JP3589333B2 - Polylactic acid demonomerization method and apparatus - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for removing low molecular substances, for example, lactide, from polylactic acid pellets.
[0002]
[Prior art]
Polylactic acid has high biological safety, and lactic acid, which is a decomposition product, is absorbed in vivo. As described above, polylactic acid is a high biosafety compound and is used for medical purposes such as surgical sutures, drug delivery (sustained release capsules), and reinforcing materials for fractures, and decomposes in the natural environment. For this reason, it has attracted attention as a degradable plastic. It is also used for various applications as uniaxially and biaxially stretched films, fibers, injection molded products and the like.
[0003]
The production method of such polylactic acid includes a direct method of obtaining a target product by directly dehydrating and condensing lactic acid, and a method of once synthesizing a cyclic lactide (dimer) from lactic acid and purifying it by a crystallization method and the like. There is a method of performing ring-opening polymerization. Lactide synthesis, purification and polymerization procedures are described, for example, in U.S. Pat. No. 4,057,537: Published European Patent Application No. 261,572: Polymer Bulletin, 14, 491-495 (1985); and Makromol. Chem. 187, 1611-1628 (1986). Japanese Patent Publication No. 56-14688 discloses that polylactic acid is produced by using two molecules of cyclic diester as an intermediate and polymerizing the intermediate with tin octylate and lauryl alcohol as catalysts. The polylactic acid thus obtained is used in advance in order to facilitate the handling in the molding process, in order to obtain a spherical, cubic, columnar, crushed, or other pellet-shaped product having a size from rice grain to bean grain. Is done.
[0004]
[Problems to be solved by the invention]
However, the melting point of high molecular weight polylactic acid having a molecular weight of 100,000 to 500,000 is as high as 175 to 200 ° C. Conventionally, such a final polymer of polylactic acid is taken out of the reactor in a molten state and heated to a temperature higher than the melting point. The polylactic acid was decomposed and colored. Further, at such temperatures, large amounts of lactide were generated in the polymer. This is presumably because at such a temperature, the equilibrium between the polymer and lactide tends to the lactide side.
[0005]
These lactides and decomposed products sublimate during injection molding or spinning using polylactic acid pellets as raw materials, and adhere to dies and nozzles, thereby hindering operation. In addition, lactide and decomposition products lower the glass transition temperature and melt viscosity of the polymer, and significantly deteriorate the moldability and thermal stability.
[0006]
For this reason, various methods for demonomerizing polylactic acid have been proposed, for example, JP-A-3-14829. It describes a method for removing residual monomers and low-molecular volatiles by maintaining the polyester at a temperature in the range of 250 ° C. or higher and a pressure of 5 mmHg or lower. It is stated that it hardly volatilizes. However, since this method removes low-molecular substances in a molten state, the equilibrium between the polymer and lactide is shifted to the lactide side, and lactide is regenerated in the polymer. Therefore, this method has a limit in removing low molecular substances.
[0007]
Japanese Patent Application Laid-Open No. 5-255488 discloses that a low molecular weight lactic acid polymer having an individual particle size of 5 μm to 5 mm is heated at a temperature higher than the glass transition temperature and lower than its melting point, so that the weight due to the dehydration reaction is increased. Methods for increasing the molecular weight by condensation are described. However, this method requires 240 hours for operation according to the examples, and it is unlikely that the method can be carried out industrially. In addition, this method is mainly intended for medical materials, and is characterized by increasing the molecular weight without a catalyst, and does not focus on reducing low molecular substances.
[0008]
Therefore, an object of the present invention is to efficiently remove volatile impurities such as low-molecular substances such as unreacted monomers even in a solid state without increasing the molecular weight.
[0009]
[Means for Solving the Problems]
The present inventors have conducted intensive research on the above problems. As a result, the inventors obtained the knowledge that if the unreacted monomers and low-molecular substances were removed after crystallization of the polylactic acid pellets, it was possible to produce polylactic acid without coloring and decomposition, and completed the present invention. Was.
That is, the present invention provides a step of crystallizing a polylactic acid pellet produced by directly polymerizing lactic acid or a ring-opening polymerization of lactide, and a step of gasifying and removing low-molecular substances from the crystallized solid polylactic acid pellet. It is characterized by comprising.
[0010]
The polylactic acid pellets of the present invention may be produced by direct polymerization of lactic acid or by ring-opening polymerization of lactide, but the latter is more preferable in order to obtain a high molecular weight product.
[0011]
In the case of the production method by ring-opening polymerization of lactide, the polymerization temperature is preferably from 140 to 210 ° C., and preferably from 150 to 180 ° C. in order to suppress the racemization and decomposition coloration of L-lactide. Polylactic acid having a weight average molecular weight of 50,000 to 300,000 is obtained by polymerization. However, the weight average molecular weight of polylactic acid in the present specification is the weight average molecular weight of only the polymer portion in the chromatogram of GPC.
[0012]
The polymerization is carried out in one or more vertical reactors to a state where they can flow easily enough. In a vertical reactor, the viscosity of the polymer increases as the polymerization proceeds. Therefore, it is preferable to perform the reaction in a plurality of reactors having blades corresponding to different viscosities. Also, when performing continuous operation, the residence time distribution becomes sharp, and a plurality of reactors are connected in series in order to increase the heat transfer area per volume. For example, first, the catalyst is uniformly stirred in a low viscosity region using a reactor equipped with inclined blades, turbine blades, full-surface blades, and the like. Next, the mixture is stirred in a reactor having a high-viscosity blade such as a helical ribbon blade. When a plurality of reactors are used, the respective reaction temperatures need not always be the same.
[0013]
Examples of the catalyst used for the polymerization include tin compounds such as tin octylate, titanium compounds such as tetraisopropyl titanate, zirconium compounds such as zirconium isopropoxide, and antimony compounds such as antimony trioxide. And a conventionally known catalyst. Further, the molecular weight of the final polymer can be adjusted by the amount of the catalyst to be added. The smaller the amount of catalyst, the slower the reaction rate but the higher the molecular weight. Further, a nucleating agent (such as talc, clay, or titanium oxide) may be added.
[0014]
The lactide used in the polymerization is selected from D-, L-, DL- or a mixture of D-, L- and the like, and lactones such as β-propiolactone, δ-valeractone, ε-caprolactone glycolide, δ- Copolymerization with butyl lactone and the like is also possible. In addition, physical properties can be controlled by a polyhydric alcohol such as glycerin. Although the polymerization reaction varies depending on the type of catalyst, when tin octylate is used, the catalyst is used in an amount of 0.0001 to 0.1% by weight, preferably 0.05 to 0.001% by weight, based on the weight of lactide. Heat and polymerize for 0 to 30 hours. The reaction is preferably carried out in an atmosphere of an inert gas such as nitrogen or air or a mixed gas thereof or in an air stream.
[0015]
The polymerized polylactic acid is formed into a pellet. The shape of the pellet does not need to be a specific shape such as a square chip, a column, or a marble for grinding, but a column or a marble is preferable. Apparatuses for this include Sandvik's strip former, rotoformer, double roll feeder, Kaiser's rotary drop former, and piston type drop former, Mitsubishi Kasei Engineering's drum cooler, and Nippon Belding's Steel belt coolers and hybrid formers. The size of the pellets is not particularly specified, but in consideration of handling in the manufacturing process such as bagging of pellets and handling during secondary molding, regardless of the shape, 0.1 to 10 g, preferably 1 to 5 g per pellet. It is.
[0016]
The produced polylactic acid pellets are crystallized. The crystallization is performed in a state of high fluidity in order to prevent crystallization heat from being generated and exceeding a predetermined temperature to fuse the pellets. In the crystallization, a jacket and / or a heated inert gas is used while flowing mechanically or with an inert gas to bring the glass transition point to above the glass transition point and below the melting point (55 to 180 ° C, preferably 80 to 120 ° C). Hold for 10 minutes to 5 hours, preferably for 1 to 2 hours. As such an apparatus, an existing conical dryer or the like can be used. For continuous operation, for example, a torus disk manufactured by Hosokawa Micron, OTWK or OTWG manufactured by Buehler, etc. may be used. These devices are used in multiple stages when the fusion property of the pellets is increased due to the molecular weight of the pellets to be crystallized, additives, the optical purity of lactic acid as a constituent unit, and the like.
[0017]
Next, the low molecular substances in the crystallized pellets are removed by gasification (removing low molecules). Removal is performed using a jacketed and / or heated inert gas or air or a mixed gas thereof, at a temperature higher than the glass transition point and lower than the melting point (100 to 180 ° C, preferably 120 to 140 ° C) for 10 to 100 hours, preferably. Hold for 20 to 50 hours. Here, the retention time varies depending on the amount of the low-molecular substance to be removed, the degree of vacuum, the flow rate of an inert gas or the like, the temperature, and the like. As a device for removal, when operating continuously, a hollow cylindrical reactor is used. A heated inert gas or the like is passed from above to below, and pellets are charged from above.
[0018]
It should be noted that the division between the operation of crystallization and the operation of removing low molecules is not always clear, and the operation of removing low molecules can be performed by operating the temperature and the reaction time using only a crystallization apparatus. The crystallization device and the demolition device depend on the difference in the ability to flow the pellet. That is, the crystallization apparatus needs to have high fluidity in order to prevent fusion due to crystallization heating. However, in the case of an apparatus that is premised on industrial production, it is advisable to use only a crystallization step for high fluidity and to use a simple hollow cylindrical reactor for the demolition step.
[0019]
The de-molecularized pellets are cooled to a glass transition temperature or lower, for example, by passing through a hollow cylindrical cooler, and are taken out. As the cooler, for example, a Hosokawa micron torus disk in which pellets flow mechanically can be used. The depolymerized polylactic acid of the present invention has a weight average molecular weight of 100,000 to 500,000 and an unreacted lactide of 1.0% or less.
[0020]
The low-molecular substance removed in the present invention is mainly unreacted lactide. The removed lactide is recovered by a cooling condenser, a cyclone, a filter, a scrubber of lactic acid or molten lactide, and used again as a raw material. In the case of using a condenser, it is conceivable whether lactide is cooled and solidified and captured, or lactide is captured in a liquid state. However, capturing in a liquid state is preferable for continuous operation.
[0021]
BEST MODE FOR CARRYING OUT THE INVENTION
An example of an apparatus for performing the method of the present invention will be described with reference to the drawings.
FIG. 1 shows an example of a demonomerization apparatus. In FIG. 1, reference numeral 1 denotes a raw material feeder, which is formed of a hollow cylindrical tank, has openings at upper and lower portions, and has an open / close valve at a lower opening (not shown).
[0022]
A crystallizer 2 is connected to a lower portion of the raw material feeder 1. The crystallizer 2 is formed of a hollow rectangular parallelepiped tank, has a gas exhaust port a at the upper part, and a gas supply port b at the lower part, and passes through a hopper type reactor 3 for performing a de-molecule removing operation described later. The heated inert gas flows. By the flow of the inert gas, for example, nitrogen gas, the inside of the crystallizer 2 is maintained at a temperature not lower than the glass transition point of the polylactic acid pellet and not higher than the melting point (55 to 180 ° C.). The gas supply port b also serves as a supply port for sending the crystallized pellets to the hopper type reactor 3. That is, the pellets fall from the supply port by the action of gravity, and only the inert gas is supplied from the gas supply port.
[0023]
The hopper type reactor 3 is a hollow cylindrical reactor having a gas supply port c and a gas exhaust port d like the crystallizer 2, and heated inert gas flows. The inert gas flows from the gas source 11 via the condenser 6 and the heater 7 through the gas supply port c of the hopper type reactor 3, and is heated to a predetermined temperature by the heater 7. The flow path 10 of the inert gas is shown by a broken line in the figure, and the inert gas is supplied from a gas source 11 → a condenser 6 → a heater 7 → a gas supply port c → a hopper type reactor 3 → a gas exhaust port d → a gas supply. The gas is discharged to the outside of the system by following the route of port b → crystallizer 2 → gas exhaust port a → condenser 6. In the condenser 6, the inert gas containing gasified low molecules discharged from the gas exhaust port a is cooled, and the low molecules are liquefied or solidified and taken out of the system. The inert gas is heated again by the heater 7 and is circulated and used, but the amount leaked out of the system is newly added from the gas source 11.
[0024]
The gas exhaust port d also serves as a supply port for crystallized pellets. Further, a control valve 12 is provided below the reactor 3.
[0025]
A hollow cylindrical cooler 4 is connected to a lower portion of the hopper type reactor 3, and the polylactic acid pellets demonomerized in the reactor 3 are solidified and taken out as a product 5. The cooler 4 cools the container itself by circulating cooling water 8 around the container. Further, a control valve 13 is provided below the cooler 4.
[0026]
With the above configuration, demonomerization of the polylactic acid pellets is performed as follows.
First, the polylactic acid pellets are supplied to the raw material feeder 1, the on-off valve at the lower opening is opened, and the polylactic acid pellets are continuously introduced into the crystallizer 2. In the crystallizer 2, the heated inert gas flows from the gas supply port b, and the heat crystallizes the polylactic acid pellets. The crystallized polylactic acid pellets are sent to the hopper type reactor 3. At this time, the control valve 12 of the hopper type reactor 3 is closed, and the crystallized pellets are held in the reactor 3 for a certain time. Since the heated inert gas is also introduced into the reactor 3, the polylactic acid pellets are melted by the heat, and only low molecular components, for example, lactide alone are gasified and removed. The gasified and removed low-molecular components ride on the flow of the inert gas, and are discharged out of the system by following the path of gas exhaust port d → gas supply port b → crystallizer 2 → gas exhaust port a → condenser 6. Here, the gasified and removed low-molecular components are cooled by the condenser 6 and liquefied and discharged (arrow 9 in the figure). The liquefied and discharged low-molecular components are mainly lactide and can be reused for polymerization.
[0027]
After the treatment for a predetermined time in the reactor 3, the polylactic acid pellets from which low molecular components have been removed are introduced into the cooler 4. The polylactic acid pellets cooled to near room temperature in the cooler 4 are taken out as products 5 from the control valve 13.
The above operations are performed continuously. That is, it is continuously supplied from the raw material feeder 1 and continuously taken out from the control valve 13. Here, the processing time means an average residence time in the container.
[0028]
In the above description, the heating of the crystallizer 2 and the hopper-type reactor 3 is performed by passing an inert gas. However, the heating is not limited to this. Is also good. Further, the steps from crystallization to de-molecule removal may be performed in a lump in the crystallizer 2 without using the hopper type reactor 3.
[0029]
【Example】
The method of the present invention was confirmed by the following experiment.
[Experimental example 1]
(material)
50 kg of in-house lactide was charged in a 50 / vertical reactor having Shinko Pantech log horn blades, and the stirrer was set at 60 rpm. After dissolving at 110 ° C., 10 ppm of tin octylate was added and reacted at 160 ° C. for 47 hours. At this time, the weight average molecular weight of polylactic acid was 193,000, the glass transition point was 40.1 ° C, and the residual lactide was 12%. This polylactic acid was formed into a 4 × 4 × 1 mm square pellet using a drum cooler (DC450) manufactured by Mitsubishi Kasei Engineering.
[0030]
(Demonomerization)
A torus disk (12-3 type) (heat transfer area: 1.9 m ² ) manufactured by Hosokawa Micron is used as a crystallization and de-molecularizer, 20 kg of pellets are charged, and nitrogen at 140 ° C. is 200 NL / min for 36 hours. Ventilated. The rotation speed of the disk was 30 rpm. Met. The remaining lactide was 970 ppm. The glass transition temperature was 60.5 ° C., the weight average molecular weight was 16,000, and the MFR was 4.18.
[0031]
[Experimental example 2]
(material)
50 kg of in-house lactide was charged in a 50 / vertical reactor having Shinko Pantech log horn blades, and the stirrer was set at 60 rpm. After dissolving at 110 ° C., 10 ppm of tin octylate was added and reacted at 160 ° C. for 47 hours. At this time, the weight average molecular weight of polylactic acid was 193,000, the glass transition point was 40.1 ° C, and the residual lactide was 12%. This polylactic acid was formed into a 4 × 4 × 1 mm square pellet using a drum cooler (DC450) manufactured by Mitsubishi Kasei Engineering.
[0032]
(Demonomerization)
A torus disk (12-3 type) (heat transfer area: 1.9 m ² ) made by Hosokawa Micron was used as a crystallizer, and a hollow cylindrical hopper type reactor having an inner diameter of 35 cm and a length of 230 cm was connected to the outlet side of the torus disk. Further, a hollow cylindrical hopper type cooler having an inner diameter of 35 cm and a length of 100 cm was connected. Using two 2.5 kW electric heaters, nitrogen was heated at 200 NL / min to 140 ° C., aerated from the lower part of the hopper type reactor, and further aerated to the crystallizer. The rotation speed of the crystallizer disk was 30 rpm. Met. Pellets were continuously supplied at a rate of 5 kg / h from a crystallizer inlet by a feeder. The cooler was cooled to 30 ° C. or less by cooling water.
[0033]
The removed lactide was captured by a carbait condenser having a heat transfer area of 3 m ² . The captured lactide was in a liquid state and was subjected to polymerization again, but there was no particular problem with the quality. The cooling temperature of the condenser at this time was 100 ° C. Further, nitrogen was used in circulation.
[0034]
The treated polylactic acid pellets had a residual lactide content of 850 ppm. The glass transition temperature was 61.5 ° C., the weight average molecular weight was 166,000, and the MFR was 4.20.
[0035]
[Experimental example 3]
By the same operation as in Experimental Example 2, the temperature of the condenser was lowered to 20 ° C., and lactide was solidified in the condenser. Similarly, nitrogen was used in circulation. The physical properties of the product were the same as in Experimental Example 2, but the condenser temperature had to be raised to 120 ° C. to melt the solidified lactide.
[0036]
The analysis conditions of the experimental example are as follows.
<Measurement of molecular weight; GPC measurement>
Detector manufactured by Shimadzu Corporation; RID-6A
Pump; LC-9A
Column oven; CTO-6A
Column: Shim-pack GPC-801C, -804C, -806C, -8025C were analyzed in series. Solvent: Chloroform flow rate: 1 ml / min
Sample volume: 200 μl
(A sample 0.5 w / w% was dissolved in chloroform.)
Column temperature; 40 ° C
<Measurement of residual lactide>
The sample was immersed in acetonitrile all day and night, and the extract was measured by a liquid chromatograph under the following conditions, and calculated by the absolute calibration curve method.
[0037]
Detector manufactured by Shimadzu Corporation; SPD-6AV (UV 210 nm)
Pump; LC-9A
Column oven; CTO-6A
Column: Asahipac GF-7MHQ (7.6 mm ID, 300 MML)
Analysis conditions Solvent; acetonitrile flow rate; 0.6 ml / min
Sample volume; 10 μl
<Measurement of glass transition temperature>
The midpoint glass transition temperature was determined according to JIS K7121.
[0038]
<Measurement of MFR (melt flow rate)>
It was measured according to JIS K7120.
[0039]
【The invention's effect】
By using the demonomerization method of the present invention, it is possible to produce polylactic acid pellets containing no low molecular components and having excellent moldability. Therefore, the polylactic acid pellets treated by the method of the present invention are excellent in moldability of films, fibers, injection molded products and the like.
[Brief description of the drawings]
FIG. 1 is a schematic diagram of an apparatus for implementing the method of the present invention.
DESCRIPTION OF SYMBOLS 1; Raw material feeder 2; Crystallizer 3; Hopper type reactor 4; Cooler 6; Condenser 7; Heater 10;

Claims (5)

A step of crystallizing a polylactic acid pellet produced by directly polymerizing lactic acid or a ring-opening polymerization of lactide, and a step of gasifying and removing low-molecular substances from the crystallized solid polylactic acid pellet. Polylactic acid demonomerization method. The method according to claim 1, wherein the inert gas or air or a mixed gas thereof is heated and aerated to gasify and remove low molecular substances from solid polylactic acid pellets. Demonomer method. A method for producing polylactic acid, comprising capturing the low-molecular substance removed in the method of claim 1 and subjecting it to polymerization. A polylactic acid having a molecular weight of 100,000 to 500,000 and an unreacted lactide of 1.0% or less, which has been demonomerized in the method of claim 1. While flowing the polylactic acid pellets, a crystallizing section for maintaining the temperature above the glass transition point of the pellets and below the melting point, and an inert gas or air or a mixture thereof in the solid polylactic acid pellets crystallized in the crystallizing section. An apparatus for demonomerizing polylactic acid, comprising a reactor for heating and aerating gas.

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