JP2002030523A - Polylactic acid fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a polylactic acid fiber having a high strength and high heat resistance. SOLUTION: This polylactic acid fiber is characterized in that the polylactic acid fiber obtained by a melt spinning method comprises a blend of a poly(L- lactic acid) and a poly(D-lactic acid) and has >=2.6 cN/dTex strength and the ratio ΔHh/ΔH1 of an entotherm ΔH1 based on the crystal fusion of single crystal of poly(L-lactic acid) and single crystal of poly(D-lactic acid) obtained by DSC measurement to entotherm ΔHh based on crystal fusion of stereocomplex crystal composed of poly(L-lactic acid) and poly(D-lactic acid) of >=10.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a polylactic acid fiber. More specifically, the present invention relates to a polylactic acid fiber having high strength and high heat resistance.

[0002]

2. Description of the Related Art Conventionally, polyethylene, polypropylene, polyester, polyamide and the like have been used as materials for fibers and molded products, and their consumption is increasing year by year.
Accordingly, the amount of waste after use is increasing. Since these wastes are currently treated by incineration or landfill, various environmental problems and problems such as securing a disposal site have occurred.

In order to cope with such problems, in recent years, various studies have been made on fibers using various biodegradable polymers that degrade resins in the natural environment by the action of microorganisms existing in soil or water. It has been done. Among them, poly L-
Fiber with lactic acid has relatively high melting point and crystallinity,
Due to its excellent characteristics such as high strength and elastic modulus when used as fibers, studies on industrial production as practical biodegradable fibers have been actively conducted.

However, the melting point of poly-L-lactic acid is 17
It is about 0 ° C, and when it is used as clothing fiber, the temperature at which ironing is possible is limited to a low temperature. When it is used as industrial fiber, it can be raised to about 150 ° C in the manufacturing process of rubber materials and resin-coated fabrics. There was a problem that it was not suitable for the application to which it was exposed.

On the other hand, lactic acid has optical isomers, and a stereocomplex crystal comprising a mixture of poly-L-lactic acid and poly-D-lactic acid, which are polymers of L-lactic acid and D-lactic acid, is formed. It is known that the melting point is higher than that of a single crystal of L-lactic acid or poly-D-lactic acid.
Japanese Patent Application Laid-Open No. 61-36321 discloses, for the first time, that the above-mentioned specific properties are industrially utilized by blending poly-L-lactic acid and poly-D-lactic acid. There is no specific technology suggestion. Japanese Patent Application Laid-Open No. 63-264913 discloses a technique in which poly-L-lactic acid and poly-D-lactic acid are blended in a solution state and then subjected to solution spinning. Is low, and the pot life is short, so that stable yarn cannot be produced, and the resulting fiber lacks quality, or the winding speed is at most several tens of m / min, and industrially efficient production cannot be performed. There was such a problem. Furthermore, Japanese Patent Application Laid-Open No. 63-241024 discloses an example of melt spinning using a composition containing equimolar amounts of poly L-lactic acid and poly D-lactic acid. Was at most about 0.5 cN / dTex, and a fiber having practical strength was not obtained.

Further, Yamane et al. Of Kyoto Institute of Technology include a stereocomplex crystal by heat-treating an undrawn yarn obtained by melt-spinning a melt blend of poly-L-lactic acid and poly-D-lactic acid or a drawn yarn obtained by drawing the same. Polylactic acid fiber is obtained (Sen-i Gakkai Preprints 1989). However, in this method, poly-L-lactic acid molecules and poly-D-lactic acid molecules are not sufficiently dispersed as internal structures of the undrawn yarn and the drawn yarn, and a partial domain structure is formed. In order to generate and grow, it is necessary to heat-treat at 200 ° C. for 2 to 10 minutes after spinning. Therefore, the molecular orientation inside the fiber is relaxed during the heat treatment, and the strength of the obtained fiber is at most 2.3 cN /
It stays at about dTex. In addition, although the drawn yarn before the heat treatment has a strength of 4.2 cN / dTex, a considerable amount of poly-L-lactic acid or poly-D-lactic acid single crystal is present and the formation of stereo complex crystals is insufficient, so that the heat resistance is high. Sex was inadequate.

Further, JP-A-9-25400 and JP-A-2000-17164 disclose a method in which a mixture of poly-L-lactic acid and poly-D-lactic acid is heated and melted, or each is heated and melted and then mixed. Techniques for obtaining a crystallized polylactic acid molded article are disclosed, but none of them specifically suggests fiberization.

As described above, various attempts have been made to obtain polylactic acid fibers having high strength and high heat resistance, but the actual situation has not been achieved yet.

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to provide a high-strength and high-heat-resistant polylactic acid fiber which could not be produced by the above-mentioned prior art and which can be industrially produced with high efficiency. It is in.

[0010]

In order to solve the above-mentioned problems, the polylactic acid fiber of the present invention mainly has the following constitution.
That is, it is a fiber comprising a blend of poly L-lactic acid and poly D-lactic acid obtained by a melt spinning method, having a strength of at least 2.6 cN / dTex, and a poly-L-lactic acid single crystal and polycrystalline obtained by DSC measurement. The ratio ΔHh / ΔHl of the endothermic amount ΔHl based on crystal melting of a single crystal of D-lactic acid and the endothermic amount ΔHh based on crystal melting of a stereocomplex crystal composed of poly-L-lactic acid and poly-D-lactic acid is 10
A polylactic acid fiber characterized by the above.

The above-mentioned polylactic acid fiber has a birefringence Δn.
Is preferably 18 or more. Further, the boiling water shrinkage Sb is preferably 3% or more.

[0012]

Next, the polylactic acid fiber of the present invention will be described in more detail.

[0013] The polylactic acid fiber of the present invention is characterized by comprising a blend of poly L-lactic acid and poly D-lactic acid.

The method for producing poly-L-lactic acid and poly-D-lactic acid includes a two-step process in which lactide, which is a cyclic dimer, is once produced from L-lactic acid or D-lactic acid as a raw material, and then ring-opening polymerization is performed. And a one-stage direct polymerization method in which the raw materials are directly subjected to dehydration condensation in a solvent. The polylactic acid used in the present invention may be obtained by any production method. In the case of the polymer obtained by the lactide method, the cyclic dimer contained in the polymer is vaporized at the time of melt-spinning and causes thread spots.
It is desirable that the content of the cyclic dimer contained in the polymer before the melt spinning be 0.3 wt% or less.
In the case of the direct polymerization method, since there is substantially no problem caused by the cyclic dimer, it is more preferable from the viewpoint of spinning properties.

The poly-L-lactic acid used in the present invention is a polymer containing L-lactic acid as a main monomer component, and is a copolymerized poly-L-lactic acid containing 15 mol% or less of a D-lactic acid component in addition to L-lactic acid. From the viewpoint of enhancing the formability of stereocomplex crystals, the D-lactic acid component in the poly-L-lactic acid is preferably as small as possible, and it is more preferable to use homopoly-L-lactic acid.

Similarly, the poly-D-lactic acid used in the present invention is D-lactic acid.
-A polymer containing lactic acid as a main monomer component, and D-
In addition to lactic acid, a copolymerized poly D-lactic acid containing 15 mol% or less of an L-lactic acid component may be used. However, from the viewpoint of enhancing the formation of stereocomplex crystals, the L-lactic acid component in the poly D-lactic acid is It is preferable that the amount is smaller, and it is more preferable to use homopoly D-lactic acid.

Further, the poly-L-lactic acid and / or poly-D-lactic acid used in the present invention may be copolymerized with another monomer component having an ester forming ability as long as the effects of the present invention are not impaired. As copolymerizable monomer components,
In addition to hydroxycarboxylic acids such as glycolic acid, 3-hydroxybutyric acid, 4-hydroxybutyric acid, 4-hydroxyvaleric acid, and 6-hydroxycaproic acid, ethylene glycol, propylene glycol, butanediol, neopentyl glycol, polyethylene glycol, glycerin, Compounds having a plurality of hydroxyl groups in a molecule such as pentaerythritol or derivatives thereof, succinic acid, adipic acid, sebacic acid, fumaric acid, terephthalic acid, isophthalic acid, 2,6-naphthalenedicarboxylic acid, 5-sodium sulfo Compounds containing a plurality of carboxylic acid groups in the molecule, such as isophthalic acid and 5-tetrabutylphosphonium sulfoisophthalic acid, and derivatives thereof are exemplified.

The weight average molecular weight of the above-mentioned poly L-lactic acid and poly D-lactic acid is preferably 50,000 or more, more preferably 100,000 or more, and more preferably 150,000 or more. If the weight average molecular weight is less than 100,000, it is difficult to improve the strength physical properties of the fiber, which is not preferable. In general, it is difficult to make the average molecular weight of poly L-lactic acid or poly D-lactic acid 500,000 or more.

The poly-L-lactic acid and poly-D-lactic acid used in the present invention may contain components other than the main polymer as long as the effects of the present invention are not impaired. For example,
Plasticizers, UV stabilizers, matting agents, deodorants, flame retardants,
If necessary, inorganic fine particles or organic compounds may be added as a yarn friction reducing agent, an antioxidant, a coloring pigment, or the like. In particular, benzophenone-based, benzotriazole-based, and hindered amine-based drugs can be preferably used as the ultraviolet light stabilizer. The blending amount at this time is preferably 0.005 to 1.0 wt% based on the fiber weight. Coloring pigments include inorganic pigments such as titanium oxide and carbon black, as well as cyanine, styrene, phthalocyanine, anthraquinone, perinone, isoindolinone, quinophthalone, quinocridone, and thioindigo. Things can be used.

The polylactic acid fiber of the present invention has a strength of 2.
6 cN / dTex or more, preferably 3.5 cN / dTex
dTex or more, more preferably 4.8 cN / dTex
That is all. When widely used for clothing or industrial use, the strength is practically insufficient if the strength is less than 2.6 cN / dTex. Further, from the same viewpoint, in the polylactic acid fiber of the present invention, the birefringence Δn is preferably 18 or more, and more preferably 22 or more.

The polylactic acid fiber of the present invention is a fiber obtained by a melt spinning method. Solution-spinning such as dry or wet spinning cannot produce industrially highly efficient products. In addition, the stability of the blended solution of poly-L-lactic acid and poly-D-lactic acid is low and the pot life is short, so it is stable. In this case, the resulting fiber cannot be produced.

In the polylactic acid fiber of the present invention, an endothermic amount ΔHl based on melting of the poly-L-lactic acid single crystal and the poly-D-lactic acid single crystal determined by DSC measurement and a stereocomplex comprising poly-L-lactic acid and poly-D-lactic acid Ratio of endothermic amount ΔHh based on crystal melting of crystal ΔHh / ΔH
1 is 10 or more, preferably ΔHh / ΔHl is 20
As described above, more preferably, ΔHh / ΔHl is 30 or more. In the case of a polylactic acid fiber composed of a blend of homopoly L-lactic acid and homopoly D-lactic acid, the peak temperature of ΔHl is around 160 to 180 ° C, and the peak temperature of ΔHh is around 210 ° C to 230 ° C. Often. If ΔHh / ΔHl is less than 10, the formation of stereocomplex crystals is insufficient, so that poly-L-lactic acid single crystal or poly-D-
The fibers are thermally deformed at the melting temperature of the lactic acid single crystal, and the heat resistance becomes insufficient.

Further, from the viewpoint of suppressing the yarn slack in processes such as stretching, relaxation and heat treatment at the time of spinning or securing the setability in the finishing process of the fabric, the boiling water shrinkage ratio Sb of the polylactic acid fiber of the present invention is 3%. % Or more, more preferably 5% or more.

The polylactic acid fiber of the present invention can be produced, for example, by the following method. The method for obtaining the polylactic acid fiber of the present invention is not limited to the method described below.

The polylactic acid fiber of the present invention can be obtained by subjecting a blend of poly L-lactic acid and poly D-lactic acid to ordinary melt spinning. As a blending method of poly-L-lactic acid and poly-D-lactic acid, for example, there is a method of subjecting a chip blend (dry blend) of poly-L-lactic acid chips and poly-D-lactic acid chips to melt spinning. Is
A conventional melt extruder such as a pressure melter type or a single-screw or twin-screw extruder can be used, but from the viewpoint of easily kneading poly-L-lactic acid and poly-D-lactic acid to easily form stereocomplex crystals. Shaft or twin-screw extruder type is preferred. Moreover,
A method of incorporating a static kneader in a polymer flow path, Poly L-
From a blend of poly-L-lactic acid and poly-D-lactic acid that has been pre-kneaded by melting and kneading a chip blend of lactic acid chips and poly-D-lactic acid chips with a twin-screw extruder-type kneader, and then forming chips. A preferred method is to prepare chips in advance and to subject the pre-kneaded chips to melt spinning. Alternatively, poly L-lactic acid and poly D-lactic acid may be mixed after being melted by separate melt extruders. In any of the above cases, kneading due to shear stress when passing through the filtration layer or the spinneret is expected. It is preferable to incorporate a static kneader after mixing from the viewpoint of strengthening.

The blend ratio of poly L-lactic acid and poly D-lactic acid used in the polylactic acid fiber of the present invention is, by weight, poly L-lactic acid: poly D-lactic acid from 30:70 to 7
The ratio is preferably from 0:30, but from the viewpoint of promoting the formation of stereocomplex crystals and improving the content ratio, it is more preferably from 40:60 to 60:40, more preferably from 50:50. preferable.

The above blend of poly-L-lactic acid and poly-D-lactic acid is melted by an extruder-type or pressure-melter-type melt extruder, weighed by a metering pump, and filtered in a spinning pack or the like. After receiving
It is discharged from a base having a desired base shape and the number of bases.
The discharged yarn is cooled and solidified by passing it through a gas having a temperature lower than the melting point of the polymer, and then oiled and taken off.However, by increasing the molecular orientation during spinning, the polylactic acid stereocomplex crystal Is preferably formed at a rate of 300 m / min or more, since it is easy to form. From the same viewpoint, the spinning draft is preferably 50 or more. In addition, it is preferable to use an air current suction device on the upstream side of the cooling or in the cooling section in order to remove sublimates from the discharged yarn. In addition, just below the spinning,
Before cooling and solidifying, it is preferable to install a heating zone to heat the yarn to a temperature equal to or higher than the melting point of the polymer from the viewpoint of increasing the strength of the fiber. For cooling, any of annular chimney and uniflo chimney can be used. The drawn undrawn yarn is then subjected to drawing. Either a two-step method of winding once before drawing or a direct spin drawing method of continuously drawing without winding after spinning may be used, but from the viewpoint of productivity, the direct spin drawing method is used. Is preferred.

The stretching step may be performed in one step or in two or more steps, but it is preferable to perform two or more steps in view of increasing strength. On the other hand, if the draw ratio is too high, the whitening phenomenon of the fiber occurs and the strength is reduced. Therefore, it is preferable to set the draw ratio so that the whitening phenomenon of the fiber does not occur. Any method may be used as the stretching heat source, such as a hot roller, a contact hot plate, a non-contact hot plate,
A heat medium bath, pins, etc. may be used.

After the stretching, a heat treatment is preferably performed at a temperature lower by about 10 to 80 ° C. than the melting point of the polymer before winding. Any method such as a hot roller, a contact hot plate, and a non-contact hot plate can be used for the heat treatment. In addition, from the viewpoint of dimensional stability, 0
Preferably, a relaxation treatment of up to 20% is performed.

In the polylactic acid fiber of the present invention, monofilament, multifilament, staple,
Any form such as non-woven fabric can be selected. When used as a multifilament, the single fiber fineness may be selected according to the use form, but is usually 0.1 dTex or more, 22 d
It is preferably set to Tex or less. The total fineness of the multifilament is preferably 5 dTex or more and 3330 dTex or less. Further, the cross-sectional shape is arbitrary such as round, flat, hollow, Y-shaped, T-shaped, and polygonal, but a round cross-section is preferable from the viewpoint of easily achieving high strength.

[0031]

Hereinafter, the present invention will be described more specifically based on examples.

The physical properties in the examples are values measured by the following methods. (1) Strength Te (cN / dtex): Measured using a “Tensilon” tensile tester type manufactured by Orientec Co., Ltd. under the conditions of a sample length of 25 cm and a tensile speed of 30 cm / min. (2) Birefringence Δn: Measured by a compensator method using a BH-2 type polarizing microscope manufactured by OLYMPUS CORPORATION with a filter at a light source wavelength of 589 nm. (3) Boiling water shrinkage ratio Sb (%): The yarn to be measured was subjected to boiling water at normal pressure in boiling water with no weight applied.
At the time of the treatment for 1 minute, the yarn length before and after the treatment was measured under a load of 0.0882 cN / dTex, and the shrinkage ratio of the yarn length after the treatment to the yarn length before the treatment was obtained. (4) Endothermic amount ΔHl (J / g) based on crystal melting of poly L-lactic acid single crystal and poly D-lactic acid single crystal and poly L
Endotherm ΔHh (J / g) based on crystal melting of stereocomplex crystal composed of -lactic acid and poly-D-lactic acid:
"SSC5200 / DSC" manufactured by Seiko Electronic Industry Co., Ltd.
Using a 120 "differential scanning calorimeter, measurement was performed at a heating rate of 10 ° C./min, and the DSC curve was obtained. (5) Heat resistance at 200 ° C .: 10 c using the fiber to be tested
An m-sided fabric was formed, and the state of the fabric after contact with an iron adjusted to a temperature of 200 ° C. for 30 seconds was observed.

：: The fabric shape before the treatment was maintained without heat fusion between the single yarns.

X: Thermal fusion between single yarns, thermal deformation or thermal fusion of the fabric was observed.

Example 1 L-lactide was mixed with 150 ppm of tin octylate, polymerized in a reaction vessel equipped with a stirrer at 192 ° C. for 10 minutes in a nitrogen atmosphere, and further formed into chips by a twin-screw kneading extruder. Thereafter, solid phase polymerization is carried out in a nitrogen atmosphere at 140 ° C. to obtain a melting point of 176 ° C. and a weight average molecular weight of 15.
10,000 poly L-lactic acid homopolymer (PLLA1) chips were obtained. Further, a poly D-lactic acid (PDLA) chip manufactured by PURAC and having a weight average molecular weight of 302,000 was used as the poly D-lactic acid homopolymer. This PLLA1 and PDL
A was blended with PLLA1: PDLA = 50/50 by weight in a chip ratio, dried under reduced pressure at 100 ° C. for 12 hours, melted and kneaded with a biaxial kneading extruder to obtain chips.
A pre-kneaded chip of a mixture comprising LLA1 (50 parts) and PLDA (50 parts) was prepared.

The pre-kneaded chips were dried under reduced pressure at 100 ° C. for 12 hours, and were melted at a spinning temperature of 260 ° C. using a pressure melter type melt spinning machine for melt extrusion.
100m length immediately after spinning from a die with 12 φ discharge holes
m, after passing through the atmosphere in the heating cylinder at a temperature of 280 ° C.
After cooling with a chimney wind at a wind speed of 30 m / min to apply an oil agent, it is taken up at a speed of 1000 m / min to obtain 84 d.
The undrawn yarn of Tex / 12fil was once wound up. The undrawn yarn was subjected to single-stage drawing at a total draw ratio of 2.8 using two pairs of Nelson rollers at 80 ° C. and 120 ° C., and the drawn yarn was wound. Table 1 shows the evaluation results of the drawn yarns. The wound stretched yarn had sufficient strength for practical use, and also had good heat resistance, as can be seen from the results of the heat resistance test at 200 ° C.

[0037]

[Table 1]

Example 2 An undrawn yarn of 84 dTex / 12fil was once wound up in the same manner as in Example 1. The undrawn yarns are respectively 80 ° C, 120 ° C, 180 ° C.
2. Using a pair of Nelson rollers at a temperature of 3.degree.
The stretched yarn was wound up at 6 times and stretched in two steps. Table 1 shows the evaluation results of the drawn yarns. The wound stretched yarn had sufficient strength for practical use, and also had good heat resistance, as can be seen from the results of the heat resistance test at 200 ° C.

(Embodiment 3) PLLA1 similar to Embodiment 1
A chip and a PDLA chip are prepared.
And PDLA were blended at a weight ratio of PLLA: PDLA = 50/50, and then dried under reduced pressure at 100 ° C. for 12 hours. Without preparing a pre-kneaded chip from this blended chip, the mixture is supplied to a single-screw extruder-type melt spinning machine with a cylinder temperature of 260 ° C. to melt and knead both, and discharge, cool, and take off under the same conditions as in Example 1. , An undrawn yarn of 84dTex / 12fil was once wound up. The undrawn yarns are each heated at 80 ° C and 120 ° C.
Using a pair of Nelson rollers at 180 ° C. and 180 ° C., the stretched yarn was wound by performing two-stage stretching at a total stretching ratio of 3.0. Table 1 shows the evaluation results of the drawn yarns. The wound stretched yarn had sufficient strength for practical use, and also had good heat resistance, as can be seen from the results of the heat resistance test at 200 ° C.

Example 4 An undrawn yarn of 84 dTex / 12fil was once wound in the same manner as in Example 1 except that the undrawn yarn was wound at a spinning speed of 250 m / min. The undrawn yarns are respectively 80 ° C, 120 ° C, 180 ° C.
1. Using a pair of Nelson rollers at 3 ° C., a total draw ratio of 2.
The drawing yarn was wound up by performing two-stage drawing at 9 times. Table 1 shows the evaluation results of the drawn yarns. The wound stretched yarn had sufficient strength for practical use, and also had good heat resistance, as can be seen from the results of the heat resistance test at 200 ° C.

Example 5 A drawn yarn was produced in the same manner as in Example 1 except that the weight ratio between PLLA1 and PDLA was PLLA1: PDLA = 60/40.
Table 1 shows the evaluation results of the drawn yarns. The wound stretched yarn had sufficient strength for practical use, and also had good heat resistance, as can be seen from the results of the heat resistance test at 200 ° C.

(Comparative Example 1) Stretching was performed in the same manner as in Example 1 except that the weight ratio between PLLA and PDLA was PLLA1: PDLA = 100/0 (the homopoly L-lactic acid chip was subjected to melt spinning). A yarn was made. Table 1 shows the evaluation results of the drawn yarns. The drawn yarn thus wound had sufficient strength for practical use, but had insufficient heat resistance as can be seen from the results of the heat resistance test at 200 ° C.

Comparative Example 2 A drawn yarn was produced in the same manner as in Example 1 except that the weight ratio between PLLA and PDLA was PLLA1: PDLA = 80/20. Table 1 shows the evaluation results of the drawn yarns. The drawn yarn thus wound had sufficient strength for practical use, but had insufficient heat resistance as can be seen from the results of the heat resistance test at 200 ° C.

Comparative Example 3 The same procedure as in Example 1 was repeated except that the undrawn yarn was wound up at a spinning speed of 50 m / min.
An undrawn yarn of 4dTex / 12fil was once wound up.
The undrawn yarn was subjected to two-stage drawing at a total draw ratio of 3.3 times using three pairs of Nelson rollers at 80 ° C., 120 ° C., and 180 ° C., and the drawn yarn was wound. Table 1 shows the evaluation results of the drawn yarns. The drawn yarn thus wound had sufficient strength for practical use, but had insufficient heat resistance as can be seen from the results of the heat resistance test at 200 ° C.

(Comparative Example 4) After the drawn yarn was wound in the same manner as in Comparative Example 3, a constant-length heat treatment was performed for 5 minutes using a dry-heat oven whose temperature in the processing chamber was adjusted to 200 ° C. Table 1 shows the evaluation results of the drawn yarns. The wound drawn yarn is 2
As can be seen from the results of the heat resistance test at 00 ° C., it had good heat resistance, but the strength was as low as 1.8 cN / dTex, which was insufficient for practical use.

[0046]

The polylactic acid fiber of the present invention is an unprecedented polylactic acid fiber having both high strength and high heat resistance and high quality, and is widely suitable and useful for clothing and industrial use.

Further, these fibers can be efficiently and industrially produced by a melt spinning method.

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Claims (3)

[Claims] 1. A fiber comprising a blend of poly-L-lactic acid and poly-D-lactic acid obtained by a melt spinning method, having a strength of at least 2.6 cN / dTex, and a poly-L-lactic acid alone determined by DSC measurement. The ratio ΔHh / ΔHl of the endothermic amount ΔHl based on the crystal melting of the crystal and the poly-D-lactic acid single crystal to the endothermic amount ΔHh based on the crystal melting of the stereocomplex crystal composed of poly L-lactic acid and poly-D-lactic acid is 10
Polylactic acid fiber characterized by the above. 2. The polylactic acid fiber according to claim 1, wherein the birefringence Δn is 18 or more. 3. The polylactic acid fiber according to claim 1, wherein the boiling water shrinkage Sb is 3% or more.