JP3251888B2 - Method and apparatus for producing high molecular weight polylactic acid - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a method and an apparatus for producing polylactic acid, which is useful as a biodegradable plastic.

[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)
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-produced in the polycondensation reaction outside the system. After suctioning with a pump to separate water and low-boiling components, the solvent is purified and circulated through 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). In addition, a patent application for making this reflux method more efficient (Japanese Patent Application No. 8-258404).
I also went.

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.

Further, as the reaction proceeds and the polymer becomes higher in molecular weight, uniform stirring cannot be achieved. In the reflux method, a high vacuum can be applied to suppress the distillation of lactide, low-boiling substances and solvent out of the system. However, there were problems that a small amount of water could not be removed in the latter stage of the reaction and the reached molecular weight of the polymer reached a plateau, and that depolymerization might occur if the reaction was performed at a high temperature for a long time.

[0010]

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a method and an apparatus capable of industrially and advantageously producing high molecular weight polylactic acid without the above-mentioned problems.

[0011]

Means for Solving the Problems In view of such a situation, the present inventors have conducted intensive studies. As a result, at first, the direct polymerization of lactic acid is carried out at a relatively high temperature using a reflux apparatus to carry out melt polymerization, and then, If solid-state polymerization is carried out by lowering the temperature to produce solid polylactic acid, the generation of lactide is suppressed, a high vacuum can be achieved, and trace amounts of by-product water can be efficiently removed. The present invention was found to be industrially advantageous, and the present invention was completed.

That is, the present invention relates to a method for producing polylactic acid from a lactic acid monomer by a direct polycondensation method. First, a low molecular weight compound of water, lactide and lactic acid generated from a reaction vessel using a multitubular heat exchange condenser. And optionally refluxing the solvent to remove water from the reaction system,
C. and melt polymerization under the conditions of 10 to 30 torr.
The present invention provides a method for producing polylactic acid, characterized in that solid-state polymerization is carried out under the following conditions.

The present invention also provides an apparatus for producing polylactic acid having a reaction vessel, a reflux device connected to the upper portion thereof, a vacuum pump connected to the reflux device, and a vacuum pump separately connected to the reaction vessel. Things.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The lactic acid used in the present invention is a D-form,
Optically active substance such as L-form or D, L having no optical activity
And any of these compounds and mixtures thereof, preferably those having a purity of 85% or more. These are firstly subjected to a polymerization reaction 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 reaction temperature for the first melt polymerization is 1
The temperature is preferably from 50 to 200C, particularly preferably from 150 to 175C. If the reaction temperature is too low, the reaction does not proceed sufficiently,
If the temperature is too high, a depolymerization reaction occurs and the production of lactide becomes significant, which is not preferable.

The pressure during the 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.

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 a 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 weight 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 clogged. Therefore, a solvent may be used to prevent 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 according to the above method and the molecular weight Mw of the polylactic acid reaches about 30,000 to 60,000, the amount of generated water 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,
Is closed, and the valve 8 is opened. A cooling trap 9 and a vacuum pump 10 are provided downstream of the valve 8 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.

After completion of the solid-phase polymerization, the reaction vessel is heated again,
The polylactic acid can be taken out of the reaction tank with a gear pump or the like.

It should be noted that the melt polymerization and the solid state polymerization are performed, for example, as shown in FIG.
As shown in the above, the reaction may be performed in separate reaction tanks.

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.

[0029]

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, in which a multi-tube heat exchange condenser 3 is provided above a reaction tank 1 provided with a stirring blade 7 for stirring a reaction solution 29. , 4 are mounted. Further, a cooling trap 5 and a vacuum pump 6 are provided downstream of the multi-tube heat exchange condensers 3 and 4. In this apparatus, initial melt polymerization is performed, and after completion of the melt polymerization, the polymerization temperature is lowered, the valve 2 is closed, the valve 8 is opened, and a cooling trap 9 and a vacuum pump 10 are provided downstream of the valve 8, The solid-state polymerization is performed by reducing the pressure in this apparatus. After that, heating is performed again to the melting point or higher, and the gear pump 11 and the die 1
2. Continuous production of high molecular weight polylactic acid via transport conveyor 13 and strand cutter 14.

Embodiment 2 FIG. 2 shows a second embodiment of the apparatus of the present invention, in which a multi-tube heat exchange condenser 3 is provided above a reaction tank 1 provided with a stirring blade 7 for stirring a reaction solution 29. , 4 are mounted. Further, a cooling trap 5 and a vacuum pump 6 are provided downstream of the multi-tube heat exchange condensers 3 and 4. In these devices, the initial melt polymerization is performed, and after the completion of the melt polymerization,
The polymerization temperature is lowered, valve 2 is closed and valve 8 is opened. Downstream of the valve 8, a cooling trap 9 and a vacuum pump 10 are provided, and solid-state polymerization is performed using this device. Thereafter, the mixture is again heated to a temperature equal to or higher than the melting point, and supplied to the twin-screw reaction extruder 16 through the gear pump 11, and the hot cut device 2
1. Pellet is continuously produced through the cooling conveyor 22. The twin-screw reaction extruder 16 is provided with deaeration holes 17 and 18, and a cooling trap 19 and a vacuum pump 2 are provided downstream thereof.
0 is installed, and a device capable of degassing the solvent, residual monomer, and volatile components is also installed.

Embodiment 3 FIG. 3 shows a third embodiment of the apparatus of the present invention, in which a multi-tube heat exchange condenser 3 is provided above a reaction vessel 1 provided with a stirring blade 7 for stirring a reaction solution 29. , 4 are mounted. Further, a cooling trap 5 and a vacuum pump 6 are provided downstream of the multi-tube heat exchange condensers 3 and 4. In these devices, the initial melt polymerization is performed, and after the completion of the melt polymerization,
The pellets are supplied to the twin-screw reaction extruder 16 through the gear pump 11, the pellets are produced by the hot cut device 21, and supplied to the reaction tank 24. The twin-screw reaction extruder 16 has a degassing hole 1
7, 18 are provided, and a cooling trap 19,
A vacuum pump 20 is installed, and a device capable of degassing the solvent, residual monomer, and volatile components is also installed. Then, the reaction tank 2
In step 4, solid-state polymerization is performed. A cooling trap 26 and a vacuum pump 27 are attached to the reaction tank.

Example 4 First, 7500 g of L-lactic acid monomer was subjected to a dehydration treatment at 100 ° C. for 4 to 5 hours using the polylactic acid producing apparatus shown in FIG. 0.5% of tin) was added thereto, and melt polymerization was carried out with stirring for 10 to 15 hours (the degree of vacuum at that time was 10 to 30 torr). Thereafter, the temperature was cooled to 130 ° C., and solid phase polymerization was carried out at a degree of vacuum of 0.1 to 2 torr for 10 to 12 hours. The obtained polylactic acid had a weight average molecular weight of 110,000.

Example 5 First, 7500 g of L-lactic acid monomer was subjected to a dehydration treatment at 100 ° C. for 4 to 5 hours using the polylactic acid production apparatus shown in FIG. Then, 0.5% of tin) and 50% of a solvent (diphenyl ether) were charged, and melt polymerization was carried out with stirring for 15 to 20 hours (the degree of vacuum at that time was 10 to 30 torr). Solid-state polymerization was performed at a vacuum of 0.1 to 2 torr for up to 12 hours. The obtained polylactic acid had a weight average molecular weight of 110,000.

Example 6 First, 7500 g of L-lactic acid monomer was dehydrated at 100 ° C. for 4 to 5 hours using the polylactic acid producing apparatus shown in FIG. Then, 0.5% of tin) and 50% of a solvent (diphenyl ether) were charged and melt polymerization was carried out with stirring for 15 to 20 hours (the degree of vacuum at that time was 10 to 30 torr). After that, the vacuum degree is 0.1 ~
Solid-state polymerization was performed at 2 torr for 18 to 20 hours. The obtained polylactic acid had a weight average molecular weight of 150,000.

Example 7 First, 7500 g of L-lactic acid monomer was subjected to a dehydration treatment at 100 ° C. for 4 to 5 hours using the polylactic acid production apparatus shown in FIG. 1, and then a catalyst (tin octylate) was applied at 160 ° C. )
And 0.5% of a solvent (diphenyl ether) was added thereto, and melt polymerization was carried out while stirring for 15 to 20 hours (the degree of vacuum at that time was 10 to 30 torr). Thereafter, the degree of vacuum is 0.1
Solid phase polymerization was performed at ~ 2 torr for 18-20 hours. The obtained polylactic acid had a weight average molecular weight of 130,000.

Example 8 Using a polylactic acid producing apparatus shown in FIG. 1, 7500 g of L-lactic acid monomer was first dehydrated at 100 ° C. for 4 to 5 hours. Then, 0.5% of tin) and 50% of a solvent (diphenyl ether) were charged and melt polymerization was carried out with stirring for 15 to 20 hours (the degree of vacuum at that time was 10 to 30 torr). Thereafter, the temperature is gradually reduced to 130 ° C. by evacuation. Then again raise the temperature to 160
° C and a vacuum of 0.1 to 2 torr.
Solid phase polymerization was performed for 20 hours. The obtained polylactic acid had a weight average molecular weight of 145,000.

[0038]

According to the present invention, high molecular weight polylactic acid can be industrially advantageously produced.

[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 view schematically showing an apparatus for producing polylactic acid of the present invention (Example 2).

FIG. 3 is a view schematically showing an apparatus for producing polylactic acid of the present invention (Example 3).

[Explanation of symbols]

 1: batch polymerization tank 2: switching valve 3: reflux pipe 4: reflux pipe 5: cooling trap 6: vacuum pump 7: stirring blade 8: switching valve 9: cooling trap 10: vacuum pump 11: gear pump 12: die 13: Transport conveyor 14: Strand cutter 15: Motor 16: Extruder for twin screw reaction 17: Degassing hole 18: Degassing hole 19: Cooling trap 20: Vacuum pump 21: Hot cutting device 22: Cooling conveyor 23: Stop valve 24: Batch type Polymerization tank (solid phase polymerization) 25: Stirring motor 26: Cooling trap 27: Vacuum pump 28: Material discharge valve 29: Reaction liquid

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Noriaki Hashimoto 1-6-1, Funakoshiminami, Aki-ku, Hiroshima-shi, Hiroshima (56) References JP-A-8-143649 (JP, A) JP-A-7-247345 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C08G 63/00-63/91

Claims (4)

(57) [Claims] In a method for producing polylactic acid from a lactic acid monomer by a direct polycondensation method, first, using a multitubular heat exchange condenser, water, lactide, low-molecular-weight compounds of lactic acid generated from a reaction vessel and, in some cases, The solvent was refluxed, melt polymerization was carried out at 150 to 200 ° C. and 10 to 30 torr while removing water from the reaction system.
A method for producing polylactic acid, wherein solid-state polymerization is carried out at 0 to 170 ° C and 0.1 to 2 torr. 2. The method according to claim 1, wherein after the completion of the melt polymerization, the polylactic acid is gradually cooled to crystallize the polylactic acid, and then heated again to a temperature lower by 10 to 20 ° C. than the melting point to carry out the solid phase polymerization. A method for producing lactic acid. 3. An apparatus for producing polylactic acid, comprising: a reaction tank, a reflux device connected to an upper portion thereof, a vacuum pump connected to the reflux device, and a vacuum pump separately connected to the reaction tank. 4. The production apparatus according to claim 3, wherein the reflux device is a multi-tube heat exchange condenser.