JPH06166908A - Ultrafine fiber and its production - Google Patents

Abstract

PURPOSE:To obtain ultrafine fiber, good in level dyeing properties and capable of remarkably improving the dyeing speck on the surface which has conventionally been considered as a disadvantage in a knitted or a woven fabric with hardly any yarn unevenness and to provide a method for producing the ultrafine fiber with hardly any fluff at a low cost. CONSTITUTION:This method for producing ultrafine fiber, having $ 0.3 denier single filament size and composed of a polyester prepared by reacting a bifunctional carboxylic acid consisting essentially of terephthalic acid with ethylene glycol is to melt the polymer having an intrinsic viscosity [eta] regulated to 0.3-0.6 and then melt spin the fiber at >=1500 to <=3500m/min take-off speed. The ultrafine fiber is characterized in that the intrinsic viscosity [eta] is 0.3-0.6; the dynamic loss tangent (tandelta) is tandelta40=0.05-0.045, tandelta60=0.044--0.036 and tandelta80=0.035-0.03; the apparent crystal size in (110) face in a wide angle X-ray diffractometry is >=55Angstrom ; the crystal growth degree is >=0.25 and the crystallinity is >=55%.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to an ultrafine fiber and a method for producing the same.

[0002]

BACKGROUND OF THE INVENTION Polyester, especially polyethylene terephthalate, has many excellent properties and is widely used in various applications, especially in fibers and films. The knitted fabric using ultrafine fibers has a soft texture, is supple and has high utility value. Conventionally well-known methods for producing ultrafine fibers include a method of forming sea-island type composite fibers of polyester and polystyrene and then removing polystyrene by a dissolution method (Japanese Patent Publication No. Sho 48-25362), a multilayer of polyamide and polyester. After forming the laminated composite fiber,
A method of peeling (Japanese Patent Publication No. 63-8207) and a method of producing ultrafine fibers by a normal spinning and drawing method (Japanese Patent Publication No. 62-62-62).
No. 3,5481, Japanese Patent Publication No. 63-8206) and the like are known.

However, among these manufacturing methods,
The sea-island type composite fiber melting method and the multi-layer bonding type composite fiber peeling method have a problem that the product cost is high due to an extremely complicated process, and the ordinary spin-drawing method requires a non-drawing step. Therefore, there is a problem that it is not possible to stably produce a product having good levelness. In general,
In the case of the spinning and drawing method, it is known that the structure of the inside of the fiber of the undrawn yarn package once wound changes with time depending on the temperature and humidity conditions of the atmosphere. This tendency is particularly noticeable with ultrafine fibers, and stress relaxation occurs at the end face of the package over time.Therefore, when the dyeability of the fiber is evaluated after drawing, the deep dyeing that matches the end face cycle of the undrawn yarn in the yarn length direction Spots occur.

The direct spinning and drawing method is known as a technique which does not pass through undrawn yarn and is widely practiced in the production of general single yarn denier fibers. As a result of trying the direct spinning and drawing method, unevenness at the time of discharging the polymer immediately leads to yarn breakage, and since the number of filaments is large, the air resistance of the running yarn increases and the tension increases,
It has been found that it is extremely difficult to produce ultrafine fibers by the direct spinning drawing method due to problems such as fluff and yarn breakage due to frictional resistance in various yarn guides.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide an ultrafine fiber having a good leveling property and an efficient and low cost production method.

[0006]

Means for Solving the Problems As a result of intensive studies on the above points, the present inventors have found that the adjustment of the intrinsic viscosity of the polymer is extremely important in the direct spinning / drawing method, and the fluff and the yarn are also related to the take-up speed. There is a range in which breakage does not occur, and the present invention was reached by investigating these relationships. That is, the present invention is as follows. One of them is a polyester having a single yarn denier of 0.3 denier or less, a polyester obtained by reacting ethylene glycol with a bifunctional carboxylic acid mainly containing terephthalic acid, and satisfying the following items (a) to (d). Ultrafine fibers characterized by that. (A) Intrinsic viscosity [η] is 0.3 to 0.6 (b) Tan δ _{40 of} mechanical loss tangent (tan δ) is 0.
05-0.045, tan δ ₆₀ is 0.044-0.03
6, tan δ ₈₀ is 0.035 to 0.03 (c) The apparent crystal size of (110) plane is 55Å or more (d) Crystal growth is 0.25 or more (e) Crystallinity is 55% or more. The other is a method for producing ultrafine fibers by direct spinning and drawing of polyester, which has a take-up speed of 1
2. The method for producing an ultrafine fiber according to claim 1, wherein melt spinning is performed at 500 m / min or more and 3000 m / min or more.

The polyester polymer obtained by reacting a difunctional carboxylic acid and ethylene glycol used in the present invention has an intrinsic viscosity (η) of 0.3 to 0.6 and stabilizes the discharge of ultrafine fibers. The porosity is preferably about 250 poise, and the polymer is preferably spun at 285 to 320 ° C. When the intrinsic viscosity is lower than this, the melt viscosity is relatively reduced and the flow at the nozzle portion of the spinneret is deteriorated. In addition, if the intrinsic viscosity is higher than this value, the spinnability is improved, but as ultrafine fibers, the texture is hard and the commercial value is reduced, so it cannot be used.

[0008] The take-up and take-up speed is 3500 to 6000 m / min, which is often used in high-speed spinning in the case of ordinary multifilaments, but in ultrafine fibers, tension increases due to air resistance of spun yarn groups. If it exceeds 3500 m / min, not only the yarn breakage and fluffing increase, but also the physical properties of the yarn centering on strength and elongation decrease. On the other hand, if it is less than 1500 m / min, it is not economically advantageous.

As explained above, according to the production method of the present invention, a yarn having few yarn breakages, fluffs, level dyeing in the yarn length direction and less yarn unevenness is obtained, and the surface of the knitted fabric which has hitherto been a defect has Dyeing spots can be remarkably improved. This is because the amorphous structure part becomes tense over time once it passes through the undrawn yarn.
Although it is difficult for the subsequent drawing to take place, it is considered that unevenness in drawing occurs periodically in the yarn length direction, especially in the side surface portion, because it changes faster in the outer layer of the package, resulting in dyeing unevenness. In high-speed spinning, a uniform product is obtained because the drawing is performed without passing through the undrawn yarn.

[0010]

EXAMPLES Hereinafter, examples of the present invention will be described in detail. The measurement conditions of the evaluation items are shown below. (1) Intrinsic viscosity (η) under O-chlorophenol solvent 3
Intrinsic viscosity determined at 5 ° C. (2) tan δ ₄₀ , tan δ ₆₀ , tan δ ₈₀ Toyo Baldwin Co., Ltd. trade name VIBRON DDV-IIc type measuring instrument at a temperature rising rate of 10 ° C./min and a frequency of 110 Hz.
an δ-temperature characteristics are measured 40 ℃, 60 ℃, 80 ℃
Tan δ value at. (3) Apparent crystal size The apparent crystal size of the (110) plane is calculated by the following formula (1) from the half width of the diffraction intensity of the equatorial diffraction curve in the wide-angle X-ray diffraction diagram.

[0011]

[Equation 1]

(4) Crystal growth degree The crystal growth degree is wide angle X.
It is determined by the following formula from the diffraction intensity of the equatorial diffraction curve in the line diffraction diagram. Crystal growth degree _{= 1- (2H 2 / H 1} + H 3) ( provided that in the above formula, H ₁ is the minimum value, maximum intensity of H ₂ are (100) reflection intensity between reflections (100) and (010) , H ₃ represents the maximum intensity of (010) reflection.) (5) Crystallinity Crystallinity is determined by the following method. Wide angle X
From the equatorial diffraction curve in the line diffraction diagram, 2θ is 7 ° and 2θ
A straight line connects the diffraction intensity curves lying between 35 ° and 35 ° to form a baseline. Next, a valley near 2θ of 20 ° is used as a vertex, and a straight line is connected along the skirts of the low-angle side and the high-angle side to separate into a crystalline portion and an amorphous portion, and the area method is calculated according to the following equation.

Crystallinity (%) = scattering intensity of crystal part / general turbulence intensity × 100 (6) Spinning stability When a 5 kg wound package was sampled, it was possible to make yarn continuously 5 times or more without yarn breakage. ◯, and less than that were marked as x. (7) Fluff The appearance of the 5 kg wound package was visually judged, and the surface fluff was 5 or less, ◯, 6 to 20 fl, and 20 or more were x. (8) Feel The sampled sample was made into a tubular knitted fabric, and the tactile sensation was determined. The texture of the conventional product obtained by the ordinary spin-drawing method in which the undrawn yarn is once wound and then stretched in the stretching step is indicated by ◯, and the other products are indicated by x. (9) Level dyeing property The sampled sample was used as a tubular knitted fabric, and then a disperse dye was used. The temperature was raised from room temperature to 100 ° C in 20 minutes at owf 3% and a bath ratio of 1: 100, and then at 100 ° C for 1 hour. Keep dyed fabric. The unevenness of the obtained knitted fabric was judged with the naked eye, and the uniform one was marked with ◯, the one with a slight difference in shade was marked with Δ, and the one with a large shade difference was marked with x. (10) Yarn spots A cross section of a fiber sample is capacitively measured at a measuring speed of 100 m / min using an electric measuring means with a measuring machine manufactured by Keisokki, and each instantaneous value of the weight per unit length of the sample is X _i , X is the average value and L is the length of integration.
As a result, the yarn unevenness was calculated using the following formula (2).

[0014]

[Equation 2]

[0015]

Example 1 Polyethylene terephthalate having an intrinsic viscosity [η] of 0.45 was melt-extruded, discharged from a spinneret nozzle, cooled, applied with an oil agent, and then heated to 70 ° C.
After being drawn by a godet roller and then stretched 1.7 times with a second godet roller heated to 150 ° C.,
Winding at 2800 m / min and winding, 110d / 1100
The thread of f was obtained. The intrinsic viscosity [η] of the obtained yarn is 0.41.
Met. Further, the physical properties and level dyeing property of this yarn were measured and shown in Table 1. As a result, a good product with few deep dyeing spots was obtained.

[0016]

Example 2 Polyethylene terephthalate having an intrinsic viscosity [η] of 0.56 was melt-extruded, discharged from a spinneret nozzle, cooled, applied with an oil agent, and then heated to 70 ° C.
After being drawn by a godet roller and then stretched 1.8 times with a second godet roller heated to 150 ° C.,
Winding and winding at 2600m / min, 105d / 700f
Got the thread. The obtained yarn had an intrinsic viscosity [η] of 0.50. In addition, the physical properties and level dyeing property of this yarn were measured and shown in Table 1. In this case, a good product with few deep dyeing spots was obtained.

[0017]

Example 3 Polyethylene terephthalate having an intrinsic viscosity [η] of 0.56 was melt-extruded, discharged from a spinneret nozzle, cooled, applied with an oil agent, and then heated to 70 ° C.
After being drawn by a godet roller and then stretched 2.5 times with a second godet roller heated to 150 ° C.,
Take-up and winding at 1500m / min, 105d / 700f
Got the thread. The obtained yarn had an intrinsic viscosity [η] of 0.50. Further, the physical properties and level dyeing property of this yarn were measured and shown in Table 1, and a good product with few deep dyeing spots was obtained here as well.

[0018]

Comparative Example 1 Polyethylene terephthalate having an intrinsic viscosity [η] of 0.45 was melt-extruded, discharged from a spinneret nozzle, cooled, applied with an oil agent, and then unwound at 1500 m / min. . This package is newly used by a twisting machine between the first roller heated to 90 ° C. and the second roller via the hot plate heated to 150 ° C. to 1.7.
Double stretching and winding at 500m / min, 110d / 1100
The thread of f was obtained. The physical properties and level dyeing property of this yarn were measured and shown in Table 1. As a result, those with large dark spots were obtained.

[0019]

[Comparative Example 2] Polyethylene terephthalate having an intrinsic viscosity [η] of 0.56 was melt extruded, discharged from a spinneret nozzle, cooled, and after applying an oil agent, an undrawn yarn was once wound at 1500 m / min. . This package was newly re-stretched by a twisting machine between the first roller heated to 90 ° C. and the second roller via a hot plate heated to 150 ° C. to be 1.8.
Double stretching and winding at 500m / min, 105d / 700f
Got the thread. The physical properties and level dyeing property of this yarn were measured and are shown in Table 1. In this case, too, a dye with large dark spots was obtained.

[0020]

Comparative Example 3 A polyethylene terephthalate having an intrinsic viscosity (η) of 0.65 was melt-extruded, discharged from a spinneret nozzle, cooled, and after applying an oil agent, heated to 70 ° C.
The yarn was taken up by a roller, then drawn 1.8 times with a second roller heated to 150 ° C., and wound at 2600 m / min to obtain a yarn of 105d / 700f. The physical properties and level dyeing property of this yarn were measured and are shown in Table 1. In this case as well, a good product with few deep dyeing spots was obtained.

When this yarn was used as a tubular knitted fabric, the texture was judged to be tactile, but it was found that the yarn was harder than the conventional product and was inferior in commercial property as an ultrafine fiber.

[0022]

[Comparative Example 4] Polyethylene terephthalate having an intrinsic viscosity [η] of 0.28 was melt extruded, discharged from a spinneret nozzle, cooled, applied with an oil agent, and then heated to 70 ° C.
It was taken up by a roller, then drawn 1.8 times with a second roller heated to 150 ° C., and then wound at 2600 m / min to obtain a yarn of 105d / 700f. It was difficult to collect the package due to many thread breaks.

[0023]

[Comparative Example 5] Polyethylene terephthalate having an intrinsic viscosity [η] of 0.56 was melt extruded, discharged from a spinneret nozzle, cooled, applied with an oil agent, and then heated to 70 ° C.
It was taken up by a roller, then drawn 1.4 times with a second roller heated to 150 ° C., and wound up at 3600 m / min to obtain a yarn of 105d / 700f. The physical properties and level dyeing property of this yarn were measured and shown in Table 1. As a result, a good product with few deep dyeing spots was obtained. On the other hand, when the appearance of the package was checked, it was found that there was a lot of fuzz and it could not be used as a product.

[0024]

[Table 1]

[0025]

EFFECTS OF THE INVENTION The ultrafine fibers of the present invention have good leveling properties and less yarn unevenness than conventional ones. In addition, the production method of the present invention can obtain ultrafine fibers efficiently and at low cost without problems such as fluff and yarn breakage.

[Brief description of drawings]

FIG. 1 is a schematic explanatory view illustrating a manufacturing method of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Base 2 Cooling device 3 Focusing guide 4 1st oiling agent application guide 5 1st confounding application device 6 1st separator roll 7 1st codet roll 8 2nd godet roll 9 2nd separator roll 10 2nd entangling application device 11 2nd oiling agent application Guide 12 Traverse guide 13 Winder

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[Procedure amendment]

[Submission date] January 27, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be amended] Claims

[Correction method] Change

[Correction content]

[Claims]

[Procedure Amendment 2]

[Document name to be amended] Statement

[Name of item to be corrected] 0003

[Correction method] Change

[Correction content]

However, among these manufacturing methods,
The sea-island type composite fiber melting method and the multi-layer bonding type composite fiber peeling method have a problem that the product cost is high due to an extremely complicated process, and the ordinary spin-drawing method requires a non-drawing step. Therefore, the level dyeing property is good and the yarn unevenness is small.
There is a problem that can not be produced in a stable manner things brewing. It is generally known that in the case of the spinning and drawing method, the structure of the inside of the fiber of the undrawn yarn package once wound changes with time depending on the temperature and humidity conditions of the atmosphere. This tendency is particularly noticeable with ultrafine fibers, and stress relaxation occurs at the end face of the package over time.Therefore, when the dyeability of the fiber is evaluated after drawing, the deep dyeing that matches the end face cycle of the undrawn yarn in the yarn length direction Spots occur.

[Procedure 3]

[Document name to be amended] Statement

[Correction target item name] 0006

[Correction method] Change

[Correction content]

[0006]

Means for Solving the Problems As a result of intensive studies on the above points, the present inventors have found that the adjustment of the intrinsic viscosity of the polymer is extremely important in the direct spinning / drawing method, and the fluff and yarn There is a range in which breakage does not occur, and the present invention was reached by investigating these relationships. That is, the present invention is as follows. One of them has a monofilament denier of 0.3 denier or less and is composed of a polyester obtained by reacting ethylene glycol with a bifunctional carboxylic acid mainly containing terephthalic acid and satisfies the following items (a) to (d). Ultrafine fibers characterized by that. (A) Intrinsic viscosity [η] is 0.3 to 0.6 (b) Tan δ _{40 of} mechanical loss tangent (tan δ) is 0.
05-0.045, tan δ ₅₀ is 0.044-0.03
6, tan δ ₆₀ is 0.035 to 0.03 (c) The apparent crystal size of (110) plane is 55Å or more (d) Crystal growth is 0.25 or more (e) Crystallinity is 55% or more Another is a method for producing ultrafine fibers by direct spinning and drawing of polyester, which has a take-up speed of 1
The method of producing ultrafine fibers of claim 1, wherein the melt spun at 500 meters / min or more 3000 m / min or less under a.

[Procedure amendment 4]

[Document name to be amended] Statement

[Correction target item name] 0007

[Correction method] Change

[Correction content]

The polyester polymer obtained by reacting ethylene glycol with a bifunctional carboxylic acid used in the present invention has an intrinsic viscosity [ η] of 0.3 to 0.6 and stabilizes the discharge of ultrafine fibers. It is desirable to set it to about 250 poise, and it is desirable to spin the polymer at 285 to 320 ° C. When the intrinsic viscosity is lower than the above range, the melt viscosity is relatively lowered, the flow in the nozzle portion of the spinneret is deteriorated, and the spinnability is lowered. In addition, if the intrinsic viscosity is higher than this, the spinnability is improved, but as ultrafine fibers, the texture is hard and the commercial value is lowered, so it cannot be used.

[Procedure Amendment 5]

[Document name to be amended] Statement

[Correction target item name] 0008

[Correction method] Change

[Correction content]

[0008] The take-up winding take rate is in the high-speed spinning is usually the case of multi-filament is in the range of 3500~6000m / minute is often used, because the tension rises in the air resistance of the spun yarn group in the ultra-fine fibers If it exceeds 3500 m / min, not only the yarn breakage and fluffing increase, but also the physical properties of the yarn centering on strength and elongation decrease. On the other hand, if it is less than 1500 m / min, it is not economically advantageous.

[Procedure correction 6]

[Document name to be amended] Statement

[Correction target item name] 0010

[Correction method] Change

[Correction content]

[0010]

EXAMPLES Hereinafter, examples of the present invention will be described in detail. The measurement conditions of the evaluation items are shown below. (1) Intrinsic viscosity [ η] Under O-chlorophenol solvent 3
Intrinsic viscosity determined at 5 ° C. (2) tan δ ₄₀ , tan δ ₅₀ , tan δ ₆₀ manufactured by Toyo Baldwin Co., Ltd. under the trade name VIBRON DDV-IIc type measuring instrument at a temperature rising rate of 10 ° C./min and a frequency of 110 Hz.
an δ-temperature characteristics are measured 40 ℃, 60 ℃, 80 ℃
Tan δ value at. (3) Apparent crystal size The apparent crystal size of the (110) plane is determined by the following formula (1) from the half-value width of the diffraction intensity of the equatorial diffraction curve in the wide-angle X-ray diffraction diagram.

[Procedure Amendment 7]

[Document name to be amended] Statement

[Name of item to be corrected] 0015

[Correction method] Change

[Correction content]

[0015]

Example 1 Polyethylene terephthalate having an intrinsic viscosity [η] of 0.45 was melt-extruded, discharged from a spinneret nozzle, cooled, applied with an oil agent, and then heated to 70 ° C.
After being drawn by a godet roller and then stretched 1.7 times with a second godet roller heated to 150 ° C.,
Winding at 2800 m / min and winding, 110d / 1100
The thread of f was obtained. The intrinsic viscosity [η] of the obtained yarn is 0.4
1, the yarn unevenness (%) was 1.3 . Also, the physical properties of this thread,
The level dyeing property was measured and is shown in Table 1. As a result, a good dyeing product with few dark spots was obtained.

[Procedure Amendment 8]

[Document name to be amended] Statement

[Correction target item name] 0016

[Correction method] Change

[Correction content]

[0016]

Example 2 Polyethylene terephthalate having an intrinsic viscosity [η] of 0.56 was melt-extruded, discharged from a spinneret nozzle, cooled, applied with an oil agent, and then heated to 70 ° C.
After being drawn by a godet roller and then stretched 1.8 times with a second godet roller heated to 150 ° C.,
Winding and winding at 2600m / min, 105d / 700f
Got the thread. The obtained yarn has an intrinsic viscosity [η] of 0.50,
The yarn unevenness (%) was 1.3 . Further, the physical properties and level dyeing property of this yarn were measured and shown in Table 1, and a good product with few deep dyeing spots was obtained here as well.

[Procedure Amendment 9]

[Document name to be amended] Statement

[Correction target item name] 0017

[Correction method] Change

[Correction content]

[0017]

Example 3 Polyethylene terephthalate having an intrinsic viscosity [η] of 0.56 was melt-extruded, discharged from a spinneret nozzle, cooled, applied with an oil agent, and then heated to 70 ° C.
After being drawn by a godet roller and then stretched 2.5 times with a second godet roller heated to 150 ° C.,
Take-up and winding at 1500m / min, 105d / 700f
Got the thread. The obtained yarn has an intrinsic viscosity [η] of 0.50,
The yarn unevenness (%) was 1.2 . Further, the physical properties and level dyeing property of this yarn were measured and shown in Table 1, and a good product with few deep dyeing spots was obtained here as well.

[Procedure Amendment 10]

[Document name to be amended] Statement

[Correction target item name] 0018

[Correction method] Change

[Correction content]

[0018]

Comparative Example 1 Polyethylene terephthalate having an intrinsic viscosity [η] of 0.45 was melt-extruded, discharged from a spinneret nozzle, cooled, applied with an oil agent, and then unwound at 1500 m / min. . This package is newly used by a twisting machine between the first roller heated to 90 ° C. and the second roller via the hot plate heated to 150 ° C. to 1.7.
Double stretching and winding at 500m / min, 110d / 1100
The thread of f was obtained. The physical properties and level dyeing property of this yarn were measured and shown in Table 1. As a result, there were large deep dye spots and large yarn spots .

[Procedure Amendment 11]

[Document name to be amended] Statement

[Correction target item name] 0019

[Correction method] Change

[Correction content]

[0019]

[Comparative Example 2] Polyethylene terephthalate having an intrinsic viscosity [η] of 0.56 was melt extruded, discharged from a spinneret nozzle, cooled, and after applying an oil agent, an undrawn yarn was once wound at 1500 m / min. . This package was newly re-stretched by a twisting machine between the first roller heated to 90 ° C. and the second roller via a hot plate heated to 150 ° C. to be 1.8.
Double stretching and winding at 500m / min, 105d / 700f
Got the thread. The physical properties and level dyeing property of this yarn were measured and are shown in Table 1. Again, there were large dark spots and large yarn spots were obtained.

[Procedure Amendment 12]

[Document name to be amended] Statement

[Correction target item name] 0020

[Correction method] Change

[Correction content]

[0020]

Comparative Example 3 Intrinsic viscosity of polyethylene terephthalate
After melt extrusion of [ η] of 0.65, the mixture was discharged from the spinneret nozzle, cooled, applied with an oil agent, and then heated to 70 ° C.
The yarn was taken up by a roller, then drawn 1.8 times with a second roller heated to 150 ° C., and wound at 2600 m / min to obtain a yarn of 105d / 700f. The physical properties and level dyeing property of this yarn were measured and shown in Table 1. Good yarns with few deep dye spots were obtained, but the yarn spots were large.

Claims (2)

[Claims] 1. A single yarn denier of 0.3 denier or less, comprising a polyester obtained by reacting a difunctional carboxylic acid mainly containing terephthalic acid with ethylene glycol, and satisfying the following items (a) to (d): Ultrafine fibers characterized by that. (A) Intrinsic viscosity (η) is 0.3 to 0.6 (b) Tan δ _{40 of} mechanical loss tangent (tan δ) is 0.
05-0.045, tan δ ₆₀ is 0.044-0.03
6, tan δ ₈₀ is 0.035 to 0.03 (c) The apparent crystal size of (110) plane is 55Å or more (d) Crystal growth is 0.25 or more (e) Crystallinity is 55% or more 2. A method for producing ultrafine fibers by direct spinning and drawing a polyester, wherein the take-up speed is 1500 m /
The method for producing ultrafine fibers according to claim 1, wherein melt spinning is performed at a speed of at least 3500 m / min.