JP7425181B2 - How to decompose polylactic acid - Google Patents

Description

The present invention relates to a method for decomposing polylactic acid.

As environmental pollution caused by resin waste has become a social issue, biodegradable resins such as polylactic acid are attracting attention. In recent years, techniques related to chemical recycling, which decomposes and reuses biodegradable resins, have been developed.

JP-A-6-279434 discloses a method for producing meso-containing lactides in which a hydroxy acid oligomer is thermally decomposed using an alkali metal salt as a catalyst. JP-A-2007-210889 discloses a method for monomerizing stereocomplex polylactic acid, in which stereocomplex polylactic acid is treated at a high temperature of 170 to 330°C for 5 to 240 minutes.

JP 2017-132730 A discloses that polylactic acid and a depolymerization catalyst are put into an extruder connected to a vent chamber maintained under reduced pressure, and the extruder melt-kneads the polylactic acid and the depolymerization catalyst. A lactide recovery method is disclosed in which a melt-kneaded product is supplied into a vent chamber, polylactic acid is depolymerized in the vent chamber, and the produced lactide is gasified and recovered from the vent chamber.

Japanese Patent Application Publication No. 6-279434 Japanese Patent Application Publication No. 2007-210889 Japanese Patent Application Publication No. 2017-132730

The method described in Japanese Patent Application Laid-open No. 6-279434 involves the reaction in a reduced pressure environment, which requires a large-scale apparatus and requires batch processing. Further, it is necessary to use a hydroxy acid having a low molecular weight as a raw material.

JP-A-2007-210889 proposes a method for obtaining lactic acid by decomposing polylactic acid. However, the method of directly synthesizing polylactic acid from lactic acid has not been put to practical use because of the problem that the molecular weight does not increase. On the other hand, in order to produce lactide, it is necessary to separate polylactic acid and water. This process requires a lot of energy, which goes against the purpose of recycling biodegradable resins, which is to reduce environmental impact.

The method described in JP 2017-132730A has low reaction reproducibility and cannot stably control the molecular weight or reaction rate of the product.

An object of the present invention is to provide a method for decomposing polylactic acid that can decompose polylactic acid while suppressing energy consumption.

A method for decomposing polylactic acid according to an embodiment of the present invention includes polylactic acid and a compound selected from the group consisting of an alkali metal salt, an alkaline earth metal salt, an alkali metal oxide, and an alkaline earth metal oxide. The above-mentioned polylactic acid and and a step of holding or kneading the additive.
Moisture content = mass of water in the reactor / (mass of polylactic acid + mass of additive + mass of water added from the outside)
However, the mass of water in the reaction apparatus includes the mass of water adsorbed on the polylactic acid and the additive, and the mass of water added from the outside, and the mass of the polylactic acid and the mass of the additive are , the mass including the mass of water adsorbed on each.

According to the present invention, polylactic acid can be decomposed while suppressing energy consumption.

FIG. 1 is a flow diagram of a method for decomposing polylactic acid according to a first embodiment of the present invention. FIG. 2 is a flow diagram of a method for decomposing polylactic acid according to a second embodiment of the present invention. FIG. 3 is a flow diagram of a method for decomposing polylactic acid according to a third embodiment of the present invention. FIG. 4 is a schematic diagram of an example of a polylactic acid decomposition device. FIG. 5 is a schematic diagram of another example of a polylactic acid decomposition device. FIG. 6 is a graph showing the relationship between water content, number average molecular weight of decomposition products, and energy consumption.

Conventionally, it was thought that a large amount of water was necessary to promote the decomposition reaction of polylactic acid. For example, the above-mentioned Japanese Patent Application Publication No. 2007-210889 describes that 5 to 100 parts by mass of water is allowed to coexist with 1 part by mass of polylactic acid.

The present inventors have discovered that under conditions in which a predetermined additive is added, the decomposition reaction of polylactic acid proceeds even if the amount of water is extremely small.

The present invention was completed based on this knowledge. Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Identical or corresponding parts in the drawings are denoted by the same reference numerals and their description will not be repeated. The dimensional ratios between the constituent members shown in each figure do not necessarily represent the actual dimensional ratios.

[First embodiment]
FIG. 1 is a flow diagram of a method for decomposing polylactic acid according to a first embodiment of the present invention. The method for decomposing polylactic acid according to the present embodiment includes a step of preparing polylactic acid and additives (step S1), a step of adjusting the water content of polylactic acid and additives (step S2), and a step of preparing polylactic acid and additives in a reaction apparatus. (step S3), holding or kneading polylactic acid and additives (step S4), and taking out decomposition products (step S5).

Polylactic acid as a raw material and additives to be added to polylactic acid are prepared (step S1). Preferably, the polylactic acid is pretreated in the form of powder or pellets. Alternatively, a pulverizer may be provided at the raw material input port of the reaction apparatus so that raw materials (for example, waste, etc.) containing polylactic acid are pulverized simultaneously with the raw material input step (step S3) to be described later.

The additive is one or a mixture of two or more selected from the group consisting of alkali metal salts, alkaline earth metal salts, alkali metal oxides, and alkaline earth metal oxides. The additive is preferably one or a mixture of two or more selected from the group consisting of sodium bicarbonate (baking soda), sodium carbonate, potassium carbonate, and magnesium oxide, and baking soda is particularly preferred.

The water content of polylactic acid and additives is adjusted as necessary (step S2). Additives are hygroscopic and may absorb moisture. Polylactic acid may also absorb moisture. Therefore, the moisture content of polylactic acid and additives may be adjusted in advance before they are introduced into the reaction apparatus.

The step of adjusting the water content (step S2) is an optional step, and may be omitted if the water content of the polylactic acid and additives to be added is stable. When adjusting the water content, the water content of at least one of polylactic acid and the additive may be adjusted.

Specific methods for adjusting the water content include drying polylactic acid and additives in a dryer, keeping them in a constant temperature and humidity tank for a predetermined period of time, and moistening them by spraying or the like.

Furthermore, the moisture content of polylactic acid and additives may be adjusted by controlling the humidity at the inlet of the reaction device. As mentioned above, the moisture content of polylactic acid and the additive is affected by the humidity at the input port of the reactor, especially since the additive has hygroscopic properties. If you want to lower the moisture content of polylactic acid and additives, you can lower the humidity at the input port of the reaction device, and if you want to increase the moisture content of polylactic acid and additives, you can lower the humidity at the input port of the reaction device. All you have to do is make it higher. Devices for controlling the humidity at the inlet of the reaction device include air conditioning equipment and a spray device.

Polylactic acid and additives are introduced into the reaction device (step S3). In this embodiment, as will be described later, polylactic acid is decomposed while controlling the moisture content in the reaction apparatus. Therefore, it is preferable that the reactor has a structure in which the inside can be sealed, except for the input and output ports for raw materials, the inlet and outlet for gas (vent ports), the inlet for moisture, and the like. The reactor is preferably made of metal, glass or ceramic, particularly preferably metal. The reaction device may be an extruder as described below.

The amount of the additive is preferably 0.2 to 20 parts by weight per 100 parts by weight of polylactic acid. However, the mass of polylactic acid and the mass of the additive include the mass of water adsorbed on each. Hereinafter, the value of (mass of additive (including mass of adsorbed water))/(mass of polylactic acid (including mass of adsorbed water)) x 100 will be referred to as "additive concentration."

If the additive concentration is too low, the rate of decomposition of polylactic acid will be slow. The lower limit of the additive concentration is more preferably 0.50 parts by mass, still more preferably 1.0 parts by mass, and even more preferably 2.0 parts by mass. On the other hand, if the additive concentration is too high, the energy required for heating increases. The upper limit of the additive concentration is more preferably 18 parts by mass, still more preferably 15 parts by mass, and even more preferably 10 parts by mass.

Polylactic acid and additives are held or kneaded (step S3). This holding or kneading step (step S3) is performed so that the water content expressed by the following formula is 0.15 to 3.0%.
Moisture content = mass of water in the reactor / (mass of polylactic acid + mass of additive + mass of water added from the outside)
However, the mass of water in the reactor includes the mass of water adsorbed on polylactic acid and additives, as well as the mass of water added from the outside. The mass includes the mass of water contained in the water.

In this holding or kneading step (step S3), moisture may be added from outside the reaction apparatus to adjust the moisture inside the reaction apparatus. Addition of moisture from the outside can be carried out, for example, by supplying moisture from an inlet of the reaction apparatus or an inlet provided separately from the inlet. Moisture may be supplied as a liquid or as a humidified gas. However, it is not essential to add water from the outside, and the mass of water added from the outside may be zero. Alternatively, the moisture content may be reduced by supplying dry gas.

If the water content is too low, the decomposition rate of polylactic acid will be slow. The lower limit of the water content is preferably 0.20%, more preferably 0.50%, and still more preferably 1.0%. On the other hand, if the water content is too high, the energy required for heating will increase. The upper limit of the water content is preferably 2.8%, more preferably 2.5%, and still more preferably 2.0%.

This holding or kneading is preferably carried out by heating the inside of the reaction apparatus. The heating temperature is preferably 120 to 340°C. The higher the heating temperature, the easier the reaction will proceed. The lower limit of the heating temperature is more preferably 160°C, still more preferably 180°C. On the other hand, if the heating temperature is too high, bonds other than the ester bonds will be broken, resulting in a decrease in the yield of the desired decomposition product. The upper limit of the heating temperature is more preferably 300°C, still more preferably 280°C, and even more preferably 260°C.

This holding or kneading is preferably carried out by pressurizing the inside of the reaction apparatus. The pressure within the reactor is preferably 0.15 to 20.0 MPa. The lower limit of the pressure within the reactor is more preferably 0.5 MPa, still more preferably 1.0 MPa, and still more preferably 3.0 MPa. The upper limit of the pressure within the reactor is more preferably 15.0 MPa, still more preferably 10.0 MPa, and still more preferably 5.0 MPa.

The holding or kneading time is not particularly limited, but is preferably 1 second or more, for example 1 second to 12 hours. The lower limit of the holding or kneading time is more preferably 30 seconds, still more preferably 1 minute, and even more preferably 2 minutes. The upper limit of the holding or kneading time is more preferably 6 hours, still more preferably 2 hours, and even more preferably 30 minutes. When kneading, the shear rate is preferably 500 s ⁻¹ or less.

After holding or kneading for a predetermined period of time, a decomposition product formed by decomposing polylactic acid is taken out from the reaction device (step S5). The decomposition products include oligomers of lactic acid, lactic acid, lactide, and the like.

The method for decomposing polylactic acid according to the first embodiment of the present invention has been described above. The method for decomposing polylactic acid according to the present embodiment includes a step of holding or kneading polylactic acid and additives under conditions such that the water content is 0.15 to 3.0%. According to this configuration, polylactic acid can be decomposed while suppressing consumption of energy required for heating water. Furthermore, according to the present embodiment, not only lactic acid oligomers and lactic acid but also lactide can be recovered.

[Second embodiment]
FIG. 2 is a flow diagram of a method for decomposing polylactic acid according to a second embodiment of the present invention. The polylactic acid decomposition method according to the present embodiment includes, in addition to each step of the first embodiment (FIG. 1), a step of measuring moisture in the reaction apparatus (step S6), and based on the measured moisture, The method further includes a step (step S7) of adjusting moisture within the reactor.

In this embodiment, in parallel with the step of holding or kneading polylactic acid and additives (step S4), the water content in the reaction apparatus is measured (step S6). The moisture content in the reactor can be measured, for example, by collecting gas components in the reactor from a vent provided in the reactor and analyzing the collected gas components with a moisture meter. The moisture meter is, for example, a Karl Fischer moisture meter or an infrared moisture meter.

Based on the measured moisture content, the moisture content in the reaction device is adjusted (step S7). Specifically, the measured moisture content is compared with a preset lower limit of moisture content, and if the measured moisture content is less than the lower limit of moisture content, an operation is performed to increase the moisture content in the reaction apparatus. Specific operations for increasing the moisture content in the reactor include, for example, injecting liquid water into the reactor or introducing humidifying gas into the reactor.

As a step (step S7) of adjusting the moisture content in the reaction device, instead of or in addition to the above operation, the measured moisture content is compared with a preset upper limit of moisture content, and the measured moisture content is adjusted. If the amount exceeds the upper limit of water content, an operation may be performed to lower the water content in the reactor. Specific operations for lowering the moisture content in the reactor include, for example, stopping water injection into the reactor and introducing dry gas into the reactor.

In this embodiment, a step of holding or kneading polylactic acid and additives (step S4), a step of measuring moisture in the reaction device (step S6), and a step of adjusting the moisture in the reaction device (step S7) are performed. , repeat until a predetermined time has elapsed. Thereby, polylactic acid and additives can be held or kneaded while the water content is controlled to be within a predetermined range.

[Third embodiment]
FIG. 3 is a flow diagram of a method for decomposing polylactic acid according to a third embodiment of the present invention. In this embodiment, the process from adding polylactic acid and additives to taking out decomposition products is performed continuously. As such a process, for example, a reaction device having an input port and an output port is used, and while polylactic acid and additives are input through the input port, the reaction product is taken out from the output port at the same time. One example is a process that continuously decomposes .

Similarly to the decomposition method of the first embodiment (FIG. 1), the decomposition method of this embodiment includes a step of preparing polylactic acid and additives (step S1), and a step of adjusting the water content of polylactic acid and additives. (Step S2), a step of charging polylactic acid and additives into a reaction device (step S3), a step of holding or kneading polylactic acid and additives (step S4), and a step of taking out decomposition products (step S5). We are prepared. As described above, in this embodiment, the process from adding polylactic acid and additives to taking out decomposition products is performed continuously. Specifically, the steps from preparing polylactic acid and additives (step S1) to removing decomposition products (step S5) are repeated until a predetermined period of time has elapsed.

Similarly to the decomposition method of the second embodiment (FIG. 2), the decomposition method of this embodiment also includes the step of measuring the moisture in the reaction apparatus (step S6), and the step of measuring the moisture in the reaction apparatus based on the measured moisture. The method further includes a step of adjusting the moisture content (step S7). However, in the decomposition method of this embodiment, the timing of implementing the step (step S7) of adjusting the moisture in the reaction apparatus is different from that of the second embodiment.

In this embodiment, the water content in the reaction apparatus is adjusted by adjusting the moisture content of at least one of polylactic acid and additives introduced into the reaction apparatus. That is, in this embodiment, the step of adjusting the moisture content of polylactic acid and additives (step S2) also serves as the step of adjusting the moisture in the reaction apparatus (step S7).

Specifically, as in the case of the second embodiment, the measured moisture content is compared with a preset lower limit of moisture content, and if the measured moisture content is less than the lower limit of moisture content, the water content in the reaction apparatus is Perform operations to increase moisture. In this embodiment, as an operation to increase the water content in the reaction apparatus, an operation is performed to increase the water content of at least one of polylactic acid and additives to be introduced into the reaction apparatus. More specifically, for example, operations such as lowering the drying temperature of polylactic acid and additives, shortening the drying time, moistening by spraying or the like, and increasing the humidity at the input port of the reaction apparatus can be mentioned.

As in the case of the second embodiment, instead of or in addition to the above operation, the measured moisture content is compared with a preset moisture content upper limit, and the measured moisture content exceeds the moisture content upper limit value. If it exceeds the water content, an operation may be performed to lower the water content in the reactor. In this embodiment, as an operation to lower the moisture in the reaction apparatus, an operation is performed to lower the moisture content of at least one of polylactic acid and additives to be introduced into the reaction apparatus. More specific examples include increasing the drying temperature of polylactic acid and additives, prolonging the drying time, stopping moistening by spraying, etc., and lowering the humidity at the input port of the reactor. .

In addition to or in place of the above operations, the moisture content within the reaction apparatus may be adjusted by injecting water into the reaction apparatus, or introducing humidified gas or dry gas. That is, in addition to or in place of adjusting the moisture content of at least one of polylactic acid and additives introduced into the reaction apparatus, Similarly, the moisture content within the reactor may be adjusted by injecting water into the reactor or introducing humidified gas or dry gas.

According to this embodiment as well, polylactic acid and additives can be held or kneaded while the water content is controlled to be within a predetermined range.

[Configuration of polylactic acid decomposition device]
Next, the configuration of the polylactic acid decomposition device will be explained. The decomposition apparatus described below is merely an example, and does not limit the method for decomposing polylactic acid according to this embodiment.

[Configuration example 1]
FIG. 4 is a schematic diagram of a decomposition device 1, which is an example of a polylactic acid decomposition device. The decomposition device 1 includes an extruder 10, a feeder 20, a moisture adjustment device 25, a gas analyzer 30, and a control device 40. Polylactic acid and additives are fed into the extruder 10 by the feeder 20, kneaded by the extruder 10, and finally taken out from the discharge port (takeout port) 11c of the extruder 10 as a low molecular weight decomposition product.

The feeder 20 feeds polylactic acid and additives into the extruder 10 in predetermined amounts. It is preferable that the feeder 20 has a structure in which the supply amounts of polylactic acid and additives can be controlled independently. The feeder 20 includes a motor 21, and by controlling the rotation speed of the motor 21, the amount of polylactic acid and additives to be supplied can be controlled. More specifically, for example, a screw feeder or a belt conveyor can be used as the feeder 20.

The extruder 10 includes a cylinder wall 11 (reactor), a screw 12, a heating device 13, a seal member 14, a die 15, and the like.

The cylinder wall 11 has an internally sealed structure except for openings (an inlet 11a, a vent 11b, and an outlet 11c) which will be described later, and functions as a reaction device for the decomposition reaction of polylactic acid.

The screw 12 kneads the contents of the cylinder wall 11. The polylactic acid and additives introduced into the cylinder wall 11 are conveyed toward the discharge port 11c while being kneaded by the screw 12. The screw 12 is equipped with a motor 121, and by controlling the rotation speed of the motor 121, the shearing speed can be adjusted.

The heating device 13 heats the cylinder wall 11 to adjust the temperature of the cylinder wall 11. In the example of FIG. 4, the heating device 13 is divided and arranged around three regions defined by the sealing member 14, so that the temperature of each region can be controlled independently. As a result, for example, the upstream region in the conveyance direction is the melting transition zone, the central region is the decomposition zone, and the downstream region is the degassing/cooling discharge zone, creating a temperature distribution in which the decomposition zone is the highest temperature. can.

The cylinder wall 11 is provided with an input port 11a, a vent port 11b, and a discharge port 11c as openings.

The input port 11a is provided at the most upstream side of the cylinder wall 11 in the conveying direction. Polylactic acid and additives are input from the feeder 20 into the input port 11a.

The vent port 11b is provided on the downstream side of the cylinder wall 11 in the conveyance direction so that only gas components can pass therethrough. It is preferable that the bend port 11b is provided near the discharge port 11c. In the example of FIG. 4, the vent port 11b is arranged in the downstream region (degassing/cooling discharge zone) of the three regions defined by the seal member 13. The vent port 11b is connected to the gas analyzer 30.

The discharge port 11c is provided at the most downstream side of the cylinder wall 11 in the conveyance direction. A die 15 is arranged at the discharge port 11c. The decomposition products formed by kneading within the cylinder wall 11 are molded into a predetermined shape by the die 15 and extruded from the discharge port 11c.

The moisture regulating device 25 regulates the moisture within the cylinder wall 11. The moisture adjustment device 25 is, for example, a spray device that adjusts the humidity of the input port 11a. The moisture adjustment device 25 may be a pump that injects water into the cylinder wall 11 or a device that introduces dry gas or humidified gas into the cylinder wall 11.

The gas analyzer 30 includes a moisture meter (not shown). The gas analyzer 30 analyzes the gas components sampled from the vent port 11b and measures the moisture within the cylinder wall 11. The measured moisture value is sent to the control device 40.

The control device 40 receives the value of moisture in the cylinder wall 11 from the gas analyzer 30, and controls the moisture adjustment device 25 based on this value. This adjusts the moisture within the cylinder wall 11. That is, the decomposition device 1 is configured such that the control device 40 can automatically perform the step of adjusting the moisture within the cylinder wall 11.

In the above example, a case has been described in which the decomposition device 1 includes the control device 40 and the control device 40 automatically performs the process of adjusting the moisture within the cylinder wall 11. However, the step of adjusting the moisture content may be performed manually, and the decomposition device may not be equipped with the control device 40.

[Configuration example 2]
FIG. 5 is a schematic diagram of a decomposition device 2 which is another example of a polylactic acid decomposition device. The decomposition device 2 includes a reaction facility 60 in place of the extruder 10 of the decomposition device 1 (FIG. 4). The reaction equipment 60 includes a reaction device 61, a screw 62, and a heating device 63.

Reactor 61 includes a main body 611 and a lid 612. The lid 612 is provided with a raw material input port 612a, a gas introduction port 612b, and a vent port 612c, which are connected to the feeder 20, the moisture adjustment device 25, and the gas analyzer 30, respectively.

The screw 62 kneads the contents of the reactor 61. The heating device 63 heats the reaction device 61.

Also in the decomposition device 2, the control device 40 receives the value of moisture in the reaction device 61 from the gas analyzer 30, and controls the moisture adjustment device 25 based on this value. This adjusts the moisture inside the reaction device 61. That is, the decomposition device 2 is configured such that the control device 40 can automatically perform the step of adjusting the water content in the reaction device 61. Note that in this example as well, the step of adjusting the moisture content may be performed manually, and the decomposition device does not need to be equipped with the control device 40.

Hereinafter, the present invention will be explained in more detail with reference to Examples. The invention is not limited to these examples.

Using a decomposition apparatus similar to the apparatus shown in Figure 4, polylactic acid with a number average molecular weight of 78,000 was decomposed by changing the water content, pressure, type of additive, and additive concentration, and the number average of the decomposition products was The molecular weight was measured. The results are shown in Tables 1 and 2.

The values in the "Energy consumption" column of Tables 1 and 2 are the energy required to heat 100 g of polylactic acid, additives, and water from room temperature (20 °C) to the reaction temperature (200 °C). and heat of fusion of additives, and heat of evaporation of water).

FIG. 6 is a graph showing the relationship between water content, number average molecular weight of decomposition products, and energy consumption, created from Example 1 and Comparative Examples 1 to 3.

As shown in Tables 1 and 2 and FIG. 6, under the conditions of Comparative Example 1, the number average molecular weight of the decomposition products was not sufficiently reduced. This is thought to be because the water content was too low, resulting in a significant decrease in the decomposition rate.

On the other hand, a comparison between Example 1 and Comparative Examples 2 and 3 shows that even if the water content is lowered to about 1.5%, the decomposition rate does not decrease much. In this example, the reaction rate of Example 1 is rather higher than that of Comparative Examples 2 and 3. On the other hand, it can be seen that energy consumption can be significantly reduced by lowering the water content.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various changes can be made within the scope of the invention.

1, 2 polylactic acid decomposition device, 10 extruder, 11 cylinder wall (reaction device), 12 screw, 13 heating device, 14 sealing member, 15 die, 20 feeder, 25 moisture adjustment device, 30 gas analyzer, 40 control Apparatus, 60 Reaction equipment, 61 Reactor, 62 Screw, 63 Heating device

Claims (9)

polylactic acid and an additive that is one or a mixture of two or more selected from the group consisting of alkali metal salts, alkaline earth metal salts, alkali metal oxides, and alkaline earth metal oxides. a step of charging into a reactor;
A step of holding or kneading the polylactic acid and the additive under conditions such that the water content expressed by the following formula is 0.15 to 3.0% ,
A method for decomposing polylactic acid, further comprising the step of adjusting the water content of at least one of the polylactic acid and the additive, before the polylactic acid and the additive are introduced into the reaction apparatus .
Moisture content = mass of water in the reactor / (mass of polylactic acid + mass of additive + mass of water added from outside)
However, the mass of water in the reaction apparatus includes the mass of water adsorbed on the polylactic acid and the additive, and the mass of water added from the outside, and the mass of the polylactic acid and the mass of the additive are , the mass including the mass of water adsorbed on each. The method for decomposing polylactic acid according to claim 1 ,
A method for decomposing polylactic acid, comprising adjusting the moisture content of at least one of the polylactic acid and the additive by controlling the humidity at the input port of the reaction apparatus. The method for decomposing polylactic acid according to claim 2 ,
A method for decomposing polylactic acid, wherein the device for adjusting the humidity at the input port is a spray device. A method for decomposing polylactic acid according to any one of claims 1 to 3 , comprising:
Measuring moisture in the reactor;
A method for decomposing polylactic acid, further comprising the step of adjusting the moisture content in the reaction device based on the measured moisture content. The method for decomposing polylactic acid according to claim 4 ,
A method for decomposing polylactic acid, the method comprising adjusting the water content in the reaction apparatus by adjusting the moisture content of at least one of the polylactic acid and the additive introduced into the reaction apparatus. A method for decomposing polylactic acid according to any one of claims 1 to 5 , comprising:
The method for decomposing polylactic acid, wherein the additive is one selected from the group consisting of sodium hydrogen carbonate, sodium carbonate, potassium carbonate, and magnesium oxide, or a mixture of two or more. The method for decomposing polylactic acid according to claim 6 ,
A method for decomposing polylactic acid, wherein the additive is sodium hydrogen carbonate. A method for decomposing polylactic acid according to any one of claims 1 to 7 , comprising:
A method for decomposing polylactic acid, wherein the amount of the additive is 0.2 to 20 parts by mass based on 100 parts by mass of the polylactic acid.
However, the mass of the polylactic acid and the mass of the additive include the mass of water adsorbed on each. A method for decomposing polylactic acid according to any one of claims 1 to 8 , comprising:
A method for decomposing polylactic acid, wherein the holding or kneading step is a step of holding or kneading at 0.15 to 20.0 MPa for 1 second or more.

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