JPH0123571B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a method for producing aromatic polyamide fibers. More specifically, a substantially para-oriented aromatic polyamide yarn consisting of a specific polymer repeating unit is drawn at a high magnification under high temperature to produce an excellent quality, high tenacity, high modulus aromatic polyamide fiber in a favorable process. The present invention relates to a method for manufacturing the same. Prior Art In recent years, the demands for industrial synthetic fibers have become more sophisticated, especially in response to the demand for high strength and high modulus.
Various new fiber materials are being developed. Among them, certain aromatic polyamide fibers, in particular fibers of substantially para-oriented aromatic copolyamides containing ether linkages in some of the polyamide repeat units, as described in British Patent No. 1501948. In order to achieve this performance, a method is adopted in which the yarn is stretched in one or two or more stages at a maximum stretching temperature of 400° C. or higher and a total stretching ratio of 6 times or higher. In this case, the yarn is drawn gradually without creating a neck, taking the form of so-called flow drawing. By the way, when an aromatic polyamide thread is drawn at a high temperature of 400°C or higher, the fibers constituting the thread soften significantly during drawing, so fusion between single fibers is unavoidable. . In particular, as the number of single fibers constituting the yarn increases, the fusion increases more and more, and the resulting drawn yarn becomes of low quality with extremely low flexibility. Furthermore, when drawing the yarn in such a significantly softened state, it is particularly important to uniformly heat the yarn, and if heating is not performed uniformly, the process performance will deteriorate. Purpose of the Invention The main purpose of the present invention is to prevent the fusion between the single fibers that occurs when drawing the aromatic polyamide yarn at high temperatures as described above, and to uniformly distribute the individual single fibers during drawing. An object of the present invention is to provide a method for producing high-grade, high-strength, high-modulus aromatic polyamide fibers with excellent process conditions by applying heat to the fibers. Structure of the Invention The above-mentioned object, according to the present invention, is to draw a yarn made of a specific aromatic polyamide in one or two or more stages at a maximum drawing temperature of 400° C. or more to a total stretching ratio of 6 times or more. In a method for producing high-strength, high-modulus aromatic polyamide fibers, when drawing is carried out at a temperature of 400°C or higher, the drawn thread is spread out into a flat shape, and the width of the thread immediately before drawing is This is achieved by opening and drawing the yarn so that its width is 1.1 to 10 times larger. In the aromatic polyamide to which the method of the present invention is applied, 80 mol% or more of the polymer repeating units are the following repeating units: -NH-Ar ₁ -NHCO-Ar ₂ -CO- [Here, Ar ₁ and Ar ₂ are ,Each

【formula】

[Formula] and

Indicates at least one aromatic residue selected from [Formula]. Note that Ar ₁ and Ar ₂ may be the same or different. Further, all or part of the hydrogen atoms in these aromatic residues may be substituted with a halogen atom or a lower alkyl group. ] Any aromatic polyamide with a substantially uneven orientation composed of the following may be used, but in particular, 80 mol% or more of the total of Ar ₁ and Ar ₂ is the following aromatic residue (A)
and (B) or (B'), and the proportion of (B) or (B') is 10 to 40%. [The hydrogen atoms of these aromatic residues may be substituted with a halogen atom and/or a lower alkyl group. ] Details of the method for producing such an aromatic copolyamide are described in British Patent No. 1,501,948, US Pat. Examples of suitable aromatic polyamides include the following three:
Mention may be made of copolyamides composed of various monomer units. Such aromatic polyamides are made into yarn by extruding the solution from a spinneret and coagulating in an aqueous coagulation bath, and the yarn (undrawn yarn) is heated to a maximum drawing temperature.
High strength and high modulus fibers are obtained by stretching in one or more stages at a high temperature of 400° C. or higher, preferably 420 to 550° C., so that the total stretching ratio is 6 times or higher. The solvent for preparing the spinning dope by dissolving the aromatic polyamide is preferably an amide solvent, such as N-methyl-2-pyrrolidone (NMP), N,N'-dimethylacetamide (DMA), N , N'-dimethylformamide (DMF), and tetramethylurea (TMU) are preferred. These solvents may contain halides of metals belonging to Group 1 or Group 3 of the periodic table. Examples of such halides include,
Particularly suitable are lithium chloride, calcium chloride, and the like. On the other hand, as the aqueous coagulation bath for coagulating the yarn extruded from the spinneret, it is preferable to use an aqueous coagulation bath containing the same kind of solvent as the amide solvent in the aromatic polyamide solution serving as the spinning stock solution, and the solvent concentration in the coagulation bath is Although it varies depending on the type of aromatic polyamide, spinning conditions, etc., it is generally preferably within the range of about 5 to 50% by weight. Incidentally, the above-mentioned halide may be contained in this coagulation bath. During spinning, a spinneret may be placed in a coagulation bath to extrude the spinning dope directly into the coagulation bath, but the spinneret may be placed several mm to several centimeters above the top surface of the coagulation bath and the spinning dope may be extruded once into the air. After extrusion, it is preferably introduced into a coagulation bath. The wet yarn pulled up from the coagulation bath is washed with water to remove the solvent. This water washing is carried out in a constant length state, or in parallel with pre-stretching to 1.05 to 2.0 times in a wet state. Performing such a slight preliminary stretching during the water washing process is effective not only for increasing the water washing efficiency but also for improving the roundness of each single fiber. The washed yarn is usually dried in a constant length state or in a slightly tensioned or slightly relaxed state. The tension or relaxation rate during drying is at most 10%. The aromatic polyamide yarn dried as described above may be further heated at a temperature of less than 400°C for 1.5~
After pre-stretching to 3 times, the temperature is 400°C or higher (preferably
It is subjected to stretching at high temperatures (420-550℃),
In this high-temperature drawing, the yarn is softened due to the high temperature, and the single fibers are fused to each other, resulting in poor process quality and furthermore, the quality of the obtained drawn yarn is also reduced. In order to solve this problem, inert inorganic fine powder is applied as a drawing aid to the wet yarn or dry yarn, and then the yarn is stretched at 400°C.
A method has been developed for suppressing the above-mentioned fusion phenomenon by stretching at a temperature above. Examples of the inorganic fine powder include aluminum silicate, magnesium silicate, graphite, talc, and the like, each having an average particle size of 20 microns or less, particularly 10 microns or less.
Fine powders of silica, mica, etc. are suitable, and these fine powders may be used as a single component, or two or more types may be used in combination. Some of these fine powders become colloidal when hydrated in an aqueous dispersion bath, while others are simply dispersed, and any of these can be used. In order to uniformly adhere the fine powder to the yarn, a bath in which the fine powder is dispersed in a dispersion medium such as water is prepared in advance, and the wet, washed yarn is immersed in the dispersion bath. It is preferable to adopt a method that involves drying. In addition, in order to uniformly disperse the fine powder, an organic or inorganic dispersion may be added to the bath, or
An antistatic agent can also be used in combination to stabilize the convergence of the yarn. The amount of the inorganic fine powder attached to the yarn is preferably 0.1 to 3% by weight based on the weight of the fiber. If the amount of adhesion is too small, the effect will be reduced, and if it is too large, the fibers will become fine powder and will likely cause trouble in the post-processing process. In this way, when inorganic fine powder is attached to aromatic polyamide yarn and high-temperature stretching is performed at a temperature of 400°C or higher, the fusion phenomenon is considerably prevented and the process condition is improved, but it still does not reach a fully satisfactory level. .
In particular, further dramatic improvements are required in order to prevent as much as possible the occurrence of fluff due to stretch yarn breakage or single yarn breakage. In the method of the present invention, in such high temperature stretching,
The yarn to be drawn is expanded into a flat shape, and the ratio of the width of the yarn immediately after stretching to the width of the yarn immediately before stretching is
By employing high-temperature flow stretching in a special widening state in which the fibers are opened and drawn to a width of 1.1 to 10, preferably 1.3 to 5, fusion is almost completely prevented and the process condition is controlled. is also significantly improved. In typical methods of drawing polyester and nylon threads that have been generally known, means are employed to converge the threads as much as possible in order to improve the drawing condition. This is because if single fibers are separated from the yarn, single fiber breakage is likely to occur, so it is preferable that the yarns are converged as closely as possible. In order to improve the convergence of the yarn, it is common knowledge that the yarn is drawn after being coated with an oil agent or interlaced by air turbulence. In addition, when the number of single fibers is extremely large, such as tow for producing staple fibers, a method is adopted in which the yarn, or tow, is expanded and drawn for the purpose of uniform heating.
The tow width immediately after stretching does not change or becomes slightly narrower than the tow width immediately before stretching. This is because the diameter of the single fibers becomes smaller when the yarn is drawn, so as long as the yarn has convergence force (adhesion force due to oil between single fibers, restraining force due to slight entanglement between single fibers, etc.), the diameter of the single fibers becomes smaller. The yarn width is usually small. On the other hand, the method of the present invention achieves good drawing using a special drawing method that was completely unthinkable in the past, in which the thread to be drawn is spread out into a flat shape and the width of the thread is further increased during the drawing process. be done. Moreover, the yarn before being supplied for drawing is substantially free from entanglement between single fibers. In the method of the present invention, the gradual opening during the drawing process reduces the pressure bond between single fibers and reduces the thickness of the yarn, so that the heating of the yarn is uniformly performed. As a result, significant improvements in prevention of fusion and process conditions are achieved. According to conventional wisdom, drawing in such a wide spread state causes deterioration of the drawing condition, but the method of the present invention has the effect of preventing fusion between single fibers and improving the process condition. is completely unpredictable. The yarn to be drawn in the method of the present invention is preferably a yarn consisting of 100 or more single fibers. The degree to which the yarn is expanded into a flat shape is better, but in reality, in the case of a yarn consisting of 1000 single fibers, 2 to 5 single fibers are stacked in the thickness direction. It is preferable to form the strip into a flat shape in which 200 to 500 single fibers are lined up in the width direction. Considering only the purpose of uniform heating, an extreme shape in which all 1000 single fibers are lined up horizontally in a row is considered to be effective.
In reality, it is difficult to implement, and the convergence of the yarn itself tends to become unstable. In the method of the present invention, in order to spread the yarn into a flat shape, simply washing and drying the spun yarn while conveying it in parallel with rollers is enough to make the yarn into a flat shape. Then, the degree of flatness may be increased by pressing the yarn with a nip roller or by pressing the moving yarn against a plate or rod having a curvature. Furthermore, in order to open the filament spread into a flat shape, the repulsive force between single fibers due to the static electricity possessed by the filament is utilized. Since this repulsive force due to static electricity is small, when conventional polyester, nylon, etc. are stretched with high tension, gradual opening as in the method of the present invention does not occur. In the method of the present invention, this gradual opening is controlled to an appropriate degree, and the ratio of the width of the yarn immediately after drawing to the width of the yarn immediately before drawing is 1.1 to 10 (preferably 1.3 to 10).
5.0). Here, the width of the yarn immediately before and after drawing refers to the width of the yarn immediately before entering a heating means such as a hot plate or gas bath, and the width immediately after leaving the heating means, respectively, in the drawing process at a temperature of 400°C or higher. refers to the width of the yarn. The ratio of the yarn widths immediately before and after drawing is 1.1.
If it is less than that, the opening during the drawing process will be insufficient and fusion between single fibers will easily occur, resulting in poor process performance. on the other hand,
If the above ratio exceeds 10, the opening of the yarn is too large, and single fibers are likely to separate from the yarn bundle, resulting in single fiber breakage, and the fuzz on the end face of the yarn package after winding increases. The quality of the product deteriorates. Adjustment of the degree of such gradual fiber opening within the above range can be achieved by appropriately combining the following means for promoting fiber opening and means for suppressing fiber opening. First, as means for promoting gradual opening, 1. A drawing aid consisting of at least one of fine talc powder, fine silica powder, and fine mica powder is applied to the yarn. 2 After drying, massage the threads to loosen them. 3. Set the conditions to lower the stretching tension. Examples of such methods include: On the other hand, as a means for suppressing gradual opening, 1. A drawing aid made of fine aluminum silicate powder is applied to the yarn. 2 Remove static electricity. Examples of such methods include: When carrying out the method of the present invention, by skillfully combining some of these means, the ratio of the yarn widths immediately before and after drawing can be maintained at an appropriate value within the range of 1.1 to 10. In the method of the present invention, to draw the yarn at a temperature of 400°C or higher, either contact drawing using a hot plate or non-contact drawing carried out in an atmosphere of air, water vapor, inert gas such as carbon dioxide gas, nitrogen gas, etc. can also be adopted. The stretching temperature is 400℃ for at least one stage of stretching.
or higher (preferably 420 to 550°C). This is because the above-mentioned aromatic polyamide has a rigid polymer chain, and therefore does not exhibit sufficient strength if the maximum stretching temperature is less than 400°C. In the method of the present invention, it is not necessary to carry out all the stretching at a temperature of 400°C or higher, but pre-stretching may be carried out by 1.05 to 2 times in a wet state, and further stretching may be carried out at a temperature of 1.5 to 2 times at a temperature below 400°C.
Additional pre-stretching to 3.5 times is also possible;
Ultimately, it is necessary to draw the fibers in an expanded state at a temperature of 400°C or higher. In addition, as mentioned above, when stretching at a temperature of 400°C or higher, if a contact method is used using a hot plate, the temperature distribution may be such that a part of the hot plate (for example, both ends) is less than 400°C. In addition, when conducting in a non-contact manner in an inert gas, it is acceptable to have a temperature distribution such that a part of the atmosphere (for example, near the inlet and outlet) is less than 400°C. In short, substantial stretching
It can be carried out at a temperature of 400℃ or higher. The stretching ratio is set so that the total stretching ratio is 6 times or more, preferably 8 to 12 times. When performing preliminary stretching, if the total stretching ratio is 6 times or more
The magnification at the stage of stretching at a temperature of 400° C. or higher may be less than 6 times. The yarn drawn according to the method of the present invention is treated with a finishing agent, a de-drawing aid, etc., if necessary, and then wound onto a bobbin. Functions and Effects of the Invention According to the method of the present invention as described above, when aromatic polyamide fibers are stretched at a high magnification in a significantly softened state with a yarn temperature of 400°C or higher to exhibit excellent physical properties, The heat necessary for drawing is uniformly distributed over the single fibers, and fusion between the single fibers during the drawing process at high temperatures is almost completely prevented, and drawing can be carried out under excellent process conditions. The obtained aromatic polyamide fiber has high strength and modulus, excellent flexibility, and has little fuzz, so it can be widely used in various applications including reinforcing materials for rubber and resin. In particular, the fibers produced under suitable conditions are 25g/
It exhibits high strength exceeding de and high modulus of over 560 g/de, and has significantly superior physical properties compared to conventional aromatic polyamide fibers. Examples Hereinafter, the method of the present invention will be explained in more detail with reference to Examples. Note that the main characteristic values used in the following examples are values measured as follows. (1) Intrinsic viscosity of the polymer () Using an Ostwald viscosity tube, the flow time of the solvent alone is to (seconds), the flow time of the dilute polymer solution is t (seconds), and the polymer concentration in the dilute solution is c
(g/dl), it is expressed as =ln(t/to)/c. Unless otherwise specified, solvents are 97.5
% sulfuric acid, c = 0.5 g/dl, and measured at 30°C. (2) Fusing degree Count the number (n) of filaments that are not fused and can be separated (i.e., filaments that can be completely taken out as single fibers) out of the total number of filaments (N) of the drawn yarn, and then The degree of fusion is expressed by the formula. Degree of fusion=N-n/2N×100 (%) Perform this measurement five times and take the average value. Example 1 The following monomer units It is composed of ．． = 3.1 aromatic copolyamide containing calcium chloride (CaCl ₂ )
A solution prepared by dissolving the polymer in NMP to a polymer concentration of 6% by weight was extruded through a spinneret with a pore diameter of 0.3 mm and a number of holes of 250 at a discharge rate of 83 g/min.
After traveling mm, it was coagulated in a coagulation bath of NMP/water (30/70 wt%) at 50°C and pulled up at a speed of 11 m/min. Subsequently, the obtained coagulated thread was pre-stretched stepwise to 1.3 times while being washed in a water bath at 50°C.
It is passed through a squeeze roller to remove water adhering to the surface, immersed in a drawing aid bath having the composition shown in Table 1 for about 5 seconds by passing it through a Nelson roller, and then passed through a squeeze roller to remove the drawing aid dispersion. A washed thread with attached water was obtained. Subsequently, the washed thread was heated to a diameter of 120℃ at a surface temperature of 120℃.
A set of a 20cm drying roller and a 3cm diameter separate roller, a 15cm diameter drying roller at 250℃, and a 1.5cm diameter drying roller.
10 turns and 5 turns on each set of separate rollers.
The yarn was wound in turns to form an almost completely dry yarn, which was stretched and wound at a total stretching ratio of 11.0 times while contacting a hot plate with a surface temperature of 500°C and a length of 1 m. A yarn spinning test was conducted for 1 hour under each condition, and the results are shown in Table 1. In Experiment No. 1, no drawing aid was applied, so the ratio of yarn width immediately after drawing to that immediately after drawing (immediately/immediately before)
was less than 1.1, the fusion between single fibers was severe, the process was poor, and fuzz and loops were often generated.
In Experiments Nos. 2 to 4, the stiction between the yarns was released due to the sliding of the talc between the filaments and the loosening action on the separation roller, and the yarn width ratio was 1.1.
, and the fusion between single fibers is drastically reduced. However, in Experiment No. 2, the yarn width ratio was too large, exceeding 10 times, and the process became unstable, resulting in yarn breakage, fluff, and increased loops.

【table】

[Table] Example 2 The same aromatic polyamide solution as in Example 1 was
It was extruded from a spinneret with a size of 0.25 mm and 1000 holes at a discharge rate of 1111 g/min, traveled 7 mm in air, and then coagulated in a coagulation bath of an aqueous NMP solution at a temperature of 50°C and a concentration of 30% by weight at 38.5 m/min. pulled up at a speed of Subsequently, the obtained coagulated thread was washed in a water bath at 50°C and stretched stepwise to 1.3 times, passed through a squeezing roller to remove water adhering to the surface, and a concentration of 2 weight containing the composition shown in Table 2 was obtained. % of a drawing aid in an aqueous dispersion bath of about 1%
The yarn was immersed for a second and passed through a squeezing roller to obtain a water-washed yarn to which the drawing aid was attached. Table 2 Composition of drawing aid Talc 85 parts Aluminum silicate 15 parts Polyethylene glycol 7 parts Sodium hexametaphosphate 3 parts Next, this water-washed yarn was placed on a drying roller at 120°C.
After winding the yarn 30 times and drying it, the dried yarn was loosened between a pair of gear rolls with triangular teeth. Each gear roll has a diameter of 84 mm and has 40 triangular teeth on its circumference, and the tips of the triangular teeth are rounded with a radius of curvature of 1 mm. The gear rolls are engaged with each other to a certain depth, and the degree of kneading changes depending on the depth of this engagement. The depth of engagement was varied from 0 mm (no engagement) to 1.5 mm. The deeper the interlocking, the stronger the loosening of the threads, which makes it easier to open the fibers during drawing at 400°C or higher, as described below. After kneading and loosening, the dried yarn is heated at a temperature of 360℃.
Stretch it twice on a 2m long hot plate and finally heat it to 500℃.
℃ and stretched 4 times on a 3 m long hot plate. (Thus, the total drawing ratio is 10.4 times.) Finishing oil was applied to the thus drawn yarn and it was wound at 400 m/min. The fineness of the obtained yarn was 1550 denier. During this drawing, we measured the width of the yarn just before it entered the hot plate at 500°C and the width of the yarn just after it left the hot plate, and reported the results along with the yarn quality, quality, and process condition. It is shown in Table 3.

[Table] As is clear from Table 3, when the meshing depth of gear rolls is less than 0.3 mm, fiber opening is insufficient, fusion prevention is not improved, strength is low, and yarn breakage rate is high. Summer (Experiment No. 5, 6). Experiment No. 7, 8,
In No. 9, the depth of engagement of the gear rolls varied from 0.6 to 1.0 mm, but the opening of the yarn under high-temperature drawing was within the range specified in the present invention, and there was almost no fusion.
It has high strength and a significantly low thread breakage rate.
Also, the number of fuzz is small. However, when the meshing of the gear rolls is as deep as 1.5 mm, the fiber opening becomes too large and the single fibers are more likely to break.As a result, the breakage rate becomes slightly higher and the fuzz of the wound fibers increases. Unfavorable (Experiment 10). Example 3 The following monomer units A solution of 6% by weight _of an aromatic polyamide composed of
It was extruded from a 250 spinneret at a rate of 93 g/min. After traveling about 10 mm in the air, the temperature of 50℃
Coagulate in a NMP/water (30/70% by weight) coagulation bath, pull up at a speed of 15 m/min, and then wash in a 50°C water bath to disperse the inert inorganic fine powder shown in Table 2 in water. The sample was immersed in a dispersion bath for 1 second, pulled out, squeezed between a rubber roller and a metal roller, and then dried. Next, it was stretched 12 times using a slit-shaped hollow pipe with a diameter of 30 mm and a length of 1 m. However, in this case, the steam atmosphere temperature was 460°C, and the yarn temperature was about 430°C. In this drawing, after the drawn yarn exited the hollow pipe and traveled approximately 30 cm, it was brought into sliding contact with a rotating roller to be coated with an antistatic agent. This rotating roller is the same as the oiling roller normally used to apply oil. After the yarn was coated with an antistatic agent, it was wound up through a codet roller. As the antistatic agent, a 2% aqueous solution of monosodium dioctyl sulfosuccinate was used. When the rotation speed of the rotating roller is increased, the amount of the antistatic agent attached increases, and a force is exerted to converge the width of the yarn immediately after drawing. Table 4 shows the results of such experiments.

[Table] In this experiment, non-contact drawing was performed using steam, so the yarn was extremely easy to open. Therefore, when the above-mentioned drawing aid was applied, the fibers were opened significantly just by drawing, and in Experiment No. 11, in which no antistatic agent was used, the yarns were opened too much, and there was a lot of fuzz on the bobbin end face, and the yarn breakage rate was also low. Slightly expensive. When an appropriate antistatic agent is applied, the opening condition is adjusted appropriately, and as shown in Experiment Nos. 12 and 13, fibers are spun with satisfactory quality, quality, and process conditions. However, if too much antistatic agent is applied, experiment No.
As shown in No. 14, fiber opening completely disappears, and as a result, fusion occurs.

Claims (1)

[Scope of Claims] 1 80 mol% or more of the polymer repeating units are the following repeating units -NH-Ar ₁ -NHCO-Ar ₂ -CO- [Here, Ar and Ar ₂ are [Formula] [Formula] [Formula] ] and [Formula] represents at least one aromatic residue selected from the following. Some or all of the hydrogen atoms in these aromatic residues may be substituted with a halogen atom or a lower alkyl group. ] The aromatic polyamide yarn composed of
In a method for producing high-strength, high-modulus aromatic polyamide fibers by drawing in one or two or more steps, when drawing is carried out at a temperature of 400°C or higher, the thread to be drawn is expanded into a flat shape and stretched. A method for producing aromatic polyamide fibers, which comprises opening and drawing the fibers so that the width of the thread immediately after drawing is 1.1 to 10 times the width of the thread immediately before drawing. 2 80 mol% or more of the total of Ar ₁ and Ar ₂ is the following aromatic residues (A) and (B) [Here, in the aromatic residues (A) and (B), all or part of the hydrogen atoms may be substituted with a halogen atom and/or a lower alkyl group. ] and the ratio of aromatic residue (B) is 10 to 40 mol%
A method for producing an aromatic polyamide fiber according to claim 1. 3 80 mol% or more of the total of Ar ₁ and Ar ₂ is the following aromatic group (A) and (B') [Here, in the aromatic residues (A) and (B'), all or part of the hydrogen atoms may be substituted with a halogen atom and/or a lower alkyl group. ] The method for producing an aromatic polyamide fiber according to claim 1, wherein the ratio of the structural unit (B') is 10 to 40 mol%. 4. The method for producing aromatic polyamide fibers according to claim 1, wherein the stretching is carried out on a hot plate at 400°C or higher. 5. The method for producing aromatic polyamide fibers according to claim 1, wherein the stretching is carried out at 400°C or higher in a gas bath. 6. The method for producing aromatic polyamide fibers according to claim 1, wherein the yarn is pre-stretched to 1.05 to 2 times in a water bath, dried, and then stretched at a temperature of 400°C or higher. 7 The yarn is first pre-stretched to 1.05 to 2 times in a water bath, dried, and then stretched to 1.5 to 2 times at a temperature below 400°C.
2. The method for producing an aromatic polyamide fiber according to claim 1, which comprises performing a second preliminary stretching to 3 times, and then stretching at a temperature of 400° C. or higher. 8. The method for producing an aromatic polyamide fiber according to claim 1, wherein the yarn is composed of 100 or more single fibers. 9. The method for producing aromatic polyamide fibers according to claim 1, which comprises adhering a drawing aid to the yarn and then drawing it at a temperature of 400°C or higher. 10. The method for producing an aromatic polyamide fiber according to claim 1, wherein the yarn is subjected to a mechanical loosening action and then stretched at a temperature of 400°C or higher. 11. The method for producing aromatic polyamide fibers according to claim 1, wherein the fibers are opened so that the width of the yarn immediately after stretching is 1.3 to 5 with respect to the width of the yarn immediately before stretching.