polystyrene for production, those having molecular weights of 184000, 427000, 791000, 1300000, 1810000 and 4240000 were used, respectively.
<Content ratio of AN in spinning solution>
4.0 g of methanol was added to 1.0 g of the spinning solution, and AN in the PAN-based polymer solution was extracted. The obtained methanol solution was determined by measuring the corresponding AN concentration from the calibration curve by gas chromatography analysis.
Gas chromatography condition apparatus: GC-2014 (manufactured by Shimadzu Corporation)
Column: Capillary column DB-1 (manufactured by Shimadzu GLC)
Sample injection volume: 0.4 μL
<Gel generation amount>
A polymer solution maintained at 50 ° C. is filled from the center of the bottom of a cylindrical container having a diameter of 6 cm and a length of 10 cm filled with a cascade mini-ring having a diameter of 1 cm through a pipe having a diameter of 1 cm and a length of 5 cm. Thereafter, the temperature of the 300 cm ³ container is raised to 120 ° C. in about 1 hour. After completion of the temperature increase, the solution is fed at 1 cc / min, and then filtered for 24 hours with a plain woven PET screen with a 28 cm ² circular 32 μm aperture diameter arranged on the outlet side of the cylindrical container. Further, DMSO is fed at 10 cc / min, and the gel and polymer solution in the pipe are washed for 24 hours and filtered through a screen bottle. The screen with the gel adhered was dried, and bright spots were counted with the naked eye while applying light with a wavelength of 354 nm.
<Tensile strength and elastic modulus of carbon fiber bundle>
Determined according to JIS R7601 (1986) “Resin-impregnated strand test method”. The carbon fiber resin-impregnated strand to be measured is 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexyl-carboxylate (100 parts by weight) / 3 boron trifluoride monoethylamine (3 parts by weight) / acetone (4 parts by weight). ) Is impregnated into carbon fiber or graphitized fiber and cured at a temperature of 130 ° C. for 30 minutes. The number of carbon fiber strands to be measured is 6, and the average value of each measurement result is the tensile strength. In this example, “Bakelite” (registered trademark) ERL4221 manufactured by Union Carbide Co., Ltd. was used as 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexyl-carboxylate.
[Example 1]
100 parts by weight of AN, 1 part by weight of itaconic acid, 0.003 part by weight of octyl mercaptan as a chain transfer agent, and 360 parts by weight of dimethyl sulfoxide were mixed and placed in a reaction vessel equipped with a reflux tube and a stirring blade. After the space in the reaction vessel was purged with nitrogen until the oxygen concentration reached 100 ppm, 0.003 part by weight of AIBN was added as a polymerization initiator, and heat treatment was performed under the following conditions while stirring.
(1) Hold at 60 ° C. for 3.5 hours Next, in the reaction vessel, 10 parts by weight of dimethyl sulfoxide, 0.4 parts by weight of AIBN as a polymerization initiator, and 0.1 parts by weight of octyl mercaptan as a chain transfer agent Then, heat treatment under the following conditions was performed with further stirring, and the remaining unreacted monomer was polymerized by a solution polymerization method to obtain a PAN polymer solution.
(2) Hold for 4 hours at a temperature of 60 ° C. (3) Increase the temperature from 60 ° C. to 80 ° C. (temperature increase rate: 10 ° C./hour)
(4) Hold for 6 hours at a temperature of 80 ° C. Trimethyl phosphate is added to the polymer solution as a 50% by weight solution of dimethyl sulfoxide, and the PAN-based polymer solution is decompressed to 80 ° C. and 0.1 kPa while stirring. Time processed. After preparing the polymer concentration to be 20% by weight, neutralize it by blowing ammonia gas so that it becomes 0.8 equivalent to the carboxyl group of itaconic acid in the polymer, and then mix with a static mixer Thus, a spinning solution having a viscosity at 45 ° C. of 45 Pa · s was obtained. The PAN-based polymer had Mw of 460,000 and Mz / Mw of 3.4. The content ratio of trimethyl phosphate in the spinning solution was 0.1% by weight, and the AN content ratio in the spinning solution was 0.01% by weight.

  The obtained spinning solution is put into a heat-resistant pressure-resistant glass bottle, stoppered, and treated in an oven at 120 ° C. for 12 hours. The viscosity at 45 ° C. is 40 Pa · s, Mw is 360,000, and Mz / Mw is 1.6. became. Furthermore, after 20 hours of treatment for 8 hours, the whole gelled, making it impossible to measure viscosity and dissolve in a solvent.

  Moreover, when the amount of gel production | generation was tested using the obtained spinning solution, the amount of gel production | generation was as very few as five pieces.

The obtained spinning solution was held at 120 ° C. in a 300 cm ³ container filled with a cascade mini-ring having a diameter of 1 cm using the same apparatus as in the gel production test described above, and fed at 1 cc / min for 48 hours. After the process of remixing into the spinning solution without filtering through a screen (this process is called forced gelation process), the filter has a pore number of 1,500 at a temperature of 40 ° C. without filtering through a filter. , Using a spinneret with a base diameter of 0.15 mm, once discharged into the air, passed through a space of about 2 mm, and then introduced into a coagulation bath consisting of an aqueous solution of 20% by weight dimethyl sulfoxide controlled at a temperature of 3 ° C. Spinning was performed by a dry and wet spinning method to obtain a coagulated yarn. The limit spinning draft rate at which yarn breakage occurred was measured by adjusting the amount of liquid fed to the die so that the discharge linear velocity was 5 m / min and changing the winding speed of the coagulated yarn. Moreover, after obtaining a coagulated yarn under the condition of a spinning draft ratio of 4, and washing with water, it was stretched at a stretching ratio of 3 times in a bath at 90 ° C. in warm water, and further provided with an amino-modified silicone-based silicone oil, at 165 ° C. Drying was performed for 30 seconds using a roller heated to a temperature, and pressure steam stretching was performed under a steam stretching ratio condition of 5 times to obtain a carbon fiber precursor fiber having a single fiber fineness of 0.8 dtex. In addition, the yarn when washed with water was only slightly cloudy, and the denseness was high. The quality of the obtained carbon fiber precursor fiber was excellent, and the yarn passing through the spinning process was stable. The obtained carbon fiber precursor fiber is combined so that the number of filaments becomes 12,000, and flameproofing is performed for 90 minutes while stretching at a draw ratio of 1.0 in air having a temperature distribution of 240 to 260 ° C. As a result, flameproofed fibers were obtained. Subsequently, in the nitrogen atmosphere having a temperature distribution of 300 to 700 ° C., the obtained flame-resistant fiber is subjected to preliminary carbonization while being drawn at a draw ratio of 1.2, and further in a nitrogen atmosphere having a maximum temperature of 1500 ° C. , Carbonization was performed with the draw ratio set to 0.97 to obtain continuous carbon fibers. In this case, the baking process passability was good.

When the strand physical property of the obtained carbon fiber bundle was measured, the strength was 6.3 GPa.
[Example 2]
Instead of the 1% by weight solution of trimethyl phosphate in dimethyl sulfoxide, a 33% by weight solution of dimethyl sulfoxide in citric acid was used, and the content ratio of citric acid in the spinning solution was changed to 0.25% by weight. Thus, a carbon fiber bundle was obtained. The obtained spinning solution was put into a heat-resistant pressure-resistant glass bottle, stoppered, and treated in an oven at 120 ° C. for 15 hours to form a gel. In addition, the amount of gel produced in the gel production amount test was as small as 16 pieces. In addition, although the yarn when washed with water was cloudy and the density was low, the processability was good in both the yarn making process and the firing process, and the obtained carbon fiber precursor fiber and carbon fiber bundle were The quality was good.
[Example 3]
A carbon fiber bundle was obtained in the same manner as in Example 1 except that trimethyl phosphate was not added and neutralization with ammonia gas was not performed. The obtained spinning solution was put into a heat-resistant pressure-resistant glass bottle, stoppered, and treated in an oven at 120 ° C. for 13 hours to form a gel. In addition, the amount of gel produced in the gel production amount test was as small as 30 pieces. In addition, although the yarn when washed with water was cloudy and the density was low, the processability was good in both the yarn making process and the firing process, and the obtained carbon fiber precursor fiber and carbon fiber bundle were The quality was good.
[Example 4]
A carbon fiber bundle was obtained in the same manner as in Example 3 except that the spinning draft ratio was changed to 20 and no pressurized steam stretching was performed. The dried coagulated yarn single fiber fineness was 2.1 dtex, the yarn when washed with water was transparent, and the density was high even without neutralization with ammonia gas. The process passability was good in both the yarn making process and the firing process, and the quality of the obtained carbon fiber precursor fiber and carbon fiber bundle was good.
[Comparative Example 1]
A spinning solution was obtained in the same manner as in Example 1 except that trimethyl phosphate was not added. The obtained spinning solution was put into a heat-resistant pressure-resistant glass bottle, stoppered, and treated in an oven at 120 ° C. for 6 hours. Further, the amount of gel produced in the gel production amount test was so large that it could not be counted. In addition, an attempt was made to obtain a carbon fiber bundle using the obtained spinning solution in the same manner as in Example 1, but sampling failed due to frequent discharge failures and yarn breakage from the die during the yarn making process.
[Example 5]
The decompression pressure was changed to 1 kPa so that the AN content ratio in the spinning solution was 0.08% by weight, and ammonia gas was adjusted to 0.3 equivalent to the carboxyl group of itaconic acid in the polymer. A carbon fiber bundle was obtained in the same manner as in Example 3 except that was neutralized by blowing. The obtained spinning solution was put into a heat-resistant pressure-resistant glass bottle, stoppered, and treated in an oven at 120 ° C. for 16 hours to form a gel. In addition, the amount of gel produced in the gel production amount test was as small as 25. In addition, the processability was good in both the yarn production process and the firing process, and the quality of the obtained carbon fiber precursor fiber and carbon fiber bundle was good.
[Comparative Example 2]
100 parts by weight of AN, 1 part by weight of itaconic acid, 0.4 part by weight of AIBN as a radical initiator, and 0.1 part by weight of octyl mercaptan as a chain transfer agent are uniformly dissolved in 370 parts by weight of dimethyl sulfoxide, which is stirred with a reflux tube. Placed in a reaction vessel with wings. After replacing the space in the reaction vessel with nitrogen, heat treatment was performed under polymerization condition A while stirring, and polymerization was performed by a solution polymerization method to obtain a PAN polymer solution.
(1) Temperature increase from 30 ° C to 60 ° C (temperature increase rate 10 ° C / hour)
(2) Hold for 4 hours at a temperature of 60 ° C. (3) Increase the temperature from 60 ° C. to 80 ° C. (temperature increase rate: 10 ° C./hour)
(4) Hold for 6 hours at a temperature of 80 ° C. The PAN-based polymer had Mw of 350,000 and Mz / Mw of 1.8. A spinning solution was obtained in the same manner as in Comparative Example 1, except that the PAN-based polymer solution was changed to that obtained as described above. The obtained spinning solution had a viscosity at 45 ° C. of 55 Pa · s. The obtained spinning solution was put into a heat-resistant pressure-resistant glass bottle, stoppered, and treated in an oven at 120 ° C. for 6 hours. The whole gelled and the gelation rate was the same even if the molecular weight distribution of the polymer changed. . Further, the amount of gel produced in the gel production amount test was so large that it could not be counted. In addition, an attempt was made to obtain a carbon fiber bundle using the obtained spinning solution in the same manner as in Example 1, but sampling failed due to frequent discharge failures and yarn breakage from the die during the yarn making process.
[Comparative Example 3]
A spinning solution was obtained in the same manner as in Comparative Example 2 except that the reduced pressure was changed to 1 kPa and the reduced pressure treatment time was changed to 1 hour so that the AN content ratio in the spinning solution was 0.4% by weight. .

  The obtained spinning solution is put into a heat-resistant pressure-resistant glass bottle, stoppered, and treated in an oven at 120 ° C. for 12 hours. The viscosity at 45 ° C. is 50 Pa · s, Mw is 300,000, and Mz / Mw is 1.8. became. Furthermore, after 20 hours of treatment for 8 hours, the whole gelled, making it impossible to measure viscosity and dissolve in a solvent.

  Moreover, when the amount of gel production | generation was tested using the obtained spinning solution, the amount of gel production | generation was as very small as one piece.

A carbon fiber bundle was obtained in the same manner as in Example 1 using the obtained spinning solution. Around the coagulation bath, there was a problem that the working environment concentration of AN was high, but the processability was good in both the yarn making process and the firing process, and the quality of the obtained carbon fiber precursor fiber and carbon fiber bundle was good. there were.
[Comparative Example 4]
100 parts by weight of AN, 1 part by weight of itaconic acid, and 0.2 part by weight of AIBN as a radical initiator were uniformly dissolved in 460 parts by weight of dimethyl sulfoxide, and this was put into a reaction vessel equipped with a reflux tube and a stirring blade. After replacing the space in the reaction vessel with nitrogen, the heat treatment was performed under the above polymerization condition A while stirring, and polymerization was performed by a solution polymerization method to obtain a PAN polymer solution. A spinning solution was obtained in the same manner as in Comparative Example 1 except that the PAN-based polymer solution was changed to the one obtained as described above and the polymer concentration was adjusted to 15% by weight. The obtained spinning solution was put into a heat-resistant pressure-resistant glass bottle, stoppered, and treated in an oven at 120 ° C. for 6 hours. The whole gelled and the gelation rate was the same even if the molecular weight distribution of the polymer changed. . Further, the amount of gel produced in the gel production amount test was so large that it could not be counted. In addition, an attempt was made to obtain a carbon fiber bundle using the obtained spinning solution in the same manner as in Example 1, but sampling failed due to frequent discharge failures and yarn breakage from the die during the yarn making process.
[Example 6]
100 parts by weight of AN, 0.3 part by weight of itaconic acid, 0.01 part by weight of 2,2′-azobisisobutylnitrile (hereinafter abbreviated as AIBN) as a radical initiator, and 200 parts by weight of dimethyl sulfoxide were mixed together. The reaction vessel was equipped with a reflux tube and a stirring blade. The space in the reaction vessel was purged with nitrogen until the oxygen concentration reached 1000 ppm, and then heat-treated under the following condition B while stirring and polymerized by a solution polymerization method to obtain a primary solution of a PAN-based polymer.
(1) Temperature increase from 30 ° C to 61 ° C (temperature increase rate 120 ° C / hour)
(2) Hold for 100 minutes at a temperature of 61 ° C. Next, in the reaction vessel, 0.7 parts by weight of itaconic acid, 10 parts by weight of dimethyl sulfoxide, 0.4 parts by weight of AIBN as a polymerization initiator, and as a chain transfer agent After 0.1 parts by weight of octyl mercaptan is metered in, the mixture is further heat-treated under the following conditions while stirring, and the remaining unreacted monomer is polymerized by a solution polymerization method to obtain a secondary solution of a PAN polymer. It was.
(2) Hold at 61 ° C. for 4 hours (3) Increase the temperature from 61 ° C. to 80 ° C. (Temperature increase rate: 10 ° C./hour)
(4) Hold for 6 hours at a temperature of 80 ° C. A carbon fiber bundle was obtained in the same manner as in Example 1 except that the PAN polymer solution was changed to the secondary solution obtained as described above. The obtained spinning solution was put into a heat-resistant pressure-resistant glass bottle, stoppered, and treated in an oven at 120 ° C. for 20 hours, whereby the whole gelled. In addition, the amount of gel produced in the gel production amount test was very small at six. In addition, the processability was good in both the yarn production process and the firing process, and the quality of the obtained carbon fiber precursor fiber and carbon fiber bundle was good.
[Example 7]
100 parts by weight of AN, 1 part by weight of itaconic acid, and 130 parts by weight of dimethyl sulfoxide were mixed and placed in a reaction vessel equipped with a reflux tube and a stirring blade. The space in the reaction vessel was purged with nitrogen until the oxygen concentration reached 100 ppm, and then 0.002 parts by weight of 2,2′-azobisisobutyronitrile (AIBN) was added as a radical initiator and stirred as follows. Was subjected to a heat treatment under the polymerization conditions D and polymerized by a solution polymerization method to obtain a primary solution of a PAN-based polymer.
Polymerization condition D
-Hold at 65 ° C for 2 hours-Decrease in temperature from 65 ° C to 30 ° C (Temperature drop rate: 120 ° C / hour)
The obtained primary solution was poured into water to precipitate a polymer, which was washed with warm water at 80 ° C. for 2 hours and then dried at 70 ° C. for 4 hours to obtain a dried polymer. Mz, Mw and Mn of the obtained dry polymer were 68,000, 5 million and 3.3 million, respectively, and the polymer concentration with respect to the solvent of the primary solution was 2.1% by weight.

Next, 240 parts by weight of dimethyl sulfoxide, 0.4 part by weight of AIBN as a radical initiator, and 0.1 part by weight of octyl mercaptan as a chain transfer agent were weighed and introduced into the primary solution, and then the following polymerization was conducted with stirring. A heat treatment under condition E was performed, and the remaining unreacted monomer was polymerized by a solution polymerization method to obtain a secondary solution of a PAN polymer.
Polymerization condition E
(1) Temperature increase from 30 ° C to 60 ° C (temperature increase rate 10 ° C / hour)
(2) Hold for 4 hours at a temperature of 60 ° C. (3) Increase the temperature from 60 ° C. to 80 ° C. (temperature increase rate: 10 ° C./hour)
(4) Hold for 6 hours at a temperature of 80 ° C. A carbon fiber bundle was obtained in the same manner as in Example 1 except that the PAN-based polymer solution was changed to the secondary solution obtained as described above. The obtained spinning solution was put into a heat-resistant pressure-resistant glass bottle, stoppered, and treated in an oven at 120 ° C. for 20 hours, whereby the whole gelled. In addition, the amount of gel produced in the gel production amount test was as small as 8 pieces. In addition, the processability was good in both the yarn production process and the firing process, and the quality of the obtained carbon fiber precursor fiber and carbon fiber bundle was good.
[Example 8]
Before discharging the spinning solution after the forced gelation treatment from the die, the filter filter medium is made of SUS316L metal fiber having an average wire diameter of 2 μm and is formed into a nonwoven fabric with a basis weight of 400 g / m ² and a filling rate of 20 g. % Non-woven fabric formed using SUS316L metal fibers having an average wire diameter of 5 μm and then sintered, and further laminated with a nonwoven fabric formed using SUS316L metal fibers having an average wire diameter of 20 μm. The filtration resistance coefficient is 20 × 10 ⁻⁵ cm ^−1, and the SUS316 metal fiber having an average wire diameter of 20 μm is used as a warp, and then the SUS316 metal fiber having an average wire diameter of 13 μm. Is used as a weft to form a twilled woven mesh and then sintered to have a filtration assurance layer basis weight of 220 g / m ² and a filling rate of 85%, and a filtration resistance coefficient of 7 × 10 ^{− A} carbon fiber bundle was obtained in the same manner as in Example 7 except that it was filtered again with a filter medium of ⁵ cm- ¹ . The process passability was good in both the yarn making process and the firing process, and the quality of the obtained carbon fiber precursor fiber and carbon fiber bundle was extremely good.

  Table 1 summarizes the results of various molecular weights of the PAN-based polymers in the above-described Examples and Comparative Examples, the limit draft magnification during yarn production, and the tensile strength of the obtained carbon fiber bundle.

Claims (6)

A polyacrylonitrile system in which Mz / Mw, which is a ratio of the weight average molecular weight Mw and the Z average molecular weight Mz, obtained by copolymerizing at least one acid monomer selected from the group consisting of acrylic acid, methacrylic acid and itaconic acid is 2 or more Spinning in which the polymer is dissolved in a solvent, the content ratio of acrylonitrile is 0.001 to 0.1% by weight, and satisfies at least one requirement selected from the following [a] to [c] A method for producing a carbon fiber precursor fiber obtained by spinning a solution to obtain a coagulated yarn, and then washing with water to form a carbon fiber precursor fiber.
[A] In the polyacrylonitrile-based polymer, the hydrogen ion of the carboxylic acid group derived from the acid monomer is 0 to 0.00% of the contained carboxyl group by at least one ion selected from the group consisting of ammonium ion and sodium ion. 3 equivalents are replaced.
[B] The spinning solution contains 0.01 to 3% by weight of an alkylating agent based on the weight of the spinning solution.
[C] The spinning solution contains 0.01 to 3% by weight of an organic carboxylic acid having a boiling point of 200 ° C. or more based on the weight of the spinning solution. The method for producing a carbon fiber precursor fiber according to claim 1, wherein the alkylating agent is trimethyl phosphate. The method for producing a carbon fiber precursor fiber according to claim 1, wherein the organic carboxylic acid is citric acid. The said polyacrylonitrile-type polymer is a manufacturing method of the carbon fiber precursor fiber in any one of Claims 1-3 whose Mw is 100,000-1 million and whose Mz / Mw is 2.5-10. The method for producing a carbon fiber precursor fiber according to any one of claims 1 to 4, wherein the spinning is wet spinning or dry wet spinning. The method for producing a carbon fiber precursor fiber according to any one of claims 1 to 4, wherein the spinning is dry and wet spinning, and the coagulated yarn has a single fiber fineness of 0.4 to 2.3 dtex when dried.