Purification method of lactide

Abstract

The present invention provides a lactide purification method, which comprises: (1) mixing an alcohol and / or ester solvent with crude lactide, and dissolving to obtain a crude lactide solution; and (2) adding water into the crude lactide solution, carrying out heat preservation, mixing and cooling to separate out lactide, carrying out solid-liquid separation to obtain a solid, and washing and drying the solid to obtain a lactide product. According to the method, the characteristic that the solubility of the lactide in an alcohol / ester solvent is in nonlinear growth is utilized, the lactide is completely dissolved firstly, then water is added for cooling and precipitation, m-lactide is hydrolyzed at the same time, then cooling is performed to enable the lactide to be continuously precipitated, and finally the powder-state lactide is obtained, so that the whole process avoids the formation of large blocks in the product, no impurity inclusion exists, and the purity of the product is greatly improved. The product quality is more uniform, the particle size range is kept at 50 + / -10 microns, the problems that a stirring paddle is damaged and a pipeline is blocked due to the existence of large blocks are avoided, and the conveying problem in engineering is solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of polymer synthesis, relates to the purification of lactide, and in particular to a purification method capable of controlling the uniformity of lactide crystals. Background Art

[0002] Polylactic acid (PLA) is a biodegradable polyester, and its degradation products are water and carbon dioxide. It is a green environmental protection material. In today's increasingly enhanced environmental awareness, polylactic acid is worthy of large-scale promotion and application. The production of polylactic acid is divided into one-step production (lactic acid → polylactic acid) and two-step production (lactic acid → lactide → polylactic acid). At present, one-step production has not been realized on a large scale due to immature technology. At present, the mainstream production method of polylactic acid in industry is the two-step method. In the two-step method, the purity of the intermediate lactide directly determines the polymerization quality of polylactic acid. Therefore, the purification of lactide has always received extensive attention.

[0003] At present, the purification methods of lactide include rectification (CN112812094A), melt crystallization (CN114133374A), solvent recrystallization (CN109400574A, CN111961028A, CN110128397A), etc. The rectification method requires high vacuum and high temperature to achieve the purpose of separation and purification. This method is more suitable for industrial production, but has high energy consumption. Moreover, lactide is a thermosensitive substance, and its configuration is extremely easy to flip at high temperature, resulting in a reduction in the yield of the target product; melt crystallization is suitable for the purification of lactide with relatively high purity, generally requiring the feed purity > 95%, and is not suitable for the purification of crude lactide; solvent recrystallization generally uses organic solvents to dissolve lactide, and then cools down to precipitate lactide crystals to achieve the purpose of purification. Compared with the previous two methods, the energy consumption is not high, but due to the certain solubility of pure organic solvents in lactide, the yield of the purified product is not high, and the economy is poor.

[0004] Those skilled in the art have actively explored new lactide purification methods. CN105646440A discloses a method for refining and purifying lactide, which uses a combination of double-solvent extraction and melt crystallization to purify lactide. In the double-solvent extraction stage, water and ethanol are selected as the double-solvent extractants. Ethanol can effectively remove pigments and lactic acid. By using the hydrolysis rate difference between L / D lactide and m-lactide in water, m-lactide can be effectively removed. The double-solvent combination can effectively purify lactide. In this method, water and ethanol are added simultaneously, and the extraction temperature is 20-50°C. If there is water and the temperature is relatively high, while m-lactide hydrolyzes, L-lactide will also inevitably hydrolyze, resulting in a certain loss of the target product and a reduction in the yield.

[0005] JP2000086652A discloses a method for purifying lactide. By contacting crude lactide in a solid state or at least partially in a molten state with ethanol to obtain a slurry-like mixture, and then separating the solid from the liquid, high-purity lactide can be obtained. Apparently, during the process of contacting the crude lactide in a molten state with ethanol to obtain a slurry-like mixture, caking is likely to occur, which affects the product purity. The technical solution also mentions that the separated solid component is washed with water to remove ethanol.

[0006] CN114478470A discloses a method for purifying lactide. First, the crude lactide is pretreated, and then water is added for a rapid reaction. The reaction time is ≤ 10 min, and the reaction temperature is below 18 °C to obtain a water-extracted stock solution. A lower-carbon alcohol solvent is added to the water-extracted stock solution, and after reaction, filtration, washing, and drying are carried out to obtain purified lactide. This method can efficiently remove impurities such as m-lactide in the crude lactide and obtain high-quality lactide products. This solution involves the pretreatment of crude lactide, which is carried out by crushing or melting. The pretreatment method is more suitable for small-scale experimental tests. In industrialization, large-scale dynamic equipment such as crushers needs to be introduced, increasing the maintenance difficulty. Molten lactide is also prone to caking when it encounters a cold extraction solution, affecting the purification effect.

[0007] The above solvent water extraction scheme for purifying crude lactide has achieved good purification effects in both the exploration and small-scale experimental stages. However, during the process of experimental scale-up, the technicians found that if solid powder feeding is adopted, there are requirements for the powder particle size. If the particle size is large, it will affect the product quality. The granulation link introduces dynamic equipment, increasing the process and bringing quite a challenge to the maintenance and continuous production of industrial production. If liquid molten state feeding is adopted, if the feeding flow rate and stirring speed are not well controlled, the molten lactide contacting the cold extraction solution will cause caking, and impurities will be included, affecting the product purity. At the same time, large cakings will damage the stirring paddle and block the pipeline, affecting subsequent transportation. Summary of the Invention

[0008] In order to solve the problems in the prior art that in the large-scale production process of the solvent water extraction and purification process of lactide, caking occurs and impurities are included, thereby affecting the product purity, and the caking will block the pipeline and damage the stirring paddle, the present invention provides a method for purifying lactide, which is suitable for the scale-up production of the lactide purification process, and the obtained lactide crystals are uniform and non-caking, and it is a practical lactide purification scheme suitable for industrial production.

[0009] The inventor of this case found during the research process that the solubility of lactide in organic solvents such as alcohols / esters shows a non-linear growth trend with the increase of temperature, and this discovery provides inspiration for solving the above existing problems.

[0010] In order to achieve the above technical objectives, the technical solution of the present invention is as follows:

[0011] The present invention provides a method for purifying lactide, comprising:

[0012] (1) Mixing an alcohol and / or ester solvent with crude lactide to dissolve it, obtaining a crude lactide solution;

[0013] (2) Adding water to the crude lactide solution, maintaining the temperature for mixing, then cooling to precipitate lactide, performing solid-liquid separation to obtain a solid, washing and drying the solid to obtain a lactide product.

[0014] Furthermore, in step (1), the alcohol and / or ester solvent is selected from one or more of methanol, ethanol, isopropanol, n-butanol, n-pentanol, ethyl acetate, ε-caprolactone, δ-valerolactone, and γ-valerolactone, preferably one or more of ethanol, isopropanol, and δ-valerolactone.

[0015] Furthermore, in step (1), the dissolution of lactide is achieved by heating to fully dissolve lactide, and lactide can be in a solid state, molten state, or semi-molten state. A more preferred implementation is to mix an alcohol and / or ester solvent with molten crude lactide to make the dissolution process easier; a more preferred implementation is to mix an alcohol and / or ester solvent at a temperature above 35°C with molten crude lactide, more preferably an alcohol and / or ester solvent at 40°C - 64°C, and most preferably at 45 - 55°C; among them, auxiliary methods commonly used in the prior art to enhance mixing, such as stirring, can also be selected.

[0016] Furthermore, in step (1), the weight ratio of the alcohol and / or ester solvent to the crude lactide is 0.25 - 2.5:1, preferably 0.4 - 0.8:1.

[0017] Furthermore, in step (1), the mixing of the solvent and the crude lactide is carried out in an atmospheric pressure environment or a positive pressure environment, and the pressure is preferably 0.1 MPa - 0.3 MPa.

[0018] Furthermore, in step (2), water is added in a weight ratio of 0.5 - 2:1 to the crude lactide, preferably 1 - 1.5:1.

[0019] Furthermore, in step (2), the temperature of the mixed solution after adding water is 10 - 35°C, preferably 15 - 25°C.

[0020] Furthermore, in step (2), the temperature of the added water is 0 - 20°C, preferably 0 - 15°C, and more preferably 2 - 8°C.

[0021] Furthermore, in step (2), the water is deionized water or treated pure water.

[0022] Further, the time for heat-preserving mixing in step (2) is 3 - 25 min, preferably 7 - 15 min.

[0023] Further, the cooling rate in step (2) is 0.1 - 20 °C / min, preferably 1 - 10 °C / min.

[0024] Further, the final temperature after cooling in step (2) is reduced to 0 - 25 °C, preferably 3 - 6 °C.

[0025] Further, the form of solid-liquid separation in step (2) can be suction filtration, plate-and-frame filtration, centrifugation or any other form of solid-liquid separation.

[0026] Further, the solvent used for washing the solid in step (2) is an alcohol and / or ester solvent, selected from one or more of methanol, ethanol, isopropanol, n-butanol, n-pentanol, ethyl acetate, ε-caprolactone, δ-valerolactone and γ-valerolactone, preferably the same as the solvent in step (1).

[0027] Further, the weight ratio of the solvent added for washing the solid to the crude lactide in step (2) is 0.05 - 2:1, preferably 0.1 - 0.3:1.

[0028] Further, the form of drying in step (2) can be vacuum drying, hot air drying, negative pressure drying or any other form of drying.

[0029] Further, the temperature of drying in step (2) is 30 - 70 °C, preferably 40 - 60 °C.

[0030] Further, the time of drying in step (2) is 0.5 - 24 h, preferably 2 - 5 h.

[0031] Further, the crude lactide in step (1) has L / D-lactide as the main component and contains various impurities such as m-lactide, lactic acid, lactic acid dimer, trimer and organic pigment, and is a product prepared from lactic acid / lactic acid ester through dehydration, polycondensation and depolymerization processes.

[0032] Further, the filtrate after solid-liquid separation in step (2) recovers the solvent, lactic acid and lactic acid polymers through rectification and returns them to the purification section and the crude lactide synthesis section for use respectively.

[0033] In the method of the present invention, after water is added in step (2), lactide starts to precipitate. Through the heat preservation process, m-lactide is hydrolyzed, and lactide continues to precipitate when the temperature is lowered. The obtained lactide product is powdery lactide. The inventor found that the lactide particles obtained in this way are very uniform, the particle size range is between 40-60μm, and the yield of the lactide product can reach more than 90%, the chemical purity of lactide can reach more than 99.0%, and the optical purity can reach more than 99.0%.

[0034] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0035] (1) Compared with the solvent water extraction process of the prior art, the present invention utilizes the characteristic that the solubility of lactide in alcohol / ester solvents shows non-linear growth. First, all lactide is dissolved and then water is added to lower the temperature for precipitation, and at the same time, m-lactide is hydrolyzed. Then, when the temperature is lowered, lactide continues to precipitate, and finally powdery lactide is obtained. The whole process avoids the formation of large chunks in the product, there is no impurity inclusion, and the product quality is more uniform.

[0036] (2) The process adopted by the present invention avoids the formation of large chunks of lactide, avoids the problems of damaging the stirring paddle and blocking the pipeline caused by the existence of large chunks, and solves the transportation problem in engineering.

[0037] (3) The particle size range of the lactide product obtained by the present invention is maintained at 50μm±10μm, the product particle size is very uniform. Compared with needle-shaped crystal lactide, the porosity of the filter cake is lower during filtration, the filter cake is more solid, which is beneficial to filtration, takes less time, and improves the solid-liquid separation efficiency. Detailed implementation mode

[0038] The present invention will be further illustrated by the following examples. The examples are implemented on the premise of the technical solution of the present invention, and the detailed implementation methods and specific operation processes are given, but the protection scope of the present invention is not limited to the following examples.

[0039] In the following examples, the experimental methods, unless otherwise specified, are all conventional methods in the art. The experimental materials used in the following examples, unless otherwise specified, can all be obtained from biochemical reagent stores.

[0040] The present invention measures the particle size of the lactide product by a laser particle size analyzer.

[0041] The present invention uses an Agilent high-performance liquid chromatograph to analyze the chemical purity of lactide. The detector is an ultraviolet detector. Phosphoric acid and acetonitrile are used as the mobile phase. The chromatographic column model is ZORBAX SB-Aq, the column length is 250mm, the column inner diameter is 4.6mm, and the filler particle size inside is 5um. Detection wavelength: 200nm, column temperature: 40°C, flow rate: 1mL / min, injection volume: 5μL.

[0042] The present invention uses an Agilent gas chromatograph to analyze the content of different optical isomers of lactide. A CYCLOSIL-B type chromatographic column is selected, the vaporization chamber temperature is 250 °C, the detector temperature is 280 °C, a hydrogen flame ionization detector is used, the initial temperature of the column temperature programmed heating is 100 °C, held for 5 min, heated to 140 °C at a rate of 4 °C / min, held for 7 min, heated to 200 °C at a rate of 8 °C / min, and held for 20 min. The carrier gas is N 2 The flow rate is 1.4 mL / min, the hydrogen flow rate is 30 mL / min, the air flow rate is 400 mL / min, and the injection volume is 0.5 μL.

[0043] The present invention uses a WZZ-2S automatic polarimeter to analyze the specific rotation of the sample, so as to characterize the optical purity of the sample. The specific rotation of pure L-lactide is -278, the specific rotation of pure D-lactide is +278, and the specific rotation of m-lactide is 0. The calculation formula for the optical purity X of the sample is as follows:

[0044]

[0045] Among them, α 纯物质 represents the specific rotation of pure lactide, and α 被测样品 represents the specific rotation of the substance to be measured.

[0046] The calculation formula for the yield Y in the lactide purification process is as follows:

[0047]

[0048] Among them, m 0 is the weight of crude lactide, y 0 is the purity of L-lactide in crude lactide, and m is the weight of the lactide product obtained after purification.

[0049] The crude lactide used in the following examples and comparative examples was prepared by the following method: Using zinc oxide and stannous octoate as catalysts, 88% L-lactic acid as raw material, and obtained through lactic acid dehydration, polycondensation and depolymerization reactions. By mass content, L-lactide is 84.2%, m-lactide is 5.6%, lactic acid is 4.8%, lactic acid dimers and trimers are 3.7%, and in addition, it also includes a small amount of water and colored impurities.

[0050] Example 1

[0051] Add 3000 g of anhydrous ethanol to the reactor, start stirring at a speed of 180 rpm, heat up to 52 °C, add 5000 g of molten crude lactide to the reactor, maintain the pressure in the reactor at 0.15 MPa until all the crude lactide is dissolved to obtain a lactide solution. Then add 5500 g of deionized water at a temperature of 4 °C, keep the temperature of the mixture at 18 °C and maintain it for 12 min. Subsequently, lower the system temperature to 3 °C at a rate of 2 °C / min to obtain the precipitated lactide solid. Perform solid-liquid separation by suction filtration for 15 min, and wash the filter cake with 1000 g of ethanol. After completion, vacuum-dry the filter cake at 55 °C for 4 h to obtain the lactide product.

[0052] The obtained lactide product is in powder form, and the particle size is between 45 μm ± 4 μm; the yield of L-lactide is 95.2%, the chemical purity of L-lactide is 99.4%, and the optical purity is 99.5%.

[0053] Example 2

[0054] Add 2000 g of isopropanol to the reactor, start stirring at a speed of 200 rpm, heat up to 55 °C, add 5000 g of molten crude lactide to the reactor, maintain the pressure in the reactor at 0.1 MPa until all the crude lactide is dissolved to obtain a lactide solution. Then add 5000 g of deionized water at a temperature of 3 °C, keep the temperature of the mixture at 25 °C and maintain it for 7 min. Subsequently, lower the system temperature to 4 °C at a rate of 10 °C / min to obtain the precipitated lactide solid. Perform solid-liquid separation by plate and frame filtration for 12 min, and wash the filter cake with 500 g of isopropanol. After completion, air-dry the filter cake at 60 °C for 2 h to obtain the lactide product.

[0055] The obtained lactide product is in powder form, and the particle size is between 42 μm ± 2 μm; the yield of L-lactide is 93.8%, the chemical purity of L-lactide is 99.0%, and the optical purity is 99.1%.

[0056] Example 3

[0057] Add 3750 g of δ-valerolactone to the reactor, start stirring at a speed of 100 rpm, heat up to 45 °C, add 5000 g of molten crude lactide to the reactor, maintain the pressure in the reactor at 0.3 MPa until all the crude lactide is dissolved to obtain a lactide solution. Then add 7500 g of deionized water at a temperature of 6 °C, keep the temperature of the mixture at 16 °C and maintain it for 15 min. Subsequently, lower the temperature of the whole reaction system to 6 °C at a rate of 1 °C / min, perform solid-liquid separation by suction filtration for 20 min, and wash the filter cake with 1500 g of ethanol. After completion, vacuum-dry the filter cake at 40 °C for 5 h to obtain the lactide product.

[0058] The obtained lactide product is in powder form, with particle sizes all between 53 μm ± 5 μm. The yield of L-lactide is 94.1%, the chemical purity of L-lactide is 99.2%, and the optical purity is 99.3%.

[0059] Comparative Example 1

[0060] Take 3000 g of anhydrous ethanol at room temperature and add it to the reactor. Start stirring at a speed of 180 rpm. Add 5000 g of molten crude lactide to the reactor, and the two form a slurry mixture. Then add 5500 g of deionized water at room temperature and maintain for 12 min. Subsequently, lower the system temperature to 4 °C at a rate of 25 °C / min to obtain the precipitated lactide solid. Filter the solid-liquid separation by suction filtration for 15 min, and wash the filter cake with 1000 g of ethanol. After completion, vacuum-dry the filter cake at 55 °C for 4 h to obtain the lactide product.

[0061] The obtained lactide product is in powder form with large chunks mixed in. Due to the non-uniform product state, the large chunks need to be ground and mixed with the powder. Inevitably, impurities are mixed in the large chunks. The yield of the L-lactide product is 81.6%, the chemical purity of L-lactide is 93.7%, and the optical purity is 92.9%.

[0062] Comparative Example 2

[0063] Take 3000 g of anhydrous ethanol at room temperature and add it to the reactor. Start stirring at a speed of 180 rpm. Add 5000 g of molten crude lactide to the reactor, and the two form a slurry mixture. Then filter the solid-liquid separation by suction filtration for 15 min, and wash the filter cake with 1000 g of ethanol. After completion, vacuum-dry the filter cake at 55 °C for 4 h to obtain the lactide product.

[0064] The obtained lactide product is in powder form with large chunks mixed in. Due to the non-uniform product state, the large chunks need to be ground and mixed with the powder. Inevitably, impurities are mixed in the large chunks. The yield of the L-lactide product is 78.7%, the chemical purity of L-lactide is 91.6%, and the optical purity is 90.4%.

[0065] Comparative Example 3

[0066] Exchange the addition sequence of ethanol and water in Example 1: Add 5500 g of deionized water into the reactor, start stirring at a speed of 180 rpm, heat up to 52 °C, add 5000 g of molten lactide into the reactor, maintain the pressure in the reactor at 0.15 MPa to obtain a lactide liquid material, then add 3000 g of absolute ethanol at 4 °C, keep the temperature of the mixed solution at 18 °C and maintain it for 12 min, then reduce the system temperature to 3 °C at a rate of 2 °C / min to obtain the precipitated lactide solid, perform solid-liquid separation by suction filtration, wash the filter cake with 1000 g of ethanol, the suction filtration time is 15 min, after completion, vacuum-dry the filter cake, and dry it at 55 °C for 4 h to obtain the lactide product.

[0067] The obtained lactide product is in a powdery state with large particles mixed in. The diameter of the large particles is 1 - 5 mm. Due to the uneven state of the product, the large particles need to be ground and mixed with the powder. Inevitably, impurities are mixed in the large particles, affecting the product purity. Moreover, the contact between water at 52 °C and molten lactide causes the hydrolysis of L-lactide, thereby reducing the product yield. The yield of the L-lactide product is 72.6%, the chemical purity of L-lactide is 90.8%, and the optical purity is 89.7%.

Claims

1. A method for purifying lactide, comprising: (1) Mixing an alcohol and / or ester solvent with crude lactide to dissolve it, obtaining a crude lactide solution; (2) Adding water to the crude lactide solution, maintaining the temperature for mixing, cooling, precipitating lactide, performing solid-liquid separation to obtain a solid, washing and drying the solid to obtain a lactide product.

2. The purification method according to claim 1, characterized in that the alcohol and / or ester solvent in step (1) is selected from one or more of methanol, ethanol, isopropanol, n-butanol, n-pentanol, ethyl acetate, ɛ-caprolactone, δ-valerolactone and γ-valerolactone.

3. The purification method according to claim 1, characterized in that in step (1), the dissolution of lactide is achieved by heating to fully dissolve lactide.

4. The purification method according to claim 1, characterized in that in step (1), the dissolution of lactide is achieved by mixing a molten crude lactide with an alcohol and / or ester solvent at a temperature above 35°C.

5. The purification method according to claim 1, characterized in that the weight ratio of the alcohol and / or ester solvent to the crude lactide in step (1) is 0.25 - 2.5:

1.

6. The purification method according to claim 1, characterized in that the pressure for mixing the solvent with the crude lactide in step (1) is 0.1 MPa - 0.3 MPa.

7. The purification method according to claim 1, characterized in that the water in step (2) is added according to a weight ratio of 0.5 - 2:1 to the crude lactide.

8. The purification method according to claim 1, characterized in that after adding water in step (2), the temperature of the mixed solution drops to 10 - 35°C.

9. The purification method according to claim 8, characterized in that the temperature of the water added in step (2) is 0 - 20°C.

10. The purification method according to claim 1, characterized in that the time for maintaining the temperature for mixing in step (2) is 3 - 25 min.

11. The purification method according to claim 1, characterized in that the cooling rate in step (2) is 0.1 - 20°C / min.

12. The purification method according to claim 1, characterized in that in step (2), the final cooling temperature is reduced to 0 - 25°C.

13. The purification method according to claim 1, characterized in that the solvent used for washing the solid in step (2) is the same as the solvent in step (1).

14. The purification method according to claim 1, characterized in that the weight ratio of the solvent added for washing the solid in step (2) to the lactide is 0.05 - 2:

1.

15. The purification method according to claim 1, characterized in that the drying temperature in step (2) is 30 - 70°C and the time is 0.5 - 24 h.

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