JP3864188B2 - High strength and high modulus fiber of polyhydroxyalkanoic acid and method for producing the same - Google Patents

Description

【００３７】
【実施例５〜８】
実施例１〜４と同様の方法で繊維を作製した。ただし、二段目の延伸を２５℃で、実施例５〜８についてそれぞれ３〜１０秒で延伸した。延伸倍率を表２に示すとおりとした。
延伸繊維を、延伸器に両端を固定したまま、温風に曝し、５０℃、５分間の熱処理を行った。
得られた繊維について、破壊強度、破壊伸び、及びヤング率を測定した。結果を表２に示す。
【００３８】
これらの結果から、本発明の方法により、繊維の物性が向上することが分かる。
【００３９】
【表２】

【００４０】
【発明の効果】
PHA類産生微生物の野生株産生物、遺伝子組換え株産生物あるいは化学合成物等、その由来によって異なる、PHA類の分子量などに関わらず、高強度かつ高弾性率である繊維が得られる方法および該方法によって得られる繊維を提供することができる。
【図面の簡単な説明】
【図１】 図１Ａは、溶融押出と氷水中での巻き取りの方法の模式図である。図１Ｂは、two roll set（２つの巻き取りローラー）を用いた氷水中での延伸方法の模式図である。図１Ｃは、延伸器を用いた二段階延伸の模式図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to fibers made from polyhydroxyalkanoic acids (hereinafter also referred to as “PHAs”) and a method for producing the same. Specifically, the present invention relates to a fiber having high fracture strength and Young's modulus, high strength and high elastic modulus, and a method for producing the same.
[0002]
[Prior art]
Since PHA has biodegradability and biocompatibility, its use for various molded products such as fibers and films has been studied.
Since fibers made from PHA have biodegradability and biocompatibility, medical devices such as surgical sutures, fishery products such as fishing lines and fishing nets, clothing materials such as fibers, non-woven fabrics, Great demand can be expected for building materials such as ropes, food and other packaging materials.
[0003]
Among PHAs, poly (3-hydroxybutanoic acid) (hereinafter also referred to as “P (3HB)”) is synthesized by many microorganisms as an intracellular storage material and accumulates in the form of granules in the cytoplasm. It is known to do (Non-Patent Document 1).
In addition, the inventors of the present invention have used P (3HB), which has a dramatically improved molecular weight, compared to a wild-type strain of a P (3HB) -producing microorganism using genetically modified Escherichia coli, a poly (3-hydroxybutanoic acid) synthetic gene It has succeeded in obtaining (Patent Document 1).
P (3HB) obtained from these P (3HB) -producing microorganisms is expected as a raw material for biodegradable products.
[0004]
So far, fibers using P (3HB) as a raw material have been manufactured by a method in which P (3HB) having a weight average molecular weight of about 600,000 (number average molecular weight of 300,000) is used as a raw material, melt-extruded, stretched at a high temperature, and heat-treated. I came. As such a method, specifically, in Non-Patent Document 2, P (3HB) having a weight average molecular weight of 300,000 is purified by chloroform, and the four-stage melting zone temperature (170 ° C-175 ° C-180 ° C-182 ° C) is obtained. ) And melt-extruded at 110 ° C. at a draw ratio of 800%, kept at 155 ° C. for 1 hour, and crystallized to produce a fiber. The physical properties of the obtained fiber are a breaking strength of 190 MPa, a breaking elongation of 54%, and a Young's modulus of 5.6 GPa. Non-Patent Document 3 discloses that without refining P (3HB) having a viscosity average molecular weight of 540,000, the pellets were once converted into pellets having a viscosity average molecular weight of 360,000, and the pellets were melt-extruded at 173 ° C., 2000-3500 m / min. Alternatively, a method is described in which a fiber is produced by winding at a winding speed of 250 m / min, stretching at 40-60 ° C., stretching at a stretch ratio of 400% or 690%, and keeping at 40-60 ° C. for crystallization. ing. The physical properties of the obtained fiber are a breaking strength of 330 MPa, a breaking elongation of 37%, and a Young's modulus of 7.7 GPa.
However, these fibers do not have physical properties comparable to general-purpose polymers and have not yet been put into practical use.
[0005]
On the other hand, Non-Patent Document 4 uses P (3HB) granules unpurified, melt-extruded at a melting temperature of 180 ° C. and a nozzle temperature of 170 ° C., wound at a winding speed of 28 m / min, 110 A method is described in which a fiber is drawn by stretching to a draw ratio of 600% at 0 ° C., and keeping it at 75, 100, 125, and 150 ° C. for 2.5 minutes under a tension of 0 MPa, 50 MPa, and 100 MPa, and crystallizing. Yes. The resulting fiber has a breaking strength of 310 MPa, a breaking elongation of 60%, and a Young's modulus of 3.8 GPa.
However, high-strength and high-modulus fibers made from purified P (3HB) and P (3HB) of any molecular weight, including high molecular weight P (3HB) with a weight average molecular weight of 600,000 or more, and production thereof No method has been found.
[0006]
Accordingly, a method for producing fibers having high strength and high elastic modulus with improved physical properties while maintaining biodegradability, including PHAs derived from wild strains of PHA-producing microorganisms and various PHAs as raw materials. Development was desired.
[0007]
[Non-Patent Document 1]
Anderson, AJ and Dawes, EA, Microbiol. Rev., 54: 450-472 (1990)
[Non-Patent Document 2]
Gordeyev et al., J. Mater. Sci. Lett., 18, 1691 (1999)
[Non-Patent Document 3]
Schmack et al., J. Polym. Sci. Polym. Phys. Ed., 38, 2841 (2000)
[Non-Patent Document 4]
Yamane et al., Polymer, 42, 3241 (2001)
[Patent Document 1]
JP-A-10-176070 [0008]
[Problems to be solved by the invention]
The subject of the present invention is high strength and high elastic modulus regardless of the molecular weight of PHAs, which differs depending on the origin, such as wild-type products of PHA-producing microorganisms, products of genetically modified strains or chemical compounds It is to provide a method for obtaining a fiber and a fiber having a high strength and a high elastic modulus obtained by the method.
[0009]
[Means for Solving the Problems]
As a result of intensive studies, the inventors of the present invention melt-extruded polyhydroxyalkanoic acid, rapidly cooled to a glass transition temperature of the polyhydroxyalkanoic acid + 15 ° C. or lower, and solidified to produce amorphous fibers. The present inventors have found that the above problems can be solved by cold-drawing a crystalline fiber at a glass transition temperature of + 20 ° C. or lower, further drawing at a glass transition temperature or higher, and further performing a tension heat treatment.
[0010]
That is, the gist of the present invention is as follows.
(1) Polyhydroxyalkanoic acid is melt-extruded, rapidly cooled to a glass transition temperature of the polyhydroxyalkanoic acid of + 15 ° C. or lower and solidified to produce amorphous fibers, and the amorphous fibers are converted to a glass transition temperature of +20. A method for producing a fiber, characterized in that it is cold-drawn at a temperature not higher than ° C., further drawn at a glass transition temperature or higher, and further subjected to tension heat treatment.
(2) The method according to (1), wherein stretching at a glass transition temperature or higher is performed in two or more stages.
(3) The method according to (2), wherein the temperature at each stage of stretching at or above the glass transition temperature is higher than the temperature at the previous stage of each stage.
(4) The method according to any one of (1) to (3), wherein the cold drawing is performed by applying tension with two winding rollers.
(5) The method according to any one of (1) to (4), wherein the polyhydroxyalkanoic acid is poly (3-hydroxybutanoic acid).
(6) Polyhydroxyalkanoic acid is melt-extruded, rapidly cooled to a glass transition temperature of the polyhydroxyalkanoic acid of + 15 ° C. or lower and solidified to produce amorphous fibers, and the amorphous fibers are converted to a glass transition temperature of +20. A fiber having a breaking strength of 350 MPa or more and a Young's modulus of 2 GPa or more, which is produced by cold drawing at a temperature of not higher than ° C., drawing at a glass transition temperature or higher, and further subjected to tension heat treatment.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below.
(1) Manufacturing method of fiber of the present invention (i) PHA used in the present invention In the manufacturing method of the present invention, polyhydroxyalkanoic acids are used as a fiber molding material. Preferable polyhydroxyalkanoic acid includes polyhydroxybutanoic acid (hereinafter also referred to as “PHB”). As methods for obtaining PHB, there are generally a fermentation synthesis method and a chemical synthesis method. The chemical synthesis method is a method of chemically synthesizing according to an ordinary organic synthesis method, and is a mixture of a poly [(R) -3-hydroxybutanoic acid] and poly [(S) -3-hydroxybutanoic acid] (racemic body). ) Is obtained. In contrast, the fermentative synthesis method is a method of culturing a microorganism having a PHB production ability and taking out PHB accumulated in the microbial cell. PHB obtained by the fermentation synthesis method is a poly [(R) -3-hydroxybutanoic acid] homopolymer.
[0012]
The microorganism that can be used in the fermentation synthesis method is not particularly limited as long as it is a microorganism having a PHB-producing ability. PHB is known to accumulate in the body of more than 60 kinds of natural microorganisms, including Algigenes genus such as Ralstonia eutropha, Alcaligenes latus and Alcaligenes faecalis. Yes. Examples of producing a high molecular weight PHB having a weight average molecular weight of 1 million (number average molecular weight 500,000) or more include strains belonging to the genus Methylobacterium, specifically Methylobacterium extorquens ATCC55366 ( Bourque, D. et al., Appl. Microbiol. Biotechnol (1995)). These strains are commercially available from the American Type Culture Collection (ATCC).
[0013]
In the fermentative synthesis method, these microorganisms are usually cultured in a normal medium containing a carbon source, a nitrogen source, inorganic ions, and other organic components as required, so that PHB can be accumulated in the cells. Collection of PHB from the microbial cells can be carried out by extraction with an organic solvent such as chloroform, a method in which the microbial components are decomposed with an enzyme such as lysozyme, and then PHB granules are separated by filtration.
[0014]
In addition, as one aspect of the fermentation synthesis method, there is a method of culturing a microorganism transformed by introducing a recombinant DNA containing a PHB synthesis gene and collecting PHB produced in the cell body. In this method, unlike directly cultivating Ralstonia, Eutropha, etc., microorganisms transformed by introducing recombinant DNA do not have PHB-degrading enzymes in the cells, and thus accumulate extremely high molecular weight PHB. be able to.
As such a transformed strain, for example, in Japanese Patent Laid-Open No. 10-176070, a transformed strain Escherichia coli XL1 obtained by introducing a plasmid pSYL105 containing the PHB synthetic gene of Ralstonia eutropha, phbCAB, into Escherichia coli XL1-Blue. -Blue (pSYL105) is disclosed. The transformant Escherichia coli XL1-Blue (pSYL105) can be obtained from Stratagene Cloning System (11011 North Torrey Pines Road La Jolla CA92037, USA).
[0015]
The transformant is cultured in a suitable medium, and PHB is accumulated in the microbial cells. Examples of the medium to be used include a normal medium containing a carbon source, a nitrogen source, inorganic ions, and other organic components as necessary. When using Escherichia coli, examples of the carbon source include glucose, and examples of the nitrogen source include those derived from natural products such as yeast extract and tryptone. In addition, inorganic nitrogen compounds such as ammonium salts may be contained. It is preferable to control the culture under aerobic conditions for 12 to 20 hours, the culture temperature at 30 to 37 ° C., and the pH during the culture at 6.0 to 8.0. Collection of PHB from the microbial cells can be carried out by extraction with an organic solvent such as chloroform, a method in which the microbial components are decomposed with an enzyme such as lysozyme, and then PHB granules are separated by filtration. Specifically, for example, PHB can be extracted from a dry cell separated and recovered from the culture solution with a suitable poor solvent and then precipitated with a precipitant.
[0016]
As PHAs used in the present invention, commercially available polyhydroxyalkanoic acids may be used.
[0017]
The molecular weight of the polyhydroxyalkanoic acid used in the present invention is not particularly limited as long as the effects of the present invention are not impaired, but is preferably a weight average molecular weight of 400,000 (number average molecular weight 200,000) or more. The upper limit of the weight average molecular weight is not particularly limited, but from the viewpoint of easy availability and moldability, the weight average molecular weight is preferably 4 million (number average molecular weight 2 million) or less, particularly preferably the weight average molecular weight 1 million (number average molecular weight 50 The following are used.
[0018]
As the polyhydroxyalkanoic acids used in the present invention, granules containing PHAs may be used without purification, or those purified and polymerized by the purification method described below may be used.
[0019]
(Ii) Production method of the present invention In the method of the present invention, the PHAs described above are melt-extruded, rapidly cooled to a glass transition temperature of the PHAs of + 15 ° C or lower and solidified to produce amorphous fibers, A crystalline fiber is cold-drawn at a glass transition temperature + 20 ° C. or lower, further drawn at a glass transition temperature or higher, and further subjected to tension heat treatment to produce a fiber.
[0020]
As a method of melt extrusion of PHAs, it can be performed by using a normal plastic fiber melting technique, for example, by heating and melting PHAs, applying a load, and extruding from an extrusion port. .
The temperature at the time of melt extrusion is usually not lower than the melting point of the polyhydroxyalkanoic acid to be melted, preferably melting point + 10 ° C. or higher, more preferably melting point +15 to 20 ° C. or higher. In the case of PHB, the melting point is 175 ° C. or higher.
[0021]
The molten polyhydroxyalkanoic acid is extruded into a cooling medium having a glass transition temperature of + 15 ° C. or lower, preferably a glass transition temperature of + 10 ° C. or lower, more preferably a glass transition temperature or lower, and rapidly cooled and fiberized. There is no particular lower limit to the temperature for rapid cooling and fiberization, but it can usually be carried out at -180 ° C or more from the viewpoint of economy. By the same quenching process, the molten polyhydroxyalkanoic acid becomes an amorphous fiber. The resulting fiber can be wound up in a cooling solvent. The glass transition temperature can be evaluated, for example, by performing dynamic viscoelasticity measurement. The dynamic viscoelasticity can be measured, for example, in a range of −100 to 120 ° C. using a Seiko Electronic DMS210 under a nitrogen atmosphere and a frequency of 1 Hz and a temperature rising rate of 2 ° C./min. For low molecular weight PHB, the glass transition temperature is 4 ° C. or lower. For high molecular weight PHB, the glass transition temperature is 10 ° C. or lower. Furthermore, in the case of high molecular weight PHB, the glass transition temperature is 20 ° C. or lower. In addition, the one where glass transition point temperature is higher is useful at the point that it is easy to process.
Examples of the cooling medium include air, water (ice water), and an inert gas. In the present invention, the rapid cooling can be carried out, for example, by extruding molten polyhydroxyalkanoic acid into air or ice water having a glass transition temperature of + 15 ° C. or lower and passing it through the same solvent while winding. The winding speed is usually 3 to 150 m / min, preferably 3 to 30 m / min.
The amorphous fiber can be confirmed by a method such as X-ray diffraction, for example. In X-ray diffraction, if a peak derived from a crystal cannot be confirmed, it can be said to be amorphous.
[0022]
The obtained amorphous fiber is cold-drawn. Preferably, the cold drawing can be performed at a glass transition temperature + 20 ° C. or lower, more preferably at a glass transition temperature + 10 ° C. or lower, and even more preferably at a glass transition temperature or lower. There is no particular lower limit to the temperature of cold drawing, but it can usually be carried out at -180 ° C or more from the economical point of view. Stretching can be performed, for example, by fixing to a stretcher and applying tension while preferably winding with two roll rollers. When the film is stretched while being fixed to a stretching machine or the like, the stretching ratio is usually 200% or more, preferably 400% or more. There is no particular upper limit for the draw ratio, and it is sufficient that it does not break. The stretching time is usually 1 to 10 seconds, and the stretching time can be determined according to the stretching ratio. When stretching while taking up with a take-up roller or the like, the draw ratio is usually 300% or more, preferably 600% or more. There is no particular upper limit for the draw ratio, and it is sufficient that it does not break. When extending | stretching, winding up with a winding roller etc., extending | stretching time is not specifically limited, It can carry out in the range of a conventional method.
[0023]
After cold drawing, further drawing is performed. Preferably, the stretching can be performed at a glass transition temperature or higher, more preferably a glass transition temperature + 5 ° C. or higher, and even more preferably a glass transition temperature + 10 ° C. or higher. There is no particular upper limit for the stretching temperature above the glass transition temperature, but stretching that can be performed usually below the melting point can be performed, for example, by tension or fixation. When the film is stretched while being fixed to a stretching machine or the like, the stretching ratio is usually 200% or more, preferably 400% or more. The stretching time is usually 1 to 10 seconds, and the stretching time can be determined according to the stretching ratio.
In the method of the present invention, the stretching after cold stretching can be performed in one stage or in multiple stages of two or more stages.
The temperature of each stage is preferably higher than the temperature of the previous stage, and can usually be performed at the temperature of the previous stage + 5 ° C. or higher, preferably at the temperature of the previous stage + 10 ° C. or higher. There is no particular upper limit on the temperature at each stage, and it can be usually performed at a melting point or lower.
[0024]
After stretching, a tension heat treatment is performed. The tension heat treatment can be performed by hot air heat treatment, dryer heat treatment, or the like. The tension heat treatment is to perform heat treatment under tension, and tension can be performed by, for example, fixing, weighting, tension, or the like. The fixed heat treatment is to perform heat treatment in a state where both ends of the fiber are fixed. In addition, when a weight is hung and weighted at the end of the fiber, the heavier is better as long as the fiber is not cut. The weight can be determined in a range up to the extent that the drawn fiber is not cut by applying a weight. The weight 0 g is a weight at which the fiber does not stretch. Moreover, when heat-treating by applying tension with a winding roller, it can be performed while applying tension by changing the roller speed of feeding and winding. The fiber is heat-treated while being drawn by tension. When heat treatment is performed by applying tension with a take-up roller, it can be usually performed at a draw ratio of 0% or more, preferably 300% or more. Note that stretching at a magnification of 0% means stretching so that the fiber does not stretch. There is no particular upper limit for the draw ratio, and it is sufficient that it does not break. When extending | stretching, winding up with a winding roller etc., extending | stretching time is not specifically limited, It can carry out in the range of a conventional method.
[0025]
Up to now, there have been reports on fiberization of low molecular weight PHB having a weight average molecular weight of about 600,000 (number average molecular weight of 300,000), but most of them have not obtained physical properties comparable to general-purpose polymers. . In addition, there is no report regarding such a method that it has been applied to a polymer PHB having a weight average molecular weight of 600,000 (number average molecular weight of 300,000) or more. However, according to the method of the present invention, it is possible to obtain a fiber having high strength and high elastic modulus regardless of the molecular weight of PHB or the presence or absence of purification.
[0026]
(2) The fiber of the present invention The fiber of the present invention melts and extrudes PHAs, rapidly cools to a glass transition temperature of the PHAs + 15 ° C. or lower and solidifies to produce amorphous fibers. Is a fiber produced by cold drawing at a glass transition temperature + 20 ° C. or lower, further drawing at a glass transition temperature or higher, and further subjected to tension heat treatment. Among such fibers, preferred is a fiber obtained by the above method, having a breaking strength of 350 MPa or more and a Young's modulus of 2 GPa or more.
[0027]
The breaking strength here is measured according to JIS-K-6301, and is 350 MPa or more, preferably 400 MPa or more for the fiber of the present invention. The Young's modulus is measured according to JIS-K-6301, and is 2 GPa or more, preferably 4 GPa or more, more preferably 6 GPa or more for the fiber of the present invention.
[0028]
The fiber of the present invention is an oriented crystalline fiber in which the orientation of the crystal part in the PHA fiber is a constant direction. Most of the fibers produced from low molecular weight PHAs obtained by conventional production methods have not obtained physical properties comparable to general-purpose polymer fibers. Moreover, such a conventional production method has never been applied to a polymer PHB having a weight average molecular weight of 600,000 (number average molecular weight of 300,000) or more. However, by the method of the present invention, oriented crystalline fibers having physical properties comparable to general-purpose polymer fibers can be obtained regardless of the molecular weight.
[0029]
As the fiber molding material in the present invention, various additives usually used for fibers other than the above PHAs, such as lubricants, ultraviolet absorbers, weathering agents, antistatic agents, antioxidants, thermal stabilizers, nucleating agents, fluids An improving agent, a coloring agent, etc. can be contained as needed.
[0030]
The fibers of the present invention are composed of PHAs having sufficient strength and flexibility as described above, and excellent in biodegradability and biocompatibility. Medical devices such as surgical sutures, fishing lines It is useful for fishery equipment such as fishing nets, clothing materials such as textiles, non-woven fabrics, building materials such as ropes, food and other packaging materials.
[0031]
【Example】
EXAMPLES The present invention will be described more specifically with reference to the following examples. However, the present invention is not limited to these examples unless it exceeds the gist.
[0032]
Examples 1 to 4
Recombinant Escherichia coli XL1-Blue (pSYL105) was prepared and cultured according to the method described in JP-A-10-176070, and PHB was obtained from the cells. As a result of measuring the weight average molecular weight of the obtained PHB according to the method described in JP-A-10-176070, the weight average molecular weight was 3 million (number average molecular weight 1.5 million).
[0033]
These PHBs were melted at 200 ° C., extruded into ice water (3 ° C.) through an extrusion port by weighted extrusion, rapidly cooled, and fiberized. The obtained fiber was wound up in ice water (3 ° C.). An example of an apparatus used for this operation is shown in FIG. 1A as a schematic diagram. The PHB granules and the polymer are melted while being heated with a heater, extruded into an ice water bath, and the resulting fibers are wound up with a roller. The extrusion diameter was 1 mm. The winding speed was 6 m / min.
The obtained fiber was drawn in ice water (3 ° C.) with a two roll set. The draw ratio was as shown in Table 1. An example of a two roll set used for this operation is shown in FIG. 1B as a schematic diagram. The fiber wound on one winding roller is stretched while being wound in an ice water bath by another roller. In such an apparatus, the fiber can be drawn at a desired draw ratio by changing the speed of the two winding rollers.
The stretched fiber was attached to a stretcher and stretched for 6-8 seconds for Examples 1-4 at 20 ° C. The draw ratio was as shown in Table 1. An example of an apparatus used for this operation is shown in FIG. 1C as a schematic diagram. The fiber attached to the stretcher is stretched while being stretched.
The stretched fiber was exposed to warm air with both ends fixed to a stretcher, and heat-treated at 70 ° C. for 5 minutes.
About the obtained fiber, breaking strength, breaking elongation, and Young's modulus were measured. The results are shown in Table 1.
[0034]
The breaking strength, breaking elongation, and Young's modulus were measured using an SV-200 type tensile and compression testing machine manufactured by Imada Seisakusho in accordance with JIS-K-6301. The tensile speed was 50 mm / min.
[0035]
From these results, it can be seen that the physical properties of the fibers are improved by the method of the present invention.
[0036]
[Table 1]

[0037]
Examples 5 to 8
Fibers were produced in the same manner as in Examples 1 to 4. However, the second-stage stretching was performed at 25 ° C., and each of Examples 5 to 8 was stretched in 3 to 10 seconds. The draw ratio was as shown in Table 2.
The stretched fiber was exposed to warm air with both ends fixed to a stretcher, and heat-treated at 50 ° C. for 5 minutes.
About the obtained fiber, breaking strength, breaking elongation, and Young's modulus were measured. The results are shown in Table 2.
[0038]
From these results, it can be seen that the physical properties of the fibers are improved by the method of the present invention.
[0039]
[Table 2]

[0040]
【The invention's effect】
A method for obtaining a fiber having high strength and high elasticity regardless of the molecular weight of PHAs, which differs depending on the origin, such as wild-type products of PHA-producing microorganisms, products of genetically modified strains or chemical compounds The fiber obtained by this method can be provided.
[Brief description of the drawings]
FIG. 1A is a schematic view of a method of melt extrusion and winding in ice water. FIG. 1B is a schematic diagram of a stretching method in ice water using two roll sets (two winding rollers). FIG. 1C is a schematic diagram of two-stage stretching using a stretcher.

Claims (6)

Polyhydroxyalkanoic acid is melt-extruded, rapidly cooled to a glass transition temperature of the polyhydroxyalkanoic acid + 15 ° C. or lower and solidified to produce amorphous fibers, and the amorphous fibers are cold-drawn at a temperature lower than the glass transition temperature. And drawing at a glass transition temperature or higher, followed by tension heat treatment.   The method according to claim 1, wherein the stretching at a glass transition temperature or higher is performed in two or more stages.   The method according to claim 2, wherein the temperature of each stage of stretching above the glass transition temperature is higher than the temperature of the previous stage of each stage.   The method according to any one of claims 1 to 3, wherein the cold drawing is performed by applying tension with two winding rollers.   The method according to any one of claims 1 to 4, wherein the polyhydroxyalkanoic acid is poly (3-hydroxybutanoic acid). Poly (3-hydroxybutanoic acid) is melt-extruded, rapidly cooled to a glass transition temperature of the poly (3-hydroxybutanoic acid) + 15 ° C. or lower, and solidified to produce amorphous fibers. A fiber having a breaking strength of 350 MPa or more and a Young's modulus of 2 GPa or more, which is produced by cold drawing at a glass transition temperature or lower , further drawing at a glass transition temperature or higher, and further subjected to tension heat treatment.

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