JP2850101B2 - Method for producing polylactic acid - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing polylactic acid, and more particularly, to a method for producing high molecular weight polylactic acid in a short time by using inexpensive lactic acid as a raw material. . The polylactic acid obtained by the present invention is suitable as a raw material for producing agricultural materials, sustained-release carriers for agricultural chemicals and agricultural chemicals, and products requiring biodegradability.

[0002]

2. Description of the Related Art Polylactic acid made from lactic acid is easily hydrolyzed and has attracted attention as a medical material such as surgical thread because of its good biodegradability, transparency and rigidity. ing. Such polylactic acid is generally obtained by dehydrating polycondensation of lactic acid to form an oligomer, depolymerizing it in the presence of a catalyst such as an antimony compound or a tin compound to obtain a cyclic diester lactide, and further, tin octylate; It is produced by ring-opening polymerization in the presence of a catalyst such as diethylzinc. Further, the ring-opening polymerization of this lactide was carried out for 3 hours.
A technique performed by an extruder having two heating zones has also been proposed (for example, see JP-A-3-502115).

[0003] However, in the case of the method for producing polylactic acid via lactide as described above, the process of producing lactide as a raw material from lactic acid and the purification process require a great deal of cost and labor. For this reason, it is a major factor that raises the manufacturing cost and hence the product price.

In order to solve such a problem, Japanese Patent Publication No. 52930/1990 discloses that lactic acid is heated and polycondensed in an inert gas atmosphere in the presence of a catalyst. A method for producing a polylactide having a molecular weight of 4,000 to 20,000 by completing a polycondensation reaction under a condition of 10 mmHg or less has been proposed.

However, this method has the following (1) to
It has the problem (4) and has not yet been established as an industrial production method. (1) In order to directly increase the molecular weight of lactic acid by a dehydration polycondensation reaction, it is necessary to react at a high temperature for a long time. However, in this case, it is difficult to control the temperature, maintain an inert gas atmosphere, and the like, and furthermore, there is a problem that the generated polylactic acid is oxidized and degraded or thermally decomposed to cause coloring and a decrease in molecular weight. (2) In the dehydration polycondensation reaction, it is necessary to forcibly dehydrate under a very high vacuum, but in this case, the reaction solution is azeotropically distilled off with water, and the yield decreases. There is a problem of doing. (3) Since lactic acid has low reactivity depending on the position of the hydroxyl group, it takes a very long time to complete the polycondensation reaction, which causes a problem that the production time is prolonged and the cost is increased. (4) If the viscosity of the reaction system increases with the progress of the reaction, it becomes practically impossible to remove water trapped inside the polylactic acid by a normal batch-type polycondensation operation. There is a problem that the molecular weight of polylactic acid is reduced.

[0006]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems (1) to (4) in a conventional manufacturing technique,
An object of the present invention is to provide a method for producing polylactic acid capable of producing high molecular weight polylactic acid in a shorter time and smoothly.

[0007]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to achieve the above object, and have found that an acetylated lactic acid oligomer having increased reactivity by acetylating a hydroxyl group of lactic acid having low reactivity is obtained. The above problem can be solved by simultaneously performing the deacetylation reaction and the polycondensation reaction using the heating kneading function and the devolatilization [removal of volatile substances (low molecular weight substances)] function of the screw type extruder. And completed the present invention.

That is, the present invention provides a method for producing polylactic acid, which comprises deacetylating and polycondensing an acetylated lactic acid oligomer in a screw-type extruder under heating and reduced pressure.

Acetylated lactic acid oligomer used in the method of the present invention (hereinafter referred to as "acetylated oligomer")
Is obtained, for example, by reacting lactic acid with an acetylating agent.

The lactic acid may be any of D-form, L-form or D, L-form and a mixture thereof, and preferably has a purity of 8%.
Use one with 5% or more. As the acetylating agent, known compounds such as acetic anhydride, glacial acetic acid, acetyl chloride, and ketene can be used, but acetic anhydride is preferable in terms of cost and ease of handling. This acetylating agent is used in an amount of about equimolar or more with lactic acid.

The reaction is preferably carried out by heating in an atmosphere of an inert gas such as nitrogen gas or argon gas. The heating temperature is preferably 100 to 150 ° C,
120-140 ° C is more preferred. If the heating temperature is too low, the reaction does not proceed, and if it is too high, depolymerization occurs, which is not preferable. The time required for the acetylation (oligomerization) reaction is about 1 to 3 hours in the above temperature range. For example, a Fourier transform infrared absorption spectrum (FT-
It is desirable to confirm the progress of the reaction by IR). That is, as the peak derived from the hydroxyl group of lactic acid, the -OH group out-of-plane bending vibration appears around 920 cm ^-1 , which disappears with the progress of acetylation, and increases with the increase of the acetyl group instead. Peaks appear at 1380 and 1460 cm ^-1 due to the bending vibration of the CH group. Therefore, it is desirable to confirm the progress of the reaction at any stage by appropriately confirming the peak by FT-IR.

In the present invention, it is preferable to sequentially remove acetic acid generated by the acetylation reaction. After the acetylation reaction is completed, it is preferable to completely remove the unreacted acetylating agent. In particular, the treatment for removing the acetylating agent is desirable because it facilitates the polycondensation reaction in the next step and does not cause inconvenience such as coloring of the polylactic acid. The acetic acid generated by the acetylation reaction is reduced in pressure in the reaction system, for example, 500 mmHg or less, preferably 350-45Hg.
By keeping the pressure at 0 mmHg, it can be distilled off.
The acetylating agent such as acetic anhydride remaining unreacted may be distilled out of the reaction system by adding an alcohol such as ethanol, propanol, or butanol to the reaction system to form a lower boiling ester. Can be. This treatment is carried out at 90-130 ° C, preferably at 100-120 ° C.
The reaction system is set to 350 to 700 mmHg, preferably 400 to 65 mmHg.
The pressure is reduced to 0 mmHg. The time required for this process is about 1
3 hours. The acetylated oligomer thus obtained usually has a Mw of about 2,000 to 3,500.

Next, the acetylated oligomer obtained as described above is supplied to a screw type extruder, and a polycondensation reaction is carried out under heating and reduced pressure in parallel with the deacetylation reaction to obtain a high molecular weight polylactic acid. Get.

First, the acetylated oligomer is supplied to a screw-type extruder. The screw-type extruder used in this step includes a screw, a cylinder, a heating unit, a drive unit, a devolatilizer, which are usually used for molding plastics. A single-screw or twin-screw extruder comprising a vent port and the like can be used. As the twin-screw extruder, any of those having the same or different rotation direction of the screw, and those having screw flights meshing with each other or not meshing with each other can be used. In the present invention, a screw has a plurality of heating units divided around the cylinder so as to be individually temperature-controllable, and a plurality of vent ports capable of devolatilizing from the inside of the cylinder in the unit. A meshing type twin screw type extruder having a ratio (L / D ratio) of the length (L) to the screw diameter (D) of 30 to 70 is preferable. It is preferable to use a twin-screw extruder having a large number of heating units and vent ports as described above because temperature control and devolatilization operation can be performed according to the reaction stage for each of different zones of the cylinder of the extruder. The supply amount of the acetylated oligomer can be set as appropriate, but is usually supplied to fill the inside of the extruder.

In this step, a catalyst can be added after the acetylated oligomer is supplied to the screw type extruder. As the catalyst in this case, manganese acetate, zinc acetate, aluminum acetate, magnesium acetate, and the like, which have high solubility in acetic acid as a by-product, are preferable. The addition amount of the catalyst is preferably 0.5 to 3.0% by weight, more preferably 1.0 to 2.0% by weight based on the supplied raw material.

Next, a deacetylation reaction and a polycondensation reaction of the acetylated oligomer are performed by heating and reducing the pressure. The heating temperature is preferably from 120 to 190 ° C, and from 150 to 180 ° C.
Is more preferred. If the heating temperature is lower than 120 ° C., the reaction rate becomes too slow, and if it exceeds 190 ° C., a depolymerization reaction occurs in parallel with the polycondensation reaction, which is not preferable. The reaction time is usually about 30 minutes within the above-mentioned temperature range. The reduced pressure is preferably 400 mmHg or less.
More preferably, the range is from 00 to 350 mmHg. If the degree of reduced pressure is too low, the generated low molecular weight substances such as acetic acid will not be sufficiently devolatilized, and the presence thereof will hinder the smooth progress of the reaction, which is not preferable. Since this reaction is performed in a screw-type extruder, kneading with the screw is performed, and the rotation speed of the screw required for the kneading is preferably 100 rpm or more,
More preferred is 150-250 rpm.

Through such a manufacturing process, Mw = 10,
000-50,000 polylactic acid can be obtained.
Next, an example of the manufacturing method of the present invention will be described with reference to FIG. 1 attached to the specification. Lactic acid and acetic anhydride are charged into a batch type reaction tank 2 equipped with a stirrer 1 and maintained at a predetermined temperature (120 to 140 ° C.) with stirring to perform an acetylation (oligomerization) reaction. At this time, the inside of the reaction tank 2 is replaced with an inert gas such as a nitrogen gas supplied from the inert gas container 3. After the acetylation reaction is completed, the pressure inside the reaction tank 2 is reduced by a vacuum pump 6 (350 to 700 mmHg).
The generated acetic acid 21 is collected and stored in the tank 4. Unreacted acetic anhydride is recovered as acetic ester 22 and stored in tank 5. The reaction tank 2 used in these treatments is desirably formed of a material having high corrosion resistance, such as titanium, a titanium alloy, Hastelloy, or the inside thereof is desirably coated with Teflon.

Next, the produced acetylated lactic acid oligomer is supplied to the hopper 7. An appropriate amount of the lactic acid oligomer supplied to the hopper 7 is supplied to the extruder 9 by the action of the gear 8. At this time, a catalyst can be added as needed from the catalyst input port 11 provided.
The lactic acid oligomer supplied to the inside of the extruder 9 is kneaded by a screw (not shown) (at 100 rpm).
As described above, in the process of moving inside the cylinder, the heating units 10 heat the respective zones (1 to 11) at different temperatures, thereby performing a deacetylation reaction and a polycondensation reaction. In this reaction process, not only the amount of heat due to heating but also the amount of heat due to shear heat generated between the polymers due to the kneading of the screw is added, so that the amount of heat that is actually added or more is supplied, and the reaction becomes smoother. Proceed to

At this time, the inside of the cylinder is depressurized by the vacuum pump 14 (350 mmHg or less). As a result, low molecular weight substances such as acetic acid pass through the plurality of vent ports 12 from the inside of the cylinder of the extruder 9. Exhausted as gas.
Low molecular weight substance 2 such as acetic acid recovered in cooling trap 13
3 is stored in a tank 19. When a twin-screw extruder is used, the residence time of the raw material in the cylinder is about 10 minutes in continuous operation, and a sufficient reaction time cannot be secured. For this reason, even in such a case, a sufficient residence time (that is, a reaction time) can be secured.
By mounting a stop valve 17 on the die portion 16 of the extruder 9 and closing and opening the stop valve 17, a sufficient raw material residence time can be ensured at any stage.

After the completion of the reaction, the produced polylactic acid in a molten state is sent out by a screw, passes through a screen 15, and
Further, it is extruded through the die 16. Thereafter, the polylactic acid 24 is stored in the storage tank 18.

[0021]

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.

Example 1 Polylactic acid was produced by the production process shown in FIG. First, D, L-lactic acid (purity 90%) was charged into a batch reactor made of titanium, and an equimolar amount of acetic anhydride was added thereto. Next, the inside of the reaction tank was replaced with nitrogen gas, and 12
The acetylation reaction (oligomerization reaction) of lactic acid was performed over about 2 hours while controlling the temperature at 0 to 130 ° C. When the IR spectrum of the obtained lactic acid oligomer was examined,
It was confirmed that the absorption around 1373 cm ^-1 and 1459 cm ^-1 increased.

Thereafter, acetic acid as a by-product was removed while reducing the pressure in the reaction vessel to 380 to 400 mmHg. Almost simultaneously, an equimolar butanol was added for the purpose of removing the remaining acetic anhydride, and butyl acetate as a reaction product of the two was distilled out of the reactor. The reaction is 420-55
The reaction was performed at 115 ° C. under a reduced pressure of 0 mmHg for about 1.8 hours. In addition, in order to confirm whether or not the reaction was proceeding smoothly, the IR spectrum of the reaction solution was examined as appropriate. As a result, the acetic acid ester (1720 to 17
Spectrum of 50 cm ^-1) and acetic acid (1790 cm ^-1) is eventually confirmed that disappeared.

Next, the obtained acetylated oligomer (M
w = 2,200) was supplied as a prepolymer until the inside of the extruder was filled. As an extruder, L / D
Ratio = 42, screw diameter 32 mm and co-rotating mesh type twin screw extruder, as shown in FIG. 1, each of a total of 11 zones of a cylinder can be set to different temperatures, and zones 4, 7 and The one having three vent ports capable of devolatilizing 10 was used. Here, the term "zone" is used only in the sense that different positions of the cylinder can be set to different heating temperatures, and does not mean that the zones are separated. The operating conditions are:
Screw rotation speed is 150rpm, cylinder temperature is zone 1
120 ° C., 170-180 ° C. in zones 2-9, and 160 ° C. in zones 10 and 11.

After that, from the inlet attached to zone 1 of the cylinder, 1.0
Manganese acetate was added in an amount of weight%, and a deacetylation reaction and a polycondensation reaction were carried out by a batch operation for about 30 minutes with a stop valve attached to an outlet of the extruder closed. In addition, the reaction is 200-250 mm inside the cylinder.
The pressure was reduced to Hg, and acetic acid generated by the reaction was removed from the vent port. Thereafter, the stop valve was opened, the screw rotation speed was increased to 200 rpm, and the polymer was extruded. The total reaction time (including the residence time in the extruder) was about 37 minutes.

Thus, the polylactic acid (Mw = 12,
800). According to this production method, the time (18) required for producing polylactic acid having the same molecular weight in the conventional method (for example, the method described in Japanese Patent Publication No. 52930/1990).
２０20 hours), the production time was reduced to about a quarter.

Example 2 Polylactic acid was produced by the production process shown in FIG. First, L-lactic acid (purity: 90%) was charged into a batch reactor made of titanium, and an equimolar amount of acetic anhydride was added thereto. Next, the inside of the reaction vessel was replaced with nitrogen gas,
The acetylation reaction of lactic acid was performed for about 2 hours while controlling the temperature at 130 ° C. IR of lactic acid oligomer obtained
Examination of the spectrum revealed that 1375 cm ^-1 and 1440
It was confirmed that the absorption around cm ^-1 increased.

Thereafter, while reducing the pressure in the reaction vessel to 350 to 380 mmHg, acetic acid as a by-product was removed. Almost simultaneously, an equimolar propanol was added for the purpose of removing the remaining acetic anhydride, and propyl acetate, a reaction product of the two, was distilled out of the reaction vessel. This reaction is 400-
The reaction was performed at 105 ° C. under a reduced pressure of 550 mmHg for about 2.4 hours.
In addition, in order to confirm whether or not the reaction was proceeding smoothly, the IR spectrum of the reaction solution was examined as appropriate. As a result, the acetic acid ester (1720 to 1
750 cm ^-1 ) and acetic acid (1790 cm ^-1 )
Eventually, it was confirmed that it had almost disappeared.

Next, the obtained acetylated oligomer (M
(w = 2,800) as a prepolymer, and supplied to the same extruder as in Example 1 until the inside of the cylinder was filled. The operating conditions of the extruder are as follows:
0 rpm, cylinder temperature is 110 ° C. in zone 1, 160-175 ° C. in zones 2-9, 150 in zones 10 and 11.
° C.

After that, from the input port attached to zone 1 of the cylinder, 1.0
By weight of zinc acetate was added, and the stop valve attached to the outlet of the extruder was closed to perform a deacetylation reaction and a polycondensation reaction by a batch operation for about 25 minutes. The reaction was performed while reducing the pressure in the cylinder to 250 to 300 mmHg and removing acetic acid generated by the reaction from the vent port. Thereafter, the stop valve was opened, the screw rotation speed was increased to 200 rpm, and the polymer was extruded. The total reaction time (including the residence time in the extruder) is
About 30 minutes.

Thus, polylactic acid (Mw = 13,
600). According to this production method, the time (18) required for producing polylactic acid having the same molecular weight in the conventional method (for example, the method described in Japanese Patent Publication No. 52930/1990).
２０20 hours), the production time was reduced to about a quarter.

Example 3 Polylactic acid was produced according to the production process shown in FIG. First, D, L-lactic acid (purity 90%) was charged into a batch reactor made of titanium, and an equimolar amount of acetic anhydride was added thereto. Next, the inside of the reaction tank was replaced with nitrogen gas, and 12
The acetylation reaction of lactic acid was performed over about 2 hours while controlling the temperature at 0 to 130 ° C. Examination of the IR spectrum of the obtained lactic acid oligomer, 1332 cm ^-1 and 14
It was confirmed that the absorption around 40 cm ^-1 increased.

Thereafter, acetic acid as a by-product was removed while reducing the pressure in the reaction vessel to 350 to 400 mmHg. Almost simultaneously, an equimolar amount of ethanol was added for the purpose of removing the remaining acetic anhydride, and ethyl acetate as a reaction product of the two was distilled out of the reaction vessel. The reaction is 400-58
The reaction was performed at 100 ° C. under a reduced pressure of 0 mmHg for about 1.6 hours. In addition, in order to confirm whether or not the reaction was proceeding smoothly, the IR spectrum of the reaction solution was examined as appropriate. As a result, the acetic acid ester (1720 to 17
Spectrum of 50 cm ^-1) and acetic acid (1790 cm ^-1) is, finally it was confirmed that almost disappeared.

Next, the obtained acetylated oligomer (M
(w = 2,500) as a prepolymer, and supplied to the same extruder as in Example 1 until the cylinder was filled. The operating conditions of the extruder are as follows.
0 rpm, cylinder temperature 110 ° C in zone 1, zone 2
155 to 170 ° C in ~ 9, 150 ° C in zones 10 and 11
Met.

After that, the supplied prepolymer was supplied to the injection port attached to zone 1 of the cylinder in an amount of 1.0 to 1.0.
Weight of aluminum acetate was added, and with the stop valve attached to the outlet of the extruder closed, about 3% was added.
By a batch operation for 0 minutes, a deacetylation reaction and a polycondensation reaction were performed. The reaction takes place in the cylinder in the range of 120-22.
The pressure was reduced to 0 mmHg, and acetic acid generated by the reaction was removed from the vent port. Thereafter, the stop valve was opened, the screw rotation speed was increased to 200 rpm, and the polymer was extruded. The total reaction time (including the residence time in the extruder) was about 37 minutes.

Thus, polylactic acid (Mw = 11,
500). According to this production method, the time (18) required for producing polylactic acid having the same molecular weight in the conventional method (for example, the method described in Japanese Patent Publication No. 52930/1990).
２０20 hours), the production time was reduced to about a quarter.

[0037]

According to the production method of the present invention, the hydroxyl group of lactic acid is substituted with a more reactive acetyl group and reacted, so that the oligomerization reaction and the subsequent polycondensation reaction are carried out.
It progresses smoothly and is completed in a short time. Further, since the high devolatilizing function of the screw type extruder is used, unnecessary substances such as acetic acid and water generated during the polycondensation reaction are easily and almost completely removed. For this reason, according to the manufacturing method of the present invention,
Compared to the conventional method, low-temperature and short-time treatment is sufficient, so that high-quality polylactic acid without inconvenience such as coloring due to oxidative deterioration or thermal decomposition and molecular weight reduction can be obtained. In addition, since unnecessary substances can be almost completely removed, the yield does not decrease accordingly. According to the production method of the present invention, Mw = satisfies the mechanical strength required for ordinary general-purpose consumables.
10,000 or more polylactic acids are obtained. Therefore, in addition to conventional medical use, this polylactic acid is used as a raw material for producing agricultural applications, for example, pots and cover films for nursery bed protection, sustained-release carriers containing agents such as agricultural chemicals and plant growing agents, and the like. Also, it is suitable as a raw material for producing products that are discarded after use, for example, food-related products such as can holders and various food packaging materials, and leisure products such as fishing lines and golf tees.

[Brief description of the drawings]

FIG. 1 is a schematic diagram for explaining a manufacturing method of the present invention.

[Explanation of symbols]

 2 batch type reaction tank 7 Hopper 9 Screw type extruder 10 Heating unit 11 Catalyst input port 12 Vent port 16 Die 17 Stop valve

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) C08G 63/06 C08G 63/78-63/87

Claims (4)

(57) [Claims] 1. A method for producing polylactic acid, comprising deacetylating and polycondensing an acetylated lactic acid oligomer in a screw-type extruder under heating and reduced pressure. 2. A method for producing polylactic acid, comprising reacting lactic acid with an acetylating agent and deacetylating and polycondensing the obtained acetylated lactic acid oligomer under heating and reduced pressure in a screw type extruder. . 3. The polylactic acid has a weight average molecular weight of 10,000.
The method for producing polylactic acid according to claim 1 or 2, wherein the amount is from 0 to 50,000. 4. The method for producing polylactic acid according to claim 2, wherein, after the reaction between lactic acid and the acetylating agent, unreacted acetylating agent and generated acetic acid are removed.