A polymer devolatilization device and a polylactic acid devolatilization method

Abstract

The invention discloses a polymer devolatilization device and a devolatilization method for polylactic acid, wherein the device comprises a devolatilization tank, wherein a feed distributor, a falling body baffle, an ultrasonic generator and other components are arranged, and ultra-high efficiency devolatilization can be achieved under vacuum and ultrasonic conditions, the material residence time is short, the heating is uniform, the internal structure of the equipment is simple, and the material is not easy to store. The use of the device for devolatilization of polylactic acid can successfully solve the problems of difficulty in removing impurities of lactide and most polymer monomers, improve the purity, and improve the molding performance and thermal stability of polylactic acid materials.

Description

Technical Field

[0001] The invention relates to the field of devolatilization, in particular to a devolatilization method and a devolatilization system for polylactic acid, and in particular to the background technology of polymer ultrasonic devolatilization. Background Art

[0002] Bulk polymerization is a common method for producing polymers. Modern bulk polymerization processes are equipped with devolatilization devices to remove volatile components such as unreacted monomers, solvent impurities and oligomers in the polymer to improve the purity of the polymer.

[0003] Environmental pollution and resource shortages have become a major threat to human society. It is a general trend to develop renewable resources and biodegradable polymer materials to replace petroleum plastic products. Among them, polylactic acid is a polymer obtained by polymerization of lactic acid as the main raw material. The raw material source is sufficient and renewable. The production process of polylactic acid is pollution-free, and the product can be biodegraded and realized in the cycle of nature. Therefore, it is an ideal green polymer material.

[0004] Among the current polylactic acid synthesis methods, the lactide ring-opening polymerization method is the most widely used and most effective method for obtaining high molecular weight polylactic acid. The lactide ring-opening polymerization process and its subsequent devolatilization process are important research contents in the field. Its research focuses on the improvement of the polymerization process on the one hand, and on the improvement of the devolatilization process in the later stage of the reaction on the other hand, removing low molecular weight volatiles as much as possible from the polymer product with high viscosity characteristics to improve the purity of the polymer. In the continuous polymerization method of producing polylactic acid using lactide as raw material, the devolatilization method and equipment are one of the key links. Since polylactic acid has a high viscosity and the boiling point of lactide monomer is relatively high, how to completely remove lactide has always been a thorny problem.

[0005] Patent CN203525331U discloses a small-volume strip-type preheating devolatilization device for polylactic acid with a flat-cover conical head. The material is easier to flow downward and is not easy to accumulate, but the devolatilization effect is not ideal. It often requires secondary devolatilization to achieve the ideal effect, which increases energy consumption and material heating time, resulting in a decrease in the quality of polyester products that are easily thermally degraded.

[0006] Patent CN110394966A discloses a devolatilizing screw and a devolatilizing extruder, in which the threads of the first threaded portion and the second threaded portion rotate in opposite directions, thereby extruding the material for conveying. The material is formed into a film, a line, a strip, etc. under the action of the threaded unit, the material channel and a porous discharge plate to achieve devolatilization. However, this method is prone to material storage, and the polymer is easily subjected to high shear loads, resulting in a decrease in the molecular weight of heat-sensitive polymers.

[0007] The polylactic acid pre-devolatilization devices in the prior art generally have the following problems: the thickness of the stagnant inner layer of the fluid near the tube wall is large, the resistance to heat transfer is large, the material flows slowly downward, it is easy to accumulate material, and the working efficiency is low. The material is heated unevenly during the devolatilization process, which will cause the color of the material to be yellowish after insufficient devolatilization; the volatile components cannot be smoothly released from the viscous material after vaporization during the devolatilization process, and the devolatilization effect is not ideal; the material under high shear load will also lead to a decrease in the quality of the final polylactic acid product and low efficiency. Therefore, an improved technology is urgently needed to solve this problem in the prior art. Summary of the invention

[0008] In view of the above problems existing in the prior art, the purpose of the present invention is to provide a devolatilization method of polylactic acid and a polymer devolatilization device. The present invention designs a polymer devolatilization device, which uses ultrasonic disentanglement to open the molecular chain, reduce the viscosity of the solution, and is conducive to the removal of small molecules. It can solve the problems of uneven heating of materials during the devolatilization process, the inability of volatiles to escape smoothly from viscous materials, and the long residence time of materials at high temperatures. The devolatilization method of polylactic acid of the present invention can successfully solve the problems of difficulty in removing impurities of lactide and most polymer monomers, improve purity, and improve the molding performance and thermal stability of polylactic acid materials.

[0009] In order to achieve the above object, the technical solution adopted by the present invention is:

[0010] The present invention provides a polymer devolatilization device, which comprises a devolatilization tank, wherein 3-6 feed ports are arranged on the top of the devolatilization tank, and the feed ports are evenly arranged in a straight line from the edge of the tank body to the central axis direction;

[0011] The feed port is connected to a feed distributor at the end thereof, wherein the feed distributor is provided with a row of uniform small holes, the hole diameter of which is 2-10 mm, preferably 3-5 mm; the number of the small holes is 4-20, preferably 6-10;

[0012] The devolatilization tank is provided with a circular notch-shaped falling baffle with a downward inclination angle of 10-45 degrees, and the notch is located at the lower end of the downwardly inclined baffle;

[0013] The area of ​​the circular segment-shaped falling body baffle is 60-75% of the cross section of the devolatilizer, where the cross section of the devolatilizer refers to the oblique cross section along the inclination angle of the circular segment-shaped falling body baffle;

[0014] The circular segment falling baffle is arranged 80-100 cm below the feed inlet and located at 40-70% of the height of the devolatilizer tank body;

[0015] The upper surface of the circular segment-shaped falling object baffle is provided with U-shaped protrusions arranged in a fish scale pattern, and the opening direction of the U-shaped protrusions is the same as the material flow direction on the falling object baffle; the height of the U-shaped protrusions is 1-10 mm, preferably 2-5 mm, and the width is 3-15 cm, preferably 5-10 cm;

[0016] An ultrasonic device is arranged below the circular segment-shaped falling object baffle;

[0017] A discharge port is arranged at the bottom of the devolatilization tank.

[0018] In the present invention, the arrangement direction of the small holes of the feed distributor is perpendicular to the material flow direction on the falling baffle.

[0019] In the present invention, the linear direction of the arrangement of the feed ports is the same as the material flow direction on the falling baffle.

[0020] In the present invention, the ultrasonic device is placed under the circular segment-shaped falling baffle and fits tightly, so that ultrasound can be effectively transmitted to the inside of the fluid through the baffle. The present invention has no special requirements for the selection of a specific type of ultrasonic device. The ultrasonic device can be placed in the tank as a whole. Limited by the internal space of the devolatilization tank, only the probe of the ultrasonic generator can be placed under the baffle, and the ultrasonic generator is located outside the devolatilization tank.

[0021] In addition, the devolatilizer of the present invention may be further provided with some conventional components according to actual needs, such as providing an exhaust port for controlling and maintaining the vacuum degree and taking away light components, providing an external jacket for the circulation of heat exchange medium, etc., but the present invention has no specific requirements.

[0022] The polymer devolatilization device of the present invention has multiple feed ports evenly distributed linearly from outside to inside on the upper end cover of the devolatilization tank. The flow time of the fluid on the falling baffle can be controlled by the different positions of the corresponding falling baffles, and then the time for the fluid to receive ultrasound can be controlled. The devolatilization of different materials such as polylactic acid can be realized by matching the ultrasound intensity. The present invention also uses the U-shaped protrusions arranged in a scale-like manner on the falling baffle to make the lower layer of the fluid flow left and right in a Z shape, and the upper layer flows up and down in a wave shape to form turbulent flow. The combination of ultrasound is more conducive to the complete overflow of volatiles in the fluid. The falling baffle is a large cut circle, and the fluid continues to flow downward from the cut line like a waterfall, which can further improve the devolatilization effect and achieve efficient devolatilization.

[0023] The polymer devolatilization device of the present invention, the polymer can be a combination of one or more of polylactic acid, polyglycolic acid, polycaprolactone, nylon, polycarbonate, polystyrene, polysulfone, polyolefin, polyacrylate, polyester, polyurethane, polymethyl methacrylate, styrene-acrylonitrile copolymer, preferably polylactic acid.

[0024] The volatile components removed by the polymer devolatilization device of the present invention are mainly unreacted monomers, by-products and solvents in the polymer solution. The content of volatile components in the polymer solution before devolatilization is 5-50wt%, and the content of volatile components can be reduced to below 1500ppm after deashing.

[0025] Preferably, the present invention provides a devolatilization method for polylactic acid, the method using the above-mentioned polymer devolatilization device, the steps comprising:

[0026] S1 adjusts the pressure in the devolatilization tank to 20-1500PaA, preferably 20-150PaA, the temperature to 170-210°C, preferably 180-200°C, the ultrasonic frequency to 20-100KHz, preferably 40-60KHz, and the power density to 0.6-3w / cm 2 , preferably 0.35-1w / cm 2 ;

[0027] S2 transports the polylactic acid material from the top feed port to the devolatilization tank. The material passes through the feed distributor and falls onto the circular segment-shaped falling baffle. Then, under the action of ultrasound, the material flows along the surface of the circular segment-shaped falling baffle to the notch position, and then falls from the notch position to the bottom of the devolatilization tank to obtain the polylactic acid product through the discharge port.

[0028] In the method of the present invention, the content of volatile components in the polylactic acid material is more than 5wt%, the volatile components are unreacted monomer impurity lactide, and the polylactic acid material can be a polylactic acid solution obtained in any process, or a polymer or copolymer thereof;

[0029] The polylactic acid material feed temperature is 170-210°C, preferably 175-190°C, and the feed flow rate is 10-100Kg / h, preferably 20-60Kg / h;

[0030] In the method of the present invention, the residence time of the polylactic acid material in the devolatilization tank is 5-40 min, preferably 10-30 min.

[0031] In the method of the present invention, the monomer content in the polylactic acid product obtained after devolatilization is below 1500 ppm, preferably below 1000 ppm.

[0032] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0033] The polymer devolatilization device of the present invention utilizes ultrasound assistance, has a simple process, a short process, is easy for volatile components to escape, and has a high devolatilization efficiency. When used for devolatilization of polymers such as polylactic acid, the material is heated evenly, the high-temperature residence time is short, the obtained polylactic acid will not degrade, and has no effect on product quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is a schematic structural diagram of the polymer devolatilization device of the present invention;

[0035] Among them: 1. devolatilization tank, 2. feed port (2, 2-1, 2-2), 3. feed distributor (3, 3-1, 3-2), 4. circular segment falling baffle, 5. ultrasonic equipment, 6. discharge port, 7. air extraction port;

[0036] Figure 2 It is a schematic diagram of the distribution of small holes of the feed distributor;

[0037] Figure 3 It is a structural schematic diagram of a circular segment-shaped falling object baffle;

[0038] Among them: 401, notch, 402, U-shaped protrusion. DETAILED DESCRIPTION

[0039] In order to deepen the understanding of the present invention, the present invention will be further described in detail below in conjunction with a polylactic acid devolatilization example and accompanying drawings. The example is only used to explain the present invention and does not constitute a limitation on the protection scope of the present invention.

[0040] Example 1

[0041] A polymer devolatilization device (such as Figure 1 As shown), it comprises a devolatilization tank 1, and three feed ports 2 (2, 2-1, 2-2) are arranged on the top of the devolatilization tank 1, and the feed ports 2 are evenly arranged in a straight line from the edge of the tank body to the central axis direction;

[0042] The end of the feed port 2 is connected to a feed distributor 3 (3, 3-1, 3-2), which is provided with a row of uniform small holes (such as Figure 2 As shown), the aperture is 3 mm and the number is 7;

[0043] A circular drop baffle 4 (such as Figure 3 As shown), the area is 70% of the oblique cross section of the devolatilizer along the 30° inclination angle, and the notch 401 is located at the lower end of the downwardly inclined baffle;

[0044] The circular segment-shaped falling baffle 4 is arranged 100 cm below the feed inlet and located at 50% of the height of the devolatilization tank 1;

[0045] The upper surface of the circular-segment falling baffle 4 is provided with U-shaped protrusions 402 arranged in a fish scale pattern, the height of the U-shaped protrusions 402 is 5 mm, the width is 5 cm, and the direction of the U-shaped opening is the same as the flow direction of the material on the falling baffle;

[0046] A tightly fitting ultrasonic device 5 is arranged below the circular segment-shaped falling object baffle 4;

[0047] The bottom of the devolatilizer 1 is also provided with a discharge port 6, and the top is provided with a suction port 7.

[0048] The arrangement direction of the small holes of the feed distributor 3 is perpendicular to the straight line direction of the feed ports 2 ; the straight line direction of the feed ports 2 is the same as the material flow direction on the falling baffle 4 .

[0049] Unless otherwise specified, the main test methods used in the embodiments and comparative examples of the present invention are all common equipment and methods in the art:

[0050] 1. Hue: Directly test with DC-P3 fully automatic colorimeter produced by Beijing Xingguang Colorimetric Instrument Co., Ltd. to measure its Hunter L value (brightness), b value (yellow-blue hue) and YI value (yellowness index). The higher the L value, the greater the brightness; the higher the b value, the yellower the polyester chips; polylactic acid is tested after tableting.

[0051] 2. Mechanical properties: The tensile strength was tested using Instron 5960 universal testing machine according to ASTM D638 standard at a tensile speed of 50 mm / min.

[0052] 3. Molecular weight and molecular weight distribution coefficient: The molecular weight was determined by using a 1515-2414 gel permeation chromatograph (GPC) from Waters, USA, with chloroform as the mobile phase, an effluent rate of 1 mL / min, a column temperature of 30°C, and polystyrene as the standard sample.

[0053] 4. Melt Index (MFR): Determined according to IS0 1133, the test conditions are 190°C, 2.16 kg, and the melting time is 5 minutes. MFR gives the extrusion flow rate of the molten plastic composition through an extrusion die of specified length and diameter under specified conditions (temperature, load, piston position), and determines the mass in the extrusion plastometer barrel sent out within a specified time.

[0054] 5. Melting point Tm: tested using Perkins Elmer's Plyris1 differential scanning calorimeter. The sample was first heated to 180°C at 20°C / min to eliminate the thermal history, then cooled to -30°C at 20°C / min, and then heated at 10°C / min to obtain the DSC curve of the sample curve.

[0055] 6. Viscosity: The intrinsic viscosity was measured using a PVS1 / 2+VRM4 viscosity tester from LAUDA of Germany in accordance with the national standard GB / T1632.5-2008 and phenol-tetrachloroethane (1:1) as solvent.

[0056] Example 2

[0057] Polylactic acid devolatilization method:

[0058] Polylactic acid material: It is prepared by polymerization of L-lactide with an optical purity of ≥95% under the action of stannous octoate and an initiator. The volatile component is the unreacted lactide monomer with a content of 5.5wt% and a molecular weight of 15±0.5x10 4 .

[0059] Using the device of Example 1, the steps are:

[0060] Adjust the pressure in the devolatilization tank to 180PaA, the temperature to 190℃, the ultrasonic frequency to 50KHz, and the power density to 0.4w / cm 2 ;

[0061] The polylactic acid material with a temperature of 190-200°C is transported to the devolatilization tank 1 from the top feed port 2 (2, 2-1, 2-2) at a flow rate of 30 Kg / h. The material passes through the feed distributor 3 (3, 3-1, 3-2) and then falls onto the circular segment-shaped falling baffle 4. Then, under the action of ultrasound, it flows along the surface of the circular segment-shaped falling baffle 4 to the position of the notch 401, and then falls from the position of the notch 401 to the bottom of the devolatilization tank 1. The material stays in the devolatilization tank 1 for 20 minutes, and the polylactic acid product is obtained through the discharge port 6.

[0062] The devolatilization device was continuously operated for 2 h, 10 h, 50 h, 100 h, 150 h, and 200 h before sampling. The polylactic acid product finally obtained was analyzed and measured. The results are shown in Table 1.

[0063] Table 1

[0064]

[0065] Example 3

[0066] Polylactic acid devolatilization method:

[0067] Polylactic acid material: prepared according to the method of Example 2, the unreacted monomer lactide in the obtained polylactic acid material is 5.08wt%, and the molecular weight is 18±0.5x10 4 .

[0068] Using the device of Example 1, the steps are:

[0069] Adjust the pressure in the devolatilization tank to 150PaA, the temperature to 195℃, the ultrasonic frequency to 70KHz, and the power density to 0.8w / cm 2 ;

[0070] The polylactic acid material with a temperature of 190-200°C is transported to the devolatilization tank 1 from the top feed port 2 (2, 2-1, 2-2) at a flow rate of 25 Kg / h. The material passes through the feed distributor 3 (3, 3-1, 3-2) and then falls onto the circular segment-shaped falling baffle 4. Then, under the action of ultrasound, it flows along the surface of the circular segment-shaped falling baffle 4 to the position of the notch 401, and then falls from the position of the notch 401 to the bottom of the devolatilization tank 1. The material stays in the devolatilization tank 1 for 25 minutes, and the polylactic acid product is obtained through the discharge port 6.

[0071] The devolatilization device was continuously operated for 2 h, 10 h, 50 h, 100 h, 150 h, and 200 h before sampling. The final polylactic acid product was analyzed and measured. The results are shown in Table 2.

[0072] Table 2

[0073]

[0074] Example 4

[0075] Polylactic acid devolatilization method:

[0076] Polylactic acid material: prepared according to the method of Example 2, the unreacted monomer lactide in the obtained polylactic acid material is 5.18wt%, and the molecular weight is 17±0.5x10 4 .

[0077] Using the device of Example 1, the steps are:

[0078] Adjust the pressure in the devolatilization tank to 100PaA, the temperature to 200℃, the ultrasonic frequency to 100KHz, and the power density to 1.5w / cm 2 ;

[0079] The polylactic acid material with a temperature of 190-200°C is simultaneously conveyed to the devolatilization tank 1 from the top feed port 2 (2, 2-1, 2-2) at a flow rate of 25 Kg / h. The material passes through the feed distributor 3 (3, 3-1, 3-2) and then falls onto the circular segment-shaped falling baffle 4. Then, under the action of ultrasound, it flows along the surface of the circular segment-shaped falling baffle 4 to the position of the notch 401, and then falls from the position of the notch 401 to the bottom of the devolatilization tank 1. The material stays in the devolatilization tank 1 for 30 minutes, and the polylactic acid product is obtained through the discharge port 6.

[0080] The devolatilization device was continuously operated for 2 h, 10 h, 50 h, 100 h, 150 h, and 200 h before sampling. The final polylactic acid product was analyzed and measured. The results are shown in Table 3.

[0081] Table 3

[0082]

[0083] Example 5

[0084] Polyglycolic acid devolatilization method:

[0085] The devolatilization method is described in Example 2, except that the material is replaced with a residual mono (glycolide) content of 6.8 wt% and a molecular weight of 9.6×10 4 of polyglycolic acid;

[0086] The devolatilization temperature was 200°C and the residence time was 30 min. The devolatilization device was continuously operated for 2 h, 10 h, 50 h, 100 h, 150 h, and 200 h, and samples were taken respectively. The polyglycolic acid product finally obtained was analyzed and determined, and the results are shown in Table 4.

[0087] Table 4

[0088]

[0089] Comparative Example 1

[0090] Referring to the polylactic acid devolatilization method of Example 2, the difference is that the circular segment-shaped falling baffle is omitted in the devolatilization device, and the other parts remain unchanged;

[0091] The devolatilization device was continuously operated for 2 h, 10 h, and 20 h before sampling. The final polylactic acid product was analyzed and measured. The results are shown in Table 5.

[0092] Table 5

[0093]

[0094] Comparative Example 2

[0095] Refer to the polylactic acid devolatilization method of Example 2, except that the U-shaped protrusions arranged in a fish scale pattern on the upper surface of the circular segment-shaped falling baffle in the devolatilization device are replaced with a lentil-shaped patterned plate, and the rest remain unchanged;

[0096] The devolatilization device was continuously operated for 2 h, 10 h, and 20 h before sampling. The polylactic acid product finally obtained was analyzed and measured. The results are shown in Table 6.

[0097] Table 6

[0098]

[0099] Comparative Example 3

[0100] Referring to the polylactic acid devolatilization method of Example 2, the difference is that the ultrasonic device is omitted in the devolatilization device, and the other parts remain unchanged;

[0101] The devolatilization device was continuously operated for 2 h, 10 h, and 20 h before sampling. The polylactic acid product finally obtained was analyzed and measured. The results are shown in Table 7.

[0102] Table 7

[0103]

[0104] Comparative Example 4

[0105] Refer to the polylactic acid devolatilization method of Example 2, except that the pressure of the devolatilization tank is controlled at 2000 PaA;

[0106] The devolatilization device was continuously operated for 1 h, 2 h, 10 h, and 20 h before sampling. The polylactic acid product finally obtained was analyzed and measured. The results are shown in Table 8.

[0107] Table 8

[0108]

Claims

1. A polymer devolatilization device, characterized in that: It comprises a devolatilizer, wherein the top of the devolatilizer is provided with 3-6 feed ports, and the feed ports are evenly arranged in a straight line from the edge of the tank body to the central axis direction; The feed port is connected to a feed distributor at the end thereof, and the feed distributor is provided with a row of uniform small holes, the hole diameter of which is 2-10 mm, and the number of small holes is 4-20; The devolatilization tank is provided with a circular notch-shaped falling baffle with a downward inclination angle of 10-45 degrees, and the notch is located at the lower end of the downwardly inclined baffle; The area of ​​the circular segment-shaped falling body baffle is 60-75% of the cross section of the devolatilizer, where the cross section of the devolatilizer refers to the oblique cross section along the inclination angle of the circular segment-shaped falling body baffle; The circular segment falling baffle is arranged 80-100 cm below the feed inlet and located at 40-70% of the height of the devolatilizer tank body; The upper surface of the circular-segment falling baffle is provided with U-shaped protrusions arranged in a fish scale pattern, and the opening direction of the U-shaped protrusions is the same as the material flow direction on the falling baffle; the height of the U-shaped protrusions is 1-10 mm, and the width is 3-15 cm; An ultrasonic device is arranged below the circular segment-shaped falling object baffle; A discharge port is arranged at the bottom of the devolatilization tank.

2. The polymer devolatilization device according to claim 1, characterized in that: The diameter of the small holes is 3-5 mm, and the number of the small holes is 6-10.

3. The polymer devolatilization device according to claim 1, characterized in that: The U-shaped protrusion has a height of 2-5 mm and a width of 5-10 cm.

4. The polymer devolatilization device according to claim 1, characterized in that: The arrangement direction of the small holes of the feed distributor is perpendicular to the material flow direction on the falling baffle; The straight line direction in which the feed ports are arranged is the same as the material flow direction on the falling baffle.

5. The polymer devolatilization device according to claim 1, characterized in that: The ultrasonic device is placed under the circular segment-shaped falling baffle and fits tightly.

6. The polymer devolatilization device according to claim 1, characterized in that: The polymer is selected from one or more combinations of polylactic acid, polyglycolic acid, nylon, polystyrene, polysulfone, polyolefin, polyester, and styrene-acrylonitrile copolymer.

7. The polymer devolatilization device according to claim 6, characterized in that: The polyester is selected from one or more of polycaprolactone, polycarbonate, polyacrylate, polyurethane and polymethyl methacrylate.

8. The polymer devolatilization device according to claim 6, characterized in that: The polymer is polylactic acid.

9. The polymer devolatilization device according to claim 1, characterized in that: The content of volatile components in the polymer solution before devolatilization is 5-50wt%, and the content of volatile components is reduced to below 500ppm after deashing.

10. A method for devolatilizing polylactic acid, characterized in that: The method adopts the polymer devolatilization device according to any one of claims 1 to 9, and the steps include: S1 adjusts the pressure in the devolatilization tank to 20-1500PaA, the temperature to 170-210℃, the ultrasonic frequency to 20-100KHz, and the power density to 0.6-3w / cm 2 ; S2 transports the polylactic acid material from the top feed port to the devolatilization tank. The material passes through the feed distributor and falls onto the circular segment-shaped falling baffle. Then, under the action of ultrasound, the material flows along the surface of the circular segment-shaped falling baffle to the notch position, and then falls from the notch position to the bottom of the devolatilization tank to obtain the polylactic acid product through the discharge port.

11. The devolatilization method according to claim 10, characterized in that: Step S1: adjust the pressure in the devolatilization tank to 20-150 PaA, the temperature to 180-200°C, the ultrasonic frequency to 40-60 KHz, and the power density to 0.35-1 w / cm 2 .

12. The devolatilization method according to claim 10, characterized in that: The polylactic acid material has a volatile component content of more than 5 wt%, and the volatile component is unreacted monomer impurity lactide; The polylactic acid material is a polylactic acid solution obtained in any process, or a polymer or copolymer thereof.

13. The devolatilization method according to claim 10, characterized in that: The feed temperature of the polylactic acid material is 170-210° C., and the feed flow rate is 10-100 Kg / h.

14. The devolatilization method according to claim 13, characterized in that: The feed temperature of the polylactic acid material is 175-190° C., and the feed flow rate is 20-60 Kg / h.

15. The devolatilization method according to claim 10, characterized in that: The residence time of the polylactic acid material in the devolatilization tank is 5-40 minutes.

16. The devolatilization method according to claim 15, characterized in that: The residence time of the polylactic acid material in the devolatilization tank is 10-30 minutes.

17. The devolatilization method according to claim 16, characterized in that: The monomer content in the polylactic acid product obtained after devolatilization is less than 1500 ppm.

18. The devolatilization method according to claim 17, characterized in that: The monomer content in the polylactic acid product obtained after devolatilization is less than 1000 ppm.

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