JPH0827255A - Production of polylactic acid - Google Patents

Abstract

PURPOSE:To efficiently produce a high-molecular polylactic acid useful for a biodegradable plastic with industrial advantage in a high yield in a short time by using a lactic acid ester as the raw material and polycondensing it according to the dealcoholation reaction method. CONSTITUTION:Production of the objective polylactic acid is carried out by conducting polycondensation reaction of (A) a lactic acid ester (especially preferably a 1 to 4C alkyl ester) preferably in a batch type polymerization reaction tank while removing the liberated alcohol and preferably further polycondensing the obtained oligomer in a screw type extruder while removing the liberated alcohol. In addition, these polycondensation reactions are preferably conducted respectively under heating and a reduced pressure (preferably at 80 to 130 deg.C and 550 to 720mmHg) in the presence of a polycondensation catalyst (especially preferably tin octylate, tetrabutyl titanate, etc.).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for industrially producing high molecular weight polylactic acid useful as a biodegradable plastic.

[0002]

2. Description of the Related Art Biodegradable plastics have recently attracted attention as a material that is decomposed in the environment after fulfilling the purpose of use and is eventually reduced to the natural world in the form of a low molecular weight compound. Among them, aliphatic polyesters have the property of being completely decomposed into monomers by microorganisms and water, and finally being reduced to the natural world as carbon dioxide and water. The development of general-purpose materials that are expected to be disposed of inside is being considered.

Polylactic acid, which is a type of aliphatic polyester, has remarkable properties such as transparency and rigidity, in addition to its decomposing property and good compatibility with other materials, which should be noted as a molding material. Therefore, as a new molding material with one function of biodegradability,
Its existence is beginning to be reviewed.

Conventionally, as a method for producing polylactic acid, lactide obtained by decomposing a lactic acid oligomer produced by dehydration and polycondensation of lactic acid by the action of a catalyst is used as a raw material, and a tin-based catalyst is added to this to open a ring. The method of polymerizing to obtain a polymer has been generally used. However, since the raw material lactide is very expensive in this process,
As a result, the price of polylactic acid products has risen sharply, which has been a factor that impedes the application of the material to general-purpose fields.

Under these circumstances, establishment of a process for synthesizing a polymer by the direct polycondensation method of lactic acid has started to be studied in recent years, but it has various problems to be solved,
It has not been established as an industrial process. These technical problems are shown below.

(1) Lactic acid as a raw material contains water as a basic composition in a proportion of 10 to 20% by weight (hereinafter referred to as "wt%"). This is because oligomers such as dimers and trimers are produced if the concentration is further increased in the production stage of lactic acid. Therefore, compared with other polymer polycondensation processes, it takes extra time to remove the free water derived from these raw materials, resulting in a longer reaction time and an industrially disadvantageous process. The current situation.

(2) Since the synthetic process of the direct polycondensation method of lactic acid is a dehydration / polycondensation reaction which proceeds by removing water molecules from lactic acid, the reaction is accelerated under severe conditions of high temperature and high vacuum. It is necessary to plan. However, under such circumstances, thermal decomposition or oxidative deterioration of the produced polymer causes a remarkable decrease in physical properties such as a decrease in molecular weight and coloring.

(3) Since the low molecular weight compounds such as lactide and lactic acid ether, which are by-produced during the reaction step, are in equilibrium with the polymer produced in the reaction solution, the molecular weight of the obtained polymer is a weight average molecular weight (Mw). The polymer stays at about 3,000 to 4,000 and cannot be a polymer having the strength for practical use.

(4) When the polymer becomes highly viscous as the reaction progresses, it becomes difficult to remove the water molecules trapped inside the polymer. It is necessary to design a device having a strong stirring function and a special structure capable of maintaining a high vacuum state inside the device. However, it is very difficult to realize such a requirement with a large-capacity batch type polymerization apparatus.

[0010]

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a method for industrially producing high molecular weight polylactic acid without the above-mentioned drawbacks.

[0011]

Means for Solving the Problems In view of such circumstances, the present inventors have conducted diligent research, and as a result, if lactic acid ester is used as a raw material instead of lactic acid itself and polycondensation is carried out in a dealcoholation reaction, The present invention has been completed by finding that high molecular weight polylactic acid can be obtained efficiently in a short time.

That is, the present invention provides a method for producing polylactic acid, which comprises subjecting a lactic acid ester to polycondensation while removing the produced alcohol.

In the present invention, lactic acid ester is used as a raw material instead of lactic acid, and as the lactic acid ester, lactic acid lower alkyl ester, particularly lactic acid C ₁ -C ₄ alkyl ester is preferable. Specific examples of the lactate ester include methyl lactate, ethyl lactate, propyl lactate, butyl lactate and the like. Among them, industrially mass-produced methyl lactate or ethyl lactate is relatively inexpensive and preferable. . Alcohol is produced from these esters during the reaction step, and methyl lactate and ethyl lactate are preferable, which give alcohols having a low boiling point even in consideration of the ease of distilling off the alcohol. The boiling points of alcohols are 64 ° C. for methanol produced from methyl lactate, 78 ° C. for ethanol produced from ethyl lactate, 97 ° C. for propanol produced from propyl lactate, and 117 ° C. for butanol produced from butyl lactate.

Lactic acid ester usually contains water in an amount of 0.5 wt% or less. When this is used as a raw material for the method of the present invention, it is necessary to remove the water beforehand in order to shorten the reaction process. preferable. It is preferable to use a desiccant for removing such water. As a desiccant, molecular sieve (synthetic zeolite), aluminum oxide,
Calcium sulfate, magnesium sulfate, sodium sulfate,
Silica gel and the like can be mentioned, but the molecular sieve is most suitable in consideration of drying (dehydration) ability, compatibility with lactate ester, ease of removal after use, propriety of reuse and the like. Of these, molecular sieves having a molecule pore diameter of 3Å or less and having a large water adsorption capacity are desirable. Since the molecular sieve has a maximum drying capacity of 20%, the required amount is calculated according to the amount of lactate ester that needs to be treated. The method of use is to add it directly to lactate and leave it for about 24 hours with occasional stirring, and a proper amount of molecular sieves (diameter 25 mm, length 300 mm).
Column method in which the lactate ester is flown at a flow rate of 2 to 3 l / h.

The polycondensation reaction is preferably carried out in the presence of a polycondensation catalyst while heating under reduced pressure while removing the produced alcohol.

As the polycondensation catalyst, tin octylate,
Stannous chloride, tetrabutyl titanate, dibutyltin oxide, tetraisopropyl titanate, zinc acetate, zinc octylate, tetrabutoxygermanium, germanium oxide and the like are preferable, but in consideration of catalytic activity and ease of handling, tin octylate, Tetrabutyl titanate and dibutyltin oxide are more preferable. Further, if a catalyst is added, it tends to turn brown during the reaction, so it is preferable to add a heat stabilizer if necessary. As a heat stabilizer, a hindered phenol type,
A phosphoric acid ester type is suitable. The amount of the catalyst added is preferably 0.01 to 1 wt%, and more preferably 0.05 to 0.5 wt% with respect to the raw material.

It is desirable that the temperature and reduced pressure conditions of the polycondensation reaction are controlled so that the produced alcohol can be removed and the boiling point of the raw material lactate ester is not higher than the reduced pressure boiling point to prevent loss due to distillation of the raw material. Such conditions vary depending on the progress of the reaction, the reaction apparatus, etc., but are usually 70 to 140.
℃, more preferably 760 mmHg or less at 80 ~ 130 ℃,
It is desirable to carry out at 550 to 720 mmHg. Further, it is desirable that the produced alcohol is removed in a vaporized state from a vent port or the like provided in the reaction apparatus, cooled, and recovered.

In the present invention, as the polycondensation reaction progresses, the viscosity of the reaction mixture increases, the stirring efficiency may decrease, and the polycondensation reaction may not proceed sufficiently. It is preferable to carry out the reaction using an apparatus that does not reduce the efficiency. From this point of view, the reaction is preferably carried out in a screw type extruder, and it is particularly preferable that the production of the lactic acid oligomer is carried out in a batch type polymerization reaction tank and the polymerization is carried out in the screw type extruder.

Therefore, in a more preferred embodiment of the present invention, the lactic acid ester is subjected to polycondensation while removing the alcohol produced in the batch type polymerization reaction tank, and the resulting oligomer is mixed with the alcohol produced in the screw type extruder. A method for producing polylactic acid by further polycondensing while removing the polylactic acid is exemplified. Hereinafter, this embodiment will be described in detail.

First, the batchwise polymerization reaction tank used for oligomer production is not particularly limited as long as it is equipped with an alcohol removing device. This reaction is usually carried out at 80 to 130 ° C. for 7
It is performed at 60 mmHg or less, more preferably 550 to 720 mmHg.

After about 8 to 12 hours have passed in such a batch operation, the lactic acid oligomer produced when the amount of evaporated and recovered alcohol is reduced is fed to the screw type extruder.

As the screw type extruder used in the present invention, a screw, a cylinder, a heating unit, a driving device, and a single screw or twin screw screw having a vent for devolatization, which are usually used for the production and molding of plastics. Type extruder. Also, for this twin-screw extruder,
It is possible to use one in which the screws rotate in the same direction or in different directions, and one in which the flights of the screws mesh with each other or do not mesh with each other. In the present invention, a plurality of heating units are separately provided around the cylinder so that the temperature can be controlled individually, and a plurality of vent ports installed so as to be devolatilized from the inside of the cylinder of those unit parts. And the ratio (L / D ratio) of the screw length (L) to the screw diameter (D) is 10 to 7
A zero mesh twin screw extruder is preferred.

The screw rotation speed of the screw type extruder is not particularly limited, but 100 rpm or more, particularly 150 to 250 rpm.
Is preferred. The cylinder temperature in the extruder is preferably 130 to 180 ° C, more preferably 140 to 170 ° C.
Set to the range of. The pressure in the extruder is preferably 300 mmHg or less, more preferably 100 to 200 mmHg.
Set to the range of. In this way, by utilizing the surface renewal force of the screw of the extruder in the final stage of the polycondensation reaction, the alcohol trapped inside the polymer is forced out of the reaction system due to the increase in the viscosity with the increase in the molecular weight of the polymer. It is possible to distill off and increase the reaction rate.

Next, the embodiment of the present invention will be described in detail with reference to FIG. 1 attached to the specification.

First, a lactate ester, a catalyst and, if necessary, a heat stabilizer are uniformly mixed in the reaction tank 1, the temperature of the reaction solution is set to a predetermined temperature, and an inert gas is supplied from the cylinder 4. , Polycondensation reaction by dealcoholization is performed.
At this time, the inside of the reaction tank 1 is depressurized by using the vacuum pump 12.

A cooling trap 13 is connected to the reaction tank 1,
Design so that the alcohol generated as the reaction progresses can be trapped sequentially. The alcohol recovered in this way is sent to the distillation apparatus 15 to be refined and then recovered in the storage tank 16.

Next, after about 8 to 12 hours have passed in such a batch operation, when the amount of evaporated and recovered alcohol decreases, the produced lactic acid oligomer is put into the hopper 6 of the twin-screw extruder 5. Then, the inside of the apparatus is depressurized by the vacuum pump 12 to advance the polycondensation reaction, and the alcohol produced and vaporized is recovered from the vent port 8.

The alcohol produced along with the progress of the polycondensation reaction in the extruder 5 is properly recovered from the vent port 8 together with lactide which is a side reaction product, and only lactide is crystallized in the cooling trap 13. After that, alcohol and lactide are filtered and separated by a filter 14, and alcohol as a filtrate is refined through a distillation apparatus 15 and stored in a storage tank 1.
Recovered in 6.

The continuous polycondensation operation of the raw materials in the extruder 5 is carried out within the residence time range of the apparatus (7 to 12 minutes).

The molten polylactic acid produced after the completion of the reaction is extruded by the screw, passed through the screen 9 and then through the die 10. These are stored in the storage tank 11 and then molded into a desired shape.

According to the method of the present invention, the weight average molecular weight Mw
It is possible to obtain a polylactic acid having a value of 10,000 to 20,000. These polylactic acids are useful as simple packaging materials, coating agents and sustained release carriers.

[0032]

In the method of the present invention, the lactic acid ester used as a raw material is an industrially mass-produced material, which is a very inexpensive material compared to lactic acid. According to the method of the present invention, lactide is used as a raw material. It has the advantage of lower manufacturing costs than conventional processes. Further, in the present invention, since the raw material is polycondensed while being dealcoholized, the reaction by-product is easily distilled off and the reaction time is about half as compared with the direct polycondensation process in which polycondensation is carried out while lactic acid is dehydrated. It is possible to synthesize a polymer having a molecular weight of As a result, polylactic acid as a molding material and its molded products can be provided to the market at low cost, and it is not necessary to have much mechanical strength and is required to have biodegradability. The growth is expected as a versatile material with promising future.

[0033]

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.

Example 1 Methyl lactate (b.p.p.) from which water was removed using a molecular sieve (pore diameter 3Å) was placed in a polymerization reaction tank of 10 liter capacity equipped with a stirrer, thermocouple, and N ₂ introduction tube. 1
(43 ° C.) was charged to 7.8 kg, and tin octylate as a catalyst was added thereto at a ratio of 0.05 wt% with respect to the raw material.
Next, under a reduced pressure of 700 to 720 mmHg, the reaction temperature is set to be equal to or lower than the reduced pressure boiling point of the raw material, preferably 85 to 110 ° C., and the produced alcohol is distilled out of the system for about 8 hours. The methyl lactate was polycondensed while carrying out.

After that, the produced lactic acid oligomer was fed to a twin-screw extruder at a rate of 3.4 kg / h, the cylinder temperature was 140 to 160 ° C., and the screw rotation speed was 2.
While setting the speed to 00 rpm, while decompressing the inside of the extruder to 180 to 200 mmHg, the methanol remaining in the raw material was efficiently devolatilized from the vent port and, at the same time, the polycondensation reaction was completed. In addition, the reaction time at that time was about 9 minutes from the residence time in the extruder.

When the basic characteristics of the polymer discharged from the extruder were examined after the reaction was completed, the weight average molecular weight Mw was 1
It was 4,500, and it was possible to achieve a value of Mw ≧ 10,000 which enables molding into a coating agent or a sustained-release carrier that does not require much mechanical strength. The polymer yield at that time was 59% as a theoretical amount.

The time required for the reaction is about 8.2 in total.
The reaction time is about 1 compared to the case where a polymer having a molecular weight equivalent to that of this example is synthesized by dehydration / polycondensation reaction using a batch-type polymerization reaction tank using lactic acid as a raw material.
It was shortened to / 2.

Example 2 7.2 kg of dried ethyl lactate (bp 154 ° C.) was charged in the same polymerization reaction tank as in Example 1 and 0.1 wt% of dibutyltin oxide as a catalyst was used as a catalyst.
Was added. Next, the temperature of the reaction liquid is reduced to the boiling point of the reduced pressure of ethyl lactate or less under a reduced pressure of 700 to 720 mmHg.
The polycondensation reaction by dealcoholization was performed while setting the temperature to 0 to 125 ° C. and distilling the produced ethanol out of the system for about 8.5 hours.

Then, the produced lactic acid oligomer was fed to the same extruder as in Example 1 at a rate of 3.2 kg / h, the cylinder temperature was set to 150 to 170 ° C., and the screw rotation speed was set to 220 rpm, 170 ~ 200mmHg in the extruder
While reducing the pressure to 2, the ethanol remaining in the reaction solution was efficiently removed from the vent port, and at the same time, the polycondensation reaction was completed. The reaction time was about 10 minutes from the residence time in the extruder.

When the basic properties of the obtained polymer were examined, the weight average molecular weight Mw was 18,100, and molding into a coating agent or sustained-release carrier that does not require much mechanical strength is possible. A value of Mw ≧ 10,000 could be achieved. The yield of the polymer discharged from the extruder after the reaction was 63% based on the theoretical amount.

The total time required for the reaction was about 8.6.
Time, compared with the case of synthesizing a polymer having a molecular weight equivalent to that of this example by directly dehydrating and polycondensing lactic acid using a batch-type polymerization reaction tank similar to the present invention,
The reaction time was shortened to about 1/2.

Example 3 Dry butyl lactate (b.
p. (185 ° C.), 7.5 kg was charged, and tetrabutyl titanate as a catalyst was added thereto at a ratio of 0.05 wt% with respect to the raw material. Next, the temperature of the reaction solution under a reduced pressure of 700 to 720 mmHg is set to 110 ° C. or lower, which is the reduced pressure boiling point of butyl lactate or less
The polycondensation reaction by dealcoholization was performed while setting the temperature to 130 ° C. and distilling the produced butanol out of the system for about 10 hours.

Then, the produced lactic acid oligomer was fed to the extruder at a rate of 3.0 kg / h, and the cylinder temperature was adjusted to 155.
~ 170 ° C and the screw rotation speed is set to 250 rpm, while the pressure inside the extruder is reduced to 170 to 190 mmHg,
Butanol remaining in the reaction solution was efficiently removed from the vent port, and at the same time, the polycondensation reaction was completed. The reaction time at that time was about 9.5 from the residence time in the extruder.
It was a minute.

When the basic properties of the obtained polymer were investigated, the weight average molecular weight Mw was 14,500, and it was possible to mold into a coating agent or sustained release carrier that does not require much mechanical strength. It was possible to achieve values of Mw ≧ 10,000. The yield of the polymer at that time was 65% based on the theoretical amount.

The total time required for the reaction was about 11.
It takes 2 hours, and the reaction time is about 5/8 as compared with the case where a polymer having the same molecular weight as in this example is synthesized by directly dehydrating and polycondensing lactic acid using the same polymerization reaction tank as in the present invention. Was shortened to.

[Brief description of drawings]

1 is a schematic diagram illustrating the steps of the method of the present invention.

[Explanation of symbols]

 1. Batch type polymerization reactor 2. Stirrer 3. Stirrer 4. Nitrogen cylinder 5. Screw type extruder 6. Hopper 7. Gear pump 8. Vent port 9. Screen 10. Die 11. Storage tank 12. Vacuum pump 13. Cooling trap 14. Filter 15. Distillation device 16. Storage tank 17. Lactate ester 18. Lactic acid oligomer (prepolymer) 19. Alcohol / lactide 20. Alcohol 21. Alcohol (purified product) 22. Polylactic acid 23. Lactide

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Kenjiro Riki, Inventor Kenjiro Funaboshi Minami 1-chome, Aki-ku, Hiroshima City, Hiroshima Prefecture, Japan 6-1 (72) Inventor Kayoko Yokota 1-6 Funakoshi-minami, Aki-ku, Hiroshima City, Hiroshima Prefecture -1 Inside Japan Steel Works, Ltd.

Claims (3)

[Claims] 1. A method for producing polylactic acid, which comprises subjecting a lactic acid ester to polycondensation while removing the produced alcohol. 2. A lactic acid ester is polycondensed in a batch-type polymerization reaction tank while removing generated alcohol, and the obtained oligomer is further polycondensed in a screw type extruder while removing generated alcohol. A method for producing polylactic acid, which comprises: 3. The production method according to claim 1, wherein the polycondensation reaction is carried out by heating under reduced pressure.