JP2821986B2 - Method for recovering lactide from polylactic acid products - 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 recovering lactide from a polylactic acid product, and more particularly to a method for recovering lactide from a polylactic acid product in a short time and at a high recovery rate using a screw extruder. .

[0002]

2. Description of the Related Art Polylactic acid is known as a biodegradable plastic. Even if it is disposed of, it is decomposed into lactic acid in the presence of water and microorganisms in the natural world, and eventually becomes water and carbon dioxide, which is reduced to ecosystems. Is done. For this reason, at present, attention has not been paid to the technology of recovering and reusing products made from the raw materials. But,
Even polylactic acid products with a large molecular weight require a long time to be biodegraded, not to mention detracting from aesthetics, and from the viewpoint of effective use of resources,
It is important to establish a technology for reuse.

[0003] To recover lactide from polylactic acid products,
It is necessary to break down polylactic acid to lactide. However, although polylactic acid is easily decomposed in the presence of heat and water, it merely cuts an ester bond to reduce the molecular weight, and the depolymerization reaction until lactide formation does not proceed very much. For this reason, the recovery rate of lactide is low as it is, which makes it difficult to commercialize the lactide recovery and reuse technology.

[0004] As this recovery and reuse technology, it is conceivable to apply a recycling process for general-purpose plastics which has already been established. However, this treatment technology is mainly performed in batch operation using a reactor, and is a continuous process. It takes a lot of processing time as compared with the method described above, and is inadequate in terms of processing capacity such as a low recovery rate, and is difficult to apply as it is.

[0005]

As described above, no industrially satisfactory technology for recovering lactide from polylactic acid products has yet been put to practical use. Therefore, an object of the present invention is to provide a method for recovering lactide from a polylactic acid product, which enables lactide to be recovered from the lactic acid product in a short time and at a high recovery rate, and the lactide can be reused.

[0006]

Means for Solving the Problems The present inventors have conducted research to achieve the above object, and found that the depolymerization reaction of polylactic acid proceeds rapidly at a temperature of 200 ° C. or more, The present inventors have found that the depolymerization reaction is further promoted by a synergistic effect, and that the lactide can be recovered in a short time and with a high yield of almost 100% by performing this process with a screw extruder. Was completed.

That is, the present invention relates to a method for preparing a polylactic acid product in a screw-type extruder in the presence of water and a catalyst.
The present invention relates to a method for recovering lactide from a polylactic acid product, wherein the lactide is heated and mixed at up to 400 ° C.

The present invention also relates to a method for recovering lactide from a polylactic acid product, further comprising a step of purifying the recovered lactide by introducing it into alcohol.

Hereinafter, the present invention will be described in detail. The term “polylactic acid product” as used in the present invention refers to a film, a sheet, a hollow container, and the like made from a cyclic dimer such as lactic acid or lactide.

[0010] The screw type extruder used in the present invention is a screw extruder which is usually used for molding plastics.
A single-screw or twin-screw extruder equipped with a cylinder, a heating unit, a driving device, a vent for deaeration, and the like can be used. Further, as the twin-screw extruder, any of those in which the rotation direction of the screw is the same or different, and those in which the flights of the screw mesh with each other or not can be used. In the present invention, a plurality of heating units divided around the cylinder so as to be individually temperature-controllable, and a plurality of vent ports installed so as to be able to deaerate from the inside of the cylinder in those unit portions. And the ratio (L / D ratio) of the screw length (L) to the screw diameter (D) is 1
A 5-70 meshing twin screw extruder is preferred. Further, among such intermeshing type twin screw type extruders, those with different rotations are particularly preferable because bulky products such as agricultural films and containers can be supplied without grinding. The polylactic acid product is washed or pulverized as required, and supplied to a screw extruder.

In the present invention, the depolymerization reaction by heating and mixing is 2
It is carried out at a temperature of from 00 to 400C, preferably from 250 to 350C.
When the temperature is lower than 200 ° C., the depolymerization reaction hardly occurs, and when the temperature is higher than 400 ° C., it is difficult to perform the reaction with an existing extruder, which is not preferable.

The hydrolysis reaction is based on the weight of polylactic acid.
Preferably it is 0.1 to 5.0% by weight, more preferably 0.1 to 5.0% by weight.
This is done by adding water in an amount of 5-2.0% by weight. When the amount of water added is less than 0.1% by weight, the progress of the reaction is slow,
If the content exceeds 5.0% by weight, the depolymerization reaction is adversely affected, such as a decrease in thermal efficiency, and is not preferred. The addition of water is preferably performed in a state where the polylactic acid is in a molten state, but may be added before the molten state. Further, the polylactic acid product is washed before being supplied to the extruder, and if the washing water remains, the reaction can be carried out with or without adding insufficient water.

Examples of the catalyst used in this reaction include alkyl aluminum, zinc oxide, tin octylate, antimony oxide, manganese acetate, tetrabutyl titanate, stannous chloride, dibutyltin dilaurate and the like. Of these, alkyl aluminum, zinc oxide, manganese acetate and the like, which have low toxicity, are preferred. By the addition of these catalysts, the reaction is promoted and the generation of by-products that dissolve lactide such as lactic acid ether and lactic acid anhydride can be suppressed. The addition amount of these catalysts is preferably from 0.1 to 1.0% by weight, more preferably from 0.3 to 0.7% by weight, based on the raw materials.

In the present invention, it is pointed out that when a nonmetallic catalyst is used in consideration of its toxicity, the reaction rate is lower than that of a tin catalyst. Therefore, in order to extend the reaction time (residence time) in the extruder, a stop valve is installed at the outlet of the extruder, and by closing or opening it, a sufficient reaction time is secured and the reaction is performed. It is completed and lactide can be recovered semi-continuously.

The screw speed of the screw type extruder in the method of the present invention is not particularly limited, but is 200 rpm.
As described above, 250 to 350 rpm is particularly preferable.

The hydrolysis reaction and the depolymerization reaction are carried out in this manner, so that polylactic acid has a low molecular weight and is finally decomposed into lactide.

Preferably, the produced lactide is recovered by vaporizing it under reduced pressure. More preferably, the generated and sublimated lactide is sucked as a gas from the vent port of the extruder and recovered by reducing the pressure in the cylinder. In this case, the reduced pressure level is preferably 100 mmHg or less, more preferably 59 mmHg or less. In addition, the vent
It is preferable to heat the lactide so that it does not precipitate upon cooling when passing therethrough. Usually, heating to about 150 ° C. is sufficient. The lactide thus recovered is cooled by an appropriate means and reused as a raw material for producing a polymer.

The lactide recovered by the above treatment contains added water and a catalyst, and trace amounts of low-molecular-weight by-products such as lactic ether and lactic anhydride generated in the reaction process.
Among them, water hydrolyzes lactide, and the low molecular weight by-product dissolves lactide. That is, the presence of these impurities impairs the properties of lactide as a raw material for polymer production. For this reason, in the present invention, after recovering lactide, a purification step for further removing impurities can be provided. In this step, the collected lactide is passed through a solvent in which lactide is hardly soluble but impurities such as water and low molecular weight by-products are soluble, i.e., lower alcohols such as methanol and ethanol. Only to dissolve and precipitate lactide. The lactide thus precipitated is filtered, dried if necessary, and reused as a monomer for a polymerization raw material.

Next, an embodiment of the method of the present invention will be described with reference to FIG. 1 attached to the specification. First, the washed or pulverized waste polylactic acid product is put into the hopper 12 and further sent into the screw type extruder 10. The waste polylactic acid product is heated by a plurality of heating units 14 in the course of traveling inside the cylinder by a rotating screw (not shown). At this time, water and a catalyst are added before and after, mixed with the waste polylactic acid product, and the hydrolysis reaction and the depolymerization reaction proceed simultaneously or before and after. As the reaction mechanism, first, the molecular chain of polylactic acid is randomly cleaved by a hydrolysis reaction, and an oligomer which is a low molecular weight substance is generated. In addition, the molecular chain of polylactic acid is also randomly cut by the shearing force applied by the screw of the screw type extruder 10. Then, by a depolymerization reaction by heating and addition of a catalyst, ester bonds are sequentially cut from the molecular chain terminals of the polylactic acid decomposed product having a reduced molecular weight, and lactide is generated. The generated and sublimated lactide is sucked from the vent port 16 by keeping the inside of the cylinder in a reduced pressure state by the vacuum pump 18, and is collected in the first trap 20 provided with a cooling means. If necessary, the first trap 20 is filled with a lower alcohol, and lactide is introduced and precipitated there, filtered with a filter 22 and dried with a drier 24. Since the filtrate obtained by the filter 22 contains lactide dissolved together with impurities without being precipitated, the lactide is crystallized again while being distilled by the reduced-pressure distillation device 23 and collected. Further, the lower alcohol obtained here as a distillation component can be reused. In addition, the waste liquid after the lactide recovery remaining in the screw type extruder 10 is sent to the tank 26 and recovered.

[0020]

EXAMPLES The present invention will be described in more detail with reference to the following Examples, but it should not be construed that the invention is limited thereto.

Example 1 Discarded polylactic acid film (Mw = 130,000)
Was washed with water to remove deposits, and then charged as it was into an interlocking twin-screw extruder. This twin-screw extruder is of a different direction rotation type having an L / D ratio of 42 and a screw diameter of 32 mm. As shown in FIG. 2, each of a total of 11 zones of a cylinder can be set to different temperatures. In addition, three zones 6, 8, and 10 have vent ports that can be degassed. Here, the zones are used in the sense that different positions of the cylinder can be set to different heating temperatures, and are not limited to the meaning that each zone is separated. Next, 11
Were heated to 250 to 300 ° C., the stop valve attached to the die outlet was closed to make the inside of the extruder hermetically closed, and the screw rotation speed was set to 300 rpm to fill the extruder with the raw material. While operating the extruder at a screw rotation speed of 300 rpm, a hydrolysis reaction was performed in the hopper and zones 1 to 6 by adding 0.5% by weight of water to the raw material from zone 1 (Mw at this stage). =
9,000), and in Zones 7 to 11, alkyl aluminum was used as a catalyst from Zone 7 at 0.5% by weight of the raw material.
After the addition, the depolymerization reaction was carried out batchwise for about 15 minutes. In parallel with this depolymerization reaction, the pressure in the extruder was reduced to 40 mmHg by a vacuum pump through a vent port provided in zones 7 to 11, and vaporized lactide was recovered (recovery rate 85 to 93). %, Based on the weight of the raw material polylactic acid product. The decomposition product remaining without being recovered as lactide was taken out with the stop valve opened after the reaction was completed, and stored in a dedicated tank. The time required for these reactions was about 25 minutes from the average residence time of the raw materials in the extruder including the batch operation. This time was about one-half that required when a similar process was studied in a conventional batch reactor. In addition, impurities such as low molecular weight by-products such as lactic acid ether and lactic anhydride generated before the depolymerization reaction are sucked from the vent port provided in the zone 6 and collected in the second trap 21. The lactide thus recovered from the extruder was introduced into ethanol in a cooling trap, precipitated as needle-like crystals while dissolving and removing impurities, and further subjected to filtration and drying steps to obtain purified lactide. The final recovery is 72-80%, and the recovery of the conventional method is about 10%.
(As a purified product), it was significantly improved. Also,
This lactide, from which impurities were also removed, was suitable as a polymer production raw material.

Example 2 Discarded polylactic acid film (Mw = 145,000)
Was washed with water, coarsely pulverized, and supplied to the same extruder as in Example 1. However, all three vent ports are connected to the trap 20. Next, all the eleventh zones were heated to 320 ° C., and the extruder was operated at a screw rotation speed of 320 rpm. A decomposition reaction was performed (Mw = 8,500 at this stage), and
In Nos. To 11, manganese acetate was added as a catalyst from zone 4 in an amount of 0.6% by weight of the raw material to perform a depolymerization reaction. In addition, the pressure inside the extruder was reduced to 80 mmHg by a vacuum pump through vent ports installed in zones 6, 8, and 10,
Evaporated lactide was recovered (recovery rate 93 to 96).
%). The time required for these reactions was about 22 minutes from the average residence time of the raw materials in the extruder including the batch operation. This time was about one-half that required when a similar process was studied in a conventional batch reactor. The lactide thus recovered from the extruder is
Needle-like crystals were precipitated while dissolving and removing impurities in methanol in a cooling trap. Further, a filtration and drying step were performed to obtain purified lactide. Final recovery rate is 68-7
0%, which is significantly improved as compared with the recovery rate of the conventional method of about 10% (as a purified product). This lactide was also free of impurities, and was suitable as a raw material for polymer production.

[0023]

The method of the present invention is a method for recovering lactide by performing a hydrolysis reaction and a depolymerization reaction in a screw type extruder. Therefore, lactide can be recovered semi-continuously in a short time and with high yield, and the processing cost can be reduced.

[Brief description of the drawings]

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

FIG. 2 is a schematic view showing an example of a screw type extruder.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Screw-type extruder 12 Hopper 14 Heating unit 16 Vent port 18 Vacuum pump 20 1st trap 21 2nd trap 22 Filter 23 Vacuum distillation apparatus 24 Dryer 26 Tank

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yukihiro Sumihiro 1-6-1, Funakoshi-minami, Aki-ku, Hiroshima-shi, Hiroshima Japan Steel Works Co., Ltd. (72) Inventor Kayoko Yokota 1-chome, Funakoshi-minami, Aki-ku, Hiroshima-shi, Hiroshima 6-1 Japan Steel Works, Ltd. (56) References JP-A-63-101378 (JP, A) JP-A-7-304859 (JP, A) JP-A-6-298754 (JP, A) (58) Survey Field (Int.Cl. ⁶ , DB name) C07D 319/12 C08G 63/06

Claims (3)

(57) [Claims] 1. A polylactic acid product is heated to 200 to 400 ° C. in a screw extruder in the presence of water and a catalyst,
A method for recovering lactide from a polylactic acid product, which comprises mixing. 2. The method according to claim 1, wherein the produced lactide is vaporized and recovered under reduced pressure. 3. A method for recovering lactide from a polylactic acid product, further comprising a step of purifying the lactide recovered by the method according to claim 1 or 2 by introducing it into alcohol.