JP7335118B2 - Method for producing polylactic acid long fiber - Google Patents

Description

The present invention relates to a method for producing polylactic acid long fibers.

In recent years, so-called wearable sensors made of polylactic acid fibers have attracted attention, and products in the form of eyeglasses and wristwatches have begun to appear. It is desirable that wearable sensors do not give the wearer the feeling of being worn, but these devices give the wearer the feeling of being worn. As the ultimate wearable device that does not give such a sensation, it is desirable to have a cloth-like shape, that is, a cloth-like shape. As such a sensor, a piezoelectric element using a polylactic acid fiber having a piezoelectric effect as a piezoelectric fiber is known.

However, polylactic acid fibers have poor process passability in manufacturing processes such as spinning, drawing, and false twisting, and for this reason, the fiber products obtained have more fluff and devitrification than products using conventional synthetic fibers. It was inferior in terms of quality.

Patent Document 1 discloses a method for producing a cheese-like package of polylactic acid fibers, in which the fiber is taken up at a spinning speed of 2500 to 7000 m/min. According to this method, a cheese-like package of polylactic acid fibers having a good winding shape can be obtained. However, at spinning speeds higher than this, spinning breakage and devitrification frequently occur, and polylactic acid long fibers of satisfactory quality cannot be obtained.

In Patent Document 2, stretching is performed between a stretching preheating roller and a heating roller, and the heating roller is used as the only heat setting means. It is disclosed for use as a means. However, even with this heat setting means, polylactic acid long fibers with a high crystallinity necessary for obtaining piezoelectricity cannot be obtained.

Japanese Patent No. 4337344 Japanese Patent No. 4803192 Japanese Patent No. 4203978

SUMMARY OF THE INVENTION An object of the present invention is to provide a polylactic acid long fiber that does not cause fluffing or devitrification, and from which a braid having excellent piezoelectric properties can be obtained.

The present invention is a method for producing a drawn polylactic acid long fiber,
(Step 1) A polylactic acid polymer containing 95 mol % or more of L-lactic acid is melted, extruded through a spinneret, cooled and solidified, wound at a spinning speed of 2000 to 2500 m/min, and the birefringence Δn is 0.005 to 0. and (Step 2) drawing the undrawn yarn at a draw ratio of 1.6 to 2.0 using at least three contact drawing rollers to obtain a drawn poly. A method for producing drawn polylactic acid long fibers, characterized by including a step of obtaining lactic acid long fibers.

According to the present invention, it is possible to provide a polylactic acid long fiber that does not cause fluffing or devitrification and from which a braid having excellent piezoelectric properties can be obtained.

It is arrangement|positioning of the roller for demonstrating the aspect for implementing invention. It is an observation example of a devitrified thread and a non-devitrified thread by stereomicroscopic observation. It is an observation example of a devitrified yarn and a non-devitrified yarn by SEM observation.

The present invention will be described in detail below.

As described above, in the method for producing a drawn polylactic acid long fiber of the present invention, in step 1, a polylactic acid polymer containing 95 mol % or more of L-lactic acid is melted, extruded through a spinneret, cooled and solidified, and spun at a spinning speed of 2,000. A step of winding at ~2500 m/min to obtain an undrawn yarn having a birefringence Δn of 0.005 to 0.0074; A step of drawing at a draw ratio of 1.6 to 2.0 to obtain a drawn polylactic acid long fiber is included in this order.

[Step 1]
In step 1, a polylactic acid polymer containing 95 mol% or more of L-lactic acid is melted, extruded through a spinneret, solidified by cooling, and wound at a spinning speed of 2000 to 2500 m/min to obtain a birefringence Δn of 0.005 to 0. This is the step of obtaining an undrawn yarn of 0.0074.

It is important that the spinning speed is 2000-2500 m/min, preferably 2000-2250 m/min. At a spinning speed of less than 2000 m/min, in order to make the physical properties of the obtained polylactic acid long fibers practical, it is essential to draw at a high magnification in the next step, and fluff and devitrification occur in the next step. occur frequently. On the other hand, take-up at a preventive speed exceeding 2500 m/min causes frequent occurrence of spun yarn breakage, and the resulting undrawn yarn has a high birefringence, which tends to cause single yarn breakage in the next drawing step, resulting in loss. Due to the occurrence of permeation and fluff, the passability in high-order processing deteriorates.

Preferably, step 1 further includes a step of applying an oil agent to the polylactic acid polymer that has been extruded through the die and solidified by cooling. That is, in step 1, a polylactic acid polymer containing 95 mol % or more of L-lactic acid is melted, extruded from a spinneret, cooled and solidified, an oil agent is applied to the cooled and solidified polylactic acid polymer, and the spinning speed is 2000 to 2500 m/min. It is preferable that the steps are winding and obtaining an undrawn yarn having a birefringence Δn of 0.005 to 0.0074.

[Polylactic acid polymer]
As the polymer used in step 1, a polylactic acid polymer containing 95 mol % or more of L-lactic acid is used. Methods for producing polylactic acid polymers include a two-step lactide method in which lactide, which is a cyclic dimer, is first produced from L-lactic acid as a raw material and then ring-opening polymerization is performed, and a direct dehydration method in a solvent from L-lactic acid as a raw material. A one-step direct polymerization process with condensation is known. The polylactic acid polymer used in the present invention may be obtained by any production method.

The weight average molecular weight of the polylactic acid polymer to be subjected to step 1 is preferably 140,000 to 170,000, more preferably 140,000 to 160,000, particularly preferably 150,000 to 160,000. If the weight-average molecular weight is less than 140,000, the strength of the fiber will be low, and it is highly likely that the fiber will break during spinning and drawing. On the other hand, if the weight-average molecular weight exceeds 170,000, it is not preferable because the resulting fiber tends to be uneven during dyeing. do not have.

The melting point of the polylactic acid polymer to be subjected to step 1 is preferably 100° C. or higher, more preferably 140° C. or higher, and particularly preferably 170° C. or higher. If the melting point is less than 100°C, the quality of the product will be low, such as poor drawability due to fusion between single yarns during spinning, and melting defects during dyeing processing, heat setting, and friction heating. I don't like it. Here, the melting point means the peak temperature of the 1st run melting peak obtained by DSC measurement.

The polylactic acid polymer to be subjected to step 1 may be a copolymerized polylactic acid polymer obtained by copolymerizing L-lactic acid and other components capable of forming an ester. Copolymerizable components include hydroxycarboxylic acids such as glycolic acid, 3-hydroxybutyric acid, 4-hydroxybutyric acid, 4-hydroxyvaleric acid, and 6-hydroxycaproic acid, as well as ethylene glycol, propylene glycol, butanediol, neo Compounds containing multiple hydroxyl groups in the molecule, such as pentyl glycol, polyethylene glycol, glycerin, and pentaerythritol, or their derivatives, and compounds containing multiple carboxylic acid groups in the molecule, such as adipic acid, sebacic acid, and fumaric acid. Alternatively, derivatives thereof can be exemplified.

Aliphatic polyester polymers such as polycaprolactone, polybutylene succinate, polyethylene succinate may be used as internal or external plasticizers to reduce the melt viscosity of the polymer. Furthermore, inorganic fine particles and organic compounds may be added as matting agents, deodorants, flame retardants, yarn friction reducing agents, antioxidants, coloring pigments, and the like.

[Step 2]
Step 2 is a step of drawing the above undrawn yarn at a draw ratio of 1.6 to 2.0 times using at least three contact drawing rollers to obtain drawn polylactic acid long fibers. In step 2, it is essential to draw using at least three contact drawing rollers.
It is preferable that step 2 include the following steps 2-1, 2-2 and 2-3 in this order.

Step 2-1 is a preheating step in which the undrawn yarn obtained in step 1 is preheated by being brought into contact with a drawing preheating roller having a surface temperature of 70 to 100°C.

In step 2-2, the undrawn yarn that has undergone the preheating step is stretched at a draw ratio of 1.5 to 1.5 between a drawing preheating roller with a surface temperature of 70 to 100°C and a drawing heating roller with a surface temperature of 140 to 160°C. This is the first stage drawing process in which drawn yarn is obtained by drawing by 6 times.

In step 2-3, the drawn yarn is drawn at a draw ratio of 1.1 to 1.3 times between a drawing heating roller having a surface temperature of 140 to 160° C. and a drawing heating roller having a surface temperature of 140 to 160° C. is the second stage drawing step for obtaining drawn polylactic acid long fibers.

This step 2 is carried out by a stretching machine having at least one contact stretching preheating roller used in step 2-1 and at least two contact stretching heating rollers used in steps 2-2 and 2-3. is preferred.

[Unstretched fiber to be subjected to step 2]
It is important that the birefringence Δn of the undrawn yarn to be subjected to step 2 is 0.005 to 0.0074, preferably 0.006 to 0.0070 or less. If the birefringence .DELTA.n is less than 0.005, the molecular orientation of the undrawn yarn is extremely low, and the orientation is not smooth during the drawing. For this reason, regardless of the drawing conditions, the drawn yarn has a large fineness unevenness, which causes frequent yarn breakage in the process and dyeing abnormalities such as uneven dyeing in fabrics made using such fibers. Become. On the other hand, when an undrawn yarn having a birefringence Δn exceeding 0.0074 is provided, the elongation of the undrawn yarn is already reduced, and devitrification occurs. It is not preferable because it occurs and the high-order processing passability is deteriorated.

[One stretching preheating roller and two heated stretching rollers]
The stretching in step 2 uses a stretching machine having at least three contact-type stretching rollers and carries out stretching in two or more stages. In step 2, if at least three contact drawing rollers are not used, but only two contact drawing rollers are used, the heating efficiency is poor, the crystallinity, which is important for the piezoelectric fiber, is low, and the piezoelectric performance is satisfactory. can't get

In step 2, as shown in FIG. 1, it is most efficient and preferable to heat the unstretched yarn supplied from the stretching preheating roller 2 by the contact type stretching heating rollers 3 and 4.

In FIG. 1, the unstretched yarn is stretched by the first stage stretching between the stretching preheating roller 2 and the contact type stretching heating roller 3 . Further, the film is stretched by the second stage stretching between the contact type stretching heating roller 3 and the contact type stretching heating roller 4 . According to this method, an undrawn yarn with satisfactory crystallinity can be obtained without generating fluff or devitrification. Between the contact-type stretching/heating roller 4 and the unheated shrinking roller 5, the roller may be under tension or under relaxation, and may be stretched or relaxed.

[Stretching]
In the method for producing the polylactic acid long fiber of the present invention, it is preferable that the fiber is drawn by double drawing from the viewpoint of good mechanical properties and drawability. The unstretched yarn is preferably 1.1 to 2 0 times, more preferably 1.4 to 1.8 times, particularly preferably 1.5 to 1.7 times. This drawing is called first-stage drawing. If the draw ratio is less than 1.1 times, the oriented crystallization of the polymer by drawing does not proceed sufficiently, and the strength cannot be improved, which is not preferable. On the other hand, when it exceeds 2.0 times, fuzz and devitrification of the product occur frequently, which is not preferable.

After the first-stage drawing, the second-stage drawing is subsequently performed between the contact-type drawing heating roller 3 for the first-stage drawing and another contact-type drawing heating roller 4 rotating at a higher speed than the contact-type drawing heating roller 3. . The ratio of the second stage drawing is preferably 1.1 to 1.4 times, particularly preferably 1.2 to 1.3 times. If the draw ratio is less than 1.1 times, the oriented crystallization of the polymer by drawing does not proceed sufficiently, and the strength cannot be improved, which is not preferable. On the other hand, if the draw ratio exceeds 1.4 times, fuzz and devitrification frequently occur in the product, which is not preferable.

[Temperature of preheating roller for stretching and temperature of contact heating roller for stretching]
The relationship between the temperature (T2) of the stretching preheating roller 2, the temperature (T3) of the contact type stretching heating roller 3, and the temperature (T4) of the contact type stretching heating roller 4 preferably satisfies all of the following equations. .
70°C≤T2≤100°C
140°C≤T3≤160°C
140°C≤T4≤160°C
If the temperature T2 of the stretching preheating roller 2 is less than 70° C., the preheating becomes insufficient, and the thermal deformation during stretching cannot be performed uniformly, resulting in uneven dyeing. On the other hand, if the temperature exceeds 100° C., yarn swaying on the rollers becomes severe and the drawing point cannot be fixed, resulting in frequent yarn breakage during the drawing process. Therefore, the temperature T2 of the stretching preheating roller 2 is preferably 70 to 100.degree. C., more preferably 80 to 95.degree. C., and particularly preferably 80 to 90.degree.

Furthermore, the temperature (T3) of the contact type stretching heating roller 3 and the temperature (T4) of the contact type stretching heating roller 4 are, for example, 140 to 160°C, preferably 140 to 155°C, and particularly preferably 140 to 150°C. If the temperatures of T3 and T4 exceed 160° C., the process stability is lacking, accompanied by frequent occurrence of stretched yarn breakage and fluff. If the temperature is less than 140° C., the degree of crystallinity cannot be advanced, and the piezoelectric performance is not exhibited or is insufficient.

〔devitrification〕
Devitrification in the present invention means a phenomenon that occurs mainly in the drawing process and is caused by microscopic irregularities formed on the surface of the polylactic acid long fiber, and as the term suggests, it loses transparency and becomes whitened. be. Here, the size of the unevenness on the surface is on the sub-μ to μ-order. If it is less than a submicron, it will not devitrify, while if it is on the order of several tens of microns, it will usually cause thread breakage or fluffing at that portion, so it is rarely recognized as devitrification. Devitrification due to surface irregularities formed on the filaments during drawing means that smooth hot drawing is not performed and excessive drawing is performed.

Smooth hot drawing means, for example, in a multistage hot drawing process, each filament is preheated at a temperature suitable for the level of orientation before drawing, drawn at a draw ratio suitable for the mobility of the molecular chains, and the molecular chains are smoothly drawn. is stretched. If the preheating temperature is too high relative to the degree of orientation before stretching, the molecular chains crystallize before they are oriented, making it difficult to stretch, resulting in unevenness on the surface. If the tension is too high, the flow of molecular chains will occur, resulting in a so-called superdraw phenomenon that is not accompanied by orientation of the molecular chains, and effective stretching will not be achieved. In this case, although unevenness is not generated on the filament surface, strength cannot be obtained. On the other hand, if the preheating before stretching is insufficient, stretching will be forced while the mobility of the molecular chains is low, resulting in unevenness on the filament surface and devitrification.

Polylactic acid has a glass transition temperature (Tg) of about 60°C at which the mobility of the molecular chain occurs, and a melting temperature (melting point: Tm) of the fiber structure crystals of about 170°C. It is as small as about 110°C. Incidentally, polyethylene terephthalate has Tg=80°C and Tm=260°C, respectively, and the difference between Tm and Tg is 180°C. Therefore, since the optimum temperature range for hot drawing polylactic acid fibers is narrow, it is easy to deviate from smooth drawing conditions, and the surface irregularities and the like easily occur, and devitrification easily occurs.

[Stretched polylactic acid long fiber]
[Intrinsic viscosity]
The intrinsic viscosity (IV) of the drawn polylactic acid long fibers obtained by the production method of the present invention is preferably 1.4 to 1.7 dL/g, more preferably 1.6 to 1.7 dL/g. If the intrinsic viscosity (IV) is less than 1.4 dL/g, the moldability during production will be insufficient, stable spinning will not be possible, and fluff and yarn breakage will easily occur, making it unsuitable for practical use. I don't like it. On the other hand, if it exceeds 1.7 dL/g, it is necessary to set the melting temperature of the polymer high as a spinning condition, which causes serious problems such as process stability and frequent breakage over time, which is not preferable.

[Crystallinity]
According to the production method of the present invention, drawn polylactic acid long fibers having a degree of crystallinity of 40% or more can be obtained. If the degree of crystallinity is less than 40%, the piezoelectricity will be extremely low and the functional performance will be insufficient. The present invention includes an embodiment in which the drawn polylactic acid long fibers obtained by the production method of the present invention have a degree of crystallinity of 40% or more.

The degree of crystallinity is an important quality for piezoelectric elements, but if only the degree of crystallinity is measured in polylactic acid fibers, fuzz and devitrification of the product will occur, and adverse effects such as process stability will occur. In the present invention, a drawn polylactic acid long fiber having a degree of crystallinity of 40% or more is obtained by performing two-step drawing in which two heat drawing steps are performed in the drawing process.

〔Strength〕
The breaking strength of the drawn polylactic acid long fibers obtained by the production method of the present invention is preferably 4.0 cN/Dtex or more, more preferably 4.1 cN/Dtex or more. If the breaking strength is less than 4.0 cN/Dtex, yarn breakage will occur during knitting and weaving, and when braided cords having piezoelectricity such as fabrics and knitted fabrics are used, the strength of the braided cord will be reduced, resulting in a decrease in product strength. I don't like it.

〔Shrinkage factor〕
The boiling water shrinkage of the drawn polylactic acid long fibers obtained by the production method of the present invention is preferably 10% or less, more preferably 9.9% or less. A boiling water shrinkage of more than 10% is not preferable because the dimensional stability of the braided cord having piezoelectric properties such as cloth or knitted fabric is lowered.

EXAMPLES The present invention will be specifically described below with reference to examples. Measurement and evaluation were performed by the following methods.
(1) molecular weight
A sample was dissolved in chloroform to a concentration of 10 mg/mL, GPC analysis was performed using Waters LC Model I plus, and the weight average molecular weight Mw and number average molecular weight Mn were measured.
(2) Intrinsic viscosity (IV)
Intrinsic viscosity (dl/g) was determined from viscosity measurements at 35°C in o-chlorophenol.
(3) Breaking strength Measured according to JIS-L-1013.
(4) Shrinkage in Boiling Water A sample with an initial length of 50 cm was suspended from a weight of 200 mg, immersed in boiling water for 15 minutes, air-dried for 5 minutes, and then the shrinkage in boiling water was determined by the following equation.
Boiling water shrinkage rate (%) = (initial sample length - sample length after shrinkage) / initial sample length x 100
(5) Crystallinity Measured using a differential scanning calorimeter. The degree of crystallinity was calculated by the following formula as the ratio of crystalline regions in the polymer. The amount of heat of fusion for a complete crystal is the amount of heat of fusion for 100% crystallization.
Crystallinity (%) = (measured heat of fusion/heat of fusion of complete crystal) x 100
(6) Birefringence (Δn)
Using the D line as a light source, the retardation and yarn diameter were measured by the Berek compensator method, and the birefringence (Δn) was obtained according to the following formula. Measurement was performed for 10 filaments, and the average value was taken.
Δn = (retardation/thread diameter)
(7) Fluff The occurrence of fluff on the yarn wound by drawing was evaluated according to the following criteria.
◯; No fluff is generated.
x; generation of fluff is observed.
(8) Spinning Process Properties Spinning was performed continuously for 7 days by melt spinning. The frequency of yarn breakage during spinning was evaluated according to the following criteria.
〇: Number of yarn breakages: 0 times/7 days △: Number of yarn breakages: 1 to 2 times/7 days ×: Number of yarn breakages: 3 or more times/7 days The frequency of occurrence was evaluated according to the following criteria.
〇: Number of thread breakages: 0 times/7 days △: Number of thread breakages: 1-2 times/7 days ×: Number of thread breakages: 3 or more times/7 days

(10) Devitrification Devitrification of each filament in Examples was confirmed by the method shown in (10-1) below. As shown in (10-2) to (10-3) below, devitrification can be observed with a stereoscopic microscope, SEM, etc. If it is difficult to determine with the naked eye, the type of fiber to be observed (filament fineness or original Evaluate by changing the observation method as appropriate depending on the yarn wearing, etc.).
(10-1) Observation with the naked eye The polylactic acid fiber is placed on black drawing paper and observed. In the case of multifilament, each single yarn is arranged so as not to overlap. When a 0.5 m filament is observed, if a white region exists, it is determined that the filament is devitrified.
(10-2) Confirmation with a stereomicroscope Polylactic acid fiber is observed with reflected light using a stereomicroscope (HFX type stereomicroscope manufactured by Nikon Corporation), photographed with a color video camera CCD-IRIS manufactured by SONY, and monitored. to observe. Randomly selected 25 points in 0.3 m of the filament were observed, and if there was a region where light was diffusely reflected due to devitrification and looked white as shown in FIG. 2, it was determined that the filament was devitrified. Note that the observation magnification can be appropriately changed depending on the fineness of the fiber. FIG. 2 shows an example of observation of devitrified threads and non-devitrified threads by stereomicroscopic observation.
(10-3) Confirmation by SEM All the filaments at arbitrary points in the polylactic acid fiber were gold-deposited using an ion coater (IB-3 manufactured by Eiko Engineering Co., Ltd.). The prepared sample was observed by SEM (ABT-55 manufactured by Topcon Corporation). If there is a filament having unevenness on the fiber surface as shown in FIG. 3, it is determined that the fiber has a devitrified region. Note that the observation magnification may be appropriately changed depending on the filament fineness. FIG. 3 shows an example of observation of devitrified threads and non-devitrified threads by SEM observation. Judgment was made based on the presence or absence of devitrification.

(11) Piezoelectric performance The obtained fiber was connected to an oscilloscope (DL6000 series DL6000 digital oscilloscope manufactured by Yokogawa Electric Corporation) using the conductive fiber B in the braided piezoelectric element 1A as a signal line, and a 100-fold amplification circuit was applied. The conductive layer 4 of the braided piezoelectric element 1A was grounded (earthed). A torsional deformation is applied to the braided piezoelectric element 1A, and the piezoelectric performance that can be applied to a clothing-shaped wearable sensor with the cloth-like piezoelectric element based on the electric signal output from the string-like piezoelectric element is determined according to the following criteria. evaluated.
〇: Piezoelectric performance is very good
×: No piezoelectric performance

[Example 1]
(Manufacturing of polylactic acid polymer chip)
0.005 parts by mass of tin octoate was added to 100 parts by mass of L-lactide (manufactured by Musashino Chemical Laboratory Co., Ltd., optical purity 100%), and the mixture was stirred in a reactor equipped with a stirring blade in a nitrogen atmosphere to 180 parts by mass. C. for 2 hours, phosphoric acid was added in an amount equivalent to 1.2 times the amount of tin octylate, and residual lactide was removed under reduced pressure at 13.3 Pa to form chips. The resulting polymer had an intrinsic viscosity (IV) of 1.68 dL/g, a weight average molecular weight of 152,000, a glass transition point of 55°C and a melting point of 175°C.

(Manufacturing of piezoelectric fibers)
The polylactic acid polymer chips obtained above are melted at 220° C., extruded into the air through a spinning nozzle having a hole diameter of 0.25 mm, 0.6 mmL, and 24 holes, and extruded at 32 g/min to form a spun yarn. After cooling with cooling air at 23° C. and 3.5 Nm ³ /min, applying an oil solution and converging, it was taken off at 2250 m/min to obtain an undrawn yarn. The birefringence of this undrawn yarn was 0.0068. This undrawn yarn was subjected to the drawing process. That is, the unstretched yarn is passed through a supply roller 1 and stretched by a stretching preheating roller 2 (temperature 85°C), a contact stretching heating roller 3 (temperature 150°C) and a contact stretching heating roller 4 (temperature 150°C). , and then heat-set with a shrinkage roller 5 at room temperature (25°C). The number of revolutions of each roller was 5 to 6 times. In this stretching step, the film was stretched at a magnification of 1.60 times between the stretching preheating roller 2 and the contact type stretching heating roller 3 as the first stage stretching. As the second stage stretching, the film was stretched by a factor of 1.15 between the contact type stretching heating roller 3 and the contact type stretching heating roller 4 . Further, the film was relaxed to 0.99 times under relaxation between the contact-type stretching/heating roller 4 and the contraction roller 5 . Therefore, the total draw ratio was set to 1.82 times. FIG. 1 shows the positional relationship of each roller.

As a result, drawn polylactic acid long fibers of 84 dTex/24 filaments were obtained. A spinning test was conducted for 7 consecutive days to evaluate the spinnability and the degree of fluffing during drawing. Table 1 shows the evaluation results.

In this production method, the drawability of the drawing step was excellent, the quality of the drawn polylactic acid long fibers obtained was excellent, and there were no problems in handling. The piezoelectric performance of the braid of piezoelectric fibers produced using this stretched polylactic acid long fiber had piezoelectric performance applicable to wearable sensors.

[Example 2]
The spinning speed for obtaining the undrawn yarn was 2400 m/min, the first stage drawing ratio was 1.55 times, the second stage drawing ratio was 1.15 times, and the total drawing ratio was 1.76 times. A drawn polylactic acid long fiber was obtained in the same manner as in Example 1, except for the above. This production method is excellent in terms of process, and the obtained polylactic acid long fibers are excellent in quality, and no problems arise in handling. The piezoelectric performance of the braid of piezoelectric fibers produced using this stretched polylactic acid long fiber had piezoelectric performance applicable to wearable sensors.

[Comparative Example 1]
Drawn polylactic acid long fibers were obtained in the same manner as in Example 1, except that the spinning speed for obtaining undrawn yarn was 1500 m/min. In this example, the spinning stability was poor, and the drawn polylactic acid long fibers obtained did not have satisfactory strength.

[Comparative Example 2]
Drawn polylactic acid long fibers were obtained in the same manner as in Example 1, except that the spinning speed for obtaining undrawn yarn was 3200 m/min. In this example, devitrification was observed, and the quality of the obtained drawn polylactic acid long fibers was significantly inferior. The spinning and drawing process properties were poor, and the resulting drawn polylactic acid long fibers had significant fluff. Also, the piezoelectric performance was not satisfactory.

[Comparative Example 3]
A stretched poly was prepared in the same manner as in Example 1, except that the total stretching ratio was 2.5 times, the first-stage stretching ratio was 1.94 times, and the second-stage stretching ratio was 1.30 times. Lactic acid long fibers were obtained.

In this example, devitrification was observed, and the quality of the obtained polylactic acid long fibers was significantly inferior. The processability of the drawing process was poor, and the resulting polylactic acid long fibers had considerable fluff. Also, the piezoelectric performance was not satisfactory.

[Comparative Example 4]
Drawn polylactic acid long fibers were obtained in the same manner as in Example 1, except that only the first drawing was performed without performing the second drawing. In this example, the number of revolutions of the contact-type stretching heating roller 3 and the contact-type stretching heating roller 4 involved in the second-stage stretching were made equal to prevent the second-stage stretching. The drawn polylactic acid long fibers obtained in this example had a low degree of crystallinity and unsatisfactory piezoelectric performance.

[Comparative Example 5]
The first-stage drawing is performed by not using a drawing machine having a drawing preheating roller 2, a contact type drawing heating roller 3 and a contact type drawing heating roller 4, but instead using a drawing machine having only two contact type drawing heating rollers. A drawn polylactic acid long fiber was obtained in the same manner as in Example 1, except that the second stage drawing was performed without drawing. The resulting stretched polylactic acid long fibers had a low degree of crystallinity and did not have satisfactory piezoelectric properties. This example corresponds to the configuration without the stretching preheating roller 2 in the first embodiment. The temperature and rotation speed of each roller were the same as in Example 1.

[Comparative Example 6]
A drawn polylactic acid long fiber was obtained in the same manner as in Example 1, except that the temperature of the drawing preheating roller was changed to 105°C. In this example, the drawability in the drawing step was poor, and the resulting drawn polylactic acid long fibers had significant fluff.

[Comparative Example 7]
A drawn polylactic acid long fiber was obtained in the same manner as in Example 1, except that the temperatures of the contact type drawing heating rollers 3 and 4 were both changed to 170°C. In this example, the drawability in the drawing step was poor, and the resulting drawn polylactic acid long fibers had significant fluff.

[Comparative Example 8]
A drawn polylactic acid long fiber was obtained in the same manner as in Example 1, except that the temperatures of the contact type drawing heating rollers 3 and 4 were changed to 120°C. In this example, devitrification was observed in the obtained drawn polylactic acid long fibers, resulting in a significant deterioration in quality. The drawability in the drawing step was poor, and the resulting drawn polylactic acid long fibers had significant fluff. Also, the piezoelectric performance was not satisfactory.

[Comparative Example 9]
Drawn polylactic acid long fibers were obtained in the same manner as in Example 1, except that the polymer chips for obtaining the undrawn fibers were changed to those having an intrinsic viscosity of 1.2. In this example, the resulting fiber was found to be devitrified, resulting in a significantly lower quality. The spinnability and the drawability in the drawing process were poor, and the resulting drawn polylactic acid long fibers had significant fluff. Also, the piezoelectric performance was not satisfactory.

[Comparative Example 10]
Drawn polylactic acid long fibers were obtained in the same manner as in Example 1, except that the polymer chips for obtaining the undrawn fibers were changed to those having a weight average molecular weight of 100,000. Devitrification was observed in this example, resulting in significantly lower quality. The spinnability and the drawability in the drawing process were poor, and the obtained drawn polylactic acid fiber had considerable fluff. Also, the piezoelectric performance was not satisfactory.

[Comparative Example 11]
A drawn polyester long fiber was obtained in the same manner as in Example 1, except that the polymer for obtaining the undrawn fiber was changed to polyethylene terephthalate (denoted as "PET" in the table). The drawn polyester fibers obtained did not have satisfactory performance in terms of piezoelectric performance.

According to the production method of the present invention, polylactic acid long fibers can be obtained with excellent productivity, and these polylactic acid long fibers can be used as piezoelectric elements by forming them into braids.

1: Supply roller 2: Stretching preheating roller 3: Contact stretching heating roller 4: Contact stretching heating roller 5: Shrink roller

Claims (5)

A method for producing a drawn polylactic acid long fiber, wherein the drawn polylactic acid long fiber has an intrinsic viscosity of 1.4 to 1.7 dL/g, a breaking strength of 4.0 cN/Dtex or more, and a boiling water shrinkage rate of 10% or less and a crystallinity of 40% or more,
(Step 1) A polylactic acid polymer containing 95 mol % or more of L-lactic acid is melted, extruded through a spinneret, cooled and solidified, wound at a spinning speed of 2000 to 2500 m/min, and the birefringence Δn is 0.005 to 0. and (Step 2) drawing the undrawn yarn at a draw ratio of 1.6 to 2.0 using at least three contact drawing rollers to obtain a drawn poly. A method for producing drawn polylactic acid long fibers, comprising a step of obtaining lactic acid long fibers. Step 2 is
(Step 2-1) A preheating step of contacting and preheating the undrawn yarn obtained in step 1 with a drawing preheating roller having a surface temperature of 70 to 100 ° C.
(Step 2-2) The undrawn yarn that has undergone the preheating step is stretched at a draw ratio of 1.5 to 1.5 between a drawing preheating roller having a surface temperature of 70 to 100°C and a drawing heating roller having a surface temperature of 140 to 160°C. A first step of drawing to obtain a drawn yarn by drawing at 6 times; 2. The drawn polylactic acid according to claim 1, comprising a second-stage drawing step of obtaining drawn polylactic acid long fibers in this order by drawing at a draw ratio of 1.1 to 1.3 times between A method for producing long fibers. Step 2 is carried out by a stretching machine having at least one contact stretching preheating roller used in Step 2-1 and at least two contact stretching heating rollers used in Steps 2-2 and 2-3. The method for producing drawn polylactic acid long fibers according to claim 2. 2. The method for producing a drawn polylactic acid long fiber according to claim 1, wherein the weight average molecular weight of the polylactic acid polymer used in step 1 is 140,000 to 170,000. 2. The method for producing a drawn polylactic acid long fiber according to claim 1, further comprising the step of applying an oil agent to the polylactic acid polymer that has been extruded through the spinneret and cooled and solidified in step 1.

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