JP3270827B2 - Method for producing polylactic acid - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a polycondensation method for producing polylactic acid by subjecting a lactic acid monomer to a polycondensation reaction, which is capable of efficiently performing solid phase polymerization in a melt polymerization step and a cooling step. The present invention relates to a method and an apparatus for producing lactic acid.

[0002]

2. Description of the Related Art Polylactic acid made from lactic acid is easily hydrolyzed or thermally decomposed into lactide, which is a cyclic duplex of lactic acid, so that it can be chemically recycled. It is decomposed into water and water, and it has excellent transparency and rigidity, and it is possible to design polymers of various grades by copolymerizing with other biodegradable substances. the film,
It is also attracting attention as a material for blow, fiber and general molded products.

Hitherto, polylactic acid has been produced by ring-opening polymerization of lactide, which is a cyclic dimer. However, this method is a major obstacle to practical use because of the large cost in the lactide production step and the purification step.

For this reason, there has been proposed a method for producing polylactic acid by directly polycondensing a lactic acid monomer in order to reduce costs. In this method, there is an equilibrium between the monomer, lactide (prepolymer) and polymer of lactic acid and water. In order to proceed with the polycondensation of lactic acid, it is necessary to first remove by-product water out of the system. In order to prevent depolymerization of the produced polymer, it is necessary to maintain the concentration of lactide at a certain level or more.

However, the conventional method (Japanese Patent Laid-Open No. 6-2)
JP-A-98913 and JP-A-6-298914) add a large amount of a solvent to the reaction system in order to efficiently remove water and low-boiling components by-product in the polycondensation reaction outside the system. After separating water and low-boiling components by suction with a vacuum pump, the solvent is purified and circulated to the reaction system, so that a large amount of solvent is required. As a result, there are problems that purification and circulation costs are large, and that a large amount of solvent remains in the product polymer after completion of the polymerization, and that separation and purification are costly. In addition, since the reaction system is evacuated, the reactor, peripheral devices and piping must be kept confidential. Furthermore, in the conventional vacuum system in the middle and final stages of the reaction, the vapor of lactide or solvent exists in the evacuation of the vacuum pump by the vapor pressure of the operating temperature (reaction temperature) and is discharged out of the system. There was a problem that the concentration of lactide in the tank was reduced and the rate of increasing the molecular weight was low.

Therefore, as a method for solving such a problem, the applicant of the present invention has proposed a method of maintaining water in a reaction system at a high vacuum, in which water, lactide, solvent and low-molecular-weight lactic acid compound other than water are discharged out of the reaction system. Has been developed by cooling and liquefying it with a reflux device and refluxing it, and a patent application (JP-A-8-14364) has already been filed.
No. 9).

However, even with these methods, the reflux of lactide was not satisfactory. That is, since lactide has a melting point of about 95 ° C., it is crystallized when it is strongly cooled in a reflux device. Therefore, mild cooling must be performed, but this inevitably causes lactide to flow out of the system together with water. As a result, it becomes difficult to obtain high molecular weight polylactic acid.

[0008] On the other hand, it is conceivable that lactic acid crystals are generated in a refluxing device by vigorously cooling and the lactide is refluxed in the reaction solution as it is. However, there is a vapor pressure due to sublimation of lactide, and lactide system It is inevitable to flow out, and there is a problem in the step of scraping the crystals, and a decrease in reflux efficiency is inevitable.

Furthermore, in the reflux method, a high vacuum cannot be applied in order to suppress the lactide, low-boiling substances and solvent from distilling out of the system, and a small amount of water cannot be removed in the latter stage of the reaction, and the reached molecular weight of the polymer reaches a peak. And that a long-time reaction at a high temperature may cause depolymerization.

[0010] In order to solve this problem, first, a direct polycondensation reaction of lactic acid is carried out by melt polymerization at a relatively high temperature using a reflux apparatus, and then the temperature is lowered to produce solid polylactic acid and solid phase polymerization. It has been found that lactide generation is suppressed, high vacuum can be achieved, and trace amounts of by-product water can be removed efficiently, so that high molecular weight polylactic acid can be obtained industrially advantageously. Was. That is, it is a two-stage polymerization method.

In this two-stage polymerization method, a single-axis stirring blade such as an anchor type or a helical ribbon blade is usually used as a stirring device in a reaction tank. These stirring blades
In a limited viscosity region, for example, an anchor type (about 1,000 poise) or a helical ribbon blade (20,000 poise) is limited, and further viscosity cannot be stirred. Further, as the viscosity increases, the polymer adheres to the stirring blade and causes co-rotation. In such a state, the polycondensation reaction does not proceed, and on the contrary, a decomposition reaction due to heat occurs, which causes a decrease in molecular weight.

As a two-stage polymerization method, a molten prepolymer is produced in a batch polymerization tank and then pelletized by an extruder. After that, solid-phase polymerization is performed in a batch polymerization tank for solid phase, and after completion of melt polymerization (after stirring is stopped), the batch polymerization tank is cooled to crystallize and solid-phase polymerization is performed at a temperature equal to or lower than the melting point. Sometimes the polymer is blocky). Thereafter, there is a method in which the batch polymerization tank is melted to a temperature equal to or higher than the melting point and pellets are produced through a die as is, or a method in which pellets are produced using an extruder. However, in these methods, solid-state polymerization takes considerably longer due to the low molecular weight of the prepolymer. However, there are problems such as that the solid state polymerization takes a long time due to the block shape, and that depolymerization may occur due to the necessity of heating for melting again at the end of the solid state polymerization.

[0013]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to solve the problems of the limit of stirring viscosity, the adhesion of polymer to stirring blades, the time required for solid-state polymerization and the decrease in molecular weight of reheating after solid-state polymerization. Another object of the present invention is to provide a method for industrially and advantageously producing high molecular weight polylactic acid.

[0014]

Means for Solving the Problems In view of such a situation, the present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, a stirrer in a reaction tank used in a two-stage polymerization method is a twin-screw stirrer. In this case, a high-viscosity polymer can be stirred, the surface renewability of the monomer or / and the polymer during the melt polymerization is good, the time of the melt polymerization can be shortened, and in the solid-phase polymerization, the melting point of the polylactic acid is lower than the melting point. It has been found that the pulverization is good even at a temperature, the surface area can be increased, the dehydration proceeds under a high vacuum, and the solid-state polymerization time can be shortened.

That is, according to the present invention, a lactic acid monomer is charged into a reaction vessel, and water, lactide, low molecular weight compounds of lactic acid and optionally a solvent generated from the reaction vessel are refluxed, and melt polymerization is carried out while removing water out of the reaction system. And then subjecting the resulting polylactic acid having a molecular weight of 30,000 to 60,000 to solid-state polymerization at a temperature equal to or lower than its melting point, in a method for producing polylactic acid, wherein stirring in the reaction vessel is performed using a twin-screw agitator. It is intended to provide a method for producing polylactic acid having an average molecular weight of 100,000 or more.

The present invention also provides a reaction tank having a twin-screw agitator therein, a reflux device connected to the upper part thereof, a vacuum pump connected to the reflux device, and a vacuum pump separately connected to the reaction tank. It is intended to provide an apparatus for producing polylactic acid having the following.

[0017]

DETAILED DESCRIPTION OF THE INVENTION The lactic acid monomer used in the present invention is:
Any of an optically active substance such as a D-form or an L-form, or a D or L-form having no optical activity, and a mixture thereof may be used. Preferably, a substance having a purity of 85% or more is used. They are,
First, a polymerization reaction is performed by a conventional method. Specifically, for example, using an apparatus as shown in FIG. 1, lactic acid as a raw material is charged into a batch-type polymerization tank, and the inside of the tank is replaced with an inert gas such as nitrogen gas or argon gas. Perform the polymerization reaction.
The polymerization tank may be a vertical type or a horizontal type, but a vertical type is preferable in consideration of discharge of materials.

The twin-screw agitator in the reaction tank used in the present invention may be one in which two screws are engaged with each other or one in which they are not engaged. It is preferable that they are arranged in a meshing state. Further, it is preferable that the two screws can rotate independently of each other and can rotate forward, backward, in the same direction, and in the opposite direction. The screw type is preferably exemplified by a full flight screw or a kneading desk. The screw type may be other special types, or a combination of several types. Further, the screw is preferably of a segment type that can be recombined. FIGS. 2 and 3 show examples of screw types.

The reaction temperature when the first melt polymerization is carried out using such a polymerization tank is preferably from 150 to 200 ° C, particularly preferably from 150 to 175 ° C. If the reaction temperature is too low, the reaction does not proceed sufficiently. If the reaction temperature is too high, a depolymerization reaction occurs and the production of lactide becomes remarkable, which is not preferable.

The pressure during melt polymerization is 10 to 30 to
The range of rr is preferable, and particularly preferably 15 to 20 torr. If the pressure is too low, the lactic acid monomer may be distilled off together with the water, and if the pressure is too high, the by-product water is difficult to be distilled off, which is not preferable. At this time, in the past, the viscosity increased as the polymerization progressed, so that the limit of stirring occurred.However, this device has the stirring limit of several tens times the capacity of the past, and the polymer adheres to the stirring blade. However, since it has excellent screening properties, it can be stirred while scraping the polymer.

In the dehydration polycondensation of lactic acid, it is important to quickly remove by-product water from the system in order to increase the reaction rate. For this reason, a vacuum is drawn from the gas phase of the polymerization tank by the vacuum pump 6 through the reflux pipes 3 and 4 and the cooling trap 5. As a result, water in the lactic acid in the polymerization tank is removed, and accordingly, the lactic acid is polycondensed and gradually polymerized. The reflux device used here may be a conventional reflux tube provided with a spiral or screw type resistor in the reverse screw direction in the tube, but it is preferable to use a multi-tube heat exchange condenser.

The multi-tube heat exchange condenser used in the present invention has a large number of condensation heat transfer tubes densely arranged vertically on a tube plate of a cylindrical main body, and mixed steam in the heat transfer tubes flowing from the reaction tank. Has a structure capable of minimizing the temperature distribution in a cross section perpendicular to the axis and enabling uniform cooling of the steam. As such a multi-tube heat exchange condenser, for example, a dephlegmator is preferable. In the present invention, only one multi-tube heat exchange condenser may be installed, but two abnormalities may be used in series in a multi-stage manner in order to make a difference in cooling temperature.

In this multi-tube heat exchange condenser, by sufficiently increasing the length of the conduction tube, the lactide, lactic acid low-molecular compound or the solvent is completely refluxed even at a high cooling temperature near the freezing point of lactide. To prevent scattering outside the reaction system and, for example, to reduce the cooling temperature to 50 to
Even if the temperature is slightly lowered to 90 ° C., lactide crystals are formed, but unlike the conventional screw type or baffle plate type, the crystals are formed on the inner wall of the vertically arranged heat transfer tube, so that the solvent (liquid ), Lactic acid (liquid), and low-molecular-weight compounds are washed away, and the inside of the conductive tube is not blocked.

In the reflux of lactide, depending on the reflux tube, if the inside of the reflux tube becomes lower than the melting point of lactide, crystallization occurs immediately and the inside of the tube is closed, so that a solvent may be used for the purpose of preventing this. As the solvent, phthalate-based solvents such as dibutyl phthalate, diethylhexyl phthalate, diethyl phthalate and dimethyl phthalate are most suitable.

The amount of the solvent to be added is preferably 5 to 25 parts by weight, more preferably 7 to 20 parts by weight, based on 100 parts by weight of the lactic acid oligomer produced by the reaction. 5 added
When the amount is less than 25 parts by weight, the solvent does not sufficiently act on the reflux of lactide and the decrease in the melt viscosity of the polymer. And quality deterioration during storage of molded articles.

When the reaction proceeds by the above method and the molecular weight Mw of the polylactic acid reaches about 30,000 to 60,000, the amount of water generated is extremely reduced even if the polycondensation proceeds. At this stage, the water to be originally removed remains in the lactide or solvent which is desired to be completely returned to the reaction system by reflux at a dissolution equilibrium concentration or an azeotropic composition concentration, and it becomes very difficult to evaporate out of the system. The rate of increase is significantly reduced. For this reason, the number average molecular weight reaches a peak at about 100,000, and it becomes difficult to obtain polylactic acid having a higher molecular weight.

Therefore, the melt polymerization is terminated, the temperature of the reaction vessel is lowered, and the temperature is set to a temperature lower than the melting point of the obtained polylactic acid, preferably 10 to 20 ° C. lower than this melting point. In particular, it is preferred that after the completion of the melt polymerization, the polylactic acid is gradually cooled to crystallize the polylactic acid into two solids, heated again to a temperature lower by 10 to 20 ° C. than this melting point, and then subjected to solid phase polymerization.

The operation for solid-phase polymerization will be described with reference to the apparatus shown in FIG. 1. After the completion of the melt polymerization, the temperature is lowered as described above,
a is closed and the valve 2b is opened. Downstream of the valve 2b,
A cooling trap 9 and a vacuum pump 10 are provided to remove water.

The solid phase polymerization is carried out at a temperature lower than the melting point of the polylactic acid, but is preferably carried out at a temperature lower by 10 to 20 ° C. than the melting point, that is, in a range of 120 to 170 ° C.
In addition, the pressure is low in by-products such as lactide because the polymerization is carried out in the solid phase, it can be carried out under a lower pressure than the melt polymerization, the effect of removing water increases, and the equilibrium is on the polylactic acid side
Suitable for producing high molecular weight polylactic acid. Specifically, it is preferable to carry out the polymerization at about 0.1 to 2 torr. Furthermore, the twin screw increases the surface area of the polylactic acid, further enhancing the water removal effect.

After the completion of the solid-phase polymerization, the polylactic acid can be taken out of the reaction tank with a screw without heating the reaction tank again.

The melt polymerization and the solid phase polymerization may be performed in separate reaction tanks. At that time, the twin screw stirring device
It is preferable to dispose in each reaction tank.

In the polymerization reaction of the present invention, a catalyst can be used. Examples of the catalyst include metal-based acid catalysts such as stannous chloride, tin octylate, and antimony oxide. These catalysts include tetrahydrofuran, butyl lactate, chloroform, acetone, xylene,
It is desirable to completely dissolve it in a solvent such as ethanol or benzene and then add it to the reaction solution. This catalyst addition operation,
Care must be taken when the reaction is performed at a high temperature, because it acts as a catalyst for depolymerizing the lactic acid oligomer, which is the reaction liquid, and promotes the production of lactide. In consideration of the high toxicity of the catalyst group, a nonmetallic acid catalyst such as manganese acetate, zinc acetate, aluminum acetate, and diethyl zinc can be used. The amount of the catalyst to be used is in the range of 0.001 to 0.5 part by weight, more preferably 0.05 to 0.1 part by weight, based on 100 parts by weight of the lactic acid oligomer as the reaction solution in the case of a metal catalyst. . In the case of a non-metallic catalyst, the catalytic activity is lower than that of a metal-based catalyst. Is preferred.

[0033]

EXAMPLES The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples.

Embodiment 1 FIG. 1 shows a first embodiment of the apparatus of the present invention, and flights 7a and 8a (FIG. 2) are provided in the center of a vertical batch polymerization tank 1.
Are arranged in such a manner that they mesh with each other.
8 and the two screws 7 and 8 are provided with a gear box 24 that can be switched in the same direction or in different directions, and a motor 25 is provided as a rotation driving means that can rotate forward and reverse.
The connecting portion 23 between the gear 8 and the gear box 24 is configured to maintain airtightness under reduced pressure by a shaft seal portion provided with a Wilson seal, a labyrinth seal, a mechanical seal, and the like. On the top of the batch polymerization tank 1, multi-tube type hot steel tube condensers 3 and 4 are attached and installed. The multi-tube heat exchange condensers 3, 4
A cooling trap 5 and a vacuum pump 6 are installed downstream of the apparatus. Initial melt polymerization is performed in these devices. After the completion of the melt polymerization, the polymerization temperature is lowered, the valve 2a is closed, and the valve 2
Open b. Downstream of the valve 2b, a cooling trap 9 and a vacuum pump 10 are installed to perform solid-state polymerization. After the solid phase polymerization is completed, the valve 2b is closed, the valve 29 is opened, and nitrogen is sealed in the vertical batch polymerization tank 1 from the nitrogen cylinder 27. Next, the material discharge hole valve 11 is opened, and the fixed amount feeder 2 is opened.
The ground polymer 13 is transferred to the hopper 12 of No. 6. At this time, the motor 25 of the vertical batch polymerization tank 1 is operated so that the polymer 22 of the vertical batch polymerization tank 1 does not remain while repeating normal rotation and reverse rotation. Thereafter, the polymer 13 is supplied to a single-screw or twin-screw extruder 28 by a fixed-quantity feeder 26, and is continuously produced through a hot-cut device 20 and a cooling conveyor 21. The twin-screw extruder 28 has a degassing hole 1
6, 17 are provided, and a cooling trap 18,
A vacuum pump 19 is installed, and a device capable of degassing the solvent, residual monomer, and volatile components is also installed.

Example 2 Using the production apparatus shown in FIG. 1, polymerization was carried out by combining the patterns of a twin-screw agitator in a vertical batch polymerization tank with a combination of 1-3 shown in FIG. First, L-lactic acid monomer 75
00 g was subjected to a dehydration treatment at 100 ° C. for 4 to 5 hours, and then 0.5% of a catalyst (stannous chloride) was added at 160 ° C., and melt polymerization was carried out while stirring for 10 to 15 hours. Vacuum of 10 to 30 Torr). Next, the temperature of the polymerization tank was cooled to 130 ° C. for crystallization (at this time, the polymer was pulverized by a twin-screw agitator). Thereafter, solid phase polymerization was performed at a degree of vacuum of 0.1 to 2 Torr for 10 to 12 hours, and pelletized by an extruder. The obtained polylactic acid had a weight average molecular weight of 110,000.

Example 3 Using the manufacturing apparatus shown in FIG. 1, polymerization was carried out by combining the patterns of a twin-screw agitator in a vertical batch polymerization tank with combinations shown in FIGS. First, L-lactic acid monomer 75
00g was subjected to a dehydration treatment at 100 ° C. for 4 to 5 hours, and then, at 160 ° C., 0.5% of a catalyst (stannous chloride) and 50% of a solvent (diphenyl ether) were charged and stirred for 15 to 20 hours. Melt polymerization was carried out (at that time, the degree of vacuum was 1
0-30 Torr). Next, the temperature of the polymerization tank was cooled to 130 ° C. for crystallization (at this time, the polymer was pulverized by a twin-screw agitator). Then 10-12
Solid phase polymerization was carried out at a degree of vacuum of 0.1 to 2 Torr for a time and pelletized by an extruder. The obtained polylactic acid had a weight average molecular weight of 110,000.

Example 4 Using the production apparatus shown in FIG. 1, polymerization was carried out by combining the patterns of a twin-screw agitator in a vertical batch polymerization tank with combinations shown in 1-3 of FIG. First, L-lactic acid monomer 75
00g was subjected to a dehydration treatment at 100 ° C. for 4 to 5 hours, and then, at 160 ° C., 0.5% of a catalyst (stannous chloride) and 50% of a solvent (diphenyl ether) were charged and stirred for 15 to 20 hours. Melt polymerization was carried out (at that time, the degree of vacuum was 1
0-30 Torr). Next, the temperature of the polymerization tank was cooled to 130 ° C. for crystallization (at this time, the polymer was pulverized by a twin-screw agitator). Then 18-20
Solid phase polymerization was carried out for 0.1 to 2 Torr in vacuum for a time, and pelletized by an extruder. The obtained polylactic acid had a weight average molecular weight of 150,000.

Example 5 Using the production apparatus shown in FIG. 1, polymerization was carried out by combining the patterns of a twin-screw agitator in a vertical batch polymerization tank with combinations shown in 1-3 shown in FIG. First, L-lactic acid monomer 75
00g was subjected to a dehydration treatment at 100 ° C. for 4 to 5 hours, and then, at 160 ° C., 0.5% of a catalyst (tin octylate) was added.
15-20 by adding 50% of solvent (diphenyl ether)
The melt polymerization was carried out with stirring for a period of time (the degree of vacuum at that time was 10 to 30 Torr). Next, in order to crystallize, the temperature of the polymerization tank was cooled to 130 ° C. (at this time, the polymer was pulverized by a twin-screw agitator).
Solid phase polymerization was carried out at a degree of vacuum of 0.1 to 2 Torr for a time and pelletized by an extruder. The obtained polylactic acid had a weight average molecular weight of 130,000.

Example 6 Using the manufacturing apparatus shown in FIG. 1, polymerization was carried out in a vertical batch type polymerization tank by combining a twin screw type stirring apparatus with a pattern 1-3 shown in FIG. First, L-lactic acid monomer 75
00g was subjected to a dehydration treatment at 100 ° C. for 4 to 5 hours, and then, at 160 ° C., 0.5% of a catalyst (stannous chloride) and 50% of a solvent (diphenyl ether) were charged and stirred for 15 to 20 hours. Melt polymerization was carried out (at that time, the degree of vacuum was 1
0-30 Torr). Next, the temperature of the polymerization tank was cooled to 130 ° C. for crystallization (at this time, the polymer was pulverized by a twin-screw agitator). Thereafter, the temperature is again raised to 160 ° C., and the vacuum degree is 0.1 to 20 hours for 18 to 20 hours.
Solid phase polymerization was performed under the conditions of 2 Torr, and pelletized by an extruder. The resulting polylactic acid had a weight average molecular weight of 14.5.
10,000 values.

Example 7 Using the production apparatus shown in FIG. 1, polymerization was carried out by combining the patterns of a twin-screw agitator in a vertical batch polymerization tank with combinations shown in 1-3 shown in FIG. First, L-lactic acid monomer 75
00g was subjected to a dehydration treatment at 100 ° C. for 4 to 5 hours, and then, at 160 ° C., 0.5% of a catalyst (stannous chloride) and 50% of a solvent (diphenyl ether) were charged and stirred for 15 to 20 hours. Melt polymerization was carried out (at that time, the degree of vacuum was 1
0-30 Torr). Next, a nucleating agent was added to promote crystallization, and the temperature of the polymerization tank was cooled to 140 ° C. (the polymer at this time was ground by a twin-screw agitator). Then, the temperature is again raised to 160 ° C.
Solid-state polymerization was performed for 20 hours under the conditions of a vacuum degree of 0.1 to 2 Torr, and pelletized by an extruder. The resulting polylactic acid is
The weight average molecular weight was 150,000.

[0041]

According to the method and the apparatus of the present invention, the polymerization time can be shortened by increasing the stirring power in the melt polymerization stage and improving the surface renewal. In solid-state polymerization, the surface area of the polymer can be increased because the pulverization can be performed simultaneously in the cooling stage during crystallization, and the solid-phase polymerization time can be shortened. However, the present invention can industrially produce high-molecular-weight polylactic acid by preventing depolymerization because it can be supplied to an extruder as it is at the end of solid-phase polymerization. .

[Brief description of the drawings]

FIG. 1 is a view schematically showing an apparatus for producing polylactic acid (Example 1) of the present invention.

FIG. 2 is a diagram schematically showing various twin-screw stirring devices of the present invention.

FIG. 3 is a view schematically showing various twin-screw stirring devices of the present invention.

[Explanation of symbols]

1. Batch polymerization tank 2a. Switching valve 1 2b. Switching valve 2 3. Reflux tube 1 4. Reflux tube 2 5. Cooling trap 1 6. Vacuum pump 1 7. Screw stirring blade 8. Screw stirring blade 9. Cooling trap 2 10. Vacuum pump 2 11. Material outlet valve 12. Hopper for quantitative fader 13. Powder material 14. chute 15. Motor 16. Deaeration hole 1 17. Deaeration hole 2 18. Cooling trap 3 19. Vacuum pump 3 20. Hot cut device 21. Cooling conveyor 22. Reaction liquid 23. Shaft seal 24. Gear box 25. Motor 26. Biaxial feeder 27. Nitrogen cylinder 28. Extruder for twin screw reaction 29. Valve 37a. Full flight screw flight section 7c. 7. Order kneading desk Reverse full flight screw 8a. Reverse full flight screw flight 8c. Kneading desk 30. Screw shaft 31. Co-rotation 32. Different direction rotation 33. Screw cap 1-1 Meshing type twin-screw stirrer for co-rotating (screw pattern: 7-7-8-8) 1-2 Meshing type twin-screw stirrer for co-rotating (screw pattern: 8-) 8-8-8) 1-3 Meshing type twin-screw agitator for co-rotating (screw pattern: 7-7b-8a-8) 1-4 Meshing type twin-screw stirrer for co-rotating (screw Pattern: 7b-7b-8b-8b) 1-5 mesh type co-rotating twin-screw agitator (screw pattern: 7b-7b-7b-8b) 1-6 mesh type co-rotating twin-screw type Stirrer (screw pattern: 7b-8b-8b-8b) 2-1 Non-meshing type biaxial screw stirrer for different direction rotation (screw pattern: right = 8-7b-7b-8 left = 7) 2-2 Full-flight Segment screw 3-2 Reverse full flight segment screw 3-3 Forward kneading desk 3-4 Reverse kneading desk

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Noriaki Hashimoto 1-6-1, Funakoshi Minami, Aki-ku, Hiroshima-shi, Hiroshima Japan Steel Works Co., Ltd. (56) References JP-A-8-143649 (JP, A) ( 58) Field surveyed (Int. Cl. ⁷ , DB name) C08G 63/00-63/91

Claims (4)

(57) [Claims] 1. A lactic acid monomer is charged into a reaction vessel, and water, lactide, a low molecular weight compound of lactic acid and optionally a solvent generated from the reaction vessel are refluxed, and melt polymerization is carried out while removing water out of the reaction system. Next, in the method for producing polylactic acid, wherein the obtained polylactic acid having a molecular weight of 30,000 to 60,000 is solid-phase polymerized at a temperature not higher than its melting point, the stirring in the reaction tank is performed using a twin-screw agitator. A method for producing polylactic acid having an average molecular weight of 100,000 or more. 2. The method according to claim 1, wherein the two screws of the twin-screw agitator are arranged in a state of meshing with each other. 3. The method according to claim 1, wherein the two screws of the twin-screw agitator rotate independently of each other. 4. A reactor having a twin-screw agitator therein, a reflux device connected to the upper portion thereof, a vacuum pump connected to the reflux device, and a poly-pump having a vacuum pump separately connected to the reaction bath. Lactic acid production equipment.