JPS6189322A - Polyester yarn and its production - Google Patents

Abstract

PURPOSE:Industrially useful polyester yarn having high strength and improved fatigue resistance, comprising ethylene terephthalate as a main constituent unit, and satisfying specific physical conditions. CONSTITUTION:Yarn obtained by melt spinning is wound at >=1,500m/min winding speed to give undrawn yarn having >=20X10<-3> double refractive index, which is drawn to give crystalline highly orientated polyester yarn having >=120X10<-3> double refractive index and >=20% crystallinity. Then, the yarn is drawn and heat-treated while applying constant tension in a range satisfying the equation I (F is tension g/d; T is drawing temperature deg.C at each drawing step) and shown by the formula F<=3.8 or tension which is reduced with rise in the drawing temperature, to the yarn, and raising the drawing temperature stepwise in a drawing temperature range satisfying the equation II (Tg is glass transition temperature of the yarn; Tm is melting point deg.C), to give the aimed yarn satisfying >=9g/d breaking strength, >=127 deg.C peak temperature of main dispersion shown in temperature dispersion of modulus of elasticity of mechanical loss, <=55 deg.C half width, and 130-160Angstrom long period.

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Industrial Application The present invention relates to industrially useful polyester fibers having high strength and excellent fatigue resistance, particularly polyester fibers suitable for reinforcing rubber structures.

(b) Prior Art Polyester fibers, especially polyethylene terephthalate fibers, have excellent mechanical and thermal properties and are therefore used not only for clothing, but also for tire cords, conveyor belts, etc.
It is also widely used in industrial applications such as v-belts, belt belts, hoses, and threads. Especially recently, compared to clothing, industrial fibers, especially fibers for reinforcing rubber structures, are becoming more popular.
As the specific gravity of 1 is becoming higher and higher, the required properties of the fibers are also becoming more demanding.

Conventionally, manufacturing methods for industrial fibers, including fibers for reinforcing rubber structures, have been directed toward improving cutting strength. A method in which undrawn yarn with a low degree of orientation obtained by discharging the polymer from a nozzle into a high-temperature 7F ambient atmosphere, then slowly cooling it, and then taking it at a low speed is drawn in multiple stages and then heat-treated at high temperature, or such undrawn yarn is heat-treated in multiple stages. In stretching, the first
A so-called steam jet method is known in which superheated steam is sprayed at high speed during stage stretching.

The fibers obtained in this way are excellent in terms of cutting strength as originally intended, but on the other hand, they cannot be said to be sufficient in terms of resistance to abrasion. At present, its uses are still limited, partly because it cannot be used.

On the other hand, many studies have been made to improve such fatigue resistance, and a representative method is the Japanese Patent Laid-Open No. 53
As described in JP-A-53031 or JP-A-53-53032, highly oriented undrawn yarn obtained by discharging a molten polymer with a high degree of polymerization, quenching it, and taking it off at high speed is multi-staged. A method of stretching and heat treatment is known. Although the rJl fibers obtained by this method achieve the desired improvements in thermal dimensional stability and fatigue resistance, on the contrary, the strength is inferior to that of the fibers obtained by the method of drawing low-oriented yarns. It has its drawbacks.

Thus, polyester fibers for industrial use manufactured using conventional methods have high strength, e! Out of the two required characteristics of fatigue resistance, only one of them has been improved, and the negative factors associated with this have also emerged, and high strength and fatigue resistance are of an antinomic nature. Ta.

(c) Purpose of the Invention The purpose of the present invention is to solve the above-mentioned trade-off problems and to produce a polyester (mfJt) having high strength and excellent y-resistance and useful for engineering purposes.

(di) Structure of the Invention The present inventor has conducted extensive research on fibers that simultaneously satisfy the above two required properties, and has developed a fiber that has high strength and excellent durability when used as fibers for reinforcing rubber structures, etc. They found that polyester fibers that meet the following characteristics (1) to (2) are suitable for exhibiting fatigue resistance.

■ Cutting strength of drawn yarn is 9.0 I/de or more ■
The peak temperature of the main dispersion that appears in the temperature dispersion of the mechanical loss modulus is 127°C or higher■The half width of the upper dispersion is 55°C or lower■The long period is 130 to t6oX The mechanical loss modulus referred to in the present invention is A static load of 0.259/de was applied to a multifilament sample with a length of 3a using a spectrometer YES-F manufactured by this company, and a strain of 0.17% t'HN was applied at a frequency of 10 Hz + a heating rate of 1. It was determined at 1ljll at 6°C/min.

The material background and manufacturing method that led the wood inventors to discover that the above characteristics (1) to (2) are suitable as a reinforcing material in rubber artifacts will be explained.

When the undrawn yarn refractive index (△n) of BOLIER hand-drawn terephthalate fiber, which is a typical polyester fiber, has a low degree of orientation such as 13 x 1o-3, it can be 99 It is relatively easy to obtain a drawn yarn with a high strength of /da or more. However, in this case, the drawn yarn is not sufficiently fatigue resistant to be used as a reinforcing material for rubber structures by cording and dipping the yarn. On the other hand, polyethylene terephthalate, for example 250
A reinforcing material for a rubber structure when the yarn is spun at a high spinning speed of 0 m/min, has a highly oriented undrawn yarn with a near birefringence ratio (△n) of 40XIO3, and is subjected to multi-stage stretching heat treatment. Although excellent fatigue resistance can be obtained, the strength remains at the level of 8-5 Fl/de at most, and does not exhibit high strength.

In order to break through the current state of the art, the wood inventors investigated the structural properties of fibers using dynamic viscoelasticity and X-gland analysis, and surprisingly found that the mechanical loss modulus of fiber The main dispersion that appears in the temperature dispersion shows a sharp shape with a half-value width of 55°C or less, and also has a long period (L ) is in the range of 130 to 160λ from the viewpoint of fatigue resistance, and the peak temperature of the main dispersion in the temperature dispersion of the mechanical loss modulus of the fiber is 127℃ or higher on the high 71 side. It was found that it is necessary to have a high strength. Furthermore, we have discovered that the purpose of the present invention, which is to achieve both high strength and fatigue resistance, can be achieved when these characteristics are achieved using a novel manufacturing method as described below.
The present invention has been achieved.

Principal dispersion during temperature dispersion of mechanical loss modulus 1. Let us consider H in more detail. 87￥1 The figure shows the principal dispersion curve;・p that appears in the temperature distribution of the mechanical loss modulus of polyester fibers produced by 11 different manufacturing methods. In the figure, the curve (1
) is the polyester fiber of the present invention, curve (2) is the polyester fiber obtained by multi-stage drawing of an undrawn yarn with a low degree of orientation at a high magnification, and curve (3) is the polyester fiber obtained by multi-stage drawing of an undrawn yarn with a high degree of orientation.
Each main dispersion of polyester J,";f:p obtained by heat treatment is schematically shown.

Dividing the distance between the peak (a) of this curve and its peak temperature (b) into two, the temperature width between
) (hereinafter simply referred to as half-value width).

The present inventors found that in such a curve, the shape of the curve represented by half-tooth 1 cold indicates the distribution of the degree of molecular chain orientation in the amorphous region of the fiber fine structure, while the peak temperature is the molecular ζ of the amorphous region, I learned that it shows a degree of orientation. Considering each curve again based on these findings, as shown in curve (2), the peak temperature is higher than 127 °C and the half-width is 55 °C, that is, the main dispersion has a broad shape. I: i; (p is the amorphous 'r() region ((It shows that even though the molecular chains in ,
7Q where the half width is below 55°C and the shape of the main dispersion is sharp, but the peak n degree is below 127°C.
fft indicates that even if the distribution of the degree of molecular chain orientation in the amorphous region is small, the degree of orientation itself is low.

From the above results, the present inventors believe that polyester 4'&fr'l, which has a high cutting strength of 9.9/de or more and excellent fatigue resistance, which is the object of the present invention, has an amorphous region. It must have a fine structure in which the degree of orientation of the molecular chains is high and the distribution of the degree of orientation of the molecular chains is small, and from this point of view it is necessary to simultaneously satisfy the characteristics (1) to (2) above. This point will be further explained in detail. The first important point in the great invention (1. The shape of the main dispersion that appears in the temperature dispersion of the mechanical loss modulus of polyester fibers is sharp. In other words, it is half the main dispersion of the mechanical loss modulus of polyester fibers. Zero value is 55℃ or less, preferably 50℃11
It is to show the bottom. Here, the half-value width of the principal variance is 5
When the temperature exceeds 5°C, the shape of the main dispersion becomes broad in relation to the present invention, and polyester fibers exhibiting such a shape of main dispersion have a fairly wide distribution of molecular chains and degree of orientation in the amorphous region. Existing. Therefore, when tension is applied to such fibers, stress concentration occurs on specific molecular chains in the amorphous region, and the molecular chains are therefore likely to be broken, resulting in poor fatigue resistance.

The second important point is that the peak temperature of the main dispersion is high. In other words, the peak temperature is at least 27°C, preferably at least 128°C. When this peak temperature is less than 127° C., it indicates that the degree of molecular chain orientation in the amorphous region in the microstructure of such polyester fiber is low. Of course, in this case, the degree of orientation of the entire KU fiber also decreases, and the cutting strength of the fiber also naturally decreases. In the case of a high cutting strength of 9.0 g/de or more, the peak temperature needs to be 127° C. or more.

The above description has mainly been about amorphous regions, but the inventors have learned that the overall structure composed of amorphous regions and crystalline regions is also important, particularly from the standpoint of fatigue resistance. That is, the long period (L) obtained from small-angle X-ray scattering is 1
30-160λ! It has been found that the surrounding area is necessary in order to obtain excellent skin resistance. In general, the long period is defined as the repeating periodic length of amorphous regions and crystalline regions that alternate in the length direction of the fiber. long period is 160
If it is X or more, there are usually only a few large crystals in the overall structure in a coarse state, resulting in a brittle structure and inferior fatigue resistance. However, if the long period is less than 130 years, the crystal structure is insufficiently developed and fatigue resistance is considered to be poor.

In the region with a long period of 130 to 160X, which is specified in the invention, crystals are sufficiently developed, and the bond between the amorphous and crystalline regions is dense, resulting in good # and fatigue resistance.

The polyester referred to in the present invention mainly refers to polyethylene terephthalate consisting of a terephthalic acid component and an ethylene glycol component, but a portion of the terephthalic acid, usually 10 mol% or less, is replaced with another dicarboxylic acid component,
The polyester may be a polyester having a diol component, and/or a polyester in which a portion of the ethylene glycol component, usually 10 mol % or less, is replaced with another diol component. However, it is also a good idea to optionally use modifiers, stabilizers, etc. in such polyesters, if necessary.

The degree of polymerization of the 4Q 4% of the present invention made of such polyester must be 0.80 or more, particularly 0.80 or more expressed in terms of critical viscosity, in order to exhibit its intended properties as a reinforcing material in the final rubber structure. 83-1. A range of OO is preferred. In addition, the intrinsic viscosity referred to in the water specification was determined by making the solution of Orthokokuchifunisol at 35°C.

In addition, the polyester fiber of the present invention has a Q/10 per 1o
A terminal carboxyl group preference of 6 grams or less of polymer is preferred. If the amount of terminal carboxyl groups becomes too high, it may cause problems in terms of durability when used as a reinforcing material in rubber structures. Decrease the amount of terminal carboxyl groups.

Any method can be used, such as polyester, phenyl glycidyl ether, linear polyester carbonate, or ethylene oxide.

Depending on the desired intrinsic viscosity and amount of terminal carboxyl groups, a method of reacting diaryl oxalates, carbodiimides, or cyclic imino ethers can be used as appropriate. A preferred method is to avoid coloring of the obtained fibers, to avoid foaming due to decomposition of additives during spinning, and to reduce the amount of terminal carboxyl groups to 10aa>106g polymer or less without reducing the degree of polymerization. It is.

Further, it is desirable that the polyester fiber of the present invention has a dry heat shrinkage rate of 10% or less at 175°C. If the dry heat shrinkage ratio of the fiber after drawing is higher than 10%, even if the heat treatment during post-processing is strengthened, the shrinkage rate will be sufficiently low, especially in the case of a high-strength yarn with a drawn yarn strength of 99/de or more. It's hard to say. Therefore, it has the disadvantage of poor thermal dimensional stability as a reinforcing material in rubber structures.

The above-mentioned polyester fibers having high strength and excellent fatigue resistance can be obtained by a new manufacturing method described below. After melting and discharging polyester having an intrinsic viscosity of preferably 0.7 or more, particularly preferably 0.85 or more, the polyester is blown with cold air and taken at a take-up speed of 1500 m/min or more, and the birefringence (△n) is 20X103 or more. An undrawn yarn is obtained. At this time, a heating cylinder is installed directly below the cap,
2 without immediately cooling the yarn discharged from the nozzle with cold air.
At a temperature of 00°C or higher, preferably 250°C or higher (ffl
It is preferable to discharge the material into a high-temperature atmosphere and then cool it with cold air. The undrawn yarn is then drawn by any drawing method such as heating O-2, heating pin, hot plate, hot bath, hot water bath, steam jet, etc.
Birefringence is 120XZO-3 by stage or multi-stage stretching.
The above is a crystalline highly oriented polyester f, &M having a crystallinity (X) of 20% or more. This stretching may be carried out continuously following spinning, or may be carried out after winding up once after spinning.

After that, this crystalline highly oriented polyester fiber was converted into the following formula [
1] and (2), preferably satisfying [1 and 2'], or applying a tension that decreases as the stretching temperature increases, while applying the following formula [3], preferably satisfying [3]. By carrying out drawing heat treatment while gradually increasing the drawing temperature in the drawing temperature range I, the polyester fiber specified in the present invention has high strength and good fatigue resistance.

2.0≦F≦:4.8-0.0 0 6T
(1) F<3.8
[2] Tg+50<T<Tm 15
(3:1 However, F: Tension (1/d
e) T: Stretching temperature (°C) at each drawing stage Tg: Glass transition temperature (°C) of the crystalline highly oriented polyester fiber Tm: Melting point (°C) of the nuclear crystalline highly oriented polyester fiber Preferably, the following formula [one It satisfies [2] [3].

2.3<:F<6.6-0.00157
[:1') F<a, 6
r2']Tg+80<T<Tm-30C3″l In this way, the stretching method in which stretching and heat treatment are carried out by gradually increasing the temperature while applying a constant tension or a tension that decreases as the stretching temperature increases is gradually increased as follows. It is called the hot drawing method.It is possible to obtain a high-strength yarn by using a so-called poy having a birefringence of 20×103 or more obtained by spinning at a high spinning speed using the sequential heating drawing method.

The birefringence index is 120X]0 as a raw yarn for sequential ectothermal drawing.
It is necessary to use highly oriented crystalline fibers having a crystallinity (X) of −3 or more, preferably 150×10 −3 or more, and a crystallinity (X) of 20% or more, preferably 30% or more. Birefringence is 120X10
If the crystallinity is less than 3 or less than 20%, it is not suitable for the sequential heating method. Stretching by the sequential heating stretching method is described in [1] above.
[2] [3] The ceremony will be completed as soon as possible.
It is regulated by the [2-[3'] formula. At a stretching tension lower than the range shown by formula [1], high-strength fibers will not be obtained even if the temperature is stretched sequentially, and conversely, at a stretching tension higher than the range shown by formulas [1] and [2], When stretched, single fiber breakage and even yarn breakage occur during stretching. In addition, when drawing at a temperature lower than the range shown by formula [3], if the tension is large, not only will single fiber breakage occur, but the fiber structure may be destroyed and the strength may actually decrease. . On the other hand, if the stretching is carried out at a temperature higher than the range shown by the formula [3], the stretching tends to be a little blown, and high strength may not be obtained or yarn breakage may easily occur.

In the case of sequential temperature stretching, it is important that the tension at each stretching stage be a constant tension or a tension that decreases with increasing stretching temperature. If the stretching tension is increased with an increase in the stretching temperature, yarn breakage is likely to occur during stretching. The sequential heating stretching method is a type of super-stretching method using a crystal stretching method, and it can be stretched while sequentially adjusting the stretching conditions to suit the state of the microstructure after stretching. Only when the stretching temperature range and stretching tension range specified in the present invention are adjusted using the above-described method, stretching at high temperature and high tension becomes possible, and therefore a high stretching ratio can be achieved and a high-strength yarn can be obtained. It is. Furthermore, in this stretching method, the stretching temperature can be raised to a higher temperature than in normal multi-stage stretching, so the heat setting effect is improved and the secondary effect is that the thermal dimensional stability is improved. It's coming.

A birefringence of 1 obtained by drawing an undrawn yarn with a birefringence of 2QX10-3 or more at a spinning speed of 1500 m/min using conventional means
When a crystalline highly oriented fiber with a crystallinity (X) of 20X10 -3 or more and a crystallinity (X) of 20% or more is subjected to sequential temperature raising stretching, the nucleus crystalline highly oriented fl#! The fiber may be once rolled up and subjected to sequential heating-up stretching, or there is no problem even if the undrawn yarn is drawn by conventional means without being wound up and then successively heated-up drawing is carried out successively.

(e) Practical Example 1 The present invention will be further explained with reference to Examples below. In addition, various measured values in the examples are based on the following methods.

(1) Birefringence (Δn) A parameter indicating the degree of orientation of molecules in a filament, which is determined by the retardation method using bromonaphthalene as a solution and a Peresoc convencator.

(For a detailed explanation of this method, please refer to Kyoritsu Shuppan ``Polymer S, h: Physical Properties of Gli Polymers IF J'') (2)
Long period (L) Long (, r, H period (L) using small-angle X-ray scattering measurement device,
It was calculated using Bragg's equation from the meridional 1,000 diffraction line obtained by using a conventionally known method, that is, using CuKa rays with a wavelength of 1.54× as a radiation source and irradiating the fiber axis at right angles.

(3) Load-unloading curve Compliant with JIS L 1017-1963 (5,4').

(4) Dry heat shrinkage rate at 175° C. Compliant with JIS L 1017-1963 (5, 1, 2).

(5) Crystallinity (X> Density d (,9
/CrI) using the following formula.

(6) Tube heat generation temperature, a parameter indicating tube life fatigue resistance, JISL 1017-19
631.32. Compliant with IA law (however, bending angle 906
). The tube heat generation temperature is determined by measuring the temperature of the tube surface with an infrared non-contact thermometer (90 minutes after the start of operation).
5AN-F, x company), and the tube life is shown as the time until the tube breaks.

(7) Glass transition temperature (Tg) and melting point (Tm) 10℃/
Tg was determined from the difference in the baseline level when measured at a heating rate of 100 min, and Tm was determined from the exothermic peak value.

Examples 1-4. Comparative Examples 1 and 2 Polyethylene terephthalate (titanium oxide content: 0.07%) having an intrinsic viscosity of 0.99 was melted at about 300°C and discharged through a spinneret having a pore diameter of 0 to 40 + 250 pores in an atmosphere. Length 20 where the temperature is maintained at 250℃
After running inside the heated cylinder of Zero Dragon, 5.
The mixture was cooled and solidified while being sprayed with a 7 m mouth, and then an oil agent was applied using an oiling roller. After that 2500 m
The yarn was wound up with a take-up roller rotating at a speed of /whip to obtain an undrawn yarn with a birefringence index (Δn) of 37×10”.

This undrawn yarn is subjected to first-stage stretching at 1.35 times between a first pair of heated Nelson rollers heated to 90°C and a second pair of heated Nelson rollers heated to 150°C,
Subsequently, a second stage of stretching was performed at 1.55 times between the second heating Nelson roller pair and the taking-off Nelson roller pair that was not actively heated, and winding was performed at a winding speed of 200 m/m. It was made into a fibril that was drawn at successive temperature increases. The fiber quality is 1
200 denier, strength 7.8 g/de.

Elongation 11.7%, △n=170X10-3. Crystallinity x
=41%, Tg: 80°C, Tm: 261°C.

This fibril was stretched in three stages with successive temperature increases using a pair of heated Nelson rollers, and wound up at a winding speed of 200+n/min. In addition, before supplying the yarn to the pair of heating Nelson rollers in the first stage, the temperature between the pair of Nelson rollers at room temperature and the pair of supplying Nelson rollers is 1.09.
/da of pretension was achieved. Table 1 shows the stretching conditions with successive temperature increases and the physical properties of the obtained yarn. Next, a green cord was created by applying a first twist and a second twist 49 times/10 cm each to this treatment system, and after applying a resorcinol-formalin adhesive, the cord was obtained in an atmosphere with a dry heat temperature of 240°C. Heat treatment was performed for 1 minute at a tension rate such that the elongation of the treated cord was 4.5 and 35% at 9 loads. The physical properties of the obtained treated cord are also shown in Table 1.

Comparative Examples 3 to 4 Spinning in the same manner as Examples 1 to 4, winding up with a pulling roller rotating at a speed of 600 m/min, birefringence (△n) 8x
It was made into an undrawn yarn of 10-3. This undrawn yarn is subjected to a first drawing at 3.3 times between a first heated Nelson roller pair heated to 90°C and a second heated Nelson roller pair heated to 120°C, and then a second heated Nelson roller pair heated to 120°C. 2 heating nelson low
The second stage stretching was performed at 1.58 times between the pair of rollers and a pair of Nelson rollers for taking off that were not actively heated, and winding was performed at a winding speed of 200 m/min. It was made into a fibril. The filament quality is 1200 denier and the strength is 8-
2 g/de, elongation 12.5%, △h=178X1
03. Crystallinity X-43% + Tg = 75°C, Tm = 26
It was 0°C.

As in Example 1, the fibrils were subjected to three stages of sequential heating-up stretching using a pair of heated Nelson rollers, and wound up at a winding speed of 200 m/min. Note that the preliminary tension before being supplied to the pair of heating Nelson rollers in the first stage is 1- as in Example 1.
09/de Toshida.

Next, this drawn yarn was made into a treated cord in the same manner as in Example 1. The physical properties of the obtained drawn yarn and treated cord are also shown in Table 1.

Comparative Examples 5 to 6 Using the same undrawn yarn as in Example 1, 90
a first pair of heated Nelson rollers which are heated to 150 °C;
The first pair of Nelson rollers is heated to
Stage stretching is performed by 1.35 times, and then second stage stretching is performed by 1.55 times between the @2 heating Nelson roller pair and the third heating Nelson roller pair heated to 200 to 230 ° C.
Subsequently, third-stage stretching was performed by a factor of 1.00 to 1.10 between the third heating Nelson roller pair and the taking-off Nelson roller pair that was not actively heated. The third stage stretching ratio is 1.
The maximum was 10 times. In Comparative Examples 5 and 6, heat setting was performed using a 300° C. gas bath between the second heating Nelson roller pair and the taking-off Nelson roller pair. In Comparative Example 7, heat setting using a gas bath was not performed. Next, this drawn yarn was made into a treated cord in the same manner as in Example 1. The physical properties of the obtained drawn yarn and treated cord are also shown in Table 2.

[Brief explanation of the drawing]

FIG. 1 shows curves of the main dispersion that appears in the temperature dispersion of the mechanical loss modulus of various polyester fibers. Song i! 1 (11 Polyester fiber curve of the present invention (2):
Polyester fiber bend N (31: Polyester with excellent fatigue resistance but low strength) W: Half value σ Figure 1 Temperature (0C)

Claims (1)

[Scope of Claims] (1) A polyester fiber containing ethylene terephthalate as a main constituent unit and satisfying the following properties [1] to [4]. [1] The cutting strength is 9.0 g/de or more [2] The peak temperature of the main dispersion that appears in the temperature dispersion of the mechanical loss modulus is 127 °C or more [3] The half width of the main dispersion is 55 °C or less [4] ] The polyester fiber according to claim (1), wherein the long period is 130 to 160 Å. (2) The half width of the main dispersion appearing in the temperature dispersion of the mechanical loss modulus is 50° C. or less. (3) The peak temperature of the main dispersion appearing in the temperature dispersion of the mechanical loss modulus is 128°C or more (
The polyester fiber described in section 1). (4) The polyester fiber according to any one of claims (1) to (3), which has a cutting strength of 9.5 g/de or more. (5) Undrawn yarn with a birefringence of 20 x 10^-^3 or more obtained by taking the melt-spun spun yarn at a take-up speed of 1500 m/min or more is drawn to have a birefringence of 120 x10^-^3 or more and crystallinity of 20% or more, and then convert the crystalline highly oriented polyester fibers into the following formulas [1] and [
Stretching and heat treatment are carried out while applying a constant tension within a range that satisfies [2] or a tension that decreases as the stretching temperature increases, while gradually increasing the stretching temperature within a stretching temperature range that satisfies the following formula [3]. A method for producing polyester fiber. 2.0≦F≦4.8-0.006T [1] F≦3.8 [2] Tg+50≦T≦Tm-15 [3] However, F: Tension (g/de) T: At each stretching stage Drawing temperature (°C) Tg: Glass transition temperature (°C) of the crystalline highly oriented polyester fiber Tm: Melting point (°C) of the crystalline highly oriented polyester fiber (6) Install a heating cylinder directly under the spinneret The method for producing polyester fibers according to claim (5), wherein the ambient temperature immediately below is 200° C. or higher. (7) Birefringence of crystalline highly oriented polyester fiber is 15
0x10^-^3 or more, the method for producing a polyester fiber according to claim (5) or (6). (8) Crystallinity of highly oriented polyester fiber is 30
% or more, the method for producing a polyester fiber according to any one of claims (5) to (7). (9) Crystalline highly oriented polyester fiber according to the following formula [1']
and [2'], or applying a tension that decreases as the stretching temperature increases, and stretching and heat treatment while increasing the stretching temperature within the stretching temperature range that satisfies [3']. The method for producing polyester fiber according to any one of items (5) to (8). 2.5≦F≦6.6-0.015T[1']F≦3.6
[2'] Tg+80≦T≦Tm-30 [3']